This is elisp.info, produced by makeinfo version 6.3 from elisp.texi.

This is the ‘GNU Emacs Lisp Reference Manual’ corresponding to Emacs
version 25.3.

   Copyright © 1990–1996, 1998–2017 Free Software Foundation, Inc.

     Permission is granted to copy, distribute and/or modify this
     document under the terms of the GNU Free Documentation License,
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     included in the section entitled “GNU Free Documentation License.”

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INFO-DIR-SECTION Emacs lisp
START-INFO-DIR-ENTRY
* Elisp: (elisp).               The Emacs Lisp Reference Manual.
END-INFO-DIR-ENTRY


File: elisp.info,  Node: Top,  Next: Introduction,  Up: (dir)

Emacs Lisp
**********

This is the ‘GNU Emacs Lisp Reference Manual’ corresponding to Emacs
version 25.3.

   Copyright © 1990–1996, 1998–2017 Free Software Foundation, Inc.

     Permission is granted to copy, distribute and/or modify this
     document under the terms of the GNU Free Documentation License,
     Version 1.3 or any later version published by the Free Software
     Foundation; with the Invariant Sections being “GNU General Public
     License,” with the Front-Cover Texts being “A GNU Manual,” and with
     the Back-Cover Texts as in (a) below.  A copy of the license is
     included in the section entitled “GNU Free Documentation License.”

     (a) The FSF’s Back-Cover Text is: “You have the freedom to copy and
     modify this GNU manual.  Buying copies from the FSF supports it in
     developing GNU and promoting software freedom.”

* Menu:

* Introduction::            Introduction and conventions used.

* Lisp Data Types::         Data types of objects in Emacs Lisp.
* Numbers::                 Numbers and arithmetic functions.
* Strings and Characters::  Strings, and functions that work on them.
* Lists::                   Lists, cons cells, and related functions.
* Sequences Arrays Vectors::  Lists, strings and vectors are called sequences.
                                Certain functions act on any kind of sequence.
                                The description of vectors is here as well.
* Hash Tables::             Very fast lookup-tables.
* Symbols::                 Symbols represent names, uniquely.

* Evaluation::              How Lisp expressions are evaluated.
* Control Structures::      Conditionals, loops, nonlocal exits.
* Variables::               Using symbols in programs to stand for values.
* Functions::               A function is a Lisp program
                              that can be invoked from other functions.
* Macros::                  Macros are a way to extend the Lisp language.
* Customization::           Making variables and faces customizable.

* Loading::                 Reading files of Lisp code into Lisp.
* Byte Compilation::        Compilation makes programs run faster.
* Debugging::               Tools and tips for debugging Lisp programs.

* Read and Print::          Converting Lisp objects to text and back.
* Minibuffers::             Using the minibuffer to read input.
* Command Loop::            How the editor command loop works,
                              and how you can call its subroutines.
* Keymaps::                 Defining the bindings from keys to commands.
* Modes::                   Defining major and minor modes.
* Documentation::           Writing and using documentation strings.

* Files::                   Accessing files.
* Backups and Auto-Saving:: Controlling how backups and auto-save
                              files are made.
* Buffers::                 Creating and using buffer objects.
* Windows::                 Manipulating windows and displaying buffers.
* Frames::                  Making multiple system-level windows.
* Positions::               Buffer positions and motion functions.
* Markers::                 Markers represent positions and update
                              automatically when the text is changed.

* Text::                    Examining and changing text in buffers.
* Non-ASCII Characters::    Non-ASCII text in buffers and strings.
* Searching and Matching::  Searching buffers for strings or regexps.
* Syntax Tables::           The syntax table controls word and list parsing.
* Abbrevs::                 How Abbrev mode works, and its data structures.

* Processes::               Running and communicating with subprocesses.
* Display::                 Features for controlling the screen display.
* System Interface::        Getting the user id, system type, environment
                              variables, and other such things.

* Packaging::               Preparing Lisp code for distribution.

Appendices

* Antinews::                Info for users downgrading to Emacs 24.
* GNU Free Documentation License:: The license for this documentation.
* GPL::                     Conditions for copying and changing GNU Emacs.
* Tips::                    Advice and coding conventions for Emacs Lisp.
* GNU Emacs Internals::     Building and dumping Emacs;
                              internal data structures.
* Standard Errors::         List of some standard error symbols.
* Standard Keymaps::        List of some standard keymaps.
* Standard Hooks::          List of some standard hook variables.

* Index::                   Index including concepts, functions, variables,
                              and other terms.



 — The Detailed Node Listing —
 ———————————

Here are other nodes that are subnodes of those already listed,
mentioned here so you can get to them in one step:

Introduction

* Caveats::                 Flaws and a request for help.
* Lisp History::            Emacs Lisp is descended from Maclisp.
* Conventions::             How the manual is formatted.
* Version Info::            Which Emacs version is running?
* Acknowledgments::         The authors, editors, and sponsors of this manual.

Conventions

* Some Terms::              Explanation of terms we use in this manual.
* nil and t::               How the symbols ‘nil’ and ‘t’ are used.
* Evaluation Notation::     The format we use for examples of evaluation.
* Printing Notation::       The format we use when examples print text.
* Error Messages::          The format we use for examples of errors.
* Buffer Text Notation::    The format we use for buffer contents in examples.
* Format of Descriptions::  Notation for describing functions, variables, etc.

Format of Descriptions

* A Sample Function Description::  A description of an imaginary
                                     function, ‘foo’.
* A Sample Variable Description::  A description of an imaginary
                                     variable, ‘electric-future-map’.

Lisp Data Types

* Printed Representation::  How Lisp objects are represented as text.
* Comments::                Comments and their formatting conventions.
* Programming Types::       Types found in all Lisp systems.
* Editing Types::           Types specific to Emacs.
* Circular Objects::        Read syntax for circular structure.
* Type Predicates::         Tests related to types.
* Equality Predicates::     Tests of equality between any two objects.

Programming Types

* Integer Type::        Numbers without fractional parts.
* Floating-Point Type:: Numbers with fractional parts and with a large range.
* Character Type::      The representation of letters, numbers and
                          control characters.
* Symbol Type::         A multi-use object that refers to a function,
                        variable, or property list, and has a unique identity.
* Sequence Type::       Both lists and arrays are classified as sequences.
* Cons Cell Type::      Cons cells, and lists (which are made from cons cells).
* Array Type::          Arrays include strings and vectors.
* String Type::         An (efficient) array of characters.
* Vector Type::         One-dimensional arrays.
* Char-Table Type::     One-dimensional sparse arrays indexed by characters.
* Bool-Vector Type::    One-dimensional arrays of ‘t’ or ‘nil’.
* Hash Table Type::     Super-fast lookup tables.
* Function Type::       A piece of executable code you can call from elsewhere.
* Macro Type::          A method of expanding an expression into another
                          expression, more fundamental but less pretty.
* Primitive Function Type::     A function written in C, callable from Lisp.
* Byte-Code Type::      A function written in Lisp, then compiled.
* Autoload Type::       A type used for automatically loading seldom-used
                          functions.
* Finalizer Type::      Runs code when no longer reachable.

Character Type

* Basic Char Syntax::       Syntax for regular characters.
* General Escape Syntax::   How to specify characters by their codes.
* Ctl-Char Syntax::         Syntax for control characters.
* Meta-Char Syntax::        Syntax for meta-characters.
* Other Char Bits::         Syntax for hyper-, super-, and alt-characters.

Cons Cell and List Types

* Box Diagrams::            Drawing pictures of lists.
* Dotted Pair Notation::    A general syntax for cons cells.
* Association List Type::   A specially constructed list.

String Type

* Syntax for Strings::      How to specify Lisp strings.
* Non-ASCII in Strings::    International characters in strings.
* Nonprinting Characters::  Literal unprintable characters in strings.
* Text Props and Strings::  Strings with text properties.

Editing Types

* Buffer Type::             The basic object of editing.
* Marker Type::             A position in a buffer.
* Window Type::             Buffers are displayed in windows.
* Frame Type::              Windows subdivide frames.
* Terminal Type::           A terminal device displays frames.
* Window Configuration Type::  Recording the way a frame is subdivided.
* Frame Configuration Type::   Recording the status of all frames.
* Process Type::            A subprocess of Emacs running on the underlying OS.
* Stream Type::             Receive or send characters.
* Keymap Type::             What function a keystroke invokes.
* Overlay Type::            How an overlay is represented.
* Font Type::               Fonts for displaying text.

Numbers

* Integer Basics::          Representation and range of integers.
* Float Basics::            Representation and range of floating point.
* Predicates on Numbers::   Testing for numbers.
* Comparison of Numbers::   Equality and inequality predicates.
* Numeric Conversions::     Converting float to integer and vice versa.
* Arithmetic Operations::   How to add, subtract, multiply and divide.
* Rounding Operations::     Explicitly rounding floating-point numbers.
* Bitwise Operations::      Logical and, or, not, shifting.
* Math Functions::          Trig, exponential and logarithmic functions.
* Random Numbers::          Obtaining random integers, predictable or not.

Strings and Characters

* String Basics::           Basic properties of strings and characters.
* Predicates for Strings::  Testing whether an object is a string or char.
* Creating Strings::        Functions to allocate new strings.
* Modifying Strings::       Altering the contents of an existing string.
* Text Comparison::         Comparing characters or strings.
* String Conversion::       Converting to and from characters and strings.
* Formatting Strings::      ‘format’: Emacs’s analogue of ‘printf’.
* Case Conversion::         Case conversion functions.
* Case Tables::             Customizing case conversion.

Lists

* Cons Cells::              How lists are made out of cons cells.
* List-related Predicates:: Is this object a list?  Comparing two lists.
* List Elements::           Extracting the pieces of a list.
* Building Lists::          Creating list structure.
* List Variables::          Modifying lists stored in variables.
* Modifying Lists::         Storing new pieces into an existing list.
* Sets And Lists::          A list can represent a finite mathematical set.
* Association Lists::       A list can represent a finite relation or mapping.
* Property Lists::          A list of paired elements.

Modifying Existing List Structure

* Setcar::                  Replacing an element in a list.
* Setcdr::                  Replacing part of the list backbone.
                              This can be used to remove or add elements.
* Rearrangement::           Reordering the elements in a list; combining lists.

Property Lists

* Plists and Alists::       Comparison of the advantages of property
                              lists and association lists.
* Plist Access::            Accessing property lists stored elsewhere.

Sequences, Arrays, and Vectors

* Sequence Functions::      Functions that accept any kind of sequence.
* Arrays::                  Characteristics of arrays in Emacs Lisp.
* Array Functions::         Functions specifically for arrays.
* Vectors::                 Special characteristics of Emacs Lisp vectors.
* Vector Functions::        Functions specifically for vectors.
* Char-Tables::             How to work with char-tables.
* Bool-Vectors::            How to work with bool-vectors.
* Rings::                   Managing a fixed-size ring of objects.

Hash Tables

* Creating Hash::           Functions to create hash tables.
* Hash Access::             Reading and writing the hash table contents.
* Defining Hash::           Defining new comparison methods.
* Other Hash::              Miscellaneous.

Symbols

* Symbol Components::       Symbols have names, values, function definitions
                              and property lists.
* Definitions::             A definition says how a symbol will be used.
* Creating Symbols::        How symbols are kept unique.
* Symbol Properties::       Each symbol has a property list
                              for recording miscellaneous information.

Symbol Properties

* Symbol Plists::           Accessing symbol properties.
* Standard Properties::     Standard meanings of symbol properties.

Evaluation

* Intro Eval::              Evaluation in the scheme of things.
* Forms::                   How various sorts of objects are evaluated.
* Quoting::                 Avoiding evaluation (to put constants in
                              the program).
* Backquote::               Easier construction of list structure.
* Eval::                    How to invoke the Lisp interpreter explicitly.

Kinds of Forms

* Self-Evaluating Forms::   Forms that evaluate to themselves.
* Symbol Forms::            Symbols evaluate as variables.
* Classifying Lists::       How to distinguish various sorts of list forms.
* Function Indirection::    When a symbol appears as the car of a list,
                              we find the real function via the symbol.
* Function Forms::          Forms that call functions.
* Macro Forms::             Forms that call macros.
* Special Forms::           Special forms are idiosyncratic primitives,
                              most of them extremely important.
* Autoloading::             Functions set up to load files
                              containing their real definitions.

Control Structures

* Sequencing::              Evaluation in textual order.
* Conditionals::            ‘if’, ‘cond’, ‘when’, ‘unless’.
* Combining Conditions::    ‘and’, ‘or’, ‘not’.
* Iteration::               ‘while’ loops.
* Generators::              Generic sequences and coroutines.
* Nonlocal Exits::          Jumping out of a sequence.

Conditionals

* Pattern matching case statement::  How to use ‘pcase’.

Nonlocal Exits

* Catch and Throw::         Nonlocal exits for the program’s own purposes.
* Examples of Catch::       Showing how such nonlocal exits can be written.
* Errors::                  How errors are signaled and handled.
* Cleanups::                Arranging to run a cleanup form if an
                              error happens.

Errors

* Signaling Errors::        How to report an error.
* Processing of Errors::    What Emacs does when you report an error.
* Handling Errors::         How you can trap errors and continue execution.
* Error Symbols::           How errors are classified for trapping them.

Variables

* Global Variables::        Variable values that exist permanently, everywhere.
* Constant Variables::      Variables that never change.
* Local Variables::         Variable values that exist only temporarily.
* Void Variables::          Symbols that lack values.
* Defining Variables::      A definition says a symbol is used as a variable.
* Tips for Defining::       Things you should think about when you
                              define a variable.
* Accessing Variables::     Examining values of variables whose names
                              are known only at run time.
* Setting Variables::       Storing new values in variables.
* Variable Scoping::        How Lisp chooses among local and global values.
* Buffer-Local Variables::  Variable values in effect only in one buffer.
* File Local Variables::    Handling local variable lists in files.
* Directory Local Variables:: Local variables common to all files in a
                                directory.
* Variable Aliases::        Variables that are aliases for other variables.
* Variables with Restricted Values::  Non-constant variables whose value can
                                        _not_ be an arbitrary Lisp object.
* Generalized Variables::   Extending the concept of variables.

Scoping Rules for Variable Bindings

* Dynamic Binding::         The default for binding local variables in Emacs.
* Dynamic Binding Tips::    Avoiding problems with dynamic binding.
* Lexical Binding::         A different type of local variable binding.
* Using Lexical Binding::   How to enable lexical binding.

Buffer-Local Variables

* Intro to Buffer-Local::   Introduction and concepts.
* Creating Buffer-Local::   Creating and destroying buffer-local bindings.
* Default Value::           The default value is seen in buffers
                              that don’t have their own buffer-local values.

Generalized Variables

* Setting Generalized Variables::   The ‘setf’ macro.
* Adding Generalized Variables::    Defining new ‘setf’ forms.

Functions

* What Is a Function::      Lisp functions vs. primitives; terminology.
* Lambda Expressions::      How functions are expressed as Lisp objects.
* Function Names::          A symbol can serve as the name of a function.
* Defining Functions::      Lisp expressions for defining functions.
* Calling Functions::       How to use an existing function.
* Mapping Functions::       Applying a function to each element of a list, etc.
* Anonymous Functions::     Lambda expressions are functions with no names.
* Generic Functions::       Polymorphism, Emacs-style.
* Function Cells::          Accessing or setting the function definition
                              of a symbol.
* Closures::                Functions that enclose a lexical environment.
* Advising Functions::      Adding to the definition of a function.
* Obsolete Functions::      Declaring functions obsolete.
* Inline Functions::        Defining functions that the compiler
                              will expand inline.
* Declare Form::          Adding additional information about a function.
* Declaring Functions::     Telling the compiler that a function is defined.
* Function Safety::         Determining whether a function is safe to call.
* Related Topics::          Cross-references to specific Lisp primitives
                              that have a special bearing on how
                              functions work.

Lambda Expressions

* Lambda Components::       The parts of a lambda expression.
* Simple Lambda::           A simple example.
* Argument List::           Details and special features of argument lists.
* Function Documentation::  How to put documentation in a function.

Advising Emacs Lisp Functions

* Core Advising Primitives::  Primitives to manipulate advice.
* Advising Named Functions::  Advising named functions.
* Advice combinators::        Ways to compose advice.
* Porting old advice::        Adapting code using the old defadvice.

Macros

* Simple Macro::            A basic example.
* Expansion::               How, when and why macros are expanded.
* Compiling Macros::        How macros are expanded by the compiler.
* Defining Macros::         How to write a macro definition.
* Problems with Macros::    Don’t evaluate the macro arguments too many times.
                              Don’t hide the user’s variables.
* Indenting Macros::        Specifying how to indent macro calls.

Common Problems Using Macros

* Wrong Time::             Do the work in the expansion, not in the macro.
* Argument Evaluation::    The expansion should evaluate each macro arg once.
* Surprising Local Vars::  Local variable bindings in the expansion
                              require special care.
* Eval During Expansion::  Don’t evaluate them; put them in the expansion.
* Repeated Expansion::     Avoid depending on how many times expansion is done.

Customization Settings

* Common Keywords::         Common keyword arguments for all kinds of
                              customization declarations.
* Group Definitions::       Writing customization group definitions.
* Variable Definitions::    Declaring user options.
* Customization Types::     Specifying the type of a user option.
* Applying Customizations:: Functions to apply customization settings.
* Custom Themes::           Writing Custom themes.

Customization Types

* Simple Types::            Simple customization types: sexp, integer, etc.
* Composite Types::         Build new types from other types or data.
* Splicing into Lists::     Splice elements into list with ‘:inline’.
* Type Keywords::           Keyword-argument pairs in a customization type.
* Defining New Types::      Give your type a name.

Loading

* How Programs Do Loading:: The ‘load’ function and others.
* Load Suffixes::           Details about the suffixes that ‘load’ tries.
* Library Search::          Finding a library to load.
* Loading Non-ASCII::       Non-ASCII characters in Emacs Lisp files.
* Autoload::                Setting up a function to autoload.
* Repeated Loading::        Precautions about loading a file twice.
* Named Features::          Loading a library if it isn’t already loaded.
* Where Defined::           Finding which file defined a certain symbol.
* Unloading::               How to unload a library that was loaded.
* Hooks for Loading::       Providing code to be run when
                              particular libraries are loaded.
* Dynamic Modules::         Modules provide additional Lisp primitives.

Byte Compilation

* Speed of Byte-Code::      An example of speedup from byte compilation.
* Compilation Functions::   Byte compilation functions.
* Docs and Compilation::    Dynamic loading of documentation strings.
* Dynamic Loading::         Dynamic loading of individual functions.
* Eval During Compile::     Code to be evaluated when you compile.
* Compiler Errors::         Handling compiler error messages.
* Byte-Code Objects::       The data type used for byte-compiled functions.
* Disassembly::             Disassembling byte-code; how to read byte-code.

Debugging Lisp Programs

* Debugger::                A debugger for the Emacs Lisp evaluator.
* Edebug::                  A source-level Emacs Lisp debugger.
* Syntax Errors::           How to find syntax errors.
* Test Coverage::           Ensuring you have tested all branches in your code.
* Profiling::               Measuring the resources that your code uses.

The Lisp Debugger

* Error Debugging::         Entering the debugger when an error happens.
* Infinite Loops::          Stopping and debugging a program that doesn’t exit.
* Function Debugging::      Entering it when a certain function is called.
* Explicit Debug::          Entering it at a certain point in the program.
* Using Debugger::          What the debugger does; what you see while in it.
* Debugger Commands::       Commands used while in the debugger.
* Invoking the Debugger::   How to call the function ‘debug’.
* Internals of Debugger::   Subroutines of the debugger, and global variables.

Edebug

* Using Edebug::            Introduction to use of Edebug.
* Instrumenting::           You must instrument your code
                              in order to debug it with Edebug.
* Edebug Execution Modes::  Execution modes, stopping more or less often.
* Jumping::                 Commands to jump to a specified place.
* Edebug Misc::             Miscellaneous commands.
* Breaks::                  Setting breakpoints to make the program stop.
* Trapping Errors::         Trapping errors with Edebug.
* Edebug Views::            Views inside and outside of Edebug.
* Edebug Eval::             Evaluating expressions within Edebug.
* Eval List::               Expressions whose values are displayed
                              each time you enter Edebug.
* Printing in Edebug::      Customization of printing.
* Trace Buffer::            How to produce trace output in a buffer.
* Coverage Testing::        How to test evaluation coverage.
* The Outside Context::     Data that Edebug saves and restores.
* Edebug and Macros::       Specifying how to handle macro calls.
* Edebug Options::          Option variables for customizing Edebug.

Breaks

* Breakpoints::             Breakpoints at stop points.
* Global Break Condition::  Breaking on an event.
* Source Breakpoints::      Embedding breakpoints in source code.

The Outside Context

* Checking Whether to Stop::When Edebug decides what to do.
* Edebug Display Update::   When Edebug updates the display.
* Edebug Recursive Edit::   When Edebug stops execution.

Edebug and Macros

* Instrumenting Macro Calls::The basic problem.
* Specification List::      How to specify complex patterns of evaluation.
* Backtracking::            What Edebug does when matching fails.
* Specification Examples::  To help understand specifications.

Debugging Invalid Lisp Syntax

* Excess Open::             How to find a spurious open paren or missing close.
* Excess Close::            How to find a spurious close paren or missing open.

Reading and Printing Lisp Objects

* Streams Intro::           Overview of streams, reading and printing.
* Input Streams::           Various data types that can be used as
                              input streams.
* Input Functions::         Functions to read Lisp objects from text.
* Output Streams::          Various data types that can be used as
                              output streams.
* Output Functions::        Functions to print Lisp objects as text.
* Output Variables::        Variables that control what the printing
                              functions do.

Minibuffers

* Intro to Minibuffers::    Basic information about minibuffers.
* Text from Minibuffer::    How to read a straight text string.
* Object from Minibuffer::  How to read a Lisp object or expression.
* Minibuffer History::      Recording previous minibuffer inputs
                              so the user can reuse them.
* Initial Input::           Specifying initial contents for the minibuffer.
* Completion::              How to invoke and customize completion.
* Yes-or-No Queries::       Asking a question with a simple answer.
* Multiple Queries::        Asking a series of similar questions.
* Reading a Password::      Reading a password from the terminal.
* Minibuffer Commands::     Commands used as key bindings in minibuffers.
* Minibuffer Windows::      Operating on the special minibuffer windows.
* Minibuffer Contents::     How such commands access the minibuffer text.
* Recursive Mini::          Whether recursive entry to minibuffer is allowed.
* Minibuffer Misc::         Various customization hooks and variables.

Completion

* Basic Completion::        Low-level functions for completing strings.
* Minibuffer Completion::   Invoking the minibuffer with completion.
* Completion Commands::     Minibuffer commands that do completion.
* High-Level Completion::   Convenient special cases of completion
                              (reading buffer names, variable names, etc.).
* Reading File Names::      Using completion to read file names and
                              shell commands.
* Completion Variables::    Variables controlling completion behavior.
* Programmed Completion::   Writing your own completion function.
* Completion in Buffers::   Completing text in ordinary buffers.

Command Loop

* Command Overview::    How the command loop reads commands.
* Defining Commands::   Specifying how a function should read arguments.
* Interactive Call::    Calling a command, so that it will read arguments.
* Distinguish Interactive::     Making a command distinguish interactive calls.
* Command Loop Info::   Variables set by the command loop for you to examine.
* Adjusting Point::     Adjustment of point after a command.
* Input Events::        What input looks like when you read it.
* Reading Input::       How to read input events from the keyboard or mouse.
* Special Events::      Events processed immediately and individually.
* Waiting::             Waiting for user input or elapsed time.
* Quitting::            How ‘C-g’ works.  How to catch or defer quitting.
* Prefix Command Arguments::    How the commands to set prefix args work.
* Recursive Editing::   Entering a recursive edit,
                          and why you usually shouldn’t.
* Disabling Commands::  How the command loop handles disabled commands.
* Command History::     How the command history is set up, and how accessed.
* Keyboard Macros::     How keyboard macros are implemented.

Defining Commands

* Using Interactive::       General rules for ‘interactive’.
* Interactive Codes::       The standard letter-codes for reading arguments
                              in various ways.
* Interactive Examples::    Examples of how to read interactive arguments.
* Generic Commands::        Select among command alternatives.


Input Events

* Keyboard Events::         Ordinary characters – keys with symbols on them.
* Function Keys::           Function keys – keys with names, not symbols.
* Mouse Events::            Overview of mouse events.
* Click Events::            Pushing and releasing a mouse button.
* Drag Events::             Moving the mouse before releasing the button.
* Button-Down Events::      A button was pushed and not yet released.
* Repeat Events::           Double and triple click (or drag, or down).
* Motion Events::           Just moving the mouse, not pushing a button.
* Focus Events::            Moving the mouse between frames.
* Misc Events::             Other events the system can generate.
* Event Examples::          Examples of the lists for mouse events.
* Classifying Events::      Finding the modifier keys in an event symbol.
                              Event types.
* Accessing Mouse::         Functions to extract info from mouse events.
* Accessing Scroll::        Functions to get info from scroll bar events.
* Strings of Events::       Special considerations for putting
                              keyboard character events in a string.

Reading Input

* Key Sequence Input::      How to read one key sequence.
* Reading One Event::       How to read just one event.
* Event Mod::               How Emacs modifies events as they are read.
* Invoking the Input Method::   How reading an event uses the input method.
* Quoted Character Input::  Asking the user to specify a character.
* Event Input Misc::        How to reread or throw away input events.

Keymaps

* Key Sequences::           Key sequences as Lisp objects.
* Keymap Basics::           Basic concepts of keymaps.
* Format of Keymaps::       What a keymap looks like as a Lisp object.
* Creating Keymaps::        Functions to create and copy keymaps.
* Inheritance and Keymaps:: How one keymap can inherit the bindings
                              of another keymap.
* Prefix Keys::             Defining a key with a keymap as its definition.
* Active Keymaps::          How Emacs searches the active keymaps
                              for a key binding.
* Searching Keymaps::       A pseudo-Lisp summary of searching active maps.
* Controlling Active Maps:: Each buffer has a local keymap
                               to override the standard (global) bindings.
                               A minor mode can also override them.
* Key Lookup::              Finding a key’s binding in one keymap.
* Functions for Key Lookup::    How to request key lookup.
* Changing Key Bindings::   Redefining a key in a keymap.
* Remapping Commands::      A keymap can translate one command to another.
* Translation Keymaps::     Keymaps for translating sequences of events.
* Key Binding Commands::    Interactive interfaces for redefining keys.
* Scanning Keymaps::        Looking through all keymaps, for printing help.
* Menu Keymaps::            Defining a menu as a keymap.

Menu Keymaps

* Defining Menus::          How to make a keymap that defines a menu.
* Mouse Menus::             How users actuate the menu with the mouse.
* Keyboard Menus::          How users actuate the menu with the keyboard.
* Menu Example::            Making a simple menu.
* Menu Bar::                How to customize the menu bar.
* Tool Bar::                A tool bar is a row of images.
* Modifying Menus::         How to add new items to a menu.
* Easy Menu::               A convenience macro for defining menus.

Defining Menus

* Simple Menu Items::       A simple kind of menu key binding.
* Extended Menu Items::     More complex menu item definitions.
* Menu Separators::         Drawing a horizontal line through a menu.
* Alias Menu Items::        Using command aliases in menu items.

Major and Minor Modes

* Hooks::              How to use hooks; how to write code that provides hooks.
* Major Modes::        Defining major modes.
* Minor Modes::        Defining minor modes.
* Mode Line Format::   Customizing the text that appears in the mode line.
* Imenu::              Providing a menu of definitions made in a buffer.
* Font Lock Mode::     How modes can highlight text according to syntax.
* Auto-Indentation::   How to teach Emacs to indent for a major mode.
* Desktop Save Mode::  How modes can have buffer state saved between
                         Emacs sessions.

Hooks

* Running Hooks::      How to run a hook.
* Setting Hooks::      How to put functions on a hook, or remove them.

Major Modes

* Major Mode Conventions::  Coding conventions for keymaps, etc.
* Auto Major Mode::         How Emacs chooses the major mode automatically.
* Mode Help::               Finding out how to use a mode.
* Derived Modes::           Defining a new major mode based on another major
                              mode.
* Basic Major Modes::       Modes that other modes are often derived from.
* Mode Hooks::              Hooks run at the end of major mode functions.
* Tabulated List Mode::     Parent mode for buffers containing tabulated data.
* Generic Modes::           Defining a simple major mode that supports
                              comment syntax and Font Lock mode.
* Example Major Modes::     Text mode and Lisp modes.

Minor Modes

* Minor Mode Conventions::  Tips for writing a minor mode.
* Keymaps and Minor Modes:: How a minor mode can have its own keymap.
* Defining Minor Modes::    A convenient facility for defining minor modes.

Mode Line Format

* Mode Line Basics::        Basic ideas of mode line control.
* Mode Line Data::          The data structure that controls the mode line.
* Mode Line Top::           The top level variable, mode-line-format.
* Mode Line Variables::     Variables used in that data structure.
* %-Constructs::            Putting information into a mode line.
* Properties in Mode::      Using text properties in the mode line.
* Header Lines::            Like a mode line, but at the top.
* Emulating Mode Line::     Formatting text as the mode line would.

Font Lock Mode

* Font Lock Basics::        Overview of customizing Font Lock.
* Search-based Fontification::  Fontification based on regexps.
* Customizing Keywords::    Customizing search-based fontification.
* Other Font Lock Variables::   Additional customization facilities.
* Levels of Font Lock::     Each mode can define alternative levels
                              so that the user can select more or less.
* Precalculated Fontification:: How Lisp programs that produce the buffer
                                  contents can also specify how to fontify it.
* Faces for Font Lock::     Special faces specifically for Font Lock.
* Syntactic Font Lock::     Fontification based on syntax tables.
* Multiline Font Lock::     How to coerce Font Lock into properly
                              highlighting multiline constructs.

Multiline Font Lock Constructs

* Font Lock Multiline::     Marking multiline chunks with a text property.
* Region to Refontify::     Controlling which region gets refontified
                              after a buffer change.

Automatic Indentation of code

* SMIE::                    A simple minded indentation engine.

Simple Minded Indentation Engine

* SMIE setup::              SMIE setup and features.
* Operator Precedence Grammars:: A very simple parsing technique.
* SMIE Grammar::            Defining the grammar of a language.
* SMIE Lexer::              Defining tokens.
* SMIE Tricks::             Working around the parser’s limitations.
* SMIE Indentation::        Specifying indentation rules.
* SMIE Indentation Helpers:: Helper functions for indentation rules.
* SMIE Indentation Example:: Sample indentation rules.
* SMIE Customization::      Customizing indentation.

Documentation

* Documentation Basics::    Where doc strings are defined and stored.
* Accessing Documentation:: How Lisp programs can access doc strings.
* Keys in Documentation::   Substituting current key bindings.
* Describing Characters::   Making printable descriptions of
                              non-printing characters and key sequences.
* Help Functions::          Subroutines used by Emacs help facilities.

Files

* Visiting Files::          Reading files into Emacs buffers for editing.
* Saving Buffers::          Writing changed buffers back into files.
* Reading from Files::      Reading files into buffers without visiting.
* Writing to Files::        Writing new files from parts of buffers.
* File Locks::              Locking and unlocking files, to prevent
                              simultaneous editing by two people.
* Information about Files:: Testing existence, accessibility, size of files.
* Changing Files::          Renaming files, changing permissions, etc.
* File Names::              Decomposing and expanding file names.
* Contents of Directories:: Getting a list of the files in a directory.
* Create/Delete Dirs::      Creating and Deleting Directories.
* Magic File Names::        Special handling for certain file names.
* Format Conversion::       Conversion to and from various file formats.

Visiting Files

* Visiting Functions::      The usual interface functions for visiting.
* Subroutines of Visiting:: Lower-level subroutines that they use.

Information about Files

* Testing Accessibility::   Is a given file readable?  Writable?
* Kinds of Files::          Is it a directory?  A symbolic link?
* Truenames::               Eliminating symbolic links from a file name.
* File Attributes::         File sizes, modification times, etc.
* Extended Attributes::     Extended file attributes for access control.
* Locating Files::          How to find a file in standard places.

File Names

* File Name Components::    The directory part of a file name, and the rest.
* Relative File Names::     Some file names are relative to a current directory.
* Directory Names::         A directory’s name as a directory
                              is different from its name as a file.
* File Name Expansion::     Converting relative file names to absolute ones.
* Unique File Names::       Generating names for temporary files.
* File Name Completion::    Finding the completions for a given file name.
* Standard File Names::     If your package uses a fixed file name,
                              how to handle various operating systems simply.

File Format Conversion

* Format Conversion Overview::   ‘insert-file-contents’ and ‘write-region’.
* Format Conversion Round-Trip:: Using ‘format-alist’.
* Format Conversion Piecemeal::  Specifying non-paired conversion.

Backups and Auto-Saving

* Backup Files::            How backup files are made; how their names
                              are chosen.
* Auto-Saving::             How auto-save files are made; how their
                              names are chosen.
* Reverting::               ‘revert-buffer’, and how to customize
                              what it does.

Backup Files

* Making Backups::          How Emacs makes backup files, and when.
* Rename or Copy::          Two alternatives: renaming the old file
                              or copying it.
* Numbered Backups::        Keeping multiple backups for each source file.
* Backup Names::            How backup file names are computed; customization.

Buffers

* Buffer Basics::           What is a buffer?
* Current Buffer::          Designating a buffer as current
                              so that primitives will access its contents.
* Buffer Names::            Accessing and changing buffer names.
* Buffer File Name::        The buffer file name indicates which file
                              is visited.
* Buffer Modification::     A buffer is “modified” if it needs to be saved.
* Modification Time::       Determining whether the visited file was changed
                              behind Emacs’s back.
* Read Only Buffers::       Modifying text is not allowed in a
                              read-only buffer.
* Buffer List::             How to look at all the existing buffers.
* Creating Buffers::        Functions that create buffers.
* Killing Buffers::         Buffers exist until explicitly killed.
* Indirect Buffers::        An indirect buffer shares text with some
                              other buffer.
* Swapping Text::           Swapping text between two buffers.
* Buffer Gap::              The gap in the buffer.

Windows

* Basic Windows::           Basic information on using windows.
* Windows and Frames::      Relating windows to the frame they appear on.
* Window Sizes::            Accessing a window’s size.
* Resizing Windows::        Changing the sizes of windows.
* Preserving Window Sizes:: Preserving the size of windows.
* Splitting Windows::       Splitting one window into two windows.
* Deleting Windows::        Deleting a window gives its space to other windows.
* Recombining Windows::     Preserving the frame layout when splitting and
                              deleting windows.
* Selecting Windows::       The selected window is the one that you edit in.
* Cyclic Window Ordering::  Moving around the existing windows.
* Buffers and Windows::     Each window displays the contents of a buffer.
* Switching Buffers::       Higher-level functions for switching to a buffer.
* Choosing Window::         How to choose a window for displaying a buffer.
* Display Action Functions:: Subroutines for ‘display-buffer’.
* Choosing Window Options:: Extra options affecting how buffers are displayed.
* Window History::          Each window remembers the buffers displayed in it.
* Dedicated Windows::       How to avoid displaying another buffer in
                              a specific window.
* Quitting Windows::        How to restore the state prior to displaying a
                              buffer.
* Window Point::            Each window has its own location of point.
* Window Start and End::    Buffer positions indicating which text is
                              on-screen in a window.
* Textual Scrolling::       Moving text up and down through the window.
* Vertical Scrolling::      Moving the contents up and down on the window.
* Horizontal Scrolling::    Moving the contents sideways on the window.
* Coordinates and Windows:: Converting coordinates to windows.
* Window Configurations::   Saving and restoring the state of the screen.
* Window Parameters::       Associating additional information with windows.
* Window Hooks::            Hooks for scrolling, window size changes,
                              redisplay going past a certain point,
                              or window configuration changes.

Frames

* Creating Frames::         Creating additional frames.
* Multiple Terminals::      Displaying on several different devices.
* Frame Geometry::          Geometric properties of frames.
* Frame Parameters::        Controlling frame size, position, font, etc.
* Terminal Parameters::     Parameters common for all frames on terminal.
* Frame Titles::            Automatic updating of frame titles.
* Deleting Frames::         Frames last until explicitly deleted.
* Finding All Frames::      How to examine all existing frames.
* Minibuffers and Frames::  How a frame finds the minibuffer to use.
* Input Focus::             Specifying the selected frame.
* Visibility of Frames::    Frames may be visible or invisible, or icons.
* Raising and Lowering::    Raising a frame makes it hide other windows;
                              lowering it makes the others hide it.
* Frame Configurations::    Saving the state of all frames.
* Mouse Tracking::          Getting events that say when the mouse moves.
* Mouse Position::          Asking where the mouse is, or moving it.
* Pop-Up Menus::            Displaying a menu for the user to select from.
* Dialog Boxes::            Displaying a box to ask yes or no.
* Pointer Shape::           Specifying the shape of the mouse pointer.
* Window System Selections::Transferring text to and from other X clients.
* Drag and Drop::               Internals of Drag-and-Drop implementation.
* Color Names::             Getting the definitions of color names.
* Text Terminal Colors::    Defining colors for text terminals.
* Resources::               Getting resource values from the server.
* Display Feature Testing:: Determining the features of a terminal.

Frame Geometry

* Frame Layout::            Basic layout of frames.
* Frame Font::              The default font of a frame and how to set it.
* Size and Position::       Changing the size and position of a frame.
* Implied Frame Resizing::  Implied resizing of frames and how to prevent it.

Frame Parameters

* Parameter Access::        How to change a frame’s parameters.
* Initial Parameters::      Specifying frame parameters when you make a frame.
* Window Frame Parameters:: List of frame parameters for window systems.
* Geometry::                Parsing geometry specifications.

Window Frame Parameters

* Basic Parameters::        Parameters that are fundamental.
* Position Parameters::     The position of the frame on the screen.
* Size Parameters::         Frame’s size.
* Layout Parameters::       Size of parts of the frame, and
                              enabling or disabling some parts.
* Buffer Parameters::       Which buffers have been or should be shown.
* Management Parameters::   Communicating with the window manager.
* Cursor Parameters::       Controlling the cursor appearance.
* Font and Color Parameters:: Fonts and colors for the frame text.

Positions

* Point::                   The special position where editing takes place.
* Motion::                  Changing point.
* Excursions::              Temporary motion and buffer changes.
* Narrowing::               Restricting editing to a portion of the buffer.

Motion

* Character Motion::        Moving in terms of characters.
* Word Motion::             Moving in terms of words.
* Buffer End Motion::       Moving to the beginning or end of the buffer.
* Text Lines::              Moving in terms of lines of text.
* Screen Lines::            Moving in terms of lines as displayed.
* List Motion::             Moving by parsing lists and sexps.
* Skipping Characters::     Skipping characters belonging to a certain set.

Markers

* Overview of Markers::     The components of a marker, and how it relocates.
* Predicates on Markers::   Testing whether an object is a marker.
* Creating Markers::        Making empty markers or markers at certain places.
* Information from Markers::Finding the marker’s buffer or character position.
* Marker Insertion Types::  Two ways a marker can relocate when you
                              insert where it points.
* Moving Markers::          Moving the marker to a new buffer or position.
* The Mark::                How the mark is implemented with a marker.
* The Region::              How to access the region.

Text

* Near Point::              Examining text in the vicinity of point.
* Buffer Contents::         Examining text in a general fashion.
* Comparing Text::          Comparing substrings of buffers.
* Insertion::               Adding new text to a buffer.
* Commands for Insertion::  User-level commands to insert text.
* Deletion::                Removing text from a buffer.
* User-Level Deletion::     User-level commands to delete text.
* The Kill Ring::           Where removed text sometimes is saved for
                              later use.
* Undo::                    Undoing changes to the text of a buffer.
* Maintaining Undo::        How to enable and disable undo information.
                              How to control how much information is kept.
* Filling::                 Functions for explicit filling.
* Margins::                 How to specify margins for filling commands.
* Adaptive Fill::           Adaptive Fill mode chooses a fill prefix
                              from context.
* Auto Filling::            How auto-fill mode is implemented to break lines.
* Sorting::                 Functions for sorting parts of the buffer.
* Columns::                 Computing horizontal positions, and using them.
* Indentation::             Functions to insert or adjust indentation.
* Case Changes::            Case conversion of parts of the buffer.
* Text Properties::         Assigning Lisp property lists to text characters.
* Substitution::            Replacing a given character wherever it appears.
* Registers::               How registers are implemented.  Accessing
                              the text or position stored in a register.
* Transposition::           Swapping two portions of a buffer.
* Decompression::           Dealing with compressed data.
* Base 64::                 Conversion to or from base 64 encoding.
* Checksum/Hash::           Computing cryptographic hashes.
* Parsing HTML/XML::        Parsing HTML and XML.
* Atomic Changes::          Installing several buffer changes atomically.
* Change Hooks::            Supplying functions to be run when text is changed.

The Kill Ring

* Kill Ring Concepts::      What text looks like in the kill ring.
* Kill Functions::          Functions that kill text.
* Yanking::                 How yanking is done.
* Yank Commands::           Commands that access the kill ring.
* Low-Level Kill Ring::     Functions and variables for kill ring access.
* Internals of Kill Ring::  Variables that hold kill ring data.

Indentation

* Primitive Indent::        Functions used to count and insert indentation.
* Mode-Specific Indent::    Customize indentation for different modes.
* Region Indent::           Indent all the lines in a region.
* Relative Indent::         Indent the current line based on previous lines.
* Indent Tabs::             Adjustable, typewriter-like tab stops.
* Motion by Indent::        Move to first non-blank character.

Text Properties

* Examining Properties::    Looking at the properties of one character.
* Changing Properties::     Setting the properties of a range of text.
* Property Search::         Searching for where a property changes value.
* Special Properties::      Particular properties with special meanings.
* Format Properties::       Properties for representing formatting of text.
* Sticky Properties::       How inserted text gets properties from
                              neighboring text.
* Lazy Properties::         Computing text properties in a lazy fashion
                              only when text is examined.
* Clickable Text::          Using text properties to make regions of text
                              do something when you click on them.
* Fields::                  The ‘field’ property defines
                              fields within the buffer.
* Not Intervals::           Why text properties do not use
                              Lisp-visible text intervals.

Parsing HTML and XML

* Document Object Model::   Access, manipulate and search the DOM.

Non-ASCII Characters

* Text Representations::    How Emacs represents text.
* Disabling Multibyte::     Controlling whether to use multibyte characters.
* Converting Representations::  Converting unibyte to multibyte and vice versa.
* Selecting a Representation::  Treating a byte sequence as unibyte or multi.
* Character Codes::         How unibyte and multibyte relate to
                                codes of individual characters.
* Character Properties::    Character attributes that define their
                                behavior and handling.
* Character Sets::          The space of possible character codes
                                is divided into various character sets.
* Scanning Charsets::       Which character sets are used in a buffer?
* Translation of Characters::   Translation tables are used for conversion.
* Coding Systems::          Coding systems are conversions for saving files.
* Input Methods::           Input methods allow users to enter various
                                non-ASCII characters without special keyboards.
* Locales::                 Interacting with the POSIX locale.

Coding Systems

* Coding System Basics::    Basic concepts.
* Encoding and I/O::        How file I/O functions handle coding systems.
* Lisp and Coding Systems:: Functions to operate on coding system names.
* User-Chosen Coding Systems::  Asking the user to choose a coding system.
* Default Coding Systems::  Controlling the default choices.
* Specifying Coding Systems::   Requesting a particular coding system
                                    for a single file operation.
* Explicit Encoding::       Encoding or decoding text without doing I/O.
* Terminal I/O Encoding::   Use of encoding for terminal I/O.

Searching and Matching

* String Search::           Search for an exact match.
* Searching and Case::      Case-independent or case-significant searching.
* Regular Expressions::     Describing classes of strings.
* Regexp Search::           Searching for a match for a regexp.
* POSIX Regexps::           Searching POSIX-style for the longest match.
* Match Data::              Finding out which part of the text matched,
                              after a string or regexp search.
* Search and Replace::      Commands that loop, searching and replacing.
* Standard Regexps::        Useful regexps for finding sentences, pages,...

Regular Expressions

* Syntax of Regexps::       Rules for writing regular expressions.
* Regexp Example::          Illustrates regular expression syntax.
* Regexp Functions::        Functions for operating on regular expressions.

Syntax of Regular Expressions

* Regexp Special::          Special characters in regular expressions.
* Char Classes::            Character classes used in regular expressions.
* Regexp Backslash::        Backslash-sequences in regular expressions.

The Match Data

* Replacing Match::         Replacing a substring that was matched.
* Simple Match Data::       Accessing single items of match data,
                              such as where a particular subexpression started.
* Entire Match Data::       Accessing the entire match data at once, as a list.
* Saving Match Data::       Saving and restoring the match data.

Syntax Tables

* Syntax Basics::           Basic concepts of syntax tables.
* Syntax Descriptors::      How characters are classified.
* Syntax Table Functions::  How to create, examine and alter syntax tables.
* Syntax Properties::       Overriding syntax with text properties.
* Motion and Syntax::       Moving over characters with certain syntaxes.
* Parsing Expressions::     Parsing balanced expressions
                              using the syntax table.
* Syntax Table Internals::  How syntax table information is stored.
* Categories::              Another way of classifying character syntax.

Syntax Descriptors

* Syntax Class Table::      Table of syntax classes.
* Syntax Flags::            Additional flags each character can have.

Parsing Expressions

* Motion via Parsing::      Motion functions that work by parsing.
* Position Parse::          Determining the syntactic state of a position.
* Parser State::            How Emacs represents a syntactic state.
* Low-Level Parsing::       Parsing across a specified region.
* Control Parsing::         Parameters that affect parsing.

Abbrevs and Abbrev Expansion

* Abbrev Tables::           Creating and working with abbrev tables.
* Defining Abbrevs::        Specifying abbreviations and their expansions.
* Abbrev Files::            Saving abbrevs in files.
* Abbrev Expansion::        Controlling expansion; expansion subroutines.
* Standard Abbrev Tables::  Abbrev tables used by various major modes.
* Abbrev Properties::       How to read and set abbrev properties.
                            Which properties have which effect.
* Abbrev Table Properties:: How to read and set abbrev table properties.
                            Which properties have which effect.

Processes

* Subprocess Creation::     Functions that start subprocesses.
* Shell Arguments::         Quoting an argument to pass it to a shell.
* Synchronous Processes::   Details of using synchronous subprocesses.
* Asynchronous Processes::  Starting up an asynchronous subprocess.
* Deleting Processes::      Eliminating an asynchronous subprocess.
* Process Information::     Accessing run-status and other attributes.
* Input to Processes::      Sending input to an asynchronous subprocess.
* Signals to Processes::    Stopping, continuing or interrupting
                              an asynchronous subprocess.
* Output from Processes::   Collecting output from an asynchronous subprocess.
* Sentinels::               Sentinels run when process run-status changes.
* Query Before Exit::       Whether to query if exiting will kill a process.
* System Processes::        Accessing other processes running on your system.
* Transaction Queues::      Transaction-based communication with subprocesses.
* Network::                 Opening network connections.
* Network Servers::         Network servers let Emacs accept net connections.
* Datagrams::               UDP network connections.
* Low-Level Network::       Lower-level but more general function
                              to create connections and servers.
* Misc Network::            Additional relevant functions for net connections.
* Serial Ports::            Communicating with serial ports.
* Byte Packing::            Using bindat to pack and unpack binary data.

Receiving Output from Processes

* Process Buffers::         By default, output is put in a buffer.
* Filter Functions::        Filter functions accept output from the process.
* Decoding Output::         Filters can get unibyte or multibyte strings.
* Accepting Output::        How to wait until process output arrives.

Low-Level Network Access

* Network Processes::       Using ‘make-network-process’.
* Network Options::         Further control over network connections.
* Network Feature Testing:: Determining which network features work on
                              the machine you are using.

Packing and Unpacking Byte Arrays

* Bindat Spec::             Describing data layout.
* Bindat Functions::        Doing the unpacking and packing.
* Bindat Examples::         Samples of what bindat.el can do for you!

Emacs Display

* Refresh Screen::          Clearing the screen and redrawing everything on it.
* Forcing Redisplay::       Forcing redisplay.
* Truncation::              Folding or wrapping long text lines.
* The Echo Area::           Displaying messages at the bottom of the screen.
* Warnings::                Displaying warning messages for the user.
* Invisible Text::          Hiding part of the buffer text.
* Selective Display::       Hiding part of the buffer text (the old way).
* Temporary Displays::      Displays that go away automatically.
* Overlays::                Use overlays to highlight parts of the buffer.
* Size of Displayed Text::  How large displayed text is.
* Line Height::             Controlling the height of lines.
* Faces::                   A face defines a graphics style
                              for text characters: font, colors, etc.
* Fringes::                 Controlling window fringes.
* Scroll Bars::             Controlling scroll bars.
* Window Dividers::         Separating windows visually.
* Display Property::        Enabling special display features.
* Images::                  Displaying images in Emacs buffers.
* Buttons::                 Adding clickable buttons to Emacs buffers.
* Abstract Display::        Emacs’s Widget for Object Collections.
* Blinking::                How Emacs shows the matching open parenthesis.
* Character Display::       How Emacs displays individual characters.
* Beeping::                 Audible signal to the user.
* Window Systems::          Which window system is being used.
* Tooltips::                Tooltip display in Emacs.
* Bidirectional Display::   Display of bidirectional scripts, such as
                              Arabic and Farsi.

The Echo Area

* Displaying Messages::     Explicitly displaying text in the echo area.
* Progress::                Informing user about progress of a long operation.
* Logging Messages::        Echo area messages are logged for the user.
* Echo Area Customization:: Controlling the echo area.

Reporting Warnings

* Warning Basics::          Warnings concepts and functions to report them.
* Warning Variables::       Variables programs bind to customize
                              their warnings.
* Warning Options::         Variables users set to control display of warnings.
* Delayed Warnings::        Deferring a warning until the end of a command.

Overlays

* Managing Overlays::       Creating and moving overlays.
* Overlay Properties::      How to read and set properties.
                              What properties do to the screen display.
* Finding Overlays::        Searching for overlays.

Faces

* Face Attributes::         What is in a face?
* Defining Faces::          How to define a face.
* Attribute Functions::     Functions to examine and set face attributes.
* Displaying Faces::        How Emacs combines the faces specified for
                              a character.
* Face Remapping::          Remapping faces to alternative definitions.
* Face Functions::          How to define and examine faces.
* Auto Faces::              Hook for automatic face assignment.
* Basic Faces::             Faces that are defined by default.
* Font Selection::          Finding the best available font for a face.
* Font Lookup::             Looking up the names of available fonts
                              and information about them.
* Fontsets::                A fontset is a collection of fonts
                              that handle a range of character sets.
* Low-Level Font::          Lisp representation for character display fonts.

Fringes

* Fringe Size/Pos::         Specifying where to put the window fringes.
* Fringe Indicators::       Displaying indicator icons in the window fringes.
* Fringe Cursors::          Displaying cursors in the right fringe.
* Fringe Bitmaps::          Specifying bitmaps for fringe indicators.
* Customizing Bitmaps::     Specifying your own bitmaps to use in the fringes.
* Overlay Arrow::           Display of an arrow to indicate position.

The ‘display’ Property

* Replacing Specs::         Display specs that replace the text.
* Specified Space::         Displaying one space with a specified width.
* Pixel Specification::     Specifying space width or height in pixels.
* Other Display Specs::     Displaying an image; adjusting the height,
                              spacing, and other properties of text.
* Display Margins::         Displaying text or images to the side of
                              the main text.

Images

* Image Formats::           Supported image formats.
* Image Descriptors::       How to specify an image for use in ‘:display’.
* XBM Images::              Special features for XBM format.
* XPM Images::              Special features for XPM format.
* PostScript Images::       Special features for PostScript format.
* ImageMagick Images::      Special features available through ImageMagick.
* Other Image Types::       Various other formats are supported.
* Defining Images::         Convenient ways to define an image for later use.
* Showing Images::          Convenient ways to display an image once
                              it is defined.
* Multi-Frame Images::      Some images contain more than one frame.
* Image Cache::             Internal mechanisms of image display.

Buttons

* Button Properties::       Button properties with special meanings.
* Button Types::            Defining common properties for classes of buttons.
* Making Buttons::          Adding buttons to Emacs buffers.
* Manipulating Buttons::    Getting and setting properties of buttons.
* Button Buffer Commands::  Buffer-wide commands and bindings for buttons.

Abstract Display

* Abstract Display Functions::  Functions in the Ewoc package.
* Abstract Display Example::    Example of using Ewoc.

Character Display

* Usual Display::       The usual conventions for displaying characters.
* Display Tables::      What a display table consists of.
* Active Display Table::  How Emacs selects a display table to use.
* Glyphs::              How to define a glyph, and what glyphs mean.
* Glyphless Chars::     How glyphless characters are drawn.

Operating System Interface

* Starting Up::             Customizing Emacs startup processing.
* Getting Out::             How exiting works (permanent or temporary).
* System Environment::      Distinguish the name and kind of system.
* User Identification::     Finding the name and user id of the user.
* Time of Day::             Getting the current time.
* Time Conversion::         Converting a time from numeric form to
                              calendrical data and vice versa.
* Time Parsing::            Converting a time from numeric form to text
                              and vice versa.
* Processor Run Time::      Getting the run time used by Emacs.
* Time Calculations::       Adding, subtracting, comparing times, etc.
* Timers::                  Setting a timer to call a function at a
                              certain time.
* Idle Timers::             Setting a timer to call a function when Emacs has
                              been idle for a certain length of time.
* Terminal Input::          Accessing and recording terminal input.
* Terminal Output::         Controlling and recording terminal output.
* Sound Output::            Playing sounds on the computer’s speaker.
* X11 Keysyms::             Operating on key symbols for X Windows.
* Batch Mode::              Running Emacs without terminal interaction.
* Session Management::      Saving and restoring state with
                              X Session Management.
* Desktop Notifications::   Desktop notifications.
* File Notifications::      File notifications.
* Dynamic Libraries::       On-demand loading of support libraries.
* Security Considerations:: Running Emacs in an unfriendly environment.

Starting Up Emacs

* Startup Summary::         Sequence of actions Emacs performs at startup.
* Init File::               Details on reading the init file.
* Terminal-Specific::       How the terminal-specific Lisp file is read.
* Command-Line Arguments::  How command-line arguments are processed,
                              and how you can customize them.

Getting Out of Emacs

* Killing Emacs::           Exiting Emacs irreversibly.
* Suspending Emacs::        Exiting Emacs reversibly.

Terminal Input

* Input Modes::             Options for how input is processed.
* Recording Input::         Saving histories of recent or all input events.

Preparing Lisp code for distribution

* Packaging Basics::        The basic concepts of Emacs Lisp packages.
* Simple Packages::         How to package a single .el file.
* Multi-file Packages::     How to package multiple files.
* Package Archives::        Maintaining package archives.

Tips and Conventions

* Coding Conventions::      Conventions for clean and robust programs.
* Key Binding Conventions:: Which keys should be bound by which programs.
* Programming Tips::        Making Emacs code fit smoothly in Emacs.
* Compilation Tips::        Making compiled code run fast.
* Warning Tips::            Turning off compiler warnings.
* Documentation Tips::      Writing readable documentation strings.
* Comment Tips::            Conventions for writing comments.
* Library Headers::         Standard headers for library packages.

GNU Emacs Internals

* Building Emacs::          How the dumped Emacs is made.
* Pure Storage::            Kludge to make preloaded Lisp functions shareable.
* Garbage Collection::      Reclaiming space for Lisp objects no longer used.
* Stack-allocated Objects:: Temporary conses and strings on C stack.
* Memory Usage::            Info about total size of Lisp objects made so far.
* C Dialect::               What C variant Emacs is written in.
* Writing Emacs Primitives::  Writing C code for Emacs.
* Object Internals::        Data formats of buffers, windows, processes.
* C Integer Types::         How C integer types are used inside Emacs.

Object Internals

* Buffer Internals::        Components of a buffer structure.
* Window Internals::        Components of a window structure.
* Process Internals::       Components of a process structure.


File: elisp.info,  Node: Introduction,  Next: Lisp Data Types,  Prev: Top,  Up: Top

1 Introduction
**************

Most of the GNU Emacs text editor is written in the programming language
called Emacs Lisp.  You can write new code in Emacs Lisp and install it
as an extension to the editor.  However, Emacs Lisp is more than a mere
extension language; it is a full computer programming language in its
own right.  You can use it as you would any other programming language.

   Because Emacs Lisp is designed for use in an editor, it has special
features for scanning and parsing text as well as features for handling
files, buffers, displays, subprocesses, and so on.  Emacs Lisp is
closely integrated with the editing facilities; thus, editing commands
are functions that can also conveniently be called from Lisp programs,
and parameters for customization are ordinary Lisp variables.

   This manual attempts to be a full description of Emacs Lisp.  For a
beginner’s introduction to Emacs Lisp, see ‘An Introduction to Emacs
Lisp Programming’, by Bob Chassell, also published by the Free Software
Foundation.  This manual presumes considerable familiarity with the use
of Emacs for editing; see ‘The GNU Emacs Manual’ for this basic
information.

   Generally speaking, the earlier chapters describe features of Emacs
Lisp that have counterparts in many programming languages, and later
chapters describe features that are peculiar to Emacs Lisp or relate
specifically to editing.

   This is the ‘GNU Emacs Lisp Reference Manual’, corresponding to Emacs
version 25.3.

* Menu:

* Caveats::             Flaws and a request for help.
* Lisp History::        Emacs Lisp is descended from Maclisp.
* Conventions::         How the manual is formatted.
* Version Info::        Which Emacs version is running?
* Acknowledgments::     The authors, editors, and sponsors of this manual.


File: elisp.info,  Node: Caveats,  Next: Lisp History,  Up: Introduction

1.1 Caveats
===========

This manual has gone through numerous drafts.  It is nearly complete but
not flawless.  There are a few topics that are not covered, either
because we consider them secondary (such as most of the individual
modes) or because they are yet to be written.  Because we are not able
to deal with them completely, we have left out several parts
intentionally.

   The manual should be fully correct in what it does cover, and it is
therefore open to criticism on anything it says—from specific examples
and descriptive text, to the ordering of chapters and sections.  If
something is confusing, or you find that you have to look at the sources
or experiment to learn something not covered in the manual, then perhaps
the manual should be fixed.  Please let us know.

   As you use this manual, we ask that you send corrections as soon as
you find them.  If you think of a simple, real life example for a
function or group of functions, please make an effort to write it up and
send it in.  Please reference any comments to the node name and function
or variable name, as appropriate.  Also state the number of the edition
you are criticizing.

   Please send comments and corrections using ‘M-x report-emacs-bug’.


File: elisp.info,  Node: Lisp History,  Next: Conventions,  Prev: Caveats,  Up: Introduction

1.2 Lisp History
================

Lisp (LISt Processing language) was first developed in the late 1950s at
the Massachusetts Institute of Technology for research in artificial
intelligence.  The great power of the Lisp language makes it ideal for
other purposes as well, such as writing editing commands.

   Dozens of Lisp implementations have been built over the years, each
with its own idiosyncrasies.  Many of them were inspired by Maclisp,
which was written in the 1960s at MIT’s Project MAC.  Eventually the
implementers of the descendants of Maclisp came together and developed a
standard for Lisp systems, called Common Lisp.  In the meantime, Gerry
Sussman and Guy Steele at MIT developed a simplified but very powerful
dialect of Lisp, called Scheme.

   GNU Emacs Lisp is largely inspired by Maclisp, and a little by Common
Lisp.  If you know Common Lisp, you will notice many similarities.
However, many features of Common Lisp have been omitted or simplified in
order to reduce the memory requirements of GNU Emacs.  Sometimes the
simplifications are so drastic that a Common Lisp user might be very
confused.  We will occasionally point out how GNU Emacs Lisp differs
from Common Lisp.  If you don’t know Common Lisp, don’t worry about it;
this manual is self-contained.

   A certain amount of Common Lisp emulation is available via the
‘cl-lib’ library.  *Note Overview: (cl)Top.

   Emacs Lisp is not at all influenced by Scheme; but the GNU project
has an implementation of Scheme, called Guile.  We use it in all new GNU
software that calls for extensibility.


File: elisp.info,  Node: Conventions,  Next: Version Info,  Prev: Lisp History,  Up: Introduction

1.3 Conventions
===============

This section explains the notational conventions that are used in this
manual.  You may want to skip this section and refer back to it later.

* Menu:

* Some Terms::               Explanation of terms we use in this manual.
* nil and t::                How the symbols ‘nil’ and ‘t’ are used.
* Evaluation Notation::      The format we use for examples of evaluation.
* Printing Notation::        The format we use when examples print text.
* Error Messages::           The format we use for examples of errors.
* Buffer Text Notation::     The format we use for buffer contents in examples.
* Format of Descriptions::   Notation for describing functions, variables, etc.


File: elisp.info,  Node: Some Terms,  Next: nil and t,  Up: Conventions

1.3.1 Some Terms
----------------

Throughout this manual, the phrases “the Lisp reader” and “the Lisp
printer” refer to those routines in Lisp that convert textual
representations of Lisp objects into actual Lisp objects, and vice
versa.  *Note Printed Representation::, for more details.  You, the
person reading this manual, are thought of as the programmer and are
addressed as “you”.  The user is the person who uses Lisp programs,
including those you write.

   Examples of Lisp code are formatted like this: ‘(list 1 2 3)’.  Names
that represent metasyntactic variables, or arguments to a function being
described, are formatted like this: FIRST-NUMBER.


File: elisp.info,  Node: nil and t,  Next: Evaluation Notation,  Prev: Some Terms,  Up: Conventions

1.3.2 ‘nil’ and ‘t’
-------------------

In Emacs Lisp, the symbol ‘nil’ has three separate meanings: it is a
symbol with the name ‘nil’; it is the logical truth value FALSE; and it
is the empty list—the list of zero elements.  When used as a variable,
‘nil’ always has the value ‘nil’.

   As far as the Lisp reader is concerned, ‘()’ and ‘nil’ are identical:
they stand for the same object, the symbol ‘nil’.  The different ways of
writing the symbol are intended entirely for human readers.  After the
Lisp reader has read either ‘()’ or ‘nil’, there is no way to determine
which representation was actually written by the programmer.

   In this manual, we write ‘()’ when we wish to emphasize that it means
the empty list, and we write ‘nil’ when we wish to emphasize that it
means the truth value FALSE.  That is a good convention to use in Lisp
programs also.

     (cons 'foo ())                ; Emphasize the empty list
     (setq foo-flag nil)           ; Emphasize the truth value FALSE

   In contexts where a truth value is expected, any non-‘nil’ value is
considered to be TRUE.  However, ‘t’ is the preferred way to represent
the truth value TRUE.  When you need to choose a value that represents
TRUE, and there is no other basis for choosing, use ‘t’.  The symbol ‘t’
always has the value ‘t’.

   In Emacs Lisp, ‘nil’ and ‘t’ are special symbols that always evaluate
to themselves.  This is so that you do not need to quote them to use
them as constants in a program.  An attempt to change their values
results in a ‘setting-constant’ error.  *Note Constant Variables::.

 -- Function: booleanp object
     Return non-‘nil’ if OBJECT is one of the two canonical boolean
     values: ‘t’ or ‘nil’.


File: elisp.info,  Node: Evaluation Notation,  Next: Printing Notation,  Prev: nil and t,  Up: Conventions

1.3.3 Evaluation Notation
-------------------------

A Lisp expression that you can evaluate is called a “form”.  Evaluating
a form always produces a result, which is a Lisp object.  In the
examples in this manual, this is indicated with ‘⇒’:

     (car '(1 2))
          ⇒ 1

You can read this as “‘(car '(1 2))’ evaluates to 1”.

   When a form is a macro call, it expands into a new form for Lisp to
evaluate.  We show the result of the expansion with ‘↦’.  We may or may
not show the result of the evaluation of the expanded form.

     (third '(a b c))
          ↦ (car (cdr (cdr '(a b c))))
          ⇒ c

   To help describe one form, we sometimes show another form that
produces identical results.  The exact equivalence of two forms is
indicated with ‘≡’.

     (make-sparse-keymap) ≡ (list 'keymap)


File: elisp.info,  Node: Printing Notation,  Next: Error Messages,  Prev: Evaluation Notation,  Up: Conventions

1.3.4 Printing Notation
-----------------------

Many of the examples in this manual print text when they are evaluated.
If you execute example code in a Lisp Interaction buffer (such as the
buffer ‘*scratch*’), the printed text is inserted into the buffer.  If
you execute the example by other means (such as by evaluating the
function ‘eval-region’), the printed text is displayed in the echo area.

   Examples in this manual indicate printed text with ‘⊣’, irrespective
of where that text goes.  The value returned by evaluating the form
follows on a separate line with ‘⇒’.

     (progn (prin1 'foo) (princ "\n") (prin1 'bar))
          ⊣ foo
          ⊣ bar
          ⇒ bar


File: elisp.info,  Node: Error Messages,  Next: Buffer Text Notation,  Prev: Printing Notation,  Up: Conventions

1.3.5 Error Messages
--------------------

Some examples signal errors.  This normally displays an error message in
the echo area.  We show the error message on a line starting with
‘error→’.  Note that ‘error→’ itself does not appear in the echo area.

     (+ 23 'x)
     error→ Wrong type argument: number-or-marker-p, x


File: elisp.info,  Node: Buffer Text Notation,  Next: Format of Descriptions,  Prev: Error Messages,  Up: Conventions

1.3.6 Buffer Text Notation
--------------------------

Some examples describe modifications to the contents of a buffer, by
showing the before and after versions of the text.  These examples show
the contents of the buffer in question between two lines of dashes
containing the buffer name.  In addition, ‘★’ indicates the location of
point.  (The symbol for point, of course, is not part of the text in the
buffer; it indicates the place _between_ two characters where point is
currently located.)

     ---------- Buffer: foo ----------
     This is the ★contents of foo.
     ---------- Buffer: foo ----------

     (insert "changed ")
          ⇒ nil
     ---------- Buffer: foo ----------
     This is the changed ★contents of foo.
     ---------- Buffer: foo ----------


File: elisp.info,  Node: Format of Descriptions,  Prev: Buffer Text Notation,  Up: Conventions

1.3.7 Format of Descriptions
----------------------------

Functions, variables, macros, commands, user options, and special forms
are described in this manual in a uniform format.  The first line of a
description contains the name of the item followed by its arguments, if
any.  The category—function, variable, or whatever—appears at the
beginning of the line.  The description follows on succeeding lines,
sometimes with examples.

* Menu:

* A Sample Function Description::       A description of an imaginary
                                          function, ‘foo’.
* A Sample Variable Description::       A description of an imaginary
                                          variable,
                                          ‘electric-future-map’.


File: elisp.info,  Node: A Sample Function Description,  Next: A Sample Variable Description,  Up: Format of Descriptions

1.3.7.1 A Sample Function Description
.....................................

In a function description, the name of the function being described
appears first.  It is followed on the same line by a list of argument
names.  These names are also used in the body of the description, to
stand for the values of the arguments.

   The appearance of the keyword ‘&optional’ in the argument list
indicates that the subsequent arguments may be omitted (omitted
arguments default to ‘nil’).  Do not write ‘&optional’ when you call the
function.

   The keyword ‘&rest’ (which must be followed by a single argument
name) indicates that any number of arguments can follow.  The single
argument name following ‘&rest’ receives, as its value, a list of all
the remaining arguments passed to the function.  Do not write ‘&rest’
when you call the function.

   Here is a description of an imaginary function ‘foo’:

 -- Function: foo integer1 &optional integer2 &rest integers
     The function ‘foo’ subtracts INTEGER1 from INTEGER2, then adds all
     the rest of the arguments to the result.  If INTEGER2 is not
     supplied, then the number 19 is used by default.

          (foo 1 5 3 9)
               ⇒ 16
          (foo 5)
               ⇒ 14

     More generally,

          (foo W X Y...)
          ≡
          (+ (- X W) Y...)

   By convention, any argument whose name contains the name of a type
(e.g., INTEGER, INTEGER1 or BUFFER) is expected to be of that type.  A
plural of a type (such as BUFFERS) often means a list of objects of that
type.  An argument named OBJECT may be of any type.  (For a list of
Emacs object types, *note Lisp Data Types::.)  An argument with any
other sort of name (e.g., NEW-FILE) is specific to the function; if the
function has a documentation string, the type of the argument should be
described there (*note Documentation::).

   *Note Lambda Expressions::, for a more complete description of
arguments modified by ‘&optional’ and ‘&rest’.

   Command, macro, and special form descriptions have the same format,
but the word ‘Function’ is replaced by ‘Command’, ‘Macro’, or ‘Special
Form’, respectively.  Commands are simply functions that may be called
interactively; macros process their arguments differently from functions
(the arguments are not evaluated), but are presented the same way.

   The descriptions of macros and special forms use a more complex
notation to specify optional and repeated arguments, because they can
break the argument list down into separate arguments in more complicated
ways.  ‘[OPTIONAL-ARG]’ means that OPTIONAL-ARG is optional and
‘REPEATED-ARGS...’ stands for zero or more arguments.  Parentheses are
used when several arguments are grouped into additional levels of list
structure.  Here is an example:

 -- Special Form: count-loop (var [from to [inc]]) body...
     This imaginary special form implements a loop that executes the
     BODY forms and then increments the variable VAR on each iteration.
     On the first iteration, the variable has the value FROM; on
     subsequent iterations, it is incremented by one (or by INC if that
     is given).  The loop exits before executing BODY if VAR equals TO.
     Here is an example:

          (count-loop (i 0 10)
            (prin1 i) (princ " ")
            (prin1 (aref vector i))
            (terpri))

     If FROM and TO are omitted, VAR is bound to ‘nil’ before the loop
     begins, and the loop exits if VAR is non-‘nil’ at the beginning of
     an iteration.  Here is an example:

          (count-loop (done)
            (if (pending)
                (fixit)
              (setq done t)))

     In this special form, the arguments FROM and TO are optional, but
     must both be present or both absent.  If they are present, INC may
     optionally be specified as well.  These arguments are grouped with
     the argument VAR into a list, to distinguish them from BODY, which
     includes all remaining elements of the form.


File: elisp.info,  Node: A Sample Variable Description,  Prev: A Sample Function Description,  Up: Format of Descriptions

1.3.7.2 A Sample Variable Description
.....................................

A “variable” is a name that can be “bound” (or “set”) to an object.  The
object to which a variable is bound is called a “value”; we say also
that variable holds that value.  Although nearly all variables can be
set by the user, certain variables exist specifically so that users can
change them; these are called “user options”.  Ordinary variables and
user options are described using a format like that for functions,
except that there are no arguments.

   Here is a description of the imaginary ‘electric-future-map’
variable.

 -- Variable: electric-future-map
     The value of this variable is a full keymap used by Electric
     Command Future mode.  The functions in this map allow you to edit
     commands you have not yet thought about executing.

   User option descriptions have the same format, but ‘Variable’ is
replaced by ‘User Option’.


File: elisp.info,  Node: Version Info,  Next: Acknowledgments,  Prev: Conventions,  Up: Introduction

1.4 Version Information
=======================

These facilities provide information about which version of Emacs is in
use.

 -- Command: emacs-version &optional here
     This function returns a string describing the version of Emacs that
     is running.  It is useful to include this string in bug reports.

          (emacs-version)
            ⇒ "GNU Emacs 24.5.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.16)
                       of 2015-06-01"

     If HERE is non-‘nil’, it inserts the text in the buffer before
     point, and returns ‘nil’.  When this function is called
     interactively, it prints the same information in the echo area, but
     giving a prefix argument makes HERE non-‘nil’.

 -- Variable: emacs-build-time
     The value of this variable indicates the time at which Emacs was
     built.  It is a list of four integers, like the value of
     ‘current-time’ (*note Time of Day::).

          emacs-build-time
               ⇒ (20614 63694 515336 438000)

 -- Variable: emacs-version
     The value of this variable is the version of Emacs being run.  It
     is a string such as ‘"23.1.1"’.  The last number in this string is
     not really part of the Emacs release version number; it is
     incremented each time Emacs is built in any given directory.  A
     value with four numeric components, such as ‘"22.0.91.1"’,
     indicates an unreleased test version.

 -- Variable: emacs-major-version
     The major version number of Emacs, as an integer.  For Emacs
     version 23.1, the value is 23.

 -- Variable: emacs-minor-version
     The minor version number of Emacs, as an integer.  For Emacs
     version 23.1, the value is 1.


File: elisp.info,  Node: Acknowledgments,  Prev: Version Info,  Up: Introduction

1.5 Acknowledgments
===================

This manual was originally written by Robert Krawitz, Bil Lewis, Dan
LaLiberte, Richard M. Stallman and Chris Welty, the volunteers of the
GNU manual group, in an effort extending over several years.  Robert J.
Chassell helped to review and edit the manual, with the support of the
Defense Advanced Research Projects Agency, ARPA Order 6082, arranged by
Warren A. Hunt, Jr. of Computational Logic, Inc.  Additional sections
have since been written by Miles Bader, Lars Brinkhoff, Chong Yidong,
Kenichi Handa, Lute Kamstra, Juri Linkov, Glenn Morris, Thien-Thi
Nguyen, Dan Nicolaescu, Martin Rudalics, Kim F. Storm, Luc Teirlinck,
and Eli Zaretskii, and others.

   Corrections were supplied by Drew Adams, Juanma Barranquero, Karl
Berry, Jim Blandy, Bard Bloom, Stephane Boucher, David Boyes, Alan
Carroll, Richard Davis, Lawrence R. Dodd, Peter Doornbosch, David A.
Duff, Chris Eich, Beverly Erlebacher, David Eckelkamp, Ralf Fassel,
Eirik Fuller, Stephen Gildea, Bob Glickstein, Eric Hanchrow, Jesper
Harder, George Hartzell, Nathan Hess, Masayuki Ida, Dan Jacobson, Jak
Kirman, Bob Knighten, Frederick M. Korz, Joe Lammens, Glenn M. Lewis, K.
Richard Magill, Brian Marick, Roland McGrath, Stefan Monnier, Skip
Montanaro, John Gardiner Myers, Thomas A. Peterson, Francesco Potortì,
Friedrich Pukelsheim, Arnold D. Robbins, Raul Rockwell, Jason Rumney,
Per Starbäck, Shinichirou Sugou, Kimmo Suominen, Edward Tharp, Bill
Trost, Rickard Westman, Jean White, Eduard Wiebe, Matthew Wilding, Carl
Witty, Dale Worley, Rusty Wright, and David D. Zuhn.

   For a more complete list of contributors, please see the relevant
change log entries in the Emacs source repository.


File: elisp.info,  Node: Lisp Data Types,  Next: Numbers,  Prev: Introduction,  Up: Top

2 Lisp Data Types
*****************

A Lisp “object” is a piece of data used and manipulated by Lisp
programs.  For our purposes, a “type” or “data type” is a set of
possible objects.

   Every object belongs to at least one type.  Objects of the same type
have similar structures and may usually be used in the same contexts.
Types can overlap, and objects can belong to two or more types.
Consequently, we can ask whether an object belongs to a particular type,
but not for _the_ type of an object.

   A few fundamental object types are built into Emacs.  These, from
which all other types are constructed, are called “primitive types”.
Each object belongs to one and only one primitive type.  These types
include “integer”, “float”, “cons”, “symbol”, “string”, “vector”,
“hash-table”, “subr”, and “byte-code function”, plus several special
types, such as “buffer”, that are related to editing.  (*Note Editing
Types::.)

   Each primitive type has a corresponding Lisp function that checks
whether an object is a member of that type.

   Lisp is unlike many other languages in that its objects are
“self-typing”: the primitive type of each object is implicit in the
object itself.  For example, if an object is a vector, nothing can treat
it as a number; Lisp knows it is a vector, not a number.

   In most languages, the programmer must declare the data type of each
variable, and the type is known by the compiler but not represented in
the data.  Such type declarations do not exist in Emacs Lisp.  A Lisp
variable can have any type of value, and it remembers whatever value you
store in it, type and all.  (Actually, a small number of Emacs Lisp
variables can only take on values of a certain type.  *Note Variables
with Restricted Values::.)

   This chapter describes the purpose, printed representation, and read
syntax of each of the standard types in GNU Emacs Lisp.  Details on how
to use these types can be found in later chapters.

* Menu:

* Printed Representation::      How Lisp objects are represented as text.
* Comments::                    Comments and their formatting conventions.
* Programming Types::           Types found in all Lisp systems.
* Editing Types::               Types specific to Emacs.
* Circular Objects::            Read syntax for circular structure.
* Type Predicates::             Tests related to types.
* Equality Predicates::         Tests of equality between any two objects.


File: elisp.info,  Node: Printed Representation,  Next: Comments,  Up: Lisp Data Types

2.1 Printed Representation and Read Syntax
==========================================

The “printed representation” of an object is the format of the output
generated by the Lisp printer (the function ‘prin1’) for that object.
Every data type has a unique printed representation.  The “read syntax”
of an object is the format of the input accepted by the Lisp reader (the
function ‘read’) for that object.  This is not necessarily unique; many
kinds of object have more than one syntax.  *Note Read and Print::.

   In most cases, an object’s printed representation is also a read
syntax for the object.  However, some types have no read syntax, since
it does not make sense to enter objects of these types as constants in a
Lisp program.  These objects are printed in “hash notation”, which
consists of the characters ‘#<’, a descriptive string (typically the
type name followed by the name of the object), and a closing ‘>’.  For
example:

     (current-buffer)
          ⇒ #<buffer objects.texi>

Hash notation cannot be read at all, so the Lisp reader signals the
error ‘invalid-read-syntax’ whenever it encounters ‘#<’.

   In other languages, an expression is text; it has no other form.  In
Lisp, an expression is primarily a Lisp object and only secondarily the
text that is the object’s read syntax.  Often there is no need to
emphasize this distinction, but you must keep it in the back of your
mind, or you will occasionally be very confused.

   When you evaluate an expression interactively, the Lisp interpreter
first reads the textual representation of it, producing a Lisp object,
and then evaluates that object (*note Evaluation::).  However,
evaluation and reading are separate activities.  Reading returns the
Lisp object represented by the text that is read; the object may or may
not be evaluated later.  *Note Input Functions::, for a description of
‘read’, the basic function for reading objects.


File: elisp.info,  Node: Comments,  Next: Programming Types,  Prev: Printed Representation,  Up: Lisp Data Types

2.2 Comments
============

A “comment” is text that is written in a program only for the sake of
humans that read the program, and that has no effect on the meaning of
the program.  In Lisp, a semicolon (‘;’) starts a comment if it is not
within a string or character constant.  The comment continues to the end
of line.  The Lisp reader discards comments; they do not become part of
the Lisp objects which represent the program within the Lisp system.

   The ‘#@COUNT’ construct, which skips the next COUNT characters, is
useful for program-generated comments containing binary data.  The Emacs
Lisp byte compiler uses this in its output files (*note Byte
Compilation::).  It isn’t meant for source files, however.

   *Note Comment Tips::, for conventions for formatting comments.


File: elisp.info,  Node: Programming Types,  Next: Editing Types,  Prev: Comments,  Up: Lisp Data Types

2.3 Programming Types
=====================

There are two general categories of types in Emacs Lisp: those having to
do with Lisp programming, and those having to do with editing.  The
former exist in many Lisp implementations, in one form or another.  The
latter are unique to Emacs Lisp.

* Menu:

* Integer Type::        Numbers without fractional parts.
* Floating-Point Type:: Numbers with fractional parts and with a large range.
* Character Type::      The representation of letters, numbers and
                        control characters.
* Symbol Type::         A multi-use object that refers to a function,
                        variable, or property list, and has a unique identity.
* Sequence Type::       Both lists and arrays are classified as sequences.
* Cons Cell Type::      Cons cells, and lists (which are made from cons cells).
* Array Type::          Arrays include strings and vectors.
* String Type::         An (efficient) array of characters.
* Vector Type::         One-dimensional arrays.
* Char-Table Type::     One-dimensional sparse arrays indexed by characters.
* Bool-Vector Type::    One-dimensional arrays of ‘t’ or ‘nil’.
* Hash Table Type::     Super-fast lookup tables.
* Function Type::       A piece of executable code you can call from elsewhere.
* Macro Type::          A method of expanding an expression into another
                          expression, more fundamental but less pretty.
* Primitive Function Type::     A function written in C, callable from Lisp.
* Byte-Code Type::      A function written in Lisp, then compiled.
* Autoload Type::       A type used for automatically loading seldom-used
                        functions.
* Finalizer Type::      Runs code when no longer reachable.


File: elisp.info,  Node: Integer Type,  Next: Floating-Point Type,  Up: Programming Types

2.3.1 Integer Type
------------------

The range of values for an integer depends on the machine.  The minimum
range is −536,870,912 to 536,870,911 (30 bits; i.e., −2**29 to 2**29 −
1) but many machines provide a wider range.  Emacs Lisp arithmetic
functions do not check for integer overflow.  Thus ‘(1+ 536870911)’ is
−536,870,912 if Emacs integers are 30 bits.

   The read syntax for integers is a sequence of (base ten) digits with
an optional sign at the beginning and an optional period at the end.
The printed representation produced by the Lisp interpreter never has a
leading ‘+’ or a final ‘.’.

     -1               ; The integer −1.
     1                ; The integer 1.
     1.               ; Also the integer 1.
     +1               ; Also the integer 1.

As a special exception, if a sequence of digits specifies an integer too
large or too small to be a valid integer object, the Lisp reader reads
it as a floating-point number (*note Floating-Point Type::).  For
instance, if Emacs integers are 30 bits, ‘536870912’ is read as the
floating-point number ‘536870912.0’.

   *Note Numbers::, for more information.


File: elisp.info,  Node: Floating-Point Type,  Next: Character Type,  Prev: Integer Type,  Up: Programming Types

2.3.2 Floating-Point Type
-------------------------

Floating-point numbers are the computer equivalent of scientific
notation; you can think of a floating-point number as a fraction
together with a power of ten.  The precise number of significant figures
and the range of possible exponents is machine-specific; Emacs uses the
C data type ‘double’ to store the value, and internally this records a
power of 2 rather than a power of 10.

   The printed representation for floating-point numbers requires either
a decimal point (with at least one digit following), an exponent, or
both.  For example, ‘1500.0’, ‘+15e2’, ‘15.0e+2’, ‘+1500000e-3’, and
‘.15e4’ are five ways of writing a floating-point number whose value is
1500.  They are all equivalent.

   *Note Numbers::, for more information.


File: elisp.info,  Node: Character Type,  Next: Symbol Type,  Prev: Floating-Point Type,  Up: Programming Types

2.3.3 Character Type
--------------------

A “character” in Emacs Lisp is nothing more than an integer.  In other
words, characters are represented by their character codes.  For
example, the character ‘A’ is represented as the integer 65.

   Individual characters are used occasionally in programs, but it is
more common to work with _strings_, which are sequences composed of
characters.  *Note String Type::.

   Characters in strings and buffers are currently limited to the range
of 0 to 4194303—twenty two bits (*note Character Codes::).  Codes 0
through 127 are ASCII codes; the rest are non-ASCII (*note Non-ASCII
Characters::).  Characters that represent keyboard input have a much
wider range, to encode modifier keys such as Control, Meta and Shift.

   There are special functions for producing a human-readable textual
description of a character for the sake of messages.  *Note Describing
Characters::.

* Menu:

* Basic Char Syntax::      Syntax for regular characters.
* General Escape Syntax::  How to specify characters by their codes.
* Ctl-Char Syntax::        Syntax for control characters.
* Meta-Char Syntax::       Syntax for meta-characters.
* Other Char Bits::        Syntax for hyper-, super-, and alt-characters.


File: elisp.info,  Node: Basic Char Syntax,  Next: General Escape Syntax,  Up: Character Type

2.3.3.1 Basic Char Syntax
.........................

Since characters are really integers, the printed representation of a
character is a decimal number.  This is also a possible read syntax for
a character, but writing characters that way in Lisp programs is not
clear programming.  You should _always_ use the special read syntax
formats that Emacs Lisp provides for characters.  These syntax formats
start with a question mark.

   The usual read syntax for alphanumeric characters is a question mark
followed by the character; thus, ‘?A’ for the character ‘A’, ‘?B’ for
the character ‘B’, and ‘?a’ for the character ‘a’.

   For example:

     ?Q ⇒ 81     ?q ⇒ 113

   You can use the same syntax for punctuation characters, but it is
often a good idea to add a ‘\’ so that the Emacs commands for editing
Lisp code don’t get confused.  For example, ‘?\(’ is the way to write
the open-paren character.  If the character is ‘\’, you _must_ use a
second ‘\’ to quote it: ‘?\\’.

   You can express the characters control-g, backspace, tab, newline,
vertical tab, formfeed, space, return, del, and escape as ‘?\a’, ‘?\b’,
‘?\t’, ‘?\n’, ‘?\v’, ‘?\f’, ‘?\s’, ‘?\r’, ‘?\d’, and ‘?\e’,
respectively.  (‘?\s’ followed by a dash has a different meaning—it
applies the Super modifier to the following character.)  Thus,

     ?\a ⇒ 7                 ; control-g, ‘C-g’
     ?\b ⇒ 8                 ; backspace, <BS>, ‘C-h’
     ?\t ⇒ 9                 ; tab, <TAB>, ‘C-i’
     ?\n ⇒ 10                ; newline, ‘C-j’
     ?\v ⇒ 11                ; vertical tab, ‘C-k’
     ?\f ⇒ 12                ; formfeed character, ‘C-l’
     ?\r ⇒ 13                ; carriage return, <RET>, ‘C-m’
     ?\e ⇒ 27                ; escape character, <ESC>, ‘C-[’
     ?\s ⇒ 32                ; space character, <SPC>
     ?\\ ⇒ 92                ; backslash character, ‘\’
     ?\d ⇒ 127               ; delete character, <DEL>

   These sequences which start with backslash are also known as “escape
sequences”, because backslash plays the role of an escape character;
this has nothing to do with the character <ESC>.  ‘\s’ is meant for use
in character constants; in string constants, just write the space.

   A backslash is allowed, and harmless, preceding any character without
a special escape meaning; thus, ‘?\+’ is equivalent to ‘?+’.  There is
no reason to add a backslash before most characters.  However, you
should add a backslash before any of the characters ‘()\|;'`"#.,’ to
avoid confusing the Emacs commands for editing Lisp code.  You can also
add a backslash before whitespace characters such as space, tab, newline
and formfeed.  However, it is cleaner to use one of the easily readable
escape sequences, such as ‘\t’ or ‘\s’, instead of an actual whitespace
character such as a tab or a space.  (If you do write backslash followed
by a space, you should write an extra space after the character constant
to separate it from the following text.)


File: elisp.info,  Node: General Escape Syntax,  Next: Ctl-Char Syntax,  Prev: Basic Char Syntax,  Up: Character Type

2.3.3.2 General Escape Syntax
.............................

In addition to the specific escape sequences for special important
control characters, Emacs provides several types of escape syntax that
you can use to specify non-ASCII text characters.

   Firstly, you can specify characters by their Unicode values.
‘?\uNNNN’ represents a character with Unicode code point ‘U+NNNN’, where
NNNN is (by convention) a hexadecimal number with exactly four digits.
The backslash indicates that the subsequent characters form an escape
sequence, and the ‘u’ specifies a Unicode escape sequence.

   There is a slightly different syntax for specifying Unicode
characters with code points higher than ‘U+FFFF’: ‘?\U00NNNNNN’
represents the character with code point ‘U+NNNNNN’, where NNNNNN is a
six-digit hexadecimal number.  The Unicode Standard only defines code
points up to ‘U+10FFFF’, so if you specify a code point higher than
that, Emacs signals an error.

   Secondly, you can specify characters by their hexadecimal character
codes.  A hexadecimal escape sequence consists of a backslash, ‘x’, and
the hexadecimal character code.  Thus, ‘?\x41’ is the character ‘A’,
‘?\x1’ is the character ‘C-a’, and ‘?\xe0’ is the character ‘à’ (‘a’
with grave accent).  You can use any number of hex digits, so you can
represent any character code in this way.

   Thirdly, you can specify characters by their character code in octal.
An octal escape sequence consists of a backslash followed by up to three
octal digits; thus, ‘?\101’ for the character ‘A’, ‘?\001’ for the
character ‘C-a’, and ‘?\002’ for the character ‘C-b’.  Only characters
up to octal code 777 can be specified this way.

   These escape sequences may also be used in strings.  *Note Non-ASCII
in Strings::.


File: elisp.info,  Node: Ctl-Char Syntax,  Next: Meta-Char Syntax,  Prev: General Escape Syntax,  Up: Character Type

2.3.3.3 Control-Character Syntax
................................

Control characters can be represented using yet another read syntax.
This consists of a question mark followed by a backslash, caret, and the
corresponding non-control character, in either upper or lower case.  For
example, both ‘?\^I’ and ‘?\^i’ are valid read syntax for the character
‘C-i’, the character whose value is 9.

   Instead of the ‘^’, you can use ‘C-’; thus, ‘?\C-i’ is equivalent to
‘?\^I’ and to ‘?\^i’:

     ?\^I ⇒ 9     ?\C-I ⇒ 9

   In strings and buffers, the only control characters allowed are those
that exist in ASCII; but for keyboard input purposes, you can turn any
character into a control character with ‘C-’.  The character codes for
these non-ASCII control characters include the 2**26 bit as well as the
code for the corresponding non-control character.  Ordinary text
terminals have no way of generating non-ASCII control characters, but
you can generate them straightforwardly using X and other window
systems.

   For historical reasons, Emacs treats the <DEL> character as the
control equivalent of ‘?’:

     ?\^? ⇒ 127     ?\C-? ⇒ 127

As a result, it is currently not possible to represent the character
‘Control-?’, which is a meaningful input character under X, using ‘\C-’.
It is not easy to change this, as various Lisp files refer to <DEL> in
this way.

   For representing control characters to be found in files or strings,
we recommend the ‘^’ syntax; for control characters in keyboard input,
we prefer the ‘C-’ syntax.  Which one you use does not affect the
meaning of the program, but may guide the understanding of people who
read it.


File: elisp.info,  Node: Meta-Char Syntax,  Next: Other Char Bits,  Prev: Ctl-Char Syntax,  Up: Character Type

2.3.3.4 Meta-Character Syntax
.............................

A “meta character” is a character typed with the <META> modifier key.
The integer that represents such a character has the 2**27 bit set.  We
use high bits for this and other modifiers to make possible a wide range
of basic character codes.

   In a string, the 2**7 bit attached to an ASCII character indicates a
meta character; thus, the meta characters that can fit in a string have
codes in the range from 128 to 255, and are the meta versions of the
ordinary ASCII characters.  *Note Strings of Events::, for details about
<META>-handling in strings.

   The read syntax for meta characters uses ‘\M-’.  For example, ‘?\M-A’
stands for ‘M-A’.  You can use ‘\M-’ together with octal character codes
(see below), with ‘\C-’, or with any other syntax for a character.
Thus, you can write ‘M-A’ as ‘?\M-A’, or as ‘?\M-\101’.  Likewise, you
can write ‘C-M-b’ as ‘?\M-\C-b’, ‘?\C-\M-b’, or ‘?\M-\002’.


File: elisp.info,  Node: Other Char Bits,  Prev: Meta-Char Syntax,  Up: Character Type

2.3.3.5 Other Character Modifier Bits
.....................................

The case of a graphic character is indicated by its character code; for
example, ASCII distinguishes between the characters ‘a’ and ‘A’.  But
ASCII has no way to represent whether a control character is upper case
or lower case.  Emacs uses the 2**25 bit to indicate that the shift key
was used in typing a control character.  This distinction is possible
only when you use X terminals or other special terminals; ordinary text
terminals do not report the distinction.  The Lisp syntax for the shift
bit is ‘\S-’; thus, ‘?\C-\S-o’ or ‘?\C-\S-O’ represents the
shifted-control-o character.

   The X Window System defines three other modifier bits that can be set
in a character: “hyper”, “super” and “alt”.  The syntaxes for these bits
are ‘\H-’, ‘\s-’ and ‘\A-’.  (Case is significant in these prefixes.)
Thus, ‘?\H-\M-\A-x’ represents ‘Alt-Hyper-Meta-x’.  (Note that ‘\s’ with
no following ‘-’ represents the space character.)  Numerically, the bit
values are 2**22 for alt, 2**23 for super and 2**24 for hyper.


File: elisp.info,  Node: Symbol Type,  Next: Sequence Type,  Prev: Character Type,  Up: Programming Types

2.3.4 Symbol Type
-----------------

A “symbol” in GNU Emacs Lisp is an object with a name.  The symbol name
serves as the printed representation of the symbol.  In ordinary Lisp
use, with one single obarray (*note Creating Symbols::), a symbol’s name
is unique—no two symbols have the same name.

   A symbol can serve as a variable, as a function name, or to hold a
property list.  Or it may serve only to be distinct from all other Lisp
objects, so that its presence in a data structure may be recognized
reliably.  In a given context, usually only one of these uses is
intended.  But you can use one symbol in all of these ways,
independently.

   A symbol whose name starts with a colon (‘:’) is called a “keyword
symbol”.  These symbols automatically act as constants, and are normally
used only by comparing an unknown symbol with a few specific
alternatives.  *Note Constant Variables::.

   A symbol name can contain any characters whatever.  Most symbol names
are written with letters, digits, and the punctuation characters
‘-+=*/’.  Such names require no special punctuation; the characters of
the name suffice as long as the name does not look like a number.  (If
it does, write a ‘\’ at the beginning of the name to force
interpretation as a symbol.)  The characters ‘_~!@$%^&:<>{}?’ are less
often used but also require no special punctuation.  Any other
characters may be included in a symbol’s name by escaping them with a
backslash.  In contrast to its use in strings, however, a backslash in
the name of a symbol simply quotes the single character that follows the
backslash.  For example, in a string, ‘\t’ represents a tab character;
in the name of a symbol, however, ‘\t’ merely quotes the letter ‘t’.  To
have a symbol with a tab character in its name, you must actually use a
tab (preceded with a backslash).  But it’s rare to do such a thing.

     Common Lisp note: In Common Lisp, lower case letters are always
     folded to upper case, unless they are explicitly escaped.  In Emacs
     Lisp, upper case and lower case letters are distinct.

   Here are several examples of symbol names.  Note that the ‘+’ in the
fourth example is escaped to prevent it from being read as a number.
This is not necessary in the sixth example because the rest of the name
makes it invalid as a number.

     foo                 ; A symbol named ‘foo’.
     FOO                 ; A symbol named ‘FOO’, different from ‘foo’.
     1+                  ; A symbol named ‘1+’
                         ;   (not ‘+1’, which is an integer).
     \+1                 ; A symbol named ‘+1’
                         ;   (not a very readable name).
     \(*\ 1\ 2\)         ; A symbol named ‘(* 1 2)’ (a worse name).
     +-*/_~!@$%^&=:<>{}  ; A symbol named ‘+-*/_~!@$%^&=:<>{}’.
                         ;   These characters need not be escaped.

   As an exception to the rule that a symbol’s name serves as its
printed representation, ‘##’ is the printed representation for an
interned symbol whose name is an empty string.  Furthermore, ‘#:FOO’ is
the printed representation for an uninterned symbol whose name is FOO.
(Normally, the Lisp reader interns all symbols; *note Creating
Symbols::.)


File: elisp.info,  Node: Sequence Type,  Next: Cons Cell Type,  Prev: Symbol Type,  Up: Programming Types

2.3.5 Sequence Types
--------------------

A “sequence” is a Lisp object that represents an ordered set of
elements.  There are two kinds of sequence in Emacs Lisp: “lists” and
“arrays”.

   Lists are the most commonly-used sequences.  A list can hold elements
of any type, and its length can be easily changed by adding or removing
elements.  See the next subsection for more about lists.

   Arrays are fixed-length sequences.  They are further subdivided into
strings, vectors, char-tables and bool-vectors.  Vectors can hold
elements of any type, whereas string elements must be characters, and
bool-vector elements must be ‘t’ or ‘nil’.  Char-tables are like vectors
except that they are indexed by any valid character code.  The
characters in a string can have text properties like characters in a
buffer (*note Text Properties::), but vectors do not support text
properties, even when their elements happen to be characters.

   Lists, strings and the other array types also share important
similarities.  For example, all have a length L, and all have elements
which can be indexed from zero to L minus one.  Several functions,
called sequence functions, accept any kind of sequence.  For example,
the function ‘length’ reports the length of any kind of sequence.  *Note
Sequences Arrays Vectors::.

   It is generally impossible to read the same sequence twice, since
sequences are always created anew upon reading.  If you read the read
syntax for a sequence twice, you get two sequences with equal contents.
There is one exception: the empty list ‘()’ always stands for the same
object, ‘nil’.


File: elisp.info,  Node: Cons Cell Type,  Next: Array Type,  Prev: Sequence Type,  Up: Programming Types

2.3.6 Cons Cell and List Types
------------------------------

A “cons cell” is an object that consists of two slots, called the CAR
slot and the CDR slot.  Each slot can “hold” any Lisp object.  We also
say that the CAR of this cons cell is whatever object its CAR slot
currently holds, and likewise for the CDR.

   A “list” is a series of cons cells, linked together so that the CDR
slot of each cons cell holds either the next cons cell or the empty
list.  The empty list is actually the symbol ‘nil’.  *Note Lists::, for
details.  Because most cons cells are used as part of lists, we refer to
any structure made out of cons cells as a “list structure”.

     A note to C programmers: a Lisp list thus works as a “linked list”
     built up of cons cells.  Because pointers in Lisp are implicit, we
     do not distinguish between a cons cell slot holding a value versus
     pointing to the value.

   Because cons cells are so central to Lisp, we also have a word for an
object which is not a cons cell.  These objects are called “atoms”.

   The read syntax and printed representation for lists are identical,
and consist of a left parenthesis, an arbitrary number of elements, and
a right parenthesis.  Here are examples of lists:

     (A 2 "A")            ; A list of three elements.
     ()                   ; A list of no elements (the empty list).
     nil                  ; A list of no elements (the empty list).
     ("A ()")             ; A list of one element: the string ‘"A ()"’.
     (A ())               ; A list of two elements: ‘A’ and the empty list.
     (A nil)              ; Equivalent to the previous.
     ((A B C))            ; A list of one element
                          ;   (which is a list of three elements).

   Upon reading, each object inside the parentheses becomes an element
of the list.  That is, a cons cell is made for each element.  The CAR
slot of the cons cell holds the element, and its CDR slot refers to the
next cons cell of the list, which holds the next element in the list.
The CDR slot of the last cons cell is set to hold ‘nil’.

   The names CAR and CDR derive from the history of Lisp.  The original
Lisp implementation ran on an IBM 704 computer which divided words into
two parts, the address and the decrement; CAR was an instruction to
extract the contents of the address part of a register, and CDR an
instruction to extract the contents of the decrement.  By contrast, cons
cells are named for the function ‘cons’ that creates them, which in turn
was named for its purpose, the construction of cells.

* Menu:

* Box Diagrams::                Drawing pictures of lists.
* Dotted Pair Notation::        A general syntax for cons cells.
* Association List Type::       A specially constructed list.


File: elisp.info,  Node: Box Diagrams,  Next: Dotted Pair Notation,  Up: Cons Cell Type

2.3.6.1 Drawing Lists as Box Diagrams
.....................................

A list can be illustrated by a diagram in which the cons cells are shown
as pairs of boxes, like dominoes.  (The Lisp reader cannot read such an
illustration; unlike the textual notation, which can be understood by
both humans and computers, the box illustrations can be understood only
by humans.)  This picture represents the three-element list ‘(rose
violet buttercup)’:

         --- ---      --- ---      --- ---
        |   |   |--> |   |   |--> |   |   |--> nil
         --- ---      --- ---      --- ---
          |            |            |
          |            |            |
           --> rose     --> violet   --> buttercup

   In this diagram, each box represents a slot that can hold or refer to
any Lisp object.  Each pair of boxes represents a cons cell.  Each arrow
represents a reference to a Lisp object, either an atom or another cons
cell.

   In this example, the first box, which holds the CAR of the first cons
cell, refers to or holds ‘rose’ (a symbol).  The second box, holding the
CDR of the first cons cell, refers to the next pair of boxes, the second
cons cell.  The CAR of the second cons cell is ‘violet’, and its CDR is
the third cons cell.  The CDR of the third (and last) cons cell is
‘nil’.

   Here is another diagram of the same list, ‘(rose violet buttercup)’,
sketched in a different manner:

      ---------------       ----------------       -------------------
     | car   | cdr   |     | car    | cdr   |     | car       | cdr   |
     | rose  |   o-------->| violet |   o-------->| buttercup |  nil  |
     |       |       |     |        |       |     |           |       |
      ---------------       ----------------       -------------------

   A list with no elements in it is the “empty list”; it is identical to
the symbol ‘nil’.  In other words, ‘nil’ is both a symbol and a list.

   Here is the list ‘(A ())’, or equivalently ‘(A nil)’, depicted with
boxes and arrows:

         --- ---      --- ---
        |   |   |--> |   |   |--> nil
         --- ---      --- ---
          |            |
          |            |
           --> A        --> nil

   Here is a more complex illustration, showing the three-element list,
‘((pine needles) oak maple)’, the first element of which is a
two-element list:

         --- ---      --- ---      --- ---
        |   |   |--> |   |   |--> |   |   |--> nil
         --- ---      --- ---      --- ---
          |            |            |
          |            |            |
          |             --> oak      --> maple
          |
          |     --- ---      --- ---
           --> |   |   |--> |   |   |--> nil
                --- ---      --- ---
                 |            |
                 |            |
                  --> pine     --> needles

   The same list represented in the second box notation looks like this:

      --------------       --------------       --------------
     | car   | cdr  |     | car   | cdr  |     | car   | cdr  |
     |   o   |   o------->| oak   |   o------->| maple |  nil |
     |   |   |      |     |       |      |     |       |      |
      -- | ---------       --------------       --------------
         |
         |
         |        --------------       ----------------
         |       | car   | cdr  |     | car     | cdr  |
          ------>| pine  |   o------->| needles |  nil |
                 |       |      |     |         |      |
                  --------------       ----------------


File: elisp.info,  Node: Dotted Pair Notation,  Next: Association List Type,  Prev: Box Diagrams,  Up: Cons Cell Type

2.3.6.2 Dotted Pair Notation
............................

“Dotted pair notation” is a general syntax for cons cells that
represents the CAR and CDR explicitly.  In this syntax, ‘(A . B)’ stands
for a cons cell whose CAR is the object A and whose CDR is the object B.
Dotted pair notation is more general than list syntax because the CDR
does not have to be a list.  However, it is more cumbersome in cases
where list syntax would work.  In dotted pair notation, the list ‘(1 2
3)’ is written as ‘(1 . (2 . (3 . nil)))’.  For ‘nil’-terminated lists,
you can use either notation, but list notation is usually clearer and
more convenient.  When printing a list, the dotted pair notation is only
used if the CDR of a cons cell is not a list.

   Here’s an example using boxes to illustrate dotted pair notation.
This example shows the pair ‘(rose . violet)’:

         --- ---
        |   |   |--> violet
         --- ---
          |
          |
           --> rose

   You can combine dotted pair notation with list notation to represent
conveniently a chain of cons cells with a non-‘nil’ final CDR.  You
write a dot after the last element of the list, followed by the CDR of
the final cons cell.  For example, ‘(rose violet . buttercup)’ is
equivalent to ‘(rose . (violet . buttercup))’.  The object looks like
this:

         --- ---      --- ---
        |   |   |--> |   |   |--> buttercup
         --- ---      --- ---
          |            |
          |            |
           --> rose     --> violet

   The syntax ‘(rose . violet . buttercup)’ is invalid because there is
nothing that it could mean.  If anything, it would say to put
‘buttercup’ in the CDR of a cons cell whose CDR is already used for
‘violet’.

   The list ‘(rose violet)’ is equivalent to ‘(rose . (violet))’, and
looks like this:

         --- ---      --- ---
        |   |   |--> |   |   |--> nil
         --- ---      --- ---
          |            |
          |            |
           --> rose     --> violet

   Similarly, the three-element list ‘(rose violet buttercup)’ is
equivalent to ‘(rose . (violet . (buttercup)))’.  It looks like this:

         --- ---      --- ---      --- ---
        |   |   |--> |   |   |--> |   |   |--> nil
         --- ---      --- ---      --- ---
          |            |            |
          |            |            |
           --> rose     --> violet   --> buttercup


File: elisp.info,  Node: Association List Type,  Prev: Dotted Pair Notation,  Up: Cons Cell Type

2.3.6.3 Association List Type
.............................

An “association list” or “alist” is a specially-constructed list whose
elements are cons cells.  In each element, the CAR is considered a
“key”, and the CDR is considered an “associated value”.  (In some cases,
the associated value is stored in the CAR of the CDR.)  Association
lists are often used as stacks, since it is easy to add or remove
associations at the front of the list.

   For example,

     (setq alist-of-colors
           '((rose . red) (lily . white) (buttercup . yellow)))

sets the variable ‘alist-of-colors’ to an alist of three elements.  In
the first element, ‘rose’ is the key and ‘red’ is the value.

   *Note Association Lists::, for a further explanation of alists and
for functions that work on alists.  *Note Hash Tables::, for another
kind of lookup table, which is much faster for handling a large number
of keys.


File: elisp.info,  Node: Array Type,  Next: String Type,  Prev: Cons Cell Type,  Up: Programming Types

2.3.7 Array Type
----------------

An “array” is composed of an arbitrary number of slots for holding or
referring to other Lisp objects, arranged in a contiguous block of
memory.  Accessing any element of an array takes approximately the same
amount of time.  In contrast, accessing an element of a list requires
time proportional to the position of the element in the list.  (Elements
at the end of a list take longer to access than elements at the
beginning of a list.)

   Emacs defines four types of array: strings, vectors, bool-vectors,
and char-tables.

   A string is an array of characters and a vector is an array of
arbitrary objects.  A bool-vector can hold only ‘t’ or ‘nil’.  These
kinds of array may have any length up to the largest integer.
Char-tables are sparse arrays indexed by any valid character code; they
can hold arbitrary objects.

   The first element of an array has index zero, the second element has
index 1, and so on.  This is called “zero-origin” indexing.  For
example, an array of four elements has indices 0, 1, 2, and 3.  The
largest possible index value is one less than the length of the array.
Once an array is created, its length is fixed.

   All Emacs Lisp arrays are one-dimensional.  (Most other programming
languages support multidimensional arrays, but they are not essential;
you can get the same effect with nested one-dimensional arrays.)  Each
type of array has its own read syntax; see the following sections for
details.

   The array type is a subset of the sequence type, and contains the
string type, the vector type, the bool-vector type, and the char-table
type.


File: elisp.info,  Node: String Type,  Next: Vector Type,  Prev: Array Type,  Up: Programming Types

2.3.8 String Type
-----------------

A “string” is an array of characters.  Strings are used for many
purposes in Emacs, as can be expected in a text editor; for example, as
the names of Lisp symbols, as messages for the user, and to represent
text extracted from buffers.  Strings in Lisp are constants: evaluation
of a string returns the same string.

   *Note Strings and Characters::, for functions that operate on
strings.

* Menu:

* Syntax for Strings::      How to specify Lisp strings.
* Non-ASCII in Strings::    International characters in strings.
* Nonprinting Characters::  Literal unprintable characters in strings.
* Text Props and Strings::  Strings with text properties.


File: elisp.info,  Node: Syntax for Strings,  Next: Non-ASCII in Strings,  Up: String Type

2.3.8.1 Syntax for Strings
..........................

The read syntax for a string is a double-quote, an arbitrary number of
characters, and another double-quote, ‘"like this"’.  To include a
double-quote in a string, precede it with a backslash; thus, ‘"\""’ is a
string containing just one double-quote character.  Likewise, you can
include a backslash by preceding it with another backslash, like this:
‘"this \\ is a single embedded backslash"’.

   The newline character is not special in the read syntax for strings;
if you write a new line between the double-quotes, it becomes a
character in the string.  But an escaped newline—one that is preceded by
‘\’—does not become part of the string; i.e., the Lisp reader ignores an
escaped newline while reading a string.  An escaped space ‘\ ’ is
likewise ignored.

     "It is useful to include newlines
     in documentation strings,
     but the newline is \
     ignored if escaped."
          ⇒ "It is useful to include newlines
     in documentation strings,
     but the newline is ignored if escaped."


File: elisp.info,  Node: Non-ASCII in Strings,  Next: Nonprinting Characters,  Prev: Syntax for Strings,  Up: String Type

2.3.8.2 Non-ASCII Characters in Strings
.......................................

There are two text representations for non-ASCII characters in Emacs
strings: multibyte and unibyte (*note Text Representations::).  Roughly
speaking, unibyte strings store raw bytes, while multibyte strings store
human-readable text.  Each character in a unibyte string is a byte,
i.e., its value is between 0 and 255.  By contrast, each character in a
multibyte string may have a value between 0 to 4194303 (*note Character
Type::).  In both cases, characters above 127 are non-ASCII.

   You can include a non-ASCII character in a string constant by writing
it literally.  If the string constant is read from a multibyte source,
such as a multibyte buffer or string, or a file that would be visited as
multibyte, then Emacs reads each non-ASCII character as a multibyte
character and automatically makes the string a multibyte string.  If the
string constant is read from a unibyte source, then Emacs reads the
non-ASCII character as unibyte, and makes the string unibyte.

   Instead of writing a character literally into a multibyte string, you
can write it as its character code using an escape sequence.  *Note
General Escape Syntax::, for details about escape sequences.

   If you use any Unicode-style escape sequence ‘\uNNNN’ or ‘\U00NNNNNN’
in a string constant (even for an ASCII character), Emacs automatically
assumes that it is multibyte.

   You can also use hexadecimal escape sequences (‘\xN’) and octal
escape sequences (‘\N’) in string constants.  *But beware:* If a string
constant contains hexadecimal or octal escape sequences, and these
escape sequences all specify unibyte characters (i.e., less than 256),
and there are no other literal non-ASCII characters or Unicode-style
escape sequences in the string, then Emacs automatically assumes that it
is a unibyte string.  That is to say, it assumes that all non-ASCII
characters occurring in the string are 8-bit raw bytes.

   In hexadecimal and octal escape sequences, the escaped character code
may contain a variable number of digits, so the first subsequent
character which is not a valid hexadecimal or octal digit terminates the
escape sequence.  If the next character in a string could be interpreted
as a hexadecimal or octal digit, write ‘\ ’ (backslash and space) to
terminate the escape sequence.  For example, ‘\xe0\ ’ represents one
character, ‘a’ with grave accent.  ‘\ ’ in a string constant is just
like backslash-newline; it does not contribute any character to the
string, but it does terminate any preceding hex escape.


File: elisp.info,  Node: Nonprinting Characters,  Next: Text Props and Strings,  Prev: Non-ASCII in Strings,  Up: String Type

2.3.8.3 Nonprinting Characters in Strings
.........................................

You can use the same backslash escape-sequences in a string constant as
in character literals (but do not use the question mark that begins a
character constant).  For example, you can write a string containing the
nonprinting characters tab and ‘C-a’, with commas and spaces between
them, like this: ‘"\t, \C-a"’.  *Note Character Type::, for a
description of the read syntax for characters.

   However, not all of the characters you can write with backslash
escape-sequences are valid in strings.  The only control characters that
a string can hold are the ASCII control characters.  Strings do not
distinguish case in ASCII control characters.

   Properly speaking, strings cannot hold meta characters; but when a
string is to be used as a key sequence, there is a special convention
that provides a way to represent meta versions of ASCII characters in a
string.  If you use the ‘\M-’ syntax to indicate a meta character in a
string constant, this sets the 2**7 bit of the character in the string.
If the string is used in ‘define-key’ or ‘lookup-key’, this numeric code
is translated into the equivalent meta character.  *Note Character
Type::.

   Strings cannot hold characters that have the hyper, super, or alt
modifiers.


File: elisp.info,  Node: Text Props and Strings,  Prev: Nonprinting Characters,  Up: String Type

2.3.8.4 Text Properties in Strings
..................................

A string can hold properties for the characters it contains, in addition
to the characters themselves.  This enables programs that copy text
between strings and buffers to copy the text’s properties with no
special effort.  *Note Text Properties::, for an explanation of what
text properties mean.  Strings with text properties use a special read
and print syntax:

     #("CHARACTERS" PROPERTY-DATA...)

where PROPERTY-DATA consists of zero or more elements, in groups of
three as follows:

     BEG END PLIST

The elements BEG and END are integers, and together specify a range of
indices in the string; PLIST is the property list for that range.  For
example,

     #("foo bar" 0 3 (face bold) 3 4 nil 4 7 (face italic))

represents a string whose textual contents are ‘foo bar’, in which the
first three characters have a ‘face’ property with value ‘bold’, and the
last three have a ‘face’ property with value ‘italic’.  (The fourth
character has no text properties, so its property list is ‘nil’.  It is
not actually necessary to mention ranges with ‘nil’ as the property
list, since any characters not mentioned in any range will default to
having no properties.)


File: elisp.info,  Node: Vector Type,  Next: Char-Table Type,  Prev: String Type,  Up: Programming Types

2.3.9 Vector Type
-----------------

A “vector” is a one-dimensional array of elements of any type.  It takes
a constant amount of time to access any element of a vector.  (In a
list, the access time of an element is proportional to the distance of
the element from the beginning of the list.)

   The printed representation of a vector consists of a left square
bracket, the elements, and a right square bracket.  This is also the
read syntax.  Like numbers and strings, vectors are considered constants
for evaluation.

     [1 "two" (three)]      ; A vector of three elements.
          ⇒ [1 "two" (three)]

   *Note Vectors::, for functions that work with vectors.


File: elisp.info,  Node: Char-Table Type,  Next: Bool-Vector Type,  Prev: Vector Type,  Up: Programming Types

2.3.10 Char-Table Type
----------------------

A “char-table” is a one-dimensional array of elements of any type,
indexed by character codes.  Char-tables have certain extra features to
make them more useful for many jobs that involve assigning information
to character codes—for example, a char-table can have a parent to
inherit from, a default value, and a small number of extra slots to use
for special purposes.  A char-table can also specify a single value for
a whole character set.

   The printed representation of a char-table is like a vector except
that there is an extra ‘#^’ at the beginning.(1)

   *Note Char-Tables::, for special functions to operate on char-tables.
Uses of char-tables include:

   • Case tables (*note Case Tables::).

   • Character category tables (*note Categories::).

   • Display tables (*note Display Tables::).

   • Syntax tables (*note Syntax Tables::).

   ---------- Footnotes ----------

   (1) You may also encounter ‘#^^’, used for sub-char-tables.


File: elisp.info,  Node: Bool-Vector Type,  Next: Hash Table Type,  Prev: Char-Table Type,  Up: Programming Types

2.3.11 Bool-Vector Type
-----------------------

A “bool-vector” is a one-dimensional array whose elements must be ‘t’ or
‘nil’.

   The printed representation of a bool-vector is like a string, except
that it begins with ‘#&’ followed by the length.  The string constant
that follows actually specifies the contents of the bool-vector as a
bitmap—each character in the string contains 8 bits, which specify the
next 8 elements of the bool-vector (1 stands for ‘t’, and 0 for ‘nil’).
The least significant bits of the character correspond to the lowest
indices in the bool-vector.

     (make-bool-vector 3 t)
          ⇒ #&3"^G"
     (make-bool-vector 3 nil)
          ⇒ #&3"^@"

These results make sense, because the binary code for ‘C-g’ is 111 and
‘C-@’ is the character with code 0.

   If the length is not a multiple of 8, the printed representation
shows extra elements, but these extras really make no difference.  For
instance, in the next example, the two bool-vectors are equal, because
only the first 3 bits are used:

     (equal #&3"\377" #&3"\007")
          ⇒ t


File: elisp.info,  Node: Hash Table Type,  Next: Function Type,  Prev: Bool-Vector Type,  Up: Programming Types

2.3.12 Hash Table Type
----------------------

A hash table is a very fast kind of lookup table, somewhat like an alist
in that it maps keys to corresponding values, but much faster.  The
printed representation of a hash table specifies its properties and
contents, like this:

     (make-hash-table)
          ⇒ #s(hash-table size 65 test eql rehash-size 1.5
                                  rehash-threshold 0.8 data ())

*Note Hash Tables::, for more information about hash tables.


File: elisp.info,  Node: Function Type,  Next: Macro Type,  Prev: Hash Table Type,  Up: Programming Types

2.3.13 Function Type
--------------------

Lisp functions are executable code, just like functions in other
programming languages.  In Lisp, unlike most languages, functions are
also Lisp objects.  A non-compiled function in Lisp is a lambda
expression: that is, a list whose first element is the symbol ‘lambda’
(*note Lambda Expressions::).

   In most programming languages, it is impossible to have a function
without a name.  In Lisp, a function has no intrinsic name.  A lambda
expression can be called as a function even though it has no name; to
emphasize this, we also call it an “anonymous function” (*note Anonymous
Functions::).  A named function in Lisp is just a symbol with a valid
function in its function cell (*note Defining Functions::).

   Most of the time, functions are called when their names are written
in Lisp expressions in Lisp programs.  However, you can construct or
obtain a function object at run time and then call it with the primitive
functions ‘funcall’ and ‘apply’.  *Note Calling Functions::.


File: elisp.info,  Node: Macro Type,  Next: Primitive Function Type,  Prev: Function Type,  Up: Programming Types

2.3.14 Macro Type
-----------------

A “Lisp macro” is a user-defined construct that extends the Lisp
language.  It is represented as an object much like a function, but with
different argument-passing semantics.  A Lisp macro has the form of a
list whose first element is the symbol ‘macro’ and whose CDR is a Lisp
function object, including the ‘lambda’ symbol.

   Lisp macro objects are usually defined with the built-in ‘defmacro’
macro, but any list that begins with ‘macro’ is a macro as far as Emacs
is concerned.  *Note Macros::, for an explanation of how to write a
macro.

   *Warning*: Lisp macros and keyboard macros (*note Keyboard Macros::)
are entirely different things.  When we use the word “macro” without
qualification, we mean a Lisp macro, not a keyboard macro.


File: elisp.info,  Node: Primitive Function Type,  Next: Byte-Code Type,  Prev: Macro Type,  Up: Programming Types

2.3.15 Primitive Function Type
------------------------------

A “primitive function” is a function callable from Lisp but written in
the C programming language.  Primitive functions are also called “subrs”
or “built-in functions”.  (The word “subr” is derived from
“subroutine”.)  Most primitive functions evaluate all their arguments
when they are called.  A primitive function that does not evaluate all
its arguments is called a “special form” (*note Special Forms::).

   It does not matter to the caller of a function whether the function
is primitive.  However, this does matter if you try to redefine a
primitive with a function written in Lisp.  The reason is that the
primitive function may be called directly from C code.  Calls to the
redefined function from Lisp will use the new definition, but calls from
C code may still use the built-in definition.  Therefore, *we discourage
redefinition of primitive functions*.

   The term “function” refers to all Emacs functions, whether written in
Lisp or C.  *Note Function Type::, for information about the functions
written in Lisp.

   Primitive functions have no read syntax and print in hash notation
with the name of the subroutine.

     (symbol-function 'car)          ; Access the function cell
                                     ;   of the symbol.
          ⇒ #<subr car>
     (subrp (symbol-function 'car))  ; Is this a primitive function?
          ⇒ t                       ; Yes.


File: elisp.info,  Node: Byte-Code Type,  Next: Autoload Type,  Prev: Primitive Function Type,  Up: Programming Types

2.3.16 Byte-Code Function Type
------------------------------

“Byte-code function objects” are produced by byte-compiling Lisp code
(*note Byte Compilation::).  Internally, a byte-code function object is
much like a vector; however, the evaluator handles this data type
specially when it appears in a function call.  *Note Byte-Code
Objects::.

   The printed representation and read syntax for a byte-code function
object is like that for a vector, with an additional ‘#’ before the
opening ‘[’.


File: elisp.info,  Node: Autoload Type,  Next: Finalizer Type,  Prev: Byte-Code Type,  Up: Programming Types

2.3.17 Autoload Type
--------------------

An “autoload object” is a list whose first element is the symbol
‘autoload’.  It is stored as the function definition of a symbol, where
it serves as a placeholder for the real definition.  The autoload object
says that the real definition is found in a file of Lisp code that
should be loaded when necessary.  It contains the name of the file, plus
some other information about the real definition.

   After the file has been loaded, the symbol should have a new function
definition that is not an autoload object.  The new definition is then
called as if it had been there to begin with.  From the user’s point of
view, the function call works as expected, using the function definition
in the loaded file.

   An autoload object is usually created with the function ‘autoload’,
which stores the object in the function cell of a symbol.  *Note
Autoload::, for more details.


File: elisp.info,  Node: Finalizer Type,  Prev: Autoload Type,  Up: Programming Types

2.3.18 Finalizer Type
---------------------

A “finalizer object” helps Lisp code clean up after objects that are no
longer needed.  A finalizer holds a Lisp function object.  When a
finalizer object becomes unreachable after a garbage collection pass,
Emacs calls the finalizer’s associated function object.  When deciding
whether a finalizer is reachable, Emacs does not count references from
finalizer objects themselves, allowing you to use finalizers without
having to worry about accidentally capturing references to finalized
objects themselves.

   Errors in finalizers are printed to ‘*Messages*’.  Emacs runs a given
finalizer object’s associated function exactly once, even if that
function fails.

 -- Function: make-finalizer function
     Make a finalizer that will run FUNCTION.  FUNCTION will be called
     after garbage collection when the returned finalizer object becomes
     unreachable.  If the finalizer object is reachable only through
     references from finalizer objects, it does not count as reachable
     for the purpose of deciding whether to run FUNCTION.  FUNCTION will
     be run once per finalizer object.


File: elisp.info,  Node: Editing Types,  Next: Circular Objects,  Prev: Programming Types,  Up: Lisp Data Types

2.4 Editing Types
=================

The types in the previous section are used for general programming
purposes, and most of them are common to most Lisp dialects.  Emacs Lisp
provides several additional data types for purposes connected with
editing.

* Menu:

* Buffer Type::         The basic object of editing.
* Marker Type::         A position in a buffer.
* Window Type::         Buffers are displayed in windows.
* Frame Type::          Windows subdivide frames.
* Terminal Type::       A terminal device displays frames.
* Window Configuration Type::   Recording the way a frame is subdivided.
* Frame Configuration Type::    Recording the status of all frames.
* Process Type::        A subprocess of Emacs running on the underlying OS.
* Stream Type::         Receive or send characters.
* Keymap Type::         What function a keystroke invokes.
* Overlay Type::        How an overlay is represented.
* Font Type::           Fonts for displaying text.


File: elisp.info,  Node: Buffer Type,  Next: Marker Type,  Up: Editing Types

2.4.1 Buffer Type
-----------------

A “buffer” is an object that holds text that can be edited (*note
Buffers::).  Most buffers hold the contents of a disk file (*note
Files::) so they can be edited, but some are used for other purposes.
Most buffers are also meant to be seen by the user, and therefore
displayed, at some time, in a window (*note Windows::).  But a buffer
need not be displayed in any window.  Each buffer has a designated
position called “point” (*note Positions::); most editing commands act
on the contents of the current buffer in the neighborhood of point.  At
any time, one buffer is the “current buffer”.

   The contents of a buffer are much like a string, but buffers are not
used like strings in Emacs Lisp, and the available operations are
different.  For example, you can insert text efficiently into an
existing buffer, altering the buffer’s contents, whereas inserting text
into a string requires concatenating substrings, and the result is an
entirely new string object.

   Many of the standard Emacs functions manipulate or test the
characters in the current buffer; a whole chapter in this manual is
devoted to describing these functions (*note Text::).

   Several other data structures are associated with each buffer:

   • a local syntax table (*note Syntax Tables::);

   • a local keymap (*note Keymaps::); and,

   • a list of buffer-local variable bindings (*note Buffer-Local
     Variables::).

   • overlays (*note Overlays::).

   • text properties for the text in the buffer (*note Text
     Properties::).

The local keymap and variable list contain entries that individually
override global bindings or values.  These are used to customize the
behavior of programs in different buffers, without actually changing the
programs.

   A buffer may be “indirect”, which means it shares the text of another
buffer, but presents it differently.  *Note Indirect Buffers::.

   Buffers have no read syntax.  They print in hash notation, showing
the buffer name.

     (current-buffer)
          ⇒ #<buffer objects.texi>


File: elisp.info,  Node: Marker Type,  Next: Window Type,  Prev: Buffer Type,  Up: Editing Types

2.4.2 Marker Type
-----------------

A “marker” denotes a position in a specific buffer.  Markers therefore
have two components: one for the buffer, and one for the position.
Changes in the buffer’s text automatically relocate the position value
as necessary to ensure that the marker always points between the same
two characters in the buffer.

   Markers have no read syntax.  They print in hash notation, giving the
current character position and the name of the buffer.

     (point-marker)
          ⇒ #<marker at 10779 in objects.texi>

   *Note Markers::, for information on how to test, create, copy, and
move markers.


File: elisp.info,  Node: Window Type,  Next: Frame Type,  Prev: Marker Type,  Up: Editing Types

2.4.3 Window Type
-----------------

A “window” describes the portion of the terminal screen that Emacs uses
to display a buffer.  Every window has one associated buffer, whose
contents appear in the window.  By contrast, a given buffer may appear
in one window, no window, or several windows.

   Though many windows may exist simultaneously, at any time one window
is designated the “selected window”.  This is the window where the
cursor is (usually) displayed when Emacs is ready for a command.  The
selected window usually displays the current buffer (*note Current
Buffer::), but this is not necessarily the case.

   Windows are grouped on the screen into frames; each window belongs to
one and only one frame.  *Note Frame Type::.

   Windows have no read syntax.  They print in hash notation, giving the
window number and the name of the buffer being displayed.  The window
numbers exist to identify windows uniquely, since the buffer displayed
in any given window can change frequently.

     (selected-window)
          ⇒ #<window 1 on objects.texi>

   *Note Windows::, for a description of the functions that work on
windows.


File: elisp.info,  Node: Frame Type,  Next: Terminal Type,  Prev: Window Type,  Up: Editing Types

2.4.4 Frame Type
----------------

A “frame” is a screen area that contains one or more Emacs windows; we
also use the term “frame” to refer to the Lisp object that Emacs uses to
refer to the screen area.

   Frames have no read syntax.  They print in hash notation, giving the
frame’s title, plus its address in core (useful to identify the frame
uniquely).

     (selected-frame)
          ⇒ #<frame emacs@psilocin.gnu.org 0xdac80>

   *Note Frames::, for a description of the functions that work on
frames.


File: elisp.info,  Node: Terminal Type,  Next: Window Configuration Type,  Prev: Frame Type,  Up: Editing Types

2.4.5 Terminal Type
-------------------

A “terminal” is a device capable of displaying one or more Emacs frames
(*note Frame Type::).

   Terminals have no read syntax.  They print in hash notation giving
the terminal’s ordinal number and its TTY device file name.

     (get-device-terminal nil)
          ⇒ #<terminal 1 on /dev/tty>


File: elisp.info,  Node: Window Configuration Type,  Next: Frame Configuration Type,  Prev: Terminal Type,  Up: Editing Types

2.4.6 Window Configuration Type
-------------------------------

A “window configuration” stores information about the positions, sizes,
and contents of the windows in a frame, so you can recreate the same
arrangement of windows later.

   Window configurations do not have a read syntax; their print syntax
looks like ‘#<window-configuration>’.  *Note Window Configurations::,
for a description of several functions related to window configurations.


File: elisp.info,  Node: Frame Configuration Type,  Next: Process Type,  Prev: Window Configuration Type,  Up: Editing Types

2.4.7 Frame Configuration Type
------------------------------

A “frame configuration” stores information about the positions, sizes,
and contents of the windows in all frames.  It is not a primitive
type—it is actually a list whose CAR is ‘frame-configuration’ and whose
CDR is an alist.  Each alist element describes one frame, which appears
as the CAR of that element.

   *Note Frame Configurations::, for a description of several functions
related to frame configurations.


File: elisp.info,  Node: Process Type,  Next: Stream Type,  Prev: Frame Configuration Type,  Up: Editing Types

2.4.8 Process Type
------------------

The word “process” usually means a running program.  Emacs itself runs
in a process of this sort.  However, in Emacs Lisp, a process is a Lisp
object that designates a subprocess created by the Emacs process.
Programs such as shells, GDB, ftp, and compilers, running in
subprocesses of Emacs, extend the capabilities of Emacs.  An Emacs
subprocess takes textual input from Emacs and returns textual output to
Emacs for further manipulation.  Emacs can also send signals to the
subprocess.

   Process objects have no read syntax.  They print in hash notation,
giving the name of the process:

     (process-list)
          ⇒ (#<process shell>)

   *Note Processes::, for information about functions that create,
delete, return information about, send input or signals to, and receive
output from processes.


File: elisp.info,  Node: Stream Type,  Next: Keymap Type,  Prev: Process Type,  Up: Editing Types

2.4.9 Stream Type
-----------------

A “stream” is an object that can be used as a source or sink for
characters—either to supply characters for input or to accept them as
output.  Many different types can be used this way: markers, buffers,
strings, and functions.  Most often, input streams (character sources)
obtain characters from the keyboard, a buffer, or a file, and output
streams (character sinks) send characters to a buffer, such as a
‘*Help*’ buffer, or to the echo area.

   The object ‘nil’, in addition to its other meanings, may be used as a
stream.  It stands for the value of the variable ‘standard-input’ or
‘standard-output’.  Also, the object ‘t’ as a stream specifies input
using the minibuffer (*note Minibuffers::) or output in the echo area
(*note The Echo Area::).

   Streams have no special printed representation or read syntax, and
print as whatever primitive type they are.

   *Note Read and Print::, for a description of functions related to
streams, including parsing and printing functions.


File: elisp.info,  Node: Keymap Type,  Next: Overlay Type,  Prev: Stream Type,  Up: Editing Types

2.4.10 Keymap Type
------------------

A “keymap” maps keys typed by the user to commands.  This mapping
controls how the user’s command input is executed.  A keymap is actually
a list whose CAR is the symbol ‘keymap’.

   *Note Keymaps::, for information about creating keymaps, handling
prefix keys, local as well as global keymaps, and changing key bindings.


File: elisp.info,  Node: Overlay Type,  Next: Font Type,  Prev: Keymap Type,  Up: Editing Types

2.4.11 Overlay Type
-------------------

An “overlay” specifies properties that apply to a part of a buffer.
Each overlay applies to a specified range of the buffer, and contains a
property list (a list whose elements are alternating property names and
values).  Overlay properties are used to present parts of the buffer
temporarily in a different display style.  Overlays have no read syntax,
and print in hash notation, giving the buffer name and range of
positions.

   *Note Overlays::, for information on how you can create and use
overlays.


File: elisp.info,  Node: Font Type,  Prev: Overlay Type,  Up: Editing Types

2.4.12 Font Type
----------------

A “font” specifies how to display text on a graphical terminal.  There
are actually three separate font types—“font objects”, “font specs”, and
“font entities”—each of which has slightly different properties.  None
of them have a read syntax; their print syntax looks like
‘#<font-object>’, ‘#<font-spec>’, and ‘#<font-entity>’ respectively.
*Note Low-Level Font::, for a description of these Lisp objects.


File: elisp.info,  Node: Circular Objects,  Next: Type Predicates,  Prev: Editing Types,  Up: Lisp Data Types

2.5 Read Syntax for Circular Objects
====================================

To represent shared or circular structures within a complex of Lisp
objects, you can use the reader constructs ‘#N=’ and ‘#N#’.

   Use ‘#N=’ before an object to label it for later reference;
subsequently, you can use ‘#N#’ to refer the same object in another
place.  Here, N is some integer.  For example, here is how to make a
list in which the first element recurs as the third element:

     (#1=(a) b #1#)

This differs from ordinary syntax such as this

     ((a) b (a))

which would result in a list whose first and third elements look alike
but are not the same Lisp object.  This shows the difference:

     (prog1 nil
       (setq x '(#1=(a) b #1#)))
     (eq (nth 0 x) (nth 2 x))
          ⇒ t
     (setq x '((a) b (a)))
     (eq (nth 0 x) (nth 2 x))
          ⇒ nil

   You can also use the same syntax to make a circular structure, which
appears as an element within itself.  Here is an example:

     #1=(a #1#)

This makes a list whose second element is the list itself.  Here’s how
you can see that it really works:

     (prog1 nil
       (setq x '#1=(a #1#)))
     (eq x (cadr x))
          ⇒ t

   The Lisp printer can produce this syntax to record circular and
shared structure in a Lisp object, if you bind the variable
‘print-circle’ to a non-‘nil’ value.  *Note Output Variables::.


File: elisp.info,  Node: Type Predicates,  Next: Equality Predicates,  Prev: Circular Objects,  Up: Lisp Data Types

2.6 Type Predicates
===================

The Emacs Lisp interpreter itself does not perform type checking on the
actual arguments passed to functions when they are called.  It could not
do so, since function arguments in Lisp do not have declared data types,
as they do in other programming languages.  It is therefore up to the
individual function to test whether each actual argument belongs to a
type that the function can use.

   All built-in functions do check the types of their actual arguments
when appropriate, and signal a ‘wrong-type-argument’ error if an
argument is of the wrong type.  For example, here is what happens if you
pass an argument to ‘+’ that it cannot handle:

     (+ 2 'a)
          error→ Wrong type argument: number-or-marker-p, a

   If you want your program to handle different types differently, you
must do explicit type checking.  The most common way to check the type
of an object is to call a “type predicate” function.  Emacs has a type
predicate for each type, as well as some predicates for combinations of
types.

   A type predicate function takes one argument; it returns ‘t’ if the
argument belongs to the appropriate type, and ‘nil’ otherwise.
Following a general Lisp convention for predicate functions, most type
predicates’ names end with ‘p’.

   Here is an example which uses the predicates ‘listp’ to check for a
list and ‘symbolp’ to check for a symbol.

     (defun add-on (x)
       (cond ((symbolp x)
              ;; If X is a symbol, put it on LIST.
              (setq list (cons x list)))
             ((listp x)
              ;; If X is a list, add its elements to LIST.
              (setq list (append x list)))
             (t
              ;; We handle only symbols and lists.
              (error "Invalid argument %s in add-on" x))))

   Here is a table of predefined type predicates, in alphabetical order,
with references to further information.

‘atom’
     *Note atom: List-related Predicates.

‘arrayp’
     *Note arrayp: Array Functions.

‘bool-vector-p’
     *Note bool-vector-p: Bool-Vectors.

‘bufferp’
     *Note bufferp: Buffer Basics.

‘byte-code-function-p’
     *Note byte-code-function-p: Byte-Code Type.

‘case-table-p’
     *Note case-table-p: Case Tables.

‘char-or-string-p’
     *Note char-or-string-p: Predicates for Strings.

‘char-table-p’
     *Note char-table-p: Char-Tables.

‘commandp’
     *Note commandp: Interactive Call.

‘consp’
     *Note consp: List-related Predicates.

‘custom-variable-p’
     *Note custom-variable-p: Variable Definitions.

‘floatp’
     *Note floatp: Predicates on Numbers.

‘fontp’
     *Note Low-Level Font::.

‘frame-configuration-p’
     *Note frame-configuration-p: Frame Configurations.

‘frame-live-p’
     *Note frame-live-p: Deleting Frames.

‘framep’
     *Note framep: Frames.

‘functionp’
     *Note functionp: Functions.

‘hash-table-p’
     *Note hash-table-p: Other Hash.

‘integer-or-marker-p’
     *Note integer-or-marker-p: Predicates on Markers.

‘integerp’
     *Note integerp: Predicates on Numbers.

‘keymapp’
     *Note keymapp: Creating Keymaps.

‘keywordp’
     *Note Constant Variables::.

‘listp’
     *Note listp: List-related Predicates.

‘markerp’
     *Note markerp: Predicates on Markers.

‘wholenump’
     *Note wholenump: Predicates on Numbers.

‘nlistp’
     *Note nlistp: List-related Predicates.

‘numberp’
     *Note numberp: Predicates on Numbers.

‘number-or-marker-p’
     *Note number-or-marker-p: Predicates on Markers.

‘overlayp’
     *Note overlayp: Overlays.

‘processp’
     *Note processp: Processes.

‘sequencep’
     *Note sequencep: Sequence Functions.

‘stringp’
     *Note stringp: Predicates for Strings.

‘subrp’
     *Note subrp: Function Cells.

‘symbolp’
     *Note symbolp: Symbols.

‘syntax-table-p’
     *Note syntax-table-p: Syntax Tables.

‘vectorp’
     *Note vectorp: Vectors.

‘window-configuration-p’
     *Note window-configuration-p: Window Configurations.

‘window-live-p’
     *Note window-live-p: Deleting Windows.

‘windowp’
     *Note windowp: Basic Windows.

‘booleanp’
     *Note booleanp: nil and t.

‘string-or-null-p’
     *Note string-or-null-p: Predicates for Strings.

   The most general way to check the type of an object is to call the
function ‘type-of’.  Recall that each object belongs to one and only one
primitive type; ‘type-of’ tells you which one (*note Lisp Data Types::).
But ‘type-of’ knows nothing about non-primitive types.  In most cases,
it is more convenient to use type predicates than ‘type-of’.

 -- Function: type-of object
     This function returns a symbol naming the primitive type of OBJECT.
     The value is one of the symbols ‘bool-vector’, ‘buffer’,
     ‘char-table’, ‘compiled-function’, ‘cons’, ‘finalizer’, ‘float’,
     ‘font-entity’, ‘font-object’, ‘font-spec’, ‘frame’, ‘hash-table’,
     ‘integer’, ‘marker’, ‘overlay’, ‘process’, ‘string’, ‘subr’,
     ‘symbol’, ‘vector’, ‘window’, or ‘window-configuration’.

          (type-of 1)
               ⇒ integer
          (type-of 'nil)
               ⇒ symbol
          (type-of '())    ; ‘()’ is ‘nil’.
               ⇒ symbol
          (type-of '(x))
               ⇒ cons


File: elisp.info,  Node: Equality Predicates,  Prev: Type Predicates,  Up: Lisp Data Types

2.7 Equality Predicates
=======================

Here we describe functions that test for equality between two objects.
Other functions test equality of contents between objects of specific
types, e.g., strings.  For these predicates, see the appropriate chapter
describing the data type.

 -- Function: eq object1 object2
     This function returns ‘t’ if OBJECT1 and OBJECT2 are the same
     object, and ‘nil’ otherwise.

     If OBJECT1 and OBJECT2 are integers with the same value, they are
     considered to be the same object (i.e., ‘eq’ returns ‘t’).  If
     OBJECT1 and OBJECT2 are symbols with the same name, they are
     normally the same object—but see *note Creating Symbols:: for
     exceptions.  For other types (e.g., lists, vectors, strings), two
     arguments with the same contents or elements are not necessarily
     ‘eq’ to each other: they are ‘eq’ only if they are the same object,
     meaning that a change in the contents of one will be reflected by
     the same change in the contents of the other.

          (eq 'foo 'foo)
               ⇒ t

          (eq 456 456)
               ⇒ t

          (eq "asdf" "asdf")
               ⇒ nil

          (eq "" "")
               ⇒ t
          ;; This exception occurs because Emacs Lisp
          ;; makes just one multibyte empty string, to save space.

          (eq '(1 (2 (3))) '(1 (2 (3))))
               ⇒ nil

          (setq foo '(1 (2 (3))))
               ⇒ (1 (2 (3)))
          (eq foo foo)
               ⇒ t
          (eq foo '(1 (2 (3))))
               ⇒ nil

          (eq [(1 2) 3] [(1 2) 3])
               ⇒ nil

          (eq (point-marker) (point-marker))
               ⇒ nil

     The ‘make-symbol’ function returns an uninterned symbol, distinct
     from the symbol that is used if you write the name in a Lisp
     expression.  Distinct symbols with the same name are not ‘eq’.
     *Note Creating Symbols::.

          (eq (make-symbol "foo") 'foo)
               ⇒ nil

 -- Function: equal object1 object2
     This function returns ‘t’ if OBJECT1 and OBJECT2 have equal
     components, and ‘nil’ otherwise.  Whereas ‘eq’ tests if its
     arguments are the same object, ‘equal’ looks inside nonidentical
     arguments to see if their elements or contents are the same.  So,
     if two objects are ‘eq’, they are ‘equal’, but the converse is not
     always true.

          (equal 'foo 'foo)
               ⇒ t

          (equal 456 456)
               ⇒ t

          (equal "asdf" "asdf")
               ⇒ t
          (eq "asdf" "asdf")
               ⇒ nil

          (equal '(1 (2 (3))) '(1 (2 (3))))
               ⇒ t
          (eq '(1 (2 (3))) '(1 (2 (3))))
               ⇒ nil

          (equal [(1 2) 3] [(1 2) 3])
               ⇒ t
          (eq [(1 2) 3] [(1 2) 3])
               ⇒ nil

          (equal (point-marker) (point-marker))
               ⇒ t

          (eq (point-marker) (point-marker))
               ⇒ nil

     Comparison of strings is case-sensitive, but does not take account
     of text properties—it compares only the characters in the strings.
     *Note Text Properties::.  Use ‘equal-including-properties’ to also
     compare text properties.  For technical reasons, a unibyte string
     and a multibyte string are ‘equal’ if and only if they contain the
     same sequence of character codes and all these codes are either in
     the range 0 through 127 (ASCII) or 160 through 255
     (‘eight-bit-graphic’).  (*note Text Representations::).

          (equal "asdf" "ASDF")
               ⇒ nil

     However, two distinct buffers are never considered ‘equal’, even if
     their textual contents are the same.

   The test for equality is implemented recursively; for example, given
two cons cells X and Y, ‘(equal X Y)’ returns ‘t’ if and only if both
the expressions below return ‘t’:

     (equal (car X) (car Y))
     (equal (cdr X) (cdr Y))

   Because of this recursive method, circular lists may therefore cause
infinite recursion (leading to an error).

 -- Function: equal-including-properties object1 object2
     This function behaves like ‘equal’ in all cases but also requires
     that for two strings to be equal, they have the same text
     properties.

          (equal "asdf" (propertize "asdf" 'asdf t))
               ⇒ t
          (equal-including-properties "asdf"
                                      (propertize "asdf" 'asdf t))
               ⇒ nil


File: elisp.info,  Node: Numbers,  Next: Strings and Characters,  Prev: Lisp Data Types,  Up: Top

3 Numbers
*********

GNU Emacs supports two numeric data types: “integers” and
“floating-point numbers”.  Integers are whole numbers such as −3, 0, 7,
13, and 511.  Floating-point numbers are numbers with fractional parts,
such as −4.5, 0.0, and 2.71828.  They can also be expressed in
exponential notation: ‘1.5e2’ is the same as ‘150.0’; here, ‘e2’ stands
for ten to the second power, and that is multiplied by 1.5.  Integer
computations are exact, though they may overflow.  Floating-point
computations often involve rounding errors, as the numbers have a fixed
amount of precision.

* Menu:

* Integer Basics::            Representation and range of integers.
* Float Basics::              Representation and range of floating point.
* Predicates on Numbers::     Testing for numbers.
* Comparison of Numbers::     Equality and inequality predicates.
* Numeric Conversions::       Converting float to integer and vice versa.
* Arithmetic Operations::     How to add, subtract, multiply and divide.
* Rounding Operations::       Explicitly rounding floating-point numbers.
* Bitwise Operations::        Logical and, or, not, shifting.
* Math Functions::            Trig, exponential and logarithmic functions.
* Random Numbers::            Obtaining random integers, predictable or not.


File: elisp.info,  Node: Integer Basics,  Next: Float Basics,  Up: Numbers

3.1 Integer Basics
==================

The range of values for an integer depends on the machine.  The minimum
range is −536,870,912 to 536,870,911 (30 bits; i.e., −2**29 to 2**29 −
1), but many machines provide a wider range.  Many examples in this
chapter assume the minimum integer width of 30 bits.

   The Lisp reader reads an integer as a sequence of digits with
optional initial sign and optional final period.  An integer that is out
of the Emacs range is treated as a floating-point number.

      1               ; The integer 1.
      1.              ; The integer 1.
     +1               ; Also the integer 1.
     -1               ; The integer −1.
      9000000000000000000
                      ; The floating-point number 9e18.
      0               ; The integer 0.
     -0               ; The integer 0.

   The syntax for integers in bases other than 10 uses ‘#’ followed by a
letter that specifies the radix: ‘b’ for binary, ‘o’ for octal, ‘x’ for
hex, or ‘RADIXr’ to specify radix RADIX.  Case is not significant for
the letter that specifies the radix.  Thus, ‘#bINTEGER’ reads INTEGER in
binary, and ‘#RADIXrINTEGER’ reads INTEGER in radix RADIX.  Allowed
values of RADIX run from 2 to 36.  For example:

     #b101100 ⇒ 44
     #o54 ⇒ 44
     #x2c ⇒ 44
     #24r1k ⇒ 44

   To understand how various functions work on integers, especially the
bitwise operators (*note Bitwise Operations::), it is often helpful to
view the numbers in their binary form.

   In 30-bit binary, the decimal integer 5 looks like this:

     0000...000101 (30 bits total)

(The ‘...’ stands for enough bits to fill out a 30-bit word; in this
case, ‘...’ stands for twenty 0 bits.  Later examples also use the ‘...’
notation to make binary integers easier to read.)

   The integer −1 looks like this:

     1111...111111 (30 bits total)

−1 is represented as 30 ones.  (This is called “two’s complement”
notation.)

   Subtracting 4 from −1 returns the negative integer −5.  In binary,
the decimal integer 4 is 100.  Consequently, −5 looks like this:

     1111...111011 (30 bits total)

   In this implementation, the largest 30-bit binary integer is
536,870,911 in decimal.  In binary, it looks like this:

     0111...111111 (30 bits total)

   Since the arithmetic functions do not check whether integers go
outside their range, when you add 1 to 536,870,911, the value is the
negative integer −536,870,912:

     (+ 1 536870911)
          ⇒ -536870912
          ⇒ 1000...000000 (30 bits total)

   Many of the functions described in this chapter accept markers for
arguments in place of numbers.  (*Note Markers::.)  Since the actual
arguments to such functions may be either numbers or markers, we often
give these arguments the name NUMBER-OR-MARKER.  When the argument value
is a marker, its position value is used and its buffer is ignored.

 -- Variable: most-positive-fixnum
     The value of this variable is the largest integer that Emacs Lisp
     can handle.  Typical values are 2**29 − 1 on 32-bit and 2**61 − 1
     on 64-bit platforms.

 -- Variable: most-negative-fixnum
     The value of this variable is the smallest integer that Emacs Lisp
     can handle.  It is negative.  Typical values are −2**29 on 32-bit
     and −2**61 on 64-bit platforms.

   In Emacs Lisp, text characters are represented by integers.  Any
integer between zero and the value of ‘(max-char)’, inclusive, is
considered to be valid as a character.  *Note Character Codes::.


File: elisp.info,  Node: Float Basics,  Next: Predicates on Numbers,  Prev: Integer Basics,  Up: Numbers

3.2 Floating-Point Basics
=========================

Floating-point numbers are useful for representing numbers that are not
integral.  The range of floating-point numbers is the same as the range
of the C data type ‘double’ on the machine you are using.  On all
computers currently supported by Emacs, this is double-precision IEEE
floating point.

   The read syntax for floating-point numbers requires either a decimal
point, an exponent, or both.  Optional signs (‘+’ or ‘-’) precede the
number and its exponent.  For example, ‘1500.0’, ‘+15e2’, ‘15.0e+2’,
‘+1500000e-3’, and ‘.15e4’ are five ways of writing a floating-point
number whose value is 1500.  They are all equivalent.  Like Common Lisp,
Emacs Lisp requires at least one digit after any decimal point in a
floating-point number; ‘1500.’ is an integer, not a floating-point
number.

   Emacs Lisp treats ‘-0.0’ as numerically equal to ordinary zero with
respect to ‘equal’ and ‘=’.  This follows the IEEE floating-point
standard, which says ‘-0.0’ and ‘0.0’ are numerically equal even though
other operations can distinguish them.

   The IEEE floating-point standard supports positive infinity and
negative infinity as floating-point values.  It also provides for a
class of values called NaN, or “not a number”; numerical functions
return such values in cases where there is no correct answer.  For
example, ‘(/ 0.0 0.0)’ returns a NaN.  Although NaN values carry a sign,
for practical purposes there is no other significant difference between
different NaN values in Emacs Lisp.

   Here are read syntaxes for these special floating-point values:

infinity
     ‘1.0e+INF’ and ‘-1.0e+INF’
not-a-number
     ‘0.0e+NaN’ and ‘-0.0e+NaN’

   The following functions are specialized for handling floating-point
numbers:

 -- Function: isnan x
     This predicate returns ‘t’ if its floating-point argument is a NaN,
     ‘nil’ otherwise.

 -- Function: frexp x
     This function returns a cons cell ‘(S . E)’, where S and E are
     respectively the significand and exponent of the floating-point
     number X.

     If X is finite, then S is a floating-point number between 0.5
     (inclusive) and 1.0 (exclusive), E is an integer, and X = S * 2**E.
     If X is zero or infinity, then S is the same as X.  If X is a NaN,
     then S is also a NaN.  If X is zero, then E is 0.

 -- Function: ldexp s e
     Given a numeric significand S and an integer exponent E, this
     function returns the floating point number S * 2**E.

 -- Function: copysign x1 x2
     This function copies the sign of X2 to the value of X1, and returns
     the result.  X1 and X2 must be floating point.

 -- Function: logb x
     This function returns the binary exponent of X.  More precisely,
     the value is the logarithm base 2 of |x|, rounded down to an
     integer.

          (logb 10)
               ⇒ 3
          (logb 10.0e20)
               ⇒ 69


File: elisp.info,  Node: Predicates on Numbers,  Next: Comparison of Numbers,  Prev: Float Basics,  Up: Numbers

3.3 Type Predicates for Numbers
===============================

The functions in this section test for numbers, or for a specific type
of number.  The functions ‘integerp’ and ‘floatp’ can take any type of
Lisp object as argument (they would not be of much use otherwise), but
the ‘zerop’ predicate requires a number as its argument.  See also
‘integer-or-marker-p’ and ‘number-or-marker-p’, in *note Predicates on
Markers::.

 -- Function: floatp object
     This predicate tests whether its argument is floating point and
     returns ‘t’ if so, ‘nil’ otherwise.

 -- Function: integerp object
     This predicate tests whether its argument is an integer, and
     returns ‘t’ if so, ‘nil’ otherwise.

 -- Function: numberp object
     This predicate tests whether its argument is a number (either
     integer or floating point), and returns ‘t’ if so, ‘nil’ otherwise.

 -- Function: natnump object
     This predicate (whose name comes from the phrase “natural number”)
     tests to see whether its argument is a nonnegative integer, and
     returns ‘t’ if so, ‘nil’ otherwise.  0 is considered non-negative.

     ‘wholenump’ is a synonym for ‘natnump’.

 -- Function: zerop number
     This predicate tests whether its argument is zero, and returns ‘t’
     if so, ‘nil’ otherwise.  The argument must be a number.

     ‘(zerop x)’ is equivalent to ‘(= x 0)’.


File: elisp.info,  Node: Comparison of Numbers,  Next: Numeric Conversions,  Prev: Predicates on Numbers,  Up: Numbers

3.4 Comparison of Numbers
=========================

To test numbers for numerical equality, you should normally use ‘=’, not
‘eq’.  There can be many distinct floating-point objects with the same
numeric value.  If you use ‘eq’ to compare them, then you test whether
two values are the same _object_.  By contrast, ‘=’ compares only the
numeric values of the objects.

   In Emacs Lisp, each integer is a unique Lisp object.  Therefore, ‘eq’
is equivalent to ‘=’ where integers are concerned.  It is sometimes
convenient to use ‘eq’ for comparing an unknown value with an integer,
because ‘eq’ does not report an error if the unknown value is not a
number—it accepts arguments of any type.  By contrast, ‘=’ signals an
error if the arguments are not numbers or markers.  However, it is
better programming practice to use ‘=’ if you can, even for comparing
integers.

   Sometimes it is useful to compare numbers with ‘equal’, which treats
two numbers as equal if they have the same data type (both integers, or
both floating point) and the same value.  By contrast, ‘=’ can treat an
integer and a floating-point number as equal.  *Note Equality
Predicates::.

   There is another wrinkle: because floating-point arithmetic is not
exact, it is often a bad idea to check for equality of floating-point
values.  Usually it is better to test for approximate equality.  Here’s
a function to do this:

     (defvar fuzz-factor 1.0e-6)
     (defun approx-equal (x y)
       (or (= x y)
           (< (/ (abs (- x y))
                 (max (abs x) (abs y)))
              fuzz-factor)))

     Common Lisp note: Comparing numbers in Common Lisp always requires
     ‘=’ because Common Lisp implements multi-word integers, and two
     distinct integer objects can have the same numeric value.  Emacs
     Lisp can have just one integer object for any given value because
     it has a limited range of integers.

 -- Function: = number-or-marker &rest number-or-markers
     This function tests whether all its arguments are numerically
     equal, and returns ‘t’ if so, ‘nil’ otherwise.

 -- Function: eql value1 value2
     This function acts like ‘eq’ except when both arguments are
     numbers.  It compares numbers by type and numeric value, so that
     ‘(eql 1.0 1)’ returns ‘nil’, but ‘(eql 1.0 1.0)’ and ‘(eql 1 1)’
     both return ‘t’.

 -- Function: /= number-or-marker1 number-or-marker2
     This function tests whether its arguments are numerically equal,
     and returns ‘t’ if they are not, and ‘nil’ if they are.

 -- Function: < number-or-marker &rest number-or-markers
     This function tests whether each argument is strictly less than the
     following argument.  It returns ‘t’ if so, ‘nil’ otherwise.

 -- Function: <= number-or-marker &rest number-or-markers
     This function tests whether each argument is less than or equal to
     the following argument.  It returns ‘t’ if so, ‘nil’ otherwise.

 -- Function: > number-or-marker &rest number-or-markers
     This function tests whether each argument is strictly greater than
     the following argument.  It returns ‘t’ if so, ‘nil’ otherwise.

 -- Function: >= number-or-marker &rest number-or-markers
     This function tests whether each argument is greater than or equal
     to the following argument.  It returns ‘t’ if so, ‘nil’ otherwise.

 -- Function: max number-or-marker &rest numbers-or-markers
     This function returns the largest of its arguments.  If any of the
     arguments is floating point, the value is returned as floating
     point, even if it was given as an integer.

          (max 20)
               ⇒ 20
          (max 1 2.5)
               ⇒ 2.5
          (max 1 3 2.5)
               ⇒ 3.0

 -- Function: min number-or-marker &rest numbers-or-markers
     This function returns the smallest of its arguments.  If any of the
     arguments is floating point, the value is returned as floating
     point, even if it was given as an integer.

          (min -4 1)
               ⇒ -4

 -- Function: abs number
     This function returns the absolute value of NUMBER.


File: elisp.info,  Node: Numeric Conversions,  Next: Arithmetic Operations,  Prev: Comparison of Numbers,  Up: Numbers

3.5 Numeric Conversions
=======================

To convert an integer to floating point, use the function ‘float’.

 -- Function: float number
     This returns NUMBER converted to floating point.  If NUMBER is
     already floating point, ‘float’ returns it unchanged.

   There are four functions to convert floating-point numbers to
integers; they differ in how they round.  All accept an argument NUMBER
and an optional argument DIVISOR.  Both arguments may be integers or
floating-point numbers.  DIVISOR may also be ‘nil’.  If DIVISOR is ‘nil’
or omitted, these functions convert NUMBER to an integer, or return it
unchanged if it already is an integer.  If DIVISOR is non-‘nil’, they
divide NUMBER by DIVISOR and convert the result to an integer.  If
DIVISOR is zero (whether integer or floating point), Emacs signals an
‘arith-error’ error.

 -- Function: truncate number &optional divisor
     This returns NUMBER, converted to an integer by rounding towards
     zero.

          (truncate 1.2)
               ⇒ 1
          (truncate 1.7)
               ⇒ 1
          (truncate -1.2)
               ⇒ -1
          (truncate -1.7)
               ⇒ -1

 -- Function: floor number &optional divisor
     This returns NUMBER, converted to an integer by rounding downward
     (towards negative infinity).

     If DIVISOR is specified, this uses the kind of division operation
     that corresponds to ‘mod’, rounding downward.

          (floor 1.2)
               ⇒ 1
          (floor 1.7)
               ⇒ 1
          (floor -1.2)
               ⇒ -2
          (floor -1.7)
               ⇒ -2
          (floor 5.99 3)
               ⇒ 1

 -- Function: ceiling number &optional divisor
     This returns NUMBER, converted to an integer by rounding upward
     (towards positive infinity).

          (ceiling 1.2)
               ⇒ 2
          (ceiling 1.7)
               ⇒ 2
          (ceiling -1.2)
               ⇒ -1
          (ceiling -1.7)
               ⇒ -1

 -- Function: round number &optional divisor
     This returns NUMBER, converted to an integer by rounding towards
     the nearest integer.  Rounding a value equidistant between two
     integers returns the even integer.

          (round 1.2)
               ⇒ 1
          (round 1.7)
               ⇒ 2
          (round -1.2)
               ⇒ -1
          (round -1.7)
               ⇒ -2


File: elisp.info,  Node: Arithmetic Operations,  Next: Rounding Operations,  Prev: Numeric Conversions,  Up: Numbers

3.6 Arithmetic Operations
=========================

Emacs Lisp provides the traditional four arithmetic operations
(addition, subtraction, multiplication, and division), as well as
remainder and modulus functions, and functions to add or subtract 1.
Except for ‘%’, each of these functions accepts both integer and
floating-point arguments, and returns a floating-point number if any
argument is floating point.

   Emacs Lisp arithmetic functions do not check for integer overflow.
Thus ‘(1+ 536870911)’ may evaluate to −536870912, depending on your
hardware.

 -- Function: 1+ number-or-marker
     This function returns NUMBER-OR-MARKER plus 1.  For example,

          (setq foo 4)
               ⇒ 4
          (1+ foo)
               ⇒ 5

     This function is not analogous to the C operator ‘++’—it does not
     increment a variable.  It just computes a sum.  Thus, if we
     continue,

          foo
               ⇒ 4

     If you want to increment the variable, you must use ‘setq’, like
     this:

          (setq foo (1+ foo))
               ⇒ 5

 -- Function: 1- number-or-marker
     This function returns NUMBER-OR-MARKER minus 1.

 -- Function: + &rest numbers-or-markers
     This function adds its arguments together.  When given no
     arguments, ‘+’ returns 0.

          (+)
               ⇒ 0
          (+ 1)
               ⇒ 1
          (+ 1 2 3 4)
               ⇒ 10

 -- Function: - &optional number-or-marker &rest more-numbers-or-markers
     The ‘-’ function serves two purposes: negation and subtraction.
     When ‘-’ has a single argument, the value is the negative of the
     argument.  When there are multiple arguments, ‘-’ subtracts each of
     the MORE-NUMBERS-OR-MARKERS from NUMBER-OR-MARKER, cumulatively.
     If there are no arguments, the result is 0.

          (- 10 1 2 3 4)
               ⇒ 0
          (- 10)
               ⇒ -10
          (-)
               ⇒ 0

 -- Function: * &rest numbers-or-markers
     This function multiplies its arguments together, and returns the
     product.  When given no arguments, ‘*’ returns 1.

          (*)
               ⇒ 1
          (* 1)
               ⇒ 1
          (* 1 2 3 4)
               ⇒ 24

 -- Function: / number &rest divisors
     With one or more DIVISORS, this function divides NUMBER by each
     divisor in DIVISORS in turn, and returns the quotient.  With no
     DIVISORS, this function returns 1/NUMBER, i.e., the multiplicative
     inverse of NUMBER.  Each argument may be a number or a marker.

     If all the arguments are integers, the result is an integer,
     obtained by rounding the quotient towards zero after each division.

          (/ 6 2)
               ⇒ 3
          (/ 5 2)
               ⇒ 2
          (/ 5.0 2)
               ⇒ 2.5
          (/ 5 2.0)
               ⇒ 2.5
          (/ 5.0 2.0)
               ⇒ 2.5
          (/ 4.0)
               ⇒ 0.25
          (/ 4)
               ⇒ 0
          (/ 25 3 2)
               ⇒ 4
          (/ -17 6)
               ⇒ -2

     If you divide an integer by the integer 0, Emacs signals an
     ‘arith-error’ error (*note Errors::).  Floating-point division of a
     nonzero number by zero yields either positive or negative infinity
     (*note Float Basics::).

 -- Function: % dividend divisor
     This function returns the integer remainder after division of
     DIVIDEND by DIVISOR.  The arguments must be integers or markers.

     For any two integers DIVIDEND and DIVISOR,

          (+ (% DIVIDEND DIVISOR)
             (* (/ DIVIDEND DIVISOR) DIVISOR))

     always equals DIVIDEND if DIVISOR is nonzero.

          (% 9 4)
               ⇒ 1
          (% -9 4)
               ⇒ -1
          (% 9 -4)
               ⇒ 1
          (% -9 -4)
               ⇒ -1

 -- Function: mod dividend divisor
     This function returns the value of DIVIDEND modulo DIVISOR; in
     other words, the remainder after division of DIVIDEND by DIVISOR,
     but with the same sign as DIVISOR.  The arguments must be numbers
     or markers.

     Unlike ‘%’, ‘mod’ permits floating-point arguments; it rounds the
     quotient downward (towards minus infinity) to an integer, and uses
     that quotient to compute the remainder.

     If DIVISOR is zero, ‘mod’ signals an ‘arith-error’ error if both
     arguments are integers, and returns a NaN otherwise.

          (mod 9 4)
               ⇒ 1
          (mod -9 4)
               ⇒ 3
          (mod 9 -4)
               ⇒ -3
          (mod -9 -4)
               ⇒ -1
          (mod 5.5 2.5)
               ⇒ .5

     For any two numbers DIVIDEND and DIVISOR,

          (+ (mod DIVIDEND DIVISOR)
             (* (floor DIVIDEND DIVISOR) DIVISOR))

     always equals DIVIDEND, subject to rounding error if either
     argument is floating point and to an ‘arith-error’ if DIVIDEND is
     an integer and DIVISOR is 0.  For ‘floor’, see *note Numeric
     Conversions::.


File: elisp.info,  Node: Rounding Operations,  Next: Bitwise Operations,  Prev: Arithmetic Operations,  Up: Numbers

3.7 Rounding Operations
=======================

The functions ‘ffloor’, ‘fceiling’, ‘fround’, and ‘ftruncate’ take a
floating-point argument and return a floating-point result whose value
is a nearby integer.  ‘ffloor’ returns the nearest integer below;
‘fceiling’, the nearest integer above; ‘ftruncate’, the nearest integer
in the direction towards zero; ‘fround’, the nearest integer.

 -- Function: ffloor float
     This function rounds FLOAT to the next lower integral value, and
     returns that value as a floating-point number.

 -- Function: fceiling float
     This function rounds FLOAT to the next higher integral value, and
     returns that value as a floating-point number.

 -- Function: ftruncate float
     This function rounds FLOAT towards zero to an integral value, and
     returns that value as a floating-point number.

 -- Function: fround float
     This function rounds FLOAT to the nearest integral value, and
     returns that value as a floating-point number.  Rounding a value
     equidistant between two integers returns the even integer.


File: elisp.info,  Node: Bitwise Operations,  Next: Math Functions,  Prev: Rounding Operations,  Up: Numbers

3.8 Bitwise Operations on Integers
==================================

In a computer, an integer is represented as a binary number, a sequence
of “bits” (digits which are either zero or one).  A bitwise operation
acts on the individual bits of such a sequence.  For example, “shifting”
moves the whole sequence left or right one or more places, reproducing
the same pattern moved over.

   The bitwise operations in Emacs Lisp apply only to integers.

 -- Function: lsh integer1 count
     ‘lsh’, which is an abbreviation for “logical shift”, shifts the
     bits in INTEGER1 to the left COUNT places, or to the right if COUNT
     is negative, bringing zeros into the vacated bits.  If COUNT is
     negative, ‘lsh’ shifts zeros into the leftmost (most-significant)
     bit, producing a positive result even if INTEGER1 is negative.
     Contrast this with ‘ash’, below.

     Here are two examples of ‘lsh’, shifting a pattern of bits one
     place to the left.  We show only the low-order eight bits of the
     binary pattern; the rest are all zero.

          (lsh 5 1)
               ⇒ 10
          ;; Decimal 5 becomes decimal 10.
          00000101 ⇒ 00001010

          (lsh 7 1)
               ⇒ 14
          ;; Decimal 7 becomes decimal 14.
          00000111 ⇒ 00001110

     As the examples illustrate, shifting the pattern of bits one place
     to the left produces a number that is twice the value of the
     previous number.

     Shifting a pattern of bits two places to the left produces results
     like this (with 8-bit binary numbers):

          (lsh 3 2)
               ⇒ 12
          ;; Decimal 3 becomes decimal 12.
          00000011 ⇒ 00001100

     On the other hand, shifting one place to the right looks like this:

          (lsh 6 -1)
               ⇒ 3
          ;; Decimal 6 becomes decimal 3.
          00000110 ⇒ 00000011

          (lsh 5 -1)
               ⇒ 2
          ;; Decimal 5 becomes decimal 2.
          00000101 ⇒ 00000010

     As the example illustrates, shifting one place to the right divides
     the value of a positive integer by two, rounding downward.

     The function ‘lsh’, like all Emacs Lisp arithmetic functions, does
     not check for overflow, so shifting left can discard significant
     bits and change the sign of the number.  For example, left shifting
     536,870,911 produces −2 in the 30-bit implementation:

          (lsh 536870911 1)          ; left shift
               ⇒ -2

     In binary, the argument looks like this:

          ;; Decimal 536,870,911
          0111...111111 (30 bits total)

     which becomes the following when left shifted:

          ;; Decimal −2
          1111...111110 (30 bits total)

 -- Function: ash integer1 count
     ‘ash’ (“arithmetic shift”) shifts the bits in INTEGER1 to the left
     COUNT places, or to the right if COUNT is negative.

     ‘ash’ gives the same results as ‘lsh’ except when INTEGER1 and
     COUNT are both negative.  In that case, ‘ash’ puts ones in the
     empty bit positions on the left, while ‘lsh’ puts zeros in those
     bit positions.

     Thus, with ‘ash’, shifting the pattern of bits one place to the
     right looks like this:

          (ash -6 -1) ⇒ -3
          ;; Decimal −6 becomes decimal −3.
          1111...111010 (30 bits total)
               ⇒
          1111...111101 (30 bits total)

     In contrast, shifting the pattern of bits one place to the right
     with ‘lsh’ looks like this:

          (lsh -6 -1) ⇒ 536870909
          ;; Decimal −6 becomes decimal 536,870,909.
          1111...111010 (30 bits total)
               ⇒
          0111...111101 (30 bits total)

     Here are other examples:

                             ;         30-bit binary values

          (lsh 5 2)          ;   5  =  0000...000101
               ⇒ 20         ;      =  0000...010100
          (ash 5 2)
               ⇒ 20
          (lsh -5 2)         ;  -5  =  1111...111011
               ⇒ -20        ;      =  1111...101100
          (ash -5 2)
               ⇒ -20
          (lsh 5 -2)         ;   5  =  0000...000101
               ⇒ 1          ;      =  0000...000001
          (ash 5 -2)
               ⇒ 1
          (lsh -5 -2)        ;  -5  =  1111...111011
               ⇒ 268435454
                             ;      =  0011...111110
          (ash -5 -2)        ;  -5  =  1111...111011
               ⇒ -2         ;      =  1111...111110

 -- Function: logand &rest ints-or-markers
     This function returns the bitwise AND of the arguments: the Nth bit
     is 1 in the result if, and only if, the Nth bit is 1 in all the
     arguments.

     For example, using 4-bit binary numbers, the bitwise AND of 13 and
     12 is 12: 1101 combined with 1100 produces 1100.  In both the
     binary numbers, the leftmost two bits are both 1 so the leftmost
     two bits of the returned value are both 1.  However, for the
     rightmost two bits, each is 0 in at least one of the arguments, so
     the rightmost two bits of the returned value are both 0.

     Therefore,

          (logand 13 12)
               ⇒ 12

     If ‘logand’ is not passed any argument, it returns a value of −1.
     This number is an identity element for ‘logand’ because its binary
     representation consists entirely of ones.  If ‘logand’ is passed
     just one argument, it returns that argument.

                             ;        30-bit binary values

          (logand 14 13)     ; 14  =  0000...001110
                             ; 13  =  0000...001101
               ⇒ 12         ; 12  =  0000...001100

          (logand 14 13 4)   ; 14  =  0000...001110
                             ; 13  =  0000...001101
                             ;  4  =  0000...000100
               ⇒ 4          ;  4  =  0000...000100

          (logand)
               ⇒ -1         ; -1  =  1111...111111

 -- Function: logior &rest ints-or-markers
     This function returns the bitwise inclusive OR of its arguments:
     the Nth bit is 1 in the result if, and only if, the Nth bit is 1 in
     at least one of the arguments.  If there are no arguments, the
     result is 0, which is an identity element for this operation.  If
     ‘logior’ is passed just one argument, it returns that argument.

                             ;        30-bit binary values

          (logior 12 5)      ; 12  =  0000...001100
                             ;  5  =  0000...000101
               ⇒ 13         ; 13  =  0000...001101

          (logior 12 5 7)    ; 12  =  0000...001100
                             ;  5  =  0000...000101
                             ;  7  =  0000...000111
               ⇒ 15         ; 15  =  0000...001111

 -- Function: logxor &rest ints-or-markers
     This function returns the bitwise exclusive OR of its arguments:
     the Nth bit is 1 in the result if, and only if, the Nth bit is 1 in
     an odd number of the arguments.  If there are no arguments, the
     result is 0, which is an identity element for this operation.  If
     ‘logxor’ is passed just one argument, it returns that argument.

                             ;        30-bit binary values

          (logxor 12 5)      ; 12  =  0000...001100
                             ;  5  =  0000...000101
               ⇒ 9          ;  9  =  0000...001001

          (logxor 12 5 7)    ; 12  =  0000...001100
                             ;  5  =  0000...000101
                             ;  7  =  0000...000111
               ⇒ 14         ; 14  =  0000...001110

 -- Function: lognot integer
     This function returns the bitwise complement of its argument: the
     Nth bit is one in the result if, and only if, the Nth bit is zero
     in INTEGER, and vice-versa.

          (lognot 5)
               ⇒ -6
          ;;  5  =  0000...000101 (30 bits total)
          ;; becomes
          ;; -6  =  1111...111010 (30 bits total)


File: elisp.info,  Node: Math Functions,  Next: Random Numbers,  Prev: Bitwise Operations,  Up: Numbers

3.9 Standard Mathematical Functions
===================================

These mathematical functions allow integers as well as floating-point
numbers as arguments.

 -- Function: sin arg
 -- Function: cos arg
 -- Function: tan arg
     These are the basic trigonometric functions, with argument ARG
     measured in radians.

 -- Function: asin arg
     The value of ‘(asin ARG)’ is a number between −pi/2 and pi/2
     (inclusive) whose sine is ARG.  If ARG is out of range (outside
     [−1, 1]), ‘asin’ returns a NaN.

 -- Function: acos arg
     The value of ‘(acos ARG)’ is a number between 0 and pi (inclusive)
     whose cosine is ARG.  If ARG is out of range (outside [−1, 1]),
     ‘acos’ returns a NaN.

 -- Function: atan y &optional x
     The value of ‘(atan Y)’ is a number between −pi/2 and pi/2
     (exclusive) whose tangent is Y.  If the optional second argument X
     is given, the value of ‘(atan y x)’ is the angle in radians between
     the vector ‘[X, Y]’ and the ‘X’ axis.

 -- Function: exp arg
     This is the exponential function; it returns e to the power ARG.

 -- Function: log arg &optional base
     This function returns the logarithm of ARG, with base BASE.  If you
     don’t specify BASE, the natural base e is used.  If ARG or BASE is
     negative, ‘log’ returns a NaN.

 -- Function: expt x y
     This function returns X raised to power Y.  If both arguments are
     integers and Y is positive, the result is an integer; in this case,
     overflow causes truncation, so watch out.  If X is a finite
     negative number and Y is a finite non-integer, ‘expt’ returns a
     NaN.

 -- Function: sqrt arg
     This returns the square root of ARG.  If ARG is finite and less
     than zero, ‘sqrt’ returns a NaN.

   In addition, Emacs defines the following common mathematical
constants:

 -- Variable: float-e
     The mathematical constant e (2.71828...).

 -- Variable: float-pi
     The mathematical constant pi (3.14159...).


File: elisp.info,  Node: Random Numbers,  Prev: Math Functions,  Up: Numbers

3.10 Random Numbers
===================

A deterministic computer program cannot generate true random numbers.
For most purposes, “pseudo-random numbers” suffice.  A series of
pseudo-random numbers is generated in a deterministic fashion.  The
numbers are not truly random, but they have certain properties that
mimic a random series.  For example, all possible values occur equally
often in a pseudo-random series.

   Pseudo-random numbers are generated from a “seed value”.  Starting
from any given seed, the ‘random’ function always generates the same
sequence of numbers.  By default, Emacs initializes the random seed at
startup, in such a way that the sequence of values of ‘random’ (with
overwhelming likelihood) differs in each Emacs run.

   Sometimes you want the random number sequence to be repeatable.  For
example, when debugging a program whose behavior depends on the random
number sequence, it is helpful to get the same behavior in each program
run.  To make the sequence repeat, execute ‘(random "")’.  This sets the
seed to a constant value for your particular Emacs executable (though it
may differ for other Emacs builds).  You can use other strings to choose
various seed values.

 -- Function: random &optional limit
     This function returns a pseudo-random integer.  Repeated calls
     return a series of pseudo-random integers.

     If LIMIT is a positive integer, the value is chosen to be
     nonnegative and less than LIMIT.  Otherwise, the value might be any
     integer representable in Lisp, i.e., an integer between
     ‘most-negative-fixnum’ and ‘most-positive-fixnum’ (*note Integer
     Basics::).

     If LIMIT is ‘t’, it means to choose a new seed as if Emacs were
     restarting, typically from the system entropy.  On systems lacking
     entropy pools, choose the seed from less-random volatile data such
     as the current time.

     If LIMIT is a string, it means to choose a new seed based on the
     string’s contents.


File: elisp.info,  Node: Strings and Characters,  Next: Lists,  Prev: Numbers,  Up: Top

4 Strings and Characters
************************

A string in Emacs Lisp is an array that contains an ordered sequence of
characters.  Strings are used as names of symbols, buffers, and files;
to send messages to users; to hold text being copied between buffers;
and for many other purposes.  Because strings are so important, Emacs
Lisp has many functions expressly for manipulating them.  Emacs Lisp
programs use strings more often than individual characters.

   *Note Strings of Events::, for special considerations for strings of
keyboard character events.

* Menu:

* Basics: String Basics.      Basic properties of strings and characters.
* Predicates for Strings::    Testing whether an object is a string or char.
* Creating Strings::          Functions to allocate new strings.
* Modifying Strings::         Altering the contents of an existing string.
* Text Comparison::           Comparing characters or strings.
* String Conversion::         Converting to and from characters and strings.
* Formatting Strings::        ‘format’: Emacs’s analogue of ‘printf’.
* Case Conversion::           Case conversion functions.
* Case Tables::               Customizing case conversion.


File: elisp.info,  Node: String Basics,  Next: Predicates for Strings,  Up: Strings and Characters

4.1 String and Character Basics
===============================

A character is a Lisp object which represents a single character of
text.  In Emacs Lisp, characters are simply integers; whether an integer
is a character or not is determined only by how it is used.  *Note
Character Codes::, for details about character representation in Emacs.

   A string is a fixed sequence of characters.  It is a type of sequence
called a “array”, meaning that its length is fixed and cannot be altered
once it is created (*note Sequences Arrays Vectors::).  Unlike in C,
Emacs Lisp strings are _not_ terminated by a distinguished character
code.

   Since strings are arrays, and therefore sequences as well, you can
operate on them with the general array and sequence functions documented
in *note Sequences Arrays Vectors::.  For example, you can access or
change individual characters in a string using the functions ‘aref’ and
‘aset’ (*note Array Functions::).  However, note that ‘length’ should
_not_ be used for computing the width of a string on display; use
‘string-width’ (*note Size of Displayed Text::) instead.

   There are two text representations for non-ASCII characters in Emacs
strings (and in buffers): unibyte and multibyte.  For most Lisp
programming, you don’t need to be concerned with these two
representations.  *Note Text Representations::, for details.

   Sometimes key sequences are represented as unibyte strings.  When a
unibyte string is a key sequence, string elements in the range 128 to
255 represent meta characters (which are large integers) rather than
character codes in the range 128 to 255.  Strings cannot hold characters
that have the hyper, super or alt modifiers; they can hold ASCII control
characters, but no other control characters.  They do not distinguish
case in ASCII control characters.  If you want to store such characters
in a sequence, such as a key sequence, you must use a vector instead of
a string.  *Note Character Type::, for more information about keyboard
input characters.

   Strings are useful for holding regular expressions.  You can also
match regular expressions against strings with ‘string-match’ (*note
Regexp Search::).  The functions ‘match-string’ (*note Simple Match
Data::) and ‘replace-match’ (*note Replacing Match::) are useful for
decomposing and modifying strings after matching regular expressions
against them.

   Like a buffer, a string can contain text properties for the
characters in it, as well as the characters themselves.  *Note Text
Properties::.  All the Lisp primitives that copy text from strings to
buffers or other strings also copy the properties of the characters
being copied.

   *Note Text::, for information about functions that display strings or
copy them into buffers.  *Note Character Type::, and *note String
Type::, for information about the syntax of characters and strings.
*Note Non-ASCII Characters::, for functions to convert between text
representations and to encode and decode character codes.


File: elisp.info,  Node: Predicates for Strings,  Next: Creating Strings,  Prev: String Basics,  Up: Strings and Characters

4.2 Predicates for Strings
==========================

For more information about general sequence and array predicates, see
*note Sequences Arrays Vectors::, and *note Arrays::.

 -- Function: stringp object
     This function returns ‘t’ if OBJECT is a string, ‘nil’ otherwise.

 -- Function: string-or-null-p object
     This function returns ‘t’ if OBJECT is a string or ‘nil’.  It
     returns ‘nil’ otherwise.

 -- Function: char-or-string-p object
     This function returns ‘t’ if OBJECT is a string or a character
     (i.e., an integer), ‘nil’ otherwise.


File: elisp.info,  Node: Creating Strings,  Next: Modifying Strings,  Prev: Predicates for Strings,  Up: Strings and Characters

4.3 Creating Strings
====================

The following functions create strings, either from scratch, or by
putting strings together, or by taking them apart.

 -- Function: make-string count character
     This function returns a string made up of COUNT repetitions of
     CHARACTER.  If COUNT is negative, an error is signaled.

          (make-string 5 ?x)
               ⇒ "xxxxx"
          (make-string 0 ?x)
               ⇒ ""

     Other functions to compare with this one include ‘make-vector’
     (*note Vectors::) and ‘make-list’ (*note Building Lists::).

 -- Function: string &rest characters
     This returns a string containing the characters CHARACTERS.

          (string ?a ?b ?c)
               ⇒ "abc"

 -- Function: substring string &optional start end
     This function returns a new string which consists of those
     characters from STRING in the range from (and including) the
     character at the index START up to (but excluding) the character at
     the index END.  The first character is at index zero.  With one
     argument, this function just copies STRING.

          (substring "abcdefg" 0 3)
               ⇒ "abc"

     In the above example, the index for ‘a’ is 0, the index for ‘b’ is
     1, and the index for ‘c’ is 2.  The index 3—which is the fourth
     character in the string—marks the character position up to which
     the substring is copied.  Thus, ‘abc’ is copied from the string
     ‘"abcdefg"’.

     A negative number counts from the end of the string, so that −1
     signifies the index of the last character of the string.  For
     example:

          (substring "abcdefg" -3 -1)
               ⇒ "ef"

     In this example, the index for ‘e’ is −3, the index for ‘f’ is −2,
     and the index for ‘g’ is −1.  Therefore, ‘e’ and ‘f’ are included,
     and ‘g’ is excluded.

     When ‘nil’ is used for END, it stands for the length of the string.
     Thus,

          (substring "abcdefg" -3 nil)
               ⇒ "efg"

     Omitting the argument END is equivalent to specifying ‘nil’.  It
     follows that ‘(substring STRING 0)’ returns a copy of all of
     STRING.

          (substring "abcdefg" 0)
               ⇒ "abcdefg"

     But we recommend ‘copy-sequence’ for this purpose (*note Sequence
     Functions::).

     If the characters copied from STRING have text properties, the
     properties are copied into the new string also.  *Note Text
     Properties::.

     ‘substring’ also accepts a vector for the first argument.  For
     example:

          (substring [a b (c) "d"] 1 3)
               ⇒ [b (c)]

     A ‘wrong-type-argument’ error is signaled if START is not an
     integer or if END is neither an integer nor ‘nil’.  An
     ‘args-out-of-range’ error is signaled if START indicates a
     character following END, or if either integer is out of range for
     STRING.

     Contrast this function with ‘buffer-substring’ (*note Buffer
     Contents::), which returns a string containing a portion of the
     text in the current buffer.  The beginning of a string is at index
     0, but the beginning of a buffer is at index 1.

 -- Function: substring-no-properties string &optional start end
     This works like ‘substring’ but discards all text properties from
     the value.  Also, START may be omitted or ‘nil’, which is
     equivalent to 0.  Thus, ‘(substring-no-properties STRING)’ returns
     a copy of STRING, with all text properties removed.

 -- Function: concat &rest sequences
     This function returns a new string consisting of the characters in
     the arguments passed to it (along with their text properties, if
     any).  The arguments may be strings, lists of numbers, or vectors
     of numbers; they are not themselves changed.  If ‘concat’ receives
     no arguments, it returns an empty string.

          (concat "abc" "-def")
               ⇒ "abc-def"
          (concat "abc" (list 120 121) [122])
               ⇒ "abcxyz"
          ;; ‘nil’ is an empty sequence.
          (concat "abc" nil "-def")
               ⇒ "abc-def"
          (concat "The " "quick brown " "fox.")
               ⇒ "The quick brown fox."
          (concat)
               ⇒ ""

     This function always constructs a new string that is not ‘eq’ to
     any existing string, except when the result is the empty string (to
     save space, Emacs makes only one empty multibyte string).

     For information about other concatenation functions, see the
     description of ‘mapconcat’ in *note Mapping Functions::, ‘vconcat’
     in *note Vector Functions::, and ‘append’ in *note Building
     Lists::.  For concatenating individual command-line arguments into
     a string to be used as a shell command, see *note
     combine-and-quote-strings: Shell Arguments.

 -- Function: split-string string &optional separators omit-nulls trim
     This function splits STRING into substrings based on the regular
     expression SEPARATORS (*note Regular Expressions::).  Each match
     for SEPARATORS defines a splitting point; the substrings between
     splitting points are made into a list, which is returned.

     If OMIT-NULLS is ‘nil’ (or omitted), the result contains null
     strings whenever there are two consecutive matches for SEPARATORS,
     or a match is adjacent to the beginning or end of STRING.  If
     OMIT-NULLS is ‘t’, these null strings are omitted from the result.

     If SEPARATORS is ‘nil’ (or omitted), the default is the value of
     ‘split-string-default-separators’.

     As a special case, when SEPARATORS is ‘nil’ (or omitted), null
     strings are always omitted from the result.  Thus:

          (split-string "  two words ")
               ⇒ ("two" "words")

     The result is not ‘("" "two" "words" "")’, which would rarely be
     useful.  If you need such a result, use an explicit value for
     SEPARATORS:

          (split-string "  two words "
                        split-string-default-separators)
               ⇒ ("" "two" "words" "")

     More examples:

          (split-string "Soup is good food" "o")
               ⇒ ("S" "up is g" "" "d f" "" "d")
          (split-string "Soup is good food" "o" t)
               ⇒ ("S" "up is g" "d f" "d")
          (split-string "Soup is good food" "o+")
               ⇒ ("S" "up is g" "d f" "d")

     Empty matches do count, except that ‘split-string’ will not look
     for a final empty match when it already reached the end of the
     string using a non-empty match or when STRING is empty:

          (split-string "aooob" "o*")
               ⇒ ("" "a" "" "b" "")
          (split-string "ooaboo" "o*")
               ⇒ ("" "" "a" "b" "")
          (split-string "" "")
               ⇒ ("")

     However, when SEPARATORS can match the empty string, OMIT-NULLS is
     usually ‘t’, so that the subtleties in the three previous examples
     are rarely relevant:

          (split-string "Soup is good food" "o*" t)
               ⇒ ("S" "u" "p" " " "i" "s" " " "g" "d" " " "f" "d")
          (split-string "Nice doggy!" "" t)
               ⇒ ("N" "i" "c" "e" " " "d" "o" "g" "g" "y" "!")
          (split-string "" "" t)
               ⇒ nil

     Somewhat odd, but predictable, behavior can occur for certain
     “non-greedy” values of SEPARATORS that can prefer empty matches
     over non-empty matches.  Again, such values rarely occur in
     practice:

          (split-string "ooo" "o*" t)
               ⇒ nil
          (split-string "ooo" "\\|o+" t)
               ⇒ ("o" "o" "o")

     If the optional argument TRIM is non-‘nil’, it should be a regular
     expression to match text to trim from the beginning and end of each
     substring.  If trimming makes the substring empty, it is treated as
     null.

     If you need to split a string into a list of individual
     command-line arguments suitable for ‘call-process’ or
     ‘start-process’, see *note split-string-and-unquote: Shell
     Arguments.

 -- Variable: split-string-default-separators
     The default value of SEPARATORS for ‘split-string’.  Its usual
     value is ‘"[ \f\t\n\r\v]+"’.


File: elisp.info,  Node: Modifying Strings,  Next: Text Comparison,  Prev: Creating Strings,  Up: Strings and Characters

4.4 Modifying Strings
=====================

The most basic way to alter the contents of an existing string is with
‘aset’ (*note Array Functions::).  ‘(aset STRING IDX CHAR)’ stores CHAR
into STRING at index IDX.  Each character occupies one or more bytes,
and if CHAR needs a different number of bytes from the character already
present at that index, ‘aset’ signals an error.

   A more powerful function is ‘store-substring’:

 -- Function: store-substring string idx obj
     This function alters part of the contents of the string STRING, by
     storing OBJ starting at index IDX.  The argument OBJ may be either
     a character or a (smaller) string.

     Since it is impossible to change the length of an existing string,
     it is an error if OBJ doesn’t fit within STRING’s actual length, or
     if any new character requires a different number of bytes from the
     character currently present at that point in STRING.

   To clear out a string that contained a password, use ‘clear-string’:

 -- Function: clear-string string
     This makes STRING a unibyte string and clears its contents to
     zeros.  It may also change STRING’s length.


File: elisp.info,  Node: Text Comparison,  Next: String Conversion,  Prev: Modifying Strings,  Up: Strings and Characters

4.5 Comparison of Characters and Strings
========================================

 -- Function: char-equal character1 character2
     This function returns ‘t’ if the arguments represent the same
     character, ‘nil’ otherwise.  This function ignores differences in
     case if ‘case-fold-search’ is non-‘nil’.

          (char-equal ?x ?x)
               ⇒ t
          (let ((case-fold-search nil))
            (char-equal ?x ?X))
               ⇒ nil

 -- Function: string= string1 string2
     This function returns ‘t’ if the characters of the two strings
     match exactly.  Symbols are also allowed as arguments, in which
     case the symbol names are used.  Case is always significant,
     regardless of ‘case-fold-search’.

     This function is equivalent to ‘equal’ for comparing two strings
     (*note Equality Predicates::).  In particular, the text properties
     of the two strings are ignored; use ‘equal-including-properties’ if
     you need to distinguish between strings that differ only in their
     text properties.  However, unlike ‘equal’, if either argument is
     not a string or symbol, ‘string=’ signals an error.

          (string= "abc" "abc")
               ⇒ t
          (string= "abc" "ABC")
               ⇒ nil
          (string= "ab" "ABC")
               ⇒ nil

     For technical reasons, a unibyte and a multibyte string are ‘equal’
     if and only if they contain the same sequence of character codes
     and all these codes are either in the range 0 through 127 (ASCII)
     or 160 through 255 (‘eight-bit-graphic’).  However, when a unibyte
     string is converted to a multibyte string, all characters with
     codes in the range 160 through 255 are converted to characters with
     higher codes, whereas ASCII characters remain unchanged.  Thus, a
     unibyte string and its conversion to multibyte are only ‘equal’ if
     the string is all ASCII.  Character codes 160 through 255 are not
     entirely proper in multibyte text, even though they can occur.  As
     a consequence, the situation where a unibyte and a multibyte string
     are ‘equal’ without both being all ASCII is a technical oddity that
     very few Emacs Lisp programmers ever get confronted with.  *Note
     Text Representations::.

 -- Function: string-equal string1 string2
     ‘string-equal’ is another name for ‘string=’.

 -- Function: string-collate-equalp string1 string2 &optional locale
          ignore-case
     This function returns ‘t’ if STRING1 and STRING2 are equal with
     respect to collation rules.  A collation rule is not only
     determined by the lexicographic order of the characters contained
     in STRING1 and STRING2, but also further rules about relations
     between these characters.  Usually, it is defined by the LOCALE
     environment Emacs is running with.

     For example, characters with different coding points but the same
     meaning might be considered as equal, like different grave accent
     Unicode characters:

          (string-collate-equalp (string ?\uFF40) (string ?\u1FEF))
               ⇒ t

     The optional argument LOCALE, a string, overrides the setting of
     your current locale identifier for collation.  The value is system
     dependent; a LOCALE ‘"en_US.UTF-8"’ is applicable on POSIX systems,
     while it would be, e.g., ‘"enu_USA.1252"’ on MS-Windows systems.

     If IGNORE-CASE is non-‘nil’, characters are converted to lower-case
     before comparing them.

     To emulate Unicode-compliant collation on MS-Windows systems, bind
     ‘w32-collate-ignore-punctuation’ to a non-‘nil’ value, since the
     codeset part of the locale cannot be ‘"UTF-8"’ on MS-Windows.

     If your system does not support a locale environment, this function
     behaves like ‘string-equal’.

     Do _not_ use this function to compare file names for equality, as
     filesystems generally don’t honor linguistic equivalence of strings
     that collation implements.

 -- Function: string< string1 string2
     This function compares two strings a character at a time.  It scans
     both the strings at the same time to find the first pair of
     corresponding characters that do not match.  If the lesser
     character of these two is the character from STRING1, then STRING1
     is less, and this function returns ‘t’.  If the lesser character is
     the one from STRING2, then STRING1 is greater, and this function
     returns ‘nil’.  If the two strings match entirely, the value is
     ‘nil’.

     Pairs of characters are compared according to their character
     codes.  Keep in mind that lower case letters have higher numeric
     values in the ASCII character set than their upper case
     counterparts; digits and many punctuation characters have a lower
     numeric value than upper case letters.  An ASCII character is less
     than any non-ASCII character; a unibyte non-ASCII character is
     always less than any multibyte non-ASCII character (*note Text
     Representations::).

          (string< "abc" "abd")
               ⇒ t
          (string< "abd" "abc")
               ⇒ nil
          (string< "123" "abc")
               ⇒ t

     When the strings have different lengths, and they match up to the
     length of STRING1, then the result is ‘t’.  If they match up to the
     length of STRING2, the result is ‘nil’.  A string of no characters
     is less than any other string.

          (string< "" "abc")
               ⇒ t
          (string< "ab" "abc")
               ⇒ t
          (string< "abc" "")
               ⇒ nil
          (string< "abc" "ab")
               ⇒ nil
          (string< "" "")
               ⇒ nil

     Symbols are also allowed as arguments, in which case their print
     names are compared.

 -- Function: string-lessp string1 string2
     ‘string-lessp’ is another name for ‘string<’.

 -- Function: string-greaterp string1 string2
     This function returns the result of comparing STRING1 and STRING2
     in the opposite order, i.e., it is equivalent to calling
     ‘(string-lessp STRING2 STRING1)’.

 -- Function: string-collate-lessp string1 string2 &optional locale
          ignore-case
     This function returns ‘t’ if STRING1 is less than STRING2 in
     collation order.  A collation order is not only determined by the
     lexicographic order of the characters contained in STRING1 and
     STRING2, but also further rules about relations between these
     characters.  Usually, it is defined by the LOCALE environment Emacs
     is running with.

     For example, punctuation and whitespace characters might be ignored
     for sorting (*note Sequence Functions::):

          (sort '("11" "12" "1 1" "1 2" "1.1" "1.2") 'string-collate-lessp)
               ⇒ ("11" "1 1" "1.1" "12" "1 2" "1.2")

     This behavior is system-dependent; e.g., punctuation and whitespace
     are never ignored on Cygwin, regardless of locale.

     The optional argument LOCALE, a string, overrides the setting of
     your current locale identifier for collation.  The value is system
     dependent; a LOCALE ‘"en_US.UTF-8"’ is applicable on POSIX systems,
     while it would be, e.g., ‘"enu_USA.1252"’ on MS-Windows systems.
     The LOCALE value of ‘"POSIX"’ or ‘"C"’ lets ‘string-collate-lessp’
     behave like ‘string-lessp’:

          (sort '("11" "12" "1 1" "1 2" "1.1" "1.2")
                (lambda (s1 s2) (string-collate-lessp s1 s2 "POSIX")))
               ⇒ ("1 1" "1 2" "1.1" "1.2" "11" "12")

     If IGNORE-CASE is non-‘nil’, characters are converted to lower-case
     before comparing them.

     To emulate Unicode-compliant collation on MS-Windows systems, bind
     ‘w32-collate-ignore-punctuation’ to a non-‘nil’ value, since the
     codeset part of the locale cannot be ‘"UTF-8"’ on MS-Windows.

     If your system does not support a locale environment, this function
     behaves like ‘string-lessp’.

 -- Function: string-prefix-p string1 string2 &optional ignore-case
     This function returns non-‘nil’ if STRING1 is a prefix of STRING2;
     i.e., if STRING2 starts with STRING1.  If the optional argument
     IGNORE-CASE is non-‘nil’, the comparison ignores case differences.

 -- Function: string-suffix-p suffix string &optional ignore-case
     This function returns non-‘nil’ if SUFFIX is a suffix of STRING;
     i.e., if STRING ends with SUFFIX.  If the optional argument
     IGNORE-CASE is non-‘nil’, the comparison ignores case differences.

 -- Function: compare-strings string1 start1 end1 string2 start2 end2
          &optional ignore-case
     This function compares a specified part of STRING1 with a specified
     part of STRING2.  The specified part of STRING1 runs from index
     START1 (inclusive) up to index END1 (exclusive); ‘nil’ for START1
     means the start of the string, while ‘nil’ for END1 means the
     length of the string.  Likewise, the specified part of STRING2 runs
     from index START2 up to index END2.

     The strings are compared by the numeric values of their characters.
     For instance, STR1 is considered less than STR2 if its first
     differing character has a smaller numeric value.  If IGNORE-CASE is
     non-‘nil’, characters are converted to upper-case before comparing
     them.  Unibyte strings are converted to multibyte for comparison
     (*note Text Representations::), so that a unibyte string and its
     conversion to multibyte are always regarded as equal.

     If the specified portions of the two strings match, the value is
     ‘t’.  Otherwise, the value is an integer which indicates how many
     leading characters agree, and which string is less.  Its absolute
     value is one plus the number of characters that agree at the
     beginning of the two strings.  The sign is negative if STRING1 (or
     its specified portion) is less.

 -- Function: assoc-string key alist &optional case-fold
     This function works like ‘assoc’, except that KEY must be a string
     or symbol, and comparison is done using ‘compare-strings’.  Symbols
     are converted to strings before testing.  If CASE-FOLD is
     non-‘nil’, KEY and the elements of ALIST are converted to
     upper-case before comparison.  Unlike ‘assoc’, this function can
     also match elements of the alist that are strings or symbols rather
     than conses.  In particular, ALIST can be a list of strings or
     symbols rather than an actual alist.  *Note Association Lists::.

   See also the function ‘compare-buffer-substrings’ in *note Comparing
Text::, for a way to compare text in buffers.  The function
‘string-match’, which matches a regular expression against a string, can
be used for a kind of string comparison; see *note Regexp Search::.


File: elisp.info,  Node: String Conversion,  Next: Formatting Strings,  Prev: Text Comparison,  Up: Strings and Characters

4.6 Conversion of Characters and Strings
========================================

This section describes functions for converting between characters,
strings and integers.  ‘format’ (*note Formatting Strings::) and
‘prin1-to-string’ (*note Output Functions::) can also convert Lisp
objects into strings.  ‘read-from-string’ (*note Input Functions::) can
convert a string representation of a Lisp object into an object.  The
functions ‘string-to-multibyte’ and ‘string-to-unibyte’ convert the text
representation of a string (*note Converting Representations::).

   *Note Documentation::, for functions that produce textual
descriptions of text characters and general input events
(‘single-key-description’ and ‘text-char-description’).  These are used
primarily for making help messages.

 -- Function: number-to-string number
     This function returns a string consisting of the printed base-ten
     representation of NUMBER.  The returned value starts with a minus
     sign if the argument is negative.

          (number-to-string 256)
               ⇒ "256"
          (number-to-string -23)
               ⇒ "-23"
          (number-to-string -23.5)
               ⇒ "-23.5"

     ‘int-to-string’ is a semi-obsolete alias for this function.

     See also the function ‘format’ in *note Formatting Strings::.

 -- Function: string-to-number string &optional base
     This function returns the numeric value of the characters in
     STRING.  If BASE is non-‘nil’, it must be an integer between 2 and
     16 (inclusive), and integers are converted in that base.  If BASE
     is ‘nil’, then base ten is used.  Floating-point conversion only
     works in base ten; we have not implemented other radices for
     floating-point numbers, because that would be much more work and
     does not seem useful.  If STRING looks like an integer but its
     value is too large to fit into a Lisp integer, ‘string-to-number’
     returns a floating-point result.

     The parsing skips spaces and tabs at the beginning of STRING, then
     reads as much of STRING as it can interpret as a number in the
     given base.  (On some systems it ignores other whitespace at the
     beginning, not just spaces and tabs.)  If STRING cannot be
     interpreted as a number, this function returns 0.

          (string-to-number "256")
               ⇒ 256
          (string-to-number "25 is a perfect square.")
               ⇒ 25
          (string-to-number "X256")
               ⇒ 0
          (string-to-number "-4.5")
               ⇒ -4.5
          (string-to-number "1e5")
               ⇒ 100000.0

     ‘string-to-int’ is an obsolete alias for this function.

 -- Function: char-to-string character
     This function returns a new string containing one character,
     CHARACTER.  This function is semi-obsolete because the function
     ‘string’ is more general.  *Note Creating Strings::.

 -- Function: string-to-char string
     This function returns the first character in STRING.  This mostly
     identical to ‘(aref string 0)’, except that it returns 0 if the
     string is empty.  (The value is also 0 when the first character of
     STRING is the null character, ASCII code 0.)  This function may be
     eliminated in the future if it does not seem useful enough to
     retain.

   Here are some other functions that can convert to or from a string:

‘concat’
     This function converts a vector or a list into a string.  *Note
     Creating Strings::.

‘vconcat’
     This function converts a string into a vector.  *Note Vector
     Functions::.

‘append’
     This function converts a string into a list.  *Note Building
     Lists::.

‘byte-to-string’
     This function converts a byte of character data into a unibyte
     string.  *Note Converting Representations::.


File: elisp.info,  Node: Formatting Strings,  Next: Case Conversion,  Prev: String Conversion,  Up: Strings and Characters

4.7 Formatting Strings
======================

“Formatting” means constructing a string by substituting computed values
at various places in a constant string.  This constant string controls
how the other values are printed, as well as where they appear; it is
called a “format string”.

   Formatting is often useful for computing messages to be displayed.
In fact, the functions ‘message’ and ‘error’ provide the same formatting
feature described here; they differ from ‘format-message’ only in how
they use the result of formatting.

 -- Function: format string &rest objects
     This function returns a new string that is made by copying STRING
     and then replacing any format specification in the copy with
     encodings of the corresponding OBJECTS.  The arguments OBJECTS are
     the computed values to be formatted.

     The characters in STRING, other than the format specifications, are
     copied directly into the output, including their text properties,
     if any.

 -- Function: format-message string &rest objects
     This function acts like ‘format’, except it also converts any
     curved single quotes in STRING as per the value of
     ‘text-quoting-style’, and treats grave accent (`) and apostrophe
     (') as if they were curved single quotes.

     A format that quotes with grave accents and apostrophes `like this'
     typically generates curved quotes ‘like this’.  In contrast, a
     format that quotes with only apostrophes 'like this' typically
     generates two closing curved quotes ’like this’, an unusual style
     in English.  *Note Keys in Documentation::, for how the
     ‘text-quoting-style’ variable affects generated quotes.

   A format specification is a sequence of characters beginning with a
‘%’.  Thus, if there is a ‘%d’ in STRING, the ‘format’ function replaces
it with the printed representation of one of the values to be formatted
(one of the arguments OBJECTS).  For example:

     (format "The value of fill-column is %d." fill-column)
          ⇒ "The value of fill-column is 72."

   Since ‘format’ interprets ‘%’ characters as format specifications,
you should _never_ pass an arbitrary string as the first argument.  This
is particularly true when the string is generated by some Lisp code.
Unless the string is _known_ to never include any ‘%’ characters, pass
‘"%s"’, described below, as the first argument, and the string as the
second, like this:

       (format "%s" ARBITRARY-STRING)

   If STRING contains more than one format specification, the format
specifications correspond to successive values from OBJECTS.  Thus, the
first format specification in STRING uses the first such value, the
second format specification uses the second such value, and so on.  Any
extra format specifications (those for which there are no corresponding
values) cause an error.  Any extra values to be formatted are ignored.

   Certain format specifications require values of particular types.  If
you supply a value that doesn’t fit the requirements, an error is
signaled.

   Here is a table of valid format specifications:

‘%s’
     Replace the specification with the printed representation of the
     object, made without quoting (that is, using ‘princ’, not
     ‘prin1’—*note Output Functions::).  Thus, strings are represented
     by their contents alone, with no ‘"’ characters, and symbols appear
     without ‘\’ characters.

     If the object is a string, its text properties are copied into the
     output.  The text properties of the ‘%s’ itself are also copied,
     but those of the object take priority.

‘%S’
     Replace the specification with the printed representation of the
     object, made with quoting (that is, using ‘prin1’—*note Output
     Functions::).  Thus, strings are enclosed in ‘"’ characters, and
     ‘\’ characters appear where necessary before special characters.

‘%o’
     Replace the specification with the base-eight representation of an
     unsigned integer.

‘%d’
     Replace the specification with the base-ten representation of a
     signed integer.

‘%x’
‘%X’
     Replace the specification with the base-sixteen representation of
     an unsigned integer.  ‘%x’ uses lower case and ‘%X’ uses upper
     case.

‘%c’
     Replace the specification with the character which is the value
     given.

‘%e’
     Replace the specification with the exponential notation for a
     floating-point number.

‘%f’
     Replace the specification with the decimal-point notation for a
     floating-point number.

‘%g’
     Replace the specification with notation for a floating-point
     number, using either exponential notation or decimal-point
     notation.  The exponential notation is used if the exponent would
     be less than -4 or greater than or equal to the precision (default:
     6).  By default, trailing zeros are removed from the fractional
     portion of the result and a decimal-point character appears only if
     it is followed by a digit.

‘%%’
     Replace the specification with a single ‘%’.  This format
     specification is unusual in that it does not use a value.  For
     example, ‘(format "%% %d" 30)’ returns ‘"% 30"’.

   Any other format character results in an ‘Invalid format operation’
error.

   Here are several examples, which assume the typical
‘text-quoting-style’ settings:

     (format "The octal value of %d is %o,
              and the hex value is %x." 18 18 18)
          ⇒ "The octal value of 18 is 22,
              and the hex value is 12."

     (format-message
      "The name of this buffer is ‘%s’." (buffer-name))
          ⇒ "The name of this buffer is ‘strings.texi’."

     (format-message
      "The buffer object prints as `%s'." (current-buffer))
          ⇒ "The buffer object prints as ‘strings.texi’."

   A specification can have a “width”, which is a decimal number between
the ‘%’ and the specification character.  If the printed representation
of the object contains fewer characters than this width, ‘format’
extends it with padding.  The width specifier is ignored for the ‘%%’
specification.  Any padding introduced by the width specifier normally
consists of spaces inserted on the left:

     (format "%5d is padded on the left with spaces" 123)
          ⇒ "  123 is padded on the left with spaces"

If the width is too small, ‘format’ does not truncate the object’s
printed representation.  Thus, you can use a width to specify a minimum
spacing between columns with no risk of losing information.  In the
following two examples, ‘%7s’ specifies a minimum width of 7.  In the
first case, the string inserted in place of ‘%7s’ has only 3 letters,
and needs 4 blank spaces as padding.  In the second case, the string
‘"specification"’ is 13 letters wide but is not truncated.

     (format "The word '%7s' has %d letters in it."
             "foo" (length "foo"))
          ⇒ "The word '    foo' has 3 letters in it."
     (format "The word '%7s' has %d letters in it."
             "specification" (length "specification"))
          ⇒ "The word 'specification' has 13 letters in it."

   Immediately after the ‘%’ and before the optional width specifier,
you can also put certain “flag characters”.

   The flag ‘+’ inserts a plus sign before a positive number, so that it
always has a sign.  A space character as flag inserts a space before a
positive number.  (Otherwise, positive numbers start with the first
digit.)  These flags are useful for ensuring that positive numbers and
negative numbers use the same number of columns.  They are ignored
except for ‘%d’, ‘%e’, ‘%f’, ‘%g’, and if both flags are used, ‘+’ takes
precedence.

   The flag ‘#’ specifies an alternate form which depends on the format
in use.  For ‘%o’, it ensures that the result begins with a ‘0’.  For
‘%x’ and ‘%X’, it prefixes the result with ‘0x’ or ‘0X’.  For ‘%e’ and
‘%f’, the ‘#’ flag means include a decimal point even if the precision
is zero.  For ‘%g’, it always includes a decimal point, and also forces
any trailing zeros after the decimal point to be left in place where
they would otherwise be removed.

   The flag ‘0’ ensures that the padding consists of ‘0’ characters
instead of spaces.  This flag is ignored for non-numerical specification
characters like ‘%s’, ‘%S’ and ‘%c’.  These specification characters
accept the ‘0’ flag, but still pad with _spaces_.

   The flag ‘-’ causes the padding inserted by the width specifier, if
any, to be inserted on the right rather than the left.  If both ‘-’ and
‘0’ are present, the ‘0’ flag is ignored.

     (format "%06d is padded on the left with zeros" 123)
          ⇒ "000123 is padded on the left with zeros"

     (format "'%-6d' is padded on the right" 123)
          ⇒ "'123   ' is padded on the right"

     (format "The word '%-7s' actually has %d letters in it."
             "foo" (length "foo"))
          ⇒ "The word 'foo    ' actually has 3 letters in it."

   All the specification characters allow an optional “precision” before
the character (after the width, if present).  The precision is a
decimal-point ‘.’ followed by a digit-string.  For the floating-point
specifications (‘%e’ and ‘%f’), the precision specifies how many digits
following the decimal point to show; if zero, the decimal-point itself
is also omitted.  For ‘%g’, the precision specifies how many significant
digits to show (significant digits are the first digit before the
decimal point and all the digits after it).  If the precision of %g is
zero or unspecified, it is treated as 1.  For ‘%s’ and ‘%S’, the
precision truncates the string to the given width, so ‘%.3s’ shows only
the first three characters of the representation for OBJECT.  For other
specification characters, the effect of precision is what the local
library functions of the ‘printf’ family produce.


File: elisp.info,  Node: Case Conversion,  Next: Case Tables,  Prev: Formatting Strings,  Up: Strings and Characters

4.8 Case Conversion in Lisp
===========================

The character case functions change the case of single characters or of
the contents of strings.  The functions normally convert only alphabetic
characters (the letters ‘A’ through ‘Z’ and ‘a’ through ‘z’, as well as
non-ASCII letters); other characters are not altered.  You can specify a
different case conversion mapping by specifying a case table (*note Case
Tables::).

   These functions do not modify the strings that are passed to them as
arguments.

   The examples below use the characters ‘X’ and ‘x’ which have ASCII
codes 88 and 120 respectively.

 -- Function: downcase string-or-char
     This function converts STRING-OR-CHAR, which should be either a
     character or a string, to lower case.

     When STRING-OR-CHAR is a string, this function returns a new string
     in which each letter in the argument that is upper case is
     converted to lower case.  When STRING-OR-CHAR is a character, this
     function returns the corresponding lower case character (an
     integer); if the original character is lower case, or is not a
     letter, the return value is equal to the original character.

          (downcase "The cat in the hat")
               ⇒ "the cat in the hat"

          (downcase ?X)
               ⇒ 120

 -- Function: upcase string-or-char
     This function converts STRING-OR-CHAR, which should be either a
     character or a string, to upper case.

     When STRING-OR-CHAR is a string, this function returns a new string
     in which each letter in the argument that is lower case is
     converted to upper case.  When STRING-OR-CHAR is a character, this
     function returns the corresponding upper case character (an
     integer); if the original character is upper case, or is not a
     letter, the return value is equal to the original character.

          (upcase "The cat in the hat")
               ⇒ "THE CAT IN THE HAT"

          (upcase ?x)
               ⇒ 88

 -- Function: capitalize string-or-char
     This function capitalizes strings or characters.  If STRING-OR-CHAR
     is a string, the function returns a new string whose contents are a
     copy of STRING-OR-CHAR in which each word has been capitalized.
     This means that the first character of each word is converted to
     upper case, and the rest are converted to lower case.

     The definition of a word is any sequence of consecutive characters
     that are assigned to the word constituent syntax class in the
     current syntax table (*note Syntax Class Table::).

     When STRING-OR-CHAR is a character, this function does the same
     thing as ‘upcase’.

          (capitalize "The cat in the hat")
               ⇒ "The Cat In The Hat"

          (capitalize "THE 77TH-HATTED CAT")
               ⇒ "The 77th-Hatted Cat"

          (capitalize ?x)
               ⇒ 88

 -- Function: upcase-initials string-or-char
     If STRING-OR-CHAR is a string, this function capitalizes the
     initials of the words in STRING-OR-CHAR, without altering any
     letters other than the initials.  It returns a new string whose
     contents are a copy of STRING-OR-CHAR, in which each word has had
     its initial letter converted to upper case.

     The definition of a word is any sequence of consecutive characters
     that are assigned to the word constituent syntax class in the
     current syntax table (*note Syntax Class Table::).

     When the argument to ‘upcase-initials’ is a character,
     ‘upcase-initials’ has the same result as ‘upcase’.

          (upcase-initials "The CAT in the hAt")
               ⇒ "The CAT In The HAt"

   *Note Text Comparison::, for functions that compare strings; some of
them ignore case differences, or can optionally ignore case differences.


File: elisp.info,  Node: Case Tables,  Prev: Case Conversion,  Up: Strings and Characters

4.9 The Case Table
==================

You can customize case conversion by installing a special “case table”.
A case table specifies the mapping between upper case and lower case
letters.  It affects both the case conversion functions for Lisp objects
(see the previous section) and those that apply to text in the buffer
(*note Case Changes::).  Each buffer has a case table; there is also a
standard case table which is used to initialize the case table of new
buffers.

   A case table is a char-table (*note Char-Tables::) whose subtype is
‘case-table’.  This char-table maps each character into the
corresponding lower case character.  It has three extra slots, which
hold related tables:

UPCASE
     The upcase table maps each character into the corresponding upper
     case character.
CANONICALIZE
     The canonicalize table maps all of a set of case-related characters
     into a particular member of that set.
EQUIVALENCES
     The equivalences table maps each one of a set of case-related
     characters into the next character in that set.

   In simple cases, all you need to specify is the mapping to
lower-case; the three related tables will be calculated automatically
from that one.

   For some languages, upper and lower case letters are not in
one-to-one correspondence.  There may be two different lower case
letters with the same upper case equivalent.  In these cases, you need
to specify the maps for both lower case and upper case.

   The extra table CANONICALIZE maps each character to a canonical
equivalent; any two characters that are related by case-conversion have
the same canonical equivalent character.  For example, since ‘a’ and ‘A’
are related by case-conversion, they should have the same canonical
equivalent character (which should be either ‘a’ for both of them, or
‘A’ for both of them).

   The extra table EQUIVALENCES is a map that cyclically permutes each
equivalence class (of characters with the same canonical equivalent).
(For ordinary ASCII, this would map ‘a’ into ‘A’ and ‘A’ into ‘a’, and
likewise for each set of equivalent characters.)

   When constructing a case table, you can provide ‘nil’ for
CANONICALIZE; then Emacs fills in this slot from the lower case and
upper case mappings.  You can also provide ‘nil’ for EQUIVALENCES; then
Emacs fills in this slot from CANONICALIZE.  In a case table that is
actually in use, those components are non-‘nil’.  Do not try to specify
EQUIVALENCES without also specifying CANONICALIZE.

   Here are the functions for working with case tables:

 -- Function: case-table-p object
     This predicate returns non-‘nil’ if OBJECT is a valid case table.

 -- Function: set-standard-case-table table
     This function makes TABLE the standard case table, so that it will
     be used in any buffers created subsequently.

 -- Function: standard-case-table
     This returns the standard case table.

 -- Function: current-case-table
     This function returns the current buffer’s case table.

 -- Function: set-case-table table
     This sets the current buffer’s case table to TABLE.

 -- Macro: with-case-table table body...
     The ‘with-case-table’ macro saves the current case table, makes
     TABLE the current case table, evaluates the BODY forms, and finally
     restores the case table.  The return value is the value of the last
     form in BODY.  The case table is restored even in case of an
     abnormal exit via ‘throw’ or error (*note Nonlocal Exits::).

   Some language environments modify the case conversions of ASCII
characters; for example, in the Turkish language environment, the ASCII
capital I is downcased into a Turkish dotless i (‘ı’).  This can
interfere with code that requires ordinary ASCII case conversion, such
as implementations of ASCII-based network protocols.  In that case, use
the ‘with-case-table’ macro with the variable ASCII-CASE-TABLE, which
stores the unmodified case table for the ASCII character set.

 -- Variable: ascii-case-table
     The case table for the ASCII character set.  This should not be
     modified by any language environment settings.

   The following three functions are convenient subroutines for packages
that define non-ASCII character sets.  They modify the specified case
table CASE-TABLE; they also modify the standard syntax table.  *Note
Syntax Tables::.  Normally you would use these functions to change the
standard case table.

 -- Function: set-case-syntax-pair uc lc case-table
     This function specifies a pair of corresponding letters, one upper
     case and one lower case.

 -- Function: set-case-syntax-delims l r case-table
     This function makes characters L and R a matching pair of
     case-invariant delimiters.

 -- Function: set-case-syntax char syntax case-table
     This function makes CHAR case-invariant, with syntax SYNTAX.

 -- Command: describe-buffer-case-table
     This command displays a description of the contents of the current
     buffer’s case table.


File: elisp.info,  Node: Lists,  Next: Sequences Arrays Vectors,  Prev: Strings and Characters,  Up: Top

5 Lists
*******

A “list” represents a sequence of zero or more elements (which may be
any Lisp objects).  The important difference between lists and vectors
is that two or more lists can share part of their structure; in
addition, you can insert or delete elements in a list without copying
the whole list.

* Menu:

* Cons Cells::          How lists are made out of cons cells.
* List-related Predicates::        Is this object a list?  Comparing two lists.
* List Elements::       Extracting the pieces of a list.
* Building Lists::      Creating list structure.
* List Variables::      Modifying lists stored in variables.
* Modifying Lists::     Storing new pieces into an existing list.
* Sets And Lists::      A list can represent a finite mathematical set.
* Association Lists::   A list can represent a finite relation or mapping.
* Property Lists::      A list of paired elements.


File: elisp.info,  Node: Cons Cells,  Next: List-related Predicates,  Up: Lists

5.1 Lists and Cons Cells
========================

Lists in Lisp are not a primitive data type; they are built up from
“cons cells” (*note Cons Cell Type::).  A cons cell is a data object
that represents an ordered pair.  That is, it has two slots, and each
slot “holds”, or “refers to”, some Lisp object.  One slot is known as
the CAR, and the other is known as the CDR.  (These names are
traditional; see *note Cons Cell Type::.)  CDR is pronounced “could-er”.

   We say that “the CAR of this cons cell is” whatever object its CAR
slot currently holds, and likewise for the CDR.

   A list is a series of cons cells chained together, so that each cell
refers to the next one.  There is one cons cell for each element of the
list.  By convention, the CARs of the cons cells hold the elements of
the list, and the CDRs are used to chain the list (this asymmetry
between CAR and CDR is entirely a matter of convention; at the level of
cons cells, the CAR and CDR slots have similar properties).  Hence, the
CDR slot of each cons cell in a list refers to the following cons cell.

   Also by convention, the CDR of the last cons cell in a list is ‘nil’.
We call such a ‘nil’-terminated structure a “true list”.  In Emacs Lisp,
the symbol ‘nil’ is both a symbol and a list with no elements.  For
convenience, the symbol ‘nil’ is considered to have ‘nil’ as its CDR
(and also as its CAR).

   Hence, the CDR of a true list is always a true list.  The CDR of a
nonempty true list is a true list containing all the elements except the
first.

   If the CDR of a list’s last cons cell is some value other than ‘nil’,
we call the structure a “dotted list”, since its printed representation
would use dotted pair notation (*note Dotted Pair Notation::).  There is
one other possibility: some cons cell’s CDR could point to one of the
previous cons cells in the list.  We call that structure a “circular
list”.

   For some purposes, it does not matter whether a list is true,
circular or dotted.  If a program doesn’t look far enough down the list
to see the CDR of the final cons cell, it won’t care.  However, some
functions that operate on lists demand true lists and signal errors if
given a dotted list.  Most functions that try to find the end of a list
enter infinite loops if given a circular list.

   Because most cons cells are used as part of lists, we refer to any
structure made out of cons cells as a “list structure”.


File: elisp.info,  Node: List-related Predicates,  Next: List Elements,  Prev: Cons Cells,  Up: Lists

5.2 Predicates on Lists
=======================

The following predicates test whether a Lisp object is an atom, whether
it is a cons cell or is a list, or whether it is the distinguished
object ‘nil’.  (Many of these predicates can be defined in terms of the
others, but they are used so often that it is worth having them.)

 -- Function: consp object
     This function returns ‘t’ if OBJECT is a cons cell, ‘nil’
     otherwise.  ‘nil’ is not a cons cell, although it _is_ a list.

 -- Function: atom object
     This function returns ‘t’ if OBJECT is an atom, ‘nil’ otherwise.
     All objects except cons cells are atoms.  The symbol ‘nil’ is an
     atom and is also a list; it is the only Lisp object that is both.

          (atom OBJECT) ≡ (not (consp OBJECT))

 -- Function: listp object
     This function returns ‘t’ if OBJECT is a cons cell or ‘nil’.
     Otherwise, it returns ‘nil’.

          (listp '(1))
               ⇒ t
          (listp '())
               ⇒ t

 -- Function: nlistp object
     This function is the opposite of ‘listp’: it returns ‘t’ if OBJECT
     is not a list.  Otherwise, it returns ‘nil’.

          (listp OBJECT) ≡ (not (nlistp OBJECT))

 -- Function: null object
     This function returns ‘t’ if OBJECT is ‘nil’, and returns ‘nil’
     otherwise.  This function is identical to ‘not’, but as a matter of
     clarity we use ‘null’ when OBJECT is considered a list and ‘not’
     when it is considered a truth value (see ‘not’ in *note Combining
     Conditions::).

          (null '(1))
               ⇒ nil
          (null '())
               ⇒ t


File: elisp.info,  Node: List Elements,  Next: Building Lists,  Prev: List-related Predicates,  Up: Lists

5.3 Accessing Elements of Lists
===============================

 -- Function: car cons-cell
     This function returns the value referred to by the first slot of
     the cons cell CONS-CELL.  In other words, it returns the CAR of
     CONS-CELL.

     As a special case, if CONS-CELL is ‘nil’, this function returns
     ‘nil’.  Therefore, any list is a valid argument.  An error is
     signaled if the argument is not a cons cell or ‘nil’.

          (car '(a b c))
               ⇒ a
          (car '())
               ⇒ nil

 -- Function: cdr cons-cell
     This function returns the value referred to by the second slot of
     the cons cell CONS-CELL.  In other words, it returns the CDR of
     CONS-CELL.

     As a special case, if CONS-CELL is ‘nil’, this function returns
     ‘nil’; therefore, any list is a valid argument.  An error is
     signaled if the argument is not a cons cell or ‘nil’.

          (cdr '(a b c))
               ⇒ (b c)
          (cdr '())
               ⇒ nil

 -- Function: car-safe object
     This function lets you take the CAR of a cons cell while avoiding
     errors for other data types.  It returns the CAR of OBJECT if
     OBJECT is a cons cell, ‘nil’ otherwise.  This is in contrast to
     ‘car’, which signals an error if OBJECT is not a list.

          (car-safe OBJECT)
          ≡
          (let ((x OBJECT))
            (if (consp x)
                (car x)
              nil))

 -- Function: cdr-safe object
     This function lets you take the CDR of a cons cell while avoiding
     errors for other data types.  It returns the CDR of OBJECT if
     OBJECT is a cons cell, ‘nil’ otherwise.  This is in contrast to
     ‘cdr’, which signals an error if OBJECT is not a list.

          (cdr-safe OBJECT)
          ≡
          (let ((x OBJECT))
            (if (consp x)
                (cdr x)
              nil))

 -- Macro: pop listname
     This macro provides a convenient way to examine the CAR of a list,
     and take it off the list, all at once.  It operates on the list
     stored in LISTNAME.  It removes the first element from the list,
     saves the CDR into LISTNAME, then returns the removed element.

     In the simplest case, LISTNAME is an unquoted symbol naming a list;
     in that case, this macro is equivalent to
     ‘(prog1 (car listname) (setq listname (cdr listname)))’.

          x
               ⇒ (a b c)
          (pop x)
               ⇒ a
          x
               ⇒ (b c)

     More generally, LISTNAME can be a generalized variable.  In that
     case, this macro saves into LISTNAME using ‘setf’.  *Note
     Generalized Variables::.

     For the ‘push’ macro, which adds an element to a list, *Note List
     Variables::.

 -- Function: nth n list
     This function returns the Nth element of LIST.  Elements are
     numbered starting with zero, so the CAR of LIST is element number
     zero.  If the length of LIST is N or less, the value is ‘nil’.

          (nth 2 '(1 2 3 4))
               ⇒ 3
          (nth 10 '(1 2 3 4))
               ⇒ nil

          (nth n x) ≡ (car (nthcdr n x))

     The function ‘elt’ is similar, but applies to any kind of sequence.
     For historical reasons, it takes its arguments in the opposite
     order.  *Note Sequence Functions::.

 -- Function: nthcdr n list
     This function returns the Nth CDR of LIST.  In other words, it
     skips past the first N links of LIST and returns what follows.

     If N is zero, ‘nthcdr’ returns all of LIST.  If the length of LIST
     is N or less, ‘nthcdr’ returns ‘nil’.

          (nthcdr 1 '(1 2 3 4))
               ⇒ (2 3 4)
          (nthcdr 10 '(1 2 3 4))
               ⇒ nil
          (nthcdr 0 '(1 2 3 4))
               ⇒ (1 2 3 4)

 -- Function: last list &optional n
     This function returns the last link of LIST.  The ‘car’ of this
     link is the list’s last element.  If LIST is null, ‘nil’ is
     returned.  If N is non-‘nil’, the Nth-to-last link is returned
     instead, or the whole of LIST if N is bigger than LIST’s length.

 -- Function: safe-length list
     This function returns the length of LIST, with no risk of either an
     error or an infinite loop.  It generally returns the number of
     distinct cons cells in the list.  However, for circular lists, the
     value is just an upper bound; it is often too large.

     If LIST is not ‘nil’ or a cons cell, ‘safe-length’ returns 0.

   The most common way to compute the length of a list, when you are not
worried that it may be circular, is with ‘length’.  *Note Sequence
Functions::.

 -- Function: caar cons-cell
     This is the same as ‘(car (car CONS-CELL))’.

 -- Function: cadr cons-cell
     This is the same as ‘(car (cdr CONS-CELL))’ or ‘(nth 1 CONS-CELL)’.

 -- Function: cdar cons-cell
     This is the same as ‘(cdr (car CONS-CELL))’.

 -- Function: cddr cons-cell
     This is the same as ‘(cdr (cdr CONS-CELL))’ or ‘(nthcdr 2
     CONS-CELL)’.

 -- Function: butlast x &optional n
     This function returns the list X with the last element, or the last
     N elements, removed.  If N is greater than zero it makes a copy of
     the list so as not to damage the original list.  In general,
     ‘(append (butlast X N) (last X N))’ will return a list equal to X.

 -- Function: nbutlast x &optional n
     This is a version of ‘butlast’ that works by destructively
     modifying the ‘cdr’ of the appropriate element, rather than making
     a copy of the list.


File: elisp.info,  Node: Building Lists,  Next: List Variables,  Prev: List Elements,  Up: Lists

5.4 Building Cons Cells and Lists
=================================

Many functions build lists, as lists reside at the very heart of Lisp.
‘cons’ is the fundamental list-building function; however, it is
interesting to note that ‘list’ is used more times in the source code
for Emacs than ‘cons’.

 -- Function: cons object1 object2
     This function is the most basic function for building new list
     structure.  It creates a new cons cell, making OBJECT1 the CAR, and
     OBJECT2 the CDR.  It then returns the new cons cell.  The arguments
     OBJECT1 and OBJECT2 may be any Lisp objects, but most often OBJECT2
     is a list.

          (cons 1 '(2))
               ⇒ (1 2)
          (cons 1 '())
               ⇒ (1)
          (cons 1 2)
               ⇒ (1 . 2)

     ‘cons’ is often used to add a single element to the front of a
     list.  This is called “consing the element onto the list”.  (1) For
     example:

          (setq list (cons newelt list))

     Note that there is no conflict between the variable named ‘list’
     used in this example and the function named ‘list’ described below;
     any symbol can serve both purposes.

 -- Function: list &rest objects
     This function creates a list with OBJECTS as its elements.  The
     resulting list is always ‘nil’-terminated.  If no OBJECTS are
     given, the empty list is returned.

          (list 1 2 3 4 5)
               ⇒ (1 2 3 4 5)
          (list 1 2 '(3 4 5) 'foo)
               ⇒ (1 2 (3 4 5) foo)
          (list)
               ⇒ nil

 -- Function: make-list length object
     This function creates a list of LENGTH elements, in which each
     element is OBJECT.  Compare ‘make-list’ with ‘make-string’ (*note
     Creating Strings::).

          (make-list 3 'pigs)
               ⇒ (pigs pigs pigs)
          (make-list 0 'pigs)
               ⇒ nil
          (setq l (make-list 3 '(a b)))
               ⇒ ((a b) (a b) (a b))
          (eq (car l) (cadr l))
               ⇒ t

 -- Function: append &rest sequences
     This function returns a list containing all the elements of
     SEQUENCES.  The SEQUENCES may be lists, vectors, bool-vectors, or
     strings, but the last one should usually be a list.  All arguments
     except the last one are copied, so none of the arguments is
     altered.  (See ‘nconc’ in *note Rearrangement::, for a way to join
     lists with no copying.)

     More generally, the final argument to ‘append’ may be any Lisp
     object.  The final argument is not copied or converted; it becomes
     the CDR of the last cons cell in the new list.  If the final
     argument is itself a list, then its elements become in effect
     elements of the result list.  If the final element is not a list,
     the result is a dotted list since its final CDR is not ‘nil’ as
     required in a true list.

   Here is an example of using ‘append’:

     (setq trees '(pine oak))
          ⇒ (pine oak)
     (setq more-trees (append '(maple birch) trees))
          ⇒ (maple birch pine oak)

     trees
          ⇒ (pine oak)
     more-trees
          ⇒ (maple birch pine oak)
     (eq trees (cdr (cdr more-trees)))
          ⇒ t

   You can see how ‘append’ works by looking at a box diagram.  The
variable ‘trees’ is set to the list ‘(pine oak)’ and then the variable
‘more-trees’ is set to the list ‘(maple birch pine oak)’.  However, the
variable ‘trees’ continues to refer to the original list:

     more-trees                trees
     |                           |
     |     --- ---      --- ---   -> --- ---      --- ---
      --> |   |   |--> |   |   |--> |   |   |--> |   |   |--> nil
           --- ---      --- ---      --- ---      --- ---
            |            |            |            |
            |            |            |            |
             --> maple    -->birch     --> pine     --> oak

   An empty sequence contributes nothing to the value returned by
‘append’.  As a consequence of this, a final ‘nil’ argument forces a
copy of the previous argument:

     trees
          ⇒ (pine oak)
     (setq wood (append trees nil))
          ⇒ (pine oak)
     wood
          ⇒ (pine oak)
     (eq wood trees)
          ⇒ nil

This once was the usual way to copy a list, before the function
‘copy-sequence’ was invented.  *Note Sequences Arrays Vectors::.

   Here we show the use of vectors and strings as arguments to ‘append’:

     (append [a b] "cd" nil)
          ⇒ (a b 99 100)

   With the help of ‘apply’ (*note Calling Functions::), we can append
all the lists in a list of lists:

     (apply 'append '((a b c) nil (x y z) nil))
          ⇒ (a b c x y z)

   If no SEQUENCES are given, ‘nil’ is returned:

     (append)
          ⇒ nil

   Here are some examples where the final argument is not a list:

     (append '(x y) 'z)
          ⇒ (x y . z)
     (append '(x y) [z])
          ⇒ (x y . [z])

The second example shows that when the final argument is a sequence but
not a list, the sequence’s elements do not become elements of the
resulting list.  Instead, the sequence becomes the final CDR, like any
other non-list final argument.

 -- Function: copy-tree tree &optional vecp
     This function returns a copy of the tree ‘tree’.  If TREE is a cons
     cell, this makes a new cons cell with the same CAR and CDR, then
     recursively copies the CAR and CDR in the same way.

     Normally, when TREE is anything other than a cons cell, ‘copy-tree’
     simply returns TREE.  However, if VECP is non-‘nil’, it copies
     vectors too (and operates recursively on their elements).

 -- Function: number-sequence from &optional to separation
     This returns a list of numbers starting with FROM and incrementing
     by SEPARATION, and ending at or just before TO.  SEPARATION can be
     positive or negative and defaults to 1.  If TO is ‘nil’ or
     numerically equal to FROM, the value is the one-element list
     ‘(FROM)’.  If TO is less than FROM with a positive SEPARATION, or
     greater than FROM with a negative SEPARATION, the value is ‘nil’
     because those arguments specify an empty sequence.

     If SEPARATION is 0 and TO is neither ‘nil’ nor numerically equal to
     FROM, ‘number-sequence’ signals an error, since those arguments
     specify an infinite sequence.

     All arguments are numbers.  Floating-point arguments can be tricky,
     because floating-point arithmetic is inexact.  For instance,
     depending on the machine, it may quite well happen that
     ‘(number-sequence 0.4 0.6 0.2)’ returns the one element list
     ‘(0.4)’, whereas ‘(number-sequence 0.4 0.8 0.2)’ returns a list
     with three elements.  The Nth element of the list is computed by
     the exact formula ‘(+ FROM (* N SEPARATION))’.  Thus, if one wants
     to make sure that TO is included in the list, one can pass an
     expression of this exact type for TO.  Alternatively, one can
     replace TO with a slightly larger value (or a slightly more
     negative value if SEPARATION is negative).

     Some examples:

          (number-sequence 4 9)
               ⇒ (4 5 6 7 8 9)
          (number-sequence 9 4 -1)
               ⇒ (9 8 7 6 5 4)
          (number-sequence 9 4 -2)
               ⇒ (9 7 5)
          (number-sequence 8)
               ⇒ (8)
          (number-sequence 8 5)
               ⇒ nil
          (number-sequence 5 8 -1)
               ⇒ nil
          (number-sequence 1.5 6 2)
               ⇒ (1.5 3.5 5.5)

   ---------- Footnotes ----------

   (1) There is no strictly equivalent way to add an element to the end
of a list.  You can use ‘(append LISTNAME (list NEWELT))’, which creates
a whole new list by copying LISTNAME and adding NEWELT to its end.  Or
you can use ‘(nconc LISTNAME (list NEWELT))’, which modifies LISTNAME by
following all the CDRs and then replacing the terminating ‘nil’.
Compare this to adding an element to the beginning of a list with
‘cons’, which neither copies nor modifies the list.


File: elisp.info,  Node: List Variables,  Next: Modifying Lists,  Prev: Building Lists,  Up: Lists

5.5 Modifying List Variables
============================

These functions, and one macro, provide convenient ways to modify a list
which is stored in a variable.

 -- Macro: push element listname
     This macro creates a new list whose CAR is ELEMENT and whose CDR is
     the list specified by LISTNAME, and saves that list in LISTNAME.
     In the simplest case, LISTNAME is an unquoted symbol naming a list,
     and this macro is equivalent to
     ‘(setq LISTNAME (cons ELEMENT LISTNAME))’.

          (setq l '(a b))
               ⇒ (a b)
          (push 'c l)
               ⇒ (c a b)
          l
               ⇒ (c a b)

     More generally, ‘listname’ can be a generalized variable.  In that
     case, this macro does the equivalent of
     ‘(setf LISTNAME (cons ELEMENT LISTNAME))’.  *Note Generalized
     Variables::.

     For the ‘pop’ macro, which removes the first element from a list,
     *Note List Elements::.

   Two functions modify lists that are the values of variables.

 -- Function: add-to-list symbol element &optional append compare-fn
     This function sets the variable SYMBOL by consing ELEMENT onto the
     old value, if ELEMENT is not already a member of that value.  It
     returns the resulting list, whether updated or not.  The value of
     SYMBOL had better be a list already before the call.  ‘add-to-list’
     uses COMPARE-FN to compare ELEMENT against existing list members;
     if COMPARE-FN is ‘nil’, it uses ‘equal’.

     Normally, if ELEMENT is added, it is added to the front of SYMBOL,
     but if the optional argument APPEND is non-‘nil’, it is added at
     the end.

     The argument SYMBOL is not implicitly quoted; ‘add-to-list’ is an
     ordinary function, like ‘set’ and unlike ‘setq’.  Quote the
     argument yourself if that is what you want.

   Here’s a scenario showing how to use ‘add-to-list’:

     (setq foo '(a b))
          ⇒ (a b)

     (add-to-list 'foo 'c)     ;; Add ‘c’.
          ⇒ (c a b)

     (add-to-list 'foo 'b)     ;; No effect.
          ⇒ (c a b)

     foo                       ;; ‘foo’ was changed.
          ⇒ (c a b)

   An equivalent expression for ‘(add-to-list 'VAR VALUE)’ is this:

     (or (member VALUE VAR)
         (setq VAR (cons VALUE VAR)))

 -- Function: add-to-ordered-list symbol element &optional order
     This function sets the variable SYMBOL by inserting ELEMENT into
     the old value, which must be a list, at the position specified by
     ORDER.  If ELEMENT is already a member of the list, its position in
     the list is adjusted according to ORDER.  Membership is tested
     using ‘eq’.  This function returns the resulting list, whether
     updated or not.

     The ORDER is typically a number (integer or float), and the
     elements of the list are sorted in non-decreasing numerical order.

     ORDER may also be omitted or ‘nil’.  Then the numeric order of
     ELEMENT stays unchanged if it already has one; otherwise, ELEMENT
     has no numeric order.  Elements without a numeric list order are
     placed at the end of the list, in no particular order.

     Any other value for ORDER removes the numeric order of ELEMENT if
     it already has one; otherwise, it is equivalent to ‘nil’.

     The argument SYMBOL is not implicitly quoted; ‘add-to-ordered-list’
     is an ordinary function, like ‘set’ and unlike ‘setq’.  Quote the
     argument yourself if necessary.

     The ordering information is stored in a hash table on SYMBOL’s
     ‘list-order’ property.

   Here’s a scenario showing how to use ‘add-to-ordered-list’:

     (setq foo '())
          ⇒ nil

     (add-to-ordered-list 'foo 'a 1)     ;; Add ‘a’.
          ⇒ (a)

     (add-to-ordered-list 'foo 'c 3)     ;; Add ‘c’.
          ⇒ (a c)

     (add-to-ordered-list 'foo 'b 2)     ;; Add ‘b’.
          ⇒ (a b c)

     (add-to-ordered-list 'foo 'b 4)     ;; Move ‘b’.
          ⇒ (a c b)

     (add-to-ordered-list 'foo 'd)       ;; Append ‘d’.
          ⇒ (a c b d)

     (add-to-ordered-list 'foo 'e)       ;; Add ‘e’.
          ⇒ (a c b e d)

     foo                       ;; ‘foo’ was changed.
          ⇒ (a c b e d)


File: elisp.info,  Node: Modifying Lists,  Next: Sets And Lists,  Prev: List Variables,  Up: Lists

5.6 Modifying Existing List Structure
=====================================

You can modify the CAR and CDR contents of a cons cell with the
primitives ‘setcar’ and ‘setcdr’.  These are destructive operations
because they change existing list structure.

     Common Lisp note: Common Lisp uses functions ‘rplaca’ and ‘rplacd’
     to alter list structure; they change structure the same way as
     ‘setcar’ and ‘setcdr’, but the Common Lisp functions return the
     cons cell while ‘setcar’ and ‘setcdr’ return the new CAR or CDR.

* Menu:

* Setcar::          Replacing an element in a list.
* Setcdr::          Replacing part of the list backbone.
                      This can be used to remove or add elements.
* Rearrangement::   Reordering the elements in a list; combining lists.


File: elisp.info,  Node: Setcar,  Next: Setcdr,  Up: Modifying Lists

5.6.1 Altering List Elements with ‘setcar’
------------------------------------------

Changing the CAR of a cons cell is done with ‘setcar’.  When used on a
list, ‘setcar’ replaces one element of a list with a different element.

 -- Function: setcar cons object
     This function stores OBJECT as the new CAR of CONS, replacing its
     previous CAR.  In other words, it changes the CAR slot of CONS to
     refer to OBJECT.  It returns the value OBJECT.  For example:

          (setq x '(1 2))
               ⇒ (1 2)
          (setcar x 4)
               ⇒ 4
          x
               ⇒ (4 2)

   When a cons cell is part of the shared structure of several lists,
storing a new CAR into the cons changes one element of each of these
lists.  Here is an example:

     ;; Create two lists that are partly shared.
     (setq x1 '(a b c))
          ⇒ (a b c)
     (setq x2 (cons 'z (cdr x1)))
          ⇒ (z b c)

     ;; Replace the CAR of a shared link.
     (setcar (cdr x1) 'foo)
          ⇒ foo
     x1                           ; Both lists are changed.
          ⇒ (a foo c)
     x2
          ⇒ (z foo c)

     ;; Replace the CAR of a link that is not shared.
     (setcar x1 'baz)
          ⇒ baz
     x1                           ; Only one list is changed.
          ⇒ (baz foo c)
     x2
          ⇒ (z foo c)

   Here is a graphical depiction of the shared structure of the two
lists in the variables ‘x1’ and ‘x2’, showing why replacing ‘b’ changes
them both:

             --- ---        --- ---      --- ---
     x1---> |   |   |----> |   |   |--> |   |   |--> nil
             --- ---        --- ---      --- ---
              |        -->   |            |
              |       |      |            |
               --> a  |       --> b        --> c
                      |
            --- ---   |
     x2--> |   |   |--
            --- ---
             |
             |
              --> z

   Here is an alternative form of box diagram, showing the same
relationship:

     x1:
      --------------       --------------       --------------
     | car   | cdr  |     | car   | cdr  |     | car   | cdr  |
     |   a   |   o------->|   b   |   o------->|   c   |  nil |
     |       |      |  -->|       |      |     |       |      |
      --------------  |    --------------       --------------
                      |
     x2:              |
      --------------  |
     | car   | cdr  | |
     |   z   |   o----
     |       |      |
      --------------


File: elisp.info,  Node: Setcdr,  Next: Rearrangement,  Prev: Setcar,  Up: Modifying Lists

5.6.2 Altering the CDR of a List
--------------------------------

The lowest-level primitive for modifying a CDR is ‘setcdr’:

 -- Function: setcdr cons object
     This function stores OBJECT as the new CDR of CONS, replacing its
     previous CDR.  In other words, it changes the CDR slot of CONS to
     refer to OBJECT.  It returns the value OBJECT.

   Here is an example of replacing the CDR of a list with a different
list.  All but the first element of the list are removed in favor of a
different sequence of elements.  The first element is unchanged, because
it resides in the CAR of the list, and is not reached via the CDR.

     (setq x '(1 2 3))
          ⇒ (1 2 3)
     (setcdr x '(4))
          ⇒ (4)
     x
          ⇒ (1 4)

   You can delete elements from the middle of a list by altering the
CDRs of the cons cells in the list.  For example, here we delete the
second element, ‘b’, from the list ‘(a b c)’, by changing the CDR of the
first cons cell:

     (setq x1 '(a b c))
          ⇒ (a b c)
     (setcdr x1 (cdr (cdr x1)))
          ⇒ (c)
     x1
          ⇒ (a c)

   Here is the result in box notation:

                        --------------------
                       |                    |
      --------------   |   --------------   |    --------------
     | car   | cdr  |  |  | car   | cdr  |   -->| car   | cdr  |
     |   a   |   o-----   |   b   |   o-------->|   c   |  nil |
     |       |      |     |       |      |      |       |      |
      --------------       --------------        --------------

The second cons cell, which previously held the element ‘b’, still
exists and its CAR is still ‘b’, but it no longer forms part of this
list.

   It is equally easy to insert a new element by changing CDRs:

     (setq x1 '(a b c))
          ⇒ (a b c)
     (setcdr x1 (cons 'd (cdr x1)))
          ⇒ (d b c)
     x1
          ⇒ (a d b c)

   Here is this result in box notation:

      --------------        -------------       -------------
     | car  | cdr   |      | car  | cdr  |     | car  | cdr  |
     |   a  |   o   |   -->|   b  |   o------->|   c  |  nil |
     |      |   |   |  |   |      |      |     |      |      |
      --------- | --   |    -------------       -------------
                |      |
          -----         --------
         |                      |
         |    ---------------   |
         |   | car   | cdr   |  |
          -->|   d   |   o------
             |       |       |
              ---------------


File: elisp.info,  Node: Rearrangement,  Prev: Setcdr,  Up: Modifying Lists

5.6.3 Functions that Rearrange Lists
------------------------------------

Here are some functions that rearrange lists destructively by modifying
the CDRs of their component cons cells.  These functions are destructive
because they chew up the original lists passed to them as arguments,
relinking their cons cells to form a new list that is the returned
value.

   See ‘delq’, in *note Sets And Lists::, for another function that
modifies cons cells.

 -- Function: nconc &rest lists
     This function returns a list containing all the elements of LISTS.
     Unlike ‘append’ (*note Building Lists::), the LISTS are _not_
     copied.  Instead, the last CDR of each of the LISTS is changed to
     refer to the following list.  The last of the LISTS is not altered.
     For example:

          (setq x '(1 2 3))
               ⇒ (1 2 3)
          (nconc x '(4 5))
               ⇒ (1 2 3 4 5)
          x
               ⇒ (1 2 3 4 5)

     Since the last argument of ‘nconc’ is not itself modified, it is
     reasonable to use a constant list, such as ‘'(4 5)’, as in the
     above example.  For the same reason, the last argument need not be
     a list:

          (setq x '(1 2 3))
               ⇒ (1 2 3)
          (nconc x 'z)
               ⇒ (1 2 3 . z)
          x
               ⇒ (1 2 3 . z)

     However, the other arguments (all but the last) must be lists.

     A common pitfall is to use a quoted constant list as a non-last
     argument to ‘nconc’.  If you do this, your program will change each
     time you run it!  Here is what happens:

          (defun add-foo (x)            ; We want this function to add
            (nconc '(foo) x))           ;   ‘foo’ to the front of its arg.

          (symbol-function 'add-foo)
               ⇒ (lambda (x) (nconc (quote (foo)) x))

          (setq xx (add-foo '(1 2)))    ; It seems to work.
               ⇒ (foo 1 2)
          (setq xy (add-foo '(3 4)))    ; What happened?
               ⇒ (foo 1 2 3 4)
          (eq xx xy)
               ⇒ t

          (symbol-function 'add-foo)
               ⇒ (lambda (x) (nconc (quote (foo 1 2 3 4) x)))


File: elisp.info,  Node: Sets And Lists,  Next: Association Lists,  Prev: Modifying Lists,  Up: Lists

5.7 Using Lists as Sets
=======================

A list can represent an unordered mathematical set—simply consider a
value an element of a set if it appears in the list, and ignore the
order of the list.  To form the union of two sets, use ‘append’ (as long
as you don’t mind having duplicate elements).  You can remove ‘equal’
duplicates using ‘delete-dups’.  Other useful functions for sets include
‘memq’ and ‘delq’, and their ‘equal’ versions, ‘member’ and ‘delete’.

     Common Lisp note: Common Lisp has functions ‘union’ (which avoids
     duplicate elements) and ‘intersection’ for set operations.
     Although standard GNU Emacs Lisp does not have them, the ‘cl-lib’
     library provides versions.  *Note (cl)Lists as Sets::.

 -- Function: memq object list
     This function tests to see whether OBJECT is a member of LIST.  If
     it is, ‘memq’ returns a list starting with the first occurrence of
     OBJECT.  Otherwise, it returns ‘nil’.  The letter ‘q’ in ‘memq’
     says that it uses ‘eq’ to compare OBJECT against the elements of
     the list.  For example:

          (memq 'b '(a b c b a))
               ⇒ (b c b a)
          (memq '(2) '((1) (2)))    ; ‘(2)’ and ‘(2)’ are not ‘eq’.
               ⇒ nil

 -- Function: delq object list
     This function destructively removes all elements ‘eq’ to OBJECT
     from LIST, and returns the resulting list.  The letter ‘q’ in
     ‘delq’ says that it uses ‘eq’ to compare OBJECT against the
     elements of the list, like ‘memq’ and ‘remq’.

     Typically, when you invoke ‘delq’, you should use the return value
     by assigning it to the variable which held the original list.  The
     reason for this is explained below.

   The ‘delq’ function deletes elements from the front of the list by
simply advancing down the list, and returning a sublist that starts
after those elements.  For example:

     (delq 'a '(a b c)) ≡ (cdr '(a b c))

When an element to be deleted appears in the middle of the list,
removing it involves changing the CDRs (*note Setcdr::).

     (setq sample-list '(a b c (4)))
          ⇒ (a b c (4))
     (delq 'a sample-list)
          ⇒ (b c (4))
     sample-list
          ⇒ (a b c (4))
     (delq 'c sample-list)
          ⇒ (a b (4))
     sample-list
          ⇒ (a b (4))

   Note that ‘(delq 'c sample-list)’ modifies ‘sample-list’ to splice
out the third element, but ‘(delq 'a sample-list)’ does not splice
anything—it just returns a shorter list.  Don’t assume that a variable
which formerly held the argument LIST now has fewer elements, or that it
still holds the original list!  Instead, save the result of ‘delq’ and
use that.  Most often we store the result back into the variable that
held the original list:

     (setq flowers (delq 'rose flowers))

   In the following example, the ‘(4)’ that ‘delq’ attempts to match and
the ‘(4)’ in the ‘sample-list’ are not ‘eq’:

     (delq '(4) sample-list)
          ⇒ (a c (4))

   If you want to delete elements that are ‘equal’ to a given value, use
‘delete’ (see below).

 -- Function: remq object list
     This function returns a copy of LIST, with all elements removed
     which are ‘eq’ to OBJECT.  The letter ‘q’ in ‘remq’ says that it
     uses ‘eq’ to compare OBJECT against the elements of ‘list’.

          (setq sample-list '(a b c a b c))
               ⇒ (a b c a b c)
          (remq 'a sample-list)
               ⇒ (b c b c)
          sample-list
               ⇒ (a b c a b c)

 -- Function: memql object list
     The function ‘memql’ tests to see whether OBJECT is a member of
     LIST, comparing members with OBJECT using ‘eql’, so floating-point
     elements are compared by value.  If OBJECT is a member, ‘memql’
     returns a list starting with its first occurrence in LIST.
     Otherwise, it returns ‘nil’.

     Compare this with ‘memq’:

          (memql 1.2 '(1.1 1.2 1.3))  ; ‘1.2’ and ‘1.2’ are ‘eql’.
               ⇒ (1.2 1.3)
          (memq 1.2 '(1.1 1.2 1.3))  ; ‘1.2’ and ‘1.2’ are not ‘eq’.
               ⇒ nil

   The following three functions are like ‘memq’, ‘delq’ and ‘remq’, but
use ‘equal’ rather than ‘eq’ to compare elements.  *Note Equality
Predicates::.

 -- Function: member object list
     The function ‘member’ tests to see whether OBJECT is a member of
     LIST, comparing members with OBJECT using ‘equal’.  If OBJECT is a
     member, ‘member’ returns a list starting with its first occurrence
     in LIST.  Otherwise, it returns ‘nil’.

     Compare this with ‘memq’:

          (member '(2) '((1) (2)))  ; ‘(2)’ and ‘(2)’ are ‘equal’.
               ⇒ ((2))
          (memq '(2) '((1) (2)))    ; ‘(2)’ and ‘(2)’ are not ‘eq’.
               ⇒ nil
          ;; Two strings with the same contents are ‘equal’.
          (member "foo" '("foo" "bar"))
               ⇒ ("foo" "bar")

 -- Function: delete object sequence
     This function removes all elements ‘equal’ to OBJECT from SEQUENCE,
     and returns the resulting sequence.

     If SEQUENCE is a list, ‘delete’ is to ‘delq’ as ‘member’ is to
     ‘memq’: it uses ‘equal’ to compare elements with OBJECT, like
     ‘member’; when it finds an element that matches, it cuts the
     element out just as ‘delq’ would.  As with ‘delq’, you should
     typically use the return value by assigning it to the variable
     which held the original list.

     If ‘sequence’ is a vector or string, ‘delete’ returns a copy of
     ‘sequence’ with all elements ‘equal’ to ‘object’ removed.

     For example:

          (setq l '((2) (1) (2)))
          (delete '(2) l)
               ⇒ ((1))
          l
               ⇒ ((2) (1))
          ;; If you want to change ‘l’ reliably,
          ;; write ‘(setq l (delete '(2) l))’.
          (setq l '((2) (1) (2)))
          (delete '(1) l)
               ⇒ ((2) (2))
          l
               ⇒ ((2) (2))
          ;; In this case, it makes no difference whether you set ‘l’,
          ;; but you should do so for the sake of the other case.
          (delete '(2) [(2) (1) (2)])
               ⇒ [(1)]

 -- Function: remove object sequence
     This function is the non-destructive counterpart of ‘delete’.  It
     returns a copy of ‘sequence’, a list, vector, or string, with
     elements ‘equal’ to ‘object’ removed.  For example:

          (remove '(2) '((2) (1) (2)))
               ⇒ ((1))
          (remove '(2) [(2) (1) (2)])
               ⇒ [(1)]

     Common Lisp note: The functions ‘member’, ‘delete’ and ‘remove’ in
     GNU Emacs Lisp are derived from Maclisp, not Common Lisp.  The
     Common Lisp versions do not use ‘equal’ to compare elements.

 -- Function: member-ignore-case object list
     This function is like ‘member’, except that OBJECT should be a
     string and that it ignores differences in letter-case and text
     representation: upper-case and lower-case letters are treated as
     equal, and unibyte strings are converted to multibyte prior to
     comparison.

 -- Function: delete-dups list
     This function destructively removes all ‘equal’ duplicates from
     LIST, stores the result in LIST and returns it.  Of several ‘equal’
     occurrences of an element in LIST, ‘delete-dups’ keeps the first
     one.

   See also the function ‘add-to-list’, in *note List Variables::, for a
way to add an element to a list stored in a variable and used as a set.


File: elisp.info,  Node: Association Lists,  Next: Property Lists,  Prev: Sets And Lists,  Up: Lists

5.8 Association Lists
=====================

An “association list”, or “alist” for short, records a mapping from keys
to values.  It is a list of cons cells called “associations”: the CAR of
each cons cell is the “key”, and the CDR is the “associated value”.(1)

   Here is an example of an alist.  The key ‘pine’ is associated with
the value ‘cones’; the key ‘oak’ is associated with ‘acorns’; and the
key ‘maple’ is associated with ‘seeds’.

     ((pine . cones)
      (oak . acorns)
      (maple . seeds))

   Both the values and the keys in an alist may be any Lisp objects.
For example, in the following alist, the symbol ‘a’ is associated with
the number ‘1’, and the string ‘"b"’ is associated with the _list_ ‘(2
3)’, which is the CDR of the alist element:

     ((a . 1) ("b" 2 3))

   Sometimes it is better to design an alist to store the associated
value in the CAR of the CDR of the element.  Here is an example of such
an alist:

     ((rose red) (lily white) (buttercup yellow))

Here we regard ‘red’ as the value associated with ‘rose’.  One advantage
of this kind of alist is that you can store other related
information—even a list of other items—in the CDR of the CDR.  One
disadvantage is that you cannot use ‘rassq’ (see below) to find the
element containing a given value.  When neither of these considerations
is important, the choice is a matter of taste, as long as you are
consistent about it for any given alist.

   The same alist shown above could be regarded as having the associated
value in the CDR of the element; the value associated with ‘rose’ would
be the list ‘(red)’.

   Association lists are often used to record information that you might
otherwise keep on a stack, since new associations may be added easily to
the front of the list.  When searching an association list for an
association with a given key, the first one found is returned, if there
is more than one.

   In Emacs Lisp, it is _not_ an error if an element of an association
list is not a cons cell.  The alist search functions simply ignore such
elements.  Many other versions of Lisp signal errors in such cases.

   Note that property lists are similar to association lists in several
respects.  A property list behaves like an association list in which
each key can occur only once.  *Note Property Lists::, for a comparison
of property lists and association lists.

 -- Function: assoc key alist
     This function returns the first association for KEY in ALIST,
     comparing KEY against the alist elements using ‘equal’ (*note
     Equality Predicates::).  It returns ‘nil’ if no association in
     ALIST has a CAR ‘equal’ to KEY.  For example:

          (setq trees '((pine . cones) (oak . acorns) (maple . seeds)))
               ⇒ ((pine . cones) (oak . acorns) (maple . seeds))
          (assoc 'oak trees)
               ⇒ (oak . acorns)
          (cdr (assoc 'oak trees))
               ⇒ acorns
          (assoc 'birch trees)
               ⇒ nil

     Here is another example, in which the keys and values are not
     symbols:

          (setq needles-per-cluster
                '((2 "Austrian Pine" "Red Pine")
                  (3 "Pitch Pine")
                  (5 "White Pine")))

          (cdr (assoc 3 needles-per-cluster))
               ⇒ ("Pitch Pine")
          (cdr (assoc 2 needles-per-cluster))
               ⇒ ("Austrian Pine" "Red Pine")

   The function ‘assoc-string’ is much like ‘assoc’ except that it
ignores certain differences between strings.  *Note Text Comparison::.

 -- Function: rassoc value alist
     This function returns the first association with value VALUE in
     ALIST.  It returns ‘nil’ if no association in ALIST has a CDR
     ‘equal’ to VALUE.

     ‘rassoc’ is like ‘assoc’ except that it compares the CDR of each
     ALIST association instead of the CAR.  You can think of this as
     reverse ‘assoc’, finding the key for a given value.

 -- Function: assq key alist
     This function is like ‘assoc’ in that it returns the first
     association for KEY in ALIST, but it makes the comparison using
     ‘eq’ instead of ‘equal’.  ‘assq’ returns ‘nil’ if no association in
     ALIST has a CAR ‘eq’ to KEY.  This function is used more often than
     ‘assoc’, since ‘eq’ is faster than ‘equal’ and most alists use
     symbols as keys.  *Note Equality Predicates::.

          (setq trees '((pine . cones) (oak . acorns) (maple . seeds)))
               ⇒ ((pine . cones) (oak . acorns) (maple . seeds))
          (assq 'pine trees)
               ⇒ (pine . cones)

     On the other hand, ‘assq’ is not usually useful in alists where the
     keys may not be symbols:

          (setq leaves
                '(("simple leaves" . oak)
                  ("compound leaves" . horsechestnut)))

          (assq "simple leaves" leaves)
               ⇒ nil
          (assoc "simple leaves" leaves)
               ⇒ ("simple leaves" . oak)

 -- Function: alist-get key alist &optional default remove
     This function is like ‘assq’, but instead of returning the entire
     association for KEY in ALIST, ‘(KEY . VALUE)’, it returns just the
     VALUE.  If KEY is not found in ALIST, it returns DEFAULT.

     This is a generalized variable (*note Generalized Variables::) that
     can be used to change a value with ‘setf’.  When using it to set a
     value, optional argument REMOVE non-‘nil’ means to remove KEY from
     ALIST if the new value is ‘eql’ to DEFAULT.

 -- Function: rassq value alist
     This function returns the first association with value VALUE in
     ALIST.  It returns ‘nil’ if no association in ALIST has a CDR ‘eq’
     to VALUE.

     ‘rassq’ is like ‘assq’ except that it compares the CDR of each
     ALIST association instead of the CAR.  You can think of this as
     reverse ‘assq’, finding the key for a given value.

     For example:

          (setq trees '((pine . cones) (oak . acorns) (maple . seeds)))

          (rassq 'acorns trees)
               ⇒ (oak . acorns)
          (rassq 'spores trees)
               ⇒ nil

     ‘rassq’ cannot search for a value stored in the CAR of the CDR of
     an element:

          (setq colors '((rose red) (lily white) (buttercup yellow)))

          (rassq 'white colors)
               ⇒ nil

     In this case, the CDR of the association ‘(lily white)’ is not the
     symbol ‘white’, but rather the list ‘(white)’.  This becomes
     clearer if the association is written in dotted pair notation:

          (lily white) ≡ (lily . (white))

 -- Function: assoc-default key alist &optional test default
     This function searches ALIST for a match for KEY.  For each element
     of ALIST, it compares the element (if it is an atom) or the
     element’s CAR (if it is a cons) against KEY, by calling TEST with
     two arguments: the element or its CAR, and KEY.  The arguments are
     passed in that order so that you can get useful results using
     ‘string-match’ with an alist that contains regular expressions
     (*note Regexp Search::).  If TEST is omitted or ‘nil’, ‘equal’ is
     used for comparison.

     If an alist element matches KEY by this criterion, then
     ‘assoc-default’ returns a value based on this element.  If the
     element is a cons, then the value is the element’s CDR.  Otherwise,
     the return value is DEFAULT.

     If no alist element matches KEY, ‘assoc-default’ returns ‘nil’.

 -- Function: copy-alist alist
     This function returns a two-level deep copy of ALIST: it creates a
     new copy of each association, so that you can alter the
     associations of the new alist without changing the old one.

          (setq needles-per-cluster
                '((2 . ("Austrian Pine" "Red Pine"))
                  (3 . ("Pitch Pine"))
                  (5 . ("White Pine"))))
          ⇒
          ((2 "Austrian Pine" "Red Pine")
           (3 "Pitch Pine")
           (5 "White Pine"))

          (setq copy (copy-alist needles-per-cluster))
          ⇒
          ((2 "Austrian Pine" "Red Pine")
           (3 "Pitch Pine")
           (5 "White Pine"))

          (eq needles-per-cluster copy)
               ⇒ nil
          (equal needles-per-cluster copy)
               ⇒ t
          (eq (car needles-per-cluster) (car copy))
               ⇒ nil
          (cdr (car (cdr needles-per-cluster)))
               ⇒ ("Pitch Pine")
          (eq (cdr (car (cdr needles-per-cluster)))
              (cdr (car (cdr copy))))
               ⇒ t

     This example shows how ‘copy-alist’ makes it possible to change the
     associations of one copy without affecting the other:

          (setcdr (assq 3 copy) '("Martian Vacuum Pine"))
          (cdr (assq 3 needles-per-cluster))
               ⇒ ("Pitch Pine")

 -- Function: assq-delete-all key alist
     This function deletes from ALIST all the elements whose CAR is ‘eq’
     to KEY, much as if you used ‘delq’ to delete each such element one
     by one.  It returns the shortened alist, and often modifies the
     original list structure of ALIST.  For correct results, use the
     return value of ‘assq-delete-all’ rather than looking at the saved
     value of ALIST.

          (setq alist '((foo 1) (bar 2) (foo 3) (lose 4)))
               ⇒ ((foo 1) (bar 2) (foo 3) (lose 4))
          (assq-delete-all 'foo alist)
               ⇒ ((bar 2) (lose 4))
          alist
               ⇒ ((foo 1) (bar 2) (lose 4))

 -- Function: rassq-delete-all value alist
     This function deletes from ALIST all the elements whose CDR is ‘eq’
     to VALUE.  It returns the shortened alist, and often modifies the
     original list structure of ALIST.  ‘rassq-delete-all’ is like
     ‘assq-delete-all’ except that it compares the CDR of each ALIST
     association instead of the CAR.

   ---------- Footnotes ----------

   (1) This usage of “key” is not related to the term “key sequence”; it
means a value used to look up an item in a table.  In this case, the
table is the alist, and the alist associations are the items.


File: elisp.info,  Node: Property Lists,  Prev: Association Lists,  Up: Lists

5.9 Property Lists
==================

A “property list” (“plist” for short) is a list of paired elements.
Each of the pairs associates a property name (usually a symbol) with a
property or value.  Here is an example of a property list:

     (pine cones numbers (1 2 3) color "blue")

This property list associates ‘pine’ with ‘cones’, ‘numbers’ with ‘(1 2
3)’, and ‘color’ with ‘"blue"’.  The property names and values can be
any Lisp objects, but the names are usually symbols (as they are in this
example).

   Property lists are used in several contexts.  For instance, the
function ‘put-text-property’ takes an argument which is a property list,
specifying text properties and associated values which are to be applied
to text in a string or buffer.  *Note Text Properties::.

   Another prominent use of property lists is for storing symbol
properties.  Every symbol possesses a list of properties, used to record
miscellaneous information about the symbol; these properties are stored
in the form of a property list.  *Note Symbol Properties::.

* Menu:

* Plists and Alists::           Comparison of the advantages of property
                                  lists and association lists.
* Plist Access::                Accessing property lists stored elsewhere.


File: elisp.info,  Node: Plists and Alists,  Next: Plist Access,  Up: Property Lists

5.9.1 Property Lists and Association Lists
------------------------------------------

Association lists (*note Association Lists::) are very similar to
property lists.  In contrast to association lists, the order of the
pairs in the property list is not significant, since the property names
must be distinct.

   Property lists are better than association lists for attaching
information to various Lisp function names or variables.  If your
program keeps all such information in one association list, it will
typically need to search that entire list each time it checks for an
association for a particular Lisp function name or variable, which could
be slow.  By contrast, if you keep the same information in the property
lists of the function names or variables themselves, each search will
scan only the length of one property list, which is usually short.  This
is why the documentation for a variable is recorded in a property named
‘variable-documentation’.  The byte compiler likewise uses properties to
record those functions needing special treatment.

   However, association lists have their own advantages.  Depending on
your application, it may be faster to add an association to the front of
an association list than to update a property.  All properties for a
symbol are stored in the same property list, so there is a possibility
of a conflict between different uses of a property name.  (For this
reason, it is a good idea to choose property names that are probably
unique, such as by beginning the property name with the program’s usual
name-prefix for variables and functions.)  An association list may be
used like a stack where associations are pushed on the front of the list
and later discarded; this is not possible with a property list.


File: elisp.info,  Node: Plist Access,  Prev: Plists and Alists,  Up: Property Lists

5.9.2 Property Lists Outside Symbols
------------------------------------

The following functions can be used to manipulate property lists.  They
all compare property names using ‘eq’.

 -- Function: plist-get plist property
     This returns the value of the PROPERTY property stored in the
     property list PLIST.  It accepts a malformed PLIST argument.  If
     PROPERTY is not found in the PLIST, it returns ‘nil’.  For example,

          (plist-get '(foo 4) 'foo)
               ⇒ 4
          (plist-get '(foo 4 bad) 'foo)
               ⇒ 4
          (plist-get '(foo 4 bad) 'bad)
               ⇒ nil
          (plist-get '(foo 4 bad) 'bar)
               ⇒ nil

 -- Function: plist-put plist property value
     This stores VALUE as the value of the PROPERTY property in the
     property list PLIST.  It may modify PLIST destructively, or it may
     construct a new list structure without altering the old.  The
     function returns the modified property list, so you can store that
     back in the place where you got PLIST.  For example,

          (setq my-plist '(bar t foo 4))
               ⇒ (bar t foo 4)
          (setq my-plist (plist-put my-plist 'foo 69))
               ⇒ (bar t foo 69)
          (setq my-plist (plist-put my-plist 'quux '(a)))
               ⇒ (bar t foo 69 quux (a))

 -- Function: lax-plist-get plist property
     Like ‘plist-get’ except that it compares properties using ‘equal’
     instead of ‘eq’.

 -- Function: lax-plist-put plist property value
     Like ‘plist-put’ except that it compares properties using ‘equal’
     instead of ‘eq’.

 -- Function: plist-member plist property
     This returns non-‘nil’ if PLIST contains the given PROPERTY.
     Unlike ‘plist-get’, this allows you to distinguish between a
     missing property and a property with the value ‘nil’.  The value is
     actually the tail of PLIST whose ‘car’ is PROPERTY.


File: elisp.info,  Node: Sequences Arrays Vectors,  Next: Hash Tables,  Prev: Lists,  Up: Top

6 Sequences, Arrays, and Vectors
********************************

The “sequence” type is the union of two other Lisp types: lists and
arrays.  In other words, any list is a sequence, and any array is a
sequence.  The common property that all sequences have is that each is
an ordered collection of elements.

   An “array” is a fixed-length object with a slot for each of its
elements.  All the elements are accessible in constant time.  The four
types of arrays are strings, vectors, char-tables and bool-vectors.

   A list is a sequence of elements, but it is not a single primitive
object; it is made of cons cells, one cell per element.  Finding the Nth
element requires looking through N cons cells, so elements farther from
the beginning of the list take longer to access.  But it is possible to
add elements to the list, or remove elements.

   The following diagram shows the relationship between these types:

               _____________________________________________
              |                                             |
              |          Sequence                           |
              |  ______   ________________________________  |
              | |      | |                                | |
              | | List | |             Array              | |
              | |      | |    ________       ________     | |
              | |______| |   |        |     |        |    | |
              |          |   | Vector |     | String |    | |
              |          |   |________|     |________|    | |
              |          |  ____________   _____________  | |
              |          | |            | |             | | |
              |          | | Char-table | | Bool-vector | | |
              |          | |____________| |_____________| | |
              |          |________________________________| |
              |_____________________________________________|

* Menu:

* Sequence Functions::    Functions that accept any kind of sequence.
* Arrays::                Characteristics of arrays in Emacs Lisp.
* Array Functions::       Functions specifically for arrays.
* Vectors::               Special characteristics of Emacs Lisp vectors.
* Vector Functions::      Functions specifically for vectors.
* Char-Tables::           How to work with char-tables.
* Bool-Vectors::          How to work with bool-vectors.
* Rings::                 Managing a fixed-size ring of objects.


File: elisp.info,  Node: Sequence Functions,  Next: Arrays,  Up: Sequences Arrays Vectors

6.1 Sequences
=============

This section describes functions that accept any kind of sequence.

 -- Function: sequencep object
     This function returns ‘t’ if OBJECT is a list, vector, string,
     bool-vector, or char-table, ‘nil’ otherwise.

 -- Function: length sequence
     This function returns the number of elements in SEQUENCE.  If
     SEQUENCE is a dotted list, a ‘wrong-type-argument’ error is
     signaled.  Circular lists may cause an infinite loop.  For a
     char-table, the value returned is always one more than the maximum
     Emacs character code.

     *Note Definition of safe-length::, for the related function
     ‘safe-length’.

          (length '(1 2 3))
              ⇒ 3
          (length ())
              ⇒ 0
          (length "foobar")
              ⇒ 6
          (length [1 2 3])
              ⇒ 3
          (length (make-bool-vector 5 nil))
              ⇒ 5

See also ‘string-bytes’, in *note Text Representations::.

   If you need to compute the width of a string on display, you should
use ‘string-width’ (*note Size of Displayed Text::), not ‘length’, since
‘length’ only counts the number of characters, but does not account for
the display width of each character.

 -- Function: elt sequence index
     This function returns the element of SEQUENCE indexed by INDEX.
     Legitimate values of INDEX are integers ranging from 0 up to one
     less than the length of SEQUENCE.  If SEQUENCE is a list,
     out-of-range values behave as for ‘nth’.  *Note Definition of
     nth::.  Otherwise, out-of-range values trigger an
     ‘args-out-of-range’ error.

          (elt [1 2 3 4] 2)
               ⇒ 3
          (elt '(1 2 3 4) 2)
               ⇒ 3
          ;; We use ‘string’ to show clearly which character ‘elt’ returns.
          (string (elt "1234" 2))
               ⇒ "3"
          (elt [1 2 3 4] 4)
               error→ Args out of range: [1 2 3 4], 4
          (elt [1 2 3 4] -1)
               error→ Args out of range: [1 2 3 4], -1

     This function generalizes ‘aref’ (*note Array Functions::) and
     ‘nth’ (*note Definition of nth::).

 -- Function: copy-sequence sequence
     This function returns a copy of SEQUENCE.  The copy is the same
     type of object as the original sequence, and it has the same
     elements in the same order.

     Storing a new element into the copy does not affect the original
     SEQUENCE, and vice versa.  However, the elements of the new
     sequence are not copies; they are identical (‘eq’) to the elements
     of the original.  Therefore, changes made within these elements, as
     found via the copied sequence, are also visible in the original
     sequence.

     If the sequence is a string with text properties, the property list
     in the copy is itself a copy, not shared with the original’s
     property list.  However, the actual values of the properties are
     shared.  *Note Text Properties::.

     This function does not work for dotted lists.  Trying to copy a
     circular list may cause an infinite loop.

     See also ‘append’ in *note Building Lists::, ‘concat’ in *note
     Creating Strings::, and ‘vconcat’ in *note Vector Functions::, for
     other ways to copy sequences.

          (setq bar '(1 2))
               ⇒ (1 2)
          (setq x (vector 'foo bar))
               ⇒ [foo (1 2)]
          (setq y (copy-sequence x))
               ⇒ [foo (1 2)]

          (eq x y)
               ⇒ nil
          (equal x y)
               ⇒ t
          (eq (elt x 1) (elt y 1))
               ⇒ t

          ;; Replacing an element of one sequence.
          (aset x 0 'quux)
          x ⇒ [quux (1 2)]
          y ⇒ [foo (1 2)]

          ;; Modifying the inside of a shared element.
          (setcar (aref x 1) 69)
          x ⇒ [quux (69 2)]
          y ⇒ [foo (69 2)]

 -- Function: reverse sequence
     This function creates a new sequence whose elements are the
     elements of SEQUENCE, but in reverse order.  The original argument
     SEQUENCE is _not_ altered.  Note that char-tables cannot be
     reversed.

          (setq x '(1 2 3 4))
               ⇒ (1 2 3 4)
          (reverse x)
               ⇒ (4 3 2 1)
          x
               ⇒ (1 2 3 4)
          (setq x [1 2 3 4])
               ⇒ [1 2 3 4]
          (reverse x)
               ⇒ [4 3 2 1]
          x
               ⇒ [1 2 3 4]
          (setq x "xyzzy")
               ⇒ "xyzzy"
          (reverse x)
               ⇒ "yzzyx"
          x
               ⇒ "xyzzy"

 -- Function: nreverse sequence
     This function reverses the order of the elements of SEQUENCE.
     Unlike ‘reverse’ the original SEQUENCE may be modified.

     For example:

          (setq x '(a b c))
               ⇒ (a b c)
          x
               ⇒ (a b c)
          (nreverse x)
               ⇒ (c b a)
          ;; The cons cell that was first is now last.
          x
               ⇒ (a)

     To avoid confusion, we usually store the result of ‘nreverse’ back
     in the same variable which held the original list:

          (setq x (nreverse x))

     Here is the ‘nreverse’ of our favorite example, ‘(a b c)’,
     presented graphically:

          Original list head:                       Reversed list:
           -------------        -------------        ------------
          | car  | cdr  |      | car  | cdr  |      | car | cdr  |
          |   a  |  nil |<--   |   b  |   o  |<--   |   c |   o  |
          |      |      |   |  |      |   |  |   |  |     |   |  |
           -------------    |   --------- | -    |   -------- | -
                            |             |      |            |
                             -------------        ------------

     For the vector, it is even simpler because you don’t need setq:

          (setq x [1 2 3 4])
               ⇒ [1 2 3 4]
          (nreverse x)
               ⇒ [4 3 2 1]
          x
               ⇒ [4 3 2 1]

     Note that unlike ‘reverse’, this function doesn’t work with
     strings.  Although you can alter string data by using ‘aset’, it is
     strongly encouraged to treat strings as immutable.

 -- Function: sort sequence predicate
     This function sorts SEQUENCE stably.  Note that this function
     doesn’t work for all sequences; it may be used only for lists and
     vectors.  If SEQUENCE is a list, it is modified destructively.
     This functions returns the sorted SEQUENCE and compares elements
     using PREDICATE.  A stable sort is one in which elements with equal
     sort keys maintain their relative order before and after the sort.
     Stability is important when successive sorts are used to order
     elements according to different criteria.

     The argument PREDICATE must be a function that accepts two
     arguments.  It is called with two elements of SEQUENCE.  To get an
     increasing order sort, the PREDICATE should return non-‘nil’ if the
     first element is “less” than the second, or ‘nil’ if not.

     The comparison function PREDICATE must give reliable results for
     any given pair of arguments, at least within a single call to
     ‘sort’.  It must be “antisymmetric”; that is, if A is less than B,
     B must not be less than A.  It must be “transitive”—that is, if A
     is less than B, and B is less than C, then A must be less than C.
     If you use a comparison function which does not meet these
     requirements, the result of ‘sort’ is unpredictable.

     The destructive aspect of ‘sort’ for lists is that it rearranges
     the cons cells forming SEQUENCE by changing CDRs.  A nondestructive
     sort function would create new cons cells to store the elements in
     their sorted order.  If you wish to make a sorted copy without
     destroying the original, copy it first with ‘copy-sequence’ and
     then sort.

     Sorting does not change the CARs of the cons cells in SEQUENCE; the
     cons cell that originally contained the element ‘a’ in SEQUENCE
     still has ‘a’ in its CAR after sorting, but it now appears in a
     different position in the list due to the change of CDRs.  For
     example:

          (setq nums '(1 3 2 6 5 4 0))
               ⇒ (1 3 2 6 5 4 0)
          (sort nums '<)
               ⇒ (0 1 2 3 4 5 6)
          nums
               ⇒ (1 2 3 4 5 6)

     *Warning*: Note that the list in ‘nums’ no longer contains 0; this
     is the same cons cell that it was before, but it is no longer the
     first one in the list.  Don’t assume a variable that formerly held
     the argument now holds the entire sorted list!  Instead, save the
     result of ‘sort’ and use that.  Most often we store the result back
     into the variable that held the original list:

          (setq nums (sort nums '<))

     For the better understanding of what stable sort is, consider the
     following vector example.  After sorting, all items whose ‘car’ is
     8 are grouped at the beginning of ‘vector’, but their relative
     order is preserved.  All items whose ‘car’ is 9 are grouped at the
     end of ‘vector’, but their relative order is also preserved:

          (setq
            vector
            (vector '(8 . "xxx") '(9 . "aaa") '(8 . "bbb") '(9 . "zzz")
                    '(9 . "ppp") '(8 . "ttt") '(8 . "eee") '(9 . "fff")))
               ⇒ [(8 . "xxx") (9 . "aaa") (8 . "bbb") (9 . "zzz")
                   (9 . "ppp") (8 . "ttt") (8 . "eee") (9 . "fff")]
          (sort vector (lambda (x y) (< (car x) (car y))))
               ⇒ [(8 . "xxx") (8 . "bbb") (8 . "ttt") (8 . "eee")
                   (9 . "aaa") (9 . "zzz") (9 . "ppp") (9 . "fff")]

     *Note Sorting::, for more functions that perform sorting.  See
     ‘documentation’ in *note Accessing Documentation::, for a useful
     example of ‘sort’.

   The ‘seq.el’ library provides the following additional sequence
manipulation macros and functions, prefixed with ‘seq-’.  To use them,
you must first load the ‘seq’ library.

   All functions defined in this library are free of side-effects; i.e.,
they do not modify any sequence (list, vector, or string) that you pass
as an argument.  Unless otherwise stated, the result is a sequence of
the same type as the input.  For those functions that take a predicate,
this should be a function of one argument.

   The ‘seq.el’ library can be extended to work with additional types of
sequential data-structures.  For that purpose, all functions are defined
using ‘cl-defgeneric’.  *Note Generic Functions::, for more details
about using ‘cl-defgeneric’ for adding extensions.

 -- Function: seq-elt sequence index
     This function returns the element of SEQUENCE at the specified
     INDEX, which is an integer whose valid value range is zero to one
     less than the length of SEQUENCE.  For out-of-range values on
     built-in sequence types, ‘seq-elt’ behaves like ‘elt’.  For the
     details, see *note Definition of elt::.

          (seq-elt [1 2 3 4] 2)
          ⇒ 3

     ‘seq-elt’ returns places settable using ‘setf’ (*note Setting
     Generalized Variables::).

          (setq vec [1 2 3 4])
          (setf (seq-elt vec 2) 5)
          vec
          ⇒ [1 2 5 4]

 -- Function: seq-length sequence
     This function returns the number of elements in SEQUENCE.  For
     built-in sequence types, ‘seq-length’ behaves like ‘length’.  *Note
     Definition of length::.

 -- Function: seqp sequence
     This function returns non-‘nil’ if SEQUENCE is a sequence (a list
     or array), or any additional type of sequence defined via ‘seq.el’
     generic functions.

          (seqp [1 2])
          ⇒ t
          (seqp 2)
          ⇒ nil

 -- Function: seq-drop sequence n
     This function returns all but the first N (an integer) elements of
     SEQUENCE.  If N is negative or zero, the result is SEQUENCE.

          (seq-drop [1 2 3 4 5 6] 3)
          ⇒ [4 5 6]
          (seq-drop "hello world" -4)
          ⇒ "hello world"

 -- Function: seq-take sequence n
     This function returns the first N (an integer) elements of
     SEQUENCE.  If N is negative or zero, the result is ‘nil’.

          (seq-take '(1 2 3 4) 3)
          ⇒ (1 2 3)
          (seq-take [1 2 3 4] 0)
          ⇒ []

 -- Function: seq-take-while predicate sequence
     This function returns the members of SEQUENCE in order, stopping
     before the first one for which PREDICATE returns ‘nil’.

          (seq-take-while (lambda (elt) (> elt 0)) '(1 2 3 -1 -2))
          ⇒ (1 2 3)
          (seq-take-while (lambda (elt) (> elt 0)) [-1 4 6])
          ⇒ []

 -- Function: seq-drop-while predicate sequence
     This function returns the members of SEQUENCE in order, starting
     from the first one for which PREDICATE returns ‘nil’.

          (seq-drop-while (lambda (elt) (> elt 0)) '(1 2 3 -1 -2))
          ⇒ (-1 -2)
          (seq-drop-while (lambda (elt) (< elt 0)) [1 4 6])
          ⇒ [1 4 6]

 -- Function: seq-do function sequence
     This function applies FUNCTION to each element of SEQUENCE in turn
     (presumably for side effects), and returns SEQUENCE.

 -- Function: seq-map function sequence
     This function returns the result of applying FUNCTION to each
     element of SEQUENCE.  The returned value is a list.

          (seq-map #'1+ '(2 4 6))
          ⇒ (3 5 7)
          (seq-map #'symbol-name [foo bar])
          ⇒ ("foo" "bar")

 -- Function: seq-mapn function &rest sequences
     This function returns the result of applying FUNCTION to each
     element of SEQUENCES.  The arity (*note sub-arity: What Is a
     Function.) of FUNCTION must match the number of sequences.  Mapping
     stops at the end of the shortest sequence, and the returned value
     is a list.

          (seq-mapn #'+ '(2 4 6) '(20 40 60))
          ⇒ (22 44 66)
          (seq-mapn #'concat '("moskito" "bite") ["bee" "sting"])
          ⇒ ("moskitobee" "bitesting")

 -- Function: seq-filter predicate sequence
     This function returns a list of all the elements in SEQUENCE for
     which PREDICATE returns non-‘nil’.

          (seq-filter (lambda (elt) (> elt 0)) [1 -1 3 -3 5])
          ⇒ (1 3 5)
          (seq-filter (lambda (elt) (> elt 0)) '(-1 -3 -5))
          ⇒ nil

 -- Function: seq-remove predicate sequence
     This function returns a list of all the elements in SEQUENCE for
     which PREDICATE returns ‘nil’.

          (seq-remove (lambda (elt) (> elt 0)) [1 -1 3 -3 5])
          ⇒ (-1 -3)
          (seq-remove (lambda (elt) (< elt 0)) '(-1 -3 -5))
          ⇒ nil

 -- Function: seq-reduce function sequence initial-value
     This function returns the result of calling FUNCTION with
     INITIAL-VALUE and the first element of SEQUENCE, then calling
     FUNCTION with that result and the second element of SEQUENCE, then
     with that result and the third element of SEQUENCE, etc.  FUNCTION
     should be a function of two arguments.  If SEQUENCE is empty, this
     returns INITIAL-VALUE without calling FUNCTION.

          (seq-reduce #'+ [1 2 3 4] 0)
          ⇒ 10
          (seq-reduce #'+ '(1 2 3 4) 5)
          ⇒ 15
          (seq-reduce #'+ '() 3)
          ⇒ 3

 -- Function: seq-some predicate sequence
     This function returns the first non-‘nil’ value returned by
     applying PREDICATE to each element of SEQUENCE in turn.

          (seq-some #'numberp ["abc" 1 nil])
          ⇒ t
          (seq-some #'numberp ["abc" "def"])
          ⇒ nil
          (seq-some #'null ["abc" 1 nil])
          ⇒ t
          (seq-some #'1+ [2 4 6])
          ⇒ 3

 -- Function: seq-find predicate sequence &optional default
     This function returns the first element in SEQUENCE for which
     PREDICATE returns non-‘nil’.  If no element matches PREDICATE, the
     function returns DEFAULT.

     Note that this function has an ambiguity if the found element is
     identical to DEFAULT, as in that case it cannot be known whether an
     element was found or not.

          (seq-find #'numberp ["abc" 1 nil])
          ⇒ 1
          (seq-find #'numberp ["abc" "def"])
          ⇒ nil

 -- Function: seq-every-p predicate sequence
     This function returns non-‘nil’ if applying PREDICATE to every
     element of SEQUENCE returns non-‘nil’.

          (seq-every-p #'numberp [2 4 6])
          ⇒ t
          (seq-some #'numberp [2 4 "6"])
          ⇒ nil

 -- Function: seq-empty-p sequence
     This function returns non-‘nil’ if SEQUENCE is empty.

          (seq-empty-p "not empty")
          ⇒ nil
          (seq-empty-p "")
          ⇒ t

 -- Function: seq-count predicate sequence
     This function returns the number of elements in SEQUENCE for which
     PREDICATE returns non-‘nil’.

          (seq-count (lambda (elt) (> elt 0)) [-1 2 0 3 -2])
          ⇒ 2

 -- Function: seq-sort function sequence
     This function returns a copy of SEQUENCE that is sorted according
     to FUNCTION, a function of two arguments that returns non-‘nil’ if
     the first argument should sort before the second.

 -- Function: seq-contains sequence elt &optional function
     This function returns the first element in SEQUENCE that is equal
     to ELT.  If the optional argument FUNCTION is non-‘nil’, it is a
     function of two arguments to use instead of the default ‘equal’.

          (seq-contains '(symbol1 symbol2) 'symbol1)
          ⇒ symbol1
          (seq-contains '(symbol1 symbol2) 'symbol3)
          ⇒ nil

 -- Function: seq-position sequence elt &optional function
     This function returns the index of the first element in SEQUENCE
     that is equal to ELT.  If the optional argument FUNCTION is
     non-‘nil’, it is a function of two arguments to use instead of the
     default ‘equal’.

          (seq-position '(a b c) 'b)
          ⇒ 1
          (seq-position '(a b c) 'd)
          ⇒ nil

 -- Function: seq-uniq sequence &optional function
     This function returns a list of the elements of SEQUENCE with
     duplicates removed.  If the optional argument FUNCTION is
     non-‘nil’, it is a function of two arguments to use instead of the
     default ‘equal’.

          (seq-uniq '(1 2 2 1 3))
          ⇒ (1 2 3)
          (seq-uniq '(1 2 2.0 1.0) #'=)
          ⇒ [3 4]

 -- Function: seq-subseq sequence start &optional end
     This function returns a subset of SEQUENCE from START to END, both
     integers (END defaults to the last element).  If START or END is
     negative, it counts from the end of SEQUENCE.

          (seq-subseq '(1 2 3 4 5) 1)
          ⇒ (2 3 4 5)
          (seq-subseq '[1 2 3 4 5] 1 3)
          ⇒ [2 3]
          (seq-subseq '[1 2 3 4 5] -3 -1)
          ⇒ [3 4]

 -- Function: seq-concatenate type &rest sequences
     This function returns a sequence of type TYPE made of the
     concatenation of SEQUENCES.  TYPE may be: ‘vector’, ‘list’ or
     ‘string’.

          (seq-concatenate 'list '(1 2) '(3 4) [5 6])
          ⇒ (1 2 3 4 5 6)
          (seq-concatenate 'string "Hello " "world")
          ⇒ "Hello world"

 -- Function: seq-mapcat function sequence &optional type
     This function returns the result of applying ‘seq-concatenate’ to
     the result of applying FUNCTION to each element of SEQUENCE.  The
     result is a sequence of type TYPE, or a list if TYPE is ‘nil’.

          (seq-mapcat #'seq-reverse '((3 2 1) (6 5 4)))
          ⇒ (1 2 3 4 5 6)

 -- Function: seq-partition sequence n
     This function returns a list of the elements of SEQUENCE grouped
     into sub-sequences of length N.  The last sequence may contain less
     elements than N.  N must be an integer.  If N is a negative integer
     or 0, the return value is ‘nil’.

          (seq-partition '(0 1 2 3 4 5 6 7) 3)
          ⇒ ((0 1 2) (3 4 5) (6 7))

 -- Function: seq-intersection sequence1 sequence2 &optional function
     This function returns a list of the elements that appear both in
     SEQUENCE1 and SEQUENCE2.  If the optional argument FUNCTION is
     non-‘nil’, it is a function of two arguments to use to compare
     elements instead of the default ‘equal’.

          (seq-intersection [2 3 4 5] [1 3 5 6 7])
          ⇒ (3 5)

 -- Function: seq-difference sequence1 sequence2 &optional function
     This function returns a list of the elements that appear in
     SEQUENCE1 but not in SEQUENCE2.  If the optional argument FUNCTION
     is non-‘nil’, it is a function of two arguments to use to compare
     elements instead of the default ‘equal’.

          (seq-difference '(2 3 4 5) [1 3 5 6 7])
          ⇒ (2 4)

 -- Function: seq-group-by function sequence
     This function separates the elements of SEQUENCE into an alist
     whose keys are the result of applying FUNCTION to each element of
     SEQUENCE.  Keys are compared using ‘equal’.

          (seq-group-by #'integerp '(1 2.1 3 2 3.2))
          ⇒ ((t 1 3 2) (nil 2.1 3.2))
          (seq-group-by #'car '((a 1) (b 2) (a 3) (c 4)))
          ⇒ ((b (b 2)) (a (a 1) (a 3)) (c (c 4)))

 -- Function: seq-into sequence type
     This function converts the sequence SEQUENCE into a sequence of
     type TYPE.  TYPE can be one of the following symbols: ‘vector’,
     ‘string’ or ‘list’.

          (seq-into [1 2 3] 'list)
          ⇒ (1 2 3)
          (seq-into nil 'vector)
          ⇒ []
          (seq-into "hello" 'vector)
          ⇒ [104 101 108 108 111]

 -- Function: seq-min sequence
     This function returns the smallest element of SEQUENCE.  The
     elements of SEQUENCE must be numbers or markers (*note Markers::).

          (seq-min [3 1 2])
          ⇒ 1
          (seq-min "Hello")
          ⇒ 72

 -- Function: seq-max sequence
     This function returns the largest element of SEQUENCE.  The
     elements of SEQUENCE must be numbers or markers.

          (seq-max [1 3 2])
          ⇒ 3
          (seq-max "Hello")
          ⇒ 111

 -- Macro: seq-doseq (var sequence) body...
     This macro is like ‘dolist’ (*note dolist: Iteration.), except that
     SEQUENCE can be a list, vector or string.  This is primarily useful
     for side-effects.

 -- Macro: seq-let arguments sequence body...
     This macro binds the variables defined in ARGUMENTS to the elements
     of SEQUENCE.  ARGUMENTS can themselves include sequences, allowing
     for nested destructuring.

     The ARGUMENTS sequence can also include the ‘&rest’ marker followed
     by a variable name to be bound to the rest of ‘sequence’.

          (seq-let [first second] [1 2 3 4]
            (list first second))
          ⇒ (1 2)
          (seq-let (_ a _ b) '(1 2 3 4)
            (list a b))
          ⇒ (2 4)
          (seq-let [a [b [c]]] [1 [2 [3]]]
            (list a b c))
          ⇒ (1 2 3)
          (seq-let [a b &rest others] [1 2 3 4]
            others)
          ⇒ [3 4]


File: elisp.info,  Node: Arrays,  Next: Array Functions,  Prev: Sequence Functions,  Up: Sequences Arrays Vectors

6.2 Arrays
==========

An “array” object has slots that hold a number of other Lisp objects,
called the elements of the array.  Any element of an array may be
accessed in constant time.  In contrast, the time to access an element
of a list is proportional to the position of that element in the list.

   Emacs defines four types of array, all one-dimensional: “strings”
(*note String Type::), “vectors” (*note Vector Type::), “bool-vectors”
(*note Bool-Vector Type::), and “char-tables” (*note Char-Table Type::).
Vectors and char-tables can hold elements of any type, but strings can
only hold characters, and bool-vectors can only hold ‘t’ and ‘nil’.

   All four kinds of array share these characteristics:

   • The first element of an array has index zero, the second element
     has index 1, and so on.  This is called “zero-origin” indexing.
     For example, an array of four elements has indices 0, 1, 2, and 3.

   • The length of the array is fixed once you create it; you cannot
     change the length of an existing array.

   • For purposes of evaluation, the array is a constant—i.e., it
     evaluates to itself.

   • The elements of an array may be referenced or changed with the
     functions ‘aref’ and ‘aset’, respectively (*note Array
     Functions::).

   When you create an array, other than a char-table, you must specify
its length.  You cannot specify the length of a char-table, because that
is determined by the range of character codes.

   In principle, if you want an array of text characters, you could use
either a string or a vector.  In practice, we always choose strings for
such applications, for four reasons:

   • They occupy one-fourth the space of a vector of the same elements.

   • Strings are printed in a way that shows the contents more clearly
     as text.

   • Strings can hold text properties.  *Note Text Properties::.

   • Many of the specialized editing and I/O facilities of Emacs accept
     only strings.  For example, you cannot insert a vector of
     characters into a buffer the way you can insert a string.  *Note
     Strings and Characters::.

   By contrast, for an array of keyboard input characters (such as a key
sequence), a vector may be necessary, because many keyboard input
characters are outside the range that will fit in a string.  *Note Key
Sequence Input::.


File: elisp.info,  Node: Array Functions,  Next: Vectors,  Prev: Arrays,  Up: Sequences Arrays Vectors

6.3 Functions that Operate on Arrays
====================================

In this section, we describe the functions that accept all types of
arrays.

 -- Function: arrayp object
     This function returns ‘t’ if OBJECT is an array (i.e., a vector, a
     string, a bool-vector or a char-table).

          (arrayp [a])
               ⇒ t
          (arrayp "asdf")
               ⇒ t
          (arrayp (syntax-table))    ;; A char-table.
               ⇒ t

 -- Function: aref array index
     This function returns the INDEXth element of ARRAY.  The first
     element is at index zero.

          (setq primes [2 3 5 7 11 13])
               ⇒ [2 3 5 7 11 13]
          (aref primes 4)
               ⇒ 11
          (aref "abcdefg" 1)
               ⇒ 98           ; ‘b’ is ASCII code 98.

     See also the function ‘elt’, in *note Sequence Functions::.

 -- Function: aset array index object
     This function sets the INDEXth element of ARRAY to be OBJECT.  It
     returns OBJECT.

          (setq w [foo bar baz])
               ⇒ [foo bar baz]
          (aset w 0 'fu)
               ⇒ fu
          w
               ⇒ [fu bar baz]

          (setq x "asdfasfd")
               ⇒ "asdfasfd"
          (aset x 3 ?Z)
               ⇒ 90
          x
               ⇒ "asdZasfd"

     If ARRAY is a string and OBJECT is not a character, a
     ‘wrong-type-argument’ error results.  The function converts a
     unibyte string to multibyte if necessary to insert a character.

 -- Function: fillarray array object
     This function fills the array ARRAY with OBJECT, so that each
     element of ARRAY is OBJECT.  It returns ARRAY.

          (setq a [a b c d e f g])
               ⇒ [a b c d e f g]
          (fillarray a 0)
               ⇒ [0 0 0 0 0 0 0]
          a
               ⇒ [0 0 0 0 0 0 0]
          (setq s "When in the course")
               ⇒ "When in the course"
          (fillarray s ?-)
               ⇒ "------------------"

     If ARRAY is a string and OBJECT is not a character, a
     ‘wrong-type-argument’ error results.

   The general sequence functions ‘copy-sequence’ and ‘length’ are often
useful for objects known to be arrays.  *Note Sequence Functions::.


File: elisp.info,  Node: Vectors,  Next: Vector Functions,  Prev: Array Functions,  Up: Sequences Arrays Vectors

6.4 Vectors
===========

A “vector” is a general-purpose array whose elements can be any Lisp
objects.  (By contrast, the elements of a string can only be characters.
*Note Strings and Characters::.)  Vectors are used in Emacs for many
purposes: as key sequences (*note Key Sequences::), as symbol-lookup
tables (*note Creating Symbols::), as part of the representation of a
byte-compiled function (*note Byte Compilation::), and more.

   Like other arrays, vectors use zero-origin indexing: the first
element has index 0.

   Vectors are printed with square brackets surrounding the elements.
Thus, a vector whose elements are the symbols ‘a’, ‘b’ and ‘a’ is
printed as ‘[a b a]’.  You can write vectors in the same way in Lisp
input.

   A vector, like a string or a number, is considered a constant for
evaluation: the result of evaluating it is the same vector.  This does
not evaluate or even examine the elements of the vector.  *Note
Self-Evaluating Forms::.

   Here are examples illustrating these principles:

     (setq avector [1 two '(three) "four" [five]])
          ⇒ [1 two (quote (three)) "four" [five]]
     (eval avector)
          ⇒ [1 two (quote (three)) "four" [five]]
     (eq avector (eval avector))
          ⇒ t


File: elisp.info,  Node: Vector Functions,  Next: Char-Tables,  Prev: Vectors,  Up: Sequences Arrays Vectors

6.5 Functions for Vectors
=========================

Here are some functions that relate to vectors:

 -- Function: vectorp object
     This function returns ‘t’ if OBJECT is a vector.

          (vectorp [a])
               ⇒ t
          (vectorp "asdf")
               ⇒ nil

 -- Function: vector &rest objects
     This function creates and returns a vector whose elements are the
     arguments, OBJECTS.

          (vector 'foo 23 [bar baz] "rats")
               ⇒ [foo 23 [bar baz] "rats"]
          (vector)
               ⇒ []

 -- Function: make-vector length object
     This function returns a new vector consisting of LENGTH elements,
     each initialized to OBJECT.

          (setq sleepy (make-vector 9 'Z))
               ⇒ [Z Z Z Z Z Z Z Z Z]

 -- Function: vconcat &rest sequences
     This function returns a new vector containing all the elements of
     SEQUENCES.  The arguments SEQUENCES may be true lists, vectors,
     strings or bool-vectors.  If no SEQUENCES are given, the empty
     vector is returned.

     The value is either the empty vector, or is a newly constructed
     nonempty vector that is not ‘eq’ to any existing vector.

          (setq a (vconcat '(A B C) '(D E F)))
               ⇒ [A B C D E F]
          (eq a (vconcat a))
               ⇒ nil
          (vconcat)
               ⇒ []
          (vconcat [A B C] "aa" '(foo (6 7)))
               ⇒ [A B C 97 97 foo (6 7)]

     The ‘vconcat’ function also allows byte-code function objects as
     arguments.  This is a special feature to make it easy to access the
     entire contents of a byte-code function object.  *Note Byte-Code
     Objects::.

     For other concatenation functions, see ‘mapconcat’ in *note Mapping
     Functions::, ‘concat’ in *note Creating Strings::, and ‘append’ in
     *note Building Lists::.

   The ‘append’ function also provides a way to convert a vector into a
list with the same elements:

     (setq avector [1 two (quote (three)) "four" [five]])
          ⇒ [1 two (quote (three)) "four" [five]]
     (append avector nil)
          ⇒ (1 two (quote (three)) "four" [five])


File: elisp.info,  Node: Char-Tables,  Next: Bool-Vectors,  Prev: Vector Functions,  Up: Sequences Arrays Vectors

6.6 Char-Tables
===============

A char-table is much like a vector, except that it is indexed by
character codes.  Any valid character code, without modifiers, can be
used as an index in a char-table.  You can access a char-table’s
elements with ‘aref’ and ‘aset’, as with any array.  In addition, a
char-table can have “extra slots” to hold additional data not associated
with particular character codes.  Like vectors, char-tables are
constants when evaluated, and can hold elements of any type.

   Each char-table has a “subtype”, a symbol, which serves two purposes:

   • The subtype provides an easy way to tell what the char-table is
     for.  For instance, display tables are char-tables with
     ‘display-table’ as the subtype, and syntax tables are char-tables
     with ‘syntax-table’ as the subtype.  The subtype can be queried
     using the function ‘char-table-subtype’, described below.

   • The subtype controls the number of “extra slots” in the char-table.
     This number is specified by the subtype’s ‘char-table-extra-slots’
     symbol property (*note Symbol Properties::), whose value should be
     an integer between 0 and 10.  If the subtype has no such symbol
     property, the char-table has no extra slots.

   A char-table can have a “parent”, which is another char-table.  If it
does, then whenever the char-table specifies ‘nil’ for a particular
character C, it inherits the value specified in the parent.  In other
words, ‘(aref CHAR-TABLE C)’ returns the value from the parent of
CHAR-TABLE if CHAR-TABLE itself specifies ‘nil’.

   A char-table can also have a “default value”.  If so, then ‘(aref
CHAR-TABLE C)’ returns the default value whenever the char-table does
not specify any other non-‘nil’ value.

 -- Function: make-char-table subtype &optional init
     Return a newly-created char-table, with subtype SUBTYPE (a symbol).
     Each element is initialized to INIT, which defaults to ‘nil’.  You
     cannot alter the subtype of a char-table after the char-table is
     created.

     There is no argument to specify the length of the char-table,
     because all char-tables have room for any valid character code as
     an index.

     If SUBTYPE has the ‘char-table-extra-slots’ symbol property, that
     specifies the number of extra slots in the char-table.  This should
     be an integer between 0 and 10; otherwise, ‘make-char-table’ raises
     an error.  If SUBTYPE has no ‘char-table-extra-slots’ symbol
     property (*note Property Lists::), the char-table has no extra
     slots.

 -- Function: char-table-p object
     This function returns ‘t’ if OBJECT is a char-table, and ‘nil’
     otherwise.

 -- Function: char-table-subtype char-table
     This function returns the subtype symbol of CHAR-TABLE.

   There is no special function to access default values in a
char-table.  To do that, use ‘char-table-range’ (see below).

 -- Function: char-table-parent char-table
     This function returns the parent of CHAR-TABLE.  The parent is
     always either ‘nil’ or another char-table.

 -- Function: set-char-table-parent char-table new-parent
     This function sets the parent of CHAR-TABLE to NEW-PARENT.

 -- Function: char-table-extra-slot char-table n
     This function returns the contents of extra slot N (zero based) of
     CHAR-TABLE.  The number of extra slots in a char-table is
     determined by its subtype.

 -- Function: set-char-table-extra-slot char-table n value
     This function stores VALUE in extra slot N (zero based) of
     CHAR-TABLE.

   A char-table can specify an element value for a single character
code; it can also specify a value for an entire character set.

 -- Function: char-table-range char-table range
     This returns the value specified in CHAR-TABLE for a range of
     characters RANGE.  Here are the possibilities for RANGE:

     ‘nil’
          Refers to the default value.

     CHAR
          Refers to the element for character CHAR (supposing CHAR is a
          valid character code).

     ‘(FROM . TO)’
          A cons cell refers to all the characters in the inclusive
          range ‘[FROM..TO]’.

 -- Function: set-char-table-range char-table range value
     This function sets the value in CHAR-TABLE for a range of
     characters RANGE.  Here are the possibilities for RANGE:

     ‘nil’
          Refers to the default value.

     ‘t’
          Refers to the whole range of character codes.

     CHAR
          Refers to the element for character CHAR (supposing CHAR is a
          valid character code).

     ‘(FROM . TO)’
          A cons cell refers to all the characters in the inclusive
          range ‘[FROM..TO]’.

 -- Function: map-char-table function char-table
     This function calls its argument FUNCTION for each element of
     CHAR-TABLE that has a non-‘nil’ value.  The call to FUNCTION is
     with two arguments, a key and a value.  The key is a possible RANGE
     argument for ‘char-table-range’—either a valid character or a cons
     cell ‘(FROM . TO)’, specifying a range of characters that share the
     same value.  The value is what ‘(char-table-range CHAR-TABLE KEY)’
     returns.

     Overall, the key-value pairs passed to FUNCTION describe all the
     values stored in CHAR-TABLE.

     The return value is always ‘nil’; to make calls to ‘map-char-table’
     useful, FUNCTION should have side effects.  For example, here is
     how to examine the elements of the syntax table:

          (let (accumulator)
             (map-char-table
              #'(lambda (key value)
                  (setq accumulator
                        (cons (list
                               (if (consp key)
                                   (list (car key) (cdr key))
                                 key)
                               value)
                              accumulator)))
              (syntax-table))
             accumulator)
          ⇒
          (((2597602 4194303) (2)) ((2597523 2597601) (3))
           ... (65379 (5 . 65378)) (65378 (4 . 65379)) (65377 (1))
           ... (12 (0)) (11 (3)) (10 (12)) (9 (0)) ((0 8) (3)))


File: elisp.info,  Node: Bool-Vectors,  Next: Rings,  Prev: Char-Tables,  Up: Sequences Arrays Vectors

6.7 Bool-vectors
================

A bool-vector is much like a vector, except that it stores only the
values ‘t’ and ‘nil’.  If you try to store any non-‘nil’ value into an
element of the bool-vector, the effect is to store ‘t’ there.  As with
all arrays, bool-vector indices start from 0, and the length cannot be
changed once the bool-vector is created.  Bool-vectors are constants
when evaluated.

   Several functions work specifically with bool-vectors; aside from
that, you manipulate them with same functions used for other kinds of
arrays.

 -- Function: make-bool-vector length initial
     Return a new bool-vector of LENGTH elements, each one initialized
     to INITIAL.

 -- Function: bool-vector &rest objects
     This function creates and returns a bool-vector whose elements are
     the arguments, OBJECTS.

 -- Function: bool-vector-p object
     This returns ‘t’ if OBJECT is a bool-vector, and ‘nil’ otherwise.

   There are also some bool-vector set operation functions, described
below:

 -- Function: bool-vector-exclusive-or a b &optional c
     Return “bitwise exclusive or” of bool vectors A and B.  If optional
     argument C is given, the result of this operation is stored into C.
     All arguments should be bool vectors of the same length.

 -- Function: bool-vector-union a b &optional c
     Return “bitwise or” of bool vectors A and B.  If optional argument
     C is given, the result of this operation is stored into C.  All
     arguments should be bool vectors of the same length.

 -- Function: bool-vector-intersection a b &optional c
     Return “bitwise and” of bool vectors A and B.  If optional argument
     C is given, the result of this operation is stored into C.  All
     arguments should be bool vectors of the same length.

 -- Function: bool-vector-set-difference a b &optional c
     Return “set difference” of bool vectors A and B.  If optional
     argument C is given, the result of this operation is stored into C.
     All arguments should be bool vectors of the same length.

 -- Function: bool-vector-not a &optional b
     Return “set complement” of bool vector A.  If optional argument B
     is given, the result of this operation is stored into B.  All
     arguments should be bool vectors of the same length.

 -- Function: bool-vector-subsetp a b
     Return ‘t’ if every ‘t’ value in A is also t in B, ‘nil’ otherwise.
     All arguments should be bool vectors of the same length.

 -- Function: bool-vector-count-consecutive a b i
     Return the number of consecutive elements in A equal B starting at
     I.  ‘a’ is a bool vector, B is ‘t’ or ‘nil’, and I is an index into
     ‘a’.

 -- Function: bool-vector-count-population a
     Return the number of elements that are ‘t’ in bool vector A.

   The printed form represents up to 8 boolean values as a single
character:

     (bool-vector t nil t nil)
          ⇒ #&4"^E"
     (bool-vector)
          ⇒ #&0""

   You can use ‘vconcat’ to print a bool-vector like other vectors:

     (vconcat (bool-vector nil t nil t))
          ⇒ [nil t nil t]

   Here is another example of creating, examining, and updating a
bool-vector:

     (setq bv (make-bool-vector 5 t))
          ⇒ #&5"^_"
     (aref bv 1)
          ⇒ t
     (aset bv 3 nil)
          ⇒ nil
     bv
          ⇒ #&5"^W"

These results make sense because the binary codes for control-_ and
control-W are 11111 and 10111, respectively.


File: elisp.info,  Node: Rings,  Prev: Bool-Vectors,  Up: Sequences Arrays Vectors

6.8 Managing a Fixed-Size Ring of Objects
=========================================

A “ring” is a fixed-size data structure that supports insertion,
deletion, rotation, and modulo-indexed reference and traversal.  An
efficient ring data structure is implemented by the ‘ring’ package.  It
provides the functions listed in this section.

   Note that several rings in Emacs, like the kill ring and the mark
ring, are actually implemented as simple lists, _not_ using the ‘ring’
package; thus the following functions won’t work on them.

 -- Function: make-ring size
     This returns a new ring capable of holding SIZE objects.  SIZE
     should be an integer.

 -- Function: ring-p object
     This returns ‘t’ if OBJECT is a ring, ‘nil’ otherwise.

 -- Function: ring-size ring
     This returns the maximum capacity of the RING.

 -- Function: ring-length ring
     This returns the number of objects that RING currently contains.
     The value will never exceed that returned by ‘ring-size’.

 -- Function: ring-elements ring
     This returns a list of the objects in RING, in order, newest first.

 -- Function: ring-copy ring
     This returns a new ring which is a copy of RING.  The new ring
     contains the same (‘eq’) objects as RING.

 -- Function: ring-empty-p ring
     This returns ‘t’ if RING is empty, ‘nil’ otherwise.

   The newest element in the ring always has index 0.  Higher indices
correspond to older elements.  Indices are computed modulo the ring
length.  Index −1 corresponds to the oldest element, −2 to the
next-oldest, and so forth.

 -- Function: ring-ref ring index
     This returns the object in RING found at index INDEX.  INDEX may be
     negative or greater than the ring length.  If RING is empty,
     ‘ring-ref’ signals an error.

 -- Function: ring-insert ring object
     This inserts OBJECT into RING, making it the newest element, and
     returns OBJECT.

     If the ring is full, insertion removes the oldest element to make
     room for the new element.

 -- Function: ring-remove ring &optional index
     Remove an object from RING, and return that object.  The argument
     INDEX specifies which item to remove; if it is ‘nil’, that means to
     remove the oldest item.  If RING is empty, ‘ring-remove’ signals an
     error.

 -- Function: ring-insert-at-beginning ring object
     This inserts OBJECT into RING, treating it as the oldest element.
     The return value is not significant.

     If the ring is full, this function removes the newest element to
     make room for the inserted element.

   If you are careful not to exceed the ring size, you can use the ring
as a first-in-first-out queue.  For example:

     (let ((fifo (make-ring 5)))
       (mapc (lambda (obj) (ring-insert fifo obj))
             '(0 one "two"))
       (list (ring-remove fifo) t
             (ring-remove fifo) t
             (ring-remove fifo)))
          ⇒ (0 t one t "two")


File: elisp.info,  Node: Hash Tables,  Next: Symbols,  Prev: Sequences Arrays Vectors,  Up: Top

7 Hash Tables
*************

A hash table is a very fast kind of lookup table, somewhat like an alist
(*note Association Lists::) in that it maps keys to corresponding
values.  It differs from an alist in these ways:

   • Lookup in a hash table is extremely fast for large tables—in fact,
     the time required is essentially _independent_ of how many elements
     are stored in the table.  For smaller tables (a few tens of
     elements) alists may still be faster because hash tables have a
     more-or-less constant overhead.

   • The correspondences in a hash table are in no particular order.

   • There is no way to share structure between two hash tables, the way
     two alists can share a common tail.

   Emacs Lisp provides a general-purpose hash table data type, along
with a series of functions for operating on them.  Hash tables have a
special printed representation, which consists of ‘#s’ followed by a
list specifying the hash table properties and contents.  *Note Creating
Hash::.  (Hash notation, the initial ‘#’ character used in the printed
representations of objects with no read representation, has nothing to
do with hash tables.  *Note Printed Representation::.)

   Obarrays are also a kind of hash table, but they are a different type
of object and are used only for recording interned symbols (*note
Creating Symbols::).

* Menu:

* Creating Hash::       Functions to create hash tables.
* Hash Access::         Reading and writing the hash table contents.
* Defining Hash::       Defining new comparison methods.
* Other Hash::          Miscellaneous.


File: elisp.info,  Node: Creating Hash,  Next: Hash Access,  Up: Hash Tables

7.1 Creating Hash Tables
========================

The principal function for creating a hash table is ‘make-hash-table’.

 -- Function: make-hash-table &rest keyword-args
     This function creates a new hash table according to the specified
     arguments.  The arguments should consist of alternating keywords
     (particular symbols recognized specially) and values corresponding
     to them.

     Several keywords make sense in ‘make-hash-table’, but the only two
     that you really need to know about are ‘:test’ and ‘:weakness’.

     ‘:test TEST’
          This specifies the method of key lookup for this hash table.
          The default is ‘eql’; ‘eq’ and ‘equal’ are other alternatives:

          ‘eql’
               Keys which are numbers are the same if they are ‘equal’,
               that is, if they are equal in value and either both are
               integers or both are floating point; otherwise, two
               distinct objects are never the same.

          ‘eq’
               Any two distinct Lisp objects are different as keys.

          ‘equal’
               Two Lisp objects are the same, as keys, if they are equal
               according to ‘equal’.

          You can use ‘define-hash-table-test’ (*note Defining Hash::)
          to define additional possibilities for TEST.

     ‘:weakness WEAK’
          The weakness of a hash table specifies whether the presence of
          a key or value in the hash table preserves it from garbage
          collection.

          The value, WEAK, must be one of ‘nil’, ‘key’, ‘value’,
          ‘key-or-value’, ‘key-and-value’, or ‘t’ which is an alias for
          ‘key-and-value’.  If WEAK is ‘key’ then the hash table does
          not prevent its keys from being collected as garbage (if they
          are not referenced anywhere else); if a particular key does
          get collected, the corresponding association is removed from
          the hash table.

          If WEAK is ‘value’, then the hash table does not prevent
          values from being collected as garbage (if they are not
          referenced anywhere else); if a particular value does get
          collected, the corresponding association is removed from the
          hash table.

          If WEAK is ‘key-and-value’ or ‘t’, both the key and the value
          must be live in order to preserve the association.  Thus, the
          hash table does not protect either keys or values from garbage
          collection; if either one is collected as garbage, that
          removes the association.

          If WEAK is ‘key-or-value’, either the key or the value can
          preserve the association.  Thus, associations are removed from
          the hash table when both their key and value would be
          collected as garbage (if not for references from weak hash
          tables).

          The default for WEAK is ‘nil’, so that all keys and values
          referenced in the hash table are preserved from garbage
          collection.

     ‘:size SIZE’
          This specifies a hint for how many associations you plan to
          store in the hash table.  If you know the approximate number,
          you can make things a little more efficient by specifying it
          this way.  If you specify too small a size, the hash table
          will grow automatically when necessary, but doing that takes
          some extra time.

          The default size is 65.

     ‘:rehash-size REHASH-SIZE’
          When you add an association to a hash table and the table is
          full, it grows automatically.  This value specifies how to
          make the hash table larger, at that time.

          If REHASH-SIZE is an integer, it should be positive, and the
          hash table grows by adding that much to the nominal size.  If
          REHASH-SIZE is floating point, it had better be greater than
          1, and the hash table grows by multiplying the old size by
          that number.

          The default value is 1.5.

     ‘:rehash-threshold THRESHOLD’
          This specifies the criterion for when the hash table is full
          (so it should be made larger).  The value, THRESHOLD, should
          be a positive floating-point number, no greater than 1.  The
          hash table is full whenever the actual number of entries
          exceeds this fraction of the nominal size.  The default for
          THRESHOLD is 0.8.

   You can also create a new hash table using the printed representation
for hash tables.  The Lisp reader can read this printed representation,
provided each element in the specified hash table has a valid read
syntax (*note Printed Representation::).  For instance, the following
specifies a new hash table containing the keys ‘key1’ and ‘key2’ (both
symbols) associated with ‘val1’ (a symbol) and ‘300’ (a number)
respectively.

     #s(hash-table size 30 data (key1 val1 key2 300))

The printed representation for a hash table consists of ‘#s’ followed by
a list beginning with ‘hash-table’.  The rest of the list should consist
of zero or more property-value pairs specifying the hash table’s
properties and initial contents.  The properties and values are read
literally.  Valid property names are ‘size’, ‘test’, ‘weakness’,
‘rehash-size’, ‘rehash-threshold’, and ‘data’.  The ‘data’ property
should be a list of key-value pairs for the initial contents; the other
properties have the same meanings as the matching ‘make-hash-table’
keywords (‘:size’, ‘:test’, etc.), described above.

   Note that you cannot specify a hash table whose initial contents
include objects that have no read syntax, such as buffers and frames.
Such objects may be added to the hash table after it is created.


File: elisp.info,  Node: Hash Access,  Next: Defining Hash,  Prev: Creating Hash,  Up: Hash Tables

7.2 Hash Table Access
=====================

This section describes the functions for accessing and storing
associations in a hash table.  In general, any Lisp object can be used
as a hash key, unless the comparison method imposes limits.  Any Lisp
object can also be used as the value.

 -- Function: gethash key table &optional default
     This function looks up KEY in TABLE, and returns its associated
     VALUE—or DEFAULT, if KEY has no association in TABLE.

 -- Function: puthash key value table
     This function enters an association for KEY in TABLE, with value
     VALUE.  If KEY already has an association in TABLE, VALUE replaces
     the old associated value.

 -- Function: remhash key table
     This function removes the association for KEY from TABLE, if there
     is one.  If KEY has no association, ‘remhash’ does nothing.

     Common Lisp note: In Common Lisp, ‘remhash’ returns non-‘nil’ if it
     actually removed an association and ‘nil’ otherwise.  In Emacs
     Lisp, ‘remhash’ always returns ‘nil’.

 -- Function: clrhash table
     This function removes all the associations from hash table TABLE,
     so that it becomes empty.  This is also called “clearing” the hash
     table.

     Common Lisp note: In Common Lisp, ‘clrhash’ returns the empty
     TABLE.  In Emacs Lisp, it returns ‘nil’.

 -- Function: maphash function table
     This function calls FUNCTION once for each of the associations in
     TABLE.  The function FUNCTION should accept two arguments—a KEY
     listed in TABLE, and its associated VALUE.  ‘maphash’ returns
     ‘nil’.


File: elisp.info,  Node: Defining Hash,  Next: Other Hash,  Prev: Hash Access,  Up: Hash Tables

7.3 Defining Hash Comparisons
=============================

You can define new methods of key lookup by means of
‘define-hash-table-test’.  In order to use this feature, you need to
understand how hash tables work, and what a “hash code” means.

   You can think of a hash table conceptually as a large array of many
slots, each capable of holding one association.  To look up a key,
‘gethash’ first computes an integer, the hash code, from the key.  It
reduces this integer modulo the length of the array, to produce an index
in the array.  Then it looks in that slot, and if necessary in other
nearby slots, to see if it has found the key being sought.

   Thus, to define a new method of key lookup, you need to specify both
a function to compute the hash code from a key, and a function to
compare two keys directly.

 -- Function: define-hash-table-test name test-fn hash-fn
     This function defines a new hash table test, named NAME.

     After defining NAME in this way, you can use it as the TEST
     argument in ‘make-hash-table’.  When you do that, the hash table
     will use TEST-FN to compare key values, and HASH-FN to compute a
     hash code from a key value.

     The function TEST-FN should accept two arguments, two keys, and
     return non-‘nil’ if they are considered the same.

     The function HASH-FN should accept one argument, a key, and return
     an integer that is the hash code of that key.  For good results,
     the function should use the whole range of integers for hash codes,
     including negative integers.

     The specified functions are stored in the property list of NAME
     under the property ‘hash-table-test’; the property value’s form is
     ‘(TEST-FN HASH-FN)’.

 -- Function: sxhash obj
     This function returns a hash code for Lisp object OBJ.  This is an
     integer which reflects the contents of OBJ and the other Lisp
     objects it points to.

     If two objects OBJ1 and OBJ2 are equal, then ‘(sxhash OBJ1)’ and
     ‘(sxhash OBJ2)’ are the same integer.

     If the two objects are not equal, the values returned by ‘sxhash’
     are usually different, but not always; once in a rare while, by
     luck, you will encounter two distinct-looking objects that give the
     same result from ‘sxhash’.

   This example creates a hash table whose keys are strings that are
compared case-insensitively.

     (defun case-fold-string= (a b)
       (eq t (compare-strings a nil nil b nil nil t)))
     (defun case-fold-string-hash (a)
       (sxhash (upcase a)))

     (define-hash-table-test 'case-fold
       'case-fold-string= 'case-fold-string-hash)

     (make-hash-table :test 'case-fold)

   Here is how you could define a hash table test equivalent to the
predefined test value ‘equal’.  The keys can be any Lisp object, and
equal-looking objects are considered the same key.

     (define-hash-table-test 'contents-hash 'equal 'sxhash)

     (make-hash-table :test 'contents-hash)


File: elisp.info,  Node: Other Hash,  Prev: Defining Hash,  Up: Hash Tables

7.4 Other Hash Table Functions
==============================

Here are some other functions for working with hash tables.

 -- Function: hash-table-p table
     This returns non-‘nil’ if TABLE is a hash table object.

 -- Function: copy-hash-table table
     This function creates and returns a copy of TABLE.  Only the table
     itself is copied—the keys and values are shared.

 -- Function: hash-table-count table
     This function returns the actual number of entries in TABLE.

 -- Function: hash-table-test table
     This returns the TEST value that was given when TABLE was created,
     to specify how to hash and compare keys.  See ‘make-hash-table’
     (*note Creating Hash::).

 -- Function: hash-table-weakness table
     This function returns the WEAK value that was specified for hash
     table TABLE.

 -- Function: hash-table-rehash-size table
     This returns the rehash size of TABLE.

 -- Function: hash-table-rehash-threshold table
     This returns the rehash threshold of TABLE.

 -- Function: hash-table-size table
     This returns the current nominal size of TABLE.


File: elisp.info,  Node: Symbols,  Next: Evaluation,  Prev: Hash Tables,  Up: Top

8 Symbols
*********

A “symbol” is an object with a unique name.  This chapter describes
symbols, their components, their property lists, and how they are
created and interned.  Separate chapters describe the use of symbols as
variables and as function names; see *note Variables::, and *note
Functions::.  For the precise read syntax for symbols, see *note Symbol
Type::.

   You can test whether an arbitrary Lisp object is a symbol with
‘symbolp’:

 -- Function: symbolp object
     This function returns ‘t’ if OBJECT is a symbol, ‘nil’ otherwise.

* Menu:

* Symbol Components::        Symbols have names, values, function definitions
                               and property lists.
* Definitions::              A definition says how a symbol will be used.
* Creating Symbols::         How symbols are kept unique.
* Symbol Properties::        Each symbol has a property list
                               for recording miscellaneous information.


File: elisp.info,  Node: Symbol Components,  Next: Definitions,  Up: Symbols

8.1 Symbol Components
=====================

Each symbol has four components (or “cells”), each of which references
another object:

Print name
     The symbol’s name.

Value
     The symbol’s current value as a variable.

Function
     The symbol’s function definition.  It can also hold a symbol, a
     keymap, or a keyboard macro.

Property list
     The symbol’s property list.

The print name cell always holds a string, and cannot be changed.  Each
of the other three cells can be set to any Lisp object.

   The print name cell holds the string that is the name of a symbol.
Since symbols are represented textually by their names, it is important
not to have two symbols with the same name.  The Lisp reader ensures
this: every time it reads a symbol, it looks for an existing symbol with
the specified name before it creates a new one.  To get a symbol’s name,
use the function ‘symbol-name’ (*note Creating Symbols::).

   The value cell holds a symbol’s value as a variable, which is what
you get if the symbol itself is evaluated as a Lisp expression.  *Note
Variables::, for details about how values are set and retrieved,
including complications such as “local bindings” and “scoping rules”.
Most symbols can have any Lisp object as a value, but certain special
symbols have values that cannot be changed; these include ‘nil’ and ‘t’,
and any symbol whose name starts with ‘:’ (those are called “keywords”).
*Note Constant Variables::.

   The function cell holds a symbol’s function definition.  Often, we
refer to “the function ‘foo’” when we really mean the function stored in
the function cell of ‘foo’; we make the distinction explicit only when
necessary.  Typically, the function cell is used to hold a function
(*note Functions::) or a macro (*note Macros::).  However, it can also
be used to hold a symbol (*note Function Indirection::), keyboard macro
(*note Keyboard Macros::), keymap (*note Keymaps::), or autoload object
(*note Autoloading::).  To get the contents of a symbol’s function cell,
use the function ‘symbol-function’ (*note Function Cells::).

   The property list cell normally should hold a correctly formatted
property list.  To get a symbol’s property list, use the function
‘symbol-plist’.  *Note Symbol Properties::.

   The function cell or the value cell may be “void”, which means that
the cell does not reference any object.  (This is not the same thing as
holding the symbol ‘void’, nor the same as holding the symbol ‘nil’.)
Examining a function or value cell that is void results in an error,
such as ‘Symbol's value as variable is void’.

   Because each symbol has separate value and function cells, variables
names and function names do not conflict.  For example, the symbol
‘buffer-file-name’ has a value (the name of the file being visited in
the current buffer) as well as a function definition (a primitive
function that returns the name of the file):

     buffer-file-name
          ⇒ "/gnu/elisp/symbols.texi"
     (symbol-function 'buffer-file-name)
          ⇒ #<subr buffer-file-name>


File: elisp.info,  Node: Definitions,  Next: Creating Symbols,  Prev: Symbol Components,  Up: Symbols

8.2 Defining Symbols
====================

A “definition” is a special kind of Lisp expression that announces your
intention to use a symbol in a particular way.  It typically specifies a
value or meaning for the symbol for one kind of use, plus documentation
for its meaning when used in this way.  Thus, when you define a symbol
as a variable, you can supply an initial value for the variable, plus
documentation for the variable.

   ‘defvar’ and ‘defconst’ are special forms that define a symbol as a
“global variable”—a variable that can be accessed at any point in a Lisp
program.  *Note Variables::, for details about variables.  To define a
customizable variable, use the ‘defcustom’ macro, which also calls
‘defvar’ as a subroutine (*note Customization::).

   In principle, you can assign a variable value to any symbol with
‘setq’, whether not it has first been defined as a variable.  However,
you ought to write a variable definition for each global variable that
you want to use; otherwise, your Lisp program may not act correctly if
it is evaluated with lexical scoping enabled (*note Variable Scoping::).

   ‘defun’ defines a symbol as a function, creating a lambda expression
and storing it in the function cell of the symbol.  This lambda
expression thus becomes the function definition of the symbol.  (The
term “function definition”, meaning the contents of the function cell,
is derived from the idea that ‘defun’ gives the symbol its definition as
a function.)  ‘defsubst’ and ‘defalias’ are two other ways of defining a
function.  *Note Functions::.

   ‘defmacro’ defines a symbol as a macro.  It creates a macro object
and stores it in the function cell of the symbol.  Note that a given
symbol can be a macro or a function, but not both at once, because both
macro and function definitions are kept in the function cell, and that
cell can hold only one Lisp object at any given time.  *Note Macros::.

   As previously noted, Emacs Lisp allows the same symbol to be defined
both as a variable (e.g., with ‘defvar’) and as a function or macro
(e.g., with ‘defun’).  Such definitions do not conflict.

   These definitions also act as guides for programming tools.  For
example, the ‘C-h f’ and ‘C-h v’ commands create help buffers containing
links to the relevant variable, function, or macro definitions.  *Note
(emacs)Name Help::.


File: elisp.info,  Node: Creating Symbols,  Next: Symbol Properties,  Prev: Definitions,  Up: Symbols

8.3 Creating and Interning Symbols
==================================

To understand how symbols are created in GNU Emacs Lisp, you must know
how Lisp reads them.  Lisp must ensure that it finds the same symbol
every time it reads the same set of characters.  Failure to do so would
cause complete confusion.

   When the Lisp reader encounters a symbol, it reads all the characters
of the name.  Then it hashes those characters to find an index in a
table called an “obarray”.  Hashing is an efficient method of looking
something up.  For example, instead of searching a telephone book cover
to cover when looking up Jan Jones, you start with the J’s and go from
there.  That is a simple version of hashing.  Each element of the
obarray is a “bucket” which holds all the symbols with a given hash
code; to look for a given name, it is sufficient to look through all the
symbols in the bucket for that name’s hash code.  (The same idea is used
for general Emacs hash tables, but they are a different data type; see
*note Hash Tables::.)

   If a symbol with the desired name is found, the reader uses that
symbol.  If the obarray does not contain a symbol with that name, the
reader makes a new symbol and adds it to the obarray.  Finding or adding
a symbol with a certain name is called “interning” it, and the symbol is
then called an “interned symbol”.

   Interning ensures that each obarray has just one symbol with any
particular name.  Other like-named symbols may exist, but not in the
same obarray.  Thus, the reader gets the same symbols for the same
names, as long as you keep reading with the same obarray.

   Interning usually happens automatically in the reader, but sometimes
other programs need to do it.  For example, after the ‘M-x’ command
obtains the command name as a string using the minibuffer, it then
interns the string, to get the interned symbol with that name.

   No obarray contains all symbols; in fact, some symbols are not in any
obarray.  They are called “uninterned symbols”.  An uninterned symbol
has the same four cells as other symbols; however, the only way to gain
access to it is by finding it in some other object or as the value of a
variable.

   Creating an uninterned symbol is useful in generating Lisp code,
because an uninterned symbol used as a variable in the code you generate
cannot clash with any variables used in other Lisp programs.

   In Emacs Lisp, an obarray is actually a vector.  Each element of the
vector is a bucket; its value is either an interned symbol whose name
hashes to that bucket, or 0 if the bucket is empty.  Each interned
symbol has an internal link (invisible to the user) to the next symbol
in the bucket.  Because these links are invisible, there is no way to
find all the symbols in an obarray except using ‘mapatoms’ (below).  The
order of symbols in a bucket is not significant.

   In an empty obarray, every element is 0, so you can create an obarray
with ‘(make-vector LENGTH 0)’.  *This is the only valid way to create an
obarray.*  Prime numbers as lengths tend to result in good hashing;
lengths one less than a power of two are also good.

   *Do not try to put symbols in an obarray yourself.*  This does not
work—only ‘intern’ can enter a symbol in an obarray properly.

     Common Lisp note: Unlike Common Lisp, Emacs Lisp does not provide
     for interning a single symbol in several obarrays.

   Most of the functions below take a name and sometimes an obarray as
arguments.  A ‘wrong-type-argument’ error is signaled if the name is not
a string, or if the obarray is not a vector.

 -- Function: symbol-name symbol
     This function returns the string that is SYMBOL’s name.  For
     example:

          (symbol-name 'foo)
               ⇒ "foo"

     *Warning:* Changing the string by substituting characters does
     change the name of the symbol, but fails to update the obarray, so
     don’t do it!

 -- Function: make-symbol name
     This function returns a newly-allocated, uninterned symbol whose
     name is NAME (which must be a string).  Its value and function
     definition are void, and its property list is ‘nil’.  In the
     example below, the value of ‘sym’ is not ‘eq’ to ‘foo’ because it
     is a distinct uninterned symbol whose name is also ‘foo’.

          (setq sym (make-symbol "foo"))
               ⇒ foo
          (eq sym 'foo)
               ⇒ nil

 -- Function: intern name &optional obarray
     This function returns the interned symbol whose name is NAME.  If
     there is no such symbol in the obarray OBARRAY, ‘intern’ creates a
     new one, adds it to the obarray, and returns it.  If OBARRAY is
     omitted, the value of the global variable ‘obarray’ is used.

          (setq sym (intern "foo"))
               ⇒ foo
          (eq sym 'foo)
               ⇒ t

          (setq sym1 (intern "foo" other-obarray))
               ⇒ foo
          (eq sym1 'foo)
               ⇒ nil

     Common Lisp note: In Common Lisp, you can intern an existing symbol
     in an obarray.  In Emacs Lisp, you cannot do this, because the
     argument to ‘intern’ must be a string, not a symbol.

 -- Function: intern-soft name &optional obarray
     This function returns the symbol in OBARRAY whose name is NAME, or
     ‘nil’ if OBARRAY has no symbol with that name.  Therefore, you can
     use ‘intern-soft’ to test whether a symbol with a given name is
     already interned.  If OBARRAY is omitted, the value of the global
     variable ‘obarray’ is used.

     The argument NAME may also be a symbol; in that case, the function
     returns NAME if NAME is interned in the specified obarray, and
     otherwise ‘nil’.

          (intern-soft "frazzle")        ; No such symbol exists.
               ⇒ nil
          (make-symbol "frazzle")        ; Create an uninterned one.
               ⇒ frazzle
          (intern-soft "frazzle")        ; That one cannot be found.
               ⇒ nil
          (setq sym (intern "frazzle"))  ; Create an interned one.
               ⇒ frazzle
          (intern-soft "frazzle")        ; That one can be found!
               ⇒ frazzle
          (eq sym 'frazzle)              ; And it is the same one.
               ⇒ t

 -- Variable: obarray
     This variable is the standard obarray for use by ‘intern’ and
     ‘read’.

 -- Function: mapatoms function &optional obarray
     This function calls FUNCTION once with each symbol in the obarray
     OBARRAY.  Then it returns ‘nil’.  If OBARRAY is omitted, it
     defaults to the value of ‘obarray’, the standard obarray for
     ordinary symbols.

          (setq count 0)
               ⇒ 0
          (defun count-syms (s)
            (setq count (1+ count)))
               ⇒ count-syms
          (mapatoms 'count-syms)
               ⇒ nil
          count
               ⇒ 1871

     See ‘documentation’ in *note Accessing Documentation::, for another
     example using ‘mapatoms’.

 -- Function: unintern symbol obarray
     This function deletes SYMBOL from the obarray OBARRAY.  If ‘symbol’
     is not actually in the obarray, ‘unintern’ does nothing.  If
     OBARRAY is ‘nil’, the current obarray is used.

     If you provide a string instead of a symbol as SYMBOL, it stands
     for a symbol name.  Then ‘unintern’ deletes the symbol (if any) in
     the obarray which has that name.  If there is no such symbol,
     ‘unintern’ does nothing.

     If ‘unintern’ does delete a symbol, it returns ‘t’.  Otherwise it
     returns ‘nil’.


File: elisp.info,  Node: Symbol Properties,  Prev: Creating Symbols,  Up: Symbols

8.4 Symbol Properties
=====================

A symbol may possess any number of “symbol properties”, which can be
used to record miscellaneous information about the symbol.  For example,
when a symbol has a ‘risky-local-variable’ property with a non-‘nil’
value, that means the variable which the symbol names is a risky
file-local variable (*note File Local Variables::).

   Each symbol’s properties and property values are stored in the
symbol’s property list cell (*note Symbol Components::), in the form of
a property list (*note Property Lists::).

* Menu:

* Symbol Plists::        Accessing symbol properties.
* Standard Properties::  Standard meanings of symbol properties.


File: elisp.info,  Node: Symbol Plists,  Next: Standard Properties,  Up: Symbol Properties

8.4.1 Accessing Symbol Properties
---------------------------------

The following functions can be used to access symbol properties.

 -- Function: get symbol property
     This function returns the value of the property named PROPERTY in
     SYMBOL’s property list.  If there is no such property, it returns
     ‘nil’.  Thus, there is no distinction between a value of ‘nil’ and
     the absence of the property.

     The name PROPERTY is compared with the existing property names
     using ‘eq’, so any object is a legitimate property.

     See ‘put’ for an example.

 -- Function: put symbol property value
     This function puts VALUE onto SYMBOL’s property list under the
     property name PROPERTY, replacing any previous property value.  The
     ‘put’ function returns VALUE.

          (put 'fly 'verb 'transitive)
               ⇒'transitive
          (put 'fly 'noun '(a buzzing little bug))
               ⇒ (a buzzing little bug)
          (get 'fly 'verb)
               ⇒ transitive
          (symbol-plist 'fly)
               ⇒ (verb transitive noun (a buzzing little bug))

 -- Function: symbol-plist symbol
     This function returns the property list of SYMBOL.

 -- Function: setplist symbol plist
     This function sets SYMBOL’s property list to PLIST.  Normally,
     PLIST should be a well-formed property list, but this is not
     enforced.  The return value is PLIST.

          (setplist 'foo '(a 1 b (2 3) c nil))
               ⇒ (a 1 b (2 3) c nil)
          (symbol-plist 'foo)
               ⇒ (a 1 b (2 3) c nil)

     For symbols in special obarrays, which are not used for ordinary
     purposes, it may make sense to use the property list cell in a
     nonstandard fashion; in fact, the abbrev mechanism does so (*note
     Abbrevs::).

     You could define ‘put’ in terms of ‘setplist’ and ‘plist-put’, as
     follows:

          (defun put (symbol prop value)
            (setplist symbol
                      (plist-put (symbol-plist symbol) prop value)))

 -- Function: function-get symbol property &optional autoload
     This function is identical to ‘get’, except that if SYMBOL is the
     name of a function alias, it looks in the property list of the
     symbol naming the actual function.  *Note Defining Functions::.  If
     the optional argument AUTOLOAD is non-‘nil’, and SYMBOL is
     auto-loaded, this function will try to autoload it, since
     autoloading might set PROPERTY of SYMBOL.  If AUTOLOAD is the
     symbol ‘macro’, only try autoloading if SYMBOL is an auto-loaded
     macro.

 -- Function: function-put function property value
     This function sets PROPERTY of FUNCTION to VALUE.  FUNCTION should
     be a symbol.  This function is preferred to calling ‘put’ for
     setting properties of a function, because it will allow us some day
     to implement remapping of old properties to new ones.


File: elisp.info,  Node: Standard Properties,  Prev: Symbol Plists,  Up: Symbol Properties

8.4.2 Standard Symbol Properties
--------------------------------

Here, we list the symbol properties which are used for special purposes
in Emacs.  In the following table, whenever we say “the named function”,
that means the function whose name is the relevant symbol; similarly for
“the named variable” etc.

‘:advertised-binding’
     This property value specifies the preferred key binding, when
     showing documentation, for the named function.  *Note Keys in
     Documentation::.

‘char-table-extra-slots’
     The value, if non-‘nil’, specifies the number of extra slots in the
     named char-table type.  *Note Char-Tables::.

‘customized-face’
‘face-defface-spec’
‘saved-face’
‘theme-face’
     These properties are used to record a face’s standard, saved,
     customized, and themed face specs.  Do not set them directly; they
     are managed by ‘defface’ and related functions.  *Note Defining
     Faces::.

‘customized-value’
‘saved-value’
‘standard-value’
‘theme-value’
     These properties are used to record a customizable variable’s
     standard value, saved value, customized-but-unsaved value, and
     themed values.  Do not set them directly; they are managed by
     ‘defcustom’ and related functions.  *Note Variable Definitions::.

‘disabled’
     If the value is non-‘nil’, the named function is disabled as a
     command.  *Note Disabling Commands::.

‘face-documentation’
     The value stores the documentation string of the named face.  This
     is set automatically by ‘defface’.  *Note Defining Faces::.

‘history-length’
     The value, if non-‘nil’, specifies the maximum minibuffer history
     length for the named history list variable.  *Note Minibuffer
     History::.

‘interactive-form’
     The value is an interactive form for the named function.  Normally,
     you should not set this directly; use the ‘interactive’ special
     form instead.  *Note Interactive Call::.

‘menu-enable’
     The value is an expression for determining whether the named menu
     item should be enabled in menus.  *Note Simple Menu Items::.

‘mode-class’
     If the value is ‘special’, the named major mode is special.  *Note
     Major Mode Conventions::.

‘permanent-local’
     If the value is non-‘nil’, the named variable is a buffer-local
     variable whose value should not be reset when changing major modes.
     *Note Creating Buffer-Local::.

‘permanent-local-hook’
     If the value is non-‘nil’, the named function should not be deleted
     from the local value of a hook variable when changing major modes.
     *Note Setting Hooks::.

‘pure’
     If the value is non-‘nil’, the named function is considered to be
     side-effect free.  Calls with constant arguments can be evaluated
     at compile time.  This may shift run time errors to compile time.

‘risky-local-variable’
     If the value is non-‘nil’, the named variable is considered risky
     as a file-local variable.  *Note File Local Variables::.

‘safe-function’
     If the value is non-‘nil’, the named function is considered
     generally safe for evaluation.  *Note Function Safety::.

‘safe-local-eval-function’
     If the value is non-‘nil’, the named function is safe to call in
     file-local evaluation forms.  *Note File Local Variables::.

‘safe-local-variable’
     The value specifies a function for determining safe file-local
     values for the named variable.  *Note File Local Variables::.

‘side-effect-free’
     A non-‘nil’ value indicates that the named function is free of
     side-effects, for determining function safety (*note Function
     Safety::) as well as for byte compiler optimizations.  Do not set
     it.

‘variable-documentation’
     If non-‘nil’, this specifies the named variable’s documentation
     string.  This is set automatically by ‘defvar’ and related
     functions.  *Note Defining Faces::.


File: elisp.info,  Node: Evaluation,  Next: Control Structures,  Prev: Symbols,  Up: Top

9 Evaluation
************

The “evaluation” of expressions in Emacs Lisp is performed by the “Lisp
interpreter”—a program that receives a Lisp object as input and computes
its “value as an expression”.  How it does this depends on the data type
of the object, according to rules described in this chapter.  The
interpreter runs automatically to evaluate portions of your program, but
can also be called explicitly via the Lisp primitive function ‘eval’.

* Menu:

* Intro Eval::  Evaluation in the scheme of things.
* Forms::       How various sorts of objects are evaluated.
* Quoting::     Avoiding evaluation (to put constants in the program).
* Backquote::   Easier construction of list structure.
* Eval::        How to invoke the Lisp interpreter explicitly.


File: elisp.info,  Node: Intro Eval,  Next: Forms,  Up: Evaluation

9.1 Introduction to Evaluation
==============================

The Lisp interpreter, or evaluator, is the part of Emacs that computes
the value of an expression that is given to it.  When a function written
in Lisp is called, the evaluator computes the value of the function by
evaluating the expressions in the function body.  Thus, running any Lisp
program really means running the Lisp interpreter.

   A Lisp object that is intended for evaluation is called a “form” or
“expression”(1).  The fact that forms are data objects and not merely
text is one of the fundamental differences between Lisp-like languages
and typical programming languages.  Any object can be evaluated, but in
practice only numbers, symbols, lists and strings are evaluated very
often.

   In subsequent sections, we will describe the details of what
evaluation means for each kind of form.

   It is very common to read a Lisp form and then evaluate the form, but
reading and evaluation are separate activities, and either can be
performed alone.  Reading per se does not evaluate anything; it converts
the printed representation of a Lisp object to the object itself.  It is
up to the caller of ‘read’ to specify whether this object is a form to
be evaluated, or serves some entirely different purpose.  *Note Input
Functions::.

   Evaluation is a recursive process, and evaluating a form often
involves evaluating parts within that form.  For instance, when you
evaluate a “function call” form such as ‘(car x)’, Emacs first evaluates
the argument (the subform ‘x’).  After evaluating the argument, Emacs
“executes” the function (‘car’), and if the function is written in Lisp,
execution works by evaluating the “body” of the function (in this
example, however, ‘car’ is not a Lisp function; it is a primitive
function implemented in C). *Note Functions::, for more information
about functions and function calls.

   Evaluation takes place in a context called the “environment”, which
consists of the current values and bindings of all Lisp variables (*note
Variables::).(2)  Whenever a form refers to a variable without creating
a new binding for it, the variable evaluates to the value given by the
current environment.  Evaluating a form may also temporarily alter the
environment by binding variables (*note Local Variables::).

   Evaluating a form may also make changes that persist; these changes
are called “side effects”.  An example of a form that produces a side
effect is ‘(setq foo 1)’.

   Do not confuse evaluation with command key interpretation.  The
editor command loop translates keyboard input into a command (an
interactively callable function) using the active keymaps, and then uses
‘call-interactively’ to execute that command.  Executing the command
usually involves evaluation, if the command is written in Lisp; however,
this step is not considered a part of command key interpretation.  *Note
Command Loop::.

   ---------- Footnotes ----------

   (1) It is sometimes also referred to as an “S-expression” or “sexp”,
but we generally do not use this terminology in this manual.

   (2) This definition of “environment” is specifically not intended to
include all the data that can affect the result of a program.


File: elisp.info,  Node: Forms,  Next: Quoting,  Prev: Intro Eval,  Up: Evaluation

9.2 Kinds of Forms
==================

A Lisp object that is intended to be evaluated is called a “form” (or an
“expression”).  How Emacs evaluates a form depends on its data type.
Emacs has three different kinds of form that are evaluated differently:
symbols, lists, and all other types.  This section describes all three
kinds, one by one, starting with the other types, which are
self-evaluating forms.

* Menu:

* Self-Evaluating Forms::   Forms that evaluate to themselves.
* Symbol Forms::            Symbols evaluate as variables.
* Classifying Lists::       How to distinguish various sorts of list forms.
* Function Indirection::    When a symbol appears as the car of a list,
                              we find the real function via the symbol.
* Function Forms::          Forms that call functions.
* Macro Forms::             Forms that call macros.
* Special Forms::           Special forms are idiosyncratic primitives,
                              most of them extremely important.
* Autoloading::             Functions set up to load files
                              containing their real definitions.


File: elisp.info,  Node: Self-Evaluating Forms,  Next: Symbol Forms,  Up: Forms

9.2.1 Self-Evaluating Forms
---------------------------

A “self-evaluating form” is any form that is not a list or symbol.
Self-evaluating forms evaluate to themselves: the result of evaluation
is the same object that was evaluated.  Thus, the number 25 evaluates to
25, and the string ‘"foo"’ evaluates to the string ‘"foo"’.  Likewise,
evaluating a vector does not cause evaluation of the elements of the
vector—it returns the same vector with its contents unchanged.

     '123               ; A number, shown without evaluation.
          ⇒ 123
     123                ; Evaluated as usual—result is the same.
          ⇒ 123
     (eval '123)        ; Evaluated "by hand"—result is the same.
          ⇒ 123
     (eval (eval '123)) ; Evaluating twice changes nothing.
          ⇒ 123

   It is common to write numbers, characters, strings, and even vectors
in Lisp code, taking advantage of the fact that they self-evaluate.
However, it is quite unusual to do this for types that lack a read
syntax, because there’s no way to write them textually.  It is possible
to construct Lisp expressions containing these types by means of a Lisp
program.  Here is an example:

     ;; Build an expression containing a buffer object.
     (setq print-exp (list 'print (current-buffer)))
          ⇒ (print #<buffer eval.texi>)
     ;; Evaluate it.
     (eval print-exp)
          ⊣ #<buffer eval.texi>
          ⇒ #<buffer eval.texi>


File: elisp.info,  Node: Symbol Forms,  Next: Classifying Lists,  Prev: Self-Evaluating Forms,  Up: Forms

9.2.2 Symbol Forms
------------------

When a symbol is evaluated, it is treated as a variable.  The result is
the variable’s value, if it has one.  If the symbol has no value as a
variable, the Lisp interpreter signals an error.  For more information
on the use of variables, see *note Variables::.

   In the following example, we set the value of a symbol with ‘setq’.
Then we evaluate the symbol, and get back the value that ‘setq’ stored.

     (setq a 123)
          ⇒ 123
     (eval 'a)
          ⇒ 123
     a
          ⇒ 123

   The symbols ‘nil’ and ‘t’ are treated specially, so that the value of
‘nil’ is always ‘nil’, and the value of ‘t’ is always ‘t’; you cannot
set or bind them to any other values.  Thus, these two symbols act like
self-evaluating forms, even though ‘eval’ treats them like any other
symbol.  A symbol whose name starts with ‘:’ also self-evaluates in the
same way; likewise, its value ordinarily cannot be changed.  *Note
Constant Variables::.


File: elisp.info,  Node: Classifying Lists,  Next: Function Indirection,  Prev: Symbol Forms,  Up: Forms

9.2.3 Classification of List Forms
----------------------------------

A form that is a nonempty list is either a function call, a macro call,
or a special form, according to its first element.  These three kinds of
forms are evaluated in different ways, described below.  The remaining
list elements constitute the “arguments” for the function, macro, or
special form.

   The first step in evaluating a nonempty list is to examine its first
element.  This element alone determines what kind of form the list is
and how the rest of the list is to be processed.  The first element is
_not_ evaluated, as it would be in some Lisp dialects such as Scheme.


File: elisp.info,  Node: Function Indirection,  Next: Function Forms,  Prev: Classifying Lists,  Up: Forms

9.2.4 Symbol Function Indirection
---------------------------------

If the first element of the list is a symbol then evaluation examines
the symbol’s function cell, and uses its contents instead of the
original symbol.  If the contents are another symbol, this process,
called “symbol function indirection”, is repeated until it obtains a
non-symbol.  *Note Function Names::, for more information about symbol
function indirection.

   One possible consequence of this process is an infinite loop, in the
event that a symbol’s function cell refers to the same symbol.
Otherwise, we eventually obtain a non-symbol, which ought to be a
function or other suitable object.

   More precisely, we should now have a Lisp function (a lambda
expression), a byte-code function, a primitive function, a Lisp macro, a
special form, or an autoload object.  Each of these types is a case
described in one of the following sections.  If the object is not one of
these types, Emacs signals an ‘invalid-function’ error.

   The following example illustrates the symbol indirection process.  We
use ‘fset’ to set the function cell of a symbol and ‘symbol-function’ to
get the function cell contents (*note Function Cells::).  Specifically,
we store the symbol ‘car’ into the function cell of ‘first’, and the
symbol ‘first’ into the function cell of ‘erste’.

     ;; Build this function cell linkage:
     ;;   -------------       -----        -------        -------
     ;;  | #<subr car> | <-- | car |  <-- | first |  <-- | erste |
     ;;   -------------       -----        -------        -------
     (symbol-function 'car)
          ⇒ #<subr car>
     (fset 'first 'car)
          ⇒ car
     (fset 'erste 'first)
          ⇒ first
     (erste '(1 2 3))   ; Call the function referenced by ‘erste’.
          ⇒ 1

   By contrast, the following example calls a function without any
symbol function indirection, because the first element is an anonymous
Lisp function, not a symbol.

     ((lambda (arg) (erste arg))
      '(1 2 3))
          ⇒ 1

Executing the function itself evaluates its body; this does involve
symbol function indirection when calling ‘erste’.

   This form is rarely used and is now deprecated.  Instead, you should
write it as:

     (funcall (lambda (arg) (erste arg))
              '(1 2 3))
   or just
     (let ((arg '(1 2 3))) (erste arg))

   The built-in function ‘indirect-function’ provides an easy way to
perform symbol function indirection explicitly.

 -- Function: indirect-function function &optional noerror
     This function returns the meaning of FUNCTION as a function.  If
     FUNCTION is a symbol, then it finds FUNCTION’s function definition
     and starts over with that value.  If FUNCTION is not a symbol, then
     it returns FUNCTION itself.

     This function returns ‘nil’ if the final symbol is unbound.  It
     signals a ‘cyclic-function-indirection’ error if there is a loop in
     the chain of symbols.

     The optional argument NOERROR is obsolete, kept for backward
     compatibility, and has no effect.

     Here is how you could define ‘indirect-function’ in Lisp:

          (defun indirect-function (function)
            (if (symbolp function)
                (indirect-function (symbol-function function))
              function))


File: elisp.info,  Node: Function Forms,  Next: Macro Forms,  Prev: Function Indirection,  Up: Forms

9.2.5 Evaluation of Function Forms
----------------------------------

If the first element of a list being evaluated is a Lisp function
object, byte-code object or primitive function object, then that list is
a “function call”.  For example, here is a call to the function ‘+’:

     (+ 1 x)

   The first step in evaluating a function call is to evaluate the
remaining elements of the list from left to right.  The results are the
actual argument values, one value for each list element.  The next step
is to call the function with this list of arguments, effectively using
the function ‘apply’ (*note Calling Functions::).  If the function is
written in Lisp, the arguments are used to bind the argument variables
of the function (*note Lambda Expressions::); then the forms in the
function body are evaluated in order, and the value of the last body
form becomes the value of the function call.


File: elisp.info,  Node: Macro Forms,  Next: Special Forms,  Prev: Function Forms,  Up: Forms

9.2.6 Lisp Macro Evaluation
---------------------------

If the first element of a list being evaluated is a macro object, then
the list is a “macro call”.  When a macro call is evaluated, the
elements of the rest of the list are _not_ initially evaluated.
Instead, these elements themselves are used as the arguments of the
macro.  The macro definition computes a replacement form, called the
“expansion” of the macro, to be evaluated in place of the original form.
The expansion may be any sort of form: a self-evaluating constant, a
symbol, or a list.  If the expansion is itself a macro call, this
process of expansion repeats until some other sort of form results.

   Ordinary evaluation of a macro call finishes by evaluating the
expansion.  However, the macro expansion is not necessarily evaluated
right away, or at all, because other programs also expand macro calls,
and they may or may not evaluate the expansions.

   Normally, the argument expressions are not evaluated as part of
computing the macro expansion, but instead appear as part of the
expansion, so they are computed when the expansion is evaluated.

   For example, given a macro defined as follows:

     (defmacro cadr (x)
       (list 'car (list 'cdr x)))

an expression such as ‘(cadr (assq 'handler list))’ is a macro call, and
its expansion is:

     (car (cdr (assq 'handler list)))

Note that the argument ‘(assq 'handler list)’ appears in the expansion.

   *Note Macros::, for a complete description of Emacs Lisp macros.


File: elisp.info,  Node: Special Forms,  Next: Autoloading,  Prev: Macro Forms,  Up: Forms

9.2.7 Special Forms
-------------------

A “special form” is a primitive function specially marked so that its
arguments are not all evaluated.  Most special forms define control
structures or perform variable bindings—things which functions cannot
do.

   Each special form has its own rules for which arguments are evaluated
and which are used without evaluation.  Whether a particular argument is
evaluated may depend on the results of evaluating other arguments.

   If an expression’s first symbol is that of a special form, the
expression should follow the rules of that special form; otherwise,
Emacs’s behavior is not well-defined (though it will not crash).  For
example, ‘((lambda (x) x . 3) 4)’ contains a subexpression that begins
with ‘lambda’ but is not a well-formed ‘lambda’ expression, so Emacs may
signal an error, or may return 3 or 4 or ‘nil’, or may behave in other
ways.

 -- Function: special-form-p object
     This predicate tests whether its argument is a special form, and
     returns ‘t’ if so, ‘nil’ otherwise.

   Here is a list, in alphabetical order, of all of the special forms in
Emacs Lisp with a reference to where each is described.

‘and’
     *note Combining Conditions::

‘catch’
     *note Catch and Throw::

‘cond’
     *note Conditionals::

‘condition-case’
     *note Handling Errors::

‘defconst’
     *note Defining Variables::

‘defvar’
     *note Defining Variables::

‘function’
     *note Anonymous Functions::

‘if’
     *note Conditionals::

‘interactive’
     *note Interactive Call::

‘lambda’
     *note Lambda Expressions::

‘let’
‘let*’
     *note Local Variables::

‘or’
     *note Combining Conditions::

‘prog1’
‘prog2’
‘progn’
     *note Sequencing::

‘quote’
     *note Quoting::

‘save-current-buffer’
     *note Current Buffer::

‘save-excursion’
     *note Excursions::

‘save-restriction’
     *note Narrowing::

‘setq’
     *note Setting Variables::

‘setq-default’
     *note Creating Buffer-Local::

‘track-mouse’
     *note Mouse Tracking::

‘unwind-protect’
     *note Nonlocal Exits::

‘while’
     *note Iteration::

     Common Lisp note: Here are some comparisons of special forms in GNU
     Emacs Lisp and Common Lisp.  ‘setq’, ‘if’, and ‘catch’ are special
     forms in both Emacs Lisp and Common Lisp.  ‘save-excursion’ is a
     special form in Emacs Lisp, but doesn’t exist in Common Lisp.
     ‘throw’ is a special form in Common Lisp (because it must be able
     to throw multiple values), but it is a function in Emacs Lisp
     (which doesn’t have multiple values).


File: elisp.info,  Node: Autoloading,  Prev: Special Forms,  Up: Forms

9.2.8 Autoloading
-----------------

The “autoload” feature allows you to call a function or macro whose
function definition has not yet been loaded into Emacs.  It specifies
which file contains the definition.  When an autoload object appears as
a symbol’s function definition, calling that symbol as a function
automatically loads the specified file; then it calls the real
definition loaded from that file.  The way to arrange for an autoload
object to appear as a symbol’s function definition is described in *note
Autoload::.


File: elisp.info,  Node: Quoting,  Next: Backquote,  Prev: Forms,  Up: Evaluation

9.3 Quoting
===========

The special form ‘quote’ returns its single argument, as written,
without evaluating it.  This provides a way to include constant symbols
and lists, which are not self-evaluating objects, in a program.  (It is
not necessary to quote self-evaluating objects such as numbers, strings,
and vectors.)

 -- Special Form: quote object
     This special form returns OBJECT, without evaluating it.

   Because ‘quote’ is used so often in programs, Lisp provides a
convenient read syntax for it.  An apostrophe character (‘'’) followed
by a Lisp object (in read syntax) expands to a list whose first element
is ‘quote’, and whose second element is the object.  Thus, the read
syntax ‘'x’ is an abbreviation for ‘(quote x)’.

   Here are some examples of expressions that use ‘quote’:

     (quote (+ 1 2))
          ⇒ (+ 1 2)
     (quote foo)
          ⇒ foo
     'foo
          ⇒ foo
     ''foo
          ⇒ (quote foo)
     '(quote foo)
          ⇒ (quote foo)
     ['foo]
          ⇒ [(quote foo)]

   Other quoting constructs include ‘function’ (*note Anonymous
Functions::), which causes an anonymous lambda expression written in
Lisp to be compiled, and ‘`’ (*note Backquote::), which is used to quote
only part of a list, while computing and substituting other parts.


File: elisp.info,  Node: Backquote,  Next: Eval,  Prev: Quoting,  Up: Evaluation

9.4 Backquote
=============

“Backquote constructs” allow you to quote a list, but selectively
evaluate elements of that list.  In the simplest case, it is identical
to the special form ‘quote’ (described in the previous section; *note
Quoting::).  For example, these two forms yield identical results:

     `(a list of (+ 2 3) elements)
          ⇒ (a list of (+ 2 3) elements)
     '(a list of (+ 2 3) elements)
          ⇒ (a list of (+ 2 3) elements)

   The special marker ‘,’ inside of the argument to backquote indicates
a value that isn’t constant.  The Emacs Lisp evaluator evaluates the
argument of ‘,’, and puts the value in the list structure:

     `(a list of ,(+ 2 3) elements)
          ⇒ (a list of 5 elements)

Substitution with ‘,’ is allowed at deeper levels of the list structure
also.  For example:

     `(1 2 (3 ,(+ 4 5)))
          ⇒ (1 2 (3 9))

   You can also “splice” an evaluated value into the resulting list,
using the special marker ‘,@’.  The elements of the spliced list become
elements at the same level as the other elements of the resulting list.
The equivalent code without using ‘`’ is often unreadable.  Here are
some examples:

     (setq some-list '(2 3))
          ⇒ (2 3)
     (cons 1 (append some-list '(4) some-list))
          ⇒ (1 2 3 4 2 3)
     `(1 ,@some-list 4 ,@some-list)
          ⇒ (1 2 3 4 2 3)

     (setq list '(hack foo bar))
          ⇒ (hack foo bar)
     (cons 'use
       (cons 'the
         (cons 'words (append (cdr list) '(as elements)))))
          ⇒ (use the words foo bar as elements)
     `(use the words ,@(cdr list) as elements)
          ⇒ (use the words foo bar as elements)


File: elisp.info,  Node: Eval,  Prev: Backquote,  Up: Evaluation

9.5 Eval
========

Most often, forms are evaluated automatically, by virtue of their
occurrence in a program being run.  On rare occasions, you may need to
write code that evaluates a form that is computed at run time, such as
after reading a form from text being edited or getting one from a
property list.  On these occasions, use the ‘eval’ function.  Often
‘eval’ is not needed and something else should be used instead.  For
example, to get the value of a variable, while ‘eval’ works,
‘symbol-value’ is preferable; or rather than store expressions in a
property list that then need to go through ‘eval’, it is better to store
functions instead that are then passed to ‘funcall’.

   The functions and variables described in this section evaluate forms,
specify limits to the evaluation process, or record recently returned
values.  Loading a file also does evaluation (*note Loading::).

   It is generally cleaner and more flexible to store a function in a
data structure, and call it with ‘funcall’ or ‘apply’, than to store an
expression in the data structure and evaluate it.  Using functions
provides the ability to pass information to them as arguments.

 -- Function: eval form &optional lexical
     This is the basic function for evaluating an expression.  It
     evaluates FORM in the current environment, and returns the result.
     The type of the FORM object determines how it is evaluated.  *Note
     Forms::.

     The argument LEXICAL specifies the scoping rule for local variables
     (*note Variable Scoping::).  If it is omitted or ‘nil’, that means
     to evaluate FORM using the default dynamic scoping rule.  If it is
     ‘t’, that means to use the lexical scoping rule.  The value of
     LEXICAL can also be a non-empty alist specifying a particular
     “lexical environment” for lexical bindings; however, this feature
     is only useful for specialized purposes, such as in Emacs Lisp
     debuggers.  *Note Lexical Binding::.

     Since ‘eval’ is a function, the argument expression that appears in
     a call to ‘eval’ is evaluated twice: once as preparation before
     ‘eval’ is called, and again by the ‘eval’ function itself.  Here is
     an example:

          (setq foo 'bar)
               ⇒ bar
          (setq bar 'baz)
               ⇒ baz
          ;; Here ‘eval’ receives argument ‘foo’
          (eval 'foo)
               ⇒ bar
          ;; Here ‘eval’ receives argument ‘bar’, which is the value of ‘foo’
          (eval foo)
               ⇒ baz

     The number of currently active calls to ‘eval’ is limited to
     ‘max-lisp-eval-depth’ (see below).

 -- Command: eval-region start end &optional stream read-function
     This function evaluates the forms in the current buffer in the
     region defined by the positions START and END.  It reads forms from
     the region and calls ‘eval’ on them until the end of the region is
     reached, or until an error is signaled and not handled.

     By default, ‘eval-region’ does not produce any output.  However, if
     STREAM is non-‘nil’, any output produced by output functions (*note
     Output Functions::), as well as the values that result from
     evaluating the expressions in the region are printed using STREAM.
     *Note Output Streams::.

     If READ-FUNCTION is non-‘nil’, it should be a function, which is
     used instead of ‘read’ to read expressions one by one.  This
     function is called with one argument, the stream for reading input.
     You can also use the variable ‘load-read-function’ (*note How
     Programs Do Loading: Definition of load-read-function.) to specify
     this function, but it is more robust to use the READ-FUNCTION
     argument.

     ‘eval-region’ does not move point.  It always returns ‘nil’.

 -- Command: eval-buffer &optional buffer-or-name stream filename
          unibyte print
     This is similar to ‘eval-region’, but the arguments provide
     different optional features.  ‘eval-buffer’ operates on the entire
     accessible portion of buffer BUFFER-OR-NAME (*note
     (emacs)Narrowing::).  BUFFER-OR-NAME can be a buffer, a buffer name
     (a string), or ‘nil’ (or omitted), which means to use the current
     buffer.  STREAM is used as in ‘eval-region’, unless STREAM is ‘nil’
     and PRINT non-‘nil’.  In that case, values that result from
     evaluating the expressions are still discarded, but the output of
     the output functions is printed in the echo area.  FILENAME is the
     file name to use for ‘load-history’ (*note Unloading::), and
     defaults to ‘buffer-file-name’ (*note Buffer File Name::).  If
     UNIBYTE is non-‘nil’, ‘read’ converts strings to unibyte whenever
     possible.

     ‘eval-current-buffer’ is an alias for this command.

 -- User Option: max-lisp-eval-depth
     This variable defines the maximum depth allowed in calls to ‘eval’,
     ‘apply’, and ‘funcall’ before an error is signaled (with error
     message ‘"Lisp nesting exceeds max-lisp-eval-depth"’).

     This limit, with the associated error when it is exceeded, is one
     way Emacs Lisp avoids infinite recursion on an ill-defined
     function.  If you increase the value of ‘max-lisp-eval-depth’ too
     much, such code can cause stack overflow instead.  On some systems,
     this overflow can be handled.  In that case, normal Lisp evaluation
     is interrupted and control is transferred back to the top level
     command loop (‘top-level’).  Note that there is no way to enter
     Emacs Lisp debugger in this situation.  *Note Error Debugging::.

     The depth limit counts internal uses of ‘eval’, ‘apply’, and
     ‘funcall’, such as for calling the functions mentioned in Lisp
     expressions, and recursive evaluation of function call arguments
     and function body forms, as well as explicit calls in Lisp code.

     The default value of this variable is 400.  If you set it to a
     value less than 100, Lisp will reset it to 100 if the given value
     is reached.  Entry to the Lisp debugger increases the value, if
     there is little room left, to make sure the debugger itself has
     room to execute.

     ‘max-specpdl-size’ provides another limit on nesting.  *Note Local
     Variables: Definition of max-specpdl-size.

 -- Variable: values
     The value of this variable is a list of the values returned by all
     the expressions that were read, evaluated, and printed from buffers
     (including the minibuffer) by the standard Emacs commands which do
     this.  (Note that this does _not_ include evaluation in ‘*ielm*’
     buffers, nor evaluation using ‘C-j’, ‘C-x C-e’, and similar
     evaluation commands in ‘lisp-interaction-mode’.)  The elements are
     ordered most recent first.

          (setq x 1)
               ⇒ 1
          (list 'A (1+ 2) auto-save-default)
               ⇒ (A 3 t)
          values
               ⇒ ((A 3 t) 1 ...)

     This variable is useful for referring back to values of forms
     recently evaluated.  It is generally a bad idea to print the value
     of ‘values’ itself, since this may be very long.  Instead, examine
     particular elements, like this:

          ;; Refer to the most recent evaluation result.
          (nth 0 values)
               ⇒ (A 3 t)
          ;; That put a new element on,
          ;;   so all elements move back one.
          (nth 1 values)
               ⇒ (A 3 t)
          ;; This gets the element that was next-to-most-recent
          ;;   before this example.
          (nth 3 values)
               ⇒ 1


File: elisp.info,  Node: Control Structures,  Next: Variables,  Prev: Evaluation,  Up: Top

10 Control Structures
*********************

A Lisp program consists of a set of “expressions”, or “forms” (*note
Forms::).  We control the order of execution of these forms by enclosing
them in “control structures”.  Control structures are special forms
which control when, whether, or how many times to execute the forms they
contain.

   The simplest order of execution is sequential execution: first form
A, then form B, and so on.  This is what happens when you write several
forms in succession in the body of a function, or at top level in a file
of Lisp code—the forms are executed in the order written.  We call this
“textual order”.  For example, if a function body consists of two forms
A and B, evaluation of the function evaluates first A and then B.  The
result of evaluating B becomes the value of the function.

   Explicit control structures make possible an order of execution other
than sequential.

   Emacs Lisp provides several kinds of control structure, including
other varieties of sequencing, conditionals, iteration, and (controlled)
jumps—all discussed below.  The built-in control structures are special
forms since their subforms are not necessarily evaluated or not
evaluated sequentially.  You can use macros to define your own control
structure constructs (*note Macros::).

* Menu:

* Sequencing::             Evaluation in textual order.
* Conditionals::           ‘if’, ‘cond’, ‘when’, ‘unless’.
* Combining Conditions::   ‘and’, ‘or’, ‘not’.
* Iteration::              ‘while’ loops.
* Generators::             Generic sequences and coroutines.
* Nonlocal Exits::         Jumping out of a sequence.


File: elisp.info,  Node: Sequencing,  Next: Conditionals,  Up: Control Structures

10.1 Sequencing
===============

Evaluating forms in the order they appear is the most common way control
passes from one form to another.  In some contexts, such as in a
function body, this happens automatically.  Elsewhere you must use a
control structure construct to do this: ‘progn’, the simplest control
construct of Lisp.

   A ‘progn’ special form looks like this:

     (progn A B C ...)

and it says to execute the forms A, B, C, and so on, in that order.
These forms are called the “body” of the ‘progn’ form.  The value of the
last form in the body becomes the value of the entire ‘progn’.
‘(progn)’ returns ‘nil’.

   In the early days of Lisp, ‘progn’ was the only way to execute two or
more forms in succession and use the value of the last of them.  But
programmers found they often needed to use a ‘progn’ in the body of a
function, where (at that time) only one form was allowed.  So the body
of a function was made into an implicit ‘progn’: several forms are
allowed just as in the body of an actual ‘progn’.  Many other control
structures likewise contain an implicit ‘progn’.  As a result, ‘progn’
is not used as much as it was many years ago.  It is needed now most
often inside an ‘unwind-protect’, ‘and’, ‘or’, or in the THEN-part of an
‘if’.

 -- Special Form: progn forms...
     This special form evaluates all of the FORMS, in textual order,
     returning the result of the final form.

          (progn (print "The first form")
                 (print "The second form")
                 (print "The third form"))
               ⊣ "The first form"
               ⊣ "The second form"
               ⊣ "The third form"
          ⇒ "The third form"

   Two other constructs likewise evaluate a series of forms but return
different values:

 -- Special Form: prog1 form1 forms...
     This special form evaluates FORM1 and all of the FORMS, in textual
     order, returning the result of FORM1.

          (prog1 (print "The first form")
                 (print "The second form")
                 (print "The third form"))
               ⊣ "The first form"
               ⊣ "The second form"
               ⊣ "The third form"
          ⇒ "The first form"

     Here is a way to remove the first element from a list in the
     variable ‘x’, then return the value of that former element:

          (prog1 (car x) (setq x (cdr x)))

 -- Special Form: prog2 form1 form2 forms...
     This special form evaluates FORM1, FORM2, and all of the following
     FORMS, in textual order, returning the result of FORM2.

          (prog2 (print "The first form")
                 (print "The second form")
                 (print "The third form"))
               ⊣ "The first form"
               ⊣ "The second form"
               ⊣ "The third form"
          ⇒ "The second form"


File: elisp.info,  Node: Conditionals,  Next: Combining Conditions,  Prev: Sequencing,  Up: Control Structures

10.2 Conditionals
=================

Conditional control structures choose among alternatives.  Emacs Lisp
has four conditional forms: ‘if’, which is much the same as in other
languages; ‘when’ and ‘unless’, which are variants of ‘if’; and ‘cond’,
which is a generalized case statement.

 -- Special Form: if condition then-form else-forms...
     ‘if’ chooses between the THEN-FORM and the ELSE-FORMS based on the
     value of CONDITION.  If the evaluated CONDITION is non-‘nil’,
     THEN-FORM is evaluated and the result returned.  Otherwise, the
     ELSE-FORMS are evaluated in textual order, and the value of the
     last one is returned.  (The ELSE part of ‘if’ is an example of an
     implicit ‘progn’.  *Note Sequencing::.)

     If CONDITION has the value ‘nil’, and no ELSE-FORMS are given, ‘if’
     returns ‘nil’.

     ‘if’ is a special form because the branch that is not selected is
     never evaluated—it is ignored.  Thus, in this example, ‘true’ is
     not printed because ‘print’ is never called:

          (if nil
              (print 'true)
            'very-false)
          ⇒ very-false

 -- Macro: when condition then-forms...
     This is a variant of ‘if’ where there are no ELSE-FORMS, and
     possibly several THEN-FORMS.  In particular,

          (when CONDITION A B C)

     is entirely equivalent to

          (if CONDITION (progn A B C) nil)

 -- Macro: unless condition forms...
     This is a variant of ‘if’ where there is no THEN-FORM:

          (unless CONDITION A B C)

     is entirely equivalent to

          (if CONDITION nil
             A B C)

 -- Special Form: cond clause...
     ‘cond’ chooses among an arbitrary number of alternatives.  Each
     CLAUSE in the ‘cond’ must be a list.  The CAR of this list is the
     CONDITION; the remaining elements, if any, the BODY-FORMS.  Thus, a
     clause looks like this:

          (CONDITION BODY-FORMS...)

     ‘cond’ tries the clauses in textual order, by evaluating the
     CONDITION of each clause.  If the value of CONDITION is non-‘nil’,
     the clause succeeds; then ‘cond’ evaluates its BODY-FORMS, and
     returns the value of the last of BODY-FORMS.  Any remaining clauses
     are ignored.

     If the value of CONDITION is ‘nil’, the clause fails, so the ‘cond’
     moves on to the following clause, trying its CONDITION.

     A clause may also look like this:

          (CONDITION)

     Then, if CONDITION is non-‘nil’ when tested, the ‘cond’ form
     returns the value of CONDITION.

     If every CONDITION evaluates to ‘nil’, so that every clause fails,
     ‘cond’ returns ‘nil’.

     The following example has four clauses, which test for the cases
     where the value of ‘x’ is a number, string, buffer and symbol,
     respectively:

          (cond ((numberp x) x)
                ((stringp x) x)
                ((bufferp x)
                 (setq temporary-hack x) ; multiple body-forms
                 (buffer-name x))        ; in one clause
                ((symbolp x) (symbol-value x)))

     Often we want to execute the last clause whenever none of the
     previous clauses was successful.  To do this, we use ‘t’ as the
     CONDITION of the last clause, like this: ‘(t BODY-FORMS)’.  The
     form ‘t’ evaluates to ‘t’, which is never ‘nil’, so this clause
     never fails, provided the ‘cond’ gets to it at all.  For example:

          (setq a 5)
          (cond ((eq a 'hack) 'foo)
                (t "default"))
          ⇒ "default"

     This ‘cond’ expression returns ‘foo’ if the value of ‘a’ is ‘hack’,
     and returns the string ‘"default"’ otherwise.

   Any conditional construct can be expressed with ‘cond’ or with ‘if’.
Therefore, the choice between them is a matter of style.  For example:

     (if A B C)
     ≡
     (cond (A B) (t C))

* Menu:

* Pattern matching case statement::


File: elisp.info,  Node: Pattern matching case statement,  Up: Conditionals

10.2.1 Pattern matching case statement
--------------------------------------

The ‘cond’ form lets you choose between alternatives using predicate
conditions that compare values of expressions against specific values
known and written in advance.  However, sometimes it is useful to select
alternatives based on more general conditions that distinguish between
broad classes of values.  The ‘pcase’ macro allows you to choose between
alternatives based on matching the value of an expression against a
series of patterns.  A pattern can be a literal value (for comparisons
to literal values you’d use ‘cond’), or it can be a more general
description of the expected structure of the expression’s value.

 -- Macro: pcase expression &rest clauses
     Evaluate EXPRESSION and choose among an arbitrary number of
     alternatives based on the value of EXPRESSION.  The possible
     alternatives are specified by CLAUSES, each of which must be a list
     of the form ‘(PATTERN BODY-FORMS...)’.  ‘pcase’ tries to match the
     value of EXPRESSION to the PATTERN of each clause, in textual
     order.  If the value matches, the clause succeeds; ‘pcase’ then
     evaluates its BODY-FORMS, and returns the value of the last of
     BODY-FORMS.  Any remaining CLAUSES are ignored.

     The PATTERN part of a clause can be of one of two types:
     “QPattern”, a pattern quoted with a backquote; or a “UPattern”,
     which is not quoted.  UPatterns are simpler, so we describe them
     first.

     Note: In the description of the patterns below, we use “the value
     being matched” to refer to the value of the EXPRESSION that is the
     first argument of ‘pcase’.

     A UPattern can have the following forms:

     ‘'VAL’
          Matches if the value being matched is ‘equal’ to VAL.
     ‘ATOM’
          Matches any ATOM, which can be a keyword, a number, or a
          string.  (These are self-quoting, so this kind of UPattern is
          actually a shorthand for ‘'ATOM’.)  Note that a string or a
          float matches any string or float with the same
          contents/value.
     ‘_’
          Matches any value.  This is known as “don’t care” or
          “wildcard”.
     ‘SYMBOL’
          Matches any value, and additionally let-binds SYMBOL to the
          value it matched, so that you can later refer to it, either in
          the BODY-FORMS or also later in the pattern.
     ‘(pred PREDFUN)’
          Matches if the predicate function PREDFUN returns non-‘nil’
          when called with the value being matched as its argument.
          PREDFUN can be one of the possible forms described below.
     ‘(guard BOOLEAN-EXPRESSION)’
          Matches if BOOLEAN-EXPRESSION evaluates to non-‘nil’.  This
          allows you to include in a UPattern boolean conditions that
          refer to symbols bound to values (including the value being
          matched) by previous UPatterns.  Typically used inside an
          ‘and’ UPattern, see below.  For example,
          ‘(and x (guard (< x 10)))’ is a pattern which matches any
          number smaller than 10 and let-binds the variable ‘x’ to that
          number.
     ‘(let UPATTERN EXPRESSION)’
          Matches if the specified EXPRESSION matches the specified
          UPATTERN.  This allows matching a pattern against the value of
          an _arbitrary_ expression, not just the expression that is the
          first argument to ‘pcase’.  (It is called ‘let’ because
          UPATTERN can bind symbols to values using the SYMBOL UPattern.
          For example: ‘((or `(key . ,val) (let val 5)) val)’.)
     ‘(app FUNCTION UPATTERN)’
          Matches if FUNCTION applied to the value being matched returns
          a value that matches UPATTERN.  This is like the ‘pred’
          UPattern, except that it tests the result against UPATTERN,
          rather than against a boolean truth value.  The FUNCTION call
          can use one of the forms described below.
     ‘(or UPATTERN1 UPATTERN2...)’
          Matches if one the argument UPatterns matches.  As soon as the
          first matching UPattern is found, the rest are not tested.
          For this reason, if any of the UPatterns let-bind symbols to
          the matched value, they should all bind the same symbols.
     ‘(and UPATTERN1 UPATTERN2...)’
          Matches if all the argument UPatterns match.

     The function calls used in the ‘pred’ and ‘app’ UPatterns can have
     one of the following forms:

     function symbol, like ‘integerp’
          In this case, the named function is applied to the value being
          matched.
     lambda-function ‘(lambda (ARG) BODY)’
          In this case, the lambda-function is called with one argument,
          the value being matched.
     ‘(FUNC ARGS...)’
          This is a function call with N specified arguments; the
          function is called with these N arguments and an additional
          N+1-th argument that is the value being matched.

     Here’s an illustrative example of using UPatterns:

          (pcase (get-return-code x)
            ('success       (message "Done!"))
            ('would-block   (message "Sorry, can't do it now"))
            ('read-only     (message "The shmliblick is read-only"))
            ('access-denied (message "You do not have the needed rights"))
            (code           (message "Unknown return code %S" code)))

     In addition, you can use backquoted patterns that are more
     powerful.  They allow matching the value of the EXPRESSION that is
     the first argument of ‘pcase’ against specifications of its
     _structure_.  For example, you can specify that the value must be a
     list of 2 elements whose first element is a specific string and the
     second element is any value with a backquoted pattern like
     ‘`("first" ,second-elem)’.

     Backquoted patterns have the form ‘`QPATTERN’ where QPATTERN can
     have the following forms:

     ‘(QPATTERN1 . QPATTERN2)’
          Matches if the value being matched is a cons cell whose ‘car’
          matches QPATTERN1 and whose ‘cdr’ matches QPATTERN2.  This
          readily generalizes to backquoted lists as in
          ‘(QPATTERN1 QPATTERN2 ...)’.
     ‘[QPATTERN1 QPATTERN2 ... QPATTERNM]’
          Matches if the value being matched is a vector of length M
          whose ‘0’..‘(M-1)’th elements match QPATTERN1, QPATTERN2 ...
          QPATTERNM, respectively.
     ‘ATOM’
          Matches if corresponding element of the value being matched is
          ‘equal’ to the specified ATOM.
     ‘,UPATTERN’
          Matches if the corresponding element of the value being
          matched matches the specified UPATTERN.

     Note that uses of QPatterns can be expressed using only UPatterns,
     as QPatterns are implemented on top of UPatterns using
     ‘pcase-defmacro’, described below.  However, using QPatterns will
     in many cases lead to a more readable code.

   Here is an example of using ‘pcase’ to implement a simple interpreter
for a little expression language (note that this example requires
lexical binding, *note Lexical Binding::):

     (defun evaluate (exp env)
       (pcase exp
         (`(add ,x ,y)       (+ (evaluate x env) (evaluate y env)))
         (`(call ,fun ,arg)  (funcall (evaluate fun env) (evaluate arg env)))
         (`(fn ,arg ,body)   (lambda (val)
                               (evaluate body (cons (cons arg val) env))))
         ((pred numberp)     exp)
         ((pred symbolp)     (cdr (assq exp env)))
         (_                  (error "Unknown expression %S" exp))))

   Here ‘`(add ,x ,y)’ is a pattern that checks that ‘exp’ is a
three-element list starting with the literal symbol ‘add’, then extracts
the second and third elements and binds them to the variables ‘x’ and
‘y’.  Then it evaluates ‘x’ and ‘y’ and adds the results.  The ‘call’
and ‘fn’ patterns similarly implement two flavors of function calls.
‘(pred numberp)’ is a pattern that simply checks that ‘exp’ is a number
and if so, evaluates it.  ‘(pred symbolp)’ matches symbols, and returns
their association.  Finally, ‘_’ is the catch-all pattern that matches
anything, so it’s suitable for reporting syntax errors.

   Here are some sample programs in this small language, including their
evaluation results:

     (evaluate '(add 1 2) nil)                 ;=> 3
     (evaluate '(add x y) '((x . 1) (y . 2)))  ;=> 3
     (evaluate '(call (fn x (add 1 x)) 2) nil) ;=> 3
     (evaluate '(sub 1 2) nil)                 ;=> error

   Additional UPatterns can be defined using the ‘pcase-defmacro’ macro.

 -- Macro: pcase-defmacro name args &rest body
     Define a new kind of UPattern for ‘pcase’.  The new UPattern will
     be invoked as ‘(NAME ACTUAL-ARGS)’.  The BODY should describe how
     to rewrite the UPattern NAME into some other UPattern.  The
     rewriting will be the result of evaluating BODY in an environment
     where ARGS are bound to ACTUAL-ARGS.


File: elisp.info,  Node: Combining Conditions,  Next: Iteration,  Prev: Conditionals,  Up: Control Structures

10.3 Constructs for Combining Conditions
========================================

This section describes three constructs that are often used together
with ‘if’ and ‘cond’ to express complicated conditions.  The constructs
‘and’ and ‘or’ can also be used individually as kinds of multiple
conditional constructs.

 -- Function: not condition
     This function tests for the falsehood of CONDITION.  It returns ‘t’
     if CONDITION is ‘nil’, and ‘nil’ otherwise.  The function ‘not’ is
     identical to ‘null’, and we recommend using the name ‘null’ if you
     are testing for an empty list.

 -- Special Form: and conditions...
     The ‘and’ special form tests whether all the CONDITIONS are true.
     It works by evaluating the CONDITIONS one by one in the order
     written.

     If any of the CONDITIONS evaluates to ‘nil’, then the result of the
     ‘and’ must be ‘nil’ regardless of the remaining CONDITIONS; so
     ‘and’ returns ‘nil’ right away, ignoring the remaining CONDITIONS.

     If all the CONDITIONS turn out non-‘nil’, then the value of the
     last of them becomes the value of the ‘and’ form.  Just ‘(and)’,
     with no CONDITIONS, returns ‘t’, appropriate because all the
     CONDITIONS turned out non-‘nil’.  (Think about it; which one did
     not?)

     Here is an example.  The first condition returns the integer 1,
     which is not ‘nil’.  Similarly, the second condition returns the
     integer 2, which is not ‘nil’.  The third condition is ‘nil’, so
     the remaining condition is never evaluated.

          (and (print 1) (print 2) nil (print 3))
               ⊣ 1
               ⊣ 2
          ⇒ nil

     Here is a more realistic example of using ‘and’:

          (if (and (consp foo) (eq (car foo) 'x))
              (message "foo is a list starting with x"))

     Note that ‘(car foo)’ is not executed if ‘(consp foo)’ returns
     ‘nil’, thus avoiding an error.

     ‘and’ expressions can also be written using either ‘if’ or ‘cond’.
     Here’s how:

          (and ARG1 ARG2 ARG3)
          ≡
          (if ARG1 (if ARG2 ARG3))
          ≡
          (cond (ARG1 (cond (ARG2 ARG3))))

 -- Special Form: or conditions...
     The ‘or’ special form tests whether at least one of the CONDITIONS
     is true.  It works by evaluating all the CONDITIONS one by one in
     the order written.

     If any of the CONDITIONS evaluates to a non-‘nil’ value, then the
     result of the ‘or’ must be non-‘nil’; so ‘or’ returns right away,
     ignoring the remaining CONDITIONS.  The value it returns is the
     non-‘nil’ value of the condition just evaluated.

     If all the CONDITIONS turn out ‘nil’, then the ‘or’ expression
     returns ‘nil’.  Just ‘(or)’, with no CONDITIONS, returns ‘nil’,
     appropriate because all the CONDITIONS turned out ‘nil’.  (Think
     about it; which one did not?)

     For example, this expression tests whether ‘x’ is either ‘nil’ or
     the integer zero:

          (or (eq x nil) (eq x 0))

     Like the ‘and’ construct, ‘or’ can be written in terms of ‘cond’.
     For example:

          (or ARG1 ARG2 ARG3)
          ≡
          (cond (ARG1)
                (ARG2)
                (ARG3))

     You could almost write ‘or’ in terms of ‘if’, but not quite:

          (if ARG1 ARG1
            (if ARG2 ARG2
              ARG3))

     This is not completely equivalent because it can evaluate ARG1 or
     ARG2 twice.  By contrast, ‘(or ARG1 ARG2 ARG3)’ never evaluates any
     argument more than once.


File: elisp.info,  Node: Iteration,  Next: Generators,  Prev: Combining Conditions,  Up: Control Structures

10.4 Iteration
==============

Iteration means executing part of a program repetitively.  For example,
you might want to repeat some computation once for each element of a
list, or once for each integer from 0 to N.  You can do this in Emacs
Lisp with the special form ‘while’:

 -- Special Form: while condition forms...
     ‘while’ first evaluates CONDITION.  If the result is non-‘nil’, it
     evaluates FORMS in textual order.  Then it reevaluates CONDITION,
     and if the result is non-‘nil’, it evaluates FORMS again.  This
     process repeats until CONDITION evaluates to ‘nil’.

     There is no limit on the number of iterations that may occur.  The
     loop will continue until either CONDITION evaluates to ‘nil’ or
     until an error or ‘throw’ jumps out of it (*note Nonlocal Exits::).

     The value of a ‘while’ form is always ‘nil’.

          (setq num 0)
               ⇒ 0
          (while (< num 4)
            (princ (format "Iteration %d." num))
            (setq num (1+ num)))
               ⊣ Iteration 0.
               ⊣ Iteration 1.
               ⊣ Iteration 2.
               ⊣ Iteration 3.
               ⇒ nil

     To write a repeat-until loop, which will execute something on each
     iteration and then do the end-test, put the body followed by the
     end-test in a ‘progn’ as the first argument of ‘while’, as shown
     here:

          (while (progn
                   (forward-line 1)
                   (not (looking-at "^$"))))

     This moves forward one line and continues moving by lines until it
     reaches an empty line.  It is peculiar in that the ‘while’ has no
     body, just the end test (which also does the real work of moving
     point).

   The ‘dolist’ and ‘dotimes’ macros provide convenient ways to write
two common kinds of loops.

 -- Macro: dolist (var list [result]) body...
     This construct executes BODY once for each element of LIST, binding
     the variable VAR locally to hold the current element.  Then it
     returns the value of evaluating RESULT, or ‘nil’ if RESULT is
     omitted.  For example, here is how you could use ‘dolist’ to define
     the ‘reverse’ function:

          (defun reverse (list)
            (let (value)
              (dolist (elt list value)
                (setq value (cons elt value)))))

 -- Macro: dotimes (var count [result]) body...
     This construct executes BODY once for each integer from 0
     (inclusive) to COUNT (exclusive), binding the variable VAR to the
     integer for the current iteration.  Then it returns the value of
     evaluating RESULT, or ‘nil’ if RESULT is omitted.  Here is an
     example of using ‘dotimes’ to do something 100 times:

          (dotimes (i 100)
            (insert "I will not obey absurd orders\n"))


File: elisp.info,  Node: Generators,  Next: Nonlocal Exits,  Prev: Iteration,  Up: Control Structures

10.5 Generators
===============

A “generator” is a function that produces a potentially-infinite stream
of values.  Each time the function produces a value, it suspends itself
and waits for a caller to request the next value.

 -- Macro: iter-defun name args [doc] [declare] [interactive] body...
     ‘iter-defun’ defines a generator function.  A generator function
     has the same signature as a normal function, but works differently.
     Instead of executing BODY when called, a generator function returns
     an iterator object.  That iterator runs BODY to generate values,
     emitting a value and pausing where ‘iter-yield’ or
     ‘iter-yield-from’ appears.  When BODY returns normally, ‘iter-next’
     signals ‘iter-end-of-sequence’ with BODY’s result as its condition
     data.

     Any kind of Lisp code is valid inside BODY, but ‘iter-yield’ and
     ‘iter-yield-from’ cannot appear inside ‘unwind-protect’ forms.

 -- Macro: iter-lambda args [doc] [interactive] body...
     ‘iter-lambda’ produces an unnamed generator function that works
     just like a generator function produced with ‘iter-defun’.

 -- Macro: iter-yield value
     When it appears inside a generator function, ‘iter-yield’ indicates
     that the current iterator should pause and return VALUE from
     ‘iter-next’.  ‘iter-yield’ evaluates to the ‘value’ parameter of
     next call to ‘iter-next’.

 -- Macro: iter-yield-from iterator
     ‘iter-yield-from’ yields all the values that ITERATOR produces and
     evaluates to the value that ITERATOR’s generator function returns
     normally.  While it has control, ITERATOR receives values sent to
     the iterator using ‘iter-next’.

   To use a generator function, first call it normally, producing a
“iterator” object.  An iterator is a specific instance of a generator.
Then use ‘iter-next’ to retrieve values from this iterator.  When there
are no more values to pull from an iterator, ‘iter-next’ raises an
‘iter-end-of-sequence’ condition with the iterator’s final value.

   It’s important to note that generator function bodies only execute
inside calls to ‘iter-next’.  A call to a function defined with
‘iter-defun’ produces an iterator; you must drive this iterator with
‘iter-next’ for anything interesting to happen.  Each call to a
generator function produces a _different_ iterator, each with its own
state.

 -- Function: iter-next iterator value
     Retrieve the next value from ITERATOR.  If there are no more values
     to be generated (because ITERATOR’s generator function returned),
     ‘iter-next’ signals the ‘iter-end-of-sequence’ condition; the data
     value associated with this condition is the value with which
     ITERATOR’s generator function returned.

     VALUE is sent into the iterator and becomes the value to which
     ‘iter-yield’ evaluates.  VALUE is ignored for the first ‘iter-next’
     call to a given iterator, since at the start of ITERATOR’s
     generator function, the generator function is not evaluating any
     ‘iter-yield’ form.

 -- Function: iter-close iterator
     If ITERATOR is suspended inside an ‘unwind-protect’’s ‘bodyform’
     and becomes unreachable, Emacs will eventually run unwind handlers
     after a garbage collection pass.  (Note that ‘iter-yield’ is
     illegal inside an ‘unwind-protect’’s ‘unwindforms’.)  To ensure
     that these handlers are run before then, use ‘iter-close’.

   Some convenience functions are provided to make working with
iterators easier:

 -- Macro: iter-do (var iterator) body ...
     Run BODY with VAR bound to each value that ITERATOR produces.

   The Common Lisp loop facility also contains features for working with
iterators.  See *Note (cl)Loop Facility::.

   The following piece of code demonstrates some important principles of
working with iterators.

     (require 'generator)
     (iter-defun my-iter (x)
       (iter-yield (1+ (iter-yield (1+ x))))
        ;; Return normally
       -1)

     (let* ((iter (my-iter 5))
            (iter2 (my-iter 0)))
       ;; Prints 6
       (print (iter-next iter))
       ;; Prints 9
       (print (iter-next iter 8))
       ;; Prints 1; iter and iter2 have distinct states
       (print (iter-next iter2 nil))

       ;; We expect the iter sequence to end now
       (condition-case x
           (iter-next iter)
         (iter-end-of-sequence
           ;; Prints -1, which my-iter returned normally
           (print (cdr x)))))


File: elisp.info,  Node: Nonlocal Exits,  Prev: Generators,  Up: Control Structures

10.6 Nonlocal Exits
===================

A “nonlocal exit” is a transfer of control from one point in a program
to another remote point.  Nonlocal exits can occur in Emacs Lisp as a
result of errors; you can also use them under explicit control.
Nonlocal exits unbind all variable bindings made by the constructs being
exited.

* Menu:

* Catch and Throw::     Nonlocal exits for the program’s own purposes.
* Examples of Catch::   Showing how such nonlocal exits can be written.
* Errors::              How errors are signaled and handled.
* Cleanups::            Arranging to run a cleanup form if an error happens.


File: elisp.info,  Node: Catch and Throw,  Next: Examples of Catch,  Up: Nonlocal Exits

10.6.1 Explicit Nonlocal Exits: ‘catch’ and ‘throw’
---------------------------------------------------

Most control constructs affect only the flow of control within the
construct itself.  The function ‘throw’ is the exception to this rule of
normal program execution: it performs a nonlocal exit on request.
(There are other exceptions, but they are for error handling only.)
‘throw’ is used inside a ‘catch’, and jumps back to that ‘catch’.  For
example:

     (defun foo-outer ()
       (catch 'foo
         (foo-inner)))

     (defun foo-inner ()
       ...
       (if x
           (throw 'foo t))
       ...)

The ‘throw’ form, if executed, transfers control straight back to the
corresponding ‘catch’, which returns immediately.  The code following
the ‘throw’ is not executed.  The second argument of ‘throw’ is used as
the return value of the ‘catch’.

   The function ‘throw’ finds the matching ‘catch’ based on the first
argument: it searches for a ‘catch’ whose first argument is ‘eq’ to the
one specified in the ‘throw’.  If there is more than one applicable
‘catch’, the innermost one takes precedence.  Thus, in the above
example, the ‘throw’ specifies ‘foo’, and the ‘catch’ in ‘foo-outer’
specifies the same symbol, so that ‘catch’ is the applicable one
(assuming there is no other matching ‘catch’ in between).

   Executing ‘throw’ exits all Lisp constructs up to the matching
‘catch’, including function calls.  When binding constructs such as
‘let’ or function calls are exited in this way, the bindings are
unbound, just as they are when these constructs exit normally (*note
Local Variables::).  Likewise, ‘throw’ restores the buffer and position
saved by ‘save-excursion’ (*note Excursions::), and the narrowing status
saved by ‘save-restriction’.  It also runs any cleanups established with
the ‘unwind-protect’ special form when it exits that form (*note
Cleanups::).

   The ‘throw’ need not appear lexically within the ‘catch’ that it
jumps to.  It can equally well be called from another function called
within the ‘catch’.  As long as the ‘throw’ takes place chronologically
after entry to the ‘catch’, and chronologically before exit from it, it
has access to that ‘catch’.  This is why ‘throw’ can be used in commands
such as ‘exit-recursive-edit’ that throw back to the editor command loop
(*note Recursive Editing::).

     Common Lisp note: Most other versions of Lisp, including Common
     Lisp, have several ways of transferring control nonsequentially:
     ‘return’, ‘return-from’, and ‘go’, for example.  Emacs Lisp has
     only ‘throw’.  The ‘cl-lib’ library provides versions of some of
     these.  *Note (cl)Blocks and Exits::.

 -- Special Form: catch tag body...
     ‘catch’ establishes a return point for the ‘throw’ function.  The
     return point is distinguished from other such return points by TAG,
     which may be any Lisp object except ‘nil’.  The argument TAG is
     evaluated normally before the return point is established.

     With the return point in effect, ‘catch’ evaluates the forms of the
     BODY in textual order.  If the forms execute normally (without
     error or nonlocal exit) the value of the last body form is returned
     from the ‘catch’.

     If a ‘throw’ is executed during the execution of BODY, specifying
     the same value TAG, the ‘catch’ form exits immediately; the value
     it returns is whatever was specified as the second argument of
     ‘throw’.

 -- Function: throw tag value
     The purpose of ‘throw’ is to return from a return point previously
     established with ‘catch’.  The argument TAG is used to choose among
     the various existing return points; it must be ‘eq’ to the value
     specified in the ‘catch’.  If multiple return points match TAG, the
     innermost one is used.

     The argument VALUE is used as the value to return from that
     ‘catch’.

     If no return point is in effect with tag TAG, then a ‘no-catch’
     error is signaled with data ‘(TAG VALUE)’.


File: elisp.info,  Node: Examples of Catch,  Next: Errors,  Prev: Catch and Throw,  Up: Nonlocal Exits

10.6.2 Examples of ‘catch’ and ‘throw’
--------------------------------------

One way to use ‘catch’ and ‘throw’ is to exit from a doubly nested loop.
(In most languages, this would be done with a ‘goto’.)  Here we compute
‘(foo I J)’ for I and J varying from 0 to 9:

     (defun search-foo ()
       (catch 'loop
         (let ((i 0))
           (while (< i 10)
             (let ((j 0))
               (while (< j 10)
                 (if (foo i j)
                     (throw 'loop (list i j)))
                 (setq j (1+ j))))
             (setq i (1+ i))))))

If ‘foo’ ever returns non-‘nil’, we stop immediately and return a list
of I and J.  If ‘foo’ always returns ‘nil’, the ‘catch’ returns
normally, and the value is ‘nil’, since that is the result of the
‘while’.

   Here are two tricky examples, slightly different, showing two return
points at once.  First, two return points with the same tag, ‘hack’:

     (defun catch2 (tag)
       (catch tag
         (throw 'hack 'yes)))
     ⇒ catch2

     (catch 'hack
       (print (catch2 'hack))
       'no)
     ⊣ yes
     ⇒ no

Since both return points have tags that match the ‘throw’, it goes to
the inner one, the one established in ‘catch2’.  Therefore, ‘catch2’
returns normally with value ‘yes’, and this value is printed.  Finally
the second body form in the outer ‘catch’, which is ‘'no’, is evaluated
and returned from the outer ‘catch’.

   Now let’s change the argument given to ‘catch2’:

     (catch 'hack
       (print (catch2 'quux))
       'no)
     ⇒ yes

We still have two return points, but this time only the outer one has
the tag ‘hack’; the inner one has the tag ‘quux’ instead.  Therefore,
‘throw’ makes the outer ‘catch’ return the value ‘yes’.  The function
‘print’ is never called, and the body-form ‘'no’ is never evaluated.


File: elisp.info,  Node: Errors,  Next: Cleanups,  Prev: Examples of Catch,  Up: Nonlocal Exits

10.6.3 Errors
-------------

When Emacs Lisp attempts to evaluate a form that, for some reason,
cannot be evaluated, it “signals” an “error”.

   When an error is signaled, Emacs’s default reaction is to print an
error message and terminate execution of the current command.  This is
the right thing to do in most cases, such as if you type ‘C-f’ at the
end of the buffer.

   In complicated programs, simple termination may not be what you want.
For example, the program may have made temporary changes in data
structures, or created temporary buffers that should be deleted before
the program is finished.  In such cases, you would use ‘unwind-protect’
to establish “cleanup expressions” to be evaluated in case of error.
(*Note Cleanups::.)  Occasionally, you may wish the program to continue
execution despite an error in a subroutine.  In these cases, you would
use ‘condition-case’ to establish “error handlers” to recover control in
case of error.

   Resist the temptation to use error handling to transfer control from
one part of the program to another; use ‘catch’ and ‘throw’ instead.
*Note Catch and Throw::.

* Menu:

* Signaling Errors::      How to report an error.
* Processing of Errors::  What Emacs does when you report an error.
* Handling Errors::       How you can trap errors and continue execution.
* Error Symbols::         How errors are classified for trapping them.


File: elisp.info,  Node: Signaling Errors,  Next: Processing of Errors,  Up: Errors

10.6.3.1 How to Signal an Error
...............................

“Signaling” an error means beginning error processing.  Error processing
normally aborts all or part of the running program and returns to a
point that is set up to handle the error (*note Processing of Errors::).
Here we describe how to signal an error.

   Most errors are signaled automatically within Lisp primitives which
you call for other purposes, such as if you try to take the CAR of an
integer or move forward a character at the end of the buffer.  You can
also signal errors explicitly with the functions ‘error’ and ‘signal’.

   Quitting, which happens when the user types ‘C-g’, is not considered
an error, but it is handled almost like an error.  *Note Quitting::.

   Every error specifies an error message, one way or another.  The
message should state what is wrong (“File does not exist”), not how
things ought to be (“File must exist”).  The convention in Emacs Lisp is
that error messages should start with a capital letter, but should not
end with any sort of punctuation.

 -- Function: error format-string &rest args
     This function signals an error with an error message constructed by
     applying ‘format-message’ (*note Formatting Strings::) to
     FORMAT-STRING and ARGS.

     These examples show typical uses of ‘error’:

          (error "That is an error -- try something else")
               error→ That is an error -- try something else

          (error "Invalid name `%s'" "A%%B")
               error→ Invalid name ‘A%%B’

     ‘error’ works by calling ‘signal’ with two arguments: the error
     symbol ‘error’, and a list containing the string returned by
     ‘format-message’.

     The ‘text-quoting-style’ variable controls what quotes are
     generated; *Note Keys in Documentation::.  A call using a format
     like "Missing `%s'" with grave accents and apostrophes typically
     generates a message like "Missing ‘foo’" with matching curved
     quotes.  In contrast, a call using a format like "Missing '%s'"
     with only apostrophes typically generates a message like "Missing
     ’foo’" with only closing curved quotes, an unusual style in
     English.

     *Warning:* If you want to use your own string as an error message
     verbatim, don’t just write ‘(error STRING)’.  If STRING STRING
     contains ‘%’, ‘`’, or ‘'’ it may be reformatted, with undesirable
     results.  Instead, use ‘(error "%s" STRING)’.

 -- Function: signal error-symbol data
     This function signals an error named by ERROR-SYMBOL.  The argument
     DATA is a list of additional Lisp objects relevant to the
     circumstances of the error.

     The argument ERROR-SYMBOL must be an “error symbol”—a symbol
     defined with ‘define-error’.  This is how Emacs Lisp classifies
     different sorts of errors.  *Note Error Symbols::, for a
     description of error symbols, error conditions and condition names.

     If the error is not handled, the two arguments are used in printing
     the error message.  Normally, this error message is provided by the
     ‘error-message’ property of ERROR-SYMBOL.  If DATA is non-‘nil’,
     this is followed by a colon and a comma separated list of the
     unevaluated elements of DATA.  For ‘error’, the error message is
     the CAR of DATA (that must be a string).  Subcategories of
     ‘file-error’ are handled specially.

     The number and significance of the objects in DATA depends on
     ERROR-SYMBOL.  For example, with a ‘wrong-type-argument’ error,
     there should be two objects in the list: a predicate that describes
     the type that was expected, and the object that failed to fit that
     type.

     Both ERROR-SYMBOL and DATA are available to any error handlers that
     handle the error: ‘condition-case’ binds a local variable to a list
     of the form ‘(ERROR-SYMBOL . DATA)’ (*note Handling Errors::).

     The function ‘signal’ never returns.

          (signal 'wrong-number-of-arguments '(x y))
               error→ Wrong number of arguments: x, y

          (signal 'no-such-error '("My unknown error condition"))
               error→ peculiar error: "My unknown error condition"

 -- Function: user-error format-string &rest args
     This function behaves exactly like ‘error’, except that it uses the
     error symbol ‘user-error’ rather than ‘error’.  As the name
     suggests, this is intended to report errors on the part of the
     user, rather than errors in the code itself.  For example, if you
     try to use the command ‘Info-history-back’ (‘l’) to move back
     beyond the start of your Info browsing history, Emacs signals a
     ‘user-error’.  Such errors do not cause entry to the debugger, even
     when ‘debug-on-error’ is non-‘nil’.  *Note Error Debugging::.

     Common Lisp note: Emacs Lisp has nothing like the Common Lisp
     concept of continuable errors.


File: elisp.info,  Node: Processing of Errors,  Next: Handling Errors,  Prev: Signaling Errors,  Up: Errors

10.6.3.2 How Emacs Processes Errors
...................................

When an error is signaled, ‘signal’ searches for an active “handler” for
the error.  A handler is a sequence of Lisp expressions designated to be
executed if an error happens in part of the Lisp program.  If the error
has an applicable handler, the handler is executed, and control resumes
following the handler.  The handler executes in the environment of the
‘condition-case’ that established it; all functions called within that
‘condition-case’ have already been exited, and the handler cannot return
to them.

   If there is no applicable handler for the error, it terminates the
current command and returns control to the editor command loop.  (The
command loop has an implicit handler for all kinds of errors.)  The
command loop’s handler uses the error symbol and associated data to
print an error message.  You can use the variable
‘command-error-function’ to control how this is done:

 -- Variable: command-error-function
     This variable, if non-‘nil’, specifies a function to use to handle
     errors that return control to the Emacs command loop.  The function
     should take three arguments: DATA, a list of the same form that
     ‘condition-case’ would bind to its variable; CONTEXT, a string
     describing the situation in which the error occurred, or (more
     often) ‘nil’; and CALLER, the Lisp function which called the
     primitive that signaled the error.

   An error that has no explicit handler may call the Lisp debugger.
The debugger is enabled if the variable ‘debug-on-error’ (*note Error
Debugging::) is non-‘nil’.  Unlike error handlers, the debugger runs in
the environment of the error, so that you can examine values of
variables precisely as they were at the time of the error.


File: elisp.info,  Node: Handling Errors,  Next: Error Symbols,  Prev: Processing of Errors,  Up: Errors

10.6.3.3 Writing Code to Handle Errors
......................................

The usual effect of signaling an error is to terminate the command that
is running and return immediately to the Emacs editor command loop.  You
can arrange to trap errors occurring in a part of your program by
establishing an error handler, with the special form ‘condition-case’.
A simple example looks like this:

     (condition-case nil
         (delete-file filename)
       (error nil))

This deletes the file named FILENAME, catching any error and returning
‘nil’ if an error occurs.  (You can use the macro ‘ignore-errors’ for a
simple case like this; see below.)

   The ‘condition-case’ construct is often used to trap errors that are
predictable, such as failure to open a file in a call to
‘insert-file-contents’.  It is also used to trap errors that are totally
unpredictable, such as when the program evaluates an expression read
from the user.

   The second argument of ‘condition-case’ is called the “protected
form”.  (In the example above, the protected form is a call to
‘delete-file’.)  The error handlers go into effect when this form begins
execution and are deactivated when this form returns.  They remain in
effect for all the intervening time.  In particular, they are in effect
during the execution of functions called by this form, in their
subroutines, and so on.  This is a good thing, since, strictly speaking,
errors can be signaled only by Lisp primitives (including ‘signal’ and
‘error’) called by the protected form, not by the protected form itself.

   The arguments after the protected form are handlers.  Each handler
lists one or more “condition names” (which are symbols) to specify which
errors it will handle.  The error symbol specified when an error is
signaled also defines a list of condition names.  A handler applies to
an error if they have any condition names in common.  In the example
above, there is one handler, and it specifies one condition name,
‘error’, which covers all errors.

   The search for an applicable handler checks all the established
handlers starting with the most recently established one.  Thus, if two
nested ‘condition-case’ forms offer to handle the same error, the inner
of the two gets to handle it.

   If an error is handled by some ‘condition-case’ form, this ordinarily
prevents the debugger from being run, even if ‘debug-on-error’ says this
error should invoke the debugger.

   If you want to be able to debug errors that are caught by a
‘condition-case’, set the variable ‘debug-on-signal’ to a non-‘nil’
value.  You can also specify that a particular handler should let the
debugger run first, by writing ‘debug’ among the conditions, like this:

     (condition-case nil
         (delete-file filename)
       ((debug error) nil))

The effect of ‘debug’ here is only to prevent ‘condition-case’ from
suppressing the call to the debugger.  Any given error will invoke the
debugger only if ‘debug-on-error’ and the other usual filtering
mechanisms say it should.  *Note Error Debugging::.

 -- Macro: condition-case-unless-debug var protected-form handlers...
     The macro ‘condition-case-unless-debug’ provides another way to
     handle debugging of such forms.  It behaves exactly like
     ‘condition-case’, unless the variable ‘debug-on-error’ is
     non-‘nil’, in which case it does not handle any errors at all.

   Once Emacs decides that a certain handler handles the error, it
returns control to that handler.  To do so, Emacs unbinds all variable
bindings made by binding constructs that are being exited, and executes
the cleanups of all ‘unwind-protect’ forms that are being exited.  Once
control arrives at the handler, the body of the handler executes
normally.

   After execution of the handler body, execution returns from the
‘condition-case’ form.  Because the protected form is exited completely
before execution of the handler, the handler cannot resume execution at
the point of the error, nor can it examine variable bindings that were
made within the protected form.  All it can do is clean up and proceed.

   Error signaling and handling have some resemblance to ‘throw’ and
‘catch’ (*note Catch and Throw::), but they are entirely separate
facilities.  An error cannot be caught by a ‘catch’, and a ‘throw’
cannot be handled by an error handler (though using ‘throw’ when there
is no suitable ‘catch’ signals an error that can be handled).

 -- Special Form: condition-case var protected-form handlers...
     This special form establishes the error handlers HANDLERS around
     the execution of PROTECTED-FORM.  If PROTECTED-FORM executes
     without error, the value it returns becomes the value of the
     ‘condition-case’ form; in this case, the ‘condition-case’ has no
     effect.  The ‘condition-case’ form makes a difference when an error
     occurs during PROTECTED-FORM.

     Each of the HANDLERS is a list of the form ‘(CONDITIONS BODY...)’.
     Here CONDITIONS is an error condition name to be handled, or a list
     of condition names (which can include ‘debug’ to allow the debugger
     to run before the handler); BODY is one or more Lisp expressions to
     be executed when this handler handles an error.  Here are examples
     of handlers:

          (error nil)

          (arith-error (message "Division by zero"))

          ((arith-error file-error)
           (message
            "Either division by zero or failure to open a file"))

     Each error that occurs has an “error symbol” that describes what
     kind of error it is, and which describes also a list of condition
     names (*note Error Symbols::).  Emacs searches all the active
     ‘condition-case’ forms for a handler that specifies one or more of
     these condition names; the innermost matching ‘condition-case’
     handles the error.  Within this ‘condition-case’, the first
     applicable handler handles the error.

     After executing the body of the handler, the ‘condition-case’
     returns normally, using the value of the last form in the handler
     body as the overall value.

     The argument VAR is a variable.  ‘condition-case’ does not bind
     this variable when executing the PROTECTED-FORM, only when it
     handles an error.  At that time, it binds VAR locally to an “error
     description”, which is a list giving the particulars of the error.
     The error description has the form ‘(ERROR-SYMBOL . DATA)’.  The
     handler can refer to this list to decide what to do.  For example,
     if the error is for failure opening a file, the file name is the
     second element of DATA—the third element of the error description.

     If VAR is ‘nil’, that means no variable is bound.  Then the error
     symbol and associated data are not available to the handler.

     Sometimes it is necessary to re-throw a signal caught by
     ‘condition-case’, for some outer-level handler to catch.  Here’s
     how to do that:

            (signal (car err) (cdr err))

     where ‘err’ is the error description variable, the first argument
     to ‘condition-case’ whose error condition you want to re-throw.
     *Note Definition of signal::.

 -- Function: error-message-string error-descriptor
     This function returns the error message string for a given error
     descriptor.  It is useful if you want to handle an error by
     printing the usual error message for that error.  *Note Definition
     of signal::.

   Here is an example of using ‘condition-case’ to handle the error that
results from dividing by zero.  The handler displays the error message
(but without a beep), then returns a very large number.

     (defun safe-divide (dividend divisor)
       (condition-case err
           ;; Protected form.
           (/ dividend divisor)
         ;; The handler.
         (arith-error                        ; Condition.
          ;; Display the usual message for this error.
          (message "%s" (error-message-string err))
          1000000)))
     ⇒ safe-divide

     (safe-divide 5 0)
          ⊣ Arithmetic error: (arith-error)
     ⇒ 1000000

The handler specifies condition name ‘arith-error’ so that it will
handle only division-by-zero errors.  Other kinds of errors will not be
handled (by this ‘condition-case’).  Thus:

     (safe-divide nil 3)
          error→ Wrong type argument: number-or-marker-p, nil

   Here is a ‘condition-case’ that catches all kinds of errors,
including those from ‘error’:

     (setq baz 34)
          ⇒ 34

     (condition-case err
         (if (eq baz 35)
             t
           ;; This is a call to the function ‘error’.
           (error "Rats!  The variable %s was %s, not 35" 'baz baz))
       ;; This is the handler; it is not a form.
       (error (princ (format "The error was: %s" err))
              2))
     ⊣ The error was: (error "Rats!  The variable baz was 34, not 35")
     ⇒ 2

 -- Macro: ignore-errors body...
     This construct executes BODY, ignoring any errors that occur during
     its execution.  If the execution is without error, ‘ignore-errors’
     returns the value of the last form in BODY; otherwise, it returns
     ‘nil’.

     Here’s the example at the beginning of this subsection rewritten
     using ‘ignore-errors’:

            (ignore-errors
             (delete-file filename))

 -- Macro: with-demoted-errors format body...
     This macro is like a milder version of ‘ignore-errors’.  Rather
     than suppressing errors altogether, it converts them into messages.
     It uses the string FORMAT to format the message.  FORMAT should
     contain a single ‘%’-sequence; e.g., ‘"Error: %S"’.  Use
     ‘with-demoted-errors’ around code that is not expected to signal
     errors, but should be robust if one does occur.  Note that this
     macro uses ‘condition-case-unless-debug’ rather than
     ‘condition-case’.


File: elisp.info,  Node: Error Symbols,  Prev: Handling Errors,  Up: Errors

10.6.3.4 Error Symbols and Condition Names
..........................................

When you signal an error, you specify an “error symbol” to specify the
kind of error you have in mind.  Each error has one and only one error
symbol to categorize it.  This is the finest classification of errors
defined by the Emacs Lisp language.

   These narrow classifications are grouped into a hierarchy of wider
classes called “error conditions”, identified by “condition names”.  The
narrowest such classes belong to the error symbols themselves: each
error symbol is also a condition name.  There are also condition names
for more extensive classes, up to the condition name ‘error’ which takes
in all kinds of errors (but not ‘quit’).  Thus, each error has one or
more condition names: ‘error’, the error symbol if that is distinct from
‘error’, and perhaps some intermediate classifications.

 -- Function: define-error name message &optional parent
     In order for a symbol to be an error symbol, it must be defined
     with ‘define-error’ which takes a parent condition (defaults to
     ‘error’).  This parent defines the conditions that this kind of
     error belongs to.  The transitive set of parents always includes
     the error symbol itself, and the symbol ‘error’.  Because quitting
     is not considered an error, the set of parents of ‘quit’ is just
     ‘(quit)’.

   In addition to its parents, the error symbol has a MESSAGE which is a
string to be printed when that error is signaled but not handled.  If
that message is not valid, the error message ‘peculiar error’ is used.
*Note Definition of signal::.

   Internally, the set of parents is stored in the ‘error-conditions’
property of the error symbol and the message is stored in the
‘error-message’ property of the error symbol.

   Here is how we define a new error symbol, ‘new-error’:

     (define-error 'new-error "A new error" 'my-own-errors)

This error has several condition names: ‘new-error’, the narrowest
classification; ‘my-own-errors’, which we imagine is a wider
classification; and all the conditions of ‘my-own-errors’ which should
include ‘error’, which is the widest of all.

   The error string should start with a capital letter but it should not
end with a period.  This is for consistency with the rest of Emacs.

   Naturally, Emacs will never signal ‘new-error’ on its own; only an
explicit call to ‘signal’ (*note Definition of signal::) in your code
can do this:

     (signal 'new-error '(x y))
          error→ A new error: x, y

   This error can be handled through any of its condition names.  This
example handles ‘new-error’ and any other errors in the class
‘my-own-errors’:

     (condition-case foo
         (bar nil t)
       (my-own-errors nil))

   The significant way that errors are classified is by their condition
names—the names used to match errors with handlers.  An error symbol
serves only as a convenient way to specify the intended error message
and list of condition names.  It would be cumbersome to give ‘signal’ a
list of condition names rather than one error symbol.

   By contrast, using only error symbols without condition names would
seriously decrease the power of ‘condition-case’.  Condition names make
it possible to categorize errors at various levels of generality when
you write an error handler.  Using error symbols alone would eliminate
all but the narrowest level of classification.

   *Note Standard Errors::, for a list of the main error symbols and
their conditions.


File: elisp.info,  Node: Cleanups,  Prev: Errors,  Up: Nonlocal Exits

10.6.4 Cleaning Up from Nonlocal Exits
--------------------------------------

The ‘unwind-protect’ construct is essential whenever you temporarily put
a data structure in an inconsistent state; it permits you to make the
data consistent again in the event of an error or throw.  (Another more
specific cleanup construct that is used only for changes in buffer
contents is the atomic change group; *note Atomic Changes::.)

 -- Special Form: unwind-protect body-form cleanup-forms...
     ‘unwind-protect’ executes BODY-FORM with a guarantee that the
     CLEANUP-FORMS will be evaluated if control leaves BODY-FORM, no
     matter how that happens.  BODY-FORM may complete normally, or
     execute a ‘throw’ out of the ‘unwind-protect’, or cause an error;
     in all cases, the CLEANUP-FORMS will be evaluated.

     If BODY-FORM finishes normally, ‘unwind-protect’ returns the value
     of BODY-FORM, after it evaluates the CLEANUP-FORMS.  If BODY-FORM
     does not finish, ‘unwind-protect’ does not return any value in the
     normal sense.

     Only BODY-FORM is protected by the ‘unwind-protect’.  If any of the
     CLEANUP-FORMS themselves exits nonlocally (via a ‘throw’ or an
     error), ‘unwind-protect’ is _not_ guaranteed to evaluate the rest
     of them.  If the failure of one of the CLEANUP-FORMS has the
     potential to cause trouble, then protect it with another
     ‘unwind-protect’ around that form.

     The number of currently active ‘unwind-protect’ forms counts,
     together with the number of local variable bindings, against the
     limit ‘max-specpdl-size’ (*note Local Variables: Definition of
     max-specpdl-size.).

   For example, here we make an invisible buffer for temporary use, and
make sure to kill it before finishing:

     (let ((buffer (get-buffer-create " *temp*")))
       (with-current-buffer buffer
         (unwind-protect
             BODY-FORM
           (kill-buffer buffer))))

You might think that we could just as well write ‘(kill-buffer
(current-buffer))’ and dispense with the variable ‘buffer’.  However,
the way shown above is safer, if BODY-FORM happens to get an error after
switching to a different buffer!  (Alternatively, you could write a
‘save-current-buffer’ around BODY-FORM, to ensure that the temporary
buffer becomes current again in time to kill it.)

   Emacs includes a standard macro called ‘with-temp-buffer’ which
expands into more or less the code shown above (*note Current Buffer:
Definition of with-temp-buffer.).  Several of the macros defined in this
manual use ‘unwind-protect’ in this way.

   Here is an actual example derived from an FTP package.  It creates a
process (*note Processes::) to try to establish a connection to a remote
machine.  As the function ‘ftp-login’ is highly susceptible to numerous
problems that the writer of the function cannot anticipate, it is
protected with a form that guarantees deletion of the process in the
event of failure.  Otherwise, Emacs might fill up with useless
subprocesses.

     (let ((win nil))
       (unwind-protect
           (progn
             (setq process (ftp-setup-buffer host file))
             (if (setq win (ftp-login process host user password))
                 (message "Logged in")
               (error "Ftp login failed")))
         (or win (and process (delete-process process)))))

   This example has a small bug: if the user types ‘C-g’ to quit, and
the quit happens immediately after the function ‘ftp-setup-buffer’
returns but before the variable ‘process’ is set, the process will not
be killed.  There is no easy way to fix this bug, but at least it is
very unlikely.


File: elisp.info,  Node: Variables,  Next: Functions,  Prev: Control Structures,  Up: Top

11 Variables
************

A “variable” is a name used in a program to stand for a value.  In Lisp,
each variable is represented by a Lisp symbol (*note Symbols::).  The
variable name is simply the symbol’s name, and the variable’s value is
stored in the symbol’s value cell(1).  *Note Symbol Components::.  In
Emacs Lisp, the use of a symbol as a variable is independent of its use
as a function name.

   As previously noted in this manual, a Lisp program is represented
primarily by Lisp objects, and only secondarily as text.  The textual
form of a Lisp program is given by the read syntax of the Lisp objects
that constitute the program.  Hence, the textual form of a variable in a
Lisp program is written using the read syntax for the symbol
representing the variable.

* Menu:

* Global Variables::            Variable values that exist permanently, everywhere.
* Constant Variables::          Variables that never change.
* Local Variables::             Variable values that exist only temporarily.
* Void Variables::              Symbols that lack values.
* Defining Variables::          A definition says a symbol is used as a variable.
* Tips for Defining::           Things you should think about when you
                            define a variable.
* Accessing Variables::         Examining values of variables whose names
                            are known only at run time.
* Setting Variables::           Storing new values in variables.
* Variable Scoping::            How Lisp chooses among local and global values.
* Buffer-Local Variables::      Variable values in effect only in one buffer.
* File Local Variables::        Handling local variable lists in files.
* Directory Local Variables::   Local variables common to all files in a directory.
* Variable Aliases::            Variables that are aliases for other variables.
* Variables with Restricted Values::  Non-constant variables whose value can
                                        _not_ be an arbitrary Lisp object.
* Generalized Variables::       Extending the concept of variables.

   ---------- Footnotes ----------

   (1) To be precise, under the default “dynamic scoping” rule, the
value cell always holds the variable’s current value, but this is not
the case under the “lexical scoping” rule.  *Note Variable Scoping::,
for details.


File: elisp.info,  Node: Global Variables,  Next: Constant Variables,  Up: Variables

11.1 Global Variables
=====================

The simplest way to use a variable is “globally”.  This means that the
variable has just one value at a time, and this value is in effect (at
least for the moment) throughout the Lisp system.  The value remains in
effect until you specify a new one.  When a new value replaces the old
one, no trace of the old value remains in the variable.

   You specify a value for a symbol with ‘setq’.  For example,

     (setq x '(a b))

gives the variable ‘x’ the value ‘(a b)’.  Note that ‘setq’ is a special
form (*note Special Forms::); it does not evaluate its first argument,
the name of the variable, but it does evaluate the second argument, the
new value.

   Once the variable has a value, you can refer to it by using the
symbol itself as an expression.  Thus,

     x ⇒ (a b)

assuming the ‘setq’ form shown above has already been executed.

   If you do set the same variable again, the new value replaces the old
one:

     x
          ⇒ (a b)
     (setq x 4)
          ⇒ 4
     x
          ⇒ 4


File: elisp.info,  Node: Constant Variables,  Next: Local Variables,  Prev: Global Variables,  Up: Variables

11.2 Variables that Never Change
================================

In Emacs Lisp, certain symbols normally evaluate to themselves.  These
include ‘nil’ and ‘t’, as well as any symbol whose name starts with ‘:’
(these are called “keywords”).  These symbols cannot be rebound, nor can
their values be changed.  Any attempt to set or bind ‘nil’ or ‘t’
signals a ‘setting-constant’ error.  The same is true for a keyword (a
symbol whose name starts with ‘:’), if it is interned in the standard
obarray, except that setting such a symbol to itself is not an error.

     nil ≡ 'nil
          ⇒ nil
     (setq nil 500)
     error→ Attempt to set constant symbol: nil

 -- Function: keywordp object
     function returns ‘t’ if OBJECT is a symbol whose name starts with
     ‘:’, interned in the standard obarray, and returns ‘nil’ otherwise.

   These constants are fundamentally different from the constants
defined using the ‘defconst’ special form (*note Defining Variables::).
A ‘defconst’ form serves to inform human readers that you do not intend
to change the value of a variable, but Emacs does not raise an error if
you actually change it.


File: elisp.info,  Node: Local Variables,  Next: Void Variables,  Prev: Constant Variables,  Up: Variables

11.3 Local Variables
====================

Global variables have values that last until explicitly superseded with
new values.  Sometimes it is useful to give a variable a “local value”—a
value that takes effect only within a certain part of a Lisp program.
When a variable has a local value, we say that it is “locally bound” to
that value, and that it is a “local variable”.

   For example, when a function is called, its argument variables
receive local values, which are the actual arguments supplied to the
function call; these local bindings take effect within the body of the
function.  To take another example, the ‘let’ special form explicitly
establishes local bindings for specific variables, which take effect
within the body of the ‘let’ form.

   We also speak of the “global binding”, which is where (conceptually)
the global value is kept.

   Establishing a local binding saves away the variable’s previous value
(or lack of one).  We say that the previous value is “shadowed”.  Both
global and local values may be shadowed.  If a local binding is in
effect, using ‘setq’ on the local variable stores the specified value in
the local binding.  When that local binding is no longer in effect, the
previously shadowed value (or lack of one) comes back.

   A variable can have more than one local binding at a time (e.g., if
there are nested ‘let’ forms that bind the variable).  The “current
binding” is the local binding that is actually in effect.  It determines
the value returned by evaluating the variable symbol, and it is the
binding acted on by ‘setq’.

   For most purposes, you can think of the current binding as the
innermost local binding, or the global binding if there is no local
binding.  To be more precise, a rule called the “scoping rule”
determines where in a program a local binding takes effect.  The default
scoping rule in Emacs Lisp is called “dynamic scoping”, which simply
states that the current binding at any given point in the execution of a
program is the most recently-created binding for that variable that
still exists.  For details about dynamic scoping, and an alternative
scoping rule called “lexical scoping”, *Note Variable Scoping::.

   The special forms ‘let’ and ‘let*’ exist to create local bindings:

 -- Special Form: let (bindings...) forms...
     This special form sets up local bindings for a certain set of
     variables, as specified by BINDINGS, and then evaluates all of the
     FORMS in textual order.  Its return value is the value of the last
     form in FORMS.

     Each of the BINDINGS is either (i) a symbol, in which case that
     symbol is locally bound to ‘nil’; or (ii) a list of the form
     ‘(SYMBOL VALUE-FORM)’, in which case SYMBOL is locally bound to the
     result of evaluating VALUE-FORM.  If VALUE-FORM is omitted, ‘nil’
     is used.

     All of the VALUE-FORMs in BINDINGS are evaluated in the order they
     appear and _before_ binding any of the symbols to them.  Here is an
     example of this: ‘z’ is bound to the old value of ‘y’, which is 2,
     not the new value of ‘y’, which is 1.

          (setq y 2)
               ⇒ 2

          (let ((y 1)
                (z y))
            (list y z))
               ⇒ (1 2)

     On the other hand, the order of _bindings_ is unspecified: in the
     following example, either 1 or 2 might be printed.

          (let ((x 1)
                (x 2))
            (print x))

     Therefore, avoid binding a variable more than once in a single
     ‘let’ form.

 -- Special Form: let* (bindings...) forms...
     This special form is like ‘let’, but it binds each variable right
     after computing its local value, before computing the local value
     for the next variable.  Therefore, an expression in BINDINGS can
     refer to the preceding symbols bound in this ‘let*’ form.  Compare
     the following example with the example above for ‘let’.

          (setq y 2)
               ⇒ 2

          (let* ((y 1)
                 (z y))    ; Use the just-established value of ‘y’.
            (list y z))
               ⇒ (1 1)

   Here is a complete list of the other facilities that create local
bindings:

   • Function calls (*note Functions::).

   • Macro calls (*note Macros::).

   • ‘condition-case’ (*note Errors::).

   Variables can also have buffer-local bindings (*note Buffer-Local
Variables::); a few variables have terminal-local bindings (*note
Multiple Terminals::).  These kinds of bindings work somewhat like
ordinary local bindings, but they are localized depending on where you
are in Emacs.

 -- User Option: max-specpdl-size
     This variable defines the limit on the total number of local
     variable bindings and ‘unwind-protect’ cleanups (see *note Cleaning
     Up from Nonlocal Exits: Cleanups.) that are allowed before Emacs
     signals an error (with data ‘"Variable binding depth exceeds
     max-specpdl-size"’).

     This limit, with the associated error when it is exceeded, is one
     way that Lisp avoids infinite recursion on an ill-defined function.
     ‘max-lisp-eval-depth’ provides another limit on depth of nesting.
     *Note Eval: Definition of max-lisp-eval-depth.

     The default value is 1300.  Entry to the Lisp debugger increases
     the value, if there is little room left, to make sure the debugger
     itself has room to execute.


File: elisp.info,  Node: Void Variables,  Next: Defining Variables,  Prev: Local Variables,  Up: Variables

11.4 When a Variable is Void
============================

We say that a variable is void if its symbol has an unassigned value
cell (*note Symbol Components::).

   Under Emacs Lisp’s default dynamic scoping rule (*note Variable
Scoping::), the value cell stores the variable’s current (local or
global) value.  Note that an unassigned value cell is _not_ the same as
having ‘nil’ in the value cell.  The symbol ‘nil’ is a Lisp object and
can be the value of a variable, just as any other object can be; but it
is still a value.  If a variable is void, trying to evaluate the
variable signals a ‘void-variable’ error, instead of returning a value.

   Under the optional lexical scoping rule, the value cell only holds
the variable’s global value—the value outside of any lexical binding
construct.  When a variable is lexically bound, the local value is
determined by the lexical environment; hence, variables can have local
values even if their symbols’ value cells are unassigned.

 -- Function: makunbound symbol
     This function empties out the value cell of SYMBOL, making the
     variable void.  It returns SYMBOL.

     If SYMBOL has a dynamic local binding, ‘makunbound’ voids the
     current binding, and this voidness lasts only as long as the local
     binding is in effect.  Afterwards, the previously shadowed local or
     global binding is reexposed; then the variable will no longer be
     void, unless the reexposed binding is void too.

     Here are some examples (assuming dynamic binding is in effect):

          (setq x 1)               ; Put a value in the global binding.
               ⇒ 1
          (let ((x 2))             ; Locally bind it.
            (makunbound 'x)        ; Void the local binding.
            x)
          error→ Symbol's value as variable is void: x
          x                        ; The global binding is unchanged.
               ⇒ 1

          (let ((x 2))             ; Locally bind it.
            (let ((x 3))           ; And again.
              (makunbound 'x)      ; Void the innermost-local binding.
              x))                  ; And refer: it’s void.
          error→ Symbol's value as variable is void: x

          (let ((x 2))
            (let ((x 3))
              (makunbound 'x))     ; Void inner binding, then remove it.
            x)                     ; Now outer ‘let’ binding is visible.
               ⇒ 2

 -- Function: boundp variable
     This function returns ‘t’ if VARIABLE (a symbol) is not void, and
     ‘nil’ if it is void.

     Here are some examples (assuming dynamic binding is in effect):

          (boundp 'abracadabra)          ; Starts out void.
               ⇒ nil
          (let ((abracadabra 5))         ; Locally bind it.
            (boundp 'abracadabra))
               ⇒ t
          (boundp 'abracadabra)          ; Still globally void.
               ⇒ nil
          (setq abracadabra 5)           ; Make it globally nonvoid.
               ⇒ 5
          (boundp 'abracadabra)
               ⇒ t


File: elisp.info,  Node: Defining Variables,  Next: Tips for Defining,  Prev: Void Variables,  Up: Variables

11.5 Defining Global Variables
==============================

A “variable definition” is a construct that announces your intention to
use a symbol as a global variable.  It uses the special forms ‘defvar’
or ‘defconst’, which are documented below.

   A variable definition serves three purposes.  First, it informs
people who read the code that the symbol is _intended_ to be used a
certain way (as a variable).  Second, it informs the Lisp system of
this, optionally supplying an initial value and a documentation string.
Third, it provides information to programming tools such as ‘etags’,
allowing them to find where the variable was defined.

   The difference between ‘defconst’ and ‘defvar’ is mainly a matter of
intent, serving to inform human readers of whether the value should ever
change.  Emacs Lisp does not actually prevent you from changing the
value of a variable defined with ‘defconst’.  One notable difference
between the two forms is that ‘defconst’ unconditionally initializes the
variable, whereas ‘defvar’ initializes it only if it is originally void.

   To define a customizable variable, you should use ‘defcustom’ (which
calls ‘defvar’ as a subroutine).  *Note Variable Definitions::.

 -- Special Form: defvar symbol [value [doc-string]]
     This special form defines SYMBOL as a variable.  Note that SYMBOL
     is not evaluated; the symbol to be defined should appear explicitly
     in the ‘defvar’ form.  The variable is marked as “special”, meaning
     that it should always be dynamically bound (*note Variable
     Scoping::).

     If VALUE is specified, and SYMBOL is void (i.e., it has no
     dynamically bound value; *note Void Variables::), then VALUE is
     evaluated and SYMBOL is set to the result.  But if SYMBOL is not
     void, VALUE is not evaluated, and SYMBOL’s value is left unchanged.
     If VALUE is omitted, the value of SYMBOL is not changed in any
     case.

     If SYMBOL has a buffer-local binding in the current buffer,
     ‘defvar’ acts on the default value, which is buffer-independent,
     rather than the buffer-local binding.  It sets the default value if
     the default value is void.  *Note Buffer-Local Variables::.

     If SYMBOL is already lexically bound (e.g., if the ‘defvar’ form
     occurs in a ‘let’ form with lexical binding enabled), then ‘defvar’
     sets the dynamic value.  The lexical binding remains in effect
     until its binding construct exits.  *Note Variable Scoping::.

     When you evaluate a top-level ‘defvar’ form with ‘C-M-x’ in Emacs
     Lisp mode (‘eval-defun’), a special feature of ‘eval-defun’
     arranges to set the variable unconditionally, without testing
     whether its value is void.

     If the DOC-STRING argument is supplied, it specifies the
     documentation string for the variable (stored in the symbol’s
     ‘variable-documentation’ property).  *Note Documentation::.

     Here are some examples.  This form defines ‘foo’ but does not
     initialize it:

          (defvar foo)
               ⇒ foo

     This example initializes the value of ‘bar’ to ‘23’, and gives it a
     documentation string:

          (defvar bar 23
            "The normal weight of a bar.")
               ⇒ bar

     The ‘defvar’ form returns SYMBOL, but it is normally used at top
     level in a file where its value does not matter.

 -- Special Form: defconst symbol value [doc-string]
     This special form defines SYMBOL as a value and initializes it.  It
     informs a person reading your code that SYMBOL has a standard
     global value, established here, that should not be changed by the
     user or by other programs.  Note that SYMBOL is not evaluated; the
     symbol to be defined must appear explicitly in the ‘defconst’.

     The ‘defconst’ form, like ‘defvar’, marks the variable as
     “special”, meaning that it should always be dynamically bound
     (*note Variable Scoping::).  In addition, it marks the variable as
     risky (*note File Local Variables::).

     ‘defconst’ always evaluates VALUE, and sets the value of SYMBOL to
     the result.  If SYMBOL does have a buffer-local binding in the
     current buffer, ‘defconst’ sets the default value, not the
     buffer-local value.  (But you should not be making buffer-local
     bindings for a symbol that is defined with ‘defconst’.)

     An example of the use of ‘defconst’ is Emacs’s definition of
     ‘float-pi’—the mathematical constant pi, which ought not to be
     changed by anyone (attempts by the Indiana State Legislature
     notwithstanding).  As the second form illustrates, however,
     ‘defconst’ is only advisory.

          (defconst float-pi 3.141592653589793 "The value of Pi.")
               ⇒ float-pi
          (setq float-pi 3)
               ⇒ float-pi
          float-pi
               ⇒ 3

   *Warning:* If you use a ‘defconst’ or ‘defvar’ special form while the
variable has a local binding (made with ‘let’, or a function argument),
it sets the local binding rather than the global binding.  This is not
what you usually want.  To prevent this, use these special forms at top
level in a file, where normally no local binding is in effect, and make
sure to load the file before making a local binding for the variable.


File: elisp.info,  Node: Tips for Defining,  Next: Accessing Variables,  Prev: Defining Variables,  Up: Variables

11.6 Tips for Defining Variables Robustly
=========================================

When you define a variable whose value is a function, or a list of
functions, use a name that ends in ‘-function’ or ‘-functions’,
respectively.

   There are several other variable name conventions; here is a complete
list:

‘...-hook’
     The variable is a normal hook (*note Hooks::).

‘...-function’
     The value is a function.

‘...-functions’
     The value is a list of functions.

‘...-form’
     The value is a form (an expression).

‘...-forms’
     The value is a list of forms (expressions).

‘...-predicate’
     The value is a predicate—a function of one argument that returns
     non-‘nil’ for success and ‘nil’ for failure.

‘...-flag’
     The value is significant only as to whether it is ‘nil’ or not.
     Since such variables often end up acquiring more values over time,
     this convention is not strongly recommended.

‘...-program’
     The value is a program name.

‘...-command’
     The value is a whole shell command.

‘...-switches’
     The value specifies options for a command.

   When you define a variable, always consider whether you should mark
it as safe or risky; see *note File Local Variables::.

   When defining and initializing a variable that holds a complicated
value (such as a keymap with bindings in it), it’s best to put the
entire computation of the value into the ‘defvar’, like this:

     (defvar my-mode-map
       (let ((map (make-sparse-keymap)))
         (define-key map "\C-c\C-a" 'my-command)
         ...
         map)
       DOCSTRING)

This method has several benefits.  First, if the user quits while
loading the file, the variable is either still uninitialized or
initialized properly, never in-between.  If it is still uninitialized,
reloading the file will initialize it properly.  Second, reloading the
file once the variable is initialized will not alter it; that is
important if the user has run hooks to alter part of the contents (such
as, to rebind keys).  Third, evaluating the ‘defvar’ form with ‘C-M-x’
will reinitialize the map completely.

   Putting so much code in the ‘defvar’ form has one disadvantage: it
puts the documentation string far away from the line which names the
variable.  Here’s a safe way to avoid that:

     (defvar my-mode-map nil
       DOCSTRING)
     (unless my-mode-map
       (let ((map (make-sparse-keymap)))
         (define-key map "\C-c\C-a" 'my-command)
         ...
         (setq my-mode-map map)))

This has all the same advantages as putting the initialization inside
the ‘defvar’, except that you must type ‘C-M-x’ twice, once on each
form, if you do want to reinitialize the variable.


File: elisp.info,  Node: Accessing Variables,  Next: Setting Variables,  Prev: Tips for Defining,  Up: Variables

11.7 Accessing Variable Values
==============================

The usual way to reference a variable is to write the symbol which names
it.  *Note Symbol Forms::.

   Occasionally, you may want to reference a variable which is only
determined at run time.  In that case, you cannot specify the variable
name in the text of the program.  You can use the ‘symbol-value’
function to extract the value.

 -- Function: symbol-value symbol
     This function returns the value stored in SYMBOL’s value cell.
     This is where the variable’s current (dynamic) value is stored.  If
     the variable has no local binding, this is simply its global value.
     If the variable is void, a ‘void-variable’ error is signaled.

     If the variable is lexically bound, the value reported by
     ‘symbol-value’ is not necessarily the same as the variable’s
     lexical value, which is determined by the lexical environment
     rather than the symbol’s value cell.  *Note Variable Scoping::.

          (setq abracadabra 5)
               ⇒ 5
          (setq foo 9)
               ⇒ 9

          ;; Here the symbol ‘abracadabra’
          ;;   is the symbol whose value is examined.
          (let ((abracadabra 'foo))
            (symbol-value 'abracadabra))
               ⇒ foo

          ;; Here, the value of ‘abracadabra’,
          ;;   which is ‘foo’,
          ;;   is the symbol whose value is examined.
          (let ((abracadabra 'foo))
            (symbol-value abracadabra))
               ⇒ 9

          (symbol-value 'abracadabra)
               ⇒ 5


File: elisp.info,  Node: Setting Variables,  Next: Variable Scoping,  Prev: Accessing Variables,  Up: Variables

11.8 Setting Variable Values
============================

The usual way to change the value of a variable is with the special form
‘setq’.  When you need to compute the choice of variable at run time,
use the function ‘set’.

 -- Special Form: setq [symbol form]...
     This special form is the most common method of changing a
     variable’s value.  Each SYMBOL is given a new value, which is the
     result of evaluating the corresponding FORM.  The current binding
     of the symbol is changed.

     ‘setq’ does not evaluate SYMBOL; it sets the symbol that you write.
     We say that this argument is “automatically quoted”.  The ‘q’ in
     ‘setq’ stands for “quoted”.

     The value of the ‘setq’ form is the value of the last FORM.

          (setq x (1+ 2))
               ⇒ 3
          x                   ; ‘x’ now has a global value.
               ⇒ 3
          (let ((x 5))
            (setq x 6)        ; The local binding of ‘x’ is set.
            x)
               ⇒ 6
          x                   ; The global value is unchanged.
               ⇒ 3

     Note that the first FORM is evaluated, then the first SYMBOL is
     set, then the second FORM is evaluated, then the second SYMBOL is
     set, and so on:

          (setq x 10          ; Notice that ‘x’ is set before
                y (1+ x))     ;   the value of ‘y’ is computed.
               ⇒ 11

 -- Function: set symbol value
     This function puts VALUE in the value cell of SYMBOL.  Since it is
     a function rather than a special form, the expression written for
     SYMBOL is evaluated to obtain the symbol to set.  The return value
     is VALUE.

     When dynamic variable binding is in effect (the default), ‘set’ has
     the same effect as ‘setq’, apart from the fact that ‘set’ evaluates
     its SYMBOL argument whereas ‘setq’ does not.  But when a variable
     is lexically bound, ‘set’ affects its _dynamic_ value, whereas
     ‘setq’ affects its current (lexical) value.  *Note Variable
     Scoping::.

          (set one 1)
          error→ Symbol's value as variable is void: one
          (set 'one 1)
               ⇒ 1
          (set 'two 'one)
               ⇒ one
          (set two 2)         ; ‘two’ evaluates to symbol ‘one’.
               ⇒ 2
          one                 ; So it is ‘one’ that was set.
               ⇒ 2
          (let ((one 1))      ; This binding of ‘one’ is set,
            (set 'one 3)      ;   not the global value.
            one)
               ⇒ 3
          one
               ⇒ 2

     If SYMBOL is not actually a symbol, a ‘wrong-type-argument’ error
     is signaled.

          (set '(x y) 'z)
          error→ Wrong type argument: symbolp, (x y)


File: elisp.info,  Node: Variable Scoping,  Next: Buffer-Local Variables,  Prev: Setting Variables,  Up: Variables

11.9 Scoping Rules for Variable Bindings
========================================

When you create a local binding for a variable, that binding takes
effect only within a limited portion of the program (*note Local
Variables::).  This section describes exactly what this means.

   Each local binding has a certain “scope” and “extent”.  “Scope”
refers to _where_ in the textual source code the binding can be
accessed.  “Extent” refers to _when_, as the program is executing, the
binding exists.

   By default, the local bindings that Emacs creates are “dynamic
bindings”.  Such a binding has “dynamic scope”, meaning that any part of
the program can potentially access the variable binding.  It also has
“dynamic extent”, meaning that the binding lasts only while the binding
construct (such as the body of a ‘let’ form) is being executed.

   Emacs can optionally create “lexical bindings”.  A lexical binding
has “lexical scope”, meaning that any reference to the variable must be
located textually within the binding construct(1).  It also has
“indefinite extent”, meaning that under some circumstances the binding
can live on even after the binding construct has finished executing, by
means of special objects called “closures”.

   The following subsections describe dynamic binding and lexical
binding in greater detail, and how to enable lexical binding in Emacs
Lisp programs.

* Menu:

* Dynamic Binding::         The default for binding local variables in Emacs.
* Dynamic Binding Tips::    Avoiding problems with dynamic binding.
* Lexical Binding::         A different type of local variable binding.
* Using Lexical Binding::   How to enable lexical binding.

   ---------- Footnotes ----------

   (1) With some exceptions; for instance, a lexical binding can also be
accessed from the Lisp debugger.


File: elisp.info,  Node: Dynamic Binding,  Next: Dynamic Binding Tips,  Up: Variable Scoping

11.9.1 Dynamic Binding
----------------------

By default, the local variable bindings made by Emacs are dynamic
bindings.  When a variable is dynamically bound, its current binding at
any point in the execution of the Lisp program is simply the most
recently-created dynamic local binding for that symbol, or the global
binding if there is no such local binding.

   Dynamic bindings have dynamic scope and extent, as shown by the
following example:

     (defvar x -99)  ; ‘x’ receives an initial value of −99.

     (defun getx ()
       x)            ; ‘x’ is used free in this function.

     (let ((x 1))    ; ‘x’ is dynamically bound.
       (getx))
          ⇒ 1

     ;; After the ‘let’ form finishes, ‘x’ reverts to its
     ;; previous value, which is −99.

     (getx)
          ⇒ -99

The function ‘getx’ refers to ‘x’.  This is a “free” reference, in the
sense that there is no binding for ‘x’ within that ‘defun’ construct
itself.  When we call ‘getx’ from within a ‘let’ form in which ‘x’ is
(dynamically) bound, it retrieves the local value (i.e., 1).  But when
we call ‘getx’ outside the ‘let’ form, it retrieves the global value
(i.e., −99).

   Here is another example, which illustrates setting a dynamically
bound variable using ‘setq’:

     (defvar x -99)      ; ‘x’ receives an initial value of −99.

     (defun addx ()
       (setq x (1+ x)))  ; Add 1 to ‘x’ and return its new value.

     (let ((x 1))
       (addx)
       (addx))
          ⇒ 3           ; The two ‘addx’ calls add to ‘x’ twice.

     ;; After the ‘let’ form finishes, ‘x’ reverts to its
     ;; previous value, which is −99.

     (addx)
          ⇒ -98

   Dynamic binding is implemented in Emacs Lisp in a simple way.  Each
symbol has a value cell, which specifies its current dynamic value (or
absence of value).  *Note Symbol Components::.  When a symbol is given a
dynamic local binding, Emacs records the contents of the value cell (or
absence thereof) in a stack, and stores the new local value in the value
cell.  When the binding construct finishes executing, Emacs pops the old
value off the stack, and puts it in the value cell.


File: elisp.info,  Node: Dynamic Binding Tips,  Next: Lexical Binding,  Prev: Dynamic Binding,  Up: Variable Scoping

11.9.2 Proper Use of Dynamic Binding
------------------------------------

Dynamic binding is a powerful feature, as it allows programs to refer to
variables that are not defined within their local textual scope.
However, if used without restraint, this can also make programs hard to
understand.  There are two clean ways to use this technique:

   • If a variable has no global definition, use it as a local variable
     only within a binding construct, such as the body of the ‘let’ form
     where the variable was bound.  If this convention is followed
     consistently throughout a program, the value of the variable will
     not affect, nor be affected by, any uses of the same variable
     symbol elsewhere in the program.

   • Otherwise, define the variable with ‘defvar’, ‘defconst’, or
     ‘defcustom’.  *Note Defining Variables::.  Usually, the definition
     should be at top-level in an Emacs Lisp file.  As far as possible,
     it should include a documentation string which explains the meaning
     and purpose of the variable.  You should also choose the variable’s
     name to avoid name conflicts (*note Coding Conventions::).

     Then you can bind the variable anywhere in a program, knowing
     reliably what the effect will be.  Wherever you encounter the
     variable, it will be easy to refer back to the definition, e.g.,
     via the ‘C-h v’ command (provided the variable definition has been
     loaded into Emacs).  *Note (emacs)Name Help::.

     For example, it is common to use local bindings for customizable
     variables like ‘case-fold-search’:

          (defun search-for-abc ()
            "Search for the string \"abc\", ignoring case differences."
            (let ((case-fold-search nil))
              (re-search-forward "abc")))


File: elisp.info,  Node: Lexical Binding,  Next: Using Lexical Binding,  Prev: Dynamic Binding Tips,  Up: Variable Scoping

11.9.3 Lexical Binding
----------------------

Lexical binding was introduced to Emacs, as an optional feature, in
version 24.1.  We expect its importance to increase in the future.
Lexical binding opens up many more opportunities for optimization, so
programs using it are likely to run faster in future Emacs versions.
Lexical binding is also more compatible with concurrency, which we want
to add to Emacs in the future.

   A lexically-bound variable has “lexical scope”, meaning that any
reference to the variable must be located textually within the binding
construct.  Here is an example (*note Using Lexical Binding::, for how
to actually enable lexical binding):

     (let ((x 1))    ; ‘x’ is lexically bound.
       (+ x 3))
          ⇒ 4

     (defun getx ()
       x)            ; ‘x’ is used free in this function.

     (let ((x 1))    ; ‘x’ is lexically bound.
       (getx))
     error→ Symbol's value as variable is void: x

Here, the variable ‘x’ has no global value.  When it is lexically bound
within a ‘let’ form, it can be used in the textual confines of that
‘let’ form.  But it can _not_ be used from within a ‘getx’ function
called from the ‘let’ form, since the function definition of ‘getx’
occurs outside the ‘let’ form itself.

   Here is how lexical binding works.  Each binding construct defines a
“lexical environment”, specifying the variables that are bound within
the construct and their local values.  When the Lisp evaluator wants the
current value of a variable, it looks first in the lexical environment;
if the variable is not specified in there, it looks in the symbol’s
value cell, where the dynamic value is stored.

   (Internally, the lexical environment is an alist of symbol-value
pairs, with the final element in the alist being the symbol ‘t’ rather
than a cons cell.  Such an alist can be passed as the second argument to
the ‘eval’ function, in order to specify a lexical environment in which
to evaluate a form.  *Note Eval::.  Most Emacs Lisp programs, however,
should not interact directly with lexical environments in this way; only
specialized programs like debuggers.)

   Lexical bindings have indefinite extent.  Even after a binding
construct has finished executing, its lexical environment can be “kept
around” in Lisp objects called “closures”.  A closure is created when
you define a named or anonymous function with lexical binding enabled.
*Note Closures::, for details.

   When a closure is called as a function, any lexical variable
references within its definition use the retained lexical environment.
Here is an example:

     (defvar my-ticker nil)   ; We will use this dynamically bound
                              ; variable to store a closure.

     (let ((x 0))             ; ‘x’ is lexically bound.
       (setq my-ticker (lambda ()
                         (setq x (1+ x)))))
         ⇒ (closure ((x . 0) t) ()
               (setq x (1+ x)))

     (funcall my-ticker)
         ⇒ 1

     (funcall my-ticker)
         ⇒ 2

     (funcall my-ticker)
         ⇒ 3

     x                        ; Note that ‘x’ has no global value.
     error→ Symbol's value as variable is void: x

The ‘let’ binding defines a lexical environment in which the variable
‘x’ is locally bound to 0.  Within this binding construct, we define a
lambda expression which increments ‘x’ by one and returns the
incremented value.  This lambda expression is automatically turned into
a closure, in which the lexical environment lives on even after the
‘let’ binding construct has exited.  Each time we evaluate the closure,
it increments ‘x’, using the binding of ‘x’ in that lexical environment.

   Note that unlike dynamic variables which are tied to the symbol
object itself, the relationship between lexical variables and symbols is
only present in the interpreter (or compiler).  Therefore, functions
which take a symbol argument (like ‘symbol-value’, ‘boundp’, and ‘set’)
can only retrieve or modify a variable’s dynamic binding (i.e., the
contents of its symbol’s value cell).


File: elisp.info,  Node: Using Lexical Binding,  Prev: Lexical Binding,  Up: Variable Scoping

11.9.4 Using Lexical Binding
----------------------------

When loading an Emacs Lisp file or evaluating a Lisp buffer, lexical
binding is enabled if the buffer-local variable ‘lexical-binding’ is
non-‘nil’:

 -- Variable: lexical-binding
     If this buffer-local variable is non-‘nil’, Emacs Lisp files and
     buffers are evaluated using lexical binding instead of dynamic
     binding.  (However, special variables are still dynamically bound;
     see below.)  If ‘nil’, dynamic binding is used for all local
     variables.  This variable is typically set for a whole Emacs Lisp
     file, as a file local variable (*note File Local Variables::).
     Note that unlike other such variables, this one must be set in the
     first line of a file.

When evaluating Emacs Lisp code directly using an ‘eval’ call, lexical
binding is enabled if the LEXICAL argument to ‘eval’ is non-‘nil’.
*Note Eval::.

   Even when lexical binding is enabled, certain variables will continue
to be dynamically bound.  These are called “special variables”.  Every
variable that has been defined with ‘defvar’, ‘defcustom’ or ‘defconst’
is a special variable (*note Defining Variables::).  All other variables
are subject to lexical binding.

 -- Function: special-variable-p symbol
     This function returns non-‘nil’ if SYMBOL is a special variable
     (i.e., it has a ‘defvar’, ‘defcustom’, or ‘defconst’ variable
     definition).  Otherwise, the return value is ‘nil’.

   The use of a special variable as a formal argument in a function is
discouraged.  Doing so gives rise to unspecified behavior when lexical
binding mode is enabled (it may use lexical binding sometimes, and
dynamic binding other times).

   Converting an Emacs Lisp program to lexical binding is easy.  First,
add a file-local variable setting of ‘lexical-binding’ to ‘t’ in the
header line of the Emacs Lisp source file (*note File Local
Variables::).  Second, check that every variable in the program which
needs to be dynamically bound has a variable definition, so that it is
not inadvertently bound lexically.

   A simple way to find out which variables need a variable definition
is to byte-compile the source file.  *Note Byte Compilation::.  If a
non-special variable is used outside of a ‘let’ form, the byte-compiler
will warn about reference or assignment to a free variable.  If a
non-special variable is bound but not used within a ‘let’ form, the
byte-compiler will warn about an unused lexical variable.  The
byte-compiler will also issue a warning if you use a special variable as
a function argument.

   (To silence byte-compiler warnings about unused variables, just use a
variable name that start with an underscore.  The byte-compiler
interprets this as an indication that this is a variable known not to be
used.)


File: elisp.info,  Node: Buffer-Local Variables,  Next: File Local Variables,  Prev: Variable Scoping,  Up: Variables

11.10 Buffer-Local Variables
============================

Global and local variable bindings are found in most programming
languages in one form or another.  Emacs, however, also supports
additional, unusual kinds of variable binding, such as “buffer-local”
bindings, which apply only in one buffer.  Having different values for a
variable in different buffers is an important customization method.
(Variables can also have bindings that are local to each terminal.
*Note Multiple Terminals::.)

* Menu:

* Intro to Buffer-Local::       Introduction and concepts.
* Creating Buffer-Local::       Creating and destroying buffer-local bindings.
* Default Value::               The default value is seen in buffers
                                 that don’t have their own buffer-local values.


File: elisp.info,  Node: Intro to Buffer-Local,  Next: Creating Buffer-Local,  Up: Buffer-Local Variables

11.10.1 Introduction to Buffer-Local Variables
----------------------------------------------

A buffer-local variable has a buffer-local binding associated with a
particular buffer.  The binding is in effect when that buffer is
current; otherwise, it is not in effect.  If you set the variable while
a buffer-local binding is in effect, the new value goes in that binding,
so its other bindings are unchanged.  This means that the change is
visible only in the buffer where you made it.

   The variable’s ordinary binding, which is not associated with any
specific buffer, is called the “default binding”.  In most cases, this
is the global binding.

   A variable can have buffer-local bindings in some buffers but not in
other buffers.  The default binding is shared by all the buffers that
don’t have their own bindings for the variable.  (This includes all
newly-created buffers.)  If you set the variable in a buffer that does
not have a buffer-local binding for it, this sets the default binding,
so the new value is visible in all the buffers that see the default
binding.

   The most common use of buffer-local bindings is for major modes to
change variables that control the behavior of commands.  For example, C
mode and Lisp mode both set the variable ‘paragraph-start’ to specify
that only blank lines separate paragraphs.  They do this by making the
variable buffer-local in the buffer that is being put into C mode or
Lisp mode, and then setting it to the new value for that mode.  *Note
Major Modes::.

   The usual way to make a buffer-local binding is with
‘make-local-variable’, which is what major mode commands typically use.
This affects just the current buffer; all other buffers (including those
yet to be created) will continue to share the default value unless they
are explicitly given their own buffer-local bindings.

   A more powerful operation is to mark the variable as “automatically
buffer-local” by calling ‘make-variable-buffer-local’.  You can think of
this as making the variable local in all buffers, even those yet to be
created.  More precisely, the effect is that setting the variable
automatically makes the variable local to the current buffer if it is
not already so.  All buffers start out by sharing the default value of
the variable as usual, but setting the variable creates a buffer-local
binding for the current buffer.  The new value is stored in the
buffer-local binding, leaving the default binding untouched.  This means
that the default value cannot be changed with ‘setq’ in any buffer; the
only way to change it is with ‘setq-default’.

   *Warning:* When a variable has buffer-local bindings in one or more
buffers, ‘let’ rebinds the binding that’s currently in effect.  For
instance, if the current buffer has a buffer-local value, ‘let’
temporarily rebinds that.  If no buffer-local bindings are in effect,
‘let’ rebinds the default value.  If inside the ‘let’ you then change to
a different current buffer in which a different binding is in effect,
you won’t see the ‘let’ binding any more.  And if you exit the ‘let’
while still in the other buffer, you won’t see the unbinding occur
(though it will occur properly).  Here is an example to illustrate:

     (setq foo 'g)
     (set-buffer "a")
     (make-local-variable 'foo)
     (setq foo 'a)
     (let ((foo 'temp))
       ;; foo ⇒ 'temp  ; let binding in buffer ‘a’
       (set-buffer "b")
       ;; foo ⇒ 'g     ; the global value since foo is not local in ‘b’
       BODY...)
     foo ⇒ 'g        ; exiting restored the local value in buffer ‘a’,
                      ; but we don’t see that in buffer ‘b’
     (set-buffer "a") ; verify the local value was restored
     foo ⇒ 'a

Note that references to ‘foo’ in BODY access the buffer-local binding of
buffer ‘b’.

   When a file specifies local variable values, these become
buffer-local values when you visit the file.  *Note (emacs)File
Variables::.

   A buffer-local variable cannot be made terminal-local (*note Multiple
Terminals::).


File: elisp.info,  Node: Creating Buffer-Local,  Next: Default Value,  Prev: Intro to Buffer-Local,  Up: Buffer-Local Variables

11.10.2 Creating and Deleting Buffer-Local Bindings
---------------------------------------------------

 -- Command: make-local-variable variable
     This function creates a buffer-local binding in the current buffer
     for VARIABLE (a symbol).  Other buffers are not affected.  The
     value returned is VARIABLE.

     The buffer-local value of VARIABLE starts out as the same value
     VARIABLE previously had.  If VARIABLE was void, it remains void.

          ;; In buffer ‘b1’:
          (setq foo 5)                ; Affects all buffers.
               ⇒ 5
          (make-local-variable 'foo)  ; Now it is local in ‘b1’.
               ⇒ foo
          foo                         ; That did not change
               ⇒ 5                   ;   the value.
          (setq foo 6)                ; Change the value
               ⇒ 6                   ;   in ‘b1’.
          foo
               ⇒ 6

          ;; In buffer ‘b2’, the value hasn’t changed.
          (with-current-buffer "b2"
            foo)
               ⇒ 5

     Making a variable buffer-local within a ‘let’-binding for that
     variable does not work reliably, unless the buffer in which you do
     this is not current either on entry to or exit from the ‘let’.
     This is because ‘let’ does not distinguish between different kinds
     of bindings; it knows only which variable the binding was made for.

     If the variable is terminal-local (*note Multiple Terminals::),
     this function signals an error.  Such variables cannot have
     buffer-local bindings as well.

     *Warning:* do not use ‘make-local-variable’ for a hook variable.
     The hook variables are automatically made buffer-local as needed if
     you use the LOCAL argument to ‘add-hook’ or ‘remove-hook’.

 -- Macro: setq-local variable value
     This macro creates a buffer-local binding in the current buffer for
     VARIABLE, and gives it the buffer-local value VALUE.  It is
     equivalent to calling ‘make-local-variable’ followed by ‘setq’.
     VARIABLE should be an unquoted symbol.

 -- Command: make-variable-buffer-local variable
     This function marks VARIABLE (a symbol) automatically buffer-local,
     so that any subsequent attempt to set it will make it local to the
     current buffer at the time.  Unlike ‘make-local-variable’, with
     which it is often confused, this cannot be undone, and affects the
     behavior of the variable in all buffers.

     A peculiar wrinkle of this feature is that binding the variable
     (with ‘let’ or other binding constructs) does not create a
     buffer-local binding for it.  Only setting the variable (with ‘set’
     or ‘setq’), while the variable does not have a ‘let’-style binding
     that was made in the current buffer, does so.

     If VARIABLE does not have a default value, then calling this
     command will give it a default value of ‘nil’.  If VARIABLE already
     has a default value, that value remains unchanged.  Subsequently
     calling ‘makunbound’ on VARIABLE will result in a void buffer-local
     value and leave the default value unaffected.

     The value returned is VARIABLE.

     *Warning:* Don’t assume that you should use
     ‘make-variable-buffer-local’ for user-option variables, simply
     because users _might_ want to customize them differently in
     different buffers.  Users can make any variable local, when they
     wish to.  It is better to leave the choice to them.

     The time to use ‘make-variable-buffer-local’ is when it is crucial
     that no two buffers ever share the same binding.  For example, when
     a variable is used for internal purposes in a Lisp program which
     depends on having separate values in separate buffers, then using
     ‘make-variable-buffer-local’ can be the best solution.

 -- Macro: defvar-local variable value &optional docstring
     This macro defines VARIABLE as a variable with initial value VALUE
     and DOCSTRING, and marks it as automatically buffer-local.  It is
     equivalent to calling ‘defvar’ followed by
     ‘make-variable-buffer-local’.  VARIABLE should be an unquoted
     symbol.

 -- Function: local-variable-p variable &optional buffer
     This returns ‘t’ if VARIABLE is buffer-local in buffer BUFFER
     (which defaults to the current buffer); otherwise, ‘nil’.

 -- Function: local-variable-if-set-p variable &optional buffer
     This returns ‘t’ if VARIABLE either has a buffer-local value in
     buffer BUFFER, or is automatically buffer-local.  Otherwise, it
     returns ‘nil’.  If omitted or ‘nil’, BUFFER defaults to the current
     buffer.

 -- Function: buffer-local-value variable buffer
     This function returns the buffer-local binding of VARIABLE (a
     symbol) in buffer BUFFER.  If VARIABLE does not have a buffer-local
     binding in buffer BUFFER, it returns the default value (*note
     Default Value::) of VARIABLE instead.

 -- Function: buffer-local-variables &optional buffer
     This function returns a list describing the buffer-local variables
     in buffer BUFFER.  (If BUFFER is omitted, the current buffer is
     used.)  Normally, each list element has the form ‘(SYM . VAL)’,
     where SYM is a buffer-local variable (a symbol) and VAL is its
     buffer-local value.  But when a variable’s buffer-local binding in
     BUFFER is void, its list element is just SYM.

          (make-local-variable 'foobar)
          (makunbound 'foobar)
          (make-local-variable 'bind-me)
          (setq bind-me 69)
          (setq lcl (buffer-local-variables))
              ;; First, built-in variables local in all buffers:
          ⇒ ((mark-active . nil)
              (buffer-undo-list . nil)
              (mode-name . "Fundamental")
              ...
              ;; Next, non-built-in buffer-local variables.
              ;; This one is buffer-local and void:
              foobar
              ;; This one is buffer-local and nonvoid:
              (bind-me . 69))

     Note that storing new values into the CDRs of cons cells in this
     list does _not_ change the buffer-local values of the variables.

 -- Command: kill-local-variable variable
     This function deletes the buffer-local binding (if any) for
     VARIABLE (a symbol) in the current buffer.  As a result, the
     default binding of VARIABLE becomes visible in this buffer.  This
     typically results in a change in the value of VARIABLE, since the
     default value is usually different from the buffer-local value just
     eliminated.

     If you kill the buffer-local binding of a variable that
     automatically becomes buffer-local when set, this makes the default
     value visible in the current buffer.  However, if you set the
     variable again, that will once again create a buffer-local binding
     for it.

     ‘kill-local-variable’ returns VARIABLE.

     This function is a command because it is sometimes useful to kill
     one buffer-local variable interactively, just as it is useful to
     create buffer-local variables interactively.

 -- Function: kill-all-local-variables
     This function eliminates all the buffer-local variable bindings of
     the current buffer except for variables marked as permanent and
     local hook functions that have a non-‘nil’ ‘permanent-local-hook’
     property (*note Setting Hooks::).  As a result, the buffer will see
     the default values of most variables.

     This function also resets certain other information pertaining to
     the buffer: it sets the local keymap to ‘nil’, the syntax table to
     the value of ‘(standard-syntax-table)’, the case table to
     ‘(standard-case-table)’, and the abbrev table to the value of
     ‘fundamental-mode-abbrev-table’.

     The very first thing this function does is run the normal hook
     ‘change-major-mode-hook’ (see below).

     Every major mode command begins by calling this function, which has
     the effect of switching to Fundamental mode and erasing most of the
     effects of the previous major mode.  To ensure that this does its
     job, the variables that major modes set should not be marked
     permanent.

     ‘kill-all-local-variables’ returns ‘nil’.

 -- Variable: change-major-mode-hook
     The function ‘kill-all-local-variables’ runs this normal hook
     before it does anything else.  This gives major modes a way to
     arrange for something special to be done if the user switches to a
     different major mode.  It is also useful for buffer-specific minor
     modes that should be forgotten if the user changes the major mode.

     For best results, make this variable buffer-local, so that it will
     disappear after doing its job and will not interfere with the
     subsequent major mode.  *Note Hooks::.

   A buffer-local variable is “permanent” if the variable name (a
symbol) has a ‘permanent-local’ property that is non-‘nil’.  Such
variables are unaffected by ‘kill-all-local-variables’, and their local
bindings are therefore not cleared by changing major modes.  Permanent
locals are appropriate for data pertaining to where the file came from
or how to save it, rather than with how to edit the contents.


File: elisp.info,  Node: Default Value,  Prev: Creating Buffer-Local,  Up: Buffer-Local Variables

11.10.3 The Default Value of a Buffer-Local Variable
----------------------------------------------------

The global value of a variable with buffer-local bindings is also called
the “default” value, because it is the value that is in effect whenever
neither the current buffer nor the selected frame has its own binding
for the variable.

   The functions ‘default-value’ and ‘setq-default’ access and change a
variable’s default value regardless of whether the current buffer has a
buffer-local binding.  For example, you could use ‘setq-default’ to
change the default setting of ‘paragraph-start’ for most buffers; and
this would work even when you are in a C or Lisp mode buffer that has a
buffer-local value for this variable.

   The special forms ‘defvar’ and ‘defconst’ also set the default value
(if they set the variable at all), rather than any buffer-local value.

 -- Function: default-value symbol
     This function returns SYMBOL’s default value.  This is the value
     that is seen in buffers and frames that do not have their own
     values for this variable.  If SYMBOL is not buffer-local, this is
     equivalent to ‘symbol-value’ (*note Accessing Variables::).

 -- Function: default-boundp symbol
     The function ‘default-boundp’ tells you whether SYMBOL’s default
     value is nonvoid.  If ‘(default-boundp 'foo)’ returns ‘nil’, then
     ‘(default-value 'foo)’ would get an error.

     ‘default-boundp’ is to ‘default-value’ as ‘boundp’ is to
     ‘symbol-value’.

 -- Special Form: setq-default [symbol form]...
     This special form gives each SYMBOL a new default value, which is
     the result of evaluating the corresponding FORM.  It does not
     evaluate SYMBOL, but does evaluate FORM.  The value of the
     ‘setq-default’ form is the value of the last FORM.

     If a SYMBOL is not buffer-local for the current buffer, and is not
     marked automatically buffer-local, ‘setq-default’ has the same
     effect as ‘setq’.  If SYMBOL is buffer-local for the current
     buffer, then this changes the value that other buffers will see (as
     long as they don’t have a buffer-local value), but not the value
     that the current buffer sees.

          ;; In buffer ‘foo’:
          (make-local-variable 'buffer-local)
               ⇒ buffer-local
          (setq buffer-local 'value-in-foo)
               ⇒ value-in-foo
          (setq-default buffer-local 'new-default)
               ⇒ new-default
          buffer-local
               ⇒ value-in-foo
          (default-value 'buffer-local)
               ⇒ new-default

          ;; In (the new) buffer ‘bar’:
          buffer-local
               ⇒ new-default
          (default-value 'buffer-local)
               ⇒ new-default
          (setq buffer-local 'another-default)
               ⇒ another-default
          (default-value 'buffer-local)
               ⇒ another-default

          ;; Back in buffer ‘foo’:
          buffer-local
               ⇒ value-in-foo
          (default-value 'buffer-local)
               ⇒ another-default

 -- Function: set-default symbol value
     This function is like ‘setq-default’, except that SYMBOL is an
     ordinary evaluated argument.

          (set-default (car '(a b c)) 23)
               ⇒ 23
          (default-value 'a)
               ⇒ 23

   A variable can be let-bound (*note Local Variables::) to a value.
This makes its global value shadowed by the binding; ‘default-value’
will then return the value from that binding, not the global value, and
‘set-default’ will be prevented from setting the global value (it will
change the let-bound value instead).  The following two functions allow
to reference the global value even if it’s shadowed by a let-binding.

 -- Function: default-toplevel-value symbol
     This function returns the “top-level” default value of SYMBOL,
     which is its value outside of any let-binding.

     (defvar variable 'global-value)
         ⇒ variable
     (let ((variable 'let-binding))
       (default-value 'variable))
         ⇒ let-binding
     (let ((variable 'let-binding))
       (default-toplevel-value 'variable))
         ⇒ global-value

 -- Function: set-default-toplevel-value symbol value
     This function sets the top-level default value of SYMBOL to the
     specified VALUE.  This comes in handy when you want to set the
     global value of SYMBOL regardless of whether your code runs in the
     context of SYMBOL’s let-binding.


File: elisp.info,  Node: File Local Variables,  Next: Directory Local Variables,  Prev: Buffer-Local Variables,  Up: Variables

11.11 File Local Variables
==========================

A file can specify local variable values; Emacs uses these to create
buffer-local bindings for those variables in the buffer visiting that
file.  *Note Local Variables in Files: (emacs)File Variables, for basic
information about file-local variables.  This section describes the
functions and variables that affect how file-local variables are
processed.

   If a file-local variable could specify an arbitrary function or Lisp
expression that would be called later, visiting a file could take over
your Emacs.  Emacs protects against this by automatically setting only
those file-local variables whose specified values are known to be safe.
Other file-local variables are set only if the user agrees.

   For additional safety, ‘read-circle’ is temporarily bound to ‘nil’
when Emacs reads file-local variables (*note Input Functions::).  This
prevents the Lisp reader from recognizing circular and shared Lisp
structures (*note Circular Objects::).

 -- User Option: enable-local-variables
     This variable controls whether to process file-local variables.
     The possible values are:

     ‘t’ (the default)
          Set the safe variables, and query (once) about any unsafe
          variables.
     ‘:safe’
          Set only the safe variables and do not query.
     ‘:all’
          Set all the variables and do not query.
     ‘nil’
          Don’t set any variables.
     anything else
          Query (once) about all the variables.

 -- Variable: inhibit-local-variables-regexps
     This is a list of regular expressions.  If a file has a name
     matching an element of this list, then it is not scanned for any
     form of file-local variable.  For examples of why you might want to
     use this, *note Auto Major Mode::.

 -- Function: hack-local-variables &optional mode-only
     This function parses, and binds or evaluates as appropriate, any
     local variables specified by the contents of the current buffer.
     The variable ‘enable-local-variables’ has its effect here.
     However, this function does not look for the ‘mode:’ local variable
     in the ‘-*-’ line.  ‘set-auto-mode’ does that, also taking
     ‘enable-local-variables’ into account (*note Auto Major Mode::).

     This function works by walking the alist stored in
     ‘file-local-variables-alist’ and applying each local variable in
     turn.  It calls ‘before-hack-local-variables-hook’ and
     ‘hack-local-variables-hook’ before and after applying the
     variables, respectively.  It only calls the before-hook if the
     alist is non-‘nil’; it always calls the other hook.  This function
     ignores a ‘mode’ element if it specifies the same major mode as the
     buffer already has.

     If the optional argument MODE-ONLY is non-‘nil’, then all this
     function does is return a symbol specifying the major mode, if the
     ‘-*-’ line or the local variables list specifies one, and ‘nil’
     otherwise.  It does not set the mode nor any other file-local
     variable.

 -- Variable: file-local-variables-alist
     This buffer-local variable holds the alist of file-local variable
     settings.  Each element of the alist is of the form
     ‘(VAR . VALUE)’, where VAR is a symbol of the local variable and
     VALUE is its value.  When Emacs visits a file, it first collects
     all the file-local variables into this alist, and then the
     ‘hack-local-variables’ function applies them one by one.

 -- Variable: before-hack-local-variables-hook
     Emacs calls this hook immediately before applying file-local
     variables stored in ‘file-local-variables-alist’.

 -- Variable: hack-local-variables-hook
     Emacs calls this hook immediately after it finishes applying
     file-local variables stored in ‘file-local-variables-alist’.

   You can specify safe values for a variable with a
‘safe-local-variable’ property.  The property has to be a function of
one argument; any value is safe if the function returns non-‘nil’ given
that value.  Many commonly-encountered file variables have
‘safe-local-variable’ properties; these include ‘fill-column’,
‘fill-prefix’, and ‘indent-tabs-mode’.  For boolean-valued variables
that are safe, use ‘booleanp’ as the property value.

   When defining a user option using ‘defcustom’, you can set its
‘safe-local-variable’ property by adding the arguments ‘:safe FUNCTION’
to ‘defcustom’ (*note Variable Definitions::).

 -- User Option: safe-local-variable-values
     This variable provides another way to mark some variable values as
     safe.  It is a list of cons cells ‘(VAR . VAL)’, where VAR is a
     variable name and VAL is a value which is safe for that variable.

     When Emacs asks the user whether or not to obey a set of file-local
     variable specifications, the user can choose to mark them as safe.
     Doing so adds those variable/value pairs to
     ‘safe-local-variable-values’, and saves it to the user’s custom
     file.

 -- Function: safe-local-variable-p sym val
     This function returns non-‘nil’ if it is safe to give SYM the value
     VAL, based on the above criteria.

   Some variables are considered “risky”.  If a variable is risky, it is
never entered automatically into ‘safe-local-variable-values’; Emacs
always queries before setting a risky variable, unless the user
explicitly allows a value by customizing ‘safe-local-variable-values’
directly.

   Any variable whose name has a non-‘nil’ ‘risky-local-variable’
property is considered risky.  When you define a user option using
‘defcustom’, you can set its ‘risky-local-variable’ property by adding
the arguments ‘:risky VALUE’ to ‘defcustom’ (*note Variable
Definitions::).  In addition, any variable whose name ends in any of
‘-command’, ‘-frame-alist’, ‘-function’, ‘-functions’, ‘-hook’,
‘-hooks’, ‘-form’, ‘-forms’, ‘-map’, ‘-map-alist’, ‘-mode-alist’,
‘-program’, or ‘-predicate’ is automatically considered risky.  The
variables ‘font-lock-keywords’, ‘font-lock-keywords’ followed by a
digit, and ‘font-lock-syntactic-keywords’ are also considered risky.

 -- Function: risky-local-variable-p sym
     This function returns non-‘nil’ if SYM is a risky variable, based
     on the above criteria.

 -- Variable: ignored-local-variables
     This variable holds a list of variables that should not be given
     local values by files.  Any value specified for one of these
     variables is completely ignored.

   The ‘Eval:’ “variable” is also a potential loophole, so Emacs
normally asks for confirmation before handling it.

 -- User Option: enable-local-eval
     This variable controls processing of ‘Eval:’ in ‘-*-’ lines or
     local variables lists in files being visited.  A value of ‘t’ means
     process them unconditionally; ‘nil’ means ignore them; anything
     else means ask the user what to do for each file.  The default
     value is ‘maybe’.

 -- User Option: safe-local-eval-forms
     This variable holds a list of expressions that are safe to evaluate
     when found in the ‘Eval:’ “variable” in a file local variables
     list.

   If the expression is a function call and the function has a
‘safe-local-eval-function’ property, the property value determines
whether the expression is safe to evaluate.  The property value can be a
predicate to call to test the expression, a list of such predicates
(it’s safe if any predicate succeeds), or ‘t’ (always safe provided the
arguments are constant).

   Text properties are also potential loopholes, since their values
could include functions to call.  So Emacs discards all text properties
from string values specified for file-local variables.


File: elisp.info,  Node: Directory Local Variables,  Next: Variable Aliases,  Prev: File Local Variables,  Up: Variables

11.12 Directory Local Variables
===============================

A directory can specify local variable values common to all files in
that directory; Emacs uses these to create buffer-local bindings for
those variables in buffers visiting any file in that directory.  This is
useful when the files in the directory belong to some “project” and
therefore share the same local variables.

   There are two different methods for specifying directory local
variables: by putting them in a special file, or by defining a “project
class” for that directory.

 -- Constant: dir-locals-file
     This constant is the name of the file where Emacs expects to find
     the directory-local variables.  The name of the file is
     ‘.dir-locals.el’(1).  A file by that name in a directory causes
     Emacs to apply its settings to any file in that directory or any of
     its subdirectories (optionally, you can exclude subdirectories; see
     below).  If some of the subdirectories have their own
     ‘.dir-locals.el’ files, Emacs uses the settings from the deepest
     file it finds starting from the file’s directory and moving up the
     directory tree.  The file specifies local variables as a specially
     formatted list; see *note Per-directory Local Variables:
     (emacs)Directory Variables, for more details.

 -- Function: hack-dir-local-variables
     This function reads the ‘.dir-locals.el’ file and stores the
     directory-local variables in ‘file-local-variables-alist’ that is
     local to the buffer visiting any file in the directory, without
     applying them.  It also stores the directory-local settings in
     ‘dir-locals-class-alist’, where it defines a special class for the
     directory in which ‘.dir-locals.el’ file was found.  This function
     works by calling ‘dir-locals-set-class-variables’ and
     ‘dir-locals-set-directory-class’, described below.

 -- Function: hack-dir-local-variables-non-file-buffer
     This function looks for directory-local variables, and immediately
     applies them in the current buffer.  It is intended to be called in
     the mode commands for non-file buffers, such as Dired buffers, to
     let them obey directory-local variable settings.  For non-file
     buffers, Emacs looks for directory-local variables in
     ‘default-directory’ and its parent directories.

 -- Function: dir-locals-set-class-variables class variables
     This function defines a set of variable settings for the named
     CLASS, which is a symbol.  You can later assign the class to one or
     more directories, and Emacs will apply those variable settings to
     all files in those directories.  The list in VARIABLES can be of
     one of the two forms: ‘(MAJOR-MODE . ALIST)’ or ‘(DIRECTORY .
     LIST)’.  With the first form, if the file’s buffer turns on a mode
     that is derived from MAJOR-MODE, then the all the variables in the
     associated ALIST are applied; ALIST should be of the form ‘(NAME .
     VALUE)’.  A special value ‘nil’ for MAJOR-MODE means the settings
     are applicable to any mode.  In ALIST, you can use a special NAME:
     ‘subdirs’.  If the associated value is ‘nil’, the alist is only
     applied to files in the relevant directory, not to those in any
     subdirectories.

     With the second form of VARIABLES, if DIRECTORY is the initial
     substring of the file’s directory, then LIST is applied recursively
     by following the above rules; LIST should be of one of the two
     forms accepted by this function in VARIABLES.

 -- Function: dir-locals-set-directory-class directory class &optional
          mtime
     This function assigns CLASS to all the files in ‘directory’ and its
     subdirectories.  Thereafter, all the variable settings specified
     for CLASS will be applied to any visited file in DIRECTORY and its
     children.  CLASS must have been already defined by
     ‘dir-locals-set-class-variables’.

     Emacs uses this function internally when it loads directory
     variables from a ‘.dir-locals.el’ file.  In that case, the optional
     argument MTIME holds the file modification time (as returned by
     ‘file-attributes’).  Emacs uses this time to check stored local
     variables are still valid.  If you are assigning a class directly,
     not via a file, this argument should be ‘nil’.

 -- Variable: dir-locals-class-alist
     This alist holds the class symbols and the associated variable
     settings.  It is updated by ‘dir-locals-set-class-variables’.

 -- Variable: dir-locals-directory-cache
     This alist holds directory names, their assigned class names, and
     modification times of the associated directory local variables file
     (if there is one).  The function ‘dir-locals-set-directory-class’
     updates this list.

 -- Variable: enable-dir-local-variables
     If ‘nil’, directory-local variables are ignored.  This variable may
     be useful for modes that want to ignore directory-locals while
     still respecting file-local variables (*note File Local
     Variables::).

   ---------- Footnotes ----------

   (1) The MS-DOS version of Emacs uses ‘_dir-locals.el’ instead, due to
limitations of the DOS filesystems.


File: elisp.info,  Node: Variable Aliases,  Next: Variables with Restricted Values,  Prev: Directory Local Variables,  Up: Variables

11.13 Variable Aliases
======================

It is sometimes useful to make two variables synonyms, so that both
variables always have the same value, and changing either one also
changes the other.  Whenever you change the name of a variable—either
because you realize its old name was not well chosen, or because its
meaning has partly changed—it can be useful to keep the old name as an
_alias_ of the new one for compatibility.  You can do this with
‘defvaralias’.

 -- Function: defvaralias new-alias base-variable &optional docstring
     This function defines the symbol NEW-ALIAS as a variable alias for
     symbol BASE-VARIABLE.  This means that retrieving the value of
     NEW-ALIAS returns the value of BASE-VARIABLE, and changing the
     value of NEW-ALIAS changes the value of BASE-VARIABLE.  The two
     aliased variable names always share the same value and the same
     bindings.

     If the DOCSTRING argument is non-‘nil’, it specifies the
     documentation for NEW-ALIAS; otherwise, the alias gets the same
     documentation as BASE-VARIABLE has, if any, unless BASE-VARIABLE is
     itself an alias, in which case NEW-ALIAS gets the documentation of
     the variable at the end of the chain of aliases.

     This function returns BASE-VARIABLE.

   Variable aliases are convenient for replacing an old name for a
variable with a new name.  ‘make-obsolete-variable’ declares that the
old name is obsolete and therefore that it may be removed at some stage
in the future.

 -- Function: make-obsolete-variable obsolete-name current-name when
          &optional access-type
     This function makes the byte compiler warn that the variable
     OBSOLETE-NAME is obsolete.  If CURRENT-NAME is a symbol, it is the
     variable’s new name; then the warning message says to use
     CURRENT-NAME instead of OBSOLETE-NAME.  If CURRENT-NAME is a
     string, this is the message and there is no replacement variable.
     WHEN should be a string indicating when the variable was first made
     obsolete (usually a version number string).

     The optional argument ACCESS-TYPE, if non-‘nil’, should specify the
     kind of access that will trigger obsolescence warnings; it can be
     either ‘get’ or ‘set’.

   You can make two variables synonyms and declare one obsolete at the
same time using the macro ‘define-obsolete-variable-alias’.

 -- Macro: define-obsolete-variable-alias obsolete-name current-name
          &optional when docstring
     This macro marks the variable OBSOLETE-NAME as obsolete and also
     makes it an alias for the variable CURRENT-NAME.  It is equivalent
     to the following:

          (defvaralias OBSOLETE-NAME CURRENT-NAME DOCSTRING)
          (make-obsolete-variable OBSOLETE-NAME CURRENT-NAME WHEN)

 -- Function: indirect-variable variable
     This function returns the variable at the end of the chain of
     aliases of VARIABLE.  If VARIABLE is not a symbol, or if VARIABLE
     is not defined as an alias, the function returns VARIABLE.

     This function signals a ‘cyclic-variable-indirection’ error if
     there is a loop in the chain of symbols.

     (defvaralias 'foo 'bar)
     (indirect-variable 'foo)
          ⇒ bar
     (indirect-variable 'bar)
          ⇒ bar
     (setq bar 2)
     bar
          ⇒ 2
     foo
          ⇒ 2
     (setq foo 0)
     bar
          ⇒ 0
     foo
          ⇒ 0


File: elisp.info,  Node: Variables with Restricted Values,  Next: Generalized Variables,  Prev: Variable Aliases,  Up: Variables

11.14 Variables with Restricted Values
======================================

Ordinary Lisp variables can be assigned any value that is a valid Lisp
object.  However, certain Lisp variables are not defined in Lisp, but in
C.  Most of these variables are defined in the C code using
‘DEFVAR_LISP’.  Like variables defined in Lisp, these can take on any
value.  However, some variables are defined using ‘DEFVAR_INT’ or
‘DEFVAR_BOOL’.  *Note Writing Emacs Primitives: Defining Lisp variables
in C, in particular the description of functions of the type
‘syms_of_FILENAME’, for a brief discussion of the C implementation.

   Variables of type ‘DEFVAR_BOOL’ can only take on the values ‘nil’ or
‘t’.  Attempting to assign them any other value will set them to ‘t’:

     (let ((display-hourglass 5))
       display-hourglass)
          ⇒ t

 -- Variable: byte-boolean-vars
     This variable holds a list of all variables of type ‘DEFVAR_BOOL’.

   Variables of type ‘DEFVAR_INT’ can take on only integer values.
Attempting to assign them any other value will result in an error:

     (setq undo-limit 1000.0)
     error→ Wrong type argument: integerp, 1000.0


File: elisp.info,  Node: Generalized Variables,  Prev: Variables with Restricted Values,  Up: Variables

11.15 Generalized Variables
===========================

A “generalized variable” or “place form” is one of the many places in
Lisp memory where values can be stored.  The simplest place form is a
regular Lisp variable.  But the CARs and CDRs of lists, elements of
arrays, properties of symbols, and many other locations are also places
where Lisp values are stored.

   Generalized variables are analogous to lvalues in the C language,
where ‘x = a[i]’ gets an element from an array and ‘a[i] = x’ stores an
element using the same notation.  Just as certain forms like ‘a[i]’ can
be lvalues in C, there is a set of forms that can be generalized
variables in Lisp.

* Menu:

* Setting Generalized Variables::   The ‘setf’ macro.
* Adding Generalized Variables::    Defining new ‘setf’ forms.


File: elisp.info,  Node: Setting Generalized Variables,  Next: Adding Generalized Variables,  Up: Generalized Variables

11.15.1 The ‘setf’ Macro
------------------------

The ‘setf’ macro is the most basic way to operate on generalized
variables.  The ‘setf’ form is like ‘setq’, except that it accepts
arbitrary place forms on the left side rather than just symbols.  For
example, ‘(setf (car a) b)’ sets the car of ‘a’ to ‘b’, doing the same
operation as ‘(setcar a b)’, but without having to remember two separate
functions for setting and accessing every type of place.

 -- Macro: setf [place form]...
     This macro evaluates FORM and stores it in PLACE, which must be a
     valid generalized variable form.  If there are several PLACE and
     FORM pairs, the assignments are done sequentially just as with
     ‘setq’.  ‘setf’ returns the value of the last FORM.

   The following Lisp forms will work as generalized variables, and so
may appear in the PLACE argument of ‘setf’:

   • A symbol naming a variable.  In other words, ‘(setf x y)’ is
     exactly equivalent to ‘(setq x y)’, and ‘setq’ itself is strictly
     speaking redundant given that ‘setf’ exists.  Many programmers
     continue to prefer ‘setq’ for setting simple variables, though,
     purely for stylistic or historical reasons.  The macro ‘(setf x y)’
     actually expands to ‘(setq x y)’, so there is no performance
     penalty for using it in compiled code.

   • A call to any of the following standard Lisp functions:

          aref      cddr      symbol-function
          car       elt       symbol-plist
          caar      get       symbol-value
          cadr      gethash
          cdr       nth
          cdar      nthcdr

   • A call to any of the following Emacs-specific functions:

          alist-get                     process-get
          frame-parameter               process-sentinel
          terminal-parameter            window-buffer
          keymap-parent                 window-display-table
          match-data                    window-dedicated-p
          overlay-get                   window-hscroll
          overlay-start                 window-parameter
          overlay-end                   window-point
          process-buffer                window-start
          process-filter                default-value

‘setf’ signals an error if you pass a PLACE form that it does not know
how to handle.

   Note that for ‘nthcdr’, the list argument of the function must itself
be a valid PLACE form.  For example, ‘(setf (nthcdr 0 foo) 7)’ will set
‘foo’ itself to 7.

   The macros ‘push’ (*note List Variables::) and ‘pop’ (*note List
Elements::) can manipulate generalized variables, not just lists.  ‘(pop
PLACE)’ removes and returns the first element of the list stored in
PLACE.  It is analogous to ‘(prog1 (car PLACE) (setf PLACE (cdr
PLACE)))’, except that it takes care to evaluate all subforms only once.
‘(push X PLACE)’ inserts X at the front of the list stored in PLACE.  It
is analogous to ‘(setf PLACE (cons X PLACE))’, except for evaluation of
the subforms.  Note that ‘push’ and ‘pop’ on an ‘nthcdr’ place can be
used to insert or delete at any position in a list.

   The ‘cl-lib’ library defines various extensions for generalized
variables, including additional ‘setf’ places.  *Note (cl)Generalized
Variables::.


File: elisp.info,  Node: Adding Generalized Variables,  Prev: Setting Generalized Variables,  Up: Generalized Variables

11.15.2 Defining new ‘setf’ forms
---------------------------------

This section describes how to define new forms that ‘setf’ can operate
on.

 -- Macro: gv-define-simple-setter name setter &optional fix-return
     This macro enables you to easily define ‘setf’ methods for simple
     cases.  NAME is the name of a function, macro, or special form.
     You can use this macro whenever NAME has a directly corresponding
     SETTER function that updates it, e.g., ‘(gv-define-simple-setter
     car setcar)’.

     This macro translates a call of the form

          (setf (NAME ARGS...) VALUE)

     into
          (SETTER ARGS... VALUE)

     Such a ‘setf’ call is documented to return VALUE.  This is no
     problem with, e.g., ‘car’ and ‘setcar’, because ‘setcar’ returns
     the value that it set.  If your SETTER function does not return
     VALUE, use a non-‘nil’ value for the FIX-RETURN argument of
     ‘gv-define-simple-setter’.  This expands into something equivalent
     to
          (let ((temp VALUE))
            (SETTER ARGS... temp)
            temp)
     so ensuring that it returns the correct result.

 -- Macro: gv-define-setter name arglist &rest body
     This macro allows for more complex ‘setf’ expansions than the
     previous form.  You may need to use this form, for example, if
     there is no simple setter function to call, or if there is one but
     it requires different arguments to the place form.

     This macro expands the form ‘(setf (NAME ARGS...) VALUE)’ by first
     binding the ‘setf’ argument forms ‘(VALUE ARGS...)’ according to
     ARGLIST, and then executing BODY.  BODY should return a Lisp form
     that does the assignment, and finally returns the value that was
     set.  An example of using this macro is:

          (gv-define-setter caar (val x) `(setcar (car ,x) ,val))

   For more control over the expansion, see the macro
‘gv-define-expander’.  The macro ‘gv-letplace’ can be useful in defining
macros that perform similarly to ‘setf’; for example, the ‘incf’ macro
of Common Lisp.  Consult the source file ‘gv.el’ for more details.

     Common Lisp note: Common Lisp defines another way to specify the
     ‘setf’ behavior of a function, namely ‘setf’ functions, whose names
     are lists ‘(setf NAME)’ rather than symbols.  For example, ‘(defun
     (setf foo) ...)’ defines the function that is used when ‘setf’ is
     applied to ‘foo’.  Emacs does not support this.  It is a
     compile-time error to use ‘setf’ on a form that has not already had
     an appropriate expansion defined.  In Common Lisp, this is not an
     error since the function ‘(setf FUNC)’ might be defined later.


File: elisp.info,  Node: Functions,  Next: Macros,  Prev: Variables,  Up: Top

12 Functions
************

A Lisp program is composed mainly of Lisp functions.  This chapter
explains what functions are, how they accept arguments, and how to
define them.

* Menu:

* What Is a Function::          Lisp functions vs. primitives; terminology.
* Lambda Expressions::          How functions are expressed as Lisp objects.
* Function Names::              A symbol can serve as the name of a function.
* Defining Functions::          Lisp expressions for defining functions.
* Calling Functions::           How to use an existing function.
* Mapping Functions::           Applying a function to each element of a list, etc.
* Anonymous Functions::         Lambda expressions are functions with no names.
* Generic Functions::           Polymorphism, Emacs-style.
* Function Cells::              Accessing or setting the function definition
                            of a symbol.
* Closures::                    Functions that enclose a lexical environment.
* Advising Functions::          Adding to the definition of a function.
* Obsolete Functions::          Declaring functions obsolete.
* Inline Functions::            Functions that the compiler will expand inline.
* Declare Form::                Adding additional information about a function.
* Declaring Functions::         Telling the compiler that a function is defined.
* Function Safety::             Determining whether a function is safe to call.
* Related Topics::              Cross-references to specific Lisp primitives
                            that have a special bearing on how functions work.


File: elisp.info,  Node: What Is a Function,  Next: Lambda Expressions,  Up: Functions

12.1 What Is a Function?
========================

In a general sense, a function is a rule for carrying out a computation
given input values called “arguments”.  The result of the computation is
called the “value” or “return value” of the function.  The computation
can also have side effects, such as lasting changes in the values of
variables or the contents of data structures.

   In most computer languages, every function has a name.  But in Lisp,
a function in the strictest sense has no name: it is an object which can
_optionally_ be associated with a symbol (e.g., ‘car’) that serves as
the function name.  *Note Function Names::.  When a function has been
given a name, we usually also refer to that symbol as a “function”
(e.g., we refer to “the function ‘car’”).  In this manual, the
distinction between a function name and the function object itself is
usually unimportant, but we will take note wherever it is relevant.

   Certain function-like objects, called “special forms” and “macros”,
also accept arguments to carry out computations.  However, as explained
below, these are not considered functions in Emacs Lisp.

   Here are important terms for functions and function-like objects:

“lambda expression”
     A function (in the strict sense, i.e., a function object) which is
     written in Lisp.  These are described in the following section.
     *Note Lambda Expressions::.

“primitive”
     A function which is callable from Lisp but is actually written in
     C.  Primitives are also called “built-in functions”, or “subrs”.
     Examples include functions like ‘car’ and ‘append’.  In addition,
     all special forms (see below) are also considered primitives.

     Usually, a function is implemented as a primitive because it is a
     fundamental part of Lisp (e.g., ‘car’), or because it provides a
     low-level interface to operating system services, or because it
     needs to run fast.  Unlike functions defined in Lisp, primitives
     can be modified or added only by changing the C sources and
     recompiling Emacs.  See *note Writing Emacs Primitives::.

“special form”
     A primitive that is like a function but does not evaluate all of
     its arguments in the usual way.  It may evaluate only some of the
     arguments, or may evaluate them in an unusual order, or several
     times.  Examples include ‘if’, ‘and’, and ‘while’.  *Note Special
     Forms::.

“macro”
     A construct defined in Lisp, which differs from a function in that
     it translates a Lisp expression into another expression which is to
     be evaluated instead of the original expression.  Macros enable
     Lisp programmers to do the sorts of things that special forms can
     do.  *Note Macros::.

“command”
     An object which can be invoked via the ‘command-execute’ primitive,
     usually due to the user typing in a key sequence “bound” to that
     command.  *Note Interactive Call::.  A command is usually a
     function; if the function is written in Lisp, it is made into a
     command by an ‘interactive’ form in the function definition (*note
     Defining Commands::).  Commands that are functions can also be
     called from Lisp expressions, just like other functions.

     Keyboard macros (strings and vectors) are commands also, even
     though they are not functions.  *Note Keyboard Macros::.  We say
     that a symbol is a command if its function cell contains a command
     (*note Symbol Components::); such a “named command” can be invoked
     with ‘M-x’.

“closure”
     A function object that is much like a lambda expression, except
     that it also encloses an environment of lexical variable bindings.
     *Note Closures::.

“byte-code function”
     A function that has been compiled by the byte compiler.  *Note
     Byte-Code Type::.

“autoload object”
     A place-holder for a real function.  If the autoload object is
     called, Emacs loads the file containing the definition of the real
     function, and then calls the real function.  *Note Autoload::.

   You can use the function ‘functionp’ to test if an object is a
function:

 -- Function: functionp object
     This function returns ‘t’ if OBJECT is any kind of function, i.e.,
     can be passed to ‘funcall’.  Note that ‘functionp’ returns ‘t’ for
     symbols that are function names, and returns ‘nil’ for special
     forms.

Unlike ‘functionp’, the next three functions do _not_ treat a symbol as
its function definition.

 -- Function: subrp object
     This function returns ‘t’ if OBJECT is a built-in function (i.e., a
     Lisp primitive).

          (subrp 'message)            ; ‘message’ is a symbol,
               ⇒ nil                 ;   not a subr object.
          (subrp (symbol-function 'message))
               ⇒ t

 -- Function: byte-code-function-p object
     This function returns ‘t’ if OBJECT is a byte-code function.  For
     example:

          (byte-code-function-p (symbol-function 'next-line))
               ⇒ t

 -- Function: subr-arity subr
     This function provides information about the argument list of a
     primitive, SUBR.  The returned value is a pair ‘(MIN . MAX)’.  MIN
     is the minimum number of args.  MAX is the maximum number or the
     symbol ‘many’, for a function with ‘&rest’ arguments, or the symbol
     ‘unevalled’ if SUBR is a special form.


File: elisp.info,  Node: Lambda Expressions,  Next: Function Names,  Prev: What Is a Function,  Up: Functions

12.2 Lambda Expressions
=======================

A lambda expression is a function object written in Lisp.  Here is an
example:

     (lambda (x)
       "Return the hyperbolic cosine of X."
       (* 0.5 (+ (exp x) (exp (- x)))))

In Emacs Lisp, such a list is a valid expression which evaluates to a
function object.

   A lambda expression, by itself, has no name; it is an “anonymous
function”.  Although lambda expressions can be used this way (*note
Anonymous Functions::), they are more commonly associated with symbols
to make “named functions” (*note Function Names::).  Before going into
these details, the following subsections describe the components of a
lambda expression and what they do.

* Menu:

* Lambda Components::           The parts of a lambda expression.
* Simple Lambda::               A simple example.
* Argument List::               Details and special features of argument lists.
* Function Documentation::      How to put documentation in a function.


File: elisp.info,  Node: Lambda Components,  Next: Simple Lambda,  Up: Lambda Expressions

12.2.1 Components of a Lambda Expression
----------------------------------------

A lambda expression is a list that looks like this:

     (lambda (ARG-VARIABLES...)
       [DOCUMENTATION-STRING]
       [INTERACTIVE-DECLARATION]
       BODY-FORMS...)

   The first element of a lambda expression is always the symbol
‘lambda’.  This indicates that the list represents a function.  The
reason functions are defined to start with ‘lambda’ is so that other
lists, intended for other uses, will not accidentally be valid as
functions.

   The second element is a list of symbols—the argument variable names.
This is called the “lambda list”.  When a Lisp function is called, the
argument values are matched up against the variables in the lambda list,
which are given local bindings with the values provided.  *Note Local
Variables::.

   The documentation string is a Lisp string object placed within the
function definition to describe the function for the Emacs help
facilities.  *Note Function Documentation::.

   The interactive declaration is a list of the form ‘(interactive
CODE-STRING)’.  This declares how to provide arguments if the function
is used interactively.  Functions with this declaration are called
“commands”; they can be called using ‘M-x’ or bound to a key.  Functions
not intended to be called in this way should not have interactive
declarations.  *Note Defining Commands::, for how to write an
interactive declaration.

   The rest of the elements are the “body” of the function: the Lisp
code to do the work of the function (or, as a Lisp programmer would say,
“a list of Lisp forms to evaluate”).  The value returned by the function
is the value returned by the last element of the body.


File: elisp.info,  Node: Simple Lambda,  Next: Argument List,  Prev: Lambda Components,  Up: Lambda Expressions

12.2.2 A Simple Lambda Expression Example
-----------------------------------------

Consider the following example:

     (lambda (a b c) (+ a b c))

We can call this function by passing it to ‘funcall’, like this:

     (funcall (lambda (a b c) (+ a b c))
              1 2 3)

This call evaluates the body of the lambda expression with the variable
‘a’ bound to 1, ‘b’ bound to 2, and ‘c’ bound to 3.  Evaluation of the
body adds these three numbers, producing the result 6; therefore, this
call to the function returns the value 6.

   Note that the arguments can be the results of other function calls,
as in this example:

     (funcall (lambda (a b c) (+ a b c))
              1 (* 2 3) (- 5 4))

This evaluates the arguments ‘1’, ‘(* 2 3)’, and ‘(- 5 4)’ from left to
right.  Then it applies the lambda expression to the argument values 1,
6 and 1 to produce the value 8.

   As these examples show, you can use a form with a lambda expression
as its CAR to make local variables and give them values.  In the old
days of Lisp, this technique was the only way to bind and initialize
local variables.  But nowadays, it is clearer to use the special form
‘let’ for this purpose (*note Local Variables::).  Lambda expressions
are mainly used as anonymous functions for passing as arguments to other
functions (*note Anonymous Functions::), or stored as symbol function
definitions to produce named functions (*note Function Names::).


File: elisp.info,  Node: Argument List,  Next: Function Documentation,  Prev: Simple Lambda,  Up: Lambda Expressions

12.2.3 Other Features of Argument Lists
---------------------------------------

Our simple sample function, ‘(lambda (a b c) (+ a b c))’, specifies
three argument variables, so it must be called with three arguments: if
you try to call it with only two arguments or four arguments, you get a
‘wrong-number-of-arguments’ error (*note Errors::).

   It is often convenient to write a function that allows certain
arguments to be omitted.  For example, the function ‘substring’ accepts
three arguments—a string, the start index and the end index—but the
third argument defaults to the LENGTH of the string if you omit it.  It
is also convenient for certain functions to accept an indefinite number
of arguments, as the functions ‘list’ and ‘+’ do.

   To specify optional arguments that may be omitted when a function is
called, simply include the keyword ‘&optional’ before the optional
arguments.  To specify a list of zero or more extra arguments, include
the keyword ‘&rest’ before one final argument.

   Thus, the complete syntax for an argument list is as follows:

     (REQUIRED-VARS...
      [&optional OPTIONAL-VARS...]
      [&rest REST-VAR])

The square brackets indicate that the ‘&optional’ and ‘&rest’ clauses,
and the variables that follow them, are optional.

   A call to the function requires one actual argument for each of the
REQUIRED-VARS.  There may be actual arguments for zero or more of the
OPTIONAL-VARS, and there cannot be any actual arguments beyond that
unless the lambda list uses ‘&rest’.  In that case, there may be any
number of extra actual arguments.

   If actual arguments for the optional and rest variables are omitted,
then they always default to ‘nil’.  There is no way for the function to
distinguish between an explicit argument of ‘nil’ and an omitted
argument.  However, the body of the function is free to consider ‘nil’
an abbreviation for some other meaningful value.  This is what
‘substring’ does; ‘nil’ as the third argument to ‘substring’ means to
use the length of the string supplied.

     Common Lisp note: Common Lisp allows the function to specify what
     default value to use when an optional argument is omitted; Emacs
     Lisp always uses ‘nil’.  Emacs Lisp does not support ‘supplied-p’
     variables that tell you whether an argument was explicitly passed.

   For example, an argument list that looks like this:

     (a b &optional c d &rest e)

binds ‘a’ and ‘b’ to the first two actual arguments, which are required.
If one or two more arguments are provided, ‘c’ and ‘d’ are bound to them
respectively; any arguments after the first four are collected into a
list and ‘e’ is bound to that list.  If there are only two arguments,
‘c’ is ‘nil’; if two or three arguments, ‘d’ is ‘nil’; if four arguments
or fewer, ‘e’ is ‘nil’.

   There is no way to have required arguments following optional ones—it
would not make sense.  To see why this must be so, suppose that ‘c’ in
the example were optional and ‘d’ were required.  Suppose three actual
arguments are given; which variable would the third argument be for?
Would it be used for the C, or for D?  One can argue for both
possibilities.  Similarly, it makes no sense to have any more arguments
(either required or optional) after a ‘&rest’ argument.

   Here are some examples of argument lists and proper calls:

     (funcall (lambda (n) (1+ n))        ; One required:
              1)                         ; requires exactly one argument.
          ⇒ 2
     (funcall (lambda (n &optional n1)   ; One required and one optional:
                (if n1 (+ n n1) (1+ n))) ; 1 or 2 arguments.
              1 2)
          ⇒ 3
     (funcall (lambda (n &rest ns)       ; One required and one rest:
                (+ n (apply '+ ns)))     ; 1 or more arguments.
              1 2 3 4 5)
          ⇒ 15


File: elisp.info,  Node: Function Documentation,  Prev: Argument List,  Up: Lambda Expressions

12.2.4 Documentation Strings of Functions
-----------------------------------------

A lambda expression may optionally have a “documentation string” just
after the lambda list.  This string does not affect execution of the
function; it is a kind of comment, but a systematized comment which
actually appears inside the Lisp world and can be used by the Emacs help
facilities.  *Note Documentation::, for how the documentation string is
accessed.

   It is a good idea to provide documentation strings for all the
functions in your program, even those that are called only from within
your program.  Documentation strings are like comments, except that they
are easier to access.

   The first line of the documentation string should stand on its own,
because ‘apropos’ displays just this first line.  It should consist of
one or two complete sentences that summarize the function’s purpose.

   The start of the documentation string is usually indented in the
source file, but since these spaces come before the starting
double-quote, they are not part of the string.  Some people make a
practice of indenting any additional lines of the string so that the
text lines up in the program source.  _That is a mistake._  The
indentation of the following lines is inside the string; what looks nice
in the source code will look ugly when displayed by the help commands.

   You may wonder how the documentation string could be optional, since
there are required components of the function that follow it (the body).
Since evaluation of a string returns that string, without any side
effects, it has no effect if it is not the last form in the body.  Thus,
in practice, there is no confusion between the first form of the body
and the documentation string; if the only body form is a string then it
serves both as the return value and as the documentation.

   The last line of the documentation string can specify calling
conventions different from the actual function arguments.  Write text
like this:

     \(fn ARGLIST)

following a blank line, at the beginning of the line, with no newline
following it inside the documentation string.  (The ‘\’ is used to avoid
confusing the Emacs motion commands.)  The calling convention specified
in this way appears in help messages in place of the one derived from
the actual arguments of the function.

   This feature is particularly useful for macro definitions, since the
arguments written in a macro definition often do not correspond to the
way users think of the parts of the macro call.


File: elisp.info,  Node: Function Names,  Next: Defining Functions,  Prev: Lambda Expressions,  Up: Functions

12.3 Naming a Function
======================

A symbol can serve as the name of a function.  This happens when the
symbol’s “function cell” (*note Symbol Components::) contains a function
object (e.g., a lambda expression).  Then the symbol itself becomes a
valid, callable function, equivalent to the function object in its
function cell.

   The contents of the function cell are also called the symbol’s
“function definition”.  The procedure of using a symbol’s function
definition in place of the symbol is called “symbol function
indirection”; see *note Function Indirection::.  If you have not given a
symbol a function definition, its function cell is said to be “void”,
and it cannot be used as a function.

   In practice, nearly all functions have names, and are referred to by
their names.  You can create a named Lisp function by defining a lambda
expression and putting it in a function cell (*note Function Cells::).
However, it is more common to use the ‘defun’ special form, described in
the next section.  *Note Defining Functions::.

   We give functions names because it is convenient to refer to them by
their names in Lisp expressions.  Also, a named Lisp function can easily
refer to itself—it can be recursive.  Furthermore, primitives can only
be referred to textually by their names, since primitive function
objects (*note Primitive Function Type::) have no read syntax.

   A function need not have a unique name.  A given function object
_usually_ appears in the function cell of only one symbol, but this is
just a convention.  It is easy to store it in several symbols using
‘fset’; then each of the symbols is a valid name for the same function.

   Note that a symbol used as a function name may also be used as a
variable; these two uses of a symbol are independent and do not
conflict.  (This is not the case in some dialects of Lisp, like Scheme.)


File: elisp.info,  Node: Defining Functions,  Next: Calling Functions,  Prev: Function Names,  Up: Functions

12.4 Defining Functions
=======================

We usually give a name to a function when it is first created.  This is
called “defining a function”, and it is done with the ‘defun’ macro.

 -- Macro: defun name args [doc] [declare] [interactive] body...
     ‘defun’ is the usual way to define new Lisp functions.  It defines
     the symbol NAME as a function with argument list ARGS and body
     forms given by BODY.  Neither NAME nor ARGS should be quoted.

     DOC, if present, should be a string specifying the function’s
     documentation string (*note Function Documentation::).  DECLARE, if
     present, should be a ‘declare’ form specifying function metadata
     (*note Declare Form::).  INTERACTIVE, if present, should be an
     ‘interactive’ form specifying how the function is to be called
     interactively (*note Interactive Call::).

     The return value of ‘defun’ is undefined.

     Here are some examples:

          (defun foo () 5)
          (foo)
               ⇒ 5

          (defun bar (a &optional b &rest c)
              (list a b c))
          (bar 1 2 3 4 5)
               ⇒ (1 2 (3 4 5))
          (bar 1)
               ⇒ (1 nil nil)
          (bar)
          error→ Wrong number of arguments.

          (defun capitalize-backwards ()
            "Upcase the last letter of the word at point."
            (interactive)
            (backward-word 1)
            (forward-word 1)
            (backward-char 1)
            (capitalize-word 1))

     Be careful not to redefine existing functions unintentionally.
     ‘defun’ redefines even primitive functions such as ‘car’ without
     any hesitation or notification.  Emacs does not prevent you from
     doing this, because redefining a function is sometimes done
     deliberately, and there is no way to distinguish deliberate
     redefinition from unintentional redefinition.

 -- Function: defalias name definition &optional doc
     This function defines the symbol NAME as a function, with
     definition DEFINITION (which can be any valid Lisp function).  Its
     return value is _undefined_.

     If DOC is non-‘nil’, it becomes the function documentation of NAME.
     Otherwise, any documentation provided by DEFINITION is used.

     Internally, ‘defalias’ normally uses ‘fset’ to set the definition.
     If NAME has a ‘defalias-fset-function’ property, however, the
     associated value is used as a function to call in place of ‘fset’.

     The proper place to use ‘defalias’ is where a specific function
     name is being defined—especially where that name appears explicitly
     in the source file being loaded.  This is because ‘defalias’
     records which file defined the function, just like ‘defun’ (*note
     Unloading::).

     By contrast, in programs that manipulate function definitions for
     other purposes, it is better to use ‘fset’, which does not keep
     such records.  *Note Function Cells::.

   You cannot create a new primitive function with ‘defun’ or
‘defalias’, but you can use them to change the function definition of
any symbol, even one such as ‘car’ or ‘x-popup-menu’ whose normal
definition is a primitive.  However, this is risky: for instance, it is
next to impossible to redefine ‘car’ without breaking Lisp completely.
Redefining an obscure function such as ‘x-popup-menu’ is less dangerous,
but it still may not work as you expect.  If there are calls to the
primitive from C code, they call the primitive’s C definition directly,
so changing the symbol’s definition will have no effect on them.

   See also ‘defsubst’, which defines a function like ‘defun’ and tells
the Lisp compiler to perform inline expansion on it.  *Note Inline
Functions::.

   Alternatively, you can define a function by providing the code which
will inline it as a compiler macro.  The following macros make this
possible.

 -- Macro: define-inline name args [doc] [declare] body...
     Define a function NAME by providing code that does its inlining, as
     a compiler macro.  The function will accept the argument list ARGS
     and will have the specified BODY.

     If present, DOC should be the function’s documentation string
     (*note Function Documentation::); DECLARE, if present, should be a
     ‘declare’ form (*note Declare Form::) specifying the function’s
     metadata.

   Functions defined via ‘define-inline’ have several advantages with
respect to macros defined by ‘defsubst’ or ‘defmacro’:

   − They can be passed to ‘mapcar’ (*note Mapping Functions::).

   − They are more efficient.

   − They can be used as “place forms” to store values (*note
     Generalized Variables::).

   − They behave in a more predictable way than ‘cl-defsubst’ (*note
     (cl)Argument Lists::).

   Like ‘defmacro’, a function inlined with ‘define-inline’ inherits the
scoping rules, either dynamic or lexical, from the call site.  *Note
Variable Scoping::.

   The following macros should be used in the body of a function defined
by ‘define-inline’.

 -- Macro: inline-quote expression
     Quote EXPRESSION for ‘define-inline’.  This is similar to the
     backquote (*note Backquote::), but quotes code and accepts only
     ‘,’, not ‘,@’.

 -- Macro: inline-letevals (bindings...) body...
     This is is similar to ‘let’ (*note Local Variables::): it sets up
     local variables as specified by BINDINGS, and then evaluates BODY
     with those bindings in effect.  Each element of BINDINGS should be
     either a symbol or a list of the form ‘(VAR EXPR)’; the result is
     to evaluate EXPR and bind VAR to the result.  The tail of BINDINGS
     can be either ‘nil’ or a symbol which should hold a list of
     arguments, in which case each argument is evaluated, and the symbol
     is bound to the resulting list.

 -- Macro: inline-const-p expression
     Return non-‘nil’ if the value of EXPRESSION is already known.

 -- Macro: inline-const-val expression
     Return the value of EXPRESSION.

 -- Macro: inline-error format &rest args
     Signal an error, formatting ARGS according to FORMAT.

   Here’s an example of using ‘define-inline’:

     (define-inline myaccessor (obj)
       (inline-letevals (obj)
         (inline-quote (if (foo-p ,obj) (aref (cdr ,obj) 3) (aref ,obj 2)))))

This is equivalent to

     (defsubst myaccessor (obj)
       (if (foo-p obj) (aref (cdr obj) 3) (aref obj 2)))


File: elisp.info,  Node: Calling Functions,  Next: Mapping Functions,  Prev: Defining Functions,  Up: Functions

12.5 Calling Functions
======================

Defining functions is only half the battle.  Functions don’t do anything
until you “call” them, i.e., tell them to run.  Calling a function is
also known as “invocation”.

   The most common way of invoking a function is by evaluating a list.
For example, evaluating the list ‘(concat "a" "b")’ calls the function
‘concat’ with arguments ‘"a"’ and ‘"b"’.  *Note Evaluation::, for a
description of evaluation.

   When you write a list as an expression in your program, you specify
which function to call, and how many arguments to give it, in the text
of the program.  Usually that’s just what you want.  Occasionally you
need to compute at run time which function to call.  To do that, use the
function ‘funcall’.  When you also need to determine at run time how
many arguments to pass, use ‘apply’.

 -- Function: funcall function &rest arguments
     ‘funcall’ calls FUNCTION with ARGUMENTS, and returns whatever
     FUNCTION returns.

     Since ‘funcall’ is a function, all of its arguments, including
     FUNCTION, are evaluated before ‘funcall’ is called.  This means
     that you can use any expression to obtain the function to be
     called.  It also means that ‘funcall’ does not see the expressions
     you write for the ARGUMENTS, only their values.  These values are
     _not_ evaluated a second time in the act of calling FUNCTION; the
     operation of ‘funcall’ is like the normal procedure for calling a
     function, once its arguments have already been evaluated.

     The argument FUNCTION must be either a Lisp function or a primitive
     function.  Special forms and macros are not allowed, because they
     make sense only when given the unevaluated argument expressions.
     ‘funcall’ cannot provide these because, as we saw above, it never
     knows them in the first place.

     If you need to use ‘funcall’ to call a command and make it behave
     as if invoked interactively, use ‘funcall-interactively’ (*note
     Interactive Call::).

          (setq f 'list)
               ⇒ list
          (funcall f 'x 'y 'z)
               ⇒ (x y z)
          (funcall f 'x 'y '(z))
               ⇒ (x y (z))
          (funcall 'and t nil)
          error→ Invalid function: #<subr and>

     Compare these examples with the examples of ‘apply’.

 -- Function: apply function &rest arguments
     ‘apply’ calls FUNCTION with ARGUMENTS, just like ‘funcall’ but with
     one difference: the last of ARGUMENTS is a list of objects, which
     are passed to FUNCTION as separate arguments, rather than a single
     list.  We say that ‘apply’ “spreads” this list so that each
     individual element becomes an argument.

     ‘apply’ returns the result of calling FUNCTION.  As with ‘funcall’,
     FUNCTION must either be a Lisp function or a primitive function;
     special forms and macros do not make sense in ‘apply’.

          (setq f 'list)
               ⇒ list
          (apply f 'x 'y 'z)
          error→ Wrong type argument: listp, z
          (apply '+ 1 2 '(3 4))
               ⇒ 10
          (apply '+ '(1 2 3 4))
               ⇒ 10

          (apply 'append '((a b c) nil (x y z) nil))
               ⇒ (a b c x y z)

     For an interesting example of using ‘apply’, see *note Definition
     of mapcar::.

   Sometimes it is useful to fix some of the function’s arguments at
certain values, and leave the rest of arguments for when the function is
actually called.  The act of fixing some of the function’s arguments is
called “partial application” of the function(1).  The result is a new
function that accepts the rest of arguments and calls the original
function with all the arguments combined.

   Here’s how to do partial application in Emacs Lisp:

 -- Function: apply-partially func &rest args
     This function returns a new function which, when called, will call
     FUNC with the list of arguments composed from ARGS and additional
     arguments specified at the time of the call.  If FUNC accepts N
     arguments, then a call to ‘apply-partially’ with ‘M < N’ arguments
     will produce a new function of ‘N - M’ arguments.

     Here’s how we could define the built-in function ‘1+’, if it didn’t
     exist, using ‘apply-partially’ and ‘+’, another built-in function:

          (defalias '1+ (apply-partially '+ 1)
            "Increment argument by one.")
          (1+ 10)
               ⇒ 11

   It is common for Lisp functions to accept functions as arguments or
find them in data structures (especially in hook variables and property
lists) and call them using ‘funcall’ or ‘apply’.  Functions that accept
function arguments are often called “functionals”.

   Sometimes, when you call a functional, it is useful to supply a no-op
function as the argument.  Here are two different kinds of no-op
function:

 -- Function: identity arg
     This function returns ARG and has no side effects.

 -- Function: ignore &rest args
     This function ignores any arguments and returns ‘nil’.

   Some functions are user-visible “commands”, which can be called
interactively (usually by a key sequence).  It is possible to invoke
such a command exactly as though it was called interactively, by using
the ‘call-interactively’ function.  *Note Interactive Call::.

   ---------- Footnotes ----------

   (1) This is related to, but different from “currying”, which
transforms a function that takes multiple arguments in such a way that
it can be called as a chain of functions, each one with a single
argument.


File: elisp.info,  Node: Mapping Functions,  Next: Anonymous Functions,  Prev: Calling Functions,  Up: Functions

12.6 Mapping Functions
======================

A “mapping function” applies a given function (_not_ a special form or
macro) to each element of a list or other collection.  Emacs Lisp has
several such functions; this section describes ‘mapcar’, ‘mapc’, and
‘mapconcat’, which map over a list.  *Note Definition of mapatoms::, for
the function ‘mapatoms’ which maps over the symbols in an obarray.
*Note Definition of maphash::, for the function ‘maphash’ which maps
over key/value associations in a hash table.

   These mapping functions do not allow char-tables because a char-table
is a sparse array whose nominal range of indices is very large.  To map
over a char-table in a way that deals properly with its sparse nature,
use the function ‘map-char-table’ (*note Char-Tables::).

 -- Function: mapcar function sequence
     ‘mapcar’ applies FUNCTION to each element of SEQUENCE in turn, and
     returns a list of the results.

     The argument SEQUENCE can be any kind of sequence except a
     char-table; that is, a list, a vector, a bool-vector, or a string.
     The result is always a list.  The length of the result is the same
     as the length of SEQUENCE.  For example:

          (mapcar 'car '((a b) (c d) (e f)))
               ⇒ (a c e)
          (mapcar '1+ [1 2 3])
               ⇒ (2 3 4)
          (mapcar 'string "abc")
               ⇒ ("a" "b" "c")

          ;; Call each function in ‘my-hooks’.
          (mapcar 'funcall my-hooks)

          (defun mapcar* (function &rest args)
            "Apply FUNCTION to successive cars of all ARGS.
          Return the list of results."
            ;; If no list is exhausted,
            (if (not (memq nil args))
                ;; apply function to CARs.
                (cons (apply function (mapcar 'car args))
                      (apply 'mapcar* function
                             ;; Recurse for rest of elements.
                             (mapcar 'cdr args)))))

          (mapcar* 'cons '(a b c) '(1 2 3 4))
               ⇒ ((a . 1) (b . 2) (c . 3))

 -- Function: mapc function sequence
     ‘mapc’ is like ‘mapcar’ except that FUNCTION is used for
     side-effects only—the values it returns are ignored, not collected
     into a list.  ‘mapc’ always returns SEQUENCE.

 -- Function: mapconcat function sequence separator
     ‘mapconcat’ applies FUNCTION to each element of SEQUENCE; the
     results, which must be sequences of characters (strings, vectors,
     or lists), are concatenated into a single string return value.
     Between each pair of result sequences, ‘mapconcat’ inserts the
     characters from SEPARATOR, which also must be a string, or a vector
     or list of characters.  *Note Sequences Arrays Vectors::.

     The argument FUNCTION must be a function that can take one argument
     and returns a sequence of characters: a string, a vector, or a
     list.  The argument SEQUENCE can be any kind of sequence except a
     char-table; that is, a list, a vector, a bool-vector, or a string.

          (mapconcat 'symbol-name
                     '(The cat in the hat)
                     " ")
               ⇒ "The cat in the hat"

          (mapconcat (function (lambda (x) (format "%c" (1+ x))))
                     "HAL-8000"
                     "")
               ⇒ "IBM.9111"


File: elisp.info,  Node: Anonymous Functions,  Next: Generic Functions,  Prev: Mapping Functions,  Up: Functions

12.7 Anonymous Functions
========================

Although functions are usually defined with ‘defun’ and given names at
the same time, it is sometimes convenient to use an explicit lambda
expression—an “anonymous function”.  Anonymous functions are valid
wherever function names are.  They are often assigned as variable
values, or as arguments to functions; for instance, you might pass one
as the FUNCTION argument to ‘mapcar’, which applies that function to
each element of a list (*note Mapping Functions::).  *Note
describe-symbols example::, for a realistic example of this.

   When defining a lambda expression that is to be used as an anonymous
function, you can in principle use any method to construct the list.
But typically you should use the ‘lambda’ macro, or the ‘function’
special form, or the ‘#'’ read syntax:

 -- Macro: lambda args [doc] [interactive] body...
     This macro returns an anonymous function with argument list ARGS,
     documentation string DOC (if any), interactive spec INTERACTIVE (if
     any), and body forms given by BODY.

     In effect, this macro makes ‘lambda’ forms self-quoting: evaluating
     a form whose CAR is ‘lambda’ yields the form itself:

          (lambda (x) (* x x))
               ⇒ (lambda (x) (* x x))

     The ‘lambda’ form has one other effect: it tells the Emacs
     evaluator and byte-compiler that its argument is a function, by
     using ‘function’ as a subroutine (see below).

 -- Special Form: function function-object
     This special form returns FUNCTION-OBJECT without evaluating it.
     In this, it is similar to ‘quote’ (*note Quoting::).  But unlike
     ‘quote’, it also serves as a note to the Emacs evaluator and
     byte-compiler that FUNCTION-OBJECT is intended to be used as a
     function.  Assuming FUNCTION-OBJECT is a valid lambda expression,
     this has two effects:

        • When the code is byte-compiled, FUNCTION-OBJECT is compiled
          into a byte-code function object (*note Byte Compilation::).

        • When lexical binding is enabled, FUNCTION-OBJECT is converted
          into a closure.  *Note Closures::.

   The read syntax ‘#'’ is a short-hand for using ‘function’.  The
following forms are all equivalent:

     (lambda (x) (* x x))
     (function (lambda (x) (* x x)))
     #'(lambda (x) (* x x))

   In the following example, we define a ‘change-property’ function that
takes a function as its third argument, followed by a ‘double-property’
function that makes use of ‘change-property’ by passing it an anonymous
function:

     (defun change-property (symbol prop function)
       (let ((value (get symbol prop)))
         (put symbol prop (funcall function value))))

     (defun double-property (symbol prop)
       (change-property symbol prop (lambda (x) (* 2 x))))

Note that we do not quote the ‘lambda’ form.

   If you compile the above code, the anonymous function is also
compiled.  This would not happen if, say, you had constructed the
anonymous function by quoting it as a list:

     (defun double-property (symbol prop)
       (change-property symbol prop '(lambda (x) (* 2 x))))

In that case, the anonymous function is kept as a lambda expression in
the compiled code.  The byte-compiler cannot assume this list is a
function, even though it looks like one, since it does not know that
‘change-property’ intends to use it as a function.


File: elisp.info,  Node: Generic Functions,  Next: Function Cells,  Prev: Anonymous Functions,  Up: Functions

12.8 Generic Functions
======================

Functions defined using ‘defun’ have a hard-coded set of assumptions
about the types and expected values of their arguments.  For example, a
function that was designed to handle values of its argument that are
either numbers or lists of numbers will fail or signal an error if
called with a value of any other type, such as a vector or a string.
This happens because the implementation of the function is not prepared
to deal with types other than those assumed during the design.

   By contrast, object-oriented programs use “polymorphic functions”: a
set of specialized functions having the same name, each one of which was
written for a certain specific set of argument types.  Which of the
functions is actually called is decided at run time based on the types
of the actual arguments.

   Emacs provides support for polymorphism.  Like other Lisp
environments, notably Common Lisp and its Common Lisp Object System
(CLOS), this support is based on “generic functions”.  The Emacs generic
functions closely follow CLOS, including use of similar names, so if you
have experience with CLOS, the rest of this section will sound very
familiar.

   A generic function specifies an abstract operation, by defining its
name and list of arguments, but (usually) no implementation.  The actual
implementation for several specific classes of arguments is provided by
“methods”, which should be defined separately.  Each method that
implements a generic function has the same name as the generic function,
but the method’s definition indicates what kinds of arguments it can
handle by “specializing” the arguments defined by the generic function.
These “argument specializers” can be more or less specific; for example,
a ‘string’ type is more specific than a more general type, such as
‘sequence’.

   Note that, unlike in message-based OO languages, such as C++ and
Simula, methods that implement generic functions don’t belong to a
class, they belong to the generic function they implement.

   When a generic function is invoked, it selects the applicable methods
by comparing the actual arguments passed by the caller with the argument
specializers of each method.  A method is applicable if the actual
arguments of the call are compatible with the method’s specializers.  If
more than one method is applicable, they are combined using certain
rules, described below, and the combination then handles the call.

 -- Macro: cl-defgeneric name arguments [documentation]
          [options-and-methods...] &rest body
     This macro defines a generic function with the specified NAME and
     ARGUMENTS.  If BODY is present, it provides the default
     implementation.  If DOCUMENTATION is present (it should always be),
     it specifies the documentation string for the generic function, in
     the form ‘(:documentation DOCSTRING)’.  The optional
     OPTIONS-AND-METHODS can be one of the following forms:

     ‘(declare DECLARATIONS)’
          A declare form, as described in *note Declare Form::.
     ‘(:argument-precedence-order &rest ARGS)’
          This form affects the sorting order for combining applicable
          methods.  Normally, when two methods are compared during
          combination, method arguments are examined left to right, and
          the first method whose argument specializer is more specific
          will come before the other one.  The order defined by this
          form overrides that, and the arguments are examined according
          to their order in this form, and not left to right.
     ‘(:method [QUALIFIERS...] args &rest body)’
          This form defines a method like ‘cl-defmethod’ does.

 -- Macro: cl-defmethod name [qualifier] arguments &rest [docstring]
          body
     This macro defines a particular implementation for the generic
     function called NAME.  The implementation code is given by BODY.
     If present, DOCSTRING is the documentation string for the method.
     The ARGUMENTS list, which must be identical in all the methods that
     implement a generic function, and must match the argument list of
     that function, provides argument specializers of the form ‘(ARG
     SPEC)’, where ARG is the argument name as specified in the
     ‘cl-defgeneric’ call, and SPEC is one of the following specializer
     forms:

     ‘TYPE’
          This specializer requires the argument to be of the given
          TYPE, one of the types from the type hierarchy described
          below.
     ‘(eql OBJECT)’
          This specializer requires the argument be ‘eql’ to the given
          OBJECT.
     ‘(head OBJECT)’
          The argument must be a cons cell whose ‘car’ is ‘eql’ to
          OBJECT.
     ‘STRUCT-TAG’
          The argument must be an instance of a class named STRUCT-TAG
          defined with ‘cl-defstruct’ (*note (cl)Structures::), or of
          one of its parent classes.

     Alternatively, the argument specializer can be of the form
     ‘&context (EXPR SPEC)’, in which case the value of EXPR must be
     compatible with the specializer provided by SPEC; SPEC can be any
     of the forms described above.  In other words, this form of
     specializer uses the value of EXPR instead of arguments for the
     decision whether the method is applicable.  For example, ‘&context
     (overwrite-mode (eql t))’ will make the method compatible only when
     ‘overwrite-mode’ is turned on.

     The type specializer, ‘(ARG TYPE)’, can specify one of the “system
     types” in the following list.  When a parent type is specified, an
     argument whose type is any of its more specific child types, as
     well as grand-children, grand-grand-children, etc.  will also be
     compatible.

     ‘integer’
          Parent type: ‘number’.
     ‘number’
     ‘null’
          Parent type: ‘symbol’
     ‘symbol’
     ‘string’
          Parent type: ‘array’.
     ‘array’
          Parent type: ‘sequence’.
     ‘cons’
          Parent type: ‘list’.
     ‘list’
          Parent type: ‘sequence’.
     ‘marker’
     ‘overlay’
     ‘float’
          Parent type: ‘number’.
     ‘window-configuration’
     ‘process’
     ‘window’
     ‘subr’
     ‘compiled-function’
     ‘buffer’
     ‘char-table’
          Parent type: ‘array’.
     ‘bool-vector’
          Parent type: ‘array’.
     ‘vector’
          Parent type: ‘array’.
     ‘frame’
     ‘hash-table’
     ‘font-spec’
     ‘font-entity’
     ‘font-object’

     The optional QUALIFIER allows combining several applicable methods.
     If it is not present, the defined method is a “primary” method,
     responsible for providing the primary implementation of the generic
     function for the specialized arguments.  You can also define
     “auxiliary methods”, by using one of the following values as
     QUALIFIER:

     ‘:before’
          This auxiliary method will run before the primary method.
          More accurately, all the ‘:before’ methods will run before the
          primary, in the most-specific-first order.
     ‘:after’
          This auxiliary method will run after the primary method.  More
          accurately, all such methods will run after the primary, in
          the most-specific-last order.
     ‘:around’
          This auxiliary method will run _instead_ of the primary
          method.  The most specific of such methods will be run before
          any other method.  Such methods normally use
          ‘cl-call-next-method’, described below, to invoke the other
          auxiliary or primary methods.
     ‘:extra STRING’
          This allows you to add more methods, distinguished by STRING,
          for the same specializers and qualifiers.

   Each time a generic function is called, it builds the “effective
method” which will handle this invocation by combining the applicable
methods defined for the function.  The process of finding the applicable
methods and producing the effective method is called “dispatch”.  The
applicable methods are those all of whose specializers are compatible
with the actual arguments of the call.  Since all of the arguments must
be compatible with the specializers, they all determine whether a method
is applicable.  Methods that explicitly specialize more than one
argument are called “multiple-dispatch methods”.

   The applicable methods are sorted into the order in which they will
be combined.  The method whose left-most argument specializer is the
most specific one will come first in the order.  (Specifying
‘:argument-precedence-order’ as part of ‘cl-defmethod’ overrides that,
as described above.)  If the method body calls ‘cl-call-next-method’,
the next most-specific method will run.  If there are applicable
‘:around’ methods, the most-specific of them will run first; it should
call ‘cl-call-next-method’ to run any of the less specific ‘:around’
methods.  Next, the ‘:before’ methods run in the order of their
specificity, followed by the primary method, and lastly the ‘:after’
methods in the reverse order of their specificity.

 -- Function: cl-call-next-method &rest args
     When invoked from within the lexical body of a primary or an
     ‘:around’ auxiliary method, call the next applicable method for the
     same generic function.  Normally, it is called with no arguments,
     which means to call the next applicable method with the same
     arguments that the calling method was invoked.  Otherwise, the
     specified arguments are used instead.

 -- Function: cl-next-method-p
     This function, when called from within the lexical body of a
     primary or an ‘:around’ auxiliary method, returns non-‘nil’ if
     there is a next method to call.


File: elisp.info,  Node: Function Cells,  Next: Closures,  Prev: Generic Functions,  Up: Functions

12.9 Accessing Function Cell Contents
=====================================

The “function definition” of a symbol is the object stored in the
function cell of the symbol.  The functions described here access, test,
and set the function cell of symbols.

   See also the function ‘indirect-function’.  *Note Definition of
indirect-function::.

 -- Function: symbol-function symbol
     This returns the object in the function cell of SYMBOL.  It does
     not check that the returned object is a legitimate function.

     If the function cell is void, the return value is ‘nil’.  To
     distinguish between a function cell that is void and one set to
     ‘nil’, use ‘fboundp’ (see below).

          (defun bar (n) (+ n 2))
          (symbol-function 'bar)
               ⇒ (lambda (n) (+ n 2))
          (fset 'baz 'bar)
               ⇒ bar
          (symbol-function 'baz)
               ⇒ bar

   If you have never given a symbol any function definition, we say that
that symbol’s function cell is “void”.  In other words, the function
cell does not have any Lisp object in it.  If you try to call the symbol
as a function, Emacs signals a ‘void-function’ error.

   Note that void is not the same as ‘nil’ or the symbol ‘void’.  The
symbols ‘nil’ and ‘void’ are Lisp objects, and can be stored into a
function cell just as any other object can be (and they can be valid
functions if you define them in turn with ‘defun’).  A void function
cell contains no object whatsoever.

   You can test the voidness of a symbol’s function definition with
‘fboundp’.  After you have given a symbol a function definition, you can
make it void once more using ‘fmakunbound’.

 -- Function: fboundp symbol
     This function returns ‘t’ if the symbol has an object in its
     function cell, ‘nil’ otherwise.  It does not check that the object
     is a legitimate function.

 -- Function: fmakunbound symbol
     This function makes SYMBOL’s function cell void, so that a
     subsequent attempt to access this cell will cause a ‘void-function’
     error.  It returns SYMBOL.  (See also ‘makunbound’, in *note Void
     Variables::.)

          (defun foo (x) x)
          (foo 1)
               ⇒1
          (fmakunbound 'foo)
               ⇒ foo
          (foo 1)
          error→ Symbol's function definition is void: foo

 -- Function: fset symbol definition
     This function stores DEFINITION in the function cell of SYMBOL.
     The result is DEFINITION.  Normally DEFINITION should be a function
     or the name of a function, but this is not checked.  The argument
     SYMBOL is an ordinary evaluated argument.

     The primary use of this function is as a subroutine by constructs
     that define or alter functions, like ‘defun’ or ‘advice-add’ (*note
     Advising Functions::).  You can also use it to give a symbol a
     function definition that is not a function, e.g., a keyboard macro
     (*note Keyboard Macros::):

          ;; Define a named keyboard macro.
          (fset 'kill-two-lines "\^u2\^k")
               ⇒ "\^u2\^k"

     It you wish to use ‘fset’ to make an alternate name for a function,
     consider using ‘defalias’ instead.  *Note Definition of defalias::.


File: elisp.info,  Node: Closures,  Next: Advising Functions,  Prev: Function Cells,  Up: Functions

12.10 Closures
==============

As explained in *note Variable Scoping::, Emacs can optionally enable
lexical binding of variables.  When lexical binding is enabled, any
named function that you create (e.g., with ‘defun’), as well as any
anonymous function that you create using the ‘lambda’ macro or the
‘function’ special form or the ‘#'’ syntax (*note Anonymous
Functions::), is automatically converted into a “closure”.

   A closure is a function that also carries a record of the lexical
environment that existed when the function was defined.  When it is
invoked, any lexical variable references within its definition use the
retained lexical environment.  In all other respects, closures behave
much like ordinary functions; in particular, they can be called in the
same way as ordinary functions.

   *Note Lexical Binding::, for an example of using a closure.

   Currently, an Emacs Lisp closure object is represented by a list with
the symbol ‘closure’ as the first element, a list representing the
lexical environment as the second element, and the argument list and
body forms as the remaining elements:

     ;; lexical binding is enabled.
     (lambda (x) (* x x))
          ⇒ (closure (t) (x) (* x x))

However, the fact that the internal structure of a closure is exposed to
the rest of the Lisp world is considered an internal implementation
detail.  For this reason, we recommend against directly examining or
altering the structure of closure objects.


File: elisp.info,  Node: Advising Functions,  Next: Obsolete Functions,  Prev: Closures,  Up: Functions

12.11 Advising Emacs Lisp Functions
===================================

When you need to modify a function defined in another library, or when
you need to modify a hook like ‘FOO-function’, a process filter, or
basically any variable or object field which holds a function value, you
can use the appropriate setter function, such as ‘fset’ or ‘defun’ for
named functions, ‘setq’ for hook variables, or ‘set-process-filter’ for
process filters, but those are often too blunt, completely throwing away
the previous value.

   The “advice” feature lets you add to the existing definition of a
function, by “advising the function”.  This is a cleaner method than
redefining the whole function.

   Emacs’s advice system provides two sets of primitives for that: the
core set, for function values held in variables and object fields (with
the corresponding primitives being ‘add-function’ and ‘remove-function’)
and another set layered on top of it for named functions (with the main
primitives being ‘advice-add’ and ‘advice-remove’).

   For example, in order to trace the calls to the process filter of a
process PROC, you could use:

     (defun my-tracing-function (proc string)
       (message "Proc %S received %S" proc string))

     (add-function :before (process-filter PROC) #'my-tracing-function)

   This will cause the process’s output to be passed to
‘my-tracing-function’ before being passed to the original process
filter.  ‘my-tracing-function’ receives the same arguments as the
original function.  When you’re done with it, you can revert to the
untraced behavior with:

     (remove-function (process-filter PROC) #'my-tracing-function)

   Similarly, if you want to trace the execution of the function named
‘display-buffer’, you could use:

     (defun his-tracing-function (orig-fun &rest args)
       (message "display-buffer called with args %S" args)
       (let ((res (apply orig-fun args)))
         (message "display-buffer returned %S" res)
         res))

     (advice-add 'display-buffer :around #'his-tracing-function)

   Here, ‘his-tracing-function’ is called instead of the original
function and receives the original function (additionally to that
function’s arguments) as argument, so it can call it if and when it
needs to.  When you’re tired of seeing this output, you can revert to
the untraced behavior with:

     (advice-remove 'display-buffer #'his-tracing-function)

   The arguments ‘:before’ and ‘:around’ used in the above examples
specify how the two functions are composed, since there are many
different ways to do it.  The added function is also called a piece of
_advice_.

* Menu:

* Core Advising Primitives::    Primitives to manipulate advice.
* Advising Named Functions::    Advising named functions.
* Advice combinators::          Ways to compose advice.
* Porting old advice::          Adapting code using the old defadvice.


File: elisp.info,  Node: Core Advising Primitives,  Next: Advising Named Functions,  Up: Advising Functions

12.11.1 Primitives to manipulate advices
----------------------------------------

 -- Macro: add-function where place function &optional props
     This macro is the handy way to add the advice FUNCTION to the
     function stored in PLACE (*note Generalized Variables::).

     WHERE determines how FUNCTION is composed with the existing
     function, e.g., whether FUNCTION should be called before, or after
     the original function.  *Note Advice combinators::, for the list of
     available ways to compose the two functions.

     When modifying a variable (whose name will usually end with
     ‘-function’), you can choose whether FUNCTION is used globally or
     only in the current buffer: if PLACE is just a symbol, then
     FUNCTION is added to the global value of PLACE.  Whereas if PLACE
     is of the form ‘(local SYMBOL)’, where SYMBOL is an expression
     which returns the variable name, then FUNCTION will only be added
     in the current buffer.  Finally, if you want to modify a lexical
     variable, you will have to use ‘(var VARIABLE)’.

     Every function added with ‘add-function’ can be accompanied by an
     association list of properties PROPS.  Currently only two of those
     properties have a special meaning:

     ‘name’
          This gives a name to the advice, which ‘remove-function’ can
          use to identify which function to remove.  Typically used when
          FUNCTION is an anonymous function.

     ‘depth’
          This specifies how to order the advice, should several pieces
          of advice be present.  By default, the depth is 0.  A depth of
          100 indicates that this piece of advice should be kept as deep
          as possible, whereas a depth of -100 indicates that it should
          stay as the outermost piece.  When two pieces of advice
          specify the same depth, the most recently added one will be
          outermost.

          For ‘:before’ advice, being outermost means that this advice
          will be run first, before any other advice, whereas being
          innermost means that it will run right before the original
          function, with no other advice run between itself and the
          original function.  Similarly, for ‘:after’ advice innermost
          means that it will run right after the original function, with
          no other advice run in between, whereas outermost means that
          it will be run right at the end after all other advice.  An
          innermost ‘:override’ piece of advice will only override the
          original function and other pieces of advice will apply to it,
          whereas an outermost ‘:override’ piece of advice will override
          not only the original function but all other advice applied to
          it as well.

     If FUNCTION is not interactive, then the combined function will
     inherit the interactive spec, if any, of the original function.
     Else, the combined function will be interactive and will use the
     interactive spec of FUNCTION.  One exception: if the interactive
     spec of FUNCTION is a function (rather than an expression or a
     string), then the interactive spec of the combined function will be
     a call to that function with as sole argument the interactive spec
     of the original function.  To interpret the spec received as
     argument, use ‘advice-eval-interactive-spec’.

     Note: The interactive spec of FUNCTION will apply to the combined
     function and should hence obey the calling convention of the
     combined function rather than that of FUNCTION.  In many cases, it
     makes no difference since they are identical, but it does matter
     for ‘:around’, ‘:filter-args’, and ‘filter-return’, where FUNCTION.

 -- Macro: remove-function place function
     This macro removes FUNCTION from the function stored in PLACE.
     This only works if FUNCTION was added to PLACE using
     ‘add-function’.

     FUNCTION is compared with functions added to PLACE using ‘equal’,
     to try and make it work also with lambda expressions.  It is
     additionally compared also with the ‘name’ property of the
     functions added to PLACE, which can be more reliable than comparing
     lambda expressions using ‘equal’.

 -- Function: advice-function-member-p advice function-def
     Return non-‘nil’ if ADVICE is already in FUNCTION-DEF.  Like for
     ‘remove-function’ above, instead of ADVICE being the actual
     function, it can also be the ‘name’ of the piece of advice.

 -- Function: advice-function-mapc f function-def
     Call the function F for every piece of advice that was added to
     FUNCTION-DEF.  F is called with two arguments: the advice function
     and its properties.

 -- Function: advice-eval-interactive-spec spec
     Evaluate the interactive SPEC just like an interactive call to a
     function with such a spec would, and then return the corresponding
     list of arguments that was built.  E.g.,
     ‘(advice-eval-interactive-spec "r\nP")’ will return a list of three
     elements, containing the boundaries of the region and the current
     prefix argument.


File: elisp.info,  Node: Advising Named Functions,  Next: Advice combinators,  Prev: Core Advising Primitives,  Up: Advising Functions

12.11.2 Advising Named Functions
--------------------------------

A common use of advice is for named functions and macros.  You could
just use ‘add-function’ as in:

     (add-function :around (symbol-function 'FUN) #'his-tracing-function)

   But you should use ‘advice-add’ and ‘advice-remove’ for that instead.
This separate set of functions to manipulate pieces of advice applied to
named functions, offers the following extra features compared to
‘add-function’: they know how to deal with macros and autoloaded
functions, they let ‘describe-function’ preserve the original docstring
as well as document the added advice, and they let you add and remove
advice before a function is even defined.

   ‘advice-add’ can be useful for altering the behavior of existing
calls to an existing function without having to redefine the whole
function.  However, it can be a source of bugs, since existing callers
to the function may assume the old behavior, and work incorrectly when
the behavior is changed by advice.  Advice can also cause confusion in
debugging, if the person doing the debugging does not notice or remember
that the function has been modified by advice.

   For these reasons, advice should be reserved for the cases where you
cannot modify a function’s behavior in any other way.  If it is possible
to do the same thing via a hook, that is preferable (*note Hooks::).  If
you simply want to change what a particular key does, it may be better
to write a new command, and remap the old command’s key bindings to the
new one (*note Remapping Commands::).  In particular, Emacs’s own source
files should not put advice on functions in Emacs.  (There are currently
a few exceptions to this convention, but we aim to correct them.)

   Special forms (*note Special Forms::) cannot be advised, however
macros can be advised, in much the same way as functions.  Of course,
this will not affect code that has already been macro-expanded, so you
need to make sure the advice is installed before the macro is expanded.

   It is possible to advise a primitive (*note What Is a Function::),
but one should typically _not_ do so, for two reasons.  Firstly, some
primitives are used by the advice mechanism, and advising them could
cause an infinite recursion.  Secondly, many primitives are called
directly from C, and such calls ignore advice; hence, one ends up in a
confusing situation where some calls (occurring from Lisp code) obey the
advice and other calls (from C code) do not.

 -- Macro: define-advice symbol (where lambda-list &optional name depth)
          &rest body
     This macro defines a piece of advice and adds it to the function
     named SYMBOL.  The advice is an anonymous function if NAME is nil
     or a function named ‘symbol@name’.  See ‘advice-add’ for
     explanation of other arguments.

 -- Function: advice-add symbol where function &optional props
     Add the advice FUNCTION to the named function SYMBOL.  WHERE and
     PROPS have the same meaning as for ‘add-function’ (*note Core
     Advising Primitives::).

 -- Function: advice-remove symbol function
     Remove the advice FUNCTION from the named function SYMBOL.
     FUNCTION can also be the ‘name’ of a piece of advice.

 -- Function: advice-member-p function symbol
     Return non-‘nil’ if the advice FUNCTION is already in the named
     function SYMBOL.  FUNCTION can also be the ‘name’ of a piece of
     advice.

 -- Function: advice-mapc function symbol
     Call FUNCTION for every piece of advice that was added to the named
     function SYMBOL.  FUNCTION is called with two arguments: the advice
     function and its properties.


File: elisp.info,  Node: Advice combinators,  Next: Porting old advice,  Prev: Advising Named Functions,  Up: Advising Functions

12.11.3 Ways to compose advice
------------------------------

Here are the different possible values for the WHERE argument of
‘add-function’ and ‘advice-add’, specifying how the advice FUNCTION and
the original function should be composed.

‘:before’
     Call FUNCTION before the old function.  Both functions receive the
     same arguments, and the return value of the composition is the
     return value of the old function.  More specifically, the
     composition of the two functions behaves like:
          (lambda (&rest r) (apply FUNCTION r) (apply OLDFUN r))
     ‘(add-function :before FUNVAR FUNCTION)’ is comparable for
     single-function hooks to ‘(add-hook 'HOOKVAR FUNCTION)’ for normal
     hooks.

‘:after’
     Call FUNCTION after the old function.  Both functions receive the
     same arguments, and the return value of the composition is the
     return value of the old function.  More specifically, the
     composition of the two functions behaves like:
          (lambda (&rest r) (prog1 (apply OLDFUN r) (apply FUNCTION r)))
     ‘(add-function :after FUNVAR FUNCTION)’ is comparable for
     single-function hooks to ‘(add-hook 'HOOKVAR FUNCTION 'append)’ for
     normal hooks.

‘:override’
     This completely replaces the old function with the new one.  The
     old function can of course be recovered if you later call
     ‘remove-function’.

‘:around’
     Call FUNCTION instead of the old function, but provide the old
     function as an extra argument to FUNCTION.  This is the most
     flexible composition.  For example, it lets you call the old
     function with different arguments, or many times, or within a
     let-binding, or you can sometimes delegate the work to the old
     function and sometimes override it completely.  More specifically,
     the composition of the two functions behaves like:
          (lambda (&rest r) (apply FUNCTION OLDFUN r))

‘:before-while’
     Call FUNCTION before the old function and don’t call the old
     function if FUNCTION returns ‘nil’.  Both functions receive the
     same arguments, and the return value of the composition is the
     return value of the old function.  More specifically, the
     composition of the two functions behaves like:
          (lambda (&rest r) (and (apply FUNCTION r) (apply OLDFUN r)))
     ‘(add-function :before-while FUNVAR FUNCTION)’ is comparable for
     single-function hooks to ‘(add-hook 'HOOKVAR FUNCTION)’ when
     HOOKVAR is run via ‘run-hook-with-args-until-failure’.

‘:before-until’
     Call FUNCTION before the old function and only call the old
     function if FUNCTION returns ‘nil’.  More specifically, the
     composition of the two functions behaves like:
          (lambda (&rest r) (or (apply FUNCTION r) (apply OLDFUN r)))
     ‘(add-function :before-until FUNVAR FUNCTION)’ is comparable for
     single-function hooks to ‘(add-hook 'HOOKVAR FUNCTION)’ when
     HOOKVAR is run via ‘run-hook-with-args-until-success’.

‘:after-while’
     Call FUNCTION after the old function and only if the old function
     returned non-‘nil’.  Both functions receive the same arguments, and
     the return value of the composition is the return value of
     FUNCTION.  More specifically, the composition of the two functions
     behaves like:
          (lambda (&rest r) (and (apply OLDFUN r) (apply FUNCTION r)))
     ‘(add-function :after-while FUNVAR FUNCTION)’ is comparable for
     single-function hooks to ‘(add-hook 'HOOKVAR FUNCTION 'append)’
     when HOOKVAR is run via ‘run-hook-with-args-until-failure’.

‘:after-until’
     Call FUNCTION after the old function and only if the old function
     returned ‘nil’.  More specifically, the composition of the two
     functions behaves like:
          (lambda (&rest r) (or  (apply OLDFUN r) (apply FUNCTION r)))
     ‘(add-function :after-until FUNVAR FUNCTION)’ is comparable for
     single-function hooks to ‘(add-hook 'HOOKVAR FUNCTION 'append)’
     when HOOKVAR is run via ‘run-hook-with-args-until-success’.

‘:filter-args’
     Call FUNCTION first and use the result (which should be a list) as
     the new arguments to pass to the old function.  More specifically,
     the composition of the two functions behaves like:
          (lambda (&rest r) (apply OLDFUN (funcall FUNCTION r)))

‘:filter-return’
     Call the old function first and pass the result to FUNCTION.  More
     specifically, the composition of the two functions behaves like:
          (lambda (&rest r) (funcall FUNCTION (apply OLDFUN r)))


File: elisp.info,  Node: Porting old advice,  Prev: Advice combinators,  Up: Advising Functions

12.11.4 Adapting code using the old defadvice
---------------------------------------------

A lot of code uses the old ‘defadvice’ mechanism, which is largely made
obsolete by the new ‘advice-add’, whose implementation and semantics is
significantly simpler.

   An old piece of advice such as:

     (defadvice previous-line (before next-line-at-end
                                      (&optional arg try-vscroll))
       "Insert an empty line when moving up from the top line."
       (if (and next-line-add-newlines (= arg 1)
                (save-excursion (beginning-of-line) (bobp)))
           (progn
             (beginning-of-line)
             (newline))))

   could be translated in the new advice mechanism into a plain
function:

     (defun previous-line--next-line-at-end (&optional arg try-vscroll)
       "Insert an empty line when moving up from the top line."
       (if (and next-line-add-newlines (= arg 1)
                (save-excursion (beginning-of-line) (bobp)))
           (progn
             (beginning-of-line)
             (newline))))

   Obviously, this does not actually modify ‘previous-line’.  For that
the old advice needed:
     (ad-activate 'previous-line)
   whereas the new advice mechanism needs:
     (advice-add 'previous-line :before #'previous-line--next-line-at-end)

   Note that ‘ad-activate’ had a global effect: it activated all pieces
of advice enabled for that specified function.  If you wanted to only
activate or deactivate a particular piece, you needed to _enable_ or
_disable_ it with ‘ad-enable-advice’ and ‘ad-disable-advice’.  The new
mechanism does away with this distinction.

   Around advice such as:

     (defadvice foo (around foo-around)
       "Ignore case in `foo'."
       (let ((case-fold-search t))
         ad-do-it))
     (ad-activate 'foo)

   could translate into:

     (defun foo--foo-around (orig-fun &rest args)
       "Ignore case in `foo'."
       (let ((case-fold-search t))
         (apply orig-fun args)))
     (advice-add 'foo :around #'foo--foo-around)

   Regarding the advice’s _class_, note that the new ‘:before’ is not
quite equivalent to the old ‘before’, because in the old advice you
could modify the function’s arguments (e.g., with ‘ad-set-arg’), and
that would affect the argument values seen by the original function,
whereas in the new ‘:before’, modifying an argument via ‘setq’ in the
advice has no effect on the arguments seen by the original function.
When porting ‘before’ advice which relied on this behavior, you’ll need
to turn it into new ‘:around’ or ‘:filter-args’ advice instead.

   Similarly old ‘after’ advice could modify the returned value by
changing ‘ad-return-value’, whereas new ‘:after’ advice cannot, so when
porting such old ‘after’ advice, you’ll need to turn it into new
‘:around’ or ‘:filter-return’ advice instead.


File: elisp.info,  Node: Obsolete Functions,  Next: Inline Functions,  Prev: Advising Functions,  Up: Functions

12.12 Declaring Functions Obsolete
==================================

You can mark a named function as “obsolete”, meaning that it may be
removed at some point in the future.  This causes Emacs to warn that the
function is obsolete whenever it byte-compiles code containing that
function, and whenever it displays the documentation for that function.
In all other respects, an obsolete function behaves like any other
function.

   The easiest way to mark a function as obsolete is to put a ‘(declare
(obsolete ...))’ form in the function’s ‘defun’ definition.  *Note
Declare Form::.  Alternatively, you can use the ‘make-obsolete’
function, described below.

   A macro (*note Macros::) can also be marked obsolete with
‘make-obsolete’; this has the same effects as for a function.  An alias
for a function or macro can also be marked as obsolete; this makes the
alias itself obsolete, not the function or macro which it resolves to.

 -- Function: make-obsolete obsolete-name current-name &optional when
     This function marks OBSOLETE-NAME as obsolete.  OBSOLETE-NAME
     should be a symbol naming a function or macro, or an alias for a
     function or macro.

     If CURRENT-NAME is a symbol, the warning message says to use
     CURRENT-NAME instead of OBSOLETE-NAME.  CURRENT-NAME does not need
     to be an alias for OBSOLETE-NAME; it can be a different function
     with similar functionality.  CURRENT-NAME can also be a string,
     which serves as the warning message.  The message should begin in
     lower case, and end with a period.  It can also be ‘nil’, in which
     case the warning message provides no additional details.

     If provided, WHEN should be a string indicating when the function
     was first made obsolete—for example, a date or a release number.

 -- Macro: define-obsolete-function-alias obsolete-name current-name
          &optional when doc
     This convenience macro marks the function OBSOLETE-NAME obsolete
     and also defines it as an alias for the function CURRENT-NAME.  It
     is equivalent to the following:

          (defalias OBSOLETE-NAME CURRENT-NAME DOC)
          (make-obsolete OBSOLETE-NAME CURRENT-NAME WHEN)

   In addition, you can mark a certain a particular calling convention
for a function as obsolete:

 -- Function: set-advertised-calling-convention function signature when
     This function specifies the argument list SIGNATURE as the correct
     way to call FUNCTION.  This causes the Emacs byte compiler to issue
     a warning whenever it comes across an Emacs Lisp program that calls
     FUNCTION any other way (however, it will still allow the code to be
     byte compiled).  WHEN should be a string indicating when the
     variable was first made obsolete (usually a version number string).

     For instance, in old versions of Emacs the ‘sit-for’ function
     accepted three arguments, like this

            (sit-for seconds milliseconds nodisp)

     However, calling ‘sit-for’ this way is considered obsolete (*note
     Waiting::).  The old calling convention is deprecated like this:

          (set-advertised-calling-convention
            'sit-for '(seconds &optional nodisp) "22.1")


File: elisp.info,  Node: Inline Functions,  Next: Declare Form,  Prev: Obsolete Functions,  Up: Functions

12.13 Inline Functions
======================

An “inline function” is a function that works just like an ordinary
function, except for one thing: when you byte-compile a call to the
function (*note Byte Compilation::), the function’s definition is
expanded into the caller.  To define an inline function, use ‘defsubst’
instead of ‘defun’.

 -- Macro: defsubst name args [doc] [declare] [interactive] body...
     This macro defines an inline function.  Its syntax is exactly the
     same as ‘defun’ (*note Defining Functions::).

   Making a function inline often makes its function calls run faster.
But it also has disadvantages.  For one thing, it reduces flexibility;
if you change the definition of the function, calls already inlined
still use the old definition until you recompile them.

   Another disadvantage is that making a large function inline can
increase the size of compiled code both in files and in memory.  Since
the speed advantage of inline functions is greatest for small functions,
you generally should not make large functions inline.

   Also, inline functions do not behave well with respect to debugging,
tracing, and advising (*note Advising Functions::).  Since ease of
debugging and the flexibility of redefining functions are important
features of Emacs, you should not make a function inline, even if it’s
small, unless its speed is really crucial, and you’ve timed the code to
verify that using ‘defun’ actually has performance problems.

   After an inline function is defined, its inline expansion can be
performed later on in the same file, just like macros.

   It’s possible to use ‘defsubst’ to define a macro to expand into the
same code that an inline function would execute (*note Macros::).  But
the macro would be limited to direct use in expressions—a macro cannot
be called with ‘apply’, ‘mapcar’ and so on.  Also, it takes some work to
convert an ordinary function into a macro.  To convert it into an inline
function is easy; just replace ‘defun’ with ‘defsubst’.  Since each
argument of an inline function is evaluated exactly once, you needn’t
worry about how many times the body uses the arguments, as you do for
macros.

   As an alternative to ‘defsubst’, you can use ‘define-inline’ to
define functions via their exhaustive compiler macro.  *Note
define-inline: Defining Functions.


File: elisp.info,  Node: Declare Form,  Next: Declaring Functions,  Prev: Inline Functions,  Up: Functions

12.14 The ‘declare’ Form
========================

‘declare’ is a special macro which can be used to add meta properties to
a function or macro: for example, marking it as obsolete, or giving its
forms a special <TAB> indentation convention in Emacs Lisp mode.

 -- Macro: declare specs...
     This macro ignores its arguments and evaluates to ‘nil’; it has no
     run-time effect.  However, when a ‘declare’ form occurs in the
     DECLARE argument of a ‘defun’ or ‘defsubst’ function definition
     (*note Defining Functions::) or a ‘defmacro’ macro definition
     (*note Defining Macros::), it appends the properties specified by
     SPECS to the function or macro.  This work is specially performed
     by ‘defun’, ‘defsubst’, and ‘defmacro’.

     Each element in SPECS should have the form ‘(PROPERTY ARGS...)’,
     which should not be quoted.  These have the following effects:

     ‘(advertised-calling-convention SIGNATURE WHEN)’
          This acts like a call to ‘set-advertised-calling-convention’
          (*note Obsolete Functions::); SIGNATURE specifies the correct
          argument list for calling the function or macro, and WHEN
          should be a string indicating when the old argument list was
          first made obsolete.

     ‘(debug EDEBUG-FORM-SPEC)’
          This is valid for macros only.  When stepping through the
          macro with Edebug, use EDEBUG-FORM-SPEC.  *Note Instrumenting
          Macro Calls::.

     ‘(doc-string N)’
          This is used when defining a function or macro which itself
          will be used to define entities like functions, macros, or
          variables.  It indicates that the Nth argument, if any, should
          be considered as a documentation string.

     ‘(indent INDENT-SPEC)’
          Indent calls to this function or macro according to
          INDENT-SPEC.  This is typically used for macros, though it
          works for functions too.  *Note Indenting Macros::.

     ‘(interactive-only VALUE)’
          Set the function’s ‘interactive-only’ property to VALUE.
          *Note The interactive-only property::.

     ‘(obsolete CURRENT-NAME WHEN)’
          Mark the function or macro as obsolete, similar to a call to
          ‘make-obsolete’ (*note Obsolete Functions::).  CURRENT-NAME
          should be a symbol (in which case the warning message says to
          use that instead), a string (specifying the warning message),
          or ‘nil’ (in which case the warning message gives no extra
          details).  WHEN should be a string indicating when the
          function or macro was first made obsolete.

     ‘(compiler-macro EXPANDER)’
          This can only be used for functions, and tells the compiler to
          use EXPANDER as an optimization function.  When encountering a
          call to the function, of the form ‘(FUNCTION ARGS...)’, the
          macro expander will call EXPANDER with that form as well as
          with ARGS..., and EXPANDER can either return a new expression
          to use instead of the function call, or it can return just the
          form unchanged, to indicate that the function call should be
          left alone.  EXPANDER can be a symbol, or it can be a form
          ‘(lambda (ARG) BODY)’ in which case ARG will hold the original
          function call expression, and the (unevaluated) arguments to
          the function can be accessed using the function’s formal
          arguments.

     ‘(gv-expander EXPANDER)’
          Declare EXPANDER to be the function to handle calls to the
          macro (or function) as a generalized variable, similarly to
          ‘gv-define-expander’.  EXPANDER can be a symbol or it can be
          of the form ‘(lambda (ARG) BODY)’ in which case that function
          will additionally have access to the macro (or function)’s
          arguments.

     ‘(gv-setter SETTER)’
          Declare SETTER to be the function to handle calls to the macro
          (or function) as a generalized variable.  SETTER can be a
          symbol in which case it will be passed to
          ‘gv-define-simple-setter’, or it can be of the form ‘(lambda
          (ARG) BODY)’ in which case that function will additionally
          have access to the macro (or function)’s arguments and it will
          passed to ‘gv-define-setter’.


File: elisp.info,  Node: Declaring Functions,  Next: Function Safety,  Prev: Declare Form,  Up: Functions

12.15 Telling the Compiler that a Function is Defined
=====================================================

Byte-compiling a file often produces warnings about functions that the
compiler doesn’t know about (*note Compiler Errors::).  Sometimes this
indicates a real problem, but usually the functions in question are
defined in other files which would be loaded if that code is run.  For
example, byte-compiling ‘fortran.el’ used to warn:

     In end of data:
     fortran.el:2152:1:Warning: the function ‘gud-find-c-expr’ is not
         known to be defined.

   In fact, ‘gud-find-c-expr’ is only used in the function that Fortran
mode uses for the local value of ‘gud-find-expr-function’, which is a
callback from GUD; if it is called, the GUD functions will be loaded.
When you know that such a warning does not indicate a real problem, it
is good to suppress the warning.  That makes new warnings which might
mean real problems more visible.  You do that with ‘declare-function’.

   All you need to do is add a ‘declare-function’ statement before the
first use of the function in question:

     (declare-function gud-find-c-expr "gud.el" nil)

   This says that ‘gud-find-c-expr’ is defined in ‘gud.el’ (the ‘.el’
can be omitted).  The compiler takes for granted that that file really
defines the function, and does not check.

   The optional third argument specifies the argument list of
‘gud-find-c-expr’.  In this case, it takes no arguments (‘nil’ is
different from not specifying a value).  In other cases, this might be
something like ‘(file &optional overwrite)’.  You don’t have to specify
the argument list, but if you do the byte compiler can check that the
calls match the declaration.

 -- Macro: declare-function function file &optional arglist fileonly
     Tell the byte compiler to assume that FUNCTION is defined, with
     arguments ARGLIST, and that the definition should come from the
     file FILE.  FILEONLY non-‘nil’ means only check that FILE exists,
     not that it actually defines FUNCTION.

   To verify that these functions really are declared where
‘declare-function’ says they are, use ‘check-declare-file’ to check all
‘declare-function’ calls in one source file, or use
‘check-declare-directory’ check all the files in and under a certain
directory.

   These commands find the file that ought to contain a function’s
definition using ‘locate-library’; if that finds no file, they expand
the definition file name relative to the directory of the file that
contains the ‘declare-function’ call.

   You can also say that a function is a primitive by specifying a file
name ending in ‘.c’ or ‘.m’.  This is useful only when you call a
primitive that is defined only on certain systems.  Most primitives are
always defined, so they will never give you a warning.

   Sometimes a file will optionally use functions from an external
package.  If you prefix the filename in the ‘declare-function’ statement
with ‘ext:’, then it will be checked if it is found, otherwise skipped
without error.

   There are some function definitions that ‘check-declare’ does not
understand (e.g., ‘defstruct’ and some other macros).  In such cases,
you can pass a non-‘nil’ FILEONLY argument to ‘declare-function’,
meaning to only check that the file exists, not that it actually defines
the function.  Note that to do this without having to specify an
argument list, you should set the ARGLIST argument to ‘t’ (because ‘nil’
means an empty argument list, as opposed to an unspecified one).


File: elisp.info,  Node: Function Safety,  Next: Related Topics,  Prev: Declaring Functions,  Up: Functions

12.16 Determining whether a Function is Safe to Call
====================================================

Some major modes, such as SES, call functions that are stored in user
files.  (*note (ses)Top::, for more information on SES.)  User files
sometimes have poor pedigrees—you can get a spreadsheet from someone
you’ve just met, or you can get one through email from someone you’ve
never met.  So it is risky to call a function whose source code is
stored in a user file until you have determined that it is safe.

 -- Function: unsafep form &optional unsafep-vars
     Returns ‘nil’ if FORM is a “safe” Lisp expression, or returns a
     list that describes why it might be unsafe.  The argument
     UNSAFEP-VARS is a list of symbols known to have temporary bindings
     at this point; it is mainly used for internal recursive calls.  The
     current buffer is an implicit argument, which provides a list of
     buffer-local bindings.

   Being quick and simple, ‘unsafep’ does a very light analysis and
rejects many Lisp expressions that are actually safe.  There are no
known cases where ‘unsafep’ returns ‘nil’ for an unsafe expression.
However, a safe Lisp expression can return a string with a ‘display’
property, containing an associated Lisp expression to be executed after
the string is inserted into a buffer.  This associated expression can be
a virus.  In order to be safe, you must delete properties from all
strings calculated by user code before inserting them into buffers.


File: elisp.info,  Node: Related Topics,  Prev: Function Safety,  Up: Functions

12.17 Other Topics Related to Functions
=======================================

Here is a table of several functions that do things related to function
calling and function definitions.  They are documented elsewhere, but we
provide cross references here.

‘apply’
     See *note Calling Functions::.

‘autoload’
     See *note Autoload::.

‘call-interactively’
     See *note Interactive Call::.

‘called-interactively-p’
     See *note Distinguish Interactive::.

‘commandp’
     See *note Interactive Call::.

‘documentation’
     See *note Accessing Documentation::.

‘eval’
     See *note Eval::.

‘funcall’
     See *note Calling Functions::.

‘function’
     See *note Anonymous Functions::.

‘ignore’
     See *note Calling Functions::.

‘indirect-function’
     See *note Function Indirection::.

‘interactive’
     See *note Using Interactive::.

‘interactive-p’
     See *note Distinguish Interactive::.

‘mapatoms’
     See *note Creating Symbols::.

‘mapcar’
     See *note Mapping Functions::.

‘map-char-table’
     See *note Char-Tables::.

‘mapconcat’
     See *note Mapping Functions::.

‘undefined’
     See *note Functions for Key Lookup::.


File: elisp.info,  Node: Macros,  Next: Customization,  Prev: Functions,  Up: Top

13 Macros
*********

“Macros” enable you to define new control constructs and other language
features.  A macro is defined much like a function, but instead of
telling how to compute a value, it tells how to compute another Lisp
expression which will in turn compute the value.  We call this
expression the “expansion” of the macro.

   Macros can do this because they operate on the unevaluated
expressions for the arguments, not on the argument values as functions
do.  They can therefore construct an expansion containing these argument
expressions or parts of them.

   If you are using a macro to do something an ordinary function could
do, just for the sake of speed, consider using an inline function
instead.  *Note Inline Functions::.

* Menu:

* Simple Macro::            A basic example.
* Expansion::               How, when and why macros are expanded.
* Compiling Macros::        How macros are expanded by the compiler.
* Defining Macros::         How to write a macro definition.
* Problems with Macros::    Don’t evaluate the macro arguments too many times.
                              Don’t hide the user’s variables.
* Indenting Macros::        Specifying how to indent macro calls.


File: elisp.info,  Node: Simple Macro,  Next: Expansion,  Up: Macros

13.1 A Simple Example of a Macro
================================

Suppose we would like to define a Lisp construct to increment a variable
value, much like the ‘++’ operator in C.  We would like to write ‘(inc
x)’ and have the effect of ‘(setq x (1+ x))’.  Here’s a macro definition
that does the job:

     (defmacro inc (var)
        (list 'setq var (list '1+ var)))

   When this is called with ‘(inc x)’, the argument VAR is the symbol
‘x’—_not_ the _value_ of ‘x’, as it would be in a function.  The body of
the macro uses this to construct the expansion, which is ‘(setq x (1+
x))’.  Once the macro definition returns this expansion, Lisp proceeds
to evaluate it, thus incrementing ‘x’.

 -- Function: macrop object
     This predicate tests whether its argument is a macro, and returns
     ‘t’ if so, ‘nil’ otherwise.


File: elisp.info,  Node: Expansion,  Next: Compiling Macros,  Prev: Simple Macro,  Up: Macros

13.2 Expansion of a Macro Call
==============================

A macro call looks just like a function call in that it is a list which
starts with the name of the macro.  The rest of the elements of the list
are the arguments of the macro.

   Evaluation of the macro call begins like evaluation of a function
call except for one crucial difference: the macro arguments are the
actual expressions appearing in the macro call.  They are not evaluated
before they are given to the macro definition.  By contrast, the
arguments of a function are results of evaluating the elements of the
function call list.

   Having obtained the arguments, Lisp invokes the macro definition just
as a function is invoked.  The argument variables of the macro are bound
to the argument values from the macro call, or to a list of them in the
case of a ‘&rest’ argument.  And the macro body executes and returns its
value just as a function body does.

   The second crucial difference between macros and functions is that
the value returned by the macro body is an alternate Lisp expression,
also known as the “expansion” of the macro.  The Lisp interpreter
proceeds to evaluate the expansion as soon as it comes back from the
macro.

   Since the expansion is evaluated in the normal manner, it may contain
calls to other macros.  It may even be a call to the same macro, though
this is unusual.

   Note that Emacs tries to expand macros when loading an uncompiled
Lisp file.  This is not always possible, but if it is, it speeds up
subsequent execution.  *Note How Programs Do Loading::.

   You can see the expansion of a given macro call by calling
‘macroexpand’.

 -- Function: macroexpand form &optional environment
     This function expands FORM, if it is a macro call.  If the result
     is another macro call, it is expanded in turn, until something
     which is not a macro call results.  That is the value returned by
     ‘macroexpand’.  If FORM is not a macro call to begin with, it is
     returned as given.

     Note that ‘macroexpand’ does not look at the subexpressions of FORM
     (although some macro definitions may do so).  Even if they are
     macro calls themselves, ‘macroexpand’ does not expand them.

     The function ‘macroexpand’ does not expand calls to inline
     functions.  Normally there is no need for that, since a call to an
     inline function is no harder to understand than a call to an
     ordinary function.

     If ENVIRONMENT is provided, it specifies an alist of macro
     definitions that shadow the currently defined macros.  Byte
     compilation uses this feature.

          (defmacro inc (var)
              (list 'setq var (list '1+ var)))

          (macroexpand '(inc r))
               ⇒ (setq r (1+ r))

          (defmacro inc2 (var1 var2)
              (list 'progn (list 'inc var1) (list 'inc var2)))

          (macroexpand '(inc2 r s))
               ⇒ (progn (inc r) (inc s))  ; ‘inc’ not expanded here.

 -- Function: macroexpand-all form &optional environment
     ‘macroexpand-all’ expands macros like ‘macroexpand’, but will look
     for and expand all macros in FORM, not just at the top-level.  If
     no macros are expanded, the return value is ‘eq’ to FORM.

     Repeating the example used for ‘macroexpand’ above with
     ‘macroexpand-all’, we see that ‘macroexpand-all’ _does_ expand the
     embedded calls to ‘inc’:

          (macroexpand-all '(inc2 r s))
               ⇒ (progn (setq r (1+ r)) (setq s (1+ s)))

 -- Function: macroexpand-1 form &optional environment
     This function expands macros like ‘macroexpand’, but it only
     performs one step of the expansion: if the result is another macro
     call, ‘macroexpand-1’ will not expand it.


File: elisp.info,  Node: Compiling Macros,  Next: Defining Macros,  Prev: Expansion,  Up: Macros

13.3 Macros and Byte Compilation
================================

You might ask why we take the trouble to compute an expansion for a
macro and then evaluate the expansion.  Why not have the macro body
produce the desired results directly?  The reason has to do with
compilation.

   When a macro call appears in a Lisp program being compiled, the Lisp
compiler calls the macro definition just as the interpreter would, and
receives an expansion.  But instead of evaluating this expansion, it
compiles the expansion as if it had appeared directly in the program.
As a result, the compiled code produces the value and side effects
intended for the macro, but executes at full compiled speed.  This would
not work if the macro body computed the value and side effects
itself—they would be computed at compile time, which is not useful.

   In order for compilation of macro calls to work, the macros must
already be defined in Lisp when the calls to them are compiled.  The
compiler has a special feature to help you do this: if a file being
compiled contains a ‘defmacro’ form, the macro is defined temporarily
for the rest of the compilation of that file.

   Byte-compiling a file also executes any ‘require’ calls at top-level
in the file, so you can ensure that necessary macro definitions are
available during compilation by requiring the files that define them
(*note Named Features::).  To avoid loading the macro definition files
when someone _runs_ the compiled program, write ‘eval-when-compile’
around the ‘require’ calls (*note Eval During Compile::).


File: elisp.info,  Node: Defining Macros,  Next: Problems with Macros,  Prev: Compiling Macros,  Up: Macros

13.4 Defining Macros
====================

A Lisp macro object is a list whose CAR is ‘macro’, and whose CDR is a
function.  Expansion of the macro works by applying the function (with
‘apply’) to the list of _unevaluated_ arguments from the macro call.

   It is possible to use an anonymous Lisp macro just like an anonymous
function, but this is never done, because it does not make sense to pass
an anonymous macro to functionals such as ‘mapcar’.  In practice, all
Lisp macros have names, and they are almost always defined with the
‘defmacro’ macro.

 -- Macro: defmacro name args [doc] [declare] body...
     ‘defmacro’ defines the symbol NAME (which should not be quoted) as
     a macro that looks like this:

          (macro lambda ARGS . BODY)

     (Note that the CDR of this list is a lambda expression.)  This
     macro object is stored in the function cell of NAME.  The meaning
     of ARGS is the same as in a function, and the keywords ‘&rest’ and
     ‘&optional’ may be used (*note Argument List::).  Neither NAME nor
     ARGS should be quoted.  The return value of ‘defmacro’ is
     undefined.

     DOC, if present, should be a string specifying the macro’s
     documentation string.  DECLARE, if present, should be a ‘declare’
     form specifying metadata for the macro (*note Declare Form::).
     Note that macros cannot have interactive declarations, since they
     cannot be called interactively.

   Macros often need to construct large list structures from a mixture
of constants and nonconstant parts.  To make this easier, use the ‘`’
syntax (*note Backquote::).  For example:

          (defmacro t-becomes-nil (variable)
            `(if (eq ,variable t)
                 (setq ,variable nil)))

          (t-becomes-nil foo)
               ≡ (if (eq foo t) (setq foo nil))


File: elisp.info,  Node: Problems with Macros,  Next: Indenting Macros,  Prev: Defining Macros,  Up: Macros

13.5 Common Problems Using Macros
=================================

Macro expansion can have counterintuitive consequences.  This section
describes some important consequences that can lead to trouble, and
rules to follow to avoid trouble.

* Menu:

* Wrong Time::             Do the work in the expansion, not in the macro.
* Argument Evaluation::    The expansion should evaluate each macro arg once.
* Surprising Local Vars::  Local variable bindings in the expansion
                              require special care.
* Eval During Expansion::  Don’t evaluate them; put them in the expansion.
* Repeated Expansion::     Avoid depending on how many times expansion is done.


File: elisp.info,  Node: Wrong Time,  Next: Argument Evaluation,  Up: Problems with Macros

13.5.1 Wrong Time
-----------------

The most common problem in writing macros is doing some of the real work
prematurely—while expanding the macro, rather than in the expansion
itself.  For instance, one real package had this macro definition:

     (defmacro my-set-buffer-multibyte (arg)
       (if (fboundp 'set-buffer-multibyte)
           (set-buffer-multibyte arg)))

   With this erroneous macro definition, the program worked fine when
interpreted but failed when compiled.  This macro definition called
‘set-buffer-multibyte’ during compilation, which was wrong, and then did
nothing when the compiled package was run.  The definition that the
programmer really wanted was this:

     (defmacro my-set-buffer-multibyte (arg)
       (if (fboundp 'set-buffer-multibyte)
           `(set-buffer-multibyte ,arg)))

This macro expands, if appropriate, into a call to
‘set-buffer-multibyte’ that will be executed when the compiled program
is actually run.


File: elisp.info,  Node: Argument Evaluation,  Next: Surprising Local Vars,  Prev: Wrong Time,  Up: Problems with Macros

13.5.2 Evaluating Macro Arguments Repeatedly
--------------------------------------------

When defining a macro you must pay attention to the number of times the
arguments will be evaluated when the expansion is executed.  The
following macro (used to facilitate iteration) illustrates the problem.
This macro allows us to write a for-loop construct.

     (defmacro for (var from init to final do &rest body)
       "Execute a simple \"for\" loop.
     For example, (for i from 1 to 10 do (print i))."
       (list 'let (list (list var init))
             (cons 'while
                   (cons (list '<= var final)
                         (append body (list (list 'inc var)))))))

     (for i from 1 to 3 do
        (setq square (* i i))
        (princ (format "\n%d %d" i square)))
     ↦
     (let ((i 1))
       (while (<= i 3)
         (setq square (* i i))
         (princ (format "\n%d %d" i square))
         (inc i)))

          ⊣1       1
          ⊣2       4
          ⊣3       9
     ⇒ nil

The arguments ‘from’, ‘to’, and ‘do’ in this macro are syntactic sugar;
they are entirely ignored.  The idea is that you will write noise words
(such as ‘from’, ‘to’, and ‘do’) in those positions in the macro call.

   Here’s an equivalent definition simplified through use of backquote:

     (defmacro for (var from init to final do &rest body)
       "Execute a simple \"for\" loop.
     For example, (for i from 1 to 10 do (print i))."
       `(let ((,var ,init))
          (while (<= ,var ,final)
            ,@body
            (inc ,var))))

   Both forms of this definition (with backquote and without) suffer
from the defect that FINAL is evaluated on every iteration.  If FINAL is
a constant, this is not a problem.  If it is a more complex form, say
‘(long-complex-calculation x)’, this can slow down the execution
significantly.  If FINAL has side effects, executing it more than once
is probably incorrect.

   A well-designed macro definition takes steps to avoid this problem by
producing an expansion that evaluates the argument expressions exactly
once unless repeated evaluation is part of the intended purpose of the
macro.  Here is a correct expansion for the ‘for’ macro:

     (let ((i 1)
           (max 3))
       (while (<= i max)
         (setq square (* i i))
         (princ (format "%d      %d" i square))
         (inc i)))

   Here is a macro definition that creates this expansion:

     (defmacro for (var from init to final do &rest body)
       "Execute a simple for loop: (for i from 1 to 10 do (print i))."
       `(let ((,var ,init)
              (max ,final))
          (while (<= ,var max)
            ,@body
            (inc ,var))))

   Unfortunately, this fix introduces another problem, described in the
following section.


File: elisp.info,  Node: Surprising Local Vars,  Next: Eval During Expansion,  Prev: Argument Evaluation,  Up: Problems with Macros

13.5.3 Local Variables in Macro Expansions
------------------------------------------

In the previous section, the definition of ‘for’ was fixed as follows to
make the expansion evaluate the macro arguments the proper number of
times:

     (defmacro for (var from init to final do &rest body)
       "Execute a simple for loop: (for i from 1 to 10 do (print i))."
       `(let ((,var ,init)
              (max ,final))
          (while (<= ,var max)
            ,@body
            (inc ,var))))

   The new definition of ‘for’ has a new problem: it introduces a local
variable named ‘max’ which the user does not expect.  This causes
trouble in examples such as the following:

     (let ((max 0))
       (for x from 0 to 10 do
         (let ((this (frob x)))
           (if (< max this)
               (setq max this)))))

The references to ‘max’ inside the body of the ‘for’, which are supposed
to refer to the user’s binding of ‘max’, really access the binding made
by ‘for’.

   The way to correct this is to use an uninterned symbol instead of
‘max’ (*note Creating Symbols::).  The uninterned symbol can be bound
and referred to just like any other symbol, but since it is created by
‘for’, we know that it cannot already appear in the user’s program.
Since it is not interned, there is no way the user can put it into the
program later.  It will never appear anywhere except where put by ‘for’.
Here is a definition of ‘for’ that works this way:

     (defmacro for (var from init to final do &rest body)
       "Execute a simple for loop: (for i from 1 to 10 do (print i))."
       (let ((tempvar (make-symbol "max")))
         `(let ((,var ,init)
                (,tempvar ,final))
            (while (<= ,var ,tempvar)
              ,@body
              (inc ,var)))))

This creates an uninterned symbol named ‘max’ and puts it in the
expansion instead of the usual interned symbol ‘max’ that appears in
expressions ordinarily.


File: elisp.info,  Node: Eval During Expansion,  Next: Repeated Expansion,  Prev: Surprising Local Vars,  Up: Problems with Macros

13.5.4 Evaluating Macro Arguments in Expansion
----------------------------------------------

Another problem can happen if the macro definition itself evaluates any
of the macro argument expressions, such as by calling ‘eval’ (*note
Eval::).  If the argument is supposed to refer to the user’s variables,
you may have trouble if the user happens to use a variable with the same
name as one of the macro arguments.  Inside the macro body, the macro
argument binding is the most local binding of this variable, so any
references inside the form being evaluated do refer to it.  Here is an
example:

     (defmacro foo (a)
       (list 'setq (eval a) t))
     (setq x 'b)
     (foo x) ↦ (setq b t)
          ⇒ t                  ; and ‘b’ has been set.
     ;; but
     (setq a 'c)
     (foo a) ↦ (setq a t)
          ⇒ t                  ; but this set ‘a’, not ‘c’.


   It makes a difference whether the user’s variable is named ‘a’ or
‘x’, because ‘a’ conflicts with the macro argument variable ‘a’.

   Another problem with calling ‘eval’ in a macro definition is that it
probably won’t do what you intend in a compiled program.  The byte
compiler runs macro definitions while compiling the program, when the
program’s own computations (which you might have wished to access with
‘eval’) don’t occur and its local variable bindings don’t exist.

   To avoid these problems, *don’t evaluate an argument expression while
computing the macro expansion*.  Instead, substitute the expression into
the macro expansion, so that its value will be computed as part of
executing the expansion.  This is how the other examples in this chapter
work.


File: elisp.info,  Node: Repeated Expansion,  Prev: Eval During Expansion,  Up: Problems with Macros

13.5.5 How Many Times is the Macro Expanded?
--------------------------------------------

Occasionally problems result from the fact that a macro call is expanded
each time it is evaluated in an interpreted function, but is expanded
only once (during compilation) for a compiled function.  If the macro
definition has side effects, they will work differently depending on how
many times the macro is expanded.

   Therefore, you should avoid side effects in computation of the macro
expansion, unless you really know what you are doing.

   One special kind of side effect can’t be avoided: constructing Lisp
objects.  Almost all macro expansions include constructed lists; that is
the whole point of most macros.  This is usually safe; there is just one
case where you must be careful: when the object you construct is part of
a quoted constant in the macro expansion.

   If the macro is expanded just once, in compilation, then the object
is constructed just once, during compilation.  But in interpreted
execution, the macro is expanded each time the macro call runs, and this
means a new object is constructed each time.

   In most clean Lisp code, this difference won’t matter.  It can matter
only if you perform side-effects on the objects constructed by the macro
definition.  Thus, to avoid trouble, *avoid side effects on objects
constructed by macro definitions*.  Here is an example of how such side
effects can get you into trouble:

     (defmacro empty-object ()
       (list 'quote (cons nil nil)))

     (defun initialize (condition)
       (let ((object (empty-object)))
         (if condition
             (setcar object condition))
         object))

If ‘initialize’ is interpreted, a new list ‘(nil)’ is constructed each
time ‘initialize’ is called.  Thus, no side effect survives between
calls.  If ‘initialize’ is compiled, then the macro ‘empty-object’ is
expanded during compilation, producing a single constant ‘(nil)’ that is
reused and altered each time ‘initialize’ is called.

   One way to avoid pathological cases like this is to think of
‘empty-object’ as a funny kind of constant, not as a memory allocation
construct.  You wouldn’t use ‘setcar’ on a constant such as ‘'(nil)’, so
naturally you won’t use it on ‘(empty-object)’ either.


File: elisp.info,  Node: Indenting Macros,  Prev: Problems with Macros,  Up: Macros

13.6 Indenting Macros
=====================

Within a macro definition, you can use the ‘declare’ form (*note
Defining Macros::) to specify how <TAB> should indent calls to the
macro.  An indentation specification is written like this:

     (declare (indent INDENT-SPEC))

This results in the ‘lisp-indent-function’ property being set on the
macro name.

Here are the possibilities for INDENT-SPEC:

‘nil’
     This is the same as no property—use the standard indentation
     pattern.
‘defun’
     Handle this function like a ‘def’ construct: treat the second line
     as the start of a “body”.
an integer, NUMBER
     The first NUMBER arguments of the function are “distinguished”
     arguments; the rest are considered the body of the expression.  A
     line in the expression is indented according to whether the first
     argument on it is distinguished or not.  If the argument is part of
     the body, the line is indented ‘lisp-body-indent’ more columns than
     the open-parenthesis starting the containing expression.  If the
     argument is distinguished and is either the first or second
     argument, it is indented _twice_ that many extra columns.  If the
     argument is distinguished and not the first or second argument, the
     line uses the standard pattern.
a symbol, SYMBOL
     SYMBOL should be a function name; that function is called to
     calculate the indentation of a line within this expression.  The
     function receives two arguments:

     POS
          The position at which the line being indented begins.
     STATE
          The value returned by ‘parse-partial-sexp’ (a Lisp primitive
          for indentation and nesting computation) when it parses up to
          the beginning of this line.

     It should return either a number, which is the number of columns of
     indentation for that line, or a list whose car is such a number.
     The difference between returning a number and returning a list is
     that a number says that all following lines at the same nesting
     level should be indented just like this one; a list says that
     following lines might call for different indentations.  This makes
     a difference when the indentation is being computed by ‘C-M-q’; if
     the value is a number, ‘C-M-q’ need not recalculate indentation for
     the following lines until the end of the list.


File: elisp.info,  Node: Customization,  Next: Loading,  Prev: Macros,  Up: Top

14 Customization Settings
*************************

Users of Emacs can customize variables and faces without writing Lisp
code, by using the Customize interface.  *Note (emacs)Easy
Customization::.  This chapter describes how to define “customization
items” that users can interact with through the Customize interface.

   Customization items include customizable variables, which are defined
with the ‘defcustom’ macro (*note Variable Definitions::); customizable
faces, which are defined with ‘defface’ (described separately in *note
Defining Faces::); and “customization groups”, defined with ‘defgroup’
(*note Group Definitions::), which act as containers for groups of
related customization items.

* Menu:

* Common Keywords::         Common keyword arguments for all kinds of
                             customization declarations.
* Group Definitions::       Writing customization group definitions.
* Variable Definitions::    Declaring user options.
* Customization Types::     Specifying the type of a user option.
* Applying Customizations:: Functions to apply customization settings.
* Custom Themes::           Writing Custom themes.


File: elisp.info,  Node: Common Keywords,  Next: Group Definitions,  Up: Customization

14.1 Common Item Keywords
=========================

The customization declarations that we will describe in the next few
sections—‘defcustom’, ‘defgroup’, etc.—all accept keyword arguments
(*note Constant Variables::) for specifying various information.  This
section describes keywords that apply to all types of customization
declarations.

   All of these keywords, except ‘:tag’, can be used more than once in a
given item.  Each use of the keyword has an independent effect.  The
keyword ‘:tag’ is an exception because any given item can only display
one name.

‘:tag LABEL’
     Use LABEL, a string, instead of the item’s name, to label the item
     in customization menus and buffers.  *Don’t use a tag which is
     substantially different from the item’s real name; that would cause
     confusion.*

‘:group GROUP’
     Put this customization item in group GROUP.  If this keyword is
     missing from a customization item, it’ll be placed in the same
     group that was last defined (in the current file).

     When you use ‘:group’ in a ‘defgroup’, it makes the new group a
     subgroup of GROUP.

     If you use this keyword more than once, you can put a single item
     into more than one group.  Displaying any of those groups will show
     this item.  Please don’t overdo this, since the result would be
     annoying.

‘:link LINK-DATA’
     Include an external link after the documentation string for this
     item.  This is a sentence containing a button that references some
     other documentation.

     There are several alternatives you can use for LINK-DATA:

     ‘(custom-manual INFO-NODE)’
          Link to an Info node; INFO-NODE is a string which specifies
          the node name, as in ‘"(emacs)Top"’.  The link appears as
          ‘[Manual]’ in the customization buffer and enters the built-in
          Info reader on INFO-NODE.

     ‘(info-link INFO-NODE)’
          Like ‘custom-manual’ except that the link appears in the
          customization buffer with the Info node name.

     ‘(url-link URL)’
          Link to a web page; URL is a string which specifies the URL.
          The link appears in the customization buffer as URL and
          invokes the WWW browser specified by
          ‘browse-url-browser-function’.

     ‘(emacs-commentary-link LIBRARY)’
          Link to the commentary section of a library; LIBRARY is a
          string which specifies the library name.  *Note Library
          Headers::.

     ‘(emacs-library-link LIBRARY)’
          Link to an Emacs Lisp library file; LIBRARY is a string which
          specifies the library name.

     ‘(file-link FILE)’
          Link to a file; FILE is a string which specifies the name of
          the file to visit with ‘find-file’ when the user invokes this
          link.

     ‘(function-link FUNCTION)’
          Link to the documentation of a function; FUNCTION is a string
          which specifies the name of the function to describe with
          ‘describe-function’ when the user invokes this link.

     ‘(variable-link VARIABLE)’
          Link to the documentation of a variable; VARIABLE is a string
          which specifies the name of the variable to describe with
          ‘describe-variable’ when the user invokes this link.

     ‘(custom-group-link GROUP)’
          Link to another customization group.  Invoking it creates a
          new customization buffer for GROUP.

     You can specify the text to use in the customization buffer by
     adding ‘:tag NAME’ after the first element of the LINK-DATA; for
     example, ‘(info-link :tag "foo" "(emacs)Top")’ makes a link to the
     Emacs manual which appears in the buffer as ‘foo’.

     You can use this keyword more than once, to add multiple links.

‘:load FILE’
     Load file FILE (a string) before displaying this customization item
     (*note Loading::).  Loading is done with ‘load’, and only if the
     file is not already loaded.

‘:require FEATURE’
     Execute ‘(require 'FEATURE)’ when your saved customizations set the
     value of this item.  FEATURE should be a symbol.

     The most common reason to use ‘:require’ is when a variable enables
     a feature such as a minor mode, and just setting the variable won’t
     have any effect unless the code which implements the mode is
     loaded.

‘:version VERSION’
     This keyword specifies that the item was first introduced in Emacs
     version VERSION, or that its default value was changed in that
     version.  The value VERSION must be a string.

‘:package-version '(PACKAGE . VERSION)’
     This keyword specifies that the item was first introduced in
     PACKAGE version VERSION, or that its meaning or default value was
     changed in that version.  This keyword takes priority over
     ‘:version’.

     PACKAGE should be the official name of the package, as a symbol
     (e.g., ‘MH-E’).  VERSION should be a string.  If the package
     PACKAGE is released as part of Emacs, PACKAGE and VERSION should
     appear in the value of ‘customize-package-emacs-version-alist’.

   Packages distributed as part of Emacs that use the ‘:package-version’
keyword must also update the ‘customize-package-emacs-version-alist’
variable.

 -- Variable: customize-package-emacs-version-alist
     This alist provides a mapping for the versions of Emacs that are
     associated with versions of a package listed in the
     ‘:package-version’ keyword.  Its elements are:

          (PACKAGE (PVERSION . EVERSION)...)

     For each PACKAGE, which is a symbol, there are one or more elements
     that contain a package version PVERSION with an associated Emacs
     version EVERSION.  These versions are strings.  For example, the
     MH-E package updates this alist with the following:

          (add-to-list 'customize-package-emacs-version-alist
                       '(MH-E ("6.0" . "22.1") ("6.1" . "22.1") ("7.0" . "22.1")
                              ("7.1" . "22.1") ("7.2" . "22.1") ("7.3" . "22.1")
                              ("7.4" . "22.1") ("8.0" . "22.1")))

     The value of PACKAGE needs to be unique and it needs to match the
     PACKAGE value appearing in the ‘:package-version’ keyword.  Since
     the user might see the value in an error message, a good choice is
     the official name of the package, such as MH-E or Gnus.


File: elisp.info,  Node: Group Definitions,  Next: Variable Definitions,  Prev: Common Keywords,  Up: Customization

14.2 Defining Customization Groups
==================================

Each Emacs Lisp package should have one main customization group which
contains all the options, faces and other groups in the package.  If the
package has a small number of options and faces, use just one group and
put everything in it.  When there are more than twenty or so options and
faces, then you should structure them into subgroups, and put the
subgroups under the package’s main customization group.  It is OK to put
some of the options and faces in the package’s main group alongside the
subgroups.

   The package’s main or only group should be a member of one or more of
the standard customization groups.  (To display the full list of them,
use ‘M-x customize’.)  Choose one or more of them (but not too many),
and add your group to each of them using the ‘:group’ keyword.

   The way to declare new customization groups is with ‘defgroup’.

 -- Macro: defgroup group members doc [keyword value]...
     Declare GROUP as a customization group containing MEMBERS.  Do not
     quote the symbol GROUP.  The argument DOC specifies the
     documentation string for the group.

     The argument MEMBERS is a list specifying an initial set of
     customization items to be members of the group.  However, most
     often MEMBERS is ‘nil’, and you specify the group’s members by
     using the ‘:group’ keyword when defining those members.

     If you want to specify group members through MEMBERS, each element
     should have the form ‘(NAME WIDGET)’.  Here NAME is a symbol, and
     WIDGET is a widget type for editing that symbol.  Useful widgets
     are ‘custom-variable’ for a variable, ‘custom-face’ for a face, and
     ‘custom-group’ for a group.

     When you introduce a new group into Emacs, use the ‘:version’
     keyword in the ‘defgroup’; then you need not use it for the
     individual members of the group.

     In addition to the common keywords (*note Common Keywords::), you
     can also use this keyword in ‘defgroup’:

     ‘:prefix PREFIX’
          If the name of an item in the group starts with PREFIX, and
          the customizable variable ‘custom-unlispify-remove-prefixes’
          is non-‘nil’, the item’s tag will omit PREFIX.  A group can
          have any number of prefixes.

 -- User Option: custom-unlispify-remove-prefixes
     If this variable is non-‘nil’, the prefixes specified by a group’s
     ‘:prefix’ keyword are omitted from tag names, whenever the user
     customizes the group.

     The default value is ‘nil’, i.e., the prefix-discarding feature is
     disabled.  This is because discarding prefixes often leads to
     confusing names for options and faces.


File: elisp.info,  Node: Variable Definitions,  Next: Customization Types,  Prev: Group Definitions,  Up: Customization

14.3 Defining Customization Variables
=====================================

“Customizable variables”, also called “user options”, are global Lisp
variables whose values can be set through the Customize interface.
Unlike other global variables, which are defined with ‘defvar’ (*note
Defining Variables::), customizable variables are defined using the
‘defcustom’ macro.  In addition to calling ‘defvar’ as a subroutine,
‘defcustom’ states how the variable should be displayed in the Customize
interface, the values it is allowed to take, etc.

 -- Macro: defcustom option standard doc [keyword value]...
     This macro declares OPTION as a user option (i.e., a customizable
     variable).  You should not quote OPTION.

     The argument STANDARD is an expression that specifies the standard
     value for OPTION.  Evaluating the ‘defcustom’ form evaluates
     STANDARD, but does not necessarily bind the option to that value.
     If OPTION already has a default value, it is left unchanged.  If
     the user has already saved a customization for OPTION, the user’s
     customized value is installed as the default value.  Otherwise, the
     result of evaluating STANDARD is installed as the default value.

     Like ‘defvar’, this macro marks ‘option’ as a special variable,
     meaning that it should always be dynamically bound.  If OPTION is
     already lexically bound, that lexical binding remains in effect
     until the binding construct exits.  *Note Variable Scoping::.

     The expression STANDARD can be evaluated at various other times,
     too—whenever the customization facility needs to know OPTION’s
     standard value.  So be sure to use an expression which is harmless
     to evaluate at any time.

     The argument DOC specifies the documentation string for the
     variable.

     If a ‘defcustom’ does not specify any ‘:group’, the last group
     defined with ‘defgroup’ in the same file will be used.  This way,
     most ‘defcustom’ do not need an explicit ‘:group’.

     When you evaluate a ‘defcustom’ form with ‘C-M-x’ in Emacs Lisp
     mode (‘eval-defun’), a special feature of ‘eval-defun’ arranges to
     set the variable unconditionally, without testing whether its value
     is void.  (The same feature applies to ‘defvar’, *note Defining
     Variables::.)  Using ‘eval-defun’ on a defcustom that is already
     defined calls the ‘:set’ function (see below), if there is one.

     If you put a ‘defcustom’ in a pre-loaded Emacs Lisp file (*note
     Building Emacs::), the standard value installed at dump time might
     be incorrect, e.g., because another variable that it depends on has
     not been assigned the right value yet.  In that case, use
     ‘custom-reevaluate-setting’, described below, to re-evaluate the
     standard value after Emacs starts up.

   In addition to the keywords listed in *note Common Keywords::, this
macro accepts the following keywords:

‘:type TYPE’
     Use TYPE as the data type for this option.  It specifies which
     values are legitimate, and how to display the value (*note
     Customization Types::).  Every ‘defcustom’ should specify a value
     for this keyword.

‘:options VALUE-LIST’
     Specify the list of reasonable values for use in this option.  The
     user is not restricted to using only these values, but they are
     offered as convenient alternatives.

     This is meaningful only for certain types, currently including
     ‘hook’, ‘plist’ and ‘alist’.  See the definition of the individual
     types for a description of how to use ‘:options’.

‘:set SETFUNCTION’
     Specify SETFUNCTION as the way to change the value of this option
     when using the Customize interface.  The function SETFUNCTION
     should take two arguments, a symbol (the option name) and the new
     value, and should do whatever is necessary to update the value
     properly for this option (which may not mean simply setting the
     option as a Lisp variable); preferably, though, it should not
     modify its value argument destructively.  The default for
     SETFUNCTION is ‘set-default’.

     If you specify this keyword, the variable’s documentation string
     should describe how to do the same job in hand-written Lisp code.

‘:get GETFUNCTION’
     Specify GETFUNCTION as the way to extract the value of this option.
     The function GETFUNCTION should take one argument, a symbol, and
     should return whatever customize should use as the current value
     for that symbol (which need not be the symbol’s Lisp value).  The
     default is ‘default-value’.

     You have to really understand the workings of Custom to use ‘:get’
     correctly.  It is meant for values that are treated in Custom as
     variables but are not actually stored in Lisp variables.  It is
     almost surely a mistake to specify GETFUNCTION for a value that
     really is stored in a Lisp variable.

‘:initialize FUNCTION’
     FUNCTION should be a function used to initialize the variable when
     the ‘defcustom’ is evaluated.  It should take two arguments, the
     option name (a symbol) and the value.  Here are some predefined
     functions meant for use in this way:

     ‘custom-initialize-set’
          Use the variable’s ‘:set’ function to initialize the variable,
          but do not reinitialize it if it is already non-void.

     ‘custom-initialize-default’
          Like ‘custom-initialize-set’, but use the function
          ‘set-default’ to set the variable, instead of the variable’s
          ‘:set’ function.  This is the usual choice for a variable
          whose ‘:set’ function enables or disables a minor mode; with
          this choice, defining the variable will not call the minor
          mode function, but customizing the variable will do so.

     ‘custom-initialize-reset’
          Always use the ‘:set’ function to initialize the variable.  If
          the variable is already non-void, reset it by calling the
          ‘:set’ function using the current value (returned by the
          ‘:get’ method).  This is the default ‘:initialize’ function.

     ‘custom-initialize-changed’
          Use the ‘:set’ function to initialize the variable, if it is
          already set or has been customized; otherwise, just use
          ‘set-default’.

     ‘custom-initialize-safe-set’
     ‘custom-initialize-safe-default’
          These functions behave like ‘custom-initialize-set’
          (‘custom-initialize-default’, respectively), but catch errors.
          If an error occurs during initialization, they set the
          variable to ‘nil’ using ‘set-default’, and signal no error.

          These functions are meant for options defined in pre-loaded
          files, where the STANDARD expression may signal an error
          because some required variable or function is not yet defined.
          The value normally gets updated in ‘startup.el’, ignoring the
          value computed by ‘defcustom’.  After startup, if one unsets
          the value and reevaluates the ‘defcustom’, the STANDARD
          expression can be evaluated without error.

‘:risky VALUE’
     Set the variable’s ‘risky-local-variable’ property to VALUE (*note
     File Local Variables::).

‘:safe FUNCTION’
     Set the variable’s ‘safe-local-variable’ property to FUNCTION
     (*note File Local Variables::).

‘:set-after VARIABLES’
     When setting variables according to saved customizations, make sure
     to set the variables VARIABLES before this one; i.e., delay setting
     this variable until after those others have been handled.  Use
     ‘:set-after’ if setting this variable won’t work properly unless
     those other variables already have their intended values.

   It is useful to specify the ‘:require’ keyword for an option that
turns on a certain feature.  This causes Emacs to load the feature, if
it is not already loaded, whenever the option is set.  *Note Common
Keywords::.  Here is an example, from the library ‘saveplace.el’:

     (defcustom save-place nil
       "Non-nil means automatically save place in each file..."
       :type 'boolean
       :require 'saveplace
       :group 'save-place)

   If a customization item has a type such as ‘hook’ or ‘alist’, which
supports ‘:options’, you can add additional values to the list from
outside the ‘defcustom’ declaration by calling
‘custom-add-frequent-value’.  For example, if you define a function
‘my-lisp-mode-initialization’ intended to be called from
‘emacs-lisp-mode-hook’, you might want to add that to the list of
reasonable values for ‘emacs-lisp-mode-hook’, but not by editing its
definition.  You can do it thus:

     (custom-add-frequent-value 'emacs-lisp-mode-hook
        'my-lisp-mode-initialization)

 -- Function: custom-add-frequent-value symbol value
     For the customization option SYMBOL, add VALUE to the list of
     reasonable values.

     The precise effect of adding a value depends on the customization
     type of SYMBOL.

   Internally, ‘defcustom’ uses the symbol property ‘standard-value’ to
record the expression for the standard value, ‘saved-value’ to record
the value saved by the user with the customization buffer, and
‘customized-value’ to record the value set by the user with the
customization buffer, but not saved.  *Note Symbol Properties::.  These
properties are lists, the car of which is an expression that evaluates
to the value.

 -- Function: custom-reevaluate-setting symbol
     This function re-evaluates the standard value of SYMBOL, which
     should be a user option declared via ‘defcustom’.  If the variable
     was customized, this function re-evaluates the saved value instead.
     Then it sets the user option to that value (using the option’s
     ‘:set’ property if that is defined).

     This is useful for customizable options that are defined before
     their value could be computed correctly.  For example, during
     startup Emacs calls this function for some user options that were
     defined in pre-loaded Emacs Lisp files, but whose initial values
     depend on information available only at run-time.

 -- Function: custom-variable-p arg
     This function returns non-‘nil’ if ARG is a customizable variable.
     A customizable variable is either a variable that has a
     ‘standard-value’ or ‘custom-autoload’ property (usually meaning it
     was declared with ‘defcustom’), or an alias for another
     customizable variable.


File: elisp.info,  Node: Customization Types,  Next: Applying Customizations,  Prev: Variable Definitions,  Up: Customization

14.4 Customization Types
========================

When you define a user option with ‘defcustom’, you must specify its
“customization type”.  That is a Lisp object which describes (1) which
values are legitimate and (2) how to display the value in the
customization buffer for editing.

   You specify the customization type in ‘defcustom’ with the ‘:type’
keyword.  The argument of ‘:type’ is evaluated, but only once when the
‘defcustom’ is executed, so it isn’t useful for the value to vary.
Normally we use a quoted constant.  For example:

     (defcustom diff-command "diff"
       "The command to use to run diff."
       :type '(string)
       :group 'diff)

   In general, a customization type is a list whose first element is a
symbol, one of the customization type names defined in the following
sections.  After this symbol come a number of arguments, depending on
the symbol.  Between the type symbol and its arguments, you can
optionally write keyword-value pairs (*note Type Keywords::).

   Some type symbols do not use any arguments; those are called “simple
types”.  For a simple type, if you do not use any keyword-value pairs,
you can omit the parentheses around the type symbol.  For example just
‘string’ as a customization type is equivalent to ‘(string)’.

   All customization types are implemented as widgets; see *note
Introduction: (widget)Top, for details.

* Menu:

* Simple Types::            Simple customization types: sexp, integer, etc.
* Composite Types::         Build new types from other types or data.
* Splicing into Lists::     Splice elements into list with ‘:inline’.
* Type Keywords::           Keyword-argument pairs in a customization type.
* Defining New Types::      Give your type a name.


File: elisp.info,  Node: Simple Types,  Next: Composite Types,  Up: Customization Types

14.4.1 Simple Types
-------------------

This section describes all the simple customization types.  For several
of these customization types, the customization widget provides inline
completion with ‘C-M-i’ or ‘M-<TAB>’.

‘sexp’
     The value may be any Lisp object that can be printed and read back.
     You can use ‘sexp’ as a fall-back for any option, if you don’t want
     to take the time to work out a more specific type to use.

‘integer’
     The value must be an integer.

‘number’
     The value must be a number (floating point or integer).

‘float’
     The value must be floating point.

‘string’
     The value must be a string.  The customization buffer shows the
     string without delimiting ‘"’ characters or ‘\’ quotes.

‘regexp’
     Like ‘string’ except that the string must be a valid regular
     expression.

‘character’
     The value must be a character code.  A character code is actually
     an integer, but this type shows the value by inserting the
     character in the buffer, rather than by showing the number.

‘file’
     The value must be a file name.  The widget provides completion.

‘(file :must-match t)’
     The value must be a file name for an existing file.  The widget
     provides completion.

‘directory’
     The value must be a directory name.  The widget provides
     completion.

‘hook’
     The value must be a list of functions.  This customization type is
     used for hook variables.  You can use the ‘:options’ keyword in a
     hook variable’s ‘defcustom’ to specify a list of functions
     recommended for use in the hook; *Note Variable Definitions::.

‘symbol’
     The value must be a symbol.  It appears in the customization buffer
     as the symbol name.  The widget provides completion.

‘function’
     The value must be either a lambda expression or a function name.
     The widget provides completion for function names.

‘variable’
     The value must be a variable name.  The widget provides completion.

‘face’
     The value must be a symbol which is a face name.  The widget
     provides completion.

‘boolean’
     The value is boolean—either ‘nil’ or ‘t’.  Note that by using
     ‘choice’ and ‘const’ together (see the next section), you can
     specify that the value must be ‘nil’ or ‘t’, but also specify the
     text to describe each value in a way that fits the specific meaning
     of the alternative.

‘key-sequence’
     The value is a key sequence.  The customization buffer shows the
     key sequence using the same syntax as the ‘kbd’ function.  *Note
     Key Sequences::.

‘coding-system’
     The value must be a coding-system name, and you can do completion
     with ‘M-<TAB>’.

‘color’
     The value must be a valid color name.  The widget provides
     completion for color names, as well as a sample and a button for
     selecting a color name from a list of color names shown in a
     ‘*Colors*’ buffer.


File: elisp.info,  Node: Composite Types,  Next: Splicing into Lists,  Prev: Simple Types,  Up: Customization Types

14.4.2 Composite Types
----------------------

When none of the simple types is appropriate, you can use composite
types, which build new types from other types or from specified data.
The specified types or data are called the “arguments” of the composite
type.  The composite type normally looks like this:

     (CONSTRUCTOR ARGUMENTS...)

but you can also add keyword-value pairs before the arguments, like
this:

     (CONSTRUCTOR {KEYWORD VALUE}... ARGUMENTS...)

   Here is a table of constructors and how to use them to write
composite types:

‘(cons CAR-TYPE CDR-TYPE)’
     The value must be a cons cell, its CAR must fit CAR-TYPE, and its
     CDR must fit CDR-TYPE.  For example, ‘(cons string symbol)’ is a
     customization type which matches values such as ‘("foo" . foo)’.

     In the customization buffer, the CAR and CDR are displayed and
     edited separately, each according to their specified type.

‘(list ELEMENT-TYPES...)’
     The value must be a list with exactly as many elements as the
     ELEMENT-TYPES given; and each element must fit the corresponding
     ELEMENT-TYPE.

     For example, ‘(list integer string function)’ describes a list of
     three elements; the first element must be an integer, the second a
     string, and the third a function.

     In the customization buffer, each element is displayed and edited
     separately, according to the type specified for it.

‘(group ELEMENT-TYPES...)’
     This works like ‘list’ except for the formatting of text in the
     Custom buffer.  ‘list’ labels each element value with its tag;
     ‘group’ does not.

‘(vector ELEMENT-TYPES...)’
     Like ‘list’ except that the value must be a vector instead of a
     list.  The elements work the same as in ‘list’.

‘(alist :key-type KEY-TYPE :value-type VALUE-TYPE)’
     The value must be a list of cons-cells, the CAR of each cell
     representing a key of customization type KEY-TYPE, and the CDR of
     the same cell representing a value of customization type
     VALUE-TYPE.  The user can add and delete key/value pairs, and edit
     both the key and the value of each pair.

     If omitted, KEY-TYPE and VALUE-TYPE default to ‘sexp’.

     The user can add any key matching the specified key type, but you
     can give some keys a preferential treatment by specifying them with
     the ‘:options’ (see *note Variable Definitions::).  The specified
     keys will always be shown in the customize buffer (together with a
     suitable value), with a checkbox to include or exclude or disable
     the key/value pair from the alist.  The user will not be able to
     edit the keys specified by the ‘:options’ keyword argument.

     The argument to the ‘:options’ keywords should be a list of
     specifications for reasonable keys in the alist.  Ordinarily, they
     are simply atoms, which stand for themselves.  For example:

          :options '("foo" "bar" "baz")

     specifies that there are three known keys, namely ‘"foo"’, ‘"bar"’
     and ‘"baz"’, which will always be shown first.

     You may want to restrict the value type for specific keys, for
     example, the value associated with the ‘"bar"’ key can only be an
     integer.  You can specify this by using a list instead of an atom
     in the list.  The first element will specify the key, like before,
     while the second element will specify the value type.  For example:

          :options '("foo" ("bar" integer) "baz")

     Finally, you may want to change how the key is presented.  By
     default, the key is simply shown as a ‘const’, since the user
     cannot change the special keys specified with the ‘:options’
     keyword.  However, you may want to use a more specialized type for
     presenting the key, like ‘function-item’ if you know it is a symbol
     with a function binding.  This is done by using a customization
     type specification instead of a symbol for the key.

          :options '("foo"
                     ((function-item some-function) integer)
                     "baz")

     Many alists use lists with two elements, instead of cons cells.
     For example,

          (defcustom list-alist
            '(("foo" 1) ("bar" 2) ("baz" 3))
            "Each element is a list of the form (KEY VALUE).")

     instead of

          (defcustom cons-alist
            '(("foo" . 1) ("bar" . 2) ("baz" . 3))
            "Each element is a cons-cell (KEY . VALUE).")

     Because of the way lists are implemented on top of cons cells, you
     can treat ‘list-alist’ in the example above as a cons cell alist,
     where the value type is a list with a single element containing the
     real value.

          (defcustom list-alist '(("foo" 1) ("bar" 2) ("baz" 3))
            "Each element is a list of the form (KEY VALUE)."
            :type '(alist :value-type (group integer)))

     The ‘group’ widget is used here instead of ‘list’ only because the
     formatting is better suited for the purpose.

     Similarly, you can have alists with more values associated with
     each key, using variations of this trick:

          (defcustom person-data '(("brian"  50 t)
                                   ("dorith" 55 nil)
                                   ("ken"    52 t))
            "Alist of basic info about people.
          Each element has the form (NAME AGE MALE-FLAG)."
            :type '(alist :value-type (group integer boolean)))

‘(plist :key-type KEY-TYPE :value-type VALUE-TYPE)’
     This customization type is similar to ‘alist’ (see above), except
     that (i) the information is stored as a property list, (*note
     Property Lists::), and (ii) KEY-TYPE, if omitted, defaults to
     ‘symbol’ rather than ‘sexp’.

‘(choice ALTERNATIVE-TYPES...)’
     The value must fit one of ALTERNATIVE-TYPES.  For example, ‘(choice
     integer string)’ allows either an integer or a string.

     In the customization buffer, the user selects an alternative using
     a menu, and can then edit the value in the usual way for that
     alternative.

     Normally the strings in this menu are determined automatically from
     the choices; however, you can specify different strings for the
     menu by including the ‘:tag’ keyword in the alternatives.  For
     example, if an integer stands for a number of spaces, while a
     string is text to use verbatim, you might write the customization
     type this way,

          (choice (integer :tag "Number of spaces")
                  (string :tag "Literal text"))

     so that the menu offers ‘Number of spaces’ and ‘Literal text’.

     In any alternative for which ‘nil’ is not a valid value, other than
     a ‘const’, you should specify a valid default for that alternative
     using the ‘:value’ keyword.  *Note Type Keywords::.

     If some values are covered by more than one of the alternatives,
     customize will choose the first alternative that the value fits.
     This means you should always list the most specific types first,
     and the most general last.  Here’s an example of proper usage:

          (choice (const :tag "Off" nil)
                  symbol (sexp :tag "Other"))

     This way, the special value ‘nil’ is not treated like other
     symbols, and symbols are not treated like other Lisp expressions.

‘(radio ELEMENT-TYPES...)’
     This is similar to ‘choice’, except that the choices are displayed
     using radio buttons rather than a menu.  This has the advantage of
     displaying documentation for the choices when applicable and so is
     often a good choice for a choice between constant functions
     (‘function-item’ customization types).

‘(const VALUE)’
     The value must be VALUE—nothing else is allowed.

     The main use of ‘const’ is inside of ‘choice’.  For example,
     ‘(choice integer (const nil))’ allows either an integer or ‘nil’.

     ‘:tag’ is often used with ‘const’, inside of ‘choice’.  For
     example,

          (choice (const :tag "Yes" t)
                  (const :tag "No" nil)
                  (const :tag "Ask" foo))

     describes a variable for which ‘t’ means yes, ‘nil’ means no, and
     ‘foo’ means “ask”.

‘(other VALUE)’
     This alternative can match any Lisp value, but if the user chooses
     this alternative, that selects the value VALUE.

     The main use of ‘other’ is as the last element of ‘choice’.  For
     example,

          (choice (const :tag "Yes" t)
                  (const :tag "No" nil)
                  (other :tag "Ask" foo))

     describes a variable for which ‘t’ means yes, ‘nil’ means no, and
     anything else means “ask”.  If the user chooses ‘Ask’ from the menu
     of alternatives, that specifies the value ‘foo’; but any other
     value (not ‘t’, ‘nil’ or ‘foo’) displays as ‘Ask’, just like ‘foo’.

‘(function-item FUNCTION)’
     Like ‘const’, but used for values which are functions.  This
     displays the documentation string as well as the function name.
     The documentation string is either the one you specify with ‘:doc’,
     or FUNCTION’s own documentation string.

‘(variable-item VARIABLE)’
     Like ‘const’, but used for values which are variable names.  This
     displays the documentation string as well as the variable name.
     The documentation string is either the one you specify with ‘:doc’,
     or VARIABLE’s own documentation string.

‘(set TYPES...)’
     The value must be a list, and each element of the list must match
     one of the TYPES specified.

     This appears in the customization buffer as a checklist, so that
     each of TYPES may have either one corresponding element or none.
     It is not possible to specify two different elements that match the
     same one of TYPES.  For example, ‘(set integer symbol)’ allows one
     integer and/or one symbol in the list; it does not allow multiple
     integers or multiple symbols.  As a result, it is rare to use
     nonspecific types such as ‘integer’ in a ‘set’.

     Most often, the TYPES in a ‘set’ are ‘const’ types, as shown here:

          (set (const :bold) (const :italic))

     Sometimes they describe possible elements in an alist:

          (set (cons :tag "Height" (const height) integer)
               (cons :tag "Width" (const width) integer))

     That lets the user specify a height value optionally and a width
     value optionally.

‘(repeat ELEMENT-TYPE)’
     The value must be a list and each element of the list must fit the
     type ELEMENT-TYPE.  This appears in the customization buffer as a
     list of elements, with ‘[INS]’ and ‘[DEL]’ buttons for adding more
     elements or removing elements.

‘(restricted-sexp :match-alternatives CRITERIA)’
     This is the most general composite type construct.  The value may
     be any Lisp object that satisfies one of CRITERIA.  CRITERIA should
     be a list, and each element should be one of these possibilities:

        • A predicate—that is, a function of one argument that has no
          side effects, and returns either ‘nil’ or non-‘nil’ according
          to the argument.  Using a predicate in the list says that
          objects for which the predicate returns non-‘nil’ are
          acceptable.

        • A quoted constant—that is, ‘'OBJECT’.  This sort of element in
          the list says that OBJECT itself is an acceptable value.

     For example,

          (restricted-sexp :match-alternatives
                           (integerp 't 'nil))

     allows integers, ‘t’ and ‘nil’ as legitimate values.

     The customization buffer shows all legitimate values using their
     read syntax, and the user edits them textually.

   Here is a table of the keywords you can use in keyword-value pairs in
a composite type:

‘:tag TAG’
     Use TAG as the name of this alternative, for user communication
     purposes.  This is useful for a type that appears inside of a
     ‘choice’.

‘:match-alternatives CRITERIA’
     Use CRITERIA to match possible values.  This is used only in
     ‘restricted-sexp’.

‘:args ARGUMENT-LIST’
     Use the elements of ARGUMENT-LIST as the arguments of the type
     construct.  For instance, ‘(const :args (foo))’ is equivalent to
     ‘(const foo)’.  You rarely need to write ‘:args’ explicitly,
     because normally the arguments are recognized automatically as
     whatever follows the last keyword-value pair.


File: elisp.info,  Node: Splicing into Lists,  Next: Type Keywords,  Prev: Composite Types,  Up: Customization Types

14.4.3 Splicing into Lists
--------------------------

The ‘:inline’ feature lets you splice a variable number of elements into
the middle of a ‘list’ or ‘vector’ customization type.  You use it by
adding ‘:inline t’ to a type specification which is contained in a
‘list’ or ‘vector’ specification.

   Normally, each entry in a ‘list’ or ‘vector’ type specification
describes a single element type.  But when an entry contains ‘:inline
t’, the value it matches is merged directly into the containing
sequence.  For example, if the entry matches a list with three elements,
those become three elements of the overall sequence.  This is analogous
to ‘,@’ in a backquote construct (*note Backquote::).

   For example, to specify a list whose first element must be ‘baz’ and
whose remaining arguments should be zero or more of ‘foo’ and ‘bar’, use
this customization type:

     (list (const baz) (set :inline t (const foo) (const bar)))

This matches values such as ‘(baz)’, ‘(baz foo)’, ‘(baz bar)’ and ‘(baz
foo bar)’.

   When the element-type is a ‘choice’, you use ‘:inline’ not in the
‘choice’ itself, but in (some of) the alternatives of the ‘choice’.  For
example, to match a list which must start with a file name, followed
either by the symbol ‘t’ or two strings, use this customization type:

     (list file
           (choice (const t)
                   (list :inline t string string)))

If the user chooses the first alternative in the choice, then the
overall list has two elements and the second element is ‘t’.  If the
user chooses the second alternative, then the overall list has three
elements and the second and third must be strings.


File: elisp.info,  Node: Type Keywords,  Next: Defining New Types,  Prev: Splicing into Lists,  Up: Customization Types

14.4.4 Type Keywords
--------------------

You can specify keyword-argument pairs in a customization type after the
type name symbol.  Here are the keywords you can use, and their
meanings:

‘:value DEFAULT’
     Provide a default value.

     If ‘nil’ is not a valid value for the alternative, then it is
     essential to specify a valid default with ‘:value’.

     If you use this for a type that appears as an alternative inside of
     ‘choice’; it specifies the default value to use, at first, if and
     when the user selects this alternative with the menu in the
     customization buffer.

     Of course, if the actual value of the option fits this alternative,
     it will appear showing the actual value, not DEFAULT.

‘:format FORMAT-STRING’
     This string will be inserted in the buffer to represent the value
     corresponding to the type.  The following ‘%’ escapes are available
     for use in FORMAT-STRING:

     ‘%[BUTTON%]’
          Display the text BUTTON marked as a button.  The ‘:action’
          attribute specifies what the button will do if the user
          invokes it; its value is a function which takes two
          arguments—the widget which the button appears in, and the
          event.

          There is no way to specify two different buttons with
          different actions.

     ‘%{SAMPLE%}’
          Show SAMPLE in a special face specified by ‘:sample-face’.

     ‘%v’
          Substitute the item’s value.  How the value is represented
          depends on the kind of item, and (for variables) on the
          customization type.

     ‘%d’
          Substitute the item’s documentation string.

     ‘%h’
          Like ‘%d’, but if the documentation string is more than one
          line, add a button to control whether to show all of it or
          just the first line.

     ‘%t’
          Substitute the tag here.  You specify the tag with the ‘:tag’
          keyword.

     ‘%%’
          Display a literal ‘%’.

‘:action ACTION’
     Perform ACTION if the user clicks on a button.

‘:button-face FACE’
     Use the face FACE (a face name or a list of face names) for button
     text displayed with ‘%[...%]’.

‘:button-prefix PREFIX’
‘:button-suffix SUFFIX’
     These specify the text to display before and after a button.  Each
     can be:

     ‘nil’
          No text is inserted.

     a string
          The string is inserted literally.

     a symbol
          The symbol’s value is used.

‘:tag TAG’
     Use TAG (a string) as the tag for the value (or part of the value)
     that corresponds to this type.

‘:doc DOC’
     Use DOC as the documentation string for this value (or part of the
     value) that corresponds to this type.  In order for this to work,
     you must specify a value for ‘:format’, and use ‘%d’ or ‘%h’ in
     that value.

     The usual reason to specify a documentation string for a type is to
     provide more information about the meanings of alternatives inside
     a ‘:choice’ type or the parts of some other composite type.

‘:help-echo MOTION-DOC’
     When you move to this item with ‘widget-forward’ or
     ‘widget-backward’, it will display the string MOTION-DOC in the
     echo area.  In addition, MOTION-DOC is used as the mouse
     ‘help-echo’ string and may actually be a function or form evaluated
     to yield a help string.  If it is a function, it is called with one
     argument, the widget.

‘:match FUNCTION’
     Specify how to decide whether a value matches the type.  The
     corresponding value, FUNCTION, should be a function that accepts
     two arguments, a widget and a value; it should return non-‘nil’ if
     the value is acceptable.

‘:validate FUNCTION’
     Specify a validation function for input.  FUNCTION takes a widget
     as an argument, and should return ‘nil’ if the widget’s current
     value is valid for the widget.  Otherwise, it should return the
     widget containing the invalid data, and set that widget’s ‘:error’
     property to a string explaining the error.


File: elisp.info,  Node: Defining New Types,  Prev: Type Keywords,  Up: Customization Types

14.4.5 Defining New Types
-------------------------

In the previous sections we have described how to construct elaborate
type specifications for ‘defcustom’.  In some cases you may want to give
such a type specification a name.  The obvious case is when you are
using the same type for many user options: rather than repeat the
specification for each option, you can give the type specification a
name, and use that name each ‘defcustom’.  The other case is when a user
option’s value is a recursive data structure.  To make it possible for a
datatype to refer to itself, it needs to have a name.

   Since custom types are implemented as widgets, the way to define a
new customize type is to define a new widget.  We are not going to
describe the widget interface here in details, see *note Introduction:
(widget)Top, for that.  Instead we are going to demonstrate the minimal
functionality needed for defining new customize types by a simple
example.

     (define-widget 'binary-tree-of-string 'lazy
       "A binary tree made of cons-cells and strings."
       :offset 4
       :tag "Node"
       :type '(choice (string :tag "Leaf" :value "")
                      (cons :tag "Interior"
                            :value ("" . "")
                            binary-tree-of-string
                            binary-tree-of-string)))

     (defcustom foo-bar ""
       "Sample variable holding a binary tree of strings."
       :type 'binary-tree-of-string)

   The function to define a new widget is called ‘define-widget’.  The
first argument is the symbol we want to make a new widget type.  The
second argument is a symbol representing an existing widget, the new
widget is going to be defined in terms of difference from the existing
widget.  For the purpose of defining new customization types, the ‘lazy’
widget is perfect, because it accepts a ‘:type’ keyword argument with
the same syntax as the keyword argument to ‘defcustom’ with the same
name.  The third argument is a documentation string for the new widget.
You will be able to see that string with the ‘M-x widget-browse <RET>
binary-tree-of-string <RET>’ command.

   After these mandatory arguments follow the keyword arguments.  The
most important is ‘:type’, which describes the data type we want to
match with this widget.  Here a ‘binary-tree-of-string’ is described as
being either a string, or a cons-cell whose car and cdr are themselves
both ‘binary-tree-of-string’.  Note the reference to the widget type we
are currently in the process of defining.  The ‘:tag’ attribute is a
string to name the widget in the user interface, and the ‘:offset’
argument is there to ensure that child nodes are indented four spaces
relative to the parent node, making the tree structure apparent in the
customization buffer.

   The ‘defcustom’ shows how the new widget can be used as an ordinary
customization type.

   The reason for the name ‘lazy’ is that the other composite widgets
convert their inferior widgets to internal form when the widget is
instantiated in a buffer.  This conversion is recursive, so the inferior
widgets will convert _their_ inferior widgets.  If the data structure is
itself recursive, this conversion is an infinite recursion.  The ‘lazy’
widget prevents the recursion: it convert its ‘:type’ argument only when
needed.


File: elisp.info,  Node: Applying Customizations,  Next: Custom Themes,  Prev: Customization Types,  Up: Customization

14.5 Applying Customizations
============================

The following functions are responsible for installing the user’s
customization settings for variables and faces, respectively.  When the
user invokes ‘Save for future sessions’ in the Customize interface, that
takes effect by writing a ‘custom-set-variables’ and/or a
‘custom-set-faces’ form into the custom file, to be evaluated the next
time Emacs starts.

 -- Function: custom-set-variables &rest args
     This function installs the variable customizations specified by
     ARGS.  Each argument in ARGS should have the form

          (VAR EXPRESSION [NOW [REQUEST [COMMENT]]])

     VAR is a variable name (a symbol), and EXPRESSION is an expression
     which evaluates to the desired customized value.

     If the ‘defcustom’ form for VAR has been evaluated prior to this
     ‘custom-set-variables’ call, EXPRESSION is immediately evaluated,
     and the variable’s value is set to the result.  Otherwise,
     EXPRESSION is stored into the variable’s ‘saved-value’ property, to
     be evaluated when the relevant ‘defcustom’ is called (usually when
     the library defining that variable is loaded into Emacs).

     The NOW, REQUEST, and COMMENT entries are for internal use only,
     and may be omitted.  NOW, if non-‘nil’, means to set the variable’s
     value now, even if the variable’s ‘defcustom’ form has not been
     evaluated.  REQUEST is a list of features to be loaded immediately
     (*note Named Features::).  COMMENT is a string describing the
     customization.

 -- Function: custom-set-faces &rest args
     This function installs the face customizations specified by ARGS.
     Each argument in ARGS should have the form

          (FACE SPEC [NOW [COMMENT]])

     FACE is a face name (a symbol), and SPEC is the customized face
     specification for that face (*note Defining Faces::).

     The NOW and COMMENT entries are for internal use only, and may be
     omitted.  NOW, if non-‘nil’, means to install the face
     specification now, even if the ‘defface’ form has not been
     evaluated.  COMMENT is a string describing the customization.


File: elisp.info,  Node: Custom Themes,  Prev: Applying Customizations,  Up: Customization

14.6 Custom Themes
==================

“Custom themes” are collections of settings that can be enabled or
disabled as a unit.  *Note (emacs)Custom Themes::.  Each Custom theme is
defined by an Emacs Lisp source file, which should follow the
conventions described in this section.  (Instead of writing a Custom
theme by hand, you can also create one using a Customize-like interface;
*note (emacs)Creating Custom Themes::.)

   A Custom theme file should be named ‘FOO-theme.el’, where FOO is the
theme name.  The first Lisp form in the file should be a call to
‘deftheme’, and the last form should be a call to ‘provide-theme’.

 -- Macro: deftheme theme &optional doc
     This macro declares THEME (a symbol) as the name of a Custom theme.
     The optional argument DOC should be a string describing the theme;
     this is the description shown when the user invokes the
     ‘describe-theme’ command or types ‘?’ in the ‘*Custom Themes*’
     buffer.

     Two special theme names are disallowed (using them causes an
     error): ‘user’ is a dummy theme that stores the user’s direct
     customization settings, and ‘changed’ is a dummy theme that stores
     changes made outside of the Customize system.

 -- Macro: provide-theme theme
     This macro declares that the theme named THEME has been fully
     specified.

   In between ‘deftheme’ and ‘provide-theme’ are Lisp forms specifying
the theme settings: usually a call to ‘custom-theme-set-variables’
and/or a call to ‘custom-theme-set-faces’.

 -- Function: custom-theme-set-variables theme &rest args
     This function specifies the Custom theme THEME’s variable settings.
     THEME should be a symbol.  Each argument in ARGS should be a list
     of the form

          (VAR EXPRESSION [NOW [REQUEST [COMMENT]]])

     where the list entries have the same meanings as in
     ‘custom-set-variables’.  *Note Applying Customizations::.

 -- Function: custom-theme-set-faces theme &rest args
     This function specifies the Custom theme THEME’s face settings.
     THEME should be a symbol.  Each argument in ARGS should be a list
     of the form

          (FACE SPEC [NOW [COMMENT]])

     where the list entries have the same meanings as in
     ‘custom-set-faces’.  *Note Applying Customizations::.

   In theory, a theme file can also contain other Lisp forms, which
would be evaluated when loading the theme, but that is bad form.  To
protect against loading themes containing malicious code, Emacs displays
the source file and asks for confirmation from the user before loading
any non-built-in theme for the first time.

   The following functions are useful for programmatically enabling and
disabling themes:

 -- Function: custom-theme-p theme
     This function return a non-‘nil’ value if THEME (a symbol) is the
     name of a Custom theme (i.e., a Custom theme which has been loaded
     into Emacs, whether or not the theme is enabled).  Otherwise, it
     returns ‘nil’.

 -- Variable: custom-known-themes
     The value of this variable is a list of themes loaded into Emacs.
     Each theme is represented by a Lisp symbol (the theme name).  The
     default value of this variable is a list containing two dummy
     themes: ‘(user changed)’.  The ‘changed’ theme stores settings made
     before any Custom themes are applied (e.g., variables set outside
     of Customize).  The ‘user’ theme stores settings the user has
     customized and saved.  Any additional themes declared with the
     ‘deftheme’ macro are added to the front of this list.

 -- Command: load-theme theme &optional no-confirm no-enable
     This function loads the Custom theme named THEME from its source
     file, looking for the source file in the directories specified by
     the variable ‘custom-theme-load-path’.  *Note (emacs)Custom
     Themes::.  It also “enables” the theme (unless the optional
     argument NO-ENABLE is non-‘nil’), causing its variable and face
     settings to take effect.  It prompts the user for confirmation
     before loading the theme, unless the optional argument NO-CONFIRM
     is non-‘nil’.

 -- Command: enable-theme theme
     This function enables the Custom theme named THEME.  It signals an
     error if no such theme has been loaded.

 -- Command: disable-theme theme
     This function disables the Custom theme named THEME.  The theme
     remains loaded, so that a subsequent call to ‘enable-theme’ will
     re-enable it.


File: elisp.info,  Node: Loading,  Next: Byte Compilation,  Prev: Customization,  Up: Top

15 Loading
**********

Loading a file of Lisp code means bringing its contents into the Lisp
environment in the form of Lisp objects.  Emacs finds and opens the
file, reads the text, evaluates each form, and then closes the file.
Such a file is also called a “Lisp library”.

   The load functions evaluate all the expressions in a file just as the
‘eval-buffer’ function evaluates all the expressions in a buffer.  The
difference is that the load functions read and evaluate the text in the
file as found on disk, not the text in an Emacs buffer.

   The loaded file must contain Lisp expressions, either as source code
or as byte-compiled code.  Each form in the file is called a “top-level
form”.  There is no special format for the forms in a loadable file; any
form in a file may equally well be typed directly into a buffer and
evaluated there.  (Indeed, most code is tested this way.)  Most often,
the forms are function definitions and variable definitions.

   Emacs can also load compiled dynamic modules: shared libraries that
provide additional functionality for use in Emacs Lisp programs, just
like a package written in Emacs Lisp would.  When a dynamic module is
loaded, Emacs calls a specially-named initialization function which the
module needs to implement, and which exposes the additional functions
and variables to Emacs Lisp programs.

   For on-demand loading of external libraries which are known in
advance to be required by certain Emacs primitives, *note Dynamic
Libraries::.

* Menu:

* How Programs Do Loading:: The ‘load’ function and others.
* Load Suffixes::           Details about the suffixes that ‘load’ tries.
* Library Search::          Finding a library to load.
* Loading Non-ASCII::       Non-ASCII characters in Emacs Lisp files.
* Autoload::                Setting up a function to autoload.
* Repeated Loading::        Precautions about loading a file twice.
* Named Features::          Loading a library if it isn’t already loaded.
* Where Defined::           Finding which file defined a certain symbol.
* Unloading::               How to unload a library that was loaded.
* Hooks for Loading::       Providing code to be run when
                              particular libraries are loaded.
* Dynamic Modules::         Modules provide additional Lisp primitives.


File: elisp.info,  Node: How Programs Do Loading,  Next: Load Suffixes,  Up: Loading

15.1 How Programs Do Loading
============================

Emacs Lisp has several interfaces for loading.  For example, ‘autoload’
creates a placeholder object for a function defined in a file; trying to
call the autoloading function loads the file to get the function’s real
definition (*note Autoload::).  ‘require’ loads a file if it isn’t
already loaded (*note Named Features::).  Ultimately, all these
facilities call the ‘load’ function to do the work.

 -- Function: load filename &optional missing-ok nomessage nosuffix
          must-suffix
     This function finds and opens a file of Lisp code, evaluates all
     the forms in it, and closes the file.

     To find the file, ‘load’ first looks for a file named
     ‘FILENAME.elc’, that is, for a file whose name is FILENAME with the
     extension ‘.elc’ appended.  If such a file exists, it is loaded.
     If there is no file by that name, then ‘load’ looks for a file
     named ‘FILENAME.el’.  If that file exists, it is loaded.  If Emacs
     was compiled with support for dynamic modules (*note Dynamic
     Modules::), ‘load’ next looks for a file named ‘FILENAME.EXT’,
     where EXT is a system-dependent file-name extension of shared
     libraries.  Finally, if neither of those names is found, ‘load’
     looks for a file named FILENAME with nothing appended, and loads it
     if it exists.  (The ‘load’ function is not clever about looking at
     FILENAME.  In the perverse case of a file named ‘foo.el.el’,
     evaluation of ‘(load "foo.el")’ will indeed find it.)

     If Auto Compression mode is enabled, as it is by default, then if
     ‘load’ can not find a file, it searches for a compressed version of
     the file before trying other file names.  It decompresses and loads
     it if it exists.  It looks for compressed versions by appending
     each of the suffixes in ‘jka-compr-load-suffixes’ to the file name.
     The value of this variable must be a list of strings.  Its standard
     value is ‘(".gz")’.

     If the optional argument NOSUFFIX is non-‘nil’, then ‘load’ does
     not try the suffixes ‘.elc’ and ‘.el’.  In this case, you must
     specify the precise file name you want, except that, if Auto
     Compression mode is enabled, ‘load’ will still use
     ‘jka-compr-load-suffixes’ to find compressed versions.  By
     specifying the precise file name and using ‘t’ for NOSUFFIX, you
     can prevent file names like ‘foo.el.el’ from being tried.

     If the optional argument MUST-SUFFIX is non-‘nil’, then ‘load’
     insists that the file name used must end in either ‘.el’ or ‘.elc’
     (possibly extended with a compression suffix) or the shared-library
     extension, unless it contains an explicit directory name.

     If the option ‘load-prefer-newer’ is non-‘nil’, then when searching
     suffixes, ‘load’ selects whichever version of a file (‘.elc’,
     ‘.el’, etc.) has been modified most recently.

     If FILENAME is a relative file name, such as ‘foo’ or
     ‘baz/foo.bar’, ‘load’ searches for the file using the variable
     ‘load-path’.  It appends FILENAME to each of the directories listed
     in ‘load-path’, and loads the first file it finds whose name
     matches.  The current default directory is tried only if it is
     specified in ‘load-path’, where ‘nil’ stands for the default
     directory.  ‘load’ tries all three possible suffixes in the first
     directory in ‘load-path’, then all three suffixes in the second
     directory, and so on.  *Note Library Search::.

     Whatever the name under which the file is eventually found, and the
     directory where Emacs found it, Emacs sets the value of the
     variable ‘load-file-name’ to that file’s name.

     If you get a warning that ‘foo.elc’ is older than ‘foo.el’, it
     means you should consider recompiling ‘foo.el’.  *Note Byte
     Compilation::.

     When loading a source file (not compiled), ‘load’ performs
     character set translation just as Emacs would do when visiting the
     file.  *Note Coding Systems::.

     When loading an uncompiled file, Emacs tries to expand any macros
     that the file contains (*note Macros::).  We refer to this as
     “eager macro expansion”.  Doing this (rather than deferring the
     expansion until the relevant code runs) can significantly speed up
     the execution of uncompiled code.  Sometimes, this macro expansion
     cannot be done, owing to a cyclic dependency.  In the simplest
     example of this, the file you are loading refers to a macro defined
     in another file, and that file in turn requires the file you are
     loading.  This is generally harmless.  Emacs prints a warning
     (‘Eager macro-expansion skipped due to cycle...’) giving details of
     the problem, but it still loads the file, just leaving the macro
     unexpanded for now.  You may wish to restructure your code so that
     this does not happen.  Loading a compiled file does not cause
     macroexpansion, because this should already have happened during
     compilation.  *Note Compiling Macros::.

     Messages like ‘Loading foo...’ and ‘Loading foo...done’ appear in
     the echo area during loading unless NOMESSAGE is non-‘nil’.

     Any unhandled errors while loading a file terminate loading.  If
     the load was done for the sake of ‘autoload’, any function
     definitions made during the loading are undone.

     If ‘load’ can’t find the file to load, then normally it signals the
     error ‘file-error’ (with ‘Cannot open load file FILENAME’).  But if
     MISSING-OK is non-‘nil’, then ‘load’ just returns ‘nil’.

     You can use the variable ‘load-read-function’ to specify a function
     for ‘load’ to use instead of ‘read’ for reading expressions.  See
     below.

     ‘load’ returns ‘t’ if the file loads successfully.

 -- Command: load-file filename
     This command loads the file FILENAME.  If FILENAME is a relative
     file name, then the current default directory is assumed.  This
     command does not use ‘load-path’, and does not append suffixes.
     However, it does look for compressed versions (if Auto Compression
     Mode is enabled).  Use this command if you wish to specify
     precisely the file name to load.

 -- Command: load-library library
     This command loads the library named LIBRARY.  It is equivalent to
     ‘load’, except for the way it reads its argument interactively.
     *Note (emacs)Lisp Libraries::.

 -- Variable: load-in-progress
     This variable is non-‘nil’ if Emacs is in the process of loading a
     file, and it is ‘nil’ otherwise.

 -- Variable: load-file-name
     When Emacs is in the process of loading a file, this variable’s
     value is the name of that file, as Emacs found it during the search
     described earlier in this section.

 -- Variable: load-read-function
     This variable specifies an alternate expression-reading function
     for ‘load’ and ‘eval-region’ to use instead of ‘read’.  The
     function should accept one argument, just as ‘read’ does.

     By default, this variable’s value is ‘read’.  *Note Input
     Functions::.

     Instead of using this variable, it is cleaner to use another, newer
     feature: to pass the function as the READ-FUNCTION argument to
     ‘eval-region’.  *Note Eval: Definition of eval-region.

   For information about how ‘load’ is used in building Emacs, see *note
Building Emacs::.


File: elisp.info,  Node: Load Suffixes,  Next: Library Search,  Prev: How Programs Do Loading,  Up: Loading

15.2 Load Suffixes
==================

We now describe some technical details about the exact suffixes that
‘load’ tries.

 -- Variable: load-suffixes
     This is a list of suffixes indicating (compiled or source) Emacs
     Lisp files.  It should not include the empty string.  ‘load’ uses
     these suffixes in order when it appends Lisp suffixes to the
     specified file name.  The standard value is ‘(".elc" ".el")’ which
     produces the behavior described in the previous section.

 -- Variable: load-file-rep-suffixes
     This is a list of suffixes that indicate representations of the
     same file.  This list should normally start with the empty string.
     When ‘load’ searches for a file it appends the suffixes in this
     list, in order, to the file name, before searching for another
     file.

     Enabling Auto Compression mode appends the suffixes in
     ‘jka-compr-load-suffixes’ to this list and disabling Auto
     Compression mode removes them again.  The standard value of
     ‘load-file-rep-suffixes’ if Auto Compression mode is disabled is
     ‘("")’.  Given that the standard value of ‘jka-compr-load-suffixes’
     is ‘(".gz")’, the standard value of ‘load-file-rep-suffixes’ if
     Auto Compression mode is enabled is ‘("" ".gz")’.

 -- Function: get-load-suffixes
     This function returns the list of all suffixes that ‘load’ should
     try, in order, when its MUST-SUFFIX argument is non-‘nil’.  This
     takes both ‘load-suffixes’ and ‘load-file-rep-suffixes’ into
     account.  If ‘load-suffixes’, ‘jka-compr-load-suffixes’ and
     ‘load-file-rep-suffixes’ all have their standard values, this
     function returns ‘(".elc" ".elc.gz" ".el" ".el.gz")’ if Auto
     Compression mode is enabled and ‘(".elc" ".el")’ if Auto
     Compression mode is disabled.

   To summarize, ‘load’ normally first tries the suffixes in the value
of ‘(get-load-suffixes)’ and then those in ‘load-file-rep-suffixes’.  If
NOSUFFIX is non-‘nil’, it skips the former group, and if MUST-SUFFIX is
non-‘nil’, it skips the latter group.

 -- User Option: load-prefer-newer
     If this option is non-‘nil’, then rather than stopping at the first
     suffix that exists, ‘load’ tests them all, and uses whichever file
     is the newest.


File: elisp.info,  Node: Library Search,  Next: Loading Non-ASCII,  Prev: Load Suffixes,  Up: Loading

15.3 Library Search
===================

When Emacs loads a Lisp library, it searches for the library in a list
of directories specified by the variable ‘load-path’.

 -- Variable: load-path
     The value of this variable is a list of directories to search when
     loading files with ‘load’.  Each element is a string (which must be
     a directory name) or ‘nil’ (which stands for the current working
     directory).

   When Emacs starts up, it sets up the value of ‘load-path’ in several
steps.  First, it initializes ‘load-path’ using default locations set
when Emacs was compiled.  Normally, this is a directory something like

     "/usr/local/share/emacs/VERSION/lisp"

   (In this and the following examples, replace ‘/usr/local’ with the
installation prefix appropriate for your Emacs.)  These directories
contain the standard Lisp files that come with Emacs.  If Emacs cannot
find them, it will not start correctly.

   If you run Emacs from the directory where it was built—that is, an
executable that has not been formally installed—Emacs instead
initializes ‘load-path’ using the ‘lisp’ directory in the directory
containing the sources from which it was built.  If you built Emacs in a
separate directory from the sources, it also adds the lisp directories
from the build directory.  (In all cases, elements are represented as
absolute file names.)

   Unless you start Emacs with the ‘--no-site-lisp’ option, it then adds
two more ‘site-lisp’ directories to the front of ‘load-path’.  These are
intended for locally installed Lisp files, and are normally of the form:

     "/usr/local/share/emacs/VERSION/site-lisp"

and

     "/usr/local/share/emacs/site-lisp"

The first one is for locally installed files for a specific Emacs
version; the second is for locally installed files meant for use with
all installed Emacs versions.  (If Emacs is running uninstalled, it also
adds ‘site-lisp’ directories from the source and build directories, if
they exist.  Normally these directories do not contain ‘site-lisp’
directories.)

   If the environment variable ‘EMACSLOADPATH’ is set, it modifies the
above initialization procedure.  Emacs initializes ‘load-path’ based on
the value of the environment variable.

   The syntax of ‘EMACSLOADPATH’ is the same as used for ‘PATH’;
directory names are separated by ‘:’ (or ‘;’, on some operating
systems).  Here is an example of how to set ‘EMACSLOADPATH’ variable
(from a ‘sh’-style shell):

     export EMACSLOADPATH=/home/foo/.emacs.d/lisp:

   An empty element in the value of the environment variable, whether
trailing (as in the above example), leading, or embedded, is replaced by
the default value of ‘load-path’ as determined by the standard
initialization procedure.  If there are no such empty elements, then
‘EMACSLOADPATH’ specifies the entire ‘load-path’.  You must include
either an empty element, or the explicit path to the directory
containing the standard Lisp files, else Emacs will not function.
(Another way to modify ‘load-path’ is to use the ‘-L’ command-line
option when starting Emacs; see below.)

   For each directory in ‘load-path’, Emacs then checks to see if it
contains a file ‘subdirs.el’, and if so, loads it.  The ‘subdirs.el’
file is created when Emacs is built/installed, and contains code that
causes Emacs to add any subdirectories of those directories to
‘load-path’.  Both immediate subdirectories and subdirectories multiple
levels down are added.  But it excludes subdirectories whose names do
not start with a letter or digit, and subdirectories named ‘RCS’ or
‘CVS’, and subdirectories containing a file named ‘.nosearch’.

   Next, Emacs adds any extra load directories that you specify using
the ‘-L’ command-line option (*note (emacs)Action Arguments::).  It also
adds the directories where optional packages are installed, if any
(*note Packaging Basics::).

   It is common to add code to one’s init file (*note Init File::) to
add one or more directories to ‘load-path’.  For example:

     (push "~/.emacs.d/lisp" load-path)

   Dumping Emacs uses a special value of ‘load-path’.  If you use a
‘site-load.el’ or ‘site-init.el’ file to customize the dumped Emacs
(*note Building Emacs::), any changes to ‘load-path’ that these files
make will be lost after dumping.

 -- Command: locate-library library &optional nosuffix path
          interactive-call
     This command finds the precise file name for library LIBRARY.  It
     searches for the library in the same way ‘load’ does, and the
     argument NOSUFFIX has the same meaning as in ‘load’: don’t add
     suffixes ‘.elc’ or ‘.el’ to the specified name LIBRARY.

     If the PATH is non-‘nil’, that list of directories is used instead
     of ‘load-path’.

     When ‘locate-library’ is called from a program, it returns the file
     name as a string.  When the user runs ‘locate-library’
     interactively, the argument INTERACTIVE-CALL is ‘t’, and this tells
     ‘locate-library’ to display the file name in the echo area.

 -- Command: list-load-path-shadows &optional stringp
     This command shows a list of “shadowed” Emacs Lisp files.  A
     shadowed file is one that will not normally be loaded, despite
     being in a directory on ‘load-path’, due to the existence of
     another similarly-named file in a directory earlier on ‘load-path’.

     For instance, suppose ‘load-path’ is set to

            ("/opt/emacs/site-lisp" "/usr/share/emacs/23.3/lisp")

     and that both these directories contain a file named ‘foo.el’.
     Then ‘(require 'foo)’ never loads the file in the second directory.
     Such a situation might indicate a problem in the way Emacs was
     installed.

     When called from Lisp, this function prints a message listing the
     shadowed files, instead of displaying them in a buffer.  If the
     optional argument ‘stringp’ is non-‘nil’, it instead returns the
     shadowed files as a string.


File: elisp.info,  Node: Loading Non-ASCII,  Next: Autoload,  Prev: Library Search,  Up: Loading

15.4 Loading Non-ASCII Characters
=================================

When Emacs Lisp programs contain string constants with non-ASCII
characters, these can be represented within Emacs either as unibyte
strings or as multibyte strings (*note Text Representations::).  Which
representation is used depends on how the file is read into Emacs.  If
it is read with decoding into multibyte representation, the text of the
Lisp program will be multibyte text, and its string constants will be
multibyte strings.  If a file containing Latin-1 characters (for
example) is read without decoding, the text of the program will be
unibyte text, and its string constants will be unibyte strings.  *Note
Coding Systems::.

   In most Emacs Lisp programs, the fact that non-ASCII strings are
multibyte strings should not be noticeable, since inserting them in
unibyte buffers converts them to unibyte automatically.  However, if
this does make a difference, you can force a particular Lisp file to be
interpreted as unibyte by writing ‘coding: raw-text’ in a local
variables section.  With that designator, the file will unconditionally
be interpreted as unibyte.  This can matter when making keybindings to
non-ASCII characters written as ‘?vLITERAL’.


File: elisp.info,  Node: Autoload,  Next: Repeated Loading,  Prev: Loading Non-ASCII,  Up: Loading

15.5 Autoload
=============

The “autoload” facility lets you register the existence of a function or
macro, but put off loading the file that defines it.  The first call to
the function automatically loads the proper library, in order to install
the real definition and other associated code, then runs the real
definition as if it had been loaded all along.  Autoloading can also be
triggered by looking up the documentation of the function or macro
(*note Documentation Basics::).

   There are two ways to set up an autoloaded function: by calling
‘autoload’, and by writing a “magic” comment in the source before the
real definition.  ‘autoload’ is the low-level primitive for autoloading;
any Lisp program can call ‘autoload’ at any time.  Magic comments are
the most convenient way to make a function autoload, for packages
installed along with Emacs.  These comments do nothing on their own, but
they serve as a guide for the command ‘update-file-autoloads’, which
constructs calls to ‘autoload’ and arranges to execute them when Emacs
is built.

 -- Function: autoload function filename &optional docstring interactive
          type
     This function defines the function (or macro) named FUNCTION so as
     to load automatically from FILENAME.  The string FILENAME specifies
     the file to load to get the real definition of FUNCTION.

     If FILENAME does not contain either a directory name, or the suffix
     ‘.el’ or ‘.elc’, this function insists on adding one of these
     suffixes, and it will not load from a file whose name is just
     FILENAME with no added suffix.  (The variable ‘load-suffixes’
     specifies the exact required suffixes.)

     The argument DOCSTRING is the documentation string for the
     function.  Specifying the documentation string in the call to
     ‘autoload’ makes it possible to look at the documentation without
     loading the function’s real definition.  Normally, this should be
     identical to the documentation string in the function definition
     itself.  If it isn’t, the function definition’s documentation
     string takes effect when it is loaded.

     If INTERACTIVE is non-‘nil’, that says FUNCTION can be called
     interactively.  This lets completion in ‘M-x’ work without loading
     FUNCTION’s real definition.  The complete interactive specification
     is not given here; it’s not needed unless the user actually calls
     FUNCTION, and when that happens, it’s time to load the real
     definition.

     You can autoload macros and keymaps as well as ordinary functions.
     Specify TYPE as ‘macro’ if FUNCTION is really a macro.  Specify
     TYPE as ‘keymap’ if FUNCTION is really a keymap.  Various parts of
     Emacs need to know this information without loading the real
     definition.

     An autoloaded keymap loads automatically during key lookup when a
     prefix key’s binding is the symbol FUNCTION.  Autoloading does not
     occur for other kinds of access to the keymap.  In particular, it
     does not happen when a Lisp program gets the keymap from the value
     of a variable and calls ‘define-key’; not even if the variable name
     is the same symbol FUNCTION.

     If FUNCTION already has a non-void function definition that is not
     an autoload object, this function does nothing and returns ‘nil’.
     Otherwise, it constructs an autoload object (*note Autoload
     Type::), and stores it as the function definition for FUNCTION.
     The autoload object has this form:

          (autoload FILENAME DOCSTRING INTERACTIVE TYPE)

     For example,

          (symbol-function 'run-prolog)
               ⇒ (autoload "prolog" 169681 t nil)

     In this case, ‘"prolog"’ is the name of the file to load, 169681
     refers to the documentation string in the ‘emacs/etc/DOC’ file
     (*note Documentation Basics::), ‘t’ means the function is
     interactive, and ‘nil’ that it is not a macro or a keymap.

 -- Function: autoloadp object
     This function returns non-‘nil’ if OBJECT is an autoload object.
     For example, to check if ‘run-prolog’ is defined as an autoloaded
     function, evaluate

          (autoloadp (symbol-function 'run-prolog))

   The autoloaded file usually contains other definitions and may
require or provide one or more features.  If the file is not completely
loaded (due to an error in the evaluation of its contents), any function
definitions or ‘provide’ calls that occurred during the load are undone.
This is to ensure that the next attempt to call any function autoloading
from this file will try again to load the file.  If not for this, then
some of the functions in the file might be defined by the aborted load,
but fail to work properly for the lack of certain subroutines not loaded
successfully because they come later in the file.

   If the autoloaded file fails to define the desired Lisp function or
macro, then an error is signaled with data ‘"Autoloading failed to
define function FUNCTION-NAME"’.

   A magic autoload comment (often called an “autoload cookie”) consists
of ‘;;;###autoload’, on a line by itself, just before the real
definition of the function in its autoloadable source file.  The command
‘M-x update-file-autoloads’ writes a corresponding ‘autoload’ call into
‘loaddefs.el’.  (The string that serves as the autoload cookie and the
name of the file generated by ‘update-file-autoloads’ can be changed
from the above defaults, see below.)  Building Emacs loads ‘loaddefs.el’
and thus calls ‘autoload’.  ‘M-x update-directory-autoloads’ is even
more powerful; it updates autoloads for all files in the current
directory.

   The same magic comment can copy any kind of form into ‘loaddefs.el’.
The form following the magic comment is copied verbatim, _except_ if it
is one of the forms which the autoload facility handles specially (e.g.,
by conversion into an ‘autoload’ call).  The forms which are not copied
verbatim are the following:

Definitions for function or function-like objects:
     ‘defun’ and ‘defmacro’; also ‘cl-defun’ and ‘cl-defmacro’ (*note
     (cl)Argument Lists::), and ‘define-overloadable-function’ (see the
     commentary in ‘mode-local.el’).

Definitions for major or minor modes:
     ‘define-minor-mode’, ‘define-globalized-minor-mode’,
     ‘define-generic-mode’, ‘define-derived-mode’,
     ‘easy-mmode-define-minor-mode’, ‘easy-mmode-define-global-mode’,
     ‘define-compilation-mode’, and ‘define-global-minor-mode’.

Other definition types:
     ‘defcustom’, ‘defgroup’, ‘defclass’ (*note EIEIO: (eieio)Top.), and
     ‘define-skeleton’ (*note Autotyping: (autotype)Top.).

   You can also use a magic comment to execute a form at build time
_without_ executing it when the file itself is loaded.  To do this,
write the form _on the same line_ as the magic comment.  Since it is in
a comment, it does nothing when you load the source file; but ‘M-x
update-file-autoloads’ copies it to ‘loaddefs.el’, where it is executed
while building Emacs.

   The following example shows how ‘doctor’ is prepared for autoloading
with a magic comment:

     ;;;###autoload
     (defun doctor ()
       "Switch to *doctor* buffer and start giving psychotherapy."
       (interactive)
       (switch-to-buffer "*doctor*")
       (doctor-mode))

Here’s what that produces in ‘loaddefs.el’:

     (autoload (quote doctor) "doctor" "\
     Switch to *doctor* buffer and start giving psychotherapy.

     \(fn)" t nil)

The backslash and newline immediately following the double-quote are a
convention used only in the preloaded uncompiled Lisp files such as
‘loaddefs.el’; they tell ‘make-docfile’ to put the documentation string
in the ‘etc/DOC’ file.  *Note Building Emacs::.  See also the commentary
in ‘lib-src/make-docfile.c’.  ‘(fn)’ in the usage part of the
documentation string is replaced with the function’s name when the
various help functions (*note Help Functions::) display it.

   If you write a function definition with an unusual macro that is not
one of the known and recognized function definition methods, use of an
ordinary magic autoload comment would copy the whole definition into
‘loaddefs.el’.  That is not desirable.  You can put the desired
‘autoload’ call into ‘loaddefs.el’ instead by writing this:

     ;;;###autoload (autoload 'foo "myfile")
     (mydefunmacro foo
       ...)

   You can use a non-default string as the autoload cookie and have the
corresponding autoload calls written into a file whose name is different
from the default ‘loaddefs.el’.  Emacs provides two variables to control
this:

 -- Variable: generate-autoload-cookie
     The value of this variable should be a string whose syntax is a
     Lisp comment.  ‘M-x update-file-autoloads’ copies the Lisp form
     that follows the cookie into the autoload file it generates.  The
     default value of this variable is ‘";;;###autoload"’.

 -- Variable: generated-autoload-file
     The value of this variable names an Emacs Lisp file where the
     autoload calls should go.  The default value is ‘loaddefs.el’, but
     you can override that, e.g., in the local variables section of a
     ‘.el’ file (*note File Local Variables::).  The autoload file is
     assumed to contain a trailer starting with a formfeed character.

   The following function may be used to explicitly load the library
specified by an autoload object:

 -- Function: autoload-do-load autoload &optional name macro-only
     This function performs the loading specified by AUTOLOAD, which
     should be an autoload object.  The optional argument NAME, if
     non-‘nil’, should be a symbol whose function value is AUTOLOAD; in
     that case, the return value of this function is the symbol’s new
     function value.  If the value of the optional argument MACRO-ONLY
     is ‘macro’, this function avoids loading a function, only a macro.


File: elisp.info,  Node: Repeated Loading,  Next: Named Features,  Prev: Autoload,  Up: Loading

15.6 Repeated Loading
=====================

You can load a given file more than once in an Emacs session.  For
example, after you have rewritten and reinstalled a function definition
by editing it in a buffer, you may wish to return to the original
version; you can do this by reloading the file it came from.

   When you load or reload files, bear in mind that the ‘load’ and
‘load-library’ functions automatically load a byte-compiled file rather
than a non-compiled file of similar name.  If you rewrite a file that
you intend to save and reinstall, you need to byte-compile the new
version; otherwise Emacs will load the older, byte-compiled file instead
of your newer, non-compiled file!  If that happens, the message
displayed when loading the file includes, ‘(compiled; note, source is
newer)’, to remind you to recompile it.

   When writing the forms in a Lisp library file, keep in mind that the
file might be loaded more than once.  For example, think about whether
each variable should be reinitialized when you reload the library;
‘defvar’ does not change the value if the variable is already
initialized.  (*Note Defining Variables::.)

   The simplest way to add an element to an alist is like this:

     (push '(leif-mode " Leif") minor-mode-alist)

But this would add multiple elements if the library is reloaded.  To
avoid the problem, use ‘add-to-list’ (*note List Variables::):

     (add-to-list 'minor-mode-alist '(leif-mode " Leif"))

   Occasionally you will want to test explicitly whether a library has
already been loaded.  If the library uses ‘provide’ to provide a named
feature, you can use ‘featurep’ earlier in the file to test whether the
‘provide’ call has been executed before (*note Named Features::).
Alternatively, you could use something like this:

     (defvar foo-was-loaded nil)

     (unless foo-was-loaded
       EXECUTE-FIRST-TIME-ONLY
       (setq foo-was-loaded t))



File: elisp.info,  Node: Named Features,  Next: Where Defined,  Prev: Repeated Loading,  Up: Loading

15.7 Features
=============

‘provide’ and ‘require’ are an alternative to ‘autoload’ for loading
files automatically.  They work in terms of named “features”.
Autoloading is triggered by calling a specific function, but a feature
is loaded the first time another program asks for it by name.

   A feature name is a symbol that stands for a collection of functions,
variables, etc.  The file that defines them should “provide” the
feature.  Another program that uses them may ensure they are defined by
“requiring” the feature.  This loads the file of definitions if it
hasn’t been loaded already.

   To require the presence of a feature, call ‘require’ with the feature
name as argument.  ‘require’ looks in the global variable ‘features’ to
see whether the desired feature has been provided already.  If not, it
loads the feature from the appropriate file.  This file should call
‘provide’ at the top level to add the feature to ‘features’; if it fails
to do so, ‘require’ signals an error.

   For example, in ‘idlwave.el’, the definition for
‘idlwave-complete-filename’ includes the following code:

     (defun idlwave-complete-filename ()
       "Use the comint stuff to complete a file name."
        (require 'comint)
        (let* ((comint-file-name-chars "~/A-Za-z0-9+@:_.$#%={}\\-")
               (comint-completion-addsuffix nil)
               ...)
            (comint-dynamic-complete-filename)))

The expression ‘(require 'comint)’ loads the file ‘comint.el’ if it has
not yet been loaded, ensuring that ‘comint-dynamic-complete-filename’ is
defined.  Features are normally named after the files that provide them,
so that ‘require’ need not be given the file name.  (Note that it is
important that the ‘require’ statement be outside the body of the ‘let’.
Loading a library while its variables are let-bound can have unintended
consequences, namely the variables becoming unbound after the let
exits.)

   The ‘comint.el’ file contains the following top-level expression:

     (provide 'comint)

This adds ‘comint’ to the global ‘features’ list, so that ‘(require
'comint)’ will henceforth know that nothing needs to be done.

   When ‘require’ is used at top level in a file, it takes effect when
you byte-compile that file (*note Byte Compilation::) as well as when
you load it.  This is in case the required package contains macros that
the byte compiler must know about.  It also avoids byte compiler
warnings for functions and variables defined in the file loaded with
‘require’.

   Although top-level calls to ‘require’ are evaluated during byte
compilation, ‘provide’ calls are not.  Therefore, you can ensure that a
file of definitions is loaded before it is byte-compiled by including a
‘provide’ followed by a ‘require’ for the same feature, as in the
following example.

     (provide 'my-feature)  ; Ignored by byte compiler,
                            ;   evaluated by ‘load’.
     (require 'my-feature)  ; Evaluated by byte compiler.

The compiler ignores the ‘provide’, then processes the ‘require’ by
loading the file in question.  Loading the file does execute the
‘provide’ call, so the subsequent ‘require’ call does nothing when the
file is loaded.

 -- Function: provide feature &optional subfeatures
     This function announces that FEATURE is now loaded, or being
     loaded, into the current Emacs session.  This means that the
     facilities associated with FEATURE are or will be available for
     other Lisp programs.

     The direct effect of calling ‘provide’ is to add FEATURE to the
     front of ‘features’ if it is not already in that list and call any
     ‘eval-after-load’ code waiting for it (*note Hooks for Loading::).
     The argument FEATURE must be a symbol.  ‘provide’ returns FEATURE.

     If provided, SUBFEATURES should be a list of symbols indicating a
     set of specific subfeatures provided by this version of FEATURE.
     You can test the presence of a subfeature using ‘featurep’.  The
     idea of subfeatures is that you use them when a package (which is
     one FEATURE) is complex enough to make it useful to give names to
     various parts or functionalities of the package, which might or
     might not be loaded, or might or might not be present in a given
     version.  *Note Network Feature Testing::, for an example.

          features
               ⇒ (bar bish)

          (provide 'foo)
               ⇒ foo
          features
               ⇒ (foo bar bish)

     When a file is loaded to satisfy an autoload, and it stops due to
     an error in the evaluation of its contents, any function
     definitions or ‘provide’ calls that occurred during the load are
     undone.  *Note Autoload::.

 -- Function: require feature &optional filename noerror
     This function checks whether FEATURE is present in the current
     Emacs session (using ‘(featurep FEATURE)’; see below).  The
     argument FEATURE must be a symbol.

     If the feature is not present, then ‘require’ loads FILENAME with
     ‘load’.  If FILENAME is not supplied, then the name of the symbol
     FEATURE is used as the base file name to load.  However, in this
     case, ‘require’ insists on finding FEATURE with an added ‘.el’ or
     ‘.elc’ suffix (possibly extended with a compression suffix); a file
     whose name is just FEATURE won’t be used.  (The variable
     ‘load-suffixes’ specifies the exact required Lisp suffixes.)

     If NOERROR is non-‘nil’, that suppresses errors from actual loading
     of the file.  In that case, ‘require’ returns ‘nil’ if loading the
     file fails.  Normally, ‘require’ returns FEATURE.

     If loading the file succeeds but does not provide FEATURE,
     ‘require’ signals an error, ‘Required feature FEATURE was not
     provided’.

 -- Function: featurep feature &optional subfeature
     This function returns ‘t’ if FEATURE has been provided in the
     current Emacs session (i.e., if FEATURE is a member of ‘features’.)
     If SUBFEATURE is non-‘nil’, then the function returns ‘t’ only if
     that subfeature is provided as well (i.e., if SUBFEATURE is a
     member of the ‘subfeature’ property of the FEATURE symbol.)

 -- Variable: features
     The value of this variable is a list of symbols that are the
     features loaded in the current Emacs session.  Each symbol was put
     in this list with a call to ‘provide’.  The order of the elements
     in the ‘features’ list is not significant.


File: elisp.info,  Node: Where Defined,  Next: Unloading,  Prev: Named Features,  Up: Loading

15.8 Which File Defined a Certain Symbol
========================================

 -- Function: symbol-file symbol &optional type
     This function returns the name of the file that defined SYMBOL.  If
     TYPE is ‘nil’, then any kind of definition is acceptable.  If TYPE
     is ‘defun’, ‘defvar’, or ‘defface’, that specifies function
     definition, variable definition, or face definition only.

     The value is normally an absolute file name.  It can also be ‘nil’,
     if the definition is not associated with any file.  If SYMBOL
     specifies an autoloaded function, the value can be a relative file
     name without extension.

   The basis for ‘symbol-file’ is the data in the variable
‘load-history’.

 -- Variable: load-history
     The value of this variable is an alist that associates the names of
     loaded library files with the names of the functions and variables
     they defined, as well as the features they provided or required.

     Each element in this alist describes one loaded library (including
     libraries that are preloaded at startup).  It is a list whose CAR
     is the absolute file name of the library (a string).  The rest of
     the list elements have these forms:

     ‘VAR’
          The symbol VAR was defined as a variable.
     ‘(defun . FUN)’
          The function FUN was defined.
     ‘(t . FUN)’
          The function FUN was previously an autoload before this
          library redefined it as a function.  The following element is
          always ‘(defun . FUN)’, which represents defining FUN as a
          function.
     ‘(autoload . FUN)’
          The function FUN was defined as an autoload.
     ‘(defface . FACE)’
          The face FACE was defined.
     ‘(require . FEATURE)’
          The feature FEATURE was required.
     ‘(provide . FEATURE)’
          The feature FEATURE was provided.
     ‘(cl-defmethod METHOD SPECIALIZERS)’
          The named METHOD was defined by using ‘cl-defmethod’, with
          SPECIALIZERS as its specializers.
     ‘(define-type . TYPE)’
          The type TYPE was defined.

     The value of ‘load-history’ may have one element whose CAR is
     ‘nil’.  This element describes definitions made with ‘eval-buffer’
     on a buffer that is not visiting a file.

   The command ‘eval-region’ updates ‘load-history’, but does so by
adding the symbols defined to the element for the file being visited,
rather than replacing that element.  *Note Eval::.


File: elisp.info,  Node: Unloading,  Next: Hooks for Loading,  Prev: Where Defined,  Up: Loading

15.9 Unloading
==============

You can discard the functions and variables loaded by a library to
reclaim memory for other Lisp objects.  To do this, use the function
‘unload-feature’:

 -- Command: unload-feature feature &optional force
     This command unloads the library that provided feature FEATURE.  It
     undefines all functions, macros, and variables defined in that
     library with ‘defun’, ‘defalias’, ‘defsubst’, ‘defmacro’,
     ‘defconst’, ‘defvar’, and ‘defcustom’.  It then restores any
     autoloads formerly associated with those symbols.  (Loading saves
     these in the ‘autoload’ property of the symbol.)

     Before restoring the previous definitions, ‘unload-feature’ runs
     ‘remove-hook’ to remove functions in the library from certain
     hooks.  These hooks include variables whose names end in ‘-hook’
     (or the deprecated suffix ‘-hooks’), plus those listed in
     ‘unload-feature-special-hooks’, as well as ‘auto-mode-alist’.  This
     is to prevent Emacs from ceasing to function because important
     hooks refer to functions that are no longer defined.

     Standard unloading activities also undoes ELP profiling of
     functions in that library, unprovides any features provided by the
     library, and cancels timers held in variables defined by the
     library.

     If these measures are not sufficient to prevent malfunction, a
     library can define an explicit unloader named
     ‘FEATURE-unload-function’.  If that symbol is defined as a
     function, ‘unload-feature’ calls it with no arguments before doing
     anything else.  It can do whatever is appropriate to unload the
     library.  If it returns ‘nil’, ‘unload-feature’ proceeds to take
     the normal unload actions.  Otherwise it considers the job to be
     done.

     Ordinarily, ‘unload-feature’ refuses to unload a library on which
     other loaded libraries depend.  (A library A depends on library B
     if A contains a ‘require’ for B.)  If the optional argument FORCE
     is non-‘nil’, dependencies are ignored and you can unload any
     library.

   The ‘unload-feature’ function is written in Lisp; its actions are
based on the variable ‘load-history’.

 -- Variable: unload-feature-special-hooks
     This variable holds a list of hooks to be scanned before unloading
     a library, to remove functions defined in the library.


File: elisp.info,  Node: Hooks for Loading,  Next: Dynamic Modules,  Prev: Unloading,  Up: Loading

15.10 Hooks for Loading
=======================

You can ask for code to be executed each time Emacs loads a library, by
using the variable ‘after-load-functions’:

 -- Variable: after-load-functions
     This abnormal hook is run after loading a file.  Each function in
     the hook is called with a single argument, the absolute filename of
     the file that was just loaded.

   If you want code to be executed when a _particular_ library is
loaded, use the macro ‘with-eval-after-load’:

 -- Macro: with-eval-after-load library body...
     This macro arranges to evaluate BODY at the end of loading the file
     LIBRARY, each time LIBRARY is loaded.  If LIBRARY is already
     loaded, it evaluates BODY right away.

     You don’t need to give a directory or extension in the file name
     LIBRARY.  Normally, you just give a bare file name, like this:

          (with-eval-after-load "edebug" (def-edebug-spec c-point t))

     To restrict which files can trigger the evaluation, include a
     directory or an extension or both in LIBRARY.  Only a file whose
     absolute true name (i.e., the name with all symbolic links chased
     out) matches all the given name components will match.  In the
     following example, ‘my_inst.elc’ or ‘my_inst.elc.gz’ in some
     directory ‘..../foo/bar’ will trigger the evaluation, but not
     ‘my_inst.el’:

          (with-eval-after-load "foo/bar/my_inst.elc" ...)

     LIBRARY can also be a feature (i.e., a symbol), in which case BODY
     is evaluated at the end of any file where ‘(provide LIBRARY)’ is
     called.

     An error in BODY does not undo the load, but does prevent execution
     of the rest of BODY.

   Normally, well-designed Lisp programs should not use
‘with-eval-after-load’.  If you need to examine and set the variables
defined in another library (those meant for outside use), you can do it
immediately—there is no need to wait until the library is loaded.  If
you need to call functions defined by that library, you should load the
library, preferably with ‘require’ (*note Named Features::).


File: elisp.info,  Node: Dynamic Modules,  Prev: Hooks for Loading,  Up: Loading

15.11 Emacs Dynamic Modules
===========================

A “dynamic Emacs module” is a shared library that provides additional
functionality for use in Emacs Lisp programs, just like a package
written in Emacs Lisp would.

   Functions that load Emacs Lisp packages can also load dynamic
modules.  They recognize dynamic modules by looking at their file-name
extension, a.k.a. “suffix”.  This suffix is platform-dependent.

 -- Variable: module-file-suffix
     This variable holds the system-dependent value of the file-name
     extension of the module files.  Its value is ‘.so’ on Posix hosts
     and ‘.dll’ on MS-Windows.

   Every dynamic module should export a C-callable function named
‘emacs_module_init’, which Emacs will call as part of the call to ‘load’
or ‘require’ which loads the module.  It should also export a symbol
named ‘plugin_is_GPL_compatible’ to indicate that its code is released
under the GPL or compatible license; Emacs will refuse to load modules
that don’t export such a symbol.

   If a module needs to call Emacs functions, it should do so through
the API defined and documented in the header file ‘emacs-module.h’ that
is part of the Emacs distribution.

   Modules can create ‘user-ptr’ Lisp objects that embed pointers to C
struct’s defined by the module.  This is useful for keeping around
complex data structures created by a module, to be passed back to the
module’s functions.  User-ptr objects can also have associated
“finalizers” – functions to be run when the object is GC’ed; this is
useful for freeing any resources allocated for the underlying data
structure, such as memory, open file descriptors, etc.

 -- Function: user-ptrp object
     This function returns ‘t’ if its argument is a ‘user-ptr’ object.

   Loadable modules in Emacs are enabled by using the ‘--with-modules’
option at configure time.


File: elisp.info,  Node: Byte Compilation,  Next: Debugging,  Prev: Loading,  Up: Top

16 Byte Compilation
*******************

Emacs Lisp has a “compiler” that translates functions written in Lisp
into a special representation called “byte-code” that can be executed
more efficiently.  The compiler replaces Lisp function definitions with
byte-code.  When a byte-code function is called, its definition is
evaluated by the “byte-code interpreter”.

   Because the byte-compiled code is evaluated by the byte-code
interpreter, instead of being executed directly by the machine’s
hardware (as true compiled code is), byte-code is completely
transportable from machine to machine without recompilation.  It is not,
however, as fast as true compiled code.

   In general, any version of Emacs can run byte-compiled code produced
by recent earlier versions of Emacs, but the reverse is not true.

   If you do not want a Lisp file to be compiled, ever, put a file-local
variable binding for ‘no-byte-compile’ into it, like this:

     ;; -*-no-byte-compile: t; -*-

* Menu:

* Speed of Byte-Code::          An example of speedup from byte compilation.
* Compilation Functions::       Byte compilation functions.
* Docs and Compilation::        Dynamic loading of documentation strings.
* Dynamic Loading::             Dynamic loading of individual functions.
* Eval During Compile::         Code to be evaluated when you compile.
* Compiler Errors::             Handling compiler error messages.
* Byte-Code Objects::           The data type used for byte-compiled functions.
* Disassembly::                 Disassembling byte-code; how to read byte-code.


File: elisp.info,  Node: Speed of Byte-Code,  Next: Compilation Functions,  Up: Byte Compilation

16.1 Performance of Byte-Compiled Code
======================================

A byte-compiled function is not as efficient as a primitive function
written in C, but runs much faster than the version written in Lisp.
Here is an example:

     (defun silly-loop (n)
       "Return the time, in seconds, to run N iterations of a loop."
       (let ((t1 (float-time)))
         (while (> (setq n (1- n)) 0))
         (- (float-time) t1)))
     ⇒ silly-loop

     (silly-loop 50000000)
     ⇒ 10.235304117202759

     (byte-compile 'silly-loop)
     ⇒ [Compiled code not shown]

     (silly-loop 50000000)
     ⇒ 3.705854892730713

   In this example, the interpreted code required 10 seconds to run,
whereas the byte-compiled code required less than 4 seconds.  These
results are representative, but actual results may vary.


File: elisp.info,  Node: Compilation Functions,  Next: Docs and Compilation,  Prev: Speed of Byte-Code,  Up: Byte Compilation

16.2 Byte-Compilation Functions
===============================

You can byte-compile an individual function or macro definition with the
‘byte-compile’ function.  You can compile a whole file with
‘byte-compile-file’, or several files with ‘byte-recompile-directory’ or
‘batch-byte-compile’.

   Sometimes, the byte compiler produces warning and/or error messages
(*note Compiler Errors::, for details).  These messages are normally
recorded in a buffer called ‘*Compile-Log*’, which uses Compilation
mode.  *Note (emacs)Compilation Mode::.  However, if the variable
‘byte-compile-debug’ is non-nil, error message will be signaled as Lisp
errors instead (*note Errors::).

   Be careful when writing macro calls in files that you intend to
byte-compile.  Since macro calls are expanded when they are compiled,
the macros need to be loaded into Emacs or the byte compiler will not do
the right thing.  The usual way to handle this is with ‘require’ forms
which specify the files containing the needed macro definitions (*note
Named Features::).  Normally, the byte compiler does not evaluate the
code that it is compiling, but it handles ‘require’ forms specially, by
loading the specified libraries.  To avoid loading the macro definition
files when someone _runs_ the compiled program, write
‘eval-when-compile’ around the ‘require’ calls (*note Eval During
Compile::).  For more details, *Note Compiling Macros::.

   Inline (‘defsubst’) functions are less troublesome; if you compile a
call to such a function before its definition is known, the call will
still work right, it will just run slower.

 -- Function: byte-compile symbol
     This function byte-compiles the function definition of SYMBOL,
     replacing the previous definition with the compiled one.  The
     function definition of SYMBOL must be the actual code for the
     function; ‘byte-compile’ does not handle function indirection.  The
     return value is the byte-code function object which is the compiled
     definition of SYMBOL (*note Byte-Code Objects::).

          (defun factorial (integer)
            "Compute factorial of INTEGER."
            (if (= 1 integer) 1
              (* integer (factorial (1- integer)))))
          ⇒ factorial

          (byte-compile 'factorial)
          ⇒
          #[(integer)
            "^H\301U\203^H^@\301\207\302^H\303^HS!\"\207"
            [integer 1 * factorial]
            4 "Compute factorial of INTEGER."]

     If SYMBOL’s definition is a byte-code function object,
     ‘byte-compile’ does nothing and returns ‘nil’.  It does not compile
     the symbol’s definition again, since the original (non-compiled)
     code has already been replaced in the symbol’s function cell by the
     byte-compiled code.

     The argument to ‘byte-compile’ can also be a ‘lambda’ expression.
     In that case, the function returns the corresponding compiled code
     but does not store it anywhere.

 -- Command: compile-defun &optional arg
     This command reads the defun containing point, compiles it, and
     evaluates the result.  If you use this on a defun that is actually
     a function definition, the effect is to install a compiled version
     of that function.

     ‘compile-defun’ normally displays the result of evaluation in the
     echo area, but if ARG is non-‘nil’, it inserts the result in the
     current buffer after the form it compiled.

 -- Command: byte-compile-file filename &optional load
     This function compiles a file of Lisp code named FILENAME into a
     file of byte-code.  The output file’s name is made by changing the
     ‘.el’ suffix into ‘.elc’; if FILENAME does not end in ‘.el’, it
     adds ‘.elc’ to the end of FILENAME.

     Compilation works by reading the input file one form at a time.  If
     it is a definition of a function or macro, the compiled function or
     macro definition is written out.  Other forms are batched together,
     then each batch is compiled, and written so that its compiled code
     will be executed when the file is read.  All comments are discarded
     when the input file is read.

     This command returns ‘t’ if there were no errors and ‘nil’
     otherwise.  When called interactively, it prompts for the file
     name.

     If LOAD is non-‘nil’, this command loads the compiled file after
     compiling it.  Interactively, LOAD is the prefix argument.

          $ ls -l push*
          -rw-r--r-- 1 lewis lewis 791 Oct  5 20:31 push.el

          (byte-compile-file "~/emacs/push.el")
               ⇒ t

          $ ls -l push*
          -rw-r--r-- 1 lewis lewis 791 Oct  5 20:31 push.el
          -rw-rw-rw- 1 lewis lewis 638 Oct  8 20:25 push.elc

 -- Command: byte-recompile-directory directory &optional flag force
     This command recompiles every ‘.el’ file in DIRECTORY (or its
     subdirectories) that needs recompilation.  A file needs
     recompilation if a ‘.elc’ file exists but is older than the ‘.el’
     file.

     When a ‘.el’ file has no corresponding ‘.elc’ file, FLAG says what
     to do.  If it is ‘nil’, this command ignores these files.  If FLAG
     is 0, it compiles them.  If it is neither ‘nil’ nor 0, it asks the
     user whether to compile each such file, and asks about each
     subdirectory as well.

     Interactively, ‘byte-recompile-directory’ prompts for DIRECTORY and
     FLAG is the prefix argument.

     If FORCE is non-‘nil’, this command recompiles every ‘.el’ file
     that has a ‘.elc’ file.

     The returned value is unpredictable.

 -- Function: batch-byte-compile &optional noforce
     This function runs ‘byte-compile-file’ on files specified on the
     command line.  This function must be used only in a batch execution
     of Emacs, as it kills Emacs on completion.  An error in one file
     does not prevent processing of subsequent files, but no output file
     will be generated for it, and the Emacs process will terminate with
     a nonzero status code.

     If NOFORCE is non-‘nil’, this function does not recompile files
     that have an up-to-date ‘.elc’ file.

          $ emacs -batch -f batch-byte-compile *.el


File: elisp.info,  Node: Docs and Compilation,  Next: Dynamic Loading,  Prev: Compilation Functions,  Up: Byte Compilation

16.3 Documentation Strings and Compilation
==========================================

When Emacs loads functions and variables from a byte-compiled file, it
normally does not load their documentation strings into memory.  Each
documentation string is dynamically loaded from the byte-compiled file
only when needed.  This saves memory, and speeds up loading by skipping
the processing of the documentation strings.

   This feature has a drawback: if you delete, move, or alter the
compiled file (such as by compiling a new version), Emacs may no longer
be able to access the documentation string of previously-loaded
functions or variables.  Such a problem normally only occurs if you
build Emacs yourself, and happen to edit and/or recompile the Lisp
source files.  To solve it, just reload each file after recompilation.

   Dynamic loading of documentation strings from byte-compiled files is
determined, at compile time, for each byte-compiled file.  It can be
disabled via the option ‘byte-compile-dynamic-docstrings’.

 -- User Option: byte-compile-dynamic-docstrings
     If this is non-‘nil’, the byte compiler generates compiled files
     that are set up for dynamic loading of documentation strings.

     To disable the dynamic loading feature for a specific file, set
     this option to ‘nil’ in its header line (*note Local Variables in
     Files: (emacs)File Variables.), like this:

          -*-byte-compile-dynamic-docstrings: nil;-*-

     This is useful mainly if you expect to change the file, and you
     want Emacs sessions that have already loaded it to keep working
     when the file changes.

   Internally, the dynamic loading of documentation strings is
accomplished by writing compiled files with a special Lisp reader
construct, ‘#@COUNT’.  This construct skips the next COUNT characters.
It also uses the ‘#$’ construct, which stands for the name of this file,
as a string.  Do not use these constructs in Lisp source files; they are
not designed to be clear to humans reading the file.


File: elisp.info,  Node: Dynamic Loading,  Next: Eval During Compile,  Prev: Docs and Compilation,  Up: Byte Compilation

16.4 Dynamic Loading of Individual Functions
============================================

When you compile a file, you can optionally enable the “dynamic function
loading” feature (also known as “lazy loading”).  With dynamic function
loading, loading the file doesn’t fully read the function definitions in
the file.  Instead, each function definition contains a place-holder
which refers to the file.  The first time each function is called, it
reads the full definition from the file, to replace the place-holder.

   The advantage of dynamic function loading is that loading the file
becomes much faster.  This is a good thing for a file which contains
many separate user-callable functions, if using one of them does not
imply you will probably also use the rest.  A specialized mode which
provides many keyboard commands often has that usage pattern: a user may
invoke the mode, but use only a few of the commands it provides.

   The dynamic loading feature has certain disadvantages:

   • If you delete or move the compiled file after loading it, Emacs can
     no longer load the remaining function definitions not already
     loaded.

   • If you alter the compiled file (such as by compiling a new
     version), then trying to load any function not already loaded will
     usually yield nonsense results.

   These problems will never happen in normal circumstances with
installed Emacs files.  But they are quite likely to happen with Lisp
files that you are changing.  The easiest way to prevent these problems
is to reload the new compiled file immediately after each recompilation.

   The byte compiler uses the dynamic function loading feature if the
variable ‘byte-compile-dynamic’ is non-‘nil’ at compilation time.  Do
not set this variable globally, since dynamic loading is desirable only
for certain files.  Instead, enable the feature for specific source
files with file-local variable bindings.  For example, you could do it
by writing this text in the source file’s first line:

     -*-byte-compile-dynamic: t;-*-

 -- Variable: byte-compile-dynamic
     If this is non-‘nil’, the byte compiler generates compiled files
     that are set up for dynamic function loading.

 -- Function: fetch-bytecode function
     If FUNCTION is a byte-code function object, this immediately
     finishes loading the byte code of FUNCTION from its byte-compiled
     file, if it is not fully loaded already.  Otherwise, it does
     nothing.  It always returns FUNCTION.


File: elisp.info,  Node: Eval During Compile,  Next: Compiler Errors,  Prev: Dynamic Loading,  Up: Byte Compilation

16.5 Evaluation During Compilation
==================================

These features permit you to write code to be evaluated during
compilation of a program.

 -- Special Form: eval-and-compile body...
     This form marks BODY to be evaluated both when you compile the
     containing code and when you run it (whether compiled or not).

     You can get a similar result by putting BODY in a separate file and
     referring to that file with ‘require’.  That method is preferable
     when BODY is large.  Effectively ‘require’ is automatically
     ‘eval-and-compile’, the package is loaded both when compiling and
     executing.

     ‘autoload’ is also effectively ‘eval-and-compile’ too.  It’s
     recognized when compiling, so uses of such a function don’t produce
     “not known to be defined” warnings.

     Most uses of ‘eval-and-compile’ are fairly sophisticated.

     If a macro has a helper function to build its result, and that
     macro is used both locally and outside the package, then
     ‘eval-and-compile’ should be used to get the helper both when
     compiling and then later when running.

     If functions are defined programmatically (with ‘fset’ say), then
     ‘eval-and-compile’ can be used to have that done at compile-time as
     well as run-time, so calls to those functions are checked (and
     warnings about “not known to be defined” suppressed).

 -- Special Form: eval-when-compile body...
     This form marks BODY to be evaluated at compile time but not when
     the compiled program is loaded.  The result of evaluation by the
     compiler becomes a constant which appears in the compiled program.
     If you load the source file, rather than compiling it, BODY is
     evaluated normally.

     If you have a constant that needs some calculation to produce,
     ‘eval-when-compile’ can do that at compile-time.  For example,

          (defvar my-regexp
            (eval-when-compile (regexp-opt '("aaa" "aba" "abb"))))

     If you’re using another package, but only need macros from it (the
     byte compiler will expand those), then ‘eval-when-compile’ can be
     used to load it for compiling, but not executing.  For example,

          (eval-when-compile
            (require 'my-macro-package))

     The same sort of thing goes for macros and ‘defsubst’ functions
     defined locally and only for use within the file.  They are needed
     for compiling the file, but in most cases they are not needed for
     execution of the compiled file.  For example,

          (eval-when-compile
            (unless (fboundp 'some-new-thing)
              (defmacro 'some-new-thing ()
                (compatibility code))))

     This is often good for code that’s only a fallback for
     compatibility with other versions of Emacs.

     *Common Lisp Note:* At top level, ‘eval-when-compile’ is analogous
     to the Common Lisp idiom ‘(eval-when (compile eval) ...)’.
     Elsewhere, the Common Lisp ‘#.’ reader macro (but not when
     interpreting) is closer to what ‘eval-when-compile’ does.


File: elisp.info,  Node: Compiler Errors,  Next: Byte-Code Objects,  Prev: Eval During Compile,  Up: Byte Compilation

16.6 Compiler Errors
====================

Error and warning messages from byte compilation are printed in a buffer
named ‘*Compile-Log*’.  These messages include file names and line
numbers identifying the location of the problem.  The usual Emacs
commands for operating on compiler output can be used on these messages.

   When an error is due to invalid syntax in the program, the byte
compiler might get confused about the error’s exact location.  One way
to investigate is to switch to the buffer ‘ *Compiler Input*’.  (This
buffer name starts with a space, so it does not show up in the Buffer
Menu.)  This buffer contains the program being compiled, and point shows
how far the byte compiler was able to read; the cause of the error might
be nearby.  *Note Syntax Errors::, for some tips for locating syntax
errors.

   A common type of warning issued by the byte compiler is for functions
and variables that were used but not defined.  Such warnings report the
line number for the end of the file, not the locations where the missing
functions or variables were used; to find these, you must search the
file manually.

   If you are sure that a warning message about a missing function or
variable is unjustified, there are several ways to suppress it:

   • You can suppress the warning for a specific call to a function FUNC
     by conditionalizing it on an ‘fboundp’ test, like this:

          (if (fboundp 'FUNC) ...(FUNC ...)...)

     The call to FUNC must be in the THEN-FORM of the ‘if’, and FUNC
     must appear quoted in the call to ‘fboundp’.  (This feature
     operates for ‘cond’ as well.)

   • Likewise, you can suppress the warning for a specific use of a
     variable VARIABLE by conditionalizing it on a ‘boundp’ test:

          (if (boundp 'VARIABLE) ...VARIABLE...)

     The reference to VARIABLE must be in the THEN-FORM of the ‘if’, and
     VARIABLE must appear quoted in the call to ‘boundp’.

   • You can tell the compiler that a function is defined using
     ‘declare-function’.  *Note Declaring Functions::.

   • Likewise, you can tell the compiler that a variable is defined
     using ‘defvar’ with no initial value.  (Note that this marks the
     variable as special.)  *Note Defining Variables::.

   You can also suppress any and all compiler warnings within a certain
expression using the construct ‘with-no-warnings’:

 -- Special Form: with-no-warnings body...
     In execution, this is equivalent to ‘(progn BODY...)’, but the
     compiler does not issue warnings for anything that occurs inside
     BODY.

     We recommend that you use this construct around the smallest
     possible piece of code, to avoid missing possible warnings other
     than one you intend to suppress.

   Byte compiler warnings can be controlled more precisely by setting
the variable ‘byte-compile-warnings’.  See its documentation string for
details.


File: elisp.info,  Node: Byte-Code Objects,  Next: Disassembly,  Prev: Compiler Errors,  Up: Byte Compilation

16.7 Byte-Code Function Objects
===============================

Byte-compiled functions have a special data type: they are “byte-code
function objects”.  Whenever such an object appears as a function to be
called, Emacs uses the byte-code interpreter to execute the byte-code.

   Internally, a byte-code function object is much like a vector; its
elements can be accessed using ‘aref’.  Its printed representation is
like that for a vector, with an additional ‘#’ before the opening ‘[’.
It must have at least four elements; there is no maximum number, but
only the first six elements have any normal use.  They are:

ARGDESC
     The descriptor of the arguments.  This can either be a list of
     arguments, as described in *note Argument List::, or an integer
     encoding the required number of arguments.  In the latter case, the
     value of the descriptor specifies the minimum number of arguments
     in the bits zero to 6, and the maximum number of arguments in bits
     8 to 14.  If the argument list uses ‘&rest’, then bit 7 is set;
     otherwise it’s cleared.

     If ARGDESC is a list, the arguments will be dynamically bound
     before executing the byte code.  If ARGDESC is an integer, the
     arguments will be instead pushed onto the stack of the byte-code
     interpreter, before executing the code.

BYTE-CODE
     The string containing the byte-code instructions.

CONSTANTS
     The vector of Lisp objects referenced by the byte code.  These
     include symbols used as function names and variable names.

STACKSIZE
     The maximum stack size this function needs.

DOCSTRING
     The documentation string (if any); otherwise, ‘nil’.  The value may
     be a number or a list, in case the documentation string is stored
     in a file.  Use the function ‘documentation’ to get the real
     documentation string (*note Accessing Documentation::).

INTERACTIVE
     The interactive spec (if any).  This can be a string or a Lisp
     expression.  It is ‘nil’ for a function that isn’t interactive.

   Here’s an example of a byte-code function object, in printed
representation.  It is the definition of the command ‘backward-sexp’.

     #[256
       "\211\204^G^@\300\262^A\301^A[!\207"
       [1 forward-sexp]
       3
       1793299
       "^p"]

   The primitive way to create a byte-code object is with
‘make-byte-code’:

 -- Function: make-byte-code &rest elements
     This function constructs and returns a byte-code function object
     with ELEMENTS as its elements.

   You should not try to come up with the elements for a byte-code
function yourself, because if they are inconsistent, Emacs may crash
when you call the function.  Always leave it to the byte compiler to
create these objects; it makes the elements consistent (we hope).


File: elisp.info,  Node: Disassembly,  Prev: Byte-Code Objects,  Up: Byte Compilation

16.8 Disassembled Byte-Code
===========================

People do not write byte-code; that job is left to the byte compiler.
But we provide a disassembler to satisfy a cat-like curiosity.  The
disassembler converts the byte-compiled code into human-readable form.

   The byte-code interpreter is implemented as a simple stack machine.
It pushes values onto a stack of its own, then pops them off to use them
in calculations whose results are themselves pushed back on the stack.
When a byte-code function returns, it pops a value off the stack and
returns it as the value of the function.

   In addition to the stack, byte-code functions can use, bind, and set
ordinary Lisp variables, by transferring values between variables and
the stack.

 -- Command: disassemble object &optional buffer-or-name
     This command displays the disassembled code for OBJECT.  In
     interactive use, or if BUFFER-OR-NAME is ‘nil’ or omitted, the
     output goes in a buffer named ‘*Disassemble*’.  If BUFFER-OR-NAME
     is non-‘nil’, it must be a buffer or the name of an existing
     buffer.  Then the output goes there, at point, and point is left
     before the output.

     The argument OBJECT can be a function name, a lambda expression
     (*note Lambda Expressions::), or a byte-code object (*note
     Byte-Code Objects::).  If it is a lambda expression, ‘disassemble’
     compiles it and disassembles the resulting compiled code.

   Here are two examples of using the ‘disassemble’ function.  We have
added explanatory comments to help you relate the byte-code to the Lisp
source; these do not appear in the output of ‘disassemble’.

     (defun factorial (integer)
       "Compute factorial of an integer."
       (if (= 1 integer) 1
         (* integer (factorial (1- integer)))))
          ⇒ factorial

     (factorial 4)
          ⇒ 24

     (disassemble 'factorial)
          ⊣ byte-code for factorial:
      doc: Compute factorial of an integer.
      args: (integer)

     0   varref   integer      ; Get the value of ‘integer’ and
                               ;   push it onto the stack.
     1   constant 1            ; Push 1 onto stack.
     2   eqlsign               ; Pop top two values off stack, compare
                               ;   them, and push result onto stack.
     3   goto-if-nil 1         ; Pop and test top of stack;
                               ;   if ‘nil’, go to 1, else continue.
     6   constant 1            ; Push 1 onto top of stack.
     7   return                ; Return the top element of the stack.
     8:1 varref   integer      ; Push value of ‘integer’ onto stack.
     9   constant factorial    ; Push ‘factorial’ onto stack.
     10  varref   integer      ; Push value of ‘integer’ onto stack.
     11  sub1                  ; Pop ‘integer’, decrement value,
                               ;   push new value onto stack.
     12  call     1            ; Call function ‘factorial’ using first
                               ;   (i.e., top) stack element as argument;
                               ;   push returned value onto stack.
     13 mult                   ; Pop top two values off stack, multiply
                               ;   them, and push result onto stack.
     14 return                 ; Return the top element of the stack.

   The ‘silly-loop’ function is somewhat more complex:

     (defun silly-loop (n)
       "Return time before and after N iterations of a loop."
       (let ((t1 (current-time-string)))
         (while (> (setq n (1- n))
                   0))
         (list t1 (current-time-string))))
          ⇒ silly-loop

     (disassemble 'silly-loop)
          ⊣ byte-code for silly-loop:
      doc: Return time before and after N iterations of a loop.
      args: (n)

     0   constant current-time-string  ; Push ‘current-time-string’
                                       ;   onto top of stack.
     1   call     0            ; Call ‘current-time-string’ with no
                               ;   argument, push result onto stack.
     2   varbind  t1           ; Pop stack and bind ‘t1’ to popped value.
     3:1 varref   n            ; Get value of ‘n’ from the environment
                               ;   and push the value on the stack.
     4   sub1                  ; Subtract 1 from top of stack.
     5   dup                   ; Duplicate top of stack; i.e., copy the top
                               ;   of the stack and push copy onto stack.
     6   varset   n            ; Pop the top of the stack,
                               ;   and bind ‘n’ to the value.

     ;; (In effect, the sequence ‘dup varset’ copies the top of the stack
     ;; into the value of ‘n’ without popping it.)

     7   constant 0            ; Push 0 onto stack.
     8   gtr                   ; Pop top two values off stack,
                               ;   test if N is greater than 0
                               ;   and push result onto stack.
     9   goto-if-not-nil 1     ; Goto 1 if ‘n’ > 0
                               ;   (this continues the while loop)
                               ;   else continue.
     12  varref   t1           ; Push value of ‘t1’ onto stack.
     13  constant current-time-string  ; Push ‘current-time-string’
                                       ;   onto the top of the stack.
     14  call     0            ; Call ‘current-time-string’ again.
     15  unbind   1            ; Unbind ‘t1’ in local environment.
     16  list2                 ; Pop top two elements off stack, create a
                               ;   list of them, and push it onto stack.
     17  return                ; Return value of the top of stack.


File: elisp.info,  Node: Debugging,  Next: Read and Print,  Prev: Byte Compilation,  Up: Top

17 Debugging Lisp Programs
**************************

There are several ways to find and investigate problems in an Emacs Lisp
program.

   • If a problem occurs when you run the program, you can use the
     built-in Emacs Lisp debugger to suspend the Lisp evaluator, and
     examine and/or alter its internal state.

   • You can use Edebug, a source-level debugger for Emacs Lisp.

   • If a syntactic problem is preventing Lisp from even reading the
     program, you can locate it using Lisp editing commands.

   • You can look at the error and warning messages produced by the byte
     compiler when it compiles the program.  *Note Compiler Errors::.

   • You can use the Testcover package to perform coverage testing on
     the program.

   • You can use the ERT package to write regression tests for the
     program.  *Note the ERT manual: (ert)Top.

   • You can profile the program to get hints about how to make it more
     efficient.

   Other useful tools for debugging input and output problems are the
dribble file (*note Terminal Input::) and the ‘open-termscript’ function
(*note Terminal Output::).

* Menu:

* Debugger::            A debugger for the Emacs Lisp evaluator.
* Edebug::              A source-level Emacs Lisp debugger.
* Syntax Errors::       How to find syntax errors.
* Test Coverage::       Ensuring you have tested all branches in your code.
* Profiling::           Measuring the resources that your code uses.


File: elisp.info,  Node: Debugger,  Next: Edebug,  Up: Debugging

17.1 The Lisp Debugger
======================

The ordinary “Lisp debugger” provides the ability to suspend evaluation
of a form.  While evaluation is suspended (a state that is commonly
known as a “break”), you may examine the run time stack, examine the
values of local or global variables, or change those values.  Since a
break is a recursive edit, all the usual editing facilities of Emacs are
available; you can even run programs that will enter the debugger
recursively.  *Note Recursive Editing::.

* Menu:

* Error Debugging::       Entering the debugger when an error happens.
* Infinite Loops::        Stopping and debugging a program that doesn’t exit.
* Function Debugging::    Entering it when a certain function is called.
* Explicit Debug::        Entering it at a certain point in the program.
* Using Debugger::        What the debugger does; what you see while in it.
* Debugger Commands::     Commands used while in the debugger.
* Invoking the Debugger:: How to call the function ‘debug’.
* Internals of Debugger:: Subroutines of the debugger, and global variables.


File: elisp.info,  Node: Error Debugging,  Next: Infinite Loops,  Up: Debugger

17.1.1 Entering the Debugger on an Error
----------------------------------------

The most important time to enter the debugger is when a Lisp error
happens.  This allows you to investigate the immediate causes of the
error.

   However, entry to the debugger is not a normal consequence of an
error.  Many commands signal Lisp errors when invoked inappropriately,
and during ordinary editing it would be very inconvenient to enter the
debugger each time this happens.  So if you want errors to enter the
debugger, set the variable ‘debug-on-error’ to non-‘nil’.  (The command
‘toggle-debug-on-error’ provides an easy way to do this.)

 -- User Option: debug-on-error
     This variable determines whether the debugger is called when an
     error is signaled and not handled.  If ‘debug-on-error’ is ‘t’, all
     kinds of errors call the debugger, except those listed in
     ‘debug-ignored-errors’ (see below).  If it is ‘nil’, none call the
     debugger.

     The value can also be a list of error conditions (*note Signaling
     Errors::).  Then the debugger is called only for error conditions
     in this list (except those also listed in ‘debug-ignored-errors’).
     For example, if you set ‘debug-on-error’ to the list
     ‘(void-variable)’, the debugger is only called for errors about a
     variable that has no value.

     Note that ‘eval-expression-debug-on-error’ overrides this variable
     in some cases; see below.

     When this variable is non-‘nil’, Emacs does not create an error
     handler around process filter functions and sentinels.  Therefore,
     errors in these functions also invoke the debugger.  *Note
     Processes::.

 -- User Option: debug-ignored-errors
     This variable specifies errors which should not enter the debugger,
     regardless of the value of ‘debug-on-error’.  Its value is a list
     of error condition symbols and/or regular expressions.  If the
     error has any of those condition symbols, or if the error message
     matches any of the regular expressions, then that error does not
     enter the debugger.

     The normal value of this variable includes ‘user-error’, as well as
     several errors that happen often during editing but rarely result
     from bugs in Lisp programs.  However, “rarely” is not “never”; if
     your program fails with an error that matches this list, you may
     try changing this list to debug the error.  The easiest way is
     usually to set ‘debug-ignored-errors’ to ‘nil’.

 -- User Option: eval-expression-debug-on-error
     If this variable has a non-‘nil’ value (the default), running the
     command ‘eval-expression’ causes ‘debug-on-error’ to be temporarily
     bound to to ‘t’.  *Note Evaluating Emacs-Lisp Expressions:
     (emacs)Lisp Eval.

     If ‘eval-expression-debug-on-error’ is ‘nil’, then the value of
     ‘debug-on-error’ is not changed during ‘eval-expression’.

 -- User Option: debug-on-signal
     Normally, errors caught by ‘condition-case’ never invoke the
     debugger.  The ‘condition-case’ gets a chance to handle the error
     before the debugger gets a chance.

     If you change ‘debug-on-signal’ to a non-‘nil’ value, the debugger
     gets the first chance at every error, regardless of the presence of
     ‘condition-case’.  (To invoke the debugger, the error must still
     fulfill the criteria specified by ‘debug-on-error’ and
     ‘debug-ignored-errors’.)

     For example, setting this variable is useful to get a backtrace
     from code evaluated by emacsclient’s ‘--eval’ option.  If Lisp code
     evaluated by emacsclient signals an error while this variable is
     non-‘nil’, the backtrace will popup in the running Emacs.

     *Warning:* Setting this variable to non-‘nil’ may have annoying
     effects.  Various parts of Emacs catch errors in the normal course
     of affairs, and you may not even realize that errors happen there.
     If you need to debug code wrapped in ‘condition-case’, consider
     using ‘condition-case-unless-debug’ (*note Handling Errors::).

 -- User Option: debug-on-event
     If you set ‘debug-on-event’ to a special event (*note Special
     Events::), Emacs will try to enter the debugger as soon as it
     receives this event, bypassing ‘special-event-map’.  At present,
     the only supported values correspond to the signals ‘SIGUSR1’ and
     ‘SIGUSR2’ (this is the default).  This can be helpful when
     ‘inhibit-quit’ is set and Emacs is not otherwise responding.

 -- Variable: debug-on-message
     If you set ‘debug-on-message’ to a regular expression, Emacs will
     enter the debugger if it displays a matching message in the echo
     area.  For example, this can be useful when trying to find the
     cause of a particular message.

   To debug an error that happens during loading of the init file, use
the option ‘--debug-init’.  This binds ‘debug-on-error’ to ‘t’ while
loading the init file, and bypasses the ‘condition-case’ which normally
catches errors in the init file.


File: elisp.info,  Node: Infinite Loops,  Next: Function Debugging,  Prev: Error Debugging,  Up: Debugger

17.1.2 Debugging Infinite Loops
-------------------------------

When a program loops infinitely and fails to return, your first problem
is to stop the loop.  On most operating systems, you can do this with
‘C-g’, which causes a “quit”.  *Note Quitting::.

   Ordinary quitting gives no information about why the program was
looping.  To get more information, you can set the variable
‘debug-on-quit’ to non-‘nil’.  Once you have the debugger running in the
middle of the infinite loop, you can proceed from the debugger using the
stepping commands.  If you step through the entire loop, you may get
enough information to solve the problem.

   Quitting with ‘C-g’ is not considered an error, and ‘debug-on-error’
has no effect on the handling of ‘C-g’.  Likewise, ‘debug-on-quit’ has
no effect on errors.

 -- User Option: debug-on-quit
     This variable determines whether the debugger is called when ‘quit’
     is signaled and not handled.  If ‘debug-on-quit’ is non-‘nil’, then
     the debugger is called whenever you quit (that is, type ‘C-g’).  If
     ‘debug-on-quit’ is ‘nil’ (the default), then the debugger is not
     called when you quit.


File: elisp.info,  Node: Function Debugging,  Next: Explicit Debug,  Prev: Infinite Loops,  Up: Debugger

17.1.3 Entering the Debugger on a Function Call
-----------------------------------------------

To investigate a problem that happens in the middle of a program, one
useful technique is to enter the debugger whenever a certain function is
called.  You can do this to the function in which the problem occurs,
and then step through the function, or you can do this to a function
called shortly before the problem, step quickly over the call to that
function, and then step through its caller.

 -- Command: debug-on-entry function-name
     This function requests FUNCTION-NAME to invoke the debugger each
     time it is called.

     Any function or macro defined as Lisp code may be set to break on
     entry, regardless of whether it is interpreted code or compiled
     code.  If the function is a command, it will enter the debugger
     when called from Lisp and when called interactively (after the
     reading of the arguments).  You can also set debug-on-entry for
     primitive functions (i.e., those written in C) this way, but it
     only takes effect when the primitive is called from Lisp code.
     Debug-on-entry is not allowed for special forms.

     When ‘debug-on-entry’ is called interactively, it prompts for
     FUNCTION-NAME in the minibuffer.  If the function is already set up
     to invoke the debugger on entry, ‘debug-on-entry’ does nothing.
     ‘debug-on-entry’ always returns FUNCTION-NAME.

     Here’s an example to illustrate use of this function:

          (defun fact (n)
            (if (zerop n) 1
                (* n (fact (1- n)))))
               ⇒ fact
          (debug-on-entry 'fact)
               ⇒ fact
          (fact 3)

          ------ Buffer: *Backtrace* ------
          Debugger entered--entering a function:
          * fact(3)
            eval((fact 3))
            eval-last-sexp-1(nil)
            eval-last-sexp(nil)
            call-interactively(eval-last-sexp)
          ------ Buffer: *Backtrace* ------


 -- Command: cancel-debug-on-entry &optional function-name
     This function undoes the effect of ‘debug-on-entry’ on
     FUNCTION-NAME.  When called interactively, it prompts for
     FUNCTION-NAME in the minibuffer.  If FUNCTION-NAME is omitted or
     ‘nil’, it cancels break-on-entry for all functions.  Calling
     ‘cancel-debug-on-entry’ does nothing to a function which is not
     currently set up to break on entry.


File: elisp.info,  Node: Explicit Debug,  Next: Using Debugger,  Prev: Function Debugging,  Up: Debugger

17.1.4 Explicit Entry to the Debugger
-------------------------------------

You can cause the debugger to be called at a certain point in your
program by writing the expression ‘(debug)’ at that point.  To do this,
visit the source file, insert the text ‘(debug)’ at the proper place,
and type ‘C-M-x’ (‘eval-defun’, a Lisp mode key binding).  *Warning:* if
you do this for temporary debugging purposes, be sure to undo this
insertion before you save the file!

   The place where you insert ‘(debug)’ must be a place where an
additional form can be evaluated and its value ignored.  (If the value
of ‘(debug)’ isn’t ignored, it will alter the execution of the program!)
The most common suitable places are inside a ‘progn’ or an implicit
‘progn’ (*note Sequencing::).

   If you don’t know exactly where in the source code you want to put
the debug statement, but you want to display a backtrace when a certain
message is displayed, you can set ‘debug-on-message’ to a regular
expression matching the desired message.


File: elisp.info,  Node: Using Debugger,  Next: Debugger Commands,  Prev: Explicit Debug,  Up: Debugger

17.1.5 Using the Debugger
-------------------------

When the debugger is entered, it displays the previously selected buffer
in one window and a buffer named ‘*Backtrace*’ in another window.  The
backtrace buffer contains one line for each level of Lisp function
execution currently going on.  At the beginning of this buffer is a
message describing the reason that the debugger was invoked (such as the
error message and associated data, if it was invoked due to an error).

   The backtrace buffer is read-only and uses a special major mode,
Debugger mode, in which letters are defined as debugger commands.  The
usual Emacs editing commands are available; thus, you can switch windows
to examine the buffer that was being edited at the time of the error,
switch buffers, visit files, or do any other sort of editing.  However,
the debugger is a recursive editing level (*note Recursive Editing::)
and it is wise to go back to the backtrace buffer and exit the debugger
(with the ‘q’ command) when you are finished with it.  Exiting the
debugger gets out of the recursive edit and buries the backtrace buffer.
(You can customize what the ‘q’ command does with the backtrace buffer
by setting the variable ‘debugger-bury-or-kill’.  For example, set it to
‘kill’ if you prefer to kill the buffer rather than bury it.  Consult
the variable’s documentation for more possibilities.)

   When the debugger has been entered, the ‘debug-on-error’ variable is
temporarily set according to ‘eval-expression-debug-on-error’.  If the
latter variable is non-‘nil’, ‘debug-on-error’ will temporarily be set
to ‘t’.  This means that any further errors that occur while doing a
debugging session will (by default) trigger another backtrace.  If this
is not what you want, you can either set
‘eval-expression-debug-on-error’ to ‘nil’, or set ‘debug-on-error’ to
‘nil’ in ‘debugger-mode-hook’.

   The backtrace buffer shows you the functions that are executing and
their argument values.  It also allows you to specify a stack frame by
moving point to the line describing that frame.  (A stack frame is the
place where the Lisp interpreter records information about a particular
invocation of a function.)  The frame whose line point is on is
considered the “current frame”.  Some of the debugger commands operate
on the current frame.  If a line starts with a star, that means that
exiting that frame will call the debugger again.  This is useful for
examining the return value of a function.

   If a function name is underlined, that means the debugger knows where
its source code is located.  You can click with the mouse on that name,
or move to it and type <RET>, to visit the source code.

   The debugger itself must be run byte-compiled, since it makes
assumptions about how many stack frames are used for the debugger
itself.  These assumptions are false if the debugger is running
interpreted.


File: elisp.info,  Node: Debugger Commands,  Next: Invoking the Debugger,  Prev: Using Debugger,  Up: Debugger

17.1.6 Debugger Commands
------------------------

The debugger buffer (in Debugger mode) provides special commands in
addition to the usual Emacs commands.  The most important use of
debugger commands is for stepping through code, so that you can see how
control flows.  The debugger can step through the control structures of
an interpreted function, but cannot do so in a byte-compiled function.
If you would like to step through a byte-compiled function, replace it
with an interpreted definition of the same function.  (To do this, visit
the source for the function and type ‘C-M-x’ on its definition.)  You
cannot use the Lisp debugger to step through a primitive function.

   Here is a list of Debugger mode commands:

‘c’
     Exit the debugger and continue execution.  This resumes execution
     of the program as if the debugger had never been entered (aside
     from any side-effects that you caused by changing variable values
     or data structures while inside the debugger).

‘d’
     Continue execution, but enter the debugger the next time any Lisp
     function is called.  This allows you to step through the
     subexpressions of an expression, seeing what values the
     subexpressions compute, and what else they do.

     The stack frame made for the function call which enters the
     debugger in this way will be flagged automatically so that the
     debugger will be called again when the frame is exited.  You can
     use the ‘u’ command to cancel this flag.

‘b’
     Flag the current frame so that the debugger will be entered when
     the frame is exited.  Frames flagged in this way are marked with
     stars in the backtrace buffer.

‘u’
     Don’t enter the debugger when the current frame is exited.  This
     cancels a ‘b’ command on that frame.  The visible effect is to
     remove the star from the line in the backtrace buffer.

‘j’
     Flag the current frame like ‘b’.  Then continue execution like ‘c’,
     but temporarily disable break-on-entry for all functions that are
     set up to do so by ‘debug-on-entry’.

‘e’
     Read a Lisp expression in the minibuffer, evaluate it (with the
     relevant lexical environment, if applicable), and print the value
     in the echo area.  The debugger alters certain important variables,
     and the current buffer, as part of its operation; ‘e’ temporarily
     restores their values from outside the debugger, so you can examine
     and change them.  This makes the debugger more transparent.  By
     contrast, ‘M-:’ does nothing special in the debugger; it shows you
     the variable values within the debugger.

‘R’
     Like ‘e’, but also save the result of evaluation in the buffer
     ‘*Debugger-record*’.

‘q’
     Terminate the program being debugged; return to top-level Emacs
     command execution.

     If the debugger was entered due to a ‘C-g’ but you really want to
     quit, and not debug, use the ‘q’ command.

‘r’
     Return a value from the debugger.  The value is computed by reading
     an expression with the minibuffer and evaluating it.

     The ‘r’ command is useful when the debugger was invoked due to exit
     from a Lisp call frame (as requested with ‘b’ or by entering the
     frame with ‘d’); then the value specified in the ‘r’ command is
     used as the value of that frame.  It is also useful if you call
     ‘debug’ and use its return value.  Otherwise, ‘r’ has the same
     effect as ‘c’, and the specified return value does not matter.

     You can’t use ‘r’ when the debugger was entered due to an error.

‘l’
     Display a list of functions that will invoke the debugger when
     called.  This is a list of functions that are set to break on entry
     by means of ‘debug-on-entry’.

‘v’
     Toggle the display of local variables of the current stack frame.


File: elisp.info,  Node: Invoking the Debugger,  Next: Internals of Debugger,  Prev: Debugger Commands,  Up: Debugger

17.1.7 Invoking the Debugger
----------------------------

Here we describe in full detail the function ‘debug’ that is used to
invoke the debugger.

 -- Command: debug &rest debugger-args
     This function enters the debugger.  It switches buffers to a buffer
     named ‘*Backtrace*’ (or ‘*Backtrace*<2>’ if it is the second
     recursive entry to the debugger, etc.), and fills it with
     information about the stack of Lisp function calls.  It then enters
     a recursive edit, showing the backtrace buffer in Debugger mode.

     The Debugger mode ‘c’, ‘d’, ‘j’, and ‘r’ commands exit the
     recursive edit; then ‘debug’ switches back to the previous buffer
     and returns to whatever called ‘debug’.  This is the only way the
     function ‘debug’ can return to its caller.

     The use of the DEBUGGER-ARGS is that ‘debug’ displays the rest of
     its arguments at the top of the ‘*Backtrace*’ buffer, so that the
     user can see them.  Except as described below, this is the _only_
     way these arguments are used.

     However, certain values for first argument to ‘debug’ have a
     special significance.  (Normally, these values are used only by the
     internals of Emacs, and not by programmers calling ‘debug’.)  Here
     is a table of these special values:

     ‘lambda’
          A first argument of ‘lambda’ means ‘debug’ was called because
          of entry to a function when ‘debug-on-next-call’ was
          non-‘nil’.  The debugger displays ‘Debugger entered--entering
          a function:’ as a line of text at the top of the buffer.

     ‘debug’
          ‘debug’ as first argument means ‘debug’ was called because of
          entry to a function that was set to debug on entry.  The
          debugger displays the string ‘Debugger entered--entering a
          function:’, just as in the ‘lambda’ case.  It also marks the
          stack frame for that function so that it will invoke the
          debugger when exited.

     ‘t’
          When the first argument is ‘t’, this indicates a call to
          ‘debug’ due to evaluation of a function call form when
          ‘debug-on-next-call’ is non-‘nil’.  The debugger displays
          ‘Debugger entered--beginning evaluation of function call
          form:’ as the top line in the buffer.

     ‘exit’
          When the first argument is ‘exit’, it indicates the exit of a
          stack frame previously marked to invoke the debugger on exit.
          The second argument given to ‘debug’ in this case is the value
          being returned from the frame.  The debugger displays
          ‘Debugger entered--returning value:’ in the top line of the
          buffer, followed by the value being returned.

     ‘error’
          When the first argument is ‘error’, the debugger indicates
          that it is being entered because an error or ‘quit’ was
          signaled and not handled, by displaying ‘Debugger
          entered--Lisp error:’ followed by the error signaled and any
          arguments to ‘signal’.  For example,

               (let ((debug-on-error t))
                 (/ 1 0))

               ------ Buffer: *Backtrace* ------
               Debugger entered--Lisp error: (arith-error)
                 /(1 0)
               ...
               ------ Buffer: *Backtrace* ------

          If an error was signaled, presumably the variable
          ‘debug-on-error’ is non-‘nil’.  If ‘quit’ was signaled, then
          presumably the variable ‘debug-on-quit’ is non-‘nil’.

     ‘nil’
          Use ‘nil’ as the first of the DEBUGGER-ARGS when you want to
          enter the debugger explicitly.  The rest of the DEBUGGER-ARGS
          are printed on the top line of the buffer.  You can use this
          feature to display messages—for example, to remind yourself of
          the conditions under which ‘debug’ is called.


File: elisp.info,  Node: Internals of Debugger,  Prev: Invoking the Debugger,  Up: Debugger

17.1.8 Internals of the Debugger
--------------------------------

This section describes functions and variables used internally by the
debugger.

 -- Variable: debugger
     The value of this variable is the function to call to invoke the
     debugger.  Its value must be a function of any number of arguments,
     or, more typically, the name of a function.  This function should
     invoke some kind of debugger.  The default value of the variable is
     ‘debug’.

     The first argument that Lisp hands to the function indicates why it
     was called.  The convention for arguments is detailed in the
     description of ‘debug’ (*note Invoking the Debugger::).

 -- Command: backtrace
     This function prints a trace of Lisp function calls currently
     active.  This is the function used by ‘debug’ to fill up the
     ‘*Backtrace*’ buffer.  It is written in C, since it must have
     access to the stack to determine which function calls are active.
     The return value is always ‘nil’.

     In the following example, a Lisp expression calls ‘backtrace’
     explicitly.  This prints the backtrace to the stream
     ‘standard-output’, which, in this case, is the buffer
     ‘backtrace-output’.

     Each line of the backtrace represents one function call.  The line
     shows the values of the function’s arguments if they are all known;
     if they are still being computed, the line says so.  The arguments
     of special forms are elided.

          (with-output-to-temp-buffer "backtrace-output"
            (let ((var 1))
              (save-excursion
                (setq var (eval '(progn
                                   (1+ var)
                                   (list 'testing (backtrace))))))))

               ⇒ (testing nil)

          ----------- Buffer: backtrace-output ------------
            backtrace()
            (list ...computing arguments...)
            (progn ...)
            eval((progn (1+ var) (list (quote testing) (backtrace))))
            (setq ...)
            (save-excursion ...)
            (let ...)
            (with-output-to-temp-buffer ...)
            eval((with-output-to-temp-buffer ...))
            eval-last-sexp-1(nil)
            eval-last-sexp(nil)
            call-interactively(eval-last-sexp)
          ----------- Buffer: backtrace-output ------------

 -- Variable: debug-on-next-call
     If this variable is non-‘nil’, it says to call the debugger before
     the next ‘eval’, ‘apply’ or ‘funcall’.  Entering the debugger sets
     ‘debug-on-next-call’ to ‘nil’.

     The ‘d’ command in the debugger works by setting this variable.

 -- Function: backtrace-debug level flag
     This function sets the debug-on-exit flag of the stack frame LEVEL
     levels down the stack, giving it the value FLAG.  If FLAG is
     non-‘nil’, this will cause the debugger to be entered when that
     frame later exits.  Even a nonlocal exit through that frame will
     enter the debugger.

     This function is used only by the debugger.

 -- Variable: command-debug-status
     This variable records the debugging status of the current
     interactive command.  Each time a command is called interactively,
     this variable is bound to ‘nil’.  The debugger can set this
     variable to leave information for future debugger invocations
     during the same command invocation.

     The advantage of using this variable rather than an ordinary global
     variable is that the data will never carry over to a subsequent
     command invocation.

     This variable is obsolete and will be removed in future versions.

 -- Function: backtrace-frame frame-number
     The function ‘backtrace-frame’ is intended for use in Lisp
     debuggers.  It returns information about what computation is
     happening in the stack frame FRAME-NUMBER levels down.

     If that frame has not evaluated the arguments yet, or is a special
     form, the value is ‘(nil FUNCTION ARG-FORMS...)’.

     If that frame has evaluated its arguments and called its function
     already, the return value is ‘(t FUNCTION ARG-VALUES...)’.

     In the return value, FUNCTION is whatever was supplied as the CAR
     of the evaluated list, or a ‘lambda’ expression in the case of a
     macro call.  If the function has a ‘&rest’ argument, that is
     represented as the tail of the list ARG-VALUES.

     If FRAME-NUMBER is out of range, ‘backtrace-frame’ returns ‘nil’.


File: elisp.info,  Node: Edebug,  Next: Syntax Errors,  Prev: Debugger,  Up: Debugging

17.2 Edebug
===========

Edebug is a source-level debugger for Emacs Lisp programs, with which
you can:

   • Step through evaluation, stopping before and after each expression.

   • Set conditional or unconditional breakpoints.

   • Stop when a specified condition is true (the global break event).

   • Trace slow or fast, stopping briefly at each stop point, or at each
     breakpoint.

   • Display expression results and evaluate expressions as if outside
     of Edebug.

   • Automatically re-evaluate a list of expressions and display their
     results each time Edebug updates the display.

   • Output trace information on function calls and returns.

   • Stop when an error occurs.

   • Display a backtrace, omitting Edebug’s own frames.

   • Specify argument evaluation for macros and defining forms.

   • Obtain rudimentary coverage testing and frequency counts.

   The first three sections below should tell you enough about Edebug to
start using it.

* Menu:

* Using Edebug::                Introduction to use of Edebug.
* Instrumenting::               You must instrument your code
                                  in order to debug it with Edebug.
* Modes: Edebug Execution Modes. Execution modes, stopping more or less often.
* Jumping::                     Commands to jump to a specified place.
* Misc: Edebug Misc.            Miscellaneous commands.
* Breaks::                      Setting breakpoints to make the program stop.
* Trapping Errors::             Trapping errors with Edebug.
* Views: Edebug Views.          Views inside and outside of Edebug.
* Eval: Edebug Eval.            Evaluating expressions within Edebug.
* Eval List::                   Expressions whose values are displayed
                                  each time you enter Edebug.
* Printing in Edebug::          Customization of printing.
* Trace Buffer::                How to produce trace output in a buffer.
* Coverage Testing::            How to test evaluation coverage.
* The Outside Context::         Data that Edebug saves and restores.
* Edebug and Macros::           Specifying how to handle macro calls.
* Options: Edebug Options.      Option variables for customizing Edebug.


File: elisp.info,  Node: Using Edebug,  Next: Instrumenting,  Up: Edebug

17.2.1 Using Edebug
-------------------

To debug a Lisp program with Edebug, you must first “instrument” the
Lisp code that you want to debug.  A simple way to do this is to first
move point into the definition of a function or macro and then do ‘C-u
C-M-x’ (‘eval-defun’ with a prefix argument).  See *note
Instrumenting::, for alternative ways to instrument code.

   Once a function is instrumented, any call to the function activates
Edebug.  Depending on which Edebug execution mode you have selected,
activating Edebug may stop execution and let you step through the
function, or it may update the display and continue execution while
checking for debugging commands.  The default execution mode is step,
which stops execution.  *Note Edebug Execution Modes::.

   Within Edebug, you normally view an Emacs buffer showing the source
of the Lisp code you are debugging.  This is referred to as the “source
code buffer”, and it is temporarily read-only.

   An arrow in the left fringe indicates the line where the function is
executing.  Point initially shows where within the line the function is
executing, but this ceases to be true if you move point yourself.

   If you instrument the definition of ‘fac’ (shown below) and then
execute ‘(fac 3)’, here is what you would normally see.  Point is at the
open-parenthesis before ‘if’.

     (defun fac (n)
     =>★(if (< 0 n)
           (* n (fac (1- n)))
         1))

   The places within a function where Edebug can stop execution are
called “stop points”.  These occur both before and after each
subexpression that is a list, and also after each variable reference.
Here we use periods to show the stop points in the function ‘fac’:

     (defun fac (n)
       .(if .(< 0 n.).
           .(* n. .(fac .(1- n.).).).
         1).)

   The special commands of Edebug are available in the source code
buffer in addition to the commands of Emacs Lisp mode.  For example, you
can type the Edebug command <SPC> to execute until the next stop point.
If you type <SPC> once after entry to ‘fac’, here is the display you
will see:

     (defun fac (n)
     =>(if ★(< 0 n)
           (* n (fac (1- n)))
         1))

   When Edebug stops execution after an expression, it displays the
expression’s value in the echo area.

   Other frequently used commands are ‘b’ to set a breakpoint at a stop
point, ‘g’ to execute until a breakpoint is reached, and ‘q’ to exit
Edebug and return to the top-level command loop.  Type ‘?’ to display a
list of all Edebug commands.


File: elisp.info,  Node: Instrumenting,  Next: Edebug Execution Modes,  Prev: Using Edebug,  Up: Edebug

17.2.2 Instrumenting for Edebug
-------------------------------

In order to use Edebug to debug Lisp code, you must first “instrument”
the code.  Instrumenting code inserts additional code into it, to invoke
Edebug at the proper places.

   When you invoke command ‘C-M-x’ (‘eval-defun’) with a prefix argument
on a function definition, it instruments the definition before
evaluating it.  (This does not modify the source code itself.)  If the
variable ‘edebug-all-defs’ is non-‘nil’, that inverts the meaning of the
prefix argument: in this case, ‘C-M-x’ instruments the definition
_unless_ it has a prefix argument.  The default value of
‘edebug-all-defs’ is ‘nil’.  The command ‘M-x edebug-all-defs’ toggles
the value of the variable ‘edebug-all-defs’.

   If ‘edebug-all-defs’ is non-‘nil’, then the commands ‘eval-region’,
‘eval-current-buffer’, and ‘eval-buffer’ also instrument any definitions
they evaluate.  Similarly, ‘edebug-all-forms’ controls whether
‘eval-region’ should instrument _any_ form, even non-defining forms.
This doesn’t apply to loading or evaluations in the minibuffer.  The
command ‘M-x edebug-all-forms’ toggles this option.

   Another command, ‘M-x edebug-eval-top-level-form’, is available to
instrument any top-level form regardless of the values of
‘edebug-all-defs’ and ‘edebug-all-forms’.  ‘edebug-defun’ is an alias
for ‘edebug-eval-top-level-form’.

   While Edebug is active, the command ‘I’ (‘edebug-instrument-callee’)
instruments the definition of the function or macro called by the list
form after point, if it is not already instrumented.  This is possible
only if Edebug knows where to find the source for that function; for
this reason, after loading Edebug, ‘eval-region’ records the position of
every definition it evaluates, even if not instrumenting it.  See also
the ‘i’ command (*note Jumping::), which steps into the call after
instrumenting the function.

   Edebug knows how to instrument all the standard special forms,
‘interactive’ forms with an expression argument, anonymous lambda
expressions, and other defining forms.  However, Edebug cannot determine
on its own what a user-defined macro will do with the arguments of a
macro call, so you must provide that information using Edebug
specifications; for details, *note Edebug and Macros::.

   When Edebug is about to instrument code for the first time in a
session, it runs the hook ‘edebug-setup-hook’, then sets it to ‘nil’.
You can use this to load Edebug specifications associated with a package
you are using, but only when you use Edebug.

   To remove instrumentation from a definition, simply re-evaluate its
definition in a way that does not instrument.  There are two ways of
evaluating forms that never instrument them: from a file with ‘load’,
and from the minibuffer with ‘eval-expression’ (‘M-:’).

   If Edebug detects a syntax error while instrumenting, it leaves point
at the erroneous code and signals an ‘invalid-read-syntax’ error.

   *Note Edebug Eval::, for other evaluation functions available inside
of Edebug.


File: elisp.info,  Node: Edebug Execution Modes,  Next: Jumping,  Prev: Instrumenting,  Up: Edebug

17.2.3 Edebug Execution Modes
-----------------------------

Edebug supports several execution modes for running the program you are
debugging.  We call these alternatives “Edebug execution modes”; do not
confuse them with major or minor modes.  The current Edebug execution
mode determines how far Edebug continues execution before
stopping—whether it stops at each stop point, or continues to the next
breakpoint, for example—and how much Edebug displays the progress of the
evaluation before it stops.

   Normally, you specify the Edebug execution mode by typing a command
to continue the program in a certain mode.  Here is a table of these
commands; all except for ‘S’ resume execution of the program, at least
for a certain distance.

‘S’
     Stop: don’t execute any more of the program, but wait for more
     Edebug commands (‘edebug-stop’).

‘<SPC>’
     Step: stop at the next stop point encountered (‘edebug-step-mode’).

‘n’
     Next: stop at the next stop point encountered after an expression
     (‘edebug-next-mode’).  Also see ‘edebug-forward-sexp’ in *note
     Jumping::.

‘t’
     Trace: pause (normally one second) at each Edebug stop point
     (‘edebug-trace-mode’).

‘T’
     Rapid trace: update the display at each stop point, but don’t
     actually pause (‘edebug-Trace-fast-mode’).

‘g’
     Go: run until the next breakpoint (‘edebug-go-mode’).  *Note
     Breakpoints::.

‘c’
     Continue: pause one second at each breakpoint, and then continue
     (‘edebug-continue-mode’).

‘C’
     Rapid continue: move point to each breakpoint, but don’t pause
     (‘edebug-Continue-fast-mode’).

‘G’
     Go non-stop: ignore breakpoints (‘edebug-Go-nonstop-mode’).  You
     can still stop the program by typing ‘S’, or any editing command.

   In general, the execution modes earlier in the above list run the
program more slowly or stop sooner than the modes later in the list.

   When you enter a new Edebug level, Edebug will normally stop at the
first instrumented function it encounters.  If you prefer to stop only
at a break point, or not at all (for example, when gathering coverage
data), change the value of ‘edebug-initial-mode’ from its default ‘step’
to ‘go’, or ‘Go-nonstop’, or one of its other values (*note Edebug
Options::).  You can do this readily with ‘C-x C-a C-m’
(‘edebug-set-initial-mode’):

 -- Command: edebug-set-initial-mode
     This command, bound to ‘C-x C-a C-m’, sets ‘edebug-initial-mode’.
     It prompts you for a key to indicate the mode.  You should enter
     one of the eight keys listed above, which sets the corresponding
     mode.

   Note that you may reenter the same Edebug level several times if, for
example, an instrumented function is called several times from one
command.

   While executing or tracing, you can interrupt the execution by typing
any Edebug command.  Edebug stops the program at the next stop point and
then executes the command you typed.  For example, typing ‘t’ during
execution switches to trace mode at the next stop point.  You can use
‘S’ to stop execution without doing anything else.

   If your function happens to read input, a character you type
intending to interrupt execution may be read by the function instead.
You can avoid such unintended results by paying attention to when your
program wants input.

   Keyboard macros containing the commands in this section do not
completely work: exiting from Edebug, to resume the program, loses track
of the keyboard macro.  This is not easy to fix.  Also, defining or
executing a keyboard macro outside of Edebug does not affect commands
inside Edebug.  This is usually an advantage.  See also the
‘edebug-continue-kbd-macro’ option in *note Edebug Options::.

 -- User Option: edebug-sit-for-seconds
     This option specifies how many seconds to wait between execution
     steps in trace mode or continue mode.  The default is 1 second.


File: elisp.info,  Node: Jumping,  Next: Edebug Misc,  Prev: Edebug Execution Modes,  Up: Edebug

17.2.4 Jumping
--------------

The commands described in this section execute until they reach a
specified location.  All except ‘i’ make a temporary breakpoint to
establish the place to stop, then switch to go mode.  Any other
breakpoint reached before the intended stop point will also stop
execution.  *Note Breakpoints::, for the details on breakpoints.

   These commands may fail to work as expected in case of nonlocal exit,
as that can bypass the temporary breakpoint where you expected the
program to stop.

‘h’
     Proceed to the stop point near where point is (‘edebug-goto-here’).

‘f’
     Run the program for one expression (‘edebug-forward-sexp’).

‘o’
     Run the program until the end of the containing sexp
     (‘edebug-step-out’).

‘i’
     Step into the function or macro called by the form after point
     (‘edebug-step-in’).

   The ‘h’ command proceeds to the stop point at or after the current
location of point, using a temporary breakpoint.

   The ‘f’ command runs the program forward over one expression.  More
precisely, it sets a temporary breakpoint at the position that
‘forward-sexp’ would reach, then executes in go mode so that the program
will stop at breakpoints.

   With a prefix argument N, the temporary breakpoint is placed N sexps
beyond point.  If the containing list ends before N more elements, then
the place to stop is after the containing expression.

   You must check that the position ‘forward-sexp’ finds is a place that
the program will really get to.  In ‘cond’, for example, this may not be
true.

   For flexibility, the ‘f’ command does ‘forward-sexp’ starting at
point, rather than at the stop point.  If you want to execute one
expression _from the current stop point_, first type ‘w’
(‘edebug-where’) to move point there, and then type ‘f’.

   The ‘o’ command continues out of an expression.  It places a
temporary breakpoint at the end of the sexp containing point.  If the
containing sexp is a function definition itself, ‘o’ continues until
just before the last sexp in the definition.  If that is where you are
now, it returns from the function and then stops.  In other words, this
command does not exit the currently executing function unless you are
positioned after the last sexp.

   The ‘i’ command steps into the function or macro called by the list
form after point, and stops at its first stop point.  Note that the form
need not be the one about to be evaluated.  But if the form is a
function call about to be evaluated, remember to use this command before
any of the arguments are evaluated, since otherwise it will be too late.

   The ‘i’ command instruments the function or macro it’s supposed to
step into, if it isn’t instrumented already.  This is convenient, but
keep in mind that the function or macro remains instrumented unless you
explicitly arrange to deinstrument it.


File: elisp.info,  Node: Edebug Misc,  Next: Breaks,  Prev: Jumping,  Up: Edebug

17.2.5 Miscellaneous Edebug Commands
------------------------------------

Some miscellaneous Edebug commands are described here.

‘?’
     Display the help message for Edebug (‘edebug-help’).

‘C-]’
     Abort one level back to the previous command level
     (‘abort-recursive-edit’).

‘q’
     Return to the top level editor command loop (‘top-level’).  This
     exits all recursive editing levels, including all levels of Edebug
     activity.  However, instrumented code protected with
     ‘unwind-protect’ or ‘condition-case’ forms may resume debugging.

‘Q’
     Like ‘q’, but don’t stop even for protected code
     (‘edebug-top-level-nonstop’).

‘r’
     Redisplay the most recently known expression result in the echo
     area (‘edebug-previous-result’).

‘d’
     Display a backtrace, excluding Edebug’s own functions for clarity
     (‘edebug-backtrace’).

     You cannot use debugger commands in the backtrace buffer in Edebug
     as you would in the standard debugger.

     The backtrace buffer is killed automatically when you continue
     execution.

   You can invoke commands from Edebug that activate Edebug again
recursively.  Whenever Edebug is active, you can quit to the top level
with ‘q’ or abort one recursive edit level with ‘C-]’.  You can display
a backtrace of all the pending evaluations with ‘d’.


File: elisp.info,  Node: Breaks,  Next: Trapping Errors,  Prev: Edebug Misc,  Up: Edebug

17.2.6 Breaks
-------------

Edebug’s step mode stops execution when the next stop point is reached.
There are three other ways to stop Edebug execution once it has started:
breakpoints, the global break condition, and source breakpoints.

* Menu:

* Breakpoints::                 Breakpoints at stop points.
* Global Break Condition::      Breaking on an event.
* Source Breakpoints::          Embedding breakpoints in source code.


File: elisp.info,  Node: Breakpoints,  Next: Global Break Condition,  Up: Breaks

17.2.6.1 Edebug Breakpoints
...........................

While using Edebug, you can specify “breakpoints” in the program you are
testing: these are places where execution should stop.  You can set a
breakpoint at any stop point, as defined in *note Using Edebug::.  For
setting and unsetting breakpoints, the stop point that is affected is
the first one at or after point in the source code buffer.  Here are the
Edebug commands for breakpoints:

‘b’
     Set a breakpoint at the stop point at or after point
     (‘edebug-set-breakpoint’).  If you use a prefix argument, the
     breakpoint is temporary—it turns off the first time it stops the
     program.

‘u’
     Unset the breakpoint (if any) at the stop point at or after point
     (‘edebug-unset-breakpoint’).

‘x CONDITION <RET>’
     Set a conditional breakpoint which stops the program only if
     evaluating CONDITION produces a non-‘nil’ value
     (‘edebug-set-conditional-breakpoint’).  With a prefix argument, the
     breakpoint is temporary.

‘B’
     Move point to the next breakpoint in the current definition
     (‘edebug-next-breakpoint’).

   While in Edebug, you can set a breakpoint with ‘b’ and unset one with
‘u’.  First move point to the Edebug stop point of your choice, then
type ‘b’ or ‘u’ to set or unset a breakpoint there.  Unsetting a
breakpoint where none has been set has no effect.

   Re-evaluating or reinstrumenting a definition removes all of its
previous breakpoints.

   A “conditional breakpoint” tests a condition each time the program
gets there.  Any errors that occur as a result of evaluating the
condition are ignored, as if the result were ‘nil’.  To set a
conditional breakpoint, use ‘x’, and specify the condition expression in
the minibuffer.  Setting a conditional breakpoint at a stop point that
has a previously established conditional breakpoint puts the previous
condition expression in the minibuffer so you can edit it.

   You can make a conditional or unconditional breakpoint “temporary” by
using a prefix argument with the command to set the breakpoint.  When a
temporary breakpoint stops the program, it is automatically unset.

   Edebug always stops or pauses at a breakpoint, except when the Edebug
mode is Go-nonstop.  In that mode, it ignores breakpoints entirely.

   To find out where your breakpoints are, use the ‘B’ command, which
moves point to the next breakpoint following point, within the same
function, or to the first breakpoint if there are no following
breakpoints.  This command does not continue execution—it just moves
point in the buffer.


File: elisp.info,  Node: Global Break Condition,  Next: Source Breakpoints,  Prev: Breakpoints,  Up: Breaks

17.2.6.2 Global Break Condition
...............................

A “global break condition” stops execution when a specified condition is
satisfied, no matter where that may occur.  Edebug evaluates the global
break condition at every stop point; if it evaluates to a non-‘nil’
value, then execution stops or pauses depending on the execution mode,
as if a breakpoint had been hit.  If evaluating the condition gets an
error, execution does not stop.

   The condition expression is stored in
‘edebug-global-break-condition’.  You can specify a new expression using
the ‘X’ command from the source code buffer while Edebug is active, or
using ‘C-x X X’ from any buffer at any time, as long as Edebug is loaded
(‘edebug-set-global-break-condition’).

   The global break condition is the simplest way to find where in your
code some event occurs, but it makes code run much more slowly.  So you
should reset the condition to ‘nil’ when not using it.


File: elisp.info,  Node: Source Breakpoints,  Prev: Global Break Condition,  Up: Breaks

17.2.6.3 Source Breakpoints
...........................

All breakpoints in a definition are forgotten each time you reinstrument
it.  If you wish to make a breakpoint that won’t be forgotten, you can
write a “source breakpoint”, which is simply a call to the function
‘edebug’ in your source code.  You can, of course, make such a call
conditional.  For example, in the ‘fac’ function, you can insert the
first line as shown below, to stop when the argument reaches zero:

     (defun fac (n)
       (if (= n 0) (edebug))
       (if (< 0 n)
           (* n (fac (1- n)))
         1))

   When the ‘fac’ definition is instrumented and the function is called,
the call to ‘edebug’ acts as a breakpoint.  Depending on the execution
mode, Edebug stops or pauses there.

   If no instrumented code is being executed when ‘edebug’ is called,
that function calls ‘debug’.


File: elisp.info,  Node: Trapping Errors,  Next: Edebug Views,  Prev: Breaks,  Up: Edebug

17.2.7 Trapping Errors
----------------------

Emacs normally displays an error message when an error is signaled and
not handled with ‘condition-case’.  While Edebug is active and executing
instrumented code, it normally responds to all unhandled errors.  You
can customize this with the options ‘edebug-on-error’ and
‘edebug-on-quit’; see *note Edebug Options::.

   When Edebug responds to an error, it shows the last stop point
encountered before the error.  This may be the location of a call to a
function which was not instrumented, and within which the error actually
occurred.  For an unbound variable error, the last known stop point
might be quite distant from the offending variable reference.  In that
case, you might want to display a full backtrace (*note Edebug Misc::).

   If you change ‘debug-on-error’ or ‘debug-on-quit’ while Edebug is
active, these changes will be forgotten when Edebug becomes inactive.
Furthermore, during Edebug’s recursive edit, these variables are bound
to the values they had outside of Edebug.


File: elisp.info,  Node: Edebug Views,  Next: Edebug Eval,  Prev: Trapping Errors,  Up: Edebug

17.2.8 Edebug Views
-------------------

These Edebug commands let you view aspects of the buffer and window
status as they were before entry to Edebug.  The outside window
configuration is the collection of windows and contents that were in
effect outside of Edebug.

‘v’
     Switch to viewing the outside window configuration
     (‘edebug-view-outside’).  Type ‘C-x X w’ to return to Edebug.

‘p’
     Temporarily display the outside current buffer with point at its
     outside position (‘edebug-bounce-point’), pausing for one second
     before returning to Edebug.  With a prefix argument N, pause for N
     seconds instead.

‘w’
     Move point back to the current stop point in the source code buffer
     (‘edebug-where’).

     If you use this command in a different window displaying the same
     buffer, that window will be used instead to display the current
     definition in the future.

‘W’
     Toggle whether Edebug saves and restores the outside window
     configuration (‘edebug-toggle-save-windows’).

     With a prefix argument, ‘W’ only toggles saving and restoring of
     the selected window.  To specify a window that is not displaying
     the source code buffer, you must use ‘C-x X W’ from the global
     keymap.

   You can view the outside window configuration with ‘v’ or just bounce
to the point in the current buffer with ‘p’, even if it is not normally
displayed.

   After moving point, you may wish to jump back to the stop point.  You
can do that with ‘w’ from a source code buffer.  You can jump back to
the stop point in the source code buffer from any buffer using ‘C-x X
w’.

   Each time you use ‘W’ to turn saving _off_, Edebug forgets the saved
outside window configuration—so that even if you turn saving back _on_,
the current window configuration remains unchanged when you next exit
Edebug (by continuing the program).  However, the automatic redisplay of
‘*edebug*’ and ‘*edebug-trace*’ may conflict with the buffers you wish
to see unless you have enough windows open.


File: elisp.info,  Node: Edebug Eval,  Next: Eval List,  Prev: Edebug Views,  Up: Edebug

17.2.9 Evaluation
-----------------

While within Edebug, you can evaluate expressions as if Edebug were not
running.  Edebug tries to be invisible to the expression’s evaluation
and printing.  Evaluation of expressions that cause side effects will
work as expected, except for changes to data that Edebug explicitly
saves and restores.  *Note The Outside Context::, for details on this
process.

‘e EXP <RET>’
     Evaluate expression EXP in the context outside of Edebug
     (‘edebug-eval-expression’).  That is, Edebug tries to minimize its
     interference with the evaluation.

‘M-: EXP <RET>’
     Evaluate expression EXP in the context of Edebug itself
     (‘eval-expression’).

‘C-x C-e’
     Evaluate the expression before point, in the context outside of
     Edebug (‘edebug-eval-last-sexp’).

   Edebug supports evaluation of expressions containing references to
lexically bound symbols created by the following constructs in ‘cl.el’:
‘lexical-let’, ‘macrolet’, and ‘symbol-macrolet’.


File: elisp.info,  Node: Eval List,  Next: Printing in Edebug,  Prev: Edebug Eval,  Up: Edebug

17.2.10 Evaluation List Buffer
------------------------------

You can use the “evaluation list buffer”, called ‘*edebug*’, to evaluate
expressions interactively.  You can also set up the “evaluation list” of
expressions to be evaluated automatically each time Edebug updates the
display.

‘E’
     Switch to the evaluation list buffer ‘*edebug*’
     (‘edebug-visit-eval-list’).

   In the ‘*edebug*’ buffer you can use the commands of Lisp Interaction
mode (*note (emacs)Lisp Interaction::) as well as these special
commands:

‘C-j’
     Evaluate the expression before point, in the outside context, and
     insert the value in the buffer (‘edebug-eval-print-last-sexp’).

‘C-x C-e’
     Evaluate the expression before point, in the context outside of
     Edebug (‘edebug-eval-last-sexp’).

‘C-c C-u’
     Build a new evaluation list from the contents of the buffer
     (‘edebug-update-eval-list’).

‘C-c C-d’
     Delete the evaluation list group that point is in
     (‘edebug-delete-eval-item’).

‘C-c C-w’
     Switch back to the source code buffer at the current stop point
     (‘edebug-where’).

   You can evaluate expressions in the evaluation list window with ‘C-j’
or ‘C-x C-e’, just as you would in ‘*scratch*’; but they are evaluated
in the context outside of Edebug.

   The expressions you enter interactively (and their results) are lost
when you continue execution; but you can set up an “evaluation list”
consisting of expressions to be evaluated each time execution stops.

   To do this, write one or more “evaluation list groups” in the
evaluation list buffer.  An evaluation list group consists of one or
more Lisp expressions.  Groups are separated by comment lines.

   The command ‘C-c C-u’ (‘edebug-update-eval-list’) rebuilds the
evaluation list, scanning the buffer and using the first expression of
each group.  (The idea is that the second expression of the group is the
value previously computed and displayed.)

   Each entry to Edebug redisplays the evaluation list by inserting each
expression in the buffer, followed by its current value.  It also
inserts comment lines so that each expression becomes its own group.
Thus, if you type ‘C-c C-u’ again without changing the buffer text, the
evaluation list is effectively unchanged.

   If an error occurs during an evaluation from the evaluation list, the
error message is displayed in a string as if it were the result.
Therefore, expressions using variables that are not currently valid do
not interrupt your debugging.

   Here is an example of what the evaluation list window looks like
after several expressions have been added to it:

     (current-buffer)
     #<buffer *scratch*>
     ;---------------------------------------------------------------
     (selected-window)
     #<window 16 on *scratch*>
     ;---------------------------------------------------------------
     (point)
     196
     ;---------------------------------------------------------------
     bad-var
     "Symbol's value as variable is void: bad-var"
     ;---------------------------------------------------------------
     (recursion-depth)
     0
     ;---------------------------------------------------------------
     this-command
     eval-last-sexp
     ;---------------------------------------------------------------

   To delete a group, move point into it and type ‘C-c C-d’, or simply
delete the text for the group and update the evaluation list with ‘C-c
C-u’.  To add a new expression to the evaluation list, insert the
expression at a suitable place, insert a new comment line, then type
‘C-c C-u’.  You need not insert dashes in the comment line—its contents
don’t matter.

   After selecting ‘*edebug*’, you can return to the source code buffer
with ‘C-c C-w’.  The ‘*edebug*’ buffer is killed when you continue
execution, and recreated next time it is needed.


File: elisp.info,  Node: Printing in Edebug,  Next: Trace Buffer,  Prev: Eval List,  Up: Edebug

17.2.11 Printing in Edebug
--------------------------

If an expression in your program produces a value containing circular
list structure, you may get an error when Edebug attempts to print it.

   One way to cope with circular structure is to set ‘print-length’ or
‘print-level’ to truncate the printing.  Edebug does this for you; it
binds ‘print-length’ and ‘print-level’ to the values of the variables
‘edebug-print-length’ and ‘edebug-print-level’ (so long as they have
non-‘nil’ values).  *Note Output Variables::.

 -- User Option: edebug-print-length
     If non-‘nil’, Edebug binds ‘print-length’ to this value while
     printing results.  The default value is ‘50’.

 -- User Option: edebug-print-level
     If non-‘nil’, Edebug binds ‘print-level’ to this value while
     printing results.  The default value is ‘50’.

   You can also print circular structures and structures that share
elements more informatively by binding ‘print-circle’ to a non-‘nil’
value.

   Here is an example of code that creates a circular structure:

     (setq a '(x y))
     (setcar a a)

Custom printing prints this as ‘Result: #1=(#1# y)’.  The ‘#1=’ notation
labels the structure that follows it with the label ‘1’, and the ‘#1#’
notation references the previously labeled structure.  This notation is
used for any shared elements of lists or vectors.

 -- User Option: edebug-print-circle
     If non-‘nil’, Edebug binds ‘print-circle’ to this value while
     printing results.  The default value is ‘t’.

   Other programs can also use custom printing; see ‘cust-print.el’ for
details.


File: elisp.info,  Node: Trace Buffer,  Next: Coverage Testing,  Prev: Printing in Edebug,  Up: Edebug

17.2.12 Trace Buffer
--------------------

Edebug can record an execution trace, storing it in a buffer named
‘*edebug-trace*’.  This is a log of function calls and returns, showing
the function names and their arguments and values.  To enable trace
recording, set ‘edebug-trace’ to a non-‘nil’ value.

   Making a trace buffer is not the same thing as using trace execution
mode (*note Edebug Execution Modes::).

   When trace recording is enabled, each function entry and exit adds
lines to the trace buffer.  A function entry record consists of ‘::::{’,
followed by the function name and argument values.  A function exit
record consists of ‘::::}’, followed by the function name and result of
the function.

   The number of ‘:’s in an entry shows its recursion depth.  You can
use the braces in the trace buffer to find the matching beginning or end
of function calls.

   You can customize trace recording for function entry and exit by
redefining the functions ‘edebug-print-trace-before’ and
‘edebug-print-trace-after’.

 -- Macro: edebug-tracing string body...
     This macro requests additional trace information around the
     execution of the BODY forms.  The argument STRING specifies text to
     put in the trace buffer, after the ‘{’ or ‘}’.  All the arguments
     are evaluated, and ‘edebug-tracing’ returns the value of the last
     form in BODY.

 -- Function: edebug-trace format-string &rest format-args
     This function inserts text in the trace buffer.  It computes the
     text with ‘(apply 'format FORMAT-STRING FORMAT-ARGS)’.  It also
     appends a newline to separate entries.

   ‘edebug-tracing’ and ‘edebug-trace’ insert lines in the trace buffer
whenever they are called, even if Edebug is not active.  Adding text to
the trace buffer also scrolls its window to show the last lines
inserted.


File: elisp.info,  Node: Coverage Testing,  Next: The Outside Context,  Prev: Trace Buffer,  Up: Edebug

17.2.13 Coverage Testing
------------------------

Edebug provides rudimentary coverage testing and display of execution
frequency.

   Coverage testing works by comparing the result of each expression
with the previous result; each form in the program is considered covered
if it has returned two different values since you began testing coverage
in the current Emacs session.  Thus, to do coverage testing on your
program, execute it under various conditions and note whether it behaves
correctly; Edebug will tell you when you have tried enough different
conditions that each form has returned two different values.

   Coverage testing makes execution slower, so it is only done if
‘edebug-test-coverage’ is non-‘nil’.  Frequency counting is performed
for all executions of an instrumented function, even if the execution
mode is Go-nonstop, and regardless of whether coverage testing is
enabled.

   Use ‘C-x X =’ (‘edebug-display-freq-count’) to display both the
coverage information and the frequency counts for a definition.  Just
‘=’ (‘edebug-temp-display-freq-count’) displays the same information
temporarily, only until you type another key.

 -- Command: edebug-display-freq-count
     This command displays the frequency count data for each line of the
     current definition.

     It inserts frequency counts as comment lines after each line of
     code.  You can undo all insertions with one ‘undo’ command.  The
     counts appear under the ‘(’ before an expression or the ‘)’ after
     an expression, or on the last character of a variable.  To simplify
     the display, a count is not shown if it is equal to the count of an
     earlier expression on the same line.

     The character ‘=’ following the count for an expression says that
     the expression has returned the same value each time it was
     evaluated.  In other words, it is not yet covered for coverage
     testing purposes.

     To clear the frequency count and coverage data for a definition,
     simply reinstrument it with ‘eval-defun’.

   For example, after evaluating ‘(fac 5)’ with a source breakpoint, and
setting ‘edebug-test-coverage’ to ‘t’, when the breakpoint is reached,
the frequency data looks like this:

     (defun fac (n)
       (if (= n 0) (edebug))
     ;#6           1      = =5
       (if (< 0 n)
     ;#5         =
           (* n (fac (1- n)))
     ;#    5               0
         1))
     ;#   0

   The comment lines show that ‘fac’ was called 6 times.  The first ‘if’
statement returned 5 times with the same result each time; the same is
true of the condition on the second ‘if’.  The recursive call of ‘fac’
did not return at all.


File: elisp.info,  Node: The Outside Context,  Next: Edebug and Macros,  Prev: Coverage Testing,  Up: Edebug

17.2.14 The Outside Context
---------------------------

Edebug tries to be transparent to the program you are debugging, but it
does not succeed completely.  Edebug also tries to be transparent when
you evaluate expressions with ‘e’ or with the evaluation list buffer, by
temporarily restoring the outside context.  This section explains
precisely what context Edebug restores, and how Edebug fails to be
completely transparent.

* Menu:

* Checking Whether to Stop::    When Edebug decides what to do.
* Edebug Display Update::       When Edebug updates the display.
* Edebug Recursive Edit::       When Edebug stops execution.


File: elisp.info,  Node: Checking Whether to Stop,  Next: Edebug Display Update,  Up: The Outside Context

17.2.14.1 Checking Whether to Stop
..................................

Whenever Edebug is entered, it needs to save and restore certain data
before even deciding whether to make trace information or stop the
program.

   • ‘max-lisp-eval-depth’ and ‘max-specpdl-size’ are both increased to
     reduce Edebug’s impact on the stack.  You could, however, still run
     out of stack space when using Edebug.

   • The state of keyboard macro execution is saved and restored.  While
     Edebug is active, ‘executing-kbd-macro’ is bound to ‘nil’ unless
     ‘edebug-continue-kbd-macro’ is non-‘nil’.


File: elisp.info,  Node: Edebug Display Update,  Next: Edebug Recursive Edit,  Prev: Checking Whether to Stop,  Up: The Outside Context

17.2.14.2 Edebug Display Update
...............................

When Edebug needs to display something (e.g., in trace mode), it saves
the current window configuration from outside Edebug (*note Window
Configurations::).  When you exit Edebug, it restores the previous
window configuration.

   Emacs redisplays only when it pauses.  Usually, when you continue
execution, the program re-enters Edebug at a breakpoint or after
stepping, without pausing or reading input in between.  In such cases,
Emacs never gets a chance to redisplay the outside configuration.
Consequently, what you see is the same window configuration as the last
time Edebug was active, with no interruption.

   Entry to Edebug for displaying something also saves and restores the
following data (though some of them are deliberately not restored if an
error or quit signal occurs).

   • Which buffer is current, and the positions of point and the mark in
     the current buffer, are saved and restored.

   • The outside window configuration is saved and restored if
     ‘edebug-save-windows’ is non-‘nil’ (*note Edebug Options::).

     The window configuration is not restored on error or quit, but the
     outside selected window _is_ reselected even on error or quit in
     case a ‘save-excursion’ is active.  If the value of
     ‘edebug-save-windows’ is a list, only the listed windows are saved
     and restored.

     The window start and horizontal scrolling of the source code buffer
     are not restored, however, so that the display remains coherent
     within Edebug.

   • The value of point in each displayed buffer is saved and restored
     if ‘edebug-save-displayed-buffer-points’ is non-‘nil’.

   • The variables ‘overlay-arrow-position’ and ‘overlay-arrow-string’
     are saved and restored, so you can safely invoke Edebug from the
     recursive edit elsewhere in the same buffer.

   • ‘cursor-in-echo-area’ is locally bound to ‘nil’ so that the cursor
     shows up in the window.


File: elisp.info,  Node: Edebug Recursive Edit,  Prev: Edebug Display Update,  Up: The Outside Context

17.2.14.3 Edebug Recursive Edit
...............................

When Edebug is entered and actually reads commands from the user, it
saves (and later restores) these additional data:

   • The current match data.  *Note Match Data::.

   • The variables ‘last-command’, ‘this-command’, ‘last-command-event’,
     ‘last-input-event’, ‘last-event-frame’, ‘last-nonmenu-event’, and
     ‘track-mouse’.  Commands in Edebug do not affect these variables
     outside of Edebug.

     Executing commands within Edebug can change the key sequence that
     would be returned by ‘this-command-keys’, and there is no way to
     reset the key sequence from Lisp.

     Edebug cannot save and restore the value of
     ‘unread-command-events’.  Entering Edebug while this variable has a
     nontrivial value can interfere with execution of the program you
     are debugging.

   • Complex commands executed while in Edebug are added to the variable
     ‘command-history’.  In rare cases this can alter execution.

   • Within Edebug, the recursion depth appears one deeper than the
     recursion depth outside Edebug.  This is not true of the
     automatically updated evaluation list window.

   • ‘standard-output’ and ‘standard-input’ are bound to ‘nil’ by the
     ‘recursive-edit’, but Edebug temporarily restores them during
     evaluations.

   • The state of keyboard macro definition is saved and restored.
     While Edebug is active, ‘defining-kbd-macro’ is bound to
     ‘edebug-continue-kbd-macro’.


File: elisp.info,  Node: Edebug and Macros,  Next: Edebug Options,  Prev: The Outside Context,  Up: Edebug

17.2.15 Edebug and Macros
-------------------------

To make Edebug properly instrument expressions that call macros, some
extra care is needed.  This subsection explains the details.

* Menu:

* Instrumenting Macro Calls::   The basic problem.
* Specification List::          How to specify complex patterns of evaluation.
* Backtracking::                What Edebug does when matching fails.
* Specification Examples::      To help understand specifications.


File: elisp.info,  Node: Instrumenting Macro Calls,  Next: Specification List,  Up: Edebug and Macros

17.2.15.1 Instrumenting Macro Calls
...................................

When Edebug instruments an expression that calls a Lisp macro, it needs
additional information about the macro to do the job properly.  This is
because there is no a-priori way to tell which subexpressions of the
macro call are forms to be evaluated.  (Evaluation may occur explicitly
in the macro body, or when the resulting expansion is evaluated, or any
time later.)

   Therefore, you must define an Edebug specification for each macro
that Edebug will encounter, to explain the format of calls to that
macro.  To do this, add a ‘debug’ declaration to the macro definition.
Here is a simple example that shows the specification for the ‘for’
example macro (*note Argument Evaluation::).

     (defmacro for (var from init to final do &rest body)
       "Execute a simple \"for\" loop.
     For example, (for i from 1 to 10 do (print i))."
       (declare (debug (symbolp "from" form "to" form "do" &rest form)))
       ...)

   The Edebug specification says which parts of a call to the macro are
forms to be evaluated.  For simple macros, the specification often looks
very similar to the formal argument list of the macro definition, but
specifications are much more general than macro arguments.  *Note
Defining Macros::, for more explanation of the ‘declare’ form.

   Take care to ensure that the specifications are known to Edebug when
you instrument code.  If you are instrumenting a function from a file
that uses ‘eval-when-compile’ to require another file containing macro
definitions, you may need to explicitly load that file.

   You can also define an edebug specification for a macro separately
from the macro definition with ‘def-edebug-spec’.  Adding ‘debug’
declarations is preferred, and more convenient, for macro definitions in
Lisp, but ‘def-edebug-spec’ makes it possible to define Edebug
specifications for special forms implemented in C.

 -- Macro: def-edebug-spec macro specification
     Specify which expressions of a call to macro MACRO are forms to be
     evaluated.  SPECIFICATION should be the edebug specification.
     Neither argument is evaluated.

     The MACRO argument can actually be any symbol, not just a macro
     name.

   Here is a table of the possibilities for SPECIFICATION and how each
directs processing of arguments.

‘t’
     All arguments are instrumented for evaluation.

‘0’
     None of the arguments is instrumented.

a symbol
     The symbol must have an Edebug specification, which is used
     instead.  This indirection is repeated until another kind of
     specification is found.  This allows you to inherit the
     specification from another macro.

a list
     The elements of the list describe the types of the arguments of a
     calling form.  The possible elements of a specification list are
     described in the following sections.

   If a macro has no Edebug specification, neither through a ‘debug’
declaration nor through a ‘def-edebug-spec’ call, the variable
‘edebug-eval-macro-args’ comes into play.

 -- User Option: edebug-eval-macro-args
     This controls the way Edebug treats macro arguments with no
     explicit Edebug specification.  If it is ‘nil’ (the default), none
     of the arguments is instrumented for evaluation.  Otherwise, all
     arguments are instrumented.


File: elisp.info,  Node: Specification List,  Next: Backtracking,  Prev: Instrumenting Macro Calls,  Up: Edebug and Macros

17.2.15.2 Specification List
............................

A “specification list” is required for an Edebug specification if some
arguments of a macro call are evaluated while others are not.  Some
elements in a specification list match one or more arguments, but others
modify the processing of all following elements.  The latter, called
“specification keywords”, are symbols beginning with ‘&’ (such as
‘&optional’).

   A specification list may contain sublists, which match arguments that
are themselves lists, or it may contain vectors used for grouping.
Sublists and groups thus subdivide the specification list into a
hierarchy of levels.  Specification keywords apply only to the remainder
of the sublist or group they are contained in.

   When a specification list involves alternatives or repetition,
matching it against an actual macro call may require backtracking.  For
more details, *note Backtracking::.

   Edebug specifications provide the power of regular expression
matching, plus some context-free grammar constructs: the matching of
sublists with balanced parentheses, recursive processing of forms, and
recursion via indirect specifications.

   Here’s a table of the possible elements of a specification list, with
their meanings (see *note Specification Examples::, for the referenced
examples):

‘sexp’
     A single unevaluated Lisp object, which is not instrumented.

‘form’
     A single evaluated expression, which is instrumented.

‘place’
     A generalized variable.  *Note Generalized Variables::.

‘body’
     Short for ‘&rest form’.  See ‘&rest’ below.

‘function-form’
     A function form: either a quoted function symbol, a quoted lambda
     expression, or a form (that should evaluate to a function symbol or
     lambda expression).  This is useful when an argument that’s a
     lambda expression might be quoted with ‘quote’ rather than
     ‘function’, since it instruments the body of the lambda expression
     either way.

‘lambda-expr’
     A lambda expression with no quoting.

‘&optional’
     All following elements in the specification list are optional; as
     soon as one does not match, Edebug stops matching at this level.

     To make just a few elements optional, followed by non-optional
     elements, use ‘[&optional SPECS...]’.  To specify that several
     elements must all match or none, use ‘&optional [SPECS...]’.  See
     the ‘defun’ example.

‘&rest’
     All following elements in the specification list are repeated zero
     or more times.  In the last repetition, however, it is not a
     problem if the expression runs out before matching all of the
     elements of the specification list.

     To repeat only a few elements, use ‘[&rest SPECS...]’.  To specify
     several elements that must all match on every repetition, use
     ‘&rest [SPECS...]’.

‘&or’
     Each of the following elements in the specification list is an
     alternative.  One of the alternatives must match, or the ‘&or’
     specification fails.

     Each list element following ‘&or’ is a single alternative.  To
     group two or more list elements as a single alternative, enclose
     them in ‘[...]’.

‘&not’
     Each of the following elements is matched as alternatives as if by
     using ‘&or’, but if any of them match, the specification fails.  If
     none of them match, nothing is matched, but the ‘&not’
     specification succeeds.

‘&define’
     Indicates that the specification is for a defining form.  The
     defining form itself is not instrumented (that is, Edebug does not
     stop before and after the defining form), but forms inside it
     typically will be instrumented.  The ‘&define’ keyword should be
     the first element in a list specification.

‘nil’
     This is successful when there are no more arguments to match at the
     current argument list level; otherwise it fails.  See sublist
     specifications and the backquote example.

‘gate’
     No argument is matched but backtracking through the gate is
     disabled while matching the remainder of the specifications at this
     level.  This is primarily used to generate more specific syntax
     error messages.  See *note Backtracking::, for more details.  Also
     see the ‘let’ example.

‘OTHER-SYMBOL’
     Any other symbol in a specification list may be a predicate or an
     indirect specification.

     If the symbol has an Edebug specification, this “indirect
     specification” should be either a list specification that is used
     in place of the symbol, or a function that is called to process the
     arguments.  The specification may be defined with ‘def-edebug-spec’
     just as for macros.  See the ‘defun’ example.

     Otherwise, the symbol should be a predicate.  The predicate is
     called with the argument, and if the predicate returns ‘nil’, the
     specification fails and the argument is not instrumented.

     Some suitable predicates include ‘symbolp’, ‘integerp’, ‘stringp’,
     ‘vectorp’, and ‘atom’.

‘[ELEMENTS...]’
     A vector of elements groups the elements into a single “group
     specification”.  Its meaning has nothing to do with vectors.

‘"STRING"’
     The argument should be a symbol named STRING.  This specification
     is equivalent to the quoted symbol, ‘'SYMBOL’, where the name of
     SYMBOL is the STRING, but the string form is preferred.

‘(vector ELEMENTS...)’
     The argument should be a vector whose elements must match the
     ELEMENTS in the specification.  See the backquote example.

‘(ELEMENTS...)’
     Any other list is a “sublist specification” and the argument must
     be a list whose elements match the specification ELEMENTS.

     A sublist specification may be a dotted list and the corresponding
     list argument may then be a dotted list.  Alternatively, the last
     CDR of a dotted list specification may be another sublist
     specification (via a grouping or an indirect specification, e.g.,
     ‘(spec . [(more specs...)])’) whose elements match the non-dotted
     list arguments.  This is useful in recursive specifications such as
     in the backquote example.  Also see the description of a ‘nil’
     specification above for terminating such recursion.

     Note that a sublist specification written as ‘(specs . nil)’ is
     equivalent to ‘(specs)’, and ‘(specs . (sublist-elements...))’ is
     equivalent to ‘(specs sublist-elements...)’.

   Here is a list of additional specifications that may appear only
after ‘&define’.  See the ‘defun’ example.

‘name’
     The argument, a symbol, is the name of the defining form.

     A defining form is not required to have a name field; and it may
     have multiple name fields.

‘:name’
     This construct does not actually match an argument.  The element
     following ‘:name’ should be a symbol; it is used as an additional
     name component for the definition.  You can use this to add a
     unique, static component to the name of the definition.  It may be
     used more than once.

‘arg’
     The argument, a symbol, is the name of an argument of the defining
     form.  However, lambda-list keywords (symbols starting with ‘&’)
     are not allowed.

‘lambda-list’
     This matches a lambda list—the argument list of a lambda
     expression.

‘def-body’
     The argument is the body of code in a definition.  This is like
     ‘body’, described above, but a definition body must be instrumented
     with a different Edebug call that looks up information associated
     with the definition.  Use ‘def-body’ for the highest level list of
     forms within the definition.

‘def-form’
     The argument is a single, highest-level form in a definition.  This
     is like ‘def-body’, except it is used to match a single form rather
     than a list of forms.  As a special case, ‘def-form’ also means
     that tracing information is not output when the form is executed.
     See the ‘interactive’ example.


File: elisp.info,  Node: Backtracking,  Next: Specification Examples,  Prev: Specification List,  Up: Edebug and Macros

17.2.15.3 Backtracking in Specifications
........................................

If a specification fails to match at some point, this does not
necessarily mean a syntax error will be signaled; instead,
“backtracking” will take place until all alternatives have been
exhausted.  Eventually every element of the argument list must be
matched by some element in the specification, and every required element
in the specification must match some argument.

   When a syntax error is detected, it might not be reported until much
later, after higher-level alternatives have been exhausted, and with the
point positioned further from the real error.  But if backtracking is
disabled when an error occurs, it can be reported immediately.  Note
that backtracking is also reenabled automatically in several situations;
when a new alternative is established by ‘&optional’, ‘&rest’, or ‘&or’,
or at the start of processing a sublist, group, or indirect
specification.  The effect of enabling or disabling backtracking is
limited to the remainder of the level currently being processed and
lower levels.

   Backtracking is disabled while matching any of the form
specifications (that is, ‘form’, ‘body’, ‘def-form’, and ‘def-body’).
These specifications will match any form so any error must be in the
form itself rather than at a higher level.

   Backtracking is also disabled after successfully matching a quoted
symbol or string specification, since this usually indicates a
recognized construct.  But if you have a set of alternative constructs
that all begin with the same symbol, you can usually work around this
constraint by factoring the symbol out of the alternatives, e.g.,
‘["foo" &or [first case] [second case] ...]’.

   Most needs are satisfied by these two ways that backtracking is
automatically disabled, but occasionally it is useful to explicitly
disable backtracking by using the ‘gate’ specification.  This is useful
when you know that no higher alternatives could apply.  See the example
of the ‘let’ specification.


File: elisp.info,  Node: Specification Examples,  Prev: Backtracking,  Up: Edebug and Macros

17.2.15.4 Specification Examples
................................

It may be easier to understand Edebug specifications by studying the
examples provided here.

   A ‘let’ special form has a sequence of bindings and a body.  Each of
the bindings is either a symbol or a sublist with a symbol and optional
expression.  In the specification below, notice the ‘gate’ inside of the
sublist to prevent backtracking once a sublist is found.

     (def-edebug-spec let
       ((&rest
         &or symbolp (gate symbolp &optional form))
        body))

   Edebug uses the following specifications for ‘defun’ and the
associated argument list and ‘interactive’ specifications.  It is
necessary to handle interactive forms specially since an expression
argument is actually evaluated outside of the function body.  (The
specification for ‘defmacro’ is very similar to that for ‘defun’, but
allows for the ‘declare’ statement.)

     (def-edebug-spec defun
       (&define name lambda-list
                [&optional stringp]   ; Match the doc string, if present.
                [&optional ("interactive" interactive)]
                def-body))

     (def-edebug-spec lambda-list
       (([&rest arg]
         [&optional ["&optional" arg &rest arg]]
         &optional ["&rest" arg]
         )))

     (def-edebug-spec interactive
       (&optional &or stringp def-form))    ; Notice: ‘def-form’

   The specification for backquote below illustrates how to match dotted
lists and use ‘nil’ to terminate recursion.  It also illustrates how
components of a vector may be matched.  (The actual specification
defined by Edebug is a little different, and does not support dotted
lists because doing so causes very deep recursion that could fail.)

     (def-edebug-spec \` (backquote-form))   ; Alias just for clarity.

     (def-edebug-spec backquote-form
       (&or ([&or "," ",@"] &or ("quote" backquote-form) form)
            (backquote-form . [&or nil backquote-form])
            (vector &rest backquote-form)
            sexp))


File: elisp.info,  Node: Edebug Options,  Prev: Edebug and Macros,  Up: Edebug

17.2.16 Edebug Options
----------------------

These options affect the behavior of Edebug:

 -- User Option: edebug-setup-hook
     Functions to call before Edebug is used.  Each time it is set to a
     new value, Edebug will call those functions once and then reset
     ‘edebug-setup-hook’ to ‘nil’.  You could use this to load up Edebug
     specifications associated with a package you are using, but only
     when you also use Edebug.  *Note Instrumenting::.

 -- User Option: edebug-all-defs
     If this is non-‘nil’, normal evaluation of defining forms such as
     ‘defun’ and ‘defmacro’ instruments them for Edebug.  This applies
     to ‘eval-defun’, ‘eval-region’, ‘eval-buffer’, and
     ‘eval-current-buffer’.

     Use the command ‘M-x edebug-all-defs’ to toggle the value of this
     option.  *Note Instrumenting::.

 -- User Option: edebug-all-forms
     If this is non-‘nil’, the commands ‘eval-defun’, ‘eval-region’,
     ‘eval-buffer’, and ‘eval-current-buffer’ instrument all forms, even
     those that don’t define anything.  This doesn’t apply to loading or
     evaluations in the minibuffer.

     Use the command ‘M-x edebug-all-forms’ to toggle the value of this
     option.  *Note Instrumenting::.

 -- User Option: edebug-save-windows
     If this is non-‘nil’, Edebug saves and restores the window
     configuration.  That takes some time, so if your program does not
     care what happens to the window configurations, it is better to set
     this variable to ‘nil’.

     If the value is a list, only the listed windows are saved and
     restored.

     You can use the ‘W’ command in Edebug to change this variable
     interactively.  *Note Edebug Display Update::.

 -- User Option: edebug-save-displayed-buffer-points
     If this is non-‘nil’, Edebug saves and restores point in all
     displayed buffers.

     Saving and restoring point in other buffers is necessary if you are
     debugging code that changes the point of a buffer that is displayed
     in a non-selected window.  If Edebug or the user then selects the
     window, point in that buffer will move to the window’s value of
     point.

     Saving and restoring point in all buffers is expensive, since it
     requires selecting each window twice, so enable this only if you
     need it.  *Note Edebug Display Update::.

 -- User Option: edebug-initial-mode
     If this variable is non-‘nil’, it specifies the initial execution
     mode for Edebug when it is first activated.  Possible values are
     ‘step’, ‘next’, ‘go’, ‘Go-nonstop’, ‘trace’, ‘Trace-fast’,
     ‘continue’, and ‘Continue-fast’.

     The default value is ‘step’.  This variable can be set
     interactively with ‘C-x C-a C-m’ (‘edebug-set-initial-mode’).
     *Note Edebug Execution Modes::.

 -- User Option: edebug-trace
     If this is non-‘nil’, trace each function entry and exit.  Tracing
     output is displayed in a buffer named ‘*edebug-trace*’, one
     function entry or exit per line, indented by the recursion level.

     Also see ‘edebug-tracing’, in *note Trace Buffer::.

 -- User Option: edebug-test-coverage
     If non-‘nil’, Edebug tests coverage of all expressions debugged.
     *Note Coverage Testing::.

 -- User Option: edebug-continue-kbd-macro
     If non-‘nil’, continue defining or executing any keyboard macro
     that is executing outside of Edebug.  Use this with caution since
     it is not debugged.  *Note Edebug Execution Modes::.

 -- User Option: edebug-unwrap-results
     If non-‘nil’, Edebug tries to remove any of its own instrumentation
     when showing the results of expressions.  This is relevant when
     debugging macros where the results of expressions are themselves
     instrumented expressions.  As a very artificial example, suppose
     that the example function ‘fac’ has been instrumented, and consider
     a macro of the form:

          (defmacro test () "Edebug example."
            (if (symbol-function 'fac)
                ...))

     If you instrument the ‘test’ macro and step through it, then by
     default the result of the ‘symbol-function’ call has numerous
     ‘edebug-after’ and ‘edebug-before’ forms, which can make it
     difficult to see the actual result.  If ‘edebug-unwrap-results’ is
     non-‘nil’, Edebug tries to remove these forms from the result.

 -- User Option: edebug-on-error
     Edebug binds ‘debug-on-error’ to this value, if ‘debug-on-error’
     was previously ‘nil’.  *Note Trapping Errors::.

 -- User Option: edebug-on-quit
     Edebug binds ‘debug-on-quit’ to this value, if ‘debug-on-quit’ was
     previously ‘nil’.  *Note Trapping Errors::.

   If you change the values of ‘edebug-on-error’ or ‘edebug-on-quit’
while Edebug is active, their values won’t be used until the _next_ time
Edebug is invoked via a new command.

 -- User Option: edebug-global-break-condition
     If non-‘nil’, an expression to test for at every stop point.  If
     the result is non-‘nil’, then break.  Errors are ignored.  *Note
     Global Break Condition::.


File: elisp.info,  Node: Syntax Errors,  Next: Test Coverage,  Prev: Edebug,  Up: Debugging

17.3 Debugging Invalid Lisp Syntax
==================================

The Lisp reader reports invalid syntax, but cannot say where the real
problem is.  For example, the error ‘End of file during parsing’ in
evaluating an expression indicates an excess of open parentheses (or
square brackets).  The reader detects this imbalance at the end of the
file, but it cannot figure out where the close parenthesis should have
been.  Likewise, ‘Invalid read syntax: ")"’ indicates an excess close
parenthesis or missing open parenthesis, but does not say where the
missing parenthesis belongs.  How, then, to find what to change?

   If the problem is not simply an imbalance of parentheses, a useful
technique is to try ‘C-M-e’ at the beginning of each defun, and see if
it goes to the place where that defun appears to end.  If it does not,
there is a problem in that defun.

   However, unmatched parentheses are the most common syntax errors in
Lisp, and we can give further advice for those cases.  (In addition,
just moving point through the code with Show Paren mode enabled might
find the mismatch.)

* Menu:

* Excess Open::     How to find a spurious open paren or missing close.
* Excess Close::    How to find a spurious close paren or missing open.


File: elisp.info,  Node: Excess Open,  Next: Excess Close,  Up: Syntax Errors

17.3.1 Excess Open Parentheses
------------------------------

The first step is to find the defun that is unbalanced.  If there is an
excess open parenthesis, the way to do this is to go to the end of the
file and type ‘C-u C-M-u’.  This will move you to the beginning of the
first defun that is unbalanced.

   The next step is to determine precisely what is wrong.  There is no
way to be sure of this except by studying the program, but often the
existing indentation is a clue to where the parentheses should have
been.  The easiest way to use this clue is to reindent with ‘C-M-q’ and
see what moves.  *But don’t do this yet!*  Keep reading, first.

   Before you do this, make sure the defun has enough close parentheses.
Otherwise, ‘C-M-q’ will get an error, or will reindent all the rest of
the file until the end.  So move to the end of the defun and insert a
close parenthesis there.  Don’t use ‘C-M-e’ to move there, since that
too will fail to work until the defun is balanced.

   Now you can go to the beginning of the defun and type ‘C-M-q’.
Usually all the lines from a certain point to the end of the function
will shift to the right.  There is probably a missing close parenthesis,
or a superfluous open parenthesis, near that point.  (However, don’t
assume this is true; study the code to make sure.)  Once you have found
the discrepancy, undo the ‘C-M-q’ with ‘C-_’, since the old indentation
is probably appropriate to the intended parentheses.

   After you think you have fixed the problem, use ‘C-M-q’ again.  If
the old indentation actually fit the intended nesting of parentheses,
and you have put back those parentheses, ‘C-M-q’ should not change
anything.


File: elisp.info,  Node: Excess Close,  Prev: Excess Open,  Up: Syntax Errors

17.3.2 Excess Close Parentheses
-------------------------------

To deal with an excess close parenthesis, first go to the beginning of
the file, then type ‘C-u -1 C-M-u’ to find the end of the first
unbalanced defun.

   Then find the actual matching close parenthesis by typing ‘C-M-f’ at
the beginning of that defun.  This will leave you somewhere short of the
place where the defun ought to end.  It is possible that you will find a
spurious close parenthesis in that vicinity.

   If you don’t see a problem at that point, the next thing to do is to
type ‘C-M-q’ at the beginning of the defun.  A range of lines will
probably shift left; if so, the missing open parenthesis or spurious
close parenthesis is probably near the first of those lines.  (However,
don’t assume this is true; study the code to make sure.)  Once you have
found the discrepancy, undo the ‘C-M-q’ with ‘C-_’, since the old
indentation is probably appropriate to the intended parentheses.

   After you think you have fixed the problem, use ‘C-M-q’ again.  If
the old indentation actually fits the intended nesting of parentheses,
and you have put back those parentheses, ‘C-M-q’ should not change
anything.


File: elisp.info,  Node: Test Coverage,  Next: Profiling,  Prev: Syntax Errors,  Up: Debugging

17.4 Test Coverage
==================

You can do coverage testing for a file of Lisp code by loading the
‘testcover’ library and using the command ‘M-x testcover-start <RET>
FILE <RET>’ to instrument the code.  Then test your code by calling it
one or more times.  Then use the command ‘M-x testcover-mark-all’ to
display colored highlights on the code to show where coverage is
insufficient.  The command ‘M-x testcover-next-mark’ will move point
forward to the next highlighted spot.

   Normally, a red highlight indicates the form was never completely
evaluated; a brown highlight means it always evaluated to the same value
(meaning there has been little testing of what is done with the result).
However, the red highlight is skipped for forms that can’t possibly
complete their evaluation, such as ‘error’.  The brown highlight is
skipped for forms that are expected to always evaluate to the same
value, such as ‘(setq x 14)’.

   For difficult cases, you can add do-nothing macros to your code to
give advice to the test coverage tool.

 -- Macro: 1value form
     Evaluate FORM and return its value, but inform coverage testing
     that FORM’s value should always be the same.

 -- Macro: noreturn form
     Evaluate FORM, informing coverage testing that FORM should never
     return.  If it ever does return, you get a run-time error.

   Edebug also has a coverage testing feature (*note Coverage
Testing::).  These features partly duplicate each other, and it would be
cleaner to combine them.


File: elisp.info,  Node: Profiling,  Prev: Test Coverage,  Up: Debugging

17.5 Profiling
==============

If your program is working correctly, but you want to make it run more
quickly or efficiently, the first thing to do is “profile” your code so
that you know how it is using resources.  If you find that one
particular function is responsible for a significant portion of the
runtime, you can start looking for ways to optimize that piece.

   Emacs has built-in support for this.  To begin profiling, type ‘M-x
profiler-start’.  You can choose to profile by processor usage, memory
usage, or both.  After doing some work, type ‘M-x profiler-report’ to
display a summary buffer for each resource that you chose to profile.
The names of the report buffers include the times at which the reports
were generated, so you can generate another report later on without
erasing previous results.  When you have finished profiling, type ‘M-x
profiler-stop’ (there is a small overhead associated with profiling).

   The profiler report buffer shows, on each line, a function that was
called, followed by how much resource (processor or memory) it used in
absolute and percentage times since profiling started.  If a given line
has a ‘+’ symbol at the left-hand side, you can expand that line by
typing <RET>, in order to see the function(s) called by the higher-level
function.  Use a prefix argument (<C-u RET>) to see the whole call tree
below a function.  Pressing <RET> again will collapse back to the
original state.

   Press ‘j’ or ‘mouse-2’ to jump to the definition of a function.
Press ‘d’ to view a function’s documentation.  You can save a profile to
a file using ‘C-x C-w’.  You can compare two profiles using ‘=’.

   The ‘elp’ library offers an alternative approach.  See the file
‘elp.el’ for instructions.

   You can check the speed of individual Emacs Lisp forms using the
‘benchmark’ library.  See the functions ‘benchmark-run’ and
‘benchmark-run-compiled’ in ‘benchmark.el’.

   To profile Emacs at the level of its C code, you can build it using
the ‘--enable-profiling’ option of ‘configure’.  When Emacs exits, it
generates a file ‘gmon.out’ that you can examine using the ‘gprof’
utility.  This feature is mainly useful for debugging Emacs.  It
actually stops the Lisp-level ‘M-x profiler-...’ commands described
above from working.


File: elisp.info,  Node: Read and Print,  Next: Minibuffers,  Prev: Debugging,  Up: Top

18 Reading and Printing Lisp Objects
************************************

“Printing” and “reading” are the operations of converting Lisp objects
to textual form and vice versa.  They use the printed representations
and read syntax described in *note Lisp Data Types::.

   This chapter describes the Lisp functions for reading and printing.
It also describes “streams”, which specify where to get the text (if
reading) or where to put it (if printing).

* Menu:

* Streams Intro::     Overview of streams, reading and printing.
* Input Streams::     Various data types that can be used as input streams.
* Input Functions::   Functions to read Lisp objects from text.
* Output Streams::    Various data types that can be used as output streams.
* Output Functions::  Functions to print Lisp objects as text.
* Output Variables::  Variables that control what the printing functions do.


File: elisp.info,  Node: Streams Intro,  Next: Input Streams,  Up: Read and Print

18.1 Introduction to Reading and Printing
=========================================

“Reading” a Lisp object means parsing a Lisp expression in textual form
and producing a corresponding Lisp object.  This is how Lisp programs
get into Lisp from files of Lisp code.  We call the text the “read
syntax” of the object.  For example, the text ‘(a . 5)’ is the read
syntax for a cons cell whose CAR is ‘a’ and whose CDR is the number 5.

   “Printing” a Lisp object means producing text that represents that
object—converting the object to its “printed representation” (*note
Printed Representation::).  Printing the cons cell described above
produces the text ‘(a . 5)’.

   Reading and printing are more or less inverse operations: printing
the object that results from reading a given piece of text often
produces the same text, and reading the text that results from printing
an object usually produces a similar-looking object.  For example,
printing the symbol ‘foo’ produces the text ‘foo’, and reading that text
returns the symbol ‘foo’.  Printing a list whose elements are ‘a’ and
‘b’ produces the text ‘(a b)’, and reading that text produces a list
(but not the same list) with elements ‘a’ and ‘b’.

   However, these two operations are not precisely inverse to each
other.  There are three kinds of exceptions:

   • Printing can produce text that cannot be read.  For example,
     buffers, windows, frames, subprocesses and markers print as text
     that starts with ‘#’; if you try to read this text, you get an
     error.  There is no way to read those data types.

   • One object can have multiple textual representations.  For example,
     ‘1’ and ‘01’ represent the same integer, and ‘(a b)’ and ‘(a .
     (b))’ represent the same list.  Reading will accept any of the
     alternatives, but printing must choose one of them.

   • Comments can appear at certain points in the middle of an object’s
     read sequence without affecting the result of reading it.


File: elisp.info,  Node: Input Streams,  Next: Input Functions,  Prev: Streams Intro,  Up: Read and Print

18.2 Input Streams
==================

Most of the Lisp functions for reading text take an “input stream” as an
argument.  The input stream specifies where or how to get the characters
of the text to be read.  Here are the possible types of input stream:

BUFFER
     The input characters are read from BUFFER, starting with the
     character directly after point.  Point advances as characters are
     read.

MARKER
     The input characters are read from the buffer that MARKER is in,
     starting with the character directly after the marker.  The marker
     position advances as characters are read.  The value of point in
     the buffer has no effect when the stream is a marker.

STRING
     The input characters are taken from STRING, starting at the first
     character in the string and using as many characters as required.

FUNCTION
     The input characters are generated by FUNCTION, which must support
     two kinds of calls:

        • When it is called with no arguments, it should return the next
          character.

        • When it is called with one argument (always a character),
          FUNCTION should save the argument and arrange to return it on
          the next call.  This is called “unreading” the character; it
          happens when the Lisp reader reads one character too many and
          wants to put it back where it came from.  In this case, it
          makes no difference what value FUNCTION returns.

‘t’
     ‘t’ used as a stream means that the input is read from the
     minibuffer.  In fact, the minibuffer is invoked once and the text
     given by the user is made into a string that is then used as the
     input stream.  If Emacs is running in batch mode, standard input is
     used instead of the minibuffer.  For example,
          (message "%s" (read t))
     will read a Lisp expression from standard input and print the
     result to standard output.

‘nil’
     ‘nil’ supplied as an input stream means to use the value of
     ‘standard-input’ instead; that value is the “default input stream”,
     and must be a non-‘nil’ input stream.

SYMBOL
     A symbol as input stream is equivalent to the symbol’s function
     definition (if any).

   Here is an example of reading from a stream that is a buffer, showing
where point is located before and after:

     ---------- Buffer: foo ----------
     This★ is the contents of foo.
     ---------- Buffer: foo ----------

     (read (get-buffer "foo"))
          ⇒ is
     (read (get-buffer "foo"))
          ⇒ the

     ---------- Buffer: foo ----------
     This is the★ contents of foo.
     ---------- Buffer: foo ----------

Note that the first read skips a space.  Reading skips any amount of
whitespace preceding the significant text.

   Here is an example of reading from a stream that is a marker,
initially positioned at the beginning of the buffer shown.  The value
read is the symbol ‘This’.


     ---------- Buffer: foo ----------
     This is the contents of foo.
     ---------- Buffer: foo ----------

     (setq m (set-marker (make-marker) 1 (get-buffer "foo")))
          ⇒ #<marker at 1 in foo>
     (read m)
          ⇒ This
     m
          ⇒ #<marker at 5 in foo>   ;; Before the first space.

   Here we read from the contents of a string:

     (read "(When in) the course")
          ⇒ (When in)

   The following example reads from the minibuffer.  The prompt is:
‘Lisp expression: ’.  (That is always the prompt used when you read from
the stream ‘t’.)  The user’s input is shown following the prompt.

     (read t)
          ⇒ 23
     ---------- Buffer: Minibuffer ----------
     Lisp expression: 23 <RET>
     ---------- Buffer: Minibuffer ----------

   Finally, here is an example of a stream that is a function, named
‘useless-stream’.  Before we use the stream, we initialize the variable
‘useless-list’ to a list of characters.  Then each call to the function
‘useless-stream’ obtains the next character in the list or unreads a
character by adding it to the front of the list.

     (setq useless-list (append "XY()" nil))
          ⇒ (88 89 40 41)

     (defun useless-stream (&optional unread)
       (if unread
           (setq useless-list (cons unread useless-list))
         (prog1 (car useless-list)
                (setq useless-list (cdr useless-list)))))
          ⇒ useless-stream

Now we read using the stream thus constructed:

     (read 'useless-stream)
          ⇒ XY

     useless-list
          ⇒ (40 41)

Note that the open and close parentheses remain in the list.  The Lisp
reader encountered the open parenthesis, decided that it ended the
input, and unread it.  Another attempt to read from the stream at this
point would read ‘()’ and return ‘nil’.


File: elisp.info,  Node: Input Functions,  Next: Output Streams,  Prev: Input Streams,  Up: Read and Print

18.3 Input Functions
====================

This section describes the Lisp functions and variables that pertain to
reading.

   In the functions below, STREAM stands for an input stream (see the
previous section).  If STREAM is ‘nil’ or omitted, it defaults to the
value of ‘standard-input’.

   An ‘end-of-file’ error is signaled if reading encounters an
unterminated list, vector, or string.

 -- Function: read &optional stream
     This function reads one textual Lisp expression from STREAM,
     returning it as a Lisp object.  This is the basic Lisp input
     function.

 -- Function: read-from-string string &optional start end
     This function reads the first textual Lisp expression from the text
     in STRING.  It returns a cons cell whose CAR is that expression,
     and whose CDR is an integer giving the position of the next
     remaining character in the string (i.e., the first one not read).

     If START is supplied, then reading begins at index START in the
     string (where the first character is at index 0).  If you specify
     END, then reading is forced to stop just before that index, as if
     the rest of the string were not there.

     For example:

          (read-from-string "(setq x 55) (setq y 5)")
               ⇒ ((setq x 55) . 11)
          (read-from-string "\"A short string\"")
               ⇒ ("A short string" . 16)

          ;; Read starting at the first character.
          (read-from-string "(list 112)" 0)
               ⇒ ((list 112) . 10)
          ;; Read starting at the second character.
          (read-from-string "(list 112)" 1)
               ⇒ (list . 5)
          ;; Read starting at the seventh character,
          ;;   and stopping at the ninth.
          (read-from-string "(list 112)" 6 8)
               ⇒ (11 . 8)

 -- Variable: standard-input
     This variable holds the default input stream—the stream that ‘read’
     uses when the STREAM argument is ‘nil’.  The default is ‘t’,
     meaning use the minibuffer.

 -- Variable: read-circle
     If non-‘nil’, this variable enables the reading of circular and
     shared structures.  *Note Circular Objects::.  Its default value is
     ‘t’.

   When reading or writing from the standard input/output streams of the
Emacs process in batch mode, it is sometimes required to make sure any
arbitrary binary data will be read/written verbatim, and/or that no
translation of newlines to or from CR-LF pairs is performed.  This issue
does not exist on Posix hosts, only on MS-Windows and MS-DOS.  The
following function allows you to control the I/O mode of any standard
stream of the Emacs process.

 -- Function: set-binary-mode stream mode
     Switch STREAM into binary or text I/O mode.  If MODE is non-‘nil’,
     switch to binary mode, otherwise switch to text mode.  The value of
     STREAM can be one of ‘stdin’, ‘stdout’, or ‘stderr’.  This function
     flushes any pending output data of STREAM as a side effect, and
     returns the previous value of I/O mode for STREAM.  On Posix hosts,
     it always returns a non-‘nil’ value and does nothing except
     flushing pending output.


File: elisp.info,  Node: Output Streams,  Next: Output Functions,  Prev: Input Functions,  Up: Read and Print

18.4 Output Streams
===================

An output stream specifies what to do with the characters produced by
printing.  Most print functions accept an output stream as an optional
argument.  Here are the possible types of output stream:

BUFFER
     The output characters are inserted into BUFFER at point.  Point
     advances as characters are inserted.

MARKER
     The output characters are inserted into the buffer that MARKER
     points into, at the marker position.  The marker position advances
     as characters are inserted.  The value of point in the buffer has
     no effect on printing when the stream is a marker, and this kind of
     printing does not move point (except that if the marker points at
     or before the position of point, point advances with the
     surrounding text, as usual).

FUNCTION
     The output characters are passed to FUNCTION, which is responsible
     for storing them away.  It is called with a single character as
     argument, as many times as there are characters to be output, and
     is responsible for storing the characters wherever you want to put
     them.

‘t’
     The output characters are displayed in the echo area.

‘nil’
     ‘nil’ specified as an output stream means to use the value of
     ‘standard-output’ instead; that value is the “default output
     stream”, and must not be ‘nil’.

SYMBOL
     A symbol as output stream is equivalent to the symbol’s function
     definition (if any).

   Many of the valid output streams are also valid as input streams.
The difference between input and output streams is therefore more a
matter of how you use a Lisp object, than of different types of object.

   Here is an example of a buffer used as an output stream.  Point is
initially located as shown immediately before the ‘h’ in ‘the’.  At the
end, point is located directly before that same ‘h’.

     ---------- Buffer: foo ----------
     This is t★he contents of foo.
     ---------- Buffer: foo ----------

     (print "This is the output" (get-buffer "foo"))
          ⇒ "This is the output"

     ---------- Buffer: foo ----------
     This is t
     "This is the output"
     ★he contents of foo.
     ---------- Buffer: foo ----------

   Now we show a use of a marker as an output stream.  Initially, the
marker is in buffer ‘foo’, between the ‘t’ and the ‘h’ in the word
‘the’.  At the end, the marker has advanced over the inserted text so
that it remains positioned before the same ‘h’.  Note that the location
of point, shown in the usual fashion, has no effect.

     ---------- Buffer: foo ----------
     This is the ★output
     ---------- Buffer: foo ----------

     (setq m (copy-marker 10))
          ⇒ #<marker at 10 in foo>

     (print "More output for foo." m)
          ⇒ "More output for foo."

     ---------- Buffer: foo ----------
     This is t
     "More output for foo."
     he ★output
     ---------- Buffer: foo ----------

     m
          ⇒ #<marker at 34 in foo>

   The following example shows output to the echo area:

     (print "Echo Area output" t)
          ⇒ "Echo Area output"
     ---------- Echo Area ----------
     "Echo Area output"
     ---------- Echo Area ----------

   Finally, we show the use of a function as an output stream.  The
function ‘eat-output’ takes each character that it is given and conses
it onto the front of the list ‘last-output’ (*note Building Lists::).
At the end, the list contains all the characters output, but in reverse
order.

     (setq last-output nil)
          ⇒ nil

     (defun eat-output (c)
       (setq last-output (cons c last-output)))
          ⇒ eat-output

     (print "This is the output" 'eat-output)
          ⇒ "This is the output"

     last-output
          ⇒ (10 34 116 117 112 116 117 111 32 101 104
         116 32 115 105 32 115 105 104 84 34 10)

Now we can put the output in the proper order by reversing the list:

     (concat (nreverse last-output))
          ⇒ "
     \"This is the output\"
     "

Calling ‘concat’ converts the list to a string so you can see its
contents more clearly.


File: elisp.info,  Node: Output Functions,  Next: Output Variables,  Prev: Output Streams,  Up: Read and Print

18.5 Output Functions
=====================

This section describes the Lisp functions for printing Lisp
objects—converting objects into their printed representation.

   Some of the Emacs printing functions add quoting characters to the
output when necessary so that it can be read properly.  The quoting
characters used are ‘"’ and ‘\’; they distinguish strings from symbols,
and prevent punctuation characters in strings and symbols from being
taken as delimiters when reading.  *Note Printed Representation::, for
full details.  You specify quoting or no quoting by the choice of
printing function.

   If the text is to be read back into Lisp, then you should print with
quoting characters to avoid ambiguity.  Likewise, if the purpose is to
describe a Lisp object clearly for a Lisp programmer.  However, if the
purpose of the output is to look nice for humans, then it is usually
better to print without quoting.

   Lisp objects can refer to themselves.  Printing a self-referential
object in the normal way would require an infinite amount of text, and
the attempt could cause infinite recursion.  Emacs detects such
recursion and prints ‘#LEVEL’ instead of recursively printing an object
already being printed.  For example, here ‘#0’ indicates a recursive
reference to the object at level 0 of the current print operation:

     (setq foo (list nil))
          ⇒ (nil)
     (setcar foo foo)
          ⇒ (#0)

   In the functions below, STREAM stands for an output stream.  (See the
previous section for a description of output streams.)  If STREAM is
‘nil’ or omitted, it defaults to the value of ‘standard-output’.

 -- Function: print object &optional stream
     The ‘print’ function is a convenient way of printing.  It outputs
     the printed representation of OBJECT to STREAM, printing in
     addition one newline before OBJECT and another after it.  Quoting
     characters are used.  ‘print’ returns OBJECT.  For example:

          (progn (print 'The\ cat\ in)
                 (print "the hat")
                 (print " came back"))
               ⊣
               ⊣ The\ cat\ in
               ⊣
               ⊣ "the hat"
               ⊣
               ⊣ " came back"
               ⇒ " came back"

 -- Function: prin1 object &optional stream
     This function outputs the printed representation of OBJECT to
     STREAM.  It does not print newlines to separate output as ‘print’
     does, but it does use quoting characters just like ‘print’.  It
     returns OBJECT.

          (progn (prin1 'The\ cat\ in)
                 (prin1 "the hat")
                 (prin1 " came back"))
               ⊣ The\ cat\ in"the hat"" came back"
               ⇒ " came back"

 -- Function: princ object &optional stream
     This function outputs the printed representation of OBJECT to
     STREAM.  It returns OBJECT.

     This function is intended to produce output that is readable by
     people, not by ‘read’, so it doesn’t insert quoting characters and
     doesn’t put double-quotes around the contents of strings.  It does
     not add any spacing between calls.

          (progn
            (princ 'The\ cat)
            (princ " in the \"hat\""))
               ⊣ The cat in the "hat"
               ⇒ " in the \"hat\""

 -- Function: terpri &optional stream ensure
     This function outputs a newline to STREAM.  The name stands for
     “terminate print”.  If ENSURE is non-‘nil’ no newline is printed if
     STREAM is already at the beginning of a line.  Note in this case
     STREAM can not be a function and an error is signalled if it is.
     This function returns ‘t’ if a newline is printed.

 -- Function: write-char character &optional stream
     This function outputs CHARACTER to STREAM.  It returns CHARACTER.

 -- Function: prin1-to-string object &optional noescape
     This function returns a string containing the text that ‘prin1’
     would have printed for the same argument.

          (prin1-to-string 'foo)
               ⇒ "foo"
          (prin1-to-string (mark-marker))
               ⇒ "#<marker at 2773 in strings.texi>"

     If NOESCAPE is non-‘nil’, that inhibits use of quoting characters
     in the output.  (This argument is supported in Emacs versions 19
     and later.)

          (prin1-to-string "foo")
               ⇒ "\"foo\""
          (prin1-to-string "foo" t)
               ⇒ "foo"

     See ‘format’, in *note Formatting Strings::, for other ways to
     obtain the printed representation of a Lisp object as a string.

 -- Macro: with-output-to-string body...
     This macro executes the BODY forms with ‘standard-output’ set up to
     feed output into a string.  Then it returns that string.

     For example, if the current buffer name is ‘foo’,

          (with-output-to-string
            (princ "The buffer is ")
            (princ (buffer-name)))

     returns ‘"The buffer is foo"’.

 -- Function: pp object &optional stream
     This function outputs OBJECT to STREAM, just like ‘prin1’, but does
     it in a prettier way.  That is, it’ll indent and fill the object to
     make it more readable for humans.

   If you need to use binary I/O in batch mode, e.g., use the functions
described in this section to write out arbitrary binary data or avoid
conversion of newlines on non-Posix hosts, see *note set-binary-mode:
Input Functions.


File: elisp.info,  Node: Output Variables,  Prev: Output Functions,  Up: Read and Print

18.6 Variables Affecting Output
===============================

 -- Variable: standard-output
     The value of this variable is the default output stream—the stream
     that print functions use when the STREAM argument is ‘nil’.  The
     default is ‘t’, meaning display in the echo area.

 -- Variable: print-quoted
     If this is non-‘nil’, that means to print quoted forms using
     abbreviated reader syntax, e.g., ‘(quote foo)’ prints as ‘'foo’,
     and ‘(function foo)’ as ‘#'foo’.

 -- Variable: print-escape-newlines
     If this variable is non-‘nil’, then newline characters in strings
     are printed as ‘\n’ and formfeeds are printed as ‘\f’.  Normally
     these characters are printed as actual newlines and formfeeds.

     This variable affects the print functions ‘prin1’ and ‘print’ that
     print with quoting.  It does not affect ‘princ’.  Here is an
     example using ‘prin1’:

          (prin1 "a\nb")
               ⊣ "a
               ⊣ b"
               ⇒ "a
          b"

          (let ((print-escape-newlines t))
            (prin1 "a\nb"))
               ⊣ "a\nb"
               ⇒ "a
          b"

     In the second expression, the local binding of
     ‘print-escape-newlines’ is in effect during the call to ‘prin1’,
     but not during the printing of the result.

 -- Variable: print-escape-nonascii
     If this variable is non-‘nil’, then unibyte non-ASCII characters in
     strings are unconditionally printed as backslash sequences by the
     print functions ‘prin1’ and ‘print’ that print with quoting.

     Those functions also use backslash sequences for unibyte non-ASCII
     characters, regardless of the value of this variable, when the
     output stream is a multibyte buffer or a marker pointing into one.

 -- Variable: print-escape-multibyte
     If this variable is non-‘nil’, then multibyte non-ASCII characters
     in strings are unconditionally printed as backslash sequences by
     the print functions ‘prin1’ and ‘print’ that print with quoting.

     Those functions also use backslash sequences for multibyte
     non-ASCII characters, regardless of the value of this variable,
     when the output stream is a unibyte buffer or a marker pointing
     into one.

 -- Variable: print-length
     The value of this variable is the maximum number of elements to
     print in any list, vector or bool-vector.  If an object being
     printed has more than this many elements, it is abbreviated with an
     ellipsis.

     If the value is ‘nil’ (the default), then there is no limit.

          (setq print-length 2)
               ⇒ 2
          (print '(1 2 3 4 5))
               ⊣ (1 2 ...)
               ⇒ (1 2 ...)

 -- Variable: print-level
     The value of this variable is the maximum depth of nesting of
     parentheses and brackets when printed.  Any list or vector at a
     depth exceeding this limit is abbreviated with an ellipsis.  A
     value of ‘nil’ (which is the default) means no limit.

 -- User Option: eval-expression-print-length
 -- User Option: eval-expression-print-level
     These are the values for ‘print-length’ and ‘print-level’ used by
     ‘eval-expression’, and thus, indirectly, by many interactive
     evaluation commands (*note Evaluating Emacs-Lisp Expressions:
     (emacs)Lisp Eval.).

   These variables are used for detecting and reporting circular and
shared structure:

 -- Variable: print-circle
     If non-‘nil’, this variable enables detection of circular and
     shared structure in printing.  *Note Circular Objects::.

 -- Variable: print-gensym
     If non-‘nil’, this variable enables detection of uninterned symbols
     (*note Creating Symbols::) in printing.  When this is enabled,
     uninterned symbols print with the prefix ‘#:’, which tells the Lisp
     reader to produce an uninterned symbol.

 -- Variable: print-continuous-numbering
     If non-‘nil’, that means number continuously across print calls.
     This affects the numbers printed for ‘#N=’ labels and ‘#M#’
     references.  Don’t set this variable with ‘setq’; you should only
     bind it temporarily to ‘t’ with ‘let’.  When you do that, you
     should also bind ‘print-number-table’ to ‘nil’.

 -- Variable: print-number-table
     This variable holds a vector used internally by printing to
     implement the ‘print-circle’ feature.  You should not use it except
     to bind it to ‘nil’ when you bind ‘print-continuous-numbering’.

 -- Variable: float-output-format
     This variable specifies how to print floating-point numbers.  The
     default is ‘nil’, meaning use the shortest output that represents
     the number without losing information.

     To control output format more precisely, you can put a string in
     this variable.  The string should hold a ‘%’-specification to be
     used in the C function ‘sprintf’.  For further restrictions on what
     you can use, see the variable’s documentation string.


File: elisp.info,  Node: Minibuffers,  Next: Command Loop,  Prev: Read and Print,  Up: Top

19 Minibuffers
**************

A “minibuffer” is a special buffer that Emacs commands use to read
arguments more complicated than the single numeric prefix argument.
These arguments include file names, buffer names, and command names (as
in ‘M-x’).  The minibuffer is displayed on the bottom line of the frame,
in the same place as the echo area (*note The Echo Area::), but only
while it is in use for reading an argument.

* Menu:

* Intro to Minibuffers::      Basic information about minibuffers.
* Text from Minibuffer::      How to read a straight text string.
* Object from Minibuffer::    How to read a Lisp object or expression.
* Minibuffer History::        Recording previous minibuffer inputs
                                so the user can reuse them.
* Initial Input::             Specifying initial contents for the minibuffer.
* Completion::                How to invoke and customize completion.
* Yes-or-No Queries::         Asking a question with a simple answer.
* Multiple Queries::          Asking a series of similar questions.
* Reading a Password::        Reading a password from the terminal.
* Minibuffer Commands::       Commands used as key bindings in minibuffers.
* Minibuffer Windows::        Operating on the special minibuffer windows.
* Minibuffer Contents::       How such commands access the minibuffer text.
* Recursive Mini::            Whether recursive entry to minibuffer is allowed.
* Minibuffer Misc::           Various customization hooks and variables.


File: elisp.info,  Node: Intro to Minibuffers,  Next: Text from Minibuffer,  Up: Minibuffers

19.1 Introduction to Minibuffers
================================

In most ways, a minibuffer is a normal Emacs buffer.  Most operations
_within_ a buffer, such as editing commands, work normally in a
minibuffer.  However, many operations for managing buffers do not apply
to minibuffers.  The name of a minibuffer always has the form
‘ *Minibuf-NUMBER*’, and it cannot be changed.  Minibuffers are
displayed only in special windows used only for minibuffers; these
windows always appear at the bottom of a frame.  (Sometimes frames have
no minibuffer window, and sometimes a special kind of frame contains
nothing but a minibuffer window; see *note Minibuffers and Frames::.)

   The text in the minibuffer always starts with the “prompt string”,
the text that was specified by the program that is using the minibuffer
to tell the user what sort of input to type.  This text is marked
read-only so you won’t accidentally delete or change it.  It is also
marked as a field (*note Fields::), so that certain motion functions,
including ‘beginning-of-line’, ‘forward-word’, ‘forward-sentence’, and
‘forward-paragraph’, stop at the boundary between the prompt and the
actual text.

   The minibuffer’s window is normally a single line; it grows
automatically if the contents require more space.  Whilst it is active,
you can explicitly resize it temporarily with the window sizing
commands; it reverts to its normal size when the minibuffer is exited.
When the minibuffer is not active, you can resize it permanently by
using the window sizing commands in the frame’s other window, or
dragging the mode line with the mouse.  (Due to details of the current
implementation, for this to work ‘resize-mini-windows’ must be ‘nil’.)
If the frame contains just a minibuffer, you can change the minibuffer’s
size by changing the frame’s size.

   Use of the minibuffer reads input events, and that alters the values
of variables such as ‘this-command’ and ‘last-command’ (*note Command
Loop Info::).  Your program should bind them around the code that uses
the minibuffer, if you do not want that to change them.

   Under some circumstances, a command can use a minibuffer even if
there is an active minibuffer; such a minibuffer is called a “recursive
minibuffer”.  The first minibuffer is named ‘ *Minibuf-1*’.  Recursive
minibuffers are named by incrementing the number at the end of the name.
(The names begin with a space so that they won’t show up in normal
buffer lists.)  Of several recursive minibuffers, the innermost (or most
recently entered) is the active minibuffer.  We usually call this _the_
minibuffer.  You can permit or forbid recursive minibuffers by setting
the variable ‘enable-recursive-minibuffers’, or by putting properties of
that name on command symbols (*Note Recursive Mini::.)

   Like other buffers, a minibuffer uses a local keymap (*note
Keymaps::) to specify special key bindings.  The function that invokes
the minibuffer also sets up its local map according to the job to be
done.  *Note Text from Minibuffer::, for the non-completion minibuffer
local maps.  *Note Completion Commands::, for the minibuffer local maps
for completion.

   When a minibuffer is inactive, its major mode is
‘minibuffer-inactive-mode’, with keymap ‘minibuffer-inactive-mode-map’.
This is only really useful if the minibuffer is in a separate frame.
*Note Minibuffers and Frames::.

   When Emacs is running in batch mode, any request to read from the
minibuffer actually reads a line from the standard input descriptor that
was supplied when Emacs was started.  This supports only basic input:
none of the special minibuffer features (history, completion, etc.) are
available in batch mode.


File: elisp.info,  Node: Text from Minibuffer,  Next: Object from Minibuffer,  Prev: Intro to Minibuffers,  Up: Minibuffers

19.2 Reading Text Strings with the Minibuffer
=============================================

The most basic primitive for minibuffer input is ‘read-from-minibuffer’,
which can be used to read either a string or a Lisp object in textual
form.  The function ‘read-regexp’ is used for reading regular
expressions (*note Regular Expressions::), which are a special kind of
string.  There are also specialized functions for reading commands,
variables, file names, etc. (*note Completion::).

   In most cases, you should not call minibuffer input functions in the
middle of a Lisp function.  Instead, do all minibuffer input as part of
reading the arguments for a command, in the ‘interactive’ specification.
*Note Defining Commands::.

 -- Function: read-from-minibuffer prompt &optional initial keymap read
          history default inherit-input-method
     This function is the most general way to get input from the
     minibuffer.  By default, it accepts arbitrary text and returns it
     as a string; however, if READ is non-‘nil’, then it uses ‘read’ to
     convert the text into a Lisp object (*note Input Functions::).

     The first thing this function does is to activate a minibuffer and
     display it with PROMPT (which must be a string) as the prompt.
     Then the user can edit text in the minibuffer.

     When the user types a command to exit the minibuffer,
     ‘read-from-minibuffer’ constructs the return value from the text in
     the minibuffer.  Normally it returns a string containing that text.
     However, if READ is non-‘nil’, ‘read-from-minibuffer’ reads the
     text and returns the resulting Lisp object, unevaluated.  (*Note
     Input Functions::, for information about reading.)

     The argument DEFAULT specifies default values to make available
     through the history commands.  It should be a string, a list of
     strings, or ‘nil’.  The string or strings become the minibuffer’s
     “future history”, available to the user with ‘M-n’.

     If READ is non-‘nil’, then DEFAULT is also used as the input to
     ‘read’, if the user enters empty input.  If DEFAULT is a list of
     strings, the first string is used as the input.  If DEFAULT is
     ‘nil’, empty input results in an ‘end-of-file’ error.  However, in
     the usual case (where READ is ‘nil’), ‘read-from-minibuffer’
     ignores DEFAULT when the user enters empty input and returns an
     empty string, ‘""’.  In this respect, it differs from all the other
     minibuffer input functions in this chapter.

     If KEYMAP is non-‘nil’, that keymap is the local keymap to use in
     the minibuffer.  If KEYMAP is omitted or ‘nil’, the value of
     ‘minibuffer-local-map’ is used as the keymap.  Specifying a keymap
     is the most important way to customize the minibuffer for various
     applications such as completion.

     The argument HISTORY specifies a history list variable to use for
     saving the input and for history commands used in the minibuffer.
     It defaults to ‘minibuffer-history’.  You can optionally specify a
     starting position in the history list as well.  *Note Minibuffer
     History::.

     If the variable ‘minibuffer-allow-text-properties’ is non-‘nil’,
     then the string that is returned includes whatever text properties
     were present in the minibuffer.  Otherwise all the text properties
     are stripped when the value is returned.

     If the argument INHERIT-INPUT-METHOD is non-‘nil’, then the
     minibuffer inherits the current input method (*note Input
     Methods::) and the setting of ‘enable-multibyte-characters’ (*note
     Text Representations::) from whichever buffer was current before
     entering the minibuffer.

     Use of INITIAL is mostly deprecated; we recommend using a non-‘nil’
     value only in conjunction with specifying a cons cell for HISTORY.
     *Note Initial Input::.

 -- Function: read-string prompt &optional initial history default
          inherit-input-method
     This function reads a string from the minibuffer and returns it.
     The arguments PROMPT, INITIAL, HISTORY and INHERIT-INPUT-METHOD are
     used as in ‘read-from-minibuffer’.  The keymap used is
     ‘minibuffer-local-map’.

     The optional argument DEFAULT is used as in ‘read-from-minibuffer’,
     except that, if non-‘nil’, it also specifies a default value to
     return if the user enters null input.  As in ‘read-from-minibuffer’
     it should be a string, a list of strings, or ‘nil’, which is
     equivalent to an empty string.  When DEFAULT is a string, that
     string is the default value.  When it is a list of strings, the
     first string is the default value.  (All these strings are
     available to the user in the “future minibuffer history”.)

     This function works by calling the ‘read-from-minibuffer’ function:

          (read-string PROMPT INITIAL HISTORY DEFAULT INHERIT)
          ≡
          (let ((value
                 (read-from-minibuffer PROMPT INITIAL nil nil
                                       HISTORY DEFAULT INHERIT)))
            (if (and (equal value "") DEFAULT)
                (if (consp DEFAULT) (car DEFAULT) DEFAULT)
              value))

 -- Function: read-regexp prompt &optional defaults history
     This function reads a regular expression as a string from the
     minibuffer and returns it.  If the minibuffer prompt string PROMPT
     does not end in ‘:’ (followed by optional whitespace), the function
     adds ‘: ’ to the end, preceded by the default return value (see
     below), if that is non-empty.

     The optional argument DEFAULTS controls the default value to return
     if the user enters null input, and should be one of: a string;
     ‘nil’, which is equivalent to an empty string; a list of strings;
     or a symbol.

     If DEFAULTS is a symbol, ‘read-regexp’ consults the value of the
     variable ‘read-regexp-defaults-function’ (see below), and if that
     is non-‘nil’ uses it in preference to DEFAULTS.  The value in this
     case should be either:

        − ‘regexp-history-last’, which means to use the first element of
          the appropriate minibuffer history list (see below).

        − A function of no arguments, whose return value (which should
          be ‘nil’, a string, or a list of strings) becomes the value of
          DEFAULTS.

     ‘read-regexp’ now ensures that the result of processing DEFAULTS is
     a list (i.e., if the value is ‘nil’ or a string, it converts it to
     a list of one element).  To this list, ‘read-regexp’ then appends a
     few potentially useful candidates for input.  These are:

        − The word or symbol at point.
        − The last regexp used in an incremental search.
        − The last string used in an incremental search.
        − The last string or pattern used in query-replace commands.

     The function now has a list of regular expressions that it passes
     to ‘read-from-minibuffer’ to obtain the user’s input.  The first
     element of the list is the default result in case of empty input.
     All elements of the list are available to the user as the “future
     minibuffer history” list (*note future list: (emacs)Minibuffer
     History.).

     The optional argument HISTORY, if non-‘nil’, is a symbol specifying
     a minibuffer history list to use (*note Minibuffer History::).  If
     it is omitted or ‘nil’, the history list defaults to
     ‘regexp-history’.

 -- User Option: read-regexp-defaults-function
     The function ‘read-regexp’ may use the value of this variable to
     determine its list of default regular expressions.  If non-‘nil’,
     the value of this variable should be either:

        − The symbol ‘regexp-history-last’.

        − A function of no arguments that returns either ‘nil’, a
          string, or a list of strings.

     See ‘read-regexp’ above for details of how these values are used.

 -- Variable: minibuffer-allow-text-properties
     If this variable is ‘nil’, then ‘read-from-minibuffer’ and
     ‘read-string’ strip all text properties from the minibuffer input
     before returning it.  However, ‘read-no-blanks-input’ (see below),
     as well as ‘read-minibuffer’ and related functions (*note Reading
     Lisp Objects With the Minibuffer: Object from Minibuffer.), and all
     functions that do minibuffer input with completion, discard text
     properties unconditionally, regardless of the value of this
     variable.

 -- Variable: minibuffer-local-map
     This is the default local keymap for reading from the minibuffer.
     By default, it makes the following bindings:

     ‘C-j’
          ‘exit-minibuffer’

     <RET>
          ‘exit-minibuffer’

     ‘C-g’
          ‘abort-recursive-edit’

     ‘M-n’
     <DOWN>
          ‘next-history-element’

     ‘M-p’
     <UP>
          ‘previous-history-element’

     ‘M-s’
          ‘next-matching-history-element’

     ‘M-r’
          ‘previous-matching-history-element’

 -- Function: read-no-blanks-input prompt &optional initial
          inherit-input-method
     This function reads a string from the minibuffer, but does not
     allow whitespace characters as part of the input: instead, those
     characters terminate the input.  The arguments PROMPT, INITIAL, and
     INHERIT-INPUT-METHOD are used as in ‘read-from-minibuffer’.

     This is a simplified interface to the ‘read-from-minibuffer’
     function, and passes the value of the ‘minibuffer-local-ns-map’
     keymap as the KEYMAP argument for that function.  Since the keymap
     ‘minibuffer-local-ns-map’ does not rebind ‘C-q’, it _is_ possible
     to put a space into the string, by quoting it.

     This function discards text properties, regardless of the value of
     ‘minibuffer-allow-text-properties’.

          (read-no-blanks-input PROMPT INITIAL)
          ≡
          (let (minibuffer-allow-text-properties)
            (read-from-minibuffer PROMPT INITIAL minibuffer-local-ns-map))

 -- Variable: minibuffer-local-ns-map
     This built-in variable is the keymap used as the minibuffer local
     keymap in the function ‘read-no-blanks-input’.  By default, it
     makes the following bindings, in addition to those of
     ‘minibuffer-local-map’:

     <SPC>
          ‘exit-minibuffer’

     <TAB>
          ‘exit-minibuffer’

     ‘?’
          ‘self-insert-and-exit’


File: elisp.info,  Node: Object from Minibuffer,  Next: Minibuffer History,  Prev: Text from Minibuffer,  Up: Minibuffers

19.3 Reading Lisp Objects with the Minibuffer
=============================================

This section describes functions for reading Lisp objects with the
minibuffer.

 -- Function: read-minibuffer prompt &optional initial
     This function reads a Lisp object using the minibuffer, and returns
     it without evaluating it.  The arguments PROMPT and INITIAL are
     used as in ‘read-from-minibuffer’.

     This is a simplified interface to the ‘read-from-minibuffer’
     function:

          (read-minibuffer PROMPT INITIAL)
          ≡
          (let (minibuffer-allow-text-properties)
            (read-from-minibuffer PROMPT INITIAL nil t))

     Here is an example in which we supply the string ‘"(testing)"’ as
     initial input:

          (read-minibuffer
           "Enter an expression: " (format "%s" '(testing)))

          ;; Here is how the minibuffer is displayed:

          ---------- Buffer: Minibuffer ----------
          Enter an expression: (testing)★
          ---------- Buffer: Minibuffer ----------

     The user can type <RET> immediately to use the initial input as a
     default, or can edit the input.

 -- Function: eval-minibuffer prompt &optional initial
     This function reads a Lisp expression using the minibuffer,
     evaluates it, then returns the result.  The arguments PROMPT and
     INITIAL are used as in ‘read-from-minibuffer’.

     This function simply evaluates the result of a call to
     ‘read-minibuffer’:

          (eval-minibuffer PROMPT INITIAL)
          ≡
          (eval (read-minibuffer PROMPT INITIAL))

 -- Function: edit-and-eval-command prompt form
     This function reads a Lisp expression in the minibuffer, evaluates
     it, then returns the result.  The difference between this command
     and ‘eval-minibuffer’ is that here the initial FORM is not optional
     and it is treated as a Lisp object to be converted to printed
     representation rather than as a string of text.  It is printed with
     ‘prin1’, so if it is a string, double-quote characters (‘"’) appear
     in the initial text.  *Note Output Functions::.

     In the following example, we offer the user an expression with
     initial text that is already a valid form:

          (edit-and-eval-command "Please edit: " '(forward-word 1))

          ;; After evaluation of the preceding expression,
          ;;   the following appears in the minibuffer:

          ---------- Buffer: Minibuffer ----------
          Please edit: (forward-word 1)★
          ---------- Buffer: Minibuffer ----------

     Typing <RET> right away would exit the minibuffer and evaluate the
     expression, thus moving point forward one word.


File: elisp.info,  Node: Minibuffer History,  Next: Initial Input,  Prev: Object from Minibuffer,  Up: Minibuffers

19.4 Minibuffer History
=======================

A “minibuffer history list” records previous minibuffer inputs so the
user can reuse them conveniently.  It is a variable whose value is a
list of strings (previous inputs), most recent first.

   There are many separate minibuffer history lists, used for different
kinds of inputs.  It’s the Lisp programmer’s job to specify the right
history list for each use of the minibuffer.

   You specify a minibuffer history list with the optional HISTORY
argument to ‘read-from-minibuffer’ or ‘completing-read’.  Here are the
possible values for it:

VARIABLE
     Use VARIABLE (a symbol) as the history list.

(VARIABLE . STARTPOS)
     Use VARIABLE (a symbol) as the history list, and assume that the
     initial history position is STARTPOS (a nonnegative integer).

     Specifying 0 for STARTPOS is equivalent to just specifying the
     symbol VARIABLE.  ‘previous-history-element’ will display the most
     recent element of the history list in the minibuffer.  If you
     specify a positive STARTPOS, the minibuffer history functions
     behave as if ‘(elt VARIABLE (1- STARTPOS))’ were the history
     element currently shown in the minibuffer.

     For consistency, you should also specify that element of the
     history as the initial minibuffer contents, using the INITIAL
     argument to the minibuffer input function (*note Initial Input::).

   If you don’t specify HISTORY, then the default history list
‘minibuffer-history’ is used.  For other standard history lists, see
below.  You can also create your own history list variable; just
initialize it to ‘nil’ before the first use.

   Both ‘read-from-minibuffer’ and ‘completing-read’ add new elements to
the history list automatically, and provide commands to allow the user
to reuse items on the list.  The only thing your program needs to do to
use a history list is to initialize it and to pass its name to the input
functions when you wish.  But it is safe to modify the list by hand when
the minibuffer input functions are not using it.

   Emacs functions that add a new element to a history list can also
delete old elements if the list gets too long.  The variable
‘history-length’ specifies the maximum length for most history lists.
To specify a different maximum length for a particular history list, put
the length in the ‘history-length’ property of the history list symbol.
The variable ‘history-delete-duplicates’ specifies whether to delete
duplicates in history.

 -- Function: add-to-history history-var newelt &optional maxelt
          keep-all
     This function adds a new element NEWELT, if it isn’t the empty
     string, to the history list stored in the variable HISTORY-VAR, and
     returns the updated history list.  It limits the list length to the
     value of MAXELT (if non-‘nil’) or ‘history-length’ (described
     below).  The possible values of MAXELT have the same meaning as the
     values of ‘history-length’.

     Normally, ‘add-to-history’ removes duplicate members from the
     history list if ‘history-delete-duplicates’ is non-‘nil’.  However,
     if KEEP-ALL is non-‘nil’, that says not to remove duplicates, and
     to add NEWELT to the list even if it is empty.

 -- Variable: history-add-new-input
     If the value of this variable is ‘nil’, standard functions that
     read from the minibuffer don’t add new elements to the history
     list.  This lets Lisp programs explicitly manage input history by
     using ‘add-to-history’.  The default value is ‘t’.

 -- User Option: history-length
     The value of this variable specifies the maximum length for all
     history lists that don’t specify their own maximum lengths.  If the
     value is ‘t’, that means there is no maximum (don’t delete old
     elements).  If a history list variable’s symbol has a non-‘nil’
     ‘history-length’ property, it overrides this variable for that
     particular history list.

 -- User Option: history-delete-duplicates
     If the value of this variable is ‘t’, that means when adding a new
     history element, all previous identical elements are deleted.

   Here are some of the standard minibuffer history list variables:

 -- Variable: minibuffer-history
     The default history list for minibuffer history input.

 -- Variable: query-replace-history
     A history list for arguments to ‘query-replace’ (and similar
     arguments to other commands).

 -- Variable: file-name-history
     A history list for file-name arguments.

 -- Variable: buffer-name-history
     A history list for buffer-name arguments.

 -- Variable: regexp-history
     A history list for regular expression arguments.

 -- Variable: extended-command-history
     A history list for arguments that are names of extended commands.

 -- Variable: shell-command-history
     A history list for arguments that are shell commands.

 -- Variable: read-expression-history
     A history list for arguments that are Lisp expressions to evaluate.

 -- Variable: face-name-history
     A history list for arguments that are faces.


File: elisp.info,  Node: Initial Input,  Next: Completion,  Prev: Minibuffer History,  Up: Minibuffers

19.5 Initial Input
==================

Several of the functions for minibuffer input have an argument called
INITIAL.  This is a mostly-deprecated feature for specifying that the
minibuffer should start out with certain text, instead of empty as
usual.

   If INITIAL is a string, the minibuffer starts out containing the text
of the string, with point at the end, when the user starts to edit the
text.  If the user simply types <RET> to exit the minibuffer, it will
use the initial input string to determine the value to return.

   *We discourage use of a non-‘nil’ value for INITIAL*, because initial
input is an intrusive interface.  History lists and default values
provide a much more convenient method to offer useful default inputs to
the user.

   There is just one situation where you should specify a string for an
INITIAL argument.  This is when you specify a cons cell for the HISTORY
argument.  *Note Minibuffer History::.

   INITIAL can also be a cons cell of the form ‘(STRING . POSITION)’.
This means to insert STRING in the minibuffer but put point at POSITION
within the string’s text.

   As a historical accident, POSITION was implemented inconsistently in
different functions.  In ‘completing-read’, POSITION’s value is
interpreted as origin-zero; that is, a value of 0 means the beginning of
the string, 1 means after the first character, etc.  In
‘read-minibuffer’, and the other non-completion minibuffer input
functions that support this argument, 1 means the beginning of the
string, 2 means after the first character, etc.

   Use of a cons cell as the value for INITIAL arguments is deprecated.


File: elisp.info,  Node: Completion,  Next: Yes-or-No Queries,  Prev: Initial Input,  Up: Minibuffers

19.6 Completion
===============

“Completion” is a feature that fills in the rest of a name starting from
an abbreviation for it.  Completion works by comparing the user’s input
against a list of valid names and determining how much of the name is
determined uniquely by what the user has typed.  For example, when you
type ‘C-x b’ (‘switch-to-buffer’) and then type the first few letters of
the name of the buffer to which you wish to switch, and then type <TAB>
(‘minibuffer-complete’), Emacs extends the name as far as it can.

   Standard Emacs commands offer completion for names of symbols, files,
buffers, and processes; with the functions in this section, you can
implement completion for other kinds of names.

   The ‘try-completion’ function is the basic primitive for completion:
it returns the longest determined completion of a given initial string,
with a given set of strings to match against.

   The function ‘completing-read’ provides a higher-level interface for
completion.  A call to ‘completing-read’ specifies how to determine the
list of valid names.  The function then activates the minibuffer with a
local keymap that binds a few keys to commands useful for completion.
Other functions provide convenient simple interfaces for reading certain
kinds of names with completion.

* Menu:

* Basic Completion::       Low-level functions for completing strings.
* Minibuffer Completion::  Invoking the minibuffer with completion.
* Completion Commands::    Minibuffer commands that do completion.
* High-Level Completion::  Convenient special cases of completion
                             (reading buffer names, variable names, etc.).
* Reading File Names::     Using completion to read file names and
                             shell commands.
* Completion Variables::   Variables controlling completion behavior.
* Programmed Completion::  Writing your own completion function.
* Completion in Buffers::  Completing text in ordinary buffers.


File: elisp.info,  Node: Basic Completion,  Next: Minibuffer Completion,  Up: Completion

19.6.1 Basic Completion Functions
---------------------------------

The following completion functions have nothing in themselves to do with
minibuffers.  We describe them here to keep them near the higher-level
completion features that do use the minibuffer.

 -- Function: try-completion string collection &optional predicate
     This function returns the longest common substring of all possible
     completions of STRING in COLLECTION.

     COLLECTION is called the “completion table”.  Its value must be a
     list of strings or cons cells, an obarray, a hash table, or a
     completion function.

     ‘try-completion’ compares STRING against each of the permissible
     completions specified by the completion table.  If no permissible
     completions match, it returns ‘nil’.  If there is just one matching
     completion, and the match is exact, it returns ‘t’.  Otherwise, it
     returns the longest initial sequence common to all possible
     matching completions.

     If COLLECTION is a list, the permissible completions are specified
     by the elements of the list, each of which should be either a
     string, or a cons cell whose CAR is either a string or a symbol (a
     symbol is converted to a string using ‘symbol-name’).  If the list
     contains elements of any other type, those are ignored.

     If COLLECTION is an obarray (*note Creating Symbols::), the names
     of all symbols in the obarray form the set of permissible
     completions.

     If COLLECTION is a hash table, then the keys that are strings or
     symbols are the possible completions.  Other keys are ignored.

     You can also use a function as COLLECTION.  Then the function is
     solely responsible for performing completion; ‘try-completion’
     returns whatever this function returns.  The function is called
     with three arguments: STRING, PREDICATE and ‘nil’ (the third
     argument is so that the same function can be used in
     ‘all-completions’ and do the appropriate thing in either case).
     *Note Programmed Completion::.

     If the argument PREDICATE is non-‘nil’, then it must be a function
     of one argument, unless COLLECTION is a hash table, in which case
     it should be a function of two arguments.  It is used to test each
     possible match, and the match is accepted only if PREDICATE returns
     non-‘nil’.  The argument given to PREDICATE is either a string or a
     cons cell (the CAR of which is a string) from the alist, or a
     symbol (_not_ a symbol name) from the obarray.  If COLLECTION is a
     hash table, PREDICATE is called with two arguments, the string key
     and the associated value.

     In addition, to be acceptable, a completion must also match all the
     regular expressions in ‘completion-regexp-list’.  (Unless
     COLLECTION is a function, in which case that function has to handle
     ‘completion-regexp-list’ itself.)

     In the first of the following examples, the string ‘foo’ is matched
     by three of the alist CARs.  All of the matches begin with the
     characters ‘fooba’, so that is the result.  In the second example,
     there is only one possible match, and it is exact, so the return
     value is ‘t’.

          (try-completion
           "foo"
           '(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4)))
               ⇒ "fooba"

          (try-completion "foo" '(("barfoo" 2) ("foo" 3)))
               ⇒ t

     In the following example, numerous symbols begin with the
     characters ‘forw’, and all of them begin with the word ‘forward’.
     In most of the symbols, this is followed with a ‘-’, but not in
     all, so no more than ‘forward’ can be completed.

          (try-completion "forw" obarray)
               ⇒ "forward"

     Finally, in the following example, only two of the three possible
     matches pass the predicate ‘test’ (the string ‘foobaz’ is too
     short).  Both of those begin with the string ‘foobar’.

          (defun test (s)
            (> (length (car s)) 6))
               ⇒ test
          (try-completion
           "foo"
           '(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4))
           'test)
               ⇒ "foobar"

 -- Function: all-completions string collection &optional predicate
     This function returns a list of all possible completions of STRING.
     The arguments to this function are the same as those of
     ‘try-completion’, and it uses ‘completion-regexp-list’ in the same
     way that ‘try-completion’ does.

     If COLLECTION is a function, it is called with three arguments:
     STRING, PREDICATE and ‘t’; then ‘all-completions’ returns whatever
     the function returns.  *Note Programmed Completion::.

     Here is an example, using the function ‘test’ shown in the example
     for ‘try-completion’:

          (defun test (s)
            (> (length (car s)) 6))
               ⇒ test

          (all-completions
           "foo"
           '(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4))
           'test)
               ⇒ ("foobar1" "foobar2")

 -- Function: test-completion string collection &optional predicate
     This function returns non-‘nil’ if STRING is a valid completion
     alternative specified by COLLECTION and PREDICATE.  The arguments
     are the same as in ‘try-completion’.  For instance, if COLLECTION
     is a list of strings, this is true if STRING appears in the list
     and PREDICATE is satisfied.

     This function uses ‘completion-regexp-list’ in the same way that
     ‘try-completion’ does.

     If PREDICATE is non-‘nil’ and if COLLECTION contains several
     strings that are equal to each other, as determined by
     ‘compare-strings’ according to ‘completion-ignore-case’, then
     PREDICATE should accept either all or none of them.  Otherwise, the
     return value of ‘test-completion’ is essentially unpredictable.

     If COLLECTION is a function, it is called with three arguments, the
     values STRING, PREDICATE and ‘lambda’; whatever it returns,
     ‘test-completion’ returns in turn.

 -- Function: completion-boundaries string collection predicate suffix
     This function returns the boundaries of the field on which
     COLLECTION will operate, assuming that STRING holds the text before
     point and SUFFIX holds the text after point.

     Normally completion operates on the whole string, so for all normal
     collections, this will always return ‘(0 . (length SUFFIX))’.  But
     more complex completion such as completion on files is done one
     field at a time.  For example, completion of ‘"/usr/sh"’ will
     include ‘"/usr/share/"’ but not ‘"/usr/share/doc"’ even if
     ‘"/usr/share/doc"’ exists.  Also ‘all-completions’ on ‘"/usr/sh"’
     will not include ‘"/usr/share/"’ but only ‘"share/"’.  So if STRING
     is ‘"/usr/sh"’ and SUFFIX is ‘"e/doc"’, ‘completion-boundaries’
     will return ‘(5 . 1)’ which tells us that the COLLECTION will only
     return completion information that pertains to the area after
     ‘"/usr/"’ and before ‘"/doc"’.

   If you store a completion alist in a variable, you should mark the
variable as risky by giving it a non-‘nil’ ‘risky-local-variable’
property.  *Note File Local Variables::.

 -- Variable: completion-ignore-case
     If the value of this variable is non-‘nil’, case is not considered
     significant in completion.  Within ‘read-file-name’, this variable
     is overridden by ‘read-file-name-completion-ignore-case’ (*note
     Reading File Names::); within ‘read-buffer’, it is overridden by
     ‘read-buffer-completion-ignore-case’ (*note High-Level
     Completion::).

 -- Variable: completion-regexp-list
     This is a list of regular expressions.  The completion functions
     only consider a completion acceptable if it matches all regular
     expressions in this list, with ‘case-fold-search’ (*note Searching
     and Case::) bound to the value of ‘completion-ignore-case’.

 -- Macro: lazy-completion-table var fun
     This macro provides a way to initialize the variable VAR as a
     collection for completion in a lazy way, not computing its actual
     contents until they are first needed.  You use this macro to
     produce a value that you store in VAR.  The actual computation of
     the proper value is done the first time you do completion using
     VAR.  It is done by calling FUN with no arguments.  The value FUN
     returns becomes the permanent value of VAR.

     Here is an example:

          (defvar foo (lazy-completion-table foo make-my-alist))

   There are several functions that take an existing completion table
and return a modified version.  ‘completion-table-case-fold’ returns a
case-insensitive table.  ‘completion-table-in-turn’ and
‘completion-table-merge’ combine multiple input tables in different
ways.  ‘completion-table-subvert’ alters a table to use a different
initial prefix.  ‘completion-table-with-quoting’ returns a table
suitable for operating on quoted text.
‘completion-table-with-predicate’ filters a table with a predicate
function.  ‘completion-table-with-terminator’ adds a terminating string.


File: elisp.info,  Node: Minibuffer Completion,  Next: Completion Commands,  Prev: Basic Completion,  Up: Completion

19.6.2 Completion and the Minibuffer
------------------------------------

This section describes the basic interface for reading from the
minibuffer with completion.

 -- Function: completing-read prompt collection &optional predicate
          require-match initial history default inherit-input-method
     This function reads a string in the minibuffer, assisting the user
     by providing completion.  It activates the minibuffer with prompt
     PROMPT, which must be a string.

     The actual completion is done by passing the completion table
     COLLECTION and the completion predicate PREDICATE to the function
     ‘try-completion’ (*note Basic Completion::).  This happens in
     certain commands bound in the local keymaps used for completion.
     Some of these commands also call ‘test-completion’.  Thus, if
     PREDICATE is non-‘nil’, it should be compatible with COLLECTION and
     ‘completion-ignore-case’.  *Note Definition of test-completion::.

     *Note Programmed Completion::, for detailed requirements when
     COLLECTION is a function.

     The value of the optional argument REQUIRE-MATCH determines how the
     user may exit the minibuffer:

        • If ‘nil’, the usual minibuffer exit commands work regardless
          of the input in the minibuffer.

        • If ‘t’, the usual minibuffer exit commands won’t exit unless
          the input completes to an element of COLLECTION.

        • If ‘confirm’, the user can exit with any input, but is asked
          for confirmation if the input is not an element of COLLECTION.

        • If ‘confirm-after-completion’, the user can exit with any
          input, but is asked for confirmation if the preceding command
          was a completion command (i.e., one of the commands in
          ‘minibuffer-confirm-exit-commands’) and the resulting input is
          not an element of COLLECTION.  *Note Completion Commands::.

        • Any other value of REQUIRE-MATCH behaves like ‘t’, except that
          the exit commands won’t exit if it performs completion.

     However, empty input is always permitted, regardless of the value
     of REQUIRE-MATCH; in that case, ‘completing-read’ returns the first
     element of DEFAULT, if it is a list; ‘""’, if DEFAULT is ‘nil’; or
     DEFAULT.  The string or strings in DEFAULT are also available to
     the user through the history commands.

     The function ‘completing-read’ uses
     ‘minibuffer-local-completion-map’ as the keymap if REQUIRE-MATCH is
     ‘nil’, and uses ‘minibuffer-local-must-match-map’ if REQUIRE-MATCH
     is non-‘nil’.  *Note Completion Commands::.

     The argument HISTORY specifies which history list variable to use
     for saving the input and for minibuffer history commands.  It
     defaults to ‘minibuffer-history’.  *Note Minibuffer History::.

     The argument INITIAL is mostly deprecated; we recommend using a
     non-‘nil’ value only in conjunction with specifying a cons cell for
     HISTORY.  *Note Initial Input::.  For default input, use DEFAULT
     instead.

     If the argument INHERIT-INPUT-METHOD is non-‘nil’, then the
     minibuffer inherits the current input method (*note Input
     Methods::) and the setting of ‘enable-multibyte-characters’ (*note
     Text Representations::) from whichever buffer was current before
     entering the minibuffer.

     If the variable ‘completion-ignore-case’ is non-‘nil’, completion
     ignores case when comparing the input against the possible matches.
     *Note Basic Completion::.  In this mode of operation, PREDICATE
     must also ignore case, or you will get surprising results.

     Here’s an example of using ‘completing-read’:

          (completing-read
           "Complete a foo: "
           '(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4))
           nil t "fo")

          ;; After evaluation of the preceding expression,
          ;;   the following appears in the minibuffer:

          ---------- Buffer: Minibuffer ----------
          Complete a foo: fo★
          ---------- Buffer: Minibuffer ----------

     If the user then types ‘<DEL> <DEL> b <RET>’, ‘completing-read’
     returns ‘barfoo’.

     The ‘completing-read’ function binds variables to pass information
     to the commands that actually do completion.  They are described in
     the following section.

 -- Variable: completing-read-function
     The value of this variable must be a function, which is called by
     ‘completing-read’ to actually do its work.  It should accept the
     same arguments as ‘completing-read’.  This can be bound to a
     different function to completely override the normal behavior of
     ‘completing-read’.


File: elisp.info,  Node: Completion Commands,  Next: High-Level Completion,  Prev: Minibuffer Completion,  Up: Completion

19.6.3 Minibuffer Commands that Do Completion
---------------------------------------------

This section describes the keymaps, commands and user options used in
the minibuffer to do completion.

 -- Variable: minibuffer-completion-table
     The value of this variable is the completion table used for
     completion in the minibuffer.  This is the global variable that
     contains what ‘completing-read’ passes to ‘try-completion’.  It is
     used by minibuffer completion commands such as
     ‘minibuffer-complete-word’.

 -- Variable: minibuffer-completion-predicate
     This variable’s value is the predicate that ‘completing-read’
     passes to ‘try-completion’.  The variable is also used by the other
     minibuffer completion functions.

 -- Variable: minibuffer-completion-confirm
     This variable determines whether Emacs asks for confirmation before
     exiting the minibuffer; ‘completing-read’ binds this variable, and
     the function ‘minibuffer-complete-and-exit’ checks the value before
     exiting.  If the value is ‘nil’, confirmation is not required.  If
     the value is ‘confirm’, the user may exit with an input that is not
     a valid completion alternative, but Emacs asks for confirmation.
     If the value is ‘confirm-after-completion’, the user may exit with
     an input that is not a valid completion alternative, but Emacs asks
     for confirmation if the user submitted the input right after any of
     the completion commands in ‘minibuffer-confirm-exit-commands’.

 -- Variable: minibuffer-confirm-exit-commands
     This variable holds a list of commands that cause Emacs to ask for
     confirmation before exiting the minibuffer, if the REQUIRE-MATCH
     argument to ‘completing-read’ is ‘confirm-after-completion’.  The
     confirmation is requested if the user attempts to exit the
     minibuffer immediately after calling any command in this list.

 -- Command: minibuffer-complete-word
     This function completes the minibuffer contents by at most a single
     word.  Even if the minibuffer contents have only one completion,
     ‘minibuffer-complete-word’ does not add any characters beyond the
     first character that is not a word constituent.  *Note Syntax
     Tables::.

 -- Command: minibuffer-complete
     This function completes the minibuffer contents as far as possible.

 -- Command: minibuffer-complete-and-exit
     This function completes the minibuffer contents, and exits if
     confirmation is not required, i.e., if
     ‘minibuffer-completion-confirm’ is ‘nil’.  If confirmation _is_
     required, it is given by repeating this command immediately—the
     command is programmed to work without confirmation when run twice
     in succession.

 -- Command: minibuffer-completion-help
     This function creates a list of the possible completions of the
     current minibuffer contents.  It works by calling ‘all-completions’
     using the value of the variable ‘minibuffer-completion-table’ as
     the COLLECTION argument, and the value of
     ‘minibuffer-completion-predicate’ as the PREDICATE argument.  The
     list of completions is displayed as text in a buffer named
     ‘*Completions*’.

 -- Function: display-completion-list completions
     This function displays COMPLETIONS to the stream in
     ‘standard-output’, usually a buffer.  (*Note Read and Print::, for
     more information about streams.)  The argument COMPLETIONS is
     normally a list of completions just returned by ‘all-completions’,
     but it does not have to be.  Each element may be a symbol or a
     string, either of which is simply printed.  It can also be a list
     of two strings, which is printed as if the strings were
     concatenated.  The first of the two strings is the actual
     completion, the second string serves as annotation.

     This function is called by ‘minibuffer-completion-help’.  A common
     way to use it is together with ‘with-output-to-temp-buffer’, like
     this:

          (with-output-to-temp-buffer "*Completions*"
            (display-completion-list
              (all-completions (buffer-string) my-alist)))

 -- User Option: completion-auto-help
     If this variable is non-‘nil’, the completion commands
     automatically display a list of possible completions whenever
     nothing can be completed because the next character is not uniquely
     determined.

 -- Variable: minibuffer-local-completion-map
     ‘completing-read’ uses this value as the local keymap when an exact
     match of one of the completions is not required.  By default, this
     keymap makes the following bindings:

     ‘?’
          ‘minibuffer-completion-help’

     <SPC>
          ‘minibuffer-complete-word’

     <TAB>
          ‘minibuffer-complete’

     and uses ‘minibuffer-local-map’ as its parent keymap (*note
     Definition of minibuffer-local-map::).

 -- Variable: minibuffer-local-must-match-map
     ‘completing-read’ uses this value as the local keymap when an exact
     match of one of the completions is required.  Therefore, no keys
     are bound to ‘exit-minibuffer’, the command that exits the
     minibuffer unconditionally.  By default, this keymap makes the
     following bindings:

     ‘C-j’
          ‘minibuffer-complete-and-exit’

     <RET>
          ‘minibuffer-complete-and-exit’

     and uses ‘minibuffer-local-completion-map’ as its parent keymap.

 -- Variable: minibuffer-local-filename-completion-map
     This is a sparse keymap that simply unbinds <SPC>; because
     filenames can contain spaces.  The function ‘read-file-name’
     combines this keymap with either ‘minibuffer-local-completion-map’
     or ‘minibuffer-local-must-match-map’.


File: elisp.info,  Node: High-Level Completion,  Next: Reading File Names,  Prev: Completion Commands,  Up: Completion

19.6.4 High-Level Completion Functions
--------------------------------------

This section describes the higher-level convenience functions for
reading certain sorts of names with completion.

   In most cases, you should not call these functions in the middle of a
Lisp function.  When possible, do all minibuffer input as part of
reading the arguments for a command, in the ‘interactive’ specification.
*Note Defining Commands::.

 -- Function: read-buffer prompt &optional default require-match
          predicate
     This function reads the name of a buffer and returns it as a
     string.  It prompts with PROMPT.  The argument DEFAULT is the
     default name to use, the value to return if the user exits with an
     empty minibuffer.  If non-‘nil’, it should be a string, a list of
     strings, or a buffer.  If it is a list, the default value is the
     first element of this list.  It is mentioned in the prompt, but is
     not inserted in the minibuffer as initial input.

     The argument PROMPT should be a string ending with a colon and a
     space.  If DEFAULT is non-‘nil’, the function inserts it in PROMPT
     before the colon to follow the convention for reading from the
     minibuffer with a default value (*note Programming Tips::).

     The optional argument REQUIRE-MATCH has the same meaning as in
     ‘completing-read’.  *Note Minibuffer Completion::.

     The optional argument PREDICATE, if non-‘nil’, specifies a function
     to filter the buffers that should be considered: the function will
     be called with every potential candidate as its argument, and
     should return ‘nil’ to reject the candidate, non-‘nil’ to accept
     it.

     In the following example, the user enters ‘minibuffer.t’, and then
     types <RET>.  The argument REQUIRE-MATCH is ‘t’, and the only
     buffer name starting with the given input is ‘minibuffer.texi’, so
     that name is the value.

          (read-buffer "Buffer name: " "foo" t)
          ;; After evaluation of the preceding expression,
          ;;   the following prompt appears,
          ;;   with an empty minibuffer:

          ---------- Buffer: Minibuffer ----------
          Buffer name (default foo): ★
          ---------- Buffer: Minibuffer ----------

          ;; The user types ‘minibuffer.t <RET>’.
               ⇒ "minibuffer.texi"

 -- User Option: read-buffer-function
     This variable, if non-‘nil’, specifies a function for reading
     buffer names.  ‘read-buffer’ calls this function instead of doing
     its usual work, with the same arguments passed to ‘read-buffer’.

 -- User Option: read-buffer-completion-ignore-case
     If this variable is non-‘nil’, ‘read-buffer’ ignores case when
     performing completion while reading the buffer name.

 -- Function: read-command prompt &optional default
     This function reads the name of a command and returns it as a Lisp
     symbol.  The argument PROMPT is used as in ‘read-from-minibuffer’.
     Recall that a command is anything for which ‘commandp’ returns ‘t’,
     and a command name is a symbol for which ‘commandp’ returns ‘t’.
     *Note Interactive Call::.

     The argument DEFAULT specifies what to return if the user enters
     null input.  It can be a symbol, a string or a list of strings.  If
     it is a string, ‘read-command’ interns it before returning it.  If
     it is a list, ‘read-command’ interns the first element of this
     list.  If DEFAULT is ‘nil’, that means no default has been
     specified; then if the user enters null input, the return value is
     ‘(intern "")’, that is, a symbol whose name is an empty string.

          (read-command "Command name? ")

          ;; After evaluation of the preceding expression,
          ;;   the following prompt appears with an empty minibuffer:

          ---------- Buffer: Minibuffer ----------
          Command name?
          ---------- Buffer: Minibuffer ----------

     If the user types ‘forward-c <RET>’, then this function returns
     ‘forward-char’.

     The ‘read-command’ function is a simplified interface to
     ‘completing-read’.  It uses the variable ‘obarray’ so as to
     complete in the set of extant Lisp symbols, and it uses the
     ‘commandp’ predicate so as to accept only command names:

          (read-command PROMPT)
          ≡
          (intern (completing-read PROMPT obarray
                                   'commandp t nil))

 -- Function: read-variable prompt &optional default
     This function reads the name of a customizable variable and returns
     it as a symbol.  Its arguments have the same form as those of
     ‘read-command’.  It behaves just like ‘read-command’, except that
     it uses the predicate ‘custom-variable-p’ instead of ‘commandp’.

 -- Command: read-color &optional prompt convert allow-empty display
     This function reads a string that is a color specification, either
     the color’s name or an RGB hex value such as ‘#RRRGGGBBB’.  It
     prompts with PROMPT (default: ‘"Color (name or #RGB triplet):"’)
     and provides completion for color names, but not for hex RGB
     values.  In addition to names of standard colors, completion
     candidates include the foreground and background colors at point.

     Valid RGB values are described in *note Color Names::.

     The function’s return value is the string typed by the user in the
     minibuffer.  However, when called interactively or if the optional
     argument CONVERT is non-‘nil’, it converts any input color name
     into the corresponding RGB value string and instead returns that.
     This function requires a valid color specification to be input.
     Empty color names are allowed when ALLOW-EMPTY is non-‘nil’ and the
     user enters null input.

     Interactively, or when DISPLAY is non-‘nil’, the return value is
     also displayed in the echo area.

   See also the functions ‘read-coding-system’ and
‘read-non-nil-coding-system’, in *note User-Chosen Coding Systems::, and
‘read-input-method-name’, in *note Input Methods::.


File: elisp.info,  Node: Reading File Names,  Next: Completion Variables,  Prev: High-Level Completion,  Up: Completion

19.6.5 Reading File Names
-------------------------

The high-level completion functions ‘read-file-name’,
‘read-directory-name’, and ‘read-shell-command’ are designed to read
file names, directory names, and shell commands, respectively.  They
provide special features, including automatic insertion of the default
directory.

 -- Function: read-file-name prompt &optional directory default
          require-match initial predicate
     This function reads a file name, prompting with PROMPT and
     providing completion.

     As an exception, this function reads a file name using a graphical
     file dialog instead of the minibuffer, if all of the following are
     true:

       1. It is invoked via a mouse command.

       2. The selected frame is on a graphical display supporting such
          dialogs.

       3. The variable ‘use-dialog-box’ is non-‘nil’.  *Note Dialog
          Boxes: (emacs)Dialog Boxes.

       4. The DIRECTORY argument, described below, does not specify a
          remote file.  *Note Remote Files: (emacs)Remote Files.

     The exact behavior when using a graphical file dialog is
     platform-dependent.  Here, we simply document the behavior when
     using the minibuffer.

     ‘read-file-name’ does not automatically expand the returned file
     name.  You must call ‘expand-file-name’ yourself if an absolute
     file name is required.

     The optional argument REQUIRE-MATCH has the same meaning as in
     ‘completing-read’.  *Note Minibuffer Completion::.

     The argument DIRECTORY specifies the directory to use for
     completing relative file names.  It should be an absolute directory
     name.  If the variable ‘insert-default-directory’ is non-‘nil’,
     DIRECTORY is also inserted in the minibuffer as initial input.  It
     defaults to the current buffer’s value of ‘default-directory’.

     If you specify INITIAL, that is an initial file name to insert in
     the buffer (after DIRECTORY, if that is inserted).  In this case,
     point goes at the beginning of INITIAL.  The default for INITIAL is
     ‘nil’—don’t insert any file name.  To see what INITIAL does, try
     the command ‘C-x C-v’ in a buffer visiting a file.  *Please note:*
     we recommend using DEFAULT rather than INITIAL in most cases.

     If DEFAULT is non-‘nil’, then the function returns DEFAULT if the
     user exits the minibuffer with the same non-empty contents that
     ‘read-file-name’ inserted initially.  The initial minibuffer
     contents are always non-empty if ‘insert-default-directory’ is
     non-‘nil’, as it is by default.  DEFAULT is not checked for
     validity, regardless of the value of REQUIRE-MATCH.  However, if
     REQUIRE-MATCH is non-‘nil’, the initial minibuffer contents should
     be a valid file (or directory) name.  Otherwise ‘read-file-name’
     attempts completion if the user exits without any editing, and does
     not return DEFAULT.  DEFAULT is also available through the history
     commands.

     If DEFAULT is ‘nil’, ‘read-file-name’ tries to find a substitute
     default to use in its place, which it treats in exactly the same
     way as if it had been specified explicitly.  If DEFAULT is ‘nil’,
     but INITIAL is non-‘nil’, then the default is the absolute file
     name obtained from DIRECTORY and INITIAL.  If both DEFAULT and
     INITIAL are ‘nil’ and the buffer is visiting a file,
     ‘read-file-name’ uses the absolute file name of that file as
     default.  If the buffer is not visiting a file, then there is no
     default.  In that case, if the user types <RET> without any
     editing, ‘read-file-name’ simply returns the pre-inserted contents
     of the minibuffer.

     If the user types <RET> in an empty minibuffer, this function
     returns an empty string, regardless of the value of REQUIRE-MATCH.
     This is, for instance, how the user can make the current buffer
     visit no file using ‘M-x set-visited-file-name’.

     If PREDICATE is non-‘nil’, it specifies a function of one argument
     that decides which file names are acceptable completion
     alternatives.  A file name is an acceptable value if PREDICATE
     returns non-‘nil’ for it.

     Here is an example of using ‘read-file-name’:

          (read-file-name "The file is ")

          ;; After evaluation of the preceding expression,
          ;;   the following appears in the minibuffer:

          ---------- Buffer: Minibuffer ----------
          The file is /gp/gnu/elisp/★
          ---------- Buffer: Minibuffer ----------

     Typing ‘manual <TAB>’ results in the following:

          ---------- Buffer: Minibuffer ----------
          The file is /gp/gnu/elisp/manual.texi★
          ---------- Buffer: Minibuffer ----------

     If the user types <RET>, ‘read-file-name’ returns the file name as
     the string ‘"/gp/gnu/elisp/manual.texi"’.

 -- Variable: read-file-name-function
     If non-‘nil’, this should be a function that accepts the same
     arguments as ‘read-file-name’.  When ‘read-file-name’ is called, it
     calls this function with the supplied arguments instead of doing
     its usual work.

 -- User Option: read-file-name-completion-ignore-case
     If this variable is non-‘nil’, ‘read-file-name’ ignores case when
     performing completion.

 -- Function: read-directory-name prompt &optional directory default
          require-match initial
     This function is like ‘read-file-name’ but allows only directory
     names as completion alternatives.

     If DEFAULT is ‘nil’ and INITIAL is non-‘nil’, ‘read-directory-name’
     constructs a substitute default by combining DIRECTORY (or the
     current buffer’s default directory if DIRECTORY is ‘nil’) and
     INITIAL.  If both DEFAULT and INITIAL are ‘nil’, this function uses
     DIRECTORY as substitute default, or the current buffer’s default
     directory if DIRECTORY is ‘nil’.

 -- User Option: insert-default-directory
     This variable is used by ‘read-file-name’, and thus, indirectly, by
     most commands reading file names.  (This includes all commands that
     use the code letters ‘f’ or ‘F’ in their interactive form.  *Note
     Code Characters for interactive: Interactive Codes.)  Its value
     controls whether ‘read-file-name’ starts by placing the name of the
     default directory in the minibuffer, plus the initial file name, if
     any.  If the value of this variable is ‘nil’, then ‘read-file-name’
     does not place any initial input in the minibuffer (unless you
     specify initial input with the INITIAL argument).  In that case,
     the default directory is still used for completion of relative file
     names, but is not displayed.

     If this variable is ‘nil’ and the initial minibuffer contents are
     empty, the user may have to explicitly fetch the next history
     element to access a default value.  If the variable is non-‘nil’,
     the initial minibuffer contents are always non-empty and the user
     can always request a default value by immediately typing <RET> in
     an unedited minibuffer.  (See above.)

     For example:

          ;; Here the minibuffer starts out with the default directory.
          (let ((insert-default-directory t))
            (read-file-name "The file is "))

          ---------- Buffer: Minibuffer ----------
          The file is ~lewis/manual/★
          ---------- Buffer: Minibuffer ----------

          ;; Here the minibuffer is empty and only the prompt
          ;;   appears on its line.
          (let ((insert-default-directory nil))
            (read-file-name "The file is "))

          ---------- Buffer: Minibuffer ----------
          The file is ★
          ---------- Buffer: Minibuffer ----------

 -- Function: read-shell-command prompt &optional initial history &rest
          args
     This function reads a shell command from the minibuffer, prompting
     with PROMPT and providing intelligent completion.  It completes the
     first word of the command using candidates that are appropriate for
     command names, and the rest of the command words as file names.

     This function uses ‘minibuffer-local-shell-command-map’ as the
     keymap for minibuffer input.  The HISTORY argument specifies the
     history list to use; if is omitted or ‘nil’, it defaults to
     ‘shell-command-history’ (*note shell-command-history: Minibuffer
     History.).  The optional argument INITIAL specifies the initial
     content of the minibuffer (*note Initial Input::).  The rest of
     ARGS, if present, are used as the DEFAULT and INHERIT-INPUT-METHOD
     arguments in ‘read-from-minibuffer’ (*note Text from Minibuffer::).

 -- Variable: minibuffer-local-shell-command-map
     This keymap is used by ‘read-shell-command’ for completing command
     and file names that are part of a shell command.  It uses
     ‘minibuffer-local-map’ as its parent keymap, and binds <TAB> to
     ‘completion-at-point’.


File: elisp.info,  Node: Completion Variables,  Next: Programmed Completion,  Prev: Reading File Names,  Up: Completion

19.6.6 Completion Variables
---------------------------

Here are some variables that can be used to alter the default completion
behavior.

 -- User Option: completion-styles
     The value of this variable is a list of completion style (symbols)
     to use for performing completion.  A “completion style” is a set of
     rules for generating completions.  Each symbol occurring this list
     must have a corresponding entry in ‘completion-styles-alist’.

 -- Variable: completion-styles-alist
     This variable stores a list of available completion styles.  Each
     element in the list has the form

          (STYLE TRY-COMPLETION ALL-COMPLETIONS DOC)

     Here, STYLE is the name of the completion style (a symbol), which
     may be used in the ‘completion-styles’ variable to refer to this
     style; TRY-COMPLETION is the function that does the completion;
     ALL-COMPLETIONS is the function that lists the completions; and DOC
     is a string describing the completion style.

     The TRY-COMPLETION and ALL-COMPLETIONS functions should each accept
     four arguments: STRING, COLLECTION, PREDICATE, and POINT.  The
     STRING, COLLECTION, and PREDICATE arguments have the same meanings
     as in ‘try-completion’ (*note Basic Completion::), and the POINT
     argument is the position of point within STRING.  Each function
     should return a non-‘nil’ value if it performed its job, and ‘nil’
     if it did not (e.g., if there is no way to complete STRING
     according to the completion style).

     When the user calls a completion command like ‘minibuffer-complete’
     (*note Completion Commands::), Emacs looks for the first style
     listed in ‘completion-styles’ and calls its TRY-COMPLETION
     function.  If this function returns ‘nil’, Emacs moves to the next
     listed completion style and calls its TRY-COMPLETION function, and
     so on until one of the TRY-COMPLETION functions successfully
     performs completion and returns a non-‘nil’ value.  A similar
     procedure is used for listing completions, via the ALL-COMPLETIONS
     functions.

     *Note (emacs)Completion Styles::, for a description of the
     available completion styles.

 -- User Option: completion-category-overrides
     This variable specifies special completion styles and other
     completion behaviors to use when completing certain types of text.
     Its value should be an alist with elements of the form ‘(CATEGORY .
     ALIST)’.  CATEGORY is a symbol describing what is being completed;
     currently, the ‘buffer’, ‘file’, and ‘unicode-name’ categories are
     defined, but others can be defined via specialized completion
     functions (*note Programmed Completion::).  ALIST is an association
     list describing how completion should behave for the corresponding
     category.  The following alist keys are supported:

     ‘styles’
          The value should be a list of completion styles (symbols).

     ‘cycle’
          The value should be a value for ‘completion-cycle-threshold’
          (*note (emacs)Completion Options::) for this category.

     Additional alist entries may be defined in the future.

 -- Variable: completion-extra-properties
     This variable is used to specify extra properties of the current
     completion command.  It is intended to be let-bound by specialized
     completion commands.  Its value should be a list of property and
     value pairs.  The following properties are supported:

     ‘:annotation-function’
          The value should be a function to add annotations in the
          completions buffer.  This function must accept one argument, a
          completion, and should either return ‘nil’ or a string to be
          displayed next to the completion.

     ‘:exit-function’
          The value should be a function to run after performing
          completion.  The function should accept two arguments, STRING
          and STATUS, where STRING is the text to which the field was
          completed, and STATUS indicates what kind of operation
          happened: ‘finished’ if text is now complete, ‘sole’ if the
          text cannot be further completed but completion is not
          finished, or ‘exact’ if the text is a valid completion but may
          be further completed.


File: elisp.info,  Node: Programmed Completion,  Next: Completion in Buffers,  Prev: Completion Variables,  Up: Completion

19.6.7 Programmed Completion
----------------------------

Sometimes it is not possible or convenient to create an alist or an
obarray containing all the intended possible completions ahead of time.
In such a case, you can supply your own function to compute the
completion of a given string.  This is called “programmed completion”.
Emacs uses programmed completion when completing file names (*note File
Name Completion::), among many other cases.

   To use this feature, pass a function as the COLLECTION argument to
‘completing-read’.  The function ‘completing-read’ arranges to pass your
completion function along to ‘try-completion’, ‘all-completions’, and
other basic completion functions, which will then let your function do
all the work.

   The completion function should accept three arguments:

   • The string to be completed.

   • A predicate function with which to filter possible matches, or
     ‘nil’ if none.  The function should call the predicate for each
     possible match, and ignore the match if the predicate returns
     ‘nil’.

   • A flag specifying the type of completion operation to perform.
     This flag may be one of the following values.

     ‘nil’
          This specifies a ‘try-completion’ operation.  The function
          should return ‘t’ if the specified string is a unique and
          exact match; if there is more than one match, it should return
          the common substring of all matches (if the string is an exact
          match for one completion alternative but also matches other
          longer alternatives, the return value is the string); if there
          are no matches, it should return ‘nil’.

     ‘t’
          This specifies an ‘all-completions’ operation.  The function
          should return a list of all possible completions of the
          specified string.

     ‘lambda’
          This specifies a ‘test-completion’ operation.  The function
          should return ‘t’ if the specified string is an exact match
          for some completion alternative; ‘nil’ otherwise.

     ‘(boundaries . SUFFIX)’
          This specifies a ‘completion-boundaries’ operation.  The
          function should return ‘(boundaries START . END)’, where START
          is the position of the beginning boundary in the specified
          string, and END is the position of the end boundary in SUFFIX.

     ‘metadata’
          This specifies a request for information about the state of
          the current completion.  The return value should have the form
          ‘(metadata . ALIST)’, where ALIST is an alist whose elements
          are described below.

     If the flag has any other value, the completion function should
     return ‘nil’.

   The following is a list of metadata entries that a completion
function may return in response to a ‘metadata’ flag argument:

‘category’
     The value should be a symbol describing what kind of text the
     completion function is trying to complete.  If the symbol matches
     one of the keys in ‘completion-category-overrides’, the usual
     completion behavior is overridden.  *Note Completion Variables::.

‘annotation-function’
     The value should be a function for “annotating” completions.  The
     function should take one argument, STRING, which is a possible
     completion.  It should return a string, which is displayed after
     the completion STRING in the ‘*Completions*’ buffer.

‘display-sort-function’
     The value should be a function for sorting completions.  The
     function should take one argument, a list of completion strings,
     and return a sorted list of completion strings.  It is allowed to
     alter the input list destructively.

‘cycle-sort-function’
     The value should be a function for sorting completions, when
     ‘completion-cycle-threshold’ is non-‘nil’ and the user is cycling
     through completion alternatives.  *Note (emacs)Completion
     Options::.  Its argument list and return value are the same as for
     ‘display-sort-function’.

 -- Function: completion-table-dynamic function &optional switch-buffer
     This function is a convenient way to write a function that can act
     as a programmed completion function.  The argument FUNCTION should
     be a function that takes one argument, a string, and returns an
     alist of possible completions of it.  It is allowed to ignore the
     argument and return a full list of all possible completions.  You
     can think of ‘completion-table-dynamic’ as a transducer between
     that interface and the interface for programmed completion
     functions.

     If the optional argument SWITCH-BUFFER is non-‘nil’, and completion
     is performed in the minibuffer, FUNCTION will be called with
     current buffer set to the buffer from which the minibuffer was
     entered.

 -- Function: completion-table-with-cache function &optional ignore-case
     This is a wrapper for ‘completion-table-dynamic’ that saves the
     last argument-result pair.  This means that multiple lookups with
     the same argument only need to call FUNCTION once.  This can be
     useful when a slow operation is involved, such as calling an
     external process.


File: elisp.info,  Node: Completion in Buffers,  Prev: Programmed Completion,  Up: Completion

19.6.8 Completion in Ordinary Buffers
-------------------------------------

Although completion is usually done in the minibuffer, the completion
facility can also be used on the text in ordinary Emacs buffers.  In
many major modes, in-buffer completion is performed by the ‘C-M-i’ or
‘M-<TAB>’ command, bound to ‘completion-at-point’.  *Note (emacs)Symbol
Completion::.  This command uses the abnormal hook variable
‘completion-at-point-functions’:

 -- Variable: completion-at-point-functions
     The value of this abnormal hook should be a list of functions,
     which are used to compute a completion table for completing the
     text at point.  It can be used by major modes to provide
     mode-specific completion tables (*note Major Mode Conventions::).

     When the command ‘completion-at-point’ runs, it calls the functions
     in the list one by one, without any argument.  Each function should
     return ‘nil’ if it is unable to produce a completion table for the
     text at point.  Otherwise it should return a list of the form

          (START END COLLECTION . PROPS)

     START and END delimit the text to complete (which should enclose
     point).  COLLECTION is a completion table for completing that text,
     in a form suitable for passing as the second argument to
     ‘try-completion’ (*note Basic Completion::); completion
     alternatives will be generated from this completion table in the
     usual way, via the completion styles defined in ‘completion-styles’
     (*note Completion Variables::).  PROPS is a property list for
     additional information; any of the properties in
     ‘completion-extra-properties’ are recognized (*note Completion
     Variables::), as well as the following additional ones:

     ‘:predicate’
          The value should be a predicate that completion candidates
          need to satisfy.

     ‘:exclusive’
          If the value is ‘no’, then if the completion table fails to
          match the text at point, ‘completion-at-point’ moves on to the
          next function in ‘completion-at-point-functions’ instead of
          reporting a completion failure.

     Supplying a function for COLLECTION is strongly recommended if
     generating the list of completions is an expensive operation.
     Emacs may internally call functions in
     ‘completion-at-point-functions’ many times, but care about the
     value of COLLECTION for only some of these calls.  By supplying a
     function for COLLECTION, Emacs can defer generating completions
     until necessary.  You can use COMPLETION-TABLE-DYNAMIC to create a
     wrapper function:

          ;; Avoid this pattern.
          (let ((beg ...) (end ...) (my-completions (my-make-completions)))
            (list beg end my-completions))

          ;; Use this instead.
          (let ((beg ...) (end ...))
            (list beg
                  end
                  (completion-table-dynamic
                    (lambda (_)
                      (my-make-completions)))))

     A function in ‘completion-at-point-functions’ may also return a
     function instead of a list as described above.  In that case, that
     returned function is called, with no argument, and it is entirely
     responsible for performing the completion.  We discourage this
     usage; it is intended to help convert old code to using
     ‘completion-at-point’.

     The first function in ‘completion-at-point-functions’ to return a
     non-‘nil’ value is used by ‘completion-at-point’.  The remaining
     functions are not called.  The exception to this is when there is
     an ‘:exclusive’ specification, as described above.

   The following function provides a convenient way to perform
completion on an arbitrary stretch of text in an Emacs buffer:

 -- Function: completion-in-region start end collection &optional
          predicate
     This function completes the text in the current buffer between the
     positions START and END, using COLLECTION.  The argument COLLECTION
     has the same meaning as in ‘try-completion’ (*note Basic
     Completion::).

     This function inserts the completion text directly into the current
     buffer.  Unlike ‘completing-read’ (*note Minibuffer Completion::),
     it does not activate the minibuffer.

     For this function to work, point must be somewhere between START
     and END.


File: elisp.info,  Node: Yes-or-No Queries,  Next: Multiple Queries,  Prev: Completion,  Up: Minibuffers

19.7 Yes-or-No Queries
======================

This section describes functions used to ask the user a yes-or-no
question.  The function ‘y-or-n-p’ can be answered with a single
character; it is useful for questions where an inadvertent wrong answer
will not have serious consequences.  ‘yes-or-no-p’ is suitable for more
momentous questions, since it requires three or four characters to
answer.

   If either of these functions is called in a command that was invoked
using the mouse—more precisely, if ‘last-nonmenu-event’ (*note Command
Loop Info::) is either ‘nil’ or a list—then it uses a dialog box or
pop-up menu to ask the question.  Otherwise, it uses keyboard input.
You can force use either of the mouse or of keyboard input by binding
‘last-nonmenu-event’ to a suitable value around the call.

   Strictly speaking, ‘yes-or-no-p’ uses the minibuffer and ‘y-or-n-p’
does not; but it seems best to describe them together.

 -- Function: y-or-n-p prompt
     This function asks the user a question, expecting input in the echo
     area.  It returns ‘t’ if the user types ‘y’, ‘nil’ if the user
     types ‘n’.  This function also accepts <SPC> to mean yes and <DEL>
     to mean no.  It accepts ‘C-]’ to quit, like ‘C-g’, because the
     question might look like a minibuffer and for that reason the user
     might try to use ‘C-]’ to get out.  The answer is a single
     character, with no <RET> needed to terminate it.  Upper and lower
     case are equivalent.

     “Asking the question” means printing PROMPT in the echo area,
     followed by the string ‘(y or n) ’.  If the input is not one of the
     expected answers (‘y’, ‘n’, ‘<SPC>’, ‘<DEL>’, or something that
     quits), the function responds ‘Please answer y or n.’, and repeats
     the request.

     This function does not actually use the minibuffer, since it does
     not allow editing of the answer.  It actually uses the echo area
     (*note The Echo Area::), which uses the same screen space as the
     minibuffer.  The cursor moves to the echo area while the question
     is being asked.

     The answers and their meanings, even ‘y’ and ‘n’, are not
     hardwired, and are specified by the keymap ‘query-replace-map’
     (*note Search and Replace::).  In particular, if the user enters
     the special responses ‘recenter’, ‘scroll-up’, ‘scroll-down’,
     ‘scroll-other-window’, or ‘scroll-other-window-down’ (respectively
     bound to ‘C-l’, ‘C-v’, ‘M-v’, ‘C-M-v’ and ‘C-M-S-v’ in
     ‘query-replace-map’), this function performs the specified window
     recentering or scrolling operation, and poses the question again.

     We show successive lines of echo area messages, but only one
     actually appears on the screen at a time.

 -- Function: y-or-n-p-with-timeout prompt seconds default
     Like ‘y-or-n-p’, except that if the user fails to answer within
     SECONDS seconds, this function stops waiting and returns DEFAULT.
     It works by setting up a timer; see *note Timers::.  The argument
     SECONDS should be a number.

 -- Function: yes-or-no-p prompt
     This function asks the user a question, expecting input in the
     minibuffer.  It returns ‘t’ if the user enters ‘yes’, ‘nil’ if the
     user types ‘no’.  The user must type <RET> to finalize the
     response.  Upper and lower case are equivalent.

     ‘yes-or-no-p’ starts by displaying PROMPT in the echo area,
     followed by ‘(yes or no) ’.  The user must type one of the expected
     responses; otherwise, the function responds ‘Please answer yes or
     no.’, waits about two seconds and repeats the request.

     ‘yes-or-no-p’ requires more work from the user than ‘y-or-n-p’ and
     is appropriate for more crucial decisions.

     Here is an example:

          (yes-or-no-p "Do you really want to remove everything? ")

          ;; After evaluation of the preceding expression,
          ;;   the following prompt appears,
          ;;   with an empty minibuffer:

          ---------- Buffer: minibuffer ----------
          Do you really want to remove everything? (yes or no)
          ---------- Buffer: minibuffer ----------

     If the user first types ‘y <RET>’, which is invalid because this
     function demands the entire word ‘yes’, it responds by displaying
     these prompts, with a brief pause between them:

          ---------- Buffer: minibuffer ----------
          Please answer yes or no.
          Do you really want to remove everything? (yes or no)
          ---------- Buffer: minibuffer ----------


File: elisp.info,  Node: Multiple Queries,  Next: Reading a Password,  Prev: Yes-or-No Queries,  Up: Minibuffers

19.8 Asking Multiple Y-or-N Questions
=====================================

When you have a series of similar questions to ask, such as “Do you want
to save this buffer?” for each buffer in turn, you should use
‘map-y-or-n-p’ to ask the collection of questions, rather than asking
each question individually.  This gives the user certain convenient
facilities such as the ability to answer the whole series at once.

 -- Function: map-y-or-n-p prompter actor list &optional help
          action-alist no-cursor-in-echo-area
     This function asks the user a series of questions, reading a
     single-character answer in the echo area for each one.

     The value of LIST specifies the objects to ask questions about.  It
     should be either a list of objects or a generator function.  If it
     is a function, it should expect no arguments, and should return
     either the next object to ask about, or ‘nil’, meaning to stop
     asking questions.

     The argument PROMPTER specifies how to ask each question.  If
     PROMPTER is a string, the question text is computed like this:

          (format PROMPTER OBJECT)

     where OBJECT is the next object to ask about (as obtained from
     LIST).

     If not a string, PROMPTER should be a function of one argument (the
     next object to ask about) and should return the question text.  If
     the value is a string, that is the question to ask the user.  The
     function can also return ‘t’, meaning do act on this object (and
     don’t ask the user), or ‘nil’, meaning ignore this object (and
     don’t ask the user).

     The argument ACTOR says how to act on the answers that the user
     gives.  It should be a function of one argument, and it is called
     with each object that the user says yes for.  Its argument is
     always an object obtained from LIST.

     If the argument HELP is given, it should be a list of this form:

          (SINGULAR PLURAL ACTION)

     where SINGULAR is a string containing a singular noun that
     describes the objects conceptually being acted on, PLURAL is the
     corresponding plural noun, and ACTION is a transitive verb
     describing what ACTOR does.

     If you don’t specify HELP, the default is ‘("object" "objects" "act
     on")’.

     Each time a question is asked, the user may enter ‘y’, ‘Y’, or
     <SPC> to act on that object; ‘n’, ‘N’, or <DEL> to skip that
     object; ‘!’ to act on all following objects; <ESC> or ‘q’ to exit
     (skip all following objects); ‘.’ (period) to act on the current
     object and then exit; or ‘C-h’ to get help.  These are the same
     answers that ‘query-replace’ accepts.  The keymap
     ‘query-replace-map’ defines their meaning for ‘map-y-or-n-p’ as
     well as for ‘query-replace’; see *note Search and Replace::.

     You can use ACTION-ALIST to specify additional possible answers and
     what they mean.  It is an alist of elements of the form ‘(CHAR
     FUNCTION HELP)’, each of which defines one additional answer.  In
     this element, CHAR is a character (the answer); FUNCTION is a
     function of one argument (an object from LIST); HELP is a string.

     When the user responds with CHAR, ‘map-y-or-n-p’ calls FUNCTION.
     If it returns non-‘nil’, the object is considered acted upon, and
     ‘map-y-or-n-p’ advances to the next object in LIST.  If it returns
     ‘nil’, the prompt is repeated for the same object.

     Normally, ‘map-y-or-n-p’ binds ‘cursor-in-echo-area’ while
     prompting.  But if NO-CURSOR-IN-ECHO-AREA is non-‘nil’, it does not
     do that.

     If ‘map-y-or-n-p’ is called in a command that was invoked using the
     mouse—more precisely, if ‘last-nonmenu-event’ (*note Command Loop
     Info::) is either ‘nil’ or a list—then it uses a dialog box or
     pop-up menu to ask the question.  In this case, it does not use
     keyboard input or the echo area.  You can force use either of the
     mouse or of keyboard input by binding ‘last-nonmenu-event’ to a
     suitable value around the call.

     The return value of ‘map-y-or-n-p’ is the number of objects acted
     on.


File: elisp.info,  Node: Reading a Password,  Next: Minibuffer Commands,  Prev: Multiple Queries,  Up: Minibuffers

19.9 Reading a Password
=======================

To read a password to pass to another program, you can use the function
‘read-passwd’.

 -- Function: read-passwd prompt &optional confirm default
     This function reads a password, prompting with PROMPT.  It does not
     echo the password as the user types it; instead, it echoes ‘.’ for
     each character in the password.  If you want to apply another
     character to hide the password, let-bind the variable
     ‘read-hide-char’ with that character.

     The optional argument CONFIRM, if non-‘nil’, says to read the
     password twice and insist it must be the same both times.  If it
     isn’t the same, the user has to type it over and over until the
     last two times match.

     The optional argument DEFAULT specifies the default password to
     return if the user enters empty input.  If DEFAULT is ‘nil’, then
     ‘read-passwd’ returns the null string in that case.


File: elisp.info,  Node: Minibuffer Commands,  Next: Minibuffer Windows,  Prev: Reading a Password,  Up: Minibuffers

19.10 Minibuffer Commands
=========================

This section describes some commands meant for use in the minibuffer.

 -- Command: exit-minibuffer
     This command exits the active minibuffer.  It is normally bound to
     keys in minibuffer local keymaps.

 -- Command: self-insert-and-exit
     This command exits the active minibuffer after inserting the last
     character typed on the keyboard (found in ‘last-command-event’;
     *note Command Loop Info::).

 -- Command: previous-history-element n
     This command replaces the minibuffer contents with the value of the
     Nth previous (older) history element.

 -- Command: next-history-element n
     This command replaces the minibuffer contents with the value of the
     Nth more recent history element.

 -- Command: previous-matching-history-element pattern n
     This command replaces the minibuffer contents with the value of the
     Nth previous (older) history element that matches PATTERN (a
     regular expression).

 -- Command: next-matching-history-element pattern n
     This command replaces the minibuffer contents with the value of the
     Nth next (newer) history element that matches PATTERN (a regular
     expression).

 -- Command: previous-complete-history-element n
     This command replaces the minibuffer contents with the value of the
     Nth previous (older) history element that completes the current
     contents of the minibuffer before the point.

 -- Command: next-complete-history-element n
     This command replaces the minibuffer contents with the value of the
     Nth next (newer) history element that completes the current
     contents of the minibuffer before the point.


File: elisp.info,  Node: Minibuffer Windows,  Next: Minibuffer Contents,  Prev: Minibuffer Commands,  Up: Minibuffers

19.11 Minibuffer Windows
========================

These functions access and select minibuffer windows, test whether they
are active and control how they get resized.

 -- Function: active-minibuffer-window
     This function returns the currently active minibuffer window, or
     ‘nil’ if there is none.

 -- Function: minibuffer-window &optional frame
     This function returns the minibuffer window used for frame FRAME.
     If FRAME is ‘nil’, that stands for the current frame.  Note that
     the minibuffer window used by a frame need not be part of that
     frame—a frame that has no minibuffer of its own necessarily uses
     some other frame’s minibuffer window.

 -- Function: set-minibuffer-window window
     This function specifies WINDOW as the minibuffer window to use.
     This affects where the minibuffer is displayed if you put text in
     it without invoking the usual minibuffer commands.  It has no
     effect on the usual minibuffer input functions because they all
     start by choosing the minibuffer window according to the current
     frame.

 -- Function: window-minibuffer-p &optional window
     This function returns non-‘nil’ if WINDOW is a minibuffer window.
     WINDOW defaults to the selected window.

   It is not correct to determine whether a given window is a minibuffer
by comparing it with the result of ‘(minibuffer-window)’, because there
can be more than one minibuffer window if there is more than one frame.

 -- Function: minibuffer-window-active-p window
     This function returns non-‘nil’ if WINDOW is the currently active
     minibuffer window.

   The following two options control whether minibuffer windows are
resized automatically and how large they can get in the process.

 -- User Option: resize-mini-windows
     This option specifies whether minibuffer windows are resized
     automatically.  The default value is ‘grow-only’, which means that
     a minibuffer window by default expands automatically to accommodate
     the text it displays and shrinks back to one line as soon as the
     minibuffer gets empty.  If the value is ‘t’, Emacs will always try
     to fit the height of a minibuffer window to the text it displays
     (with a minimum of one line).  If the value is ‘nil’, a minibuffer
     window never changes size automatically.  In that case the window
     resizing commands (*note Resizing Windows::) can be used to adjust
     its height.

 -- User Option: max-mini-window-height
     This option provides a maximum height for resizing minibuffer
     windows automatically.  A floating-point number specifies a
     fraction of the frame’s height; an integer specifies the maximum
     number of lines.  The default value is 0.25.


File: elisp.info,  Node: Minibuffer Contents,  Next: Recursive Mini,  Prev: Minibuffer Windows,  Up: Minibuffers

19.12 Minibuffer Contents
=========================

These functions access the minibuffer prompt and contents.

 -- Function: minibuffer-prompt
     This function returns the prompt string of the currently active
     minibuffer.  If no minibuffer is active, it returns ‘nil’.

 -- Function: minibuffer-prompt-end
     This function returns the current position of the end of the
     minibuffer prompt, if a minibuffer is current.  Otherwise, it
     returns the minimum valid buffer position.

 -- Function: minibuffer-prompt-width
     This function returns the current display-width of the minibuffer
     prompt, if a minibuffer is current.  Otherwise, it returns zero.

 -- Function: minibuffer-contents
     This function returns the editable contents of the minibuffer (that
     is, everything except the prompt) as a string, if a minibuffer is
     current.  Otherwise, it returns the entire contents of the current
     buffer.

 -- Function: minibuffer-contents-no-properties
     This is like ‘minibuffer-contents’, except that it does not copy
     text properties, just the characters themselves.  *Note Text
     Properties::.

 -- Command: delete-minibuffer-contents
     This command erases the editable contents of the minibuffer (that
     is, everything except the prompt), if a minibuffer is current.
     Otherwise, it erases the entire current buffer.


File: elisp.info,  Node: Recursive Mini,  Next: Minibuffer Misc,  Prev: Minibuffer Contents,  Up: Minibuffers

19.13 Recursive Minibuffers
===========================

These functions and variables deal with recursive minibuffers (*note
Recursive Editing::):

 -- Function: minibuffer-depth
     This function returns the current depth of activations of the
     minibuffer, a nonnegative integer.  If no minibuffers are active,
     it returns zero.

 -- User Option: enable-recursive-minibuffers
     If this variable is non-‘nil’, you can invoke commands (such as
     ‘find-file’) that use minibuffers even while the minibuffer window
     is active.  Such invocation produces a recursive editing level for
     a new minibuffer.  The outer-level minibuffer is invisible while
     you are editing the inner one.

     If this variable is ‘nil’, you cannot invoke minibuffer commands
     when the minibuffer window is active, not even if you switch to
     another window to do it.

   If a command name has a property ‘enable-recursive-minibuffers’ that
is non-‘nil’, then the command can use the minibuffer to read arguments
even if it is invoked from the minibuffer.  A command can also achieve
this by binding ‘enable-recursive-minibuffers’ to ‘t’ in the interactive
declaration (*note Using Interactive::).  The minibuffer command
‘next-matching-history-element’ (normally ‘M-s’ in the minibuffer) does
the latter.


File: elisp.info,  Node: Minibuffer Misc,  Prev: Recursive Mini,  Up: Minibuffers

19.14 Minibuffer Miscellany
===========================

 -- Function: minibufferp &optional buffer-or-name
     This function returns non-‘nil’ if BUFFER-OR-NAME is a minibuffer.
     If BUFFER-OR-NAME is omitted, it tests the current buffer.

 -- Variable: minibuffer-setup-hook
     This is a normal hook that is run whenever the minibuffer is
     entered.  *Note Hooks::.

 -- Variable: minibuffer-exit-hook
     This is a normal hook that is run whenever the minibuffer is
     exited.  *Note Hooks::.

 -- Variable: minibuffer-help-form
     The current value of this variable is used to rebind ‘help-form’
     locally inside the minibuffer (*note Help Functions::).

 -- Variable: minibuffer-scroll-window
     If the value of this variable is non-‘nil’, it should be a window
     object.  When the function ‘scroll-other-window’ is called in the
     minibuffer, it scrolls this window.

 -- Function: minibuffer-selected-window
     This function returns the window that was selected when the
     minibuffer was entered.  If selected window is not a minibuffer
     window, it returns ‘nil’.

 -- User Option: max-mini-window-height
     This variable specifies the maximum height for resizing minibuffer
     windows.  If a float, it specifies a fraction of the height of the
     frame.  If an integer, it specifies a number of lines.

 -- Function: minibuffer-message string &rest args
     This function displays STRING temporarily at the end of the
     minibuffer text, for a few seconds, or until the next input event
     arrives, whichever comes first.  The variable
     ‘minibuffer-message-timeout’ specifies the number of seconds to
     wait in the absence of input.  It defaults to 2.  If ARGS is
     non-‘nil’, the actual message is obtained by passing STRING and
     ARGS through ‘format-message’.  *Note Formatting Strings::.

 -- Command: minibuffer-inactive-mode
     This is the major mode used in inactive minibuffers.  It uses
     keymap ‘minibuffer-inactive-mode-map’.  This can be useful if the
     minibuffer is in a separate frame.  *Note Minibuffers and Frames::.


File: elisp.info,  Node: Command Loop,  Next: Keymaps,  Prev: Minibuffers,  Up: Top

20 Command Loop
***************

When you run Emacs, it enters the “editor command loop” almost
immediately.  This loop reads key sequences, executes their definitions,
and displays the results.  In this chapter, we describe how these things
are done, and the subroutines that allow Lisp programs to do them.

* Menu:

* Command Overview::    How the command loop reads commands.
* Defining Commands::   Specifying how a function should read arguments.
* Interactive Call::    Calling a command, so that it will read arguments.
* Distinguish Interactive::     Making a command distinguish interactive calls.
* Command Loop Info::   Variables set by the command loop for you to examine.
* Adjusting Point::     Adjustment of point after a command.
* Input Events::        What input looks like when you read it.
* Reading Input::       How to read input events from the keyboard or mouse.
* Special Events::      Events processed immediately and individually.
* Waiting::             Waiting for user input or elapsed time.
* Quitting::            How ‘C-g’ works.  How to catch or defer quitting.
* Prefix Command Arguments::    How the commands to set prefix args work.
* Recursive Editing::   Entering a recursive edit,
                          and why you usually shouldn’t.
* Disabling Commands::  How the command loop handles disabled commands.
* Command History::     How the command history is set up, and how accessed.
* Keyboard Macros::     How keyboard macros are implemented.


File: elisp.info,  Node: Command Overview,  Next: Defining Commands,  Up: Command Loop

20.1 Command Loop Overview
==========================

The first thing the command loop must do is read a key sequence, which
is a sequence of input events that translates into a command.  It does
this by calling the function ‘read-key-sequence’.  Lisp programs can
also call this function (*note Key Sequence Input::).  They can also
read input at a lower level with ‘read-key’ or ‘read-event’ (*note
Reading One Event::), or discard pending input with ‘discard-input’
(*note Event Input Misc::).

   The key sequence is translated into a command through the currently
active keymaps.  *Note Key Lookup::, for information on how this is
done.  The result should be a keyboard macro or an interactively
callable function.  If the key is ‘M-x’, then it reads the name of
another command, which it then calls.  This is done by the command
‘execute-extended-command’ (*note Interactive Call::).

   Prior to executing the command, Emacs runs ‘undo-boundary’ to create
an undo boundary.  *Note Maintaining Undo::.

   To execute a command, Emacs first reads its arguments by calling
‘command-execute’ (*note Interactive Call::).  For commands written in
Lisp, the ‘interactive’ specification says how to read the arguments.
This may use the prefix argument (*note Prefix Command Arguments::) or
may read with prompting in the minibuffer (*note Minibuffers::).  For
example, the command ‘find-file’ has an ‘interactive’ specification
which says to read a file name using the minibuffer.  The function body
of ‘find-file’ does not use the minibuffer, so if you call ‘find-file’
as a function from Lisp code, you must supply the file name string as an
ordinary Lisp function argument.

   If the command is a keyboard macro (i.e., a string or vector), Emacs
executes it using ‘execute-kbd-macro’ (*note Keyboard Macros::).

 -- Variable: pre-command-hook
     This normal hook is run by the editor command loop before it
     executes each command.  At that time, ‘this-command’ contains the
     command that is about to run, and ‘last-command’ describes the
     previous command.  *Note Command Loop Info::.

 -- Variable: post-command-hook
     This normal hook is run by the editor command loop after it
     executes each command (including commands terminated prematurely by
     quitting or by errors).  At that time, ‘this-command’ refers to the
     command that just ran, and ‘last-command’ refers to the command
     before that.

     This hook is also run when Emacs first enters the command loop (at
     which point ‘this-command’ and ‘last-command’ are both ‘nil’).

   Quitting is suppressed while running ‘pre-command-hook’ and
‘post-command-hook’.  If an error happens while executing one of these
hooks, it does not terminate execution of the hook; instead the error is
silenced and the function in which the error occurred is removed from
the hook.

   A request coming into the Emacs server (*note (emacs)Emacs Server::)
runs these two hooks just as a keyboard command does.


File: elisp.info,  Node: Defining Commands,  Next: Interactive Call,  Prev: Command Overview,  Up: Command Loop

20.2 Defining Commands
======================

The special form ‘interactive’ turns a Lisp function into a command.
The ‘interactive’ form must be located at top-level in the function
body, usually as the first form in the body; this applies to both lambda
expressions (*note Lambda Expressions::) and ‘defun’ forms (*note
Defining Functions::).  This form does nothing during the actual
execution of the function; its presence serves as a flag, telling the
Emacs command loop that the function can be called interactively.  The
argument of the ‘interactive’ form specifies how the arguments for an
interactive call should be read.

   Alternatively, an ‘interactive’ form may be specified in a function
symbol’s ‘interactive-form’ property.  A non-‘nil’ value for this
property takes precedence over any ‘interactive’ form in the function
body itself.  This feature is seldom used.

   Sometimes, a function is only intended to be called interactively,
never directly from Lisp.  In that case, give the function a non-‘nil’
‘interactive-only’ property, either directly or via ‘declare’ (*note
Declare Form::).  This causes the byte compiler to warn if the command
is called from Lisp.  The output of ‘describe-function’ will include
similar information.  The value of the property can be: a string, which
the byte-compiler will use directly in its warning (it should end with a
period, and not start with a capital, e.g., ‘"use (system-name)
instead."’); ‘t’; any other symbol, which should be an alternative
function to use in Lisp code.

* Menu:

* Using Interactive::     General rules for ‘interactive’.
* Interactive Codes::     The standard letter-codes for reading arguments
                             in various ways.
* Interactive Examples::  Examples of how to read interactive arguments.
* Generic Commands::      Select among command alternatives.


File: elisp.info,  Node: Using Interactive,  Next: Interactive Codes,  Up: Defining Commands

20.2.1 Using ‘interactive’
--------------------------

This section describes how to write the ‘interactive’ form that makes a
Lisp function an interactively-callable command, and how to examine a
command’s ‘interactive’ form.

 -- Special Form: interactive arg-descriptor
     This special form declares that a function is a command, and that
     it may therefore be called interactively (via ‘M-x’ or by entering
     a key sequence bound to it).  The argument ARG-DESCRIPTOR declares
     how to compute the arguments to the command when the command is
     called interactively.

     A command may be called from Lisp programs like any other function,
     but then the caller supplies the arguments and ARG-DESCRIPTOR has
     no effect.

     The ‘interactive’ form must be located at top-level in the function
     body, or in the function symbol’s ‘interactive-form’ property
     (*note Symbol Properties::).  It has its effect because the command
     loop looks for it before calling the function (*note Interactive
     Call::).  Once the function is called, all its body forms are
     executed; at this time, if the ‘interactive’ form occurs within the
     body, the form simply returns ‘nil’ without even evaluating its
     argument.

     By convention, you should put the ‘interactive’ form in the
     function body, as the first top-level form.  If there is an
     ‘interactive’ form in both the ‘interactive-form’ symbol property
     and the function body, the former takes precedence.  The
     ‘interactive-form’ symbol property can be used to add an
     interactive form to an existing function, or change how its
     arguments are processed interactively, without redefining the
     function.

   There are three possibilities for the argument ARG-DESCRIPTOR:

   • It may be omitted or ‘nil’; then the command is called with no
     arguments.  This leads quickly to an error if the command requires
     one or more arguments.

   • It may be a string; its contents are a sequence of elements
     separated by newlines, one for each argument(1).  Each element
     consists of a code character (*note Interactive Codes::) optionally
     followed by a prompt (which some code characters use and some
     ignore).  Here is an example:

          (interactive "P\nbFrobnicate buffer: ")

     The code letter ‘P’ sets the command’s first argument to the raw
     command prefix (*note Prefix Command Arguments::).  ‘bFrobnicate
     buffer: ’ prompts the user with ‘Frobnicate buffer: ’ to enter the
     name of an existing buffer, which becomes the second and final
     argument.

     The prompt string can use ‘%’ to include previous argument values
     (starting with the first argument) in the prompt.  This is done
     using ‘format-message’ (*note Formatting Strings::).  For example,
     here is how you could read the name of an existing buffer followed
     by a new name to give to that buffer:

          (interactive "bBuffer to rename: \nsRename buffer %s to: ")

     If ‘*’ appears at the beginning of the string, then an error is
     signaled if the buffer is read-only.

     If ‘@’ appears at the beginning of the string, and if the key
     sequence used to invoke the command includes any mouse events, then
     the window associated with the first of those events is selected
     before the command is run.

     If ‘^’ appears at the beginning of the string, and if the command
     was invoked through “shift-translation”, set the mark and activate
     the region temporarily, or extend an already active region, before
     the command is run.  If the command was invoked without
     shift-translation, and the region is temporarily active, deactivate
     the region before the command is run.  Shift-translation is
     controlled on the user level by ‘shift-select-mode’; see *note
     (emacs)Shift Selection::.

     You can use ‘*’, ‘@’, and ‘^’ together; the order does not matter.
     Actual reading of arguments is controlled by the rest of the prompt
     string (starting with the first character that is not ‘*’, ‘@’, or
     ‘^’).

   • It may be a Lisp expression that is not a string; then it should be
     a form that is evaluated to get a list of arguments to pass to the
     command.  Usually this form will call various functions to read
     input from the user, most often through the minibuffer (*note
     Minibuffers::) or directly from the keyboard (*note Reading
     Input::).

     Providing point or the mark as an argument value is also common,
     but if you do this _and_ read input (whether using the minibuffer
     or not), be sure to get the integer values of point or the mark
     after reading.  The current buffer may be receiving subprocess
     output; if subprocess output arrives while the command is waiting
     for input, it could relocate point and the mark.

     Here’s an example of what _not_ to do:

          (interactive
           (list (region-beginning) (region-end)
                 (read-string "Foo: " nil 'my-history)))

     Here’s how to avoid the problem, by examining point and the mark
     after reading the keyboard input:

          (interactive
           (let ((string (read-string "Foo: " nil 'my-history)))
             (list (region-beginning) (region-end) string)))

     *Warning:* the argument values should not include any data types
     that can’t be printed and then read.  Some facilities save
     ‘command-history’ in a file to be read in the subsequent sessions;
     if a command’s arguments contain a data type that prints using
     ‘#<...>’ syntax, those facilities won’t work.

     There are, however, a few exceptions: it is ok to use a limited set
     of expressions such as ‘(point)’, ‘(mark)’, ‘(region-beginning)’,
     and ‘(region-end)’, because Emacs recognizes them specially and
     puts the expression (rather than its value) into the command
     history.  To see whether the expression you wrote is one of these
     exceptions, run the command, then examine ‘(car command-history)’.

 -- Function: interactive-form function
     This function returns the ‘interactive’ form of FUNCTION.  If
     FUNCTION is an interactively callable function (*note Interactive
     Call::), the value is the command’s ‘interactive’ form
     ‘(interactive SPEC)’, which specifies how to compute its arguments.
     Otherwise, the value is ‘nil’.  If FUNCTION is a symbol, its
     function definition is used.

   ---------- Footnotes ----------

   (1) Some elements actually supply two arguments.


File: elisp.info,  Node: Interactive Codes,  Next: Interactive Examples,  Prev: Using Interactive,  Up: Defining Commands

20.2.2 Code Characters for ‘interactive’
----------------------------------------

The code character descriptions below contain a number of key words,
defined here as follows:

Completion
     Provide completion.  <TAB>, <SPC>, and <RET> perform name
     completion because the argument is read using ‘completing-read’
     (*note Completion::).  ‘?’ displays a list of possible completions.

Existing
     Require the name of an existing object.  An invalid name is not
     accepted; the commands to exit the minibuffer do not exit if the
     current input is not valid.

Default
     A default value of some sort is used if the user enters no text in
     the minibuffer.  The default depends on the code character.

No I/O
     This code letter computes an argument without reading any input.
     Therefore, it does not use a prompt string, and any prompt string
     you supply is ignored.

     Even though the code letter doesn’t use a prompt string, you must
     follow it with a newline if it is not the last code character in
     the string.

Prompt
     A prompt immediately follows the code character.  The prompt ends
     either with the end of the string or with a newline.

Special
     This code character is meaningful only at the beginning of the
     interactive string, and it does not look for a prompt or a newline.
     It is a single, isolated character.

   Here are the code character descriptions for use with ‘interactive’:

‘*’
     Signal an error if the current buffer is read-only.  Special.

‘@’
     Select the window mentioned in the first mouse event in the key
     sequence that invoked this command.  Special.

‘^’
     If the command was invoked through shift-translation, set the mark
     and activate the region temporarily, or extend an already active
     region, before the command is run.  If the command was invoked
     without shift-translation, and the region is temporarily active,
     deactivate the region before the command is run.  Special.

‘a’
     A function name (i.e., a symbol satisfying ‘fboundp’).  Existing,
     Completion, Prompt.

‘b’
     The name of an existing buffer.  By default, uses the name of the
     current buffer (*note Buffers::).  Existing, Completion, Default,
     Prompt.

‘B’
     A buffer name.  The buffer need not exist.  By default, uses the
     name of a recently used buffer other than the current buffer.
     Completion, Default, Prompt.

‘c’
     A character.  The cursor does not move into the echo area.  Prompt.

‘C’
     A command name (i.e., a symbol satisfying ‘commandp’).  Existing,
     Completion, Prompt.

‘d’
     The position of point, as an integer (*note Point::).  No I/O.

‘D’
     A directory name.  The default is the current default directory of
     the current buffer, ‘default-directory’ (*note File Name
     Expansion::).  Existing, Completion, Default, Prompt.

‘e’
     The first or next non-keyboard event in the key sequence that
     invoked the command.  More precisely, ‘e’ gets events that are
     lists, so you can look at the data in the lists.  *Note Input
     Events::.  No I/O.

     You use ‘e’ for mouse events and for special system events (*note
     Misc Events::).  The event list that the command receives depends
     on the event.  *Note Input Events::, which describes the forms of
     the list for each event in the corresponding subsections.

     You can use ‘e’ more than once in a single command’s interactive
     specification.  If the key sequence that invoked the command has N
     events that are lists, the Nth ‘e’ provides the Nth such event.
     Events that are not lists, such as function keys and ASCII
     characters, do not count where ‘e’ is concerned.

‘f’
     A file name of an existing file (*note File Names::).  The default
     directory is ‘default-directory’.  Existing, Completion, Default,
     Prompt.

‘F’
     A file name.  The file need not exist.  Completion, Default,
     Prompt.

‘G’
     A file name.  The file need not exist.  If the user enters just a
     directory name, then the value is just that directory name, with no
     file name within the directory added.  Completion, Default, Prompt.

‘i’
     An irrelevant argument.  This code always supplies ‘nil’ as the
     argument’s value.  No I/O.

‘k’
     A key sequence (*note Key Sequences::).  This keeps reading events
     until a command (or undefined command) is found in the current key
     maps.  The key sequence argument is represented as a string or
     vector.  The cursor does not move into the echo area.  Prompt.

     If ‘k’ reads a key sequence that ends with a down-event, it also
     reads and discards the following up-event.  You can get access to
     that up-event with the ‘U’ code character.

     This kind of input is used by commands such as ‘describe-key’ and
     ‘global-set-key’.

‘K’
     A key sequence, whose definition you intend to change.  This works
     like ‘k’, except that it suppresses, for the last input event in
     the key sequence, the conversions that are normally used (when
     necessary) to convert an undefined key into a defined one.

‘m’
     The position of the mark, as an integer.  No I/O.

‘M’
     Arbitrary text, read in the minibuffer using the current buffer’s
     input method, and returned as a string (*note (emacs)Input
     Methods::).  Prompt.

‘n’
     A number, read with the minibuffer.  If the input is not a number,
     the user has to try again.  ‘n’ never uses the prefix argument.
     Prompt.

‘N’
     The numeric prefix argument; but if there is no prefix argument,
     read a number as with ‘n’.  The value is always a number.  *Note
     Prefix Command Arguments::.  Prompt.

‘p’
     The numeric prefix argument.  (Note that this ‘p’ is lower case.)
     No I/O.

‘P’
     The raw prefix argument.  (Note that this ‘P’ is upper case.)  No
     I/O.

‘r’
     Point and the mark, as two numeric arguments, smallest first.  This
     is the only code letter that specifies two successive arguments
     rather than one.  This will signal an error if the mark is not set
     in the buffer which is current when the command is invoked.  No
     I/O.

‘s’
     Arbitrary text, read in the minibuffer and returned as a string
     (*note Text from Minibuffer::).  Terminate the input with either
     ‘C-j’ or <RET>.  (‘C-q’ may be used to include either of these
     characters in the input.)  Prompt.

‘S’
     An interned symbol whose name is read in the minibuffer.  Terminate
     the input with either ‘C-j’ or <RET>.  Other characters that
     normally terminate a symbol (e.g., whitespace, parentheses and
     brackets) do not do so here.  Prompt.

‘U’
     A key sequence or ‘nil’.  Can be used after a ‘k’ or ‘K’ argument
     to get the up-event that was discarded (if any) after ‘k’ or ‘K’
     read a down-event.  If no up-event has been discarded, ‘U’ provides
     ‘nil’ as the argument.  No I/O.

‘v’
     A variable declared to be a user option (i.e., satisfying the
     predicate ‘custom-variable-p’).  This reads the variable using
     ‘read-variable’.  *Note Definition of read-variable::.  Existing,
     Completion, Prompt.

‘x’
     A Lisp object, specified with its read syntax, terminated with a
     ‘C-j’ or <RET>.  The object is not evaluated.  *Note Object from
     Minibuffer::.  Prompt.

‘X’
     A Lisp form’s value.  ‘X’ reads as ‘x’ does, then evaluates the
     form so that its value becomes the argument for the command.
     Prompt.

‘z’
     A coding system name (a symbol).  If the user enters null input,
     the argument value is ‘nil’.  *Note Coding Systems::.  Completion,
     Existing, Prompt.

‘Z’
     A coding system name (a symbol)—but only if this command has a
     prefix argument.  With no prefix argument, ‘Z’ provides ‘nil’ as
     the argument value.  Completion, Existing, Prompt.


File: elisp.info,  Node: Interactive Examples,  Next: Generic Commands,  Prev: Interactive Codes,  Up: Defining Commands

20.2.3 Examples of Using ‘interactive’
--------------------------------------

Here are some examples of ‘interactive’:

     (defun foo1 ()              ; ‘foo1’ takes no arguments,
         (interactive)           ;   just moves forward two words.
         (forward-word 2))
          ⇒ foo1

     (defun foo2 (n)             ; ‘foo2’ takes one argument,
         (interactive "^p")      ;   which is the numeric prefix.
                                 ; under ‘shift-select-mode’,
                                 ;   will activate or extend region.
         (forward-word (* 2 n)))
          ⇒ foo2

     (defun foo3 (n)             ; ‘foo3’ takes one argument,
         (interactive "nCount:") ;   which is read with the Minibuffer.
         (forward-word (* 2 n)))
          ⇒ foo3

     (defun three-b (b1 b2 b3)
       "Select three existing buffers.
     Put them into three windows, selecting the last one."
         (interactive "bBuffer1:\nbBuffer2:\nbBuffer3:")
         (delete-other-windows)
         (split-window (selected-window) 8)
         (switch-to-buffer b1)
         (other-window 1)
         (split-window (selected-window) 8)
         (switch-to-buffer b2)
         (other-window 1)
         (switch-to-buffer b3))
          ⇒ three-b
     (three-b "*scratch*" "declarations.texi" "*mail*")
          ⇒ nil


File: elisp.info,  Node: Generic Commands,  Prev: Interactive Examples,  Up: Defining Commands

20.2.4 Select among Command Alternatives
----------------------------------------

The macro ‘define-alternatives’ can be used to define “generic
commands”.  These are interactive functions whose implementation can be
selected from several alternatives, as a matter of user preference.

 -- Macro: define-alternatives command &rest customizations
     Define the new command COMMAND, a symbol.

     When a user runs ‘M-x COMMAND <RET>’ for the first time, Emacs
     prompts for which real form of the command to use, and records the
     selection by way of a custom variable.  Using a prefix argument
     repeats this process of choosing an alternative.

     The variable ‘COMMAND-alternatives’ should contain an alist with
     alternative implementations of COMMAND.  Until this variable is
     set, ‘define-alternatives’ has no effect.

     If CUSTOMIZATIONS is non-‘nil’, it should consist of alternating
     ‘defcustom’ keywords (typically ‘:group’ and ‘:version’) and values
     to add to the declaration of ‘COMMAND-alternatives’.


File: elisp.info,  Node: Interactive Call,  Next: Distinguish Interactive,  Prev: Defining Commands,  Up: Command Loop

20.3 Interactive Call
=====================

After the command loop has translated a key sequence into a command, it
invokes that command using the function ‘command-execute’.  If the
command is a function, ‘command-execute’ calls ‘call-interactively’,
which reads the arguments and calls the command.  You can also call
these functions yourself.

   Note that the term “command”, in this context, refers to an
interactively callable function (or function-like object), or a keyboard
macro.  It does not refer to the key sequence used to invoke a command
(*note Keymaps::).

 -- Function: commandp object &optional for-call-interactively
     This function returns ‘t’ if OBJECT is a command.  Otherwise, it
     returns ‘nil’.

     Commands include strings and vectors (which are treated as keyboard
     macros), lambda expressions that contain a top-level ‘interactive’
     form (*note Using Interactive::), byte-code function objects made
     from such lambda expressions, autoload objects that are declared as
     interactive (non-‘nil’ fourth argument to ‘autoload’), and some
     primitive functions.  Also, a symbol is considered a command if it
     has a non-‘nil’ ‘interactive-form’ property, or if its function
     definition satisfies ‘commandp’.

     If FOR-CALL-INTERACTIVELY is non-‘nil’, then ‘commandp’ returns ‘t’
     only for objects that ‘call-interactively’ could call—thus, not for
     keyboard macros.

     See ‘documentation’ in *note Accessing Documentation::, for a
     realistic example of using ‘commandp’.

 -- Function: call-interactively command &optional record-flag keys
     This function calls the interactively callable function COMMAND,
     providing arguments according to its interactive calling
     specifications.  It returns whatever COMMAND returns.

     If, for instance, you have a function with the following signature:

          (defun foo (begin end)
            (interactive "r")
            ...)

     then saying

          (call-interactively 'foo)

     will call ‘foo’ with the region (‘point’ and ‘mark’) as the
     arguments.

     An error is signaled if COMMAND is not a function or if it cannot
     be called interactively (i.e., is not a command).  Note that
     keyboard macros (strings and vectors) are not accepted, even though
     they are considered commands, because they are not functions.  If
     COMMAND is a symbol, then ‘call-interactively’ uses its function
     definition.

     If RECORD-FLAG is non-‘nil’, then this command and its arguments
     are unconditionally added to the list ‘command-history’.
     Otherwise, the command is added only if it uses the minibuffer to
     read an argument.  *Note Command History::.

     The argument KEYS, if given, should be a vector which specifies the
     sequence of events to supply if the command inquires which events
     were used to invoke it.  If KEYS is omitted or ‘nil’, the default
     is the return value of ‘this-command-keys-vector’.  *Note
     Definition of this-command-keys-vector::.

 -- Function: funcall-interactively function &rest arguments
     This function works like ‘funcall’ (*note Calling Functions::), but
     it makes the call look like an interactive invocation: a call to
     ‘called-interactively-p’ inside FUNCTION will return ‘t’.  If
     FUNCTION is not a command, it is called without signaling an error.

 -- Function: command-execute command &optional record-flag keys special
     This function executes COMMAND.  The argument COMMAND must satisfy
     the ‘commandp’ predicate; i.e., it must be an interactively
     callable function or a keyboard macro.

     A string or vector as COMMAND is executed with ‘execute-kbd-macro’.
     A function is passed to ‘call-interactively’ (see above), along
     with the RECORD-FLAG and KEYS arguments.

     If COMMAND is a symbol, its function definition is used in its
     place.  A symbol with an ‘autoload’ definition counts as a command
     if it was declared to stand for an interactively callable function.
     Such a definition is handled by loading the specified library and
     then rechecking the definition of the symbol.

     The argument SPECIAL, if given, means to ignore the prefix argument
     and not clear it.  This is used for executing special events (*note
     Special Events::).

 -- Command: execute-extended-command prefix-argument
     This function reads a command name from the minibuffer using
     ‘completing-read’ (*note Completion::).  Then it uses
     ‘command-execute’ to call the specified command.  Whatever that
     command returns becomes the value of ‘execute-extended-command’.

     If the command asks for a prefix argument, it receives the value
     PREFIX-ARGUMENT.  If ‘execute-extended-command’ is called
     interactively, the current raw prefix argument is used for
     PREFIX-ARGUMENT, and thus passed on to whatever command is run.

     ‘execute-extended-command’ is the normal definition of ‘M-x’, so it
     uses the string ‘M-x ’ as a prompt.  (It would be better to take
     the prompt from the events used to invoke
     ‘execute-extended-command’, but that is painful to implement.)  A
     description of the value of the prefix argument, if any, also
     becomes part of the prompt.

          (execute-extended-command 3)
          ---------- Buffer: Minibuffer ----------
          3 M-x forward-word RET
          ---------- Buffer: Minibuffer ----------
               ⇒ t


File: elisp.info,  Node: Distinguish Interactive,  Next: Command Loop Info,  Prev: Interactive Call,  Up: Command Loop

20.4 Distinguish Interactive Calls
==================================

Sometimes a command should display additional visual feedback (such as
an informative message in the echo area) for interactive calls only.
There are three ways to do this.  The recommended way to test whether
the function was called using ‘call-interactively’ is to give it an
optional argument ‘print-message’ and use the ‘interactive’ spec to make
it non-‘nil’ in interactive calls.  Here’s an example:

     (defun foo (&optional print-message)
       (interactive "p")
       (when print-message
         (message "foo")))

We use ‘"p"’ because the numeric prefix argument is never ‘nil’.
Defined in this way, the function does display the message when called
from a keyboard macro.

   The above method with the additional argument is usually best,
because it allows callers to say “treat this call as interactive”.  But
you can also do the job by testing ‘called-interactively-p’.

 -- Function: called-interactively-p kind
     This function returns ‘t’ when the calling function was called
     using ‘call-interactively’.

     The argument KIND should be either the symbol ‘interactive’ or the
     symbol ‘any’.  If it is ‘interactive’, then
     ‘called-interactively-p’ returns ‘t’ only if the call was made
     directly by the user—e.g., if the user typed a key sequence bound
     to the calling function, but _not_ if the user ran a keyboard macro
     that called the function (*note Keyboard Macros::).  If KIND is
     ‘any’, ‘called-interactively-p’ returns ‘t’ for any kind of
     interactive call, including keyboard macros.

     If in doubt, use ‘any’; the only known proper use of ‘interactive’
     is if you need to decide whether to display a helpful message while
     a function is running.

     A function is never considered to be called interactively if it was
     called via Lisp evaluation (or with ‘apply’ or ‘funcall’).

Here is an example of using ‘called-interactively-p’:

     (defun foo ()
       (interactive)
       (when (called-interactively-p 'any)
         (message "Interactive!")
         'foo-called-interactively))

     ;; Type ‘M-x foo’.
          ⊣ Interactive!

     (foo)
          ⇒ nil

Here is another example that contrasts direct and indirect calls to
‘called-interactively-p’.

     (defun bar ()
       (interactive)
       (message "%s" (list (foo) (called-interactively-p 'any))))

     ;; Type ‘M-x bar’.
          ⊣ (nil t)


File: elisp.info,  Node: Command Loop Info,  Next: Adjusting Point,  Prev: Distinguish Interactive,  Up: Command Loop

20.5 Information from the Command Loop
======================================

The editor command loop sets several Lisp variables to keep status
records for itself and for commands that are run.  With the exception of
‘this-command’ and ‘last-command’ it’s generally a bad idea to change
any of these variables in a Lisp program.

 -- Variable: last-command
     This variable records the name of the previous command executed by
     the command loop (the one before the current command).  Normally
     the value is a symbol with a function definition, but this is not
     guaranteed.

     The value is copied from ‘this-command’ when a command returns to
     the command loop, except when the command has specified a prefix
     argument for the following command.

     This variable is always local to the current terminal and cannot be
     buffer-local.  *Note Multiple Terminals::.

 -- Variable: real-last-command
     This variable is set up by Emacs just like ‘last-command’, but
     never altered by Lisp programs.

 -- Variable: last-repeatable-command
     This variable stores the most recently executed command that was
     not part of an input event.  This is the command ‘repeat’ will try
     to repeat, *Note (emacs)Repeating::.

 -- Variable: this-command
     This variable records the name of the command now being executed by
     the editor command loop.  Like ‘last-command’, it is normally a
     symbol with a function definition.

     The command loop sets this variable just before running a command,
     and copies its value into ‘last-command’ when the command finishes
     (unless the command specified a prefix argument for the following
     command).

     Some commands set this variable during their execution, as a flag
     for whatever command runs next.  In particular, the functions for
     killing text set ‘this-command’ to ‘kill-region’ so that any kill
     commands immediately following will know to append the killed text
     to the previous kill.

   If you do not want a particular command to be recognized as the
previous command in the case where it got an error, you must code that
command to prevent this.  One way is to set ‘this-command’ to ‘t’ at the
beginning of the command, and set ‘this-command’ back to its proper
value at the end, like this:

     (defun foo (args...)
       (interactive ...)
       (let ((old-this-command this-command))
         (setq this-command t)
         ...do the work...
         (setq this-command old-this-command)))

We do not bind ‘this-command’ with ‘let’ because that would restore the
old value in case of error—a feature of ‘let’ which in this case does
precisely what we want to avoid.

 -- Variable: this-original-command
     This has the same value as ‘this-command’ except when command
     remapping occurs (*note Remapping Commands::).  In that case,
     ‘this-command’ gives the command actually run (the result of
     remapping), and ‘this-original-command’ gives the command that was
     specified to run but remapped into another command.

 -- Function: this-command-keys
     This function returns a string or vector containing the key
     sequence that invoked the present command, plus any previous
     commands that generated the prefix argument for this command.  Any
     events read by the command using ‘read-event’ without a timeout get
     tacked on to the end.

     However, if the command has called ‘read-key-sequence’, it returns
     the last read key sequence.  *Note Key Sequence Input::.  The value
     is a string if all events in the sequence were characters that fit
     in a string.  *Note Input Events::.

          (this-command-keys)
          ;; Now use ‘C-u C-x C-e’ to evaluate that.
               ⇒ "^U^X^E"

 -- Function: this-command-keys-vector
     Like ‘this-command-keys’, except that it always returns the events
     in a vector, so you don’t need to deal with the complexities of
     storing input events in a string (*note Strings of Events::).

 -- Function: clear-this-command-keys &optional keep-record
     This function empties out the table of events for
     ‘this-command-keys’ to return.  Unless KEEP-RECORD is non-‘nil’, it
     also empties the records that the function ‘recent-keys’ (*note
     Recording Input::) will subsequently return.  This is useful after
     reading a password, to prevent the password from echoing
     inadvertently as part of the next command in certain cases.

 -- Variable: last-nonmenu-event
     This variable holds the last input event read as part of a key
     sequence, not counting events resulting from mouse menus.

     One use of this variable is for telling ‘x-popup-menu’ where to pop
     up a menu.  It is also used internally by ‘y-or-n-p’ (*note
     Yes-or-No Queries::).

 -- Variable: last-command-event
     This variable is set to the last input event that was read by the
     command loop as part of a command.  The principal use of this
     variable is in ‘self-insert-command’, which uses it to decide which
     character to insert.

          last-command-event
          ;; Now use ‘C-u C-x C-e’ to evaluate that.
               ⇒ 5

     The value is 5 because that is the ASCII code for ‘C-e’.

 -- Variable: last-event-frame
     This variable records which frame the last input event was directed
     to.  Usually this is the frame that was selected when the event was
     generated, but if that frame has redirected input focus to another
     frame, the value is the frame to which the event was redirected.
     *Note Input Focus::.

     If the last event came from a keyboard macro, the value is ‘macro’.


File: elisp.info,  Node: Adjusting Point,  Next: Input Events,  Prev: Command Loop Info,  Up: Command Loop

20.6 Adjusting Point After Commands
===================================

It is not easy to display a value of point in the middle of a sequence
of text that has the ‘display’, ‘composition’ or is invisible.
Therefore, after a command finishes and returns to the command loop, if
point is within such a sequence, the command loop normally moves point
to the edge of the sequence.

   A command can inhibit this feature by setting the variable
‘disable-point-adjustment’:

 -- Variable: disable-point-adjustment
     If this variable is non-‘nil’ when a command returns to the command
     loop, then the command loop does not check for those text
     properties, and does not move point out of sequences that have
     them.

     The command loop sets this variable to ‘nil’ before each command,
     so if a command sets it, the effect applies only to that command.

 -- Variable: global-disable-point-adjustment
     If you set this variable to a non-‘nil’ value, the feature of
     moving point out of these sequences is completely turned off.


File: elisp.info,  Node: Input Events,  Next: Reading Input,  Prev: Adjusting Point,  Up: Command Loop

20.7 Input Events
=================

The Emacs command loop reads a sequence of “input events” that represent
keyboard or mouse activity, or system events sent to Emacs.  The events
for keyboard activity are characters or symbols; other events are always
lists.  This section describes the representation and meaning of input
events in detail.

 -- Function: eventp object
     This function returns non-‘nil’ if OBJECT is an input event or
     event type.

     Note that any symbol might be used as an event or an event type.
     ‘eventp’ cannot distinguish whether a symbol is intended by Lisp
     code to be used as an event.  Instead, it distinguishes whether the
     symbol has actually been used in an event that has been read as
     input in the current Emacs session.  If a symbol has not yet been
     so used, ‘eventp’ returns ‘nil’.

* Menu:

* Keyboard Events::             Ordinary characters – keys with symbols on them.
* Function Keys::               Function keys – keys with names, not symbols.
* Mouse Events::                Overview of mouse events.
* Click Events::                Pushing and releasing a mouse button.
* Drag Events::                 Moving the mouse before releasing the button.
* Button-Down Events::          A button was pushed and not yet released.
* Repeat Events::               Double and triple click (or drag, or down).
* Motion Events::               Just moving the mouse, not pushing a button.
* Focus Events::                Moving the mouse between frames.
* Misc Events::                 Other events the system can generate.
* Event Examples::              Examples of the lists for mouse events.
* Classifying Events::          Finding the modifier keys in an event symbol.
                                Event types.
* Accessing Mouse::             Functions to extract info from mouse events.
* Accessing Scroll::            Functions to get info from scroll bar events.
* Strings of Events::           Special considerations for putting
                                  keyboard character events in a string.


File: elisp.info,  Node: Keyboard Events,  Next: Function Keys,  Up: Input Events

20.7.1 Keyboard Events
----------------------

There are two kinds of input you can get from the keyboard: ordinary
keys, and function keys.  Ordinary keys correspond to characters; the
events they generate are represented in Lisp as characters.  The event
type of a character event is the character itself (an integer); see
*note Classifying Events::.

   An input character event consists of a “basic code” between 0 and
524287, plus any or all of these “modifier bits”:

meta
     The 2**27 bit in the character code indicates a character typed
     with the meta key held down.

control
     The 2**26 bit in the character code indicates a non-ASCII control
     character.

     ASCII control characters such as ‘C-a’ have special basic codes of
     their own, so Emacs needs no special bit to indicate them.  Thus,
     the code for ‘C-a’ is just 1.

     But if you type a control combination not in ASCII, such as ‘%’
     with the control key, the numeric value you get is the code for ‘%’
     plus 2**26 (assuming the terminal supports non-ASCII control
     characters).

shift
     The 2**25 bit in the character code indicates an ASCII control
     character typed with the shift key held down.

     For letters, the basic code itself indicates upper versus lower
     case; for digits and punctuation, the shift key selects an entirely
     different character with a different basic code.  In order to keep
     within the ASCII character set whenever possible, Emacs avoids
     using the 2**25 bit for those characters.

     However, ASCII provides no way to distinguish ‘C-A’ from ‘C-a’, so
     Emacs uses the 2**25 bit in ‘C-A’ and not in ‘C-a’.

hyper
     The 2**24 bit in the character code indicates a character typed
     with the hyper key held down.

super
     The 2**23 bit in the character code indicates a character typed
     with the super key held down.

alt
     The 2**22 bit in the character code indicates a character typed
     with the alt key held down.  (The key labeled <Alt> on most
     keyboards is actually treated as the meta key, not this.)

   It is best to avoid mentioning specific bit numbers in your program.
To test the modifier bits of a character, use the function
‘event-modifiers’ (*note Classifying Events::).  When making key
bindings, you can use the read syntax for characters with modifier bits
(‘\C-’, ‘\M-’, and so on).  For making key bindings with ‘define-key’,
you can use lists such as ‘(control hyper ?x)’ to specify the characters
(*note Changing Key Bindings::).  The function ‘event-convert-list’
converts such a list into an event type (*note Classifying Events::).


File: elisp.info,  Node: Function Keys,  Next: Mouse Events,  Prev: Keyboard Events,  Up: Input Events

20.7.2 Function Keys
--------------------

Most keyboards also have “function keys”—keys that have names or symbols
that are not characters.  Function keys are represented in Emacs Lisp as
symbols; the symbol’s name is the function key’s label, in lower case.
For example, pressing a key labeled <F1> generates an input event
represented by the symbol ‘f1’.

   The event type of a function key event is the event symbol itself.
*Note Classifying Events::.

   Here are a few special cases in the symbol-naming convention for
function keys:

‘backspace’, ‘tab’, ‘newline’, ‘return’, ‘delete’
     These keys correspond to common ASCII control characters that have
     special keys on most keyboards.

     In ASCII, ‘C-i’ and <TAB> are the same character.  If the terminal
     can distinguish between them, Emacs conveys the distinction to Lisp
     programs by representing the former as the integer 9, and the
     latter as the symbol ‘tab’.

     Most of the time, it’s not useful to distinguish the two.  So
     normally ‘local-function-key-map’ (*note Translation Keymaps::) is
     set up to map ‘tab’ into 9.  Thus, a key binding for character code
     9 (the character ‘C-i’) also applies to ‘tab’.  Likewise for the
     other symbols in this group.  The function ‘read-char’ likewise
     converts these events into characters.

     In ASCII, <BS> is really ‘C-h’.  But ‘backspace’ converts into the
     character code 127 (<DEL>), not into code 8 (<BS>).  This is what
     most users prefer.

‘left’, ‘up’, ‘right’, ‘down’
     Cursor arrow keys
‘kp-add’, ‘kp-decimal’, ‘kp-divide’, ...
     Keypad keys (to the right of the regular keyboard).
‘kp-0’, ‘kp-1’, ...
     Keypad keys with digits.
‘kp-f1’, ‘kp-f2’, ‘kp-f3’, ‘kp-f4’
     Keypad PF keys.
‘kp-home’, ‘kp-left’, ‘kp-up’, ‘kp-right’, ‘kp-down’
     Keypad arrow keys.  Emacs normally translates these into the
     corresponding non-keypad keys ‘home’, ‘left’, ...
‘kp-prior’, ‘kp-next’, ‘kp-end’, ‘kp-begin’, ‘kp-insert’, ‘kp-delete’
     Additional keypad duplicates of keys ordinarily found elsewhere.
     Emacs normally translates these into the like-named non-keypad
     keys.

   You can use the modifier keys <ALT>, <CTRL>, <HYPER>, <META>,
<SHIFT>, and <SUPER> with function keys.  The way to represent them is
with prefixes in the symbol name:

‘A-’
     The alt modifier.
‘C-’
     The control modifier.
‘H-’
     The hyper modifier.
‘M-’
     The meta modifier.
‘S-’
     The shift modifier.
‘s-’
     The super modifier.

   Thus, the symbol for the key <F3> with <META> held down is ‘M-f3’.
When you use more than one prefix, we recommend you write them in
alphabetical order; but the order does not matter in arguments to the
key-binding lookup and modification functions.


File: elisp.info,  Node: Mouse Events,  Next: Click Events,  Prev: Function Keys,  Up: Input Events

20.7.3 Mouse Events
-------------------

Emacs supports four kinds of mouse events: click events, drag events,
button-down events, and motion events.  All mouse events are represented
as lists.  The CAR of the list is the event type; this says which mouse
button was involved, and which modifier keys were used with it.  The
event type can also distinguish double or triple button presses (*note
Repeat Events::).  The rest of the list elements give position and time
information.

   For key lookup, only the event type matters: two events of the same
type necessarily run the same command.  The command can access the full
values of these events using the ‘e’ interactive code.  *Note
Interactive Codes::.

   A key sequence that starts with a mouse event is read using the
keymaps of the buffer in the window that the mouse was in, not the
current buffer.  This does not imply that clicking in a window selects
that window or its buffer—that is entirely under the control of the
command binding of the key sequence.


File: elisp.info,  Node: Click Events,  Next: Drag Events,  Prev: Mouse Events,  Up: Input Events

20.7.4 Click Events
-------------------

When the user presses a mouse button and releases it at the same
location, that generates a “click” event.  All mouse click event share
the same format:

     (EVENT-TYPE POSITION CLICK-COUNT)

EVENT-TYPE
     This is a symbol that indicates which mouse button was used.  It is
     one of the symbols ‘mouse-1’, ‘mouse-2’, ..., where the buttons are
     numbered left to right.

     You can also use prefixes ‘A-’, ‘C-’, ‘H-’, ‘M-’, ‘S-’ and ‘s-’ for
     modifiers alt, control, hyper, meta, shift and super, just as you
     would with function keys.

     This symbol also serves as the event type of the event.  Key
     bindings describe events by their types; thus, if there is a key
     binding for ‘mouse-1’, that binding would apply to all events whose
     EVENT-TYPE is ‘mouse-1’.

POSITION
     This is a “mouse position list” specifying where the mouse click
     occurred; see below for details.

CLICK-COUNT
     This is the number of rapid repeated presses so far of the same
     mouse button.  *Note Repeat Events::.

   To access the contents of a mouse position list in the POSITION slot
of a click event, you should typically use the functions documented in
*note Accessing Mouse::.  The explicit format of the list depends on
where the click occurred.  For clicks in the text area, mode line,
header line, or in the fringe or marginal areas, the mouse position list
has the form

     (WINDOW POS-OR-AREA (X . Y) TIMESTAMP
      OBJECT TEXT-POS (COL . ROW)
      IMAGE (DX . DY) (WIDTH . HEIGHT))

The meanings of these list elements are as follows:

WINDOW
     The window in which the click occurred.

POS-OR-AREA
     The buffer position of the character clicked on in the text area;
     or, if the click was outside the text area, the window area where
     it occurred.  It is one of the symbols ‘mode-line’, ‘header-line’,
     ‘vertical-line’, ‘left-margin’, ‘right-margin’, ‘left-fringe’, or
     ‘right-fringe’.

     In one special case, POS-OR-AREA is a list containing a symbol (one
     of the symbols listed above) instead of just the symbol.  This
     happens after the imaginary prefix keys for the event are
     registered by Emacs.  *Note Key Sequence Input::.

X, Y
     The relative pixel coordinates of the click.  For clicks in the
     text area of a window, the coordinate origin ‘(0 . 0)’ is taken to
     be the top left corner of the text area.  *Note Window Sizes::.
     For clicks in a mode line or header line, the coordinate origin is
     the top left corner of the window itself.  For fringes, margins,
     and the vertical border, X does not have meaningful data.  For
     fringes and margins, Y is relative to the bottom edge of the header
     line.  In all cases, the X and Y coordinates increase rightward and
     downward respectively.

TIMESTAMP
     The time at which the event occurred, as an integer number of
     milliseconds since a system-dependent initial time.

OBJECT
     Either ‘nil’ if there is no string-type text property at the click
     position, or a cons cell of the form (STRING .  STRING-POS) if
     there is one:

     STRING
          The string which was clicked on, including any properties.

     STRING-POS
          The position in the string where the click occurred.

TEXT-POS
     For clicks on a marginal area or on a fringe, this is the buffer
     position of the first visible character in the corresponding line
     in the window.  For clicks on the mode line or the header line,
     this is ‘nil’.  For other events, it is the buffer position closest
     to the click.

COL, ROW
     These are the actual column and row coordinate numbers of the glyph
     under the X, Y position.  If X lies beyond the last column of
     actual text on its line, COL is reported by adding fictional extra
     columns that have the default character width.  Row 0 is taken to
     be the header line if the window has one, or the topmost row of the
     text area otherwise.  Column 0 is taken to be the leftmost column
     of the text area for clicks on a window text area, or the leftmost
     mode line or header line column for clicks there.  For clicks on
     fringes or vertical borders, these have no meaningful data.  For
     clicks on margins, COL is measured from the left edge of the margin
     area and ROW is measured from the top of the margin area.

IMAGE
     This is the image object on which the click occurred.  It is either
     ‘nil’ if there is no image at the position clicked on, or it is an
     image object as returned by ‘find-image’ if click was in an image.

DX, DY
     These are the pixel coordinates of the click, relative to the top
     left corner of OBJECT, which is ‘(0 . 0)’.  If OBJECT is ‘nil’, the
     coordinates are relative to the top left corner of the character
     glyph clicked on.

WIDTH, HEIGHT
     These are the pixel width and height of OBJECT or, if this is
     ‘nil’, those of the character glyph clicked on.

   For clicks on a scroll bar, POSITION has this form:

     (WINDOW AREA (PORTION . WHOLE) TIMESTAMP PART)

WINDOW
     The window whose scroll bar was clicked on.

AREA
     This is the symbol ‘vertical-scroll-bar’.

PORTION
     The number of pixels from the top of the scroll bar to the click
     position.  On some toolkits, including GTK+, Emacs cannot extract
     this data, so the value is always ‘0’.

WHOLE
     The total length, in pixels, of the scroll bar.  On some toolkits,
     including GTK+, Emacs cannot extract this data, so the value is
     always ‘0’.

TIMESTAMP
     The time at which the event occurred, in milliseconds.  On some
     toolkits, including GTK+, Emacs cannot extract this data, so the
     value is always ‘0’.

PART
     The part of the scroll bar on which the click occurred.  It is one
     of the symbols ‘handle’ (the scroll bar handle), ‘above-handle’
     (the area above the handle), ‘below-handle’ (the area below the
     handle), ‘up’ (the up arrow at one end of the scroll bar), or
     ‘down’ (the down arrow at one end of the scroll bar).


File: elisp.info,  Node: Drag Events,  Next: Button-Down Events,  Prev: Click Events,  Up: Input Events

20.7.5 Drag Events
------------------

With Emacs, you can have a drag event without even changing your
clothes.  A “drag event” happens every time the user presses a mouse
button and then moves the mouse to a different character position before
releasing the button.  Like all mouse events, drag events are
represented in Lisp as lists.  The lists record both the starting mouse
position and the final position, like this:

     (EVENT-TYPE
      (WINDOW1 START-POSITION)
      (WINDOW2 END-POSITION))

   For a drag event, the name of the symbol EVENT-TYPE contains the
prefix ‘drag-’.  For example, dragging the mouse with button 2 held down
generates a ‘drag-mouse-2’ event.  The second and third elements of the
event give the starting and ending position of the drag, as mouse
position lists (*note Click Events::).  You can access the second
element of any mouse event in the same way.  However, the drag event may
end outside the boundaries of the frame that was initially selected.  In
that case, the third element’s position list contains that frame in
place of a window.

   The ‘drag-’ prefix follows the modifier key prefixes such as ‘C-’ and
‘M-’.

   If ‘read-key-sequence’ receives a drag event that has no key binding,
and the corresponding click event does have a binding, it changes the
drag event into a click event at the drag’s starting position.  This
means that you don’t have to distinguish between click and drag events
unless you want to.


File: elisp.info,  Node: Button-Down Events,  Next: Repeat Events,  Prev: Drag Events,  Up: Input Events

20.7.6 Button-Down Events
-------------------------

Click and drag events happen when the user releases a mouse button.
They cannot happen earlier, because there is no way to distinguish a
click from a drag until the button is released.

   If you want to take action as soon as a button is pressed, you need
to handle “button-down” events.(1)  These occur as soon as a button is
pressed.  They are represented by lists that look exactly like click
events (*note Click Events::), except that the EVENT-TYPE symbol name
contains the prefix ‘down-’.  The ‘down-’ prefix follows modifier key
prefixes such as ‘C-’ and ‘M-’.

   The function ‘read-key-sequence’ ignores any button-down events that
don’t have command bindings; therefore, the Emacs command loop ignores
them too.  This means that you need not worry about defining button-down
events unless you want them to do something.  The usual reason to define
a button-down event is so that you can track mouse motion (by reading
motion events) until the button is released.  *Note Motion Events::.

   ---------- Footnotes ----------

   (1) Button-down is the conservative antithesis of drag.


File: elisp.info,  Node: Repeat Events,  Next: Motion Events,  Prev: Button-Down Events,  Up: Input Events

20.7.7 Repeat Events
--------------------

If you press the same mouse button more than once in quick succession
without moving the mouse, Emacs generates special “repeat” mouse events
for the second and subsequent presses.

   The most common repeat events are “double-click” events.  Emacs
generates a double-click event when you click a button twice; the event
happens when you release the button (as is normal for all click events).

   The event type of a double-click event contains the prefix ‘double-’.
Thus, a double click on the second mouse button with <meta> held down
comes to the Lisp program as ‘M-double-mouse-2’.  If a double-click
event has no binding, the binding of the corresponding ordinary click
event is used to execute it.  Thus, you need not pay attention to the
double click feature unless you really want to.

   When the user performs a double click, Emacs generates first an
ordinary click event, and then a double-click event.  Therefore, you
must design the command binding of the double click event to assume that
the single-click command has already run.  It must produce the desired
results of a double click, starting from the results of a single click.

   This is convenient, if the meaning of a double click somehow builds
on the meaning of a single click—which is recommended user interface
design practice for double clicks.

   If you click a button, then press it down again and start moving the
mouse with the button held down, then you get a “double-drag” event when
you ultimately release the button.  Its event type contains
‘double-drag’ instead of just ‘drag’.  If a double-drag event has no
binding, Emacs looks for an alternate binding as if the event were an
ordinary drag.

   Before the double-click or double-drag event, Emacs generates a
“double-down” event when the user presses the button down for the second
time.  Its event type contains ‘double-down’ instead of just ‘down’.  If
a double-down event has no binding, Emacs looks for an alternate binding
as if the event were an ordinary button-down event.  If it finds no
binding that way either, the double-down event is ignored.

   To summarize, when you click a button and then press it again right
away, Emacs generates a down event and a click event for the first
click, a double-down event when you press the button again, and finally
either a double-click or a double-drag event.

   If you click a button twice and then press it again, all in quick
succession, Emacs generates a “triple-down” event, followed by either a
“triple-click” or a “triple-drag”.  The event types of these events
contain ‘triple’ instead of ‘double’.  If any triple event has no
binding, Emacs uses the binding that it would use for the corresponding
double event.

   If you click a button three or more times and then press it again,
the events for the presses beyond the third are all triple events.
Emacs does not have separate event types for quadruple, quintuple, etc.
events.  However, you can look at the event list to find out precisely
how many times the button was pressed.

 -- Function: event-click-count event
     This function returns the number of consecutive button presses that
     led up to EVENT.  If EVENT is a double-down, double-click or
     double-drag event, the value is 2.  If EVENT is a triple event, the
     value is 3 or greater.  If EVENT is an ordinary mouse event (not a
     repeat event), the value is 1.

 -- User Option: double-click-fuzz
     To generate repeat events, successive mouse button presses must be
     at approximately the same screen position.  The value of
     ‘double-click-fuzz’ specifies the maximum number of pixels the
     mouse may be moved (horizontally or vertically) between two
     successive clicks to make a double-click.

     This variable is also the threshold for motion of the mouse to
     count as a drag.

 -- User Option: double-click-time
     To generate repeat events, the number of milliseconds between
     successive button presses must be less than the value of
     ‘double-click-time’.  Setting ‘double-click-time’ to ‘nil’ disables
     multi-click detection entirely.  Setting it to ‘t’ removes the time
     limit; Emacs then detects multi-clicks by position only.


File: elisp.info,  Node: Motion Events,  Next: Focus Events,  Prev: Repeat Events,  Up: Input Events

20.7.8 Motion Events
--------------------

Emacs sometimes generates “mouse motion” events to describe motion of
the mouse without any button activity.  Mouse motion events are
represented by lists that look like this:

     (mouse-movement POSITION)

POSITION is a mouse position list (*note Click Events::), specifying the
current position of the mouse cursor.  As with the end-position of a
drag event, this position list may represent a location outside the
boundaries of the initially selected frame, in which case the list
contains that frame in place of a window.

   The special form ‘track-mouse’ enables generation of motion events
within its body.  Outside of ‘track-mouse’ forms, Emacs does not
generate events for mere motion of the mouse, and these events do not
appear.  *Note Mouse Tracking::.


File: elisp.info,  Node: Focus Events,  Next: Misc Events,  Prev: Motion Events,  Up: Input Events

20.7.9 Focus Events
-------------------

Window systems provide general ways for the user to control which window
gets keyboard input.  This choice of window is called the “focus”.  When
the user does something to switch between Emacs frames, that generates a
“focus event”.  The normal definition of a focus event, in the global
keymap, is to select a new frame within Emacs, as the user would expect.
*Note Input Focus::, which also describes hooks related to focus events.

   Focus events are represented in Lisp as lists that look like this:

     (switch-frame NEW-FRAME)

where NEW-FRAME is the frame switched to.

   Some X window managers are set up so that just moving the mouse into
a window is enough to set the focus there.  Usually, there is no need
for a Lisp program to know about the focus change until some other kind
of input arrives.  Emacs generates a focus event only when the user
actually types a keyboard key or presses a mouse button in the new
frame; just moving the mouse between frames does not generate a focus
event.

   A focus event in the middle of a key sequence would garble the
sequence.  So Emacs never generates a focus event in the middle of a key
sequence.  If the user changes focus in the middle of a key
sequence—that is, after a prefix key—then Emacs reorders the events so
that the focus event comes either before or after the multi-event key
sequence, and not within it.


File: elisp.info,  Node: Misc Events,  Next: Event Examples,  Prev: Focus Events,  Up: Input Events

20.7.10 Miscellaneous System Events
-----------------------------------

A few other event types represent occurrences within the system.

‘(delete-frame (FRAME))’
     This kind of event indicates that the user gave the window manager
     a command to delete a particular window, which happens to be an
     Emacs frame.

     The standard definition of the ‘delete-frame’ event is to delete
     FRAME.

‘(iconify-frame (FRAME))’
     This kind of event indicates that the user iconified FRAME using
     the window manager.  Its standard definition is ‘ignore’; since the
     frame has already been iconified, Emacs has no work to do.  The
     purpose of this event type is so that you can keep track of such
     events if you want to.

‘(make-frame-visible (FRAME))’
     This kind of event indicates that the user deiconified FRAME using
     the window manager.  Its standard definition is ‘ignore’; since the
     frame has already been made visible, Emacs has no work to do.

‘(wheel-up POSITION)’
‘(wheel-down POSITION)’
     These kinds of event are generated by moving a mouse wheel.  The
     POSITION element is a mouse position list (*note Click Events::),
     specifying the position of the mouse cursor when the event
     occurred.

     This kind of event is generated only on some kinds of systems.  On
     some systems, ‘mouse-4’ and ‘mouse-5’ are used instead.  For
     portable code, use the variables ‘mouse-wheel-up-event’ and
     ‘mouse-wheel-down-event’ defined in ‘mwheel.el’ to determine what
     event types to expect for the mouse wheel.

‘(drag-n-drop POSITION FILES)’
     This kind of event is generated when a group of files is selected
     in an application outside of Emacs, and then dragged and dropped
     onto an Emacs frame.

     The element POSITION is a list describing the position of the
     event, in the same format as used in a mouse-click event (*note
     Click Events::), and FILES is the list of file names that were
     dragged and dropped.  The usual way to handle this event is by
     visiting these files.

     This kind of event is generated, at present, only on some kinds of
     systems.

‘help-echo’
     This kind of event is generated when a mouse pointer moves onto a
     portion of buffer text which has a ‘help-echo’ text property.  The
     generated event has this form:

          (help-echo FRAME HELP WINDOW OBJECT POS)

     The precise meaning of the event parameters and the way these
     parameters are used to display the help-echo text are described in
     *note Text help-echo::.

‘sigusr1’
‘sigusr2’
     These events are generated when the Emacs process receives the
     signals ‘SIGUSR1’ and ‘SIGUSR2’.  They contain no additional data
     because signals do not carry additional information.  They can be
     useful for debugging (*note Error Debugging::).

     To catch a user signal, bind the corresponding event to an
     interactive command in the ‘special-event-map’ (*note Active
     Keymaps::).  The command is called with no arguments, and the
     specific signal event is available in ‘last-input-event’.  For
     example:

          (defun sigusr-handler ()
            (interactive)
            (message "Caught signal %S" last-input-event))

          (define-key special-event-map [sigusr1] 'sigusr-handler)

     To test the signal handler, you can make Emacs send a signal to
     itself:

          (signal-process (emacs-pid) 'sigusr1)

‘language-change’
     This kind of event is generated on MS-Windows when the input
     language has changed.  This typically means that the keyboard keys
     will send to Emacs characters from a different language.  The
     generated event has this form:

          (language-change FRAME CODEPAGE LANGUAGE-ID)

     Here FRAME is the frame which was current when the input language
     changed; CODEPAGE is the new codepage number; and LANGUAGE-ID is
     the numerical ID of the new input language.  The coding-system
     (*note Coding Systems::) that corresponds to CODEPAGE is
     ‘cpCODEPAGE’ or ‘windows-CODEPAGE’.  To convert LANGUAGE-ID to a
     string (e.g., to use it for various language-dependent features,
     such as ‘set-language-environment’), use the ‘w32-get-locale-info’
     function, like this:

          ;; Get the abbreviated language name, such as "ENU" for English
          (w32-get-locale-info language-id)
          ;; Get the full English name of the language,
          ;; such as "English (United States)"
          (w32-get-locale-info language-id 4097)
          ;; Get the full localized name of the language
          (w32-get-locale-info language-id t)

   If one of these events arrives in the middle of a key sequence—that
is, after a prefix key—then Emacs reorders the events so that this event
comes either before or after the multi-event key sequence, not within
it.


File: elisp.info,  Node: Event Examples,  Next: Classifying Events,  Prev: Misc Events,  Up: Input Events

20.7.11 Event Examples
----------------------

If the user presses and releases the left mouse button over the same
location, that generates a sequence of events like this:

     (down-mouse-1 (#<window 18 on NEWS> 2613 (0 . 38) -864320))
     (mouse-1      (#<window 18 on NEWS> 2613 (0 . 38) -864180))

   While holding the control key down, the user might hold down the
second mouse button, and drag the mouse from one line to the next.  That
produces two events, as shown here:

     (C-down-mouse-2 (#<window 18 on NEWS> 3440 (0 . 27) -731219))
     (C-drag-mouse-2 (#<window 18 on NEWS> 3440 (0 . 27) -731219)
                     (#<window 18 on NEWS> 3510 (0 . 28) -729648))

   While holding down the meta and shift keys, the user might press the
second mouse button on the window’s mode line, and then drag the mouse
into another window.  That produces a pair of events like these:

     (M-S-down-mouse-2 (#<window 18 on NEWS> mode-line (33 . 31) -457844))
     (M-S-drag-mouse-2 (#<window 18 on NEWS> mode-line (33 . 31) -457844)
                       (#<window 20 on carlton-sanskrit.tex> 161 (33 . 3)
                        -453816))

   The frame with input focus might not take up the entire screen, and
the user might move the mouse outside the scope of the frame.  Inside
the ‘track-mouse’ special form, that produces an event like this:

     (mouse-movement (#<frame *ielm* 0x102849a30> nil (563 . 205) 532301936))

   To handle a SIGUSR1 signal, define an interactive function, and bind
it to the ‘signal usr1’ event sequence:

     (defun usr1-handler ()
       (interactive)
       (message "Got USR1 signal"))
     (global-set-key [signal usr1] 'usr1-handler)


File: elisp.info,  Node: Classifying Events,  Next: Accessing Mouse,  Prev: Event Examples,  Up: Input Events

20.7.12 Classifying Events
--------------------------

Every event has an “event type”, which classifies the event for key
binding purposes.  For a keyboard event, the event type equals the event
value; thus, the event type for a character is the character, and the
event type for a function key symbol is the symbol itself.  For events
that are lists, the event type is the symbol in the CAR of the list.
Thus, the event type is always a symbol or a character.

   Two events of the same type are equivalent where key bindings are
concerned; thus, they always run the same command.  That does not
necessarily mean they do the same things, however, as some commands look
at the whole event to decide what to do.  For example, some commands use
the location of a mouse event to decide where in the buffer to act.

   Sometimes broader classifications of events are useful.  For example,
you might want to ask whether an event involved the <META> key,
regardless of which other key or mouse button was used.

   The functions ‘event-modifiers’ and ‘event-basic-type’ are provided
to get such information conveniently.

 -- Function: event-modifiers event
     This function returns a list of the modifiers that EVENT has.  The
     modifiers are symbols; they include ‘shift’, ‘control’, ‘meta’,
     ‘alt’, ‘hyper’ and ‘super’.  In addition, the modifiers list of a
     mouse event symbol always contains one of ‘click’, ‘drag’, and
     ‘down’.  For double or triple events, it also contains ‘double’ or
     ‘triple’.

     The argument EVENT may be an entire event object, or just an event
     type.  If EVENT is a symbol that has never been used in an event
     that has been read as input in the current Emacs session, then
     ‘event-modifiers’ can return ‘nil’, even when EVENT actually has
     modifiers.

     Here are some examples:

          (event-modifiers ?a)
               ⇒ nil
          (event-modifiers ?A)
               ⇒ (shift)
          (event-modifiers ?\C-a)
               ⇒ (control)
          (event-modifiers ?\C-%)
               ⇒ (control)
          (event-modifiers ?\C-\S-a)
               ⇒ (control shift)
          (event-modifiers 'f5)
               ⇒ nil
          (event-modifiers 's-f5)
               ⇒ (super)
          (event-modifiers 'M-S-f5)
               ⇒ (meta shift)
          (event-modifiers 'mouse-1)
               ⇒ (click)
          (event-modifiers 'down-mouse-1)
               ⇒ (down)

     The modifiers list for a click event explicitly contains ‘click’,
     but the event symbol name itself does not contain ‘click’.

 -- Function: event-basic-type event
     This function returns the key or mouse button that EVENT describes,
     with all modifiers removed.  The EVENT argument is as in
     ‘event-modifiers’.  For example:

          (event-basic-type ?a)
               ⇒ 97
          (event-basic-type ?A)
               ⇒ 97
          (event-basic-type ?\C-a)
               ⇒ 97
          (event-basic-type ?\C-\S-a)
               ⇒ 97
          (event-basic-type 'f5)
               ⇒ f5
          (event-basic-type 's-f5)
               ⇒ f5
          (event-basic-type 'M-S-f5)
               ⇒ f5
          (event-basic-type 'down-mouse-1)
               ⇒ mouse-1

 -- Function: mouse-movement-p object
     This function returns non-‘nil’ if OBJECT is a mouse movement
     event.  *Note Motion Events::.

 -- Function: event-convert-list list
     This function converts a list of modifier names and a basic event
     type to an event type which specifies all of them.  The basic event
     type must be the last element of the list.  For example,

          (event-convert-list '(control ?a))
               ⇒ 1
          (event-convert-list '(control meta ?a))
               ⇒ -134217727
          (event-convert-list '(control super f1))
               ⇒ C-s-f1


File: elisp.info,  Node: Accessing Mouse,  Next: Accessing Scroll,  Prev: Classifying Events,  Up: Input Events

20.7.13 Accessing Mouse Events
------------------------------

This section describes convenient functions for accessing the data in a
mouse button or motion event.  Keyboard event data can be accessed using
the same functions, but data elements that aren’t applicable to keyboard
events are zero or ‘nil’.

   The following two functions return a mouse position list (*note Click
Events::), specifying the position of a mouse event.

 -- Function: event-start event
     This returns the starting position of EVENT.

     If EVENT is a click or button-down event, this returns the location
     of the event.  If EVENT is a drag event, this returns the drag’s
     starting position.

 -- Function: event-end event
     This returns the ending position of EVENT.

     If EVENT is a drag event, this returns the position where the user
     released the mouse button.  If EVENT is a click or button-down
     event, the value is actually the starting position, which is the
     only position such events have.

 -- Function: posnp object
     This function returns non-‘nil’ if OBJECT is a mouse position list,
     in either of the formats documented in *note Click Events::); and
     ‘nil’ otherwise.

   These functions take a mouse position list as argument, and return
various parts of it:

 -- Function: posn-window position
     Return the window that POSITION is in.  If POSITION represents a
     location outside the frame where the event was initiated, return
     that frame instead.

 -- Function: posn-area position
     Return the window area recorded in POSITION.  It returns ‘nil’ when
     the event occurred in the text area of the window; otherwise, it is
     a symbol identifying the area in which the event occurred.

 -- Function: posn-point position
     Return the buffer position in POSITION.  When the event occurred in
     the text area of the window, in a marginal area, or on a fringe,
     this is an integer specifying a buffer position.  Otherwise, the
     value is undefined.

 -- Function: posn-x-y position
     Return the pixel-based x and y coordinates in POSITION, as a cons
     cell ‘(X . Y)’.  These coordinates are relative to the window given
     by ‘posn-window’.

     This example shows how to convert the window-relative coordinates
     in the text area of a window into frame-relative coordinates:

          (defun frame-relative-coordinates (position)
            "Return frame-relative coordinates from POSITION.
          POSITION is assumed to lie in a window text area."
            (let* ((x-y (posn-x-y position))
                   (window (posn-window position))
                   (edges (window-inside-pixel-edges window)))
              (cons (+ (car x-y) (car edges))
                    (+ (cdr x-y) (cadr edges)))))

 -- Function: posn-col-row position
     This function returns a cons cell ‘(COL . ROW)’, containing the
     estimated column and row corresponding to buffer position in
     POSITION.  The return value is given in units of the frame’s
     default character width and default line height (including
     spacing), as computed from the X and Y values corresponding to
     POSITION.  (So, if the actual characters have non-default sizes,
     the actual row and column may differ from these computed values.)

     Note that ROW is counted from the top of the text area.  If the
     window given by POSITION possesses a header line (*note Header
     Lines::), it is _not_ included in the ROW count.

 -- Function: posn-actual-col-row position
     Return the actual row and column in POSITION, as a cons cell ‘(COL
     . ROW)’.  The values are the actual row and column numbers in the
     window given by POSITION.  *Note Click Events::, for details.  The
     function returns ‘nil’ if POSITION does not include actual position
     values; in that case ‘posn-col-row’ can be used to get approximate
     values.

     Note that this function doesn’t account for the visual width of
     characters on display, like the number of visual columns taken by a
     tab character or an image.  If you need the coordinates in
     canonical character units, use ‘posn-col-row’ instead.

 -- Function: posn-string position
     Return the string object in POSITION, either ‘nil’, or a cons cell
     ‘(STRING . STRING-POS)’.

 -- Function: posn-image position
     Return the image object in POSITION, either ‘nil’, or an image
     ‘(image ...)’.

 -- Function: posn-object position
     Return the image or string object in POSITION, either ‘nil’, an
     image ‘(image ...)’, or a cons cell ‘(STRING . STRING-POS)’.

 -- Function: posn-object-x-y position
     Return the pixel-based x and y coordinates relative to the upper
     left corner of the object in POSITION as a cons cell ‘(DX . DY)’.
     If the POSITION is on buffer text, return the relative position of
     the buffer-text character closest to that position.

 -- Function: posn-object-width-height position
     Return the pixel width and height of the object in POSITION as a
     cons cell ‘(WIDTH . HEIGHT)’.  If the POSITION is a buffer
     position, return the size of the character at that position.

 -- Function: posn-timestamp position
     Return the timestamp in POSITION.  This is the time at which the
     event occurred, in milliseconds.

   These functions compute a position list given particular buffer
position or screen position.  You can access the data in this position
list with the functions described above.

 -- Function: posn-at-point &optional pos window
     This function returns a position list for position POS in WINDOW.
     POS defaults to point in WINDOW; WINDOW defaults to the selected
     window.

     ‘posn-at-point’ returns ‘nil’ if POS is not visible in WINDOW.

 -- Function: posn-at-x-y x y &optional frame-or-window whole
     This function returns position information corresponding to pixel
     coordinates X and Y in a specified frame or window,
     FRAME-OR-WINDOW, which defaults to the selected window.  The
     coordinates X and Y are relative to the frame or window used.  If
     WHOLE is ‘nil’, the coordinates are relative to the window text
     area, otherwise they are relative to the entire window area
     including scroll bars, margins and fringes.


File: elisp.info,  Node: Accessing Scroll,  Next: Strings of Events,  Prev: Accessing Mouse,  Up: Input Events

20.7.14 Accessing Scroll Bar Events
-----------------------------------

These functions are useful for decoding scroll bar events.

 -- Function: scroll-bar-event-ratio event
     This function returns the fractional vertical position of a scroll
     bar event within the scroll bar.  The value is a cons cell
     ‘(PORTION . WHOLE)’ containing two integers whose ratio is the
     fractional position.

 -- Function: scroll-bar-scale ratio total
     This function multiplies (in effect) RATIO by TOTAL, rounding the
     result to an integer.  The argument RATIO is not a number, but
     rather a pair ‘(NUM . DENOM)’—typically a value returned by
     ‘scroll-bar-event-ratio’.

     This function is handy for scaling a position on a scroll bar into
     a buffer position.  Here’s how to do that:

          (+ (point-min)
             (scroll-bar-scale
                (posn-x-y (event-start event))
                (- (point-max) (point-min))))

     Recall that scroll bar events have two integers forming a ratio, in
     place of a pair of x and y coordinates.


File: elisp.info,  Node: Strings of Events,  Prev: Accessing Scroll,  Up: Input Events

20.7.15 Putting Keyboard Events in Strings
------------------------------------------

In most of the places where strings are used, we conceptualize the
string as containing text characters—the same kind of characters found
in buffers or files.  Occasionally Lisp programs use strings that
conceptually contain keyboard characters; for example, they may be key
sequences or keyboard macro definitions.  However, storing keyboard
characters in a string is a complex matter, for reasons of historical
compatibility, and it is not always possible.

   We recommend that new programs avoid dealing with these complexities
by not storing keyboard events in strings.  Here is how to do that:

   • Use vectors instead of strings for key sequences, when you plan to
     use them for anything other than as arguments to ‘lookup-key’ and
     ‘define-key’.  For example, you can use ‘read-key-sequence-vector’
     instead of ‘read-key-sequence’, and ‘this-command-keys-vector’
     instead of ‘this-command-keys’.

   • Use vectors to write key sequence constants containing meta
     characters, even when passing them directly to ‘define-key’.

   • When you have to look at the contents of a key sequence that might
     be a string, use ‘listify-key-sequence’ (*note Event Input Misc::)
     first, to convert it to a list.

   The complexities stem from the modifier bits that keyboard input
characters can include.  Aside from the Meta modifier, none of these
modifier bits can be included in a string, and the Meta modifier is
allowed only in special cases.

   The earliest GNU Emacs versions represented meta characters as codes
in the range of 128 to 255.  At that time, the basic character codes
ranged from 0 to 127, so all keyboard character codes did fit in a
string.  Many Lisp programs used ‘\M-’ in string constants to stand for
meta characters, especially in arguments to ‘define-key’ and similar
functions, and key sequences and sequences of events were always
represented as strings.

   When we added support for larger basic character codes beyond 127,
and additional modifier bits, we had to change the representation of
meta characters.  Now the flag that represents the Meta modifier in a
character is 2**27 and such numbers cannot be included in a string.

   To support programs with ‘\M-’ in string constants, there are special
rules for including certain meta characters in a string.  Here are the
rules for interpreting a string as a sequence of input characters:

   • If the keyboard character value is in the range of 0 to 127, it can
     go in the string unchanged.

   • The meta variants of those characters, with codes in the range of
     2**27 to 2**27+127, can also go in the string, but you must change
     their numeric values.  You must set the 2**7 bit instead of the
     2**27 bit, resulting in a value between 128 and 255.  Only a
     unibyte string can include these codes.

   • Non-ASCII characters above 256 can be included in a multibyte
     string.

   • Other keyboard character events cannot fit in a string.  This
     includes keyboard events in the range of 128 to 255.

   Functions such as ‘read-key-sequence’ that construct strings of
keyboard input characters follow these rules: they construct vectors
instead of strings, when the events won’t fit in a string.

   When you use the read syntax ‘\M-’ in a string, it produces a code in
the range of 128 to 255—the same code that you get if you modify the
corresponding keyboard event to put it in the string.  Thus, meta events
in strings work consistently regardless of how they get into the
strings.

   However, most programs would do well to avoid these issues by
following the recommendations at the beginning of this section.


File: elisp.info,  Node: Reading Input,  Next: Special Events,  Prev: Input Events,  Up: Command Loop

20.8 Reading Input
==================

The editor command loop reads key sequences using the function
‘read-key-sequence’, which uses ‘read-event’.  These and other functions
for event input are also available for use in Lisp programs.  See also
‘momentary-string-display’ in *note Temporary Displays::, and ‘sit-for’
in *note Waiting::.  *Note Terminal Input::, for functions and variables
for controlling terminal input modes and debugging terminal input.

   For higher-level input facilities, see *note Minibuffers::.

* Menu:

* Key Sequence Input::          How to read one key sequence.
* Reading One Event::           How to read just one event.
* Event Mod::                   How Emacs modifies events as they are read.
* Invoking the Input Method::   How reading an event uses the input method.
* Quoted Character Input::      Asking the user to specify a character.
* Event Input Misc::            How to reread or throw away input events.


File: elisp.info,  Node: Key Sequence Input,  Next: Reading One Event,  Up: Reading Input

20.8.1 Key Sequence Input
-------------------------

The command loop reads input a key sequence at a time, by calling
‘read-key-sequence’.  Lisp programs can also call this function; for
example, ‘describe-key’ uses it to read the key to describe.

 -- Function: read-key-sequence prompt &optional continue-echo
          dont-downcase-last switch-frame-ok command-loop
     This function reads a key sequence and returns it as a string or
     vector.  It keeps reading events until it has accumulated a
     complete key sequence; that is, enough to specify a non-prefix
     command using the currently active keymaps.  (Remember that a key
     sequence that starts with a mouse event is read using the keymaps
     of the buffer in the window that the mouse was in, not the current
     buffer.)

     If the events are all characters and all can fit in a string, then
     ‘read-key-sequence’ returns a string (*note Strings of Events::).
     Otherwise, it returns a vector, since a vector can hold all kinds
     of events—characters, symbols, and lists.  The elements of the
     string or vector are the events in the key sequence.

     Reading a key sequence includes translating the events in various
     ways.  *Note Translation Keymaps::.

     The argument PROMPT is either a string to be displayed in the echo
     area as a prompt, or ‘nil’, meaning not to display a prompt.  The
     argument CONTINUE-ECHO, if non-‘nil’, means to echo this key as a
     continuation of the previous key.

     Normally any upper case event is converted to lower case if the
     original event is undefined and the lower case equivalent is
     defined.  The argument DONT-DOWNCASE-LAST, if non-‘nil’, means do
     not convert the last event to lower case.  This is appropriate for
     reading a key sequence to be defined.

     The argument SWITCH-FRAME-OK, if non-‘nil’, means that this
     function should process a ‘switch-frame’ event if the user switches
     frames before typing anything.  If the user switches frames in the
     middle of a key sequence, or at the start of the sequence but
     SWITCH-FRAME-OK is ‘nil’, then the event will be put off until
     after the current key sequence.

     The argument COMMAND-LOOP, if non-‘nil’, means that this key
     sequence is being read by something that will read commands one
     after another.  It should be ‘nil’ if the caller will read just one
     key sequence.

     In the following example, Emacs displays the prompt ‘?’ in the echo
     area, and then the user types ‘C-x C-f’.

          (read-key-sequence "?")

          ---------- Echo Area ----------
          ?C-x C-f
          ---------- Echo Area ----------

               ⇒ "^X^F"

     The function ‘read-key-sequence’ suppresses quitting: ‘C-g’ typed
     while reading with this function works like any other character,
     and does not set ‘quit-flag’.  *Note Quitting::.

 -- Function: read-key-sequence-vector prompt &optional continue-echo
          dont-downcase-last switch-frame-ok command-loop
     This is like ‘read-key-sequence’ except that it always returns the
     key sequence as a vector, never as a string.  *Note Strings of
     Events::.

   If an input character is upper-case (or has the shift modifier) and
has no key binding, but its lower-case equivalent has one, then
‘read-key-sequence’ converts the character to lower case.  Note that
‘lookup-key’ does not perform case conversion in this way.

   When reading input results in such a “shift-translation”, Emacs sets
the variable ‘this-command-keys-shift-translated’ to a non-‘nil’ value.
Lisp programs can examine this variable if they need to modify their
behavior when invoked by shift-translated keys.  For example, the
function ‘handle-shift-selection’ examines the value of this variable to
determine how to activate or deactivate the region (*note
handle-shift-selection: The Mark.).

   The function ‘read-key-sequence’ also transforms some mouse events.
It converts unbound drag events into click events, and discards unbound
button-down events entirely.  It also reshuffles focus events and
miscellaneous window events so that they never appear in a key sequence
with any other events.

   When mouse events occur in special parts of a window, such as a mode
line or a scroll bar, the event type shows nothing special—it is the
same symbol that would normally represent that combination of mouse
button and modifier keys.  The information about the window part is kept
elsewhere in the event—in the coordinates.  But ‘read-key-sequence’
translates this information into imaginary prefix keys, all of which are
symbols: ‘header-line’, ‘horizontal-scroll-bar’, ‘menu-bar’,
‘mode-line’, ‘vertical-line’, and ‘vertical-scroll-bar’.  You can define
meanings for mouse clicks in special window parts by defining key
sequences using these imaginary prefix keys.

   For example, if you call ‘read-key-sequence’ and then click the mouse
on the window’s mode line, you get two events, like this:

     (read-key-sequence "Click on the mode line: ")
          ⇒ [mode-line
              (mouse-1
               (#<window 6 on NEWS> mode-line
                (40 . 63) 5959987))]

 -- Variable: num-input-keys
     This variable’s value is the number of key sequences processed so
     far in this Emacs session.  This includes key sequences read from
     the terminal and key sequences read from keyboard macros being
     executed.


File: elisp.info,  Node: Reading One Event,  Next: Event Mod,  Prev: Key Sequence Input,  Up: Reading Input

20.8.2 Reading One Event
------------------------

The lowest level functions for command input are ‘read-event’,
‘read-char’, and ‘read-char-exclusive’.

 -- Function: read-event &optional prompt inherit-input-method seconds
     This function reads and returns the next event of command input,
     waiting if necessary until an event is available.

     The returned event may come directly from the user, or from a
     keyboard macro.  It is not decoded by the keyboard’s input coding
     system (*note Terminal I/O Encoding::).

     If the optional argument PROMPT is non-‘nil’, it should be a string
     to display in the echo area as a prompt.  Otherwise, ‘read-event’
     does not display any message to indicate it is waiting for input;
     instead, it prompts by echoing: it displays descriptions of the
     events that led to or were read by the current command.  *Note The
     Echo Area::.

     If INHERIT-INPUT-METHOD is non-‘nil’, then the current input method
     (if any) is employed to make it possible to enter a non-ASCII
     character.  Otherwise, input method handling is disabled for
     reading this event.

     If ‘cursor-in-echo-area’ is non-‘nil’, then ‘read-event’ moves the
     cursor temporarily to the echo area, to the end of any message
     displayed there.  Otherwise ‘read-event’ does not move the cursor.

     If SECONDS is non-‘nil’, it should be a number specifying the
     maximum time to wait for input, in seconds.  If no input arrives
     within that time, ‘read-event’ stops waiting and returns ‘nil’.  A
     floating point SECONDS means to wait for a fractional number of
     seconds.  Some systems support only a whole number of seconds; on
     these systems, SECONDS is rounded down.  If SECONDS is ‘nil’,
     ‘read-event’ waits as long as necessary for input to arrive.

     If SECONDS is ‘nil’, Emacs is considered idle while waiting for
     user input to arrive.  Idle timers—those created with
     ‘run-with-idle-timer’ (*note Idle Timers::)—can run during this
     period.  However, if SECONDS is non-‘nil’, the state of idleness
     remains unchanged.  If Emacs is non-idle when ‘read-event’ is
     called, it remains non-idle throughout the operation of
     ‘read-event’; if Emacs is idle (which can happen if the call
     happens inside an idle timer), it remains idle.

     If ‘read-event’ gets an event that is defined as a help character,
     then in some cases ‘read-event’ processes the event directly
     without returning.  *Note Help Functions::.  Certain other events,
     called “special events”, are also processed directly within
     ‘read-event’ (*note Special Events::).

     Here is what happens if you call ‘read-event’ and then press the
     right-arrow function key:

          (read-event)
               ⇒ right

 -- Function: read-char &optional prompt inherit-input-method seconds
     This function reads and returns a character of command input.  If
     the user generates an event which is not a character (i.e., a mouse
     click or function key event), ‘read-char’ signals an error.  The
     arguments work as in ‘read-event’.

     In the first example, the user types the character ‘1’ (ASCII code
     49).  The second example shows a keyboard macro definition that
     calls ‘read-char’ from the minibuffer using ‘eval-expression’.
     ‘read-char’ reads the keyboard macro’s very next character, which
     is ‘1’.  Then ‘eval-expression’ displays its return value in the
     echo area.

          (read-char)
               ⇒ 49

          ;; We assume here you use ‘M-:’ to evaluate this.
          (symbol-function 'foo)
               ⇒ "^[:(read-char)^M1"
          (execute-kbd-macro 'foo)
               ⊣ 49
               ⇒ nil

 -- Function: read-char-exclusive &optional prompt inherit-input-method
          seconds
     This function reads and returns a character of command input.  If
     the user generates an event which is not a character,
     ‘read-char-exclusive’ ignores it and reads another event, until it
     gets a character.  The arguments work as in ‘read-event’.

   None of the above functions suppress quitting.

 -- Variable: num-nonmacro-input-events
     This variable holds the total number of input events received so
     far from the terminal—not counting those generated by keyboard
     macros.

   We emphasize that, unlike ‘read-key-sequence’, the functions
‘read-event’, ‘read-char’, and ‘read-char-exclusive’ do not perform the
translations described in *note Translation Keymaps::.  If you wish to
read a single key taking these translations into account, use the
function ‘read-key’:

 -- Function: read-key &optional prompt
     This function reads a single key.  It is intermediate between
     ‘read-key-sequence’ and ‘read-event’.  Unlike the former, it reads
     a single key, not a key sequence.  Unlike the latter, it does not
     return a raw event, but decodes and translates the user input
     according to ‘input-decode-map’, ‘local-function-key-map’, and
     ‘key-translation-map’ (*note Translation Keymaps::).

     The argument PROMPT is either a string to be displayed in the echo
     area as a prompt, or ‘nil’, meaning not to display a prompt.

 -- Function: read-char-choice prompt chars &optional inhibit-quit
     This function uses ‘read-key’ to read and return a single
     character.  It ignores any input that is not a member of CHARS, a
     list of accepted characters.  Optionally, it will also ignore
     keyboard-quit events while it is waiting for valid input.  If you
     bind ‘help-form’ (*note Help Functions::) to a non-‘nil’ value
     while calling ‘read-char-choice’, then pressing ‘help-char’ causes
     it to evaluate ‘help-form’ and display the result.  It then
     continues to wait for a valid input character, or keyboard-quit.


File: elisp.info,  Node: Event Mod,  Next: Invoking the Input Method,  Prev: Reading One Event,  Up: Reading Input

20.8.3 Modifying and Translating Input Events
---------------------------------------------

Emacs modifies every event it reads according to
‘extra-keyboard-modifiers’, then translates it through
‘keyboard-translate-table’ (if applicable), before returning it from
‘read-event’.

 -- Variable: extra-keyboard-modifiers
     This variable lets Lisp programs “press” the modifier keys on the
     keyboard.  The value is a character.  Only the modifiers of the
     character matter.  Each time the user types a keyboard key, it is
     altered as if those modifier keys were held down.  For instance, if
     you bind ‘extra-keyboard-modifiers’ to ‘?\C-\M-a’, then all
     keyboard input characters typed during the scope of the binding
     will have the control and meta modifiers applied to them.  The
     character ‘?\C-@’, equivalent to the integer 0, does not count as a
     control character for this purpose, but as a character with no
     modifiers.  Thus, setting ‘extra-keyboard-modifiers’ to zero
     cancels any modification.

     When using a window system, the program can press any of the
     modifier keys in this way.  Otherwise, only the <CTL> and <META>
     keys can be virtually pressed.

     Note that this variable applies only to events that really come
     from the keyboard, and has no effect on mouse events or any other
     events.

 -- Variable: keyboard-translate-table
     This terminal-local variable is the translate table for keyboard
     characters.  It lets you reshuffle the keys on the keyboard without
     changing any command bindings.  Its value is normally a char-table,
     or else ‘nil’.  (It can also be a string or vector, but this is
     considered obsolete.)

     If ‘keyboard-translate-table’ is a char-table (*note
     Char-Tables::), then each character read from the keyboard is
     looked up in this char-table.  If the value found there is
     non-‘nil’, then it is used instead of the actual input character.

     Note that this translation is the first thing that happens to a
     character after it is read from the terminal.  Record-keeping
     features such as ‘recent-keys’ and dribble files record the
     characters after translation.

     Note also that this translation is done before the characters are
     supplied to input methods (*note Input Methods::).  Use
     ‘translation-table-for-input’ (*note Translation of Characters::),
     if you want to translate characters after input methods operate.

 -- Function: keyboard-translate from to
     This function modifies ‘keyboard-translate-table’ to translate
     character code FROM into character code TO.  It creates the
     keyboard translate table if necessary.

   Here’s an example of using the ‘keyboard-translate-table’ to make
‘C-x’, ‘C-c’ and ‘C-v’ perform the cut, copy and paste operations:

     (keyboard-translate ?\C-x 'control-x)
     (keyboard-translate ?\C-c 'control-c)
     (keyboard-translate ?\C-v 'control-v)
     (global-set-key [control-x] 'kill-region)
     (global-set-key [control-c] 'kill-ring-save)
     (global-set-key [control-v] 'yank)

On a graphical terminal that supports extended ASCII input, you can
still get the standard Emacs meanings of one of those characters by
typing it with the shift key.  That makes it a different character as
far as keyboard translation is concerned, but it has the same usual
meaning.

   *Note Translation Keymaps::, for mechanisms that translate event
sequences at the level of ‘read-key-sequence’.


File: elisp.info,  Node: Invoking the Input Method,  Next: Quoted Character Input,  Prev: Event Mod,  Up: Reading Input

20.8.4 Invoking the Input Method
--------------------------------

The event-reading functions invoke the current input method, if any
(*note Input Methods::).  If the value of ‘input-method-function’ is
non-‘nil’, it should be a function; when ‘read-event’ reads a printing
character (including <SPC>) with no modifier bits, it calls that
function, passing the character as an argument.

 -- Variable: input-method-function
     If this is non-‘nil’, its value specifies the current input method
     function.

     *Warning:* don’t bind this variable with ‘let’.  It is often
     buffer-local, and if you bind it around reading input (which is
     exactly when you _would_ bind it), switching buffers asynchronously
     while Emacs is waiting will cause the value to be restored in the
     wrong buffer.

   The input method function should return a list of events which should
be used as input.  (If the list is ‘nil’, that means there is no input,
so ‘read-event’ waits for another event.)  These events are processed
before the events in ‘unread-command-events’ (*note Event Input Misc::).
Events returned by the input method function are not passed to the input
method function again, even if they are printing characters with no
modifier bits.

   If the input method function calls ‘read-event’ or
‘read-key-sequence’, it should bind ‘input-method-function’ to ‘nil’
first, to prevent recursion.

   The input method function is not called when reading the second and
subsequent events of a key sequence.  Thus, these characters are not
subject to input method processing.  The input method function should
test the values of ‘overriding-local-map’ and
‘overriding-terminal-local-map’; if either of these variables is
non-‘nil’, the input method should put its argument into a list and
return that list with no further processing.


File: elisp.info,  Node: Quoted Character Input,  Next: Event Input Misc,  Prev: Invoking the Input Method,  Up: Reading Input

20.8.5 Quoted Character Input
-----------------------------

You can use the function ‘read-quoted-char’ to ask the user to specify a
character, and allow the user to specify a control or meta character
conveniently, either literally or as an octal character code.  The
command ‘quoted-insert’ uses this function.

 -- Function: read-quoted-char &optional prompt
     This function is like ‘read-char’, except that if the first
     character read is an octal digit (0–7), it reads any number of
     octal digits (but stopping if a non-octal digit is found), and
     returns the character represented by that numeric character code.
     If the character that terminates the sequence of octal digits is
     <RET>, it is discarded.  Any other terminating character is used as
     input after this function returns.

     Quitting is suppressed when the first character is read, so that
     the user can enter a ‘C-g’.  *Note Quitting::.

     If PROMPT is supplied, it specifies a string for prompting the
     user.  The prompt string is always displayed in the echo area,
     followed by a single ‘-’.

     In the following example, the user types in the octal number 177
     (which is 127 in decimal).

          (read-quoted-char "What character")

          ---------- Echo Area ----------
          What character 1 7 7-
          ---------- Echo Area ----------

               ⇒ 127


File: elisp.info,  Node: Event Input Misc,  Prev: Quoted Character Input,  Up: Reading Input

20.8.6 Miscellaneous Event Input Features
-----------------------------------------

This section describes how to peek ahead at events without using them
up, how to check for pending input, and how to discard pending input.
See also the function ‘read-passwd’ (*note Reading a Password::).

 -- Variable: unread-command-events
     This variable holds a list of events waiting to be read as command
     input.  The events are used in the order they appear in the list,
     and removed one by one as they are used.

     The variable is needed because in some cases a function reads an
     event and then decides not to use it.  Storing the event in this
     variable causes it to be processed normally, by the command loop or
     by the functions to read command input.

     For example, the function that implements numeric prefix arguments
     reads any number of digits.  When it finds a non-digit event, it
     must unread the event so that it can be read normally by the
     command loop.  Likewise, incremental search uses this feature to
     unread events with no special meaning in a search, because these
     events should exit the search and then execute normally.

     The reliable and easy way to extract events from a key sequence so
     as to put them in ‘unread-command-events’ is to use
     ‘listify-key-sequence’ (see below).

     Normally you add events to the front of this list, so that the
     events most recently unread will be reread first.

     Events read from this list are not normally added to the current
     command’s key sequence (as returned by, e.g., ‘this-command-keys’),
     as the events will already have been added once as they were read
     for the first time.  An element of the form ‘(t . EVENT)’ forces
     EVENT to be added to the current command’s key sequence.

 -- Function: listify-key-sequence key
     This function converts the string or vector KEY to a list of
     individual events, which you can put in ‘unread-command-events’.

 -- Function: input-pending-p &optional check-timers
     This function determines whether any command input is currently
     available to be read.  It returns immediately, with value ‘t’ if
     there is available input, ‘nil’ otherwise.  On rare occasions it
     may return ‘t’ when no input is available.

     If the optional argument CHECK-TIMERS is non-‘nil’, then if no
     input is available, Emacs runs any timers which are ready.  *Note
     Timers::.

 -- Variable: last-input-event
     This variable records the last terminal input event read, whether
     as part of a command or explicitly by a Lisp program.

     In the example below, the Lisp program reads the character ‘1’,
     ASCII code 49.  It becomes the value of ‘last-input-event’, while
     ‘C-e’ (we assume ‘C-x C-e’ command is used to evaluate this
     expression) remains the value of ‘last-command-event’.

          (progn (print (read-char))
                 (print last-command-event)
                 last-input-event)
               ⊣ 49
               ⊣ 5
               ⇒ 49

 -- Macro: while-no-input body...
     This construct runs the BODY forms and returns the value of the
     last one—but only if no input arrives.  If any input arrives during
     the execution of the BODY forms, it aborts them (working much like
     a quit).  The ‘while-no-input’ form returns ‘nil’ if aborted by a
     real quit, and returns ‘t’ if aborted by arrival of other input.

     If a part of BODY binds ‘inhibit-quit’ to non-‘nil’, arrival of
     input during those parts won’t cause an abort until the end of that
     part.

     If you want to be able to distinguish all possible values computed
     by BODY from both kinds of abort conditions, write the code like
     this:

          (while-no-input
            (list
              (progn . BODY)))

 -- Function: discard-input
     This function discards the contents of the terminal input buffer
     and cancels any keyboard macro that might be in the process of
     definition.  It returns ‘nil’.

     In the following example, the user may type a number of characters
     right after starting the evaluation of the form.  After the
     ‘sleep-for’ finishes sleeping, ‘discard-input’ discards any
     characters typed during the sleep.

          (progn (sleep-for 2)
                 (discard-input))
               ⇒ nil


File: elisp.info,  Node: Special Events,  Next: Waiting,  Prev: Reading Input,  Up: Command Loop

20.9 Special Events
===================

Certain “special events” are handled at a very low level—as soon as they
are read.  The ‘read-event’ function processes these events itself, and
never returns them.  Instead, it keeps waiting for the first event that
is not special and returns that one.

   Special events do not echo, they are never grouped into key
sequences, and they never appear in the value of ‘last-command-event’ or
‘(this-command-keys)’.  They do not discard a numeric argument, they
cannot be unread with ‘unread-command-events’, they may not appear in a
keyboard macro, and they are not recorded in a keyboard macro while you
are defining one.

   Special events do, however, appear in ‘last-input-event’ immediately
after they are read, and this is the way for the event’s definition to
find the actual event.

   The events types ‘iconify-frame’, ‘make-frame-visible’,
‘delete-frame’, ‘drag-n-drop’, ‘language-change’, and user signals like
‘sigusr1’ are normally handled in this way.  The keymap which defines
how to handle special events—and which events are special—is in the
variable ‘special-event-map’ (*note Active Keymaps::).


File: elisp.info,  Node: Waiting,  Next: Quitting,  Prev: Special Events,  Up: Command Loop

20.10 Waiting for Elapsed Time or Input
=======================================

The wait functions are designed to wait for a certain amount of time to
pass or until there is input.  For example, you may wish to pause in the
middle of a computation to allow the user time to view the display.
‘sit-for’ pauses and updates the screen, and returns immediately if
input comes in, while ‘sleep-for’ pauses without updating the screen.

 -- Function: sit-for seconds &optional nodisp
     This function performs redisplay (provided there is no pending
     input from the user), then waits SECONDS seconds, or until input is
     available.  The usual purpose of ‘sit-for’ is to give the user time
     to read text that you display.  The value is ‘t’ if ‘sit-for’
     waited the full time with no input arriving (*note Event Input
     Misc::).  Otherwise, the value is ‘nil’.

     The argument SECONDS need not be an integer.  If it is floating
     point, ‘sit-for’ waits for a fractional number of seconds.  Some
     systems support only a whole number of seconds; on these systems,
     SECONDS is rounded down.

     The expression ‘(sit-for 0)’ is equivalent to ‘(redisplay)’, i.e.,
     it requests a redisplay, without any delay, if there is no pending
     input.  *Note Forcing Redisplay::.

     If NODISP is non-‘nil’, then ‘sit-for’ does not redisplay, but it
     still returns as soon as input is available (or when the timeout
     elapses).

     In batch mode (*note Batch Mode::), ‘sit-for’ cannot be
     interrupted, even by input from the standard input descriptor.  It
     is thus equivalent to ‘sleep-for’, which is described below.

     It is also possible to call ‘sit-for’ with three arguments, as
     ‘(sit-for SECONDS MILLISEC NODISP)’, but that is considered
     obsolete.

 -- Function: sleep-for seconds &optional millisec
     This function simply pauses for SECONDS seconds without updating
     the display.  It pays no attention to available input.  It returns
     ‘nil’.

     The argument SECONDS need not be an integer.  If it is floating
     point, ‘sleep-for’ waits for a fractional number of seconds.  Some
     systems support only a whole number of seconds; on these systems,
     SECONDS is rounded down.

     The optional argument MILLISEC specifies an additional waiting
     period measured in milliseconds.  This adds to the period specified
     by SECONDS.  If the system doesn’t support waiting fractions of a
     second, you get an error if you specify nonzero MILLISEC.

     Use ‘sleep-for’ when you wish to guarantee a delay.

   *Note Time of Day::, for functions to get the current time.


File: elisp.info,  Node: Quitting,  Next: Prefix Command Arguments,  Prev: Waiting,  Up: Command Loop

20.11 Quitting
==============

Typing ‘C-g’ while a Lisp function is running causes Emacs to “quit”
whatever it is doing.  This means that control returns to the innermost
active command loop.

   Typing ‘C-g’ while the command loop is waiting for keyboard input
does not cause a quit; it acts as an ordinary input character.  In the
simplest case, you cannot tell the difference, because ‘C-g’ normally
runs the command ‘keyboard-quit’, whose effect is to quit.  However,
when ‘C-g’ follows a prefix key, they combine to form an undefined key.
The effect is to cancel the prefix key as well as any prefix argument.

   In the minibuffer, ‘C-g’ has a different definition: it aborts out of
the minibuffer.  This means, in effect, that it exits the minibuffer and
then quits.  (Simply quitting would return to the command loop _within_
the minibuffer.)  The reason why ‘C-g’ does not quit directly when the
command reader is reading input is so that its meaning can be redefined
in the minibuffer in this way.  ‘C-g’ following a prefix key is not
redefined in the minibuffer, and it has its normal effect of canceling
the prefix key and prefix argument.  This too would not be possible if
‘C-g’ always quit directly.

   When ‘C-g’ does directly quit, it does so by setting the variable
‘quit-flag’ to ‘t’.  Emacs checks this variable at appropriate times and
quits if it is not ‘nil’.  Setting ‘quit-flag’ non-‘nil’ in any way thus
causes a quit.

   At the level of C code, quitting cannot happen just anywhere; only at
the special places that check ‘quit-flag’.  The reason for this is that
quitting at other places might leave an inconsistency in Emacs’s
internal state.  Because quitting is delayed until a safe place,
quitting cannot make Emacs crash.

   Certain functions such as ‘read-key-sequence’ or ‘read-quoted-char’
prevent quitting entirely even though they wait for input.  Instead of
quitting, ‘C-g’ serves as the requested input.  In the case of
‘read-key-sequence’, this serves to bring about the special behavior of
‘C-g’ in the command loop.  In the case of ‘read-quoted-char’, this is
so that ‘C-q’ can be used to quote a ‘C-g’.

   You can prevent quitting for a portion of a Lisp function by binding
the variable ‘inhibit-quit’ to a non-‘nil’ value.  Then, although ‘C-g’
still sets ‘quit-flag’ to ‘t’ as usual, the usual result of this—a
quit—is prevented.  Eventually, ‘inhibit-quit’ will become ‘nil’ again,
such as when its binding is unwound at the end of a ‘let’ form.  At that
time, if ‘quit-flag’ is still non-‘nil’, the requested quit happens
immediately.  This behavior is ideal when you wish to make sure that
quitting does not happen within a critical section of the program.

   In some functions (such as ‘read-quoted-char’), ‘C-g’ is handled in a
special way that does not involve quitting.  This is done by reading the
input with ‘inhibit-quit’ bound to ‘t’, and setting ‘quit-flag’ to ‘nil’
before ‘inhibit-quit’ becomes ‘nil’ again.  This excerpt from the
definition of ‘read-quoted-char’ shows how this is done; it also shows
that normal quitting is permitted after the first character of input.

     (defun read-quoted-char (&optional prompt)
       "...DOCUMENTATION..."
       (let ((message-log-max nil) done (first t) (code 0) char)
         (while (not done)
           (let ((inhibit-quit first)
                 ...)
             (and prompt (message "%s-" prompt))
             (setq char (read-event))
             (if inhibit-quit (setq quit-flag nil)))
           ...set the variable ‘code’...)
         code))

 -- Variable: quit-flag
     If this variable is non-‘nil’, then Emacs quits immediately, unless
     ‘inhibit-quit’ is non-‘nil’.  Typing ‘C-g’ ordinarily sets
     ‘quit-flag’ non-‘nil’, regardless of ‘inhibit-quit’.

 -- Variable: inhibit-quit
     This variable determines whether Emacs should quit when ‘quit-flag’
     is set to a value other than ‘nil’.  If ‘inhibit-quit’ is
     non-‘nil’, then ‘quit-flag’ has no special effect.

 -- Macro: with-local-quit body...
     This macro executes BODY forms in sequence, but allows quitting, at
     least locally, within BODY even if ‘inhibit-quit’ was non-‘nil’
     outside this construct.  It returns the value of the last form in
     BODY, unless exited by quitting, in which case it returns ‘nil’.

     If ‘inhibit-quit’ is ‘nil’ on entry to ‘with-local-quit’, it only
     executes the BODY, and setting ‘quit-flag’ causes a normal quit.
     However, if ‘inhibit-quit’ is non-‘nil’ so that ordinary quitting
     is delayed, a non-‘nil’ ‘quit-flag’ triggers a special kind of
     local quit.  This ends the execution of BODY and exits the
     ‘with-local-quit’ body with ‘quit-flag’ still non-‘nil’, so that
     another (ordinary) quit will happen as soon as that is allowed.  If
     ‘quit-flag’ is already non-‘nil’ at the beginning of BODY, the
     local quit happens immediately and the body doesn’t execute at all.

     This macro is mainly useful in functions that can be called from
     timers, process filters, process sentinels, ‘pre-command-hook’,
     ‘post-command-hook’, and other places where ‘inhibit-quit’ is
     normally bound to ‘t’.

 -- Command: keyboard-quit
     This function signals the ‘quit’ condition with ‘(signal 'quit
     nil)’.  This is the same thing that quitting does.  (See ‘signal’
     in *note Errors::.)

   You can specify a character other than ‘C-g’ to use for quitting.
See the function ‘set-input-mode’ in *note Input Modes::.


File: elisp.info,  Node: Prefix Command Arguments,  Next: Recursive Editing,  Prev: Quitting,  Up: Command Loop

20.12 Prefix Command Arguments
==============================

Most Emacs commands can use a “prefix argument”, a number specified
before the command itself.  (Don’t confuse prefix arguments with prefix
keys.)  The prefix argument is at all times represented by a value,
which may be ‘nil’, meaning there is currently no prefix argument.  Each
command may use the prefix argument or ignore it.

   There are two representations of the prefix argument: “raw” and
“numeric”.  The editor command loop uses the raw representation
internally, and so do the Lisp variables that store the information, but
commands can request either representation.

   Here are the possible values of a raw prefix argument:

   • ‘nil’, meaning there is no prefix argument.  Its numeric value is
     1, but numerous commands make a distinction between ‘nil’ and the
     integer 1.

   • An integer, which stands for itself.

   • A list of one element, which is an integer.  This form of prefix
     argument results from one or a succession of ‘C-u’s with no digits.
     The numeric value is the integer in the list, but some commands
     make a distinction between such a list and an integer alone.

   • The symbol ‘-’.  This indicates that ‘M--’ or ‘C-u -’ was typed,
     without following digits.  The equivalent numeric value is −1, but
     some commands make a distinction between the integer −1 and the
     symbol ‘-’.

   We illustrate these possibilities by calling the following function
with various prefixes:

     (defun display-prefix (arg)
       "Display the value of the raw prefix arg."
       (interactive "P")
       (message "%s" arg))

Here are the results of calling ‘display-prefix’ with various raw prefix
arguments:

             M-x display-prefix  ⊣ nil

     C-u     M-x display-prefix  ⊣ (4)

     C-u C-u M-x display-prefix  ⊣ (16)

     C-u 3   M-x display-prefix  ⊣ 3

     M-3     M-x display-prefix  ⊣ 3      ; (Same as ‘C-u 3’.)

     C-u -   M-x display-prefix  ⊣ -

     M--     M-x display-prefix  ⊣ -      ; (Same as ‘C-u -’.)

     C-u - 7 M-x display-prefix  ⊣ -7

     M-- 7   M-x display-prefix  ⊣ -7     ; (Same as ‘C-u -7’.)

   Emacs uses two variables to store the prefix argument: ‘prefix-arg’
and ‘current-prefix-arg’.  Commands such as ‘universal-argument’ that
set up prefix arguments for other commands store them in ‘prefix-arg’.
In contrast, ‘current-prefix-arg’ conveys the prefix argument to the
current command, so setting it has no effect on the prefix arguments for
future commands.

   Normally, commands specify which representation to use for the prefix
argument, either numeric or raw, in the ‘interactive’ specification.
(*Note Using Interactive::.)  Alternatively, functions may look at the
value of the prefix argument directly in the variable
‘current-prefix-arg’, but this is less clean.

 -- Function: prefix-numeric-value arg
     This function returns the numeric meaning of a valid raw prefix
     argument value, ARG.  The argument may be a symbol, a number, or a
     list.  If it is ‘nil’, the value 1 is returned; if it is ‘-’, the
     value −1 is returned; if it is a number, that number is returned;
     if it is a list, the CAR of that list (which should be a number) is
     returned.

 -- Variable: current-prefix-arg
     This variable holds the raw prefix argument for the _current_
     command.  Commands may examine it directly, but the usual method
     for accessing it is with ‘(interactive "P")’.

 -- Variable: prefix-arg
     The value of this variable is the raw prefix argument for the
     _next_ editing command.  Commands such as ‘universal-argument’ that
     specify prefix arguments for the following command work by setting
     this variable.

 -- Variable: last-prefix-arg
     The raw prefix argument value used by the previous command.

   The following commands exist to set up prefix arguments for the
following command.  Do not call them for any other reason.

 -- Command: universal-argument
     This command reads input and specifies a prefix argument for the
     following command.  Don’t call this command yourself unless you
     know what you are doing.

 -- Command: digit-argument arg
     This command adds to the prefix argument for the following command.
     The argument ARG is the raw prefix argument as it was before this
     command; it is used to compute the updated prefix argument.  Don’t
     call this command yourself unless you know what you are doing.

 -- Command: negative-argument arg
     This command adds to the numeric argument for the next command.
     The argument ARG is the raw prefix argument as it was before this
     command; its value is negated to form the new prefix argument.
     Don’t call this command yourself unless you know what you are
     doing.


File: elisp.info,  Node: Recursive Editing,  Next: Disabling Commands,  Prev: Prefix Command Arguments,  Up: Command Loop

20.13 Recursive Editing
=======================

The Emacs command loop is entered automatically when Emacs starts up.
This top-level invocation of the command loop never exits; it keeps
running as long as Emacs does.  Lisp programs can also invoke the
command loop.  Since this makes more than one activation of the command
loop, we call it “recursive editing”.  A recursive editing level has the
effect of suspending whatever command invoked it and permitting the user
to do arbitrary editing before resuming that command.

   The commands available during recursive editing are the same ones
available in the top-level editing loop and defined in the keymaps.
Only a few special commands exit the recursive editing level; the others
return to the recursive editing level when they finish.  (The special
commands for exiting are always available, but they do nothing when
recursive editing is not in progress.)

   All command loops, including recursive ones, set up all-purpose error
handlers so that an error in a command run from the command loop will
not exit the loop.

   Minibuffer input is a special kind of recursive editing.  It has a
few special wrinkles, such as enabling display of the minibuffer and the
minibuffer window, but fewer than you might suppose.  Certain keys
behave differently in the minibuffer, but that is only because of the
minibuffer’s local map; if you switch windows, you get the usual Emacs
commands.

   To invoke a recursive editing level, call the function
‘recursive-edit’.  This function contains the command loop; it also
contains a call to ‘catch’ with tag ‘exit’, which makes it possible to
exit the recursive editing level by throwing to ‘exit’ (*note Catch and
Throw::).  If you throw a value other than ‘t’, then ‘recursive-edit’
returns normally to the function that called it.  The command ‘C-M-c’
(‘exit-recursive-edit’) does this.  Throwing a ‘t’ value causes
‘recursive-edit’ to quit, so that control returns to the command loop
one level up.  This is called “aborting”, and is done by ‘C-]’
(‘abort-recursive-edit’).

   Most applications should not use recursive editing, except as part of
using the minibuffer.  Usually it is more convenient for the user if you
change the major mode of the current buffer temporarily to a special
major mode, which should have a command to go back to the previous mode.
(The ‘e’ command in Rmail uses this technique.)  Or, if you wish to give
the user different text to edit recursively, create and select a new
buffer in a special mode.  In this mode, define a command to complete
the processing and go back to the previous buffer.  (The ‘m’ command in
Rmail does this.)

   Recursive edits are useful in debugging.  You can insert a call to
‘debug’ into a function definition as a sort of breakpoint, so that you
can look around when the function gets there.  ‘debug’ invokes a
recursive edit but also provides the other features of the debugger.

   Recursive editing levels are also used when you type ‘C-r’ in
‘query-replace’ or use ‘C-x q’ (‘kbd-macro-query’).

 -- Command: recursive-edit
     This function invokes the editor command loop.  It is called
     automatically by the initialization of Emacs, to let the user begin
     editing.  When called from a Lisp program, it enters a recursive
     editing level.

     If the current buffer is not the same as the selected window’s
     buffer, ‘recursive-edit’ saves and restores the current buffer.
     Otherwise, if you switch buffers, the buffer you switched to is
     current after ‘recursive-edit’ returns.

     In the following example, the function ‘simple-rec’ first advances
     point one word, then enters a recursive edit, printing out a
     message in the echo area.  The user can then do any editing
     desired, and then type ‘C-M-c’ to exit and continue executing
     ‘simple-rec’.

          (defun simple-rec ()
            (forward-word 1)
            (message "Recursive edit in progress")
            (recursive-edit)
            (forward-word 1))
               ⇒ simple-rec
          (simple-rec)
               ⇒ nil

 -- Command: exit-recursive-edit
     This function exits from the innermost recursive edit (including
     minibuffer input).  Its definition is effectively ‘(throw 'exit
     nil)’.

 -- Command: abort-recursive-edit
     This function aborts the command that requested the innermost
     recursive edit (including minibuffer input), by signaling ‘quit’
     after exiting the recursive edit.  Its definition is effectively
     ‘(throw 'exit t)’.  *Note Quitting::.

 -- Command: top-level
     This function exits all recursive editing levels; it does not
     return a value, as it jumps completely out of any computation
     directly back to the main command loop.

 -- Function: recursion-depth
     This function returns the current depth of recursive edits.  When
     no recursive edit is active, it returns 0.


File: elisp.info,  Node: Disabling Commands,  Next: Command History,  Prev: Recursive Editing,  Up: Command Loop

20.14 Disabling Commands
========================

“Disabling a command” marks the command as requiring user confirmation
before it can be executed.  Disabling is used for commands which might
be confusing to beginning users, to prevent them from using the commands
by accident.

   The low-level mechanism for disabling a command is to put a non-‘nil’
‘disabled’ property on the Lisp symbol for the command.  These
properties are normally set up by the user’s init file (*note Init
File::) with Lisp expressions such as this:

     (put 'upcase-region 'disabled t)

For a few commands, these properties are present by default (you can
remove them in your init file if you wish).

   If the value of the ‘disabled’ property is a string, the message
saying the command is disabled includes that string.  For example:

     (put 'delete-region 'disabled
          "Text deleted this way cannot be yanked back!\n")

   *Note (emacs)Disabling::, for the details on what happens when a
disabled command is invoked interactively.  Disabling a command has no
effect on calling it as a function from Lisp programs.

 -- Command: enable-command command
     Allow COMMAND (a symbol) to be executed without special
     confirmation from now on, and alter the user’s init file (*note
     Init File::) so that this will apply to future sessions.

 -- Command: disable-command command
     Require special confirmation to execute COMMAND from now on, and
     alter the user’s init file so that this will apply to future
     sessions.

 -- Variable: disabled-command-function
     The value of this variable should be a function.  When the user
     invokes a disabled command interactively, this function is called
     instead of the disabled command.  It can use ‘this-command-keys’ to
     determine what the user typed to run the command, and thus find the
     command itself.

     The value may also be ‘nil’.  Then all commands work normally, even
     disabled ones.

     By default, the value is a function that asks the user whether to
     proceed.


File: elisp.info,  Node: Command History,  Next: Keyboard Macros,  Prev: Disabling Commands,  Up: Command Loop

20.15 Command History
=====================

The command loop keeps a history of the complex commands that have been
executed, to make it convenient to repeat these commands.  A “complex
command” is one for which the interactive argument reading uses the
minibuffer.  This includes any ‘M-x’ command, any ‘M-:’ command, and any
command whose ‘interactive’ specification reads an argument from the
minibuffer.  Explicit use of the minibuffer during the execution of the
command itself does not cause the command to be considered complex.

 -- Variable: command-history
     This variable’s value is a list of recent complex commands, each
     represented as a form to evaluate.  It continues to accumulate all
     complex commands for the duration of the editing session, but when
     it reaches the maximum size (*note Minibuffer History::), the
     oldest elements are deleted as new ones are added.

          command-history
          ⇒ ((switch-to-buffer "chistory.texi")
              (describe-key "^X^[")
              (visit-tags-table "~/emacs/src/")
              (find-tag "repeat-complex-command"))

   This history list is actually a special case of minibuffer history
(*note Minibuffer History::), with one special twist: the elements are
expressions rather than strings.

   There are a number of commands devoted to the editing and recall of
previous commands.  The commands ‘repeat-complex-command’, and
‘list-command-history’ are described in the user manual (*note
(emacs)Repetition::).  Within the minibuffer, the usual minibuffer
history commands are available.


File: elisp.info,  Node: Keyboard Macros,  Prev: Command History,  Up: Command Loop

20.16 Keyboard Macros
=====================

A “keyboard macro” is a canned sequence of input events that can be
considered a command and made the definition of a key.  The Lisp
representation of a keyboard macro is a string or vector containing the
events.  Don’t confuse keyboard macros with Lisp macros (*note
Macros::).

 -- Function: execute-kbd-macro kbdmacro &optional count loopfunc
     This function executes KBDMACRO as a sequence of events.  If
     KBDMACRO is a string or vector, then the events in it are executed
     exactly as if they had been input by the user.  The sequence is
     _not_ expected to be a single key sequence; normally a keyboard
     macro definition consists of several key sequences concatenated.

     If KBDMACRO is a symbol, then its function definition is used in
     place of KBDMACRO.  If that is another symbol, this process
     repeats.  Eventually the result should be a string or vector.  If
     the result is not a symbol, string, or vector, an error is
     signaled.

     The argument COUNT is a repeat count; KBDMACRO is executed that
     many times.  If COUNT is omitted or ‘nil’, KBDMACRO is executed
     once.  If it is 0, KBDMACRO is executed over and over until it
     encounters an error or a failing search.

     If LOOPFUNC is non-‘nil’, it is a function that is called, without
     arguments, prior to each iteration of the macro.  If LOOPFUNC
     returns ‘nil’, then this stops execution of the macro.

     *Note Reading One Event::, for an example of using
     ‘execute-kbd-macro’.

 -- Variable: executing-kbd-macro
     This variable contains the string or vector that defines the
     keyboard macro that is currently executing.  It is ‘nil’ if no
     macro is currently executing.  A command can test this variable so
     as to behave differently when run from an executing macro.  Do not
     set this variable yourself.

 -- Variable: defining-kbd-macro
     This variable is non-‘nil’ if and only if a keyboard macro is being
     defined.  A command can test this variable so as to behave
     differently while a macro is being defined.  The value is ‘append’
     while appending to the definition of an existing macro.  The
     commands ‘start-kbd-macro’, ‘kmacro-start-macro’ and
     ‘end-kbd-macro’ set this variable—do not set it yourself.

     The variable is always local to the current terminal and cannot be
     buffer-local.  *Note Multiple Terminals::.

 -- Variable: last-kbd-macro
     This variable is the definition of the most recently defined
     keyboard macro.  Its value is a string or vector, or ‘nil’.

     The variable is always local to the current terminal and cannot be
     buffer-local.  *Note Multiple Terminals::.

 -- Variable: kbd-macro-termination-hook
     This normal hook is run when a keyboard macro terminates,
     regardless of what caused it to terminate (reaching the macro end
     or an error which ended the macro prematurely).


File: elisp.info,  Node: Keymaps,  Next: Modes,  Prev: Command Loop,  Up: Top

21 Keymaps
**********

The command bindings of input events are recorded in data structures
called “keymaps”.  Each entry in a keymap associates (or “binds”) an
individual event type, either to another keymap or to a command.  When
an event type is bound to a keymap, that keymap is used to look up the
next input event; this continues until a command is found.  The whole
process is called “key lookup”.

* Menu:

* Key Sequences::               Key sequences as Lisp objects.
* Keymap Basics::               Basic concepts of keymaps.
* Format of Keymaps::           What a keymap looks like as a Lisp object.
* Creating Keymaps::            Functions to create and copy keymaps.
* Inheritance and Keymaps::     How one keymap can inherit the bindings
                                   of another keymap.
* Prefix Keys::                 Defining a key with a keymap as its definition.
* Active Keymaps::              How Emacs searches the active keymaps
                                   for a key binding.
* Searching Keymaps::           A pseudo-Lisp summary of searching active maps.
* Controlling Active Maps::     Each buffer has a local keymap
                                   to override the standard (global) bindings.
                                   A minor mode can also override them.
* Key Lookup::                  Finding a key’s binding in one keymap.
* Functions for Key Lookup::    How to request key lookup.
* Changing Key Bindings::       Redefining a key in a keymap.
* Remapping Commands::          A keymap can translate one command to another.
* Translation Keymaps::         Keymaps for translating sequences of events.
* Key Binding Commands::        Interactive interfaces for redefining keys.
* Scanning Keymaps::            Looking through all keymaps, for printing help.
* Menu Keymaps::                Defining a menu as a keymap.


File: elisp.info,  Node: Key Sequences,  Next: Keymap Basics,  Up: Keymaps

21.1 Key Sequences
==================

A “key sequence”, or “key” for short, is a sequence of one or more input
events that form a unit.  Input events include characters, function
keys, mouse actions, or system events external to Emacs, such as
‘iconify-frame’ (*note Input Events::).  The Emacs Lisp representation
for a key sequence is a string or vector.  Unless otherwise stated, any
Emacs Lisp function that accepts a key sequence as an argument can
handle both representations.

   In the string representation, alphanumeric characters ordinarily
stand for themselves; for example, ‘"a"’ represents ‘a’ and ‘"2"’
represents ‘2’.  Control character events are prefixed by the substring
‘"\C-"’, and meta characters by ‘"\M-"’; for example, ‘"\C-x"’
represents the key ‘C-x’.  In addition, the <TAB>, <RET>, <ESC>, and
<DEL> events are represented by ‘"\t"’, ‘"\r"’, ‘"\e"’, and ‘"\d"’
respectively.  The string representation of a complete key sequence is
the concatenation of the string representations of the constituent
events; thus, ‘"\C-xl"’ represents the key sequence ‘C-x l’.

   Key sequences containing function keys, mouse button events, system
events, or non-ASCII characters such as ‘C-=’ or ‘H-a’ cannot be
represented as strings; they have to be represented as vectors.

   In the vector representation, each element of the vector represents
an input event, in its Lisp form.  *Note Input Events::.  For example,
the vector ‘[?\C-x ?l]’ represents the key sequence ‘C-x l’.

   For examples of key sequences written in string and vector
representations, *note (emacs)Init Rebinding::.

 -- Function: kbd keyseq-text
     This function converts the text KEYSEQ-TEXT (a string constant)
     into a key sequence (a string or vector constant).  The contents of
     KEYSEQ-TEXT should use the same syntax as in the buffer invoked by
     the ‘C-x C-k <RET>’ (‘kmacro-edit-macro’) command; in particular,
     you must surround function key names with ‘<...>’.  *Note
     (emacs)Edit Keyboard Macro::.

          (kbd "C-x") ⇒ "\C-x"
          (kbd "C-x C-f") ⇒ "\C-x\C-f"
          (kbd "C-x 4 C-f") ⇒ "\C-x4\C-f"
          (kbd "X") ⇒ "X"
          (kbd "RET") ⇒ "\^M"
          (kbd "C-c SPC") ⇒ "\C-c "
          (kbd "<f1> SPC") ⇒ [f1 32]
          (kbd "C-M-<down>") ⇒ [C-M-down]


File: elisp.info,  Node: Keymap Basics,  Next: Format of Keymaps,  Prev: Key Sequences,  Up: Keymaps

21.2 Keymap Basics
==================

A keymap is a Lisp data structure that specifies “key bindings” for
various key sequences.

   A single keymap directly specifies definitions for individual events.
When a key sequence consists of a single event, its binding in a keymap
is the keymap’s definition for that event.  The binding of a longer key
sequence is found by an iterative process: first find the definition of
the first event (which must itself be a keymap); then find the second
event’s definition in that keymap, and so on until all the events in the
key sequence have been processed.

   If the binding of a key sequence is a keymap, we call the key
sequence a “prefix key”.  Otherwise, we call it a “complete key”
(because no more events can be added to it).  If the binding is ‘nil’,
we call the key “undefined”.  Examples of prefix keys are ‘C-c’, ‘C-x’,
and ‘C-x 4’.  Examples of defined complete keys are ‘X’, <RET>, and ‘C-x
4 C-f’.  Examples of undefined complete keys are ‘C-x C-g’, and ‘C-c 3’.
*Note Prefix Keys::, for more details.

   The rule for finding the binding of a key sequence assumes that the
intermediate bindings (found for the events before the last) are all
keymaps; if this is not so, the sequence of events does not form a
unit—it is not really one key sequence.  In other words, removing one or
more events from the end of any valid key sequence must always yield a
prefix key.  For example, ‘C-f C-n’ is not a key sequence; ‘C-f’ is not
a prefix key, so a longer sequence starting with ‘C-f’ cannot be a key
sequence.

   The set of possible multi-event key sequences depends on the bindings
for prefix keys; therefore, it can be different for different keymaps,
and can change when bindings are changed.  However, a one-event sequence
is always a key sequence, because it does not depend on any prefix keys
for its well-formedness.

   At any time, several primary keymaps are “active”—that is, in use for
finding key bindings.  These are the “global map”, which is shared by
all buffers; the “local keymap”, which is usually associated with a
specific major mode; and zero or more “minor mode keymaps”, which belong
to currently enabled minor modes.  (Not all minor modes have keymaps.)
The local keymap bindings shadow (i.e., take precedence over) the
corresponding global bindings.  The minor mode keymaps shadow both local
and global keymaps.  *Note Active Keymaps::, for details.


File: elisp.info,  Node: Format of Keymaps,  Next: Creating Keymaps,  Prev: Keymap Basics,  Up: Keymaps

21.3 Format of Keymaps
======================

Each keymap is a list whose CAR is the symbol ‘keymap’.  The remaining
elements of the list define the key bindings of the keymap.  A symbol
whose function definition is a keymap is also a keymap.  Use the
function ‘keymapp’ (see below) to test whether an object is a keymap.

   Several kinds of elements may appear in a keymap, after the symbol
‘keymap’ that begins it:

‘(TYPE . BINDING)’
     This specifies one binding, for events of type TYPE.  Each ordinary
     binding applies to events of a particular “event type”, which is
     always a character or a symbol.  *Note Classifying Events::.  In
     this kind of binding, BINDING is a command.

‘(TYPE ITEM-NAME . BINDING)’
     This specifies a binding which is also a simple menu item that
     displays as ITEM-NAME in the menu.  *Note Simple Menu Items::.

‘(TYPE ITEM-NAME HELP-STRING . BINDING)’
     This is a simple menu item with help string HELP-STRING.

‘(TYPE menu-item . DETAILS)’
     This specifies a binding which is also an extended menu item.  This
     allows use of other features.  *Note Extended Menu Items::.

‘(t . BINDING)’
     This specifies a “default key binding”; any event not bound by
     other elements of the keymap is given BINDING as its binding.
     Default bindings allow a keymap to bind all possible event types
     without having to enumerate all of them.  A keymap that has a
     default binding completely masks any lower-precedence keymap,
     except for events explicitly bound to ‘nil’ (see below).

‘CHAR-TABLE’
     If an element of a keymap is a char-table, it counts as holding
     bindings for all character events with no modifier bits (*note
     modifier bits::): the element whose index is C is the binding for
     the character C.  This is a compact way to record lots of bindings.
     A keymap with such a char-table is called a “full keymap”.  Other
     keymaps are called “sparse keymaps”.

‘VECTOR’
     This kind of element is similar to a char-table: the element whose
     index is C is the binding for the character C.  Since the range of
     characters that can be bound this way is limited by the vector
     size, and vector creation allocates space for all character codes
     from 0 up, this format should not be used except for creating menu
     keymaps (*note Menu Keymaps::), where the bindings themselves don’t
     matter.

‘STRING’
     Aside from elements that specify bindings for keys, a keymap can
     also have a string as an element.  This is called the “overall
     prompt string” and makes it possible to use the keymap as a menu.
     *Note Defining Menus::.

‘(keymap ...)’
     If an element of a keymap is itself a keymap, it counts as if this
     inner keymap were inlined in the outer keymap.  This is used for
     multiple-inheritance, such as in ‘make-composed-keymap’.

   When the binding is ‘nil’, it doesn’t constitute a definition but it
does take precedence over a default binding or a binding in the parent
keymap.  On the other hand, a binding of ‘nil’ does _not_ override
lower-precedence keymaps; thus, if the local map gives a binding of
‘nil’, Emacs uses the binding from the global map.

   Keymaps do not directly record bindings for the meta characters.
Instead, meta characters are regarded for purposes of key lookup as
sequences of two characters, the first of which is <ESC> (or whatever is
currently the value of ‘meta-prefix-char’).  Thus, the key ‘M-a’ is
internally represented as ‘<ESC> a’, and its global binding is found at
the slot for ‘a’ in ‘esc-map’ (*note Prefix Keys::).

   This conversion applies only to characters, not to function keys or
other input events; thus, ‘M-<end>’ has nothing to do with ‘<ESC>
<end>’.

   Here as an example is the local keymap for Lisp mode, a sparse
keymap.  It defines bindings for <DEL>, ‘C-c C-z’, ‘C-M-q’, and ‘C-M-x’
(the actual value also contains a menu binding, which is omitted here
for the sake of brevity).

     lisp-mode-map
     ⇒
     (keymap
      (3 keymap
         ;; C-c C-z
         (26 . run-lisp))
      (27 keymap
          ;; ‘C-M-x’, treated as ‘<ESC> C-x’
          (24 . lisp-send-defun))
      ;; This part is inherited from ‘lisp-mode-shared-map’.
      keymap
      ;; <DEL>
      (127 . backward-delete-char-untabify)
      (27 keymap
          ;; ‘C-M-q’, treated as ‘<ESC> C-q’
          (17 . indent-sexp)))

 -- Function: keymapp object
     This function returns ‘t’ if OBJECT is a keymap, ‘nil’ otherwise.
     More precisely, this function tests for a list whose CAR is
     ‘keymap’, or for a symbol whose function definition satisfies
     ‘keymapp’.

          (keymapp '(keymap))
              ⇒ t
          (fset 'foo '(keymap))
          (keymapp 'foo)
              ⇒ t
          (keymapp (current-global-map))
              ⇒ t


File: elisp.info,  Node: Creating Keymaps,  Next: Inheritance and Keymaps,  Prev: Format of Keymaps,  Up: Keymaps

21.4 Creating Keymaps
=====================

Here we describe the functions for creating keymaps.

 -- Function: make-sparse-keymap &optional prompt
     This function creates and returns a new sparse keymap with no
     entries.  (A sparse keymap is the kind of keymap you usually want.)
     The new keymap does not contain a char-table, unlike ‘make-keymap’,
     and does not bind any events.

          (make-sparse-keymap)
              ⇒ (keymap)

     If you specify PROMPT, that becomes the overall prompt string for
     the keymap.  You should specify this only for menu keymaps (*note
     Defining Menus::).  A keymap with an overall prompt string will
     always present a mouse menu or a keyboard menu if it is active for
     looking up the next input event.  Don’t specify an overall prompt
     string for the main map of a major or minor mode, because that
     would cause the command loop to present a keyboard menu every time.

 -- Function: make-keymap &optional prompt
     This function creates and returns a new full keymap.  That keymap
     contains a char-table (*note Char-Tables::) with slots for all
     characters without modifiers.  The new keymap initially binds all
     these characters to ‘nil’, and does not bind any other kind of
     event.  The argument PROMPT specifies a prompt string, as in
     ‘make-sparse-keymap’.

          (make-keymap)
              ⇒ (keymap #^[nil nil keymap nil nil nil ...])

     A full keymap is more efficient than a sparse keymap when it holds
     lots of bindings; for just a few, the sparse keymap is better.

 -- Function: copy-keymap keymap
     This function returns a copy of KEYMAP.  Any keymaps that appear
     directly as bindings in KEYMAP are also copied recursively, and so
     on to any number of levels.  However, recursive copying does not
     take place when the definition of a character is a symbol whose
     function definition is a keymap; the same symbol appears in the new
     copy.

          (setq map (copy-keymap (current-local-map)))
          ⇒ (keymap
               ;; (This implements meta characters.)
               (27 keymap
                   (83 . center-paragraph)
                   (115 . center-line))
               (9 . tab-to-tab-stop))

          (eq map (current-local-map))
              ⇒ nil
          (equal map (current-local-map))
              ⇒ t


File: elisp.info,  Node: Inheritance and Keymaps,  Next: Prefix Keys,  Prev: Creating Keymaps,  Up: Keymaps

21.5 Inheritance and Keymaps
============================

A keymap can inherit the bindings of another keymap, which we call the
“parent keymap”.  Such a keymap looks like this:

     (keymap ELEMENTS... . PARENT-KEYMAP)

The effect is that this keymap inherits all the bindings of
PARENT-KEYMAP, whatever they may be at the time a key is looked up, but
can add to them or override them with ELEMENTS.

   If you change the bindings in PARENT-KEYMAP using ‘define-key’ or
other key-binding functions, these changed bindings are visible in the
inheriting keymap, unless shadowed by the bindings made by ELEMENTS.
The converse is not true: if you use ‘define-key’ to change bindings in
the inheriting keymap, these changes are recorded in ELEMENTS, but have
no effect on PARENT-KEYMAP.

   The proper way to construct a keymap with a parent is to use
‘set-keymap-parent’; if you have code that directly constructs a keymap
with a parent, please convert the program to use ‘set-keymap-parent’
instead.

 -- Function: keymap-parent keymap
     This returns the parent keymap of KEYMAP.  If KEYMAP has no parent,
     ‘keymap-parent’ returns ‘nil’.

 -- Function: set-keymap-parent keymap parent
     This sets the parent keymap of KEYMAP to PARENT, and returns
     PARENT.  If PARENT is ‘nil’, this function gives KEYMAP no parent
     at all.

     If KEYMAP has submaps (bindings for prefix keys), they too receive
     new parent keymaps that reflect what PARENT specifies for those
     prefix keys.

   Here is an example showing how to make a keymap that inherits from
‘text-mode-map’:

     (let ((map (make-sparse-keymap)))
       (set-keymap-parent map text-mode-map)
       map)

   A non-sparse keymap can have a parent too, but this is not very
useful.  A non-sparse keymap always specifies something as the binding
for every numeric character code without modifier bits, even if it is
‘nil’, so these character’s bindings are never inherited from the parent
keymap.

   Sometimes you want to make a keymap that inherits from more than one
map.  You can use the function ‘make-composed-keymap’ for this.

 -- Function: make-composed-keymap maps &optional parent
     This function returns a new keymap composed of the existing
     keymap(s) MAPS, and optionally inheriting from a parent keymap
     PARENT.  MAPS can be a single keymap or a list of more than one.
     When looking up a key in the resulting new map, Emacs searches in
     each of the MAPS in turn, and then in PARENT, stopping at the first
     match.  A ‘nil’ binding in any one of MAPS overrides any binding in
     PARENT, but it does not override any non-‘nil’ binding in any other
     of the MAPS.

For example, here is how Emacs sets the parent of ‘help-mode-map’, such
that it inherits from both ‘button-buffer-map’ and ‘special-mode-map’:

     (defvar help-mode-map
       (let ((map (make-sparse-keymap)))
         (set-keymap-parent map
           (make-composed-keymap button-buffer-map special-mode-map))
         ... map) ... )


File: elisp.info,  Node: Prefix Keys,  Next: Active Keymaps,  Prev: Inheritance and Keymaps,  Up: Keymaps

21.6 Prefix Keys
================

A “prefix key” is a key sequence whose binding is a keymap.  The keymap
defines what to do with key sequences that extend the prefix key.  For
example, ‘C-x’ is a prefix key, and it uses a keymap that is also stored
in the variable ‘ctl-x-map’.  This keymap defines bindings for key
sequences starting with ‘C-x’.

   Some of the standard Emacs prefix keys use keymaps that are also
found in Lisp variables:

   • ‘esc-map’ is the global keymap for the <ESC> prefix key.  Thus, the
     global definitions of all meta characters are actually found here.
     This map is also the function definition of ‘ESC-prefix’.

   • ‘help-map’ is the global keymap for the ‘C-h’ prefix key.

   • ‘mode-specific-map’ is the global keymap for the prefix key ‘C-c’.
     This map is actually global, not mode-specific, but its name
     provides useful information about ‘C-c’ in the output of ‘C-h b’
     (‘display-bindings’), since the main use of this prefix key is for
     mode-specific bindings.

   • ‘ctl-x-map’ is the global keymap used for the ‘C-x’ prefix key.
     This map is found via the function cell of the symbol
     ‘Control-X-prefix’.

   • ‘mule-keymap’ is the global keymap used for the ‘C-x <RET>’ prefix
     key.

   • ‘ctl-x-4-map’ is the global keymap used for the ‘C-x 4’ prefix key.

   • ‘ctl-x-5-map’ is the global keymap used for the ‘C-x 5’ prefix key.

   • ‘2C-mode-map’ is the global keymap used for the ‘C-x 6’ prefix key.

   • ‘vc-prefix-map’ is the global keymap used for the ‘C-x v’ prefix
     key.

   • ‘goto-map’ is the global keymap used for the ‘M-g’ prefix key.

   • ‘search-map’ is the global keymap used for the ‘M-s’ prefix key.

   • ‘facemenu-keymap’ is the global keymap used for the ‘M-o’ prefix
     key.

   • The other Emacs prefix keys are ‘C-x @’, ‘C-x a i’, ‘C-x <ESC>’ and
     ‘<ESC> <ESC>’.  They use keymaps that have no special names.

   The keymap binding of a prefix key is used for looking up the event
that follows the prefix key.  (It may instead be a symbol whose function
definition is a keymap.  The effect is the same, but the symbol serves
as a name for the prefix key.)  Thus, the binding of ‘C-x’ is the symbol
‘Control-X-prefix’, whose function cell holds the keymap for ‘C-x’
commands.  (The same keymap is also the value of ‘ctl-x-map’.)

   Prefix key definitions can appear in any active keymap.  The
definitions of ‘C-c’, ‘C-x’, ‘C-h’ and <ESC> as prefix keys appear in
the global map, so these prefix keys are always available.  Major and
minor modes can redefine a key as a prefix by putting a prefix key
definition for it in the local map or the minor mode’s map.  *Note
Active Keymaps::.

   If a key is defined as a prefix in more than one active map, then its
various definitions are in effect merged: the commands defined in the
minor mode keymaps come first, followed by those in the local map’s
prefix definition, and then by those from the global map.

   In the following example, we make ‘C-p’ a prefix key in the local
keymap, in such a way that ‘C-p’ is identical to ‘C-x’.  Then the
binding for ‘C-p C-f’ is the function ‘find-file’, just like ‘C-x C-f’.
The key sequence ‘C-p 6’ is not found in any active keymap.

     (use-local-map (make-sparse-keymap))
         ⇒ nil
     (local-set-key "\C-p" ctl-x-map)
         ⇒ nil
     (key-binding "\C-p\C-f")
         ⇒ find-file

     (key-binding "\C-p6")
         ⇒ nil

 -- Function: define-prefix-command symbol &optional mapvar prompt
     This function prepares SYMBOL for use as a prefix key’s binding: it
     creates a sparse keymap and stores it as SYMBOL’s function
     definition.  Subsequently binding a key sequence to SYMBOL will
     make that key sequence into a prefix key.  The return value is
     ‘symbol’.

     This function also sets SYMBOL as a variable, with the keymap as
     its value.  But if MAPVAR is non-‘nil’, it sets MAPVAR as a
     variable instead.

     If PROMPT is non-‘nil’, that becomes the overall prompt string for
     the keymap.  The prompt string should be given for menu keymaps
     (*note Defining Menus::).


File: elisp.info,  Node: Active Keymaps,  Next: Searching Keymaps,  Prev: Prefix Keys,  Up: Keymaps

21.7 Active Keymaps
===================

Emacs contains many keymaps, but at any time only a few keymaps are
“active”.  When Emacs receives user input, it translates the input event
(*note Translation Keymaps::), and looks for a key binding in the active
keymaps.

   Usually, the active keymaps are: (i) the keymap specified by the
‘keymap’ property, (ii) the keymaps of enabled minor modes, (iii) the
current buffer’s local keymap, and (iv) the global keymap, in that
order.  Emacs searches for each input key sequence in all these keymaps.

   Of these usual keymaps, the highest-precedence one is specified by
the ‘keymap’ text or overlay property at point, if any.  (For a mouse
input event, Emacs uses the event position instead of point; *note
Searching Keymaps::.)

   Next in precedence are keymaps specified by enabled minor modes.
These keymaps, if any, are specified by the variables
‘emulation-mode-map-alists’, ‘minor-mode-overriding-map-alist’, and
‘minor-mode-map-alist’.  *Note Controlling Active Maps::.

   Next in precedence is the buffer’s “local keymap”, containing key
bindings specific to the buffer.  The minibuffer also has a local keymap
(*note Intro to Minibuffers::).  If there is a ‘local-map’ text or
overlay property at point, that specifies the local keymap to use, in
place of the buffer’s default local keymap.

   The local keymap is normally set by the buffer’s major mode, and
every buffer with the same major mode shares the same local keymap.
Hence, if you call ‘local-set-key’ (*note Key Binding Commands::) to
change the local keymap in one buffer, that also affects the local
keymaps in other buffers with the same major mode.

   Finally, the “global keymap” contains key bindings that are defined
regardless of the current buffer, such as ‘C-f’.  It is always active,
and is bound to the variable ‘global-map’.

   Apart from the above usual keymaps, Emacs provides special ways for
programs to make other keymaps active.  Firstly, the variable
‘overriding-local-map’ specifies a keymap that replaces the usual active
keymaps, except for the global keymap.  Secondly, the terminal-local
variable ‘overriding-terminal-local-map’ specifies a keymap that takes
precedence over _all_ other keymaps (including ‘overriding-local-map’);
this is normally used for modal/transient keybindings (the function
‘set-transient-map’ provides a convenient interface for this).  *Note
Controlling Active Maps::, for details.

   Making keymaps active is not the only way to use them.  Keymaps are
also used in other ways, such as for translating events within
‘read-key-sequence’.  *Note Translation Keymaps::.

   *Note Standard Keymaps::, for a list of some standard keymaps.

 -- Function: current-active-maps &optional olp position
     This returns the list of active keymaps that would be used by the
     command loop in the current circumstances to look up a key
     sequence.  Normally it ignores ‘overriding-local-map’ and
     ‘overriding-terminal-local-map’, but if OLP is non-‘nil’ then it
     pays attention to them.  POSITION can optionally be either an event
     position as returned by ‘event-start’ or a buffer position, and may
     change the keymaps as described for ‘key-binding’.

 -- Function: key-binding key &optional accept-defaults no-remap
          position
     This function returns the binding for KEY according to the current
     active keymaps.  The result is ‘nil’ if KEY is undefined in the
     keymaps.

     The argument ACCEPT-DEFAULTS controls checking for default
     bindings, as in ‘lookup-key’ (*note Functions for Key Lookup::).

     When commands are remapped (*note Remapping Commands::),
     ‘key-binding’ normally processes command remappings so as to return
     the remapped command that will actually be executed.  However, if
     NO-REMAP is non-‘nil’, ‘key-binding’ ignores remappings and returns
     the binding directly specified for KEY.

     If KEY starts with a mouse event (perhaps following a prefix
     event), the maps to be consulted are determined based on the
     event’s position.  Otherwise, they are determined based on the
     value of point.  However, you can override either of them by
     specifying POSITION.  If POSITION is non-‘nil’, it should be either
     a buffer position or an event position like the value of
     ‘event-start’.  Then the maps consulted are determined based on
     POSITION.

     Emacs signals an error if KEY is not a string or a vector.

          (key-binding "\C-x\C-f")
              ⇒ find-file


File: elisp.info,  Node: Searching Keymaps,  Next: Controlling Active Maps,  Prev: Active Keymaps,  Up: Keymaps

21.8 Searching the Active Keymaps
=================================

Here is a pseudo-Lisp summary of how Emacs searches the active keymaps:

     (or (if overriding-terminal-local-map
             (FIND-IN overriding-terminal-local-map))
         (if overriding-local-map
             (FIND-IN overriding-local-map)
           (or (FIND-IN (get-char-property (point) 'keymap))
               (FIND-IN-ANY emulation-mode-map-alists)
               (FIND-IN-ANY minor-mode-overriding-map-alist)
               (FIND-IN-ANY minor-mode-map-alist)
               (if (get-text-property (point) 'local-map)
                   (FIND-IN (get-char-property (point) 'local-map))
                 (FIND-IN (current-local-map)))))
         (FIND-IN (current-global-map)))

Here, FIND-IN and FIND-IN-ANY are pseudo functions that search in one
keymap and in an alist of keymaps, respectively.  Note that the
‘set-transient-map’ function works by setting
‘overriding-terminal-local-map’ (*note Controlling Active Maps::).

   In the above pseudo-code, if a key sequence starts with a mouse event
(*note Mouse Events::), that event’s position is used instead of point,
and the event’s buffer is used instead of the current buffer.  In
particular, this affects how the ‘keymap’ and ‘local-map’ properties are
looked up.  If a mouse event occurs on a string embedded with a
‘display’, ‘before-string’, or ‘after-string’ property (*note Special
Properties::), and the string has a non-‘nil’ ‘keymap’ or ‘local-map’
property, that overrides the corresponding property in the underlying
buffer text (i.e., the property specified by the underlying text is
ignored).

   When a key binding is found in one of the active keymaps, and that
binding is a command, the search is over—the command is executed.
However, if the binding is a symbol with a value or a string, Emacs
replaces the input key sequences with the variable’s value or the
string, and restarts the search of the active keymaps.  *Note Key
Lookup::.

   The command which is finally found might also be remapped.  *Note
Remapping Commands::.


File: elisp.info,  Node: Controlling Active Maps,  Next: Key Lookup,  Prev: Searching Keymaps,  Up: Keymaps

21.9 Controlling the Active Keymaps
===================================

 -- Variable: global-map
     This variable contains the default global keymap that maps Emacs
     keyboard input to commands.  The global keymap is normally this
     keymap.  The default global keymap is a full keymap that binds
     ‘self-insert-command’ to all of the printing characters.

     It is normal practice to change the bindings in the global keymap,
     but you should not assign this variable any value other than the
     keymap it starts out with.

 -- Function: current-global-map
     This function returns the current global keymap.  This is the same
     as the value of ‘global-map’ unless you change one or the other.
     The return value is a reference, not a copy; if you use
     ‘define-key’ or other functions on it you will alter global
     bindings.

          (current-global-map)
          ⇒ (keymap [set-mark-command beginning-of-line ...
                      delete-backward-char])

 -- Function: current-local-map
     This function returns the current buffer’s local keymap, or ‘nil’
     if it has none.  In the following example, the keymap for the
     ‘*scratch*’ buffer (using Lisp Interaction mode) is a sparse keymap
     in which the entry for <ESC>, ASCII code 27, is another sparse
     keymap.

          (current-local-map)
          ⇒ (keymap
              (10 . eval-print-last-sexp)
              (9 . lisp-indent-line)
              (127 . backward-delete-char-untabify)
              (27 keymap
                  (24 . eval-defun)
                  (17 . indent-sexp)))

   ‘current-local-map’ returns a reference to the local keymap, not a
copy of it; if you use ‘define-key’ or other functions on it you will
alter local bindings.

 -- Function: current-minor-mode-maps
     This function returns a list of the keymaps of currently enabled
     minor modes.

 -- Function: use-global-map keymap
     This function makes KEYMAP the new current global keymap.  It
     returns ‘nil’.

     It is very unusual to change the global keymap.

 -- Function: use-local-map keymap
     This function makes KEYMAP the new local keymap of the current
     buffer.  If KEYMAP is ‘nil’, then the buffer has no local keymap.
     ‘use-local-map’ returns ‘nil’.  Most major mode commands use this
     function.

 -- Variable: minor-mode-map-alist
     This variable is an alist describing keymaps that may or may not be
     active according to the values of certain variables.  Its elements
     look like this:

          (VARIABLE . KEYMAP)

     The keymap KEYMAP is active whenever VARIABLE has a non-‘nil’
     value.  Typically VARIABLE is the variable that enables or disables
     a minor mode.  *Note Keymaps and Minor Modes::.

     Note that elements of ‘minor-mode-map-alist’ do not have the same
     structure as elements of ‘minor-mode-alist’.  The map must be the
     CDR of the element; a list with the map as the second element will
     not do.  The CDR can be either a keymap (a list) or a symbol whose
     function definition is a keymap.

     When more than one minor mode keymap is active, the earlier one in
     ‘minor-mode-map-alist’ takes priority.  But you should design minor
     modes so that they don’t interfere with each other.  If you do this
     properly, the order will not matter.

     See *note Keymaps and Minor Modes::, for more information about
     minor modes.  See also ‘minor-mode-key-binding’ (*note Functions
     for Key Lookup::).

 -- Variable: minor-mode-overriding-map-alist
     This variable allows major modes to override the key bindings for
     particular minor modes.  The elements of this alist look like the
     elements of ‘minor-mode-map-alist’: ‘(VARIABLE . KEYMAP)’.

     If a variable appears as an element of
     ‘minor-mode-overriding-map-alist’, the map specified by that
     element totally replaces any map specified for the same variable in
     ‘minor-mode-map-alist’.

     ‘minor-mode-overriding-map-alist’ is automatically buffer-local in
     all buffers.

 -- Variable: overriding-local-map
     If non-‘nil’, this variable holds a keymap to use instead of the
     buffer’s local keymap, any text property or overlay keymaps, and
     any minor mode keymaps.  This keymap, if specified, overrides all
     other maps that would have been active, except for the current
     global map.

 -- Variable: overriding-terminal-local-map
     If non-‘nil’, this variable holds a keymap to use instead of
     ‘overriding-local-map’, the buffer’s local keymap, text property or
     overlay keymaps, and all the minor mode keymaps.

     This variable is always local to the current terminal and cannot be
     buffer-local.  *Note Multiple Terminals::.  It is used to implement
     incremental search mode.

 -- Variable: overriding-local-map-menu-flag
     If this variable is non-‘nil’, the value of ‘overriding-local-map’
     or ‘overriding-terminal-local-map’ can affect the display of the
     menu bar.  The default value is ‘nil’, so those map variables have
     no effect on the menu bar.

     Note that these two map variables do affect the execution of key
     sequences entered using the menu bar, even if they do not affect
     the menu bar display.  So if a menu bar key sequence comes in, you
     should clear the variables before looking up and executing that key
     sequence.  Modes that use the variables would typically do this
     anyway; normally they respond to events that they do not handle by
     “unreading” them and exiting.

 -- Variable: special-event-map
     This variable holds a keymap for special events.  If an event type
     has a binding in this keymap, then it is special, and the binding
     for the event is run directly by ‘read-event’.  *Note Special
     Events::.

 -- Variable: emulation-mode-map-alists
     This variable holds a list of keymap alists to use for emulation
     modes.  It is intended for modes or packages using multiple
     minor-mode keymaps.  Each element is a keymap alist which has the
     same format and meaning as ‘minor-mode-map-alist’, or a symbol with
     a variable binding which is such an alist.  The active keymaps in
     each alist are used before ‘minor-mode-map-alist’ and
     ‘minor-mode-overriding-map-alist’.

 -- Function: set-transient-map keymap &optional keep-pred on-exit
     This function adds KEYMAP as a “transient” keymap, which takes
     precedence over other keymaps for one (or more) subsequent keys.

     Normally, KEYMAP is used just once, to look up the very next key.
     If the optional argument KEEP-PRED is ‘t’, the map stays active as
     long as the user types keys defined in KEYMAP; when the user types
     a key that is not in KEYMAP, the transient keymap is deactivated
     and normal key lookup continues for that key.

     The KEEP-PRED argument can also be a function.  In that case, the
     function is called with no arguments, prior to running each
     command, while KEYMAP is active; it should return non-‘nil’ if
     KEYMAP should stay active.

     The optional argument ON-EXIT, if non-nil, specifies a function
     that is called, with no arguments, after KEYMAP is deactivated.

     This function works by adding and removing KEYMAP from the variable
     ‘overriding-terminal-local-map’, which takes precedence over all
     other active keymaps (*note Searching Keymaps::).


File: elisp.info,  Node: Key Lookup,  Next: Functions for Key Lookup,  Prev: Controlling Active Maps,  Up: Keymaps

21.10 Key Lookup
================

“Key lookup” is the process of finding the binding of a key sequence
from a given keymap.  The execution or use of the binding is not part of
key lookup.

   Key lookup uses just the event type of each event in the key
sequence; the rest of the event is ignored.  In fact, a key sequence
used for key lookup may designate a mouse event with just its types (a
symbol) instead of the entire event (a list).  *Note Input Events::.
Such a key sequence is insufficient for ‘command-execute’ to run, but it
is sufficient for looking up or rebinding a key.

   When the key sequence consists of multiple events, key lookup
processes the events sequentially: the binding of the first event is
found, and must be a keymap; then the second event’s binding is found in
that keymap, and so on until all the events in the key sequence are used
up.  (The binding thus found for the last event may or may not be a
keymap.)  Thus, the process of key lookup is defined in terms of a
simpler process for looking up a single event in a keymap.  How that is
done depends on the type of object associated with the event in that
keymap.

   Let’s use the term “keymap entry” to describe the value found by
looking up an event type in a keymap.  (This doesn’t include the item
string and other extra elements in a keymap element for a menu item,
because ‘lookup-key’ and other key lookup functions don’t include them
in the returned value.)  While any Lisp object may be stored in a keymap
as a keymap entry, not all make sense for key lookup.  Here is a table
of the meaningful types of keymap entries:

‘nil’
     ‘nil’ means that the events used so far in the lookup form an
     undefined key.  When a keymap fails to mention an event type at
     all, and has no default binding, that is equivalent to a binding of
     ‘nil’ for that event type.

COMMAND
     The events used so far in the lookup form a complete key, and
     COMMAND is its binding.  *Note What Is a Function::.

ARRAY
     The array (either a string or a vector) is a keyboard macro.  The
     events used so far in the lookup form a complete key, and the array
     is its binding.  See *note Keyboard Macros::, for more information.

KEYMAP
     The events used so far in the lookup form a prefix key.  The next
     event of the key sequence is looked up in KEYMAP.

LIST
     The meaning of a list depends on what it contains:

        • If the CAR of LIST is the symbol ‘keymap’, then the list is a
          keymap, and is treated as a keymap (see above).

        • If the CAR of LIST is ‘lambda’, then the list is a lambda
          expression.  This is presumed to be a function, and is treated
          as such (see above).  In order to execute properly as a key
          binding, this function must be a command—it must have an
          ‘interactive’ specification.  *Note Defining Commands::.

SYMBOL
     The function definition of SYMBOL is used in place of SYMBOL.  If
     that too is a symbol, then this process is repeated, any number of
     times.  Ultimately this should lead to an object that is a keymap,
     a command, or a keyboard macro.

     Note that keymaps and keyboard macros (strings and vectors) are not
     valid functions, so a symbol with a keymap, string, or vector as
     its function definition is invalid as a function.  It is, however,
     valid as a key binding.  If the definition is a keyboard macro,
     then the symbol is also valid as an argument to ‘command-execute’
     (*note Interactive Call::).

     The symbol ‘undefined’ is worth special mention: it means to treat
     the key as undefined.  Strictly speaking, the key is defined, and
     its binding is the command ‘undefined’; but that command does the
     same thing that is done automatically for an undefined key: it
     rings the bell (by calling ‘ding’) but does not signal an error.

     ‘undefined’ is used in local keymaps to override a global key
     binding and make the key undefined locally.  A local binding of
     ‘nil’ would fail to do this because it would not override the
     global binding.

ANYTHING ELSE
     If any other type of object is found, the events used so far in the
     lookup form a complete key, and the object is its binding, but the
     binding is not executable as a command.

   In short, a keymap entry may be a keymap, a command, a keyboard
macro, a symbol that leads to one of them, or ‘nil’.


File: elisp.info,  Node: Functions for Key Lookup,  Next: Changing Key Bindings,  Prev: Key Lookup,  Up: Keymaps

21.11 Functions for Key Lookup
==============================

Here are the functions and variables pertaining to key lookup.

 -- Function: lookup-key keymap key &optional accept-defaults
     This function returns the definition of KEY in KEYMAP.  All the
     other functions described in this chapter that look up keys use
     ‘lookup-key’.  Here are examples:

          (lookup-key (current-global-map) "\C-x\C-f")
              ⇒ find-file
          (lookup-key (current-global-map) (kbd "C-x C-f"))
              ⇒ find-file
          (lookup-key (current-global-map) "\C-x\C-f12345")
              ⇒ 2

     If the string or vector KEY is not a valid key sequence according
     to the prefix keys specified in KEYMAP, it must be too long and
     have extra events at the end that do not fit into a single key
     sequence.  Then the value is a number, the number of events at the
     front of KEY that compose a complete key.

     If ACCEPT-DEFAULTS is non-‘nil’, then ‘lookup-key’ considers
     default bindings as well as bindings for the specific events in
     KEY.  Otherwise, ‘lookup-key’ reports only bindings for the
     specific sequence KEY, ignoring default bindings except when you
     explicitly ask about them.  (To do this, supply ‘t’ as an element
     of KEY; see *note Format of Keymaps::.)

     If KEY contains a meta character (not a function key), that
     character is implicitly replaced by a two-character sequence: the
     value of ‘meta-prefix-char’, followed by the corresponding non-meta
     character.  Thus, the first example below is handled by conversion
     into the second example.

          (lookup-key (current-global-map) "\M-f")
              ⇒ forward-word
          (lookup-key (current-global-map) "\ef")
              ⇒ forward-word

     Unlike ‘read-key-sequence’, this function does not modify the
     specified events in ways that discard information (*note Key
     Sequence Input::).  In particular, it does not convert letters to
     lower case and it does not change drag events to clicks.

 -- Command: undefined
     Used in keymaps to undefine keys.  It calls ‘ding’, but does not
     cause an error.

 -- Function: local-key-binding key &optional accept-defaults
     This function returns the binding for KEY in the current local
     keymap, or ‘nil’ if it is undefined there.

     The argument ACCEPT-DEFAULTS controls checking for default
     bindings, as in ‘lookup-key’ (above).

 -- Function: global-key-binding key &optional accept-defaults
     This function returns the binding for command KEY in the current
     global keymap, or ‘nil’ if it is undefined there.

     The argument ACCEPT-DEFAULTS controls checking for default
     bindings, as in ‘lookup-key’ (above).

 -- Function: minor-mode-key-binding key &optional accept-defaults
     This function returns a list of all the active minor mode bindings
     of KEY.  More precisely, it returns an alist of pairs ‘(MODENAME .
     BINDING)’, where MODENAME is the variable that enables the minor
     mode, and BINDING is KEY’s binding in that mode.  If KEY has no
     minor-mode bindings, the value is ‘nil’.

     If the first binding found is not a prefix definition (a keymap or
     a symbol defined as a keymap), all subsequent bindings from other
     minor modes are omitted, since they would be completely shadowed.
     Similarly, the list omits non-prefix bindings that follow prefix
     bindings.

     The argument ACCEPT-DEFAULTS controls checking for default
     bindings, as in ‘lookup-key’ (above).

 -- User Option: meta-prefix-char
     This variable is the meta-prefix character code.  It is used for
     translating a meta character to a two-character sequence so it can
     be looked up in a keymap.  For useful results, the value should be
     a prefix event (*note Prefix Keys::).  The default value is 27,
     which is the ASCII code for <ESC>.

     As long as the value of ‘meta-prefix-char’ remains 27, key lookup
     translates ‘M-b’ into ‘<ESC> b’, which is normally defined as the
     ‘backward-word’ command.  However, if you were to set
     ‘meta-prefix-char’ to 24, the code for ‘C-x’, then Emacs will
     translate ‘M-b’ into ‘C-x b’, whose standard binding is the
     ‘switch-to-buffer’ command.  (Don’t actually do this!)  Here is an
     illustration of what would happen:

          meta-prefix-char                    ; The default value.
               ⇒ 27
          (key-binding "\M-b")
               ⇒ backward-word
          ?\C-x                               ; The print representation
               ⇒ 24                          ;   of a character.
          (setq meta-prefix-char 24)
               ⇒ 24
          (key-binding "\M-b")
               ⇒ switch-to-buffer            ; Now, typing ‘M-b’ is
                                              ;   like typing ‘C-x b’.

          (setq meta-prefix-char 27)          ; Avoid confusion!
               ⇒ 27                          ; Restore the default value!

     This translation of one event into two happens only for characters,
     not for other kinds of input events.  Thus, ‘M-<F1>’, a function
     key, is not converted into ‘<ESC> <F1>’.


File: elisp.info,  Node: Changing Key Bindings,  Next: Remapping Commands,  Prev: Functions for Key Lookup,  Up: Keymaps

21.12 Changing Key Bindings
===========================

The way to rebind a key is to change its entry in a keymap.  If you
change a binding in the global keymap, the change is effective in all
buffers (though it has no direct effect in buffers that shadow the
global binding with a local one).  If you change the current buffer’s
local map, that usually affects all buffers using the same major mode.
The ‘global-set-key’ and ‘local-set-key’ functions are convenient
interfaces for these operations (*note Key Binding Commands::).  You can
also use ‘define-key’, a more general function; then you must explicitly
specify the map to change.

   When choosing the key sequences for Lisp programs to rebind, please
follow the Emacs conventions for use of various keys (*note Key Binding
Conventions::).

   In writing the key sequence to rebind, it is good to use the special
escape sequences for control and meta characters (*note String Type::).
The syntax ‘\C-’ means that the following character is a control
character and ‘\M-’ means that the following character is a meta
character.  Thus, the string ‘"\M-x"’ is read as containing a single
‘M-x’, ‘"\C-f"’ is read as containing a single ‘C-f’, and ‘"\M-\C-x"’
and ‘"\C-\M-x"’ are both read as containing a single ‘C-M-x’.  You can
also use this escape syntax in vectors, as well as others that aren’t
allowed in strings; one example is ‘[?\C-\H-x home]’.  *Note Character
Type::.

   The key definition and lookup functions accept an alternate syntax
for event types in a key sequence that is a vector: you can use a list
containing modifier names plus one base event (a character or function
key name).  For example, ‘(control ?a)’ is equivalent to ‘?\C-a’ and
‘(hyper control left)’ is equivalent to ‘C-H-left’.  One advantage of
such lists is that the precise numeric codes for the modifier bits don’t
appear in compiled files.

   The functions below signal an error if KEYMAP is not a keymap, or if
KEY is not a string or vector representing a key sequence.  You can use
event types (symbols) as shorthand for events that are lists.  The ‘kbd’
function (*note Key Sequences::) is a convenient way to specify the key
sequence.

 -- Function: define-key keymap key binding
     This function sets the binding for KEY in KEYMAP.  (If KEY is more
     than one event long, the change is actually made in another keymap
     reached from KEYMAP.)  The argument BINDING can be any Lisp object,
     but only certain types are meaningful.  (For a list of meaningful
     types, see *note Key Lookup::.)  The value returned by ‘define-key’
     is BINDING.

     If KEY is ‘[t]’, this sets the default binding in KEYMAP.  When an
     event has no binding of its own, the Emacs command loop uses the
     keymap’s default binding, if there is one.

     Every prefix of KEY must be a prefix key (i.e., bound to a keymap)
     or undefined; otherwise an error is signaled.  If some prefix of
     KEY is undefined, then ‘define-key’ defines it as a prefix key so
     that the rest of KEY can be defined as specified.

     If there was previously no binding for KEY in KEYMAP, the new
     binding is added at the beginning of KEYMAP.  The order of bindings
     in a keymap makes no difference for keyboard input, but it does
     matter for menu keymaps (*note Menu Keymaps::).

   This example creates a sparse keymap and makes a number of bindings
in it:

     (setq map (make-sparse-keymap))
         ⇒ (keymap)
     (define-key map "\C-f" 'forward-char)
         ⇒ forward-char
     map
         ⇒ (keymap (6 . forward-char))

     ;; Build sparse submap for ‘C-x’ and bind ‘f’ in that.
     (define-key map (kbd "C-x f") 'forward-word)
         ⇒ forward-word
     map
     ⇒ (keymap
         (24 keymap                ; C-x
             (102 . forward-word)) ;      f
         (6 . forward-char))       ; C-f

     ;; Bind ‘C-p’ to the ‘ctl-x-map’.
     (define-key map (kbd "C-p") ctl-x-map)
     ;; ctl-x-map
     ⇒ [nil ... find-file ... backward-kill-sentence]

     ;; Bind ‘C-f’ to ‘foo’ in the ‘ctl-x-map’.
     (define-key map (kbd "C-p C-f") 'foo)
     ⇒ 'foo
     map
     ⇒ (keymap     ; Note ‘foo’ in ‘ctl-x-map’.
         (16 keymap [nil ... foo ... backward-kill-sentence])
         (24 keymap
             (102 . forward-word))
         (6 . forward-char))

Note that storing a new binding for ‘C-p C-f’ actually works by changing
an entry in ‘ctl-x-map’, and this has the effect of changing the
bindings of both ‘C-p C-f’ and ‘C-x C-f’ in the default global map.

   The function ‘substitute-key-definition’ scans a keymap for keys that
have a certain binding and rebinds them with a different binding.
Another feature which is cleaner and can often produce the same results
is to remap one command into another (*note Remapping Commands::).

 -- Function: substitute-key-definition olddef newdef keymap &optional
          oldmap
     This function replaces OLDDEF with NEWDEF for any keys in KEYMAP
     that were bound to OLDDEF.  In other words, OLDDEF is replaced with
     NEWDEF wherever it appears.  The function returns ‘nil’.

     For example, this redefines ‘C-x C-f’, if you do it in an Emacs
     with standard bindings:

          (substitute-key-definition
           'find-file 'find-file-read-only (current-global-map))

     If OLDMAP is non-‘nil’, that changes the behavior of
     ‘substitute-key-definition’: the bindings in OLDMAP determine which
     keys to rebind.  The rebindings still happen in KEYMAP, not in
     OLDMAP.  Thus, you can change one map under the control of the
     bindings in another.  For example,

          (substitute-key-definition
            'delete-backward-char 'my-funny-delete
            my-map global-map)

     puts the special deletion command in ‘my-map’ for whichever keys
     are globally bound to the standard deletion command.

     Here is an example showing a keymap before and after substitution:

          (setq map '(keymap
                      (?1 . olddef-1)
                      (?2 . olddef-2)
                      (?3 . olddef-1)))
          ⇒ (keymap (49 . olddef-1) (50 . olddef-2) (51 . olddef-1))

          (substitute-key-definition 'olddef-1 'newdef map)
          ⇒ nil
          map
          ⇒ (keymap (49 . newdef) (50 . olddef-2) (51 . newdef))

 -- Function: suppress-keymap keymap &optional nodigits
     This function changes the contents of the full keymap KEYMAP by
     remapping ‘self-insert-command’ to the command ‘undefined’ (*note
     Remapping Commands::).  This has the effect of undefining all
     printing characters, thus making ordinary insertion of text
     impossible.  ‘suppress-keymap’ returns ‘nil’.

     If NODIGITS is ‘nil’, then ‘suppress-keymap’ defines digits to run
     ‘digit-argument’, and ‘-’ to run ‘negative-argument’.  Otherwise it
     makes them undefined like the rest of the printing characters.

     The ‘suppress-keymap’ function does not make it impossible to
     modify a buffer, as it does not suppress commands such as ‘yank’
     and ‘quoted-insert’.  To prevent any modification of a buffer, make
     it read-only (*note Read Only Buffers::).

     Since this function modifies KEYMAP, you would normally use it on a
     newly created keymap.  Operating on an existing keymap that is used
     for some other purpose is likely to cause trouble; for example,
     suppressing ‘global-map’ would make it impossible to use most of
     Emacs.

     This function can be used to initialize the local keymap of a major
     mode for which insertion of text is not desirable.  But usually
     such a mode should be derived from ‘special-mode’ (*note Basic
     Major Modes::); then its keymap will automatically inherit from
     ‘special-mode-map’, which is already suppressed.  Here is how
     ‘special-mode-map’ is defined:

          (defvar special-mode-map
            (let ((map (make-sparse-keymap)))
              (suppress-keymap map)
              (define-key map "q" 'quit-window)
              ...
              map))


File: elisp.info,  Node: Remapping Commands,  Next: Translation Keymaps,  Prev: Changing Key Bindings,  Up: Keymaps

21.13 Remapping Commands
========================

A special kind of key binding can be used to “remap” one command to
another, without having to refer to the key sequence(s) bound to the
original command.  To use this feature, make a key binding for a key
sequence that starts with the dummy event ‘remap’, followed by the
command name you want to remap; for the binding, specify the new
definition (usually a command name, but possibly any other valid
definition for a key binding).

   For example, suppose My mode provides a special command
‘my-kill-line’, which should be invoked instead of ‘kill-line’.  To
establish this, its mode keymap should contain the following remapping:

     (define-key my-mode-map [remap kill-line] 'my-kill-line)

Then, whenever ‘my-mode-map’ is active, if the user types ‘C-k’ (the
default global key sequence for ‘kill-line’) Emacs will instead run
‘my-kill-line’.

   Note that remapping only takes place through active keymaps; for
example, putting a remapping in a prefix keymap like ‘ctl-x-map’
typically has no effect, as such keymaps are not themselves active.  In
addition, remapping only works through a single level; in the following
example,

     (define-key my-mode-map [remap kill-line] 'my-kill-line)
     (define-key my-mode-map [remap my-kill-line] 'my-other-kill-line)

‘kill-line’ is _not_ remapped to ‘my-other-kill-line’.  Instead, if an
ordinary key binding specifies ‘kill-line’, it is remapped to
‘my-kill-line’; if an ordinary binding specifies ‘my-kill-line’, it is
remapped to ‘my-other-kill-line’.

   To undo the remapping of a command, remap it to ‘nil’; e.g.,

     (define-key my-mode-map [remap kill-line] nil)

 -- Function: command-remapping command &optional position keymaps
     This function returns the remapping for COMMAND (a symbol), given
     the current active keymaps.  If COMMAND is not remapped (which is
     the usual situation), or not a symbol, the function returns ‘nil’.
     ‘position’ can optionally specify a buffer position or an event
     position to determine the keymaps to use, as in ‘key-binding’.

     If the optional argument ‘keymaps’ is non-‘nil’, it specifies a
     list of keymaps to search in.  This argument is ignored if
     ‘position’ is non-‘nil’.


File: elisp.info,  Node: Translation Keymaps,  Next: Key Binding Commands,  Prev: Remapping Commands,  Up: Keymaps

21.14 Keymaps for Translating Sequences of Events
=================================================

When the ‘read-key-sequence’ function reads a key sequence (*note Key
Sequence Input::), it uses “translation keymaps” to translate certain
event sequences into others.  The translation keymaps are
‘input-decode-map’, ‘local-function-key-map’, and ‘key-translation-map’
(in order of priority).

   Translation keymaps have the same structure as other keymaps, but are
used differently: they specify translations to make while reading key
sequences, rather than bindings for complete key sequences.  As each key
sequence is read, it is checked against each translation keymap.  If one
of the translation keymaps binds K to a vector V, then whenever K
appears as a sub-sequence _anywhere_ in a key sequence, that
sub-sequence is replaced with the events in V.

   For example, VT100 terminals send ‘<ESC> O P’ when the keypad key
<PF1> is pressed.  On such terminals, Emacs must translate that sequence
of events into a single event ‘pf1’.  This is done by binding ‘<ESC> O
P’ to ‘[pf1]’ in ‘input-decode-map’.  Thus, when you type ‘C-c <PF1>’ on
the terminal, the terminal emits the character sequence ‘C-c <ESC> O P’,
and ‘read-key-sequence’ translates this back into ‘C-c <PF1>’ and
returns it as the vector ‘[?\C-c pf1]’.

   Translation keymaps take effect only after Emacs has decoded the
keyboard input (via the input coding system specified by
‘keyboard-coding-system’).  *Note Terminal I/O Encoding::.

 -- Variable: input-decode-map
     This variable holds a keymap that describes the character sequences
     sent by function keys on an ordinary character terminal.

     The value of ‘input-decode-map’ is usually set up automatically
     according to the terminal’s Terminfo or Termcap entry, but
     sometimes those need help from terminal-specific Lisp files.  Emacs
     comes with terminal-specific files for many common terminals; their
     main purpose is to make entries in ‘input-decode-map’ beyond those
     that can be deduced from Termcap and Terminfo.  *Note
     Terminal-Specific::.

 -- Variable: local-function-key-map
     This variable holds a keymap similar to ‘input-decode-map’ except
     that it describes key sequences which should be translated to
     alternative interpretations that are usually preferred.  It applies
     after ‘input-decode-map’ and before ‘key-translation-map’.

     Entries in ‘local-function-key-map’ are ignored if they conflict
     with bindings made in the minor mode, local, or global keymaps.
     I.e., the remapping only applies if the original key sequence would
     otherwise not have any binding.

     ‘local-function-key-map’ inherits from ‘function-key-map’, but the
     latter should not be used directly.

 -- Variable: key-translation-map
     This variable is another keymap used just like ‘input-decode-map’
     to translate input events into other events.  It differs from
     ‘input-decode-map’ in that it goes to work after
     ‘local-function-key-map’ is finished rather than before; it
     receives the results of translation by ‘local-function-key-map’.

     Just like ‘input-decode-map’, but unlike ‘local-function-key-map’,
     this keymap is applied regardless of whether the input key-sequence
     has a normal binding.  Note however that actual key bindings can
     have an effect on ‘key-translation-map’, even though they are
     overridden by it.  Indeed, actual key bindings override
     ‘local-function-key-map’ and thus may alter the key sequence that
     ‘key-translation-map’ receives.  Clearly, it is better to avoid
     this type of situation.

     The intent of ‘key-translation-map’ is for users to map one
     character set to another, including ordinary characters normally
     bound to ‘self-insert-command’.

   You can use ‘input-decode-map’, ‘local-function-key-map’, and
‘key-translation-map’ for more than simple aliases, by using a function,
instead of a key sequence, as the translation of a key.  Then this
function is called to compute the translation of that key.

   The key translation function receives one argument, which is the
prompt that was specified in ‘read-key-sequence’—or ‘nil’ if the key
sequence is being read by the editor command loop.  In most cases you
can ignore the prompt value.

   If the function reads input itself, it can have the effect of
altering the event that follows.  For example, here’s how to define ‘C-c
h’ to turn the character that follows into a Hyper character:

     (defun hyperify (prompt)
       (let ((e (read-event)))
         (vector (if (numberp e)
                     (logior (lsh 1 24) e)
                   (if (memq 'hyper (event-modifiers e))
                       e
                     (add-event-modifier "H-" e))))))

     (defun add-event-modifier (string e)
       (let ((symbol (if (symbolp e) e (car e))))
         (setq symbol (intern (concat string
                                      (symbol-name symbol))))
         (if (symbolp e)
             symbol
           (cons symbol (cdr e)))))

     (define-key local-function-key-map "\C-ch" 'hyperify)

21.14.1 Interaction with normal keymaps
---------------------------------------

The end of a key sequence is detected when that key sequence either is
bound to a command, or when Emacs determines that no additional event
can lead to a sequence that is bound to a command.

   This means that, while ‘input-decode-map’ and ‘key-translation-map’
apply regardless of whether the original key sequence would have a
binding, the presence of such a binding can still prevent translation
from taking place.  For example, let us return to our VT100 example
above and add a binding for ‘C-c <ESC>’ to the global map; now when the
user hits ‘C-c <PF1>’ Emacs will fail to decode ‘C-c <ESC> O P’ into
‘C-c <PF1>’ because it will stop reading keys right after ‘C-x <ESC>’,
leaving ‘O P’ for later.  This is in case the user really hit ‘C-c
<ESC>’, in which case Emacs should not sit there waiting for the next
key to decide whether the user really pressed ‘<ESC>’ or ‘<PF1>’.

   For that reason, it is better to avoid binding commands to key
sequences where the end of the key sequence is a prefix of a key
translation.  The main such problematic suffixes/prefixes are ‘<ESC>’,
‘M-O’ (which is really ‘<ESC> O’) and ‘M-[’ (which is really ‘<ESC> [’).


File: elisp.info,  Node: Key Binding Commands,  Next: Scanning Keymaps,  Prev: Translation Keymaps,  Up: Keymaps

21.15 Commands for Binding Keys
===============================

This section describes some convenient interactive interfaces for
changing key bindings.  They work by calling ‘define-key’.

   People often use ‘global-set-key’ in their init files (*note Init
File::) for simple customization.  For example,

     (global-set-key (kbd "C-x C-\\") 'next-line)

or

     (global-set-key [?\C-x ?\C-\\] 'next-line)

or

     (global-set-key [(control ?x) (control ?\\)] 'next-line)

redefines ‘C-x C-\’ to move down a line.

     (global-set-key [M-mouse-1] 'mouse-set-point)

redefines the first (leftmost) mouse button, entered with the Meta key,
to set point where you click.

   Be careful when using non-ASCII text characters in Lisp
specifications of keys to bind.  If these are read as multibyte text, as
they usually will be in a Lisp file (*note Loading Non-ASCII::), you
must type the keys as multibyte too.  For instance, if you use this:

     (global-set-key "ö" 'my-function) ; bind o-umlaut

or

     (global-set-key ?ö 'my-function) ; bind o-umlaut

and your language environment is multibyte Latin-1, these commands
actually bind the multibyte character with code 246, not the byte code
246 (‘M-v’) sent by a Latin-1 terminal.  In order to use this binding,
you need to teach Emacs how to decode the keyboard by using an
appropriate input method (*note Input Methods: (emacs)Input Methods.).

 -- Command: global-set-key key binding
     This function sets the binding of KEY in the current global map to
     BINDING.

          (global-set-key KEY BINDING)
          ≡
          (define-key (current-global-map) KEY BINDING)

 -- Command: global-unset-key key
     This function removes the binding of KEY from the current global
     map.

     One use of this function is in preparation for defining a longer
     key that uses KEY as a prefix—which would not be allowed if KEY has
     a non-prefix binding.  For example:

          (global-unset-key "\C-l")
              ⇒ nil
          (global-set-key "\C-l\C-l" 'redraw-display)
              ⇒ nil

     This function is equivalent to using ‘define-key’ as follows:

          (global-unset-key KEY)
          ≡
          (define-key (current-global-map) KEY nil)

 -- Command: local-set-key key binding
     This function sets the binding of KEY in the current local keymap
     to BINDING.

          (local-set-key KEY BINDING)
          ≡
          (define-key (current-local-map) KEY BINDING)

 -- Command: local-unset-key key
     This function removes the binding of KEY from the current local
     map.

          (local-unset-key KEY)
          ≡
          (define-key (current-local-map) KEY nil)


File: elisp.info,  Node: Scanning Keymaps,  Next: Menu Keymaps,  Prev: Key Binding Commands,  Up: Keymaps

21.16 Scanning Keymaps
======================

This section describes functions used to scan all the current keymaps
for the sake of printing help information.

 -- Function: accessible-keymaps keymap &optional prefix
     This function returns a list of all the keymaps that can be reached
     (via zero or more prefix keys) from KEYMAP.  The value is an
     association list with elements of the form ‘(KEY . MAP)’, where KEY
     is a prefix key whose definition in KEYMAP is MAP.

     The elements of the alist are ordered so that the KEY increases in
     length.  The first element is always ‘([] . KEYMAP)’, because the
     specified keymap is accessible from itself with a prefix of no
     events.

     If PREFIX is given, it should be a prefix key sequence; then
     ‘accessible-keymaps’ includes only the submaps whose prefixes start
     with PREFIX.  These elements look just as they do in the value of
     ‘(accessible-keymaps)’; the only difference is that some elements
     are omitted.

     In the example below, the returned alist indicates that the key
     <ESC>, which is displayed as ‘^[’, is a prefix key whose definition
     is the sparse keymap ‘(keymap (83 . center-paragraph) (115 .
     foo))’.

          (accessible-keymaps (current-local-map))
          ⇒(([] keymap
                (27 keymap   ; Note this keymap for <ESC> is repeated below.
                    (83 . center-paragraph)
                    (115 . center-line))
                (9 . tab-to-tab-stop))

             ("^[" keymap
              (83 . center-paragraph)
              (115 . foo)))

     In the following example, ‘C-h’ is a prefix key that uses a sparse
     keymap starting with ‘(keymap (118 . describe-variable)...)’.
     Another prefix, ‘C-x 4’, uses a keymap which is also the value of
     the variable ‘ctl-x-4-map’.  The event ‘mode-line’ is one of
     several dummy events used as prefixes for mouse actions in special
     parts of a window.

          (accessible-keymaps (current-global-map))
          ⇒ (([] keymap [set-mark-command beginning-of-line ...
                             delete-backward-char])
              ("^H" keymap (118 . describe-variable) ...
               (8 . help-for-help))
              ("^X" keymap [x-flush-mouse-queue ...
               backward-kill-sentence])
              ("^[" keymap [mark-sexp backward-sexp ...
               backward-kill-word])
              ("^X4" keymap (15 . display-buffer) ...)
              ([mode-line] keymap
               (S-mouse-2 . mouse-split-window-horizontally) ...))

     These are not all the keymaps you would see in actuality.

 -- Function: map-keymap function keymap
     The function ‘map-keymap’ calls FUNCTION once for each binding in
     KEYMAP.  It passes two arguments, the event type and the value of
     the binding.  If KEYMAP has a parent, the parent’s bindings are
     included as well.  This works recursively: if the parent has itself
     a parent, then the grandparent’s bindings are also included and so
     on.

     This function is the cleanest way to examine all the bindings in a
     keymap.

 -- Function: where-is-internal command &optional keymap firstonly
          noindirect no-remap
     This function is a subroutine used by the ‘where-is’ command (*note
     Help: (emacs)Help.).  It returns a list of all key sequences (of
     any length) that are bound to COMMAND in a set of keymaps.

     The argument COMMAND can be any object; it is compared with all
     keymap entries using ‘eq’.

     If KEYMAP is ‘nil’, then the maps used are the current active
     keymaps, disregarding ‘overriding-local-map’ (that is, pretending
     its value is ‘nil’).  If KEYMAP is a keymap, then the maps searched
     are KEYMAP and the global keymap.  If KEYMAP is a list of keymaps,
     only those keymaps are searched.

     Usually it’s best to use ‘overriding-local-map’ as the expression
     for KEYMAP.  Then ‘where-is-internal’ searches precisely the
     keymaps that are active.  To search only the global map, pass the
     value ‘(keymap)’ (an empty keymap) as KEYMAP.

     If FIRSTONLY is ‘non-ascii’, then the value is a single vector
     representing the first key sequence found, rather than a list of
     all possible key sequences.  If FIRSTONLY is ‘t’, then the value is
     the first key sequence, except that key sequences consisting
     entirely of ASCII characters (or meta variants of ASCII characters)
     are preferred to all other key sequences and that the return value
     can never be a menu binding.

     If NOINDIRECT is non-‘nil’, ‘where-is-internal’ doesn’t look inside
     menu-items to find their commands.  This makes it possible to
     search for a menu-item itself.

     The fifth argument, NO-REMAP, determines how this function treats
     command remappings (*note Remapping Commands::).  There are two
     cases of interest:

     If a command OTHER-COMMAND is remapped to COMMAND:
          If NO-REMAP is ‘nil’, find the bindings for OTHER-COMMAND and
          treat them as though they are also bindings for COMMAND.  If
          NO-REMAP is non-‘nil’, include the vector ‘[remap
          OTHER-COMMAND]’ in the list of possible key sequences, instead
          of finding those bindings.

     If COMMAND is remapped to OTHER-COMMAND:
          If NO-REMAP is ‘nil’, return the bindings for OTHER-COMMAND
          rather than COMMAND.  If NO-REMAP is non-‘nil’, return the
          bindings for COMMAND, ignoring the fact that it is remapped.

 -- Command: describe-bindings &optional prefix buffer-or-name
     This function creates a listing of all current key bindings, and
     displays it in a buffer named ‘*Help*’.  The text is grouped by
     modes—minor modes first, then the major mode, then global bindings.

     If PREFIX is non-‘nil’, it should be a prefix key; then the listing
     includes only keys that start with PREFIX.

     When several characters with consecutive ASCII codes have the same
     definition, they are shown together, as ‘FIRSTCHAR..LASTCHAR’.  In
     this instance, you need to know the ASCII codes to understand which
     characters this means.  For example, in the default global map, the
     characters ‘<SPC> .. ~’ are described by a single line.  <SPC> is
     ASCII 32, ‘~’ is ASCII 126, and the characters between them include
     all the normal printing characters, (e.g., letters, digits,
     punctuation, etc.); all these characters are bound to
     ‘self-insert-command’.

     If BUFFER-OR-NAME is non-‘nil’, it should be a buffer or a buffer
     name.  Then ‘describe-bindings’ lists that buffer’s bindings,
     instead of the current buffer’s.


File: elisp.info,  Node: Menu Keymaps,  Prev: Scanning Keymaps,  Up: Keymaps

21.17 Menu Keymaps
==================

A keymap can operate as a menu as well as defining bindings for keyboard
keys and mouse buttons.  Menus are usually actuated with the mouse, but
they can function with the keyboard also.  If a menu keymap is active
for the next input event, that activates the keyboard menu feature.

* Menu:

* Defining Menus::     How to make a keymap that defines a menu.
* Mouse Menus::        How users actuate the menu with the mouse.
* Keyboard Menus::     How users actuate the menu with the keyboard.
* Menu Example::       Making a simple menu.
* Menu Bar::           How to customize the menu bar.
* Tool Bar::           A tool bar is a row of images.
* Modifying Menus::    How to add new items to a menu.
* Easy Menu::      A convenience macro for making menus.


File: elisp.info,  Node: Defining Menus,  Next: Mouse Menus,  Up: Menu Keymaps

21.17.1 Defining Menus
----------------------

A keymap acts as a menu if it has an “overall prompt string”, which is a
string that appears as an element of the keymap.  (*Note Format of
Keymaps::.)  The string should describe the purpose of the menu’s
commands.  Emacs displays the overall prompt string as the menu title in
some cases, depending on the toolkit (if any) used for displaying
menus.(1)  Keyboard menus also display the overall prompt string.

   The easiest way to construct a keymap with a prompt string is to
specify the string as an argument when you call ‘make-keymap’,
‘make-sparse-keymap’ (*note Creating Keymaps::), or
‘define-prefix-command’ (*note Definition of define-prefix-command::).
If you do not want the keymap to operate as a menu, don’t specify a
prompt string for it.

 -- Function: keymap-prompt keymap
     This function returns the overall prompt string of KEYMAP, or ‘nil’
     if it has none.

   The menu’s items are the bindings in the keymap.  Each binding
associates an event type to a definition, but the event types have no
significance for the menu appearance.  (Usually we use pseudo-events,
symbols that the keyboard cannot generate, as the event types for menu
item bindings.)  The menu is generated entirely from the bindings that
correspond in the keymap to these events.

   The order of items in the menu is the same as the order of bindings
in the keymap.  Since ‘define-key’ puts new bindings at the front, you
should define the menu items starting at the bottom of the menu and
moving to the top, if you care about the order.  When you add an item to
an existing menu, you can specify its position in the menu using
‘define-key-after’ (*note Modifying Menus::).

* Menu:

* Simple Menu Items::       A simple kind of menu key binding.
* Extended Menu Items::     More complex menu item definitions.
* Menu Separators::         Drawing a horizontal line through a menu.
* Alias Menu Items::        Using command aliases in menu items.

   ---------- Footnotes ----------

   (1) It is required for menus which do not use a toolkit, e.g., on a
text terminal.


File: elisp.info,  Node: Simple Menu Items,  Next: Extended Menu Items,  Up: Defining Menus

21.17.1.1 Simple Menu Items
...........................

The simpler (and original) way to define a menu item is to bind some
event type (it doesn’t matter what event type) to a binding like this:

     (ITEM-STRING . REAL-BINDING)

The CAR, ITEM-STRING, is the string to be displayed in the menu.  It
should be short—preferably one to three words.  It should describe the
action of the command it corresponds to.  Note that not all graphical
toolkits can display non-ASCII text in menus (it will work for keyboard
menus and will work to a large extent with the GTK+ toolkit).

   You can also supply a second string, called the help string, as
follows:

     (ITEM-STRING HELP . REAL-BINDING)

HELP specifies a help-echo string to display while the mouse is on that
item in the same way as ‘help-echo’ text properties (*note Help
display::).

   As far as ‘define-key’ is concerned, ITEM-STRING and HELP-STRING are
part of the event’s binding.  However, ‘lookup-key’ returns just
REAL-BINDING, and only REAL-BINDING is used for executing the key.

   If REAL-BINDING is ‘nil’, then ITEM-STRING appears in the menu but
cannot be selected.

   If REAL-BINDING is a symbol and has a non-‘nil’ ‘menu-enable’
property, that property is an expression that controls whether the menu
item is enabled.  Every time the keymap is used to display a menu, Emacs
evaluates the expression, and it enables the menu item only if the
expression’s value is non-‘nil’.  When a menu item is disabled, it is
displayed in a fuzzy fashion, and cannot be selected.

   The menu bar does not recalculate which items are enabled every time
you look at a menu.  This is because the X toolkit requires the whole
tree of menus in advance.  To force recalculation of the menu bar, call
‘force-mode-line-update’ (*note Mode Line Format::).


File: elisp.info,  Node: Extended Menu Items,  Next: Menu Separators,  Prev: Simple Menu Items,  Up: Defining Menus

21.17.1.2 Extended Menu Items
.............................

An extended-format menu item is a more flexible and also cleaner
alternative to the simple format.  You define an event type with a
binding that’s a list starting with the symbol ‘menu-item’.  For a
non-selectable string, the binding looks like this:

     (menu-item ITEM-NAME)

A string starting with two or more dashes specifies a separator line;
see *note Menu Separators::.

   To define a real menu item which can be selected, the extended format
binding looks like this:

     (menu-item ITEM-NAME REAL-BINDING
         . ITEM-PROPERTY-LIST)

Here, ITEM-NAME is an expression which evaluates to the menu item
string.  Thus, the string need not be a constant.  The third element,
REAL-BINDING, is the command to execute.  The tail of the list,
ITEM-PROPERTY-LIST, has the form of a property list which contains other
information.

   Here is a table of the properties that are supported:

‘:enable FORM’
     The result of evaluating FORM determines whether the item is
     enabled (non-‘nil’ means yes).  If the item is not enabled, you
     can’t really click on it.

‘:visible FORM’
     The result of evaluating FORM determines whether the item should
     actually appear in the menu (non-‘nil’ means yes).  If the item
     does not appear, then the menu is displayed as if this item were
     not defined at all.

‘:help HELP’
     The value of this property, HELP, specifies a help-echo string to
     display while the mouse is on that item.  This is displayed in the
     same way as ‘help-echo’ text properties (*note Help display::).
     Note that this must be a constant string, unlike the ‘help-echo’
     property for text and overlays.

‘:button (TYPE . SELECTED)’
     This property provides a way to define radio buttons and toggle
     buttons.  The CAR, TYPE, says which: it should be ‘:toggle’ or
     ‘:radio’.  The CDR, SELECTED, should be a form; the result of
     evaluating it says whether this button is currently selected.

     A “toggle” is a menu item which is labeled as either on or off
     according to the value of SELECTED.  The command itself should
     toggle SELECTED, setting it to ‘t’ if it is ‘nil’, and to ‘nil’ if
     it is ‘t’.  Here is how the menu item to toggle the
     ‘debug-on-error’ flag is defined:

          (menu-item "Debug on Error" toggle-debug-on-error
                     :button (:toggle
                              . (and (boundp 'debug-on-error)
                                     debug-on-error)))

     This works because ‘toggle-debug-on-error’ is defined as a command
     which toggles the variable ‘debug-on-error’.

     “Radio buttons” are a group of menu items, in which at any time one
     and only one is selected.  There should be a variable whose value
     says which one is selected at any time.  The SELECTED form for each
     radio button in the group should check whether the variable has the
     right value for selecting that button.  Clicking on the button
     should set the variable so that the button you clicked on becomes
     selected.

‘:key-sequence KEY-SEQUENCE’
     This property specifies which key sequence is likely to be bound to
     the same command invoked by this menu item.  If you specify the
     right key sequence, that makes preparing the menu for display run
     much faster.

     If you specify the wrong key sequence, it has no effect; before
     Emacs displays KEY-SEQUENCE in the menu, it verifies that
     KEY-SEQUENCE is really equivalent to this menu item.

‘:key-sequence nil’
     This property indicates that there is normally no key binding which
     is equivalent to this menu item.  Using this property saves time in
     preparing the menu for display, because Emacs does not need to
     search the keymaps for a keyboard equivalent for this menu item.

     However, if the user has rebound this item’s definition to a key
     sequence, Emacs ignores the ‘:keys’ property and finds the keyboard
     equivalent anyway.

‘:keys STRING’
     This property specifies that STRING is the string to display as the
     keyboard equivalent for this menu item.  You can use the ‘\\[...]’
     documentation construct in STRING.

‘:filter FILTER-FN’
     This property provides a way to compute the menu item dynamically.
     The property value FILTER-FN should be a function of one argument;
     when it is called, its argument will be REAL-BINDING.  The function
     should return the binding to use instead.

     Emacs can call this function at any time that it does redisplay or
     operates on menu data structures, so you should write it so it can
     safely be called at any time.


File: elisp.info,  Node: Menu Separators,  Next: Alias Menu Items,  Prev: Extended Menu Items,  Up: Defining Menus

21.17.1.3 Menu Separators
.........................

A menu separator is a kind of menu item that doesn’t display any
text—instead, it divides the menu into subparts with a horizontal line.
A separator looks like this in the menu keymap:

     (menu-item SEPARATOR-TYPE)

where SEPARATOR-TYPE is a string starting with two or more dashes.

   In the simplest case, SEPARATOR-TYPE consists of only dashes.  That
specifies the default kind of separator.  (For compatibility, ‘""’ and
‘-’ also count as separators.)

   Certain other values of SEPARATOR-TYPE specify a different style of
separator.  Here is a table of them:

‘"--no-line"’
‘"--space"’
     An extra vertical space, with no actual line.

‘"--single-line"’
     A single line in the menu’s foreground color.

‘"--double-line"’
     A double line in the menu’s foreground color.

‘"--single-dashed-line"’
     A single dashed line in the menu’s foreground color.

‘"--double-dashed-line"’
     A double dashed line in the menu’s foreground color.

‘"--shadow-etched-in"’
     A single line with a 3D sunken appearance.  This is the default,
     used separators consisting of dashes only.

‘"--shadow-etched-out"’
     A single line with a 3D raised appearance.

‘"--shadow-etched-in-dash"’
     A single dashed line with a 3D sunken appearance.

‘"--shadow-etched-out-dash"’
     A single dashed line with a 3D raised appearance.

‘"--shadow-double-etched-in"’
     Two lines with a 3D sunken appearance.

‘"--shadow-double-etched-out"’
     Two lines with a 3D raised appearance.

‘"--shadow-double-etched-in-dash"’
     Two dashed lines with a 3D sunken appearance.

‘"--shadow-double-etched-out-dash"’
     Two dashed lines with a 3D raised appearance.

   You can also give these names in another style, adding a colon after
the double-dash and replacing each single dash with capitalization of
the following word.  Thus, ‘"--:singleLine"’, is equivalent to
‘"--single-line"’.

   You can use a longer form to specify keywords such as ‘:enable’ and
‘:visible’ for a menu separator:

   ‘(menu-item SEPARATOR-TYPE nil . ITEM-PROPERTY-LIST)’

   For example:

     (menu-item "--" nil :visible (boundp 'foo))

   Some systems and display toolkits don’t really handle all of these
separator types.  If you use a type that isn’t supported, the menu
displays a similar kind of separator that is supported.


File: elisp.info,  Node: Alias Menu Items,  Prev: Menu Separators,  Up: Defining Menus

21.17.1.4 Alias Menu Items
..........................

Sometimes it is useful to make menu items that use the same command but
with different enable conditions.  The best way to do this in Emacs now
is with extended menu items; before that feature existed, it could be
done by defining alias commands and using them in menu items.  Here’s an
example that makes two aliases for ‘read-only-mode’ and gives them
different enable conditions:

     (defalias 'make-read-only 'read-only-mode)
     (put 'make-read-only 'menu-enable '(not buffer-read-only))
     (defalias 'make-writable 'read-only-mode)
     (put 'make-writable 'menu-enable 'buffer-read-only)

   When using aliases in menus, often it is useful to display the
equivalent key bindings for the real command name, not the aliases
(which typically don’t have any key bindings except for the menu
itself).  To request this, give the alias symbol a non-‘nil’
‘menu-alias’ property.  Thus,

     (put 'make-read-only 'menu-alias t)
     (put 'make-writable 'menu-alias t)

causes menu items for ‘make-read-only’ and ‘make-writable’ to show the
keyboard bindings for ‘read-only-mode’.


File: elisp.info,  Node: Mouse Menus,  Next: Keyboard Menus,  Prev: Defining Menus,  Up: Menu Keymaps

21.17.2 Menus and the Mouse
---------------------------

The usual way to make a menu keymap produce a menu is to make it the
definition of a prefix key.  (A Lisp program can explicitly pop up a
menu and receive the user’s choice—see *note Pop-Up Menus::.)

   If the prefix key ends with a mouse event, Emacs handles the menu
keymap by popping up a visible menu, so that the user can select a
choice with the mouse.  When the user clicks on a menu item, the event
generated is whatever character or symbol has the binding that brought
about that menu item.  (A menu item may generate a series of events if
the menu has multiple levels or comes from the menu bar.)

   It’s often best to use a button-down event to trigger the menu.  Then
the user can select a menu item by releasing the button.

   If the menu keymap contains a binding to a nested keymap, the nested
keymap specifies a “submenu”.  There will be a menu item, labeled by the
nested keymap’s item string, and clicking on this item automatically
pops up the specified submenu.  As a special exception, if the menu
keymap contains a single nested keymap and no other menu items, the menu
shows the contents of the nested keymap directly, not as a submenu.

   However, if Emacs is compiled without X toolkit support, or on text
terminals, submenus are not supported.  Each nested keymap is shown as a
menu item, but clicking on it does not automatically pop up the submenu.
If you wish to imitate the effect of submenus, you can do that by giving
a nested keymap an item string which starts with ‘@’.  This causes Emacs
to display the nested keymap using a separate “menu pane”; the rest of
the item string after the ‘@’ is the pane label.  If Emacs is compiled
without X toolkit support, or if a menu is displayed on a text terminal,
menu panes are not used; in that case, a ‘@’ at the beginning of an item
string is omitted when the menu label is displayed, and has no other
effect.


File: elisp.info,  Node: Keyboard Menus,  Next: Menu Example,  Prev: Mouse Menus,  Up: Menu Keymaps

21.17.3 Menus and the Keyboard
------------------------------

When a prefix key ending with a keyboard event (a character or function
key) has a definition that is a menu keymap, the keymap operates as a
keyboard menu; the user specifies the next event by choosing a menu item
with the keyboard.

   Emacs displays the keyboard menu with the map’s overall prompt
string, followed by the alternatives (the item strings of the map’s
bindings), in the echo area.  If the bindings don’t all fit at once, the
user can type <SPC> to see the next line of alternatives.  Successive
uses of <SPC> eventually get to the end of the menu and then cycle
around to the beginning.  (The variable ‘menu-prompt-more-char’
specifies which character is used for this; <SPC> is the default.)

   When the user has found the desired alternative from the menu, he or
she should type the corresponding character—the one whose binding is
that alternative.

 -- Variable: menu-prompt-more-char
     This variable specifies the character to use to ask to see the next
     line of a menu.  Its initial value is 32, the code for <SPC>.


File: elisp.info,  Node: Menu Example,  Next: Menu Bar,  Prev: Keyboard Menus,  Up: Menu Keymaps

21.17.4 Menu Example
--------------------

Here is a complete example of defining a menu keymap.  It is the
definition of the ‘Replace’ submenu in the ‘Edit’ menu in the menu bar,
and it uses the extended menu item format (*note Extended Menu Items::).
First we create the keymap, and give it a name:

     (defvar menu-bar-replace-menu (make-sparse-keymap "Replace"))

Next we define the menu items:

     (define-key menu-bar-replace-menu [tags-repl-continue]
       '(menu-item "Continue Replace" tags-loop-continue
                   :help "Continue last tags replace operation"))
     (define-key menu-bar-replace-menu [tags-repl]
       '(menu-item "Replace in tagged files" tags-query-replace
                   :help "Interactively replace a regexp in all tagged files"))
     (define-key menu-bar-replace-menu [separator-replace-tags]
       '(menu-item "--"))
     ;; ...

Note the symbols which the bindings are made for; these appear inside
square brackets, in the key sequence being defined.  In some cases, this
symbol is the same as the command name; sometimes it is different.
These symbols are treated as function keys, but they are not real
function keys on the keyboard.  They do not affect the functioning of
the menu itself, but they are echoed in the echo area when the user
selects from the menu, and they appear in the output of ‘where-is’ and
‘apropos’.

   The menu in this example is intended for use with the mouse.  If a
menu is intended for use with the keyboard, that is, if it is bound to a
key sequence ending with a keyboard event, then the menu items should be
bound to characters or real function keys, that can be typed with the
keyboard.

   The binding whose definition is ‘("--")’ is a separator line.  Like a
real menu item, the separator has a key symbol, in this case
‘separator-replace-tags’.  If one menu has two separators, they must
have two different key symbols.

   Here is how we make this menu appear as an item in the parent menu:

     (define-key menu-bar-edit-menu [replace]
       (list 'menu-item "Replace" menu-bar-replace-menu))

Note that this incorporates the submenu keymap, which is the value of
the variable ‘menu-bar-replace-menu’, rather than the symbol
‘menu-bar-replace-menu’ itself.  Using that symbol in the parent menu
item would be meaningless because ‘menu-bar-replace-menu’ is not a
command.

   If you wanted to attach the same replace menu to a mouse click, you
can do it this way:

     (define-key global-map [C-S-down-mouse-1]
        menu-bar-replace-menu)


File: elisp.info,  Node: Menu Bar,  Next: Tool Bar,  Prev: Menu Example,  Up: Menu Keymaps

21.17.5 The Menu Bar
--------------------

Emacs usually shows a “menu bar” at the top of each frame.  *Note
(emacs)Menu Bars::.  Menu bar items are subcommands of the fake function
key ‘menu-bar’, as defined in the active keymaps.

   To add an item to the menu bar, invent a fake function key of your
own (let’s call it KEY), and make a binding for the key sequence
‘[menu-bar KEY]’.  Most often, the binding is a menu keymap, so that
pressing a button on the menu bar item leads to another menu.

   When more than one active keymap defines the same function key for
the menu bar, the item appears just once.  If the user clicks on that
menu bar item, it brings up a single, combined menu containing all the
subcommands of that item—the global subcommands, the local subcommands,
and the minor mode subcommands.

   The variable ‘overriding-local-map’ is normally ignored when
determining the menu bar contents.  That is, the menu bar is computed
from the keymaps that would be active if ‘overriding-local-map’ were
‘nil’.  *Note Active Keymaps::.

   Here’s an example of setting up a menu bar item:

     ;; Make a menu keymap (with a prompt string)
     ;; and make it the menu bar item’s definition.
     (define-key global-map [menu-bar words]
       (cons "Words" (make-sparse-keymap "Words")))

     ;; Define specific subcommands in this menu.
     (define-key global-map
       [menu-bar words forward]
       '("Forward word" . forward-word))
     (define-key global-map
       [menu-bar words backward]
       '("Backward word" . backward-word))

   A local keymap can cancel a menu bar item made by the global keymap
by rebinding the same fake function key with ‘undefined’ as the binding.
For example, this is how Dired suppresses the ‘Edit’ menu bar item:

     (define-key dired-mode-map [menu-bar edit] 'undefined)

Here, ‘edit’ is the fake function key used by the global map for the
‘Edit’ menu bar item.  The main reason to suppress a global menu bar
item is to regain space for mode-specific items.

 -- Variable: menu-bar-final-items
     Normally the menu bar shows global items followed by items defined
     by the local maps.

     This variable holds a list of fake function keys for items to
     display at the end of the menu bar rather than in normal sequence.
     The default value is ‘(help-menu)’; thus, the ‘Help’ menu item
     normally appears at the end of the menu bar, following local menu
     items.

 -- Variable: menu-bar-update-hook
     This normal hook is run by redisplay to update the menu bar
     contents, before redisplaying the menu bar.  You can use it to
     update menus whose contents should vary.  Since this hook is run
     frequently, we advise you to ensure that the functions it calls do
     not take much time in the usual case.

   Next to every menu bar item, Emacs displays a key binding that runs
the same command (if such a key binding exists).  This serves as a
convenient hint for users who do not know the key binding.  If a command
has multiple bindings, Emacs normally displays the first one it finds.
You can specify one particular key binding by assigning an
‘:advertised-binding’ symbol property to the command.  *Note Keys in
Documentation::.


File: elisp.info,  Node: Tool Bar,  Next: Modifying Menus,  Prev: Menu Bar,  Up: Menu Keymaps

21.17.6 Tool bars
-----------------

A “tool bar” is a row of clickable icons at the top of a frame, just
below the menu bar.  *Note (emacs)Tool Bars::.  Emacs normally shows a
tool bar on graphical displays.

   On each frame, the frame parameter ‘tool-bar-lines’ controls how many
lines’ worth of height to reserve for the tool bar.  A zero value
suppresses the tool bar.  If the value is nonzero, and
‘auto-resize-tool-bars’ is non-‘nil’, the tool bar expands and contracts
automatically as needed to hold the specified contents.  If the value is
‘grow-only’, the tool bar expands automatically, but does not contract
automatically.

   The tool bar contents are controlled by a menu keymap attached to a
fake function key called ‘tool-bar’ (much like the way the menu bar is
controlled).  So you define a tool bar item using ‘define-key’, like
this:

     (define-key global-map [tool-bar KEY] ITEM)

where KEY is a fake function key to distinguish this item from other
items, and ITEM is a menu item key binding (*note Extended Menu
Items::), which says how to display this item and how it behaves.

   The usual menu keymap item properties, ‘:visible’, ‘:enable’,
‘:button’, and ‘:filter’, are useful in tool bar bindings and have their
normal meanings.  The REAL-BINDING in the item must be a command, not a
keymap; in other words, it does not work to define a tool bar icon as a
prefix key.

   The ‘:help’ property specifies a help-echo string to display while
the mouse is on that item.  This is displayed in the same way as
‘help-echo’ text properties (*note Help display::).

   In addition, you should use the ‘:image’ property; this is how you
specify the image to display in the tool bar:

‘:image IMAGE’
     IMAGE is either a single image specification (*note Images::) or a
     vector of four image specifications.  If you use a vector of four,
     one of them is used, depending on circumstances:

     item 0
          Used when the item is enabled and selected.
     item 1
          Used when the item is enabled and deselected.
     item 2
          Used when the item is disabled and selected.
     item 3
          Used when the item is disabled and deselected.

   The GTK+ and NS versions of Emacs ignores items 1 to 3, because
disabled and/or deselected images are autocomputed from item 0.

   If IMAGE is a single image specification, Emacs draws the tool bar
button in disabled state by applying an edge-detection algorithm to the
image.

   The ‘:rtl’ property specifies an alternative image to use for
right-to-left languages.  Only the GTK+ version of Emacs supports this
at present.

   Like the menu bar, the tool bar can display separators (*note Menu
Separators::).  Tool bar separators are vertical rather than horizontal,
though, and only a single style is supported.  They are represented in
the tool bar keymap by ‘(menu-item "--")’ entries; properties like
‘:visible’ are not supported for tool bar separators.  Separators are
rendered natively in GTK+ and Nextstep tool bars; in the other cases,
they are rendered using an image of a vertical line.

   The default tool bar is defined so that items specific to editing do
not appear for major modes whose command symbol has a ‘mode-class’
property of ‘special’ (*note Major Mode Conventions::).  Major modes may
add items to the global bar by binding ‘[tool-bar FOO]’ in their local
map.  It makes sense for some major modes to replace the default tool
bar items completely, since not many can be accommodated conveniently,
and the default bindings make this easy by using an indirection through
‘tool-bar-map’.

 -- Variable: tool-bar-map
     By default, the global map binds ‘[tool-bar]’ as follows:

          (global-set-key [tool-bar]
                          `(menu-item ,(purecopy "tool bar") ignore
                                      :filter tool-bar-make-keymap))

     The function ‘tool-bar-make-keymap’, in turn, derives the actual
     tool bar map dynamically from the value of the variable
     ‘tool-bar-map’.  Hence, you should normally adjust the default
     (global) tool bar by changing that map.  Some major modes, such as
     Info mode, completely replace the global tool bar by making
     ‘tool-bar-map’ buffer-local and setting it to a different keymap.

   There are two convenience functions for defining tool bar items, as
follows.

 -- Function: tool-bar-add-item icon def key &rest props
     This function adds an item to the tool bar by modifying
     ‘tool-bar-map’.  The image to use is defined by ICON, which is the
     base name of an XPM, XBM or PBM image file to be located by
     ‘find-image’.  Given a value ‘"exit"’, say, ‘exit.xpm’, ‘exit.pbm’
     and ‘exit.xbm’ would be searched for in that order on a color
     display.  On a monochrome display, the search order is ‘.pbm’,
     ‘.xbm’ and ‘.xpm’.  The binding to use is the command DEF, and KEY
     is the fake function key symbol in the prefix keymap.  The
     remaining arguments PROPS are additional property list elements to
     add to the menu item specification.

     To define items in some local map, bind ‘tool-bar-map’ with ‘let’
     around calls of this function:
          (defvar foo-tool-bar-map
            (let ((tool-bar-map (make-sparse-keymap)))
              (tool-bar-add-item ...)
              ...
              tool-bar-map))

 -- Function: tool-bar-add-item-from-menu command icon &optional map
          &rest props
     This function is a convenience for defining tool bar items which
     are consistent with existing menu bar bindings.  The binding of
     COMMAND is looked up in the menu bar in MAP (default ‘global-map’)
     and modified to add an image specification for ICON, which is found
     in the same way as by ‘tool-bar-add-item’.  The resulting binding
     is then placed in ‘tool-bar-map’, so use this function only for
     global tool bar items.

     MAP must contain an appropriate keymap bound to ‘[menu-bar]’.  The
     remaining arguments PROPS are additional property list elements to
     add to the menu item specification.

 -- Function: tool-bar-local-item-from-menu command icon in-map
          &optional from-map &rest props
     This function is used for making non-global tool bar items.  Use it
     like ‘tool-bar-add-item-from-menu’ except that IN-MAP specifies the
     local map to make the definition in.  The argument FROM-MAP is like
     the MAP argument of ‘tool-bar-add-item-from-menu’.

 -- Variable: auto-resize-tool-bars
     If this variable is non-‘nil’, the tool bar automatically resizes
     to show all defined tool bar items—but not larger than a quarter of
     the frame’s height.

     If the value is ‘grow-only’, the tool bar expands automatically,
     but does not contract automatically.  To contract the tool bar, the
     user has to redraw the frame by entering ‘C-l’.

     If Emacs is built with GTK or Nextstep, the tool bar can only show
     one line, so this variable has no effect.

 -- Variable: auto-raise-tool-bar-buttons
     If this variable is non-‘nil’, tool bar items display in raised
     form when the mouse moves over them.

 -- Variable: tool-bar-button-margin
     This variable specifies an extra margin to add around tool bar
     items.  The value is an integer, a number of pixels.  The default
     is 4.

 -- Variable: tool-bar-button-relief
     This variable specifies the shadow width for tool bar items.  The
     value is an integer, a number of pixels.  The default is 1.

 -- Variable: tool-bar-border
     This variable specifies the height of the border drawn below the
     tool bar area.  An integer specifies height as a number of pixels.
     If the value is one of ‘internal-border-width’ (the default) or
     ‘border-width’, the tool bar border height corresponds to the
     corresponding frame parameter.

   You can define a special meaning for clicking on a tool bar item with
the shift, control, meta, etc., modifiers.  You do this by setting up
additional items that relate to the original item through the fake
function keys.  Specifically, the additional items should use the
modified versions of the same fake function key used to name the
original item.

   Thus, if the original item was defined this way,

     (define-key global-map [tool-bar shell]
       '(menu-item "Shell" shell
                   :image (image :type xpm :file "shell.xpm")))

then here is how you can define clicking on the same tool bar image with
the shift modifier:

     (define-key global-map [tool-bar S-shell] 'some-command)

   *Note Function Keys::, for more information about how to add
modifiers to function keys.


File: elisp.info,  Node: Modifying Menus,  Next: Easy Menu,  Prev: Tool Bar,  Up: Menu Keymaps

21.17.7 Modifying Menus
-----------------------

When you insert a new item in an existing menu, you probably want to put
it in a particular place among the menu’s existing items.  If you use
‘define-key’ to add the item, it normally goes at the front of the menu.
To put it elsewhere in the menu, use ‘define-key-after’:

 -- Function: define-key-after map key binding &optional after
     Define a binding in MAP for KEY, with value BINDING, just like
     ‘define-key’, but position the binding in MAP after the binding for
     the event AFTER.  The argument KEY should be of length one—a vector
     or string with just one element.  But AFTER should be a single
     event type—a symbol or a character, not a sequence.  The new
     binding goes after the binding for AFTER.  If AFTER is ‘t’ or is
     omitted, then the new binding goes last, at the end of the keymap.
     However, new bindings are added before any inherited keymap.

     Here is an example:

          (define-key-after my-menu [drink]
            '("Drink" . drink-command) 'eat)

     makes a binding for the fake function key <DRINK> and puts it right
     after the binding for <EAT>.

     Here is how to insert an item called ‘Work’ in the ‘Signals’ menu
     of Shell mode, after the item ‘break’:

          (define-key-after
            (lookup-key shell-mode-map [menu-bar signals])
            [work] '("Work" . work-command) 'break)


File: elisp.info,  Node: Easy Menu,  Prev: Modifying Menus,  Up: Menu Keymaps

21.17.8 Easy Menu
-----------------

The following macro provides a convenient way to define pop-up menus
and/or menu bar menus.

 -- Macro: easy-menu-define symbol maps doc menu
     This macro defines a pop-up menu and/or menu bar submenu, whose
     contents are given by MENU.

     If SYMBOL is non-‘nil’, it should be a symbol; then this macro
     defines SYMBOL as a function for popping up the menu (*note Pop-Up
     Menus::), with DOC as its documentation string.  SYMBOL should not
     be quoted.

     Regardless of the value of SYMBOL, if MAPS is a keymap, the menu is
     added to that keymap, as a top-level menu for the menu bar (*note
     Menu Bar::).  It can also be a list of keymaps, in which case the
     menu is added separately to each of those keymaps.

     The first element of MENU must be a string, which serves as the
     menu label.  It may be followed by any number of the following
     keyword-argument pairs:

     ‘:filter FUNCTION’
          FUNCTION must be a function which, if called with one
          argument—the list of the other menu items—returns the actual
          items to be displayed in the menu.

     ‘:visible INCLUDE’
          INCLUDE is an expression; if it evaluates to ‘nil’, the menu
          is made invisible.  ‘:included’ is an alias for ‘:visible’.

     ‘:active ENABLE’
          ENABLE is an expression; if it evaluates to ‘nil’, the menu is
          not selectable.  ‘:enable’ is an alias for ‘:active’.

     The remaining elements in MENU are menu items.

     A menu item can be a vector of three elements, ‘[NAME CALLBACK
     ENABLE]’.  NAME is the menu item name (a string).  CALLBACK is a
     command to run, or an expression to evaluate, when the item is
     chosen.  ENABLE is an expression; if it evaluates to ‘nil’, the
     item is disabled for selection.

     Alternatively, a menu item may have the form:

             [ NAME CALLBACK [ KEYWORD ARG ]... ]

     where NAME and CALLBACK have the same meanings as above, and each
     optional KEYWORD and ARG pair should be one of the following:

     ‘:keys KEYS’
          KEYS is a keyboard equivalent to the menu item (a string).
          This is normally not needed, as keyboard equivalents are
          computed automatically.  KEYS is expanded with
          ‘substitute-command-keys’ before it is displayed (*note Keys
          in Documentation::).

     ‘:key-sequence KEYS’
          KEYS is a hint for speeding up Emacs’s first display of the
          menu.  It should be ‘nil’ if you know that the menu item has
          no keyboard equivalent; otherwise it should be a string or
          vector specifying a keyboard equivalent for the menu item.

     ‘:active ENABLE’
          ENABLE is an expression; if it evaluates to ‘nil’, the item is
          make unselectable..  ‘:enable’ is an alias for ‘:active’.

     ‘:visible INCLUDE’
          INCLUDE is an expression; if it evaluates to ‘nil’, the item
          is made invisible.  ‘:included’ is an alias for ‘:visible’.

     ‘:label FORM’
          FORM is an expression that is evaluated to obtain a value
          which serves as the menu item’s label (the default is NAME).

     ‘:suffix FORM’
          FORM is an expression that is dynamically evaluated and whose
          value is concatenated with the menu entry’s label.

     ‘:style STYLE’
          STYLE is a symbol describing the type of menu item; it should
          be ‘toggle’ (a checkbox), or ‘radio’ (a radio button), or
          anything else (meaning an ordinary menu item).

     ‘:selected SELECTED’
          SELECTED is an expression; the checkbox or radio button is
          selected whenever the expression’s value is non-‘nil’.

     ‘:help HELP’
          HELP is a string describing the menu item.

     Alternatively, a menu item can be a string.  Then that string
     appears in the menu as unselectable text.  A string consisting of
     dashes is displayed as a separator (*note Menu Separators::).

     Alternatively, a menu item can be a list with the same format as
     MENU.  This is a submenu.

   Here is an example of using ‘easy-menu-define’ to define a menu
similar to the one defined in the example in *note Menu Bar:::

     (easy-menu-define words-menu global-map
       "Menu for word navigation commands."
       '("Words"
          ["Forward word" forward-word]
          ["Backward word" backward-word]))


File: elisp.info,  Node: Modes,  Next: Documentation,  Prev: Keymaps,  Up: Top

22 Major and Minor Modes
************************

A “mode” is a set of definitions that customize Emacs behavior in useful
ways.  There are two varieties of modes: “minor modes”, which provide
features that users can turn on and off while editing; and “major
modes”, which are used for editing or interacting with a particular kind
of text.  Each buffer has exactly one “major mode” at a time.

   This chapter describes how to write both major and minor modes, how
to indicate them in the mode line, and how they run hooks supplied by
the user.  For related topics such as keymaps and syntax tables, see
*note Keymaps::, and *note Syntax Tables::.

* Menu:

* Hooks::             How to use hooks; how to write code that provides hooks.
* Major Modes::       Defining major modes.
* Minor Modes::       Defining minor modes.
* Mode Line Format::  Customizing the text that appears in the mode line.
* Imenu::             Providing a menu of definitions made in a buffer.
* Font Lock Mode::    How modes can highlight text according to syntax.
* Auto-Indentation::  How to teach Emacs to indent for a major mode.
* Desktop Save Mode:: How modes can have buffer state saved between
                        Emacs sessions.


File: elisp.info,  Node: Hooks,  Next: Major Modes,  Up: Modes

22.1 Hooks
==========

A “hook” is a variable where you can store a function or functions to be
called on a particular occasion by an existing program.  Emacs provides
hooks for the sake of customization.  Most often, hooks are set up in
the init file (*note Init File::), but Lisp programs can set them also.
*Note Standard Hooks::, for a list of some standard hook variables.

   Most of the hooks in Emacs are “normal hooks”.  These variables
contain lists of functions to be called with no arguments.  By
convention, whenever the hook name ends in ‘-hook’, that tells you it is
normal.  We try to make all hooks normal, as much as possible, so that
you can use them in a uniform way.

   Every major mode command is supposed to run a normal hook called the
“mode hook” as one of the last steps of initialization.  This makes it
easy for a user to customize the behavior of the mode, by overriding the
buffer-local variable assignments already made by the mode.  Most minor
mode functions also run a mode hook at the end.  But hooks are used in
other contexts too.  For example, the hook ‘suspend-hook’ runs just
before Emacs suspends itself (*note Suspending Emacs::).

   The recommended way to add a hook function to a hook is by calling
‘add-hook’ (*note Setting Hooks::).  The hook functions may be any of
the valid kinds of functions that ‘funcall’ accepts (*note What Is a
Function::).  Most normal hook variables are initially void; ‘add-hook’
knows how to deal with this.  You can add hooks either globally or
buffer-locally with ‘add-hook’.

   If the hook variable’s name does not end with ‘-hook’, that indicates
it is probably an “abnormal hook”.  That means the hook functions are
called with arguments, or their return values are used in some way.  The
hook’s documentation says how the functions are called.  You can use
‘add-hook’ to add a function to an abnormal hook, but you must write the
function to follow the hook’s calling convention.  By convention,
abnormal hook names end in ‘-functions’.

   If the variable’s name ends in ‘-function’, then its value is just a
single function, not a list of functions.  ‘add-hook’ cannot be used to
modify such a _single function hook_, and you have to use ‘add-function’
instead (*note Advising Functions::).

* Menu:

* Running Hooks::    How to run a hook.
* Setting Hooks::    How to put functions on a hook, or remove them.


File: elisp.info,  Node: Running Hooks,  Next: Setting Hooks,  Up: Hooks

22.1.1 Running Hooks
--------------------

In this section, we document the ‘run-hooks’ function, which is used to
run a normal hook.  We also document the functions for running various
kinds of abnormal hooks.

 -- Function: run-hooks &rest hookvars
     This function takes one or more normal hook variable names as
     arguments, and runs each hook in turn.  Each argument should be a
     symbol that is a normal hook variable.  These arguments are
     processed in the order specified.

     If a hook variable has a non-‘nil’ value, that value should be a
     list of functions.  ‘run-hooks’ calls all the functions, one by
     one, with no arguments.

     The hook variable’s value can also be a single function—either a
     lambda expression or a symbol with a function definition—which
     ‘run-hooks’ calls.  But this usage is obsolete.

     If the hook variable is buffer-local, the buffer-local variable
     will be used instead of the global variable.  However, if the
     buffer-local variable contains the element ‘t’, the global hook
     variable will be run as well.

 -- Function: run-hook-with-args hook &rest args
     This function runs an abnormal hook by calling all the hook
     functions in HOOK, passing each one the arguments ARGS.

 -- Function: run-hook-with-args-until-failure hook &rest args
     This function runs an abnormal hook by calling each hook function
     in turn, stopping if one of them fails by returning ‘nil’.  Each
     hook function is passed the arguments ARGS.  If this function stops
     because one of the hook functions fails, it returns ‘nil’;
     otherwise it returns a non-‘nil’ value.

 -- Function: run-hook-with-args-until-success hook &rest args
     This function runs an abnormal hook by calling each hook function,
     stopping if one of them succeeds by returning a non-‘nil’ value.
     Each hook function is passed the arguments ARGS.  If this function
     stops because one of the hook functions returns a non-‘nil’ value,
     it returns that value; otherwise it returns ‘nil’.


File: elisp.info,  Node: Setting Hooks,  Prev: Running Hooks,  Up: Hooks

22.1.2 Setting Hooks
--------------------

Here’s an example that uses a mode hook to turn on Auto Fill mode when
in Lisp Interaction mode:

     (add-hook 'lisp-interaction-mode-hook 'auto-fill-mode)

 -- Function: add-hook hook function &optional append local
     This function is the handy way to add function FUNCTION to hook
     variable HOOK.  You can use it for abnormal hooks as well as for
     normal hooks.  FUNCTION can be any Lisp function that can accept
     the proper number of arguments for HOOK.  For example,

          (add-hook 'text-mode-hook 'my-text-hook-function)

     adds ‘my-text-hook-function’ to the hook called ‘text-mode-hook’.

     If FUNCTION is already present in HOOK (comparing using ‘equal’),
     then ‘add-hook’ does not add it a second time.

     If FUNCTION has a non-‘nil’ property ‘permanent-local-hook’, then
     ‘kill-all-local-variables’ (or changing major modes) won’t delete
     it from the hook variable’s local value.

     For a normal hook, hook functions should be designed so that the
     order in which they are executed does not matter.  Any dependence
     on the order is asking for trouble.  However, the order is
     predictable: normally, FUNCTION goes at the front of the hook list,
     so it is executed first (barring another ‘add-hook’ call).  If the
     optional argument APPEND is non-‘nil’, the new hook function goes
     at the end of the hook list and is executed last.

     ‘add-hook’ can handle the cases where HOOK is void or its value is
     a single function; it sets or changes the value to a list of
     functions.

     If LOCAL is non-‘nil’, that says to add FUNCTION to the
     buffer-local hook list instead of to the global hook list.  This
     makes the hook buffer-local and adds ‘t’ to the buffer-local value.
     The latter acts as a flag to run the hook functions in the default
     value as well as in the local value.

 -- Function: remove-hook hook function &optional local
     This function removes FUNCTION from the hook variable HOOK.  It
     compares FUNCTION with elements of HOOK using ‘equal’, so it works
     for both symbols and lambda expressions.

     If LOCAL is non-‘nil’, that says to remove FUNCTION from the
     buffer-local hook list instead of from the global hook list.


File: elisp.info,  Node: Major Modes,  Next: Minor Modes,  Prev: Hooks,  Up: Modes

22.2 Major Modes
================

Major modes specialize Emacs for editing or interacting with particular
kinds of text.  Each buffer has exactly one major mode at a time.  Every
major mode is associated with a “major mode command”, whose name should
end in ‘-mode’.  This command takes care of switching to that mode in
the current buffer, by setting various buffer-local variables such as a
local keymap.  *Note Major Mode Conventions::.  Note that unlike minor
modes there is no way to “turn off” a major mode, instead the buffer
must be switched to a different one.

   The least specialized major mode is called “Fundamental mode”, which
has no mode-specific definitions or variable settings.

 -- Command: fundamental-mode
     This is the major mode command for Fundamental mode.  Unlike other
     mode commands, it does _not_ run any mode hooks (*note Major Mode
     Conventions::), since you are not supposed to customize this mode.

   The easiest way to write a major mode is to use the macro
‘define-derived-mode’, which sets up the new mode as a variant of an
existing major mode.  *Note Derived Modes::.  We recommend using
‘define-derived-mode’ even if the new mode is not an obvious derivative
of another mode, as it automatically enforces many coding conventions
for you.  *Note Basic Major Modes::, for common modes to derive from.

   The standard GNU Emacs Lisp directory tree contains the code for
several major modes, in files such as ‘text-mode.el’, ‘texinfo.el’,
‘lisp-mode.el’, and ‘rmail.el’.  You can study these libraries to see
how modes are written.

 -- User Option: major-mode
     The buffer-local value of this variable holds the symbol for the
     current major mode.  Its default value holds the default major mode
     for new buffers.  The standard default value is ‘fundamental-mode’.

     If the default value is ‘nil’, then whenever Emacs creates a new
     buffer via a command such as ‘C-x b’ (‘switch-to-buffer’), the new
     buffer is put in the major mode of the previously current buffer.
     As an exception, if the major mode of the previous buffer has a
     ‘mode-class’ symbol property with value ‘special’, the new buffer
     is put in Fundamental mode (*note Major Mode Conventions::).

* Menu:

* Major Mode Conventions::  Coding conventions for keymaps, etc.
* Auto Major Mode::         How Emacs chooses the major mode automatically.
* Mode Help::               Finding out how to use a mode.
* Derived Modes::           Defining a new major mode based on another major
                              mode.
* Basic Major Modes::       Modes that other modes are often derived from.
* Mode Hooks::              Hooks run at the end of major mode functions.
* Tabulated List Mode::     Parent mode for buffers containing tabulated data.
* Generic Modes::           Defining a simple major mode that supports
                              comment syntax and Font Lock mode.
* Example Major Modes::     Text mode and Lisp modes.


File: elisp.info,  Node: Major Mode Conventions,  Next: Auto Major Mode,  Up: Major Modes

22.2.1 Major Mode Conventions
-----------------------------

The code for every major mode should follow various coding conventions,
including conventions for local keymap and syntax table initialization,
function and variable names, and hooks.

   If you use the ‘define-derived-mode’ macro, it will take care of many
of these conventions automatically.  *Note Derived Modes::.  Note also
that Fundamental mode is an exception to many of these conventions,
because it represents the default state of Emacs.

   The following list of conventions is only partial.  Each major mode
should aim for consistency in general with other Emacs major modes, as
this makes Emacs as a whole more coherent.  It is impossible to list
here all the possible points where this issue might come up; if the
Emacs developers point out an area where your major mode deviates from
the usual conventions, please make it compatible.

   • Define a major mode command whose name ends in ‘-mode’.  When
     called with no arguments, this command should switch to the new
     mode in the current buffer by setting up the keymap, syntax table,
     and buffer-local variables in an existing buffer.  It should not
     change the buffer’s contents.

   • Write a documentation string for this command that describes the
     special commands available in this mode.  *Note Mode Help::.

     The documentation string may include the special documentation
     substrings, ‘\[COMMAND]’, ‘\{KEYMAP}’, and ‘\<KEYMAP>’, which allow
     the help display to adapt automatically to the user’s own key
     bindings.  *Note Keys in Documentation::.

   • The major mode command should start by calling
     ‘kill-all-local-variables’.  This runs the normal hook
     ‘change-major-mode-hook’, then gets rid of the buffer-local
     variables of the major mode previously in effect.  *Note Creating
     Buffer-Local::.

   • The major mode command should set the variable ‘major-mode’ to the
     major mode command symbol.  This is how ‘describe-mode’ discovers
     which documentation to print.

   • The major mode command should set the variable ‘mode-name’ to the
     “pretty” name of the mode, usually a string (but see *note Mode
     Line Data::, for other possible forms).  The name of the mode
     appears in the mode line.

   • Since all global names are in the same name space, all the global
     variables, constants, and functions that are part of the mode
     should have names that start with the major mode name (or with an
     abbreviation of it if the name is long).  *Note Coding
     Conventions::.

   • In a major mode for editing some kind of structured text, such as a
     programming language, indentation of text according to structure is
     probably useful.  So the mode should set ‘indent-line-function’ to
     a suitable function, and probably customize other variables for
     indentation.  *Note Auto-Indentation::.

   • The major mode should usually have its own keymap, which is used as
     the local keymap in all buffers in that mode.  The major mode
     command should call ‘use-local-map’ to install this local map.
     *Note Active Keymaps::, for more information.

     This keymap should be stored permanently in a global variable named
     ‘MODENAME-mode-map’.  Normally the library that defines the mode
     sets this variable.

     *Note Tips for Defining::, for advice about how to write the code
     to set up the mode’s keymap variable.

   • The key sequences bound in a major mode keymap should usually start
     with ‘C-c’, followed by a control character, a digit, or ‘{’, ‘}’,
     ‘<’, ‘>’, ‘:’ or ‘;’.  The other punctuation characters are
     reserved for minor modes, and ordinary letters are reserved for
     users.

     A major mode can also rebind the keys ‘M-n’, ‘M-p’ and ‘M-s’.  The
     bindings for ‘M-n’ and ‘M-p’ should normally be some kind of moving
     forward and backward, but this does not necessarily mean cursor
     motion.

     It is legitimate for a major mode to rebind a standard key sequence
     if it provides a command that does the same job in a way better
     suited to the text this mode is used for.  For example, a major
     mode for editing a programming language might redefine ‘C-M-a’ to
     move to the beginning of a function in a way that works better for
     that language.

     It is also legitimate for a major mode to rebind a standard key
     sequence whose standard meaning is rarely useful in that mode.  For
     instance, minibuffer modes rebind ‘M-r’, whose standard meaning is
     rarely of any use in the minibuffer.  Major modes such as Dired or
     Rmail that do not allow self-insertion of text can reasonably
     redefine letters and other printing characters as special commands.

   • Major modes for editing text should not define <RET> to do anything
     other than insert a newline.  However, it is ok for specialized
     modes for text that users don’t directly edit, such as Dired and
     Info modes, to redefine <RET> to do something entirely different.

   • Major modes should not alter options that are primarily a matter of
     user preference, such as whether Auto-Fill mode is enabled.  Leave
     this to each user to decide.  However, a major mode should
     customize other variables so that Auto-Fill mode will work usefully
     _if_ the user decides to use it.

   • The mode may have its own syntax table or may share one with other
     related modes.  If it has its own syntax table, it should store
     this in a variable named ‘MODENAME-mode-syntax-table’.  *Note
     Syntax Tables::.

   • If the mode handles a language that has a syntax for comments, it
     should set the variables that define the comment syntax.  *Note
     Options Controlling Comments: (emacs)Options for Comments.

   • The mode may have its own abbrev table or may share one with other
     related modes.  If it has its own abbrev table, it should store
     this in a variable named ‘MODENAME-mode-abbrev-table’.  If the
     major mode command defines any abbrevs itself, it should pass ‘t’
     for the SYSTEM-FLAG argument to ‘define-abbrev’.  *Note Defining
     Abbrevs::.

   • The mode should specify how to do highlighting for Font Lock mode,
     by setting up a buffer-local value for the variable
     ‘font-lock-defaults’ (*note Font Lock Mode::).

   • Each face that the mode defines should, if possible, inherit from
     an existing Emacs face.  *Note Basic Faces::, and *note Faces for
     Font Lock::.

   • The mode should specify how Imenu should find the definitions or
     sections of a buffer, by setting up a buffer-local value for the
     variable ‘imenu-generic-expression’, for the two variables
     ‘imenu-prev-index-position-function’ and
     ‘imenu-extract-index-name-function’, or for the variable
     ‘imenu-create-index-function’ (*note Imenu::).

   • The mode can specify a local value for
     ‘eldoc-documentation-function’ to tell ElDoc mode how to handle
     this mode.

   • The mode can specify how to complete various keywords by adding one
     or more buffer-local entries to the special hook
     ‘completion-at-point-functions’.  *Note Completion in Buffers::.

   • To make a buffer-local binding for an Emacs customization variable,
     use ‘make-local-variable’ in the major mode command, not
     ‘make-variable-buffer-local’.  The latter function would make the
     variable local to every buffer in which it is subsequently set,
     which would affect buffers that do not use this mode.  It is
     undesirable for a mode to have such global effects.  *Note
     Buffer-Local Variables::.

     With rare exceptions, the only reasonable way to use
     ‘make-variable-buffer-local’ in a Lisp package is for a variable
     which is used only within that package.  Using it on a variable
     used by other packages would interfere with them.

   • Each major mode should have a normal “mode hook” named
     ‘MODENAME-mode-hook’.  The very last thing the major mode command
     should do is to call ‘run-mode-hooks’.  This runs the normal hook
     ‘change-major-mode-after-body-hook’, the mode hook, and then the
     normal hook ‘after-change-major-mode-hook’.  *Note Mode Hooks::.

   • The major mode command may start by calling some other major mode
     command (called the “parent mode”) and then alter some of its
     settings.  A mode that does this is called a “derived mode”.  The
     recommended way to define one is to use the ‘define-derived-mode’
     macro, but this is not required.  Such a mode should call the
     parent mode command inside a ‘delay-mode-hooks’ form.  (Using
     ‘define-derived-mode’ does this automatically.)  *Note Derived
     Modes::, and *note Mode Hooks::.

   • If something special should be done if the user switches a buffer
     from this mode to any other major mode, this mode can set up a
     buffer-local value for ‘change-major-mode-hook’ (*note Creating
     Buffer-Local::).

   • If this mode is appropriate only for specially-prepared text
     produced by the mode itself (rather than by the user typing at the
     keyboard or by an external file), then the major mode command
     symbol should have a property named ‘mode-class’ with value
     ‘special’, put on as follows:

          (put 'funny-mode 'mode-class 'special)

     This tells Emacs that new buffers created while the current buffer
     is in Funny mode should not be put in Funny mode, even though the
     default value of ‘major-mode’ is ‘nil’.  By default, the value of
     ‘nil’ for ‘major-mode’ means to use the current buffer’s major mode
     when creating new buffers (*note Auto Major Mode::), but with such
     ‘special’ modes, Fundamental mode is used instead.  Modes such as
     Dired, Rmail, and Buffer List use this feature.

     The function ‘view-buffer’ does not enable View mode in buffers
     whose mode-class is special, because such modes usually provide
     their own View-like bindings.

     The ‘define-derived-mode’ macro automatically marks the derived
     mode as special if the parent mode is special.  Special mode is a
     convenient parent for such modes to inherit from; *Note Basic Major
     Modes::.

   • If you want to make the new mode the default for files with certain
     recognizable names, add an element to ‘auto-mode-alist’ to select
     the mode for those file names (*note Auto Major Mode::).  If you
     define the mode command to autoload, you should add this element in
     the same file that calls ‘autoload’.  If you use an autoload cookie
     for the mode command, you can also use an autoload cookie for the
     form that adds the element (*note autoload cookie::).  If you do
     not autoload the mode command, it is sufficient to add the element
     in the file that contains the mode definition.

   • The top-level forms in the file defining the mode should be written
     so that they may be evaluated more than once without adverse
     consequences.  For instance, use ‘defvar’ or ‘defcustom’ to set
     mode-related variables, so that they are not reinitialized if they
     already have a value (*note Defining Variables::).


File: elisp.info,  Node: Auto Major Mode,  Next: Mode Help,  Prev: Major Mode Conventions,  Up: Major Modes

22.2.2 How Emacs Chooses a Major Mode
-------------------------------------

When Emacs visits a file, it automatically selects a major mode for the
buffer based on information in the file name or in the file itself.  It
also processes local variables specified in the file text.

 -- Command: normal-mode &optional find-file
     This function establishes the proper major mode and buffer-local
     variable bindings for the current buffer.  First it calls
     ‘set-auto-mode’ (see below), then it runs ‘hack-local-variables’ to
     parse, and bind or evaluate as appropriate, the file’s local
     variables (*note File Local Variables::).

     If the FIND-FILE argument to ‘normal-mode’ is non-‘nil’,
     ‘normal-mode’ assumes that the ‘find-file’ function is calling it.
     In this case, it may process local variables in the ‘-*-’ line or
     at the end of the file.  The variable ‘enable-local-variables’
     controls whether to do so.  *Note Local Variables in Files:
     (emacs)File Variables, for the syntax of the local variables
     section of a file.

     If you run ‘normal-mode’ interactively, the argument FIND-FILE is
     normally ‘nil’.  In this case, ‘normal-mode’ unconditionally
     processes any file local variables.

     The function calls ‘set-auto-mode’ to choose a major mode.  If this
     does not specify a mode, the buffer stays in the major mode
     determined by the default value of ‘major-mode’ (see below).

     ‘normal-mode’ uses ‘condition-case’ around the call to the major
     mode command, so errors are caught and reported as a ‘File mode
     specification error’, followed by the original error message.

 -- Function: set-auto-mode &optional keep-mode-if-same
     This function selects the major mode that is appropriate for the
     current buffer.  It bases its decision (in order of precedence) on
     the ‘-*-’ line, on any ‘mode:’ local variable near the end of a
     file, on the ‘#!’ line (using ‘interpreter-mode-alist’), on the
     text at the beginning of the buffer (using ‘magic-mode-alist’), and
     finally on the visited file name (using ‘auto-mode-alist’).  *Note
     How Major Modes are Chosen: (emacs)Choosing Modes.  If
     ‘enable-local-variables’ is ‘nil’, ‘set-auto-mode’ does not check
     the ‘-*-’ line, or near the end of the file, for any mode tag.

     There are some file types where it is not appropriate to scan the
     file contents for a mode specifier.  For example, a tar archive may
     happen to contain, near the end of the file, a member file that has
     a local variables section specifying a mode for that particular
     file.  This should not be applied to the containing tar file.
     Similarly, a tiff image file might just happen to contain a first
     line that seems to match the ‘-*-’ pattern.  For these reasons,
     both these file extensions are members of the list
     ‘inhibit-local-variables-regexps’.  Add patterns to this list to
     prevent Emacs searching them for local variables of any kind (not
     just mode specifiers).

     If KEEP-MODE-IF-SAME is non-‘nil’, this function does not call the
     mode command if the buffer is already in the proper major mode.
     For instance, ‘set-visited-file-name’ sets this to ‘t’ to avoid
     killing buffer local variables that the user may have set.

 -- Function: set-buffer-major-mode buffer
     This function sets the major mode of BUFFER to the default value of
     ‘major-mode’; if that is ‘nil’, it uses the current buffer’s major
     mode (if that is suitable).  As an exception, if BUFFER’s name is
     ‘*scratch*’, it sets the mode to ‘initial-major-mode’.

     The low-level primitives for creating buffers do not use this
     function, but medium-level commands such as ‘switch-to-buffer’ and
     ‘find-file-noselect’ use it whenever they create buffers.

 -- User Option: initial-major-mode
     The value of this variable determines the major mode of the initial
     ‘*scratch*’ buffer.  The value should be a symbol that is a major
     mode command.  The default value is ‘lisp-interaction-mode’.

 -- Variable: interpreter-mode-alist
     This variable specifies major modes to use for scripts that specify
     a command interpreter in a ‘#!’ line.  Its value is an alist with
     elements of the form ‘(REGEXP . MODE)’; this says to use mode MODE
     if the file specifies an interpreter which matches ‘\\`REGEXP\\'’.
     For example, one of the default elements is ‘("python[0-9.]*" .
     python-mode)’.

 -- Variable: magic-mode-alist
     This variable’s value is an alist with elements of the form
     ‘(REGEXP . FUNCTION)’, where REGEXP is a regular expression and
     FUNCTION is a function or ‘nil’.  After visiting a file,
     ‘set-auto-mode’ calls FUNCTION if the text at the beginning of the
     buffer matches REGEXP and FUNCTION is non-‘nil’; if FUNCTION is
     ‘nil’, ‘auto-mode-alist’ gets to decide the mode.

 -- Variable: magic-fallback-mode-alist
     This works like ‘magic-mode-alist’, except that it is handled only
     if ‘auto-mode-alist’ does not specify a mode for this file.

 -- Variable: auto-mode-alist
     This variable contains an association list of file name patterns
     (regular expressions) and corresponding major mode commands.
     Usually, the file name patterns test for suffixes, such as ‘.el’
     and ‘.c’, but this need not be the case.  An ordinary element of
     the alist looks like ‘(REGEXP . MODE-FUNCTION)’.

     For example,

          (("\\`/tmp/fol/" . text-mode)
           ("\\.texinfo\\'" . texinfo-mode)
           ("\\.texi\\'" . texinfo-mode)
           ("\\.el\\'" . emacs-lisp-mode)
           ("\\.c\\'" . c-mode)
           ("\\.h\\'" . c-mode)
           ...)

     When you visit a file whose expanded file name (*note File Name
     Expansion::), with version numbers and backup suffixes removed
     using ‘file-name-sans-versions’ (*note File Name Components::),
     matches a REGEXP, ‘set-auto-mode’ calls the corresponding
     MODE-FUNCTION.  This feature enables Emacs to select the proper
     major mode for most files.

     If an element of ‘auto-mode-alist’ has the form ‘(REGEXP FUNCTION
     t)’, then after calling FUNCTION, Emacs searches ‘auto-mode-alist’
     again for a match against the portion of the file name that did not
     match before.  This feature is useful for uncompression packages:
     an entry of the form ‘("\\.gz\\'" FUNCTION t)’ can uncompress the
     file and then put the uncompressed file in the proper mode
     according to the name sans ‘.gz’.

     Here is an example of how to prepend several pattern pairs to
     ‘auto-mode-alist’.  (You might use this sort of expression in your
     init file.)

          (setq auto-mode-alist
            (append
             ;; File name (within directory) starts with a dot.
             '(("/\\.[^/]*\\'" . fundamental-mode)
               ;; File name has no dot.
               ("/[^\\./]*\\'" . fundamental-mode)
               ;; File name ends in ‘.C’.
               ("\\.C\\'" . c++-mode))
             auto-mode-alist))


File: elisp.info,  Node: Mode Help,  Next: Derived Modes,  Prev: Auto Major Mode,  Up: Major Modes

22.2.3 Getting Help about a Major Mode
--------------------------------------

The ‘describe-mode’ function provides information about major modes.  It
is normally bound to ‘C-h m’.  It uses the value of the variable
‘major-mode’ (*note Major Modes::), which is why every major mode
command needs to set that variable.

 -- Command: describe-mode &optional buffer
     This command displays the documentation of the current buffer’s
     major mode and minor modes.  It uses the ‘documentation’ function
     to retrieve the documentation strings of the major and minor mode
     commands (*note Accessing Documentation::).

     If called from Lisp with a non-‘nil’ BUFFER argument, this function
     displays the documentation for that buffer’s major and minor modes,
     rather than those of the current buffer.


File: elisp.info,  Node: Derived Modes,  Next: Basic Major Modes,  Prev: Mode Help,  Up: Major Modes

22.2.4 Defining Derived Modes
-----------------------------

The recommended way to define a new major mode is to derive it from an
existing one using ‘define-derived-mode’.  If there is no closely
related mode, you should inherit from either ‘text-mode’,
‘special-mode’, or ‘prog-mode’.  *Note Basic Major Modes::.  If none of
these are suitable, you can inherit from ‘fundamental-mode’ (*note Major
Modes::).

 -- Macro: define-derived-mode variant parent name docstring
          keyword-args... body...
     This macro defines VARIANT as a major mode command, using NAME as
     the string form of the mode name.  VARIANT and PARENT should be
     unquoted symbols.

     The new command VARIANT is defined to call the function PARENT,
     then override certain aspects of that parent mode:

        • The new mode has its own sparse keymap, named ‘VARIANT-map’.
          ‘define-derived-mode’ makes the parent mode’s keymap the
          parent of the new map, unless ‘VARIANT-map’ is already set and
          already has a parent.

        • The new mode has its own syntax table, kept in the variable
          ‘VARIANT-syntax-table’, unless you override this using the
          ‘:syntax-table’ keyword (see below).  ‘define-derived-mode’
          makes the parent mode’s syntax-table the parent of
          ‘VARIANT-syntax-table’, unless the latter is already set and
          already has a parent different from the standard syntax table.

        • The new mode has its own abbrev table, kept in the variable
          ‘VARIANT-abbrev-table’, unless you override this using the
          ‘:abbrev-table’ keyword (see below).

        • The new mode has its own mode hook, ‘VARIANT-hook’.  It runs
          this hook, after running the hooks of its ancestor modes, with
          ‘run-mode-hooks’, as the last thing it does.  *Note Mode
          Hooks::.

     In addition, you can specify how to override other aspects of
     PARENT with BODY.  The command VARIANT evaluates the forms in BODY
     after setting up all its usual overrides, just before running the
     mode hooks.

     If PARENT has a non-‘nil’ ‘mode-class’ symbol property, then
     ‘define-derived-mode’ sets the ‘mode-class’ property of VARIANT to
     the same value.  This ensures, for example, that if PARENT is a
     special mode, then VARIANT is also a special mode (*note Major Mode
     Conventions::).

     You can also specify ‘nil’ for PARENT.  This gives the new mode no
     parent.  Then ‘define-derived-mode’ behaves as described above,
     but, of course, omits all actions connected with PARENT.

     The argument DOCSTRING specifies the documentation string for the
     new mode.  ‘define-derived-mode’ adds some general information
     about the mode’s hook, followed by the mode’s keymap, at the end of
     this documentation string.  If you omit DOCSTRING,
     ‘define-derived-mode’ generates a documentation string.

     The KEYWORD-ARGS are pairs of keywords and values.  The values are
     evaluated.  The following keywords are currently supported:

     ‘:syntax-table’
          You can use this to explicitly specify a syntax table for the
          new mode.  If you specify a ‘nil’ value, the new mode uses the
          same syntax table as PARENT, or the standard syntax table if
          PARENT is ‘nil’.  (Note that this does _not_ follow the
          convention used for non-keyword arguments that a ‘nil’ value
          is equivalent with not specifying the argument.)

     ‘:abbrev-table’
          You can use this to explicitly specify an abbrev table for the
          new mode.  If you specify a ‘nil’ value, the new mode uses the
          same abbrev table as PARENT, or
          ‘fundamental-mode-abbrev-table’ if PARENT is ‘nil’.  (Again, a
          ‘nil’ value is _not_ equivalent to not specifying this
          keyword.)

     ‘:group’
          If this is specified, the value should be the customization
          group for this mode.  (Not all major modes have one.)  The
          command ‘customize-mode’ uses this.  ‘define-derived-mode’
          does _not_ automatically define the specified customization
          group.

     Here is a hypothetical example:

          (defvar hypertext-mode-map
            (let ((map (make-sparse-keymap)))
              (define-key map [down-mouse-3] 'do-hyper-link)
              map))

          (define-derived-mode hypertext-mode
            text-mode "Hypertext"
            "Major mode for hypertext."
            (setq-local case-fold-search nil))

     Do not write an ‘interactive’ spec in the definition;
     ‘define-derived-mode’ does that automatically.

 -- Function: derived-mode-p &rest modes
     This function returns non-‘nil’ if the current major mode is
     derived from any of the major modes given by the symbols MODES.


File: elisp.info,  Node: Basic Major Modes,  Next: Mode Hooks,  Prev: Derived Modes,  Up: Major Modes

22.2.5 Basic Major Modes
------------------------

Apart from Fundamental mode, there are three major modes that other
major modes commonly derive from: Text mode, Prog mode, and Special
mode.  While Text mode is useful in its own right (e.g., for editing
files ending in ‘.txt’), Prog mode and Special mode exist mainly to let
other modes derive from them.

   As far as possible, new major modes should be derived, either
directly or indirectly, from one of these three modes.  One reason is
that this allows users to customize a single mode hook (e.g.,
‘prog-mode-hook’) for an entire family of relevant modes (e.g., all
programming language modes).

 -- Command: text-mode
     Text mode is a major mode for editing human languages.  It defines
     the ‘"’ and ‘\’ characters as having punctuation syntax (*note
     Syntax Class Table::), and binds ‘M-<TAB>’ to
     ‘ispell-complete-word’ (*note (emacs)Spelling::).

     An example of a major mode derived from Text mode is HTML mode.
     *Note SGML and HTML Modes: (emacs)HTML Mode.

 -- Command: prog-mode
     Prog mode is a basic major mode for buffers containing programming
     language source code.  Most of the programming language major modes
     built into Emacs are derived from it.

     Prog mode binds ‘parse-sexp-ignore-comments’ to ‘t’ (*note Motion
     via Parsing::) and ‘bidi-paragraph-direction’ to ‘left-to-right’
     (*note Bidirectional Display::).

 -- Command: special-mode
     Special mode is a basic major mode for buffers containing text that
     is produced specially by Emacs, rather than directly from a file.
     Major modes derived from Special mode are given a ‘mode-class’
     property of ‘special’ (*note Major Mode Conventions::).

     Special mode sets the buffer to read-only.  Its keymap defines
     several common bindings, including ‘q’ for ‘quit-window’ and ‘g’
     for ‘revert-buffer’ (*note Reverting::).

     An example of a major mode derived from Special mode is Buffer Menu
     mode, which is used by the ‘*Buffer List*’ buffer.  *Note Listing
     Existing Buffers: (emacs)List Buffers.

   In addition, modes for buffers of tabulated data can inherit from
Tabulated List mode, which is in turn derived from Special mode.  *Note
Tabulated List Mode::.


File: elisp.info,  Node: Mode Hooks,  Next: Tabulated List Mode,  Prev: Basic Major Modes,  Up: Major Modes

22.2.6 Mode Hooks
-----------------

Every major mode command should finish by running the mode-independent
normal hook ‘change-major-mode-after-body-hook’, its mode hook, and the
normal hook ‘after-change-major-mode-hook’.  It does this by calling
‘run-mode-hooks’.  If the major mode is a derived mode, that is if it
calls another major mode (the parent mode) in its body, it should do
this inside ‘delay-mode-hooks’ so that the parent won’t run these hooks
itself.  Instead, the derived mode’s call to ‘run-mode-hooks’ runs the
parent’s mode hook too.  *Note Major Mode Conventions::.

   Emacs versions before Emacs 22 did not have ‘delay-mode-hooks’.
Versions before 24 did not have ‘change-major-mode-after-body-hook’.
When user-implemented major modes do not use ‘run-mode-hooks’ and have
not been updated to use these newer features, they won’t entirely follow
these conventions: they may run the parent’s mode hook too early, or
fail to run ‘after-change-major-mode-hook’.  If you encounter such a
major mode, please correct it to follow these conventions.

   When you define a major mode using ‘define-derived-mode’, it
automatically makes sure these conventions are followed.  If you define
a major mode “by hand”, not using ‘define-derived-mode’, use the
following functions to handle these conventions automatically.

 -- Function: run-mode-hooks &rest hookvars
     Major modes should run their mode hook using this function.  It is
     similar to ‘run-hooks’ (*note Hooks::), but it also runs
     ‘change-major-mode-after-body-hook’ and
     ‘after-change-major-mode-hook’.

     When this function is called during the execution of a
     ‘delay-mode-hooks’ form, it does not run the hooks immediately.
     Instead, it arranges for the next call to ‘run-mode-hooks’ to run
     them.

 -- Macro: delay-mode-hooks body...
     When one major mode command calls another, it should do so inside
     of ‘delay-mode-hooks’.

     This macro executes BODY, but tells all ‘run-mode-hooks’ calls
     during the execution of BODY to delay running their hooks.  The
     hooks will actually run during the next call to ‘run-mode-hooks’
     after the end of the ‘delay-mode-hooks’ construct.

 -- Variable: change-major-mode-after-body-hook
     This is a normal hook run by ‘run-mode-hooks’.  It is run before
     the mode hooks.

 -- Variable: after-change-major-mode-hook
     This is a normal hook run by ‘run-mode-hooks’.  It is run at the
     very end of every properly-written major mode command.


File: elisp.info,  Node: Tabulated List Mode,  Next: Generic Modes,  Prev: Mode Hooks,  Up: Major Modes

22.2.7 Tabulated List mode
--------------------------

Tabulated List mode is a major mode for displaying tabulated data, i.e.,
data consisting of “entries”, each entry occupying one row of text with
its contents divided into columns.  Tabulated List mode provides
facilities for pretty-printing rows and columns, and sorting the rows
according to the values in each column.  It is derived from Special mode
(*note Basic Major Modes::).

   Tabulated List mode is intended to be used as a parent mode by a more
specialized major mode.  Examples include Process Menu mode (*note
Process Information::) and Package Menu mode (*note (emacs)Package
Menu::).

   Such a derived mode should use ‘define-derived-mode’ in the usual
way, specifying ‘tabulated-list-mode’ as the second argument (*note
Derived Modes::).  The body of the ‘define-derived-mode’ form should
specify the format of the tabulated data, by assigning values to the
variables documented below; optionally, it can then call the function
‘tabulated-list-init-header’, which will populate a header with the
names of the columns.

   The derived mode should also define a “listing command”.  This, not
the mode command, is what the user calls (e.g., ‘M-x list-processes’).
The listing command should create or switch to a buffer, turn on the
derived mode, specify the tabulated data, and finally call
‘tabulated-list-print’ to populate the buffer.

 -- Variable: tabulated-list-format
     This buffer-local variable specifies the format of the Tabulated
     List data.  Its value should be a vector.  Each element of the
     vector represents a data column, and should be a list ‘(NAME WIDTH
     SORT)’, where

        • NAME is the column’s name (a string).

        • WIDTH is the width to reserve for the column (an integer).
          This is meaningless for the last column, which runs to the end
          of each line.

        • SORT specifies how to sort entries by the column.  If ‘nil’,
          the column cannot be used for sorting.  If ‘t’, the column is
          sorted by comparing string values.  Otherwise, this should be
          a predicate function for ‘sort’ (*note Rearrangement::), which
          accepts two arguments with the same form as the elements of
          ‘tabulated-list-entries’ (see below).

 -- Variable: tabulated-list-entries
     This buffer-local variable specifies the entries displayed in the
     Tabulated List buffer.  Its value should be either a list, or a
     function.

     If the value is a list, each list element corresponds to one entry,
     and should have the form ‘(ID CONTENTS)’, where

        • ID is either ‘nil’, or a Lisp object that identifies the
          entry.  If the latter, the cursor stays on the same entry when
          re-sorting entries.  Comparison is done with ‘equal’.

        • CONTENTS is a vector with the same number of elements as
          ‘tabulated-list-format’.  Each vector element is either a
          string, which is inserted into the buffer as-is, or a list
          ‘(LABEL . PROPERTIES)’, which means to insert a text button by
          calling ‘insert-text-button’ with LABEL and PROPERTIES as
          arguments (*note Making Buttons::).

          There should be no newlines in any of these strings.

     Otherwise, the value should be a function which returns a list of
     the above form when called with no arguments.

 -- Variable: tabulated-list-revert-hook
     This normal hook is run prior to reverting a Tabulated List buffer.
     A derived mode can add a function to this hook to recompute
     ‘tabulated-list-entries’.

 -- Variable: tabulated-list-printer
     The value of this variable is the function called to insert an
     entry at point, including its terminating newline.  The function
     should accept two arguments, ID and CONTENTS, having the same
     meanings as in ‘tabulated-list-entries’.  The default value is a
     function which inserts an entry in a straightforward way; a mode
     which uses Tabulated List mode in a more complex way can specify
     another function.

 -- Variable: tabulated-list-sort-key
     The value of this variable specifies the current sort key for the
     Tabulated List buffer.  If it is ‘nil’, no sorting is done.
     Otherwise, it should have the form ‘(NAME . FLIP)’, where NAME is a
     string matching one of the column names in ‘tabulated-list-format’,
     and FLIP, if non-‘nil’, means to invert the sort order.

 -- Function: tabulated-list-init-header
     This function computes and sets ‘header-line-format’ for the
     Tabulated List buffer (*note Header Lines::), and assigns a keymap
     to the header line to allow sorting entries by clicking on column
     headers.

     Modes derived from Tabulated List mode should call this after
     setting the above variables (in particular, only after setting
     ‘tabulated-list-format’).

 -- Function: tabulated-list-print &optional remember-pos update
     This function populates the current buffer with entries.  It should
     be called by the listing command.  It erases the buffer, sorts the
     entries specified by ‘tabulated-list-entries’ according to
     ‘tabulated-list-sort-key’, then calls the function specified by
     ‘tabulated-list-printer’ to insert each entry.

     If the optional argument REMEMBER-POS is non-‘nil’, this function
     looks for the ID element on the current line, if any, and tries to
     move to that entry after all the entries are (re)inserted.

     If the optional argument UPDATE is non-‘nil’, this function will
     only erase or add entries that have changed since the last print.
     This is several times faster if most entries haven’t changed since
     the last time this function was called.  The only difference in
     outcome is that tags placed via ‘tabulated-list-put-tag’ will not
     be removed from entries that haven’t changed (normally all tags are
     removed).


File: elisp.info,  Node: Generic Modes,  Next: Example Major Modes,  Prev: Tabulated List Mode,  Up: Major Modes

22.2.8 Generic Modes
--------------------

“Generic modes” are simple major modes with basic support for comment
syntax and Font Lock mode.  To define a generic mode, use the macro
‘define-generic-mode’.  See the file ‘generic-x.el’ for some examples of
the use of ‘define-generic-mode’.

 -- Macro: define-generic-mode mode comment-list keyword-list
          font-lock-list auto-mode-list function-list &optional
          docstring
     This macro defines a generic mode command named MODE (a symbol, not
     quoted).  The optional argument DOCSTRING is the documentation for
     the mode command.  If you do not supply it, ‘define-generic-mode’
     generates one by default.

     The argument COMMENT-LIST is a list in which each element is either
     a character, a string of one or two characters, or a cons cell.  A
     character or a string is set up in the mode’s syntax table as a
     comment starter.  If the entry is a cons cell, the CAR is set up as
     a comment starter and the CDR as a comment ender.  (Use ‘nil’ for
     the latter if you want comments to end at the end of the line.)
     Note that the syntax table mechanism has limitations about what
     comment starters and enders are actually possible.  *Note Syntax
     Tables::.

     The argument KEYWORD-LIST is a list of keywords to highlight with
     ‘font-lock-keyword-face’.  Each keyword should be a string.
     Meanwhile, FONT-LOCK-LIST is a list of additional expressions to
     highlight.  Each element of this list should have the same form as
     an element of ‘font-lock-keywords’.  *Note Search-based
     Fontification::.

     The argument AUTO-MODE-LIST is a list of regular expressions to add
     to the variable ‘auto-mode-alist’.  They are added by the execution
     of the ‘define-generic-mode’ form, not by expanding the macro call.

     Finally, FUNCTION-LIST is a list of functions for the mode command
     to call for additional setup.  It calls these functions just before
     it runs the mode hook variable ‘MODE-hook’.


File: elisp.info,  Node: Example Major Modes,  Prev: Generic Modes,  Up: Major Modes

22.2.9 Major Mode Examples
--------------------------

Text mode is perhaps the simplest mode besides Fundamental mode.  Here
are excerpts from ‘text-mode.el’ that illustrate many of the conventions
listed above:

     ;; Create the syntax table for this mode.
     (defvar text-mode-syntax-table
       (let ((st (make-syntax-table)))
         (modify-syntax-entry ?\" ".   " st)
         (modify-syntax-entry ?\\ ".   " st)
         ;; Add 'p' so M-c on 'hello' leads to 'Hello', not 'hello'.
         (modify-syntax-entry ?' "w p" st)
         st)
       "Syntax table used while in `text-mode'.")

     ;; Create the keymap for this mode.
     (defvar text-mode-map
       (let ((map (make-sparse-keymap)))
         (define-key map "\e\t" 'ispell-complete-word)
         map)
       "Keymap for `text-mode'.
     Many other modes, such as `mail-mode', `outline-mode' and
     `indented-text-mode', inherit all the commands defined in this map.")

   Here is how the actual mode command is defined now:

     (define-derived-mode text-mode nil "Text"
       "Major mode for editing text written for humans to read.
     In this mode, paragraphs are delimited only by blank or white lines.
     You can thus get the full benefit of adaptive filling
      (see the variable `adaptive-fill-mode').
     \\{text-mode-map}
     Turning on Text mode runs the normal hook `text-mode-hook'."
       (set (make-local-variable 'text-mode-variant) t)
       (set (make-local-variable 'require-final-newline)
            mode-require-final-newline)
       (set (make-local-variable 'indent-line-function) 'indent-relative))

(The last line is redundant nowadays, since ‘indent-relative’ is the
default value, and we’ll delete it in a future version.)

   The three Lisp modes (Lisp mode, Emacs Lisp mode, and Lisp
Interaction mode) have more features than Text mode and the code is
correspondingly more complicated.  Here are excerpts from ‘lisp-mode.el’
that illustrate how these modes are written.

   Here is how the Lisp mode syntax and abbrev tables are defined:

     ;; Create mode-specific table variables.
     (defvar lisp-mode-abbrev-table nil)
     (define-abbrev-table 'lisp-mode-abbrev-table ())

     (defvar lisp-mode-syntax-table
       (let ((table (copy-syntax-table emacs-lisp-mode-syntax-table)))
         (modify-syntax-entry ?\[ "_   " table)
         (modify-syntax-entry ?\] "_   " table)
         (modify-syntax-entry ?# "' 14" table)
         (modify-syntax-entry ?| "\" 23bn" table)
         table)
       "Syntax table used in `lisp-mode'.")

   The three modes for Lisp share much of their code.  For instance,
each calls the following function to set various variables:

     (defun lisp-mode-variables (&optional syntax keywords-case-insensitive)
       (when syntax
         (set-syntax-table lisp-mode-syntax-table))
       (setq local-abbrev-table lisp-mode-abbrev-table)
       ...

Amongst other things, this function sets up the ‘comment-start’ variable
to handle Lisp comments:

       (make-local-variable 'comment-start)
       (setq comment-start ";")
       ...

   Each of the different Lisp modes has a slightly different keymap.
For example, Lisp mode binds ‘C-c C-z’ to ‘run-lisp’, but the other Lisp
modes do not.  However, all Lisp modes have some commands in common.
The following code sets up the common commands:

     (defvar lisp-mode-shared-map
       (let ((map (make-sparse-keymap)))
         (define-key map "\e\C-q" 'indent-sexp)
         (define-key map "\177" 'backward-delete-char-untabify)
         map)
       "Keymap for commands shared by all sorts of Lisp modes.")

And here is the code to set up the keymap for Lisp mode:

     (defvar lisp-mode-map
       (let ((map (make-sparse-keymap))
     	(menu-map (make-sparse-keymap "Lisp")))
         (set-keymap-parent map lisp-mode-shared-map)
         (define-key map "\e\C-x" 'lisp-eval-defun)
         (define-key map "\C-c\C-z" 'run-lisp)
         ...
         map)
       "Keymap for ordinary Lisp mode.
     All commands in `lisp-mode-shared-map' are inherited by this map.")

Finally, here is the major mode command for Lisp mode:

     (define-derived-mode lisp-mode prog-mode "Lisp"
       "Major mode for editing Lisp code for Lisps other than GNU Emacs Lisp.
     Commands:
     Delete converts tabs to spaces as it moves back.
     Blank lines separate paragraphs.  Semicolons start comments.

     \\{lisp-mode-map}
     Note that `run-lisp' may be used either to start an inferior Lisp job
     or to switch back to an existing one.

     Entry to this mode calls the value of `lisp-mode-hook'
     if that value is non-nil."
       (lisp-mode-variables nil t)
       (set (make-local-variable 'find-tag-default-function)
            'lisp-find-tag-default)
       (set (make-local-variable 'comment-start-skip)
            "\\(\\(^\\|[^\\\\\n]\\)\\(\\\\\\\\\\)*\\)\\(;+\\|#|\\) *")
       (setq imenu-case-fold-search t))


File: elisp.info,  Node: Minor Modes,  Next: Mode Line Format,  Prev: Major Modes,  Up: Modes

22.3 Minor Modes
================

A “minor mode” provides optional features that users may enable or
disable independently of the choice of major mode.  Minor modes can be
enabled individually or in combination.

   Most minor modes implement features that are independent of the major
mode, and can thus be used with most major modes.  For example, Auto
Fill mode works with any major mode that permits text insertion.  A few
minor modes, however, are specific to a particular major mode.  For
example, Diff Auto Refine mode is a minor mode that is intended to be
used only with Diff mode.

   Ideally, a minor mode should have its desired effect regardless of
the other minor modes in effect.  It should be possible to activate and
deactivate minor modes in any order.

 -- Variable: minor-mode-list
     The value of this variable is a list of all minor mode commands.

* Menu:

* Minor Mode Conventions::      Tips for writing a minor mode.
* Keymaps and Minor Modes::     How a minor mode can have its own keymap.
* Defining Minor Modes::        A convenient facility for defining minor modes.


File: elisp.info,  Node: Minor Mode Conventions,  Next: Keymaps and Minor Modes,  Up: Minor Modes

22.3.1 Conventions for Writing Minor Modes
------------------------------------------

There are conventions for writing minor modes just as there are for
major modes.  These conventions are described below.  The easiest way to
follow them is to use the macro ‘define-minor-mode’.  *Note Defining
Minor Modes::.

   • Define a variable whose name ends in ‘-mode’.  We call this the
     “mode variable”.  The minor mode command should set this variable.
     The value will be ‘nil’ if the mode is disabled, and non-‘nil’ if
     the mode is enabled.  The variable should be buffer-local if the
     minor mode is buffer-local.

     This variable is used in conjunction with the ‘minor-mode-alist’ to
     display the minor mode name in the mode line.  It also determines
     whether the minor mode keymap is active, via ‘minor-mode-map-alist’
     (*note Controlling Active Maps::).  Individual commands or hooks
     can also check its value.

   • Define a command, called the “mode command”, whose name is the same
     as the mode variable.  Its job is to set the value of the mode
     variable, plus anything else that needs to be done to actually
     enable or disable the mode’s features.

     The mode command should accept one optional argument.  If called
     interactively with no prefix argument, it should toggle the mode
     (i.e., enable if it is disabled, and disable if it is enabled).  If
     called interactively with a prefix argument, it should enable the
     mode if the argument is positive and disable it otherwise.

     If the mode command is called from Lisp (i.e., non-interactively),
     it should enable the mode if the argument is omitted or ‘nil’; it
     should toggle the mode if the argument is the symbol ‘toggle’;
     otherwise it should treat the argument in the same way as for an
     interactive call with a numeric prefix argument, as described
     above.

     The following example shows how to implement this behavior (it is
     similar to the code generated by the ‘define-minor-mode’ macro):

          (interactive (list (or current-prefix-arg 'toggle)))
          (let ((enable (if (eq arg 'toggle)
                            (not foo-mode) ; this mode’s mode variable
                          (> (prefix-numeric-value arg) 0))))
            (if enable
                DO-ENABLE
              DO-DISABLE))

     The reason for this somewhat complex behavior is that it lets users
     easily toggle the minor mode interactively, and also lets the minor
     mode be easily enabled in a mode hook, like this:

          (add-hook 'text-mode-hook 'foo-mode)

     This behaves correctly whether or not ‘foo-mode’ was already
     enabled, since the ‘foo-mode’ mode command unconditionally enables
     the minor mode when it is called from Lisp with no argument.
     Disabling a minor mode in a mode hook is a little uglier:

          (add-hook 'text-mode-hook (lambda () (foo-mode -1)))

     However, this is not very commonly done.

   • Add an element to ‘minor-mode-alist’ for each minor mode (*note
     Definition of minor-mode-alist::), if you want to indicate the
     minor mode in the mode line.  This element should be a list of the
     following form:

          (MODE-VARIABLE STRING)

     Here MODE-VARIABLE is the variable that controls enabling of the
     minor mode, and STRING is a short string, starting with a space, to
     represent the mode in the mode line.  These strings must be short
     so that there is room for several of them at once.

     When you add an element to ‘minor-mode-alist’, use ‘assq’ to check
     for an existing element, to avoid duplication.  For example:

          (unless (assq 'leif-mode minor-mode-alist)
            (push '(leif-mode " Leif") minor-mode-alist))

     or like this, using ‘add-to-list’ (*note List Variables::):

          (add-to-list 'minor-mode-alist '(leif-mode " Leif"))

   In addition, several major mode conventions apply to minor modes as
well: those regarding the names of global symbols, the use of a hook at
the end of the initialization function, and the use of keymaps and other
tables.

   The minor mode should, if possible, support enabling and disabling
via Custom (*note Customization::).  To do this, the mode variable
should be defined with ‘defcustom’, usually with ‘:type 'boolean’.  If
just setting the variable is not sufficient to enable the mode, you
should also specify a ‘:set’ method which enables the mode by invoking
the mode command.  Note in the variable’s documentation string that
setting the variable other than via Custom may not take effect.  Also,
mark the definition with an autoload cookie (*note autoload cookie::),
and specify a ‘:require’ so that customizing the variable will load the
library that defines the mode.  For example:

     ;;;###autoload
     (defcustom msb-mode nil
       "Toggle msb-mode.
     Setting this variable directly does not take effect;
     use either \\[customize] or the function `msb-mode'."
       :set 'custom-set-minor-mode
       :initialize 'custom-initialize-default
       :version "20.4"
       :type    'boolean
       :group   'msb
       :require 'msb)


File: elisp.info,  Node: Keymaps and Minor Modes,  Next: Defining Minor Modes,  Prev: Minor Mode Conventions,  Up: Minor Modes

22.3.2 Keymaps and Minor Modes
------------------------------

Each minor mode can have its own keymap, which is active when the mode
is enabled.  To set up a keymap for a minor mode, add an element to the
alist ‘minor-mode-map-alist’.  *Note Definition of
minor-mode-map-alist::.

   One use of minor mode keymaps is to modify the behavior of certain
self-inserting characters so that they do something else as well as
self-insert.  (Another way to customize ‘self-insert-command’ is through
‘post-self-insert-hook’.  Apart from this, the facilities for
customizing ‘self-insert-command’ are limited to special cases, designed
for abbrevs and Auto Fill mode.  Do not try substituting your own
definition of ‘self-insert-command’ for the standard one.  The editor
command loop handles this function specially.)

   Minor modes may bind commands to key sequences consisting of ‘C-c’
followed by a punctuation character.  However, sequences consisting of
‘C-c’ followed by one of ‘{}<>:;’, or a control character or digit, are
reserved for major modes.  Also, ‘C-c LETTER’ is reserved for users.
*Note Key Binding Conventions::.


File: elisp.info,  Node: Defining Minor Modes,  Prev: Keymaps and Minor Modes,  Up: Minor Modes

22.3.3 Defining Minor Modes
---------------------------

The macro ‘define-minor-mode’ offers a convenient way of implementing a
mode in one self-contained definition.

 -- Macro: define-minor-mode mode doc [init-value [lighter [keymap]]]
          keyword-args... body...
     This macro defines a new minor mode whose name is MODE (a symbol).
     It defines a command named MODE to toggle the minor mode, with DOC
     as its documentation string.

     The toggle command takes one optional (prefix) argument.  If called
     interactively with no argument it toggles the mode on or off.  A
     positive prefix argument enables the mode, any other prefix
     argument disables it.  From Lisp, an argument of ‘toggle’ toggles
     the mode, whereas an omitted or ‘nil’ argument enables the mode.
     This makes it easy to enable the minor mode in a major mode hook,
     for example.  If DOC is ‘nil’, the macro supplies a default
     documentation string explaining the above.

     By default, it also defines a variable named MODE, which is set to
     ‘t’ or ‘nil’ by enabling or disabling the mode.  The variable is
     initialized to INIT-VALUE.  Except in unusual circumstances (see
     below), this value must be ‘nil’.

     The string LIGHTER says what to display in the mode line when the
     mode is enabled; if it is ‘nil’, the mode is not displayed in the
     mode line.

     The optional argument KEYMAP specifies the keymap for the minor
     mode.  If non-‘nil’, it should be a variable name (whose value is a
     keymap), a keymap, or an alist of the form

          (KEY-SEQUENCE . DEFINITION)

     where each KEY-SEQUENCE and DEFINITION are arguments suitable for
     passing to ‘define-key’ (*note Changing Key Bindings::).  If KEYMAP
     is a keymap or an alist, this also defines the variable ‘MODE-map’.

     The above three arguments INIT-VALUE, LIGHTER, and KEYMAP can be
     (partially) omitted when KEYWORD-ARGS are used.  The KEYWORD-ARGS
     consist of keywords followed by corresponding values.  A few
     keywords have special meanings:

     ‘:group GROUP’
          Custom group name to use in all generated ‘defcustom’ forms.
          Defaults to MODE without the possible trailing ‘-mode’.
          *Warning:* don’t use this default group name unless you have
          written a ‘defgroup’ to define that group properly.  *Note
          Group Definitions::.

     ‘:global GLOBAL’
          If non-‘nil’, this specifies that the minor mode should be
          global rather than buffer-local.  It defaults to ‘nil’.

          One of the effects of making a minor mode global is that the
          MODE variable becomes a customization variable.  Toggling it
          through the Customize interface turns the mode on and off, and
          its value can be saved for future Emacs sessions (*note
          (emacs)Saving Customizations::.  For the saved variable to
          work, you should ensure that the ‘define-minor-mode’ form is
          evaluated each time Emacs starts; for packages that are not
          part of Emacs, the easiest way to do this is to specify a
          ‘:require’ keyword.

     ‘:init-value INIT-VALUE’
          This is equivalent to specifying INIT-VALUE positionally.

     ‘:lighter LIGHTER’
          This is equivalent to specifying LIGHTER positionally.

     ‘:keymap KEYMAP’
          This is equivalent to specifying KEYMAP positionally.

     ‘:variable PLACE’
          This replaces the default variable MODE, used to store the
          state of the mode.  If you specify this, the MODE variable is
          not defined, and any INIT-VALUE argument is unused.  PLACE can
          be a different named variable (which you must define
          yourself), or anything that can be used with the ‘setf’
          function (*note Generalized Variables::).  PLACE can also be a
          cons ‘(GET . SET)’, where GET is an expression that returns
          the current state, and SET is a function of one argument (a
          state) that sets it.

     ‘:after-hook AFTER-HOOK’
          This defines a single Lisp form which is evaluated after the
          mode hooks have run.  It should not be quoted.

     Any other keyword arguments are passed directly to the ‘defcustom’
     generated for the variable MODE.

     The command named MODE first performs the standard actions such as
     setting the variable named MODE and then executes the BODY forms,
     if any.  It then runs the mode hook variable ‘MODE-hook’ and
     finishes by evaluating any form in ‘:after-hook’.

   The initial value must be ‘nil’ except in cases where (1) the mode is
preloaded in Emacs, or (2) it is painless for loading to enable the mode
even though the user did not request it.  For instance, if the mode has
no effect unless something else is enabled, and will always be loaded by
that time, enabling it by default is harmless.  But these are unusual
circumstances.  Normally, the initial value must be ‘nil’.

   The name ‘easy-mmode-define-minor-mode’ is an alias for this macro.

   Here is an example of using ‘define-minor-mode’:

     (define-minor-mode hungry-mode
       "Toggle Hungry mode.
     Interactively with no argument, this command toggles the mode.
     A positive prefix argument enables the mode, any other prefix
     argument disables it.  From Lisp, argument omitted or nil enables
     the mode, `toggle' toggles the state.

     When Hungry mode is enabled, the control delete key
     gobbles all preceding whitespace except the last.
     See the command \\[hungry-electric-delete]."
      ;; The initial value.
      nil
      ;; The indicator for the mode line.
      " Hungry"
      ;; The minor mode bindings.
      '(([C-backspace] . hungry-electric-delete))
      :group 'hunger)

This defines a minor mode named “Hungry mode”, a command named
‘hungry-mode’ to toggle it, a variable named ‘hungry-mode’ which
indicates whether the mode is enabled, and a variable named
‘hungry-mode-map’ which holds the keymap that is active when the mode is
enabled.  It initializes the keymap with a key binding for ‘C-<DEL>’.
It puts the variable ‘hungry-mode’ into custom group ‘hunger’.  There
are no BODY forms—many minor modes don’t need any.

   Here’s an equivalent way to write it:

     (define-minor-mode hungry-mode
       "Toggle Hungry mode.
     ...rest of documentation as before..."
      ;; The initial value.
      :init-value nil
      ;; The indicator for the mode line.
      :lighter " Hungry"
      ;; The minor mode bindings.
      :keymap
      '(([C-backspace] . hungry-electric-delete)
        ([C-M-backspace]
         . (lambda ()
             (interactive)
             (hungry-electric-delete t))))
      :group 'hunger)

 -- Macro: define-globalized-minor-mode global-mode mode turn-on
          keyword-args...
     This defines a global toggle named GLOBAL-MODE whose meaning is to
     enable or disable the buffer-local minor mode MODE in all buffers.
     To turn on the minor mode in a buffer, it uses the function
     TURN-ON; to turn off the minor mode, it calls MODE with −1 as
     argument.

     Globally enabling the mode also affects buffers subsequently
     created by visiting files, and buffers that use a major mode other
     than Fundamental mode; but it does not detect the creation of a new
     buffer in Fundamental mode.

     This defines the customization option GLOBAL-MODE (*note
     Customization::), which can be toggled in the Customize interface
     to turn the minor mode on and off.  As with ‘define-minor-mode’,
     you should ensure that the ‘define-globalized-minor-mode’ form is
     evaluated each time Emacs starts, for example by providing a
     ‘:require’ keyword.

     Use ‘:group GROUP’ in KEYWORD-ARGS to specify the custom group for
     the mode variable of the global minor mode.

     Generally speaking, when you define a globalized minor mode, you
     should also define a non-globalized version, so that people can use
     (or disable) it in individual buffers.  This also allows them to
     disable a globally enabled minor mode in a specific major mode, by
     using that mode’s hook.


File: elisp.info,  Node: Mode Line Format,  Next: Imenu,  Prev: Minor Modes,  Up: Modes

22.4 Mode Line Format
=====================

Each Emacs window (aside from minibuffer windows) typically has a mode
line at the bottom, which displays status information about the buffer
displayed in the window.  The mode line contains information about the
buffer, such as its name, associated file, depth of recursive editing,
and major and minor modes.  A window can also have a “header line”,
which is much like the mode line but appears at the top of the window.

   This section describes how to control the contents of the mode line
and header line.  We include it in this chapter because much of the
information displayed in the mode line relates to the enabled major and
minor modes.

* Menu:

* Base: Mode Line Basics.       Basic ideas of mode line control.
* Data: Mode Line Data.         The data structure that controls the mode line.
* Top: Mode Line Top.           The top level variable, mode-line-format.
* Mode Line Variables::         Variables used in that data structure.
* %-Constructs::                Putting information into a mode line.
* Properties in Mode::          Using text properties in the mode line.
* Header Lines::                Like a mode line, but at the top.
* Emulating Mode Line::         Formatting text as the mode line would.


File: elisp.info,  Node: Mode Line Basics,  Next: Mode Line Data,  Up: Mode Line Format

22.4.1 Mode Line Basics
-----------------------

The contents of each mode line are specified by the buffer-local
variable ‘mode-line-format’ (*note Mode Line Top::).  This variable
holds a “mode line construct”: a template that controls what is
displayed on the buffer’s mode line.  The value of ‘header-line-format’
specifies the buffer’s header line in the same way.  All windows for the
same buffer use the same ‘mode-line-format’ and ‘header-line-format’.

   For efficiency, Emacs does not continuously recompute each window’s
mode line and header line.  It does so when circumstances appear to call
for it—for instance, if you change the window configuration, switch
buffers, narrow or widen the buffer, scroll, or modify the buffer.  If
you alter any of the variables referenced by ‘mode-line-format’ or
‘header-line-format’ (*note Mode Line Variables::), or any other data
structures that affect how text is displayed (*note Display::), you
should use the function ‘force-mode-line-update’ to update the display.

 -- Function: force-mode-line-update &optional all
     This function forces Emacs to update the current buffer’s mode line
     and header line, based on the latest values of all relevant
     variables, during its next redisplay cycle.  If the optional
     argument ALL is non-‘nil’, it forces an update for all mode lines
     and header lines.

     This function also forces an update of the menu bar and frame
     title.

   The selected window’s mode line is usually displayed in a different
color using the face ‘mode-line’.  Other windows’ mode lines appear in
the face ‘mode-line-inactive’ instead.  *Note Faces::.


File: elisp.info,  Node: Mode Line Data,  Next: Mode Line Top,  Prev: Mode Line Basics,  Up: Mode Line Format

22.4.2 The Data Structure of the Mode Line
------------------------------------------

The mode line contents are controlled by a data structure called a “mode
line construct”, made up of lists, strings, symbols, and numbers kept in
buffer-local variables.  Each data type has a specific meaning for the
mode line appearance, as described below.  The same data structure is
used for constructing frame titles (*note Frame Titles::) and header
lines (*note Header Lines::).

   A mode line construct may be as simple as a fixed string of text, but
it usually specifies how to combine fixed strings with variables’ values
to construct the text.  Many of these variables are themselves defined
to have mode line constructs as their values.

   Here are the meanings of various data types as mode line constructs:

‘STRING’
     A string as a mode line construct appears verbatim except for
     “‘%’-constructs” in it.  These stand for substitution of other
     data; see *note %-Constructs::.

     If parts of the string have ‘face’ properties, they control display
     of the text just as they would text in the buffer.  Any characters
     which have no ‘face’ properties are displayed, by default, in the
     face ‘mode-line’ or ‘mode-line-inactive’ (*note (emacs)Standard
     Faces::).  The ‘help-echo’ and ‘keymap’ properties in STRING have
     special meanings.  *Note Properties in Mode::.

‘SYMBOL’
     A symbol as a mode line construct stands for its value.  The value
     of SYMBOL is used as a mode line construct, in place of SYMBOL.
     However, the symbols ‘t’ and ‘nil’ are ignored, as is any symbol
     whose value is void.

     There is one exception: if the value of SYMBOL is a string, it is
     displayed verbatim: the ‘%’-constructs are not recognized.

     Unless SYMBOL is marked as risky (i.e., it has a non-‘nil’
     ‘risky-local-variable’ property), all text properties specified in
     SYMBOL’s value are ignored.  This includes the text properties of
     strings in SYMBOL’s value, as well as all ‘:eval’ and ‘:propertize’
     forms in it.  (The reason for this is security: non-risky variables
     could be set automatically from file variables without prompting
     the user.)

‘(STRING REST...)’
‘(LIST REST...)’
     A list whose first element is a string or list means to process all
     the elements recursively and concatenate the results.  This is the
     most common form of mode line construct.

‘(:eval FORM)’
     A list whose first element is the symbol ‘:eval’ says to evaluate
     FORM, and use the result as a string to display.  Make sure this
     evaluation cannot load any files, as doing so could cause infinite
     recursion.

‘(:propertize ELT PROPS...)’
     A list whose first element is the symbol ‘:propertize’ says to
     process the mode line construct ELT recursively, then add the text
     properties specified by PROPS to the result.  The argument PROPS
     should consist of zero or more pairs TEXT-PROPERTY VALUE.

‘(SYMBOL THEN ELSE)’
     A list whose first element is a symbol that is not a keyword
     specifies a conditional.  Its meaning depends on the value of
     SYMBOL.  If SYMBOL has a non-‘nil’ value, the second element, THEN,
     is processed recursively as a mode line construct.  Otherwise, the
     third element, ELSE, is processed recursively.  You may omit ELSE;
     then the mode line construct displays nothing if the value of
     SYMBOL is ‘nil’ or void.

‘(WIDTH REST...)’
     A list whose first element is an integer specifies truncation or
     padding of the results of REST.  The remaining elements REST are
     processed recursively as mode line constructs and concatenated
     together.  When WIDTH is positive, the result is space filled on
     the right if its width is less than WIDTH.  When WIDTH is negative,
     the result is truncated on the right to −WIDTH columns if its width
     exceeds −WIDTH.

     For example, the usual way to show what percentage of a buffer is
     above the top of the window is to use a list like this: ‘(-3
     "%p")’.


File: elisp.info,  Node: Mode Line Top,  Next: Mode Line Variables,  Prev: Mode Line Data,  Up: Mode Line Format

22.4.3 The Top Level of Mode Line Control
-----------------------------------------

The variable in overall control of the mode line is ‘mode-line-format’.

 -- User Option: mode-line-format
     The value of this variable is a mode line construct that controls
     the contents of the mode-line.  It is always buffer-local in all
     buffers.

     If you set this variable to ‘nil’ in a buffer, that buffer does not
     have a mode line.  (A window that is just one line tall also does
     not display a mode line.)

   The default value of ‘mode-line-format’ is designed to use the values
of other variables such as ‘mode-line-position’ and ‘mode-line-modes’
(which in turn incorporates the values of the variables ‘mode-name’ and
‘minor-mode-alist’).  Very few modes need to alter ‘mode-line-format’
itself.  For most purposes, it is sufficient to alter some of the
variables that ‘mode-line-format’ either directly or indirectly refers
to.

   If you do alter ‘mode-line-format’ itself, the new value should use
the same variables that appear in the default value (*note Mode Line
Variables::), rather than duplicating their contents or displaying the
information in another fashion.  This way, customizations made by the
user or by Lisp programs (such as ‘display-time’ and major modes) via
changes to those variables remain effective.

   Here is a hypothetical example of a ‘mode-line-format’ that might be
useful for Shell mode (in reality, Shell mode does not set
‘mode-line-format’):

     (setq mode-line-format
       (list "-"
        'mode-line-mule-info
        'mode-line-modified
        'mode-line-frame-identification
        "%b--"
        ;; Note that this is evaluated while making the list.
        ;; It makes a mode line construct which is just a string.
        (getenv "HOST")
        ":"
        'default-directory
        "   "
        'global-mode-string
        "   %[("
        '(:eval (mode-line-mode-name))
        'mode-line-process
        'minor-mode-alist
        "%n"
        ")%]--"
        '(which-func-mode ("" which-func-format "--"))
        '(line-number-mode "L%l--")
        '(column-number-mode "C%c--")
        '(-3 "%p")))

(The variables ‘line-number-mode’, ‘column-number-mode’ and
‘which-func-mode’ enable particular minor modes; as usual, these
variable names are also the minor mode command names.)


File: elisp.info,  Node: Mode Line Variables,  Next: %-Constructs,  Prev: Mode Line Top,  Up: Mode Line Format

22.4.4 Variables Used in the Mode Line
--------------------------------------

This section describes variables incorporated by the standard value of
‘mode-line-format’ into the text of the mode line.  There is nothing
inherently special about these variables; any other variables could have
the same effects on the mode line if the value of ‘mode-line-format’ is
changed to use them.  However, various parts of Emacs set these
variables on the understanding that they will control parts of the mode
line; therefore, practically speaking, it is essential for the mode line
to use them.  Also see *note (emacs)Optional Mode Line::.

 -- Variable: mode-line-mule-info
     This variable holds the value of the mode line construct that
     displays information about the language environment, buffer coding
     system, and current input method.  *Note Non-ASCII Characters::.

 -- Variable: mode-line-modified
     This variable holds the value of the mode line construct that
     displays whether the current buffer is modified.  Its default value
     displays ‘**’ if the buffer is modified, ‘--’ if the buffer is not
     modified, ‘%%’ if the buffer is read only, and ‘%*’ if the buffer
     is read only and modified.

     Changing this variable does not force an update of the mode line.

 -- Variable: mode-line-frame-identification
     This variable identifies the current frame.  Its default value
     displays ‘" "’ if you are using a window system which can show
     multiple frames, or ‘"-%F "’ on an ordinary terminal which shows
     only one frame at a time.

 -- Variable: mode-line-buffer-identification
     This variable identifies the buffer being displayed in the window.
     Its default value displays the buffer name, padded with spaces to
     at least 12 columns.

 -- Variable: mode-line-position
     This variable indicates the position in the buffer.  Its default
     value displays the buffer percentage and, optionally, the buffer
     size, the line number and the column number.

 -- Variable: vc-mode
     The variable ‘vc-mode’, buffer-local in each buffer, records
     whether the buffer’s visited file is maintained with version
     control, and, if so, which kind.  Its value is a string that
     appears in the mode line, or ‘nil’ for no version control.

 -- Variable: mode-line-modes
     This variable displays the buffer’s major and minor modes.  Its
     default value also displays the recursive editing level,
     information on the process status, and whether narrowing is in
     effect.

 -- Variable: mode-line-remote
     This variable is used to show whether ‘default-directory’ for the
     current buffer is remote.

 -- Variable: mode-line-client
     This variable is used to identify ‘emacsclient’ frames.

   The following three variables are used in ‘mode-line-modes’:

 -- Variable: mode-name
     This buffer-local variable holds the “pretty” name of the current
     buffer’s major mode.  Each major mode should set this variable so
     that the mode name will appear in the mode line.  The value does
     not have to be a string, but can use any of the data types valid in
     a mode-line construct (*note Mode Line Data::).  To compute the
     string that will identify the mode name in the mode line, use
     ‘format-mode-line’ (*note Emulating Mode Line::).

 -- Variable: mode-line-process
     This buffer-local variable contains the mode line information on
     process status in modes used for communicating with subprocesses.
     It is displayed immediately following the major mode name, with no
     intervening space.  For example, its value in the ‘*shell*’ buffer
     is ‘(":%s")’, which allows the shell to display its status along
     with the major mode as: ‘(Shell:run)’.  Normally this variable is
     ‘nil’.

 -- Variable: mode-line-front-space
     This variable is displayed at the front of the mode line.  By
     default, this construct is displayed right at the beginning of the
     mode line, except that if there is a memory-full message, it is
     displayed first.

 -- Variable: mode-line-end-spaces
     This variable is displayed at the end of the mode line.

 -- Variable: mode-line-misc-info
     Mode line construct for miscellaneous information.  By default,
     this shows the information specified by ‘global-mode-string’.

 -- Variable: minor-mode-alist
     This variable holds an association list whose elements specify how
     the mode line should indicate that a minor mode is active.  Each
     element of the ‘minor-mode-alist’ should be a two-element list:

          (MINOR-MODE-VARIABLE MODE-LINE-STRING)

     More generally, MODE-LINE-STRING can be any mode line construct.
     It appears in the mode line when the value of MINOR-MODE-VARIABLE
     is non-‘nil’, and not otherwise.  These strings should begin with
     spaces so that they don’t run together.  Conventionally, the
     MINOR-MODE-VARIABLE for a specific mode is set to a non-‘nil’ value
     when that minor mode is activated.

     ‘minor-mode-alist’ itself is not buffer-local.  Each variable
     mentioned in the alist should be buffer-local if its minor mode can
     be enabled separately in each buffer.

 -- Variable: global-mode-string
     This variable holds a mode line construct that, by default, appears
     in the mode line just after the ‘which-func-mode’ minor mode if
     set, else after ‘mode-line-modes’.  The command ‘display-time’ sets
     ‘global-mode-string’ to refer to the variable
     ‘display-time-string’, which holds a string containing the time and
     load information.

     The ‘%M’ construct substitutes the value of ‘global-mode-string’,
     but that is obsolete, since the variable is included in the mode
     line from ‘mode-line-format’.

   Here is a simplified version of the default value of
‘mode-line-format’.  The real default value also specifies addition of
text properties.

     ("-"
      mode-line-mule-info
      mode-line-modified
      mode-line-frame-identification
      mode-line-buffer-identification
      "   "
      mode-line-position
      (vc-mode vc-mode)
      "   "
      mode-line-modes
      (which-func-mode ("" which-func-format "--"))
      (global-mode-string ("--" global-mode-string))
      "-%-")


File: elisp.info,  Node: %-Constructs,  Next: Properties in Mode,  Prev: Mode Line Variables,  Up: Mode Line Format

22.4.5 ‘%’-Constructs in the Mode Line
--------------------------------------

Strings used as mode line constructs can use certain ‘%’-constructs to
substitute various kinds of data.  The following is a list of the
defined ‘%’-constructs, and what they mean.

   In any construct except ‘%%’, you can add a decimal integer after the
‘%’ to specify a minimum field width.  If the width is less, the field
is padded to that width.  Purely numeric constructs (‘c’, ‘i’, ‘I’, and
‘l’) are padded by inserting spaces to the left, and others are padded
by inserting spaces to the right.

‘%b’
     The current buffer name, obtained with the ‘buffer-name’ function.
     *Note Buffer Names::.

‘%c’
     The current column number of point.

‘%e’
     When Emacs is nearly out of memory for Lisp objects, a brief
     message saying so.  Otherwise, this is empty.

‘%f’
     The visited file name, obtained with the ‘buffer-file-name’
     function.  *Note Buffer File Name::.

‘%F’
     The title (only on a window system) or the name of the selected
     frame.  *Note Basic Parameters::.

‘%i’
     The size of the accessible part of the current buffer; basically
     ‘(- (point-max) (point-min))’.

‘%I’
     Like ‘%i’, but the size is printed in a more readable way by using
     ‘k’ for 10^3, ‘M’ for 10^6, ‘G’ for 10^9, etc., to abbreviate.

‘%l’
     The current line number of point, counting within the accessible
     portion of the buffer.

‘%n’
     ‘Narrow’ when narrowing is in effect; nothing otherwise (see
     ‘narrow-to-region’ in *note Narrowing::).

‘%p’
     The percentage of the buffer text above the *top* of window, or
     ‘Top’, ‘Bottom’ or ‘All’.  Note that the default mode line
     construct truncates this to three characters.

‘%P’
     The percentage of the buffer text that is above the *bottom* of the
     window (which includes the text visible in the window, as well as
     the text above the top), plus ‘Top’ if the top of the buffer is
     visible on screen; or ‘Bottom’ or ‘All’.

‘%s’
     The status of the subprocess belonging to the current buffer,
     obtained with ‘process-status’.  *Note Process Information::.

‘%z’
     The mnemonics of keyboard, terminal, and buffer coding systems.

‘%Z’
     Like ‘%z’, but including the end-of-line format.

‘%*’
     ‘%’ if the buffer is read only (see ‘buffer-read-only’);
     ‘*’ if the buffer is modified (see ‘buffer-modified-p’);
     ‘-’ otherwise.  *Note Buffer Modification::.

‘%+’
     ‘*’ if the buffer is modified (see ‘buffer-modified-p’);
     ‘%’ if the buffer is read only (see ‘buffer-read-only’);
     ‘-’ otherwise.  This differs from ‘%*’ only for a modified
     read-only buffer.  *Note Buffer Modification::.

‘%&’
     ‘*’ if the buffer is modified, and ‘-’ otherwise.

‘%[’
     An indication of the depth of recursive editing levels (not
     counting minibuffer levels): one ‘[’ for each editing level.  *Note
     Recursive Editing::.

‘%]’
     One ‘]’ for each recursive editing level (not counting minibuffer
     levels).

‘%-’
     Dashes sufficient to fill the remainder of the mode line.

‘%%’
     The character ‘%’—this is how to include a literal ‘%’ in a string
     in which ‘%’-constructs are allowed.

   The following two ‘%’-constructs are still supported, but they are
obsolete, since you can get the same results with the variables
‘mode-name’ and ‘global-mode-string’.

‘%m’
     The value of ‘mode-name’.

‘%M’
     The value of ‘global-mode-string’.


File: elisp.info,  Node: Properties in Mode,  Next: Header Lines,  Prev: %-Constructs,  Up: Mode Line Format

22.4.6 Properties in the Mode Line
----------------------------------

Certain text properties are meaningful in the mode line.  The ‘face’
property affects the appearance of text; the ‘help-echo’ property
associates help strings with the text, and ‘keymap’ can make the text
mouse-sensitive.

   There are four ways to specify text properties for text in the mode
line:

  1. Put a string with a text property directly into the mode line data
     structure.

  2. Put a text property on a mode line %-construct such as ‘%12b’; then
     the expansion of the %-construct will have that same text property.

  3. Use a ‘(:propertize ELT PROPS...)’ construct to give ELT a text
     property specified by PROPS.

  4. Use a list containing ‘:eval FORM’ in the mode line data structure,
     and make FORM evaluate to a string that has a text property.

   You can use the ‘keymap’ property to specify a keymap.  This keymap
only takes real effect for mouse clicks; binding character keys and
function keys to it has no effect, since it is impossible to move point
into the mode line.

   When the mode line refers to a variable which does not have a
non-‘nil’ ‘risky-local-variable’ property, any text properties given or
specified within that variable’s values are ignored.  This is because
such properties could otherwise specify functions to be called, and
those functions could come from file local variables.


File: elisp.info,  Node: Header Lines,  Next: Emulating Mode Line,  Prev: Properties in Mode,  Up: Mode Line Format

22.4.7 Window Header Lines
--------------------------

A window can have a “header line” at the top, just as it can have a mode
line at the bottom.  The header line feature works just like the mode
line feature, except that it’s controlled by ‘header-line-format’:

 -- Variable: header-line-format
     This variable, local in every buffer, specifies how to display the
     header line, for windows displaying the buffer.  The format of the
     value is the same as for ‘mode-line-format’ (*note Mode Line
     Data::).  It is normally ‘nil’, so that ordinary buffers have no
     header line.

 -- Function: window-header-line-height &optional window
     This function returns the height in pixels of WINDOW’s header line.
     WINDOW must be a live window, and defaults to the selected window.

   A window that is just one line tall never displays a header line.  A
window that is two lines tall cannot display both a mode line and a
header line at once; if it has a mode line, then it does not display a
header line.


File: elisp.info,  Node: Emulating Mode Line,  Prev: Header Lines,  Up: Mode Line Format

22.4.8 Emulating Mode Line Formatting
-------------------------------------

You can use the function ‘format-mode-line’ to compute the text that
would appear in a mode line or header line based on a certain mode line
construct.

 -- Function: format-mode-line format &optional face window buffer
     This function formats a line of text according to FORMAT as if it
     were generating the mode line for WINDOW, but it also returns the
     text as a string.  The argument WINDOW defaults to the selected
     window.  If BUFFER is non-‘nil’, all the information used is taken
     from BUFFER; by default, it comes from WINDOW’s buffer.

     The value string normally has text properties that correspond to
     the faces, keymaps, etc., that the mode line would have.  Any
     character for which no ‘face’ property is specified by FORMAT gets
     a default value determined by FACE.  If FACE is ‘t’, that stands
     for either ‘mode-line’ if WINDOW is selected, otherwise
     ‘mode-line-inactive’.  If FACE is ‘nil’ or omitted, that stands for
     the default face.  If FACE is an integer, the value returned by
     this function will have no text properties.

     You can also specify other valid faces as the value of FACE.  If
     specified, that face provides the ‘face’ property for characters
     whose face is not specified by FORMAT.

     Note that using ‘mode-line’, ‘mode-line-inactive’, or ‘header-line’
     as FACE will actually redisplay the mode line or the header line,
     respectively, using the current definitions of the corresponding
     face, in addition to returning the formatted string.  (Other faces
     do not cause redisplay.)

     For example, ‘(format-mode-line header-line-format)’ returns the
     text that would appear in the selected window’s header line (‘""’
     if it has no header line).  ‘(format-mode-line header-line-format
     'header-line)’ returns the same text, with each character carrying
     the face that it will have in the header line itself, and also
     redraws the header line.


File: elisp.info,  Node: Imenu,  Next: Font Lock Mode,  Prev: Mode Line Format,  Up: Modes

22.5 Imenu
==========

“Imenu” is a feature that lets users select a definition or section in
the buffer, from a menu which lists all of them, to go directly to that
location in the buffer.  Imenu works by constructing a buffer index
which lists the names and buffer positions of the definitions, or other
named portions of the buffer; then the user can choose one of them and
move point to it.  Major modes can add a menu bar item to use Imenu
using ‘imenu-add-to-menubar’.

 -- Command: imenu-add-to-menubar name
     This function defines a local menu bar item named NAME to run
     Imenu.

   The user-level commands for using Imenu are described in the Emacs
Manual (*note Imenu: (emacs)Imenu.).  This section explains how to
customize Imenu’s method of finding definitions or buffer portions for a
particular major mode.

   The usual and simplest way is to set the variable
‘imenu-generic-expression’:

 -- Variable: imenu-generic-expression
     This variable, if non-‘nil’, is a list that specifies regular
     expressions for finding definitions for Imenu.  Simple elements of
     ‘imenu-generic-expression’ look like this:

          (MENU-TITLE REGEXP INDEX)

     Here, if MENU-TITLE is non-‘nil’, it says that the matches for this
     element should go in a submenu of the buffer index; MENU-TITLE
     itself specifies the name for the submenu.  If MENU-TITLE is ‘nil’,
     the matches for this element go directly in the top level of the
     buffer index.

     The second item in the list, REGEXP, is a regular expression (*note
     Regular Expressions::); anything in the buffer that it matches is
     considered a definition, something to mention in the buffer index.
     The third item, INDEX, is a non-negative integer that indicates
     which subexpression in REGEXP matches the definition’s name.

     An element can also look like this:

          (MENU-TITLE REGEXP INDEX FUNCTION ARGUMENTS...)

     Each match for this element creates an index item, and when the
     index item is selected by the user, it calls FUNCTION with
     arguments consisting of the item name, the buffer position, and
     ARGUMENTS.

     For Emacs Lisp mode, ‘imenu-generic-expression’ could look like
     this:

          ((nil "^\\s-*(def\\(un\\|subst\\|macro\\|advice\\)\
          \\s-+\\([-A-Za-z0-9+]+\\)" 2)
           ("*Vars*" "^\\s-*(def\\(var\\|const\\)\
          \\s-+\\([-A-Za-z0-9+]+\\)" 2)
           ("*Types*"
            "^\\s-*\
          (def\\(type\\|struct\\|class\\|ine-condition\\)\
          \\s-+\\([-A-Za-z0-9+]+\\)" 2))

     Setting this variable makes it buffer-local in the current buffer.

 -- Variable: imenu-case-fold-search
     This variable controls whether matching against the regular
     expressions in the value of ‘imenu-generic-expression’ is
     case-sensitive: ‘t’, the default, means matching should ignore
     case.

     Setting this variable makes it buffer-local in the current buffer.

 -- Variable: imenu-syntax-alist
     This variable is an alist of syntax table modifiers to use while
     processing ‘imenu-generic-expression’, to override the syntax table
     of the current buffer.  Each element should have this form:

          (CHARACTERS . SYNTAX-DESCRIPTION)

     The CAR, CHARACTERS, can be either a character or a string.  The
     element says to give that character or characters the syntax
     specified by SYNTAX-DESCRIPTION, which is passed to
     ‘modify-syntax-entry’ (*note Syntax Table Functions::).

     This feature is typically used to give word syntax to characters
     which normally have symbol syntax, and thus to simplify
     ‘imenu-generic-expression’ and speed up matching.  For example,
     Fortran mode uses it this way:

          (setq imenu-syntax-alist '(("_$" . "w")))

     The ‘imenu-generic-expression’ regular expressions can then use
     ‘\\sw+’ instead of ‘\\(\\sw\\|\\s_\\)+’.  Note that this technique
     may be inconvenient when the mode needs to limit the initial
     character of a name to a smaller set of characters than are allowed
     in the rest of a name.

     Setting this variable makes it buffer-local in the current buffer.

   Another way to customize Imenu for a major mode is to set the
variables ‘imenu-prev-index-position-function’ and
‘imenu-extract-index-name-function’:

 -- Variable: imenu-prev-index-position-function
     If this variable is non-‘nil’, its value should be a function that
     finds the next definition to put in the buffer index, scanning
     backward in the buffer from point.  It should return ‘nil’ if it
     doesn’t find another definition before point.  Otherwise it should
     leave point at the place it finds a definition and return any
     non-‘nil’ value.

     Setting this variable makes it buffer-local in the current buffer.

 -- Variable: imenu-extract-index-name-function
     If this variable is non-‘nil’, its value should be a function to
     return the name for a definition, assuming point is in that
     definition as the ‘imenu-prev-index-position-function’ function
     would leave it.

     Setting this variable makes it buffer-local in the current buffer.

   The last way to customize Imenu for a major mode is to set the
variable ‘imenu-create-index-function’:

 -- Variable: imenu-create-index-function
     This variable specifies the function to use for creating a buffer
     index.  The function should take no arguments, and return an index
     alist for the current buffer.  It is called within
     ‘save-excursion’, so where it leaves point makes no difference.

     The index alist can have three types of elements.  Simple elements
     look like this:

          (INDEX-NAME . INDEX-POSITION)

     Selecting a simple element has the effect of moving to position
     INDEX-POSITION in the buffer.  Special elements look like this:

          (INDEX-NAME INDEX-POSITION FUNCTION ARGUMENTS...)

     Selecting a special element performs:

          (funcall FUNCTION
                   INDEX-NAME INDEX-POSITION ARGUMENTS...)

     A nested sub-alist element looks like this:

          (MENU-TITLE . SUB-ALIST)

     It creates the submenu MENU-TITLE specified by SUB-ALIST.

     The default value of ‘imenu-create-index-function’ is
     ‘imenu-default-create-index-function’.  This function calls the
     value of ‘imenu-prev-index-position-function’ and the value of
     ‘imenu-extract-index-name-function’ to produce the index alist.
     However, if either of these two variables is ‘nil’, the default
     function uses ‘imenu-generic-expression’ instead.

     Setting this variable makes it buffer-local in the current buffer.


File: elisp.info,  Node: Font Lock Mode,  Next: Auto-Indentation,  Prev: Imenu,  Up: Modes

22.6 Font Lock Mode
===================

“Font Lock mode” is a buffer-local minor mode that automatically
attaches ‘face’ properties to certain parts of the buffer based on their
syntactic role.  How it parses the buffer depends on the major mode;
most major modes define syntactic criteria for which faces to use in
which contexts.  This section explains how to customize Font Lock for a
particular major mode.

   Font Lock mode finds text to highlight in two ways: through syntactic
parsing based on the syntax table, and through searching (usually for
regular expressions).  Syntactic fontification happens first; it finds
comments and string constants and highlights them.  Search-based
fontification happens second.

* Menu:

* Font Lock Basics::            Overview of customizing Font Lock.
* Search-based Fontification::  Fontification based on regexps.
* Customizing Keywords::        Customizing search-based fontification.
* Other Font Lock Variables::   Additional customization facilities.
* Levels of Font Lock::         Each mode can define alternative levels
                                  so that the user can select more or less.
* Precalculated Fontification::  How Lisp programs that produce the buffer
                                  contents can also specify how to fontify it.
* Faces for Font Lock::         Special faces specifically for Font Lock.
* Syntactic Font Lock::         Fontification based on syntax tables.
* Multiline Font Lock::         How to coerce Font Lock into properly
                                  highlighting multiline constructs.


File: elisp.info,  Node: Font Lock Basics,  Next: Search-based Fontification,  Up: Font Lock Mode

22.6.1 Font Lock Basics
-----------------------

The Font Lock functionality is based on several basic functions.  Each
of these calls the function specified by the corresponding variable.
This indirection allows major and minor modes to modify the way
fontification works in the buffers of that mode, and even use the Font
Lock mechanisms for features that have nothing to do with fontification.
(This is why the description below says “should” when it describes what
the functions do: the mode can customize the values of the corresponding
variables to do something entirely different.)  The variables mentioned
below are described in *note Other Font Lock Variables::.

‘font-lock-fontify-buffer’
     This function should fontify the current buffer’s accessible
     portion, by calling the function specified by
     ‘font-lock-fontify-buffer-function’.

‘font-lock-unfontify-buffer’
     Used when turning Font Lock off to remove the fontification.  Calls
     the function specified by ‘font-lock-unfontify-buffer-function’.

‘font-lock-fontify-region beg end &optional loudly’
     Should fontify the region between BEG and END.  If LOUDLY is
     non-‘nil’, should display status messages while fontifying.  Calls
     the function specified by ‘font-lock-fontify-region-function’.

‘font-lock-unfontify-region beg end’
     Should remove fontification from the region between BEG and END.
     Calls the function specified by
     ‘font-lock-unfontify-region-function’.

‘font-lock-flush &optional beg end’
     This function should mark the fontification of the region between
     BEG and END as outdated.  If not specified or ‘nil’, BEG and END
     default to the beginning and end of the buffer’s accessible
     portion.  Calls the function specified by
     ‘font-lock-flush-function’.

‘font-lock-ensure &optional beg end’
     This function should make sure the region between BEG and END has
     been fontified.  The optional arguments BEG and END default to the
     beginning and the end of the buffer’s accessible portion.  Calls
     the function specified by ‘font-lock-ensure-function’.

   There are several variables that control how Font Lock mode
highlights text.  But major modes should not set any of these variables
directly.  Instead, they should set ‘font-lock-defaults’ as a
buffer-local variable.  The value assigned to this variable is used, if
and when Font Lock mode is enabled, to set all the other variables.

 -- Variable: font-lock-defaults
     This variable is set by modes to specify how to fontify text in
     that mode.  It automatically becomes buffer-local when set.  If its
     value is ‘nil’, Font Lock mode does no highlighting, and you can
     use the ‘Faces’ menu (under ‘Edit’ and then ‘Text Properties’ in
     the menu bar) to assign faces explicitly to text in the buffer.

     If non-‘nil’, the value should look like this:

          (KEYWORDS [KEYWORDS-ONLY [CASE-FOLD
           [SYNTAX-ALIST OTHER-VARS...]]])

     The first element, KEYWORDS, indirectly specifies the value of
     ‘font-lock-keywords’ which directs search-based fontification.  It
     can be a symbol, a variable or a function whose value is the list
     to use for ‘font-lock-keywords’.  It can also be a list of several
     such symbols, one for each possible level of fontification.  The
     first symbol specifies the ‘mode default’ level of fontification,
     the next symbol level 1 fontification, the next level 2, and so on.
     The ‘mode default’ level is normally the same as level 1.  It is
     used when ‘font-lock-maximum-decoration’ has a ‘nil’ value.  *Note
     Levels of Font Lock::.

     The second element, KEYWORDS-ONLY, specifies the value of the
     variable ‘font-lock-keywords-only’.  If this is omitted or ‘nil’,
     syntactic fontification (of strings and comments) is also
     performed.  If this is non-‘nil’, syntactic fontification is not
     performed.  *Note Syntactic Font Lock::.

     The third element, CASE-FOLD, specifies the value of
     ‘font-lock-keywords-case-fold-search’.  If it is non-‘nil’, Font
     Lock mode ignores case during search-based fontification.

     If the fourth element, SYNTAX-ALIST, is non-‘nil’, it should be a
     list of cons cells of the form ‘(CHAR-OR-STRING . STRING)’.  These
     are used to set up a syntax table for syntactic fontification; the
     resulting syntax table is stored in ‘font-lock-syntax-table’.  If
     SYNTAX-ALIST is omitted or ‘nil’, syntactic fontification uses the
     syntax table returned by the ‘syntax-table’ function.  *Note Syntax
     Table Functions::.

     All the remaining elements (if any) are collectively called
     OTHER-VARS.  Each of these elements should have the form ‘(VARIABLE
     . VALUE)’—which means, make VARIABLE buffer-local and then set it
     to VALUE.  You can use these OTHER-VARS to set other variables that
     affect fontification, aside from those you can control with the
     first five elements.  *Note Other Font Lock Variables::.

   If your mode fontifies text explicitly by adding ‘font-lock-face’
properties, it can specify ‘(nil t)’ for ‘font-lock-defaults’ to turn
off all automatic fontification.  However, this is not required; it is
possible to fontify some things using ‘font-lock-face’ properties and
set up automatic fontification for other parts of the text.


File: elisp.info,  Node: Search-based Fontification,  Next: Customizing Keywords,  Prev: Font Lock Basics,  Up: Font Lock Mode

22.6.2 Search-based Fontification
---------------------------------

The variable which directly controls search-based fontification is
‘font-lock-keywords’, which is typically specified via the KEYWORDS
element in ‘font-lock-defaults’.

 -- Variable: font-lock-keywords
     The value of this variable is a list of the keywords to highlight.
     Lisp programs should not set this variable directly.  Normally, the
     value is automatically set by Font Lock mode, using the KEYWORDS
     element in ‘font-lock-defaults’.  The value can also be altered
     using the functions ‘font-lock-add-keywords’ and
     ‘font-lock-remove-keywords’ (*note Customizing Keywords::).

   Each element of ‘font-lock-keywords’ specifies how to find certain
cases of text, and how to highlight those cases.  Font Lock mode
processes the elements of ‘font-lock-keywords’ one by one, and for each
element, it finds and handles all matches.  Ordinarily, once part of the
text has been fontified already, this cannot be overridden by a
subsequent match in the same text; but you can specify different
behavior using the OVERRIDE element of a SUBEXP-HIGHLIGHTER.

   Each element of ‘font-lock-keywords’ should have one of these forms:

‘REGEXP’
     Highlight all matches for REGEXP using ‘font-lock-keyword-face’.
     For example,

          ;; Highlight occurrences of the word ‘foo’
          ;; using ‘font-lock-keyword-face’.
          "\\<foo\\>"

     Be careful when composing these regular expressions; a poorly
     written pattern can dramatically slow things down!  The function
     ‘regexp-opt’ (*note Regexp Functions::) is useful for calculating
     optimal regular expressions to match several keywords.

‘FUNCTION’
     Find text by calling FUNCTION, and highlight the matches it finds
     using ‘font-lock-keyword-face’.

     When FUNCTION is called, it receives one argument, the limit of the
     search; it should begin searching at point, and not search beyond
     the limit.  It should return non-‘nil’ if it succeeds, and set the
     match data to describe the match that was found.  Returning ‘nil’
     indicates failure of the search.

     Fontification will call FUNCTION repeatedly with the same limit,
     and with point where the previous invocation left it, until
     FUNCTION fails.  On failure, FUNCTION need not reset point in any
     particular way.

‘(MATCHER . SUBEXP)’
     In this kind of element, MATCHER is either a regular expression or
     a function, as described above.  The CDR, SUBEXP, specifies which
     subexpression of MATCHER should be highlighted (instead of the
     entire text that MATCHER matched).

          ;; Highlight the ‘bar’ in each occurrence of ‘fubar’,
          ;; using ‘font-lock-keyword-face’.
          ("fu\\(bar\\)" . 1)

     If you use ‘regexp-opt’ to produce the regular expression MATCHER,
     you can use ‘regexp-opt-depth’ (*note Regexp Functions::) to
     calculate the value for SUBEXP.

‘(MATCHER . FACESPEC)’
     In this kind of element, FACESPEC is an expression whose value
     specifies the face to use for highlighting.  In the simplest case,
     FACESPEC is a Lisp variable (a symbol) whose value is a face name.

          ;; Highlight occurrences of ‘fubar’,
          ;; using the face which is the value of ‘fubar-face’.
          ("fubar" . fubar-face)

     However, FACESPEC can also evaluate to a list of this form:

          (face FACE PROP1 VAL1 PROP2 VAL2...)

     to specify the face FACE and various additional text properties to
     put on the text that matches.  If you do this, be sure to add the
     other text property names that you set in this way to the value of
     ‘font-lock-extra-managed-props’ so that the properties will also be
     cleared out when they are no longer appropriate.  Alternatively,
     you can set the variable ‘font-lock-unfontify-region-function’ to a
     function that clears these properties.  *Note Other Font Lock
     Variables::.

‘(MATCHER . SUBEXP-HIGHLIGHTER)’
     In this kind of element, SUBEXP-HIGHLIGHTER is a list which
     specifies how to highlight matches found by MATCHER.  It has the
     form:

          (SUBEXP FACESPEC [OVERRIDE [LAXMATCH]])

     The CAR, SUBEXP, is an integer specifying which subexpression of
     the match to fontify (0 means the entire matching text).  The
     second subelement, FACESPEC, is an expression whose value specifies
     the face, as described above.

     The last two values in SUBEXP-HIGHLIGHTER, OVERRIDE and LAXMATCH,
     are optional flags.  If OVERRIDE is ‘t’, this element can override
     existing fontification made by previous elements of
     ‘font-lock-keywords’.  If it is ‘keep’, then each character is
     fontified if it has not been fontified already by some other
     element.  If it is ‘prepend’, the face specified by FACESPEC is
     added to the beginning of the ‘font-lock-face’ property.  If it is
     ‘append’, the face is added to the end of the ‘font-lock-face’
     property.

     If LAXMATCH is non-‘nil’, it means there should be no error if
     there is no subexpression numbered SUBEXP in MATCHER.  Obviously,
     fontification of the subexpression numbered SUBEXP will not occur.
     However, fontification of other subexpressions (and other regexps)
     will continue.  If LAXMATCH is ‘nil’, and the specified
     subexpression is missing, then an error is signaled which
     terminates search-based fontification.

     Here are some examples of elements of this kind, and what they do:

          ;; Highlight occurrences of either ‘foo’ or ‘bar’, using
          ;; ‘foo-bar-face’, even if they have already been highlighted.
          ;; ‘foo-bar-face’ should be a variable whose value is a face.
          ("foo\\|bar" 0 foo-bar-face t)

          ;; Highlight the first subexpression within each occurrence
          ;; that the function ‘fubar-match’ finds,
          ;; using the face which is the value of ‘fubar-face’.
          (fubar-match 1 fubar-face)

‘(MATCHER . ANCHORED-HIGHLIGHTER)’
     In this kind of element, ANCHORED-HIGHLIGHTER specifies how to
     highlight text that follows a match found by MATCHER.  So a match
     found by MATCHER acts as the anchor for further searches specified
     by ANCHORED-HIGHLIGHTER.  ANCHORED-HIGHLIGHTER is a list of the
     following form:

          (ANCHORED-MATCHER PRE-FORM POST-FORM
                                  SUBEXP-HIGHLIGHTERS...)

     Here, ANCHORED-MATCHER, like MATCHER, is either a regular
     expression or a function.  After a match of MATCHER is found, point
     is at the end of the match.  Now, Font Lock evaluates the form
     PRE-FORM.  Then it searches for matches of ANCHORED-MATCHER and
     uses SUBEXP-HIGHLIGHTERS to highlight these.  A SUBEXP-HIGHLIGHTER
     is as described above.  Finally, Font Lock evaluates POST-FORM.

     The forms PRE-FORM and POST-FORM can be used to initialize before,
     and cleanup after, ANCHORED-MATCHER is used.  Typically, PRE-FORM
     is used to move point to some position relative to the match of
     MATCHER, before starting with ANCHORED-MATCHER.  POST-FORM might be
     used to move back, before resuming with MATCHER.

     After Font Lock evaluates PRE-FORM, it does not search for
     ANCHORED-MATCHER beyond the end of the line.  However, if PRE-FORM
     returns a buffer position that is greater than the position of
     point after PRE-FORM is evaluated, then the position returned by
     PRE-FORM is used as the limit of the search instead.  It is
     generally a bad idea to return a position greater than the end of
     the line; in other words, the ANCHORED-MATCHER search should not
     span lines.

     For example,

          ;; Highlight occurrences of the word ‘item’ following
          ;; an occurrence of the word ‘anchor’ (on the same line)
          ;; in the value of ‘item-face’.
          ("\\<anchor\\>" "\\<item\\>" nil nil (0 item-face))

     Here, PRE-FORM and POST-FORM are ‘nil’.  Therefore searching for
     ‘item’ starts at the end of the match of ‘anchor’, and searching
     for subsequent instances of ‘anchor’ resumes from where searching
     for ‘item’ concluded.

‘(MATCHER HIGHLIGHTERS...)’
     This sort of element specifies several HIGHLIGHTER lists for a
     single MATCHER.  A HIGHLIGHTER list can be of the type
     SUBEXP-HIGHLIGHTER or ANCHORED-HIGHLIGHTER as described above.

     For example,

          ;; Highlight occurrences of the word ‘anchor’ in the value
          ;; of ‘anchor-face’, and subsequent occurrences of the word
          ;; ‘item’ (on the same line) in the value of ‘item-face’.
          ("\\<anchor\\>" (0 anchor-face)
                          ("\\<item\\>" nil nil (0 item-face)))

‘(eval . FORM)’
     Here FORM is an expression to be evaluated the first time this
     value of ‘font-lock-keywords’ is used in a buffer.  Its value
     should have one of the forms described in this table.

   *Warning:* Do not design an element of ‘font-lock-keywords’ to match
text which spans lines; this does not work reliably.  For details, see
*Note Multiline Font Lock::.

   You can use CASE-FOLD in ‘font-lock-defaults’ to specify the value of
‘font-lock-keywords-case-fold-search’ which says whether search-based
fontification should be case-insensitive.

 -- Variable: font-lock-keywords-case-fold-search
     Non-‘nil’ means that regular expression matching for the sake of
     ‘font-lock-keywords’ should be case-insensitive.


File: elisp.info,  Node: Customizing Keywords,  Next: Other Font Lock Variables,  Prev: Search-based Fontification,  Up: Font Lock Mode

22.6.3 Customizing Search-Based Fontification
---------------------------------------------

You can use ‘font-lock-add-keywords’ to add additional search-based
fontification rules to a major mode, and ‘font-lock-remove-keywords’ to
remove rules.

 -- Function: font-lock-add-keywords mode keywords &optional how
     This function adds highlighting KEYWORDS, for the current buffer or
     for major mode MODE.  The argument KEYWORDS should be a list with
     the same format as the variable ‘font-lock-keywords’.

     If MODE is a symbol which is a major mode command name, such as
     ‘c-mode’, the effect is that enabling Font Lock mode in MODE will
     add KEYWORDS to ‘font-lock-keywords’.  Calling with a non-‘nil’
     value of MODE is correct only in your ‘~/.emacs’ file.

     If MODE is ‘nil’, this function adds KEYWORDS to
     ‘font-lock-keywords’ in the current buffer.  This way of calling
     ‘font-lock-add-keywords’ is usually used in mode hook functions.

     By default, KEYWORDS are added at the beginning of
     ‘font-lock-keywords’.  If the optional argument HOW is ‘set’, they
     are used to replace the value of ‘font-lock-keywords’.  If HOW is
     any other non-‘nil’ value, they are added at the end of
     ‘font-lock-keywords’.

     Some modes provide specialized support you can use in additional
     highlighting patterns.  See the variables
     ‘c-font-lock-extra-types’, ‘c++-font-lock-extra-types’, and
     ‘java-font-lock-extra-types’, for example.

     *Warning:* Major mode commands must not call
     ‘font-lock-add-keywords’ under any circumstances, either directly
     or indirectly, except through their mode hooks.  (Doing so would
     lead to incorrect behavior for some minor modes.)  They should set
     up their rules for search-based fontification by setting
     ‘font-lock-keywords’.

 -- Function: font-lock-remove-keywords mode keywords
     This function removes KEYWORDS from ‘font-lock-keywords’ for the
     current buffer or for major mode MODE.  As in
     ‘font-lock-add-keywords’, MODE should be a major mode command name
     or ‘nil’.  All the caveats and requirements for
     ‘font-lock-add-keywords’ apply here too.  The argument KEYWORDS
     must exactly match the one used by the corresponding
     ‘font-lock-add-keywords’.

   For example, the following code adds two fontification patterns for C
mode: one to fontify the word ‘FIXME’, even in comments, and another to
fontify the words ‘and’, ‘or’ and ‘not’ as keywords.

     (font-lock-add-keywords 'c-mode
      '(("\\<\\(FIXME\\):" 1 font-lock-warning-face prepend)
        ("\\<\\(and\\|or\\|not\\)\\>" . font-lock-keyword-face)))

This example affects only C mode proper.  To add the same patterns to C
mode _and_ all modes derived from it, do this instead:

     (add-hook 'c-mode-hook
      (lambda ()
       (font-lock-add-keywords nil
        '(("\\<\\(FIXME\\):" 1 font-lock-warning-face prepend)
          ("\\<\\(and\\|or\\|not\\)\\>" .
           font-lock-keyword-face)))))


File: elisp.info,  Node: Other Font Lock Variables,  Next: Levels of Font Lock,  Prev: Customizing Keywords,  Up: Font Lock Mode

22.6.4 Other Font Lock Variables
--------------------------------

This section describes additional variables that a major mode can set by
means of OTHER-VARS in ‘font-lock-defaults’ (*note Font Lock Basics::).

 -- Variable: font-lock-mark-block-function
     If this variable is non-‘nil’, it should be a function that is
     called with no arguments, to choose an enclosing range of text for
     refontification for the command ‘M-o M-o’
     (‘font-lock-fontify-block’).

     The function should report its choice by placing the region around
     it.  A good choice is a range of text large enough to give proper
     results, but not too large so that refontification becomes slow.
     Typical values are ‘mark-defun’ for programming modes or
     ‘mark-paragraph’ for textual modes.

 -- Variable: font-lock-extra-managed-props
     This variable specifies additional properties (other than
     ‘font-lock-face’) that are being managed by Font Lock mode.  It is
     used by ‘font-lock-default-unfontify-region’, which normally only
     manages the ‘font-lock-face’ property.  If you want Font Lock to
     manage other properties as well, you must specify them in a
     FACESPEC in ‘font-lock-keywords’ as well as add them to this list.
     *Note Search-based Fontification::.

 -- Variable: font-lock-fontify-buffer-function
     Function to use for fontifying the buffer.  The default value is
     ‘font-lock-default-fontify-buffer’.

 -- Variable: font-lock-unfontify-buffer-function
     Function to use for unfontifying the buffer.  This is used when
     turning off Font Lock mode.  The default value is
     ‘font-lock-default-unfontify-buffer’.

 -- Variable: font-lock-fontify-region-function
     Function to use for fontifying a region.  It should take two
     arguments, the beginning and end of the region, and an optional
     third argument VERBOSE.  If VERBOSE is non-‘nil’, the function
     should print status messages.  The default value is
     ‘font-lock-default-fontify-region’.

 -- Variable: font-lock-unfontify-region-function
     Function to use for unfontifying a region.  It should take two
     arguments, the beginning and end of the region.  The default value
     is ‘font-lock-default-unfontify-region’.

 -- Variable: font-lock-flush-function
     Function to use for declaring that a region’s fontification is out
     of date.  It takes two arguments, the beginning and end of the
     region.  The default value of this variable is
     ‘font-lock-after-change-function’.

 -- Variable: font-lock-ensure-function
     Function to use for making sure a region of the current buffer has
     been fontified.  It is called with two arguments, the beginning and
     end of the region.  The default value of this variable is a
     function that calls ‘font-lock-default-fontify-buffer’ if the
     buffer is not fontified; the effect is to make sure the entire
     accessible portion of the buffer is fontified.

 -- Function: jit-lock-register function &optional contextual
     This function tells Font Lock mode to run the Lisp function
     FUNCTION any time it has to fontify or refontify part of the
     current buffer.  It calls FUNCTION before calling the default
     fontification functions, and gives it two arguments, START and END,
     which specify the region to be fontified or refontified.

     The optional argument CONTEXTUAL, if non-‘nil’, forces Font Lock
     mode to always refontify a syntactically relevant part of the
     buffer, and not just the modified lines.  This argument can usually
     be omitted.

 -- Function: jit-lock-unregister function
     If FUNCTION was previously registered as a fontification function
     using ‘jit-lock-register’, this function unregisters it.


File: elisp.info,  Node: Levels of Font Lock,  Next: Precalculated Fontification,  Prev: Other Font Lock Variables,  Up: Font Lock Mode

22.6.5 Levels of Font Lock
--------------------------

Some major modes offer three different levels of fontification.  You can
define multiple levels by using a list of symbols for KEYWORDS in
‘font-lock-defaults’.  Each symbol specifies one level of fontification;
it is up to the user to choose one of these levels, normally by setting
‘font-lock-maximum-decoration’ (*note (emacs)Font Lock::).  The chosen
level’s symbol value is used to initialize ‘font-lock-keywords’.

   Here are the conventions for how to define the levels of
fontification:

   • Level 1: highlight function declarations, file directives (such as
     include or import directives), strings and comments.  The idea is
     speed, so only the most important and top-level components are
     fontified.

   • Level 2: in addition to level 1, highlight all language keywords,
     including type names that act like keywords, as well as named
     constant values.  The idea is that all keywords (either syntactic
     or semantic) should be fontified appropriately.

   • Level 3: in addition to level 2, highlight the symbols being
     defined in function and variable declarations, and all builtin
     function names, wherever they appear.


File: elisp.info,  Node: Precalculated Fontification,  Next: Faces for Font Lock,  Prev: Levels of Font Lock,  Up: Font Lock Mode

22.6.6 Precalculated Fontification
----------------------------------

Some major modes such as ‘list-buffers’ and ‘occur’ construct the buffer
text programmatically.  The easiest way for them to support Font Lock
mode is to specify the faces of text when they insert the text in the
buffer.

   The way to do this is to specify the faces in the text with the
special text property ‘font-lock-face’ (*note Special Properties::).
When Font Lock mode is enabled, this property controls the display, just
like the ‘face’ property.  When Font Lock mode is disabled,
‘font-lock-face’ has no effect on the display.

   It is ok for a mode to use ‘font-lock-face’ for some text and also
use the normal Font Lock machinery.  But if the mode does not use the
normal Font Lock machinery, it should not set the variable
‘font-lock-defaults’.


File: elisp.info,  Node: Faces for Font Lock,  Next: Syntactic Font Lock,  Prev: Precalculated Fontification,  Up: Font Lock Mode

22.6.7 Faces for Font Lock
--------------------------

Font Lock mode can highlight using any face, but Emacs defines several
faces specifically for Font Lock to use to highlight text.  These “Font
Lock faces” are listed below.  They can also be used by major modes for
syntactic highlighting outside of Font Lock mode (*note Major Mode
Conventions::).

   Each of these symbols is both a face name, and a variable whose
default value is the symbol itself.  Thus, the default value of
‘font-lock-comment-face’ is ‘font-lock-comment-face’.

   The faces are listed with descriptions of their typical usage, and in
order of greater to lesser prominence.  If a mode’s syntactic categories
do not fit well with the usage descriptions, the faces can be assigned
using the ordering as a guide.

‘font-lock-warning-face’
     for a construct that is peculiar, or that greatly changes the
     meaning of other text, like ‘;;;###autoload’ in Emacs Lisp and
     ‘#error’ in C.

‘font-lock-function-name-face’
     for the name of a function being defined or declared.

‘font-lock-variable-name-face’
     for the name of a variable being defined or declared.

‘font-lock-keyword-face’
     for a keyword with special syntactic significance, like ‘for’ and
     ‘if’ in C.

‘font-lock-comment-face’
     for comments.

‘font-lock-comment-delimiter-face’
     for comments delimiters, like ‘/*’ and ‘*/’ in C.  On most
     terminals, this inherits from ‘font-lock-comment-face’.

‘font-lock-type-face’
     for the names of user-defined data types.

‘font-lock-constant-face’
     for the names of constants, like ‘NULL’ in C.

‘font-lock-builtin-face’
     for the names of built-in functions.

‘font-lock-preprocessor-face’
     for preprocessor commands.  This inherits, by default, from
     ‘font-lock-builtin-face’.

‘font-lock-string-face’
     for string constants.

‘font-lock-doc-face’
     for documentation strings in the code.  This inherits, by default,
     from ‘font-lock-string-face’.

‘font-lock-negation-char-face’
     for easily-overlooked negation characters.


File: elisp.info,  Node: Syntactic Font Lock,  Next: Multiline Font Lock,  Prev: Faces for Font Lock,  Up: Font Lock Mode

22.6.8 Syntactic Font Lock
--------------------------

Syntactic fontification uses a syntax table (*note Syntax Tables::) to
find and highlight syntactically relevant text.  If enabled, it runs
prior to search-based fontification.  The variable
‘font-lock-syntactic-face-function’, documented below, determines which
syntactic constructs to highlight.  There are several variables that
affect syntactic fontification; you should set them by means of
‘font-lock-defaults’ (*note Font Lock Basics::).

   Whenever Font Lock mode performs syntactic fontification on a stretch
of text, it first calls the function specified by
‘syntax-propertize-function’.  Major modes can use this to apply
‘syntax-table’ text properties to override the buffer’s syntax table in
special cases.  *Note Syntax Properties::.

 -- Variable: font-lock-keywords-only
     If the value of this variable is non-‘nil’, Font Lock does not do
     syntactic fontification, only search-based fontification based on
     ‘font-lock-keywords’.  It is normally set by Font Lock mode based
     on the KEYWORDS-ONLY element in ‘font-lock-defaults’.

 -- Variable: font-lock-syntax-table
     This variable holds the syntax table to use for fontification of
     comments and strings.  It is normally set by Font Lock mode based
     on the SYNTAX-ALIST element in ‘font-lock-defaults’.  If this value
     is ‘nil’, syntactic fontification uses the buffer’s syntax table
     (the value returned by the function ‘syntax-table’; *note Syntax
     Table Functions::).

 -- Variable: font-lock-syntactic-face-function
     If this variable is non-‘nil’, it should be a function to determine
     which face to use for a given syntactic element (a string or a
     comment).  The value is normally set through an OTHER-VARS element
     in ‘font-lock-defaults’.

     The function is called with one argument, the parse state at point
     returned by ‘parse-partial-sexp’, and should return a face.  The
     default value returns ‘font-lock-comment-face’ for comments and
     ‘font-lock-string-face’ for strings (*note Faces for Font Lock::).


File: elisp.info,  Node: Multiline Font Lock,  Prev: Syntactic Font Lock,  Up: Font Lock Mode

22.6.9 Multiline Font Lock Constructs
-------------------------------------

Normally, elements of ‘font-lock-keywords’ should not match across
multiple lines; that doesn’t work reliably, because Font Lock usually
scans just part of the buffer, and it can miss a multi-line construct
that crosses the line boundary where the scan starts.  (The scan
normally starts at the beginning of a line.)

   Making elements that match multiline constructs work properly has two
aspects: correct _identification_ and correct _rehighlighting_.  The
first means that Font Lock finds all multiline constructs.  The second
means that Font Lock will correctly rehighlight all the relevant text
when a multiline construct is changed—for example, if some of the text
that was previously part of a multiline construct ceases to be part of
it.  The two aspects are closely related, and often getting one of them
to work will appear to make the other also work.  However, for reliable
results you must attend explicitly to both aspects.

   There are three ways to ensure correct identification of multiline
constructs:

   • Add a function to ‘font-lock-extend-region-functions’ that does the
     _identification_ and extends the scan so that the scanned text
     never starts or ends in the middle of a multiline construct.
   • Use the ‘font-lock-fontify-region-function’ hook similarly to
     extend the scan so that the scanned text never starts or ends in
     the middle of a multiline construct.
   • Somehow identify the multiline construct right when it gets
     inserted into the buffer (or at any point after that but before
     font-lock tries to highlight it), and mark it with a
     ‘font-lock-multiline’ which will instruct font-lock not to start or
     end the scan in the middle of the construct.

   There are three ways to do rehighlighting of multiline constructs:

   • Place a ‘font-lock-multiline’ property on the construct.  This will
     rehighlight the whole construct if any part of it is changed.  In
     some cases you can do this automatically by setting the
     ‘font-lock-multiline’ variable, which see.
   • Make sure ‘jit-lock-contextually’ is set and rely on it doing its
     job.  This will only rehighlight the part of the construct that
     follows the actual change, and will do it after a short delay.
     This only works if the highlighting of the various parts of your
     multiline construct never depends on text in subsequent lines.
     Since ‘jit-lock-contextually’ is activated by default, this can be
     an attractive solution.
   • Place a ‘jit-lock-defer-multiline’ property on the construct.  This
     works only if ‘jit-lock-contextually’ is used, and with the same
     delay before rehighlighting, but like ‘font-lock-multiline’, it
     also handles the case where highlighting depends on subsequent
     lines.

* Menu:

* Font Lock Multiline::         Marking multiline chunks with a text property.
* Region to Refontify::         Controlling which region gets refontified
                                  after a buffer change.


File: elisp.info,  Node: Font Lock Multiline,  Next: Region to Refontify,  Up: Multiline Font Lock

22.6.9.1 Font Lock Multiline
............................

One way to ensure reliable rehighlighting of multiline Font Lock
constructs is to put on them the text property ‘font-lock-multiline’.
It should be present and non-‘nil’ for text that is part of a multiline
construct.

   When Font Lock is about to highlight a range of text, it first
extends the boundaries of the range as necessary so that they do not
fall within text marked with the ‘font-lock-multiline’ property.  Then
it removes any ‘font-lock-multiline’ properties from the range, and
highlights it.  The highlighting specification (mostly
‘font-lock-keywords’) must reinstall this property each time, whenever
it is appropriate.

   *Warning:* don’t use the ‘font-lock-multiline’ property on large
ranges of text, because that will make rehighlighting slow.

 -- Variable: font-lock-multiline
     If the ‘font-lock-multiline’ variable is set to ‘t’, Font Lock will
     try to add the ‘font-lock-multiline’ property automatically on
     multiline constructs.  This is not a universal solution, however,
     since it slows down Font Lock somewhat.  It can miss some multiline
     constructs, or make the property larger or smaller than necessary.

     For elements whose MATCHER is a function, the function should
     ensure that submatch 0 covers the whole relevant multiline
     construct, even if only a small subpart will be highlighted.  It is
     often just as easy to add the ‘font-lock-multiline’ property by
     hand.

   The ‘font-lock-multiline’ property is meant to ensure proper
refontification; it does not automatically identify new multiline
constructs.  Identifying the requires that Font Lock mode operate on
large enough chunks at a time.  This will happen by accident on many
cases, which may give the impression that multiline constructs magically
work.  If you set the ‘font-lock-multiline’ variable non-‘nil’, this
impression will be even stronger, since the highlighting of those
constructs which are found will be properly updated from then on.  But
that does not work reliably.

   To find multiline constructs reliably, you must either manually place
the ‘font-lock-multiline’ property on the text before Font Lock mode
looks at it, or use ‘font-lock-fontify-region-function’.


File: elisp.info,  Node: Region to Refontify,  Prev: Font Lock Multiline,  Up: Multiline Font Lock

22.6.9.2 Region to Fontify after a Buffer Change
................................................

When a buffer is changed, the region that Font Lock refontifies is by
default the smallest sequence of whole lines that spans the change.
While this works well most of the time, sometimes it doesn’t—for
example, when a change alters the syntactic meaning of text on an
earlier line.

   You can enlarge (or even reduce) the region to refontify by setting
the following variable:

 -- Variable: font-lock-extend-after-change-region-function
     This buffer-local variable is either ‘nil’ or a function for Font
     Lock mode to call to determine the region to scan and fontify.

     The function is given three parameters, the standard BEG, END, and
     OLD-LEN from ‘after-change-functions’ (*note Change Hooks::).  It
     should return either a cons of the beginning and end buffer
     positions (in that order) of the region to fontify, or ‘nil’ (which
     means choose the region in the standard way).  This function needs
     to preserve point, the match-data, and the current restriction.
     The region it returns may start or end in the middle of a line.

     Since this function is called after every buffer change, it should
     be reasonably fast.


File: elisp.info,  Node: Auto-Indentation,  Next: Desktop Save Mode,  Prev: Font Lock Mode,  Up: Modes

22.7 Automatic Indentation of code
==================================

For programming languages, an important feature of a major mode is to
provide automatic indentation.  There are two parts: one is to decide
what is the right indentation of a line, and the other is to decide when
to reindent a line.  By default, Emacs reindents a line whenever you
type a character in ‘electric-indent-chars’, which by default only
includes Newline.  Major modes can add chars to ‘electric-indent-chars’
according to the syntax of the language.

   Deciding what is the right indentation is controlled in Emacs by
‘indent-line-function’ (*note Mode-Specific Indent::).  For some modes,
the _right_ indentation cannot be known reliably, typically because
indentation is significant so several indentations are valid but with
different meanings.  In that case, the mode should set
‘electric-indent-inhibit’ to make sure the line is not constantly
re-indented against the user’s wishes.

   Writing a good indentation function can be difficult and to a large
extent it is still a black art.  Many major mode authors will start by
writing a simple indentation function that works for simple cases, for
example by comparing with the indentation of the previous text line.
For most programming languages that are not really line-based, this
tends to scale very poorly: improving such a function to let it handle
more diverse situations tends to become more and more difficult,
resulting in the end with a large, complex, unmaintainable indentation
function which nobody dares to touch.

   A good indentation function will usually need to actually parse the
text, according to the syntax of the language.  Luckily, it is not
necessary to parse the text in as much detail as would be needed for a
compiler, but on the other hand, the parser embedded in the indentation
code will want to be somewhat friendly to syntactically incorrect code.

   Good maintainable indentation functions usually fall into two
categories: either parsing forward from some safe starting point until
the position of interest, or parsing backward from the position of
interest.  Neither of the two is a clearly better choice than the other:
parsing backward is often more difficult than parsing forward because
programming languages are designed to be parsed forward, but for the
purpose of indentation it has the advantage of not needing to guess a
safe starting point, and it generally enjoys the property that only a
minimum of text will be analyzed to decide the indentation of a line, so
indentation will tend to be less affected by syntax errors in some
earlier unrelated piece of code.  Parsing forward on the other hand is
usually easier and has the advantage of making it possible to reindent
efficiently a whole region at a time, with a single parse.

   Rather than write your own indentation function from scratch, it is
often preferable to try and reuse some existing ones or to rely on a
generic indentation engine.  There are sadly few such engines.  The
CC-mode indentation code (used with C, C++, Java, Awk and a few other
such modes) has been made more generic over the years, so if your
language seems somewhat similar to one of those languages, you might try
to use that engine.  Another one is SMIE which takes an approach in the
spirit of Lisp sexps and adapts it to non-Lisp languages.

* Menu:

* SMIE::                        A simple minded indentation engine.


File: elisp.info,  Node: SMIE,  Up: Auto-Indentation

22.7.1 Simple Minded Indentation Engine
---------------------------------------

SMIE is a package that provides a generic navigation and indentation
engine.  Based on a very simple parser using an operator precedence
grammar, it lets major modes extend the sexp-based navigation of Lisp to
non-Lisp languages as well as provide a simple to use but reliable
auto-indentation.

   Operator precedence grammar is a very primitive technology for
parsing compared to some of the more common techniques used in
compilers.  It has the following characteristics: its parsing power is
very limited, and it is largely unable to detect syntax errors, but it
has the advantage of being algorithmically efficient and able to parse
forward just as well as backward.  In practice that means that SMIE can
use it for indentation based on backward parsing, that it can provide
both ‘forward-sexp’ and ‘backward-sexp’ functionality, and that it will
naturally work on syntactically incorrect code without any extra effort.
The downside is that it also means that most programming languages
cannot be parsed correctly using SMIE, at least not without resorting to
some special tricks (*note SMIE Tricks::).

* Menu:

* SMIE setup::                  SMIE setup and features.
* Operator Precedence Grammars::  A very simple parsing technique.
* SMIE Grammar::                Defining the grammar of a language.
* SMIE Lexer::                  Defining tokens.
* SMIE Tricks::                 Working around the parser’s limitations.
* SMIE Indentation::            Specifying indentation rules.
* SMIE Indentation Helpers::    Helper functions for indentation rules.
* SMIE Indentation Example::    Sample indentation rules.
* SMIE Customization::          Customizing indentation.


File: elisp.info,  Node: SMIE setup,  Next: Operator Precedence Grammars,  Up: SMIE

22.7.1.1 SMIE Setup and Features
................................

SMIE is meant to be a one-stop shop for structural navigation and
various other features which rely on the syntactic structure of code, in
particular automatic indentation.  The main entry point is ‘smie-setup’
which is a function typically called while setting up a major mode.

 -- Function: smie-setup grammar rules-function &rest keywords
     Setup SMIE navigation and indentation.  GRAMMAR is a grammar table
     generated by ‘smie-prec2->grammar’.  RULES-FUNCTION is a set of
     indentation rules for use on ‘smie-rules-function’.  KEYWORDS are
     additional arguments, which can include the following keywords:
        • ‘:forward-token’ FUN: Specify the forward lexer to use.
        • ‘:backward-token’ FUN: Specify the backward lexer to use.

   Calling this function is sufficient to make commands such as
‘forward-sexp’, ‘backward-sexp’, and ‘transpose-sexps’ be able to
properly handle structural elements other than just the paired
parentheses already handled by syntax tables.  For example, if the
provided grammar is precise enough, ‘transpose-sexps’ can correctly
transpose the two arguments of a ‘+’ operator, taking into account the
precedence rules of the language.

   Calling ‘smie-setup’ is also sufficient to make TAB indentation work
in the expected way, extends ‘blink-matching-paren’ to apply to elements
like ‘begin...end’, and provides some commands that you can bind in the
major mode keymap.

 -- Command: smie-close-block
     This command closes the most recently opened (and not yet closed)
     block.

 -- Command: smie-down-list &optional arg
     This command is like ‘down-list’ but it also pays attention to
     nesting of tokens other than parentheses, such as ‘begin...end’.


File: elisp.info,  Node: Operator Precedence Grammars,  Next: SMIE Grammar,  Prev: SMIE setup,  Up: SMIE

22.7.1.2 Operator Precedence Grammars
.....................................

SMIE’s precedence grammars simply give to each token a pair of
precedences: the left-precedence and the right-precedence.  We say ‘T1 <
T2’ if the right-precedence of token ‘T1’ is less than the
left-precedence of token ‘T2’.  A good way to read this ‘<’ is as a kind
of parenthesis: if we find ‘... T1 something T2 ...’ then that should be
parsed as ‘... T1 (something T2 ...’ rather than as ‘... T1 something)
T2 ...’.  The latter interpretation would be the case if we had ‘T1 >
T2’.  If we have ‘T1 = T2’, it means that token T2 follows token T1 in
the same syntactic construction, so typically we have ‘"begin" = "end"’.
Such pairs of precedences are sufficient to express left-associativity
or right-associativity of infix operators, nesting of tokens like
parentheses and many other cases.

 -- Function: smie-prec2->grammar table
     This function takes a _prec2_ grammar TABLE and returns an alist
     suitable for use in ‘smie-setup’.  The _prec2_ TABLE is itself
     meant to be built by one of the functions below.

 -- Function: smie-merge-prec2s &rest tables
     This function takes several _prec2_ TABLES and merges them into a
     new _prec2_ table.

 -- Function: smie-precs->prec2 precs
     This function builds a _prec2_ table from a table of precedences
     PRECS.  PRECS should be a list, sorted by precedence (for example
     ‘"+"’ will come before ‘"*"’), of elements of the form ‘(ASSOC OP
     ...)’, where each OP is a token that acts as an operator; ASSOC is
     their associativity, which can be either ‘left’, ‘right’, ‘assoc’,
     or ‘nonassoc’.  All operators in a given element share the same
     precedence level and associativity.

 -- Function: smie-bnf->prec2 bnf &rest resolvers
     This function lets you specify the grammar using a BNF notation.
     It accepts a BNF description of the grammar along with a set of
     conflict resolution rules RESOLVERS, and returns a _prec2_ table.

     BNF is a list of nonterminal definitions of the form ‘(NONTERM RHS1
     RHS2 ...)’ where each RHS is a (non-empty) list of terminals (aka
     tokens) or non-terminals.

     Not all grammars are accepted:
        • An RHS cannot be an empty list (an empty list is never needed,
          since SMIE allows all non-terminals to match the empty string
          anyway).
        • An RHS cannot have 2 consecutive non-terminals: each pair of
          non-terminals needs to be separated by a terminal (aka token).
          This is a fundamental limitation of operator precedence
          grammars.

     Additionally, conflicts can occur:
        • The returned _prec2_ table holds constraints between pairs of
          tokens, and for any given pair only one constraint can be
          present: T1 < T2, T1 = T2, or T1 > T2.
        • A token can be an ‘opener’ (something similar to an
          open-paren), a ‘closer’ (like a close-paren), or ‘neither’ of
          the two (e.g., an infix operator, or an inner token like
          ‘"else"’).

     Precedence conflicts can be resolved via RESOLVERS, which is a list
     of _precs_ tables (see ‘smie-precs->prec2’): for each precedence
     conflict, if those ‘precs’ tables specify a particular constraint,
     then the conflict is resolved by using this constraint instead,
     else a conflict is reported and one of the conflicting constraints
     is picked arbitrarily and the others are simply ignored.


File: elisp.info,  Node: SMIE Grammar,  Next: SMIE Lexer,  Prev: Operator Precedence Grammars,  Up: SMIE

22.7.1.3 Defining the Grammar of a Language
...........................................

The usual way to define the SMIE grammar of a language is by defining a
new global variable that holds the precedence table by giving a set of
BNF rules.  For example, the grammar definition for a small Pascal-like
language could look like:
     (require 'smie)
     (defvar sample-smie-grammar
       (smie-prec2->grammar
        (smie-bnf->prec2
         '((id)
           (inst ("begin" insts "end")
                 ("if" exp "then" inst "else" inst)
                 (id ":=" exp)
                 (exp))
           (insts (insts ";" insts) (inst))
           (exp (exp "+" exp)
                (exp "*" exp)
                ("(" exps ")"))
           (exps (exps "," exps) (exp)))
         '((assoc ";"))
         '((assoc ","))
         '((assoc "+") (assoc "*")))))

A few things to note:

   • The above grammar does not explicitly mention the syntax of
     function calls: SMIE will automatically allow any sequence of
     sexps, such as identifiers, balanced parentheses, or ‘begin ...
     end’ blocks to appear anywhere anyway.
   • The grammar category ‘id’ has no right hand side: this does not
     mean that it can match only the empty string, since as mentioned
     any sequence of sexps can appear anywhere anyway.
   • Because non terminals cannot appear consecutively in the BNF
     grammar, it is difficult to correctly handle tokens that act as
     terminators, so the above grammar treats ‘";"’ as a statement
     _separator_ instead, which SMIE can handle very well.
   • Separators used in sequences (such as ‘","’ and ‘";"’ above) are
     best defined with BNF rules such as ‘(foo (foo "separator" foo)
     ...)’ which generate precedence conflicts which are then resolved
     by giving them an explicit ‘(assoc "separator")’.
   • The ‘("(" exps ")")’ rule was not needed to pair up parens, since
     SMIE will pair up any characters that are marked as having paren
     syntax in the syntax table.  What this rule does instead (together
     with the definition of ‘exps’) is to make it clear that ‘","’
     should not appear outside of parentheses.
   • Rather than have a single _precs_ table to resolve conflicts, it is
     preferable to have several tables, so as to let the BNF part of the
     grammar specify relative precedences where possible.
   • Unless there is a very good reason to prefer ‘left’ or ‘right’, it
     is usually preferable to mark operators as associative, using
     ‘assoc’.  For that reason ‘"+"’ and ‘"*"’ are defined above as
     ‘assoc’, although the language defines them formally as left
     associative.


File: elisp.info,  Node: SMIE Lexer,  Next: SMIE Tricks,  Prev: SMIE Grammar,  Up: SMIE

22.7.1.4 Defining Tokens
........................

SMIE comes with a predefined lexical analyzer which uses syntax tables
in the following way: any sequence of characters that have word or
symbol syntax is considered a token, and so is any sequence of
characters that have punctuation syntax.  This default lexer is often a
good starting point but is rarely actually correct for any given
language.  For example, it will consider ‘"2,+3"’ to be composed of 3
tokens: ‘"2"’, ‘",+"’, and ‘"3"’.

   To describe the lexing rules of your language to SMIE, you need 2
functions, one to fetch the next token, and another to fetch the
previous token.  Those functions will usually first skip whitespace and
comments and then look at the next chunk of text to see if it is a
special token.  If so it should skip the token and return a description
of this token.  Usually this is simply the string extracted from the
buffer, but it can be anything you want.  For example:
     (defvar sample-keywords-regexp
       (regexp-opt '("+" "*" "," ";" ">" ">=" "<" "<=" ":=" "=")))
     (defun sample-smie-forward-token ()
       (forward-comment (point-max))
       (cond
        ((looking-at sample-keywords-regexp)
         (goto-char (match-end 0))
         (match-string-no-properties 0))
        (t (buffer-substring-no-properties
            (point)
            (progn (skip-syntax-forward "w_")
                   (point))))))
     (defun sample-smie-backward-token ()
       (forward-comment (- (point)))
       (cond
        ((looking-back sample-keywords-regexp (- (point) 2) t)
         (goto-char (match-beginning 0))
         (match-string-no-properties 0))
        (t (buffer-substring-no-properties
            (point)
            (progn (skip-syntax-backward "w_")
                   (point))))))

   Notice how those lexers return the empty string when in front of
parentheses.  This is because SMIE automatically takes care of the
parentheses defined in the syntax table.  More specifically if the lexer
returns ‘nil’ or an empty string, SMIE tries to handle the corresponding
text as a sexp according to syntax tables.


File: elisp.info,  Node: SMIE Tricks,  Next: SMIE Indentation,  Prev: SMIE Lexer,  Up: SMIE

22.7.1.5 Living With a Weak Parser
..................................

The parsing technique used by SMIE does not allow tokens to behave
differently in different contexts.  For most programming languages, this
manifests itself by precedence conflicts when converting the BNF
grammar.

   Sometimes, those conflicts can be worked around by expressing the
grammar slightly differently.  For example, for Modula-2 it might seem
natural to have a BNF grammar that looks like this:

       ...
       (inst ("IF" exp "THEN" insts "ELSE" insts "END")
             ("CASE" exp "OF" cases "END")
             ...)
       (cases (cases "|" cases)
              (caselabel ":" insts)
              ("ELSE" insts))
       ...

   But this will create conflicts for ‘"ELSE"’: on the one hand, the IF
rule implies (among many other things) that ‘"ELSE" = "END"’; but on the
other hand, since ‘"ELSE"’ appears within ‘cases’, which appears left of
‘"END"’, we also have ‘"ELSE" > "END"’.  We can solve the conflict
either by using:
       ...
       (inst ("IF" exp "THEN" insts "ELSE" insts "END")
             ("CASE" exp "OF" cases "END")
             ("CASE" exp "OF" cases "ELSE" insts "END")
             ...)
       (cases (cases "|" cases) (caselabel ":" insts))
       ...
   or
       ...
       (inst ("IF" exp "THEN" else "END")
             ("CASE" exp "OF" cases "END")
             ...)
       (else (insts "ELSE" insts))
       (cases (cases "|" cases) (caselabel ":" insts) (else))
       ...

   Reworking the grammar to try and solve conflicts has its downsides,
tho, because SMIE assumes that the grammar reflects the logical
structure of the code, so it is preferable to keep the BNF closer to the
intended abstract syntax tree.

   Other times, after careful consideration you may conclude that those
conflicts are not serious and simply resolve them via the RESOLVERS
argument of ‘smie-bnf->prec2’.  Usually this is because the grammar is
simply ambiguous: the conflict does not affect the set of programs
described by the grammar, but only the way those programs are parsed.
This is typically the case for separators and associative infix
operators, where you want to add a resolver like ‘'((assoc "|"))’.
Another case where this can happen is for the classic _dangling else_
problem, where you will use ‘'((assoc "else" "then"))’.  It can also
happen for cases where the conflict is real and cannot really be
resolved, but it is unlikely to pose a problem in practice.

   Finally, in many cases some conflicts will remain despite all efforts
to restructure the grammar.  Do not despair: while the parser cannot be
made more clever, you can make the lexer as smart as you want.  So, the
solution is then to look at the tokens involved in the conflict and to
split one of those tokens into 2 (or more) different tokens.  E.g., if
the grammar needs to distinguish between two incompatible uses of the
token ‘"begin"’, make the lexer return different tokens (say
‘"begin-fun"’ and ‘"begin-plain"’) depending on which kind of ‘"begin"’
it finds.  This pushes the work of distinguishing the different cases to
the lexer, which will thus have to look at the surrounding text to find
ad-hoc clues.


File: elisp.info,  Node: SMIE Indentation,  Next: SMIE Indentation Helpers,  Prev: SMIE Tricks,  Up: SMIE

22.7.1.6 Specifying Indentation Rules
.....................................

Based on the provided grammar, SMIE will be able to provide automatic
indentation without any extra effort.  But in practice, this default
indentation style will probably not be good enough.  You will want to
tweak it in many different cases.

   SMIE indentation is based on the idea that indentation rules should
be as local as possible.  To this end, it relies on the idea of
_virtual_ indentation, which is the indentation that a particular
program point would have if it were at the beginning of a line.  Of
course, if that program point is indeed at the beginning of a line, its
virtual indentation is its current indentation.  But if not, then SMIE
uses the indentation algorithm to compute the virtual indentation of
that point.  Now in practice, the virtual indentation of a program point
does not have to be identical to the indentation it would have if we
inserted a newline before it.  To see how this works, the SMIE rule for
indentation after a ‘{’ in C does not care whether the ‘{’ is standing
on a line of its own or is at the end of the preceding line.  Instead,
these different cases are handled in the indentation rule that decides
how to indent before a ‘{’.

   Another important concept is the notion of _parent_: The _parent_ of
a token, is the head token of the nearest enclosing syntactic construct.
For example, the parent of an ‘else’ is the ‘if’ to which it belongs,
and the parent of an ‘if’, in turn, is the lead token of the surrounding
construct.  The command ‘backward-sexp’ jumps from a token to its
parent, but there are some caveats: for _openers_ (tokens which start a
construct, like ‘if’), you need to start with point before the token,
while for others you need to start with point after the token.
‘backward-sexp’ stops with point before the parent token if that is the
_opener_ of the token of interest, and otherwise it stops with point
after the parent token.

   SMIE indentation rules are specified using a function that takes two
arguments METHOD and ARG where the meaning of ARG and the expected
return value depend on METHOD.

   METHOD can be:
   • ‘:after’, in which case ARG is a token and the function should
     return the OFFSET to use for indentation after ARG.
   • ‘:before’, in which case ARG is a token and the function should
     return the OFFSET to use to indent ARG itself.
   • ‘:elem’, in which case the function should return either the offset
     to use to indent function arguments (if ARG is the symbol ‘arg’) or
     the basic indentation step (if ARG is the symbol ‘basic’).
   • ‘:list-intro’, in which case ARG is a token and the function should
     return non-‘nil’ if the token is followed by a list of expressions
     (not separated by any token) rather than an expression.

   When ARG is a token, the function is called with point just before
that token.  A return value of ‘nil’ always means to fallback on the
default behavior, so the function should return ‘nil’ for arguments it
does not expect.

   OFFSET can be:
   • ‘nil’: use the default indentation rule.
   • ‘(column . COLUMN)’: indent to column COLUMN.
   • NUMBER: offset by NUMBER, relative to a base token which is the
     current token for ‘:after’ and its parent for ‘:before’.


File: elisp.info,  Node: SMIE Indentation Helpers,  Next: SMIE Indentation Example,  Prev: SMIE Indentation,  Up: SMIE

22.7.1.7 Helper Functions for Indentation Rules
...............................................

SMIE provides various functions designed specifically for use in the
indentation rules function (several of those functions break if used in
another context).  These functions all start with the prefix
‘smie-rule-’.

 -- Function: smie-rule-bolp
     Return non-‘nil’ if the current token is the first on the line.

 -- Function: smie-rule-hanging-p
     Return non-‘nil’ if the current token is _hanging_.  A token is
     _hanging_ if it is the last token on the line and if it is preceded
     by other tokens: a lone token on a line is not hanging.

 -- Function: smie-rule-next-p &rest tokens
     Return non-‘nil’ if the next token is among TOKENS.

 -- Function: smie-rule-prev-p &rest tokens
     Return non-‘nil’ if the previous token is among TOKENS.

 -- Function: smie-rule-parent-p &rest parents
     Return non-‘nil’ if the current token’s parent is among PARENTS.

 -- Function: smie-rule-sibling-p
     Return non-‘nil’ if the current token’s parent is actually a
     sibling.  This is the case for example when the parent of a ‘","’
     is just the previous ‘","’.

 -- Function: smie-rule-parent &optional offset
     Return the proper offset to align the current token with the
     parent.  If non-‘nil’, OFFSET should be an integer giving an
     additional offset to apply.

 -- Function: smie-rule-separator method
     Indent current token as a _separator_.

     By _separator_, we mean here a token whose sole purpose is to
     separate various elements within some enclosing syntactic
     construct, and which does not have any semantic significance in
     itself (i.e., it would typically not exist as a node in an abstract
     syntax tree).

     Such a token is expected to have an associative syntax and be
     closely tied to its syntactic parent.  Typical examples are ‘","’
     in lists of arguments (enclosed inside parentheses), or ‘";"’ in
     sequences of instructions (enclosed in a ‘{...}’ or ‘begin...end’
     block).

     METHOD should be the method name that was passed to
     ‘smie-rules-function’.


File: elisp.info,  Node: SMIE Indentation Example,  Next: SMIE Customization,  Prev: SMIE Indentation Helpers,  Up: SMIE

22.7.1.8 Sample Indentation Rules
.................................

Here is an example of an indentation function:

     (defun sample-smie-rules (kind token)
       (pcase (cons kind token)
         (`(:elem . basic) sample-indent-basic)
         (`(,_ . ",") (smie-rule-separator kind))
         (`(:after . ":=") sample-indent-basic)
         (`(:before . ,(or `"begin" `"(" `"{")))
          (if (smie-rule-hanging-p) (smie-rule-parent)))
         (`(:before . "if")
          (and (not (smie-rule-bolp)) (smie-rule-prev-p "else")
               (smie-rule-parent)))))

A few things to note:

   • The first case indicates the basic indentation increment to use.
     If ‘sample-indent-basic’ is ‘nil’, then SMIE uses the global
     setting ‘smie-indent-basic’.  The major mode could have set
     ‘smie-indent-basic’ buffer-locally instead, but that is
     discouraged.

   • The rule for the token ‘","’ make SMIE try to be more clever when
     the comma separator is placed at the beginning of lines.  It tries
     to outdent the separator so as to align the code after the comma;
     for example:

          x = longfunctionname (
                  arg1
                , arg2
              );

   • The rule for indentation after ‘":="’ exists because otherwise SMIE
     would treat ‘":="’ as an infix operator and would align the right
     argument with the left one.

   • The rule for indentation before ‘"begin"’ is an example of the use
     of virtual indentation: This rule is used only when ‘"begin"’ is
     hanging, which can happen only when ‘"begin"’ is not at the
     beginning of a line.  So this is not used when indenting ‘"begin"’
     itself but only when indenting something relative to this
     ‘"begin"’.  Concretely, this rule changes the indentation from:

              if x > 0 then begin
                      dosomething(x);
                  end
     to
              if x > 0 then begin
                  dosomething(x);
              end

   • The rule for indentation before ‘"if"’ is similar to the one for
     ‘"begin"’, but where the purpose is to treat ‘"else if"’ as a
     single unit, so as to align a sequence of tests rather than indent
     each test further to the right.  This function does this only in
     the case where the ‘"if"’ is not placed on a separate line, hence
     the ‘smie-rule-bolp’ test.

     If we know that the ‘"else"’ is always aligned with its ‘"if"’ and
     is always at the beginning of a line, we can use a more efficient
     rule:
          ((equal token "if")
           (and (not (smie-rule-bolp))
                (smie-rule-prev-p "else")
                (save-excursion
                  (sample-smie-backward-token)
                  (cons 'column (current-column)))))

     The advantage of this formulation is that it reuses the indentation
     of the previous ‘"else"’, rather than going all the way back to the
     first ‘"if"’ of the sequence.


File: elisp.info,  Node: SMIE Customization,  Prev: SMIE Indentation Example,  Up: SMIE

22.7.1.9 Customizing Indentation
................................

If you are using a mode whose indentation is provided by SMIE, you can
customize the indentation to suit your preferences.  You can do this on
a per-mode basis (using the option ‘smie-config’), or a per-file basis
(using the function ‘smie-config-local’ in a file-local variable
specification).

 -- User Option: smie-config
     This option lets you customize indentation on a per-mode basis.  It
     is an alist with elements of the form ‘(MODE . RULES)’.  For the
     precise form of rules, see the variable’s documentation; but you
     may find it easier to use the command ‘smie-config-guess’.

 -- Command: smie-config-guess
     This command tries to work out appropriate settings to produce your
     preferred style of indentation.  Simply call the command while
     visiting a file that is indented with your style.

 -- Command: smie-config-save
     Call this command after using ‘smie-config-guess’, to save your
     settings for future sessions.

 -- Command: smie-config-show-indent &optional move
     This command displays the rules that are used to indent the current
     line.

 -- Command: smie-config-set-indent
     This command adds a local rule to adjust the indentation of the
     current line.

 -- Function: smie-config-local rules
     This function adds RULES as indentation rules for the current
     buffer.  These add to any mode-specific rules defined by the
     ‘smie-config’ option.  To specify custom indentation rules for a
     specific file, add an entry to the file’s local variables of the
     form: ‘eval: (smie-config-local '(RULES))’.


File: elisp.info,  Node: Desktop Save Mode,  Prev: Auto-Indentation,  Up: Modes

22.8 Desktop Save Mode
======================

“Desktop Save Mode” is a feature to save the state of Emacs from one
session to another.  The user-level commands for using Desktop Save Mode
are described in the GNU Emacs Manual (*note (emacs)Saving Emacs
Sessions::).  Modes whose buffers visit a file, don’t have to do
anything to use this feature.

   For buffers not visiting a file to have their state saved, the major
mode must bind the buffer local variable ‘desktop-save-buffer’ to a
non-‘nil’ value.

 -- Variable: desktop-save-buffer
     If this buffer-local variable is non-‘nil’, the buffer will have
     its state saved in the desktop file at desktop save.  If the value
     is a function, it is called at desktop save with argument
     DESKTOP-DIRNAME, and its value is saved in the desktop file along
     with the state of the buffer for which it was called.  When file
     names are returned as part of the auxiliary information, they
     should be formatted using the call

          (desktop-file-name FILE-NAME DESKTOP-DIRNAME)

   For buffers not visiting a file to be restored, the major mode must
define a function to do the job, and that function must be listed in the
alist ‘desktop-buffer-mode-handlers’.

 -- Variable: desktop-buffer-mode-handlers
     Alist with elements

          (MAJOR-MODE . RESTORE-BUFFER-FUNCTION)

     The function RESTORE-BUFFER-FUNCTION will be called with argument
     list

          (BUFFER-FILE-NAME BUFFER-NAME DESKTOP-BUFFER-MISC)

     and it should return the restored buffer.  Here DESKTOP-BUFFER-MISC
     is the value returned by the function optionally bound to
     ‘desktop-save-buffer’.


File: elisp.info,  Node: Documentation,  Next: Files,  Prev: Modes,  Up: Top

23 Documentation
****************

GNU Emacs has convenient built-in help facilities, most of which derive
their information from documentation strings associated with functions
and variables.  This chapter describes how to access documentation
strings in Lisp programs.

   The contents of a documentation string should follow certain
conventions.  In particular, its first line should be a complete
sentence (or two complete sentences) that briefly describes what the
function or variable does.  *Note Documentation Tips::, for how to write
good documentation strings.

   Note that the documentation strings for Emacs are not the same thing
as the Emacs manual.  Manuals have their own source files, written in
the Texinfo language; documentation strings are specified in the
definitions of the functions and variables they apply to.  A collection
of documentation strings is not sufficient as a manual because a good
manual is not organized in that fashion; it is organized in terms of
topics of discussion.

   For commands to display documentation strings, see *note Help:
(emacs)Help.

* Menu:

* Documentation Basics::      Where doc strings are defined and stored.
* Accessing Documentation::   How Lisp programs can access doc strings.
* Keys in Documentation::     Substituting current key bindings.
* Describing Characters::     Making printable descriptions of
                                non-printing characters and key sequences.
* Help Functions::            Subroutines used by Emacs help facilities.


File: elisp.info,  Node: Documentation Basics,  Next: Accessing Documentation,  Up: Documentation

23.1 Documentation Basics
=========================

A documentation string is written using the Lisp syntax for strings,
with double-quote characters surrounding the text.  It is, in fact, an
actual Lisp string.  When the string appears in the proper place in a
function or variable definition, it serves as the function’s or
variable’s documentation.

   In a function definition (a ‘lambda’ or ‘defun’ form), the
documentation string is specified after the argument list, and is
normally stored directly in the function object.  *Note Function
Documentation::.  You can also put function documentation in the
‘function-documentation’ property of a function name (*note Accessing
Documentation::).

   In a variable definition (a ‘defvar’ form), the documentation string
is specified after the initial value.  *Note Defining Variables::.  The
string is stored in the variable’s ‘variable-documentation’ property.

   Sometimes, Emacs does not keep documentation strings in memory.
There are two such circumstances.  Firstly, to save memory, the
documentation for preloaded functions and variables (including
primitives) is kept in a file named ‘DOC’, in the directory specified by
‘doc-directory’ (*note Accessing Documentation::).  Secondly, when a
function or variable is loaded from a byte-compiled file, Emacs avoids
loading its documentation string (*note Docs and Compilation::).  In
both cases, Emacs looks up the documentation string from the file only
when needed, such as when the user calls ‘C-h f’ (‘describe-function’)
for a function.

   Documentation strings can contain special “key substitution
sequences”, referring to key bindings which are looked up only when the
user views the documentation.  This allows the help commands to display
the correct keys even if a user rearranges the default key bindings.
*Note Keys in Documentation::.

   In the documentation string of an autoloaded command (*note
Autoload::), these key-substitution sequences have an additional special
effect: they cause ‘C-h f’ on the command to trigger autoloading.  (This
is needed for correctly setting up the hyperlinks in the ‘*Help*’
buffer.)


File: elisp.info,  Node: Accessing Documentation,  Next: Keys in Documentation,  Prev: Documentation Basics,  Up: Documentation

23.2 Access to Documentation Strings
====================================

 -- Function: documentation-property symbol property &optional verbatim
     This function returns the documentation string recorded in SYMBOL’s
     property list under property PROPERTY.  It is most often used to
     look up the documentation strings of variables, for which PROPERTY
     is ‘variable-documentation’.  However, it can also be used to look
     up other kinds of documentation, such as for customization groups
     (but for function documentation, use the ‘documentation’ function,
     below).

     If the property value refers to a documentation string stored in
     the ‘DOC’ file or a byte-compiled file, this function looks up that
     string and returns it.

     If the property value isn’t ‘nil’, isn’t a string, and doesn’t
     refer to text in a file, then it is evaluated as a Lisp expression
     to obtain a string.

     Finally, this function passes the string through
     ‘substitute-command-keys’ to substitute key bindings (*note Keys in
     Documentation::).  It skips this step if VERBATIM is non-‘nil’.

          (documentation-property 'command-line-processed
             'variable-documentation)
               ⇒ "Non-nil once command line has been processed"
          (symbol-plist 'command-line-processed)
               ⇒ (variable-documentation 188902)
          (documentation-property 'emacs 'group-documentation)
               ⇒ "Customization of the One True Editor."

 -- Function: documentation function &optional verbatim
     This function returns the documentation string of FUNCTION.  It
     handles macros, named keyboard macros, and special forms, as well
     as ordinary functions.

     If FUNCTION is a symbol, this function first looks for the
     ‘function-documentation’ property of that symbol; if that has a
     non-‘nil’ value, the documentation comes from that value (if the
     value is not a string, it is evaluated).

     If FUNCTION is not a symbol, or if it has no
     ‘function-documentation’ property, then ‘documentation’ extracts
     the documentation string from the actual function definition,
     reading it from a file if called for.

     Finally, unless VERBATIM is non-‘nil’, this function calls
     ‘substitute-command-keys’.  The result is the documentation string
     to return.

     The ‘documentation’ function signals a ‘void-function’ error if
     FUNCTION has no function definition.  However, it is OK if the
     function definition has no documentation string.  In that case,
     ‘documentation’ returns ‘nil’.

 -- Function: face-documentation face
     This function returns the documentation string of FACE as a face.

   Here is an example of using the two functions, ‘documentation’ and
‘documentation-property’, to display the documentation strings for
several symbols in a ‘*Help*’ buffer.

     (defun describe-symbols (pattern)
       "Describe the Emacs Lisp symbols matching PATTERN.
     All symbols that have PATTERN in their name are described
     in the *Help* buffer."
       (interactive "sDescribe symbols matching: ")
       (let ((describe-func
              (function
               (lambda (s)
                 ;; Print description of symbol.
                 (if (fboundp s)             ; It is a function.
                     (princ
                      (format "%s\t%s\n%s\n\n" s
                        (if (commandp s)
                            (let ((keys (where-is-internal s)))
                              (if keys
                                  (concat
                                   "Keys: "
                                   (mapconcat 'key-description
                                              keys " "))
                                "Keys: none"))
                          "Function")
                        (or (documentation s)
                            "not documented"))))

                 (if (boundp s)              ; It is a variable.
                     (princ
                      (format "%s\t%s\n%s\n\n" s
                        (if (custom-variable-p s)
                            "Option " "Variable")
                        (or (documentation-property
                              s 'variable-documentation)
                            "not documented")))))))
             sym-list)

         ;; Build a list of symbols that match pattern.
         (mapatoms (function
                    (lambda (sym)
                      (if (string-match pattern (symbol-name sym))
                          (setq sym-list (cons sym sym-list))))))

         ;; Display the data.
         (help-setup-xref (list 'describe-symbols pattern) (interactive-p))
         (with-help-window (help-buffer)
           (mapcar describe-func (sort sym-list 'string<)))))

   The ‘describe-symbols’ function works like ‘apropos’, but provides
more information.

     (describe-symbols "goal")

     ---------- Buffer: *Help* ----------
     goal-column     Option
     Semipermanent goal column for vertical motion, as set by ...

     minibuffer-temporary-goal-position      Variable
     not documented

     set-goal-column Keys: C-x C-n
     Set the current horizontal position as a goal for C-n and C-p.
     Those commands will move to this position in the line moved to
     rather than trying to keep the same horizontal position.
     With a non-nil argument ARG, clears out the goal column
     so that C-n and C-p resume vertical motion.
     The goal column is stored in the variable ‘goal-column’.

     (fn ARG)

     temporary-goal-column   Variable
     Current goal column for vertical motion.
     It is the column where point was at the start of the current run
     of vertical motion commands.

     When moving by visual lines via the function ‘line-move-visual’, it is a cons
     cell (COL . HSCROLL), where COL is the x-position, in pixels,
     divided by the default column width, and HSCROLL is the number of
     columns by which window is scrolled from left margin.

     When the ‘track-eol’ feature is doing its job, the value is
     ‘most-positive-fixnum’.
     ---------- Buffer: *Help* ----------

 -- Function: Snarf-documentation filename
     This function is used when building Emacs, just before the runnable
     Emacs is dumped.  It finds the positions of the documentation
     strings stored in the file FILENAME, and records those positions
     into memory in the function definitions and variable property
     lists.  *Note Building Emacs::.

     Emacs reads the file FILENAME from the ‘emacs/etc’ directory.  When
     the dumped Emacs is later executed, the same file will be looked
     for in the directory ‘doc-directory’.  Usually FILENAME is ‘"DOC"’.

 -- Variable: doc-directory
     This variable holds the name of the directory which should contain
     the file ‘"DOC"’ that contains documentation strings for built-in
     and preloaded functions and variables.

     In most cases, this is the same as ‘data-directory’.  They may be
     different when you run Emacs from the directory where you built it,
     without actually installing it.  *Note Definition of
     data-directory::.


File: elisp.info,  Node: Keys in Documentation,  Next: Describing Characters,  Prev: Accessing Documentation,  Up: Documentation

23.3 Substituting Key Bindings in Documentation
===============================================

When documentation strings refer to key sequences, they should use the
current, actual key bindings.  They can do so using certain special text
sequences described below.  Accessing documentation strings in the usual
way substitutes current key binding information for these special
sequences.  This works by calling ‘substitute-command-keys’.  You can
also call that function yourself.

   Here is a list of the special sequences and what they mean:

‘\[COMMAND]’
     stands for a key sequence that will invoke COMMAND, or ‘M-x
     COMMAND’ if COMMAND has no key bindings.

‘\{MAPVAR}’
     stands for a summary of the keymap which is the value of the
     variable MAPVAR.  The summary is made using ‘describe-bindings’.

‘\<MAPVAR>’
     stands for no text itself.  It is used only for a side effect: it
     specifies MAPVAR’s value as the keymap for any following
     ‘\[COMMAND]’ sequences in this documentation string.

‘‘’
‘`’
     (left single quotation mark and grave accent) both stand for a left
     quote.  This generates a left single quotation mark, an apostrophe,
     or a grave accent depending on the value of ‘text-quoting-style’.

‘’’
‘'’
     (right single quotation mark and apostrophe) both stand for a right
     quote.  This generates a right single quotation mark or an
     apostrophe depending on the value of ‘text-quoting-style’.

‘\=’
     quotes the following character and is discarded; thus, ‘\=`’ puts
     ‘`’ into the output, ‘\=\[’ puts ‘\[’ into the output, and ‘\=\=’
     puts ‘\=’ into the output.

   *Please note:* Each ‘\’ must be doubled when written in a string in
Emacs Lisp.

 -- Variable: text-quoting-style
     The value of this variable is a symbol that specifies the style
     Emacs should use for single quotes in the wording of help and
     messages.  If the variable’s value is ‘curve’, the style is ‘like
     this’ with curved single quotes.  If the value is ‘straight’, the
     style is 'like this' with straight apostrophes.  If the value is
     ‘grave’, the style is `like this' with grave accent and apostrophe,
     the standard style before Emacs version 25.  The default value
     ‘nil’ acts like ‘curve’ if curved single quotes are displayable,
     and like ‘grave’ otherwise.

     This variable can be used by experts on platforms that have
     problems with curved quotes.  As it is not intended for casual use,
     it is not a user option.

 -- Function: substitute-command-keys string
     This function scans STRING for the above special sequences and
     replaces them by what they stand for, returning the result as a
     string.  This permits display of documentation that refers
     accurately to the user’s own customized key bindings.

     If a command has multiple bindings, this function normally uses the
     first one it finds.  You can specify one particular key binding by
     assigning an ‘:advertised-binding’ symbol property to the command,
     like this:

          (put 'undo :advertised-binding [?\C-/])

     The ‘:advertised-binding’ property also affects the binding shown
     in menu items (*note Menu Bar::).  The property is ignored if it
     specifies a key binding that the command does not actually have.

   Here are examples of the special sequences:

     (substitute-command-keys
        "To abort recursive edit, type `\\[abort-recursive-edit]'.")
     ⇒ "To abort recursive edit, type ‘C-]’."

     (substitute-command-keys
        "The keys that are defined for the minibuffer here are:
       \\{minibuffer-local-must-match-map}")
     ⇒ "The keys that are defined for the minibuffer here are:

     ?               minibuffer-completion-help
     SPC             minibuffer-complete-word
     TAB             minibuffer-complete
     C-j             minibuffer-complete-and-exit
     RET             minibuffer-complete-and-exit
     C-g             abort-recursive-edit
     "

     (substitute-command-keys
        "To abort a recursive edit from the minibuffer, type \
     `\\<minibuffer-local-must-match-map>\\[abort-recursive-edit]'.")
     ⇒ "To abort a recursive edit from the minibuffer, type ‘C-g’."

   There are other special conventions for the text in documentation
strings—for instance, you can refer to functions, variables, and
sections of this manual.  *Note Documentation Tips::, for details.


File: elisp.info,  Node: Describing Characters,  Next: Help Functions,  Prev: Keys in Documentation,  Up: Documentation

23.4 Describing Characters for Help Messages
============================================

These functions convert events, key sequences, or characters to textual
descriptions.  These descriptions are useful for including arbitrary
text characters or key sequences in messages, because they convert
non-printing and whitespace characters to sequences of printing
characters.  The description of a non-whitespace printing character is
the character itself.

 -- Function: key-description sequence &optional prefix
     This function returns a string containing the Emacs standard
     notation for the input events in SEQUENCE.  If PREFIX is non-‘nil’,
     it is a sequence of input events leading up to SEQUENCE and is
     included in the return value.  Both arguments may be strings,
     vectors or lists.  *Note Input Events::, for more information about
     valid events.

          (key-description [?\M-3 delete])
               ⇒ "M-3 <delete>"
          (key-description [delete] "\M-3")
               ⇒ "M-3 <delete>"

     See also the examples for ‘single-key-description’, below.

 -- Function: single-key-description event &optional no-angles
     This function returns a string describing EVENT in the standard
     Emacs notation for keyboard input.  A normal printing character
     appears as itself, but a control character turns into a string
     starting with ‘C-’, a meta character turns into a string starting
     with ‘M-’, and space, tab, etc., appear as ‘SPC’, ‘TAB’, etc.  A
     function key symbol appears inside angle brackets ‘<...>’.  An
     event that is a list appears as the name of the symbol in the CAR
     of the list, inside angle brackets.

     If the optional argument NO-ANGLES is non-‘nil’, the angle brackets
     around function keys and event symbols are omitted; this is for
     compatibility with old versions of Emacs which didn’t use the
     brackets.

          (single-key-description ?\C-x)
               ⇒ "C-x"
          (key-description "\C-x \M-y \n \t \r \f123")
               ⇒ "C-x SPC M-y SPC C-j SPC TAB SPC RET SPC C-l 1 2 3"
          (single-key-description 'delete)
               ⇒ "<delete>"
          (single-key-description 'C-mouse-1)
               ⇒ "<C-mouse-1>"
          (single-key-description 'C-mouse-1 t)
               ⇒ "C-mouse-1"

 -- Function: text-char-description character
     This function returns a string describing CHARACTER in the standard
     Emacs notation for characters that appear in text—like
     ‘single-key-description’, except that control characters are
     represented with a leading caret (which is how control characters
     in Emacs buffers are usually displayed).  Another difference is
     that ‘text-char-description’ recognizes the 2**7 bit as the Meta
     character, whereas ‘single-key-description’ uses the 2**27 bit for
     Meta.

          (text-char-description ?\C-c)
               ⇒ "^C"
          (text-char-description ?\M-m)
               ⇒ "\xed"
          (text-char-description ?\C-\M-m)
               ⇒ "\x8d"
          (text-char-description (+ 128 ?m))
               ⇒ "M-m"
          (text-char-description (+ 128 ?\C-m))
               ⇒ "M-^M"

 -- Command: read-kbd-macro string &optional need-vector
     This function is used mainly for operating on keyboard macros, but
     it can also be used as a rough inverse for ‘key-description’.  You
     call it with a string containing key descriptions, separated by
     spaces; it returns a string or vector containing the corresponding
     events.  (This may or may not be a single valid key sequence,
     depending on what events you use; *note Key Sequences::.)  If
     NEED-VECTOR is non-‘nil’, the return value is always a vector.


File: elisp.info,  Node: Help Functions,  Prev: Describing Characters,  Up: Documentation

23.5 Help Functions
===================

Emacs provides a variety of built-in help functions, all accessible to
the user as subcommands of the prefix ‘C-h’.  For more information about
them, see *note Help: (emacs)Help.  Here we describe some program-level
interfaces to the same information.

 -- Command: apropos pattern &optional do-all
     This function finds all meaningful symbols whose names contain a
     match for the apropos pattern PATTERN.  An apropos pattern is
     either a word to match, a space-separated list of words of which at
     least two must match, or a regular expression (if any special
     regular expression characters occur).  A symbol is meaningful if it
     has a definition as a function, variable, or face, or has
     properties.

     The function returns a list of elements that look like this:

          (SYMBOL SCORE FUNCTION-DOC VARIABLE-DOC
           PLIST-DOC WIDGET-DOC FACE-DOC GROUP-DOC)

     Here, SCORE is an integer measure of how important the symbol seems
     to be as a match.  Each of the remaining elements is a
     documentation string, or ‘nil’, for SYMBOL as a function, variable,
     etc.

     It also displays the symbols in a buffer named ‘*Apropos*’, each
     with a one-line description taken from the beginning of its
     documentation string.

     If DO-ALL is non-‘nil’, or if the user option ‘apropos-do-all’ is
     non-‘nil’, then ‘apropos’ also shows key bindings for the functions
     that are found; it also shows _all_ interned symbols, not just
     meaningful ones (and it lists them in the return value as well).

 -- Variable: help-map
     The value of this variable is a local keymap for characters
     following the Help key, ‘C-h’.

 -- Prefix Command: help-command
     This symbol is not a function; its function definition cell holds
     the keymap known as ‘help-map’.  It is defined in ‘help.el’ as
     follows:

          (define-key global-map (string help-char) 'help-command)
          (fset 'help-command help-map)

 -- User Option: help-char
     The value of this variable is the help character—the character that
     Emacs recognizes as meaning Help.  By default, its value is 8,
     which stands for ‘C-h’.  When Emacs reads this character, if
     ‘help-form’ is a non-‘nil’ Lisp expression, it evaluates that
     expression, and displays the result in a window if it is a string.

     Usually the value of ‘help-form’ is ‘nil’.  Then the help character
     has no special meaning at the level of command input, and it
     becomes part of a key sequence in the normal way.  The standard key
     binding of ‘C-h’ is a prefix key for several general-purpose help
     features.

     The help character is special after prefix keys, too.  If it has no
     binding as a subcommand of the prefix key, it runs
     ‘describe-prefix-bindings’, which displays a list of all the
     subcommands of the prefix key.

 -- User Option: help-event-list
     The value of this variable is a list of event types that serve as
     alternative help characters.  These events are handled just like
     the event specified by ‘help-char’.

 -- Variable: help-form
     If this variable is non-‘nil’, its value is a form to evaluate
     whenever the character ‘help-char’ is read.  If evaluating the form
     produces a string, that string is displayed.

     A command that calls ‘read-event’, ‘read-char-choice’, or
     ‘read-char’ probably should bind ‘help-form’ to a non-‘nil’
     expression while it does input.  (The time when you should not do
     this is when ‘C-h’ has some other meaning.)  Evaluating this
     expression should result in a string that explains what the input
     is for and how to enter it properly.

     Entry to the minibuffer binds this variable to the value of
     ‘minibuffer-help-form’ (*note Definition of
     minibuffer-help-form::).

 -- Variable: prefix-help-command
     This variable holds a function to print help for a prefix key.  The
     function is called when the user types a prefix key followed by the
     help character, and the help character has no binding after that
     prefix.  The variable’s default value is
     ‘describe-prefix-bindings’.

 -- Command: describe-prefix-bindings
     This function calls ‘describe-bindings’ to display a list of all
     the subcommands of the prefix key of the most recent key sequence.
     The prefix described consists of all but the last event of that key
     sequence.  (The last event is, presumably, the help character.)

   The following two functions are meant for modes that want to provide
help without relinquishing control, such as the electric modes.  Their
names begin with ‘Helper’ to distinguish them from the ordinary help
functions.

 -- Command: Helper-describe-bindings
     This command pops up a window displaying a help buffer containing a
     listing of all of the key bindings from both the local and global
     keymaps.  It works by calling ‘describe-bindings’.

 -- Command: Helper-help
     This command provides help for the current mode.  It prompts the
     user in the minibuffer with the message ‘Help (Type ? for further
     options)’, and then provides assistance in finding out what the key
     bindings are, and what the mode is intended for.  It returns ‘nil’.

     This can be customized by changing the map ‘Helper-help-map’.

 -- Variable: data-directory
     This variable holds the name of the directory in which Emacs finds
     certain documentation and text files that come with Emacs.

 -- Function: help-buffer
     This function returns the name of the help buffer, which is
     normally ‘*Help*’; if such a buffer does not exist, it is first
     created.

 -- Macro: with-help-window buffer-name body...
     This macro evaluates BODY like ‘with-output-to-temp-buffer’ (*note
     Temporary Displays::), inserting any output produced by its forms
     into a buffer named BUFFER-NAME.  (Usually, BUFFER-NAME should be
     the value returned by the function ‘help-buffer’.)  It also puts
     the specified buffer into Help mode and displays a message telling
     the user how to quit and scroll the help window.  It selects the
     help window if the current value of the user option
     ‘help-window-select’ has been set accordingly.  It returns the last
     value in BODY.

 -- Function: help-setup-xref item interactive-p
     This function updates the cross reference data in the ‘*Help*’
     buffer, which is used to regenerate the help information when the
     user clicks on the ‘Back’ or ‘Forward’ buttons.  Most commands that
     use the ‘*Help*’ buffer should invoke this function before clearing
     the buffer.  The ITEM argument should have the form ‘(FUNCTION .
     ARGS)’, where FUNCTION is a function to call, with argument list
     ARGS, to regenerate the help buffer.  The INTERACTIVE-P argument is
     non-‘nil’ if the calling command was invoked interactively; in that
     case, the stack of items for the ‘*Help*’ buffer’s ‘Back’ buttons
     is cleared.

   *Note describe-symbols example::, for an example of using
‘help-buffer’, ‘with-help-window’, and ‘help-setup-xref’.

 -- Macro: make-help-screen fname help-line help-text help-map
     This macro defines a help command named FNAME that acts like a
     prefix key that shows a list of the subcommands it offers.

     When invoked, FNAME displays HELP-TEXT in a window, then reads and
     executes a key sequence according to HELP-MAP.  The string
     HELP-TEXT should describe the bindings available in HELP-MAP.

     The command FNAME is defined to handle a few events itself, by
     scrolling the display of HELP-TEXT.  When FNAME reads one of those
     special events, it does the scrolling and then reads another event.
     When it reads an event that is not one of those few, and which has
     a binding in HELP-MAP, it executes that key’s binding and then
     returns.

     The argument HELP-LINE should be a single-line summary of the
     alternatives in HELP-MAP.  In the current version of Emacs, this
     argument is used only if you set the option ‘three-step-help’ to
     ‘t’.

     This macro is used in the command ‘help-for-help’ which is the
     binding of ‘C-h C-h’.

 -- User Option: three-step-help
     If this variable is non-‘nil’, commands defined with
     ‘make-help-screen’ display their HELP-LINE strings in the echo area
     at first, and display the longer HELP-TEXT strings only if the user
     types the help character again.


File: elisp.info,  Node: Files,  Next: Backups and Auto-Saving,  Prev: Documentation,  Up: Top

24 Files
********

This chapter describes the Emacs Lisp functions and variables to find,
create, view, save, and otherwise work with files and directories.  A
few other file-related functions are described in *note Buffers::, and
those related to backups and auto-saving are described in *note Backups
and Auto-Saving::.

   Many of the file functions take one or more arguments that are file
names.  A file name is a string.  Most of these functions expand file
name arguments using the function ‘expand-file-name’, so that ‘~’ is
handled correctly, as are relative file names (including ‘../’).  *Note
File Name Expansion::.

   In addition, certain “magic” file names are handled specially.  For
example, when a remote file name is specified, Emacs accesses the file
over the network via an appropriate protocol.  *Note Remote Files:
(emacs)Remote Files.  This handling is done at a very low level, so you
may assume that all the functions described in this chapter accept magic
file names as file name arguments, except where noted.  *Note Magic File
Names::, for details.

   When file I/O functions signal Lisp errors, they usually use the
condition ‘file-error’ (*note Handling Errors::).  The error message is
in most cases obtained from the operating system, according to locale
‘system-messages-locale’, and decoded using coding system
‘locale-coding-system’ (*note Locales::).

* Menu:

* Visiting Files::           Reading files into Emacs buffers for editing.
* Saving Buffers::           Writing changed buffers back into files.
* Reading from Files::       Reading files into buffers without visiting.
* Writing to Files::         Writing new files from parts of buffers.
* File Locks::               Locking and unlocking files, to prevent
                               simultaneous editing by two people.
* Information about Files::  Testing existence, accessibility, size of files.
* Changing Files::           Renaming files, changing permissions, etc.
* File Names::               Decomposing and expanding file names.
* Contents of Directories::  Getting a list of the files in a directory.
* Create/Delete Dirs::       Creating and Deleting Directories.
* Magic File Names::         Special handling for certain file names.
* Format Conversion::        Conversion to and from various file formats.


File: elisp.info,  Node: Visiting Files,  Next: Saving Buffers,  Up: Files

24.1 Visiting Files
===================

Visiting a file means reading a file into a buffer.  Once this is done,
we say that the buffer is “visiting” that file, and call the file “the
visited file” of the buffer.

   A file and a buffer are two different things.  A file is information
recorded permanently in the computer (unless you delete it).  A buffer,
on the other hand, is information inside of Emacs that will vanish at
the end of the editing session (or when you kill the buffer).  When a
buffer is visiting a file, it contains information copied from the file.
The copy in the buffer is what you modify with editing commands.
Changes to the buffer do not change the file; to make the changes
permanent, you must “save” the buffer, which means copying the altered
buffer contents back into the file.

   Despite the distinction between files and buffers, people often refer
to a file when they mean a buffer and vice-versa.  Indeed, we say, “I am
editing a file”, rather than, “I am editing a buffer that I will soon
save as a file of the same name”.  Humans do not usually need to make
the distinction explicit.  When dealing with a computer program,
however, it is good to keep the distinction in mind.

* Menu:

* Visiting Functions::         The usual interface functions for visiting.
* Subroutines of Visiting::    Lower-level subroutines that they use.


File: elisp.info,  Node: Visiting Functions,  Next: Subroutines of Visiting,  Up: Visiting Files

24.1.1 Functions for Visiting Files
-----------------------------------

This section describes the functions normally used to visit files.  For
historical reasons, these functions have names starting with ‘find-’
rather than ‘visit-’.  *Note Buffer File Name::, for functions and
variables that access the visited file name of a buffer or that find an
existing buffer by its visited file name.

   In a Lisp program, if you want to look at the contents of a file but
not alter it, the fastest way is to use ‘insert-file-contents’ in a
temporary buffer.  Visiting the file is not necessary and takes longer.
*Note Reading from Files::.

 -- Command: find-file filename &optional wildcards
     This command selects a buffer visiting the file FILENAME, using an
     existing buffer if there is one, and otherwise creating a new
     buffer and reading the file into it.  It also returns that buffer.

     Aside from some technical details, the body of the ‘find-file’
     function is basically equivalent to:

          (switch-to-buffer (find-file-noselect filename nil nil wildcards))

     (See ‘switch-to-buffer’ in *note Switching Buffers::.)

     If WILDCARDS is non-‘nil’, which is always true in an interactive
     call, then ‘find-file’ expands wildcard characters in FILENAME and
     visits all the matching files.

     When ‘find-file’ is called interactively, it prompts for FILENAME
     in the minibuffer.

 -- Command: find-file-literally filename
     This command visits FILENAME, like ‘find-file’ does, but it does
     not perform any format conversions (*note Format Conversion::),
     character code conversions (*note Coding Systems::), or end-of-line
     conversions (*note End of line conversion: Coding System Basics.).
     The buffer visiting the file is made unibyte, and its major mode is
     Fundamental mode, regardless of the file name.  File local variable
     specifications in the file (*note File Local Variables::) are
     ignored, and automatic decompression and adding a newline at the
     end of the file due to ‘require-final-newline’ (*note
     require-final-newline: Saving Buffers.) are also disabled.

     Note that if Emacs already has a buffer visiting the same file
     non-literally, it will not visit the same file literally, but
     instead just switch to the existing buffer.  If you want to be sure
     of accessing a file’s contents literally, you should create a
     temporary buffer and then read the file contents into it using
     ‘insert-file-contents-literally’ (*note Reading from Files::).

 -- Function: find-file-noselect filename &optional nowarn rawfile
          wildcards
     This function is the guts of all the file-visiting functions.  It
     returns a buffer visiting the file FILENAME.  You may make the
     buffer current or display it in a window if you wish, but this
     function does not do so.

     The function returns an existing buffer if there is one; otherwise
     it creates a new buffer and reads the file into it.  When
     ‘find-file-noselect’ uses an existing buffer, it first verifies
     that the file has not changed since it was last visited or saved in
     that buffer.  If the file has changed, this function asks the user
     whether to reread the changed file.  If the user says ‘yes’, any
     edits previously made in the buffer are lost.

     Reading the file involves decoding the file’s contents (*note
     Coding Systems::), including end-of-line conversion, and format
     conversion (*note Format Conversion::).  If WILDCARDS is non-‘nil’,
     then ‘find-file-noselect’ expands wildcard characters in FILENAME
     and visits all the matching files.

     This function displays warning or advisory messages in various
     peculiar cases, unless the optional argument NOWARN is non-‘nil’.
     For example, if it needs to create a buffer, and there is no file
     named FILENAME, it displays the message ‘(New file)’ in the echo
     area, and leaves the buffer empty.

     The ‘find-file-noselect’ function normally calls ‘after-find-file’
     after reading the file (*note Subroutines of Visiting::).  That
     function sets the buffer major mode, parses local variables, warns
     the user if there exists an auto-save file more recent than the
     file just visited, and finishes by running the functions in
     ‘find-file-hook’.

     If the optional argument RAWFILE is non-‘nil’, then
     ‘after-find-file’ is not called, and the
     ‘find-file-not-found-functions’ are not run in case of failure.
     What’s more, a non-‘nil’ RAWFILE value suppresses coding system
     conversion and format conversion.

     The ‘find-file-noselect’ function usually returns the buffer that
     is visiting the file FILENAME.  But, if wildcards are actually used
     and expanded, it returns a list of buffers that are visiting the
     various files.

          (find-file-noselect "/etc/fstab")
               ⇒ #<buffer fstab>

 -- Command: find-file-other-window filename &optional wildcards
     This command selects a buffer visiting the file FILENAME, but does
     so in a window other than the selected window.  It may use another
     existing window or split a window; see *note Switching Buffers::.

     When this command is called interactively, it prompts for FILENAME.

 -- Command: find-file-read-only filename &optional wildcards
     This command selects a buffer visiting the file FILENAME, like
     ‘find-file’, but it marks the buffer as read-only.  *Note Read Only
     Buffers::, for related functions and variables.

     When this command is called interactively, it prompts for FILENAME.

 -- User Option: find-file-wildcards
     If this variable is non-‘nil’, then the various ‘find-file’
     commands check for wildcard characters and visit all the files that
     match them (when invoked interactively or when their WILDCARDS
     argument is non-‘nil’).  If this option is ‘nil’, then the
     ‘find-file’ commands ignore their WILDCARDS argument and never
     treat wildcard characters specially.

 -- User Option: find-file-hook
     The value of this variable is a list of functions to be called
     after a file is visited.  The file’s local-variables specification
     (if any) will have been processed before the hooks are run.  The
     buffer visiting the file is current when the hook functions are
     run.

     This variable is a normal hook.  *Note Hooks::.

 -- Variable: find-file-not-found-functions
     The value of this variable is a list of functions to be called when
     ‘find-file’ or ‘find-file-noselect’ is passed a nonexistent file
     name.  ‘find-file-noselect’ calls these functions as soon as it
     detects a nonexistent file.  It calls them in the order of the
     list, until one of them returns non-‘nil’.  ‘buffer-file-name’ is
     already set up.

     This is not a normal hook because the values of the functions are
     used, and in many cases only some of the functions are called.

 -- Variable: find-file-literally
     This buffer-local variable, if set to a non-‘nil’ value, makes
     ‘save-buffer’ behave as if the buffer were visiting its file
     literally, i.e., without conversions of any kind.  The command
     ‘find-file-literally’ sets this variable’s local value, but other
     equivalent functions and commands can do that as well, e.g., to
     avoid automatic addition of a newline at the end of the file.  This
     variable is permanent local, so it is unaffected by changes of
     major modes.


File: elisp.info,  Node: Subroutines of Visiting,  Prev: Visiting Functions,  Up: Visiting Files

24.1.2 Subroutines of Visiting
------------------------------

The ‘find-file-noselect’ function uses two important subroutines which
are sometimes useful in user Lisp code: ‘create-file-buffer’ and
‘after-find-file’.  This section explains how to use them.

 -- Function: create-file-buffer filename
     This function creates a suitably named buffer for visiting
     FILENAME, and returns it.  It uses FILENAME (sans directory) as the
     name if that name is free; otherwise, it appends a string such as
     ‘<2>’ to get an unused name.  See also *note Creating Buffers::.
     Note that the ‘uniquify’ library affects the result of this
     function.  *Note (emacs)Uniquify::.

     *Please note:* ‘create-file-buffer’ does _not_ associate the new
     buffer with a file and does not select the buffer.  It also does
     not use the default major mode.

          (create-file-buffer "foo")
               ⇒ #<buffer foo>
          (create-file-buffer "foo")
               ⇒ #<buffer foo<2>>
          (create-file-buffer "foo")
               ⇒ #<buffer foo<3>>

     This function is used by ‘find-file-noselect’.  It uses
     ‘generate-new-buffer’ (*note Creating Buffers::).

 -- Function: after-find-file &optional error warn noauto
          after-find-file-from-revert-buffer nomodes
     This function sets the buffer major mode, and parses local
     variables (*note Auto Major Mode::).  It is called by
     ‘find-file-noselect’ and by the default revert function (*note
     Reverting::).

     If reading the file got an error because the file does not exist,
     but its directory does exist, the caller should pass a non-‘nil’
     value for ERROR.  In that case, ‘after-find-file’ issues a warning:
     ‘(New file)’.  For more serious errors, the caller should usually
     not call ‘after-find-file’.

     If WARN is non-‘nil’, then this function issues a warning if an
     auto-save file exists and is more recent than the visited file.

     If NOAUTO is non-‘nil’, that says not to enable or disable
     Auto-Save mode.  The mode remains enabled if it was enabled before.

     If AFTER-FIND-FILE-FROM-REVERT-BUFFER is non-‘nil’, that means this
     call was from ‘revert-buffer’.  This has no direct effect, but some
     mode functions and hook functions check the value of this variable.

     If NOMODES is non-‘nil’, that means don’t alter the buffer’s major
     mode, don’t process local variables specifications in the file, and
     don’t run ‘find-file-hook’.  This feature is used by
     ‘revert-buffer’ in some cases.

     The last thing ‘after-find-file’ does is call all the functions in
     the list ‘find-file-hook’.


File: elisp.info,  Node: Saving Buffers,  Next: Reading from Files,  Prev: Visiting Files,  Up: Files

24.2 Saving Buffers
===================

When you edit a file in Emacs, you are actually working on a buffer that
is visiting that file—that is, the contents of the file are copied into
the buffer and the copy is what you edit.  Changes to the buffer do not
change the file until you “save” the buffer, which means copying the
contents of the buffer into the file.

 -- Command: save-buffer &optional backup-option
     This function saves the contents of the current buffer in its
     visited file if the buffer has been modified since it was last
     visited or saved.  Otherwise it does nothing.

     ‘save-buffer’ is responsible for making backup files.  Normally,
     BACKUP-OPTION is ‘nil’, and ‘save-buffer’ makes a backup file only
     if this is the first save since visiting the file.  Other values
     for BACKUP-OPTION request the making of backup files in other
     circumstances:

        • With an argument of 4 or 64, reflecting 1 or 3 ‘C-u’’s, the
          ‘save-buffer’ function marks this version of the file to be
          backed up when the buffer is next saved.

        • With an argument of 16 or 64, reflecting 2 or 3 ‘C-u’’s, the
          ‘save-buffer’ function unconditionally backs up the previous
          version of the file before saving it.

        • With an argument of 0, unconditionally do _not_ make any
          backup file.

 -- Command: save-some-buffers &optional save-silently-p pred
     This command saves some modified file-visiting buffers.  Normally
     it asks the user about each buffer.  But if SAVE-SILENTLY-P is
     non-‘nil’, it saves all the file-visiting buffers without querying
     the user.

     The optional PRED argument controls which buffers to ask about (or
     to save silently if SAVE-SILENTLY-P is non-‘nil’).  If it is ‘nil’,
     that means to ask only about file-visiting buffers.  If it is ‘t’,
     that means also offer to save certain other non-file buffers—those
     that have a non-‘nil’ buffer-local value of ‘buffer-offer-save’
     (*note Killing Buffers::).  A user who says ‘yes’ to saving a
     non-file buffer is asked to specify the file name to use.  The
     ‘save-buffers-kill-emacs’ function passes the value ‘t’ for PRED.

     If PRED is neither ‘t’ nor ‘nil’, then it should be a function of
     no arguments.  It will be called in each buffer to decide whether
     to offer to save that buffer.  If it returns a non-‘nil’ value in a
     certain buffer, that means do offer to save that buffer.

 -- Command: write-file filename &optional confirm
     This function writes the current buffer into file FILENAME, makes
     the buffer visit that file, and marks it not modified.  Then it
     renames the buffer based on FILENAME, appending a string like ‘<2>’
     if necessary to make a unique buffer name.  It does most of this
     work by calling ‘set-visited-file-name’ (*note Buffer File Name::)
     and ‘save-buffer’.

     If CONFIRM is non-‘nil’, that means to ask for confirmation before
     overwriting an existing file.  Interactively, confirmation is
     required, unless the user supplies a prefix argument.

     If FILENAME is an existing directory, or a symbolic link to one,
     ‘write-file’ uses the name of the visited file, in directory
     FILENAME.  If the buffer is not visiting a file, it uses the buffer
     name instead.

   Saving a buffer runs several hooks.  It also performs format
conversion (*note Format Conversion::).  Note that these hooks,
described below, are only run by ‘save-buffer’, they are not run by
other primitives and functions that write buffer text to files, and in
particular auto-saving (*note Auto-Saving::) doesn’t run these hooks.

 -- Variable: write-file-functions
     The value of this variable is a list of functions to be called
     before writing out a buffer to its visited file.  If one of them
     returns non-‘nil’, the file is considered already written and the
     rest of the functions are not called, nor is the usual code for
     writing the file executed.

     If a function in ‘write-file-functions’ returns non-‘nil’, it is
     responsible for making a backup file (if that is appropriate).  To
     do so, execute the following code:

          (or buffer-backed-up (backup-buffer))

     You might wish to save the file modes value returned by
     ‘backup-buffer’ and use that (if non-‘nil’) to set the mode bits of
     the file that you write.  This is what ‘save-buffer’ normally does.
     *Note Making Backup Files: Making Backups.

     The hook functions in ‘write-file-functions’ are also responsible
     for encoding the data (if desired): they must choose a suitable
     coding system and end-of-line conversion (*note Lisp and Coding
     Systems::), perform the encoding (*note Explicit Encoding::), and
     set ‘last-coding-system-used’ to the coding system that was used
     (*note Encoding and I/O::).

     If you set this hook locally in a buffer, it is assumed to be
     associated with the file or the way the contents of the buffer were
     obtained.  Thus the variable is marked as a permanent local, so
     that changing the major mode does not alter a buffer-local value.
     On the other hand, calling ‘set-visited-file-name’ will reset it.
     If this is not what you want, you might like to use
     ‘write-contents-functions’ instead.

     Even though this is not a normal hook, you can use ‘add-hook’ and
     ‘remove-hook’ to manipulate the list.  *Note Hooks::.

 -- Variable: write-contents-functions
     This works just like ‘write-file-functions’, but it is intended for
     hooks that pertain to the buffer’s contents, not to the particular
     visited file or its location.  Such hooks are usually set up by
     major modes, as buffer-local bindings for this variable.  This
     variable automatically becomes buffer-local whenever it is set;
     switching to a new major mode always resets this variable, but
     calling ‘set-visited-file-name’ does not.

     If any of the functions in this hook returns non-‘nil’, the file is
     considered already written and the rest are not called and neither
     are the functions in ‘write-file-functions’.

 -- User Option: before-save-hook
     This normal hook runs before a buffer is saved in its visited file,
     regardless of whether that is done normally or by one of the hooks
     described above.  For instance, the ‘copyright.el’ program uses
     this hook to make sure the file you are saving has the current year
     in its copyright notice.

 -- User Option: after-save-hook
     This normal hook runs after a buffer has been saved in its visited
     file.  One use of this hook is in Fast Lock mode; it uses this hook
     to save the highlighting information in a cache file.

 -- User Option: file-precious-flag
     If this variable is non-‘nil’, then ‘save-buffer’ protects against
     I/O errors while saving by writing the new file to a temporary name
     instead of the name it is supposed to have, and then renaming it to
     the intended name after it is clear there are no errors.  This
     procedure prevents problems such as a lack of disk space from
     resulting in an invalid file.

     As a side effect, backups are necessarily made by copying.  *Note
     Rename or Copy::.  Yet, at the same time, saving a precious file
     always breaks all hard links between the file you save and other
     file names.

     Some modes give this variable a non-‘nil’ buffer-local value in
     particular buffers.

 -- User Option: require-final-newline
     This variable determines whether files may be written out that do
     _not_ end with a newline.  If the value of the variable is ‘t’,
     then ‘save-buffer’ silently adds a newline at the end of the buffer
     whenever it does not already end in one.  If the value is ‘visit’,
     Emacs adds a missing newline just after it visits the file.  If the
     value is ‘visit-save’, Emacs adds a missing newline both on
     visiting and on saving.  For any other non-‘nil’ value,
     ‘save-buffer’ asks the user whether to add a newline each time the
     case arises.

     If the value of the variable is ‘nil’, then ‘save-buffer’ doesn’t
     add newlines at all.  ‘nil’ is the default value, but a few major
     modes set it to ‘t’ in particular buffers.

   See also the function ‘set-visited-file-name’ (*note Buffer File
Name::).


File: elisp.info,  Node: Reading from Files,  Next: Writing to Files,  Prev: Saving Buffers,  Up: Files

24.3 Reading from Files
=======================

To copy the contents of a file into a buffer, use the function
‘insert-file-contents’.  (Don’t use the command ‘insert-file’ in a Lisp
program, as that sets the mark.)

 -- Function: insert-file-contents filename &optional visit beg end
          replace
     This function inserts the contents of file FILENAME into the
     current buffer after point.  It returns a list of the absolute file
     name and the length of the data inserted.  An error is signaled if
     FILENAME is not the name of a file that can be read.

     This function checks the file contents against the defined file
     formats, and converts the file contents if appropriate and also
     calls the functions in the list ‘after-insert-file-functions’.
     *Note Format Conversion::.  Normally, one of the functions in the
     ‘after-insert-file-functions’ list determines the coding system
     (*note Coding Systems::) used for decoding the file’s contents,
     including end-of-line conversion.  However, if the file contains
     null bytes, it is by default visited without any code conversions.
     *Note inhibit-null-byte-detection: Lisp and Coding Systems.

     If VISIT is non-‘nil’, this function additionally marks the buffer
     as unmodified and sets up various fields in the buffer so that it
     is visiting the file FILENAME: these include the buffer’s visited
     file name and its last save file modtime.  This feature is used by
     ‘find-file-noselect’ and you probably should not use it yourself.

     If BEG and END are non-‘nil’, they should be numbers that are byte
     offsets specifying the portion of the file to insert.  In this
     case, VISIT must be ‘nil’.  For example,

          (insert-file-contents filename nil 0 500)

     inserts the first 500 characters of a file.

     If the argument REPLACE is non-‘nil’, it means to replace the
     contents of the buffer (actually, just the accessible portion) with
     the contents of the file.  This is better than simply deleting the
     buffer contents and inserting the whole file, because (1) it
     preserves some marker positions and (2) it puts less data in the
     undo list.

     It is possible to read a special file (such as a FIFO or an I/O
     device) with ‘insert-file-contents’, as long as REPLACE and VISIT
     are ‘nil’.

 -- Function: insert-file-contents-literally filename &optional visit
          beg end replace
     This function works like ‘insert-file-contents’ except that it does
     not run ‘find-file-hook’, and does not do format decoding,
     character code conversion, automatic uncompression, and so on.

   If you want to pass a file name to another process so that another
program can read the file, use the function ‘file-local-copy’; see *note
Magic File Names::.


File: elisp.info,  Node: Writing to Files,  Next: File Locks,  Prev: Reading from Files,  Up: Files

24.4 Writing to Files
=====================

You can write the contents of a buffer, or part of a buffer, directly to
a file on disk using the ‘append-to-file’ and ‘write-region’ functions.
Don’t use these functions to write to files that are being visited; that
could cause confusion in the mechanisms for visiting.

 -- Command: append-to-file start end filename
     This function appends the contents of the region delimited by START
     and END in the current buffer to the end of file FILENAME.  If that
     file does not exist, it is created.  This function returns ‘nil’.

     An error is signaled if FILENAME specifies a nonwritable file, or a
     nonexistent file in a directory where files cannot be created.

     When called from Lisp, this function is completely equivalent to:

          (write-region start end filename t)

 -- Command: write-region start end filename &optional append visit
          lockname mustbenew
     This function writes the region delimited by START and END in the
     current buffer into the file specified by FILENAME.

     If START is ‘nil’, then the command writes the entire buffer
     contents (_not_ just the accessible portion) to the file and
     ignores END.

     If START is a string, then ‘write-region’ writes or appends that
     string, rather than text from the buffer.  END is ignored in this
     case.

     If APPEND is non-‘nil’, then the specified text is appended to the
     existing file contents (if any).  If APPEND is a number,
     ‘write-region’ seeks to that byte offset from the start of the file
     and writes the data from there.

     If MUSTBENEW is non-‘nil’, then ‘write-region’ asks for
     confirmation if FILENAME names an existing file.  If MUSTBENEW is
     the symbol ‘excl’, then ‘write-region’ does not ask for
     confirmation, but instead it signals an error ‘file-already-exists’
     if the file already exists.

     The test for an existing file, when MUSTBENEW is ‘excl’, uses a
     special system feature.  At least for files on a local disk, there
     is no chance that some other program could create a file of the
     same name before Emacs does, without Emacs’s noticing.

     If VISIT is ‘t’, then Emacs establishes an association between the
     buffer and the file: the buffer is then visiting that file.  It
     also sets the last file modification time for the current buffer to
     FILENAME’s modtime, and marks the buffer as not modified.  This
     feature is used by ‘save-buffer’, but you probably should not use
     it yourself.

     If VISIT is a string, it specifies the file name to visit.  This
     way, you can write the data to one file (FILENAME) while recording
     the buffer as visiting another file (VISIT).  The argument VISIT is
     used in the echo area message and also for file locking; VISIT is
     stored in ‘buffer-file-name’.  This feature is used to implement
     ‘file-precious-flag’; don’t use it yourself unless you really know
     what you’re doing.

     The optional argument LOCKNAME, if non-‘nil’, specifies the file
     name to use for purposes of locking and unlocking, overriding
     FILENAME and VISIT for that purpose.

     The function ‘write-region’ converts the data which it writes to
     the appropriate file formats specified by ‘buffer-file-format’ and
     also calls the functions in the list
     ‘write-region-annotate-functions’.  *Note Format Conversion::.

     Normally, ‘write-region’ displays the message ‘Wrote FILENAME’ in
     the echo area.  This message is inhibited if VISIT is neither ‘t’
     nor ‘nil’ nor a string, or if Emacs is operating in batch mode
     (*note Batch Mode::).  This feature is useful for programs that use
     files for internal purposes, files that the user does not need to
     know about.

 -- Macro: with-temp-file file body...
     The ‘with-temp-file’ macro evaluates the BODY forms with a
     temporary buffer as the current buffer; then, at the end, it writes
     the buffer contents into file FILE.  It kills the temporary buffer
     when finished, restoring the buffer that was current before the
     ‘with-temp-file’ form.  Then it returns the value of the last form
     in BODY.

     The current buffer is restored even in case of an abnormal exit via
     ‘throw’ or error (*note Nonlocal Exits::).

     See also ‘with-temp-buffer’ in *note The Current Buffer: Definition
     of with-temp-buffer.


File: elisp.info,  Node: File Locks,  Next: Information about Files,  Prev: Writing to Files,  Up: Files

24.5 File Locks
===============

When two users edit the same file at the same time, they are likely to
interfere with each other.  Emacs tries to prevent this situation from
arising by recording a “file lock” when a file is being modified.  Emacs
can then detect the first attempt to modify a buffer visiting a file
that is locked by another Emacs job, and ask the user what to do.  The
file lock is really a file, a symbolic link with a special name, stored
in the same directory as the file you are editing.  (On file systems
that do not support symbolic links, a regular file is used.)

   When you access files using NFS, there may be a small probability
that you and another user will both lock the same file simultaneously.
If this happens, it is possible for the two users to make changes
simultaneously, but Emacs will still warn the user who saves second.
Also, the detection of modification of a buffer visiting a file changed
on disk catches some cases of simultaneous editing; see *note
Modification Time::.

 -- Function: file-locked-p filename
     This function returns ‘nil’ if the file FILENAME is not locked.  It
     returns ‘t’ if it is locked by this Emacs process, and it returns
     the name of the user who has locked it if it is locked by some
     other job.

          (file-locked-p "foo")
               ⇒ nil

 -- Function: lock-buffer &optional filename
     This function locks the file FILENAME, if the current buffer is
     modified.  The argument FILENAME defaults to the current buffer’s
     visited file.  Nothing is done if the current buffer is not
     visiting a file, or is not modified, or if the option
     ‘create-lockfiles’ is ‘nil’.

 -- Function: unlock-buffer
     This function unlocks the file being visited in the current buffer,
     if the buffer is modified.  If the buffer is not modified, then the
     file should not be locked, so this function does nothing.  It also
     does nothing if the current buffer is not visiting a file, or is
     not locked.

 -- User Option: create-lockfiles
     If this variable is ‘nil’, Emacs does not lock files.

 -- Function: ask-user-about-lock file other-user
     This function is called when the user tries to modify FILE, but it
     is locked by another user named OTHER-USER.  The default definition
     of this function asks the user to say what to do.  The value this
     function returns determines what Emacs does next:

        • A value of ‘t’ says to grab the lock on the file.  Then this
          user may edit the file and OTHER-USER loses the lock.

        • A value of ‘nil’ says to ignore the lock and let this user
          edit the file anyway.

        • This function may instead signal a ‘file-locked’ error, in
          which case the change that the user was about to make does not
          take place.

          The error message for this error looks like this:

               error→ File is locked: FILE OTHER-USER

          where ‘file’ is the name of the file and OTHER-USER is the
          name of the user who has locked the file.

     If you wish, you can replace the ‘ask-user-about-lock’ function
     with your own version that makes the decision in another way.


File: elisp.info,  Node: Information about Files,  Next: Changing Files,  Prev: File Locks,  Up: Files

24.6 Information about Files
============================

This section describes the functions for retrieving various types of
information about files (or directories or symbolic links), such as
whether a file is readable or writable, and its size.  These functions
all take arguments which are file names.  Except where noted, these
arguments need to specify existing files, or an error is signaled.

   Be careful with file names that end in spaces.  On some filesystems
(notably, MS-Windows), trailing whitespace characters in file names are
silently and automatically ignored.

* Menu:

* Testing Accessibility::   Is a given file readable?  Writable?
* Kinds of Files::          Is it a directory?  A symbolic link?
* Truenames::               Eliminating symbolic links from a file name.
* File Attributes::         File sizes, modification times, etc.
* Extended Attributes::     Extended file attributes for access control.
* Locating Files::          How to find a file in standard places.


File: elisp.info,  Node: Testing Accessibility,  Next: Kinds of Files,  Up: Information about Files

24.6.1 Testing Accessibility
----------------------------

These functions test for permission to access a file for reading,
writing, or execution.  Unless explicitly stated otherwise, they
recursively follow symbolic links for their file name arguments, at all
levels (at the level of the file itself and at all levels of parent
directories).

   On some operating systems, more complex sets of access permissions
can be specified, via mechanisms such as Access Control Lists (ACLs).
*Note Extended Attributes::, for how to query and set those permissions.

 -- Function: file-exists-p filename
     This function returns ‘t’ if a file named FILENAME appears to
     exist.  This does not mean you can necessarily read the file, only
     that you can find out its attributes.  (On Unix and GNU/Linux, this
     is true if the file exists and you have execute permission on the
     containing directories, regardless of the permissions of the file
     itself.)

     If the file does not exist, or if access control policies prevent
     you from finding its attributes, this function returns ‘nil’.

     Directories are files, so ‘file-exists-p’ returns ‘t’ when given a
     directory name.  However, symbolic links are treated specially;
     ‘file-exists-p’ returns ‘t’ for a symbolic link name only if the
     target file exists.

 -- Function: file-readable-p filename
     This function returns ‘t’ if a file named FILENAME exists and you
     can read it.  It returns ‘nil’ otherwise.

 -- Function: file-executable-p filename
     This function returns ‘t’ if a file named FILENAME exists and you
     can execute it.  It returns ‘nil’ otherwise.  On Unix and
     GNU/Linux, if the file is a directory, execute permission means you
     can check the existence and attributes of files inside the
     directory, and open those files if their modes permit.

 -- Function: file-writable-p filename
     This function returns ‘t’ if the file FILENAME can be written or
     created by you, and ‘nil’ otherwise.  A file is writable if the
     file exists and you can write it.  It is creatable if it does not
     exist, but the specified directory does exist and you can write in
     that directory.

     In the example below, ‘foo’ is not writable because the parent
     directory does not exist, even though the user could create such a
     directory.

          (file-writable-p "~/no-such-dir/foo")
               ⇒ nil

 -- Function: file-accessible-directory-p dirname
     This function returns ‘t’ if you have permission to open existing
     files in the directory whose name as a file is DIRNAME; otherwise
     (or if there is no such directory), it returns ‘nil’.  The value of
     DIRNAME may be either a directory name (such as ‘/foo/’) or the
     file name of a file which is a directory (such as ‘/foo’, without
     the final slash).

     For example, from the following we deduce that any attempt to read
     a file in ‘/foo/’ will give an error:

          (file-accessible-directory-p "/foo")
               ⇒ nil

 -- Function: access-file filename string
     This function opens file FILENAME for reading, then closes it and
     returns ‘nil’.  However, if the open fails, it signals an error
     using STRING as the error message text.

 -- Function: file-ownership-preserved-p filename &optional group
     This function returns ‘t’ if deleting the file FILENAME and then
     creating it anew would keep the file’s owner unchanged.  It also
     returns ‘t’ for nonexistent files.

     If the optional argument GROUP is non-‘nil’, this function also
     checks that the file’s group would be unchanged.

     If FILENAME is a symbolic link, then, unlike the other functions
     discussed here, ‘file-ownership-preserved-p’ does _not_ replace
     FILENAME with its target.  However, it does recursively follow
     symbolic links at all levels of parent directories.

 -- Function: file-modes filename
     This function returns the “mode bits” of FILENAME—an integer
     summarizing its read, write, and execution permissions.  Symbolic
     links in FILENAME are recursively followed at all levels.  If the
     file does not exist, the return value is ‘nil’.

     *Note (coreutils)File permissions::, for a description of mode
     bits.  For example, if the low-order bit is 1, the file is
     executable by all users; if the second-lowest-order bit is 1, the
     file is writable by all users; etc.  The highest possible value is
     4095 (7777 octal), meaning that everyone has read, write, and
     execute permission, the SUID bit is set for both others and group,
     and the sticky bit is set.

     *Note Changing Files::, for the ‘set-file-modes’ function, which
     can be used to set these permissions.

          (file-modes "~/junk/diffs")
               ⇒ 492               ; Decimal integer.
          (format "%o" 492)
               ⇒ "754"             ; Convert to octal.

          (set-file-modes "~/junk/diffs" #o666)
               ⇒ nil

          $ ls -l diffs
          -rw-rw-rw- 1 lewis lewis 3063 Oct 30 16:00 diffs

     *MS-DOS note:* On MS-DOS, there is no such thing as an executable
     file mode bit.  So ‘file-modes’ considers a file executable if its
     name ends in one of the standard executable extensions, such as
     ‘.com’, ‘.bat’, ‘.exe’, and some others.  Files that begin with the
     Unix-standard ‘#!’ signature, such as shell and Perl scripts, are
     also considered executable.  Directories are also reported as
     executable, for compatibility with Unix.  These conventions are
     also followed by ‘file-attributes’ (*note File Attributes::).


File: elisp.info,  Node: Kinds of Files,  Next: Truenames,  Prev: Testing Accessibility,  Up: Information about Files

24.6.2 Distinguishing Kinds of Files
------------------------------------

This section describes how to distinguish various kinds of files, such
as directories, symbolic links, and ordinary files.

 -- Function: file-symlink-p filename
     If the file FILENAME is a symbolic link, the ‘file-symlink-p’
     function returns its (non-recursive) link target as a string.  (The
     link target string is not necessarily the full absolute file name
     of the target; determining the full file name that the link points
     to is nontrivial, see below.)  If the leading directories of
     FILENAME include symbolic links, this function recursively follows
     them.

     If the file FILENAME is not a symbolic link, or does not exist,
     ‘file-symlink-p’ returns ‘nil’.

     Here are a few examples of using this function:

          (file-symlink-p "not-a-symlink")
               ⇒ nil
          (file-symlink-p "sym-link")
               ⇒ "not-a-symlink"
          (file-symlink-p "sym-link2")
               ⇒ "sym-link"
          (file-symlink-p "/bin")
               ⇒ "/pub/bin"

     Note that in the third example, the function returned ‘sym-link’,
     but did not proceed to resolve it, although that file is itself a
     symbolic link.  This is what we meant by “non-recursive” above—the
     process of following the symbolic links does not recurse if the
     link target is itself a link.

     The string that this function returns is what is recorded in the
     symbolic link; it may or may not include any leading directories.
     This function does _not_ expand the link target to produce a
     fully-qualified file name, and in particular does not use the
     leading directories, if any, of the FILENAME argument if the link
     target is not an absolute file name.  Here’s an example:

          (file-symlink-p "/foo/bar/baz")
               ⇒ "some-file"

     Here, although ‘/foo/bar/baz’ was given as a fully-qualified file
     name, the result is not, and in fact does not have any leading
     directories at all.  And since ‘some-file’ might itself be a
     symbolic link, you cannot simply prepend leading directories to it,
     nor even naively use ‘expand-file-name’ (*note File Name
     Expansion::) to produce its absolute file name.

     For this reason, this function is seldom useful if you need to
     determine more than just the fact that a file is or isn’t a
     symbolic link.  If you actually need the file name of the link
     target, use ‘file-chase-links’ or ‘file-truename’, described in
     *note Truenames::.

   The next two functions recursively follow symbolic links at all
levels for FILENAME.

 -- Function: file-directory-p filename
     This function returns ‘t’ if FILENAME is the name of an existing
     directory, ‘nil’ otherwise.

          (file-directory-p "~rms")
               ⇒ t
          (file-directory-p "~rms/lewis/files.texi")
               ⇒ nil
          (file-directory-p "~rms/lewis/no-such-file")
               ⇒ nil
          (file-directory-p "$HOME")
               ⇒ nil
          (file-directory-p
           (substitute-in-file-name "$HOME"))
               ⇒ t

 -- Function: file-regular-p filename
     This function returns ‘t’ if the file FILENAME exists and is a
     regular file (not a directory, named pipe, terminal, or other I/O
     device).


File: elisp.info,  Node: Truenames,  Next: File Attributes,  Prev: Kinds of Files,  Up: Information about Files

24.6.3 Truenames
----------------

The “truename” of a file is the name that you get by following symbolic
links at all levels until none remain, then simplifying away ‘.’ and
‘..’ appearing as name components.  This results in a sort of canonical
name for the file.  A file does not always have a unique truename; the
number of distinct truenames a file has is equal to the number of hard
links to the file.  However, truenames are useful because they eliminate
symbolic links as a cause of name variation.

 -- Function: file-truename filename
     This function returns the truename of the file FILENAME.  If the
     argument is not an absolute file name, this function first expands
     it against ‘default-directory’.

     This function does not expand environment variables.  Only
     ‘substitute-in-file-name’ does that.  *Note Definition of
     substitute-in-file-name::.

     If you may need to follow symbolic links preceding ‘..’ appearing
     as a name component, call ‘file-truename’ without prior direct or
     indirect calls to ‘expand-file-name’.  Otherwise, the file name
     component immediately preceding ‘..’ will be simplified away before
     ‘file-truename’ is called.  To eliminate the need for a call to
     ‘expand-file-name’, ‘file-truename’ handles ‘~’ in the same way
     that ‘expand-file-name’ does.  *Note Functions that Expand
     Filenames: File Name Expansion.

 -- Function: file-chase-links filename &optional limit
     This function follows symbolic links, starting with FILENAME, until
     it finds a file name which is not the name of a symbolic link.
     Then it returns that file name.  This function does _not_ follow
     symbolic links at the level of parent directories.

     If you specify a number for LIMIT, then after chasing through that
     many links, the function just returns what it has even if that is
     still a symbolic link.

   To illustrate the difference between ‘file-chase-links’ and
‘file-truename’, suppose that ‘/usr/foo’ is a symbolic link to the
directory ‘/home/foo’, and ‘/home/foo/hello’ is an ordinary file (or at
least, not a symbolic link) or nonexistent.  Then we would have:

     (file-chase-links "/usr/foo/hello")
          ;; This does not follow the links in the parent directories.
          ⇒ "/usr/foo/hello"
     (file-truename "/usr/foo/hello")
          ;; Assuming that ‘/home’ is not a symbolic link.
          ⇒ "/home/foo/hello"

 -- Function: file-equal-p file1 file2
     This function returns ‘t’ if the files FILE1 and FILE2 name the
     same file.  This is similar to comparing their truenames, except
     that remote file names are also handled in an appropriate manner.
     If FILE1 or FILE2 does not exist, the return value is unspecified.

 -- Function: file-in-directory-p file dir
     This function returns ‘t’ if FILE is a file in directory DIR, or in
     a subdirectory of DIR.  It also returns ‘t’ if FILE and DIR are the
     same directory.  It compares the truenames of the two directories.
     If DIR does not name an existing directory, the return value is
     ‘nil’.

 -- Function: vc-responsible-backend file
     This function determines the responsible VC backend of the given
     FILE.  For example, if ‘emacs.c’ is a file tracked by Git,
     ‘(vc-responsible-backend "emacs.c")’ returns ‘Git’.  Note that if
     FILE is a symbolic link, ‘vc-responsible-backend’ will not resolve
     it—the backend of the symbolic link file itself is reported.  To
     get the backend VC of the file to which FILE refers, wrap FILE with
     a symbolic link resolving function such as ‘file-chase-links’:

          (vc-responsible-backend (file-chase-links "emacs.c"))


File: elisp.info,  Node: File Attributes,  Next: Extended Attributes,  Prev: Truenames,  Up: Information about Files

24.6.4 File Attributes
----------------------

This section describes the functions for getting detailed information
about a file, including the owner and group numbers, the number of
names, the inode number, the size, and the times of access and
modification.

 -- Function: file-newer-than-file-p filename1 filename2
     This function returns ‘t’ if the file FILENAME1 is newer than file
     FILENAME2.  If FILENAME1 does not exist, it returns ‘nil’.  If
     FILENAME1 does exist, but FILENAME2 does not, it returns ‘t’.

     In the following example, assume that the file ‘aug-19’ was written
     on the 19th, ‘aug-20’ was written on the 20th, and the file
     ‘no-file’ doesn’t exist at all.

          (file-newer-than-file-p "aug-19" "aug-20")
               ⇒ nil
          (file-newer-than-file-p "aug-20" "aug-19")
               ⇒ t
          (file-newer-than-file-p "aug-19" "no-file")
               ⇒ t
          (file-newer-than-file-p "no-file" "aug-19")
               ⇒ nil

   If the FILENAME argument to the next two functions is a symbolic
link, then these function do _not_ replace it with its target.  However,
they both recursively follow symbolic links at all levels of parent
directories.

 -- Function: file-attributes filename &optional id-format
     This function returns a list of attributes of file FILENAME.  If
     the specified file cannot be opened, it returns ‘nil’.  The
     optional parameter ID-FORMAT specifies the preferred format of
     attributes UID and GID (see below)—the valid values are ‘'string’
     and ‘'integer’.  The latter is the default, but we plan to change
     that, so you should specify a non-‘nil’ value for ID-FORMAT if you
     use the returned UID or GID.

     The elements of the list, in order, are:

       0. ‘t’ for a directory, a string for a symbolic link (the name
          linked to), or ‘nil’ for a text file.

       1. The number of names the file has.  Alternate names, also known
          as hard links, can be created by using the ‘add-name-to-file’
          function (*note Changing Files::).

       2. The file’s UID, normally as a string.  However, if it does not
          correspond to a named user, the value is a number.

       3. The file’s GID, likewise.

       4. The time of last access, as a list of four integers ‘(SEC-HIGH
          SEC-LOW MICROSEC PICOSEC)’.  (This is similar to the value of
          ‘current-time’; see *note Time of Day::.)  Note that on some
          FAT-based filesystems, only the date of last access is
          recorded, so this time will always hold the midnight of the
          day of last access.

       5. The time of last modification as a list of four integers (as
          above).  This is the last time when the file’s contents were
          modified.

       6. The time of last status change as a list of four integers (as
          above).  This is the time of the last change to the file’s
          access mode bits, its owner and group, and other information
          recorded in the filesystem for the file, beyond the file’s
          contents.

       7. The size of the file in bytes.  This is floating point if the
          size is too large to fit in a Lisp integer.

       8. The file’s modes, as a string of ten letters or dashes, as in
          ‘ls -l’.

       9. An unspecified value, present for backward compatibility.

       10. The file’s inode number.  If possible, this is an integer.
          If the inode number is too large to be represented as an
          integer in Emacs Lisp but dividing it by 2^{16} yields a
          representable integer, then the value has the form ‘(HIGH .
          LOW)’, where LOW holds the low 16 bits.  If the inode number
          is too wide for even that, the value is of the form ‘(HIGH
          MIDDLE . LOW)’, where ‘high’ holds the high bits, MIDDLE the
          middle 24 bits, and LOW the low 16 bits.

       11. The filesystem number of the device that the file is on.
          Depending on the magnitude of the value, this can be either an
          integer or a cons cell, in the same manner as the inode
          number.  This element and the file’s inode number together
          give enough information to distinguish any two files on the
          system—no two files can have the same values for both of these
          numbers.

     For example, here are the file attributes for ‘files.texi’:

          (file-attributes "files.texi" 'string)
               ⇒  (nil 1 "lh" "users"
                    (20614 64019 50040 152000)
                    (20000 23 0 0)
                    (20614 64555 902289 872000)
                    122295 "-rw-rw-rw-"
                    t (5888 2 . 43978)
                    (15479 . 46724))

     and here is how the result is interpreted:

     ‘nil’
          is neither a directory nor a symbolic link.

     ‘1’
          has only one name (the name ‘files.texi’ in the current
          default directory).

     ‘"lh"’
          is owned by the user with name ‘lh’.

     ‘"users"’
          is in the group with name ‘users’.

     ‘(20614 64019 50040 152000)’
          was last accessed on October 23, 2012, at 20:12:03.050040152
          UTC.

     ‘(20000 23 0 0)’
          was last modified on July 15, 2001, at 08:53:43 UTC.

     ‘(20614 64555 902289 872000)’
          last had its status changed on October 23, 2012, at
          20:20:59.902289872 UTC.

     ‘122295’
          is 122295 bytes long.  (It may not contain 122295 characters,
          though, if some of the bytes belong to multibyte sequences,
          and also if the end-of-line format is CR-LF.)

     ‘"-rw-rw-rw-"’
          has a mode of read and write access for the owner, group, and
          world.

     ‘t’
          is merely a placeholder; it carries no information.

     ‘(5888 2 . 43978)’
          has an inode number of 6473924464520138.

     ‘(15479 . 46724)’
          is on the file-system device whose number is 1014478468.

 -- Function: file-nlinks filename
     This function returns the number of names (i.e., hard links) that
     file FILENAME has.  If the file does not exist, this function
     returns ‘nil’.  Note that symbolic links have no effect on this
     function, because they are not considered to be names of the files
     they link to.

          $ ls -l foo*
          -rw-rw-rw- 2 rms rms 4 Aug 19 01:27 foo
          -rw-rw-rw- 2 rms rms 4 Aug 19 01:27 foo1

          (file-nlinks "foo")
               ⇒ 2
          (file-nlinks "doesnt-exist")
               ⇒ nil


File: elisp.info,  Node: Extended Attributes,  Next: Locating Files,  Prev: File Attributes,  Up: Information about Files

24.6.5 Extended File Attributes
-------------------------------

On some operating systems, each file can be associated with arbitrary
“extended file attributes”.  At present, Emacs supports querying and
setting two specific sets of extended file attributes: Access Control
Lists (ACLs) and SELinux contexts.  These extended file attributes are
used, on some systems, to impose more sophisticated file access controls
than the basic Unix-style permissions discussed in the previous
sections.

   A detailed explanation of ACLs and SELinux is beyond the scope of
this manual.  For our purposes, each file can be associated with an
“ACL”, which specifies its properties under an ACL-based file control
system, and/or an “SELinux context”, which specifies its properties
under the SELinux system.

 -- Function: file-acl filename
     This function returns the ACL for the file FILENAME.  The exact
     Lisp representation of the ACL is unspecified (and may change in
     future Emacs versions), but it is the same as what ‘set-file-acl’
     takes for its ACL argument (*note Changing Files::).

     The underlying ACL implementation is platform-specific; on
     GNU/Linux and BSD, Emacs uses the POSIX ACL interface, while on
     MS-Windows Emacs emulates the POSIX ACL interface with native file
     security APIs.

     If Emacs was not compiled with ACL support, or the file does not
     exist or is inaccessible, or Emacs was unable to determine the ACL
     entries for any other reason, then the return value is ‘nil’.

 -- Function: file-selinux-context filename
     This function returns the SELinux context of the file FILENAME, as
     a list of the form ‘(USER ROLE TYPE RANGE)’.  The list elements are
     the context’s user, role, type, and range respectively, as Lisp
     strings; see the SELinux documentation for details about what these
     actually mean.  The return value has the same form as what
     ‘set-file-selinux-context’ takes for its CONTEXT argument (*note
     Changing Files::).

     If Emacs was not compiled with SELinux support, or the file does
     not exist or is inaccessible, or if the system does not support
     SELinux, then the return value is ‘(nil nil nil nil)’.

 -- Function: file-extended-attributes filename
     This function returns an alist of the Emacs-recognized extended
     attributes of file FILENAME.  Currently, it serves as a convenient
     way to retrieve both the ACL and SELinux context; you can then call
     the function ‘set-file-extended-attributes’, with the returned
     alist as its second argument, to apply the same file access
     attributes to another file (*note Changing Files::).

     One of the elements is ‘(acl . ACL)’, where ACL has the same form
     returned by ‘file-acl’.

     Another element is ‘(selinux-context . CONTEXT)’, where CONTEXT is
     the SELinux context, in the same form returned by
     ‘file-selinux-context’.


File: elisp.info,  Node: Locating Files,  Prev: Extended Attributes,  Up: Information about Files

24.6.6 Locating Files in Standard Places
----------------------------------------

This section explains how to search for a file in a list of directories
(a “path”), or for an executable file in the standard list of executable
file directories.

   To search for a user-specific configuration file, *Note Standard File
Names::, for the ‘locate-user-emacs-file’ function.

 -- Function: locate-file filename path &optional suffixes predicate
     This function searches for a file whose name is FILENAME in a list
     of directories given by PATH, trying the suffixes in SUFFIXES.  If
     it finds such a file, it returns the file’s absolute file name
     (*note Relative File Names::); otherwise it returns ‘nil’.

     The optional argument SUFFIXES gives the list of file-name suffixes
     to append to FILENAME when searching.  ‘locate-file’ tries each
     possible directory with each of these suffixes.  If SUFFIXES is
     ‘nil’, or ‘("")’, then there are no suffixes, and FILENAME is used
     only as-is.  Typical values of SUFFIXES are ‘exec-suffixes’ (*note
     Subprocess Creation::), ‘load-suffixes’, ‘load-file-rep-suffixes’
     and the return value of the function ‘get-load-suffixes’ (*note
     Load Suffixes::).

     Typical values for PATH are ‘exec-path’ (*note Subprocess
     Creation::) when looking for executable programs, or ‘load-path’
     (*note Library Search::) when looking for Lisp files.  If FILENAME
     is absolute, PATH has no effect, but the suffixes in SUFFIXES are
     still tried.

     The optional argument PREDICATE, if non-‘nil’, specifies a
     predicate function for testing whether a candidate file is
     suitable.  The predicate is passed the candidate file name as its
     single argument.  If PREDICATE is ‘nil’ or omitted, ‘locate-file’
     uses ‘file-readable-p’ as the predicate.  *Note Kinds of Files::,
     for other useful predicates, e.g., ‘file-executable-p’ and
     ‘file-directory-p’.

     For compatibility, PREDICATE can also be one of the symbols
     ‘executable’, ‘readable’, ‘writable’, ‘exists’, or a list of one or
     more of these symbols.

 -- Function: executable-find program
     This function searches for the executable file of the named PROGRAM
     and returns the absolute file name of the executable, including its
     file-name extensions, if any.  It returns ‘nil’ if the file is not
     found.  The functions searches in all the directories in
     ‘exec-path’, and tries all the file-name extensions in
     ‘exec-suffixes’ (*note Subprocess Creation::).


File: elisp.info,  Node: Changing Files,  Next: File Names,  Prev: Information about Files,  Up: Files

24.7 Changing File Names and Attributes
=======================================

The functions in this section rename, copy, delete, link, and set the
modes (permissions) of files.  They all signal a ‘file-error’ error if
they fail to perform their function, reporting the system-dependent
error message that describes the reason for the failure.

   In the functions that have an argument NEWNAME, if a file by the name
of NEWNAME already exists, the actions taken depend on the value of the
argument OK-IF-ALREADY-EXISTS:

   • Signal a ‘file-already-exists’ error if OK-IF-ALREADY-EXISTS is
     ‘nil’.

   • Request confirmation if OK-IF-ALREADY-EXISTS is a number.

   • Replace the old file without confirmation if OK-IF-ALREADY-EXISTS
     is any other value.

   The next four commands all recursively follow symbolic links at all
levels of parent directories for their first argument, but, if that
argument is itself a symbolic link, then only ‘copy-file’ replaces it
with its (recursive) target.

 -- Command: add-name-to-file oldname newname &optional
          ok-if-already-exists
     This function gives the file named OLDNAME the additional name
     NEWNAME.  This means that NEWNAME becomes a new hard link to
     OLDNAME.

     In the first part of the following example, we list two files,
     ‘foo’ and ‘foo3’.

          $ ls -li fo*
          81908 -rw-rw-rw- 1 rms rms 29 Aug 18 20:32 foo
          84302 -rw-rw-rw- 1 rms rms 24 Aug 18 20:31 foo3

     Now we create a hard link, by calling ‘add-name-to-file’, then list
     the files again.  This shows two names for one file, ‘foo’ and
     ‘foo2’.

          (add-name-to-file "foo" "foo2")
               ⇒ nil

          $ ls -li fo*
          81908 -rw-rw-rw- 2 rms rms 29 Aug 18 20:32 foo
          81908 -rw-rw-rw- 2 rms rms 29 Aug 18 20:32 foo2
          84302 -rw-rw-rw- 1 rms rms 24 Aug 18 20:31 foo3

     Finally, we evaluate the following:

          (add-name-to-file "foo" "foo3" t)

     and list the files again.  Now there are three names for one file:
     ‘foo’, ‘foo2’, and ‘foo3’.  The old contents of ‘foo3’ are lost.

          (add-name-to-file "foo1" "foo3")
               ⇒ nil

          $ ls -li fo*
          81908 -rw-rw-rw- 3 rms rms 29 Aug 18 20:32 foo
          81908 -rw-rw-rw- 3 rms rms 29 Aug 18 20:32 foo2
          81908 -rw-rw-rw- 3 rms rms 29 Aug 18 20:32 foo3

     This function is meaningless on operating systems where multiple
     names for one file are not allowed.  Some systems implement
     multiple names by copying the file instead.

     See also ‘file-nlinks’ in *note File Attributes::.

 -- Command: rename-file filename newname &optional ok-if-already-exists
     This command renames the file FILENAME as NEWNAME.

     If FILENAME has additional names aside from FILENAME, it continues
     to have those names.  In fact, adding the name NEWNAME with
     ‘add-name-to-file’ and then deleting FILENAME has the same effect
     as renaming, aside from momentary intermediate states.

 -- Command: copy-file oldname newname &optional ok-if-exists time
          preserve-uid-gid preserve-extended-attributes
     This command copies the file OLDNAME to NEWNAME.  An error is
     signaled if OLDNAME does not exist.  If NEWNAME names a directory,
     it copies OLDNAME into that directory, preserving its final name
     component.

     If TIME is non-‘nil’, then this function gives the new file the
     same last-modified time that the old one has.  (This works on only
     some operating systems.)  If setting the time gets an error,
     ‘copy-file’ signals a ‘file-date-error’ error.  In an interactive
     call, a prefix argument specifies a non-‘nil’ value for TIME.

     If argument PRESERVE-UID-GID is ‘nil’, we let the operating system
     decide the user and group ownership of the new file (this is
     usually set to the user running Emacs).  If PRESERVE-UID-GID is
     non-‘nil’, we attempt to copy the user and group ownership of the
     file.  This works only on some operating systems, and only if you
     have the correct permissions to do so.

     If the optional argument PRESERVE-PERMISSIONS is non-‘nil’, this
     function copies the file modes (or “permissions”) of OLDNAME to
     NEWNAME, as well as the Access Control List and SELinux context (if
     any).  *Note Information about Files::.

     Otherwise, the file modes of NEWNAME are left unchanged if it is an
     existing file, and set to those of OLDNAME, masked by the default
     file permissions (see ‘set-default-file-modes’ below), if NEWNAME
     is to be newly created.  The Access Control List or SELinux context
     are not copied over in either case.

 -- Command: make-symbolic-link filename newname &optional ok-if-exists
     This command makes a symbolic link to FILENAME, named NEWNAME.
     This is like the shell command ‘ln -s FILENAME NEWNAME’.

     This function is not available on systems that don’t support
     symbolic links.

 -- Command: delete-file filename &optional trash
     This command deletes the file FILENAME.  If the file has multiple
     names, it continues to exist under the other names.  If FILENAME is
     a symbolic link, ‘delete-file’ deletes only the symbolic link and
     not its target (though it does follow symbolic links at all levels
     of parent directories).

     A suitable kind of ‘file-error’ error is signaled if the file does
     not exist, or is not deletable.  (On Unix and GNU/Linux, a file is
     deletable if its directory is writable.)

     If the optional argument TRASH is non-‘nil’ and the variable
     ‘delete-by-moving-to-trash’ is non-‘nil’, this command moves the
     file into the system Trash instead of deleting it.  *Note
     Miscellaneous File Operations: (emacs)Misc File Ops.  When called
     interactively, TRASH is ‘t’ if no prefix argument is given, and
     ‘nil’ otherwise.

     See also ‘delete-directory’ in *note Create/Delete Dirs::.

 -- Command: set-file-modes filename mode
     This function sets the “file mode” (or “permissions”) of FILENAME
     to MODE.  It recursively follows symbolic links at all levels for
     FILENAME.

     If called non-interactively, MODE must be an integer.  Only the
     lowest 12 bits of the integer are used; on most systems, only the
     lowest 9 bits are meaningful.  You can use the Lisp construct for
     octal numbers to enter MODE.  For example,

          (set-file-modes #o644)

     specifies that the file should be readable and writable for its
     owner, readable for group members, and readable for all other
     users.  *Note (coreutils)File permissions::, for a description of
     mode bit specifications.

     Interactively, MODE is read from the minibuffer using
     ‘read-file-modes’ (see below), which lets the user type in either
     an integer or a string representing the permissions symbolically.

     *Note File Attributes::, for the function ‘file-modes’, which
     returns the permissions of a file.

 -- Function: set-default-file-modes mode
     This function sets the default permissions for new files created by
     Emacs and its subprocesses.  Every file created with Emacs
     initially has these permissions, or a subset of them
     (‘write-region’ will not grant execute permissions even if the
     default file permissions allow execution).  On Unix and GNU/Linux,
     the default permissions are given by the bitwise complement of the
     ‘umask’ value.

     The argument MODE should be an integer which specifies the
     permissions, similar to ‘set-file-modes’ above.  Only the lowest 9
     bits are meaningful.

     The default file permissions have no effect when you save a
     modified version of an existing file; saving a file preserves its
     existing permissions.

 -- Macro: with-file-modes mode body...
     This macro evaluates the BODY forms with the default permissions
     for new files temporarily set to MODES (whose value is as for
     ‘set-file-modes’ above).  When finished, it restores the original
     default file permissions, and returns the value of the last form in
     BODY.

     This is useful for creating private files, for example.

 -- Function: default-file-modes
     This function returns the default file permissions, as an integer.

 -- Function: read-file-modes &optional prompt base-file
     This function reads a set of file mode bits from the minibuffer.
     The first optional argument PROMPT specifies a non-default prompt.
     Second second optional argument BASE-FILE is the name of a file on
     whose permissions to base the mode bits that this function returns,
     if what the user types specifies mode bits relative to permissions
     of an existing file.

     If user input represents an octal number, this function returns
     that number.  If it is a complete symbolic specification of mode
     bits, as in ‘"u=rwx"’, the function converts it to the equivalent
     numeric value using ‘file-modes-symbolic-to-number’ and returns the
     result.  If the specification is relative, as in ‘"o+g"’, then the
     permissions on which the specification is based are taken from the
     mode bits of BASE-FILE.  If BASE-FILE is omitted or ‘nil’, the
     function uses ‘0’ as the base mode bits.  The complete and relative
     specifications can be combined, as in ‘"u+r,g+rx,o+r,g-w"’.  *Note
     (coreutils)File permissions::, for a description of file mode
     specifications.

 -- Function: file-modes-symbolic-to-number modes &optional base-modes
     This function converts a symbolic file mode specification in MODES
     into the equivalent integer.  If the symbolic specification is
     based on an existing file, that file’s mode bits are taken from the
     optional argument BASE-MODES; if that argument is omitted or ‘nil’,
     it defaults to 0, i.e., no access rights at all.

 -- Function: set-file-times filename &optional time
     This function sets the access and modification times of FILENAME to
     TIME.  The return value is ‘t’ if the times are successfully set,
     otherwise it is ‘nil’.  TIME defaults to the current time and must
     be in the format returned by ‘current-time’ (*note Time of Day::).

 -- Function: set-file-extended-attributes filename attribute-alist
     This function sets the Emacs-recognized extended file attributes
     for ‘filename’.  The second argument ATTRIBUTE-ALIST should be an
     alist of the same form returned by ‘file-extended-attributes’.  The
     return value is ‘t’ if the attributes are successfully set,
     otherwise it is ‘nil’.  *Note Extended Attributes::.

 -- Function: set-file-selinux-context filename context
     This function sets the SELinux security context for FILENAME to
     CONTEXT.  The CONTEXT argument should be a list ‘(USER ROLE TYPE
     RANGE)’, where each element is a string.  *Note Extended
     Attributes::.

     The function returns ‘t’ if it succeeds in setting the SELinux
     context of FILENAME.  It returns ‘nil’ if the context was not set
     (e.g., if SELinux is disabled, or if Emacs was compiled without
     SELinux support).

 -- Function: set-file-acl filename acl
     This function sets the Access Control List for FILENAME to ACL.
     The ACL argument should have the same form returned by the function
     ‘file-acl’.  *Note Extended Attributes::.

     The function returns ‘t’ if it successfully sets the ACL of
     FILENAME, ‘nil’ otherwise.


File: elisp.info,  Node: File Names,  Next: Contents of Directories,  Prev: Changing Files,  Up: Files

24.8 File Names
===============

Files are generally referred to by their names, in Emacs as elsewhere.
File names in Emacs are represented as strings.  The functions that
operate on a file all expect a file name argument.

   In addition to operating on files themselves, Emacs Lisp programs
often need to operate on file names; i.e., to take them apart and to use
part of a name to construct related file names.  This section describes
how to manipulate file names.

   The functions in this section do not actually access files, so they
can operate on file names that do not refer to an existing file or
directory.

   On MS-DOS and MS-Windows, these functions (like the function that
actually operate on files) accept MS-DOS or MS-Windows file-name syntax,
where backslashes separate the components, as well as Unix syntax; but
they always return Unix syntax.  This enables Lisp programs to specify
file names in Unix syntax and work properly on all systems without
change.(1)

* Menu:

* File Name Components::  The directory part of a file name, and the rest.
* Relative File Names::   Some file names are relative to a current directory.
* Directory Names::       A directory’s name as a directory
                            is different from its name as a file.
* File Name Expansion::   Converting relative file names to absolute ones.
* Unique File Names::     Generating names for temporary files.
* File Name Completion::  Finding the completions for a given file name.
* Standard File Names::   If your package uses a fixed file name,
                            how to handle various operating systems simply.

   ---------- Footnotes ----------

   (1) In MS-Windows versions of Emacs compiled for the Cygwin
environment, you can use the functions
‘cygwin-convert-file-name-to-windows’ and
‘cygwin-convert-file-name-from-windows’ to convert between the two
file-name syntaxes.


File: elisp.info,  Node: File Name Components,  Next: Relative File Names,  Up: File Names

24.8.1 File Name Components
---------------------------

The operating system groups files into directories.  To specify a file,
you must specify the directory and the file’s name within that
directory.  Therefore, Emacs considers a file name as having two main
parts: the “directory name” part, and the “nondirectory” part (or “file
name within the directory”).  Either part may be empty.  Concatenating
these two parts reproduces the original file name.

   On most systems, the directory part is everything up to and including
the last slash (backslash is also allowed in input on MS-DOS or
MS-Windows); the nondirectory part is the rest.

   For some purposes, the nondirectory part is further subdivided into
the name proper and the “version number”.  On most systems, only backup
files have version numbers in their names.

 -- Function: file-name-directory filename
     This function returns the directory part of FILENAME, as a
     directory name (*note Directory Names::), or ‘nil’ if FILENAME does
     not include a directory part.

     On GNU and Unix systems, a string returned by this function always
     ends in a slash.  On MS-DOS it can also end in a colon.

          (file-name-directory "lewis/foo")  ; Unix example
               ⇒ "lewis/"
          (file-name-directory "foo")        ; Unix example
               ⇒ nil

 -- Function: file-name-nondirectory filename
     This function returns the nondirectory part of FILENAME.

          (file-name-nondirectory "lewis/foo")
               ⇒ "foo"
          (file-name-nondirectory "foo")
               ⇒ "foo"
          (file-name-nondirectory "lewis/")
               ⇒ ""

 -- Function: file-name-sans-versions filename &optional
          keep-backup-version
     This function returns FILENAME with any file version numbers,
     backup version numbers, or trailing tildes discarded.

     If KEEP-BACKUP-VERSION is non-‘nil’, then true file version numbers
     understood as such by the file system are discarded from the return
     value, but backup version numbers are kept.

          (file-name-sans-versions "~rms/foo.~1~")
               ⇒ "~rms/foo"
          (file-name-sans-versions "~rms/foo~")
               ⇒ "~rms/foo"
          (file-name-sans-versions "~rms/foo")
               ⇒ "~rms/foo"

 -- Function: file-name-extension filename &optional period
     This function returns FILENAME’s final extension, if any, after
     applying ‘file-name-sans-versions’ to remove any version/backup
     part.  The extension, in a file name, is the part that follows the
     last ‘.’ in the last name component (minus any version/backup
     part).

     This function returns ‘nil’ for extensionless file names such as
     ‘foo’.  It returns ‘""’ for null extensions, as in ‘foo.’.  If the
     last component of a file name begins with a ‘.’, that ‘.’ doesn’t
     count as the beginning of an extension.  Thus, ‘.emacs’’s extension
     is ‘nil’, not ‘.emacs’.

     If PERIOD is non-‘nil’, then the returned value includes the period
     that delimits the extension, and if FILENAME has no extension, the
     value is ‘""’.

 -- Function: file-name-sans-extension filename
     This function returns FILENAME minus its extension, if any.  The
     version/backup part, if present, is only removed if the file has an
     extension.  For example,

          (file-name-sans-extension "foo.lose.c")
               ⇒ "foo.lose"
          (file-name-sans-extension "big.hack/foo")
               ⇒ "big.hack/foo"
          (file-name-sans-extension "/my/home/.emacs")
               ⇒ "/my/home/.emacs"
          (file-name-sans-extension "/my/home/.emacs.el")
               ⇒ "/my/home/.emacs"
          (file-name-sans-extension "~/foo.el.~3~")
               ⇒ "~/foo"
          (file-name-sans-extension "~/foo.~3~")
               ⇒ "~/foo.~3~"

     Note that the ‘.~3~’ in the two last examples is the backup part,
     not an extension.

 -- Function: file-name-base &optional filename
     This function is the composition of ‘file-name-sans-extension’ and
     ‘file-name-nondirectory’.  For example,

          (file-name-base "/my/home/foo.c")
              ⇒ "foo"

     The FILENAME argument defaults to ‘buffer-file-name’.


File: elisp.info,  Node: Relative File Names,  Next: Directory Names,  Prev: File Name Components,  Up: File Names

24.8.2 Absolute and Relative File Names
---------------------------------------

All the directories in the file system form a tree starting at the root
directory.  A file name can specify all the directory names starting
from the root of the tree; then it is called an “absolute” file name.
Or it can specify the position of the file in the tree relative to a
default directory; then it is called a “relative” file name.  On Unix
and GNU/Linux, an absolute file name starts with a ‘/’ or a ‘~’ (*note
abbreviate-file-name::), and a relative one does not.  On MS-DOS and
MS-Windows, an absolute file name starts with a slash or a backslash, or
with a drive specification ‘X:/’, where X is the “drive letter”.

 -- Function: file-name-absolute-p filename
     This function returns ‘t’ if file FILENAME is an absolute file
     name, ‘nil’ otherwise.

          (file-name-absolute-p "~rms/foo")
               ⇒ t
          (file-name-absolute-p "rms/foo")
               ⇒ nil
          (file-name-absolute-p "/user/rms/foo")
               ⇒ t

   Given a possibly relative file name, you can convert it to an
absolute name using ‘expand-file-name’ (*note File Name Expansion::).
This function converts absolute file names to relative names:

 -- Function: file-relative-name filename &optional directory
     This function tries to return a relative name that is equivalent to
     FILENAME, assuming the result will be interpreted relative to
     DIRECTORY (an absolute directory name or directory file name).  If
     DIRECTORY is omitted or ‘nil’, it defaults to the current buffer’s
     default directory.

     On some operating systems, an absolute file name begins with a
     device name.  On such systems, FILENAME has no relative equivalent
     based on DIRECTORY if they start with two different device names.
     In this case, ‘file-relative-name’ returns FILENAME in absolute
     form.

          (file-relative-name "/foo/bar" "/foo/")
               ⇒ "bar"
          (file-relative-name "/foo/bar" "/hack/")
               ⇒ "../foo/bar"


File: elisp.info,  Node: Directory Names,  Next: File Name Expansion,  Prev: Relative File Names,  Up: File Names

24.8.3 Directory Names
----------------------

A “directory name” is the name of a directory.  A directory is actually
a kind of file, so it has a file name (called the “directory file name”,
which is related to the directory name but not identical to it.  (This
is not quite the same as the usual Unix terminology.)  These two
different names for the same entity are related by a syntactic
transformation.  On GNU and Unix systems, this is simple: a directory
name ends in a slash, whereas the directory file name lacks that slash.
On MS-DOS the relationship is more complicated.

   The difference between directory name and directory file name is
subtle but crucial.  When an Emacs variable or function argument is
described as being a directory name, a directory file name is not
acceptable.  When ‘file-name-directory’ returns a string, that is always
a directory name.

   The following two functions convert between directory names and
directory file names.  They do nothing special with environment variable
substitutions such as ‘$HOME’, and the constructs ‘~’, ‘.’ and ‘..’.

 -- Function: file-name-as-directory filename
     This function returns a string representing FILENAME in a form that
     the operating system will interpret as the name of a directory (a
     directory name).  On most systems, this means appending a slash to
     the string (if it does not already end in one).

          (file-name-as-directory "~rms/lewis")
               ⇒ "~rms/lewis/"

 -- Function: directory-name-p filename
     This function returns non-‘nil’ if FILENAME ends with a directory
     separator character.  This is the forward slash ‘/’ on Unix and GNU
     systems; MS-Windows and MS-DOS recognize both the forward slash and
     the backslash ‘\’ as directory separators.

 -- Function: directory-file-name dirname
     This function returns a string representing DIRNAME in a form that
     the operating system will interpret as the name of a file (a
     directory file name).  On most systems, this means removing the
     final slash (or backslash) from the string.

          (directory-file-name "~lewis/")
               ⇒ "~lewis"

   Given a directory name, you can combine it with a relative file name
using ‘concat’:

     (concat DIRNAME RELFILE)

Be sure to verify that the file name is relative before doing that.  If
you use an absolute file name, the results could be syntactically
invalid or refer to the wrong file.

   If you want to use a directory file name in making such a
combination, you must first convert it to a directory name using
‘file-name-as-directory’:

     (concat (file-name-as-directory DIRFILE) RELFILE)

Don’t try concatenating a slash by hand, as in

     ;;; Wrong!
     (concat DIRFILE "/" RELFILE)

because this is not portable.  Always use ‘file-name-as-directory’.

   To avoid the issues mentioned above, or if the DIRNAME value might be
nil (for example, from an element of ‘load-path’), use:

     (expand-file-name RELFILE DIRNAME)

   To convert a directory name to its abbreviation, use this function:

 -- Function: abbreviate-file-name filename
     This function returns an abbreviated form of FILENAME.  It applies
     the abbreviations specified in ‘directory-abbrev-alist’ (*note File
     Aliases: (emacs)File Aliases.), then substitutes ‘~’ for the user’s
     home directory if the argument names a file in the home directory
     or one of its subdirectories.  If the home directory is a root
     directory, it is not replaced with ‘~’, because this does not make
     the result shorter on many systems.

     You can use this function for directory names and for file names,
     because it recognizes abbreviations even as part of the name.


File: elisp.info,  Node: File Name Expansion,  Next: Unique File Names,  Prev: Directory Names,  Up: File Names

24.8.4 Functions that Expand Filenames
--------------------------------------

“Expanding” a file name means converting a relative file name to an
absolute one.  Since this is done relative to a default directory, you
must specify the default directory name as well as the file name to be
expanded.  It also involves expanding abbreviations like ‘~/’ (*note
abbreviate-file-name::), and eliminating redundancies like ‘./’ and
‘NAME/../’.

 -- Function: expand-file-name filename &optional directory
     This function converts FILENAME to an absolute file name.  If
     DIRECTORY is supplied, it is the default directory to start with if
     FILENAME is relative.  (The value of DIRECTORY should itself be an
     absolute directory name or directory file name; it may start with
     ‘~’.)  Otherwise, the current buffer’s value of ‘default-directory’
     is used.  For example:

          (expand-file-name "foo")
               ⇒ "/xcssun/users/rms/lewis/foo"
          (expand-file-name "../foo")
               ⇒ "/xcssun/users/rms/foo"
          (expand-file-name "foo" "/usr/spool/")
               ⇒ "/usr/spool/foo"

     If the part of the combined file name before the first slash is
     ‘~’, it expands to the value of the ‘HOME’ environment variable
     (usually your home directory).  If the part before the first slash
     is ‘~USER’ and if USER is a valid login name, it expands to USER’s
     home directory.

     Filenames containing ‘.’ or ‘..’ are simplified to their canonical
     form:

          (expand-file-name "bar/../foo")
               ⇒ "/xcssun/users/rms/lewis/foo"

     In some cases, a leading ‘..’ component can remain in the output:

          (expand-file-name "../home" "/")
               ⇒ "/../home"

     This is for the sake of filesystems that have the concept of a
     superroot above the root directory ‘/’.  On other filesystems,
     ‘/../’ is interpreted exactly the same as ‘/’.

     Note that ‘expand-file-name’ does _not_ expand environment
     variables; only ‘substitute-in-file-name’ does that:

          (expand-file-name "$HOME/foo")
               ⇒ "/xcssun/users/rms/lewis/$HOME/foo"

     Note also that ‘expand-file-name’ does not follow symbolic links at
     any level.  This results in a difference between the way
     ‘file-truename’ and ‘expand-file-name’ treat ‘..’.  Assuming that
     ‘/tmp/bar’ is a symbolic link to the directory ‘/tmp/foo/bar’ we
     get:

          (file-truename "/tmp/bar/../myfile")
               ⇒ "/tmp/foo/myfile"
          (expand-file-name "/tmp/bar/../myfile")
               ⇒ "/tmp/myfile"

     If you may need to follow symbolic links preceding ‘..’, you should
     make sure to call ‘file-truename’ without prior direct or indirect
     calls to ‘expand-file-name’.  *Note Truenames::.

 -- Variable: default-directory
     The value of this buffer-local variable is the default directory
     for the current buffer.  It should be an absolute directory name;
     it may start with ‘~’.  This variable is buffer-local in every
     buffer.

     ‘expand-file-name’ uses the default directory when its second
     argument is ‘nil’.

     The value is always a string ending with a slash.

          default-directory
               ⇒ "/user/lewis/manual/"

 -- Function: substitute-in-file-name filename
     This function replaces environment variable references in FILENAME
     with the environment variable values.  Following standard Unix
     shell syntax, ‘$’ is the prefix to substitute an environment
     variable value.  If the input contains ‘$$’, that is converted to
     ‘$’; this gives the user a way to quote a ‘$’.

     The environment variable name is the series of alphanumeric
     characters (including underscores) that follow the ‘$’.  If the
     character following the ‘$’ is a ‘{’, then the variable name is
     everything up to the matching ‘}’.

     Calling ‘substitute-in-file-name’ on output produced by
     ‘substitute-in-file-name’ tends to give incorrect results.  For
     instance, use of ‘$$’ to quote a single ‘$’ won’t work properly,
     and ‘$’ in an environment variable’s value could lead to repeated
     substitution.  Therefore, programs that call this function and put
     the output where it will be passed to this function need to double
     all ‘$’ characters to prevent subsequent incorrect results.

     Here we assume that the environment variable ‘HOME’, which holds
     the user’s home directory name, has value ‘/xcssun/users/rms’.

          (substitute-in-file-name "$HOME/foo")
               ⇒ "/xcssun/users/rms/foo"

     After substitution, if a ‘~’ or a ‘/’ appears immediately after
     another ‘/’, the function discards everything before it (up through
     the immediately preceding ‘/’).

          (substitute-in-file-name "bar/~/foo")
               ⇒ "~/foo"
          (substitute-in-file-name "/usr/local/$HOME/foo")
               ⇒ "/xcssun/users/rms/foo"
               ;; ‘/usr/local/’ has been discarded.


File: elisp.info,  Node: Unique File Names,  Next: File Name Completion,  Prev: File Name Expansion,  Up: File Names

24.8.5 Generating Unique File Names
-----------------------------------

Some programs need to write temporary files.  Here is the usual way to
construct a name for such a file:

     (make-temp-file NAME-OF-APPLICATION)

The job of ‘make-temp-file’ is to prevent two different users or two
different jobs from trying to use the exact same file name.

 -- Function: make-temp-file prefix &optional dir-flag suffix
     This function creates a temporary file and returns its name.  Emacs
     creates the temporary file’s name by adding to PREFIX some random
     characters that are different in each Emacs job.  The result is
     guaranteed to be a newly created empty file.  On MS-DOS, this
     function can truncate the STRING prefix to fit into the 8+3
     file-name limits.  If PREFIX is a relative file name, it is
     expanded against ‘temporary-file-directory’.

          (make-temp-file "foo")
               ⇒ "/tmp/foo232J6v"

     When ‘make-temp-file’ returns, the file has been created and is
     empty.  At that point, you should write the intended contents into
     the file.

     If DIR-FLAG is non-‘nil’, ‘make-temp-file’ creates an empty
     directory instead of an empty file.  It returns the file name, not
     the directory name, of that directory.  *Note Directory Names::.

     If SUFFIX is non-‘nil’, ‘make-temp-file’ adds it at the end of the
     file name.

     To prevent conflicts among different libraries running in the same
     Emacs, each Lisp program that uses ‘make-temp-file’ should have its
     own PREFIX.  The number added to the end of PREFIX distinguishes
     between the same application running in different Emacs jobs.
     Additional added characters permit a large number of distinct names
     even in one Emacs job.

   The default directory for temporary files is controlled by the
variable ‘temporary-file-directory’.  This variable gives the user a
uniform way to specify the directory for all temporary files.  Some
programs use ‘small-temporary-file-directory’ instead, if that is
non-‘nil’.  To use it, you should expand the prefix against the proper
directory before calling ‘make-temp-file’.

 -- User Option: temporary-file-directory
     This variable specifies the directory name for creating temporary
     files.  Its value should be a directory name (*note Directory
     Names::), but it is good for Lisp programs to cope if the value is
     a directory’s file name instead.  Using the value as the second
     argument to ‘expand-file-name’ is a good way to achieve that.

     The default value is determined in a reasonable way for your
     operating system; it is based on the ‘TMPDIR’, ‘TMP’ and ‘TEMP’
     environment variables, with a fall-back to a system-dependent name
     if none of these variables is defined.

     Even if you do not use ‘make-temp-file’ to create the temporary
     file, you should still use this variable to decide which directory
     to put the file in.  However, if you expect the file to be small,
     you should use ‘small-temporary-file-directory’ first if that is
     non-‘nil’.

 -- User Option: small-temporary-file-directory
     This variable specifies the directory name for creating certain
     temporary files, which are likely to be small.

     If you want to write a temporary file which is likely to be small,
     you should compute the directory like this:

          (make-temp-file
            (expand-file-name PREFIX
                              (or small-temporary-file-directory
                                  temporary-file-directory)))

 -- Function: make-temp-name base-name
     This function generates a string that can be used as a unique file
     name.  The name starts with BASE-NAME, and has several random
     characters appended to it, which are different in each Emacs job.
     It is like ‘make-temp-file’ except that (i) it just constructs a
     name, and does not create a file, and (ii) BASE-NAME should be an
     absolute file name (on MS-DOS, this function can truncate BASE-NAME
     to fit into the 8+3 file-name limits).

     *Warning:* In most cases, you should not use this function; use
     ‘make-temp-file’ instead!  This function is susceptible to a race
     condition, between the ‘make-temp-name’ call and the creation of
     the file, which in some cases may cause a security hole.


File: elisp.info,  Node: File Name Completion,  Next: Standard File Names,  Prev: Unique File Names,  Up: File Names

24.8.6 File Name Completion
---------------------------

This section describes low-level subroutines for completing a file name.
For higher level functions, see *note Reading File Names::.

 -- Function: file-name-all-completions partial-filename directory
     This function returns a list of all possible completions for a file
     whose name starts with PARTIAL-FILENAME in directory DIRECTORY.
     The order of the completions is the order of the files in the
     directory, which is unpredictable and conveys no useful
     information.

     The argument PARTIAL-FILENAME must be a file name containing no
     directory part and no slash (or backslash on some systems).  The
     current buffer’s default directory is prepended to DIRECTORY, if
     DIRECTORY is not absolute.

     In the following example, suppose that ‘~rms/lewis’ is the current
     default directory, and has five files whose names begin with ‘f’:
     ‘foo’, ‘file~’, ‘file.c’, ‘file.c.~1~’, and ‘file.c.~2~’.

          (file-name-all-completions "f" "")
               ⇒ ("foo" "file~" "file.c.~2~"
                          "file.c.~1~" "file.c")

          (file-name-all-completions "fo" "")
               ⇒ ("foo")

 -- Function: file-name-completion filename directory &optional
          predicate
     This function completes the file name FILENAME in directory
     DIRECTORY.  It returns the longest prefix common to all file names
     in directory DIRECTORY that start with FILENAME.  If PREDICATE is
     non-‘nil’ then it ignores possible completions that don’t satisfy
     PREDICATE, after calling that function with one argument, the
     expanded absolute file name.

     If only one match exists and FILENAME matches it exactly, the
     function returns ‘t’.  The function returns ‘nil’ if directory
     DIRECTORY contains no name starting with FILENAME.

     In the following example, suppose that the current default
     directory has five files whose names begin with ‘f’: ‘foo’,
     ‘file~’, ‘file.c’, ‘file.c.~1~’, and ‘file.c.~2~’.

          (file-name-completion "fi" "")
               ⇒ "file"

          (file-name-completion "file.c.~1" "")
               ⇒ "file.c.~1~"

          (file-name-completion "file.c.~1~" "")
               ⇒ t

          (file-name-completion "file.c.~3" "")
               ⇒ nil

 -- User Option: completion-ignored-extensions
     ‘file-name-completion’ usually ignores file names that end in any
     string in this list.  It does not ignore them when all the possible
     completions end in one of these suffixes.  This variable has no
     effect on ‘file-name-all-completions’.

     A typical value might look like this:

          completion-ignored-extensions
               ⇒ (".o" ".elc" "~" ".dvi")

     If an element of ‘completion-ignored-extensions’ ends in a slash
     ‘/’, it signals a directory.  The elements which do _not_ end in a
     slash will never match a directory; thus, the above value will not
     filter out a directory named ‘foo.elc’.


File: elisp.info,  Node: Standard File Names,  Prev: File Name Completion,  Up: File Names

24.8.7 Standard File Names
--------------------------

Sometimes, an Emacs Lisp program needs to specify a standard file name
for a particular use—typically, to hold configuration data specified by
the current user.  Usually, such files should be located in the
directory specified by ‘user-emacs-directory’, which is ‘~/.emacs.d’ by
default (*note Init File::).  For example, abbrev definitions are stored
by default in ‘~/.emacs.d/abbrev_defs’.  The easiest way to specify such
a file name is to use the function ‘locate-user-emacs-file’.

 -- Function: locate-user-emacs-file base-name &optional old-name
     This function returns an absolute file name for an Emacs-specific
     configuration or data file.  The argument ‘base-name’ should be a
     relative file name.  The return value is the absolute name of a
     file in the directory specified by ‘user-emacs-directory’; if that
     directory does not exist, this function creates it.

     If the optional argument OLD-NAME is non-‘nil’, it specifies a file
     in the user’s home directory, ‘~/OLD-NAME’.  If such a file exists,
     the return value is the absolute name of that file, instead of the
     file specified by BASE-NAME.  This argument is intended to be used
     by Emacs packages to provide backward compatibility.  For instance,
     prior to the introduction of ‘user-emacs-directory’, the abbrev
     file was located in ‘~/.abbrev_defs’.  Here is the definition of
     ‘abbrev-file-name’:

          (defcustom abbrev-file-name
            (locate-user-emacs-file "abbrev_defs" ".abbrev_defs")
            "Default name of file from which to read abbrevs."
            ...
            :type 'file)

   A lower-level function for standardizing file names, which
‘locate-user-emacs-file’ uses as a subroutine, is
‘convert-standard-filename’.

 -- Function: convert-standard-filename filename
     This function returns a file name based on FILENAME, which fits the
     conventions of the current operating system.

     On GNU and Unix systems, this simply returns FILENAME.  On other
     operating systems, it may enforce system-specific file name
     conventions; for example, on MS-DOS this function performs a
     variety of changes to enforce MS-DOS file name limitations,
     including converting any leading ‘.’ to ‘_’ and truncating to three
     characters after the ‘.’.

     The recommended way to use this function is to specify a name which
     fits the conventions of GNU and Unix systems, and pass it to
     ‘convert-standard-filename’.


File: elisp.info,  Node: Contents of Directories,  Next: Create/Delete Dirs,  Prev: File Names,  Up: Files

24.9 Contents of Directories
============================

A directory is a kind of file that contains other files entered under
various names.  Directories are a feature of the file system.

   Emacs can list the names of the files in a directory as a Lisp list,
or display the names in a buffer using the ‘ls’ shell command.  In the
latter case, it can optionally display information about each file,
depending on the options passed to the ‘ls’ command.

 -- Function: directory-files directory &optional full-name match-regexp
          nosort
     This function returns a list of the names of the files in the
     directory DIRECTORY.  By default, the list is in alphabetical
     order.

     If FULL-NAME is non-‘nil’, the function returns the files’ absolute
     file names.  Otherwise, it returns the names relative to the
     specified directory.

     If MATCH-REGEXP is non-‘nil’, this function returns only those file
     names that contain a match for that regular expression—the other
     file names are excluded from the list.  On case-insensitive
     filesystems, the regular expression matching is case-insensitive.

     If NOSORT is non-‘nil’, ‘directory-files’ does not sort the list,
     so you get the file names in no particular order.  Use this if you
     want the utmost possible speed and don’t care what order the files
     are processed in.  If the order of processing is visible to the
     user, then the user will probably be happier if you do sort the
     names.

          (directory-files "~lewis")
               ⇒ ("#foo#" "#foo.el#" "." ".."
                   "dired-mods.el" "files.texi"
                   "files.texi.~1~")

     An error is signaled if DIRECTORY is not the name of a directory
     that can be read.

 -- Function: directory-files-recursively directory regexp &optional
          include-directories
     Return all files under DIRECTORY whose names match REGEXP.  This
     function searches the specified DIRECTORY and its sub-directories,
     recursively, for files whose basenames (i.e., without the leading
     directories) match the specified REGEXP, and returns a list of the
     absolute file names of the matching files (*note absolute file
     names: Relative File Names.).  The file names are returned in
     depth-first order, meaning that files in some sub-directory are
     returned before the files in its parent directory.  In addition,
     matching files found in each subdirectory are sorted alphabetically
     by their basenames.  By default, directories whose names match
     REGEXP are omitted from the list, but if the optional argument
     INCLUDE-DIRECTORIES is non-‘nil’, they are included.

 -- Function: directory-files-and-attributes directory &optional
          full-name match-regexp nosort id-format
     This is similar to ‘directory-files’ in deciding which files to
     report on and how to report their names.  However, instead of
     returning a list of file names, it returns for each file a list
     ‘(FILENAME . ATTRIBUTES)’, where ATTRIBUTES is what
     ‘file-attributes’ would return for that file.  The optional
     argument ID-FORMAT has the same meaning as the corresponding
     argument to ‘file-attributes’ (*note Definition of
     file-attributes::).

 -- Function: file-expand-wildcards pattern &optional full
     This function expands the wildcard pattern PATTERN, returning a
     list of file names that match it.

     If PATTERN is written as an absolute file name, the values are
     absolute also.

     If PATTERN is written as a relative file name, it is interpreted
     relative to the current default directory.  The file names returned
     are normally also relative to the current default directory.
     However, if FULL is non-‘nil’, they are absolute.

 -- Function: insert-directory file switches &optional wildcard
          full-directory-p
     This function inserts (in the current buffer) a directory listing
     for directory FILE, formatted with ‘ls’ according to SWITCHES.  It
     leaves point after the inserted text.  SWITCHES may be a string of
     options, or a list of strings representing individual options.

     The argument FILE may be either a directory name or a file
     specification including wildcard characters.  If WILDCARD is
     non-‘nil’, that means treat FILE as a file specification with
     wildcards.

     If FULL-DIRECTORY-P is non-‘nil’, that means the directory listing
     is expected to show the full contents of a directory.  You should
     specify ‘t’ when FILE is a directory and switches do not contain
     ‘-d’.  (The ‘-d’ option to ‘ls’ says to describe a directory itself
     as a file, rather than showing its contents.)

     On most systems, this function works by running a directory listing
     program whose name is in the variable ‘insert-directory-program’.
     If WILDCARD is non-‘nil’, it also runs the shell specified by
     ‘shell-file-name’, to expand the wildcards.

     MS-DOS and MS-Windows systems usually lack the standard Unix
     program ‘ls’, so this function emulates the standard Unix program
     ‘ls’ with Lisp code.

     As a technical detail, when SWITCHES contains the long ‘--dired’
     option, ‘insert-directory’ treats it specially, for the sake of
     dired.  However, the normally equivalent short ‘-D’ option is just
     passed on to ‘insert-directory-program’, as any other option.

 -- Variable: insert-directory-program
     This variable’s value is the program to run to generate a directory
     listing for the function ‘insert-directory’.  It is ignored on
     systems which generate the listing with Lisp code.


File: elisp.info,  Node: Create/Delete Dirs,  Next: Magic File Names,  Prev: Contents of Directories,  Up: Files

24.10 Creating, Copying and Deleting Directories
================================================

Most Emacs Lisp file-manipulation functions get errors when used on
files that are directories.  For example, you cannot delete a directory
with ‘delete-file’.  These special functions exist to create and delete
directories.

 -- Command: make-directory dirname &optional parents
     This command creates a directory named DIRNAME.  If PARENTS is
     non-‘nil’, as is always the case in an interactive call, that means
     to create the parent directories first, if they don’t already
     exist.

     ‘mkdir’ is an alias for this.

 -- Command: copy-directory dirname newname &optional keep-time parents
          copy-contents
     This command copies the directory named DIRNAME to NEWNAME.  If
     NEWNAME names an existing directory, DIRNAME will be copied to a
     subdirectory there.

     It always sets the file modes of the copied files to match the
     corresponding original file.

     The third argument KEEP-TIME non-‘nil’ means to preserve the
     modification time of the copied files.  A prefix arg makes
     KEEP-TIME non-‘nil’.

     The fourth argument PARENTS says whether to create parent
     directories if they don’t exist.  Interactively, this happens by
     default.

     The fifth argument COPY-CONTENTS, if non-‘nil’, means to copy the
     contents of DIRNAME directly into NEWNAME if the latter is an
     existing directory, instead of copying DIRNAME into it as a
     subdirectory.

 -- Command: delete-directory dirname &optional recursive trash
     This command deletes the directory named DIRNAME.  The function
     ‘delete-file’ does not work for files that are directories; you
     must use ‘delete-directory’ for them.  If RECURSIVE is ‘nil’, and
     the directory contains any files, ‘delete-directory’ signals an
     error.

     ‘delete-directory’ only follows symbolic links at the level of
     parent directories.

     If the optional argument TRASH is non-‘nil’ and the variable
     ‘delete-by-moving-to-trash’ is non-‘nil’, this command moves the
     file into the system Trash instead of deleting it.  *Note
     Miscellaneous File Operations: (emacs)Misc File Ops.  When called
     interactively, TRASH is ‘t’ if no prefix argument is given, and
     ‘nil’ otherwise.


File: elisp.info,  Node: Magic File Names,  Next: Format Conversion,  Prev: Create/Delete Dirs,  Up: Files

24.11 Making Certain File Names “Magic”
=======================================

You can implement special handling for certain file names.  This is
called making those names “magic”.  The principal use for this feature
is in implementing access to remote files (*note Remote Files:
(emacs)Remote Files.).

   To define a kind of magic file name, you must supply a regular
expression to define the class of names (all those that match the
regular expression), plus a handler that implements all the primitive
Emacs file operations for file names that match.

   The variable ‘file-name-handler-alist’ holds a list of handlers,
together with regular expressions that determine when to apply each
handler.  Each element has this form:

     (REGEXP . HANDLER)

All the Emacs primitives for file access and file name transformation
check the given file name against ‘file-name-handler-alist’.  If the
file name matches REGEXP, the primitives handle that file by calling
HANDLER.

   The first argument given to HANDLER is the name of the primitive, as
a symbol; the remaining arguments are the arguments that were passed to
that primitive.  (The first of these arguments is most often the file
name itself.)  For example, if you do this:

     (file-exists-p FILENAME)

and FILENAME has handler HANDLER, then HANDLER is called like this:

     (funcall HANDLER 'file-exists-p FILENAME)

   When a function takes two or more arguments that must be file names,
it checks each of those names for a handler.  For example, if you do
this:

     (expand-file-name FILENAME DIRNAME)

then it checks for a handler for FILENAME and then for a handler for
DIRNAME.  In either case, the HANDLER is called like this:

     (funcall HANDLER 'expand-file-name FILENAME DIRNAME)

The HANDLER then needs to figure out whether to handle FILENAME or
DIRNAME.

   If the specified file name matches more than one handler, the one
whose match starts last in the file name gets precedence.  This rule is
chosen so that handlers for jobs such as uncompression are handled
first, before handlers for jobs such as remote file access.

   Here are the operations that a magic file name handler gets to
handle:

‘access-file’, ‘add-name-to-file’, ‘byte-compiler-base-file-name’,
‘copy-directory’, ‘copy-file’, ‘delete-directory’, ‘delete-file’,
‘diff-latest-backup-file’, ‘directory-file-name’, ‘directory-files’,
‘directory-files-and-attributes’, ‘dired-compress-file’,
‘dired-uncache’,
‘expand-file-name’, ‘file-accessible-directory-p’, ‘file-acl’,
‘file-attributes’, ‘file-directory-p’, ‘file-equal-p’,
‘file-executable-p’, ‘file-exists-p’, ‘file-in-directory-p’,
‘file-local-copy’, ‘file-modes’, ‘file-name-all-completions’,
‘file-name-as-directory’, ‘file-name-completion’, ‘file-name-directory’,
‘file-name-nondirectory’, ‘file-name-sans-versions’,
‘file-newer-than-file-p’, ‘file-notify-add-watch’,
‘file-notify-rm-watch’, ‘file-notify-valid-p’,
‘file-ownership-preserved-p’, ‘file-readable-p’, ‘file-regular-p’,
‘file-remote-p’, ‘file-selinux-context’, ‘file-symlink-p’,
‘file-truename’, ‘file-writable-p’, ‘find-backup-file-name’,
‘get-file-buffer’, ‘insert-directory’, ‘insert-file-contents’,
‘load’, ‘make-auto-save-file-name’, ‘make-directory’,
‘make-directory-internal’, ‘make-symbolic-link’,
‘process-file’, ‘rename-file’, ‘set-file-acl’, ‘set-file-modes’,
‘set-file-selinux-context’, ‘set-file-times’,
‘set-visited-file-modtime’, ‘shell-command’, ‘start-file-process’,
‘substitute-in-file-name’,
‘unhandled-file-name-directory’, ‘vc-registered’,
‘verify-visited-file-modtime’,
‘write-region’.

   Handlers for ‘insert-file-contents’ typically need to clear the
buffer’s modified flag, with ‘(set-buffer-modified-p nil)’, if the VISIT
argument is non-‘nil’.  This also has the effect of unlocking the buffer
if it is locked.

   The handler function must handle all of the above operations, and
possibly others to be added in the future.  It need not implement all
these operations itself—when it has nothing special to do for a certain
operation, it can reinvoke the primitive, to handle the operation in the
usual way.  It should always reinvoke the primitive for an operation it
does not recognize.  Here’s one way to do this:

     (defun my-file-handler (operation &rest args)
       ;; First check for the specific operations
       ;; that we have special handling for.
       (cond ((eq operation 'insert-file-contents) ...)
             ((eq operation 'write-region) ...)
             ...
             ;; Handle any operation we don’t know about.
             (t (let ((inhibit-file-name-handlers
                       (cons 'my-file-handler
                             (and (eq inhibit-file-name-operation operation)
                                  inhibit-file-name-handlers)))
                      (inhibit-file-name-operation operation))
                  (apply operation args)))))

   When a handler function decides to call the ordinary Emacs primitive
for the operation at hand, it needs to prevent the primitive from
calling the same handler once again, thus leading to an infinite
recursion.  The example above shows how to do this, with the variables
‘inhibit-file-name-handlers’ and ‘inhibit-file-name-operation’.  Be
careful to use them exactly as shown above; the details are crucial for
proper behavior in the case of multiple handlers, and for operations
that have two file names that may each have handlers.

   Handlers that don’t really do anything special for actual access to
the file—such as the ones that implement completion of host names for
remote file names—should have a non-‘nil’ ‘safe-magic’ property.  For
instance, Emacs normally protects directory names it finds in ‘PATH’
from becoming magic, if they look like magic file names, by prefixing
them with ‘/:’.  But if the handler that would be used for them has a
non-‘nil’ ‘safe-magic’ property, the ‘/:’ is not added.

   A file name handler can have an ‘operations’ property to declare
which operations it handles in a nontrivial way.  If this property has a
non-‘nil’ value, it should be a list of operations; then only those
operations will call the handler.  This avoids inefficiency, but its
main purpose is for autoloaded handler functions, so that they won’t be
loaded except when they have real work to do.

   Simply deferring all operations to the usual primitives does not
work.  For instance, if the file name handler applies to
‘file-exists-p’, then it must handle ‘load’ itself, because the usual
‘load’ code won’t work properly in that case.  However, if the handler
uses the ‘operations’ property to say it doesn’t handle ‘file-exists-p’,
then it need not handle ‘load’ nontrivially.

 -- Variable: inhibit-file-name-handlers
     This variable holds a list of handlers whose use is presently
     inhibited for a certain operation.

 -- Variable: inhibit-file-name-operation
     The operation for which certain handlers are presently inhibited.

 -- Function: find-file-name-handler file operation
     This function returns the handler function for file name FILE, or
     ‘nil’ if there is none.  The argument OPERATION should be the
     operation to be performed on the file—the value you will pass to
     the handler as its first argument when you call it.  If OPERATION
     equals ‘inhibit-file-name-operation’, or if it is not found in the
     ‘operations’ property of the handler, this function returns ‘nil’.

 -- Function: file-local-copy filename
     This function copies file FILENAME to an ordinary non-magic file on
     the local machine, if it isn’t on the local machine already.  Magic
     file names should handle the ‘file-local-copy’ operation if they
     refer to files on other machines.  A magic file name that is used
     for other purposes than remote file access should not handle
     ‘file-local-copy’; then this function will treat the file as local.

     If FILENAME is local, whether magic or not, this function does
     nothing and returns ‘nil’.  Otherwise it returns the file name of
     the local copy file.

 -- Function: file-remote-p filename &optional identification connected
     This function tests whether FILENAME is a remote file.  If FILENAME
     is local (not remote), the return value is ‘nil’.  If FILENAME is
     indeed remote, the return value is a string that identifies the
     remote system.

     This identifier string can include a host name and a user name, as
     well as characters designating the method used to access the remote
     system.  For example, the remote identifier string for the filename
     ‘/sudo::/some/file’ is ‘/sudo:root@localhost:’.

     If ‘file-remote-p’ returns the same identifier for two different
     filenames, that means they are stored on the same file system and
     can be accessed locally with respect to each other.  This means,
     for example, that it is possible to start a remote process
     accessing both files at the same time.  Implementers of file
     handlers need to ensure this principle is valid.

     IDENTIFICATION specifies which part of the identifier shall be
     returned as string.  IDENTIFICATION can be the symbol ‘method’,
     ‘user’ or ‘host’; any other value is handled like ‘nil’ and means
     to return the complete identifier string.  In the example above,
     the remote ‘user’ identifier string would be ‘root’.

     If CONNECTED is non-‘nil’, this function returns ‘nil’ even if
     FILENAME is remote, if Emacs has no network connection to its host.
     This is useful when you want to avoid the delay of making
     connections when they don’t exist.

 -- Function: unhandled-file-name-directory filename
     This function returns the name of a directory that is not magic.
     For a non-magic FILENAME it returns the corresponding directory
     name (*note Directory Names::).  For a magic FILENAME, it invokes
     the file name handler, which therefore decides what value to
     return.  If FILENAME is not accessible from a local process, then
     the file name handler should indicate that by returning ‘nil’.

     This is useful for running a subprocess; every subprocess must have
     a non-magic directory to serve as its current directory, and this
     function is a good way to come up with one.

 -- User Option: remote-file-name-inhibit-cache
     The attributes of remote files can be cached for better
     performance.  If they are changed outside of Emacs’s control, the
     cached values become invalid, and must be reread.

     When this variable is set to ‘nil’, cached values are never
     expired.  Use this setting with caution, only if you are sure
     nothing other than Emacs ever changes the remote files.  If it is
     set to ‘t’, cached values are never used.  This is the safest
     value, but could result in performance degradation.

     A compromise is to set it to a positive number.  This means that
     cached values are used for that amount of seconds since they were
     cached.  If a remote file is checked regularly, it might be a good
     idea to let-bind this variable to a value less than the time period
     between consecutive checks.  For example:

          (defun display-time-file-nonempty-p (file)
            (let ((remote-file-name-inhibit-cache
                   (- display-time-interval 5)))
              (and (file-exists-p file)
                   (< 0 (nth 7 (file-attributes
                                 (file-chase-links file)))))))


File: elisp.info,  Node: Format Conversion,  Prev: Magic File Names,  Up: Files

24.12 File Format Conversion
============================

Emacs performs several steps to convert the data in a buffer (text, text
properties, and possibly other information) to and from a representation
suitable for storing into a file.  This section describes the
fundamental functions that perform this “format conversion”, namely
‘insert-file-contents’ for reading a file into a buffer, and
‘write-region’ for writing a buffer into a file.

* Menu:

* Overview: Format Conversion Overview.     ‘insert-file-contents’ and ‘write-region’.
* Round-Trip: Format Conversion Round-Trip. Using ‘format-alist’.
* Piecemeal: Format Conversion Piecemeal.   Specifying non-paired conversion.


File: elisp.info,  Node: Format Conversion Overview,  Next: Format Conversion Round-Trip,  Up: Format Conversion

24.12.1 Overview
----------------

The function ‘insert-file-contents’:

   • initially, inserts bytes from the file into the buffer;
   • decodes bytes to characters as appropriate;
   • processes formats as defined by entries in ‘format-alist’; and
   • calls functions in ‘after-insert-file-functions’.

The function ‘write-region’:

   • initially, calls functions in ‘write-region-annotate-functions’;
   • processes formats as defined by entries in ‘format-alist’;
   • encodes characters to bytes as appropriate; and
   • modifies the file with the bytes.

   This shows the symmetry of the lowest-level operations; reading and
writing handle things in opposite order.  The rest of this section
describes the two facilities surrounding the three variables named
above, as well as some related functions.  *note Coding Systems::, for
details on character encoding and decoding.


File: elisp.info,  Node: Format Conversion Round-Trip,  Next: Format Conversion Piecemeal,  Prev: Format Conversion Overview,  Up: Format Conversion

24.12.2 Round-Trip Specification
--------------------------------

The most general of the two facilities is controlled by the variable
‘format-alist’, a list of “file format” specifications, which describe
textual representations used in files for the data in an Emacs buffer.
The descriptions for reading and writing are paired, which is why we
call this “round-trip” specification (*note Format Conversion
Piecemeal::, for non-paired specification).

 -- Variable: format-alist
     This list contains one format definition for each defined file
     format.  Each format definition is a list of this form:

          (NAME DOC-STRING REGEXP FROM-FN TO-FN MODIFY MODE-FN PRESERVE)

Here is what the elements in a format definition mean:

NAME
     The name of this format.

DOC-STRING
     A documentation string for the format.

REGEXP
     A regular expression which is used to recognize files represented
     in this format.  If ‘nil’, the format is never applied
     automatically.

FROM-FN
     A shell command or function to decode data in this format (to
     convert file data into the usual Emacs data representation).

     A shell command is represented as a string; Emacs runs the command
     as a filter to perform the conversion.

     If FROM-FN is a function, it is called with two arguments, BEGIN
     and END, which specify the part of the buffer it should convert.
     It should convert the text by editing it in place.  Since this can
     change the length of the text, FROM-FN should return the modified
     end position.

     One responsibility of FROM-FN is to make sure that the beginning of
     the file no longer matches REGEXP.  Otherwise it is likely to get
     called again.

TO-FN
     A shell command or function to encode data in this format—that is,
     to convert the usual Emacs data representation into this format.

     If TO-FN is a string, it is a shell command; Emacs runs the command
     as a filter to perform the conversion.

     If TO-FN is a function, it is called with three arguments: BEGIN
     and END, which specify the part of the buffer it should convert,
     and BUFFER, which specifies which buffer.  There are two ways it
     can do the conversion:

        • By editing the buffer in place.  In this case, TO-FN should
          return the end-position of the range of text, as modified.

        • By returning a list of annotations.  This is a list of
          elements of the form ‘(POSITION . STRING)’, where POSITION is
          an integer specifying the relative position in the text to be
          written, and STRING is the annotation to add there.  The list
          must be sorted in order of position when TO-FN returns it.

          When ‘write-region’ actually writes the text from the buffer
          to the file, it intermixes the specified annotations at the
          corresponding positions.  All this takes place without
          modifying the buffer.

MODIFY
     A flag, ‘t’ if the encoding function modifies the buffer, and ‘nil’
     if it works by returning a list of annotations.

MODE-FN
     A minor-mode function to call after visiting a file converted from
     this format.  The function is called with one argument, the integer
     1; that tells a minor-mode function to enable the mode.

PRESERVE
     A flag, ‘t’ if ‘format-write-file’ should not remove this format
     from ‘buffer-file-format’.

   The function ‘insert-file-contents’ automatically recognizes file
formats when it reads the specified file.  It checks the text of the
beginning of the file against the regular expressions of the format
definitions, and if it finds a match, it calls the decoding function for
that format.  Then it checks all the known formats over again.  It keeps
checking them until none of them is applicable.

   Visiting a file, with ‘find-file-noselect’ or the commands that use
it, performs conversion likewise (because it calls
‘insert-file-contents’); it also calls the mode function for each format
that it decodes.  It stores a list of the format names in the
buffer-local variable ‘buffer-file-format’.

 -- Variable: buffer-file-format
     This variable states the format of the visited file.  More
     precisely, this is a list of the file format names that were
     decoded in the course of visiting the current buffer’s file.  It is
     always buffer-local in all buffers.

   When ‘write-region’ writes data into a file, it first calls the
encoding functions for the formats listed in ‘buffer-file-format’, in
the order of appearance in the list.

 -- Command: format-write-file file format &optional confirm
     This command writes the current buffer contents into the file FILE
     in a format based on FORMAT, which is a list of format names.  It
     constructs the actual format starting from FORMAT, then appending
     any elements from the value of ‘buffer-file-format’ with a
     non-‘nil’ PRESERVE flag (see above), if they are not already
     present in FORMAT.  It then updates ‘buffer-file-format’ with this
     format, making it the default for future saves.  Except for the
     FORMAT argument, this command is similar to ‘write-file’.  In
     particular, CONFIRM has the same meaning and interactive treatment
     as the corresponding argument to ‘write-file’.  *Note Definition of
     write-file::.

 -- Command: format-find-file file format
     This command finds the file FILE, converting it according to format
     FORMAT.  It also makes FORMAT the default if the buffer is saved
     later.

     The argument FORMAT is a list of format names.  If FORMAT is ‘nil’,
     no conversion takes place.  Interactively, typing just <RET> for
     FORMAT specifies ‘nil’.

 -- Command: format-insert-file file format &optional beg end
     This command inserts the contents of file FILE, converting it
     according to format FORMAT.  If BEG and END are non-‘nil’, they
     specify which part of the file to read, as in
     ‘insert-file-contents’ (*note Reading from Files::).

     The return value is like what ‘insert-file-contents’ returns: a
     list of the absolute file name and the length of the data inserted
     (after conversion).

     The argument FORMAT is a list of format names.  If FORMAT is ‘nil’,
     no conversion takes place.  Interactively, typing just <RET> for
     FORMAT specifies ‘nil’.

 -- Variable: buffer-auto-save-file-format
     This variable specifies the format to use for auto-saving.  Its
     value is a list of format names, just like the value of
     ‘buffer-file-format’; however, it is used instead of
     ‘buffer-file-format’ for writing auto-save files.  If the value is
     ‘t’, the default, auto-saving uses the same format as a regular
     save in the same buffer.  This variable is always buffer-local in
     all buffers.


File: elisp.info,  Node: Format Conversion Piecemeal,  Prev: Format Conversion Round-Trip,  Up: Format Conversion

24.12.3 Piecemeal Specification
-------------------------------

In contrast to the round-trip specification described in the previous
subsection (*note Format Conversion Round-Trip::), you can use the
variables ‘after-insert-file-functions’ and
‘write-region-annotate-functions’ to separately control the respective
reading and writing conversions.

   Conversion starts with one representation and produces another
representation.  When there is only one conversion to do, there is no
conflict about what to start with.  However, when there are multiple
conversions involved, conflict may arise when two conversions need to
start with the same data.

   This situation is best understood in the context of converting text
properties during ‘write-region’.  For example, the character at
position 42 in a buffer is ‘X’ with a text property ‘foo’.  If the
conversion for ‘foo’ is done by inserting into the buffer, say, ‘FOO:’,
then that changes the character at position 42 from ‘X’ to ‘F’.  The
next conversion will start with the wrong data straight away.

   To avoid conflict, cooperative conversions do not modify the buffer,
but instead specify “annotations”, a list of elements of the form
‘(POSITION . STRING)’, sorted in order of increasing POSITION.

   If there is more than one conversion, ‘write-region’ merges their
annotations destructively into one sorted list.  Later, when the text
from the buffer is actually written to the file, it intermixes the
specified annotations at the corresponding positions.  All this takes
place without modifying the buffer.

   In contrast, when reading, the annotations intermixed with the text
are handled immediately.  ‘insert-file-contents’ sets point to the
beginning of some text to be converted, then calls the conversion
functions with the length of that text.  These functions should always
return with point at the beginning of the inserted text.  This approach
makes sense for reading because annotations removed by the first
converter can’t be mistakenly processed by a later converter.  Each
conversion function should scan for the annotations it recognizes,
remove the annotation, modify the buffer text (to set a text property,
for example), and return the updated length of the text, as it stands
after those changes.  The value returned by one function becomes the
argument to the next function.

 -- Variable: write-region-annotate-functions
     A list of functions for ‘write-region’ to call.  Each function in
     the list is called with two arguments: the start and end of the
     region to be written.  These functions should not alter the
     contents of the buffer.  Instead, they should return annotations.

     As a special case, a function may return with a different buffer
     current.  Emacs takes this to mean that the current buffer contains
     altered text to be output.  It therefore changes the START and END
     arguments of the ‘write-region’ call, giving them the values of
     ‘point-min’ and ‘point-max’ in the new buffer, respectively.  It
     also discards all previous annotations, because they should have
     been dealt with by this function.

 -- Variable: write-region-post-annotation-function
     The value of this variable, if non-‘nil’, should be a function.
     This function is called, with no arguments, after ‘write-region’
     has completed.

     If any function in ‘write-region-annotate-functions’ returns with a
     different buffer current, Emacs calls
     ‘write-region-post-annotation-function’ more than once.  Emacs
     calls it with the last buffer that was current, and again with the
     buffer before that, and so on back to the original buffer.

     Thus, a function in ‘write-region-annotate-functions’ can create a
     buffer, give this variable the local value of ‘kill-buffer’ in that
     buffer, set up the buffer with altered text, and make the buffer
     current.  The buffer will be killed after ‘write-region’ is done.

 -- Variable: after-insert-file-functions
     Each function in this list is called by ‘insert-file-contents’ with
     one argument, the number of characters inserted, and with point at
     the beginning of the inserted text.  Each function should leave
     point unchanged, and return the new character count describing the
     inserted text as modified by the function.

   We invite users to write Lisp programs to store and retrieve text
properties in files, using these hooks, and thus to experiment with
various data formats and find good ones.  Eventually we hope users will
produce good, general extensions we can install in Emacs.

   We suggest not trying to handle arbitrary Lisp objects as text
property names or values—because a program that general is probably
difficult to write, and slow.  Instead, choose a set of possible data
types that are reasonably flexible, and not too hard to encode.


File: elisp.info,  Node: Backups and Auto-Saving,  Next: Buffers,  Prev: Files,  Up: Top

25 Backups and Auto-Saving
**************************

Backup files and auto-save files are two methods by which Emacs tries to
protect the user from the consequences of crashes or of the user’s own
errors.  Auto-saving preserves the text from earlier in the current
editing session; backup files preserve file contents prior to the
current session.

* Menu:

* Backup Files::   How backup files are made; how their names are chosen.
* Auto-Saving::    How auto-save files are made; how their names are chosen.
* Reverting::      ‘revert-buffer’, and how to customize what it does.


File: elisp.info,  Node: Backup Files,  Next: Auto-Saving,  Up: Backups and Auto-Saving

25.1 Backup Files
=================

A “backup file” is a copy of the old contents of a file you are editing.
Emacs makes a backup file the first time you save a buffer into its
visited file.  Thus, normally, the backup file contains the contents of
the file as it was before the current editing session.  The contents of
the backup file normally remain unchanged once it exists.

   Backups are usually made by renaming the visited file to a new name.
Optionally, you can specify that backup files should be made by copying
the visited file.  This choice makes a difference for files with
multiple names; it also can affect whether the edited file remains owned
by the original owner or becomes owned by the user editing it.

   By default, Emacs makes a single backup file for each file edited.
You can alternatively request numbered backups; then each new backup
file gets a new name.  You can delete old numbered backups when you
don’t want them any more, or Emacs can delete them automatically.

* Menu:

* Making Backups::     How Emacs makes backup files, and when.
* Rename or Copy::     Two alternatives: renaming the old file or copying it.
* Numbered Backups::   Keeping multiple backups for each source file.
* Backup Names::       How backup file names are computed; customization.


File: elisp.info,  Node: Making Backups,  Next: Rename or Copy,  Up: Backup Files

25.1.1 Making Backup Files
--------------------------

 -- Function: backup-buffer
     This function makes a backup of the file visited by the current
     buffer, if appropriate.  It is called by ‘save-buffer’ before
     saving the buffer the first time.

     If a backup was made by renaming, the return value is a cons cell
     of the form (MODES EXTRA-ALIST BACKUPNAME), where MODES are the
     mode bits of the original file, as returned by ‘file-modes’ (*note
     Testing Accessibility::), EXTRA-ALIST is an alist describing the
     original file’s extended attributes, as returned by
     ‘file-extended-attributes’ (*note Extended Attributes::), and
     BACKUPNAME is the name of the backup.

     In all other cases (i.e., if a backup was made by copying or if no
     backup was made), this function returns ‘nil’.

 -- Variable: buffer-backed-up
     This buffer-local variable says whether this buffer’s file has been
     backed up on account of this buffer.  If it is non-‘nil’, the
     backup file has been written.  Otherwise, the file should be backed
     up when it is next saved (if backups are enabled).  This is a
     permanent local; ‘kill-all-local-variables’ does not alter it.

 -- User Option: make-backup-files
     This variable determines whether or not to make backup files.  If
     it is non-‘nil’, then Emacs creates a backup of each file when it
     is saved for the first time—provided that ‘backup-inhibited’ is
     ‘nil’ (see below).

     The following example shows how to change the ‘make-backup-files’
     variable only in the Rmail buffers and not elsewhere.  Setting it
     ‘nil’ stops Emacs from making backups of these files, which may
     save disk space.  (You would put this code in your init file.)

          (add-hook 'rmail-mode-hook
                    (lambda () (setq-local make-backup-files nil)))

 -- Variable: backup-enable-predicate
     This variable’s value is a function to be called on certain
     occasions to decide whether a file should have backup files.  The
     function receives one argument, an absolute file name to consider.
     If the function returns ‘nil’, backups are disabled for that file.
     Otherwise, the other variables in this section say whether and how
     to make backups.

     The default value is ‘normal-backup-enable-predicate’, which checks
     for files in ‘temporary-file-directory’ and
     ‘small-temporary-file-directory’.

 -- Variable: backup-inhibited
     If this variable is non-‘nil’, backups are inhibited.  It records
     the result of testing ‘backup-enable-predicate’ on the visited file
     name.  It can also coherently be used by other mechanisms that
     inhibit backups based on which file is visited.  For example, VC
     sets this variable non-‘nil’ to prevent making backups for files
     managed with a version control system.

     This is a permanent local, so that changing the major mode does not
     lose its value.  Major modes should not set this variable—they
     should set ‘make-backup-files’ instead.

 -- User Option: backup-directory-alist
     This variable’s value is an alist of filename patterns and backup
     directory names.  Each element looks like
          (REGEXP . DIRECTORY)

     Backups of files with names matching REGEXP will be made in
     DIRECTORY.  DIRECTORY may be relative or absolute.  If it is
     absolute, so that all matching files are backed up into the same
     directory, the file names in this directory will be the full name
     of the file backed up with all directory separators changed to ‘!’
     to prevent clashes.  This will not work correctly if your
     filesystem truncates the resulting name.

     For the common case of all backups going into one directory, the
     alist should contain a single element pairing ‘"."’ with the
     appropriate directory name.

     If this variable is ‘nil’ (the default), or it fails to match a
     filename, the backup is made in the original file’s directory.

     On MS-DOS filesystems without long names this variable is always
     ignored.

 -- User Option: make-backup-file-name-function
     This variable’s value is a function to use for making backup file
     names.  The function ‘make-backup-file-name’ calls it.  *Note
     Naming Backup Files: Backup Names.

     This could be buffer-local to do something special for specific
     files.  If you change it, you may need to change
     ‘backup-file-name-p’ and ‘file-name-sans-versions’ too.


File: elisp.info,  Node: Rename or Copy,  Next: Numbered Backups,  Prev: Making Backups,  Up: Backup Files

25.1.2 Backup by Renaming or by Copying?
----------------------------------------

There are two ways that Emacs can make a backup file:

   • Emacs can rename the original file so that it becomes a backup
     file, and then write the buffer being saved into a new file.  After
     this procedure, any other names (i.e., hard links) of the original
     file now refer to the backup file.  The new file is owned by the
     user doing the editing, and its group is the default for new files
     written by the user in that directory.

   • Emacs can copy the original file into a backup file, and then
     overwrite the original file with new contents.  After this
     procedure, any other names (i.e., hard links) of the original file
     continue to refer to the current (updated) version of the file.
     The file’s owner and group will be unchanged.

   The first method, renaming, is the default.

   The variable ‘backup-by-copying’, if non-‘nil’, says to use the
second method, which is to copy the original file and overwrite it with
the new buffer contents.  The variable ‘file-precious-flag’, if
non-‘nil’, also has this effect (as a sideline of its main
significance).  *Note Saving Buffers::.

 -- User Option: backup-by-copying
     If this variable is non-‘nil’, Emacs always makes backup files by
     copying.  The default is ‘nil’.

   The following three variables, when non-‘nil’, cause the second
method to be used in certain special cases.  They have no effect on the
treatment of files that don’t fall into the special cases.

 -- User Option: backup-by-copying-when-linked
     If this variable is non-‘nil’, Emacs makes backups by copying for
     files with multiple names (hard links).  The default is ‘nil’.

     This variable is significant only if ‘backup-by-copying’ is ‘nil’,
     since copying is always used when that variable is non-‘nil’.

 -- User Option: backup-by-copying-when-mismatch
     If this variable is non-‘nil’ (the default), Emacs makes backups by
     copying in cases where renaming would change either the owner or
     the group of the file.

     The value has no effect when renaming would not alter the owner or
     group of the file; that is, for files which are owned by the user
     and whose group matches the default for a new file created there by
     the user.

     This variable is significant only if ‘backup-by-copying’ is ‘nil’,
     since copying is always used when that variable is non-‘nil’.

 -- User Option: backup-by-copying-when-privileged-mismatch
     This variable, if non-‘nil’, specifies the same behavior as
     ‘backup-by-copying-when-mismatch’, but only for certain user-id
     values: namely, those less than or equal to a certain number.  You
     set this variable to that number.

     Thus, if you set ‘backup-by-copying-when-privileged-mismatch’ to 0,
     backup by copying is done for the superuser only, when necessary to
     prevent a change in the owner of the file.

     The default is 200.


File: elisp.info,  Node: Numbered Backups,  Next: Backup Names,  Prev: Rename or Copy,  Up: Backup Files

25.1.3 Making and Deleting Numbered Backup Files
------------------------------------------------

If a file’s name is ‘foo’, the names of its numbered backup versions are
‘foo.~V~’, for various integers V, like this: ‘foo.~1~’, ‘foo.~2~’,
‘foo.~3~’, ..., ‘foo.~259~’, and so on.

 -- User Option: version-control
     This variable controls whether to make a single non-numbered backup
     file or multiple numbered backups.

     ‘nil’
          Make numbered backups if the visited file already has numbered
          backups; otherwise, do not.  This is the default.

     ‘never’
          Do not make numbered backups.

     ANYTHING ELSE
          Make numbered backups.

   The use of numbered backups ultimately leads to a large number of
backup versions, which must then be deleted.  Emacs can do this
automatically or it can ask the user whether to delete them.

 -- User Option: kept-new-versions
     The value of this variable is the number of newest versions to keep
     when a new numbered backup is made.  The newly made backup is
     included in the count.  The default value is 2.

 -- User Option: kept-old-versions
     The value of this variable is the number of oldest versions to keep
     when a new numbered backup is made.  The default value is 2.

   If there are backups numbered 1, 2, 3, 5, and 7, and both of these
variables have the value 2, then the backups numbered 1 and 2 are kept
as old versions and those numbered 5 and 7 are kept as new versions;
backup version 3 is excess.  The function ‘find-backup-file-name’ (*note
Backup Names::) is responsible for determining which backup versions to
delete, but does not delete them itself.

 -- User Option: delete-old-versions
     If this variable is ‘t’, then saving a file deletes excess backup
     versions silently.  If it is ‘nil’, that means to ask for
     confirmation before deleting excess backups.  Otherwise, they are
     not deleted at all.

 -- User Option: dired-kept-versions
     This variable specifies how many of the newest backup versions to
     keep in the Dired command ‘.’ (‘dired-clean-directory’).  That’s
     the same thing ‘kept-new-versions’ specifies when you make a new
     backup file.  The default is 2.


File: elisp.info,  Node: Backup Names,  Prev: Numbered Backups,  Up: Backup Files

25.1.4 Naming Backup Files
--------------------------

The functions in this section are documented mainly because you can
customize the naming conventions for backup files by redefining them.
If you change one, you probably need to change the rest.

 -- Function: backup-file-name-p filename
     This function returns a non-‘nil’ value if FILENAME is a possible
     name for a backup file.  It just checks the name, not whether a
     file with the name FILENAME exists.

          (backup-file-name-p "foo")
               ⇒ nil
          (backup-file-name-p "foo~")
               ⇒ 3

     The standard definition of this function is as follows:

          (defun backup-file-name-p (file)
            "Return non-nil if FILE is a backup file \
          name (numeric or not)..."
            (string-match "~\\'" file))

     Thus, the function returns a non-‘nil’ value if the file name ends
     with a ‘~’.  (We use a backslash to split the documentation
     string’s first line into two lines in the text, but produce just
     one line in the string itself.)

     This simple expression is placed in a separate function to make it
     easy to redefine for customization.

 -- Function: make-backup-file-name filename
     This function returns a string that is the name to use for a
     non-numbered backup file for file FILENAME.  On Unix, this is just
     FILENAME with a tilde appended.

     The standard definition of this function, on most operating
     systems, is as follows:

          (defun make-backup-file-name (file)
            "Create the non-numeric backup file name for FILE..."
            (concat file "~"))

     You can change the backup-file naming convention by redefining this
     function.  The following example redefines ‘make-backup-file-name’
     to prepend a ‘.’ in addition to appending a tilde:

          (defun make-backup-file-name (filename)
            (expand-file-name
              (concat "." (file-name-nondirectory filename) "~")
              (file-name-directory filename)))

          (make-backup-file-name "backups.texi")
               ⇒ ".backups.texi~"

     Some parts of Emacs, including some Dired commands, assume that
     backup file names end with ‘~’.  If you do not follow that
     convention, it will not cause serious problems, but these commands
     may give less-than-desirable results.

 -- Function: find-backup-file-name filename
     This function computes the file name for a new backup file for
     FILENAME.  It may also propose certain existing backup files for
     deletion.  ‘find-backup-file-name’ returns a list whose CAR is the
     name for the new backup file and whose CDR is a list of backup
     files whose deletion is proposed.  The value can also be ‘nil’,
     which means not to make a backup.

     Two variables, ‘kept-old-versions’ and ‘kept-new-versions’,
     determine which backup versions should be kept.  This function
     keeps those versions by excluding them from the CDR of the value.
     *Note Numbered Backups::.

     In this example, the value says that ‘~rms/foo.~5~’ is the name to
     use for the new backup file, and ‘~rms/foo.~3~’ is an excess
     version that the caller should consider deleting now.

          (find-backup-file-name "~rms/foo")
               ⇒ ("~rms/foo.~5~" "~rms/foo.~3~")

 -- Function: file-newest-backup filename
     This function returns the name of the most recent backup file for
     FILENAME, or ‘nil’ if that file has no backup files.

     Some file comparison commands use this function so that they can
     automatically compare a file with its most recent backup.


File: elisp.info,  Node: Auto-Saving,  Next: Reverting,  Prev: Backup Files,  Up: Backups and Auto-Saving

25.2 Auto-Saving
================

Emacs periodically saves all files that you are visiting; this is called
“auto-saving”.  Auto-saving prevents you from losing more than a limited
amount of work if the system crashes.  By default, auto-saves happen
every 300 keystrokes, or after around 30 seconds of idle time.  *Note
Auto Save: (emacs)Auto Save, for information on auto-save for users.
Here we describe the functions used to implement auto-saving and the
variables that control them.

 -- Variable: buffer-auto-save-file-name
     This buffer-local variable is the name of the file used for
     auto-saving the current buffer.  It is ‘nil’ if the buffer should
     not be auto-saved.

          buffer-auto-save-file-name
               ⇒ "/xcssun/users/rms/lewis/#backups.texi#"

 -- Command: auto-save-mode arg
     This is the mode command for Auto Save mode, a buffer-local minor
     mode.  When Auto Save mode is enabled, auto-saving is enabled in
     the buffer.  The calling convention is the same as for other minor
     mode commands (*note Minor Mode Conventions::).

     Unlike most minor modes, there is no ‘auto-save-mode’ variable.
     Auto Save mode is enabled if ‘buffer-auto-save-file-name’ is
     non-‘nil’ and ‘buffer-saved-size’ (see below) is non-zero.

 -- Function: auto-save-file-name-p filename
     This function returns a non-‘nil’ value if FILENAME is a string
     that could be the name of an auto-save file.  It assumes the usual
     naming convention for auto-save files: a name that begins and ends
     with hash marks (‘#’) is a possible auto-save file name.  The
     argument FILENAME should not contain a directory part.

          (make-auto-save-file-name)
               ⇒ "/xcssun/users/rms/lewis/#backups.texi#"
          (auto-save-file-name-p "#backups.texi#")
               ⇒ 0
          (auto-save-file-name-p "backups.texi")
               ⇒ nil

     The standard definition of this function is as follows:

          (defun auto-save-file-name-p (filename)
            "Return non-nil if FILENAME can be yielded by..."
            (string-match "^#.*#$" filename))

     This function exists so that you can customize it if you wish to
     change the naming convention for auto-save files.  If you redefine
     it, be sure to redefine the function ‘make-auto-save-file-name’
     correspondingly.

 -- Function: make-auto-save-file-name
     This function returns the file name to use for auto-saving the
     current buffer.  This is just the file name with hash marks (‘#’)
     prepended and appended to it.  This function does not look at the
     variable ‘auto-save-visited-file-name’ (described below); callers
     of this function should check that variable first.

          (make-auto-save-file-name)
               ⇒ "/xcssun/users/rms/lewis/#backups.texi#"

     Here is a simplified version of the standard definition of this
     function:

          (defun make-auto-save-file-name ()
            "Return file name to use for auto-saves \
          of current buffer.."
            (if buffer-file-name
                (concat
                 (file-name-directory buffer-file-name)
                 "#"
                 (file-name-nondirectory buffer-file-name)
                 "#")
              (expand-file-name
               (concat "#%" (buffer-name) "#"))))

     This exists as a separate function so that you can redefine it to
     customize the naming convention for auto-save files.  Be sure to
     change ‘auto-save-file-name-p’ in a corresponding way.

 -- User Option: auto-save-visited-file-name
     If this variable is non-‘nil’, Emacs auto-saves buffers in the
     files they are visiting.  That is, the auto-save is done in the
     same file that you are editing.  Normally, this variable is ‘nil’,
     so auto-save files have distinct names that are created by
     ‘make-auto-save-file-name’.

     When you change the value of this variable, the new value does not
     take effect in an existing buffer until the next time auto-save
     mode is reenabled in it.  If auto-save mode is already enabled,
     auto-saves continue to go in the same file name until
     ‘auto-save-mode’ is called again.

     Note that setting this variable to a non-‘nil’ value does not
     change the fact that auto-saving is different from saving the
     buffer; e.g., the hooks described in *note Saving Buffers:: are
     _not_ run when a buffer is auto-saved.

 -- Function: recent-auto-save-p
     This function returns ‘t’ if the current buffer has been auto-saved
     since the last time it was read in or saved.

 -- Function: set-buffer-auto-saved
     This function marks the current buffer as auto-saved.  The buffer
     will not be auto-saved again until the buffer text is changed
     again.  The function returns ‘nil’.

 -- User Option: auto-save-interval
     The value of this variable specifies how often to do auto-saving,
     in terms of number of input events.  Each time this many additional
     input events are read, Emacs does auto-saving for all buffers in
     which that is enabled.  Setting this to zero disables autosaving
     based on the number of characters typed.

 -- User Option: auto-save-timeout
     The value of this variable is the number of seconds of idle time
     that should cause auto-saving.  Each time the user pauses for this
     long, Emacs does auto-saving for all buffers in which that is
     enabled.  (If the current buffer is large, the specified timeout is
     multiplied by a factor that increases as the size increases; for a
     million-byte buffer, the factor is almost 4.)

     If the value is zero or ‘nil’, then auto-saving is not done as a
     result of idleness, only after a certain number of input events as
     specified by ‘auto-save-interval’.

 -- Variable: auto-save-hook
     This normal hook is run whenever an auto-save is about to happen.

 -- User Option: auto-save-default
     If this variable is non-‘nil’, buffers that are visiting files have
     auto-saving enabled by default.  Otherwise, they do not.

 -- Command: do-auto-save &optional no-message current-only
     This function auto-saves all buffers that need to be auto-saved.
     It saves all buffers for which auto-saving is enabled and that have
     been changed since the previous auto-save.

     If any buffers are auto-saved, ‘do-auto-save’ normally displays a
     message saying ‘Auto-saving...’ in the echo area while auto-saving
     is going on.  However, if NO-MESSAGE is non-‘nil’, the message is
     inhibited.

     If CURRENT-ONLY is non-‘nil’, only the current buffer is
     auto-saved.

 -- Function: delete-auto-save-file-if-necessary &optional force
     This function deletes the current buffer’s auto-save file if
     ‘delete-auto-save-files’ is non-‘nil’.  It is called every time a
     buffer is saved.

     Unless FORCE is non-‘nil’, this function only deletes the file if
     it was written by the current Emacs session since the last true
     save.

 -- User Option: delete-auto-save-files
     This variable is used by the function
     ‘delete-auto-save-file-if-necessary’.  If it is non-‘nil’, Emacs
     deletes auto-save files when a true save is done (in the visited
     file).  This saves disk space and unclutters your directory.

 -- Function: rename-auto-save-file
     This function adjusts the current buffer’s auto-save file name if
     the visited file name has changed.  It also renames an existing
     auto-save file, if it was made in the current Emacs session.  If
     the visited file name has not changed, this function does nothing.

 -- Variable: buffer-saved-size
     The value of this buffer-local variable is the length of the
     current buffer, when it was last read in, saved, or auto-saved.
     This is used to detect a substantial decrease in size, and turn off
     auto-saving in response.

     If it is −1, that means auto-saving is temporarily shut off in this
     buffer due to a substantial decrease in size.  Explicitly saving
     the buffer stores a positive value in this variable, thus
     reenabling auto-saving.  Turning auto-save mode off or on also
     updates this variable, so that the substantial decrease in size is
     forgotten.

     If it is −2, that means this buffer should disregard changes in
     buffer size; in particular, it should not shut off auto-saving
     temporarily due to changes in buffer size.

 -- Variable: auto-save-list-file-name
     This variable (if non-‘nil’) specifies a file for recording the
     names of all the auto-save files.  Each time Emacs does
     auto-saving, it writes two lines into this file for each buffer
     that has auto-saving enabled.  The first line gives the name of the
     visited file (it’s empty if the buffer has none), and the second
     gives the name of the auto-save file.

     When Emacs exits normally, it deletes this file; if Emacs crashes,
     you can look in the file to find all the auto-save files that might
     contain work that was otherwise lost.  The ‘recover-session’
     command uses this file to find them.

     The default name for this file specifies your home directory and
     starts with ‘.saves-’.  It also contains the Emacs process ID and
     the host name.

 -- User Option: auto-save-list-file-prefix
     After Emacs reads your init file, it initializes
     ‘auto-save-list-file-name’ (if you have not already set it
     non-‘nil’) based on this prefix, adding the host name and process
     ID.  If you set this to ‘nil’ in your init file, then Emacs does
     not initialize ‘auto-save-list-file-name’.


File: elisp.info,  Node: Reverting,  Prev: Auto-Saving,  Up: Backups and Auto-Saving

25.3 Reverting
==============

If you have made extensive changes to a file and then change your mind
about them, you can get rid of them by reading in the previous version
of the file with the ‘revert-buffer’ command.  *Note Reverting a Buffer:
(emacs)Reverting.

 -- Command: revert-buffer &optional ignore-auto noconfirm
          preserve-modes
     This command replaces the buffer text with the text of the visited
     file on disk.  This action undoes all changes since the file was
     visited or saved.

     By default, if the latest auto-save file is more recent than the
     visited file, and the argument IGNORE-AUTO is ‘nil’,
     ‘revert-buffer’ asks the user whether to use that auto-save
     instead.  When you invoke this command interactively, IGNORE-AUTO
     is ‘t’ if there is no numeric prefix argument; thus, the
     interactive default is not to check the auto-save file.

     Normally, ‘revert-buffer’ asks for confirmation before it changes
     the buffer; but if the argument NOCONFIRM is non-‘nil’,
     ‘revert-buffer’ does not ask for confirmation.

     Normally, this command reinitializes the buffer’s major and minor
     modes using ‘normal-mode’.  But if PRESERVE-MODES is non-‘nil’, the
     modes remain unchanged.

     Reverting tries to preserve marker positions in the buffer by using
     the replacement feature of ‘insert-file-contents’.  If the buffer
     contents and the file contents are identical before the revert
     operation, reverting preserves all the markers.  If they are not
     identical, reverting does change the buffer; in that case, it
     preserves the markers in the unchanged text (if any) at the
     beginning and end of the buffer.  Preserving any additional markers
     would be problematical.

 -- Variable: revert-buffer-in-progress-p
     ‘revert-buffer’ binds this variable to a non-‘nil’ value while it
     is working.

   You can customize how ‘revert-buffer’ does its work by setting the
variables described in the rest of this section.

 -- User Option: revert-without-query
     This variable holds a list of files that should be reverted without
     query.  The value is a list of regular expressions.  If the visited
     file name matches one of these regular expressions, and the file
     has changed on disk but the buffer is not modified, then
     ‘revert-buffer’ reverts the file without asking the user for
     confirmation.

   Some major modes customize ‘revert-buffer’ by making buffer-local
bindings for these variables:

 -- Variable: revert-buffer-function
     The value of this variable is the function to use to revert this
     buffer.  It should be a function with two optional arguments to do
     the work of reverting.  The two optional arguments, IGNORE-AUTO and
     NOCONFIRM, are the arguments that ‘revert-buffer’ received.

     Modes such as Dired mode, in which the text being edited does not
     consist of a file’s contents but can be regenerated in some other
     fashion, can give this variable a buffer-local value that is a
     special function to regenerate the contents.

 -- Variable: revert-buffer-insert-file-contents-function
     The value of this variable specifies the function to use to insert
     the updated contents when reverting this buffer.  The function
     receives two arguments: first the file name to use; second, ‘t’ if
     the user has asked to read the auto-save file.

     The reason for a mode to change this variable instead of
     ‘revert-buffer-function’ is to avoid duplicating or replacing the
     rest of what ‘revert-buffer’ does: asking for confirmation,
     clearing the undo list, deciding the proper major mode, and running
     the hooks listed below.

 -- Variable: before-revert-hook
     This normal hook is run by the default ‘revert-buffer-function’
     before inserting the modified contents.  A custom
     ‘revert-buffer-function’ may or may not run this hook.

 -- Variable: after-revert-hook
     This normal hook is run by the default ‘revert-buffer-function’
     after inserting the modified contents.  A custom
     ‘revert-buffer-function’ may or may not run this hook.

 -- Variable: buffer-stale-function
     The value of this variable specifies a function to call to check
     whether a buffer needs reverting.  The default value only handles
     buffers that are visiting files, by checking their modification
     time.  Buffers that are not visiting files require a custom
     function (*note (emacs)Supporting additional buffers::).


File: elisp.info,  Node: Buffers,  Next: Windows,  Prev: Backups and Auto-Saving,  Up: Top

26 Buffers
**********

A “buffer” is a Lisp object containing text to be edited.  Buffers are
used to hold the contents of files that are being visited; there may
also be buffers that are not visiting files.  While several buffers may
exist at one time, only one buffer is designated the “current buffer” at
any time.  Most editing commands act on the contents of the current
buffer.  Each buffer, including the current buffer, may or may not be
displayed in any windows.

* Menu:

* Buffer Basics::       What is a buffer?
* Current Buffer::      Designating a buffer as current
                          so that primitives will access its contents.
* Buffer Names::        Accessing and changing buffer names.
* Buffer File Name::    The buffer file name indicates which file is visited.
* Buffer Modification:: A buffer is “modified” if it needs to be saved.
* Modification Time::   Determining whether the visited file was changed
                         behind Emacs’s back.
* Read Only Buffers::   Modifying text is not allowed in a read-only buffer.
* Buffer List::         How to look at all the existing buffers.
* Creating Buffers::    Functions that create buffers.
* Killing Buffers::     Buffers exist until explicitly killed.
* Indirect Buffers::    An indirect buffer shares text with some other buffer.
* Swapping Text::       Swapping text between two buffers.
* Buffer Gap::          The gap in the buffer.


File: elisp.info,  Node: Buffer Basics,  Next: Current Buffer,  Up: Buffers

26.1 Buffer Basics
==================

A “buffer” is a Lisp object containing text to be edited.  Buffers are
used to hold the contents of files that are being visited; there may
also be buffers that are not visiting files.  Although several buffers
normally exist, only one buffer is designated the “current buffer” at
any time.  Most editing commands act on the contents of the current
buffer.  Each buffer, including the current buffer, may or may not be
displayed in any windows.

   Buffers in Emacs editing are objects that have distinct names and
hold text that can be edited.  Buffers appear to Lisp programs as a
special data type.  You can think of the contents of a buffer as a
string that you can extend; insertions and deletions may occur in any
part of the buffer.  *Note Text::.

   A Lisp buffer object contains numerous pieces of information.  Some
of this information is directly accessible to the programmer through
variables, while other information is accessible only through
special-purpose functions.  For example, the visited file name is
directly accessible through a variable, while the value of point is
accessible only through a primitive function.

   Buffer-specific information that is directly accessible is stored in
“buffer-local” variable bindings, which are variable values that are
effective only in a particular buffer.  This feature allows each buffer
to override the values of certain variables.  Most major modes override
variables such as ‘fill-column’ or ‘comment-column’ in this way.  For
more information about buffer-local variables and functions related to
them, see *note Buffer-Local Variables::.

   For functions and variables related to visiting files in buffers, see
*note Visiting Files:: and *note Saving Buffers::.  For functions and
variables related to the display of buffers in windows, see *note
Buffers and Windows::.

 -- Function: bufferp object
     This function returns ‘t’ if OBJECT is a buffer, ‘nil’ otherwise.


File: elisp.info,  Node: Current Buffer,  Next: Buffer Names,  Prev: Buffer Basics,  Up: Buffers

26.2 The Current Buffer
=======================

There are, in general, many buffers in an Emacs session.  At any time,
one of them is designated the “current buffer”—the buffer in which most
editing takes place.  Most of the primitives for examining or changing
text operate implicitly on the current buffer (*note Text::).

   Normally, the buffer displayed in the selected window is the current
buffer, but this is not always so: a Lisp program can temporarily
designate any buffer as current in order to operate on its contents,
without changing what is displayed on the screen.  The most basic
function for designating a current buffer is ‘set-buffer’.

 -- Function: current-buffer
     This function returns the current buffer.

          (current-buffer)
               ⇒ #<buffer buffers.texi>

 -- Function: set-buffer buffer-or-name
     This function makes BUFFER-OR-NAME the current buffer.
     BUFFER-OR-NAME must be an existing buffer or the name of an
     existing buffer.  The return value is the buffer made current.

     This function does not display the buffer in any window, so the
     user cannot necessarily see the buffer.  But Lisp programs will now
     operate on it.

   When an editing command returns to the editor command loop, Emacs
automatically calls ‘set-buffer’ on the buffer shown in the selected
window.  This is to prevent confusion: it ensures that the buffer that
the cursor is in, when Emacs reads a command, is the buffer to which
that command applies (*note Command Loop::).  Thus, you should not use
‘set-buffer’ to switch visibly to a different buffer; for that, use the
functions described in *note Switching Buffers::.

   When writing a Lisp function, do _not_ rely on this behavior of the
command loop to restore the current buffer after an operation.  Editing
commands can also be called as Lisp functions by other programs, not
just from the command loop; it is convenient for the caller if the
subroutine does not change which buffer is current (unless, of course,
that is the subroutine’s purpose).

   To operate temporarily on another buffer, put the ‘set-buffer’ within
a ‘save-current-buffer’ form.  Here, as an example, is a simplified
version of the command ‘append-to-buffer’:

     (defun append-to-buffer (buffer start end)
       "Append the text of the region to BUFFER."
       (interactive "BAppend to buffer: \nr")
       (let ((oldbuf (current-buffer)))
         (save-current-buffer
           (set-buffer (get-buffer-create buffer))
           (insert-buffer-substring oldbuf start end))))

Here, we bind a local variable to record the current buffer, and then
‘save-current-buffer’ arranges to make it current again later.  Next,
‘set-buffer’ makes the specified buffer current, and
‘insert-buffer-substring’ copies the string from the original buffer to
the specified (and now current) buffer.

   Alternatively, we can use the ‘with-current-buffer’ macro:

     (defun append-to-buffer (buffer start end)
       "Append the text of the region to BUFFER."
       (interactive "BAppend to buffer: \nr")
       (let ((oldbuf (current-buffer)))
         (with-current-buffer (get-buffer-create buffer)
           (insert-buffer-substring oldbuf start end))))

   In either case, if the buffer appended to happens to be displayed in
some window, the next redisplay will show how its text has changed.  If
it is not displayed in any window, you will not see the change
immediately on the screen.  The command causes the buffer to become
current temporarily, but does not cause it to be displayed.

   If you make local bindings (with ‘let’ or function arguments) for a
variable that may also have buffer-local bindings, make sure that the
same buffer is current at the beginning and at the end of the local
binding’s scope.  Otherwise you might bind it in one buffer and unbind
it in another!

   Do not rely on using ‘set-buffer’ to change the current buffer back,
because that won’t do the job if a quit happens while the wrong buffer
is current.  For instance, in the previous example, it would have been
wrong to do this:

       (let ((oldbuf (current-buffer)))
         (set-buffer (get-buffer-create buffer))
         (insert-buffer-substring oldbuf start end)
         (set-buffer oldbuf))

Using ‘save-current-buffer’ or ‘with-current-buffer’, as we did,
correctly handles quitting, errors, and ‘throw’, as well as ordinary
evaluation.

 -- Special Form: save-current-buffer body...
     The ‘save-current-buffer’ special form saves the identity of the
     current buffer, evaluates the BODY forms, and finally restores that
     buffer as current.  The return value is the value of the last form
     in BODY.  The current buffer is restored even in case of an
     abnormal exit via ‘throw’ or error (*note Nonlocal Exits::).

     If the buffer that used to be current has been killed by the time
     of exit from ‘save-current-buffer’, then it is not made current
     again, of course.  Instead, whichever buffer was current just
     before exit remains current.

 -- Macro: with-current-buffer buffer-or-name body...
     The ‘with-current-buffer’ macro saves the identity of the current
     buffer, makes BUFFER-OR-NAME current, evaluates the BODY forms, and
     finally restores the current buffer.  BUFFER-OR-NAME must specify
     an existing buffer or the name of an existing buffer.

     The return value is the value of the last form in BODY.  The
     current buffer is restored even in case of an abnormal exit via
     ‘throw’ or error (*note Nonlocal Exits::).

 -- Macro: with-temp-buffer body...
     The ‘with-temp-buffer’ macro evaluates the BODY forms with a
     temporary buffer as the current buffer.  It saves the identity of
     the current buffer, creates a temporary buffer and makes it
     current, evaluates the BODY forms, and finally restores the
     previous current buffer while killing the temporary buffer.  By
     default, undo information (*note Undo::) is not recorded in the
     buffer created by this macro (but BODY can enable that, if needed).

     The return value is the value of the last form in BODY.  You can
     return the contents of the temporary buffer by using
     ‘(buffer-string)’ as the last form.

     The current buffer is restored even in case of an abnormal exit via
     ‘throw’ or error (*note Nonlocal Exits::).

     See also ‘with-temp-file’ in *note Writing to Files: Definition of
     with-temp-file.


File: elisp.info,  Node: Buffer Names,  Next: Buffer File Name,  Prev: Current Buffer,  Up: Buffers

26.3 Buffer Names
=================

Each buffer has a unique name, which is a string.  Many of the functions
that work on buffers accept either a buffer or a buffer name as an
argument.  Any argument called BUFFER-OR-NAME is of this sort, and an
error is signaled if it is neither a string nor a buffer.  Any argument
called BUFFER must be an actual buffer object, not a name.

   Buffers that are ephemeral and generally uninteresting to the user
have names starting with a space, so that the ‘list-buffers’ and
‘buffer-menu’ commands don’t mention them (but if such a buffer visits a
file, it *is* mentioned).  A name starting with space also initially
disables recording undo information; see *note Undo::.

 -- Function: buffer-name &optional buffer
     This function returns the name of BUFFER as a string.  BUFFER
     defaults to the current buffer.

     If ‘buffer-name’ returns ‘nil’, it means that BUFFER has been
     killed.  *Note Killing Buffers::.

          (buffer-name)
               ⇒ "buffers.texi"

          (setq foo (get-buffer "temp"))
               ⇒ #<buffer temp>
          (kill-buffer foo)
               ⇒ nil
          (buffer-name foo)
               ⇒ nil
          foo
               ⇒ #<killed buffer>

 -- Command: rename-buffer newname &optional unique
     This function renames the current buffer to NEWNAME.  An error is
     signaled if NEWNAME is not a string.

     Ordinarily, ‘rename-buffer’ signals an error if NEWNAME is already
     in use.  However, if UNIQUE is non-‘nil’, it modifies NEWNAME to
     make a name that is not in use.  Interactively, you can make UNIQUE
     non-‘nil’ with a numeric prefix argument.  (This is how the command
     ‘rename-uniquely’ is implemented.)

     This function returns the name actually given to the buffer.

 -- Function: get-buffer buffer-or-name
     This function returns the buffer specified by BUFFER-OR-NAME.  If
     BUFFER-OR-NAME is a string and there is no buffer with that name,
     the value is ‘nil’.  If BUFFER-OR-NAME is a buffer, it is returned
     as given; that is not very useful, so the argument is usually a
     name.  For example:

          (setq b (get-buffer "lewis"))
               ⇒ #<buffer lewis>
          (get-buffer b)
               ⇒ #<buffer lewis>
          (get-buffer "Frazzle-nots")
               ⇒ nil

     See also the function ‘get-buffer-create’ in *note Creating
     Buffers::.

 -- Function: generate-new-buffer-name starting-name &optional ignore
     This function returns a name that would be unique for a new
     buffer—but does not create the buffer.  It starts with
     STARTING-NAME, and produces a name not currently in use for any
     buffer by appending a number inside of ‘<...>’.  It starts at 2 and
     keeps incrementing the number until it is not the name of an
     existing buffer.

     If the optional second argument IGNORE is non-‘nil’, it should be a
     string, a potential buffer name.  It means to consider that
     potential buffer acceptable, if it is tried, even it is the name of
     an existing buffer (which would normally be rejected).  Thus, if
     buffers named ‘foo’, ‘foo<2>’, ‘foo<3>’ and ‘foo<4>’ exist,

          (generate-new-buffer-name "foo")
               ⇒ "foo<5>"
          (generate-new-buffer-name "foo" "foo<3>")
               ⇒ "foo<3>"
          (generate-new-buffer-name "foo" "foo<6>")
               ⇒ "foo<5>"

     See the related function ‘generate-new-buffer’ in *note Creating
     Buffers::.


File: elisp.info,  Node: Buffer File Name,  Next: Buffer Modification,  Prev: Buffer Names,  Up: Buffers

26.4 Buffer File Name
=====================

The “buffer file name” is the name of the file that is visited in that
buffer.  When a buffer is not visiting a file, its buffer file name is
‘nil’.  Most of the time, the buffer name is the same as the
nondirectory part of the buffer file name, but the buffer file name and
the buffer name are distinct and can be set independently.  *Note
Visiting Files::.

 -- Function: buffer-file-name &optional buffer
     This function returns the absolute file name of the file that
     BUFFER is visiting.  If BUFFER is not visiting any file,
     ‘buffer-file-name’ returns ‘nil’.  If BUFFER is not supplied, it
     defaults to the current buffer.

          (buffer-file-name (other-buffer))
               ⇒ "/usr/user/lewis/manual/files.texi"

 -- Variable: buffer-file-name
     This buffer-local variable contains the name of the file being
     visited in the current buffer, or ‘nil’ if it is not visiting a
     file.  It is a permanent local variable, unaffected by
     ‘kill-all-local-variables’.

          buffer-file-name
               ⇒ "/usr/user/lewis/manual/buffers.texi"

     It is risky to change this variable’s value without doing various
     other things.  Normally it is better to use ‘set-visited-file-name’
     (see below); some of the things done there, such as changing the
     buffer name, are not strictly necessary, but others are essential
     to avoid confusing Emacs.

 -- Variable: buffer-file-truename
     This buffer-local variable holds the abbreviated truename of the
     file visited in the current buffer, or ‘nil’ if no file is visited.
     It is a permanent local, unaffected by ‘kill-all-local-variables’.
     *Note Truenames::, and *note abbreviate-file-name::.

 -- Variable: buffer-file-number
     This buffer-local variable holds the file number and directory
     device number of the file visited in the current buffer, or ‘nil’
     if no file or a nonexistent file is visited.  It is a permanent
     local, unaffected by ‘kill-all-local-variables’.

     The value is normally a list of the form ‘(FILENUM DEVNUM)’.  This
     pair of numbers uniquely identifies the file among all files
     accessible on the system.  See the function ‘file-attributes’, in
     *note File Attributes::, for more information about them.

     If ‘buffer-file-name’ is the name of a symbolic link, then both
     numbers refer to the recursive target.

 -- Function: get-file-buffer filename
     This function returns the buffer visiting file FILENAME.  If there
     is no such buffer, it returns ‘nil’.  The argument FILENAME, which
     must be a string, is expanded (*note File Name Expansion::), then
     compared against the visited file names of all live buffers.  Note
     that the buffer’s ‘buffer-file-name’ must match the expansion of
     FILENAME exactly.  This function will not recognize other names for
     the same file.

          (get-file-buffer "buffers.texi")
              ⇒ #<buffer buffers.texi>

     In unusual circumstances, there can be more than one buffer
     visiting the same file name.  In such cases, this function returns
     the first such buffer in the buffer list.

 -- Function: find-buffer-visiting filename &optional predicate
     This is like ‘get-file-buffer’, except that it can return any
     buffer visiting the file _possibly under a different name_.  That
     is, the buffer’s ‘buffer-file-name’ does not need to match the
     expansion of FILENAME exactly, it only needs to refer to the same
     file.  If PREDICATE is non-‘nil’, it should be a function of one
     argument, a buffer visiting FILENAME.  The buffer is only
     considered a suitable return value if PREDICATE returns non-‘nil’.
     If it can not find a suitable buffer to return,
     ‘find-buffer-visiting’ returns ‘nil’.

 -- Command: set-visited-file-name filename &optional no-query
          along-with-file
     If FILENAME is a non-empty string, this function changes the name
     of the file visited in the current buffer to FILENAME.  (If the
     buffer had no visited file, this gives it one.)  The _next time_
     the buffer is saved it will go in the newly-specified file.

     This command marks the buffer as modified, since it does not (as
     far as Emacs knows) match the contents of FILENAME, even if it
     matched the former visited file.  It also renames the buffer to
     correspond to the new file name, unless the new name is already in
     use.

     If FILENAME is ‘nil’ or the empty string, that stands for “no
     visited file”.  In this case, ‘set-visited-file-name’ marks the
     buffer as having no visited file, without changing the buffer’s
     modified flag.

     Normally, this function asks the user for confirmation if there
     already is a buffer visiting FILENAME.  If NO-QUERY is non-‘nil’,
     that prevents asking this question.  If there already is a buffer
     visiting FILENAME, and the user confirms or NO-QUERY is non-‘nil’,
     this function makes the new buffer name unique by appending a
     number inside of ‘<...>’ to FILENAME.

     If ALONG-WITH-FILE is non-‘nil’, that means to assume that the
     former visited file has been renamed to FILENAME.  In this case,
     the command does not change the buffer’s modified flag, nor the
     buffer’s recorded last file modification time as reported by
     ‘visited-file-modtime’ (*note Modification Time::).  If
     ALONG-WITH-FILE is ‘nil’, this function clears the recorded last
     file modification time, after which ‘visited-file-modtime’ returns
     zero.

     When the function ‘set-visited-file-name’ is called interactively,
     it prompts for FILENAME in the minibuffer.

 -- Variable: list-buffers-directory
     This buffer-local variable specifies a string to display in a
     buffer listing where the visited file name would go, for buffers
     that don’t have a visited file name.  Dired buffers use this
     variable.


File: elisp.info,  Node: Buffer Modification,  Next: Modification Time,  Prev: Buffer File Name,  Up: Buffers

26.5 Buffer Modification
========================

Emacs keeps a flag called the “modified flag” for each buffer, to record
whether you have changed the text of the buffer.  This flag is set to
‘t’ whenever you alter the contents of the buffer, and cleared to ‘nil’
when you save it.  Thus, the flag shows whether there are unsaved
changes.  The flag value is normally shown in the mode line (*note Mode
Line Variables::), and controls saving (*note Saving Buffers::) and
auto-saving (*note Auto-Saving::).

   Some Lisp programs set the flag explicitly.  For example, the
function ‘set-visited-file-name’ sets the flag to ‘t’, because the text
does not match the newly-visited file, even if it is unchanged from the
file formerly visited.

   The functions that modify the contents of buffers are described in
*note Text::.

 -- Function: buffer-modified-p &optional buffer
     This function returns ‘t’ if the buffer BUFFER has been modified
     since it was last read in from a file or saved, or ‘nil’ otherwise.
     If BUFFER is not supplied, the current buffer is tested.

 -- Function: set-buffer-modified-p flag
     This function marks the current buffer as modified if FLAG is
     non-‘nil’, or as unmodified if the flag is ‘nil’.

     Another effect of calling this function is to cause unconditional
     redisplay of the mode line for the current buffer.  In fact, the
     function ‘force-mode-line-update’ works by doing this:

          (set-buffer-modified-p (buffer-modified-p))

 -- Function: restore-buffer-modified-p flag
     Like ‘set-buffer-modified-p’, but does not force redisplay of mode
     lines.

 -- Command: not-modified &optional arg
     This command marks the current buffer as unmodified, and not
     needing to be saved.  If ARG is non-‘nil’, it marks the buffer as
     modified, so that it will be saved at the next suitable occasion.
     Interactively, ARG is the prefix argument.

     Don’t use this function in programs, since it prints a message in
     the echo area; use ‘set-buffer-modified-p’ (above) instead.

 -- Function: buffer-modified-tick &optional buffer
     This function returns BUFFER’s modification-count.  This is a
     counter that increments every time the buffer is modified.  If
     BUFFER is ‘nil’ (or omitted), the current buffer is used.  The
     counter can wrap around occasionally.

 -- Function: buffer-chars-modified-tick &optional buffer
     This function returns BUFFER’s character-change modification-count.
     Changes to text properties leave this counter unchanged; however,
     each time text is inserted or removed from the buffer, the counter
     is reset to the value that would be returned by
     ‘buffer-modified-tick’.  By comparing the values returned by two
     ‘buffer-chars-modified-tick’ calls, you can tell whether a
     character change occurred in that buffer in between the calls.  If
     BUFFER is ‘nil’ (or omitted), the current buffer is used.


File: elisp.info,  Node: Modification Time,  Next: Read Only Buffers,  Prev: Buffer Modification,  Up: Buffers

26.6 Buffer Modification Time
=============================

Suppose that you visit a file and make changes in its buffer, and
meanwhile the file itself is changed on disk.  At this point, saving the
buffer would overwrite the changes in the file.  Occasionally this may
be what you want, but usually it would lose valuable information.  Emacs
therefore checks the file’s modification time using the functions
described below before saving the file.  (*Note File Attributes::, for
how to examine a file’s modification time.)

 -- Function: verify-visited-file-modtime &optional buffer
     This function compares what BUFFER (by default, the current-buffer)
     has recorded for the modification time of its visited file against
     the actual modification time of the file as recorded by the
     operating system.  The two should be the same unless some other
     process has written the file since Emacs visited or saved it.

     The function returns ‘t’ if the last actual modification time and
     Emacs’s recorded modification time are the same, ‘nil’ otherwise.
     It also returns ‘t’ if the buffer has no recorded last modification
     time, that is if ‘visited-file-modtime’ would return zero.

     It always returns ‘t’ for buffers that are not visiting a file,
     even if ‘visited-file-modtime’ returns a non-zero value.  For
     instance, it always returns ‘t’ for dired buffers.  It returns ‘t’
     for buffers that are visiting a file that does not exist and never
     existed, but ‘nil’ for file-visiting buffers whose file has been
     deleted.

 -- Function: clear-visited-file-modtime
     This function clears out the record of the last modification time
     of the file being visited by the current buffer.  As a result, the
     next attempt to save this buffer will not complain of a discrepancy
     in file modification times.

     This function is called in ‘set-visited-file-name’ and other
     exceptional places where the usual test to avoid overwriting a
     changed file should not be done.

 -- Function: visited-file-modtime
     This function returns the current buffer’s recorded last file
     modification time, as a list of the form ‘(HIGH LOW MICROSEC
     PICOSEC)’.  (This is the same format that ‘file-attributes’ uses to
     return time values; *note File Attributes::.)

     If the buffer has no recorded last modification time, this function
     returns zero.  This case occurs, for instance, if the buffer is not
     visiting a file or if the time has been explicitly cleared by
     ‘clear-visited-file-modtime’.  Note, however, that
     ‘visited-file-modtime’ returns a list for some non-file buffers
     too.  For instance, in a Dired buffer listing a directory, it
     returns the last modification time of that directory, as recorded
     by Dired.

     If the buffer is not visiting a file, this function returns -1.

 -- Function: set-visited-file-modtime &optional time
     This function updates the buffer’s record of the last modification
     time of the visited file, to the value specified by TIME if TIME is
     not ‘nil’, and otherwise to the last modification time of the
     visited file.

     If TIME is neither ‘nil’ nor zero, it should have the form ‘(HIGH
     LOW MICROSEC PICOSEC)’, the format used by ‘current-time’ (*note
     Time of Day::).

     This function is useful if the buffer was not read from the file
     normally, or if the file itself has been changed for some known
     benign reason.

 -- Function: ask-user-about-supersession-threat filename
     This function is used to ask a user how to proceed after an attempt
     to modify an buffer visiting file FILENAME when the file is newer
     than the buffer text.  Emacs detects this because the modification
     time of the file on disk is newer than the last save-time of the
     buffer.  This means some other program has probably altered the
     file.

     Depending on the user’s answer, the function may return normally,
     in which case the modification of the buffer proceeds, or it may
     signal a ‘file-supersession’ error with data ‘(FILENAME)’, in which
     case the proposed buffer modification is not allowed.

     This function is called automatically by Emacs on the proper
     occasions.  It exists so you can customize Emacs by redefining it.
     See the file ‘userlock.el’ for the standard definition.

     See also the file locking mechanism in *note File Locks::.


File: elisp.info,  Node: Read Only Buffers,  Next: Buffer List,  Prev: Modification Time,  Up: Buffers

26.7 Read-Only Buffers
======================

If a buffer is “read-only”, then you cannot change its contents,
although you may change your view of the contents by scrolling and
narrowing.

   Read-only buffers are used in two kinds of situations:

   • A buffer visiting a write-protected file is normally read-only.

     Here, the purpose is to inform the user that editing the buffer
     with the aim of saving it in the file may be futile or undesirable.
     The user who wants to change the buffer text despite this can do so
     after clearing the read-only flag with ‘C-x C-q’.

   • Modes such as Dired and Rmail make buffers read-only when altering
     the contents with the usual editing commands would probably be a
     mistake.

     The special commands of these modes bind ‘buffer-read-only’ to
     ‘nil’ (with ‘let’) or bind ‘inhibit-read-only’ to ‘t’ around the
     places where they themselves change the text.

 -- Variable: buffer-read-only
     This buffer-local variable specifies whether the buffer is
     read-only.  The buffer is read-only if this variable is non-‘nil’.
     However, characters that have the ‘inhibit-read-only’ text property
     can still be modified.  *Note inhibit-read-only: Special
     Properties.

 -- Variable: inhibit-read-only
     If this variable is non-‘nil’, then read-only buffers and,
     depending on the actual value, some or all read-only characters may
     be modified.  Read-only characters in a buffer are those that have
     a non-‘nil’ ‘read-only’ text property.  *Note Special Properties::,
     for more information about text properties.

     If ‘inhibit-read-only’ is ‘t’, all ‘read-only’ character properties
     have no effect.  If ‘inhibit-read-only’ is a list, then ‘read-only’
     character properties have no effect if they are members of the list
     (comparison is done with ‘eq’).

 -- Command: read-only-mode &optional arg
     This is the mode command for Read Only minor mode, a buffer-local
     minor mode.  When the mode is enabled, ‘buffer-read-only’ is
     non-‘nil’ in the buffer; when disabled, ‘buffer-read-only’ is ‘nil’
     in the buffer.  The calling convention is the same as for other
     minor mode commands (*note Minor Mode Conventions::).

     This minor mode mainly serves as a wrapper for ‘buffer-read-only’;
     unlike most minor modes, there is no separate ‘read-only-mode’
     variable.  Even when Read Only mode is disabled, characters with
     non-‘nil’ ‘read-only’ text properties remain read-only.  To
     temporarily ignore all read-only states, bind ‘inhibit-read-only’,
     as described above.

     When enabling Read Only mode, this mode command also enables View
     mode if the option ‘view-read-only’ is non-‘nil’.  *Note
     Miscellaneous Buffer Operations: (emacs)Misc Buffer.  When
     disabling Read Only mode, it disables View mode if View mode was
     enabled.

 -- Function: barf-if-buffer-read-only &optional position
     This function signals a ‘buffer-read-only’ error if the current
     buffer is read-only.  If the text at POSITION (which defaults to
     point) has the ‘inhibit-read-only’ text property set, the error
     will not be raised.

     *Note Using Interactive::, for another way to signal an error if
     the current buffer is read-only.


File: elisp.info,  Node: Buffer List,  Next: Creating Buffers,  Prev: Read Only Buffers,  Up: Buffers

26.8 The Buffer List
====================

The “buffer list” is a list of all live buffers.  The order of the
buffers in this list is based primarily on how recently each buffer has
been displayed in a window.  Several functions, notably ‘other-buffer’,
use this ordering.  A buffer list displayed for the user also follows
this order.

   Creating a buffer adds it to the end of the buffer list, and killing
a buffer removes it from that list.  A buffer moves to the front of this
list whenever it is chosen for display in a window (*note Switching
Buffers::) or a window displaying it is selected (*note Selecting
Windows::).  A buffer moves to the end of the list when it is buried
(see ‘bury-buffer’, below).  There are no functions available to the
Lisp programmer which directly manipulate the buffer list.

   In addition to the fundamental buffer list just described, Emacs
maintains a local buffer list for each frame, in which the buffers that
have been displayed (or had their windows selected) in that frame come
first.  (This order is recorded in the frame’s ‘buffer-list’ frame
parameter; see *note Buffer Parameters::.)  Buffers never displayed in
that frame come afterward, ordered according to the fundamental buffer
list.

 -- Function: buffer-list &optional frame
     This function returns the buffer list, including all buffers, even
     those whose names begin with a space.  The elements are actual
     buffers, not their names.

     If FRAME is a frame, this returns FRAME’s local buffer list.  If
     FRAME is ‘nil’ or omitted, the fundamental buffer list is used: the
     buffers appear in order of most recent display or selection,
     regardless of which frames they were displayed on.

          (buffer-list)
               ⇒ (#<buffer buffers.texi>
                   #<buffer  *Minibuf-1*> #<buffer buffer.c>
                   #<buffer *Help*> #<buffer TAGS>)

          ;; Note that the name of the minibuffer
          ;;   begins with a space!
          (mapcar (function buffer-name) (buffer-list))
              ⇒ ("buffers.texi" " *Minibuf-1*"
                  "buffer.c" "*Help*" "TAGS")

   The list returned by ‘buffer-list’ is constructed specifically; it is
not an internal Emacs data structure, and modifying it has no effect on
the order of buffers.  If you want to change the order of buffers in the
fundamental buffer list, here is an easy way:

     (defun reorder-buffer-list (new-list)
       (while new-list
         (bury-buffer (car new-list))
         (setq new-list (cdr new-list))))

   With this method, you can specify any order for the list, but there
is no danger of losing a buffer or adding something that is not a valid
live buffer.

   To change the order or value of a specific frame’s buffer list, set
that frame’s ‘buffer-list’ parameter with ‘modify-frame-parameters’
(*note Parameter Access::).

 -- Function: other-buffer &optional buffer visible-ok frame
     This function returns the first buffer in the buffer list other
     than BUFFER.  Usually, this is the buffer appearing in the most
     recently selected window (in frame FRAME or else the selected
     frame, *note Input Focus::), aside from BUFFER.  Buffers whose
     names start with a space are not considered at all.

     If BUFFER is not supplied (or if it is not a live buffer), then
     ‘other-buffer’ returns the first buffer in the selected frame’s
     local buffer list.  (If FRAME is non-‘nil’, it returns the first
     buffer in FRAME’s local buffer list instead.)

     If FRAME has a non-‘nil’ ‘buffer-predicate’ parameter, then
     ‘other-buffer’ uses that predicate to decide which buffers to
     consider.  It calls the predicate once for each buffer, and if the
     value is ‘nil’, that buffer is ignored.  *Note Buffer Parameters::.

     If VISIBLE-OK is ‘nil’, ‘other-buffer’ avoids returning a buffer
     visible in any window on any visible frame, except as a last
     resort.  If VISIBLE-OK is non-‘nil’, then it does not matter
     whether a buffer is displayed somewhere or not.

     If no suitable buffer exists, the buffer ‘*scratch*’ is returned
     (and created, if necessary).

 -- Function: last-buffer &optional buffer visible-ok frame
     This function returns the last buffer in FRAME’s buffer list other
     than BUFFER.  If FRAME is omitted or ‘nil’, it uses the selected
     frame’s buffer list.

     The argument VISIBLE-OK is handled as with ‘other-buffer’, see
     above.  If no suitable buffer can be found, the buffer ‘*scratch*’
     is returned.

 -- Command: bury-buffer &optional buffer-or-name
     This command puts BUFFER-OR-NAME at the end of the buffer list,
     without changing the order of any of the other buffers on the list.
     This buffer therefore becomes the least desirable candidate for
     ‘other-buffer’ to return.  The argument can be either a buffer
     itself or the name of one.

     This function operates on each frame’s ‘buffer-list’ parameter as
     well as the fundamental buffer list; therefore, the buffer that you
     bury will come last in the value of ‘(buffer-list FRAME)’ and in
     the value of ‘(buffer-list)’.  In addition, it also puts the buffer
     at the end of the list of buffer of the selected window (*note
     Window History::) provided it is shown in that window.

     If BUFFER-OR-NAME is ‘nil’ or omitted, this means to bury the
     current buffer.  In addition, if the current buffer is displayed in
     the selected window, this makes sure that the window is either
     deleted or another buffer is shown in it.  More precisely, if the
     selected window is dedicated (*note Dedicated Windows::) and there
     are other windows on its frame, the window is deleted.  If it is
     the only window on its frame and that frame is not the only frame
     on its terminal, the frame is dismissed by calling the function
     specified by ‘frame-auto-hide-function’ (*note Quitting Windows::).
     Otherwise, it calls ‘switch-to-prev-buffer’ (*note Window
     History::) to show another buffer in that window.  If
     BUFFER-OR-NAME is displayed in some other window, it remains
     displayed there.

     To replace a buffer in all the windows that display it, use
     ‘replace-buffer-in-windows’, *Note Buffers and Windows::.

 -- Command: unbury-buffer
     This command switches to the last buffer in the local buffer list
     of the selected frame.  More precisely, it calls the function
     ‘switch-to-buffer’ (*note Switching Buffers::), to display the
     buffer returned by ‘last-buffer’ (see above), in the selected
     window.

 -- Variable: buffer-list-update-hook
     This is a normal hook run whenever the buffer list changes.
     Functions (implicitly) running this hook are ‘get-buffer-create’
     (*note Creating Buffers::), ‘rename-buffer’ (*note Buffer Names::),
     ‘kill-buffer’ (*note Killing Buffers::), ‘bury-buffer’ (see above)
     and ‘select-window’ (*note Selecting Windows::).


File: elisp.info,  Node: Creating Buffers,  Next: Killing Buffers,  Prev: Buffer List,  Up: Buffers

26.9 Creating Buffers
=====================

This section describes the two primitives for creating buffers.
‘get-buffer-create’ creates a buffer if it finds no existing buffer with
the specified name; ‘generate-new-buffer’ always creates a new buffer
and gives it a unique name.

   Other functions you can use to create buffers include
‘with-output-to-temp-buffer’ (*note Temporary Displays::) and
‘create-file-buffer’ (*note Visiting Files::).  Starting a subprocess
can also create a buffer (*note Processes::).

 -- Function: get-buffer-create buffer-or-name
     This function returns a buffer named BUFFER-OR-NAME.  The buffer
     returned does not become the current buffer—this function does not
     change which buffer is current.

     BUFFER-OR-NAME must be either a string or an existing buffer.  If
     it is a string and a live buffer with that name already exists,
     ‘get-buffer-create’ returns that buffer.  If no such buffer exists,
     it creates a new buffer.  If BUFFER-OR-NAME is a buffer instead of
     a string, it is returned as given, even if it is dead.

          (get-buffer-create "foo")
               ⇒ #<buffer foo>

     The major mode for a newly created buffer is set to Fundamental
     mode.  (The default value of the variable ‘major-mode’ is handled
     at a higher level; see *note Auto Major Mode::.)  If the name
     begins with a space, the buffer initially disables undo information
     recording (*note Undo::).

 -- Function: generate-new-buffer name
     This function returns a newly created, empty buffer, but does not
     make it current.  The name of the buffer is generated by passing
     NAME to the function ‘generate-new-buffer-name’ (*note Buffer
     Names::).  Thus, if there is no buffer named NAME, then that is the
     name of the new buffer; if that name is in use, a suffix of the
     form ‘<N>’, where N is an integer, is appended to NAME.

     An error is signaled if NAME is not a string.

          (generate-new-buffer "bar")
               ⇒ #<buffer bar>
          (generate-new-buffer "bar")
               ⇒ #<buffer bar<2>>
          (generate-new-buffer "bar")
               ⇒ #<buffer bar<3>>

     The major mode for the new buffer is set to Fundamental mode.  The
     default value of the variable ‘major-mode’ is handled at a higher
     level.  *Note Auto Major Mode::.


File: elisp.info,  Node: Killing Buffers,  Next: Indirect Buffers,  Prev: Creating Buffers,  Up: Buffers

26.10 Killing Buffers
=====================

“Killing a buffer” makes its name unknown to Emacs and makes the memory
space it occupied available for other use.

   The buffer object for the buffer that has been killed remains in
existence as long as anything refers to it, but it is specially marked
so that you cannot make it current or display it.  Killed buffers retain
their identity, however; if you kill two distinct buffers, they remain
distinct according to ‘eq’ although both are dead.

   If you kill a buffer that is current or displayed in a window, Emacs
automatically selects or displays some other buffer instead.  This means
that killing a buffer can change the current buffer.  Therefore, when
you kill a buffer, you should also take the precautions associated with
changing the current buffer (unless you happen to know that the buffer
being killed isn’t current).  *Note Current Buffer::.

   If you kill a buffer that is the base buffer of one or more indirect
buffers (*note Indirect Buffers::), the indirect buffers are
automatically killed as well.

   The ‘buffer-name’ of a buffer is ‘nil’ if, and only if, the buffer is
killed.  A buffer that has not been killed is called a “live” buffer.
To test whether a buffer is live or killed, use the function
‘buffer-live-p’ (see below).

 -- Command: kill-buffer &optional buffer-or-name
     This function kills the buffer BUFFER-OR-NAME, freeing all its
     memory for other uses or to be returned to the operating system.
     If BUFFER-OR-NAME is ‘nil’ or omitted, it kills the current buffer.

     Any processes that have this buffer as the ‘process-buffer’ are
     sent the ‘SIGHUP’ (hangup) signal, which normally causes them to
     terminate.  *Note Signals to Processes::.

     If the buffer is visiting a file and contains unsaved changes,
     ‘kill-buffer’ asks the user to confirm before the buffer is killed.
     It does this even if not called interactively.  To prevent the
     request for confirmation, clear the modified flag before calling
     ‘kill-buffer’.  *Note Buffer Modification::.

     This function calls ‘replace-buffer-in-windows’ for cleaning up all
     windows currently displaying the buffer to be killed.

     Killing a buffer that is already dead has no effect.

     This function returns ‘t’ if it actually killed the buffer.  It
     returns ‘nil’ if the user refuses to confirm or if BUFFER-OR-NAME
     was already dead.

          (kill-buffer "foo.unchanged")
               ⇒ t
          (kill-buffer "foo.changed")

          ---------- Buffer: Minibuffer ----------
          Buffer foo.changed modified; kill anyway? (yes or no) yes
          ---------- Buffer: Minibuffer ----------

               ⇒ t

 -- Variable: kill-buffer-query-functions
     Before confirming unsaved changes, ‘kill-buffer’ calls the
     functions in the list ‘kill-buffer-query-functions’, in order of
     appearance, with no arguments.  The buffer being killed is the
     current buffer when they are called.  The idea of this feature is
     that these functions will ask for confirmation from the user.  If
     any of them returns ‘nil’, ‘kill-buffer’ spares the buffer’s life.

 -- Variable: kill-buffer-hook
     This is a normal hook run by ‘kill-buffer’ after asking all the
     questions it is going to ask, just before actually killing the
     buffer.  The buffer to be killed is current when the hook functions
     run.  *Note Hooks::.  This variable is a permanent local, so its
     local binding is not cleared by changing major modes.

 -- User Option: buffer-offer-save
     This variable, if non-‘nil’ in a particular buffer, tells
     ‘save-buffers-kill-emacs’ and ‘save-some-buffers’ (if the second
     optional argument to that function is ‘t’) to offer to save that
     buffer, just as they offer to save file-visiting buffers.  *Note
     Definition of save-some-buffers::.  The variable
     ‘buffer-offer-save’ automatically becomes buffer-local when set for
     any reason.  *Note Buffer-Local Variables::.

 -- Variable: buffer-save-without-query
     This variable, if non-‘nil’ in a particular buffer, tells
     ‘save-buffers-kill-emacs’ and ‘save-some-buffers’ to save this
     buffer (if it’s modified) without asking the user.  The variable
     automatically becomes buffer-local when set for any reason.

 -- Function: buffer-live-p object
     This function returns ‘t’ if OBJECT is a live buffer (a buffer
     which has not been killed), ‘nil’ otherwise.


File: elisp.info,  Node: Indirect Buffers,  Next: Swapping Text,  Prev: Killing Buffers,  Up: Buffers

26.11 Indirect Buffers
======================

An “indirect buffer” shares the text of some other buffer, which is
called the “base buffer” of the indirect buffer.  In some ways it is the
analogue, for buffers, of a symbolic link among files.  The base buffer
may not itself be an indirect buffer.

   The text of the indirect buffer is always identical to the text of
its base buffer; changes made by editing either one are visible
immediately in the other.  This includes the text properties as well as
the characters themselves.

   In all other respects, the indirect buffer and its base buffer are
completely separate.  They have different names, independent values of
point, independent narrowing, independent markers and overlays (though
inserting or deleting text in either buffer relocates the markers and
overlays for both), independent major modes, and independent
buffer-local variable bindings.

   An indirect buffer cannot visit a file, but its base buffer can.  If
you try to save the indirect buffer, that actually saves the base
buffer.

   Killing an indirect buffer has no effect on its base buffer.  Killing
the base buffer effectively kills the indirect buffer in that it cannot
ever again be the current buffer.

 -- Command: make-indirect-buffer base-buffer name &optional clone
     This creates and returns an indirect buffer named NAME whose base
     buffer is BASE-BUFFER.  The argument BASE-BUFFER may be a live
     buffer or the name (a string) of an existing buffer.  If NAME is
     the name of an existing buffer, an error is signaled.

     If CLONE is non-‘nil’, then the indirect buffer originally shares
     the state of BASE-BUFFER such as major mode, minor modes, buffer
     local variables and so on.  If CLONE is omitted or ‘nil’ the
     indirect buffer’s state is set to the default state for new
     buffers.

     If BASE-BUFFER is an indirect buffer, its base buffer is used as
     the base for the new buffer.  If, in addition, CLONE is non-‘nil’,
     the initial state is copied from the actual base buffer, not from
     BASE-BUFFER.

 -- Command: clone-indirect-buffer newname display-flag &optional
          norecord
     This function creates and returns a new indirect buffer that shares
     the current buffer’s base buffer and copies the rest of the current
     buffer’s attributes.  (If the current buffer is not indirect, it is
     used as the base buffer.)

     If DISPLAY-FLAG is non-‘nil’, that means to display the new buffer
     by calling ‘pop-to-buffer’.  If NORECORD is non-‘nil’, that means
     not to put the new buffer to the front of the buffer list.

 -- Function: buffer-base-buffer &optional buffer
     This function returns the base buffer of BUFFER, which defaults to
     the current buffer.  If BUFFER is not indirect, the value is ‘nil’.
     Otherwise, the value is another buffer, which is never an indirect
     buffer.


File: elisp.info,  Node: Swapping Text,  Next: Buffer Gap,  Prev: Indirect Buffers,  Up: Buffers

26.12 Swapping Text Between Two Buffers
=======================================

Specialized modes sometimes need to let the user access from the same
buffer several vastly different types of text.  For example, you may
need to display a summary of the buffer text, in addition to letting the
user access the text itself.

   This could be implemented with multiple buffers (kept in sync when
the user edits the text), or with narrowing (*note Narrowing::).  But
these alternatives might sometimes become tedious or prohibitively
expensive, especially if each type of text requires expensive
buffer-global operations in order to provide correct display and editing
commands.

   Emacs provides another facility for such modes: you can quickly swap
buffer text between two buffers with ‘buffer-swap-text’.  This function
is very fast because it doesn’t move any text, it only changes the
internal data structures of the buffer object to point to a different
chunk of text.  Using it, you can pretend that a group of two or more
buffers are actually a single virtual buffer that holds the contents of
all the individual buffers together.

 -- Function: buffer-swap-text buffer
     This function swaps the text of the current buffer and that of its
     argument BUFFER.  It signals an error if one of the two buffers is
     an indirect buffer (*note Indirect Buffers::) or is a base buffer
     of an indirect buffer.

     All the buffer properties that are related to the buffer text are
     swapped as well: the positions of point and mark, all the markers,
     the overlays, the text properties, the undo list, the value of the
     ‘enable-multibyte-characters’ flag (*note
     enable-multibyte-characters: Text Representations.), etc.

     *Warning:* If this function is called from within a
     ‘save-excursion’ form, the current buffer will be set to BUFFER
     upon leaving the form, since the marker used by ‘save-excursion’ to
     save the position and buffer will be swapped as well.

   If you use ‘buffer-swap-text’ on a file-visiting buffer, you should
set up a hook to save the buffer’s original text rather than what it was
swapped with.  ‘write-region-annotate-functions’ works for this purpose.
You should probably set ‘buffer-saved-size’ to −2 in the buffer, so that
changes in the text it is swapped with will not interfere with
auto-saving.


File: elisp.info,  Node: Buffer Gap,  Prev: Swapping Text,  Up: Buffers

26.13 The Buffer Gap
====================

Emacs buffers are implemented using an invisible “gap” to make insertion
and deletion faster.  Insertion works by filling in part of the gap, and
deletion adds to the gap.  Of course, this means that the gap must first
be moved to the locus of the insertion or deletion.  Emacs moves the gap
only when you try to insert or delete.  This is why your first editing
command in one part of a large buffer, after previously editing in
another far-away part, sometimes involves a noticeable delay.

   This mechanism works invisibly, and Lisp code should never be
affected by the gap’s current location, but these functions are
available for getting information about the gap status.

 -- Function: gap-position
     This function returns the current gap position in the current
     buffer.

 -- Function: gap-size
     This function returns the current gap size of the current buffer.


File: elisp.info,  Node: Windows,  Next: Frames,  Prev: Buffers,  Up: Top

27 Windows
**********

This chapter describes the functions and variables related to Emacs
windows.  *Note Frames::, for how windows are assigned an area of screen
available for Emacs to use.  *Note Display::, for information on how
text is displayed in windows.

* Menu:

* Basic Windows::           Basic information on using windows.
* Windows and Frames::      Relating windows to the frame they appear on.
* Window Sizes::            Accessing a window’s size.
* Resizing Windows::        Changing the sizes of windows.
* Preserving Window Sizes:: Preserving the size of windows.
* Splitting Windows::       Creating a new window.
* Deleting Windows::        Removing a window from its frame.
* Recombining Windows::     Preserving the frame layout when splitting and
                              deleting windows.
* Selecting Windows::       The selected window is the one that you edit in.
* Cyclic Window Ordering::  Moving around the existing windows.
* Buffers and Windows::     Each window displays the contents of a buffer.
* Switching Buffers::       Higher-level functions for switching to a buffer.
* Choosing Window::         How to choose a window for displaying a buffer.
* Display Action Functions:: Subroutines for ‘display-buffer’.
* Choosing Window Options:: Extra options affecting how buffers are displayed.
* Window History::          Each window remembers the buffers displayed in it.
* Dedicated Windows::       How to avoid displaying another buffer in
                              a specific window.
* Quitting Windows::        How to restore the state prior to displaying a
                              buffer.
* Window Point::            Each window has its own location of point.
* Window Start and End::    Buffer positions indicating which text is
                              on-screen in a window.
* Textual Scrolling::       Moving text up and down through the window.
* Vertical Scrolling::      Moving the contents up and down on the window.
* Horizontal Scrolling::    Moving the contents sideways on the window.
* Coordinates and Windows:: Converting coordinates to windows.
* Window Configurations::   Saving and restoring the state of the screen.
* Window Parameters::       Associating additional information with windows.
* Window Hooks::            Hooks for scrolling, window size changes,
                              redisplay going past a certain point,
                              or window configuration changes.


File: elisp.info,  Node: Basic Windows,  Next: Windows and Frames,  Up: Windows

27.1 Basic Concepts of Emacs Windows
====================================

A “window” is an area of the screen that is used to display a buffer
(*note Buffers::).  In Emacs Lisp, windows are represented by a special
Lisp object type.

   Windows are grouped into frames (*note Frames::).  Each frame
contains at least one window; the user can subdivide it into multiple,
non-overlapping windows to view several buffers at once.  Lisp programs
can use multiple windows for a variety of purposes.  In Rmail, for
example, you can view a summary of message titles in one window, and the
contents of the selected message in another window.

   Emacs uses the word “window” with a different meaning than in
graphical desktop environments and window systems, such as the X Window
System.  When Emacs is run on X, each of its graphical X windows is an
Emacs frame (containing one or more Emacs windows).  When Emacs is run
on a text terminal, the frame fills the entire terminal screen.

   Unlike X windows, Emacs windows are “tiled”; they never overlap
within the area of the frame.  When a window is created, resized, or
deleted, the change in window space is taken from or given to the
adjacent windows, so that the total area of the frame is unchanged.

 -- Function: windowp object
     This function returns ‘t’ if OBJECT is a window (whether or not it
     displays a buffer).  Otherwise, it returns ‘nil’.

   A “live window” is one that is actually displaying a buffer in a
frame.

 -- Function: window-live-p object
     This function returns ‘t’ if OBJECT is a live window and ‘nil’
     otherwise.  A live window is one that displays a buffer.

   The windows in each frame are organized into a “window tree”.  *Note
Windows and Frames::.  The leaf nodes of each window tree are live
windows—the ones actually displaying buffers.  The internal nodes of the
window tree are “internal windows”, which are not live.

   A “valid window” is one that is either live or internal.  A valid
window can be “deleted”, i.e., removed from its frame (*note Deleting
Windows::); then it is no longer valid, but the Lisp object representing
it might be still referenced from other Lisp objects.  A deleted window
may be made valid again by restoring a saved window configuration (*note
Window Configurations::).

   You can distinguish valid windows from deleted windows with
‘window-valid-p’.

 -- Function: window-valid-p object
     This function returns ‘t’ if OBJECT is a live window, or an
     internal window in a window tree.  Otherwise, it returns ‘nil’,
     including for the case where OBJECT is a deleted window.

   In each frame, at any time, exactly one Emacs window is designated as
“selected within the frame”.  For the selected frame, that window is
called the “selected window”—the one in which most editing takes place,
and in which the cursor for selected windows appears (*note Cursor
Parameters::).  The selected window’s buffer is usually also the current
buffer, except when ‘set-buffer’ has been used (*note Current Buffer::).
As for non-selected frames, the window selected within the frame becomes
the selected window if the frame is ever selected.  *Note Selecting
Windows::.

 -- Function: selected-window
     This function returns the selected window (which is always a live
     window).

   Sometimes several windows collectively and cooperatively display a
buffer, for example, under the management of Follow Mode (*note
(emacs)Follow Mode::), where the windows together display a bigger
portion of the buffer than one window could alone.  It is often useful
to consider such a “window group” as a single entity.  Several functions
such as ‘window-group-start’ (*note Window Start and End::) allow you to
do this by supplying, as an argument, one of the windows as a stand in
for the whole group.

 -- Function: selected-window-group
     When the selected window is a member of a group of windows, this
     function returns a list of the windows in the group, ordered such
     that the first window in the list is displaying the earliest part
     of the buffer, and so on.  Otherwise the function returns a list
     containing just the selected window.

     The selected window is considered part of a group when the buffer
     local variable ‘selected-window-group-function’ is set to a
     function.  In this case, ‘selected-window-group’ calls it with no
     arguments and returns its result (which should be the list of
     windows in the group).


File: elisp.info,  Node: Windows and Frames,  Next: Window Sizes,  Prev: Basic Windows,  Up: Windows

27.2 Windows and Frames
=======================

Each window belongs to exactly one frame (*note Frames::).

 -- Function: window-frame &optional window
     This function returns the frame that the window WINDOW belongs to.
     If WINDOW is ‘nil’, it defaults to the selected window.

 -- Function: window-list &optional frame minibuffer window
     This function returns a list of live windows belonging to the frame
     FRAME.  If FRAME is omitted or ‘nil’, it defaults to the selected
     frame.

     The optional argument MINIBUFFER specifies whether to include the
     minibuffer window in the returned list.  If MINIBUFFER is ‘t’, the
     minibuffer window is included.  If MINIBUFFER is ‘nil’ or omitted,
     the minibuffer window is included only if it is active.  If
     MINIBUFFER is neither ‘nil’ nor ‘t’, the minibuffer window is never
     included.

     The optional argument WINDOW, if non-‘nil’, should be a live window
     on the specified frame; then WINDOW will be the first element in
     the returned list.  If WINDOW is omitted or ‘nil’, the window
     selected within the frame is the first element.

   Windows in the same frame are organized into a “window tree”, whose
leaf nodes are the live windows.  The internal nodes of a window tree
are not live; they exist for the purpose of organizing the relationships
between live windows.  The root node of a window tree is called the
“root window”.  It can be either a live window (if the frame has just
one window), or an internal window.

   A minibuffer window (*note Minibuffer Windows::) is not part of its
frame’s window tree unless the frame is a minibuffer-only frame.
Nonetheless, most of the functions in this section accept the minibuffer
window as an argument.  Also, the function ‘window-tree’ described at
the end of this section lists the minibuffer window alongside the actual
window tree.

 -- Function: frame-root-window &optional frame-or-window
     This function returns the root window for FRAME-OR-WINDOW.  The
     argument FRAME-OR-WINDOW should be either a window or a frame; if
     omitted or ‘nil’, it defaults to the selected frame.  If
     FRAME-OR-WINDOW is a window, the return value is the root window of
     that window’s frame.

   When a window is split, there are two live windows where previously
there was one.  One of these is represented by the same Lisp window
object as the original window, and the other is represented by a
newly-created Lisp window object.  Both of these live windows become
leaf nodes of the window tree, as “child windows” of a single internal
window.  If necessary, Emacs automatically creates this internal window,
which is also called the “parent window”, and assigns it to the
appropriate position in the window tree.  A set of windows that share
the same parent are called “siblings”.

 -- Function: window-parent &optional window
     This function returns the parent window of WINDOW.  If WINDOW is
     omitted or ‘nil’, it defaults to the selected window.  The return
     value is ‘nil’ if WINDOW has no parent (i.e., it is a minibuffer
     window or the root window of its frame).

   Each internal window always has at least two child windows.  If this
number falls to one as a result of window deletion, Emacs automatically
deletes the internal window, and its sole remaining child window takes
its place in the window tree.

   Each child window can be either a live window, or an internal window
(which in turn would have its own child windows).  Therefore, each
internal window can be thought of as occupying a certain rectangular
“screen area”—the union of the areas occupied by the live windows that
are ultimately descended from it.

   For each internal window, the screen areas of the immediate children
are arranged either vertically or horizontally (never both).  If the
child windows are arranged one above the other, they are said to form a
“vertical combination”; if they are arranged side by side, they are said
to form a “horizontal combination”.  Consider the following example:

          ______________________________________
         | ______  ____________________________ |
         ||      || __________________________ ||
         ||      |||                          |||
         ||      |||                          |||
         ||      |||                          |||
         ||      |||____________W4____________|||
         ||      || __________________________ ||
         ||      |||                          |||
         ||      |||                          |||
         ||      |||____________W5____________|||
         ||__W2__||_____________W3_____________ |
         |__________________W1__________________|


The root window of this frame is an internal window, W1.  Its child
windows form a horizontal combination, consisting of the live window W2
and the internal window W3.  The child windows of W3 form a vertical
combination, consisting of the live windows W4 and W5.  Hence, the live
windows in this window tree are W2, W4, and W5.

   The following functions can be used to retrieve a child window of an
internal window, and the siblings of a child window.

 -- Function: window-top-child &optional window
     This function returns the topmost child window of WINDOW, if WINDOW
     is an internal window whose children form a vertical combination.
     For any other type of window, the return value is ‘nil’.

 -- Function: window-left-child &optional window
     This function returns the leftmost child window of WINDOW, if
     WINDOW is an internal window whose children form a horizontal
     combination.  For any other type of window, the return value is
     ‘nil’.

 -- Function: window-child window
     This function returns the first child window of the internal window
     WINDOW—the topmost child window for a vertical combination, or the
     leftmost child window for a horizontal combination.  If WINDOW is a
     live window, the return value is ‘nil’.

 -- Function: window-combined-p &optional window horizontal
     This function returns a non-‘nil’ value if and only if WINDOW is
     part of a vertical combination.  If WINDOW is omitted or ‘nil’, it
     defaults to the selected one.

     If the optional argument HORIZONTAL is non-‘nil’, this means to
     return non-‘nil’ if and only if WINDOW is part of a horizontal
     combination.

 -- Function: window-next-sibling &optional window
     This function returns the next sibling of the window WINDOW.  If
     omitted or ‘nil’, WINDOW defaults to the selected window.  The
     return value is ‘nil’ if WINDOW is the last child of its parent.

 -- Function: window-prev-sibling &optional window
     This function returns the previous sibling of the window WINDOW.
     If omitted or ‘nil’, WINDOW defaults to the selected window.  The
     return value is ‘nil’ if WINDOW is the first child of its parent.

   The functions ‘window-next-sibling’ and ‘window-prev-sibling’ should
not be confused with the functions ‘next-window’ and ‘previous-window’,
which return the next and previous window, respectively, in the cyclic
ordering of windows (*note Cyclic Window Ordering::).

   You can use the following functions to find the first live window on
a frame and the window nearest to a given window.

 -- Function: frame-first-window &optional frame-or-window
     This function returns the live window at the upper left corner of
     the frame specified by FRAME-OR-WINDOW.  The argument
     FRAME-OR-WINDOW must denote a window or a live frame and defaults
     to the selected frame.  If FRAME-OR-WINDOW specifies a window, this
     function returns the first window on that window’s frame.  Under
     the assumption that the frame from our canonical example is
     selected ‘(frame-first-window)’ returns W2.

 -- Function: window-in-direction direction &optional window ignore sign
          wrap mini
     This function returns the nearest live window in direction
     DIRECTION as seen from the position of ‘window-point’ in window
     WINDOW.  The argument DIRECTION must be one of ‘above’, ‘below’,
     ‘left’ or ‘right’.  The optional argument WINDOW must denote a live
     window and defaults to the selected one.

     This function does not return a window whose ‘no-other-window’
     parameter is non-‘nil’ (*note Window Parameters::).  If the nearest
     window’s ‘no-other-window’ parameter is non-‘nil’, this function
     tries to find another window in the indicated direction whose
     ‘no-other-window’ parameter is ‘nil’.  If the optional argument
     IGNORE is non-‘nil’, a window may be returned even if its
     ‘no-other-window’ parameter is non-‘nil’.

     If the optional argument SIGN is a negative number, it means to use
     the right or bottom edge of WINDOW as reference position instead of
     ‘window-point’.  If SIGN is a positive number, it means to use the
     left or top edge of WINDOW as reference position.

     If the optional argument WRAP is non-‘nil’, this means to wrap
     DIRECTION around frame borders.  For example, if WINDOW is at the
     top of the frame and DIRECTION is ‘above’, then this function
     usually returns the frame’s minibuffer window if it’s active and a
     window at the bottom of the frame otherwise.

     If the optional argument MINI is ‘nil’, this means to return the
     minibuffer window if and only if it is currently active.  If MINI
     is non-‘nil’, this function may return the minibuffer window even
     when it’s not active.  However, if WRAP is non-‘nil’, it always
     acts as if MINI were ‘nil’.

     If it doesn’t find a suitable window, this function returns ‘nil’.

   The following function allows the entire window tree of a frame to be
retrieved:

 -- Function: window-tree &optional frame
     This function returns a list representing the window tree for frame
     FRAME.  If FRAME is omitted or ‘nil’, it defaults to the selected
     frame.

     The return value is a list of the form ‘(ROOT MINI)’, where ROOT
     represents the window tree of the frame’s root window, and MINI is
     the frame’s minibuffer window.

     If the root window is live, ROOT is that window itself.  Otherwise,
     ROOT is a list ‘(DIR EDGES W1 W2 ...)’ where DIR is ‘nil’ for a
     horizontal combination and ‘t’ for a vertical combination, EDGES
     gives the size and position of the combination, and the remaining
     elements are the child windows.  Each child window may again be a
     window object (for a live window) or a list with the same format as
     above (for an internal window).  The EDGES element is a list ‘(LEFT
     TOP RIGHT BOTTOM)’, similar to the value returned by ‘window-edges’
     (*note Coordinates and Windows::).


File: elisp.info,  Node: Window Sizes,  Next: Resizing Windows,  Prev: Windows and Frames,  Up: Windows

27.3 Window Sizes
=================

The following schematic shows the structure of a live window:

             ____________________________________________
            |______________ Header Line ______________|RD| ^
          ^ |LS|LM|LF|                       |RF|RM|RS|  | |
          | |  |  |  |                       |  |  |  |  | |
     Window |  |  |  |       Text Area       |  |  |  |  | Window
     Body | |  |  |  |     (Window Body)     |  |  |  |  | Total
     Height |  |  |  |                       |  |  |  |  | Height
          | |  |  |  |<- Window Body Width ->|  |  |  |  | |
          v |__|__|__|_______________________|__|__|__|  | |
            |_________ Horizontal Scroll Bar _________|  | |
            |_______________ Mode Line _______________|__| |
            |_____________ Bottom Divider _______________| v
             <---------- Window Total Width ------------>


   At the center of the window is the “text area”, or “body”, where the
buffer text is displayed.  The text area can be surrounded by a series
of optional areas.  On the left and right, from innermost to outermost,
these are the left and right fringes, denoted by LF and RF (*note
Fringes::); the left and right margins, denoted by LM and RM in the
schematic (*note Display Margins::); the left or right vertical scroll
bar, only one of which is present at any time, denoted by LS and RS
(*note Scroll Bars::); and the right divider, denoted by RD (*note
Window Dividers::).  At the top of the window is the header line (*note
Header Lines::).  At the bottom of the window are the horizontal scroll
bar (*note Scroll Bars::); the mode line (*note Mode Line Format::); and
the bottom divider (*note Window Dividers::).

   Emacs provides miscellaneous functions for finding the height and
width of a window.  The return value of many of these functions can be
specified either in units of pixels or in units of lines and columns.
On a graphical display, the latter actually correspond to the height and
width of a default character specified by the frame’s default font as
returned by ‘frame-char-height’ and ‘frame-char-width’ (*note Frame
Font::).  Thus, if a window is displaying text with a different font or
size, the reported line height and column width for that window may
differ from the actual number of text lines or columns displayed within
it.

   The “total height” of a window is the number of lines comprising the
window’s body, the header line, the horizontal scroll bar, the mode line
and the bottom divider (if any).

 -- Function: window-total-height &optional window round
     This function returns the total height, in lines, of the window
     WINDOW.  If WINDOW is omitted or ‘nil’, it defaults to the selected
     window.  If WINDOW is an internal window, the return value is the
     total height occupied by its descendant windows.

     If a window’s pixel height is not an integral multiple of its
     frame’s default character height, the number of lines occupied by
     the window is rounded internally.  This is done in a way such that,
     if the window is a parent window, the sum of the total heights of
     all its child windows internally equals the total height of their
     parent.  This means that although two windows have the same pixel
     height, their internal total heights may differ by one line.  This
     means also, that if window is vertically combined and has a next
     sibling, the topmost row of that sibling can be calculated as the
     sum of this window’s topmost row and total height (*note
     Coordinates and Windows::)

     If the optional argument ROUND is ‘ceiling’, this function returns
     the smallest integer larger than WINDOW’s pixel height divided by
     the character height of its frame; if it is ‘floor’, it returns the
     largest integer smaller than said value; with any other ROUND it
     returns the internal value of WINDOWS’s total height.

   The “total width” of a window is the number of lines comprising the
window’s body, its margins, fringes, scroll bars and a right divider (if
any).

 -- Function: window-total-width &optional window round
     This function returns the total width, in columns, of the window
     WINDOW.  If WINDOW is omitted or ‘nil’, it defaults to the selected
     window.  If WINDOW is internal, the return value is the total width
     occupied by its descendant windows.

     If a window’s pixel width is not an integral multiple of its
     frame’s character width, the number of lines occupied by the window
     is rounded internally.  This is done in a way such that, if the
     window is a parent window, the sum of the total widths of all its
     children internally equals the total width of their parent.  This
     means that although two windows have the same pixel width, their
     internal total widths may differ by one column.  This means also,
     that if this window is horizontally combined and has a next
     sibling, the leftmost column of that sibling can be calculated as
     the sum of this window’s leftmost column and total width (*note
     Coordinates and Windows::).  The optional argument ROUND behaves as
     it does for ‘window-total-height’.

 -- Function: window-total-size &optional window horizontal round
     This function returns either the total height in lines or the total
     width in columns of the window WINDOW.  If HORIZONTAL is omitted or
     ‘nil’, this is equivalent to calling ‘window-total-height’ for
     WINDOW; otherwise it is equivalent to calling ‘window-total-width’
     for WINDOW.  The optional argument ROUND behaves as it does for
     ‘window-total-height’.

   The following two functions can be used to return the total size of a
window in units of pixels.

 -- Function: window-pixel-height &optional window
     This function returns the total height of window WINDOW in pixels.
     WINDOW must be a valid window and defaults to the selected one.

     The return value includes mode and header line, a horizontal scroll
     bar and a bottom divider, if any.  If WINDOW is an internal window,
     its pixel height is the pixel height of the screen areas spanned by
     its children.

 -- Function: window-pixel-width &optional window
     This function returns the width of window WINDOW in pixels.  WINDOW
     must be a valid window and defaults to the selected one.

     The return value includes the fringes and margins of WINDOW as well
     as any vertical dividers or scroll bars belonging to WINDOW.  If
     WINDOW is an internal window, its pixel width is the width of the
     screen areas spanned by its children.

   The following functions can be used to determine whether a given
window has any adjacent windows.

 -- Function: window-full-height-p &optional window
     This function returns non-‘nil’ if WINDOW has no other window above
     or below it in its frame.  More precisely, this means that the
     total height of WINDOW equals the total height of the root window
     on that frame.  The minibuffer window does not count in this
     regard.  If WINDOW is omitted or ‘nil’, it defaults to the selected
     window.

 -- Function: window-full-width-p &optional window
     This function returns non-‘nil’ if WINDOW has no other window to
     the left or right in its frame, i.e., its total width equals that
     of the root window on that frame.  If WINDOW is omitted or ‘nil’,
     it defaults to the selected window.

   The “body height” of a window is the height of its text area, which
does not include a mode or header line, a horizontal scroll bar, or a
bottom divider.

 -- Function: window-body-height &optional window pixelwise
     This function returns the height, in lines, of the body of window
     WINDOW.  If WINDOW is omitted or ‘nil’, it defaults to the selected
     window; otherwise it must be a live window.

     If the optional argument PIXELWISE is non-‘nil’, this function
     returns the body height of WINDOW counted in pixels.

     If PIXELWISE is ‘nil’, the return value is rounded down to the
     nearest integer, if necessary.  This means that if a line at the
     bottom of the text area is only partially visible, that line is not
     counted.  It also means that the height of a window’s body can
     never exceed its total height as returned by ‘window-total-height’.

   The “body width” of a window is the width of its text area, which
does not include the scroll bar, fringes, margins or a right divider.
Note that when one or both fringes are removed (by setting their width
to zero), the display engine reserves two character cells, one on each
side of the window, for displaying the continuation and truncation
glyphs, which leaves 2 columns less for text display.  (The function
‘window-max-chars-per-line’, described below, takes this peculiarity
into account.)

 -- Function: window-body-width &optional window pixelwise
     This function returns the width, in columns, of the body of window
     WINDOW.  If WINDOW is omitted or ‘nil’, it defaults to the selected
     window; otherwise it must be a live window.

     If the optional argument PIXELWISE is non-‘nil’, this function
     returns the body width of WINDOW in units of pixels.

     If PIXELWISE is ‘nil’, the return value is rounded down to the
     nearest integer, if necessary.  This means that if a column on the
     right of the text area is only partially visible, that column is
     not counted.  It also means that the width of a window’s body can
     never exceed its total width as returned by ‘window-total-width’.

 -- Function: window-body-size &optional window horizontal pixelwise
     This function returns the body height or body width of WINDOW.  If
     HORIZONTAL is omitted or ‘nil’, it is equivalent to calling
     ‘window-body-height’ for WINDOW; otherwise it is equivalent to
     calling ‘window-body-width’.  In either case, the optional argument
     PIXELWISE is passed to the function called.

   For compatibility with previous versions of Emacs, ‘window-height’ is
an alias for ‘window-total-height’, and ‘window-width’ is an alias for
‘window-body-width’.  These aliases are considered obsolete and will be
removed in the future.

   The pixel heights of a window’s mode and header line can be retrieved
with the functions given below.  Their return value is usually accurate
unless the window has not been displayed before: In that case, the
return value is based on an estimate of the font used for the window’s
frame.

 -- Function: window-mode-line-height &optional window
     This function returns the height in pixels of WINDOW’s mode line.
     WINDOW must be a live window and defaults to the selected one.  If
     WINDOW has no mode line, the return value is zero.

 -- Function: window-header-line-height &optional window
     This function returns the height in pixels of WINDOW’s header line.
     WINDOW must be a live window and defaults to the selected one.  If
     WINDOW has no header line, the return value is zero.

   Functions for retrieving the height and/or width of window dividers
(*note Window Dividers::), fringes (*note Fringes::), scroll bars (*note
Scroll Bars::), and display margins (*note Display Margins::) are
described in the corresponding sections.

   If your Lisp program needs to make layout decisions, you will find
the following function useful:

 -- Function: window-max-chars-per-line &optional window face
     This function returns the number of characters displayed in the
     specified face FACE in the specified window WINDOW (which must be a
     live window).  If FACE was remapped (*note Face Remapping::), the
     information is returned for the remapped face.  If omitted or
     ‘nil’, FACE defaults to the default face, and WINDOW defaults to
     the selected window.

     Unlike ‘window-body-width’, this function accounts for the actual
     size of FACE’s font, instead of working in units of the canonical
     character width of WINDOW’s frame (*note Frame Font::).  It also
     accounts for space used by the continuation glyph, if WINDOW lacks
     one or both of its fringes.

   Commands that change the size of windows (*note Resizing Windows::),
or split them (*note Splitting Windows::), obey the variables
‘window-min-height’ and ‘window-min-width’, which specify the smallest
allowable window height and width.  They also obey the variable
‘window-size-fixed’, with which a window can be “fixed” in size (*note
Preserving Window Sizes::).

 -- User Option: window-min-height
     This option specifies the minimum total height, in lines, of any
     window.  Its value has to accommodate at least one text line as
     well as a mode and header line, a horizontal scroll bar and a
     bottom divider, if present.

 -- User Option: window-min-width
     This option specifies the minimum total width, in columns, of any
     window.  Its value has to accommodate two text columns as well as
     margins, fringes, a scroll bar and a right divider, if present.

   The following function tells how small a specific window can get
taking into account the sizes of its areas and the values of
‘window-min-height’, ‘window-min-width’ and ‘window-size-fixed’ (*note
Preserving Window Sizes::).

 -- Function: window-min-size &optional window horizontal ignore
          pixelwise
     This function returns the minimum size of WINDOW.  WINDOW must be a
     valid window and defaults to the selected one.  The optional
     argument HORIZONTAL non-‘nil’ means to return the minimum number of
     columns of WINDOW; otherwise return the minimum number of WINDOW’s
     lines.

     The return value makes sure that all components of WINDOW remain
     fully visible if WINDOW’s size were actually set to it.  With
     HORIZONTAL ‘nil’ it includes the mode and header line, the
     horizontal scroll bar and the bottom divider, if present.  With
     HORIZONTAL non-‘nil’ it includes the margins and fringes, the
     vertical scroll bar and the right divider, if present.

     The optional argument IGNORE, if non-‘nil’, means ignore
     restrictions imposed by fixed size windows, ‘window-min-height’ or
     ‘window-min-width’ settings.  If IGNORE equals ‘safe’, live windows
     may get as small as ‘window-safe-min-height’ lines and
     ‘window-safe-min-width’ columns.  If IGNORE is a window, ignore
     restrictions for that window only.  Any other non-‘nil’ value means
     ignore all of the above restrictions for all windows.

     The optional argument PIXELWISE non-‘nil’ means to return the
     minimum size of WINDOW counted in pixels.


File: elisp.info,  Node: Resizing Windows,  Next: Preserving Window Sizes,  Prev: Window Sizes,  Up: Windows

27.4 Resizing Windows
=====================

This section describes functions for resizing a window without changing
the size of its frame.  Because live windows do not overlap, these
functions are meaningful only on frames that contain two or more
windows: resizing a window also changes the size of a neighboring
window.  If there is just one window on a frame, its size cannot be
changed except by resizing the frame (*note Size and Position::).

   Except where noted, these functions also accept internal windows as
arguments.  Resizing an internal window causes its child windows to be
resized to fit the same space.

 -- Function: window-resizable window delta &optional horizontal ignore
          pixelwise
     This function returns DELTA if the size of WINDOW can be changed
     vertically by DELTA lines.  If the optional argument HORIZONTAL is
     non-‘nil’, it instead returns DELTA if WINDOW can be resized
     horizontally by DELTA columns.  It does not actually change the
     window size.

     If WINDOW is ‘nil’, it defaults to the selected window.

     A positive value of DELTA means to check whether the window can be
     enlarged by that number of lines or columns; a negative value of
     DELTA means to check whether the window can be shrunk by that many
     lines or columns.  If DELTA is non-zero, a return value of 0 means
     that the window cannot be resized.

     Normally, the variables ‘window-min-height’ and ‘window-min-width’
     specify the smallest allowable window size (*note Window Sizes::).
     However, if the optional argument IGNORE is non-‘nil’, this
     function ignores ‘window-min-height’ and ‘window-min-width’, as
     well as ‘window-size-fixed’.  Instead, it considers the
     minimum-height window to be one consisting of a header and a mode
     line, a horizontal scrollbar and a bottom divider (if any), plus a
     text area one line tall; and a minimum-width window as one
     consisting of fringes, margins, a scroll bar and a right divider
     (if any), plus a text area two columns wide.

     If the optional argument PIXELWISE is non-‘nil’, DELTA is
     interpreted as pixels.

 -- Function: window-resize window delta &optional horizontal ignore
          pixelwise
     This function resizes WINDOW by DELTA increments.  If HORIZONTAL is
     ‘nil’, it changes the height by DELTA lines; otherwise, it changes
     the width by DELTA columns.  A positive DELTA means to enlarge the
     window, and a negative DELTA means to shrink it.

     If WINDOW is ‘nil’, it defaults to the selected window.  If the
     window cannot be resized as demanded, an error is signaled.

     The optional argument IGNORE has the same meaning as for the
     function ‘window-resizable’ above.

     If the optional argument PIXELWISE is non-‘nil’, DELTA will be
     interpreted as pixels.

     The choice of which window edges this function alters depends on
     the values of the option ‘window-combination-resize’ and the
     combination limits of the involved windows; in some cases, it may
     alter both edges.  *Note Recombining Windows::.  To resize by
     moving only the bottom or right edge of a window, use the function
     ‘adjust-window-trailing-edge’.

 -- Function: adjust-window-trailing-edge window delta &optional
          horizontal pixelwise
     This function moves WINDOW’s bottom edge by DELTA lines.  If
     optional argument HORIZONTAL is non-‘nil’, it instead moves the
     right edge by DELTA columns.  If WINDOW is ‘nil’, it defaults to
     the selected window.

     If the optional argument PIXELWISE is non-‘nil’, DELTA is
     interpreted as pixels.

     A positive DELTA moves the edge downwards or to the right; a
     negative DELTA moves it upwards or to the left.  If the edge cannot
     be moved as far as specified by DELTA, this function moves it as
     far as possible but does not signal a error.

     This function tries to resize windows adjacent to the edge that is
     moved.  If this is not possible for some reason (e.g., if that
     adjacent window is fixed-size), it may resize other windows.

 -- User Option: window-resize-pixelwise
     If the value of this option is non-‘nil’, Emacs resizes windows in
     units of pixels.  This currently affects functions like
     ‘split-window’ (*note Splitting Windows::), ‘maximize-window’,
     ‘minimize-window’, ‘fit-window-to-buffer’, ‘fit-frame-to-buffer’
     and ‘shrink-window-if-larger-than-buffer’ (all listed below).

     Note that when a frame’s pixel size is not a multiple of its
     character size, at least one window may get resized pixelwise even
     if this option is ‘nil’.  The default value is ‘nil’.

   The following commands resize windows in more specific ways.  When
called interactively, they act on the selected window.

 -- Command: fit-window-to-buffer &optional window max-height min-height
          max-width min-width preserve-size
     This command adjusts the height or width of WINDOW to fit the text
     in it.  It returns non-‘nil’ if it was able to resize WINDOW, and
     ‘nil’ otherwise.  If WINDOW is omitted or ‘nil’, it defaults to the
     selected window.  Otherwise, it should be a live window.

     If WINDOW is part of a vertical combination, this function adjusts
     WINDOW’s height.  The new height is calculated from the actual
     height of the accessible portion of its buffer.  The optional
     argument MAX-HEIGHT, if non-‘nil’, specifies the maximum total
     height that this function can give WINDOW.  The optional argument
     MIN-HEIGHT, if non-‘nil’, specifies the minimum total height that
     it can give, which overrides the variable ‘window-min-height’.
     Both MAX-HEIGHT and MIN-HEIGHT are specified in lines and include
     mode and header line and a bottom divider, if any.

     If WINDOW is part of a horizontal combination and the value of the
     option ‘fit-window-to-buffer-horizontally’ (see below) is
     non-‘nil’, this function adjusts WINDOW’s height.  The new width of
     WINDOW is calculated from the maximum length of its buffer’s lines
     that follow the current start position of WINDOW.  The optional
     argument MAX-WIDTH specifies a maximum width and defaults to the
     width of WINDOW’s frame.  The optional argument MIN-WIDTH specifies
     a minimum width and defaults to ‘window-min-width’.  Both MAX-WIDTH
     and MIN-WIDTH are specified in columns and include fringes, margins
     and scrollbars, if any.

     The optional argument PRESERVE-SIZE, if non-‘nil’, will install a
     parameter to preserve the size of WINDOW during future resize
     operations (*note Preserving Window Sizes::).

     If the option ‘fit-frame-to-buffer’ (see below) is non-‘nil’, this
     function will try to resize the frame of WINDOW to fit its contents
     by calling ‘fit-frame-to-buffer’ (see below).

 -- User Option: fit-window-to-buffer-horizontally
     If this is non-‘nil’, ‘fit-window-to-buffer’ can resize windows
     horizontally.  If this is ‘nil’ (the default)
     ‘fit-window-to-buffer’ never resizes windows horizontally.  If this
     is ‘only’, it can resize windows horizontally only.  Any other
     value means ‘fit-window-to-buffer’ can resize windows in both
     dimensions.

 -- User Option: fit-frame-to-buffer
     If this option is non-‘nil’, ‘fit-window-to-buffer’ can fit a frame
     to its buffer.  A frame is fit if and only if its root window is a
     live window and this option is non-‘nil’.  If this is
     ‘horizontally’, frames are fit horizontally only.  If this is
     ‘vertically’, frames are fit vertically only.  Any other non-‘nil’
     value means frames can be resized in both dimensions.

   If you have a frame that displays only one window, you can fit that
frame to its buffer using the command ‘fit-frame-to-buffer’.

 -- Command: fit-frame-to-buffer &optional frame max-height min-height
          max-width min-width only
     This command adjusts the size of FRAME to display the contents of
     its buffer exactly.  FRAME can be any live frame and defaults to
     the selected one.  Fitting is done only if FRAME’s root window is
     live.  The arguments MAX-HEIGHT, MIN-HEIGHT, MAX-WIDTH and
     MIN-WIDTH specify bounds on the new total size of FRAME’s root
     window.  MIN-HEIGHT and MIN-WIDTH default to the values of
     ‘window-min-height’ and ‘window-min-width’ respectively.

     If the optional argument ONLY is ‘vertically’, this function may
     resize the frame vertically only.  If ONLY is ‘horizontally’, it
     may resize the frame horizontally only.

   The behavior of ‘fit-frame-to-buffer’ can be controlled with the help
of the two options listed next.

 -- User Option: fit-frame-to-buffer-margins
     This option can be used to specify margins around frames to be fit
     by ‘fit-frame-to-buffer’.  Such margins can be useful to avoid, for
     example, that such frames overlap the taskbar.

     It specifies the numbers of pixels to be left free on the left,
     above, the right, and below a frame that shall be fit.  The default
     specifies ‘nil’ for each which means to use no margins.  The value
     specified here can be overridden for a specific frame by that
     frame’s ‘fit-frame-to-buffer-margins’ parameter, if present.

 -- User Option: fit-frame-to-buffer-sizes
     This option specifies size boundaries for ‘fit-frame-to-buffer’.
     It specifies the total maximum and minimum lines and maximum and
     minimum columns of the root window of any frame that shall be fit
     to its buffer.  If any of these values is non-‘nil’, it overrides
     the corresponding argument of ‘fit-frame-to-buffer’.

 -- Command: shrink-window-if-larger-than-buffer &optional window
     This command attempts to reduce WINDOW’s height as much as possible
     while still showing its full buffer, but no less than
     ‘window-min-height’ lines.  The return value is non-‘nil’ if the
     window was resized, and ‘nil’ otherwise.  If WINDOW is omitted or
     ‘nil’, it defaults to the selected window.  Otherwise, it should be
     a live window.

     This command does nothing if the window is already too short to
     display all of its buffer, or if any of the buffer is scrolled
     off-screen, or if the window is the only live window in its frame.

     This command calls ‘fit-window-to-buffer’ (see above) to do its
     work.

 -- Command: balance-windows &optional window-or-frame
     This function balances windows in a way that gives more space to
     full-width and/or full-height windows.  If WINDOW-OR-FRAME
     specifies a frame, it balances all windows on that frame.  If
     WINDOW-OR-FRAME specifies a window, it balances only that window
     and its siblings (*note Windows and Frames::).

 -- Command: balance-windows-area
     This function attempts to give all windows on the selected frame
     approximately the same share of the screen area.  Full-width or
     full-height windows are not given more space than other windows.

 -- Command: maximize-window &optional window
     This function attempts to make WINDOW as large as possible, in both
     dimensions, without resizing its frame or deleting other windows.
     If WINDOW is omitted or ‘nil’, it defaults to the selected window.

 -- Command: minimize-window &optional window
     This function attempts to make WINDOW as small as possible, in both
     dimensions, without deleting it or resizing its frame.  If WINDOW
     is omitted or ‘nil’, it defaults to the selected window.


File: elisp.info,  Node: Preserving Window Sizes,  Next: Splitting Windows,  Prev: Resizing Windows,  Up: Windows

27.5 Preserving Window Sizes
============================

A window can get resized explicitly by using one of the functions from
the preceding section or implicitly, for example, when resizing an
adjacent window, when splitting or deleting a window (*note Splitting
Windows::, *note Deleting Windows::) or when resizing the window’s frame
(*note Size and Position::).

   It is possible to avoid implicit resizing of a specific window when
there are one or more other resizable windows on the same frame.  For
this purpose, Emacs must be advised to “preserve” the size of that
window.  There are two basic ways to do that.

 -- Variable: window-size-fixed
     If this buffer-local variable is non-‘nil’, the size of any window
     displaying the buffer cannot normally be changed.  Deleting a
     window or changing the frame’s size may still change the window’s
     size, if there is no choice.

     If the value is ‘height’, then only the window’s height is fixed;
     if the value is ‘width’, then only the window’s width is fixed.
     Any other non-‘nil’ value fixes both the width and the height.

     If this variable is ‘nil’, this does not necessarily mean that any
     window showing the buffer can be resized in the desired direction.
     To determine that, use the function ‘window-resizable’.  *Note
     Resizing Windows::.

   Often ‘window-size-fixed’ is overly aggressive because it inhibits
any attempt to explicitly resize or split an affected window as well.
This may even happen after the window has been resized implicitly, for
example, when deleting an adjacent window or resizing the window’s
frame.  The following function tries hard to never disallow resizing
such a window explicitly:

 -- Function: window-preserve-size &optional window horizontal preserve
     This function (un-)marks the height of window WINDOW as preserved
     for future resize operations.  WINDOW must be a live window and
     defaults to the selected one.  If the optional argument HORIZONTAL
     is non-‘nil’, it (un-)marks the width of WINDOW as preserved.

     If the optional argument PRESERVE is ‘t’, this means to preserve
     the current height/width of WINDOW’s body.  The height/width of
     WINDOW will change only if Emacs has no better choice.  Resizing a
     window whose height/width is preserved by this function never
     throws an error.

     If PRESERVE is ‘nil’, this means to stop preserving the
     height/width of WINDOW, lifting any respective restraint induced by
     a previous call of this function for WINDOW.  Calling
     ‘enlarge-window’, ‘shrink-window’ or ‘fit-window-to-buffer’ with
     WINDOW as argument may also remove the respective restraint.

   ‘window-preserve-size’ is currently invoked by the following
functions:

‘fit-window-to-buffer’
     If the optional argument PRESERVE-SIZE of that function (*note
     Resizing Windows::) is non-‘nil’, the size established by that
     function is preserved.

‘display-buffer’
     If the ALIST argument of that function (*note Choosing Window::)
     contains a ‘preserve-size’ entry, the size of the window produced
     by that function is preserved.

   ‘window-preserve-size’ installs a window parameter (*note Window
Parameters::) called ‘preserved-size’ which is consulted by the window
resizing functions.  This parameter will not prevent resizing the window
when the window shows another buffer than the one when
‘window-preserve-size’ was invoked or if its size has changed since
then.

   The following function can be used to check whether the height of a
particular window is preserved:

 -- Function: window-preserved-size &optional window horizontal
     This function returns the preserved height of window WINDOW in
     pixels.  WINDOW must be a live window and defaults to the selected
     one.  If the optional argument HORIZONTAL is non-‘nil’, it returns
     the preserved width of WINDOW.  It returns ‘nil’ if the size of
     WINDOW is not preserved.


File: elisp.info,  Node: Splitting Windows,  Next: Deleting Windows,  Prev: Preserving Window Sizes,  Up: Windows

27.6 Splitting Windows
======================

This section describes functions for creating a new window by
“splitting” an existing one.

 -- Function: split-window &optional window size side pixelwise
     This function creates a new live window next to the window WINDOW.
     If WINDOW is omitted or ‘nil’, it defaults to the selected window.
     That window is split, and reduced in size.  The space is taken up
     by the new window, which is returned.

     The optional second argument SIZE determines the sizes of WINDOW
     and/or the new window.  If it is omitted or ‘nil’, both windows are
     given equal sizes; if there is an odd line, it is allocated to the
     new window.  If SIZE is a positive number, WINDOW is given SIZE
     lines (or columns, depending on the value of SIDE).  If SIZE is a
     negative number, the new window is given −SIZE lines (or columns).

     If SIZE is ‘nil’, this function obeys the variables
     ‘window-min-height’ and ‘window-min-width’ (*note Window Sizes::).
     Thus, it signals an error if splitting would result in making a
     window smaller than those variables specify.  However, a non-‘nil’
     value for SIZE causes those variables to be ignored; in that case,
     the smallest allowable window is considered to be one that has
     space for a text area one line tall and/or two columns wide.

     Hence, if SIZE is specified, it’s the caller’s responsibility to
     check whether the emanating windows are large enough to encompass
     all areas like a mode line or a scroll bar.  The function
     ‘window-min-size’ (*note Window Sizes::) can be used to determine
     the minimum requirements of WINDOW in this regard.  Since the new
     window usually inherits areas like the mode line or the scroll bar
     from WINDOW, that function is also a good guess for the minimum
     size of the new window.  The caller should specify a smaller size
     only if it correspondingly removes an inherited area before the
     next redisplay.

     The optional third argument SIDE determines the position of the new
     window relative to WINDOW.  If it is ‘nil’ or ‘below’, the new
     window is placed below WINDOW.  If it is ‘above’, the new window is
     placed above WINDOW.  In both these cases, SIZE specifies a total
     window height, in lines.

     If SIDE is ‘t’ or ‘right’, the new window is placed on the right of
     WINDOW.  If SIDE is ‘left’, the new window is placed on the left of
     WINDOW.  In both these cases, SIZE specifies a total window width,
     in columns.

     The optional fourth argument PIXELWISE, if non-‘nil’, means to
     interpret SIZE in units of pixels, instead of lines and columns.

     If WINDOW is a live window, the new window inherits various
     properties from it, including margins and scroll bars.  If WINDOW
     is an internal window, the new window inherits the properties of
     the window selected within WINDOW’s frame.

     The behavior of this function may be altered by the window
     parameters of WINDOW, so long as the variable
     ‘ignore-window-parameters’ is ‘nil’.  If the value of the
     ‘split-window’ window parameter is ‘t’, this function ignores all
     other window parameters.  Otherwise, if the value of the
     ‘split-window’ window parameter is a function, that function is
     called with the arguments WINDOW, SIZE, and SIDE, in lieu of the
     usual action of ‘split-window’.  Otherwise, this function obeys the
     ‘window-atom’ or ‘window-side’ window parameter, if any.  *Note
     Window Parameters::.

   As an example, here is a sequence of ‘split-window’ calls that yields
the window configuration discussed in *note Windows and Frames::.  This
example demonstrates splitting a live window as well as splitting an
internal window.  We begin with a frame containing a single window (a
live root window), which we denote by W4.  Calling ‘(split-window W4)’
yields this window configuration:

          ______________________________________
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||_________________W4_________________||
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||_________________W5_________________||
         |__________________W3__________________|


The ‘split-window’ call has created a new live window, denoted by W5.
It has also created a new internal window, denoted by W3, which becomes
the root window and the parent of both W4 and W5.

   Next, we call ‘(split-window W3 nil 'left)’, passing the internal
window W3 as the argument.  The result:

          ______________________________________
         | ______  ____________________________ |
         ||      || __________________________ ||
         ||      |||                          |||
         ||      |||                          |||
         ||      |||                          |||
         ||      |||____________W4____________|||
         ||      || __________________________ ||
         ||      |||                          |||
         ||      |||                          |||
         ||      |||____________W5____________|||
         ||__W2__||_____________W3_____________ |
         |__________________W1__________________|

A new live window W2 is created, to the left of the internal window W3.
A new internal window W1 is created, becoming the new root window.

   For interactive use, Emacs provides two commands which always split
the selected window.  These call ‘split-window’ internally.

 -- Command: split-window-right &optional size
     This function splits the selected window into two side-by-side
     windows, putting the selected window on the left.  If SIZE is
     positive, the left window gets SIZE columns; if SIZE is negative,
     the right window gets −SIZE columns.

 -- Command: split-window-below &optional size
     This function splits the selected window into two windows, one
     above the other, leaving the upper window selected.  If SIZE is
     positive, the upper window gets SIZE lines; if SIZE is negative,
     the lower window gets −SIZE lines.

 -- User Option: split-window-keep-point
     If the value of this variable is non-‘nil’ (the default),
     ‘split-window-below’ behaves as described above.

     If it is ‘nil’, ‘split-window-below’ adjusts point in each of the
     two windows to minimize redisplay.  (This is useful on slow
     terminals.)  It selects whichever window contains the screen line
     that point was previously on.  Note that this only affects
     ‘split-window-below’, not the lower-level ‘split-window’ function.


File: elisp.info,  Node: Deleting Windows,  Next: Recombining Windows,  Prev: Splitting Windows,  Up: Windows

27.7 Deleting Windows
=====================

“Deleting” a window removes it from the frame’s window tree.  If the
window is a live window, it disappears from the screen.  If the window
is an internal window, its child windows are deleted too.

   Even after a window is deleted, it continues to exist as a Lisp
object, until there are no more references to it.  Window deletion can
be reversed, by restoring a saved window configuration (*note Window
Configurations::).

 -- Command: delete-window &optional window
     This function removes WINDOW from display and returns ‘nil’.  If
     WINDOW is omitted or ‘nil’, it defaults to the selected window.  If
     deleting the window would leave no more windows in the window tree
     (e.g., if it is the only live window in the frame), an error is
     signaled.

     By default, the space taken up by WINDOW is given to one of its
     adjacent sibling windows, if any.  However, if the variable
     ‘window-combination-resize’ is non-‘nil’, the space is
     proportionally distributed among any remaining windows in the same
     window combination.  *Note Recombining Windows::.

     The behavior of this function may be altered by the window
     parameters of WINDOW, so long as the variable
     ‘ignore-window-parameters’ is ‘nil’.  If the value of the
     ‘delete-window’ window parameter is ‘t’, this function ignores all
     other window parameters.  Otherwise, if the value of the
     ‘delete-window’ window parameter is a function, that function is
     called with the argument WINDOW, in lieu of the usual action of
     ‘delete-window’.  Otherwise, this function obeys the ‘window-atom’
     or ‘window-side’ window parameter, if any.  *Note Window
     Parameters::.

 -- Command: delete-other-windows &optional window
     This function makes WINDOW fill its frame, by deleting other
     windows as necessary.  If WINDOW is omitted or ‘nil’, it defaults
     to the selected window.  The return value is ‘nil’.

     The behavior of this function may be altered by the window
     parameters of WINDOW, so long as the variable
     ‘ignore-window-parameters’ is ‘nil’.  If the value of the
     ‘delete-other-windows’ window parameter is ‘t’, this function
     ignores all other window parameters.  Otherwise, if the value of
     the ‘delete-other-windows’ window parameter is a function, that
     function is called with the argument WINDOW, in lieu of the usual
     action of ‘delete-other-windows’.  Otherwise, this function obeys
     the ‘window-atom’ or ‘window-side’ window parameter, if any.  *Note
     Window Parameters::.

 -- Command: delete-windows-on &optional buffer-or-name frame
     This function deletes all windows showing BUFFER-OR-NAME, by
     calling ‘delete-window’ on those windows.  BUFFER-OR-NAME should be
     a buffer, or the name of a buffer; if omitted or ‘nil’, it defaults
     to the current buffer.  If there are no windows showing the
     specified buffer, this function does nothing.  If the specified
     buffer is a minibuffer, an error is signaled.

     If there is a dedicated window showing the buffer, and that window
     is the only one on its frame, this function also deletes that frame
     if it is not the only frame on the terminal.

     The optional argument FRAME specifies which frames to operate on:

        • ‘nil’ means operate on all frames.
        • ‘t’ means operate on the selected frame.
        • ‘visible’ means operate on all visible frames.
        • ‘0’ means operate on all visible or iconified frames.
        • A frame means operate on that frame.

     Note that this argument does not have the same meaning as in other
     functions which scan all live windows (*note Cyclic Window
     Ordering::).  Specifically, the meanings of ‘t’ and ‘nil’ here are
     the opposite of what they are in those other functions.


File: elisp.info,  Node: Recombining Windows,  Next: Selecting Windows,  Prev: Deleting Windows,  Up: Windows

27.8 Recombining Windows
========================

When deleting the last sibling of a window W, its parent window is
deleted too, with W replacing it in the window tree.  This means that W
must be recombined with its parent’s siblings to form a new window
combination (*note Windows and Frames::).  In some occasions, deleting a
live window may even entail the deletion of two internal windows.

          ______________________________________
         | ______  ____________________________ |
         ||      || __________________________ ||
         ||      ||| ___________  ___________ |||
         ||      ||||           ||           ||||
         ||      ||||____W6_____||_____W7____||||
         ||      |||____________W4____________|||
         ||      || __________________________ ||
         ||      |||                          |||
         ||      |||                          |||
         ||      |||____________W5____________|||
         ||__W2__||_____________W3_____________ |
         |__________________W1__________________|


Deleting W5 in this configuration normally causes the deletion of W3 and
W4.  The remaining live windows W2, W6 and W7 are recombined to form a
new horizontal combination with parent W1.

   Sometimes, however, it makes sense to not delete a parent window like
W4.  In particular, a parent window should not be removed when it was
used to preserve a combination embedded in a combination of the same
type.  Such embeddings make sense to assure that when you split a window
and subsequently delete the new window, Emacs reestablishes the layout
of the associated frame as it existed before the splitting.

   Consider a scenario starting with two live windows W2 and W3 and
their parent W1.

          ______________________________________
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||_________________W2_________________||
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||_________________W3_________________||
         |__________________W1__________________|


Split W2 to make a new window W4 as follows.

          ______________________________________
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||_________________W2_________________||
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||_________________W4_________________||
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||_________________W3_________________||
         |__________________W1__________________|


Now, when enlarging a window vertically, Emacs tries to obtain the
corresponding space from its lower sibling, provided such a window
exists.  In our scenario, enlarging W4 will steal space from W3.

          ______________________________________
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||_________________W2_________________||
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||_________________W4_________________||
         | ____________________________________ |
         ||_________________W3_________________||
         |__________________W1__________________|


Deleting W4 will now give its entire space to W2, including the space
earlier stolen from W3.

          ______________________________________
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||_________________W2_________________||
         | ____________________________________ |
         ||_________________W3_________________||
         |__________________W1__________________|


This can be counterintuitive, in particular if W4 were used for
displaying a buffer only temporarily (*note Temporary Displays::), and
you want to continue working with the initial layout.

   The behavior can be fixed by making a new parent window when
splitting W2.  The variable described next allows that to be done.

 -- User Option: window-combination-limit
     This variable controls whether splitting a window shall make a new
     parent window.  The following values are recognized:

     ‘nil’
          This means that the new live window is allowed to share the
          existing parent window, if one exists, provided the split
          occurs in the same direction as the existing window
          combination (otherwise, a new internal window is created
          anyway).

     ‘window-size’
          In this case ‘display-buffer’ makes a new parent window if it
          is passed a ‘window-height’ or ‘window-width’ entry in the
          ALIST argument (*note Display Action Functions::).

     ‘temp-buffer’
          This value causes the creation of a new parent window when a
          window is split for showing a temporary buffer (*note
          Temporary Displays::) only.

     ‘display-buffer’
          This means that when ‘display-buffer’ (*note Choosing
          Window::) splits a window it always makes a new parent window.

     ‘t’
          In this case a new parent window is always created when
          splitting a window.  Thus, if the value of this variable is at
          all times ‘t’, then at all times every window tree is a binary
          tree (a tree where each window except the root window has
          exactly one sibling).

     The default is ‘nil’.  Other values are reserved for future use.

     If, as a consequence of this variable’s setting, ‘split-window’
     makes a new parent window, it also calls
     ‘set-window-combination-limit’ (see below) on the newly-created
     internal window.  This affects how the window tree is rearranged
     when the child windows are deleted (see below).

   If ‘window-combination-limit’ is ‘t’, splitting W2 in the initial
configuration of our scenario would have produced this:

          ______________________________________
         | ____________________________________ |
         || __________________________________ ||
         |||                                  |||
         |||________________W2________________|||
         || __________________________________ ||
         |||                                  |||
         |||________________W4________________|||
         ||_________________W5_________________||
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||_________________W3_________________||
         |__________________W1__________________|


A new internal window W5 has been created; its children are W2 and the
new live window W4.  Now, W2 is the only sibling of W4, so enlarging W4
will try to shrink W2, leaving W3 unaffected.  Observe that W5
represents a vertical combination of two windows embedded in the
vertical combination W1.

 -- Function: set-window-combination-limit window limit
     This function sets the “combination limit” of the window WINDOW to
     LIMIT.  This value can be retrieved via the function
     ‘window-combination-limit’.  See below for its effects; note that
     it is only meaningful for internal windows.  The ‘split-window’
     function automatically calls this function, passing it ‘t’ as
     LIMIT, provided the value of the variable
     ‘window-combination-limit’ is ‘t’ when it is called.

 -- Function: window-combination-limit window
     This function returns the combination limit for WINDOW.

     The combination limit is meaningful only for an internal window.
     If it is ‘nil’, then Emacs is allowed to automatically delete
     WINDOW, in response to a window deletion, in order to group the
     child windows of WINDOW with its sibling windows to form a new
     window combination.  If the combination limit is ‘t’, the child
     windows of WINDOW are never automatically recombined with its
     siblings.

     If, in the configuration shown at the beginning of this section,
     the combination limit of W4 (the parent window of W6 and W7) is
     ‘t’, deleting W5 will not implicitly delete W4 too.

   Alternatively, the problems sketched above can be avoided by always
resizing all windows in the same combination whenever one of its windows
is split or deleted.  This also permits splitting windows that would be
otherwise too small for such an operation.

 -- User Option: window-combination-resize
     If this variable is ‘nil’, ‘split-window’ can only split a window
     (denoted by WINDOW) if WINDOW’s screen area is large enough to
     accommodate both itself and the new window.

     If this variable is ‘t’, ‘split-window’ tries to resize all windows
     that are part of the same combination as WINDOW, in order to
     accommodate the new window.  In particular, this may allow
     ‘split-window’ to succeed even if WINDOW is a fixed-size window or
     too small to ordinarily split.  Furthermore, subsequently resizing
     or deleting WINDOW may resize all other windows in its combination.

     The default is ‘nil’.  Other values are reserved for future use.  A
     specific split operation may ignore the value of this variable if
     it is affected by a non-‘nil’ value of ‘window-combination-limit’.

   To illustrate the effect of ‘window-combination-resize’, consider the
following frame layout.

          ______________________________________
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||_________________W2_________________||
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||_________________W3_________________||
         |__________________W1__________________|


If ‘window-combination-resize’ is ‘nil’, splitting window W3 leaves the
size of W2 unchanged:

          ______________________________________
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||                                    ||
         ||_________________W2_________________||
         | ____________________________________ |
         ||                                    ||
         ||_________________W3_________________||
         | ____________________________________ |
         ||                                    ||
         ||_________________W4_________________||
         |__________________W1__________________|


If ‘window-combination-resize’ is ‘t’, splitting W3 instead leaves all
three live windows with approximately the same height:

          ______________________________________
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||_________________W2_________________||
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||_________________W3_________________||
         | ____________________________________ |
         ||                                    ||
         ||                                    ||
         ||_________________W4_________________||
         |__________________W1__________________|


Deleting any of the live windows W2, W3 or W4 will distribute its space
proportionally among the two remaining live windows.


File: elisp.info,  Node: Selecting Windows,  Next: Cyclic Window Ordering,  Prev: Recombining Windows,  Up: Windows

27.9 Selecting Windows
======================

 -- Function: select-window window &optional norecord
     This function makes WINDOW the selected window and the window
     selected within its frame (*note Basic Windows::) and selects that
     frame.  It also makes WINDOW’s buffer (*note Buffers and Windows::)
     current and sets that buffer’s value of ‘point’ to the value of
     ‘window-point’ (*note Window Point::) in WINDOW.  WINDOW must be a
     live window.  The return value is WINDOW.

     By default, this function also moves WINDOW’s buffer to the front
     of the buffer list (*note Buffer List::), and makes WINDOW the most
     recently selected window.  However, if the optional argument
     NORECORD is non-‘nil’, these additional actions are omitted.

     This function runs ‘buffer-list-update-hook’ (*note Buffer List::)
     unless NORECORD is non-‘nil’.  Note that applications and internal
     routines often temporarily select a window in order to simplify
     coding.  As a rule, such selections (including those made by the
     macros ‘save-selected-window’ and ‘with-selected-window’ below) are
     not recorded thus avoiding to pollute ‘buffer-list-update-hook’.
     Selections that really count are those causing a visible change in
     the next redisplay of WINDOW’s frame and should be always recorded.
     This also means that to run a function each time a window gets
     selected, putting it on ‘buffer-list-update-hook’ should be the
     right choice.

   The sequence of calls to ‘select-window’ with a non-‘nil’ NORECORD
argument determines an ordering of windows by their selection time.  The
function ‘get-lru-window’ can be used to retrieve the least recently
selected live window (*note Cyclic Window Ordering::).

 -- Macro: save-selected-window forms...
     This macro records the selected frame, as well as the selected
     window of each frame, executes FORMS in sequence, then restores the
     earlier selected frame and windows.  It also saves and restores the
     current buffer.  It returns the value of the last form in FORMS.

     This macro does not save or restore anything about the sizes,
     arrangement or contents of windows; therefore, if FORMS change
     them, the change persists.  If the previously selected window of
     some frame is no longer live at the time of exit from FORMS, that
     frame’s selected window is left alone.  If the previously selected
     window is no longer live, then whatever window is selected at the
     end of FORMS remains selected.  The current buffer is restored if
     and only if it is still live when exiting FORMS.

     This macro changes neither the ordering of recently selected
     windows nor the buffer list.

 -- Macro: with-selected-window window forms...
     This macro selects WINDOW, executes FORMS in sequence, then
     restores the previously selected window and current buffer.  The
     ordering of recently selected windows and the buffer list remain
     unchanged unless you deliberately change them within FORMS; for
     example, by calling ‘select-window’ with argument NORECORD ‘nil’.

     This macro does not change the order of recently selected windows
     or the buffer list.

 -- Function: frame-selected-window &optional frame
     This function returns the window on FRAME that is selected within
     that frame.  FRAME should be a live frame; if omitted or ‘nil’, it
     defaults to the selected frame.

 -- Function: set-frame-selected-window frame window &optional norecord
     This function makes WINDOW the window selected within the frame
     FRAME.  FRAME should be a live frame; if ‘nil’, it defaults to the
     selected frame.  WINDOW should be a live window; if ‘nil’, it
     defaults to the selected window.

     If FRAME is the selected frame, this makes WINDOW the selected
     window.

     If the optional argument NORECORD is non-‘nil’, this function does
     not alter the list of most recently selected windows, nor the
     buffer list.

 -- Function: window-use-time &optional window
     This functions returns the use time of window WINDOW.  WINDOW must
     be a live window and defaults to the selected one.

     The “use time” of a window is not really a time value, but an
     integer that does increase monotonically with each call of
     ‘select-window’ with a ‘nil’ NORECORD argument.  The window with
     the lowest use time is usually called the least recently used
     window while the window with the highest use time is called the
     most recently used one (*note Cyclic Window Ordering::).


File: elisp.info,  Node: Cyclic Window Ordering,  Next: Buffers and Windows,  Prev: Selecting Windows,  Up: Windows

27.10 Cyclic Ordering of Windows
================================

When you use the command ‘C-x o’ (‘other-window’) to select some other
window, it moves through live windows in a specific order.  For any
given configuration of windows, this order never varies.  It is called
the “cyclic ordering of windows”.

   The ordering is determined by a depth-first traversal of each frame’s
window tree, retrieving the live windows which are the leaf nodes of the
tree (*note Windows and Frames::).  If the minibuffer is active, the
minibuffer window is included too.  The ordering is cyclic, so the last
window in the sequence is followed by the first one.

 -- Function: next-window &optional window minibuf all-frames
     This function returns a live window, the one following WINDOW in
     the cyclic ordering of windows.  WINDOW should be a live window; if
     omitted or ‘nil’, it defaults to the selected window.

     The optional argument MINIBUF specifies whether minibuffer windows
     should be included in the cyclic ordering.  Normally, when MINIBUF
     is ‘nil’, a minibuffer window is included only if it is currently
     active; this matches the behavior of ‘C-x o’.  (Note that a
     minibuffer window is active as long as its minibuffer is in use;
     see *note Minibuffers::).

     If MINIBUF is ‘t’, the cyclic ordering includes all minibuffer
     windows.  If MINIBUF is neither ‘t’ nor ‘nil’, minibuffer windows
     are not included even if they are active.

     The optional argument ALL-FRAMES specifies which frames to
     consider:

        • ‘nil’ means to consider windows on WINDOW’s frame.  If the
          minibuffer window is considered (as specified by the MINIBUF
          argument), then frames that share the minibuffer window are
          considered too.

        • ‘t’ means to consider windows on all existing frames.

        • ‘visible’ means to consider windows on all visible frames.

        • 0 means to consider windows on all visible or iconified
          frames.

        • A frame means to consider windows on that specific frame.

        • Anything else means to consider windows on WINDOW’s frame, and
          no others.

     If more than one frame is considered, the cyclic ordering is
     obtained by appending the orderings for those frames, in the same
     order as the list of all live frames (*note Finding All Frames::).

 -- Function: previous-window &optional window minibuf all-frames
     This function returns a live window, the one preceding WINDOW in
     the cyclic ordering of windows.  The other arguments are handled
     like in ‘next-window’.

 -- Command: other-window count &optional all-frames
     This function selects a live window, one COUNT places from the
     selected window in the cyclic ordering of windows.  If COUNT is a
     positive number, it skips COUNT windows forwards; if COUNT is
     negative, it skips −COUNT windows backwards; if COUNT is zero, that
     simply re-selects the selected window.  When called interactively,
     COUNT is the numeric prefix argument.

     The optional argument ALL-FRAMES has the same meaning as in
     ‘next-window’, like a ‘nil’ MINIBUF argument to ‘next-window’.

     This function does not select a window that has a non-‘nil’
     ‘no-other-window’ window parameter (*note Window Parameters::).

 -- Function: walk-windows fun &optional minibuf all-frames
     This function calls the function FUN once for each live window,
     with the window as the argument.

     It follows the cyclic ordering of windows.  The optional arguments
     MINIBUF and ALL-FRAMES specify the set of windows included; these
     have the same arguments as in ‘next-window’.  If ALL-FRAMES
     specifies a frame, the first window walked is the first window on
     that frame (the one returned by ‘frame-first-window’), not
     necessarily the selected window.

     If FUN changes the window configuration by splitting or deleting
     windows, that does not alter the set of windows walked, which is
     determined prior to calling FUN for the first time.

 -- Function: one-window-p &optional no-mini all-frames
     This function returns ‘t’ if the selected window is the only live
     window, and ‘nil’ otherwise.

     If the minibuffer window is active, it is normally considered (so
     that this function returns ‘nil’).  However, if the optional
     argument NO-MINI is non-‘nil’, the minibuffer window is ignored
     even if active.  The optional argument ALL-FRAMES has the same
     meaning as for ‘next-window’.

   The following functions return a window which satisfies some
criterion, without selecting it:

 -- Function: get-lru-window &optional all-frames dedicated not-selected
     This function returns a live window which is heuristically the
     least recently used.  The optional argument ALL-FRAMES has the same
     meaning as in ‘next-window’.

     If any full-width windows are present, only those windows are
     considered.  A minibuffer window is never a candidate.  A dedicated
     window (*note Dedicated Windows::) is never a candidate unless the
     optional argument DEDICATED is non-‘nil’.  The selected window is
     never returned, unless it is the only candidate.  However, if the
     optional argument NOT-SELECTED is non-‘nil’, this function returns
     ‘nil’ in that case.

 -- Function: get-mru-window &optional all-frames dedicated not-selected
     This function is like ‘get-lru-window’, but it returns the most
     recently used window instead.  The meaning of the arguments is the
     same as described for ‘get-lru-window’.

 -- Function: get-largest-window &optional all-frames dedicated
          not-selected
     This function returns the window with the largest area (height
     times width).  The optional argument ALL-FRAMES specifies the
     windows to search, and has the same meaning as in ‘next-window’.

     A minibuffer window is never a candidate.  A dedicated window
     (*note Dedicated Windows::) is never a candidate unless the
     optional argument DEDICATED is non-‘nil’.  The selected window is
     not a candidate if the optional argument NOT-SELECTED is non-‘nil’.
     If the optional argument NOT-SELECTED is non-‘nil’ and the selected
     window is the only candidate, this function returns ‘nil’.

     If there are two candidate windows of the same size, this function
     prefers the one that comes first in the cyclic ordering of windows,
     starting from the selected window.

 -- Function: get-window-with-predicate predicate &optional minibuf
          all-frames default
     This function calls the function PREDICATE for each of the windows
     in the cyclic order of windows in turn, passing it the window as an
     argument.  If the predicate returns non-‘nil’ for any window, this
     function stops and returns that window.  If no such window is
     found, the return value is DEFAULT (which defaults to ‘nil’).

     The optional arguments MINIBUF and ALL-FRAMES specify the windows
     to search, and have the same meanings as in ‘next-window’.


File: elisp.info,  Node: Buffers and Windows,  Next: Switching Buffers,  Prev: Cyclic Window Ordering,  Up: Windows

27.11 Buffers and Windows
=========================

This section describes low-level functions for examining and setting the
contents of windows.  *Note Switching Buffers::, for higher-level
functions for displaying a specific buffer in a window.

 -- Function: window-buffer &optional window
     This function returns the buffer that WINDOW is displaying.  If
     WINDOW is omitted or ‘nil’ it defaults to the selected window.  If
     WINDOW is an internal window, this function returns ‘nil’.

 -- Function: set-window-buffer window buffer-or-name &optional
          keep-margins
     This function makes WINDOW display BUFFER-OR-NAME.  WINDOW should
     be a live window; if ‘nil’, it defaults to the selected window.
     BUFFER-OR-NAME should be a buffer, or the name of an existing
     buffer.  This function does not change which window is selected,
     nor does it directly change which buffer is current (*note Current
     Buffer::).  Its return value is ‘nil’.

     If WINDOW is “strongly dedicated” to a buffer and BUFFER-OR-NAME
     does not specify that buffer, this function signals an error.
     *Note Dedicated Windows::.

     By default, this function resets WINDOW’s position, display
     margins, fringe widths, and scroll bar settings, based on the local
     variables in the specified buffer.  However, if the optional
     argument KEEP-MARGINS is non-‘nil’, it leaves the display margins
     and fringe widths unchanged.

     When writing an application, you should normally use the
     higher-level functions described in *note Switching Buffers::,
     instead of calling ‘set-window-buffer’ directly.

     This runs ‘window-scroll-functions’, followed by
     ‘window-configuration-change-hook’.  *Note Window Hooks::.

 -- Variable: buffer-display-count
     This buffer-local variable records the number of times a buffer has
     been displayed in a window.  It is incremented each time
     ‘set-window-buffer’ is called for the buffer.

 -- Variable: buffer-display-time
     This buffer-local variable records the time at which a buffer was
     last displayed in a window.  The value is ‘nil’ if the buffer has
     never been displayed.  It is updated each time ‘set-window-buffer’
     is called for the buffer, with the value returned by ‘current-time’
     (*note Time of Day::).

 -- Function: get-buffer-window &optional buffer-or-name all-frames
     This function returns the first window displaying BUFFER-OR-NAME in
     the cyclic ordering of windows, starting from the selected window
     (*note Cyclic Window Ordering::).  If no such window exists, the
     return value is ‘nil’.

     BUFFER-OR-NAME should be a buffer or the name of a buffer; if
     omitted or ‘nil’, it defaults to the current buffer.  The optional
     argument ALL-FRAMES specifies which windows to consider:

        • ‘t’ means consider windows on all existing frames.
        • ‘visible’ means consider windows on all visible frames.
        • 0 means consider windows on all visible or iconified frames.
        • A frame means consider windows on that frame only.
        • Any other value means consider windows on the selected frame.

     Note that these meanings differ slightly from those of the
     ALL-FRAMES argument to ‘next-window’ (*note Cyclic Window
     Ordering::).  This function may be changed in a future version of
     Emacs to eliminate this discrepancy.

 -- Function: get-buffer-window-list &optional buffer-or-name minibuf
          all-frames
     This function returns a list of all windows currently displaying
     BUFFER-OR-NAME.  BUFFER-OR-NAME should be a buffer or the name of
     an existing buffer.  If omitted or ‘nil’, it defaults to the
     current buffer.  If the currently selected window displays
     BUFFER-OR-NAME, it will be the first in the list returned by this
     function.

     The arguments MINIBUF and ALL-FRAMES have the same meanings as in
     the function ‘next-window’ (*note Cyclic Window Ordering::).  Note
     that the ALL-FRAMES argument does _not_ behave exactly like in
     ‘get-buffer-window’.

 -- Command: replace-buffer-in-windows &optional buffer-or-name
     This command replaces BUFFER-OR-NAME with some other buffer, in all
     windows displaying it.  BUFFER-OR-NAME should be a buffer, or the
     name of an existing buffer; if omitted or ‘nil’, it defaults to the
     current buffer.

     The replacement buffer in each window is chosen via
     ‘switch-to-prev-buffer’ (*note Window History::).  Any dedicated
     window displaying BUFFER-OR-NAME is deleted if possible (*note
     Dedicated Windows::).  If such a window is the only window on its
     frame and there are other frames on the same terminal, the frame is
     deleted as well.  If the dedicated window is the only window on the
     only frame on its terminal, the buffer is replaced anyway.


File: elisp.info,  Node: Switching Buffers,  Next: Choosing Window,  Prev: Buffers and Windows,  Up: Windows

27.12 Switching to a Buffer in a Window
=======================================

This section describes high-level functions for switching to a specified
buffer in some window.  In general, “switching to a buffer” means to (1)
show the buffer in some window, (2) make that window the selected window
(and its frame the selected frame), and (3) make the buffer the current
buffer.

   Do _not_ use these functions to make a buffer temporarily current
just so a Lisp program can access or modify it.  They have side-effects,
such as changing window histories (*note Window History::), which will
surprise the user if used that way.  If you want to make a buffer
current to modify it in Lisp, use ‘with-current-buffer’,
‘save-current-buffer’, or ‘set-buffer’.  *Note Current Buffer::.

 -- Command: switch-to-buffer buffer-or-name &optional norecord
          force-same-window
     This command attempts to display BUFFER-OR-NAME in the selected
     window and make it the current buffer.  It is often used
     interactively (as the binding of ‘C-x b’), as well as in Lisp
     programs.  The return value is the buffer switched to.

     If BUFFER-OR-NAME is ‘nil’, it defaults to the buffer returned by
     ‘other-buffer’ (*note Buffer List::).  If BUFFER-OR-NAME is a
     string that is not the name of any existing buffer, this function
     creates a new buffer with that name; the new buffer’s major mode is
     determined by the variable ‘major-mode’ (*note Major Modes::).

     Normally, the specified buffer is put at the front of the buffer
     list—both the global buffer list and the selected frame’s buffer
     list (*note Buffer List::).  However, this is not done if the
     optional argument NORECORD is non-‘nil’.

     Sometimes, the selected window may not be suitable for displaying
     the buffer.  This happens if the selected window is a minibuffer
     window, or if the selected window is strongly dedicated to its
     buffer (*note Dedicated Windows::).  In such cases, the command
     normally tries to display the buffer in some other window, by
     invoking ‘pop-to-buffer’ (see below).

     If the optional argument FORCE-SAME-WINDOW is non-‘nil’ and the
     selected window is not suitable for displaying the buffer, this
     function always signals an error when called non-interactively.  In
     interactive use, if the selected window is a minibuffer window,
     this function will try to use some other window instead.  If the
     selected window is strongly dedicated to its buffer, the option
     ‘switch-to-buffer-in-dedicated-window’ described next can be used
     to proceed.

 -- User Option: switch-to-buffer-in-dedicated-window
     This option, if non-‘nil’, allows ‘switch-to-buffer’ to proceed
     when called interactively and the selected window is strongly
     dedicated to its buffer.

     The following values are respected:

     ‘nil’
          Disallows switching and signals an error as in non-interactive
          use.

     ‘prompt’
          Prompts the user whether to allow switching.

     ‘pop’
          Invokes ‘pop-to-buffer’ to proceed.

     ‘t’
          Marks the selected window as non-dedicated and proceeds.

     This option does not affect non-interactive calls of
     ‘switch-to-buffer’.

   By default, ‘switch-to-buffer’ shows the buffer at its position of
‘point’.  This behavior can be tuned using the following option.

 -- User Option: switch-to-buffer-preserve-window-point
     If this variable is ‘nil’, ‘switch-to-buffer’ displays the buffer
     specified by BUFFER-OR-NAME at the position of that buffer’s
     ‘point’.  If this variable is ‘already-displayed’, it tries to
     display the buffer at its previous position in the selected window,
     provided the buffer is currently displayed in some other window on
     any visible or iconified frame.  If this variable is ‘t’,
     ‘switch-to-buffer’ unconditionally tries to display the buffer at
     its previous position in the selected window.

     This variable is ignored if the buffer is already displayed in the
     selected window or never appeared in it before, or if
     ‘switch-to-buffer’ calls ‘pop-to-buffer’ to display the buffer.

   The next two commands are similar to ‘switch-to-buffer’, except for
the described features.

 -- Command: switch-to-buffer-other-window buffer-or-name &optional
          norecord
     This function displays the buffer specified by BUFFER-OR-NAME in
     some window other than the selected window.  It uses the function
     ‘pop-to-buffer’ internally (see below).

     If the selected window already displays the specified buffer, it
     continues to do so, but another window is nonetheless found to
     display it as well.

     The BUFFER-OR-NAME and NORECORD arguments have the same meanings as
     in ‘switch-to-buffer’.

 -- Command: switch-to-buffer-other-frame buffer-or-name &optional
          norecord
     This function displays the buffer specified by BUFFER-OR-NAME in a
     new frame.  It uses the function ‘pop-to-buffer’ internally (see
     below).

     If the specified buffer is already displayed in another window, in
     any frame on the current terminal, this switches to that window
     instead of creating a new frame.  However, the selected window is
     never used for this.

     The BUFFER-OR-NAME and NORECORD arguments have the same meanings as
     in ‘switch-to-buffer’.

   The above commands use the function ‘pop-to-buffer’, which flexibly
displays a buffer in some window and selects that window for editing.
In turn, ‘pop-to-buffer’ uses ‘display-buffer’ for displaying the
buffer.  Hence, all the variables affecting ‘display-buffer’ will affect
it as well.  *Note Choosing Window::, for the documentation of
‘display-buffer’.

 -- Command: pop-to-buffer buffer-or-name &optional action norecord
     This function makes BUFFER-OR-NAME the current buffer and displays
     it in some window, preferably not the window currently selected.
     It then selects the displaying window.  If that window is on a
     different graphical frame, that frame is given input focus if
     possible (*note Input Focus::).  The return value is the buffer
     that was switched to.

     If BUFFER-OR-NAME is ‘nil’, it defaults to the buffer returned by
     ‘other-buffer’ (*note Buffer List::).  If BUFFER-OR-NAME is a
     string that is not the name of any existing buffer, this function
     creates a new buffer with that name; the new buffer’s major mode is
     determined by the variable ‘major-mode’ (*note Major Modes::).

     If ACTION is non-‘nil’, it should be a display action to pass to
     ‘display-buffer’ (*note Choosing Window::).  Alternatively, a
     non-‘nil’, non-list value means to pop to a window other than the
     selected one—even if the buffer is already displayed in the
     selected window.

     Like ‘switch-to-buffer’, this function updates the buffer list
     unless NORECORD is non-‘nil’.


File: elisp.info,  Node: Choosing Window,  Next: Display Action Functions,  Prev: Switching Buffers,  Up: Windows

27.13 Choosing a Window for Display
===================================

The command ‘display-buffer’ flexibly chooses a window for display, and
displays a specified buffer in that window.  It can be called
interactively, via the key binding ‘C-x 4 C-o’.  It is also used as a
subroutine by many functions and commands, including ‘switch-to-buffer’
and ‘pop-to-buffer’ (*note Switching Buffers::).

   This command performs several complex steps to find a window to
display in.  These steps are described by means of “display actions”,
which have the form ‘(FUNCTION . ALIST)’.  Here, FUNCTION is either a
function or a list of functions, which we refer to as “action
functions”; ALIST is an association list, which we refer to as an
“action alist”.

   An action function accepts two arguments: the buffer to display and
an action alist.  It attempts to display the buffer in some window,
picking or creating a window according to its own criteria.  If
successful, it returns the window; otherwise, it returns ‘nil’.  *Note
Display Action Functions::, for a list of predefined action functions.

   ‘display-buffer’ works by combining display actions from several
sources, and calling the action functions in turn, until one of them
manages to display the buffer and returns a non-‘nil’ value.

 -- Command: display-buffer buffer-or-name &optional action frame
     This command makes BUFFER-OR-NAME appear in some window, without
     selecting the window or making the buffer current.  The argument
     BUFFER-OR-NAME must be a buffer or the name of an existing buffer.
     The return value is the window chosen to display the buffer.

     The optional argument ACTION, if non-‘nil’, should normally be a
     display action (described above).  ‘display-buffer’ builds a list
     of action functions and an action alist, by consolidating display
     actions from the following sources (in order):

        • The variable ‘display-buffer-overriding-action’.

        • The user option ‘display-buffer-alist’.

        • The ACTION argument.

        • The user option ‘display-buffer-base-action’.

        • The constant ‘display-buffer-fallback-action’.

     Each action function is called in turn, passing the buffer as the
     first argument and the combined action alist as the second
     argument, until one of the functions returns non-‘nil’.  The caller
     can pass ‘(allow-no-window . t)’ as an element of the action alist
     to indicate its readiness to handle the case of not displaying the
     buffer in a window.

     The argument ACTION can also have a non-‘nil’, non-list value.
     This has the special meaning that the buffer should be displayed in
     a window other than the selected one, even if the selected window
     is already displaying it.  If called interactively with a prefix
     argument, ACTION is ‘t’.

     The optional argument FRAME, if non-‘nil’, specifies which frames
     to check when deciding whether the buffer is already displayed.  It
     is equivalent to adding an element ‘(reusable-frames . FRAME)’ to
     the action alist of ACTION.  *Note Display Action Functions::.

 -- Variable: display-buffer-overriding-action
     The value of this variable should be a display action, which is
     treated with the highest priority by ‘display-buffer’.  The default
     value is empty, i.e., ‘(nil . nil)’.

 -- User Option: display-buffer-alist
     The value of this option is an alist mapping conditions to display
     actions.  Each condition may be either a regular expression
     matching a buffer name or a function that takes two arguments: a
     buffer name and the ACTION argument passed to ‘display-buffer’.  If
     the name of the buffer passed to ‘display-buffer’ either matches a
     regular expression in this alist or the function specified by a
     condition returns non-‘nil’, then ‘display-buffer’ uses the
     corresponding display action to display the buffer.

 -- User Option: display-buffer-base-action
     The value of this option should be a display action.  This option
     can be used to define a standard display action for calls to
     ‘display-buffer’.

 -- Constant: display-buffer-fallback-action
     This display action specifies the fallback behavior for
     ‘display-buffer’ if no other display actions are given.


File: elisp.info,  Node: Display Action Functions,  Next: Choosing Window Options,  Prev: Choosing Window,  Up: Windows

27.14 Action Functions for ‘display-buffer’
===========================================

The following basic action functions are defined in Emacs.  Each of
these functions takes two arguments: BUFFER, the buffer to display, and
ALIST, an action alist.  Each action function returns the window if it
succeeds, and ‘nil’ if it fails.

 -- Function: display-buffer-same-window buffer alist
     This function tries to display BUFFER in the selected window.  It
     fails if the selected window is a minibuffer window or is dedicated
     to another buffer (*note Dedicated Windows::).  It also fails if
     ALIST has a non-‘nil’ ‘inhibit-same-window’ entry.

 -- Function: display-buffer-reuse-window buffer alist
     This function tries to display BUFFER by finding a window that is
     already displaying it.

     If ALIST has a non-‘nil’ ‘inhibit-same-window’ entry, the selected
     window is not eligible for reuse.  If ALIST contains a
     ‘reusable-frames’ entry, its value determines which frames to
     search for a reusable window:

        • ‘nil’ means consider windows on the selected frame.
          (Actually, the last non-minibuffer frame.)
        • ‘t’ means consider windows on all frames.
        • ‘visible’ means consider windows on all visible frames.
        • 0 means consider windows on all visible or iconified frames.
        • A frame means consider windows on that frame only.

     Note that these meanings differ slightly from those of the
     ALL-FRAMES argument to ‘next-window’ (*note Cyclic Window
     Ordering::).

     If ALIST contains no ‘reusable-frames’ entry, this function
     normally searches just the selected frame; however, if the variable
     ‘pop-up-frames’ is non-‘nil’, it searches all frames on the current
     terminal.  *Note Choosing Window Options::.

     If this function chooses a window on another frame, it makes that
     frame visible and, unless ALIST contains an ‘inhibit-switch-frame’
     entry (*note Choosing Window Options::), raises that frame if
     necessary.

 -- Function: display-buffer-pop-up-frame buffer alist
     This function creates a new frame, and displays the buffer in that
     frame’s window.  It actually performs the frame creation by calling
     the function specified in ‘pop-up-frame-function’ (*note Choosing
     Window Options::).  If ALIST contains a ‘pop-up-frame-parameters’
     entry, the associated value is added to the newly created frame’s
     parameters.

 -- Function: display-buffer-use-some-frame buffer alist
     This function tries to display BUFFER by trying to find a frame
     that meets a predicate (by default any frame other than the current
     frame).

     If this function chooses a window on another frame, it makes that
     frame visible and, unless ALIST contains an ‘inhibit-switch-frame’
     entry (*note Choosing Window Options::), raises that frame if
     necessary.

     If ALIST has a non-‘nil’ ‘frame-predicate’ entry, its value is a
     function taking one argument (a frame), returning non-‘nil’ if the
     frame is a candidate; this function replaces the default predicate.

     If ALIST has a non-‘nil’ ‘inhibit-same-window’ entry, the selected
     window is used; thus if the selected frame has a single window, it
     is not used.

 -- Function: display-buffer-pop-up-window buffer alist
     This function tries to display BUFFER by splitting the largest or
     least recently-used window (typically one on the selected frame).
     It actually performs the split by calling the function specified in
     ‘split-window-preferred-function’ (*note Choosing Window
     Options::).

     The size of the new window can be adjusted by supplying
     ‘window-height’ and ‘window-width’ entries in ALIST.  To adjust the
     window’s height, use an entry whose CAR is ‘window-height’ and
     whose CDR is one of:

        • ‘nil’ means to leave the height of the new window alone.

        • A number specifies the desired height of the new window.  An
          integer specifies the number of lines of the window.  A
          floating-point number gives the fraction of the window’s
          height with respect to the height of the frame’s root window.

        • If the CDR specifies a function, that function is called with
          one argument: the new window.  The function is supposed to
          adjust the height of the window; its return value is ignored.
          Suitable functions are ‘shrink-window-if-larger-than-buffer’
          and ‘fit-window-to-buffer’, see *note Resizing Windows::.

     To adjust the window’s width, use an entry whose CAR is
     ‘window-width’ and whose CDR is one of:

        • ‘nil’ means to leave the width of the new window alone.

        • A number specifies the desired width of the new window.  An
          integer specifies the number of columns of the window.  A
          floating-point number gives the fraction of the window’s width
          with respect to the width of the frame’s root window.

        • If the CDR specifies a function, that function is called with
          one argument: the new window.  The function is supposed to
          adjust the width of the window; its return value is ignored.

     If ALIST contains a ‘preserve-size’ entry, Emacs will try to
     preserve the size of the new window during future resize operations
     (*note Preserving Window Sizes::).  The CDR of that entry must be a
     cons cell whose CAR, if non-‘nil’, means to preserve the width of
     the window and whose CDR, if non-‘nil’, means to preserve the
     height of the window.

     This function can fail if no window splitting can be performed for
     some reason (e.g., if the selected frame has an ‘unsplittable’
     frame parameter; *note Buffer Parameters::).

 -- Function: display-buffer-below-selected buffer alist
     This function tries to display BUFFER in a window below the
     selected window.  This means to either split the selected window or
     use the window below the selected one.  If it does create a new
     window, it will also adjust its size provided ALIST contains a
     suitable ‘window-height’ or ‘window-width’ entry, see above.

 -- Function: display-buffer-in-previous-window buffer alist
     This function tries to display BUFFER in a window previously
     showing it.  If ALIST has a non-‘nil’ ‘inhibit-same-window’ entry,
     the selected window is not eligible for reuse.  If ALIST contains a
     ‘reusable-frames’ entry, its value determines which frames to
     search for a suitable window as with ‘display-buffer-reuse-window’.

     If ALIST has a ‘previous-window’ entry, the window specified by
     that entry will override any other window found by the methods
     above, even if that window never showed BUFFER before.

 -- Function: display-buffer-at-bottom buffer alist
     This function tries to display BUFFER in a window at the bottom of
     the selected frame.

     This either splits the window at the bottom of the frame or the
     frame’s root window, or reuses an existing window at the bottom of
     the selected frame.

 -- Function: display-buffer-use-some-window buffer alist
     This function tries to display BUFFER by choosing an existing
     window and displaying the buffer in that window.  It can fail if
     all windows are dedicated to another buffer (*note Dedicated
     Windows::).

 -- Function: display-buffer-no-window buffer alist
     If ALIST has a non-‘nil’ ‘allow-no-window’ entry, then this
     function does not display ‘buffer’.  This allows you to override
     the default action and avoid displaying the buffer.  It is assumed
     that when the caller specifies a non-‘nil’ ‘allow-no-window’ value
     it can handle a ‘nil’ value returned from ‘display-buffer’ in this
     case.

   To illustrate the use of action functions, consider the following
example.

     (display-buffer
      (get-buffer-create "*foo*")
      '((display-buffer-reuse-window
         display-buffer-pop-up-window
         display-buffer-pop-up-frame)
        (reusable-frames . 0)
        (window-height . 10) (window-width . 40)))

Evaluating the form above will cause ‘display-buffer’ to proceed as
follows: If a buffer called *foo* already appears on a visible or
iconified frame, it will reuse its window.  Otherwise, it will try to
pop up a new window or, if that is impossible, a new frame and show the
buffer there.  If all these steps fail, it will proceed using whatever
‘display-buffer-base-action’ and ‘display-buffer-fallback-action’
prescribe.

   Furthermore, ‘display-buffer’ will try to adjust a reused window
(provided *foo* was put by ‘display-buffer’ there before) or a popped-up
window as follows: If the window is part of a vertical combination, it
will set its height to ten lines.  Note that if, instead of the number
10, we specified the function ‘fit-window-to-buffer’, ‘display-buffer’
would come up with a one-line window to fit the empty buffer.  If the
window is part of a horizontal combination, it sets its width to 40
columns.  Whether a new window is vertically or horizontally combined
depends on the shape of the window split and the values of
‘split-window-preferred-function’, ‘split-height-threshold’ and
‘split-width-threshold’ (*note Choosing Window Options::).

   Now suppose we combine this call with a preexisting setup for
‘display-buffer-alist’ as follows.

     (let ((display-buffer-alist
            (cons
             '("\\*foo\\*"
               (display-buffer-reuse-window display-buffer-below-selected)
               (reusable-frames)
               (window-height . 5))
             display-buffer-alist)))
       (display-buffer
        (get-buffer-create "*foo*")
        '((display-buffer-reuse-window
           display-buffer-pop-up-window
           display-buffer-pop-up-frame)
          (reusable-frames . 0)
          (window-height . 10) (window-width . 40))))

This form will have ‘display-buffer’ first try reusing a window that
shows *foo* on the selected frame.  If there’s no such window, it will
try to split the selected window or, if that is impossible, use the
window below the selected window.

   If there’s no window below the selected one, or the window below the
selected one is dedicated to its buffer, ‘display-buffer’ will proceed
as described in the previous example.  Note, however, that when it tries
to adjust the height of any reused or popped-up window, it will in any
case try to set its number of lines to 5 since that value overrides the
corresponding specification in the ACTION argument of ‘display-buffer’.


File: elisp.info,  Node: Choosing Window Options,  Next: Window History,  Prev: Display Action Functions,  Up: Windows

27.15 Additional Options for Displaying Buffers
===============================================

The behavior of the standard display actions of ‘display-buffer’ (*note
Choosing Window::) can be modified by a variety of user options.

 -- User Option: pop-up-windows
     If the value of this variable is non-‘nil’, ‘display-buffer’ is
     allowed to split an existing window to make a new window for
     displaying in.  This is the default.

     This variable is provided mainly for backward compatibility.  It is
     obeyed by ‘display-buffer’ via a special mechanism in
     ‘display-buffer-fallback-action’, which only calls the action
     function ‘display-buffer-pop-up-window’ (*note Display Action
     Functions::) when the value is ‘nil’.  It is not consulted by
     ‘display-buffer-pop-up-window’ itself, which the user may specify
     directly in ‘display-buffer-alist’ etc.

 -- User Option: split-window-preferred-function
     This variable specifies a function for splitting a window, in order
     to make a new window for displaying a buffer.  It is used by the
     ‘display-buffer-pop-up-window’ action function to actually split
     the window (*note Display Action Functions::).

     The default value is ‘split-window-sensibly’, which is documented
     below.  The value must be a function that takes one argument, a
     window, and return either a new window (which will be used to
     display the desired buffer) or ‘nil’ (which means the splitting
     failed).

 -- Function: split-window-sensibly &optional window
     This function tries to split WINDOW, and return the newly created
     window.  If WINDOW cannot be split, it returns ‘nil’.  If WINDOW is
     omitted or ‘nil’, it defaults to the selected window.

     This function obeys the usual rules that determine when a window
     may be split (*note Splitting Windows::).  It first tries to split
     by placing the new window below, subject to the restriction imposed
     by ‘split-height-threshold’ (see below), in addition to any other
     restrictions.  If that fails, it tries to split by placing the new
     window to the right, subject to ‘split-width-threshold’ (see
     below).  If that fails, and the window is the only window on its
     frame, this function again tries to split and place the new window
     below, disregarding ‘split-height-threshold’.  If this fails as
     well, this function gives up and returns ‘nil’.

 -- User Option: split-height-threshold
     This variable, used by ‘split-window-sensibly’, specifies whether
     to split the window placing the new window below.  If it is an
     integer, that means to split only if the original window has at
     least that many lines.  If it is ‘nil’, that means not to split
     this way.

 -- User Option: split-width-threshold
     This variable, used by ‘split-window-sensibly’, specifies whether
     to split the window placing the new window to the right.  If the
     value is an integer, that means to split only if the original
     window has at least that many columns.  If the value is ‘nil’, that
     means not to split this way.

 -- User Option: even-window-sizes
     This variable, if non-‘nil’, causes ‘display-buffer’ to even window
     sizes whenever it reuses an existing window and that window is
     adjacent to the selected one.

     If its value is ‘width-only’, sizes are evened only if the reused
     window is on the left or right of the selected one and the selected
     window is wider than the reused one.  If its value is ‘height-only’
     sizes are evened only if the reused window is above or beneath the
     selected window and the selected window is higher than the reused
     one.  Any other non-‘nil’ value means to even sizes in any of these
     cases provided the selected window is larger than the reused one in
     the sense of their combination.

 -- User Option: pop-up-frames
     If the value of this variable is non-‘nil’, that means
     ‘display-buffer’ may display buffers by making new frames.  The
     default is ‘nil’.

     A non-‘nil’ value also means that when ‘display-buffer’ is looking
     for a window already displaying BUFFER-OR-NAME, it can search any
     visible or iconified frame, not just the selected frame.

     This variable is provided mainly for backward compatibility.  It is
     obeyed by ‘display-buffer’ via a special mechanism in
     ‘display-buffer-fallback-action’, which calls the action function
     ‘display-buffer-pop-up-frame’ (*note Display Action Functions::) if
     the value is non-‘nil’.  (This is done before attempting to split a
     window.)  This variable is not consulted by
     ‘display-buffer-pop-up-frame’ itself, which the user may specify
     directly in ‘display-buffer-alist’ etc.

 -- User Option: pop-up-frame-function
     This variable specifies a function for creating a new frame, in
     order to make a new window for displaying a buffer.  It is used by
     the ‘display-buffer-pop-up-frame’ action function (*note Display
     Action Functions::).

     The value should be a function that takes no arguments and returns
     a frame, or ‘nil’ if no frame could be created.  The default value
     is a function that creates a frame using the parameters specified
     by ‘pop-up-frame-alist’ (see below).

 -- User Option: pop-up-frame-alist
     This variable holds an alist of frame parameters (*note Frame
     Parameters::), which is used by the default function in
     ‘pop-up-frame-function’ to make a new frame.  The default is ‘nil’.

 -- User Option: same-window-buffer-names
     A list of buffer names for buffers that should be displayed in the
     selected window.  If a buffer’s name is in this list,
     ‘display-buffer’ handles the buffer by showing it in the selected
     window.

 -- User Option: same-window-regexps
     A list of regular expressions that specify buffers that should be
     displayed in the selected window.  If the buffer’s name matches any
     of the regular expressions in this list, ‘display-buffer’ handles
     the buffer by showing it in the selected window.

 -- Function: same-window-p buffer-name
     This function returns ‘t’ if displaying a buffer named BUFFER-NAME
     with ‘display-buffer’ would put it in the selected window.


File: elisp.info,  Node: Window History,  Next: Dedicated Windows,  Prev: Choosing Window Options,  Up: Windows

27.16 Window History
====================

Each window remembers in a list the buffers it has previously displayed,
and the order in which these buffers were removed from it.  This history
is used, for example, by ‘replace-buffer-in-windows’ (*note Buffers and
Windows::), and when quitting windows (*note Quitting Windows::).  The
list is automatically maintained by Emacs, but you can use the following
functions to explicitly inspect or alter it:

 -- Function: window-prev-buffers &optional window
     This function returns a list specifying the previous contents of
     WINDOW.  The optional argument WINDOW should be a live window and
     defaults to the selected one.

     Each list element has the form ‘(BUFFER WINDOW-START WINDOW-POS)’,
     where BUFFER is a buffer previously shown in the window,
     WINDOW-START is the window start position (*note Window Start and
     End::) when that buffer was last shown, and WINDOW-POS is the point
     position (*note Window Point::) when that buffer was last shown in
     WINDOW.

     The list is ordered so that earlier elements correspond to more
     recently-shown buffers, and the first element usually corresponds
     to the buffer most recently removed from the window.

 -- Function: set-window-prev-buffers window prev-buffers
     This function sets WINDOW’s previous buffers to the value of
     PREV-BUFFERS.  The argument WINDOW must be a live window and
     defaults to the selected one.  The argument PREV-BUFFERS should be
     a list of the same form as that returned by ‘window-prev-buffers’.

   In addition, each buffer maintains a list of “next buffers”, which is
a list of buffers re-shown by ‘switch-to-prev-buffer’ (see below).  This
list is mainly used by ‘switch-to-prev-buffer’ and
‘switch-to-next-buffer’ for choosing buffers to switch to.

 -- Function: window-next-buffers &optional window
     This function returns the list of buffers recently re-shown in
     WINDOW via ‘switch-to-prev-buffer’.  The WINDOW argument must
     denote a live window or ‘nil’ (meaning the selected window).

 -- Function: set-window-next-buffers window next-buffers
     This function sets the next buffer list of WINDOW to NEXT-BUFFERS.
     The WINDOW argument should be a live window or ‘nil’ (meaning the
     selected window).  The argument NEXT-BUFFERS should be a list of
     buffers.

   The following commands can be used to cycle through the global buffer
list, much like ‘bury-buffer’ and ‘unbury-buffer’.  However, they cycle
according to the specified window’s history list, rather than the global
buffer list.  In addition, they restore window-specific window start and
point positions, and may show a buffer even if it is already shown in
another window.  The ‘switch-to-prev-buffer’ command, in particular, is
used by ‘replace-buffer-in-windows’, ‘bury-buffer’ and ‘quit-window’ to
find a replacement buffer for a window.

 -- Command: switch-to-prev-buffer &optional window bury-or-kill
     This command displays the previous buffer in WINDOW.  The argument
     WINDOW should be a live window or ‘nil’ (meaning the selected
     window).  If the optional argument BURY-OR-KILL is non-‘nil’, this
     means that the buffer currently shown in WINDOW is about to be
     buried or killed and consequently should not be switched to in
     future invocations of this command.

     The previous buffer is usually the buffer shown before the buffer
     currently shown in WINDOW.  However, a buffer that has been buried
     or killed, or has been already shown by a recent invocation of
     ‘switch-to-prev-buffer’, does not qualify as previous buffer.

     If repeated invocations of this command have already shown all
     buffers previously shown in WINDOW, further invocations will show
     buffers from the buffer list of the frame WINDOW appears on (*note
     Buffer List::), trying to skip buffers that are already shown in
     another window on that frame.

 -- Command: switch-to-next-buffer &optional window
     This command switches to the next buffer in WINDOW, thus undoing
     the effect of the last ‘switch-to-prev-buffer’ command in WINDOW.
     The argument WINDOW must be a live window and defaults to the
     selected one.

     If there is no recent invocation of ‘switch-to-prev-buffer’ that
     can be undone, this function tries to show a buffer from the buffer
     list of the frame WINDOW appears on (*note Buffer List::).

   By default ‘switch-to-prev-buffer’ and ‘switch-to-next-buffer’ can
switch to a buffer that is already shown in another window on the same
frame.  The following option can be used to override this behavior.

 -- User Option: switch-to-visible-buffer
     If this variable is non-‘nil’, ‘switch-to-prev-buffer’ and
     ‘switch-to-next-buffer’ may switch to a buffer that is already
     visible on the same frame, provided the buffer was shown in the
     relevant window before.  If it is ‘nil’, ‘switch-to-prev-buffer’
     and ‘switch-to-next-buffer’ always try to avoid switching to a
     buffer that is already visible in another window on the same frame.
     The default is ‘t’.


File: elisp.info,  Node: Dedicated Windows,  Next: Quitting Windows,  Prev: Window History,  Up: Windows

27.17 Dedicated Windows
=======================

Functions for displaying a buffer can be told to not use specific
windows by marking these windows as “dedicated” to their buffers.
‘display-buffer’ (*note Choosing Window::) never uses a dedicated window
for displaying another buffer in it.  ‘get-lru-window’ and
‘get-largest-window’ (*note Cyclic Window Ordering::) do not consider
dedicated windows as candidates when their DEDICATED argument is
non-‘nil’.  The behavior of ‘set-window-buffer’ (*note Buffers and
Windows::) with respect to dedicated windows is slightly different, see
below.

   Functions supposed to remove a buffer from a window or a window from
a frame can behave specially when a window they operate on is dedicated.
We will distinguish three basic cases, namely where (1) the window is
not the only window on its frame, (2) the window is the only window on
its frame but there are other frames on the same terminal left, and (3)
the window is the only window on the only frame on the same terminal.

   In particular, ‘delete-windows-on’ (*note Deleting Windows::) handles
case (2) by deleting the associated frame and case (3) by showing
another buffer in that frame’s only window.  The function
‘replace-buffer-in-windows’ (*note Buffers and Windows::) which is
called when a buffer gets killed, deletes the window in case (1) and
behaves like ‘delete-windows-on’ otherwise.

   When ‘bury-buffer’ (*note Buffer List::) operates on the selected
window (which shows the buffer that shall be buried), it handles case
(2) by calling ‘frame-auto-hide-function’ (*note Quitting Windows::) to
deal with the selected frame.  The other two cases are handled as with
‘replace-buffer-in-windows’.

 -- Function: window-dedicated-p &optional window
     This function returns non-‘nil’ if WINDOW is dedicated to its
     buffer and ‘nil’ otherwise.  More precisely, the return value is
     the value assigned by the last call of ‘set-window-dedicated-p’ for
     WINDOW, or ‘nil’ if that function was never called with WINDOW as
     its argument.  The default for WINDOW is the selected window.

 -- Function: set-window-dedicated-p window flag
     This function marks WINDOW as dedicated to its buffer if FLAG is
     non-‘nil’, and non-dedicated otherwise.

     As a special case, if FLAG is ‘t’, WINDOW becomes “strongly”
     dedicated to its buffer.  ‘set-window-buffer’ signals an error when
     the window it acts upon is strongly dedicated to its buffer and
     does not already display the buffer it is asked to display.  Other
     functions do not treat ‘t’ differently from any non-‘nil’ value.


File: elisp.info,  Node: Quitting Windows,  Next: Window Point,  Prev: Dedicated Windows,  Up: Windows

27.18 Quitting Windows
======================

When you want to get rid of a window used for displaying a buffer, you
can call ‘delete-window’ or ‘delete-windows-on’ (*note Deleting
Windows::) to remove that window from its frame.  If the buffer is shown
on a separate frame, you might want to call ‘delete-frame’ (*note
Deleting Frames::) instead.  If, on the other hand, a window has been
reused for displaying the buffer, you might prefer showing the buffer
previously shown in that window, by calling the function
‘switch-to-prev-buffer’ (*note Window History::).  Finally, you might
want to either bury (*note Buffer List::) or kill (*note Killing
Buffers::) the window’s buffer.

   The following command uses information on how the window for
displaying the buffer was obtained in the first place, thus attempting
to automate the above decisions for you.

 -- Command: quit-window &optional kill window
     This command quits WINDOW and buries its buffer.  The argument
     WINDOW must be a live window and defaults to the selected one.
     With prefix argument KILL non-‘nil’, it kills the buffer instead of
     burying it.  It calls the function ‘quit-restore-window’ described
     next to deal with the window and its buffer.

 -- Function: quit-restore-window &optional window bury-or-kill
     This function handles WINDOW and its buffer after quitting.  The
     optional argument WINDOW must be a live window and defaults to the
     selected one.  The function’s behavior is determined by the four
     elements of the ‘quit-restore’ window parameter (*note Window
     Parameters::), which is set to nil afterwards.

     The window is deleted entirely if: 1) the first element of the
     ‘quit-restore’ parameter is one of ’window or ’frame, 2) the window
     has no history of previously-displayed buffers, and 3) the
     displayed buffer matches the one in the fourth element of the
     ‘quit-restore’ parameter.  If WINDOW is the only window on its
     frame and there are other frames on the frame’s terminal, the value
     of the optional argument BURY-OR-KILL determines how to proceed
     with the window.  If BURY-OR-KILL equals ‘kill’, the frame is
     deleted unconditionally.  Otherwise, the fate of the frame is
     determined by calling ‘frame-auto-hide-function’ (see below) with
     that frame as sole argument.

     If the third element of the ‘quit-restore’ parameter is a list of
     buffer, window start (*note Window Start and End::), and point
     (*note Window Point::), and that buffer is still live, the buffer
     will be displayed, and start and point set accordingly.  If, in
     addition, WINDOW’s buffer was temporarily resized, this function
     will also try to restore the original height of WINDOW.

     Otherwise, if WINDOW was previously used for displaying other
     buffers (*note Window History::), the most recent buffer in that
     history will be displayed.

     The optional argument BURY-OR-KILL specifies how to deal with
     WINDOW’s buffer.  The following values are handled:

     ‘nil’
          This means to not deal with the buffer in any particular way.
          As a consequence, if WINDOW is not deleted, invoking
          ‘switch-to-prev-buffer’ will usually show the buffer again.

     ‘append’
          This means that if WINDOW is not deleted, its buffer is moved
          to the end of WINDOW’s list of previous buffers, so it’s less
          likely that a future invocation of ‘switch-to-prev-buffer’
          will switch to it.  Also, it moves the buffer to the end of
          the frame’s buffer list.

     ‘bury’
          This means that if WINDOW is not deleted, its buffer is
          removed from WINDOW’s list of previous buffers.  Also, it
          moves the buffer to the end of the frame’s buffer list.  This
          value provides the most reliable remedy to not have
          ‘switch-to-prev-buffer’ switch to this buffer again without
          killing the buffer.

     ‘kill’
          This means to kill WINDOW’s buffer.

     Typically, the display routines run by ‘display-buffer’ will set
     the ‘quit-restore’ window parameter correctly.  It’s also possible
     to set it manually, using the following code for displaying BUFFER
     in WINDOW:

          (display-buffer-record-window type window buffer)

          (set-window-buffer window buffer)

          (set-window-prev-buffers window nil)

     Setting the window history to nil ensures that a future call to
     ‘quit-window’ can delete the window altogether.

   The following option specifies how to deal with a frame containing
just one window that should be either quit, or whose buffer should be
buried.

 -- User Option: frame-auto-hide-function
     The function specified by this option is called to automatically
     hide frames.  This function is called with one argument—a frame.

     The function specified here is called by ‘bury-buffer’ (*note
     Buffer List::) when the selected window is dedicated and shows the
     buffer to bury.  It is also called by ‘quit-restore-window’ (see
     above) when the frame of the window to quit has been specially
     created for displaying that window’s buffer and the buffer is not
     killed.

     The default is to call ‘iconify-frame’ (*note Visibility of
     Frames::).  Alternatively, you may specify either ‘delete-frame’
     (*note Deleting Frames::) to remove the frame from its display,
     ‘ignore’ to leave the frame unchanged, or any other function that
     can take a frame as its sole argument.

     Note that the function specified by this option is called only if
     the specified frame contains just one live window and there is at
     least one other frame on the same terminal.


File: elisp.info,  Node: Window Point,  Next: Window Start and End,  Prev: Quitting Windows,  Up: Windows

27.19 Windows and Point
=======================

Each window has its own value of point (*note Point::), independent of
the value of point in other windows displaying the same buffer.  This
makes it useful to have multiple windows showing one buffer.

   • The window point is established when a window is first created; it
     is initialized from the buffer’s point, or from the window point of
     another window opened on the buffer if such a window exists.

   • Selecting a window sets the value of point in its buffer from the
     window’s value of point.  Conversely, deselecting a window sets the
     window’s value of point from that of the buffer.  Thus, when you
     switch between windows that display a given buffer, the point value
     for the selected window is in effect in the buffer, while the point
     values for the other windows are stored in those windows.

   • As long as the selected window displays the current buffer, the
     window’s point and the buffer’s point always move together; they
     remain equal.

   Emacs displays the cursor, by default as a rectangular block, in each
window at the position of that window’s point.  When the user switches
to another buffer in a window, Emacs moves that window’s cursor to where
point is in that buffer.  If the exact position of point is hidden
behind some display element, such as a display string or an image, Emacs
displays the cursor immediately before or after that display element.

 -- Function: window-point &optional window
     This function returns the current position of point in WINDOW.  For
     a nonselected window, this is the value point would have (in that
     window’s buffer) if that window were selected.  The default for
     WINDOW is the selected window.

     When WINDOW is the selected window, the value returned is the value
     of point in that window’s buffer.  Strictly speaking, it would be
     more correct to return the top-level value of point, outside of any
     ‘save-excursion’ forms.  But that value is hard to find.

 -- Function: set-window-point window position
     This function positions point in WINDOW at position POSITION in
     WINDOW’s buffer.  It returns POSITION.

     If WINDOW is selected, this simply does ‘goto-char’ in WINDOW’s
     buffer.

 -- Variable: window-point-insertion-type
     This variable specifies the marker insertion type (*note Marker
     Insertion Types::) of ‘window-point’.  The default is ‘nil’, so
     ‘window-point’ will stay behind text inserted there.


File: elisp.info,  Node: Window Start and End,  Next: Textual Scrolling,  Prev: Window Point,  Up: Windows

27.20 The Window Start and End Positions
========================================

Each window maintains a marker used to keep track of a buffer position
that specifies where in the buffer display should start.  This position
is called the “display-start” position of the window (or just the
“start”).  The character after this position is the one that appears at
the upper left corner of the window.  It is usually, but not inevitably,
at the beginning of a text line.

   After switching windows or buffers, and in some other cases, if the
window start is in the middle of a line, Emacs adjusts the window start
to the start of a line.  This prevents certain operations from leaving
the window start at a meaningless point within a line.  This feature may
interfere with testing some Lisp code by executing it using the commands
of Lisp mode, because they trigger this readjustment.  To test such
code, put it into a command and bind the command to a key.

 -- Function: window-start &optional window
     This function returns the display-start position of window WINDOW.
     If WINDOW is ‘nil’, the selected window is used.

     When you create a window, or display a different buffer in it, the
     display-start position is set to a display-start position recently
     used for the same buffer, or to ‘point-min’ if the buffer doesn’t
     have any.

     Redisplay updates the window-start position (if you have not
     specified it explicitly since the previous redisplay)—to make sure
     point appears on the screen.  Nothing except redisplay
     automatically changes the window-start position; if you move point,
     do not expect the window-start position to change in response until
     after the next redisplay.

 -- Function: window-group-start &optional window
     This function is like ‘window-start’, except that when WINDOW is a
     part of a group of windows (*note Window Group::),
     ‘window-group-start’ returns the start position of the entire
     group.  This condition holds when the buffer local variable
     ‘window-group-start-function’ is set to a function.  In this case,
     ‘window-group-start’ calls the function with the single argument
     WINDOW, then returns its result.

 -- Function: window-end &optional window update
     This function returns the position where display of its buffer ends
     in WINDOW.  The default for WINDOW is the selected window.

     Simply changing the buffer text or moving point does not update the
     value that ‘window-end’ returns.  The value is updated only when
     Emacs redisplays and redisplay completes without being preempted.

     If the last redisplay of WINDOW was preempted, and did not finish,
     Emacs does not know the position of the end of display in that
     window.  In that case, this function returns ‘nil’.

     If UPDATE is non-‘nil’, ‘window-end’ always returns an up-to-date
     value for where display ends, based on the current ‘window-start’
     value.  If a previously saved value of that position is still
     valid, ‘window-end’ returns that value; otherwise it computes the
     correct value by scanning the buffer text.

     Even if UPDATE is non-‘nil’, ‘window-end’ does not attempt to
     scroll the display if point has moved off the screen, the way real
     redisplay would do.  It does not alter the ‘window-start’ value.
     In effect, it reports where the displayed text will end if
     scrolling is not required.

 -- Function: window-group-end &optional window update
     This function is like ‘window-end’, except that when WINDOW is a
     part of a group of windows (*note Window Group::),
     ‘window-group-end’ returns the end position of the entire group.
     This condition holds when the buffer local variable
     ‘window-group-end-function’ is set to a function.  In this case,
     ‘window-group-end’ calls the function with the two arguments WINDOW
     and UPDATE, then returns its result.  The argument UPDATE has the
     same meaning as in ‘window-end’.

 -- Function: set-window-start window position &optional noforce
     This function sets the display-start position of WINDOW to POSITION
     in WINDOW’s buffer.  It returns POSITION.

     The display routines insist that the position of point be visible
     when a buffer is displayed.  Normally, they change the
     display-start position (that is, scroll the window) whenever
     necessary to make point visible.  However, if you specify the start
     position with this function using ‘nil’ for NOFORCE, it means you
     want display to start at POSITION even if that would put the
     location of point off the screen.  If this does place point off
     screen, the display routines move point to the left margin on the
     middle line in the window.

     For example, if point is 1 and you set the start of the window
     to 37, the start of the next line, point will be above the top of
     the window.  The display routines will automatically move point if
     it is still 1 when redisplay occurs.  Here is an example:

          ;; Here is what ‘foo’ looks like before executing
          ;;   the ‘set-window-start’ expression.

          ---------- Buffer: foo ----------
          ★This is the contents of buffer foo.
          2
          3
          4
          5
          6
          ---------- Buffer: foo ----------

          (set-window-start
           (selected-window)
           (save-excursion
             (goto-char 1)
             (forward-line 1)
             (point)))
          ⇒ 37

          ;; Here is what ‘foo’ looks like after executing
          ;;   the ‘set-window-start’ expression.
          ---------- Buffer: foo ----------
          2
          3
          ★4
          5
          6
          ---------- Buffer: foo ----------

     If NOFORCE is non-‘nil’, and POSITION would place point off screen
     at the next redisplay, then redisplay computes a new window-start
     position that works well with point, and thus POSITION is not used.

 -- Function: set-window-group-start window position &optional noforce
     This function is like ‘set-window-start’, except that when WINDOW
     is a part of a group of windows (*note Window Group::),
     ‘set-window-group-start’ sets the start position of the entire
     group.  This condition holds when the buffer local variable
     ‘set-window-group-start-function’ is set to a function.  In this
     case, ‘set-window-group-start’ calls the function with the three
     arguments WINDOW, POSITION, and NOFORCE, then returns its result.
     The arguments POSITION and NOFORCE in this function have the same
     meaning as in ‘set-window-start’.

 -- Function: pos-visible-in-window-p &optional position window
          partially
     This function returns non-‘nil’ if POSITION is within the range of
     text currently visible on the screen in WINDOW.  It returns ‘nil’
     if POSITION is scrolled vertically out of view.  Locations that are
     partially obscured are not considered visible unless PARTIALLY is
     non-‘nil’.  The argument POSITION defaults to the current position
     of point in WINDOW; WINDOW defaults to the selected window.  If
     POSITION is ‘t’, that means to check either the first visible
     position of the last screen line in WINDOW, or the end-of-buffer
     position, whichever comes first.

     This function considers only vertical scrolling.  If POSITION is
     out of view only because WINDOW has been scrolled horizontally,
     ‘pos-visible-in-window-p’ returns non-‘nil’ anyway.  *Note
     Horizontal Scrolling::.

     If POSITION is visible, ‘pos-visible-in-window-p’ returns ‘t’ if
     PARTIALLY is ‘nil’; if PARTIALLY is non-‘nil’, and the character
     following POSITION is fully visible, it returns a list of the form
     ‘(X Y)’, where X and Y are the pixel coordinates relative to the
     top left corner of the window; otherwise it returns an extended
     list of the form ‘(X Y RTOP RBOT ROWH VPOS)’, where RTOP and RBOT
     specify the number of off-window pixels at the top and bottom of
     the row at POSITION, ROWH specifies the visible height of that row,
     and VPOS specifies the vertical position (zero-based row number) of
     that row.

     Here is an example:

          ;; If point is off the screen now, recenter it now.
          (or (pos-visible-in-window-p
               (point) (selected-window))
              (recenter 0))

 -- Function: pos-visible-in-window-group-p &optional position window
          partially
     This function is like ‘pos-visible-in-window-p’, except that when
     WINDOW is a part of a group of windows (*note Window Group::),
     ‘pos-visible-in-window-group-p’ tests the visibility of POS in the
     entire group, not just in the single WINDOW.  This condition holds
     when the buffer local variable
     ‘pos-visible-in-window-group-p-function’ is set to a function.  In
     this case ‘pos-visible-in-window-group-p’ calls the function with
     the three arguments POSITION, WINDOW, and PARTIALLY, then returns
     its result.  The arguments POSITION and PARTIALLY have the same
     meaning as in ‘pos-visible-in-window-p’.

 -- Function: window-line-height &optional line window
     This function returns the height of text line LINE in WINDOW.  If
     LINE is one of ‘header-line’ or ‘mode-line’, ‘window-line-height’
     returns information about the corresponding line of the window.
     Otherwise, LINE is a text line number starting from 0.  A negative
     number counts from the end of the window.  The default for LINE is
     the current line in WINDOW; the default for WINDOW is the selected
     window.

     If the display is not up to date, ‘window-line-height’ returns
     ‘nil’.  In that case, ‘pos-visible-in-window-p’ may be used to
     obtain related information.

     If there is no line corresponding to the specified LINE,
     ‘window-line-height’ returns ‘nil’.  Otherwise, it returns a list
     ‘(HEIGHT VPOS YPOS OFFBOT)’, where HEIGHT is the height in pixels
     of the visible part of the line, VPOS and YPOS are the vertical
     position in lines and pixels of the line relative to the top of the
     first text line, and OFFBOT is the number of off-window pixels at
     the bottom of the text line.  If there are off-window pixels at the
     top of the (first) text line, YPOS is negative.


File: elisp.info,  Node: Textual Scrolling,  Next: Vertical Scrolling,  Prev: Window Start and End,  Up: Windows

27.21 Textual Scrolling
=======================

“Textual scrolling” means moving the text up or down through a window.
It works by changing the window’s display-start location.  It may also
change the value of ‘window-point’ to keep point on the screen (*note
Window Point::).

   The basic textual scrolling functions are ‘scroll-up’ (which scrolls
forward) and ‘scroll-down’ (which scrolls backward).  In these function
names, “up” and “down” refer to the direction of motion of the buffer
text relative to the window.  Imagine that the text is written on a long
roll of paper and that the scrolling commands move the paper up and
down.  Thus, if you are looking at the middle of a buffer and repeatedly
call ‘scroll-down’, you will eventually see the beginning of the buffer.

   Unfortunately, this sometimes causes confusion, because some people
tend to think in terms of the opposite convention: they imagine the
window moving over text that remains in place, so that “down” commands
take you to the end of the buffer.  This convention is consistent with
fact that such a command is bound to a key named <PageDown> on modern
keyboards.

   Textual scrolling functions (aside from ‘scroll-other-window’) have
unpredictable results if the current buffer is not the one displayed in
the selected window.  *Note Current Buffer::.

   If the window contains a row taller than the height of the window
(for example in the presence of a large image), the scroll functions
will adjust the window’s vertical scroll position to scroll the
partially visible row.  Lisp callers can disable this feature by binding
the variable ‘auto-window-vscroll’ to ‘nil’ (*note Vertical
Scrolling::).

 -- Command: scroll-up &optional count
     This function scrolls forward by COUNT lines in the selected
     window.

     If COUNT is negative, it scrolls backward instead.  If COUNT is
     ‘nil’ (or omitted), the distance scrolled is
     ‘next-screen-context-lines’ lines less than the height of the
     window’s text area.

     If the selected window cannot be scrolled any further, this
     function signals an error.  Otherwise, it returns ‘nil’.

 -- Command: scroll-down &optional count
     This function scrolls backward by COUNT lines in the selected
     window.

     If COUNT is negative, it scrolls forward instead.  In other
     respects, it behaves the same way as ‘scroll-up’ does.

 -- Command: scroll-up-command &optional count
     This behaves like ‘scroll-up’, except that if the selected window
     cannot be scrolled any further and the value of the variable
     ‘scroll-error-top-bottom’ is ‘t’, it tries to move to the end of
     the buffer instead.  If point is already there, it signals an
     error.

 -- Command: scroll-down-command &optional count
     This behaves like ‘scroll-down’, except that if the selected window
     cannot be scrolled any further and the value of the variable
     ‘scroll-error-top-bottom’ is ‘t’, it tries to move to the beginning
     of the buffer instead.  If point is already there, it signals an
     error.

 -- Command: scroll-other-window &optional count
     This function scrolls the text in another window upward COUNT
     lines.  Negative values of COUNT, or ‘nil’, are handled as in
     ‘scroll-up’.

     You can specify which buffer to scroll by setting the variable
     ‘other-window-scroll-buffer’ to a buffer.  If that buffer isn’t
     already displayed, ‘scroll-other-window’ displays it in some
     window.

     When the selected window is the minibuffer, the next window is
     normally the leftmost one immediately above it.  You can specify a
     different window to scroll, when the minibuffer is selected, by
     setting the variable ‘minibuffer-scroll-window’.  This variable has
     no effect when any other window is selected.  When it is non-‘nil’
     and the minibuffer is selected, it takes precedence over
     ‘other-window-scroll-buffer’.  *Note Definition of
     minibuffer-scroll-window::.

     When the minibuffer is active, it is the next window if the
     selected window is the one at the bottom right corner.  In this
     case, ‘scroll-other-window’ attempts to scroll the minibuffer.  If
     the minibuffer contains just one line, it has nowhere to scroll to,
     so the line reappears after the echo area momentarily displays the
     message ‘End of buffer’.

 -- Variable: other-window-scroll-buffer
     If this variable is non-‘nil’, it tells ‘scroll-other-window’ which
     buffer’s window to scroll.

 -- User Option: scroll-margin
     This option specifies the size of the scroll margin—a minimum
     number of lines between point and the top or bottom of a window.
     Whenever point gets within this many lines of the top or bottom of
     the window, redisplay scrolls the text automatically (if possible)
     to move point out of the margin, closer to the center of the
     window.

 -- User Option: scroll-conservatively
     This variable controls how scrolling is done automatically when
     point moves off the screen (or into the scroll margin).  If the
     value is a positive integer N, then redisplay scrolls the text up
     to N lines in either direction, if that will bring point back into
     proper view.  This behavior is called “conservative scrolling”.
     Otherwise, scrolling happens in the usual way, under the control of
     other variables such as ‘scroll-up-aggressively’ and
     ‘scroll-down-aggressively’.

     The default value is zero, which means that conservative scrolling
     never happens.

 -- User Option: scroll-down-aggressively
     The value of this variable should be either ‘nil’ or a fraction F
     between 0 and 1.  If it is a fraction, that specifies where on the
     screen to put point when scrolling down.  More precisely, when a
     window scrolls down because point is above the window start, the
     new start position is chosen to put point F part of the window
     height from the top.  The larger F, the more aggressive the
     scrolling.

     A value of ‘nil’ is equivalent to .5, since its effect is to center
     point.  This variable automatically becomes buffer-local when set
     in any fashion.

 -- User Option: scroll-up-aggressively
     Likewise, for scrolling up.  The value, F, specifies how far point
     should be placed from the bottom of the window; thus, as with
     ‘scroll-up-aggressively’, a larger value scrolls more aggressively.

 -- User Option: scroll-step
     This variable is an older variant of ‘scroll-conservatively’.  The
     difference is that if its value is N, that permits scrolling only
     by precisely N lines, not a smaller number.  This feature does not
     work with ‘scroll-margin’.  The default value is zero.

 -- User Option: scroll-preserve-screen-position
     If this option is ‘t’, whenever a scrolling command moves point
     off-window, Emacs tries to adjust point to keep the cursor at its
     old vertical position in the window, rather than the window edge.

     If the value is non-‘nil’ and not ‘t’, Emacs adjusts point to keep
     the cursor at the same vertical position, even if the scrolling
     command didn’t move point off-window.

     This option affects all scroll commands that have a non-‘nil’
     ‘scroll-command’ symbol property.

 -- User Option: next-screen-context-lines
     The value of this variable is the number of lines of continuity to
     retain when scrolling by full screens.  For example, ‘scroll-up’
     with an argument of ‘nil’ scrolls so that this many lines at the
     bottom of the window appear instead at the top.  The default value
     is ‘2’.

 -- User Option: scroll-error-top-bottom
     If this option is ‘nil’ (the default), ‘scroll-up-command’ and
     ‘scroll-down-command’ simply signal an error when no more scrolling
     is possible.

     If the value is ‘t’, these commands instead move point to the
     beginning or end of the buffer (depending on scrolling direction);
     only if point is already on that position do they signal an error.

 -- Command: recenter &optional count
     This function scrolls the text in the selected window so that point
     is displayed at a specified vertical position within the window.
     It does not move point with respect to the text.

     If COUNT is a non-negative number, that puts the line containing
     point COUNT lines down from the top of the window.  If COUNT is a
     negative number, then it counts upward from the bottom of the
     window, so that −1 stands for the last usable line in the window.

     If COUNT is ‘nil’ (or a non-‘nil’ list), ‘recenter’ puts the line
     containing point in the middle of the window.  If COUNT is ‘nil’,
     this function may redraw the frame, according to the value of
     ‘recenter-redisplay’.

     When ‘recenter’ is called interactively, COUNT is the raw prefix
     argument.  Thus, typing ‘C-u’ as the prefix sets the COUNT to a
     non-‘nil’ list, while typing ‘C-u 4’ sets COUNT to 4, which
     positions the current line four lines from the top.

     With an argument of zero, ‘recenter’ positions the current line at
     the top of the window.  The command ‘recenter-top-bottom’ offers a
     more convenient way to achieve this.

 -- Function: recenter-window-group &optional count
     This function is like ‘recenter’, except that when the selected
     window is part of a group of windows (*note Window Group::),
     ‘recenter-window-group’ scrolls the entire group.  This condition
     holds when the buffer local variable
     ‘recenter-window-group-function’ is set to a function.  In this
     case, ‘recenter-window-group’ calls the function with the argument
     COUNT, then returns its result.  The argument COUNT has the same
     meaning as in ‘recenter’, but with respect to the entire window
     group.

 -- User Option: recenter-redisplay
     If this variable is non-‘nil’, calling ‘recenter’ with a ‘nil’
     argument redraws the frame.  The default value is ‘tty’, which
     means only redraw the frame if it is a tty frame.

 -- Command: recenter-top-bottom &optional count
     This command, which is the default binding for ‘C-l’, acts like
     ‘recenter’, except if called with no argument.  In that case,
     successive calls place point according to the cycling order defined
     by the variable ‘recenter-positions’.

 -- User Option: recenter-positions
     This variable controls how ‘recenter-top-bottom’ behaves when
     called with no argument.  The default value is ‘(middle top
     bottom)’, which means that successive calls of
     ‘recenter-top-bottom’ with no argument cycle between placing point
     at the middle, top, and bottom of the window.


File: elisp.info,  Node: Vertical Scrolling,  Next: Horizontal Scrolling,  Prev: Textual Scrolling,  Up: Windows

27.22 Vertical Fractional Scrolling
===================================

“Vertical fractional scrolling” means shifting text in a window up or
down by a specified multiple or fraction of a line.  Each window has a
“vertical scroll position”, which is a number, never less than zero.  It
specifies how far to raise the contents of the window.  Raising the
window contents generally makes all or part of some lines disappear off
the top, and all or part of some other lines appear at the bottom.  The
usual value is zero.

   The vertical scroll position is measured in units of the normal line
height, which is the height of the default font.  Thus, if the value is
.5, that means the window contents are scrolled up half the normal line
height.  If it is 3.3, that means the window contents are scrolled up
somewhat over three times the normal line height.

   What fraction of a line the vertical scrolling covers, or how many
lines, depends on what the lines contain.  A value of .5 could scroll a
line whose height is very short off the screen, while a value of 3.3
could scroll just part of the way through a tall line or an image.

 -- Function: window-vscroll &optional window pixels-p
     This function returns the current vertical scroll position of
     WINDOW.  The default for WINDOW is the selected window.  If
     PIXELS-P is non-‘nil’, the return value is measured in pixels,
     rather than in units of the normal line height.

          (window-vscroll)
               ⇒ 0

 -- Function: set-window-vscroll window lines &optional pixels-p
     This function sets WINDOW’s vertical scroll position to LINES.  If
     WINDOW is ‘nil’, the selected window is used.  The argument LINES
     should be zero or positive; if not, it is taken as zero.

     The actual vertical scroll position must always correspond to an
     integral number of pixels, so the value you specify is rounded
     accordingly.

     The return value is the result of this rounding.

          (set-window-vscroll (selected-window) 1.2)
               ⇒ 1.13

     If PIXELS-P is non-‘nil’, LINES specifies a number of pixels.  In
     this case, the return value is LINES.

 -- Variable: auto-window-vscroll
     If this variable is non-‘nil’, the ‘line-move’, ‘scroll-up’, and
     ‘scroll-down’ functions will automatically modify the vertical
     scroll position to scroll through display rows that are taller than
     the height of the window, for example in the presence of large
     images.


File: elisp.info,  Node: Horizontal Scrolling,  Next: Coordinates and Windows,  Prev: Vertical Scrolling,  Up: Windows

27.23 Horizontal Scrolling
==========================

“Horizontal scrolling” means shifting the image in the window left or
right by a specified multiple of the normal character width.  Each
window has a “horizontal scroll position”, which is a number, never less
than zero.  It specifies how far to shift the contents left.  Shifting
the window contents left generally makes all or part of some characters
disappear off the left, and all or part of some other characters appear
at the right.  The usual value is zero.

   The horizontal scroll position is measured in units of the normal
character width, which is the width of space in the default font.  Thus,
if the value is 5, that means the window contents are scrolled left by 5
times the normal character width.  How many characters actually
disappear off to the left depends on their width, and could vary from
line to line.

   Because we read from side to side in the inner loop, and from top to
bottom in the outer loop, the effect of horizontal scrolling is not like
that of textual or vertical scrolling.  Textual scrolling involves
selection of a portion of text to display, and vertical scrolling moves
the window contents contiguously; but horizontal scrolling causes part
of _each line_ to go off screen.

   Usually, no horizontal scrolling is in effect; then the leftmost
column is at the left edge of the window.  In this state, scrolling to
the right is meaningless, since there is no data to the left of the edge
to be revealed by it; so this is not allowed.  Scrolling to the left is
allowed; it scrolls the first columns of text off the edge of the window
and can reveal additional columns on the right that were truncated
before.  Once a window has a nonzero amount of leftward horizontal
scrolling, you can scroll it back to the right, but only so far as to
reduce the net horizontal scroll to zero.  There is no limit to how far
left you can scroll, but eventually all the text will disappear off the
left edge.

   If ‘auto-hscroll-mode’ is set, redisplay automatically alters the
horizontal scrolling of a window as necessary to ensure that point is
always visible.  However, you can still set the horizontal scrolling
value explicitly.  The value you specify serves as a lower bound for
automatic scrolling, i.e., automatic scrolling will not scroll a window
to a column less than the specified one.

 -- Command: scroll-left &optional count set-minimum
     This function scrolls the selected window COUNT columns to the left
     (or to the right if COUNT is negative).  The default for COUNT is
     the window width, minus 2.

     The return value is the total amount of leftward horizontal
     scrolling in effect after the change—just like the value returned
     by ‘window-hscroll’ (below).

     Note that text in paragraphs whose base direction is right-to-left
     (*note Bidirectional Display::) moves in the opposite direction:
     e.g., it moves to the right when ‘scroll-left’ is invoked with a
     positive value of COUNT.

     Once you scroll a window as far right as it can go, back to its
     normal position where the total leftward scrolling is zero,
     attempts to scroll any farther right have no effect.

     If SET-MINIMUM is non-‘nil’, the new scroll amount becomes the
     lower bound for automatic scrolling; that is, automatic scrolling
     will not scroll a window to a column less than the value returned
     by this function.  Interactive calls pass non-‘nil’ for
     SET-MINIMUM.

 -- Command: scroll-right &optional count set-minimum
     This function scrolls the selected window COUNT columns to the
     right (or to the left if COUNT is negative).  The default for COUNT
     is the window width, minus 2.  Aside from the direction of
     scrolling, this works just like ‘scroll-left’.

 -- Function: window-hscroll &optional window
     This function returns the total leftward horizontal scrolling of
     WINDOW—the number of columns by which the text in WINDOW is
     scrolled left past the left margin.  (In right-to-left paragraphs,
     the value is the total amount of the rightward scrolling instead.)
     The default for WINDOW is the selected window.

     The return value is never negative.  It is zero when no horizontal
     scrolling has been done in WINDOW (which is usually the case).

          (window-hscroll)
               ⇒ 0
          (scroll-left 5)
               ⇒ 5
          (window-hscroll)
               ⇒ 5

 -- Function: set-window-hscroll window columns
     This function sets horizontal scrolling of WINDOW.  The value of
     COLUMNS specifies the amount of scrolling, in terms of columns from
     the left margin (right margin in right-to-left paragraphs).  The
     argument COLUMNS should be zero or positive; if not, it is taken as
     zero.  Fractional values of COLUMNS are not supported at present.

     Note that ‘set-window-hscroll’ may appear not to work if you test
     it by evaluating a call with ‘M-:’ in a simple way.  What happens
     is that the function sets the horizontal scroll value and returns,
     but then redisplay adjusts the horizontal scrolling to make point
     visible, and this overrides what the function did.  You can observe
     the function’s effect if you call it while point is sufficiently
     far from the left margin that it will remain visible.

     The value returned is COLUMNS.

          (set-window-hscroll (selected-window) 10)
               ⇒ 10

   Here is how you can determine whether a given position POSITION is
off the screen due to horizontal scrolling:

     (defun hscroll-on-screen (window position)
       (save-excursion
         (goto-char position)
         (and
          (>= (- (current-column) (window-hscroll window)) 0)
          (< (- (current-column) (window-hscroll window))
             (window-width window)))))


File: elisp.info,  Node: Coordinates and Windows,  Next: Window Configurations,  Prev: Horizontal Scrolling,  Up: Windows

27.24 Coordinates and Windows
=============================

This section describes functions that report the position of a window.
Most of these functions report positions relative to an origin at the
native position of the window’s frame (*note Frame Geometry::).  Some
functions report positions relative to the origin of the display of the
window’s frame.  In any case, the origin has the coordinates (0, 0) and
X and Y coordinates increase rightward and downward respectively.

   For the following functions, X and Y coordinates are reported in
integer character units, i.e., numbers of lines and columns
respectively.  On a graphical display, each “line” and “column”
corresponds to the height and width of the default character specified
by the frame’s default font (*note Frame Font::).

 -- Function: window-edges &optional window body absolute pixelwise
     This function returns a list of the edge coordinates of WINDOW.  If
     WINDOW is omitted or ‘nil’, it defaults to the selected window.

     The return value has the form ‘(LEFT TOP RIGHT BOTTOM)’.  These
     list elements are, respectively, the X coordinate of the leftmost
     column occupied by the window, the Y coordinate of the topmost row,
     the X coordinate one column to the right of the rightmost column,
     and the Y coordinate one row down from the bottommost row.

     Note that these are the actual outer edges of the window, including
     any header line, mode line, scroll bar, fringes, window divider and
     display margins.  On a text terminal, if the window has a neighbor
     on its right, its right edge includes the separator line between
     the window and its neighbor.

     If the optional argument BODY is ‘nil’, this means to return the
     edges corresponding to the total size of WINDOW.  BODY non-‘nil’
     means to return the edges of WINDOW’s body (aka text area).  If
     BODY is non-‘nil’, WINDOW must specify a live window.

     If the optional argument ABSOLUTE is ‘nil’, this means to return
     edges relative to the native position of WINDOW’s frame.  ABSOLUTE
     non-‘nil’ means to return coordinates relative to the origin (0, 0)
     of WINDOW’s display.  On non-graphical systems this argument has no
     effect.

     If the optional argument PIXELWISE is ‘nil’, this means to return
     the coordinates in terms of the default character width and height
     of WINDOW’s frame (*note Frame Font::), rounded if necessary.
     PIXELWISE non-‘nil’ means to return the coordinates in pixels.
     Note that the pixel specified by RIGHT and BOTTOM is immediately
     outside of these edges.  If ABSOLUTE is non-‘nil’, PIXELWISE is
     implicitly non-‘nil’ too.

 -- Function: window-body-edges &optional window
     This function returns the edges of WINDOW’s body (*note Window
     Sizes::).  Calling ‘(window-body-edges window)’ is equivalent to
     calling ‘(window-edges window t)’, see above.

   The following functions can be used to relate a set of frame-relative
coordinates to a window:

 -- Function: window-at x y &optional frame
     This function returns the live window at the coordinates X and Y
     given in default character sizes (*note Frame Font::) relative to
     the native position of FRAME (*note Frame Geometry::).

     If there is no window at that position, the return value is ‘nil’.
     If FRAME is omitted or ‘nil’, it defaults to the selected frame.

 -- Function: coordinates-in-window-p coordinates window
     This function checks whether a window WINDOW occupies the frame
     relative coordinates COORDINATES, and if so, which part of the
     window that is.  WINDOW should be a live window.

     COORDINATES should be a cons cell of the form ‘(X . Y)’, where X
     and Y are given in default character sizes (*note Frame Font::)
     relative to the native position of WINDOW’s frame (*note Frame
     Geometry::).

     If there is no window at the specified position, the return value
     is ‘nil’ .  Otherwise, the return value is one of the following:

     ‘(RELX . RELY)’
          The coordinates are inside WINDOW.  The numbers RELX and RELY
          are the equivalent window-relative coordinates for the
          specified position, counting from 0 at the top left corner of
          the window.

     ‘mode-line’
          The coordinates are in the mode line of WINDOW.

     ‘header-line’
          The coordinates are in the header line of WINDOW.

     ‘right-divider’
          The coordinates are in the divider separating WINDOW from a
          window on the right.

     ‘bottom-divider’
          The coordinates are in the divider separating WINDOW from a
          window beneath.

     ‘vertical-line’
          The coordinates are in the vertical line between WINDOW and
          its neighbor to the right.  This value occurs only if the
          window doesn’t have a scroll bar; positions in a scroll bar
          are considered outside the window for these purposes.

     ‘left-fringe’
     ‘right-fringe’
          The coordinates are in the left or right fringe of the window.

     ‘left-margin’
     ‘right-margin’
          The coordinates are in the left or right margin of the window.

     ‘nil’
          The coordinates are not in any part of WINDOW.

     The function ‘coordinates-in-window-p’ does not require a frame as
     argument because it always uses the frame that WINDOW is on.

   The following functions return window positions in pixels, rather
than character units.  Though mostly useful on graphical displays, they
can also be called on text terminals, where the screen area of each text
character is taken to be one pixel.

 -- Function: window-pixel-edges &optional window
     This function returns a list of pixel coordinates for the edges of
     WINDOW.  Calling ‘(window-pixel-edges window)’ is equivalent to
     calling ‘(window-edges window nil nil t)’, see above.

 -- Function: window-body-pixel-edges &optional window
     This function returns the pixel edges of WINDOW’s body.  Calling
     ‘(window-body-pixel-edges window)’ is equivalent to calling
     ‘(window-edges window t nil t)’, see above.

   The following functions return window positions in pixels, relative
to the origin of the display screen rather than that of the frame:

 -- Function: window-absolute-pixel-edges &optional window
     This function returns the pixel coordinates of WINDOW relative to
     an origin at (0, 0) of the display of WINDOW’s frame.  Calling
     ‘(window-absolute-pixel-edges)’ is equivalent to calling
     ‘(window-edges window nil t t)’, see above.

 -- Function: window-absolute-body-pixel-edges &optional window
     This function returns the pixel coordinates of WINDOW’s body
     relative to an origin at (0, 0) of the display of WINDOW’s frame.
     Calling ‘(window-absolute-body-pixel-edges window)’ is equivalent
     to calling ‘(window-edges window t t t)’, see above.

     Combined with ‘set-mouse-absolute-pixel-position’, this function
     can be used to move the mouse pointer to an arbitrary buffer
     position visible in some window:

          (let ((edges (window-absolute-body-pixel-edges))
                (position (pos-visible-in-window-p nil nil t)))
            (set-mouse-absolute-pixel-position
             (+ (nth 0 edges) (nth 0 position))
             (+ (nth 1 edges) (nth 1 position))))

     On a graphical terminal this form “warps” the mouse cursor to the
     upper left corner of the glyph at the selected window’s point.  A
     position calculated this way can be also used to show a tooltip
     window there.

   The following function returns the screen coordinates of a buffer
position visible in a window:

 -- Function: window-absolute-pixel-position &optional position window
     If the buffer position POSITION is visible in window WINDOW, this
     function returns the display coordinates of the upper/left corner
     of the glyph at POSITION.  The return value is a cons of the X- and
     Y-coordinates of that corner, relative to an origin at (0, 0) of
     WINDOW’s display.  It returns ‘nil’ if POSITION is not visible in
     WINDOW.

     WINDOW must be a live window and defaults to the selected window.
     POSITION defaults to the value of ‘window-point’ of WINDOW.

     This means that in order to move the mouse pointer to the position
     of point in the selected window, it’s sufficient to write:

          (let ((position (window-absolute-pixel-position)))
            (set-mouse-absolute-pixel-position
             (car position) (cdr position)))


File: elisp.info,  Node: Window Configurations,  Next: Window Parameters,  Prev: Coordinates and Windows,  Up: Windows

27.25 Window Configurations
===========================

A “window configuration” records the entire layout of one frame—all
windows, their sizes, which buffers they contain, how those buffers are
scrolled, and their value of point; also their fringes, margins, and
scroll bar settings.  It also includes the value of
‘minibuffer-scroll-window’.  As a special exception, the window
configuration does not record the value of point in the selected window
for the current buffer.

   You can bring back an entire frame layout by restoring a previously
saved window configuration.  If you want to record the layout of all
frames instead of just one, use a frame configuration instead of a
window configuration.  *Note Frame Configurations::.

 -- Function: current-window-configuration &optional frame
     This function returns a new object representing FRAME’s current
     window configuration.  The default for FRAME is the selected frame.
     The variable ‘window-persistent-parameters’ specifies which window
     parameters (if any) are saved by this function.  *Note Window
     Parameters::.

 -- Function: set-window-configuration configuration
     This function restores the configuration of windows and buffers as
     specified by CONFIGURATION, for the frame that CONFIGURATION was
     created for.

     The argument CONFIGURATION must be a value that was previously
     returned by ‘current-window-configuration’.  The configuration is
     restored in the frame from which CONFIGURATION was made, whether
     that frame is selected or not.  This always counts as a window size
     change and triggers execution of the ‘window-size-change-functions’
     (*note Window Hooks::), because ‘set-window-configuration’ doesn’t
     know how to tell whether the new configuration actually differs
     from the old one.

     If the frame from which CONFIGURATION was saved is dead, all this
     function does is restore the three variables ‘window-min-height’,
     ‘window-min-width’ and ‘minibuffer-scroll-window’.  In this case,
     the function returns ‘nil’.  Otherwise, it returns ‘t’.

     Here is a way of using this function to get the same effect as
     ‘save-window-excursion’:

          (let ((config (current-window-configuration)))
            (unwind-protect
                (progn (split-window-below nil)
                       ...)
              (set-window-configuration config)))

 -- Macro: save-window-excursion forms...
     This macro records the window configuration of the selected frame,
     executes FORMS in sequence, then restores the earlier window
     configuration.  The return value is the value of the final form in
     FORMS.

     Most Lisp code should not use this macro; ‘save-selected-window’ is
     typically sufficient.  In particular, this macro cannot reliably
     prevent the code in FORMS from opening new windows, because new
     windows might be opened in other frames (*note Choosing Window::),
     and ‘save-window-excursion’ only saves and restores the window
     configuration on the current frame.

     Do not use this macro in ‘window-size-change-functions’; exiting
     the macro triggers execution of ‘window-size-change-functions’,
     leading to an endless loop.

 -- Function: window-configuration-p object
     This function returns ‘t’ if OBJECT is a window configuration.

 -- Function: compare-window-configurations config1 config2
     This function compares two window configurations as regards the
     structure of windows, but ignores the values of point and the saved
     scrolling positions—it can return ‘t’ even if those aspects differ.

     The function ‘equal’ can also compare two window configurations; it
     regards configurations as unequal if they differ in any respect,
     even a saved point.

 -- Function: window-configuration-frame config
     This function returns the frame for which the window configuration
     CONFIG was made.

   Other primitives to look inside of window configurations would make
sense, but are not implemented because we did not need them.  See the
file ‘winner.el’ for some more operations on windows configurations.

   The objects returned by ‘current-window-configuration’ die together
with the Emacs process.  In order to store a window configuration on
disk and read it back in another Emacs session, you can use the
functions described next.  These functions are also useful to clone the
state of a frame into an arbitrary live window
(‘set-window-configuration’ effectively clones the windows of a frame
into the root window of that very frame only).

 -- Function: window-state-get &optional window writable
     This function returns the state of WINDOW as a Lisp object.  The
     argument WINDOW must be a valid window and defaults to the root
     window of the selected frame.

     If the optional argument WRITABLE is non-‘nil’, this means to not
     use markers for sampling positions like ‘window-point’ or
     ‘window-start’.  This argument should be non-‘nil’ when the state
     will be written to disk and read back in another session.

     Together, the argument WRITABLE and the variable
     ‘window-persistent-parameters’ specify which window parameters are
     saved by this function.  *Note Window Parameters::.

   The value returned by ‘window-state-get’ can be used in the same
session to make a clone of a window in another window.  It can be also
written to disk and read back in another session.  In either case, use
the following function to restore the state of the window.

 -- Function: window-state-put state &optional window ignore
     This function puts the window state STATE into WINDOW.  The
     argument STATE should be the state of a window returned by an
     earlier invocation of ‘window-state-get’, see above.  The optional
     argument WINDOW can be either a live window or an internal window
     (*note Windows and Frames::) and defaults to the selected one.  If
     WINDOW is not live, it is replaced by a live window before putting
     STATE into it.

     If the optional argument IGNORE is non-‘nil’, it means to ignore
     minimum window sizes and fixed-size restrictions.  If IGNORE is
     ‘safe’, this means windows can get as small as one line and/or two
     columns.


File: elisp.info,  Node: Window Parameters,  Next: Window Hooks,  Prev: Window Configurations,  Up: Windows

27.26 Window Parameters
=======================

This section describes how window parameters can be used to associate
additional information with windows.

 -- Function: window-parameter window parameter
     This function returns WINDOW’s value for PARAMETER.  The default
     for WINDOW is the selected window.  If WINDOW has no setting for
     PARAMETER, this function returns ‘nil’.

 -- Function: window-parameters &optional window
     This function returns all parameters of WINDOW and their values.
     The default for WINDOW is the selected window.  The return value is
     either ‘nil’, or an association list whose elements have the form
     ‘(PARAMETER . VALUE)’.

 -- Function: set-window-parameter window parameter value
     This function sets WINDOW’s value of PARAMETER to VALUE and returns
     VALUE.  The default for WINDOW is the selected window.

   By default, the functions that save and restore window configurations
or the states of windows (*note Window Configurations::) do not care
about window parameters.  This means that when you change the value of a
parameter within the body of a ‘save-window-excursion’, the previous
value is not restored when that macro exits.  It also means that when
you restore via ‘window-state-put’ a window state saved earlier by
‘window-state-get’, all cloned windows have their parameters reset to
‘nil’.  The following variable allows you to override the standard
behavior:

 -- Variable: window-persistent-parameters
     This variable is an alist specifying which parameters get saved by
     ‘current-window-configuration’ and ‘window-state-get’, and
     subsequently restored by ‘set-window-configuration’ and
     ‘window-state-put’.  *Note Window Configurations::.

     The CAR of each entry of this alist is a symbol specifying the
     parameter.  The CDR should be one of the following:

     ‘nil’
          This value means the parameter is saved neither by
          ‘window-state-get’ nor by ‘current-window-configuration’.

     ‘t’
          This value specifies that the parameter is saved by
          ‘current-window-configuration’ and (provided its WRITABLE
          argument is ‘nil’) by ‘window-state-get’.

     ‘writable’
          This means that the parameter is saved unconditionally by both
          ‘current-window-configuration’ and ‘window-state-get’.  This
          value should not be used for parameters whose values do not
          have a read syntax.  Otherwise, invoking ‘window-state-put’ in
          another session may fail with an ‘invalid-read-syntax’ error.

   Some functions (notably ‘delete-window’, ‘delete-other-windows’ and
‘split-window’), may behave specially when their WINDOW argument has a
parameter set.  You can override such special behavior by binding the
following variable to a non-‘nil’ value:

 -- Variable: ignore-window-parameters
     If this variable is non-‘nil’, some standard functions do not
     process window parameters.  The functions currently affected by
     this are ‘split-window’, ‘delete-window’, ‘delete-other-windows’,
     and ‘other-window’.

     An application can bind this variable to a non-‘nil’ value around
     calls to these functions.  If it does so, the application is fully
     responsible for correctly assigning the parameters of all involved
     windows when exiting that function.

   The following parameters are currently used by the window management
code:

‘delete-window’
     This parameter affects the execution of ‘delete-window’ (*note
     Deleting Windows::).

‘delete-other-windows’
     This parameter affects the execution of ‘delete-other-windows’
     (*note Deleting Windows::).

‘split-window’
     This parameter affects the execution of ‘split-window’ (*note
     Splitting Windows::).

‘other-window’
     This parameter affects the execution of ‘other-window’ (*note
     Cyclic Window Ordering::).

‘no-other-window’
     This parameter marks the window as not selectable by ‘other-window’
     (*note Cyclic Window Ordering::).

‘clone-of’
     This parameter specifies the window that this one has been cloned
     from.  It is installed by ‘window-state-get’ (*note Window
     Configurations::).

‘preserved-size’
     This parameter specifies a buffer, a direction where ‘nil’ means
     vertical and ‘t’ horizontal, and a size in pixels.  If this window
     displays the specified buffer and its size in the indicated
     direction equals the size specified by this parameter, then Emacs
     will try to preserve the size of this window in the indicated
     direction.  This parameter is installed and updated by the function
     ‘window-preserve-size’ (*note Preserving Window Sizes::).

‘quit-restore’
     This parameter is installed by the buffer display functions (*note
     Choosing Window::) and consulted by ‘quit-restore-window’ (*note
     Quitting Windows::).  It contains four elements:

     The first element is one of the symbols ‘window’, meaning that the
     window has been specially created by ‘display-buffer’; ‘frame’, a
     separate frame has been created; ‘same’, the window has only ever
     displayed this buffer; or ‘other’, the window showed another buffer
     before.  ‘frame’ and ‘window’ affect how the window is quit, while
     ‘same’ and ‘other’ affect the redisplay of buffers previously shown
     in this window.

     The second element is either one of the symbols ‘window’ or
     ‘frame’, or a list whose elements are the buffer shown in the
     window before, that buffer’s window start and window point
     positions, and the window’s height at that time.  If that buffer is
     still live when the window is quit, then the function
     ‘quit-restore-window’ reuses the window to display the buffer.

     The third element is the window selected at the time the parameter
     was created.  If ‘quit-restore-window’ deletes the window passed to
     it as argument, it then tries to reselect this window.

     The fourth element is the buffer whose display caused the creation
     of this parameter.  ‘quit-restore-window’ deletes the specified
     window only if it still shows that buffer.

‘min-margins’
     The value of this parameter is a cons cell whose CAR and CDR, if
     non-‘nil’, specify the minimum values (in columns) for the left and
     right margin of this window.  When present, Emacs will use these
     values instead of the actual margin widths for determining whether
     a window can be split or shrunk horizontally.

     Emacs never auto-adjusts the margins of any window after splitting
     or resizing it.  It is the sole responsibility of any application
     setting this parameter to adjust the margins of this window as well
     as those of any new window that inherits this window’s margins due
     to a split.  Both ‘window-configuration-change-hook’ and
     ‘window-size-change-functions’ (*note Window Hooks::) should be
     employed for this purpose.

     This parameter was introduced in Emacs version 25.1 to support
     applications that use large margins to center buffer text within a
     window and should be used, with due care, exclusively by those
     applications.  It might be replaced by an improved solution in
     future versions of Emacs.

   There are additional parameters ‘window-atom’ and ‘window-side’;
these are reserved and should not be used by applications.


File: elisp.info,  Node: Window Hooks,  Prev: Window Parameters,  Up: Windows

27.27 Hooks for Window Scrolling and Changes
============================================

This section describes how a Lisp program can take action whenever a
window displays a different part of its buffer or a different buffer.
There are three actions that can change this: scrolling the window,
switching buffers in the window, and changing the size of the window.
The first two actions run ‘window-scroll-functions’; the last runs
‘window-size-change-functions’.

 -- Variable: window-scroll-functions
     This variable holds a list of functions that Emacs should call
     before redisplaying a window with scrolling.  Displaying a
     different buffer in the window also runs these functions.

     This variable is not a normal hook, because each function is called
     with two arguments: the window, and its new display-start position.

     These functions must take care when using ‘window-end’ (*note
     Window Start and End::); if you need an up-to-date value, you must
     use the UPDATE argument to ensure you get it.

     *Warning:* don’t use this feature to alter the way the window is
     scrolled.  It’s not designed for that, and such use probably won’t
     work.

 -- Variable: window-size-change-functions
     This variable holds a list of functions to be called if the size of
     any window changes for any reason.  The functions are called at the
     beginning of a redisplay cycle, and just once for each frame on
     which size changes have occurred.

     Each function receives the frame as its sole argument.  There is no
     direct way to find out which windows on that frame have changed
     size, or precisely how.  However, if a size-change function
     records, at each call, the existing windows and their sizes, it can
     also compare the present sizes and the previous sizes.

     Creating or deleting windows counts as a size change, and therefore
     causes these functions to be called.  Changing the frame size also
     counts, because it changes the sizes of the existing windows.

     You may use ‘save-selected-window’ in these functions (*note
     Selecting Windows::).  However, do not use ‘save-window-excursion’
     (*note Window Configurations::); exiting that macro counts as a
     size change, which would cause these functions to be called over
     and over.

 -- Variable: window-configuration-change-hook
     A normal hook that is run every time you change the window
     configuration of an existing frame.  This includes splitting or
     deleting windows, changing the sizes of windows, or displaying a
     different buffer in a window.

     The buffer-local part of this hook is run once for each window on
     the affected frame, with the relevant window selected and its
     buffer current.  The global part is run once for the modified
     frame, with that frame selected.

   In addition, you can use ‘jit-lock-register’ to register a Font Lock
fontification function, which will be called whenever parts of a buffer
are (re)fontified because a window was scrolled or its size changed.
*Note Other Font Lock Variables::.


File: elisp.info,  Node: Frames,  Next: Positions,  Prev: Windows,  Up: Top

28 Frames
*********

A “frame” is a screen object that contains one or more Emacs windows
(*note Windows::).  It is the kind of object called a “window” in the
terminology of graphical environments; but we can’t call it a “window”
here, because Emacs uses that word in a different way.  In Emacs Lisp, a
“frame object” is a Lisp object that represents a frame on the screen.
*Note Frame Type::.

   A frame initially contains a single main window and/or a minibuffer
window; you can subdivide the main window vertically or horizontally
into smaller windows.  *Note Splitting Windows::.

   A “terminal” is a display device capable of displaying one or more
Emacs frames.  In Emacs Lisp, a “terminal object” is a Lisp object that
represents a terminal.  *Note Terminal Type::.

   There are two classes of terminals: “text terminals” and “graphical
terminals”.  Text terminals are non-graphics-capable displays, including
‘xterm’ and other terminal emulators.  On a text terminal, each Emacs
frame occupies the terminal’s entire screen; although you can create
additional frames and switch between them, the terminal only shows one
frame at a time.  Graphical terminals, on the other hand, are managed by
graphical display systems such as the X Window System, which allow Emacs
to show multiple frames simultaneously on the same display.

   On GNU and Unix systems, you can create additional frames on any
available terminal, within a single Emacs session, regardless of whether
Emacs was started on a text or graphical terminal.  Emacs can display on
both graphical and text terminals simultaneously.  This comes in handy,
for instance, when you connect to the same session from several remote
locations.  *Note Multiple Terminals::.

 -- Function: framep object
     This predicate returns a non-‘nil’ value if OBJECT is a frame, and
     ‘nil’ otherwise.  For a frame, the value indicates which kind of
     display the frame uses:

     ‘t’
          The frame is displayed on a text terminal.
     ‘x’
          The frame is displayed on an X graphical terminal.
     ‘w32’
          The frame is displayed on a MS-Windows graphical terminal.
     ‘ns’
          The frame is displayed on a GNUstep or Macintosh Cocoa
          graphical terminal.
     ‘pc’
          The frame is displayed on an MS-DOS terminal.

 -- Function: frame-terminal &optional frame
     This function returns the terminal object that displays FRAME.  If
     FRAME is ‘nil’ or unspecified, it defaults to the selected frame.

 -- Function: terminal-live-p object
     This predicate returns a non-‘nil’ value if OBJECT is a terminal
     that is live (i.e., not deleted), and ‘nil’ otherwise.  For live
     terminals, the return value indicates what kind of frames are
     displayed on that terminal; the list of possible values is the same
     as for ‘framep’ above.

* Menu:

* Creating Frames::             Creating additional frames.
* Multiple Terminals::          Displaying on several different devices.
* Frame Geometry::              Geometric properties of frames.
* Frame Parameters::            Controlling frame size, position, font, etc.
* Terminal Parameters::         Parameters common for all frames on terminal.
* Frame Titles::                Automatic updating of frame titles.
* Deleting Frames::             Frames last until explicitly deleted.
* Finding All Frames::          How to examine all existing frames.
* Minibuffers and Frames::      How a frame finds the minibuffer to use.
* Input Focus::                 Specifying the selected frame.
* Visibility of Frames::        Frames may be visible or invisible, or icons.
* Raising and Lowering::        Raising a frame makes it hide other windows;
                                  lowering it makes the others hide it.
* Frame Configurations::        Saving the state of all frames.
* Mouse Tracking::              Getting events that say when the mouse moves.
* Mouse Position::              Asking where the mouse is, or moving it.
* Pop-Up Menus::                Displaying a menu for the user to select from.
* Dialog Boxes::                Displaying a box to ask yes or no.
* Pointer Shape::               Specifying the shape of the mouse pointer.
* Window System Selections::    Transferring text to and from other X clients.
* Drag and Drop::               Internals of Drag-and-Drop implementation.
* Color Names::                 Getting the definitions of color names.
* Text Terminal Colors::        Defining colors for text terminals.
* Resources::                   Getting resource values from the server.
* Display Feature Testing::     Determining the features of a terminal.


File: elisp.info,  Node: Creating Frames,  Next: Multiple Terminals,  Up: Frames

28.1 Creating Frames
====================

To create a new frame, call the function ‘make-frame’.

 -- Command: make-frame &optional alist
     This function creates and returns a new frame, displaying the
     current buffer.

     The ALIST argument is an alist that specifies frame parameters for
     the new frame.  *Note Frame Parameters::.  If you specify the
     ‘terminal’ parameter in ALIST, the new frame is created on that
     terminal.  Otherwise, if you specify the ‘window-system’ frame
     parameter in ALIST, that determines whether the frame should be
     displayed on a text terminal or a graphical terminal.  *Note Window
     Systems::.  If neither is specified, the new frame is created in
     the same terminal as the selected frame.

     Any parameters not mentioned in ALIST default to the values in the
     alist ‘default-frame-alist’ (*note Initial Parameters::);
     parameters not specified there default from the X resources or its
     equivalent on your operating system (*note X Resources: (emacs)X
     Resources.).  After the frame is created, Emacs applies any
     parameters listed in ‘frame-inherited-parameters’ (see below) and
     not present in the argument, taking the values from the frame that
     was selected when ‘make-frame’ was called.

     Note that on multi-monitor displays (*note Multiple Terminals::),
     the window manager might position the frame differently than
     specified by the positional parameters in ALIST (*note Position
     Parameters::).  For example, some window managers have a policy of
     displaying the frame on the monitor that contains the largest part
     of the window (a.k.a. the “dominating” monitor).

     This function itself does not make the new frame the selected
     frame.  *Note Input Focus::.  The previously selected frame remains
     selected.  On graphical terminals, however, the windowing system
     may select the new frame for its own reasons.

 -- Variable: before-make-frame-hook
     A normal hook run by ‘make-frame’ before it creates the frame.

 -- Variable: after-make-frame-functions
     An abnormal hook run by ‘make-frame’ after it creates the frame.
     Each function in ‘after-make-frame-functions’ receives one
     argument, the frame just created.

 -- Variable: frame-inherited-parameters
     This variable specifies the list of frame parameters that a newly
     created frame inherits from the currently selected frame.  For each
     parameter (a symbol) that is an element in the list and is not
     present in the argument to ‘make-frame’, the function sets the
     value of that parameter in the created frame to its value in the
     selected frame.


File: elisp.info,  Node: Multiple Terminals,  Next: Frame Geometry,  Prev: Creating Frames,  Up: Frames

28.2 Multiple Terminals
=======================

Emacs represents each terminal as a “terminal object” data type (*note
Terminal Type::).  On GNU and Unix systems, Emacs can use multiple
terminals simultaneously in each session.  On other systems, it can only
use a single terminal.  Each terminal object has the following
attributes:

   • The name of the device used by the terminal (e.g., ‘:0.0’ or
     ‘/dev/tty’).

   • The terminal and keyboard coding systems used on the terminal.
     *Note Terminal I/O Encoding::.

   • The kind of display associated with the terminal.  This is the
     symbol returned by the function ‘terminal-live-p’ (i.e., ‘x’, ‘t’,
     ‘w32’, ‘ns’, or ‘pc’).  *Note Frames::.

   • A list of terminal parameters.  *Note Terminal Parameters::.

   There is no primitive for creating terminal objects.  Emacs creates
them as needed, such as when you call ‘make-frame-on-display’ (described
below).

 -- Function: terminal-name &optional terminal
     This function returns the file name of the device used by TERMINAL.
     If TERMINAL is omitted or ‘nil’, it defaults to the selected
     frame’s terminal.  TERMINAL can also be a frame, meaning that
     frame’s terminal.

 -- Function: terminal-list
     This function returns a list of all live terminal objects.

 -- Function: get-device-terminal device
     This function returns a terminal whose device name is given by
     DEVICE.  If DEVICE is a string, it can be either the file name of a
     terminal device, or the name of an X display of the form
     ‘HOST:SERVER.SCREEN’.  If DEVICE is a frame, this function returns
     that frame’s terminal; ‘nil’ means the selected frame.  Finally, if
     DEVICE is a terminal object that represents a live terminal, that
     terminal is returned.  The function signals an error if its
     argument is none of the above.

 -- Function: delete-terminal &optional terminal force
     This function deletes all frames on TERMINAL and frees the
     resources used by it.  It runs the abnormal hook
     ‘delete-terminal-functions’, passing TERMINAL as the argument to
     each function.

     If TERMINAL is omitted or ‘nil’, it defaults to the selected
     frame’s terminal.  TERMINAL can also be a frame, meaning that
     frame’s terminal.

     Normally, this function signals an error if you attempt to delete
     the sole active terminal, but if FORCE is non-‘nil’, you are
     allowed to do so.  Emacs automatically calls this function when the
     last frame on a terminal is deleted (*note Deleting Frames::).

 -- Variable: delete-terminal-functions
     An abnormal hook run by ‘delete-terminal’.  Each function receives
     one argument, the TERMINAL argument passed to ‘delete-terminal’.
     Due to technical details, the functions may be called either just
     before the terminal is deleted, or just afterwards.

   A few Lisp variables are “terminal-local”; that is, they have a
separate binding for each terminal.  The binding in effect at any time
is the one for the terminal that the currently selected frame belongs
to.  These variables include ‘default-minibuffer-frame’,
‘defining-kbd-macro’, ‘last-kbd-macro’, and ‘system-key-alist’.  They
are always terminal-local, and can never be buffer-local (*note
Buffer-Local Variables::).

   On GNU and Unix systems, each X display is a separate graphical
terminal.  When Emacs is started from within the X window system, it
uses the X display specified by the ‘DISPLAY’ environment variable, or
by the ‘--display’ option (*note (emacs)Initial Options::).  Emacs can
connect to other X displays via the command ‘make-frame-on-display’.
Each X display has its own selected frame and its own minibuffer
windows; however, only one of those frames is _the_ selected frame at
any given moment (*note Input Focus::).  Emacs can even connect to other
text terminals, by interacting with the ‘emacsclient’ program.  *Note
(emacs)Emacs Server::.

   A single X server can handle more than one display.  Each X display
has a three-part name, ‘HOSTNAME:DISPLAYNUMBER.SCREENNUMBER’.  The first
part, HOSTNAME, specifies the name of the machine to which the display
is physically connected.  The second part, DISPLAYNUMBER, is a
zero-based number that identifies one or more monitors connected to that
machine that share a common keyboard and pointing device (mouse, tablet,
etc.).  The third part, SCREENNUMBER, identifies a zero-based screen
number (a separate monitor) that is part of a single monitor collection
on that X server.  When you use two or more screens belonging to one
server, Emacs knows by the similarity in their names that they share a
single keyboard.

   Systems that don’t use the X window system, such as MS-Windows, don’t
support the notion of X displays, and have only one display on each
host.  The display name on these systems doesn’t follow the above 3-part
format; for example, the display name on MS-Windows systems is a
constant string ‘w32’, and exists for compatibility, so that you could
pass it to functions that expect a display name.

 -- Command: make-frame-on-display display &optional parameters
     This function creates and returns a new frame on DISPLAY, taking
     the other frame parameters from the alist PARAMETERS.  DISPLAY
     should be the name of an X display (a string).

     Before creating the frame, this function ensures that Emacs is set
     up to display graphics.  For instance, if Emacs has not processed X
     resources (e.g., if it was started on a text terminal), it does so
     at this time.  In all other respects, this function behaves like
     ‘make-frame’ (*note Creating Frames::).

 -- Function: x-display-list
     This function returns a list that indicates which X displays Emacs
     has a connection to.  The elements of the list are strings, and
     each one is a display name.

 -- Function: x-open-connection display &optional xrm-string
          must-succeed
     This function opens a connection to the X display DISPLAY, without
     creating a frame on that display.  Normally, Emacs Lisp programs
     need not call this function, as ‘make-frame-on-display’ calls it
     automatically.  The only reason for calling it is to check whether
     communication can be established with a given X display.

     The optional argument XRM-STRING, if not ‘nil’, is a string of
     resource names and values, in the same format used in the
     ‘.Xresources’ file.  *Note X Resources: (emacs)X Resources.  These
     values apply to all Emacs frames created on this display,
     overriding the resource values recorded in the X server.  Here’s an
     example of what this string might look like:

          "*BorderWidth: 3\n*InternalBorder: 2\n"

     If MUST-SUCCEED is non-‘nil’, failure to open the connection
     terminates Emacs.  Otherwise, it is an ordinary Lisp error.

 -- Function: x-close-connection display
     This function closes the connection to display DISPLAY.  Before you
     can do this, you must first delete all the frames that were open on
     that display (*note Deleting Frames::).

   On some multi-monitor setups, a single X display outputs to more than
one physical monitor.  You can use the functions
‘display-monitor-attributes-list’ and ‘frame-monitor-attributes’ to
obtain information about such setups.

 -- Function: display-monitor-attributes-list &optional display
     This function returns a list of physical monitor attributes on
     DISPLAY, which can be a display name (a string), a terminal, or a
     frame; if omitted or ‘nil’, it defaults to the selected frame’s
     display.  Each element of the list is an association list,
     representing the attributes of a physical monitor.  The first
     element corresponds to the primary monitor.  The attribute keys and
     values are:

     ‘geometry’
          Position of the top-left corner of the monitor’s screen and
          its size, in pixels, as ‘(X Y WIDTH HEIGHT)’.  Note that, if
          the monitor is not the primary monitor, some of the
          coordinates might be negative.

     ‘workarea’
          Position of the top-left corner and size of the work area
          (usable space) in pixels as ‘(X Y WIDTH HEIGHT)’.  This may be
          different from ‘geometry’ in that space occupied by various
          window manager features (docks, taskbars, etc.) may be
          excluded from the work area.  Whether or not such features
          actually subtract from the work area depends on the platform
          and environment.  Again, if the monitor is not the primary
          monitor, some of the coordinates might be negative.

     ‘mm-size’
          Width and height in millimeters as ‘(WIDTH HEIGHT)’

     ‘frames’
          List of frames that this physical monitor dominates (see
          below).

     ‘name’
          Name of the physical monitor as STRING.

     ‘source’
          Source of the multi-monitor information as STRING; e.g.,
          ‘XRandr’ or ‘Xinerama’.

     X, Y, WIDTH, and HEIGHT are integers.  ‘name’ and ‘source’ may be
     absent.

     A frame is “dominated” by a physical monitor when either the
     largest area of the frame resides in that monitor, or (if the frame
     does not intersect any physical monitors) that monitor is the
     closest to the frame.  Every (non-tooltip) frame (whether visible
     or not) in a graphical display is dominated by exactly one physical
     monitor at a time, though the frame can span multiple (or no)
     physical monitors.

     Here’s an example of the data produced by this function on a
     2-monitor display:

            (display-monitor-attributes-list)
            ⇒
            (((geometry 0 0 1920 1080) ;; Left-hand, primary monitor
              (workarea 0 0 1920 1050) ;; A taskbar occupies some of the height
              (mm-size 677 381)
              (name . "DISPLAY1")
              (frames #<frame emacs@host *Messages* 0x11578c0>
                      #<frame emacs@host *scratch* 0x114b838>))
             ((geometry 1920 0 1680 1050) ;; Right-hand monitor
              (workarea 1920 0 1680 1050) ;; Whole screen can be used
              (mm-size 593 370)
              (name . "DISPLAY2")
              (frames)))

 -- Function: frame-monitor-attributes &optional frame
     This function returns the attributes of the physical monitor
     dominating (see above) FRAME, which defaults to the selected frame.


File: elisp.info,  Node: Frame Geometry,  Next: Frame Parameters,  Prev: Multiple Terminals,  Up: Frames

28.3 Frame Geometry
===================

The geometry of a frame depends on the toolkit that was used to build
this instance of Emacs and the terminal that displays the frame.  This
chapter describes these dependencies and some of the functions to deal
with them.  Note that the FRAME argument of all of these functions has
to specify a live frame (*note Deleting Frames::).  If omitted or ‘nil’,
it specifies the selected frame (*note Input Focus::).

* Menu:

* Frame Layout::            Basic layout of frames.
* Frame Font::              The default font of a frame and how to set it.
* Size and Position::       Changing the size and position of a frame.
* Implied Frame Resizing::  Implied resizing of frames and how to prevent it.


File: elisp.info,  Node: Frame Layout,  Next: Frame Font,  Up: Frame Geometry

28.3.1 Frame Layout
-------------------

The drawing below sketches the layout of a frame on a graphical
terminal:

             <------------ Outer Frame Width ----------->
             ___________________________________________
          ^(0)  ___________ External Border __________   |
          | |  |_____________ Title Bar ______________|  |
          | | (1)_____________ Menu Bar ______________|  | ^
          | | (2)_____________ Tool Bar ______________|  | ^
          | | (3) _________ Internal Border ________  |  | ^
          | |  | |   ^                              | |  | |
          | |  | |   |                              | |  | |
     Outer  |  | | Inner                            | |  | Native
     Frame  |  | | Frame                            | |  | Frame
     Height |  | | Height                           | |  | Height
          | |  | |   |                              | |  | |
          | |  | |<--+--- Inner Frame Width ------->| |  | |
          | |  | |   |                              | |  | |
          | |  | |___v______________________________| |  | |
          | |  |___________ Internal Border __________|  | v
          v |______________ External Border _____________|
                <-------- Native Frame Width -------->


   In practice not all of the areas shown in the drawing will or may be
present.  The meaning of these areas is:

‘Outer Frame’
     The “outer frame” is a rectangle comprising all areas shown in the
     drawing.  The edges of that rectangle are called the “outer edges”
     of the frame.  The “outer width” and “outer height” of the frame
     specify the size of that rectangle.

     The upper left corner of the outer frame (indicated by ‘(0)’ in the
     drawing above) is the “outer position” or the frame.  It is
     specified by and settable via the ‘left’ and ‘top’ frame parameters
     (*note Position Parameters::) as well as the functions
     ‘frame-position’ and ‘set-frame-position’ (*note Size and
     Position::).

‘External Border’
     The “external border” is part of the decorations supplied by the
     window manager.  It’s typically used for resizing the frame with
     the mouse.  The external border is normally not shown on “fullboth”
     and maximized frames (*note Size Parameters::) and doesn’t exist
     for text terminal frames.

     The external border should not be confused with the “outer border”
     specified by the ‘border-width’ frame parameter (*note Layout
     Parameters::).  Since the outer border is usually ignored on most
     platforms it is not covered here.

‘Title Bar’
     The “title bar” is also part of the window manager’s decorations
     and typically displays the title of the frame (*note Frame
     Titles::) as well as buttons for minimizing, maximizing and
     deleting the frame.  The title bar is usually not displayed on
     fullboth (*note Size Parameters::) or tooltip frames.  Title bars
     don’t exist for text terminal frames.

‘Menu Bar’
     The menu bar (*note Menu Bar::) can be either internal (drawn by
     Emacs itself) or external (drawn by a toolkit).  Most builds (GTK+,
     Lucid, Motif and Windows) rely on an external menu bar.  NS also
     uses an external menu bar which, however, is not part of the outer
     frame.  Non-toolkit builds can provide an internal menu bar.  On
     text terminal frames, the menu bar is part of the frame’s root
     window (*note Windows and Frames::).

‘Tool Bar’
     Like the menu bar, the tool bar (*note Tool Bar::) can be either
     internal (drawn by Emacs itself) or external (drawn by a toolkit).
     The GTK+ and NS builds have the tool bar drawn by the toolkit.  The
     remaining builds use internal tool bars.  With GTK+ the tool bar
     can be located on either side of the frame, immediately outside the
     internal border, see below.

‘Native Frame’
     The “native frame” is a rectangle located entirely within the outer
     frame.  It excludes the areas occupied by the external border, the
     title bar and any external menu or external tool bar.  The area
     enclosed by the native frame is sometimes also referred to as the
     “display area” of the frame.  The edges of the native frame are
     called the “native edges” of the frame.  The “native width” and
     “native height” of the frame specify the size of the rectangle.

     The top left corner of the native frame specifies the “native
     position” of the frame.  (1)–(3) in the drawing above indicate that
     position for the various builds:

          (1) non-toolkit and terminal frames

          (2) Lucid, Motif and Windows frames

          (3) GTK+ and NS frames

     Accordingly, the native height of a frame includes the height of
     the tool bar but not that of the menu bar (Lucid, Motif, Windows)
     or those of the menu bar and the tool bar (non-toolkit and text
     terminal frames).

     The native position of a frame is the reference position of
     functions that set or return the current position of the mouse
     (*note Mouse Position::) and for functions dealing with the
     position of windows like ‘window-edges’, ‘window-at’ or
     ‘coordinates-in-window-p’ (*note Coordinates and Windows::).

‘Internal Border’
     The internal border (*note Layout Parameters::) is a border drawn
     by Emacs around the inner frame (see below).

‘Inner Frame’
     The “inner frame” is the rectangle reserved for the frame’s
     windows.  It’s enclosed by the internal border which, however, is
     not part of the inner frame.  Its edges are called the “inner
     edges” of the frame.  The “inner width” and “inner height” specify
     the size of the rectangle.

     As a rule, the inner frame is subdivided into the frame’s root
     window (*note Windows and Frames::) and the frame’s minibuffer
     window (*note Minibuffer Windows::).  There are two notable
     exceptions to this rule: A “minibuffer-less frame” contains a root
     window only and does not contain a minibuffer window.  A
     “minibuffer-only frame” contains only a minibuffer window which
     also serves as that frame’s root window.  See *note Initial
     Parameters:: for how to create such frame configurations.

‘Text Area’
     The “text area” of a frame is a somewhat fictitious area located
     entirely within the native frame.  It can be obtained by removing
     from the native frame any internal borders, one vertical and one
     horizontal scroll bar, and one left and one right fringe as
     specified for this frame, see *note Layout Parameters::.

   The “absolute position” of a frame or its edges is usually given in
terms of pixels counted from an origin at position (0, 0) of the frame’s
display.  Note that with multiple monitors the origin does not
necessarily coincide with the top left corner of the entire usable
display area.  Hence the absolute outer position of a frame or the
absolute positions of the edges of the outer, native or inner frame can
be negative in such an environment even when that frame is completely
visible.

   For a frame on a graphical terminal the following function returns
the sizes of the areas described above:

 -- Function: frame-geometry &optional frame
     This function returns geometric attributes of FRAME.  The return
     value is an association list of the attributes listed below.  All
     coordinate, height and width values are integers counting pixels.

     ‘outer-position’
          A cons of the absolute X- and Y-coordinates of the outer
          position of FRAME, relative to the origin at position (0, 0)
          of FRAME’s display.

     ‘outer-size’
          A cons of the outer width and height of FRAME.

     ‘external-border-size’
          A cons of the horizontal and vertical width of FRAME’s
          external borders as supplied by the window manager.  If the
          window manager doesn’t supply these values, Emacs will try to
          guess them from the coordinates of the outer and inner frame.

     ‘title-bar-size’
          A cons of the width and height of the title bar of FRAME as
          supplied by the window manager or operating system.  If both
          of them are zero, the frame has no title bar.  If only the
          width is zero, Emacs was not able to retrieve the width
          information.

     ‘menu-bar-external’
          If non-‘nil’, this means the menu bar is external (not part of
          the native frame of FRAME).

     ‘menu-bar-size’
          A cons of the width and height of the menu bar of FRAME.

     ‘tool-bar-external’
          If non-‘nil’, this means the tool bar is external (not part of
          the native frame of FRAME).

     ‘tool-bar-position’
          This tells on which side the tool bar on FRAME is and can be
          one of ‘left’, ‘top’, ‘right’ or ‘bottom’.  The only toolkit
          that currently supports a value other than ‘top’ is GTK+.

     ‘tool-bar-size’
          A cons of the width and height of the tool bar of FRAME.

     ‘internal-border-width’
          The width of the internal border of FRAME.

   The following function can be used to retrieve the edges of the
outer, native and inner frame.

 -- Function: frame-edges &optional frame type
     This function returns the edges of the outer, native or inner frame
     of FRAME.  FRAME must be a live frame and defaults to the selected
     one.  The list returned has the form (LEFT TOP RIGHT BOTTOM) where
     all values are in pixels relative to the position (0, 0) of FRAME’s
     display.  For terminal frames LEFT and TOP are both zero.

     Optional argument TYPE specifies the type of the edges to return:
     TYPE ‘outer-edges’ means to return the outer edges of FRAME,
     ‘native-edges’ (or ‘nil’) means to return its native edges and
     ‘inner-edges’ means to return its inner edges.

     Notice that the pixels at the positions BOTTOM and RIGHT lie
     immediately outside the corresponding frame.  This means that if
     you have, for example, two side-by-side frames positioned such that
     the right outer edge of the frame on the left equals the left outer
     edge of the frame on the right, the pixels representing that edge
     are part of the frame on the right.


File: elisp.info,  Node: Frame Font,  Next: Size and Position,  Prev: Frame Layout,  Up: Frame Geometry

28.3.2 Frame Font
-----------------

Each frame has a “default font” which specifies the default character
size for that frame.  This size is meant when retrieving or changing the
size of a frame in terms of columns or lines (*note Size Parameters::).
It is also used when resizing (*note Window Sizes::) or splitting (*note
Splitting Windows::) windows.

   The terms “line height” and “canonical character height” are
sometimes used instead of “default character height”.  Similarly, the
terms “column width” and “canonical character width” are used instead of
“default character width”.

 -- Function: frame-char-height &optional frame
 -- Function: frame-char-width &optional frame
     These functions return the default height and width of a character
     in FRAME, measured in pixels.  Together, these values establish the
     size of the default font on FRAME.  The values depend on the choice
     of font for FRAME, see *note Font and Color Parameters::.

   The default font can be also set directly with the following
function:

 -- Command: set-frame-font font &optional keep-size frames
     This sets the default font to FONT.  When called interactively, it
     prompts for the name of a font, and uses that font on the selected
     frame.  When called from Lisp, FONT should be a font name (a
     string), a font object, font entity, or a font spec.

     If the optional argument KEEP-SIZE is ‘nil’, this keeps the number
     of frame lines and columns fixed.  (If non-‘nil’, the option
     ‘frame-inhibit-implied-resize’ described in the next section will
     override this.)  If KEEP-SIZE is non-‘nil’ (or with a prefix
     argument), it tries to keep the size of the display area of the
     current frame fixed by adjusting the number of lines and columns.

     If the optional argument FRAMES is ‘nil’, this applies the font to
     the selected frame only.  If FRAMES is non-‘nil’, it should be a
     list of frames to act upon, or ‘t’ meaning all existing and all
     future graphical frames.


File: elisp.info,  Node: Size and Position,  Next: Implied Frame Resizing,  Prev: Frame Font,  Up: Frame Geometry

28.3.3 Size and Position
------------------------

You can read or change the position of a frame using the frame
parameters ‘left’ and ‘top’ (*note Position Parameters::) and its size
using the ‘height’ and ‘width’ parameters (*note Size Parameters::).
Here are some special features for working with sizes and positions.
For all of these functions the argument FRAME must denote a live frame
and defaults to the selected frame.

 -- Function: frame-position &optional frame
     This function returns the outer position (*note Frame Layout::) of
     FRAME in pixels.  The value is a cons giving the coordinates of the
     top left corner of the outer frame of FRAME relative to an origin
     at the position (0, 0) of the frame’s display.  On a text terminal
     frame both values are zero.

 -- Function: set-frame-position frame x y
     This function sets the outer frame position of FRAME to X and Y.
     The latter arguments specify pixels and normally count from an
     origin at the position (0, 0) of FRAME’s display.

     A negative parameter value positions the right edge of the outer
     frame by -X pixels left from the right edge of the screen or the
     bottom edge by -Y pixels up from the bottom edge of the screen.

     This function has no effect on text terminal frames.

 -- Function: frame-pixel-height &optional frame
 -- Function: frame-pixel-width &optional frame
     These functions return the inner height and width (the height and
     width of the display area, see *note Frame Layout::) of FRAME in
     pixels.  For a text terminal, the results are in characters rather
     than pixels.

 -- Function: frame-text-height &optional frame
 -- Function: frame-text-width &optional frame
     These functions return the height and width of the text area of
     FRAME (*note Frame Layout::), measured in pixels.  For a text
     terminal, the results are in characters rather than pixels.

     The value returned by ‘frame-text-height’ differs from that
     returned by ‘frame-pixel-height’ by not including the heights of
     any internal tool bar or menu bar, the height of one horizontal
     scroll bar and the widths of the internal border.

     The value returned by ‘frame-text-width’ differs from that returned
     by ‘frame-pixel-width’ by not including the width of one vertical
     scroll bar, the widths of one left and one right fringe and the
     widths of the internal border.

 -- Function: frame-height &optional frame
 -- Function: frame-width &optional frame
     These functions return the height and width of the text area of
     FRAME, measured in units of the default font height and width of
     FRAME (*note Frame Font::).  These functions are plain shorthands
     for writing ‘(frame-parameter frame 'height)’ and ‘(frame-parameter
     frame 'width)’.

     If the text area of FRAME measured in pixels is not a multiple of
     its default font size, the values returned by these functions are
     rounded down to the number of characters of the default font that
     fully fit into the text area.

 -- User Option: frame-resize-pixelwise
     If this option is ‘nil’, a frame’s size is usually rounded to a
     multiple of the current values of that frame’s ‘frame-char-height’
     and ‘frame-char-width’ whenever the frame is resized.  If this is
     non-‘nil’, no rounding occurs, hence frame sizes can
     increase/decrease by one pixel.

     Setting this variable usually causes the next resize operation to
     pass the corresponding size hints to the window manager.  This
     means that this variable should be set only in a user’s initial
     file; applications should never bind it temporarily.

     The precise meaning of a value of ‘nil’ for this option depends on
     the toolkit used.  Dragging the external border with the mouse is
     done character-wise provided the window manager is willing to
     process the corresponding size hints.  Calling ‘set-frame-size’
     (see below) with arguments that do not specify the frame size as an
     integer multiple of its character size, however, may: be ignored,
     cause a rounding (GTK+), or be accepted (Lucid, Motif, MS-Windows).

     With some window managers you may have to set this to non-‘nil’ in
     order to make a frame appear truly maximized or full-screen.

 -- Function: set-frame-size frame width height &optional pixelwise
     This function sets the size of the text area of FRAME, measured in
     terms of the canonical height and width of a character on FRAME
     (*note Frame Font::).

     The optional argument PIXELWISE non-‘nil’ means to measure the new
     width and height in units of pixels instead.  Note that if
     ‘frame-resize-pixelwise’ is ‘nil’, some toolkits may refuse to
     fully honor the request if it does not increase/decrease the frame
     size to a multiple of its character size.

 -- Function: set-frame-height frame height &optional pretend pixelwise
     This function resizes the text area of FRAME to a height of HEIGHT
     lines.  The sizes of existing windows in FRAME are altered
     proportionally to fit.

     If PRETEND is non-‘nil’, then Emacs displays HEIGHT lines of output
     in FRAME, but does not change its value for the actual height of
     the frame.  This is only useful on text terminals.  Using a smaller
     height than the terminal actually implements may be useful to
     reproduce behavior observed on a smaller screen, or if the terminal
     malfunctions when using its whole screen.  Setting the frame height
     directly does not always work, because knowing the correct actual
     size may be necessary for correct cursor positioning on text
     terminals.

     The optional fourth argument PIXELWISE non-‘nil’ means that FRAME
     should be HEIGHT pixels high.  Note that if
     ‘frame-resize-pixelwise’ is ‘nil’, some toolkits may refuse to
     fully honor the request if it does not increase/decrease the frame
     height to a multiple of its character height.

 -- Function: set-frame-width frame width &optional pretend pixelwise
     This function sets the width of the text area of FRAME, measured in
     characters.  The argument PRETEND has the same meaning as in
     ‘set-frame-height’.

     The optional fourth argument PIXELWISE non-‘nil’ means that FRAME
     should be WIDTH pixels wide.  Note that if ‘frame-resize-pixelwise’
     is ‘nil’, some toolkits may refuse to fully honor the request if it
     does not increase/decrease the frame width to a multiple of its
     character width.

   None of these three functions will make a frame smaller than needed
to display all of its windows together with their scroll bars, fringes,
margins, dividers, mode and header lines.  This contrasts with requests
by the window manager triggered, for example, by dragging the external
border of a frame with the mouse.  Such requests are always honored by
clipping, if necessary, portions that cannot be displayed at the right,
bottom corner of the frame.


File: elisp.info,  Node: Implied Frame Resizing,  Prev: Size and Position,  Up: Frame Geometry

28.3.4 Implied Frame Resizing
-----------------------------

By default, Emacs tries to keep the number of lines and columns of a
frame’s text area unaltered when, for example, adding or removing the
menu bar, changing the default font or setting the width of the frame’s
scroll bars.  This means, however, that in such case Emacs must ask the
window manager to resize the outer frame in order to accommodate the
size change.  Note that wrapping a menu or tool bar usually does not
resize the frame’s outer size, hence this will alter the number of
displayed lines.

   Occasionally, such “implied frame resizing” may be unwanted, for
example, when the frame is maximized or made full-screen (where it’s
turned off by default).  In other cases you can disable implied resizing
with the following option:

 -- User Option: frame-inhibit-implied-resize
     If this option is ‘nil’, changing font, menu bar, tool bar,
     internal borders, fringes or scroll bars of a specific frame may
     implicitly resize the frame’s display area in order to preserve the
     number of columns or lines the frame displays.  If this option is
     non-‘nil’, no implied resizing is done.

     The value of this option can be also be a list of frame parameters.
     In that case, implied resizing is inhibited when changing a
     parameter that appears in this list.  The frame parameters
     currently handled by this option are: ‘font’, ‘font-backend’,
     ‘internal-border-width’, ‘menu-bar-lines’ and ‘tool-bar-lines’.

     Changing any of the ‘scroll-bar-width’, ‘scroll-bar-height’,
     ‘vertical-scroll-bars’, ‘horizontal-scroll-bars’, ‘left-fringe’ and
     ‘right-fringe’ frame parameters is handled as if the frame
     contained just one live window.  This means, for example, that
     removing vertical scroll bars on a frame containing several side by
     side windows will shrink the outer frame width by the width of one
     scroll bar provided this option is ‘nil’ and keep it unchanged if
     this option is either ‘t’ or a list containing
     ‘vertical-scroll-bars’.

     The default value is ‘'(tool-bar-lines)’ for Lucid, Motif and
     Windows (which means that adding/removing a tool bar there does not
     change the outer frame height), ‘nil’ on all other window systems
     including GTK+ (which means that changing any of the parameters
     listed above may change the size of the outer frame), and ‘t’
     otherwise (which means the outer frame size never changes
     implicitly when there’s no window system support).

     Note that when a frame is not large enough to accommodate a change
     of any of the parameters listed above, Emacs may try to enlarge the
     frame even if this option is non-‘nil’.


File: elisp.info,  Node: Frame Parameters,  Next: Terminal Parameters,  Prev: Frame Geometry,  Up: Frames

28.4 Frame Parameters
=====================

A frame has many parameters that control its appearance and behavior.
Just what parameters a frame has depends on what display mechanism it
uses.

   Frame parameters exist mostly for the sake of graphical displays.
Most frame parameters have no effect when applied to a frame on a text
terminal; only the ‘height’, ‘width’, ‘name’, ‘title’, ‘menu-bar-lines’,
‘buffer-list’ and ‘buffer-predicate’ parameters do something special.
If the terminal supports colors, the parameters ‘foreground-color’,
‘background-color’, ‘background-mode’ and ‘display-type’ are also
meaningful.  If the terminal supports frame transparency, the parameter
‘alpha’ is also meaningful.

* Menu:

* Parameter Access::       How to change a frame’s parameters.
* Initial Parameters::     Specifying frame parameters when you make a frame.
* Window Frame Parameters:: List of frame parameters for window systems.
* Geometry::               Parsing geometry specifications.


File: elisp.info,  Node: Parameter Access,  Next: Initial Parameters,  Up: Frame Parameters

28.4.1 Access to Frame Parameters
---------------------------------

These functions let you read and change the parameter values of a frame.

 -- Function: frame-parameter frame parameter
     This function returns the value of the parameter PARAMETER (a
     symbol) of FRAME.  If FRAME is ‘nil’, it returns the selected
     frame’s parameter.  If FRAME has no setting for PARAMETER, this
     function returns ‘nil’.

 -- Function: frame-parameters &optional frame
     The function ‘frame-parameters’ returns an alist listing all the
     parameters of FRAME and their values.  If FRAME is ‘nil’ or
     omitted, this returns the selected frame’s parameters

 -- Function: modify-frame-parameters frame alist
     This function alters the frame FRAME based on the elements of
     ALIST.  Each element of ALIST has the form ‘(PARM . VALUE)’, where
     PARM is a symbol naming a parameter.  If you don’t mention a
     parameter in ALIST, its value doesn’t change.  If FRAME is ‘nil’,
     it defaults to the selected frame.

     Some parameters are only meaningful for frames on certain kinds of
     display (*note Frames::).  If ALIST includes parameters that are
     not meaningful for the FRAME’s display, this function will change
     its value in the frame’s parameter list, but will otherwise ignore
     it.

     When ALIST specifies more than one parameter whose value can affect
     the new size of FRAME, the final size of the frame may differ
     according to the toolkit used.  For example, specifying that a
     frame should from now on have a menu and/or tool bar instead of
     none and simultaneously specifying the new height of the frame will
     inevitably lead to a recalculation of the frame’s height.
     Conceptually, in such case, this function will try to have the
     explicit height specification prevail.  It cannot be excluded,
     however, that the addition (or removal) of the menu or tool bar,
     when eventually performed by the toolkit, will defeat this
     intention.

     Sometimes, binding ‘frame-inhibit-implied-resize’ (*note Implied
     Frame Resizing::) to a non-‘nil’ value around calls to this
     function may fix the problem sketched here.  Sometimes, however,
     exactly such binding may be hit by the problem.

 -- Function: set-frame-parameter frame parm value
     This function sets the frame parameter PARM to the specified VALUE.
     If FRAME is ‘nil’, it defaults to the selected frame.

 -- Function: modify-all-frames-parameters alist
     This function alters the frame parameters of all existing frames
     according to ALIST, then modifies ‘default-frame-alist’ (and, if
     necessary, ‘initial-frame-alist’) to apply the same parameter
     values to frames that will be created henceforth.


File: elisp.info,  Node: Initial Parameters,  Next: Window Frame Parameters,  Prev: Parameter Access,  Up: Frame Parameters

28.4.2 Initial Frame Parameters
-------------------------------

You can specify the parameters for the initial startup frame by setting
‘initial-frame-alist’ in your init file (*note Init File::).

 -- User Option: initial-frame-alist
     This variable’s value is an alist of parameter values used when
     creating the initial frame.  You can set this variable to specify
     the appearance of the initial frame without altering subsequent
     frames.  Each element has the form:

          (PARAMETER . VALUE)

     Emacs creates the initial frame before it reads your init file.
     After reading that file, Emacs checks ‘initial-frame-alist’, and
     applies the parameter settings in the altered value to the already
     created initial frame.

     If these settings affect the frame geometry and appearance, you’ll
     see the frame appear with the wrong ones and then change to the
     specified ones.  If that bothers you, you can specify the same
     geometry and appearance with X resources; those do take effect
     before the frame is created.  *Note X Resources: (emacs)X
     Resources.

     X resource settings typically apply to all frames.  If you want to
     specify some X resources solely for the sake of the initial frame,
     and you don’t want them to apply to subsequent frames, here’s how
     to achieve this.  Specify parameters in ‘default-frame-alist’ to
     override the X resources for subsequent frames; then, to prevent
     these from affecting the initial frame, specify the same parameters
     in ‘initial-frame-alist’ with values that match the X resources.

   If these parameters include ‘(minibuffer . nil)’, that indicates that
the initial frame should have no minibuffer.  In this case, Emacs
creates a separate “minibuffer-only frame” as well.

 -- User Option: minibuffer-frame-alist
     This variable’s value is an alist of parameter values used when
     creating an initial minibuffer-only frame (i.e., the
     minibuffer-only frame that Emacs creates if ‘initial-frame-alist’
     specifies a frame with no minibuffer).

 -- User Option: default-frame-alist
     This is an alist specifying default values of frame parameters for
     all Emacs frames—the first frame, and subsequent frames.  When
     using the X Window System, you can get the same results by means of
     X resources in many cases.

     Setting this variable does not affect existing frames.
     Furthermore, functions that display a buffer in a separate frame
     may override the default parameters by supplying their own
     parameters.

   If you invoke Emacs with command-line options that specify frame
appearance, those options take effect by adding elements to either
‘initial-frame-alist’ or ‘default-frame-alist’.  Options which affect
just the initial frame, such as ‘--geometry’ and ‘--maximized’, add to
‘initial-frame-alist’; the others add to ‘default-frame-alist’.  *note
Command Line Arguments for Emacs Invocation: (emacs)Emacs Invocation.


File: elisp.info,  Node: Window Frame Parameters,  Next: Geometry,  Prev: Initial Parameters,  Up: Frame Parameters

28.4.3 Window Frame Parameters
------------------------------

Just what parameters a frame has depends on what display mechanism it
uses.  This section describes the parameters that have special meanings
on some or all kinds of terminals.  Of these, ‘name’, ‘title’, ‘height’,
‘width’, ‘buffer-list’ and ‘buffer-predicate’ provide meaningful
information in terminal frames, and ‘tty-color-mode’ is meaningful only
for frames on text terminals.

* Menu:

* Basic Parameters::            Parameters that are fundamental.
* Position Parameters::         The position of the frame on the screen.
* Size Parameters::             Frame’s size.
* Layout Parameters::           Size of parts of the frame, and
                                  enabling or disabling some parts.
* Buffer Parameters::           Which buffers have been or should be shown.
* Management Parameters::       Communicating with the window manager.
* Cursor Parameters::           Controlling the cursor appearance.
* Font and Color Parameters::   Fonts and colors for the frame text.


File: elisp.info,  Node: Basic Parameters,  Next: Position Parameters,  Up: Window Frame Parameters

28.4.3.1 Basic Parameters
.........................

These frame parameters give the most basic information about the frame.
‘title’ and ‘name’ are meaningful on all terminals.

‘display’
     The display on which to open this frame.  It should be a string of
     the form ‘HOST:DPY.SCREEN’, just like the ‘DISPLAY’ environment
     variable.  *Note Multiple Terminals::, for more details about
     display names.

‘display-type’
     This parameter describes the range of possible colors that can be
     used in this frame.  Its value is ‘color’, ‘grayscale’ or ‘mono’.

‘title’
     If a frame has a non-‘nil’ title, it appears in the window system’s
     title bar at the top of the frame, and also in the mode line of
     windows in that frame if ‘mode-line-frame-identification’ uses ‘%F’
     (*note %-Constructs::).  This is normally the case when Emacs is
     not using a window system, and can only display one frame at a
     time.  *Note Frame Titles::.

‘name’
     The name of the frame.  The frame name serves as a default for the
     frame title, if the ‘title’ parameter is unspecified or ‘nil’.  If
     you don’t specify a name, Emacs sets the frame name automatically
     (*note Frame Titles::).

     If you specify the frame name explicitly when you create the frame,
     the name is also used (instead of the name of the Emacs executable)
     when looking up X resources for the frame.

‘explicit-name’
     If the frame name was specified explicitly when the frame was
     created, this parameter will be that name.  If the frame wasn’t
     explicitly named, this parameter will be ‘nil’.


File: elisp.info,  Node: Position Parameters,  Next: Size Parameters,  Prev: Basic Parameters,  Up: Window Frame Parameters

28.4.3.2 Position Parameters
............................

Position parameters’ values are measured in pixels.  (Note that none of
these parameters exist on TTY frames.)

‘left’
     The position, in pixels, of the left (or right) edge of the frame
     with respect to the left (or right) edge of the screen.  The value
     may be:

     an integer
          A positive integer relates the left edge of the frame to the
          left edge of the screen.  A negative integer relates the right
          frame edge to the right screen edge.

     ‘(+ POS)’
          This specifies the position of the left frame edge relative to
          the left screen edge.  The integer POS may be positive or
          negative; a negative value specifies a position outside the
          screen or on a monitor other than the primary one (for
          multi-monitor displays).

     ‘(- POS)’
          This specifies the position of the right frame edge relative
          to the right screen edge.  The integer POS may be positive or
          negative; a negative value specifies a position outside the
          screen or on a monitor other than the primary one (for
          multi-monitor displays).

     Some window managers ignore program-specified positions.  If you
     want to be sure the position you specify is not ignored, specify a
     non-‘nil’ value for the ‘user-position’ parameter as well.

     If the window manager refuses to align a frame at the left or top
     screen edge, combining position notation and ‘user-position’ as in

          (modify-frame-parameters
            nil '((user-position . t) (left . (+ -4))))

     may help to override that.

‘top’
     The screen position of the top (or bottom) edge, in pixels, with
     respect to the top (or bottom) edge of the screen.  It works just
     like ‘left’, except vertically instead of horizontally.

‘icon-left’
     The screen position of the left edge of the frame’s icon, in
     pixels, counting from the left edge of the screen.  This takes
     effect when the frame is iconified, if the window manager supports
     this feature.  If you specify a value for this parameter, then you
     must also specify a value for ‘icon-top’ and vice versa.

‘icon-top’
     The screen position of the top edge of the frame’s icon, in pixels,
     counting from the top edge of the screen.  This takes effect when
     the frame is iconified, if the window manager supports this
     feature.

‘user-position’
     When you create a frame and specify its screen position with the
     ‘left’ and ‘top’ parameters, use this parameter to say whether the
     specified position was user-specified (explicitly requested in some
     way by a human user) or merely program-specified (chosen by a
     program).  A non-‘nil’ value says the position was user-specified.

     Window managers generally heed user-specified positions, and some
     heed program-specified positions too.  But many ignore
     program-specified positions, placing the window in a default
     fashion or letting the user place it with the mouse.  Some window
     managers, including ‘twm’, let the user specify whether to obey
     program-specified positions or ignore them.

     When you call ‘make-frame’, you should specify a non-‘nil’ value
     for this parameter if the values of the ‘left’ and ‘top’ parameters
     represent the user’s stated preference; otherwise, use ‘nil’.


File: elisp.info,  Node: Size Parameters,  Next: Layout Parameters,  Prev: Position Parameters,  Up: Window Frame Parameters

28.4.3.3 Size Parameters
........................

Frame parameters specify frame sizes in character units.  On graphical
displays, the ‘default’ face determines the actual pixel sizes of these
character units (*note Face Attributes::).

‘height’
     The height of the frame’s text area (*note Frame Geometry::), in
     characters.

‘width’
     The width of the frame’s text area (*note Frame Geometry::), in
     characters.

‘user-size’
     This does for the size parameters ‘height’ and ‘width’ what the
     ‘user-position’ parameter (*note user-position: Position
     Parameters.) does for the position parameters ‘top’ and ‘left’.

‘fullscreen’
     This parameter specifies whether to maximize the frame’s width,
     height or both.  Its value can be ‘fullwidth’, ‘fullheight’,
     ‘fullboth’, or ‘maximized’.  A “fullwidth” frame is as wide as
     possible, a “fullheight” frame is as tall as possible, and a
     “fullboth” frame is both as wide and as tall as possible.  A
     “maximized” frame is like a “fullboth” frame, except that it
     usually keeps its title bar and the buttons for resizing and
     closing the frame.  Also, maximized frames typically avoid hiding
     any task bar or panels displayed on the desktop.  A “fullboth”
     frame, on the other hand, usually omits the title bar and occupies
     the entire available screen space.

     Full-height and full-width frames are more similar to maximized
     frames in this regard.  However, these typically display an
     external border which might be absent with maximized frames.  Hence
     the heights of maximized and full-height frames and the widths of
     maximized and full-width frames often differ by a few pixels.

     With some window managers you may have to customize the variable
     ‘frame-resize-pixelwise’ (*note Size and Position::) in order to
     make a frame truly appear maximized or full-screen.  Moreover, some
     window managers might not support smooth transition between the
     various full-screen or maximization states.  Customizing the
     variable ‘x-frame-normalize-before-maximize’ can help to overcome
     that.

‘fullscreen-restore’
     This parameter specifies the desired fullscreen state of the frame
     after invoking the ‘toggle-frame-fullscreen’ command (*note
     (emacs)Frame Commands::) in the “fullboth” state.  Normally this
     parameter is installed automatically by that command when toggling
     the state to fullboth.  If, however, you start Emacs in the
     “fullboth” state, you have to specify the desired behavior in your
     initial file as, for example

          (setq default-frame-alist
              '((fullscreen . fullboth) (fullscreen-restore . fullheight)))

     This will give a new frame full height after typing in it <F11> for
     the first time.


File: elisp.info,  Node: Layout Parameters,  Next: Buffer Parameters,  Prev: Size Parameters,  Up: Window Frame Parameters

28.4.3.4 Layout Parameters
..........................

These frame parameters enable or disable various parts of the frame, or
control their sizes.

‘border-width’
     The width in pixels of the frame’s border.

‘internal-border-width’
     The distance in pixels between text (or fringe) and the frame’s
     border.

‘vertical-scroll-bars’
     Whether the frame has scroll bars for vertical scrolling, and which
     side of the frame they should be on.  The possible values are
     ‘left’, ‘right’, and ‘nil’ for no scroll bars.

‘horizontal-scroll-bars’
     Whether the frame has scroll bars for horizontal scrolling (‘t’ and
     ‘bottom’ mean yes, ‘nil’ means no).

‘scroll-bar-width’
     The width of vertical scroll bars, in pixels, or ‘nil’ meaning to
     use the default width.

‘scroll-bar-height’
     The height of horizontal scroll bars, in pixels, or ‘nil’ meaning
     to use the default height.

‘left-fringe’
‘right-fringe’
     The default width of the left and right fringes of windows in this
     frame (*note Fringes::).  If either of these is zero, that
     effectively removes the corresponding fringe.

     When you use ‘frame-parameter’ to query the value of either of
     these two frame parameters, the return value is always an integer.
     When using ‘set-frame-parameter’, passing a ‘nil’ value imposes an
     actual default value of 8 pixels.

‘right-divider-width’
     The width (thickness) reserved for the right divider (*note Window
     Dividers::) of any window on the frame, in pixels.  A value of zero
     means to not draw right dividers.

‘bottom-divider-width’
     The width (thickness) reserved for the bottom divider (*note Window
     Dividers::) of any window on the frame, in pixels.  A value of zero
     means to not draw bottom dividers.

‘menu-bar-lines’
     The number of lines to allocate at the top of the frame for a menu
     bar.  The default is 1 if Menu Bar mode is enabled, and 0
     otherwise.  *Note (emacs)Menu Bars::.

‘tool-bar-lines’
     The number of lines to use for the tool bar.  The default is 1 if
     Tool Bar mode is enabled, and 0 otherwise.  *Note (emacs)Tool
     Bars::.

‘tool-bar-position’
     The position of the tool bar.  Currently only for the GTK tool bar.
     Value can be one of ‘top’, ‘bottom’ ‘left’, ‘right’.  The default
     is ‘top’.

‘line-spacing’
     Additional space to leave below each text line, in pixels (a
     positive integer).  *Note Line Height::, for more information.


File: elisp.info,  Node: Buffer Parameters,  Next: Management Parameters,  Prev: Layout Parameters,  Up: Window Frame Parameters

28.4.3.5 Buffer Parameters
..........................

These frame parameters, meaningful on all kinds of terminals, deal with
which buffers have been, or should, be displayed in the frame.

‘minibuffer’
     Whether this frame has its own minibuffer.  The value ‘t’ means
     yes, ‘nil’ means no, ‘only’ means this frame is just a minibuffer.
     If the value is a minibuffer window (in some other frame), the
     frame uses that minibuffer.

     This frame parameter takes effect when the frame is created, and
     can not be changed afterwards.

‘buffer-predicate’
     The buffer-predicate function for this frame.  The function
     ‘other-buffer’ uses this predicate (from the selected frame) to
     decide which buffers it should consider, if the predicate is not
     ‘nil’.  It calls the predicate with one argument, a buffer, once
     for each buffer; if the predicate returns a non-‘nil’ value, it
     considers that buffer.

‘buffer-list’
     A list of buffers that have been selected in this frame, ordered
     most-recently-selected first.

‘unsplittable’
     If non-‘nil’, this frame’s window is never split automatically.


File: elisp.info,  Node: Management Parameters,  Next: Cursor Parameters,  Prev: Buffer Parameters,  Up: Window Frame Parameters

28.4.3.6 Window Management Parameters
.....................................

The following frame parameters control various aspects of the frame’s
interaction with the window manager.  They have no effect on text
terminals.

‘visibility’
     The state of visibility of the frame.  There are three
     possibilities: ‘nil’ for invisible, ‘t’ for visible, and ‘icon’ for
     iconified.  *Note Visibility of Frames::.

‘auto-raise’
     If non-‘nil’, Emacs automatically raises the frame when it is
     selected.  Some window managers do not allow this.

‘auto-lower’
     If non-‘nil’, Emacs automatically lowers the frame when it is
     deselected.  Some window managers do not allow this.

‘icon-type’
     The type of icon to use for this frame.  If the value is a string,
     that specifies a file containing a bitmap to use; ‘nil’ specifies
     no icon (in which case the window manager decides what to show);
     any other non-‘nil’ value specifies the default Emacs icon.

‘icon-name’
     The name to use in the icon for this frame, when and if the icon
     appears.  If this is ‘nil’, the frame’s title is used.

‘window-id’
     The ID number which the graphical display uses for this frame.
     Emacs assigns this parameter when the frame is created; changing
     the parameter has no effect on the actual ID number.

‘outer-window-id’
     The ID number of the outermost window-system window in which the
     frame exists.  As with ‘window-id’, changing this parameter has no
     actual effect.

‘wait-for-wm’
     If non-‘nil’, tell Xt to wait for the window manager to confirm
     geometry changes.  Some window managers, including versions of
     Fvwm2 and KDE, fail to confirm, so Xt hangs.  Set this to ‘nil’ to
     prevent hanging with those window managers.

‘sticky’
     If non-‘nil’, the frame is visible on all virtual desktops on
     systems with virtual desktops.


File: elisp.info,  Node: Cursor Parameters,  Next: Font and Color Parameters,  Prev: Management Parameters,  Up: Window Frame Parameters

28.4.3.7 Cursor Parameters
..........................

This frame parameter controls the way the cursor looks.

‘cursor-type’
     How to display the cursor.  Legitimate values are:

     ‘box’
          Display a filled box.  (This is the default.)
     ‘hollow’
          Display a hollow box.
     ‘nil’
          Don’t display a cursor.
     ‘bar’
          Display a vertical bar between characters.
     ‘(bar . WIDTH)’
          Display a vertical bar WIDTH pixels wide between characters.
     ‘hbar’
          Display a horizontal bar.
     ‘(hbar . HEIGHT)’
          Display a horizontal bar HEIGHT pixels high.

   The ‘cursor-type’ frame parameter may be overridden by the variables
‘cursor-type’ and ‘cursor-in-non-selected-windows’:

 -- Variable: cursor-type
     This buffer-local variable controls how the cursor looks in a
     selected window showing the buffer.  If its value is ‘t’, that
     means to use the cursor specified by the ‘cursor-type’ frame
     parameter.  Otherwise, the value should be one of the cursor types
     listed above, and it overrides the ‘cursor-type’ frame parameter.

 -- User Option: cursor-in-non-selected-windows
     This buffer-local variable controls how the cursor looks in a
     window that is not selected.  It supports the same values as the
     ‘cursor-type’ frame parameter; also, ‘nil’ means don’t display a
     cursor in nonselected windows, and ‘t’ (the default) means use a
     standard modification of the usual cursor type (solid box becomes
     hollow box, and bar becomes a narrower bar).

 -- User Option: x-stretch-cursor
     This variable controls the width of the block cursor displayed on
     extra-wide glyphs such as a tab or a stretch of white space.  By
     default, the block cursor is only as wide as the font’s default
     character, and will not cover all of the width of the glyph under
     it if that glyph is extra-wide.  A non-‘nil’ value of this variable
     means draw the block cursor as wide as the glyph under it.  The
     default value is ‘nil’.

     This variable has no effect on text-mode frames, since the
     text-mode cursor is drawn by the terminal out of Emacs’s control.

 -- User Option: blink-cursor-alist
     This variable specifies how to blink the cursor.  Each element has
     the form ‘(ON-STATE . OFF-STATE)’.  Whenever the cursor type equals
     ON-STATE (comparing using ‘equal’), the corresponding OFF-STATE
     specifies what the cursor looks like when it blinks off.  Both
     ON-STATE and OFF-STATE should be suitable values for the
     ‘cursor-type’ frame parameter.

     There are various defaults for how to blink each type of cursor, if
     the type is not mentioned as an ON-STATE here.  Changes in this
     variable do not take effect immediately, only when you specify the
     ‘cursor-type’ frame parameter.


File: elisp.info,  Node: Font and Color Parameters,  Prev: Cursor Parameters,  Up: Window Frame Parameters

28.4.3.8 Font and Color Parameters
..................................

These frame parameters control the use of fonts and colors.

‘font-backend’
     A list of symbols, specifying the “font backends” to use for
     drawing fonts in the frame, in order of priority.  On X, there are
     currently two available font backends: ‘x’ (the X core font driver)
     and ‘xft’ (the Xft font driver).  On MS-Windows, there are
     currently two available font backends: ‘gdi’ and ‘uniscribe’ (*note
     (emacs)Windows Fonts::).  On other systems, there is only one
     available font backend, so it does not make sense to modify this
     frame parameter.

‘background-mode’
     This parameter is either ‘dark’ or ‘light’, according to whether
     the background color is a light one or a dark one.

‘tty-color-mode’
     This parameter overrides the terminal’s color support as given by
     the system’s terminal capabilities database in that this
     parameter’s value specifies the color mode to use on a text
     terminal.  The value can be either a symbol or a number.  A number
     specifies the number of colors to use (and, indirectly, what
     commands to issue to produce each color).  For example,
     ‘(tty-color-mode . 8)’ specifies use of the ANSI escape sequences
     for 8 standard text colors.  A value of -1 turns off color support.

     If the parameter’s value is a symbol, it specifies a number through
     the value of ‘tty-color-mode-alist’, and the associated number is
     used instead.

‘screen-gamma’
     If this is a number, Emacs performs gamma correction which adjusts
     the brightness of all colors.  The value should be the screen gamma
     of your display.

     Usual PC monitors have a screen gamma of 2.2, so color values in
     Emacs, and in X windows generally, are calibrated to display
     properly on a monitor with that gamma value.  If you specify 2.2
     for ‘screen-gamma’, that means no correction is needed.  Other
     values request correction, designed to make the corrected colors
     appear on your screen the way they would have appeared without
     correction on an ordinary monitor with a gamma value of 2.2.

     If your monitor displays colors too light, you should specify a
     ‘screen-gamma’ value smaller than 2.2.  This requests correction
     that makes colors darker.  A screen gamma value of 1.5 may give
     good results for LCD color displays.

‘alpha’
     This parameter specifies the opacity of the frame, on graphical
     displays that support variable opacity.  It should be an integer
     between 0 and 100, where 0 means completely transparent and 100
     means completely opaque.  It can also have a ‘nil’ value, which
     tells Emacs not to set the frame opacity (leaving it to the window
     manager).

     To prevent the frame from disappearing completely from view, the
     variable ‘frame-alpha-lower-limit’ defines a lower opacity limit.
     If the value of the frame parameter is less than the value of this
     variable, Emacs uses the latter.  By default,
     ‘frame-alpha-lower-limit’ is 20.

     The ‘alpha’ frame parameter can also be a cons cell ‘(ACTIVE .
     INACTIVE)’, where ACTIVE is the opacity of the frame when it is
     selected, and INACTIVE is the opacity when it is not selected.

   The following frame parameters are semi-obsolete in that they are
automatically equivalent to particular face attributes of particular
faces (*note (emacs)Standard Faces::):

‘font’
     The name of the font for displaying text in the frame.  This is a
     string, either a valid font name for your system or the name of an
     Emacs fontset (*note Fontsets::).  It is equivalent to the ‘font’
     attribute of the ‘default’ face.

‘foreground-color’
     The color to use for the image of a character.  It is equivalent to
     the ‘:foreground’ attribute of the ‘default’ face.

‘background-color’
     The color to use for the background of characters.  It is
     equivalent to the ‘:background’ attribute of the ‘default’ face.

‘mouse-color’
     The color for the mouse pointer.  It is equivalent to the
     ‘:background’ attribute of the ‘mouse’ face.

‘cursor-color’
     The color for the cursor that shows point.  It is equivalent to the
     ‘:background’ attribute of the ‘cursor’ face.

‘border-color’
     The color for the border of the frame.  It is equivalent to the
     ‘:background’ attribute of the ‘border’ face.

‘scroll-bar-foreground’
     If non-‘nil’, the color for the foreground of scroll bars.  It is
     equivalent to the ‘:foreground’ attribute of the ‘scroll-bar’ face.

‘scroll-bar-background’
     If non-‘nil’, the color for the background of scroll bars.  It is
     equivalent to the ‘:background’ attribute of the ‘scroll-bar’ face.


File: elisp.info,  Node: Geometry,  Prev: Window Frame Parameters,  Up: Frame Parameters

28.4.4 Geometry
---------------

Here’s how to examine the data in an X-style window geometry
specification:

 -- Function: x-parse-geometry geom
     The function ‘x-parse-geometry’ converts a standard X window
     geometry string to an alist that you can use as part of the
     argument to ‘make-frame’.

     The alist describes which parameters were specified in GEOM, and
     gives the values specified for them.  Each element looks like
     ‘(PARAMETER . VALUE)’.  The possible PARAMETER values are ‘left’,
     ‘top’, ‘width’, and ‘height’.

     For the size parameters, the value must be an integer.  The
     position parameter names ‘left’ and ‘top’ are not totally accurate,
     because some values indicate the position of the right or bottom
     edges instead.  The VALUE possibilities for the position parameters
     are: an integer, a list ‘(+ POS)’, or a list ‘(- POS)’; as
     previously described (*note Position Parameters::).

     Here is an example:

          (x-parse-geometry "35x70+0-0")
               ⇒ ((height . 70) (width . 35)
                   (top - 0) (left . 0))


File: elisp.info,  Node: Terminal Parameters,  Next: Frame Titles,  Prev: Frame Parameters,  Up: Frames

28.5 Terminal Parameters
========================

Each terminal has a list of associated parameters.  These “terminal
parameters” are mostly a convenient way of storage for terminal-local
variables, but some terminal parameters have a special meaning.

   This section describes functions to read and change the parameter
values of a terminal.  They all accept as their argument either a
terminal or a frame; the latter means use that frame’s terminal.  An
argument of ‘nil’ means the selected frame’s terminal.

 -- Function: terminal-parameters &optional terminal
     This function returns an alist listing all the parameters of
     TERMINAL and their values.

 -- Function: terminal-parameter terminal parameter
     This function returns the value of the parameter PARAMETER (a
     symbol) of TERMINAL.  If TERMINAL has no setting for PARAMETER,
     this function returns ‘nil’.

 -- Function: set-terminal-parameter terminal parameter value
     This function sets the parameter PARAMETER of TERMINAL to the
     specified VALUE, and returns the previous value of that parameter.

   Here’s a list of a few terminal parameters that have a special
meaning:

‘background-mode’
     The classification of the terminal’s background color, either
     ‘light’ or ‘dark’.
‘normal-erase-is-backspace’
     Value is either 1 or 0, depending on whether
     ‘normal-erase-is-backspace-mode’ is turned on or off on this
     terminal.  *Note (emacs)DEL Does Not Delete::.
‘terminal-initted’
     After the terminal is initialized, this is set to the
     terminal-specific initialization function.
‘tty-mode-set-strings’
     When present, a list of strings containing escape sequences that
     Emacs will output while configuring a tty for rendering.  Emacs
     emits these strings only when configuring a terminal: if you want
     to enable a mode on a terminal that is already active (for example,
     while in ‘tty-setup-hook’), explicitly output the necessary escape
     sequence using ‘send-string-to-terminal’ in addition to adding the
     sequence to ‘tty-mode-set-strings’.
‘tty-mode-reset-strings’
     When present, a list of strings that undo the effects of the
     strings in ‘tty-mode-set-strings’.  Emacs emits these strings when
     exiting, deleting a terminal, or suspending itself.


File: elisp.info,  Node: Frame Titles,  Next: Deleting Frames,  Prev: Terminal Parameters,  Up: Frames

28.6 Frame Titles
=================

Every frame has a ‘name’ parameter; this serves as the default for the
frame title which window systems typically display at the top of the
frame.  You can specify a name explicitly by setting the ‘name’ frame
property.

   Normally you don’t specify the name explicitly, and Emacs computes
the frame name automatically based on a template stored in the variable
‘frame-title-format’.  Emacs recomputes the name each time the frame is
redisplayed.

 -- Variable: frame-title-format
     This variable specifies how to compute a name for a frame when you
     have not explicitly specified one.  The variable’s value is
     actually a mode line construct, just like ‘mode-line-format’,
     except that the ‘%c’ and ‘%l’ constructs are ignored.  *Note Mode
     Line Data::.

 -- Variable: icon-title-format
     This variable specifies how to compute the name for an iconified
     frame, when you have not explicitly specified the frame title.
     This title appears in the icon itself.

 -- Variable: multiple-frames
     This variable is set automatically by Emacs.  Its value is ‘t’ when
     there are two or more frames (not counting minibuffer-only frames
     or invisible frames).  The default value of ‘frame-title-format’
     uses ‘multiple-frames’ so as to put the buffer name in the frame
     title only when there is more than one frame.

     The value of this variable is not guaranteed to be accurate except
     while processing ‘frame-title-format’ or ‘icon-title-format’.


File: elisp.info,  Node: Deleting Frames,  Next: Finding All Frames,  Prev: Frame Titles,  Up: Frames

28.7 Deleting Frames
====================

A “live frame” is one that has not been deleted.  When a frame is
deleted, it is removed from its terminal display, although it may
continue to exist as a Lisp object until there are no more references to
it.

 -- Command: delete-frame &optional frame force
     This function deletes the frame FRAME.  Unless FRAME is a tooltip,
     it first runs the hook ‘delete-frame-functions’ (each function gets
     one argument, FRAME).  By default, FRAME is the selected frame.

     A frame cannot be deleted as long as its minibuffer serves as
     surrogate minibuffer for another frame (*note Minibuffers and
     Frames::).  Normally, you cannot delete a frame if all other frames
     are invisible, but if FORCE is non-‘nil’, then you are allowed to
     do so.

 -- Function: frame-live-p frame
     The function ‘frame-live-p’ returns non-‘nil’ if the frame FRAME
     has not been deleted.  The possible non-‘nil’ return values are
     like those of ‘framep’.  *Note Frames::.

   Some window managers provide a command to delete a window.  These
work by sending a special message to the program that operates the
window.  When Emacs gets one of these commands, it generates a
‘delete-frame’ event, whose normal definition is a command that calls
the function ‘delete-frame’.  *Note Misc Events::.


File: elisp.info,  Node: Finding All Frames,  Next: Minibuffers and Frames,  Prev: Deleting Frames,  Up: Frames

28.8 Finding All Frames
=======================

 -- Function: frame-list
     This function returns a list of all the live frames, i.e., those
     that have not been deleted.  It is analogous to ‘buffer-list’ for
     buffers, and includes frames on all terminals.  The list that you
     get is newly created, so modifying the list doesn’t have any effect
     on the internals of Emacs.

 -- Function: visible-frame-list
     This function returns a list of just the currently visible frames.
     *Note Visibility of Frames::.  Frames on text terminals always
     count as visible, even though only the selected one is actually
     displayed.

 -- Function: next-frame &optional frame minibuf
     This function lets you cycle conveniently through all the frames on
     the current display from an arbitrary starting point.  It returns
     the next frame after FRAME in the cycle.  If FRAME is omitted or
     ‘nil’, it defaults to the selected frame (*note Input Focus::).

     The second argument, MINIBUF, says which frames to consider:

     ‘nil’
          Exclude minibuffer-only frames.
     ‘visible’
          Consider all visible frames.
     0
          Consider all visible or iconified frames.
     a window
          Consider only the frames using that particular window as their
          minibuffer.
     anything else
          Consider all frames.

 -- Function: previous-frame &optional frame minibuf
     Like ‘next-frame’, but cycles through all frames in the opposite
     direction.

   See also ‘next-window’ and ‘previous-window’, in *note Cyclic Window
Ordering::.


File: elisp.info,  Node: Minibuffers and Frames,  Next: Input Focus,  Prev: Finding All Frames,  Up: Frames

28.9 Minibuffers and Frames
===========================

Normally, each frame has its own minibuffer window at the bottom, which
is used whenever that frame is selected.  If the frame has a minibuffer,
you can get it with ‘minibuffer-window’ (*note Minibuffer Windows::).

   However, you can also create a frame without a minibuffer.  Such a
frame must use the minibuffer window of some other frame.  That other
frame will serve as “surrogate minibuffer frame” for this frame and
cannot be deleted via ‘delete-frame’ (*note Deleting Frames::) as long
as this frame is live.

   When you create the frame, you can explicitly specify the minibuffer
window to use (in some other frame).  If you don’t, then the minibuffer
is found in the frame which is the value of the variable
‘default-minibuffer-frame’.  Its value should be a frame that does have
a minibuffer.

   If you use a minibuffer-only frame, you might want that frame to
raise when you enter the minibuffer.  If so, set the variable
‘minibuffer-auto-raise’ to ‘t’.  *Note Raising and Lowering::.

 -- Variable: default-minibuffer-frame
     This variable specifies the frame to use for the minibuffer window,
     by default.  It does not affect existing frames.  It is always
     local to the current terminal and cannot be buffer-local.  *Note
     Multiple Terminals::.


File: elisp.info,  Node: Input Focus,  Next: Visibility of Frames,  Prev: Minibuffers and Frames,  Up: Frames

28.10 Input Focus
=================

At any time, one frame in Emacs is the “selected frame”.  The selected
window always resides on the selected frame.

   When Emacs displays its frames on several terminals (*note Multiple
Terminals::), each terminal has its own selected frame.  But only one of
these is _the_ selected frame: it’s the frame that belongs to the
terminal from which the most recent input came.  That is, when Emacs
runs a command that came from a certain terminal, the selected frame is
the one of that terminal.  Since Emacs runs only a single command at any
given time, it needs to consider only one selected frame at a time; this
frame is what we call “the selected frame” in this manual.  The display
on which the selected frame is shown is the “selected frame’s display”.

 -- Function: selected-frame
     This function returns the selected frame.

   Some window systems and window managers direct keyboard input to the
window object that the mouse is in; others require explicit clicks or
commands to “shift the focus” to various window objects.  Either way,
Emacs automatically keeps track of which frame has the focus.  To
explicitly switch to a different frame from a Lisp function, call
‘select-frame-set-input-focus’.

   Lisp programs can also switch frames temporarily by calling the
function ‘select-frame’.  This does not alter the window system’s
concept of focus; rather, it escapes from the window manager’s control
until that control is somehow reasserted.

   When using a text terminal, only one frame can be displayed at a time
on the terminal, so after a call to ‘select-frame’, the next redisplay
actually displays the newly selected frame.  This frame remains selected
until a subsequent call to ‘select-frame’.  Each frame on a text
terminal has a number which appears in the mode line before the buffer
name (*note Mode Line Variables::).

 -- Function: select-frame-set-input-focus frame &optional norecord
     This function selects FRAME, raises it (should it happen to be
     obscured by other frames) and tries to give it the X server’s
     focus.  On a text terminal, the next redisplay displays the new
     frame on the entire terminal screen.  The optional argument
     NORECORD has the same meaning as for ‘select-frame’ (see below).
     The return value of this function is not significant.

 -- Command: select-frame frame &optional norecord
     This function selects frame FRAME, temporarily disregarding the
     focus of the X server if any.  The selection of FRAME lasts until
     the next time the user does something to select a different frame,
     or until the next time this function is called.  (If you are using
     a window system, the previously selected frame may be restored as
     the selected frame after return to the command loop, because it
     still may have the window system’s input focus.)

     The specified FRAME becomes the selected frame, and its terminal
     becomes the selected terminal.  This function then calls
     ‘select-window’ as a subroutine, passing the window selected within
     FRAME as its first argument and NORECORD as its second argument
     (hence, if NORECORD is non-‘nil’, this avoids changing the order of
     recently selected windows nor the buffer list).  *Note Selecting
     Windows::.

     This function returns FRAME, or ‘nil’ if FRAME has been deleted.

     In general, you should never use ‘select-frame’ in a way that could
     switch to a different terminal without switching back when you’re
     done.

   Emacs cooperates with the window system by arranging to select frames
as the server and window manager request.  It does so by generating a
special kind of input event, called a “focus” event, when appropriate.
The command loop handles a focus event by calling ‘handle-switch-frame’.
*Note Focus Events::.

 -- Command: handle-switch-frame frame
     This function handles a focus event by selecting frame FRAME.

     Focus events normally do their job by invoking this command.  Don’t
     call it for any other reason.

 -- Function: redirect-frame-focus frame &optional focus-frame
     This function redirects focus from FRAME to FOCUS-FRAME.  This
     means that FOCUS-FRAME will receive subsequent keystrokes and
     events intended for FRAME.  After such an event, the value of
     ‘last-event-frame’ will be FOCUS-FRAME.  Also, switch-frame events
     specifying FRAME will instead select FOCUS-FRAME.

     If FOCUS-FRAME is omitted or ‘nil’, that cancels any existing
     redirection for FRAME, which therefore once again receives its own
     events.

     One use of focus redirection is for frames that don’t have
     minibuffers.  These frames use minibuffers on other frames.
     Activating a minibuffer on another frame redirects focus to that
     frame.  This puts the focus on the minibuffer’s frame, where it
     belongs, even though the mouse remains in the frame that activated
     the minibuffer.

     Selecting a frame can also change focus redirections.  Selecting
     frame ‘bar’, when ‘foo’ had been selected, changes any redirections
     pointing to ‘foo’ so that they point to ‘bar’ instead.  This allows
     focus redirection to work properly when the user switches from one
     frame to another using ‘select-window’.

     This means that a frame whose focus is redirected to itself is
     treated differently from a frame whose focus is not redirected.
     ‘select-frame’ affects the former but not the latter.

     The redirection lasts until ‘redirect-frame-focus’ is called to
     change it.

 -- Variable: focus-in-hook
     This is a normal hook run when an Emacs frame gains input focus.

 -- Variable: focus-out-hook
     This is a normal hook run when an Emacs frame loses input focus.

 -- User Option: focus-follows-mouse
     This option is how you inform Emacs whether the window manager
     transfers focus when the user moves the mouse.  Non-‘nil’ says that
     it does.  When this is so, the command ‘other-frame’ moves the
     mouse to a position consistent with the new selected frame.


File: elisp.info,  Node: Visibility of Frames,  Next: Raising and Lowering,  Prev: Input Focus,  Up: Frames

28.11 Visibility of Frames
==========================

A frame on a graphical display may be “visible”, “invisible”, or
“iconified”.  If it is visible, its contents are displayed in the usual
manner.  If it is iconified, its contents are not displayed, but there
is a little icon somewhere to bring the frame back into view (some
window managers refer to this state as “minimized” rather than
“iconified”, but from Emacs’ point of view they are the same thing).  If
a frame is invisible, it is not displayed at all.

   Visibility is meaningless on text terminals, since only the selected
one is actually displayed in any case.

 -- Function: frame-visible-p frame
     This function returns the visibility status of frame FRAME.  The
     value is ‘t’ if FRAME is visible, ‘nil’ if it is invisible, and
     ‘icon’ if it is iconified.

     On a text terminal, all frames are considered visible for the
     purposes of this function, even though only one frame is displayed.
     *Note Raising and Lowering::.

 -- Command: iconify-frame &optional frame
     This function iconifies frame FRAME.  If you omit FRAME, it
     iconifies the selected frame.

 -- Command: make-frame-visible &optional frame
     This function makes frame FRAME visible.  If you omit FRAME, it
     makes the selected frame visible.  This does not raise the frame,
     but you can do that with ‘raise-frame’ if you wish (*note Raising
     and Lowering::).

 -- Command: make-frame-invisible &optional frame force
     This function makes frame FRAME invisible.  If you omit FRAME, it
     makes the selected frame invisible.

     Unless FORCE is non-‘nil’, this function refuses to make FRAME
     invisible if all other frames are invisible..

   The visibility status of a frame is also available as a frame
parameter.  You can read or change it as such.  *Note Management
Parameters::.  The user can also iconify and deiconify frames with the
window manager.  This happens below the level at which Emacs can exert
any control, but Emacs does provide events that you can use to keep
track of such changes.  *Note Misc Events::.


File: elisp.info,  Node: Raising and Lowering,  Next: Frame Configurations,  Prev: Visibility of Frames,  Up: Frames

28.12 Raising and Lowering Frames
=================================

Most window systems use a desktop metaphor.  Part of this metaphor is
the idea that system-level windows (e.g., Emacs frames) are stacked in a
notional third dimension perpendicular to the screen surface.  Where two
overlap, the one higher up covers the one underneath.  You can “raise”
or “lower” a frame using the functions ‘raise-frame’ and ‘lower-frame’.

 -- Command: raise-frame &optional frame
     This function raises frame FRAME (default, the selected frame).  If
     FRAME is invisible or iconified, this makes it visible.

 -- Command: lower-frame &optional frame
     This function lowers frame FRAME (default, the selected frame).

 -- User Option: minibuffer-auto-raise
     If this is non-‘nil’, activation of the minibuffer raises the frame
     that the minibuffer window is in.

   On window systems, you can also enable auto-raising (on frame
selection) or auto-lowering (on frame deselection) using frame
parameters.  *Note Management Parameters::.

   The concept of raising and lowering frames also applies to text
terminal frames.  On each text terminal, only the top frame is displayed
at any one time.

 -- Function: tty-top-frame &optional terminal
     This function returns the top frame on TERMINAL.  TERMINAL should
     be a terminal object, a frame (meaning that frame’s terminal), or
     ‘nil’ (meaning the selected frame’s terminal).  If it does not
     refer to a text terminal, the return value is ‘nil’.


File: elisp.info,  Node: Frame Configurations,  Next: Mouse Tracking,  Prev: Raising and Lowering,  Up: Frames

28.13 Frame Configurations
==========================

A “frame configuration” records the current arrangement of frames, all
their properties, and the window configuration of each one.  (*Note
Window Configurations::.)

 -- Function: current-frame-configuration
     This function returns a frame configuration list that describes the
     current arrangement of frames and their contents.

 -- Function: set-frame-configuration configuration &optional nodelete
     This function restores the state of frames described in
     CONFIGURATION.  However, this function does not restore deleted
     frames.

     Ordinarily, this function deletes all existing frames not listed in
     CONFIGURATION.  But if NODELETE is non-‘nil’, the unwanted frames
     are iconified instead.


File: elisp.info,  Node: Mouse Tracking,  Next: Mouse Position,  Prev: Frame Configurations,  Up: Frames

28.14 Mouse Tracking
====================

Sometimes it is useful to “track” the mouse, which means to display
something to indicate where the mouse is and move the indicator as the
mouse moves.  For efficient mouse tracking, you need a way to wait until
the mouse actually moves.

   The convenient way to track the mouse is to ask for events to
represent mouse motion.  Then you can wait for motion by waiting for an
event.  In addition, you can easily handle any other sorts of events
that may occur.  That is useful, because normally you don’t want to
track the mouse forever—only until some other event, such as the release
of a button.

 -- Special Form: track-mouse body...
     This special form executes BODY, with generation of mouse motion
     events enabled.  Typically, BODY would use ‘read-event’ to read the
     motion events and modify the display accordingly.  *Note Motion
     Events::, for the format of mouse motion events.

     The value of ‘track-mouse’ is that of the last form in BODY.  You
     should design BODY to return when it sees the up-event that
     indicates the release of the button, or whatever kind of event
     means it is time to stop tracking.

     The ‘track-mouse’ form causes Emacs to generate mouse motion events
     by binding the variable ‘track-mouse’ to a non-‘nil’ value.  If
     that variable has the special value ‘dragging’, it additionally
     instructs the display engine to refrain from changing the shape of
     the mouse pointer.  This is desirable in Lisp programs that require
     mouse dragging across large portions of Emacs display, which might
     otherwise cause the mouse pointer to change its shape according to
     the display portion it hovers on (*note Pointer Shape::).
     Therefore, Lisp programs that need the mouse pointer to retain its
     original shape during dragging should bind ‘track-mouse’ to the
     value ‘dragging’ at the beginning of their BODY.

   The usual purpose of tracking mouse motion is to indicate on the
screen the consequences of pushing or releasing a button at the current
position.

   In many cases, you can avoid the need to track the mouse by using the
‘mouse-face’ text property (*note Special Properties::).  That works at
a much lower level and runs more smoothly than Lisp-level mouse
tracking.


File: elisp.info,  Node: Mouse Position,  Next: Pop-Up Menus,  Prev: Mouse Tracking,  Up: Frames

28.15 Mouse Position
====================

The functions ‘mouse-position’ and ‘set-mouse-position’ give access to
the current position of the mouse.

 -- Function: mouse-position
     This function returns a description of the position of the mouse.
     The value looks like ‘(FRAME X . Y)’, where X and Y are integers
     giving the (possibly rounded) position in multiples of the default
     character size of FRAME (*note Frame Font::) relative to the native
     position of FRAME (*note Frame Geometry::).

 -- Variable: mouse-position-function
     If non-‘nil’, the value of this variable is a function for
     ‘mouse-position’ to call.  ‘mouse-position’ calls this function
     just before returning, with its normal return value as the sole
     argument, and it returns whatever this function returns to it.

     This abnormal hook exists for the benefit of packages like
     ‘xt-mouse.el’ that need to do mouse handling at the Lisp level.

 -- Function: set-mouse-position frame x y
     This function “warps the mouse” to position X, Y in frame FRAME.
     The arguments X and Y are integers, giving the position in
     multiples of the default character size of FRAME (*note Frame
     Font::) relative to the native position of FRAME (*note Frame
     Geometry::).

     The resulting mouse position is constrained to the native frame of
     FRAME.  If FRAME is not visible, this function does nothing.  The
     return value is not significant.

 -- Function: mouse-pixel-position
     This function is like ‘mouse-position’ except that it returns
     coordinates in units of pixels rather than units of characters.

 -- Function: set-mouse-pixel-position frame x y
     This function warps the mouse like ‘set-mouse-position’ except that
     X and Y are in units of pixels rather than units of characters.

     The resulting mouse position is not constrained to the native frame
     of FRAME.  If FRAME is not visible, this function does nothing.
     The return value is not significant.

   On a graphical terminal the following two functions allow the
absolute position of the mouse cursor to be retrieved and set.

 -- Function: mouse-absolute-pixel-position
     This function returns a cons cell (X .  Y) of the coordinates of
     the mouse cursor position in pixels, relative to a position (0, 0)
     of the selected frame’s display.

 -- Function: set-mouse-absolute-pixel-position x y
     This function moves the mouse cursor to the position (X, Y).  The
     coordinates X and Y are interpreted in pixels relative to a
     position (0, 0) of the selected frame’s display.

   The following function can tell whether the mouse cursor is currently
visible on a frame:

 -- Function: frame-pointer-visible-p &optional frame
     This predicate function returns non-‘nil’ if the mouse pointer
     displayed on FRAME is visible; otherwise it returns ‘nil’.  FRAME
     omitted or ‘nil’ means the selected frame.  This is useful when
     ‘make-pointer-invisible’ is set to ‘t’: it allows you to know if
     the pointer has been hidden.  *Note (emacs)Mouse Avoidance::.


File: elisp.info,  Node: Pop-Up Menus,  Next: Dialog Boxes,  Prev: Mouse Position,  Up: Frames

28.16 Pop-Up Menus
==================

A Lisp program can pop up a menu so that the user can choose an
alternative with the mouse.  On a text terminal, if the mouse is not
available, the user can choose an alternative using the keyboard motion
keys—‘C-n’, ‘C-p’, or up- and down-arrow keys.

 -- Function: x-popup-menu position menu
     This function displays a pop-up menu and returns an indication of
     what selection the user makes.

     The argument POSITION specifies where on the screen to put the top
     left corner of the menu.  It can be either a mouse button event
     (which says to put the menu where the user actuated the button) or
     a list of this form:

          ((XOFFSET YOFFSET) WINDOW)

     where XOFFSET and YOFFSET are coordinates, measured in pixels,
     counting from the top left corner of WINDOW.  WINDOW may be a
     window or a frame.

     If POSITION is ‘t’, it means to use the current mouse position (or
     the top-left corner of the frame if the mouse is not available on a
     text terminal).  If POSITION is ‘nil’, it means to precompute the
     key binding equivalents for the keymaps specified in MENU, without
     actually displaying or popping up the menu.

     The argument MENU says what to display in the menu.  It can be a
     keymap or a list of keymaps (*note Menu Keymaps::).  In this case,
     the return value is the list of events corresponding to the user’s
     choice.  This list has more than one element if the choice occurred
     in a submenu.  (Note that ‘x-popup-menu’ does not actually execute
     the command bound to that sequence of events.)  On text terminals
     and toolkits that support menu titles, the title is taken from the
     prompt string of MENU if MENU is a keymap, or from the prompt
     string of the first keymap in MENU if it is a list of keymaps
     (*note Defining Menus::).

     Alternatively, MENU can have the following form:

          (TITLE PANE1 PANE2...)

     where each pane is a list of form

          (TITLE ITEM1 ITEM2...)

     Each ITEM should be a cons cell, ‘(LINE . VALUE)’, where LINE is a
     string and VALUE is the value to return if that LINE is chosen.
     Unlike in a menu keymap, a ‘nil’ VALUE does not make the menu item
     non-selectable.  Alternatively, each ITEM can be a string rather
     than a cons cell; this makes a non-selectable menu item.

     If the user gets rid of the menu without making a valid choice, for
     instance by clicking the mouse away from a valid choice or by
     typing ‘C-g’, then this normally results in a quit and
     ‘x-popup-menu’ does not return.  But if POSITION is a mouse button
     event (indicating that the user invoked the menu with the mouse)
     then no quit occurs and ‘x-popup-menu’ returns ‘nil’.

   *Usage note:* Don’t use ‘x-popup-menu’ to display a menu if you could
do the job with a prefix key defined with a menu keymap.  If you use a
menu keymap to implement a menu, ‘C-h c’ and ‘C-h a’ can see the
individual items in that menu and provide help for them.  If instead you
implement the menu by defining a command that calls ‘x-popup-menu’, the
help facilities cannot know what happens inside that command, so they
cannot give any help for the menu’s items.

   The menu bar mechanism, which lets you switch between submenus by
moving the mouse, cannot look within the definition of a command to see
that it calls ‘x-popup-menu’.  Therefore, if you try to implement a
submenu using ‘x-popup-menu’, it cannot work with the menu bar in an
integrated fashion.  This is why all menu bar submenus are implemented
with menu keymaps within the parent menu, and never with ‘x-popup-menu’.
*Note Menu Bar::.

   If you want a menu bar submenu to have contents that vary, you should
still use a menu keymap to implement it.  To make the contents vary, add
a hook function to ‘menu-bar-update-hook’ to update the contents of the
menu keymap as necessary.


File: elisp.info,  Node: Dialog Boxes,  Next: Pointer Shape,  Prev: Pop-Up Menus,  Up: Frames

28.17 Dialog Boxes
==================

A dialog box is a variant of a pop-up menu—it looks a little different,
it always appears in the center of a frame, and it has just one level
and one or more buttons.  The main use of dialog boxes is for asking
questions that the user can answer with “yes”, “no”, and a few other
alternatives.  With a single button, they can also force the user to
acknowledge important information.  The functions ‘y-or-n-p’ and
‘yes-or-no-p’ use dialog boxes instead of the keyboard, when called from
commands invoked by mouse clicks.

 -- Function: x-popup-dialog position contents &optional header
     This function displays a pop-up dialog box and returns an
     indication of what selection the user makes.  The argument CONTENTS
     specifies the alternatives to offer; it has this format:

          (TITLE (STRING . VALUE)...)

     which looks like the list that specifies a single pane for
     ‘x-popup-menu’.

     The return value is VALUE from the chosen alternative.

     As for ‘x-popup-menu’, an element of the list may be just a string
     instead of a cons cell ‘(STRING . VALUE)’.  That makes a box that
     cannot be selected.

     If ‘nil’ appears in the list, it separates the left-hand items from
     the right-hand items; items that precede the ‘nil’ appear on the
     left, and items that follow the ‘nil’ appear on the right.  If you
     don’t include a ‘nil’ in the list, then approximately half the
     items appear on each side.

     Dialog boxes always appear in the center of a frame; the argument
     POSITION specifies which frame.  The possible values are as in
     ‘x-popup-menu’, but the precise coordinates or the individual
     window don’t matter; only the frame matters.

     If HEADER is non-‘nil’, the frame title for the box is
     ‘Information’, otherwise it is ‘Question’.  The former is used for
     ‘message-box’ (*note message-box::).  (On text terminals, the box
     title is not displayed.)

     In some configurations, Emacs cannot display a real dialog box; so
     instead it displays the same items in a pop-up menu in the center
     of the frame.

     If the user gets rid of the dialog box without making a valid
     choice, for instance using the window manager, then this produces a
     quit and ‘x-popup-dialog’ does not return.


File: elisp.info,  Node: Pointer Shape,  Next: Window System Selections,  Prev: Dialog Boxes,  Up: Frames

28.18 Pointer Shape
===================

You can specify the mouse pointer style for particular text or images
using the ‘pointer’ text property, and for images with the ‘:pointer’
and ‘:map’ image properties.  The values you can use in these properties
are ‘text’ (or ‘nil’), ‘arrow’, ‘hand’, ‘vdrag’, ‘hdrag’, ‘modeline’,
and ‘hourglass’.  ‘text’ stands for the usual mouse pointer style used
over text.

   Over void parts of the window (parts that do not correspond to any of
the buffer contents), the mouse pointer usually uses the ‘arrow’ style,
but you can specify a different style (one of those above) by setting
‘void-text-area-pointer’.

 -- User Option: void-text-area-pointer
     This variable specifies the mouse pointer style for void text
     areas.  These include the areas after the end of a line or below
     the last line in the buffer.  The default is to use the ‘arrow’
     (non-text) pointer style.

   When using X, you can specify what the ‘text’ pointer style really
looks like by setting the variable ‘x-pointer-shape’.

 -- Variable: x-pointer-shape
     This variable specifies the pointer shape to use ordinarily in the
     Emacs frame, for the ‘text’ pointer style.

 -- Variable: x-sensitive-text-pointer-shape
     This variable specifies the pointer shape to use when the mouse is
     over mouse-sensitive text.

   These variables affect newly created frames.  They do not normally
affect existing frames; however, if you set the mouse color of a frame,
that also installs the current value of those two variables.  *Note Font
and Color Parameters::.

   The values you can use, to specify either of these pointer shapes,
are defined in the file ‘lisp/term/x-win.el’.  Use ‘M-x apropos <RET>
x-pointer <RET>’ to see a list of them.


File: elisp.info,  Node: Window System Selections,  Next: Drag and Drop,  Prev: Pointer Shape,  Up: Frames

28.19 Window System Selections
==============================

In window systems, such as X, data can be transferred between different
applications by means of “selections”.  X defines an arbitrary number of
“selection types”, each of which can store its own data; however, only
three are commonly used: the “clipboard”, “primary selection”, and
“secondary selection”.  Other window systems support only the clipboard.
*Note Cut and Paste: (emacs)Cut and Paste, for Emacs commands that make
use of these selections.  This section documents the low-level functions
for reading and setting window-system selections.

 -- Command: gui-set-selection type data
     This function sets a window-system selection.  It takes two
     arguments: a selection type TYPE, and the value to assign to it,
     DATA.

     TYPE should be a symbol; it is usually one of ‘PRIMARY’,
     ‘SECONDARY’ or ‘CLIPBOARD’.  These are symbols with upper-case
     names, in accord with X Window System conventions.  If TYPE is
     ‘nil’, that stands for ‘PRIMARY’.

     If DATA is ‘nil’, it means to clear out the selection.  Otherwise,
     DATA may be a string, a symbol, an integer (or a cons of two
     integers or list of two integers), an overlay, or a cons of two
     markers pointing to the same buffer.  An overlay or a pair of
     markers stands for text in the overlay or between the markers.  The
     argument DATA may also be a vector of valid non-vector selection
     values.

     This function returns DATA.

 -- Function: gui-get-selection &optional type data-type
     This function accesses selections set up by Emacs or by other
     programs.  It takes two optional arguments, TYPE and DATA-TYPE.
     The default for TYPE, the selection type, is ‘PRIMARY’.

     The DATA-TYPE argument specifies the form of data conversion to
     use, to convert the raw data obtained from another program into
     Lisp data.  Meaningful values include ‘TEXT’, ‘STRING’,
     ‘UTF8_STRING’, ‘TARGETS’, ‘LENGTH’, ‘DELETE’, ‘FILE_NAME’,
     ‘CHARACTER_POSITION’, ‘NAME’, ‘LINE_NUMBER’, ‘COLUMN_NUMBER’,
     ‘OWNER_OS’, ‘HOST_NAME’, ‘USER’, ‘CLASS’, ‘ATOM’, and ‘INTEGER’.
     (These are symbols with upper-case names in accord with X
     conventions.)  The default for DATA-TYPE is ‘STRING’.  Window
     systems other than X usually support only a small subset of these
     types, in addition to ‘STRING’.

 -- User Option: selection-coding-system
     This variable specifies the coding system to use when reading and
     writing selections or the clipboard.  *Note Coding Systems::.  The
     default is ‘compound-text-with-extensions’, which converts to the
     text representation that X11 normally uses.

   When Emacs runs on MS-Windows, it does not implement X selections in
general, but it does support the clipboard.  ‘gui-get-selection’ and
‘gui-set-selection’ on MS-Windows support the text data type only; if
the clipboard holds other types of data, Emacs treats the clipboard as
empty.  The supported data type is ‘STRING’.

   For backward compatibility, there are obsolete aliases
‘x-get-selection’ and ‘x-set-selection’, which were the names of
‘gui-get-selection’ and ‘gui-set-selection’ before Emacs 25.1.


File: elisp.info,  Node: Drag and Drop,  Next: Color Names,  Prev: Window System Selections,  Up: Frames

28.20 Drag and Drop
===================

When a user drags something from another application over Emacs, that
other application expects Emacs to tell it if Emacs can handle the data
that is dragged.  The variable ‘x-dnd-test-function’ is used by Emacs to
determine what to reply.  The default value is
‘x-dnd-default-test-function’ which accepts drops if the type of the
data to be dropped is present in ‘x-dnd-known-types’.  You can customize
‘x-dnd-test-function’ and/or ‘x-dnd-known-types’ if you want Emacs to
accept or reject drops based on some other criteria.

   If you want to change the way Emacs handles drop of different types
or add a new type, customize ‘x-dnd-types-alist’.  This requires
detailed knowledge of what types other applications use for drag and
drop.

   When an URL is dropped on Emacs it may be a file, but it may also be
another URL type (ftp, http, etc.).  Emacs first checks
‘dnd-protocol-alist’ to determine what to do with the URL.  If there is
no match there and if ‘browse-url-browser-function’ is an alist, Emacs
looks for a match there.  If no match is found the text for the URL is
inserted.  If you want to alter Emacs behavior, you can customize these
variables.


File: elisp.info,  Node: Color Names,  Next: Text Terminal Colors,  Prev: Drag and Drop,  Up: Frames

28.21 Color Names
=================

A color name is text (usually in a string) that specifies a color.
Symbolic names such as ‘black’, ‘white’, ‘red’, etc., are allowed; use
‘M-x list-colors-display’ to see a list of defined names.  You can also
specify colors numerically in forms such as ‘#RGB’ and ‘RGB:R/G/B’,
where R specifies the red level, G specifies the green level, and B
specifies the blue level.  You can use either one, two, three, or four
hex digits for R; then you must use the same number of hex digits for
all G and B as well, making either 3, 6, 9 or 12 hex digits in all.
(See the documentation of the X Window System for more details about
numerical RGB specification of colors.)

   These functions provide a way to determine which color names are
valid, and what they look like.  In some cases, the value depends on the
“selected frame”, as described below; see *note Input Focus::, for the
meaning of the term “selected frame”.

   To read user input of color names with completion, use ‘read-color’
(*note read-color: High-Level Completion.).

 -- Function: color-defined-p color &optional frame
     This function reports whether a color name is meaningful.  It
     returns ‘t’ if so; otherwise, ‘nil’.  The argument FRAME says which
     frame’s display to ask about; if FRAME is omitted or ‘nil’, the
     selected frame is used.

     Note that this does not tell you whether the display you are using
     really supports that color.  When using X, you can ask for any
     defined color on any kind of display, and you will get some
     result—typically, the closest it can do.  To determine whether a
     frame can really display a certain color, use ‘color-supported-p’
     (see below).

     This function used to be called ‘x-color-defined-p’, and that name
     is still supported as an alias.

 -- Function: defined-colors &optional frame
     This function returns a list of the color names that are defined
     and supported on frame FRAME (default, the selected frame).  If
     FRAME does not support colors, the value is ‘nil’.

     This function used to be called ‘x-defined-colors’, and that name
     is still supported as an alias.

 -- Function: color-supported-p color &optional frame background-p
     This returns ‘t’ if FRAME can really display the color COLOR (or at
     least something close to it).  If FRAME is omitted or ‘nil’, the
     question applies to the selected frame.

     Some terminals support a different set of colors for foreground and
     background.  If BACKGROUND-P is non-‘nil’, that means you are
     asking whether COLOR can be used as a background; otherwise you are
     asking whether it can be used as a foreground.

     The argument COLOR must be a valid color name.

 -- Function: color-gray-p color &optional frame
     This returns ‘t’ if COLOR is a shade of gray, as defined on FRAME’s
     display.  If FRAME is omitted or ‘nil’, the question applies to the
     selected frame.  If COLOR is not a valid color name, this function
     returns ‘nil’.

 -- Function: color-values color &optional frame
     This function returns a value that describes what COLOR should
     ideally look like on FRAME.  If COLOR is defined, the value is a
     list of three integers, which give the amount of red, the amount of
     green, and the amount of blue.  Each integer ranges in principle
     from 0 to 65535, but some displays may not use the full range.
     This three-element list is called the “rgb values” of the color.

     If COLOR is not defined, the value is ‘nil’.

          (color-values "black")
               ⇒ (0 0 0)
          (color-values "white")
               ⇒ (65280 65280 65280)
          (color-values "red")
               ⇒ (65280 0 0)
          (color-values "pink")
               ⇒ (65280 49152 51968)
          (color-values "hungry")
               ⇒ nil

     The color values are returned for FRAME’s display.  If FRAME is
     omitted or ‘nil’, the information is returned for the selected
     frame’s display.  If the frame cannot display colors, the value is
     ‘nil’.

     This function used to be called ‘x-color-values’, and that name is
     still supported as an alias.


File: elisp.info,  Node: Text Terminal Colors,  Next: Resources,  Prev: Color Names,  Up: Frames

28.22 Text Terminal Colors
==========================

Text terminals usually support only a small number of colors, and the
computer uses small integers to select colors on the terminal.  This
means that the computer cannot reliably tell what the selected color
looks like; instead, you have to inform your application which small
integers correspond to which colors.  However, Emacs does know the
standard set of colors and will try to use them automatically.

   The functions described in this section control how terminal colors
are used by Emacs.

   Several of these functions use or return “rgb values”, described in
*note Color Names::.

   These functions accept a display (either a frame or the name of a
terminal) as an optional argument.  We hope in the future to make Emacs
support different colors on different text terminals; then this argument
will specify which terminal to operate on (the default being the
selected frame’s terminal; *note Input Focus::).  At present, though,
the FRAME argument has no effect.

 -- Function: tty-color-define name number &optional rgb frame
     This function associates the color name NAME with color number
     NUMBER on the terminal.

     The optional argument RGB, if specified, is an rgb value, a list of
     three numbers that specify what the color actually looks like.  If
     you do not specify RGB, then this color cannot be used by
     ‘tty-color-approximate’ to approximate other colors, because Emacs
     will not know what it looks like.

 -- Function: tty-color-clear &optional frame
     This function clears the table of defined colors for a text
     terminal.

 -- Function: tty-color-alist &optional frame
     This function returns an alist recording the known colors supported
     by a text terminal.

     Each element has the form ‘(NAME NUMBER . RGB)’ or ‘(NAME NUMBER)’.
     Here, NAME is the color name, NUMBER is the number used to specify
     it to the terminal.  If present, RGB is a list of three color
     values (for red, green, and blue) that says what the color actually
     looks like.

 -- Function: tty-color-approximate rgb &optional frame
     This function finds the closest color, among the known colors
     supported for DISPLAY, to that described by the rgb value RGB (a
     list of color values).  The return value is an element of
     ‘tty-color-alist’.

 -- Function: tty-color-translate color &optional frame
     This function finds the closest color to COLOR among the known
     colors supported for DISPLAY and returns its index (an integer).
     If the name COLOR is not defined, the value is ‘nil’.


File: elisp.info,  Node: Resources,  Next: Display Feature Testing,  Prev: Text Terminal Colors,  Up: Frames

28.23 X Resources
=================

This section describes some of the functions and variables for querying
and using X resources, or their equivalent on your operating system.
*Note X Resources: (emacs)X Resources, for more information about X
resources.

 -- Function: x-get-resource attribute class &optional component
          subclass
     The function ‘x-get-resource’ retrieves a resource value from the X
     Window defaults database.

     Resources are indexed by a combination of a “key” and a “class”.
     This function searches using a key of the form ‘INSTANCE.ATTRIBUTE’
     (where INSTANCE is the name under which Emacs was invoked), and
     using ‘Emacs.CLASS’ as the class.

     The optional arguments COMPONENT and SUBCLASS add to the key and
     the class, respectively.  You must specify both of them or neither.
     If you specify them, the key is ‘INSTANCE.COMPONENT.ATTRIBUTE’, and
     the class is ‘Emacs.CLASS.SUBCLASS’.

 -- Variable: x-resource-class
     This variable specifies the application name that ‘x-get-resource’
     should look up.  The default value is ‘"Emacs"’.  You can examine X
     resources for other application names by binding this variable to
     some other string, around a call to ‘x-get-resource’.

 -- Variable: x-resource-name
     This variable specifies the instance name that ‘x-get-resource’
     should look up.  The default value is the name Emacs was invoked
     with, or the value specified with the ‘-name’ or ‘-rn’ switches.

   To illustrate some of the above, suppose that you have the line:

     xterm.vt100.background: yellow

in your X resources file (whose name is usually ‘~/.Xdefaults’ or
‘~/.Xresources’).  Then:

     (let ((x-resource-class "XTerm") (x-resource-name "xterm"))
       (x-get-resource "vt100.background" "VT100.Background"))
          ⇒ "yellow"
     (let ((x-resource-class "XTerm") (x-resource-name "xterm"))
       (x-get-resource "background" "VT100" "vt100" "Background"))
          ⇒ "yellow"

 -- Variable: inhibit-x-resources
     If this variable is non-‘nil’, Emacs does not look up X resources,
     and X resources do not have any effect when creating new frames.


File: elisp.info,  Node: Display Feature Testing,  Prev: Resources,  Up: Frames

28.24 Display Feature Testing
=============================

The functions in this section describe the basic capabilities of a
particular display.  Lisp programs can use them to adapt their behavior
to what the display can do.  For example, a program that ordinarily uses
a popup menu could use the minibuffer if popup menus are not supported.

   The optional argument DISPLAY in these functions specifies which
display to ask the question about.  It can be a display name, a frame
(which designates the display that frame is on), or ‘nil’ (which refers
to the selected frame’s display, *note Input Focus::).

   *Note Color Names::, *note Text Terminal Colors::, for other
functions to obtain information about displays.

 -- Function: display-popup-menus-p &optional display
     This function returns ‘t’ if popup menus are supported on DISPLAY,
     ‘nil’ if not.  Support for popup menus requires that the mouse be
     available, since the menu is popped up by clicking the mouse on
     some portion of the Emacs display.

 -- Function: display-graphic-p &optional display
     This function returns ‘t’ if DISPLAY is a graphic display capable
     of displaying several frames and several different fonts at once.
     This is true for displays that use a window system such as X, and
     false for text terminals.

 -- Function: display-mouse-p &optional display
     This function returns ‘t’ if DISPLAY has a mouse available, ‘nil’
     if not.

 -- Function: display-color-p &optional display
     This function returns ‘t’ if the screen is a color screen.  It used
     to be called ‘x-display-color-p’, and that name is still supported
     as an alias.

 -- Function: display-grayscale-p &optional display
     This function returns ‘t’ if the screen can display shades of gray.
     (All color displays can do this.)

 -- Function: display-supports-face-attributes-p attributes &optional
          display
     This function returns non-‘nil’ if all the face attributes in
     ATTRIBUTES are supported (*note Face Attributes::).

     The definition of “supported” is somewhat heuristic, but basically
     means that a face containing all the attributes in ATTRIBUTES, when
     merged with the default face for display, can be represented in a
     way that’s

       1. different in appearance than the default face, and

       2. close in spirit to what the attributes specify, if not exact.

     Point (2) implies that a ‘:weight black’ attribute will be
     satisfied by any display that can display bold, as will
     ‘:foreground "yellow"’ as long as some yellowish color can be
     displayed, but ‘:slant italic’ will _not_ be satisfied by the tty
     display code’s automatic substitution of a dim face for italic.

 -- Function: display-selections-p &optional display
     This function returns ‘t’ if DISPLAY supports selections.  Windowed
     displays normally support selections, but they may also be
     supported in some other cases.

 -- Function: display-images-p &optional display
     This function returns ‘t’ if DISPLAY can display images.  Windowed
     displays ought in principle to handle images, but some systems lack
     the support for that.  On a display that does not support images,
     Emacs cannot display a tool bar.

 -- Function: display-screens &optional display
     This function returns the number of screens associated with the
     display.

 -- Function: display-pixel-height &optional display
     This function returns the height of the screen in pixels.  On a
     character terminal, it gives the height in characters.

     For graphical terminals, note that on multi-monitor setups this
     refers to the pixel height for all physical monitors associated
     with DISPLAY.  *Note Multiple Terminals::.

 -- Function: display-pixel-width &optional display
     This function returns the width of the screen in pixels.  On a
     character terminal, it gives the width in characters.

     For graphical terminals, note that on multi-monitor setups this
     refers to the pixel width for all physical monitors associated with
     DISPLAY.  *Note Multiple Terminals::.

 -- Function: display-mm-height &optional display
     This function returns the height of the screen in millimeters, or
     ‘nil’ if Emacs cannot get that information.

     For graphical terminals, note that on multi-monitor setups this
     refers to the height for all physical monitors associated with
     DISPLAY.  *Note Multiple Terminals::.

 -- Function: display-mm-width &optional display
     This function returns the width of the screen in millimeters, or
     ‘nil’ if Emacs cannot get that information.

     For graphical terminals, note that on multi-monitor setups this
     refers to the width for all physical monitors associated with
     DISPLAY.  *Note Multiple Terminals::.

 -- User Option: display-mm-dimensions-alist
     This variable allows the user to specify the dimensions of
     graphical displays returned by ‘display-mm-height’ and
     ‘display-mm-width’ in case the system provides incorrect values.

 -- Function: display-backing-store &optional display
     This function returns the backing store capability of the display.
     Backing store means recording the pixels of windows (and parts of
     windows) that are not exposed, so that when exposed they can be
     displayed very quickly.

     Values can be the symbols ‘always’, ‘when-mapped’, or ‘not-useful’.
     The function can also return ‘nil’ when the question is
     inapplicable to a certain kind of display.

 -- Function: display-save-under &optional display
     This function returns non-‘nil’ if the display supports the
     SaveUnder feature.  That feature is used by pop-up windows to save
     the pixels they obscure, so that they can pop down quickly.

 -- Function: display-planes &optional display
     This function returns the number of planes the display supports.
     This is typically the number of bits per pixel.  For a tty display,
     it is log to base two of the number of colors supported.

 -- Function: display-visual-class &optional display
     This function returns the visual class for the screen.  The value
     is one of the symbols ‘static-gray’ (a limited, unchangeable number
     of grays), ‘gray-scale’ (a full range of grays), ‘static-color’ (a
     limited, unchangeable number of colors), ‘pseudo-color’ (a limited
     number of colors), ‘true-color’ (a full range of colors), and
     ‘direct-color’ (a full range of colors).

 -- Function: display-color-cells &optional display
     This function returns the number of color cells the screen
     supports.

   These functions obtain additional information about the window system
in use where Emacs shows the specified DISPLAY.  (Their names begin with
‘x-’ for historical reasons.)

 -- Function: x-server-version &optional display
     This function returns the list of version numbers of the GUI window
     system running on DISPLAY, such as the X server on GNU and Unix
     systems.  The value is a list of three integers: the major and
     minor version numbers of the protocol, and the distributor-specific
     release number of the window system software itself.  On GNU and
     Unix systems, these are normally the version of the X protocol and
     the distributor-specific release number of the X server software.
     On MS-Windows, this is the version of the Windows OS.

 -- Function: x-server-vendor &optional display
     This function returns the vendor that provided the window system
     software (as a string).  On GNU and Unix systems this really means
     whoever distributes the X server.  On MS-Windows this is the vendor
     ID string of the Windows OS (Microsoft).

     When the developers of X labeled software distributors as
     “vendors”, they showed their false assumption that no system could
     ever be developed and distributed noncommercially.


File: elisp.info,  Node: Positions,  Next: Markers,  Prev: Frames,  Up: Top

29 Positions
************

A “position” is the index of a character in the text of a buffer.  More
precisely, a position identifies the place between two characters (or
before the first character, or after the last character), so we can
speak of the character before or after a given position.  However, we
often speak of the character “at” a position, meaning the character
after that position.

   Positions are usually represented as integers starting from 1, but
can also be represented as “markers”—special objects that relocate
automatically when text is inserted or deleted so they stay with the
surrounding characters.  Functions that expect an argument to be a
position (an integer), but accept a marker as a substitute, normally
ignore which buffer the marker points into; they convert the marker to
an integer, and use that integer, exactly as if you had passed the
integer as the argument, even if the marker points to the wrong buffer.
A marker that points nowhere cannot convert to an integer; using it
instead of an integer causes an error.  *Note Markers::.

   See also the field feature (*note Fields::), which provides functions
that are used by many cursor-motion commands.

* Menu:

* Point::         The special position where editing takes place.
* Motion::        Changing point.
* Excursions::    Temporary motion and buffer changes.
* Narrowing::     Restricting editing to a portion of the buffer.


File: elisp.info,  Node: Point,  Next: Motion,  Up: Positions

29.1 Point
==========

“Point” is a special buffer position used by many editing commands,
including the self-inserting typed characters and text insertion
functions.  Other commands move point through the text to allow editing
and insertion at different places.

   Like other positions, point designates a place between two characters
(or before the first character, or after the last character), rather
than a particular character.  Usually terminals display the cursor over
the character that immediately follows point; point is actually before
the character on which the cursor sits.

   The value of point is a number no less than 1, and no greater than
the buffer size plus 1.  If narrowing is in effect (*note Narrowing::),
then point is constrained to fall within the accessible portion of the
buffer (possibly at one end of it).

   Each buffer has its own value of point, which is independent of the
value of point in other buffers.  Each window also has a value of point,
which is independent of the value of point in other windows on the same
buffer.  This is why point can have different values in various windows
that display the same buffer.  When a buffer appears in only one window,
the buffer’s point and the window’s point normally have the same value,
so the distinction is rarely important.  *Note Window Point::, for more
details.

 -- Function: point
     This function returns the value of point in the current buffer, as
     an integer.

          (point)
               ⇒ 175

 -- Function: point-min
     This function returns the minimum accessible value of point in the
     current buffer.  This is normally 1, but if narrowing is in effect,
     it is the position of the start of the region that you narrowed to.
     (*Note Narrowing::.)

 -- Function: point-max
     This function returns the maximum accessible value of point in the
     current buffer.  This is ‘(1+ (buffer-size))’, unless narrowing is
     in effect, in which case it is the position of the end of the
     region that you narrowed to.  (*Note Narrowing::.)

 -- Function: buffer-end flag
     This function returns ‘(point-max)’ if FLAG is greater than 0,
     ‘(point-min)’ otherwise.  The argument FLAG must be a number.

 -- Function: buffer-size &optional buffer
     This function returns the total number of characters in the current
     buffer.  In the absence of any narrowing (*note Narrowing::),
     ‘point-max’ returns a value one larger than this.

     If you specify a buffer, BUFFER, then the value is the size of
     BUFFER.

          (buffer-size)
               ⇒ 35
          (point-max)
               ⇒ 36


File: elisp.info,  Node: Motion,  Next: Excursions,  Prev: Point,  Up: Positions

29.2 Motion
===========

Motion functions change the value of point, either relative to the
current value of point, relative to the beginning or end of the buffer,
or relative to the edges of the selected window.  *Note Point::.

* Menu:

* Character Motion::       Moving in terms of characters.
* Word Motion::            Moving in terms of words.
* Buffer End Motion::      Moving to the beginning or end of the buffer.
* Text Lines::             Moving in terms of lines of text.
* Screen Lines::           Moving in terms of lines as displayed.
* List Motion::            Moving by parsing lists and sexps.
* Skipping Characters::    Skipping characters belonging to a certain set.


File: elisp.info,  Node: Character Motion,  Next: Word Motion,  Up: Motion

29.2.1 Motion by Characters
---------------------------

These functions move point based on a count of characters.  ‘goto-char’
is the fundamental primitive; the other functions use that.

 -- Command: goto-char position
     This function sets point in the current buffer to the value
     POSITION.

     If narrowing is in effect, POSITION still counts from the beginning
     of the buffer, but point cannot go outside the accessible portion.
     If POSITION is out of range, ‘goto-char’ moves point to the
     beginning or the end of the accessible portion.

     When this function is called interactively, POSITION is the numeric
     prefix argument, if provided; otherwise it is read from the
     minibuffer.

     ‘goto-char’ returns POSITION.

 -- Command: forward-char &optional count
     This function moves point COUNT characters forward, towards the end
     of the buffer (or backward, towards the beginning of the buffer, if
     COUNT is negative).  If COUNT is ‘nil’, the default is 1.

     If this attempts to move past the beginning or end of the buffer
     (or the limits of the accessible portion, when narrowing is in
     effect), it signals an error with error symbol
     ‘beginning-of-buffer’ or ‘end-of-buffer’.

     In an interactive call, COUNT is the numeric prefix argument.

 -- Command: backward-char &optional count
     This is just like ‘forward-char’ except that it moves in the
     opposite direction.


File: elisp.info,  Node: Word Motion,  Next: Buffer End Motion,  Prev: Character Motion,  Up: Motion

29.2.2 Motion by Words
----------------------

The functions for parsing words described below use the syntax table and
‘char-script-table’ to decide whether a given character is part of a
word.  *Note Syntax Tables::, and see *note Character Properties::.

 -- Command: forward-word &optional count
     This function moves point forward COUNT words (or backward if COUNT
     is negative).  If COUNT is omitted or ‘nil’, it defaults to 1.  In
     an interactive call, COUNT is specified by the numeric prefix
     argument.

     “Moving one word” means moving until point crosses a
     word-constituent character, which indicates the beginning of a
     word, and then continue moving until the word ends.  By default,
     characters that begin and end words, known as “word boundaries”,
     are defined by the current buffer’s syntax table (*note Syntax
     Class Table::), but modes can override that by setting up a
     suitable ‘find-word-boundary-function-table’, described below.
     Characters that belong to different scripts (as defined by
     ‘char-syntax-table’), also define a word boundary (*note Character
     Properties::).  In any case, this function cannot move point past
     the boundary of the accessible portion of the buffer, or across a
     field boundary (*note Fields::).  The most common case of a field
     boundary is the end of the prompt in the minibuffer.

     If it is possible to move COUNT words, without being stopped
     prematurely by the buffer boundary or a field boundary, the value
     is ‘t’.  Otherwise, the return value is ‘nil’ and point stops at
     the buffer boundary or field boundary.

     If ‘inhibit-field-text-motion’ is non-‘nil’, this function ignores
     field boundaries.

 -- Command: backward-word &optional count
     This function is just like ‘forward-word’, except that it moves
     backward until encountering the front of a word, rather than
     forward.

 -- User Option: words-include-escapes
     This variable affects the behavior of ‘forward-word’ and
     ‘backward-word’, and everything that uses them.  If it is
     non-‘nil’, then characters in the escape and character-quote syntax
     classes count as part of words.  Otherwise, they do not.

 -- Variable: inhibit-field-text-motion
     If this variable is non-‘nil’, certain motion functions including
     ‘forward-word’, ‘forward-sentence’, and ‘forward-paragraph’ ignore
     field boundaries.

 -- Variable: find-word-boundary-function-table
     This variable affects the behavior of ‘forward-word’ and
     ‘backward-word’, and everything that uses them.  Its value is a
     char-table (*note Char-Tables::) of functions to search for word
     boundaries.  If a character has a non-‘nil’ entry in this table,
     then when a word starts or ends with that character, the
     corresponding function will be called with 2 arguments: POS and
     LIMIT.  The function should return the position of the other word
     boundary.  Specifically, if POS is smaller than LIMIT, then POS is
     at the beginning of a word, and the function should return the
     position after the last character of the word; otherwise, POS is at
     the last character of a word, and the function should return the
     position of that word’s first character.

 -- Function: forward-word-strictly &optional count
     This function is like ‘forward-word’, but it is not affected by
     ‘find-word-boundary-function-table’.  Lisp programs that should not
     change behavior when word movement is modified by modes which set
     that table, such as ‘subword-mode’, should use this function
     instead of ‘forward-word’.

 -- Function: backward-word-strictly &optional count
     This function is like ‘backward-word’, but it is not affected by
     ‘find-word-boundary-function-table’.  Like with
     ‘forward-word-strictly’, use this function instead of
     ‘backward-word’ when movement by words should only consider syntax
     tables.


File: elisp.info,  Node: Buffer End Motion,  Next: Text Lines,  Prev: Word Motion,  Up: Motion

29.2.3 Motion to an End of the Buffer
-------------------------------------

To move point to the beginning of the buffer, write:

     (goto-char (point-min))

Likewise, to move to the end of the buffer, use:

     (goto-char (point-max))

   Here are two commands that users use to do these things.  They are
documented here to warn you not to use them in Lisp programs, because
they set the mark and display messages in the echo area.

 -- Command: beginning-of-buffer &optional n
     This function moves point to the beginning of the buffer (or the
     limits of the accessible portion, when narrowing is in effect),
     setting the mark at the previous position (except in Transient Mark
     mode, if the mark is already active, it does not set the mark.)

     If N is non-‘nil’, then it puts point N tenths of the way from the
     beginning of the accessible portion of the buffer.  In an
     interactive call, N is the numeric prefix argument, if provided;
     otherwise N defaults to ‘nil’.

     *Warning:* Don’t use this function in Lisp programs!

 -- Command: end-of-buffer &optional n
     This function moves point to the end of the buffer (or the limits
     of the accessible portion, when narrowing is in effect), setting
     the mark at the previous position (except in Transient Mark mode
     when the mark is already active).  If N is non-‘nil’, then it puts
     point N tenths of the way from the end of the accessible portion of
     the buffer.

     In an interactive call, N is the numeric prefix argument, if
     provided; otherwise N defaults to ‘nil’.

     *Warning:* Don’t use this function in Lisp programs!


File: elisp.info,  Node: Text Lines,  Next: Screen Lines,  Prev: Buffer End Motion,  Up: Motion

29.2.4 Motion by Text Lines
---------------------------

Text lines are portions of the buffer delimited by newline characters,
which are regarded as part of the previous line.  The first text line
begins at the beginning of the buffer, and the last text line ends at
the end of the buffer whether or not the last character is a newline.
The division of the buffer into text lines is not affected by the width
of the window, by line continuation in display, or by how tabs and
control characters are displayed.

 -- Command: beginning-of-line &optional count
     This function moves point to the beginning of the current line.
     With an argument COUNT not ‘nil’ or 1, it moves forward COUNT−1
     lines and then to the beginning of the line.

     This function does not move point across a field boundary (*note
     Fields::) unless doing so would move beyond there to a different
     line; therefore, if COUNT is ‘nil’ or 1, and point starts at a
     field boundary, point does not move.  To ignore field boundaries,
     either bind ‘inhibit-field-text-motion’ to ‘t’, or use the
     ‘forward-line’ function instead.  For instance, ‘(forward-line 0)’
     does the same thing as ‘(beginning-of-line)’, except that it
     ignores field boundaries.

     If this function reaches the end of the buffer (or of the
     accessible portion, if narrowing is in effect), it positions point
     there.  No error is signaled.

 -- Function: line-beginning-position &optional count
     Return the position that ‘(beginning-of-line COUNT)’ would move to.

 -- Command: end-of-line &optional count
     This function moves point to the end of the current line.  With an
     argument COUNT not ‘nil’ or 1, it moves forward COUNT−1 lines and
     then to the end of the line.

     This function does not move point across a field boundary (*note
     Fields::) unless doing so would move beyond there to a different
     line; therefore, if COUNT is ‘nil’ or 1, and point starts at a
     field boundary, point does not move.  To ignore field boundaries,
     bind ‘inhibit-field-text-motion’ to ‘t’.

     If this function reaches the end of the buffer (or of the
     accessible portion, if narrowing is in effect), it positions point
     there.  No error is signaled.

 -- Function: line-end-position &optional count
     Return the position that ‘(end-of-line COUNT)’ would move to.

 -- Command: forward-line &optional count
     This function moves point forward COUNT lines, to the beginning of
     the line following that.  If COUNT is negative, it moves point
     −COUNT lines backward, to the beginning of a line preceding that.
     If COUNT is zero, it moves point to the beginning of the current
     line.  If COUNT is ‘nil’, that means 1.

     If ‘forward-line’ encounters the beginning or end of the buffer (or
     of the accessible portion) before finding that many lines, it sets
     point there.  No error is signaled.

     ‘forward-line’ returns the difference between COUNT and the number
     of lines actually moved.  If you attempt to move down five lines
     from the beginning of a buffer that has only three lines, point
     stops at the end of the last line, and the value will be 2.  As an
     explicit exception, if the last accessible line is non-empty, but
     has no newline (e.g., if the buffer ends without a newline), the
     function sets point to the end of that line, and the value returned
     by the function counts that line as one line successfully moved.

     In an interactive call, COUNT is the numeric prefix argument.

 -- Function: count-lines start end
     This function returns the number of lines between the positions
     START and END in the current buffer.  If START and END are equal,
     then it returns 0.  Otherwise it returns at least 1, even if START
     and END are on the same line.  This is because the text between
     them, considered in isolation, must contain at least one line
     unless it is empty.

 -- Command: count-words start end
     This function returns the number of words between the positions
     START and END in the current buffer.

     This function can also be called interactively.  In that case, it
     prints a message reporting the number of lines, words, and
     characters in the buffer, or in the region if the region is active.

 -- Function: line-number-at-pos &optional pos
     This function returns the line number in the current buffer
     corresponding to the buffer position POS.  If POS is ‘nil’ or
     omitted, the current buffer position is used.

   Also see the functions ‘bolp’ and ‘eolp’ in *note Near Point::.
These functions do not move point, but test whether it is already at the
beginning or end of a line.


File: elisp.info,  Node: Screen Lines,  Next: List Motion,  Prev: Text Lines,  Up: Motion

29.2.5 Motion by Screen Lines
-----------------------------

The line functions in the previous section count text lines, delimited
only by newline characters.  By contrast, these functions count screen
lines, which are defined by the way the text appears on the screen.  A
text line is a single screen line if it is short enough to fit the width
of the selected window, but otherwise it may occupy several screen
lines.

   In some cases, text lines are truncated on the screen rather than
continued onto additional screen lines.  In these cases,
‘vertical-motion’ moves point much like ‘forward-line’.  *Note
Truncation::.

   Because the width of a given string depends on the flags that control
the appearance of certain characters, ‘vertical-motion’ behaves
differently, for a given piece of text, depending on the buffer it is
in, and even on the selected window (because the width, the truncation
flag, and display table may vary between windows).  *Note Usual
Display::.

   These functions scan text to determine where screen lines break, and
thus take time proportional to the distance scanned.

 -- Function: vertical-motion count &optional window cur-col
     This function moves point to the start of the screen line COUNT
     screen lines down from the screen line containing point.  If COUNT
     is negative, it moves up instead.

     The COUNT argument can be a cons cell, ‘(COLS . LINES)’, instead of
     an integer.  Then the function moves by LINES screen lines, and
     puts point COLS columns from the visual start of that screen line.
     Note that COLS are counted from the _visual_ start of the line; if
     the window is scrolled horizontally (*note Horizontal Scrolling::),
     the column on which point will end is in addition to the number of
     columns by which the text is scrolled.

     The return value is the number of screen lines over which point was
     moved.  The value may be less in absolute value than COUNT if the
     beginning or end of the buffer was reached.

     The window WINDOW is used for obtaining parameters such as the
     width, the horizontal scrolling, and the display table.  But
     ‘vertical-motion’ always operates on the current buffer, even if
     WINDOW currently displays some other buffer.

     The optional argument CUR-COL specifies the current column when the
     function is called.  This is the window-relative horizontal
     coordinate of point, measured in units of font width of the frame’s
     default face.  Providing it speeds up the function, especially in
     very long lines, because it doesn’t have to go back in the buffer
     in order to determine the current column.  Note that CUR-COL is
     also counted from the visual start of the line.

 -- Function: count-screen-lines &optional beg end count-final-newline
          window
     This function returns the number of screen lines in the text from
     BEG to END.  The number of screen lines may be different from the
     number of actual lines, due to line continuation, the display
     table, etc.  If BEG and END are ‘nil’ or omitted, they default to
     the beginning and end of the accessible portion of the buffer.

     If the region ends with a newline, that is ignored unless the
     optional third argument COUNT-FINAL-NEWLINE is non-‘nil’.

     The optional fourth argument WINDOW specifies the window for
     obtaining parameters such as width, horizontal scrolling, and so
     on.  The default is to use the selected window’s parameters.

     Like ‘vertical-motion’, ‘count-screen-lines’ always uses the
     current buffer, regardless of which buffer is displayed in WINDOW.
     This makes possible to use ‘count-screen-lines’ in any buffer,
     whether or not it is currently displayed in some window.

 -- Command: move-to-window-line count
     This function moves point with respect to the text currently
     displayed in the selected window.  It moves point to the beginning
     of the screen line COUNT screen lines from the top of the window.
     If COUNT is negative, that specifies a position −COUNT lines from
     the bottom (or the last line of the buffer, if the buffer ends
     above the specified screen position).

     If COUNT is ‘nil’, then point moves to the beginning of the line in
     the middle of the window.  If the absolute value of COUNT is
     greater than the size of the window, then point moves to the place
     that would appear on that screen line if the window were tall
     enough.  This will probably cause the next redisplay to scroll to
     bring that location onto the screen.

     In an interactive call, COUNT is the numeric prefix argument.

     The value returned is the window line number point has moved to,
     with the top line in the window numbered 0.

 -- Function: move-to-window-group-line count
     This function is like ‘move-to-window-line’, except that when the
     selected window is a part of a group of windows (*note Window
     Group::), ‘move-to-window-group-line’ will move to a position with
     respect to the entire group, not just the single window.  This
     condition holds when the buffer local variable
     ‘move-to-window-group-line-function’ is set to a function.  In this
     case, ‘move-to-window-group-line’ calls the function with the
     argument COUNT, then returns its result.

 -- Function: compute-motion from frompos to topos width offsets window
     This function scans the current buffer, calculating screen
     positions.  It scans the buffer forward from position FROM,
     assuming that is at screen coordinates FROMPOS, to position TO or
     coordinates TOPOS, whichever comes first.  It returns the ending
     buffer position and screen coordinates.

     The coordinate arguments FROMPOS and TOPOS are cons cells of the
     form ‘(HPOS . VPOS)’.

     The argument WIDTH is the number of columns available to display
     text; this affects handling of continuation lines.  ‘nil’ means the
     actual number of usable text columns in the window, which is
     equivalent to the value returned by ‘(window-width window)’.

     The argument OFFSETS is either ‘nil’ or a cons cell of the form
     ‘(HSCROLL . TAB-OFFSET)’.  Here HSCROLL is the number of columns
     not being displayed at the left margin; most callers get this by
     calling ‘window-hscroll’.  Meanwhile, TAB-OFFSET is the offset
     between column numbers on the screen and column numbers in the
     buffer.  This can be nonzero in a continuation line, when the
     previous screen lines’ widths do not add up to a multiple of
     ‘tab-width’.  It is always zero in a non-continuation line.

     The window WINDOW serves only to specify which display table to
     use.  ‘compute-motion’ always operates on the current buffer,
     regardless of what buffer is displayed in WINDOW.

     The return value is a list of five elements:

          (POS HPOS VPOS PREVHPOS CONTIN)

     Here POS is the buffer position where the scan stopped, VPOS is the
     vertical screen position, and HPOS is the horizontal screen
     position.

     The result PREVHPOS is the horizontal position one character back
     from POS.  The result CONTIN is ‘t’ if the last line was continued
     after (or within) the previous character.

     For example, to find the buffer position of column COL of screen
     line LINE of a certain window, pass the window’s display start
     location as FROM and the window’s upper-left coordinates as
     FROMPOS.  Pass the buffer’s ‘(point-max)’ as TO, to limit the scan
     to the end of the accessible portion of the buffer, and pass LINE
     and COL as TOPOS.  Here’s a function that does this:

          (defun coordinates-of-position (col line)
            (car (compute-motion (window-start)
                                 '(0 . 0)
                                 (point-max)
                                 (cons col line)
                                 (window-width)
                                 (cons (window-hscroll) 0)
                                 (selected-window))))

     When you use ‘compute-motion’ for the minibuffer, you need to use
     ‘minibuffer-prompt-width’ to get the horizontal position of the
     beginning of the first screen line.  *Note Minibuffer Contents::.


File: elisp.info,  Node: List Motion,  Next: Skipping Characters,  Prev: Screen Lines,  Up: Motion

29.2.6 Moving over Balanced Expressions
---------------------------------------

Here are several functions concerned with balanced-parenthesis
expressions (also called “sexps” in connection with moving across them
in Emacs).  The syntax table controls how these functions interpret
various characters; see *note Syntax Tables::.  *Note Parsing
Expressions::, for lower-level primitives for scanning sexps or parts of
sexps.  For user-level commands, see *note Commands for Editing with
Parentheses: (emacs)Parentheses.

 -- Command: forward-list &optional arg
     This function moves forward across ARG (default 1) balanced groups
     of parentheses.  (Other syntactic entities such as words or paired
     string quotes are ignored.)

 -- Command: backward-list &optional arg
     This function moves backward across ARG (default 1) balanced groups
     of parentheses.  (Other syntactic entities such as words or paired
     string quotes are ignored.)

 -- Command: up-list &optional arg escape-strings no-syntax-crossing
     This function moves forward out of ARG (default 1) levels of
     parentheses.  A negative argument means move backward but still to
     a less deep spot.  If ESCAPE-STRINGS is non-‘nil’ (as it is
     interactively), move out of enclosing strings as well.  If
     NO-SYNTAX-CROSSING is non-‘nil’ (as it is interactively), prefer to
     break out of any enclosing string instead of moving to the start of
     a list broken across multiple strings.  On error, location of point
     is unspecified.

 -- Command: backward-up-list &optional arg escape-strings
          no-syntax-crossing
     This function is just like ‘up-list’, but with a negated argument.

 -- Command: down-list &optional arg
     This function moves forward into ARG (default 1) levels of
     parentheses.  A negative argument means move backward but still go
     deeper in parentheses (−ARG levels).

 -- Command: forward-sexp &optional arg
     This function moves forward across ARG (default 1) balanced
     expressions.  Balanced expressions include both those delimited by
     parentheses and other kinds, such as words and string constants.
     *Note Parsing Expressions::.  For example,

          ---------- Buffer: foo ----------
          (concat★ "foo " (car x) y z)
          ---------- Buffer: foo ----------

          (forward-sexp 3)
               ⇒ nil

          ---------- Buffer: foo ----------
          (concat "foo " (car x) y★ z)
          ---------- Buffer: foo ----------

 -- Command: backward-sexp &optional arg
     This function moves backward across ARG (default 1) balanced
     expressions.

 -- Command: beginning-of-defun &optional arg
     This function moves back to the ARGth beginning of a defun.  If ARG
     is negative, this actually moves forward, but it still moves to the
     beginning of a defun, not to the end of one.  ARG defaults to 1.

 -- Command: end-of-defun &optional arg
     This function moves forward to the ARGth end of a defun.  If ARG is
     negative, this actually moves backward, but it still moves to the
     end of a defun, not to the beginning of one.  ARG defaults to 1.

 -- User Option: defun-prompt-regexp
     If non-‘nil’, this buffer-local variable holds a regular expression
     that specifies what text can appear before the open-parenthesis
     that starts a defun.  That is to say, a defun begins on a line that
     starts with a match for this regular expression, followed by a
     character with open-parenthesis syntax.

 -- User Option: open-paren-in-column-0-is-defun-start
     If this variable’s value is non-‘nil’, an open parenthesis in
     column 0 is considered to be the start of a defun.  If it is ‘nil’,
     an open parenthesis in column 0 has no special meaning.  The
     default is ‘t’.

 -- Variable: beginning-of-defun-function
     If non-‘nil’, this variable holds a function for finding the
     beginning of a defun.  The function ‘beginning-of-defun’ calls this
     function instead of using its normal method, passing it its
     optional argument.  If the argument is non-‘nil’, the function
     should move back by that many functions, like ‘beginning-of-defun’
     does.

 -- Variable: end-of-defun-function
     If non-‘nil’, this variable holds a function for finding the end of
     a defun.  The function ‘end-of-defun’ calls this function instead
     of using its normal method.


File: elisp.info,  Node: Skipping Characters,  Prev: List Motion,  Up: Motion

29.2.7 Skipping Characters
--------------------------

The following two functions move point over a specified set of
characters.  For example, they are often used to skip whitespace.  For
related functions, see *note Motion and Syntax::.

   These functions convert the set string to multibyte if the buffer is
multibyte, and they convert it to unibyte if the buffer is unibyte, as
the search functions do (*note Searching and Matching::).

 -- Function: skip-chars-forward character-set &optional limit
     This function moves point in the current buffer forward, skipping
     over a given set of characters.  It examines the character
     following point, then advances point if the character matches
     CHARACTER-SET.  This continues until it reaches a character that
     does not match.  The function returns the number of characters
     moved over.

     The argument CHARACTER-SET is a string, like the inside of a
     ‘[...]’ in a regular expression except that ‘]’ does not terminate
     it, and ‘\’ quotes ‘^’, ‘-’ or ‘\’.  Thus, ‘"a-zA-Z"’ skips over
     all letters, stopping before the first nonletter, and ‘"^a-zA-Z"’
     skips nonletters stopping before the first letter.  See *Note
     Regular Expressions::.  Character classes can also be used, e.g.,
     ‘"[:alnum:]"’.  See *note Char Classes::.

     If LIMIT is supplied (it must be a number or a marker), it
     specifies the maximum position in the buffer that point can be
     skipped to.  Point will stop at or before LIMIT.

     In the following example, point is initially located directly
     before the ‘T’.  After the form is evaluated, point is located at
     the end of that line (between the ‘t’ of ‘hat’ and the newline).
     The function skips all letters and spaces, but not newlines.

          ---------- Buffer: foo ----------
          I read "★The cat in the hat
          comes back" twice.
          ---------- Buffer: foo ----------

          (skip-chars-forward "a-zA-Z ")
               ⇒ 18

          ---------- Buffer: foo ----------
          I read "The cat in the hat★
          comes back" twice.
          ---------- Buffer: foo ----------

 -- Function: skip-chars-backward character-set &optional limit
     This function moves point backward, skipping characters that match
     CHARACTER-SET, until LIMIT.  It is just like ‘skip-chars-forward’
     except for the direction of motion.

     The return value indicates the distance traveled.  It is an integer
     that is zero or less.


File: elisp.info,  Node: Excursions,  Next: Narrowing,  Prev: Motion,  Up: Positions

29.3 Excursions
===============

It is often useful to move point temporarily within a localized portion
of the program.  This is called an “excursion”, and it is done with the
‘save-excursion’ special form.  This construct remembers the initial
identity of the current buffer, and its value of point, and restores
them after the excursion completes.  It is the standard way to move
point within one part of a program and avoid affecting the rest of the
program, and is used thousands of times in the Lisp sources of Emacs.

   If you only need to save and restore the identity of the current
buffer, use ‘save-current-buffer’ or ‘with-current-buffer’ instead
(*note Current Buffer::).  If you need to save or restore window
configurations, see the forms described in *note Window Configurations::
and in *note Frame Configurations::.

 -- Special Form: save-excursion body...
     This special form saves the identity of the current buffer and the
     value of point in it, evaluates BODY, and finally restores the
     buffer and its saved value of point.  Both saved values are
     restored even in case of an abnormal exit via ‘throw’ or error
     (*note Nonlocal Exits::).

     The value returned by ‘save-excursion’ is the result of the last
     form in BODY, or ‘nil’ if no body forms were given.

   Because ‘save-excursion’ only saves point for the buffer that was
current at the start of the excursion, any changes made to point in
other buffers, during the excursion, will remain in effect afterward.
This frequently leads to unintended consequences, so the byte compiler
warns if you call ‘set-buffer’ during an excursion:

     Warning: Use ‘with-current-buffer’ rather than
              save-excursion+set-buffer

To avoid such problems, you should call ‘save-excursion’ only after
setting the desired current buffer, as in the following example:

     (defun append-string-to-buffer (string buffer)
       "Append STRING to the end of BUFFER."
       (with-current-buffer buffer
         (save-excursion
           (goto-char (point-max))
           (insert string))))

   Likewise, ‘save-excursion’ does not restore window-buffer
correspondences altered by functions such as ‘switch-to-buffer’.

   *Warning:* Ordinary insertion of text adjacent to the saved point
value relocates the saved value, just as it relocates all markers.  More
precisely, the saved value is a marker with insertion type ‘nil’.  *Note
Marker Insertion Types::.  Therefore, when the saved point value is
restored, it normally comes before the inserted text.

 -- Macro: save-mark-and-excursion body...
     This macro is like ‘save-excursion’, but also saves and restores
     the mark location and ‘mark-active’.  This macro does what
     ‘save-excursion’ did before Emacs 25.1.


File: elisp.info,  Node: Narrowing,  Prev: Excursions,  Up: Positions

29.4 Narrowing
==============

“Narrowing” means limiting the text addressable by Emacs editing
commands to a limited range of characters in a buffer.  The text that
remains addressable is called the “accessible portion” of the buffer.

   Narrowing is specified with two buffer positions, which become the
beginning and end of the accessible portion.  For most editing commands
and primitives, these positions replace the values of the beginning and
end of the buffer.  While narrowing is in effect, no text outside the
accessible portion is displayed, and point cannot move outside the
accessible portion.  Note that narrowing does not alter actual buffer
positions (*note Point::); it only determines which positions are
considered the accessible portion of the buffer.  Most functions refuse
to operate on text that is outside the accessible portion.

   Commands for saving buffers are unaffected by narrowing; they save
the entire buffer regardless of any narrowing.

   If you need to display in a single buffer several very different
types of text, consider using an alternative facility described in *note
Swapping Text::.

 -- Command: narrow-to-region start end
     This function sets the accessible portion of the current buffer to
     start at START and end at END.  Both arguments should be character
     positions.

     In an interactive call, START and END are set to the bounds of the
     current region (point and the mark, with the smallest first).

 -- Command: narrow-to-page &optional move-count
     This function sets the accessible portion of the current buffer to
     include just the current page.  An optional first argument
     MOVE-COUNT non-‘nil’ means to move forward or backward by
     MOVE-COUNT pages and then narrow to one page.  The variable
     ‘page-delimiter’ specifies where pages start and end (*note
     Standard Regexps::).

     In an interactive call, MOVE-COUNT is set to the numeric prefix
     argument.

 -- Command: widen
     This function cancels any narrowing in the current buffer, so that
     the entire contents are accessible.  This is called “widening”.  It
     is equivalent to the following expression:

          (narrow-to-region 1 (1+ (buffer-size)))

 -- Function: buffer-narrowed-p
     This function returns non-‘nil’ if the buffer is narrowed, and
     ‘nil’ otherwise.

 -- Special Form: save-restriction body...
     This special form saves the current bounds of the accessible
     portion, evaluates the BODY forms, and finally restores the saved
     bounds, thus restoring the same state of narrowing (or absence
     thereof) formerly in effect.  The state of narrowing is restored
     even in the event of an abnormal exit via ‘throw’ or error (*note
     Nonlocal Exits::).  Therefore, this construct is a clean way to
     narrow a buffer temporarily.

     The value returned by ‘save-restriction’ is that returned by the
     last form in BODY, or ‘nil’ if no body forms were given.

     *Caution:* it is easy to make a mistake when using the
     ‘save-restriction’ construct.  Read the entire description here
     before you try it.

     If BODY changes the current buffer, ‘save-restriction’ still
     restores the restrictions on the original buffer (the buffer whose
     restrictions it saved from), but it does not restore the identity
     of the current buffer.

     ‘save-restriction’ does _not_ restore point; use ‘save-excursion’
     for that.  If you use both ‘save-restriction’ and ‘save-excursion’
     together, ‘save-excursion’ should come first (on the outside).
     Otherwise, the old point value would be restored with temporary
     narrowing still in effect.  If the old point value were outside the
     limits of the temporary narrowing, this would fail to restore it
     accurately.

     Here is a simple example of correct use of ‘save-restriction’:

          ---------- Buffer: foo ----------
          This is the contents of foo
          This is the contents of foo
          This is the contents of foo★
          ---------- Buffer: foo ----------

          (save-excursion
            (save-restriction
              (goto-char 1)
              (forward-line 2)
              (narrow-to-region 1 (point))
              (goto-char (point-min))
              (replace-string "foo" "bar")))

          ---------- Buffer: foo ----------
          This is the contents of bar
          This is the contents of bar
          This is the contents of foo★
          ---------- Buffer: foo ----------


File: elisp.info,  Node: Markers,  Next: Text,  Prev: Positions,  Up: Top

30 Markers
**********

A “marker” is a Lisp object used to specify a position in a buffer
relative to the surrounding text.  A marker changes its offset from the
beginning of the buffer automatically whenever text is inserted or
deleted, so that it stays with the two characters on either side of it.

* Menu:

* Overview of Markers::      The components of a marker, and how it relocates.
* Predicates on Markers::    Testing whether an object is a marker.
* Creating Markers::         Making empty markers or markers at certain places.
* Information from Markers:: Finding the marker’s buffer or character position.
* Marker Insertion Types::   Two ways a marker can relocate when you
                               insert where it points.
* Moving Markers::           Moving the marker to a new buffer or position.
* The Mark::                 How the mark is implemented with a marker.
* The Region::               How to access the region.


File: elisp.info,  Node: Overview of Markers,  Next: Predicates on Markers,  Up: Markers

30.1 Overview of Markers
========================

A marker specifies a buffer and a position in that buffer.  A marker can
be used to represent a position in functions that require one, just as
an integer could be used.  In that case, the marker’s buffer is normally
ignored.  Of course, a marker used in this way usually points to a
position in the buffer that the function operates on, but that is
entirely the programmer’s responsibility.  *Note Positions::, for a
complete description of positions.

   A marker has three attributes: the marker position, the marker
buffer, and the insertion type.  The marker position is an integer that
is equivalent (at a given time) to the marker as a position in that
buffer.  But the marker’s position value can change during the life of
the marker, and often does.  Insertion and deletion of text in the
buffer relocate the marker.  The idea is that a marker positioned
between two characters remains between those two characters despite
insertion and deletion elsewhere in the buffer.  Relocation changes the
integer equivalent of the marker.

   Deleting text around a marker’s position leaves the marker between
the characters immediately before and after the deleted text.  Inserting
text at the position of a marker normally leaves the marker either in
front of or after the new text, depending on the marker’s “insertion
type” (*note Marker Insertion Types::)—unless the insertion is done with
‘insert-before-markers’ (*note Insertion::).

   Insertion and deletion in a buffer must check all the markers and
relocate them if necessary.  This slows processing in a buffer with a
large number of markers.  For this reason, it is a good idea to make a
marker point nowhere if you are sure you don’t need it any more.
Markers that can no longer be accessed are eventually removed (*note
Garbage Collection::).

   Because it is common to perform arithmetic operations on a marker
position, most of these operations (including ‘+’ and ‘-’) accept
markers as arguments.  In such cases, the marker stands for its current
position.

   Here are examples of creating markers, setting markers, and moving
point to markers:

     ;; Make a new marker that initially does not point anywhere:
     (setq m1 (make-marker))
          ⇒ #<marker in no buffer>

     ;; Set ‘m1’ to point between the 99th and 100th characters
     ;;   in the current buffer:
     (set-marker m1 100)
          ⇒ #<marker at 100 in markers.texi>

     ;; Now insert one character at the beginning of the buffer:
     (goto-char (point-min))
          ⇒ 1
     (insert "Q")
          ⇒ nil

     ;; ‘m1’ is updated appropriately.
     m1
          ⇒ #<marker at 101 in markers.texi>

     ;; Two markers that point to the same position
     ;;   are not ‘eq’, but they are ‘equal’.
     (setq m2 (copy-marker m1))
          ⇒ #<marker at 101 in markers.texi>
     (eq m1 m2)
          ⇒ nil
     (equal m1 m2)
          ⇒ t

     ;; When you are finished using a marker, make it point nowhere.
     (set-marker m1 nil)
          ⇒ #<marker in no buffer>


File: elisp.info,  Node: Predicates on Markers,  Next: Creating Markers,  Prev: Overview of Markers,  Up: Markers

30.2 Predicates on Markers
==========================

You can test an object to see whether it is a marker, or whether it is
either an integer or a marker.  The latter test is useful in connection
with the arithmetic functions that work with both markers and integers.

 -- Function: markerp object
     This function returns ‘t’ if OBJECT is a marker, ‘nil’ otherwise.
     Note that integers are not markers, even though many functions will
     accept either a marker or an integer.

 -- Function: integer-or-marker-p object
     This function returns ‘t’ if OBJECT is an integer or a marker,
     ‘nil’ otherwise.

 -- Function: number-or-marker-p object
     This function returns ‘t’ if OBJECT is a number (either integer or
     floating point) or a marker, ‘nil’ otherwise.


File: elisp.info,  Node: Creating Markers,  Next: Information from Markers,  Prev: Predicates on Markers,  Up: Markers

30.3 Functions that Create Markers
==================================

When you create a new marker, you can make it point nowhere, or point to
the present position of point, or to the beginning or end of the
accessible portion of the buffer, or to the same place as another given
marker.

   The next four functions all return markers with insertion type ‘nil’.
*Note Marker Insertion Types::.

 -- Function: make-marker
     This function returns a newly created marker that does not point
     anywhere.

          (make-marker)
               ⇒ #<marker in no buffer>

 -- Function: point-marker
     This function returns a new marker that points to the present
     position of point in the current buffer.  *Note Point::.  For an
     example, see ‘copy-marker’, below.

 -- Function: point-min-marker
     This function returns a new marker that points to the beginning of
     the accessible portion of the buffer.  This will be the beginning
     of the buffer unless narrowing is in effect.  *Note Narrowing::.

 -- Function: point-max-marker
     This function returns a new marker that points to the end of the
     accessible portion of the buffer.  This will be the end of the
     buffer unless narrowing is in effect.  *Note Narrowing::.

     Here are examples of this function and ‘point-min-marker’, shown in
     a buffer containing a version of the source file for the text of
     this chapter.

          (point-min-marker)
               ⇒ #<marker at 1 in markers.texi>
          (point-max-marker)
               ⇒ #<marker at 24080 in markers.texi>

          (narrow-to-region 100 200)
               ⇒ nil
          (point-min-marker)
               ⇒ #<marker at 100 in markers.texi>
          (point-max-marker)
               ⇒ #<marker at 200 in markers.texi>

 -- Function: copy-marker &optional marker-or-integer insertion-type
     If passed a marker as its argument, ‘copy-marker’ returns a new
     marker that points to the same place and the same buffer as does
     MARKER-OR-INTEGER.  If passed an integer as its argument,
     ‘copy-marker’ returns a new marker that points to position
     MARKER-OR-INTEGER in the current buffer.

     The new marker’s insertion type is specified by the argument
     INSERTION-TYPE.  *Note Marker Insertion Types::.

          (copy-marker 0)
               ⇒ #<marker at 1 in markers.texi>

          (copy-marker 90000)
               ⇒ #<marker at 24080 in markers.texi>

     An error is signaled if MARKER is neither a marker nor an integer.

   Two distinct markers are considered ‘equal’ (even though not ‘eq’) to
each other if they have the same position and buffer, or if they both
point nowhere.

     (setq p (point-marker))
          ⇒ #<marker at 2139 in markers.texi>

     (setq q (copy-marker p))
          ⇒ #<marker at 2139 in markers.texi>

     (eq p q)
          ⇒ nil

     (equal p q)
          ⇒ t


File: elisp.info,  Node: Information from Markers,  Next: Marker Insertion Types,  Prev: Creating Markers,  Up: Markers

30.4 Information from Markers
=============================

This section describes the functions for accessing the components of a
marker object.

 -- Function: marker-position marker
     This function returns the position that MARKER points to, or ‘nil’
     if it points nowhere.

 -- Function: marker-buffer marker
     This function returns the buffer that MARKER points into, or ‘nil’
     if it points nowhere.

          (setq m (make-marker))
               ⇒ #<marker in no buffer>
          (marker-position m)
               ⇒ nil
          (marker-buffer m)
               ⇒ nil

          (set-marker m 3770 (current-buffer))
               ⇒ #<marker at 3770 in markers.texi>
          (marker-buffer m)
               ⇒ #<buffer markers.texi>
          (marker-position m)
               ⇒ 3770


File: elisp.info,  Node: Marker Insertion Types,  Next: Moving Markers,  Prev: Information from Markers,  Up: Markers

30.5 Marker Insertion Types
===========================

When you insert text directly at the place where a marker points, there
are two possible ways to relocate that marker: it can point before the
inserted text, or point after it.  You can specify which one a given
marker should do by setting its “insertion type”.  Note that use of
‘insert-before-markers’ ignores markers’ insertion types, always
relocating a marker to point after the inserted text.

 -- Function: set-marker-insertion-type marker type
     This function sets the insertion type of marker MARKER to TYPE.  If
     TYPE is ‘t’, MARKER will advance when text is inserted at its
     position.  If TYPE is ‘nil’, MARKER does not advance when text is
     inserted there.

 -- Function: marker-insertion-type marker
     This function reports the current insertion type of MARKER.

   All functions that create markers without accepting an argument that
specifies the insertion type, create them with insertion type ‘nil’
(*note Creating Markers::).  Also, the mark has, by default, insertion
type ‘nil’.


File: elisp.info,  Node: Moving Markers,  Next: The Mark,  Prev: Marker Insertion Types,  Up: Markers

30.6 Moving Marker Positions
============================

This section describes how to change the position of an existing marker.
When you do this, be sure you know whether the marker is used outside of
your program, and, if so, what effects will result from moving
it—otherwise, confusing things may happen in other parts of Emacs.

 -- Function: set-marker marker position &optional buffer
     This function moves MARKER to POSITION in BUFFER.  If BUFFER is not
     provided, it defaults to the current buffer.

     If POSITION is ‘nil’ or a marker that points nowhere, then MARKER
     is set to point nowhere.

     The value returned is MARKER.

          (setq m (point-marker))
               ⇒ #<marker at 4714 in markers.texi>
          (set-marker m 55)
               ⇒ #<marker at 55 in markers.texi>
          (setq b (get-buffer "foo"))
               ⇒ #<buffer foo>
          (set-marker m 0 b)
               ⇒ #<marker at 1 in foo>

 -- Function: move-marker marker position &optional buffer
     This is another name for ‘set-marker’.


File: elisp.info,  Node: The Mark,  Next: The Region,  Prev: Moving Markers,  Up: Markers

30.7 The Mark
=============

Each buffer has a special marker, which is designated “the mark”.  When
a buffer is newly created, this marker exists but does not point
anywhere; this means that the mark doesn’t exist in that buffer yet.
Subsequent commands can set the mark.

   The mark specifies a position to bound a range of text for many
commands, such as ‘kill-region’ and ‘indent-rigidly’.  These commands
typically act on the text between point and the mark, which is called
the “region”.  If you are writing a command that operates on the region,
don’t examine the mark directly; instead, use ‘interactive’ with the ‘r’
specification.  This provides the values of point and the mark as
arguments to the command in an interactive call, but permits other Lisp
programs to specify arguments explicitly.  *Note Interactive Codes::.

   Some commands set the mark as a side-effect.  Commands should do this
only if it has a potential use to the user, and never for their own
internal purposes.  For example, the ‘replace-regexp’ command sets the
mark to the value of point before doing any replacements, because this
enables the user to move back there conveniently after the replace is
finished.

   Once the mark exists in a buffer, it normally never ceases to exist.
However, it may become “inactive”, if Transient Mark mode is enabled.
The buffer-local variable ‘mark-active’, if non-‘nil’, means that the
mark is active.  A command can call the function ‘deactivate-mark’ to
deactivate the mark directly, or it can request deactivation of the mark
upon return to the editor command loop by setting the variable
‘deactivate-mark’ to a non-‘nil’ value.

   If Transient Mark mode is enabled, certain editing commands that
normally apply to text near point, apply instead to the region when the
mark is active.  This is the main motivation for using Transient Mark
mode.  (Another is that this enables highlighting of the region when the
mark is active.  *Note Display::.)

   In addition to the mark, each buffer has a “mark ring” which is a
list of markers containing previous values of the mark.  When editing
commands change the mark, they should normally save the old value of the
mark on the mark ring.  The variable ‘mark-ring-max’ specifies the
maximum number of entries in the mark ring; once the list becomes this
long, adding a new element deletes the last element.

   There is also a separate global mark ring, but that is used only in a
few particular user-level commands, and is not relevant to Lisp
programming.  So we do not describe it here.

 -- Function: mark &optional force
     This function returns the current buffer’s mark position as an
     integer, or ‘nil’ if no mark has ever been set in this buffer.

     If Transient Mark mode is enabled, and ‘mark-even-if-inactive’ is
     ‘nil’, ‘mark’ signals an error if the mark is inactive.  However,
     if FORCE is non-‘nil’, then ‘mark’ disregards inactivity of the
     mark, and returns the mark position (or ‘nil’) anyway.

 -- Function: mark-marker
     This function returns the marker that represents the current
     buffer’s mark.  It is not a copy, it is the marker used internally.
     Therefore, changing this marker’s position will directly affect the
     buffer’s mark.  Don’t do that unless that is the effect you want.

          (setq m (mark-marker))
               ⇒ #<marker at 3420 in markers.texi>
          (set-marker m 100)
               ⇒ #<marker at 100 in markers.texi>
          (mark-marker)
               ⇒ #<marker at 100 in markers.texi>

     Like any marker, this marker can be set to point at any buffer you
     like.  If you make it point at any buffer other than the one of
     which it is the mark, it will yield perfectly consistent, but
     rather odd, results.  We recommend that you not do it!

 -- Function: set-mark position
     This function sets the mark to POSITION, and activates the mark.
     The old value of the mark is _not_ pushed onto the mark ring.

     *Please note:* Use this function only if you want the user to see
     that the mark has moved, and you want the previous mark position to
     be lost.  Normally, when a new mark is set, the old one should go
     on the ‘mark-ring’.  For this reason, most applications should use
     ‘push-mark’ and ‘pop-mark’, not ‘set-mark’.

     Novice Emacs Lisp programmers often try to use the mark for the
     wrong purposes.  The mark saves a location for the user’s
     convenience.  An editing command should not alter the mark unless
     altering the mark is part of the user-level functionality of the
     command.  (And, in that case, this effect should be documented.)
     To remember a location for internal use in the Lisp program, store
     it in a Lisp variable.  For example:

          (let ((beg (point)))
            (forward-line 1)
            (delete-region beg (point))).

 -- Function: push-mark &optional position nomsg activate
     This function sets the current buffer’s mark to POSITION, and
     pushes a copy of the previous mark onto ‘mark-ring’.  If POSITION
     is ‘nil’, then the value of point is used.

     The function ‘push-mark’ normally _does not_ activate the mark.  To
     do that, specify ‘t’ for the argument ACTIVATE.

     A ‘Mark set’ message is displayed unless NOMSG is non-‘nil’.

 -- Function: pop-mark
     This function pops off the top element of ‘mark-ring’ and makes
     that mark become the buffer’s actual mark.  This does not move
     point in the buffer, and it does nothing if ‘mark-ring’ is empty.
     It deactivates the mark.

 -- User Option: transient-mark-mode
     This variable, if non-‘nil’, enables Transient Mark mode.  In
     Transient Mark mode, every buffer-modifying primitive sets
     ‘deactivate-mark’.  As a consequence, most commands that modify the
     buffer also deactivate the mark.

     When Transient Mark mode is enabled and the mark is active, many
     commands that normally apply to the text near point instead apply
     to the region.  Such commands should use the function
     ‘use-region-p’ to test whether they should operate on the region.
     *Note The Region::.

     Lisp programs can set ‘transient-mark-mode’ to non-‘nil’, non-‘t’
     values to enable Transient Mark mode temporarily.  If the value is
     ‘lambda’, Transient Mark mode is automatically turned off after any
     action, such as buffer modification, that would normally deactivate
     the mark.  If the value is ‘(only . OLDVAL)’, then
     ‘transient-mark-mode’ is set to the value OLDVAL after any
     subsequent command that moves point and is not shift-translated
     (*note shift-translation: Key Sequence Input.), or after any other
     action that would normally deactivate the mark.

 -- User Option: mark-even-if-inactive
     If this is non-‘nil’, Lisp programs and the Emacs user can use the
     mark even when it is inactive.  This option affects the behavior of
     Transient Mark mode.  When the option is non-‘nil’, deactivation of
     the mark turns off region highlighting, but commands that use the
     mark behave as if the mark were still active.

 -- Variable: deactivate-mark
     If an editor command sets this variable non-‘nil’, then the editor
     command loop deactivates the mark after the command returns (if
     Transient Mark mode is enabled).  All the primitives that change
     the buffer set ‘deactivate-mark’, to deactivate the mark when the
     command is finished.  Setting this variable makes it buffer-local.

     To write Lisp code that modifies the buffer without causing
     deactivation of the mark at the end of the command, bind
     ‘deactivate-mark’ to ‘nil’ around the code that does the
     modification.  For example:

          (let (deactivate-mark)
            (insert " "))

 -- Function: deactivate-mark &optional force
     If Transient Mark mode is enabled or FORCE is non-‘nil’, this
     function deactivates the mark and runs the normal hook
     ‘deactivate-mark-hook’.  Otherwise, it does nothing.

 -- Variable: mark-active
     The mark is active when this variable is non-‘nil’.  This variable
     is always buffer-local in each buffer.  Do _not_ use the value of
     this variable to decide whether a command that normally operates on
     text near point should operate on the region instead.  Use the
     function ‘use-region-p’ for that (*note The Region::).

 -- Variable: activate-mark-hook
 -- Variable: deactivate-mark-hook
     These normal hooks are run, respectively, when the mark becomes
     active and when it becomes inactive.  The hook ‘activate-mark-hook’
     is also run at the end of the command loop if the mark is active
     and it is possible that the region may have changed.

 -- Function: handle-shift-selection
     This function implements the shift-selection behavior of
     point-motion commands.  *Note (emacs)Shift Selection::.  It is
     called automatically by the Emacs command loop whenever a command
     with a ‘^’ character in its ‘interactive’ spec is invoked, before
     the command itself is executed (*note ^: Interactive Codes.).

     If ‘shift-select-mode’ is non-‘nil’ and the current command was
     invoked via shift translation (*note shift-translation: Key
     Sequence Input.), this function sets the mark and temporarily
     activates the region, unless the region was already temporarily
     activated in this way.  Otherwise, if the region has been activated
     temporarily, it deactivates the mark and restores the variable
     ‘transient-mark-mode’ to its earlier value.

 -- Variable: mark-ring
     The value of this buffer-local variable is the list of saved former
     marks of the current buffer, most recent first.

          mark-ring
          ⇒ (#<marker at 11050 in markers.texi>
              #<marker at 10832 in markers.texi>
              ...)

 -- User Option: mark-ring-max
     The value of this variable is the maximum size of ‘mark-ring’.  If
     more marks than this are pushed onto the ‘mark-ring’, ‘push-mark’
     discards an old mark when it adds a new one.

   When Delete Selection mode (*note Delete Selection: (emacs)Using
Region.) is enabled, commands that operate on the active region (a.k.a.
“selection”) behave slightly differently.  This works by adding the
function ‘delete-selection-pre-hook’ to the ‘pre-command-hook’ (*note
Command Overview::).  That function calls ‘delete-selection-helper’ to
delete the selection as appropriate for the command.  If you want to
adapt a command to Delete Selection mode, put the ‘delete-selection’
property on the function’s symbol (*note Symbol Plists::); commands that
don’t have this property on their symbol won’t delete the selection.
This property can have one of several values to tailor the behavior to
what the command is supposed to do; see the doc strings of
‘delete-selection-pre-hook’ and ‘delete-selection-helper’ for the
details.


File: elisp.info,  Node: The Region,  Prev: The Mark,  Up: Markers

30.8 The Region
===============

The text between point and the mark is known as “the region”.  Various
functions operate on text delimited by point and the mark, but only
those functions specifically related to the region itself are described
here.

   The next two functions signal an error if the mark does not point
anywhere.  If Transient Mark mode is enabled and ‘mark-even-if-inactive’
is ‘nil’, they also signal an error if the mark is inactive.

 -- Function: region-beginning
     This function returns the position of the beginning of the region
     (as an integer).  This is the position of either point or the mark,
     whichever is smaller.

 -- Function: region-end
     This function returns the position of the end of the region (as an
     integer).  This is the position of either point or the mark,
     whichever is larger.

   Instead of using ‘region-beginning’ and ‘region-end’, a command
designed to operate on a region should normally use ‘interactive’ with
the ‘r’ specification to find the beginning and end of the region.  This
lets other Lisp programs specify the bounds explicitly as arguments.
*Note Interactive Codes::.

 -- Function: use-region-p
     This function returns ‘t’ if Transient Mark mode is enabled, the
     mark is active, and there is a valid region in the buffer.  This
     function is intended to be used by commands that operate on the
     region, instead of on text near point, when the mark is active.

     A region is valid if it has a non-zero size, or if the user option
     ‘use-empty-active-region’ is non-‘nil’ (by default, it is ‘nil’).
     The function ‘region-active-p’ is similar to ‘use-region-p’, but
     considers all regions as valid.  In most cases, you should not use
     ‘region-active-p’, since if the region is empty it is often more
     appropriate to operate on point.


File: elisp.info,  Node: Text,  Next: Non-ASCII Characters,  Prev: Markers,  Up: Top

31 Text
*******

This chapter describes the functions that deal with the text in a
buffer.  Most examine, insert, or delete text in the current buffer,
often operating at point or on text adjacent to point.  Many are
interactive.  All the functions that change the text provide for undoing
the changes (*note Undo::).

   Many text-related functions operate on a region of text defined by
two buffer positions passed in arguments named START and END.  These
arguments should be either markers (*note Markers::) or numeric
character positions (*note Positions::).  The order of these arguments
does not matter; it is all right for START to be the end of the region
and END the beginning.  For example, ‘(delete-region 1 10)’ and
‘(delete-region 10 1)’ are equivalent.  An ‘args-out-of-range’ error is
signaled if either START or END is outside the accessible portion of the
buffer.  In an interactive call, point and the mark are used for these
arguments.

   Throughout this chapter, “text” refers to the characters in the
buffer, together with their properties (when relevant).  Keep in mind
that point is always between two characters, and the cursor appears on
the character after point.

* Menu:

* Near Point::       Examining text in the vicinity of point.
* Buffer Contents::  Examining text in a general fashion.
* Comparing Text::   Comparing substrings of buffers.
* Insertion::        Adding new text to a buffer.
* Commands for Insertion::  User-level commands to insert text.
* Deletion::         Removing text from a buffer.
* User-Level Deletion::     User-level commands to delete text.
* The Kill Ring::    Where removed text sometimes is saved for later use.
* Undo::             Undoing changes to the text of a buffer.
* Maintaining Undo:: How to enable and disable undo information.
                        How to control how much information is kept.
* Filling::          Functions for explicit filling.
* Margins::          How to specify margins for filling commands.
* Adaptive Fill::    Adaptive Fill mode chooses a fill prefix from context.
* Auto Filling::     How auto-fill mode is implemented to break lines.
* Sorting::          Functions for sorting parts of the buffer.
* Columns::          Computing horizontal positions, and using them.
* Indentation::      Functions to insert or adjust indentation.
* Case Changes::     Case conversion of parts of the buffer.
* Text Properties::  Assigning Lisp property lists to text characters.
* Substitution::     Replacing a given character wherever it appears.
* Registers::        How registers are implemented.  Accessing the text or
                       position stored in a register.
* Transposition::    Swapping two portions of a buffer.
* Decompression::    Dealing with compressed data.
* Base 64::          Conversion to or from base 64 encoding.
* Checksum/Hash::    Computing cryptographic hashes.
* Parsing HTML/XML:: Parsing HTML and XML.
* Atomic Changes::   Installing several buffer changes atomically.
* Change Hooks::     Supplying functions to be run when text is changed.


File: elisp.info,  Node: Near Point,  Next: Buffer Contents,  Up: Text

31.1 Examining Text Near Point
==============================

Many functions are provided to look at the characters around point.
Several simple functions are described here.  See also ‘looking-at’ in
*note Regexp Search::.

   In the following four functions, “beginning” or “end” of buffer
refers to the beginning or end of the accessible portion.

 -- Function: char-after &optional position
     This function returns the character in the current buffer at (i.e.,
     immediately after) position POSITION.  If POSITION is out of range
     for this purpose, either before the beginning of the buffer, or at
     or beyond the end, then the value is ‘nil’.  The default for
     POSITION is point.

     In the following example, assume that the first character in the
     buffer is ‘@’:

          (string (char-after 1))
               ⇒ "@"

 -- Function: char-before &optional position
     This function returns the character in the current buffer
     immediately before position POSITION.  If POSITION is out of range
     for this purpose, either at or before the beginning of the buffer,
     or beyond the end, then the value is ‘nil’.  The default for
     POSITION is point.

 -- Function: following-char
     This function returns the character following point in the current
     buffer.  This is similar to ‘(char-after (point))’.  However, if
     point is at the end of the buffer, then ‘following-char’ returns 0.

     Remember that point is always between characters, and the cursor
     normally appears over the character following point.  Therefore,
     the character returned by ‘following-char’ is the character the
     cursor is over.

     In this example, point is between the ‘a’ and the ‘c’.

          ---------- Buffer: foo ----------
          Gentlemen may cry ``Pea★ce! Peace!,''
          but there is no peace.
          ---------- Buffer: foo ----------

          (string (preceding-char))
               ⇒ "a"
          (string (following-char))
               ⇒ "c"

 -- Function: preceding-char
     This function returns the character preceding point in the current
     buffer.  See above, under ‘following-char’, for an example.  If
     point is at the beginning of the buffer, ‘preceding-char’ returns
     0.

 -- Function: bobp
     This function returns ‘t’ if point is at the beginning of the
     buffer.  If narrowing is in effect, this means the beginning of the
     accessible portion of the text.  See also ‘point-min’ in *note
     Point::.

 -- Function: eobp
     This function returns ‘t’ if point is at the end of the buffer.  If
     narrowing is in effect, this means the end of accessible portion of
     the text.  See also ‘point-max’ in *Note Point::.

 -- Function: bolp
     This function returns ‘t’ if point is at the beginning of a line.
     *Note Text Lines::.  The beginning of the buffer (or of its
     accessible portion) always counts as the beginning of a line.

 -- Function: eolp
     This function returns ‘t’ if point is at the end of a line.  The
     end of the buffer (or of its accessible portion) is always
     considered the end of a line.


File: elisp.info,  Node: Buffer Contents,  Next: Comparing Text,  Prev: Near Point,  Up: Text

31.2 Examining Buffer Contents
==============================

This section describes functions that allow a Lisp program to convert
any portion of the text in the buffer into a string.

 -- Function: buffer-substring start end
     This function returns a string containing a copy of the text of the
     region defined by positions START and END in the current buffer.
     If the arguments are not positions in the accessible portion of the
     buffer, ‘buffer-substring’ signals an ‘args-out-of-range’ error.

     Here’s an example which assumes Font-Lock mode is not enabled:

          ---------- Buffer: foo ----------
          This is the contents of buffer foo

          ---------- Buffer: foo ----------

          (buffer-substring 1 10)
               ⇒ "This is t"
          (buffer-substring (point-max) 10)
               ⇒ "he contents of buffer foo\n"

     If the text being copied has any text properties, these are copied
     into the string along with the characters they belong to.  *Note
     Text Properties::.  However, overlays (*note Overlays::) in the
     buffer and their properties are ignored, not copied.

     For example, if Font-Lock mode is enabled, you might get results
     like these:

          (buffer-substring 1 10)
               ⇒ #("This is t" 0 1 (fontified t) 1 9 (fontified t))

 -- Function: buffer-substring-no-properties start end
     This is like ‘buffer-substring’, except that it does not copy text
     properties, just the characters themselves.  *Note Text
     Properties::.

 -- Function: buffer-string
     This function returns the contents of the entire accessible portion
     of the current buffer, as a string.

   If you need to make sure the resulting string, when copied to a
different location, will not change its visual appearance due to
reordering of bidirectional text, use the
‘buffer-substring-with-bidi-context’ function (*note
buffer-substring-with-bidi-context: Bidirectional Display.).

 -- Function: filter-buffer-substring start end &optional delete
     This function filters the buffer text between START and END using a
     function specified by the variable
     ‘filter-buffer-substring-function’, and returns the result.

     The default filter function consults the obsolete wrapper hook
     ‘filter-buffer-substring-functions’ (see the documentation string
     of the macro ‘with-wrapper-hook’ for the details about this
     obsolete facility), and the obsolete variable
     ‘buffer-substring-filters’.  If both of these are ‘nil’, it returns
     the unaltered text from the buffer, i.e., what ‘buffer-substring’
     would return.

     If DELETE is non-‘nil’, the function deletes the text between START
     and END after copying it, like ‘delete-and-extract-region’.

     Lisp code should use this function instead of ‘buffer-substring’,
     ‘buffer-substring-no-properties’, or ‘delete-and-extract-region’
     when copying into user-accessible data structures such as the
     kill-ring, X clipboard, and registers.  Major and minor modes can
     modify ‘filter-buffer-substring-function’ to alter such text as it
     is copied out of the buffer.

 -- Variable: filter-buffer-substring-function
     The value of this variable is a function that
     ‘filter-buffer-substring’ will call to do the actual work.  The
     function receives three arguments, the same as those of
     ‘filter-buffer-substring’, which it should treat as per the
     documentation of that function.  It should return the filtered text
     (and optionally delete the source text).

The following two variables are obsoleted by
‘filter-buffer-substring-function’, but are still supported for backward
compatibility.

 -- Variable: filter-buffer-substring-functions
     This obsolete variable is a wrapper hook, whose members should be
     functions that accept four arguments: FUN, START, END, and DELETE.
     FUN is a function that takes three arguments (START, END, and
     DELETE), and returns a string.  In both cases, the START, END, and
     DELETE arguments are the same as those of
     ‘filter-buffer-substring’.

     The first hook function is passed a FUN that is equivalent to the
     default operation of ‘filter-buffer-substring’, i.e., it returns
     the buffer-substring between START and END (processed by any
     ‘buffer-substring-filters’) and optionally deletes the original
     text from the buffer.  In most cases, the hook function will call
     FUN once, and then do its own processing of the result.  The next
     hook function receives a FUN equivalent to this, and so on.  The
     actual return value is the result of all the hook functions acting
     in sequence.

 -- Variable: buffer-substring-filters
     The value of this obsolete variable should be a list of functions
     that accept a single string argument and return another string.
     The default ‘filter-buffer-substring’ function passes the buffer
     substring to the first function in this list, and the return value
     of each function is passed to the next function.  The return value
     of the last function is passed to
     ‘filter-buffer-substring-functions’.

 -- Function: current-word &optional strict really-word
     This function returns the symbol (or word) at or near point, as a
     string.  The return value includes no text properties.

     If the optional argument REALLY-WORD is non-‘nil’, it finds a word;
     otherwise, it finds a symbol (which includes both word characters
     and symbol constituent characters).

     If the optional argument STRICT is non-‘nil’, then point must be in
     or next to the symbol or word—if no symbol or word is there, the
     function returns ‘nil’.  Otherwise, a nearby symbol or word on the
     same line is acceptable.

 -- Function: thing-at-point thing &optional no-properties
     Return the THING around or next to point, as a string.

     The argument THING is a symbol which specifies a kind of syntactic
     entity.  Possibilities include ‘symbol’, ‘list’, ‘sexp’, ‘defun’,
     ‘filename’, ‘url’, ‘word’, ‘sentence’, ‘whitespace’, ‘line’,
     ‘page’, and others.

     When the optional argument NO-PROPERTIES is non-‘nil’, this
     function strips text properties from the return value.

          ---------- Buffer: foo ----------
          Gentlemen may cry ``Pea★ce! Peace!,''
          but there is no peace.
          ---------- Buffer: foo ----------

          (thing-at-point 'word)
               ⇒ "Peace"
          (thing-at-point 'line)
               ⇒ "Gentlemen may cry ``Peace! Peace!,''\n"
          (thing-at-point 'whitespace)
               ⇒ nil


File: elisp.info,  Node: Comparing Text,  Next: Insertion,  Prev: Buffer Contents,  Up: Text

31.3 Comparing Text
===================

This function lets you compare portions of the text in a buffer, without
copying them into strings first.

 -- Function: compare-buffer-substrings buffer1 start1 end1 buffer2
          start2 end2
     This function lets you compare two substrings of the same buffer or
     two different buffers.  The first three arguments specify one
     substring, giving a buffer (or a buffer name) and two positions
     within the buffer.  The last three arguments specify the other
     substring in the same way.  You can use ‘nil’ for BUFFER1, BUFFER2,
     or both to stand for the current buffer.

     The value is negative if the first substring is less, positive if
     the first is greater, and zero if they are equal.  The absolute
     value of the result is one plus the index of the first differing
     characters within the substrings.

     This function ignores case when comparing characters if
     ‘case-fold-search’ is non-‘nil’.  It always ignores text
     properties.

     Suppose you have the text ‘foobarbar haha!rara!’ in the current
     buffer; then in this example the two substrings are ‘rbar ’ and
     ‘rara!’.  The value is 2 because the first substring is greater at
     the second character.

          (compare-buffer-substrings nil 6 11 nil 16 21)
               ⇒ 2


File: elisp.info,  Node: Insertion,  Next: Commands for Insertion,  Prev: Comparing Text,  Up: Text

31.4 Inserting Text
===================

“Insertion” means adding new text to a buffer.  The inserted text goes
at point—between the character before point and the character after
point.  Some insertion functions leave point before the inserted text,
while other functions leave it after.  We call the former insertion
“after point” and the latter insertion “before point”.

   Insertion moves markers located at positions after the insertion
point, so that they stay with the surrounding text (*note Markers::).
When a marker points at the place of insertion, insertion may or may not
relocate the marker, depending on the marker’s insertion type (*note
Marker Insertion Types::).  Certain special functions such as
‘insert-before-markers’ relocate all such markers to point after the
inserted text, regardless of the markers’ insertion type.

   Insertion functions signal an error if the current buffer is
read-only (*note Read Only Buffers::) or if they insert within read-only
text (*note Special Properties::).

   These functions copy text characters from strings and buffers along
with their properties.  The inserted characters have exactly the same
properties as the characters they were copied from.  By contrast,
characters specified as separate arguments, not part of a string or
buffer, inherit their text properties from the neighboring text.

   The insertion functions convert text from unibyte to multibyte in
order to insert in a multibyte buffer, and vice versa—if the text comes
from a string or from a buffer.  However, they do not convert unibyte
character codes 128 through 255 to multibyte characters, not even if the
current buffer is a multibyte buffer.  *Note Converting
Representations::.

 -- Function: insert &rest args
     This function inserts the strings and/or characters ARGS into the
     current buffer, at point, moving point forward.  In other words, it
     inserts the text before point.  An error is signaled unless all
     ARGS are either strings or characters.  The value is ‘nil’.

 -- Function: insert-before-markers &rest args
     This function inserts the strings and/or characters ARGS into the
     current buffer, at point, moving point forward.  An error is
     signaled unless all ARGS are either strings or characters.  The
     value is ‘nil’.

     This function is unlike the other insertion functions in that it
     relocates markers initially pointing at the insertion point, to
     point after the inserted text.  If an overlay begins at the
     insertion point, the inserted text falls outside the overlay; if a
     nonempty overlay ends at the insertion point, the inserted text
     falls inside that overlay.

 -- Command: insert-char character &optional count inherit
     This command inserts COUNT instances of CHARACTER into the current
     buffer before point.  The argument COUNT must be an integer, and
     CHARACTER must be a character.

     If called interactively, this command prompts for CHARACTER using
     its Unicode name or its code point.  *Note (emacs)Inserting Text::.

     This function does not convert unibyte character codes 128 through
     255 to multibyte characters, not even if the current buffer is a
     multibyte buffer.  *Note Converting Representations::.

     If INHERIT is non-‘nil’, the inserted characters inherit sticky
     text properties from the two characters before and after the
     insertion point.  *Note Sticky Properties::.

 -- Function: insert-buffer-substring from-buffer-or-name &optional
          start end
     This function inserts a portion of buffer FROM-BUFFER-OR-NAME into
     the current buffer before point.  The text inserted is the region
     between START (inclusive) and END (exclusive).  (These arguments
     default to the beginning and end of the accessible portion of that
     buffer.)  This function returns ‘nil’.

     In this example, the form is executed with buffer ‘bar’ as the
     current buffer.  We assume that buffer ‘bar’ is initially empty.

          ---------- Buffer: foo ----------
          We hold these truths to be self-evident, that all
          ---------- Buffer: foo ----------

          (insert-buffer-substring "foo" 1 20)
               ⇒ nil

          ---------- Buffer: bar ----------
          We hold these truth★
          ---------- Buffer: bar ----------

 -- Function: insert-buffer-substring-no-properties from-buffer-or-name
          &optional start end
     This is like ‘insert-buffer-substring’ except that it does not copy
     any text properties.

   *Note Sticky Properties::, for other insertion functions that inherit
text properties from the nearby text in addition to inserting it.
Whitespace inserted by indentation functions also inherits text
properties.


File: elisp.info,  Node: Commands for Insertion,  Next: Deletion,  Prev: Insertion,  Up: Text

31.5 User-Level Insertion Commands
==================================

This section describes higher-level commands for inserting text,
commands intended primarily for the user but useful also in Lisp
programs.

 -- Command: insert-buffer from-buffer-or-name
     This command inserts the entire accessible contents of
     FROM-BUFFER-OR-NAME (which must exist) into the current buffer
     after point.  It leaves the mark after the inserted text.  The
     value is ‘nil’.

 -- Command: self-insert-command count
     This command inserts the last character typed; it does so COUNT
     times, before point, and returns ‘nil’.  Most printing characters
     are bound to this command.  In routine use, ‘self-insert-command’
     is the most frequently called function in Emacs, but programs
     rarely use it except to install it on a keymap.

     In an interactive call, COUNT is the numeric prefix argument.

     Self-insertion translates the input character through
     ‘translation-table-for-input’.  *Note Translation of Characters::.

     This command calls ‘auto-fill-function’ whenever that is non-‘nil’
     and the character inserted is in the table ‘auto-fill-chars’ (*note
     Auto Filling::).

     This command performs abbrev expansion if Abbrev mode is enabled
     and the inserted character does not have word-constituent syntax.
     (*Note Abbrevs::, and *note Syntax Class Table::.)  It is also
     responsible for calling ‘blink-paren-function’ when the inserted
     character has close parenthesis syntax (*note Blinking::).

     The final thing this command does is to run the hook
     ‘post-self-insert-hook’.  You could use this to automatically
     reindent text as it is typed, for example.

     Do not try substituting your own definition of
     ‘self-insert-command’ for the standard one.  The editor command
     loop handles this function specially.

 -- Command: newline &optional number-of-newlines
     This command inserts newlines into the current buffer before point.
     If NUMBER-OF-NEWLINES is supplied, that many newline characters are
     inserted.

     This function calls ‘auto-fill-function’ if the current column
     number is greater than the value of ‘fill-column’ and
     NUMBER-OF-NEWLINES is ‘nil’.  Typically what ‘auto-fill-function’
     does is insert a newline; thus, the overall result in this case is
     to insert two newlines at different places: one at point, and
     another earlier in the line.  ‘newline’ does not auto-fill if
     NUMBER-OF-NEWLINES is non-‘nil’.

     This command indents to the left margin if that is not zero.  *Note
     Margins::.

     The value returned is ‘nil’.  In an interactive call, COUNT is the
     numeric prefix argument.

 -- Variable: overwrite-mode
     This variable controls whether overwrite mode is in effect.  The
     value should be ‘overwrite-mode-textual’, ‘overwrite-mode-binary’,
     or ‘nil’.  ‘overwrite-mode-textual’ specifies textual overwrite
     mode (treats newlines and tabs specially), and
     ‘overwrite-mode-binary’ specifies binary overwrite mode (treats
     newlines and tabs like any other characters).


File: elisp.info,  Node: Deletion,  Next: User-Level Deletion,  Prev: Commands for Insertion,  Up: Text

31.6 Deleting Text
==================

Deletion means removing part of the text in a buffer, without saving it
in the kill ring (*note The Kill Ring::).  Deleted text can’t be yanked,
but can be reinserted using the undo mechanism (*note Undo::).  Some
deletion functions do save text in the kill ring in some special cases.

   All of the deletion functions operate on the current buffer.

 -- Command: erase-buffer
     This function deletes the entire text of the current buffer (_not_
     just the accessible portion), leaving it empty.  If the buffer is
     read-only, it signals a ‘buffer-read-only’ error; if some of the
     text in it is read-only, it signals a ‘text-read-only’ error.
     Otherwise, it deletes the text without asking for any confirmation.
     It returns ‘nil’.

     Normally, deleting a large amount of text from a buffer inhibits
     further auto-saving of that buffer because it has shrunk.  However,
     ‘erase-buffer’ does not do this, the idea being that the future
     text is not really related to the former text, and its size should
     not be compared with that of the former text.

 -- Command: delete-region start end
     This command deletes the text between positions START and END in
     the current buffer, and returns ‘nil’.  If point was inside the
     deleted region, its value afterward is START.  Otherwise, point
     relocates with the surrounding text, as markers do.

 -- Function: delete-and-extract-region start end
     This function deletes the text between positions START and END in
     the current buffer, and returns a string containing the text just
     deleted.

     If point was inside the deleted region, its value afterward is
     START.  Otherwise, point relocates with the surrounding text, as
     markers do.

 -- Command: delete-char count &optional killp
     This command deletes COUNT characters directly after point, or
     before point if COUNT is negative.  If KILLP is non-‘nil’, then it
     saves the deleted characters in the kill ring.

     In an interactive call, COUNT is the numeric prefix argument, and
     KILLP is the unprocessed prefix argument.  Therefore, if a prefix
     argument is supplied, the text is saved in the kill ring.  If no
     prefix argument is supplied, then one character is deleted, but not
     saved in the kill ring.

     The value returned is always ‘nil’.

 -- Command: delete-backward-char count &optional killp
     This command deletes COUNT characters directly before point, or
     after point if COUNT is negative.  If KILLP is non-‘nil’, then it
     saves the deleted characters in the kill ring.

     In an interactive call, COUNT is the numeric prefix argument, and
     KILLP is the unprocessed prefix argument.  Therefore, if a prefix
     argument is supplied, the text is saved in the kill ring.  If no
     prefix argument is supplied, then one character is deleted, but not
     saved in the kill ring.

     The value returned is always ‘nil’.

 -- Command: backward-delete-char-untabify count &optional killp
     This command deletes COUNT characters backward, changing tabs into
     spaces.  When the next character to be deleted is a tab, it is
     first replaced with the proper number of spaces to preserve
     alignment and then one of those spaces is deleted instead of the
     tab.  If KILLP is non-‘nil’, then the command saves the deleted
     characters in the kill ring.

     Conversion of tabs to spaces happens only if COUNT is positive.  If
     it is negative, exactly −COUNT characters after point are deleted.

     In an interactive call, COUNT is the numeric prefix argument, and
     KILLP is the unprocessed prefix argument.  Therefore, if a prefix
     argument is supplied, the text is saved in the kill ring.  If no
     prefix argument is supplied, then one character is deleted, but not
     saved in the kill ring.

     The value returned is always ‘nil’.

 -- User Option: backward-delete-char-untabify-method
     This option specifies how ‘backward-delete-char-untabify’ should
     deal with whitespace.  Possible values include ‘untabify’, the
     default, meaning convert a tab to many spaces and delete one;
     ‘hungry’, meaning delete all tabs and spaces before point with one
     command; ‘all’ meaning delete all tabs, spaces and newlines before
     point, and ‘nil’, meaning do nothing special for whitespace
     characters.


File: elisp.info,  Node: User-Level Deletion,  Next: The Kill Ring,  Prev: Deletion,  Up: Text

31.7 User-Level Deletion Commands
=================================

This section describes higher-level commands for deleting text, commands
intended primarily for the user but useful also in Lisp programs.

 -- Command: delete-horizontal-space &optional backward-only
     This function deletes all spaces and tabs around point.  It returns
     ‘nil’.

     If BACKWARD-ONLY is non-‘nil’, the function deletes spaces and tabs
     before point, but not after point.

     In the following examples, we call ‘delete-horizontal-space’ four
     times, once on each line, with point between the second and third
     characters on the line each time.

          ---------- Buffer: foo ----------
          I ★thought
          I ★     thought
          We★ thought
          Yo★u thought
          ---------- Buffer: foo ----------

          (delete-horizontal-space)   ; Four times.
               ⇒ nil

          ---------- Buffer: foo ----------
          Ithought
          Ithought
          Wethought
          You thought
          ---------- Buffer: foo ----------

 -- Command: delete-indentation &optional join-following-p
     This function joins the line point is on to the previous line,
     deleting any whitespace at the join and in some cases replacing it
     with one space.  If JOIN-FOLLOWING-P is non-‘nil’,
     ‘delete-indentation’ joins this line to the following line instead.
     The function returns ‘nil’.

     If there is a fill prefix, and the second of the lines being joined
     starts with the prefix, then ‘delete-indentation’ deletes the fill
     prefix before joining the lines.  *Note Margins::.

     In the example below, point is located on the line starting
     ‘events’, and it makes no difference if there are trailing spaces
     in the preceding line.

          ---------- Buffer: foo ----------
          When in the course of human
          ★    events, it becomes necessary
          ---------- Buffer: foo ----------

          (delete-indentation)
               ⇒ nil

          ---------- Buffer: foo ----------
          When in the course of human★ events, it becomes necessary
          ---------- Buffer: foo ----------

     After the lines are joined, the function ‘fixup-whitespace’ is
     responsible for deciding whether to leave a space at the junction.

 -- Command: fixup-whitespace
     This function replaces all the horizontal whitespace surrounding
     point with either one space or no space, according to the context.
     It returns ‘nil’.

     At the beginning or end of a line, the appropriate amount of space
     is none.  Before a character with close parenthesis syntax, or
     after a character with open parenthesis or expression-prefix
     syntax, no space is also appropriate.  Otherwise, one space is
     appropriate.  *Note Syntax Class Table::.

     In the example below, ‘fixup-whitespace’ is called the first time
     with point before the word ‘spaces’ in the first line.  For the
     second invocation, point is directly after the ‘(’.

          ---------- Buffer: foo ----------
          This has too many     ★spaces
          This has too many spaces at the start of (★   this list)
          ---------- Buffer: foo ----------

          (fixup-whitespace)
               ⇒ nil
          (fixup-whitespace)
               ⇒ nil

          ---------- Buffer: foo ----------
          This has too many spaces
          This has too many spaces at the start of (this list)
          ---------- Buffer: foo ----------

 -- Command: just-one-space &optional n
     This command replaces any spaces and tabs around point with a
     single space, or N spaces if N is specified.  It returns ‘nil’.

 -- Command: delete-blank-lines
     This function deletes blank lines surrounding point.  If point is
     on a blank line with one or more blank lines before or after it,
     then all but one of them are deleted.  If point is on an isolated
     blank line, then it is deleted.  If point is on a nonblank line,
     the command deletes all blank lines immediately following it.

     A blank line is defined as a line containing only tabs and spaces.

     ‘delete-blank-lines’ returns ‘nil’.

 -- Command: delete-trailing-whitespace &optional start end
     Delete trailing whitespace in the region defined by START and END.

     This command deletes whitespace characters after the last
     non-whitespace character in each line in the region.

     If this command acts on the entire buffer (i.e., if called
     interactively with the mark inactive, or called from Lisp with END
     ‘nil’), it also deletes all trailing lines at the end of the buffer
     if the variable ‘delete-trailing-lines’ is non-‘nil’.


File: elisp.info,  Node: The Kill Ring,  Next: Undo,  Prev: User-Level Deletion,  Up: Text

31.8 The Kill Ring
==================

“Kill functions” delete text like the deletion functions, but save it so
that the user can reinsert it by “yanking”.  Most of these functions
have ‘kill-’ in their name.  By contrast, the functions whose names
start with ‘delete-’ normally do not save text for yanking (though they
can still be undone); these are deletion functions.

   Most of the kill commands are primarily for interactive use, and are
not described here.  What we do describe are the functions provided for
use in writing such commands.  You can use these functions to write
commands for killing text.  When you need to delete text for internal
purposes within a Lisp function, you should normally use deletion
functions, so as not to disturb the kill ring contents.  *Note
Deletion::.

   Killed text is saved for later yanking in the “kill ring”.  This is a
list that holds a number of recent kills, not just the last text kill.
We call this a “ring” because yanking treats it as having elements in a
cyclic order.  The list is kept in the variable ‘kill-ring’, and can be
operated on with the usual functions for lists; there are also
specialized functions, described in this section, that treat it as a
ring.

   Some people think this use of the word “kill” is unfortunate, since
it refers to operations that specifically _do not_ destroy the entities
killed.  This is in sharp contrast to ordinary life, in which death is
permanent and killed entities do not come back to life.  Therefore,
other metaphors have been proposed.  For example, the term “cut ring”
makes sense to people who, in pre-computer days, used scissors and paste
to cut up and rearrange manuscripts.  However, it would be difficult to
change the terminology now.

* Menu:

* Kill Ring Concepts::     What text looks like in the kill ring.
* Kill Functions::         Functions that kill text.
* Yanking::                How yanking is done.
* Yank Commands::          Commands that access the kill ring.
* Low-Level Kill Ring::    Functions and variables for kill ring access.
* Internals of Kill Ring:: Variables that hold kill ring data.


File: elisp.info,  Node: Kill Ring Concepts,  Next: Kill Functions,  Up: The Kill Ring

31.8.1 Kill Ring Concepts
-------------------------

The kill ring records killed text as strings in a list, most recent
first.  A short kill ring, for example, might look like this:

     ("some text" "a different piece of text" "even older text")

When the list reaches ‘kill-ring-max’ entries in length, adding a new
entry automatically deletes the last entry.

   When kill commands are interwoven with other commands, each kill
command makes a new entry in the kill ring.  Multiple kill commands in
succession build up a single kill ring entry, which would be yanked as a
unit; the second and subsequent consecutive kill commands add text to
the entry made by the first one.

   For yanking, one entry in the kill ring is designated the front of
the ring.  Some yank commands rotate the ring by designating a different
element as the front.  But this virtual rotation doesn’t change the list
itself—the most recent entry always comes first in the list.


File: elisp.info,  Node: Kill Functions,  Next: Yanking,  Prev: Kill Ring Concepts,  Up: The Kill Ring

31.8.2 Functions for Killing
----------------------------

‘kill-region’ is the usual subroutine for killing text.  Any command
that calls this function is a kill command (and should probably have
‘kill’ in its name).  ‘kill-region’ puts the newly killed text in a new
element at the beginning of the kill ring or adds it to the most recent
element.  It determines automatically (using ‘last-command’) whether the
previous command was a kill command, and if so appends the killed text
to the most recent entry.

   The commands described below can filter the killed text before they
save it in the kill ring.  They call ‘filter-buffer-substring’ (*note
Buffer Contents::) to perform the filtering.  By default, there’s no
filtering, but major and minor modes and hook functions can set up
filtering, so that text saved in the kill ring is different from what
was in the buffer.

 -- Command: kill-region start end &optional region
     This function kills the stretch of text between START and END; but
     if the optional argument REGION is non-‘nil’, it ignores START and
     END, and kills the text in the current region instead.  The text is
     deleted but saved in the kill ring, along with its text properties.
     The value is always ‘nil’.

     In an interactive call, START and END are point and the mark, and
     REGION is always non-‘nil’, so the command always kills the text in
     the current region.

     If the buffer or text is read-only, ‘kill-region’ modifies the kill
     ring just the same, then signals an error without modifying the
     buffer.  This is convenient because it lets the user use a series
     of kill commands to copy text from a read-only buffer into the kill
     ring.

 -- User Option: kill-read-only-ok
     If this option is non-‘nil’, ‘kill-region’ does not signal an error
     if the buffer or text is read-only.  Instead, it simply returns,
     updating the kill ring but not changing the buffer.

 -- Command: copy-region-as-kill start end &optional region
     This function saves the stretch of text between START and END on
     the kill ring (including text properties), but does not delete the
     text from the buffer.  However, if the optional argument REGION is
     non-‘nil’, the function ignores START and END, and saves the
     current region instead.  It always returns ‘nil’.

     In an interactive call, START and END are point and the mark, and
     REGION is always non-‘nil’, so the command always saves the text in
     the current region.

     The command does not set ‘this-command’ to ‘kill-region’, so a
     subsequent kill command does not append to the same kill ring
     entry.


File: elisp.info,  Node: Yanking,  Next: Yank Commands,  Prev: Kill Functions,  Up: The Kill Ring

31.8.3 Yanking
--------------

Yanking means inserting text from the kill ring, but it does not insert
the text blindly.  The ‘yank’ command, and related commands, use
‘insert-for-yank’ to perform special processing on the text before it is
inserted.

 -- Function: insert-for-yank string
     This function works like ‘insert’, except that it processes the
     text in STRING according to the ‘yank-handler’ text property, as
     well as the variables ‘yank-handled-properties’ and
     ‘yank-excluded-properties’ (see below), before inserting the result
     into the current buffer.

 -- Function: insert-buffer-substring-as-yank buf &optional start end
     This function resembles ‘insert-buffer-substring’, except that it
     processes the text according to ‘yank-handled-properties’ and
     ‘yank-excluded-properties’.  (It does not handle the ‘yank-handler’
     property, which does not normally occur in buffer text anyway.)

   If you put a ‘yank-handler’ text property on all or part of a string,
that alters how ‘insert-for-yank’ inserts the string.  If different
parts of the string have different ‘yank-handler’ values (comparison
being done with ‘eq’), each substring is handled separately.  The
property value must be a list of one to four elements, with the
following format (where elements after the first may be omitted):

     (FUNCTION PARAM NOEXCLUDE UNDO)

   Here is what the elements do:

FUNCTION
     When FUNCTION is non-‘nil’, it is called instead of ‘insert’ to
     insert the string, with one argument—the string to insert.

PARAM
     If PARAM is present and non-‘nil’, it replaces STRING (or the
     substring of STRING being processed) as the object passed to
     FUNCTION (or ‘insert’).  For example, if FUNCTION is
     ‘yank-rectangle’, PARAM should be a list of strings to insert as a
     rectangle.

NOEXCLUDE
     If NOEXCLUDE is present and non-‘nil’, that disables the normal
     action of ‘yank-handled-properties’ and ‘yank-excluded-properties’
     on the inserted string.

UNDO
     If UNDO is present and non-‘nil’, it is a function that will be
     called by ‘yank-pop’ to undo the insertion of the current object.
     It is called with two arguments, the start and end of the current
     region.  FUNCTION can set ‘yank-undo-function’ to override the UNDO
     value.

 -- User Option: yank-handled-properties
     This variable specifies special text property handling conditions
     for yanked text.  It takes effect after the text has been inserted
     (either normally, or via the ‘yank-handler’ property), and prior to
     ‘yank-excluded-properties’ taking effect.

     The value should be an alist of elements ‘(PROP . FUN)’.  Each
     alist element is handled in order.  The inserted text is scanned
     for stretches of text having text properties ‘eq’ to PROP; for each
     such stretch, FUN is called with three arguments: the value of the
     property, and the start and end positions of the text.

 -- User Option: yank-excluded-properties
     The value of this variable is the list of properties to remove from
     inserted text.  Its default value contains properties that might
     lead to annoying results, such as causing the text to respond to
     the mouse or specifying key bindings.  It takes effect after
     ‘yank-handled-properties’.


File: elisp.info,  Node: Yank Commands,  Next: Low-Level Kill Ring,  Prev: Yanking,  Up: The Kill Ring

31.8.4 Functions for Yanking
----------------------------

This section describes higher-level commands for yanking, which are
intended primarily for the user but useful also in Lisp programs.  Both
‘yank’ and ‘yank-pop’ honor the ‘yank-excluded-properties’ variable and
‘yank-handler’ text property (*note Yanking::).

 -- Command: yank &optional arg
     This command inserts before point the text at the front of the kill
     ring.  It sets the mark at the beginning of that text, using
     ‘push-mark’ (*note The Mark::), and puts point at the end.

     If ARG is a non-‘nil’ list (which occurs interactively when the
     user types ‘C-u’ with no digits), then ‘yank’ inserts the text as
     described above, but puts point before the yanked text and sets the
     mark after it.

     If ARG is a number, then ‘yank’ inserts the ARGth most recently
     killed text—the ARGth element of the kill ring list, counted
     cyclically from the front, which is considered the first element
     for this purpose.

     ‘yank’ does not alter the contents of the kill ring, unless it used
     text provided by another program, in which case it pushes that text
     onto the kill ring.  However if ARG is an integer different from
     one, it rotates the kill ring to place the yanked string at the
     front.

     ‘yank’ returns ‘nil’.

 -- Command: yank-pop &optional arg
     This command replaces the just-yanked entry from the kill ring with
     a different entry from the kill ring.

     This is allowed only immediately after a ‘yank’ or another
     ‘yank-pop’.  At such a time, the region contains text that was just
     inserted by yanking.  ‘yank-pop’ deletes that text and inserts in
     its place a different piece of killed text.  It does not add the
     deleted text to the kill ring, since it is already in the kill ring
     somewhere.  It does however rotate the kill ring to place the newly
     yanked string at the front.

     If ARG is ‘nil’, then the replacement text is the previous element
     of the kill ring.  If ARG is numeric, the replacement is the ARGth
     previous kill.  If ARG is negative, a more recent kill is the
     replacement.

     The sequence of kills in the kill ring wraps around, so that after
     the oldest one comes the newest one, and before the newest one goes
     the oldest.

     The return value is always ‘nil’.

 -- Variable: yank-undo-function
     If this variable is non-‘nil’, the function ‘yank-pop’ uses its
     value instead of ‘delete-region’ to delete the text inserted by the
     previous ‘yank’ or ‘yank-pop’ command.  The value must be a
     function of two arguments, the start and end of the current region.

     The function ‘insert-for-yank’ automatically sets this variable
     according to the UNDO element of the ‘yank-handler’ text property,
     if there is one.


File: elisp.info,  Node: Low-Level Kill Ring,  Next: Internals of Kill Ring,  Prev: Yank Commands,  Up: The Kill Ring

31.8.5 Low-Level Kill Ring
--------------------------

These functions and variables provide access to the kill ring at a lower
level, but are still convenient for use in Lisp programs, because they
take care of interaction with window system selections (*note Window
System Selections::).

 -- Function: current-kill n &optional do-not-move
     The function ‘current-kill’ rotates the yanking pointer, which
     designates the front of the kill ring, by N places (from newer
     kills to older ones), and returns the text at that place in the
     ring.

     If the optional second argument DO-NOT-MOVE is non-‘nil’, then
     ‘current-kill’ doesn’t alter the yanking pointer; it just returns
     the Nth kill, counting from the current yanking pointer.

     If N is zero, indicating a request for the latest kill,
     ‘current-kill’ calls the value of ‘interprogram-paste-function’
     (documented below) before consulting the kill ring.  If that value
     is a function and calling it returns a string or a list of several
     string, ‘current-kill’ pushes the strings onto the kill ring and
     returns the first string.  It also sets the yanking pointer to
     point to the kill-ring entry of the first string returned by
     ‘interprogram-paste-function’, regardless of the value of
     DO-NOT-MOVE.  Otherwise, ‘current-kill’ does not treat a zero value
     for N specially: it returns the entry pointed at by the yanking
     pointer and does not move the yanking pointer.

 -- Function: kill-new string &optional replace
     This function pushes the text STRING onto the kill ring and makes
     the yanking pointer point to it.  It discards the oldest entry if
     appropriate.  It also invokes the value of
     ‘interprogram-cut-function’ (see below).

     If REPLACE is non-‘nil’, then ‘kill-new’ replaces the first element
     of the kill ring with STRING, rather than pushing STRING onto the
     kill ring.

 -- Function: kill-append string before-p
     This function appends the text STRING to the first entry in the
     kill ring and makes the yanking pointer point to the combined
     entry.  Normally STRING goes at the end of the entry, but if
     BEFORE-P is non-‘nil’, it goes at the beginning.  This function
     also invokes the value of ‘interprogram-cut-function’ (see below).

 -- Variable: interprogram-paste-function
     This variable provides a way of transferring killed text from other
     programs, when you are using a window system.  Its value should be
     ‘nil’ or a function of no arguments.

     If the value is a function, ‘current-kill’ calls it to get the most
     recent kill.  If the function returns a non-‘nil’ value, then that
     value is used as the most recent kill.  If it returns ‘nil’, then
     the front of the kill ring is used.

     To facilitate support for window systems that support multiple
     selections, this function may also return a list of strings.  In
     that case, the first string is used as the most recent kill, and
     all the other strings are pushed onto the kill ring, for easy
     access by ‘yank-pop’.

     The normal use of this function is to get the window system’s
     clipboard as the most recent kill, even if the selection belongs to
     another application.  *Note Window System Selections::.  However,
     if the clipboard contents come from the current Emacs session, this
     function should return ‘nil’.

 -- Variable: interprogram-cut-function
     This variable provides a way of communicating killed text to other
     programs, when you are using a window system.  Its value should be
     ‘nil’ or a function of one required argument.

     If the value is a function, ‘kill-new’ and ‘kill-append’ call it
     with the new first element of the kill ring as the argument.

     The normal use of this function is to put newly killed text in the
     window system’s clipboard.  *Note Window System Selections::.


File: elisp.info,  Node: Internals of Kill Ring,  Prev: Low-Level Kill Ring,  Up: The Kill Ring

31.8.6 Internals of the Kill Ring
---------------------------------

The variable ‘kill-ring’ holds the kill ring contents, in the form of a
list of strings.  The most recent kill is always at the front of the
list.

   The ‘kill-ring-yank-pointer’ variable points to a link in the kill
ring list, whose CAR is the text to yank next.  We say it identifies the
front of the ring.  Moving ‘kill-ring-yank-pointer’ to a different link
is called “rotating the kill ring”.  We call the kill ring a “ring”
because the functions that move the yank pointer wrap around from the
end of the list to the beginning, or vice-versa.  Rotation of the kill
ring is virtual; it does not change the value of ‘kill-ring’.

   Both ‘kill-ring’ and ‘kill-ring-yank-pointer’ are Lisp variables
whose values are normally lists.  The word “pointer” in the name of the
‘kill-ring-yank-pointer’ indicates that the variable’s purpose is to
identify one element of the list for use by the next yank command.

   The value of ‘kill-ring-yank-pointer’ is always ‘eq’ to one of the
links in the kill ring list.  The element it identifies is the CAR of
that link.  Kill commands, which change the kill ring, also set this
variable to the value of ‘kill-ring’.  The effect is to rotate the ring
so that the newly killed text is at the front.

   Here is a diagram that shows the variable ‘kill-ring-yank-pointer’
pointing to the second entry in the kill ring ‘("some text" "a different
piece of text" "yet older text")’.

     kill-ring                  ---- kill-ring-yank-pointer
       |                       |
       |                       v
       |     --- ---          --- ---      --- ---
        --> |   |   |------> |   |   |--> |   |   |--> nil
             --- ---          --- ---      --- ---
              |                |            |
              |                |            |
              |                |             -->"yet older text"
              |                |
              |                 --> "a different piece of text"
              |
               --> "some text"

This state of affairs might occur after ‘C-y’ (‘yank’) immediately
followed by ‘M-y’ (‘yank-pop’).

 -- Variable: kill-ring
     This variable holds the list of killed text sequences, most
     recently killed first.

 -- Variable: kill-ring-yank-pointer
     This variable’s value indicates which element of the kill ring is
     at the front of the ring for yanking.  More precisely, the value is
     a tail of the value of ‘kill-ring’, and its CAR is the kill string
     that ‘C-y’ should yank.

 -- User Option: kill-ring-max
     The value of this variable is the maximum length to which the kill
     ring can grow, before elements are thrown away at the end.  The
     default value for ‘kill-ring-max’ is 60.


File: elisp.info,  Node: Undo,  Next: Maintaining Undo,  Prev: The Kill Ring,  Up: Text

31.9 Undo
=========

Most buffers have an “undo list”, which records all changes made to the
buffer’s text so that they can be undone.  (The buffers that don’t have
one are usually special-purpose buffers for which Emacs assumes that
undoing is not useful.  In particular, any buffer whose name begins with
a space has its undo recording off by default; see *note Buffer
Names::.)  All the primitives that modify the text in the buffer
automatically add elements to the front of the undo list, which is in
the variable ‘buffer-undo-list’.

 -- Variable: buffer-undo-list
     This buffer-local variable’s value is the undo list of the current
     buffer.  A value of ‘t’ disables the recording of undo information.

   Here are the kinds of elements an undo list can have:

‘POSITION’
     This kind of element records a previous value of point; undoing
     this element moves point to POSITION.  Ordinary cursor motion does
     not make any sort of undo record, but deletion operations use these
     entries to record where point was before the command.

‘(BEG . END)’
     This kind of element indicates how to delete text that was
     inserted.  Upon insertion, the text occupied the range BEG–END in
     the buffer.

‘(TEXT . POSITION)’
     This kind of element indicates how to reinsert text that was
     deleted.  The deleted text itself is the string TEXT.  The place to
     reinsert it is ‘(abs POSITION)’.  If POSITION is positive, point
     was at the beginning of the deleted text, otherwise it was at the
     end.  Zero or more (MARKER .  ADJUSTMENT) elements follow
     immediately after this element.

‘(t . TIME-FLAG)’
     This kind of element indicates that an unmodified buffer became
     modified.  A TIME-FLAG of the form ‘(SEC-HIGH SEC-LOW MICROSEC
     PICOSEC)’ represents the visited file’s modification time as of
     when it was previously visited or saved, using the same format as
     ‘current-time’; see *note Time of Day::.  A TIME-FLAG of 0 means
     the buffer does not correspond to any file; −1 means the visited
     file previously did not exist.  ‘primitive-undo’ uses these values
     to determine whether to mark the buffer as unmodified once again;
     it does so only if the file’s status matches that of TIME-FLAG.

‘(nil PROPERTY VALUE BEG . END)’
     This kind of element records a change in a text property.  Here’s
     how you might undo the change:

          (put-text-property BEG END PROPERTY VALUE)

‘(MARKER . ADJUSTMENT)’
     This kind of element records the fact that the marker MARKER was
     relocated due to deletion of surrounding text, and that it moved
     ADJUSTMENT character positions.  If the marker’s location is
     consistent with the (TEXT .  POSITION) element preceding it in the
     undo list, then undoing this element moves MARKER − ADJUSTMENT
     characters.

‘(apply FUNNAME . ARGS)’
     This is an extensible undo item, which is undone by calling FUNNAME
     with arguments ARGS.

‘(apply DELTA BEG END FUNNAME . ARGS)’
     This is an extensible undo item, which records a change limited to
     the range BEG to END, which increased the size of the buffer by
     DELTA characters.  It is undone by calling FUNNAME with arguments
     ARGS.

     This kind of element enables undo limited to a region to determine
     whether the element pertains to that region.

‘nil’
     This element is a boundary.  The elements between two boundaries
     are called a “change group”; normally, each change group
     corresponds to one keyboard command, and undo commands normally
     undo an entire group as a unit.

 -- Function: undo-boundary
     This function places a boundary element in the undo list.  The undo
     command stops at such a boundary, and successive undo commands undo
     to earlier and earlier boundaries.  This function returns ‘nil’.

     Calling this function explicitly is useful for splitting the
     effects of a command into more than one unit.  For example,
     ‘query-replace’ calls ‘undo-boundary’ after each replacement, so
     that the user can undo individual replacements one by one.

     Mostly, however, this function is called automatically at an
     appropriate time.

 -- Function: undo-auto-amalgamate
     The editor command loop automatically calls ‘undo-boundary’ just
     before executing each key sequence, so that each undo normally
     undoes the effects of one command.  A few exceptional commands are
     “amalgamating”: these commands generally cause small changes to
     buffers, so with these a boundary is inserted only every 20th
     command, allowing the changes to be undone as a group.  By default,
     the commands ‘self-insert-command’, which produces self-inserting
     input characters (*note Commands for Insertion::), and
     ‘delete-char’, which deletes characters (*note Deletion::), are
     amalgamating.  Where a command affects the contents of several
     buffers, as may happen, for example, when a function on the
     ‘post-command-hook’ affects a buffer other than the
     ‘current-buffer’, then ‘undo-boundary’ will be called in each of
     the affected buffers.

 -- Variable: undo-auto-current-boundary-timer
     Some buffers, such as process buffers, can change even when no
     commands are executing.  In these cases, ‘undo-boundary’ is
     normally called periodically by the timer in this variable.
     Setting this variable to non-‘nil’ prevents this behavior.

 -- Variable: undo-in-progress
     This variable is normally ‘nil’, but the undo commands bind it to
     ‘t’.  This is so that various kinds of change hooks can tell when
     they’re being called for the sake of undoing.

 -- Function: primitive-undo count list
     This is the basic function for undoing elements of an undo list.
     It undoes the first COUNT elements of LIST, returning the rest of
     LIST.

     ‘primitive-undo’ adds elements to the buffer’s undo list when it
     changes the buffer.  Undo commands avoid confusion by saving the
     undo list value at the beginning of a sequence of undo operations.
     Then the undo operations use and update the saved value.  The new
     elements added by undoing are not part of this saved value, so they
     don’t interfere with continuing to undo.

     This function does not bind ‘undo-in-progress’.


File: elisp.info,  Node: Maintaining Undo,  Next: Filling,  Prev: Undo,  Up: Text

31.10 Maintaining Undo Lists
============================

This section describes how to enable and disable undo information for a
given buffer.  It also explains how the undo list is truncated
automatically so it doesn’t get too big.

   Recording of undo information in a newly created buffer is normally
enabled to start with; but if the buffer name starts with a space, the
undo recording is initially disabled.  You can explicitly enable or
disable undo recording with the following two functions, or by setting
‘buffer-undo-list’ yourself.

 -- Command: buffer-enable-undo &optional buffer-or-name
     This command enables recording undo information for buffer
     BUFFER-OR-NAME, so that subsequent changes can be undone.  If no
     argument is supplied, then the current buffer is used.  This
     function does nothing if undo recording is already enabled in the
     buffer.  It returns ‘nil’.

     In an interactive call, BUFFER-OR-NAME is the current buffer.  You
     cannot specify any other buffer.

 -- Command: buffer-disable-undo &optional buffer-or-name
     This function discards the undo list of BUFFER-OR-NAME, and
     disables further recording of undo information.  As a result, it is
     no longer possible to undo either previous changes or any
     subsequent changes.  If the undo list of BUFFER-OR-NAME is already
     disabled, this function has no effect.

     In an interactive call, BUFFER-OR-NAME is the current buffer.  You
     cannot specify any other buffer.  This function returns ‘nil’.

   As editing continues, undo lists get longer and longer.  To prevent
them from using up all available memory space, garbage collection trims
them back to size limits you can set.  (For this purpose, the size of an
undo list measures the cons cells that make up the list, plus the
strings of deleted text.)  Three variables control the range of
acceptable sizes: ‘undo-limit’, ‘undo-strong-limit’ and
‘undo-outer-limit’.  In these variables, size is counted as the number
of bytes occupied, which includes both saved text and other data.

 -- User Option: undo-limit
     This is the soft limit for the acceptable size of an undo list.
     The change group at which this size is exceeded is the last one
     kept.

 -- User Option: undo-strong-limit
     This is the upper limit for the acceptable size of an undo list.
     The change group at which this size is exceeded is discarded itself
     (along with all older change groups).  There is one exception: the
     very latest change group is only discarded if it exceeds
     ‘undo-outer-limit’.

 -- User Option: undo-outer-limit
     If at garbage collection time the undo info for the current command
     exceeds this limit, Emacs discards the info and displays a warning.
     This is a last ditch limit to prevent memory overflow.

 -- User Option: undo-ask-before-discard
     If this variable is non-‘nil’, when the undo info exceeds
     ‘undo-outer-limit’, Emacs asks in the echo area whether to discard
     the info.  The default value is ‘nil’, which means to discard it
     automatically.

     This option is mainly intended for debugging.  Garbage collection
     is inhibited while the question is asked, which means that Emacs
     might leak memory if the user waits too long before answering the
     question.


File: elisp.info,  Node: Filling,  Next: Margins,  Prev: Maintaining Undo,  Up: Text

31.11 Filling
=============

“Filling” means adjusting the lengths of lines (by moving the line
breaks) so that they are nearly (but no greater than) a specified
maximum width.  Additionally, lines can be “justified”, which means
inserting spaces to make the left and/or right margins line up
precisely.  The width is controlled by the variable ‘fill-column’.  For
ease of reading, lines should be no longer than 70 or so columns.

   You can use Auto Fill mode (*note Auto Filling::) to fill text
automatically as you insert it, but changes to existing text may leave
it improperly filled.  Then you must fill the text explicitly.

   Most of the commands in this section return values that are not
meaningful.  All the functions that do filling take note of the current
left margin, current right margin, and current justification style
(*note Margins::).  If the current justification style is ‘none’, the
filling functions don’t actually do anything.

   Several of the filling functions have an argument JUSTIFY.  If it is
non-‘nil’, that requests some kind of justification.  It can be ‘left’,
‘right’, ‘full’, or ‘center’, to request a specific style of
justification.  If it is ‘t’, that means to use the current
justification style for this part of the text (see
‘current-justification’, below).  Any other value is treated as ‘full’.

   When you call the filling functions interactively, using a prefix
argument implies the value ‘full’ for JUSTIFY.

 -- Command: fill-paragraph &optional justify region
     This command fills the paragraph at or after point.  If JUSTIFY is
     non-‘nil’, each line is justified as well.  It uses the ordinary
     paragraph motion commands to find paragraph boundaries.  *Note
     (emacs)Paragraphs::.

     When REGION is non-‘nil’, then if Transient Mark mode is enabled
     and the mark is active, this command calls ‘fill-region’ to fill
     all the paragraphs in the region, instead of filling only the
     current paragraph.  When this command is called interactively,
     REGION is ‘t’.

 -- Command: fill-region start end &optional justify nosqueeze to-eop
     This command fills each of the paragraphs in the region from START
     to END.  It justifies as well if JUSTIFY is non-‘nil’.

     If NOSQUEEZE is non-‘nil’, that means to leave whitespace other
     than line breaks untouched.  If TO-EOP is non-‘nil’, that means to
     keep filling to the end of the paragraph—or the next hard newline,
     if ‘use-hard-newlines’ is enabled (see below).

     The variable ‘paragraph-separate’ controls how to distinguish
     paragraphs.  *Note Standard Regexps::.

 -- Command: fill-individual-paragraphs start end &optional justify
          citation-regexp
     This command fills each paragraph in the region according to its
     individual fill prefix.  Thus, if the lines of a paragraph were
     indented with spaces, the filled paragraph will remain indented in
     the same fashion.

     The first two arguments, START and END, are the beginning and end
     of the region to be filled.  The third and fourth arguments,
     JUSTIFY and CITATION-REGEXP, are optional.  If JUSTIFY is
     non-‘nil’, the paragraphs are justified as well as filled.  If
     CITATION-REGEXP is non-‘nil’, it means the function is operating on
     a mail message and therefore should not fill the header lines.  If
     CITATION-REGEXP is a string, it is used as a regular expression; if
     it matches the beginning of a line, that line is treated as a
     citation marker.

     Ordinarily, ‘fill-individual-paragraphs’ regards each change in
     indentation as starting a new paragraph.  If
     ‘fill-individual-varying-indent’ is non-‘nil’, then only separator
     lines separate paragraphs.  That mode can handle indented
     paragraphs with additional indentation on the first line.

 -- User Option: fill-individual-varying-indent
     This variable alters the action of ‘fill-individual-paragraphs’ as
     described above.

 -- Command: fill-region-as-paragraph start end &optional justify
          nosqueeze squeeze-after
     This command considers a region of text as a single paragraph and
     fills it.  If the region was made up of many paragraphs, the blank
     lines between paragraphs are removed.  This function justifies as
     well as filling when JUSTIFY is non-‘nil’.

     If NOSQUEEZE is non-‘nil’, that means to leave whitespace other
     than line breaks untouched.  If SQUEEZE-AFTER is non-‘nil’, it
     specifies a position in the region, and means don’t canonicalize
     spaces before that position.

     In Adaptive Fill mode, this command calls ‘fill-context-prefix’ to
     choose a fill prefix by default.  *Note Adaptive Fill::.

 -- Command: justify-current-line &optional how eop nosqueeze
     This command inserts spaces between the words of the current line
     so that the line ends exactly at ‘fill-column’.  It returns ‘nil’.

     The argument HOW, if non-‘nil’ specifies explicitly the style of
     justification.  It can be ‘left’, ‘right’, ‘full’, ‘center’, or
     ‘none’.  If it is ‘t’, that means to do follow specified
     justification style (see ‘current-justification’, below).  ‘nil’
     means to do full justification.

     If EOP is non-‘nil’, that means do only left-justification if
     ‘current-justification’ specifies full justification.  This is used
     for the last line of a paragraph; even if the paragraph as a whole
     is fully justified, the last line should not be.

     If NOSQUEEZE is non-‘nil’, that means do not change interior
     whitespace.

 -- User Option: default-justification
     This variable’s value specifies the style of justification to use
     for text that doesn’t specify a style with a text property.  The
     possible values are ‘left’, ‘right’, ‘full’, ‘center’, or ‘none’.
     The default value is ‘left’.

 -- Function: current-justification
     This function returns the proper justification style to use for
     filling the text around point.

     This returns the value of the ‘justification’ text property at
     point, or the variable DEFAULT-JUSTIFICATION if there is no such
     text property.  However, it returns ‘nil’ rather than ‘none’ to
     mean “don’t justify”.

 -- User Option: sentence-end-double-space
     If this variable is non-‘nil’, a period followed by just one space
     does not count as the end of a sentence, and the filling functions
     avoid breaking the line at such a place.

 -- User Option: sentence-end-without-period
     If this variable is non-‘nil’, a sentence can end without a period.
     This is used for languages like Thai, where sentences end with a
     double space but without a period.

 -- User Option: sentence-end-without-space
     If this variable is non-‘nil’, it should be a string of characters
     that can end a sentence without following spaces.

 -- Variable: fill-paragraph-function
     This variable provides a way to override the filling of paragraphs.
     If its value is non-‘nil’, ‘fill-paragraph’ calls this function to
     do the work.  If the function returns a non-‘nil’ value,
     ‘fill-paragraph’ assumes the job is done, and immediately returns
     that value.

     The usual use of this feature is to fill comments in programming
     language modes.  If the function needs to fill a paragraph in the
     usual way, it can do so as follows:

          (let ((fill-paragraph-function nil))
            (fill-paragraph arg))

 -- Variable: fill-forward-paragraph-function
     This variable provides a way to override how the filling functions,
     such as ‘fill-region’ and ‘fill-paragraph’, move forward to the
     next paragraph.  Its value should be a function, which is called
     with a single argument N, the number of paragraphs to move, and
     should return the difference between N and the number of paragraphs
     actually moved.  The default value of this variable is
     ‘forward-paragraph’.  *Note (emacs)Paragraphs::.

 -- Variable: use-hard-newlines
     If this variable is non-‘nil’, the filling functions do not delete
     newlines that have the ‘hard’ text property.  These hard newlines
     act as paragraph separators.  *Note Hard and Soft Newlines:
     (emacs)Hard and Soft Newlines.


File: elisp.info,  Node: Margins,  Next: Adaptive Fill,  Prev: Filling,  Up: Text

31.12 Margins for Filling
=========================

 -- User Option: fill-prefix
     This buffer-local variable, if non-‘nil’, specifies a string of
     text that appears at the beginning of normal text lines and should
     be disregarded when filling them.  Any line that fails to start
     with the fill prefix is considered the start of a paragraph; so is
     any line that starts with the fill prefix followed by additional
     whitespace.  Lines that start with the fill prefix but no
     additional whitespace are ordinary text lines that can be filled
     together.  The resulting filled lines also start with the fill
     prefix.

     The fill prefix follows the left margin whitespace, if any.

 -- User Option: fill-column
     This buffer-local variable specifies the maximum width of filled
     lines.  Its value should be an integer, which is a number of
     columns.  All the filling, justification, and centering commands
     are affected by this variable, including Auto Fill mode (*note Auto
     Filling::).

     As a practical matter, if you are writing text for other people to
     read, you should set ‘fill-column’ to no more than 70.  Otherwise
     the line will be too long for people to read comfortably, and this
     can make the text seem clumsy.

     The default value for ‘fill-column’ is 70.

 -- Command: set-left-margin from to margin
     This sets the ‘left-margin’ property on the text from FROM to TO to
     the value MARGIN.  If Auto Fill mode is enabled, this command also
     refills the region to fit the new margin.

 -- Command: set-right-margin from to margin
     This sets the ‘right-margin’ property on the text from FROM to TO
     to the value MARGIN.  If Auto Fill mode is enabled, this command
     also refills the region to fit the new margin.

 -- Function: current-left-margin
     This function returns the proper left margin value to use for
     filling the text around point.  The value is the sum of the
     ‘left-margin’ property of the character at the start of the current
     line (or zero if none), and the value of the variable
     ‘left-margin’.

 -- Function: current-fill-column
     This function returns the proper fill column value to use for
     filling the text around point.  The value is the value of the
     ‘fill-column’ variable, minus the value of the ‘right-margin’
     property of the character after point.

 -- Command: move-to-left-margin &optional n force
     This function moves point to the left margin of the current line.
     The column moved to is determined by calling the function
     ‘current-left-margin’.  If the argument N is non-‘nil’,
     ‘move-to-left-margin’ moves forward N−1 lines first.

     If FORCE is non-‘nil’, that says to fix the line’s indentation if
     that doesn’t match the left margin value.

 -- Function: delete-to-left-margin &optional from to
     This function removes left margin indentation from the text between
     FROM and TO.  The amount of indentation to delete is determined by
     calling ‘current-left-margin’.  In no case does this function
     delete non-whitespace.  If FROM and TO are omitted, they default to
     the whole buffer.

 -- Function: indent-to-left-margin
     This function adjusts the indentation at the beginning of the
     current line to the value specified by the variable ‘left-margin’.
     (That may involve either inserting or deleting whitespace.)  This
     function is value of ‘indent-line-function’ in Paragraph-Indent
     Text mode.

 -- User Option: left-margin
     This variable specifies the base left margin column.  In
     Fundamental mode, <RET> indents to this column.  This variable
     automatically becomes buffer-local when set in any fashion.

 -- User Option: fill-nobreak-predicate
     This variable gives major modes a way to specify not to break a
     line at certain places.  Its value should be a list of functions.
     Whenever filling considers breaking the line at a certain place in
     the buffer, it calls each of these functions with no arguments and
     with point located at that place.  If any of the functions returns
     non-‘nil’, then the line won’t be broken there.


File: elisp.info,  Node: Adaptive Fill,  Next: Auto Filling,  Prev: Margins,  Up: Text

31.13 Adaptive Fill Mode
========================

When “Adaptive Fill Mode” is enabled, Emacs determines the fill prefix
automatically from the text in each paragraph being filled rather than
using a predetermined value.  During filling, this fill prefix gets
inserted at the start of the second and subsequent lines of the
paragraph as described in *note Filling::, and in *note Auto Filling::.

 -- User Option: adaptive-fill-mode
     Adaptive Fill mode is enabled when this variable is non-‘nil’.  It
     is ‘t’ by default.

 -- Function: fill-context-prefix from to
     This function implements the heart of Adaptive Fill mode; it
     chooses a fill prefix based on the text between FROM and TO,
     typically the start and end of a paragraph.  It does this by
     looking at the first two lines of the paragraph, based on the
     variables described below.

     Usually, this function returns the fill prefix, a string.  However,
     before doing this, the function makes a final check (not specially
     mentioned in the following) that a line starting with this prefix
     wouldn’t look like the start of a paragraph.  Should this happen,
     the function signals the anomaly by returning ‘nil’ instead.

     In detail, ‘fill-context-prefix’ does this:

       1. It takes a candidate for the fill prefix from the first
          line—it tries first the function in ‘adaptive-fill-function’
          (if any), then the regular expression ‘adaptive-fill-regexp’
          (see below).  The first non-‘nil’ result of these, or the
          empty string if they’re both ‘nil’, becomes the first line’s
          candidate.
       2. If the paragraph has as yet only one line, the function tests
          the validity of the prefix candidate just found.  The function
          then returns the candidate if it’s valid, or a string of
          spaces otherwise.  (see the description of
          ‘adaptive-fill-first-line-regexp’ below).
       3. When the paragraph already has two lines, the function next
          looks for a prefix candidate on the second line, in just the
          same way it did for the first line.  If it doesn’t find one,
          it returns ‘nil’.
       4. The function now compares the two candidate prefixes
          heuristically: if the non-whitespace characters in the line 2
          candidate occur in the same order in the line 1 candidate, the
          function returns the line 2 candidate.  Otherwise, it returns
          the largest initial substring which is common to both
          candidates (which might be the empty string).

 -- User Option: adaptive-fill-regexp
     Adaptive Fill mode matches this regular expression against the text
     starting after the left margin whitespace (if any) on a line; the
     characters it matches are that line’s candidate for the fill
     prefix.

     The default value matches whitespace with certain punctuation
     characters intermingled.

 -- User Option: adaptive-fill-first-line-regexp
     Used only in one-line paragraphs, this regular expression acts as
     an additional check of the validity of the one available candidate
     fill prefix: the candidate must match this regular expression, or
     match ‘comment-start-skip’.  If it doesn’t, ‘fill-context-prefix’
     replaces the candidate with a string of spaces of the same width as
     it.

     The default value of this variable is ‘"\\`[ \t]*\\'"’, which
     matches only a string of whitespace.  The effect of this default is
     to force the fill prefixes found in one-line paragraphs always to
     be pure whitespace.

 -- User Option: adaptive-fill-function
     You can specify more complex ways of choosing a fill prefix
     automatically by setting this variable to a function.  The function
     is called with point after the left margin (if any) of a line, and
     it must preserve point.  It should return either that line’s fill
     prefix or ‘nil’, meaning it has failed to determine a prefix.


File: elisp.info,  Node: Auto Filling,  Next: Sorting,  Prev: Adaptive Fill,  Up: Text

31.14 Auto Filling
==================

Auto Fill mode is a minor mode that fills lines automatically as text is
inserted.  This section describes the hook used by Auto Fill mode.  For
a description of functions that you can call explicitly to fill and
justify existing text, see *note Filling::.

   Auto Fill mode also enables the functions that change the margins and
justification style to refill portions of the text.  *Note Margins::.

 -- Variable: auto-fill-function
     The value of this buffer-local variable should be a function (of no
     arguments) to be called after self-inserting a character from the
     table ‘auto-fill-chars’.  It may be ‘nil’, in which case nothing
     special is done in that case.

     The value of ‘auto-fill-function’ is ‘do-auto-fill’ when Auto-Fill
     mode is enabled.  That is a function whose sole purpose is to
     implement the usual strategy for breaking a line.

 -- Variable: normal-auto-fill-function
     This variable specifies the function to use for
     ‘auto-fill-function’, if and when Auto Fill is turned on.  Major
     modes can set buffer-local values for this variable to alter how
     Auto Fill works.

 -- Variable: auto-fill-chars
     A char table of characters which invoke ‘auto-fill-function’ when
     self-inserted—space and newline in most language environments.
     They have an entry ‘t’ in the table.


File: elisp.info,  Node: Sorting,  Next: Columns,  Prev: Auto Filling,  Up: Text

31.15 Sorting Text
==================

The sorting functions described in this section all rearrange text in a
buffer.  This is in contrast to the function ‘sort’, which rearranges
the order of the elements of a list (*note Rearrangement::).  The values
returned by these functions are not meaningful.

 -- Function: sort-subr reverse nextrecfun endrecfun &optional
          startkeyfun endkeyfun predicate
     This function is the general text-sorting routine that subdivides a
     buffer into records and then sorts them.  Most of the commands in
     this section use this function.

     To understand how ‘sort-subr’ works, consider the whole accessible
     portion of the buffer as being divided into disjoint pieces called
     “sort records”.  The records may or may not be contiguous, but they
     must not overlap.  A portion of each sort record (perhaps all of
     it) is designated as the sort key.  Sorting rearranges the records
     in order by their sort keys.

     Usually, the records are rearranged in order of ascending sort key.
     If the first argument to the ‘sort-subr’ function, REVERSE, is
     non-‘nil’, the sort records are rearranged in order of descending
     sort key.

     The next four arguments to ‘sort-subr’ are functions that are
     called to move point across a sort record.  They are called many
     times from within ‘sort-subr’.

       1. NEXTRECFUN is called with point at the end of a record.  This
          function moves point to the start of the next record.  The
          first record is assumed to start at the position of point when
          ‘sort-subr’ is called.  Therefore, you should usually move
          point to the beginning of the buffer before calling
          ‘sort-subr’.

          This function can indicate there are no more sort records by
          leaving point at the end of the buffer.

       2. ENDRECFUN is called with point within a record.  It moves
          point to the end of the record.

       3. STARTKEYFUN is called to move point from the start of a record
          to the start of the sort key.  This argument is optional; if
          it is omitted, the whole record is the sort key.  If supplied,
          the function should either return a non-‘nil’ value to be used
          as the sort key, or return ‘nil’ to indicate that the sort key
          is in the buffer starting at point.  In the latter case,
          ENDKEYFUN is called to find the end of the sort key.

       4. ENDKEYFUN is called to move point from the start of the sort
          key to the end of the sort key.  This argument is optional.
          If STARTKEYFUN returns ‘nil’ and this argument is omitted (or
          ‘nil’), then the sort key extends to the end of the record.
          There is no need for ENDKEYFUN if STARTKEYFUN returns a
          non-‘nil’ value.

     The argument PREDICATE is the function to use to compare keys.  If
     keys are numbers, it defaults to ‘<’; otherwise it defaults to
     ‘string<’.

     As an example of ‘sort-subr’, here is the complete function
     definition for ‘sort-lines’:

          ;; Note that the first two lines of doc string
          ;; are effectively one line when viewed by a user.
          (defun sort-lines (reverse beg end)
            "Sort lines in region alphabetically;\
           argument means descending order.
          Called from a program, there are three arguments:
          REVERSE (non-nil means reverse order),\
           BEG and END (region to sort).
          The variable `sort-fold-case' determines\
           whether alphabetic case affects
          the sort order."
            (interactive "P\nr")
            (save-excursion
              (save-restriction
                (narrow-to-region beg end)
                (goto-char (point-min))
                (let ((inhibit-field-text-motion t))
                  (sort-subr reverse 'forward-line 'end-of-line)))))

     Here ‘forward-line’ moves point to the start of the next record,
     and ‘end-of-line’ moves point to the end of record.  We do not pass
     the arguments STARTKEYFUN and ENDKEYFUN, because the entire record
     is used as the sort key.

     The ‘sort-paragraphs’ function is very much the same, except that
     its ‘sort-subr’ call looks like this:

          (sort-subr reverse
                     (function
                      (lambda ()
                        (while (and (not (eobp))
                                    (looking-at paragraph-separate))
                          (forward-line 1))))
                     'forward-paragraph)

     Markers pointing into any sort records are left with no useful
     position after ‘sort-subr’ returns.

 -- User Option: sort-fold-case
     If this variable is non-‘nil’, ‘sort-subr’ and the other buffer
     sorting functions ignore case when comparing strings.

 -- Command: sort-regexp-fields reverse record-regexp key-regexp start
          end
     This command sorts the region between START and END alphabetically
     as specified by RECORD-REGEXP and KEY-REGEXP.  If REVERSE is a
     negative integer, then sorting is in reverse order.

     Alphabetical sorting means that two sort keys are compared by
     comparing the first characters of each, the second characters of
     each, and so on.  If a mismatch is found, it means that the sort
     keys are unequal; the sort key whose character is less at the point
     of first mismatch is the lesser sort key.  The individual
     characters are compared according to their numerical character
     codes in the Emacs character set.

     The value of the RECORD-REGEXP argument specifies how to divide the
     buffer into sort records.  At the end of each record, a search is
     done for this regular expression, and the text that matches it is
     taken as the next record.  For example, the regular expression
     ‘^.+$’, which matches lines with at least one character besides a
     newline, would make each such line into a sort record.  *Note
     Regular Expressions::, for a description of the syntax and meaning
     of regular expressions.

     The value of the KEY-REGEXP argument specifies what part of each
     record is the sort key.  The KEY-REGEXP could match the whole
     record, or only a part.  In the latter case, the rest of the record
     has no effect on the sorted order of records, but it is carried
     along when the record moves to its new position.

     The KEY-REGEXP argument can refer to the text matched by a
     subexpression of RECORD-REGEXP, or it can be a regular expression
     on its own.

     If KEY-REGEXP is:

     ‘\DIGIT’
          then the text matched by the DIGITth ‘\(...\)’ parenthesis
          grouping in RECORD-REGEXP is the sort key.

     ‘\&’
          then the whole record is the sort key.

     a regular expression
          then ‘sort-regexp-fields’ searches for a match for the regular
          expression within the record.  If such a match is found, it is
          the sort key.  If there is no match for KEY-REGEXP within a
          record then that record is ignored, which means its position
          in the buffer is not changed.  (The other records may move
          around it.)

     For example, if you plan to sort all the lines in the region by the
     first word on each line starting with the letter ‘f’, you should
     set RECORD-REGEXP to ‘^.*$’ and set KEY-REGEXP to ‘\<f\w*\>’.  The
     resulting expression looks like this:

          (sort-regexp-fields nil "^.*$" "\\<f\\w*\\>"
                              (region-beginning)
                              (region-end))

     If you call ‘sort-regexp-fields’ interactively, it prompts for
     RECORD-REGEXP and KEY-REGEXP in the minibuffer.

 -- Command: sort-lines reverse start end
     This command alphabetically sorts lines in the region between START
     and END.  If REVERSE is non-‘nil’, the sort is in reverse order.

 -- Command: sort-paragraphs reverse start end
     This command alphabetically sorts paragraphs in the region between
     START and END.  If REVERSE is non-‘nil’, the sort is in reverse
     order.

 -- Command: sort-pages reverse start end
     This command alphabetically sorts pages in the region between START
     and END.  If REVERSE is non-‘nil’, the sort is in reverse order.

 -- Command: sort-fields field start end
     This command sorts lines in the region between START and END,
     comparing them alphabetically by the FIELDth field of each line.
     Fields are separated by whitespace and numbered starting from 1.
     If FIELD is negative, sorting is by the −FIELDth field from the end
     of the line.  This command is useful for sorting tables.

 -- Command: sort-numeric-fields field start end
     This command sorts lines in the region between START and END,
     comparing them numerically by the FIELDth field of each line.
     Fields are separated by whitespace and numbered starting from 1.
     The specified field must contain a number in each line of the
     region.  Numbers starting with 0 are treated as octal, and numbers
     starting with ‘0x’ are treated as hexadecimal.

     If FIELD is negative, sorting is by the −FIELDth field from the end
     of the line.  This command is useful for sorting tables.

 -- User Option: sort-numeric-base
     This variable specifies the default radix for ‘sort-numeric-fields’
     to parse numbers.

 -- Command: sort-columns reverse &optional beg end
     This command sorts the lines in the region between BEG and END,
     comparing them alphabetically by a certain range of columns.  The
     column positions of BEG and END bound the range of columns to sort
     on.

     If REVERSE is non-‘nil’, the sort is in reverse order.

     One unusual thing about this command is that the entire line
     containing position BEG, and the entire line containing position
     END, are included in the region sorted.

     Note that ‘sort-columns’ rejects text that contains tabs, because
     tabs could be split across the specified columns.  Use ‘M-x
     untabify’ to convert tabs to spaces before sorting.

     When possible, this command actually works by calling the ‘sort’
     utility program.


File: elisp.info,  Node: Columns,  Next: Indentation,  Prev: Sorting,  Up: Text

31.16 Counting Columns
======================

The column functions convert between a character position (counting
characters from the beginning of the buffer) and a column position
(counting screen characters from the beginning of a line).

   These functions count each character according to the number of
columns it occupies on the screen.  This means control characters count
as occupying 2 or 4 columns, depending upon the value of ‘ctl-arrow’,
and tabs count as occupying a number of columns that depends on the
value of ‘tab-width’ and on the column where the tab begins.  *Note
Usual Display::.

   Column number computations ignore the width of the window and the
amount of horizontal scrolling.  Consequently, a column value can be
arbitrarily high.  The first (or leftmost) column is numbered 0.  They
also ignore overlays and text properties, aside from invisibility.

 -- Function: current-column
     This function returns the horizontal position of point, measured in
     columns, counting from 0 at the left margin.  The column position
     is the sum of the widths of all the displayed representations of
     the characters between the start of the current line and point.

 -- Command: move-to-column column &optional force
     This function moves point to COLUMN in the current line.  The
     calculation of COLUMN takes into account the widths of the
     displayed representations of the characters between the start of
     the line and point.

     When called interactively, COLUMN is the value of prefix numeric
     argument.  If COLUMN is not an integer, an error is signaled.

     If it is impossible to move to column COLUMN because that is in the
     middle of a multicolumn character such as a tab, point moves to the
     end of that character.  However, if FORCE is non-‘nil’, and COLUMN
     is in the middle of a tab, then ‘move-to-column’ converts the tab
     into spaces so that it can move precisely to column COLUMN.  Other
     multicolumn characters can cause anomalies despite FORCE, since
     there is no way to split them.

     The argument FORCE also has an effect if the line isn’t long enough
     to reach column COLUMN; if it is ‘t’, that means to add whitespace
     at the end of the line to reach that column.

     The return value is the column number actually moved to.


File: elisp.info,  Node: Indentation,  Next: Case Changes,  Prev: Columns,  Up: Text

31.17 Indentation
=================

The indentation functions are used to examine, move to, and change
whitespace that is at the beginning of a line.  Some of the functions
can also change whitespace elsewhere on a line.  Columns and indentation
count from zero at the left margin.

* Menu:

* Primitive Indent::      Functions used to count and insert indentation.
* Mode-Specific Indent::  Customize indentation for different modes.
* Region Indent::         Indent all the lines in a region.
* Relative Indent::       Indent the current line based on previous lines.
* Indent Tabs::           Adjustable, typewriter-like tab stops.
* Motion by Indent::      Move to first non-blank character.


File: elisp.info,  Node: Primitive Indent,  Next: Mode-Specific Indent,  Up: Indentation

31.17.1 Indentation Primitives
------------------------------

This section describes the primitive functions used to count and insert
indentation.  The functions in the following sections use these
primitives.  *Note Size of Displayed Text::, for related functions.

 -- Function: current-indentation
     This function returns the indentation of the current line, which is
     the horizontal position of the first nonblank character.  If the
     contents are entirely blank, then this is the horizontal position
     of the end of the line.

 -- Command: indent-to column &optional minimum
     This function indents from point with tabs and spaces until COLUMN
     is reached.  If MINIMUM is specified and non-‘nil’, then at least
     that many spaces are inserted even if this requires going beyond
     COLUMN.  Otherwise the function does nothing if point is already
     beyond COLUMN.  The value is the column at which the inserted
     indentation ends.

     The inserted whitespace characters inherit text properties from the
     surrounding text (usually, from the preceding text only).  *Note
     Sticky Properties::.

 -- User Option: indent-tabs-mode
     If this variable is non-‘nil’, indentation functions can insert
     tabs as well as spaces.  Otherwise, they insert only spaces.
     Setting this variable automatically makes it buffer-local in the
     current buffer.


File: elisp.info,  Node: Mode-Specific Indent,  Next: Region Indent,  Prev: Primitive Indent,  Up: Indentation

31.17.2 Indentation Controlled by Major Mode
--------------------------------------------

An important function of each major mode is to customize the <TAB> key
to indent properly for the language being edited.  This section
describes the mechanism of the <TAB> key and how to control it.  The
functions in this section return unpredictable values.

 -- Command: indent-for-tab-command &optional rigid
     This is the command bound to <TAB> in most editing modes.  Its
     usual action is to indent the current line, but it can
     alternatively insert a tab character or indent a region.

     Here is what it does:

        • First, it checks whether Transient Mark mode is enabled and
          the region is active.  If so, it called ‘indent-region’ to
          indent all the text in the region (*note Region Indent::).

        • Otherwise, if the indentation function in
          ‘indent-line-function’ is ‘indent-to-left-margin’ (a trivial
          command that inserts a tab character), or if the variable
          ‘tab-always-indent’ specifies that a tab character ought to be
          inserted (see below), then it inserts a tab character.

        • Otherwise, it indents the current line; this is done by
          calling the function in ‘indent-line-function’.  If the line
          is already indented, and the value of ‘tab-always-indent’ is
          ‘complete’ (see below), it tries completing the text at point.

     If RIGID is non-‘nil’ (interactively, with a prefix argument), then
     after this command indents a line or inserts a tab, it also rigidly
     indents the entire balanced expression which starts at the
     beginning of the current line, in order to reflect the new
     indentation.  This argument is ignored if the command indents the
     region.

 -- Variable: indent-line-function
     This variable’s value is the function to be used by
     ‘indent-for-tab-command’, and various other indentation commands,
     to indent the current line.  It is usually assigned by the major
     mode; for instance, Lisp mode sets it to ‘lisp-indent-line’, C mode
     sets it to ‘c-indent-line’, and so on.  The default value is
     ‘indent-relative’.  *Note Auto-Indentation::.

 -- Command: indent-according-to-mode
     This command calls the function in ‘indent-line-function’ to indent
     the current line in a way appropriate for the current major mode.

 -- Command: newline-and-indent
     This function inserts a newline, then indents the new line (the one
     following the newline just inserted) according to the major mode.
     It does indentation by calling ‘indent-according-to-mode’.

 -- Command: reindent-then-newline-and-indent
     This command reindents the current line, inserts a newline at
     point, and then indents the new line (the one following the newline
     just inserted).  It does indentation on both lines by calling
     ‘indent-according-to-mode’.

 -- User Option: tab-always-indent
     This variable can be used to customize the behavior of the <TAB>
     (‘indent-for-tab-command’) command.  If the value is ‘t’ (the
     default), the command normally just indents the current line.  If
     the value is ‘nil’, the command indents the current line only if
     point is at the left margin or in the line’s indentation;
     otherwise, it inserts a tab character.  If the value is ‘complete’,
     the command first tries to indent the current line, and if the line
     was already indented, it calls ‘completion-at-point’ to complete
     the text at point (*note Completion in Buffers::).


File: elisp.info,  Node: Region Indent,  Next: Relative Indent,  Prev: Mode-Specific Indent,  Up: Indentation

31.17.3 Indenting an Entire Region
----------------------------------

This section describes commands that indent all the lines in the region.
They return unpredictable values.

 -- Command: indent-region start end &optional to-column
     This command indents each nonblank line starting between START
     (inclusive) and END (exclusive).  If TO-COLUMN is ‘nil’,
     ‘indent-region’ indents each nonblank line by calling the current
     mode’s indentation function, the value of ‘indent-line-function’.

     If TO-COLUMN is non-‘nil’, it should be an integer specifying the
     number of columns of indentation; then this function gives each
     line exactly that much indentation, by either adding or deleting
     whitespace.

     If there is a fill prefix, ‘indent-region’ indents each line by
     making it start with the fill prefix.

 -- Variable: indent-region-function
     The value of this variable is a function that can be used by
     ‘indent-region’ as a short cut.  It should take two arguments, the
     start and end of the region.  You should design the function so
     that it will produce the same results as indenting the lines of the
     region one by one, but presumably faster.

     If the value is ‘nil’, there is no short cut, and ‘indent-region’
     actually works line by line.

     A short-cut function is useful in modes such as C mode and Lisp
     mode, where the ‘indent-line-function’ must scan from the beginning
     of the function definition: applying it to each line would be
     quadratic in time.  The short cut can update the scan information
     as it moves through the lines indenting them; this takes linear
     time.  In a mode where indenting a line individually is fast, there
     is no need for a short cut.

     ‘indent-region’ with a non-‘nil’ argument TO-COLUMN has a different
     meaning and does not use this variable.

 -- Command: indent-rigidly start end count
     This function indents all lines starting between START (inclusive)
     and END (exclusive) sideways by COUNT columns.  This preserves the
     shape of the affected region, moving it as a rigid unit.

     This is useful not only for indenting regions of unindented text,
     but also for indenting regions of formatted code.  For example, if
     COUNT is 3, this command adds 3 columns of indentation to every
     line that begins in the specified region.

     If called interactively with no prefix argument, this command
     invokes a transient mode for adjusting indentation rigidly.  *Note
     (emacs)Indentation Commands::.

 -- Command: indent-code-rigidly start end columns &optional
          nochange-regexp
     This is like ‘indent-rigidly’, except that it doesn’t alter lines
     that start within strings or comments.

     In addition, it doesn’t alter a line if NOCHANGE-REGEXP matches at
     the beginning of the line (if NOCHANGE-REGEXP is non-‘nil’).


File: elisp.info,  Node: Relative Indent,  Next: Indent Tabs,  Prev: Region Indent,  Up: Indentation

31.17.4 Indentation Relative to Previous Lines
----------------------------------------------

This section describes two commands that indent the current line based
on the contents of previous lines.

 -- Command: indent-relative &optional unindented-ok
     This command inserts whitespace at point, extending to the same
     column as the next “indent point” of the previous nonblank line.
     An indent point is a non-whitespace character following whitespace.
     The next indent point is the first one at a column greater than the
     current column of point.  For example, if point is underneath and
     to the left of the first non-blank character of a line of text, it
     moves to that column by inserting whitespace.

     If the previous nonblank line has no next indent point (i.e., none
     at a great enough column position), ‘indent-relative’ either does
     nothing (if UNINDENTED-OK is non-‘nil’) or calls ‘tab-to-tab-stop’.
     Thus, if point is underneath and to the right of the last column of
     a short line of text, this command ordinarily moves point to the
     next tab stop by inserting whitespace.

     The return value of ‘indent-relative’ is unpredictable.

     In the following example, point is at the beginning of the second
     line:

                      This line is indented twelve spaces.
          ★The quick brown fox jumped.

     Evaluation of the expression ‘(indent-relative nil)’ produces the
     following:

                      This line is indented twelve spaces.
                      ★The quick brown fox jumped.

     In this next example, point is between the ‘m’ and ‘p’ of ‘jumped’:

                      This line is indented twelve spaces.
          The quick brown fox jum★ped.

     Evaluation of the expression ‘(indent-relative nil)’ produces the
     following:

                      This line is indented twelve spaces.
          The quick brown fox jum  ★ped.

 -- Command: indent-relative-maybe
     This command indents the current line like the previous nonblank
     line, by calling ‘indent-relative’ with ‘t’ as the UNINDENTED-OK
     argument.  The return value is unpredictable.

     If the previous nonblank line has no indent points beyond the
     current column, this command does nothing.


File: elisp.info,  Node: Indent Tabs,  Next: Motion by Indent,  Prev: Relative Indent,  Up: Indentation

31.17.5 Adjustable Tab Stops
----------------------------

This section explains the mechanism for user-specified tab stops and the
mechanisms that use and set them.  The name “tab stops” is used because
the feature is similar to that of the tab stops on a typewriter.  The
feature works by inserting an appropriate number of spaces and tab
characters to reach the next tab stop column; it does not affect the
display of tab characters in the buffer (*note Usual Display::).  Note
that the <TAB> character as input uses this tab stop feature only in a
few major modes, such as Text mode.  *Note (emacs)Tab Stops::.

 -- Command: tab-to-tab-stop
     This command inserts spaces or tabs before point, up to the next
     tab stop column defined by ‘tab-stop-list’.

 -- User Option: tab-stop-list
     This variable defines the tab stop columns used by
     ‘tab-to-tab-stop’.  It should be either ‘nil’, or a list of
     increasing integers, which need not be evenly spaced.  The list is
     implicitly extended to infinity through repetition of the interval
     between the last and penultimate elements (or ‘tab-width’ if the
     list has fewer than two elements).  A value of ‘nil’ means a tab
     stop every ‘tab-width’ columns.

     Use ‘M-x edit-tab-stops’ to edit the location of tab stops
     interactively.


File: elisp.info,  Node: Motion by Indent,  Prev: Indent Tabs,  Up: Indentation

31.17.6 Indentation-Based Motion Commands
-----------------------------------------

These commands, primarily for interactive use, act based on the
indentation in the text.

 -- Command: back-to-indentation
     This command moves point to the first non-whitespace character in
     the current line (which is the line in which point is located).  It
     returns ‘nil’.

 -- Command: backward-to-indentation &optional arg
     This command moves point backward ARG lines and then to the first
     nonblank character on that line.  It returns ‘nil’.  If ARG is
     omitted or ‘nil’, it defaults to 1.

 -- Command: forward-to-indentation &optional arg
     This command moves point forward ARG lines and then to the first
     nonblank character on that line.  It returns ‘nil’.  If ARG is
     omitted or ‘nil’, it defaults to 1.


File: elisp.info,  Node: Case Changes,  Next: Text Properties,  Prev: Indentation,  Up: Text

31.18 Case Changes
==================

The case change commands described here work on text in the current
buffer.  *Note Case Conversion::, for case conversion functions that
work on strings and characters.  *Note Case Tables::, for how to
customize which characters are upper or lower case and how to convert
them.

 -- Command: capitalize-region start end
     This function capitalizes all words in the region defined by START
     and END.  To capitalize means to convert each word’s first
     character to upper case and convert the rest of each word to lower
     case.  The function returns ‘nil’.

     If one end of the region is in the middle of a word, the part of
     the word within the region is treated as an entire word.

     When ‘capitalize-region’ is called interactively, START and END are
     point and the mark, with the smallest first.

          ---------- Buffer: foo ----------
          This is the contents of the 5th foo.
          ---------- Buffer: foo ----------

          (capitalize-region 1 37)
          ⇒ nil

          ---------- Buffer: foo ----------
          This Is The Contents Of The 5th Foo.
          ---------- Buffer: foo ----------

 -- Command: downcase-region start end
     This function converts all of the letters in the region defined by
     START and END to lower case.  The function returns ‘nil’.

     When ‘downcase-region’ is called interactively, START and END are
     point and the mark, with the smallest first.

 -- Command: upcase-region start end
     This function converts all of the letters in the region defined by
     START and END to upper case.  The function returns ‘nil’.

     When ‘upcase-region’ is called interactively, START and END are
     point and the mark, with the smallest first.

 -- Command: capitalize-word count
     This function capitalizes COUNT words after point, moving point
     over as it does.  To capitalize means to convert each word’s first
     character to upper case and convert the rest of each word to lower
     case.  If COUNT is negative, the function capitalizes the −COUNT
     previous words but does not move point.  The value is ‘nil’.

     If point is in the middle of a word, the part of the word before
     point is ignored when moving forward.  The rest is treated as an
     entire word.

     When ‘capitalize-word’ is called interactively, COUNT is set to the
     numeric prefix argument.

 -- Command: downcase-word count
     This function converts the COUNT words after point to all lower
     case, moving point over as it does.  If COUNT is negative, it
     converts the −COUNT previous words but does not move point.  The
     value is ‘nil’.

     When ‘downcase-word’ is called interactively, COUNT is set to the
     numeric prefix argument.

 -- Command: upcase-word count
     This function converts the COUNT words after point to all upper
     case, moving point over as it does.  If COUNT is negative, it
     converts the −COUNT previous words but does not move point.  The
     value is ‘nil’.

     When ‘upcase-word’ is called interactively, COUNT is set to the
     numeric prefix argument.


File: elisp.info,  Node: Text Properties,  Next: Substitution,  Prev: Case Changes,  Up: Text

31.19 Text Properties
=====================

Each character position in a buffer or a string can have a “text
property list”, much like the property list of a symbol (*note Property
Lists::).  The properties belong to a particular character at a
particular place, such as, the letter ‘T’ at the beginning of this
sentence or the first ‘o’ in ‘foo’—if the same character occurs in two
different places, the two occurrences in general have different
properties.

   Each property has a name and a value.  Both of these can be any Lisp
object, but the name is normally a symbol.  Typically each property name
symbol is used for a particular purpose; for instance, the text property
‘face’ specifies the faces for displaying the character (*note Special
Properties::).  The usual way to access the property list is to specify
a name and ask what value corresponds to it.

   If a character has a ‘category’ property, we call it the “property
category” of the character.  It should be a symbol.  The properties of
the symbol serve as defaults for the properties of the character.

   Copying text between strings and buffers preserves the properties
along with the characters; this includes such diverse functions as
‘substring’, ‘insert’, and ‘buffer-substring’.

* Menu:

* Examining Properties::   Looking at the properties of one character.
* Changing Properties::    Setting the properties of a range of text.
* Property Search::        Searching for where a property changes value.
* Special Properties::     Particular properties with special meanings.
* Format Properties::      Properties for representing formatting of text.
* Sticky Properties::      How inserted text gets properties from
                             neighboring text.
* Lazy Properties::        Computing text properties in a lazy fashion
                             only when text is examined.
* Clickable Text::         Using text properties to make regions of text
                             do something when you click on them.
* Fields::                 The ‘field’ property defines
                             fields within the buffer.
* Not Intervals::          Why text properties do not use
                             Lisp-visible text intervals.


File: elisp.info,  Node: Examining Properties,  Next: Changing Properties,  Up: Text Properties

31.19.1 Examining Text Properties
---------------------------------

The simplest way to examine text properties is to ask for the value of a
particular property of a particular character.  For that, use
‘get-text-property’.  Use ‘text-properties-at’ to get the entire
property list of a character.  *Note Property Search::, for functions to
examine the properties of a number of characters at once.

   These functions handle both strings and buffers.  Keep in mind that
positions in a string start from 0, whereas positions in a buffer start
from 1.

 -- Function: get-text-property pos prop &optional object
     This function returns the value of the PROP property of the
     character after position POS in OBJECT (a buffer or string).  The
     argument OBJECT is optional and defaults to the current buffer.

     If there is no PROP property strictly speaking, but the character
     has a property category that is a symbol, then ‘get-text-property’
     returns the PROP property of that symbol.

 -- Function: get-char-property position prop &optional object
     This function is like ‘get-text-property’, except that it checks
     overlays first and then text properties.  *Note Overlays::.

     The argument OBJECT may be a string, a buffer, or a window.  If it
     is a window, then the buffer displayed in that window is used for
     text properties and overlays, but only the overlays active for that
     window are considered.  If OBJECT is a buffer, then overlays in
     that buffer are considered first, in order of decreasing priority,
     followed by the text properties.  If OBJECT is a string, only text
     properties are considered, since strings never have overlays.

 -- Function: get-pos-property position prop &optional object
     This function is like ‘get-char-property’, except that it pays
     attention to properties’ stickiness and overlays’ advancement
     settings instead of the property of the character at (i.e., right
     after) POSITION.

 -- Function: get-char-property-and-overlay position prop &optional
          object
     This is like ‘get-char-property’, but gives extra information about
     the overlay that the property value comes from.

     Its value is a cons cell whose CAR is the property value, the same
     value ‘get-char-property’ would return with the same arguments.
     Its CDR is the overlay in which the property was found, or ‘nil’,
     if it was found as a text property or not found at all.

     If POSITION is at the end of OBJECT, both the CAR and the CDR of
     the value are ‘nil’.

 -- Variable: char-property-alias-alist
     This variable holds an alist which maps property names to a list of
     alternative property names.  If a character does not specify a
     direct value for a property, the alternative property names are
     consulted in order; the first non-‘nil’ value is used.  This
     variable takes precedence over ‘default-text-properties’, and
     ‘category’ properties take precedence over this variable.

 -- Function: text-properties-at position &optional object
     This function returns the entire property list of the character at
     POSITION in the string or buffer OBJECT.  If OBJECT is ‘nil’, it
     defaults to the current buffer.

 -- Variable: default-text-properties
     This variable holds a property list giving default values for text
     properties.  Whenever a character does not specify a value for a
     property, neither directly, through a category symbol, or through
     ‘char-property-alias-alist’, the value stored in this list is used
     instead.  Here is an example:

          (setq default-text-properties '(foo 69)
                char-property-alias-alist nil)
          ;; Make sure character 1 has no properties of its own.
          (set-text-properties 1 2 nil)
          ;; What we get, when we ask, is the default value.
          (get-text-property 1 'foo)
               ⇒ 69


File: elisp.info,  Node: Changing Properties,  Next: Property Search,  Prev: Examining Properties,  Up: Text Properties

31.19.2 Changing Text Properties
--------------------------------

The primitives for changing properties apply to a specified range of
text in a buffer or string.  The function ‘set-text-properties’ (see end
of section) sets the entire property list of the text in that range;
more often, it is useful to add, change, or delete just certain
properties specified by name.

   Since text properties are considered part of the contents of the
buffer (or string), and can affect how a buffer looks on the screen, any
change in buffer text properties marks the buffer as modified.  Buffer
text property changes are undoable also (*note Undo::).  Positions in a
string start from 0, whereas positions in a buffer start from 1.

 -- Function: put-text-property start end prop value &optional object
     This function sets the PROP property to VALUE for the text between
     START and END in the string or buffer OBJECT.  If OBJECT is ‘nil’,
     it defaults to the current buffer.

 -- Function: add-text-properties start end props &optional object
     This function adds or overrides text properties for the text
     between START and END in the string or buffer OBJECT.  If OBJECT is
     ‘nil’, it defaults to the current buffer.

     The argument PROPS specifies which properties to add.  It should
     have the form of a property list (*note Property Lists::): a list
     whose elements include the property names followed alternately by
     the corresponding values.

     The return value is ‘t’ if the function actually changed some
     property’s value; ‘nil’ otherwise (if PROPS is ‘nil’ or its values
     agree with those in the text).

     For example, here is how to set the ‘comment’ and ‘face’ properties
     of a range of text:

          (add-text-properties START END
                               '(comment t face highlight))

 -- Function: remove-text-properties start end props &optional object
     This function deletes specified text properties from the text
     between START and END in the string or buffer OBJECT.  If OBJECT is
     ‘nil’, it defaults to the current buffer.

     The argument PROPS specifies which properties to delete.  It should
     have the form of a property list (*note Property Lists::): a list
     whose elements are property names alternating with corresponding
     values.  But only the names matter—the values that accompany them
     are ignored.  For example, here’s how to remove the ‘face’
     property.

          (remove-text-properties START END '(face nil))

     The return value is ‘t’ if the function actually changed some
     property’s value; ‘nil’ otherwise (if PROPS is ‘nil’ or if no
     character in the specified text had any of those properties).

     To remove all text properties from certain text, use
     ‘set-text-properties’ and specify ‘nil’ for the new property list.

 -- Function: remove-list-of-text-properties start end
          list-of-properties &optional object
     Like ‘remove-text-properties’ except that LIST-OF-PROPERTIES is a
     list of property names only, not an alternating list of property
     names and values.

 -- Function: set-text-properties start end props &optional object
     This function completely replaces the text property list for the
     text between START and END in the string or buffer OBJECT.  If
     OBJECT is ‘nil’, it defaults to the current buffer.

     The argument PROPS is the new property list.  It should be a list
     whose elements are property names alternating with corresponding
     values.

     After ‘set-text-properties’ returns, all the characters in the
     specified range have identical properties.

     If PROPS is ‘nil’, the effect is to get rid of all properties from
     the specified range of text.  Here’s an example:

          (set-text-properties START END nil)

     Do not rely on the return value of this function.

 -- Function: add-face-text-property start end face &optional appendp
          object
     This function acts on the text between START and END, adding the
     face FACE to the ‘face’ text property.  FACE should be a valid
     value for the ‘face’ property (*note Special Properties::), such as
     a face name or an anonymous face (*note Faces::).

     If any text in the region already has a non-‘nil’ ‘face’ property,
     those face(s) are retained.  This function sets the ‘face’ property
     to a list of faces, with FACE as the first element (by default) and
     the pre-existing faces as the remaining elements.  If the optional
     argument APPEND is non-‘nil’, FACE is appended to the end of the
     list instead.  Note that in a face list, the first occurring value
     for each attribute takes precedence.

     For example, the following code would assign a italicized green
     face to the text between START and END:

          (add-face-text-property START END 'italic)
          (add-face-text-property START END '(:foreground "red"))
          (add-face-text-property START END '(:foreground "green"))

     The optional argument OBJECT, if non-‘nil’, specifies a buffer or
     string to act on, rather than the current buffer.  If OBJECT is a
     string, then START and END are zero-based indices into the string.

   The easiest way to make a string with text properties is with
‘propertize’:

 -- Function: propertize string &rest properties
     This function returns a copy of STRING with the text properties
     PROPERTIES added.  These properties apply to all the characters in
     the string that is returned.  Here is an example that constructs a
     string with a ‘face’ property and a ‘mouse-face’ property:

          (propertize "foo" 'face 'italic
                      'mouse-face 'bold-italic)
               ⇒ #("foo" 0 3 (mouse-face bold-italic face italic))

     To put different properties on various parts of a string, you can
     construct each part with ‘propertize’ and then combine them with
     ‘concat’:

          (concat
           (propertize "foo" 'face 'italic
                       'mouse-face 'bold-italic)
           " and "
           (propertize "bar" 'face 'italic
                       'mouse-face 'bold-italic))
               ⇒ #("foo and bar"
                           0 3 (face italic mouse-face bold-italic)
                           3 8 nil
                           8 11 (face italic mouse-face bold-italic))

   *Note Buffer Contents::, for the function
‘buffer-substring-no-properties’, which copies text from the buffer but
does not copy its properties.

   If you wish to add or remove text properties to a buffer without
marking the buffer as modified, you can wrap the calls above in the
‘with-silent-modifications’ macro.


File: elisp.info,  Node: Property Search,  Next: Special Properties,  Prev: Changing Properties,  Up: Text Properties

31.19.3 Text Property Search Functions
--------------------------------------

In typical use of text properties, most of the time several or many
consecutive characters have the same value for a property.  Rather than
writing your programs to examine characters one by one, it is much
faster to process chunks of text that have the same property value.

   Here are functions you can use to do this.  They use ‘eq’ for
comparing property values.  In all cases, OBJECT defaults to the current
buffer.

   For good performance, it’s very important to use the LIMIT argument
to these functions, especially the ones that search for a single
property—otherwise, they may spend a long time scanning to the end of
the buffer, if the property you are interested in does not change.

   These functions do not move point; instead, they return a position
(or ‘nil’).  Remember that a position is always between two characters;
the position returned by these functions is between two characters with
different properties.

 -- Function: next-property-change pos &optional object limit
     The function scans the text forward from position POS in the string
     or buffer OBJECT until it finds a change in some text property,
     then returns the position of the change.  In other words, it
     returns the position of the first character beyond POS whose
     properties are not identical to those of the character just after
     POS.

     If LIMIT is non-‘nil’, then the scan ends at position LIMIT.  If
     there is no property change before that point, this function
     returns LIMIT.

     The value is ‘nil’ if the properties remain unchanged all the way
     to the end of OBJECT and LIMIT is ‘nil’.  If the value is
     non-‘nil’, it is a position greater than or equal to POS.  The
     value equals POS only when LIMIT equals POS.

     Here is an example of how to scan the buffer by chunks of text
     within which all properties are constant:

          (while (not (eobp))
            (let ((plist (text-properties-at (point)))
                  (next-change
                   (or (next-property-change (point) (current-buffer))
                       (point-max))))
              Process text from point to NEXT-CHANGE...
              (goto-char next-change)))

 -- Function: previous-property-change pos &optional object limit
     This is like ‘next-property-change’, but scans back from POS
     instead of forward.  If the value is non-‘nil’, it is a position
     less than or equal to POS; it equals POS only if LIMIT equals POS.

 -- Function: next-single-property-change pos prop &optional object
          limit
     The function scans text for a change in the PROP property, then
     returns the position of the change.  The scan goes forward from
     position POS in the string or buffer OBJECT.  In other words, this
     function returns the position of the first character beyond POS
     whose PROP property differs from that of the character just after
     POS.

     If LIMIT is non-‘nil’, then the scan ends at position LIMIT.  If
     there is no property change before that point,
     ‘next-single-property-change’ returns LIMIT.

     The value is ‘nil’ if the property remains unchanged all the way to
     the end of OBJECT and LIMIT is ‘nil’.  If the value is non-‘nil’,
     it is a position greater than or equal to POS; it equals POS only
     if LIMIT equals POS.

 -- Function: previous-single-property-change pos prop &optional object
          limit
     This is like ‘next-single-property-change’, but scans back from POS
     instead of forward.  If the value is non-‘nil’, it is a position
     less than or equal to POS; it equals POS only if LIMIT equals POS.

 -- Function: next-char-property-change pos &optional limit
     This is like ‘next-property-change’ except that it considers
     overlay properties as well as text properties, and if no change is
     found before the end of the buffer, it returns the maximum buffer
     position rather than ‘nil’ (in this sense, it resembles the
     corresponding overlay function ‘next-overlay-change’, rather than
     ‘next-property-change’).  There is no OBJECT operand because this
     function operates only on the current buffer.  It returns the next
     address at which either kind of property changes.

 -- Function: previous-char-property-change pos &optional limit
     This is like ‘next-char-property-change’, but scans back from POS
     instead of forward, and returns the minimum buffer position if no
     change is found.

 -- Function: next-single-char-property-change pos prop &optional object
          limit
     This is like ‘next-single-property-change’ except that it considers
     overlay properties as well as text properties, and if no change is
     found before the end of the OBJECT, it returns the maximum valid
     position in OBJECT rather than ‘nil’.  Unlike
     ‘next-char-property-change’, this function _does_ have an OBJECT
     operand; if OBJECT is not a buffer, only text-properties are
     considered.

 -- Function: previous-single-char-property-change pos prop &optional
          object limit
     This is like ‘next-single-char-property-change’, but scans back
     from POS instead of forward, and returns the minimum valid position
     in OBJECT if no change is found.

 -- Function: text-property-any start end prop value &optional object
     This function returns non-‘nil’ if at least one character between
     START and END has a property PROP whose value is VALUE.  More
     precisely, it returns the position of the first such character.
     Otherwise, it returns ‘nil’.

     The optional fifth argument, OBJECT, specifies the string or buffer
     to scan.  Positions are relative to OBJECT.  The default for OBJECT
     is the current buffer.

 -- Function: text-property-not-all start end prop value &optional
          object
     This function returns non-‘nil’ if at least one character between
     START and END does not have a property PROP with value VALUE.  More
     precisely, it returns the position of the first such character.
     Otherwise, it returns ‘nil’.

     The optional fifth argument, OBJECT, specifies the string or buffer
     to scan.  Positions are relative to OBJECT.  The default for OBJECT
     is the current buffer.


File: elisp.info,  Node: Special Properties,  Next: Format Properties,  Prev: Property Search,  Up: Text Properties

31.19.4 Properties with Special Meanings
----------------------------------------

Here is a table of text property names that have special built-in
meanings.  The following sections list a few additional special property
names that control filling and property inheritance.  All other names
have no standard meaning, and you can use them as you like.

   Note: the properties ‘composition’, ‘display’, ‘invisible’ and
‘intangible’ can also cause point to move to an acceptable place, after
each Emacs command.  *Note Adjusting Point::.

‘category’
     If a character has a ‘category’ property, we call it the “property
     category” of the character.  It should be a symbol.  The properties
     of this symbol serve as defaults for the properties of the
     character.

‘face’
     The ‘face’ property controls the appearance of the character (*note
     Faces::).  The value of the property can be the following:

        • A face name (a symbol or string).

        • An anonymous face: a property list of the form ‘(KEYWORD VALUE
          ...)’, where each KEYWORD is a face attribute name and VALUE
          is a value for that attribute.

        • A list of faces.  Each list element should be either a face
          name or an anonymous face.  This specifies a face which is an
          aggregate of the attributes of each of the listed faces.
          Faces occurring earlier in the list have higher priority.

        • A cons cell of the form ‘(foreground-color . COLOR-NAME)’ or
          ‘(background-color . COLOR-NAME)’.  This specifies the
          foreground or background color, similar to ‘(:foreground
          COLOR-NAME)’ or ‘(:background COLOR-NAME)’.  This form is
          supported for backward compatibility only, and should be
          avoided.

     Font Lock mode (*note Font Lock Mode::) works in most buffers by
     dynamically updating the ‘face’ property of characters based on the
     context.

     The ‘add-face-text-property’ function provides a convenient way to
     set this text property.  *Note Changing Properties::.

‘font-lock-face’
     This property specifies a value for the ‘face’ property that Font
     Lock mode should apply to the underlying text.  It is one of the
     fontification methods used by Font Lock mode, and is useful for
     special modes that implement their own highlighting.  *Note
     Precalculated Fontification::.  When Font Lock mode is disabled,
     ‘font-lock-face’ has no effect.

‘mouse-face’
     This property is used instead of ‘face’ when the mouse is on or
     near the character.  For this purpose, “near” means that all text
     between the character and where the mouse is have the same
     ‘mouse-face’ property value.

     Emacs ignores all face attributes from the ‘mouse-face’ property
     that alter the text size (e.g., ‘:height’, ‘:weight’, and
     ‘:slant’).  Those attributes are always the same as for the
     unhighlighted text.

‘fontified’
     This property says whether the text is ready for display.  If
     ‘nil’, Emacs’s redisplay routine calls the functions in
     ‘fontification-functions’ (*note Auto Faces::) to prepare this part
     of the buffer before it is displayed.  It is used internally by the
     just-in-time font locking code.

‘display’
     This property activates various features that change the way text
     is displayed.  For example, it can make text appear taller or
     shorter, higher or lower, wider or narrow, or replaced with an
     image.  *Note Display Property::.

‘help-echo’
     If text has a string as its ‘help-echo’ property, then when you
     move the mouse onto that text, Emacs displays that string in the
     echo area, or in the tooltip window (*note Tooltips::).

     If the value of the ‘help-echo’ property is a function, that
     function is called with three arguments, WINDOW, OBJECT and POS and
     should return a help string or ‘nil’ for none.  The first argument,
     WINDOW is the window in which the help was found.  The second,
     OBJECT, is the buffer, overlay or string which had the ‘help-echo’
     property.  The POS argument is as follows:

        • If OBJECT is a buffer, POS is the position in the buffer.
        • If OBJECT is an overlay, that overlay has a ‘help-echo’
          property, and POS is the position in the overlay’s buffer.
        • If OBJECT is a string (an overlay string or a string displayed
          with the ‘display’ property), POS is the position in that
          string.

     If the value of the ‘help-echo’ property is neither a function nor
     a string, it is evaluated to obtain a help string.

     You can alter the way help text is displayed by setting the
     variable ‘show-help-function’ (*note Help display::).

     This feature is used in the mode line and for other active text.

‘keymap’
     The ‘keymap’ property specifies an additional keymap for commands.
     When this keymap applies, it is used for key lookup before the
     minor mode keymaps and before the buffer’s local map.  *Note Active
     Keymaps::.  If the property value is a symbol, the symbol’s
     function definition is used as the keymap.

     The property’s value for the character before point applies if it
     is non-‘nil’ and rear-sticky, and the property’s value for the
     character after point applies if it is non-‘nil’ and front-sticky.
     (For mouse clicks, the position of the click is used instead of the
     position of point.)

‘local-map’
     This property works like ‘keymap’ except that it specifies a keymap
     to use _instead of_ the buffer’s local map.  For most purposes
     (perhaps all purposes), it is better to use the ‘keymap’ property.

‘syntax-table’
     The ‘syntax-table’ property overrides what the syntax table says
     about this particular character.  *Note Syntax Properties::.

‘read-only’
     If a character has the property ‘read-only’, then modifying that
     character is not allowed.  Any command that would do so gets an
     error, ‘text-read-only’.  If the property value is a string, that
     string is used as the error message.

     Insertion next to a read-only character is an error if inserting
     ordinary text there would inherit the ‘read-only’ property due to
     stickiness.  Thus, you can control permission to insert next to
     read-only text by controlling the stickiness.  *Note Sticky
     Properties::.

     Since changing properties counts as modifying the buffer, it is not
     possible to remove a ‘read-only’ property unless you know the
     special trick: bind ‘inhibit-read-only’ to a non-‘nil’ value and
     then remove the property.  *Note Read Only Buffers::.

‘inhibit-read-only’
     Characters that have the property ‘inhibit-read-only’ can be edited
     even in read-only buffers.  *Note Read Only Buffers::.

‘invisible’
     A non-‘nil’ ‘invisible’ property can make a character invisible on
     the screen.  *Note Invisible Text::, for details.

‘intangible’
     If a group of consecutive characters have equal and non-‘nil’
     ‘intangible’ properties, then you cannot place point between them.
     If you try to move point forward into the group, point actually
     moves to the end of the group.  If you try to move point backward
     into the group, point actually moves to the start of the group.

     If consecutive characters have unequal non-‘nil’ ‘intangible’
     properties, they belong to separate groups; each group is
     separately treated as described above.

     When the variable ‘inhibit-point-motion-hooks’ is non-‘nil’ (as it
     is by default), the ‘intangible’ property is ignored.

     Beware: this property operates at a very low level, and affects a
     lot of code in unexpected ways.  So use it with extreme caution.  A
     common misuse is to put an intangible property on invisible text,
     which is actually unnecessary since the command loop will move
     point outside of the invisible text at the end of each command
     anyway.  *Note Adjusting Point::.  For these reasons, this property
     is obsolete; use the ‘cursor-intangible’ property instead.

‘cursor-intangible’
     When the minor mode ‘cursor-intangible-mode’ is turned on, point is
     moved away of any position that has a non-‘nil’ ‘cursor-intangible’
     property, just before redisplay happens.

‘field’
     Consecutive characters with the same ‘field’ property constitute a
     “field”.  Some motion functions including ‘forward-word’ and
     ‘beginning-of-line’ stop moving at a field boundary.  *Note
     Fields::.

‘cursor’
     Normally, the cursor is displayed at the beginning or the end of
     any overlay and text property strings present at the current buffer
     position.  You can place the cursor on any desired character of
     these strings by giving that character a non-‘nil’ ‘cursor’ text
     property.  In addition, if the value of the ‘cursor’ property is an
     integer, it specifies the number of buffer’s character positions,
     starting with the position where the overlay or the ‘display’
     property begins, for which the cursor should be displayed on that
     character.  Specifically, if the value of the ‘cursor’ property of
     a character is the number N, the cursor will be displayed on this
     character for any buffer position in the range ‘[OVPOS..OVPOS+N)’,
     where OVPOS is the overlay’s starting position given by
     ‘overlay-start’ (*note Managing Overlays::), or the position where
     the ‘display’ text property begins in the buffer.

     In other words, the string character with the ‘cursor’ property of
     any non-‘nil’ value is the character where to display the cursor.
     The value of the property says for which buffer positions to
     display the cursor there.  If the value is an integer N, the cursor
     is displayed there when point is anywhere between the beginning of
     the overlay or ‘display’ property and N positions after that.  If
     the value is anything else and non-‘nil’, the cursor is displayed
     there only when point is at the beginning of the ‘display’ property
     or at ‘overlay-start’.

     When the buffer has many overlay strings (e.g., *note
     before-string: Overlay Properties.) that conceal some of the buffer
     text or ‘display’ properties that are strings, it is a good idea to
     use the ‘cursor’ property on these strings to cue the Emacs display
     about the places where to put the cursor while traversing these
     strings.  This directly communicates to the display engine where
     the Lisp program wants to put the cursor, or where the user would
     expect the cursor, when point is located on some buffer position
     that is “covered” by the display or overlay string.

‘pointer’
     This specifies a specific pointer shape when the mouse pointer is
     over this text or image.  *Note Pointer Shape::, for possible
     pointer shapes.

‘line-spacing’
     A newline can have a ‘line-spacing’ text or overlay property that
     controls the height of the display line ending with that newline.
     The property value overrides the default frame line spacing and the
     buffer local ‘line-spacing’ variable.  *Note Line Height::.

‘line-height’
     A newline can have a ‘line-height’ text or overlay property that
     controls the total height of the display line ending in that
     newline.  *Note Line Height::.

‘wrap-prefix’
     If text has a ‘wrap-prefix’ property, the prefix it defines will be
     added at display time to the beginning of every continuation line
     due to text wrapping (so if lines are truncated, the wrap-prefix is
     never used).  It may be a string or an image (*note Other Display
     Specs::), or a stretch of whitespace such as specified by the
     ‘:width’ or ‘:align-to’ display properties (*note Specified
     Space::).

     A wrap-prefix may also be specified for an entire buffer using the
     ‘wrap-prefix’ buffer-local variable (however, a ‘wrap-prefix’
     text-property takes precedence over the value of the ‘wrap-prefix’
     variable).  *Note Truncation::.

‘line-prefix’
     If text has a ‘line-prefix’ property, the prefix it defines will be
     added at display time to the beginning of every non-continuation
     line.  It may be a string or an image (*note Other Display
     Specs::), or a stretch of whitespace such as specified by the
     ‘:width’ or ‘:align-to’ display properties (*note Specified
     Space::).

     A line-prefix may also be specified for an entire buffer using the
     ‘line-prefix’ buffer-local variable (however, a ‘line-prefix’
     text-property takes precedence over the value of the ‘line-prefix’
     variable).  *Note Truncation::.

‘modification-hooks’
     If a character has the property ‘modification-hooks’, then its
     value should be a list of functions; modifying that character calls
     all of those functions before the actual modification.  Each
     function receives two arguments: the beginning and end of the part
     of the buffer being modified.  Note that if a particular
     modification hook function appears on several characters being
     modified by a single primitive, you can’t predict how many times
     the function will be called.  Furthermore, insertion will not
     modify any existing character, so this hook will only be run when
     removing some characters, replacing them with others, or changing
     their text-properties.

     If these functions modify the buffer, they should bind
     ‘inhibit-modification-hooks’ to ‘t’ around doing so, to avoid
     confusing the internal mechanism that calls these hooks.

     Overlays also support the ‘modification-hooks’ property, but the
     details are somewhat different (*note Overlay Properties::).

‘insert-in-front-hooks’
‘insert-behind-hooks’
     The operation of inserting text in a buffer also calls the
     functions listed in the ‘insert-in-front-hooks’ property of the
     following character and in the ‘insert-behind-hooks’ property of
     the preceding character.  These functions receive two arguments,
     the beginning and end of the inserted text.  The functions are
     called _after_ the actual insertion takes place.

     See also *note Change Hooks::, for other hooks that are called when
     you change text in a buffer.

‘point-entered’
‘point-left’
     The special properties ‘point-entered’ and ‘point-left’ record hook
     functions that report motion of point.  Each time point moves,
     Emacs compares these two property values:

        • the ‘point-left’ property of the character after the old
          location, and
        • the ‘point-entered’ property of the character after the new
          location.

     If these two values differ, each of them is called (if not ‘nil’)
     with two arguments: the old value of point, and the new one.

     The same comparison is made for the characters before the old and
     new locations.  The result may be to execute two ‘point-left’
     functions (which may be the same function) and/or two
     ‘point-entered’ functions (which may be the same function).  In any
     case, all the ‘point-left’ functions are called first, followed by
     all the ‘point-entered’ functions.

     It is possible to use ‘char-after’ to examine characters at various
     buffer positions without moving point to those positions.  Only an
     actual change in the value of point runs these hook functions.

     The variable ‘inhibit-point-motion-hooks’ by default inhibits
     running the ‘point-left’ and ‘point-entered’ hooks, see *note
     Inhibit point motion hooks::.

     These properties are obsolete; please use ‘cursor-sensor-functions’
     instead.

‘cursor-sensor-functions’
     This special property records a list of functions that react to
     cursor motion.  Each function in the list is called, just before
     redisplay, with 3 arguments: the affected window, the previous
     known position of the cursor, and one of the symbols ‘entered’ or
     ‘left’, depending on whether the cursor is entering the text that
     has this property or leaving it.  The functions are called only
     when the minor mode ‘cursor-sensor-mode’ is turned on.

‘composition’
     This text property is used to display a sequence of characters as a
     single glyph composed from components.  But the value of the
     property itself is completely internal to Emacs and should not be
     manipulated directly by, for instance, ‘put-text-property’.

 -- Variable: inhibit-point-motion-hooks
     When this obsolete variable is non-‘nil’, ‘point-left’ and
     ‘point-entered’ hooks are not run, and the ‘intangible’ property
     has no effect.  Do not set this variable globally; bind it with
     ‘let’.  Since the affected properties are obsolete, this variable’s
     default value is ‘t’, to effectively disable them.

 -- Variable: show-help-function
     If this variable is non-‘nil’, it specifies a function called to
     display help strings.  These may be ‘help-echo’ properties, menu
     help strings (*note Simple Menu Items::, *note Extended Menu
     Items::), or tool bar help strings (*note Tool Bar::).  The
     specified function is called with one argument, the help string to
     display, which is passed through ‘substitute-command-keys’ before
     being given to the function; see *note Keys in Documentation::.
     Tooltip mode (*note (emacs)Tooltips::) provides an example.


File: elisp.info,  Node: Format Properties,  Next: Sticky Properties,  Prev: Special Properties,  Up: Text Properties

31.19.5 Formatted Text Properties
---------------------------------

These text properties affect the behavior of the fill commands.  They
are used for representing formatted text.  *Note Filling::, and *note
Margins::.

‘hard’
     If a newline character has this property, it is a “hard” newline.
     The fill commands do not alter hard newlines and do not move words
     across them.  However, this property takes effect only if the
     ‘use-hard-newlines’ minor mode is enabled.  *Note Hard and Soft
     Newlines: (emacs)Hard and Soft Newlines.

‘right-margin’
     This property specifies an extra right margin for filling this part
     of the text.

‘left-margin’
     This property specifies an extra left margin for filling this part
     of the text.

‘justification’
     This property specifies the style of justification for filling this
     part of the text.


File: elisp.info,  Node: Sticky Properties,  Next: Lazy Properties,  Prev: Format Properties,  Up: Text Properties

31.19.6 Stickiness of Text Properties
-------------------------------------

Self-inserting characters, the ones that get inserted into a buffer when
the user types them (*note Commands for Insertion::), normally take on
the same properties as the preceding character.  This is called
“inheritance” of properties.

   By contrast, a Lisp program can do insertion with inheritance or
without, depending on the choice of insertion primitive.  The ordinary
text insertion functions, such as ‘insert’, do not inherit any
properties.  They insert text with precisely the properties of the
string being inserted, and no others.  This is correct for programs that
copy text from one context to another—for example, into or out of the
kill ring.  To insert with inheritance, use the special primitives
described in this section.  Self-inserting characters inherit properties
because they work using these primitives.

   When you do insertion with inheritance, _which_ properties are
inherited, and from where, depends on which properties are “sticky”.
Insertion after a character inherits those of its properties that are
“rear-sticky”.  Insertion before a character inherits those of its
properties that are “front-sticky”.  When both sides offer different
sticky values for the same property, the previous character’s value
takes precedence.

   By default, a text property is rear-sticky but not front-sticky;
thus, the default is to inherit all the properties of the preceding
character, and nothing from the following character.

   You can control the stickiness of various text properties with two
specific text properties, ‘front-sticky’ and ‘rear-nonsticky’, and with
the variable ‘text-property-default-nonsticky’.  You can use the
variable to specify a different default for a given property.  You can
use those two text properties to make any specific properties sticky or
nonsticky in any particular part of the text.

   If a character’s ‘front-sticky’ property is ‘t’, then all its
properties are front-sticky.  If the ‘front-sticky’ property is a list,
then the sticky properties of the character are those whose names are in
the list.  For example, if a character has a ‘front-sticky’ property
whose value is ‘(face read-only)’, then insertion before the character
can inherit its ‘face’ property and its ‘read-only’ property, but no
others.

   The ‘rear-nonsticky’ property works the opposite way.  Most
properties are rear-sticky by default, so the ‘rear-nonsticky’ property
says which properties are _not_ rear-sticky.  If a character’s
‘rear-nonsticky’ property is ‘t’, then none of its properties are
rear-sticky.  If the ‘rear-nonsticky’ property is a list, properties are
rear-sticky _unless_ their names are in the list.

 -- Variable: text-property-default-nonsticky
     This variable holds an alist which defines the default
     rear-stickiness of various text properties.  Each element has the
     form ‘(PROPERTY . NONSTICKINESS)’, and it defines the stickiness of
     a particular text property, PROPERTY.

     If NONSTICKINESS is non-‘nil’, this means that the property
     PROPERTY is rear-nonsticky by default.  Since all properties are
     front-nonsticky by default, this makes PROPERTY nonsticky in both
     directions by default.

     The text properties ‘front-sticky’ and ‘rear-nonsticky’, when used,
     take precedence over the default NONSTICKINESS specified in
     ‘text-property-default-nonsticky’.

   Here are the functions that insert text with inheritance of
properties:

 -- Function: insert-and-inherit &rest strings
     Insert the strings STRINGS, just like the function ‘insert’, but
     inherit any sticky properties from the adjoining text.

 -- Function: insert-before-markers-and-inherit &rest strings
     Insert the strings STRINGS, just like the function
     ‘insert-before-markers’, but inherit any sticky properties from the
     adjoining text.

   *Note Insertion::, for the ordinary insertion functions which do not
inherit.


File: elisp.info,  Node: Lazy Properties,  Next: Clickable Text,  Prev: Sticky Properties,  Up: Text Properties

31.19.7 Lazy Computation of Text Properties
-------------------------------------------

Instead of computing text properties for all the text in the buffer, you
can arrange to compute the text properties for parts of the text when
and if something depends on them.

   The primitive that extracts text from the buffer along with its
properties is ‘buffer-substring’.  Before examining the properties, this
function runs the abnormal hook ‘buffer-access-fontify-functions’.

 -- Variable: buffer-access-fontify-functions
     This variable holds a list of functions for computing text
     properties.  Before ‘buffer-substring’ copies the text and text
     properties for a portion of the buffer, it calls all the functions
     in this list.  Each of the functions receives two arguments that
     specify the range of the buffer being accessed.  (The buffer itself
     is always the current buffer.)

   The function ‘buffer-substring-no-properties’ does not call these
functions, since it ignores text properties anyway.

   In order to prevent the hook functions from being called more than
once for the same part of the buffer, you can use the variable
‘buffer-access-fontified-property’.

 -- Variable: buffer-access-fontified-property
     If this variable’s value is non-‘nil’, it is a symbol which is used
     as a text property name.  A non-‘nil’ value for that text property
     means the other text properties for this character have already
     been computed.

     If all the characters in the range specified for ‘buffer-substring’
     have a non-‘nil’ value for this property, ‘buffer-substring’ does
     not call the ‘buffer-access-fontify-functions’ functions.  It
     assumes these characters already have the right text properties,
     and just copies the properties they already have.

     The normal way to use this feature is that the
     ‘buffer-access-fontify-functions’ functions add this property, as
     well as others, to the characters they operate on.  That way, they
     avoid being called over and over for the same text.


File: elisp.info,  Node: Clickable Text,  Next: Fields,  Prev: Lazy Properties,  Up: Text Properties

31.19.8 Defining Clickable Text
-------------------------------

“Clickable text” is text that can be clicked, with either the mouse or
via a keyboard command, to produce some result.  Many major modes use
clickable text to implement textual hyper-links, or “links” for short.

   The easiest way to insert and manipulate links is to use the ‘button’
package.  *Note Buttons::.  In this section, we will explain how to
manually set up clickable text in a buffer, using text properties.  For
simplicity, we will refer to the clickable text as a “link”.

   Implementing a link involves three separate steps: (1) indicating
clickability when the mouse moves over the link; (2) making <RET> or
‘mouse-2’ on that link do something; and (3) setting up a ‘follow-link’
condition so that the link obeys ‘mouse-1-click-follows-link’.

   To indicate clickability, add the ‘mouse-face’ text property to the
text of the link; then Emacs will highlight the link when the mouse
moves over it.  In addition, you should define a tooltip or echo area
message, using the ‘help-echo’ text property.  *Note Special
Properties::.  For instance, here is how Dired indicates that file names
are clickable:

      (if (dired-move-to-filename)
          (add-text-properties
            (point)
            (save-excursion
              (dired-move-to-end-of-filename)
              (point))
            '(mouse-face highlight
              help-echo "mouse-2: visit this file in other window")))

   To make the link clickable, bind <RET> and ‘mouse-2’ to commands that
perform the desired action.  Each command should check to see whether it
was called on a link, and act accordingly.  For instance, Dired’s major
mode keymap binds ‘mouse-2’ to the following command:

     (defun dired-mouse-find-file-other-window (event)
       "In Dired, visit the file or directory name you click on."
       (interactive "e")
       (let ((window (posn-window (event-end event)))
             (pos (posn-point (event-end event)))
             file)
         (if (not (windowp window))
             (error "No file chosen"))
         (with-current-buffer (window-buffer window)
           (goto-char pos)
           (setq file (dired-get-file-for-visit)))
         (if (file-directory-p file)
             (or (and (cdr dired-subdir-alist)
                      (dired-goto-subdir file))
                 (progn
                   (select-window window)
                   (dired-other-window file)))
           (select-window window)
           (find-file-other-window (file-name-sans-versions file t)))))

This command uses the functions ‘posn-window’ and ‘posn-point’ to
determine where the click occurred, and ‘dired-get-file-for-visit’ to
determine which file to visit.

   Instead of binding the mouse command in a major mode keymap, you can
bind it within the link text, using the ‘keymap’ text property (*note
Special Properties::).  For instance:

     (let ((map (make-sparse-keymap)))
       (define-key map [mouse-2] 'operate-this-button)
       (put-text-property link-start link-end 'keymap map))

With this method, you can easily define different commands for different
links.  Furthermore, the global definition of <RET> and ‘mouse-2’ remain
available for the rest of the text in the buffer.

   The basic Emacs command for clicking on links is ‘mouse-2’.  However,
for compatibility with other graphical applications, Emacs also
recognizes ‘mouse-1’ clicks on links, provided the user clicks on the
link quickly without moving the mouse.  This behavior is controlled by
the user option ‘mouse-1-click-follows-link’.  *Note (emacs)Mouse
References::.

   To set up the link so that it obeys ‘mouse-1-click-follows-link’, you
must either (1) apply a ‘follow-link’ text or overlay property to the
link text, or (2) bind the ‘follow-link’ event to a keymap (which can be
a major mode keymap or a local keymap specified via the ‘keymap’ text
property).  The value of the ‘follow-link’ property, or the binding for
the ‘follow-link’ event, acts as a condition for the link action.  This
condition tells Emacs two things: the circumstances under which a
‘mouse-1’ click should be regarded as occurring inside the link, and how
to compute an action code that says what to translate the ‘mouse-1’
click into.  The link action condition can be one of the following:

‘mouse-face’
     If the condition is the symbol ‘mouse-face’, a position is inside a
     link if there is a non-‘nil’ ‘mouse-face’ property at that
     position.  The action code is always ‘t’.

     For example, here is how Info mode handles <mouse-1>:

          (define-key Info-mode-map [follow-link] 'mouse-face)

a function
     If the condition is a function, FUNC, then a position POS is inside
     a link if ‘(FUNC POS)’ evaluates to non-‘nil’.  The value returned
     by FUNC serves as the action code.

     For example, here is how pcvs enables ‘mouse-1’ to follow links on
     file names only:

          (define-key map [follow-link]
            (lambda (pos)
              (eq (get-char-property pos 'face) 'cvs-filename-face)))

anything else
     If the condition value is anything else, then the position is
     inside a link and the condition itself is the action code.
     Clearly, you should specify this kind of condition only when
     applying the condition via a text or property overlay on the link
     text (so that it does not apply to the entire buffer).

The action code tells ‘mouse-1’ how to follow the link:

a string or vector
     If the action code is a string or vector, the ‘mouse-1’ event is
     translated into the first element of the string or vector; i.e.,
     the action of the ‘mouse-1’ click is the local or global binding of
     that character or symbol.  Thus, if the action code is ‘"foo"’,
     ‘mouse-1’ translates into ‘f’.  If it is ‘[foo]’, ‘mouse-1’
     translates into <foo>.

anything else
     For any other non-‘nil’ action code, the ‘mouse-1’ event is
     translated into a ‘mouse-2’ event at the same position.

   To define ‘mouse-1’ to activate a button defined with
‘define-button-type’, give the button a ‘follow-link’ property.  The
property value should be a link action condition, as described above.
*Note Buttons::.  For example, here is how Help mode handles ‘mouse-1’:

     (define-button-type 'help-xref
       'follow-link t
       'action #'help-button-action)

   To define ‘mouse-1’ on a widget defined with ‘define-widget’, give
the widget a ‘:follow-link’ property.  The property value should be a
link action condition, as described above.  For example, here is how the
‘link’ widget specifies that a <mouse-1> click shall be translated to
<RET>:

     (define-widget 'link 'item
       "An embedded link."
       :button-prefix 'widget-link-prefix
       :button-suffix 'widget-link-suffix
       :follow-link "\C-m"
       :help-echo "Follow the link."
       :format "%[%t%]")

 -- Function: mouse-on-link-p pos
     This function returns non-‘nil’ if position POS in the current
     buffer is on a link.  POS can also be a mouse event location, as
     returned by ‘event-start’ (*note Accessing Mouse::).


File: elisp.info,  Node: Fields,  Next: Not Intervals,  Prev: Clickable Text,  Up: Text Properties

31.19.9 Defining and Using Fields
---------------------------------

A field is a range of consecutive characters in the buffer that are
identified by having the same value (comparing with ‘eq’) of the ‘field’
property (either a text-property or an overlay property).  This section
describes special functions that are available for operating on fields.

   You specify a field with a buffer position, POS.  We think of each
field as containing a range of buffer positions, so the position you
specify stands for the field containing that position.

   When the characters before and after POS are part of the same field,
there is no doubt which field contains POS: the one those characters
both belong to.  When POS is at a boundary between fields, which field
it belongs to depends on the stickiness of the ‘field’ properties of the
two surrounding characters (*note Sticky Properties::).  The field whose
property would be inherited by text inserted at POS is the field that
contains POS.

   There is an anomalous case where newly inserted text at POS would not
inherit the ‘field’ property from either side.  This happens if the
previous character’s ‘field’ property is not rear-sticky, and the
following character’s ‘field’ property is not front-sticky.  In this
case, POS belongs to neither the preceding field nor the following
field; the field functions treat it as belonging to an empty field whose
beginning and end are both at POS.

   In all of these functions, if POS is omitted or ‘nil’, the value of
point is used by default.  If narrowing is in effect, then POS should
fall within the accessible portion.  *Note Narrowing::.

 -- Function: field-beginning &optional pos escape-from-edge limit
     This function returns the beginning of the field specified by POS.

     If POS is at the beginning of its field, and ESCAPE-FROM-EDGE is
     non-‘nil’, then the return value is always the beginning of the
     preceding field that _ends_ at POS, regardless of the stickiness of
     the ‘field’ properties around POS.

     If LIMIT is non-‘nil’, it is a buffer position; if the beginning of
     the field is before LIMIT, then LIMIT will be returned instead.

 -- Function: field-end &optional pos escape-from-edge limit
     This function returns the end of the field specified by POS.

     If POS is at the end of its field, and ESCAPE-FROM-EDGE is
     non-‘nil’, then the return value is always the end of the following
     field that _begins_ at POS, regardless of the stickiness of the
     ‘field’ properties around POS.

     If LIMIT is non-‘nil’, it is a buffer position; if the end of the
     field is after LIMIT, then LIMIT will be returned instead.

 -- Function: field-string &optional pos
     This function returns the contents of the field specified by POS,
     as a string.

 -- Function: field-string-no-properties &optional pos
     This function returns the contents of the field specified by POS,
     as a string, discarding text properties.

 -- Function: delete-field &optional pos
     This function deletes the text of the field specified by POS.

 -- Function: constrain-to-field new-pos old-pos &optional
          escape-from-edge only-in-line inhibit-capture-property
     This function constrains NEW-POS to the field that OLD-POS belongs
     to—in other words, it returns the position closest to NEW-POS that
     is in the same field as OLD-POS.

     If NEW-POS is ‘nil’, then ‘constrain-to-field’ uses the value of
     point instead, and moves point to the resulting position in
     addition to returning that position.

     If OLD-POS is at the boundary of two fields, then the acceptable
     final positions depend on the argument ESCAPE-FROM-EDGE.  If
     ESCAPE-FROM-EDGE is ‘nil’, then NEW-POS must be in the field whose
     ‘field’ property equals what new characters inserted at OLD-POS
     would inherit.  (This depends on the stickiness of the ‘field’
     property for the characters before and after OLD-POS.)  If
     ESCAPE-FROM-EDGE is non-‘nil’, NEW-POS can be anywhere in the two
     adjacent fields.  Additionally, if two fields are separated by
     another field with the special value ‘boundary’, then any point
     within this special field is also considered to be on the boundary.

     Commands like ‘C-a’ with no argument, that normally move backward
     to a specific kind of location and stay there once there, probably
     should specify ‘nil’ for ESCAPE-FROM-EDGE.  Other motion commands
     that check fields should probably pass ‘t’.

     If the optional argument ONLY-IN-LINE is non-‘nil’, and
     constraining NEW-POS in the usual way would move it to a different
     line, NEW-POS is returned unconstrained.  This used in commands
     that move by line, such as ‘next-line’ and ‘beginning-of-line’, so
     that they respect field boundaries only in the case where they can
     still move to the right line.

     If the optional argument INHIBIT-CAPTURE-PROPERTY is non-‘nil’, and
     OLD-POS has a non-‘nil’ property of that name, then any field
     boundaries are ignored.

     You can cause ‘constrain-to-field’ to ignore all field boundaries
     (and so never constrain anything) by binding the variable
     ‘inhibit-field-text-motion’ to a non-‘nil’ value.


File: elisp.info,  Node: Not Intervals,  Prev: Fields,  Up: Text Properties

31.19.10 Why Text Properties are not Intervals
----------------------------------------------

Some editors that support adding attributes to text in the buffer do so
by letting the user specify intervals within the text, and adding the
properties to the intervals.  Those editors permit the user or the
programmer to determine where individual intervals start and end.  We
deliberately provided a different sort of interface in Emacs Lisp to
avoid certain paradoxical behavior associated with text modification.

   If the actual subdivision into intervals is meaningful, that means
you can distinguish between a buffer that is just one interval with a
certain property, and a buffer containing the same text subdivided into
two intervals, both of which have that property.

   Suppose you take the buffer with just one interval and kill part of
the text.  The text remaining in the buffer is one interval, and the
copy in the kill ring (and the undo list) becomes a separate interval.
Then if you yank back the killed text, you get two intervals with the
same properties.  Thus, editing does not preserve the distinction
between one interval and two.

   Suppose we attempt to fix this problem by coalescing the two
intervals when the text is inserted.  That works fine if the buffer
originally was a single interval.  But suppose instead that we have two
adjacent intervals with the same properties, and we kill the text of one
interval and yank it back.  The same interval-coalescence feature that
rescues the other case causes trouble in this one: after yanking, we
have just one interval.  Once again, editing does not preserve the
distinction between one interval and two.

   Insertion of text at the border between intervals also raises
questions that have no satisfactory answer.

   However, it is easy to arrange for editing to behave consistently for
questions of the form, “What are the properties of text at this buffer
or string position?” So we have decided these are the only questions
that make sense; we have not implemented asking questions about where
intervals start or end.

   In practice, you can usually use the text property search functions
in place of explicit interval boundaries.  You can think of them as
finding the boundaries of intervals, assuming that intervals are always
coalesced whenever possible.  *Note Property Search::.

   Emacs also provides explicit intervals as a presentation feature; see
*note Overlays::.


File: elisp.info,  Node: Substitution,  Next: Registers,  Prev: Text Properties,  Up: Text

31.20 Substituting for a Character Code
=======================================

The following functions replace characters within a specified region
based on their character codes.

 -- Function: subst-char-in-region start end old-char new-char &optional
          noundo
     This function replaces all occurrences of the character OLD-CHAR
     with the character NEW-CHAR in the region of the current buffer
     defined by START and END.

     If NOUNDO is non-‘nil’, then ‘subst-char-in-region’ does not record
     the change for undo and does not mark the buffer as modified.  This
     was useful for controlling the old selective display feature (*note
     Selective Display::).

     ‘subst-char-in-region’ does not move point and returns ‘nil’.

          ---------- Buffer: foo ----------
          This is the contents of the buffer before.
          ---------- Buffer: foo ----------

          (subst-char-in-region 1 20 ?i ?X)
               ⇒ nil

          ---------- Buffer: foo ----------
          ThXs Xs the contents of the buffer before.
          ---------- Buffer: foo ----------

 -- Command: translate-region start end table
     This function applies a translation table to the characters in the
     buffer between positions START and END.

     The translation table TABLE is a string or a char-table; ‘(aref
     TABLE OCHAR)’ gives the translated character corresponding to
     OCHAR.  If TABLE is a string, any characters with codes larger than
     the length of TABLE are not altered by the translation.

     The return value of ‘translate-region’ is the number of characters
     that were actually changed by the translation.  This does not count
     characters that were mapped into themselves in the translation
     table.


File: elisp.info,  Node: Registers,  Next: Transposition,  Prev: Substitution,  Up: Text

31.21 Registers
===============

A register is a sort of variable used in Emacs editing that can hold a
variety of different kinds of values.  Each register is named by a
single character.  All ASCII characters and their meta variants (but
with the exception of ‘C-g’) can be used to name registers.  Thus, there
are 255 possible registers.  A register is designated in Emacs Lisp by
the character that is its name.

 -- Variable: register-alist
     This variable is an alist of elements of the form ‘(NAME .
     CONTENTS)’.  Normally, there is one element for each Emacs register
     that has been used.

     The object NAME is a character (an integer) identifying the
     register.

   The CONTENTS of a register can have several possible types:

a number
     A number stands for itself.  If ‘insert-register’ finds a number in
     the register, it converts the number to decimal.

a marker
     A marker represents a buffer position to jump to.

a string
     A string is text saved in the register.

a rectangle
     A rectangle is represented by a list of strings.

‘(WINDOW-CONFIGURATION POSITION)’
     This represents a window configuration to restore in one frame, and
     a position to jump to in the current buffer.

‘(FRAME-CONFIGURATION POSITION)’
     This represents a frame configuration to restore, and a position to
     jump to in the current buffer.

(file FILENAME)
     This represents a file to visit; jumping to this value visits file
     FILENAME.

(file-query FILENAME POSITION)
     This represents a file to visit and a position in it; jumping to
     this value visits file FILENAME and goes to buffer position
     POSITION.  Restoring this type of position asks the user for
     confirmation first.

   The functions in this section return unpredictable values unless
otherwise stated.

 -- Function: get-register reg
     This function returns the contents of the register REG, or ‘nil’ if
     it has no contents.

 -- Function: set-register reg value
     This function sets the contents of register REG to VALUE.  A
     register can be set to any value, but the other register functions
     expect only certain data types.  The return value is VALUE.

 -- Command: view-register reg
     This command displays what is contained in register REG.

 -- Command: insert-register reg &optional beforep
     This command inserts contents of register REG into the current
     buffer.

     Normally, this command puts point before the inserted text, and the
     mark after it.  However, if the optional second argument BEFOREP is
     non-‘nil’, it puts the mark before and point after.

     When called interactively, the command defaults to putting point
     after text, and a prefix argument inverts this behavior.

     If the register contains a rectangle, then the rectangle is
     inserted with its upper left corner at point.  This means that text
     is inserted in the current line and underneath it on successive
     lines.

     If the register contains something other than saved text (a string)
     or a rectangle (a list), currently useless things happen.  This may
     be changed in the future.

 -- Function: register-read-with-preview prompt
     This function reads and returns a register name, prompting with
     PROMPT and possibly showing a preview of the existing registers and
     their contents.  The preview is shown in a temporary window, after
     the delay specified by the user option ‘register-preview-delay’, if
     its value and ‘register-alist’ are both non-‘nil’.  The preview is
     also shown if the user requests help (e.g., by typing the help
     character).  We recommend that all interactive commands which read
     register names use this function.


File: elisp.info,  Node: Transposition,  Next: Decompression,  Prev: Registers,  Up: Text

31.22 Transposition of Text
===========================

This function can be used to transpose stretches of text:

 -- Function: transpose-regions start1 end1 start2 end2 &optional
          leave-markers
     This function exchanges two nonoverlapping portions of the buffer.
     Arguments START1 and END1 specify the bounds of one portion and
     arguments START2 and END2 specify the bounds of the other portion.

     Normally, ‘transpose-regions’ relocates markers with the transposed
     text; a marker previously positioned within one of the two
     transposed portions moves along with that portion, thus remaining
     between the same two characters in their new position.  However, if
     LEAVE-MARKERS is non-‘nil’, ‘transpose-regions’ does not do this—it
     leaves all markers unrelocated.


File: elisp.info,  Node: Decompression,  Next: Base 64,  Prev: Transposition,  Up: Text

31.23 Dealing With Compressed Data
==================================

When ‘auto-compression-mode’ is enabled, Emacs automatically
uncompresses compressed files when you visit them, and automatically
recompresses them if you alter and save them.  *Note (emacs)Compressed
Files::.

   The above feature works by calling an external executable (e.g.,
‘gzip’).  Emacs can also be compiled with support for built-in
decompression using the zlib library, which is faster than calling an
external program.

 -- Function: zlib-available-p
     This function returns non-‘nil’ if built-in zlib decompression is
     available.

 -- Function: zlib-decompress-region start end
     This function decompresses the region between START and END, using
     built-in zlib decompression.  The region should contain data that
     were compressed with gzip or zlib.  On success, the function
     replaces the contents of the region with the decompressed data.  On
     failure, the function leaves the region unchanged and returns
     ‘nil’.  This function can be called only in unibyte buffers.


File: elisp.info,  Node: Base 64,  Next: Checksum/Hash,  Prev: Decompression,  Up: Text

31.24 Base 64 Encoding
======================

Base 64 code is used in email to encode a sequence of 8-bit bytes as a
longer sequence of ASCII graphic characters.  It is defined in Internet
RFC(1)2045.  This section describes the functions for converting to and
from this code.

 -- Command: base64-encode-region beg end &optional no-line-break
     This function converts the region from BEG to END into base 64
     code.  It returns the length of the encoded text.  An error is
     signaled if a character in the region is multibyte, i.e., in a
     multibyte buffer the region must contain only characters from the
     charsets ‘ascii’, ‘eight-bit-control’ and ‘eight-bit-graphic’.

     Normally, this function inserts newline characters into the encoded
     text, to avoid overlong lines.  However, if the optional argument
     NO-LINE-BREAK is non-‘nil’, these newlines are not added, so the
     output is just one long line.

 -- Function: base64-encode-string string &optional no-line-break
     This function converts the string STRING into base 64 code.  It
     returns a string containing the encoded text.  As for
     ‘base64-encode-region’, an error is signaled if a character in the
     string is multibyte.

     Normally, this function inserts newline characters into the encoded
     text, to avoid overlong lines.  However, if the optional argument
     NO-LINE-BREAK is non-‘nil’, these newlines are not added, so the
     result string is just one long line.

 -- Command: base64-decode-region beg end
     This function converts the region from BEG to END from base 64 code
     into the corresponding decoded text.  It returns the length of the
     decoded text.

     The decoding functions ignore newline characters in the encoded
     text.

 -- Function: base64-decode-string string
     This function converts the string STRING from base 64 code into the
     corresponding decoded text.  It returns a unibyte string containing
     the decoded text.

     The decoding functions ignore newline characters in the encoded
     text.

   ---------- Footnotes ----------

   (1) An RFC, an acronym for “Request for Comments”, is a numbered
Internet informational document describing a standard.  RFCs are usually
written by technical experts acting on their own initiative, and are
traditionally written in a pragmatic, experience-driven manner.


File: elisp.info,  Node: Checksum/Hash,  Next: Parsing HTML/XML,  Prev: Base 64,  Up: Text

31.25 Checksum/Hash
===================

Emacs has built-in support for computing “cryptographic hashes”.  A
cryptographic hash, or “checksum”, is a digital fingerprint of a piece
of data (e.g., a block of text) which can be used to check that you have
an unaltered copy of that data.

   Emacs supports several common cryptographic hash algorithms: MD5,
SHA-1, SHA-2, SHA-224, SHA-256, SHA-384 and SHA-512.  MD5 is the oldest
of these algorithms, and is commonly used in “message digests” to check
the integrity of messages transmitted over a network.  MD5 is not
collision resistant (i.e., it is possible to deliberately design
different pieces of data which have the same MD5 hash), so you should
not used it for anything security-related.  A similar theoretical
weakness also exists in SHA-1.  Therefore, for security-related
applications you should use the other hash types, such as SHA-2.

 -- Function: secure-hash algorithm object &optional start end binary
     This function returns a hash for OBJECT.  The argument ALGORITHM is
     a symbol stating which hash to compute: one of ‘md5’, ‘sha1’,
     ‘sha224’, ‘sha256’, ‘sha384’ or ‘sha512’.  The argument OBJECT
     should be a buffer or a string.

     The optional arguments START and END are character positions
     specifying the portion of OBJECT to compute the message digest for.
     If they are ‘nil’ or omitted, the hash is computed for the whole of
     OBJECT.

     If the argument BINARY is omitted or ‘nil’, the function returns
     the “text form” of the hash, as an ordinary Lisp string.  If BINARY
     is non-‘nil’, it returns the hash in “binary form”, as a sequence
     of bytes stored in a unibyte string.

     This function does not compute the hash directly from the internal
     representation of OBJECT’s text (*note Text Representations::).
     Instead, it encodes the text using a coding system (*note Coding
     Systems::), and computes the hash from that encoded text.  If
     OBJECT is a buffer, the coding system used is the one which would
     be chosen by default for writing the text into a file.  If OBJECT
     is a string, the user’s preferred coding system is used (*note
     (emacs)Recognize Coding::).

 -- Function: md5 object &optional start end coding-system noerror
     This function returns an MD5 hash.  It is semi-obsolete, since for
     most purposes it is equivalent to calling ‘secure-hash’ with ‘md5’
     as the ALGORITHM argument.  The OBJECT, START and END arguments
     have the same meanings as in ‘secure-hash’.

     If CODING-SYSTEM is non-‘nil’, it specifies a coding system to use
     to encode the text; if omitted or ‘nil’, the default coding system
     is used, like in ‘secure-hash’.

     Normally, ‘md5’ signals an error if the text can’t be encoded using
     the specified or chosen coding system.  However, if NOERROR is
     non-‘nil’, it silently uses ‘raw-text’ coding instead.


File: elisp.info,  Node: Parsing HTML/XML,  Next: Atomic Changes,  Prev: Checksum/Hash,  Up: Text

31.26 Parsing HTML and XML
==========================

When Emacs is compiled with libxml2 support, the following functions are
available to parse HTML or XML text into Lisp object trees.

 -- Function: libxml-parse-html-region start end &optional base-url
          discard-comments
     This function parses the text between START and END as HTML, and
     returns a list representing the HTML “parse tree”.  It attempts to
     handle real-world HTML by robustly coping with syntax mistakes.

     The optional argument BASE-URL, if non-‘nil’, should be a string
     specifying the base URL for relative URLs occurring in links.

     If the optional argument DISCARD-COMMENTS is non-‘nil’, then the
     parse tree is created without any comments.

     In the parse tree, each HTML node is represented by a list in which
     the first element is a symbol representing the node name, the
     second element is an alist of node attributes, and the remaining
     elements are the subnodes.

     The following example demonstrates this.  Given this (malformed)
     HTML document:

          <html><head></head><body width=101><div class=thing>Foo<div>Yes

     A call to ‘libxml-parse-html-region’ returns this DOM (document
     object model):

          (html nil
           (head nil)
           (body ((width . "101"))
            (div ((class . "thing"))
             "Foo"
             (div nil
              "Yes"))))

 -- Function: shr-insert-document dom
     This function renders the parsed HTML in DOM into the current
     buffer.  The argument DOM should be a list as generated by
     ‘libxml-parse-html-region’.  This function is, e.g., used by *note
     EWW: (eww)Top.

 -- Function: libxml-parse-xml-region start end &optional base-url
          discard-comments
     This function is the same as ‘libxml-parse-html-region’, except
     that it parses the text as XML rather than HTML (so it is stricter
     about syntax).

* Menu:

* Document Object Model:: Access, manipulate and search the DOM.


File: elisp.info,  Node: Document Object Model,  Up: Parsing HTML/XML

31.26.1 Document Object Model
-----------------------------

The DOM returned by ‘libxml-parse-html-region’ (and the other XML
parsing functions) is a tree structure where each node has a node name
(called a “tag”), and optional key/value “attribute” list, and then a
list of “child nodes”.  The child nodes are either strings or DOM
objects.

     (body ((width . "101"))
      (div ((class . "thing"))
       "Foo"
       (div nil
        "Yes")))

 -- Function: dom-node tag &optional attributes &rest children
     This function creates a DOM node of type TAG.  If given, ATTRIBUTES
     should be a key/value pair list.  If given, CHILDREN should be DOM
     nodes.

   The following functions can be used to work with this structure.
Each function takes a DOM node, or a list of nodes.  In the latter case,
only the first node in the list is used.

   Simple accessors:

‘dom-tag NODE’
     Return the “tag” (also called “node name”) of the node.

‘dom-attr NODE ATTRIBUTE’
     Return the value of ATTRIBUTE in the node.  A common usage would
     be:

          (dom-attr img 'href)
          => "http://fsf.org/logo.png"

‘dom-children NODE’
     Return all the children of the node.

‘dom-non-text-children NODE’
     Return all the non-string children of the node.

‘dom-attributes NODE’
     Return the key/value pair list of attributes of the node.

‘dom-text NODE’
     Return all the textual elements of the node as a concatenated
     string.

‘dom-texts NODE’
     Return all the textual elements of the node, as well as the textual
     elements of all the children of the node, recursively, as a
     concatenated string.  This function also takes an optional
     separator to be inserted between the textual elements.

‘dom-parent DOM NODE’
     Return the parent of NODE in DOM.

   The following are functions for altering the DOM.

‘dom-set-attribute NODE ATTRIBUTE VALUE’
     Set the ATTRIBUTE of the node to VALUE.

‘dom-append-child NODE CHILD’
     Append CHILD as the last child of NODE.

‘dom-add-child-before NODE CHILD BEFORE’
     Add CHILD to NODE’s child list before the BEFORE node.  If BEFORE
     is ‘nil’, make CHILD the first child.

‘dom-set-attributes NODE ATTRIBUTES’
     Replace all the attributes of the node with a new key/value list.

   The following are functions for searching for elements in the DOM.
They all return lists of matching nodes.

‘dom-by-tag DOM TAG’
     Return all nodes in DOM that are of type TAG.  A typical use would
     be:

          (dom-by-tag dom 'td)
          => '((td ...) (td ...) (td ...))

‘dom-by-class DOM MATCH’
     Return all nodes in DOM that have class names that match MATCH,
     which is a regular expression.

‘dom-by-style DOM STYLE’
     Return all nodes in DOM that have styles that match MATCH, which is
     a regular expression.

‘dom-by-id DOM STYLE’
     Return all nodes in DOM that have IDs that match MATCH, which is a
     regular expression.

‘dom-strings DOM’
     Return all strings in DOM.

   Utility functions:

‘dom-pp DOM &optional REMOVE-EMPTY’
     Pretty-print DOM at point.  If REMOVE-EMPTY, don’t print textual
     nodes that just contain white-space.


File: elisp.info,  Node: Atomic Changes,  Next: Change Hooks,  Prev: Parsing HTML/XML,  Up: Text

31.27 Atomic Change Groups
==========================

In database terminology, an “atomic” change is an indivisible change—it
can succeed entirely or it can fail entirely, but it cannot partly
succeed.  A Lisp program can make a series of changes to one or several
buffers as an “atomic change group”, meaning that either the entire
series of changes will be installed in their buffers or, in case of an
error, none of them will be.

   To do this for one buffer, the one already current, simply write a
call to ‘atomic-change-group’ around the code that makes the changes,
like this:

     (atomic-change-group
       (insert foo)
       (delete-region x y))

If an error (or other nonlocal exit) occurs inside the body of
‘atomic-change-group’, it unmakes all the changes in that buffer that
were during the execution of the body.  This kind of change group has no
effect on any other buffers—any such changes remain.

   If you need something more sophisticated, such as to make changes in
various buffers constitute one atomic group, you must directly call
lower-level functions that ‘atomic-change-group’ uses.

 -- Function: prepare-change-group &optional buffer
     This function sets up a change group for buffer BUFFER, which
     defaults to the current buffer.  It returns a handle that
     represents the change group.  You must use this handle to activate
     the change group and subsequently to finish it.

   To use the change group, you must “activate” it.  You must do this
before making any changes in the text of BUFFER.

 -- Function: activate-change-group handle
     This function activates the change group that HANDLE designates.

   After you activate the change group, any changes you make in that
buffer become part of it.  Once you have made all the desired changes in
the buffer, you must “finish” the change group.  There are two ways to
do this: you can either accept (and finalize) all the changes, or cancel
them all.

 -- Function: accept-change-group handle
     This function accepts all the changes in the change group specified
     by HANDLE, making them final.

 -- Function: cancel-change-group handle
     This function cancels and undoes all the changes in the change
     group specified by HANDLE.

   Your code should use ‘unwind-protect’ to make sure the group is
always finished.  The call to ‘activate-change-group’ should be inside
the ‘unwind-protect’, in case the user types ‘C-g’ just after it runs.
(This is one reason why ‘prepare-change-group’ and
‘activate-change-group’ are separate functions, because normally you
would call ‘prepare-change-group’ before the start of that
‘unwind-protect’.)  Once you finish the group, don’t use the handle
again—in particular, don’t try to finish the same group twice.

   To make a multibuffer change group, call ‘prepare-change-group’ once
for each buffer you want to cover, then use ‘nconc’ to combine the
returned values, like this:

     (nconc (prepare-change-group buffer-1)
            (prepare-change-group buffer-2))

   You can then activate the multibuffer change group with a single call
to ‘activate-change-group’, and finish it with a single call to
‘accept-change-group’ or ‘cancel-change-group’.

   Nested use of several change groups for the same buffer works as you
would expect.  Non-nested use of change groups for the same buffer will
get Emacs confused, so don’t let it happen; the first change group you
start for any given buffer should be the last one finished.


File: elisp.info,  Node: Change Hooks,  Prev: Atomic Changes,  Up: Text

31.28 Change Hooks
==================

These hook variables let you arrange to take notice of changes in
buffers (or in a particular buffer, if you make them buffer-local).  See
also *note Special Properties::, for how to detect changes to specific
parts of the text.

   The functions you use in these hooks should save and restore the
match data if they do anything that uses regular expressions; otherwise,
they will interfere in bizarre ways with the editing operations that
call them.

 -- Variable: before-change-functions
     This variable holds a list of functions to call when Emacs is about
     to modify a buffer.  Each function gets two arguments, the
     beginning and end of the region that is about to change,
     represented as integers.  The buffer that is about to change is
     always the current buffer when the function is called.

 -- Variable: after-change-functions
     This variable holds a list of functions to call after Emacs
     modifies a buffer.  Each function receives three arguments: the
     beginning and end of the region just changed, and the length of the
     text that existed before the change.  All three arguments are
     integers.  The buffer that has been changed is always the current
     buffer when the function is called.

     The length of the old text is the difference between the buffer
     positions before and after that text as it was before the change.
     As for the changed text, its length is simply the difference
     between the first two arguments.

   Output of messages into the ‘*Messages*’ buffer does not call these
functions, and neither do certain internal buffer changes, such as
changes in buffers created by Emacs internally for certain jobs, that
should not be visible to Lisp programs.

   Do _not_ expect the before-change hooks and the after-change hooks be
called in balanced pairs around each buffer change.  Also don’t expect
the before-change hooks to be called for every chunk of text Emacs is
about to delete.  These hooks are provided on the assumption that Lisp
programs will use either before- or the after-change hooks, but not
both, and the boundaries of the region where the changes happen might
include more than just the actual changed text, or even lump together
several changes done piecemeal.

 -- Macro: combine-after-change-calls body...
     The macro executes BODY normally, but arranges to call the
     after-change functions just once for a series of several changes—if
     that seems safe.

     If a program makes several text changes in the same area of the
     buffer, using the macro ‘combine-after-change-calls’ around that
     part of the program can make it run considerably faster when
     after-change hooks are in use.  When the after-change hooks are
     ultimately called, the arguments specify a portion of the buffer
     including all of the changes made within the
     ‘combine-after-change-calls’ body.

     *Warning:* You must not alter the values of
     ‘after-change-functions’ within the body of a
     ‘combine-after-change-calls’ form.

     *Warning:* if the changes you combine occur in widely scattered
     parts of the buffer, this will still work, but it is not advisable,
     because it may lead to inefficient behavior for some change hook
     functions.

 -- Variable: first-change-hook
     This variable is a normal hook that is run whenever a buffer is
     changed that was previously in the unmodified state.

 -- Variable: inhibit-modification-hooks
     If this variable is non-‘nil’, all of the change hooks are
     disabled; none of them run.  This affects all the hook variables
     described above in this section, as well as the hooks attached to
     certain special text properties (*note Special Properties::) and
     overlay properties (*note Overlay Properties::).

     Also, this variable is bound to non-‘nil’ while running those same
     hook variables, so that by default modifying the buffer from a
     modification hook does not cause other modification hooks to be
     run.  If you do want modification hooks to be run in a particular
     piece of code that is itself run from a modification hook, then
     rebind locally ‘inhibit-modification-hooks’ to ‘nil’.


File: elisp.info,  Node: Non-ASCII Characters,  Next: Searching and Matching,  Prev: Text,  Up: Top

32 Non-ASCII Characters
***********************

This chapter covers the special issues relating to characters and how
they are stored in strings and buffers.

* Menu:

* Text Representations::    How Emacs represents text.
* Disabling Multibyte::     Controlling whether to use multibyte characters.
* Converting Representations::  Converting unibyte to multibyte and vice versa.
* Selecting a Representation::  Treating a byte sequence as unibyte or multi.
* Character Codes::         How unibyte and multibyte relate to
                                codes of individual characters.
* Character Properties::    Character attributes that define their
                                behavior and handling.
* Character Sets::          The space of possible character codes
                                is divided into various character sets.
* Scanning Charsets::       Which character sets are used in a buffer?
* Translation of Characters::   Translation tables are used for conversion.
* Coding Systems::          Coding systems are conversions for saving files.
* Input Methods::           Input methods allow users to enter various
                                non-ASCII characters without special keyboards.
* Locales::                 Interacting with the POSIX locale.


File: elisp.info,  Node: Text Representations,  Next: Disabling Multibyte,  Up: Non-ASCII Characters

32.1 Text Representations
=========================

Emacs buffers and strings support a large repertoire of characters from
many different scripts, allowing users to type and display text in
almost any known written language.

   To support this multitude of characters and scripts, Emacs closely
follows the “Unicode Standard”.  The Unicode Standard assigns a unique
number, called a “codepoint”, to each and every character.  The range of
codepoints defined by Unicode, or the Unicode “codespace”, is
‘0..#x10FFFF’ (in hexadecimal notation), inclusive.  Emacs extends this
range with codepoints in the range ‘#x110000..#x3FFFFF’, which it uses
for representing characters that are not unified with Unicode and “raw
8-bit bytes” that cannot be interpreted as characters.  Thus, a
character codepoint in Emacs is a 22-bit integer.

   To conserve memory, Emacs does not hold fixed-length 22-bit numbers
that are codepoints of text characters within buffers and strings.
Rather, Emacs uses a variable-length internal representation of
characters, that stores each character as a sequence of 1 to 5 8-bit
bytes, depending on the magnitude of its codepoint(1).  For example, any
ASCII character takes up only 1 byte, a Latin-1 character takes up 2
bytes, etc.  We call this representation of text “multibyte”.

   Outside Emacs, characters can be represented in many different
encodings, such as ISO-8859-1, GB-2312, Big-5, etc.  Emacs converts
between these external encodings and its internal representation, as
appropriate, when it reads text into a buffer or a string, or when it
writes text to a disk file or passes it to some other process.

   Occasionally, Emacs needs to hold and manipulate encoded text or
binary non-text data in its buffers or strings.  For example, when Emacs
visits a file, it first reads the file’s text verbatim into a buffer,
and only then converts it to the internal representation.  Before the
conversion, the buffer holds encoded text.

   Encoded text is not really text, as far as Emacs is concerned, but
rather a sequence of raw 8-bit bytes.  We call buffers and strings that
hold encoded text “unibyte” buffers and strings, because Emacs treats
them as a sequence of individual bytes.  Usually, Emacs displays unibyte
buffers and strings as octal codes such as ‘\237’.  We recommend that
you never use unibyte buffers and strings except for manipulating
encoded text or binary non-text data.

   In a buffer, the buffer-local value of the variable
‘enable-multibyte-characters’ specifies the representation used.  The
representation for a string is determined and recorded in the string
when the string is constructed.

 -- Variable: enable-multibyte-characters
     This variable specifies the current buffer’s text representation.
     If it is non-‘nil’, the buffer contains multibyte text; otherwise,
     it contains unibyte encoded text or binary non-text data.

     You cannot set this variable directly; instead, use the function
     ‘set-buffer-multibyte’ to change a buffer’s representation.

 -- Function: position-bytes position
     Buffer positions are measured in character units.  This function
     returns the byte-position corresponding to buffer position POSITION
     in the current buffer.  This is 1 at the start of the buffer, and
     counts upward in bytes.  If POSITION is out of range, the value is
     ‘nil’.

 -- Function: byte-to-position byte-position
     Return the buffer position, in character units, corresponding to
     given BYTE-POSITION in the current buffer.  If BYTE-POSITION is out
     of range, the value is ‘nil’.  In a multibyte buffer, an arbitrary
     value of BYTE-POSITION can be not at character boundary, but inside
     a multibyte sequence representing a single character; in this case,
     this function returns the buffer position of the character whose
     multibyte sequence includes BYTE-POSITION.  In other words, the
     value does not change for all byte positions that belong to the
     same character.

   The following two functions are useful when a Lisp program needs to
map buffer positions to byte offsets in a file visited by the buffer.

 -- Function: bufferpos-to-filepos position &optional quality
          coding-system
     This function is similar to ‘position-bytes’, but instead of byte
     position in the current buffer it returns the offset from the
     beginning of the current buffer’s file of the byte that corresponds
     to the given character POSITION in the buffer.  The conversion
     requires to know how the text is encoded in the buffer’s file; this
     is what the CODING-SYSTEM argument is for, defaulting to the value
     of ‘buffer-file-coding-system’.  The optional argument QUALITY
     specifies how accurate the result should be; it should be one of
     the following:

     ‘exact’
          The result must be accurate.  The function may need to encode
          and decode a large part of the buffer, which is expensive and
          can be slow.
     ‘approximate’
          The value can be an approximation.  The function may avoid
          expensive processing and return an inexact result.
     ‘nil’
          If the exact result needs expensive processing, the function
          will return ‘nil’ rather than an approximation.  This is the
          default if the argument is omitted.

 -- Function: filepos-to-bufferpos byte &optional quality coding-system
     This function returns the buffer position corresponding to a file
     position specified by BYTE, a zero-base byte offset from the file’s
     beginning.  The function performs the conversion opposite to what
     ‘bufferpos-to-filepos’ does.  Optional arguments QUALITY and
     CODING-SYSTEM have the same meaning and values as for
     ‘bufferpos-to-filepos’.

 -- Function: multibyte-string-p string
     Return ‘t’ if STRING is a multibyte string, ‘nil’ otherwise.  This
     function also returns ‘nil’ if STRING is some object other than a
     string.

 -- Function: string-bytes string
     This function returns the number of bytes in STRING.  If STRING is
     a multibyte string, this can be greater than ‘(length STRING)’.

 -- Function: unibyte-string &rest bytes
     This function concatenates all its argument BYTES and makes the
     result a unibyte string.

   ---------- Footnotes ----------

   (1) This internal representation is based on one of the encodings
defined by the Unicode Standard, called “UTF-8”, for representing any
Unicode codepoint, but Emacs extends UTF-8 to represent the additional
codepoints it uses for raw 8-bit bytes and characters not unified with
Unicode.


File: elisp.info,  Node: Disabling Multibyte,  Next: Converting Representations,  Prev: Text Representations,  Up: Non-ASCII Characters

32.2 Disabling Multibyte Characters
===================================

By default, Emacs starts in multibyte mode: it stores the contents of
buffers and strings using an internal encoding that represents non-ASCII
characters using multi-byte sequences.  Multibyte mode allows you to use
all the supported languages and scripts without limitations.

   Under very special circumstances, you may want to disable multibyte
character support, for a specific buffer.  When multibyte characters are
disabled in a buffer, we call that “unibyte mode”.  In unibyte mode,
each character in the buffer has a character code ranging from 0 through
255 (0377 octal); 0 through 127 (0177 octal) represent ASCII characters,
and 128 (0200 octal) through 255 (0377 octal) represent non-ASCII
characters.

   To edit a particular file in unibyte representation, visit it using
‘find-file-literally’.  *Note Visiting Functions::.  You can convert a
multibyte buffer to unibyte by saving it to a file, killing the buffer,
and visiting the file again with ‘find-file-literally’.  Alternatively,
you can use ‘C-x <RET> c’ (‘universal-coding-system-argument’) and
specify ‘raw-text’ as the coding system with which to visit or save a
file.  *Note Specifying a Coding System for File Text: (emacs)Text
Coding.  Unlike ‘find-file-literally’, finding a file as ‘raw-text’
doesn’t disable format conversion, uncompression, or auto mode
selection.

   The buffer-local variable ‘enable-multibyte-characters’ is non-‘nil’
in multibyte buffers, and ‘nil’ in unibyte ones.  The mode line also
indicates whether a buffer is multibyte or not.  With a graphical
display, in a multibyte buffer, the portion of the mode line that
indicates the character set has a tooltip that (amongst other things)
says that the buffer is multibyte.  In a unibyte buffer, the character
set indicator is absent.  Thus, in a unibyte buffer (when using a
graphical display) there is normally nothing before the indication of
the visited file’s end-of-line convention (colon, backslash, etc.),
unless you are using an input method.

   You can turn off multibyte support in a specific buffer by invoking
the command ‘toggle-enable-multibyte-characters’ in that buffer.


File: elisp.info,  Node: Converting Representations,  Next: Selecting a Representation,  Prev: Disabling Multibyte,  Up: Non-ASCII Characters

32.3 Converting Text Representations
====================================

Emacs can convert unibyte text to multibyte; it can also convert
multibyte text to unibyte, provided that the multibyte text contains
only ASCII and 8-bit raw bytes.  In general, these conversions happen
when inserting text into a buffer, or when putting text from several
strings together in one string.  You can also explicitly convert a
string’s contents to either representation.

   Emacs chooses the representation for a string based on the text from
which it is constructed.  The general rule is to convert unibyte text to
multibyte text when combining it with other multibyte text, because the
multibyte representation is more general and can hold whatever
characters the unibyte text has.

   When inserting text into a buffer, Emacs converts the text to the
buffer’s representation, as specified by ‘enable-multibyte-characters’
in that buffer.  In particular, when you insert multibyte text into a
unibyte buffer, Emacs converts the text to unibyte, even though this
conversion cannot in general preserve all the characters that might be
in the multibyte text.  The other natural alternative, to convert the
buffer contents to multibyte, is not acceptable because the buffer’s
representation is a choice made by the user that cannot be overridden
automatically.

   Converting unibyte text to multibyte text leaves ASCII characters
unchanged, and converts bytes with codes 128 through 255 to the
multibyte representation of raw eight-bit bytes.

   Converting multibyte text to unibyte converts all ASCII and eight-bit
characters to their single-byte form, but loses information for
non-ASCII characters by discarding all but the low 8 bits of each
character’s codepoint.  Converting unibyte text to multibyte and back to
unibyte reproduces the original unibyte text.

   The next two functions either return the argument STRING, or a newly
created string with no text properties.

 -- Function: string-to-multibyte string
     This function returns a multibyte string containing the same
     sequence of characters as STRING.  If STRING is a multibyte string,
     it is returned unchanged.  The function assumes that STRING
     includes only ASCII characters and raw 8-bit bytes; the latter are
     converted to their multibyte representation corresponding to the
     codepoints ‘#x3FFF80’ through ‘#x3FFFFF’, inclusive (*note
     codepoints: Text Representations.).

 -- Function: string-to-unibyte string
     This function returns a unibyte string containing the same sequence
     of characters as STRING.  It signals an error if STRING contains a
     non-ASCII character.  If STRING is a unibyte string, it is returned
     unchanged.  Use this function for STRING arguments that contain
     only ASCII and eight-bit characters.

 -- Function: byte-to-string byte
     This function returns a unibyte string containing a single byte of
     character data, CHARACTER.  It signals an error if CHARACTER is not
     an integer between 0 and 255.

 -- Function: multibyte-char-to-unibyte char
     This converts the multibyte character CHAR to a unibyte character,
     and returns that character.  If CHAR is neither ASCII nor
     eight-bit, the function returns −1.

 -- Function: unibyte-char-to-multibyte char
     This convert the unibyte character CHAR to a multibyte character,
     assuming CHAR is either ASCII or raw 8-bit byte.


File: elisp.info,  Node: Selecting a Representation,  Next: Character Codes,  Prev: Converting Representations,  Up: Non-ASCII Characters

32.4 Selecting a Representation
===============================

Sometimes it is useful to examine an existing buffer or string as
multibyte when it was unibyte, or vice versa.

 -- Function: set-buffer-multibyte multibyte
     Set the representation type of the current buffer.  If MULTIBYTE is
     non-‘nil’, the buffer becomes multibyte.  If MULTIBYTE is ‘nil’,
     the buffer becomes unibyte.

     This function leaves the buffer contents unchanged when viewed as a
     sequence of bytes.  As a consequence, it can change the contents
     viewed as characters; for instance, a sequence of three bytes which
     is treated as one character in multibyte representation will count
     as three characters in unibyte representation.  Eight-bit
     characters representing raw bytes are an exception.  They are
     represented by one byte in a unibyte buffer, but when the buffer is
     set to multibyte, they are converted to two-byte sequences, and
     vice versa.

     This function sets ‘enable-multibyte-characters’ to record which
     representation is in use.  It also adjusts various data in the
     buffer (including overlays, text properties and markers) so that
     they cover the same text as they did before.

     This function signals an error if the buffer is narrowed, since the
     narrowing might have occurred in the middle of multibyte character
     sequences.

     This function also signals an error if the buffer is an indirect
     buffer.  An indirect buffer always inherits the representation of
     its base buffer.

 -- Function: string-as-unibyte string
     If STRING is already a unibyte string, this function returns STRING
     itself.  Otherwise, it returns a new string with the same bytes as
     STRING, but treating each byte as a separate character (so that the
     value may have more characters than STRING); as an exception, each
     eight-bit character representing a raw byte is converted into a
     single byte.  The newly-created string contains no text properties.

 -- Function: string-as-multibyte string
     If STRING is a multibyte string, this function returns STRING
     itself.  Otherwise, it returns a new string with the same bytes as
     STRING, but treating each multibyte sequence as one character.
     This means that the value may have fewer characters than STRING
     has.  If a byte sequence in STRING is invalid as a multibyte
     representation of a single character, each byte in the sequence is
     treated as a raw 8-bit byte.  The newly-created string contains no
     text properties.


File: elisp.info,  Node: Character Codes,  Next: Character Properties,  Prev: Selecting a Representation,  Up: Non-ASCII Characters

32.5 Character Codes
====================

The unibyte and multibyte text representations use different character
codes.  The valid character codes for unibyte representation range from
0 to ‘#xFF’ (255)—the values that can fit in one byte.  The valid
character codes for multibyte representation range from 0 to ‘#x3FFFFF’.
In this code space, values 0 through ‘#x7F’ (127) are for ASCII
characters, and values ‘#x80’ (128) through ‘#x3FFF7F’ (4194175) are for
non-ASCII characters.

   Emacs character codes are a superset of the Unicode standard.  Values
0 through ‘#x10FFFF’ (1114111) correspond to Unicode characters of the
same codepoint; values ‘#x110000’ (1114112) through ‘#x3FFF7F’ (4194175)
represent characters that are not unified with Unicode; and values
‘#x3FFF80’ (4194176) through ‘#x3FFFFF’ (4194303) represent eight-bit
raw bytes.

 -- Function: characterp charcode
     This returns ‘t’ if CHARCODE is a valid character, and ‘nil’
     otherwise.

          (characterp 65)
               ⇒ t
          (characterp 4194303)
               ⇒ t
          (characterp 4194304)
               ⇒ nil

 -- Function: max-char
     This function returns the largest value that a valid character
     codepoint can have.

          (characterp (max-char))
               ⇒ t
          (characterp (1+ (max-char)))
               ⇒ nil

 -- Function: get-byte &optional pos string
     This function returns the byte at character position POS in the
     current buffer.  If the current buffer is unibyte, this is
     literally the byte at that position.  If the buffer is multibyte,
     byte values of ASCII characters are the same as character
     codepoints, whereas eight-bit raw bytes are converted to their
     8-bit codes.  The function signals an error if the character at POS
     is non-ASCII.

     The optional argument STRING means to get a byte value from that
     string instead of the current buffer.


File: elisp.info,  Node: Character Properties,  Next: Character Sets,  Prev: Character Codes,  Up: Non-ASCII Characters

32.6 Character Properties
=========================

A “character property” is a named attribute of a character that
specifies how the character behaves and how it should be handled during
text processing and display.  Thus, character properties are an
important part of specifying the character’s semantics.

   On the whole, Emacs follows the Unicode Standard in its
implementation of character properties.  In particular, Emacs supports
the Unicode Character Property Model
(http://www.unicode.org/reports/tr23/), and the Emacs character property
database is derived from the Unicode Character Database (UCD).  See the
Character Properties chapter of the Unicode Standard
(http://www.unicode.org/versions/Unicode6.2.0/ch04.pdf), for a detailed
description of Unicode character properties and their meaning.  This
section assumes you are already familiar with that chapter of the
Unicode Standard, and want to apply that knowledge to Emacs Lisp
programs.

   In Emacs, each property has a name, which is a symbol, and a set of
possible values, whose types depend on the property; if a character does
not have a certain property, the value is ‘nil’.  As a general rule, the
names of character properties in Emacs are produced from the
corresponding Unicode properties by downcasing them and replacing each
‘_’ character with a dash ‘-’.  For example, ‘Canonical_Combining_Class’
becomes ‘canonical-combining-class’.  However, sometimes we shorten the
names to make their use easier.

   Some codepoints are left “unassigned” by the UCD—they don’t
correspond to any character.  The Unicode Standard defines default
values of properties for such codepoints; they are mentioned below for
each property.

   Here is the full list of value types for all the character properties
that Emacs knows about:

‘name’
     Corresponds to the ‘Name’ Unicode property.  The value is a string
     consisting of upper-case Latin letters A to Z, digits, spaces, and
     hyphen ‘-’ characters.  For unassigned codepoints, the value is
     ‘nil’.

‘general-category’
     Corresponds to the ‘General_Category’ Unicode property.  The value
     is a symbol whose name is a 2-letter abbreviation of the
     character’s classification.  For unassigned codepoints, the value
     is ‘Cn’.

‘canonical-combining-class’
     Corresponds to the ‘Canonical_Combining_Class’ Unicode property.
     The value is an integer.  For unassigned codepoints, the value is
     zero.

‘bidi-class’
     Corresponds to the Unicode ‘Bidi_Class’ property.  The value is a
     symbol whose name is the Unicode “directional type” of the
     character.  Emacs uses this property when it reorders bidirectional
     text for display (*note Bidirectional Display::).  For unassigned
     codepoints, the value depends on the code blocks to which the
     codepoint belongs: most unassigned codepoints get the value of ‘L’
     (strong L), but some get values of ‘AL’ (Arabic letter) or ‘R’
     (strong R).

‘decomposition’
     Corresponds to the Unicode properties ‘Decomposition_Type’ and
     ‘Decomposition_Value’.  The value is a list, whose first element
     may be a symbol representing a compatibility formatting tag, such
     as ‘small’(1); the other elements are characters that give the
     compatibility decomposition sequence of this character.  For
     characters that don’t have decomposition sequences, and for
     unassigned codepoints, the value is a list with a single member,
     the character itself.

‘decimal-digit-value’
     Corresponds to the Unicode ‘Numeric_Value’ property for characters
     whose ‘Numeric_Type’ is ‘Decimal’.  The value is an integer, or
     ‘nil’ if the character has no decimal digit value.  For unassigned
     codepoints, the value is ‘nil’, which means NaN, or “not a number”.

‘digit-value’
     Corresponds to the Unicode ‘Numeric_Value’ property for characters
     whose ‘Numeric_Type’ is ‘Digit’.  The value is an integer.
     Examples of such characters include compatibility subscript and
     superscript digits, for which the value is the corresponding
     number.  For characters that don’t have any numeric value, and for
     unassigned codepoints, the value is ‘nil’, which means NaN.

‘numeric-value’
     Corresponds to the Unicode ‘Numeric_Value’ property for characters
     whose ‘Numeric_Type’ is ‘Numeric’.  The value of this property is a
     number.  Examples of characters that have this property include
     fractions, subscripts, superscripts, Roman numerals, currency
     numerators, and encircled numbers.  For example, the value of this
     property for the character ‘U+2155’ (VULGAR FRACTION ONE FIFTH) is
     ‘0.2’.  For characters that don’t have any numeric value, and for
     unassigned codepoints, the value is ‘nil’, which means NaN.

‘mirrored’
     Corresponds to the Unicode ‘Bidi_Mirrored’ property.  The value of
     this property is a symbol, either ‘Y’ or ‘N’.  For unassigned
     codepoints, the value is ‘N’.

‘mirroring’
     Corresponds to the Unicode ‘Bidi_Mirroring_Glyph’ property.  The
     value of this property is a character whose glyph represents the
     mirror image of the character’s glyph, or ‘nil’ if there’s no
     defined mirroring glyph.  All the characters whose ‘mirrored’
     property is ‘N’ have ‘nil’ as their ‘mirroring’ property; however,
     some characters whose ‘mirrored’ property is ‘Y’ also have ‘nil’
     for ‘mirroring’, because no appropriate characters exist with
     mirrored glyphs.  Emacs uses this property to display mirror images
     of characters when appropriate (*note Bidirectional Display::).
     For unassigned codepoints, the value is ‘nil’.

‘paired-bracket’
     Corresponds to the Unicode ‘Bidi_Paired_Bracket’ property.  The
     value of this property is the codepoint of a character’s “paired
     bracket”, or ‘nil’ if the character is not a bracket character.
     This establishes a mapping between characters that are treated as
     bracket pairs by the Unicode Bidirectional Algorithm; Emacs uses
     this property when it decides how to reorder for display
     parentheses, braces, and other similar characters (*note
     Bidirectional Display::).

‘bracket-type’
     Corresponds to the Unicode ‘Bidi_Paired_Bracket_Type’ property.
     For characters whose ‘paired-bracket’ property is non-‘nil’, the
     value of this property is a symbol, either ‘o’ (for opening bracket
     characters) or ‘c’ (for closing bracket characters).  For
     characters whose ‘paired-bracket’ property is ‘nil’, the value is
     the symbol ‘n’ (None).  Like ‘paired-bracket’, this property is
     used for bidirectional display.

‘old-name’
     Corresponds to the Unicode ‘Unicode_1_Name’ property.  The value is
     a string.  For unassigned codepoints, and characters that have no
     value for this property, the value is ‘nil’.

‘iso-10646-comment’
     Corresponds to the Unicode ‘ISO_Comment’ property.  The value is
     either a string or ‘nil’.  For unassigned codepoints, the value is
     ‘nil’.

‘uppercase’
     Corresponds to the Unicode ‘Simple_Uppercase_Mapping’ property.
     The value of this property is a single character.  For unassigned
     codepoints, the value is ‘nil’, which means the character itself.

‘lowercase’
     Corresponds to the Unicode ‘Simple_Lowercase_Mapping’ property.
     The value of this property is a single character.  For unassigned
     codepoints, the value is ‘nil’, which means the character itself.

‘titlecase’
     Corresponds to the Unicode ‘Simple_Titlecase_Mapping’ property.
     “Title case” is a special form of a character used when the first
     character of a word needs to be capitalized.  The value of this
     property is a single character.  For unassigned codepoints, the
     value is ‘nil’, which means the character itself.

 -- Function: get-char-code-property char propname
     This function returns the value of CHAR’s PROPNAME property.

          (get-char-code-property ?\s 'general-category)
               ⇒ Zs
          (get-char-code-property ?1 'general-category)
               ⇒ Nd
          ;; U+2084 SUBSCRIPT FOUR
          (get-char-code-property ?\u2084 'digit-value)
               ⇒ 4
          ;; U+2155 VULGAR FRACTION ONE FIFTH
          (get-char-code-property ?\u2155 'numeric-value)
               ⇒ 0.2
          ;; U+2163 ROMAN NUMERAL FOUR
          (get-char-code-property ?\u2163 'numeric-value)
               ⇒ 4
          (get-char-code-property ?\( 'paired-bracket)
               ⇒ 41  ;; closing parenthesis
          (get-char-code-property ?\) 'bracket-type)
               ⇒ c

 -- Function: char-code-property-description prop value
     This function returns the description string of property PROP’s
     VALUE, or ‘nil’ if VALUE has no description.

          (char-code-property-description 'general-category 'Zs)
               ⇒ "Separator, Space"
          (char-code-property-description 'general-category 'Nd)
               ⇒ "Number, Decimal Digit"
          (char-code-property-description 'numeric-value '1/5)
               ⇒ nil

 -- Function: put-char-code-property char propname value
     This function stores VALUE as the value of the property PROPNAME
     for the character CHAR.

 -- Variable: unicode-category-table
     The value of this variable is a char-table (*note Char-Tables::)
     that specifies, for each character, its Unicode ‘General_Category’
     property as a symbol.

 -- Variable: char-script-table
     The value of this variable is a char-table that specifies, for each
     character, a symbol whose name is the script to which the character
     belongs, according to the Unicode Standard classification of the
     Unicode code space into script-specific blocks.  This char-table
     has a single extra slot whose value is the list of all script
     symbols.

 -- Variable: char-width-table
     The value of this variable is a char-table that specifies the width
     of each character in columns that it will occupy on the screen.

 -- Variable: printable-chars
     The value of this variable is a char-table that specifies, for each
     character, whether it is printable or not.  That is, if evaluating
     ‘(aref printable-chars char)’ results in ‘t’, the character is
     printable, and if it results in ‘nil’, it is not.

   ---------- Footnotes ----------

   (1) The Unicode specification writes these tag names inside ‘<..>’
brackets, but the tag names in Emacs do not include the brackets; e.g.,
Unicode specifies ‘<small>’ where Emacs uses ‘small’.


File: elisp.info,  Node: Character Sets,  Next: Scanning Charsets,  Prev: Character Properties,  Up: Non-ASCII Characters

32.7 Character Sets
===================

An Emacs “character set”, or “charset”, is a set of characters in which
each character is assigned a numeric code point.  (The Unicode Standard
calls this a “coded character set”.)  Each Emacs charset has a name
which is a symbol.  A single character can belong to any number of
different character sets, but it will generally have a different code
point in each charset.  Examples of character sets include ‘ascii’,
‘iso-8859-1’, ‘greek-iso8859-7’, and ‘windows-1255’.  The code point
assigned to a character in a charset is usually different from its code
point used in Emacs buffers and strings.

   Emacs defines several special character sets.  The character set
‘unicode’ includes all the characters whose Emacs code points are in the
range ‘0..#x10FFFF’.  The character set ‘emacs’ includes all ASCII and
non-ASCII characters.  Finally, the ‘eight-bit’ charset includes the
8-bit raw bytes; Emacs uses it to represent raw bytes encountered in
text.

 -- Function: charsetp object
     Returns ‘t’ if OBJECT is a symbol that names a character set, ‘nil’
     otherwise.

 -- Variable: charset-list
     The value is a list of all defined character set names.

 -- Function: charset-priority-list &optional highestp
     This function returns a list of all defined character sets ordered
     by their priority.  If HIGHESTP is non-‘nil’, the function returns
     a single character set of the highest priority.

 -- Function: set-charset-priority &rest charsets
     This function makes CHARSETS the highest priority character sets.

 -- Function: char-charset character &optional restriction
     This function returns the name of the character set of highest
     priority that CHARACTER belongs to.  ASCII characters are an
     exception: for them, this function always returns ‘ascii’.

     If RESTRICTION is non-‘nil’, it should be a list of charsets to
     search.  Alternatively, it can be a coding system, in which case
     the returned charset must be supported by that coding system (*note
     Coding Systems::).

 -- Function: charset-plist charset
     This function returns the property list of the character set
     CHARSET.  Although CHARSET is a symbol, this is not the same as the
     property list of that symbol.  Charset properties include important
     information about the charset, such as its documentation string,
     short name, etc.

 -- Function: put-charset-property charset propname value
     This function sets the PROPNAME property of CHARSET to the given
     VALUE.

 -- Function: get-charset-property charset propname
     This function returns the value of CHARSETs property PROPNAME.

 -- Command: list-charset-chars charset
     This command displays a list of characters in the character set
     CHARSET.

   Emacs can convert between its internal representation of a character
and the character’s codepoint in a specific charset.  The following two
functions support these conversions.

 -- Function: decode-char charset code-point
     This function decodes a character that is assigned a CODE-POINT in
     CHARSET, to the corresponding Emacs character, and returns it.  If
     CHARSET doesn’t contain a character of that code point, the value
     is ‘nil’.  If CODE-POINT doesn’t fit in a Lisp integer (*note
     most-positive-fixnum: Integer Basics.), it can be specified as a
     cons cell ‘(HIGH . LOW)’, where LOW are the lower 16 bits of the
     value and HIGH are the high 16 bits.

 -- Function: encode-char char charset
     This function returns the code point assigned to the character CHAR
     in CHARSET.  If the result does not fit in a Lisp integer, it is
     returned as a cons cell ‘(HIGH . LOW)’ that fits the second
     argument of ‘decode-char’ above.  If CHARSET doesn’t have a
     codepoint for CHAR, the value is ‘nil’.

   The following function comes in handy for applying a certain function
to all or part of the characters in a charset:

 -- Function: map-charset-chars function charset &optional arg from-code
          to-code
     Call FUNCTION for characters in CHARSET.  FUNCTION is called with
     two arguments.  The first one is a cons cell ‘(FROM . TO)’, where
     FROM and TO indicate a range of characters contained in charset.
     The second argument passed to FUNCTION is ARG.

     By default, the range of codepoints passed to FUNCTION includes all
     the characters in CHARSET, but optional arguments FROM-CODE and
     TO-CODE limit that to the range of characters between these two
     codepoints of CHARSET.  If either of them is ‘nil’, it defaults to
     the first or last codepoint of CHARSET, respectively.


File: elisp.info,  Node: Scanning Charsets,  Next: Translation of Characters,  Prev: Character Sets,  Up: Non-ASCII Characters

32.8 Scanning for Character Sets
================================

Sometimes it is useful to find out which character set a particular
character belongs to.  One use for this is in determining which coding
systems (*note Coding Systems::) are capable of representing all of the
text in question; another is to determine the font(s) for displaying
that text.

 -- Function: charset-after &optional pos
     This function returns the charset of highest priority containing
     the character at position POS in the current buffer.  If POS is
     omitted or ‘nil’, it defaults to the current value of point.  If
     POS is out of range, the value is ‘nil’.

 -- Function: find-charset-region beg end &optional translation
     This function returns a list of the character sets of highest
     priority that contain characters in the current buffer between
     positions BEG and END.

     The optional argument TRANSLATION specifies a translation table to
     use for scanning the text (*note Translation of Characters::).  If
     it is non-‘nil’, then each character in the region is translated
     through this table, and the value returned describes the translated
     characters instead of the characters actually in the buffer.

 -- Function: find-charset-string string &optional translation
     This function returns a list of character sets of highest priority
     that contain characters in STRING.  It is just like
     ‘find-charset-region’, except that it applies to the contents of
     STRING instead of part of the current buffer.


File: elisp.info,  Node: Translation of Characters,  Next: Coding Systems,  Prev: Scanning Charsets,  Up: Non-ASCII Characters

32.9 Translation of Characters
==============================

A “translation table” is a char-table (*note Char-Tables::) that
specifies a mapping of characters into characters.  These tables are
used in encoding and decoding, and for other purposes.  Some coding
systems specify their own particular translation tables; there are also
default translation tables which apply to all other coding systems.

   A translation table has two extra slots.  The first is either ‘nil’
or a translation table that performs the reverse translation; the second
is the maximum number of characters to look up for translating sequences
of characters (see the description of
‘make-translation-table-from-alist’ below).

 -- Function: make-translation-table &rest translations
     This function returns a translation table based on the argument
     TRANSLATIONS.  Each element of TRANSLATIONS should be a list of
     elements of the form ‘(FROM . TO)’; this says to translate the
     character FROM into TO.

     The arguments and the forms in each argument are processed in
     order, and if a previous form already translates TO to some other
     character, say TO-ALT, FROM is also translated to TO-ALT.

   During decoding, the translation table’s translations are applied to
the characters that result from ordinary decoding.  If a coding system
has the property ‘:decode-translation-table’, that specifies the
translation table to use, or a list of translation tables to apply in
sequence.  (This is a property of the coding system, as returned by
‘coding-system-get’, not a property of the symbol that is the coding
system’s name.  *Note Basic Concepts of Coding Systems: Coding System
Basics.)  Finally, if ‘standard-translation-table-for-decode’ is
non-‘nil’, the resulting characters are translated by that table.

   During encoding, the translation table’s translations are applied to
the characters in the buffer, and the result of translation is actually
encoded.  If a coding system has property ‘:encode-translation-table’,
that specifies the translation table to use, or a list of translation
tables to apply in sequence.  In addition, if the variable
‘standard-translation-table-for-encode’ is non-‘nil’, it specifies the
translation table to use for translating the result.

 -- Variable: standard-translation-table-for-decode
     This is the default translation table for decoding.  If a coding
     systems specifies its own translation tables, the table that is the
     value of this variable, if non-‘nil’, is applied after them.

 -- Variable: standard-translation-table-for-encode
     This is the default translation table for encoding.  If a coding
     systems specifies its own translation tables, the table that is the
     value of this variable, if non-‘nil’, is applied after them.

 -- Variable: translation-table-for-input
     Self-inserting characters are translated through this translation
     table before they are inserted.  Search commands also translate
     their input through this table, so they can compare more reliably
     with what’s in the buffer.

     This variable automatically becomes buffer-local when set.

 -- Function: make-translation-table-from-vector vec
     This function returns a translation table made from VEC that is an
     array of 256 elements to map bytes (values 0 through #xFF) to
     characters.  Elements may be ‘nil’ for untranslated bytes.  The
     returned table has a translation table for reverse mapping in the
     first extra slot, and the value ‘1’ in the second extra slot.

     This function provides an easy way to make a private coding system
     that maps each byte to a specific character.  You can specify the
     returned table and the reverse translation table using the
     properties ‘:decode-translation-table’ and
     ‘:encode-translation-table’ respectively in the PROPS argument to
     ‘define-coding-system’.

 -- Function: make-translation-table-from-alist alist
     This function is similar to ‘make-translation-table’ but returns a
     complex translation table rather than a simple one-to-one mapping.
     Each element of ALIST is of the form ‘(FROM . TO)’, where FROM and
     TO are either characters or vectors specifying a sequence of
     characters.  If FROM is a character, that character is translated
     to TO (i.e., to a character or a character sequence).  If FROM is a
     vector of characters, that sequence is translated to TO.  The
     returned table has a translation table for reverse mapping in the
     first extra slot, and the maximum length of all the FROM character
     sequences in the second extra slot.


File: elisp.info,  Node: Coding Systems,  Next: Input Methods,  Prev: Translation of Characters,  Up: Non-ASCII Characters

32.10 Coding Systems
====================

When Emacs reads or writes a file, and when Emacs sends text to a
subprocess or receives text from a subprocess, it normally performs
character code conversion and end-of-line conversion as specified by a
particular “coding system”.

   How to define a coding system is an arcane matter, and is not
documented here.

* Menu:

* Coding System Basics::        Basic concepts.
* Encoding and I/O::            How file I/O functions handle coding systems.
* Lisp and Coding Systems::     Functions to operate on coding system names.
* User-Chosen Coding Systems::  Asking the user to choose a coding system.
* Default Coding Systems::      Controlling the default choices.
* Specifying Coding Systems::   Requesting a particular coding system
                                    for a single file operation.
* Explicit Encoding::           Encoding or decoding text without doing I/O.
* Terminal I/O Encoding::       Use of encoding for terminal I/O.


File: elisp.info,  Node: Coding System Basics,  Next: Encoding and I/O,  Up: Coding Systems

32.10.1 Basic Concepts of Coding Systems
----------------------------------------

“Character code conversion” involves conversion between the internal
representation of characters used inside Emacs and some other encoding.
Emacs supports many different encodings, in that it can convert to and
from them.  For example, it can convert text to or from encodings such
as Latin 1, Latin 2, Latin 3, Latin 4, Latin 5, and several variants of
ISO 2022.  In some cases, Emacs supports several alternative encodings
for the same characters; for example, there are three coding systems for
the Cyrillic (Russian) alphabet: ISO, Alternativnyj, and KOI8.

   Every coding system specifies a particular set of character code
conversions, but the coding system ‘undecided’ is special: it leaves the
choice unspecified, to be chosen heuristically for each file, based on
the file’s data.  The coding system ‘prefer-utf-8’ is like ‘undecided’,
but it prefers to choose ‘utf-8’ when possible.

   In general, a coding system doesn’t guarantee roundtrip identity:
decoding a byte sequence using coding system, then encoding the
resulting text in the same coding system, can produce a different byte
sequence.  But some coding systems do guarantee that the byte sequence
will be the same as what you originally decoded.  Here are a few
examples:

     iso-8859-1, utf-8, big5, shift_jis, euc-jp

   Encoding buffer text and then decoding the result can also fail to
reproduce the original text.  For instance, if you encode a character
with a coding system which does not support that character, the result
is unpredictable, and thus decoding it using the same coding system may
produce a different text.  Currently, Emacs can’t report errors that
result from encoding unsupported characters.

   “End of line conversion” handles three different conventions used on
various systems for representing end of line in files.  The Unix
convention, used on GNU and Unix systems, is to use the linefeed
character (also called newline).  The DOS convention, used on MS-Windows
and MS-DOS systems, is to use a carriage-return and a linefeed at the
end of a line.  The Mac convention is to use just carriage-return.
(This was the convention used in Classic Mac OS.)

   “Base coding systems” such as ‘latin-1’ leave the end-of-line
conversion unspecified, to be chosen based on the data.  “Variant coding
systems” such as ‘latin-1-unix’, ‘latin-1-dos’ and ‘latin-1-mac’ specify
the end-of-line conversion explicitly as well.  Most base coding systems
have three corresponding variants whose names are formed by adding
‘-unix’, ‘-dos’ and ‘-mac’.

   The coding system ‘raw-text’ is special in that it prevents character
code conversion, and causes the buffer visited with this coding system
to be a unibyte buffer.  For historical reasons, you can save both
unibyte and multibyte text with this coding system.  When you use
‘raw-text’ to encode multibyte text, it does perform one character code
conversion: it converts eight-bit characters to their single-byte
external representation.  ‘raw-text’ does not specify the end-of-line
conversion, allowing that to be determined as usual by the data, and has
the usual three variants which specify the end-of-line conversion.

   ‘no-conversion’ (and its alias ‘binary’) is equivalent to
‘raw-text-unix’: it specifies no conversion of either character codes or
end-of-line.

   The coding system ‘utf-8-emacs’ specifies that the data is
represented in the internal Emacs encoding (*note Text
Representations::).  This is like ‘raw-text’ in that no code conversion
happens, but different in that the result is multibyte data.  The name
‘emacs-internal’ is an alias for ‘utf-8-emacs’.

 -- Function: coding-system-get coding-system property
     This function returns the specified property of the coding system
     CODING-SYSTEM.  Most coding system properties exist for internal
     purposes, but one that you might find useful is ‘:mime-charset’.
     That property’s value is the name used in MIME for the character
     coding which this coding system can read and write.  Examples:

          (coding-system-get 'iso-latin-1 :mime-charset)
               ⇒ iso-8859-1
          (coding-system-get 'iso-2022-cn :mime-charset)
               ⇒ iso-2022-cn
          (coding-system-get 'cyrillic-koi8 :mime-charset)
               ⇒ koi8-r

     The value of the ‘:mime-charset’ property is also defined as an
     alias for the coding system.

 -- Function: coding-system-aliases coding-system
     This function returns the list of aliases of CODING-SYSTEM.


File: elisp.info,  Node: Encoding and I/O,  Next: Lisp and Coding Systems,  Prev: Coding System Basics,  Up: Coding Systems

32.10.2 Encoding and I/O
------------------------

The principal purpose of coding systems is for use in reading and
writing files.  The function ‘insert-file-contents’ uses a coding system
to decode the file data, and ‘write-region’ uses one to encode the
buffer contents.

   You can specify the coding system to use either explicitly (*note
Specifying Coding Systems::), or implicitly using a default mechanism
(*note Default Coding Systems::).  But these methods may not completely
specify what to do.  For example, they may choose a coding system such
as ‘undecided’ which leaves the character code conversion to be
determined from the data.  In these cases, the I/O operation finishes
the job of choosing a coding system.  Very often you will want to find
out afterwards which coding system was chosen.

 -- Variable: buffer-file-coding-system
     This buffer-local variable records the coding system used for
     saving the buffer and for writing part of the buffer with
     ‘write-region’.  If the text to be written cannot be safely encoded
     using the coding system specified by this variable, these
     operations select an alternative encoding by calling the function
     ‘select-safe-coding-system’ (*note User-Chosen Coding Systems::).
     If selecting a different encoding requires to ask the user to
     specify a coding system, ‘buffer-file-coding-system’ is updated to
     the newly selected coding system.

     ‘buffer-file-coding-system’ does _not_ affect sending text to a
     subprocess.

 -- Variable: save-buffer-coding-system
     This variable specifies the coding system for saving the buffer (by
     overriding ‘buffer-file-coding-system’).  Note that it is not used
     for ‘write-region’.

     When a command to save the buffer starts out to use
     ‘buffer-file-coding-system’ (or ‘save-buffer-coding-system’), and
     that coding system cannot handle the actual text in the buffer, the
     command asks the user to choose another coding system (by calling
     ‘select-safe-coding-system’).  After that happens, the command also
     updates ‘buffer-file-coding-system’ to represent the coding system
     that the user specified.

 -- Variable: last-coding-system-used
     I/O operations for files and subprocesses set this variable to the
     coding system name that was used.  The explicit encoding and
     decoding functions (*note Explicit Encoding::) set it too.

     *Warning:* Since receiving subprocess output sets this variable, it
     can change whenever Emacs waits; therefore, you should copy the
     value shortly after the function call that stores the value you are
     interested in.

   The variable ‘selection-coding-system’ specifies how to encode
selections for the window system.  *Note Window System Selections::.

 -- Variable: file-name-coding-system
     The variable ‘file-name-coding-system’ specifies the coding system
     to use for encoding file names.  Emacs encodes file names using
     that coding system for all file operations.  If
     ‘file-name-coding-system’ is ‘nil’, Emacs uses a default coding
     system determined by the selected language environment.  In the
     default language environment, any non-ASCII characters in file
     names are not encoded specially; they appear in the file system
     using the internal Emacs representation.

   *Warning:* if you change ‘file-name-coding-system’ (or the language
environment) in the middle of an Emacs session, problems can result if
you have already visited files whose names were encoded using the
earlier coding system and are handled differently under the new coding
system.  If you try to save one of these buffers under the visited file
name, saving may use the wrong file name, or it may get an error.  If
such a problem happens, use ‘C-x C-w’ to specify a new file name for
that buffer.

   On Windows 2000 and later, Emacs by default uses Unicode APIs to pass
file names to the OS, so the value of ‘file-name-coding-system’ is
largely ignored.  Lisp applications that need to encode or decode file
names on the Lisp level should use ‘utf-8’ coding-system when
‘system-type’ is ‘windows-nt’; the conversion of UTF-8 encoded file
names to the encoding appropriate for communicating with the OS is
performed internally by Emacs.


File: elisp.info,  Node: Lisp and Coding Systems,  Next: User-Chosen Coding Systems,  Prev: Encoding and I/O,  Up: Coding Systems

32.10.3 Coding Systems in Lisp
------------------------------

Here are the Lisp facilities for working with coding systems:

 -- Function: coding-system-list &optional base-only
     This function returns a list of all coding system names (symbols).
     If BASE-ONLY is non-‘nil’, the value includes only the base coding
     systems.  Otherwise, it includes alias and variant coding systems
     as well.

 -- Function: coding-system-p object
     This function returns ‘t’ if OBJECT is a coding system name or
     ‘nil’.

 -- Function: check-coding-system coding-system
     This function checks the validity of CODING-SYSTEM.  If that is
     valid, it returns CODING-SYSTEM.  If CODING-SYSTEM is ‘nil’, the
     function return ‘nil’.  For any other values, it signals an error
     whose ‘error-symbol’ is ‘coding-system-error’ (*note signal:
     Signaling Errors.).

 -- Function: coding-system-eol-type coding-system
     This function returns the type of end-of-line (a.k.a. “eol”)
     conversion used by CODING-SYSTEM.  If CODING-SYSTEM specifies a
     certain eol conversion, the return value is an integer 0, 1, or 2,
     standing for ‘unix’, ‘dos’, and ‘mac’, respectively.  If
     CODING-SYSTEM doesn’t specify eol conversion explicitly, the return
     value is a vector of coding systems, each one with one of the
     possible eol conversion types, like this:

          (coding-system-eol-type 'latin-1)
               ⇒ [latin-1-unix latin-1-dos latin-1-mac]

     If this function returns a vector, Emacs will decide, as part of
     the text encoding or decoding process, what eol conversion to use.
     For decoding, the end-of-line format of the text is auto-detected,
     and the eol conversion is set to match it (e.g., DOS-style CRLF
     format will imply ‘dos’ eol conversion).  For encoding, the eol
     conversion is taken from the appropriate default coding system
     (e.g., default value of ‘buffer-file-coding-system’ for
     ‘buffer-file-coding-system’), or from the default eol conversion
     appropriate for the underlying platform.

 -- Function: coding-system-change-eol-conversion coding-system eol-type
     This function returns a coding system which is like CODING-SYSTEM
     except for its eol conversion, which is specified by ‘eol-type’.
     EOL-TYPE should be ‘unix’, ‘dos’, ‘mac’, or ‘nil’.  If it is ‘nil’,
     the returned coding system determines the end-of-line conversion
     from the data.

     EOL-TYPE may also be 0, 1 or 2, standing for ‘unix’, ‘dos’ and
     ‘mac’, respectively.

 -- Function: coding-system-change-text-conversion eol-coding
          text-coding
     This function returns a coding system which uses the end-of-line
     conversion of EOL-CODING, and the text conversion of TEXT-CODING.
     If TEXT-CODING is ‘nil’, it returns ‘undecided’, or one of its
     variants according to EOL-CODING.

 -- Function: find-coding-systems-region from to
     This function returns a list of coding systems that could be used
     to encode a text between FROM and TO.  All coding systems in the
     list can safely encode any multibyte characters in that portion of
     the text.

     If the text contains no multibyte characters, the function returns
     the list ‘(undecided)’.

 -- Function: find-coding-systems-string string
     This function returns a list of coding systems that could be used
     to encode the text of STRING.  All coding systems in the list can
     safely encode any multibyte characters in STRING.  If the text
     contains no multibyte characters, this returns the list
     ‘(undecided)’.

 -- Function: find-coding-systems-for-charsets charsets
     This function returns a list of coding systems that could be used
     to encode all the character sets in the list CHARSETS.

 -- Function: check-coding-systems-region start end coding-system-list
     This function checks whether coding systems in the list
     ‘coding-system-list’ can encode all the characters in the region
     between START and END.  If all of the coding systems in the list
     can encode the specified text, the function returns ‘nil’.  If some
     coding systems cannot encode some of the characters, the value is
     an alist, each element of which has the form ‘(CODING-SYSTEM1 POS1
     POS2 ...)’, meaning that CODING-SYSTEM1 cannot encode characters at
     buffer positions POS1, POS2, ....

     START may be a string, in which case END is ignored and the
     returned value references string indices instead of buffer
     positions.

 -- Function: detect-coding-region start end &optional highest
     This function chooses a plausible coding system for decoding the
     text from START to END.  This text should be a byte sequence, i.e.,
     unibyte text or multibyte text with only ASCII and eight-bit
     characters (*note Explicit Encoding::).

     Normally this function returns a list of coding systems that could
     handle decoding the text that was scanned.  They are listed in
     order of decreasing priority.  But if HIGHEST is non-‘nil’, then
     the return value is just one coding system, the one that is highest
     in priority.

     If the region contains only ASCII characters except for such
     ISO-2022 control characters ISO-2022 as ‘ESC’, the value is
     ‘undecided’ or ‘(undecided)’, or a variant specifying end-of-line
     conversion, if that can be deduced from the text.

     If the region contains null bytes, the value is ‘no-conversion’,
     even if the region contains text encoded in some coding system.

 -- Function: detect-coding-string string &optional highest
     This function is like ‘detect-coding-region’ except that it
     operates on the contents of STRING instead of bytes in the buffer.

 -- Variable: inhibit-null-byte-detection
     If this variable has a non-‘nil’ value, null bytes are ignored when
     detecting the encoding of a region or a string.  This allows the
     encoding of text that contains null bytes to be correctly detected,
     such as Info files with Index nodes.

 -- Variable: inhibit-iso-escape-detection
     If this variable has a non-‘nil’ value, ISO-2022 escape sequences
     are ignored when detecting the encoding of a region or a string.
     The result is that no text is ever detected as encoded in some
     ISO-2022 encoding, and all escape sequences become visible in a
     buffer.  *Warning:* _Use this variable with extreme caution,
     because many files in the Emacs distribution use ISO-2022
     encoding._

 -- Function: coding-system-charset-list coding-system
     This function returns the list of character sets (*note Character
     Sets::) supported by CODING-SYSTEM.  Some coding systems that
     support too many character sets to list them all yield special
     values:
        • If CODING-SYSTEM supports all Emacs characters, the value is
          ‘(emacs)’.
        • If CODING-SYSTEM supports all Unicode characters, the value is
          ‘(unicode)’.
        • If CODING-SYSTEM supports all ISO-2022 charsets, the value is
          ‘iso-2022’.
        • If CODING-SYSTEM supports all the characters in the internal
          coding system used by Emacs version 21 (prior to the
          implementation of internal Unicode support), the value is
          ‘emacs-mule’.

   *Note Process Information: Coding systems for a subprocess, in
particular the description of the functions ‘process-coding-system’ and
‘set-process-coding-system’, for how to examine or set the coding
systems used for I/O to a subprocess.


File: elisp.info,  Node: User-Chosen Coding Systems,  Next: Default Coding Systems,  Prev: Lisp and Coding Systems,  Up: Coding Systems

32.10.4 User-Chosen Coding Systems
----------------------------------

 -- Function: select-safe-coding-system from to &optional
          default-coding-system accept-default-p file
     This function selects a coding system for encoding specified text,
     asking the user to choose if necessary.  Normally the specified
     text is the text in the current buffer between FROM and TO.  If
     FROM is a string, the string specifies the text to encode, and TO
     is ignored.

     If the specified text includes raw bytes (*note Text
     Representations::), ‘select-safe-coding-system’ suggests ‘raw-text’
     for its encoding.

     If DEFAULT-CODING-SYSTEM is non-‘nil’, that is the first coding
     system to try; if that can handle the text,
     ‘select-safe-coding-system’ returns that coding system.  It can
     also be a list of coding systems; then the function tries each of
     them one by one.  After trying all of them, it next tries the
     current buffer’s value of ‘buffer-file-coding-system’ (if it is not
     ‘undecided’), then the default value of ‘buffer-file-coding-system’
     and finally the user’s most preferred coding system, which the user
     can set using the command ‘prefer-coding-system’ (*note Recognizing
     Coding Systems: (emacs)Recognize Coding.).

     If one of those coding systems can safely encode all the specified
     text, ‘select-safe-coding-system’ chooses it and returns it.
     Otherwise, it asks the user to choose from a list of coding systems
     which can encode all the text, and returns the user’s choice.

     DEFAULT-CODING-SYSTEM can also be a list whose first element is ‘t’
     and whose other elements are coding systems.  Then, if no coding
     system in the list can handle the text, ‘select-safe-coding-system’
     queries the user immediately, without trying any of the three
     alternatives described above.  This is handy for checking only the
     coding systems in the list.

     The optional argument ACCEPT-DEFAULT-P determines whether a coding
     system selected without user interaction is acceptable.  If it’s
     omitted or ‘nil’, such a silent selection is always acceptable.  If
     it is non-‘nil’, it should be a function;
     ‘select-safe-coding-system’ calls this function with one argument,
     the base coding system of the selected coding system.  If the
     function returns ‘nil’, ‘select-safe-coding-system’ rejects the
     silently selected coding system, and asks the user to select a
     coding system from a list of possible candidates.

     If the variable ‘select-safe-coding-system-accept-default-p’ is
     non-‘nil’, it should be a function taking a single argument.  It is
     used in place of ACCEPT-DEFAULT-P, overriding any value supplied
     for this argument.

     As a final step, before returning the chosen coding system,
     ‘select-safe-coding-system’ checks whether that coding system is
     consistent with what would be selected if the contents of the
     region were read from a file.  (If not, this could lead to data
     corruption in a file subsequently re-visited and edited.)
     Normally, ‘select-safe-coding-system’ uses ‘buffer-file-name’ as
     the file for this purpose, but if FILE is non-‘nil’, it uses that
     file instead (this can be relevant for ‘write-region’ and similar
     functions).  If it detects an apparent inconsistency,
     ‘select-safe-coding-system’ queries the user before selecting the
     coding system.

 -- Variable: select-safe-coding-system-function
     This variable names the function to be called to request the user
     to select a proper coding system for encoding text when the default
     coding system for an output operation cannot safely encode that
     text.  The default value of this variable is
     ‘select-safe-coding-system’.  Emacs primitives that write text to
     files, such as ‘write-region’, or send text to other processes,
     such as ‘process-send-region’, normally call the value of this
     variable, unless ‘coding-system-for-write’ is bound to a non-‘nil’
     value (*note Specifying Coding Systems::).

   Here are two functions you can use to let the user specify a coding
system, with completion.  *Note Completion::.

 -- Function: read-coding-system prompt &optional default
     This function reads a coding system using the minibuffer, prompting
     with string PROMPT, and returns the coding system name as a symbol.
     If the user enters null input, DEFAULT specifies which coding
     system to return.  It should be a symbol or a string.

 -- Function: read-non-nil-coding-system prompt
     This function reads a coding system using the minibuffer, prompting
     with string PROMPT, and returns the coding system name as a symbol.
     If the user tries to enter null input, it asks the user to try
     again.  *Note Coding Systems::.


File: elisp.info,  Node: Default Coding Systems,  Next: Specifying Coding Systems,  Prev: User-Chosen Coding Systems,  Up: Coding Systems

32.10.5 Default Coding Systems
------------------------------

This section describes variables that specify the default coding system
for certain files or when running certain subprograms, and the function
that I/O operations use to access them.

   The idea of these variables is that you set them once and for all to
the defaults you want, and then do not change them again.  To specify a
particular coding system for a particular operation in a Lisp program,
don’t change these variables; instead, override them using
‘coding-system-for-read’ and ‘coding-system-for-write’ (*note Specifying
Coding Systems::).

 -- User Option: auto-coding-regexp-alist
     This variable is an alist of text patterns and corresponding coding
     systems.  Each element has the form ‘(REGEXP . CODING-SYSTEM)’; a
     file whose first few kilobytes match REGEXP is decoded with
     CODING-SYSTEM when its contents are read into a buffer.  The
     settings in this alist take priority over ‘coding:’ tags in the
     files and the contents of ‘file-coding-system-alist’ (see below).
     The default value is set so that Emacs automatically recognizes
     mail files in Babyl format and reads them with no code conversions.

 -- User Option: file-coding-system-alist
     This variable is an alist that specifies the coding systems to use
     for reading and writing particular files.  Each element has the
     form ‘(PATTERN . CODING)’, where PATTERN is a regular expression
     that matches certain file names.  The element applies to file names
     that match PATTERN.

     The CDR of the element, CODING, should be either a coding system, a
     cons cell containing two coding systems, or a function name (a
     symbol with a function definition).  If CODING is a coding system,
     that coding system is used for both reading the file and writing
     it.  If CODING is a cons cell containing two coding systems, its
     CAR specifies the coding system for decoding, and its CDR specifies
     the coding system for encoding.

     If CODING is a function name, the function should take one
     argument, a list of all arguments passed to
     ‘find-operation-coding-system’.  It must return a coding system or
     a cons cell containing two coding systems.  This value has the same
     meaning as described above.

     If CODING (or what returned by the above function) is ‘undecided’,
     the normal code-detection is performed.

 -- User Option: auto-coding-alist
     This variable is an alist that specifies the coding systems to use
     for reading and writing particular files.  Its form is like that of
     ‘file-coding-system-alist’, but, unlike the latter, this variable
     takes priority over any ‘coding:’ tags in the file.

 -- Variable: process-coding-system-alist
     This variable is an alist specifying which coding systems to use
     for a subprocess, depending on which program is running in the
     subprocess.  It works like ‘file-coding-system-alist’, except that
     PATTERN is matched against the program name used to start the
     subprocess.  The coding system or systems specified in this alist
     are used to initialize the coding systems used for I/O to the
     subprocess, but you can specify other coding systems later using
     ‘set-process-coding-system’.

   *Warning:* Coding systems such as ‘undecided’, which determine the
coding system from the data, do not work entirely reliably with
asynchronous subprocess output.  This is because Emacs handles
asynchronous subprocess output in batches, as it arrives.  If the coding
system leaves the character code conversion unspecified, or leaves the
end-of-line conversion unspecified, Emacs must try to detect the proper
conversion from one batch at a time, and this does not always work.

   Therefore, with an asynchronous subprocess, if at all possible, use a
coding system which determines both the character code conversion and
the end of line conversion—that is, one like ‘latin-1-unix’, rather than
‘undecided’ or ‘latin-1’.

 -- Variable: network-coding-system-alist
     This variable is an alist that specifies the coding system to use
     for network streams.  It works much like
     ‘file-coding-system-alist’, with the difference that the PATTERN in
     an element may be either a port number or a regular expression.  If
     it is a regular expression, it is matched against the network
     service name used to open the network stream.

 -- Variable: default-process-coding-system
     This variable specifies the coding systems to use for subprocess
     (and network stream) input and output, when nothing else specifies
     what to do.

     The value should be a cons cell of the form ‘(INPUT-CODING .
     OUTPUT-CODING)’.  Here INPUT-CODING applies to input from the
     subprocess, and OUTPUT-CODING applies to output to it.

 -- User Option: auto-coding-functions
     This variable holds a list of functions that try to determine a
     coding system for a file based on its undecoded contents.

     Each function in this list should be written to look at text in the
     current buffer, but should not modify it in any way.  The buffer
     will contain undecoded text of parts of the file.  Each function
     should take one argument, SIZE, which tells it how many characters
     to look at, starting from point.  If the function succeeds in
     determining a coding system for the file, it should return that
     coding system.  Otherwise, it should return ‘nil’.

     If a file has a ‘coding:’ tag, that takes precedence, so these
     functions won’t be called.

 -- Function: find-auto-coding filename size
     This function tries to determine a suitable coding system for
     FILENAME.  It examines the buffer visiting the named file, using
     the variables documented above in sequence, until it finds a match
     for one of the rules specified by these variables.  It then returns
     a cons cell of the form ‘(CODING . SOURCE)’, where CODING is the
     coding system to use and SOURCE is a symbol, one of
     ‘auto-coding-alist’, ‘auto-coding-regexp-alist’, ‘:coding’, or
     ‘auto-coding-functions’, indicating which one supplied the matching
     rule.  The value ‘:coding’ means the coding system was specified by
     the ‘coding:’ tag in the file (*note coding tag: (emacs)Specify
     Coding.).  The order of looking for a matching rule is
     ‘auto-coding-alist’ first, then ‘auto-coding-regexp-alist’, then
     the ‘coding:’ tag, and lastly ‘auto-coding-functions’.  If no
     matching rule was found, the function returns ‘nil’.

     The second argument SIZE is the size of text, in characters,
     following point.  The function examines text only within SIZE
     characters after point.  Normally, the buffer should be positioned
     at the beginning when this function is called, because one of the
     places for the ‘coding:’ tag is the first one or two lines of the
     file; in that case, SIZE should be the size of the buffer.

 -- Function: set-auto-coding filename size
     This function returns a suitable coding system for file FILENAME.
     It uses ‘find-auto-coding’ to find the coding system.  If no coding
     system could be determined, the function returns ‘nil’.  The
     meaning of the argument SIZE is like in ‘find-auto-coding’.

 -- Function: find-operation-coding-system operation &rest arguments
     This function returns the coding system to use (by default) for
     performing OPERATION with ARGUMENTS.  The value has this form:

          (DECODING-SYSTEM . ENCODING-SYSTEM)

     The first element, DECODING-SYSTEM, is the coding system to use for
     decoding (in case OPERATION does decoding), and ENCODING-SYSTEM is
     the coding system for encoding (in case OPERATION does encoding).

     The argument OPERATION is a symbol; it should be one of
     ‘write-region’, ‘start-process’, ‘call-process’,
     ‘call-process-region’, ‘insert-file-contents’, or
     ‘open-network-stream’.  These are the names of the Emacs I/O
     primitives that can do character code and eol conversion.

     The remaining arguments should be the same arguments that might be
     given to the corresponding I/O primitive.  Depending on the
     primitive, one of those arguments is selected as the “target”.  For
     example, if OPERATION does file I/O, whichever argument specifies
     the file name is the target.  For subprocess primitives, the
     process name is the target.  For ‘open-network-stream’, the target
     is the service name or port number.

     Depending on OPERATION, this function looks up the target in
     ‘file-coding-system-alist’, ‘process-coding-system-alist’, or
     ‘network-coding-system-alist’.  If the target is found in the
     alist, ‘find-operation-coding-system’ returns its association in
     the alist; otherwise it returns ‘nil’.

     If OPERATION is ‘insert-file-contents’, the argument corresponding
     to the target may be a cons cell of the form ‘(FILENAME . BUFFER)’.
     In that case, FILENAME is a file name to look up in
     ‘file-coding-system-alist’, and BUFFER is a buffer that contains
     the file’s contents (not yet decoded).  If
     ‘file-coding-system-alist’ specifies a function to call for this
     file, and that function needs to examine the file’s contents (as it
     usually does), it should examine the contents of BUFFER instead of
     reading the file.


File: elisp.info,  Node: Specifying Coding Systems,  Next: Explicit Encoding,  Prev: Default Coding Systems,  Up: Coding Systems

32.10.6 Specifying a Coding System for One Operation
----------------------------------------------------

You can specify the coding system for a specific operation by binding
the variables ‘coding-system-for-read’ and/or ‘coding-system-for-write’.

 -- Variable: coding-system-for-read
     If this variable is non-‘nil’, it specifies the coding system to
     use for reading a file, or for input from a synchronous subprocess.

     It also applies to any asynchronous subprocess or network stream,
     but in a different way: the value of ‘coding-system-for-read’ when
     you start the subprocess or open the network stream specifies the
     input decoding method for that subprocess or network stream.  It
     remains in use for that subprocess or network stream unless and
     until overridden.

     The right way to use this variable is to bind it with ‘let’ for a
     specific I/O operation.  Its global value is normally ‘nil’, and
     you should not globally set it to any other value.  Here is an
     example of the right way to use the variable:

          ;; Read the file with no character code conversion.
          (let ((coding-system-for-read 'no-conversion))
            (insert-file-contents filename))

     When its value is non-‘nil’, this variable takes precedence over
     all other methods of specifying a coding system to use for input,
     including ‘file-coding-system-alist’, ‘process-coding-system-alist’
     and ‘network-coding-system-alist’.

 -- Variable: coding-system-for-write
     This works much like ‘coding-system-for-read’, except that it
     applies to output rather than input.  It affects writing to files,
     as well as sending output to subprocesses and net connections.  It
     also applies to encoding command-line arguments with which Emacs
     invokes subprocesses.

     When a single operation does both input and output, as do
     ‘call-process-region’ and ‘start-process’, both
     ‘coding-system-for-read’ and ‘coding-system-for-write’ affect it.

 -- Variable: coding-system-require-warning
     Binding ‘coding-system-for-write’ to a non-‘nil’ value prevents
     output primitives from calling the function specified by
     ‘select-safe-coding-system-function’ (*note User-Chosen Coding
     Systems::).  This is because ‘C-x RET c’
     (‘universal-coding-system-argument’) works by binding
     ‘coding-system-for-write’, and Emacs should obey user selection.
     If a Lisp program binds ‘coding-system-for-write’ to a value that
     might not be safe for encoding the text to be written, it can also
     bind ‘coding-system-require-warning’ to a non-‘nil’ value, which
     will force the output primitives to check the encoding by calling
     the value of ‘select-safe-coding-system-function’ even though
     ‘coding-system-for-write’ is non-‘nil’.  Alternatively, call
     ‘select-safe-coding-system’ explicitly before using the specified
     encoding.

 -- User Option: inhibit-eol-conversion
     When this variable is non-‘nil’, no end-of-line conversion is done,
     no matter which coding system is specified.  This applies to all
     the Emacs I/O and subprocess primitives, and to the explicit
     encoding and decoding functions (*note Explicit Encoding::).

   Sometimes, you need to prefer several coding systems for some
operation, rather than fix a single one.  Emacs lets you specify a
priority order for using coding systems.  This ordering affects the
sorting of lists of coding systems returned by functions such as
‘find-coding-systems-region’ (*note Lisp and Coding Systems::).

 -- Function: coding-system-priority-list &optional highestp
     This function returns the list of coding systems in the order of
     their current priorities.  Optional argument HIGHESTP, if
     non-‘nil’, means return only the highest priority coding system.

 -- Function: set-coding-system-priority &rest coding-systems
     This function puts CODING-SYSTEMS at the beginning of the priority
     list for coding systems, thus making their priority higher than all
     the rest.

 -- Macro: with-coding-priority coding-systems &rest body...
     This macro execute BODY, like ‘progn’ does (*note progn:
     Sequencing.), with CODING-SYSTEMS at the front of the priority list
     for coding systems.  CODING-SYSTEMS should be a list of coding
     systems to prefer during execution of BODY.


File: elisp.info,  Node: Explicit Encoding,  Next: Terminal I/O Encoding,  Prev: Specifying Coding Systems,  Up: Coding Systems

32.10.7 Explicit Encoding and Decoding
--------------------------------------

All the operations that transfer text in and out of Emacs have the
ability to use a coding system to encode or decode the text.  You can
also explicitly encode and decode text using the functions in this
section.

   The result of encoding, and the input to decoding, are not ordinary
text.  They logically consist of a series of byte values; that is, a
series of ASCII and eight-bit characters.  In unibyte buffers and
strings, these characters have codes in the range 0 through #xFF (255).
In a multibyte buffer or string, eight-bit characters have character
codes higher than #xFF (*note Text Representations::), but Emacs
transparently converts them to their single-byte values when you encode
or decode such text.

   The usual way to read a file into a buffer as a sequence of bytes, so
you can decode the contents explicitly, is with
‘insert-file-contents-literally’ (*note Reading from Files::);
alternatively, specify a non-‘nil’ RAWFILE argument when visiting a file
with ‘find-file-noselect’.  These methods result in a unibyte buffer.

   The usual way to use the byte sequence that results from explicitly
encoding text is to copy it to a file or process—for example, to write
it with ‘write-region’ (*note Writing to Files::), and suppress encoding
by binding ‘coding-system-for-write’ to ‘no-conversion’.

   Here are the functions to perform explicit encoding or decoding.  The
encoding functions produce sequences of bytes; the decoding functions
are meant to operate on sequences of bytes.  All of these functions
discard text properties.  They also set ‘last-coding-system-used’ to the
precise coding system they used.

 -- Command: encode-coding-region start end coding-system &optional
          destination
     This command encodes the text from START to END according to coding
     system CODING-SYSTEM.  Normally, the encoded text replaces the
     original text in the buffer, but the optional argument DESTINATION
     can change that.  If DESTINATION is a buffer, the encoded text is
     inserted in that buffer after point (point does not move); if it is
     ‘t’, the command returns the encoded text as a unibyte string
     without inserting it.

     If encoded text is inserted in some buffer, this command returns
     the length of the encoded text.

     The result of encoding is logically a sequence of bytes, but the
     buffer remains multibyte if it was multibyte before, and any 8-bit
     bytes are converted to their multibyte representation (*note Text
     Representations::).

     Do _not_ use ‘undecided’ for CODING-SYSTEM when encoding text,
     since that may lead to unexpected results.  Instead, use
     ‘select-safe-coding-system’ (*note select-safe-coding-system:
     User-Chosen Coding Systems.) to suggest a suitable encoding, if
     there’s no obvious pertinent value for CODING-SYSTEM.

 -- Function: encode-coding-string string coding-system &optional nocopy
          buffer
     This function encodes the text in STRING according to coding system
     CODING-SYSTEM.  It returns a new string containing the encoded
     text, except when NOCOPY is non-‘nil’, in which case the function
     may return STRING itself if the encoding operation is trivial.  The
     result of encoding is a unibyte string.

 -- Command: decode-coding-region start end coding-system &optional
          destination
     This command decodes the text from START to END according to coding
     system CODING-SYSTEM.  To make explicit decoding useful, the text
     before decoding ought to be a sequence of byte values, but both
     multibyte and unibyte buffers are acceptable (in the multibyte
     case, the raw byte values should be represented as eight-bit
     characters).  Normally, the decoded text replaces the original text
     in the buffer, but the optional argument DESTINATION can change
     that.  If DESTINATION is a buffer, the decoded text is inserted in
     that buffer after point (point does not move); if it is ‘t’, the
     command returns the decoded text as a multibyte string without
     inserting it.

     If decoded text is inserted in some buffer, this command returns
     the length of the decoded text.

     This command puts a ‘charset’ text property on the decoded text.
     The value of the property states the character set used to decode
     the original text.

 -- Function: decode-coding-string string coding-system &optional nocopy
          buffer
     This function decodes the text in STRING according to
     CODING-SYSTEM.  It returns a new string containing the decoded
     text, except when NOCOPY is non-‘nil’, in which case the function
     may return STRING itself if the decoding operation is trivial.  To
     make explicit decoding useful, the contents of STRING ought to be a
     unibyte string with a sequence of byte values, but a multibyte
     string is also acceptable (assuming it contains 8-bit bytes in
     their multibyte form).

     If optional argument BUFFER specifies a buffer, the decoded text is
     inserted in that buffer after point (point does not move).  In this
     case, the return value is the length of the decoded text.

     This function puts a ‘charset’ text property on the decoded text.
     The value of the property states the character set used to decode
     the original text:

          (decode-coding-string "Gr\374ss Gott" 'latin-1)
               ⇒ #("Grüss Gott" 0 9 (charset iso-8859-1))

 -- Function: decode-coding-inserted-region from to filename &optional
          visit beg end replace
     This function decodes the text from FROM to TO as if it were being
     read from file FILENAME using ‘insert-file-contents’ using the rest
     of the arguments provided.

     The normal way to use this function is after reading text from a
     file without decoding, if you decide you would rather have decoded
     it.  Instead of deleting the text and reading it again, this time
     with decoding, you can call this function.


File: elisp.info,  Node: Terminal I/O Encoding,  Prev: Explicit Encoding,  Up: Coding Systems

32.10.8 Terminal I/O Encoding
-----------------------------

Emacs can use coding systems to decode keyboard input and encode
terminal output.  This is useful for terminals that transmit or display
text using a particular encoding, such as Latin-1.  Emacs does not set
‘last-coding-system-used’ when encoding or decoding terminal I/O.

 -- Function: keyboard-coding-system &optional terminal
     This function returns the coding system used for decoding keyboard
     input from TERMINAL.  A value of ‘no-conversion’ means no decoding
     is done.  If TERMINAL is omitted or ‘nil’, it means the selected
     frame’s terminal.  *Note Multiple Terminals::.

 -- Command: set-keyboard-coding-system coding-system &optional terminal
     This command specifies CODING-SYSTEM as the coding system to use
     for decoding keyboard input from TERMINAL.  If CODING-SYSTEM is
     ‘nil’, that means not to decode keyboard input.  If TERMINAL is a
     frame, it means that frame’s terminal; if it is ‘nil’, that means
     the currently selected frame’s terminal.  *Note Multiple
     Terminals::.

 -- Function: terminal-coding-system &optional terminal
     This function returns the coding system that is in use for encoding
     terminal output from TERMINAL.  A value of ‘no-conversion’ means no
     encoding is done.  If TERMINAL is a frame, it means that frame’s
     terminal; if it is ‘nil’, that means the currently selected frame’s
     terminal.

 -- Command: set-terminal-coding-system coding-system &optional terminal
     This command specifies CODING-SYSTEM as the coding system to use
     for encoding terminal output from TERMINAL.  If CODING-SYSTEM is
     ‘nil’, that means not to encode terminal output.  If TERMINAL is a
     frame, it means that frame’s terminal; if it is ‘nil’, that means
     the currently selected frame’s terminal.


File: elisp.info,  Node: Input Methods,  Next: Locales,  Prev: Coding Systems,  Up: Non-ASCII Characters

32.11 Input Methods
===================

“Input methods” provide convenient ways of entering non-ASCII characters
from the keyboard.  Unlike coding systems, which translate non-ASCII
characters to and from encodings meant to be read by programs, input
methods provide human-friendly commands.  (*Note (emacs)Input Methods::,
for information on how users use input methods to enter text.)  How to
define input methods is not yet documented in this manual, but here we
describe how to use them.

   Each input method has a name, which is currently a string; in the
future, symbols may also be usable as input method names.

 -- Variable: current-input-method
     This variable holds the name of the input method now active in the
     current buffer.  (It automatically becomes local in each buffer
     when set in any fashion.)  It is ‘nil’ if no input method is active
     in the buffer now.

 -- User Option: default-input-method
     This variable holds the default input method for commands that
     choose an input method.  Unlike ‘current-input-method’, this
     variable is normally global.

 -- Command: set-input-method input-method
     This command activates input method INPUT-METHOD for the current
     buffer.  It also sets ‘default-input-method’ to INPUT-METHOD.  If
     INPUT-METHOD is ‘nil’, this command deactivates any input method
     for the current buffer.

 -- Function: read-input-method-name prompt &optional default
          inhibit-null
     This function reads an input method name with the minibuffer,
     prompting with PROMPT.  If DEFAULT is non-‘nil’, that is returned
     by default, if the user enters empty input.  However, if
     INHIBIT-NULL is non-‘nil’, empty input signals an error.

     The returned value is a string.

 -- Variable: input-method-alist
     This variable defines all the supported input methods.  Each
     element defines one input method, and should have the form:

          (INPUT-METHOD LANGUAGE-ENV ACTIVATE-FUNC
           TITLE DESCRIPTION ARGS...)

     Here INPUT-METHOD is the input method name, a string; LANGUAGE-ENV
     is another string, the name of the language environment this input
     method is recommended for.  (That serves only for documentation
     purposes.)

     ACTIVATE-FUNC is a function to call to activate this method.  The
     ARGS, if any, are passed as arguments to ACTIVATE-FUNC.  All told,
     the arguments to ACTIVATE-FUNC are INPUT-METHOD and the ARGS.

     TITLE is a string to display in the mode line while this method is
     active.  DESCRIPTION is a string describing this method and what it
     is good for.

   The fundamental interface to input methods is through the variable
‘input-method-function’.  *Note Reading One Event::, and *note Invoking
the Input Method::.


File: elisp.info,  Node: Locales,  Prev: Input Methods,  Up: Non-ASCII Characters

32.12 Locales
=============

In POSIX, locales control which language to use in language-related
features.  These Emacs variables control how Emacs interacts with these
features.

 -- Variable: locale-coding-system
     This variable specifies the coding system to use for decoding
     system error messages and—on X Window system only—keyboard input,
     for sending batch output to the standard output and error streams,
     for encoding the format argument to ‘format-time-string’, and for
     decoding the return value of ‘format-time-string’.

 -- Variable: system-messages-locale
     This variable specifies the locale to use for generating system
     error messages.  Changing the locale can cause messages to come out
     in a different language or in a different orthography.  If the
     variable is ‘nil’, the locale is specified by environment variables
     in the usual POSIX fashion.

 -- Variable: system-time-locale
     This variable specifies the locale to use for formatting time
     values.  Changing the locale can cause messages to appear according
     to the conventions of a different language.  If the variable is
     ‘nil’, the locale is specified by environment variables in the
     usual POSIX fashion.

 -- Function: locale-info item
     This function returns locale data ITEM for the current POSIX
     locale, if available.  ITEM should be one of these symbols:

     ‘codeset’
          Return the character set as a string (locale item ‘CODESET’).

     ‘days’
          Return a 7-element vector of day names (locale items ‘DAY_1’
          through ‘DAY_7’);

     ‘months’
          Return a 12-element vector of month names (locale items
          ‘MON_1’ through ‘MON_12’).

     ‘paper’
          Return a list ‘(WIDTH HEIGHT)’ for the default paper size
          measured in millimeters (locale items ‘PAPER_WIDTH’ and
          ‘PAPER_HEIGHT’).

     If the system can’t provide the requested information, or if ITEM
     is not one of those symbols, the value is ‘nil’.  All strings in
     the return value are decoded using ‘locale-coding-system’.  *Note
     (libc)Locales::, for more information about locales and locale
     items.


File: elisp.info,  Node: Searching and Matching,  Next: Syntax Tables,  Prev: Non-ASCII Characters,  Up: Top

33 Searching and Matching
*************************

GNU Emacs provides two ways to search through a buffer for specified
text: exact string searches and regular expression searches.  After a
regular expression search, you can examine the “match data” to determine
which text matched the whole regular expression or various portions of
it.

* Menu:

* String Search::         Search for an exact match.
* Searching and Case::    Case-independent or case-significant searching.
* Regular Expressions::   Describing classes of strings.
* Regexp Search::         Searching for a match for a regexp.
* POSIX Regexps::         Searching POSIX-style for the longest match.
* Match Data::            Finding out which part of the text matched,
                            after a string or regexp search.
* Search and Replace::    Commands that loop, searching and replacing.
* Standard Regexps::      Useful regexps for finding sentences, pages,...

   The ‘skip-chars...’ functions also perform a kind of searching.
*Note Skipping Characters::.  To search for changes in character
properties, see *note Property Search::.


File: elisp.info,  Node: String Search,  Next: Searching and Case,  Up: Searching and Matching

33.1 Searching for Strings
==========================

These are the primitive functions for searching through the text in a
buffer.  They are meant for use in programs, but you may call them
interactively.  If you do so, they prompt for the search string; the
arguments LIMIT and NOERROR are ‘nil’, and REPEAT is 1.  For more
details on interactive searching, *note Searching and Replacement:
(emacs)Search.

   These search functions convert the search string to multibyte if the
buffer is multibyte; they convert the search string to unibyte if the
buffer is unibyte.  *Note Text Representations::.

 -- Command: search-forward string &optional limit noerror count
     This function searches forward from point for an exact match for
     STRING.  If successful, it sets point to the end of the occurrence
     found, and returns the new value of point.  If no match is found,
     the value and side effects depend on NOERROR (see below).

     In the following example, point is initially at the beginning of
     the line.  Then ‘(search-forward "fox")’ moves point after the last
     letter of ‘fox’:

          ---------- Buffer: foo ----------
          ★The quick brown fox jumped over the lazy dog.
          ---------- Buffer: foo ----------

          (search-forward "fox")
               ⇒ 20

          ---------- Buffer: foo ----------
          The quick brown fox★ jumped over the lazy dog.
          ---------- Buffer: foo ----------

     The argument LIMIT specifies the bound to the search, and should be
     a position in the current buffer.  No match extending after that
     position is accepted.  If LIMIT is omitted or ‘nil’, it defaults to
     the end of the accessible portion of the buffer.

     What happens when the search fails depends on the value of NOERROR.
     If NOERROR is ‘nil’, a ‘search-failed’ error is signaled.  If
     NOERROR is ‘t’, ‘search-forward’ returns ‘nil’ and does nothing.
     If NOERROR is neither ‘nil’ nor ‘t’, then ‘search-forward’ moves
     point to the upper bound and returns ‘nil’.

     The argument NOERROR only affects valid searches which fail to find
     a match.  Invalid arguments cause errors regardless of NOERROR.

     If COUNT is a positive number N, the search is done N times; each
     successive search starts at the end of the previous match.  If all
     these successive searches succeed, the function call succeeds,
     moving point and returning its new value.  Otherwise the function
     call fails, with results depending on the value of NOERROR, as
     described above.  If COUNT is a negative number -N, the search is
     done N times in the opposite (backward) direction.

 -- Command: search-backward string &optional limit noerror count
     This function searches backward from point for STRING.  It is like
     ‘search-forward’, except that it searches backwards rather than
     forwards.  Backward searches leave point at the beginning of the
     match.

 -- Command: word-search-forward string &optional limit noerror count
     This function searches forward from point for a word match for
     STRING.  If it finds a match, it sets point to the end of the match
     found, and returns the new value of point.

     Word matching regards STRING as a sequence of words, disregarding
     punctuation that separates them.  It searches the buffer for the
     same sequence of words.  Each word must be distinct in the buffer
     (searching for the word ‘ball’ does not match the word ‘balls’),
     but the details of punctuation and spacing are ignored (searching
     for ‘ball boy’ does match ‘ball. Boy!’).

     In this example, point is initially at the beginning of the buffer;
     the search leaves it between the ‘y’ and the ‘!’.

          ---------- Buffer: foo ----------
          ★He said "Please!  Find
          the ball boy!"
          ---------- Buffer: foo ----------

          (word-search-forward "Please find the ball, boy.")
               ⇒ 39

          ---------- Buffer: foo ----------
          He said "Please!  Find
          the ball boy★!"
          ---------- Buffer: foo ----------

     If LIMIT is non-‘nil’, it must be a position in the current buffer;
     it specifies the upper bound to the search.  The match found must
     not extend after that position.

     If NOERROR is ‘nil’, then ‘word-search-forward’ signals an error if
     the search fails.  If NOERROR is ‘t’, then it returns ‘nil’ instead
     of signaling an error.  If NOERROR is neither ‘nil’ nor ‘t’, it
     moves point to LIMIT (or the end of the accessible portion of the
     buffer) and returns ‘nil’.

     If COUNT is a positive number, it specifies how many successive
     occurrences to search for.  Point is positioned at the end of the
     last match.  If COUNT is a negative number, the search is backward
     and point is positioned at the beginning of the last match.

     Internally, ‘word-search-forward’ and related functions use the
     function ‘word-search-regexp’ to convert STRING to a regular
     expression that ignores punctuation.

 -- Command: word-search-forward-lax string &optional limit noerror
          count
     This command is identical to ‘word-search-forward’, except that the
     beginning or the end of STRING need not match a word boundary,
     unless STRING begins or ends in whitespace.  For instance,
     searching for ‘ball boy’ matches ‘ball boyee’, but does not match
     ‘balls boy’.

 -- Command: word-search-backward string &optional limit noerror count
     This function searches backward from point for a word match to
     STRING.  This function is just like ‘word-search-forward’ except
     that it searches backward and normally leaves point at the
     beginning of the match.

 -- Command: word-search-backward-lax string &optional limit noerror
          count
     This command is identical to ‘word-search-backward’, except that
     the beginning or the end of STRING need not match a word boundary,
     unless STRING begins or ends in whitespace.


File: elisp.info,  Node: Searching and Case,  Next: Regular Expressions,  Prev: String Search,  Up: Searching and Matching

33.2 Searching and Case
=======================

By default, searches in Emacs ignore the case of the text they are
searching through; if you specify searching for ‘FOO’, then ‘Foo’ or
‘foo’ is also considered a match.  This applies to regular expressions,
too; thus, ‘[aB]’ would match ‘a’ or ‘A’ or ‘b’ or ‘B’.

   If you do not want this feature, set the variable ‘case-fold-search’
to ‘nil’.  Then all letters must match exactly, including case.  This is
a buffer-local variable; altering the variable affects only the current
buffer.  (*Note Intro to Buffer-Local::.)  Alternatively, you may change
the default value.  In Lisp code, you will more typically use ‘let’ to
bind ‘case-fold-search’ to the desired value.

   Note that the user-level incremental search feature handles case
distinctions differently.  When the search string contains only lower
case letters, the search ignores case, but when the search string
contains one or more upper case letters, the search becomes
case-sensitive.  But this has nothing to do with the searching functions
used in Lisp code.  *Note (emacs)Incremental Search::.

 -- User Option: case-fold-search
     This buffer-local variable determines whether searches should
     ignore case.  If the variable is ‘nil’ they do not ignore case;
     otherwise (and by default) they do ignore case.

 -- User Option: case-replace
     This variable determines whether the higher-level replacement
     functions should preserve case.  If the variable is ‘nil’, that
     means to use the replacement text verbatim.  A non-‘nil’ value
     means to convert the case of the replacement text according to the
     text being replaced.

     This variable is used by passing it as an argument to the function
     ‘replace-match’.  *Note Replacing Match::.


File: elisp.info,  Node: Regular Expressions,  Next: Regexp Search,  Prev: Searching and Case,  Up: Searching and Matching

33.3 Regular Expressions
========================

A “regular expression”, or “regexp” for short, is a pattern that denotes
a (possibly infinite) set of strings.  Searching for matches for a
regexp is a very powerful operation.  This section explains how to write
regexps; the following section says how to search for them.

   For interactive development of regular expressions, you can use the
‘M-x re-builder’ command.  It provides a convenient interface for
creating regular expressions, by giving immediate visual feedback in a
separate buffer.  As you edit the regexp, all its matches in the target
buffer are highlighted.  Each parenthesized sub-expression of the regexp
is shown in a distinct face, which makes it easier to verify even very
complex regexps.

* Menu:

* Syntax of Regexps::       Rules for writing regular expressions.
* Regexp Example::          Illustrates regular expression syntax.
* Regexp Functions::        Functions for operating on regular expressions.


File: elisp.info,  Node: Syntax of Regexps,  Next: Regexp Example,  Up: Regular Expressions

33.3.1 Syntax of Regular Expressions
------------------------------------

Regular expressions have a syntax in which a few characters are special
constructs and the rest are “ordinary”.  An ordinary character is a
simple regular expression that matches that character and nothing else.
The special characters are ‘.’, ‘*’, ‘+’, ‘?’, ‘[’, ‘^’, ‘$’, and ‘\’;
no new special characters will be defined in the future.  The character
‘]’ is special if it ends a character alternative (see later).  The
character ‘-’ is special inside a character alternative.  A ‘[:’ and
balancing ‘:]’ enclose a character class inside a character alternative.
Any other character appearing in a regular expression is ordinary,
unless a ‘\’ precedes it.

   For example, ‘f’ is not a special character, so it is ordinary, and
therefore ‘f’ is a regular expression that matches the string ‘f’ and no
other string.  (It does _not_ match the string ‘fg’, but it does match a
_part_ of that string.)  Likewise, ‘o’ is a regular expression that
matches only ‘o’.

   Any two regular expressions A and B can be concatenated.  The result
is a regular expression that matches a string if A matches some amount
of the beginning of that string and B matches the rest of the string.

   As a simple example, we can concatenate the regular expressions ‘f’
and ‘o’ to get the regular expression ‘fo’, which matches only the
string ‘fo’.  Still trivial.  To do something more powerful, you need to
use one of the special regular expression constructs.

* Menu:

* Regexp Special::      Special characters in regular expressions.
* Char Classes::        Character classes used in regular expressions.
* Regexp Backslash::    Backslash-sequences in regular expressions.


File: elisp.info,  Node: Regexp Special,  Next: Char Classes,  Up: Syntax of Regexps

33.3.1.1 Special Characters in Regular Expressions
..................................................

Here is a list of the characters that are special in a regular
expression.

‘.’ (Period)
     is a special character that matches any single character except a
     newline.  Using concatenation, we can make regular expressions like
     ‘a.b’, which matches any three-character string that begins with
     ‘a’ and ends with ‘b’.

‘*’
     is not a construct by itself; it is a postfix operator that means
     to match the preceding regular expression repetitively as many
     times as possible.  Thus, ‘o*’ matches any number of ‘o’s
     (including no ‘o’s).

     ‘*’ always applies to the _smallest_ possible preceding expression.
     Thus, ‘fo*’ has a repeating ‘o’, not a repeating ‘fo’.  It matches
     ‘f’, ‘fo’, ‘foo’, and so on.

     The matcher processes a ‘*’ construct by matching, immediately, as
     many repetitions as can be found.  Then it continues with the rest
     of the pattern.  If that fails, backtracking occurs, discarding
     some of the matches of the ‘*’-modified construct in the hope that
     that will make it possible to match the rest of the pattern.  For
     example, in matching ‘ca*ar’ against the string ‘caaar’, the ‘a*’
     first tries to match all three ‘a’s; but the rest of the pattern is
     ‘ar’ and there is only ‘r’ left to match, so this try fails.  The
     next alternative is for ‘a*’ to match only two ‘a’s.  With this
     choice, the rest of the regexp matches successfully.

     *Warning:* Nested repetition operators can run for an indefinitely
     long time, if they lead to ambiguous matching.  For example, trying
     to match the regular expression ‘\(x+y*\)*a’ against the string
     ‘xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxz’ could take hours before it
     ultimately fails.  Emacs must try each way of grouping the ‘x’s
     before concluding that none of them can work.  Even worse,
     ‘\(x*\)*’ can match the null string in infinitely many ways, so it
     causes an infinite loop.  To avoid these problems, check nested
     repetitions carefully, to make sure that they do not cause
     combinatorial explosions in backtracking.

‘+’
     is a postfix operator, similar to ‘*’ except that it must match the
     preceding expression at least once.  So, for example, ‘ca+r’
     matches the strings ‘car’ and ‘caaaar’ but not the string ‘cr’,
     whereas ‘ca*r’ matches all three strings.

‘?’
     is a postfix operator, similar to ‘*’ except that it must match the
     preceding expression either once or not at all.  For example,
     ‘ca?r’ matches ‘car’ or ‘cr’; nothing else.

‘*?’, ‘+?’, ‘??’
     These are “non-greedy” variants of the operators ‘*’, ‘+’ and ‘?’.
     Where those operators match the largest possible substring
     (consistent with matching the entire containing expression), the
     non-greedy variants match the smallest possible substring
     (consistent with matching the entire containing expression).

     For example, the regular expression ‘c[ad]*a’ when applied to the
     string ‘cdaaada’ matches the whole string; but the regular
     expression ‘c[ad]*?a’, applied to that same string, matches just
     ‘cda’.  (The smallest possible match here for ‘[ad]*?’ that permits
     the whole expression to match is ‘d’.)

‘[ ... ]’
     is a “character alternative”, which begins with ‘[’ and is
     terminated by ‘]’.  In the simplest case, the characters between
     the two brackets are what this character alternative can match.

     Thus, ‘[ad]’ matches either one ‘a’ or one ‘d’, and ‘[ad]*’ matches
     any string composed of just ‘a’s and ‘d’s (including the empty
     string).  It follows that ‘c[ad]*r’ matches ‘cr’, ‘car’, ‘cdr’,
     ‘caddaar’, etc.

     You can also include character ranges in a character alternative,
     by writing the starting and ending characters with a ‘-’ between
     them.  Thus, ‘[a-z]’ matches any lower-case ASCII letter.  Ranges
     may be intermixed freely with individual characters, as in
     ‘[a-z$%.]’, which matches any lower case ASCII letter or ‘$’, ‘%’
     or period.

     If ‘case-fold-search’ is non-‘nil’, ‘[a-z]’ also matches upper-case
     letters.  Note that a range like ‘[a-z]’ is not affected by the
     locale’s collation sequence, it always represents a sequence in
     ASCII order.

     Note also that the usual regexp special characters are not special
     inside a character alternative.  A completely different set of
     characters is special inside character alternatives: ‘]’, ‘-’ and
     ‘^’.

     To include a ‘]’ in a character alternative, you must make it the
     first character.  For example, ‘[]a]’ matches ‘]’ or ‘a’.  To
     include a ‘-’, write ‘-’ as the first or last character of the
     character alternative, or put it after a range.  Thus, ‘[]-]’
     matches both ‘]’ and ‘-’.  (As explained below, you cannot use ‘\]’
     to include a ‘]’ inside a character alternative, since ‘\’ is not
     special there.)

     To include ‘^’ in a character alternative, put it anywhere but at
     the beginning.

     If a range starts with a unibyte character C and ends with a
     multibyte character C2, the range is divided into two parts: one
     spans the unibyte characters ‘C..?\377’, the other the multibyte
     characters ‘C1..C2’, where C1 is the first character of the charset
     to which C2 belongs.

     A character alternative can also specify named character classes
     (*note Char Classes::).  This is a POSIX feature.  For example,
     ‘[[:ascii:]]’ matches any ASCII character.  Using a character class
     is equivalent to mentioning each of the characters in that class;
     but the latter is not feasible in practice, since some classes
     include thousands of different characters.

‘[^ ... ]’
     ‘[^’ begins a “complemented character alternative”.  This matches
     any character except the ones specified.  Thus, ‘[^a-z0-9A-Z]’
     matches all characters _except_ letters and digits.

     ‘^’ is not special in a character alternative unless it is the
     first character.  The character following the ‘^’ is treated as if
     it were first (in other words, ‘-’ and ‘]’ are not special there).

     A complemented character alternative can match a newline, unless
     newline is mentioned as one of the characters not to match.  This
     is in contrast to the handling of regexps in programs such as
     ‘grep’.

     You can specify named character classes, just like in character
     alternatives.  For instance, ‘[^[:ascii:]]’ matches any non-ASCII
     character.  *Note Char Classes::.

‘^’
     When matching a buffer, ‘^’ matches the empty string, but only at
     the beginning of a line in the text being matched (or the beginning
     of the accessible portion of the buffer).  Otherwise it fails to
     match anything.  Thus, ‘^foo’ matches a ‘foo’ that occurs at the
     beginning of a line.

     When matching a string instead of a buffer, ‘^’ matches at the
     beginning of the string or after a newline character.

     For historical compatibility reasons, ‘^’ can be used only at the
     beginning of the regular expression, or after ‘\(’, ‘\(?:’ or ‘\|’.

‘$’
     is similar to ‘^’ but matches only at the end of a line (or the end
     of the accessible portion of the buffer).  Thus, ‘x+$’ matches a
     string of one ‘x’ or more at the end of a line.

     When matching a string instead of a buffer, ‘$’ matches at the end
     of the string or before a newline character.

     For historical compatibility reasons, ‘$’ can be used only at the
     end of the regular expression, or before ‘\)’ or ‘\|’.

‘\’
     has two functions: it quotes the special characters (including
     ‘\’), and it introduces additional special constructs.

     Because ‘\’ quotes special characters, ‘\$’ is a regular expression
     that matches only ‘$’, and ‘\[’ is a regular expression that
     matches only ‘[’, and so on.

     Note that ‘\’ also has special meaning in the read syntax of Lisp
     strings (*note String Type::), and must be quoted with ‘\’.  For
     example, the regular expression that matches the ‘\’ character is
     ‘\\’.  To write a Lisp string that contains the characters ‘\\’,
     Lisp syntax requires you to quote each ‘\’ with another ‘\’.
     Therefore, the read syntax for a regular expression matching ‘\’ is
     ‘"\\\\"’.

   *Please note:* For historical compatibility, special characters are
treated as ordinary ones if they are in contexts where their special
meanings make no sense.  For example, ‘*foo’ treats ‘*’ as ordinary
since there is no preceding expression on which the ‘*’ can act.  It is
poor practice to depend on this behavior; quote the special character
anyway, regardless of where it appears.

   As a ‘\’ is not special inside a character alternative, it can never
remove the special meaning of ‘-’ or ‘]’.  So you should not quote these
characters when they have no special meaning either.  This would not
clarify anything, since backslashes can legitimately precede these
characters where they _have_ special meaning, as in ‘[^\]’ (‘"[^\\]"’
for Lisp string syntax), which matches any single character except a
backslash.

   In practice, most ‘]’ that occur in regular expressions close a
character alternative and hence are special.  However, occasionally a
regular expression may try to match a complex pattern of literal ‘[’ and
‘]’.  In such situations, it sometimes may be necessary to carefully
parse the regexp from the start to determine which square brackets
enclose a character alternative.  For example, ‘[^][]]’ consists of the
complemented character alternative ‘[^][]’ (which matches any single
character that is not a square bracket), followed by a literal ‘]’.

   The exact rules are that at the beginning of a regexp, ‘[’ is special
and ‘]’ not.  This lasts until the first unquoted ‘[’, after which we
are in a character alternative; ‘[’ is no longer special (except when it
starts a character class) but ‘]’ is special, unless it immediately
follows the special ‘[’ or that ‘[’ followed by a ‘^’.  This lasts until
the next special ‘]’ that does not end a character class.  This ends the
character alternative and restores the ordinary syntax of regular
expressions; an unquoted ‘[’ is special again and a ‘]’ not.


File: elisp.info,  Node: Char Classes,  Next: Regexp Backslash,  Prev: Regexp Special,  Up: Syntax of Regexps

33.3.1.2 Character Classes
..........................

Here is a table of the classes you can use in a character alternative,
and what they mean:

‘[:ascii:]’
     This matches any ASCII character (codes 0–127).
‘[:alnum:]’
     This matches any letter or digit.  For multibyte characters, it
     matches characters whose Unicode ‘general-category’ property (*note
     Character Properties::) indicates they are alphabetic or decimal
     number characters.
‘[:alpha:]’
     This matches any letter.  For multibyte characters, it matches
     characters whose Unicode ‘general-category’ property (*note
     Character Properties::) indicates they are alphabetic characters.
‘[:blank:]’
     This matches space and tab only.
‘[:cntrl:]’
     This matches any ASCII control character.
‘[:digit:]’
     This matches ‘0’ through ‘9’.  Thus, ‘[-+[:digit:]]’ matches any
     digit, as well as ‘+’ and ‘-’.
‘[:graph:]’
     This matches graphic characters—everything except whitespace, ASCII
     and non-ASCII control characters, surrogates, and codepoints
     unassigned by Unicode, as indicated by the Unicode
     ‘general-category’ property (*note Character Properties::).
‘[:lower:]’
     This matches any lower-case letter, as determined by the current
     case table (*note Case Tables::).  If ‘case-fold-search’ is
     non-‘nil’, this also matches any upper-case letter.
‘[:multibyte:]’
     This matches any multibyte character (*note Text
     Representations::).
‘[:nonascii:]’
     This matches any non-ASCII character.
‘[:print:]’
     This matches any printing character—either whitespace, or a graphic
     character matched by ‘[:graph:]’.
‘[:punct:]’
     This matches any punctuation character.  (At present, for multibyte
     characters, it matches anything that has non-word syntax.)
‘[:space:]’
     This matches any character that has whitespace syntax (*note Syntax
     Class Table::).
‘[:unibyte:]’
     This matches any unibyte character (*note Text Representations::).
‘[:upper:]’
     This matches any upper-case letter, as determined by the current
     case table (*note Case Tables::).  If ‘case-fold-search’ is
     non-‘nil’, this also matches any lower-case letter.
‘[:word:]’
     This matches any character that has word syntax (*note Syntax Class
     Table::).
‘[:xdigit:]’
     This matches the hexadecimal digits: ‘0’ through ‘9’, ‘a’ through
     ‘f’ and ‘A’ through ‘F’.


File: elisp.info,  Node: Regexp Backslash,  Prev: Char Classes,  Up: Syntax of Regexps

33.3.1.3 Backslash Constructs in Regular Expressions
....................................................

For the most part, ‘\’ followed by any character matches only that
character.  However, there are several exceptions: certain sequences
starting with ‘\’ that have special meanings.  Here is a table of the
special ‘\’ constructs.

‘\|’
     specifies an alternative.  Two regular expressions A and B with
     ‘\|’ in between form an expression that matches anything that
     either A or B matches.

     Thus, ‘foo\|bar’ matches either ‘foo’ or ‘bar’ but no other string.

     ‘\|’ applies to the largest possible surrounding expressions.  Only
     a surrounding ‘\( ... \)’ grouping can limit the grouping power of
     ‘\|’.

     If you need full backtracking capability to handle multiple uses of
     ‘\|’, use the POSIX regular expression functions (*note POSIX
     Regexps::).

‘\{M\}’
     is a postfix operator that repeats the previous pattern exactly M
     times.  Thus, ‘x\{5\}’ matches the string ‘xxxxx’ and nothing else.
     ‘c[ad]\{3\}r’ matches string such as ‘caaar’, ‘cdddr’, ‘cadar’, and
     so on.

‘\{M,N\}’
     is a more general postfix operator that specifies repetition with a
     minimum of M repeats and a maximum of N repeats.  If M is omitted,
     the minimum is 0; if N is omitted, there is no maximum.

     For example, ‘c[ad]\{1,2\}r’ matches the strings ‘car’, ‘cdr’,
     ‘caar’, ‘cadr’, ‘cdar’, and ‘cddr’, and nothing else.
     ‘\{0,1\}’ or ‘\{,1\}’ is equivalent to ‘?’.
     ‘\{0,\}’ or ‘\{,\}’ is equivalent to ‘*’.
     ‘\{1,\}’ is equivalent to ‘+’.

‘\( ... \)’
     is a grouping construct that serves three purposes:

       1. To enclose a set of ‘\|’ alternatives for other operations.
          Thus, the regular expression ‘\(foo\|bar\)x’ matches either
          ‘foox’ or ‘barx’.

       2. To enclose a complicated expression for the postfix operators
          ‘*’, ‘+’ and ‘?’ to operate on.  Thus, ‘ba\(na\)*’ matches
          ‘ba’, ‘bana’, ‘banana’, ‘bananana’, etc., with any number
          (zero or more) of ‘na’ strings.

       3. To record a matched substring for future reference with
          ‘\DIGIT’ (see below).

     This last application is not a consequence of the idea of a
     parenthetical grouping; it is a separate feature that was assigned
     as a second meaning to the same ‘\( ... \)’ construct because, in
     practice, there was usually no conflict between the two meanings.
     But occasionally there is a conflict, and that led to the
     introduction of shy groups.

‘\(?: ... \)’
     is the “shy group” construct.  A shy group serves the first two
     purposes of an ordinary group (controlling the nesting of other
     operators), but it does not get a number, so you cannot refer back
     to its value with ‘\DIGIT’.  Shy groups are particularly useful for
     mechanically-constructed regular expressions, because they can be
     added automatically without altering the numbering of ordinary,
     non-shy groups.

     Shy groups are also called “non-capturing” or “unnumbered groups”.

‘\(?NUM: ... \)’
     is the “explicitly numbered group” construct.  Normal groups get
     their number implicitly, based on their position, which can be
     inconvenient.  This construct allows you to force a particular
     group number.  There is no particular restriction on the numbering,
     e.g., you can have several groups with the same number in which
     case the last one to match (i.e., the rightmost match) will win.
     Implicitly numbered groups always get the smallest integer larger
     than the one of any previous group.

‘\DIGIT’
     matches the same text that matched the DIGITth occurrence of a
     grouping (‘\( ... \)’) construct.

     In other words, after the end of a group, the matcher remembers the
     beginning and end of the text matched by that group.  Later on in
     the regular expression you can use ‘\’ followed by DIGIT to match
     that same text, whatever it may have been.

     The strings matching the first nine grouping constructs appearing
     in the entire regular expression passed to a search or matching
     function are assigned numbers 1 through 9 in the order that the
     open parentheses appear in the regular expression.  So you can use
     ‘\1’ through ‘\9’ to refer to the text matched by the corresponding
     grouping constructs.

     For example, ‘\(.*\)\1’ matches any newline-free string that is
     composed of two identical halves.  The ‘\(.*\)’ matches the first
     half, which may be anything, but the ‘\1’ that follows must match
     the same exact text.

     If a ‘\( ... \)’ construct matches more than once (which can
     happen, for instance, if it is followed by ‘*’), only the last
     match is recorded.

     If a particular grouping construct in the regular expression was
     never matched—for instance, if it appears inside of an alternative
     that wasn’t used, or inside of a repetition that repeated zero
     times—then the corresponding ‘\DIGIT’ construct never matches
     anything.  To use an artificial example, ‘\(foo\(b*\)\|lose\)\2’
     cannot match ‘lose’: the second alternative inside the larger group
     matches it, but then ‘\2’ is undefined and can’t match anything.
     But it can match ‘foobb’, because the first alternative matches
     ‘foob’ and ‘\2’ matches ‘b’.

‘\w’
     matches any word-constituent character.  The editor syntax table
     determines which characters these are.  *Note Syntax Tables::.

‘\W’
     matches any character that is not a word constituent.

‘\sCODE’
     matches any character whose syntax is CODE.  Here CODE is a
     character that represents a syntax code: thus, ‘w’ for word
     constituent, ‘-’ for whitespace, ‘(’ for open parenthesis, etc.  To
     represent whitespace syntax, use either ‘-’ or a space character.
     *Note Syntax Class Table::, for a list of syntax codes and the
     characters that stand for them.

‘\SCODE’
     matches any character whose syntax is not CODE.

‘\cC’
     matches any character whose category is C.  Here C is a character
     that represents a category: thus, ‘c’ for Chinese characters or ‘g’
     for Greek characters in the standard category table.  You can see
     the list of all the currently defined categories with ‘M-x
     describe-categories <RET>’.  You can also define your own
     categories in addition to the standard ones using the
     ‘define-category’ function (*note Categories::).

‘\CC’
     matches any character whose category is not C.

   The following regular expression constructs match the empty
string—that is, they don’t use up any characters—but whether they match
depends on the context.  For all, the beginning and end of the
accessible portion of the buffer are treated as if they were the actual
beginning and end of the buffer.

‘\`’
     matches the empty string, but only at the beginning of the buffer
     or string being matched against.

‘\'’
     matches the empty string, but only at the end of the buffer or
     string being matched against.

‘\=’
     matches the empty string, but only at point.  (This construct is
     not defined when matching against a string.)

‘\b’
     matches the empty string, but only at the beginning or end of a
     word.  Thus, ‘\bfoo\b’ matches any occurrence of ‘foo’ as a
     separate word.  ‘\bballs?\b’ matches ‘ball’ or ‘balls’ as a
     separate word.

     ‘\b’ matches at the beginning or end of the buffer (or string)
     regardless of what text appears next to it.

‘\B’
     matches the empty string, but _not_ at the beginning or end of a
     word, nor at the beginning or end of the buffer (or string).

‘\<’
     matches the empty string, but only at the beginning of a word.
     ‘\<’ matches at the beginning of the buffer (or string) only if a
     word-constituent character follows.

‘\>’
     matches the empty string, but only at the end of a word.  ‘\>’
     matches at the end of the buffer (or string) only if the contents
     end with a word-constituent character.

‘\_<’
     matches the empty string, but only at the beginning of a symbol.  A
     symbol is a sequence of one or more word or symbol constituent
     characters.  ‘\_<’ matches at the beginning of the buffer (or
     string) only if a symbol-constituent character follows.

‘\_>’
     matches the empty string, but only at the end of a symbol.  ‘\_>’
     matches at the end of the buffer (or string) only if the contents
     end with a symbol-constituent character.

   Not every string is a valid regular expression.  For example, a
string that ends inside a character alternative without a terminating
‘]’ is invalid, and so is a string that ends with a single ‘\’.  If an
invalid regular expression is passed to any of the search functions, an
‘invalid-regexp’ error is signaled.


File: elisp.info,  Node: Regexp Example,  Next: Regexp Functions,  Prev: Syntax of Regexps,  Up: Regular Expressions

33.3.2 Complex Regexp Example
-----------------------------

Here is a complicated regexp which was formerly used by Emacs to
recognize the end of a sentence together with any whitespace that
follows.  (Nowadays Emacs uses a similar but more complex default regexp
constructed by the function ‘sentence-end’.  *Note Standard Regexps::.)

   Below, we show first the regexp as a string in Lisp syntax (to
distinguish spaces from tab characters), and then the result of
evaluating it.  The string constant begins and ends with a double-quote.
‘\"’ stands for a double-quote as part of the string, ‘\\’ for a
backslash as part of the string, ‘\t’ for a tab and ‘\n’ for a newline.

     "[.?!][]\"')}]*\\($\\| $\\|\t\\|  \\)[ \t\n]*"
          ⇒ "[.?!][]\"')}]*\\($\\| $\\|  \\|  \\)[
     ]*"

In the output, tab and newline appear as themselves.

   This regular expression contains four parts in succession and can be
deciphered as follows:

‘[.?!]’
     The first part of the pattern is a character alternative that
     matches any one of three characters: period, question mark, and
     exclamation mark.  The match must begin with one of these three
     characters.  (This is one point where the new default regexp used
     by Emacs differs from the old.  The new value also allows some
     non-ASCII characters that end a sentence without any following
     whitespace.)

‘[]\"')}]*’
     The second part of the pattern matches any closing braces and
     quotation marks, zero or more of them, that may follow the period,
     question mark or exclamation mark.  The ‘\"’ is Lisp syntax for a
     double-quote in a string.  The ‘*’ at the end indicates that the
     immediately preceding regular expression (a character alternative,
     in this case) may be repeated zero or more times.

‘\\($\\| $\\|\t\\|  \\)’
     The third part of the pattern matches the whitespace that follows
     the end of a sentence: the end of a line (optionally with a space),
     or a tab, or two spaces.  The double backslashes mark the
     parentheses and vertical bars as regular expression syntax; the
     parentheses delimit a group and the vertical bars separate
     alternatives.  The dollar sign is used to match the end of a line.

‘[ \t\n]*’
     Finally, the last part of the pattern matches any additional
     whitespace beyond the minimum needed to end a sentence.


File: elisp.info,  Node: Regexp Functions,  Prev: Regexp Example,  Up: Regular Expressions

33.3.3 Regular Expression Functions
-----------------------------------

These functions operate on regular expressions.

 -- Function: regexp-quote string
     This function returns a regular expression whose only exact match
     is STRING.  Using this regular expression in ‘looking-at’ will
     succeed only if the next characters in the buffer are STRING; using
     it in a search function will succeed if the text being searched
     contains STRING.  *Note Regexp Search::.

     This allows you to request an exact string match or search when
     calling a function that wants a regular expression.

          (regexp-quote "^The cat$")
               ⇒ "\\^The cat\\$"

     One use of ‘regexp-quote’ is to combine an exact string match with
     context described as a regular expression.  For example, this
     searches for the string that is the value of STRING, surrounded by
     whitespace:

          (re-search-forward
           (concat "\\s-" (regexp-quote string) "\\s-"))

 -- Function: regexp-opt strings &optional paren
     This function returns an efficient regular expression that will
     match any of the strings in the list STRINGS.  This is useful when
     you need to make matching or searching as fast as possible—for
     example, for Font Lock mode(1).

     The optional argument PAREN can be any of the following:

     a string
          The resulting regexp is preceded by PAREN and followed by
          ‘\)’, e.g.  use ‘"\\(?1:"’ to produce an explicitly numbered
          group.

     ‘words’
          The resulting regexp is surrounded by ‘\<\(’ and ‘\)\>’.

     ‘symbols’
          The resulting regexp is surrounded by ‘\_<\(’ and ‘\)\_>’
          (this is often appropriate when matching programming-language
          keywords and the like).

     non-‘nil’
          The resulting regexp is surrounded by ‘\(’ and ‘\)’.

     ‘nil’
          The resulting regexp is surrounded by ‘\(?:’ and ‘\)’, if it
          is necessary to ensure that a postfix operator appended to it
          will apply to the whole expression.

     The resulting regexp of ‘regexp-opt’ is equivalent to but usually
     more efficient than that of a simplified version:

          (defun simplified-regexp-opt (strings &optional paren)
           (let ((parens
                  (cond
                   ((stringp paren)       (cons paren "\\)"))
                   ((eq paren 'words)    '("\\<\\(" . "\\)\\>"))
                   ((eq paren 'symbols) '("\\_<\\(" . "\\)\\_>"))
                   ((null paren)          '("\\(?:" . "\\)"))
                   (t                       '("\\(" . "\\)")))))
             (concat (car paren)
                     (mapconcat 'regexp-quote strings "\\|")
                     (cdr paren))))

 -- Function: regexp-opt-depth regexp
     This function returns the total number of grouping constructs
     (parenthesized expressions) in REGEXP.  This does not include shy
     groups (*note Regexp Backslash::).

 -- Function: regexp-opt-charset chars
     This function returns a regular expression matching a character in
     the list of characters CHARS.

          (regexp-opt-charset '(?a ?b ?c ?d ?e))
               ⇒ "[a-e]"

   ---------- Footnotes ----------

   (1) Note that ‘regexp-opt’ does not guarantee that its result is
absolutely the most efficient form possible.  A hand-tuned regular
expression can sometimes be slightly more efficient, but is almost never
worth the effort.


File: elisp.info,  Node: Regexp Search,  Next: POSIX Regexps,  Prev: Regular Expressions,  Up: Searching and Matching

33.4 Regular Expression Searching
=================================

In GNU Emacs, you can search for the next match for a regular expression
(*note Syntax of Regexps::) either incrementally or not.  For
incremental search commands, see *note Regular Expression Search:
(emacs)Regexp Search.  Here we describe only the search functions useful
in programs.  The principal one is ‘re-search-forward’.

   These search functions convert the regular expression to multibyte if
the buffer is multibyte; they convert the regular expression to unibyte
if the buffer is unibyte.  *Note Text Representations::.

 -- Command: re-search-forward regexp &optional limit noerror count
     This function searches forward in the current buffer for a string
     of text that is matched by the regular expression REGEXP.  The
     function skips over any amount of text that is not matched by
     REGEXP, and leaves point at the end of the first match found.  It
     returns the new value of point.

     If LIMIT is non-‘nil’, it must be a position in the current buffer.
     It specifies the upper bound to the search.  No match extending
     after that position is accepted.  If LIMIT is omitted or ‘nil’, it
     defaults to the end of the accessible portion of the buffer.

     What ‘re-search-forward’ does when the search fails depends on the
     value of NOERROR:

     ‘nil’
          Signal a ‘search-failed’ error.
     ‘t’
          Do nothing and return ‘nil’.
     anything else
          Move point to LIMIT (or the end of the accessible portion of
          the buffer) and return ‘nil’.

     The argument NOERROR only affects valid searches which fail to find
     a match.  Invalid arguments cause errors regardless of NOERROR.

     If COUNT is a positive number N, the search is done N times; each
     successive search starts at the end of the previous match.  If all
     these successive searches succeed, the function call succeeds,
     moving point and returning its new value.  Otherwise the function
     call fails, with results depending on the value of NOERROR, as
     described above.  If COUNT is a negative number -N, the search is
     done N times in the opposite (backward) direction.

     In the following example, point is initially before the ‘T’.
     Evaluating the search call moves point to the end of that line
     (between the ‘t’ of ‘hat’ and the newline).

          ---------- Buffer: foo ----------
          I read "★The cat in the hat
          comes back" twice.
          ---------- Buffer: foo ----------

          (re-search-forward "[a-z]+" nil t 5)
               ⇒ 27

          ---------- Buffer: foo ----------
          I read "The cat in the hat★
          comes back" twice.
          ---------- Buffer: foo ----------

 -- Command: re-search-backward regexp &optional limit noerror count
     This function searches backward in the current buffer for a string
     of text that is matched by the regular expression REGEXP, leaving
     point at the beginning of the first text found.

     This function is analogous to ‘re-search-forward’, but they are not
     simple mirror images.  ‘re-search-forward’ finds the match whose
     beginning is as close as possible to the starting point.  If
     ‘re-search-backward’ were a perfect mirror image, it would find the
     match whose end is as close as possible.  However, in fact it finds
     the match whose beginning is as close as possible (and yet ends
     before the starting point).  The reason for this is that matching a
     regular expression at a given spot always works from beginning to
     end, and starts at a specified beginning position.

     A true mirror-image of ‘re-search-forward’ would require a special
     feature for matching regular expressions from end to beginning.
     It’s not worth the trouble of implementing that.

 -- Function: string-match regexp string &optional start
     This function returns the index of the start of the first match for
     the regular expression REGEXP in STRING, or ‘nil’ if there is no
     match.  If START is non-‘nil’, the search starts at that index in
     STRING.

     For example,

          (string-match
           "quick" "The quick brown fox jumped quickly.")
               ⇒ 4
          (string-match
           "quick" "The quick brown fox jumped quickly." 8)
               ⇒ 27

     The index of the first character of the string is 0, the index of
     the second character is 1, and so on.

     If this function finds a match, the index of the first character
     beyond the match is available as ‘(match-end 0)’.  *Note Match
     Data::.

          (string-match
           "quick" "The quick brown fox jumped quickly." 8)
               ⇒ 27

          (match-end 0)
               ⇒ 32

 -- Function: string-match-p regexp string &optional start
     This predicate function does what ‘string-match’ does, but it
     avoids modifying the match data.

 -- Function: looking-at regexp
     This function determines whether the text in the current buffer
     directly following point matches the regular expression REGEXP.
     “Directly following” means precisely that: the search is “anchored”
     and it can succeed only starting with the first character following
     point.  The result is ‘t’ if so, ‘nil’ otherwise.

     This function does not move point, but it does update the match
     data.  *Note Match Data::.  If you need to test for a match without
     modifying the match data, use ‘looking-at-p’, described below.

     In this example, point is located directly before the ‘T’.  If it
     were anywhere else, the result would be ‘nil’.

          ---------- Buffer: foo ----------
          I read "★The cat in the hat
          comes back" twice.
          ---------- Buffer: foo ----------

          (looking-at "The cat in the hat$")
               ⇒ t

 -- Function: looking-back regexp limit &optional greedy
     This function returns ‘t’ if REGEXP matches the text immediately
     before point (i.e., ending at point), and ‘nil’ otherwise.

     Because regular expression matching works only going forward, this
     is implemented by searching backwards from point for a match that
     ends at point.  That can be quite slow if it has to search a long
     distance.  You can bound the time required by specifying a
     non-‘nil’ value for LIMIT, which says not to search before LIMIT.
     In this case, the match that is found must begin at or after LIMIT.
     Here’s an example:

          ---------- Buffer: foo ----------
          I read "★The cat in the hat
          comes back" twice.
          ---------- Buffer: foo ----------

          (looking-back "read \"" 3)
               ⇒ t
          (looking-back "read \"" 4)
               ⇒ nil

     If GREEDY is non-‘nil’, this function extends the match backwards
     as far as possible, stopping when a single additional previous
     character cannot be part of a match for REGEXP.  When the match is
     extended, its starting position is allowed to occur before LIMIT.

     As a general recommendation, try to avoid using ‘looking-back’
     wherever possible, since it is slow.  For this reason, there are no
     plans to add a ‘looking-back-p’ function.

 -- Function: looking-at-p regexp
     This predicate function works like ‘looking-at’, but without
     updating the match data.

 -- Variable: search-spaces-regexp
     If this variable is non-‘nil’, it should be a regular expression
     that says how to search for whitespace.  In that case, any group of
     spaces in a regular expression being searched for stands for use of
     this regular expression.  However, spaces inside of constructs such
     as ‘[...]’ and ‘*’, ‘+’, ‘?’ are not affected by
     ‘search-spaces-regexp’.

     Since this variable affects all regular expression search and match
     constructs, you should bind it temporarily for as small as possible
     a part of the code.


File: elisp.info,  Node: POSIX Regexps,  Next: Match Data,  Prev: Regexp Search,  Up: Searching and Matching

33.5 POSIX Regular Expression Searching
=======================================

The usual regular expression functions do backtracking when necessary to
handle the ‘\|’ and repetition constructs, but they continue this only
until they find _some_ match.  Then they succeed and report the first
match found.

   This section describes alternative search functions which perform the
full backtracking specified by the POSIX standard for regular expression
matching.  They continue backtracking until they have tried all
possibilities and found all matches, so they can report the longest
match, as required by POSIX.  This is much slower, so use these
functions only when you really need the longest match.

   The POSIX search and match functions do not properly support the
non-greedy repetition operators (*note non-greedy: Regexp Special.).
This is because POSIX backtracking conflicts with the semantics of
non-greedy repetition.

 -- Command: posix-search-forward regexp &optional limit noerror count
     This is like ‘re-search-forward’ except that it performs the full
     backtracking specified by the POSIX standard for regular expression
     matching.

 -- Command: posix-search-backward regexp &optional limit noerror count
     This is like ‘re-search-backward’ except that it performs the full
     backtracking specified by the POSIX standard for regular expression
     matching.

 -- Function: posix-looking-at regexp
     This is like ‘looking-at’ except that it performs the full
     backtracking specified by the POSIX standard for regular expression
     matching.

 -- Function: posix-string-match regexp string &optional start
     This is like ‘string-match’ except that it performs the full
     backtracking specified by the POSIX standard for regular expression
     matching.


File: elisp.info,  Node: Match Data,  Next: Search and Replace,  Prev: POSIX Regexps,  Up: Searching and Matching

33.6 The Match Data
===================

Emacs keeps track of the start and end positions of the segments of text
found during a search; this is called the “match data”.  Thanks to the
match data, you can search for a complex pattern, such as a date in a
mail message, and then extract parts of the match under control of the
pattern.

   Because the match data normally describe the most recent search only,
you must be careful not to do another search inadvertently between the
search you wish to refer back to and the use of the match data.  If you
can’t avoid another intervening search, you must save and restore the
match data around it, to prevent it from being overwritten.

   Notice that all functions are allowed to overwrite the match data
unless they’re explicitly documented not to do so.  A consequence is
that functions that are run implicitly in the background (*note
Timers::, and *note Idle Timers::) should likely save and restore the
match data explicitly.

* Menu:

* Replacing Match::       Replacing a substring that was matched.
* Simple Match Data::     Accessing single items of match data,
                            such as where a particular subexpression started.
* Entire Match Data::     Accessing the entire match data at once, as a list.
* Saving Match Data::     Saving and restoring the match data.


File: elisp.info,  Node: Replacing Match,  Next: Simple Match Data,  Up: Match Data

33.6.1 Replacing the Text that Matched
--------------------------------------

This function replaces all or part of the text matched by the last
search.  It works by means of the match data.

 -- Function: replace-match replacement &optional fixedcase literal
          string subexp
     This function performs a replacement operation on a buffer or
     string.

     If you did the last search in a buffer, you should omit the STRING
     argument or specify ‘nil’ for it, and make sure that the current
     buffer is the one in which you performed the last search.  Then
     this function edits the buffer, replacing the matched text with
     REPLACEMENT.  It leaves point at the end of the replacement text.

     If you performed the last search on a string, pass the same string
     as STRING.  Then this function returns a new string, in which the
     matched text is replaced by REPLACEMENT.

     If FIXEDCASE is non-‘nil’, then ‘replace-match’ uses the
     replacement text without case conversion; otherwise, it converts
     the replacement text depending upon the capitalization of the text
     to be replaced.  If the original text is all upper case, this
     converts the replacement text to upper case.  If all words of the
     original text are capitalized, this capitalizes all the words of
     the replacement text.  If all the words are one-letter and they are
     all upper case, they are treated as capitalized words rather than
     all-upper-case words.

     If LITERAL is non-‘nil’, then REPLACEMENT is inserted exactly as it
     is, the only alterations being case changes as needed.  If it is
     ‘nil’ (the default), then the character ‘\’ is treated specially.
     If a ‘\’ appears in REPLACEMENT, then it must be part of one of the
     following sequences:

     ‘\&’
          This stands for the entire text being replaced.

     ‘\N’, where N is a digit
          This stands for the text that matched the Nth subexpression in
          the original regexp.  Subexpressions are those expressions
          grouped inside ‘\(...\)’.  If the Nth subexpression never
          matched, an empty string is substituted.

     ‘\\’
          This stands for a single ‘\’ in the replacement text.

     ‘\?’
          This stands for itself (for compatibility with
          ‘replace-regexp’ and related commands; *note (emacs)Regexp
          Replace::).

     Any other character following ‘\’ signals an error.

     The substitutions performed by ‘\&’ and ‘\N’ occur after case
     conversion, if any.  Therefore, the strings they substitute are
     never case-converted.

     If SUBEXP is non-‘nil’, that says to replace just subexpression
     number SUBEXP of the regexp that was matched, not the entire match.
     For example, after matching ‘foo \(ba*r\)’, calling ‘replace-match’
     with 1 as SUBEXP means to replace just the text that matched
     ‘\(ba*r\)’.

 -- Function: match-substitute-replacement replacement &optional
          fixedcase literal string subexp
     This function returns the text that would be inserted into the
     buffer by ‘replace-match’, but without modifying the buffer.  It is
     useful if you want to present the user with actual replacement
     result, with constructs like ‘\N’ or ‘\&’ substituted with matched
     groups.  Arguments REPLACEMENT and optional FIXEDCASE, LITERAL,
     STRING and SUBEXP have the same meaning as for ‘replace-match’.


File: elisp.info,  Node: Simple Match Data,  Next: Entire Match Data,  Prev: Replacing Match,  Up: Match Data

33.6.2 Simple Match Data Access
-------------------------------

This section explains how to use the match data to find out what was
matched by the last search or match operation, if it succeeded.

   You can ask about the entire matching text, or about a particular
parenthetical subexpression of a regular expression.  The COUNT argument
in the functions below specifies which.  If COUNT is zero, you are
asking about the entire match.  If COUNT is positive, it specifies which
subexpression you want.

   Recall that the subexpressions of a regular expression are those
expressions grouped with escaped parentheses, ‘\(...\)’.  The COUNTth
subexpression is found by counting occurrences of ‘\(’ from the
beginning of the whole regular expression.  The first subexpression is
numbered 1, the second 2, and so on.  Only regular expressions can have
subexpressions—after a simple string search, the only information
available is about the entire match.

   Every successful search sets the match data.  Therefore, you should
query the match data immediately after searching, before calling any
other function that might perform another search.  Alternatively, you
may save and restore the match data (*note Saving Match Data::) around
the call to functions that could perform another search.  Or use the
functions that explicitly do not modify the match data; e.g.,
‘string-match-p’.

   A search which fails may or may not alter the match data.  In the
current implementation, it does not, but we may change it in the future.
Don’t try to rely on the value of the match data after a failing search.

 -- Function: match-string count &optional in-string
     This function returns, as a string, the text matched in the last
     search or match operation.  It returns the entire text if COUNT is
     zero, or just the portion corresponding to the COUNTth
     parenthetical subexpression, if COUNT is positive.

     If the last such operation was done against a string with
     ‘string-match’, then you should pass the same string as the
     argument IN-STRING.  After a buffer search or match, you should
     omit IN-STRING or pass ‘nil’ for it; but you should make sure that
     the current buffer when you call ‘match-string’ is the one in which
     you did the searching or matching.  Failure to follow this advice
     will lead to incorrect results.

     The value is ‘nil’ if COUNT is out of range, or for a subexpression
     inside a ‘\|’ alternative that wasn’t used or a repetition that
     repeated zero times.

 -- Function: match-string-no-properties count &optional in-string
     This function is like ‘match-string’ except that the result has no
     text properties.

 -- Function: match-beginning count
     If the last regular expression search found a match, this function
     returns the position of the start of the matching text or of a
     subexpression of it.

     If COUNT is zero, then the value is the position of the start of
     the entire match.  Otherwise, COUNT specifies a subexpression in
     the regular expression, and the value of the function is the
     starting position of the match for that subexpression.

     The value is ‘nil’ for a subexpression inside a ‘\|’ alternative
     that wasn’t used or a repetition that repeated zero times.

 -- Function: match-end count
     This function is like ‘match-beginning’ except that it returns the
     position of the end of the match, rather than the position of the
     beginning.

   Here is an example of using the match data, with a comment showing
the positions within the text:

     (string-match "\\(qu\\)\\(ick\\)"
                   "The quick fox jumped quickly.")
                   ;0123456789
          ⇒ 4

     (match-string 0 "The quick fox jumped quickly.")
          ⇒ "quick"
     (match-string 1 "The quick fox jumped quickly.")
          ⇒ "qu"
     (match-string 2 "The quick fox jumped quickly.")
          ⇒ "ick"

     (match-beginning 1)       ; The beginning of the match
          ⇒ 4                 ;   with ‘qu’ is at index 4.

     (match-beginning 2)       ; The beginning of the match
          ⇒ 6                 ;   with ‘ick’ is at index 6.

     (match-end 1)             ; The end of the match
          ⇒ 6                 ;   with ‘qu’ is at index 6.

     (match-end 2)             ; The end of the match
          ⇒ 9                 ;   with ‘ick’ is at index 9.

   Here is another example.  Point is initially located at the beginning
of the line.  Searching moves point to between the space and the word
‘in’.  The beginning of the entire match is at the 9th character of the
buffer (‘T’), and the beginning of the match for the first subexpression
is at the 13th character (‘c’).

     (list
       (re-search-forward "The \\(cat \\)")
       (match-beginning 0)
       (match-beginning 1))
         ⇒ (17 9 13)

     ---------- Buffer: foo ----------
     I read "The cat ★in the hat comes back" twice.
             ^   ^
             9  13
     ---------- Buffer: foo ----------

(In this case, the index returned is a buffer position; the first
character of the buffer counts as 1.)


File: elisp.info,  Node: Entire Match Data,  Next: Saving Match Data,  Prev: Simple Match Data,  Up: Match Data

33.6.3 Accessing the Entire Match Data
--------------------------------------

The functions ‘match-data’ and ‘set-match-data’ read or write the entire
match data, all at once.

 -- Function: match-data &optional integers reuse reseat
     This function returns a list of positions (markers or integers)
     that record all the information on the text that the last search
     matched.  Element zero is the position of the beginning of the
     match for the whole expression; element one is the position of the
     end of the match for the expression.  The next two elements are the
     positions of the beginning and end of the match for the first
     subexpression, and so on.  In general, element number 2N
     corresponds to ‘(match-beginning N)’; and element number 2N + 1
     corresponds to ‘(match-end N)’.

     Normally all the elements are markers or ‘nil’, but if INTEGERS is
     non-‘nil’, that means to use integers instead of markers.  (In that
     case, the buffer itself is appended as an additional element at the
     end of the list, to facilitate complete restoration of the match
     data.)  If the last match was done on a string with ‘string-match’,
     then integers are always used, since markers can’t point into a
     string.

     If REUSE is non-‘nil’, it should be a list.  In that case,
     ‘match-data’ stores the match data in REUSE.  That is, REUSE is
     destructively modified.  REUSE does not need to have the right
     length.  If it is not long enough to contain the match data, it is
     extended.  If it is too long, the length of REUSE stays the same,
     but the elements that were not used are set to ‘nil’.  The purpose
     of this feature is to reduce the need for garbage collection.

     If RESEAT is non-‘nil’, all markers on the REUSE list are reseated
     to point to nowhere.

     As always, there must be no possibility of intervening searches
     between the call to a search function and the call to ‘match-data’
     that is intended to access the match data for that search.

          (match-data)
               ⇒  (#<marker at 9 in foo>
                    #<marker at 17 in foo>
                    #<marker at 13 in foo>
                    #<marker at 17 in foo>)

 -- Function: set-match-data match-list &optional reseat
     This function sets the match data from the elements of MATCH-LIST,
     which should be a list that was the value of a previous call to
     ‘match-data’.  (More precisely, anything that has the same format
     will work.)

     If MATCH-LIST refers to a buffer that doesn’t exist, you don’t get
     an error; that sets the match data in a meaningless but harmless
     way.

     If RESEAT is non-‘nil’, all markers on the MATCH-LIST list are
     reseated to point to nowhere.

     ‘store-match-data’ is a semi-obsolete alias for ‘set-match-data’.


File: elisp.info,  Node: Saving Match Data,  Prev: Entire Match Data,  Up: Match Data

33.6.4 Saving and Restoring the Match Data
------------------------------------------

When you call a function that may search, you may need to save and
restore the match data around that call, if you want to preserve the
match data from an earlier search for later use.  Here is an example
that shows the problem that arises if you fail to save the match data:

     (re-search-forward "The \\(cat \\)")
          ⇒ 48
     (foo)                   ; ‘foo’ does more searching.
     (match-end 0)
          ⇒ 61              ; Unexpected result—not 48!

   You can save and restore the match data with ‘save-match-data’:

 -- Macro: save-match-data body...
     This macro executes BODY, saving and restoring the match data
     around it.  The return value is the value of the last form in BODY.

   You could use ‘set-match-data’ together with ‘match-data’ to imitate
the effect of the special form ‘save-match-data’.  Here is how:

     (let ((data (match-data)))
       (unwind-protect
           ...   ; Ok to change the original match data.
         (set-match-data data)))

   Emacs automatically saves and restores the match data when it runs
process filter functions (*note Filter Functions::) and process
sentinels (*note Sentinels::).


File: elisp.info,  Node: Search and Replace,  Next: Standard Regexps,  Prev: Match Data,  Up: Searching and Matching

33.7 Search and Replace
=======================

If you want to find all matches for a regexp in part of the buffer, and
replace them, the best way is to write an explicit loop using
‘re-search-forward’ and ‘replace-match’, like this:

     (while (re-search-forward "foo[ \t]+bar" nil t)
       (replace-match "foobar"))

*Note Replacing the Text that Matched: Replacing Match, for a
description of ‘replace-match’.

   However, replacing matches in a string is more complex, especially if
you want to do it efficiently.  So Emacs provides a function to do this.

 -- Function: replace-regexp-in-string regexp rep string &optional
          fixedcase literal subexp start
     This function copies STRING and searches it for matches for REGEXP,
     and replaces them with REP.  It returns the modified copy.  If
     START is non-‘nil’, the search for matches starts at that index in
     STRING, so matches starting before that index are not changed.

     This function uses ‘replace-match’ to do the replacement, and it
     passes the optional arguments FIXEDCASE, LITERAL and SUBEXP along
     to ‘replace-match’.

     Instead of a string, REP can be a function.  In that case,
     ‘replace-regexp-in-string’ calls REP for each match, passing the
     text of the match as its sole argument.  It collects the value REP
     returns and passes that to ‘replace-match’ as the replacement
     string.  The match data at this point are the result of matching
     REGEXP against a substring of STRING.

   If you want to write a command along the lines of ‘query-replace’,
you can use ‘perform-replace’ to do the work.

 -- Function: perform-replace from-string replacements query-flag
          regexp-flag delimited-flag &optional repeat-count map start
          end
     This function is the guts of ‘query-replace’ and related commands.
     It searches for occurrences of FROM-STRING in the text between
     positions START and END and replaces some or all of them.  If START
     is ‘nil’ (or omitted), point is used instead, and the end of the
     buffer’s accessible portion is used for END.

     If QUERY-FLAG is ‘nil’, it replaces all occurrences; otherwise, it
     asks the user what to do about each one.

     If REGEXP-FLAG is non-‘nil’, then FROM-STRING is considered a
     regular expression; otherwise, it must match literally.  If
     DELIMITED-FLAG is non-‘nil’, then only replacements surrounded by
     word boundaries are considered.

     The argument REPLACEMENTS specifies what to replace occurrences
     with.  If it is a string, that string is used.  It can also be a
     list of strings, to be used in cyclic order.

     If REPLACEMENTS is a cons cell, ‘(FUNCTION . DATA)’, this means to
     call FUNCTION after each match to get the replacement text.  This
     function is called with two arguments: DATA, and the number of
     replacements already made.

     If REPEAT-COUNT is non-‘nil’, it should be an integer.  Then it
     specifies how many times to use each of the strings in the
     REPLACEMENTS list before advancing cyclically to the next one.

     If FROM-STRING contains upper-case letters, then ‘perform-replace’
     binds ‘case-fold-search’ to ‘nil’, and it uses the REPLACEMENTS
     without altering their case.

     Normally, the keymap ‘query-replace-map’ defines the possible user
     responses for queries.  The argument MAP, if non-‘nil’, specifies a
     keymap to use instead of ‘query-replace-map’.

     This function uses one of two functions to search for the next
     occurrence of FROM-STRING.  These functions are specified by the
     values of two variables: ‘replace-re-search-function’ and
     ‘replace-search-function’.  The former is called when the argument
     REGEXP-FLAG is non-‘nil’, the latter when it is ‘nil’.

 -- Variable: query-replace-map
     This variable holds a special keymap that defines the valid user
     responses for ‘perform-replace’ and the commands that use it, as
     well as ‘y-or-n-p’ and ‘map-y-or-n-p’.  This map is unusual in two
     ways:

        • The key bindings are not commands, just symbols that are
          meaningful to the functions that use this map.

        • Prefix keys are not supported; each key binding must be for a
          single-event key sequence.  This is because the functions
          don’t use ‘read-key-sequence’ to get the input; instead, they
          read a single event and look it up “by hand”.

   Here are the meaningful bindings for ‘query-replace-map’.  Several of
them are meaningful only for ‘query-replace’ and friends.

‘act’
     Do take the action being considered—in other words, “yes”.

‘skip’
     Do not take action for this question—in other words, “no”.

‘exit’
     Answer this question “no”, and give up on the entire series of
     questions, assuming that the answers will be “no”.

‘exit-prefix’
     Like ‘exit’, but add the key that was pressed to
     ‘unread-command-events’ (*note Event Input Misc::).

‘act-and-exit’
     Answer this question “yes”, and give up on the entire series of
     questions, assuming that subsequent answers will be “no”.

‘act-and-show’
     Answer this question “yes”, but show the results—don’t advance yet
     to the next question.

‘automatic’
     Answer this question and all subsequent questions in the series
     with “yes”, without further user interaction.

‘backup’
     Move back to the previous place that a question was asked about.

‘edit’
     Enter a recursive edit to deal with this question—instead of any
     other action that would normally be taken.

‘edit-replacement’
     Edit the replacement for this question in the minibuffer.

‘delete-and-edit’
     Delete the text being considered, then enter a recursive edit to
     replace it.

‘recenter’
‘scroll-up’
‘scroll-down’
‘scroll-other-window’
‘scroll-other-window-down’
     Perform the specified window scroll operation, then ask the same
     question again.  Only ‘y-or-n-p’ and related functions use this
     answer.

‘quit’
     Perform a quit right away.  Only ‘y-or-n-p’ and related functions
     use this answer.

‘help’
     Display some help, then ask again.

 -- Variable: multi-query-replace-map
     This variable holds a keymap that extends ‘query-replace-map’ by
     providing additional keybindings that are useful in multi-buffer
     replacements.  The additional bindings are:

     ‘automatic-all’
          Answer this question and all subsequent questions in the
          series with “yes”, without further user interaction, for all
          remaining buffers.

     ‘exit-current’
          Answer this question “no”, and give up on the entire series of
          questions for the current buffer.  Continue to the next buffer
          in the sequence.

 -- Variable: replace-search-function
     This variable specifies a function that ‘perform-replace’ calls to
     search for the next string to replace.  Its default value is
     ‘search-forward’.  Any other value should name a function of 3
     arguments: the first 3 arguments of ‘search-forward’ (*note String
     Search::).

 -- Variable: replace-re-search-function
     This variable specifies a function that ‘perform-replace’ calls to
     search for the next regexp to replace.  Its default value is
     ‘re-search-forward’.  Any other value should name a function of 3
     arguments: the first 3 arguments of ‘re-search-forward’ (*note
     Regexp Search::).


File: elisp.info,  Node: Standard Regexps,  Prev: Search and Replace,  Up: Searching and Matching

33.8 Standard Regular Expressions Used in Editing
=================================================

This section describes some variables that hold regular expressions used
for certain purposes in editing:

 -- User Option: page-delimiter
     This is the regular expression describing line-beginnings that
     separate pages.  The default value is ‘"^\014"’ (i.e., ‘"^^L"’ or
     ‘"^\C-l"’); this matches a line that starts with a formfeed
     character.

   The following two regular expressions should _not_ assume the match
always starts at the beginning of a line; they should not use ‘^’ to
anchor the match.  Most often, the paragraph commands do check for a
match only at the beginning of a line, which means that ‘^’ would be
superfluous.  When there is a nonzero left margin, they accept matches
that start after the left margin.  In that case, a ‘^’ would be
incorrect.  However, a ‘^’ is harmless in modes where a left margin is
never used.

 -- User Option: paragraph-separate
     This is the regular expression for recognizing the beginning of a
     line that separates paragraphs.  (If you change this, you may have
     to change ‘paragraph-start’ also.)  The default value is
     ‘"[ \t\f]*$"’, which matches a line that consists entirely of
     spaces, tabs, and form feeds (after its left margin).

 -- User Option: paragraph-start
     This is the regular expression for recognizing the beginning of a
     line that starts _or_ separates paragraphs.  The default value is
     ‘"\f\\|[ \t]*$"’, which matches a line containing only whitespace
     or starting with a form feed (after its left margin).

 -- User Option: sentence-end
     If non-‘nil’, the value should be a regular expression describing
     the end of a sentence, including the whitespace following the
     sentence.  (All paragraph boundaries also end sentences,
     regardless.)

     If the value is ‘nil’, as it is by default, then the function
     ‘sentence-end’ constructs the regexp.  That is why you should
     always call the function ‘sentence-end’ to obtain the regexp to be
     used to recognize the end of a sentence.

 -- Function: sentence-end
     This function returns the value of the variable ‘sentence-end’, if
     non-‘nil’.  Otherwise it returns a default value based on the
     values of the variables ‘sentence-end-double-space’ (*note
     Definition of sentence-end-double-space::),
     ‘sentence-end-without-period’, and ‘sentence-end-without-space’.


File: elisp.info,  Node: Syntax Tables,  Next: Abbrevs,  Prev: Searching and Matching,  Up: Top

34 Syntax Tables
****************

A “syntax table” specifies the syntactic role of each character in a
buffer.  It can be used to determine where words, symbols, and other
syntactic constructs begin and end.  This information is used by many
Emacs facilities, including Font Lock mode (*note Font Lock Mode::) and
the various complex movement commands (*note Motion::).

* Menu:

* Basics: Syntax Basics.     Basic concepts of syntax tables.
* Syntax Descriptors::       How characters are classified.
* Syntax Table Functions::   How to create, examine and alter syntax tables.
* Syntax Properties::        Overriding syntax with text properties.
* Motion and Syntax::        Moving over characters with certain syntaxes.
* Parsing Expressions::      Parsing balanced expressions
                                using the syntax table.
* Syntax Table Internals::   How syntax table information is stored.
* Categories::               Another way of classifying character syntax.


File: elisp.info,  Node: Syntax Basics,  Next: Syntax Descriptors,  Up: Syntax Tables

34.1 Syntax Table Concepts
==========================

A syntax table is a data structure which can be used to look up the
“syntax class” and other syntactic properties of each character.  Syntax
tables are used by Lisp programs for scanning and moving across text.

   Internally, a syntax table is a char-table (*note Char-Tables::).
The element at index C describes the character with code C; its value is
a cons cell which specifies the syntax of the character in question.
*Note Syntax Table Internals::, for details.  However, instead of using
‘aset’ and ‘aref’ to modify and inspect syntax table contents, you
should usually use the higher-level functions ‘char-syntax’ and
‘modify-syntax-entry’, which are described in *note Syntax Table
Functions::.

 -- Function: syntax-table-p object
     This function returns ‘t’ if OBJECT is a syntax table.

   Each buffer has its own major mode, and each major mode has its own
idea of the syntax class of various characters.  For example, in Lisp
mode, the character ‘;’ begins a comment, but in C mode, it terminates a
statement.  To support these variations, the syntax table is local to
each buffer.  Typically, each major mode has its own syntax table, which
it installs in all buffers that use that mode.  For example, the
variable ‘emacs-lisp-mode-syntax-table’ holds the syntax table used by
Emacs Lisp mode, and ‘c-mode-syntax-table’ holds the syntax table used
by C mode.  Changing a major mode’s syntax table alters the syntax in
all of that mode’s buffers, as well as in any buffers subsequently put
in that mode.  Occasionally, several similar modes share one syntax
table.  *Note Example Major Modes::, for an example of how to set up a
syntax table.

   A syntax table can “inherit” from another syntax table, which is
called its “parent syntax table”.  A syntax table can leave the syntax
class of some characters unspecified, by giving them the “inherit”
syntax class; such a character then acquires the syntax class specified
by the parent syntax table (*note Syntax Class Table::).  Emacs defines
a “standard syntax table”, which is the default parent syntax table, and
is also the syntax table used by Fundamental mode.

 -- Function: standard-syntax-table
     This function returns the standard syntax table, which is the
     syntax table used in Fundamental mode.

   Syntax tables are not used by the Emacs Lisp reader, which has its
own built-in syntactic rules which cannot be changed.  (Some Lisp
systems provide ways to redefine the read syntax, but we decided to
leave this feature out of Emacs Lisp for simplicity.)


File: elisp.info,  Node: Syntax Descriptors,  Next: Syntax Table Functions,  Prev: Syntax Basics,  Up: Syntax Tables

34.2 Syntax Descriptors
=======================

The “syntax class” of a character describes its syntactic role.  Each
syntax table specifies the syntax class of each character.  There is no
necessary relationship between the class of a character in one syntax
table and its class in any other table.

   Each syntax class is designated by a mnemonic character, which serves
as the name of the class when you need to specify a class.  Usually,
this designator character is one that is often assigned that class;
however, its meaning as a designator is unvarying and independent of
what syntax that character currently has.  Thus, ‘\’ as a designator
character always stands for escape character syntax, regardless of
whether the ‘\’ character actually has that syntax in the current syntax
table.  *Note Syntax Class Table::, for a list of syntax classes and
their designator characters.

   A “syntax descriptor” is a Lisp string that describes the syntax
class and other syntactic properties of a character.  When you want to
modify the syntax of a character, that is done by calling the function
‘modify-syntax-entry’ and passing a syntax descriptor as one of its
arguments (*note Syntax Table Functions::).

   The first character in a syntax descriptor must be a syntax class
designator character.  The second character, if present, specifies a
matching character (e.g., in Lisp, the matching character for ‘(’ is
‘)’); a space specifies that there is no matching character.  Then come
characters specifying additional syntax properties (*note Syntax
Flags::).

   If no matching character or flags are needed, only one character
(specifying the syntax class) is sufficient.

   For example, the syntax descriptor for the character ‘*’ in C mode is
‘". 23"’ (i.e., punctuation, matching character slot unused, second
character of a comment-starter, first character of a comment-ender), and
the entry for ‘/’ is ‘. 14’ (i.e., punctuation, matching character slot
unused, first character of a comment-starter, second character of a
comment-ender).

   Emacs also defines “raw syntax descriptors”, which are used to
describe syntax classes at a lower level.  *Note Syntax Table
Internals::.

* Menu:

* Syntax Class Table::      Table of syntax classes.
* Syntax Flags::            Additional flags each character can have.


File: elisp.info,  Node: Syntax Class Table,  Next: Syntax Flags,  Up: Syntax Descriptors

34.2.1 Table of Syntax Classes
------------------------------

Here is a table of syntax classes, the characters that designate them,
their meanings, and examples of their use.

Whitespace characters: ‘ ’ or ‘-’
     Characters that separate symbols and words from each other.
     Typically, whitespace characters have no other syntactic
     significance, and multiple whitespace characters are syntactically
     equivalent to a single one.  Space, tab, and formfeed are
     classified as whitespace in almost all major modes.

     This syntax class can be designated by either ‘ ’ or ‘-’.  Both
     designators are equivalent.

Word constituents: ‘w’
     Parts of words in human languages.  These are typically used in
     variable and command names in programs.  All upper- and lower-case
     letters, and the digits, are typically word constituents.

Symbol constituents: ‘_’
     Extra characters used in variable and command names along with word
     constituents.  Examples include the characters ‘$&*+-_<>’ in Lisp
     mode, which may be part of a symbol name even though they are not
     part of English words.  In standard C, the only
     non-word-constituent character that is valid in symbols is
     underscore (‘_’).

Punctuation characters: ‘.’
     Characters used as punctuation in a human language, or used in a
     programming language to separate symbols from one another.  Some
     programming language modes, such as Emacs Lisp mode, have no
     characters in this class since the few characters that are not
     symbol or word constituents all have other uses.  Other programming
     language modes, such as C mode, use punctuation syntax for
     operators.

Open parenthesis characters: ‘(’
Close parenthesis characters: ‘)’
     Characters used in dissimilar pairs to surround sentences or
     expressions.  Such a grouping is begun with an open parenthesis
     character and terminated with a close.  Each open parenthesis
     character matches a particular close parenthesis character, and
     vice versa.  Normally, Emacs indicates momentarily the matching
     open parenthesis when you insert a close parenthesis.  *Note
     Blinking::.

     In human languages, and in C code, the parenthesis pairs are ‘()’,
     ‘[]’, and ‘{}’.  In Emacs Lisp, the delimiters for lists and
     vectors (‘()’ and ‘[]’) are classified as parenthesis characters.

String quotes: ‘"’
     Characters used to delimit string constants.  The same string quote
     character appears at the beginning and the end of a string.  Such
     quoted strings do not nest.

     The parsing facilities of Emacs consider a string as a single
     token.  The usual syntactic meanings of the characters in the
     string are suppressed.

     The Lisp modes have two string quote characters: double-quote (‘"’)
     and vertical bar (‘|’).  ‘|’ is not used in Emacs Lisp, but it is
     used in Common Lisp.  C also has two string quote characters:
     double-quote for strings, and apostrophe (‘'’) for character
     constants.

     Human text has no string quote characters.  We do not want
     quotation marks to turn off the usual syntactic properties of other
     characters in the quotation.

Escape-syntax characters: ‘\’
     Characters that start an escape sequence, such as is used in string
     and character constants.  The character ‘\’ belongs to this class
     in both C and Lisp.  (In C, it is used thus only inside strings,
     but it turns out to cause no trouble to treat it this way
     throughout C code.)

     Characters in this class count as part of words if
     ‘words-include-escapes’ is non-‘nil’.  *Note Word Motion::.

Character quotes: ‘/’
     Characters used to quote the following character so that it loses
     its normal syntactic meaning.  This differs from an escape
     character in that only the character immediately following is ever
     affected.

     Characters in this class count as part of words if
     ‘words-include-escapes’ is non-‘nil’.  *Note Word Motion::.

     This class is used for backslash in TeX mode.

Paired delimiters: ‘$’
     Similar to string quote characters, except that the syntactic
     properties of the characters between the delimiters are not
     suppressed.  Only TeX mode uses a paired delimiter presently—the
     ‘$’ that both enters and leaves math mode.

Expression prefixes: ‘'’
     Characters used for syntactic operators that are considered as part
     of an expression if they appear next to one.  In Lisp modes, these
     characters include the apostrophe, ‘'’ (used for quoting), the
     comma, ‘,’ (used in macros), and ‘#’ (used in the read syntax for
     certain data types).

Comment starters: ‘<’
Comment enders: ‘>’
     Characters used in various languages to delimit comments.  Human
     text has no comment characters.  In Lisp, the semicolon (‘;’)
     starts a comment and a newline or formfeed ends one.

Inherit standard syntax: ‘@’
     This syntax class does not specify a particular syntax.  It says to
     look in the standard syntax table to find the syntax of this
     character.

Generic comment delimiters: ‘!’
     Characters that start or end a special kind of comment.  _Any_
     generic comment delimiter matches _any_ generic comment delimiter,
     but they cannot match a comment starter or comment ender; generic
     comment delimiters can only match each other.

     This syntax class is primarily meant for use with the
     ‘syntax-table’ text property (*note Syntax Properties::).  You can
     mark any range of characters as forming a comment, by giving the
     first and last characters of the range ‘syntax-table’ properties
     identifying them as generic comment delimiters.

Generic string delimiters: ‘|’
     Characters that start or end a string.  This class differs from the
     string quote class in that _any_ generic string delimiter can match
     any other generic string delimiter; but they do not match ordinary
     string quote characters.

     This syntax class is primarily meant for use with the
     ‘syntax-table’ text property (*note Syntax Properties::).  You can
     mark any range of characters as forming a string constant, by
     giving the first and last characters of the range ‘syntax-table’
     properties identifying them as generic string delimiters.


File: elisp.info,  Node: Syntax Flags,  Prev: Syntax Class Table,  Up: Syntax Descriptors

34.2.2 Syntax Flags
-------------------

In addition to the classes, entries for characters in a syntax table can
specify flags.  There are eight possible flags, represented by the
characters ‘1’, ‘2’, ‘3’, ‘4’, ‘b’, ‘c’, ‘n’, and ‘p’.

   All the flags except ‘p’ are used to describe comment delimiters.
The digit flags are used for comment delimiters made up of 2 characters.
They indicate that a character can _also_ be part of a comment sequence,
in addition to the syntactic properties associated with its character
class.  The flags are independent of the class and each other for the
sake of characters such as ‘*’ in C mode, which is a punctuation
character, _and_ the second character of a start-of-comment sequence
(‘/*’), _and_ the first character of an end-of-comment sequence (‘*/’).
The flags ‘b’, ‘c’, and ‘n’ are used to qualify the corresponding
comment delimiter.

   Here is a table of the possible flags for a character C, and what
they mean:

   • ‘1’ means C is the start of a two-character comment-start sequence.

   • ‘2’ means C is the second character of such a sequence.

   • ‘3’ means C is the start of a two-character comment-end sequence.

   • ‘4’ means C is the second character of such a sequence.

   • ‘b’ means that C as a comment delimiter belongs to the alternative
     “b” comment style.  For a two-character comment starter, this flag
     is only significant on the second char, and for a 2-character
     comment ender it is only significant on the first char.

   • ‘c’ means that C as a comment delimiter belongs to the alternative
     “c” comment style.  For a two-character comment delimiter, ‘c’ on
     either character makes it of style “c”.

   • ‘n’ on a comment delimiter character specifies that this kind of
     comment can be nested.  For a two-character comment delimiter, ‘n’
     on either character makes it nestable.

     Emacs supports several comment styles simultaneously in any one
     syntax table.  A comment style is a set of flags ‘b’, ‘c’, and ‘n’,
     so there can be up to 8 different comment styles.  Each comment
     delimiter has a style and only matches comment delimiters of the
     same style.  Thus if a comment starts with the comment-start
     sequence of style “bn”, it will extend until the next matching
     comment-end sequence of style “bn”.

     The appropriate comment syntax settings for C++ can be as follows:

     ‘/’
          ‘124’
     ‘*’
          ‘23b’
     newline
          ‘>’

     This defines four comment-delimiting sequences:

     ‘/*’
          This is a comment-start sequence for “b” style because the
          second character, ‘*’, has the ‘b’ flag.

     ‘//’
          This is a comment-start sequence for “a” style because the
          second character, ‘/’, does not have the ‘b’ flag.

     ‘*/’
          This is a comment-end sequence for “b” style because the first
          character, ‘*’, has the ‘b’ flag.

     newline
          This is a comment-end sequence for “a” style, because the
          newline character does not have the ‘b’ flag.

   • ‘p’ identifies an additional prefix character for Lisp syntax.
     These characters are treated as whitespace when they appear between
     expressions.  When they appear within an expression, they are
     handled according to their usual syntax classes.

     The function ‘backward-prefix-chars’ moves back over these
     characters, as well as over characters whose primary syntax class
     is prefix (‘'’).  *Note Motion and Syntax::.


File: elisp.info,  Node: Syntax Table Functions,  Next: Syntax Properties,  Prev: Syntax Descriptors,  Up: Syntax Tables

34.3 Syntax Table Functions
===========================

In this section we describe functions for creating, accessing and
altering syntax tables.

 -- Function: make-syntax-table &optional table
     This function creates a new syntax table.  If TABLE is non-‘nil’,
     the parent of the new syntax table is TABLE; otherwise, the parent
     is the standard syntax table.

     In the new syntax table, all characters are initially given the
     “inherit” (‘@’) syntax class, i.e., their syntax is inherited from
     the parent table (*note Syntax Class Table::).

 -- Function: copy-syntax-table &optional table
     This function constructs a copy of TABLE and returns it.  If TABLE
     is omitted or ‘nil’, it returns a copy of the standard syntax
     table.  Otherwise, an error is signaled if TABLE is not a syntax
     table.

 -- Command: modify-syntax-entry char syntax-descriptor &optional table
     This function sets the syntax entry for CHAR according to
     SYNTAX-DESCRIPTOR.  CHAR must be a character, or a cons cell of the
     form ‘(MIN . MAX)’; in the latter case, the function sets the
     syntax entries for all characters in the range between MIN and MAX,
     inclusive.

     The syntax is changed only for TABLE, which defaults to the current
     buffer’s syntax table, and not in any other syntax table.

     The argument SYNTAX-DESCRIPTOR is a syntax descriptor, i.e., a
     string whose first character is a syntax class designator and whose
     second and subsequent characters optionally specify a matching
     character and syntax flags.  *Note Syntax Descriptors::.  An error
     is signaled if SYNTAX-DESCRIPTOR is not a valid syntax descriptor.

     This function always returns ‘nil’.  The old syntax information in
     the table for this character is discarded.

     Examples:

          ;; Put the space character in class whitespace.
          (modify-syntax-entry ?\s " ")
               ⇒ nil

          ;; Make ‘$’ an open parenthesis character,
          ;;   with ‘^’ as its matching close.
          (modify-syntax-entry ?$ "(^")
               ⇒ nil

          ;; Make ‘^’ a close parenthesis character,
          ;;   with ‘$’ as its matching open.
          (modify-syntax-entry ?^ ")$")
               ⇒ nil

          ;; Make ‘/’ a punctuation character,
          ;;   the first character of a start-comment sequence,
          ;;   and the second character of an end-comment sequence.
          ;;   This is used in C mode.
          (modify-syntax-entry ?/ ". 14")
               ⇒ nil

 -- Function: char-syntax character
     This function returns the syntax class of CHARACTER, represented by
     its designator character (*note Syntax Class Table::).  This
     returns _only_ the class, not its matching character or syntax
     flags.

     The following examples apply to C mode.  (We use ‘string’ to make
     it easier to see the character returned by ‘char-syntax’.)

          ;; Space characters have whitespace syntax class.
          (string (char-syntax ?\s))
               ⇒ " "

          ;; Forward slash characters have punctuation syntax.
          ;; Note that this char-syntax call does not reveal
          ;; that it is also part of comment-start and -end sequences.
          (string (char-syntax ?/))
               ⇒ "."

          ;; Open parenthesis characters have open parenthesis syntax.
          ;; Note that this char-syntax call does not reveal that
          ;; it has a matching character, ‘)’.
          (string (char-syntax ?\())
               ⇒ "("

 -- Function: set-syntax-table table
     This function makes TABLE the syntax table for the current buffer.
     It returns TABLE.

 -- Function: syntax-table
     This function returns the current syntax table, which is the table
     for the current buffer.

 -- Command: describe-syntax &optional buffer
     This command displays the contents of the syntax table of BUFFER
     (by default, the current buffer) in a help buffer.

 -- Macro: with-syntax-table table body...
     This macro executes BODY using TABLE as the current syntax table.
     It returns the value of the last form in BODY, after restoring the
     old current syntax table.

     Since each buffer has its own current syntax table, we should make
     that more precise: ‘with-syntax-table’ temporarily alters the
     current syntax table of whichever buffer is current at the time the
     macro execution starts.  Other buffers are not affected.


File: elisp.info,  Node: Syntax Properties,  Next: Motion and Syntax,  Prev: Syntax Table Functions,  Up: Syntax Tables

34.4 Syntax Properties
======================

When the syntax table is not flexible enough to specify the syntax of a
language, you can override the syntax table for specific character
occurrences in the buffer, by applying a ‘syntax-table’ text property.
*Note Text Properties::, for how to apply text properties.

   The valid values of ‘syntax-table’ text property are:

SYNTAX-TABLE
     If the property value is a syntax table, that table is used instead
     of the current buffer’s syntax table to determine the syntax for
     the underlying text character.

‘(SYNTAX-CODE . MATCHING-CHAR)’
     A cons cell of this format is a raw syntax descriptor (*note Syntax
     Table Internals::), which directly specifies a syntax class for the
     underlying text character.

‘nil’
     If the property is ‘nil’, the character’s syntax is determined from
     the current syntax table in the usual way.

 -- Variable: parse-sexp-lookup-properties
     If this is non-‘nil’, the syntax scanning functions, like
     ‘forward-sexp’, pay attention to syntax text properties.  Otherwise
     they use only the current syntax table.

 -- Variable: syntax-propertize-function
     This variable, if non-‘nil’, should store a function for applying
     ‘syntax-table’ properties to a specified stretch of text.  It is
     intended to be used by major modes to install a function which
     applies ‘syntax-table’ properties in some mode-appropriate way.

     The function is called by ‘syntax-ppss’ (*note Position Parse::),
     and by Font Lock mode during syntactic fontification (*note
     Syntactic Font Lock::).  It is called with two arguments, START and
     END, which are the starting and ending positions of the text on
     which it should act.  It is allowed to call ‘syntax-ppss’ on any
     position before END.  However, it should not call
     ‘syntax-ppss-flush-cache’; so, it is not allowed to call
     ‘syntax-ppss’ on some position and later modify the buffer at an
     earlier position.

 -- Variable: syntax-propertize-extend-region-functions
     This abnormal hook is run by the syntax parsing code prior to
     calling ‘syntax-propertize-function’.  Its role is to help locate
     safe starting and ending buffer positions for passing to
     ‘syntax-propertize-function’.  For example, a major mode can add a
     function to this hook to identify multi-line syntactic constructs,
     and ensure that the boundaries do not fall in the middle of one.

     Each function in this hook should accept two arguments, START and
     END.  It should return either a cons cell of two adjusted buffer
     positions, ‘(NEW-START . NEW-END)’, or ‘nil’ if no adjustment is
     necessary.  The hook functions are run in turn, repeatedly, until
     they all return ‘nil’.


File: elisp.info,  Node: Motion and Syntax,  Next: Parsing Expressions,  Prev: Syntax Properties,  Up: Syntax Tables

34.5 Motion and Syntax
======================

This section describes functions for moving across characters that have
certain syntax classes.

 -- Function: skip-syntax-forward syntaxes &optional limit
     This function moves point forward across characters having syntax
     classes mentioned in SYNTAXES (a string of syntax class
     characters).  It stops when it encounters the end of the buffer, or
     position LIMIT (if specified), or a character it is not supposed to
     skip.

     If SYNTAXES starts with ‘^’, then the function skips characters
     whose syntax is _not_ in SYNTAXES.

     The return value is the distance traveled, which is a nonnegative
     integer.

 -- Function: skip-syntax-backward syntaxes &optional limit
     This function moves point backward across characters whose syntax
     classes are mentioned in SYNTAXES.  It stops when it encounters the
     beginning of the buffer, or position LIMIT (if specified), or a
     character it is not supposed to skip.

     If SYNTAXES starts with ‘^’, then the function skips characters
     whose syntax is _not_ in SYNTAXES.

     The return value indicates the distance traveled.  It is an integer
     that is zero or less.

 -- Function: backward-prefix-chars
     This function moves point backward over any number of characters
     with expression prefix syntax.  This includes both characters in
     the expression prefix syntax class, and characters with the ‘p’
     flag.


File: elisp.info,  Node: Parsing Expressions,  Next: Syntax Table Internals,  Prev: Motion and Syntax,  Up: Syntax Tables

34.6 Parsing Expressions
========================

This section describes functions for parsing and scanning balanced
expressions.  We will refer to such expressions as “sexps”, following
the terminology of Lisp, even though these functions can act on
languages other than Lisp.  Basically, a sexp is either a balanced
parenthetical grouping, a string, or a symbol (i.e., a sequence of
characters whose syntax is either word constituent or symbol
constituent).  However, characters in the expression prefix syntax class
(*note Syntax Class Table::) are treated as part of the sexp if they
appear next to it.

   The syntax table controls the interpretation of characters, so these
functions can be used for Lisp expressions when in Lisp mode and for C
expressions when in C mode.  *Note List Motion::, for convenient
higher-level functions for moving over balanced expressions.

   A character’s syntax controls how it changes the state of the parser,
rather than describing the state itself.  For example, a string
delimiter character toggles the parser state between in-string and
in-code, but the syntax of characters does not directly say whether they
are inside a string.  For example (note that 15 is the syntax code for
generic string delimiters),

     (put-text-property 1 9 'syntax-table '(15 . nil))

does not tell Emacs that the first eight chars of the current buffer are
a string, but rather that they are all string delimiters.  As a result,
Emacs treats them as four consecutive empty string constants.

* Menu:

* Motion via Parsing::       Motion functions that work by parsing.
* Position Parse::           Determining the syntactic state of a position.
* Parser State::             How Emacs represents a syntactic state.
* Low-Level Parsing::        Parsing across a specified region.
* Control Parsing::          Parameters that affect parsing.


File: elisp.info,  Node: Motion via Parsing,  Next: Position Parse,  Up: Parsing Expressions

34.6.1 Motion Commands Based on Parsing
---------------------------------------

This section describes simple point-motion functions that operate based
on parsing expressions.

 -- Function: scan-lists from count depth
     This function scans forward COUNT balanced parenthetical groupings
     from position FROM.  It returns the position where the scan stops.
     If COUNT is negative, the scan moves backwards.

     If DEPTH is nonzero, treat the starting position as being DEPTH
     parentheses deep.  The scanner moves forward or backward through
     the buffer until the depth changes to zero COUNT times.  Hence, a
     positive value for DEPTH has the effect of moving out DEPTH levels
     of parenthesis from the starting position, while a negative DEPTH
     has the effect of moving deeper by -DEPTH levels of parenthesis.

     Scanning ignores comments if ‘parse-sexp-ignore-comments’ is
     non-‘nil’.

     If the scan reaches the beginning or end of the accessible part of
     the buffer before it has scanned over COUNT parenthetical
     groupings, the return value is ‘nil’ if the depth at that point is
     zero; if the depth is non-zero, a ‘scan-error’ error is signaled.

 -- Function: scan-sexps from count
     This function scans forward COUNT sexps from position FROM.  It
     returns the position where the scan stops.  If COUNT is negative,
     the scan moves backwards.

     Scanning ignores comments if ‘parse-sexp-ignore-comments’ is
     non-‘nil’.

     If the scan reaches the beginning or end of (the accessible part
     of) the buffer while in the middle of a parenthetical grouping, an
     error is signaled.  If it reaches the beginning or end between
     groupings but before count is used up, ‘nil’ is returned.

 -- Function: forward-comment count
     This function moves point forward across COUNT complete comments
     (that is, including the starting delimiter and the terminating
     delimiter if any), plus any whitespace encountered on the way.  It
     moves backward if COUNT is negative.  If it encounters anything
     other than a comment or whitespace, it stops, leaving point at the
     place where it stopped.  This includes (for instance) finding the
     end of a comment when moving forward and expecting the beginning of
     one.  The function also stops immediately after moving over the
     specified number of complete comments.  If COUNT comments are found
     as expected, with nothing except whitespace between them, it
     returns ‘t’; otherwise it returns ‘nil’.

     This function cannot tell whether the comments it traverses are
     embedded within a string.  If they look like comments, it treats
     them as comments.

     To move forward over all comments and whitespace following point,
     use ‘(forward-comment (buffer-size))’.  ‘(buffer-size)’ is a good
     argument to use, because the number of comments in the buffer
     cannot exceed that many.


File: elisp.info,  Node: Position Parse,  Next: Parser State,  Prev: Motion via Parsing,  Up: Parsing Expressions

34.6.2 Finding the Parse State for a Position
---------------------------------------------

For syntactic analysis, such as in indentation, often the useful thing
is to compute the syntactic state corresponding to a given buffer
position.  This function does that conveniently.

 -- Function: syntax-ppss &optional pos
     This function returns the parser state that the parser would reach
     at position POS starting from the beginning of the buffer.  *Note
     Parser State::, for a description of the parser state.

     The return value is the same as if you call the low-level parsing
     function ‘parse-partial-sexp’ to parse from the beginning of the
     buffer to POS (*note Low-Level Parsing::).  However, ‘syntax-ppss’
     uses a cache to speed up the computation.  Due to this
     optimization, the second value (previous complete subexpression)
     and sixth value (minimum parenthesis depth) in the returned parser
     state are not meaningful.

     This function has a side effect: it adds a buffer-local entry to
     ‘before-change-functions’ (*note Change Hooks::) for
     ‘syntax-ppss-flush-cache’ (see below).  This entry keeps the cache
     consistent as the buffer is modified.  However, the cache might not
     be updated if ‘syntax-ppss’ is called while
     ‘before-change-functions’ is temporarily let-bound, or if the
     buffer is modified without running the hook, such as when using
     ‘inhibit-modification-hooks’.  In those cases, it is necessary to
     call ‘syntax-ppss-flush-cache’ explicitly.

 -- Function: syntax-ppss-flush-cache beg &rest ignored-args
     This function flushes the cache used by ‘syntax-ppss’, starting at
     position BEG.  The remaining arguments, IGNORED-ARGS, are ignored;
     this function accepts them so that it can be directly used on hooks
     such as ‘before-change-functions’ (*note Change Hooks::).


File: elisp.info,  Node: Parser State,  Next: Low-Level Parsing,  Prev: Position Parse,  Up: Parsing Expressions

34.6.3 Parser State
-------------------

A “parser state” is a list of ten elements describing the state of the
syntactic parser, after it parses the text between a specified starting
point and a specified end point in the buffer.  Parsing functions such
as ‘syntax-ppss’ (*note Position Parse::) return a parser state as the
value.  Some parsing functions accept a parser state as an argument, for
resuming parsing.

   Here are the meanings of the elements of the parser state:

  0. The depth in parentheses, counting from 0.  *Warning:* this can be
     negative if there are more close parens than open parens between
     the parser’s starting point and end point.

  1. The character position of the start of the innermost parenthetical
     grouping containing the stopping point; ‘nil’ if none.

  2. The character position of the start of the last complete
     subexpression terminated; ‘nil’ if none.

  3. Non-‘nil’ if inside a string.  More precisely, this is the
     character that will terminate the string, or ‘t’ if a generic
     string delimiter character should terminate it.

  4. ‘t’ if inside a non-nestable comment (of any comment style; *note
     Syntax Flags::); or the comment nesting level if inside a comment
     that can be nested.

  5. ‘t’ if the end point is just after a quote character.

  6. The minimum parenthesis depth encountered during this scan.

  7. What kind of comment is active: ‘nil’ if not in a comment or in a
     comment of style ‘a’; 1 for a comment of style ‘b’; 2 for a comment
     of style ‘c’; and ‘syntax-table’ for a comment that should be ended
     by a generic comment delimiter character.

  8. The string or comment start position.  While inside a comment, this
     is the position where the comment began; while inside a string,
     this is the position where the string began.  When outside of
     strings and comments, this element is ‘nil’.

  9. Internal data for continuing the parsing.  The meaning of this data
     is subject to change; it is used if you pass this list as the STATE
     argument to another call.

   Elements 1, 2, and 6 are ignored in a state which you pass as an
argument to continue parsing, and elements 8 and 9 are used only in
trivial cases.  Those elements are mainly used internally by the parser
code.

   One additional piece of useful information is available from a parser
state using this function:

 -- Function: syntax-ppss-toplevel-pos state
     This function extracts, from parser state STATE, the last position
     scanned in the parse which was at top level in grammatical
     structure.  “At top level” means outside of any parentheses,
     comments, or strings.

     The value is ‘nil’ if STATE represents a parse which has arrived at
     a top level position.


File: elisp.info,  Node: Low-Level Parsing,  Next: Control Parsing,  Prev: Parser State,  Up: Parsing Expressions

34.6.4 Low-Level Parsing
------------------------

The most basic way to use the expression parser is to tell it to start
at a given position with a certain state, and parse up to a specified
end position.

 -- Function: parse-partial-sexp start limit &optional target-depth
          stop-before state stop-comment
     This function parses a sexp in the current buffer starting at
     START, not scanning past LIMIT.  It stops at position LIMIT or when
     certain criteria described below are met, and sets point to the
     location where parsing stops.  It returns a parser state (*note
     Parser State::) describing the status of the parse at the point
     where it stops.

     If the third argument TARGET-DEPTH is non-‘nil’, parsing stops if
     the depth in parentheses becomes equal to TARGET-DEPTH.  The depth
     starts at 0, or at whatever is given in STATE.

     If the fourth argument STOP-BEFORE is non-‘nil’, parsing stops when
     it comes to any character that starts a sexp.  If STOP-COMMENT is
     non-‘nil’, parsing stops when it comes to the start of a comment.
     If STOP-COMMENT is the symbol ‘syntax-table’, parsing stops after
     the start of a comment or a string, or the end of a comment or a
     string, whichever comes first.

     If STATE is ‘nil’, START is assumed to be at the top level of
     parenthesis structure, such as the beginning of a function
     definition.  Alternatively, you might wish to resume parsing in the
     middle of the structure.  To do this, you must provide a STATE
     argument that describes the initial status of parsing.  The value
     returned by a previous call to ‘parse-partial-sexp’ will do nicely.


File: elisp.info,  Node: Control Parsing,  Prev: Low-Level Parsing,  Up: Parsing Expressions

34.6.5 Parameters to Control Parsing
------------------------------------

 -- Variable: multibyte-syntax-as-symbol
     If this variable is non-‘nil’, ‘scan-sexps’ treats all non-ASCII
     characters as symbol constituents regardless of what the syntax
     table says about them.  (However, text properties can still
     override the syntax.)

 -- User Option: parse-sexp-ignore-comments
     If the value is non-‘nil’, then comments are treated as whitespace
     by the functions in this section and by ‘forward-sexp’,
     ‘scan-lists’ and ‘scan-sexps’.

   The behavior of ‘parse-partial-sexp’ is also affected by
‘parse-sexp-lookup-properties’ (*note Syntax Properties::).

 -- Variable: comment-end-can-be-escaped
     If this buffer local variable is non-‘nil’, a single character
     which usually terminates a comment doesn’t do so when that
     character is escaped.  This is used in C and C++ Modes, where line
     comments starting with ‘//’ can be continued onto the next line by
     escaping the newline with ‘\’.

   You can use ‘forward-comment’ to move forward or backward over one
comment or several comments.


File: elisp.info,  Node: Syntax Table Internals,  Next: Categories,  Prev: Parsing Expressions,  Up: Syntax Tables

34.7 Syntax Table Internals
===========================

Syntax tables are implemented as char-tables (*note Char-Tables::), but
most Lisp programs don’t work directly with their elements.  Syntax
tables do not store syntax data as syntax descriptors (*note Syntax
Descriptors::); they use an internal format, which is documented in this
section.  This internal format can also be assigned as syntax properties
(*note Syntax Properties::).

   Each entry in a syntax table is a “raw syntax descriptor”: a cons
cell of the form ‘(SYNTAX-CODE . MATCHING-CHAR)’.  SYNTAX-CODE is an
integer which encodes the syntax class and syntax flags, according to
the table below.  MATCHING-CHAR, if non-‘nil’, specifies a matching
character (similar to the second character in a syntax descriptor).

   Here are the syntax codes corresponding to the various syntax
classes:

Code           Class                  Code           Class
0              whitespace             8              paired delimiter
1              punctuation            9              escape
2              word                   10             character quote
3              symbol                 11             comment-start
4              open parenthesis       12             comment-end
5              close parenthesis      13             inherit
6              expression prefix      14             generic comment
7              string quote           15             generic string

For example, in the standard syntax table, the entry for ‘(’ is ‘(4 .
41)’.  41 is the character code for ‘)’.

   Syntax flags are encoded in higher order bits, starting 16 bits from
the least significant bit.  This table gives the power of two which
corresponds to each syntax flag.

Prefix      Flag                   Prefix      Flag
‘1’         ‘(lsh 1 16)’           ‘p’         ‘(lsh 1 20)’
‘2’         ‘(lsh 1 17)’           ‘b’         ‘(lsh 1 21)’
‘3’         ‘(lsh 1 18)’           ‘n’         ‘(lsh 1 22)’
‘4’         ‘(lsh 1 19)’

 -- Function: string-to-syntax desc
     Given a syntax descriptor DESC (a string), this function returns
     the corresponding raw syntax descriptor.

 -- Function: syntax-after pos
     This function returns the raw syntax descriptor for the character
     in the buffer after position POS, taking account of syntax
     properties as well as the syntax table.  If POS is outside the
     buffer’s accessible portion (*note accessible portion: Narrowing.),
     the return value is ‘nil’.

 -- Function: syntax-class syntax
     This function returns the syntax code for the raw syntax descriptor
     SYNTAX.  More precisely, it takes the raw syntax descriptor’s
     SYNTAX-CODE component, masks off the high 16 bits which record the
     syntax flags, and returns the resulting integer.

     If SYNTAX is ‘nil’, the return value is returns ‘nil’.  This is so
     that the expression

          (syntax-class (syntax-after pos))

     evaluates to ‘nil’ if ‘pos’ is outside the buffer’s accessible
     portion, without throwing errors or returning an incorrect code.


File: elisp.info,  Node: Categories,  Prev: Syntax Table Internals,  Up: Syntax Tables

34.8 Categories
===============

“Categories” provide an alternate way of classifying characters
syntactically.  You can define several categories as needed, then
independently assign each character to one or more categories.  Unlike
syntax classes, categories are not mutually exclusive; it is normal for
one character to belong to several categories.

   Each buffer has a “category table” which records which categories are
defined and also which characters belong to each category.  Each
category table defines its own categories, but normally these are
initialized by copying from the standard categories table, so that the
standard categories are available in all modes.

   Each category has a name, which is an ASCII printing character in the
range ‘ ’ to ‘~’.  You specify the name of a category when you define it
with ‘define-category’.

   The category table is actually a char-table (*note Char-Tables::).
The element of the category table at index C is a “category set”—a
bool-vector—that indicates which categories character C belongs to.  In
this category set, if the element at index CAT is ‘t’, that means
category CAT is a member of the set, and that character C belongs to
category CAT.

   For the next three functions, the optional argument TABLE defaults to
the current buffer’s category table.

 -- Function: define-category char docstring &optional table
     This function defines a new category, with name CHAR and
     documentation DOCSTRING, for the category table TABLE.

     Here’s an example of defining a new category for characters that
     have strong right-to-left directionality (*note Bidirectional
     Display::) and using it in a special category table.  To obtain the
     information about the directionality of characters, the example
     code uses the ‘bidi-class’ Unicode property (*note bidi-class:
     Character Properties.).

          (defvar special-category-table-for-bidi
            ;;     Make an empty category-table.
            (let ((category-table (make-category-table))
                  ;; Create a char-table which gives the 'bidi-class' Unicode
                  ;; property for each character.
                  (uniprop-table (unicode-property-table-internal 'bidi-class)))
              (define-category ?R "Characters of bidi-class R, AL, or RLO"
                               category-table)
              ;; Modify the category entry of each character whose 'bidi-class'
              ;; Unicode property is R, AL, or RLO -- these have a
              ;; right-to-left directionality.
              (map-char-table
               #'(lambda (key val)
                   (if (memq val '(R AL RLO))
                       (modify-category-entry key ?R category-table)))
               uniprop-table)
              category-table))

 -- Function: category-docstring category &optional table
     This function returns the documentation string of category CATEGORY
     in category table TABLE.

          (category-docstring ?a)
               ⇒ "ASCII"
          (category-docstring ?l)
               ⇒ "Latin"

 -- Function: get-unused-category &optional table
     This function returns a category name (a character) which is not
     currently defined in TABLE.  If all possible categories are in use
     in TABLE, it returns ‘nil’.

 -- Function: category-table
     This function returns the current buffer’s category table.

 -- Function: category-table-p object
     This function returns ‘t’ if OBJECT is a category table, otherwise
     ‘nil’.

 -- Function: standard-category-table
     This function returns the standard category table.

 -- Function: copy-category-table &optional table
     This function constructs a copy of TABLE and returns it.  If TABLE
     is not supplied (or is ‘nil’), it returns a copy of the standard
     category table.  Otherwise, an error is signaled if TABLE is not a
     category table.

 -- Function: set-category-table table
     This function makes TABLE the category table for the current
     buffer.  It returns TABLE.

 -- Function: make-category-table
     This creates and returns an empty category table.  In an empty
     category table, no categories have been allocated, and no
     characters belong to any categories.

 -- Function: make-category-set categories
     This function returns a new category set—a bool-vector—whose
     initial contents are the categories listed in the string
     CATEGORIES.  The elements of CATEGORIES should be category names;
     the new category set has ‘t’ for each of those categories, and
     ‘nil’ for all other categories.

          (make-category-set "al")
               ⇒ #&128"\0\0\0\0\0\0\0\0\0\0\0\0\2\20\0\0"

 -- Function: char-category-set char
     This function returns the category set for character CHAR in the
     current buffer’s category table.  This is the bool-vector which
     records which categories the character CHAR belongs to.  The
     function ‘char-category-set’ does not allocate storage, because it
     returns the same bool-vector that exists in the category table.

          (char-category-set ?a)
               ⇒ #&128"\0\0\0\0\0\0\0\0\0\0\0\0\2\20\0\0"

 -- Function: category-set-mnemonics category-set
     This function converts the category set CATEGORY-SET into a string
     containing the characters that designate the categories that are
     members of the set.

          (category-set-mnemonics (char-category-set ?a))
               ⇒ "al"

 -- Function: modify-category-entry char category &optional table reset
     This function modifies the category set of CHAR in category table
     TABLE (which defaults to the current buffer’s category table).
     CHAR can be a character, or a cons cell of the form ‘(MIN . MAX)’;
     in the latter case, the function modifies the category sets of all
     characters in the range between MIN and MAX, inclusive.

     Normally, it modifies a category set by adding CATEGORY to it.  But
     if RESET is non-‘nil’, then it deletes CATEGORY instead.

 -- Command: describe-categories &optional buffer-or-name
     This function describes the category specifications in the current
     category table.  It inserts the descriptions in a buffer, and then
     displays that buffer.  If BUFFER-OR-NAME is non-‘nil’, it describes
     the category table of that buffer instead.


File: elisp.info,  Node: Abbrevs,  Next: Processes,  Prev: Syntax Tables,  Up: Top

35 Abbrevs and Abbrev Expansion
*******************************

An abbreviation or “abbrev” is a string of characters that may be
expanded to a longer string.  The user can insert the abbrev string and
find it replaced automatically with the expansion of the abbrev.  This
saves typing.

   The set of abbrevs currently in effect is recorded in an “abbrev
table”.  Each buffer has a local abbrev table, but normally all buffers
in the same major mode share one abbrev table.  There is also a global
abbrev table.  Normally both are used.

   An abbrev table is represented as an obarray.  *Note Creating
Symbols::, for information about obarrays.  Each abbreviation is
represented by a symbol in the obarray.  The symbol’s name is the
abbreviation; its value is the expansion; its function definition is the
hook function for performing the expansion (*note Defining Abbrevs::);
and its property list cell contains various additional properties,
including the use count and the number of times the abbreviation has
been expanded (*note Abbrev Properties::).

   Certain abbrevs, called “system abbrevs”, are defined by a major mode
instead of the user.  A system abbrev is identified by its non-‘nil’
‘:system’ property (*note Abbrev Properties::).  When abbrevs are saved
to an abbrev file, system abbrevs are omitted.  *Note Abbrev Files::.

   Because the symbols used for abbrevs are not interned in the usual
obarray, they will never appear as the result of reading a Lisp
expression; in fact, normally they are never used except by the code
that handles abbrevs.  Therefore, it is safe to use them in a
nonstandard way.

   If the minor mode Abbrev mode is enabled, the buffer-local variable
‘abbrev-mode’ is non-‘nil’, and abbrevs are automatically expanded in
the buffer.  For the user-level commands for abbrevs, see *note Abbrev
Mode: (emacs)Abbrevs.

* Menu:

* Tables: Abbrev Tables.        Creating and working with abbrev tables.
* Defining Abbrevs::            Specifying abbreviations and their expansions.
* Files: Abbrev Files.          Saving abbrevs in files.
* Expansion: Abbrev Expansion.  Controlling expansion; expansion subroutines.
* Standard Abbrev Tables::      Abbrev tables used by various major modes.
* Abbrev Properties::           How to read and set abbrev properties.
                                Which properties have which effect.
* Abbrev Table Properties::     How to read and set abbrev table properties.
                                Which properties have which effect.


File: elisp.info,  Node: Abbrev Tables,  Next: Defining Abbrevs,  Up: Abbrevs

35.1 Abbrev Tables
==================

This section describes how to create and manipulate abbrev tables.

 -- Function: make-abbrev-table &optional props
     This function creates and returns a new, empty abbrev table—an
     obarray containing no symbols.  It is a vector filled with zeros.
     PROPS is a property list that is applied to the new table (*note
     Abbrev Table Properties::).

 -- Function: abbrev-table-p object
     This function returns a non-‘nil’ value if OBJECT is an abbrev
     table.

 -- Function: clear-abbrev-table abbrev-table
     This function undefines all the abbrevs in ABBREV-TABLE, leaving it
     empty.

 -- Function: copy-abbrev-table abbrev-table
     This function returns a copy of ABBREV-TABLE—a new abbrev table
     containing the same abbrev definitions.  It does _not_ copy any
     property lists; only the names, values, and functions.

 -- Function: define-abbrev-table tabname definitions &optional
          docstring &rest props
     This function defines TABNAME (a symbol) as an abbrev table name,
     i.e., as a variable whose value is an abbrev table.  It defines
     abbrevs in the table according to DEFINITIONS, a list of elements
     of the form ‘(ABBREVNAME EXPANSION [HOOK] [PROPS...])’.  These
     elements are passed as arguments to ‘define-abbrev’.

     The optional string DOCSTRING is the documentation string of the
     variable TABNAME.  The property list PROPS is applied to the abbrev
     table (*note Abbrev Table Properties::).

     If this function is called more than once for the same TABNAME,
     subsequent calls add the definitions in DEFINITIONS to TABNAME,
     rather than overwriting the entire original contents.  (A
     subsequent call only overrides abbrevs explicitly redefined or
     undefined in DEFINITIONS.)

 -- Variable: abbrev-table-name-list
     This is a list of symbols whose values are abbrev tables.
     ‘define-abbrev-table’ adds the new abbrev table name to this list.

 -- Function: insert-abbrev-table-description name &optional human
     This function inserts before point a description of the abbrev
     table named NAME.  The argument NAME is a symbol whose value is an
     abbrev table.

     If HUMAN is non-‘nil’, the description is human-oriented.  System
     abbrevs are listed and identified as such.  Otherwise the
     description is a Lisp expression—a call to ‘define-abbrev-table’
     that would define NAME as it is currently defined, but without the
     system abbrevs.  (The mode or package using NAME is supposed to add
     these to NAME separately.)


File: elisp.info,  Node: Defining Abbrevs,  Next: Abbrev Files,  Prev: Abbrev Tables,  Up: Abbrevs

35.2 Defining Abbrevs
=====================

‘define-abbrev’ is the low-level basic function for defining an abbrev
in an abbrev table.

   When a major mode defines a system abbrev, it should call
‘define-abbrev’ and specify ‘t’ for the ‘:system’ property.  Be aware
that any saved non-system abbrevs are restored at startup, i.e., before
some major modes are loaded.  Therefore, major modes should not assume
that their abbrev tables are empty when they are first loaded.

 -- Function: define-abbrev abbrev-table name expansion &optional hook
          &rest props
     This function defines an abbrev named NAME, in ABBREV-TABLE, to
     expand to EXPANSION and call HOOK, with properties PROPS (*note
     Abbrev Properties::).  The return value is NAME.  The ‘:system’
     property in PROPS is treated specially here: if it has the value
     ‘force’, then it will overwrite an existing definition even for a
     non-system abbrev of the same name.

     NAME should be a string.  The argument EXPANSION is normally the
     desired expansion (a string), or ‘nil’ to undefine the abbrev.  If
     it is anything but a string or ‘nil’, then the abbreviation expands
     solely by running HOOK.

     The argument HOOK is a function or ‘nil’.  If HOOK is non-‘nil’,
     then it is called with no arguments after the abbrev is replaced
     with EXPANSION; point is located at the end of EXPANSION when HOOK
     is called.

     If HOOK is a non-‘nil’ symbol whose ‘no-self-insert’ property is
     non-‘nil’, HOOK can explicitly control whether to insert the
     self-inserting input character that triggered the expansion.  If
     HOOK returns non-‘nil’ in this case, that inhibits insertion of the
     character.  By contrast, if HOOK returns ‘nil’, ‘expand-abbrev’ (or
     ‘abbrev-insert’) also returns ‘nil’, as if expansion had not really
     occurred.

     Normally, ‘define-abbrev’ sets the variable ‘abbrevs-changed’ to
     ‘t’, if it actually changes the abbrev.  This is so that some
     commands will offer to save the abbrevs.  It does not do this for a
     system abbrev, since those aren’t saved anyway.

 -- User Option: only-global-abbrevs
     If this variable is non-‘nil’, it means that the user plans to use
     global abbrevs only.  This tells the commands that define
     mode-specific abbrevs to define global ones instead.  This variable
     does not alter the behavior of the functions in this section; it is
     examined by their callers.


File: elisp.info,  Node: Abbrev Files,  Next: Abbrev Expansion,  Prev: Defining Abbrevs,  Up: Abbrevs

35.3 Saving Abbrevs in Files
============================

A file of saved abbrev definitions is actually a file of Lisp code.  The
abbrevs are saved in the form of a Lisp program to define the same
abbrev tables with the same contents.  Therefore, you can load the file
with ‘load’ (*note How Programs Do Loading::).  However, the function
‘quietly-read-abbrev-file’ is provided as a more convenient interface.
Emacs automatically calls this function at startup.

   User-level facilities such as ‘save-some-buffers’ can save abbrevs in
a file automatically, under the control of variables described here.

 -- User Option: abbrev-file-name
     This is the default file name for reading and saving abbrevs.  By
     default, Emacs will look for ‘~/.emacs.d/abbrev_defs’, and, if not
     found, for ‘~/.abbrev_defs’; if neither file exists, Emacs will
     create ‘~/.emacs.d/abbrev_defs’.

 -- Function: quietly-read-abbrev-file &optional filename
     This function reads abbrev definitions from a file named FILENAME,
     previously written with ‘write-abbrev-file’.  If FILENAME is
     omitted or ‘nil’, the file specified in ‘abbrev-file-name’ is used.

     As the name implies, this function does not display any messages.

 -- User Option: save-abbrevs
     A non-‘nil’ value for ‘save-abbrevs’ means that Emacs should offer
     to save abbrevs (if any have changed) when files are saved.  If the
     value is ‘silently’, Emacs saves the abbrevs without asking the
     user.  ‘abbrev-file-name’ specifies the file to save the abbrevs
     in.  The default value is ‘t’.

 -- Variable: abbrevs-changed
     This variable is set non-‘nil’ by defining or altering any abbrevs
     (except system abbrevs).  This serves as a flag for various Emacs
     commands to offer to save your abbrevs.

 -- Command: write-abbrev-file &optional filename
     Save all abbrev definitions (except system abbrevs), for all abbrev
     tables listed in ‘abbrev-table-name-list’, in the file FILENAME, in
     the form of a Lisp program that when loaded will define the same
     abbrevs.  If FILENAME is ‘nil’ or omitted, ‘abbrev-file-name’ is
     used.  This function returns ‘nil’.


File: elisp.info,  Node: Abbrev Expansion,  Next: Standard Abbrev Tables,  Prev: Abbrev Files,  Up: Abbrevs

35.4 Looking Up and Expanding Abbreviations
===========================================

Abbrevs are usually expanded by certain interactive commands, including
‘self-insert-command’.  This section describes the subroutines used in
writing such commands, as well as the variables they use for
communication.

 -- Function: abbrev-symbol abbrev &optional table
     This function returns the symbol representing the abbrev named
     ABBREV.  It returns ‘nil’ if that abbrev is not defined.  The
     optional second argument TABLE is the abbrev table in which to look
     it up.  If TABLE is ‘nil’, this function tries first the current
     buffer’s local abbrev table, and second the global abbrev table.

 -- Function: abbrev-expansion abbrev &optional table
     This function returns the string that ABBREV would expand into (as
     defined by the abbrev tables used for the current buffer).  It
     returns ‘nil’ if ABBREV is not a valid abbrev.  The optional
     argument TABLE specifies the abbrev table to use, as in
     ‘abbrev-symbol’.

 -- Command: expand-abbrev
     This command expands the abbrev before point, if any.  If point
     does not follow an abbrev, this command does nothing.  To do the
     expansion, it calls the function that is the value of the
     ‘abbrev-expand-function’ variable, with no arguments, and returns
     whatever that function does.

     The default expansion function returns the abbrev symbol if it did
     expansion, and ‘nil’ otherwise.  If the abbrev symbol has a hook
     function that is a symbol whose ‘no-self-insert’ property is
     non-‘nil’, and if the hook function returns ‘nil’ as its value,
     then the default expansion function returns ‘nil’, even though
     expansion did occur.

 -- Function: abbrev-insert abbrev &optional name start end
     This function inserts the abbrev expansion of ‘abbrev’, replacing
     the text between ‘start’ and ‘end’.  If ‘start’ is omitted, it
     defaults to point.  ‘name’, if non-‘nil’, should be the name by
     which this abbrev was found (a string); it is used to figure out
     whether to adjust the capitalization of the expansion.  The
     function returns ‘abbrev’ if the abbrev was successfully inserted,
     otherwise it returns ‘nil’.

 -- Command: abbrev-prefix-mark &optional arg
     This command marks the current location of point as the beginning
     of an abbrev.  The next call to ‘expand-abbrev’ will use the text
     from here to point (where it is then) as the abbrev to expand,
     rather than using the previous word as usual.

     First, this command expands any abbrev before point, unless ARG is
     non-‘nil’.  (Interactively, ARG is the prefix argument.)  Then it
     inserts a hyphen before point, to indicate the start of the next
     abbrev to be expanded.  The actual expansion removes the hyphen.

 -- User Option: abbrev-all-caps
     When this is set non-‘nil’, an abbrev entered entirely in upper
     case is expanded using all upper case.  Otherwise, an abbrev
     entered entirely in upper case is expanded by capitalizing each
     word of the expansion.

 -- Variable: abbrev-start-location
     The value of this variable is a buffer position (an integer or a
     marker) for ‘expand-abbrev’ to use as the start of the next abbrev
     to be expanded.  The value can also be ‘nil’, which means to use
     the word before point instead.  ‘abbrev-start-location’ is set to
     ‘nil’ each time ‘expand-abbrev’ is called.  This variable is also
     set by ‘abbrev-prefix-mark’.

 -- Variable: abbrev-start-location-buffer
     The value of this variable is the buffer for which
     ‘abbrev-start-location’ has been set.  Trying to expand an abbrev
     in any other buffer clears ‘abbrev-start-location’.  This variable
     is set by ‘abbrev-prefix-mark’.

 -- Variable: last-abbrev
     This is the ‘abbrev-symbol’ of the most recent abbrev expanded.
     This information is left by ‘expand-abbrev’ for the sake of the
     ‘unexpand-abbrev’ command (*note Expanding Abbrevs:
     (emacs)Expanding Abbrevs.).

 -- Variable: last-abbrev-location
     This is the location of the most recent abbrev expanded.  This
     contains information left by ‘expand-abbrev’ for the sake of the
     ‘unexpand-abbrev’ command.

 -- Variable: last-abbrev-text
     This is the exact expansion text of the most recent abbrev
     expanded, after case conversion (if any).  Its value is ‘nil’ if
     the abbrev has already been unexpanded.  This contains information
     left by ‘expand-abbrev’ for the sake of the ‘unexpand-abbrev’
     command.

 -- Variable: abbrev-expand-function
     The value of this variable is a function that ‘expand-abbrev’ will
     call with no arguments to do the expansion.  The function can do
     anything it wants before and after performing the expansion.  It
     should return the abbrev symbol if expansion took place.

   The following sample code shows a simple use of
‘abbrev-expand-function’.  It assumes that ‘foo-mode’ is a mode for
editing certain files in which lines that start with ‘#’ are comments.
You want to use Text mode abbrevs for those lines.  The regular local
abbrev table, ‘foo-mode-abbrev-table’ is appropriate for all other
lines.  *Note Standard Abbrev Tables::, for the definitions of
‘local-abbrev-table’ and ‘text-mode-abbrev-table’.  *Note Advising
Functions::, for details of ‘add-function’.

     (defun foo-mode-abbrev-expand-function (expand)
       (if (not (save-excursion (forward-line 0) (eq (char-after) ?#)))
           ;; Performs normal expansion.
           (funcall expand)
         ;; We're inside a comment: use the text-mode abbrevs.
         (let ((local-abbrev-table text-mode-abbrev-table))
           (funcall expand))))

     (add-hook 'foo-mode-hook
               #'(lambda ()
                   (add-function :around (local 'abbrev-expand-function)
                                 #'foo-mode-abbrev-expand-function)))


File: elisp.info,  Node: Standard Abbrev Tables,  Next: Abbrev Properties,  Prev: Abbrev Expansion,  Up: Abbrevs

35.5 Standard Abbrev Tables
===========================

Here we list the variables that hold the abbrev tables for the preloaded
major modes of Emacs.

 -- Variable: global-abbrev-table
     This is the abbrev table for mode-independent abbrevs.  The abbrevs
     defined in it apply to all buffers.  Each buffer may also have a
     local abbrev table, whose abbrev definitions take precedence over
     those in the global table.

 -- Variable: local-abbrev-table
     The value of this buffer-local variable is the (mode-specific)
     abbreviation table of the current buffer.  It can also be a list of
     such tables.

 -- Variable: abbrev-minor-mode-table-alist
     The value of this variable is a list of elements of the form ‘(MODE
     . ABBREV-TABLE)’ where MODE is the name of a variable: if the
     variable is bound to a non-‘nil’ value, then the ABBREV-TABLE is
     active, otherwise it is ignored.  ABBREV-TABLE can also be a list
     of abbrev tables.

 -- Variable: fundamental-mode-abbrev-table
     This is the local abbrev table used in Fundamental mode; in other
     words, it is the local abbrev table in all buffers in Fundamental
     mode.

 -- Variable: text-mode-abbrev-table
     This is the local abbrev table used in Text mode.

 -- Variable: lisp-mode-abbrev-table
     This is the local abbrev table used in Lisp mode.  It is the parent
     of the local abbrev table used in Emacs Lisp mode.  *Note Abbrev
     Table Properties::.


File: elisp.info,  Node: Abbrev Properties,  Next: Abbrev Table Properties,  Prev: Standard Abbrev Tables,  Up: Abbrevs

35.6 Abbrev Properties
======================

Abbrevs have properties, some of which influence the way they work.  You
can provide them as arguments to ‘define-abbrev’, and manipulate them
with the following functions:

 -- Function: abbrev-put abbrev prop val
     Set the property PROP of ABBREV to value VAL.

 -- Function: abbrev-get abbrev prop
     Return the property PROP of ABBREV, or ‘nil’ if the abbrev has no
     such property.

   The following properties have special meanings:

‘:count’
     This property counts the number of times the abbrev has been
     expanded.  If not explicitly set, it is initialized to 0 by
     ‘define-abbrev’.

‘:system’
     If non-‘nil’, this property marks the abbrev as a system abbrev.
     Such abbrevs are not saved (*note Abbrev Files::).

‘:enable-function’
     If non-‘nil’, this property should be a function of no arguments
     which returns ‘nil’ if the abbrev should not be used and ‘t’
     otherwise.

‘:case-fixed’
     If non-‘nil’, this property indicates that the case of the abbrev’s
     name is significant and should only match a text with the same
     pattern of capitalization.  It also disables the code that modifies
     the capitalization of the expansion.


File: elisp.info,  Node: Abbrev Table Properties,  Prev: Abbrev Properties,  Up: Abbrevs

35.7 Abbrev Table Properties
============================

Like abbrevs, abbrev tables have properties, some of which influence the
way they work.  You can provide them as arguments to
‘define-abbrev-table’, and manipulate them with the functions:

 -- Function: abbrev-table-put table prop val
     Set the property PROP of abbrev table TABLE to value VAL.

 -- Function: abbrev-table-get table prop
     Return the property PROP of abbrev table TABLE, or ‘nil’ if the
     abbrev has no such property.

   The following properties have special meaning:

‘:enable-function’
     This is like the ‘:enable-function’ abbrev property except that it
     applies to all abbrevs in the table.  It is used before even trying
     to find the abbrev before point, so it can dynamically modify the
     abbrev table.

‘:case-fixed’
     This is like the ‘:case-fixed’ abbrev property except that it
     applies to all abbrevs in the table.

‘:regexp’
     If non-‘nil’, this property is a regular expression that indicates
     how to extract the name of the abbrev before point, before looking
     it up in the table.  When the regular expression matches before
     point, the abbrev name is expected to be in submatch 1.  If this
     property is ‘nil’, the default is to use ‘backward-word’ and
     ‘forward-word’ to find the name.  This property allows the use of
     abbrevs whose name contains characters of non-word syntax.

‘:parents’
     This property holds a list of tables from which to inherit other
     abbrevs.

‘:abbrev-table-modiff’
     This property holds a counter incremented each time a new abbrev is
     added to the table.


File: elisp.info,  Node: Processes,  Next: Display,  Prev: Abbrevs,  Up: Top

36 Processes
************

In the terminology of operating systems, a “process” is a space in which
a program can execute.  Emacs runs in a process.  Emacs Lisp programs
can invoke other programs in processes of their own.  These are called
“subprocesses” or “child processes” of the Emacs process, which is their
“parent process”.

   A subprocess of Emacs may be “synchronous” or “asynchronous”,
depending on how it is created.  When you create a synchronous
subprocess, the Lisp program waits for the subprocess to terminate
before continuing execution.  When you create an asynchronous
subprocess, it can run in parallel with the Lisp program.  This kind of
subprocess is represented within Emacs by a Lisp object which is also
called a “process”.  Lisp programs can use this object to communicate
with the subprocess or to control it.  For example, you can send
signals, obtain status information, receive output from the process, or
send input to it.

   In addition to processes that run programs, Lisp programs can open
connections of several types to devices or processes running on the same
machine or on other machines.  The supported connection types are: TCP
and UDP network connections, serial port connections, and pipe
connections.  Each such connection is also represented by a process
object.

 -- Function: processp object
     This function returns ‘t’ if OBJECT represents an Emacs process
     object, ‘nil’ otherwise.  The process object can represent a
     subprocess running a program or a connection of any supported type.

   In addition to subprocesses of the current Emacs session, you can
also access other processes running on your machine.  *Note System
Processes::.

* Menu:

* Subprocess Creation::      Functions that start subprocesses.
* Shell Arguments::          Quoting an argument to pass it to a shell.
* Synchronous Processes::    Details of using synchronous subprocesses.
* Asynchronous Processes::   Starting up an asynchronous subprocess.
* Deleting Processes::       Eliminating an asynchronous subprocess.
* Process Information::      Accessing run-status and other attributes.
* Input to Processes::       Sending input to an asynchronous subprocess.
* Signals to Processes::     Stopping, continuing or interrupting
                               an asynchronous subprocess.
* Output from Processes::    Collecting output from an asynchronous subprocess.
* Sentinels::                Sentinels run when process run-status changes.
* Query Before Exit::        Whether to query if exiting will kill a process.
* System Processes::         Accessing other processes running on your system.
* Transaction Queues::       Transaction-based communication with subprocesses.
* Network::                  Opening network connections.
* Network Servers::          Network servers let Emacs accept net connections.
* Datagrams::                UDP network connections.
* Low-Level Network::        Lower-level but more general function
                               to create connections and servers.
* Misc Network::             Additional relevant functions for net connections.
* Serial Ports::             Communicating with serial ports.
* Byte Packing::             Using bindat to pack and unpack binary data.


File: elisp.info,  Node: Subprocess Creation,  Next: Shell Arguments,  Up: Processes

36.1 Functions that Create Subprocesses
=======================================

There are three primitives that create a new subprocess in which to run
a program.  One of them, ‘make-process’, creates an asynchronous process
and returns a process object (*note Asynchronous Processes::).  The
other two, ‘call-process’ and ‘call-process-region’, create a
synchronous process and do not return a process object (*note
Synchronous Processes::).  There are various higher-level functions that
make use of these primitives to run particular types of process.

   Synchronous and asynchronous processes are explained in the following
sections.  Since the three functions are all called in a similar
fashion, their common arguments are described here.

   In all cases, the functions specify the program to be run.  An error
is signaled if the file is not found or cannot be executed.  If the file
name is relative, the variable ‘exec-path’ contains a list of
directories to search.  Emacs initializes ‘exec-path’ when it starts up,
based on the value of the environment variable ‘PATH’.  The standard
file name constructs, ‘~’, ‘.’, and ‘..’, are interpreted as usual in
‘exec-path’, but environment variable substitutions (‘$HOME’, etc.) are
not recognized; use ‘substitute-in-file-name’ to perform them (*note
File Name Expansion::).  ‘nil’ in this list refers to
‘default-directory’.

   Executing a program can also try adding suffixes to the specified
name:

 -- User Option: exec-suffixes
     This variable is a list of suffixes (strings) to try adding to the
     specified program file name.  The list should include ‘""’ if you
     want the name to be tried exactly as specified.  The default value
     is system-dependent.

   *Please note:* The argument PROGRAM contains only the name of the
program file; it may not contain any command-line arguments.  You must
use a separate argument, ARGS, to provide those, as described below.

   Each of the subprocess-creating functions has a BUFFER-OR-NAME
argument that specifies where the output from the program will go.  It
should be a buffer or a buffer name; if it is a buffer name, that will
create the buffer if it does not already exist.  It can also be ‘nil’,
which says to discard the output, unless a custom filter function
handles it.  (*Note Filter Functions::, and *note Read and Print::.)
Normally, you should avoid having multiple processes send output to the
same buffer because their output would be intermixed randomly.  For
synchronous processes, you can send the output to a file instead of a
buffer (and the corresponding argument is therefore more appropriately
called DESTINATION).  By default, both standard output and standard
error streams go to the same destination, but all the 3 primitives allow
optionally to direct the standard error stream to a different
destination.

   All three of the subprocess-creating functions allow to specify
command-line arguments for the process to run.  For ‘call-process’ and
‘call-process-region’, these come in the form of a ‘&rest’ argument,
ARGS.  For ‘make-process’, both the program to run and its command-line
arguments are specified as a list of strings.  The command-line
arguments must all be strings, and they are supplied to the program as
separate argument strings.  Wildcard characters and other shell
constructs have no special meanings in these strings, since the strings
are passed directly to the specified program.

   The subprocess inherits its environment from Emacs, but you can
specify overrides for it with ‘process-environment’.  *Note System
Environment::.  The subprocess gets its current directory from the value
of ‘default-directory’.

 -- Variable: exec-directory
     The value of this variable is a string, the name of a directory
     that contains programs that come with GNU Emacs and are intended
     for Emacs to invoke.  The program ‘movemail’ is an example of such
     a program; Rmail uses it to fetch new mail from an inbox.

 -- User Option: exec-path
     The value of this variable is a list of directories to search for
     programs to run in subprocesses.  Each element is either the name
     of a directory (i.e., a string), or ‘nil’, which stands for the
     default directory (which is the value of ‘default-directory’).
     *Note executable-find: Locating Files, for the details of this
     search.

     The value of ‘exec-path’ is used by ‘call-process’ and
     ‘start-process’ when the PROGRAM argument is not an absolute file
     name.

     Generally, you should not modify ‘exec-path’ directly.  Instead,
     ensure that your ‘PATH’ environment variable is set appropriately
     before starting Emacs.  Trying to modify ‘exec-path’ independently
     of ‘PATH’ can lead to confusing results.


File: elisp.info,  Node: Shell Arguments,  Next: Synchronous Processes,  Prev: Subprocess Creation,  Up: Processes

36.2 Shell Arguments
====================

Lisp programs sometimes need to run a shell and give it a command that
contains file names that were specified by the user.  These programs
ought to be able to support any valid file name.  But the shell gives
special treatment to certain characters, and if these characters occur
in the file name, they will confuse the shell.  To handle these
characters, use the function ‘shell-quote-argument’:

 -- Function: shell-quote-argument argument
     This function returns a string that represents, in shell syntax, an
     argument whose actual contents are ARGUMENT.  It should work
     reliably to concatenate the return value into a shell command and
     then pass it to a shell for execution.

     Precisely what this function does depends on your operating system.
     The function is designed to work with the syntax of your system’s
     standard shell; if you use an unusual shell, you will need to
     redefine this function.  *Note Security Considerations::.

          ;; This example shows the behavior on GNU and Unix systems.
          (shell-quote-argument "foo > bar")
               ⇒ "foo\\ \\>\\ bar"

          ;; This example shows the behavior on MS-DOS and MS-Windows.
          (shell-quote-argument "foo > bar")
               ⇒ "\"foo > bar\""

     Here’s an example of using ‘shell-quote-argument’ to construct a
     shell command:

          (concat "diff -u "
                  (shell-quote-argument oldfile)
                  " "
                  (shell-quote-argument newfile))

   The following two functions are useful for combining a list of
individual command-line argument strings into a single string, and
taking a string apart into a list of individual command-line arguments.
These functions are mainly intended for converting user input in the
minibuffer, a Lisp string, into a list of string arguments to be passed
to ‘make-process’, ‘call-process’ or ‘start-process’, or for converting
such lists of arguments into a single Lisp string to be presented in the
minibuffer or echo area.  Note that if a shell is involved (e.g., if
using ‘call-process-shell-command’), arguments should still be protected
by ‘shell-quote-argument’; ‘combine-and-quote-strings’ is _not_ intended
to protect special characters from shell evaluation.

 -- Function: split-string-and-unquote string &optional separators
     This function splits STRING into substrings at matches for the
     regular expression SEPARATORS, like ‘split-string’ does (*note
     Creating Strings::); in addition, it removes quoting from the
     substrings.  It then makes a list of the substrings and returns it.

     If SEPARATORS is omitted or ‘nil’, it defaults to ‘"\\s-+"’, which
     is a regular expression that matches one or more characters with
     whitespace syntax (*note Syntax Class Table::).

     This function supports two types of quoting: enclosing a whole
     string in double quotes ‘"..."’, and quoting individual characters
     with a backslash escape ‘\’.  The latter is also used in Lisp
     strings, so this function can handle those as well.

 -- Function: combine-and-quote-strings list-of-strings &optional
          separator
     This function concatenates LIST-OF-STRINGS into a single string,
     quoting each string as necessary.  It also sticks the SEPARATOR
     string between each pair of strings; if SEPARATOR is omitted or
     ‘nil’, it defaults to ‘" "’.  The return value is the resulting
     string.

     The strings in LIST-OF-STRINGS that need quoting are those that
     include SEPARATOR as their substring.  Quoting a string encloses it
     in double quotes ‘"..."’.  In the simplest case, if you are consing
     a command from the individual command-line arguments, every
     argument that includes embedded blanks will be quoted.


File: elisp.info,  Node: Synchronous Processes,  Next: Asynchronous Processes,  Prev: Shell Arguments,  Up: Processes

36.3 Creating a Synchronous Process
===================================

After a “synchronous process” is created, Emacs waits for the process to
terminate before continuing.  Starting Dired on GNU or Unix(1) is an
example of this: it runs ‘ls’ in a synchronous process, then modifies
the output slightly.  Because the process is synchronous, the entire
directory listing arrives in the buffer before Emacs tries to do
anything with it.

   While Emacs waits for the synchronous subprocess to terminate, the
user can quit by typing ‘C-g’.  The first ‘C-g’ tries to kill the
subprocess with a ‘SIGINT’ signal; but it waits until the subprocess
actually terminates before quitting.  If during that time the user types
another ‘C-g’, that kills the subprocess instantly with ‘SIGKILL’ and
quits immediately (except on MS-DOS, where killing other processes
doesn’t work).  *Note Quitting::.

   The synchronous subprocess functions return an indication of how the
process terminated.

   The output from a synchronous subprocess is generally decoded using a
coding system, much like text read from a file.  The input sent to a
subprocess by ‘call-process-region’ is encoded using a coding system,
much like text written into a file.  *Note Coding Systems::.

 -- Function: call-process program &optional infile destination display
          &rest args
     This function calls PROGRAM and waits for it to finish.

     The current working directory of the subprocess is set to the
     current buffer’s value of ‘default-directory’ if that is local (as
     determined by ‘unhandled-file-name-directory’), or "~" otherwise.
     If you want to run a process in a remote directory use
     ‘process-file’.

     The standard input for the new process comes from file INFILE if
     INFILE is not ‘nil’, and from the null device otherwise.  The
     argument DESTINATION says where to put the process output.  Here
     are the possibilities:

     a buffer
          Insert the output in that buffer, before point.  This includes
          both the standard output stream and the standard error stream
          of the process.

     a buffer name (a string)
          Insert the output in a buffer with that name, before point.

     ‘t’
          Insert the output in the current buffer, before point.

     ‘nil’
          Discard the output.

     0
          Discard the output, and return ‘nil’ immediately without
          waiting for the subprocess to finish.

          In this case, the process is not truly synchronous, since it
          can run in parallel with Emacs; but you can think of it as
          synchronous in that Emacs is essentially finished with the
          subprocess as soon as this function returns.

          MS-DOS doesn’t support asynchronous subprocesses, so this
          option doesn’t work there.

     ‘(:file FILE-NAME)’
          Send the output to the file name specified, overwriting it if
          it already exists.

     ‘(REAL-DESTINATION ERROR-DESTINATION)’
          Keep the standard output stream separate from the standard
          error stream; deal with the ordinary output as specified by
          REAL-DESTINATION, and dispose of the error output according to
          ERROR-DESTINATION.  If ERROR-DESTINATION is ‘nil’, that means
          to discard the error output, ‘t’ means mix it with the
          ordinary output, and a string specifies a file name to
          redirect error output into.

          You can’t directly specify a buffer to put the error output
          in; that is too difficult to implement.  But you can achieve
          this result by sending the error output to a temporary file
          and then inserting the file into a buffer when the subprocess
          finishes.

     If DISPLAY is non-‘nil’, then ‘call-process’ redisplays the buffer
     as output is inserted.  (However, if the coding system chosen for
     decoding output is ‘undecided’, meaning deduce the encoding from
     the actual data, then redisplay sometimes cannot continue once
     non-ASCII characters are encountered.  There are fundamental
     reasons why it is hard to fix this; see *note Output from
     Processes::.)

     Otherwise the function ‘call-process’ does no redisplay, and the
     results become visible on the screen only when Emacs redisplays
     that buffer in the normal course of events.

     The remaining arguments, ARGS, are strings that specify command
     line arguments for the program.  Each string is passed to PROGRAM
     as a separate argument.

     The value returned by ‘call-process’ (unless you told it not to
     wait) indicates the reason for process termination.  A number gives
     the exit status of the subprocess; 0 means success, and any other
     value means failure.  If the process terminated with a signal,
     ‘call-process’ returns a string describing the signal.  If you told
     ‘call-process’ not to wait, it returns ‘nil’.

     In the examples below, the buffer ‘foo’ is current.

          (call-process "pwd" nil t)
               ⇒ 0

          ---------- Buffer: foo ----------
          /home/lewis/manual
          ---------- Buffer: foo ----------

          (call-process "grep" nil "bar" nil "lewis" "/etc/passwd")
               ⇒ 0

          ---------- Buffer: bar ----------
          lewis:x:1001:1001:Bil Lewis,,,,:/home/lewis:/bin/bash

          ---------- Buffer: bar ----------

     Here is an example of the use of ‘call-process’, as used to be
     found in the definition of the ‘insert-directory’ function:

          (call-process insert-directory-program nil t nil switches
                        (if full-directory-p
                            (concat (file-name-as-directory file) ".")
                          file))

 -- Function: process-file program &optional infile buffer display &rest
          args
     This function processes files synchronously in a separate process.
     It is similar to ‘call-process’, but may invoke a file handler
     based on the value of the variable ‘default-directory’, which
     specifies the current working directory of the subprocess.

     The arguments are handled in almost the same way as for
     ‘call-process’, with the following differences:

     Some file handlers may not support all combinations and forms of
     the arguments INFILE, BUFFER, and DISPLAY.  For example, some file
     handlers might behave as if DISPLAY were ‘nil’, regardless of the
     value actually passed.  As another example, some file handlers
     might not support separating standard output and error output by
     way of the BUFFER argument.

     If a file handler is invoked, it determines the program to run
     based on the first argument PROGRAM.  For instance, suppose that a
     handler for remote files is invoked.  Then the path that is used
     for searching for the program might be different from ‘exec-path’.

     The second argument INFILE may invoke a file handler.  The file
     handler could be different from the handler chosen for the
     ‘process-file’ function itself.  (For example, ‘default-directory’
     could be on one remote host, and INFILE on a different remote host.
     Or ‘default-directory’ could be non-special, whereas INFILE is on a
     remote host.)

     If BUFFER is a list of the form ‘(REAL-DESTINATION
     ERROR-DESTINATION)’, and ERROR-DESTINATION names a file, then the
     same remarks as for INFILE apply.

     The remaining arguments (ARGS) will be passed to the process
     verbatim.  Emacs is not involved in processing file names that are
     present in ARGS.  To avoid confusion, it may be best to avoid
     absolute file names in ARGS, but rather to specify all file names
     as relative to ‘default-directory’.  The function
     ‘file-relative-name’ is useful for constructing such relative file
     names.

 -- Variable: process-file-side-effects
     This variable indicates whether a call of ‘process-file’ changes
     remote files.

     By default, this variable is always set to ‘t’, meaning that a call
     of ‘process-file’ could potentially change any file on a remote
     host.  When set to ‘nil’, a file handler could optimize its
     behavior with respect to remote file attribute caching.

     You should only ever change this variable with a let-binding; never
     with ‘setq’.

 -- Function: call-process-region start end program &optional delete
          destination display &rest args
     This function sends the text from START to END as standard input to
     a process running PROGRAM.  It deletes the text sent if DELETE is
     non-‘nil’; this is useful when DESTINATION is ‘t’, to insert the
     output in the current buffer in place of the input.

     The arguments DESTINATION and DISPLAY control what to do with the
     output from the subprocess, and whether to update the display as it
     comes in.  For details, see the description of ‘call-process’,
     above.  If DESTINATION is the integer 0, ‘call-process-region’
     discards the output and returns ‘nil’ immediately, without waiting
     for the subprocess to finish (this only works if asynchronous
     subprocesses are supported; i.e., not on MS-DOS).

     The remaining arguments, ARGS, are strings that specify command
     line arguments for the program.

     The return value of ‘call-process-region’ is just like that of
     ‘call-process’: ‘nil’ if you told it to return without waiting;
     otherwise, a number or string which indicates how the subprocess
     terminated.

     In the following example, we use ‘call-process-region’ to run the
     ‘cat’ utility, with standard input being the first five characters
     in buffer ‘foo’ (the word ‘input’).  ‘cat’ copies its standard
     input into its standard output.  Since the argument DESTINATION is
     ‘t’, this output is inserted in the current buffer.

          ---------- Buffer: foo ----------
          input★
          ---------- Buffer: foo ----------

          (call-process-region 1 6 "cat" nil t)
               ⇒ 0

          ---------- Buffer: foo ----------
          inputinput★
          ---------- Buffer: foo ----------

     For example, the ‘shell-command-on-region’ command uses
     ‘call-process-region’ in a manner similar to this:

          (call-process-region
           start end
           shell-file-name      ; name of program
           nil                  ; do not delete region
           buffer               ; send output to ‘buffer’
           nil                  ; no redisplay during output
           "-c" command)        ; arguments for the shell

 -- Function: call-process-shell-command command &optional infile
          destination display
     This function executes the shell command COMMAND synchronously.
     The other arguments are handled as in ‘call-process’.  An old
     calling convention allowed passing any number of additional
     arguments after DISPLAY, which were concatenated to COMMAND; this
     is still supported, but strongly discouraged.

 -- Function: process-file-shell-command command &optional infile
          destination display
     This function is like ‘call-process-shell-command’, but uses
     ‘process-file’ internally.  Depending on ‘default-directory’,
     COMMAND can be executed also on remote hosts.  An old calling
     convention allowed passing any number of additional arguments after
     DISPLAY, which were concatenated to COMMAND; this is still
     supported, but strongly discouraged.

 -- Function: shell-command-to-string command
     This function executes COMMAND (a string) as a shell command, then
     returns the command’s output as a string.

 -- Function: process-lines program &rest args
     This function runs PROGRAM, waits for it to finish, and returns its
     output as a list of strings.  Each string in the list holds a
     single line of text output by the program; the end-of-line
     characters are stripped from each line.  The arguments beyond
     PROGRAM, ARGS, are strings that specify command-line arguments with
     which to run the program.

     If PROGRAM exits with a non-zero exit status, this function signals
     an error.

     This function works by calling ‘call-process’, so program output is
     decoded in the same way as for ‘call-process’.

   ---------- Footnotes ----------

   (1) On other systems, Emacs uses a Lisp emulation of ‘ls’; see *note
Contents of Directories::.


File: elisp.info,  Node: Asynchronous Processes,  Next: Deleting Processes,  Prev: Synchronous Processes,  Up: Processes

36.4 Creating an Asynchronous Process
=====================================

In this section, we describe how to create an “asynchronous process”.
After an asynchronous process is created, it runs in parallel with
Emacs, and Emacs can communicate with it using the functions described
in the following sections (*note Input to Processes::, and *note Output
from Processes::).  Note that process communication is only partially
asynchronous: Emacs sends data to the process only when certain
functions are called, and Emacs accepts data from the process only while
waiting for input or for a time delay.

   An asynchronous process is controlled either via a “pty”
(pseudo-terminal) or a “pipe”.  The choice of pty or pipe is made when
creating the process, by default based on the value of the variable
‘process-connection-type’ (see below).  If available, ptys are usually
preferable for processes visible to the user, as in Shell mode, because
they allow for job control (‘C-c’, ‘C-z’, etc.) between the process and
its children, and because interactive programs treat ptys as terminal
devices, whereas pipes don’t support these features.  However, for
subprocesses used by Lisp programs for internal purposes, it is often
better to use a pipe, because pipes are more efficient, and because they
are immune to stray character injections that ptys introduce for large
(around 500 byte) messages.  Also, the total number of ptys is limited
on many systems and it is good not to waste them.

 -- Function: make-process &rest args
     This function is the basic low-level primitive for starting
     asynchronous subprocesses.  It returns a process object
     representing the subprocess.  Compared to the more high-level
     ‘start-process’, described below, it takes keyword arguments, is
     more flexible, and allows to specify process filters and sentinels
     in a single call.

     The arguments ARGS are a list of keyword/argument pairs.  Omitting
     a keyword is always equivalent to specifying it with value ‘nil’.
     Here are the meaningful keywords:

     :name NAME
          Use the string NAME as the process name; if a process with
          this name already exists, then NAME is modified (by appending
          ‘<1>’, etc.) to be unique.

     :buffer BUFFER
          Use BUFFER as the process buffer.  If the value is ‘nil’, the
          subprocess is not associated with any buffer.

     :command COMMAND
          Use COMMAND as the command line of the process.  The value
          should be a list starting with the program’s executable file
          name, followed by strings to give to the program as its
          arguments.  If the first element of the list is ‘nil’, Emacs
          opens a new pseudoterminal (pty) and associates its input and
          output with BUFFER, without actually running any program; the
          rest of the list elements are ignored in that case.

     :coding CODING
          If CODING is a symbol, it specifies the coding system to be
          used for both reading and writing of data from and to the
          connection.  If CODING is a cons cell ‘(DECODING . ENCODING)’,
          then DECODING will be used for reading and ENCODING for
          writing.  The coding system used for encoding the data written
          to the program is also used for encoding the command-line
          arguments (but not the program itself, whose file name is
          encoded as any other file name; *note file-name-coding-system:
          Encoding and I/O.).

          If CODING is ‘nil’, the default rules for finding the coding
          system will apply.  *Note Default Coding Systems::.

     :connection-type TYPE
          Initialize the type of device used to communicate with the
          subprocess.  Possible values are ‘pty’ to use a pty, ‘pipe’ to
          use a pipe, or ‘nil’ to use the default derived from the value
          of the ‘process-connection-type’ variable.  This parameter and
          the value of ‘process-connection-type’ are ignored if a
          non-‘nil’ value is specified for the ‘:stderr’ parameter; in
          that case, the type will always be ‘pipe’.

     :noquery QUERY-FLAG
          Initialize the process query flag to QUERY-FLAG.  *Note Query
          Before Exit::.

     :stop STOPPED
          If STOPPED is non-‘nil’, start the process in the stopped
          state.

     :filter FILTER
          Initialize the process filter to FILTER.  If not specified, a
          default filter will be provided, which can be overridden
          later.  *Note Filter Functions::.

     :sentinel SENTINEL
          Initialize the process sentinel to SENTINEL.  If not
          specified, a default sentinel will be used, which can be
          overridden later.  *Note Sentinels::.

     :stderr STDERR
          Associate STDERR with the standard error of the process.  A
          non-‘nil’ value should be either a buffer or a pipe process
          created with ‘make-pipe-process’, described below.

     The original argument list, modified with the actual connection
     information, is available via the ‘process-contact’ function.

     The current working directory of the subprocess is set to the
     current buffer’s value of ‘default-directory’ if that is local (as
     determined by ‘unhandled-file-name-directory’), or "~" otherwise.
     If you want to run a process in a remote direcotry use
     ‘start-file-process’.

 -- Function: make-pipe-process &rest args
     This function creates a bidirectional pipe which can be attached to
     a child process.  This is useful with the ‘:stderr’ keyword of
     ‘make-process’.  The function returns a process object.

     The arguments ARGS are a list of keyword/argument pairs.  Omitting
     a keyword is always equivalent to specifying it with value ‘nil’.

     Here are the meaningful keywords:

     :name NAME
          Use the string NAME as the process name.  As with
          ‘make-process’, it is modified if necessary to make it unique.

     :buffer BUFFER
          Use BUFFER as the process buffer.

     :coding CODING
          If CODING is a symbol, it specifies the coding system to be
          used for both reading and writing of data from and to the
          connection.  If CODING is a cons cell ‘(DECODING . ENCODING)’,
          then DECODING will be used for reading and ENCODING for
          writing.

          If CODING is ‘nil’, the default rules for finding the coding
          system will apply.  *Note Default Coding Systems::.

     :noquery QUERY-FLAG
          Initialize the process query flag to QUERY-FLAG.  *Note Query
          Before Exit::.

     :stop STOPPED
          If STOPPED is non-‘nil’, start the process in the stopped
          state.

     :filter FILTER
          Initialize the process filter to FILTER.  If not specified, a
          default filter will be provided, which can be changed later.
          *Note Filter Functions::.

     :sentinel SENTINEL
          Initialize the process sentinel to SENTINEL.  If not
          specified, a default sentinel will be used, which can be
          changed later.  *Note Sentinels::.

     The original argument list, modified with the actual connection
     information, is available via the ‘process-contact’ function.

 -- Function: start-process name buffer-or-name program &rest args
     This function is a higher-level wrapper around ‘make-process’,
     exposing an interface that is similar to ‘call-process’.  It
     creates a new asynchronous subprocess and starts the specified
     PROGRAM running in it.  It returns a process object that stands for
     the new subprocess in Lisp.  The argument NAME specifies the name
     for the process object; as with ‘make-process’, it is modified if
     necessary to make it unique.  The buffer BUFFER-OR-NAME is the
     buffer to associate with the process.

     If PROGRAM is ‘nil’, Emacs opens a new pseudoterminal (pty) and
     associates its input and output with BUFFER-OR-NAME, without
     creating a subprocess.  In that case, the remaining arguments ARGS
     are ignored.

     The rest of ARGS are strings that specify command line arguments
     for the subprocess.

     In the example below, the first process is started and runs
     (rather, sleeps) for 100 seconds (the output buffer ‘foo’ is
     created immediately).  Meanwhile, the second process is started,
     and given the name ‘my-process<1>’ for the sake of uniqueness.  It
     inserts the directory listing at the end of the buffer ‘foo’,
     before the first process finishes.  Then it finishes, and a message
     to that effect is inserted in the buffer.  Much later, the first
     process finishes, and another message is inserted in the buffer for
     it.

          (start-process "my-process" "foo" "sleep" "100")
               ⇒ #<process my-process>

          (start-process "my-process" "foo" "ls" "-l" "/bin")
               ⇒ #<process my-process<1>>

          ---------- Buffer: foo ----------
          total 8336
          -rwxr-xr-x 1 root root 971384 Mar 30 10:14 bash
          -rwxr-xr-x 1 root root 146920 Jul  5  2011 bsd-csh
          ...
          -rwxr-xr-x 1 root root 696880 Feb 28 15:55 zsh4

          Process my-process<1> finished

          Process my-process finished
          ---------- Buffer: foo ----------

 -- Function: start-file-process name buffer-or-name program &rest args
     Like ‘start-process’, this function starts a new asynchronous
     subprocess running PROGRAM in it, and returns its process object.

     The difference from ‘start-process’ is that this function may
     invoke a file handler based on the value of ‘default-directory’.
     This handler ought to run PROGRAM, perhaps on the local host,
     perhaps on a remote host that corresponds to ‘default-directory’.
     In the latter case, the local part of ‘default-directory’ becomes
     the working directory of the process.

     This function does not try to invoke file name handlers for PROGRAM
     or for the rest of ARGS.

     Depending on the implementation of the file handler, it might not
     be possible to apply ‘process-filter’ or ‘process-sentinel’ to the
     resulting process object.  *Note Filter Functions::, and *note
     Sentinels::.

     Some file handlers may not support ‘start-file-process’ (for
     example the function ‘ange-ftp-hook-function’).  In such cases,
     this function does nothing and returns ‘nil’.

 -- Function: start-process-shell-command name buffer-or-name command
     This function is like ‘start-process’, except that it uses a shell
     to execute the specified COMMAND.  The argument COMMAND is a shell
     command string.  The variable ‘shell-file-name’ specifies which
     shell to use.

     The point of running a program through the shell, rather than
     directly with ‘make-process’ or ‘start-process’, is so that you can
     employ shell features such as wildcards in the arguments.  It
     follows that if you include any arbitrary user-specified arguments
     in the command, you should quote them with ‘shell-quote-argument’
     first, so that any special shell characters do _not_ have their
     special shell meanings.  *Note Shell Arguments::.  Of course, when
     executing commands based on user input you should also consider the
     security implications.

 -- Function: start-file-process-shell-command name buffer-or-name
          command
     This function is like ‘start-process-shell-command’, but uses
     ‘start-file-process’ internally.  Because of this, COMMAND can also
     be executed on remote hosts, depending on ‘default-directory’.

 -- Variable: process-connection-type
     This variable controls the type of device used to communicate with
     asynchronous subprocesses.  If it is non-‘nil’, then ptys are used,
     when available.  Otherwise, pipes are used.

     The value of ‘process-connection-type’ takes effect when
     ‘make-process’ or ‘start-process’ is called.  So you can specify
     how to communicate with one subprocess by binding the variable
     around the call to these functions.

     Note that the value of this variable is ignored when ‘make-process’
     is called with a non-‘nil’ value of the ‘:stderr’ parameter; in
     that case, Emacs will communicate with the process using pipes.

          (let ((process-connection-type nil))  ; use a pipe
            (start-process ...))

     To determine whether a given subprocess actually got a pipe or a
     pty, use the function ‘process-tty-name’ (*note Process
     Information::).


File: elisp.info,  Node: Deleting Processes,  Next: Process Information,  Prev: Asynchronous Processes,  Up: Processes

36.5 Deleting Processes
=======================

“Deleting a process” disconnects Emacs immediately from the subprocess.
Processes are deleted automatically after they terminate, but not
necessarily right away.  You can delete a process explicitly at any
time.  If you explicitly delete a terminated process before it is
deleted automatically, no harm results.  Deleting a running process
sends a signal to terminate it (and its child processes, if any), and
calls the process sentinel.  *Note Sentinels::.

   When a process is deleted, the process object itself continues to
exist as long as other Lisp objects point to it.  All the Lisp
primitives that work on process objects accept deleted processes, but
those that do I/O or send signals will report an error.  The process
mark continues to point to the same place as before, usually into a
buffer where output from the process was being inserted.

 -- User Option: delete-exited-processes
     This variable controls automatic deletion of processes that have
     terminated (due to calling ‘exit’ or to a signal).  If it is ‘nil’,
     then they continue to exist until the user runs ‘list-processes’.
     Otherwise, they are deleted immediately after they exit.

 -- Function: delete-process process
     This function deletes a process, killing it with a ‘SIGKILL’ signal
     if the process was running a program.  The argument may be a
     process, the name of a process, a buffer, or the name of a buffer.
     (A buffer or buffer-name stands for the process that
     ‘get-buffer-process’ returns.)  Calling ‘delete-process’ on a
     running process terminates it, updates the process status, and runs
     the sentinel immediately.  If the process has already terminated,
     calling ‘delete-process’ has no effect on its status, or on the
     running of its sentinel (which will happen sooner or later).

     If the process object represents a network, serial, or pipe
     connection, its status changes to ‘closed’; otherwise, it changes
     to ‘signal’, unless the process already exited.  *Note
     process-status: Process Information.

          (delete-process "*shell*")
               ⇒ nil


File: elisp.info,  Node: Process Information,  Next: Input to Processes,  Prev: Deleting Processes,  Up: Processes

36.6 Process Information
========================

Several functions return information about processes.

 -- Command: list-processes &optional query-only buffer
     This command displays a listing of all living processes.  In
     addition, it finally deletes any process whose status was ‘Exited’
     or ‘Signaled’.  It returns ‘nil’.

     The processes are shown in a buffer named ‘*Process List*’ (unless
     you specify otherwise using the optional argument BUFFER), whose
     major mode is Process Menu mode.

     If QUERY-ONLY is non-‘nil’, it only lists processes whose query
     flag is non-‘nil’.  *Note Query Before Exit::.

 -- Function: process-list
     This function returns a list of all processes that have not been
     deleted.

          (process-list)
               ⇒ (#<process display-time> #<process shell>)

 -- Function: get-process name
     This function returns the process named NAME (a string), or ‘nil’
     if there is none.  The argument NAME can also be a process object,
     in which case it is returned.

          (get-process "shell")
               ⇒ #<process shell>

 -- Function: process-command process
     This function returns the command that was executed to start
     PROCESS.  This is a list of strings, the first string being the
     program executed and the rest of the strings being the arguments
     that were given to the program.  For a network, serial, or pipe
     connection, this is either ‘nil’, which means the process is
     running or ‘t’ (process is stopped).

          (process-command (get-process "shell"))
               ⇒ ("bash" "-i")

 -- Function: process-contact process &optional key
     This function returns information about how a network, a serial, or
     a pipe connection was set up.  When KEY is ‘nil’, it returns
     ‘(HOSTNAME SERVICE)’ for a network connection, ‘(PORT SPEED)’ for a
     serial connection, and ‘t’ for a pipe connection.  For an ordinary
     child process, this function always returns ‘t’ when called with a
     ‘nil’ KEY.

     If KEY is ‘t’, the value is the complete status information for the
     connection, server, serial port, or pipe; that is, the list of
     keywords and values specified in ‘make-network-process’,
     ‘make-serial-process’, or ‘make-pipe-process’, except that some of
     the values represent the current status instead of what you
     specified.

     For a network process, the values include (see
     ‘make-network-process’ for a complete list):

     ‘:buffer’
          The associated value is the process buffer.
     ‘:filter’
          The associated value is the process filter function.  *Note
          Filter Functions::.
     ‘:sentinel’
          The associated value is the process sentinel function.  *Note
          Sentinels::.
     ‘:remote’
          In a connection, the address in internal format of the remote
          peer.
     ‘:local’
          The local address, in internal format.
     ‘:service’
          In a server, if you specified ‘t’ for SERVICE, this value is
          the actual port number.

     ‘:local’ and ‘:remote’ are included even if they were not specified
     explicitly in ‘make-network-process’.

     For a serial connection, see ‘make-serial-process’ and
     ‘serial-process-configure’ for the list of keys.  For a pipe
     connection, see ‘make-pipe-process’ for the list of keys.

     If KEY is a keyword, the function returns the value corresponding
     to that keyword.

 -- Function: process-id process
     This function returns the PID of PROCESS.  This is an integral
     number that distinguishes the process PROCESS from all other
     processes running on the same computer at the current time.  The
     PID of a process is chosen by the operating system kernel when the
     process is started and remains constant as long as the process
     exists.  For network, serial, and pipe connections, this function
     returns ‘nil’.

 -- Function: process-name process
     This function returns the name of PROCESS, as a string.

 -- Function: process-status process-name
     This function returns the status of PROCESS-NAME as a symbol.  The
     argument PROCESS-NAME must be a process, a buffer, or a process
     name (a string).

     The possible values for an actual subprocess are:

     ‘run’
          for a process that is running.
     ‘stop’
          for a process that is stopped but continuable.
     ‘exit’
          for a process that has exited.
     ‘signal’
          for a process that has received a fatal signal.
     ‘open’
          for a network, serial, or pipe connection that is open.
     ‘closed’
          for a network, serial, or pipe connection that is closed.
          Once a connection is closed, you cannot reopen it, though you
          might be able to open a new connection to the same place.
     ‘connect’
          for a non-blocking connection that is waiting to complete.
     ‘failed’
          for a non-blocking connection that has failed to complete.
     ‘listen’
          for a network server that is listening.
     ‘nil’
          if PROCESS-NAME is not the name of an existing process.

          (process-status (get-buffer "*shell*"))
               ⇒ run

     For a network, serial, or pipe connection, ‘process-status’ returns
     one of the symbols ‘open’, ‘stop’, or ‘closed’.  The latter means
     that the other side closed the connection, or Emacs did
     ‘delete-process’.  The value ‘stop’ means that ‘stop-process’ was
     called on the connection.

 -- Function: process-live-p process
     This function returns non-‘nil’ if PROCESS is alive.  A process is
     considered alive if its status is ‘run’, ‘open’, ‘listen’,
     ‘connect’ or ‘stop’.

 -- Function: process-type process
     This function returns the symbol ‘network’ for a network connection
     or server, ‘serial’ for a serial port connection, ‘pipe’ for a pipe
     connection, or ‘real’ for a subprocess created for running a
     program.

 -- Function: process-exit-status process
     This function returns the exit status of PROCESS or the signal
     number that killed it.  (Use the result of ‘process-status’ to
     determine which of those it is.)  If PROCESS has not yet
     terminated, the value is 0.  For network, serial, and pipe
     connections that are already closed, the value is either 0 or 256,
     depending on whether the connection was closed normally or
     abnormally.

 -- Function: process-tty-name process
     This function returns the terminal name that PROCESS is using for
     its communication with Emacs—or ‘nil’ if it is using pipes instead
     of a pty (see ‘process-connection-type’ in *note Asynchronous
     Processes::).  If PROCESS represents a program running on a remote
     host, the terminal name used by that program on the remote host is
     provided as process property ‘remote-tty’.  If PROCESS represents a
     network, serial, or pipe connection, the value is ‘nil’.

 -- Function: process-coding-system process
     This function returns a cons cell ‘(DECODE . ENCODE)’, describing
     the coding systems in use for decoding output from, and encoding
     input to, PROCESS (*note Coding Systems::).

 -- Function: set-process-coding-system process &optional
          decoding-system encoding-system
     This function specifies the coding systems to use for subsequent
     output from and input to PROCESS.  It will use DECODING-SYSTEM to
     decode subprocess output, and ENCODING-SYSTEM to encode subprocess
     input.

   Every process also has a property list that you can use to store
miscellaneous values associated with the process.

 -- Function: process-get process propname
     This function returns the value of the PROPNAME property of
     PROCESS.

 -- Function: process-put process propname value
     This function sets the value of the PROPNAME property of PROCESS to
     VALUE.

 -- Function: process-plist process
     This function returns the process plist of PROCESS.

 -- Function: set-process-plist process plist
     This function sets the process plist of PROCESS to PLIST.


File: elisp.info,  Node: Input to Processes,  Next: Signals to Processes,  Prev: Process Information,  Up: Processes

36.7 Sending Input to Processes
===============================

Asynchronous subprocesses receive input when it is sent to them by
Emacs, which is done with the functions in this section.  You must
specify the process to send input to, and the input data to send.  If
the subprocess runs a program, the data appears on the standard input of
that program; for connections, the data is sent to the connected device
or program.

   Some operating systems have limited space for buffered input in a
pty.  On these systems, Emacs sends an EOF periodically amidst the other
characters, to force them through.  For most programs, these EOFs do no
harm.

   Subprocess input is normally encoded using a coding system before the
subprocess receives it, much like text written into a file.  You can use
‘set-process-coding-system’ to specify which coding system to use (*note
Process Information::).  Otherwise, the coding system comes from
‘coding-system-for-write’, if that is non-‘nil’; or else from the
defaulting mechanism (*note Default Coding Systems::).

   Sometimes the system is unable to accept input for that process,
because the input buffer is full.  When this happens, the send functions
wait a short while, accepting output from subprocesses, and then try
again.  This gives the subprocess a chance to read more of its pending
input and make space in the buffer.  It also allows filters, sentinels
and timers to run—so take account of that in writing your code.

   In these functions, the PROCESS argument can be a process or the name
of a process, or a buffer or buffer name (which stands for a process via
‘get-buffer-process’).  ‘nil’ means the current buffer’s process.

 -- Function: process-send-string process string
     This function sends PROCESS the contents of STRING as standard
     input.  It returns ‘nil’.  For example, to make a Shell buffer list
     files:

          (process-send-string "shell<1>" "ls\n")
               ⇒ nil

 -- Function: process-send-region process start end
     This function sends the text in the region defined by START and END
     as standard input to PROCESS.

     An error is signaled unless both START and END are integers or
     markers that indicate positions in the current buffer.  (It is
     unimportant which number is larger.)

 -- Function: process-send-eof &optional process
     This function makes PROCESS see an end-of-file in its input.  The
     EOF comes after any text already sent to it.  The function returns
     PROCESS.

          (process-send-eof "shell")
               ⇒ "shell"

 -- Function: process-running-child-p &optional process
     This function will tell you whether a PROCESS, which must not be a
     connection but a real subprocess, has given control of its terminal
     to a child process of its own.  If this is true, the function
     returns the numeric ID of the foreground process group of PROCESS;
     it returns ‘nil’ if Emacs can be certain that this is not so.  The
     value is ‘t’ if Emacs cannot tell whether this is true.  This
     function signals an error if PROCESS is a network, serial, or pipe
     connection, or is the subprocess is not active.


File: elisp.info,  Node: Signals to Processes,  Next: Output from Processes,  Prev: Input to Processes,  Up: Processes

36.8 Sending Signals to Processes
=================================

“Sending a signal” to a subprocess is a way of interrupting its
activities.  There are several different signals, each with its own
meaning.  The set of signals and their names is defined by the operating
system.  For example, the signal ‘SIGINT’ means that the user has typed
‘C-c’, or that some analogous thing has happened.

   Each signal has a standard effect on the subprocess.  Most signals
kill the subprocess, but some stop (or resume) execution instead.  Most
signals can optionally be handled by programs; if the program handles
the signal, then we can say nothing in general about its effects.

   You can send signals explicitly by calling the functions in this
section.  Emacs also sends signals automatically at certain times:
killing a buffer sends a ‘SIGHUP’ signal to all its associated
processes; killing Emacs sends a ‘SIGHUP’ signal to all remaining
processes.  (‘SIGHUP’ is a signal that usually indicates that the user
“hung up the phone”, i.e., disconnected.)

   Each of the signal-sending functions takes two optional arguments:
PROCESS and CURRENT-GROUP.

   The argument PROCESS must be either a process, a process name, a
buffer, a buffer name, or ‘nil’.  A buffer or buffer name stands for a
process through ‘get-buffer-process’.  ‘nil’ stands for the process
associated with the current buffer.  Except with ‘stop-process’ and
‘continue-process’, an error is signaled if PROCESS does not identify an
active process, or if it represents a network, serial, or pipe
connection.

   The argument CURRENT-GROUP is a flag that makes a difference when you
are running a job-control shell as an Emacs subprocess.  If it is
non-‘nil’, then the signal is sent to the current process-group of the
terminal that Emacs uses to communicate with the subprocess.  If the
process is a job-control shell, this means the shell’s current subjob.
If CURRENT-GROUP is ‘nil’, the signal is sent to the process group of
the immediate subprocess of Emacs.  If the subprocess is a job-control
shell, this is the shell itself.  If CURRENT-GROUP is ‘lambda’, the
signal is sent to the process-group that owns the terminal, but only if
it is not the shell itself.

   The flag CURRENT-GROUP has no effect when a pipe is used to
communicate with the subprocess, because the operating system does not
support the distinction in the case of pipes.  For the same reason,
job-control shells won’t work when a pipe is used.  See
‘process-connection-type’ in *note Asynchronous Processes::.

 -- Function: interrupt-process &optional process current-group
     This function interrupts the process PROCESS by sending the signal
     ‘SIGINT’.  Outside of Emacs, typing the interrupt character
     (normally ‘C-c’ on some systems, and <DEL> on others) sends this
     signal.  When the argument CURRENT-GROUP is non-‘nil’, you can
     think of this function as typing ‘C-c’ on the terminal by which
     Emacs talks to the subprocess.

 -- Function: kill-process &optional process current-group
     This function kills the process PROCESS by sending the signal
     ‘SIGKILL’.  This signal kills the subprocess immediately, and
     cannot be handled by the subprocess.

 -- Function: quit-process &optional process current-group
     This function sends the signal ‘SIGQUIT’ to the process PROCESS.
     This signal is the one sent by the quit character (usually ‘C-\’)
     when you are not inside Emacs.

 -- Function: stop-process &optional process current-group
     This function stops the specified PROCESS.  If it is a real
     subprocess running a program, it sends the signal ‘SIGTSTP’ to that
     subprocess.  If PROCESS represents a network, serial, or pipe
     connection, this function inhibits handling of the incoming data
     from the connection; for a network server, this means not accepting
     new connections.  Use ‘continue-process’ to resume normal
     execution.

     Outside of Emacs, on systems with job control, the stop character
     (usually ‘C-z’) normally sends the ‘SIGTSTP’ signal to a
     subprocess.  When CURRENT-GROUP is non-‘nil’, you can think of this
     function as typing ‘C-z’ on the terminal Emacs uses to communicate
     with the subprocess.

 -- Function: continue-process &optional process current-group
     This function resumes execution of the process PROCESS.  If it is a
     real subprocess running a program, it sends the signal ‘SIGCONT’ to
     that subprocess; this presumes that PROCESS was stopped previously.
     If PROCESS represents a network, serial, or pipe connection, this
     function resumes handling of the incoming data from the connection.
     For serial connections, data that arrived during the time the
     process was stopped might be lost.

 -- Command: signal-process process signal
     This function sends a signal to process PROCESS.  The argument
     SIGNAL specifies which signal to send; it should be an integer, or
     a symbol whose name is a signal.

     The PROCESS argument can be a system process ID (an integer); that
     allows you to send signals to processes that are not children of
     Emacs.  *Note System Processes::.


File: elisp.info,  Node: Output from Processes,  Next: Sentinels,  Prev: Signals to Processes,  Up: Processes

36.9 Receiving Output from Processes
====================================

The output that an asynchronous subprocess writes to its standard output
stream is passed to a function called the “filter function”.  The
default filter function simply inserts the output into a buffer, which
is called the associated buffer of the process (*note Process
Buffers::).  If the process has no buffer then the default filter
discards the output.

   If the subprocess writes to its standard error stream, by default the
error output is also passed to the process filter function.  If Emacs
uses a pseudo-TTY (pty) for communication with the subprocess, then it
is impossible to separate the standard output and standard error streams
of the subprocess, because a pseudo-TTY has only one output channel.  In
that case, if you want to keep the output to those streams separate, you
should redirect one of them to a file—for example, by using an
appropriate shell command via ‘start-process-shell-command’ or a similar
function.

   Alternatively, you could use the ‘:stderr’ parameter with a non-‘nil’
value in a call to ‘make-process’ (*note make-process: Asynchronous
Processes.) to make the destination of the error output separate from
the standard output; in that case, Emacs will use pipes for
communicating with the subprocess.

   When a subprocess terminates, Emacs reads any pending output, then
stops reading output from that subprocess.  Therefore, if the subprocess
has children that are still live and still producing output, Emacs won’t
receive that output.

   Output from a subprocess can arrive only while Emacs is waiting: when
reading terminal input (see the function ‘waiting-for-user-input-p’), in
‘sit-for’ and ‘sleep-for’ (*note Waiting::), and in
‘accept-process-output’ (*note Accepting Output::).  This minimizes the
problem of timing errors that usually plague parallel programming.  For
example, you can safely create a process and only then specify its
buffer or filter function; no output can arrive before you finish, if
the code in between does not call any primitive that waits.

 -- Variable: process-adaptive-read-buffering
     On some systems, when Emacs reads the output from a subprocess, the
     output data is read in very small blocks, potentially resulting in
     very poor performance.  This behavior can be remedied to some
     extent by setting the variable ‘process-adaptive-read-buffering’ to
     a non-‘nil’ value (the default), as it will automatically delay
     reading from such processes, thus allowing them to produce more
     output before Emacs tries to read it.

* Menu:

* Process Buffers::         By default, output is put in a buffer.
* Filter Functions::        Filter functions accept output from the process.
* Decoding Output::         Filters can get unibyte or multibyte strings.
* Accepting Output::        How to wait until process output arrives.


File: elisp.info,  Node: Process Buffers,  Next: Filter Functions,  Up: Output from Processes

36.9.1 Process Buffers
----------------------

A process can (and usually does) have an “associated buffer”, which is
an ordinary Emacs buffer that is used for two purposes: storing the
output from the process, and deciding when to kill the process.  You can
also use the buffer to identify a process to operate on, since in normal
practice only one process is associated with any given buffer.  Many
applications of processes also use the buffer for editing input to be
sent to the process, but this is not built into Emacs Lisp.

   By default, process output is inserted in the associated buffer.
(You can change this by defining a custom filter function, *note Filter
Functions::.)  The position to insert the output is determined by the
‘process-mark’, which is then updated to point to the end of the text
just inserted.  Usually, but not always, the ‘process-mark’ is at the
end of the buffer.

   Killing the associated buffer of a process also kills the process.
Emacs asks for confirmation first, if the process’s
‘process-query-on-exit-flag’ is non-‘nil’ (*note Query Before Exit::).
This confirmation is done by the function
‘process-kill-buffer-query-function’, which is run from
‘kill-buffer-query-functions’ (*note Killing Buffers::).

 -- Function: process-buffer process
     This function returns the associated buffer of the specified
     PROCESS.

          (process-buffer (get-process "shell"))
               ⇒ #<buffer *shell*>

 -- Function: process-mark process
     This function returns the process marker for PROCESS, which is the
     marker that says where to insert output from the process.

     If PROCESS does not have a buffer, ‘process-mark’ returns a marker
     that points nowhere.

     The default filter function uses this marker to decide where to
     insert process output, and updates it to point after the inserted
     text.  That is why successive batches of output are inserted
     consecutively.

     Custom filter functions normally should use this marker in the same
     fashion.  For an example of a filter function that uses
     ‘process-mark’, *note Process Filter Example::.

     When the user is expected to enter input in the process buffer for
     transmission to the process, the process marker separates the new
     input from previous output.

 -- Function: set-process-buffer process buffer
     This function sets the buffer associated with PROCESS to BUFFER.
     If BUFFER is ‘nil’, the process becomes associated with no buffer.

 -- Function: get-buffer-process buffer-or-name
     This function returns a nondeleted process associated with the
     buffer specified by BUFFER-OR-NAME.  If there are several processes
     associated with it, this function chooses one (currently, the one
     most recently created, but don’t count on that).  Deletion of a
     process (see ‘delete-process’) makes it ineligible for this
     function to return.

     It is usually a bad idea to have more than one process associated
     with the same buffer.

          (get-buffer-process "*shell*")
               ⇒ #<process shell>

     Killing the process’s buffer deletes the process, which kills the
     subprocess with a ‘SIGHUP’ signal (*note Signals to Processes::).

   If the process’s buffer is displayed in a window, your Lisp program
may wish to tell the process the dimensions of that window, so that the
process could adapt its output to those dimensions, much as it adapts to
the screen dimensions.  The following functions allow communicating this
kind of information to processes; however, not all systems support the
underlying functionality, so it is best to provide fallbacks, e.g., via
command-line arguments or environment variables.

 -- Function: set-process-window-size process height width
     Tell PROCESS that its logical window size has dimensions WIDTH by
     HEIGHT, in character units.  If this function succeeds in
     communicating this information to the process, it returns ‘t’;
     otherwise it returns ‘nil’.

   When windows that display buffers associated with process change
their dimensions, the affected processes should be told about these
changes.  By default, when the window configuration changes, Emacs will
automatically call ‘set-process-window-size’ on behalf of every process
whose buffer is displayed in a window, passing it the smallest
dimensions of all the windows displaying the process’s buffer.  This
works via ‘window-configuration-change-hook’ (*note Window Hooks::),
which is told to invoke the function that is the value of the variable
‘window-adjust-process-window-size-function’ for each process whose
buffer is displayed in at least one window.  You can customize this
behavior by setting the value of that variable.

 -- User Option: window-adjust-process-window-size-function
     The value of this variable should be a function of two arguments: a
     process and the list of windows displaying the process’s buffer.
     When the function is called, the process’s buffer is the current
     buffer.  The function should return a cons cell ‘(WIDTH . HEIGHT)’
     that describes the dimensions of the logical process window to be
     passed via a call to ‘set-process-window-size’.  The function can
     also return ‘nil’, in which case Emacs will not call
     ‘set-process-window-size’ for this process.

     Emacs supplies two predefined values for this variable:
     ‘window-adjust-process-window-size-smallest’, which returns the
     smallest of all the dimensions of the windows that display a
     process’s buffer; and ‘window-adjust-process-window-size-largest’,
     which returns the largest dimensions.  For more complex strategies,
     write your own function.

     This variable can be buffer-local.

   If the process has the ‘adjust-window-size-function’ property (*note
Process Information::), its value overrides the global and buffer-local
values of ‘window-adjust-process-window-size-function’.


File: elisp.info,  Node: Filter Functions,  Next: Decoding Output,  Prev: Process Buffers,  Up: Output from Processes

36.9.2 Process Filter Functions
-------------------------------

A process “filter function” is a function that receives the standard
output from the associated process.  _All_ output from that process is
passed to the filter.  The default filter simply outputs directly to the
process buffer.

   By default, the error output from the process, if any, is also passed
to the filter function, unless the destination for the standard error
stream of the process was separated from the standard output when the
process was created (*note Output from Processes::).

   The filter function can only be called when Emacs is waiting for
something, because process output arrives only at such times.  Emacs
waits when reading terminal input (see the function
‘waiting-for-user-input-p’), in ‘sit-for’ and ‘sleep-for’ (*note
Waiting::), and in ‘accept-process-output’ (*note Accepting Output::).

   A filter function must accept two arguments: the associated process
and a string, which is output just received from it.  The function is
then free to do whatever it chooses with the output.

   Quitting is normally inhibited within a filter function—otherwise,
the effect of typing ‘C-g’ at command level or to quit a user command
would be unpredictable.  If you want to permit quitting inside a filter
function, bind ‘inhibit-quit’ to ‘nil’.  In most cases, the right way to
do this is with the macro ‘with-local-quit’.  *Note Quitting::.

   If an error happens during execution of a filter function, it is
caught automatically, so that it doesn’t stop the execution of whatever
program was running when the filter function was started.  However, if
‘debug-on-error’ is non-‘nil’, errors are not caught.  This makes it
possible to use the Lisp debugger to debug filter functions.  *Note
Debugger::.

   Many filter functions sometimes (or always) insert the output in the
process’s buffer, mimicking the actions of the default filter.  Such
filter functions need to make sure that they save the current buffer,
select the correct buffer (if different) before inserting output, and
then restore the original buffer.  They should also check whether the
buffer is still alive, update the process marker, and in some cases
update the value of point.  Here is how to do these things:

     (defun ordinary-insertion-filter (proc string)
       (when (buffer-live-p (process-buffer proc))
         (with-current-buffer (process-buffer proc)
           (let ((moving (= (point) (process-mark proc))))
             (save-excursion
               ;; Insert the text, advancing the process marker.
               (goto-char (process-mark proc))
               (insert string)
               (set-marker (process-mark proc) (point)))
             (if moving (goto-char (process-mark proc)))))))

   To make the filter force the process buffer to be visible whenever
new text arrives, you could insert a line like the following just before
the ‘with-current-buffer’ construct:

     (display-buffer (process-buffer proc))

   To force point to the end of the new output, no matter where it was
previously, eliminate the variable ‘moving’ from the example and call
‘goto-char’ unconditionally.

   Note that Emacs automatically saves and restores the match data while
executing filter functions.  *Note Match Data::.

   The output to the filter may come in chunks of any size.  A program
that produces the same output twice in a row may send it as one batch of
200 characters one time, and five batches of 40 characters the next.  If
the filter looks for certain text strings in the subprocess output, make
sure to handle the case where one of these strings is split across two
or more batches of output; one way to do this is to insert the received
text into a temporary buffer, which can then be searched.

 -- Function: set-process-filter process filter
     This function gives PROCESS the filter function FILTER.  If FILTER
     is ‘nil’, it gives the process the default filter, which inserts
     the process output into the process buffer.

 -- Function: process-filter process
     This function returns the filter function of PROCESS.

   In case the process’s output needs to be passed to several filters,
you can use ‘add-function’ to combine an existing filter with a new one.
*Note Advising Functions::.

   Here is an example of the use of a filter function:

     (defun keep-output (process output)
        (setq kept (cons output kept)))
          ⇒ keep-output
     (setq kept nil)
          ⇒ nil
     (set-process-filter (get-process "shell") 'keep-output)
          ⇒ keep-output
     (process-send-string "shell" "ls ~/other\n")
          ⇒ nil
     kept
          ⇒ ("lewis@slug:$ "
     "FINAL-W87-SHORT.MSS    backup.otl              kolstad.mss~
     address.txt             backup.psf              kolstad.psf
     backup.bib~             david.mss               resume-Dec-86.mss~
     backup.err              david.psf               resume-Dec.psf
     backup.mss              dland                   syllabus.mss
     "
     "#backups.mss#          backup.mss~             kolstad.mss
     ")


File: elisp.info,  Node: Decoding Output,  Next: Accepting Output,  Prev: Filter Functions,  Up: Output from Processes

36.9.3 Decoding Process Output
------------------------------

When Emacs writes process output directly into a multibyte buffer, it
decodes the output according to the process output coding system.  If
the coding system is ‘raw-text’ or ‘no-conversion’, Emacs converts the
unibyte output to multibyte using ‘string-to-multibyte’, and inserts the
resulting multibyte text.

   You can use ‘set-process-coding-system’ to specify which coding
system to use (*note Process Information::).  Otherwise, the coding
system comes from ‘coding-system-for-read’, if that is non-‘nil’; or
else from the defaulting mechanism (*note Default Coding Systems::).  If
the text output by a process contains null bytes, Emacs by default uses
‘no-conversion’ for it; see *note inhibit-null-byte-detection: Lisp and
Coding Systems, for how to control this behavior.

   *Warning:* Coding systems such as ‘undecided’, which determine the
coding system from the data, do not work entirely reliably with
asynchronous subprocess output.  This is because Emacs has to process
asynchronous subprocess output in batches, as it arrives.  Emacs must
try to detect the proper coding system from one batch at a time, and
this does not always work.  Therefore, if at all possible, specify a
coding system that determines both the character code conversion and the
end of line conversion—that is, one like ‘latin-1-unix’, rather than
‘undecided’ or ‘latin-1’.

   When Emacs calls a process filter function, it provides the process
output as a multibyte string or as a unibyte string according to the
process’s filter coding system.  Emacs decodes the output according to
the process output coding system, which usually produces a multibyte
string, except for coding systems such as ‘binary’ and ‘raw-text’.


File: elisp.info,  Node: Accepting Output,  Prev: Decoding Output,  Up: Output from Processes

36.9.4 Accepting Output from Processes
--------------------------------------

Output from asynchronous subprocesses normally arrives only while Emacs
is waiting for some sort of external event, such as elapsed time or
terminal input.  Occasionally it is useful in a Lisp program to
explicitly permit output to arrive at a specific point, or even to wait
until output arrives from a process.

 -- Function: accept-process-output &optional process seconds millisec
          just-this-one
     This function allows Emacs to read pending output from processes.
     The output is given to their filter functions.  If PROCESS is
     non-‘nil’ then this function does not return until some output has
     been received from PROCESS.

     The arguments SECONDS and MILLISEC let you specify timeout periods.
     The former specifies a period measured in seconds and the latter
     specifies one measured in milliseconds.  The two time periods thus
     specified are added together, and ‘accept-process-output’ returns
     after that much time, even if there is no subprocess output.

     The argument MILLISEC is obsolete (and should not be used), because
     SECONDS can be floating point to specify waiting a fractional
     number of seconds.  If SECONDS is 0, the function accepts whatever
     output is pending but does not wait.

     If PROCESS is a process, and the argument JUST-THIS-ONE is
     non-‘nil’, only output from that process is handled, suspending
     output from other processes until some output has been received
     from that process or the timeout expires.  If JUST-THIS-ONE is an
     integer, also inhibit running timers.  This feature is generally
     not recommended, but may be necessary for specific applications,
     such as speech synthesis.

     The function ‘accept-process-output’ returns non-‘nil’ if it got
     output from PROCESS, or from any process if PROCESS is ‘nil’.  It
     returns ‘nil’ if the timeout expired before output arrived.


File: elisp.info,  Node: Sentinels,  Next: Query Before Exit,  Prev: Output from Processes,  Up: Processes

36.10 Sentinels: Detecting Process Status Changes
=================================================

A “process sentinel” is a function that is called whenever the
associated process changes status for any reason, including signals
(whether sent by Emacs or caused by the process’s own actions) that
terminate, stop, or continue the process.  The process sentinel is also
called if the process exits.  The sentinel receives two arguments: the
process for which the event occurred, and a string describing the type
of event.

   If no sentinel function was specified for a process, it will use the
default sentinel function, which inserts a message in the process’s
buffer with the process name and the string describing the event.

   The string describing the event looks like one of the following:

   • ‘"finished\n"’.

   • ‘"deleted\n"’.

   • ‘"exited abnormally with code EXITCODE (core dumped)\n"’.  The
     “core dumped” part is optional, and only appears if the process
     dumped core.

   • ‘"failed with code FAIL-CODE\n"’.

   • ‘"SIGNAL-DESCRIPTION (core dumped)\n"’.  The SIGNAL-DESCRIPTION is
     a system-dependent textual description of a signal, e.g.,
     ‘"killed"’ for ‘SIGKILL’.  The “core dumped” part is optional, and
     only appears if the process dumped core.

   • ‘"open from HOST-NAME\n"’.

   • ‘"open\n"’.

   • ‘"connection broken by remote peer\n"’.

   A sentinel runs only while Emacs is waiting (e.g., for terminal
input, or for time to elapse, or for process output).  This avoids the
timing errors that could result from running sentinels at random places
in the middle of other Lisp programs.  A program can wait, so that
sentinels will run, by calling ‘sit-for’ or ‘sleep-for’ (*note
Waiting::), or ‘accept-process-output’ (*note Accepting Output::).
Emacs also allows sentinels to run when the command loop is reading
input.  ‘delete-process’ calls the sentinel when it terminates a running
process.

   Emacs does not keep a queue of multiple reasons to call the sentinel
of one process; it records just the current status and the fact that
there has been a change.  Therefore two changes in status, coming in
quick succession, can call the sentinel just once.  However, process
termination will always run the sentinel exactly once.  This is because
the process status can’t change again after termination.

   Emacs explicitly checks for output from the process before running
the process sentinel.  Once the sentinel runs due to process
termination, no further output can arrive from the process.

   A sentinel that writes the output into the buffer of the process
should check whether the buffer is still alive.  If it tries to insert
into a dead buffer, it will get an error.  If the buffer is dead,
‘(buffer-name (process-buffer PROCESS))’ returns ‘nil’.

   Quitting is normally inhibited within a sentinel—otherwise, the
effect of typing ‘C-g’ at command level or to quit a user command would
be unpredictable.  If you want to permit quitting inside a sentinel,
bind ‘inhibit-quit’ to ‘nil’.  In most cases, the right way to do this
is with the macro ‘with-local-quit’.  *Note Quitting::.

   If an error happens during execution of a sentinel, it is caught
automatically, so that it doesn’t stop the execution of whatever
programs was running when the sentinel was started.  However, if
‘debug-on-error’ is non-‘nil’, errors are not caught.  This makes it
possible to use the Lisp debugger to debug the sentinel.  *Note
Debugger::.

   While a sentinel is running, the process sentinel is temporarily set
to ‘nil’ so that the sentinel won’t run recursively.  For this reason it
is not possible for a sentinel to specify a new sentinel.

   Note that Emacs automatically saves and restores the match data while
executing sentinels.  *Note Match Data::.

 -- Function: set-process-sentinel process sentinel
     This function associates SENTINEL with PROCESS.  If SENTINEL is
     ‘nil’, then the process will have the default sentinel, which
     inserts a message in the process’s buffer when the process status
     changes.

     Changes in process sentinels take effect immediately—if the
     sentinel is slated to be run but has not been called yet, and you
     specify a new sentinel, the eventual call to the sentinel will use
     the new one.

          (defun msg-me (process event)
             (princ
               (format "Process: %s had the event '%s'" process event)))
          (set-process-sentinel (get-process "shell") 'msg-me)
               ⇒ msg-me
          (kill-process (get-process "shell"))
               ⊣ Process: #<process shell> had the event 'killed'
               ⇒ #<process shell>

 -- Function: process-sentinel process
     This function returns the sentinel of PROCESS.

   In case a process status changes need to be passed to several
sentinels, you can use ‘add-function’ to combine an existing sentinel
with a new one.  *Note Advising Functions::.

 -- Function: waiting-for-user-input-p
     While a sentinel or filter function is running, this function
     returns non-‘nil’ if Emacs was waiting for keyboard input from the
     user at the time the sentinel or filter function was called, or
     ‘nil’ if it was not.


File: elisp.info,  Node: Query Before Exit,  Next: System Processes,  Prev: Sentinels,  Up: Processes

36.11 Querying Before Exit
==========================

When Emacs exits, it terminates all its subprocesses.  For subprocesses
that run a program, it sends them the ‘SIGHUP’ signal; connections are
simply closed.  Because subprocesses may be doing valuable work, Emacs
normally asks the user to confirm that it is ok to terminate them.  Each
process has a query flag, which, if non-‘nil’, says that Emacs should
ask for confirmation before exiting and thus killing that process.  The
default for the query flag is ‘t’, meaning _do_ query.

 -- Function: process-query-on-exit-flag process
     This returns the query flag of PROCESS.

 -- Function: set-process-query-on-exit-flag process flag
     This function sets the query flag of PROCESS to FLAG.  It returns
     FLAG.

     Here is an example of using ‘set-process-query-on-exit-flag’ on a
     shell process to avoid querying:

          (set-process-query-on-exit-flag (get-process "shell") nil)
               ⇒ nil


File: elisp.info,  Node: System Processes,  Next: Transaction Queues,  Prev: Query Before Exit,  Up: Processes

36.12 Accessing Other Processes
===============================

In addition to accessing and manipulating processes that are
subprocesses of the current Emacs session, Emacs Lisp programs can also
access other processes running on the same machine.  We call these
“system processes”, to distinguish them from Emacs subprocesses.

   Emacs provides several primitives for accessing system processes.
Not all platforms support these primitives; on those which don’t, these
primitives return ‘nil’.

 -- Function: list-system-processes
     This function returns a list of all the processes running on the
     system.  Each process is identified by its PID, a numerical process
     ID that is assigned by the OS and distinguishes the process from
     all the other processes running on the same machine at the same
     time.

 -- Function: process-attributes pid
     This function returns an alist of attributes for the process
     specified by its process ID PID.  Each association in the alist is
     of the form ‘(KEY . VALUE)’, where KEY designates the attribute and
     VALUE is the value of that attribute.  The various attribute KEYs
     that this function can return are listed below.  Not all platforms
     support all of these attributes; if an attribute is not supported,
     its association will not appear in the returned alist.  Values that
     are numbers can be either integer or floating point, depending on
     the magnitude of the value.

     ‘euid’
          The effective user ID of the user who invoked the process.
          The corresponding VALUE is a number.  If the process was
          invoked by the same user who runs the current Emacs session,
          the value is identical to what ‘user-uid’ returns (*note User
          Identification::).

     ‘user’
          User name corresponding to the process’s effective user ID, a
          string.

     ‘egid’
          The group ID of the effective user ID, a number.

     ‘group’
          Group name corresponding to the effective user’s group ID, a
          string.

     ‘comm’
          The name of the command that runs in the process.  This is a
          string that usually specifies the name of the executable file
          of the process, without the leading directories.  However,
          some special system processes can report strings that do not
          correspond to an executable file of a program.

     ‘state’
          The state code of the process.  This is a short string that
          encodes the scheduling state of the process.  Here’s a list of
          the most frequently seen codes:

          ‘"D"’
               uninterruptible sleep (usually I/O)
          ‘"R"’
               running
          ‘"S"’
               interruptible sleep (waiting for some event)
          ‘"T"’
               stopped, e.g., by a job control signal
          ‘"Z"’
               zombie: a process that terminated, but was not reaped by
               its parent

          For the full list of the possible states, see the manual page
          of the ‘ps’ command.

     ‘ppid’
          The process ID of the parent process, a number.

     ‘pgrp’
          The process group ID of the process, a number.

     ‘sess’
          The session ID of the process.  This is a number that is the
          process ID of the process’s “session leader”.

     ‘ttname’
          A string that is the name of the process’s controlling
          terminal.  On Unix and GNU systems, this is normally the file
          name of the corresponding terminal device, such as
          ‘/dev/pts65’.

     ‘tpgid’
          The numerical process group ID of the foreground process group
          that uses the process’s terminal.

     ‘minflt’
          The number of minor page faults caused by the process since
          its beginning.  (Minor page faults are those that don’t
          involve reading from disk.)

     ‘majflt’
          The number of major page faults caused by the process since
          its beginning.  (Major page faults require a disk to be read,
          and are thus more expensive than minor page faults.)

     ‘cminflt’
     ‘cmajflt’
          Like ‘minflt’ and ‘majflt’, but include the number of page
          faults for all the child processes of the given process.

     ‘utime’
          Time spent by the process in the user context, for running the
          application’s code.  The corresponding VALUE is in the
          ‘(HIGH LOW MICROSEC PICOSEC)’ format, the same format used by
          functions ‘current-time’ (*note current-time: Time of Day.)
          and ‘file-attributes’ (*note File Attributes::).

     ‘stime’
          Time spent by the process in the system (kernel) context, for
          processing system calls.  The corresponding VALUE is in the
          same format as for ‘utime’.

     ‘time’
          The sum of ‘utime’ and ‘stime’.  The corresponding VALUE is in
          the same format as for ‘utime’.

     ‘cutime’
     ‘cstime’
     ‘ctime’
          Like ‘utime’, ‘stime’, and ‘time’, but include the times of
          all the child processes of the given process.

     ‘pri’
          The numerical priority of the process.

     ‘nice’
          The “nice value” of the process, a number.  (Processes with
          smaller nice values get scheduled more favorably.)

     ‘thcount’
          The number of threads in the process.

     ‘start’
          The time when the process was started, in the same ‘(HIGH LOW
          MICROSEC PICOSEC)’ format used by ‘file-attributes’ and
          ‘current-time’.

     ‘etime’
          The time elapsed since the process started, in the format
          ‘(HIGH LOW MICROSEC PICOSEC)’.

     ‘vsize’
          The virtual memory size of the process, measured in kilobytes.

     ‘rss’
          The size of the process’s “resident set”, the number of
          kilobytes occupied by the process in the machine’s physical
          memory.

     ‘pcpu’
          The percentage of the CPU time used by the process since it
          started.  The corresponding VALUE is a floating-point number
          between 0 and 100.

     ‘pmem’
          The percentage of the total physical memory installed on the
          machine used by the process’s resident set.  The value is a
          floating-point number between 0 and 100.

     ‘args’
          The command-line with which the process was invoked.  This is
          a string in which individual command-line arguments are
          separated by blanks; whitespace characters that are embedded
          in the arguments are quoted as appropriate for the system’s
          shell: escaped by backslash characters on GNU and Unix, and
          enclosed in double quote characters on Windows.  Thus, this
          command-line string can be directly used in primitives such as
          ‘shell-command’.


File: elisp.info,  Node: Transaction Queues,  Next: Network,  Prev: System Processes,  Up: Processes

36.13 Transaction Queues
========================

You can use a “transaction queue” to communicate with a subprocess using
transactions.  First use ‘tq-create’ to create a transaction queue
communicating with a specified process.  Then you can call ‘tq-enqueue’
to send a transaction.

 -- Function: tq-create process
     This function creates and returns a transaction queue communicating
     with PROCESS.  The argument PROCESS should be a subprocess capable
     of sending and receiving streams of bytes.  It may be a child
     process, or it may be a TCP connection to a server, possibly on
     another machine.

 -- Function: tq-enqueue queue question regexp closure fn &optional
          delay-question
     This function sends a transaction to queue QUEUE.  Specifying the
     queue has the effect of specifying the subprocess to talk to.

     The argument QUESTION is the outgoing message that starts the
     transaction.  The argument FN is the function to call when the
     corresponding answer comes back; it is called with two arguments:
     CLOSURE, and the answer received.

     The argument REGEXP is a regular expression that should match text
     at the end of the entire answer, but nothing before; that’s how
     ‘tq-enqueue’ determines where the answer ends.

     If the argument DELAY-QUESTION is non-‘nil’, delay sending this
     question until the process has finished replying to any previous
     questions.  This produces more reliable results with some
     processes.

 -- Function: tq-close queue
     Shut down transaction queue QUEUE, waiting for all pending
     transactions to complete, and then terminate the connection or
     child process.

   Transaction queues are implemented by means of a filter function.
*Note Filter Functions::.


File: elisp.info,  Node: Network,  Next: Network Servers,  Prev: Transaction Queues,  Up: Processes

36.14 Network Connections
=========================

Emacs Lisp programs can open stream (TCP) and datagram (UDP) network
connections (*note Datagrams::) to other processes on the same machine
or other machines.  A network connection is handled by Lisp much like a
subprocess, and is represented by a process object.  However, the
process you are communicating with is not a child of the Emacs process,
has no process ID, and you can’t kill it or send it signals.  All you
can do is send and receive data.  ‘delete-process’ closes the
connection, but does not kill the program at the other end; that program
must decide what to do about closure of the connection.

   Lisp programs can listen for connections by creating network servers.
A network server is also represented by a kind of process object, but
unlike a network connection, the network server never transfers data
itself.  When it receives a connection request, it creates a new network
connection to represent the connection just made.  (The network
connection inherits certain information, including the process plist,
from the server.)  The network server then goes back to listening for
more connection requests.

   Network connections and servers are created by calling
‘make-network-process’ with an argument list consisting of
keyword/argument pairs, for example ‘:server t’ to create a server
process, or ‘:type 'datagram’ to create a datagram connection.  *Note
Low-Level Network::, for details.  You can also use the
‘open-network-stream’ function described below.

   To distinguish the different types of processes, the ‘process-type’
function returns the symbol ‘network’ for a network connection or
server, ‘serial’ for a serial port connection, ‘pipe’ for a pipe
connection, or ‘real’ for a real subprocess.

   The ‘process-status’ function returns ‘open’, ‘closed’, ‘connect’,
‘stop’, or ‘failed’ for network connections.  For a network server, the
status is always ‘listen’.  Except for ‘stop’, none of those values is
possible for a real subprocess.  *Note Process Information::.

   You can stop and resume operation of a network process by calling
‘stop-process’ and ‘continue-process’.  For a server process, being
stopped means not accepting new connections.  (Up to 5 connection
requests will be queued for when you resume the server; you can increase
this limit, unless it is imposed by the operating system—see the
‘:server’ keyword of ‘make-network-process’, *note Network Processes::.)
For a network stream connection, being stopped means not processing
input (any arriving input waits until you resume the connection).  For a
datagram connection, some number of packets may be queued but input may
be lost.  You can use the function ‘process-command’ to determine
whether a network connection or server is stopped; a non-‘nil’ value
means yes.

   Emacs can create encrypted network connections, using either built-in
or external support.  The built-in support uses the GnuTLS Transport
Layer Security Library; see the GnuTLS project page
(http://www.gnu.org/software/gnutls/).  If your Emacs was compiled with
GnuTLS support, the function ‘gnutls-available-p’ is defined and returns
non-‘nil’.  For more details, *note Overview: (emacs-gnutls)Top.  The
external support uses the ‘starttls.el’ library, which requires a helper
utility such as ‘gnutls-cli’ to be installed on the system.  The
‘open-network-stream’ function can transparently handle the details of
creating encrypted connections for you, using whatever support is
available.

 -- Function: open-network-stream name buffer host service &rest
          parameters
     This function opens a TCP connection, with optional encryption, and
     returns a process object that represents the connection.

     The NAME argument specifies the name for the process object.  It is
     modified as necessary to make it unique.

     The BUFFER argument is the buffer to associate with the connection.
     Output from the connection is inserted in the buffer, unless you
     specify your own filter function to handle the output.  If BUFFER
     is ‘nil’, it means that the connection is not associated with any
     buffer.

     The arguments HOST and SERVICE specify where to connect to; HOST is
     the host name (a string), and SERVICE is the name of a defined
     network service (a string) or a port number (an integer).

     The remaining arguments PARAMETERS are keyword/argument pairs that
     are mainly relevant to encrypted connections:

     ‘:nowait BOOLEAN’
          If non-‘nil’, try to make an asynchronous connection.

     ‘:type TYPE’
          The type of connection.  Options are:

          ‘plain’
               An ordinary, unencrypted connection.
          ‘tls’
          ‘ssl’
               A TLS (Transport Layer Security) connection.
          ‘nil’
          ‘network’
               Start with a plain connection, and if parameters
               ‘:success’ and ‘:capability-command’ are supplied, try to
               upgrade to an encrypted connection via STARTTLS.  If that
               fails, retain the unencrypted connection.
          ‘starttls’
               As for ‘nil’, but if STARTTLS fails drop the connection.
          ‘shell’
               A shell connection.

     ‘:always-query-capabilities BOOLEAN’
          If non-‘nil’, always ask for the server’s capabilities, even
          when doing a ‘plain’ connection.

     ‘:capability-command CAPABILITY-COMMAND’
          Command string to query the host capabilities.

     ‘:end-of-command REGEXP’
     ‘:end-of-capability REGEXP’
          Regular expression matching the end of a command, or the end
          of the command CAPABILITY-COMMAND.  The latter defaults to the
          former.

     ‘:starttls-function FUNCTION’
          Function of one argument (the response to CAPABILITY-COMMAND),
          which returns either ‘nil’, or the command to activate
          STARTTLS if supported.

     ‘:success REGEXP’
          Regular expression matching a successful STARTTLS negotiation.

     ‘:use-starttls-if-possible BOOLEAN’
          If non-‘nil’, do opportunistic STARTTLS upgrades even if Emacs
          doesn’t have built-in TLS support.

     ‘:warn-unless-encrypted BOOLEAN’
          If non-‘nil’, and ‘:return-value’ is also non-‘nil’, Emacs
          will warn if the connection isn’t encrypted.  This is useful
          for protocols like IMAP and the like, where most users would
          expect the network traffic to be encrypted.

     ‘:client-certificate LIST-OR-T’
          Either a list of the form ‘(KEY-FILE CERT-FILE)’, naming the
          certificate key file and certificate file itself, or ‘t’,
          meaning to query ‘auth-source’ for this information (*note
          Overview: (auth)Top.).  Only used for TLS or STARTTLS.

     ‘:return-list CONS-OR-NIL’
          The return value of this function.  If omitted or ‘nil’,
          return a process object.  Otherwise, a cons of the form
          ‘(PROCESS-OBJECT . PLIST)’, where PLIST has keywords:

          ‘:greeting STRING-OR-NIL’
               If non-‘nil’, the greeting string returned by the host.
          ‘:capabilities STRING-OR-NIL’
               If non-‘nil’, the host’s capability string.
          ‘:type SYMBOL’
               The connection type: ‘plain’ or ‘tls’.


File: elisp.info,  Node: Network Servers,  Next: Datagrams,  Prev: Network,  Up: Processes

36.15 Network Servers
=====================

You create a server by calling ‘make-network-process’ (*note Network
Processes::) with ‘:server t’.  The server will listen for connection
requests from clients.  When it accepts a client connection request,
that creates a new network connection, itself a process object, with the
following parameters:

   • The connection’s process name is constructed by concatenating the
     server process’s NAME with a client identification string.  The
     client identification string for an IPv4 connection looks like
     ‘<A.B.C.D:P>’, which represents an address and port number.
     Otherwise, it is a unique number in brackets, as in ‘<NNN>’.  The
     number is unique for each connection in the Emacs session.

   • If the server has a non-default filter, the connection process does
     not get a separate process buffer; otherwise, Emacs creates a new
     buffer for the purpose.  The buffer name is the server’s buffer
     name or process name, concatenated with the client identification
     string.

     The server’s process buffer value is never used directly, but the
     log function can retrieve it and use it to log connections by
     inserting text there.

   • The communication type and the process filter and sentinel are
     inherited from those of the server.  The server never directly uses
     its filter and sentinel; their sole purpose is to initialize
     connections made to the server.

   • The connection’s process contact information is set according to
     the client’s addressing information (typically an IP address and a
     port number).  This information is associated with the
     ‘process-contact’ keywords ‘:host’, ‘:service’, ‘:remote’.

   • The connection’s local address is set up according to the port
     number used for the connection.

   • The client process’s plist is initialized from the server’s plist.


File: elisp.info,  Node: Datagrams,  Next: Low-Level Network,  Prev: Network Servers,  Up: Processes

36.16 Datagrams
===============

A “datagram” connection communicates with individual packets rather than
streams of data.  Each call to ‘process-send’ sends one datagram packet
(*note Input to Processes::), and each datagram received results in one
call to the filter function.

   The datagram connection doesn’t have to talk with the same remote
peer all the time.  It has a “remote peer address” which specifies where
to send datagrams to.  Each time an incoming datagram is passed to the
filter function, the peer address is set to the address that datagram
came from; that way, if the filter function sends a datagram, it will go
back to that place.  You can specify the remote peer address when you
create the datagram connection using the ‘:remote’ keyword.  You can
change it later on by calling ‘set-process-datagram-address’.

 -- Function: process-datagram-address process
     If PROCESS is a datagram connection or server, this function
     returns its remote peer address.

 -- Function: set-process-datagram-address process address
     If PROCESS is a datagram connection or server, this function sets
     its remote peer address to ADDRESS.


File: elisp.info,  Node: Low-Level Network,  Next: Misc Network,  Prev: Datagrams,  Up: Processes

36.17 Low-Level Network Access
==============================

You can also create network connections by operating at a lower level
than that of ‘open-network-stream’, using ‘make-network-process’.

* Menu:

* Proc: Network Processes.   Using ‘make-network-process’.
* Options: Network Options.  Further control over network connections.
* Features: Network Feature Testing.
                             Determining which network features work on
                               the machine you are using.


File: elisp.info,  Node: Network Processes,  Next: Network Options,  Up: Low-Level Network

36.17.1 ‘make-network-process’
------------------------------

The basic function for creating network connections and network servers
is ‘make-network-process’.  It can do either of those jobs, depending on
the arguments you give it.

 -- Function: make-network-process &rest args
     This function creates a network connection or server and returns
     the process object that represents it.  The arguments ARGS are a
     list of keyword/argument pairs.  Omitting a keyword is always
     equivalent to specifying it with value ‘nil’, except for ‘:coding’,
     ‘:filter-multibyte’, and ‘:reuseaddr’.  Here are the meaningful
     keywords (those corresponding to network options are listed in the
     following section):

     :name NAME
          Use the string NAME as the process name.  It is modified if
          necessary to make it unique.

     :type TYPE
          Specify the communication type.  A value of ‘nil’ specifies a
          stream connection (the default); ‘datagram’ specifies a
          datagram connection; ‘seqpacket’ specifies a sequenced packet
          stream connection.  Both connections and servers can be of
          these types.

     :server SERVER-FLAG
          If SERVER-FLAG is non-‘nil’, create a server.  Otherwise,
          create a connection.  For a stream type server, SERVER-FLAG
          may be an integer, which then specifies the length of the
          queue of pending connections to the server.  The default queue
          length is 5.

     :host HOST
          Specify the host to connect to.  HOST should be a host name or
          Internet address, as a string, or the symbol ‘local’ to
          specify the local host.  If you specify HOST for a server, it
          must specify a valid address for the local host, and only
          clients connecting to that address will be accepted.

     :service SERVICE
          SERVICE specifies a port number to connect to; or, for a
          server, the port number to listen on.  It should be a service
          name that translates to a port number, or an integer
          specifying the port number directly.  For a server, it can
          also be ‘t’, which means to let the system select an unused
          port number.

     :family FAMILY
          FAMILY specifies the address (and protocol) family for
          communication.  ‘nil’ means determine the proper address
          family automatically for the given HOST and SERVICE.  ‘local’
          specifies a Unix socket, in which case HOST is ignored.
          ‘ipv4’ and ‘ipv6’ specify to use IPv4 and IPv6, respectively.

     :local LOCAL-ADDRESS
          For a server process, LOCAL-ADDRESS is the address to listen
          on.  It overrides FAMILY, HOST and SERVICE, so you might as
          well not specify them.

     :remote REMOTE-ADDRESS
          For a connection, REMOTE-ADDRESS is the address to connect to.
          It overrides FAMILY, HOST and SERVICE, so you might as well
          not specify them.

          For a datagram server, REMOTE-ADDRESS specifies the initial
          setting of the remote datagram address.

          The format of LOCAL-ADDRESS or REMOTE-ADDRESS depends on the
          address family:

             - An IPv4 address is represented as a five-element vector
               of four 8-bit integers and one 16-bit integer ‘[A B C D
               P]’ corresponding to numeric IPv4 address A.B.C.D and
               port number P.

             - An IPv6 address is represented as a nine-element vector
               of 16-bit integers ‘[A B C D E F G H P]’ corresponding to
               numeric IPv6 address A:B:C:D:E:F:G:H and port number P.

             - A local address is represented as a string, which
               specifies the address in the local address space.

             - An unsupported-family address is represented by a cons
               ‘(F . AV)’, where F is the family number and AV is a
               vector specifying the socket address using one element
               per address data byte.  Do not rely on this format in
               portable code, as it may depend on implementation defined
               constants, data sizes, and data structure alignment.

     :nowait BOOL
          If BOOL is non-‘nil’ for a stream connection, return without
          waiting for the connection to complete.  When the connection
          succeeds or fails, Emacs will call the sentinel function, with
          a second argument matching ‘"open"’ (if successful) or
          ‘"failed"’.  The default is to block, so that
          ‘make-network-process’ does not return until the connection
          has succeeded or failed.

     :stop STOPPED
          If STOPPED is non-‘nil’, start the network connection or
          server in the stopped state.

     :buffer BUFFER
          Use BUFFER as the process buffer.

     :coding CODING
          Use CODING as the coding system for this process.  To specify
          different coding systems for decoding data from the connection
          and for encoding data sent to it, specify ‘(DECODING .
          ENCODING)’ for CODING.

          If you don’t specify this keyword at all, the default is to
          determine the coding systems from the data.

     :noquery QUERY-FLAG
          Initialize the process query flag to QUERY-FLAG.  *Note Query
          Before Exit::.

     :filter FILTER
          Initialize the process filter to FILTER.

     :filter-multibyte MULTIBYTE
          If MULTIBYTE is non-‘nil’, strings given to the process filter
          are multibyte, otherwise they are unibyte.  The default is the
          default value of ‘enable-multibyte-characters’.

     :sentinel SENTINEL
          Initialize the process sentinel to SENTINEL.

     :log LOG
          Initialize the log function of a server process to LOG.  The
          log function is called each time the server accepts a network
          connection from a client.  The arguments passed to the log
          function are SERVER, CONNECTION, and MESSAGE; where SERVER is
          the server process, CONNECTION is the new process for the
          connection, and MESSAGE is a string describing what has
          happened.

     :plist PLIST
          Initialize the process plist to PLIST.

     The original argument list, modified with the actual connection
     information, is available via the ‘process-contact’ function.


File: elisp.info,  Node: Network Options,  Next: Network Feature Testing,  Prev: Network Processes,  Up: Low-Level Network

36.17.2 Network Options
-----------------------

The following network options can be specified when you create a network
process.  Except for ‘:reuseaddr’, you can also set or modify these
options later, using ‘set-network-process-option’.

   For a server process, the options specified with
‘make-network-process’ are not inherited by the client connections, so
you will need to set the necessary options for each child connection as
it is created.

:bindtodevice DEVICE-NAME
     If DEVICE-NAME is a non-empty string identifying a network
     interface name (see ‘network-interface-list’), only handle packets
     received on that interface.  If DEVICE-NAME is ‘nil’ (the default),
     handle packets received on any interface.

     Using this option may require special privileges on some systems.

:broadcast BROADCAST-FLAG
     If BROADCAST-FLAG is non-‘nil’ for a datagram process, the process
     will receive datagram packet sent to a broadcast address, and be
     able to send packets to a broadcast address.  This is ignored for a
     stream connection.

:dontroute DONTROUTE-FLAG
     If DONTROUTE-FLAG is non-‘nil’, the process can only send to hosts
     on the same network as the local host.

:keepalive KEEPALIVE-FLAG
     If KEEPALIVE-FLAG is non-‘nil’ for a stream connection, enable
     exchange of low-level keep-alive messages.

:linger LINGER-ARG
     If LINGER-ARG is non-‘nil’, wait for successful transmission of all
     queued packets on the connection before it is deleted (see
     ‘delete-process’).  If LINGER-ARG is an integer, it specifies the
     maximum time in seconds to wait for queued packets to be sent
     before closing the connection.  The default is ‘nil’, which means
     to discard unsent queued packets when the process is deleted.

:oobinline OOBINLINE-FLAG
     If OOBINLINE-FLAG is non-‘nil’ for a stream connection, receive
     out-of-band data in the normal data stream.  Otherwise, ignore
     out-of-band data.

:priority PRIORITY
     Set the priority for packets sent on this connection to the integer
     PRIORITY.  The interpretation of this number is protocol specific;
     such as setting the TOS (type of service) field on IP packets sent
     on this connection.  It may also have system dependent effects,
     such as selecting a specific output queue on the network interface.

:reuseaddr REUSEADDR-FLAG
     If REUSEADDR-FLAG is non-‘nil’ (the default) for a stream server
     process, allow this server to reuse a specific port number (see
     ‘:service’), unless another process on this host is already
     listening on that port.  If REUSEADDR-FLAG is ‘nil’, there may be a
     period of time after the last use of that port (by any process on
     the host) where it is not possible to make a new server on that
     port.

 -- Function: set-network-process-option process option value &optional
          no-error
     This function sets or modifies a network option for network process
     PROCESS.  The accepted options and values are as for
     ‘make-network-process’.  If NO-ERROR is non-‘nil’, this function
     returns ‘nil’ instead of signaling an error if OPTION is not a
     supported option.  If the function successfully completes, it
     returns ‘t’.

     The current setting of an option is available via the
     ‘process-contact’ function.


File: elisp.info,  Node: Network Feature Testing,  Prev: Network Options,  Up: Low-Level Network

36.17.3 Testing Availability of Network Features
------------------------------------------------

To test for the availability of a given network feature, use ‘featurep’
like this:

     (featurep 'make-network-process '(KEYWORD VALUE))

The result of this form is ‘t’ if it works to specify KEYWORD with value
VALUE in ‘make-network-process’.  Here are some of the KEYWORD—VALUE
pairs you can test in this way.

‘(:nowait t)’
     Non-‘nil’ if non-blocking connect is supported.
‘(:type datagram)’
     Non-‘nil’ if datagrams are supported.
‘(:family local)’
     Non-‘nil’ if local (a.k.a. “UNIX domain”) sockets are supported.
‘(:family ipv6)’
     Non-‘nil’ if IPv6 is supported.
‘(:service t)’
     Non-‘nil’ if the system can select the port for a server.

   To test for the availability of a given network option, use
‘featurep’ like this:

     (featurep 'make-network-process 'KEYWORD)

The accepted KEYWORD values are ‘:bindtodevice’, etc.  For the complete
list, *note Network Options::.  This form returns non-‘nil’ if that
particular network option is supported by ‘make-network-process’ (or
‘set-network-process-option’).


File: elisp.info,  Node: Misc Network,  Next: Serial Ports,  Prev: Low-Level Network,  Up: Processes

36.18 Misc Network Facilities
=============================

These additional functions are useful for creating and operating on
network connections.  Note that they are supported only on some systems.

 -- Function: network-interface-list
     This function returns a list describing the network interfaces of
     the machine you are using.  The value is an alist whose elements
     have the form ‘(NAME . ADDRESS)’.  ADDRESS has the same form as the
     LOCAL-ADDRESS and REMOTE-ADDRESS arguments to
     ‘make-network-process’.

 -- Function: network-interface-info ifname
     This function returns information about the network interface named
     IFNAME.  The value is a list of the form ‘(ADDR BCAST NETMASK
     HWADDR FLAGS)’.

     ADDR
          The Internet protocol address.
     BCAST
          The broadcast address.
     NETMASK
          The network mask.
     HWADDR
          The layer 2 address (Ethernet MAC address, for instance).
     FLAGS
          The current flags of the interface.

 -- Function: format-network-address address &optional omit-port
     This function converts the Lisp representation of a network address
     to a string.

     A five-element vector ‘[A B C D P]’ represents an IPv4 address
     A.B.C.D and port number P.  ‘format-network-address’ converts that
     to the string ‘"A.B.C.D:P"’.

     A nine-element vector ‘[A B C D E F G H P]’ represents an IPv6
     address along with a port number.  ‘format-network-address’
     converts that to the string ‘"[A:B:C:D:E:F:G:H]:P"’.

     If the vector does not include the port number, P, or if OMIT-PORT
     is non-‘nil’, the result does not include the ‘:P’ suffix.


File: elisp.info,  Node: Serial Ports,  Next: Byte Packing,  Prev: Misc Network,  Up: Processes

36.19 Communicating with Serial Ports
=====================================

Emacs can communicate with serial ports.  For interactive use, ‘M-x
serial-term’ opens a terminal window.  In a Lisp program,
‘make-serial-process’ creates a process object.

   The serial port can be configured at run-time, without having to
close and re-open it.  The function ‘serial-process-configure’ lets you
change the speed, bytesize, and other parameters.  In a terminal window
created by ‘serial-term’, you can click on the mode line for
configuration.

   A serial connection is represented by a process object, which can be
used in a similar way to a subprocess or network process.  You can send
and receive data, and configure the serial port.  A serial process
object has no process ID, however, and you can’t send signals to it, and
the status codes are different from other types of processes.
‘delete-process’ on the process object or ‘kill-buffer’ on the process
buffer close the connection, but this does not affect the device
connected to the serial port.

   The function ‘process-type’ returns the symbol ‘serial’ for a process
object representing a serial port connection.

   Serial ports are available on GNU/Linux, Unix, and MS Windows
systems.

 -- Command: serial-term port speed
     Start a terminal-emulator for a serial port in a new buffer.  PORT
     is the name of the serial port to connect to.  For example, this
     could be ‘/dev/ttyS0’ on Unix.  On MS Windows, this could be
     ‘COM1’, or ‘\\.\COM10’ (double the backslashes in Lisp strings).

     SPEED is the speed of the serial port in bits per second.  9600 is
     a common value.  The buffer is in Term mode; see *note (emacs)Term
     Mode::, for the commands to use in that buffer.  You can change the
     speed and the configuration in the mode line menu.

 -- Function: make-serial-process &rest args
     This function creates a process and a buffer.  Arguments are
     specified as keyword/argument pairs.  Here’s the list of the
     meaningful keywords, with the first two (PORT and SPEED) being
     mandatory:

     ‘:port PORT’
          This is the name of the serial port.  On Unix and GNU systems,
          this is a file name such as ‘/dev/ttyS0’.  On Windows, this
          could be ‘COM1’, or ‘\\.\COM10’ for ports higher than ‘COM9’
          (double the backslashes in Lisp strings).

     ‘:speed SPEED’
          The speed of the serial port in bits per second.  This
          function calls ‘serial-process-configure’ to handle the speed;
          see the following documentation of that function for more
          details.

     ‘:name NAME’
          The name of the process.  If NAME is not given, PORT will
          serve as the process name as well.

     ‘:buffer BUFFER’
          The buffer to associate with the process.  The value can be
          either a buffer or a string that names a buffer.  Process
          output goes at the end of that buffer, unless you specify an
          output stream or filter function to handle the output.  If
          BUFFER is not given, the process buffer’s name is taken from
          the value of the ‘:name’ keyword.

     ‘:coding CODING’
          If CODING is a symbol, it specifies the coding system used for
          both reading and writing for this process.  If CODING is a
          cons ‘(DECODING . ENCODING)’, DECODING is used for reading,
          and ENCODING is used for writing.  If not specified, the
          default is to determine the coding systems from the data
          itself.

     ‘:noquery QUERY-FLAG’
          Initialize the process query flag to QUERY-FLAG.  *Note Query
          Before Exit::.  The flags defaults to ‘nil’ if unspecified.

     ‘:stop BOOL’
          Start process in the stopped state if BOOL is non-‘nil’.  In
          the stopped state, a serial process does not accept incoming
          data, but you can send outgoing data.  The stopped state is
          cleared by ‘continue-process’ and set by ‘stop-process’.

     ‘:filter FILTER’
          Install FILTER as the process filter.

     ‘:sentinel SENTINEL’
          Install SENTINEL as the process sentinel.

     ‘:plist PLIST’
          Install PLIST as the initial plist of the process.

     ‘:bytesize’
     ‘:parity’
     ‘:stopbits’
     ‘:flowcontrol’
          These are handled by ‘serial-process-configure’, which is
          called by ‘make-serial-process’.

     The original argument list, possibly modified by later
     configuration, is available via the function ‘process-contact’.

     Here is an example:

          (make-serial-process :port "/dev/ttyS0" :speed 9600)

 -- Function: serial-process-configure &rest args

     This function configures a serial port connection.  Arguments are
     specified as keyword/argument pairs.  Attributes that are not given
     are re-initialized from the process’s current configuration
     (available via the function ‘process-contact’), or set to
     reasonable default values.  The following arguments are defined:

     ‘:process PROCESS’
     ‘:name NAME’
     ‘:buffer BUFFER’
     ‘:port PORT’
          Any of these arguments can be given to identify the process
          that is to be configured.  If none of these arguments is
          given, the current buffer’s process is used.

     ‘:speed SPEED’
          The speed of the serial port in bits per second, a.k.a. “baud
          rate”.  The value can be any number, but most serial ports
          work only at a few defined values between 1200 and 115200,
          with 9600 being the most common value.  If SPEED is ‘nil’, the
          function ignores all other arguments and does not configure
          the port.  This may be useful for special serial ports such as
          Bluetooth-to-serial converters, which can only be configured
          through ‘AT’ commands sent through the connection.  The value
          of ‘nil’ for SPEED is valid only for connections that were
          already opened by a previous call to ‘make-serial-process’ or
          ‘serial-term’.

     ‘:bytesize BYTESIZE’
          The number of bits per byte, which can be 7 or 8.  If BYTESIZE
          is not given or ‘nil’, it defaults to 8.

     ‘:parity PARITY’
          The value can be ‘nil’ (don’t use parity), the symbol ‘odd’
          (use odd parity), or the symbol ‘even’ (use even parity).  If
          PARITY is not given, it defaults to no parity.

     ‘:stopbits STOPBITS’
          The number of stopbits used to terminate a transmission of
          each byte.  STOPBITS can be 1 or 2.  If STOPBITS is not given
          or ‘nil’, it defaults to 1.

     ‘:flowcontrol FLOWCONTROL’
          The type of flow control to use for this connection, which is
          either ‘nil’ (don’t use flow control), the symbol ‘hw’ (use
          RTS/CTS hardware flow control), or the symbol ‘sw’ (use
          XON/XOFF software flow control).  If FLOWCONTROL is not given,
          it defaults to no flow control.

     Internally, ‘make-serial-process’ calls ‘serial-process-configure’
     for the initial configuration of the serial port.


File: elisp.info,  Node: Byte Packing,  Prev: Serial Ports,  Up: Processes

36.20 Packing and Unpacking Byte Arrays
=======================================

This section describes how to pack and unpack arrays of bytes, usually
for binary network protocols.  These functions convert byte arrays to
alists, and vice versa.  The byte array can be represented as a unibyte
string or as a vector of integers, while the alist associates symbols
either with fixed-size objects or with recursive sub-alists.  To use the
functions referred to in this section, load the ‘bindat’ library.

   Conversion from byte arrays to nested alists is also known as
“deserializing” or “unpacking”, while going in the opposite direction is
also known as “serializing” or “packing”.

* Menu:

* Bindat Spec::         Describing data layout.
* Bindat Functions::    Doing the unpacking and packing.
* Bindat Examples::     Samples of what bindat.el can do for you!


File: elisp.info,  Node: Bindat Spec,  Next: Bindat Functions,  Up: Byte Packing

36.20.1 Describing Data Layout
------------------------------

To control unpacking and packing, you write a “data layout
specification”, a special nested list describing named and typed
“fields”.  This specification controls the length of each field to be
processed, and how to pack or unpack it.  We normally keep bindat specs
in variables whose names end in ‘-bindat-spec’; that kind of name is
automatically recognized as risky.

   A field’s “type” describes the size (in bytes) of the object that the
field represents and, in the case of multibyte fields, how the bytes are
ordered within the field.  The two possible orderings are “big endian”
(also known as “network byte ordering”) and “little endian”.  For
instance, the number ‘#x23cd’ (decimal 9165) in big endian would be the
two bytes ‘#x23’ ‘#xcd’; and in little endian, ‘#xcd’ ‘#x23’.  Here are
the possible type values:

‘u8’
‘byte’
     Unsigned byte, with length 1.

‘u16’
‘word’
‘short’
     Unsigned integer in network byte order, with length 2.

‘u24’
     Unsigned integer in network byte order, with length 3.

‘u32’
‘dword’
‘long’
     Unsigned integer in network byte order, with length 4.  Note: These
     values may be limited by Emacs’s integer implementation limits.

‘u16r’
‘u24r’
‘u32r’
     Unsigned integer in little endian order, with length 2, 3 and 4,
     respectively.

‘str LEN’
     String of length LEN.

‘strz LEN’
     Zero-terminated string, in a fixed-size field with length LEN.

‘vec LEN [TYPE]’
     Vector of LEN elements of type TYPE, defaulting to bytes.  The TYPE
     is any of the simple types above, or another vector specified as a
     list of the form ‘(vec LEN [TYPE])’.

‘ip’
     Four-byte vector representing an Internet address.  For example:
     ‘[127 0 0 1]’ for localhost.

‘bits LEN’
     List of set bits in LEN bytes.  The bytes are taken in big endian
     order and the bits are numbered starting with ‘8 * LEN − 1’ and
     ending with zero.  For example: ‘bits 2’ unpacks ‘#x28’ ‘#x1c’ to
     ‘(2 3 4 11 13)’ and ‘#x1c’ ‘#x28’ to ‘(3 5 10 11 12)’.

‘(eval FORM)’
     FORM is a Lisp expression evaluated at the moment the field is
     unpacked or packed.  The result of the evaluation should be one of
     the above-listed type specifications.

   For a fixed-size field, the length LEN is given as an integer
specifying the number of bytes in the field.

   When the length of a field is not fixed, it typically depends on the
value of a preceding field.  In this case, the length LEN can be given
either as a list ‘(NAME ...)’ identifying a “field name” in the format
specified for ‘bindat-get-field’ below, or by an expression ‘(eval
FORM)’ where FORM should evaluate to an integer, specifying the field
length.

   A field specification generally has the form ‘([NAME] HANDLER)’,
where NAME is optional.  Don’t use names that are symbols meaningful as
type specifications (above) or handler specifications (below), since
that would be ambiguous.  NAME can be a symbol or an expression ‘(eval
FORM)’, in which case FORM should evaluate to a symbol.

   HANDLER describes how to unpack or pack the field and can be one of
the following:

‘TYPE’
     Unpack/pack this field according to the type specification TYPE.

‘eval FORM’
     Evaluate FORM, a Lisp expression, for side-effect only.  If the
     field name is specified, the value is bound to that field name.

‘fill LEN’
     Skip LEN bytes.  In packing, this leaves them unchanged, which
     normally means they remain zero.  In unpacking, this means they are
     ignored.

‘align LEN’
     Skip to the next multiple of LEN bytes.

‘struct SPEC-NAME’
     Process SPEC-NAME as a sub-specification.  This describes a
     structure nested within another structure.

‘union FORM (TAG SPEC)...’
     Evaluate FORM, a Lisp expression, find the first TAG that matches
     it, and process its associated data layout specification SPEC.
     Matching can occur in one of three ways:

        • If a TAG has the form ‘(eval EXPR)’, evaluate EXPR with the
          variable ‘tag’ dynamically bound to the value of FORM.  A
          non-‘nil’ result indicates a match.

        • TAG matches if it is ‘equal’ to the value of FORM.

        • TAG matches unconditionally if it is ‘t’.

‘repeat COUNT FIELD-SPECS...’
     Process the FIELD-SPECS recursively, in order, then repeat starting
     from the first one, processing all the specifications COUNT times
     overall.  The COUNT is given using the same formats as a field
     length—if an ‘eval’ form is used, it is evaluated just once.  For
     correct operation, each specification in FIELD-SPECS must include a
     name.

   For the ‘(eval FORM)’ forms used in a bindat specification, the FORM
can access and update these dynamically bound variables during
evaluation:

‘last’
     Value of the last field processed.

‘bindat-raw’
     The data as a byte array.

‘bindat-idx’
     Current index (within ‘bindat-raw’) for unpacking or packing.

‘struct’
     The alist containing the structured data that have been unpacked so
     far, or the entire structure being packed.  You can use
     ‘bindat-get-field’ to access specific fields of this structure.

‘count’
‘index’
     Inside a ‘repeat’ block, these contain the maximum number of
     repetitions (as specified by the COUNT parameter), and the current
     repetition number (counting from 0).  Setting ‘count’ to zero will
     terminate the inner-most repeat block after the current repetition
     has completed.


File: elisp.info,  Node: Bindat Functions,  Next: Bindat Examples,  Prev: Bindat Spec,  Up: Byte Packing

36.20.2 Functions to Unpack and Pack Bytes
------------------------------------------

In the following documentation, SPEC refers to a data layout
specification, ‘bindat-raw’ to a byte array, and STRUCT to an alist
representing unpacked field data.

 -- Function: bindat-unpack spec bindat-raw &optional bindat-idx
     This function unpacks data from the unibyte string or byte array
     ‘bindat-raw’ according to SPEC.  Normally, this starts unpacking at
     the beginning of the byte array, but if BINDAT-IDX is non-‘nil’, it
     specifies a zero-based starting position to use instead.

     The value is an alist or nested alist in which each element
     describes one unpacked field.

 -- Function: bindat-get-field struct &rest name
     This function selects a field’s data from the nested alist STRUCT.
     Usually STRUCT was returned by ‘bindat-unpack’.  If NAME
     corresponds to just one argument, that means to extract a top-level
     field value.  Multiple NAME arguments specify repeated lookup of
     sub-structures.  An integer name acts as an array index.

     For example, if NAME is ‘(a b 2 c)’, that means to find field ‘c’
     in the third element of subfield ‘b’ of field ‘a’.  (This
     corresponds to ‘struct.a.b[2].c’ in C.)

   Although packing and unpacking operations change the organization of
data (in memory), they preserve the data’s “total length”, which is the
sum of all the fields’ lengths, in bytes.  This value is not generally
inherent in either the specification or alist alone; instead, both
pieces of information contribute to its calculation.  Likewise, the
length of a string or array being unpacked may be longer than the data’s
total length as described by the specification.

 -- Function: bindat-length spec struct
     This function returns the total length of the data in STRUCT,
     according to SPEC.

 -- Function: bindat-pack spec struct &optional bindat-raw bindat-idx
     This function returns a byte array packed according to SPEC from
     the data in the alist STRUCT.  It normally creates and fills a new
     byte array starting at the beginning.  However, if BINDAT-RAW is
     non-‘nil’, it specifies a pre-allocated unibyte string or vector to
     pack into.  If BINDAT-IDX is non-‘nil’, it specifies the starting
     offset for packing into ‘bindat-raw’.

     When pre-allocating, you should make sure ‘(length BINDAT-RAW)’
     meets or exceeds the total length to avoid an out-of-range error.

 -- Function: bindat-ip-to-string ip
     Convert the Internet address vector IP to a string in the usual
     dotted notation.

          (bindat-ip-to-string [127 0 0 1])
               ⇒ "127.0.0.1"


File: elisp.info,  Node: Bindat Examples,  Prev: Bindat Functions,  Up: Byte Packing

36.20.3 Examples of Byte Unpacking and Packing
----------------------------------------------

Here is a complete example of byte unpacking and packing:

     (require 'bindat)

     (defvar fcookie-index-spec
       '((:version  u32)
         (:count    u32)
         (:longest  u32)
         (:shortest u32)
         (:flags    u32)
         (:delim    u8)
         (:ignored  fill 3)
         (:offset   repeat (:count) (:foo u32)))
       "Description of a fortune cookie index file's contents.")

     (defun fcookie (cookies &optional index)
       "Display a random fortune cookie from file COOKIES.
     Optional second arg INDEX specifies the associated index
     filename, by default \"COOKIES.dat\".  Display cookie text
     in buffer \"*Fortune Cookie: BASENAME*\", where BASENAME
     is COOKIES without the directory part."
       (interactive "fCookies file: ")
       (let* ((info (with-temp-buffer
                      (insert-file-contents-literally
                       (or index (concat cookies ".dat")))
                      (bindat-unpack fcookie-index-spec
                                     (buffer-string))))
              (sel (random (bindat-get-field info :count)))
              (beg (cdar (bindat-get-field info :offset sel)))
              (end (or (cdar (bindat-get-field info
                                               :offset (1+ sel)))
                       (nth 7 (file-attributes cookies)))))
         (switch-to-buffer
          (get-buffer-create
           (format "*Fortune Cookie: %s*"
                   (file-name-nondirectory cookies))))
         (erase-buffer)
         (insert-file-contents-literally
          cookies nil beg (- end 3))))

     (defun fcookie-create-index (cookies &optional index delim)
       "Scan file COOKIES, and write out its index file.
     Optional arg INDEX specifies the index filename, which by
     default is \"COOKIES.dat\".  Optional arg DELIM specifies the
     unibyte character that, when found on a line of its own in
     COOKIES, indicates the border between entries."
       (interactive "fCookies file: ")
       (setq delim (or delim ?%))
       (let ((delim-line (format "\n%c\n" delim))
             (count 0)
             (max 0)
             min p q len offsets)
         (unless (= 3 (string-bytes delim-line))
           (error "Delimiter cannot be represented in one byte"))
         (with-temp-buffer
           (insert-file-contents-literally cookies)
           (while (and (setq p (point))
                       (search-forward delim-line (point-max) t)
                       (setq len (- (point) 3 p)))
             (setq count (1+ count)
                   max (max max len)
                   min (min (or min max) len)
                   offsets (cons (1- p) offsets))))
         (with-temp-buffer
           (set-buffer-multibyte nil)
           (insert
            (bindat-pack
             fcookie-index-spec
             `((:version . 2)
               (:count . ,count)
               (:longest . ,max)
               (:shortest . ,min)
               (:flags . 0)
               (:delim . ,delim)
               (:offset . ,(mapcar (lambda (o)
                                     (list (cons :foo o)))
                                   (nreverse offsets))))))
           (let ((coding-system-for-write 'raw-text-unix))
             (write-file (or index (concat cookies ".dat")))))))

   The following is an example of defining and unpacking a complex
structure.  Consider the following C structures:

     struct header {
         unsigned long    dest_ip;
         unsigned long    src_ip;
         unsigned short   dest_port;
         unsigned short   src_port;
     };

     struct data {
         unsigned char    type;
         unsigned char    opcode;
         unsigned short   length;  /* in network byte order  */
         unsigned char    id[8];   /* null-terminated string  */
         unsigned char    data[/* (length + 3) & ~3 */];
     };

     struct packet {
         struct header    header;
         unsigned long    counters[2];  /* in little endian order  */
         unsigned char    items;
         unsigned char    filler[3];
         struct data      item[/* items */];

     };

   The corresponding data layout specification is:

     (setq header-spec
           '((dest-ip   ip)
             (src-ip    ip)
             (dest-port u16)
             (src-port  u16)))

     (setq data-spec
           '((type      u8)
             (opcode    u8)
             (length    u16)  ; network byte order
             (id        strz 8)
             (data      vec (length))
             (align     4)))

     (setq packet-spec
           '((header    struct header-spec)
             (counters  vec 2 u32r)   ; little endian order
             (items     u8)
             (fill      3)
             (item      repeat (items)
                        (struct data-spec))))

   A binary data representation is:

     (setq binary-data
           [ 192 168 1 100 192 168 1 101 01 28 21 32
             160 134 1 0 5 1 0 0 2 0 0 0
             2 3 0 5 ?A ?B ?C ?D ?E ?F 0 0 1 2 3 4 5 0 0 0
             1 4 0 7 ?B ?C ?D ?E ?F ?G 0 0 6 7 8 9 10 11 12 0 ])

   The corresponding decoded structure is:

     (setq decoded (bindat-unpack packet-spec binary-data))
          ⇒
     ((header
       (dest-ip   . [192 168 1 100])
       (src-ip    . [192 168 1 101])
       (dest-port . 284)
       (src-port  . 5408))
      (counters . [100000 261])
      (items . 2)
      (item ((data . [1 2 3 4 5])
             (id . "ABCDEF")
             (length . 5)
             (opcode . 3)
             (type . 2))
            ((data . [6 7 8 9 10 11 12])
             (id . "BCDEFG")
             (length . 7)
             (opcode . 4)
             (type . 1))))

   An example of fetching data from this structure:

     (bindat-get-field decoded 'item 1 'id)
          ⇒ "BCDEFG"


File: elisp.info,  Node: Display,  Next: System Interface,  Prev: Processes,  Up: Top

37 Emacs Display
****************

This chapter describes a number of features related to the display that
Emacs presents to the user.

* Menu:

* Refresh Screen::      Clearing the screen and redrawing everything on it.
* Forcing Redisplay::   Forcing redisplay.
* Truncation::          Folding or wrapping long text lines.
* The Echo Area::       Displaying messages at the bottom of the screen.
* Warnings::            Displaying warning messages for the user.
* Invisible Text::      Hiding part of the buffer text.
* Selective Display::   Hiding part of the buffer text (the old way).
* Temporary Displays::  Displays that go away automatically.
* Overlays::            Use overlays to highlight parts of the buffer.
* Size of Displayed Text::  How large displayed text is.
* Line Height::         Controlling the height of lines.
* Faces::               A face defines a graphics style for text characters:
                          font, colors, etc.
* Fringes::             Controlling window fringes.
* Scroll Bars::         Controlling scroll bars.
* Window Dividers::     Separating windows visually.
* Display Property::    Enabling special display features.
* Images::              Displaying images in Emacs buffers.
* Xwidgets::            Displaying native widgets in Emacs buffers.
* Buttons::             Adding clickable buttons to Emacs buffers.
* Abstract Display::    Emacs’s Widget for Object Collections.
* Blinking::            How Emacs shows the matching open parenthesis.
* Character Display::   How Emacs displays individual characters.
* Beeping::             Audible signal to the user.
* Window Systems::      Which window system is being used.
* Tooltips::            Tooltip display in Emacs.
* Bidirectional Display:: Display of bidirectional scripts, such as
                             Arabic and Farsi.


File: elisp.info,  Node: Refresh Screen,  Next: Forcing Redisplay,  Up: Display

37.1 Refreshing the Screen
==========================

The function ‘redraw-frame’ clears and redisplays the entire contents of
a given frame (*note Frames::).  This is useful if the screen is
corrupted.

 -- Function: redraw-frame &optional frame
     This function clears and redisplays frame FRAME.  If FRAME is
     omitted or nil, it redraws the selected frame.

   Even more powerful is ‘redraw-display’:

 -- Command: redraw-display
     This function clears and redisplays all visible frames.

   In Emacs, processing user input takes priority over redisplay.  If
you call these functions when input is available, they don’t redisplay
immediately, but the requested redisplay does happen eventually—after
all the input has been processed.

   On text terminals, suspending and resuming Emacs normally also
refreshes the screen.  Some terminal emulators record separate contents
for display-oriented programs such as Emacs and for ordinary sequential
display.  If you are using such a terminal, you might want to inhibit
the redisplay on resumption.

 -- User Option: no-redraw-on-reenter
     This variable controls whether Emacs redraws the entire screen
     after it has been suspended and resumed.  Non-‘nil’ means there is
     no need to redraw, ‘nil’ means redrawing is needed.  The default is
     ‘nil’.


File: elisp.info,  Node: Forcing Redisplay,  Next: Truncation,  Prev: Refresh Screen,  Up: Display

37.2 Forcing Redisplay
======================

Emacs normally tries to redisplay the screen whenever it waits for
input.  With the following function, you can request an immediate
attempt to redisplay, in the middle of Lisp code, without actually
waiting for input.

 -- Function: redisplay &optional force
     This function tries immediately to redisplay.  The optional
     argument FORCE, if non-‘nil’, forces the redisplay to be performed,
     instead of being preempted if input is pending.

     The function returns ‘t’ if it actually tried to redisplay, and
     ‘nil’ otherwise.  A value of ‘t’ does not mean that redisplay
     proceeded to completion; it could have been preempted by newly
     arriving input.

   Although ‘redisplay’ tries immediately to redisplay, it does not
change how Emacs decides which parts of its frame(s) to redisplay.  By
contrast, the following function adds certain windows to the pending
redisplay work (as if their contents had completely changed), but does
not immediately try to perform redisplay.

 -- Function: force-window-update &optional object
     This function forces some or all windows to be updated the next
     time Emacs does a redisplay.  If OBJECT is a window, that window is
     to be updated.  If OBJECT is a buffer or buffer name, all windows
     displaying that buffer are to be updated.  If OBJECT is ‘nil’ (or
     omitted), all windows are to be updated.

     This function does not do a redisplay immediately; Emacs does that
     as it waits for input, or when the function ‘redisplay’ is called.

 -- Variable: pre-redisplay-function
     A function run just before redisplay.  It is called with one
     argument, the set of windows to be redisplayed.  The set can be
     ‘nil’, meaning only the selected window, or ‘t’, meaning all the
     windows.

 -- Variable: pre-redisplay-functions
     This hook is run just before redisplay.  It is called once in each
     window that is about to be redisplayed, with ‘current-buffer’ set
     to the buffer displayed in that window.


File: elisp.info,  Node: Truncation,  Next: The Echo Area,  Prev: Forcing Redisplay,  Up: Display

37.3 Truncation
===============

When a line of text extends beyond the right edge of a window, Emacs can
“continue” the line (make it wrap to the next screen line), or
“truncate” the line (limit it to one screen line).  The additional
screen lines used to display a long text line are called “continuation”
lines.  Continuation is not the same as filling; continuation happens on
the screen only, not in the buffer contents, and it breaks a line
precisely at the right margin, not at a word boundary.  *Note Filling::.

   On a graphical display, tiny arrow images in the window fringes
indicate truncated and continued lines (*note Fringes::).  On a text
terminal, a ‘$’ in the rightmost column of the window indicates
truncation; a ‘\’ on the rightmost column indicates a line that wraps.
(The display table can specify alternate characters to use for this;
*note Display Tables::).

 -- User Option: truncate-lines
     If this buffer-local variable is non-‘nil’, lines that extend
     beyond the right edge of the window are truncated; otherwise, they
     are continued.  As a special exception, the variable
     ‘truncate-partial-width-windows’ takes precedence in
     “partial-width” windows (i.e., windows that do not occupy the
     entire frame width).

 -- User Option: truncate-partial-width-windows
     This variable controls line truncation in “partial-width” windows.
     A partial-width window is one that does not occupy the entire frame
     width (*note Splitting Windows::).  If the value is ‘nil’, line
     truncation is determined by the variable ‘truncate-lines’ (see
     above).  If the value is an integer N, lines are truncated if the
     partial-width window has fewer than N columns, regardless of the
     value of ‘truncate-lines’; if the partial-width window has N or
     more columns, line truncation is determined by ‘truncate-lines’.
     For any other non-‘nil’ value, lines are truncated in every
     partial-width window, regardless of the value of ‘truncate-lines’.

   When horizontal scrolling (*note Horizontal Scrolling::) is in use in
a window, that forces truncation.

 -- Variable: wrap-prefix
     If this buffer-local variable is non-‘nil’, it defines a “wrap
     prefix” which Emacs displays at the start of every continuation
     line.  (If lines are truncated, ‘wrap-prefix’ is never used.)  Its
     value may be a string or an image (*note Other Display Specs::), or
     a stretch of whitespace such as specified by the ‘:width’ or
     ‘:align-to’ display properties (*note Specified Space::).  The
     value is interpreted in the same way as a ‘display’ text property.
     *Note Display Property::.

     A wrap prefix may also be specified for regions of text, using the
     ‘wrap-prefix’ text or overlay property.  This takes precedence over
     the ‘wrap-prefix’ variable.  *Note Special Properties::.

 -- Variable: line-prefix
     If this buffer-local variable is non-‘nil’, it defines a “line
     prefix” which Emacs displays at the start of every non-continuation
     line.  Its value may be a string or an image (*note Other Display
     Specs::), or a stretch of whitespace such as specified by the
     ‘:width’ or ‘:align-to’ display properties (*note Specified
     Space::).  The value is interpreted in the same way as a ‘display’
     text property.  *Note Display Property::.

     A line prefix may also be specified for regions of text using the
     ‘line-prefix’ text or overlay property.  This takes precedence over
     the ‘line-prefix’ variable.  *Note Special Properties::.


File: elisp.info,  Node: The Echo Area,  Next: Warnings,  Prev: Truncation,  Up: Display

37.4 The Echo Area
==================

The “echo area” is used for displaying error messages (*note Errors::),
for messages made with the ‘message’ primitive, and for echoing
keystrokes.  It is not the same as the minibuffer, despite the fact that
the minibuffer appears (when active) in the same place on the screen as
the echo area.  *Note The Minibuffer: (emacs)Minibuffer.

   Apart from the functions documented in this section, you can print
Lisp objects to the echo area by specifying ‘t’ as the output stream.
*Note Output Streams::.

* Menu:

* Displaying Messages:: Explicitly displaying text in the echo area.
* Progress::            Informing user about progress of a long operation.
* Logging Messages::    Echo area messages are logged for the user.
* Echo Area Customization:: Controlling the echo area.


File: elisp.info,  Node: Displaying Messages,  Next: Progress,  Up: The Echo Area

37.4.1 Displaying Messages in the Echo Area
-------------------------------------------

This section describes the standard functions for displaying messages in
the echo area.

 -- Function: message format-string &rest arguments
     This function displays a message in the echo area.  FORMAT-STRING
     is a format string, and ARGUMENTS are the objects for its format
     specifications, like in the ‘format-message’ function (*note
     Formatting Strings::).  The resulting formatted string is displayed
     in the echo area; if it contains ‘face’ text properties, it is
     displayed with the specified faces (*note Faces::).  The string is
     also added to the ‘*Messages*’ buffer, but without text properties
     (*note Logging Messages::).

     The ‘text-quoting-style’ variable controls what quotes are
     generated; *Note Keys in Documentation::.  A call using a format
     like "Missing `%s'" with grave accents and apostrophes typically
     generates a message like "Missing ‘foo’" with matching curved
     quotes.  In contrast, a call using a format like "Missing '%s'"
     with only apostrophes typically generates a message like "Missing
     ’foo’" with only closing curved quotes, an unusual style in
     English.

     In batch mode, the message is printed to the standard error stream,
     followed by a newline.

     When ‘inhibit-message’ is non-‘nil’, no message will be displayed
     in the echo area, it will only be logged to ‘*Messages*’.

     If FORMAT-STRING is ‘nil’ or the empty string, ‘message’ clears the
     echo area; if the echo area has been expanded automatically, this
     brings it back to its normal size.  If the minibuffer is active,
     this brings the minibuffer contents back onto the screen
     immediately.

          (message "Reverting `%s'..." (buffer-name))
           ⊣ Reverting ‘subr.el’...
          ⇒ "Reverting ‘subr.el’..."

          ---------- Echo Area ----------
          Reverting ‘subr.el’...
          ---------- Echo Area ----------

     To automatically display a message in the echo area or in a
     pop-buffer, depending on its size, use ‘display-message-or-buffer’
     (see below).

     *Warning:* If you want to use your own string as a message
     verbatim, don’t just write ‘(message STRING)’.  If STRING contains
     ‘%’, ‘`’, or ‘'’ it may be reformatted, with undesirable results.
     Instead, use ‘(message "%s" STRING)’.

 -- Variable: inhibit-message
     When this variable is non-‘nil’, ‘message’ and related functions
     will not use the Echo Area to display messages.

 -- Macro: with-temp-message message &rest body
     This construct displays a message in the echo area temporarily,
     during the execution of BODY.  It displays MESSAGE, executes BODY,
     then returns the value of the last body form while restoring the
     previous echo area contents.

 -- Function: message-or-box format-string &rest arguments
     This function displays a message like ‘message’, but may display it
     in a dialog box instead of the echo area.  If this function is
     called in a command that was invoked using the mouse—more
     precisely, if ‘last-nonmenu-event’ (*note Command Loop Info::) is
     either ‘nil’ or a list—then it uses a dialog box or pop-up menu to
     display the message.  Otherwise, it uses the echo area.  (This is
     the same criterion that ‘y-or-n-p’ uses to make a similar decision;
     see *note Yes-or-No Queries::.)

     You can force use of the mouse or of the echo area by binding
     ‘last-nonmenu-event’ to a suitable value around the call.

 -- Function: message-box format-string &rest arguments
     This function displays a message like ‘message’, but uses a dialog
     box (or a pop-up menu) whenever that is possible.  If it is
     impossible to use a dialog box or pop-up menu, because the terminal
     does not support them, then ‘message-box’ uses the echo area, like
     ‘message’.

 -- Function: display-message-or-buffer message &optional buffer-name
          action frame
     This function displays the message MESSAGE, which may be either a
     string or a buffer.  If it is shorter than the maximum height of
     the echo area, as defined by ‘max-mini-window-height’, it is
     displayed in the echo area, using ‘message’.  Otherwise,
     ‘display-buffer’ is used to show it in a pop-up buffer.

     Returns either the string shown in the echo area, or when a pop-up
     buffer is used, the window used to display it.

     If MESSAGE is a string, then the optional argument BUFFER-NAME is
     the name of the buffer used to display it when a pop-up buffer is
     used, defaulting to ‘*Message*’.  In the case where MESSAGE is a
     string and displayed in the echo area, it is not specified whether
     the contents are inserted into the buffer anyway.

     The optional arguments ACTION and FRAME are as for
     ‘display-buffer’, and only used if a buffer is displayed.

 -- Function: current-message
     This function returns the message currently being displayed in the
     echo area, or ‘nil’ if there is none.


File: elisp.info,  Node: Progress,  Next: Logging Messages,  Prev: Displaying Messages,  Up: The Echo Area

37.4.2 Reporting Operation Progress
-----------------------------------

When an operation can take a while to finish, you should inform the user
about the progress it makes.  This way the user can estimate remaining
time and clearly see that Emacs is busy working, not hung.  A convenient
way to do this is to use a “progress reporter”.

   Here is a working example that does nothing useful:

     (let ((progress-reporter
            (make-progress-reporter "Collecting mana for Emacs..."
                                    0  500)))
       (dotimes (k 500)
         (sit-for 0.01)
         (progress-reporter-update progress-reporter k))
       (progress-reporter-done progress-reporter))

 -- Function: make-progress-reporter message &optional min-value
          max-value current-value min-change min-time
     This function creates and returns a progress reporter object, which
     you will use as an argument for the other functions listed below.
     The idea is to precompute as much data as possible to make progress
     reporting very fast.

     When this progress reporter is subsequently used, it will display
     MESSAGE in the echo area, followed by progress percentage.  MESSAGE
     is treated as a simple string.  If you need it to depend on a
     filename, for instance, use ‘format-message’ before calling this
     function.

     The arguments MIN-VALUE and MAX-VALUE should be numbers standing
     for the starting and final states of the operation.  For instance,
     an operation that scans a buffer should set these to the results of
     ‘point-min’ and ‘point-max’ correspondingly.  MAX-VALUE should be
     greater than MIN-VALUE.

     Alternatively, you can set MIN-VALUE and MAX-VALUE to ‘nil’.  In
     that case, the progress reporter does not report process
     percentages; it instead displays a “spinner” that rotates a notch
     each time you update the progress reporter.

     If MIN-VALUE and MAX-VALUE are numbers, you can give the argument
     CURRENT-VALUE a numerical value specifying the initial progress; if
     omitted, this defaults to MIN-VALUE.

     The remaining arguments control the rate of echo area updates.  The
     progress reporter will wait for at least MIN-CHANGE more percents
     of the operation to be completed before printing next message; the
     default is one percent.  MIN-TIME specifies the minimum time in
     seconds to pass between successive prints; the default is 0.2
     seconds.  (On some operating systems, the progress reporter may
     handle fractions of seconds with varying precision).

     This function calls ‘progress-reporter-update’, so the first
     message is printed immediately.

 -- Function: progress-reporter-update reporter &optional value
     This function does the main work of reporting progress of your
     operation.  It displays the message of REPORTER, followed by
     progress percentage determined by VALUE.  If percentage is zero, or
     close enough according to the MIN-CHANGE and MIN-TIME arguments,
     then it is omitted from the output.

     REPORTER must be the result of a call to ‘make-progress-reporter’.
     VALUE specifies the current state of your operation and must be
     between MIN-VALUE and MAX-VALUE (inclusive) as passed to
     ‘make-progress-reporter’.  For instance, if you scan a buffer, then
     VALUE should be the result of a call to ‘point’.

     This function respects MIN-CHANGE and MIN-TIME as passed to
     ‘make-progress-reporter’ and so does not output new messages on
     every invocation.  It is thus very fast and normally you should not
     try to reduce the number of calls to it: resulting overhead will
     most likely negate your effort.

 -- Function: progress-reporter-force-update reporter &optional value
          new-message
     This function is similar to ‘progress-reporter-update’ except that
     it prints a message in the echo area unconditionally.

     The first two arguments have the same meaning as for
     ‘progress-reporter-update’.  Optional NEW-MESSAGE allows you to
     change the message of the REPORTER.  Since this function always
     updates the echo area, such a change will be immediately presented
     to the user.

 -- Function: progress-reporter-done reporter
     This function should be called when the operation is finished.  It
     prints the message of REPORTER followed by word ‘done’ in the echo
     area.

     You should always call this function and not hope for
     ‘progress-reporter-update’ to print ‘100%’.  Firstly, it may never
     print it, there are many good reasons for this not to happen.
     Secondly, ‘done’ is more explicit.

 -- Macro: dotimes-with-progress-reporter (var count [result]) message
          body...
     This is a convenience macro that works the same way as ‘dotimes’
     does, but also reports loop progress using the functions described
     above.  It allows you to save some typing.

     You can rewrite the example in the beginning of this node using
     this macro this way:

          (dotimes-with-progress-reporter
              (k 500)
              "Collecting some mana for Emacs..."
            (sit-for 0.01))


File: elisp.info,  Node: Logging Messages,  Next: Echo Area Customization,  Prev: Progress,  Up: The Echo Area

37.4.3 Logging Messages in ‘*Messages*’
---------------------------------------

Almost all the messages displayed in the echo area are also recorded in
the ‘*Messages*’ buffer so that the user can refer back to them.  This
includes all the messages that are output with ‘message’.  By default,
this buffer is read-only and uses the major mode ‘messages-buffer-mode’.
Nothing prevents the user from killing the ‘*Messages*’ buffer, but the
next display of a message recreates it.  Any Lisp code that needs to
access the ‘*Messages*’ buffer directly and wants to ensure that it
exists should use the function ‘messages-buffer’.

 -- Function: messages-buffer
     This function returns the ‘*Messages*’ buffer.  If it does not
     exist, it creates it, and switches it to ‘messages-buffer-mode’.

 -- User Option: message-log-max
     This variable specifies how many lines to keep in the ‘*Messages*’
     buffer.  The value ‘t’ means there is no limit on how many lines to
     keep.  The value ‘nil’ disables message logging entirely.  Here’s
     how to display a message and prevent it from being logged:

          (let (message-log-max)
            (message ...))

   To make ‘*Messages*’ more convenient for the user, the logging
facility combines successive identical messages.  It also combines
successive related messages for the sake of two cases: question followed
by answer, and a series of progress messages.

   A question followed by an answer has two messages like the ones
produced by ‘y-or-n-p’: the first is ‘QUESTION’, and the second is
‘QUESTION...ANSWER’.  The first message conveys no additional
information beyond what’s in the second, so logging the second message
discards the first from the log.

   A series of progress messages has successive messages like those
produced by ‘make-progress-reporter’.  They have the form
‘BASE...HOW-FAR’, where BASE is the same each time, while HOW-FAR
varies.  Logging each message in the series discards the previous one,
provided they are consecutive.

   The functions ‘make-progress-reporter’ and ‘y-or-n-p’ don’t have to
do anything special to activate the message log combination feature.  It
operates whenever two consecutive messages are logged that share a
common prefix ending in ‘...’.


File: elisp.info,  Node: Echo Area Customization,  Prev: Logging Messages,  Up: The Echo Area

37.4.4 Echo Area Customization
------------------------------

These variables control details of how the echo area works.

 -- Variable: cursor-in-echo-area
     This variable controls where the cursor appears when a message is
     displayed in the echo area.  If it is non-‘nil’, then the cursor
     appears at the end of the message.  Otherwise, the cursor appears
     at point—not in the echo area at all.

     The value is normally ‘nil’; Lisp programs bind it to ‘t’ for brief
     periods of time.

 -- Variable: echo-area-clear-hook
     This normal hook is run whenever the echo area is cleared—either by
     ‘(message nil)’ or for any other reason.

 -- User Option: echo-keystrokes
     This variable determines how much time should elapse before command
     characters echo.  Its value must be a number, and specifies the
     number of seconds to wait before echoing.  If the user types a
     prefix key (such as ‘C-x’) and then delays this many seconds before
     continuing, the prefix key is echoed in the echo area.  (Once
     echoing begins in a key sequence, all subsequent characters in the
     same key sequence are echoed immediately.)

     If the value is zero, then command input is not echoed.

 -- Variable: message-truncate-lines
     Normally, displaying a long message resizes the echo area to
     display the entire message.  But if the variable
     ‘message-truncate-lines’ is non-‘nil’, the echo area does not
     resize, and the message is truncated to fit it.

   The variable ‘max-mini-window-height’, which specifies the maximum
height for resizing minibuffer windows, also applies to the echo area
(which is really a special use of the minibuffer window; *note
Minibuffer Misc::).


File: elisp.info,  Node: Warnings,  Next: Invisible Text,  Prev: The Echo Area,  Up: Display

37.5 Reporting Warnings
=======================

“Warnings” are a facility for a program to inform the user of a possible
problem, but continue running.

* Menu:

* Warning Basics::      Warnings concepts and functions to report them.
* Warning Variables::   Variables programs bind to customize their warnings.
* Warning Options::     Variables users set to control display of warnings.
* Delayed Warnings::    Deferring a warning until the end of a command.


File: elisp.info,  Node: Warning Basics,  Next: Warning Variables,  Up: Warnings

37.5.1 Warning Basics
---------------------

Every warning has a textual message, which explains the problem for the
user, and a “severity level” which is a symbol.  Here are the possible
severity levels, in order of decreasing severity, and their meanings:

‘:emergency’
     A problem that will seriously impair Emacs operation soon if you do
     not attend to it promptly.
‘:error’
     A report of data or circumstances that are inherently wrong.
‘:warning’
     A report of data or circumstances that are not inherently wrong,
     but raise suspicion of a possible problem.
‘:debug’
     A report of information that may be useful if you are debugging.

   When your program encounters invalid input data, it can either signal
a Lisp error by calling ‘error’ or ‘signal’ or report a warning with
severity ‘:error’.  Signaling a Lisp error is the easiest thing to do,
but it means the program cannot continue processing.  If you want to
take the trouble to implement a way to continue processing despite the
bad data, then reporting a warning of severity ‘:error’ is the right way
to inform the user of the problem.  For instance, the Emacs Lisp byte
compiler can report an error that way and continue compiling other
functions.  (If the program signals a Lisp error and then handles it
with ‘condition-case’, the user won’t see the error message; it could
show the message to the user by reporting it as a warning.)

   Each warning has a “warning type” to classify it.  The type is a list
of symbols.  The first symbol should be the custom group that you use
for the program’s user options.  For example, byte compiler warnings use
the warning type ‘(bytecomp)’.  You can also subcategorize the warnings,
if you wish, by using more symbols in the list.

 -- Function: display-warning type message &optional level buffer-name
     This function reports a warning, using MESSAGE as the message and
     TYPE as the warning type.  LEVEL should be the severity level, with
     ‘:warning’ being the default.

     BUFFER-NAME, if non-‘nil’, specifies the name of the buffer for
     logging the warning.  By default, it is ‘*Warnings*’.

 -- Function: lwarn type level message &rest args
     This function reports a warning using the value of ‘(format-message
     MESSAGE ARGS...)’ as the message in the ‘*Warnings*’ buffer.  In
     other respects it is equivalent to ‘display-warning’.

 -- Function: warn message &rest args
     This function reports a warning using the value of ‘(format-message
     MESSAGE ARGS...)’ as the message, ‘(emacs)’ as the type, and
     ‘:warning’ as the severity level.  It exists for compatibility
     only; we recommend not using it, because you should specify a
     specific warning type.


File: elisp.info,  Node: Warning Variables,  Next: Warning Options,  Prev: Warning Basics,  Up: Warnings

37.5.2 Warning Variables
------------------------

Programs can customize how their warnings appear by binding the
variables described in this section.

 -- Variable: warning-levels
     This list defines the meaning and severity order of the warning
     severity levels.  Each element defines one severity level, and they
     are arranged in order of decreasing severity.

     Each element has the form ‘(LEVEL STRING FUNCTION)’, where LEVEL is
     the severity level it defines.  STRING specifies the textual
     description of this level.  STRING should use ‘%s’ to specify where
     to put the warning type information, or it can omit the ‘%s’ so as
     not to include that information.

     The optional FUNCTION, if non-‘nil’, is a function to call with no
     arguments, to get the user’s attention.

     Normally you should not change the value of this variable.

 -- Variable: warning-prefix-function
     If non-‘nil’, the value is a function to generate prefix text for
     warnings.  Programs can bind the variable to a suitable function.
     ‘display-warning’ calls this function with the warnings buffer
     current, and the function can insert text in it.  That text becomes
     the beginning of the warning message.

     The function is called with two arguments, the severity level and
     its entry in ‘warning-levels’.  It should return a list to use as
     the entry (this value need not be an actual member of
     ‘warning-levels’).  By constructing this value, the function can
     change the severity of the warning, or specify different handling
     for a given severity level.

     If the variable’s value is ‘nil’ then there is no function to call.

 -- Variable: warning-series
     Programs can bind this variable to ‘t’ to say that the next warning
     should begin a series.  When several warnings form a series, that
     means to leave point on the first warning of the series, rather
     than keep moving it for each warning so that it appears on the last
     one.  The series ends when the local binding is unbound and
     ‘warning-series’ becomes ‘nil’ again.

     The value can also be a symbol with a function definition.  That is
     equivalent to ‘t’, except that the next warning will also call the
     function with no arguments with the warnings buffer current.  The
     function can insert text which will serve as a header for the
     series of warnings.

     Once a series has begun, the value is a marker which points to the
     buffer position in the warnings buffer of the start of the series.

     The variable’s normal value is ‘nil’, which means to handle each
     warning separately.

 -- Variable: warning-fill-prefix
     When this variable is non-‘nil’, it specifies a fill prefix to use
     for filling each warning’s text.

 -- Variable: warning-type-format
     This variable specifies the format for displaying the warning type
     in the warning message.  The result of formatting the type this way
     gets included in the message under the control of the string in the
     entry in ‘warning-levels’.  The default value is ‘" (%s)"’.  If you
     bind it to ‘""’ then the warning type won’t appear at all.


File: elisp.info,  Node: Warning Options,  Next: Delayed Warnings,  Prev: Warning Variables,  Up: Warnings

37.5.3 Warning Options
----------------------

These variables are used by users to control what happens when a Lisp
program reports a warning.

 -- User Option: warning-minimum-level
     This user option specifies the minimum severity level that should
     be shown immediately to the user.  The default is ‘:warning’, which
     means to immediately display all warnings except ‘:debug’ warnings.

 -- User Option: warning-minimum-log-level
     This user option specifies the minimum severity level that should
     be logged in the warnings buffer.  The default is ‘:warning’, which
     means to log all warnings except ‘:debug’ warnings.

 -- User Option: warning-suppress-types
     This list specifies which warning types should not be displayed
     immediately for the user.  Each element of the list should be a
     list of symbols.  If its elements match the first elements in a
     warning type, then that warning is not displayed immediately.

 -- User Option: warning-suppress-log-types
     This list specifies which warning types should not be logged in the
     warnings buffer.  Each element of the list should be a list of
     symbols.  If it matches the first few elements in a warning type,
     then that warning is not logged.


File: elisp.info,  Node: Delayed Warnings,  Prev: Warning Options,  Up: Warnings

37.5.4 Delayed Warnings
-----------------------

Sometimes, you may wish to avoid showing a warning while a command is
running, and only show it only after the end of the command.  You can
use the variable ‘delayed-warnings-list’ for this.

 -- Variable: delayed-warnings-list
     The value of this variable is a list of warnings to be displayed
     after the current command has finished.  Each element must be a
     list

          (TYPE MESSAGE [LEVEL [BUFFER-NAME]])

     with the same form, and the same meanings, as the argument list of
     ‘display-warning’ (*note Warning Basics::).  Immediately after
     running ‘post-command-hook’ (*note Command Overview::), the Emacs
     command loop displays all the warnings specified by this variable,
     then resets it to ‘nil’.

   Programs which need to further customize the delayed warnings
mechanism can change the variable ‘delayed-warnings-hook’:

 -- Variable: delayed-warnings-hook
     This is a normal hook which is run by the Emacs command loop, after
     ‘post-command-hook’, in order to to process and display delayed
     warnings.

     Its default value is a list of two functions:

          (collapse-delayed-warnings display-delayed-warnings)

     The function ‘collapse-delayed-warnings’ removes repeated entries
     from ‘delayed-warnings-list’.  The function
     ‘display-delayed-warnings’ calls ‘display-warning’ on each of the
     entries in ‘delayed-warnings-list’, in turn, and then sets
     ‘delayed-warnings-list’ to ‘nil’.


File: elisp.info,  Node: Invisible Text,  Next: Selective Display,  Prev: Warnings,  Up: Display

37.6 Invisible Text
===================

You can make characters “invisible”, so that they do not appear on the
screen, with the ‘invisible’ property.  This can be either a text
property (*note Text Properties::) or an overlay property (*note
Overlays::).  Cursor motion also partly ignores these characters; if the
command loop finds that point is inside a range of invisible text after
a command, it relocates point to the other side of the text.

   In the simplest case, any non-‘nil’ ‘invisible’ property makes a
character invisible.  This is the default case—if you don’t alter the
default value of ‘buffer-invisibility-spec’, this is how the ‘invisible’
property works.  You should normally use ‘t’ as the value of the
‘invisible’ property if you don’t plan to set ‘buffer-invisibility-spec’
yourself.

   More generally, you can use the variable ‘buffer-invisibility-spec’
to control which values of the ‘invisible’ property make text invisible.
This permits you to classify the text into different subsets in advance,
by giving them different ‘invisible’ values, and subsequently make
various subsets visible or invisible by changing the value of
‘buffer-invisibility-spec’.

   Controlling visibility with ‘buffer-invisibility-spec’ is especially
useful in a program to display the list of entries in a database.  It
permits the implementation of convenient filtering commands to view just
a part of the entries in the database.  Setting this variable is very
fast, much faster than scanning all the text in the buffer looking for
properties to change.

 -- Variable: buffer-invisibility-spec
     This variable specifies which kinds of ‘invisible’ properties
     actually make a character invisible.  Setting this variable makes
     it buffer-local.

     ‘t’
          A character is invisible if its ‘invisible’ property is
          non-‘nil’.  This is the default.

     a list
          Each element of the list specifies a criterion for
          invisibility; if a character’s ‘invisible’ property fits any
          one of these criteria, the character is invisible.  The list
          can have two kinds of elements:

          ‘ATOM’
               A character is invisible if its ‘invisible’ property
               value is ATOM or if it is a list with ATOM as a member;
               comparison is done with ‘eq’.

          ‘(ATOM . t)’
               A character is invisible if its ‘invisible’ property
               value is ATOM or if it is a list with ATOM as a member;
               comparison is done with ‘eq’.  Moreover, a sequence of
               such characters displays as an ellipsis.

   Two functions are specifically provided for adding elements to
‘buffer-invisibility-spec’ and removing elements from it.

 -- Function: add-to-invisibility-spec element
     This function adds the element ELEMENT to
     ‘buffer-invisibility-spec’.  If ‘buffer-invisibility-spec’ was ‘t’,
     it changes to a list, ‘(t)’, so that text whose ‘invisible’
     property is ‘t’ remains invisible.

 -- Function: remove-from-invisibility-spec element
     This removes the element ELEMENT from ‘buffer-invisibility-spec’.
     This does nothing if ELEMENT is not in the list.

   A convention for use of ‘buffer-invisibility-spec’ is that a major
mode should use the mode’s own name as an element of
‘buffer-invisibility-spec’ and as the value of the ‘invisible’ property:

     ;; If you want to display an ellipsis:
     (add-to-invisibility-spec '(my-symbol . t))
     ;; If you don’t want ellipsis:
     (add-to-invisibility-spec 'my-symbol)

     (overlay-put (make-overlay beginning end)
                  'invisible 'my-symbol)

     ;; When done with the invisibility:
     (remove-from-invisibility-spec '(my-symbol . t))
     ;; Or respectively:
     (remove-from-invisibility-spec 'my-symbol)

   You can check for invisibility using the following function:

 -- Function: invisible-p pos-or-prop
     If POS-OR-PROP is a marker or number, this function returns a
     non-‘nil’ value if the text at that position is invisible.

     If POS-OR-PROP is any other kind of Lisp object, that is taken to
     mean a possible value of the ‘invisible’ text or overlay property.
     In that case, this function returns a non-‘nil’ value if that value
     would cause text to become invisible, based on the current value of
     ‘buffer-invisibility-spec’.

   Ordinarily, functions that operate on text or move point do not care
whether the text is invisible, they process invisible characters and
visible characters alike.  The user-level line motion commands, such as
‘next-line’, ‘previous-line’, ignore invisible newlines if
‘line-move-ignore-invisible’ is non-‘nil’ (the default), i.e., behave
like these invisible newlines didn’t exist in the buffer, but only
because they are explicitly programmed to do so.

   If a command ends with point inside or at the boundary of invisible
text, the main editing loop relocates point to one of the two ends of
the invisible text.  Emacs chooses the direction of relocation so that
it is the same as the overall movement direction of the command; if in
doubt, it prefers a position where an inserted char would not inherit
the ‘invisible’ property.  Additionally, if the text is not replaced by
an ellipsis and the command only moved within the invisible text, then
point is moved one extra character so as to try and reflect the
command’s movement by a visible movement of the cursor.

   Thus, if the command moved point back to an invisible range (with the
usual stickiness), Emacs moves point back to the beginning of that
range.  If the command moved point forward into an invisible range,
Emacs moves point forward to the first visible character that follows
the invisible text and then forward one more character.

   These “adjustments” of point that ended up in the middle of invisible
text can be disabled by setting ‘disable-point-adjustment’ to a
non-‘nil’ value.  *Note Adjusting Point::.

   Incremental search can make invisible overlays visible temporarily
and/or permanently when a match includes invisible text.  To enable
this, the overlay should have a non-‘nil’ ‘isearch-open-invisible’
property.  The property value should be a function to be called with the
overlay as an argument.  This function should make the overlay visible
permanently; it is used when the match overlaps the overlay on exit from
the search.

   During the search, such overlays are made temporarily visible by
temporarily modifying their invisible and intangible properties.  If you
want this to be done differently for a certain overlay, give it an
‘isearch-open-invisible-temporary’ property which is a function.  The
function is called with two arguments: the first is the overlay, and the
second is ‘nil’ to make the overlay visible, or ‘t’ to make it invisible
again.


File: elisp.info,  Node: Selective Display,  Next: Temporary Displays,  Prev: Invisible Text,  Up: Display

37.7 Selective Display
======================

“Selective display” refers to a pair of related features for hiding
certain lines on the screen.

   The first variant, explicit selective display, was designed for use
in a Lisp program: it controls which lines are hidden by altering the
text.  This kind of hiding is now obsolete; instead you can get the same
effect with the ‘invisible’ property (*note Invisible Text::).

   In the second variant, the choice of lines to hide is made
automatically based on indentation.  This variant is designed to be a
user-level feature.

   The way you control explicit selective display is by replacing a
newline (control-j) with a carriage return (control-m).  The text that
was formerly a line following that newline is now hidden.  Strictly
speaking, it is temporarily no longer a line at all, since only newlines
can separate lines; it is now part of the previous line.

   Selective display does not directly affect editing commands.  For
example, ‘C-f’ (‘forward-char’) moves point unhesitatingly into hidden
text.  However, the replacement of newline characters with carriage
return characters affects some editing commands.  For example,
‘next-line’ skips hidden lines, since it searches only for newlines.
Modes that use selective display can also define commands that take
account of the newlines, or that control which parts of the text are
hidden.

   When you write a selectively displayed buffer into a file, all the
control-m’s are output as newlines.  This means that when you next read
in the file, it looks OK, with nothing hidden.  The selective display
effect is seen only within Emacs.

 -- Variable: selective-display
     This buffer-local variable enables selective display.  This means
     that lines, or portions of lines, may be made hidden.

        • If the value of ‘selective-display’ is ‘t’, then the character
          control-m marks the start of hidden text; the control-m, and
          the rest of the line following it, are not displayed.  This is
          explicit selective display.

        • If the value of ‘selective-display’ is a positive integer,
          then lines that start with more than that many columns of
          indentation are not displayed.

     When some portion of a buffer is hidden, the vertical movement
     commands operate as if that portion did not exist, allowing a
     single ‘next-line’ command to skip any number of hidden lines.
     However, character movement commands (such as ‘forward-char’) do
     not skip the hidden portion, and it is possible (if tricky) to
     insert or delete text in an hidden portion.

     In the examples below, we show the _display appearance_ of the
     buffer ‘foo’, which changes with the value of ‘selective-display’.
     The _contents_ of the buffer do not change.

          (setq selective-display nil)
               ⇒ nil

          ---------- Buffer: foo ----------
          1 on this column
           2on this column
            3n this column
            3n this column
           2on this column
          1 on this column
          ---------- Buffer: foo ----------

          (setq selective-display 2)
               ⇒ 2

          ---------- Buffer: foo ----------
          1 on this column
           2on this column
           2on this column
          1 on this column
          ---------- Buffer: foo ----------

 -- User Option: selective-display-ellipses
     If this buffer-local variable is non-‘nil’, then Emacs displays
     ‘...’ at the end of a line that is followed by hidden text.  This
     example is a continuation of the previous one.

          (setq selective-display-ellipses t)
               ⇒ t

          ---------- Buffer: foo ----------
          1 on this column
           2on this column ...
           2on this column
          1 on this column
          ---------- Buffer: foo ----------

     You can use a display table to substitute other text for the
     ellipsis (‘...’).  *Note Display Tables::.


File: elisp.info,  Node: Temporary Displays,  Next: Overlays,  Prev: Selective Display,  Up: Display

37.8 Temporary Displays
=======================

Temporary displays are used by Lisp programs to put output into a buffer
and then present it to the user for perusal rather than for editing.
Many help commands use this feature.

 -- Macro: with-output-to-temp-buffer buffer-name body...
     This function executes the forms in BODY while arranging to insert
     any output they print into the buffer named BUFFER-NAME, which is
     first created if necessary, and put into Help mode.  (See the
     similar form ‘with-temp-buffer-window’ below.)  Finally, the buffer
     is displayed in some window, but that window is not selected.

     If the forms in BODY do not change the major mode in the output
     buffer, so that it is still Help mode at the end of their
     execution, then ‘with-output-to-temp-buffer’ makes this buffer
     read-only at the end, and also scans it for function and variable
     names to make them into clickable cross-references.  *Note Tips for
     Documentation Strings: Docstring hyperlinks, in particular the item
     on hyperlinks in documentation strings, for more details.

     The string BUFFER-NAME specifies the temporary buffer, which need
     not already exist.  The argument must be a string, not a buffer.
     The buffer is erased initially (with no questions asked), and it is
     marked as unmodified after ‘with-output-to-temp-buffer’ exits.

     ‘with-output-to-temp-buffer’ binds ‘standard-output’ to the
     temporary buffer, then it evaluates the forms in BODY.  Output
     using the Lisp output functions within BODY goes by default to that
     buffer (but screen display and messages in the echo area, although
     they are “output” in the general sense of the word, are not
     affected).  *Note Output Functions::.

     Several hooks are available for customizing the behavior of this
     construct; they are listed below.

     The value of the last form in BODY is returned.

          ---------- Buffer: foo ----------
           This is the contents of foo.
          ---------- Buffer: foo ----------

          (with-output-to-temp-buffer "foo"
              (print 20)
              (print standard-output))
          ⇒ #<buffer foo>

          ---------- Buffer: foo ----------

          20

          #<buffer foo>

          ---------- Buffer: foo ----------

 -- User Option: temp-buffer-show-function
     If this variable is non-‘nil’, ‘with-output-to-temp-buffer’ calls
     it as a function to do the job of displaying a help buffer.  The
     function gets one argument, which is the buffer it should display.

     It is a good idea for this function to run ‘temp-buffer-show-hook’
     just as ‘with-output-to-temp-buffer’ normally would, inside of
     ‘save-selected-window’ and with the chosen window and buffer
     selected.

 -- Variable: temp-buffer-setup-hook
     This normal hook is run by ‘with-output-to-temp-buffer’ before
     evaluating BODY.  When the hook runs, the temporary buffer is
     current.  This hook is normally set up with a function to put the
     buffer in Help mode.

 -- Variable: temp-buffer-show-hook
     This normal hook is run by ‘with-output-to-temp-buffer’ after
     displaying the temporary buffer.  When the hook runs, the temporary
     buffer is current, and the window it was displayed in is selected.

 -- Macro: with-temp-buffer-window buffer-or-name action quit-function
          body...
     This macro is similar to ‘with-output-to-temp-buffer’.  Like that
     construct, it executes BODY while arranging to insert any output it
     prints into the buffer named BUFFER-OR-NAME and displays that
     buffer in some window.  Unlike ‘with-output-to-temp-buffer’,
     however, it does not automatically switch that buffer to Help mode.

     The argument BUFFER-OR-NAME specifies the temporary buffer.  It can
     be either a buffer, which must already exist, or a string, in which
     case a buffer of that name is created, if necessary.  The buffer is
     marked as unmodified and read-only when ‘with-temp-buffer-window’
     exits.

     This macro does not call ‘temp-buffer-show-function’.  Rather, it
     passes the ACTION argument to ‘display-buffer’ (*note Choosing
     Window::) in order to display the buffer.

     The value of the last form in BODY is returned, unless the argument
     QUIT-FUNCTION is specified.  In that case, it is called with two
     arguments: the window showing the buffer and the result of BODY.
     The final return value is then whatever QUIT-FUNCTION returns.

     This macro uses the normal hooks ‘temp-buffer-window-setup-hook’
     and ‘temp-buffer-window-show-hook’ in place of the analogous hooks
     run by ‘with-output-to-temp-buffer’.

   The two constructs described next are mostly identical to
‘with-temp-buffer-window’ but differ from it as specified:

 -- Macro: with-current-buffer-window buffer-or-name action
          quit-function &rest body
     This macro is like ‘with-temp-buffer-window’ but unlike that makes
     the buffer specified by BUFFER-OR-NAME current for running BODY.

 -- Macro: with-displayed-buffer-window buffer-or-name action
          quit-function &rest body
     This macro is like ‘with-current-buffer-window’ but unlike that
     displays the buffer specified by BUFFER-OR-NAME _before_ running
     BODY.

   A window showing a temporary buffer can be fit to the size of that
buffer using the following mode:

 -- User Option: temp-buffer-resize-mode
     When this minor mode is enabled, windows showing a temporary buffer
     are automatically resized to fit their buffer’s contents.

     A window is resized if and only if it has been specially created
     for the buffer.  In particular, windows that have shown another
     buffer before are not resized.  By default, this mode uses
     ‘fit-window-to-buffer’ (*note Resizing Windows::) for resizing.
     You can specify a different function by customizing the options
     ‘temp-buffer-max-height’ and ‘temp-buffer-max-width’ below.

 -- User Option: temp-buffer-max-height
     This option specifies the maximum height (in lines) of a window
     displaying a temporary buffer when ‘temp-buffer-resize-mode’ is
     enabled.  It can also be a function to be called to choose the
     height for such a buffer.  It gets one argument, the buffer, and
     should return a positive integer.  At the time the function is
     called, the window to be resized is selected.

 -- User Option: temp-buffer-max-width
     This option specifies the maximum width of a window (in columns)
     displaying a temporary buffer when ‘temp-buffer-resize-mode’ is
     enabled.  It can also be a function to be called to choose the
     width for such a buffer.  It gets one argument, the buffer, and
     should return a positive integer.  At the time the function is
     called, the window to be resized is selected.

   The following function uses the current buffer for temporal display:

 -- Function: momentary-string-display string position &optional char
          message
     This function momentarily displays STRING in the current buffer at
     POSITION.  It has no effect on the undo list or on the buffer’s
     modification status.

     The momentary display remains until the next input event.  If the
     next input event is CHAR, ‘momentary-string-display’ ignores it and
     returns.  Otherwise, that event remains buffered for subsequent use
     as input.  Thus, typing CHAR will simply remove the string from the
     display, while typing (say) ‘C-f’ will remove the string from the
     display and later (presumably) move point forward.  The argument
     CHAR is a space by default.

     The return value of ‘momentary-string-display’ is not meaningful.

     If the string STRING does not contain control characters, you can
     do the same job in a more general way by creating (and then
     subsequently deleting) an overlay with a ‘before-string’ property.
     *Note Overlay Properties::.

     If MESSAGE is non-‘nil’, it is displayed in the echo area while
     STRING is displayed in the buffer.  If it is ‘nil’, a default
     message says to type CHAR to continue.

     In this example, point is initially located at the beginning of the
     second line:

          ---------- Buffer: foo ----------
          This is the contents of foo.
          ★Second line.
          ---------- Buffer: foo ----------

          (momentary-string-display
            "**** Important Message! ****"
            (point) ?\r
            "Type RET when done reading")
          ⇒ t

          ---------- Buffer: foo ----------
          This is the contents of foo.
          **** Important Message! ****Second line.
          ---------- Buffer: foo ----------

          ---------- Echo Area ----------
          Type RET when done reading
          ---------- Echo Area ----------


File: elisp.info,  Node: Overlays,  Next: Size of Displayed Text,  Prev: Temporary Displays,  Up: Display

37.9 Overlays
=============

You can use “overlays” to alter the appearance of a buffer’s text on the
screen, for the sake of presentation features.  An overlay is an object
that belongs to a particular buffer, and has a specified beginning and
end.  It also has properties that you can examine and set; these affect
the display of the text within the overlay.

   The visual effect of an overlay is the same as of the corresponding
text property (*note Text Properties::).  However, due to a different
implementation, overlays generally don’t scale well (many operations
take a time that is proportional to the number of overlays in the
buffer).  If you need to affect the visual appearance of many portions
in the buffer, we recommend using text properties.

   An overlay uses markers to record its beginning and end; thus,
editing the text of the buffer adjusts the beginning and end of each
overlay so that it stays with the text.  When you create the overlay,
you can specify whether text inserted at the beginning should be inside
the overlay or outside, and likewise for the end of the overlay.

* Menu:

* Managing Overlays::   Creating and moving overlays.
* Overlay Properties::  How to read and set properties.
                          What properties do to the screen display.
* Finding Overlays::    Searching for overlays.


File: elisp.info,  Node: Managing Overlays,  Next: Overlay Properties,  Up: Overlays

37.9.1 Managing Overlays
------------------------

This section describes the functions to create, delete and move
overlays, and to examine their contents.  Overlay changes are not
recorded in the buffer’s undo list, since the overlays are not part of
the buffer’s contents.

 -- Function: overlayp object
     This function returns ‘t’ if OBJECT is an overlay.

 -- Function: make-overlay start end &optional buffer front-advance
          rear-advance
     This function creates and returns an overlay that belongs to BUFFER
     and ranges from START to END.  Both START and END must specify
     buffer positions; they may be integers or markers.  If BUFFER is
     omitted, the overlay is created in the current buffer.

     An overlay whose START and END specify the same buffer position is
     known as “empty”.  A non-empty overlay can become empty if the text
     between its START and END is deleted.  When that happens, the
     overlay is by default not deleted, but you can cause it to be
     deleted by giving it the ‘evaporate’ property (*note evaporate
     property: Overlay Properties.).

     The arguments FRONT-ADVANCE and REAR-ADVANCE specify the marker
     insertion type for the start of the overlay and for the end of the
     overlay, respectively.  *Note Marker Insertion Types::.  If they
     are both ‘nil’, the default, then the overlay extends to include
     any text inserted at the beginning, but not text inserted at the
     end.  If FRONT-ADVANCE is non-‘nil’, text inserted at the beginning
     of the overlay is excluded from the overlay.  If REAR-ADVANCE is
     non-‘nil’, text inserted at the end of the overlay is included in
     the overlay.

 -- Function: overlay-start overlay
     This function returns the position at which OVERLAY starts, as an
     integer.

 -- Function: overlay-end overlay
     This function returns the position at which OVERLAY ends, as an
     integer.

 -- Function: overlay-buffer overlay
     This function returns the buffer that OVERLAY belongs to.  It
     returns ‘nil’ if OVERLAY has been deleted.

 -- Function: delete-overlay overlay
     This function deletes OVERLAY.  The overlay continues to exist as a
     Lisp object, and its property list is unchanged, but it ceases to
     be attached to the buffer it belonged to, and ceases to have any
     effect on display.

     A deleted overlay is not permanently disconnected.  You can give it
     a position in a buffer again by calling ‘move-overlay’.

 -- Function: move-overlay overlay start end &optional buffer
     This function moves OVERLAY to BUFFER, and places its bounds at
     START and END.  Both arguments START and END must specify buffer
     positions; they may be integers or markers.

     If BUFFER is omitted, OVERLAY stays in the same buffer it was
     already associated with; if OVERLAY was deleted, it goes into the
     current buffer.

     The return value is OVERLAY.

     This is the only valid way to change the endpoints of an overlay.
     Do not try modifying the markers in the overlay by hand, as that
     fails to update other vital data structures and can cause some
     overlays to be lost.

 -- Function: remove-overlays &optional start end name value
     This function removes all the overlays between START and END whose
     property NAME has the value VALUE.  It can move the endpoints of
     the overlays in the region, or split them.

     If NAME is omitted or ‘nil’, it means to delete all overlays in the
     specified region.  If START and/or END are omitted or ‘nil’, that
     means the beginning and end of the buffer respectively.  Therefore,
     ‘(remove-overlays)’ removes all the overlays in the current buffer.

 -- Function: copy-overlay overlay
     This function returns a copy of OVERLAY.  The copy has the same
     endpoints and properties as OVERLAY.  However, the marker insertion
     type for the start of the overlay and for the end of the overlay
     are set to their default values (*note Marker Insertion Types::).

   Here are some examples:

     ;; Create an overlay.
     (setq foo (make-overlay 1 10))
          ⇒ #<overlay from 1 to 10 in display.texi>
     (overlay-start foo)
          ⇒ 1
     (overlay-end foo)
          ⇒ 10
     (overlay-buffer foo)
          ⇒ #<buffer display.texi>
     ;; Give it a property we can check later.
     (overlay-put foo 'happy t)
          ⇒ t
     ;; Verify the property is present.
     (overlay-get foo 'happy)
          ⇒ t
     ;; Move the overlay.
     (move-overlay foo 5 20)
          ⇒ #<overlay from 5 to 20 in display.texi>
     (overlay-start foo)
          ⇒ 5
     (overlay-end foo)
          ⇒ 20
     ;; Delete the overlay.
     (delete-overlay foo)
          ⇒ nil
     ;; Verify it is deleted.
     foo
          ⇒ #<overlay in no buffer>
     ;; A deleted overlay has no position.
     (overlay-start foo)
          ⇒ nil
     (overlay-end foo)
          ⇒ nil
     (overlay-buffer foo)
          ⇒ nil
     ;; Undelete the overlay.
     (move-overlay foo 1 20)
          ⇒ #<overlay from 1 to 20 in display.texi>
     ;; Verify the results.
     (overlay-start foo)
          ⇒ 1
     (overlay-end foo)
          ⇒ 20
     (overlay-buffer foo)
          ⇒ #<buffer display.texi>
     ;; Moving and deleting the overlay does not change its properties.
     (overlay-get foo 'happy)
          ⇒ t

   Emacs stores the overlays of each buffer in two lists, divided around
an arbitrary center position.  One list extends backwards through the
buffer from that center position, and the other extends forwards from
that center position.  The center position can be anywhere in the
buffer.

 -- Function: overlay-recenter pos
     This function recenters the overlays of the current buffer around
     position POS.  That makes overlay lookup faster for positions near
     POS, but slower for positions far away from POS.

   A loop that scans the buffer forwards, creating overlays, can run
faster if you do ‘(overlay-recenter (point-max))’ first.


File: elisp.info,  Node: Overlay Properties,  Next: Finding Overlays,  Prev: Managing Overlays,  Up: Overlays

37.9.2 Overlay Properties
-------------------------

Overlay properties are like text properties in that the properties that
alter how a character is displayed can come from either source.  But in
most respects they are different.  *Note Text Properties::, for
comparison.

   Text properties are considered a part of the text; overlays and their
properties are specifically considered not to be part of the text.
Thus, copying text between various buffers and strings preserves text
properties, but does not try to preserve overlays.  Changing a buffer’s
text properties marks the buffer as modified, while moving an overlay or
changing its properties does not.  Unlike text property changes, overlay
property changes are not recorded in the buffer’s undo list.

   Since more than one overlay can specify a property value for the same
character, Emacs lets you specify a priority value of each overlay.  The
priority value is used to decide which of the overlapping overlays will
“win”.

   These functions read and set the properties of an overlay:

 -- Function: overlay-get overlay prop
     This function returns the value of property PROP recorded in
     OVERLAY, if any.  If OVERLAY does not record any value for that
     property, but it does have a ‘category’ property which is a symbol,
     that symbol’s PROP property is used.  Otherwise, the value is
     ‘nil’.

 -- Function: overlay-put overlay prop value
     This function sets the value of property PROP recorded in OVERLAY
     to VALUE.  It returns VALUE.

 -- Function: overlay-properties overlay
     This returns a copy of the property list of OVERLAY.

   See also the function ‘get-char-property’ which checks both overlay
properties and text properties for a given character.  *Note Examining
Properties::.

   Many overlay properties have special meanings; here is a table of
them:

‘priority’
     This property’s value determines the priority of the overlay.  If
     you want to specify a priority value, use either ‘nil’ (or zero),
     or a positive integer.  Any other value has undefined behavior.

     The priority matters when two or more overlays cover the same
     character and both specify the same property; the one whose
     ‘priority’ value is larger overrides the other.  (For the ‘face’
     property, the higher priority overlay’s value does not completely
     override the other value; instead, its face attributes override the
     face attributes of the lower priority ‘face’ property.)  If two
     overlays have the same priority value, and one is nested in the
     other, then the inner one will prevail over the outer one.  If
     neither is nested in the other then you should not make assumptions
     about which overlay will prevail.

     Currently, all overlays take priority over text properties.

     Note that Emacs sometimes uses non-numeric priority values for some
     of its internal overlays, so do not try to do arithmetic on the
     priority of an overlay (unless it is one that you created).  In
     particular, the overlay used for showing the region uses a priority
     value of the form ‘(PRIMARY . SECONDARY)’, where the PRIMARY value
     is used as described above, and SECONDARY is the fallback value
     used when PRIMARY and the nesting considerations fail to resolve
     the precedence between overlays.  However, you are advised not to
     design Lisp programs based on this implementation detail; if you
     need to put overlays in priority order, use the SORTED argument of
     ‘overlays-at’.  *Note Finding Overlays::.

‘window’
     If the ‘window’ property is non-‘nil’, then the overlay applies
     only on that window.

‘category’
     If an overlay has a ‘category’ property, we call it the “category”
     of the overlay.  It should be a symbol.  The properties of the
     symbol serve as defaults for the properties of the overlay.

‘face’
     This property controls the appearance of the text (*note Faces::).
     The value of the property can be the following:

        • A face name (a symbol or string).

        • An anonymous face: a property list of the form ‘(KEYWORD VALUE
          ...)’, where each KEYWORD is a face attribute name and VALUE
          is a value for that attribute.

        • A list of faces.  Each list element should be either a face
          name or an anonymous face.  This specifies a face which is an
          aggregate of the attributes of each of the listed faces.
          Faces occurring earlier in the list have higher priority.

        • A cons cell of the form ‘(foreground-color . COLOR-NAME)’ or
          ‘(background-color . COLOR-NAME)’.  This specifies the
          foreground or background color, similar to ‘(:foreground
          COLOR-NAME)’ or ‘(:background COLOR-NAME)’.  This form is
          supported for backward compatibility only, and should be
          avoided.

‘mouse-face’
     This property is used instead of ‘face’ when the mouse is within
     the range of the overlay.  However, Emacs ignores all face
     attributes from this property that alter the text size (e.g.,
     ‘:height’, ‘:weight’, and ‘:slant’).  Those attributes are always
     the same as in the unhighlighted text.

‘display’
     This property activates various features that change the way text
     is displayed.  For example, it can make text appear taller or
     shorter, higher or lower, wider or narrower, or replaced with an
     image.  *Note Display Property::.

‘help-echo’
     If an overlay has a ‘help-echo’ property, then when you move the
     mouse onto the text in the overlay, Emacs displays a help string in
     the echo area, or in the tooltip window.  For details see *note
     Text help-echo::.

‘field’
     Consecutive characters with the same ‘field’ property constitute a
     _field_.  Some motion functions including ‘forward-word’ and
     ‘beginning-of-line’ stop moving at a field boundary.  *Note
     Fields::.

‘modification-hooks’
     This property’s value is a list of functions to be called if any
     character within the overlay is changed or if text is inserted
     strictly within the overlay.

     The hook functions are called both before and after each change.
     If the functions save the information they receive, and compare
     notes between calls, they can determine exactly what change has
     been made in the buffer text.

     When called before a change, each function receives four arguments:
     the overlay, ‘nil’, and the beginning and end of the text range to
     be modified.

     When called after a change, each function receives five arguments:
     the overlay, ‘t’, the beginning and end of the text range just
     modified, and the length of the pre-change text replaced by that
     range.  (For an insertion, the pre-change length is zero; for a
     deletion, that length is the number of characters deleted, and the
     post-change beginning and end are equal.)

     If these functions modify the buffer, they should bind
     ‘inhibit-modification-hooks’ to ‘t’ around doing so, to avoid
     confusing the internal mechanism that calls these hooks.

     Text properties also support the ‘modification-hooks’ property, but
     the details are somewhat different (*note Special Properties::).

‘insert-in-front-hooks’
     This property’s value is a list of functions to be called before
     and after inserting text right at the beginning of the overlay.
     The calling conventions are the same as for the
     ‘modification-hooks’ functions.

‘insert-behind-hooks’
     This property’s value is a list of functions to be called before
     and after inserting text right at the end of the overlay.  The
     calling conventions are the same as for the ‘modification-hooks’
     functions.

‘invisible’
     The ‘invisible’ property can make the text in the overlay
     invisible, which means that it does not appear on the screen.
     *Note Invisible Text::, for details.

‘intangible’
     The ‘intangible’ property on an overlay works just like the
     ‘intangible’ text property.  It is obsolete.  *Note Special
     Properties::, for details.

‘isearch-open-invisible’
     This property tells incremental search how to make an invisible
     overlay visible, permanently, if the final match overlaps it.
     *Note Invisible Text::.

‘isearch-open-invisible-temporary’
     This property tells incremental search how to make an invisible
     overlay visible, temporarily, during the search.  *Note Invisible
     Text::.

‘before-string’
     This property’s value is a string to add to the display at the
     beginning of the overlay.  The string does not appear in the buffer
     in any sense—only on the screen.

‘after-string’
     This property’s value is a string to add to the display at the end
     of the overlay.  The string does not appear in the buffer in any
     sense—only on the screen.

‘line-prefix’
     This property specifies a display spec to prepend to each
     non-continuation line at display-time.  *Note Truncation::.

‘wrap-prefix’
     This property specifies a display spec to prepend to each
     continuation line at display-time.  *Note Truncation::.

‘evaporate’
     If this property is non-‘nil’, the overlay is deleted automatically
     if it becomes empty (i.e., if its length becomes zero).  If you
     give an empty overlay (*note empty overlay: Managing Overlays.) a
     non-‘nil’ ‘evaporate’ property, that deletes it immediately.  Note
     that, unless an overlay has this property, it will not be deleted
     when the text between its starting and ending positions is deleted
     from the buffer.

‘keymap’
     If this property is non-‘nil’, it specifies a keymap for a portion
     of the text.  This keymap is used when the character after point is
     within the overlay, and takes precedence over most other keymaps.
     *Note Active Keymaps::.

‘local-map’
     The ‘local-map’ property is similar to ‘keymap’ but replaces the
     buffer’s local map rather than augmenting existing keymaps.  This
     also means it has lower precedence than minor mode keymaps.

   The ‘keymap’ and ‘local-map’ properties do not affect a string
displayed by the ‘before-string’, ‘after-string’, or ‘display’
properties.  This is only relevant for mouse clicks and other mouse
events that fall on the string, since point is never on the string.  To
bind special mouse events for the string, assign it a ‘keymap’ or
‘local-map’ text property.  *Note Special Properties::.


File: elisp.info,  Node: Finding Overlays,  Prev: Overlay Properties,  Up: Overlays

37.9.3 Searching for Overlays
-----------------------------

 -- Function: overlays-at pos &optional sorted
     This function returns a list of all the overlays that cover the
     character at position POS in the current buffer.  If SORTED is
     non-‘nil’, the list is in decreasing order of priority, otherwise
     it is in no particular order.  An overlay contains position POS if
     it begins at or before POS, and ends after POS.

     To illustrate usage, here is a Lisp function that returns a list of
     the overlays that specify property PROP for the character at point:

          (defun find-overlays-specifying (prop)
            (let ((overlays (overlays-at (point)))
                  found)
              (while overlays
                (let ((overlay (car overlays)))
                  (if (overlay-get overlay prop)
                      (setq found (cons overlay found))))
                (setq overlays (cdr overlays)))
              found))

 -- Function: overlays-in beg end
     This function returns a list of the overlays that overlap the
     region BEG through END.  An overlay overlaps with a region if it
     contains one or more characters in the region; empty overlays
     (*note empty overlay: Managing Overlays.) overlap if they are at
     BEG, strictly between BEG and END, or at END when END denotes the
     position at the end of the buffer.

 -- Function: next-overlay-change pos
     This function returns the buffer position of the next beginning or
     end of an overlay, after POS.  If there is none, it returns
     ‘(point-max)’.

 -- Function: previous-overlay-change pos
     This function returns the buffer position of the previous beginning
     or end of an overlay, before POS.  If there is none, it returns
     ‘(point-min)’.

   As an example, here’s a simplified (and inefficient) version of the
primitive function ‘next-single-char-property-change’ (*note Property
Search::).  It searches forward from position POS for the next position
where the value of a given property ‘prop’, as obtained from either
overlays or text properties, changes.

     (defun next-single-char-property-change (position prop)
       (save-excursion
         (goto-char position)
         (let ((propval (get-char-property (point) prop)))
           (while (and (not (eobp))
                       (eq (get-char-property (point) prop) propval))
             (goto-char (min (next-overlay-change (point))
                             (next-single-property-change (point) prop)))))
         (point)))


File: elisp.info,  Node: Size of Displayed Text,  Next: Line Height,  Prev: Overlays,  Up: Display

37.10 Size of Displayed Text
============================

Since not all characters have the same width, these functions let you
check the width of a character.  *Note Primitive Indent::, and *note
Screen Lines::, for related functions.

 -- Function: char-width char
     This function returns the width in columns of the character CHAR,
     if it were displayed in the current buffer (i.e., taking into
     account the buffer’s display table, if any; *note Display
     Tables::).  The width of a tab character is usually ‘tab-width’
     (*note Usual Display::).

 -- Function: string-width string
     This function returns the width in columns of the string STRING, if
     it were displayed in the current buffer and the selected window.

 -- Function: truncate-string-to-width string width &optional
          start-column padding ellipsis
     This function returns the part of STRING that fits within WIDTH
     columns, as a new string.

     If STRING does not reach WIDTH, then the result ends where STRING
     ends.  If one multi-column character in STRING extends across the
     column WIDTH, that character is not included in the result.  Thus,
     the result can fall short of WIDTH but cannot go beyond it.

     The optional argument START-COLUMN specifies the starting column.
     If this is non-‘nil’, then the first START-COLUMN columns of the
     string are omitted from the value.  If one multi-column character
     in STRING extends across the column START-COLUMN, that character is
     not included.

     The optional argument PADDING, if non-‘nil’, is a padding character
     added at the beginning and end of the result string, to extend it
     to exactly WIDTH columns.  The padding character is used at the end
     of the result if it falls short of WIDTH.  It is also used at the
     beginning of the result if one multi-column character in STRING
     extends across the column START-COLUMN.

     If ELLIPSIS is non-‘nil’, it should be a string which will replace
     the end of STRING (including any padding) if it extends beyond
     WIDTH, unless the display width of STRING is equal to or less than
     the display width of ELLIPSIS.  If ELLIPSIS is non-‘nil’ and not a
     string, it stands for the value of the variable
     ‘truncate-string-ellipsis’.

          (truncate-string-to-width "\tab\t" 12 4)
               ⇒ "ab"
          (truncate-string-to-width "\tab\t" 12 4 ?\s)
               ⇒ "    ab  "

   The following function returns the size in pixels of text as if it
were displayed in a given window.  This function is used by
‘fit-window-to-buffer’ and ‘fit-frame-to-buffer’ (*note Resizing
Windows::) to make a window exactly as large as the text it contains.

 -- Function: window-text-pixel-size &optional window from to x-limit
          y-limit mode-and-header-line
     This function returns the size of the text of WINDOW’s buffer in
     pixels.  WINDOW must be a live window and defaults to the selected
     one.  The return value is a cons of the maximum pixel-width of any
     text line and the maximum pixel-height of all text lines.

     The optional argument FROM, if non-‘nil’, specifies the first text
     position to consider and defaults to the minimum accessible
     position of the buffer.  If FROM is ‘t’, it uses the minimum
     accessible position that is not a newline character.  The optional
     argument TO, if non-‘nil’, specifies the last text position to
     consider and defaults to the maximum accessible position of the
     buffer.  If TO is ‘t’, it uses the maximum accessible position that
     is not a newline character.

     The optional argument X-LIMIT, if non-‘nil’, specifies the maximum
     pixel-width that can be returned.  X-LIMIT ‘nil’ or omitted, means
     to use the pixel-width of WINDOW’s body (*note Window Sizes::);
     this is useful when the caller does not intend to change the width
     of WINDOW.  Otherwise, the caller should specify here the maximum
     width WINDOW’s body may assume.  Text whose x-coordinate is beyond
     X-LIMIT is ignored.  Since calculating the width of long lines can
     take some time, it’s always a good idea to make this argument as
     small as needed; in particular, if the buffer might contain long
     lines that will be truncated anyway.

     The optional argument Y-LIMIT, if non-‘nil’, specifies the maximum
     pixel-height that can be returned.  Text lines whose y-coordinate
     is beyond Y-LIMIT are ignored.  Since calculating the pixel-height
     of a large buffer can take some time, it makes sense to specify
     this argument; in particular, if the caller does not know the size
     of the buffer.

     The optional argument MODE-AND-HEADER-LINE ‘nil’ or omitted means
     to not include the height of the mode- or header-line of WINDOW in
     the return value.  If it is either the symbol ‘mode-line’ or
     ‘header-line’, include only the height of that line, if present, in
     the return value.  If it is ‘t’, include the height of both, if
     present, in the return value.

 -- Function: line-pixel-height
     This function returns the height in pixels of the line at point in
     the selected window.  The value includes the line spacing of the
     line (*note Line Height::).


File: elisp.info,  Node: Line Height,  Next: Faces,  Prev: Size of Displayed Text,  Up: Display

37.11 Line Height
=================

The total height of each display line consists of the height of the
contents of the line, plus optional additional vertical line spacing
above or below the display line.

   The height of the line contents is the maximum height of any
character or image on that display line, including the final newline if
there is one.  (A display line that is continued doesn’t include a final
newline.)  That is the default line height, if you do nothing to specify
a greater height.  (In the most common case, this equals the height of
the corresponding frame’s default font, see *note Frame Font::.)

   There are several ways to explicitly specify a larger line height,
either by specifying an absolute height for the display line, or by
specifying vertical space.  However, no matter what you specify, the
actual line height can never be less than the default.

   A newline can have a ‘line-height’ text or overlay property that
controls the total height of the display line ending in that newline.

   If the property value is ‘t’, the newline character has no effect on
the displayed height of the line—the visible contents alone determine
the height.  The ‘line-spacing’ property, described below, is also
ignored in this case.  This is useful for tiling small images (or image
slices) without adding blank areas between the images.

   If the property value is a list of the form ‘(HEIGHT TOTAL)’, that
adds extra space _below_ the display line.  First Emacs uses HEIGHT as a
height spec to control extra space _above_ the line; then it adds enough
space _below_ the line to bring the total line height up to TOTAL.  In
this case, any value of ‘line-spacing’ property for the newline is
ignored.

   Any other kind of property value is a height spec, which translates
into a number—the specified line height.  There are several ways to
write a height spec; here’s how each of them translates into a number:

‘INTEGER’
     If the height spec is a positive integer, the height value is that
     integer.
‘FLOAT’
     If the height spec is a float, FLOAT, the numeric height value is
     FLOAT times the frame’s default line height.
‘(FACE . RATIO)’
     If the height spec is a cons of the format shown, the numeric
     height is RATIO times the height of face FACE.  RATIO can be any
     type of number, or ‘nil’ which means a ratio of 1.  If FACE is ‘t’,
     it refers to the current face.
‘(nil . RATIO)’
     If the height spec is a cons of the format shown, the numeric
     height is RATIO times the height of the contents of the line.

   Thus, any valid height spec determines the height in pixels, one way
or another.  If the line contents’ height is less than that, Emacs adds
extra vertical space above the line to achieve the specified total
height.

   If you don’t specify the ‘line-height’ property, the line’s height
consists of the contents’ height plus the line spacing.  There are
several ways to specify the line spacing for different parts of Emacs
text.

   On graphical terminals, you can specify the line spacing for all
lines in a frame, using the ‘line-spacing’ frame parameter (*note Layout
Parameters::).  However, if the default value of ‘line-spacing’ is
non-‘nil’, it overrides the frame’s ‘line-spacing’ parameter.  An
integer specifies the number of pixels put below lines.  A
floating-point number specifies the spacing relative to the frame’s
default line height.

   You can specify the line spacing for all lines in a buffer via the
buffer-local ‘line-spacing’ variable.  An integer specifies the number
of pixels put below lines.  A floating-point number specifies the
spacing relative to the default frame line height.  This overrides line
spacings specified for the frame.

   Finally, a newline can have a ‘line-spacing’ text or overlay property
that can enlarge the default frame line spacing and the buffer local
‘line-spacing’ variable: if its value is larger than the buffer or frame
defaults, that larger value is used instead, for the display line ending
in that newline.

   One way or another, these mechanisms specify a Lisp value for the
spacing of each line.  The value is a height spec, and it translates
into a Lisp value as described above.  However, in this case the numeric
height value specifies the line spacing, rather than the line height.

   On text terminals, the line spacing cannot be altered.


File: elisp.info,  Node: Faces,  Next: Fringes,  Prev: Line Height,  Up: Display

37.12 Faces
===========

A “face” is a collection of graphical attributes for displaying text:
font, foreground color, background color, optional underlining, etc.
Faces control how Emacs displays text in buffers, as well as other parts
of the frame such as the mode line.

   One way to represent a face is as a property list of attributes, like
‘(:foreground "red" :weight bold)’.  Such a list is called an “anonymous
face”.  For example, you can assign an anonymous face as the value of
the ‘face’ text property, and Emacs will display the underlying text
with the specified attributes.  *Note Special Properties::.

   More commonly, a face is referred to via a “face name”: a Lisp symbol
associated with a set of face attributes(1).  Named faces are defined
using the ‘defface’ macro (*note Defining Faces::).  Emacs comes with
several standard named faces (*note Basic Faces::).

   Many parts of Emacs required named faces, and do not accept anonymous
faces.  These include the functions documented in *note Attribute
Functions::, and the variable ‘font-lock-keywords’ (*note Search-based
Fontification::).  Unless otherwise stated, we will use the term “face”
to refer only to named faces.

 -- Function: facep object
     This function returns a non-‘nil’ value if OBJECT is a named face:
     a Lisp symbol or string which serves as a face name.  Otherwise, it
     returns ‘nil’.

* Menu:

* Face Attributes::     What is in a face?
* Defining Faces::      How to define a face.
* Attribute Functions::  Functions to examine and set face attributes.
* Displaying Faces::     How Emacs combines the faces specified for a character.
* Face Remapping::      Remapping faces to alternative definitions.
* Face Functions::      How to define and examine faces.
* Auto Faces::          Hook for automatic face assignment.
* Basic Faces::         Faces that are defined by default.
* Font Selection::      Finding the best available font for a face.
* Font Lookup::         Looking up the names of available fonts
                          and information about them.
* Fontsets::            A fontset is a collection of fonts
                          that handle a range of character sets.
* Low-Level Font::      Lisp representation for character display fonts.

   ---------- Footnotes ----------

   (1) For backward compatibility, you can also use a string to specify
a face name; that is equivalent to a Lisp symbol with the same name.


File: elisp.info,  Node: Face Attributes,  Next: Defining Faces,  Up: Faces

37.12.1 Face Attributes
-----------------------

“Face attributes” determine the visual appearance of a face.  The
following table lists all the face attributes, their possible values,
and their effects.

   Apart from the values given below, each face attribute can have the
value ‘unspecified’.  This special value means that the face doesn’t
specify that attribute directly.  An ‘unspecified’ attribute tells Emacs
to refer instead to a parent face (see the description ‘:inherit’
attribute below); or, failing that, to an underlying face (*note
Displaying Faces::).  The ‘default’ face must specify all attributes.

   Some of these attributes are meaningful only on certain kinds of
displays.  If your display cannot handle a certain attribute, the
attribute is ignored.

‘:family’
     Font family or fontset (a string).  *Note (emacs)Fonts::, for more
     information about font families.  The function ‘font-family-list’
     (see below) returns a list of available family names.  *Note
     Fontsets::, for information about fontsets.

‘:foundry’
     The name of the “font foundry” for the font family specified by the
     ‘:family’ attribute (a string).  *Note (emacs)Fonts::.

‘:width’
     Relative character width.  This should be one of the symbols
     ‘ultra-condensed’, ‘extra-condensed’, ‘condensed’,
     ‘semi-condensed’, ‘normal’, ‘semi-expanded’, ‘expanded’,
     ‘extra-expanded’, or ‘ultra-expanded’.

‘:height’
     The height of the font.  In the simplest case, this is an integer
     in units of 1/10 point.

     The value can also be floating point or a function, which specifies
     the height relative to an “underlying face” (*note Displaying
     Faces::).  A floating-point value specifies the amount by which to
     scale the height of the underlying face.  A function value is
     called with one argument, the height of the underlying face, and
     returns the height of the new face.  If the function is passed an
     integer argument, it must return an integer.

     The height of the default face must be specified using an integer;
     floating point and function values are not allowed.

‘:weight’
     Font weight—one of the symbols (from densest to faintest)
     ‘ultra-bold’, ‘extra-bold’, ‘bold’, ‘semi-bold’, ‘normal’,
     ‘semi-light’, ‘light’, ‘extra-light’, or ‘ultra-light’.  On text
     terminals which support variable-brightness text, any weight
     greater than normal is displayed as extra bright, and any weight
     less than normal is displayed as half-bright.

‘:slant’
     Font slant—one of the symbols ‘italic’, ‘oblique’, ‘normal’,
     ‘reverse-italic’, or ‘reverse-oblique’.  On text terminals that
     support variable-brightness text, slanted text is displayed as
     half-bright.

‘:foreground’
     Foreground color, a string.  The value can be a system-defined
     color name, or a hexadecimal color specification.  *Note Color
     Names::.  On black-and-white displays, certain shades of gray are
     implemented by stipple patterns.

‘:distant-foreground’
     Alternative foreground color, a string.  This is like ‘:foreground’
     but the color is only used as a foreground when the background
     color is near to the foreground that would have been used.  This is
     useful for example when marking text (i.e., the region face).  If
     the text has a foreground that is visible with the region face,
     that foreground is used.  If the foreground is near the region face
     background, ‘:distant-foreground’ is used instead so the text is
     readable.

‘:background’
     Background color, a string.  The value can be a system-defined
     color name, or a hexadecimal color specification.  *Note Color
     Names::.

‘:underline’
     Whether or not characters should be underlined, and in what way.
     The possible values of the ‘:underline’ attribute are:

     ‘nil’
          Don’t underline.

     ‘t’
          Underline with the foreground color of the face.

     COLOR
          Underline in color COLOR, a string specifying a color.

     ‘(:color COLOR :style STYLE)’
          COLOR is either a string, or the symbol ‘foreground-color’,
          meaning the foreground color of the face.  Omitting the
          attribute ‘:color’ means to use the foreground color of the
          face.  STYLE should be a symbol ‘line’ or ‘wave’, meaning to
          use a straight or wavy line.  Omitting the attribute ‘:style’
          means to use a straight line.

‘:overline’
     Whether or not characters should be overlined, and in what color.
     If the value is ‘t’, overlining uses the foreground color of the
     face.  If the value is a string, overlining uses that color.  The
     value ‘nil’ means do not overline.

‘:strike-through’
     Whether or not characters should be strike-through, and in what
     color.  The value is used like that of ‘:overline’.

‘:box’
     Whether or not a box should be drawn around characters, its color,
     the width of the box lines, and 3D appearance.  Here are the
     possible values of the ‘:box’ attribute, and what they mean:

     ‘nil’
          Don’t draw a box.

     ‘t’
          Draw a box with lines of width 1, in the foreground color.

     COLOR
          Draw a box with lines of width 1, in color COLOR.

     ‘(:line-width WIDTH :color COLOR :style STYLE)’
          This way you can explicitly specify all aspects of the box.
          The value WIDTH specifies the width of the lines to draw; it
          defaults to 1.  A negative width -N means to draw a line of
          width N that occupies the space of the underlying text, thus
          avoiding any increase in the character height or width.

          The value COLOR specifies the color to draw with.  The default
          is the foreground color of the face for simple boxes, and the
          background color of the face for 3D boxes.

          The value STYLE specifies whether to draw a 3D box.  If it is
          ‘released-button’, the box looks like a 3D button that is not
          being pressed.  If it is ‘pressed-button’, the box looks like
          a 3D button that is being pressed.  If it is ‘nil’ or omitted,
          a plain 2D box is used.

‘:inverse-video’
     Whether or not characters should be displayed in inverse video.
     The value should be ‘t’ (yes) or ‘nil’ (no).

‘:stipple’
     The background stipple, a bitmap.

     The value can be a string; that should be the name of a file
     containing external-format X bitmap data.  The file is found in the
     directories listed in the variable ‘x-bitmap-file-path’.

     Alternatively, the value can specify the bitmap directly, with a
     list of the form ‘(WIDTH HEIGHT DATA)’.  Here, WIDTH and HEIGHT
     specify the size in pixels, and DATA is a string containing the raw
     bits of the bitmap, row by row.  Each row occupies (WIDTH + 7) / 8
     consecutive bytes in the string (which should be a unibyte string
     for best results).  This means that each row always occupies at
     least one whole byte.

     If the value is ‘nil’, that means use no stipple pattern.

     Normally you do not need to set the stipple attribute, because it
     is used automatically to handle certain shades of gray.

‘:font’
     The font used to display the face.  Its value should be a font
     object.  *Note Low-Level Font::, for information about font
     objects, font specs, and font entities.

     When specifying this attribute using ‘set-face-attribute’ (*note
     Attribute Functions::), you may also supply a font spec, a font
     entity, or a string.  Emacs converts such values to an appropriate
     font object, and stores that font object as the actual attribute
     value.  If you specify a string, the contents of the string should
     be a font name (*note (emacs)Fonts::); if the font name is an XLFD
     containing wildcards, Emacs chooses the first font matching those
     wildcards.  Specifying this attribute also changes the values of
     the ‘:family’, ‘:foundry’, ‘:width’, ‘:height’, ‘:weight’, and
     ‘:slant’ attributes.

‘:inherit’
     The name of a face from which to inherit attributes, or a list of
     face names.  Attributes from inherited faces are merged into the
     face like an underlying face would be, with higher priority than
     underlying faces (*note Displaying Faces::).  If a list of faces is
     used, attributes from faces earlier in the list override those from
     later faces.

 -- Function: font-family-list &optional frame
     This function returns a list of available font family names.  The
     optional argument FRAME specifies the frame on which the text is to
     be displayed; if it is ‘nil’, the selected frame is used.

 -- User Option: underline-minimum-offset
     This variable specifies the minimum distance between the baseline
     and the underline, in pixels, when displaying underlined text.

 -- User Option: x-bitmap-file-path
     This variable specifies a list of directories for searching for
     bitmap files, for the ‘:stipple’ attribute.

 -- Function: bitmap-spec-p object
     This returns ‘t’ if OBJECT is a valid bitmap specification,
     suitable for use with ‘:stipple’ (see above).  It returns ‘nil’
     otherwise.


File: elisp.info,  Node: Defining Faces,  Next: Attribute Functions,  Prev: Face Attributes,  Up: Faces

37.12.2 Defining Faces
----------------------

The usual way to define a face is through the ‘defface’ macro.  This
macro associates a face name (a symbol) with a default “face spec”.  A
face spec is a construct which specifies what attributes a face should
have on any given terminal; for example, a face spec might specify one
foreground color on high-color terminals, and a different foreground
color on low-color terminals.

   People are sometimes tempted to create a variable whose value is a
face name.  In the vast majority of cases, this is not necessary; the
usual procedure is to define a face with ‘defface’, and then use its
name directly.

 -- Macro: defface face spec doc [keyword value]...
     This macro declares FACE as a named face whose default face spec is
     given by SPEC.  You should not quote the symbol FACE, and it should
     not end in ‘-face’ (that would be redundant).  The argument DOC is
     a documentation string for the face.  The additional KEYWORD
     arguments have the same meanings as in ‘defgroup’ and ‘defcustom’
     (*note Common Keywords::).

     If FACE already has a default face spec, this macro does nothing.

     The default face spec determines FACE’s appearance when no
     customizations are in effect (*note Customization::).  If FACE has
     already been customized (via Custom themes or via customizations
     read from the init file), its appearance is determined by the
     custom face spec(s), which override the default face spec SPEC.
     However, if the customizations are subsequently removed, the
     appearance of FACE will again be determined by its default face
     spec.

     As an exception, if you evaluate a ‘defface’ form with ‘C-M-x’ in
     Emacs Lisp mode (‘eval-defun’), a special feature of ‘eval-defun’
     overrides any custom face specs on the face, causing the face to
     reflect exactly what the ‘defface’ says.

     The SPEC argument is a “face spec”, which states how the face
     should appear on different kinds of terminals.  It should be an
     alist whose elements each have the form

          (DISPLAY . PLIST)

     DISPLAY specifies a class of terminals (see below).  PLIST is a
     property list of face attributes and their values, specifying how
     the face appears on such terminals.  For backward compatibility,
     you can also write an element as ‘(DISPLAY PLIST)’.

     The DISPLAY part of an element of SPEC determines which terminals
     the element matches.  If more than one element of SPEC matches a
     given terminal, the first element that matches is the one used for
     that terminal.  There are three possibilities for DISPLAY:

     ‘default’
          This element of SPEC doesn’t match any terminal; instead, it
          specifies defaults that apply to all terminals.  This element,
          if used, must be the first element of SPEC.  Each of the
          following elements can override any or all of these defaults.

     ‘t’
          This element of SPEC matches all terminals.  Therefore, any
          subsequent elements of SPEC are never used.  Normally ‘t’ is
          used in the last (or only) element of SPEC.

     a list
          If DISPLAY is a list, each element should have the form
          ‘(CHARACTERISTIC VALUE...)’.  Here CHARACTERISTIC specifies a
          way of classifying terminals, and the VALUEs are possible
          classifications which DISPLAY should apply to.  Here are the
          possible values of CHARACTERISTIC:

          ‘type’
               The kind of window system the terminal uses—either
               ‘graphic’ (any graphics-capable display), ‘x’, ‘pc’ (for
               the MS-DOS console), ‘w32’ (for MS Windows 9X/NT/2K/XP),
               or ‘tty’ (a non-graphics-capable display).  *Note
               window-system: Window Systems.

          ‘class’
               What kinds of colors the terminal supports—either
               ‘color’, ‘grayscale’, or ‘mono’.

          ‘background’
               The kind of background—either ‘light’ or ‘dark’.

          ‘min-colors’
               An integer that represents the minimum number of colors
               the terminal should support.  This matches a terminal if
               its ‘display-color-cells’ value is at least the specified
               integer.

          ‘supports’
               Whether or not the terminal can display the face
               attributes given in VALUE... (*note Face Attributes::).
               *Note Display Face Attribute Testing::, for more
               information on exactly how this testing is done.

          If an element of DISPLAY specifies more than one VALUE for a
          given CHARACTERISTIC, any of those values is acceptable.  If
          DISPLAY has more than one element, each element should specify
          a different CHARACTERISTIC; then _each_ characteristic of the
          terminal must match one of the VALUEs specified for it in
          DISPLAY.

   For example, here’s the definition of the standard face ‘highlight’:

     (defface highlight
       '((((class color) (min-colors 88) (background light))
          :background "darkseagreen2")
         (((class color) (min-colors 88) (background dark))
          :background "darkolivegreen")
         (((class color) (min-colors 16) (background light))
          :background "darkseagreen2")
         (((class color) (min-colors 16) (background dark))
          :background "darkolivegreen")
         (((class color) (min-colors 8))
          :background "green" :foreground "black")
         (t :inverse-video t))
       "Basic face for highlighting."
       :group 'basic-faces)

   Internally, Emacs stores each face’s default spec in its
‘face-defface-spec’ symbol property (*note Symbol Properties::).  The
‘saved-face’ property stores any face spec saved by the user using the
customization buffer; the ‘customized-face’ property stores the face
spec customized for the current session, but not saved; and the
‘theme-face’ property stores an alist associating the active
customization settings and Custom themes with the face specs for that
face.  The face’s documentation string is stored in the
‘face-documentation’ property.

   Normally, a face is declared just once, using ‘defface’, and any
further changes to its appearance are applied using the Customize
framework (e.g., via the Customize user interface or via the
‘custom-set-faces’ function; *note Applying Customizations::), or by
face remapping (*note Face Remapping::).  In the rare event that you
need to change a face spec directly from Lisp, you can use the
‘face-spec-set’ function.

 -- Function: face-spec-set face spec &optional spec-type
     This function applies SPEC as a face spec for ‘face’.  SPEC should
     be a face spec, as described in the above documentation for
     ‘defface’.

     This function also defines FACE as a valid face name if it is not
     already one, and (re)calculates its attributes on existing frames.

     The argument SPEC-TYPE determines which spec to set.  If it is
     ‘nil’ or ‘face-override-spec’, this function sets the “override
     spec”, which overrides over all other face specs on FACE.  If it is
     ‘customized-face’ or ‘saved-face’, this function sets the
     customized spec or the saved custom spec.  If it is
     ‘face-defface-spec’, this function sets the default face spec (the
     same one set by ‘defface’).  If it is ‘reset’, this function clears
     out all customization specs and override specs from FACE (in this
     case, the value of SPEC is ignored).  Any other value of SPEC-TYPE
     is reserved for internal use.


File: elisp.info,  Node: Attribute Functions,  Next: Displaying Faces,  Prev: Defining Faces,  Up: Faces

37.12.3 Face Attribute Functions
--------------------------------

This section describes functions for directly accessing and modifying
the attributes of a named face.

 -- Function: face-attribute face attribute &optional frame inherit
     This function returns the value of the ATTRIBUTE attribute for FACE
     on FRAME.

     If FRAME is omitted or ‘nil’, that means the selected frame (*note
     Input Focus::).  If FRAME is ‘t’, this function returns the value
     of the specified attribute for newly-created frames (this is
     normally ‘unspecified’, unless you have specified some value using
     ‘set-face-attribute’; see below).

     If INHERIT is ‘nil’, only attributes directly defined by FACE are
     considered, so the return value may be ‘unspecified’, or a relative
     value.  If INHERIT is non-‘nil’, FACE’s definition of ATTRIBUTE is
     merged with the faces specified by its ‘:inherit’ attribute;
     however the return value may still be ‘unspecified’ or relative.
     If INHERIT is a face or a list of faces, then the result is further
     merged with that face (or faces), until it becomes specified and
     absolute.

     To ensure that the return value is always specified and absolute,
     use a value of ‘default’ for INHERIT; this will resolve any
     unspecified or relative values by merging with the ‘default’ face
     (which is always completely specified).

     For example,

          (face-attribute 'bold :weight)
               ⇒ bold

 -- Function: face-attribute-relative-p attribute value
     This function returns non-‘nil’ if VALUE, when used as the value of
     the face attribute ATTRIBUTE, is relative.  This means it would
     modify, rather than completely override, any value that comes from
     a subsequent face in the face list or that is inherited from
     another face.

     ‘unspecified’ is a relative value for all attributes.  For
     ‘:height’, floating point and function values are also relative.

     For example:

          (face-attribute-relative-p :height 2.0)
               ⇒ t

 -- Function: face-all-attributes face &optional frame
     This function returns an alist of attributes of FACE.  The elements
     of the result are name-value pairs of the form
     ‘(ATTR-NAME . ATTR-VALUE)’.  Optional argument FRAME specifies the
     frame whose definition of FACE to return; if omitted or ‘nil’, the
     returned value describes the default attributes of FACE for newly
     created frames.

 -- Function: merge-face-attribute attribute value1 value2
     If VALUE1 is a relative value for the face attribute ATTRIBUTE,
     returns it merged with the underlying value VALUE2; otherwise, if
     VALUE1 is an absolute value for the face attribute ATTRIBUTE,
     returns VALUE1 unchanged.

   Normally, Emacs uses the face specs of each face to automatically
calculate its attributes on each frame (*note Defining Faces::).  The
function ‘set-face-attribute’ can override this calculation by directly
assigning attributes to a face, either on a specific frame or for all
frames.  This function is mostly intended for internal usage.

 -- Function: set-face-attribute face frame &rest arguments
     This function sets one or more attributes of FACE for FRAME.  The
     attributes specifies in this way override the face spec(s)
     belonging to FACE.

     The extra arguments ARGUMENTS specify the attributes to set, and
     the values for them.  They should consist of alternating attribute
     names (such as ‘:family’ or ‘:underline’) and values.  Thus,

          (set-face-attribute 'foo nil :weight 'bold :slant 'italic)

     sets the attribute ‘:weight’ to ‘bold’ and the attribute ‘:slant’
     to ‘italic’.

     If FRAME is ‘t’, this function sets the default attributes for
     newly created frames.  If FRAME is ‘nil’, this function sets the
     attributes for all existing frames, as well as for newly created
     frames.

   The following commands and functions mostly provide compatibility
with old versions of Emacs.  They work by calling ‘set-face-attribute’.
Values of ‘t’ and ‘nil’ (or omitted) for their FRAME argument are
handled just like ‘set-face-attribute’ and ‘face-attribute’.  The
commands read their arguments using the minibuffer, if called
interactively.

 -- Command: set-face-foreground face color &optional frame
 -- Command: set-face-background face color &optional frame
     These set the ‘:foreground’ attribute (or ‘:background’ attribute,
     respectively) of FACE to COLOR.

 -- Command: set-face-stipple face pattern &optional frame
     This sets the ‘:stipple’ attribute of FACE to PATTERN.

 -- Command: set-face-font face font &optional frame
     This sets the ‘:font’ attribute of FACE to FONT.

 -- Function: set-face-bold face bold-p &optional frame
     This sets the ‘:weight’ attribute of FACE to NORMAL if BOLD-P is
     ‘nil’, and to BOLD otherwise.

 -- Function: set-face-italic face italic-p &optional frame
     This sets the ‘:slant’ attribute of FACE to NORMAL if ITALIC-P is
     ‘nil’, and to ITALIC otherwise.

 -- Function: set-face-underline face underline &optional frame
     This sets the ‘:underline’ attribute of FACE to UNDERLINE.

 -- Function: set-face-inverse-video face inverse-video-p &optional
          frame
     This sets the ‘:inverse-video’ attribute of FACE to
     INVERSE-VIDEO-P.

 -- Command: invert-face face &optional frame
     This swaps the foreground and background colors of face FACE.

   The following functions examine the attributes of a face.  They
mostly provide compatibility with old versions of Emacs.  If you don’t
specify FRAME, they refer to the selected frame; ‘t’ refers to the
default data for new frames.  They return ‘unspecified’ if the face
doesn’t define any value for that attribute.  If INHERIT is ‘nil’, only
an attribute directly defined by the face is returned.  If INHERIT is
non-‘nil’, any faces specified by its ‘:inherit’ attribute are
considered as well, and if INHERIT is a face or a list of faces, then
they are also considered, until a specified attribute is found.  To
ensure that the return value is always specified, use a value of
‘default’ for INHERIT.

 -- Function: face-font face &optional frame character
     This function returns the name of the font of face FACE.

     If the optional argument FRAME is specified, it returns the name of
     the font of FACE for that frame.  If FRAME is omitted or ‘nil’, the
     selected frame is used.  And, in this case, if the optional third
     argument CHARACTER is supplied, it returns the font name used for
     CHARACTER.

 -- Function: face-foreground face &optional frame inherit
 -- Function: face-background face &optional frame inherit
     These functions return the foreground color (or background color,
     respectively) of face FACE, as a string.  If the color is
     unspecified, they return ‘nil’.

 -- Function: face-stipple face &optional frame inherit
     This function returns the name of the background stipple pattern of
     face FACE, or ‘nil’ if it doesn’t have one.

 -- Function: face-bold-p face &optional frame inherit
     This function returns a non-‘nil’ value if the ‘:weight’ attribute
     of FACE is bolder than normal (i.e., one of ‘semi-bold’, ‘bold’,
     ‘extra-bold’, or ‘ultra-bold’).  Otherwise, it returns ‘nil’.

 -- Function: face-italic-p face &optional frame inherit
     This function returns a non-‘nil’ value if the ‘:slant’ attribute
     of FACE is ‘italic’ or ‘oblique’, and ‘nil’ otherwise.

 -- Function: face-underline-p face &optional frame inherit
     This function returns non-‘nil’ if face FACE specifies a non-‘nil’
     ‘:underline’ attribute.

 -- Function: face-inverse-video-p face &optional frame inherit
     This function returns non-‘nil’ if face FACE specifies a non-‘nil’
     ‘:inverse-video’ attribute.


File: elisp.info,  Node: Displaying Faces,  Next: Face Remapping,  Prev: Attribute Functions,  Up: Faces

37.12.4 Displaying Faces
------------------------

When Emacs displays a given piece of text, the visual appearance of the
text may be determined by faces drawn from different sources.  If these
various sources together specify more than one face for a particular
character, Emacs merges the attributes of the various faces.  Here is
the order in which Emacs merges the faces, from highest to lowest
priority:

   • If the text consists of a special glyph, the glyph can specify a
     particular face.  *Note Glyphs::.

   • If the text lies within an active region, Emacs highlights it using
     the ‘region’ face.  *Note (emacs)Standard Faces::.

   • If the text lies within an overlay with a non-‘nil’ ‘face’
     property, Emacs applies the face(s) specified by that property.  If
     the overlay has a ‘mouse-face’ property and the mouse is near
     enough to the overlay, Emacs applies the face or face attributes
     specified by the ‘mouse-face’ property instead.  *Note Overlay
     Properties::.

     When multiple overlays cover one character, an overlay with higher
     priority overrides those with lower priority.  *Note Overlays::.

   • If the text contains a ‘face’ or ‘mouse-face’ property, Emacs
     applies the specified faces and face attributes.  *Note Special
     Properties::.  (This is how Font Lock mode faces are applied.
     *Note Font Lock Mode::.)

   • If the text lies within the mode line of the selected window, Emacs
     applies the ‘mode-line’ face.  For the mode line of a non-selected
     window, Emacs applies the ‘mode-line-inactive’ face.  For a header
     line, Emacs applies the ‘header-line’ face.

   • If any given attribute has not been specified during the preceding
     steps, Emacs applies the attribute of the ‘default’ face.

   At each stage, if a face has a valid ‘:inherit’ attribute, Emacs
treats any attribute with an ‘unspecified’ value as having the
corresponding value drawn from the parent face(s).  *note Face
Attributes::.  Note that the parent face(s) may also leave the attribute
unspecified; in that case, the attribute remains unspecified at the next
level of face merging.


File: elisp.info,  Node: Face Remapping,  Next: Face Functions,  Prev: Displaying Faces,  Up: Faces

37.12.5 Face Remapping
----------------------

The variable ‘face-remapping-alist’ is used for buffer-local or global
changes in the appearance of a face.  For instance, it is used to
implement the ‘text-scale-adjust’ command (*note (emacs)Text Scale::).

 -- Variable: face-remapping-alist
     The value of this variable is an alist whose elements have the form
     ‘(FACE . REMAPPING)’.  This causes Emacs to display any text having
     the face FACE with REMAPPING, rather than the ordinary definition
     of FACE.

     REMAPPING may be any face spec suitable for a ‘face’ text property:
     either a face (i.e., a face name or a property list of
     attribute/value pairs), or a list of faces.  For details, see the
     description of the ‘face’ text property in *note Special
     Properties::.  REMAPPING serves as the complete specification for
     the remapped face—it replaces the normal definition of FACE,
     instead of modifying it.

     If ‘face-remapping-alist’ is buffer-local, its local value takes
     effect only within that buffer.

     Note: face remapping is non-recursive.  If REMAPPING references the
     same face name FACE, either directly or via the ‘:inherit’
     attribute of some other face in REMAPPING, that reference uses the
     normal definition of FACE.  For instance, if the ‘mode-line’ face
     is remapped using this entry in ‘face-remapping-alist’:

          (mode-line italic mode-line)

     then the new definition of the ‘mode-line’ face inherits from the
     ‘italic’ face, and the _normal_ (non-remapped) definition of
     ‘mode-line’ face.

   The following functions implement a higher-level interface to
‘face-remapping-alist’.  Most Lisp code should use these functions
instead of setting ‘face-remapping-alist’ directly, to avoid trampling
on remappings applied elsewhere.  These functions are intended for
buffer-local remappings, so they all make ‘face-remapping-alist’
buffer-local as a side-effect.  They manage ‘face-remapping-alist’
entries of the form

       (FACE RELATIVE-SPEC-1 RELATIVE-SPEC-2 ... BASE-SPEC)

where, as explained above, each of the RELATIVE-SPEC-N and BASE-SPEC is
either a face name, or a property list of attribute/value pairs.  Each
of the “relative remapping” entries, RELATIVE-SPEC-N, is managed by the
‘face-remap-add-relative’ and ‘face-remap-remove-relative’ functions;
these are intended for simple modifications like changing the text size.
The “base remapping” entry, BASE-SPEC, has the lowest priority and is
managed by the ‘face-remap-set-base’ and ‘face-remap-reset-base’
functions; it is intended for major modes to remap faces in the buffers
they control.

 -- Function: face-remap-add-relative face &rest specs
     This function adds the face spec in SPECS as relative remappings
     for face FACE in the current buffer.  The remaining arguments,
     SPECS, should form either a list of face names, or a property list
     of attribute/value pairs.

     The return value is a Lisp object that serves as a cookie; you can
     pass this object as an argument to ‘face-remap-remove-relative’ if
     you need to remove the remapping later.

          ;; Remap the 'escape-glyph' face into a combination
          ;; of the 'highlight' and 'italic' faces:
          (face-remap-add-relative 'escape-glyph 'highlight 'italic)

          ;; Increase the size of the 'default' face by 50%:
          (face-remap-add-relative 'default :height 1.5)

 -- Function: face-remap-remove-relative cookie
     This function removes a relative remapping previously added by
     ‘face-remap-add-relative’.  COOKIE should be the Lisp object
     returned by ‘face-remap-add-relative’ when the remapping was added.

 -- Function: face-remap-set-base face &rest specs
     This function sets the base remapping of FACE in the current buffer
     to SPECS.  If SPECS is empty, the default base remapping is
     restored, similar to calling ‘face-remap-reset-base’ (see below);
     note that this is different from SPECS containing a single value
     ‘nil’, which has the opposite result (the global definition of FACE
     is ignored).

     This overwrites the default BASE-SPEC, which inherits the global
     face definition, so it is up to the caller to add such inheritance
     if so desired.

 -- Function: face-remap-reset-base face
     This function sets the base remapping of FACE to its default value,
     which inherits from FACE’s global definition.


File: elisp.info,  Node: Face Functions,  Next: Auto Faces,  Prev: Face Remapping,  Up: Faces

37.12.6 Functions for Working with Faces
----------------------------------------

Here are additional functions for creating and working with faces.

 -- Function: face-list
     This function returns a list of all defined face names.

 -- Function: face-id face
     This function returns the “face number” of face FACE.  This is a
     number that uniquely identifies a face at low levels within Emacs.
     It is seldom necessary to refer to a face by its face number.

 -- Function: face-documentation face
     This function returns the documentation string of face FACE, or
     ‘nil’ if none was specified for it.

 -- Function: face-equal face1 face2 &optional frame
     This returns ‘t’ if the faces FACE1 and FACE2 have the same
     attributes for display.

 -- Function: face-differs-from-default-p face &optional frame
     This returns non-‘nil’ if the face FACE displays differently from
     the default face.

   A “face alias” provides an equivalent name for a face.  You can
define a face alias by giving the alias symbol the ‘face-alias’
property, with a value of the target face name.  The following example
makes ‘modeline’ an alias for the ‘mode-line’ face.

     (put 'modeline 'face-alias 'mode-line)

 -- Macro: define-obsolete-face-alias obsolete-face current-face when
     This macro defines ‘obsolete-face’ as an alias for CURRENT-FACE,
     and also marks it as obsolete, indicating that it may be removed in
     future.  WHEN should be a string indicating when ‘obsolete-face’
     was made obsolete (usually a version number string).


File: elisp.info,  Node: Auto Faces,  Next: Basic Faces,  Prev: Face Functions,  Up: Faces

37.12.7 Automatic Face Assignment
---------------------------------

This hook is used for automatically assigning faces to text in the
buffer.  It is part of the implementation of Jit-Lock mode, used by
Font-Lock.

 -- Variable: fontification-functions
     This variable holds a list of functions that are called by Emacs
     redisplay as needed, just before doing redisplay.  They are called
     even when Font Lock Mode isn’t enabled.  When Font Lock Mode is
     enabled, this variable usually holds just one function,
     ‘jit-lock-function’.

     The functions are called in the order listed, with one argument, a
     buffer position POS.  Collectively they should attempt to assign
     faces to the text in the current buffer starting at POS.

     The functions should record the faces they assign by setting the
     ‘face’ property.  They should also add a non-‘nil’ ‘fontified’
     property to all the text they have assigned faces to.  That
     property tells redisplay that faces have been assigned to that text
     already.

     It is probably a good idea for the functions to do nothing if the
     character after POS already has a non-‘nil’ ‘fontified’ property,
     but this is not required.  If one function overrides the
     assignments made by a previous one, the properties after the last
     function finishes are the ones that really matter.

     For efficiency, we recommend writing these functions so that they
     usually assign faces to around 400 to 600 characters at each call.


File: elisp.info,  Node: Basic Faces,  Next: Font Selection,  Prev: Auto Faces,  Up: Faces

37.12.8 Basic Faces
-------------------

If your Emacs Lisp program needs to assign some faces to text, it is
often a good idea to use certain existing faces or inherit from them,
rather than defining entirely new faces.  This way, if other users have
customized the basic faces to give Emacs a certain look, your program
will fit in without additional customization.

   Some of the basic faces defined in Emacs are listed below.  In
addition to these, you might want to make use of the Font Lock faces for
syntactic highlighting, if highlighting is not already handled by Font
Lock mode, or if some Font Lock faces are not in use.  *Note Faces for
Font Lock::.

‘default’
     The default face, whose attributes are all specified.  All other
     faces implicitly inherit from it: any unspecified attribute
     defaults to the attribute on this face (*note Face Attributes::).

‘bold’
‘italic’
‘bold-italic’
‘underline’
‘fixed-pitch’
‘fixed-pitch-serif’
‘variable-pitch’
     These have the attributes indicated by their names (e.g., ‘bold’
     has a bold ‘:weight’ attribute), with all other attributes
     unspecified (and so given by ‘default’).

‘shadow’
     For dimmed-out text.  For example, it is used for the ignored part
     of a filename in the minibuffer (*note Minibuffers for File Names:
     (emacs)Minibuffer File.).

‘link’
‘link-visited’
     For clickable text buttons that send the user to a different buffer
     or location.

‘highlight’
     For stretches of text that should temporarily stand out.  For
     example, it is commonly assigned to the ‘mouse-face’ property for
     cursor highlighting (*note Special Properties::).

‘match’
‘isearch’
‘lazy-highlight’
     For text matching (respectively) permanent search matches,
     interactive search matches, and lazy highlighting other matches
     than the current interactive one.

‘error’
‘warning’
‘success’
     For text concerning errors, warnings, or successes.  For example,
     these are used for messages in ‘*Compilation*’ buffers.


File: elisp.info,  Node: Font Selection,  Next: Font Lookup,  Prev: Basic Faces,  Up: Faces

37.12.9 Font Selection
----------------------

Before Emacs can draw a character on a graphical display, it must select
a “font” for that character(1).  *Note (emacs)Fonts::.  Normally, Emacs
automatically chooses a font based on the faces assigned to that
character—specifically, the face attributes ‘:family’, ‘:weight’,
‘:slant’, and ‘:width’ (*note Face Attributes::).  The choice of font
also depends on the character to be displayed; some fonts can only
display a limited set of characters.  If no available font exactly fits
the requirements, Emacs looks for the “closest matching font”.  The
variables in this section control how Emacs makes this selection.

 -- User Option: face-font-family-alternatives
     If a given family is specified but does not exist, this variable
     specifies alternative font families to try.  Each element should
     have this form:

          (FAMILY ALTERNATE-FAMILIES...)

     If FAMILY is specified but not available, Emacs will try the other
     families given in ALTERNATE-FAMILIES, one by one, until it finds a
     family that does exist.

 -- User Option: face-font-selection-order
     If there is no font that exactly matches all desired face
     attributes (‘:width’, ‘:height’, ‘:weight’, and ‘:slant’), this
     variable specifies the order in which these attributes should be
     considered when selecting the closest matching font.  The value
     should be a list containing those four attribute symbols, in order
     of decreasing importance.  The default is ‘(:width :height :weight
     :slant)’.

     Font selection first finds the best available matches for the first
     attribute in the list; then, among the fonts which are best in that
     way, it searches for the best matches in the second attribute, and
     so on.

     The attributes ‘:weight’ and ‘:width’ have symbolic values in a
     range centered around ‘normal’.  Matches that are more extreme
     (farther from ‘normal’) are somewhat preferred to matches that are
     less extreme (closer to ‘normal’); this is designed to ensure that
     non-normal faces contrast with normal ones, whenever possible.

     One example of a case where this variable makes a difference is
     when the default font has no italic equivalent.  With the default
     ordering, the ‘italic’ face will use a non-italic font that is
     similar to the default one.  But if you put ‘:slant’ before
     ‘:height’, the ‘italic’ face will use an italic font, even if its
     height is not quite right.

 -- User Option: face-font-registry-alternatives
     This variable lets you specify alternative font registries to try,
     if a given registry is specified and doesn’t exist.  Each element
     should have this form:

          (REGISTRY ALTERNATE-REGISTRIES...)

     If REGISTRY is specified but not available, Emacs will try the
     other registries given in ALTERNATE-REGISTRIES, one by one, until
     it finds a registry that does exist.

   Emacs can make use of scalable fonts, but by default it does not use
them.

 -- User Option: scalable-fonts-allowed
     This variable controls which scalable fonts to use.  A value of
     ‘nil’, the default, means do not use scalable fonts.  ‘t’ means to
     use any scalable font that seems appropriate for the text.

     Otherwise, the value must be a list of regular expressions.  Then a
     scalable font is enabled for use if its name matches any regular
     expression in the list.  For example,

          (setq scalable-fonts-allowed '("iso10646-1$"))

     allows the use of scalable fonts with registry ‘iso10646-1’.

 -- Variable: face-font-rescale-alist
     This variable specifies scaling for certain faces.  Its value
     should be a list of elements of the form

          (FONTNAME-REGEXP . SCALE-FACTOR)

     If FONTNAME-REGEXP matches the font name that is about to be used,
     this says to choose a larger similar font according to the factor
     SCALE-FACTOR.  You would use this feature to normalize the font
     size if certain fonts are bigger or smaller than their nominal
     heights and widths would suggest.

   ---------- Footnotes ----------

   (1) In this context, the term “font” has nothing to do with Font Lock
(*note Font Lock Mode::).


File: elisp.info,  Node: Font Lookup,  Next: Fontsets,  Prev: Font Selection,  Up: Faces

37.12.10 Looking Up Fonts
-------------------------

 -- Function: x-list-fonts name &optional reference-face frame maximum
          width
     This function returns a list of available font names that match
     NAME.  NAME should be a string containing a font name in either the
     Fontconfig, GTK, or XLFD format (*note (emacs)Fonts::).  Within an
     XLFD string, wildcard characters may be used: the ‘*’ character
     matches any substring, and the ‘?’ character matches any single
     character.  Case is ignored when matching font names.

     If the optional arguments REFERENCE-FACE and FRAME are specified,
     the returned list includes only fonts that are the same size as
     REFERENCE-FACE (a face name) currently is on the frame FRAME.

     The optional argument MAXIMUM sets a limit on how many fonts to
     return.  If it is non-‘nil’, then the return value is truncated
     after the first MAXIMUM matching fonts.  Specifying a small value
     for MAXIMUM can make this function much faster, in cases where many
     fonts match the pattern.

     The optional argument WIDTH specifies a desired font width.  If it
     is non-‘nil’, the function only returns those fonts whose
     characters are (on average) WIDTH times as wide as REFERENCE-FACE.

 -- Function: x-family-fonts &optional family frame
     This function returns a list describing the available fonts for
     family FAMILY on FRAME.  If FAMILY is omitted or ‘nil’, this list
     applies to all families, and therefore, it contains all available
     fonts.  Otherwise, FAMILY must be a string; it may contain the
     wildcards ‘?’ and ‘*’.

     The list describes the display that FRAME is on; if FRAME is
     omitted or ‘nil’, it applies to the selected frame’s display (*note
     Input Focus::).

     Each element in the list is a vector of the following form:

          [FAMILY WIDTH POINT-SIZE WEIGHT SLANT
           FIXED-P FULL REGISTRY-AND-ENCODING]

     The first five elements correspond to face attributes; if you
     specify these attributes for a face, it will use this font.

     The last three elements give additional information about the font.
     FIXED-P is non-‘nil’ if the font is fixed-pitch.  FULL is the full
     name of the font, and REGISTRY-AND-ENCODING is a string giving the
     registry and encoding of the font.


File: elisp.info,  Node: Fontsets,  Next: Low-Level Font,  Prev: Font Lookup,  Up: Faces

37.12.11 Fontsets
-----------------

A “fontset” is a list of fonts, each assigned to a range of character
codes.  An individual font cannot display the whole range of characters
that Emacs supports, but a fontset can.  Fontsets have names, just as
fonts do, and you can use a fontset name in place of a font name when
you specify the font for a frame or a face.  Here is information about
defining a fontset under Lisp program control.

 -- Function: create-fontset-from-fontset-spec fontset-spec &optional
          style-variant-p noerror
     This function defines a new fontset according to the specification
     string FONTSET-SPEC.  The string should have this format:

          FONTPATTERN, [CHARSET:FONT]...

     Whitespace characters before and after the commas are ignored.

     The first part of the string, FONTPATTERN, should have the form of
     a standard X font name, except that the last two fields should be
     ‘fontset-ALIAS’.

     The new fontset has two names, one long and one short.  The long
     name is FONTPATTERN in its entirety.  The short name is
     ‘fontset-ALIAS’.  You can refer to the fontset by either name.  If
     a fontset with the same name already exists, an error is signaled,
     unless NOERROR is non-‘nil’, in which case this function does
     nothing.

     If optional argument STYLE-VARIANT-P is non-‘nil’, that says to
     create bold, italic and bold-italic variants of the fontset as
     well.  These variant fontsets do not have a short name, only a long
     one, which is made by altering FONTPATTERN to indicate the bold
     and/or italic status.

     The specification string also says which fonts to use in the
     fontset.  See below for the details.

   The construct ‘CHARSET:FONT’ specifies which font to use (in this
fontset) for one particular character set.  Here, CHARSET is the name of
a character set, and FONT is the font to use for that character set.
You can use this construct any number of times in the specification
string.

   For the remaining character sets, those that you don’t specify
explicitly, Emacs chooses a font based on FONTPATTERN: it replaces
‘fontset-ALIAS’ with a value that names one character set.  For the
ASCII character set, ‘fontset-ALIAS’ is replaced with ‘ISO8859-1’.

   In addition, when several consecutive fields are wildcards, Emacs
collapses them into a single wildcard.  This is to prevent use of
auto-scaled fonts.  Fonts made by scaling larger fonts are not usable
for editing, and scaling a smaller font is not useful because it is
better to use the smaller font in its own size, which Emacs does.

   Thus if FONTPATTERN is this,

     -*-fixed-medium-r-normal-*-24-*-*-*-*-*-fontset-24

the font specification for ASCII characters would be this:

     -*-fixed-medium-r-normal-*-24-*-ISO8859-1

and the font specification for Chinese GB2312 characters would be this:

     -*-fixed-medium-r-normal-*-24-*-gb2312*-*

   You may not have any Chinese font matching the above font
specification.  Most X distributions include only Chinese fonts that
have ‘song ti’ or ‘fangsong ti’ in the FAMILY field.  In such a case,
‘Fontset-N’ can be specified as below:

     Emacs.Fontset-0: -*-fixed-medium-r-normal-*-24-*-*-*-*-*-fontset-24,\
             chinese-gb2312:-*-*-medium-r-normal-*-24-*-gb2312*-*

Then, the font specifications for all but Chinese GB2312 characters have
‘fixed’ in the FAMILY field, and the font specification for Chinese
GB2312 characters has a wild card ‘*’ in the FAMILY field.

 -- Function: set-fontset-font name character font-spec &optional frame
          add
     This function modifies the existing fontset NAME to use the font
     matching with FONT-SPEC for the specified CHARACTER.

     If NAME is ‘nil’, this function modifies the fontset of the
     selected frame or that of FRAME if FRAME is not ‘nil’.

     If NAME is ‘t’, this function modifies the default fontset, whose
     short name is ‘fontset-default’.

     In addition to specifying a single codepoint, CHARACTER may be a
     cons ‘(FROM . TO)’, where FROM and TO are character codepoints.  In
     that case, use FONT-SPEC for all the characters in the range FROM
     and TO (inclusive).

     CHARACTER may be a charset.  In that case, use FONT-SPEC for all
     character in the charsets.

     CHARACTER may be a script name.  In that case, use FONT-SPEC for
     all character in the charsets.

     FONT-SPEC may be a font-spec object created by the function
     ‘font-spec’ (*note Low-Level Font::).

     FONT-SPEC may be a cons; ‘(FAMILY . REGISTRY)’, where FAMILY is a
     family name of a font (possibly including a foundry name at the
     head), REGISTRY is a registry name of a font (possibly including an
     encoding name at the tail).

     FONT-SPEC may be a font name string.

     FONT-SPEC may be ‘nil’, which explicitly specifies that there’s no
     font for the specified CHARACTER.  This is useful, for example, to
     avoid expensive system-wide search for fonts for characters that
     have no glyphs, like those from the Unicode Private Use Area (PUA).

     The optional argument ADD, if non-‘nil’, specifies how to add
     FONT-SPEC to the font specifications previously set.  If it is
     ‘prepend’, FONT-SPEC is prepended.  If it is ‘append’, FONT-SPEC is
     appended.  By default, FONT-SPEC overrides the previous settings.

     For instance, this changes the default fontset to use a font of
     which family name is ‘Kochi Gothic’ for all characters belonging to
     the charset ‘japanese-jisx0208’.

          (set-fontset-font t 'japanese-jisx0208
                            (font-spec :family "Kochi Gothic"))

 -- Function: char-displayable-p char
     This function returns ‘t’ if Emacs ought to be able to display
     CHAR.  More precisely, if the selected frame’s fontset has a font
     to display the character set that CHAR belongs to.

     Fontsets can specify a font on a per-character basis; when the
     fontset does that, this function’s value may not be accurate.


File: elisp.info,  Node: Low-Level Font,  Prev: Fontsets,  Up: Faces

37.12.12 Low-Level Font Representation
--------------------------------------

Normally, it is not necessary to manipulate fonts directly.  In case you
need to do so, this section explains how.

   In Emacs Lisp, fonts are represented using three different Lisp
object types: “font objects”, “font specs”, and “font entities”.

 -- Function: fontp object &optional type
     Return ‘t’ if OBJECT is a font object, font spec, or font entity.
     Otherwise, return ‘nil’.

     The optional argument TYPE, if non-‘nil’, determines the exact type
     of Lisp object to check for.  In that case, TYPE should be one of
     ‘font-object’, ‘font-spec’, or ‘font-entity’.

   A font object is a Lisp object that represents a font that Emacs has
“opened”.  Font objects cannot be modified in Lisp, but they can be
inspected.

 -- Function: font-at position &optional window string
     Return the font object that is being used to display the character
     at position POSITION in the window WINDOW.  If WINDOW is ‘nil’, it
     defaults to the selected window.  If STRING is ‘nil’, POSITION
     specifies a position in the current buffer; otherwise, STRING
     should be a string, and POSITION specifies a position in that
     string.

   A font spec is a Lisp object that contains a set of specifications
that can be used to find a font.  More than one font may match the
specifications in a font spec.

 -- Function: font-spec &rest arguments
     Return a new font spec using the specifications in ARGUMENTS, which
     should come in ‘property’-‘value’ pairs.  The possible
     specifications are as follows:

     ‘:name’
          The font name (a string), in either XLFD, Fontconfig, or GTK
          format.  *Note (emacs)Fonts::.

     ‘:family’
     ‘:foundry’
     ‘:weight’
     ‘:slant’
     ‘:width’
          These have the same meanings as the face attributes of the
          same name.  *Note Face Attributes::.

     ‘:size’
          The font size—either a non-negative integer that specifies the
          pixel size, or a floating-point number that specifies the
          point size.

     ‘:adstyle’
          Additional typographic style information for the font, such as
          ‘sans’.  The value should be a string or a symbol.

     ‘:registry’
          The charset registry and encoding of the font, such as
          ‘iso8859-1’.  The value should be a string or a symbol.

     ‘:script’
          The script that the font must support (a symbol).

     ‘:lang’
          The language that the font should support.  The value should
          be a symbol whose name is a two-letter ISO-639 language name.
          On X, the value is matched against the “Additional Style”
          field of the XLFD name of a font, if it is non-empty.  On
          MS-Windows, fonts matching the spec are required to support
          codepages needed for the language.  Currently, only a small
          set of CJK languages is supported with this property: ‘ja’,
          ‘ko’, and ‘zh’.

     ‘:otf’
          The font must be an OpenType font that supports these OpenType
          features, provided Emacs is compiled with a library, such as
          ‘libotf’ on GNU/Linux, that supports complex text layout for
          scripts which need that.  The value must be a list of the form

               (SCRIPT-TAG LANGSYS-TAG GSUB GPOS)

          where SCRIPT-TAG is the OpenType script tag symbol;
          LANGSYS-TAG is the OpenType language system tag symbol, or
          ‘nil’ to use the default language system; ‘gsub’ is a list of
          OpenType GSUB feature tag symbols, or ‘nil’ if none is
          required; and ‘gpos’ is a list of OpenType GPOS feature tag
          symbols, or ‘nil’ if none is required.  If ‘gsub’ or ‘gpos’ is
          a list, a ‘nil’ element in that list means that the font must
          not match any of the remaining tag symbols.  The ‘gpos’
          element may be omitted.

 -- Function: font-put font-spec property value
     Set the font property PROPERTY in the font-spec FONT-SPEC to VALUE.

   A font entity is a reference to a font that need not be open.  Its
properties are intermediate between a font object and a font spec: like
a font object, and unlike a font spec, it refers to a single, specific
font.  Unlike a font object, creating a font entity does not load the
contents of that font into computer memory.  Emacs may open multiple
font objects of different sizes from a single font entity referring to a
scalable font.

 -- Function: find-font font-spec &optional frame
     This function returns a font entity that best matches the font spec
     FONT-SPEC on frame FRAME.  If FRAME is ‘nil’, it defaults to the
     selected frame.

 -- Function: list-fonts font-spec &optional frame num prefer
     This function returns a list of all font entities that match the
     font spec FONT-SPEC.

     The optional argument FRAME, if non-‘nil’, specifies the frame on
     which the fonts are to be displayed.  The optional argument NUM, if
     non-‘nil’, should be an integer that specifies the maximum length
     of the returned list.  The optional argument PREFER, if non-‘nil’,
     should be another font spec, which is used to control the order of
     the returned list; the returned font entities are sorted in order
     of decreasing closeness to that font spec.

   If you call ‘set-face-attribute’ and pass a font spec, font entity,
or font name string as the value of the ‘:font’ attribute, Emacs opens
the best matching font that is available for display.  It then stores
the corresponding font object as the actual value of the ‘:font’
attribute for that face.

   The following functions can be used to obtain information about a
font.  For these functions, the FONT argument can be a font object, a
font entity, or a font spec.

 -- Function: font-get font property
     This function returns the value of the font property PROPERTY for
     FONT.

     If FONT is a font spec and the font spec does not specify PROPERTY,
     the return value is ‘nil’.  If FONT is a font object or font
     entity, the value for the :SCRIPT property may be a list of scripts
     supported by the font.

 -- Function: font-face-attributes font &optional frame
     This function returns a list of face attributes corresponding to
     FONT.  The optional argument FRAME specifies the frame on which the
     font is to be displayed.  If it is ‘nil’, the selected frame is
     used.  The return value has the form

          (:family FAMILY :height HEIGHT :weight WEIGHT
             :slant SLANT :width WIDTH)

     where the values of FAMILY, HEIGHT, WEIGHT, SLANT, and WIDTH are
     face attribute values.  Some of these key-attribute pairs may be
     omitted from the list if they are not specified by FONT.

 -- Function: font-xlfd-name font &optional fold-wildcards
     This function returns the XLFD (X Logical Font Descriptor), a
     string, matching FONT.  *Note (emacs)Fonts::, for information about
     XLFDs.  If the name is too long for an XLFD (which can contain at
     most 255 characters), the function returns ‘nil’.

     If the optional argument FOLD-WILDCARDS is non-‘nil’, consecutive
     wildcards in the XLFD are folded into one.

   The following two functions return important information about a
font.

 -- Function: font-info name &optional frame
     This function returns information about a font specified by its
     NAME, a string, as it is used on FRAME.  If FRAME is omitted or
     ‘nil’, it defaults to the selected frame.

     The value returned by the function is a vector of the form
     ‘[OPENED-NAME FULL-NAME SIZE HEIGHT BASELINE-OFFSET
     RELATIVE-COMPOSE DEFAULT-ASCENT MAX-WIDTH ASCENT DESCENT
     SPACE-WIDTH AVERAGE-WIDTH FILENAME CAPABILITY]’.  Here’s the
     description of each components of this vector:

     OPENED-NAME
          The name used to open the font, a string.

     FULL-NAME
          The full name of the font, a string.

     SIZE
          The pixel size of the font.

     HEIGHT
          The height of the font in pixels.

     BASELINE-OFFSET
          The offset in pixels from the ASCII baseline, positive upward.

     RELATIVE-COMPOSE
     DEFAULT-ASCENT
          Numbers controlling how to compose characters.

     ASCENT
     DESCENT
          The ascent and descent of this font.  The sum of these two
          numbers should be equal to the value of HEIGHT above.

     SPACE-WIDTH
          The width, in pixels, of the font’s space character.

     AVERAGE-WIDTH
          The average width of the font characters.  If this is zero,
          Emacs uses the value of SPACE-WIDTH instead, when it
          calculates text layout on display.

     FILENAME
          The file name of the font as a string.  This can be ‘nil’ if
          the font back-end does not provide a way to find out the
          font’s file name.

     CAPABILITY
          A list whose first element is a symbol representing the font
          type, one of ‘x’, ‘opentype’, ‘truetype’, ‘type1’, ‘pcf’, or
          ‘bdf’.  For OpenType fonts, the list includes 2 additional
          elements describing the GSUB and GPOS features supported by
          the font.  Each of these elements is a list of the form
          ‘((SCRIPT (LANGSYS FEATURE ...) ...) ...)’, where SCRIPT is a
          symbol representing an OpenType script tag, LANGSYS is a
          symbol representing an OpenType langsys tag (or ‘nil’, which
          stands for the default langsys), and each FEATURE is a symbol
          representing an OpenType feature tag.

 -- Function: query-font font-object
     This function returns information about a FONT-OBJECT.  (This is in
     contrast to ‘font-info’, which takes the font name, a string, as
     its argument.)

     The value returned by the function is a vector of the form ‘[NAME
     FILENAME PIXEL-SIZE MAX-WIDTH ASCENT DESCENT SPACE-WIDTH
     AVERAGE-WIDTH CAPABILITY]’.  Here’s the description of each
     components of this vector:

     NAME
          The font name, a string.

     FILENAME
          The file name of the font as a string.  This can be ‘nil’ if
          the font back-end does not provide a way to find out the
          font’s file name.

     PIXEL-SIZE
          The pixel size of the font used to open the font.

     MAX-WIDTH
          The maximum advance width of the font.

     ASCENT
     DESCENT
          The ascent and descent of this font.  The sum of these two
          numbers gives the font height.

     SPACE-WIDTH
          The width, in pixels, of the font’s space character.

     AVERAGE-WIDTH
          The average width of the font characters.  If this is zero,
          Emacs uses the value of SPACE-WIDTH instead, when it
          calculates text layout on display.

     CAPABILITY
          A list whose first element is a symbol representing the font
          type, one of ‘x’, ‘opentype’, ‘truetype’, ‘type1’, ‘pcf’, or
          ‘bdf’.  For OpenType fonts, the list includes 2 additional
          elements describing the GSUB and GPOS features supported by
          the font.  Each of these elements is a list of the form
          ‘((SCRIPT (LANGSYS FEATURE ...) ...) ...)’, where SCRIPT is a
          symbol representing an OpenType script tag, LANGSYS is a
          symbol representing an OpenType langsys tag (or ‘nil’, which
          stands for the default langsys), and each FEATURE is a symbol
          representing an OpenType feature tag.

   The following four functions return size information about fonts used
by various faces, allowing various layout considerations in Lisp
programs.  These functions take face remapping into consideration,
returning information about the remapped face, if the face in question
was remapped.  *Note Face Remapping::.

 -- Function: default-font-width
     This function returns the average width in pixels of the font used
     by the current buffer’s default face.

 -- Function: default-font-height
     This function returns the height in pixels of the font used by the
     current buffer’s default face.

 -- Function: window-font-width &optional window face
     This function returns the average width in pixels for the font used
     by FACE in WINDOW.  The specified WINDOW must be a live window.  If
     ‘nil’ or omitted, WINDOW defaults to the selected window, and FACE
     defaults to the default face in WINDOW.

 -- Function: window-font-height &optional window face
     This function returns the height in pixels for the font used by
     FACE in WINDOW.  The specified WINDOW must be a live window.  If
     ‘nil’ or omitted, WINDOW defaults to the selected window, and FACE
     defaults to the default face in WINDOW.


File: elisp.info,  Node: Fringes,  Next: Scroll Bars,  Prev: Faces,  Up: Display

37.13 Fringes
=============

On graphical displays, Emacs draws “fringes” next to each window: thin
vertical strips down the sides which can display bitmaps indicating
truncation, continuation, horizontal scrolling, and so on.

* Menu:

* Fringe Size/Pos::     Specifying where to put the window fringes.
* Fringe Indicators::   Displaying indicator icons in the window fringes.
* Fringe Cursors::      Displaying cursors in the right fringe.
* Fringe Bitmaps::      Specifying bitmaps for fringe indicators.
* Customizing Bitmaps:: Specifying your own bitmaps to use in the fringes.
* Overlay Arrow::       Display of an arrow to indicate position.


File: elisp.info,  Node: Fringe Size/Pos,  Next: Fringe Indicators,  Up: Fringes

37.13.1 Fringe Size and Position
--------------------------------

The following buffer-local variables control the position and width of
fringes in windows showing that buffer.

 -- Variable: fringes-outside-margins
     The fringes normally appear between the display margins and the
     window text.  If the value is non-‘nil’, they appear outside the
     display margins.  *Note Display Margins::.

 -- Variable: left-fringe-width
     This variable, if non-‘nil’, specifies the width of the left fringe
     in pixels.  A value of ‘nil’ means to use the left fringe width
     from the window’s frame.

 -- Variable: right-fringe-width
     This variable, if non-‘nil’, specifies the width of the right
     fringe in pixels.  A value of ‘nil’ means to use the right fringe
     width from the window’s frame.

   Any buffer which does not specify values for these variables uses the
values specified by the ‘left-fringe’ and ‘right-fringe’ frame
parameters (*note Layout Parameters::).

   The above variables actually take effect via the function
‘set-window-buffer’ (*note Buffers and Windows::), which calls
‘set-window-fringes’ as a subroutine.  If you change one of these
variables, the fringe display is not updated in existing windows showing
the buffer, unless you call ‘set-window-buffer’ again in each affected
window.  You can also use ‘set-window-fringes’ to control the fringe
display in individual windows.

 -- Function: set-window-fringes window left &optional right
          outside-margins
     This function sets the fringe widths of window WINDOW.  If WINDOW
     is ‘nil’, the selected window is used.

     The argument LEFT specifies the width in pixels of the left fringe,
     and likewise RIGHT for the right fringe.  A value of ‘nil’ for
     either one stands for the default width.  If OUTSIDE-MARGINS is
     non-‘nil’, that specifies that fringes should appear outside of the
     display margins.

 -- Function: window-fringes &optional window
     This function returns information about the fringes of a window
     WINDOW.  If WINDOW is omitted or ‘nil’, the selected window is
     used.  The value has the form ‘(LEFT-WIDTH RIGHT-WIDTH
     OUTSIDE-MARGINS)’.


File: elisp.info,  Node: Fringe Indicators,  Next: Fringe Cursors,  Prev: Fringe Size/Pos,  Up: Fringes

37.13.2 Fringe Indicators
-------------------------

“Fringe indicators” are tiny icons displayed in the window fringe to
indicate truncated or continued lines, buffer boundaries, etc.

 -- User Option: indicate-empty-lines
     When this is non-‘nil’, Emacs displays a special glyph in the
     fringe of each empty line at the end of the buffer, on graphical
     displays.  *Note Fringes::.  This variable is automatically
     buffer-local in every buffer.

 -- User Option: indicate-buffer-boundaries
     This buffer-local variable controls how the buffer boundaries and
     window scrolling are indicated in the window fringes.

     Emacs can indicate the buffer boundaries—that is, the first and
     last line in the buffer—with angle icons when they appear on the
     screen.  In addition, Emacs can display an up-arrow in the fringe
     to show that there is text above the screen, and a down-arrow to
     show there is text below the screen.

     There are three kinds of basic values:

     ‘nil’
          Don’t display any of these fringe icons.
     ‘left’
          Display the angle icons and arrows in the left fringe.
     ‘right’
          Display the angle icons and arrows in the right fringe.
     any non-alist
          Display the angle icons in the left fringe and don’t display
          the arrows.

     Otherwise the value should be an alist that specifies which fringe
     indicators to display and where.  Each element of the alist should
     have the form ‘(INDICATOR . POSITION)’.  Here, INDICATOR is one of
     ‘top’, ‘bottom’, ‘up’, ‘down’, and ‘t’ (which covers all the icons
     not yet specified), while POSITION is one of ‘left’, ‘right’ and
     ‘nil’.

     For example, ‘((top . left) (t . right))’ places the top angle
     bitmap in left fringe, and the bottom angle bitmap as well as both
     arrow bitmaps in right fringe.  To show the angle bitmaps in the
     left fringe, and no arrow bitmaps, use ‘((top . left) (bottom .
     left))’.

 -- Variable: fringe-indicator-alist
     This buffer-local variable specifies the mapping from logical
     fringe indicators to the actual bitmaps displayed in the window
     fringes.  The value is an alist of elements ‘(INDICATOR .
     BITMAPS)’, where INDICATOR specifies a logical indicator type and
     BITMAPS specifies the fringe bitmaps to use for that indicator.

     Each INDICATOR should be one of the following symbols:

     ‘truncation’, ‘continuation’.
          Used for truncation and continuation lines.

     ‘up’, ‘down’, ‘top’, ‘bottom’, ‘top-bottom’
          Used when ‘indicate-buffer-boundaries’ is non-‘nil’: ‘up’ and
          ‘down’ indicate a buffer boundary lying above or below the
          window edge; ‘top’ and ‘bottom’ indicate the topmost and
          bottommost buffer text line; and ‘top-bottom’ indicates where
          there is just one line of text in the buffer.

     ‘empty-line’
          Used to indicate empty lines when ‘indicate-empty-lines’ is
          non-‘nil’.

     ‘overlay-arrow’
          Used for overlay arrows (*note Overlay Arrow::).

     Each BITMAPS value may be a list of symbols ‘(LEFT RIGHT [LEFT1
     RIGHT1])’.  The LEFT and RIGHT symbols specify the bitmaps shown in
     the left and/or right fringe, for the specific indicator.  LEFT1
     and RIGHT1 are specific to the ‘bottom’ and ‘top-bottom’
     indicators, and are used to indicate that the last text line has no
     final newline.  Alternatively, BITMAPS may be a single symbol which
     is used in both left and right fringes.

     *Note Fringe Bitmaps::, for a list of standard bitmap symbols and
     how to define your own.  In addition, ‘nil’ represents the empty
     bitmap (i.e., an indicator that is not shown).

     When ‘fringe-indicator-alist’ has a buffer-local value, and there
     is no bitmap defined for a logical indicator, or the bitmap is ‘t’,
     the corresponding value from the default value of
     ‘fringe-indicator-alist’ is used.


File: elisp.info,  Node: Fringe Cursors,  Next: Fringe Bitmaps,  Prev: Fringe Indicators,  Up: Fringes

37.13.3 Fringe Cursors
----------------------

When a line is exactly as wide as the window, Emacs displays the cursor
in the right fringe instead of using two lines.  Different bitmaps are
used to represent the cursor in the fringe depending on the current
buffer’s cursor type.

 -- User Option: overflow-newline-into-fringe
     If this is non-‘nil’, lines exactly as wide as the window (not
     counting the final newline character) are not continued.  Instead,
     when point is at the end of the line, the cursor appears in the
     right fringe.

 -- Variable: fringe-cursor-alist
     This variable specifies the mapping from logical cursor type to the
     actual fringe bitmaps displayed in the right fringe.  The value is
     an alist where each element has the form ‘(CURSOR-TYPE . BITMAP)’,
     which means to use the fringe bitmap BITMAP to display cursors of
     type CURSOR-TYPE.

     Each CURSOR-TYPE should be one of ‘box’, ‘hollow’, ‘bar’, ‘hbar’,
     or ‘hollow-small’.  The first four have the same meanings as in the
     ‘cursor-type’ frame parameter (*note Cursor Parameters::).  The
     ‘hollow-small’ type is used instead of ‘hollow’ when the normal
     ‘hollow-rectangle’ bitmap is too tall to fit on a specific display
     line.

     Each BITMAP should be a symbol specifying the fringe bitmap to be
     displayed for that logical cursor type.  *Note Fringe Bitmaps::.

     When ‘fringe-cursor-alist’ has a buffer-local value, and there is
     no bitmap defined for a cursor type, the corresponding value from
     the default value of ‘fringes-indicator-alist’ is used.


File: elisp.info,  Node: Fringe Bitmaps,  Next: Customizing Bitmaps,  Prev: Fringe Cursors,  Up: Fringes

37.13.4 Fringe Bitmaps
----------------------

The “fringe bitmaps” are the actual bitmaps which represent the logical
fringe indicators for truncated or continued lines, buffer boundaries,
overlay arrows, etc.  Each bitmap is represented by a symbol.  These
symbols are referred to by the variable ‘fringe-indicator-alist’, which
maps fringe indicators to bitmaps (*note Fringe Indicators::), and the
variable ‘fringe-cursor-alist’, which maps fringe cursors to bitmaps
(*note Fringe Cursors::).

   Lisp programs can also directly display a bitmap in the left or right
fringe, by using a ‘display’ property for one of the characters
appearing in the line (*note Other Display Specs::).  Such a display
specification has the form

     (FRINGE BITMAP [FACE])

FRINGE is either the symbol ‘left-fringe’ or ‘right-fringe’.  BITMAP is
a symbol identifying the bitmap to display.  The optional FACE names a
face whose foreground color is used to display the bitmap; this face is
automatically merged with the ‘fringe’ face.

   Here is a list of the standard fringe bitmaps defined in Emacs, and
how they are currently used in Emacs (via ‘fringe-indicator-alist’ and
‘fringe-cursor-alist’):

‘left-arrow’, ‘right-arrow’
     Used to indicate truncated lines.

‘left-curly-arrow’, ‘right-curly-arrow’
     Used to indicate continued lines.

‘right-triangle’, ‘left-triangle’
     The former is used by overlay arrows.  The latter is unused.

‘up-arrow’, ‘down-arrow’, ‘top-left-angle’ ‘top-right-angle’
‘bottom-left-angle’, ‘bottom-right-angle’
‘top-right-angle’, ‘top-left-angle’
‘left-bracket’, ‘right-bracket’, ‘top-right-angle’, ‘top-left-angle’
     Used to indicate buffer boundaries.

‘filled-rectangle’, ‘hollow-rectangle’
‘filled-square’, ‘hollow-square’
‘vertical-bar’, ‘horizontal-bar’
     Used for different types of fringe cursors.

‘empty-line’, ‘exclamation-mark’, ‘question-mark’, ‘exclamation-mark’
     Not used by core Emacs features.

The next subsection describes how to define your own fringe bitmaps.

 -- Function: fringe-bitmaps-at-pos &optional pos window
     This function returns the fringe bitmaps of the display line
     containing position POS in window WINDOW.  The return value has the
     form ‘(LEFT RIGHT OV)’, where LEFT is the symbol for the fringe
     bitmap in the left fringe (or ‘nil’ if no bitmap), RIGHT is similar
     for the right fringe, and OV is non-‘nil’ if there is an overlay
     arrow in the left fringe.

     The value is ‘nil’ if POS is not visible in WINDOW.  If WINDOW is
     ‘nil’, that stands for the selected window.  If POS is ‘nil’, that
     stands for the value of point in WINDOW.


File: elisp.info,  Node: Customizing Bitmaps,  Next: Overlay Arrow,  Prev: Fringe Bitmaps,  Up: Fringes

37.13.5 Customizing Fringe Bitmaps
----------------------------------

 -- Function: define-fringe-bitmap bitmap bits &optional height width
          align
     This function defines the symbol BITMAP as a new fringe bitmap, or
     replaces an existing bitmap with that name.

     The argument BITS specifies the image to use.  It should be either
     a string or a vector of integers, where each element (an integer)
     corresponds to one row of the bitmap.  Each bit of an integer
     corresponds to one pixel of the bitmap, where the low bit
     corresponds to the rightmost pixel of the bitmap.

     The height is normally the length of BITS.  However, you can
     specify a different height with non-‘nil’ HEIGHT.  The width is
     normally 8, but you can specify a different width with non-‘nil’
     WIDTH.  The width must be an integer between 1 and 16.

     The argument ALIGN specifies the positioning of the bitmap relative
     to the range of rows where it is used; the default is to center the
     bitmap.  The allowed values are ‘top’, ‘center’, or ‘bottom’.

     The ALIGN argument may also be a list ‘(ALIGN PERIODIC)’ where
     ALIGN is interpreted as described above.  If PERIODIC is non-‘nil’,
     it specifies that the rows in ‘bits’ should be repeated enough
     times to reach the specified height.

 -- Function: destroy-fringe-bitmap bitmap
     This function destroy the fringe bitmap identified by BITMAP.  If
     BITMAP identifies a standard fringe bitmap, it actually restores
     the standard definition of that bitmap, instead of eliminating it
     entirely.

 -- Function: set-fringe-bitmap-face bitmap &optional face
     This sets the face for the fringe bitmap BITMAP to FACE.  If FACE
     is ‘nil’, it selects the ‘fringe’ face.  The bitmap’s face controls
     the color to draw it in.

     FACE is merged with the ‘fringe’ face, so normally FACE should
     specify only the foreground color.


File: elisp.info,  Node: Overlay Arrow,  Prev: Customizing Bitmaps,  Up: Fringes

37.13.6 The Overlay Arrow
-------------------------

The “overlay arrow” is useful for directing the user’s attention to a
particular line in a buffer.  For example, in the modes used for
interface to debuggers, the overlay arrow indicates the line of code
about to be executed.  This feature has nothing to do with “overlays”
(*note Overlays::).

 -- Variable: overlay-arrow-string
     This variable holds the string to display to call attention to a
     particular line, or ‘nil’ if the arrow feature is not in use.  On a
     graphical display the contents of the string are ignored; instead a
     glyph is displayed in the fringe area to the left of the display
     area.

 -- Variable: overlay-arrow-position
     This variable holds a marker that indicates where to display the
     overlay arrow.  It should point at the beginning of a line.  On a
     non-graphical display the arrow text appears at the beginning of
     that line, overlaying any text that would otherwise appear.  Since
     the arrow is usually short, and the line usually begins with
     indentation, normally nothing significant is overwritten.

     The overlay-arrow string is displayed in any given buffer if the
     value of ‘overlay-arrow-position’ in that buffer points into that
     buffer.  Thus, it is possible to display multiple overlay arrow
     strings by creating buffer-local bindings of
     ‘overlay-arrow-position’.  However, it is usually cleaner to use
     ‘overlay-arrow-variable-list’ to achieve this result.

   You can do a similar job by creating an overlay with a
‘before-string’ property.  *Note Overlay Properties::.

   You can define multiple overlay arrows via the variable
‘overlay-arrow-variable-list’.

 -- Variable: overlay-arrow-variable-list
     This variable’s value is a list of variables, each of which
     specifies the position of an overlay arrow.  The variable
     ‘overlay-arrow-position’ has its normal meaning because it is on
     this list.

   Each variable on this list can have properties ‘overlay-arrow-string’
and ‘overlay-arrow-bitmap’ that specify an overlay arrow string (for
text terminals) or fringe bitmap (for graphical terminals) to display at
the corresponding overlay arrow position.  If either property is not
set, the default ‘overlay-arrow-string’ or ‘overlay-arrow’ fringe
indicator is used.


File: elisp.info,  Node: Scroll Bars,  Next: Window Dividers,  Prev: Fringes,  Up: Display

37.14 Scroll Bars
=================

Normally the frame parameter ‘vertical-scroll-bars’ controls whether the
windows in the frame have vertical scroll bars, and whether they are on
the left or right.  The frame parameter ‘scroll-bar-width’ specifies how
wide they are (‘nil’ meaning the default).

   The frame parameter ‘horizontal-scroll-bars’ controls whether the
windows in the frame have horizontal scroll bars.  The frame parameter
‘scroll-bar-height’ specifies how high they are (‘nil’ meaning the
default).  *Note Layout Parameters::.

   Horizontal scroll bars are not available on all platforms.  The
function ‘horizontal-scroll-bars-available-p’ which takes no argument
returns non-‘nil’ if they are available on your system.

   The following three functions take as argument a live frame which
defaults to the selected one.

 -- Function: frame-current-scroll-bars &optional frame
     This function reports the scroll bar types for frame FRAME.  The
     value is a cons cell ‘(VERTICAL-TYPE . HORIZONTAL-TYPE)’, where
     VERTICAL-TYPE is either ‘left’, ‘right’, or ‘nil’ (which means no
     vertical scroll bar.)  HORIZONTAL-TYPE is either ‘bottom’ or ‘nil’
     (which means no horizontal scroll bar).

 -- Function: frame-scroll-bar-width &optional frame
     This function returns the width of vertical scroll bars of FRAME in
     pixels.

 -- Function: frame-scroll-bar-height &optional frame
     This function returns the height of horizontal scroll bars of FRAME
     in pixels.

   You can override the frame specific settings for individual windows
by using the following function:

 -- Function: set-window-scroll-bars window &optional width
          vertical-type height horizontal-type
     This function sets the width and/or height and the types of scroll
     bars for window WINDOW.

     WIDTH specifies the width of the vertical scroll bar in pixels
     (‘nil’ means use the width specified for the frame).  VERTICAL-TYPE
     specifies whether to have a vertical scroll bar and, if so, where.
     The possible values are ‘left’, ‘right’, ‘t’, which means to use
     the frame’s default, and ‘nil’ for no vertical scroll bar.

     HEIGHT specifies the height of the horizontal scroll bar in pixels
     (‘nil’ means use the height specified for the frame).
     HORIZONTAL-TYPE specifies whether to have a horizontal scroll bar.
     The possible values are ‘bottom’, ‘t’, which means to use the
     frame’s default, and ‘nil’ for no horizontal scroll bar.

     If WINDOW is ‘nil’, the selected window is used.

   The following four functions take as argument a live window which
defaults to the selected one.

 -- Function: window-scroll-bars &optional window
     This function returns a list of the form ‘(WIDTH COLUMNS
     VERTICAL-TYPE HEIGHT LINES HORIZONTAL-TYPE)’.

     The value WIDTH is the value that was specified for the width of
     the vertical scroll bar (which may be ‘nil’); COLUMNS is the
     (possibly rounded) number of columns that the vertical scroll bar
     actually occupies.

     The value HEIGHT is the value that was specified for the height of
     the horizontal scroll bar (which may be ‘nil’); LINES is the
     (possibly rounded) number of lines that the horizontally scroll bar
     actually occupies.

 -- Function: window-current-scroll-bars &optional window
     This function reports the scroll bar type for window WINDOW.  The
     value is a cons cell ‘(VERTICAL-TYPE . HORIZONTAL-TYPE)’.  Unlike
     ‘window-scroll-bars’, this reports the scroll bar type actually
     used, once frame defaults and ‘scroll-bar-mode’ are taken into
     account.

 -- Function: window-scroll-bar-width &optional window
     This function returns the width in pixels of WINDOW’s vertical
     scrollbar.

 -- Function: window-scroll-bar-height &optional window
     This function returns the height in pixels of WINDOW’s horizontal
     scrollbar.

   If you don’t specify these values for a window with
‘set-window-scroll-bars’, the buffer-local variables
‘vertical-scroll-bar’, ‘horizontal-scroll-bar’, ‘scroll-bar-width’ and
‘scroll-bar-height’ in the buffer being displayed control the window’s
scroll bars.  The function ‘set-window-buffer’ examines these variables.
If you change them in a buffer that is already visible in a window, you
can make the window take note of the new values by calling
‘set-window-buffer’ specifying the same buffer that is already
displayed.

   You can control the appearance of scroll bars for a particular buffer
by setting the following variables which automatically become
buffer-local when set.

 -- Variable: vertical-scroll-bar
     This variable specifies the location of the vertical scroll bar.
     The possible values are ‘left’, ‘right’, ‘t’, which means to use
     the frame’s default, and ‘nil’ for no scroll bar.

 -- Variable: horizontal-scroll-bar
     This variable specifies the location of the horizontal scroll bar.
     The possible values are ‘bottom’, ‘t’, which means to use the
     frame’s default, and ‘nil’ for no scroll bar.

 -- Variable: scroll-bar-width
     This variable specifies the width of the buffer’s vertical scroll
     bars, measured in pixels.  A value of ‘nil’ means to use the value
     specified by the frame.

 -- Variable: scroll-bar-height
     This variable specifies the height of the buffer’s horizontal
     scroll bar, measured in pixels.  A value of ‘nil’ means to use the
     value specified by the frame.

   Finally you can toggle the display of scroll bars on all frames by
customizing the variables ‘scroll-bar-mode’ and
‘horizontal-scroll-bar-mode’.

 -- User Option: scroll-bar-mode
     This variable controls whether and where to put vertical scroll
     bars in all frames.  The possible values are ‘nil’ for no scroll
     bars, ‘left’ to put scroll bars on the left and ‘right’ to put
     scroll bars on the right.

 -- User Option: horizontal-scroll-bar-mode
     This variable controls whether to display horizontal scroll bars on
     all frames.


File: elisp.info,  Node: Window Dividers,  Next: Display Property,  Prev: Scroll Bars,  Up: Display

37.15 Window Dividers
=====================

Window dividers are bars drawn between a frame’s windows.  A right
divider is drawn between a window and any adjacent windows on the right.
Its width (thickness) is specified by the frame parameter
‘right-divider-width’.  A bottom divider is drawn between a window and
adjacent windows on the bottom or the echo area.  Its width is specified
by the frame parameter ‘bottom-divider-width’.  In either case,
specifying a width of zero means to not draw such dividers.  *Note
Layout Parameters::.

   Technically, a right divider belongs to the window on its left, which
means that its width contributes to the total width of that window.  A
bottom divider belongs to the window above it, which means that its
width contributes to the total height of that window.  *Note Window
Sizes::.  When a window has both, a right and a bottom divider, the
bottom divider prevails.  This means that a bottom divider is drawn over
the full total width of its window while the right divider ends above
the bottom divider.

   Dividers can be dragged with the mouse and are therefore useful for
adjusting the sizes of adjacent windows with the mouse.  They also serve
to visually set apart adjacent windows when no scroll bars or mode lines
are present.  The following three faces allow the customization of the
appearance of dividers:

‘window-divider’
     When a divider is less than three pixels wide, it is drawn solidly
     with the foreground of this face.  For larger dividers this face is
     used for the inner part only, excluding the first and last pixel.

‘window-divider-first-pixel’
     This is the face used for drawing the first pixel of a divider that
     is at least three pixels wide.  To obtain a solid appearance, set
     this to the same value used for the ‘window-divider’ face.

‘window-divider-last-pixel’
     This is the face used for drawing the last pixel of a divider that
     is at least three pixels wide.  To obtain a solid appearance, set
     this to the same value used for the ‘window-divider’ face.

   You can get the sizes of the dividers of a specific window with the
following two functions.

 -- Function: window-right-divider-width &optional window
     Return the width (thickness) in pixels of WINDOW’s right divider.
     WINDOW must be a live window and defaults to the selected one.  The
     return value is always zero for a rightmost window.

 -- Function: window-bottom-divider-width &optional window
     Return the width (thickness) in pixels of WINDOW’s bottom divider.
     WINDOW must be a live window and defaults to the selected one.  The
     return value is zero for the minibuffer window or a bottommost
     window on a minibuffer-less frame.


File: elisp.info,  Node: Display Property,  Next: Images,  Prev: Window Dividers,  Up: Display

37.16 The ‘display’ Property
============================

The ‘display’ text property (or overlay property) is used to insert
images into text, and to control other aspects of how text displays.
The value of the ‘display’ property should be a display specification,
or a list or vector containing several display specifications.  Display
specifications in the same ‘display’ property value generally apply in
parallel to the text they cover.

   If several sources (overlays and/or a text property) specify values
for the ‘display’ property, only one of the values takes effect,
following the rules of ‘get-char-property’.  *Note Examining
Properties::.

   The rest of this section describes several kinds of display
specifications and what they mean.

* Menu:

* Replacing Specs::      Display specs that replace the text.
* Specified Space::      Displaying one space with a specified width.
* Pixel Specification::  Specifying space width or height in pixels.
* Other Display Specs::     Displaying an image; adjusting the height,
                              spacing, and other properties of text.
* Display Margins::     Displaying text or images to the side of the main text.


File: elisp.info,  Node: Replacing Specs,  Next: Specified Space,  Up: Display Property

37.16.1 Display Specs That Replace The Text
-------------------------------------------

Some kinds of display specifications specify something to display
instead of the text that has the property.  These are called “replacing”
display specifications.  Emacs does not allow the user to interactively
move point into the middle of buffer text that is replaced in this way.

   If a list of display specifications includes more than one replacing
display specification, the first overrides the rest.  Replacing display
specifications make most other display specifications irrelevant, since
those don’t apply to the replacement.

   For replacing display specifications, “the text that has the
property” means all the consecutive characters that have the same Lisp
object as their ‘display’ property; these characters are replaced as a
single unit.  If two characters have different Lisp objects as their
‘display’ properties (i.e., objects which are not ‘eq’), they are
handled separately.

   Here is an example which illustrates this point.  A string serves as
a replacing display specification, which replaces the text that has the
property with the specified string (*note Other Display Specs::).
Consider the following function:

     (defun foo ()
       (dotimes (i 5)
         (let ((string (concat "A"))
               (start (+ i i (point-min))))
           (put-text-property start (1+ start) 'display string)
           (put-text-property start (+ 2 start) 'display string))))

This function gives each of the first ten characters in the buffer a
‘display’ property which is a string ‘"A"’, but they don’t all get the
same string object.  The first two characters get the same string
object, so they are replaced with one ‘A’; the fact that the display
property was assigned in two separate calls to ‘put-text-property’ is
irrelevant.  Similarly, the next two characters get a second string
(‘concat’ creates a new string object), so they are replaced with one
‘A’; and so on.  Thus, the ten characters appear as five A’s.


File: elisp.info,  Node: Specified Space,  Next: Pixel Specification,  Prev: Replacing Specs,  Up: Display Property

37.16.2 Specified Spaces
------------------------

To display a space of specified width and/or height, use a display
specification of the form ‘(space . PROPS)’, where PROPS is a property
list (a list of alternating properties and values).  You can put this
property on one or more consecutive characters; a space of the specified
height and width is displayed in place of _all_ of those characters.
These are the properties you can use in PROPS to specify the weight of
the space:

‘:width WIDTH’
     If WIDTH is a number, it specifies that the space width should be
     WIDTH times the normal character width.  WIDTH can also be a “pixel
     width” specification (*note Pixel Specification::).

‘:relative-width FACTOR’
     Specifies that the width of the stretch should be computed from the
     first character in the group of consecutive characters that have
     the same ‘display’ property.  The space width is the pixel width of
     that character, multiplied by FACTOR.  (On text-mode terminals, the
     “pixel width” of a character is usually 1, but it could be more for
     TABs and double-width CJK characters.)

‘:align-to HPOS’
     Specifies that the space should be wide enough to reach HPOS.  If
     HPOS is a number, it is measured in units of the normal character
     width.  HPOS can also be a “pixel width” specification (*note Pixel
     Specification::).

   You should use one and only one of the above properties.  You can
also specify the height of the space, with these properties:

‘:height HEIGHT’
     Specifies the height of the space.  If HEIGHT is a number, it
     specifies that the space height should be HEIGHT times the normal
     character height.  The HEIGHT may also be a “pixel height”
     specification (*note Pixel Specification::).

‘:relative-height FACTOR’
     Specifies the height of the space, multiplying the ordinary height
     of the text having this display specification by FACTOR.

‘:ascent ASCENT’
     If the value of ASCENT is a non-negative number no greater than
     100, it specifies that ASCENT percent of the height of the space
     should be considered as the ascent of the space—that is, the part
     above the baseline.  The ascent may also be specified in pixel
     units with a “pixel ascent” specification (*note Pixel
     Specification::).

   Don’t use both ‘:height’ and ‘:relative-height’ together.

   The ‘:width’ and ‘:align-to’ properties are supported on non-graphic
terminals, but the other space properties in this section are not.

   Note that space properties are treated as paragraph separators for
the purposes of reordering bidirectional text for display.  *Note
Bidirectional Display::, for the details.


File: elisp.info,  Node: Pixel Specification,  Next: Other Display Specs,  Prev: Specified Space,  Up: Display Property

37.16.3 Pixel Specification for Spaces
--------------------------------------

The value of the ‘:width’, ‘:align-to’, ‘:height’, and ‘:ascent’
properties can be a special kind of expression that is evaluated during
redisplay.  The result of the evaluation is used as an absolute number
of pixels.

   The following expressions are supported:

       EXPR ::= NUM | (NUM) | UNIT | ELEM | POS | IMAGE | FORM
       NUM  ::= INTEGER | FLOAT | SYMBOL
       UNIT ::= in | mm | cm | width | height
       ELEM ::= left-fringe | right-fringe | left-margin | right-margin
             |  scroll-bar | text
       POS  ::= left | center | right
       FORM ::= (NUM . EXPR) | (OP EXPR ...)
       OP   ::= + | -

   The form NUM specifies a fraction of the default frame font height or
width.  The form ‘(NUM)’ specifies an absolute number of pixels.  If NUM
is a symbol, SYMBOL, its buffer-local variable binding is used.

   The ‘in’, ‘mm’, and ‘cm’ units specify the number of pixels per inch,
millimeter, and centimeter, respectively.  The ‘width’ and ‘height’
units correspond to the default width and height of the current face.
An image specification ‘image’ corresponds to the width or height of the
image.

   The elements ‘left-fringe’, ‘right-fringe’, ‘left-margin’,
‘right-margin’, ‘scroll-bar’, and ‘text’ specify to the width of the
corresponding area of the window.

   The ‘left’, ‘center’, and ‘right’ positions can be used with
‘:align-to’ to specify a position relative to the left edge, center, or
right edge of the text area.

   Any of the above window elements (except ‘text’) can also be used
with ‘:align-to’ to specify that the position is relative to the left
edge of the given area.  Once the base offset for a relative position
has been set (by the first occurrence of one of these symbols), further
occurrences of these symbols are interpreted as the width of the
specified area.  For example, to align to the center of the left-margin,
use

     :align-to (+ left-margin (0.5 . left-margin))

   If no specific base offset is set for alignment, it is always
relative to the left edge of the text area.  For example, ‘:align-to 0’
in a header-line aligns with the first text column in the text area.

   A value of the form ‘(NUM . EXPR)’ stands for the product of the
values of NUM and EXPR.  For example, ‘(2 . in)’ specifies a width of 2
inches, while ‘(0.5 . IMAGE)’ specifies half the width (or height) of
the specified image.

   The form ‘(+ EXPR ...)’ adds up the value of the expressions.  The
form ‘(- EXPR ...)’ negates or subtracts the value of the expressions.


File: elisp.info,  Node: Other Display Specs,  Next: Display Margins,  Prev: Pixel Specification,  Up: Display Property

37.16.4 Other Display Specifications
------------------------------------

Here are the other sorts of display specifications that you can use in
the ‘display’ text property.

‘STRING’
     Display STRING instead of the text that has this property.

     Recursive display specifications are not supported—STRING’s
     ‘display’ properties, if any, are not used.

‘(image . IMAGE-PROPS)’
     This kind of display specification is an image descriptor (*note
     Images::).  When used as a display specification, it means to
     display the image instead of the text that has the display
     specification.

‘(slice X Y WIDTH HEIGHT)’
     This specification together with ‘image’ specifies a “slice” (a
     partial area) of the image to display.  The elements Y and X
     specify the top left corner of the slice, within the image; WIDTH
     and HEIGHT specify the width and height of the slice.  Integers are
     numbers of pixels.  A floating-point number in the range 0.0–1.0
     stands for that fraction of the width or height of the entire
     image.

‘((margin nil) STRING)’
     A display specification of this form means to display STRING
     instead of the text that has the display specification, at the same
     position as that text.  It is equivalent to using just STRING, but
     it is done as a special case of marginal display (*note Display
     Margins::).

‘(left-fringe BITMAP [FACE])’
‘(right-fringe BITMAP [FACE])’
     This display specification on any character of a line of text
     causes the specified BITMAP be displayed in the left or right
     fringes for that line, instead of the characters that have the
     display specification.  The optional FACE specifies the colors to
     be used for the bitmap.  *Note Fringe Bitmaps::, for the details.

‘(space-width FACTOR)’
     This display specification affects all the space characters within
     the text that has the specification.  It displays all of these
     spaces FACTOR times as wide as normal.  The element FACTOR should
     be an integer or float.  Characters other than spaces are not
     affected at all; in particular, this has no effect on tab
     characters.

‘(height HEIGHT)’
     This display specification makes the text taller or shorter.  Here
     are the possibilities for HEIGHT:

     ‘(+ N)’
          This means to use a font that is N steps larger.  A “step” is
          defined by the set of available fonts—specifically, those that
          match what was otherwise specified for this text, in all
          attributes except height.  Each size for which a suitable font
          is available counts as another step.  N should be an integer.

     ‘(- N)’
          This means to use a font that is N steps smaller.

     a number, FACTOR
          A number, FACTOR, means to use a font that is FACTOR times as
          tall as the default font.

     a symbol, FUNCTION
          A symbol is a function to compute the height.  It is called
          with the current height as argument, and should return the new
          height to use.

     anything else, FORM
          If the HEIGHT value doesn’t fit the previous possibilities, it
          is a form.  Emacs evaluates it to get the new height, with the
          symbol ‘height’ bound to the current specified font height.

‘(raise FACTOR)’
     This kind of display specification raises or lowers the text it
     applies to, relative to the baseline of the line.  It is mainly
     meant to support display of subscripts and superscripts.

     The FACTOR must be a number, which is interpreted as a multiple of
     the height of the affected text.  If it is positive, that means to
     display the characters raised.  If it is negative, that means to
     display them lower down.

     Note that if the text also has a ‘height’ display specification,
     which was specified before (i.e. to the left of) ‘raise’, the
     latter will affect the amount of raising or lowering in pixels,
     because that is based on the height of the text being raised.
     Therefore, if you want to display a sub- or superscript that is
     smaller than the normal text height, consider specifying ‘raise’
     before ‘height’.

   You can make any display specification conditional.  To do that,
package it in another list of the form ‘(when CONDITION . SPEC)’.  Then
the specification SPEC applies only when CONDITION evaluates to a
non-‘nil’ value.  During the evaluation, ‘object’ is bound to the string
or buffer having the conditional ‘display’ property.  ‘position’ and
‘buffer-position’ are bound to the position within ‘object’ and the
buffer position where the ‘display’ property was found, respectively.
Both positions can be different when ‘object’ is a string.


File: elisp.info,  Node: Display Margins,  Prev: Other Display Specs,  Up: Display Property

37.16.5 Displaying in the Margins
---------------------------------

A buffer can have blank areas called “display margins” on the left and
on the right.  Ordinary text never appears in these areas, but you can
put things into the display margins using the ‘display’ property.  There
is currently no way to make text or images in the margin
mouse-sensitive.

   The way to display something in the margins is to specify it in a
margin display specification in the ‘display’ property of some text.
This is a replacing display specification, meaning that the text you put
it on does not get displayed; the margin display appears, but that text
does not.

   A margin display specification looks like ‘((margin right-margin)
SPEC)’ or ‘((margin left-margin) SPEC)’.  Here, SPEC is another display
specification that says what to display in the margin.  Typically it is
a string of text to display, or an image descriptor.

   To display something in the margin _in association with_ certain
buffer text, without altering or preventing the display of that text,
put a ‘before-string’ property on the text and put the margin display
specification on the contents of the before-string.

   Before the display margins can display anything, you must give them a
nonzero width.  The usual way to do that is to set these variables:

 -- Variable: left-margin-width
     This variable specifies the width of the left margin, in character
     cell (a.k.a. “column”) units.  It is buffer-local in all buffers.
     A value of ‘nil’ means no left marginal area.

 -- Variable: right-margin-width
     This variable specifies the width of the right margin, in character
     cell units.  It is buffer-local in all buffers.  A value of ‘nil’
     means no right marginal area.

   Setting these variables does not immediately affect the window.
These variables are checked when a new buffer is displayed in the
window.  Thus, you can make changes take effect by calling
‘set-window-buffer’.

   You can also set the margin widths immediately.

 -- Function: set-window-margins window left &optional right
     This function specifies the margin widths for window WINDOW, in
     character cell units.  The argument LEFT controls the left margin,
     and RIGHT controls the right margin (default ‘0’).

 -- Function: window-margins &optional window
     This function returns the width of the left and right margins of
     WINDOW as a cons cell of the form ‘(LEFT . RIGHT)’.  If one of the
     two marginal areas does not exist, its width is returned as ‘nil’;
     if neither of the two margins exist, the function returns ‘(nil)’.
     If WINDOW is ‘nil’, the selected window is used.


File: elisp.info,  Node: Images,  Next: Xwidgets,  Prev: Display Property,  Up: Display

37.17 Images
============

To display an image in an Emacs buffer, you must first create an image
descriptor, then use it as a display specifier in the ‘display’ property
of text that is displayed (*note Display Property::).

   Emacs is usually able to display images when it is run on a graphical
terminal.  Images cannot be displayed in a text terminal, on certain
graphical terminals that lack the support for this, or if Emacs is
compiled without image support.  You can use the function
‘display-images-p’ to determine if images can in principle be displayed
(*note Display Feature Testing::).

* Menu:

* Image Formats::       Supported image formats.
* Image Descriptors::   How to specify an image for use in ‘:display’.
* XBM Images::          Special features for XBM format.
* XPM Images::          Special features for XPM format.
* PostScript Images::   Special features for PostScript format.
* ImageMagick Images::  Special features available through ImageMagick.
* Other Image Types::   Various other formats are supported.
* Defining Images::     Convenient ways to define an image for later use.
* Showing Images::      Convenient ways to display an image once it is defined.
* Multi-Frame Images::  Some images contain more than one frame.
* Image Cache::         Internal mechanisms of image display.


File: elisp.info,  Node: Image Formats,  Next: Image Descriptors,  Up: Images

37.17.1 Image Formats
---------------------

Emacs can display a number of different image formats.  Some of these
image formats are supported only if particular support libraries are
installed.  On some platforms, Emacs can load support libraries on
demand; if so, the variable ‘dynamic-library-alist’ can be used to
modify the set of known names for these dynamic libraries.  *Note
Dynamic Libraries::.

   Supported image formats (and the required support libraries) include
PBM and XBM (which do not depend on support libraries and are always
available), XPM (‘libXpm’), GIF (‘libgif’ or ‘libungif’), PostScript
(‘gs’), JPEG (‘libjpeg’), TIFF (‘libtiff’), PNG (‘libpng’), and SVG
(‘librsvg’).

   Each of these image formats is associated with an “image type
symbol”.  The symbols for the above formats are, respectively, ‘pbm’,
‘xbm’, ‘xpm’, ‘gif’, ‘postscript’, ‘jpeg’, ‘tiff’, ‘png’, and ‘svg’.

   Furthermore, if you build Emacs with ImageMagick (‘libMagickWand’)
support, Emacs can display any image format that ImageMagick can.  *Note
ImageMagick Images::.  All images displayed via ImageMagick have type
symbol ‘imagemagick’.

 -- Variable: image-types
     This variable contains a list of type symbols for image formats
     which are potentially supported in the current configuration.

     “Potentially” means that Emacs knows about the image types, not
     necessarily that they can be used (for example, they could depend
     on unavailable dynamic libraries).  To know which image types are
     really available, use ‘image-type-available-p’.

 -- Function: image-type-available-p type
     This function returns non-‘nil’ if images of type TYPE can be
     loaded and displayed.  TYPE must be an image type symbol.

     For image types whose support libraries are statically linked, this
     function always returns ‘t’.  For image types whose support
     libraries are dynamically loaded, it returns ‘t’ if the library
     could be loaded and ‘nil’ otherwise.


File: elisp.info,  Node: Image Descriptors,  Next: XBM Images,  Prev: Image Formats,  Up: Images

37.17.2 Image Descriptors
-------------------------

An “image descriptor” is a list which specifies the underlying data for
an image, and how to display it.  It is typically used as the value of a
‘display’ overlay or text property (*note Other Display Specs::); but
*Note Showing Images::, for convenient helper functions to insert images
into buffers.

   Each image descriptor has the form ‘(image . PROPS)’, where PROPS is
a property list of alternating keyword symbols and values, including at
least the pair ‘:type TYPE’ that specifies the image type.

   The following is a list of properties that are meaningful for all
image types (there are also properties which are meaningful only for
certain image types, as documented in the following subsections):

‘:type TYPE’
     The image type.  *Note Image Formats::.  Every image descriptor
     must include this property.

‘:file FILE’
     This says to load the image from file FILE.  If FILE is not an
     absolute file name, it is expanded in ‘data-directory’.

‘:data DATA’
     This specifies the raw image data.  Each image descriptor must have
     either ‘:data’ or ‘:file’, but not both.

     For most image types, the value of a ‘:data’ property should be a
     string containing the image data.  Some image types do not support
     ‘:data’; for some others, ‘:data’ alone is not enough, so you need
     to use other image properties along with ‘:data’.  See the
     following subsections for details.

‘:margin MARGIN’
     This specifies how many pixels to add as an extra margin around the
     image.  The value, MARGIN, must be a non-negative number, or a pair
     ‘(X . Y)’ of such numbers.  If it is a pair, X specifies how many
     pixels to add horizontally, and Y specifies how many pixels to add
     vertically.  If ‘:margin’ is not specified, the default is zero.

‘:ascent ASCENT’
     This specifies the amount of the image’s height to use for its
     ascent—that is, the part above the baseline.  The value, ASCENT,
     must be a number in the range 0 to 100, or the symbol ‘center’.

     If ASCENT is a number, that percentage of the image’s height is
     used for its ascent.

     If ASCENT is ‘center’, the image is vertically centered around a
     centerline which would be the vertical centerline of text drawn at
     the position of the image, in the manner specified by the text
     properties and overlays that apply to the image.

     If this property is omitted, it defaults to 50.

‘:relief RELIEF’
     This adds a shadow rectangle around the image.  The value, RELIEF,
     specifies the width of the shadow lines, in pixels.  If RELIEF is
     negative, shadows are drawn so that the image appears as a pressed
     button; otherwise, it appears as an unpressed button.

‘:conversion ALGORITHM’
     This specifies a conversion algorithm that should be applied to the
     image before it is displayed; the value, ALGORITHM, specifies which
     algorithm.

     ‘laplace’
     ‘emboss’
          Specifies the Laplace edge detection algorithm, which blurs
          out small differences in color while highlighting larger
          differences.  People sometimes consider this useful for
          displaying the image for a disabled button.

     ‘(edge-detection :matrix MATRIX :color-adjust ADJUST)’
          Specifies a general edge-detection algorithm.  MATRIX must be
          either a nine-element list or a nine-element vector of
          numbers.  A pixel at position x/y in the transformed image is
          computed from original pixels around that position.  MATRIX
          specifies, for each pixel in the neighborhood of x/y, a factor
          with which that pixel will influence the transformed pixel;
          element 0 specifies the factor for the pixel at x-1/y-1,
          element 1 the factor for the pixel at x/y-1 etc., as shown
          below:
                 (x-1/y-1  x/y-1  x+1/y-1
                  x-1/y    x/y    x+1/y
                  x-1/y+1  x/y+1  x+1/y+1)

          The resulting pixel is computed from the color intensity of
          the color resulting from summing up the RGB values of
          surrounding pixels, multiplied by the specified factors, and
          dividing that sum by the sum of the factors’ absolute values.

          Laplace edge-detection currently uses a matrix of
                 (1  0  0
                  0  0  0
                  0  0 -1)

          Emboss edge-detection uses a matrix of
                 ( 2 -1  0
                  -1  0  1
                   0  1 -2)

     ‘disabled’
          Specifies transforming the image so that it looks disabled.

‘:mask MASK’
     If MASK is ‘heuristic’ or ‘(heuristic BG)’, build a clipping mask
     for the image, so that the background of a frame is visible behind
     the image.  If BG is not specified, or if BG is ‘t’, determine the
     background color of the image by looking at the four corners of the
     image, assuming the most frequently occurring color from the
     corners is the background color of the image.  Otherwise, BG must
     be a list ‘(RED GREEN BLUE)’ specifying the color to assume for the
     background of the image.

     If MASK is ‘nil’, remove a mask from the image, if it has one.
     Images in some formats include a mask which can be removed by
     specifying ‘:mask nil’.

‘:pointer SHAPE’
     This specifies the pointer shape when the mouse pointer is over
     this image.  *Note Pointer Shape::, for available pointer shapes.

‘:map MAP’
     This associates an image map of “hot spots” with this image.

     An image map is an alist where each element has the format ‘(AREA
     ID PLIST)’.  An AREA is specified as either a rectangle, a circle,
     or a polygon.

     A rectangle is a cons ‘(rect . ((X0 . Y0) . (X1 . Y1)))’ which
     specifies the pixel coordinates of the upper left and bottom right
     corners of the rectangle area.

     A circle is a cons ‘(circle . ((X0 . Y0) . R))’ which specifies the
     center and the radius of the circle; R may be a float or integer.

     A polygon is a cons ‘(poly . [X0 Y0 X1 Y1 ...])’ where each pair in
     the vector describes one corner in the polygon.

     When the mouse pointer lies on a hot-spot area of an image, the
     PLIST of that hot-spot is consulted; if it contains a ‘help-echo’
     property, that defines a tool-tip for the hot-spot, and if it
     contains a ‘pointer’ property, that defines the shape of the mouse
     cursor when it is on the hot-spot.  *Note Pointer Shape::, for
     available pointer shapes.

     When you click the mouse when the mouse pointer is over a hot-spot,
     an event is composed by combining the ID of the hot-spot with the
     mouse event; for instance, ‘[area4 mouse-1]’ if the hot-spot’s ID
     is ‘area4’.

 -- Function: image-mask-p spec &optional frame
     This function returns ‘t’ if image SPEC has a mask bitmap.  FRAME
     is the frame on which the image will be displayed.  FRAME ‘nil’ or
     omitted means to use the selected frame (*note Input Focus::).


File: elisp.info,  Node: XBM Images,  Next: XPM Images,  Prev: Image Descriptors,  Up: Images

37.17.3 XBM Images
------------------

To use XBM format, specify ‘xbm’ as the image type.  This image format
doesn’t require an external library, so images of this type are always
supported.

   Additional image properties supported for the ‘xbm’ image type are:

‘:foreground FOREGROUND’
     The value, FOREGROUND, should be a string specifying the image
     foreground color, or ‘nil’ for the default color.  This color is
     used for each pixel in the XBM that is 1.  The default is the
     frame’s foreground color.

‘:background BACKGROUND’
     The value, BACKGROUND, should be a string specifying the image
     background color, or ‘nil’ for the default color.  This color is
     used for each pixel in the XBM that is 0.  The default is the
     frame’s background color.

   If you specify an XBM image using data within Emacs instead of an
external file, use the following three properties:

‘:data DATA’
     The value, DATA, specifies the contents of the image.  There are
     three formats you can use for DATA:

        • A vector of strings or bool-vectors, each specifying one line
          of the image.  Do specify ‘:height’ and ‘:width’.

        • A string containing the same byte sequence as an XBM file
          would contain.  You must not specify ‘:height’ and ‘:width’ in
          this case, because omitting them is what indicates the data
          has the format of an XBM file.  The file contents specify the
          height and width of the image.

        • A string or a bool-vector containing the bits of the image
          (plus perhaps some extra bits at the end that will not be
          used).  It should contain at least WIDTH * ‘height’ bits.  In
          this case, you must specify ‘:height’ and ‘:width’, both to
          indicate that the string contains just the bits rather than a
          whole XBM file, and to specify the size of the image.

‘:width WIDTH’
     The value, WIDTH, specifies the width of the image, in pixels.

‘:height HEIGHT’
     The value, HEIGHT, specifies the height of the image, in pixels.


File: elisp.info,  Node: XPM Images,  Next: PostScript Images,  Prev: XBM Images,  Up: Images

37.17.4 XPM Images
------------------

To use XPM format, specify ‘xpm’ as the image type.  The additional
image property ‘:color-symbols’ is also meaningful with the ‘xpm’ image
type:

‘:color-symbols SYMBOLS’
     The value, SYMBOLS, should be an alist whose elements have the form
     ‘(NAME . COLOR)’.  In each element, NAME is the name of a color as
     it appears in the image file, and COLOR specifies the actual color
     to use for displaying that name.


File: elisp.info,  Node: PostScript Images,  Next: ImageMagick Images,  Prev: XPM Images,  Up: Images

37.17.5 PostScript Images
-------------------------

To use PostScript for an image, specify image type ‘postscript’.  This
works only if you have Ghostscript installed.  You must always use these
three properties:

‘:pt-width WIDTH’
     The value, WIDTH, specifies the width of the image measured in
     points (1/72 inch).  WIDTH must be an integer.

‘:pt-height HEIGHT’
     The value, HEIGHT, specifies the height of the image in points
     (1/72 inch).  HEIGHT must be an integer.

‘:bounding-box BOX’
     The value, BOX, must be a list or vector of four integers, which
     specifying the bounding box of the PostScript image, analogous to
     the ‘BoundingBox’ comment found in PostScript files.

          %%BoundingBox: 22 171 567 738


File: elisp.info,  Node: ImageMagick Images,  Next: Other Image Types,  Prev: PostScript Images,  Up: Images

37.17.6 ImageMagick Images
--------------------------

If you build Emacs with ImageMagick support, you can use the ImageMagick
library to load many image formats (*note (emacs)File Conveniences::).
The image type symbol for images loaded via ImageMagick is
‘imagemagick’, regardless of the actual underlying image format.

   To check for ImageMagick support, use the following:

     (image-type-available-p 'imagemagick)

 -- Function: imagemagick-types
     This function returns a list of image file extensions supported by
     the current ImageMagick installation.  Each list element is a
     symbol representing an internal ImageMagick name for an image type,
     such as ‘BMP’ for ‘.bmp’ images.

 -- User Option: imagemagick-enabled-types
     The value of this variable is a list of ImageMagick image types
     which Emacs may attempt to render using ImageMagick.  Each list
     element should be one of the symbols in the list returned by
     ‘imagemagick-types’, or an equivalent string.  Alternatively, a
     value of ‘t’ enables ImageMagick for all possible image types.
     Regardless of the value of this variable,
     ‘imagemagick-types-inhibit’ (see below) takes precedence.

 -- User Option: imagemagick-types-inhibit
     The value of this variable lists the ImageMagick image types which
     should never be rendered using ImageMagick, regardless of the value
     of ‘imagemagick-enabled-types’.  A value of ‘t’ disables
     ImageMagick entirely.

 -- Variable: image-format-suffixes
     This variable is an alist mapping image types to file name
     extensions.  Emacs uses this in conjunction with the ‘:format’
     image property (see below) to give a hint to the ImageMagick
     library as to the type of an image.  Each element has the form
     ‘(TYPE EXTENSION)’, where TYPE is a symbol specifying an image
     content-type, and EXTENSION is a string that specifies the
     associated file name extension.

   Images loaded with ImageMagick support the following additional image
descriptor properties:

‘:background BACKGROUND’
     BACKGROUND, if non-‘nil’, should be a string specifying a color,
     which is used as the image’s background color if the image supports
     transparency.  If the value is ‘nil’, it defaults to the frame’s
     background color.

‘:width WIDTH, :height HEIGHT’
     The ‘:width’ and ‘:height’ keywords are used for scaling the image.
     If only one of them is specified, the other one will be calculated
     so as to preserve the aspect ratio.  If both are specified, aspect
     ratio may not be preserved.

‘:max-width MAX-WIDTH, :max-height MAX-HEIGHT’
     The ‘:max-width’ and ‘:max-height’ keywords are used for scaling if
     the size of the image of the image exceeds these values.  If
     ‘:width’ is set it will have precedence over ‘max-width’, and if
     ‘:height’ is set it will have precedence over ‘max-height’, but you
     can otherwise mix these keywords as you wish.  ‘:max-width’ and
     ‘:max-height’ will always preserve the aspect ratio.

‘:format TYPE’
     The value, TYPE, should be a symbol specifying the type of the
     image data, as found in ‘image-format-suffixes’.  This is used when
     the image does not have an associated file name, to provide a hint
     to ImageMagick to help it detect the image type.

‘:rotation ANGLE’
     Specifies a rotation angle in degrees.

‘:index FRAME’
     *Note Multi-Frame Images::.


File: elisp.info,  Node: Other Image Types,  Next: Defining Images,  Prev: ImageMagick Images,  Up: Images

37.17.7 Other Image Types
-------------------------

For PBM images, specify image type ‘pbm’.  Color, gray-scale and
monochromatic images are supported.  For mono PBM images, two additional
image properties are supported.

‘:foreground FOREGROUND’
     The value, FOREGROUND, should be a string specifying the image
     foreground color, or ‘nil’ for the default color.  This color is
     used for each pixel in the PBM that is 1.  The default is the
     frame’s foreground color.

‘:background BACKGROUND’
     The value, BACKGROUND, should be a string specifying the image
     background color, or ‘nil’ for the default color.  This color is
     used for each pixel in the PBM that is 0.  The default is the
     frame’s background color.

The remaining image types that Emacs can support are:

GIF
     Image type ‘gif’.  Supports the ‘:index’ property.  *Note
     Multi-Frame Images::.

JPEG
     Image type ‘jpeg’.

PNG
     Image type ‘png’.

SVG
     Image type ‘svg’.

TIFF
     Image type ‘tiff’.  Supports the ‘:index’ property.  *Note
     Multi-Frame Images::.


File: elisp.info,  Node: Defining Images,  Next: Showing Images,  Prev: Other Image Types,  Up: Images

37.17.8 Defining Images
-----------------------

The functions ‘create-image’, ‘defimage’ and ‘find-image’ provide
convenient ways to create image descriptors.

 -- Function: create-image file-or-data &optional type data-p &rest
          props
     This function creates and returns an image descriptor which uses
     the data in FILE-OR-DATA.  FILE-OR-DATA can be a file name or a
     string containing the image data; DATA-P should be ‘nil’ for the
     former case, non-‘nil’ for the latter case.

     The optional argument TYPE is a symbol specifying the image type.
     If TYPE is omitted or ‘nil’, ‘create-image’ tries to determine the
     image type from the file’s first few bytes, or else from the file’s
     name.

     The remaining arguments, PROPS, specify additional image
     properties—for example,

          (create-image "foo.xpm" 'xpm nil :heuristic-mask t)

     The function returns ‘nil’ if images of this type are not
     supported.  Otherwise it returns an image descriptor.

 -- Macro: defimage symbol specs &optional doc
     This macro defines SYMBOL as an image name.  The arguments SPECS is
     a list which specifies how to display the image.  The third
     argument, DOC, is an optional documentation string.

     Each argument in SPECS has the form of a property list, and each
     one should specify at least the ‘:type’ property and either the
     ‘:file’ or the ‘:data’ property.  The value of ‘:type’ should be a
     symbol specifying the image type, the value of ‘:file’ is the file
     to load the image from, and the value of ‘:data’ is a string
     containing the actual image data.  Here is an example:

          (defimage test-image
            ((:type xpm :file "~/test1.xpm")
             (:type xbm :file "~/test1.xbm")))

     ‘defimage’ tests each argument, one by one, to see if it is
     usable—that is, if the type is supported and the file exists.  The
     first usable argument is used to make an image descriptor which is
     stored in SYMBOL.

     If none of the alternatives will work, then SYMBOL is defined as
     ‘nil’.

 -- Function: find-image specs
     This function provides a convenient way to find an image satisfying
     one of a list of image specifications SPECS.

     Each specification in SPECS is a property list with contents
     depending on image type.  All specifications must at least contain
     the properties ‘:type TYPE’ and either ‘:file FILE’ or
     ‘:data DATA’, where TYPE is a symbol specifying the image type,
     e.g., ‘xbm’, FILE is the file to load the image from, and DATA is a
     string containing the actual image data.  The first specification
     in the list whose TYPE is supported, and FILE exists, is used to
     construct the image specification to be returned.  If no
     specification is satisfied, ‘nil’ is returned.

     The image is looked for in ‘image-load-path’.

 -- User Option: image-load-path
     This variable’s value is a list of locations in which to search for
     image files.  If an element is a string or a variable symbol whose
     value is a string, the string is taken to be the name of a
     directory to search.  If an element is a variable symbol whose
     value is a list, that is taken to be a list of directory names to
     search.

     The default is to search in the ‘images’ subdirectory of the
     directory specified by ‘data-directory’, then the directory
     specified by ‘data-directory’, and finally in the directories in
     ‘load-path’.  Subdirectories are not automatically included in the
     search, so if you put an image file in a subdirectory, you have to
     supply the subdirectory name explicitly.  For example, to find the
     image ‘images/foo/bar.xpm’ within ‘data-directory’, you should
     specify the image as follows:

          (defimage foo-image '((:type xpm :file "foo/bar.xpm")))

 -- Function: image-load-path-for-library library image &optional path
          no-error
     This function returns a suitable search path for images used by the
     Lisp package LIBRARY.

     The function searches for IMAGE first using ‘image-load-path’,
     excluding ‘data-directory/images’, and then in ‘load-path’,
     followed by a path suitable for LIBRARY, which includes
     ‘../../etc/images’ and ‘../etc/images’ relative to the library file
     itself, and finally in ‘data-directory/images’.

     Then this function returns a list of directories which contains
     first the directory in which IMAGE was found, followed by the value
     of ‘load-path’.  If PATH is given, it is used instead of
     ‘load-path’.

     If NO-ERROR is non-‘nil’ and a suitable path can’t be found, don’t
     signal an error.  Instead, return a list of directories as before,
     except that ‘nil’ appears in place of the image directory.

     Here is an example of using ‘image-load-path-for-library’:

          (defvar image-load-path) ; shush compiler
          (let* ((load-path (image-load-path-for-library
                              "mh-e" "mh-logo.xpm"))
                 (image-load-path (cons (car load-path)
                                        image-load-path)))
            (mh-tool-bar-folder-buttons-init))


File: elisp.info,  Node: Showing Images,  Next: Multi-Frame Images,  Prev: Defining Images,  Up: Images

37.17.9 Showing Images
----------------------

You can use an image descriptor by setting up the ‘display’ property
yourself, but it is easier to use the functions in this section.

 -- Function: insert-image image &optional string area slice
     This function inserts IMAGE in the current buffer at point.  The
     value IMAGE should be an image descriptor; it could be a value
     returned by ‘create-image’, or the value of a symbol defined with
     ‘defimage’.  The argument STRING specifies the text to put in the
     buffer to hold the image.  If it is omitted or ‘nil’,
     ‘insert-image’ uses ‘" "’ by default.

     The argument AREA specifies whether to put the image in a margin.
     If it is ‘left-margin’, the image appears in the left margin;
     ‘right-margin’ specifies the right margin.  If AREA is ‘nil’ or
     omitted, the image is displayed at point within the buffer’s text.

     The argument SLICE specifies a slice of the image to insert.  If
     SLICE is ‘nil’ or omitted the whole image is inserted.  Otherwise,
     SLICE is a list ‘(X Y WIDTH HEIGHT)’ which specifies the X and Y
     positions and WIDTH and HEIGHT of the image area to insert.
     Integer values are in units of pixels.  A floating-point number in
     the range 0.0–1.0 stands for that fraction of the width or height
     of the entire image.

     Internally, this function inserts STRING in the buffer, and gives
     it a ‘display’ property which specifies IMAGE.  *Note Display
     Property::.

 -- Function: insert-sliced-image image &optional string area rows cols
     This function inserts IMAGE in the current buffer at point, like
     ‘insert-image’, but splits the image into ROWSxCOLS equally sized
     slices.

     Emacs displays each slice as a separate image, and allows more
     intuitive scrolling up/down, instead of jumping up/down the entire
     image when paging through a buffer that displays (large) images.

 -- Function: put-image image pos &optional string area
     This function puts image IMAGE in front of POS in the current
     buffer.  The argument POS should be an integer or a marker.  It
     specifies the buffer position where the image should appear.  The
     argument STRING specifies the text that should hold the image as an
     alternative to the default.

     The argument IMAGE must be an image descriptor, perhaps returned by
     ‘create-image’ or stored by ‘defimage’.

     The argument AREA specifies whether to put the image in a margin.
     If it is ‘left-margin’, the image appears in the left margin;
     ‘right-margin’ specifies the right margin.  If AREA is ‘nil’ or
     omitted, the image is displayed at point within the buffer’s text.

     Internally, this function creates an overlay, and gives it a
     ‘before-string’ property containing text that has a ‘display’
     property whose value is the image.  (Whew!)

 -- Function: remove-images start end &optional buffer
     This function removes images in BUFFER between positions START and
     END.  If BUFFER is omitted or ‘nil’, images are removed from the
     current buffer.

     This removes only images that were put into BUFFER the way
     ‘put-image’ does it, not images that were inserted with
     ‘insert-image’ or in other ways.

 -- Function: image-size spec &optional pixels frame
     This function returns the size of an image as a pair
     ‘(WIDTH . HEIGHT)’.  SPEC is an image specification.  PIXELS
     non-‘nil’ means return sizes measured in pixels, otherwise return
     sizes measured in the default character size of FRAME (*note Frame
     Font::).  FRAME is the frame on which the image will be displayed.
     FRAME null or omitted means use the selected frame (*note Input
     Focus::).

 -- Variable: max-image-size
     This variable is used to define the maximum size of image that
     Emacs will load.  Emacs will refuse to load (and display) any image
     that is larger than this limit.

     If the value is an integer, it directly specifies the maximum image
     height and width, measured in pixels.  If it is floating point, it
     specifies the maximum image height and width as a ratio to the
     frame height and width.  If the value is non-numeric, there is no
     explicit limit on the size of images.

     The purpose of this variable is to prevent unreasonably large
     images from accidentally being loaded into Emacs.  It only takes
     effect the first time an image is loaded.  Once an image is placed
     in the image cache, it can always be displayed, even if the value
     of ‘max-image-size’ is subsequently changed (*note Image Cache::).


File: elisp.info,  Node: Multi-Frame Images,  Next: Image Cache,  Prev: Showing Images,  Up: Images

37.17.10 Multi-Frame Images
---------------------------

Some image files can contain more than one image.  We say that there are
multiple “frames” in the image.  At present, Emacs supports multiple
frames for GIF, TIFF, and certain ImageMagick formats such as DJVM.

   The frames can be used either to represent multiple pages (this is
usually the case with multi-frame TIFF files, for example), or to create
animation (usually the case with multi-frame GIF files).

   A multi-frame image has a property ‘:index’, whose value is an
integer (counting from 0) that specifies which frame is being displayed.

 -- Function: image-multi-frame-p image
     This function returns non-‘nil’ if IMAGE contains more than one
     frame.  The actual return value is a cons ‘(NIMAGES . DELAY)’,
     where NIMAGES is the number of frames and DELAY is the delay in
     seconds between them, or ‘nil’ if the image does not specify a
     delay.  Images that are intended to be animated usually specify a
     frame delay, whereas ones that are intended to be treated as
     multiple pages do not.

 -- Function: image-current-frame image
     This function returns the index of the current frame number for
     IMAGE, counting from 0.

 -- Function: image-show-frame image n &optional nocheck
     This function switches IMAGE to frame number N.  It replaces a
     frame number outside the valid range with that of the end of the
     range, unless NOCHECK is non-‘nil’.  If IMAGE does not contain a
     frame with the specified number, the image displays as a hollow
     box.

 -- Function: image-animate image &optional index limit
     This function animates IMAGE.  The optional integer INDEX specifies
     the frame from which to start (default 0).  The optional argument
     LIMIT controls the length of the animation.  If omitted or ‘nil’,
     the image animates once only; if ‘t’ it loops forever; if a number
     animation stops after that many seconds.

Animation operates by means of a timer.  Note that Emacs imposes a
minimum frame delay of 0.01 (‘image-minimum-frame-delay’) seconds.  If
the image itself does not specify a delay, Emacs uses
‘image-default-frame-delay’.

 -- Function: image-animate-timer image
     This function returns the timer responsible for animating IMAGE, if
     there is one.


File: elisp.info,  Node: Image Cache,  Prev: Multi-Frame Images,  Up: Images

37.17.11 Image Cache
--------------------

Emacs caches images so that it can display them again more efficiently.
When Emacs displays an image, it searches the image cache for an
existing image specification ‘equal’ to the desired specification.  If a
match is found, the image is displayed from the cache.  Otherwise, Emacs
loads the image normally.

 -- Function: image-flush spec &optional frame
     This function removes the image with specification SPEC from the
     image cache of frame FRAME.  Image specifications are compared
     using ‘equal’.  If FRAME is ‘nil’, it defaults to the selected
     frame.  If FRAME is ‘t’, the image is flushed on all existing
     frames.

     In Emacs’s current implementation, each graphical terminal
     possesses an image cache, which is shared by all the frames on that
     terminal (*note Multiple Terminals::).  Thus, refreshing an image
     in one frame also refreshes it in all other frames on the same
     terminal.

   One use for ‘image-flush’ is to tell Emacs about a change in an image
file.  If an image specification contains a ‘:file’ property, the image
is cached based on the file’s contents when the image is first
displayed.  Even if the file subsequently changes, Emacs continues
displaying the old version of the image.  Calling ‘image-flush’ flushes
the image from the cache, forcing Emacs to re-read the file the next
time it needs to display that image.

   Another use for ‘image-flush’ is for memory conservation.  If your
Lisp program creates a large number of temporary images over a period
much shorter than ‘image-cache-eviction-delay’ (see below), you can opt
to flush unused images yourself, instead of waiting for Emacs to do it
automatically.

 -- Function: clear-image-cache &optional filter
     This function clears an image cache, removing all the images stored
     in it.  If FILTER is omitted or ‘nil’, it clears the cache for the
     selected frame.  If FILTER is a frame, it clears the cache for that
     frame.  If FILTER is ‘t’, all image caches are cleared.  Otherwise,
     FILTER is taken to be a file name, and all images associated with
     that file name are removed from all image caches.

   If an image in the image cache has not been displayed for a specified
period of time, Emacs removes it from the cache and frees the associated
memory.

 -- Variable: image-cache-eviction-delay
     This variable specifies the number of seconds an image can remain
     in the cache without being displayed.  When an image is not
     displayed for this length of time, Emacs removes it from the image
     cache.

     Under some circumstances, if the number of images in the cache
     grows too large, the actual eviction delay may be shorter than
     this.

     If the value is ‘nil’, Emacs does not remove images from the cache
     except when you explicitly clear it.  This mode can be useful for
     debugging.


File: elisp.info,  Node: Xwidgets,  Next: Buttons,  Prev: Images,  Up: Display

37.18 Embedded Native Widgets
=============================

Emacs is able to display native widgets, such as GTK WebKit widgets, in
Emacs buffers when it was built with the necessary support libraries and
is running on a graphical terminal.  To test whether Emacs supports
display of embedded widgets, check that the ‘xwidget-internal’ feature
is available (*note Named Features::).

   To display an embedded widget in a buffer, you must first create an
xwidget object, and then use that object as the display specifier in a
‘display’ text or overlay property (*note Display Property::).

 -- Function: make-xwidget type title width height arguments &optional
          buffer
     This creates and returns an xwidget object.  If BUFFER is omitted
     or ‘nil’, it defaults to the current buffer.  If BUFFER names a
     buffer that doesn’t exist, it will be created.  The TYPE identifies
     the type of the xwidget component, it can be one of the following:

     ‘webkit’
          The WebKit component.

     The WIDTH and HEIGHT arguments specify the widget size in pixels,
     and TITLE, a string, specifies its title.

 -- Function: xwidgetp object
     This function returns ‘t’ if OBJECT is an xwidget, ‘nil’ otherwise.

 -- Function: xwidget-plist xwidget
     This function returns the property list of XWIDGET.

 -- Function: set-xwidget-plist xwidget plist
     This function replaces the property list of XWIDGET with a new
     property list given by PLIST.

 -- Function: xwidget-buffer xwidget
     This function returns the buffer of XWIDGET.

 -- Function: get-buffer-xwidgets buffer
     This function returns a list of xwidget objects associated with the
     BUFFER, which can be specified as a buffer object or a name of an
     existing buffer, a string.  The value is ‘nil’ if BUFFER contains
     no xwidgets.

 -- Function: xwidget-webkit-goto-uri xwidget uri
     This function browses the specified URI in the given XWIDGET.  The
     URI is a string that specifies the name of a file or a URL.

 -- Function: xwidget-webkit-execute-script xwidget script
     This function causes the browser widget specified by XWIDGET to
     execute the specified JavaScript ‘script’.

 -- Function: xwidget-webkit-execute-script-rv xwidget script &optional
          default
     This function executes the specified SCRIPT like
     ‘xwidget-webkit-execute-script’ does, but it also returns the
     script’s return value as a string.  If SCRIPT doesn’t return a
     value, this function returns DEFAULT, or ‘nil’ if DEFAULT was
     omitted.

 -- Function: xwidget-webkit-get-title xwidget
     This function returns the title of XWIDGET as a string.

 -- Function: xwidget-resize xwidget width height
     This function resizes the specified XWIDGET to the size
     WIDTHxHEIGHT pixels.

 -- Function: xwidget-size-request xwidget
     This function returns the desired size of XWIDGET as a list of the
     form ‘(WIDTH HEIGHT)’.  The dimensions are in pixels.

 -- Function: xwidget-info xwidget
     This function returns the attributes of XWIDGET as a vector of the
     form ‘[TYPE TITLE WIDTH HEIGHT]’.  The attributes are usually
     determined by ‘make-xwidget’ when the xwidget is created.

 -- Function: set-xwidget-query-on-exit-flag xwidget flag
     This function allows you to arrange that Emacs will ask the user
     for confirmation before exiting or before killing a buffer that has
     XWIDGET associated with it.  If FLAG is non-‘nil’, Emacs will query
     the user, otherwise it will not.

 -- Function: xwidget-query-on-exit-flag xwidget
     This function returns the current setting of XWIDGETs query-on-exit
     flag, either ‘t’ or ‘nil’.


File: elisp.info,  Node: Buttons,  Next: Abstract Display,  Prev: Xwidgets,  Up: Display

37.19 Buttons
=============

The Button package defines functions for inserting and manipulating
“buttons” that can be activated with the mouse or via keyboard commands.
These buttons are typically used for various kinds of hyperlinks.

   A button is essentially a set of text or overlay properties, attached
to a stretch of text in a buffer.  These properties are called “button
properties”.  One of these properties, the “action property”, specifies
a function which is called when the user invokes the button using the
keyboard or the mouse.  The action function may examine the button and
use its other properties as desired.

   In some ways, the Button package duplicates the functionality in the
Widget package.  *Note Introduction: (widget)Top.  The advantage of the
Button package is that it is faster, smaller, and simpler to program.
From the point of view of the user, the interfaces produced by the two
packages are very similar.

* Menu:

* Button Properties::      Button properties with special meanings.
* Button Types::           Defining common properties for classes of buttons.
* Making Buttons::         Adding buttons to Emacs buffers.
* Manipulating Buttons::   Getting and setting properties of buttons.
* Button Buffer Commands:: Buffer-wide commands and bindings for buttons.


File: elisp.info,  Node: Button Properties,  Next: Button Types,  Up: Buttons

37.19.1 Button Properties
-------------------------

Each button has an associated list of properties defining its appearance
and behavior, and other arbitrary properties may be used for application
specific purposes.  The following properties have special meaning to the
Button package:

‘action’
     The function to call when the user invokes the button, which is
     passed the single argument BUTTON.  By default this is ‘ignore’,
     which does nothing.

‘mouse-action’
     This is similar to ‘action’, and when present, will be used instead
     of ‘action’ for button invocations resulting from mouse-clicks
     (instead of the user hitting <RET>).  If not present, mouse-clicks
     use ‘action’ instead.

‘face’
     This is an Emacs face controlling how buttons of this type are
     displayed; by default this is the ‘button’ face.

‘mouse-face’
     This is an additional face which controls appearance during
     mouse-overs (merged with the usual button face); by default this is
     the usual Emacs ‘highlight’ face.

‘keymap’
     The button’s keymap, defining bindings active within the button
     region.  By default this is the usual button region keymap, stored
     in the variable ‘button-map’, which defines <RET> and <mouse-2> to
     invoke the button.

‘type’
     The button type.  *Note Button Types::.

‘help-echo’
     A string displayed by the Emacs tool-tip help system; by default,
     ‘"mouse-2, RET: Push this button"’.

‘follow-link’
     The follow-link property, defining how a <mouse-1> click behaves on
     this button, *Note Clickable Text::.

‘button’
     All buttons have a non-‘nil’ ‘button’ property, which may be useful
     in finding regions of text that comprise buttons (which is what the
     standard button functions do).

   There are other properties defined for the regions of text in a
button, but these are not generally interesting for typical uses.


File: elisp.info,  Node: Button Types,  Next: Making Buttons,  Prev: Button Properties,  Up: Buttons

37.19.2 Button Types
--------------------

Every button has a “button type”, which defines default values for the
button’s properties.  Button types are arranged in a hierarchy, with
specialized types inheriting from more general types, so that it’s easy
to define special-purpose types of buttons for specific tasks.

 -- Function: define-button-type name &rest properties
     Define a button type called NAME (a symbol).  The remaining
     arguments form a sequence of PROPERTY VALUE pairs, specifying
     default property values for buttons with this type (a button’s type
     may be set by giving it a ‘type’ property when creating the button,
     using the ‘:type’ keyword argument).

     In addition, the keyword argument ‘:supertype’ may be used to
     specify a button-type from which NAME inherits its default property
     values.  Note that this inheritance happens only when NAME is
     defined; subsequent changes to a supertype are not reflected in its
     subtypes.

   Using ‘define-button-type’ to define default properties for buttons
is not necessary—buttons without any specified type use the built-in
button-type ‘button’—but it is encouraged, since doing so usually makes
the resulting code clearer and more efficient.


File: elisp.info,  Node: Making Buttons,  Next: Manipulating Buttons,  Prev: Button Types,  Up: Buttons

37.19.3 Making Buttons
----------------------

Buttons are associated with a region of text, using an overlay or text
properties to hold button-specific information, all of which are
initialized from the button’s type (which defaults to the built-in
button type ‘button’).  Like all Emacs text, the appearance of the
button is governed by the ‘face’ property; by default (via the ‘face’
property inherited from the ‘button’ button-type) this is a simple
underline, like a typical web-page link.

   For convenience, there are two sorts of button-creation functions,
those that add button properties to an existing region of a buffer,
called ‘make-...button’, and those that also insert the button text,
called ‘insert-...button’.

   The button-creation functions all take the ‘&rest’ argument
PROPERTIES, which should be a sequence of PROPERTY VALUE pairs,
specifying properties to add to the button; see *note Button
Properties::.  In addition, the keyword argument ‘:type’ may be used to
specify a button-type from which to inherit other properties; see *note
Button Types::.  Any properties not explicitly specified during creation
will be inherited from the button’s type (if the type defines such a
property).

   The following functions add a button using an overlay (*note
Overlays::) to hold the button properties:

 -- Function: make-button beg end &rest properties
     This makes a button from BEG to END in the current buffer, and
     returns it.

 -- Function: insert-button label &rest properties
     This insert a button with the label LABEL at point, and returns it.

   The following functions are similar, but using text properties (*note
Text Properties::) to hold the button properties.  Such buttons do not
add markers to the buffer, so editing in the buffer does not slow down
if there is an extremely large numbers of buttons.  However, if there is
an existing face text property on the text (e.g., a face assigned by
Font Lock mode), the button face may not be visible.  Both of these
functions return the starting position of the new button.

 -- Function: make-text-button beg end &rest properties
     This makes a button from BEG to END in the current buffer, using
     text properties.

 -- Function: insert-text-button label &rest properties
     This inserts a button with the label LABEL at point, using text
     properties.


File: elisp.info,  Node: Manipulating Buttons,  Next: Button Buffer Commands,  Prev: Making Buttons,  Up: Buttons

37.19.4 Manipulating Buttons
----------------------------

These are functions for getting and setting properties of buttons.
Often these are used by a button’s invocation function to determine what
to do.

   Where a BUTTON parameter is specified, it means an object referring
to a specific button, either an overlay (for overlay buttons), or a
buffer-position or marker (for text property buttons).  Such an object
is passed as the first argument to a button’s invocation function when
it is invoked.

 -- Function: button-start button
     Return the position at which BUTTON starts.

 -- Function: button-end button
     Return the position at which BUTTON ends.

 -- Function: button-get button prop
     Get the property of button BUTTON named PROP.

 -- Function: button-put button prop val
     Set BUTTON’s PROP property to VAL.

 -- Function: button-activate button &optional use-mouse-action
     Call BUTTON’s ‘action’ property (i.e., invoke the function that is
     the value of that property, passing it the single argument BUTTON).
     If USE-MOUSE-ACTION is non-‘nil’, try to invoke the button’s
     ‘mouse-action’ property instead of ‘action’; if the button has no
     ‘mouse-action’ property, use ‘action’ as normal.

 -- Function: button-label button
     Return BUTTON’s text label.

 -- Function: button-type button
     Return BUTTON’s button-type.

 -- Function: button-has-type-p button type
     Return ‘t’ if BUTTON has button-type TYPE, or one of TYPE’s
     subtypes.

 -- Function: button-at pos
     Return the button at position POS in the current buffer, or ‘nil’.
     If the button at POS is a text property button, the return value is
     a marker pointing to POS.

 -- Function: button-type-put type prop val
     Set the button-type TYPE’s PROP property to VAL.

 -- Function: button-type-get type prop
     Get the property of button-type TYPE named PROP.

 -- Function: button-type-subtype-p type supertype
     Return ‘t’ if button-type TYPE is a subtype of SUPERTYPE.


File: elisp.info,  Node: Button Buffer Commands,  Prev: Manipulating Buttons,  Up: Buttons

37.19.5 Button Buffer Commands
------------------------------

These are commands and functions for locating and operating on buttons
in an Emacs buffer.

   ‘push-button’ is the command that a user uses to actually push a
button, and is bound by default in the button itself to <RET> and to
<mouse-2> using a local keymap in the button’s overlay or text
properties.  Commands that are useful outside the buttons itself, such
as ‘forward-button’ and ‘backward-button’ are additionally available in
the keymap stored in ‘button-buffer-map’; a mode which uses buttons may
want to use ‘button-buffer-map’ as a parent keymap for its keymap.

   If the button has a non-‘nil’ ‘follow-link’ property, and
‘mouse-1-click-follows-link’ is set, a quick <mouse-1> click will also
activate the ‘push-button’ command.  *Note Clickable Text::.

 -- Command: push-button &optional pos use-mouse-action
     Perform the action specified by a button at location POS.  POS may
     be either a buffer position or a mouse-event.  If USE-MOUSE-ACTION
     is non-‘nil’, or POS is a mouse-event (*note Mouse Events::), try
     to invoke the button’s ‘mouse-action’ property instead of ‘action’;
     if the button has no ‘mouse-action’ property, use ‘action’ as
     normal.  POS defaults to point, except when ‘push-button’ is
     invoked interactively as the result of a mouse-event, in which
     case, the mouse event’s position is used.  If there’s no button at
     POS, do nothing and return ‘nil’, otherwise return ‘t’.

 -- Command: forward-button n &optional wrap display-message
     Move to the Nth next button, or Nth previous button if N is
     negative.  If N is zero, move to the start of any button at point.
     If WRAP is non-‘nil’, moving past either end of the buffer
     continues from the other end.  If DISPLAY-MESSAGE is non-‘nil’, the
     button’s help-echo string is displayed.  Any button with a
     non-‘nil’ ‘skip’ property is skipped over.  Returns the button
     found.

 -- Command: backward-button n &optional wrap display-message
     Move to the Nth previous button, or Nth next button if N is
     negative.  If N is zero, move to the start of any button at point.
     If WRAP is non-‘nil’, moving past either end of the buffer
     continues from the other end.  If DISPLAY-MESSAGE is non-‘nil’, the
     button’s help-echo string is displayed.  Any button with a
     non-‘nil’ ‘skip’ property is skipped over.  Returns the button
     found.

 -- Function: next-button pos &optional count-current
 -- Function: previous-button pos &optional count-current
     Return the next button after (for ‘next-button’) or before (for
     ‘previous-button’) position POS in the current buffer.  If
     COUNT-CURRENT is non-‘nil’, count any button at POS in the search,
     instead of starting at the next button.


File: elisp.info,  Node: Abstract Display,  Next: Blinking,  Prev: Buttons,  Up: Display

37.20 Abstract Display
======================

The Ewoc package constructs buffer text that represents a structure of
Lisp objects, and updates the text to follow changes in that structure.
This is like the “view” component in the “model–view–controller” design
paradigm.  Ewoc means “Emacs’s Widget for Object Collections”.

   An “ewoc” is a structure that organizes information required to
construct buffer text that represents certain Lisp data.  The buffer
text of the ewoc has three parts, in order: first, fixed “header” text;
next, textual descriptions of a series of data elements (Lisp objects
that you specify); and last, fixed “footer” text.  Specifically, an ewoc
contains information on:

   • The buffer which its text is generated in.

   • The text’s start position in the buffer.

   • The header and footer strings.

   • A doubly-linked chain of “nodes”, each of which contains:

        • A “data element”, a single Lisp object.

        • Links to the preceding and following nodes in the chain.

   • A “pretty-printer” function which is responsible for inserting the
     textual representation of a data element value into the current
     buffer.

   Typically, you define an ewoc with ‘ewoc-create’, and then pass the
resulting ewoc structure to other functions in the Ewoc package to build
nodes within it, and display it in the buffer.  Once it is displayed in
the buffer, other functions determine the correspondence between buffer
positions and nodes, move point from one node’s textual representation
to another, and so forth.  *Note Abstract Display Functions::.

   A node “encapsulates” a data element much the way a variable holds a
value.  Normally, encapsulation occurs as a part of adding a node to the
ewoc.  You can retrieve the data element value and place a new value in
its place, like so:

     (ewoc-data NODE)
     ⇒ value

     (ewoc-set-data NODE NEW-VALUE)
     ⇒ NEW-VALUE

You can also use, as the data element value, a Lisp object (list or
vector) that is a container for the real value, or an index into some
other structure.  The example (*note Abstract Display Example::) uses
the latter approach.

   When the data changes, you will want to update the text in the
buffer.  You can update all nodes by calling ‘ewoc-refresh’, or just
specific nodes using ‘ewoc-invalidate’, or all nodes satisfying a
predicate using ‘ewoc-map’.  Alternatively, you can delete invalid nodes
using ‘ewoc-delete’ or ‘ewoc-filter’, and add new nodes in their place.
Deleting a node from an ewoc deletes its associated textual description
from buffer, as well.

* Menu:

* Abstract Display Functions::  Functions in the Ewoc package.
* Abstract Display Example::    Example of using Ewoc.


File: elisp.info,  Node: Abstract Display Functions,  Next: Abstract Display Example,  Up: Abstract Display

37.20.1 Abstract Display Functions
----------------------------------

In this subsection, EWOC and NODE stand for the structures described
above (*note Abstract Display::), while DATA stands for an arbitrary
Lisp object used as a data element.

 -- Function: ewoc-create pretty-printer &optional header footer nosep
     This constructs and returns a new ewoc, with no nodes (and thus no
     data elements).  PRETTY-PRINTER should be a function that takes one
     argument, a data element of the sort you plan to use in this ewoc,
     and inserts its textual description at point using ‘insert’ (and
     never ‘insert-before-markers’, because that would interfere with
     the Ewoc package’s internal mechanisms).

     Normally, a newline is automatically inserted after the header, the
     footer and every node’s textual description.  If NOSEP is
     non-‘nil’, no newline is inserted.  This may be useful for
     displaying an entire ewoc on a single line, for example, or for
     making nodes invisible by arranging for PRETTY-PRINTER to do
     nothing for those nodes.

     An ewoc maintains its text in the buffer that is current when you
     create it, so switch to the intended buffer before calling
     ‘ewoc-create’.

 -- Function: ewoc-buffer ewoc
     This returns the buffer where EWOC maintains its text.

 -- Function: ewoc-get-hf ewoc
     This returns a cons cell ‘(HEADER . FOOTER)’ made from EWOC’s
     header and footer.

 -- Function: ewoc-set-hf ewoc header footer
     This sets the header and footer of EWOC to the strings HEADER and
     FOOTER, respectively.

 -- Function: ewoc-enter-first ewoc data
 -- Function: ewoc-enter-last ewoc data
     These add a new node encapsulating DATA, putting it, respectively,
     at the beginning or end of EWOC’s chain of nodes.

 -- Function: ewoc-enter-before ewoc node data
 -- Function: ewoc-enter-after ewoc node data
     These add a new node encapsulating DATA, adding it to EWOC before
     or after NODE, respectively.

 -- Function: ewoc-prev ewoc node
 -- Function: ewoc-next ewoc node
     These return, respectively, the previous node and the next node of
     NODE in EWOC.

 -- Function: ewoc-nth ewoc n
     This returns the node in EWOC found at zero-based index N.  A
     negative N means count from the end.  ‘ewoc-nth’ returns ‘nil’ if N
     is out of range.

 -- Function: ewoc-data node
     This extracts the data encapsulated by NODE and returns it.

 -- Function: ewoc-set-data node data
     This sets the data encapsulated by NODE to DATA.

 -- Function: ewoc-locate ewoc &optional pos guess
     This determines the node in EWOC which contains point (or POS if
     specified), and returns that node.  If EWOC has no nodes, it
     returns ‘nil’.  If POS is before the first node, it returns the
     first node; if POS is after the last node, it returns the last
     node.  The optional third arg GUESS should be a node that is likely
     to be near POS; this doesn’t alter the result, but makes the
     function run faster.

 -- Function: ewoc-location node
     This returns the start position of NODE.

 -- Function: ewoc-goto-prev ewoc arg
 -- Function: ewoc-goto-next ewoc arg
     These move point to the previous or next, respectively, ARGth node
     in EWOC.  ‘ewoc-goto-prev’ does not move if it is already at the
     first node or if EWOC is empty, whereas ‘ewoc-goto-next’ moves past
     the last node, returning ‘nil’.  Excepting this special case, these
     functions return the node moved to.

 -- Function: ewoc-goto-node ewoc node
     This moves point to the start of NODE in EWOC.

 -- Function: ewoc-refresh ewoc
     This function regenerates the text of EWOC.  It works by deleting
     the text between the header and the footer, i.e., all the data
     elements’ representations, and then calling the pretty-printer
     function for each node, one by one, in order.

 -- Function: ewoc-invalidate ewoc &rest nodes
     This is similar to ‘ewoc-refresh’, except that only NODES in EWOC
     are updated instead of the entire set.

 -- Function: ewoc-delete ewoc &rest nodes
     This deletes each node in NODES from EWOC.

 -- Function: ewoc-filter ewoc predicate &rest args
     This calls PREDICATE for each data element in EWOC and deletes
     those nodes for which PREDICATE returns ‘nil’.  Any ARGS are passed
     to PREDICATE.

 -- Function: ewoc-collect ewoc predicate &rest args
     This calls PREDICATE for each data element in EWOC and returns a
     list of those elements for which PREDICATE returns non-‘nil’.  The
     elements in the list are ordered as in the buffer.  Any ARGS are
     passed to PREDICATE.

 -- Function: ewoc-map map-function ewoc &rest args
     This calls MAP-FUNCTION for each data element in EWOC and updates
     those nodes for which MAP-FUNCTION returns non-‘nil’.  Any ARGS are
     passed to MAP-FUNCTION.


File: elisp.info,  Node: Abstract Display Example,  Prev: Abstract Display Functions,  Up: Abstract Display

37.20.2 Abstract Display Example
--------------------------------

Here is a simple example using functions of the ewoc package to
implement a “color components” display, an area in a buffer that
represents a vector of three integers (itself representing a 24-bit RGB
value) in various ways.

     (setq colorcomp-ewoc nil
           colorcomp-data nil
           colorcomp-mode-map nil
           colorcomp-labels ["Red" "Green" "Blue"])

     (defun colorcomp-pp (data)
       (if data
           (let ((comp (aref colorcomp-data data)))
             (insert (aref colorcomp-labels data) "\t: #x"
                     (format "%02X" comp) " "
                     (make-string (ash comp -2) ?#) "\n"))
         (let ((cstr (format "#%02X%02X%02X"
                             (aref colorcomp-data 0)
                             (aref colorcomp-data 1)
                             (aref colorcomp-data 2)))
               (samp " (sample text) "))
           (insert "Color\t: "
                   (propertize samp 'face
                               `(foreground-color . ,cstr))
                   (propertize samp 'face
                               `(background-color . ,cstr))
                   "\n"))))

     (defun colorcomp (color)
       "Allow fiddling with COLOR in a new buffer.
     The buffer is in Color Components mode."
       (interactive "sColor (name or #RGB or #RRGGBB): ")
       (when (string= "" color)
         (setq color "green"))
       (unless (color-values color)
         (error "No such color: %S" color))
       (switch-to-buffer
        (generate-new-buffer (format "originally: %s" color)))
       (kill-all-local-variables)
       (setq major-mode 'colorcomp-mode
             mode-name "Color Components")
       (use-local-map colorcomp-mode-map)
       (erase-buffer)
       (buffer-disable-undo)
       (let ((data (apply 'vector (mapcar (lambda (n) (ash n -8))
                                          (color-values color))))
             (ewoc (ewoc-create 'colorcomp-pp
                                "\nColor Components\n\n"
                                (substitute-command-keys
                                 "\n\\{colorcomp-mode-map}"))))
         (set (make-local-variable 'colorcomp-data) data)
         (set (make-local-variable 'colorcomp-ewoc) ewoc)
         (ewoc-enter-last ewoc 0)
         (ewoc-enter-last ewoc 1)
         (ewoc-enter-last ewoc 2)
         (ewoc-enter-last ewoc nil)))

   This example can be extended to be a color selection widget (in other
words, the “controller” part of the “model–view–controller” design
paradigm) by defining commands to modify ‘colorcomp-data’ and to finish
the selection process, and a keymap to tie it all together conveniently.

     (defun colorcomp-mod (index limit delta)
       (let ((cur (aref colorcomp-data index)))
         (unless (= limit cur)
           (aset colorcomp-data index (+ cur delta)))
         (ewoc-invalidate
          colorcomp-ewoc
          (ewoc-nth colorcomp-ewoc index)
          (ewoc-nth colorcomp-ewoc -1))))

     (defun colorcomp-R-more () (interactive) (colorcomp-mod 0 255 1))
     (defun colorcomp-G-more () (interactive) (colorcomp-mod 1 255 1))
     (defun colorcomp-B-more () (interactive) (colorcomp-mod 2 255 1))
     (defun colorcomp-R-less () (interactive) (colorcomp-mod 0 0 -1))
     (defun colorcomp-G-less () (interactive) (colorcomp-mod 1 0 -1))
     (defun colorcomp-B-less () (interactive) (colorcomp-mod 2 0 -1))

     (defun colorcomp-copy-as-kill-and-exit ()
       "Copy the color components into the kill ring and kill the buffer.
     The string is formatted #RRGGBB (hash followed by six hex digits)."
       (interactive)
       (kill-new (format "#%02X%02X%02X"
                         (aref colorcomp-data 0)
                         (aref colorcomp-data 1)
                         (aref colorcomp-data 2)))
       (kill-buffer nil))

     (setq colorcomp-mode-map
           (let ((m (make-sparse-keymap)))
             (suppress-keymap m)
             (define-key m "i" 'colorcomp-R-less)
             (define-key m "o" 'colorcomp-R-more)
             (define-key m "k" 'colorcomp-G-less)
             (define-key m "l" 'colorcomp-G-more)
             (define-key m "," 'colorcomp-B-less)
             (define-key m "." 'colorcomp-B-more)
             (define-key m " " 'colorcomp-copy-as-kill-and-exit)
             m))

   Note that we never modify the data in each node, which is fixed when
the ewoc is created to be either ‘nil’ or an index into the vector
‘colorcomp-data’, the actual color components.


File: elisp.info,  Node: Blinking,  Next: Character Display,  Prev: Abstract Display,  Up: Display

37.21 Blinking Parentheses
==========================

This section describes the mechanism by which Emacs shows a matching
open parenthesis when the user inserts a close parenthesis.

 -- Variable: blink-paren-function
     The value of this variable should be a function (of no arguments)
     to be called whenever a character with close parenthesis syntax is
     inserted.  The value of ‘blink-paren-function’ may be ‘nil’, in
     which case nothing is done.

 -- User Option: blink-matching-paren
     If this variable is ‘nil’, then ‘blink-matching-open’ does nothing.

 -- User Option: blink-matching-paren-distance
     This variable specifies the maximum distance to scan for a matching
     parenthesis before giving up.

 -- User Option: blink-matching-delay
     This variable specifies the number of seconds to keep indicating
     the matching parenthesis.  A fraction of a second often gives good
     results, but the default is 1, which works on all systems.

 -- Command: blink-matching-open
     This function is the default value of ‘blink-paren-function’.  It
     assumes that point follows a character with close parenthesis
     syntax and applies the appropriate effect momentarily to the
     matching opening character.  If that character is not already on
     the screen, it displays the character’s context in the echo area.
     To avoid long delays, this function does not search farther than
     ‘blink-matching-paren-distance’ characters.

     Here is an example of calling this function explicitly.

          (defun interactive-blink-matching-open ()
            "Indicate momentarily the start of parenthesized sexp before point."
            (interactive)
            (let ((blink-matching-paren-distance
                   (buffer-size))
                  (blink-matching-paren t))
              (blink-matching-open)))


File: elisp.info,  Node: Character Display,  Next: Beeping,  Prev: Blinking,  Up: Display

37.22 Character Display
=======================

This section describes how characters are actually displayed by Emacs.
Typically, a character is displayed as a “glyph” (a graphical symbol
which occupies one character position on the screen), whose appearance
corresponds to the character itself.  For example, the character ‘a’
(character code 97) is displayed as ‘a’.  Some characters, however, are
displayed specially.  For example, the formfeed character (character
code 12) is usually displayed as a sequence of two glyphs, ‘^L’, while
the newline character (character code 10) starts a new screen line.

   You can modify how each character is displayed by defining a “display
table”, which maps each character code into a sequence of glyphs.  *Note
Display Tables::.

* Menu:

* Usual Display::       The usual conventions for displaying characters.
* Display Tables::      What a display table consists of.
* Active Display Table::  How Emacs selects a display table to use.
* Glyphs::              How to define a glyph, and what glyphs mean.
* Glyphless Chars::     How glyphless characters are drawn.


File: elisp.info,  Node: Usual Display,  Next: Display Tables,  Up: Character Display

37.22.1 Usual Display Conventions
---------------------------------

Here are the conventions for displaying each character code (in the
absence of a display table, which can override these conventions; *note
Display Tables::).

   • The “printable ASCII characters”, character codes 32 through 126
     (consisting of numerals, English letters, and symbols like ‘#’) are
     displayed literally.

   • The tab character (character code 9) displays as whitespace
     stretching up to the next tab stop column.  *Note (emacs)Text
     Display::.  The variable ‘tab-width’ controls the number of spaces
     per tab stop (see below).

   • The newline character (character code 10) has a special effect: it
     ends the preceding line and starts a new line.

   • The non-printable “ASCII control characters”—character codes 0
     through 31, as well as the <DEL> character (character code
     127)—display in one of two ways according to the variable
     ‘ctl-arrow’.  If this variable is non-‘nil’ (the default), these
     characters are displayed as sequences of two glyphs, where the
     first glyph is ‘^’ (a display table can specify a glyph to use
     instead of ‘^’); e.g., the <DEL> character is displayed as ‘^?’.

     If ‘ctl-arrow’ is ‘nil’, these characters are displayed as octal
     escapes (see below).

     This rule also applies to carriage return (character code 13), if
     that character appears in the buffer.  But carriage returns usually
     do not appear in buffer text; they are eliminated as part of
     end-of-line conversion (*note Coding System Basics::).

   • “Raw bytes” are non-ASCII characters with codes 128 through 255
     (*note Text Representations::).  These characters display as “octal
     escapes”: sequences of four glyphs, where the first glyph is the
     ASCII code for ‘\’, and the others are digit characters
     representing the character code in octal.  (A display table can
     specify a glyph to use instead of ‘\’.)

   • Each non-ASCII character with code above 255 is displayed
     literally, if the terminal supports it.  If the terminal does not
     support it, the character is said to be “glyphless”, and it is
     usually displayed using a placeholder glyph.  For example, if a
     graphical terminal has no font for a character, Emacs usually
     displays a box containing the character code in hexadecimal.  *Note
     Glyphless Chars::.

   The above display conventions apply even when there is a display
table, for any character whose entry in the active display table is
‘nil’.  Thus, when you set up a display table, you need only specify the
characters for which you want special behavior.

   The following variables affect how certain characters are displayed
on the screen.  Since they change the number of columns the characters
occupy, they also affect the indentation functions.  They also affect
how the mode line is displayed; if you want to force redisplay of the
mode line using the new values, call the function
‘force-mode-line-update’ (*note Mode Line Format::).

 -- User Option: ctl-arrow
     This buffer-local variable controls how control characters are
     displayed.  If it is non-‘nil’, they are displayed as a caret
     followed by the character: ‘^A’.  If it is ‘nil’, they are
     displayed as octal escapes: a backslash followed by three octal
     digits, as in ‘\001’.

 -- User Option: tab-width
     The value of this buffer-local variable is the spacing between tab
     stops used for displaying tab characters in Emacs buffers.  The
     value is in units of columns, and the default is 8.  Note that this
     feature is completely independent of the user-settable tab stops
     used by the command ‘tab-to-tab-stop’.  *Note Indent Tabs::.


File: elisp.info,  Node: Display Tables,  Next: Active Display Table,  Prev: Usual Display,  Up: Character Display

37.22.2 Display Tables
----------------------

A display table is a special-purpose char-table (*note Char-Tables::),
with ‘display-table’ as its subtype, which is used to override the usual
character display conventions.  This section describes how to make,
inspect, and assign elements to a display table object.

 -- Function: make-display-table
     This creates and returns a display table.  The table initially has
     ‘nil’ in all elements.

   The ordinary elements of the display table are indexed by character
codes; the element at index C says how to display the character code C.
The value should be ‘nil’ (which means to display the character C
according to the usual display conventions; *note Usual Display::), or a
vector of glyph codes (which means to display the character C as those
glyphs; *note Glyphs::).

   *Warning:* if you use the display table to change the display of
newline characters, the whole buffer will be displayed as one long line.

   The display table also has six “extra slots” which serve special
purposes.  Here is a table of their meanings; ‘nil’ in any slot means to
use the default for that slot, as stated below.

0
     The glyph for the end of a truncated screen line (the default for
     this is ‘$’).  *Note Glyphs::.  On graphical terminals, Emacs uses
     arrows in the fringes to indicate truncation, so the display table
     has no effect.

1
     The glyph for the end of a continued line (the default is ‘\’).  On
     graphical terminals, Emacs uses curved arrows in the fringes to
     indicate continuation, so the display table has no effect.

2
     The glyph for indicating a character displayed as an octal
     character code (the default is ‘\’).

3
     The glyph for indicating a control character (the default is ‘^’).

4
     A vector of glyphs for indicating the presence of invisible lines
     (the default is ‘...’).  *Note Selective Display::.

5
     The glyph used to draw the border between side-by-side windows (the
     default is ‘|’).  *Note Splitting Windows::.  This takes effect
     only when there are no scroll bars; if scroll bars are supported
     and in use, a scroll bar separates the two windows.

   For example, here is how to construct a display table that mimics the
effect of setting ‘ctl-arrow’ to a non-‘nil’ value (*note Glyphs::, for
the function ‘make-glyph-code’):

     (setq disptab (make-display-table))
     (dotimes (i 32)
       (or (= i ?\t)
           (= i ?\n)
           (aset disptab i
                 (vector (make-glyph-code ?^ 'escape-glyph)
                         (make-glyph-code (+ i 64) 'escape-glyph)))))
     (aset disptab 127
           (vector (make-glyph-code ?^ 'escape-glyph)
                   (make-glyph-code ?? 'escape-glyph)))))

 -- Function: display-table-slot display-table slot
     This function returns the value of the extra slot SLOT of
     DISPLAY-TABLE.  The argument SLOT may be a number from 0 to 5
     inclusive, or a slot name (symbol).  Valid symbols are
     ‘truncation’, ‘wrap’, ‘escape’, ‘control’, ‘selective-display’, and
     ‘vertical-border’.

 -- Function: set-display-table-slot display-table slot value
     This function stores VALUE in the extra slot SLOT of DISPLAY-TABLE.
     The argument SLOT may be a number from 0 to 5 inclusive, or a slot
     name (symbol).  Valid symbols are ‘truncation’, ‘wrap’, ‘escape’,
     ‘control’, ‘selective-display’, and ‘vertical-border’.

 -- Function: describe-display-table display-table
     This function displays a description of the display table
     DISPLAY-TABLE in a help buffer.

 -- Command: describe-current-display-table
     This command displays a description of the current display table in
     a help buffer.


File: elisp.info,  Node: Active Display Table,  Next: Glyphs,  Prev: Display Tables,  Up: Character Display

37.22.3 Active Display Table
----------------------------

Each window can specify a display table, and so can each buffer.  The
window’s display table, if there is one, takes precedence over the
buffer’s display table.  If neither exists, Emacs tries to use the
standard display table; if that is ‘nil’, Emacs uses the usual character
display conventions (*note Usual Display::).

   Note that display tables affect how the mode line is displayed, so if
you want to force redisplay of the mode line using a new display table,
call ‘force-mode-line-update’ (*note Mode Line Format::).

 -- Function: window-display-table &optional window
     This function returns WINDOW’s display table, or ‘nil’ if there is
     none.  The default for WINDOW is the selected window.

 -- Function: set-window-display-table window table
     This function sets the display table of WINDOW to TABLE.  The
     argument TABLE should be either a display table or ‘nil’.

 -- Variable: buffer-display-table
     This variable is automatically buffer-local in all buffers; its
     value specifies the buffer’s display table.  If it is ‘nil’, there
     is no buffer display table.

 -- Variable: standard-display-table
     The value of this variable is the standard display table, which is
     used when Emacs is displaying a buffer in a window with neither a
     window display table nor a buffer display table defined, or when
     Emacs is outputting text to the standard output or error streams.
     Although its default is typically ‘nil’, in an interactive session
     if the terminal cannot display curved quotes, its default maps
     curved quotes to ASCII approximations.  *Note Keys in
     Documentation::.

   The ‘disp-table’ library defines several functions for changing the
standard display table.


File: elisp.info,  Node: Glyphs,  Next: Glyphless Chars,  Prev: Active Display Table,  Up: Character Display

37.22.4 Glyphs
--------------

A “glyph” is a graphical symbol which occupies a single character
position on the screen.  Each glyph is represented in Lisp as a “glyph
code”, which specifies a character and optionally a face to display it
in (*note Faces::).  The main use of glyph codes is as the entries of
display tables (*note Display Tables::).  The following functions are
used to manipulate glyph codes:

 -- Function: make-glyph-code char &optional face
     This function returns a glyph code representing char CHAR with face
     FACE.  If FACE is omitted or ‘nil’, the glyph uses the default
     face; in that case, the glyph code is an integer.  If FACE is
     non-‘nil’, the glyph code is not necessarily an integer object.

 -- Function: glyph-char glyph
     This function returns the character of glyph code GLYPH.

 -- Function: glyph-face glyph
     This function returns face of glyph code GLYPH, or ‘nil’ if GLYPH
     uses the default face.

   You can set up a “glyph table” to change how glyph codes are actually
displayed on text terminals.  This feature is semi-obsolete; use
‘glyphless-char-display’ instead (*note Glyphless Chars::).

 -- Variable: glyph-table
     The value of this variable, if non-‘nil’, is the current glyph
     table.  It takes effect only on character terminals; on graphical
     displays, all glyphs are displayed literally.  The glyph table
     should be a vector whose Gth element specifies how to display glyph
     code G, where G is the glyph code for a glyph whose face is
     unspecified.  Each element should be one of the following:

     ‘nil’
          Display this glyph literally.

     a string
          Display this glyph by sending the specified string to the
          terminal.

     a glyph code
          Display the specified glyph code instead.

     Any integer glyph code greater than or equal to the length of the
     glyph table is displayed literally.


File: elisp.info,  Node: Glyphless Chars,  Prev: Glyphs,  Up: Character Display

37.22.5 Glyphless Character Display
-----------------------------------

“Glyphless characters” are characters which are displayed in a special
way, e.g., as a box containing a hexadecimal code, instead of being
displayed literally.  These include characters which are explicitly
defined to be glyphless, as well as characters for which there is no
available font (on a graphical display), and characters which cannot be
encoded by the terminal’s coding system (on a text terminal).

 -- Variable: glyphless-char-display
     The value of this variable is a char-table which defines glyphless
     characters and how they are displayed.  Each entry must be one of
     the following display methods:

     ‘nil’
          Display the character in the usual way.

     ‘zero-width’
          Don’t display the character.

     ‘thin-space’
          Display a thin space, 1-pixel wide on graphical displays, or
          1-character wide on text terminals.

     ‘empty-box’
          Display an empty box.

     ‘hex-code’
          Display a box containing the Unicode codepoint of the
          character, in hexadecimal notation.

     an ASCII string
          Display a box containing that string.  The string should
          contain at most 6 ASCII characters.

     a cons cell ‘(GRAPHICAL . TEXT)’
          Display with GRAPHICAL on graphical displays, and with TEXT on
          text terminals.  Both GRAPHICAL and TEXT must be one of the
          display methods described above.

     The ‘thin-space’, ‘empty-box’, ‘hex-code’, and ASCII string display
     methods are drawn with the ‘glyphless-char’ face.  On text
     terminals, a box is emulated by square brackets, ‘[]’.

     The char-table has one extra slot, which determines how to display
     any character that cannot be displayed with any available font, or
     cannot be encoded by the terminal’s coding system.  Its value
     should be one of the above display methods, except ‘zero-width’ or
     a cons cell.

     If a character has a non-‘nil’ entry in an active display table,
     the display table takes effect; in this case, Emacs does not
     consult ‘glyphless-char-display’ at all.

 -- User Option: glyphless-char-display-control
     This user option provides a convenient way to set
     ‘glyphless-char-display’ for groups of similar characters.  Do not
     set its value directly from Lisp code; the value takes effect only
     via a custom ‘:set’ function (*note Variable Definitions::), which
     updates ‘glyphless-char-display’.

     Its value should be an alist of elements ‘(GROUP . METHOD)’, where
     GROUP is a symbol specifying a group of characters, and METHOD is a
     symbol specifying how to display them.

     GROUP should be one of the following:

     ‘c0-control’
          ASCII control characters ‘U+0000’ to ‘U+001F’, excluding the
          newline and tab characters (normally displayed as escape
          sequences like ‘^A’; *note How Text Is Displayed: (emacs)Text
          Display.).

     ‘c1-control’
          Non-ASCII, non-printing characters ‘U+0080’ to ‘U+009F’
          (normally displayed as octal escape sequences like ‘\230’).

     ‘format-control’
          Characters of Unicode General Category [Cf], such as ‘U+200E’
          (Left-to-Right Mark), but excluding characters that have
          graphic images, such as ‘U+00AD’ (Soft Hyphen).

     ‘no-font’
          Characters for there is no suitable font, or which cannot be
          encoded by the terminal’s coding system.

     The METHOD symbol should be one of ‘zero-width’, ‘thin-space’,
     ‘empty-box’, or ‘hex-code’.  These have the same meanings as in
     ‘glyphless-char-display’, above.


File: elisp.info,  Node: Beeping,  Next: Window Systems,  Prev: Character Display,  Up: Display

37.23 Beeping
=============

This section describes how to make Emacs ring the bell (or blink the
screen) to attract the user’s attention.  Be conservative about how
often you do this; frequent bells can become irritating.  Also be
careful not to use just beeping when signaling an error is more
appropriate (*note Errors::).

 -- Function: ding &optional do-not-terminate
     This function beeps, or flashes the screen (see ‘visible-bell’
     below).  It also terminates any keyboard macro currently executing
     unless DO-NOT-TERMINATE is non-‘nil’.

 -- Function: beep &optional do-not-terminate
     This is a synonym for ‘ding’.

 -- User Option: visible-bell
     This variable determines whether Emacs should flash the screen to
     represent a bell.  Non-‘nil’ means yes, ‘nil’ means no.  This is
     effective on graphical displays, and on text terminals provided the
     terminal’s Termcap entry defines the visible bell capability
     (‘vb’).

 -- User Option: ring-bell-function
     If this is non-‘nil’, it specifies how Emacs should ring the bell.
     Its value should be a function of no arguments.  If this is
     non-‘nil’, it takes precedence over the ‘visible-bell’ variable.


File: elisp.info,  Node: Window Systems,  Next: Tooltips,  Prev: Beeping,  Up: Display

37.24 Window Systems
====================

Emacs works with several window systems, most notably the X Window
System.  Both Emacs and X use the term “window”, but use it differently.
An Emacs frame is a single window as far as X is concerned; the
individual Emacs windows are not known to X at all.

 -- Variable: window-system
     This terminal-local variable tells Lisp programs what window system
     Emacs is using for displaying the frame.  The possible values are

     ‘x’
          Emacs is displaying the frame using X.
     ‘w32’
          Emacs is displaying the frame using native MS-Windows GUI.
     ‘ns’
          Emacs is displaying the frame using the Nextstep interface
          (used on GNUstep and macOS).
     ‘pc’
          Emacs is displaying the frame using MS-DOS direct screen
          writes.
     ‘nil’
          Emacs is displaying the frame on a character-based terminal.

 -- Variable: initial-window-system
     This variable holds the value of ‘window-system’ used for the first
     frame created by Emacs during startup.  (When Emacs is invoked with
     the ‘--daemon’ option, it does not create any initial frames, so
     ‘initial-window-system’ is ‘nil’, except on MS-Windows, where it is
     still ‘w32’.  *Note daemon: (emacs)Initial Options.)

 -- Function: window-system &optional frame
     This function returns a symbol whose name tells what window system
     is used for displaying FRAME (which defaults to the currently
     selected frame).  The list of possible symbols it returns is the
     same one documented for the variable ‘window-system’ above.

   Do _not_ use ‘window-system’ and ‘initial-window-system’ as
predicates or boolean flag variables, if you want to write code that
works differently on text terminals and graphic displays.  That is
because ‘window-system’ is not a good indicator of Emacs capabilities on
a given display type.  Instead, use ‘display-graphic-p’ or any of the
other ‘display-*-p’ predicates described in *note Display Feature
Testing::.


File: elisp.info,  Node: Tooltips,  Next: Bidirectional Display,  Prev: Window Systems,  Up: Display

37.25 Tooltips
==============

“Tooltips” are special frames (*note Frames::) that are used to display
helpful hints (a.k.a. “tips”) related to the current position of the
mouse pointer.  Emacs uses tooltips to display help strings about active
portions of text (*note Special Properties::) and about various UI
elements, such as menu items (*note Extended Menu Items::) and tool-bar
buttons (*note Tool Bar::).

 -- Function: tooltip-mode
     Tooltip Mode is a minor mode that enables display of tooltips.
     Turning off this mode causes the tooltips be displayed in the echo
     area.  On text-mode (a.k.a. “TTY”) frames, tooltips are always
     displayed in the echo area.

   When Emacs is built with GTK+ support, it by default displays
tooltips using GTK+ functions, and the appearance of the tooltips is
then controlled by GTK+ settings.  GTK+ tooltips can be disabled by
changing the value of the variable ‘x-gtk-use-system-tooltips’ to ‘nil’.
The rest of this subsection describes how to control non-GTK+ tooltips,
which are presented by Emacs itself.

   Since tooltips are special frames, they have their frame parameters
(*note Frame Parameters::).  Unlike other frames, the frame parameters
for tooltips are stored in a special variable.

 -- Variable: tooltip-frame-parameters
     This customizable option holds the frame parameters used for
     displaying tooltips.  Any font and color parameters are ignored,
     and the corresponding attributes of the ‘tooltip’ face are used
     instead.  If ‘left’ or ‘top’ parameters are included, they are used
     as absolute frame-relative coordinates where the tooltip should be
     shown.  (Mouse-relative position of the tooltip can be customized
     using the variables described in *note (emacs)Tooltips::.)  Note
     that the ‘left’ and ‘top’ parameters, if present, override the
     values of mouse-relative offsets.

   The ‘tooltip’ face determines the appearance of text shown in
tooltips.  It should generally use a variable-pitch font of size that is
preferably smaller than the default frame font.

 -- Variable: tooltip-functions
     This abnormal hook is a list of functions to call when Emacs needs
     to display a tooltip.  Each function is called with a single
     argument EVENT which is a copy of the last mouse movement event.
     If a function on this list actually displays the tooltip, it should
     return non-‘nil’, and then the rest of the functions will not be
     called.  The default value of this variable is a single function
     ‘tooltip-help-tips’.

   If you write your own function to be put on the ‘tooltip-functions’
list, you may need to know the buffer of the mouse event that triggered
the tooltip display.  The following function provides that information.

 -- Function: tooltip-event-buffer event
     This function returns the buffer over which EVENT occurred.  Call
     it with the argument of the function from ‘tooltip-functions’ to
     obtain the buffer whose text triggered the tooltip.  Note that the
     event might occur not over a buffer (e.g., over the tool bar), in
     which case this function will return ‘nil’.

   Other aspects of tooltip display are controlled by several
customizable settings; see *note (emacs)Tooltips::.


File: elisp.info,  Node: Bidirectional Display,  Prev: Tooltips,  Up: Display

37.26 Bidirectional Display
===========================

Emacs can display text written in scripts, such as Arabic, Farsi, and
Hebrew, whose natural ordering for horizontal text display runs from
right to left.  Furthermore, segments of Latin script and digits
embedded in right-to-left text are displayed left-to-right, while
segments of right-to-left script embedded in left-to-right text (e.g.,
Arabic or Hebrew text in comments or strings in a program source file)
are appropriately displayed right-to-left.  We call such mixtures of
left-to-right and right-to-left text “bidirectional text”.  This section
describes the facilities and options for editing and displaying
bidirectional text.

   Text is stored in Emacs buffers and strings in “logical” (or
“reading”) order, i.e., the order in which a human would read each
character.  In right-to-left and bidirectional text, the order in which
characters are displayed on the screen (called “visual order”) is not
the same as logical order; the characters’ screen positions do not
increase monotonically with string or buffer position.  In performing
this “bidirectional reordering”, Emacs follows the Unicode Bidirectional
Algorithm (a.k.a. UBA), which is described in Annex #9 of the Unicode
standard (<http://www.unicode.org/reports/tr9/>).  Emacs provides a
“Full Bidirectionality” class implementation of the UBA, consistent with
the requirements of the Unicode Standard v8.0.

 -- Variable: bidi-display-reordering
     If the value of this buffer-local variable is non-‘nil’ (the
     default), Emacs performs bidirectional reordering for display.  The
     reordering affects buffer text, as well as display strings and
     overlay strings from text and overlay properties in the buffer
     (*note Overlay Properties::, and *note Display Property::).  If the
     value is ‘nil’, Emacs does not perform bidirectional reordering in
     the buffer.

     The default value of ‘bidi-display-reordering’ controls the
     reordering of strings which are not directly supplied by a buffer,
     including the text displayed in mode lines (*note Mode Line
     Format::) and header lines (*note Header Lines::).

   Emacs never reorders the text of a unibyte buffer, even if
‘bidi-display-reordering’ is non-‘nil’ in the buffer.  This is because
unibyte buffers contain raw bytes, not characters, and thus lack the
directionality properties required for reordering.  Therefore, to test
whether text in a buffer will be reordered for display, it is not enough
to test the value of ‘bidi-display-reordering’ alone.  The correct test
is this:

      (if (and enable-multibyte-characters
               bidi-display-reordering)
          ;; Buffer is being reordered for display
        )

   However, unibyte display and overlay strings _are_ reordered if their
parent buffer is reordered.  This is because plain-ASCII strings are
stored by Emacs as unibyte strings.  If a unibyte display or overlay
string includes non-ASCII characters, these characters are assumed to
have left-to-right direction.

   Text covered by ‘display’ text properties, by overlays with ‘display’
properties whose value is a string, and by any other properties that
replace buffer text, is treated as a single unit when it is reordered
for display.  That is, the entire chunk of text covered by these
properties is reordered together.  Moreover, the bidirectional
properties of the characters in such a chunk of text are ignored, and
Emacs reorders them as if they were replaced with a single character
‘U+FFFC’, known as the “Object Replacement Character”.  This means that
placing a display property over a portion of text may change the way
that the surrounding text is reordered for display.  To prevent this
unexpected effect, always place such properties on text whose
directionality is identical with text that surrounds it.

   Each paragraph of bidirectional text has a “base direction”, either
right-to-left or left-to-right.  Left-to-right paragraphs are displayed
beginning at the left margin of the window, and are truncated or
continued when the text reaches the right margin.  Right-to-left
paragraphs are displayed beginning at the right margin, and are
continued or truncated at the left margin.

   By default, Emacs determines the base direction of each paragraph by
looking at the text at its beginning.  The precise method of determining
the base direction is specified by the UBA; in a nutshell, the first
character in a paragraph that has an explicit directionality determines
the base direction of the paragraph.  However, sometimes a buffer may
need to force a certain base direction for its paragraphs.  For example,
buffers containing program source code should force all paragraphs to be
displayed left-to-right.  You can use following variable to do this:

 -- Variable: bidi-paragraph-direction
     If the value of this buffer-local variable is the symbol
     ‘right-to-left’ or ‘left-to-right’, all paragraphs in the buffer
     are assumed to have that specified direction.  Any other value is
     equivalent to ‘nil’ (the default), which means to determine the
     base direction of each paragraph from its contents.

     Modes for program source code should set this to ‘left-to-right’.
     Prog mode does this by default, so modes derived from Prog mode do
     not need to set this explicitly (*note Basic Major Modes::).

 -- Function: current-bidi-paragraph-direction &optional buffer
     This function returns the paragraph direction at point in the named
     BUFFER.  The returned value is a symbol, either ‘left-to-right’ or
     ‘right-to-left’.  If BUFFER is omitted or ‘nil’, it defaults to the
     current buffer.  If the buffer-local value of the variable
     ‘bidi-paragraph-direction’ is non-‘nil’, the returned value will be
     identical to that value; otherwise, the returned value reflects the
     paragraph direction determined dynamically by Emacs.  For buffers
     whose value of ‘bidi-display-reordering’ is ‘nil’ as well as
     unibyte buffers, this function always returns ‘left-to-right’.

   Sometimes there’s a need to move point in strict visual order, either
to the left or to the right of its current screen position.  Emacs
provides a primitive to do that.

 -- Function: move-point-visually direction
     This function moves point of the currently selected window to the
     buffer position that appears immediately to the right or to the
     left of point on the screen.  If DIRECTION is positive, point will
     move one screen position to the right, otherwise it will move one
     screen position to the left.  Note that, depending on the
     surrounding bidirectional context, this could potentially move
     point many buffer positions away.  If invoked at the end of a
     screen line, the function moves point to the rightmost or leftmost
     screen position of the next or previous screen line, as appropriate
     for the value of DIRECTION.

     The function returns the new buffer position as its value.

   Bidirectional reordering can have surprising and unpleasant effects
when two strings with bidirectional content are juxtaposed in a buffer,
or otherwise programmatically concatenated into a string of text.  A
typical problematic case is when a buffer consists of sequences of text
fields separated by whitespace or punctuation characters, like Buffer
Menu mode or Rmail Summary Mode.  Because the punctuation characters
used as separators have “weak directionality”, they take on the
directionality of surrounding text.  As result, a numeric field that
follows a field with bidirectional content can be displayed _to the
left_ of the preceding field, messing up the expected layout.  There are
several ways to avoid this problem:

   − Append the special character ‘U+200E’, LEFT-TO-RIGHT MARK, or LRM,
     to the end of each field that may have bidirectional content, or
     prepend it to the beginning of the following field.  The function
     ‘bidi-string-mark-left-to-right’, described below, comes in handy
     for this purpose.  (In a right-to-left paragraph, use ‘U+200F’,
     RIGHT-TO-LEFT MARK, or RLM, instead.)  This is one of the solutions
     recommended by the UBA.

   − Include the tab character in the field separator.  The tab
     character plays the role of “segment separator” in bidirectional
     reordering, causing the text on either side to be reordered
     separately.

   − Separate fields with a ‘display’ property or overlay with a
     property value of the form ‘(space . PROPS)’ (*note Specified
     Space::).  Emacs treats this display specification as a “paragraph
     separator”, and reorders the text on either side separately.

 -- Function: bidi-string-mark-left-to-right string
     This function returns its argument STRING, possibly modified, such
     that the result can be safely concatenated with another string, or
     juxtaposed with another string in a buffer, without disrupting the
     relative layout of this string and the next one on display.  If the
     string returned by this function is displayed as part of a
     left-to-right paragraph, it will always appear on display to the
     left of the text that follows it.  The function works by examining
     the characters of its argument, and if any of those characters
     could cause reordering on display, the function appends the LRM
     character to the string.  The appended LRM character is made
     invisible by giving it an ‘invisible’ text property of ‘t’ (*note
     Invisible Text::).

   The reordering algorithm uses the bidirectional properties of the
characters stored as their ‘bidi-class’ property (*note Character
Properties::).  Lisp programs can change these properties by calling the
‘put-char-code-property’ function.  However, doing this requires a
thorough understanding of the UBA, and is therefore not recommended.
Any changes to the bidirectional properties of a character have global
effect: they affect all Emacs frames and windows.

   Similarly, the ‘mirroring’ property is used to display the
appropriate mirrored character in the reordered text.  Lisp programs can
affect the mirrored display by changing this property.  Again, any such
changes affect all of Emacs display.

   The bidirectional properties of characters can be overridden by
inserting into the text special directional control characters,
LEFT-TO-RIGHT OVERRIDE (LRO) and RIGHT-TO-LEFT OVERRIDE (RLO).  Any
characters between a RLO and the following newline or POP DIRECTIONAL
FORMATTING (PDF) control character, whichever comes first, will be
displayed as if they were strong right-to-left characters, i.e. they
will be reversed on display.  Similarly, any characters between LRO and
PDF or newline will display as if they were strong left-to-right, and
will _not_ be reversed even if they are strong right-to-left characters.

   These overrides are useful when you want to make some text unaffected
by the reordering algorithm, and instead directly control the display
order.  But they can also be used for malicious purposes, known as
“phishing”.  Specifically, a URL on a Web page or a link in an email
message can be manipulated to make its visual appearance unrecognizable,
or similar to some popular benign location, while the real location,
interpreted by a browser in the logical order, is very different.

   Emacs provides a primitive that applications can use to detect
instances of text whose bidirectional properties were overridden so as
to make a left-to-right character display as if it were a right-to-left
character, or vise versa.

 -- Function: bidi-find-overridden-directionality from to &optional
          object
     This function looks at the text of the specified OBJECT between
     positions FROM (inclusive) and TO (exclusive), and returns the
     first position where it finds a strong left-to-right character
     whose directional properties were forced to display the character
     as right-to-left, or for a strong right-to-left character that was
     forced to display as left-to-right.  If it finds no such characters
     in the specified region of text, it returns ‘nil’.

     The optional argument OBJECT specifies which text to search, and
     defaults to the current buffer.  If OBJECT is non-‘nil’, it can be
     some other buffer, or it can be a string or a window.  If it is a
     string, the function searches that string.  If it is a window, the
     function searches the buffer displayed in that window.  If a buffer
     whose text you want to examine is displayed in some window, we
     recommend to specify it by that window, rather than pass the buffer
     to the function.  This is because telling the function about the
     window allows it to correctly account for window-specific overlays,
     which might change the result of the function if some text in the
     buffer is covered by overlays.

   When text that includes mixed right-to-left and left-to-right
characters and bidirectional controls is copied into a different
location, it can change its visual appearance, and also can affect the
visual appearance of the surrounding text at destination.  This is
because reordering of bidirectional text specified by the UBA has
non-trivial context-dependent effects both on the copied text and on the
text at copy destination that will surround it.

   Sometimes, a Lisp program may need to preserve the exact visual
appearance of the copied text at destination, and of the text that
surrounds the copy.  Lisp programs can use the following function to
achieve that effect.

 -- Function: buffer-substring-with-bidi-context start end &optional
          no-properties
     This function works similar to ‘buffer-substring’ (*note Buffer
     Contents::), but it prepends and appends to the copied text bidi
     directional control characters necessary to preserve the visual
     appearance of the text when it is inserted at another place.
     Optional argument NO-PROPERTIES, if non-‘nil’, means remove the
     text properties from the copy of the text.


File: elisp.info,  Node: System Interface,  Next: Packaging,  Prev: Display,  Up: Top

38 Operating System Interface
*****************************

This chapter is about starting and getting out of Emacs, access to
values in the operating system environment, and terminal input, output.

   *Note Building Emacs::, for related information.  *Note Display::,
for additional operating system status information pertaining to the
terminal and the screen.

* Menu:

* Starting Up::         Customizing Emacs startup processing.
* Getting Out::         How exiting works (permanent or temporary).
* System Environment::  Distinguish the name and kind of system.
* User Identification:: Finding the name and user id of the user.
* Time of Day::         Getting the current time.
* Time Zone Rules::     Rules for time zones and daylight saving time.
* Time Conversion::     Converting a time from numeric form to
                          calendrical data and vice versa.
* Time Parsing::        Converting a time from numeric form to text
                          and vice versa.
* Processor Run Time::  Getting the run time used by Emacs.
* Time Calculations::   Adding, subtracting, comparing times, etc.
* Timers::              Setting a timer to call a function at a certain time.
* Idle Timers::         Setting a timer to call a function when Emacs has
                          been idle for a certain length of time.
* Terminal Input::      Accessing and recording terminal input.
* Terminal Output::     Controlling and recording terminal output.
* Sound Output::        Playing sounds on the computer’s speaker.
* X11 Keysyms::         Operating on key symbols for X Windows.
* Batch Mode::          Running Emacs without terminal interaction.
* Session Management::  Saving and restoring state with X Session Management.
* Desktop Notifications:: Desktop notifications.
* File Notifications::  File notifications.
* Dynamic Libraries::   On-demand loading of support libraries.
* Security Considerations:: Running Emacs in an unfriendly environment.


File: elisp.info,  Node: Starting Up,  Next: Getting Out,  Up: System Interface

38.1 Starting Up Emacs
======================

This section describes what Emacs does when it is started, and how you
can customize these actions.

* Menu:

* Startup Summary::         Sequence of actions Emacs performs at startup.
* Init File::               Details on reading the init file.
* Terminal-Specific::       How the terminal-specific Lisp file is read.
* Command-Line Arguments::  How command-line arguments are processed,
                              and how you can customize them.


File: elisp.info,  Node: Startup Summary,  Next: Init File,  Up: Starting Up

38.1.1 Summary: Sequence of Actions at Startup
----------------------------------------------

When Emacs is started up, it performs the following operations (see
‘normal-top-level’ in ‘startup.el’):

  1. It adds subdirectories to ‘load-path’, by running the file named
     ‘subdirs.el’ in each directory in the list.  Normally, this file
     adds the directory’s subdirectories to the list, and those are
     scanned in their turn.  The files ‘subdirs.el’ are normally
     generated automatically when Emacs is installed.

  2. It loads any ‘leim-list.el’ that it finds in the ‘load-path’
     directories.  This file is intended for registering input methods.
     The search is only for any personal ‘leim-list.el’ files that you
     may have created; it skips the directories containing the standard
     Emacs libraries (these should contain only a single ‘leim-list.el’
     file, which is compiled into the Emacs executable).

  3. It sets the variable ‘before-init-time’ to the value of
     ‘current-time’ (*note Time of Day::).  It also sets
     ‘after-init-time’ to ‘nil’, which signals to Lisp programs that
     Emacs is being initialized.

  4. It sets the language environment and the terminal coding system, if
     requested by environment variables such as ‘LANG’.

  5. It does some basic parsing of the command-line arguments.

  6. If not running in batch mode, it initializes the window system that
     the variable ‘initial-window-system’ specifies (*note
     initial-window-system: Window Systems.).  The initialization
     function for each supported window system is specified by
     ‘window-system-initialization-alist’.  If the value of
     ‘initial-window-system’ is WINDOWSYSTEM, then the appropriate
     initialization function is defined in the file
     ‘term/WINDOWSYSTEM-win.el’.  This file should have been compiled
     into the Emacs executable when it was built.

  7. It runs the normal hook ‘before-init-hook’.

  8. If appropriate, it creates a graphical frame.  This is not done if
     the options ‘--batch’ or ‘--daemon’ were specified.

  9. It initializes the initial frame’s faces, and sets up the menu bar
     and tool bar if needed.  If graphical frames are supported, it sets
     up the tool bar even if the current frame is not a graphical one,
     since a graphical frame may be created later on.

  10. It use ‘custom-reevaluate-setting’ to re-initialize the members of
     the list ‘custom-delayed-init-variables’.  These are any pre-loaded
     user options whose default value depends on the run-time, rather
     than build-time, context.  *Note custom-initialize-delay: Building
     Emacs.

  11. It loads the library ‘site-start’, if it exists.  This is not done
     if the options ‘-Q’ or ‘--no-site-file’ were specified.

  12. It loads your init file (*note Init File::).  This is not done if
     the options ‘-q’, ‘-Q’, or ‘--batch’ were specified.  If the ‘-u’
     option was specified, Emacs looks for the init file in that user’s
     home directory instead.

  13. It loads the library ‘default’, if it exists.  This is not done if
     ‘inhibit-default-init’ is non-‘nil’, nor if the options ‘-q’, ‘-Q’,
     or ‘--batch’ were specified.

  14. It loads your abbrevs from the file specified by
     ‘abbrev-file-name’, if that file exists and can be read (*note
     abbrev-file-name: Abbrev Files.).  This is not done if the option
     ‘--batch’ was specified.

  15. It calls the function ‘package-initialize’ to activate any
     optional Emacs Lisp package that has been installed.  *Note
     Packaging Basics::.  However, Emacs doesn’t initialize packages
     when ‘package-enable-at-startup’ is ‘nil’ or when it’s started with
     one of the options ‘-q’, ‘-Q’, or ‘--batch’.  To initialize
     packages in the latter case, ‘package-initialize’ should be called
     explicitly (e.g., via the ‘--funcall’ option).

  16. It sets the variable ‘after-init-time’ to the value of
     ‘current-time’.  This variable was set to ‘nil’ earlier; setting it
     to the current time signals that the initialization phase is over,
     and, together with ‘before-init-time’, provides the measurement of
     how long it took.

  17. It runs the normal hook ‘after-init-hook’.

  18. If the buffer ‘*scratch*’ exists and is still in Fundamental mode
     (as it should be by default), it sets its major mode according to
     ‘initial-major-mode’.

  19. If started on a text terminal, it loads the terminal-specific Lisp
     library (*note Terminal-Specific::), and runs the hook
     ‘tty-setup-hook’.  This is not done in ‘--batch’ mode, nor if
     ‘term-file-prefix’ is ‘nil’.

  20. It displays the initial echo area message, unless you have
     suppressed that with ‘inhibit-startup-echo-area-message’.

  21. It processes any command-line options that were not handled
     earlier.

  22. It now exits if the option ‘--batch’ was specified.

  23. If the ‘*scratch*’ buffer exists and is empty, it inserts
     ‘(substitute-command-keys initial-scratch-message)’ into that
     buffer.

  24. If ‘initial-buffer-choice’ is a string, it visits the file (or
     directory) with that name.  If it is a function, it calls the
     function with no arguments and selects the buffer that it returns.
     If one file is given as a command line argument, that file is
     visited and its buffer displayed alongside ‘initial-buffer-choice’.
     If more than one file is given, all of the files are visited and
     the ‘*Buffer List*’ buffer is displayed alongside
     ‘initial-buffer-choice’.

  25. It runs ‘emacs-startup-hook’.

  26. It calls ‘frame-notice-user-settings’, which modifies the
     parameters of the selected frame according to whatever the init
     files specify.

  27. It runs ‘window-setup-hook’.  The only difference between this
     hook and ‘emacs-startup-hook’ is that this one runs after the
     previously mentioned modifications to the frame parameters.

  28. It displays the “startup screen”, which is a special buffer that
     contains information about copyleft and basic Emacs usage.  This is
     not done if ‘inhibit-startup-screen’ or ‘initial-buffer-choice’ are
     non-‘nil’, or if the ‘--no-splash’ or ‘-Q’ command-line options
     were specified.

  29. If the option ‘--daemon’ was specified, it calls ‘server-start’,
     and on Posix systems also detaches from the controlling terminal.
     *Note (emacs)Emacs Server::.

  30. If started by the X session manager, it calls
     ‘emacs-session-restore’ passing it as argument the ID of the
     previous session.  *Note Session Management::.

The following options affect some aspects of the startup sequence.

 -- User Option: inhibit-startup-screen
     This variable, if non-‘nil’, inhibits the startup screen.  In that
     case, Emacs typically displays the ‘*scratch*’ buffer; but see
     ‘initial-buffer-choice’, below.

     Do not set this variable in the init file of a new user, or in a
     way that affects more than one user, as that would prevent new
     users from receiving information about copyleft and basic Emacs
     usage.

     ‘inhibit-startup-message’ and ‘inhibit-splash-screen’ are aliases
     for this variable.

 -- User Option: initial-buffer-choice
     If non-‘nil’, this variable is a string that specifies a file or
     directory for Emacs to display after starting up, instead of the
     startup screen.  If its value is a function, Emacs calls that
     function which must return a buffer which is then displayed.  If
     its value is ‘t’, Emacs displays the ‘*scratch*’ buffer.

 -- User Option: inhibit-startup-echo-area-message
     This variable controls the display of the startup echo area
     message.  You can suppress the startup echo area message by adding
     text with this form to your init file:

          (setq inhibit-startup-echo-area-message
                "YOUR-LOGIN-NAME")

     Emacs explicitly checks for an expression as shown above in your
     init file; your login name must appear in the expression as a Lisp
     string constant.  You can also use the Customize interface.  Other
     methods of setting ‘inhibit-startup-echo-area-message’ to the same
     value do not inhibit the startup message.  This way, you can easily
     inhibit the message for yourself if you wish, but thoughtless
     copying of your init file will not inhibit the message for someone
     else.

 -- User Option: initial-scratch-message
     This variable, if non-‘nil’, should be a string, which is treated
     as documentation to be inserted into the ‘*scratch*’ buffer when
     Emacs starts up.  If it is ‘nil’, the ‘*scratch*’ buffer is empty.

The following command-line options affect some aspects of the startup
sequence.  *Note (emacs)Initial Options::.

‘--no-splash’
     Do not display a splash screen.

‘--batch’
     Run without an interactive terminal.  *Note Batch Mode::.

‘--daemon’
     Do not initialize any display; just start a server in the
     background.

‘--no-init-file’
‘-q’
     Do not load either the init file, or the ‘default’ library.

‘--no-site-file’
     Do not load the ‘site-start’ library.

‘--quick’
‘-Q’
     Equivalent to ‘-q --no-site-file --no-splash’.


File: elisp.info,  Node: Init File,  Next: Terminal-Specific,  Prev: Startup Summary,  Up: Starting Up

38.1.2 The Init File
--------------------

When you start Emacs, it normally attempts to load your “init file”.
This is either a file named ‘.emacs’ or ‘.emacs.el’ in your home
directory, or a file named ‘init.el’ in a subdirectory named ‘.emacs.d’
in your home directory.

   The command-line switches ‘-q’, ‘-Q’, and ‘-u’ control whether and
where to find the init file; ‘-q’ (and the stronger ‘-Q’) says not to
load an init file, while ‘-u USER’ says to load USER’s init file instead
of yours.  *Note (emacs)Entering Emacs::.  If neither option is
specified, Emacs uses the ‘LOGNAME’ environment variable, or the ‘USER’
(most systems) or ‘USERNAME’ (MS systems) variable, to find your home
directory and thus your init file; this way, even if you have su’d,
Emacs still loads your own init file.  If those environment variables
are absent, though, Emacs uses your user-id to find your home directory.

   An Emacs installation may have a “default init file”, which is a Lisp
library named ‘default.el’.  Emacs finds this file through the standard
search path for libraries (*note How Programs Do Loading::).  The Emacs
distribution does not come with this file; it is intended for local
customizations.  If the default init file exists, it is loaded whenever
you start Emacs.  But your own personal init file, if any, is loaded
first; if it sets ‘inhibit-default-init’ to a non-‘nil’ value, then
Emacs does not subsequently load the ‘default.el’ file.  In batch mode,
or if you specify ‘-q’ (or ‘-Q’), Emacs loads neither your personal init
file nor the default init file.

   Another file for site-customization is ‘site-start.el’.  Emacs loads
this _before_ the user’s init file.  You can inhibit the loading of this
file with the option ‘--no-site-file’.

 -- User Option: site-run-file
     This variable specifies the site-customization file to load before
     the user’s init file.  Its normal value is ‘"site-start"’.  The
     only way you can change it with real effect is to do so before
     dumping Emacs.

   *Note Init File Examples: (emacs)Init Examples, for examples of how
to make various commonly desired customizations in your ‘.emacs’ file.

 -- User Option: inhibit-default-init
     If this variable is non-‘nil’, it prevents Emacs from loading the
     default initialization library file.  The default value is ‘nil’.

 -- Variable: before-init-hook
     This normal hook is run, once, just before loading all the init
     files (‘site-start.el’, your init file, and ‘default.el’).  (The
     only way to change it with real effect is before dumping Emacs.)

 -- Variable: after-init-hook
     This normal hook is run, once, just after loading all the init
     files (‘site-start.el’, your init file, and ‘default.el’), before
     loading the terminal-specific library (if started on a text
     terminal) and processing the command-line action arguments.

 -- Variable: emacs-startup-hook
     This normal hook is run, once, just after handling the command line
     arguments.  In batch mode, Emacs does not run this hook.

 -- Variable: window-setup-hook
     This normal hook is very similar to ‘emacs-startup-hook’.  The only
     difference is that it runs slightly later, after setting of the
     frame parameters.  *Note window-setup-hook: Startup Summary.

 -- Variable: user-init-file
     This variable holds the absolute file name of the user’s init file.
     If the actual init file loaded is a compiled file, such as
     ‘.emacs.elc’, the value refers to the corresponding source file.

 -- Variable: user-emacs-directory
     This variable holds the name of the ‘.emacs.d’ directory.  It is
     ‘~/.emacs.d’ on all platforms but MS-DOS.


File: elisp.info,  Node: Terminal-Specific,  Next: Command-Line Arguments,  Prev: Init File,  Up: Starting Up

38.1.3 Terminal-Specific Initialization
---------------------------------------

Each terminal type can have its own Lisp library that Emacs loads when
run on that type of terminal.  The library’s name is constructed by
concatenating the value of the variable ‘term-file-prefix’ and the
terminal type (specified by the environment variable ‘TERM’).  Normally,
‘term-file-prefix’ has the value ‘"term/"’; changing this is not
recommended.  If there is an entry matching ‘TERM’ in the
‘term-file-aliases’ association list, Emacs uses the associated value in
place of ‘TERM’.  Emacs finds the file in the normal manner, by
searching the ‘load-path’ directories, and trying the ‘.elc’ and ‘.el’
suffixes.

   The usual role of a terminal-specific library is to enable special
keys to send sequences that Emacs can recognize.  It may also need to
set or add to ‘input-decode-map’ if the Termcap or Terminfo entry does
not specify all the terminal’s function keys.  *Note Terminal Input::.

   When the name of the terminal type contains a hyphen or underscore,
and no library is found whose name is identical to the terminal’s name,
Emacs strips from the terminal’s name the last hyphen or underscore and
everything that follows it, and tries again.  This process is repeated
until Emacs finds a matching library, or until there are no more hyphens
or underscores in the name (i.e., there is no terminal-specific
library).  For example, if the terminal name is ‘xterm-256color’ and
there is no ‘term/xterm-256color.el’ library, Emacs tries to load
‘term/xterm.el’.  If necessary, the terminal library can evaluate
‘(getenv "TERM")’ to find the full name of the terminal type.

   Your init file can prevent the loading of the terminal-specific
library by setting the variable ‘term-file-prefix’ to ‘nil’.

   You can also arrange to override some of the actions of the
terminal-specific library by using ‘tty-setup-hook’.  This is a normal
hook that Emacs runs after initializing a new text terminal.  You could
use this hook to define initializations for terminals that do not have
their own libraries.  *Note Hooks::.

 -- User Option: term-file-prefix
     If the value of this variable is non-‘nil’, Emacs loads a
     terminal-specific initialization file as follows:

          (load (concat term-file-prefix (getenv "TERM")))

     You may set the ‘term-file-prefix’ variable to ‘nil’ in your init
     file if you do not wish to load the terminal-initialization file.

     On MS-DOS, Emacs sets the ‘TERM’ environment variable to
     ‘internal’.

 -- User Option: term-file-aliases
     This variable is an an association list mapping terminal types to
     their aliases.  For example, an element of the form ‘("vt102" .
     "vt100")’ means to treat a terminal of type ‘vt102’ like one of
     type ‘vt100’.

 -- Variable: tty-setup-hook
     This variable is a normal hook that Emacs runs after initializing a
     new text terminal.  (This applies when Emacs starts up in
     non-windowed mode, and when making a tty ‘emacsclient’ connection.)
     The hook runs after loading your init file (if applicable) and the
     terminal-specific Lisp file, so you can use it to adjust the
     definitions made by that file.

     For a related feature, *note window-setup-hook: Init File.


File: elisp.info,  Node: Command-Line Arguments,  Prev: Terminal-Specific,  Up: Starting Up

38.1.4 Command-Line Arguments
-----------------------------

You can use command-line arguments to request various actions when you
start Emacs.  Note that the recommended way of using Emacs is to start
it just once, after logging in, and then do all editing in the same
Emacs session (*note (emacs)Entering Emacs::).  For this reason, you
might not use command-line arguments very often; nonetheless, they can
be useful when invoking Emacs from session scripts or debugging Emacs.
This section describes how Emacs processes command-line arguments.

 -- Function: command-line
     This function parses the command line that Emacs was called with,
     processes it, and (amongst other things) loads the user’s init file
     and displays the startup messages.

 -- Variable: command-line-processed
     The value of this variable is ‘t’ once the command line has been
     processed.

     If you redump Emacs by calling ‘dump-emacs’ (*note Building
     Emacs::), you may wish to set this variable to ‘nil’ first in order
     to cause the new dumped Emacs to process its new command-line
     arguments.

 -- Variable: command-switch-alist
     This variable is an alist of user-defined command-line options and
     associated handler functions.  By default it is empty, but you can
     add elements if you wish.

     A “command-line option” is an argument on the command line, which
     has the form:

          -OPTION

     The elements of the ‘command-switch-alist’ look like this:

          (OPTION . HANDLER-FUNCTION)

     The CAR, OPTION, is a string, the name of a command-line option
     (not including the initial hyphen).  The HANDLER-FUNCTION is called
     to handle OPTION, and receives the option name as its sole
     argument.

     In some cases, the option is followed in the command line by an
     argument.  In these cases, the HANDLER-FUNCTION can find all the
     remaining command-line arguments in the variable
     ‘command-line-args-left’ (see below).  (The entire list of
     command-line arguments is in ‘command-line-args’.)

     The command-line arguments are parsed by the ‘command-line-1’
     function in the ‘startup.el’ file.  See also *note Command Line
     Arguments for Emacs Invocation: (emacs)Emacs Invocation.

 -- Variable: command-line-args
     The value of this variable is the list of command-line arguments
     passed to Emacs.

 -- Variable: command-line-args-left
     The value of this variable is the list of command-line arguments
     that have not yet been processed.

 -- Variable: command-line-functions
     This variable’s value is a list of functions for handling an
     unrecognized command-line argument.  Each time the next argument to
     be processed has no special meaning, the functions in this list are
     called, in order of appearance, until one of them returns a
     non-‘nil’ value.

     These functions are called with no arguments.  They can access the
     command-line argument under consideration through the variable
     ‘argi’, which is bound temporarily at this point.  The remaining
     arguments (not including the current one) are in the variable
     ‘command-line-args-left’.

     When a function recognizes and processes the argument in ‘argi’, it
     should return a non-‘nil’ value to say it has dealt with that
     argument.  If it has also dealt with some of the following
     arguments, it can indicate that by deleting them from
     ‘command-line-args-left’.

     If all of these functions return ‘nil’, then the argument is
     treated as a file name to visit.


File: elisp.info,  Node: Getting Out,  Next: System Environment,  Prev: Starting Up,  Up: System Interface

38.2 Getting Out of Emacs
=========================

There are two ways to get out of Emacs: you can kill the Emacs job,
which exits permanently, or you can suspend it, which permits you to
reenter the Emacs process later.  (In a graphical environment, you can
of course simply switch to another application without doing anything
special to Emacs, then switch back to Emacs when you want.)

* Menu:

* Killing Emacs::        Exiting Emacs irreversibly.
* Suspending Emacs::     Exiting Emacs reversibly.


File: elisp.info,  Node: Killing Emacs,  Next: Suspending Emacs,  Up: Getting Out

38.2.1 Killing Emacs
--------------------

Killing Emacs means ending the execution of the Emacs process.  If you
started Emacs from a terminal, the parent process normally resumes
control.  The low-level primitive for killing Emacs is ‘kill-emacs’.

 -- Command: kill-emacs &optional exit-data
     This command calls the hook ‘kill-emacs-hook’, then exits the Emacs
     process and kills it.

     If EXIT-DATA is an integer, that is used as the exit status of the
     Emacs process.  (This is useful primarily in batch operation; see
     *note Batch Mode::.)

     If EXIT-DATA is a string, its contents are stuffed into the
     terminal input buffer so that the shell (or whatever program next
     reads input) can read them.

   The ‘kill-emacs’ function is normally called via the higher-level
command ‘C-x C-c’ (‘save-buffers-kill-terminal’).  *Note
(emacs)Exiting::.  It is also called automatically if Emacs receives a
‘SIGTERM’ or ‘SIGHUP’ operating system signal (e.g., when the
controlling terminal is disconnected), or if it receives a ‘SIGINT’
signal while running in batch mode (*note Batch Mode::).

 -- Variable: kill-emacs-hook
     This normal hook is run by ‘kill-emacs’, before it kills Emacs.

     Because ‘kill-emacs’ can be called in situations where user
     interaction is impossible (e.g., when the terminal is
     disconnected), functions on this hook should not attempt to
     interact with the user.  If you want to interact with the user when
     Emacs is shutting down, use ‘kill-emacs-query-functions’, described
     below.

   When Emacs is killed, all the information in the Emacs process, aside
from files that have been saved, is lost.  Because killing Emacs
inadvertently can lose a lot of work, the ‘save-buffers-kill-terminal’
command queries for confirmation if you have buffers that need saving or
subprocesses that are running.  It also runs the abnormal hook
‘kill-emacs-query-functions’:

 -- Variable: kill-emacs-query-functions
     When ‘save-buffers-kill-terminal’ is killing Emacs, it calls the
     functions in this hook, after asking the standard questions and
     before calling ‘kill-emacs’.  The functions are called in order of
     appearance, with no arguments.  Each function can ask for
     additional confirmation from the user.  If any of them returns
     ‘nil’, ‘save-buffers-kill-emacs’ does not kill Emacs, and does not
     run the remaining functions in this hook.  Calling ‘kill-emacs’
     directly does not run this hook.


File: elisp.info,  Node: Suspending Emacs,  Prev: Killing Emacs,  Up: Getting Out

38.2.2 Suspending Emacs
-----------------------

On text terminals, it is possible to “suspend Emacs”, which means
stopping Emacs temporarily and returning control to its superior
process, which is usually the shell.  This allows you to resume editing
later in the same Emacs process, with the same buffers, the same kill
ring, the same undo history, and so on.  To resume Emacs, use the
appropriate command in the parent shell—most likely ‘fg’.

   Suspending works only on a terminal device from which the Emacs
session was started.  We call that device the “controlling terminal” of
the session.  Suspending is not allowed if the controlling terminal is a
graphical terminal.  Suspending is usually not relevant in graphical
environments, since you can simply switch to another application without
doing anything special to Emacs.

   Some operating systems (those without ‘SIGTSTP’, or MS-DOS) do not
support suspension of jobs; on these systems, suspension actually
creates a new shell temporarily as a subprocess of Emacs.  Then you
would exit the shell to return to Emacs.

 -- Command: suspend-emacs &optional string
     This function stops Emacs and returns control to the superior
     process.  If and when the superior process resumes Emacs,
     ‘suspend-emacs’ returns ‘nil’ to its caller in Lisp.

     This function works only on the controlling terminal of the Emacs
     session; to relinquish control of other tty devices, use
     ‘suspend-tty’ (see below).  If the Emacs session uses more than one
     terminal, you must delete the frames on all the other terminals
     before suspending Emacs, or this function signals an error.  *Note
     Multiple Terminals::.

     If STRING is non-‘nil’, its characters are sent to Emacs’s superior
     shell, to be read as terminal input.  The characters in STRING are
     not echoed by the superior shell; only the results appear.

     Before suspending, ‘suspend-emacs’ runs the normal hook
     ‘suspend-hook’.  After the user resumes Emacs, ‘suspend-emacs’ runs
     the normal hook ‘suspend-resume-hook’.  *Note Hooks::.

     The next redisplay after resumption will redraw the entire screen,
     unless the variable ‘no-redraw-on-reenter’ is non-‘nil’.  *Note
     Refresh Screen::.

     Here is an example of how you could use these hooks:

          (add-hook 'suspend-hook
                    (lambda () (or (y-or-n-p "Really suspend? ")
                                   (error "Suspend canceled"))))
          (add-hook 'suspend-resume-hook (lambda () (message "Resumed!")
                                           (sit-for 2)))

     Here is what you would see upon evaluating ‘(suspend-emacs "pwd")’:

          ---------- Buffer: Minibuffer ----------
          Really suspend? y
          ---------- Buffer: Minibuffer ----------

          ---------- Parent Shell ----------
          bash$ /home/username
          bash$ fg

          ---------- Echo Area ----------
          Resumed!

     Note that ‘pwd’ is not echoed after Emacs is suspended.  But it is
     read and executed by the shell.

 -- Variable: suspend-hook
     This variable is a normal hook that Emacs runs before suspending.

 -- Variable: suspend-resume-hook
     This variable is a normal hook that Emacs runs on resuming after a
     suspension.

 -- Function: suspend-tty &optional tty
     If TTY specifies a terminal device used by Emacs, this function
     relinquishes the device and restores it to its prior state.  Frames
     that used the device continue to exist, but are not updated and
     Emacs doesn’t read input from them.  TTY can be a terminal object,
     a frame (meaning the terminal for that frame), or ‘nil’ (meaning
     the terminal for the selected frame).  *Note Multiple Terminals::.

     If TTY is already suspended, this function does nothing.

     This function runs the hook ‘suspend-tty-functions’, passing the
     terminal object as an argument to each function.

 -- Function: resume-tty &optional tty
     This function resumes the previously suspended terminal device TTY;
     where TTY has the same possible values as it does for
     ‘suspend-tty’.

     This function reopens the terminal device, re-initializes it, and
     redraws it with that terminal’s selected frame.  It then runs the
     hook ‘resume-tty-functions’, passing the terminal object as an
     argument to each function.

     If the same device is already used by another Emacs terminal, this
     function signals an error.  If TTY is not suspended, this function
     does nothing.

 -- Function: controlling-tty-p &optional tty
     This function returns non-‘nil’ if TTY is the controlling terminal
     of the Emacs session; TTY can be a terminal object, a frame
     (meaning the terminal for that frame), or ‘nil’ (meaning the
     terminal for the selected frame).

 -- Command: suspend-frame
     This command “suspends” a frame.  For GUI frames, it calls
     ‘iconify-frame’ (*note Visibility of Frames::); for frames on text
     terminals, it calls either ‘suspend-emacs’ or ‘suspend-tty’,
     depending on whether the frame is displayed on the controlling
     terminal device or not.


File: elisp.info,  Node: System Environment,  Next: User Identification,  Prev: Getting Out,  Up: System Interface

38.3 Operating System Environment
=================================

Emacs provides access to variables in the operating system environment
through various functions.  These variables include the name of the
system, the user’s UID, and so on.

 -- Variable: system-configuration
     This variable holds the standard GNU configuration name for the
     hardware/software configuration of your system, as a string.  For
     example, a typical value for a 64-bit GNU/Linux system is
     ‘"x86_64-unknown-linux-gnu"’.

 -- Variable: system-type
     The value of this variable is a symbol indicating the type of
     operating system Emacs is running on.  The possible values are:

     ‘aix’
          IBM’s AIX.

     ‘berkeley-unix’
          Berkeley BSD and its variants.

     ‘cygwin’
          Cygwin, a Posix layer on top of MS-Windows.

     ‘darwin’
          Darwin (macOS).

     ‘gnu’
          The GNU system (using the GNU kernel, which consists of the
          HURD and Mach).

     ‘gnu/linux’
          A GNU/Linux system—that is, a variant GNU system, using the
          Linux kernel.  (These systems are the ones people often call
          “Linux”, but actually Linux is just the kernel, not the whole
          system.)

     ‘gnu/kfreebsd’
          A GNU (glibc-based) system with a FreeBSD kernel.

     ‘hpux’
          Hewlett-Packard HPUX operating system.

     ‘irix’
          Silicon Graphics Irix system.

     ‘nacl’
          Google Native Client (NaCl) sandboxing system.

     ‘ms-dos’
          Microsoft’s DOS.  Emacs compiled with DJGPP for MS-DOS binds
          ‘system-type’ to ‘ms-dos’ even when you run it on MS-Windows.

     ‘usg-unix-v’
          AT&T Unix System V.

     ‘windows-nt’
          Microsoft Windows NT, 9X and later.  The value of
          ‘system-type’ is always ‘windows-nt’, e.g., even on Windows
          10.

     We do not wish to add new symbols to make finer distinctions unless
     it is absolutely necessary!  In fact, we hope to eliminate some of
     these alternatives in the future.  If you need to make a finer
     distinction than ‘system-type’ allows for, you can test
     ‘system-configuration’, e.g., against a regexp.

 -- Function: system-name
     This function returns the name of the machine you are running on,
     as a string.

 -- User Option: mail-host-address
     If this variable is non-‘nil’, it is used instead of ‘system-name’
     for purposes of generating email addresses.  For example, it is
     used when constructing the default value of ‘user-mail-address’.
     *Note User Identification::.  (Since this is done when Emacs starts
     up, the value actually used is the one saved when Emacs was dumped.
     *Note Building Emacs::.)

 -- Command: getenv var &optional frame
     This function returns the value of the environment variable VAR, as
     a string.  VAR should be a string.  If VAR is undefined in the
     environment, ‘getenv’ returns ‘nil’.  It returns ‘""’ if VAR is set
     but null.  Within Emacs, a list of environment variables and their
     values is kept in the variable ‘process-environment’.

          (getenv "USER")
               ⇒ "lewis"

     The shell command ‘printenv’ prints all or part of the environment:

          bash$ printenv
          PATH=/usr/local/bin:/usr/bin:/bin
          USER=lewis
          TERM=xterm
          SHELL=/bin/bash
          HOME=/home/lewis
          ...

 -- Command: setenv variable &optional value substitute
     This command sets the value of the environment variable named
     VARIABLE to VALUE.  VARIABLE should be a string.  Internally, Emacs
     Lisp can handle any string.  However, normally VARIABLE should be a
     valid shell identifier, that is, a sequence of letters, digits and
     underscores, starting with a letter or underscore.  Otherwise,
     errors may occur if subprocesses of Emacs try to access the value
     of VARIABLE.  If VALUE is omitted or ‘nil’ (or, interactively, with
     a prefix argument), ‘setenv’ removes VARIABLE from the environment.
     Otherwise, VALUE should be a string.

     If the optional argument SUBSTITUTE is non-‘nil’, Emacs calls the
     function ‘substitute-env-vars’ to expand any environment variables
     in VALUE.

     ‘setenv’ works by modifying ‘process-environment’; binding that
     variable with ‘let’ is also reasonable practice.

     ‘setenv’ returns the new value of VARIABLE, or ‘nil’ if it removed
     VARIABLE from the environment.

 -- Variable: process-environment
     This variable is a list of strings, each describing one environment
     variable.  The functions ‘getenv’ and ‘setenv’ work by means of
     this variable.

          process-environment
          ⇒ ("PATH=/usr/local/bin:/usr/bin:/bin"
              "USER=lewis"
              "TERM=xterm"
              "SHELL=/bin/bash"
              "HOME=/home/lewis"
              ...)

     If ‘process-environment’ contains multiple elements that specify
     the same environment variable, the first of these elements
     specifies the variable, and the others are ignored.

 -- Variable: initial-environment
     This variable holds the list of environment variables Emacs
     inherited from its parent process when Emacs started.

 -- Variable: path-separator
     This variable holds a string that says which character separates
     directories in a search path (as found in an environment variable).
     Its value is ‘":"’ for Unix and GNU systems, and ‘";"’ for MS
     systems.

 -- Function: parse-colon-path path
     This function takes a search path string such as the value of the
     ‘PATH’ environment variable, and splits it at the separators,
     returning a list of directory names.  ‘nil’ in this list means the
     current directory.  Although the function’s name says “colon”, it
     actually uses the value of ‘path-separator’.

          (parse-colon-path ":/foo:/bar")
               ⇒ (nil "/foo/" "/bar/")

 -- Variable: invocation-name
     This variable holds the program name under which Emacs was invoked.
     The value is a string, and does not include a directory name.

 -- Variable: invocation-directory
     This variable holds the directory in which the Emacs executable was
     located when it was run, or ‘nil’ if that directory cannot be
     determined.

 -- Variable: installation-directory
     If non-‘nil’, this is a directory within which to look for the
     ‘lib-src’ and ‘etc’ subdirectories.  In an installed Emacs, it is
     normally ‘nil’.  It is non-‘nil’ when Emacs can’t find those
     directories in their standard installed locations, but can find
     them in a directory related somehow to the one containing the Emacs
     executable (i.e., ‘invocation-directory’).

 -- Function: load-average &optional use-float
     This function returns the current 1-minute, 5-minute, and 15-minute
     system load averages, in a list.  The load average indicates the
     number of processes trying to run on the system.

     By default, the values are integers that are 100 times the system
     load averages, but if USE-FLOAT is non-‘nil’, then they are
     returned as floating-point numbers without multiplying by 100.

     If it is impossible to obtain the load average, this function
     signals an error.  On some platforms, access to load averages
     requires installing Emacs as setuid or setgid so that it can read
     kernel information, and that usually isn’t advisable.

     If the 1-minute load average is available, but the 5- or 15-minute
     averages are not, this function returns a shortened list containing
     the available averages.

          (load-average)
               ⇒ (169 48 36)
          (load-average t)
               ⇒ (1.69 0.48 0.36)

     The shell command ‘uptime’ returns similar information.

 -- Function: emacs-pid
     This function returns the process ID of the Emacs process, as an
     integer.

 -- Variable: tty-erase-char
     This variable holds the erase character that was selected in the
     system’s terminal driver, before Emacs was started.


File: elisp.info,  Node: User Identification,  Next: Time of Day,  Prev: System Environment,  Up: System Interface

38.4 User Identification
========================

 -- Variable: init-file-user
     This variable says which user’s init files should be used by
     Emacs—or ‘nil’ if none.  ‘""’ stands for the user who originally
     logged in.  The value reflects command-line options such as ‘-q’ or
     ‘-u USER’.

     Lisp packages that load files of customizations, or any other sort
     of user profile, should obey this variable in deciding where to
     find it.  They should load the profile of the user name found in
     this variable.  If ‘init-file-user’ is ‘nil’, meaning that the
     ‘-q’, ‘-Q’, or ‘-batch’ option was used, then Lisp packages should
     not load any customization files or user profile.

 -- User Option: user-mail-address
     This holds the nominal email address of the user who is using
     Emacs.  Emacs normally sets this variable to a default value after
     reading your init files, but not if you have already set it.  So
     you can set the variable to some other value in your init file if
     you do not want to use the default value.

 -- Function: user-login-name &optional uid
     This function returns the name under which the user is logged in.
     It uses the environment variables ‘LOGNAME’ or ‘USER’ if either is
     set.  Otherwise, the value is based on the effective UID, not the
     real UID.

     If you specify UID (a number), the result is the user name that
     corresponds to UID, or ‘nil’ if there is no such user.

 -- Function: user-real-login-name
     This function returns the user name corresponding to Emacs’s real
     UID.  This ignores the effective UID, and the environment variables
     ‘LOGNAME’ and ‘USER’.

 -- Function: user-full-name &optional uid
     This function returns the full name of the logged-in user—or the
     value of the environment variable ‘NAME’, if that is set.

     If the Emacs process’s user-id does not correspond to any known
     user (and provided ‘NAME’ is not set), the result is ‘"unknown"’.

     If UID is non-‘nil’, then it should be a number (a user-id) or a
     string (a login name).  Then ‘user-full-name’ returns the full name
     corresponding to that user-id or login name.  If you specify a
     user-id or login name that isn’t defined, it returns ‘nil’.

   The symbols ‘user-login-name’, ‘user-real-login-name’ and
‘user-full-name’ are variables as well as functions.  The functions
return the same values that the variables hold.  These variables allow
you to fake out Emacs by telling the functions what to return.  The
variables are also useful for constructing frame titles (*note Frame
Titles::).

 -- Function: user-real-uid
     This function returns the real UID of the user.  The value may be
     floating point, in the (unlikely) event that the UID is too large
     to fit in a Lisp integer.

 -- Function: user-uid
     This function returns the effective UID of the user.  The value may
     be floating point.

 -- Function: group-gid
     This function returns the effective GID of the Emacs process.  The
     value may be floating point.

 -- Function: group-real-gid
     This function returns the real GID of the Emacs process.  The value
     may be floating point.

 -- Function: system-users
     This function returns a list of strings, listing the user names on
     the system.  If Emacs cannot retrieve this information, the return
     value is a list containing just the value of
     ‘user-real-login-name’.

 -- Function: system-groups
     This function returns a list of strings, listing the names of user
     groups on the system.  If Emacs cannot retrieve this information,
     the return value is ‘nil’.


File: elisp.info,  Node: Time of Day,  Next: Time Zone Rules,  Prev: User Identification,  Up: System Interface

38.5 Time of Day
================

This section explains how to determine the current time and time zone.

   Most of these functions represent time as a list of four integers
‘(SEC-HIGH SEC-LOW MICROSEC PICOSEC)’.  This represents the number of
seconds from the “epoch” (January 1, 1970 at 00:00 UTC), using the
formula: HIGH * 2**16 + LOW + MICRO * 10**−6 + PICO * 10**−12.  The
return value of ‘current-time’ represents time using this form, as do
the timestamps in the return values of other functions such as
‘file-attributes’ (*note Definition of file-attributes::).  In some
cases, functions may return two- or three-element lists, with omitted
MICROSEC and PICOSEC components defaulting to zero.

   Function arguments, e.g., the TIME argument to ‘current-time-string’,
accept a more-general “time value” format, which can be a list of
integers as above, or a single number for seconds since the epoch, or
‘nil’ for the current time.  You can convert a time value into a
human-readable string using ‘current-time-string’ and
‘format-time-string’, into a list of integers using ‘seconds-to-time’,
and into other forms using ‘decode-time’ and ‘float-time’.  These
functions are described in the following sections.

 -- Function: current-time-string &optional time zone
     This function returns the current time and date as a human-readable
     string.  The format does not vary for the initial part of the
     string, which contains the day of week, month, day of month, and
     time of day in that order: the number of characters used for these
     fields is always the same, so you can reliably use ‘substring’ to
     extract them.  You should count characters from the beginning of
     the string rather than from the end, as the year might not have
     exactly four digits, and additional information may some day be
     added at the end.

     The argument TIME, if given, specifies a time to format, instead of
     the current time.  The optional argument ZONE defaults to the
     current time zone rule.  *Note Time Zone Rules::.

          (current-time-string)
               ⇒ "Wed Oct 14 22:21:05 1987"

 -- Function: current-time
     This function returns the current time, represented as a list of
     four integers ‘(SEC-HIGH SEC-LOW MICROSEC PICOSEC)’.  These
     integers have trailing zeros on systems that return time with lower
     resolutions.  On all current machines PICOSEC is a multiple of
     1000, but this may change as higher-resolution clocks become
     available.

 -- Function: float-time &optional time
     This function returns the current time as a floating-point number
     of seconds since the epoch.  The optional argument TIME, if given,
     specifies a time to convert instead of the current time.

     _Warning_: Since the result is floating point, it may not be exact.
     Do not use this function if precise time stamps are required.

     ‘time-to-seconds’ is an alias for this function.

 -- Function: seconds-to-time time
     This function converts a time value to list-of-integer form.  For
     example, if TIME is a number, ‘(time-to-seconds (seconds-to-time
     TIME))’ equals the number unless overflow or rounding errors occur.


File: elisp.info,  Node: Time Zone Rules,  Next: Time Conversion,  Prev: Time of Day,  Up: System Interface

38.6 Time Zone Rules
====================

The default time zone is determined by the ‘TZ’ environment variable.
*Note System Environment::.  For example, you can tell Emacs to default
to Universal Time with ‘(setenv "TZ" "UTC0")’.  If ‘TZ’ is not in the
environment, Emacs uses system wall clock time, which is a
platform-dependent default time zone.

   The set of supported ‘TZ’ strings is system-dependent.  GNU and many
other systems support the tzdata database, e.g., ‘"America/New_York"’
specifies the time zone and daylight saving time history for locations
near New York City.  GNU and most other systems support POSIX-style ‘TZ’
strings, e.g., ‘"EST+5EDT,M4.1.0/2,M10.5.0/2"’ specifies the rules used
in New York from 1987 through 2006.  All systems support the string
‘"UTC0"’ meaning Universal Time.

   Functions that convert to and from local time accept an optional
“time zone rule” argument, which specifies the conversion’s time zone
and daylight saving time history.  If the time zone rule is omitted or
‘nil’, the conversion uses Emacs’s default time zone.  If it is ‘t’, the
conversion uses Universal Time.  If it is ‘wall’, the conversion uses
the system wall clock time.  If it is a string, the conversion uses the
time zone rule equivalent to setting ‘TZ’ to that string.

 -- Function: current-time-zone &optional time zone
     This function returns a list describing the time zone that the user
     is in.

     The value has the form ‘(OFFSET ABBR)’.  Here OFFSET is an integer
     giving the number of seconds ahead of Universal Time (east of
     Greenwich).  A negative value means west of Greenwich.  The second
     element, ABBR, is a string giving an abbreviation for the time
     zone, e.g., ‘"CST"’ for China Standard Time or for U.S. Central
     Standard Time.  Both elements can change when daylight saving time
     begins or ends; if the user has specified a time zone that does not
     use a seasonal time adjustment, then the value is constant through
     time.

     If the operating system doesn’t supply all the information
     necessary to compute the value, the unknown elements of the list
     are ‘nil’.

     The argument TIME, if given, specifies a time value to analyze
     instead of the current time.  The optional argument ZONE defaults
     to the current time zone rule.


File: elisp.info,  Node: Time Conversion,  Next: Time Parsing,  Prev: Time Zone Rules,  Up: System Interface

38.7 Time Conversion
====================

These functions convert time values (*note Time of Day::) into
calendrical information and vice versa.

   Many 32-bit operating systems are limited to system times containing
32 bits of information in their seconds component; these systems
typically handle only the times from 1901-12-13 20:45:52 through
2038-01-19 03:14:07 Universal Time.  However, 64-bit and some 32-bit
operating systems have larger seconds components, and can represent
times far in the past or future.

   Time conversion functions always use the Gregorian calendar, even for
dates before the Gregorian calendar was introduced.  Year numbers count
the number of years since the year 1 B.C., and do not skip zero as
traditional Gregorian years do; for example, the year number −37
represents the Gregorian year 38 B.C.

 -- Function: decode-time &optional time zone
     This function converts a time value into calendrical information.
     If you don’t specify TIME, it decodes the current time, and
     similarly ZONE defaults to the current time zone rule.  *Note Time
     Zone Rules::.  The return value is a list of nine elements, as
     follows:

          (SECONDS MINUTES HOUR DAY MONTH YEAR DOW DST UTCOFF)

     Here is what the elements mean:

     SECONDS
          The number of seconds past the minute, as an integer between 0
          and 59.  On some operating systems, this is 60 for leap
          seconds.
     MINUTES
          The number of minutes past the hour, as an integer between 0
          and 59.
     HOUR
          The hour of the day, as an integer between 0 and 23.
     DAY
          The day of the month, as an integer between 1 and 31.
     MONTH
          The month of the year, as an integer between 1 and 12.
     YEAR
          The year, an integer typically greater than 1900.
     DOW
          The day of week, as an integer between 0 and 6, where 0 stands
          for Sunday.
     DST
          ‘t’ if daylight saving time is effect, otherwise ‘nil’.
     UTCOFF
          An integer indicating the Universal Time offset in seconds,
          i.e., the number of seconds east of Greenwich.

     *Common Lisp Note:* Common Lisp has different meanings for DOW and
     UTCOFF.

 -- Function: encode-time seconds minutes hour day month year &optional
          zone
     This function is the inverse of ‘decode-time’.  It converts seven
     items of calendrical data into a list-of-integer time value.  For
     the meanings of the arguments, see the table above under
     ‘decode-time’.

     Year numbers less than 100 are not treated specially.  If you want
     them to stand for years above 1900, or years above 2000, you must
     alter them yourself before you call ‘encode-time’.

     The optional argument ZONE defaults to the current time zone rule.
     *Note Time Zone Rules::.  In addition to the usual time zone rule
     values, it can also be a list (as you would get from
     ‘current-time-zone’) or an integer (as from ‘decode-time’), applied
     without any further alteration for daylight saving time.

     If you pass more than seven arguments to ‘encode-time’, the first
     six are used as SECONDS through YEAR, the last argument is used as
     ZONE, and the arguments in between are ignored.  This feature makes
     it possible to use the elements of a list returned by ‘decode-time’
     as the arguments to ‘encode-time’, like this:

          (apply 'encode-time (decode-time ...))

     You can perform simple date arithmetic by using out-of-range values
     for the SECONDS, MINUTES, HOUR, DAY, and MONTH arguments; for
     example, day 0 means the day preceding the given month.

     The operating system puts limits on the range of possible time
     values; if you try to encode a time that is out of range, an error
     results.  For instance, years before 1970 do not work on some
     systems; on others, years as early as 1901 do work.


File: elisp.info,  Node: Time Parsing,  Next: Processor Run Time,  Prev: Time Conversion,  Up: System Interface

38.8 Parsing and Formatting Times
=================================

These functions convert time values to text in a string, and vice versa.
Time values are lists of two to four integers (*note Time of Day::).

 -- Function: date-to-time string
     This function parses the time-string STRING and returns the
     corresponding time value.

 -- Function: format-time-string format-string &optional time zone

     This function converts TIME (or the current time, if TIME is
     omitted or ‘nil’) to a string according to FORMAT-STRING.  The
     conversion uses the time zone rule ZONE, which defaults to the
     current time zone rule.  *Note Time Zone Rules::.  The argument
     FORMAT-STRING may contain ‘%’-sequences which say to substitute
     parts of the time.  Here is a table of what the ‘%’-sequences mean:

     ‘%a’
          This stands for the abbreviated name of the day of week.
     ‘%A’
          This stands for the full name of the day of week.
     ‘%b’
          This stands for the abbreviated name of the month.
     ‘%B’
          This stands for the full name of the month.
     ‘%c’
          This is a synonym for ‘%x %X’.
     ‘%C’
          This has a locale-specific meaning.  In the default locale
          (named C), it is equivalent to ‘%A, %B %e, %Y’.
     ‘%d’
          This stands for the day of month, zero-padded.
     ‘%D’
          This is a synonym for ‘%m/%d/%y’.
     ‘%e’
          This stands for the day of month, blank-padded.
     ‘%F’
          This stands for the ISO 8601 date format, i.e., ‘"%Y-%m-%d"’.
     ‘%g’
          This stands for the year corresponding to the ISO week within
          the century.
     ‘%G’
          This stands for the year corresponding to the ISO week.
     ‘%h’
          This is a synonym for ‘%b’.
     ‘%H’
          This stands for the hour (00–23).
     ‘%I’
          This stands for the hour (01–12).
     ‘%j’
          This stands for the day of the year (001–366).
     ‘%k’
          This stands for the hour (0–23), blank padded.
     ‘%l’
          This stands for the hour (1–12), blank padded.
     ‘%m’
          This stands for the month (01–12).
     ‘%M’
          This stands for the minute (00–59).
     ‘%n’
          This stands for a newline.
     ‘%N’
          This stands for the nanoseconds (000000000–999999999).  To ask
          for fewer digits, use ‘%3N’ for milliseconds, ‘%6N’ for
          microseconds, etc.  Any excess digits are discarded, without
          rounding.
     ‘%p’
          This stands for ‘AM’ or ‘PM’, as appropriate.
     ‘%r’
          This is a synonym for ‘%I:%M:%S %p’.
     ‘%R’
          This is a synonym for ‘%H:%M’.
     ‘%S’
          This stands for the seconds (00–59).
     ‘%t’
          This stands for a tab character.
     ‘%T’
          This is a synonym for ‘%H:%M:%S’.
     ‘%u’
          This stands for the numeric day of week (1–7).  Monday is day
          1.
     ‘%U’
          This stands for the week of the year (01–52), assuming that
          weeks start on Sunday.
     ‘%V’
          This stands for the week of the year according to ISO 8601.
     ‘%w’
          This stands for the numeric day of week (0–6).  Sunday is day
          0.
     ‘%W’
          This stands for the week of the year (01–52), assuming that
          weeks start on Monday.
     ‘%x’
          This has a locale-specific meaning.  In the default locale
          (named ‘C’), it is equivalent to ‘%D’.
     ‘%X’
          This has a locale-specific meaning.  In the default locale
          (named ‘C’), it is equivalent to ‘%T’.
     ‘%y’
          This stands for the year without century (00–99).
     ‘%Y’
          This stands for the year with century.
     ‘%Z’
          This stands for the time zone abbreviation (e.g., ‘EST’).
     ‘%z’
          This stands for the time zone numerical offset (e.g.,
          ‘-0500’).

     You can also specify the field width and type of padding for any of
     these ‘%’-sequences.  This works as in ‘printf’: you write the
     field width as digits in the middle of a ‘%’-sequences.  If you
     start the field width with ‘0’, it means to pad with zeros.  If you
     start the field width with ‘_’, it means to pad with spaces.

     For example, ‘%S’ specifies the number of seconds since the minute;
     ‘%03S’ means to pad this with zeros to 3 positions, ‘%_3S’ to pad
     with spaces to 3 positions.  Plain ‘%3S’ pads with zeros, because
     that is how ‘%S’ normally pads to two positions.

     The characters ‘E’ and ‘O’ act as modifiers when used between ‘%’
     and one of the letters in the table above.  ‘E’ specifies using the
     current locale’s alternative version of the date and time.  In a
     Japanese locale, for example, ‘%Ex’ might yield a date format based
     on the Japanese Emperors’ reigns.  ‘E’ is allowed in ‘%Ec’, ‘%EC’,
     ‘%Ex’, ‘%EX’, ‘%Ey’, and ‘%EY’.

     ‘O’ means to use the current locale’s alternative representation of
     numbers, instead of the ordinary decimal digits.  This is allowed
     with most letters, all the ones that output numbers.

     This function uses the C library function ‘strftime’ (*note
     (libc)Formatting Calendar Time::) to do most of the work.  In order
     to communicate with that function, it first encodes its argument
     using the coding system specified by ‘locale-coding-system’ (*note
     Locales::); after ‘strftime’ returns the resulting string,
     ‘format-time-string’ decodes the string using that same coding
     system.

 -- Function: format-seconds format-string seconds
     This function converts its argument SECONDS into a string of years,
     days, hours, etc., according to FORMAT-STRING.  The argument
     FORMAT-STRING may contain ‘%’-sequences which control the
     conversion.  Here is a table of what the ‘%’-sequences mean:

     ‘%y’
     ‘%Y’
          The integer number of 365-day years.
     ‘%d’
     ‘%D’
          The integer number of days.
     ‘%h’
     ‘%H’
          The integer number of hours.
     ‘%m’
     ‘%M’
          The integer number of minutes.
     ‘%s’
     ‘%S’
          The integer number of seconds.
     ‘%z’
          Non-printing control flag.  When it is used, other specifiers
          must be given in the order of decreasing size, i.e., years
          before days, hours before minutes, etc.  Nothing will be
          produced in the result string to the left of ‘%z’ until the
          first non-zero conversion is encountered.  For example, the
          default format used by ‘emacs-uptime’ (*note emacs-uptime:
          Processor Run Time.) ‘"%Y, %D, %H, %M, %z%S"’ means that the
          number of seconds will always be produced, but years, days,
          hours, and minutes will only be shown if they are non-zero.
     ‘%%’
          Produces a literal ‘%’.

     Upper-case format sequences produce the units in addition to the
     numbers, lower-case formats produce only the numbers.

     You can also specify the field width by following the ‘%’ with a
     number; shorter numbers will be padded with blanks.  An optional
     period before the width requests zero-padding instead.  For
     example, ‘"%.3Y"’ might produce ‘"004 years"’.

     _Warning:_ This function works only with values of SECONDS that
     don’t exceed ‘most-positive-fixnum’ (*note most-positive-fixnum:
     Integer Basics.).


File: elisp.info,  Node: Processor Run Time,  Next: Time Calculations,  Prev: Time Parsing,  Up: System Interface

38.9 Processor Run time
=======================

Emacs provides several functions and primitives that return time, both
elapsed and processor time, used by the Emacs process.

 -- Command: emacs-uptime &optional format
     This function returns a string representing the Emacs “uptime”—the
     elapsed wall-clock time this instance of Emacs is running.  The
     string is formatted by ‘format-seconds’ according to the optional
     argument FORMAT.  For the available format descriptors, see *note
     format-seconds: Time Parsing.  If FORMAT is ‘nil’ or omitted, it
     defaults to ‘"%Y, %D, %H, %M, %z%S"’.

     When called interactively, it prints the uptime in the echo area.

 -- Function: get-internal-run-time
     This function returns the processor run time used by Emacs as a
     list of four integers: ‘(SEC-HIGH SEC-LOW MICROSEC PICOSEC)’, using
     the same format as ‘current-time’ (*note Time of Day::).

     Note that the time returned by this function excludes the time
     Emacs was not using the processor, and if the Emacs process has
     several threads, the returned value is the sum of the processor
     times used up by all Emacs threads.

     If the system doesn’t provide a way to determine the processor run
     time, ‘get-internal-run-time’ returns the same time as
     ‘current-time’.

 -- Command: emacs-init-time
     This function returns the duration of the Emacs initialization
     (*note Startup Summary::) in seconds, as a string.  When called
     interactively, it prints the duration in the echo area.


File: elisp.info,  Node: Time Calculations,  Next: Timers,  Prev: Processor Run Time,  Up: System Interface

38.10 Time Calculations
=======================

These functions perform calendrical computations using time values
(*note Time of Day::).  A value of ‘nil’ for any of their time-value
arguments stands for the current system time, and a single integer
number stands for the number of seconds since the epoch.

 -- Function: time-less-p t1 t2
     This returns ‘t’ if time value T1 is less than time value T2.

 -- Function: time-subtract t1 t2
     This returns the time difference T1 − T2 between two time values,
     as a time value.  If you need the difference in units of elapsed
     seconds, use ‘float-time’ (*note float-time: Time of Day.) to
     convert the result into seconds.

 -- Function: time-add t1 t2
     This returns the sum of two time values, as a time value.  One
     argument should represent a time difference rather than a point in
     time, either as a list or as a single number of elapsed seconds.
     Here is how to add a number of seconds to a time value:

          (time-add TIME SECONDS)

 -- Function: time-to-days time-value
     This function returns the number of days between the beginning of
     year 1 and TIME-VALUE.

 -- Function: time-to-day-in-year time-value
     This returns the day number within the year corresponding to
     TIME-VALUE.

 -- Function: date-leap-year-p year
     This function returns ‘t’ if YEAR is a leap year.


File: elisp.info,  Node: Timers,  Next: Idle Timers,  Prev: Time Calculations,  Up: System Interface

38.11 Timers for Delayed Execution
==================================

You can set up a “timer” to call a function at a specified future time
or after a certain length of idleness.  A timer is a special object that
stores the information about the next invocation times and the function
to invoke.

 -- Function: timerp object
     This predicate function returns non-‘nil’ of ‘object’ is a timer.

   Emacs cannot run timers at any arbitrary point in a Lisp program; it
can run them only when Emacs could accept output from a subprocess:
namely, while waiting or inside certain primitive functions such as
‘sit-for’ or ‘read-event’ which _can_ wait.  Therefore, a timer’s
execution may be delayed if Emacs is busy.  However, the time of
execution is very precise if Emacs is idle.

   Emacs binds ‘inhibit-quit’ to ‘t’ before calling the timer function,
because quitting out of many timer functions can leave things in an
inconsistent state.  This is normally unproblematical because most timer
functions don’t do a lot of work.  Indeed, for a timer to call a
function that takes substantial time to run is likely to be annoying.
If a timer function needs to allow quitting, it should use
‘with-local-quit’ (*note Quitting::).  For example, if a timer function
calls ‘accept-process-output’ to receive output from an external
process, that call should be wrapped inside ‘with-local-quit’, to ensure
that ‘C-g’ works if the external process hangs.

   It is usually a bad idea for timer functions to alter buffer
contents.  When they do, they usually should call ‘undo-boundary’ both
before and after changing the buffer, to separate the timer’s changes
from user commands’ changes and prevent a single undo entry from growing
to be quite large.

   Timer functions should also avoid calling functions that cause Emacs
to wait, such as ‘sit-for’ (*note Waiting::).  This can lead to
unpredictable effects, since other timers (or even the same timer) can
run while waiting.  If a timer function needs to perform an action after
a certain time has elapsed, it can do this by scheduling a new timer.

   If a timer function calls functions that can change the match data,
it should save and restore the match data.  *Note Saving Match Data::.

 -- Command: run-at-time time repeat function &rest args
     This sets up a timer that calls the function FUNCTION with
     arguments ARGS at time TIME.  If REPEAT is a number (integer or
     floating point), the timer is scheduled to run again every REPEAT
     seconds after TIME.  If REPEAT is ‘nil’, the timer runs only once.

     TIME may specify an absolute or a relative time.

     Absolute times may be specified using a string with a limited
     variety of formats, and are taken to be times _today_, even if
     already in the past.  The recognized forms are ‘XXXX’, ‘X:XX’, or
     ‘XX:XX’ (military time), and ‘XXam’, ‘XXAM’, ‘XXpm’, ‘XXPM’,
     ‘XX:XXam’, ‘XX:XXAM’, ‘XX:XXpm’, or ‘XX:XXPM’.  A period can be
     used instead of a colon to separate the hour and minute parts.

     To specify a relative time as a string, use numbers followed by
     units.  For example:

     ‘1 min’
          denotes 1 minute from now.
     ‘1 min 5 sec’
          denotes 65 seconds from now.
     ‘1 min 2 sec 3 hour 4 day 5 week 6 fortnight 7 month 8 year’
          denotes exactly 103 months, 123 days, and 10862 seconds from
          now.

     For relative time values, Emacs considers a month to be exactly
     thirty days, and a year to be exactly 365.25 days.

     Not all convenient formats are strings.  If TIME is a number
     (integer or floating point), that specifies a relative time
     measured in seconds.  The result of ‘encode-time’ can also be used
     to specify an absolute value for TIME.

     In most cases, REPEAT has no effect on when _first_ call takes
     place—TIME alone specifies that.  There is one exception: if TIME
     is ‘t’, then the timer runs whenever the time is a multiple of
     REPEAT seconds after the epoch.  This is useful for functions like
     ‘display-time’.

     The function ‘run-at-time’ returns a timer value that identifies
     the particular scheduled future action.  You can use this value to
     call ‘cancel-timer’ (see below).

   A repeating timer nominally ought to run every REPEAT seconds, but
remember that any invocation of a timer can be late.  Lateness of one
repetition has no effect on the scheduled time of the next repetition.
For instance, if Emacs is busy computing for long enough to cover three
scheduled repetitions of the timer, and then starts to wait, it will
immediately call the timer function three times in immediate succession
(presuming no other timers trigger before or between them).  If you want
a timer to run again no less than N seconds after the last invocation,
don’t use the REPEAT argument.  Instead, the timer function should
explicitly reschedule the timer.

 -- User Option: timer-max-repeats
     This variable’s value specifies the maximum number of times to
     repeat calling a timer function in a row, when many previously
     scheduled calls were unavoidably delayed.

 -- Macro: with-timeout (seconds timeout-forms...) body...
     Execute BODY, but give up after SECONDS seconds.  If BODY finishes
     before the time is up, ‘with-timeout’ returns the value of the last
     form in BODY.  If, however, the execution of BODY is cut short by
     the timeout, then ‘with-timeout’ executes all the TIMEOUT-FORMS and
     returns the value of the last of them.

     This macro works by setting a timer to run after SECONDS seconds.
     If BODY finishes before that time, it cancels the timer.  If the
     timer actually runs, it terminates execution of BODY, then executes
     TIMEOUT-FORMS.

     Since timers can run within a Lisp program only when the program
     calls a primitive that can wait, ‘with-timeout’ cannot stop
     executing BODY while it is in the midst of a computation—only when
     it calls one of those primitives.  So use ‘with-timeout’ only with
     a BODY that waits for input, not one that does a long computation.

   The function ‘y-or-n-p-with-timeout’ provides a simple way to use a
timer to avoid waiting too long for an answer.  *Note Yes-or-No
Queries::.

 -- Function: cancel-timer timer
     This cancels the requested action for TIMER, which should be a
     timer—usually, one previously returned by ‘run-at-time’ or
     ‘run-with-idle-timer’.  This cancels the effect of that call to one
     of these functions; the arrival of the specified time will not
     cause anything special to happen.


File: elisp.info,  Node: Idle Timers,  Next: Terminal Input,  Prev: Timers,  Up: System Interface

38.12 Idle Timers
=================

Here is how to set up a timer that runs when Emacs is idle for a certain
length of time.  Aside from how to set them up, idle timers work just
like ordinary timers.

 -- Command: run-with-idle-timer secs repeat function &rest args
     Set up a timer which runs the next time Emacs is idle for SECS
     seconds.  The value of SECS may be a number or a value of the type
     returned by ‘current-idle-time’.

     If REPEAT is ‘nil’, the timer runs just once, the first time Emacs
     remains idle for a long enough time.  More often REPEAT is
     non-‘nil’, which means to run the timer _each time_ Emacs remains
     idle for SECS seconds.

     The function ‘run-with-idle-timer’ returns a timer value which you
     can use in calling ‘cancel-timer’ (*note Timers::).

   Emacs becomes “idle” when it starts waiting for user input, and it
remains idle until the user provides some input.  If a timer is set for
five seconds of idleness, it runs approximately five seconds after Emacs
first becomes idle.  Even if REPEAT is non-‘nil’, this timer will not
run again as long as Emacs remains idle, because the duration of
idleness will continue to increase and will not go down to five seconds
again.

   Emacs can do various things while idle: garbage collect, autosave or
handle data from a subprocess.  But these interludes during idleness do
not interfere with idle timers, because they do not reset the clock of
idleness to zero.  An idle timer set for 600 seconds will run when ten
minutes have elapsed since the last user command was finished, even if
subprocess output has been accepted thousands of times within those ten
minutes, and even if there have been garbage collections and autosaves.

   When the user supplies input, Emacs becomes non-idle while executing
the input.  Then it becomes idle again, and all the idle timers that are
set up to repeat will subsequently run another time, one by one.

   Do not write an idle timer function containing a loop which does a
certain amount of processing each time around, and exits when
‘(input-pending-p)’ is non-‘nil’.  This approach seems very natural but
has two problems:

   • It blocks out all process output (since Emacs accepts process
     output only while waiting).

   • It blocks out any idle timers that ought to run during that time.

Similarly, do not write an idle timer function that sets up another idle
timer (including the same idle timer) with SECS argument less than or
equal to the current idleness time.  Such a timer will run almost
immediately, and continue running again and again, instead of waiting
for the next time Emacs becomes idle.  The correct approach is to
reschedule with an appropriate increment of the current value of the
idleness time, as described below.

 -- Function: current-idle-time
     If Emacs is idle, this function returns the length of time Emacs
     has been idle, as a list of four integers: ‘(SEC-HIGH SEC-LOW
     MICROSEC PICOSEC)’, using the same format as ‘current-time’ (*note
     Time of Day::).

     When Emacs is not idle, ‘current-idle-time’ returns ‘nil’.  This is
     a convenient way to test whether Emacs is idle.

   The main use of ‘current-idle-time’ is when an idle timer function
wants to “take a break” for a while.  It can set up another idle timer
to call the same function again, after a few seconds more idleness.
Here’s an example:

     (defvar my-resume-timer nil
       "Timer for `my-timer-function' to reschedule itself, or nil.")

     (defun my-timer-function ()
       ;; If the user types a command while ‘my-resume-timer’
       ;; is active, the next time this function is called from
       ;; its main idle timer, deactivate ‘my-resume-timer’.
       (when my-resume-timer
         (cancel-timer my-resume-timer))
       ...DO THE WORK FOR A WHILE...
       (when TAKING-A-BREAK
         (setq my-resume-timer
               (run-with-idle-timer
                 ;; Compute an idle time BREAK-LENGTH
                 ;; more than the current value.
                 (time-add (current-idle-time) BREAK-LENGTH)
                 nil
                 'my-timer-function))))


File: elisp.info,  Node: Terminal Input,  Next: Terminal Output,  Prev: Idle Timers,  Up: System Interface

38.13 Terminal Input
====================

This section describes functions and variables for recording or
manipulating terminal input.  See *note Display::, for related
functions.

* Menu:

* Input Modes::         Options for how input is processed.
* Recording Input::     Saving histories of recent or all input events.


File: elisp.info,  Node: Input Modes,  Next: Recording Input,  Up: Terminal Input

38.13.1 Input Modes
-------------------

 -- Function: set-input-mode interrupt flow meta &optional quit-char
     This function sets the mode for reading keyboard input.  If
     INTERRUPT is non-‘nil’, then Emacs uses input interrupts.  If it is
     ‘nil’, then it uses CBREAK mode.  The default setting is
     system-dependent.  Some systems always use CBREAK mode regardless
     of what is specified.

     When Emacs communicates directly with X, it ignores this argument
     and uses interrupts if that is the way it knows how to communicate.

     If FLOW is non-‘nil’, then Emacs uses XON/XOFF (‘C-q’, ‘C-s’) flow
     control for output to the terminal.  This has no effect except in
     CBREAK mode.

     The argument META controls support for input character codes above
     127.  If META is ‘t’, Emacs converts characters with the 8th bit
     set into Meta characters.  If META is ‘nil’, Emacs disregards the
     8th bit; this is necessary when the terminal uses it as a parity
     bit.  If META is neither ‘t’ nor ‘nil’, Emacs uses all 8 bits of
     input unchanged.  This is good for terminals that use 8-bit
     character sets.

     If QUIT-CHAR is non-‘nil’, it specifies the character to use for
     quitting.  Normally this character is ‘C-g’.  *Note Quitting::.

   The ‘current-input-mode’ function returns the input mode settings
Emacs is currently using.

 -- Function: current-input-mode
     This function returns the current mode for reading keyboard input.
     It returns a list, corresponding to the arguments of
     ‘set-input-mode’, of the form ‘(INTERRUPT FLOW META QUIT)’ in
     which:
     INTERRUPT
          is non-‘nil’ when Emacs is using interrupt-driven input.  If
          ‘nil’, Emacs is using CBREAK mode.
     FLOW
          is non-‘nil’ if Emacs uses XON/XOFF (‘C-q’, ‘C-s’) flow
          control for output to the terminal.  This value is meaningful
          only when INTERRUPT is ‘nil’.
     META
          is ‘t’ if Emacs treats the eighth bit of input characters as
          the meta bit; ‘nil’ means Emacs clears the eighth bit of every
          input character; any other value means Emacs uses all eight
          bits as the basic character code.
     QUIT
          is the character Emacs currently uses for quitting, usually
          ‘C-g’.


File: elisp.info,  Node: Recording Input,  Prev: Input Modes,  Up: Terminal Input

38.13.2 Recording Input
-----------------------

 -- Function: recent-keys
     This function returns a vector containing the last 300 input events
     from the keyboard or mouse.  All input events are included, whether
     or not they were used as parts of key sequences.  Thus, you always
     get the last 300 input events, not counting events generated by
     keyboard macros.  (These are excluded because they are less
     interesting for debugging; it should be enough to see the events
     that invoked the macros.)

     A call to ‘clear-this-command-keys’ (*note Command Loop Info::)
     causes this function to return an empty vector immediately
     afterward.

 -- Command: open-dribble-file filename
     This function opens a “dribble file” named FILENAME.  When a
     dribble file is open, each input event from the keyboard or mouse
     (but not those from keyboard macros) is written in that file.  A
     non-character event is expressed using its printed representation
     surrounded by ‘<...>’.  Be aware that sensitive information (such
     as passwords) may end up recorded in the dribble file.

     You close the dribble file by calling this function with an
     argument of ‘nil’.

   See also the ‘open-termscript’ function (*note Terminal Output::).


File: elisp.info,  Node: Terminal Output,  Next: Sound Output,  Prev: Terminal Input,  Up: System Interface

38.14 Terminal Output
=====================

The terminal output functions send output to a text terminal, or keep
track of output sent to the terminal.  The variable ‘baud-rate’ tells
you what Emacs thinks is the output speed of the terminal.

 -- User Option: baud-rate
     This variable’s value is the output speed of the terminal, as far
     as Emacs knows.  Setting this variable does not change the speed of
     actual data transmission, but the value is used for calculations
     such as padding.

     It also affects decisions about whether to scroll part of the
     screen or repaint on text terminals.  *Note Forcing Redisplay::,
     for the corresponding functionality on graphical terminals.

     The value is measured in baud.

   If you are running across a network, and different parts of the
network work at different baud rates, the value returned by Emacs may be
different from the value used by your local terminal.  Some network
protocols communicate the local terminal speed to the remote machine, so
that Emacs and other programs can get the proper value, but others do
not.  If Emacs has the wrong value, it makes decisions that are less
than optimal.  To fix the problem, set ‘baud-rate’.

 -- Function: send-string-to-terminal string &optional terminal
     This function sends STRING to TERMINAL without alteration.  Control
     characters in STRING have terminal-dependent effects.  (If you need
     to display non-ASCII text on the terminal, encode it using one of
     the functions described in *note Explicit Encoding::.)  This
     function operates only on text terminals.  TERMINAL may be a
     terminal object, a frame, or ‘nil’ for the selected frame’s
     terminal.  In batch mode, STRING is sent to ‘stdout’ when TERMINAL
     is ‘nil’.

     One use of this function is to define function keys on terminals
     that have downloadable function key definitions.  For example, this
     is how (on certain terminals) to define function key 4 to move
     forward four characters (by transmitting the characters ‘C-u C-f’
     to the computer):

          (send-string-to-terminal "\eF4\^U\^F")
               ⇒ nil

 -- Command: open-termscript filename
     This function is used to open a “termscript file” that will record
     all the characters sent by Emacs to the terminal.  It returns
     ‘nil’.  Termscript files are useful for investigating problems
     where Emacs garbles the screen, problems that are due to incorrect
     Termcap entries or to undesirable settings of terminal options more
     often than to actual Emacs bugs.  Once you are certain which
     characters were actually output, you can determine reliably whether
     they correspond to the Termcap specifications in use.

          (open-termscript "../junk/termscript")
               ⇒ nil

     You close the termscript file by calling this function with an
     argument of ‘nil’.

     See also ‘open-dribble-file’ in *note Recording Input::.


File: elisp.info,  Node: Sound Output,  Next: X11 Keysyms,  Prev: Terminal Output,  Up: System Interface

38.15 Sound Output
==================

To play sound using Emacs, use the function ‘play-sound’.  Only certain
systems are supported; if you call ‘play-sound’ on a system which cannot
really do the job, it gives an error.

   The sound must be stored as a file in RIFF-WAVE format (‘.wav’) or
Sun Audio format (‘.au’).

 -- Function: play-sound sound
     This function plays a specified sound.  The argument, SOUND, has
     the form ‘(sound PROPERTIES...)’, where the PROPERTIES consist of
     alternating keywords (particular symbols recognized specially) and
     values corresponding to them.

     Here is a table of the keywords that are currently meaningful in
     SOUND, and their meanings:

     ‘:file FILE’
          This specifies the file containing the sound to play.  If the
          file name is not absolute, it is expanded against the
          directory ‘data-directory’.

     ‘:data DATA’
          This specifies the sound to play without need to refer to a
          file.  The value, DATA, should be a string containing the same
          bytes as a sound file.  We recommend using a unibyte string.

     ‘:volume VOLUME’
          This specifies how loud to play the sound.  It should be a
          number in the range of 0 to 1.  The default is to use whatever
          volume has been specified before.

     ‘:device DEVICE’
          This specifies the system device on which to play the sound,
          as a string.  The default device is system-dependent.

     Before actually playing the sound, ‘play-sound’ calls the functions
     in the list ‘play-sound-functions’.  Each function is called with
     one argument, SOUND.

 -- Command: play-sound-file file &optional volume device
     This function is an alternative interface to playing a sound FILE
     specifying an optional VOLUME and DEVICE.

 -- Variable: play-sound-functions
     A list of functions to be called before playing a sound.  Each
     function is called with one argument, a property list that
     describes the sound.


File: elisp.info,  Node: X11 Keysyms,  Next: Batch Mode,  Prev: Sound Output,  Up: System Interface

38.16 Operating on X11 Keysyms
==============================

To define system-specific X11 keysyms, set the variable
‘system-key-alist’.

 -- Variable: system-key-alist
     This variable’s value should be an alist with one element for each
     system-specific keysym.  Each element has the form ‘(CODE .
     SYMBOL)’, where CODE is the numeric keysym code (not including the
     vendor-specific bit, −2**28), and SYMBOL is the name for the
     function key.

     For example ‘(168 . mute-acute)’ defines a system-specific key
     (used by HP X servers) whose numeric code is −2**28 + 168.

     It is not crucial to exclude from the alist the keysyms of other X
     servers; those do no harm, as long as they don’t conflict with the
     ones used by the X server actually in use.

     The variable is always local to the current terminal, and cannot be
     buffer-local.  *Note Multiple Terminals::.

   You can specify which keysyms Emacs should use for the Meta, Alt,
Hyper, and Super modifiers by setting these variables:

 -- Variable: x-alt-keysym
 -- Variable: x-meta-keysym
 -- Variable: x-hyper-keysym
 -- Variable: x-super-keysym
     The name of the keysym that should stand for the Alt modifier
     (respectively, for Meta, Hyper, and Super).  For example, here is
     how to swap the Meta and Alt modifiers within Emacs:
          (setq x-alt-keysym 'meta)
          (setq x-meta-keysym 'alt)


File: elisp.info,  Node: Batch Mode,  Next: Session Management,  Prev: X11 Keysyms,  Up: System Interface

38.17 Batch Mode
================

The command-line option ‘-batch’ causes Emacs to run noninteractively.
In this mode, Emacs does not read commands from the terminal, it does
not alter the terminal modes, and it does not expect to be outputting to
an erasable screen.  The idea is that you specify Lisp programs to run;
when they are finished, Emacs should exit.  The way to specify the
programs to run is with ‘-l FILE’, which loads the library named FILE,
or ‘-f FUNCTION’, which calls FUNCTION with no arguments, or ‘--eval
FORM’.

   Any Lisp program output that would normally go to the echo area,
either using ‘message’, or using ‘prin1’, etc., with ‘t’ as the stream,
goes instead to Emacs’s standard descriptors when in batch mode:
‘message’ writes to the standard error descriptor, while ‘prin1’ and
other print functions write to the standard output.  Similarly, input
that would normally come from the minibuffer is read from the standard
input descriptor.  Thus, Emacs behaves much like a noninteractive
application program.  (The echo area output that Emacs itself normally
generates, such as command echoing, is suppressed entirely.)

   Non-ASCII text written to the standard output or error descriptors is
by default encoded using ‘locale-coding-system’ (*note Locales::) if it
is non-‘nil’; this can be overridden by binding
‘coding-system-for-write’ to a coding system of you choice (*note
Explicit Encoding::).

 -- Variable: noninteractive
     This variable is non-‘nil’ when Emacs is running in batch mode.


File: elisp.info,  Node: Session Management,  Next: Desktop Notifications,  Prev: Batch Mode,  Up: System Interface

38.18 Session Management
========================

Emacs supports the X Session Management Protocol, which is used to
suspend and restart applications.  In the X Window System, a program
called the “session manager” is responsible for keeping track of the
applications that are running.  When the X server shuts down, the
session manager asks applications to save their state, and delays the
actual shutdown until they respond.  An application can also cancel the
shutdown.

   When the session manager restarts a suspended session, it directs
these applications to individually reload their saved state.  It does
this by specifying a special command-line argument that says what saved
session to restore.  For Emacs, this argument is ‘--smid SESSION’.

 -- Variable: emacs-save-session-functions
     Emacs supports saving state via a hook called
     ‘emacs-save-session-functions’.  Emacs runs this hook when the
     session manager tells it that the window system is shutting down.
     The functions are called with no arguments, and with the current
     buffer set to a temporary buffer.  Each function can use ‘insert’
     to add Lisp code to this buffer.  At the end, Emacs saves the
     buffer in a file, called the “session file”.

     Subsequently, when the session manager restarts Emacs, it loads the
     session file automatically (*note Loading::).  This is performed by
     a function named ‘emacs-session-restore’, which is called during
     startup.  *Note Startup Summary::.

     If a function in ‘emacs-save-session-functions’ returns non-‘nil’,
     Emacs tells the session manager to cancel the shutdown.

   Here is an example that just inserts some text into ‘*scratch*’ when
Emacs is restarted by the session manager.

     (add-hook 'emacs-save-session-functions 'save-yourself-test)

     (defun save-yourself-test ()
       (insert "(save-current-buffer
       (switch-to-buffer \"*scratch*\")
       (insert \"I am restored\"))")
       nil)


File: elisp.info,  Node: Desktop Notifications,  Next: File Notifications,  Prev: Session Management,  Up: System Interface

38.19 Desktop Notifications
===========================

Emacs is able to send “notifications” on systems that support the
freedesktop.org Desktop Notifications Specification and on MS-Windows.
In order to use this functionality on Posix hosts, Emacs must have been
compiled with D-Bus support, and the ‘notifications’ library must be
loaded.  *Note D-Bus: (dbus)Top.  The following function is supported
when D-Bus support is available:

 -- Function: notifications-notify &rest params
     This function sends a notification to the desktop via D-Bus,
     consisting of the parameters specified by the PARAMS arguments.
     These arguments should consist of alternating keyword and value
     pairs.  The supported keywords and values are as follows:

     ‘:bus BUS’
          The D-Bus bus.  This argument is needed only if a bus other
          than ‘:session’ shall be used.

     ‘:title TITLE’
          The notification title.

     ‘:body TEXT’
          The notification body text.  Depending on the implementation
          of the notification server, the text could contain HTML
          markups, like ‘"<b>bold text</b>"’, hyperlinks, or images.
          Special HTML characters must be encoded, as ‘"Contact
          &lt;postmaster@localhost&gt;!"’.

     ‘:app-name NAME’
          The name of the application sending the notification.  The
          default is ‘notifications-application-name’.

     ‘:replaces-id ID’
          The notification ID that this notification replaces.  ID must
          be the result of a previous ‘notifications-notify’ call.

     ‘:app-icon ICON-FILE’
          The file name of the notification icon.  If set to ‘nil’, no
          icon is displayed.  The default is
          ‘notifications-application-icon’.

     ‘:actions (KEY TITLE KEY TITLE ...)’
          A list of actions to be applied.  KEY and TITLE are both
          strings.  The default action (usually invoked by clicking the
          notification) should have a key named ‘"default"’.  The title
          can be anything, though implementations are free not to
          display it.

     ‘:timeout TIMEOUT’
          The timeout time in milliseconds since the display of the
          notification at which the notification should automatically
          close.  If −1, the notification’s expiration time is dependent
          on the notification server’s settings, and may vary for the
          type of notification.  If 0, the notification never expires.
          Default value is −1.

     ‘:urgency URGENCY’
          The urgency level.  It can be ‘low’, ‘normal’, or ‘critical’.

     ‘:action-items’
          When this keyword is given, the TITLE string of the actions is
          interpreted as icon name.

     ‘:category CATEGORY’
          The type of notification this is, a string.  See the Desktop
          Notifications Specification
          (http://developer.gnome.org/notification-spec/#categories) for
          a list of standard categories.

     ‘:desktop-entry FILENAME’
          This specifies the name of the desktop filename representing
          the calling program, like ‘"emacs"’.

     ‘:image-data (WIDTH HEIGHT ROWSTRIDE HAS-ALPHA BITS CHANNELS DATA)’
          This is a raw data image format that describes the width,
          height, rowstride, whether there is an alpha channel, bits per
          sample, channels and image data, respectively.

     ‘:image-path PATH’
          This is represented either as a URI (‘file://’ is the only URI
          schema supported right now) or a name in a
          freedesktop.org-compliant icon theme from
          ‘$XDG_DATA_DIRS/icons’.

     ‘:sound-file FILENAME’
          The path to a sound file to play when the notification pops
          up.

     ‘:sound-name NAME’
          A themable named sound from the freedesktop.org sound naming
          specification from ‘$XDG_DATA_DIRS/sounds’, to play when the
          notification pops up.  Similar to the icon name, only for
          sounds.  An example would be ‘"message-new-instant"’.

     ‘:suppress-sound’
          Causes the server to suppress playing any sounds, if it has
          that ability.

     ‘:resident’
          When set the server will not automatically remove the
          notification when an action has been invoked.  The
          notification will remain resident in the server until it is
          explicitly removed by the user or by the sender.  This hint is
          likely only useful when the server has the ‘:persistence’
          capability.

     ‘:transient’
          When set the server will treat the notification as transient
          and by-pass the server’s persistence capability, if it should
          exist.

     ‘:x POSITION’
     ‘:y POSITION’
          Specifies the X, Y location on the screen that the
          notification should point to.  Both arguments must be used
          together.

     ‘:on-action FUNCTION’
          Function to call when an action is invoked.  The notification
          ID and the KEY of the action are passed as arguments to the
          function.

     ‘:on-close FUNCTION’
          Function to call when the notification has been closed by
          timeout or by the user.  The function receive the notification
          ID and the closing REASON as arguments:

             • ‘expired’ if the notification has expired
             • ‘dismissed’ if the notification was dismissed by the user
             • ‘close-notification’ if the notification was closed by a
               call to ‘notifications-close-notification’
             • ‘undefined’ if the notification server hasn’t provided a
               reason

     Which parameters are accepted by the notification server can be
     checked via ‘notifications-get-capabilities’.

     This function returns a notification id, an integer, which can be
     used to manipulate the notification item with
     ‘notifications-close-notification’ or the ‘:replaces-id’ argument
     of another ‘notifications-notify’ call.  For example:

          (defun my-on-action-function (id key)
            (message "Message %d, key \"%s\" pressed" id key))
               ⇒ my-on-action-function

          (defun my-on-close-function (id reason)
            (message "Message %d, closed due to \"%s\"" id reason))
               ⇒ my-on-close-function

          (notifications-notify
           :title "Title"
           :body "This is <b>important</b>."
           :actions '("Confirm" "I agree" "Refuse" "I disagree")
           :on-action 'my-on-action-function
           :on-close 'my-on-close-function)
               ⇒ 22

          A message window opens on the desktop.  Press ``I agree''.
               ⇒ Message 22, key "Confirm" pressed
                  Message 22, closed due to "dismissed"

 -- Function: notifications-close-notification id &optional bus
     This function closes a notification with identifier ID.  BUS can be
     a string denoting a D-Bus connection, the default is ‘:session’.

 -- Function: notifications-get-capabilities &optional bus
     Returns the capabilities of the notification server, a list of
     symbols.  BUS can be a string denoting a D-Bus connection, the
     default is ‘:session’.  The following capabilities can be expected:

     ‘:actions’
          The server will provide the specified actions to the user.

     ‘:body’
          Supports body text.

     ‘:body-hyperlinks’
          The server supports hyperlinks in the notifications.

     ‘:body-images’
          The server supports images in the notifications.

     ‘:body-markup’
          Supports markup in the body text.

     ‘:icon-multi’
          The server will render an animation of all the frames in a
          given image array.

     ‘:icon-static’
          Supports display of exactly 1 frame of any given image array.
          This value is mutually exclusive with ‘:icon-multi’.

     ‘:persistence’
          The server supports persistence of notifications.

     ‘:sound’
          The server supports sounds on notifications.

     Further vendor-specific caps start with ‘:x-vendor’, like
     ‘:x-gnome-foo-cap’.

 -- Function: notifications-get-server-information &optional bus
     Return information on the notification server, a list of strings.
     BUS can be a string denoting a D-Bus connection, the default is
     ‘:session’.  The returned list is ‘(NAME VENDOR VERSION
     SPEC-VERSION)’.

     NAME
          The product name of the server.

     VENDOR
          The vendor name.  For example, ‘"KDE"’, ‘"GNOME"’.

     VERSION
          The server’s version number.

     SPEC-VERSION
          The specification version the server is compliant with.

     If SPEC_VERSION is ‘nil’, the server supports a specification prior
     to ‘"1.0"’.

   When Emacs runs on MS-Windows as a GUI session, it supports a small
subset of the D-Bus notifications functionality via a native primitive:

 -- Function: w32-notification-notify &rest params
     This function displays an MS-Windows tray notification as specified
     by PARAMS.  MS-Windows tray notifications are displayed in a
     balloon from an icon in the notification area of the taskbar.

     Value is the integer unique ID of the notification that can be used
     to remove the notification using ‘w32-notification-close’,
     described below.  If the function fails, the return value is ‘nil’.

     The arguments PARAMS are specified as keyword/value pairs.  All the
     parameters are optional, but if no parameters are specified, the
     function will do nothing and return ‘nil’.

     The following parameters are supported:

     ‘:icon ICON’
          Display ICON in the system tray.  If ICON is a string, it
          should specify a file name from which to load the icon; the
          specified file should be a ‘.ico’ Windows icon file.  If ICON
          is not a string, or if this parameter is not specified, the
          standard Emacs icon will be used.

     ‘:tip TIP’
          Use TIP as the tooltip for the notification.  If TIP is a
          string, this is the text of a tooltip that will be shown when
          the mouse pointer hovers over the tray icon added by the
          notification.  If TIP is not a string, or if this parameter is
          not specified, the default tooltip text is ‘Emacs
          notification’.  The tooltip text can be up to 127 characters
          long (63 on Windows versions before W2K). Longer strings will
          be truncated.

     ‘:level LEVEL’
          Notification severity level, one of ‘info’, ‘warning’, or
          ‘error’.  If given, the value determines the icon displayed to
          the left of the notification title, but only if the ‘:title’
          parameter (see below) is also specified and is a string.

     ‘:title TITLE’
          The title of the notification.  If TITLE is a string, it is
          displayed in a larger font immediately above the body text.
          The title text can be up to 63 characters long; longer text
          will be truncated.

     ‘:body BODY’
          The body of the notification.  If BODY is a string, it
          specifies the text of the notification message.  Use embedded
          newlines to control how the text is broken into lines.  The
          body text can be up to 255 characters long, and will be
          truncated if it’s longer.  Unlike with D-Bus, the body text
          should be plain text, with no markup.

     Note that versions of Windows before W2K support only ‘:icon’ and
     ‘:tip’.  The other parameters can be passed, but they will be
     ignored on those old systems.

     There can be at most one active notification at any given time.  An
     active notification must be removed by calling
     ‘w32-notification-close’ before a new one can be shown.

   To remove the notification and its icon from the taskbar, use the
following function:

 -- Function: w32-notification-close id
     This function removes the tray notification given by its unique ID.


File: elisp.info,  Node: File Notifications,  Next: Dynamic Libraries,  Prev: Desktop Notifications,  Up: System Interface

38.20 Notifications on File Changes
===================================

Several operating systems support watching of filesystems for changes of
files.  If configured properly, Emacs links a respective library like
‘inotify’, ‘kqueue’, ‘gfilenotify’, or ‘w32notify’ statically.  These
libraries enable watching of filesystems on the local machine.

   It is also possible to watch filesystems on remote machines, *note
Remote Files: (emacs)Remote Files. This does not depend on one of the
libraries linked to Emacs.

   Since all these libraries emit different events on notified file
changes, there is the Emacs library ‘filenotify’ which provides a unique
interface.

 -- Function: file-notify-add-watch file flags callback
     Add a watch for filesystem events pertaining to FILE.  This
     arranges for filesystem events pertaining to FILE to be reported to
     Emacs.

     The returned value is a descriptor for the added watch.  Its type
     depends on the underlying library, it cannot be assumed to be an
     integer as in the example below.  It should be used for comparison
     by ‘equal’ only.

     If the FILE cannot be watched for some reason, this function
     signals a ‘file-notify-error’ error.

     Sometimes, mounted filesystems cannot be watched for file changes.
     This is not detected by this function, a non-‘nil’ return value
     does not guarantee that changes on FILE will be notified.

     FLAGS is a list of conditions to set what will be watched for.  It
     can include the following symbols:

     ‘change’
          watch for file changes
     ‘attribute-change’
          watch for file attribute changes, like permissions or
          modification time

     If FILE is a directory, changes for all files in that directory
     will be notified.  This does not work recursively.

     When any event happens, Emacs will call the CALLBACK function
     passing it a single argument EVENT, which is of the form

          (DESCRIPTOR ACTION FILE [FILE1])

     DESCRIPTOR is the same object as the one returned by this function.
     ACTION is the description of the event.  It could be any one of the
     following symbols:

     ‘created’
          FILE was created
     ‘deleted’
          FILE was deleted
     ‘changed’
          FILE’s contents has changed; with ‘w32notify’ library, reports
          attribute changes as well
     ‘renamed’
          FILE has been renamed to FILE1
     ‘attribute-changed’
          a FILE attribute was changed
     ‘stopped’
          watching FILE has been stopped

     Note that the ‘w32notify’ library does not report
     ‘attribute-changed’ events.  When some file’s attribute, like
     permissions or modification time, has changed, this library reports
     a ‘changed’ event.  Likewise, the ‘kqueue’ library does not report
     reliably file attribute changes when watching a directory.

     The ‘stopped’ event reports, that watching the file has been
     stopped.  This could be because ‘file-notify-rm-watch’ was called
     (see below), or because the file being watched was deleted, or due
     to another error reported from the underlying library.

     FILE and FILE1 are the name of the file(s) whose event is being
     reported.  For example:

          (require 'filenotify)
               ⇒ filenotify

          (defun my-notify-callback (event)
            (message "Event %S" event))
               ⇒ my-notify-callback

          (file-notify-add-watch
            "/tmp" '(change attribute-change) 'my-notify-callback)
               ⇒ 35025468

          (write-region "foo" nil "/tmp/foo")
               ⇒ Event (35025468 created "/tmp/.#foo")
                  Event (35025468 created "/tmp/foo")
                  Event (35025468 changed "/tmp/foo")
                  Event (35025468 deleted "/tmp/.#foo")

          (write-region "bla" nil "/tmp/foo")
               ⇒ Event (35025468 created "/tmp/.#foo")
                  Event (35025468 changed "/tmp/foo")
                  Event (35025468 deleted "/tmp/.#foo")

          (set-file-modes "/tmp/foo" (default-file-modes))
               ⇒ Event (35025468 attribute-changed "/tmp/foo")

     Whether the action ‘renamed’ is returned, depends on the used watch
     library.  Otherwise, the actions ‘deleted’ and ‘created’ could be
     returned in a random order.

          (rename-file "/tmp/foo" "/tmp/bla")
               ⇒ Event (35025468 renamed "/tmp/foo" "/tmp/bla")

          (delete-file "/tmp/bla")
               ⇒ Event (35025468 deleted "/tmp/bla")

 -- Function: file-notify-rm-watch descriptor
     Removes an existing file watch specified by its DESCRIPTOR.
     DESCRIPTOR should be an object returned by ‘file-notify-add-watch’.

 -- Function: file-notify-valid-p descriptor
     Checks a watch specified by its DESCRIPTOR for validity.
     DESCRIPTOR should be an object returned by ‘file-notify-add-watch’.

     A watch can become invalid if the file or directory it watches is
     deleted, or if the watcher thread exits abnormally for any other
     reason.  Removing the watch by calling ‘file-notify-rm-watch’ also
     makes it invalid.

          (make-directory "/tmp/foo")
               ⇒ Event (35025468 created "/tmp/foo")

          (setq desc
                (file-notify-add-watch
                  "/tmp/foo" '(change) 'my-notify-callback))
               ⇒ 11359632

          (file-notify-valid-p desc)
               ⇒ t

          (write-region "bla" nil "/tmp/foo/bla")
               ⇒ Event (11359632 created "/tmp/foo/.#bla")
                  Event (11359632 created "/tmp/foo/bla")
                  Event (11359632 changed "/tmp/foo/bla")
                  Event (11359632 deleted "/tmp/foo/.#bla")

          ;; Deleting a file in the directory doesn't invalidate the watch.
          (delete-file "/tmp/foo/bla")
               ⇒ Event (11359632 deleted "/tmp/foo/bla")

          (write-region "bla" nil "/tmp/foo/bla")
               ⇒ Event (11359632 created "/tmp/foo/.#bla")
                  Event (11359632 created "/tmp/foo/bla")
                  Event (11359632 changed "/tmp/foo/bla")
                  Event (11359632 deleted "/tmp/foo/.#bla")

          ;; Deleting the directory invalidates the watch.
          ;; Events arrive for different watch descriptors.
          (delete-directory "/tmp/foo" 'recursive)
               ⇒ Event (35025468 deleted "/tmp/foo")
                  Event (11359632 deleted "/tmp/foo/bla")
                  Event (11359632 deleted "/tmp/foo")
                  Event (11359632 stopped "/tmp/foo")

          (file-notify-valid-p desc)
               ⇒ nil


File: elisp.info,  Node: Dynamic Libraries,  Next: Security Considerations,  Prev: File Notifications,  Up: System Interface

38.21 Dynamically Loaded Libraries
==================================

A “dynamically loaded library” is a library that is loaded on demand,
when its facilities are first needed.  Emacs supports such on-demand
loading of support libraries for some of its features.

 -- Variable: dynamic-library-alist
     This is an alist of dynamic libraries and external library files
     implementing them.

     Each element is a list of the form ‘(LIBRARY FILES...)’, where the
     ‘car’ is a symbol representing a supported external library, and
     the rest are strings giving alternate filenames for that library.

     Emacs tries to load the library from the files in the order they
     appear in the list; if none is found, the Emacs session won’t have
     access to that library, and the features it provides will be
     unavailable.

     Image support on some platforms uses this facility.  Here’s an
     example of setting this variable for supporting images on
     MS-Windows:

          (setq dynamic-library-alist
                '((xpm "libxpm.dll" "xpm4.dll" "libXpm-nox4.dll")
                  (png "libpng12d.dll" "libpng12.dll" "libpng.dll"
                       "libpng13d.dll" "libpng13.dll")
                  (jpeg "jpeg62.dll" "libjpeg.dll" "jpeg-62.dll"
                        "jpeg.dll")
                  (tiff "libtiff3.dll" "libtiff.dll")
                  (gif "giflib4.dll" "libungif4.dll" "libungif.dll")
                  (svg "librsvg-2-2.dll")
                  (gdk-pixbuf "libgdk_pixbuf-2.0-0.dll")
                  (glib "libglib-2.0-0.dll")
                  (gobject "libgobject-2.0-0.dll")))

     Note that image types ‘pbm’ and ‘xbm’ do not need entries in this
     variable because they do not depend on external libraries and are
     always available in Emacs.

     Also note that this variable is not meant to be a generic facility
     for accessing external libraries; only those already known by Emacs
     can be loaded through it.

     This variable is ignored if the given LIBRARY is statically linked
     into Emacs.


File: elisp.info,  Node: Security Considerations,  Prev: Dynamic Libraries,  Up: System Interface

38.22 Security Considerations
=============================

Like any application, Emacs can be run in a secure environment, where
the operating system enforces rules about access and the like.  With
some care, Emacs-based applications can also be part of a security
perimeter that checks such rules.  Although the default settings for
Emacs work well for a typical software development environment, they may
require adjustment in environments containing untrusted users that may
include attackers.  Here is a compendium of security issues that may be
helpful if you are developing such applications.  It is by no means
complete; it is intended to give you an idea of the security issues
involved, rather than to be a security checklist.

File local variables
     A file that Emacs visits can contain variable settings that affects
     the buffer visiting that file; *Note File Local Variables::.
     Similarly, a directory can specify local variable values common to
     all files in that directory; *Note Directory Local Variables::.
     Although Emacs takes some effort to protect against misuse of these
     variables, a security hole can be created merely by a package
     setting ‘safe-local-variable’ too optimistically, a problem that is
     all too common.  To disable this feature for both files and
     directories, set ‘enable-local-variables’ to ‘nil’.

Access control
     Although Emacs normally respects access permissions of the
     underlying operating system, in some cases it handles accesses
     specially.  For example, file names can have handlers that treat
     the files specially, with their own access checking.  *Note Magic
     File Names::.  Also, a buffer can be read-only even if the
     corresponding file is writeable, and vice versa, which can result
     in messages such as ‘File passwd is write-protected; try to save
     anyway? (yes or no)’.  *Note Read Only Buffers::.

Authentication
     Emacs has several functions that deal with passwords, e.g.,
     ‘read-passwd’.  *Note Reading a Password::.  Although these
     functions do not attempt to broadcast passwords to the world, their
     implementations are not proof against determined attackers with
     access to Emacs internals.  For example, even if Elisp code uses
     ‘clear-string’ to scrub a password from its memory after using it,
     remnants of the password may still reside in the garbage-collected
     free list.  *Note Modifying Strings::.

Code injection
     Emacs can send commands to many other applications, and
     applications should take care that strings sent as operands of
     these commands are not misinterpreted as directives.  For example,
     when using a shell command to rename a file A to B, do not simply
     use the string ‘mv A B’, because either file name might start with
     ‘-’, or might contain shell metacharacters like ‘;’.  Although
     functions like ‘shell-quote-argument’ can help avoid this sort of
     problem, they are not panaceas; for example, on a POSIX platform
     ‘shell-quote-argument’ quotes shell metacharacters but not leading
     ‘-’.  *Note Shell Arguments::.  Typically it is safer to use
     ‘call-process’ than a subshell.  *Note Synchronous Processes::.
     And it is safer yet to use builtin Emacs functions; for example,
     use ‘(rename-file "A" "B" t)’ instead of invoking ‘mv’.  *Note
     Changing Files::.

Coding systems
     Emacs attempts to infer the coding systems of the files and network
     connections it accesses.  *Note Coding Systems::.  If Emacs infers
     incorrectly, or if the other parties to the network connection
     disagree with Emacs’s inferences, the resulting system could be
     unreliable.  Also, even when it infers correctly, Emacs often can
     use bytes that other programs cannot.  For example, although to
     Emacs the null byte is just a character like any other, many other
     applications treat it as a string terminator and mishandle strings
     or files containing null bytes.

Environment and configuration variables
     POSIX specifies several environment variables that can affect how
     Emacs behaves.  Any environment variable whose name consists
     entirely of uppercase ASCII letters, digits, and the underscore may
     affect the internal behavior of Emacs.  Emacs uses several such
     variables, e.g., ‘EMACSLOADPATH’.  *Note Library Search::.  On some
     platforms some environment variables (e.g., ‘PATH’,
     ‘POSIXLY_CORRECT’, ‘SHELL’, ‘TMPDIR’) need to have
     properly-configured values in order to get standard behavior for
     any utility Emacs might invoke.  Even seemingly-benign variables
     like ‘TZ’ may have security implications.  *Note System
     Environment::.

     Emacs has customization and other variables with similar
     considerations.  For example, if the variable ‘shell-file-name’
     specifies a shell with nonstandard behavior, an Emacs-based
     application may misbehave.

Installation
     When Emacs is installed, if the installation directory hierarchy
     can be modified by untrusted users, the application cannot be
     trusted.  This applies also to the directory hierarchies of the
     programs that Emacs uses, and of the files that Emacs reads and
     writes.

Network access
     Emacs often accesses the network, and you may want to configure it
     to avoid network accesses that it would normally do.  For example,
     unless you set ‘tramp-mode’ to ‘nil’, file names using a certain
     syntax are interpreted as being network files, and are retrieved
     across the network.  *Note The Tramp Manual: (tramp)Top.

Race conditions
     Emacs applications have the same sort of race-condition issues that
     other applications do.  For example, even when ‘(file-readable-p
     "foo.txt")’ returns ‘t’, it could be that ‘foo.txt’ is unreadable
     because some other program changed the file’s permissions between
     the call to ‘file-readable-p’ and now.  *Note Testing
     Accessibility::.

Resource limits
     When Emacs exhausts memory or other operating system resources, its
     behavior can be less reliable, in that computations that ordinarily
     run to completion may abort back to the top level.  This may cause
     Emacs to neglect operations that it normally would have done.


File: elisp.info,  Node: Packaging,  Next: Antinews,  Prev: System Interface,  Up: Top

39 Preparing Lisp code for distribution
***************************************

Emacs provides a standard way to distribute Emacs Lisp code to users.  A
“package” is a collection of one or more files, formatted and bundled in
such a way that users can easily download, install, uninstall, and
upgrade it.

   The following sections describe how to create a package, and how to
put it in a “package archive” for others to download.  *Note
(emacs)Packages::, for a description of user-level features of the
packaging system.

* Menu:

* Packaging Basics::        The basic concepts of Emacs Lisp packages.
* Simple Packages::         How to package a single .el file.
* Multi-file Packages::     How to package multiple files.
* Package Archives::        Maintaining package archives.


File: elisp.info,  Node: Packaging Basics,  Next: Simple Packages,  Up: Packaging

39.1 Packaging Basics
=====================

A package is either a “simple package” or a “multi-file package”.  A
simple package is stored in a package archive as a single Emacs Lisp
file, while a multi-file package is stored as a tar file (containing
multiple Lisp files, and possibly non-Lisp files such as a manual).

   In ordinary usage, the difference between simple packages and
multi-file packages is relatively unimportant; the Package Menu
interface makes no distinction between them.  However, the procedure for
creating them differs, as explained in the following sections.

   Each package (whether simple or multi-file) has certain “attributes”:

Name
     A short word (e.g., ‘auctex’).  This is usually also the symbol
     prefix used in the program (*note Coding Conventions::).

Version
     A version number, in a form that the function ‘version-to-list’
     understands (e.g., ‘11.86’).  Each release of a package should be
     accompanied by an increase in the version number so that it will be
     recognized as an upgrade by users querying the package archive.

Brief description
     This is shown when the package is listed in the Package Menu.  It
     should occupy a single line, ideally in 36 characters or less.

Long description
     This is shown in the buffer created by ‘C-h P’
     (‘describe-package’), following the package’s brief description and
     installation status.  It normally spans multiple lines, and should
     fully describe the package’s capabilities and how to begin using it
     once it is installed.

Dependencies
     A list of other packages (possibly including minimal acceptable
     version numbers) on which this package depends.  The list may be
     empty, meaning this package has no dependencies.  Otherwise,
     installing this package also automatically installs its
     dependencies, recursively; if any dependency cannot be found, the
     package cannot be installed.

   Installing a package, either via the command ‘package-install-file’,
or via the Package Menu, creates a subdirectory of ‘package-user-dir’
named ‘NAME-VERSION’, where NAME is the package’s name and VERSION its
version (e.g., ‘~/.emacs.d/elpa/auctex-11.86/’).  We call this the
package’s “content directory”.  It is where Emacs puts the package’s
contents (the single Lisp file for a simple package, or the files
extracted from a multi-file package).

   Emacs then searches every Lisp file in the content directory for
autoload magic comments (*note Autoload::).  These autoload definitions
are saved to a file named ‘NAME-autoloads.el’ in the content directory.
They are typically used to autoload the principal user commands defined
in the package, but they can also perform other tasks, such as adding an
element to ‘auto-mode-alist’ (*note Auto Major Mode::).  Note that a
package typically does _not_ autoload every function and variable
defined within it—only the handful of commands typically called to begin
using the package.  Emacs then byte-compiles every Lisp file in the
package.

   After installation, the installed package is “loaded”: Emacs adds the
package’s content directory to ‘load-path’, and evaluates the autoload
definitions in ‘NAME-autoloads.el’.

   Whenever Emacs starts up, it automatically calls the function
‘package-initialize’ to load installed packages.  This is done after
loading the init file and abbrev file (if any) and before running
‘after-init-hook’ (*note Startup Summary::).  Automatic package loading
is disabled if the user option ‘package-enable-at-startup’ is ‘nil’.

 -- Command: package-initialize &optional no-activate
     This function initializes Emacs’ internal record of which packages
     are installed, and loads them.  The user option ‘package-load-list’
     specifies which packages to load; by default, all installed
     packages are loaded.  If called during startup, this function also
     sets ‘package-enable-at-startup’ to ‘nil’, to avoid accidentally
     loading the packages twice.  *Note (emacs)Package Installation::.

     The optional argument NO-ACTIVATE, if non-‘nil’, causes Emacs to
     update its record of installed packages without actually loading
     them; it is for internal use only.


File: elisp.info,  Node: Simple Packages,  Next: Multi-file Packages,  Prev: Packaging Basics,  Up: Packaging

39.2 Simple Packages
====================

A simple package consists of a single Emacs Lisp source file.  The file
must conform to the Emacs Lisp library header conventions (*note Library
Headers::).  The package’s attributes are taken from the various
headers, as illustrated by the following example:

     ;;; superfrobnicator.el --- Frobnicate and bifurcate flanges

     ;; Copyright (C) 2011 Free Software Foundation, Inc.

     ;; Author: J. R. Hacker <jrh@example.com>
     ;; Version: 1.3
     ;; Package-Requires: ((flange "1.0"))
     ;; Keywords: multimedia, frobnicate
     ;; URL: http://example.com/jrhacker/superfrobnicate

     ...

     ;;; Commentary:

     ;; This package provides a minor mode to frobnicate and/or
     ;; bifurcate any flanges you desire.  To activate it, just type
     ...

     ;;;###autoload
     (define-minor-mode superfrobnicator-mode
     ...

   The name of the package is the same as the base name of the file, as
written on the first line.  Here, it is ‘superfrobnicator’.

   The brief description is also taken from the first line.  Here, it is
‘Frobnicate and bifurcate flanges’.

   The version number comes from the ‘Package-Version’ header, if it
exists, or from the ‘Version’ header otherwise.  One or the other _must_
be present.  Here, the version number is 1.3.

   If the file has a ‘;;; Commentary:’ section, this section is used as
the long description.  (When displaying the description, Emacs omits the
‘;;; Commentary:’ line, as well as the leading comment characters in the
commentary itself.)

   If the file has a ‘Package-Requires’ header, that is used as the
package dependencies.  In the above example, the package depends on the
‘flange’ package, version 1.0 or higher.  *Note Library Headers::, for a
description of the ‘Package-Requires’ header.  If the header is omitted,
the package has no dependencies.

   The ‘Keywords’ and ‘URL’ headers are optional, but recommended.  The
command ‘describe-package’ uses these to add links to its output.  The
‘Keywords’ header should contain at least one standard keyword from the
‘finder-known-keywords’ list.

   The file ought to also contain one or more autoload magic comments,
as explained in *note Packaging Basics::.  In the above example, a magic
comment autoloads ‘superfrobnicator-mode’.

   *Note Package Archives::, for a explanation of how to add a
single-file package to a package archive.


File: elisp.info,  Node: Multi-file Packages,  Next: Package Archives,  Prev: Simple Packages,  Up: Packaging

39.3 Multi-file Packages
========================

A multi-file package is less convenient to create than a single-file
package, but it offers more features: it can include multiple Emacs Lisp
files, an Info manual, and other file types (such as images).

   Prior to installation, a multi-file package is stored in a package
archive as a tar file.  The tar file must be named ‘NAME-VERSION.tar’,
where NAME is the package name and VERSION is the version number.  Its
contents, once extracted, must all appear in a directory named
‘NAME-VERSION’, the “content directory” (*note Packaging Basics::).
Files may also extract into subdirectories of the content directory.

   One of the files in the content directory must be named
‘NAME-pkg.el’.  It must contain a single Lisp form, consisting of a call
to the function ‘define-package’, described below.  This defines the
package’s attributes: version, brief description, and requirements.

   For example, if we distribute version 1.3 of the superfrobnicator as
a multi-file package, the tar file would be ‘superfrobnicator-1.3.tar’.
Its contents would extract into the directory ‘superfrobnicator-1.3’,
and one of these would be the file ‘superfrobnicator-pkg.el’.

 -- Function: define-package name version &optional docstring
          requirements
     This function defines a package.  NAME is the package name, a
     string.  VERSION is the version, as a string of a form that can be
     understood by the function ‘version-to-list’.  DOCSTRING is the
     brief description.

     REQUIREMENTS is a list of required packages and their versions.
     Each element in this list should have the form ‘(DEP-NAME
     DEP-VERSION)’, where DEP-NAME is a symbol whose name is the
     dependency’s package name, and DEP-VERSION is the dependency’s
     version (a string).

   If the content directory contains a file named ‘README’, this file is
used as the long description.

   If the content directory contains a file named ‘dir’, this is assumed
to be an Info directory file made with ‘install-info’.  *Note Invoking
install-info: (texinfo)Invoking install-info.  The relevant Info files
should also be present in the content directory.  In this case, Emacs
will automatically add the content directory to ‘Info-directory-list’
when the package is activated.

   Do not include any ‘.elc’ files in the package.  Those are created
when the package is installed.  Note that there is no way to control the
order in which files are byte-compiled.

   Do not include any file named ‘NAME-autoloads.el’.  This file is
reserved for the package’s autoload definitions (*note Packaging
Basics::).  It is created automatically when the package is installed,
by searching all the Lisp files in the package for autoload magic
comments.

   If the multi-file package contains auxiliary data files (such as
images), the package’s Lisp code can refer to these files via the
variable ‘load-file-name’ (*note Loading::).  Here is an example:

     (defconst superfrobnicator-base (file-name-directory load-file-name))

     (defun superfrobnicator-fetch-image (file)
       (expand-file-name file superfrobnicator-base))


File: elisp.info,  Node: Package Archives,  Prev: Multi-file Packages,  Up: Packaging

39.4 Creating and Maintaining Package Archives
==============================================

Via the Package Menu, users may download packages from “package
archives”.  Such archives are specified by the variable
‘package-archives’, whose default value contains a single entry: the
archive hosted by the GNU project at <http://elpa.gnu.org>.  This
section describes how to set up and maintain a package archive.

 -- User Option: package-archives
     The value of this variable is an alist of package archives
     recognized by the Emacs package manager.

     Each alist element corresponds to one archive, and should have the
     form ‘(ID . LOCATION)’, where ID is the name of the archive (a
     string) and LOCATION is its “base location” (a string).

     If the base location starts with ‘http:’, it is treated as a HTTP
     URL, and packages are downloaded from this archive via HTTP (as is
     the case for the default GNU archive).

     Otherwise, the base location should be a directory name.  In this
     case, Emacs retrieves packages from this archive via ordinary file
     access.  Such local archives are mainly useful for testing.

   A package archive is simply a directory in which the package files,
and associated files, are stored.  If you want the archive to be
reachable via HTTP, this directory must be accessible to a web server.
How to accomplish this is beyond the scope of this manual.

   A convenient way to set up and update a package archive is via the
‘package-x’ library.  This is included with Emacs, but not loaded by
default; type ‘M-x load-library <RET> package-x <RET>’ to load it, or
add ‘(require 'package-x)’ to your init file.  *Note Lisp Libraries:
(emacs)Lisp Libraries.  Once loaded, you can make use of the following:

 -- User Option: package-archive-upload-base
     The value of this variable is the base location of a package
     archive, as a directory name.  The commands in the ‘package-x’
     library will use this base location.

     The directory name should be absolute.  You may specify a remote
     name, such as ‘/ssh:foo@example.com:/var/www/packages/’, if the
     package archive is on a different machine.  *Note Remote Files:
     (emacs)Remote Files.

 -- Command: package-upload-file filename
     This command prompts for FILENAME, a file name, and uploads that
     file to ‘package-archive-upload-base’.  The file must be either a
     simple package (a ‘.el’ file) or a multi-file package (a ‘.tar’
     file); otherwise, an error is raised.  The package attributes are
     automatically extracted, and the archive’s contents list is updated
     with this information.

     If ‘package-archive-upload-base’ does not specify a valid
     directory, the function prompts interactively for one.  If the
     directory does not exist, it is created.  The directory need not
     have any initial contents (i.e., you can use this command to
     populate an initially empty archive).

 -- Command: package-upload-buffer
     This command is similar to ‘package-upload-file’, but instead of
     prompting for a package file, it uploads the contents of the
     current buffer.  The current buffer must be visiting a simple
     package (a ‘.el’ file) or a multi-file package (a ‘.tar’ file);
     otherwise, an error is raised.

After you create an archive, remember that it is not accessible in the
Package Menu interface unless it is in ‘package-archives’.

   Maintaining a public package archive entails a degree of
responsibility.  When Emacs users install packages from your archive,
those packages can cause Emacs to run arbitrary code with the
permissions of the installing user.  (This is true for Emacs code in
general, not just for packages.)  So you should ensure that your archive
is well-maintained and keep the hosting system secure.

   One way to increase the security of your packages is to “sign” them
using a cryptographic key.  If you have generated a private/public gpg
key pair, you can use gpg to sign the package like this:

     gpg -ba -o FILE.sig FILE

For a single-file package, FILE is the package Lisp file; for a
multi-file package, it is the package tar file.  You can also sign the
archive’s contents file in the same way.  Make the ‘.sig’ files
available in the same location as the packages.  You should also make
your public key available for people to download; e.g., by uploading it
to a key server such as <http://pgp.mit.edu/>.  When people install
packages from your archive, they can use your public key to verify the
signatures.

   A full explanation of these matters is outside the scope of this
manual.  For more information on cryptographic keys and signing, *note
GnuPG: (gnupg)Top.  Emacs comes with an interface to GNU Privacy Guard,
*note EasyPG: (epa)Top.


File: elisp.info,  Node: Antinews,  Next: GNU Free Documentation License,  Prev: Packaging,  Up: Top

Appendix A Emacs 24 Antinews
****************************

For those users who live backwards in time, here is information about
downgrading to Emacs version 24.5.  We hope you will enjoy the greater
simplicity that results from the absence of many Emacs 25.3 features.

A.1 Old Lisp Features in Emacs 24
=================================

   • The requirement that ‘setq’ and ‘setf’ must be called with an even
     number of arguments has been removed.  You can now call them with
     an odd number of arguments, and Emacs will helpfully supply a ‘nil’
     for the missing one.  Simplicity rules!

   • ‘M-x shell’ and ‘M-x compile’ set the ‘EMACS’ environment variable,
     as they should, to indicate that the subprocess is run by Emacs.
     This is so packages that took years to learn how to work around
     that setting could continue using their code to that effect.

   • The ‘save-excursion’ form saves and restores the mark, as expected.
     No more need for the new ‘save-mark-and-excursion’, which has been
     deleted.

   • We have removed the ‘text-quoting-style’ variable and the
     associated functionality that translates quote characters in
     messages displayed to the user and in help buffers.  Emacs now
     shows exactly the same quote characters as you wrote in your code!
     Likewise, ‘substitute-command-keys’ leaves the quote characters
     alone.  As you move back in time, Unicode support becomes less and
     less important, so no need to display those fancy new quotes the
     Unicode Standard invented.

   • Regular expressions have been simplified by removing support for
     Unicode character properties in regexp classes.  As result,
     ‘[:alpha:]’ and ‘[:alnum:]’ will match any character with a word
     syntax, and ‘[:graph:]’ and ‘[:print:]’ will match any multibyte
     character, including surrogates and unassigned codepoints.  Once
     again, this is in line with diminishing importance of Unicode as
     you move back in time.

   • Evaluating ‘(/ N)’ will now yield N.  We have realized that
     interpreting that as in Common Lisp was a bad mistake that needed
     to be corrected.

   • The ‘pcase’ form was significantly simplified by removing the
     UPatterns ‘quote’ and ‘app’.  To further simplify this facility,
     we’ve removed ‘pcase-defmacro’, since we found no need for letting
     Lisp programs define new UPatterns.

   • We’ve removed the text properties ‘cursor-intangible’ and
     ‘cursor-sensor-functions’, replacing them by the much simpler
     ‘intangible’, ‘point-entered’, and ‘point-left’ properties.  The
     latter are implemented on a much lower level, and therefore are
     better integrated with user expectations.  For similar reasons,
     ‘cursor-intangible-mode’ and ‘cursor-sensor-mode’ were removed; use
     the hook variable ‘inhibit-point-motion-hooks’ which is no longer
     obsolete.

   • Process creation and management functions were significantly
     improved and simplified by removing ‘make-process’ and the ‘pipe’
     connection type.  Redirecting ‘stderr’ of a subprocess should be
     done with shell facilities, not by Emacs.

   • We decided that shutting up informative messages is bad for user
     interaction, so we’ve removed the ‘inhibit-message’ variable which
     could be used to that effect.

   • Support for generators and for finalizers has been removed, as we
     found no real need for these facilities.

   • Due to excessive complexity and the diminishing need for Unicode
     support, the functions ‘string-collate-lessp’ and
     ‘string-collate-equalp’ were removed.  Their locale-independent
     counterparts ‘string-lessp’ and ‘string-equal’ are so much more
     simple and yield predictable results that we don’t see any
     situation where the locale-dependent collation could be useful in
     Emacs.  As result, the ‘ls-lisp.el’ package sorts files in a
     locale-independent manner.

   • In preparation for removal in some past version of Emacs of the
     bidirectional editing support, we started by deleting two functions
     ‘bidi-find-overridden-directionality’ and
     ‘buffer-substring-with-bidi-context’.

   • Time conversion functions, such as ‘current-time-string’, no longer
     accept an optional ZONE argument.  If you need to change the
     current time zone (why?), do that explicitly with
     ‘set-time-zone-rule’.

   • As part of the ongoing quest for simplicity, many other functions
     and variables have been eliminated.


File: elisp.info,  Node: GNU Free Documentation License,  Next: GPL,  Prev: Antinews,  Up: Top

Appendix B GNU Free Documentation License
*****************************************

                     Version 1.3, 3 November 2008

     Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
     <http://fsf.org/>

     Everyone is permitted to copy and distribute verbatim copies
     of this license document, but changing it is not allowed.

  0. PREAMBLE

     The purpose of this License is to make a manual, textbook, or other
     functional and useful document “free” in the sense of freedom: to
     assure everyone the effective freedom to copy and redistribute it,
     with or without modifying it, either commercially or
     noncommercially.  Secondarily, this License preserves for the
     author and publisher a way to get credit for their work, while not
     being considered responsible for modifications made by others.

     This License is a kind of “copyleft”, which means that derivative
     works of the document must themselves be free in the same sense.
     It complements the GNU General Public License, which is a copyleft
     license designed for free software.

     We have designed this License in order to use it for manuals for
     free software, because free software needs free documentation: a
     free program should come with manuals providing the same freedoms
     that the software does.  But this License is not limited to
     software manuals; it can be used for any textual work, regardless
     of subject matter or whether it is published as a printed book.  We
     recommend this License principally for works whose purpose is
     instruction or reference.

  1. APPLICABILITY AND DEFINITIONS

     This License applies to any manual or other work, in any medium,
     that contains a notice placed by the copyright holder saying it can
     be distributed under the terms of this License.  Such a notice
     grants a world-wide, royalty-free license, unlimited in duration,
     to use that work under the conditions stated herein.  The
     “Document”, below, refers to any such manual or work.  Any member
     of the public is a licensee, and is addressed as “you”.  You accept
     the license if you copy, modify or distribute the work in a way
     requiring permission under copyright law.

     A “Modified Version” of the Document means any work containing the
     Document or a portion of it, either copied verbatim, or with
     modifications and/or translated into another language.

     A “Secondary Section” is a named appendix or a front-matter section
     of the Document that deals exclusively with the relationship of the
     publishers or authors of the Document to the Document’s overall
     subject (or to related matters) and contains nothing that could
     fall directly within that overall subject.  (Thus, if the Document
     is in part a textbook of mathematics, a Secondary Section may not
     explain any mathematics.)  The relationship could be a matter of
     historical connection with the subject or with related matters, or
     of legal, commercial, philosophical, ethical or political position
     regarding them.

     The “Invariant Sections” are certain Secondary Sections whose
     titles are designated, as being those of Invariant Sections, in the
     notice that says that the Document is released under this License.
     If a section does not fit the above definition of Secondary then it
     is not allowed to be designated as Invariant.  The Document may
     contain zero Invariant Sections.  If the Document does not identify
     any Invariant Sections then there are none.

     The “Cover Texts” are certain short passages of text that are
     listed, as Front-Cover Texts or Back-Cover Texts, in the notice
     that says that the Document is released under this License.  A
     Front-Cover Text may be at most 5 words, and a Back-Cover Text may
     be at most 25 words.

     A “Transparent” copy of the Document means a machine-readable copy,
     represented in a format whose specification is available to the
     general public, that is suitable for revising the document
     straightforwardly with generic text editors or (for images composed
     of pixels) generic paint programs or (for drawings) some widely
     available drawing editor, and that is suitable for input to text
     formatters or for automatic translation to a variety of formats
     suitable for input to text formatters.  A copy made in an otherwise
     Transparent file format whose markup, or absence of markup, has
     been arranged to thwart or discourage subsequent modification by
     readers is not Transparent.  An image format is not Transparent if
     used for any substantial amount of text.  A copy that is not
     “Transparent” is called “Opaque”.

     Examples of suitable formats for Transparent copies include plain
     ASCII without markup, Texinfo input format, LaTeX input format,
     SGML or XML using a publicly available DTD, and standard-conforming
     simple HTML, PostScript or PDF designed for human modification.
     Examples of transparent image formats include PNG, XCF and JPG.
     Opaque formats include proprietary formats that can be read and
     edited only by proprietary word processors, SGML or XML for which
     the DTD and/or processing tools are not generally available, and
     the machine-generated HTML, PostScript or PDF produced by some word
     processors for output purposes only.

     The “Title Page” means, for a printed book, the title page itself,
     plus such following pages as are needed to hold, legibly, the
     material this License requires to appear in the title page.  For
     works in formats which do not have any title page as such, “Title
     Page” means the text near the most prominent appearance of the
     work’s title, preceding the beginning of the body of the text.

     The “publisher” means any person or entity that distributes copies
     of the Document to the public.

     A section “Entitled XYZ” means a named subunit of the Document
     whose title either is precisely XYZ or contains XYZ in parentheses
     following text that translates XYZ in another language.  (Here XYZ
     stands for a specific section name mentioned below, such as
     “Acknowledgements”, “Dedications”, “Endorsements”, or “History”.)
     To “Preserve the Title” of such a section when you modify the
     Document means that it remains a section “Entitled XYZ” according
     to this definition.

     The Document may include Warranty Disclaimers next to the notice
     which states that this License applies to the Document.  These
     Warranty Disclaimers are considered to be included by reference in
     this License, but only as regards disclaiming warranties: any other
     implication that these Warranty Disclaimers may have is void and
     has no effect on the meaning of this License.

  2. VERBATIM COPYING

     You may copy and distribute the Document in any medium, either
     commercially or noncommercially, provided that this License, the
     copyright notices, and the license notice saying this License
     applies to the Document are reproduced in all copies, and that you
     add no other conditions whatsoever to those of this License.  You
     may not use technical measures to obstruct or control the reading
     or further copying of the copies you make or distribute.  However,
     you may accept compensation in exchange for copies.  If you
     distribute a large enough number of copies you must also follow the
     conditions in section 3.

     You may also lend copies, under the same conditions stated above,
     and you may publicly display copies.

  3. COPYING IN QUANTITY

     If you publish printed copies (or copies in media that commonly
     have printed covers) of the Document, numbering more than 100, and
     the Document’s license notice requires Cover Texts, you must
     enclose the copies in covers that carry, clearly and legibly, all
     these Cover Texts: Front-Cover Texts on the front cover, and
     Back-Cover Texts on the back cover.  Both covers must also clearly
     and legibly identify you as the publisher of these copies.  The
     front cover must present the full title with all words of the title
     equally prominent and visible.  You may add other material on the
     covers in addition.  Copying with changes limited to the covers, as
     long as they preserve the title of the Document and satisfy these
     conditions, can be treated as verbatim copying in other respects.

     If the required texts for either cover are too voluminous to fit
     legibly, you should put the first ones listed (as many as fit
     reasonably) on the actual cover, and continue the rest onto
     adjacent pages.

     If you publish or distribute Opaque copies of the Document
     numbering more than 100, you must either include a machine-readable
     Transparent copy along with each Opaque copy, or state in or with
     each Opaque copy a computer-network location from which the general
     network-using public has access to download using public-standard
     network protocols a complete Transparent copy of the Document, free
     of added material.  If you use the latter option, you must take
     reasonably prudent steps, when you begin distribution of Opaque
     copies in quantity, to ensure that this Transparent copy will
     remain thus accessible at the stated location until at least one
     year after the last time you distribute an Opaque copy (directly or
     through your agents or retailers) of that edition to the public.

     It is requested, but not required, that you contact the authors of
     the Document well before redistributing any large number of copies,
     to give them a chance to provide you with an updated version of the
     Document.

  4. MODIFICATIONS

     You may copy and distribute a Modified Version of the Document
     under the conditions of sections 2 and 3 above, provided that you
     release the Modified Version under precisely this License, with the
     Modified Version filling the role of the Document, thus licensing
     distribution and modification of the Modified Version to whoever
     possesses a copy of it.  In addition, you must do these things in
     the Modified Version:

       A. Use in the Title Page (and on the covers, if any) a title
          distinct from that of the Document, and from those of previous
          versions (which should, if there were any, be listed in the
          History section of the Document).  You may use the same title
          as a previous version if the original publisher of that
          version gives permission.

       B. List on the Title Page, as authors, one or more persons or
          entities responsible for authorship of the modifications in
          the Modified Version, together with at least five of the
          principal authors of the Document (all of its principal
          authors, if it has fewer than five), unless they release you
          from this requirement.

       C. State on the Title page the name of the publisher of the
          Modified Version, as the publisher.

       D. Preserve all the copyright notices of the Document.

       E. Add an appropriate copyright notice for your modifications
          adjacent to the other copyright notices.

       F. Include, immediately after the copyright notices, a license
          notice giving the public permission to use the Modified
          Version under the terms of this License, in the form shown in
          the Addendum below.

       G. Preserve in that license notice the full lists of Invariant
          Sections and required Cover Texts given in the Document’s
          license notice.

       H. Include an unaltered copy of this License.

       I. Preserve the section Entitled “History”, Preserve its Title,
          and add to it an item stating at least the title, year, new
          authors, and publisher of the Modified Version as given on the
          Title Page.  If there is no section Entitled “History” in the
          Document, create one stating the title, year, authors, and
          publisher of the Document as given on its Title Page, then add
          an item describing the Modified Version as stated in the
          previous sentence.

       J. Preserve the network location, if any, given in the Document
          for public access to a Transparent copy of the Document, and
          likewise the network locations given in the Document for
          previous versions it was based on.  These may be placed in the
          “History” section.  You may omit a network location for a work
          that was published at least four years before the Document
          itself, or if the original publisher of the version it refers
          to gives permission.

       K. For any section Entitled “Acknowledgements” or “Dedications”,
          Preserve the Title of the section, and preserve in the section
          all the substance and tone of each of the contributor
          acknowledgements and/or dedications given therein.

       L. Preserve all the Invariant Sections of the Document, unaltered
          in their text and in their titles.  Section numbers or the
          equivalent are not considered part of the section titles.

       M. Delete any section Entitled “Endorsements”.  Such a section
          may not be included in the Modified Version.

       N. Do not retitle any existing section to be Entitled
          “Endorsements” or to conflict in title with any Invariant
          Section.

       O. Preserve any Warranty Disclaimers.

     If the Modified Version includes new front-matter sections or
     appendices that qualify as Secondary Sections and contain no
     material copied from the Document, you may at your option designate
     some or all of these sections as invariant.  To do this, add their
     titles to the list of Invariant Sections in the Modified Version’s
     license notice.  These titles must be distinct from any other
     section titles.

     You may add a section Entitled “Endorsements”, provided it contains
     nothing but endorsements of your Modified Version by various
     parties—for example, statements of peer review or that the text has
     been approved by an organization as the authoritative definition of
     a standard.

     You may add a passage of up to five words as a Front-Cover Text,
     and a passage of up to 25 words as a Back-Cover Text, to the end of
     the list of Cover Texts in the Modified Version.  Only one passage
     of Front-Cover Text and one of Back-Cover Text may be added by (or
     through arrangements made by) any one entity.  If the Document
     already includes a cover text for the same cover, previously added
     by you or by arrangement made by the same entity you are acting on
     behalf of, you may not add another; but you may replace the old
     one, on explicit permission from the previous publisher that added
     the old one.

     The author(s) and publisher(s) of the Document do not by this
     License give permission to use their names for publicity for or to
     assert or imply endorsement of any Modified Version.

  5. COMBINING DOCUMENTS

     You may combine the Document with other documents released under
     this License, under the terms defined in section 4 above for
     modified versions, provided that you include in the combination all
     of the Invariant Sections of all of the original documents,
     unmodified, and list them all as Invariant Sections of your
     combined work in its license notice, and that you preserve all
     their Warranty Disclaimers.

     The combined work need only contain one copy of this License, and
     multiple identical Invariant Sections may be replaced with a single
     copy.  If there are multiple Invariant Sections with the same name
     but different contents, make the title of each such section unique
     by adding at the end of it, in parentheses, the name of the
     original author or publisher of that section if known, or else a
     unique number.  Make the same adjustment to the section titles in
     the list of Invariant Sections in the license notice of the
     combined work.

     In the combination, you must combine any sections Entitled
     “History” in the various original documents, forming one section
     Entitled “History”; likewise combine any sections Entitled
     “Acknowledgements”, and any sections Entitled “Dedications”.  You
     must delete all sections Entitled “Endorsements.”

  6. COLLECTIONS OF DOCUMENTS

     You may make a collection consisting of the Document and other
     documents released under this License, and replace the individual
     copies of this License in the various documents with a single copy
     that is included in the collection, provided that you follow the
     rules of this License for verbatim copying of each of the documents
     in all other respects.

     You may extract a single document from such a collection, and
     distribute it individually under this License, provided you insert
     a copy of this License into the extracted document, and follow this
     License in all other respects regarding verbatim copying of that
     document.

  7. AGGREGATION WITH INDEPENDENT WORKS

     A compilation of the Document or its derivatives with other
     separate and independent documents or works, in or on a volume of a
     storage or distribution medium, is called an “aggregate” if the
     copyright resulting from the compilation is not used to limit the
     legal rights of the compilation’s users beyond what the individual
     works permit.  When the Document is included in an aggregate, this
     License does not apply to the other works in the aggregate which
     are not themselves derivative works of the Document.

     If the Cover Text requirement of section 3 is applicable to these
     copies of the Document, then if the Document is less than one half
     of the entire aggregate, the Document’s Cover Texts may be placed
     on covers that bracket the Document within the aggregate, or the
     electronic equivalent of covers if the Document is in electronic
     form.  Otherwise they must appear on printed covers that bracket
     the whole aggregate.

  8. TRANSLATION

     Translation is considered a kind of modification, so you may
     distribute translations of the Document under the terms of section
     4.  Replacing Invariant Sections with translations requires special
     permission from their copyright holders, but you may include
     translations of some or all Invariant Sections in addition to the
     original versions of these Invariant Sections.  You may include a
     translation of this License, and all the license notices in the
     Document, and any Warranty Disclaimers, provided that you also
     include the original English version of this License and the
     original versions of those notices and disclaimers.  In case of a
     disagreement between the translation and the original version of
     this License or a notice or disclaimer, the original version will
     prevail.

     If a section in the Document is Entitled “Acknowledgements”,
     “Dedications”, or “History”, the requirement (section 4) to
     Preserve its Title (section 1) will typically require changing the
     actual title.

  9. TERMINATION

     You may not copy, modify, sublicense, or distribute the Document
     except as expressly provided under this License.  Any attempt
     otherwise to copy, modify, sublicense, or distribute it is void,
     and will automatically terminate your rights under this License.

     However, if you cease all violation of this License, then your
     license from a particular copyright holder is reinstated (a)
     provisionally, unless and until the copyright holder explicitly and
     finally terminates your license, and (b) permanently, if the
     copyright holder fails to notify you of the violation by some
     reasonable means prior to 60 days after the cessation.

     Moreover, your license from a particular copyright holder is
     reinstated permanently if the copyright holder notifies you of the
     violation by some reasonable means, this is the first time you have
     received notice of violation of this License (for any work) from
     that copyright holder, and you cure the violation prior to 30 days
     after your receipt of the notice.

     Termination of your rights under this section does not terminate
     the licenses of parties who have received copies or rights from you
     under this License.  If your rights have been terminated and not
     permanently reinstated, receipt of a copy of some or all of the
     same material does not give you any rights to use it.

  10. FUTURE REVISIONS OF THIS LICENSE

     The Free Software Foundation may publish new, revised versions of
     the GNU Free Documentation License from time to time.  Such new
     versions will be similar in spirit to the present version, but may
     differ in detail to address new problems or concerns.  See
     <http://www.gnu.org/copyleft/>.

     Each version of the License is given a distinguishing version
     number.  If the Document specifies that a particular numbered
     version of this License “or any later version” applies to it, you
     have the option of following the terms and conditions either of
     that specified version or of any later version that has been
     published (not as a draft) by the Free Software Foundation.  If the
     Document does not specify a version number of this License, you may
     choose any version ever published (not as a draft) by the Free
     Software Foundation.  If the Document specifies that a proxy can
     decide which future versions of this License can be used, that
     proxy’s public statement of acceptance of a version permanently
     authorizes you to choose that version for the Document.

  11. RELICENSING

     “Massive Multiauthor Collaboration Site” (or “MMC Site”) means any
     World Wide Web server that publishes copyrightable works and also
     provides prominent facilities for anybody to edit those works.  A
     public wiki that anybody can edit is an example of such a server.
     A “Massive Multiauthor Collaboration” (or “MMC”) contained in the
     site means any set of copyrightable works thus published on the MMC
     site.

     “CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0
     license published by Creative Commons Corporation, a not-for-profit
     corporation with a principal place of business in San Francisco,
     California, as well as future copyleft versions of that license
     published by that same organization.

     “Incorporate” means to publish or republish a Document, in whole or
     in part, as part of another Document.

     An MMC is “eligible for relicensing” if it is licensed under this
     License, and if all works that were first published under this
     License somewhere other than this MMC, and subsequently
     incorporated in whole or in part into the MMC, (1) had no cover
     texts or invariant sections, and (2) were thus incorporated prior
     to November 1, 2008.

     The operator of an MMC Site may republish an MMC contained in the
     site under CC-BY-SA on the same site at any time before August 1,
     2009, provided the MMC is eligible for relicensing.

ADDENDUM: How to use this License for your documents
====================================================

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and license
notices just after the title page:

       Copyright (C)  YEAR  YOUR NAME.
       Permission is granted to copy, distribute and/or modify this document
       under the terms of the GNU Free Documentation License, Version 1.3
       or any later version published by the Free Software Foundation;
       with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
       Texts.  A copy of the license is included in the section entitled ``GNU
       Free Documentation License''.

   If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the “with...Texts.” line with this:

         with the Invariant Sections being LIST THEIR TITLES, with
         the Front-Cover Texts being LIST, and with the Back-Cover Texts
         being LIST.

   If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

   If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of free
software license, such as the GNU General Public License, to permit
their use in free software.


File: elisp.info,  Node: GPL,  Next: Tips,  Prev: GNU Free Documentation License,  Up: Top

Appendix C GNU General Public License
*************************************

                        Version 3, 29 June 2007

     Copyright © 2007 Free Software Foundation, Inc. <http://fsf.org/>

     Everyone is permitted to copy and distribute verbatim copies of this
     license document, but changing it is not allowed.

Preamble
========

The GNU General Public License is a free, copyleft license for software
and other kinds of works.

   The licenses for most software and other practical works are designed
to take away your freedom to share and change the works.  By contrast,
the GNU General Public License is intended to guarantee your freedom to
share and change all versions of a program—to make sure it remains free
software for all its users.  We, the Free Software Foundation, use the
GNU General Public License for most of our software; it applies also to
any other work released this way by its authors.  You can apply it to
your programs, too.

   When we speak of free software, we are referring to freedom, not
price.  Our General Public Licenses are designed to make sure that you
have the freedom to distribute copies of free software (and charge for
them if you wish), that you receive source code or can get it if you
want it, that you can change the software or use pieces of it in new
free programs, and that you know you can do these things.

   To protect your rights, we need to prevent others from denying you
these rights or asking you to surrender the rights.  Therefore, you have
certain responsibilities if you distribute copies of the software, or if
you modify it: responsibilities to respect the freedom of others.

   For example, if you distribute copies of such a program, whether
gratis or for a fee, you must pass on to the recipients the same
freedoms that you received.  You must make sure that they, too, receive
or can get the source code.  And you must show them these terms so they
know their rights.

   Developers that use the GNU GPL protect your rights with two steps:
(1) assert copyright on the software, and (2) offer you this License
giving you legal permission to copy, distribute and/or modify it.

   For the developers’ and authors’ protection, the GPL clearly explains
that there is no warranty for this free software.  For both users’ and
authors’ sake, the GPL requires that modified versions be marked as
changed, so that their problems will not be attributed erroneously to
authors of previous versions.

   Some devices are designed to deny users access to install or run
modified versions of the software inside them, although the manufacturer
can do so.  This is fundamentally incompatible with the aim of
protecting users’ freedom to change the software.  The systematic
pattern of such abuse occurs in the area of products for individuals to
use, which is precisely where it is most unacceptable.  Therefore, we
have designed this version of the GPL to prohibit the practice for those
products.  If such problems arise substantially in other domains, we
stand ready to extend this provision to those domains in future versions
of the GPL, as needed to protect the freedom of users.

   Finally, every program is threatened constantly by software patents.
States should not allow patents to restrict development and use of
software on general-purpose computers, but in those that do, we wish to
avoid the special danger that patents applied to a free program could
make it effectively proprietary.  To prevent this, the GPL assures that
patents cannot be used to render the program non-free.

   The precise terms and conditions for copying, distribution and
modification follow.

TERMS AND CONDITIONS
====================

  0. Definitions.

     “This License” refers to version 3 of the GNU General Public
     License.

     “Copyright” also means copyright-like laws that apply to other
     kinds of works, such as semiconductor masks.

     “The Program” refers to any copyrightable work licensed under this
     License.  Each licensee is addressed as “you”.  “Licensees” and
     “recipients” may be individuals or organizations.

     To “modify” a work means to copy from or adapt all or part of the
     work in a fashion requiring copyright permission, other than the
     making of an exact copy.  The resulting work is called a “modified
     version” of the earlier work or a work “based on” the earlier work.

     A “covered work” means either the unmodified Program or a work
     based on the Program.

     To “propagate” a work means to do anything with it that, without
     permission, would make you directly or secondarily liable for
     infringement under applicable copyright law, except executing it on
     a computer or modifying a private copy.  Propagation includes
     copying, distribution (with or without modification), making
     available to the public, and in some countries other activities as
     well.

     To “convey” a work means any kind of propagation that enables other
     parties to make or receive copies.  Mere interaction with a user
     through a computer network, with no transfer of a copy, is not
     conveying.

     An interactive user interface displays “Appropriate Legal Notices”
     to the extent that it includes a convenient and prominently visible
     feature that (1) displays an appropriate copyright notice, and (2)
     tells the user that there is no warranty for the work (except to
     the extent that warranties are provided), that licensees may convey
     the work under this License, and how to view a copy of this
     License.  If the interface presents a list of user commands or
     options, such as a menu, a prominent item in the list meets this
     criterion.

  1. Source Code.

     The “source code” for a work means the preferred form of the work
     for making modifications to it.  “Object code” means any non-source
     form of a work.

     A “Standard Interface” means an interface that either is an
     official standard defined by a recognized standards body, or, in
     the case of interfaces specified for a particular programming
     language, one that is widely used among developers working in that
     language.

     The “System Libraries” of an executable work include anything,
     other than the work as a whole, that (a) is included in the normal
     form of packaging a Major Component, but which is not part of that
     Major Component, and (b) serves only to enable use of the work with
     that Major Component, or to implement a Standard Interface for
     which an implementation is available to the public in source code
     form.  A “Major Component”, in this context, means a major
     essential component (kernel, window system, and so on) of the
     specific operating system (if any) on which the executable work
     runs, or a compiler used to produce the work, or an object code
     interpreter used to run it.

     The “Corresponding Source” for a work in object code form means all
     the source code needed to generate, install, and (for an executable
     work) run the object code and to modify the work, including scripts
     to control those activities.  However, it does not include the
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     The Corresponding Source need not include anything that users can
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     The Corresponding Source for a work in source code form is that
     same work.

  2. Basic Permissions.

     All rights granted under this License are granted for the term of
     copyright on the Program, and are irrevocable provided the stated
     conditions are met.  This License explicitly affirms your unlimited
     permission to run the unmodified Program.  The output from running
     a covered work is covered by this License only if the output, given
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     You may make, run and propagate covered works that you do not
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     Conveying under any other circumstances is permitted solely under
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  3. Protecting Users’ Legal Rights From Anti-Circumvention Law.

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  4. Conveying Verbatim Copies.

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  5. Conveying Modified Source Versions.

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       d. If the work has interactive user interfaces, each must display
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       d. Convey the object code by offering access from a designated
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       e. Convey the object code using peer-to-peer transmission,
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     A separable portion of the object code, whose source code is
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     A “User Product” is either (1) a “consumer product”, which means
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     If you convey an object code work under this section in, or with,
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     this requirement does not apply if neither you nor any third party
     retains the ability to install modified object code on the User
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     The requirement to provide Installation Information does not
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     Corresponding Source conveyed, and Installation Information
     provided, in accord with this section must be in a format that is
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     public in source code form), and must require no special password
     or key for unpacking, reading or copying.

  7. Additional Terms.

     “Additional permissions” are terms that supplement the terms of
     this License by making exceptions from one or more of its
     conditions.  Additional permissions that are applicable to the
     entire Program shall be treated as though they were included in
     this License, to the extent that they are valid under applicable
     law.  If additional permissions apply only to part of the Program,
     that part may be used separately under those permissions, but the
     entire Program remains governed by this License without regard to
     the additional permissions.

     When you convey a copy of a covered work, you may at your option
     remove any additional permissions from that copy, or from any part
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     Notwithstanding any other provision of this License, for material
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     holders of that material) supplement the terms of this License with
     terms:

       a. Disclaiming warranty or limiting liability differently from
          the terms of sections 15 and 16 of this License; or

       b. Requiring preservation of specified reasonable legal notices
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       d. Limiting the use for publicity purposes of names of licensors
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       f. Requiring indemnification of licensors and authors of that
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     All other non-permissive additional terms are considered “further
     restrictions” within the meaning of section 10.  If the Program as
     you received it, or any part of it, contains a notice stating that
     it is governed by this License along with a term that is a further
     restriction, you may remove that term.  If a license document
     contains a further restriction but permits relicensing or conveying
     under this License, you may add to a covered work material governed
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     restriction does not survive such relicensing or conveying.

     If you add terms to a covered work in accord with this section, you
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     Additional terms, permissive or non-permissive, may be stated in
     the form of a separately written license, or stated as exceptions;
     the above requirements apply either way.

  8. Termination.

     You may not propagate or modify a covered work except as expressly
     provided under this License.  Any attempt otherwise to propagate or
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     under this License (including any patent licenses granted under the
     third paragraph of section 11).

     However, if you cease all violation of this License, then your
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     copyright holder fails to notify you of the violation by some
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     Moreover, your license from a particular copyright holder is
     reinstated permanently if the copyright holder notifies you of the
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     received notice of violation of this License (for any work) from
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     after your receipt of the notice.

     Termination of your rights under this section does not terminate
     the licenses of parties who have received copies or rights from you
     under this License.  If your rights have been terminated and not
     permanently reinstated, you do not qualify to receive new licenses
     for the same material under section 10.

  9. Acceptance Not Required for Having Copies.

     You are not required to accept this License in order to receive or
     run a copy of the Program.  Ancillary propagation of a covered work
     occurring solely as a consequence of using peer-to-peer
     transmission to receive a copy likewise does not require
     acceptance.  However, nothing other than this License grants you
     permission to propagate or modify any covered work.  These actions
     infringe copyright if you do not accept this License.  Therefore,
     by modifying or propagating a covered work, you indicate your
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  10. Automatic Licensing of Downstream Recipients.

     Each time you convey a covered work, the recipient automatically
     receives a license from the original licensors, to run, modify and
     propagate that work, subject to this License.  You are not
     responsible for enforcing compliance by third parties with this
     License.

     An “entity transaction” is a transaction transferring control of an
     organization, or substantially all assets of one, or subdividing an
     organization, or merging organizations.  If propagation of a
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     efforts.

     You may not impose any further restrictions on the exercise of the
     rights granted or affirmed under this License.  For example, you
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     of rights granted under this License, and you may not initiate
     litigation (including a cross-claim or counterclaim in a lawsuit)
     alleging that any patent claim is infringed by making, using,
     selling, offering for sale, or importing the Program or any portion
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  11. Patents.

     A “contributor” is a copyright holder who authorizes use under this
     License of the Program or a work on which the Program is based.
     The work thus licensed is called the contributor’s “contributor
     version”.

     A contributor’s “essential patent claims” are all patent claims
     owned or controlled by the contributor, whether already acquired or
     hereafter acquired, that would be infringed by some manner,
     permitted by this License, of making, using, or selling its
     contributor version, but do not include claims that would be
     infringed only as a consequence of further modification of the
     contributor version.  For purposes of this definition, “control”
     includes the right to grant patent sublicenses in a manner
     consistent with the requirements of this License.

     Each contributor grants you a non-exclusive, worldwide,
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     In the following three paragraphs, a “patent license” is any
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     enforce a patent (such as an express permission to practice a
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     such a patent license to a party means to make such an agreement or
     commitment not to enforce a patent against the party.

     If you convey a covered work, knowingly relying on a patent
     license, and the Corresponding Source of the work is not available
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     readily accessible means, then you must either (1) cause the
     Corresponding Source to be so available, or (2) arrange to deprive
     yourself of the benefit of the patent license for this particular
     work, or (3) arrange, in a manner consistent with the requirements
     of this License, to extend the patent license to downstream
     recipients.  “Knowingly relying” means you have actual knowledge
     that, but for the patent license, your conveying the covered work
     in a country, or your recipient’s use of the covered work in a
     country, would infringe one or more identifiable patents in that
     country that you have reason to believe are valid.

     If, pursuant to or in connection with a single transaction or
     arrangement, you convey, or propagate by procuring conveyance of, a
     covered work, and grant a patent license to some of the parties
     receiving the covered work authorizing them to use, propagate,
     modify or convey a specific copy of the covered work, then the
     patent license you grant is automatically extended to all
     recipients of the covered work and works based on it.

     A patent license is “discriminatory” if it does not include within
     the scope of its coverage, prohibits the exercise of, or is
     conditioned on the non-exercise of one or more of the rights that
     are specifically granted under this License.  You may not convey a
     covered work if you are a party to an arrangement with a third
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     you make payment to the third party based on the extent of your
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     grants, to any of the parties who would receive the covered work
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     entered into that arrangement, or that patent license was granted,
     prior to 28 March 2007.

     Nothing in this License shall be construed as excluding or limiting
     any implied license or other defenses to infringement that may
     otherwise be available to you under applicable patent law.

  12. No Surrender of Others’ Freedom.

     If conditions are imposed on you (whether by court order, agreement
     or otherwise) that contradict the conditions of this License, they
     do not excuse you from the conditions of this License.  If you
     cannot convey a covered work so as to satisfy simultaneously your
     obligations under this License and any other pertinent obligations,
     then as a consequence you may not convey it at all.  For example,
     if you agree to terms that obligate you to collect a royalty for
     further conveying from those to whom you convey the Program, the
     only way you could satisfy both those terms and this License would
     be to refrain entirely from conveying the Program.

  13. Use with the GNU Affero General Public License.

     Notwithstanding any other provision of this License, you have
     permission to link or combine any covered work with a work licensed
     under version 3 of the GNU Affero General Public License into a
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     of this License will continue to apply to the part which is the
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     General Public License, section 13, concerning interaction through
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  14. Revised Versions of this License.

     The Free Software Foundation may publish revised and/or new
     versions of the GNU General Public License from time to time.  Such
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     may differ in detail to address new problems or concerns.

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     If the Program specifies that a proxy can decide which future
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     Later license versions may give you additional or different
     permissions.  However, no additional obligations are imposed on any
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  15. Disclaimer of Warranty.

     THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY
     APPLICABLE LAW.  EXCEPT WHEN OTHERWISE STATED IN WRITING THE
     COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM “AS IS”
     WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED,
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     SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL
     NECESSARY SERVICING, REPAIR OR CORRECTION.

  16. Limitation of Liability.

     IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN
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     BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD
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  17. Interpretation of Sections 15 and 16.

     If the disclaimer of warranty and limitation of liability provided
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     approximates an absolute waiver of all civil liability in
     connection with the Program, unless a warranty or assumption of
     liability accompanies a copy of the Program in return for a fee.

END OF TERMS AND CONDITIONS
===========================

How to Apply These Terms to Your New Programs
=============================================

If you develop a new program, and you want it to be of the greatest
possible use to the public, the best way to achieve this is to make it
free software which everyone can redistribute and change under these
terms.

   To do so, attach the following notices to the program.  It is safest
to attach them to the start of each source file to most effectively
state the exclusion of warranty; and each file should have at least the
“copyright” line and a pointer to where the full notice is found.

     ONE LINE TO GIVE THE PROGRAM'S NAME AND A BRIEF IDEA OF WHAT IT DOES.
     Copyright (C) YEAR NAME OF AUTHOR

     This program is free software: you can redistribute it and/or modify
     it under the terms of the GNU General Public License as published by
     the Free Software Foundation, either version 3 of the License, or (at
     your option) any later version.

     This program is distributed in the hope that it will be useful, but
     WITHOUT ANY WARRANTY; without even the implied warranty of
     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
     General Public License for more details.

     You should have received a copy of the GNU General Public License
     along with this program.  If not, see <http://www.gnu.org/licenses/>.

   Also add information on how to contact you by electronic and paper
mail.

   If the program does terminal interaction, make it output a short
notice like this when it starts in an interactive mode:

     PROGRAM Copyright (C) YEAR NAME OF AUTHOR
     This program comes with ABSOLUTELY NO WARRANTY; for details type ‘show w’.
     This is free software, and you are welcome to redistribute it
     under certain conditions; type ‘show c’ for details.

   The hypothetical commands ‘show w’ and ‘show c’ should show the
appropriate parts of the General Public License.  Of course, your
program’s commands might be different; for a GUI interface, you would
use an “about box”.

   You should also get your employer (if you work as a programmer) or
school, if any, to sign a “copyright disclaimer” for the program, if
necessary.  For more information on this, and how to apply and follow
the GNU GPL, see <http://www.gnu.org/licenses/>.

   The GNU General Public License does not permit incorporating your
program into proprietary programs.  If your program is a subroutine
library, you may consider it more useful to permit linking proprietary
applications with the library.  If this is what you want to do, use the
GNU Lesser General Public License instead of this License.  But first,
please read <http://www.gnu.org/philosophy/why-not-lgpl.html>.


File: elisp.info,  Node: Tips,  Next: GNU Emacs Internals,  Prev: GPL,  Up: Top

Appendix D Tips and Conventions
*******************************

This chapter describes no additional features of Emacs Lisp.  Instead it
gives advice on making effective use of the features described in the
previous chapters, and describes conventions Emacs Lisp programmers
should follow.

   You can automatically check some of the conventions described below
by running the command ‘M-x checkdoc RET’ when visiting a Lisp file.  It
cannot check all of the conventions, and not all the warnings it gives
necessarily correspond to problems, but it is worth examining them all.
Alternatively, use the command ‘M-x checkdoc-current-buffer RET’ to
check the conventions in the current buffer, or ‘checkdoc-file’ when you
want to check a file in batch mode, e.g., with a command run by
‘M-x compile RET’.

* Menu:

* Coding Conventions::        Conventions for clean and robust programs.
* Key Binding Conventions::   Which keys should be bound by which programs.
* Programming Tips::          Making Emacs code fit smoothly in Emacs.
* Compilation Tips::          Making compiled code run fast.
* Warning Tips::              Turning off compiler warnings.
* Documentation Tips::        Writing readable documentation strings.
* Comment Tips::              Conventions for writing comments.
* Library Headers::           Standard headers for library packages.


File: elisp.info,  Node: Coding Conventions,  Next: Key Binding Conventions,  Up: Tips

D.1 Emacs Lisp Coding Conventions
=================================

Here are conventions that you should follow when writing Emacs Lisp code
intended for widespread use:

   • Simply loading a package should not change Emacs’s editing
     behavior.  Include a command or commands to enable and disable the
     feature, or to invoke it.

     This convention is mandatory for any file that includes custom
     definitions.  If fixing such a file to follow this convention
     requires an incompatible change, go ahead and make the incompatible
     change; don’t postpone it.

   • You should choose a short word to distinguish your program from
     other Lisp programs.  The names of all global symbols in your
     program, that is the names of variables, constants, and functions,
     should begin with that chosen prefix.  Separate the prefix from the
     rest of the name with a hyphen, ‘-’.  This practice helps avoid
     name conflicts, since all global variables in Emacs Lisp share the
     same name space, and all functions share another name space(1).
     Use two hyphens to separate prefix and name if the symbol is not
     meant to be used by other packages.

     Occasionally, for a command name intended for users to use, it is
     more convenient if some words come before the package’s name
     prefix.  And constructs that define functions, variables, etc.,
     work better if they start with ‘defun’ or ‘defvar’, so put the name
     prefix later on in the name.

     This recommendation applies even to names for traditional Lisp
     primitives that are not primitives in Emacs Lisp—such as
     ‘copy-list’.  Believe it or not, there is more than one plausible
     way to define ‘copy-list’.  Play it safe; append your name prefix
     to produce a name like ‘foo-copy-list’ or ‘mylib-copy-list’
     instead.

     If you write a function that you think ought to be added to Emacs
     under a certain name, such as ‘twiddle-files’, don’t call it by
     that name in your program.  Call it ‘mylib-twiddle-files’ in your
     program, and send mail to ‘bug-gnu-emacs@gnu.org’ suggesting we add
     it to Emacs.  If and when we do, we can change the name easily
     enough.

     If one prefix is insufficient, your package can use two or three
     alternative common prefixes, so long as they make sense.

   • Put a call to ‘provide’ at the end of each separate Lisp file.
     *Note Named Features::.

   • If a file requires certain other Lisp programs to be loaded
     beforehand, then the comments at the beginning of the file should
     say so.  Also, use ‘require’ to make sure they are loaded.  *Note
     Named Features::.

   • If a file FOO uses a macro defined in another file BAR, but does
     not use any functions or variables defined in BAR, then FOO should
     contain the following expression:

          (eval-when-compile (require 'BAR))

     This tells Emacs to load BAR just before byte-compiling FOO, so
     that the macro definition is available during compilation.  Using
     ‘eval-when-compile’ avoids loading BAR when the compiled version of
     FOO is _used_.  It should be called before the first use of the
     macro in the file.  *Note Compiling Macros::.

   • Avoid loading additional libraries at run time unless they are
     really needed.  If your file simply cannot work without some other
     library, then just ‘require’ that library at the top-level and be
     done with it.  But if your file contains several independent
     features, and only one or two require the extra library, then
     consider putting ‘require’ statements inside the relevant functions
     rather than at the top-level.  Or use ‘autoload’ statements to load
     the extra library when needed.  This way people who don’t use those
     aspects of your file do not need to load the extra library.

   • If you need Common Lisp extensions, use the ‘cl-lib’ library rather
     than the old ‘cl’ library.  The latter does not use a clean
     namespace (i.e., its definitions do not start with a ‘cl-’ prefix).
     If your package loads ‘cl’ at run time, that could cause name
     clashes for users who don’t use that package.

     There is no problem with using the ‘cl’ package at _compile_ time,
     with ‘(eval-when-compile (require 'cl))’.  That’s sufficient for
     using the macros in the ‘cl’ package, because the compiler expands
     them before generating the byte-code.  It is still better to use
     the more modern ‘cl-lib’ in this case, though.

   • When defining a major mode, please follow the major mode
     conventions.  *Note Major Mode Conventions::.

   • When defining a minor mode, please follow the minor mode
     conventions.  *Note Minor Mode Conventions::.

   • If the purpose of a function is to tell you whether a certain
     condition is true or false, give the function a name that ends in
     ‘p’ (which stands for “predicate”).  If the name is one word, add
     just ‘p’; if the name is multiple words, add ‘-p’.  Examples are
     ‘framep’ and ‘frame-live-p’.

   • If the purpose of a variable is to store a single function, give it
     a name that ends in ‘-function’.  If the purpose of a variable is
     to store a list of functions (i.e., the variable is a hook), please
     follow the naming conventions for hooks.  *Note Hooks::.

   • If loading the file adds functions to hooks, define a function
     ‘FEATURE-unload-function’, where FEATURE is the name of the feature
     the package provides, and make it undo any such changes.  Using
     ‘unload-feature’ to unload the file will run this function.  *Note
     Unloading::.

   • It is a bad idea to define aliases for the Emacs primitives.
     Normally you should use the standard names instead.  The case where
     an alias may be useful is where it facilitates backwards
     compatibility or portability.

   • If a package needs to define an alias or a new function for
     compatibility with some other version of Emacs, name it with the
     package prefix, not with the raw name with which it occurs in the
     other version.  Here is an example from Gnus, which provides many
     examples of such compatibility issues.

          (defalias 'gnus-point-at-bol
            (if (fboundp 'point-at-bol)
                'point-at-bol
              'line-beginning-position))

   • Redefining or advising an Emacs primitive is a bad idea.  It may do
     the right thing for a particular program, but there is no telling
     what other programs might break as a result.

   • It is likewise a bad idea for one Lisp package to advise a function
     in another Lisp package (*note Advising Functions::).

   • Avoid using ‘eval-after-load’ and ‘with-eval-after-load’ in
     libraries and packages (*note Hooks for Loading::).  This feature
     is meant for personal customizations; using it in a Lisp program is
     unclean, because it modifies the behavior of another Lisp file in a
     way that’s not visible in that file.  This is an obstacle for
     debugging, much like advising a function in the other package.

   • If a file does replace any of the standard functions or library
     programs of Emacs, prominent comments at the beginning of the file
     should say which functions are replaced, and how the behavior of
     the replacements differs from that of the originals.

   • Constructs that define a function or variable should be macros, not
     functions, and their names should start with ‘define-’.  The macro
     should receive the name to be defined as the first argument.  That
     will help various tools find the definition automatically.  Avoid
     constructing the names in the macro itself, since that would
     confuse these tools.

   • In some other systems there is a convention of choosing variable
     names that begin and end with ‘*’.  We don’t use that convention in
     Emacs Lisp, so please don’t use it in your programs.  (Emacs uses
     such names only for special-purpose buffers.)  People will find
     Emacs more coherent if all libraries use the same conventions.

   • The default file coding system for Emacs Lisp source files is UTF-8
     (*note Text Representations::).  In the rare event that your
     program contains characters which are _not_ in UTF-8, you should
     specify an appropriate coding system in the source file’s ‘-*-’
     line or local variables list.  *Note Local Variables in Files:
     (emacs)File Variables.

   • Indent the file using the default indentation parameters.

   • Don’t make a habit of putting close-parentheses on lines by
     themselves; Lisp programmers find this disconcerting.

   • Please put a copyright notice and copying permission notice on the
     file if you distribute copies.  *Note Library Headers::.

   ---------- Footnotes ----------

   (1) The benefits of a Common Lisp-style package system are considered
not to outweigh the costs.


File: elisp.info,  Node: Key Binding Conventions,  Next: Programming Tips,  Prev: Coding Conventions,  Up: Tips

D.2 Key Binding Conventions
===========================

   • Many special major modes, like Dired, Info, Compilation, and Occur,
     are designed to handle read-only text that contains “hyper-links”.
     Such a major mode should redefine ‘mouse-2’ and <RET> to follow the
     links.  It should also set up a ‘follow-link’ condition, so that
     the link obeys ‘mouse-1-click-follows-link’.  *Note Clickable
     Text::.  *Note Buttons::, for an easy method of implementing such
     clickable links.

   • Don’t define ‘C-c LETTER’ as a key in Lisp programs.  Sequences
     consisting of ‘C-c’ and a letter (either upper or lower case) are
     reserved for users; they are the *only* sequences reserved for
     users, so do not block them.

     Changing all the Emacs major modes to respect this convention was a
     lot of work; abandoning this convention would make that work go to
     waste, and inconvenience users.  Please comply with it.

   • Function keys <F5> through <F9> without modifier keys are also
     reserved for users to define.

   • Sequences consisting of ‘C-c’ followed by a control character or a
     digit are reserved for major modes.

   • Sequences consisting of ‘C-c’ followed by ‘{’, ‘}’, ‘<’, ‘>’, ‘:’
     or ‘;’ are also reserved for major modes.

   • Sequences consisting of ‘C-c’ followed by any other ASCII
     punctuation or symbol character are allocated for minor modes.
     Using them in a major mode is not absolutely prohibited, but if you
     do that, the major mode binding may be shadowed from time to time
     by minor modes.

   • Don’t bind ‘C-h’ following any prefix character (including ‘C-c’).
     If you don’t bind ‘C-h’, it is automatically available as a help
     character for listing the subcommands of the prefix character.

   • Don’t bind a key sequence ending in <ESC> except following another
     <ESC>.  (That is, it is OK to bind a sequence ending in ‘<ESC>
     <ESC>’.)

     The reason for this rule is that a non-prefix binding for <ESC> in
     any context prevents recognition of escape sequences as function
     keys in that context.

   • Similarly, don’t bind a key sequence ending in <C-g>, since that is
     commonly used to cancel a key sequence.

   • Anything that acts like a temporary mode or state that the user can
     enter and leave should define ‘<ESC> <ESC>’ or ‘<ESC> <ESC> <ESC>’
     as a way to escape.

     For a state that accepts ordinary Emacs commands, or more generally
     any kind of state in which <ESC> followed by a function key or
     arrow key is potentially meaningful, then you must not define
     ‘<ESC> <ESC>’, since that would preclude recognizing an escape
     sequence after <ESC>.  In these states, you should define ‘<ESC>
     <ESC> <ESC>’ as the way to escape.  Otherwise, define ‘<ESC> <ESC>’
     instead.


File: elisp.info,  Node: Programming Tips,  Next: Compilation Tips,  Prev: Key Binding Conventions,  Up: Tips

D.3 Emacs Programming Tips
==========================

Following these conventions will make your program fit better into Emacs
when it runs.

   • Don’t use ‘next-line’ or ‘previous-line’ in programs; nearly
     always, ‘forward-line’ is more convenient as well as more
     predictable and robust.  *Note Text Lines::.

   • Don’t call functions that set the mark, unless setting the mark is
     one of the intended features of your program.  The mark is a
     user-level feature, so it is incorrect to change the mark except to
     supply a value for the user’s benefit.  *Note The Mark::.

     In particular, don’t use any of these functions:

        • ‘beginning-of-buffer’, ‘end-of-buffer’
        • ‘replace-string’, ‘replace-regexp’
        • ‘insert-file’, ‘insert-buffer’

     If you just want to move point, or replace a certain string, or
     insert a file or buffer’s contents, without any of the other
     features intended for interactive users, you can replace these
     functions with one or two lines of simple Lisp code.

   • Use lists rather than vectors, except when there is a particular
     reason to use a vector.  Lisp has more facilities for manipulating
     lists than for vectors, and working with lists is usually more
     convenient.

     Vectors are advantageous for tables that are substantial in size
     and are accessed in random order (not searched front to back),
     provided there is no need to insert or delete elements (only lists
     allow that).

   • The recommended way to show a message in the echo area is with the
     ‘message’ function, not ‘princ’.  *Note The Echo Area::.

   • When you encounter an error condition, call the function ‘error’
     (or ‘signal’).  The function ‘error’ does not return.  *Note
     Signaling Errors::.

     Don’t use ‘message’, ‘throw’, ‘sleep-for’, or ‘beep’ to report
     errors.

   • An error message should start with a capital letter but should not
     end with a period.

   • A question asked in the minibuffer with ‘yes-or-no-p’ or ‘y-or-n-p’
     should start with a capital letter and end with ‘? ’.

   • When you mention a default value in a minibuffer prompt, put it and
     the word ‘default’ inside parentheses.  It should look like this:

          Enter the answer (default 42):

   • In ‘interactive’, if you use a Lisp expression to produce a list of
     arguments, don’t try to provide the correct default values for
     region or position arguments.  Instead, provide ‘nil’ for those
     arguments if they were not specified, and have the function body
     compute the default value when the argument is ‘nil’.  For
     instance, write this:

          (defun foo (pos)
            (interactive
             (list (if SPECIFIED SPECIFIED-POS)))
            (unless pos (setq pos DEFAULT-POS))
            ...)

     rather than this:

          (defun foo (pos)
            (interactive
             (list (if SPECIFIED SPECIFIED-POS
                       DEFAULT-POS)))
            ...)

     This is so that repetition of the command will recompute these
     defaults based on the current circumstances.

     You do not need to take such precautions when you use interactive
     specs ‘d’, ‘m’ and ‘r’, because they make special arrangements to
     recompute the argument values on repetition of the command.

   • Many commands that take a long time to execute display a message
     that says something like ‘Operating...’ when they start, and change
     it to ‘Operating...done’ when they finish.  Please keep the style
     of these messages uniform: _no_ space around the ellipsis, and _no_
     period after ‘done’.  *Note Progress::, for an easy way to generate
     such messages.

   • Try to avoid using recursive edits.  Instead, do what the Rmail ‘e’
     command does: use a new local keymap that contains a command
     defined to switch back to the old local keymap.  Or simply switch
     to another buffer and let the user switch back at will.  *Note
     Recursive Editing::.


File: elisp.info,  Node: Compilation Tips,  Next: Warning Tips,  Prev: Programming Tips,  Up: Tips

D.4 Tips for Making Compiled Code Fast
======================================

Here are ways of improving the execution speed of byte-compiled Lisp
programs.

   • Profile your program, to find out where the time is being spent.
     *Note Profiling::.

   • Use iteration rather than recursion whenever possible.  Function
     calls are slow in Emacs Lisp even when a compiled function is
     calling another compiled function.

   • Using the primitive list-searching functions ‘memq’, ‘member’,
     ‘assq’, or ‘assoc’ is even faster than explicit iteration.  It can
     be worth rearranging a data structure so that one of these
     primitive search functions can be used.

   • Certain built-in functions are handled specially in byte-compiled
     code, avoiding the need for an ordinary function call.  It is a
     good idea to use these functions rather than alternatives.  To see
     whether a function is handled specially by the compiler, examine
     its ‘byte-compile’ property.  If the property is non-‘nil’, then
     the function is handled specially.

     For example, the following input will show you that ‘aref’ is
     compiled specially (*note Array Functions::):

          (get 'aref 'byte-compile)
               ⇒ byte-compile-two-args

     Note that in this case (and many others), you must first load the
     ‘bytecomp’ library, which defines the ‘byte-compile’ property.

   • If calling a small function accounts for a substantial part of your
     program’s running time, make the function inline.  This eliminates
     the function call overhead.  Since making a function inline reduces
     the flexibility of changing the program, don’t do it unless it
     gives a noticeable speedup in something slow enough that users care
     about the speed.  *Note Inline Functions::.


File: elisp.info,  Node: Warning Tips,  Next: Documentation Tips,  Prev: Compilation Tips,  Up: Tips

D.5 Tips for Avoiding Compiler Warnings
=======================================

   • Try to avoid compiler warnings about undefined free variables, by
     adding dummy ‘defvar’ definitions for these variables, like this:

          (defvar foo)

     Such a definition has no effect except to tell the compiler not to
     warn about uses of the variable ‘foo’ in this file.

   • Similarly, to avoid a compiler warning about an undefined function
     that you know _will_ be defined, use a ‘declare-function’ statement
     (*note Declaring Functions::).

   • If you use many functions and variables from a certain file, you
     can add a ‘require’ for that package to avoid compilation warnings
     for them.  For instance,

          (eval-when-compile
            (require 'foo))

   • If you bind a variable in one function, and use it or set it in
     another function, the compiler warns about the latter function
     unless the variable has a definition.  But adding a definition
     would be unclean if the variable has a short name, since Lisp
     packages should not define short variable names.  The right thing
     to do is to rename this variable to start with the name prefix used
     for the other functions and variables in your package.

   • The last resort for avoiding a warning, when you want to do
     something that is usually a mistake but you know is not a mistake
     in your usage, is to put it inside ‘with-no-warnings’.  *Note
     Compiler Errors::.


File: elisp.info,  Node: Documentation Tips,  Next: Comment Tips,  Prev: Warning Tips,  Up: Tips

D.6 Tips for Documentation Strings
==================================

Here are some tips and conventions for the writing of documentation
strings.  You can check many of these conventions by running the command
‘M-x checkdoc-minor-mode’.

   • Every command, function, or variable intended for users to know
     about should have a documentation string.

   • An internal variable or subroutine of a Lisp program might as well
     have a documentation string.  Documentation strings take up very
     little space in a running Emacs.

   • Format the documentation string so that it fits in an Emacs window
     on an 80-column screen.  It is a good idea for most lines to be no
     wider than 60 characters.  The first line should not be wider than
     67 characters or it will look bad in the output of ‘apropos’.

     You can fill the text if that looks good.  Emacs Lisp mode fills
     documentation strings to the width specified by
     ‘emacs-lisp-docstring-fill-column’.  However, you can sometimes
     make a documentation string much more readable by adjusting its
     line breaks with care.  Use blank lines between sections if the
     documentation string is long.

   • The first line of the documentation string should consist of one or
     two complete sentences that stand on their own as a summary.  ‘M-x
     apropos’ displays just the first line, and if that line’s contents
     don’t stand on their own, the result looks bad.  In particular,
     start the first line with a capital letter and end it with a
     period.

     For a function, the first line should briefly answer the question,
     “What does this function do?” For a variable, the first line should
     briefly answer the question, “What does this value mean?”

     Don’t limit the documentation string to one line; use as many lines
     as you need to explain the details of how to use the function or
     variable.  Please use complete sentences for the rest of the text
     too.

   • When the user tries to use a disabled command, Emacs displays just
     the first paragraph of its documentation string—everything through
     the first blank line.  If you wish, you can choose which
     information to include before the first blank line so as to make
     this display useful.

   • The first line should mention all the important arguments of the
     function, and should mention them in the order that they are
     written in a function call.  If the function has many arguments,
     then it is not feasible to mention them all in the first line; in
     that case, the first line should mention the first few arguments,
     including the most important arguments.

   • When a function’s documentation string mentions the value of an
     argument of the function, use the argument name in capital letters
     as if it were a name for that value.  Thus, the documentation
     string of the function ‘eval’ refers to its first argument as
     ‘FORM’, because the actual argument name is ‘form’:

          Evaluate FORM and return its value.

     Also write metasyntactic variables in capital letters, such as when
     you show the decomposition of a list or vector into subunits, some
     of which may vary.  ‘KEY’ and ‘VALUE’ in the following example
     illustrate this practice:

          The argument TABLE should be an alist whose elements
          have the form (KEY . VALUE).  Here, KEY is ...

   • Never change the case of a Lisp symbol when you mention it in a doc
     string.  If the symbol’s name is ‘foo’, write “foo”, not “Foo”
     (which is a different symbol).

     This might appear to contradict the policy of writing function
     argument values, but there is no real contradiction; the argument
     _value_ is not the same thing as the _symbol_ that the function
     uses to hold the value.

     If this puts a lower-case letter at the beginning of a sentence and
     that annoys you, rewrite the sentence so that the symbol is not at
     the start of it.

   • Do not start or end a documentation string with whitespace.

   • *Do not* indent subsequent lines of a documentation string so that
     the text is lined up in the source code with the text of the first
     line.  This looks nice in the source code, but looks bizarre when
     users view the documentation.  Remember that the indentation before
     the starting double-quote is not part of the string!

   • When a documentation string refers to a Lisp symbol, write it as it
     would be printed (which usually means in lower case), surrounding
     it with curved single quotes (‘ and ’).  There are two exceptions:
     write ‘t’ and ‘nil’ without surrounding punctuation.  For example:
     ‘CODE can be ‘lambda’, nil, or t’.  *Note (emacs)Quotation Marks::,
     for how to enter curved single quotes.

     Documentation strings can also use an older single-quoting
     convention, which quotes symbols with grave accent ` and apostrophe
     ': `like-this' rather than ‘like-this’.  This older convention was
     designed for now-obsolete displays in which grave accent and
     apostrophe were mirror images.

     Documentation using either convention is converted to the user’s
     preferred format when it is copied into a help buffer.  *Note Keys
     in Documentation::.

     Help mode automatically creates a hyperlink when a documentation
     string uses a single-quoted symbol name, if the symbol has either a
     function or a variable definition.  You do not need to do anything
     special to make use of this feature.  However, when a symbol has
     both a function definition and a variable definition, and you want
     to refer to just one of them, you can specify which one by writing
     one of the words ‘variable’, ‘option’, ‘function’, or ‘command’,
     immediately before the symbol name.  (Case makes no difference in
     recognizing these indicator words.)  For example, if you write

          This function sets the variable `buffer-file-name'.

     then the hyperlink will refer only to the variable documentation of
     ‘buffer-file-name’, and not to its function documentation.

     If a symbol has a function definition and/or a variable definition,
     but those are irrelevant to the use of the symbol that you are
     documenting, you can write the words ‘symbol’ or ‘program’ before
     the symbol name to prevent making any hyperlink.  For example,

          If the argument KIND-OF-RESULT is the symbol `list',
          this function returns a list of all the objects
          that satisfy the criterion.

     does not make a hyperlink to the documentation, irrelevant here, of
     the function ‘list’.

     Normally, no hyperlink is made for a variable without variable
     documentation.  You can force a hyperlink for such variables by
     preceding them with one of the words ‘variable’ or ‘option’.

     Hyperlinks for faces are only made if the face name is preceded or
     followed by the word ‘face’.  In that case, only the face
     documentation will be shown, even if the symbol is also defined as
     a variable or as a function.

     To make a hyperlink to Info documentation, write the single-quoted
     name of the Info node (or anchor), preceded by ‘info node’, ‘Info
     node’, ‘info anchor’ or ‘Info anchor’.  The Info file name defaults
     to ‘emacs’.  For example,

          See Info node `Font Lock' and Info node `(elisp)Font Lock Basics'.

     Finally, to create a hyperlink to URLs, write the single-quoted
     URL, preceded by ‘URL’.  For example,

          The home page for the GNU project has more information (see URL
          `http://www.gnu.org/').

   • Don’t write key sequences directly in documentation strings.
     Instead, use the ‘\\[...]’ construct to stand for them.  For
     example, instead of writing ‘C-f’, write the construct
     ‘\\[forward-char]’.  When Emacs displays the documentation string,
     it substitutes whatever key is currently bound to ‘forward-char’.
     (This is normally ‘C-f’, but it may be some other character if the
     user has moved key bindings.)  *Note Keys in Documentation::.

   • In documentation strings for a major mode, you will want to refer
     to the key bindings of that mode’s local map, rather than global
     ones.  Therefore, use the construct ‘\\<...>’ once in the
     documentation string to specify which key map to use.  Do this
     before the first use of ‘\\[...]’.  The text inside the ‘\\<...>’
     should be the name of the variable containing the local keymap for
     the major mode.

     It is not practical to use ‘\\[...]’ very many times, because
     display of the documentation string will become slow.  So use this
     to describe the most important commands in your major mode, and
     then use ‘\\{...}’ to display the rest of the mode’s keymap.

   • For consistency, phrase the verb in the first sentence of a
     function’s documentation string as an imperative—for instance, use
     “Return the cons of A and B.” in preference to “Returns the cons of
     A and B.” Usually it looks good to do likewise for the rest of the
     first paragraph.  Subsequent paragraphs usually look better if each
     sentence is indicative and has a proper subject.

   • The documentation string for a function that is a yes-or-no
     predicate should start with words such as “Return t if”, to
     indicate explicitly what constitutes truth.  The word “return”
     avoids starting the sentence with lower-case “t”, which could be
     somewhat distracting.

   • If a line in a documentation string begins with an
     open-parenthesis, write a backslash before the open-parenthesis,
     like this:

          The argument FOO can be either a number
          \(a buffer position) or a string (a file name).

     This prevents the open-parenthesis from being treated as the start
     of a defun (*note Defuns: (emacs)Defuns.).

   • Write documentation strings in the active voice, not the passive,
     and in the present tense, not the future.  For instance, use
     “Return a list containing A and B.” instead of “A list containing A
     and B will be returned.”

   • Avoid using the word “cause” (or its equivalents) unnecessarily.
     Instead of, “Cause Emacs to display text in boldface”, write just
     “Display text in boldface”.

   • Avoid using “iff” (a mathematics term meaning “if and only if”),
     since many people are unfamiliar with it and mistake it for a typo.
     In most cases, the meaning is clear with just “if”.  Otherwise, try
     to find an alternate phrasing that conveys the meaning.

   • When a command is meaningful only in a certain mode or situation,
     do mention that in the documentation string.  For example, the
     documentation of ‘dired-find-file’ is:

          In Dired, visit the file or directory named on this line.

   • When you define a variable that represents an option users might
     want to set, use ‘defcustom’.  *Note Defining Variables::.

   • The documentation string for a variable that is a yes-or-no flag
     should start with words such as “Non-nil means”, to make it clear
     that all non-‘nil’ values are equivalent and indicate explicitly
     what ‘nil’ and non-‘nil’ mean.


File: elisp.info,  Node: Comment Tips,  Next: Library Headers,  Prev: Documentation Tips,  Up: Tips

D.7 Tips on Writing Comments
============================

We recommend these conventions for comments:

‘;’
     Comments that start with a single semicolon, ‘;’, should all be
     aligned to the same column on the right of the source code.  Such
     comments usually explain how the code on that line does its job.
     For example:

          (setq base-version-list                 ; There was a base
                (assoc (substring fn 0 start-vn)  ; version to which
                       file-version-assoc-list))  ; this looks like
                                                  ; a subversion.

‘;;’
     Comments that start with two semicolons, ‘;;’, should be aligned to
     the same level of indentation as the code.  Such comments usually
     describe the purpose of the following lines or the state of the
     program at that point.  For example:

          (prog1 (setq auto-fill-function
                       ...
                       ...
            ;; Update mode line.
            (force-mode-line-update)))

     We also normally use two semicolons for comments outside functions.

          ;; This Lisp code is run in Emacs when it is to operate as
          ;; a server for other processes.

     If a function has no documentation string, it should instead have a
     two-semicolon comment right before the function, explaining what
     the function does and how to call it properly.  Explain precisely
     what each argument means and how the function interprets its
     possible values.  It is much better to convert such comments to
     documentation strings, though.

‘;;;’
     Comments that start with three semicolons, ‘;;;’, should start at
     the left margin.  We use them for comments which should be
     considered a heading by Outline minor mode.  By default, comments
     starting with at least three semicolons (followed by a single space
     and a non-whitespace character) are considered headings, comments
     starting with two or fewer are not.  Historically, triple-semicolon
     comments have also been used for commenting out lines within a
     function, but this use is discouraged.

     When commenting out entire functions, use two semicolons.

‘;;;;’
     Comments that start with four semicolons, ‘;;;;’, should be aligned
     to the left margin and are used for headings of major sections of a
     program.  For example:

          ;;;; The kill ring

Generally speaking, the ‘M-;’ (‘comment-dwim’) command automatically
starts a comment of the appropriate type; or indents an existing comment
to the right place, depending on the number of semicolons.  *Note
Manipulating Comments: (emacs)Comments.


File: elisp.info,  Node: Library Headers,  Prev: Comment Tips,  Up: Tips

D.8 Conventional Headers for Emacs Libraries
============================================

Emacs has conventions for using special comments in Lisp libraries to
divide them into sections and give information such as who wrote them.
Using a standard format for these items makes it easier for tools (and
people) to extract the relevant information.  This section explains
these conventions, starting with an example:

     ;;; foo.el --- Support for the Foo programming language

     ;; Copyright (C) 2010-2017 Your Name

     ;; Author: Your Name <yourname@example.com>
     ;; Maintainer: Someone Else <someone@example.com>
     ;; Created: 14 Jul 2010
     ;; Keywords: languages
     ;; Homepage: http://example.com/foo

     ;; This file is not part of GNU Emacs.

     ;; This file is free software...
     ...
     ;; along with this file.  If not, see <http://www.gnu.org/licenses/>.

   The very first line should have this format:

     ;;; FILENAME --- DESCRIPTION

The description should be contained in one line.  If the file needs a
‘-*-’ specification, put it after DESCRIPTION.  If this would make the
first line too long, use a Local Variables section at the end of the
file.

   The copyright notice usually lists your name (if you wrote the file).
If you have an employer who claims copyright on your work, you might
need to list them instead.  Do not say that the copyright holder is the
Free Software Foundation (or that the file is part of GNU Emacs) unless
your file has been accepted into the Emacs distribution.  For more
information on the form of copyright and license notices, see the guide
on the GNU website (http://www.gnu.org/licenses/gpl-howto.html).

   After the copyright notice come several “header comment” lines, each
beginning with ‘;; HEADER-NAME:’.  Here is a table of the conventional
possibilities for HEADER-NAME:

‘Author’
     This line states the name and email address of at least the
     principal author of the library.  If there are multiple authors,
     list them on continuation lines led by ‘;;’ and a tab or at least
     two spaces.  We recommend including a contact email address, of the
     form ‘<...>’.  For example:

          ;; Author: Your Name <yourname@example.com>
          ;;      Someone Else <someone@example.com>
          ;;      Another Person <another@example.com>

‘Maintainer’
     This header has the same format as the Author header.  It lists the
     person(s) who currently maintain(s) the file (respond to bug
     reports, etc.).

     If there is no maintainer line, the person(s) in the Author field
     is/are presumed to be the maintainers.  Some files in Emacs use
     ‘FSF’ for the maintainer.  This means that the original author is
     no longer responsible for the file, and that it is maintained as
     part of Emacs.

‘Created’
     This optional line gives the original creation date of the file,
     and is for historical interest only.

‘Version’
     If you wish to record version numbers for the individual Lisp
     program, put them in this line.  Lisp files distributed with Emacs
     generally do not have a ‘Version’ header, since the version number
     of Emacs itself serves the same purpose.  If you are distributing a
     collection of multiple files, we recommend not writing the version
     in every file, but only the main one.

‘Keywords’
     This line lists keywords for the ‘finder-by-keyword’ help command.
     Please use that command to see a list of the meaningful keywords.
     The command ‘M-x checkdoc-package-keywords RET’ will find and
     display any keywords that are not in ‘finder-known-keywords’.  If
     you set the variable ‘checkdoc-package-keywords-flag’ non-‘nil’,
     checkdoc commands will include the keyword verification in its
     checks.

     This field is how people will find your package when they’re
     looking for things by topic.  To separate the keywords, you can use
     spaces, commas, or both.

     The name of this field is unfortunate, since people often assume it
     is the place to write arbitrary keywords that describe their
     package, rather than just the relevant Finder keywords.

‘Homepage’
     This line states the homepage of the library.

‘Package-Version’
     If ‘Version’ is not suitable for use by the package manager, then a
     package can define ‘Package-Version’; it will be used instead.
     This is handy if ‘Version’ is an RCS id or something else that
     cannot be parsed by ‘version-to-list’.  *Note Packaging Basics::.

‘Package-Requires’
     If this exists, it names packages on which the current package
     depends for proper operation.  *Note Packaging Basics::.  This is
     used by the package manager both at download time (to ensure that a
     complete set of packages is downloaded) and at activation time (to
     ensure that a package is only activated if all its dependencies
     have been).

     Its format is a list of lists on a single line.  The ‘car’ of each
     sub-list is the name of a package, as a symbol.  The ‘cadr’ of each
     sub-list is the minimum acceptable version number, as a string that
     can be parse by ‘version-to-list’.  An entry that lacks a version
     (i.e., an entry which is just a symbol, or a sub-list of one
     element) is equivalent to entry with version "0".  For instance:

          ;; Package-Requires: ((gnus "1.0") (bubbles "2.7.2") cl-lib (seq))

     The package code automatically defines a package named ‘emacs’ with
     the version number of the currently running Emacs.  This can be
     used to require a minimal version of Emacs for a package.

   Just about every Lisp library ought to have the ‘Author’ and
‘Keywords’ header comment lines.  Use the others if they are
appropriate.  You can also put in header lines with other header
names—they have no standard meanings, so they can’t do any harm.

   We use additional stylized comments to subdivide the contents of the
library file.  These should be separated from anything else by blank
lines.  Here is a table of them:

‘;;; Commentary:’
     This begins introductory comments that explain how the library
     works.  It should come right after the copying permissions,
     terminated by a ‘Change Log’, ‘History’ or ‘Code’ comment line.
     This text is used by the Finder package, so it should make sense in
     that context.

‘;;; Change Log:’
     This begins an optional log of changes to the file over time.
     Don’t put too much information in this section—it is better to keep
     the detailed logs in a version control system (as Emacs does) or in
     a separate ‘ChangeLog’ file.  ‘History’ is an alternative to
     ‘Change Log’.

‘;;; Code:’
     This begins the actual code of the program.

‘;;; FILENAME ends here’
     This is the “footer line”; it appears at the very end of the file.
     Its purpose is to enable people to detect truncated versions of the
     file from the lack of a footer line.


File: elisp.info,  Node: GNU Emacs Internals,  Next: Standard Errors,  Prev: Tips,  Up: Top

Appendix E GNU Emacs Internals
******************************

This chapter describes how the runnable Emacs executable is dumped with
the preloaded Lisp libraries in it, how storage is allocated, and some
internal aspects of GNU Emacs that may be of interest to C programmers.

* Menu:

* Building Emacs::      How the dumped Emacs is made.
* Pure Storage::        Kludge to make preloaded Lisp functions shareable.
* Garbage Collection::  Reclaiming space for Lisp objects no longer used.
* Stack-allocated Objects::    Temporary conses and strings on C stack.
* Memory Usage::        Info about total size of Lisp objects made so far.
* C Dialect::           What C variant Emacs is written in.
* Writing Emacs Primitives::   Writing C code for Emacs.
* Object Internals::    Data formats of buffers, windows, processes.
* C Integer Types::     How C integer types are used inside Emacs.


File: elisp.info,  Node: Building Emacs,  Next: Pure Storage,  Up: GNU Emacs Internals

E.1 Building Emacs
==================

This section explains the steps involved in building the Emacs
executable.  You don’t have to know this material to build and install
Emacs, since the makefiles do all these things automatically.  This
information is pertinent to Emacs developers.

   Building Emacs requires GNU Make version 3.81 or later.

   Compilation of the C source files in the ‘src’ directory produces an
executable file called ‘temacs’, also called a “bare impure Emacs”.  It
contains the Emacs Lisp interpreter and I/O routines, but not the
editing commands.

   The command ‘temacs -l loadup’ would run ‘temacs’ and direct it to
load ‘loadup.el’.  The ‘loadup’ library loads additional Lisp libraries,
which set up the normal Emacs editing environment.  After this step, the
Emacs executable is no longer “bare”.

   Because it takes some time to load the standard Lisp files, the
‘temacs’ executable usually isn’t run directly by users.  Instead, as
one of the last steps of building Emacs, the command ‘temacs -batch -l
loadup dump’ is run.  The special ‘dump’ argument causes ‘temacs’ to
dump out an executable program, called ‘emacs’, which has all the
standard Lisp files preloaded.  (The ‘-batch’ argument prevents ‘temacs’
from trying to initialize any of its data on the terminal, so that the
tables of terminal information are empty in the dumped Emacs.)

   The dumped ‘emacs’ executable (also called a “pure” Emacs) is the one
which is installed.  The variable ‘preloaded-file-list’ stores a list of
the Lisp files preloaded into the dumped Emacs.  If you port Emacs to a
new operating system, and are not able to implement dumping, then Emacs
must load ‘loadup.el’ each time it starts.

   You can specify additional files to preload by writing a library
named ‘site-load.el’ that loads them.  You may need to rebuild Emacs
with an added definition

     #define SITELOAD_PURESIZE_EXTRA N

to make N added bytes of pure space to hold the additional files; see
‘src/puresize.h’.  (Try adding increments of 20000 until it is big
enough.)  However, the advantage of preloading additional files
decreases as machines get faster.  On modern machines, it is usually not
advisable.

   After ‘loadup.el’ reads ‘site-load.el’, it finds the documentation
strings for primitive and preloaded functions (and variables) in the
file ‘etc/DOC’ where they are stored, by calling ‘Snarf-documentation’
(*note Accessing Documentation: Definition of Snarf-documentation.).

   You can specify other Lisp expressions to execute just before dumping
by putting them in a library named ‘site-init.el’.  This file is
executed after the documentation strings are found.

   If you want to preload function or variable definitions, there are
three ways you can do this and make their documentation strings
accessible when you subsequently run Emacs:

   • Arrange to scan these files when producing the ‘etc/DOC’ file, and
     load them with ‘site-load.el’.

   • Load the files with ‘site-init.el’, then copy the files into the
     installation directory for Lisp files when you install Emacs.

   • Specify a ‘nil’ value for ‘byte-compile-dynamic-docstrings’ as a
     local variable in each of these files, and load them with either
     ‘site-load.el’ or ‘site-init.el’.  (This method has the drawback
     that the documentation strings take up space in Emacs all the
     time.)

   It is not advisable to put anything in ‘site-load.el’ or
‘site-init.el’ that would alter any of the features that users expect in
an ordinary unmodified Emacs.  If you feel you must override normal
features for your site, do it with ‘default.el’, so that users can
override your changes if they wish.  *Note Startup Summary::.  Note that
if either ‘site-load.el’ or ‘site-init.el’ changes ‘load-path’, the
changes will be lost after dumping.  *Note Library Search::.  To make a
permanent change to ‘load-path’, use the ‘--enable-locallisppath’ option
of ‘configure’.

   In a package that can be preloaded, it is sometimes necessary (or
useful) to delay certain evaluations until Emacs subsequently starts up.
The vast majority of such cases relate to the values of customizable
variables.  For example, ‘tutorial-directory’ is a variable defined in
‘startup.el’, which is preloaded.  The default value is set based on
‘data-directory’.  The variable needs to access the value of
‘data-directory’ when Emacs starts, not when it is dumped, because the
Emacs executable has probably been installed in a different location
since it was dumped.

 -- Function: custom-initialize-delay symbol value
     This function delays the initialization of SYMBOL to the next Emacs
     start.  You normally use this function by specifying it as the
     ‘:initialize’ property of a customizable variable.  (The argument
     VALUE is unused, and is provided only for compatibility with the
     form Custom expects.)

   In the unlikely event that you need a more general functionality than
‘custom-initialize-delay’ provides, you can use ‘before-init-hook’
(*note Startup Summary::).

 -- Function: dump-emacs to-file from-file
     This function dumps the current state of Emacs into an executable
     file TO-FILE.  It takes symbols from FROM-FILE (this is normally
     the executable file ‘temacs’).

     If you want to use this function in an Emacs that was already
     dumped, you must run Emacs with ‘-batch’.


File: elisp.info,  Node: Pure Storage,  Next: Garbage Collection,  Prev: Building Emacs,  Up: GNU Emacs Internals

E.2 Pure Storage
================

Emacs Lisp uses two kinds of storage for user-created Lisp objects:
“normal storage” and “pure storage”.  Normal storage is where all the
new data created during an Emacs session are kept (*note Garbage
Collection::).  Pure storage is used for certain data in the preloaded
standard Lisp files—data that should never change during actual use of
Emacs.

   Pure storage is allocated only while ‘temacs’ is loading the standard
preloaded Lisp libraries.  In the file ‘emacs’, it is marked as
read-only (on operating systems that permit this), so that the memory
space can be shared by all the Emacs jobs running on the machine at
once.  Pure storage is not expandable; a fixed amount is allocated when
Emacs is compiled, and if that is not sufficient for the preloaded
libraries, ‘temacs’ allocates dynamic memory for the part that didn’t
fit.  The resulting image will work, but garbage collection (*note
Garbage Collection::) is disabled in this situation, causing a memory
leak.  Such an overflow normally won’t happen unless you try to preload
additional libraries or add features to the standard ones.  Emacs will
display a warning about the overflow when it starts.  If this happens,
you should increase the compilation parameter ‘SYSTEM_PURESIZE_EXTRA’ in
the file ‘src/puresize.h’ and rebuild Emacs.

 -- Function: purecopy object
     This function makes a copy in pure storage of OBJECT, and returns
     it.  It copies a string by simply making a new string with the same
     characters, but without text properties, in pure storage.  It
     recursively copies the contents of vectors and cons cells.  It does
     not make copies of other objects such as symbols, but just returns
     them unchanged.  It signals an error if asked to copy markers.

     This function is a no-op except while Emacs is being built and
     dumped; it is usually called only in preloaded Lisp files.

 -- Variable: pure-bytes-used
     The value of this variable is the number of bytes of pure storage
     allocated so far.  Typically, in a dumped Emacs, this number is
     very close to the total amount of pure storage available—if it were
     not, we would preallocate less.

 -- Variable: purify-flag
     This variable determines whether ‘defun’ should make a copy of the
     function definition in pure storage.  If it is non-‘nil’, then the
     function definition is copied into pure storage.

     This flag is ‘t’ while loading all of the basic functions for
     building Emacs initially (allowing those functions to be shareable
     and non-collectible).  Dumping Emacs as an executable always writes
     ‘nil’ in this variable, regardless of the value it actually has
     before and after dumping.

     You should not change this flag in a running Emacs.


File: elisp.info,  Node: Garbage Collection,  Next: Stack-allocated Objects,  Prev: Pure Storage,  Up: GNU Emacs Internals

E.3 Garbage Collection
======================

When a program creates a list or the user defines a new function (such
as by loading a library), that data is placed in normal storage.  If
normal storage runs low, then Emacs asks the operating system to
allocate more memory.  Different types of Lisp objects, such as symbols,
cons cells, small vectors, markers, etc., are segregated in distinct
blocks in memory.  (Large vectors, long strings, buffers and certain
other editing types, which are fairly large, are allocated in individual
blocks, one per object; small strings are packed into blocks of 8k
bytes, and small vectors are packed into blocks of 4k bytes).

   Beyond the basic vector, a lot of objects like window, buffer, and
frame are managed as if they were vectors.  The corresponding C data
structures include the ‘struct vectorlike_header’ field whose ‘size’
member contains the subtype enumerated by ‘enum pvec_type’ and an
information about how many ‘Lisp_Object’ fields this structure contains
and what the size of the rest data is.  This information is needed to
calculate the memory footprint of an object, and used by the vector
allocation code while iterating over the vector blocks.

   It is quite common to use some storage for a while, then release it
by (for example) killing a buffer or deleting the last pointer to an
object.  Emacs provides a “garbage collector” to reclaim this abandoned
storage.  The garbage collector operates by finding and marking all Lisp
objects that are still accessible to Lisp programs.  To begin with, it
assumes all the symbols, their values and associated function
definitions, and any data presently on the stack, are accessible.  Any
objects that can be reached indirectly through other accessible objects
are also accessible.

   When marking is finished, all objects still unmarked are garbage.  No
matter what the Lisp program or the user does, it is impossible to refer
to them, since there is no longer a way to reach them.  Their space
might as well be reused, since no one will miss them.  The second
(sweep) phase of the garbage collector arranges to reuse them.

   The sweep phase puts unused cons cells onto a “free list” for future
allocation; likewise for symbols and markers.  It compacts the
accessible strings so they occupy fewer 8k blocks; then it frees the
other 8k blocks.  Unreachable vectors from vector blocks are coalesced
to create largest possible free areas; if a free area spans a complete
4k block, that block is freed.  Otherwise, the free area is recorded in
a free list array, where each entry corresponds to a free list of areas
of the same size.  Large vectors, buffers, and other large objects are
allocated and freed individually.

     Common Lisp note: Unlike other Lisps, GNU Emacs Lisp does not call
     the garbage collector when the free list is empty.  Instead, it
     simply requests the operating system to allocate more storage, and
     processing continues until ‘gc-cons-threshold’ bytes have been
     used.

     This means that you can make sure that the garbage collector will
     not run during a certain portion of a Lisp program by calling the
     garbage collector explicitly just before it (provided that portion
     of the program does not use so much space as to force a second
     garbage collection).

 -- Command: garbage-collect
     This command runs a garbage collection, and returns information on
     the amount of space in use.  (Garbage collection can also occur
     spontaneously if you use more than ‘gc-cons-threshold’ bytes of
     Lisp data since the previous garbage collection.)

     ‘garbage-collect’ returns a list with information on amount of
     space in use, where each entry has the form ‘(NAME SIZE USED)’ or
     ‘(NAME SIZE USED FREE)’.  In the entry, NAME is a symbol describing
     the kind of objects this entry represents, SIZE is the number of
     bytes used by each one, USED is the number of those objects that
     were found live in the heap, and optional FREE is the number of
     those objects that are not live but that Emacs keeps around for
     future allocations.  So an overall result is:

          ((conses CONS-SIZE USED-CONSES FREE-CONSES)
           (symbols SYMBOL-SIZE USED-SYMBOLS FREE-SYMBOLS)
           (miscs MISC-SIZE USED-MISCS FREE-MISCS)
           (strings STRING-SIZE USED-STRINGS FREE-STRINGS)
           (string-bytes BYTE-SIZE USED-BYTES)
           (vectors VECTOR-SIZE USED-VECTORS)
           (vector-slots SLOT-SIZE USED-SLOTS FREE-SLOTS)
           (floats FLOAT-SIZE USED-FLOATS FREE-FLOATS)
           (intervals INTERVAL-SIZE USED-INTERVALS FREE-INTERVALS)
           (buffers BUFFER-SIZE USED-BUFFERS)
           (heap UNIT-SIZE TOTAL-SIZE FREE-SIZE))

     Here is an example:

          (garbage-collect)
                ⇒ ((conses 16 49126 8058) (symbols 48 14607 0)
                           (miscs 40 34 56) (strings 32 2942 2607)
                           (string-bytes 1 78607) (vectors 16 7247)
                           (vector-slots 8 341609 29474) (floats 8 71 102)
                           (intervals 56 27 26) (buffers 944 8)
                           (heap 1024 11715 2678))

     Below is a table explaining each element.  Note that last ‘heap’
     entry is optional and present only if an underlying ‘malloc’
     implementation provides ‘mallinfo’ function.

     CONS-SIZE
          Internal size of a cons cell, i.e., ‘sizeof (struct
          Lisp_Cons)’.

     USED-CONSES
          The number of cons cells in use.

     FREE-CONSES
          The number of cons cells for which space has been obtained
          from the operating system, but that are not currently being
          used.

     SYMBOL-SIZE
          Internal size of a symbol, i.e., ‘sizeof (struct
          Lisp_Symbol)’.

     USED-SYMBOLS
          The number of symbols in use.

     FREE-SYMBOLS
          The number of symbols for which space has been obtained from
          the operating system, but that are not currently being used.

     MISC-SIZE
          Internal size of a miscellaneous entity, i.e., ‘sizeof (union
          Lisp_Misc)’, which is a size of the largest type enumerated in
          ‘enum Lisp_Misc_Type’.

     USED-MISCS
          The number of miscellaneous objects in use.  These include
          markers and overlays, plus certain objects not visible to
          users.

     FREE-MISCS
          The number of miscellaneous objects for which space has been
          obtained from the operating system, but that are not currently
          being used.

     STRING-SIZE
          Internal size of a string header, i.e., ‘sizeof (struct
          Lisp_String)’.

     USED-STRINGS
          The number of string headers in use.

     FREE-STRINGS
          The number of string headers for which space has been obtained
          from the operating system, but that are not currently being
          used.

     BYTE-SIZE
          This is used for convenience and equals to ‘sizeof (char)’.

     USED-BYTES
          The total size of all string data in bytes.

     VECTOR-SIZE
          Internal size of a vector header, i.e., ‘sizeof (struct
          Lisp_Vector)’.

     USED-VECTORS
          The number of vector headers allocated from the vector blocks.

     SLOT-SIZE
          Internal size of a vector slot, always equal to ‘sizeof
          (Lisp_Object)’.

     USED-SLOTS
          The number of slots in all used vectors.

     FREE-SLOTS
          The number of free slots in all vector blocks.

     FLOAT-SIZE
          Internal size of a float object, i.e., ‘sizeof (struct
          Lisp_Float)’.  (Do not confuse it with the native platform
          ‘float’ or ‘double’.)

     USED-FLOATS
          The number of floats in use.

     FREE-FLOATS
          The number of floats for which space has been obtained from
          the operating system, but that are not currently being used.

     INTERVAL-SIZE
          Internal size of an interval object, i.e., ‘sizeof (struct
          interval)’.

     USED-INTERVALS
          The number of intervals in use.

     FREE-INTERVALS
          The number of intervals for which space has been obtained from
          the operating system, but that are not currently being used.

     BUFFER-SIZE
          Internal size of a buffer, i.e., ‘sizeof (struct buffer)’.
          (Do not confuse with the value returned by ‘buffer-size’
          function.)

     USED-BUFFERS
          The number of buffer objects in use.  This includes killed
          buffers invisible to users, i.e., all buffers in ‘all_buffers’
          list.

     UNIT-SIZE
          The unit of heap space measurement, always equal to 1024
          bytes.

     TOTAL-SIZE
          Total heap size, in UNIT-SIZE units.

     FREE-SIZE
          Heap space which is not currently used, in UNIT-SIZE units.

     If there was overflow in pure space (*note Pure Storage::),
     ‘garbage-collect’ returns ‘nil’, because a real garbage collection
     cannot be done.

 -- User Option: garbage-collection-messages
     If this variable is non-‘nil’, Emacs displays a message at the
     beginning and end of garbage collection.  The default value is
     ‘nil’.

 -- Variable: post-gc-hook
     This is a normal hook that is run at the end of garbage collection.
     Garbage collection is inhibited while the hook functions run, so be
     careful writing them.

 -- User Option: gc-cons-threshold
     The value of this variable is the number of bytes of storage that
     must be allocated for Lisp objects after one garbage collection in
     order to trigger another garbage collection.  You can use the
     result returned by ‘garbage-collect’ to get an information about
     size of the particular object type; space allocated to the contents
     of buffers does not count.  Note that the subsequent garbage
     collection does not happen immediately when the threshold is
     exhausted, but only the next time the Lisp interpreter is called.

     The initial threshold value is ‘GC_DEFAULT_THRESHOLD’, defined in
     ‘alloc.c’.  Since it’s defined in ‘word_size’ units, the value is
     400,000 for the default 32-bit configuration and 800,000 for the
     64-bit one.  If you specify a larger value, garbage collection will
     happen less often.  This reduces the amount of time spent garbage
     collecting, but increases total memory use.  You may want to do
     this when running a program that creates lots of Lisp data.

     You can make collections more frequent by specifying a smaller
     value, down to 1/10th of ‘GC_DEFAULT_THRESHOLD’.  A value less than
     this minimum will remain in effect only until the subsequent
     garbage collection, at which time ‘garbage-collect’ will set the
     threshold back to the minimum.

 -- User Option: gc-cons-percentage
     The value of this variable specifies the amount of consing before a
     garbage collection occurs, as a fraction of the current heap size.
     This criterion and ‘gc-cons-threshold’ apply in parallel, and
     garbage collection occurs only when both criteria are satisfied.

     As the heap size increases, the time to perform a garbage
     collection increases.  Thus, it can be desirable to do them less
     frequently in proportion.

   The value returned by ‘garbage-collect’ describes the amount of
memory used by Lisp data, broken down by data type.  By contrast, the
function ‘memory-limit’ provides information on the total amount of
memory Emacs is currently using.

 -- Function: memory-limit
     This function returns the address of the last byte Emacs has
     allocated, divided by 1024.  We divide the value by 1024 to make
     sure it fits in a Lisp integer.

     You can use this to get a general idea of how your actions affect
     the memory usage.

 -- Variable: memory-full
     This variable is ‘t’ if Emacs is nearly out of memory for Lisp
     objects, and ‘nil’ otherwise.

 -- Function: memory-use-counts
     This returns a list of numbers that count the number of objects
     created in this Emacs session.  Each of these counters increments
     for a certain kind of object.  See the documentation string for
     details.

 -- Function: memory-info
     This functions returns an amount of total system memory and how
     much of it is free.  On an unsupported system, the value may be
     ‘nil’.

 -- Variable: gcs-done
     This variable contains the total number of garbage collections done
     so far in this Emacs session.

 -- Variable: gc-elapsed
     This variable contains the total number of seconds of elapsed time
     during garbage collection so far in this Emacs session, as a
     floating-point number.


File: elisp.info,  Node: Stack-allocated Objects,  Next: Memory Usage,  Prev: Garbage Collection,  Up: GNU Emacs Internals

E.4 Stack-allocated Objects
===========================

The garbage collector described above is used to manage data visible
from Lisp programs, as well as most of the data internally used by the
Lisp interpreter.  Sometimes it may be useful to allocate temporary
internal objects using the C stack of the interpreter.  This can help
performance, as stack allocation is typically faster than using heap
memory to allocate and the garbage collector to free.  The downside is
that using such objects after they are freed results in undefined
behavior, so uses should be well thought out and carefully debugged by
using the ‘GC_CHECK_MARKED_OBJECTS’ feature (see ‘src/alloc.c’).  In
particular, stack-allocated objects should never be made visible to user
Lisp code.

   Currently, cons cells and strings can be allocated this way.  This is
implemented by C macros like ‘AUTO_CONS’ and ‘AUTO_STRING’ that define a
named ‘Lisp_Object’ with block lifetime.  These objects are not freed by
the garbage collector; instead, they have automatic storage duration,
i.e., they are allocated like local variables and are automatically
freed at the end of execution of the C block that defined the object.

   For performance reasons, stack-allocated strings are limited to ASCII
characters, and many of these strings are immutable, i.e., calling
‘ASET’ on them produces undefined behavior.


File: elisp.info,  Node: Memory Usage,  Next: C Dialect,  Prev: Stack-allocated Objects,  Up: GNU Emacs Internals

E.5 Memory Usage
================

These functions and variables give information about the total amount of
memory allocation that Emacs has done, broken down by data type.  Note
the difference between these and the values returned by
‘garbage-collect’; those count objects that currently exist, but these
count the number or size of all allocations, including those for objects
that have since been freed.

 -- Variable: cons-cells-consed
     The total number of cons cells that have been allocated so far in
     this Emacs session.

 -- Variable: floats-consed
     The total number of floats that have been allocated so far in this
     Emacs session.

 -- Variable: vector-cells-consed
     The total number of vector cells that have been allocated so far in
     this Emacs session.

 -- Variable: symbols-consed
     The total number of symbols that have been allocated so far in this
     Emacs session.

 -- Variable: string-chars-consed
     The total number of string characters that have been allocated so
     far in this session.

 -- Variable: misc-objects-consed
     The total number of miscellaneous objects that have been allocated
     so far in this session.  These include markers and overlays, plus
     certain objects not visible to users.

 -- Variable: intervals-consed
     The total number of intervals that have been allocated so far in
     this Emacs session.

 -- Variable: strings-consed
     The total number of strings that have been allocated so far in this
     Emacs session.


File: elisp.info,  Node: C Dialect,  Next: Writing Emacs Primitives,  Prev: Memory Usage,  Up: GNU Emacs Internals

E.6 C Dialect
=============

The C part of Emacs is portable to C99 or later: C11-specific features
such as ‘<stdalign.h>’ and ‘_Noreturn’ are not used without a check,
typically at configuration time, and the Emacs build procedure provides
a substitute implementation if necessary.  Some C11 features, such as
anonymous structures and unions, are too difficult to emulate, so they
are avoided entirely.

   At some point in the future the base C dialect will no doubt change
to C11.


File: elisp.info,  Node: Writing Emacs Primitives,  Next: Object Internals,  Prev: C Dialect,  Up: GNU Emacs Internals

E.7 Writing Emacs Primitives
============================

Lisp primitives are Lisp functions implemented in C.  The details of
interfacing the C function so that Lisp can call it are handled by a few
C macros.  The only way to really understand how to write new C code is
to read the source, but we can explain some things here.

   An example of a special form is the definition of ‘or’, from
‘eval.c’.  (An ordinary function would have the same general
appearance.)

     DEFUN ("or", For, Sor, 0, UNEVALLED, 0,
       doc: /* Eval args until one of them yields non-nil, then return
     that value.
     The remaining args are not evalled at all.
     If all args return nil, return nil.
     usage: (or CONDITIONS...)  */)
       (Lisp_Object args)
     {
       Lisp_Object val = Qnil;

       while (CONSP (args))
         {
           val = eval_sub (XCAR (args));
           if (!NILP (val))
             break;
           args = XCDR (args);
           QUIT;
         }

       return val;
     }

   Let’s start with a precise explanation of the arguments to the
‘DEFUN’ macro.  Here is a template for them:

     DEFUN (LNAME, FNAME, SNAME, MIN, MAX, INTERACTIVE, DOC)

LNAME
     This is the name of the Lisp symbol to define as the function name;
     in the example above, it is ‘or’.

FNAME
     This is the C function name for this function.  This is the name
     that is used in C code for calling the function.  The name is, by
     convention, ‘F’ prepended to the Lisp name, with all dashes (‘-’)
     in the Lisp name changed to underscores.  Thus, to call this
     function from C code, call ‘For’.

SNAME
     This is a C variable name to use for a structure that holds the
     data for the subr object that represents the function in Lisp.
     This structure conveys the Lisp symbol name to the initialization
     routine that will create the symbol and store the subr object as
     its definition.  By convention, this name is always FNAME with ‘F’
     replaced with ‘S’.

MIN
     This is the minimum number of arguments that the function requires.
     The function ‘or’ allows a minimum of zero arguments.

MAX
     This is the maximum number of arguments that the function accepts,
     if there is a fixed maximum.  Alternatively, it can be ‘UNEVALLED’,
     indicating a special form that receives unevaluated arguments, or
     ‘MANY’, indicating an unlimited number of evaluated arguments (the
     equivalent of ‘&rest’).  Both ‘UNEVALLED’ and ‘MANY’ are macros.
     If MAX is a number, it must be more than MIN but less than 8.

INTERACTIVE
     This is an interactive specification, a string such as might be
     used as the argument of ‘interactive’ in a Lisp function.  In the
     case of ‘or’, it is 0 (a null pointer), indicating that ‘or’ cannot
     be called interactively.  A value of ‘""’ indicates a function that
     should receive no arguments when called interactively.  If the
     value begins with a ‘"(’, the string is evaluated as a Lisp form.
     For example:

          DEFUN ("foo", Ffoo, Sfoo, 0, UNEVALLED,
                 "(list (read-char-by-name \"Insert character: \")\
                        (prefix-numeric-value current-prefix-arg)\
                        t))",
            doc: /* ... /*)

DOC
     This is the documentation string.  It uses C comment syntax rather
     than C string syntax because comment syntax requires nothing
     special to include multiple lines.  The ‘doc:’ identifies the
     comment that follows as the documentation string.  The ‘/*’ and
     ‘*/’ delimiters that begin and end the comment are not part of the
     documentation string.

     If the last line of the documentation string begins with the
     keyword ‘usage:’, the rest of the line is treated as the argument
     list for documentation purposes.  This way, you can use different
     argument names in the documentation string from the ones used in
     the C code.  ‘usage:’ is required if the function has an unlimited
     number of arguments.

     All the usual rules for documentation strings in Lisp code (*note
     Documentation Tips::) apply to C code documentation strings too.

   After the call to the ‘DEFUN’ macro, you must write the argument list
for the C function, including the types for the arguments.  If the
primitive accepts a fixed maximum number of Lisp arguments, there must
be one C argument for each Lisp argument, and each argument must be of
type ‘Lisp_Object’.  (Various macros and functions for creating values
of type ‘Lisp_Object’ are declared in the file ‘lisp.h’.)  If the
primitive has no upper limit on the number of Lisp arguments, it must
have exactly two C arguments: the first is the number of Lisp arguments,
and the second is the address of a block containing their values.  These
have types ‘int’ and ‘Lisp_Object *’ respectively.  Since ‘Lisp_Object’
can hold any Lisp object of any data type, you can determine the actual
data type only at run time; so if you want a primitive to accept only a
certain type of argument, you must check the type explicitly using a
suitable predicate (*note Type Predicates::).

   Within the function ‘For’ itself, the local variable ‘args’ refers to
objects controlled by Emacs’s stack-marking garbage collector.  Although
the garbage collector does not reclaim objects reachable from C
‘Lisp_Object’ stack variables, it may move non-object components of an
object, such as string contents; so functions that access non-object
components must take care to refetch their addresses after performing
Lisp evaluation.  Lisp evaluation can occur via calls to ‘eval_sub’ or
‘Feval’, either directly or indirectly.

   Note the call to the ‘QUIT’ macro inside the loop: this macro checks
whether the user pressed ‘C-g’, and if so, aborts the processing.  You
should do that in any loop that can potentially require a large number
of iterations; in this case, the list of arguments could be very long.
This increases Emacs responsiveness and improves user experience.

   You must not use C initializers for static or global variables unless
the variables are never written once Emacs is dumped.  These variables
with initializers are allocated in an area of memory that becomes
read-only (on certain operating systems) as a result of dumping Emacs.
*Note Pure Storage::.

   Defining the C function is not enough to make a Lisp primitive
available; you must also create the Lisp symbol for the primitive and
store a suitable subr object in its function cell.  The code looks like
this:

     defsubr (&SNAME);

Here SNAME is the name you used as the third argument to ‘DEFUN’.

   If you add a new primitive to a file that already has Lisp primitives
defined in it, find the function (near the end of the file) named
‘syms_of_SOMETHING’, and add the call to ‘defsubr’ there.  If the file
doesn’t have this function, or if you create a new file, add to it a
‘syms_of_FILENAME’ (e.g., ‘syms_of_myfile’).  Then find the spot in
‘emacs.c’ where all of these functions are called, and add a call to
‘syms_of_FILENAME’ there.

   The function ‘syms_of_FILENAME’ is also the place to define any C
variables that are to be visible as Lisp variables.  ‘DEFVAR_LISP’ makes
a C variable of type ‘Lisp_Object’ visible in Lisp.  ‘DEFVAR_INT’ makes
a C variable of type ‘int’ visible in Lisp with a value that is always
an integer.  ‘DEFVAR_BOOL’ makes a C variable of type ‘int’ visible in
Lisp with a value that is either ‘t’ or ‘nil’.  Note that variables
defined with ‘DEFVAR_BOOL’ are automatically added to the list
‘byte-boolean-vars’ used by the byte compiler.

   If you want to make a Lisp variables that is defined in C behave like
one declared with ‘defcustom’, add an appropriate entry to
‘cus-start.el’.

   If you define a file-scope C variable of type ‘Lisp_Object’, you must
protect it from garbage-collection by calling ‘staticpro’ in
‘syms_of_FILENAME’, like this:

     staticpro (&VARIABLE);

   Here is another example function, with more complicated arguments.
This comes from the code in ‘window.c’, and it demonstrates the use of
macros and functions to manipulate Lisp objects.

     DEFUN ("coordinates-in-window-p", Fcoordinates_in_window_p,
       Scoordinates_in_window_p, 2, 2, 0,
       doc: /* Return non-nil if COORDINATES are in WINDOW.
       ...
       or `right-margin' is returned.  */)
       (register Lisp_Object coordinates, Lisp_Object window)
     {
       struct window *w;
       struct frame *f;
       int x, y;
       Lisp_Object lx, ly;

       CHECK_LIVE_WINDOW (window);
       w = XWINDOW (window);
       f = XFRAME (w->frame);
       CHECK_CONS (coordinates);
       lx = Fcar (coordinates);
       ly = Fcdr (coordinates);
       CHECK_NUMBER_OR_FLOAT (lx);
       CHECK_NUMBER_OR_FLOAT (ly);
       x = FRAME_PIXEL_X_FROM_CANON_X (f, lx) + FRAME_INTERNAL_BORDER_WIDTH(f);
       y = FRAME_PIXEL_Y_FROM_CANON_Y (f, ly) + FRAME_INTERNAL_BORDER_WIDTH(f);

       switch (coordinates_in_window (w, x, y))
         {
         case ON_NOTHING:            /* NOT in window at all.  */
           return Qnil;

         ...

         case ON_MODE_LINE:          /* In mode line of window.  */
           return Qmode_line;

         ...

         case ON_SCROLL_BAR:         /* On scroll-bar of window.  */
           /* Historically we are supposed to return nil in this case.  */
           return Qnil;

         default:
           abort ();
         }
     }

   Note that C code cannot call functions by name unless they are
defined in C.  The way to call a function written in Lisp is to use
‘Ffuncall’, which embodies the Lisp function ‘funcall’.  Since the Lisp
function ‘funcall’ accepts an unlimited number of arguments, in C it
takes two: the number of Lisp-level arguments, and a one-dimensional
array containing their values.  The first Lisp-level argument is the
Lisp function to call, and the rest are the arguments to pass to it.

   The C functions ‘call0’, ‘call1’, ‘call2’, and so on, provide handy
ways to call a Lisp function conveniently with a fixed number of
arguments.  They work by calling ‘Ffuncall’.

   ‘eval.c’ is a very good file to look through for examples; ‘lisp.h’
contains the definitions for some important macros and functions.

   If you define a function which is side-effect free, update the code
in ‘byte-opt.el’ that binds ‘side-effect-free-fns’ and
‘side-effect-and-error-free-fns’ so that the compiler optimizer knows
about it.


File: elisp.info,  Node: Object Internals,  Next: C Integer Types,  Prev: Writing Emacs Primitives,  Up: GNU Emacs Internals

E.8 Object Internals
====================

Emacs Lisp provides a rich set of the data types.  Some of them, like
cons cells, integers and strings, are common to nearly all Lisp
dialects.  Some others, like markers and buffers, are quite special and
needed to provide the basic support to write editor commands in Lisp.
To implement such a variety of object types and provide an efficient way
to pass objects between the subsystems of an interpreter, there is a set
of C data structures and a special type to represent the pointers to all
of them, which is known as “tagged pointer”.

   In C, the tagged pointer is an object of type ‘Lisp_Object’.  Any
initialized variable of such a type always holds the value of one of the
following basic data types: integer, symbol, string, cons cell, float,
vectorlike or miscellaneous object.  Each of these data types has the
corresponding tag value.  All tags are enumerated by ‘enum Lisp_Type’
and placed into a 3-bit bitfield of the ‘Lisp_Object’.  The rest of the
bits is the value itself.  Integers are immediate, i.e., directly
represented by those “value bits”, and all other objects are represented
by the C pointers to a corresponding object allocated from the heap.
Width of the ‘Lisp_Object’ is platform- and configuration-dependent:
usually it’s equal to the width of an underlying platform pointer (i.e.,
32-bit on a 32-bit machine and 64-bit on a 64-bit one), but also there
is a special configuration where ‘Lisp_Object’ is 64-bit but all
pointers are 32-bit.  The latter trick was designed to overcome the
limited range of values for Lisp integers on a 32-bit system by using
64-bit ‘long long’ type for ‘Lisp_Object’.

   The following C data structures are defined in ‘lisp.h’ to represent
the basic data types beyond integers:

‘struct Lisp_Cons’
     Cons cell, an object used to construct lists.

‘struct Lisp_String’
     String, the basic object to represent a sequence of characters.

‘struct Lisp_Vector’
     Array, a fixed-size set of Lisp objects which may be accessed by an
     index.

‘struct Lisp_Symbol’
     Symbol, the unique-named entity commonly used as an identifier.

‘struct Lisp_Float’
     Floating-point value.

‘union Lisp_Misc’
     Miscellaneous kinds of objects which don’t fit into any of the
     above.

   These types are the first-class citizens of an internal type system.
Since the tag space is limited, all other types are the subtypes of
either ‘Lisp_Vectorlike’ or ‘Lisp_Misc’.  Vector subtypes are enumerated
by ‘enum pvec_type’, and nearly all complex objects like windows,
buffers, frames, and processes fall into this category.  The rest of
special types, including markers and overlays, are enumerated by ‘enum
Lisp_Misc_Type’ and form the set of subtypes of ‘Lisp_Misc’.

   Below there is a description of a few subtypes of ‘Lisp_Vectorlike’.
Buffer object represents the text to display and edit.  Window is the
part of display structure which shows the buffer or used as a container
to recursively place other windows on the same frame.  (Do not confuse
Emacs Lisp window object with the window as an entity managed by the
user interface system like X; in Emacs terminology, the latter is called
frame.)  Finally, process object is used to manage the subprocesses.

* Menu:

* Buffer Internals::    Components of a buffer structure.
* Window Internals::    Components of a window structure.
* Process Internals::   Components of a process structure.


File: elisp.info,  Node: Buffer Internals,  Next: Window Internals,  Up: Object Internals

E.8.1 Buffer Internals
----------------------

Two structures (see ‘buffer.h’) are used to represent buffers in C.  The
‘buffer_text’ structure contains fields describing the text of a buffer;
the ‘buffer’ structure holds other fields.  In the case of indirect
buffers, two or more ‘buffer’ structures reference the same
‘buffer_text’ structure.

   Here are some of the fields in ‘struct buffer_text’:

‘beg’
     The address of the buffer contents.

‘gpt’
‘gpt_byte’
     The character and byte positions of the buffer gap.  *Note Buffer
     Gap::.

‘z’
‘z_byte’
     The character and byte positions of the end of the buffer text.

‘gap_size’
     The size of buffer’s gap.  *Note Buffer Gap::.

‘modiff’
‘save_modiff’
‘chars_modiff’
‘overlay_modiff’
     These fields count the number of buffer-modification events
     performed in this buffer.  ‘modiff’ is incremented after each
     buffer-modification event, and is never otherwise changed;
     ‘save_modiff’ contains the value of ‘modiff’ the last time the
     buffer was visited or saved; ‘chars_modiff’ counts only
     modifications to the characters in the buffer, ignoring all other
     kinds of changes; and ‘overlay_modiff’ counts only modifications to
     the overlays.

‘beg_unchanged’
‘end_unchanged’
     The number of characters at the start and end of the text that are
     known to be unchanged since the last complete redisplay.

‘unchanged_modified’
‘overlay_unchanged_modified’
     The values of ‘modiff’ and ‘overlay_modiff’, respectively, after
     the last complete redisplay.  If their current values match
     ‘modiff’ or ‘overlay_modiff’, that means ‘beg_unchanged’ and
     ‘end_unchanged’ contain no useful information.

‘markers’
     The markers that refer to this buffer.  This is actually a single
     marker, and successive elements in its marker ‘chain’ are the other
     markers referring to this buffer text.

‘intervals’
     The interval tree which records the text properties of this buffer.

   Some of the fields of ‘struct buffer’ are:

‘header’
     A header of type ‘struct vectorlike_header’ is common to all
     vectorlike objects.

‘own_text’
     A ‘struct buffer_text’ structure that ordinarily holds the buffer
     contents.  In indirect buffers, this field is not used.

‘text’
     A pointer to the ‘buffer_text’ structure for this buffer.  In an
     ordinary buffer, this is the ‘own_text’ field above.  In an
     indirect buffer, this is the ‘own_text’ field of the base buffer.

‘next’
     A pointer to the next buffer, in the chain of all buffers,
     including killed buffers.  This chain is used only for allocation
     and garbage collection, in order to collect killed buffers
     properly.

‘pt’
‘pt_byte’
     The character and byte positions of point in a buffer.

‘begv’
‘begv_byte’
     The character and byte positions of the beginning of the accessible
     range of text in the buffer.

‘zv’
‘zv_byte’
     The character and byte positions of the end of the accessible range
     of text in the buffer.

‘base_buffer’
     In an indirect buffer, this points to the base buffer.  In an
     ordinary buffer, it is null.

‘local_flags’
     This field contains flags indicating that certain variables are
     local in this buffer.  Such variables are declared in the C code
     using ‘DEFVAR_PER_BUFFER’, and their buffer-local bindings are
     stored in fields in the buffer structure itself.  (Some of these
     fields are described in this table.)

‘modtime’
     The modification time of the visited file.  It is set when the file
     is written or read.  Before writing the buffer into a file, this
     field is compared to the modification time of the file to see if
     the file has changed on disk.  *Note Buffer Modification::.

‘auto_save_modified’
     The time when the buffer was last auto-saved.

‘last_window_start’
     The ‘window-start’ position in the buffer as of the last time the
     buffer was displayed in a window.

‘clip_changed’
     This flag indicates that narrowing has changed in the buffer.
     *Note Narrowing::.

‘prevent_redisplay_optimizations_p’
     This flag indicates that redisplay optimizations should not be used
     to display this buffer.

‘overlay_center’
     This field holds the current overlay center position.  *Note
     Managing Overlays::.

‘overlays_before’
‘overlays_after’
     These fields hold, respectively, a list of overlays that end at or
     before the current overlay center, and a list of overlays that end
     after the current overlay center.  *Note Managing Overlays::.
     ‘overlays_before’ is sorted in order of decreasing end position,
     and ‘overlays_after’ is sorted in order of increasing beginning
     position.

‘name’
     A Lisp string that names the buffer.  It is guaranteed to be
     unique.  *Note Buffer Names::.

‘save_length’
     The length of the file this buffer is visiting, when last read or
     saved.  This and other fields concerned with saving are not kept in
     the ‘buffer_text’ structure because indirect buffers are never
     saved.

‘directory’
     The directory for expanding relative file names.  This is the value
     of the buffer-local variable ‘default-directory’ (*note File Name
     Expansion::).

‘filename’
     The name of the file visited in this buffer, or ‘nil’.  This is the
     value of the buffer-local variable ‘buffer-file-name’ (*note Buffer
     File Name::).

‘undo_list’
‘backed_up’
‘auto_save_file_name’
‘auto_save_file_format’
‘read_only’
‘file_format’
‘file_truename’
‘invisibility_spec’
‘display_count’
‘display_time’
     These fields store the values of Lisp variables that are
     automatically buffer-local (*note Buffer-Local Variables::), whose
     corresponding variable names have the additional prefix ‘buffer-’
     and have underscores replaced with dashes.  For instance,
     ‘undo_list’ stores the value of ‘buffer-undo-list’.

‘mark’
     The mark for the buffer.  The mark is a marker, hence it is also
     included on the list ‘markers’.  *Note The Mark::.

‘local_var_alist’
     The association list describing the buffer-local variable bindings
     of this buffer, not including the built-in buffer-local bindings
     that have special slots in the buffer object.  (Those slots are
     omitted from this table.)  *Note Buffer-Local Variables::.

‘major_mode’
     Symbol naming the major mode of this buffer, e.g., ‘lisp-mode’.

‘mode_name’
     Pretty name of the major mode, e.g., ‘"Lisp"’.

‘keymap’
‘abbrev_table’
‘syntax_table’
‘category_table’
‘display_table’
     These fields store the buffer’s local keymap (*note Keymaps::),
     abbrev table (*note Abbrev Tables::), syntax table (*note Syntax
     Tables::), category table (*note Categories::), and display table
     (*note Display Tables::).

‘downcase_table’
‘upcase_table’
‘case_canon_table’
     These fields store the conversion tables for converting text to
     lower case, upper case, and for canonicalizing text for case-fold
     search.  *Note Case Tables::.

‘minor_modes’
     An alist of the minor modes of this buffer.

‘pt_marker’
‘begv_marker’
‘zv_marker’
     These fields are only used in an indirect buffer, or in a buffer
     that is the base of an indirect buffer.  Each holds a marker that
     records ‘pt’, ‘begv’, and ‘zv’ respectively, for this buffer when
     the buffer is not current.

‘mode_line_format’
‘header_line_format’
‘case_fold_search’
‘tab_width’
‘fill_column’
‘left_margin’
‘auto_fill_function’
‘truncate_lines’
‘word_wrap’
‘ctl_arrow’
‘bidi_display_reordering’
‘bidi_paragraph_direction’
‘selective_display’
‘selective_display_ellipses’
‘overwrite_mode’
‘abbrev_mode’
‘mark_active’
‘enable_multibyte_characters’
‘buffer_file_coding_system’
‘cache_long_line_scans’
‘point_before_scroll’
‘left_fringe_width’
‘right_fringe_width’
‘fringes_outside_margins’
‘scroll_bar_width’
‘indicate_empty_lines’
‘indicate_buffer_boundaries’
‘fringe_indicator_alist’
‘fringe_cursor_alist’
‘scroll_up_aggressively’
‘scroll_down_aggressively’
‘cursor_type’
‘cursor_in_non_selected_windows’
     These fields store the values of Lisp variables that are
     automatically buffer-local (*note Buffer-Local Variables::), whose
     corresponding variable names have underscores replaced with dashes.
     For instance, ‘mode_line_format’ stores the value of
     ‘mode-line-format’.

‘last_selected_window’
     This is the last window that was selected with this buffer in it,
     or ‘nil’ if that window no longer displays this buffer.


File: elisp.info,  Node: Window Internals,  Next: Process Internals,  Prev: Buffer Internals,  Up: Object Internals

E.8.2 Window Internals
----------------------

The fields of a window (for a complete list, see the definition of
‘struct window’ in ‘window.h’) include:

‘frame’
     The frame that this window is on.

‘mini_p’
     Non-‘nil’ if this window is a minibuffer window.

‘parent’
     Internally, Emacs arranges windows in a tree; each group of
     siblings has a parent window whose area includes all the siblings.
     This field points to a window’s parent.

     Parent windows do not display buffers, and play little role in
     display except to shape their child windows.  Emacs Lisp programs
     usually have no access to the parent windows; they operate on the
     windows at the leaves of the tree, which actually display buffers.

‘hchild’
‘vchild’
     These fields contain the window’s leftmost child and its topmost
     child respectively.  ‘hchild’ is used if the window is subdivided
     horizontally by child windows, and ‘vchild’ if it is subdivided
     vertically.  In a live window, only one of ‘hchild’, ‘vchild’, and
     ‘buffer’ (q.v.) is non-‘nil’.

‘next’
‘prev’
     The next sibling and previous sibling of this window.  ‘next’ is
     ‘nil’ if the window is the right-most or bottom-most in its group;
     ‘prev’ is ‘nil’ if it is the left-most or top-most in its group.

‘left_col’
     The left-hand edge of the window, measured in columns, relative to
     the leftmost column in the frame (column 0).

‘top_line’
     The top edge of the window, measured in lines, relative to the
     topmost line in the frame (line 0).

‘total_cols’
‘total_lines’
     The width and height of the window, measured in columns and lines
     respectively.  The width includes the scroll bar and fringes,
     and/or the separator line on the right of the window (if any).

‘buffer’
     The buffer that the window is displaying.

‘start’
     A marker pointing to the position in the buffer that is the first
     character displayed in the window.

‘pointm’
     This is the value of point in the current buffer when this window
     is selected; when it is not selected, it retains its previous
     value.

‘force_start’
     If this flag is non-‘nil’, it says that the window has been
     scrolled explicitly by the Lisp program.  This affects what the
     next redisplay does if point is off the screen: instead of
     scrolling the window to show the text around point, it moves point
     to a location that is on the screen.

‘frozen_window_start_p’
     This field is set temporarily to 1 to indicate to redisplay that
     ‘start’ of this window should not be changed, even if point gets
     invisible.

‘start_at_line_beg’
     Non-‘nil’ means current value of ‘start’ was the beginning of a
     line when it was chosen.

‘use_time’
     This is the last time that the window was selected.  The function
     ‘get-lru-window’ uses this field.

‘sequence_number’
     A unique number assigned to this window when it was created.

‘last_modified’
     The ‘modiff’ field of the window’s buffer, as of the last time a
     redisplay completed in this window.

‘last_overlay_modified’
     The ‘overlay_modiff’ field of the window’s buffer, as of the last
     time a redisplay completed in this window.

‘last_point’
     The buffer’s value of point, as of the last time a redisplay
     completed in this window.

‘last_had_star’
     A non-‘nil’ value means the window’s buffer was modified when the
     window was last updated.

‘vertical_scroll_bar’
     This window’s vertical scroll bar.

‘left_margin_cols’
‘right_margin_cols’
     The widths of the left and right margins in this window.  A value
     of ‘nil’ means no margin.

‘left_fringe_width’
‘right_fringe_width’
     The widths of the left and right fringes in this window.  A value
     of ‘nil’ or ‘t’ means use the values of the frame.

‘fringes_outside_margins’
     A non-‘nil’ value means the fringes outside the display margins;
     othersize they are between the margin and the text.

‘window_end_pos’
     This is computed as ‘z’ minus the buffer position of the last glyph
     in the current matrix of the window.  The value is only valid if
     ‘window_end_valid’ is not ‘nil’.

‘window_end_bytepos’
     The byte position corresponding to ‘window_end_pos’.

‘window_end_vpos’
     The window-relative vertical position of the line containing
     ‘window_end_pos’.

‘window_end_valid’
     This field is set to a non-‘nil’ value if ‘window_end_pos’ is truly
     valid.  This is ‘nil’ if nontrivial redisplay is pre-empted, since
     in that case the display that ‘window_end_pos’ was computed for did
     not get onto the screen.

‘cursor’
     A structure describing where the cursor is in this window.

‘last_cursor’
     The value of ‘cursor’ as of the last redisplay that finished.

‘phys_cursor’
     A structure describing where the cursor of this window physically
     is.

‘phys_cursor_type’
‘phys_cursor_height’
‘phys_cursor_width’
     The type, height, and width of the cursor that was last displayed
     on this window.

‘phys_cursor_on_p’
     This field is non-zero if the cursor is physically on.

‘cursor_off_p’
     Non-zero means the cursor in this window is logically off.  This is
     used for blinking the cursor.

‘last_cursor_off_p’
     This field contains the value of ‘cursor_off_p’ as of the time of
     the last redisplay.

‘must_be_updated_p’
     This is set to 1 during redisplay when this window must be updated.

‘hscroll’
     This is the number of columns that the display in the window is
     scrolled horizontally to the left.  Normally, this is 0.

‘vscroll’
     Vertical scroll amount, in pixels.  Normally, this is 0.

‘dedicated’
     Non-‘nil’ if this window is dedicated to its buffer.

‘display_table’
     The window’s display table, or ‘nil’ if none is specified for it.

‘update_mode_line’
     Non-‘nil’ means this window’s mode line needs to be updated.

‘base_line_number’
     The line number of a certain position in the buffer, or ‘nil’.
     This is used for displaying the line number of point in the mode
     line.

‘base_line_pos’
     The position in the buffer for which the line number is known, or
     ‘nil’ meaning none is known.  If it is a buffer, don’t display the
     line number as long as the window shows that buffer.

‘column_number_displayed’
     The column number currently displayed in this window’s mode line,
     or ‘nil’ if column numbers are not being displayed.

‘current_matrix’
‘desired_matrix’
     Glyph matrices describing the current and desired display of this
     window.


File: elisp.info,  Node: Process Internals,  Prev: Window Internals,  Up: Object Internals

E.8.3 Process Internals
-----------------------

The fields of a process (for a complete list, see the definition of
‘struct Lisp_Process’ in ‘process.h’) include:

‘name’
     A string, the name of the process.

‘command’
     A list containing the command arguments that were used to start
     this process.  For a network or serial process, it is ‘nil’ if the
     process is running or ‘t’ if the process is stopped.

‘filter’
     A function used to accept output from the process.

‘sentinel’
     A function called whenever the state of the process changes.

‘buffer’
     The associated buffer of the process.

‘pid’
     An integer, the operating system’s process ID.  Pseudo-processes
     such as network or serial connections use a value of 0.

‘childp’
     A flag, ‘t’ if this is really a child process.  For a network or
     serial connection, it is a plist based on the arguments to
     ‘make-network-process’ or ‘make-serial-process’.

‘mark’
     A marker indicating the position of the end of the last output from
     this process inserted into the buffer.  This is often but not
     always the end of the buffer.

‘kill_without_query’
     If this is non-zero, killing Emacs while this process is still
     running does not ask for confirmation about killing the process.

‘raw_status’
     The raw process status, as returned by the ‘wait’ system call.

‘status’
     The process status, as ‘process-status’ should return it.

‘tick’
‘update_tick’
     If these two fields are not equal, a change in the status of the
     process needs to be reported, either by running the sentinel or by
     inserting a message in the process buffer.

‘pty_flag’
     Non-‘nil’ if communication with the subprocess uses a pty; ‘nil’ if
     it uses a pipe.

‘infd’
     The file descriptor for input from the process.

‘outfd’
     The file descriptor for output to the process.

‘tty_name’
     The name of the terminal that the subprocess is using, or ‘nil’ if
     it is using pipes.

‘decode_coding_system’
     Coding-system for decoding the input from this process.

‘decoding_buf’
     A working buffer for decoding.

‘decoding_carryover’
     Size of carryover in decoding.

‘encode_coding_system’
     Coding-system for encoding the output to this process.

‘encoding_buf’
     A working buffer for encoding.

‘inherit_coding_system_flag’
     Flag to set ‘coding-system’ of the process buffer from the coding
     system used to decode process output.

‘type’
     Symbol indicating the type of process: ‘real’, ‘network’, ‘serial’.


File: elisp.info,  Node: C Integer Types,  Prev: Object Internals,  Up: GNU Emacs Internals

E.9 C Integer Types
===================

Here are some guidelines for use of integer types in the Emacs C source
code.  These guidelines sometimes give competing advice; common sense is
advised.

   • Avoid arbitrary limits.  For example, avoid ‘int len = strlen (s);’
     unless the length of ‘s’ is required for other reasons to fit in
     ‘int’ range.

   • Do not assume that signed integer arithmetic wraps around on
     overflow.  This is no longer true of Emacs porting targets: signed
     integer overflow has undefined behavior in practice, and can dump
     core or even cause earlier or later code to behave illogically.
     Unsigned overflow does wrap around reliably, modulo a power of two.

   • Prefer signed types to unsigned, as code gets confusing when signed
     and unsigned types are combined.  Many other guidelines assume that
     types are signed; in the rarer cases where unsigned types are
     needed, similar advice may apply to the unsigned counterparts
     (e.g., ‘size_t’ instead of ‘ptrdiff_t’, or ‘uintptr_t’ instead of
     ‘intptr_t’).

   • Prefer ‘int’ for Emacs character codes, in the range 0 .. 0x3FFFFF.
     More generally, prefer ‘int’ for integers known to be in ‘int’
     range, e.g., screen column counts.

   • Prefer ‘ptrdiff_t’ for sizes, i.e., for integers bounded by the
     maximum size of any individual C object or by the maximum number of
     elements in any C array.  This is part of Emacs’s general
     preference for signed types.  Using ‘ptrdiff_t’ limits objects to
     ‘PTRDIFF_MAX’ bytes, but larger objects would cause trouble anyway
     since they would break pointer subtraction, so this does not impose
     an arbitrary limit.

   • Avoid ‘ssize_t’ except when communicating to low-level APIs that
     have ‘ssize_t’-related limitations.  Although it’s equivalent to
     ‘ptrdiff_t’ on typical platforms, ‘ssize_t’ is occasionally
     narrower, so using it for size-related calculations could overflow.
     Also, ‘ptrdiff_t’ is more ubiquitous and better-standardized, has
     standard ‘printf’ formats, and is the basis for Emacs’s internal
     size-overflow checking.  When using ‘ssize_t’, please note that
     POSIX requires support only for values in the range −1 ..
     ‘SSIZE_MAX’.

   • Prefer ‘intptr_t’ for internal representations of pointers, or for
     integers bounded only by the number of objects that can exist at
     any given time or by the total number of bytes that can be
     allocated.  Currently Emacs sometimes uses other types when
     ‘intptr_t’ would be better; fixing this is lower priority, as the
     code works as-is on Emacs’s current porting targets.

   • Prefer the Emacs-defined type ‘EMACS_INT’ for representing values
     converted to or from Emacs Lisp fixnums, as fixnum arithmetic is
     based on ‘EMACS_INT’.

   • When representing a system value (such as a file size or a count of
     seconds since the Epoch), prefer the corresponding system type
     (e.g., ‘off_t’, ‘time_t’).  Do not assume that a system type is
     signed, unless this assumption is known to be safe.  For example,
     although ‘off_t’ is always signed, ‘time_t’ need not be.

   • Prefer the Emacs-defined type ‘printmax_t’ for representing values
     that might be any signed integer that can be printed, using a
     ‘printf’-family function.

   • Prefer ‘intmax_t’ for representing values that might be any signed
     integer value.

   • Prefer ‘bool’, ‘false’ and ‘true’ for booleans.  Using ‘bool’ can
     make programs easier to read and a bit faster than using ‘int’.
     Although it is also OK to use ‘int’, ‘0’ and ‘1’, this older style
     is gradually being phased out.  When using ‘bool’, respect the
     limitations of the replacement implementation of ‘bool’, as
     documented in the source file ‘lib/stdbool.in.h’.  In particular,
     boolean bitfields should be of type ‘bool_bf’, not ‘bool’, so that
     they work correctly even when compiling Objective C with standard
     GCC.

   • In bitfields, prefer ‘unsigned int’ or ‘signed int’ to ‘int’, as
     ‘int’ is less portable: it might be signed, and might not be.
     Single-bit bit fields should be ‘unsigned int’ or ‘bool_bf’ so that
     their values are 0 or 1.


File: elisp.info,  Node: Standard Errors,  Next: Standard Keymaps,  Prev: GNU Emacs Internals,  Up: Top

Appendix F Standard Errors
**************************

Here is a list of the more important error symbols in standard Emacs,
grouped by concept.  The list includes each symbol’s message and a cross
reference to a description of how the error can occur.

   Each error symbol has an set of parent error conditions that is a
list of symbols.  Normally this list includes the error symbol itself
and the symbol ‘error’.  Occasionally it includes additional symbols,
which are intermediate classifications, narrower than ‘error’ but
broader than a single error symbol.  For example, all the errors in
accessing files have the condition ‘file-error’.  If we do not say here
that a certain error symbol has additional error conditions, that means
it has none.

   As a special exception, the error symbol ‘quit’ does not have the
condition ‘error’, because quitting is not considered an error.

   Most of these error symbols are defined in C (mainly ‘data.c’), but
some are defined in Lisp.  For example, the file ‘userlock.el’ defines
the ‘file-locked’ and ‘file-supersession’ errors.  Several of the
specialized Lisp libraries distributed with Emacs define their own error
symbols.  We do not attempt to list of all those here.

   *Note Errors::, for an explanation of how errors are generated and
handled.

‘error’
     The message is ‘error’.  *Note Errors::.

‘quit’
     The message is ‘Quit’.  *Note Quitting::.

‘args-out-of-range’
     The message is ‘Args out of range’.  This happens when trying to
     access an element beyond the range of a sequence, buffer, or other
     container-like object.  *Note Sequences Arrays Vectors::, and *Note
     Text::.

‘arith-error’
     The message is ‘Arithmetic error’.  This occurs when trying to
     perform integer division by zero.  *Note Numeric Conversions::, and
     *Note Arithmetic Operations::.

‘beginning-of-buffer’
     The message is ‘Beginning of buffer’.  *Note Character Motion::.

‘buffer-read-only’
     The message is ‘Buffer is read-only’.  *Note Read Only Buffers::.

‘circular-list’
     The message is ‘List contains a loop’.  This happens when a
     circular structure is encountered.  *Note Circular Objects::.

‘cl-assertion-failed’
     The message is ‘Assertion failed’.  This happens when the
     ‘cl-assert’ macro fails a test.  *Note (cl)Assertions::.

‘coding-system-error’
     The message is ‘Invalid coding system’.  *Note Lisp and Coding
     Systems::.

‘cyclic-function-indirection’
     The message is ‘Symbol's chain of function indirections contains a
     loop’.  *Note Function Indirection::.

‘cyclic-variable-indirection’
     The message is ‘Symbol's chain of variable indirections contains a
     loop’.  *Note Variable Aliases::.

‘dbus-error’
     The message is ‘D-Bus error’.  This is only defined if Emacs was
     compiled with D-Bus support.  *Note (dbus)Errors and Events::.

‘end-of-buffer’
     The message is ‘End of buffer’.  *Note Character Motion::.

‘end-of-file’
     The message is ‘End of file during parsing’.  Note that this is not
     a subcategory of ‘file-error’, because it pertains to the Lisp
     reader, not to file I/O.  *Note Input Functions::.

‘file-already-exists’
     This is a subcategory of ‘file-error’.  *Note Writing to Files::.

‘file-date-error’
     This is a subcategory of ‘file-error’.  It occurs when ‘copy-file’
     tries and fails to set the last-modification time of the output
     file.  *Note Changing Files::.

‘file-error’
     We do not list the error-strings of this error and its
     subcategories, because the error message is normally constructed
     from the data items alone when the error condition ‘file-error’ is
     present.  Thus, the error-strings are not very relevant.  However,
     these error symbols do have ‘error-message’ properties, and if no
     data is provided, the ‘error-message’ property _is_ used.  *Note
     Files::.

‘compression-error’
     This is a subcategory of ‘file-error’, which results from problems
     handling a compressed file.  *Note How Programs Do Loading::.

‘file-locked’
     This is a subcategory of ‘file-error’.  *Note File Locks::.

‘file-supersession’
     This is a subcategory of ‘file-error’.  *Note Modification Time::.

‘file-notify-error’
     This is a subcategory of ‘file-error’.  It happens, when a file
     could not be watched for changes.  *Note File Notifications::.

‘ftp-error’
     This is a subcategory of ‘file-error’, which results from problems
     in accessing a remote file using ftp.  *Note (emacs)Remote Files::.

‘invalid-function’
     The message is ‘Invalid function’.  *Note Function Indirection::.

‘invalid-read-syntax’
     The message is ‘Invalid read syntax’.  *Note Printed
     Representation::.

‘invalid-regexp’
     The message is ‘Invalid regexp’.  *Note Regular Expressions::.

‘mark-inactive’
     The message is ‘The mark is not active now’.  *Note The Mark::.

‘no-catch’
     The message is ‘No catch for tag’.  *Note Catch and Throw::.

‘scan-error’
     The message is ‘Scan error’.  This happens when certain
     syntax-parsing functions find invalid syntax or mismatched
     parentheses.  Conventionally raised with three argument: a
     human-readable error message, the start of the obstacle that cannot
     be moved over, and the end of the obstacle.  *Note List Motion::,
     and *Note Parsing Expressions::.

‘search-failed’
     The message is ‘Search failed’.  *Note Searching and Matching::.

‘setting-constant’
     The message is ‘Attempt to set a constant symbol’.  This happens
     when attempting to assign values to ‘nil’, ‘t’, and keyword
     symbols.  *Note Constant Variables::.

‘text-read-only’
     The message is ‘Text is read-only’.  This is a subcategory of
     ‘buffer-read-only’.  *Note Special Properties::.

‘undefined-color’
     The message is ‘Undefined color’.  *Note Color Names::.

‘user-error’
     The message is the empty string.  *Note Signaling Errors::.

‘void-function’
     The message is ‘Symbol's function definition is void’.  *Note
     Function Cells::.

‘void-variable’
     The message is ‘Symbol's value as variable is void’.  *Note
     Accessing Variables::.

‘wrong-number-of-arguments’
     The message is ‘Wrong number of arguments’.  *Note Argument List::.

‘wrong-type-argument’
     The message is ‘Wrong type argument’.  *Note Type Predicates::.


File: elisp.info,  Node: Standard Keymaps,  Next: Standard Hooks,  Prev: Standard Errors,  Up: Top

Appendix G Standard Keymaps
***************************

In this section we list some of the more general keymaps.  Many of these
exist when Emacs is first started, but some are loaded only when the
respective feature is accessed.

   There are many other, more specialized, maps than these; in
particular those associated with major and minor modes.  The minibuffer
uses several keymaps (*note Completion Commands::).  For more details on
keymaps, *note Keymaps::.

‘2C-mode-map’
     A sparse keymap for subcommands of the prefix ‘C-x 6’.
     *Note Two-Column Editing: (emacs)Two-Column.

‘abbrev-map’
     A sparse keymap for subcommands of the prefix ‘C-x a’.
     *Note (emacs)Defining Abbrevs::.

‘button-buffer-map’
     A sparse keymap useful for buffers containing buffers.
     You may want to use this as a parent keymap.  *Note Buttons::.

‘button-map’
     A sparse keymap used by buttons.

‘ctl-x-4-map’
     A sparse keymap for subcommands of the prefix ‘C-x 4’.

‘ctl-x-5-map’
     A sparse keymap for subcommands of the prefix ‘C-x 5’.

‘ctl-x-map’
     A full keymap for ‘C-x’ commands.

‘ctl-x-r-map’
     A sparse keymap for subcommands of the prefix ‘C-x r’.
     *Note (emacs)Registers::.

‘esc-map’
     A full keymap for ‘ESC’ (or ‘Meta’) commands.

‘facemenu-keymap’
     A sparse keymap used for the ‘M-o’ prefix key.

‘function-key-map’
     The parent keymap of all ‘local-function-key-map’ (q.v.) instances.

‘global-map’
     The full keymap containing default global key bindings.
     Modes should not modify the Global map.

‘goto-map’
     A sparse keymap used for the ‘M-g’ prefix key.

‘help-map’
     A sparse keymap for the keys following the help character ‘C-h’.
     *Note Help Functions::.

‘Helper-help-map’
     A full keymap used by the help utility package.
     It has the same keymap in its value cell and in its function cell.

‘input-decode-map’
     The keymap for translating keypad and function keys.
     If there are none, then it contains an empty sparse keymap.  *Note
     Translation Keymaps::.

‘key-translation-map’
     A keymap for translating keys.  This one overrides ordinary key
     bindings, unlike ‘local-function-key-map’.  *Note Translation
     Keymaps::.

‘kmacro-keymap’
     A sparse keymap for keys that follows the ‘C-x C-k’ prefix search.
     *Note (emacs)Keyboard Macros::.

‘local-function-key-map’
     The keymap for translating key sequences to preferred alternatives.
     If there are none, then it contains an empty sparse keymap.  *Note
     Translation Keymaps::.

‘menu-bar-file-menu’
‘menu-bar-edit-menu’
‘menu-bar-options-menu’
‘global-buffers-menu-map’
‘menu-bar-tools-menu’
‘menu-bar-help-menu’
     These keymaps display the main, top-level menus in the menu bar.
     Some of them contain sub-menus.  For example, the Edit menu
     contains ‘menu-bar-search-menu’, etc.  *Note Menu Bar::.

‘minibuffer-inactive-mode-map’
     A full keymap used in the minibuffer when it is not active.
     *Note Editing in the Minibuffer: (emacs)Minibuffer Edit.

‘mode-line-coding-system-map’
‘mode-line-input-method-map’
‘mode-line-column-line-number-mode-map’
     These keymaps control various areas of the mode line.
     *Note Mode Line Format::.

‘mode-specific-map’
     The keymap for characters following ‘C-c’.  Note, this is in the
     global map.  This map is not actually mode-specific: its name was
     chosen to be informative in ‘C-h b’ (‘display-bindings’), where it
     describes the main use of the ‘C-c’ prefix key.

‘mouse-appearance-menu-map’
     A sparse keymap used for the ‘S-mouse-1’ key.

‘mule-keymap’
     The global keymap used for the ‘C-x <RET>’ prefix key.

‘narrow-map’
     A sparse keymap for subcommands of the prefix ‘C-x n’.

‘prog-mode-map’
     The keymap used by Prog mode.
     *Note Basic Major Modes::.

‘query-replace-map’
‘multi-query-replace-map’
     A sparse keymap used for responses in ‘query-replace’ and related
     commands; also for ‘y-or-n-p’ and ‘map-y-or-n-p’.  The functions
     that use this map do not support prefix keys; they look up one
     event at a time.  ‘multi-query-replace-map’ extends
     ‘query-replace-map’ for multi-buffer replacements.  *Note
     query-replace-map: Search and Replace.

‘search-map’
     A sparse keymap that provides global bindings for search-related
     commands.

‘special-mode-map’
     The keymap used by Special mode.
     *Note Basic Major Modes::.

‘tool-bar-map’
     The keymap defining the contents of the tool bar.
     *Note Tool Bar::.

‘universal-argument-map’
     A sparse keymap used while processing ‘C-u’.
     *Note Prefix Command Arguments::.

‘vc-prefix-map’
     The global keymap used for the ‘C-x v’ prefix key.

‘x-alternatives-map’
     A sparse keymap used to map certain keys under graphical frames.
     The function ‘x-setup-function-keys’ uses this.


File: elisp.info,  Node: Standard Hooks,  Next: Index,  Prev: Standard Keymaps,  Up: Top

Appendix H Standard Hooks
*************************

The following is a list of some hook variables that let you provide
functions to be called from within Emacs on suitable occasions.

   Most of these variables have names ending with ‘-hook’.  They are
“normal hooks”, run by means of ‘run-hooks’.  The value of such a hook
is a list of functions; the functions are called with no arguments and
their values are completely ignored.  The recommended way to put a new
function on such a hook is to call ‘add-hook’.  *Note Hooks::, for more
information about using hooks.

   The variables whose names end in ‘-functions’ are usually “abnormal
hooks” (some old code may also use the deprecated ‘-hooks’ suffix);
their values are lists of functions, but these functions are called in a
special way (they are passed arguments, or their return values are
used).  The variables whose names end in ‘-function’ have single
functions as their values.

   This is not an exhaustive list, it only covers the more general
hooks.  For example, every major mode defines a hook named
‘MODENAME-mode-hook’.  The major mode command runs this normal hook with
‘run-mode-hooks’ as the very last thing it does.  *Note Mode Hooks::.
Most minor modes have mode hooks too.

   A special feature allows you to specify expressions to evaluate if
and when a file is loaded (*note Hooks for Loading::).  That feature is
not exactly a hook, but does a similar job.

‘activate-mark-hook’
‘deactivate-mark-hook’
     *Note The Mark::.

‘after-change-functions’
‘before-change-functions’
‘first-change-hook’
     *Note Change Hooks::.

‘after-change-major-mode-hook’
‘change-major-mode-after-body-hook’
     *Note Mode Hooks::.

‘after-init-hook’
‘before-init-hook’
‘emacs-startup-hook’
‘window-setup-hook’
     *Note Init File::.

‘after-insert-file-functions’
‘write-region-annotate-functions’
‘write-region-post-annotation-function’
     *Note Format Conversion::.

‘after-make-frame-functions’
‘before-make-frame-hook’
     *Note Creating Frames::.

‘after-save-hook’
‘before-save-hook’
‘write-contents-functions’
‘write-file-functions’
     *Note Saving Buffers::.

‘after-setting-font-hook’
     Hook run after a frame’s font changes.

‘auto-save-hook’
     *Note Auto-Saving::.

‘before-hack-local-variables-hook’
‘hack-local-variables-hook’
     *Note File Local Variables::.

‘buffer-access-fontify-functions’
     *Note Lazy Properties::.

‘buffer-list-update-hook’
     Hook run when the buffer list changes (*note Buffer List::).

‘buffer-quit-function’
     Function to call to quit the current buffer.

‘change-major-mode-hook’
     *Note Creating Buffer-Local::.

‘command-line-functions’
     *Note Command-Line Arguments::.

‘delayed-warnings-hook’
     The command loop runs this soon after ‘post-command-hook’ (q.v.).

‘focus-in-hook’
‘focus-out-hook’
     *Note Input Focus::.

‘delete-frame-functions’
     *Note Deleting Frames::.

‘delete-terminal-functions’
     *Note Multiple Terminals::.

‘pop-up-frame-function’
‘split-window-preferred-function’
     *Note Choosing Window Options::.

‘echo-area-clear-hook’
     *Note Echo Area Customization::.

‘find-file-hook’
‘find-file-not-found-functions’
     *Note Visiting Functions::.

‘font-lock-extend-after-change-region-function’
     *Note Region to Refontify::.

‘font-lock-extend-region-functions’
     *Note Multiline Font Lock::.

‘font-lock-fontify-buffer-function’
‘font-lock-fontify-region-function’
‘font-lock-mark-block-function’
‘font-lock-unfontify-buffer-function’
‘font-lock-unfontify-region-function’
     *Note Other Font Lock Variables::.

‘fontification-functions’
     *Note Automatic Face Assignment: Auto Faces.

‘frame-auto-hide-function’
     *Note Quitting Windows::.

‘kill-buffer-hook’
‘kill-buffer-query-functions’
     *Note Killing Buffers::.

‘kill-emacs-hook’
‘kill-emacs-query-functions’
     *Note Killing Emacs::.

‘menu-bar-update-hook’
     *Note Menu Bar::.

‘minibuffer-setup-hook’
‘minibuffer-exit-hook’
     *Note Minibuffer Misc::.

‘mouse-leave-buffer-hook’
     Hook run when about to switch windows with a mouse command.

‘mouse-position-function’
     *Note Mouse Position::.

‘prefix-command-echo-keystrokes-functions’
     An abnormal hook run by prefix commands (such as ‘C-u’) which
     should return a string describing the current prefix state.  For
     example, ‘C-u’ produces ‘C-u-’ and ‘C-u 1 2 3-’.  Each hook
     function is called with no arguments and should return a string
     describing the current prefix state, or ‘nil’ if there’s no prefix
     state.  *Note Prefix Command Arguments::.

‘prefix-command-preserve-state-hook’
     Hook run when a prefix command needs to preserve the prefix by
     passing the current prefix command state to the next command.  For
     example, ‘C-u’ needs to pass the state to the next command when the
     user types ‘C-u -’ or follows ‘C-u’ with a digit.

‘pre-redisplay-functions’
     Hook run in each window just before redisplaying it.  *Note Forcing
     Redisplay::.

‘post-command-hook’
‘pre-command-hook’
     *Note Command Overview::.

‘post-gc-hook’
     *Note Garbage Collection::.

‘post-self-insert-hook’
     *Note Keymaps and Minor Modes::.

‘suspend-hook’
‘suspend-resume-hook’
‘suspend-tty-functions’
‘resume-tty-functions’
     *Note Suspending Emacs::.

‘syntax-begin-function’
‘syntax-propertize-extend-region-functions’
‘syntax-propertize-function’
‘font-lock-syntactic-face-function’
     *Note Syntactic Font Lock::.  *Note Syntax Properties::.

‘temp-buffer-setup-hook’
‘temp-buffer-show-function’
‘temp-buffer-show-hook’
     *Note Temporary Displays::.

‘tty-setup-hook’
     *Note Terminal-Specific::.

‘window-configuration-change-hook’
‘window-scroll-functions’
‘window-size-change-functions’
     *Note Window Hooks::.

‘window-text-change-functions’
     Functions to call in redisplay when text in the window might
     change.


File: elisp.info,  Node: Index,  Prev: Standard Hooks,  Up: Top

Index
*****

[index]
* Menu:

* " in printing:                         Output Functions.    (line   9)
* " in strings:                          Syntax for Strings.  (line   6)
* ## read syntax:                        Symbol Type.         (line  57)
* #$:                                    Docs and Compilation.
                                                              (line  37)
* #' syntax:                             Anonymous Functions. (line  49)
* #( read syntax:                        Text Props and Strings.
                                                              (line   6)
* #@COUNT:                               Docs and Compilation.
                                                              (line  37)
* #COLON read syntax:                    Symbol Type.         (line  57)
* #N# read syntax:                       Circular Objects.    (line   6)
* #N= read syntax:                       Circular Objects.    (line   6)
* #^ read syntax:                        Char-Table Type.     (line  14)
* $ in display:                          Truncation.          (line   6)
* $ in regexp:                           Regexp Special.      (line 154)
* %:                                     Arithmetic Operations.
                                                              (line 110)
* % in format:                           Formatting Strings.  (line  39)
* & in replacement:                      Replacing Match.     (line  41)
* &optional:                             Argument List.       (line  18)
* &rest:                                 Argument List.       (line  18)
* ' for quoting:                         Quoting.             (line  15)
* ( in regexp:                           Regexp Backslash.    (line  44)
* (?: in regexp:                         Regexp Backslash.    (line  66)
* (...) in lists:                        Cons Cell Type.      (line  25)
* ) in regexp:                           Regexp Backslash.    (line  44)
* *:                                     Arithmetic Operations.
                                                              (line  66)
* * in interactive:                      Using Interactive.   (line  67)
* * in regexp:                           Regexp Special.      (line  16)
* *scratch*:                             Auto Major Mode.     (line  80)
* +:                                     Arithmetic Operations.
                                                              (line  41)
* + in regexp:                           Regexp Special.      (line  48)
* , (with backquote):                    Backquote.           (line  16)
* ,@ (with backquote):                   Backquote.           (line  29)
* -:                                     Arithmetic Operations.
                                                              (line  52)
* --enable-locallisppath option to configure: Building Emacs. (line  75)
* –enable-profiling option of configure: Profiling.           (line  41)
* . in lists:                            Dotted Pair Notation.
                                                              (line   6)
* . in regexp:                           Regexp Special.      (line  10)
* .emacs:                                Init File.           (line   6)
* /:                                     Arithmetic Operations.
                                                              (line  77)
* /=:                                    Comparison of Numbers.
                                                              (line  55)
* /dev/tty:                              Serial Ports.        (line   6)
* 1+:                                    Arithmetic Operations.
                                                              (line  17)
* 1-:                                    Arithmetic Operations.
                                                              (line  38)
* 1value:                                Test Coverage.       (line  25)
* 2C-mode-map:                           Prefix Keys.         (line  38)
* 2D box:                                Face Attributes.     (line 118)
* 3D box:                                Face Attributes.     (line 118)
* ; in comment:                          Comments.            (line   6)
* <:                                     Comparison of Numbers.
                                                              (line  59)
* <=:                                    Comparison of Numbers.
                                                              (line  63)
* =:                                     Comparison of Numbers.
                                                              (line  45)
* >:                                     Comparison of Numbers.
                                                              (line  67)
* >=:                                    Comparison of Numbers.
                                                              (line  71)
* ? in character constant:               Basic Char Syntax.   (line   6)
* ? in minibuffer:                       Text from Minibuffer.
                                                              (line 233)
* ? in regexp:                           Regexp Special.      (line  54)
* @ in interactive:                      Using Interactive.   (line  70)
* [ in regexp:                           Regexp Special.      (line  72)
* [...] (Edebug):                        Specification List.  (line 126)
* \ in character constant:               General Escape Syntax.
                                                              (line  10)
* \ in display:                          Truncation.          (line   6)
* \ in printing:                         Output Functions.    (line   9)
* \ in regexp:                           Regexp Special.      (line 165)
* \ in replacement:                      Replacing Match.     (line  50)
* \ in strings:                          Syntax for Strings.  (line   6)
* \ in symbols:                          Symbol Type.         (line  23)
* \' in regexp:                          Regexp Backslash.    (line 162)
* \< in regexp:                          Regexp Backslash.    (line 183)
* \= in regexp:                          Regexp Backslash.    (line 166)
* \> in regexp:                          Regexp Backslash.    (line 188)
* \a:                                    Basic Char Syntax.   (line  27)
* \b:                                    Basic Char Syntax.   (line  27)
* \b in regexp:                          Regexp Backslash.    (line 170)
* \B in regexp:                          Regexp Backslash.    (line 179)
* \e:                                    Basic Char Syntax.   (line  27)
* \f:                                    Basic Char Syntax.   (line  27)
* \n:                                    Basic Char Syntax.   (line  27)
* \n in print:                           Output Variables.    (line  17)
* \N in replacement:                     Replacing Match.     (line  44)
* \r:                                    Basic Char Syntax.   (line  27)
* \s:                                    Basic Char Syntax.   (line  27)
* \s in regexp:                          Regexp Backslash.    (line 129)
* \S in regexp:                          Regexp Backslash.    (line 137)
* \t:                                    Basic Char Syntax.   (line  27)
* \v:                                    Basic Char Syntax.   (line  27)
* \w in regexp:                          Regexp Backslash.    (line 122)
* \W in regexp:                          Regexp Backslash.    (line 126)
* \_< in regexp:                         Regexp Backslash.    (line 193)
* \_> in regexp:                         Regexp Backslash.    (line 199)
* \` in regexp:                          Regexp Backslash.    (line 158)
* ] in regexp:                           Regexp Special.      (line  72)
* ^ in interactive:                      Using Interactive.   (line  75)
* ^ in regexp:                           Regexp Special.      (line 123)
* `:                                     Backquote.           (line   6)
* ‘ (list substitution):                 Backquote.           (line   6)
* | in regexp:                           Regexp Backslash.    (line  12)
* abbrev:                                Abbrevs.             (line   6)
* abbrev properties:                     Abbrev Properties.   (line   6)
* abbrev table properties:               Abbrev Table Properties.
                                                              (line   6)
* abbrev tables:                         Abbrev Tables.       (line   6)
* abbrev tables in modes:                Major Mode Conventions.
                                                              (line 120)
* abbrev-all-caps:                       Abbrev Expansion.    (line  59)
* abbrev-expand-function:                Abbrev Expansion.    (line  97)
* abbrev-expansion:                      Abbrev Expansion.    (line  18)
* abbrev-file-name:                      Abbrev Files.        (line  16)
* abbrev-get:                            Abbrev Properties.   (line  13)
* abbrev-insert:                         Abbrev Expansion.    (line  39)
* abbrev-map:                            Standard Keymaps.    (line  20)
* abbrev-minor-mode-table-alist:         Standard Abbrev Tables.
                                                              (line  20)
* abbrev-prefix-mark:                    Abbrev Expansion.    (line  48)
* abbrev-put:                            Abbrev Properties.   (line  10)
* abbrev-start-location:                 Abbrev Expansion.    (line  65)
* abbrev-start-location-buffer:          Abbrev Expansion.    (line  73)
* abbrev-symbol:                         Abbrev Expansion.    (line  11)
* abbrev-table-get:                      Abbrev Table Properties.
                                                              (line  13)
* abbrev-table-name-list:                Abbrev Tables.       (line  45)
* abbrev-table-p:                        Abbrev Tables.       (line  14)
* abbrev-table-put:                      Abbrev Table Properties.
                                                              (line  10)
* abbreviate-file-name:                  Directory Names.     (line  78)
* abbreviated file names:                Directory Names.     (line  78)
* abbrevs, looking up and expanding:     Abbrev Expansion.    (line   6)
* abbrevs-changed:                       Abbrev Files.        (line  36)
* abnormal hook:                         Hooks.               (line  33)
* abort-recursive-edit:                  Recursive Editing.   (line  92)
* aborting:                              Recursive Editing.   (line  32)
* abs:                                   Comparison of Numbers.
                                                              (line  95)
* absolute file name:                    Relative File Names. (line   6)
* absolute position:                     Frame Layout.        (line 140)
* accept input from processes:           Accepting Output.    (line   6)
* accept-change-group:                   Atomic Changes.      (line  48)
* accept-process-output:                 Accepting Output.    (line  12)
* access control list:                   Extended Attributes. (line  14)
* access minibuffer contents:            Minibuffer Contents. (line   6)
* access-file:                           Testing Accessibility.
                                                              (line  71)
* accessibility of a file:               Testing Accessibility.
                                                              (line   6)
* accessible portion (of a buffer):      Narrowing.           (line   6)
* accessible-keymaps:                    Scanning Keymaps.    (line   9)
* accessing documentation strings:       Accessing Documentation.
                                                              (line   6)
* accessing hash tables:                 Hash Access.         (line   6)
* accessing plist properties:            Plist Access.        (line   6)
* ACL entries:                           Extended Attributes. (line  14)
* acos:                                  Math Functions.      (line  20)
* action (button property):              Button Properties.   (line  12)
* action alist, for display-buffer:      Choosing Window.     (line  12)
* action function, for display-buffer:   Choosing Window.     (line  12)
* action, customization keyword:         Type Keywords.       (line  63)
* activate-change-group:                 Atomic Changes.      (line  39)
* activate-mark-hook:                    The Mark.            (line 174)
* active display table:                  Active Display Table.
                                                              (line   6)
* active keymap:                         Active Keymaps.      (line   6)
* active keymap, controlling:            Controlling Active Maps.
                                                              (line   6)
* active-minibuffer-window:              Minibuffer Windows.  (line   9)
* adaptive-fill-first-line-regexp:       Adaptive Fill.       (line  62)
* adaptive-fill-function:                Adaptive Fill.       (line  75)
* adaptive-fill-mode:                    Adaptive Fill.       (line  12)
* adaptive-fill-regexp:                  Adaptive Fill.       (line  53)
* add-face-text-property:                Changing Properties. (line  89)
* add-function:                          Core Advising Primitives.
                                                              (line   6)
* add-hook:                              Setting Hooks.       (line  11)
* add-name-to-file:                      Changing Files.      (line  28)
* add-text-properties:                   Changing Properties. (line  23)
* add-to-history:                        Minibuffer History.  (line  56)
* add-to-invisibility-spec:              Invisible Text.      (line  63)
* add-to-list:                           List Variables.      (line  34)
* add-to-ordered-list:                   List Variables.      (line  69)
* address field of register:             Cons Cell Type.      (line   6)
* adjust-window-trailing-edge:           Resizing Windows.    (line  70)
* adjusting point:                       Adjusting Point.     (line   6)
* advertised binding:                    Keys in Documentation.
                                                              (line  69)
* advice, add and remove:                Core Advising Primitives.
                                                              (line   6)
* advice-add:                            Advising Named Functions.
                                                              (line  56)
* advice-eval-interactive-spec:          Core Advising Primitives.
                                                              (line  93)
* advice-function-mapc:                  Core Advising Primitives.
                                                              (line  88)
* advice-function-member-p:              Core Advising Primitives.
                                                              (line  83)
* advice-mapc:                           Advising Named Functions.
                                                              (line  70)
* advice-member-p:                       Advising Named Functions.
                                                              (line  65)
* advice-remove:                         Advising Named Functions.
                                                              (line  61)
* advising functions:                    Advising Functions.  (line   6)
* advising named functions:              Advising Named Functions.
                                                              (line   6)
* after-change-functions:                Change Hooks.        (line  23)
* after-change-major-mode-hook:          Mode Hooks.          (line  52)
* after-find-file:                       Subroutines of Visiting.
                                                              (line  32)
* after-init-hook:                       Init File.           (line  54)
* after-init-time:                       Startup Summary.     (line  83)
* after-insert-file-functions:           Format Conversion Piecemeal.
                                                              (line  78)
* after-load-functions:                  Hooks for Loading.   (line   9)
* after-make-frame-functions:            Creating Frames.     (line  45)
* after-revert-hook:                     Reverting.           (line  87)
* after-save-hook:                       Saving Buffers.      (line 133)
* after-setting-font-hook:               Standard Hooks.      (line  68)
* after-string (overlay property):       Overlay Properties.  (line 200)
* alias, for coding systems:             Coding System Basics.
                                                              (line  89)
* alias, for faces:                      Face Functions.      (line  28)
* alias, for functions:                  Defining Functions.  (line  53)
* alias, for variables:                  Variable Aliases.    (line   6)
* alist:                                 Association Lists.   (line   6)
* alist vs. plist:                       Plists and Alists.   (line   6)
* alist-get:                             Association Lists.   (line 123)
* all-completions:                       Basic Completion.    (line  96)
* alpha, a frame parameter:              Font and Color Parameters.
                                                              (line  54)
* alt characters:                        Other Char Bits.     (line  16)
* alternatives, defining:                Generic Commands.    (line   6)
* amalgamating commands, and undo:       Undo.                (line  98)
* and:                                   Combining Conditions.
                                                              (line  17)
* animation:                             Multi-Frame Images.  (line   6)
* anonymous face:                        Faces.               (line  11)
* anonymous function:                    Anonymous Functions. (line   6)
* apostrophe for quoting:                Quoting.             (line  15)
* append:                                Building Lists.      (line  61)
* append-to-file:                        Writing to Files.    (line  11)
* apply:                                 Calling Functions.   (line  56)
* apply, and debugging:                  Internals of Debugger.
                                                              (line  62)
* apply-partially:                       Calling Functions.   (line  91)
* applying customizations:               Applying Customizations.
                                                              (line   6)
* apropos:                               Help Functions.      (line  11)
* aref:                                  Array Functions.     (line  20)
* args, customization keyword:           Composite Types.     (line 290)
* argument:                              What Is a Function.  (line   6)
* argument binding:                      Argument List.       (line   6)
* argument lists, features:              Argument List.       (line   6)
* arguments for shell commands:          Shell Arguments.     (line   6)
* arguments, interactive entry:          Using Interactive.   (line   6)
* arguments, reading:                    Minibuffers.         (line   6)
* argv:                                  Command-Line Arguments.
                                                              (line  62)
* arith-error example:                   Handling Errors.     (line 155)
* arith-error in division:               Arithmetic Operations.
                                                              (line 105)
* arithmetic operations:                 Arithmetic Operations.
                                                              (line   6)
* arithmetic shift:                      Bitwise Operations.  (line  82)
* array:                                 Arrays.              (line   6)
* array elements:                        Array Functions.     (line  21)
* arrayp:                                Array Functions.     (line   9)
* ASCII character codes:                 Character Type.      (line   6)
* ASCII control characters:              Usual Display.       (line  22)
* ascii-case-table:                      Case Tables.         (line  90)
* aset:                                  Array Functions.     (line  33)
* ash:                                   Bitwise Operations.  (line  81)
* asin:                                  Math Functions.      (line  15)
* ask-user-about-lock:                   File Locks.          (line  49)
* ask-user-about-supersession-threat:    Modification Time.   (line  74)
* asking the user questions:             Yes-or-No Queries.   (line   6)
* assoc:                                 Association Lists.   (line  58)
* assoc-default:                         Association Lists.   (line 165)
* assoc-string:                          Text Comparison.     (line 216)
* association list:                      Association Lists.   (line   6)
* assq:                                  Association Lists.   (line  98)
* assq-delete-all:                       Association Lists.   (line 221)
* asynchronous subprocess:               Asynchronous Processes.
                                                              (line   6)
* atan:                                  Math Functions.      (line  25)
* atom:                                  List-related Predicates.
                                                              (line  15)
* atomic changes:                        Atomic Changes.      (line   6)
* atoms:                                 Cons Cell Type.      (line  22)
* attributes of text:                    Text Properties.     (line   6)
* Auto Fill mode:                        Auto Filling.        (line   6)
* auto-coding-alist:                     Default Coding Systems.
                                                              (line  51)
* auto-coding-functions:                 Default Coding Systems.
                                                              (line  97)
* auto-coding-regexp-alist:              Default Coding Systems.
                                                              (line  17)
* auto-fill-chars:                       Auto Filling.        (line  30)
* auto-fill-function:                    Auto Filling.        (line  14)
* auto-hscroll-mode:                     Horizontal Scrolling.
                                                              (line  40)
* auto-lower, a frame parameter:         Management Parameters.
                                                              (line  19)
* auto-mode-alist:                       Auto Major Mode.     (line 104)
* auto-raise, a frame parameter:         Management Parameters.
                                                              (line  15)
* auto-raise-tool-bar-buttons:           Tool Bar.            (line 154)
* auto-resize-tool-bars:                 Tool Bar.            (line 142)
* auto-save-default:                     Auto-Saving.         (line 135)
* auto-save-file-name-p:                 Auto-Saving.         (line  32)
* auto-save-hook:                        Auto-Saving.         (line 132)
* auto-save-interval:                    Auto-Saving.         (line 113)
* auto-save-list-file-name:              Auto-Saving.         (line 190)
* auto-save-list-file-prefix:            Auto-Saving.         (line 207)
* auto-save-mode:                        Auto-Saving.         (line  22)
* auto-save-timeout:                     Auto-Saving.         (line 120)
* auto-save-visited-file-name:           Auto-Saving.         (line  86)
* auto-window-vscroll:                   Vertical Scrolling.  (line  51)
* autoload:                              Autoload.            (line  24)
* autoload <1>:                          Autoload.            (line   6)
* autoload cookie:                       Autoload.            (line 103)
* autoload errors:                       Autoload.            (line  89)
* autoload object:                       What Is a Function.  (line  84)
* autoload-do-load:                      Autoload.            (line 198)
* autoloadp:                             Autoload.            (line  82)
* automatic face assignment:             Auto Faces.          (line   6)
* automatically buffer-local:            Intro to Buffer-Local.
                                                              (line  39)
* back-to-indentation:                   Motion by Indent.    (line   9)
* background-color, a frame parameter:   Font and Color Parameters.
                                                              (line  86)
* background-mode, a frame parameter:    Font and Color Parameters.
                                                              (line  18)
* backing store:                         Display Feature Testing.
                                                              (line 116)
* backquote (list substitution):         Backquote.           (line   6)
* backslash in character constants:      General Escape Syntax.
                                                              (line  10)
* backslash in regular expressions:      Regexp Backslash.    (line   6)
* backslash in strings:                  Syntax for Strings.  (line   6)
* backslash in symbols:                  Symbol Type.         (line  23)
* backspace:                             Basic Char Syntax.   (line  27)
* backtrace:                             Internals of Debugger.
                                                              (line  20)
* backtrace from emacsclient’s --eval:   Error Debugging.     (line  74)
* backtrace-debug:                       Internals of Debugger.
                                                              (line  68)
* backtrace-frame:                       Internals of Debugger.
                                                              (line  90)
* backtracking:                          Backtracking.        (line   6)
* backtracking and POSIX regular expressions: POSIX Regexps.  (line   6)
* backtracking and regular expressions:  Regexp Special.      (line  25)
* backup file:                           Backup Files.        (line   6)
* backup files, rename or copy:          Rename or Copy.      (line   6)
* backup-buffer:                         Making Backups.      (line   6)
* backup-by-copying:                     Rename or Copy.      (line  29)
* backup-by-copying-when-linked:         Rename or Copy.      (line  37)
* backup-by-copying-when-mismatch:       Rename or Copy.      (line  44)
* backup-by-copying-when-privileged-mismatch: Rename or Copy. (line  57)
* backup-directory-alist:                Making Backups.      (line  67)
* backup-enable-predicate:               Making Backups.      (line  43)
* backup-file-name-p:                    Backup Names.        (line  10)
* backup-inhibited:                      Making Backups.      (line  55)
* backups and auto-saving:               Backups and Auto-Saving.
                                                              (line   6)
* backward-button:                       Button Buffer Commands.
                                                              (line  41)
* backward-char:                         Character Motion.    (line  36)
* backward-delete-char-untabify:         Deletion.            (line  68)
* backward-delete-char-untabify-method:  Deletion.            (line  87)
* backward-list:                         List Motion.         (line  19)
* backward-prefix-chars:                 Motion and Syntax.   (line  34)
* backward-sexp:                         List Motion.         (line  60)
* backward-to-indentation:               Motion by Indent.    (line  14)
* backward-up-list:                      List Motion.         (line  34)
* backward-word:                         Word Motion.         (line  38)
* backward-word-strictly:                Word Motion.         (line  75)
* balance-windows:                       Resizing Windows.    (line 208)
* balance-windows-area:                  Resizing Windows.    (line 215)
* balanced parenthesis motion:           List Motion.         (line   6)
* balancing parentheses:                 Blinking.            (line   6)
* balancing window sizes:                Resizing Windows.    (line 208)
* barf-if-buffer-read-only:              Read Only Buffers.   (line  66)
* base 64 encoding:                      Base 64.             (line   6)
* base buffer:                           Indirect Buffers.    (line   6)
* base coding system:                    Coding System Basics.
                                                              (line  45)
* base direction of a paragraph:         Bidirectional Display.
                                                              (line  75)
* base for reading an integer:           Integer Basics.      (line  24)
* base location, package archive:        Package Archives.    (line  12)
* base remapping, faces:                 Face Remapping.      (line  39)
* base64-decode-region:                  Base 64.             (line  34)
* base64-decode-string:                  Base 64.             (line  42)
* base64-encode-region:                  Base 64.             (line  11)
* base64-encode-string:                  Base 64.             (line  23)
* basic code (of input character):       Keyboard Events.     (line  12)
* basic faces:                           Basic Faces.         (line   6)
* batch mode:                            Batch Mode.          (line   6)
* batch-byte-compile:                    Compilation Functions.
                                                              (line 125)
* baud, in serial connections:           Serial Ports.        (line 114)
* baud-rate:                             Terminal Output.     (line  10)
* beep:                                  Beeping.             (line  17)
* before point, insertion:               Insertion.           (line   6)
* before-change-functions:               Change Hooks.        (line  16)
* before-hack-local-variables-hook:      File Local Variables.
                                                              (line  79)
* before-init-hook:                      Init File.           (line  49)
* before-init-time:                      Startup Summary.     (line  22)
* before-make-frame-hook:                Creating Frames.     (line  42)
* before-revert-hook:                    Reverting.           (line  82)
* before-save-hook:                      Saving Buffers.      (line 126)
* before-string (overlay property):      Overlay Properties.  (line 195)
* beginning of line:                     Text Lines.          (line  54)
* beginning of line in regexp:           Regexp Special.      (line 141)
* beginning-of-buffer:                   Buffer End Motion.   (line  18)
* beginning-of-defun:                    List Motion.         (line  64)
* beginning-of-defun-function:           List Motion.         (line  87)
* beginning-of-line:                     Text Lines.          (line  14)
* bell:                                  Beeping.             (line   6)
* bell character:                        Basic Char Syntax.   (line  27)
* benchmark.el:                          Profiling.           (line  37)
* benchmarking:                          Profiling.           (line  37)
* bidi-display-reordering:               Bidirectional Display.
                                                              (line  29)
* bidi-find-overridden-directionality:   Bidirectional Display.
                                                              (line 211)
* bidi-paragraph-direction:              Bidirectional Display.
                                                              (line  91)
* bidi-string-mark-left-to-right:        Bidirectional Display.
                                                              (line 161)
* bidirectional class of characters:     Character Properties.
                                                              (line  57)
* bidirectional display:                 Bidirectional Display.
                                                              (line   6)
* bidirectional reordering:              Bidirectional Display.
                                                              (line  17)
* big endian:                            Bindat Spec.         (line  13)
* binary coding system:                  Coding System Basics.
                                                              (line  62)
* binary I/O in batch mode:              Input Functions.     (line  60)
* bindat-get-field:                      Bindat Functions.    (line  19)
* bindat-ip-to-string:                   Bindat Functions.    (line  53)
* bindat-length:                         Bindat Functions.    (line  38)
* bindat-pack:                           Bindat Functions.    (line  42)
* bindat-unpack:                         Bindat Functions.    (line  10)
* binding arguments:                     Argument List.       (line   6)
* binding local variables:               Local Variables.     (line   6)
* binding of a key:                      Keymap Basics.       (line   6)
* bitmap-spec-p:                         Face Attributes.     (line 210)
* bitmaps, fringe:                       Fringe Bitmaps.      (line   6)
* bitwise arithmetic:                    Bitwise Operations.  (line   6)
* blink-cursor-alist:                    Cursor Parameters.   (line  56)
* blink-matching-delay:                  Blinking.            (line  22)
* blink-matching-open:                   Blinking.            (line  27)
* blink-matching-paren:                  Blinking.            (line  15)
* blink-matching-paren-distance:         Blinking.            (line  18)
* blink-paren-function:                  Blinking.            (line   9)
* blinking parentheses:                  Blinking.            (line   6)
* bobp:                                  Near Point.          (line  61)
* body height of a window:               Window Sizes.        (line 144)
* body of a window:                      Window Sizes.        (line  23)
* body of function:                      Lambda Components.   (line  37)
* body size of a window:                 Window Sizes.        (line 185)
* body width of a window:                Window Sizes.        (line 162)
* bolp:                                  Near Point.          (line  72)
* bool-vector:                           Bool-Vectors.        (line  21)
* bool-vector-count-consecutive:         Bool-Vectors.        (line  60)
* bool-vector-count-population:          Bool-Vectors.        (line  65)
* bool-vector-exclusive-or:              Bool-Vectors.        (line  31)
* bool-vector-intersection:              Bool-Vectors.        (line  41)
* bool-vector-not:                       Bool-Vectors.        (line  51)
* bool-vector-p:                         Bool-Vectors.        (line  25)
* bool-vector-set-difference:            Bool-Vectors.        (line  46)
* bool-vector-subsetp:                   Bool-Vectors.        (line  56)
* bool-vector-union:                     Bool-Vectors.        (line  36)
* Bool-vectors:                          Bool-Vectors.        (line   6)
* boolean:                               nil and t.           (line   6)
* booleanp:                              nil and t.           (line  36)
* border-color, a frame parameter:       Font and Color Parameters.
                                                              (line  98)
* border-width, a frame parameter:       Layout Parameters.   (line   9)
* bottom dividers:                       Window Dividers.     (line   6)
* bottom-divider-width, a frame parameter: Layout Parameters. (line  49)
* boundp:                                Void Variables.      (line  56)
* box diagrams, for lists:               Box Diagrams.        (line   6)
* break:                                 Debugger.            (line   6)
* breakpoints (Edebug):                  Breakpoints.         (line   6)
* bucket (in obarray):                   Creating Symbols.    (line  11)
* buffer:                                Buffers.             (line   6)
* buffer boundaries, indicating:         Fringe Indicators.   (line  16)
* buffer contents:                       Text.                (line  23)
* buffer file name:                      Buffer File Name.    (line   6)
* buffer gap:                            Buffer Gap.          (line   6)
* buffer input stream:                   Input Streams.       (line  11)
* buffer internals:                      Buffer Internals.    (line   6)
* buffer list:                           Buffer List.         (line   6)
* buffer modification:                   Buffer Modification. (line   6)
* buffer names:                          Buffer Names.        (line   6)
* buffer output stream:                  Output Streams.      (line  11)
* buffer portion as string:              Buffer Contents.     (line   6)
* buffer position:                       Positions.           (line   6)
* buffer text notation:                  Buffer Text Notation.
                                                              (line   6)
* buffer, read-only:                     Read Only Buffers.   (line   6)
* buffer-access-fontified-property:      Lazy Properties.     (line  29)
* buffer-access-fontify-functions:       Lazy Properties.     (line  14)
* buffer-auto-save-file-format:          Format Conversion Round-Trip.
                                                              (line 147)
* buffer-auto-save-file-name:            Auto-Saving.         (line  14)
* buffer-backed-up:                      Making Backups.      (line  22)
* buffer-base-buffer:                    Indirect Buffers.    (line  59)
* buffer-chars-modified-tick:            Buffer Modification. (line  56)
* buffer-disable-undo:                   Maintaining Undo.    (line  26)
* buffer-display-count:                  Buffers and Windows. (line  41)
* buffer-display-table:                  Active Display Table.
                                                              (line  24)
* buffer-display-time:                   Buffers and Windows. (line  46)
* buffer-enable-undo:                    Maintaining Undo.    (line  16)
* buffer-end:                            Point.               (line  50)
* buffer-file-coding-system:             Encoding and I/O.    (line  20)
* buffer-file-format:                    Format Conversion Round-Trip.
                                                              (line 101)
* buffer-file-name:                      Buffer File Name.    (line  22)
* buffer-file-name <1>:                  Buffer File Name.    (line  13)
* buffer-file-number:                    Buffer File Name.    (line  43)
* buffer-file-truename:                  Buffer File Name.    (line  37)
* buffer-invisibility-spec:              Invisible Text.      (line  34)
* buffer-list:                           Buffer List.         (line  28)
* buffer-list, a frame parameter:        Buffer Parameters.   (line  26)
* buffer-list-update-hook:               Buffer List.         (line 139)
* buffer-list-update-hook <1>:           Standard Hooks.      (line  81)
* buffer-live-p:                         Killing Buffers.     (line  95)
* buffer-local variables:                Buffer-Local Variables.
                                                              (line   6)
* buffer-local variables in modes:       Major Mode Conventions.
                                                              (line 150)
* buffer-local-value:                    Creating Buffer-Local.
                                                              (line 101)
* buffer-local-variables:                Creating Buffer-Local.
                                                              (line 107)
* buffer-modified-p:                     Buffer Modification. (line  22)
* buffer-modified-tick:                  Buffer Modification. (line  50)
* buffer-name:                           Buffer Names.        (line  18)
* buffer-name-history:                   Minibuffer History.  (line 100)
* buffer-narrowed-p:                     Narrowing.           (line  53)
* buffer-offer-save:                     Killing Buffers.     (line  80)
* buffer-predicate, a frame parameter:   Buffer Parameters.   (line  18)
* buffer-quit-function:                  Standard Hooks.      (line  84)
* buffer-read-only:                      Read Only Buffers.   (line  27)
* buffer-save-without-query:             Killing Buffers.     (line  89)
* buffer-saved-size:                     Auto-Saving.         (line 173)
* buffer-size:                           Point.               (line  54)
* buffer-stale-function:                 Reverting.           (line  92)
* buffer-string:                         Buffer Contents.     (line  43)
* buffer-substring:                      Buffer Contents.     (line   9)
* buffer-substring-filters:              Buffer Contents.     (line 106)
* buffer-substring-no-properties:        Buffer Contents.     (line  38)
* buffer-substring-with-bidi-context:    Bidirectional Display.
                                                              (line 246)
* buffer-swap-text:                      Swapping Text.       (line  26)
* buffer-undo-list:                      Undo.                (line  15)
* bufferp:                               Buffer Basics.       (line  40)
* bufferpos-to-filepos:                  Text Representations.
                                                              (line  82)
* buffers to display on frame:           Buffer Parameters.   (line   6)
* buffers without undo information:      Buffer Names.        (line  12)
* buffers, controlled in windows:        Buffers and Windows. (line   6)
* buffers, creating:                     Creating Buffers.    (line   6)
* buffers, killing:                      Killing Buffers.     (line   6)
* bugs:                                  Caveats.             (line  27)
* bugs in this manual:                   Caveats.             (line   6)
* building Emacs:                        Building Emacs.      (line   6)
* building lists:                        Building Lists.      (line   6)
* built-in function:                     What Is a Function.  (line  33)
* bury-buffer:                           Buffer List.         (line 101)
* butlast:                               List Elements.       (line 146)
* button (button property):              Button Properties.   (line  49)
* button buffer commands:                Button Buffer Commands.
                                                              (line   6)
* button properties:                     Button Properties.   (line   6)
* button types:                          Button Types.        (line   6)
* button-activate:                       Manipulating Buttons.
                                                              (line  28)
* button-at:                             Manipulating Buttons.
                                                              (line  45)
* button-down event:                     Button-Down Events.  (line   6)
* button-end:                            Manipulating Buttons.
                                                              (line  19)
* button-face, customization keyword:    Type Keywords.       (line  66)
* button-get:                            Manipulating Buttons.
                                                              (line  22)
* button-has-type-p:                     Manipulating Buttons.
                                                              (line  41)
* button-label:                          Manipulating Buttons.
                                                              (line  35)
* button-prefix, customization keyword:  Type Keywords.       (line  71)
* button-put:                            Manipulating Buttons.
                                                              (line  25)
* button-start:                          Manipulating Buttons.
                                                              (line  16)
* button-suffix, customization keyword:  Type Keywords.       (line  71)
* button-type:                           Manipulating Buttons.
                                                              (line  38)
* button-type-get:                       Manipulating Buttons.
                                                              (line  53)
* button-type-put:                       Manipulating Buttons.
                                                              (line  50)
* button-type-subtype-p:                 Manipulating Buttons.
                                                              (line  56)
* buttons in buffers:                    Buttons.             (line   6)
* byte compilation:                      Byte Compilation.    (line   6)
* byte compiler warnings, how to avoid:  Warning Tips.        (line   6)
* byte packing and unpacking:            Byte Packing.        (line   6)
* byte to string:                        Converting Representations.
                                                              (line  59)
* byte-boolean-vars:                     Variables with Restricted Values.
                                                              (line  22)
* byte-boolean-vars <1>:                 Writing Emacs Primitives.
                                                              (line 159)
* byte-code:                             Byte Compilation.    (line   6)
* byte-code function:                    Byte-Code Objects.   (line   6)
* byte-code object:                      Byte-Code Objects.   (line   6)
* byte-code-function-p:                  What Is a Function.  (line 109)
* byte-compile:                          Compilation Functions.
                                                              (line  34)
* byte-compile-debug:                    Compilation Functions.
                                                              (line  11)
* byte-compile-dynamic:                  Dynamic Loading.     (line  44)
* byte-compile-dynamic-docstrings:       Docs and Compilation.
                                                              (line  23)
* byte-compile-file:                     Compilation Functions.
                                                              (line  75)
* byte-compiling macros:                 Compiling Macros.    (line   6)
* byte-compiling require:                Named Features.      (line  51)
* byte-recompile-directory:              Compilation Functions.
                                                              (line 105)
* byte-to-position:                      Text Representations.
                                                              (line  68)
* byte-to-string:                        Converting Representations.
                                                              (line  58)
* bytes:                                 Strings and Characters.
                                                              (line   6)
* bytesize, in serial connections:       Serial Ports.        (line 114)
* C programming language:                C Dialect.           (line   6)
* C-c:                                   Prefix Keys.         (line  21)
* C-g:                                   Quitting.            (line   6)
* C-h:                                   Prefix Keys.         (line  19)
* C-M-x:                                 Instrumenting.       (line  10)
* C-x:                                   Prefix Keys.         (line  27)
* C-x 4:                                 Prefix Keys.         (line  34)
* C-x 5:                                 Prefix Keys.         (line  36)
* C-x 6:                                 Prefix Keys.         (line  38)
* C-x C-a C-m:                           Edebug Execution Modes.
                                                              (line  67)
* C-x <RET>:                             Prefix Keys.         (line  31)
* C-x v:                                 Prefix Keys.         (line  40)
* C-x X =:                               Coverage Testing.    (line  23)
* caar:                                  List Elements.       (line 133)
* cadr:                                  List Elements.       (line 136)
* calendrical computations:              Time Calculations.   (line   6)
* calendrical information:               Time Conversion.     (line   6)
* call stack:                            Internals of Debugger.
                                                              (line  21)
* call-interactively:                    Interactive Call.    (line  37)
* call-process:                          Synchronous Processes.
                                                              (line  29)
* call-process, command-line arguments from minibuffer: Shell Arguments.
                                                              (line  40)
* call-process-region:                   Synchronous Processes.
                                                              (line 190)
* call-process-shell-command:            Synchronous Processes.
                                                              (line 241)
* called-interactively-p:                Distinguish Interactive.
                                                              (line  26)
* calling a function:                    Calling Functions.   (line   6)
* cancel-change-group:                   Atomic Changes.      (line  52)
* cancel-debug-on-entry:                 Function Debugging.  (line  51)
* cancel-timer:                          Timers.              (line 129)
* canonical character height:            Frame Font.          (line  12)
* canonical character width:             Frame Font.          (line  12)
* capitalization:                        Case Conversion.     (line  54)
* capitalize:                            Case Conversion.     (line  53)
* capitalize-region:                     Case Changes.        (line  12)
* capitalize-word:                       Case Changes.        (line  49)
* car:                                   List Elements.       (line   6)
* car-safe:                              List Elements.       (line  34)
* case conversion in buffers:            Case Changes.        (line   6)
* case conversion in Lisp:               Case Conversion.     (line   6)
* case in replacements:                  Replacing Match.     (line   9)
* case-fold-search:                      Searching and Case.  (line  25)
* case-replace:                          Searching and Case.  (line  30)
* case-table-p:                          Case Tables.         (line  59)
* catch:                                 Catch and Throw.     (line  60)
* categories of characters:              Categories.          (line   6)
* category (overlay property):           Overlay Properties.  (line  83)
* category (text property):              Special Properties.  (line  15)
* category set:                          Categories.          (line  23)
* category table:                        Categories.          (line  12)
* category, regexp search for:           Regexp Backslash.    (line 139)
* category-docstring:                    Categories.          (line  62)
* category-set-mnemonics:                Categories.          (line 121)
* category-table:                        Categories.          (line  76)
* category-table-p:                      Categories.          (line  79)
* cdar:                                  List Elements.       (line 139)
* cddr:                                  List Elements.       (line 142)
* cdr:                                   List Elements.       (line  20)
* cdr-safe:                              List Elements.       (line  47)
* ceiling:                               Numeric Conversions. (line  53)
* centering point:                       Textual Scrolling.   (line 174)
* change hooks:                          Change Hooks.        (line   6)
* change hooks for a character:          Special Properties.  (line 269)
* change load-path at configure time:    Building Emacs.      (line  75)
* change-major-mode-after-body-hook:     Mode Hooks.          (line  48)
* change-major-mode-hook:                Creating Buffer-Local.
                                                              (line 179)
* changing key bindings:                 Changing Key Bindings.
                                                              (line   6)
* changing text properties:              Changing Properties. (line   6)
* changing to another buffer:            Current Buffer.      (line   6)
* changing window size:                  Resizing Windows.    (line   6)
* char-after:                            Near Point.          (line  13)
* char-before:                           Near Point.          (line  26)
* char-category-set:                     Categories.          (line 111)
* char-charset:                          Character Sets.      (line  38)
* char-code-property-description:        Character Properties.
                                                              (line 185)
* char-displayable-p:                    Fontsets.            (line 132)
* char-equal:                            Text Comparison.     (line   6)
* char-or-string-p:                      Predicates for Strings.
                                                              (line  16)
* char-property-alias-alist:             Examining Properties.
                                                              (line  56)
* char-script-table:                     Character Properties.
                                                              (line 205)
* char-syntax:                           Syntax Table Functions.
                                                              (line  66)
* char-table length:                     Sequence Functions.  (line  13)
* char-table-extra-slot:                 Char-Tables.         (line  72)
* char-table-p:                          Char-Tables.         (line  55)
* char-table-parent:                     Char-Tables.         (line  65)
* char-table-range:                      Char-Tables.         (line  84)
* char-table-subtype:                    Char-Tables.         (line  59)
* char-tables:                           Char-Tables.         (line   6)
* char-to-string:                        String Conversion.   (line  65)
* char-width:                            Size of Displayed Text.
                                                              (line  10)
* char-width-table:                      Character Properties.
                                                              (line 213)
* character alternative (in regexp):     Regexp Special.      (line  72)
* character arrays:                      Strings and Characters.
                                                              (line   6)
* character case:                        Case Conversion.     (line   6)
* character categories:                  Categories.          (line   6)
* character classes in regexp:           Char Classes.        (line   6)
* character code conversion:             Coding System Basics.
                                                              (line   6)
* character codepoint:                   Text Representations.
                                                              (line  10)
* character codes:                       Character Codes.     (line   6)
* character insertion:                   Commands for Insertion.
                                                              (line  17)
* character printing:                    Describing Characters.
                                                              (line  29)
* character properties:                  Character Properties.
                                                              (line   6)
* character set, searching:              Scanning Charsets.   (line   6)
* character sets:                        Character Sets.      (line   6)
* character to string:                   String Conversion.   (line  66)
* character translation tables:          Translation of Characters.
                                                              (line   6)
* character width on display:            Size of Displayed Text.
                                                              (line   6)
* characterp:                            Character Codes.     (line  21)
* characters:                            Strings and Characters.
                                                              (line   6)
* characters for interactive codes:      Interactive Codes.   (line   6)
* characters, multi-byte:                Non-ASCII Characters.
                                                              (line   6)
* characters, representation in buffers and strings: Text Representations.
                                                              (line  20)
* charset:                               Character Sets.      (line   6)
* charset, coding systems to encode:     Lisp and Coding Systems.
                                                              (line  80)
* charset, text property:                Explicit Encoding.   (line 105)
* charset-after:                         Scanning Charsets.   (line  12)
* charset-list:                          Character Sets.      (line  27)
* charset-plist:                         Character Sets.      (line  48)
* charset-priority-list:                 Character Sets.      (line  30)
* charsetp:                              Character Sets.      (line  23)
* charsets supported by a coding system: Lisp and Coding Systems.
                                                              (line 137)
* check-coding-system:                   Lisp and Coding Systems.
                                                              (line  18)
* check-coding-systems-region:           Lisp and Coding Systems.
                                                              (line  84)
* checkdoc:                              Tips.                (line  11)
* checkdoc-current-buffer:               Tips.                (line  11)
* checkdoc-file:                         Tips.                (line  11)
* checkdoc-minor-mode:                   Documentation Tips.  (line   6)
* checkdoc-package-keywords:             Library Headers.     (line  84)
* checkdoc-package-keywords-flag:        Library Headers.     (line  84)
* child process:                         Processes.           (line   6)
* child window:                          Windows and Frames.  (line  50)
* choice, customization types:           Splicing into Lists. (line  27)
* circular list:                         Cons Cells.          (line  34)
* circular structure, read syntax:       Circular Objects.    (line   6)
* cl:                                    Lisp History.        (line  28)
* CL note—allocate more storage:         Garbage Collection.  (line  51)
* CL note—case of letters:               Symbol Type.         (line  38)
* CL note—default optional arg:          Argument List.       (line  46)
* CL note—integers vrs eq:               Comparison of Numbers.
                                                              (line  39)
* CL note—interning existing symbol:     Creating Symbols.    (line 111)
* CL note—lack union, intersection:      Sets And Lists.      (line  13)
* CL note—no continuable errors:         Signaling Errors.    (line 102)
* CL note—no setf functions:             Adding Generalized Variables.
                                                              (line  53)
* CL note—only throw in Emacs:           Catch and Throw.     (line  54)
* CL note—rplaca vs setcar:              Modifying Lists.     (line  10)
* CL note—special forms compared:        Special Forms.       (line  99)
* CL note—symbol in obarrays:            Creating Symbols.    (line  65)
* cl-call-next-method:                   Generic Functions.   (line 196)
* cl-defgeneric:                         Generic Functions.   (line  49)
* cl-defmethod:                          Generic Functions.   (line  71)
* cl-next-method-p:                      Generic Functions.   (line 204)
* classification of file types:          Kinds of Files.      (line   6)
* classifying events:                    Classifying Events.  (line   6)
* cleanup forms:                         Cleanups.            (line  13)
* clear-abbrev-table:                    Abbrev Tables.       (line  18)
* clear-image-cache:                     Image Cache.         (line  39)
* clear-string:                          Modifying Strings.   (line  26)
* clear-this-command-keys:               Command Loop Info.   (line  94)
* clear-visited-file-modtime:            Modification Time.   (line  33)
* click event:                           Click Events.        (line   6)
* clickable buttons in buffers:          Buttons.             (line   6)
* clickable text:                        Clickable Text.      (line   6)
* clipboard:                             Window System Selections.
                                                              (line   6)
* clipboard support (for MS-Windows):    Window System Selections.
                                                              (line  57)
* clone-indirect-buffer:                 Indirect Buffers.    (line  48)
* CLOS:                                  Generic Functions.   (line  20)
* closure:                               Closures.            (line  13)
* closures, example of using:            Lexical Binding.     (line  50)
* clrhash:                               Hash Access.         (line  28)
* coded character set:                   Character Sets.      (line   6)
* codepoint, largest value:              Character Codes.     (line  32)
* codes, interactive, description of:    Interactive Codes.   (line   6)
* codespace:                             Text Representations.
                                                              (line  10)
* coding conventions in Emacs Lisp:      Coding Conventions.  (line   6)
* coding standards:                      Tips.                (line   6)
* coding system:                         Coding Systems.      (line   6)
* coding system for operation:           Specifying Coding Systems.
                                                              (line   6)
* coding system, automatically determined: Default Coding Systems.
                                                              (line   6)
* coding system, validity check:         Lisp and Coding Systems.
                                                              (line  18)
* coding systems for encoding a string:  Lisp and Coding Systems.
                                                              (line  73)
* coding systems for encoding region:    Lisp and Coding Systems.
                                                              (line  64)
* coding systems, priority:              Specifying Coding Systems.
                                                              (line  67)
* coding-system-aliases:                 Coding System Basics.
                                                              (line  89)
* coding-system-change-eol-conversion:   Lisp and Coding Systems.
                                                              (line  47)
* coding-system-change-text-conversion:  Lisp and Coding Systems.
                                                              (line  57)
* coding-system-charset-list:            Lisp and Coding Systems.
                                                              (line 137)
* coding-system-eol-type:                Lisp and Coding Systems.
                                                              (line  25)
* coding-system-for-read:                Specifying Coding Systems.
                                                              (line   9)
* coding-system-for-write:               Specifying Coding Systems.
                                                              (line  34)
* coding-system-get:                     Coding System Basics.
                                                              (line  72)
* coding-system-list:                    Lisp and Coding Systems.
                                                              (line   8)
* coding-system-p:                       Lisp and Coding Systems.
                                                              (line  14)
* coding-system-priority-list:           Specifying Coding Systems.
                                                              (line  73)
* coding-system-require-warning:         Specifying Coding Systems.
                                                              (line  45)
* collapse-delayed-warnings:             Delayed Warnings.    (line  35)
* color names:                           Color Names.         (line   6)
* color-defined-p:                       Color Names.         (line  25)
* color-gray-p:                          Color Names.         (line  61)
* color-supported-p:                     Color Names.         (line  49)
* color-values:                          Color Names.         (line  67)
* colors on text terminals:              Text Terminal Colors.
                                                              (line   6)
* column width:                          Frame Font.          (line  12)
* columns:                               Columns.             (line   6)
* COM1:                                  Serial Ports.        (line   6)
* combine-after-change-calls:            Change Hooks.        (line  50)
* combine-and-quote-strings:             Shell Arguments.     (line  67)
* combining conditions:                  Combining Conditions.
                                                              (line   6)
* command:                               What Is a Function.  (line  60)
* command descriptions:                  A Sample Function Description.
                                                              (line   6)
* command history:                       Command History.     (line   6)
* command in keymap:                     Key Lookup.          (line  42)
* command loop:                          Command Loop.        (line   6)
* command loop variables:                Command Loop Info.   (line   6)
* command loop, recursive:               Recursive Editing.   (line   6)
* command-debug-status:                  Internals of Debugger.
                                                              (line  77)
* command-error-function:                Processing of Errors.
                                                              (line  22)
* command-execute:                       Interactive Call.    (line  79)
* command-history:                       Command History.     (line  14)
* command-line:                          Command-Line Arguments.
                                                              (line  14)
* command-line arguments:                Command-Line Arguments.
                                                              (line   6)
* command-line options:                  Command-Line Arguments.
                                                              (line  29)
* command-line-args:                     Command-Line Arguments.
                                                              (line  57)
* command-line-args-left:                Command-Line Arguments.
                                                              (line  61)
* command-line-functions:                Command-Line Arguments.
                                                              (line  65)
* command-line-processed:                Command-Line Arguments.
                                                              (line  19)
* command-remapping:                     Remapping Commands.  (line  42)
* command-switch-alist:                  Command-Line Arguments.
                                                              (line  28)
* commandp:                              Interactive Call.    (line  17)
* commandp example:                      High-Level Completion.
                                                              (line  96)
* commands, defining:                    Defining Commands.   (line   6)
* comment style:                         Syntax Flags.        (line  45)
* comment syntax:                        Syntax Class Table.  (line 110)
* comment-end-can-be-escaped:            Control Parsing.     (line  20)
* commentary, in a Lisp library:         Library Headers.     (line 139)
* comments:                              Comments.            (line   6)
* comments, Lisp convention for:         Comment Tips.        (line   6)
* Common Lisp:                           Lisp History.        (line  11)
* compare-buffer-substrings:             Comparing Text.      (line   9)
* compare-strings:                       Text Comparison.     (line 192)
* compare-window-configurations:         Window Configurations.
                                                              (line  74)
* comparing buffer text:                 Comparing Text.      (line   6)
* comparing file modification time:      Modification Time.   (line   6)
* comparing numbers:                     Comparison of Numbers.
                                                              (line   6)
* comparing time values:                 Time Calculations.   (line   6)
* compilation (Emacs Lisp):              Byte Compilation.    (line   6)
* compilation functions:                 Compilation Functions.
                                                              (line   6)
* compile-defun:                         Compilation Functions.
                                                              (line  65)
* compile-time constant:                 Eval During Compile. (line  42)
* compiled function:                     Byte-Code Objects.   (line   6)
* compiler errors:                       Compiler Errors.     (line   6)
* complete key:                          Keymap Basics.       (line   6)
* completing-read:                       Minibuffer Completion.
                                                              (line   9)
* completing-read-function:              Minibuffer Completion.
                                                              (line  99)
* completion:                            Completion.          (line   6)
* completion styles:                     Completion Variables.
                                                              (line   9)
* completion table:                      Basic Completion.    (line  14)
* completion table, modifying:           Basic Completion.    (line 187)
* completion tables, combining:          Basic Completion.    (line 187)
* completion, file name:                 File Name Completion.
                                                              (line   6)
* completion-at-point:                   Completion in Buffers.
                                                              (line   6)
* completion-at-point-functions:         Completion in Buffers.
                                                              (line  13)
* completion-auto-help:                  Completion Commands. (line  86)
* completion-boundaries:                 Basic Completion.    (line 139)
* completion-category-overrides:         Completion Variables.
                                                              (line  49)
* completion-extra-properties:           Completion Variables.
                                                              (line  69)
* completion-ignore-case:                Basic Completion.    (line 160)
* completion-ignored-extensions:         File Name Completion.
                                                              (line  61)
* completion-in-region:                  Completion in Buffers.
                                                              (line  83)
* completion-regexp-list:                Basic Completion.    (line 168)
* completion-styles:                     Completion Variables.
                                                              (line   9)
* completion-styles-alist:               Completion Variables.
                                                              (line  15)
* completion-table-case-fold:            Basic Completion.    (line 187)
* completion-table-dynamic:              Programmed Completion.
                                                              (line  93)
* completion-table-in-turn:              Basic Completion.    (line 187)
* completion-table-merge:                Basic Completion.    (line 187)
* completion-table-subvert:              Basic Completion.    (line 187)
* completion-table-with-cache:           Programmed Completion.
                                                              (line 108)
* completion-table-with-predicate:       Basic Completion.    (line 187)
* completion-table-with-quoting:         Basic Completion.    (line 187)
* completion-table-with-terminator:      Basic Completion.    (line 187)
* complex arguments:                     Minibuffers.         (line   6)
* complex command:                       Command History.     (line   6)
* composite types (customization):       Composite Types.     (line   6)
* composition (text property):           Special Properties.  (line 342)
* composition property, and point display: Adjusting Point.   (line   6)
* compute-motion:                        Screen Lines.        (line 108)
* concat:                                Creating Strings.    (line  98)
* concatenating bidirectional strings:   Bidirectional Display.
                                                              (line 131)
* concatenating lists:                   Rearrangement.       (line  16)
* concatenating strings:                 Creating Strings.    (line  99)
* cond:                                  Conditionals.        (line  51)
* condition name:                        Error Symbols.       (line   6)
* condition-case:                        Handling Errors.     (line  92)
* condition-case-unless-debug:           Handling Errors.     (line  67)
* conditional evaluation:                Conditionals.        (line   6)
* conditional selection of windows:      Cyclic Window Ordering.
                                                              (line 139)
* cons:                                  Building Lists.      (line  11)
* cons cells:                            Building Lists.      (line   6)
* cons-cells-consed:                     Memory Usage.        (line  13)
* consing:                               Building Lists.      (line  25)
* consp:                                 List-related Predicates.
                                                              (line  11)
* constant variables:                    Constant Variables.  (line   6)
* constant variables <1>:                Defining Variables.  (line  76)
* constrain-to-field:                    Fields.              (line  68)
* content directory, package:            Packaging Basics.    (line  47)
* continuation lines:                    Truncation.          (line   6)
* continue-process:                      Signals to Processes.
                                                              (line  85)
* control character key constants:       Changing Key Bindings.
                                                              (line  20)
* control character printing:            Describing Characters.
                                                              (line  29)
* control characters:                    Ctl-Char Syntax.     (line   6)
* control characters in display:         Usual Display.       (line  66)
* control characters, reading:           Quoted Character Input.
                                                              (line  12)
* control structures:                    Control Structures.  (line   6)
* Control-X-prefix:                      Prefix Keys.         (line  27)
* controller part, model/view/controller: Abstract Display Example.
                                                              (line  63)
* controlling terminal:                  Suspending Emacs.    (line  13)
* controlling-tty-p:                     Suspending Emacs.    (line 107)
* conventions for writing major modes:   Major Mode Conventions.
                                                              (line   6)
* conventions for writing minor modes:   Minor Mode Conventions.
                                                              (line   6)
* conversion of strings:                 String Conversion.   (line   6)
* convert buffer position to file byte:  Text Representations.
                                                              (line  79)
* convert file byte to buffer position:  Text Representations.
                                                              (line  79)
* convert-standard-filename:             Standard File Names. (line  40)
* converting file names from/to MS-Windows syntax: File Names.
                                                              (line  19)
* converting numbers:                    Numeric Conversions. (line   6)
* coordinate, relative to frame:         Coordinates and Windows.
                                                              (line   6)
* coordinates-in-window-p:               Coordinates and Windows.
                                                              (line  70)
* copy-abbrev-table:                     Abbrev Tables.       (line  22)
* copy-alist:                            Association Lists.   (line 182)
* copy-category-table:                   Categories.          (line  86)
* copy-directory:                        Create/Delete Dirs.  (line  19)
* copy-file:                             Changing Files.      (line  82)
* copy-hash-table:                       Other Hash.          (line  11)
* copy-keymap:                           Creating Keymaps.    (line  39)
* copy-marker:                           Creating Markers.    (line  52)
* copy-overlay:                          Managing Overlays.   (line  85)
* copy-region-as-kill:                   Kill Functions.      (line  43)
* copy-sequence:                         Sequence Functions.  (line  63)
* copy-syntax-table:                     Syntax Table Functions.
                                                              (line  18)
* copy-tree:                             Building Lists.      (line 149)
* copying alists:                        Association Lists.   (line 183)
* copying bidirectional text, preserve visual order: Bidirectional Display.
                                                              (line 233)
* copying files:                         Changing Files.      (line   6)
* copying lists:                         Building Lists.      (line  62)
* copying sequences:                     Sequence Functions.  (line  64)
* copying strings:                       Creating Strings.    (line  99)
* copying vectors:                       Vector Functions.    (line  33)
* copysign:                              Float Basics.        (line  62)
* cos:                                   Math Functions.      (line  10)
* count-lines:                           Text Lines.          (line  75)
* count-loop:                            A Sample Function Description.
                                                              (line  66)
* count-screen-lines:                    Screen Lines.        (line  58)
* count-words:                           Text Lines.          (line  83)
* counting columns:                      Columns.             (line   6)
* coverage testing:                      Test Coverage.       (line   6)
* coverage testing (Edebug):             Coverage Testing.    (line   6)
* create subprocess:                     Subprocess Creation. (line   6)
* create-file-buffer:                    Subroutines of Visiting.
                                                              (line  10)
* create-fontset-from-fontset-spec:      Fontsets.            (line  13)
* create-image:                          Defining Images.     (line   9)
* create-lockfiles:                      File Locks.          (line  46)
* creating buffers:                      Creating Buffers.    (line   6)
* creating hash tables:                  Creating Hash.       (line   6)
* creating keymaps:                      Creating Keymaps.    (line   6)
* creating markers:                      Creating Markers.    (line   6)
* creating strings:                      Creating Strings.    (line   6)
* creating, copying and deleting directories: Create/Delete Dirs.
                                                              (line   6)
* cryptographic hash:                    Checksum/Hash.       (line   6)
* ctl-arrow:                             Usual Display.       (line  65)
* ctl-x-4-map:                           Prefix Keys.         (line  34)
* ctl-x-5-map:                           Prefix Keys.         (line  36)
* ctl-x-map:                             Prefix Keys.         (line  27)
* ctl-x-r-map:                           Standard Keymaps.    (line  40)
* curly quotes:                          Formatting Strings.  (line  27)
* curly quotes <1>:                      Keys in Documentation.
                                                              (line  49)
* curly quotes <2>:                      Documentation Tips.  (line  95)
* current binding:                       Local Variables.     (line  29)
* current buffer:                        Current Buffer.      (line   6)
* current buffer mark:                   The Mark.            (line  53)
* current buffer point and mark (Edebug): Edebug Display Update.
                                                              (line  22)
* current buffer position:               Point.               (line  32)
* current command:                       Command Loop Info.   (line  34)
* current stack frame:                   Using Debugger.      (line  36)
* current-active-maps:                   Active Keymaps.      (line  58)
* current-bidi-paragraph-direction:      Bidirectional Display.
                                                              (line 102)
* current-buffer:                        Current Buffer.      (line  17)
* current-case-table:                    Case Tables.         (line  69)
* current-column:                        Columns.             (line  22)
* current-fill-column:                   Margins.             (line  50)
* current-frame-configuration:           Frame Configurations.
                                                              (line  10)
* current-global-map:                    Controlling Active Maps.
                                                              (line  16)
* current-idle-time:                     Idle Timers.         (line  61)
* current-indentation:                   Primitive Indent.    (line  10)
* current-input-method:                  Input Methods.       (line  17)
* current-input-mode:                    Input Modes.         (line  34)
* current-justification:                 Filling.             (line 123)
* current-kill:                          Low-Level Kill Ring. (line  11)
* current-left-margin:                   Margins.             (line  43)
* current-local-map:                     Controlling Active Maps.
                                                              (line  27)
* current-message:                       Displaying Messages. (line 107)
* current-minor-mode-maps:               Controlling Active Maps.
                                                              (line  47)
* current-prefix-arg:                    Prefix Command Arguments.
                                                              (line  85)
* current-time:                          Time of Day.         (line  45)
* current-time-string:                   Time of Day.         (line  27)
* current-time-zone:                     Time Zone Rules.     (line  28)
* current-window-configuration:          Window Configurations.
                                                              (line  19)
* current-word:                          Buffer Contents.     (line 115)
* currying:                              Calling Functions.   (line  82)
* cursor:                                Window Point.        (line  25)
* cursor (text property):                Special Properties.  (line 190)
* cursor position for display properties and overlays: Special Properties.
                                                              (line 215)
* cursor, and frame parameters:          Cursor Parameters.   (line   6)
* cursor, fringe:                        Fringe Cursors.      (line   6)
* cursor-color, a frame parameter:       Font and Color Parameters.
                                                              (line  94)
* cursor-in-echo-area:                   Echo Area Customization.
                                                              (line   8)
* cursor-in-non-selected-windows:        Cursor Parameters.   (line  36)
* cursor-intangible (text property):     Special Properties.  (line 179)
* cursor-intangible-mode:                Special Properties.  (line 179)
* cursor-sensor-functions (text property): Special Properties.
                                                              (line 333)
* cursor-sensor-mode:                    Special Properties.  (line 333)
* cursor-type:                           Cursor Parameters.   (line  26)
* cursor-type <1>:                       Cursor Parameters.   (line  29)
* cursor-type, a frame parameter:        Cursor Parameters.   (line   8)
* curved quotes:                         Formatting Strings.  (line  27)
* curved quotes <1>:                     Keys in Documentation.
                                                              (line  49)
* curved quotes <2>:                     Documentation Tips.  (line  95)
* cust-print:                            Printing in Edebug.  (line   6)
* custom themes:                         Custom Themes.       (line   6)
* custom-add-frequent-value:             Variable Definitions.
                                                              (line 183)
* custom-initialize-delay:               Building Emacs.      (line  95)
* custom-known-themes:                   Custom Themes.       (line  72)
* custom-reevaluate-setting:             Variable Definitions.
                                                              (line 198)
* custom-set-faces:                      Applying Customizations.
                                                              (line  36)
* custom-set-variables:                  Applying Customizations.
                                                              (line  13)
* custom-theme-p:                        Custom Themes.       (line  66)
* custom-theme-set-faces:                Custom Themes.       (line  47)
* custom-theme-set-variables:            Custom Themes.       (line  37)
* custom-unlispify-remove-prefixes:      Group Definitions.   (line  51)
* custom-variable-p:                     Variable Definitions.
                                                              (line 211)
* customizable variables, how to define: Variable Definitions.
                                                              (line   6)
* customization groups, defining:        Group Definitions.   (line   6)
* customization item:                    Customization.       (line   6)
* customization keywords:                Common Keywords.     (line   6)
* customization types:                   Customization Types. (line   6)
* customization types, define new:       Defining New Types.  (line   6)
* customize-package-emacs-version-alist: Common Keywords.     (line 128)
* cyclic ordering of windows:            Cyclic Window Ordering.
                                                              (line   6)
* cygwin-convert-file-name-from-windows: File Names.          (line  19)
* cygwin-convert-file-name-to-windows:   File Names.          (line  19)
* data type:                             Lisp Data Types.     (line   6)
* data-directory:                        Help Functions.      (line 121)
* datagrams:                             Datagrams.           (line   6)
* date-leap-year-p:                      Time Calculations.   (line  36)
* date-to-time:                          Time Parsing.        (line   9)
* deactivate-mark:                       The Mark.            (line 147)
* deactivate-mark <1>:                   The Mark.            (line 162)
* deactivate-mark-hook:                  The Mark.            (line 175)
* debug:                                 Invoking the Debugger.
                                                              (line   9)
* debug-ignored-errors:                  Error Debugging.     (line  39)
* debug-on-entry:                        Function Debugging.  (line  13)
* debug-on-error:                        Error Debugging.     (line  17)
* debug-on-error use:                    Processing of Errors.
                                                              (line  31)
* debug-on-event:                        Error Debugging.     (line  85)
* debug-on-message:                      Error Debugging.     (line  93)
* debug-on-next-call:                    Internals of Debugger.
                                                              (line  61)
* debug-on-quit:                         Infinite Loops.      (line  21)
* debug-on-signal:                       Error Debugging.     (line  63)
* debugger:                              Internals of Debugger.
                                                              (line   9)
* debugger command list:                 Debugger Commands.   (line   6)
* debugger for Emacs Lisp:               Debugger.            (line   6)
* debugger, explicit entry:              Explicit Debug.      (line   6)
* debugger-bury-or-kill:                 Using Debugger.      (line  13)
* debugging errors:                      Error Debugging.     (line   6)
* debugging invalid Lisp syntax:         Syntax Errors.       (line   6)
* debugging lisp programs:               Debugging.           (line   6)
* debugging specific functions:          Function Debugging.  (line   6)
* declare:                               Declare Form.        (line   6)
* declare <1>:                           Declare Form.        (line  10)
* declare-function:                      Declaring Functions. (line   6)
* declare-function <1>:                  Declaring Functions. (line  39)
* declaring functions:                   Declaring Functions. (line   6)
* decode process output:                 Decoding Output.     (line   6)
* decode-char:                           Character Sets.      (line  70)
* decode-coding-inserted-region:         Explicit Encoding.   (line 112)
* decode-coding-region:                  Explicit Encoding.   (line  69)
* decode-coding-string:                  Explicit Encoding.   (line  90)
* decode-time:                           Time Conversion.     (line  22)
* decoding file formats:                 Format Conversion.   (line   6)
* decoding in coding systems:            Explicit Encoding.   (line   6)
* decrement field of register:           Cons Cell Type.      (line   6)
* dedicated window:                      Dedicated Windows.   (line   6)
* def-edebug-spec:                       Instrumenting Macro Calls.
                                                              (line  42)
* defalias:                              Defining Functions.  (line  53)
* defalias-fset-function property:       Defining Functions.  (line  61)
* default argument string:               Interactive Codes.   (line  20)
* default character height:              Frame Font.          (line   6)
* default character size:                Frame Font.          (line   6)
* default character width:               Frame Font.          (line   6)
* default coding system:                 Default Coding Systems.
                                                              (line   6)
* default coding system, functions to determine: Default Coding Systems.
                                                              (line  97)
* default filter function of a process:  Filter Functions.    (line   6)
* default font:                          Frame Font.          (line   6)
* default height of character:           Frame Font.          (line   6)
* default init file:                     Init File.           (line  21)
* default key binding:                   Format of Keymaps.   (line  32)
* default sentinel function of a process: Sentinels.          (line  14)
* default value:                         Default Value.       (line   6)
* default value of char-table:           Char-Tables.         (line  34)
* default width of character:            Frame Font.          (line   6)
* default-boundp:                        Default Value.       (line  27)
* default-directory:                     File Name Expansion. (line  70)
* default-file-modes:                    Changing Files.      (line 190)
* default-font-height:                   Low-Level Font.      (line 288)
* default-font-width:                    Low-Level Font.      (line 284)
* default-frame-alist:                   Initial Parameters.  (line  47)
* default-input-method:                  Input Methods.       (line  23)
* default-justification:                 Filling.             (line 117)
* default-minibuffer-frame:              Minibuffers and Frames.
                                                              (line  26)
* default-process-coding-system:         Default Coding Systems.
                                                              (line  88)
* default-text-properties:               Examining Properties.
                                                              (line  69)
* default-toplevel-value:                Default Value.       (line  92)
* default-value:                         Default Value.       (line  21)
* default.el:                            Startup Summary.     (line  68)
* defconst:                              Defining Variables.  (line  76)
* defcustom:                             Variable Definitions.
                                                              (line  14)
* defface:                               Defining Faces.      (line  18)
* defgroup:                              Group Definitions.   (line  22)
* defimage:                              Defining Images.     (line  29)
* define customization group:            Group Definitions.   (line   6)
* define customization options:          Variable Definitions.
                                                              (line   6)
* define hash comparisons:               Defining Hash.       (line   6)
* define image:                          Defining Images.     (line   6)
* define new customization types:        Defining New Types.  (line   6)
* define-abbrev:                         Defining Abbrevs.    (line  15)
* define-abbrev-table:                   Abbrev Tables.       (line  27)
* define-advice:                         Advising Named Functions.
                                                              (line  49)
* define-alternatives:                   Generic Commands.    (line  10)
* define-button-type:                    Button Types.        (line  11)
* define-category:                       Categories.          (line  33)
* define-derived-mode:                   Derived Modes.       (line  13)
* define-error:                          Error Symbols.       (line   6)
* define-error <1>:                      Error Symbols.       (line  20)
* define-fringe-bitmap:                  Customizing Bitmaps. (line   6)
* define-generic-mode:                   Generic Modes.       (line  11)
* define-globalized-minor-mode:          Defining Minor Modes.
                                                              (line 156)
* define-hash-table-test:                Defining Hash.       (line  21)
* define-inline:                         Defining Functions.  (line  93)
* define-key:                            Changing Key Bindings.
                                                              (line  45)
* define-key-after:                      Modifying Menus.     (line  11)
* define-minor-mode:                     Defining Minor Modes.
                                                              (line   9)
* define-obsolete-face-alias:            Face Functions.      (line  35)
* define-obsolete-function-alias:        Obsolete Functions.  (line  39)
* define-obsolete-variable-alias:        Variable Aliases.    (line  52)
* define-package:                        Multi-file Packages. (line  27)
* define-prefix-command:                 Prefix Keys.         (line  87)
* defined-colors:                        Color Names.         (line  41)
* defining a function:                   Defining Functions.  (line   6)
* defining abbrevs:                      Defining Abbrevs.    (line   6)
* defining commands:                     Defining Commands.   (line   6)
* defining customization variables in C: Writing Emacs Primitives.
                                                              (line 168)
* defining faces:                        Defining Faces.      (line   6)
* defining Lisp variables in C:          Writing Emacs Primitives.
                                                              (line 159)
* defining macros:                       Defining Macros.     (line   6)
* defining menus:                        Defining Menus.      (line   6)
* defining tokens, SMIE:                 SMIE Lexer.          (line   6)
* defining-kbd-macro:                    Keyboard Macros.     (line  44)
* definitions of symbols:                Definitions.         (line   6)
* defmacro:                              Defining Macros.     (line  16)
* defsubr, Lisp symbol for a primitive:  Writing Emacs Primitives.
                                                              (line 142)
* defsubst:                              Inline Functions.    (line  12)
* deftheme:                              Custom Themes.       (line  17)
* defun:                                 Defining Functions.  (line   9)
* DEFUN, C macro to define Lisp primitives: Writing Emacs Primitives.
                                                              (line  37)
* defun-prompt-regexp:                   List Motion.         (line  74)
* defvar:                                Defining Variables.  (line  27)
* defvar-local:                          Creating Buffer-Local.
                                                              (line  84)
* defvaralias:                           Variable Aliases.    (line  14)
* DEFVAR_INT, DEFVAR_LISP, DEFVAR_BOOL:  Writing Emacs Primitives.
                                                              (line 159)
* delay-mode-hooks:                      Mode Hooks.          (line  39)
* delayed warnings:                      Delayed Warnings.    (line   6)
* delayed-warnings-hook:                 Delayed Warnings.    (line  26)
* delayed-warnings-hook <1>:             Standard Hooks.      (line  93)
* delayed-warnings-list:                 Delayed Warnings.    (line  10)
* delete:                                Sets And Lists.      (line 125)
* delete-and-extract-region:             Deletion.            (line  33)
* delete-auto-save-file-if-necessary:    Auto-Saving.         (line 152)
* delete-auto-save-files:                Auto-Saving.         (line 161)
* delete-backward-char:                  Deletion.            (line  55)
* delete-blank-lines:                    User-Level Deletion. (line 101)
* delete-by-moving-to-trash:             Changing Files.      (line 120)
* delete-by-moving-to-trash <1>:         Create/Delete Dirs.  (line  41)
* delete-char:                           Deletion.            (line  42)
* delete-directory:                      Create/Delete Dirs.  (line  41)
* delete-dups:                           Sets And Lists.      (line 179)
* delete-exited-processes:               Deleting Processes.  (line  21)
* delete-field:                          Fields.              (line  65)
* delete-file:                           Changing Files.      (line 120)
* delete-frame:                          Deleting Frames.     (line  11)
* delete-frame event:                    Misc Events.         (line   8)
* delete-frame-functions:                Deleting Frames.     (line  12)
* delete-horizontal-space:               User-Level Deletion. (line   9)
* delete-indentation:                    User-Level Deletion. (line  37)
* delete-minibuffer-contents:            Minibuffer Contents. (line  32)
* delete-old-versions:                   Numbered Backups.    (line  44)
* delete-other-windows:                  Deleting Windows.    (line  39)
* delete-overlay:                        Managing Overlays.   (line  50)
* delete-process:                        Deleting Processes.  (line  27)
* delete-region:                         Deletion.            (line  27)
* delete-selection, symbol property:     The Mark.            (line 210)
* delete-selection-helper:               The Mark.            (line 210)
* delete-selection-pre-hook:             The Mark.            (line 210)
* delete-terminal:                       Multiple Terminals.  (line  47)
* delete-terminal-functions:             Multiple Terminals.  (line  62)
* delete-to-left-margin:                 Margins.             (line  65)
* delete-trailing-whitespace:            User-Level Deletion. (line 112)
* delete-window:                         Deleting Windows.    (line  15)
* delete-windows-on:                     Deleting Windows.    (line  55)
* deleting files:                        Changing Files.      (line   6)
* deleting frames:                       Deleting Frames.     (line   6)
* deleting list elements:                Sets And Lists.      (line  31)
* deleting previous char:                Deletion.            (line  56)
* deleting processes:                    Deleting Processes.  (line   6)
* deleting text vs killing:              Deletion.            (line   6)
* deleting whitespace:                   User-Level Deletion. (line  10)
* deleting windows:                      Deleting Windows.    (line   6)
* delq:                                  Sets And Lists.      (line  30)
* dependencies:                          Packaging Basics.    (line   6)
* derived mode:                          Derived Modes.       (line   6)
* derived-mode-p:                        Derived Modes.       (line 105)
* describe characters and events:        Describing Characters.
                                                              (line   6)
* describe-bindings:                     Scanning Keymaps.    (line 123)
* describe-buffer-case-table:            Case Tables.         (line 111)
* describe-categories:                   Categories.          (line 139)
* describe-current-display-table:        Display Tables.      (line  89)
* describe-display-table:                Display Tables.      (line  85)
* describe-mode:                         Mode Help.           (line  11)
* describe-prefix-bindings:              Help Functions.      (line  97)
* describe-syntax:                       Syntax Table Functions.
                                                              (line  99)
* description for interactive codes:     Interactive Codes.   (line   6)
* description format:                    Format of Descriptions.
                                                              (line   6)
* deserializing:                         Byte Packing.        (line  13)
* desktop notifications:                 Desktop Notifications.
                                                              (line   6)
* desktop save mode:                     Desktop Save Mode.   (line   6)
* desktop-buffer-mode-handlers:          Desktop Save Mode.   (line  31)
* desktop-save-buffer:                   Desktop Save Mode.   (line  16)
* destroy-fringe-bitmap:                 Customizing Bitmaps. (line  31)
* destructive list operations:           Modifying Lists.     (line   6)
* detect-coding-region:                  Lisp and Coding Systems.
                                                              (line  98)
* detect-coding-string:                  Lisp and Coding Systems.
                                                              (line 118)
* diagrams, boxed, for lists:            Box Diagrams.        (line   6)
* dialog boxes:                          Dialog Boxes.        (line   6)
* digit-argument:                        Prefix Command Arguments.
                                                              (line 107)
* ding:                                  Beeping.             (line  12)
* dir-locals-class-alist:                Directory Local Variables.
                                                              (line  82)
* dir-locals-directory-cache:            Directory Local Variables.
                                                              (line  86)
* dir-locals-file:                       Directory Local Variables.
                                                              (line  16)
* dir-locals-set-class-variables:        Directory Local Variables.
                                                              (line  47)
* dir-locals-set-directory-class:        Directory Local Variables.
                                                              (line  67)
* directional overrides:                 Bidirectional Display.
                                                              (line 188)
* directory file name:                   Directory Names.     (line   6)
* directory local variables:             Directory Local Variables.
                                                              (line   6)
* directory name:                        Directory Names.     (line   6)
* directory part (of file name):         File Name Components.
                                                              (line   6)
* directory-abbrev-alist:                Directory Names.     (line  78)
* directory-file-name:                   Directory Names.     (line  40)
* directory-files:                       Contents of Directories.
                                                              (line  14)
* directory-files-and-attributes:        Contents of Directories.
                                                              (line  59)
* directory-files-recursively:           Contents of Directories.
                                                              (line  44)
* directory-name-p:                      Directory Names.     (line  34)
* directory-oriented functions:          Contents of Directories.
                                                              (line   6)
* dired-kept-versions:                   Numbered Backups.    (line  50)
* disable-command:                       Disabling Commands.  (line  36)
* disable-point-adjustment:              Adjusting Point.     (line  15)
* disable-theme:                         Custom Themes.       (line  96)
* disabled:                              Disabling Commands.  (line  11)
* disabled command:                      Disabling Commands.  (line   6)
* disabled-command-function:             Disabling Commands.  (line  41)
* disabling multibyte:                   Disabling Multibyte. (line   6)
* disabling undo:                        Maintaining Undo.    (line  27)
* disassemble:                           Disassembly.         (line  20)
* disassembled byte-code:                Disassembly.         (line   6)
* discard-input:                         Event Input Misc.    (line  89)
* discarding input:                      Event Input Misc.    (line  90)
* dispatch of methods for generic function: Generic Functions.
                                                              (line 174)
* display (overlay property):            Overlay Properties.  (line 117)
* display (text property):               Display Property.    (line   6)
* display action:                        Choosing Window.     (line  12)
* display area:                          Frame Layout.        (line  84)
* display feature testing:               Display Feature Testing.
                                                              (line   6)
* display margins:                       Display Margins.     (line   6)
* display message in echo area:          Displaying Messages. (line   6)
* display name on X:                     Multiple Terminals.  (line  87)
* display properties, and bidi reordering of text: Bidirectional Display.
                                                              (line  62)
* display property, and point display:   Adjusting Point.     (line   6)
* display specification:                 Display Property.    (line   6)
* display table:                         Display Tables.      (line   6)
* display, a frame parameter:            Basic Parameters.    (line   9)
* display, abstract:                     Abstract Display.    (line   6)
* display, arbitrary objects:            Abstract Display.    (line   6)
* display-backing-store:                 Display Feature Testing.
                                                              (line 116)
* display-buffer:                        Choosing Window.     (line  29)
* display-buffer-alist:                  Choosing Window.     (line  73)
* display-buffer-at-bottom:              Display Action Functions.
                                                              (line 141)
* display-buffer-base-action:            Choosing Window.     (line  83)
* display-buffer-below-selected:         Display Action Functions.
                                                              (line 123)
* display-buffer-fallback-action:        Choosing Window.     (line  88)
* display-buffer-in-previous-window:     Display Action Functions.
                                                              (line 130)
* display-buffer-no-window:              Display Action Functions.
                                                              (line 155)
* display-buffer-overriding-action:      Choosing Window.     (line  68)
* display-buffer-pop-up-frame:           Display Action Functions.
                                                              (line  47)
* display-buffer-pop-up-window:          Display Action Functions.
                                                              (line  73)
* display-buffer-reuse-window:           Display Action Functions.
                                                              (line  17)
* display-buffer-same-window:            Display Action Functions.
                                                              (line  11)
* display-buffer-use-some-frame:         Display Action Functions.
                                                              (line  55)
* display-buffer-use-some-window:        Display Action Functions.
                                                              (line 149)
* display-color-cells:                   Display Feature Testing.
                                                              (line 144)
* display-color-p:                       Display Feature Testing.
                                                              (line  35)
* display-completion-list:               Completion Commands. (line  67)
* display-delayed-warnings:              Delayed Warnings.    (line  35)
* display-graphic-p:                     Display Feature Testing.
                                                              (line  25)
* display-grayscale-p:                   Display Feature Testing.
                                                              (line  40)
* display-images-p:                      Display Feature Testing.
                                                              (line  69)
* display-message-or-buffer:             Displaying Messages. (line  87)
* display-mm-dimensions-alist:           Display Feature Testing.
                                                              (line 111)
* display-mm-height:                     Display Feature Testing.
                                                              (line  95)
* display-mm-width:                      Display Feature Testing.
                                                              (line 103)
* display-monitor-attributes-list:       Multiple Terminals.  (line 152)
* display-mouse-p:                       Display Feature Testing.
                                                              (line  31)
* display-pixel-height:                  Display Feature Testing.
                                                              (line  79)
* display-pixel-width:                   Display Feature Testing.
                                                              (line  87)
* display-planes:                        Display Feature Testing.
                                                              (line 131)
* display-popup-menus-p:                 Display Feature Testing.
                                                              (line  19)
* display-save-under:                    Display Feature Testing.
                                                              (line 126)
* display-screens:                       Display Feature Testing.
                                                              (line  75)
* display-selections-p:                  Display Feature Testing.
                                                              (line  64)
* display-start position:                Window Start and End.
                                                              (line   6)
* display-supports-face-attributes-p:    Display Feature Testing.
                                                              (line  44)
* display-table-slot:                    Display Tables.      (line  72)
* display-type, a frame parameter:       Basic Parameters.    (line  15)
* display-visual-class:                  Display Feature Testing.
                                                              (line 136)
* display-warning:                       Warning Basics.      (line  39)
* displaying a buffer:                   Switching Buffers.   (line   6)
* displaying faces:                      Displaying Faces.    (line   6)
* displays, multiple:                    Multiple Terminals.  (line   6)
* distinguish interactive calls:         Distinguish Interactive.
                                                              (line   6)
* dnd-protocol-alist:                    Drag and Drop.       (line  20)
* do-auto-save:                          Auto-Saving.         (line 139)
* DOC (documentation) file:              Documentation Basics.
                                                              (line  23)
* doc, customization keyword:            Type Keywords.       (line  88)
* doc-directory:                         Accessing Documentation.
                                                              (line 158)
* Document Object Model:                 Document Object Model.
                                                              (line   6)
* documentation:                         Accessing Documentation.
                                                              (line  35)
* documentation conventions:             Documentation Basics.
                                                              (line   6)
* documentation for major mode:          Mode Help.           (line   6)
* documentation notation:                Evaluation Notation. (line   6)
* documentation of function:             Function Documentation.
                                                              (line   6)
* documentation strings:                 Documentation.       (line   6)
* documentation strings, conventions and tips: Documentation Tips.
                                                              (line   6)
* documentation, keys in:                Keys in Documentation.
                                                              (line   6)
* documentation-property:                Accessing Documentation.
                                                              (line   6)
* dolist:                                Iteration.           (line  51)
* DOM:                                   Document Object Model.
                                                              (line   6)
* dom-node:                              Document Object Model.
                                                              (line  18)
* dotimes:                               Iteration.           (line  63)
* dotimes-with-progress-reporter:        Progress.            (line 100)
* dotted list:                           Cons Cells.          (line  34)
* dotted lists (Edebug):                 Specification List.  (line 142)
* dotted pair notation:                  Dotted Pair Notation.
                                                              (line   6)
* double-click events:                   Repeat Events.       (line   6)
* double-click-fuzz:                     Repeat Events.       (line  70)
* double-click-time:                     Repeat Events.       (line  80)
* double-quote in strings:               Syntax for Strings.  (line   6)
* down-list:                             List Motion.         (line  38)
* downcase:                              Case Conversion.     (line  19)
* downcase-region:                       Case Changes.        (line  35)
* downcase-word:                         Case Changes.        (line  63)
* downcasing in lookup-key:              Key Sequence Input.  (line  73)
* drag and drop:                         Drag and Drop.       (line   6)
* drag event:                            Drag Events.         (line   6)
* drag-n-drop event:                     Misc Events.         (line  41)
* dribble file:                          Recording Input.     (line  20)
* dump-emacs:                            Building Emacs.      (line 106)
* dumping Emacs:                         Building Emacs.      (line  23)
* dynamic binding:                       Variable Scoping.    (line  15)
* dynamic extent:                        Variable Scoping.    (line  15)
* dynamic libraries:                     Dynamic Libraries.   (line   6)
* dynamic loading of documentation:      Docs and Compilation.
                                                              (line   6)
* dynamic loading of functions:          Dynamic Loading.     (line   6)
* dynamic modules:                       Dynamic Modules.     (line   6)
* dynamic scope:                         Variable Scoping.    (line  15)
* dynamic-library-alist:                 Dynamic Libraries.   (line  10)
* eager macro expansion:                 How Programs Do Loading.
                                                              (line  79)
* easy-menu-define:                      Easy Menu.           (line   9)
* easy-mmode-define-minor-mode:          Defining Minor Modes.
                                                              (line 108)
* echo area:                             The Echo Area.       (line   6)
* echo area customization:               Echo Area Customization.
                                                              (line   6)
* echo-area-clear-hook:                  Echo Area Customization.
                                                              (line  17)
* echo-keystrokes:                       Echo Area Customization.
                                                              (line  21)
* edebug:                                Source Breakpoints.  (line   6)
* Edebug debugging facility:             Edebug.              (line   6)
* Edebug execution modes:                Edebug Execution Modes.
                                                              (line   6)
* Edebug specification list:             Specification List.  (line   6)
* edebug-all-defs:                       Edebug Options.      (line  15)
* edebug-all-forms:                      Edebug Options.      (line  24)
* edebug-continue-kbd-macro:             Edebug Options.      (line  80)
* edebug-defun:                          Instrumenting.       (line  26)
* edebug-display-freq-count:             Coverage Testing.    (line  28)
* edebug-eval-macro-args:                Instrumenting Macro Calls.
                                                              (line  74)
* edebug-eval-top-level-form:            Instrumenting.       (line  26)
* edebug-global-break-condition:         Edebug Options.      (line 115)
* edebug-initial-mode:                   Edebug Options.      (line  59)
* edebug-on-error:                       Edebug Options.      (line 103)
* edebug-on-quit:                        Edebug Options.      (line 107)
* edebug-print-circle:                   Printing in Edebug.  (line  37)
* edebug-print-length:                   Printing in Edebug.  (line  15)
* edebug-print-level:                    Printing in Edebug.  (line  19)
* edebug-print-trace-after:              Trace Buffer.        (line  24)
* edebug-print-trace-before:             Trace Buffer.        (line  24)
* edebug-save-displayed-buffer-points:   Edebug Options.      (line  45)
* edebug-save-windows:                   Edebug Options.      (line  33)
* edebug-set-global-break-condition:     Global Break Condition.
                                                              (line  13)
* edebug-set-initial-mode:               Edebug Execution Modes.
                                                              (line  66)
* edebug-setup-hook:                     Edebug Options.      (line   8)
* edebug-sit-for-seconds:                Edebug Execution Modes.
                                                              (line  94)
* edebug-temp-display-freq-count:        Coverage Testing.    (line  23)
* edebug-test-coverage:                  Edebug Options.      (line  76)
* edebug-trace:                          Edebug Options.      (line  69)
* edebug-trace <1>:                      Trace Buffer.        (line  35)
* edebug-tracing:                        Trace Buffer.        (line  28)
* edebug-unwrap-results:                 Edebug Options.      (line  85)
* edge detection, images:                Image Descriptors.   (line  79)
* edit-and-eval-command:                 Object from Minibuffer.
                                                              (line  49)
* editing types:                         Editing Types.       (line   6)
* editor command loop:                   Command Loop.        (line   6)
* eight-bit, a charset:                  Character Sets.      (line  16)
* electric-future-map:                   A Sample Variable Description.
                                                              (line  17)
* element (of list):                     Lists.               (line   6)
* elements of sequences:                 Sequence Functions.  (line  41)
* elp.el:                                Profiling.           (line  34)
* elt:                                   Sequence Functions.  (line  40)
* Emacs event standard notation:         Describing Characters.
                                                              (line  14)
* Emacs process run time:                Processor Run Time.  (line   6)
* emacs, a charset:                      Character Sets.      (line  16)
* emacs-build-time:                      Version Info.        (line  22)
* emacs-init-time:                       Processor Run Time.  (line  33)
* emacs-internal coding system:          Coding System Basics.
                                                              (line  66)
* emacs-lisp-docstring-fill-column:      Documentation Tips.  (line  22)
* emacs-major-version:                   Version Info.        (line  38)
* emacs-minor-version:                   Version Info.        (line  42)
* emacs-pid:                             System Environment.  (line 204)
* emacs-save-session-functions:          Session Management.  (line  19)
* emacs-session-restore:                 Session Management.  (line  28)
* emacs-startup-hook:                    Init File.           (line  60)
* emacs-uptime:                          Processor Run Time.  (line   9)
* emacs-version:                         Version Info.        (line  30)
* emacs-version <1>:                     Version Info.        (line   9)
* emacsclient, getting a backtrace:      Error Debugging.     (line  74)
* EMACSLOADPATH environment variable:    Library Search.      (line  51)
* emacs_module_init:                     Dynamic Modules.     (line  19)
* embedded widgets:                      Xwidgets.            (line   6)
* empty lines, indicating:               Fringe Indicators.   (line  10)
* empty list:                            Box Diagrams.        (line  41)
* empty overlay:                         Managing Overlays.   (line  21)
* empty region:                          The Region.          (line  37)
* emulation-mode-map-alists:             Controlling Active Maps.
                                                              (line 138)
* enable-command:                        Disabling Commands.  (line  31)
* enable-dir-local-variables:            Directory Local Variables.
                                                              (line  92)
* enable-local-eval:                     File Local Variables.
                                                              (line 143)
* enable-local-variables:                File Local Variables.
                                                              (line  24)
* enable-multibyte-characters:           Text Representations.
                                                              (line  53)
* enable-multibyte-characters <1>:       Disabling Multibyte. (line  30)
* enable-recursive-minibuffers:          Recursive Mini.      (line  14)
* enable-theme:                          Custom Themes.       (line  92)
* encapsulation, ewoc:                   Abstract Display.    (line  41)
* encode-char:                           Character Sets.      (line  79)
* encode-coding-region:                  Explicit Encoding.   (line  37)
* encode-coding-string:                  Explicit Encoding.   (line  61)
* encode-time:                           Time Conversion.     (line  60)
* encoding file formats:                 Format Conversion.   (line   6)
* encoding in coding systems:            Explicit Encoding.   (line   6)
* encrypted network connections:         Network.             (line  55)
* end of line in regexp:                 Regexp Special.      (line 154)
* end-of-buffer:                         Buffer End Motion.   (line  31)
* end-of-defun:                          List Motion.         (line  69)
* end-of-defun-function:                 List Motion.         (line  95)
* end-of-file:                           Input Functions.     (line  13)
* end-of-line:                           Text Lines.          (line  35)
* end-of-line conversion:                Coding System Basics.
                                                              (line  37)
* endianness:                            Bindat Spec.         (line  13)
* environment:                           Intro Eval.          (line  40)
* environment variable access:           System Environment.  (line  85)
* environment variables, subprocesses:   Subprocess Creation. (line  67)
* eobp:                                  Near Point.          (line  67)
* EOL conversion:                        Coding System Basics.
                                                              (line  37)
* eol conversion of coding system:       Lisp and Coding Systems.
                                                              (line  47)
* eol type of coding system:             Lisp and Coding Systems.
                                                              (line  25)
* eolp:                                  Near Point.          (line  77)
* epoch:                                 Time of Day.         (line   8)
* eq:                                    Equality Predicates. (line  11)
* eql:                                   Comparison of Numbers.
                                                              (line  49)
* equal:                                 Equality Predicates. (line  63)
* equal-including-properties:            Equality Predicates. (line 123)
* equality:                              Equality Predicates. (line   6)
* erase-buffer:                          Deletion.            (line  13)
* error:                                 Signaling Errors.    (line  25)
* error cleanup:                         Cleanups.            (line  13)
* error debugging:                       Error Debugging.     (line   6)
* error description:                     Handling Errors.     (line 127)
* error display:                         The Echo Area.       (line   6)
* error handler:                         Handling Errors.     (line   6)
* error in debug:                        Invoking the Debugger.
                                                              (line  61)
* error message notation:                Error Messages.      (line   6)
* error name:                            Error Symbols.       (line   6)
* error symbol:                          Error Symbols.       (line   6)
* error-conditions:                      Error Symbols.       (line   6)
* error-message-string:                  Handling Errors.     (line 149)
* errors:                                Errors.              (line   6)
* <ESC>:                                 Functions for Key Lookup.
                                                              (line  84)
* esc-map:                               Prefix Keys.         (line  15)
* ESC-prefix:                            Prefix Keys.         (line  15)
* escape (ASCII character):              Basic Char Syntax.   (line  27)
* escape characters:                     Output Variables.    (line  17)
* escape characters in printing:         Output Functions.    (line   9)
* escape sequence:                       Basic Char Syntax.   (line  45)
* eval:                                  Eval.                (line  26)
* eval during compilation:               Eval During Compile. (line   6)
* eval, and debugging:                   Internals of Debugger.
                                                              (line  62)
* eval-and-compile:                      Eval During Compile. (line   9)
* eval-buffer:                           Eval.                (line  82)
* eval-buffer (Edebug):                  Instrumenting.       (line  19)
* eval-current-buffer:                   Eval.                (line  98)
* eval-current-buffer (Edebug):          Instrumenting.       (line  19)
* eval-defun (Edebug):                   Instrumenting.       (line  10)
* eval-expression (Edebug):              Instrumenting.       (line  52)
* eval-expression-debug-on-error:        Error Debugging.     (line  54)
* eval-expression-print-length:          Output Variables.    (line  80)
* eval-expression-print-level:           Output Variables.    (line  81)
* eval-minibuffer:                       Object from Minibuffer.
                                                              (line  37)
* eval-region:                           Eval.                (line  60)
* eval-region (Edebug):                  Instrumenting.       (line  19)
* eval-when-compile:                     Eval During Compile. (line  35)
* evaluated expression argument:         Interactive Codes.   (line 203)
* evaluation:                            Evaluation.          (line   6)
* evaluation error:                      Local Variables.     (line 113)
* evaluation list group:                 Eval List.           (line  47)
* evaluation notation:                   Evaluation Notation. (line   6)
* evaluation of buffer contents:         Eval.                (line  82)
* evaluation of special forms:           Special Forms.       (line   6)
* evaporate (overlay property):          Overlay Properties.  (line 213)
* even-window-sizes:                     Choosing Window Options.
                                                              (line  64)
* event printing:                        Describing Characters.
                                                              (line  29)
* event translation:                     Event Mod.           (line   6)
* event type:                            Classifying Events.  (line   6)
* event, reading only one:               Reading One Event.   (line   6)
* event-basic-type:                      Classifying Events.  (line  66)
* event-click-count:                     Repeat Events.       (line  63)
* event-convert-list:                    Classifying Events.  (line  92)
* event-end:                             Accessing Mouse.     (line  21)
* event-modifiers:                       Classifying Events.  (line  26)
* event-start:                           Accessing Mouse.     (line  14)
* eventp:                                Input Events.        (line  12)
* events:                                Input Events.        (line   6)
* ewoc:                                  Abstract Display.    (line   6)
* ewoc-buffer:                           Abstract Display Functions.
                                                              (line  29)
* ewoc-collect:                          Abstract Display Functions.
                                                              (line 107)
* ewoc-create:                           Abstract Display Functions.
                                                              (line  10)
* ewoc-data:                             Abstract Display Functions.
                                                              (line  60)
* ewoc-delete:                           Abstract Display Functions.
                                                              (line  99)
* ewoc-enter-after:                      Abstract Display Functions.
                                                              (line  46)
* ewoc-enter-before:                     Abstract Display Functions.
                                                              (line  45)
* ewoc-enter-first:                      Abstract Display Functions.
                                                              (line  40)
* ewoc-enter-last:                       Abstract Display Functions.
                                                              (line  41)
* ewoc-filter:                           Abstract Display Functions.
                                                              (line 102)
* ewoc-get-hf:                           Abstract Display Functions.
                                                              (line  32)
* ewoc-goto-next:                        Abstract Display Functions.
                                                              (line  79)
* ewoc-goto-node:                        Abstract Display Functions.
                                                              (line  86)
* ewoc-goto-prev:                        Abstract Display Functions.
                                                              (line  78)
* ewoc-invalidate:                       Abstract Display Functions.
                                                              (line  95)
* ewoc-locate:                           Abstract Display Functions.
                                                              (line  66)
* ewoc-location:                         Abstract Display Functions.
                                                              (line  75)
* ewoc-map:                              Abstract Display Functions.
                                                              (line 113)
* ewoc-next:                             Abstract Display Functions.
                                                              (line  51)
* ewoc-nth:                              Abstract Display Functions.
                                                              (line  55)
* ewoc-prev:                             Abstract Display Functions.
                                                              (line  50)
* ewoc-refresh:                          Abstract Display Functions.
                                                              (line  89)
* ewoc-set-data:                         Abstract Display Functions.
                                                              (line  63)
* ewoc-set-hf:                           Abstract Display Functions.
                                                              (line  36)
* examining text properties:             Examining Properties.
                                                              (line   6)
* examining the interactive form:        Using Interactive.   (line 129)
* examining windows:                     Buffers and Windows. (line   6)
* examples of using interactive:         Interactive Examples.
                                                              (line   6)
* excess close parentheses:              Excess Close.        (line   6)
* excess open parentheses:               Excess Open.         (line   6)
* excursion:                             Excursions.          (line   6)
* exec-directory:                        Subprocess Creation. (line  72)
* exec-path:                             Subprocess Creation. (line  78)
* exec-suffixes:                         Subprocess Creation. (line  32)
* executable-find:                       Locating Files.      (line  46)
* execute program:                       Subprocess Creation. (line  18)
* execute with prefix argument:          Interactive Call.    (line 104)
* execute-extended-command:              Interactive Call.    (line  98)
* execute-kbd-macro:                     Keyboard Macros.     (line  12)
* executing-kbd-macro:                   Keyboard Macros.     (line  37)
* execution speed:                       Compilation Tips.    (line   6)
* exit:                                  Recursive Editing.   (line  32)
* exit recursive editing:                Recursive Editing.   (line  32)
* exit-minibuffer:                       Minibuffer Commands. (line   8)
* exit-recursive-edit:                   Recursive Editing.   (line  87)
* exiting Emacs:                         Getting Out.         (line   6)
* exp:                                   Math Functions.      (line  31)
* expand-abbrev:                         Abbrev Expansion.    (line  25)
* expand-file-name:                      File Name Expansion. (line  13)
* expanding abbrevs:                     Abbrev Expansion.    (line   6)
* expansion of file names:               File Name Expansion. (line   6)
* expansion of macros:                   Expansion.           (line   6)
* explicit selective display:            Selective Display.   (line   9)
* expression:                            Intro Eval.          (line  12)
* expt:                                  Math Functions.      (line  39)
* extended file attributes:              Extended Attributes. (line   6)
* extended menu item:                    Extended Menu Items. (line   6)
* extended-command-history:              Minibuffer History.  (line 106)
* extent:                                Variable Scoping.    (line  10)
* external border:                       Frame Layout.        (line  47)
* external menu bar:                     Frame Layout.        (line  67)
* external tool bar:                     Frame Layout.        (line  76)
* extra slots of char-table:             Char-Tables.         (line   6)
* extra-keyboard-modifiers:              Event Mod.           (line  11)
* face (button property):                Button Properties.   (line  23)
* face (overlay property):               Overlay Properties.  (line  88)
* face (text property):                  Special Properties.  (line  22)
* face alias:                            Face Functions.      (line  28)
* face attributes:                       Face Attributes.     (line   6)
* face attributes, access and modification: Attribute Functions.
                                                              (line   6)
* face codes of text:                    Special Properties.  (line  22)
* face merging:                          Displaying Faces.    (line   6)
* face name:                             Faces.               (line  17)
* face remapping:                        Face Remapping.      (line   6)
* face spec:                             Defining Faces.      (line   6)
* face-all-attributes:                   Attribute Functions. (line  53)
* face-attribute:                        Attribute Functions. (line   9)
* face-attribute-relative-p:             Attribute Functions. (line  38)
* face-background:                       Attribute Functions. (line 152)
* face-bold-p:                           Attribute Functions. (line 161)
* face-differs-from-default-p:           Face Functions.      (line  24)
* face-documentation:                    Accessing Documentation.
                                                              (line  59)
* face-documentation <1>:                Face Functions.      (line  16)
* face-equal:                            Face Functions.      (line  20)
* face-font:                             Attribute Functions. (line 142)
* face-font-family-alternatives:         Font Selection.      (line  16)
* face-font-registry-alternatives:       Font Selection.      (line  54)
* face-font-rescale-alist:               Font Selection.      (line  81)
* face-font-selection-order:             Font Selection.      (line  27)
* face-foreground:                       Attribute Functions. (line 151)
* face-id:                               Face Functions.      (line  11)
* face-inverse-video-p:                  Attribute Functions. (line 174)
* face-italic-p:                         Attribute Functions. (line 166)
* face-list:                             Face Functions.      (line   8)
* face-name-history:                     Minibuffer History.  (line 115)
* face-remap-add-relative:               Face Remapping.      (line  59)
* face-remap-remove-relative:            Face Remapping.      (line  76)
* face-remap-reset-base:                 Face Remapping.      (line  93)
* face-remap-set-base:                   Face Remapping.      (line  81)
* face-remapping-alist:                  Face Remapping.      (line  10)
* face-spec-set:                         Defining Faces.      (line 144)
* face-stipple:                          Attribute Functions. (line 157)
* face-underline-p:                      Attribute Functions. (line 170)
* facemenu-keymap:                       Prefix Keys.         (line  47)
* facep:                                 Faces.               (line  28)
* faces:                                 Faces.               (line   6)
* faces for font lock:                   Faces for Font Lock. (line   6)
* faces, automatic choice:               Auto Faces.          (line   6)
* false:                                 nil and t.           (line   6)
* fboundp:                               Function Cells.      (line  44)
* fceiling:                              Rounding Operations. (line  16)
* FEATURE-unload-function:               Unloading.           (line  31)
* featurep:                              Named Features.      (line 127)
* features:                              Named Features.      (line 134)
* features <1>:                          Named Features.      (line   6)
* fetch-bytecode:                        Dynamic Loading.     (line  48)
* ffloor:                                Rounding Operations. (line  12)
* field (overlay property):              Overlay Properties.  (line 129)
* field (text property):                 Special Properties.  (line 184)
* field width:                           Formatting Strings.  (line 145)
* field-beginning:                       Fields.              (line  35)
* field-end:                             Fields.              (line  46)
* field-string:                          Fields.              (line  57)
* field-string-no-properties:            Fields.              (line  61)
* fields:                                Fields.              (line   6)
* fifo data structure:                   Rings.               (line  69)
* file accessibility:                    Testing Accessibility.
                                                              (line   6)
* file age:                              File Attributes.     (line  12)
* file attributes:                       File Attributes.     (line   6)
* file classification:                   Kinds of Files.      (line   6)
* file contents, and default coding system: Default Coding Systems.
                                                              (line  17)
* file format conversion:                Format Conversion.   (line   6)
* file handler:                          Magic File Names.    (line  16)
* file hard link:                        Changing Files.      (line  30)
* file local variables:                  File Local Variables.
                                                              (line   6)
* file local variables <1>:              Security Considerations.
                                                              (line  18)
* file locks:                            File Locks.          (line   6)
* file mode specification error:         Auto Major Mode.     (line  33)
* file modes:                            Testing Accessibility.
                                                              (line  90)
* file modes and MS-DOS:                 Testing Accessibility.
                                                              (line 117)
* file modes, setting:                   Changing Files.      (line 140)
* file modification time:                File Attributes.     (line  12)
* file name abbreviations:               Directory Names.     (line  78)
* file name completion subroutines:      File Name Completion.
                                                              (line   6)
* file name of buffer:                   Buffer File Name.    (line   6)
* file name of directory:                Directory Names.     (line   6)
* file name, and default coding system:  Default Coding Systems.
                                                              (line  27)
* file names:                            File Names.          (line   6)
* file names in directory:               Contents of Directories.
                                                              (line   6)
* file names, trailing whitespace:       Information about Files.
                                                              (line  12)
* file notifications:                    File Notifications.  (line   6)
* file open error:                       Subroutines of Visiting.
                                                              (line  39)
* file permissions:                      Testing Accessibility.
                                                              (line  90)
* file permissions, setting:             Changing Files.      (line 140)
* file symbolic links:                   Kinds of Files.      (line  10)
* file with multiple names:              Changing Files.      (line  30)
* file, information about:               Information about Files.
                                                              (line   6)
* file-accessible-directory-p:           Testing Accessibility.
                                                              (line  57)
* file-acl:                              Extended Attributes. (line  20)
* file-already-exists:                   Changing Files.      (line 114)
* file-attributes:                       File Attributes.     (line  34)
* file-chase-links:                      Truenames.           (line  32)
* file-coding-system-alist:              Default Coding Systems.
                                                              (line  27)
* file-directory-p:                      Kinds of Files.      (line  64)
* file-equal-p:                          Truenames.           (line  54)
* file-error:                            How Programs Do Loading.
                                                              (line 102)
* file-executable-p:                     Testing Accessibility.
                                                              (line  36)
* file-exists-p:                         Testing Accessibility.
                                                              (line  16)
* file-expand-wildcards:                 Contents of Directories.
                                                              (line  70)
* file-extended-attributes:              Extended Attributes. (line  48)
* file-in-directory-p:                   Truenames.           (line  60)
* file-local-copy:                       Magic File Names.    (line 159)
* file-local-variables-alist:            File Local Variables.
                                                              (line  70)
* file-locked:                           File Locks.          (line  61)
* file-locked-p:                         File Locks.          (line  23)
* file-modes:                            Testing Accessibility.
                                                              (line  89)
* file-modes-symbolic-to-number:         Changing Files.      (line 213)
* file-name encoding, MS-Windows:        Encoding and I/O.    (line  79)
* file-name-absolute-p:                  Relative File Names. (line  16)
* file-name-all-completions:             File Name Completion.
                                                              (line   9)
* file-name-as-directory:                Directory Names.     (line  25)
* file-name-base:                        File Name Components.
                                                              (line  98)
* file-name-coding-system:               Encoding and I/O.    (line  60)
* file-name-completion:                  File Name Completion.
                                                              (line  32)
* file-name-directory:                   File Name Components.
                                                              (line  21)
* file-name-extension:                   File Name Components.
                                                              (line  60)
* file-name-handler-alist:               Magic File Names.    (line  16)
* file-name-history:                     Minibuffer History.  (line  97)
* file-name-nondirectory:                File Name Components.
                                                              (line  34)
* file-name-sans-extension:              File Name Components.
                                                              (line  77)
* file-name-sans-versions:               File Name Components.
                                                              (line  44)
* file-newer-than-file-p:                File Attributes.     (line  11)
* file-newest-backup:                    Backup Names.        (line  84)
* file-nlinks:                           File Attributes.     (line 154)
* file-notify-add-watch:                 File Notifications.  (line  19)
* file-notify-rm-watch:                  File Notifications.  (line 120)
* file-notify-valid-p:                   File Notifications.  (line 124)
* file-ownership-preserved-p:            Testing Accessibility.
                                                              (line  76)
* file-precious-flag:                    Saving Buffers.      (line 138)
* file-readable-p:                       Testing Accessibility.
                                                              (line  32)
* file-regular-p:                        Kinds of Files.      (line  80)
* file-relative-name:                    Relative File Names. (line  31)
* file-remote-p:                         Magic File Names.    (line 171)
* file-selinux-context:                  Extended Attributes. (line  35)
* file-supersession:                     Modification Time.   (line  82)
* file-symlink-p:                        Kinds of Files.      (line   9)
* file-truename:                         Truenames.           (line  14)
* file-writable-p:                       Testing Accessibility.
                                                              (line  43)
* filepos-to-bufferpos:                  Text Representations.
                                                              (line 106)
* fill-column:                           Margins.             (line  19)
* fill-context-prefix:                   Adaptive Fill.       (line  16)
* fill-forward-paragraph-function:       Filling.             (line 160)
* fill-individual-paragraphs:            Filling.             (line  57)
* fill-individual-varying-indent:        Filling.             (line  80)
* fill-nobreak-predicate:                Margins.             (line  84)
* fill-paragraph:                        Filling.             (line  33)
* fill-paragraph-function:               Filling.             (line 146)
* fill-prefix:                           Margins.             (line   6)
* fill-region:                           Filling.             (line  45)
* fill-region-as-paragraph:              Filling.             (line  84)
* fillarray:                             Array Functions.     (line  55)
* filling text:                          Filling.             (line   6)
* filling, automatic:                    Auto Filling.        (line   6)
* filter function:                       Filter Functions.    (line   6)
* filter multibyte flag, of process:     Decoding Output.     (line  30)
* filter-buffer-substring:               Buffer Contents.     (line  53)
* filter-buffer-substring-function:      Buffer Contents.     (line  76)
* filter-buffer-substring-functions:     Buffer Contents.     (line  88)
* filtering killed text:                 Kill Functions.      (line  14)
* filtering sequences:                   Sequence Functions.  (line 364)
* find file in path:                     Locating Files.      (line   6)
* find library:                          Library Search.      (line   6)
* find-auto-coding:                      Default Coding Systems.
                                                              (line 112)
* find-backup-file-name:                 Backup Names.        (line  64)
* find-buffer-visiting:                  Buffer File Name.    (line  73)
* find-charset-region:                   Scanning Charsets.   (line  18)
* find-charset-string:                   Scanning Charsets.   (line  29)
* find-coding-systems-for-charsets:      Lisp and Coding Systems.
                                                              (line  80)
* find-coding-systems-region:            Lisp and Coding Systems.
                                                              (line  64)
* find-coding-systems-string:            Lisp and Coding Systems.
                                                              (line  73)
* find-file:                             Visiting Functions.  (line  17)
* find-file-hook:                        Visiting Functions.  (line 125)
* find-file-literally:                   Visiting Functions.  (line 145)
* find-file-literally <1>:               Visiting Functions.  (line  36)
* find-file-name-handler:                Magic File Names.    (line 151)
* find-file-noselect:                    Visiting Functions.  (line  55)
* find-file-not-found-functions:         Visiting Functions.  (line 134)
* find-file-other-window:                Visiting Functions.  (line 103)
* find-file-read-only:                   Visiting Functions.  (line 110)
* find-file-wildcards:                   Visiting Functions.  (line 117)
* find-font:                             Low-Level Font.      (line 108)
* find-image:                            Defining Images.     (line  53)
* find-operation-coding-system:          Default Coding Systems.
                                                              (line 141)
* find-word-boundary-function-table:     Word Motion.         (line  54)
* finding files:                         Visiting Files.      (line   6)
* finding windows:                       Cyclic Window Ordering.
                                                              (line 101)
* first-change-hook:                     Change Hooks.        (line  72)
* fit-frame-to-buffer:                   Resizing Windows.    (line 147)
* fit-frame-to-buffer <1>:               Resizing Windows.    (line 158)
* fit-frame-to-buffer-margins:           Resizing Windows.    (line 175)
* fit-frame-to-buffer-sizes:             Resizing Windows.    (line 186)
* fit-window-to-buffer:                  Resizing Windows.    (line 103)
* fit-window-to-buffer-horizontally:     Resizing Windows.    (line 139)
* fixed-size window:                     Window Sizes.        (line 235)
* fixup-whitespace:                      User-Level Deletion. (line  67)
* flags in format specifications:        Formatting Strings.  (line 170)
* float:                                 Numeric Conversions. (line   8)
* float-e:                               Math Functions.      (line  53)
* float-output-format:                   Output Variables.    (line 112)
* float-pi:                              Math Functions.      (line  56)
* float-time:                            Time of Day.         (line  53)
* floating-point functions:              Math Functions.      (line   6)
* floatp:                                Predicates on Numbers.
                                                              (line  13)
* floats-consed:                         Memory Usage.        (line  17)
* floor:                                 Numeric Conversions. (line  35)
* flowcontrol, in serial connections:    Serial Ports.        (line 114)
* flushing input:                        Event Input Misc.    (line  90)
* fmakunbound:                           Function Cells.      (line  49)
* fn in function’s documentation string: Autoload.            (line 159)
* focus event:                           Focus Events.        (line   6)
* focus-follows-mouse:                   Input Focus.         (line 121)
* focus-in-hook:                         Input Focus.         (line 115)
* focus-in-hook <1>:                     Standard Hooks.      (line  96)
* focus-out-hook:                        Input Focus.         (line 118)
* focus-out-hook <1>:                    Standard Hooks.      (line  97)
* follow links:                          Clickable Text.      (line   6)
* follow-link (button property):         Button Properties.   (line  45)
* follow-link (text or overlay property): Clickable Text.     (line  84)
* following-char:                        Near Point.          (line  33)
* font and color, frame parameters:      Font and Color Parameters.
                                                              (line   6)
* font entity:                           Low-Level Font.      (line 100)
* font information for layout:           Low-Level Font.      (line 278)
* font lock faces:                       Faces for Font Lock. (line   6)
* Font Lock mode:                        Font Lock Mode.      (line   6)
* font lookup:                           Font Lookup.         (line   6)
* font object:                           Low-Level Font.      (line  20)
* font property:                         Low-Level Font.      (line   6)
* font registry:                         Low-Level Font.      (line  62)
* font selection:                        Font Selection.      (line   6)
* font spec:                             Low-Level Font.      (line  32)
* font, a frame parameter:               Font and Color Parameters.
                                                              (line  76)
* font-at:                               Low-Level Font.      (line  24)
* font-backend, a frame parameter:       Font and Color Parameters.
                                                              (line   8)
* font-face-attributes:                  Low-Level Font.      (line 144)
* font-family-list:                      Face Attributes.     (line 197)
* font-get:                              Low-Level Font.      (line 135)
* font-info:                             Low-Level Font.      (line 169)
* font-lock-add-keywords:                Customizing Keywords.
                                                              (line  10)
* font-lock-builtin-face:                Faces for Font Lock. (line  50)
* font-lock-comment-delimiter-face:      Faces for Font Lock. (line  40)
* font-lock-comment-face:                Faces for Font Lock. (line  37)
* font-lock-constant-face:               Faces for Font Lock. (line  47)
* font-lock-defaults:                    Font Lock Basics.    (line  54)
* font-lock-doc-face:                    Faces for Font Lock. (line  60)
* font-lock-ensure &optional beg end:    Font Lock Basics.    (line  42)
* font-lock-ensure-function:             Other Font Lock Variables.
                                                              (line  57)
* font-lock-extend-after-change-region-function: Region to Refontify.
                                                              (line  15)
* font-lock-extra-managed-props:         Other Font Lock Variables.
                                                              (line  21)
* font-lock-face (text property):        Special Properties.  (line  51)
* font-lock-flush &optional beg end:     Font Lock Basics.    (line  35)
* font-lock-flush-function:              Other Font Lock Variables.
                                                              (line  51)
* font-lock-fontify-buffer:              Font Lock Basics.    (line  16)
* font-lock-fontify-buffer-function:     Other Font Lock Variables.
                                                              (line  30)
* font-lock-fontify-region beg end &optional loudly: Font Lock Basics.
                                                              (line  25)
* font-lock-fontify-region-function:     Other Font Lock Variables.
                                                              (line  39)
* font-lock-function-name-face:          Faces for Font Lock. (line  27)
* font-lock-keyword-face:                Faces for Font Lock. (line  33)
* font-lock-keywords:                    Search-based Fontification.
                                                              (line  10)
* font-lock-keywords-case-fold-search:   Search-based Fontification.
                                                              (line 204)
* font-lock-keywords-only:               Syntactic Font Lock. (line  20)
* font-lock-mark-block-function:         Other Font Lock Variables.
                                                              (line   9)
* font-lock-multiline:                   Font Lock Multiline. (line  22)
* font-lock-negation-char-face:          Faces for Font Lock. (line  64)
* font-lock-preprocessor-face:           Faces for Font Lock. (line  53)
* font-lock-remove-keywords:             Customizing Keywords.
                                                              (line  42)
* font-lock-string-face:                 Faces for Font Lock. (line  57)
* font-lock-syntactic-face-function:     Syntactic Font Lock. (line  34)
* font-lock-syntax-table:                Syntactic Font Lock. (line  26)
* font-lock-type-face:                   Faces for Font Lock. (line  44)
* font-lock-unfontify-buffer:            Font Lock Basics.    (line  21)
* font-lock-unfontify-buffer-function:   Other Font Lock Variables.
                                                              (line  34)
* font-lock-unfontify-region beg end:    Font Lock Basics.    (line  30)
* font-lock-unfontify-region-function:   Other Font Lock Variables.
                                                              (line  46)
* font-lock-variable-name-face:          Faces for Font Lock. (line  30)
* font-lock-warning-face:                Faces for Font Lock. (line  22)
* font-put:                              Low-Level Font.      (line  97)
* font-spec:                             Low-Level Font.      (line  36)
* font-xlfd-name:                        Low-Level Font.      (line 157)
* fontification-functions:               Auto Faces.          (line  10)
* fontified (text property):             Special Properties.  (line  70)
* fontp:                                 Low-Level Font.      (line  12)
* fontset:                               Fontsets.            (line   6)
* foo:                                   A Sample Function Description.
                                                              (line  24)
* for:                                   Argument Evaluation. (line  11)
* force coding system for operation:     Specifying Coding Systems.
                                                              (line   6)
* force entry to debugger:               Explicit Debug.      (line   6)
* force-mode-line-update:                Mode Line Basics.    (line  22)
* force-window-update:                   Forcing Redisplay.   (line  27)
* forcing redisplay:                     Forcing Redisplay.   (line   6)
* foreground-color, a frame parameter:   Font and Color Parameters.
                                                              (line  82)
* form:                                  Intro Eval.          (line  12)
* format:                                Formatting Strings.  (line  16)
* format definition:                     Format Conversion Round-Trip.
                                                              (line  19)
* format of keymaps:                     Format of Keymaps.   (line   6)
* format specification:                  Formatting Strings.  (line  39)
* format, customization keyword:         Type Keywords.       (line  25)
* format-alist:                          Format Conversion Round-Trip.
                                                              (line  13)
* format-find-file:                      Format Conversion Round-Trip.
                                                              (line 124)
* format-insert-file:                    Format Conversion Round-Trip.
                                                              (line 133)
* format-message:                        Formatting Strings.  (line  26)
* format-mode-line:                      Emulating Mode Line. (line  10)
* format-network-address:                Misc Network.        (line  32)
* format-seconds:                        Time Parsing.        (line 143)
* format-time-string:                    Time Parsing.        (line  13)
* format-write-file:                     Format Conversion Round-Trip.
                                                              (line 111)
* formatting strings:                    Formatting Strings.  (line   6)
* formatting time values:                Time Parsing.        (line   6)
* formfeed:                              Basic Char Syntax.   (line  27)
* forward-button:                        Button Buffer Commands.
                                                              (line  32)
* forward-char:                          Character Motion.    (line  24)
* forward-comment:                       Motion via Parsing.  (line  42)
* forward-line:                          Text Lines.          (line  53)
* forward-list:                          List Motion.         (line  14)
* forward-sexp:                          List Motion.         (line  43)
* forward-to-indentation:                Motion by Indent.    (line  19)
* forward-word:                          Word Motion.         (line  10)
* forward-word-strictly:                 Word Motion.         (line  68)
* frame:                                 Frames.              (line   6)
* frame configuration:                   Frame Configurations.
                                                              (line   6)
* frame creation:                        Creating Frames.     (line   6)
* frame geometry:                        Frame Geometry.      (line   6)
* frame layout:                          Frame Layout.        (line   6)
* frame layout parameters:               Layout Parameters.   (line   6)
* frame parameters:                      Frame Parameters.    (line   6)
* frame parameters for windowed displays: Window Frame Parameters.
                                                              (line   6)
* frame position:                        Frame Geometry.      (line   6)
* frame position <1>:                    Size and Position.   (line   6)
* frame position <2>:                    Position Parameters. (line   6)
* frame size:                            Frame Geometry.      (line   6)
* frame size <1>:                        Size and Position.   (line   6)
* frame title:                           Frame Titles.        (line   6)
* frame visibility:                      Visibility of Frames.
                                                              (line   6)
* frame without a minibuffer:            Minibuffers and Frames.
                                                              (line  10)
* frame, which buffers to display:       Buffer Parameters.   (line   6)
* frame-alpha-lower-limit:               Font and Color Parameters.
                                                              (line  55)
* frame-auto-hide-function:              Quitting Windows.    (line 102)
* frame-char-height:                     Frame Font.          (line  17)
* frame-char-width:                      Frame Font.          (line  18)
* frame-current-scroll-bars:             Scroll Bars.         (line  23)
* frame-edges:                           Frame Layout.        (line 203)
* frame-first-window:                    Windows and Frames.  (line 151)
* frame-geometry:                        Frame Layout.        (line 152)
* frame-height:                          Size and Position.   (line  54)
* frame-inherited-parameters:            Creating Frames.     (line  50)
* frame-inhibit-implied-resize:          Implied Frame Resizing.
                                                              (line  20)
* frame-list:                            Finding All Frames.  (line   6)
* frame-live-p:                          Deleting Frames.     (line  22)
* frame-monitor-attributes:              Multiple Terminals.  (line 219)
* frame-parameter:                       Parameter Access.    (line   8)
* frame-parameters:                      Parameter Access.    (line  14)
* frame-pixel-height:                    Size and Position.   (line  31)
* frame-pixel-width:                     Size and Position.   (line  32)
* frame-pointer-visible-p:               Mouse Position.      (line  64)
* frame-position:                        Size and Position.   (line  13)
* frame-relative coordinate:             Coordinates and Windows.
                                                              (line   6)
* frame-resize-pixelwise:                Size and Position.   (line  67)
* frame-root-window:                     Windows and Frames.  (line  43)
* frame-scroll-bar-height:               Scroll Bars.         (line  34)
* frame-scroll-bar-width:                Scroll Bars.         (line  30)
* frame-selected-window:                 Selecting Windows.   (line  64)
* frame-terminal:                        Frames.              (line  54)
* frame-text-height:                     Size and Position.   (line  38)
* frame-text-width:                      Size and Position.   (line  39)
* frame-title-format:                    Frame Titles.        (line  16)
* frame-visible-p:                       Visibility of Frames.
                                                              (line  17)
* frame-width:                           Size and Position.   (line  55)
* framep:                                Frames.              (line  37)
* frames, scanning all:                  Finding All Frames.  (line   6)
* free list:                             Garbage Collection.  (line  41)
* free variable:                         Using Lexical Binding.
                                                              (line  47)
* frequency counts:                      Coverage Testing.    (line   6)
* frexp:                                 Float Basics.        (line  48)
* fringe bitmaps:                        Fringe Bitmaps.      (line   6)
* fringe bitmaps, customizing:           Customizing Bitmaps. (line   6)
* fringe cursors:                        Fringe Cursors.      (line   6)
* fringe indicators:                     Fringe Indicators.   (line   6)
* fringe-bitmaps-at-pos:                 Fringe Bitmaps.      (line  55)
* fringe-cursor-alist:                   Fringe Cursors.      (line  17)
* fringe-indicator-alist:                Fringe Indicators.   (line  50)
* fringes:                               Fringes.             (line   6)
* fringes, and empty line indication:    Fringe Indicators.   (line  10)
* fringes-outside-margins:               Fringe Size/Pos.     (line   9)
* fround:                                Rounding Operations. (line  24)
* fset:                                  Function Cells.      (line  63)
* ftp-login:                             Cleanups.            (line  57)
* ftruncate:                             Rounding Operations. (line  20)
* full keymap:                           Format of Keymaps.   (line   6)
* full-height window:                    Window Sizes.        (line 127)
* full-width window:                     Window Sizes.        (line 127)
* fullboth frames:                       Size Parameters.     (line  23)
* fullheight frames:                     Size Parameters.     (line  23)
* fullscreen, a frame parameter:         Size Parameters.     (line  23)
* fullscreen-restore, a frame parameter: Size Parameters.     (line  50)
* fullwidth frames:                      Size Parameters.     (line  23)
* funcall:                               Calling Functions.   (line  22)
* funcall, and debugging:                Internals of Debugger.
                                                              (line  62)
* funcall-interactively:                 Interactive Call.    (line  73)
* function:                              Anonymous Functions. (line  35)
* function aliases:                      Defining Functions.  (line  53)
* function call:                         Function Forms.      (line   6)
* function call debugging:               Function Debugging.  (line   6)
* function cell:                         Symbol Components.   (line  16)
* function cell in autoload:             Autoload.            (line  64)
* function declaration:                  Declaring Functions. (line   6)
* function definition:                   Function Names.      (line   6)
* function descriptions:                 A Sample Function Description.
                                                              (line   6)
* function form evaluation:              Function Forms.      (line   6)
* function input stream:                 Input Streams.       (line  26)
* function invocation:                   Calling Functions.   (line   6)
* function keys:                         Function Keys.       (line   6)
* function name:                         Function Names.      (line   6)
* function output stream:                Output Streams.      (line  24)
* function quoting:                      Anonymous Functions. (line  36)
* function safety:                       Function Safety.     (line   6)
* function-documentation property:       Documentation Basics.
                                                              (line  12)
* function-get:                          Symbol Plists.       (line  58)
* function-put:                          Symbol Plists.       (line  68)
* functionals:                           Calling Functions.   (line 107)
* functionp:                             What Is a Function.  (line  91)
* functions in modes:                    Major Mode Conventions.
                                                              (line  51)
* functions, making them interactive:    Defining Commands.   (line   6)
* fundamental-mode:                      Major Modes.         (line  18)
* fundamental-mode-abbrev-table:         Standard Abbrev Tables.
                                                              (line  27)
* gamma correction:                      Font and Color Parameters.
                                                              (line  37)
* gap-position:                          Buffer Gap.          (line  18)
* gap-size:                              Buffer Gap.          (line  22)
* garbage collection:                    Garbage Collection.  (line  25)
* garbage collection protection:         Writing Emacs Primitives.
                                                              (line 121)
* garbage-collect:                       Garbage Collection.  (line  63)
* garbage-collection-messages:           Garbage Collection.  (line 224)
* gc-cons-percentage:                    Garbage Collection.  (line 258)
* gc-cons-threshold:                     Garbage Collection.  (line 234)
* gc-elapsed:                            Garbage Collection.  (line 300)
* gcs-done:                              Garbage Collection.  (line 296)
* generalized variable:                  Generalized Variables.
                                                              (line   6)
* generate-autoload-cookie:              Autoload.            (line 182)
* generate-new-buffer:                   Creating Buffers.    (line  36)
* generate-new-buffer-name:              Buffer Names.        (line  66)
* generated-autoload-file:               Autoload.            (line 188)
* generators:                            Generators.          (line   6)
* generic commands:                      Generic Commands.    (line   6)
* generic functions:                     Generic Functions.   (line   6)
* generic mode:                          Generic Modes.       (line   6)
* geometry specification:                Geometry.            (line  10)
* get:                                   Symbol Plists.       (line   8)
* get, defcustom keyword:                Variable Definitions.
                                                              (line  89)
* get-buffer:                            Buffer Names.        (line  49)
* get-buffer-create:                     Creating Buffers.    (line  16)
* get-buffer-process:                    Process Buffers.     (line  59)
* get-buffer-window:                     Buffers and Windows. (line  53)
* get-buffer-window-list:                Buffers and Windows. (line  74)
* get-buffer-xwidgets:                   Xwidgets.            (line  42)
* get-byte:                              Character Codes.     (line  41)
* get-char-code-property:                Character Properties.
                                                              (line 164)
* get-char-property:                     Examining Properties.
                                                              (line  25)
* get-char-property-and-overlay:         Examining Properties.
                                                              (line  43)
* get-charset-property:                  Character Sets.      (line  59)
* get-device-terminal:                   Multiple Terminals.  (line  37)
* get-file-buffer:                       Buffer File Name.    (line  57)
* get-internal-run-time:                 Processor Run Time.  (line  19)
* get-largest-window:                    Cyclic Window Ordering.
                                                              (line 122)
* get-load-suffixes:                     Load Suffixes.       (line  31)
* get-lru-window:                        Cyclic Window Ordering.
                                                              (line 104)
* get-mru-window:                        Cyclic Window Ordering.
                                                              (line 117)
* get-pos-property:                      Examining Properties.
                                                              (line  37)
* get-process:                           Process Information. (line  27)
* get-register:                          Registers.           (line  57)
* get-text-property:                     Examining Properties.
                                                              (line  16)
* get-unused-category:                   Categories.          (line  71)
* get-window-with-predicate:             Cyclic Window Ordering.
                                                              (line 139)
* getenv:                                System Environment.  (line  84)
* gethash:                               Hash Access.         (line  11)
* GID:                                   User Identification. (line  68)
* global binding:                        Local Variables.     (line   6)
* global break condition:                Global Break Condition.
                                                              (line   6)
* global keymap:                         Active Keymaps.      (line  38)
* global variable:                       Global Variables.    (line   6)
* global-abbrev-table:                   Standard Abbrev Tables.
                                                              (line   9)
* global-buffers-menu-map:               Standard Keymaps.    (line  92)
* global-disable-point-adjustment:       Adjusting Point.     (line  24)
* global-key-binding:                    Functions for Key Lookup.
                                                              (line  60)
* global-map:                            Controlling Active Maps.
                                                              (line   6)
* global-mode-string:                    Mode Line Variables. (line 115)
* global-set-key:                        Key Binding Commands.
                                                              (line  46)
* global-unset-key:                      Key Binding Commands.
                                                              (line  54)
* glyph:                                 Glyphs.              (line   6)
* glyph code:                            Glyphs.              (line   6)
* glyph-char:                            Glyphs.              (line  19)
* glyph-face:                            Glyphs.              (line  22)
* glyph-table:                           Glyphs.              (line  30)
* glyphless characters:                  Glyphless Chars.     (line   6)
* glyphless-char-display:                Glyphless Chars.     (line  13)
* glyphless-char-display-control:        Glyphless Chars.     (line  58)
* goto-char:                             Character Motion.    (line   9)
* goto-map:                              Prefix Keys.         (line  43)
* grammar, SMIE:                         SMIE Grammar.        (line   6)
* graphical display:                     Frames.              (line  21)
* graphical terminal:                    Frames.              (line  21)
* group, customization keyword:          Common Keywords.     (line  23)
* group-gid:                             User Identification. (line  68)
* group-real-gid:                        User Identification. (line  72)
* gui-get-selection:                     Window System Selections.
                                                              (line  35)
* gui-set-selection:                     Window System Selections.
                                                              (line  15)
* gv-define-expander:                    Adding Generalized Variables.
                                                              (line  48)
* gv-define-setter:                      Adding Generalized Variables.
                                                              (line  34)
* gv-define-simple-setter:               Adding Generalized Variables.
                                                              (line   9)
* gv-letplace:                           Adding Generalized Variables.
                                                              (line  48)
* hack-dir-local-variables:              Directory Local Variables.
                                                              (line  29)
* hack-dir-local-variables-non-file-buffer: Directory Local Variables.
                                                              (line  39)
* hack-local-variables:                  File Local Variables.
                                                              (line  46)
* hack-local-variables-hook:             File Local Variables.
                                                              (line  83)
* handle-shift-selection:                The Mark.            (line 181)
* handle-switch-frame:                   Input Focus.         (line  78)
* handling errors:                       Handling Errors.     (line   6)
* hardening:                             Security Considerations.
                                                              (line   6)
* hash code:                             Defining Hash.       (line   6)
* hash notation:                         Printed Representation.
                                                              (line  13)
* hash table access:                     Hash Access.         (line   6)
* hash tables:                           Hash Tables.         (line   6)
* hash, cryptographic:                   Checksum/Hash.       (line   6)
* hash-table-count:                      Other Hash.          (line  15)
* hash-table-p:                          Other Hash.          (line   8)
* hash-table-rehash-size:                Other Hash.          (line  27)
* hash-table-rehash-threshold:           Other Hash.          (line  30)
* hash-table-size:                       Other Hash.          (line  33)
* hash-table-test:                       Other Hash.          (line  18)
* hash-table-weakness:                   Other Hash.          (line  23)
* hashing:                               Creating Symbols.    (line  11)
* header comments:                       Library Headers.     (line   6)
* header line (of a window):             Header Lines.        (line   6)
* header-line prefix key:                Key Sequence Input.  (line  92)
* header-line-format:                    Header Lines.        (line  10)
* height of a line:                      Line Height.         (line   6)
* height of a window:                    Window Sizes.        (line  47)
* height spec:                           Line Height.         (line  38)
* height, a frame parameter:             Size Parameters.     (line  10)
* help for major mode:                   Mode Help.           (line   6)
* help functions:                        Help Functions.      (line   6)
* help-buffer:                           Help Functions.      (line 125)
* help-char:                             Help Functions.      (line  51)
* help-command:                          Help Functions.      (line  43)
* help-echo (overlay property):          Overlay Properties.  (line 123)
* help-echo (text property):             Special Properties.  (line  83)
* help-echo event:                       Misc Events.         (line  55)
* help-echo, customization keyword:      Type Keywords.       (line  98)
* help-event-list:                       Help Functions.      (line  69)
* help-form:                             Help Functions.      (line  74)
* help-index (button property):          Button Properties.   (line  41)
* help-map:                              Help Functions.      (line  39)
* help-setup-xref:                       Help Functions.      (line 141)
* help-window-select:                    Help Functions.      (line 130)
* Helper-describe-bindings:              Help Functions.      (line 108)
* Helper-help:                           Help Functions.      (line 113)
* Helper-help-map:                       Help Functions.      (line 119)
* hex numbers:                           Integer Basics.      (line  24)
* hidden buffers:                        Buffer Names.        (line  12)
* history list:                          Minibuffer History.  (line   6)
* history of commands:                   Command History.     (line   6)
* history-add-new-input:                 Minibuffer History.  (line  70)
* history-delete-duplicates:             Minibuffer History.  (line  84)
* history-length:                        Minibuffer History.  (line  76)
* HOME environment variable:             Subprocess Creation. (line  18)
* hook variables, list of:               Standard Hooks.      (line   6)
* hooks:                                 Hooks.               (line   6)
* hooks for changing a character:        Special Properties.  (line 269)
* hooks for loading:                     Hooks for Loading.   (line   6)
* hooks for motion of point:             Special Properties.  (line 302)
* hooks for text changes:                Change Hooks.        (line   6)
* hooks for window operations:           Window Hooks.        (line   6)
* horizontal combination:                Windows and Frames.  (line  77)
* horizontal position:                   Columns.             (line   6)
* horizontal scrolling:                  Horizontal Scrolling.
                                                              (line   6)
* horizontal-scroll-bar:                 Scroll Bars.         (line 111)
* horizontal-scroll-bar prefix key:      Key Sequence Input.  (line  92)
* horizontal-scroll-bar-mode:            Scroll Bars.         (line 136)
* horizontal-scroll-bars, a frame parameter: Layout Parameters.
                                                              (line  21)
* horizontal-scroll-bars-available-p:    Scroll Bars.         (line  16)
* how to visit files:                    Visiting Functions.  (line   6)
* HTML DOM:                              Document Object Model.
                                                              (line   6)
* hyper characters:                      Other Char Bits.     (line  16)
* hyperlinks in documentation strings:   Documentation Tips.  (line 112)
* icon-left, a frame parameter:          Position Parameters. (line  50)
* icon-name, a frame parameter:          Management Parameters.
                                                              (line  29)
* icon-title-format:                     Frame Titles.        (line  23)
* icon-top, a frame parameter:           Position Parameters. (line  57)
* icon-type, a frame parameter:          Management Parameters.
                                                              (line  23)
* iconified frame:                       Visibility of Frames.
                                                              (line   6)
* iconify-frame:                         Visibility of Frames.
                                                              (line  26)
* iconify-frame event:                   Misc Events.         (line  16)
* identity:                              Calling Functions.   (line 116)
* idle timers:                           Idle Timers.         (line   6)
* idleness:                              Idle Timers.         (line  23)
* IEEE floating point:                   Float Basics.        (line   6)
* if:                                    Conditionals.        (line  11)
* ignore:                                Calling Functions.   (line 119)
* ignore-errors:                         Handling Errors.     (line 198)
* ignore-window-parameters:              Window Parameters.   (line  64)
* ignored-local-variables:               File Local Variables.
                                                              (line 135)
* image animation:                       Multi-Frame Images.  (line   6)
* image cache:                           Image Cache.         (line   6)
* image descriptor:                      Image Descriptors.   (line   6)
* image formats:                         Image Formats.       (line   6)
* image frames:                          Multi-Frame Images.  (line   6)
* image maps:                            Image Descriptors.   (line 129)
* image slice:                           Showing Images.      (line  34)
* image types:                           Image Formats.       (line   6)
* image-animate:                         Multi-Frame Images.  (line  37)
* image-animate-timer:                   Multi-Frame Images.  (line  49)
* image-cache-eviction-delay:            Image Cache.         (line  51)
* image-current-frame:                   Multi-Frame Images.  (line  26)
* image-default-frame-delay:             Multi-Frame Images.  (line  44)
* image-flush:                           Image Cache.         (line  12)
* image-format-suffixes:                 ImageMagick Images.  (line  36)
* image-load-path:                       Defining Images.     (line  69)
* image-load-path-for-library:           Defining Images.     (line  88)
* image-mask-p:                          Image Descriptors.   (line 157)
* image-minimum-frame-delay:             Multi-Frame Images.  (line  44)
* image-multi-frame-p:                   Multi-Frame Images.  (line  17)
* image-show-frame:                      Multi-Frame Images.  (line  30)
* image-size:                            Showing Images.      (line  71)
* image-type-available-p:                Image Formats.       (line  37)
* image-types:                           Image Formats.       (line  28)
* ImageMagick images:                    ImageMagick Images.  (line   6)
* imagemagick-enabled-types:             ImageMagick Images.  (line  21)
* imagemagick-types:                     ImageMagick Images.  (line  15)
* imagemagick-types-inhibit:             ImageMagick Images.  (line  30)
* images in buffers:                     Images.              (line   6)
* images, support for more formats:      ImageMagick Images.  (line   6)
* Imenu:                                 Imenu.               (line   6)
* imenu-add-to-menubar:                  Imenu.               (line  14)
* imenu-case-fold-search:                Imenu.               (line  68)
* imenu-create-index-function:           Imenu.               (line 128)
* imenu-extract-index-name-function:     Imenu.               (line 117)
* imenu-generic-expression:              Imenu.               (line  26)
* imenu-prev-index-position-function:    Imenu.               (line 107)
* imenu-syntax-alist:                    Imenu.               (line  76)
* implicit progn:                        Sequencing.          (line  21)
* implied frame resizing:                Implied Frame Resizing.
                                                              (line   6)
* implied resizing of frame:             Implied Frame Resizing.
                                                              (line   6)
* inactive minibuffer:                   Intro to Minibuffers.
                                                              (line  60)
* inc:                                   Simple Macro.        (line  11)
* indefinite extent:                     Variable Scoping.    (line  21)
* indent-according-to-mode:              Mode-Specific Indent.
                                                              (line  48)
* indent-code-rigidly:                   Region Indent.       (line  58)
* indent-for-tab-command:                Mode-Specific Indent.
                                                              (line  11)
* indent-line-function:                  Mode-Specific Indent.
                                                              (line  40)
* indent-region:                         Region Indent.       (line   9)
* indent-region-function:                Region Indent.       (line  23)
* indent-relative:                       Relative Indent.     (line   9)
* indent-relative-maybe:                 Relative Indent.     (line  50)
* indent-rigidly:                        Region Indent.       (line  44)
* indent-tabs-mode:                      Primitive Indent.    (line  28)
* indent-to:                             Primitive Indent.    (line  16)
* indent-to-left-margin:                 Margins.             (line  72)
* indentation:                           Indentation.         (line   6)
* indentation rules, SMIE:               SMIE Indentation.    (line   6)
* indicate-buffer-boundaries:            Fringe Indicators.   (line  15)
* indicate-empty-lines:                  Fringe Indicators.   (line   9)
* indicators, fringe:                    Fringe Indicators.   (line   6)
* indirect buffers:                      Indirect Buffers.    (line   6)
* indirect specifications:               Specification List.  (line 109)
* indirect-function:                     Function Indirection.
                                                              (line  65)
* indirect-variable:                     Variable Aliases.    (line  61)
* indirection for functions:             Function Indirection.
                                                              (line   6)
* infinite loops:                        Infinite Loops.      (line   6)
* infinite recursion:                    Local Variables.     (line 113)
* infinity:                              Float Basics.        (line  26)
* inheritance, for faces:                Face Attributes.     (line 189)
* inheritance, keymap:                   Inheritance and Keymaps.
                                                              (line   6)
* inheritance, syntax table:             Syntax Basics.       (line  36)
* inheritance, text property:            Sticky Properties.   (line   6)
* inhibit-default-init:                  Init File.           (line  45)
* inhibit-eol-conversion:                Specifying Coding Systems.
                                                              (line  61)
* inhibit-field-text-motion:             Word Motion.         (line  49)
* inhibit-file-name-handlers:            Magic File Names.    (line 144)
* inhibit-file-name-operation:           Magic File Names.    (line 148)
* inhibit-iso-escape-detection:          Lisp and Coding Systems.
                                                              (line 128)
* inhibit-local-variables-regexps:       File Local Variables.
                                                              (line  40)
* inhibit-local-variables-regexps <1>:   Auto Major Mode.     (line  52)
* inhibit-message:                       Displaying Messages. (line  57)
* inhibit-modification-hooks:            Change Hooks.        (line  76)
* inhibit-null-byte-detection:           Lisp and Coding Systems.
                                                              (line 122)
* inhibit-point-motion-hooks:            Special Properties.  (line 347)
* inhibit-quit:                          Quitting.            (line  78)
* inhibit-read-only:                     Read Only Buffers.   (line  34)
* inhibit-read-only (text property):     Special Properties.  (line 149)
* inhibit-splash-screen:                 Startup Summary.     (line 157)
* inhibit-startup-echo-area-message:     Startup Summary.     (line 167)
* inhibit-startup-message:               Startup Summary.     (line 157)
* inhibit-startup-screen:                Startup Summary.     (line 147)
* inhibit-x-resources:                   Resources.           (line  51)
* init file:                             Init File.           (line   6)
* init-file-user:                        User Identification. (line   6)
* init.el:                               Init File.           (line   6)
* initial-buffer-choice:                 Startup Summary.     (line 160)
* initial-environment:                   System Environment.  (line 142)
* initial-frame-alist:                   Initial Parameters.  (line   9)
* initial-major-mode:                    Auto Major Mode.     (line  79)
* initial-scratch-message:               Startup Summary.     (line 184)
* initial-window-system:                 Window Systems.      (line  28)
* initial-window-system, and startup:    Startup Summary.     (line  32)
* initialization of Emacs:               Startup Summary.     (line   6)
* initialize, defcustom keyword:         Variable Definitions.
                                                              (line 102)
* inline completion:                     Completion in Buffers.
                                                              (line   6)
* inline functions:                      Inline Functions.    (line   6)
* inline-const-p:                        Defining Functions.  (line 139)
* inline-const-val:                      Defining Functions.  (line 142)
* inline-error:                          Defining Functions.  (line 145)
* inline-letevals:                       Defining Functions.  (line 128)
* inline-quote:                          Defining Functions.  (line 123)
* inner edges:                           Frame Layout.        (line 118)
* inner frame:                           Frame Layout.        (line 118)
* inner height:                          Frame Layout.        (line 118)
* inner width:                           Frame Layout.        (line 118)
* innermost containing parentheses:      Parser State.        (line  19)
* input events:                          Input Events.        (line   6)
* input focus:                           Input Focus.         (line   6)
* input methods:                         Input Methods.       (line   6)
* input modes:                           Input Modes.         (line   6)
* input stream:                          Input Streams.       (line   6)
* input-decode-map:                      Translation Keymaps. (line  32)
* input-method-alist:                    Input Methods.       (line  43)
* input-method-function:                 Invoking the Input Method.
                                                              (line  12)
* input-pending-p:                       Event Input Misc.    (line  44)
* insert:                                Insertion.           (line  37)
* insert-abbrev-table-description:       Abbrev Tables.       (line  49)
* insert-and-inherit:                    Sticky Properties.   (line  73)
* insert-before-markers:                 Insertion.           (line  43)
* insert-before-markers-and-inherit:     Sticky Properties.   (line  77)
* insert-behind-hooks (overlay property): Overlay Properties. (line 169)
* insert-behind-hooks (text property):   Special Properties.  (line 290)
* insert-buffer:                         Commands for Insertion.
                                                              (line  10)
* insert-buffer-substring:               Insertion.           (line  72)
* insert-buffer-substring-as-yank:       Yanking.             (line  18)
* insert-buffer-substring-no-properties: Insertion.           (line  94)
* insert-button:                         Making Buttons.      (line  35)
* insert-char:                           Insertion.           (line  56)
* insert-default-directory:              Reading File Names.  (line 132)
* insert-directory:                      Contents of Directories.
                                                              (line  82)
* insert-directory-program:              Contents of Directories.
                                                              (line 114)
* insert-file-contents:                  Reading from Files.  (line  10)
* insert-file-contents-literally:        Reading from Files.  (line  52)
* insert-for-yank:                       Yanking.             (line  11)
* insert-image:                          Showing Images.      (line   9)
* insert-in-front-hooks (overlay property): Overlay Properties.
                                                              (line 163)
* insert-in-front-hooks (text property): Special Properties.  (line 290)
* insert-register:                       Registers.           (line  69)
* insert-sliced-image:                   Showing Images.      (line  34)
* insert-text-button:                    Making Buttons.      (line  50)
* inserting killed text:                 Yank Commands.       (line  12)
* insertion before point:                Insertion.           (line   6)
* insertion of text:                     Insertion.           (line   6)
* insertion type of a marker:            Marker Insertion Types.
                                                              (line   6)
* inside comment:                        Parser State.        (line  29)
* inside string:                         Parser State.        (line  25)
* installation-directory:                System Environment.  (line 171)
* instrumenting for Edebug:              Instrumenting.       (line   6)
* int-to-string:                         String Conversion.   (line  31)
* intangible (overlay property):         Overlay Properties.  (line 180)
* intangible (text property):            Special Properties.  (line 157)
* integer to decimal:                    String Conversion.   (line  20)
* integer to hexadecimal:                Formatting Strings.  (line  96)
* integer to octal:                      Formatting Strings.  (line  87)
* integer to string:                     String Conversion.   (line  20)
* integer types (C programming language): C Integer Types.    (line   6)
* integer-or-marker-p:                   Predicates on Markers.
                                                              (line  15)
* integerp:                              Predicates on Numbers.
                                                              (line  17)
* integers:                              Numbers.             (line   6)
* integers in specific radix:            Integer Basics.      (line  24)
* interactive:                           Using Interactive.   (line  10)
* interactive call:                      Interactive Call.    (line   6)
* interactive code description:          Interactive Codes.   (line   6)
* interactive completion:                Interactive Codes.   (line  10)
* interactive function:                  Defining Commands.   (line   6)
* interactive spec, using:               Using Interactive.   (line   6)
* interactive specification in primitives: Writing Emacs Primitives.
                                                              (line  73)
* interactive, examples of using:        Interactive Examples.
                                                              (line   6)
* interactive-form:                      Using Interactive.   (line 129)
* interactive-form property:             Defining Commands.   (line  16)
* interactive-form, symbol property:     Using Interactive.   (line  21)
* interactive-only property:             Defining Commands.   (line  21)
* intern:                                Creating Symbols.    (line  95)
* intern-soft:                           Creating Symbols.    (line 115)
* internal menu bar:                     Frame Layout.        (line  67)
* internal representation of characters: Text Representations.
                                                              (line  20)
* internal tool bar:                     Frame Layout.        (line  76)
* internal windows:                      Basic Windows.       (line  39)
* internal-border-width, a frame parameter: Layout Parameters.
                                                              (line  12)
* internals, of buffer:                  Buffer Internals.    (line   6)
* internals, of process:                 Process Internals.   (line   6)
* internals, of window:                  Window Internals.    (line   6)
* interning:                             Creating Symbols.    (line  23)
* interpreter:                           Evaluation.          (line   6)
* interpreter <1>:                       Evaluation.          (line   6)
* interpreter-mode-alist:                Auto Major Mode.     (line  84)
* interprogram-cut-function:             Low-Level Kill Ring. (line  72)
* interprogram-paste-function:           Low-Level Kill Ring. (line  50)
* interrupt Lisp functions:              Quitting.            (line   6)
* interrupt-process:                     Signals to Processes.
                                                              (line  52)
* intervals:                             Not Intervals.       (line   6)
* intervals-consed:                      Memory Usage.        (line  38)
* invalid prefix key error:              Changing Key Bindings.
                                                              (line  57)
* invalid-function:                      Function Indirection.
                                                              (line  18)
* invalid-read-syntax:                   Printed Representation.
                                                              (line  25)
* invalid-regexp:                        Regexp Backslash.    (line 203)
* invert-face:                           Attribute Functions. (line 127)
* invisible (overlay property):          Overlay Properties.  (line 175)
* invisible (text property):             Special Properties.  (line 153)
* invisible frame:                       Visibility of Frames.
                                                              (line   6)
* invisible text:                        Invisible Text.      (line   6)
* invisible-p:                           Invisible Text.      (line  92)
* invisible/intangible text, and point:  Adjusting Point.     (line   6)
* invocation-directory:                  System Environment.  (line 166)
* invocation-name:                       System Environment.  (line 162)
* invoking input method:                 Invoking the Input Method.
                                                              (line   6)
* invoking lisp debugger:                Invoking the Debugger.
                                                              (line   6)
* is this call interactive:              Distinguish Interactive.
                                                              (line   6)
* isnan:                                 Float Basics.        (line  44)
* italic text:                           Face Attributes.     (line  60)
* iter-close:                            Generators.          (line  65)
* iter-defun:                            Generators.          (line  10)
* iter-do:                               Generators.          (line  75)
* iter-lambda:                           Generators.          (line  23)
* iter-next:                             Generators.          (line  52)
* iter-yield:                            Generators.          (line  27)
* iter-yield-from:                       Generators.          (line  33)
* iteration:                             Iteration.           (line   6)
* jit-lock-register:                     Other Font Lock Variables.
                                                              (line  65)
* jit-lock-unregister:                   Other Font Lock Variables.
                                                              (line  77)
* joining lists:                         Rearrangement.       (line  16)
* jumbled display of bidirectional text: Bidirectional Display.
                                                              (line 131)
* just-one-space:                        User-Level Deletion. (line  97)
* justify-current-line:                  Filling.             (line  99)
* kbd:                                   Key Sequences.       (line  35)
* kbd-macro-termination-hook:            Keyboard Macros.     (line  62)
* kept-new-versions:                     Numbered Backups.    (line  28)
* kept-old-versions:                     Numbered Backups.    (line  33)
* key:                                   Key Sequences.       (line   6)
* key binding:                           Keymap Basics.       (line   6)
* key binding, conventions for:          Key Binding Conventions.
                                                              (line   6)
* key lookup:                            Key Lookup.          (line   6)
* key sequence:                          Key Sequences.       (line   6)
* key sequence error:                    Changing Key Bindings.
                                                              (line  57)
* key sequence input:                    Key Sequence Input.  (line   6)
* key substitution sequence:             Keys in Documentation.
                                                              (line   6)
* key translation function:              Translation Keymaps. (line  78)
* key-binding:                           Active Keymaps.      (line  67)
* key-description:                       Describing Characters.
                                                              (line  13)
* key-translation-map:                   Translation Keymaps. (line  58)
* keyboard events:                       Keyboard Events.     (line   6)
* keyboard events in strings:            Strings of Events.   (line   6)
* keyboard events, data in:              Accessing Mouse.     (line   6)
* keyboard input:                        Reading Input.       (line   6)
* keyboard input decoding on X:          Locales.             (line  11)
* keyboard macro execution:              Interactive Call.    (line  80)
* keyboard macro termination:            Beeping.             (line  13)
* keyboard macro, terminating:           Event Input Misc.    (line  90)
* keyboard macros:                       Keyboard Macros.     (line   6)
* keyboard macros (Edebug):              Edebug Execution Modes.
                                                              (line  87)
* keyboard-coding-system:                Terminal I/O Encoding.
                                                              (line  11)
* keyboard-quit:                         Quitting.            (line 104)
* keyboard-translate:                    Event Mod.           (line  54)
* keyboard-translate-table:              Event Mod.           (line  32)
* keymap:                                Keymaps.             (line   6)
* keymap (button property):              Button Properties.   (line  32)
* keymap (overlay property):             Overlay Properties.  (line 222)
* keymap (text property):                Special Properties.  (line 110)
* keymap entry:                          Key Lookup.          (line   6)
* keymap format:                         Format of Keymaps.   (line   6)
* keymap in keymap:                      Key Lookup.          (line  51)
* keymap inheritance:                    Inheritance and Keymaps.
                                                              (line   6)
* keymap inheritance from multiple maps: Inheritance and Keymaps.
                                                              (line  53)
* keymap of character:                   Special Properties.  (line 110)
* keymap of character (and overlays):    Overlay Properties.  (line 222)
* keymap prompt string:                  Format of Keymaps.   (line  57)
* keymap-parent:                         Inheritance and Keymaps.
                                                              (line  27)
* keymap-prompt:                         Defining Menus.      (line  20)
* keymapp:                               Format of Keymaps.   (line 106)
* keymaps for translating events:        Translation Keymaps. (line   6)
* keymaps in modes:                      Major Mode Conventions.
                                                              (line  63)
* keymaps, scanning:                     Scanning Keymaps.    (line   6)
* keymaps, standard:                     Standard Keymaps.    (line   6)
* keys in documentation strings:         Keys in Documentation.
                                                              (line   6)
* keys, reserved:                        Key Binding Conventions.
                                                              (line  14)
* keystroke:                             Key Sequences.       (line   6)
* keyword symbol:                        Constant Variables.  (line   6)
* keywordp:                              Constant Variables.  (line  19)
* kill command repetition:               Command Loop Info.   (line  43)
* kill ring:                             The Kill Ring.       (line   6)
* kill-all-local-variables:              Creating Buffer-Local.
                                                              (line 155)
* kill-append:                           Low-Level Kill Ring. (line  43)
* kill-buffer:                           Killing Buffers.     (line  31)
* kill-buffer-hook:                      Killing Buffers.     (line  73)
* kill-buffer-query-functions:           Killing Buffers.     (line  65)
* kill-emacs:                            Killing Emacs.       (line  10)
* kill-emacs-hook:                       Killing Emacs.       (line  29)
* kill-emacs-query-functions:            Killing Emacs.       (line  46)
* kill-local-variable:                   Creating Buffer-Local.
                                                              (line 135)
* kill-new:                              Low-Level Kill Ring. (line  33)
* kill-process:                          Signals to Processes.
                                                              (line  60)
* kill-read-only-ok:                     Kill Functions.      (line  38)
* kill-region:                           Kill Functions.      (line  21)
* kill-ring:                             Internals of Kill Ring.
                                                              (line  50)
* kill-ring-max:                         Internals of Kill Ring.
                                                              (line  60)
* kill-ring-yank-pointer:                Internals of Kill Ring.
                                                              (line  54)
* killing buffers:                       Killing Buffers.     (line   6)
* killing Emacs:                         Killing Emacs.       (line   6)
* kmacro-keymap:                         Standard Keymaps.    (line  78)
* lambda:                                Anonymous Functions. (line  20)
* lambda expression:                     Lambda Expressions.  (line   6)
* lambda in debug:                       Invoking the Debugger.
                                                              (line  32)
* lambda in keymap:                      Key Lookup.          (line  60)
* lambda list:                           Lambda Components.   (line  13)
* lambda-list (Edebug):                  Specification List.  (line 177)
* language-change event:                 Misc Events.         (line  90)
* largest Lisp integer:                  Integer Basics.      (line  79)
* largest window:                        Cyclic Window Ordering.
                                                              (line 122)
* last:                                  List Elements.       (line 115)
* last-abbrev:                           Abbrev Expansion.    (line  79)
* last-abbrev-location:                  Abbrev Expansion.    (line  85)
* last-abbrev-text:                      Abbrev Expansion.    (line  90)
* last-buffer:                           Buffer List.         (line  92)
* last-coding-system-used:               Encoding and I/O.    (line  47)
* last-command:                          Command Loop Info.   (line  11)
* last-command-event:                    Command Loop Info.   (line 110)
* last-event-frame:                      Command Loop Info.   (line 122)
* last-input-event:                      Event Input Misc.    (line  54)
* last-kbd-macro:                        Keyboard Macros.     (line  55)
* last-nonmenu-event:                    Command Loop Info.   (line 102)
* last-prefix-arg:                       Prefix Command Arguments.
                                                              (line  96)
* last-repeatable-command:               Command Loop Info.   (line  28)
* lax-plist-get:                         Plist Access.        (line  37)
* lax-plist-put:                         Plist Access.        (line  41)
* layout of frame:                       Frame Layout.        (line   6)
* layout on display, and bidirectional text: Bidirectional Display.
                                                              (line 131)
* layout parameters of frames:           Layout Parameters.   (line   6)
* lazy loading:                          Dynamic Loading.     (line   6)
* lazy-completion-table:                 Basic Completion.    (line 174)
* ldexp:                                 Float Basics.        (line  58)
* least recently used window:            Cyclic Window Ordering.
                                                              (line 104)
* left, a frame parameter:               Position Parameters. (line   9)
* left-fringe, a frame parameter:        Layout Parameters.   (line  33)
* left-fringe-width:                     Fringe Size/Pos.     (line  14)
* left-margin:                           Margins.             (line  79)
* left-margin-width:                     Display Margins.     (line  31)
* length:                                Sequence Functions.  (line  12)
* let:                                   Local Variables.     (line  47)
* let*:                                  Local Variables.     (line  82)
* lexical binding:                       Variable Scoping.    (line  21)
* lexical binding (Edebug):              Edebug Eval.         (line  26)
* lexical comparison of strings:         Text Comparison.     (line  90)
* lexical environment:                   Lexical Binding.     (line  35)
* lexical scope:                         Variable Scoping.    (line  21)
* lexical-binding:                       Using Lexical Binding.
                                                              (line  10)
* library:                               Loading.             (line   6)
* library compilation:                   Compilation Functions.
                                                              (line 106)
* library header comments:               Library Headers.     (line   6)
* library search:                        Library Search.      (line   6)
* libxml-parse-html-region:              Parsing HTML/XML.    (line   9)
* libxml-parse-xml-region:               Parsing HTML/XML.    (line  48)
* line end conversion:                   Coding System Basics.
                                                              (line  37)
* line height:                           Frame Font.          (line  12)
* line height <1>:                       Line Height.         (line   6)
* line number:                           Text Lines.          (line  92)
* line truncation:                       Truncation.          (line   6)
* line wrapping:                         Truncation.          (line   6)
* line-beginning-position:               Text Lines.          (line  32)
* line-end-position:                     Text Lines.          (line  50)
* line-height (text property):           Special Properties.  (line 237)
* line-height (text property) <1>:       Line Height.         (line  22)
* line-move-ignore-invisible:            Invisible Text.      (line 102)
* line-number-at-pos:                    Text Lines.          (line  91)
* line-pixel-height:                     Size of Displayed Text.
                                                              (line 102)
* line-prefix:                           Truncation.          (line  58)
* line-spacing:                          Line Height.         (line  75)
* line-spacing (text property):          Special Properties.  (line 231)
* line-spacing (text property) <1>:      Line Height.         (line  81)
* line-spacing, a frame parameter:       Layout Parameters.   (line  69)
* lines:                                 Text Lines.          (line   6)
* lines in region:                       Text Lines.          (line  76)
* link, customization keyword:           Common Keywords.     (line  37)
* linked list:                           Cons Cell Type.      (line  17)
* linking files:                         Changing Files.      (line   6)
* Lisp debugger:                         Debugger.            (line   6)
* Lisp expression motion:                List Motion.         (line   6)
* Lisp history:                          Lisp History.        (line   6)
* Lisp library:                          Loading.             (line   6)
* Lisp nesting error:                    Eval.                (line 114)
* Lisp object:                           Lisp Data Types.     (line   6)
* Lisp objects, stack-allocated:         Stack-allocated Objects.
                                                              (line   6)
* Lisp package:                          Packaging.           (line   6)
* Lisp printer:                          Output Functions.    (line  40)
* Lisp reader:                           Streams Intro.       (line   6)
* lisp variables defined in C, restrictions: Variables with Restricted Values.
                                                              (line   6)
* lisp-indent-function property:         Indenting Macros.    (line  12)
* lisp-mode-abbrev-table:                Standard Abbrev Tables.
                                                              (line  35)
* lisp-mode.el:                          Example Major Modes. (line  46)
* list:                                  Building Lists.      (line  35)
* list all coding systems:               Lisp and Coding Systems.
                                                              (line   8)
* list elements:                         List Elements.       (line   6)
* list form evaluation:                  Classifying Lists.   (line   6)
* list in keymap:                        Key Lookup.          (line  55)
* list length:                           Sequence Functions.  (line  13)
* list modification:                     List Variables.      (line   6)
* list motion:                           List Motion.         (line   6)
* list predicates:                       List-related Predicates.
                                                              (line   6)
* list reverse:                          Sequence Functions.  (line 112)
* list structure:                        Cons Cell Type.      (line  11)
* list structure <1>:                    Cons Cells.          (line  48)
* list, replace element:                 Setcar.              (line   6)
* list-buffers-directory:                Buffer File Name.    (line 121)
* list-charset-chars:                    Character Sets.      (line  62)
* list-fonts:                            Low-Level Font.      (line 113)
* list-load-path-shadows:                Library Search.      (line 111)
* list-processes:                        Process Information. (line   8)
* list-system-processes:                 System Processes.    (line  15)
* listify-key-sequence:                  Event Input Misc.    (line  40)
* listing all buffers:                   Buffer List.         (line   6)
* listp:                                 List-related Predicates.
                                                              (line  22)
* lists:                                 Lists.               (line   6)
* lists and cons cells:                  Cons Cells.          (line   6)
* lists as sets:                         Sets And Lists.      (line   6)
* literal evaluation:                    Self-Evaluating Forms.
                                                              (line   6)
* little endian:                         Bindat Spec.         (line  13)
* live buffer:                           Killing Buffers.     (line  26)
* live windows:                          Basic Windows.       (line  32)
* ln:                                    Changing Files.      (line 114)
* load:                                  How Programs Do Loading.
                                                              (line  13)
* load error with require:               Named Features.      (line  17)
* load errors:                           How Programs Do Loading.
                                                              (line  98)
* load, customization keyword:           Common Keywords.     (line  95)
* load-average:                          System Environment.  (line 179)
* load-file:                             How Programs Do Loading.
                                                              (line 112)
* load-file-name:                        How Programs Do Loading.
                                                              (line 129)
* load-file-rep-suffixes:                Load Suffixes.       (line  16)
* load-history:                          Where Defined.       (line  20)
* load-in-progress:                      How Programs Do Loading.
                                                              (line 125)
* load-library:                          How Programs Do Loading.
                                                              (line 120)
* load-path:                             Library Search.      (line   9)
* load-prefer-newer:                     Load Suffixes.       (line  46)
* load-read-function:                    How Programs Do Loading.
                                                              (line 134)
* load-suffixes:                         Load Suffixes.       (line   9)
* load-theme:                            Custom Themes.       (line  82)
* loading:                               Loading.             (line   6)
* loading hooks:                         Hooks for Loading.   (line   6)
* loading, and non-ASCII characters:     Loading Non-ASCII.   (line   6)
* loadup.el:                             Building Emacs.      (line  18)
* local binding:                         Local Variables.     (line   6)
* local keymap:                          Active Keymaps.      (line  26)
* local variables:                       Local Variables.     (line   6)
* local variables, killed by major mode: Creating Buffer-Local.
                                                              (line 155)
* local-abbrev-table:                    Standard Abbrev Tables.
                                                              (line  15)
* local-function-key-map:                Translation Keymaps. (line  44)
* local-key-binding:                     Functions for Key Lookup.
                                                              (line  53)
* local-map (overlay property):          Overlay Properties.  (line 228)
* local-map (text property):             Special Properties.  (line 123)
* local-set-key:                         Key Binding Commands.
                                                              (line  73)
* local-unset-key:                       Key Binding Commands.
                                                              (line  81)
* local-variable-if-set-p:               Creating Buffer-Local.
                                                              (line  95)
* local-variable-p:                      Creating Buffer-Local.
                                                              (line  91)
* locale:                                Locales.             (line   6)
* locale-coding-system:                  Locales.             (line  10)
* locale-dependent string comparison:    Text Comparison.     (line 143)
* locale-dependent string equivalence:   Text Comparison.     (line  55)
* locale-info:                           Locales.             (line  31)
* locate file in path:                   Locating Files.      (line   6)
* locate-file:                           Locating Files.      (line  13)
* locate-library:                        Library Search.      (line  96)
* locate-user-emacs-file:                Standard File Names. (line  14)
* lock file:                             File Locks.          (line   6)
* lock-buffer:                           File Locks.          (line  32)
* log:                                   Math Functions.      (line  34)
* logand:                                Bitwise Operations.  (line 130)
* logb:                                  Float Basics.        (line  66)
* logging echo-area messages:            Logging Messages.    (line   6)
* logical arithmetic:                    Bitwise Operations.  (line   6)
* logical order:                         Bidirectional Display.
                                                              (line  17)
* logical shift:                         Bitwise Operations.  (line  15)
* logior:                                Bitwise Operations.  (line 166)
* lognot:                                Bitwise Operations.  (line 202)
* logxor:                                Bitwise Operations.  (line 184)
* looking up abbrevs:                    Abbrev Expansion.    (line   6)
* looking up fonts:                      Font Lookup.         (line   6)
* looking-at:                            Regexp Search.       (line 119)
* looking-at-p:                          Regexp Search.       (line 172)
* looking-back:                          Regexp Search.       (line 141)
* lookup tables:                         Hash Tables.         (line   6)
* lookup-key:                            Functions for Key Lookup.
                                                              (line   8)
* loops, infinite:                       Infinite Loops.      (line   6)
* lower case:                            Case Conversion.     (line   6)
* lower-frame:                           Raising and Lowering.
                                                              (line  16)
* lowering a frame:                      Raising and Lowering.
                                                              (line   6)
* LRO:                                   Bidirectional Display.
                                                              (line 188)
* lsh:                                   Bitwise Operations.  (line  14)
* lwarn:                                 Warning Basics.      (line  47)
* M-g:                                   Prefix Keys.         (line  43)
* M-o:                                   Prefix Keys.         (line  47)
* M-s:                                   Prefix Keys.         (line  45)
* M-x:                                   Interactive Call.    (line 109)
* Maclisp:                               Lisp History.        (line  11)
* macro:                                 What Is a Function.  (line  53)
* macro argument evaluation:             Argument Evaluation. (line  55)
* macro call:                            Expansion.           (line   6)
* macro call evaluation:                 Macro Forms.         (line   6)
* macro caveats:                         Problems with Macros.
                                                              (line   6)
* macro compilation:                     Compilation Functions.
                                                              (line  18)
* macro descriptions:                    A Sample Function Description.
                                                              (line   6)
* macro expansion:                       Expansion.           (line  41)
* macro, how to define:                  Defining Macros.     (line   6)
* macroexpand:                           Expansion.           (line  40)
* macroexpand-1:                         Expansion.           (line  84)
* macroexpand-all:                       Expansion.           (line  72)
* macrop:                                Simple Macro.        (line  20)
* macros:                                Macros.              (line   6)
* macros, at compile time:               Eval During Compile. (line  48)
* magic autoload comment:                Autoload.            (line 103)
* magic file names:                      Magic File Names.    (line   6)
* magic-fallback-mode-alist:             Auto Major Mode.     (line 100)
* magic-mode-alist:                      Auto Major Mode.     (line  92)
* mail-host-address:                     System Environment.  (line  76)
* major mode:                            Major Modes.         (line   6)
* major mode command:                    Major Modes.         (line   6)
* major mode conventions:                Major Mode Conventions.
                                                              (line   6)
* major mode hook:                       Major Mode Conventions.
                                                              (line 163)
* major mode keymap:                     Active Keymaps.      (line  32)
* major mode, automatic selection:       Auto Major Mode.     (line   6)
* major-mode:                            Major Modes.         (line  35)
* make-abbrev-table:                     Abbrev Tables.       (line   8)
* make-auto-save-file-name:              Auto-Saving.         (line  57)
* make-backup-file-name:                 Backup Names.        (line  35)
* make-backup-file-name-function:        Making Backups.      (line  90)
* make-backup-files:                     Making Backups.      (line  29)
* make-bool-vector:                      Bool-Vectors.        (line  17)
* make-button:                           Making Buttons.      (line  31)
* make-byte-code:                        Byte-Code Objects.   (line  63)
* make-category-set:                     Categories.          (line 101)
* make-category-table:                   Categories.          (line  96)
* make-char-table:                       Char-Tables.         (line  38)
* make-composed-keymap:                  Inheritance and Keymaps.
                                                              (line  56)
* make-directory:                        Create/Delete Dirs.  (line  11)
* make-display-table:                    Display Tables.      (line  11)
* make-finalizer:                        Finalizer Type.      (line  19)
* make-frame:                            Creating Frames.     (line   8)
* make-frame-invisible:                  Visibility of Frames.
                                                              (line  36)
* make-frame-on-display:                 Multiple Terminals.  (line 106)
* make-frame-visible:                    Visibility of Frames.
                                                              (line  30)
* make-frame-visible event:              Misc Events.         (line  23)
* make-glyph-code:                       Glyphs.              (line  13)
* make-hash-table:                       Creating Hash.       (line   8)
* make-help-screen:                      Help Functions.      (line 156)
* make-indirect-buffer:                  Indirect Buffers.    (line  31)
* make-keymap:                           Creating Keymaps.    (line  25)
* make-list:                             Building Lists.      (line  47)
* make-local-variable:                   Creating Buffer-Local.
                                                              (line   6)
* make-marker:                           Creating Markers.    (line  14)
* make-network-process:                  Network Processes.   (line  10)
* make-obsolete:                         Obsolete Functions.  (line  23)
* make-obsolete-variable:                Variable Aliases.    (line  35)
* make-overlay:                          Managing Overlays.   (line  14)
* make-pipe-process:                     Asynchronous Processes.
                                                              (line 114)
* make-process:                          Asynchronous Processes.
                                                              (line  29)
* make-progress-reporter:                Progress.            (line  21)
* make-ring:                             Rings.               (line  15)
* make-serial-process:                   Serial Ports.        (line  42)
* make-sparse-keymap:                    Creating Keymaps.    (line   8)
* make-string:                           Creating Strings.    (line   9)
* make-symbol:                           Creating Symbols.    (line  83)
* make-symbolic-link:                    Changing Files.      (line 113)
* make-syntax-table:                     Syntax Table Functions.
                                                              (line   9)
* make-temp-file:                        Unique File Names.   (line  14)
* make-temp-name:                        Unique File Names.   (line  81)
* make-text-button:                      Making Buttons.      (line  46)
* make-translation-table:                Translation of Characters.
                                                              (line  18)
* make-translation-table-from-alist:     Translation of Characters.
                                                              (line  78)
* make-translation-table-from-vector:    Translation of Characters.
                                                              (line  64)
* make-variable-buffer-local:            Creating Buffer-Local.
                                                              (line  51)
* make-vector:                           Vector Functions.    (line  25)
* make-xwidget:                          Xwidgets.            (line  16)
* making backup files:                   Making Backups.      (line   6)
* making buttons:                        Making Buttons.      (line   6)
* makunbound:                            Void Variables.      (line  23)
* malicious use of directional overrides: Bidirectional Display.
                                                              (line 198)
* managing overlays:                     Managing Overlays.   (line   6)
* manipulating buttons:                  Manipulating Buttons.
                                                              (line   6)
* map-char-table:                        Char-Tables.         (line 117)
* map-charset-chars:                     Character Sets.      (line  89)
* map-keymap:                            Scanning Keymaps.    (line  64)
* map-y-or-n-p:                          Multiple Queries.    (line  12)
* mapatoms:                              Creating Symbols.    (line 143)
* mapc:                                  Mapping Functions.   (line  52)
* mapcar:                                Mapping Functions.   (line  19)
* mapconcat:                             Mapping Functions.   (line  57)
* maphash:                               Hash Access.         (line  36)
* mapping functions:                     Mapping Functions.   (line   6)
* margins, display:                      Display Margins.     (line   6)
* margins, filling:                      Margins.             (line   6)
* mark:                                  The Mark.            (line  52)
* mark excursion:                        Excursions.          (line  59)
* mark ring:                             The Mark.            (line  41)
* mark, the:                             The Mark.            (line   6)
* mark-active:                           The Mark.            (line 167)
* mark-even-if-inactive:                 The Mark.            (line 140)
* mark-marker:                           The Mark.            (line  61)
* mark-ring:                             The Mark.            (line 196)
* mark-ring-max:                         The Mark.            (line 205)
* marker argument:                       Interactive Codes.   (line 141)
* marker creation:                       Creating Markers.    (line   6)
* marker garbage collection:             Overview of Markers. (line  31)
* marker information:                    Information from Markers.
                                                              (line   6)
* marker input stream:                   Input Streams.       (line  16)
* marker output stream:                  Output Streams.      (line  15)
* marker relocation:                     Overview of Markers. (line  24)
* marker, how to move position:          Moving Markers.      (line   6)
* marker-buffer:                         Information from Markers.
                                                              (line  13)
* marker-insertion-type:                 Marker Insertion Types.
                                                              (line  19)
* marker-position:                       Information from Markers.
                                                              (line   9)
* markerp:                               Predicates on Markers.
                                                              (line  10)
* markers:                               Markers.             (line   6)
* markers as numbers:                    Overview of Markers. (line  38)
* markers, predicates for:               Predicates on Markers.
                                                              (line   6)
* match data:                            Match Data.          (line   6)
* match, customization keyword:          Type Keywords.       (line 106)
* match-alternatives, customization keyword: Composite Types. (line 286)
* match-beginning:                       Simple Match Data.   (line  57)
* match-data:                            Entire Match Data.   (line   9)
* match-end:                             Simple Match Data.   (line  70)
* match-string:                          Simple Match Data.   (line  35)
* match-string-no-properties:            Simple Match Data.   (line  53)
* match-substitute-replacement:          Replacing Match.     (line  69)
* mathematical functions:                Math Functions.      (line   6)
* max:                                   Comparison of Numbers.
                                                              (line  75)
* max-char:                              Character Codes.     (line  32)
* max-image-size:                        Showing Images.      (line  80)
* max-lisp-eval-depth:                   Eval.                (line 100)
* max-mini-window-height:                Minibuffer Windows.  (line  55)
* max-mini-window-height <1>:            Minibuffer Misc.     (line  32)
* max-specpdl-size:                      Local Variables.     (line 112)
* maximize-window:                       Resizing Windows.    (line 220)
* maximized frames:                      Size Parameters.     (line  23)
* maximizing windows:                    Resizing Windows.    (line 220)
* maximum Lisp integer:                  Integer Basics.      (line  79)
* maximum value of character codepoint:  Character Codes.     (line  32)
* md5:                                   Checksum/Hash.       (line  46)
* MD5 checksum:                          Checksum/Hash.       (line   6)
* measuring resource usage:              Profiling.           (line   6)
* member:                                Sets And Lists.      (line 109)
* member-ignore-case:                    Sets And Lists.      (line 172)
* membership in a list:                  Sets And Lists.      (line  19)
* memory allocation:                     Garbage Collection.  (line   6)
* memory usage:                          Profiling.           (line   6)
* memory usage <1>:                      Memory Usage.        (line   6)
* memory-full:                           Garbage Collection.  (line 281)
* memory-info:                           Garbage Collection.  (line 291)
* memory-limit:                          Garbage Collection.  (line 273)
* memory-use-counts:                     Garbage Collection.  (line 285)
* memq:                                  Sets And Lists.      (line  18)
* memql:                                 Sets And Lists.      (line  91)
* menu bar:                              Menu Bar.            (line   6)
* menu bar keymaps:                      Standard Keymaps.    (line  92)
* menu definition example:               Menu Example.        (line   6)
* menu item:                             Defining Menus.      (line   6)
* menu keymaps:                          Menu Keymaps.        (line   6)
* menu modification:                     Modifying Menus.     (line   6)
* menu prompt string:                    Defining Menus.      (line   6)
* menu separators:                       Menu Separators.     (line   6)
* menu-bar prefix key:                   Key Sequence Input.  (line  92)
* menu-bar-file-menu:                    Standard Keymaps.    (line  92)
* menu-bar-final-items:                  Menu Bar.            (line  51)
* menu-bar-help-menu:                    Standard Keymaps.    (line  92)
* menu-bar-lines frame parameter:        Layout Parameters.   (line  54)
* menu-bar-options-menu:                 Standard Keymaps.    (line  92)
* menu-bar-tools-menu:                   Standard Keymaps.    (line  92)
* menu-bar-update-hook:                  Menu Bar.            (line  61)
* menu-item:                             Extended Menu Items. (line   6)
* menu-prompt-more-char:                 Keyboard Menus.      (line  23)
* menus, popup:                          Pop-Up Menus.        (line   6)
* merge-face-attribute:                  Attribute Functions. (line  62)
* message:                               Displaying Messages. (line   9)
* message digest:                        Checksum/Hash.       (line  11)
* message, finding what causes a particular message: Error Debugging.
                                                              (line  93)
* message-box:                           Displaying Messages. (line  80)
* message-log-max:                       Logging Messages.    (line  19)
* message-or-box:                        Displaying Messages. (line  67)
* message-truncate-lines:                Echo Area Customization.
                                                              (line  32)
* messages-buffer:                       Logging Messages.    (line  15)
* meta character key constants:          Changing Key Bindings.
                                                              (line  20)
* meta character printing:               Describing Characters.
                                                              (line  29)
* meta characters:                       Meta-Char Syntax.    (line   6)
* meta characters lookup:                Format of Keymaps.   (line  73)
* meta-prefix-char:                      Functions for Key Lookup.
                                                              (line  83)
* min:                                   Comparison of Numbers.
                                                              (line  87)
* minibuffer:                            Minibuffers.         (line   6)
* minibuffer completion:                 Minibuffer Completion.
                                                              (line   6)
* minibuffer contents, accessing:        Minibuffer Contents. (line   6)
* minibuffer history:                    Minibuffer History.  (line   6)
* minibuffer input:                      Recursive Editing.   (line  25)
* minibuffer input, and command-line arguments: Shell Arguments.
                                                              (line  40)
* minibuffer input, reading lisp objects: Object from Minibuffer.
                                                              (line   6)
* minibuffer input, reading text strings: Text from Minibuffer.
                                                              (line   6)
* minibuffer window, and next-window:    Cyclic Window Ordering.
                                                              (line  18)
* minibuffer windows:                    Minibuffer Windows.  (line   6)
* minibuffer, a frame parameter:         Buffer Parameters.   (line   9)
* minibuffer-allow-text-properties:      Text from Minibuffer.
                                                              (line 162)
* minibuffer-auto-raise:                 Raising and Lowering.
                                                              (line  19)
* minibuffer-complete:                   Completion Commands. (line  47)
* minibuffer-complete-and-exit:          Completion Commands. (line  50)
* minibuffer-complete-word:              Completion Commands. (line  40)
* minibuffer-completion-confirm:         Completion Commands. (line  21)
* minibuffer-completion-help:            Completion Commands. (line  58)
* minibuffer-completion-predicate:       Completion Commands. (line  16)
* minibuffer-completion-table:           Completion Commands. (line   9)
* minibuffer-confirm-exit-commands:      Completion Commands. (line  33)
* minibuffer-contents:                   Minibuffer Contents. (line  21)
* minibuffer-contents-no-properties:     Minibuffer Contents. (line  27)
* minibuffer-depth:                      Recursive Mini.      (line   9)
* minibuffer-exit-hook:                  Minibuffer Misc.     (line  14)
* minibuffer-frame-alist:                Initial Parameters.  (line  41)
* minibuffer-help-form:                  Minibuffer Misc.     (line  18)
* minibuffer-history:                    Minibuffer History.  (line  90)
* minibuffer-inactive-mode:              Minibuffer Misc.     (line  46)
* minibuffer-less frame:                 Frame Layout.        (line 124)
* minibuffer-local-completion-map:       Completion Commands. (line  92)
* minibuffer-local-filename-completion-map: Completion Commands.
                                                              (line 124)
* minibuffer-local-map:                  Text from Minibuffer.
                                                              (line 172)
* minibuffer-local-must-match-map:       Completion Commands. (line 109)
* minibuffer-local-ns-map:               Text from Minibuffer.
                                                              (line 220)
* minibuffer-local-shell-command-map:    Reading File Names.  (line 187)
* minibuffer-message:                    Minibuffer Misc.     (line  37)
* minibuffer-message-timeout:            Minibuffer Misc.     (line  37)
* minibuffer-only frame:                 Frame Layout.        (line 124)
* minibuffer-only frame <1>:             Initial Parameters.  (line  37)
* minibuffer-prompt:                     Minibuffer Contents. (line   8)
* minibuffer-prompt-end:                 Minibuffer Contents. (line  12)
* minibuffer-prompt-width:               Minibuffer Contents. (line  17)
* minibuffer-scroll-window:              Minibuffer Misc.     (line  22)
* minibuffer-selected-window:            Minibuffer Misc.     (line  27)
* minibuffer-setup-hook:                 Minibuffer Misc.     (line  10)
* minibuffer-window:                     Minibuffer Windows.  (line  13)
* minibuffer-window-active-p:            Minibuffer Windows.  (line  36)
* minibufferp:                           Minibuffer Misc.     (line   6)
* minimize-window:                       Resizing Windows.    (line 225)
* minimized frame:                       Visibility of Frames.
                                                              (line   6)
* minimizing windows:                    Resizing Windows.    (line 225)
* minimum Lisp integer:                  Integer Basics.      (line  84)
* minor mode:                            Minor Modes.         (line   6)
* minor mode conventions:                Minor Mode Conventions.
                                                              (line   6)
* minor-mode-alist:                      Mode Line Variables. (line  97)
* minor-mode-key-binding:                Functions for Key Lookup.
                                                              (line  67)
* minor-mode-list:                       Minor Modes.         (line  21)
* minor-mode-map-alist:                  Controlling Active Maps.
                                                              (line  63)
* minor-mode-overriding-map-alist:       Controlling Active Maps.
                                                              (line  89)
* mirroring of characters:               Character Properties.
                                                              (line 101)
* misc-objects-consed:                   Memory Usage.        (line  33)
* mkdir:                                 Create/Delete Dirs.  (line  11)
* mod:                                   Arithmetic Operations.
                                                              (line 130)
* mode:                                  Modes.               (line   6)
* mode bits:                             Testing Accessibility.
                                                              (line  90)
* mode help:                             Mode Help.           (line   6)
* mode hook:                             Major Mode Conventions.
                                                              (line 163)
* mode line:                             Mode Line Format.    (line   6)
* mode line construct:                   Mode Line Data.      (line   6)
* mode loading:                          Major Mode Conventions.
                                                              (line 218)
* mode variable:                         Minor Mode Conventions.
                                                              (line  11)
* mode-class (property):                 Major Mode Conventions.
                                                              (line 189)
* mode-line prefix key:                  Key Sequence Input.  (line  92)
* mode-line-buffer-identification:       Mode Line Variables. (line  35)
* mode-line-client:                      Mode Line Variables. (line  61)
* mode-line-coding-system-map:           Standard Keymaps.    (line 103)
* mode-line-column-line-number-mode-map: Standard Keymaps.    (line 103)
* mode-line-end-spaces:                  Mode Line Variables. (line  90)
* mode-line-format:                      Mode Line Top.       (line   8)
* mode-line-frame-identification:        Mode Line Variables. (line  29)
* mode-line-front-space:                 Mode Line Variables. (line  84)
* mode-line-input-method-map:            Standard Keymaps.    (line 103)
* mode-line-misc-info:                   Mode Line Variables. (line  93)
* mode-line-modes:                       Mode Line Variables. (line  51)
* mode-line-modified:                    Mode Line Variables. (line  20)
* mode-line-mule-info:                   Mode Line Variables. (line  15)
* mode-line-position:                    Mode Line Variables. (line  40)
* mode-line-process:                     Mode Line Variables. (line  75)
* mode-line-remote:                      Mode Line Variables. (line  57)
* mode-name:                             Mode Line Variables. (line  66)
* mode-specific-map:                     Prefix Keys.         (line  21)
* model/view/controller:                 Abstract Display.    (line   6)
* modification flag (of buffer):         Buffer Modification. (line   6)
* modification of lists:                 Rearrangement.       (line   6)
* modification time of buffer:           Modification Time.   (line   6)
* modification time of file:             File Attributes.     (line  64)
* modification-hooks (overlay property): Overlay Properties.  (line 135)
* modification-hooks (text property):    Special Properties.  (line 269)
* modifier bits (of input character):    Keyboard Events.     (line  12)
* modifiers of events:                   Event Mod.           (line   6)
* modify a list:                         List Variables.      (line   6)
* modify-all-frames-parameters:          Parameter Access.    (line  53)
* modify-category-entry:                 Categories.          (line 129)
* modify-frame-parameters:               Parameter Access.    (line  19)
* modify-syntax-entry:                   Syntax Table Functions.
                                                              (line  24)
* modifying strings:                     Modifying Strings.   (line   6)
* module-file-suffix:                    Dynamic Modules.     (line  14)
* modulus:                               Arithmetic Operations.
                                                              (line 131)
* momentary-string-display:              Temporary Displays.  (line 153)
* most recently selected windows:        Selecting Windows.   (line  31)
* most recently used window:             Cyclic Window Ordering.
                                                              (line 117)
* most-negative-fixnum:                  Integer Basics.      (line  84)
* most-positive-fixnum:                  Integer Basics.      (line  79)
* motion based on parsing:               Motion via Parsing.  (line   6)
* motion by chars, words, lines, lists:  Motion.              (line   6)
* motion event:                          Motion Events.       (line   6)
* mouse click event:                     Click Events.        (line   6)
* mouse drag event:                      Drag Events.         (line   6)
* mouse events, data in:                 Accessing Mouse.     (line   6)
* mouse events, in special parts of frame: Key Sequence Input.
                                                              (line  92)
* mouse events, repeated:                Repeat Events.       (line   6)
* mouse motion events:                   Motion Events.       (line   6)
* mouse pointer shape:                   Pointer Shape.       (line   6)
* mouse position:                        Mouse Position.      (line   6)
* mouse position list:                   Click Events.        (line  27)
* mouse position list, accessing:        Accessing Mouse.     (line  34)
* mouse tracking:                        Mouse Tracking.      (line   6)
* mouse, availability:                   Display Feature Testing.
                                                              (line  32)
* mouse-1:                               Clickable Text.      (line   6)
* mouse-1-click-follows-link:            Clickable Text.      (line  77)
* mouse-2:                               Key Binding Conventions.
                                                              (line   6)
* mouse-absolute-pixel-position:         Mouse Position.      (line  51)
* mouse-action (button property):        Button Properties.   (line  17)
* mouse-appearance-menu-map:             Standard Keymaps.    (line 113)
* mouse-color, a frame parameter:        Font and Color Parameters.
                                                              (line  90)
* mouse-face (button property):          Button Properties.   (line  27)
* mouse-face (overlay property):         Overlay Properties.  (line 110)
* mouse-face (text property):            Special Properties.  (line  59)
* mouse-leave-buffer-hook:               Standard Hooks.      (line 151)
* mouse-movement-p:                      Classifying Events.  (line  88)
* mouse-on-link-p:                       Clickable Text.      (line 160)
* mouse-pixel-position:                  Mouse Position.      (line  36)
* mouse-position:                        Mouse Position.      (line   9)
* mouse-position-function:               Mouse Position.      (line  16)
* mouse-wheel-down-event:                Misc Events.         (line  35)
* mouse-wheel-up-event:                  Misc Events.         (line  35)
* move to beginning or end of buffer:    Buffer End Motion.   (line   6)
* move-marker:                           Moving Markers.      (line  29)
* move-overlay:                          Managing Overlays.   (line  59)
* move-point-visually:                   Bidirectional Display.
                                                              (line 117)
* move-to-column:                        Columns.             (line  28)
* move-to-left-margin:                   Margins.             (line  56)
* move-to-window-group-line:             Screen Lines.        (line  98)
* move-to-window-group-line-function:    Screen Lines.        (line  98)
* move-to-window-line:                   Screen Lines.        (line  78)
* movemail:                              Subprocess Creation. (line  73)
* moving across syntax classes:          Motion and Syntax.   (line   6)
* moving markers:                        Moving Markers.      (line   6)
* MS-DOS and file modes:                 Testing Accessibility.
                                                              (line 117)
* MS-Windows file-name syntax:           File Names.          (line  19)
* mule-keymap:                           Prefix Keys.         (line  31)
* multi-file package:                    Multi-file Packages. (line   6)
* multi-frame images:                    Multi-Frame Images.  (line   6)
* multi-monitor:                         Multiple Terminals.  (line 147)
* multi-query-replace-map:               Search and Replace.  (line 157)
* multi-tty:                             Multiple Terminals.  (line   6)
* multibyte characters:                  Non-ASCII Characters.
                                                              (line   6)
* multibyte text:                        Text Representations.
                                                              (line  20)
* multibyte-char-to-unibyte:             Converting Representations.
                                                              (line  63)
* multibyte-string-p:                    Text Representations.
                                                              (line 114)
* multibyte-syntax-as-symbol:            Control Parsing.     (line   6)
* multiline font lock:                   Multiline Font Lock. (line   6)
* multiple terminals:                    Multiple Terminals.  (line   6)
* multiple windows:                      Basic Windows.       (line  10)
* multiple X displays:                   Multiple Terminals.  (line   6)
* multiple yes-or-no questions:          Multiple Queries.    (line   6)
* multiple-dispatch methods:             Generic Functions.   (line 174)
* multiple-frames:                       Frame Titles.        (line  28)
* name, a frame parameter:               Basic Parameters.    (line  27)
* named function:                        Function Names.      (line   6)
* naming backup files:                   Backup Names.        (line   6)
* NaN:                                   Float Basics.        (line  26)
* narrow-map:                            Standard Keymaps.    (line 119)
* narrow-to-page:                        Narrowing.           (line  35)
* narrow-to-region:                      Narrowing.           (line  27)
* narrowing:                             Narrowing.           (line   6)
* native edges:                          Frame Layout.        (line  84)
* native frame:                          Frame Layout.        (line  84)
* native height:                         Frame Layout.        (line  84)
* native position:                       Frame Layout.        (line  92)
* native width:                          Frame Layout.        (line  84)
* natnump:                               Predicates on Numbers.
                                                              (line  25)
* natural numbers:                       Predicates on Numbers.
                                                              (line  26)
* nbutlast:                              List Elements.       (line 152)
* nconc:                                 Rearrangement.       (line  15)
* negative infinity:                     Float Basics.        (line  26)
* negative-argument:                     Prefix Command Arguments.
                                                              (line 113)
* network byte ordering:                 Bindat Spec.         (line  13)
* network connection:                    Network.             (line   6)
* network connection, encrypted:         Network.             (line  55)
* network servers:                       Network Servers.     (line   6)
* network service name, and default coding system: Default Coding Systems.
                                                              (line  80)
* network-coding-system-alist:           Default Coding Systems.
                                                              (line  80)
* network-interface-info:                Misc Network.        (line  16)
* network-interface-list:                Misc Network.        (line   9)
* new file message:                      Subroutines of Visiting.
                                                              (line  39)
* newline:                               Basic Char Syntax.   (line  27)
* newline <1>:                           Commands for Insertion.
                                                              (line  46)
* newline and Auto Fill mode:            Commands for Insertion.
                                                              (line  51)
* newline in print:                      Output Functions.    (line  84)
* newline in strings:                    Syntax for Strings.  (line  13)
* newline-and-indent:                    Mode-Specific Indent.
                                                              (line  52)
* next input:                            Event Input Misc.    (line  11)
* next-button:                           Button Buffer Commands.
                                                              (line  50)
* next-char-property-change:             Property Search.     (line  82)
* next-complete-history-element:         Minibuffer Commands. (line  40)
* next-frame:                            Finding All Frames.  (line  19)
* next-history-element:                  Minibuffer Commands. (line  21)
* next-matching-history-element:         Minibuffer Commands. (line  30)
* next-overlay-change:                   Finding Overlays.    (line  34)
* next-property-change:                  Property Search.     (line  25)
* next-screen-context-lines:             Textual Scrolling.   (line 157)
* next-single-char-property-change:      Property Search.     (line  97)
* next-single-property-change:           Property Search.     (line  58)
* next-window:                           Cyclic Window Ordering.
                                                              (line  17)
* nil:                                   nil and t.           (line   6)
* nil as a list:                         Box Diagrams.        (line  41)
* nil in keymap:                         Key Lookup.          (line  36)
* nil input stream:                      Input Streams.       (line  50)
* nil output stream:                     Output Streams.      (line  34)
* nlistp:                                List-related Predicates.
                                                              (line  31)
* no-byte-compile:                       Byte Compilation.    (line  21)
* no-catch:                              Catch and Throw.     (line  86)
* no-conversion coding system:           Coding System Basics.
                                                              (line  62)
* no-redraw-on-reenter:                  Refresh Screen.      (line  30)
* no-self-insert property:               Defining Abbrevs.    (line  34)
* node, ewoc:                            Abstract Display.    (line  24)
* non-ASCII characters:                  Non-ASCII Characters.
                                                              (line   6)
* non-ASCII characters in loaded files:  Loading Non-ASCII.   (line   6)
* non-ASCII text in keybindings:         Key Binding Commands.
                                                              (line  29)
* non-capturing group:                   Regexp Backslash.    (line  66)
* non-greedy repetition characters in regexp: Regexp Special. (line  59)
* nondirectory part (of file name):      File Name Components.
                                                              (line   6)
* noninteractive:                        Batch Mode.          (line  31)
* nonlocal exits:                        Nonlocal Exits.      (line   6)
* nonlocal exits, cleaning up:           Cleanups.            (line   6)
* nonprinting characters, reading:       Quoted Character Input.
                                                              (line  12)
* noreturn:                              Test Coverage.       (line  29)
* normal hook:                           Hooks.               (line  12)
* normal-auto-fill-function:             Auto Filling.        (line  24)
* normal-backup-enable-predicate:        Making Backups.      (line  51)
* normal-mode:                           Auto Major Mode.     (line  10)
* not:                                   Combining Conditions.
                                                              (line  11)
* not-modified:                          Buffer Modification. (line  41)
* notation:                              Evaluation Notation. (line   6)
* notifications, on desktop:             Desktop Notifications.
                                                              (line   6)
* notifications-close-notification:      Desktop Notifications.
                                                              (line 167)
* notifications-get-capabilities:        Desktop Notifications.
                                                              (line 171)
* notifications-get-server-information:  Desktop Notifications.
                                                              (line 208)
* notifications-notify:                  Desktop Notifications.
                                                              (line  13)
* nreverse:                              Sequence Functions.  (line 136)
* nth:                                   List Elements.       (line  85)
* nthcdr:                                List Elements.       (line 101)
* null:                                  List-related Predicates.
                                                              (line  37)
* null bytes, and decoding text:         Lisp and Coding Systems.
                                                              (line 122)
* num-input-keys:                        Key Sequence Input.  (line 112)
* num-nonmacro-input-events:             Reading One Event.   (line  94)
* number comparison:                     Comparison of Numbers.
                                                              (line   6)
* number conversions:                    Numeric Conversions. (line   6)
* number-or-marker-p:                    Predicates on Markers.
                                                              (line  19)
* number-sequence:                       Building Lists.      (line 158)
* number-to-string:                      String Conversion.   (line  19)
* numbered backups:                      Numbered Backups.    (line   6)
* numberp:                               Predicates on Numbers.
                                                              (line  21)
* numbers:                               Numbers.             (line   6)
* numeric prefix argument:               Prefix Command Arguments.
                                                              (line   6)
* numeric prefix argument usage:         Interactive Codes.   (line 159)
* numerical RGB color specification:     Color Names.         (line   6)
* obarray:                               Creating Symbols.    (line 139)
* obarray <1>:                           Creating Symbols.    (line  11)
* obarray in completion:                 Basic Completion.    (line  31)
* object:                                Lisp Data Types.     (line   6)
* object internals:                      Object Internals.    (line   6)
* object to string:                      Output Functions.    (line  94)
* obsolete functions:                    Obsolete Functions.  (line   6)
* octal character code:                  General Escape Syntax.
                                                              (line  30)
* octal character input:                 Quoted Character Input.
                                                              (line  12)
* octal escapes:                         Usual Display.       (line  38)
* octal numbers:                         Integer Basics.      (line  24)
* old advices, porting:                  Porting old advice.  (line   6)
* one-window-p:                          Cyclic Window Ordering.
                                                              (line  91)
* only-global-abbrevs:                   Defining Abbrevs.    (line  47)
* opacity, frame:                        Font and Color Parameters.
                                                              (line  55)
* open-dribble-file:                     Recording Input.     (line  19)
* open-network-stream:                   Network.             (line  67)
* open-paren-in-column-0-is-defun-start: List Motion.         (line  81)
* open-termscript:                       Terminal Output.     (line  49)
* OpenType font:                         Low-Level Font.      (line  80)
* operating system environment:          System Environment.  (line   6)
* operating system signal:               Killing Emacs.       (line  22)
* operations (property):                 Magic File Names.    (line 130)
* optimize regexp:                       Regexp Functions.    (line  29)
* option descriptions:                   A Sample Variable Description.
                                                              (line   6)
* optional arguments:                    Argument List.       (line  18)
* options on command line:               Command-Line Arguments.
                                                              (line  29)
* options, defcustom keyword:            Variable Definitions.
                                                              (line  67)
* or:                                    Combining Conditions.
                                                              (line  59)
* ordering of windows, cyclic:           Cyclic Window Ordering.
                                                              (line   6)
* other-buffer:                          Buffer List.         (line  67)
* other-window:                          Cyclic Window Ordering.
                                                              (line  62)
* other-window-scroll-buffer:            Textual Scrolling.   (line  96)
* outer edges:                           Frame Layout.        (line  34)
* outer frame:                           Frame Layout.        (line  34)
* outer height:                          Frame Layout.        (line  34)
* outer position:                        Frame Layout.        (line  39)
* outer width:                           Frame Layout.        (line  34)
* outer-window-id, a frame parameter:    Management Parameters.
                                                              (line  38)
* output from processes:                 Output from Processes.
                                                              (line   6)
* output stream:                         Output Streams.      (line   6)
* output-controlling variables:          Output Variables.    (line   6)
* overall prompt string:                 Format of Keymaps.   (line  57)
* overflow:                              Integer Basics.      (line   9)
* overflow-newline-into-fringe:          Fringe Cursors.      (line  11)
* overlay properties:                    Overlay Properties.  (line   6)
* overlay, empty:                        Managing Overlays.   (line  21)
* overlay-arrow-position:                Overlay Arrow.       (line  19)
* overlay-arrow-string:                  Overlay Arrow.       (line  12)
* overlay-arrow-variable-list:           Overlay Arrow.       (line  40)
* overlay-buffer:                        Managing Overlays.   (line  46)
* overlay-end:                           Managing Overlays.   (line  42)
* overlay-get:                           Overlay Properties.  (line  26)
* overlay-properties:                    Overlay Properties.  (line  37)
* overlay-put:                           Overlay Properties.  (line  33)
* overlay-recenter:                      Managing Overlays.   (line 148)
* overlay-start:                         Managing Overlays.   (line  38)
* overlayp:                              Managing Overlays.   (line  11)
* overlays:                              Overlays.            (line   6)
* overlays, managing:                    Managing Overlays.   (line   6)
* overlays, scalability:                 Overlays.            (line  12)
* overlays, searching for:               Finding Overlays.    (line   6)
* overlays-at:                           Finding Overlays.    (line   6)
* overlays-in:                           Finding Overlays.    (line  26)
* overlined text:                        Face Attributes.     (line 108)
* override spec (for a face):            Defining Faces.      (line 152)
* overriding bidirectional properties:   Bidirectional Display.
                                                              (line 188)
* overriding-local-map:                  Controlling Active Maps.
                                                              (line 102)
* overriding-local-map-menu-flag:        Controlling Active Maps.
                                                              (line 118)
* overriding-terminal-local-map:         Controlling Active Maps.
                                                              (line 109)
* overwrite-mode:                        Commands for Insertion.
                                                              (line  65)
* package:                               Packaging.           (line   6)
* package archive:                       Package Archives.    (line   6)
* package archive security:              Package Archives.    (line  73)
* package attributes:                    Packaging Basics.    (line   6)
* package autoloads:                     Packaging Basics.    (line  55)
* package dependencies:                  Packaging Basics.    (line   6)
* package name:                          Packaging Basics.    (line   6)
* package signing:                       Package Archives.    (line  73)
* package version:                       Packaging Basics.    (line   6)
* package-archive-upload-base:           Package Archives.    (line  39)
* package-archives:                      Package Archives.    (line  12)
* package-initialize:                    Packaging Basics.    (line  76)
* package-upload-buffer:                 Package Archives.    (line  63)
* package-upload-file:                   Package Archives.    (line  49)
* package-version, customization keyword: Common Keywords.    (line 114)
* packing:                               Byte Packing.        (line  13)
* padding:                               Formatting Strings.  (line 145)
* page-delimiter:                        Standard Regexps.    (line   9)
* paragraph-separate:                    Standard Regexps.    (line  24)
* paragraph-start:                       Standard Regexps.    (line  31)
* parameters of initial frame:           Initial Parameters.  (line   6)
* parent of char-table:                  Char-Tables.         (line  28)
* parent process:                        Processes.           (line   6)
* parent window:                         Windows and Frames.  (line  50)
* parent window <1>:                     Windows and Frames.  (line  60)
* parenthesis:                           Cons Cell Type.      (line  25)
* parenthesis depth:                     Low-Level Parsing.   (line  19)
* parenthesis matching:                  Blinking.            (line   6)
* parenthesis mismatch, debugging:       Syntax Errors.       (line  20)
* parity, in serial connections:         Serial Ports.        (line 114)
* parse state for a position:            Position Parse.      (line   6)
* parse-colon-path:                      System Environment.  (line 152)
* parse-partial-sexp:                    Low-Level Parsing.   (line  10)
* parse-sexp-ignore-comments:            Control Parsing.     (line  12)
* parse-sexp-lookup-properties:          Syntax Properties.   (line  27)
* parse-sexp-lookup-properties <1>:      Control Parsing.     (line  17)
* parser state:                          Parser State.        (line   6)
* parsing buffer text:                   Syntax Tables.       (line   6)
* parsing expressions:                   Parsing Expressions. (line   6)
* parsing html:                          Parsing HTML/XML.    (line   6)
* parsing xml:                           Parsing HTML/XML.    (line  48)
* parsing, control parameters:           Control Parsing.     (line   6)
* partial application of functions:      Calling Functions.   (line  82)
* partial-width windows:                 Truncation.          (line  30)
* passwords, reading:                    Reading a Password.  (line   6)
* PATH environment variable:             Subprocess Creation. (line  18)
* path-separator:                        System Environment.  (line 146)
* pattern matching:                      Pattern matching case statement.
                                                              (line   6)
* PBM:                                   Other Image Types.   (line   6)
* pcase:                                 Pattern matching case statement.
                                                              (line  16)
* pcase <1>:                             Pattern matching case statement.
                                                              (line   6)
* pcase-defmacro:                        Pattern matching case statement.
                                                              (line 177)
* peculiar error:                        Error Symbols.       (line  29)
* peeking at input:                      Event Input Misc.    (line  11)
* percent symbol in mode line:           Mode Line Data.      (line  20)
* perform-replace:                       Search and Replace.  (line  40)
* performance analysis:                  Coverage Testing.    (line   6)
* permanent local variable:              Creating Buffer-Local.
                                                              (line 190)
* permissions, file:                     Testing Accessibility.
                                                              (line  90)
* permissions, file <1>:                 Changing Files.      (line 140)
* phishing using directional overrides:  Bidirectional Display.
                                                              (line 198)
* piece of advice:                       Advising Functions.  (line   6)
* pipe:                                  Asynchronous Processes.
                                                              (line  15)
* pixel height of a window:              Window Sizes.        (line 109)
* pixel width of a window:               Window Sizes.        (line 118)
* pixelwise, resizing windows:           Resizing Windows.    (line  89)
* place form:                            Generalized Variables.
                                                              (line   6)
* play-sound:                            Sound Output.        (line  13)
* play-sound-file:                       Sound Output.        (line  45)
* play-sound-functions:                  Sound Output.        (line  49)
* plist:                                 Property Lists.      (line   6)
* plist access:                          Plist Access.        (line   6)
* plist vs. alist:                       Plists and Alists.   (line   6)
* plist-get:                             Plist Access.        (line   9)
* plist-member:                          Plist Access.        (line  45)
* plist-put:                             Plist Access.        (line  23)
* plugin_is_GPL_compatible:              Dynamic Modules.     (line  19)
* point:                                 Point.               (line  31)
* point <1>:                             Point.               (line   6)
* point excursion:                       Excursions.          (line  21)
* point excursion <1>:                   Excursions.          (line  59)
* point in window:                       Window Point.        (line   6)
* point with narrowing:                  Point.               (line  17)
* point-entered (text property):         Special Properties.  (line 302)
* point-left (text property):            Special Properties.  (line 302)
* point-marker:                          Creating Markers.    (line  21)
* point-max:                             Point.               (line  44)
* point-max-marker:                      Creating Markers.    (line  31)
* point-min:                             Point.               (line  38)
* point-min-marker:                      Creating Markers.    (line  26)
* pointer (text property):               Special Properties.  (line 226)
* pointer shape:                         Pointer Shape.       (line   6)
* pointers:                              Cons Cell Type.      (line   6)
* polymorphism:                          Generic Functions.   (line   6)
* pop:                                   List Elements.       (line  60)
* pop-mark:                              The Mark.            (line 111)
* pop-to-buffer:                         Switching Buffers.   (line 130)
* pop-up-frame-alist:                    Choosing Window Options.
                                                              (line 107)
* pop-up-frame-function:                 Choosing Window Options.
                                                              (line  96)
* pop-up-frames:                         Choosing Window Options.
                                                              (line  78)
* pop-up-windows:                        Choosing Window Options.
                                                              (line   9)
* port number, and default coding system: Default Coding Systems.
                                                              (line  80)
* pos-visible-in-window-group-p:         Window Start and End.
                                                              (line 180)
* pos-visible-in-window-group-p-function: Window Start and End.
                                                              (line 180)
* pos-visible-in-window-p:               Window Start and End.
                                                              (line 145)
* position (in buffer):                  Positions.           (line   6)
* position argument:                     Interactive Codes.   (line  79)
* position in window:                    Window Point.        (line   6)
* position of frame:                     Frame Geometry.      (line   6)
* position of frame <1>:                 Size and Position.   (line   6)
* position of mouse:                     Mouse Position.      (line   6)
* position-bytes:                        Text Representations.
                                                              (line  61)
* positive infinity:                     Float Basics.        (line  26)
* posix-looking-at:                      POSIX Regexps.       (line  33)
* posix-search-backward:                 POSIX Regexps.       (line  28)
* posix-search-forward:                  POSIX Regexps.       (line  23)
* posix-string-match:                    POSIX Regexps.       (line  38)
* posn-actual-col-row:                   Accessing Mouse.     (line  83)
* posn-area:                             Accessing Mouse.     (line  42)
* posn-at-point:                         Accessing Mouse.     (line 127)
* posn-at-x-y:                           Accessing Mouse.     (line 134)
* posn-col-row:                          Accessing Mouse.     (line  70)
* posn-image:                            Accessing Mouse.     (line 100)
* posn-object:                           Accessing Mouse.     (line 104)
* posn-object-width-height:              Accessing Mouse.     (line 114)
* posn-object-x-y:                       Accessing Mouse.     (line 108)
* posn-point:                            Accessing Mouse.     (line  47)
* posn-string:                           Accessing Mouse.     (line  96)
* posn-timestamp:                        Accessing Mouse.     (line 119)
* posn-window:                           Accessing Mouse.     (line  37)
* posn-x-y:                              Accessing Mouse.     (line  53)
* posnp:                                 Accessing Mouse.     (line  29)
* post-command-hook:                     Command Overview.    (line  44)
* post-gc-hook:                          Garbage Collection.  (line 229)
* post-self-insert-hook:                 Commands for Insertion.
                                                              (line  38)
* postscript images:                     PostScript Images.   (line   6)
* pp:                                    Output Functions.    (line 126)
* pre-command-hook:                      Command Overview.    (line  38)
* pre-redisplay-function:                Forcing Redisplay.   (line  37)
* pre-redisplay-functions:               Forcing Redisplay.   (line  43)
* preceding-char:                        Near Point.          (line  55)
* precision in format specifications:    Formatting Strings.  (line 208)
* predicates for lists:                  List-related Predicates.
                                                              (line   6)
* predicates for markers:                Predicates on Markers.
                                                              (line   6)
* predicates for numbers:                Predicates on Numbers.
                                                              (line   6)
* predicates for strings:                Predicates for Strings.
                                                              (line   6)
* prefer-utf-8 coding system:            Coding System Basics.
                                                              (line  15)
* prefix argument:                       Prefix Command Arguments.
                                                              (line   6)
* prefix argument unreading:             Event Input Misc.    (line  20)
* prefix command:                        Prefix Keys.         (line  88)
* prefix key:                            Prefix Keys.         (line   6)
* prefix, defgroup keyword:              Group Definitions.   (line  46)
* prefix-arg:                            Prefix Command Arguments.
                                                              (line  90)
* prefix-command-echo-keystrokes-functions: Standard Hooks.   (line 157)
* prefix-command-preserve-state-hook:    Standard Hooks.      (line 165)
* prefix-help-command:                   Help Functions.      (line  90)
* prefix-numeric-value:                  Prefix Command Arguments.
                                                              (line  77)
* preloaded Lisp files:                  Building Emacs.      (line  32)
* preloaded-file-list:                   Building Emacs.      (line  32)
* preloading additional functions and variables: Building Emacs.
                                                              (line  55)
* prepare-change-group:                  Atomic Changes.      (line  30)
* preserving window sizes:               Preserving Window Sizes.
                                                              (line   6)
* preventing backtracking:               Specification List.  (line 102)
* preventing prefix key:                 Key Lookup.          (line  85)
* preventing quitting:                   Quitting.            (line  45)
* previous complete subexpression:       Parser State.        (line  22)
* previous-button:                       Button Buffer Commands.
                                                              (line  51)
* previous-char-property-change:         Property Search.     (line  92)
* previous-complete-history-element:     Minibuffer Commands. (line  35)
* previous-frame:                        Finding All Frames.  (line  39)
* previous-history-element:              Minibuffer Commands. (line  17)
* previous-matching-history-element:     Minibuffer Commands. (line  25)
* previous-overlay-change:               Finding Overlays.    (line  39)
* previous-property-change:              Property Search.     (line  53)
* previous-single-char-property-change:  Property Search.     (line 107)
* previous-single-property-change:       Property Search.     (line  76)
* previous-window:                       Cyclic Window Ordering.
                                                              (line  57)
* primary selection:                     Window System Selections.
                                                              (line   6)
* primitive:                             What Is a Function.  (line  33)
* primitive function:                    Primitive Function Type.
                                                              (line   6)
* primitive function internals:          Writing Emacs Primitives.
                                                              (line   6)
* primitive type:                        Lisp Data Types.     (line  16)
* primitive-undo:                        Undo.                (line 124)
* prin1:                                 Output Functions.    (line  56)
* prin1-to-string:                       Output Functions.    (line  93)
* princ:                                 Output Functions.    (line  68)
* print:                                 Output Functions.    (line  39)
* print example:                         Output Streams.      (line  50)
* print name cell:                       Symbol Components.   (line  10)
* print-circle:                          Output Variables.    (line  90)
* print-continuous-numbering:            Output Variables.    (line 100)
* print-escape-multibyte:                Output Variables.    (line  50)
* print-escape-newlines:                 Output Variables.    (line  16)
* print-escape-nonascii:                 Output Variables.    (line  41)
* print-gensym:                          Output Variables.    (line  94)
* print-length:                          Output Variables.    (line  60)
* print-level:                           Output Variables.    (line  74)
* print-number-table:                    Output Variables.    (line 107)
* print-quoted:                          Output Variables.    (line  11)
* printable ASCII characters:            Usual Display.       (line  10)
* printable-chars:                       Character Properties.
                                                              (line 217)
* printed representation:                Printed Representation.
                                                              (line   6)
* printed representation for characters: Basic Char Syntax.   (line   6)
* printing:                              Streams Intro.       (line   6)
* printing (Edebug):                     Printing in Edebug.  (line   6)
* printing circular structures:          Printing in Edebug.  (line   6)
* printing limits:                       Output Variables.    (line  61)
* printing notation:                     Printing Notation.   (line   6)
* priority (overlay property):           Overlay Properties.  (line  48)
* priority order of coding systems:      Specifying Coding Systems.
                                                              (line  67)
* process:                               Processes.           (line   6)
* process creation:                      Subprocess Creation. (line   6)
* process filter:                        Filter Functions.    (line   6)
* process filter multibyte flag:         Decoding Output.     (line  30)
* process information:                   Process Information. (line   6)
* process input:                         Input to Processes.  (line   6)
* process internals:                     Process Internals.   (line   6)
* process output:                        Output from Processes.
                                                              (line   6)
* process sentinel:                      Sentinels.           (line   6)
* process signals:                       Signals to Processes.
                                                              (line   6)
* process-adaptive-read-buffering:       Output from Processes.
                                                              (line  43)
* process-attributes:                    System Processes.    (line  22)
* process-buffer:                        Process Buffers.     (line  28)
* process-coding-system:                 Process Information. (line 171)
* process-coding-system-alist:           Default Coding Systems.
                                                              (line  57)
* process-command:                       Process Information. (line  35)
* process-connection-type:               Asynchronous Processes.
                                                              (line 253)
* process-contact:                       Process Information. (line  46)
* process-datagram-address:              Datagrams.           (line  20)
* process-environment:                   System Environment.  (line 125)
* process-exit-status:                   Process Information. (line 153)
* process-file:                          Synchronous Processes.
                                                              (line 137)
* process-file-shell-command:            Synchronous Processes.
                                                              (line 249)
* process-file-side-effects:             Synchronous Processes.
                                                              (line 178)
* process-filter:                        Filter Functions.    (line  84)
* process-get:                           Process Information. (line 186)
* process-id:                            Process Information. (line  91)
* process-kill-buffer-query-function:    Process Buffers.     (line  21)
* process-lines:                         Synchronous Processes.
                                                              (line 262)
* process-list:                          Process Information. (line  20)
* process-live-p:                        Process Information. (line 142)
* process-mark:                          Process Buffers.     (line  35)
* process-name:                          Process Information. (line 100)
* process-plist:                         Process Information. (line 194)
* process-put:                           Process Information. (line 190)
* process-query-on-exit-flag:            Query Before Exit.   (line  14)
* process-running-child-p:               Input to Processes.  (line  60)
* process-send-eof:                      Input to Processes.  (line  52)
* process-send-region:                   Input to Processes.  (line  44)
* process-send-string:                   Input to Processes.  (line  36)
* process-sentinel:                      Sentinels.           (line 107)
* process-status:                        Process Information. (line 103)
* process-tty-name:                      Process Information. (line 162)
* process-type:                          Process Information. (line 147)
* processing of errors:                  Processing of Errors.
                                                              (line   6)
* processor run time:                    Processor Run Time.  (line   6)
* processp:                              Processes.           (line  30)
* profile:                               Profiling.           (line   6)
* profiling:                             Profiling.           (line   6)
* prog-mode:                             Basic Major Modes.   (line  27)
* prog-mode, and bidi-paragraph-direction: Bidirectional Display.
                                                              (line  98)
* prog-mode-hook:                        Basic Major Modes.   (line  12)
* prog1:                                 Sequencing.          (line  47)
* prog2:                                 Sequencing.          (line  64)
* progn:                                 Sequencing.          (line  32)
* program arguments:                     Subprocess Creation. (line  57)
* program directories:                   Subprocess Creation. (line  84)
* program name, and default coding system: Default Coding Systems.
                                                              (line  57)
* programmed completion:                 Programmed Completion.
                                                              (line   6)
* programming conventions:               Programming Tips.    (line   6)
* programming types:                     Programming Types.   (line   6)
* progress reporting:                    Progress.            (line   6)
* progress-reporter-done:                Progress.            (line  90)
* progress-reporter-force-update:        Progress.            (line  79)
* progress-reporter-update:              Progress.            (line  60)
* prompt for file name:                  Reading File Names.  (line   6)
* prompt string (of menu):               Defining Menus.      (line   6)
* prompt string of keymap:               Format of Keymaps.   (line  57)
* properties of text:                    Text Properties.     (line   6)
* propertize:                            Changing Properties. (line 118)
* property category of text character:   Special Properties.  (line  15)
* property list:                         Property Lists.      (line   6)
* property list cell:                    Symbol Components.   (line  20)
* property lists vs association lists:   Plists and Alists.   (line   6)
* protect C variables from garbage collection: Writing Emacs Primitives.
                                                              (line 121)
* protected forms:                       Cleanups.            (line  13)
* provide:                               Named Features.      (line  73)
* provide-theme:                         Custom Themes.       (line  29)
* providing features:                    Named Features.      (line   6)
* pty:                                   Asynchronous Processes.
                                                              (line  15)
* pure storage:                          Pure Storage.        (line   6)
* pure-bytes-used:                       Pure Storage.        (line  39)
* purecopy:                              Pure Storage.        (line  28)
* purify-flag:                           Pure Storage.        (line  45)
* push:                                  List Variables.      (line   9)
* push-button:                           Button Buffer Commands.
                                                              (line  21)
* push-mark:                             The Mark.            (line 101)
* put:                                   Symbol Plists.       (line  19)
* put-char-code-property:                Character Properties.
                                                              (line 196)
* put-charset-property:                  Character Sets.      (line  55)
* put-image:                             Showing Images.      (line  43)
* put-text-property:                     Changing Properties. (line  18)
* puthash:                               Hash Access.         (line  15)
* query-font:                            Low-Level Font.      (line 229)
* query-replace-history:                 Minibuffer History.  (line  93)
* query-replace-map:                     Search and Replace.  (line  84)
* querying the user:                     Yes-or-No Queries.   (line   6)
* question mark in character constant:   Basic Char Syntax.   (line   6)
* quietly-read-abbrev-file:              Abbrev Files.        (line  22)
* QUIT, use in Lisp primitives:          Writing Emacs Primitives.
                                                              (line 130)
* quit-flag:                             Quitting.            (line  73)
* quit-process:                          Signals to Processes.
                                                              (line  65)
* quit-restore-window:                   Quitting Windows.    (line  28)
* quit-window:                           Quitting Windows.    (line  21)
* quitting:                              Quitting.            (line   6)
* quitting from infinite loop:           Infinite Loops.      (line   6)
* quote:                                 Quoting.             (line  12)
* quote character:                       Parser State.        (line  33)
* quote special characters in regexp:    Regexp Functions.    (line   8)
* quoted character input:                Quoted Character Input.
                                                              (line   6)
* quoted-insert suppression:             Changing Key Bindings.
                                                              (line 159)
* quoting and unquoting command-line arguments: Shell Arguments.
                                                              (line  40)
* quoting characters in printing:        Output Functions.    (line   9)
* quoting using apostrophe:              Quoting.             (line  15)
* radio, customization types:            Composite Types.     (line 172)
* radix for reading an integer:          Integer Basics.      (line  24)
* raise-frame:                           Raising and Lowering.
                                                              (line  12)
* raising a frame:                       Raising and Lowering.
                                                              (line   6)
* random:                                Random Numbers.      (line  27)
* random numbers:                        Random Numbers.      (line   6)
* rassoc:                                Association Lists.   (line  89)
* rassq:                                 Association Lists.   (line 133)
* rassq-delete-all:                      Association Lists.   (line 236)
* raw prefix argument:                   Prefix Command Arguments.
                                                              (line   6)
* raw prefix argument usage:             Interactive Codes.   (line 163)
* raw syntax descriptor:                 Syntax Table Internals.
                                                              (line  13)
* raw-text coding system:                Coding System Basics.
                                                              (line  52)
* re-builder:                            Regular Expressions. (line  11)
* re-search-backward:                    Regexp Search.       (line  67)
* re-search-forward:                     Regexp Search.       (line  16)
* read:                                  Input Functions.     (line  16)
* read command name:                     Interactive Call.    (line  99)
* read file names:                       Reading File Names.  (line   6)
* read input:                            Reading Input.       (line   6)
* read syntax:                           Printed Representation.
                                                              (line   6)
* read syntax for characters:            Basic Char Syntax.   (line   6)
* read-buffer:                           High-Level Completion.
                                                              (line  14)
* read-buffer-completion-ignore-case:    High-Level Completion.
                                                              (line  60)
* read-buffer-function:                  High-Level Completion.
                                                              (line  55)
* read-char:                             Reading One Event.   (line  62)
* read-char-choice:                      Reading One Event.   (line 116)
* read-char-exclusive:                   Reading One Event.   (line  85)
* read-circle:                           Input Functions.     (line  55)
* read-coding-system:                    User-Chosen Coding Systems.
                                                              (line  82)
* read-color:                            High-Level Completion.
                                                              (line 107)
* read-command:                          High-Level Completion.
                                                              (line  64)
* read-directory-name:                   Reading File Names.  (line 120)
* read-event:                            Reading One Event.   (line   9)
* read-expression-history:               Minibuffer History.  (line 112)
* read-file-modes:                       Changing Files.      (line 193)
* read-file-name:                        Reading File Names.  (line  12)
* read-file-name-completion-ignore-case: Reading File Names.  (line 116)
* read-file-name-function:               Reading File Names.  (line 110)
* read-from-minibuffer:                  Text from Minibuffer.
                                                              (line  18)
* read-from-string:                      Input Functions.     (line  21)
* read-input-method-name:                Input Methods.       (line  34)
* read-kbd-macro:                        Describing Characters.
                                                              (line  75)
* read-key:                              Reading One Event.   (line 105)
* read-key-sequence:                     Key Sequence Input.  (line  10)
* read-key-sequence-vector:              Key Sequence Input.  (line  67)
* read-minibuffer:                       Object from Minibuffer.
                                                              (line   9)
* read-no-blanks-input:                  Text from Minibuffer.
                                                              (line 199)
* read-non-nil-coding-system:            User-Chosen Coding Systems.
                                                              (line  88)
* read-only (text property):             Special Properties.  (line 132)
* read-only buffer:                      Read Only Buffers.   (line   6)
* read-only buffers in interactive:      Using Interactive.   (line  67)
* read-only character:                   Special Properties.  (line 132)
* read-only-mode:                        Read Only Buffers.   (line  46)
* read-passwd:                           Reading a Password.  (line   9)
* read-quoted-char:                      Quoted Character Input.
                                                              (line  11)
* read-quoted-char quitting:             Quitting.            (line  54)
* read-regexp:                           Text from Minibuffer.
                                                              (line 104)
* read-regexp-defaults-function:         Text from Minibuffer.
                                                              (line 150)
* read-shell-command:                    Reading File Names.  (line 171)
* read-string:                           Text from Minibuffer.
                                                              (line  77)
* read-variable:                         High-Level Completion.
                                                              (line 101)
* reading:                               Streams Intro.       (line   6)
* reading a single event:                Reading One Event.   (line   6)
* reading from files:                    Reading from Files.  (line   6)
* reading from minibuffer with completion: Minibuffer Completion.
                                                              (line   6)
* reading interactive arguments:         Interactive Codes.   (line  41)
* reading numbers in hex, octal, and binary: Integer Basics.  (line  24)
* reading order:                         Bidirectional Display.
                                                              (line  17)
* reading symbols:                       Creating Symbols.    (line   6)
* real-last-command:                     Command Loop Info.   (line  24)
* rearrangement of lists:                Rearrangement.       (line   6)
* rebinding:                             Changing Key Bindings.
                                                              (line   6)
* recent-auto-save-p:                    Auto-Saving.         (line 104)
* recent-keys:                           Recording Input.     (line   6)
* recenter:                              Textual Scrolling.   (line 173)
* recenter-positions:                    Textual Scrolling.   (line 219)
* recenter-redisplay:                    Textual Scrolling.   (line 208)
* recenter-top-bottom:                   Textual Scrolling.   (line 213)
* recenter-window-group:                 Textual Scrolling.   (line 197)
* recenter-window-group-function:        Textual Scrolling.   (line 197)
* recombining windows:                   Recombining Windows. (line   6)
* record command history:                Interactive Call.    (line  62)
* recording input:                       Recording Input.     (line   6)
* recursion:                             Iteration.           (line   6)
* recursion-depth:                       Recursive Editing.   (line 103)
* recursive command loop:                Recursive Editing.   (line   6)
* recursive editing level:               Recursive Editing.   (line   6)
* recursive evaluation:                  Intro Eval.          (line  30)
* recursive minibuffers:                 Recursive Mini.      (line   6)
* recursive-edit:                        Recursive Editing.   (line  61)
* redirect-frame-focus:                  Input Focus.         (line  84)
* redisplay:                             Forcing Redisplay.   (line  11)
* redo:                                  Undo.                (line   6)
* redraw-display:                        Refresh Screen.      (line  16)
* redraw-frame:                          Refresh Screen.      (line  10)
* reducing sequences:                    Sequence Functions.  (line 382)
* references, following:                 Key Binding Conventions.
                                                              (line   6)
* refresh the screen:                    Refresh Screen.      (line   6)
* regexp:                                Regular Expressions. (line   6)
* regexp alternative:                    Regexp Backslash.    (line  12)
* regexp grouping:                       Regexp Backslash.    (line  44)
* regexp searching:                      Regexp Search.       (line   6)
* regexp syntax:                         Syntax of Regexps.   (line   6)
* regexp, special characters in:         Regexp Special.      (line   6)
* regexp-history:                        Minibuffer History.  (line 103)
* regexp-opt:                            Regexp Functions.    (line  29)
* regexp-opt-charset:                    Regexp Functions.    (line  78)
* regexp-opt-depth:                      Regexp Functions.    (line  73)
* regexp-quote:                          Regexp Functions.    (line   8)
* regexps used standardly in editing:    Standard Regexps.    (line   6)
* region:                                The Region.          (line   6)
* region argument:                       Interactive Codes.   (line 167)
* region-beginning:                      The Region.          (line  15)
* region-end:                            The Region.          (line  20)
* register preview:                      Registers.           (line  90)
* register-alist:                        Registers.           (line  13)
* register-read-with-preview:            Registers.           (line  89)
* registers:                             Registers.           (line   6)
* regular expression:                    Regular Expressions. (line   6)
* regular expression searching:          Regexp Search.       (line   6)
* regular expressions, developing:       Regular Expressions. (line  11)
* reindent-then-newline-and-indent:      Mode-Specific Indent.
                                                              (line  57)
* relative file name:                    Relative File Names. (line   6)
* relative remapping, faces:             Face Remapping.      (line  39)
* remainder:                             Arithmetic Operations.
                                                              (line 111)
* remapping commands:                    Remapping Commands.  (line   6)
* remhash:                               Hash Access.         (line  20)
* remote-file-name-inhibit-cache:        Magic File Names.    (line 212)
* remove:                                Sets And Lists.      (line 158)
* remove-from-invisibility-spec:         Invisible Text.      (line  69)
* remove-function:                       Core Advising Primitives.
                                                              (line  72)
* remove-hook:                           Setting Hooks.       (line  46)
* remove-images:                         Showing Images.      (line  62)
* remove-list-of-text-properties:        Changing Properties. (line  64)
* remove-overlays:                       Managing Overlays.   (line  75)
* remove-text-properties:                Changing Properties. (line  43)
* removing from sequences:               Sequence Functions.  (line 373)
* remq:                                  Sets And Lists.      (line  79)
* rename-auto-save-file:                 Auto-Saving.         (line 167)
* rename-buffer:                         Buffer Names.        (line  37)
* rename-file:                           Changing Files.      (line  74)
* rendering html:                        Parsing HTML/XML.    (line  42)
* reordering, of bidirectional text:     Bidirectional Display.
                                                              (line  17)
* reordering, of elements in lists:      Rearrangement.       (line   6)
* repeat events:                         Repeat Events.       (line   6)
* repeated loading:                      Repeated Loading.    (line   6)
* replace bindings:                      Changing Key Bindings.
                                                              (line 112)
* replace characters:                    Substitution.        (line  11)
* replace characters in region:          Substitution.        (line   6)
* replace list element:                  Setcar.              (line   6)
* replace matched text:                  Replacing Match.     (line   6)
* replace part of list:                  Setcdr.              (line   6)
* replace-buffer-in-windows:             Buffers and Windows. (line  88)
* replace-match:                         Replacing Match.     (line   9)
* replace-re-search-function:            Search and Replace.  (line 179)
* replace-regexp-in-string:              Search and Replace.  (line  19)
* replace-search-function:               Search and Replace.  (line 172)
* replacement after search:              Search and Replace.  (line   6)
* replacing display specs:               Replacing Specs.     (line   6)
* require:                               Named Features.      (line 106)
* require, customization keyword:        Common Keywords.     (line 100)
* require-final-newline:                 Saving Buffers.      (line 154)
* requiring features:                    Named Features.      (line   6)
* reserved keys:                         Key Binding Conventions.
                                                              (line  14)
* resize window:                         Resizing Windows.    (line   6)
* resize-mini-windows:                   Minibuffer Windows.  (line  43)
* rest arguments:                        Argument List.       (line  18)
* restore-buffer-modified-p:             Buffer Modification. (line  37)
* restricted-sexp, customization types:  Composite Types.     (line 253)
* restriction (in a buffer):             Narrowing.           (line   6)
* resume (cf. no-redraw-on-reenter):     Refresh Screen.      (line  31)
* resume-tty:                            Suspending Emacs.    (line  93)
* resume-tty-functions:                  Suspending Emacs.    (line  98)
* rethrow a signal:                      Handling Errors.     (line 139)
* return (ASCII character):              Basic Char Syntax.   (line  27)
* return value:                          What Is a Function.  (line   6)
* reverse:                               Sequence Functions.  (line 111)
* reversing a list:                      Sequence Functions.  (line 137)
* reversing a string:                    Sequence Functions.  (line 137)
* reversing a vector:                    Sequence Functions.  (line 137)
* revert-buffer:                         Reverting.           (line  11)
* revert-buffer-function:                Reverting.           (line  59)
* revert-buffer-in-progress-p:           Reverting.           (line  41)
* revert-buffer-insert-file-contents-function: Reverting.     (line  70)
* revert-without-query:                  Reverting.           (line  48)
* reverting buffers:                     Reverting.           (line   6)
* rgb value:                             Color Names.         (line  68)
* right dividers:                        Window Dividers.     (line   6)
* right-divider-width, a frame parameter: Layout Parameters.  (line  44)
* right-fringe, a frame parameter:       Layout Parameters.   (line  33)
* right-fringe-width:                    Fringe Size/Pos.     (line  19)
* right-margin-width:                    Display Margins.     (line  36)
* right-to-left text:                    Bidirectional Display.
                                                              (line   6)
* ring data structure:                   Rings.               (line   6)
* ring-bell-function:                    Beeping.             (line  27)
* ring-copy:                             Rings.               (line  32)
* ring-elements:                         Rings.               (line  29)
* ring-empty-p:                          Rings.               (line  36)
* ring-insert:                           Rings.               (line  49)
* ring-insert-at-beginning:              Rings.               (line  62)
* ring-length:                           Rings.               (line  25)
* ring-p:                                Rings.               (line  19)
* ring-ref:                              Rings.               (line  44)
* ring-remove:                           Rings.               (line  56)
* ring-size:                             Rings.               (line  22)
* risky, defcustom keyword:              Variable Definitions.
                                                              (line 146)
* risky-local-variable-p:                File Local Variables.
                                                              (line 131)
* RLO:                                   Bidirectional Display.
                                                              (line 188)
* rm:                                    Changing Files.      (line 121)
* root window:                           Windows and Frames.  (line  29)
* round:                                 Numeric Conversions. (line  66)
* rounding in conversions:               Numeric Conversions. (line   6)
* rounding without conversion:           Rounding Operations. (line   6)
* rplaca:                                Modifying Lists.     (line  10)
* rplacd:                                Modifying Lists.     (line  10)
* run time stack:                        Internals of Debugger.
                                                              (line  21)
* run-at-time:                           Timers.              (line  47)
* run-hook-with-args:                    Running Hooks.       (line  29)
* run-hook-with-args-until-failure:      Running Hooks.       (line  33)
* run-hook-with-args-until-success:      Running Hooks.       (line  40)
* run-hooks:                             Running Hooks.       (line  10)
* run-mode-hooks:                        Mode Hooks.          (line  28)
* run-with-idle-timer:                   Idle Timers.         (line  10)
* S-expression:                          Intro Eval.          (line  12)
* safe local variable:                   File Local Variables.
                                                              (line  87)
* safe, defcustom keyword:               Variable Definitions.
                                                              (line 150)
* safe-length:                           List Elements.       (line 121)
* safe-local-eval-forms:                 File Local Variables.
                                                              (line 150)
* safe-local-variable-p:                 File Local Variables.
                                                              (line 110)
* safe-local-variable-values:            File Local Variables.
                                                              (line  99)
* safe-magic (property):                 Magic File Names.    (line 122)
* safely encode a string:                Lisp and Coding Systems.
                                                              (line  73)
* safely encode characters in a charset: Lisp and Coding Systems.
                                                              (line  80)
* safely encode region:                  Lisp and Coding Systems.
                                                              (line  64)
* safety of functions:                   Function Safety.     (line   6)
* same-window-buffer-names:              Choosing Window Options.
                                                              (line 112)
* same-window-p:                         Choosing Window Options.
                                                              (line 124)
* same-window-regexps:                   Choosing Window Options.
                                                              (line 118)
* save abbrevs in files:                 Abbrev Files.        (line   6)
* save-abbrevs:                          Abbrev Files.        (line  29)
* save-buffer:                           Saving Buffers.      (line  12)
* save-buffer-coding-system:             Encoding and I/O.    (line  34)
* save-current-buffer:                   Current Buffer.      (line 100)
* save-excursion:                        Excursions.          (line  20)
* save-mark-and-excursion:               Excursions.          (line  58)
* save-match-data:                       Saving Match Data.   (line  19)
* save-restriction:                      Narrowing.           (line  57)
* save-selected-window:                  Selecting Windows.   (line  36)
* save-some-buffers:                     Saving Buffers.      (line  34)
* save-window-excursion:                 Window Configurations.
                                                              (line  54)
* SaveUnder feature:                     Display Feature Testing.
                                                              (line 126)
* saving buffers:                        Saving Buffers.      (line   6)
* saving text properties:                Format Conversion.   (line   6)
* saving window information:             Window Configurations.
                                                              (line   6)
* scalability of overlays:               Overlays.            (line  12)
* scalable fonts:                        Font Selection.      (line  65)
* scalable-fonts-allowed:                Font Selection.      (line  68)
* scan-lists:                            Motion via Parsing.  (line   9)
* scan-sexps:                            Motion via Parsing.  (line  29)
* scanning expressions:                  Parsing Expressions. (line   6)
* scanning for character sets:           Scanning Charsets.   (line   6)
* scanning keymaps:                      Scanning Keymaps.    (line   6)
* scope:                                 Variable Scoping.    (line  10)
* scoping rule:                          Variable Scoping.    (line   6)
* screen layout:                         Frame Configuration Type.
                                                              (line   6)
* screen lines, moving by:               Screen Lines.        (line   6)
* screen of terminal:                    Basic Windows.       (line  17)
* screen refresh:                        Refresh Screen.      (line   6)
* screen-gamma, a frame parameter:       Font and Color Parameters.
                                                              (line  36)
* script symbols:                        Character Properties.
                                                              (line 206)
* scroll bar events, data in:            Accessing Scroll.    (line   6)
* scroll bars:                           Scroll Bars.         (line   6)
* scroll-bar-background, a frame parameter: Font and Color Parameters.
                                                              (line 106)
* scroll-bar-event-ratio:                Accessing Scroll.    (line   8)
* scroll-bar-foreground, a frame parameter: Font and Color Parameters.
                                                              (line 102)
* scroll-bar-height:                     Scroll Bars.         (line 121)
* scroll-bar-height, a frame parameter:  Layout Parameters.   (line  29)
* scroll-bar-mode:                       Scroll Bars.         (line 130)
* scroll-bar-scale:                      Accessing Scroll.    (line  14)
* scroll-bar-width:                      Scroll Bars.         (line 116)
* scroll-bar-width, a frame parameter:   Layout Parameters.   (line  25)
* scroll-command property:               Textual Scrolling.   (line 145)
* scroll-conservatively:                 Textual Scrolling.   (line 108)
* scroll-down:                           Textual Scrolling.   (line  49)
* scroll-down-aggressively:              Textual Scrolling.   (line 121)
* scroll-down-command:                   Textual Scrolling.   (line  63)
* scroll-error-top-bottom:               Textual Scrolling.   (line 164)
* scroll-left:                           Horizontal Scrolling.
                                                              (line  47)
* scroll-margin:                         Textual Scrolling.   (line 100)
* scroll-other-window:                   Textual Scrolling.   (line  70)
* scroll-preserve-screen-position:       Textual Scrolling.   (line 145)
* scroll-right:                          Horizontal Scrolling.
                                                              (line  71)
* scroll-step:                           Textual Scrolling.   (line 139)
* scroll-up:                             Textual Scrolling.   (line  37)
* scroll-up-aggressively:                Textual Scrolling.   (line 134)
* scroll-up-command:                     Textual Scrolling.   (line  56)
* scrolling textually:                   Textual Scrolling.   (line   6)
* search-backward:                       String Search.       (line  60)
* search-failed:                         String Search.       (line  43)
* search-forward:                        String Search.       (line  17)
* search-map:                            Prefix Keys.         (line  45)
* search-spaces-regexp:                  Regexp Search.       (line 176)
* searching:                             Searching and Matching.
                                                              (line   6)
* searching active keymaps for keys:     Searching Keymaps.   (line   6)
* searching and case:                    Searching and Case.  (line   6)
* searching and replacing:               Search and Replace.  (line   6)
* searching for overlays:                Finding Overlays.    (line   6)
* searching for regexp:                  Regexp Search.       (line   6)
* searching text properties:             Property Search.     (line   6)
* secondary selection:                   Window System Selections.
                                                              (line   6)
* seconds-to-time:                       Time of Day.         (line  63)
* secure-hash:                           Checksum/Hash.       (line  21)
* security:                              Security Considerations.
                                                              (line   6)
* seed, for random number generation:    Random Numbers.      (line  13)
* select safe coding system:             User-Chosen Coding Systems.
                                                              (line   6)
* select-frame:                          Input Focus.         (line  49)
* select-frame-set-input-focus:          Input Focus.         (line  41)
* select-safe-coding-system:             User-Chosen Coding Systems.
                                                              (line   6)
* select-safe-coding-system-accept-default-p: User-Chosen Coding Systems.
                                                              (line  51)
* select-safe-coding-system-function:    User-Chosen Coding Systems.
                                                              (line  68)
* select-window:                         Selecting Windows.   (line   6)
* selected window:                       Basic Windows.       (line  59)
* selected-frame:                        Input Focus.         (line  19)
* selected-window:                       Basic Windows.       (line  69)
* selected-window-group:                 Basic Windows.       (line  82)
* selected-window-group-function:        Basic Windows.       (line  83)
* selecting a buffer:                    Current Buffer.      (line   6)
* selecting a font:                      Font Selection.      (line   6)
* selecting a window:                    Selecting Windows.   (line   6)
* selection (for window systems):        Window System Selections.
                                                              (line   6)
* selection-coding-system:               Window System Selections.
                                                              (line  51)
* selective-display:                     Selective Display.   (line  38)
* selective-display-ellipses:            Selective Display.   (line  84)
* self-evaluating form:                  Self-Evaluating Forms.
                                                              (line   6)
* self-insert-and-exit:                  Minibuffer Commands. (line  12)
* self-insert-command:                   Commands for Insertion.
                                                              (line  16)
* self-insert-command override:          Changing Key Bindings.
                                                              (line 149)
* self-insert-command, minor modes:      Keymaps and Minor Modes.
                                                              (line  11)
* self-insertion:                        Commands for Insertion.
                                                              (line  17)
* SELinux context:                       Extended Attributes. (line  14)
* send-string-to-terminal:               Terminal Output.     (line  30)
* sending signals:                       Signals to Processes.
                                                              (line   6)
* sentence-end:                          Standard Regexps.    (line  37)
* sentence-end <1>:                      Standard Regexps.    (line  48)
* sentence-end-double-space:             Filling.             (line 132)
* sentence-end-without-period:           Filling.             (line 137)
* sentence-end-without-space:            Filling.             (line 142)
* sentinel (of process):                 Sentinels.           (line   6)
* seq library:                           Sequence Functions.  (line 254)
* seq-concatenate:                       Sequence Functions.  (line 496)
* seq-contains:                          Sequence Functions.  (line 452)
* seq-count:                             Sequence Functions.  (line 440)
* seq-difference:                        Sequence Functions.  (line 532)
* seq-do:                                Sequence Functions.  (line 338)
* seq-doseq:                             Sequence Functions.  (line 581)
* seq-drop:                              Sequence Functions.  (line 302)
* seq-drop-while:                        Sequence Functions.  (line 329)
* seq-elt:                               Sequence Functions.  (line 269)
* seq-empty-p:                           Sequence Functions.  (line 432)
* seq-every-p:                           Sequence Functions.  (line 423)
* seq-filter:                            Sequence Functions.  (line 363)
* seq-find:                              Sequence Functions.  (line 409)
* seq-group-by:                          Sequence Functions.  (line 541)
* seq-intersection:                      Sequence Functions.  (line 523)
* seq-into:                              Sequence Functions.  (line 551)
* seq-length:                            Sequence Functions.  (line 287)
* seq-let:                               Sequence Functions.  (line 586)
* seq-map:                               Sequence Functions.  (line 342)
* seq-mapcat:                            Sequence Functions.  (line 506)
* seq-mapn:                              Sequence Functions.  (line 351)
* seq-max:                               Sequence Functions.  (line 572)
* seq-min:                               Sequence Functions.  (line 563)
* seq-partition:                         Sequence Functions.  (line 514)
* seq-position:                          Sequence Functions.  (line 462)
* seq-reduce:                            Sequence Functions.  (line 381)
* seq-remove:                            Sequence Functions.  (line 372)
* seq-some:                              Sequence Functions.  (line 396)
* seq-sort:                              Sequence Functions.  (line 447)
* seq-subseq:                            Sequence Functions.  (line 484)
* seq-take:                              Sequence Functions.  (line 311)
* seq-take-while:                        Sequence Functions.  (line 320)
* seq-uniq:                              Sequence Functions.  (line 473)
* seqp:                                  Sequence Functions.  (line 292)
* sequence:                              Sequences Arrays Vectors.
                                                              (line   6)
* sequence destructuring:                Sequence Functions.  (line 587)
* sequence functions in seq:             Sequence Functions.  (line 254)
* sequence iteration:                    Sequence Functions.  (line 582)
* sequence length:                       Sequence Functions.  (line  13)
* sequence reverse:                      Sequence Functions.  (line 112)
* sequencep:                             Sequence Functions.  (line   8)
* sequencing:                            Sequencing.          (line   6)
* sequential execution:                  Sequencing.          (line   6)
* serial connections:                    Serial Ports.        (line   6)
* serial-process-configure:              Serial Ports.        (line 113)
* serial-term:                           Serial Ports.        (line  31)
* serializing:                           Byte Packing.        (line  13)
* session file:                          Session Management.  (line  20)
* session manager:                       Session Management.  (line   6)
* set:                                   Setting Variables.   (line  41)
* set, defcustom keyword:                Variable Definitions.
                                                              (line  76)
* set-advertised-calling-convention:     Obsolete Functions.  (line  51)
* set-after, defcustom keyword:          Variable Definitions.
                                                              (line 154)
* set-auto-coding:                       Default Coding Systems.
                                                              (line 135)
* set-auto-mode:                         Auto Major Mode.     (line  37)
* set-binary-mode:                       Input Functions.     (line  68)
* set-buffer:                            Current Buffer.      (line  23)
* set-buffer-auto-saved:                 Auto-Saving.         (line 108)
* set-buffer-major-mode:                 Auto Major Mode.     (line  69)
* set-buffer-modified-p:                 Buffer Modification. (line  27)
* set-buffer-multibyte:                  Selecting a Representation.
                                                              (line   9)
* set-case-syntax:                       Case Tables.         (line 108)
* set-case-syntax-delims:                Case Tables.         (line 104)
* set-case-syntax-pair:                  Case Tables.         (line 100)
* set-case-table:                        Case Tables.         (line  72)
* set-category-table:                    Categories.          (line  92)
* set-char-table-extra-slot:             Char-Tables.         (line  77)
* set-char-table-parent:                 Char-Tables.         (line  69)
* set-char-table-range:                  Char-Tables.         (line  99)
* set-charset-priority:                  Character Sets.      (line  35)
* set-coding-system-priority:            Specifying Coding Systems.
                                                              (line  78)
* set-default:                           Default Value.       (line  76)
* set-default-file-modes:                Changing Files.      (line 164)
* set-default-toplevel-value:            Default Value.       (line 105)
* set-display-table-slot:                Display Tables.      (line  79)
* set-face-attribute:                    Attribute Functions. (line  74)
* set-face-background:                   Attribute Functions. (line 101)
* set-face-bold:                         Attribute Functions. (line 111)
* set-face-font:                         Attribute Functions. (line 108)
* set-face-foreground:                   Attribute Functions. (line 100)
* set-face-inverse-video:                Attribute Functions. (line 122)
* set-face-italic:                       Attribute Functions. (line 115)
* set-face-stipple:                      Attribute Functions. (line 105)
* set-face-underline:                    Attribute Functions. (line 119)
* set-file-acl:                          Changing Files.      (line 244)
* set-file-extended-attributes:          Changing Files.      (line 226)
* set-file-modes:                        Changing Files.      (line 140)
* set-file-selinux-context:              Changing Files.      (line 233)
* set-file-times:                        Changing Files.      (line 220)
* set-fontset-font:                      Fontsets.            (line  83)
* set-frame-configuration:               Frame Configurations.
                                                              (line  14)
* set-frame-font:                        Frame Font.          (line  27)
* set-frame-height:                      Size and Position.   (line 101)
* set-frame-parameter:                   Parameter Access.    (line  49)
* set-frame-position:                    Size and Position.   (line  20)
* set-frame-selected-window:             Selecting Windows.   (line  69)
* set-frame-size:                        Size and Position.   (line  90)
* set-frame-width:                       Size and Position.   (line 122)
* set-fringe-bitmap-face:                Customizing Bitmaps. (line  37)
* set-input-method:                      Input Methods.       (line  28)
* set-input-mode:                        Input Modes.         (line   6)
* set-keyboard-coding-system:            Terminal I/O Encoding.
                                                              (line  17)
* set-keymap-parent:                     Inheritance and Keymaps.
                                                              (line  31)
* set-left-margin:                       Margins.             (line  33)
* set-mark:                              The Mark.            (line  79)
* set-marker:                            Moving Markers.      (line  11)
* set-marker-insertion-type:             Marker Insertion Types.
                                                              (line  13)
* set-match-data:                        Entire Match Data.   (line  49)
* set-minibuffer-window:                 Minibuffer Windows.  (line  20)
* set-mouse-absolute-pixel-position:     Mouse Position.      (line  56)
* set-mouse-pixel-position:              Mouse Position.      (line  40)
* set-mouse-position:                    Mouse Position.      (line  25)
* set-network-process-option:            Network Options.     (line  66)
* set-process-buffer:                    Process Buffers.     (line  55)
* set-process-coding-system:             Process Information. (line 176)
* set-process-datagram-address:          Datagrams.           (line  24)
* set-process-filter:                    Filter Functions.    (line  79)
* set-process-plist:                     Process Information. (line 197)
* set-process-query-on-exit-flag:        Query Before Exit.   (line  17)
* set-process-sentinel:                  Sentinels.           (line  87)
* set-process-window-size:               Process Buffers.     (line  84)
* set-register:                          Registers.           (line  61)
* set-right-margin:                      Margins.             (line  38)
* set-standard-case-table:               Case Tables.         (line  62)
* set-syntax-table:                      Syntax Table Functions.
                                                              (line  91)
* set-terminal-coding-system:            Terminal I/O Encoding.
                                                              (line  32)
* set-terminal-parameter:                Terminal Parameters. (line  24)
* set-text-properties:                   Changing Properties. (line  70)
* set-transient-map:                     Controlling Active Maps.
                                                              (line 147)
* set-visited-file-modtime:              Modification Time.   (line  60)
* set-visited-file-name:                 Buffer File Name.    (line  84)
* set-window-buffer:                     Buffers and Windows. (line  15)
* set-window-combination-limit:          Recombining Windows. (line 186)
* set-window-configuration:              Window Configurations.
                                                              (line  26)
* set-window-dedicated-p:                Dedicated Windows.   (line  43)
* set-window-display-table:              Active Display Table.
                                                              (line  20)
* set-window-fringes:                    Fringe Size/Pos.     (line  36)
* set-window-group-start:                Window Start and End.
                                                              (line 134)
* set-window-group-start-function:       Window Start and End.
                                                              (line 134)
* set-window-hscroll:                    Horizontal Scrolling.
                                                              (line  94)
* set-window-margins:                    Display Margins.     (line  48)
* set-window-next-buffers:               Window History.      (line  45)
* set-window-parameter:                  Window Parameters.   (line  20)
* set-window-point:                      Window Point.        (line  43)
* set-window-prev-buffers:               Window History.      (line  29)
* set-window-scroll-bars:                Scroll Bars.         (line  41)
* set-window-start:                      Window Start and End.
                                                              (line  80)
* set-window-vscroll:                    Vertical Scrolling.  (line  34)
* set-xwidget-plist:                     Xwidgets.            (line  35)
* set-xwidget-query-on-exit-flag:        Xwidgets.            (line  80)
* setcar:                                Setcar.              (line   9)
* setcdr:                                Setcdr.              (line   8)
* setenv:                                System Environment.  (line 104)
* setf:                                  Setting Generalized Variables.
                                                              (line  13)
* setplist:                              Symbol Plists.       (line  36)
* setq:                                  Setting Variables.   (line  10)
* setq-default:                          Default Value.       (line  35)
* setq-local:                            Creating Buffer-Local.
                                                              (line  45)
* sets:                                  Sets And Lists.      (line   6)
* setting modes of files:                Changing Files.      (line   6)
* setting-constant error:                Constant Variables.  (line   6)
* severity level:                        Warning Basics.      (line   6)
* sexp:                                  Intro Eval.          (line  12)
* sexp motion:                           List Motion.         (line   6)
* SHA hash:                              Checksum/Hash.       (line   6)
* shadowed Lisp files:                   Library Search.      (line 111)
* shadowing of variables:                Local Variables.     (line  22)
* shared structure, read syntax:         Circular Objects.    (line   6)
* shell command arguments:               Shell Arguments.     (line   6)
* shell-command-history:                 Minibuffer History.  (line 109)
* shell-command-to-string:               Synchronous Processes.
                                                              (line 258)
* shell-quote-argument:                  Shell Arguments.     (line  13)
* shift-selection, and interactive spec: Using Interactive.   (line  75)
* shift-translation:                     Key Sequence Input.  (line  73)
* show image:                            Showing Images.      (line   6)
* show-help-function:                    Special Properties.  (line 354)
* shr-insert-document:                   Parsing HTML/XML.    (line  42)
* shrink-window-if-larger-than-buffer:   Resizing Windows.    (line 193)
* shy groups:                            Regexp Backslash.    (line  66)
* sibling window:                        Windows and Frames.  (line  50)
* side effect:                           Intro Eval.          (line  47)
* SIGHUP:                                Killing Emacs.       (line  22)
* SIGINT:                                Killing Emacs.       (line  22)
* signal:                                Signaling Errors.    (line  56)
* signal-process:                        Signals to Processes.
                                                              (line  94)
* signaling errors:                      Signaling Errors.    (line   6)
* signals:                               Signals to Processes.
                                                              (line   6)
* SIGTERM:                               Killing Emacs.       (line  22)
* SIGTSTP:                               Suspending Emacs.    (line  20)
* sigusr1 event:                         Misc Events.         (line  66)
* sigusr2 event:                         Misc Events.         (line  66)
* simple package:                        Simple Packages.     (line   6)
* sin:                                   Math Functions.      (line   9)
* single file package:                   Simple Packages.     (line   6)
* single-function hook:                  Hooks.               (line  41)
* single-key-description:                Describing Characters.
                                                              (line  28)
* sit-for:                               Waiting.             (line  12)
* site-init.el:                          Building Emacs.      (line  55)
* site-lisp directories:                 Library Search.      (line  34)
* site-load.el:                          Building Emacs.      (line  38)
* site-run-file:                         Init File.           (line  36)
* site-start.el:                         Startup Summary.     (line  59)
* size of frame:                         Frame Geometry.      (line   6)
* size of image:                         Showing Images.      (line  72)
* size of text on display:               Size of Displayed Text.
                                                              (line   6)
* size of window:                        Window Sizes.        (line   6)
* skip-chars-backward:                   Skipping Characters. (line  52)
* skip-chars-forward:                    Skipping Characters. (line  14)
* skip-syntax-backward:                  Motion and Syntax.   (line  22)
* skip-syntax-forward:                   Motion and Syntax.   (line   9)
* skipping characters:                   Skipping Characters. (line   6)
* skipping characters of certain syntax: Motion and Syntax.   (line   6)
* skipping comments:                     Control Parsing.     (line  13)
* sleep-for:                             Waiting.             (line  41)
* slice, image:                          Showing Images.      (line  34)
* small-temporary-file-directory:        Unique File Names.   (line  69)
* smallest Lisp integer:                 Integer Basics.      (line  84)
* SMIE:                                  SMIE.                (line   6)
* SMIE grammar:                          SMIE Grammar.        (line   6)
* SMIE lexer:                            SMIE Lexer.          (line   6)
* smie-bnf->prec2:                       Operator Precedence Grammars.
                                                              (line  37)
* smie-close-block:                      SMIE setup.          (line  32)
* smie-config:                           SMIE Customization.  (line  12)
* smie-config-guess:                     SMIE Customization.  (line  18)
* smie-config-local:                     SMIE Customization.  (line  35)
* smie-config-save:                      SMIE Customization.  (line  23)
* smie-config-set-indent:                SMIE Customization.  (line  31)
* smie-config-show-indent:               SMIE Customization.  (line  27)
* smie-down-list:                        SMIE setup.          (line  36)
* smie-merge-prec2s:                     Operator Precedence Grammars.
                                                              (line  24)
* smie-prec2->grammar:                   Operator Precedence Grammars.
                                                              (line  19)
* smie-precs->prec2:                     Operator Precedence Grammars.
                                                              (line  28)
* smie-rule-bolp:                        SMIE Indentation Helpers.
                                                              (line  11)
* smie-rule-hanging-p:                   SMIE Indentation Helpers.
                                                              (line  14)
* smie-rule-next-p:                      SMIE Indentation Helpers.
                                                              (line  19)
* smie-rule-parent:                      SMIE Indentation Helpers.
                                                              (line  33)
* smie-rule-parent-p:                    SMIE Indentation Helpers.
                                                              (line  25)
* smie-rule-prev-p:                      SMIE Indentation Helpers.
                                                              (line  22)
* smie-rule-separator:                   SMIE Indentation Helpers.
                                                              (line  38)
* smie-rule-sibling-p:                   SMIE Indentation Helpers.
                                                              (line  28)
* smie-setup:                            SMIE setup.          (line  11)
* Snarf-documentation:                   Accessing Documentation.
                                                              (line 147)
* sort:                                  Sequence Functions.  (line 182)
* sort-columns:                          Sorting.             (line 207)
* sort-fields:                           Sorting.             (line 185)
* sort-fold-case:                        Sorting.             (line 106)
* sort-lines:                            Sorting.             (line 172)
* sort-numeric-base:                     Sorting.             (line 203)
* sort-numeric-fields:                   Sorting.             (line 192)
* sort-pages:                            Sorting.             (line 181)
* sort-paragraphs:                       Sorting.             (line 176)
* sort-regexp-fields:                    Sorting.             (line 110)
* sort-subr:                             Sorting.             (line  11)
* sorting lists:                         Sequence Functions.  (line 183)
* sorting sequences:                     Sequence Functions.  (line 447)
* sorting text:                          Sorting.             (line   6)
* sorting vectors:                       Sequence Functions.  (line 183)
* sound:                                 Sound Output.        (line   6)
* source breakpoints:                    Source Breakpoints.  (line   6)
* space (ASCII character):               Basic Char Syntax.   (line  27)
* space display spec, and bidirectional text: Bidirectional Display.
                                                              (line 156)
* spaces, pixel specification:           Pixel Specification. (line   6)
* spaces, specified height or width:     Specified Space.     (line   6)
* sparse keymap:                         Format of Keymaps.   (line   6)
* <SPC> in minibuffer:                   Text from Minibuffer.
                                                              (line 227)
* special events:                        Special Events.      (line   6)
* special form descriptions:             A Sample Function Description.
                                                              (line   6)
* special forms:                         Special Forms.       (line   6)
* special forms for control structures:  Control Structures.  (line   6)
* special modes:                         Major Mode Conventions.
                                                              (line 189)
* special variables:                     Using Lexical Binding.
                                                              (line  24)
* special-event-map:                     Controlling Active Maps.
                                                              (line 132)
* special-form-p:                        Special Forms.       (line  23)
* special-mode:                          Basic Major Modes.   (line  36)
* special-variable-p:                    Using Lexical Binding.
                                                              (line  30)
* specify coding system:                 Specifying Coding Systems.
                                                              (line   6)
* specify color:                         Color Names.         (line   6)
* speedups:                              Compilation Tips.    (line   6)
* splicing (with backquote):             Backquote.           (line  29)
* split-height-threshold:                Choosing Window Options.
                                                              (line  50)
* split-string:                          Creating Strings.    (line 128)
* split-string-and-unquote:              Shell Arguments.     (line  52)
* split-string-default-separators:       Creating Strings.    (line 207)
* split-width-threshold:                 Choosing Window Options.
                                                              (line  57)
* split-window:                          Splitting Windows.   (line   9)
* split-window-below:                    Splitting Windows.   (line 125)
* split-window-keep-point:               Splitting Windows.   (line 131)
* split-window-preferred-function:       Choosing Window Options.
                                                              (line  22)
* split-window-right:                    Splitting Windows.   (line 119)
* split-window-sensibly:                 Choosing Window Options.
                                                              (line  34)
* splitting windows:                     Splitting Windows.   (line   6)
* sqrt:                                  Math Functions.      (line  46)
* stable sort:                           Sequence Functions.  (line 183)
* stack allocated Lisp objects:          Stack-allocated Objects.
                                                              (line   6)
* standard abbrev tables:                Standard Abbrev Tables.
                                                              (line   6)
* standard colors for character terminals: Font and Color Parameters.
                                                              (line  23)
* standard errors:                       Standard Errors.     (line   6)
* standard hooks:                        Standard Hooks.      (line   6)
* standard regexps used in editing:      Standard Regexps.    (line   6)
* standard syntax table:                 Syntax Basics.       (line  36)
* standard-case-table:                   Case Tables.         (line  66)
* standard-category-table:               Categories.          (line  83)
* standard-display-table:                Active Display Table.
                                                              (line  29)
* standard-input:                        Input Functions.     (line  50)
* standard-output:                       Output Variables.    (line   6)
* standard-syntax-table:                 Syntax Basics.       (line  44)
* standard-translation-table-for-decode: Translation of Characters.
                                                              (line  46)
* standard-translation-table-for-encode: Translation of Characters.
                                                              (line  51)
* standards of coding style:             Tips.                (line   6)
* start-file-process:                    Asynchronous Processes.
                                                              (line 208)
* start-file-process-shell-command:      Asynchronous Processes.
                                                              (line 247)
* start-process:                         Asynchronous Processes.
                                                              (line 162)
* start-process, command-line arguments from minibuffer: Shell Arguments.
                                                              (line  40)
* start-process-shell-command:           Asynchronous Processes.
                                                              (line 231)
* STARTTLS network connections:          Network.             (line  55)
* startup of Emacs:                      Startup Summary.     (line   6)
* startup screen:                        Startup Summary.     (line 131)
* startup.el:                            Startup Summary.     (line   6)
* staticpro, protection from GC:         Writing Emacs Primitives.
                                                              (line 172)
* sticky text properties:                Sticky Properties.   (line   6)
* sticky, a frame parameter:             Management Parameters.
                                                              (line  49)
* stop points:                           Using Edebug.        (line  36)
* stop-process:                          Signals to Processes.
                                                              (line  70)
* stopbits, in serial connections:       Serial Ports.        (line 114)
* stopping an infinite loop:             Infinite Loops.      (line   6)
* stopping on events:                    Global Break Condition.
                                                              (line   6)
* storage of vector-like Lisp objects:   Garbage Collection.  (line  16)
* store-match-data:                      Entire Match Data.   (line  62)
* store-substring:                       Modifying Strings.   (line  14)
* stream (for printing):                 Output Streams.      (line   6)
* stream (for reading):                  Input Streams.       (line   6)
* strike-through text:                   Face Attributes.     (line 114)
* string:                                Creating Strings.    (line  21)
* string creation:                       Creating Strings.    (line   6)
* string equality:                       Text Comparison.     (line   6)
* string in keymap:                      Key Lookup.          (line  46)
* string input stream:                   Input Streams.       (line  22)
* string length:                         Sequence Functions.  (line  13)
* string modification:                   Modifying Strings.   (line   6)
* string predicates:                     Predicates for Strings.
                                                              (line   6)
* string reverse:                        Sequence Functions.  (line 112)
* string search:                         String Search.       (line   6)
* string to number:                      String Conversion.   (line  36)
* string to object:                      Input Functions.     (line  22)
* string, number of bytes:               Text Representations.
                                                              (line 120)
* string, writing a doc string:          Documentation Basics.
                                                              (line   6)
* string-as-multibyte:                   Selecting a Representation.
                                                              (line  45)
* string-as-unibyte:                     Selecting a Representation.
                                                              (line  37)
* string-bytes:                          Text Representations.
                                                              (line 119)
* string-chars-consed:                   Memory Usage.        (line  29)
* string-collate-equalp:                 Text Comparison.     (line  55)
* string-collate-lessp:                  Text Comparison.     (line 143)
* string-equal:                          Text Comparison.     (line  52)
* string-greaterp:                       Text Comparison.     (line 138)
* string-lessp:                          Text Comparison.     (line 135)
* string-match:                          Regexp Search.       (line  86)
* string-match-p:                        Regexp Search.       (line 115)
* string-or-null-p:                      Predicates for Strings.
                                                              (line  12)
* string-prefix-p:                       Text Comparison.     (line 182)
* string-suffix-p:                       Text Comparison.     (line 187)
* string-to-char:                        String Conversion.   (line  70)
* string-to-int:                         String Conversion.   (line  63)
* string-to-multibyte:                   Converting Representations.
                                                              (line  42)
* string-to-number:                      String Conversion.   (line  35)
* string-to-syntax:                      Syntax Table Internals.
                                                              (line  45)
* string-to-unibyte:                     Converting Representations.
                                                              (line  51)
* string-width:                          Size of Displayed Text.
                                                              (line  17)
* string<:                               Text Comparison.     (line  90)
* string=:                               Text Comparison.     (line  17)
* stringp:                               Predicates for Strings.
                                                              (line   9)
* strings:                               Strings and Characters.
                                                              (line   6)
* strings with keyboard events:          Strings of Events.   (line   6)
* strings, formatting them:              Formatting Strings.  (line   6)
* strings-consed:                        Memory Usage.        (line  42)
* submenu:                               Mouse Menus.         (line  20)
* subprocess:                            Processes.           (line   6)
* subr:                                  What Is a Function.  (line  33)
* subr-arity:                            What Is a Function.  (line 116)
* subrp:                                 What Is a Function.  (line 100)
* subst-char-in-region:                  Substitution.        (line   9)
* substitute characters:                 Substitution.        (line   6)
* substitute-command-keys:               Keys in Documentation.
                                                              (line  63)
* substitute-in-file-name:               File Name Expansion. (line  84)
* substitute-key-definition:             Changing Key Bindings.
                                                              (line 110)
* substituting keys in documentation:    Keys in Documentation.
                                                              (line   6)
* substring:                             Creating Strings.    (line  27)
* substring-no-properties:               Creating Strings.    (line  91)
* subtype of char-table:                 Char-Tables.         (line  14)
* suggestions:                           Caveats.             (line  27)
* super characters:                      Other Char Bits.     (line  16)
* suppress-keymap:                       Changing Key Bindings.
                                                              (line 148)
* surrogate minibuffer frame:            Minibuffers and Frames.
                                                              (line  10)
* suspend (cf. no-redraw-on-reenter):    Refresh Screen.      (line  31)
* suspend evaluation:                    Recursive Editing.   (line  62)
* suspend-emacs:                         Suspending Emacs.    (line  25)
* suspend-frame:                         Suspending Emacs.    (line 113)
* suspend-hook:                          Suspending Emacs.    (line  73)
* suspend-resume-hook:                   Suspending Emacs.    (line  76)
* suspend-tty:                           Suspending Emacs.    (line  80)
* suspend-tty-functions:                 Suspending Emacs.    (line  90)
* suspending Emacs:                      Suspending Emacs.    (line   6)
* swap text between buffers:             Swapping Text.       (line   6)
* switch-to-buffer:                      Switching Buffers.   (line  19)
* switch-to-buffer-in-dedicated-window:  Switching Buffers.   (line  53)
* switch-to-buffer-other-frame:          Switching Buffers.   (line 109)
* switch-to-buffer-other-window:         Switching Buffers.   (line  96)
* switch-to-buffer-preserve-window-point: Switching Buffers.  (line  79)
* switch-to-next-buffer:                 Window History.      (line  79)
* switch-to-prev-buffer:                 Window History.      (line  60)
* switch-to-visible-buffer:              Window History.      (line  93)
* switches on command line:              Command-Line Arguments.
                                                              (line  29)
* switching to a buffer:                 Switching Buffers.   (line   6)
* sxhash:                                Defining Hash.       (line  41)
* symbol:                                Symbols.             (line   6)
* symbol components:                     Symbol Components.   (line   6)
* symbol equality:                       Creating Symbols.    (line  39)
* symbol evaluation:                     Symbol Forms.        (line   6)
* symbol function indirection:           Function Indirection.
                                                              (line   6)
* symbol in keymap:                      Key Lookup.          (line  67)
* symbol name hashing:                   Creating Symbols.    (line  11)
* symbol property:                       Symbol Properties.   (line   6)
* symbol that evaluates to itself:       Constant Variables.  (line   6)
* symbol with constant value:            Constant Variables.  (line   6)
* symbol, where defined:                 Where Defined.       (line   6)
* symbol-file:                           Where Defined.       (line   6)
* symbol-function:                       Function Cells.      (line  13)
* symbol-name:                           Creating Symbols.    (line  72)
* symbol-plist:                          Symbol Plists.       (line  33)
* symbol-value:                          Accessing Variables. (line  14)
* symbolp:                               Symbols.             (line  16)
* symbols-consed:                        Memory Usage.        (line  25)
* synchronous subprocess:                Synchronous Processes.
                                                              (line   6)
* syntactic font lock:                   Syntactic Font Lock. (line   6)
* syntax class:                          Syntax Descriptors.  (line   6)
* syntax class table:                    Syntax Class Table.  (line   6)
* syntax code:                           Syntax Table Internals.
                                                              (line  13)
* syntax descriptor:                     Syntax Descriptors.  (line  21)
* syntax entry, setting:                 Syntax Table Functions.
                                                              (line  25)
* syntax error (Edebug):                 Backtracking.        (line   6)
* syntax flags:                          Syntax Flags.        (line   6)
* syntax for characters:                 Basic Char Syntax.   (line   6)
* syntax of regular expressions:         Syntax of Regexps.   (line   6)
* syntax table:                          Syntax Tables.       (line   6)
* syntax table example:                  Example Major Modes. (line  53)
* syntax table internals:                Syntax Table Internals.
                                                              (line   6)
* syntax tables in modes:                Major Mode Conventions.
                                                              (line 111)
* syntax-after:                          Syntax Table Internals.
                                                              (line  49)
* syntax-class:                          Syntax Table Internals.
                                                              (line  56)
* syntax-ppss:                           Position Parse.      (line  10)
* syntax-ppss-flush-cache:               Position Parse.      (line  33)
* syntax-ppss-toplevel-pos:              Parser State.        (line  59)
* syntax-propertize-extend-region-functions: Syntax Properties.
                                                              (line  48)
* syntax-propertize-function:            Syntax Properties.   (line  32)
* syntax-table:                          Syntax Table Functions.
                                                              (line  95)
* syntax-table (text property):          Syntax Properties.   (line   6)
* syntax-table-p:                        Syntax Basics.       (line  19)
* system abbrev:                         Abbrevs.             (line  25)
* system processes:                      System Processes.    (line   6)
* system type and name:                  System Environment.  (line  16)
* system-configuration:                  System Environment.  (line  10)
* system-groups:                         User Identification. (line  82)
* system-key-alist:                      X11 Keysyms.         (line   9)
* system-messages-locale:                Locales.             (line  17)
* system-name:                           System Environment.  (line  72)
* system-time-locale:                    Locales.             (line  24)
* system-type:                           System Environment.  (line  16)
* system-users:                          User Identification. (line  76)
* t:                                     nil and t.           (line  25)
* t input stream:                        Input Streams.       (line  40)
* t output stream:                       Output Streams.      (line  31)
* tab (ASCII character):                 Basic Char Syntax.   (line  27)
* tab deletion:                          Deletion.            (line  69)
* <TAB> in minibuffer:                   Text from Minibuffer.
                                                              (line 230)
* tab-always-indent:                     Mode-Specific Indent.
                                                              (line  63)
* tab-stop-list:                         Indent Tabs.         (line  19)
* tab-to-tab-stop:                       Indent Tabs.         (line  15)
* tab-width:                             Usual Display.       (line  72)
* tabs stops for indentation:            Indent Tabs.         (line   6)
* Tabulated List mode:                   Tabulated List Mode. (line   6)
* tabulated-list-entries:                Tabulated List Mode. (line  51)
* tabulated-list-format:                 Tabulated List Mode. (line  32)
* tabulated-list-init-header:            Tabulated List Mode. (line  96)
* tabulated-list-mode:                   Tabulated List Mode. (line  18)
* tabulated-list-print:                  Tabulated List Mode. (line 106)
* tabulated-list-printer:                Tabulated List Mode. (line  80)
* tabulated-list-revert-hook:            Tabulated List Mode. (line  75)
* tabulated-list-sort-key:               Tabulated List Mode. (line  89)
* tag on run time stack:                 Catch and Throw.     (line  61)
* tag, customization keyword:            Common Keywords.     (line  18)
* tan:                                   Math Functions.      (line  11)
* TCP:                                   Network.             (line   6)
* temacs:                                Building Emacs.      (line   6)
* TEMP environment variable:             Unique File Names.   (line  52)
* temp-buffer-max-height:                Temporary Displays.  (line 135)
* temp-buffer-max-width:                 Temporary Displays.  (line 143)
* temp-buffer-resize-mode:               Temporary Displays.  (line 124)
* temp-buffer-setup-hook:                Temporary Displays.  (line  69)
* temp-buffer-show-function:             Temporary Displays.  (line  59)
* temp-buffer-show-hook:                 Temporary Displays.  (line  75)
* temp-buffer-window-setup-hook:         Temporary Displays.  (line 103)
* temp-buffer-window-show-hook:          Temporary Displays.  (line 103)
* temporary buffer display:              Temporary Displays.  (line   6)
* temporary display:                     Temporary Displays.  (line   6)
* temporary files:                       Unique File Names.   (line   6)
* temporary-file-directory:              Unique File Names.   (line  51)
* TERM environment variable:             Terminal-Specific.   (line  43)
* term-file-aliases:                     Terminal-Specific.   (line  54)
* term-file-prefix:                      Terminal-Specific.   (line  42)
* Termcap:                               Terminal-Specific.   (line  17)
* terminal:                              Frames.              (line  17)
* terminal input:                        Terminal Input.      (line   6)
* terminal input modes:                  Input Modes.         (line   6)
* terminal output:                       Terminal Output.     (line   6)
* terminal parameters:                   Terminal Parameters. (line   6)
* terminal screen:                       Basic Windows.       (line  17)
* terminal type:                         Terminal Type.       (line   6)
* terminal-coding-system:                Terminal I/O Encoding.
                                                              (line  25)
* terminal-list:                         Multiple Terminals.  (line  34)
* terminal-live-p:                       Frames.              (line  58)
* terminal-local variables:              Multiple Terminals.  (line  68)
* terminal-name:                         Multiple Terminals.  (line  28)
* terminal-parameter:                    Terminal Parameters. (line  19)
* terminal-parameters:                   Terminal Parameters. (line  15)
* terminal-specific initialization:      Terminal-Specific.   (line   6)
* termscript file:                       Terminal Output.     (line  50)
* terpri:                                Output Functions.    (line  83)
* test-completion:                       Basic Completion.    (line 119)
* testcover-mark-all:                    Test Coverage.       (line   6)
* testcover-next-mark:                   Test Coverage.       (line   6)
* testcover-start:                       Test Coverage.       (line   6)
* testing types:                         Type Predicates.     (line  21)
* text:                                  Text.                (line   6)
* text area:                             Frame Layout.        (line 134)
* text area of a window:                 Window Sizes.        (line  23)
* text comparison:                       Text Comparison.     (line   6)
* text conversion of coding system:      Lisp and Coding Systems.
                                                              (line  57)
* text deletion:                         Deletion.            (line   6)
* text insertion:                        Insertion.           (line   6)
* text near point:                       Near Point.          (line   6)
* text parsing:                          Syntax Tables.       (line   6)
* text properties:                       Text Properties.     (line   6)
* text properties in files:              Format Conversion.   (line   6)
* text properties in the mode line:      Properties in Mode.  (line   6)
* text properties, changing:             Changing Properties. (line   6)
* text properties, examining:            Examining Properties.
                                                              (line   6)
* text properties, read syntax:          Text Props and Strings.
                                                              (line   6)
* text properties, searching:            Property Search.     (line   6)
* text representation:                   Text Representations.
                                                              (line   6)
* text terminal:                         Frames.              (line  21)
* text-char-description:                 Describing Characters.
                                                              (line  54)
* text-mode:                             Basic Major Modes.   (line  18)
* text-mode-abbrev-table:                Standard Abbrev Tables.
                                                              (line  32)
* text-properties-at:                    Examining Properties.
                                                              (line  64)
* text-property-any:                     Property Search.     (line 113)
* text-property-default-nonsticky:       Sticky Properties.   (line  55)
* text-property-not-all:                 Property Search.     (line 123)
* text-quoting-style:                    Keys in Documentation.
                                                              (line  48)
* textual order:                         Control Structures.  (line  12)
* textual scrolling:                     Textual Scrolling.   (line   6)
* thing-at-point:                        Buffer Contents.     (line 128)
* this-command:                          Command Loop Info.   (line  33)
* this-command-keys:                     Command Loop Info.   (line  73)
* this-command-keys-shift-translated:    Key Sequence Input.  (line  78)
* this-command-keys-vector:              Command Loop Info.   (line  89)
* this-original-command:                 Command Loop Info.   (line  66)
* three-step-help:                       Help Functions.      (line 179)
* throw:                                 Catch and Throw.     (line  76)
* throw example:                         Recursive Editing.   (line  32)
* tiled windows:                         Basic Windows.       (line  23)
* time calculations:                     Time Calculations.   (line   6)
* time conversion:                       Time Conversion.     (line   6)
* time formatting:                       Time Parsing.        (line   6)
* time of day:                           Time of Day.         (line   6)
* time parsing:                          Time Parsing.        (line   6)
* time value:                            Time of Day.         (line  18)
* time zone rule:                        Time Zone Rules.     (line  20)
* time zone rules:                       Time Zone Rules.     (line   6)
* time zone, current:                    Time Zone Rules.     (line  29)
* time-add:                              Time Calculations.   (line  20)
* time-less-p:                           Time Calculations.   (line  11)
* time-subtract:                         Time Calculations.   (line  14)
* time-to-day-in-year:                   Time Calculations.   (line  32)
* time-to-days:                          Time Calculations.   (line  28)
* timer-max-repeats:                     Timers.              (line 102)
* timerp:                                Timers.              (line  11)
* timers:                                Timers.              (line   6)
* timestamp of a mouse event:            Accessing Mouse.     (line 119)
* timing programs:                       Profiling.           (line  34)
* tips for writing Lisp:                 Tips.                (line   6)
* title bar:                             Frame Layout.        (line  59)
* title, a frame parameter:              Basic Parameters.    (line  19)
* TLS network connections:               Network.             (line  55)
* TMP environment variable:              Unique File Names.   (line  52)
* TMPDIR environment variable:           Unique File Names.   (line  52)
* toggle-enable-multibyte-characters:    Disabling Multibyte. (line  41)
* tool bar:                              Tool Bar.            (line   6)
* tool-bar-add-item:                     Tool Bar.            (line 101)
* tool-bar-add-item-from-menu:           Tool Bar.            (line 121)
* tool-bar-border:                       Tool Bar.            (line 167)
* tool-bar-button-margin:                Tool Bar.            (line 158)
* tool-bar-button-relief:                Tool Bar.            (line 163)
* tool-bar-lines frame parameter:        Layout Parameters.   (line  59)
* tool-bar-local-item-from-menu:         Tool Bar.            (line 135)
* tool-bar-map:                          Tool Bar.            (line  84)
* tool-bar-position frame parameter:     Layout Parameters.   (line  64)
* tooltip face:                          Tooltips.            (line  41)
* tooltip for help strings:              Special Properties.  (line  83)
* tooltip-event-buffer:                  Tooltips.            (line  58)
* tooltip-frame-parameters:              Tooltips.            (line  30)
* tooltip-functions:                     Tooltips.            (line  45)
* tooltip-help-tips:                     Tooltips.            (line  45)
* tooltip-mode:                          Tooltips.            (line  13)
* tooltips:                              Tooltips.            (line   6)
* top frame:                             Raising and Lowering.
                                                              (line  27)
* top, a frame parameter:                Position Parameters. (line  45)
* top-level:                             Recursive Editing.   (line  98)
* top-level default value:               Default Value.       (line  92)
* top-level form:                        Loading.             (line  16)
* total height of a window:              Window Sizes.        (line  47)
* total pixel height of a window:        Window Sizes.        (line 109)
* total pixel width of a window:         Window Sizes.        (line 118)
* total width of a window:               Window Sizes.        (line  75)
* tq-close:                              Transaction Queues.  (line  37)
* tq-create:                             Transaction Queues.  (line  11)
* tq-enqueue:                            Transaction Queues.  (line  18)
* trace buffer:                          Trace Buffer.        (line   6)
* track-mouse:                           Mouse Tracking.      (line  18)
* trailing blanks in file names:         Information about Files.
                                                              (line  12)
* transaction queue:                     Transaction Queues.  (line   6)
* transcendental functions:              Math Functions.      (line   6)
* transient keymap:                      Controlling Active Maps.
                                                              (line 147)
* transient-mark-mode:                   The Mark.            (line 117)
* translate-region:                      Substitution.        (line  33)
* translating input events:              Event Mod.           (line   6)
* translation keymap:                    Translation Keymaps. (line   6)
* translation tables:                    Translation of Characters.
                                                              (line   6)
* translation-table-for-input:           Translation of Characters.
                                                              (line  56)
* transparency, frame:                   Font and Color Parameters.
                                                              (line  55)
* transpose-regions:                     Transposition.       (line   8)
* trash:                                 Changing Files.      (line 120)
* trash <1>:                             Create/Delete Dirs.  (line  41)
* tray notifications, MS-Windows:        Desktop Notifications.
                                                              (line 229)
* triple-click events:                   Repeat Events.       (line   6)
* true:                                  nil and t.           (line  25)
* true list:                             Cons Cells.          (line  24)
* truename (of file):                    Truenames.           (line   6)
* truncate:                              Numeric Conversions. (line  22)
* truncate-lines:                        Truncation.          (line  21)
* truncate-partial-width-windows:        Truncation.          (line  29)
* truncate-string-ellipsis:              Size of Displayed Text.
                                                              (line  44)
* truncate-string-to-width:              Size of Displayed Text.
                                                              (line  21)
* truth value:                           nil and t.           (line   6)
* try-completion:                        Basic Completion.    (line  10)
* tty-color-alist:                       Text Terminal Colors.
                                                              (line  40)
* tty-color-approximate:                 Text Terminal Colors.
                                                              (line  50)
* tty-color-clear:                       Text Terminal Colors.
                                                              (line  36)
* tty-color-define:                      Text Terminal Colors.
                                                              (line  26)
* tty-color-mode, a frame parameter:     Font and Color Parameters.
                                                              (line  22)
* tty-color-translate:                   Text Terminal Colors.
                                                              (line  56)
* tty-erase-char:                        System Environment.  (line 208)
* tty-setup-hook:                        Terminal-Specific.   (line  60)
* tty-top-frame:                         Raising and Lowering.
                                                              (line  31)
* turn multibyte support on or off:      Disabling Multibyte. (line  11)
* two’s complement:                      Integer Basics.      (line  52)
* type:                                  Lisp Data Types.     (line   6)
* type (button property):                Button Properties.   (line  38)
* type checking:                         Type Predicates.     (line   6)
* type checking internals:               Writing Emacs Primitives.
                                                              (line 119)
* type predicates:                       Type Predicates.     (line  21)
* type, defcustom keyword:               Customization Types. (line  11)
* type-of:                               Type Predicates.     (line 178)
* typographic conventions:               Some Terms.          (line  14)
* TZ, environment variable:              Time Zone Rules.     (line   6)
* UBA:                                   Bidirectional Display.
                                                              (line  17)
* UDP:                                   Network.             (line   6)
* UID:                                   User Identification. (line  59)
* umask:                                 Changing Files.      (line 165)
* unassigned character codepoints:       Character Properties.
                                                              (line  32)
* unbalanced parentheses:                Syntax Errors.       (line  20)
* unbinding keys:                        Key Binding Commands.
                                                              (line  55)
* unbury-buffer:                         Buffer List.         (line 132)
* undecided coding system:               Coding System Basics.
                                                              (line  15)
* undecided coding-system, when encoding: Explicit Encoding.  (line  55)
* undefined:                             Functions for Key Lookup.
                                                              (line  49)
* undefined in keymap:                   Key Lookup.          (line  79)
* undefined key:                         Keymap Basics.       (line   6)
* underline-minimum-offset:              Face Attributes.     (line 202)
* underlined text:                       Face Attributes.     (line  87)
* undo avoidance:                        Substitution.        (line  15)
* undo-ask-before-discard:               Maintaining Undo.    (line  62)
* undo-auto-amalgamate:                  Undo.                (line  97)
* undo-auto-current-boundary-timer:      Undo.                (line 113)
* undo-boundary:                         Undo.                (line  84)
* undo-in-progress:                      Undo.                (line 119)
* undo-limit:                            Maintaining Undo.    (line  45)
* undo-outer-limit:                      Maintaining Undo.    (line  57)
* undo-strong-limit:                     Maintaining Undo.    (line  50)
* unexec:                                Building Emacs.      (line 107)
* unhandled-file-name-directory:         Magic File Names.    (line 200)
* unibyte buffers, and bidi reordering:  Bidirectional Display.
                                                              (line  43)
* unibyte text:                          Text Representations.
                                                              (line  40)
* unibyte-char-to-multibyte:             Converting Representations.
                                                              (line  68)
* unibyte-string:                        Text Representations.
                                                              (line 123)
* Unicode:                               Text Representations.
                                                              (line  10)
* unicode bidirectional algorithm:       Bidirectional Display.
                                                              (line  17)
* unicode character escape:              General Escape Syntax.
                                                              (line  10)
* unicode general category:              Character Properties.
                                                              (line  46)
* unicode, a charset:                    Character Sets.      (line  16)
* unicode-category-table:                Character Properties.
                                                              (line 200)
* unintern:                              Creating Symbols.    (line 162)
* uninterned symbol:                     Creating Symbols.    (line  39)
* unique file names:                     Unique File Names.   (line   6)
* universal-argument:                    Prefix Command Arguments.
                                                              (line 102)
* universal-argument-map:                Standard Keymaps.    (line 147)
* unless:                                Conditionals.        (line  41)
* unload-feature:                        Unloading.           (line  10)
* unload-feature-special-hooks:          Unloading.           (line  49)
* unloading packages:                    Unloading.           (line   6)
* unloading packages, preparing for:     Coding Conventions.  (line 110)
* unlock-buffer:                         File Locks.          (line  39)
* unnumbered group:                      Regexp Backslash.    (line  66)
* unpacking:                             Byte Packing.        (line  13)
* unread-command-events:                 Event Input Misc.    (line  10)
* unsafep:                               Function Safety.     (line  13)
* unsplittable, a frame parameter:       Buffer Parameters.   (line  30)
* unused lexical variable:               Using Lexical Binding.
                                                              (line  47)
* unwind-protect:                        Cleanups.            (line  12)
* unwinding:                             Cleanups.            (line  13)
* up-list:                               List Motion.         (line  24)
* upcase:                                Case Conversion.     (line  36)
* upcase-initials:                       Case Conversion.     (line  76)
* upcase-region:                         Case Changes.        (line  42)
* upcase-word:                           Case Changes.        (line  72)
* update-directory-autoloads:            Autoload.            (line 103)
* update-file-autoloads:                 Autoload.            (line 103)
* upper case:                            Case Conversion.     (line   6)
* upper case key sequence:               Key Sequence Input.  (line  73)
* uptime of Emacs:                       Processor Run Time.  (line  10)
* use time of window:                    Selecting Windows.   (line  82)
* use-empty-active-region:               The Region.          (line  37)
* use-global-map:                        Controlling Active Maps.
                                                              (line  51)
* use-hard-newlines:                     Filling.             (line 169)
* use-local-map:                         Controlling Active Maps.
                                                              (line  57)
* use-region-p:                          The Region.          (line  31)
* user errors, signaling:                Signaling Errors.    (line  92)
* user groups:                           User Identification. (line  82)
* user identification:                   User Identification. (line   6)
* user options, how to define:           Variable Definitions.
                                                              (line   6)
* user signals:                          Misc Events.         (line  66)
* user-defined error:                    Error Symbols.       (line   6)
* user-emacs-directory:                  Init File.           (line  74)
* user-error:                            Signaling Errors.    (line  92)
* user-full-name:                        User Identification. (line  52)
* user-full-name <1>:                    User Identification. (line  40)
* user-init-file:                        Init File.           (line  69)
* user-login-name:                       User Identification. (line  52)
* user-login-name <1>:                   User Identification. (line  26)
* user-mail-address:                     User Identification. (line  19)
* user-position, a frame parameter:      Position Parameters. (line  63)
* user-ptr object:                       Dynamic Modules.     (line  30)
* user-ptrp:                             Dynamic Modules.     (line  38)
* user-real-login-name:                  User Identification. (line  52)
* user-real-login-name <1>:              User Identification. (line  35)
* user-real-uid:                         User Identification. (line  59)
* user-size, a frame parameter:          Size Parameters.     (line  18)
* user-uid:                              User Identification. (line  64)
* utf-8-emacs coding system:             Coding System Basics.
                                                              (line  66)
* valid windows:                         Basic Windows.       (line  44)
* validity of coding system:             Lisp and Coding Systems.
                                                              (line  18)
* value cell:                            Symbol Components.   (line  13)
* value of expression:                   Evaluation.          (line   6)
* value of function:                     What Is a Function.  (line   6)
* values:                                Eval.                (line 128)
* variable:                              Variables.           (line   6)
* variable aliases:                      Variable Aliases.    (line   6)
* variable definition:                   Defining Variables.  (line   6)
* variable descriptions:                 A Sample Variable Description.
                                                              (line   6)
* variable limit error:                  Local Variables.     (line 113)
* variable with constant value:          Constant Variables.  (line   6)
* variable, buffer-local:                Buffer-Local Variables.
                                                              (line   6)
* variable-documentation property:       Documentation Basics.
                                                              (line  19)
* variable-width spaces:                 Specified Space.     (line   6)
* variant coding system:                 Coding System Basics.
                                                              (line  45)
* vc-mode:                               Mode Line Variables. (line  45)
* vc-prefix-map:                         Prefix Keys.         (line  40)
* vc-responsible-backend:                Truenames.           (line  67)
* vconcat:                               Vector Functions.    (line  32)
* vector:                                Vector Functions.    (line  16)
* vector (type):                         Vectors.             (line   6)
* vector evaluation:                     Self-Evaluating Forms.
                                                              (line   6)
* vector length:                         Sequence Functions.  (line  13)
* vector reverse:                        Sequence Functions.  (line 112)
* vector-cells-consed:                   Memory Usage.        (line  21)
* vector-like objects, storage:          Garbage Collection.  (line  16)
* vectorp:                               Vector Functions.    (line   8)
* verify-visited-file-modtime:           Modification Time.   (line  14)
* version number (in file name):         File Name Components.
                                                              (line   6)
* version, customization keyword:        Common Keywords.     (line 109)
* version-control:                       Numbered Backups.    (line  10)
* vertical combination:                  Windows and Frames.  (line  77)
* vertical fractional scrolling:         Vertical Scrolling.  (line   6)
* vertical scroll position:              Vertical Scrolling.  (line   6)
* vertical tab:                          Basic Char Syntax.   (line  27)
* vertical-line prefix key:              Key Sequence Input.  (line  92)
* vertical-motion:                       Screen Lines.        (line  28)
* vertical-scroll-bar:                   Scroll Bars.         (line 106)
* vertical-scroll-bar prefix key:        Key Sequence Input.  (line  92)
* vertical-scroll-bars, a frame parameter: Layout Parameters. (line  16)
* view part, model/view/controller:      Abstract Display.    (line   6)
* view-register:                         Registers.           (line  66)
* virtual buffers:                       Swapping Text.       (line   6)
* visibility, a frame parameter:         Management Parameters.
                                                              (line  10)
* visible frame:                         Visibility of Frames.
                                                              (line   6)
* visible-bell:                          Beeping.             (line  20)
* visible-frame-list:                    Finding All Frames.  (line  13)
* visited file:                          Buffer File Name.    (line   6)
* visited file mode:                     Auto Major Mode.     (line  38)
* visited-file-modtime:                  Modification Time.   (line  43)
* visiting files:                        Visiting Files.      (line   6)
* visiting files, functions for:         Visiting Functions.  (line   6)
* visual order:                          Bidirectional Display.
                                                              (line  17)
* visual order, preserve when copying bidirectional text: Bidirectional Display.
                                                              (line 233)
* visual-order cursor motion:            Bidirectional Display.
                                                              (line 113)
* void function:                         Function Indirection.
                                                              (line   6)
* void function cell:                    Function Cells.      (line  29)
* void variable:                         Void Variables.      (line   6)
* void-function:                         Function Cells.      (line  14)
* void-text-area-pointer:                Pointer Shape.       (line  18)
* void-variable error:                   Void Variables.      (line   6)
* w32-collate-ignore-punctuation:        Text Comparison.     (line  79)
* w32-notification-close:                Desktop Notifications.
                                                              (line 295)
* w32-notification-notify:               Desktop Notifications.
                                                              (line 232)
* wait-for-wm, a frame parameter:        Management Parameters.
                                                              (line  43)
* waiting:                               Waiting.             (line   6)
* waiting for command key input:         Event Input Misc.    (line  45)
* waiting-for-user-input-p:              Sentinels.           (line 114)
* walk-windows:                          Cyclic Window Ordering.
                                                              (line  76)
* warn:                                  Warning Basics.      (line  52)
* warning options:                       Warning Options.     (line   6)
* warning type:                          Warning Basics.      (line  33)
* warning variables:                     Warning Variables.   (line   6)
* warning-fill-prefix:                   Warning Variables.   (line  61)
* warning-levels:                        Warning Variables.   (line   9)
* warning-minimum-level:                 Warning Options.     (line   9)
* warning-minimum-log-level:             Warning Options.     (line  14)
* warning-prefix-function:               Warning Variables.   (line  25)
* warning-series:                        Warning Variables.   (line  41)
* warning-suppress-log-types:            Warning Options.     (line  25)
* warning-suppress-types:                Warning Options.     (line  19)
* warning-type-format:                   Warning Variables.   (line  65)
* warnings:                              Warnings.            (line   6)
* watch, for filesystem events:          File Notifications.  (line   6)
* webkit browser widget:                 Xwidgets.            (line   6)
* wheel-down event:                      Misc Events.         (line  28)
* wheel-up event:                        Misc Events.         (line  28)
* when:                                  Conditionals.        (line  31)
* where was a symbol defined:            Where Defined.       (line   6)
* where-is-internal:                     Scanning Keymaps.    (line  75)
* while:                                 Iteration.           (line  11)
* while-no-input:                        Event Input Misc.    (line  70)
* whitespace:                            Basic Char Syntax.   (line  27)
* wholenump:                             Predicates on Numbers.
                                                              (line  30)
* widen:                                 Narrowing.           (line  46)
* widening:                              Narrowing.           (line  47)
* width of a window:                     Window Sizes.        (line  75)
* width, a frame parameter:              Size Parameters.     (line  14)
* window:                                Basic Windows.       (line   6)
* window (overlay property):             Overlay Properties.  (line  79)
* window body:                           Window Sizes.        (line  23)
* window body height:                    Window Sizes.        (line 144)
* window body size:                      Window Sizes.        (line 185)
* window body width:                     Window Sizes.        (line 162)
* window combination:                    Windows and Frames.  (line  77)
* window combination limit:              Recombining Windows. (line 186)
* window configuration (Edebug):         Edebug Display Update.
                                                              (line  25)
* window configurations:                 Window Configurations.
                                                              (line   6)
* window dividers:                       Window Dividers.     (line   6)
* window end position:                   Window Start and End.
                                                              (line  46)
* window excursions:                     Excursions.          (line  49)
* window header line:                    Header Lines.        (line   6)
* window height:                         Window Sizes.        (line  47)
* window history:                        Window History.      (line   6)
* window in direction:                   Windows and Frames.  (line 160)
* window internals:                      Window Internals.    (line   6)
* window layout in a frame:              Window Configuration Type.
                                                              (line   6)
* window layout, all frames:             Frame Configuration Type.
                                                              (line   6)
* window manager interaction, and frame parameters: Management Parameters.
                                                              (line   6)
* window order by time of last use:      Selecting Windows.   (line  82)
* window ordering, cyclic:               Cyclic Window Ordering.
                                                              (line   6)
* window parameters:                     Window Parameters.   (line   6)
* window pixel height:                   Window Sizes.        (line 109)
* window pixel width:                    Window Sizes.        (line 118)
* window point:                          Window Point.        (line   6)
* window point internals:                Window Internals.    (line  61)
* window position:                       Window Point.        (line   6)
* window position <1>:                   Coordinates and Windows.
                                                              (line   6)
* window position on display:            Position Parameters. (line   6)
* window positions and window managers:  Position Parameters. (line  70)
* window resizing:                       Resizing Windows.    (line   6)
* window selected within a frame:        Basic Windows.       (line  59)
* window size:                           Window Sizes.        (line   6)
* window size on display:                Size Parameters.     (line   6)
* window size, changing:                 Resizing Windows.    (line   6)
* window splitting:                      Splitting Windows.   (line   6)
* window start position:                 Window Start and End.
                                                              (line   6)
* window state:                          Window Configurations.
                                                              (line  99)
* window that satisfies a predicate:     Cyclic Window Ordering.
                                                              (line 139)
* window top line:                       Window Start and End.
                                                              (line  22)
* window tree:                           Windows and Frames.  (line  29)
* window use time:                       Selecting Windows.   (line  82)
* window width:                          Window Sizes.        (line  75)
* window-absolute-body-pixel-edges:      Coordinates and Windows.
                                                              (line 147)
* window-absolute-pixel-edges:           Coordinates and Windows.
                                                              (line 141)
* window-absolute-pixel-position:        Coordinates and Windows.
                                                              (line 171)
* window-adjust-process-window-size-function: Process Buffers.
                                                              (line 102)
* window-at:                             Coordinates and Windows.
                                                              (line  62)
* window-body-edges:                     Coordinates and Windows.
                                                              (line  54)
* window-body-height:                    Window Sizes.        (line 148)
* window-body-pixel-edges:               Coordinates and Windows.
                                                              (line 133)
* window-body-size:                      Window Sizes.        (line 185)
* window-body-width:                     Window Sizes.        (line 171)
* window-bottom-divider-width:           Window Dividers.     (line  53)
* window-buffer:                         Buffers and Windows. (line  10)
* window-child:                          Windows and Frames.  (line 118)
* window-combination-limit:              Recombining Windows. (line 121)
* window-combination-limit <1>:          Recombining Windows. (line 195)
* window-combination-resize:             Recombining Windows. (line 215)
* window-combined-p:                     Windows and Frames.  (line 124)
* window-configuration-change-hook:      Window Hooks.        (line  51)
* window-configuration-frame:            Window Configurations.
                                                              (line  83)
* window-configuration-p:                Window Configurations.
                                                              (line  71)
* window-current-scroll-bars:            Scroll Bars.         (line  77)
* window-dedicated-p:                    Dedicated Windows.   (line  36)
* window-display-table:                  Active Display Table.
                                                              (line  16)
* window-edges:                          Coordinates and Windows.
                                                              (line  19)
* window-end:                            Window Start and End.
                                                              (line  46)
* window-font-height:                    Low-Level Font.      (line 298)
* window-font-width:                     Low-Level Font.      (line 292)
* window-frame:                          Windows and Frames.  (line   8)
* window-fringes:                        Fringe Size/Pos.     (line  47)
* window-full-height-p:                  Window Sizes.        (line 130)
* window-full-width-p:                   Window Sizes.        (line 138)
* window-group-end:                      Window Start and End.
                                                              (line  70)
* window-group-end-function:             Window Start and End.
                                                              (line  70)
* window-group-start:                    Window Start and End.
                                                              (line  37)
* window-group-start-function:           Window Start and End.
                                                              (line  38)
* window-header-line-height:             Header Lines.        (line  17)
* window-header-line-height <1>:         Window Sizes.        (line 208)
* window-hscroll:                        Horizontal Scrolling.
                                                              (line  77)
* window-id, a frame parameter:          Management Parameters.
                                                              (line  33)
* window-in-direction:                   Windows and Frames.  (line 160)
* window-left-child:                     Windows and Frames.  (line 112)
* window-line-height:                    Window Start and End.
                                                              (line 193)
* window-list:                           Windows and Frames.  (line  12)
* window-live-p:                         Basic Windows.       (line  35)
* window-margins:                        Display Margins.     (line  53)
* window-max-chars-per-line:             Window Sizes.        (line 221)
* window-min-height:                     Window Sizes.        (line 235)
* window-min-height <1>:                 Window Sizes.        (line 242)
* window-min-size:                       Window Sizes.        (line 258)
* window-min-width:                      Window Sizes.        (line 235)
* window-min-width <1>:                  Window Sizes.        (line 248)
* window-minibuffer-p:                   Minibuffer Windows.  (line  28)
* window-mode-line-height:               Window Sizes.        (line 203)
* window-next-buffers:                   Window History.      (line  40)
* window-next-sibling:                   Windows and Frames.  (line 133)
* window-parameter:                      Window Parameters.   (line   9)
* window-parameters:                     Window Parameters.   (line  14)
* window-parent:                         Windows and Frames.  (line  60)
* window-persistent-parameters:          Window Parameters.   (line  34)
* window-pixel-edges:                    Coordinates and Windows.
                                                              (line 128)
* window-pixel-height:                   Window Sizes.        (line 109)
* window-pixel-width:                    Window Sizes.        (line 118)
* window-point:                          Window Point.        (line  32)
* window-point-insertion-type:           Window Point.        (line  50)
* window-preserve-size:                  Preserving Window Sizes.
                                                              (line  39)
* window-preserved-size:                 Preserving Window Sizes.
                                                              (line  80)
* window-prev-buffers:                   Window History.      (line  13)
* window-prev-sibling:                   Windows and Frames.  (line 138)
* window-resizable:                      Resizing Windows.    (line  17)
* window-resize:                         Resizing Windows.    (line  47)
* window-resize-pixelwise:               Resizing Windows.    (line  89)
* window-right-divider-width:            Window Dividers.     (line  48)
* window-scroll-bar-height:              Scroll Bars.         (line  88)
* window-scroll-bar-width:               Scroll Bars.         (line  84)
* window-scroll-bars:                    Scroll Bars.         (line  63)
* window-scroll-functions:               Window Hooks.        (line  13)
* window-setup-hook:                     Init File.           (line  64)
* window-size-change-functions:          Window Hooks.        (line  29)
* window-size-fixed:                     Preserving Window Sizes.
                                                              (line  17)
* window-start:                          Window Start and End.
                                                              (line  21)
* window-state-get:                      Window Configurations.
                                                              (line  99)
* window-state-put:                      Window Configurations.
                                                              (line 118)
* window-system:                         Window Systems.      (line  11)
* window-system <1>:                     Window Systems.      (line  35)
* window-system-initialization-alist:    Startup Summary.     (line  32)
* window-text-change-functions:          Standard Hooks.      (line 210)
* window-text-pixel-size:                Size of Displayed Text.
                                                              (line  61)
* window-top-child:                      Windows and Frames.  (line 107)
* window-total-height:                   Window Sizes.        (line  51)
* window-total-size:                     Window Sizes.        (line  98)
* window-total-width:                    Window Sizes.        (line  79)
* window-tree:                           Windows and Frames.  (line 198)
* window-use-time:                       Selecting Windows.   (line  82)
* window-valid-p:                        Basic Windows.       (line  54)
* window-vscroll:                        Vertical Scrolling.  (line  25)
* windowp:                               Basic Windows.       (line  28)
* windows, controlling precisely:        Buffers and Windows. (line   6)
* windows, recombining:                  Recombining Windows. (line   6)
* with-case-table:                       Case Tables.         (line  75)
* with-coding-priority:                  Specifying Coding Systems.
                                                              (line  83)
* with-current-buffer:                   Current Buffer.      (line 112)
* with-current-buffer-window:            Temporary Displays.  (line 110)
* with-demoted-errors:                   Handling Errors.     (line 210)
* with-displayed-buffer-window:          Temporary Displays.  (line 115)
* with-eval-after-load:                  Hooks for Loading.   (line  17)
* with-file-modes:                       Changing Files.      (line 181)
* with-help-window:                      Help Functions.      (line 130)
* with-local-quit:                       Quitting.            (line  83)
* with-no-warnings:                      Compiler Errors.     (line  57)
* with-output-to-string:                 Output Functions.    (line 114)
* with-output-to-temp-buffer:            Temporary Displays.  (line  10)
* with-selected-window:                  Selecting Windows.   (line  54)
* with-silent-modifications:             Changing Properties. (line 147)
* with-syntax-table:                     Syntax Table Functions.
                                                              (line 103)
* with-temp-buffer:                      Current Buffer.      (line 122)
* with-temp-buffer-window:               Temporary Displays.  (line  80)
* with-temp-file:                        Writing to Files.    (line  83)
* with-temp-message:                     Displaying Messages. (line  61)
* with-timeout:                          Timers.              (line 107)
* word-search-backward:                  String Search.       (line 121)
* word-search-backward-lax:              String Search.       (line 127)
* word-search-forward:                   String Search.       (line  66)
* word-search-forward-lax:               String Search.       (line 113)
* word-search-regexp:                    String Search.       (line 109)
* words in region:                       Text Lines.          (line  84)
* words-include-escapes:                 Word Motion.         (line  43)
* wrap-prefix:                           Truncation.          (line  44)
* write-abbrev-file:                     Abbrev Files.        (line  41)
* write-char:                            Output Functions.    (line  90)
* write-contents-functions:              Saving Buffers.      (line 113)
* write-file:                            Saving Buffers.      (line  54)
* write-file-functions:                  Saving Buffers.      (line  77)
* write-region:                          Writing to Files.    (line  23)
* write-region-annotate-functions:       Format Conversion Piecemeal.
                                                              (line  48)
* write-region-post-annotation-function: Format Conversion Piecemeal.
                                                              (line  62)
* writing a documentation string:        Documentation Basics.
                                                              (line   6)
* writing Emacs primitives:              Writing Emacs Primitives.
                                                              (line   6)
* writing to files:                      Writing to Files.    (line   6)
* wrong-number-of-arguments:             Argument List.       (line   6)
* wrong-type-argument:                   Type Predicates.     (line   6)
* X display names:                       Multiple Terminals.  (line  87)
* X Window System:                       Window Systems.      (line  16)
* x-alt-keysym:                          X11 Keysyms.         (line  29)
* x-alternatives-map:                    Standard Keymaps.    (line 154)
* x-bitmap-file-path:                    Face Attributes.     (line 206)
* x-close-connection:                    Multiple Terminals.  (line 142)
* x-color-defined-p:                     Color Names.         (line  38)
* x-color-values:                        Color Names.         (line  93)
* x-defined-colors:                      Color Names.         (line  46)
* x-display-color-p:                     Display Feature Testing.
                                                              (line  36)
* x-display-list:                        Multiple Terminals.  (line 117)
* x-dnd-known-types:                     Drag and Drop.       (line   6)
* x-dnd-test-function:                   Drag and Drop.       (line   6)
* x-dnd-types-alist:                     Drag and Drop.       (line  15)
* x-family-fonts:                        Font Lookup.         (line  29)
* x-get-resource:                        Resources.           (line  11)
* x-gtk-use-system-tooltips:             Tooltips.            (line  19)
* x-hyper-keysym:                        X11 Keysyms.         (line  31)
* x-list-fonts:                          Font Lookup.         (line   6)
* x-meta-keysym:                         X11 Keysyms.         (line  30)
* x-open-connection:                     Multiple Terminals.  (line 122)
* x-parse-geometry:                      Geometry.            (line   9)
* x-pointer-shape:                       Pointer Shape.       (line  27)
* x-popup-dialog:                        Dialog Boxes.        (line  15)
* x-popup-menu:                          Pop-Up Menus.        (line  11)
* x-resource-class:                      Resources.           (line  26)
* x-resource-name:                       Resources.           (line  32)
* x-sensitive-text-pointer-shape:        Pointer Shape.       (line  31)
* x-server-vendor:                       Display Feature Testing.
                                                              (line 162)
* x-server-version:                      Display Feature Testing.
                                                              (line 152)
* x-setup-function-keys:                 Standard Keymaps.    (line 154)
* x-stretch-cursor:                      Cursor Parameters.   (line  44)
* x-super-keysym:                        X11 Keysyms.         (line  32)
* X11 keysyms:                           X11 Keysyms.         (line   6)
* XBM:                                   XBM Images.          (line   6)
* XML DOM:                               Document Object Model.
                                                              (line   6)
* XPM:                                   XPM Images.          (line   6)
* xwidget:                               Xwidgets.            (line   6)
* xwidget-buffer:                        Xwidgets.            (line  39)
* xwidget-info:                          Xwidgets.            (line  75)
* xwidget-plist:                         Xwidgets.            (line  32)
* xwidget-query-on-exit-flag:            Xwidgets.            (line  86)
* xwidget-resize:                        Xwidgets.            (line  67)
* xwidget-size-request:                  Xwidgets.            (line  71)
* xwidget-webkit-execute-script:         Xwidgets.            (line  52)
* xwidget-webkit-execute-script-rv:      Xwidgets.            (line  56)
* xwidget-webkit-get-title:              Xwidgets.            (line  64)
* xwidget-webkit-goto-uri:               Xwidgets.            (line  48)
* xwidgetp:                              Xwidgets.            (line  29)
* y-or-n-p:                              Yes-or-No Queries.   (line  23)
* y-or-n-p-with-timeout:                 Yes-or-No Queries.   (line  58)
* yank:                                  Yank Commands.       (line  11)
* yank suppression:                      Changing Key Bindings.
                                                              (line 159)
* yank-excluded-properties:              Yanking.             (line  70)
* yank-handled-properties:               Yanking.             (line  58)
* yank-pop:                              Yank Commands.       (line  34)
* yank-undo-function:                    Yank Commands.       (line  57)
* yanking and text properties:           Yanking.             (line  58)
* yes-or-no questions:                   Yes-or-No Queries.   (line   6)
* yes-or-no-p:                           Yes-or-No Queries.   (line  64)
* zerop:                                 Predicates on Numbers.
                                                              (line  32)
* zlib-available-p:                      Decompression.       (line  16)
* zlib-decompress-region:                Decompression.       (line  20)



Tag Table:
Node: Top1072
Node: Introduction71317
Node: Caveats73218
Node: Lisp History74534
Node: Conventions76222
Node: Some Terms77038
Node: nil and t77791
Node: Evaluation Notation79709
Node: Printing Notation80667
Node: Error Messages81489
Node: Buffer Text Notation81944
Node: Format of Descriptions82854
Node: A Sample Function Description83725
Node: A Sample Variable Description87898
Node: Version Info88989
Node: Acknowledgments90797
Node: Lisp Data Types92593
Node: Printed Representation95173
Node: Comments97230
Node: Programming Types98145
Node: Integer Type100010
Node: Floating-Point Type101267
Node: Character Type102204
Node: Basic Char Syntax103572
Node: General Escape Syntax106797
Node: Ctl-Char Syntax108772
Node: Meta-Char Syntax110612
Node: Other Char Bits111741
Ref: modifier bits112560
Node: Symbol Type112983
Node: Sequence Type116381
Node: Cons Cell Type118128
Node: Box Diagrams121050
Node: Dotted Pair Notation124722
Node: Association List Type127304
Node: Array Type128340
Node: String Type130086
Node: Syntax for Strings130883
Node: Non-ASCII in Strings132067
Node: Nonprinting Characters134821
Node: Text Props and Strings136289
Node: Vector Type137661
Node: Char-Table Type138445
Ref: Char-Table Type-Footnote-1139513
Node: Bool-Vector Type139581
Node: Hash Table Type140818
Node: Function Type141422
Node: Macro Type142580
Node: Primitive Function Type143506
Node: Byte-Code Type145111
Node: Autoload Type145743
Node: Finalizer Type146790
Node: Editing Types148036
Node: Buffer Type149117
Node: Marker Type151291
Node: Window Type152028
Node: Frame Type153277
Node: Terminal Type153901
Node: Window Configuration Type154361
Node: Frame Configuration Type154950
Node: Process Type155567
Node: Stream Type156534
Node: Keymap Type157689
Node: Overlay Type158163
Node: Font Type158815
Node: Circular Objects159369
Node: Type Predicates160895
Node: Equality Predicates166504
Node: Numbers171172
Node: Integer Basics172588
Node: Float Basics176230
Node: Predicates on Numbers179340
Node: Comparison of Numbers180906
Node: Numeric Conversions185241
Node: Arithmetic Operations187795
Node: Rounding Operations192950
Node: Bitwise Operations194177
Node: Math Functions202304
Node: Random Numbers204439
Node: Strings and Characters206531
Node: String Basics207823
Node: Predicates for Strings210968
Node: Creating Strings211689
Node: Modifying Strings220161
Node: Text Comparison221472
Node: String Conversion232581
Node: Formatting Strings236592
Node: Case Conversion246910
Node: Case Tables250876
Node: Lists256025
Node: Cons Cells257029
Node: List-related Predicates259607
Node: List Elements261403
Ref: Definition of nth264333
Ref: Definition of safe-length265704
Node: Building Lists267139
Ref: Building Lists-Footnote-1274921
Node: List Variables275410
Node: Modifying Lists279809
Node: Setcar280735
Node: Setcdr283332
Node: Rearrangement285959
Node: Sets And Lists288206
Node: Association Lists296103
Ref: Association Lists-Footnote-1306309
Node: Property Lists306522
Node: Plists and Alists307914
Node: Plist Access309774
Node: Sequences Arrays Vectors311825
Node: Sequence Functions314364
Ref: Definition of length314742
Ref: Definition of elt315746
Node: Arrays337664
Node: Array Functions340184
Node: Vectors342550
Node: Vector Functions343932
Node: Char-Tables346209
Node: Bool-Vectors352606
Node: Rings356232
Node: Hash Tables359302
Node: Creating Hash361008
Node: Hash Access367017
Ref: Definition of maphash368526
Node: Defining Hash368756
Node: Other Hash371867
Node: Symbols373055
Node: Symbol Components374113
Node: Definitions377339
Node: Creating Symbols379874
Ref: Definition of mapatoms386465
Node: Symbol Properties387648
Node: Symbol Plists388433
Node: Standard Properties391477
Node: Evaluation395621
Node: Intro Eval396497
Ref: Intro Eval-Footnote-1399579
Ref: Intro Eval-Footnote-2399721
Node: Forms399860
Node: Self-Evaluating Forms401081
Node: Symbol Forms402627
Node: Classifying Lists403764
Node: Function Indirection404531
Ref: Definition of indirect-function407234
Node: Function Forms408007
Node: Macro Forms409023
Node: Special Forms410645
Node: Autoloading413452
Node: Quoting414066
Node: Backquote415491
Node: Eval417282
Ref: Definition of eval-region420127
Ref: Definition of max-lisp-eval-depth422313
Node: Control Structures425120
Node: Sequencing426903
Node: Conditionals429884
Node: Pattern matching case statement434031
Node: Combining Conditions443362
Node: Iteration447196
Node: Generators450167
Node: Nonlocal Exits454881
Node: Catch and Throw455593
Node: Examples of Catch459914
Node: Errors461963
Node: Signaling Errors463496
Ref: Definition of signal466157
Node: Processing of Errors468636
Node: Handling Errors470587
Node: Error Symbols480833
Node: Cleanups484540
Node: Variables488340
Ref: Variables-Footnote-1490554
Node: Global Variables490786
Node: Constant Variables491951
Node: Local Variables493265
Ref: Definition of max-specpdl-size498112
Node: Void Variables498870
Node: Defining Variables502059
Node: Tips for Defining507604
Node: Accessing Variables510504
Node: Setting Variables512219
Node: Variable Scoping515158
Ref: Variable Scoping-Footnote-1517037
Node: Dynamic Binding517144
Node: Dynamic Binding Tips519484
Node: Lexical Binding521423
Node: Using Lexical Binding525714
Node: Buffer-Local Variables528694
Node: Intro to Buffer-Local529615
Node: Creating Buffer-Local533837
Node: Default Value543328
Node: File Local Variables548025
Node: Directory Local Variables556116
Ref: Directory Local Variables-Footnote-1561430
Node: Variable Aliases561544
Node: Variables with Restricted Values565108
Node: Generalized Variables566444
Node: Setting Generalized Variables567372
Node: Adding Generalized Variables570871
Node: Functions573773
Node: What Is a Function575438
Node: Lambda Expressions581060
Node: Lambda Components582163
Node: Simple Lambda584007
Node: Argument List585593
Node: Function Documentation589680
Node: Function Names592328
Node: Defining Functions594358
Ref: Definition of defalias596441
Node: Calling Functions601036
Ref: Calling Functions-Footnote-1606654
Node: Mapping Functions606869
Ref: Definition of mapcar607838
Node: Anonymous Functions610362
Node: Generic Functions613950
Node: Function Cells624089
Node: Closures627498
Node: Advising Functions629100
Node: Core Advising Primitives632170
Node: Advising Named Functions637512
Node: Advice combinators641353
Node: Porting old advice646152
Node: Obsolete Functions649188
Node: Inline Functions652530
Node: Declare Form655041
Ref: Definition of declare655421
Node: Declaring Functions659621
Node: Function Safety663368
Node: Related Topics665007
Node: Macros666325
Node: Simple Macro667629
Node: Expansion668573
Node: Compiling Macros672473
Node: Defining Macros674156
Node: Problems with Macros676126
Node: Wrong Time676919
Node: Argument Evaluation677984
Node: Surprising Local Vars680924
Node: Eval During Expansion683056
Node: Repeated Expansion684895
Node: Indenting Macros687324
Node: Customization689824
Node: Common Keywords691079
Node: Group Definitions697692
Node: Variable Definitions700604
Node: Customization Types711545
Node: Simple Types713457
Node: Composite Types716618
Node: Splicing into Lists729485
Node: Type Keywords731354
Node: Defining New Types735684
Node: Applying Customizations739166
Node: Custom Themes741492
Node: Loading746139
Node: How Programs Do Loading748568
Ref: Definition of load-read-function755774
Node: Load Suffixes756379
Node: Library Search758865
Node: Loading Non-ASCII765123
Node: Autoload766469
Ref: autoload cookie771668
Node: Repeated Loading776724
Node: Named Features778774
Node: Where Defined785571
Node: Unloading788229
Node: Hooks for Loading790801
Node: Dynamic Modules793025
Node: Byte Compilation795030
Node: Speed of Byte-Code796703
Node: Compilation Functions797634
Node: Docs and Compilation804055
Node: Dynamic Loading806226
Node: Eval During Compile808866
Node: Compiler Errors812127
Node: Byte-Code Objects815207
Node: Disassembly818150
Node: Debugging824022
Node: Debugger825593
Node: Error Debugging826763
Node: Infinite Loops832048
Node: Function Debugging833368
Node: Explicit Debug835914
Node: Using Debugger837083
Node: Debugger Commands840148
Node: Invoking the Debugger844212
Node: Internals of Debugger848392
Node: Edebug853021
Node: Using Edebug855339
Node: Instrumenting857993
Node: Edebug Execution Modes861291
Node: Jumping865432
Node: Edebug Misc868492
Node: Breaks869989
Node: Breakpoints870517
Node: Global Break Condition873266
Node: Source Breakpoints874355
Node: Trapping Errors875343
Node: Edebug Views876499
Node: Edebug Eval878701
Node: Eval List879836
Node: Printing in Edebug883921
Node: Trace Buffer885701
Node: Coverage Testing887696
Node: The Outside Context890536
Node: Checking Whether to Stop891283
Node: Edebug Display Update892020
Node: Edebug Recursive Edit894200
Node: Edebug and Macros895892
Node: Instrumenting Macro Calls896464
Node: Specification List899965
Node: Backtracking908326
Node: Specification Examples910527
Node: Edebug Options912683
Node: Syntax Errors918040
Node: Excess Open919403
Node: Excess Close921211
Node: Test Coverage922510
Node: Profiling924147
Node: Read and Print926588
Node: Streams Intro927578
Node: Input Streams929736
Node: Input Functions934680
Node: Output Streams937981
Node: Output Functions942274
Node: Output Variables947852
Node: Minibuffers953078
Node: Intro to Minibuffers954680
Node: Text from Minibuffer958546
Ref: Definition of minibuffer-local-map967436
Node: Object from Minibuffer969392
Node: Minibuffer History972240
Node: Initial Input977561
Node: Completion979314
Node: Basic Completion981420
Ref: Definition of test-completion986780
Node: Minibuffer Completion990935
Node: Completion Commands995899
Node: High-Level Completion1001895
Ref: Definition of read-variable1006628
Node: Reading File Names1008248
Node: Completion Variables1017575
Node: Programmed Completion1022085
Node: Completion in Buffers1027562
Node: Yes-or-No Queries1032121
Node: Multiple Queries1036950
Node: Reading a Password1041316
Node: Minibuffer Commands1042403
Node: Minibuffer Windows1044219
Ref: Definition of minibuffer-window1044700
Node: Minibuffer Contents1047109
Node: Recursive Mini1048611
Node: Minibuffer Misc1050075
Ref: Definition of minibuffer-help-form1050708
Ref: Definition of minibuffer-scroll-window1050883
Node: Command Loop1052305
Node: Command Overview1053892
Node: Defining Commands1057066
Ref: The interactive-only property1058099
Node: Using Interactive1059107
Ref: Using Interactive-Footnote-11065901
Node: Interactive Codes1065954
Node: Interactive Examples1074275
Node: Generic Commands1075768
Node: Interactive Call1076954
Node: Distinguish Interactive1082730
Node: Command Loop Info1085435
Ref: Definition of this-command-keys-vector1089455
Node: Adjusting Point1091352
Node: Input Events1092536
Node: Keyboard Events1094744
Node: Function Keys1097539
Node: Mouse Events1100617
Node: Click Events1101746
Node: Drag Events1108102
Node: Button-Down Events1109710
Ref: Button-Down Events-Footnote-11110932
Node: Repeat Events1110992
Node: Motion Events1115437
Node: Focus Events1116364
Node: Misc Events1117897
Node: Event Examples1122993
Node: Classifying Events1124799
Node: Accessing Mouse1128892
Node: Accessing Scroll1135386
Node: Strings of Events1136592
Node: Reading Input1140497
Node: Key Sequence Input1141571
Node: Reading One Event1147268
Node: Event Mod1153468
Node: Invoking the Input Method1157184
Node: Quoted Character Input1159214
Node: Event Input Misc1160767
Node: Special Events1165354
Node: Waiting1166670
Node: Quitting1169494
Node: Prefix Command Arguments1175445
Node: Recursive Editing1180504
Node: Disabling Commands1185681
Node: Command History1187881
Node: Keyboard Macros1189611
Node: Keymaps1192718
Node: Key Sequences1194685
Node: Keymap Basics1197179
Node: Format of Keymaps1199797
Node: Creating Keymaps1204935
Node: Inheritance and Keymaps1207463
Node: Prefix Keys1210663
Ref: Definition of define-prefix-command1214524
Node: Active Keymaps1215162
Node: Searching Keymaps1219941
Node: Controlling Active Maps1222190
Ref: Definition of minor-mode-map-alist1224719
Node: Key Lookup1229895
Node: Functions for Key Lookup1234544
Node: Changing Key Bindings1240018
Node: Remapping Commands1248450
Node: Translation Keymaps1250925
Node: Key Binding Commands1257677
Node: Scanning Keymaps1260511
Node: Menu Keymaps1267498
Node: Defining Menus1268376
Ref: Defining Menus-Footnote-11270520
Node: Simple Menu Items1270607
Node: Extended Menu Items1272552
Node: Menu Separators1277483
Node: Alias Menu Items1280069
Node: Mouse Menus1281327
Node: Keyboard Menus1283410
Node: Menu Example1284636
Node: Menu Bar1287311
Node: Tool Bar1290693
Node: Modifying Menus1299649
Node: Easy Menu1301204
Node: Modes1305862
Node: Hooks1307183
Node: Running Hooks1309717
Node: Setting Hooks1311908
Node: Major Modes1314352
Node: Major Mode Conventions1317492
Node: Auto Major Mode1329139
Node: Mode Help1336627
Node: Derived Modes1337571
Node: Basic Major Modes1342695
Node: Mode Hooks1345143
Node: Tabulated List Mode1347875
Node: Generic Modes1354079
Node: Example Major Modes1356283
Node: Minor Modes1361340
Node: Minor Mode Conventions1362542
Node: Keymaps and Minor Modes1367921
Node: Defining Minor Modes1369215
Node: Mode Line Format1377708
Node: Mode Line Basics1379078
Node: Mode Line Data1380878
Node: Mode Line Top1385202
Node: Mode Line Variables1387743
Ref: Definition of minor-mode-alist1392321
Node: %-Constructs1394281
Node: Properties in Mode1398180
Node: Header Lines1399744
Node: Emulating Mode Line1400909
Node: Imenu1403120
Node: Font Lock Mode1410011
Node: Font Lock Basics1411703
Node: Search-based Fontification1417350
Node: Customizing Keywords1427254
Node: Other Font Lock Variables1430539
Node: Levels of Font Lock1434520
Node: Precalculated Fontification1435900
Node: Faces for Font Lock1436893
Node: Syntactic Font Lock1439207
Node: Multiline Font Lock1441505
Node: Font Lock Multiline1444748
Node: Region to Refontify1447193
Node: Auto-Indentation1448580
Node: SMIE1452150
Node: SMIE setup1453979
Node: Operator Precedence Grammars1455922
Node: SMIE Grammar1459633
Node: SMIE Lexer1462494
Node: SMIE Tricks1464729
Node: SMIE Indentation1468080
Node: SMIE Indentation Helpers1471601
Node: SMIE Indentation Example1473940
Node: SMIE Customization1477109
Node: Desktop Save Mode1478888
Node: Documentation1480661
Node: Documentation Basics1482264
Node: Accessing Documentation1484563
Ref: describe-symbols example1487677
Ref: Definition of Snarf-documentation1490993
Node: Keys in Documentation1492030
Node: Describing Characters1496749
Node: Help Functions1500697
Ref: Definition of data-directory1506341
Node: Files1509552
Node: Visiting Files1512001
Node: Visiting Functions1513469
Node: Subroutines of Visiting1521285
Node: Saving Buffers1524162
Ref: Definition of save-some-buffers1525749
Ref: Definition of write-file1526920
Node: Reading from Files1532943
Node: Writing to Files1535945
Ref: Definition of with-temp-file1540018
Node: File Locks1540621
Node: Information about Files1543999
Node: Testing Accessibility1545106
Node: Kinds of Files1551020
Node: Truenames1554585
Node: File Attributes1558526
Ref: Definition of file-attributes1559960
Node: Extended Attributes1565410
Node: Locating Files1568523
Node: Changing Files1571288
Node: File Names1583137
Ref: File Names-Footnote-11584907
Node: File Name Components1585147
Node: Relative File Names1589611
Node: Directory Names1591848
Ref: abbreviate-file-name1595142
Node: File Name Expansion1595766
Ref: Definition of substitute-in-file-name1599346
Node: Unique File Names1601132
Node: File Name Completion1605715
Node: Standard File Names1608964
Node: Contents of Directories1611676
Node: Create/Delete Dirs1617587
Node: Magic File Names1620110
Node: Format Conversion1632168
Node: Format Conversion Overview1632962
Node: Format Conversion Round-Trip1634003
Node: Format Conversion Piecemeal1641121
Node: Backups and Auto-Saving1646227
Node: Backup Files1646908
Node: Making Backups1648301
Node: Rename or Copy1653012
Node: Numbered Backups1656209
Node: Backup Names1658607
Node: Auto-Saving1662396
Node: Reverting1672334
Ref: Definition of revert-buffer-function1675053
Node: Buffers1677061
Node: Buffer Basics1678596
Node: Current Buffer1680684
Ref: Definition of with-temp-buffer1686481
Node: Buffer Names1687362
Node: Buffer File Name1691073
Node: Buffer Modification1697316
Node: Modification Time1700465
Node: Read Only Buffers1705144
Node: Buffer List1708694
Node: Creating Buffers1715956
Node: Killing Buffers1718474
Node: Indirect Buffers1723216
Node: Swapping Text1726279
Node: Buffer Gap1728784
Node: Windows1729790
Node: Basic Windows1732346
Ref: Window Group1735821
Node: Windows and Frames1737012
Node: Window Sizes1748158
Node: Resizing Windows1763169
Node: Preserving Window Sizes1775160
Node: Splitting Windows1779374
Node: Deleting Windows1786530
Node: Recombining Windows1790656
Node: Selecting Windows1803909
Node: Cyclic Window Ordering1808728
Node: Buffers and Windows1816130
Node: Switching Buffers1821243
Node: Choosing Window1828550
Node: Display Action Functions1833127
Node: Choosing Window Options1844179
Node: Window History1850803
Node: Dedicated Windows1856189
Node: Quitting Windows1859016
Node: Window Point1865032
Node: Window Start and End1867721
Node: Textual Scrolling1878487
Node: Vertical Scrolling1889668
Node: Horizontal Scrolling1892320
Node: Coordinates and Windows1898368
Node: Window Configurations1907288
Node: Window Parameters1913829
Node: Window Hooks1921633
Node: Frames1924862
Node: Creating Frames1929686
Node: Multiple Terminals1932512
Node: Frame Geometry1943310
Node: Frame Layout1944161
Node: Frame Font1954792
Node: Size and Position1956985
Node: Implied Frame Resizing1964249
Node: Frame Parameters1967185
Node: Parameter Access1968338
Node: Initial Parameters1971272
Node: Window Frame Parameters1974465
Node: Basic Parameters1975669
Node: Position Parameters1977474
Node: Size Parameters1981134
Node: Layout Parameters1984204
Node: Buffer Parameters1986954
Node: Management Parameters1988282
Node: Cursor Parameters1990407
Node: Font and Color Parameters1993505
Node: Geometry1998611
Node: Terminal Parameters1999861
Node: Frame Titles2002346
Node: Deleting Frames2004034
Node: Finding All Frames2005522
Node: Minibuffers and Frames2007269
Node: Input Focus2008742
Node: Visibility of Frames2015086
Node: Raising and Lowering2017353
Node: Frame Configurations2019020
Node: Mouse Tracking2019922
Node: Mouse Position2022397
Node: Pop-Up Menus2025671
Node: Dialog Boxes2029803
Node: Pointer Shape2032312
Node: Window System Selections2034275
Node: Drag and Drop2037761
Node: Color Names2039108
Node: Text Terminal Colors2043550
Node: Resources2046298
Node: Display Feature Testing2048661
Ref: Display Face Attribute Testing2050707
Node: Positions2056830
Node: Point2058349
Node: Motion2061084
Node: Character Motion2061856
Node: Word Motion2063414
Node: Buffer End Motion2067625
Node: Text Lines2069395
Ref: Definition of count-lines2073184
Node: Screen Lines2074326
Node: List Motion2082860
Node: Skipping Characters2087447
Node: Excursions2090100
Node: Narrowing2093035
Node: Markers2097713
Node: Overview of Markers2098742
Node: Predicates on Markers2101970
Node: Creating Markers2102895
Node: Information from Markers2105975
Node: Marker Insertion Types2106931
Node: Moving Markers2108153
Node: The Mark2109336
Node: The Region2120691
Node: Text2122673
Node: Near Point2125849
Node: Buffer Contents2129148
Node: Comparing Text2136074
Node: Insertion2137537
Node: Commands for Insertion2142464
Node: Deletion2145809
Node: User-Level Deletion2150416
Node: The Kill Ring2155339
Node: Kill Ring Concepts2157596
Node: Kill Functions2158653
Node: Yanking2161497
Node: Yank Commands2165054
Node: Low-Level Kill Ring2168125
Node: Internals of Kill Ring2172293
Node: Undo2175282
Node: Maintaining Undo2181874
Node: Filling2185329
Ref: Definition of sentence-end-double-space2191960
Node: Margins2193991
Node: Adaptive Fill2198363
Node: Auto Filling2202535
Node: Sorting2204042
Node: Columns2214554
Node: Indentation2216996
Node: Primitive Indent2217782
Node: Mode-Specific Indent2219281
Node: Region Indent2223071
Node: Relative Indent2226167
Node: Indent Tabs2228614
Node: Motion by Indent2230077
Node: Case Changes2231015
Node: Text Properties2234320
Node: Examining Properties2236701
Node: Changing Properties2240806
Node: Property Search2247791
Node: Special Properties2254356
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Ref: Inhibit point motion hooks2271691
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Node: Format Properties2272644
Node: Sticky Properties2273668
Node: Lazy Properties2277900
Node: Clickable Text2280135
Node: Fields2287613
Node: Not Intervals2293126
Node: Substitution2295667
Node: Registers2297557
Node: Transposition2301437
Node: Decompression2302356
Node: Base 642303547
Ref: Base 64-Footnote-12305768
Node: Checksum/Hash2306047
Node: Parsing HTML/XML2309170
Node: Document Object Model2311321
Node: Atomic Changes2314672
Node: Change Hooks2318353
Node: Non-ASCII Characters2322721
Node: Text Representations2324110
Ref: Text Representations-Footnote-12330678
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Node: Converting Representations2333380
Node: Selecting a Representation2336985
Node: Character Codes2339723
Node: Character Properties2341851
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Node: Character Sets2352997
Node: Scanning Charsets2357896
Node: Translation of Characters2359595
Node: Coding Systems2364462
Node: Coding System Basics2365583
Node: Encoding and I/O2370383
Node: Lisp and Coding Systems2374896
Node: User-Chosen Coding Systems2382772
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Node: Terminal I/O Encoding2408657
Node: Input Methods2410662
Node: Locales2413598
Node: Searching and Matching2415954
Node: String Search2417192
Node: Searching and Case2423498
Node: Regular Expressions2425481
Node: Syntax of Regexps2426606
Node: Regexp Special2428531
Node: Char Classes2439678
Node: Regexp Backslash2442356
Node: Regexp Example2451803
Node: Regexp Functions2454347
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Node: POSIX Regexps2466281
Node: Match Data2468221
Node: Replacing Match2469684
Node: Simple Match Data2473320
Node: Entire Match Data2478682
Node: Saving Match Data2481736
Node: Search and Replace2483099
Node: Standard Regexps2490998
Node: Syntax Tables2493659
Node: Syntax Basics2494744
Node: Syntax Descriptors2497487
Node: Syntax Class Table2499982
Node: Syntax Flags2506619
Node: Syntax Table Functions2510477
Node: Syntax Properties2515169
Node: Motion and Syntax2518165
Node: Parsing Expressions2519771
Node: Motion via Parsing2521770
Node: Position Parse2524868
Node: Parser State2526916
Node: Low-Level Parsing2529886
Node: Control Parsing2531718
Node: Syntax Table Internals2533001
Node: Categories2536297
Node: Abbrevs2542837
Node: Abbrev Tables2545472
Node: Defining Abbrevs2548182
Node: Abbrev Files2550862
Node: Abbrev Expansion2553226
Node: Standard Abbrev Tables2559504
Node: Abbrev Properties2561101
Node: Abbrev Table Properties2562512
Node: Processes2564304
Node: Subprocess Creation2567681
Node: Shell Arguments2572676
Node: Synchronous Processes2576708
Ref: Synchronous Processes-Footnote-12589505
Node: Asynchronous Processes2589609
Node: Deleting Processes2602660
Node: Process Information2604998
Ref: Coding systems for a subprocess2612437
Node: Input to Processes2613539
Node: Signals to Processes2616873
Node: Output from Processes2622323
Node: Process Buffers2625395
Node: Filter Functions2631580
Ref: Process Filter Example2634025
Node: Decoding Output2636890
Node: Accepting Output2638843
Node: Sentinels2640960
Node: Query Before Exit2646456
Node: System Processes2647556
Node: Transaction Queues2654846
Node: Network2656764
Node: Network Servers2664520
Node: Datagrams2666589
Node: Low-Level Network2667877
Node: Network Processes2668497
Node: Network Options2675155
Node: Network Feature Testing2678714
Node: Misc Network2680031
Node: Serial Ports2681855
Node: Byte Packing2689383
Node: Bindat Spec2690347
Node: Bindat Functions2696248
Node: Bindat Examples2699105
Node: Display2705112
Node: Refresh Screen2707046
Node: Forcing Redisplay2708475
Node: Truncation2710675
Node: The Echo Area2714471
Node: Displaying Messages2715395
Ref: message-box2719253
Node: Progress2720743
Node: Logging Messages2726120
Node: Echo Area Customization2728588
Node: Warnings2730454
Node: Warning Basics2731015
Node: Warning Variables2733923
Node: Warning Options2737319
Node: Delayed Warnings2738702
Node: Invisible Text2740357
Node: Selective Display2747538
Node: Temporary Displays2751731
Node: Overlays2760887
Node: Managing Overlays2762344
Node: Overlay Properties2768559
Node: Finding Overlays2779526
Node: Size of Displayed Text2782180
Node: Line Height2787665
Node: Faces2792262
Ref: Faces-Footnote-12794692
Node: Face Attributes2794834
Node: Defining Faces2804542
Node: Attribute Functions2812539
Node: Displaying Faces2820802
Node: Face Remapping2823131
Node: Face Functions2827815
Node: Auto Faces2829523
Node: Basic Faces2831168
Node: Font Selection2833383
Ref: Font Selection-Footnote-12837736
Node: Font Lookup2837840
Node: Fontsets2840325
Node: Low-Level Font2846597
Node: Fringes2859753
Node: Fringe Size/Pos2860492
Node: Fringe Indicators2862851
Node: Fringe Cursors2867139
Node: Fringe Bitmaps2868913
Node: Customizing Bitmaps2871845
Node: Overlay Arrow2873952
Node: Scroll Bars2876445
Node: Window Dividers2882800
Node: Display Property2885680
Node: Replacing Specs2886988
Node: Specified Space2889164
Node: Pixel Specification2892066
Node: Other Display Specs2894903
Node: Display Margins2899918
Node: Images2902748
Node: Image Formats2904174
Node: Image Descriptors2906353
Node: XBM Images2913732
Node: XPM Images2915986
Node: PostScript Images2916570
Node: ImageMagick Images2917446
Node: Other Image Types2921124
Node: Defining Images2922367
Node: Showing Images2927866
Node: Multi-Frame Images2932731
Node: Image Cache2935192
Node: Xwidgets2938248
Node: Buttons2942096
Node: Button Properties2943505
Node: Button Types2945585
Node: Making Buttons2946974
Node: Manipulating Buttons2949480
Node: Button Buffer Commands2951666
Node: Abstract Display2954719
Node: Abstract Display Functions2957627
Node: Abstract Display Example2962706
Node: Blinking2967432
Node: Character Display2969424
Node: Usual Display2970650
Node: Display Tables2974616
Node: Active Display Table2978578
Node: Glyphs2980527
Node: Glyphless Chars2982614
Node: Beeping2986558
Node: Window Systems2987904
Node: Tooltips2990092
Node: Bidirectional Display2993528
Node: System Interface3007906
Node: Starting Up3009968
Node: Startup Summary3010551
Node: Init File3020304
Node: Terminal-Specific3024248
Node: Command-Line Arguments3027771
Node: Getting Out3031520
Node: Killing Emacs3032136
Node: Suspending Emacs3034801
Node: System Environment3040176
Node: User Identification3048645
Node: Time of Day3052546
Node: Time Zone Rules3055943
Node: Time Conversion3058462
Node: Time Parsing3062570
Node: Processor Run Time3070557
Node: Time Calculations3072269
Node: Timers3073784
Node: Idle Timers3080680
Node: Terminal Input3085028
Node: Input Modes3085462
Node: Recording Input3087958
Node: Terminal Output3089353
Node: Sound Output3092493
Node: X11 Keysyms3094687
Node: Batch Mode3096231
Node: Session Management3097925
Node: Desktop Notifications3100061
Node: File Notifications3112652
Node: Dynamic Libraries3119574
Node: Security Considerations3121803
Node: Packaging3128352
Node: Packaging Basics3129235
Node: Simple Packages3133666
Node: Multi-file Packages3136263
Node: Package Archives3139615
Node: Antinews3144583
Node: GNU Free Documentation License3149407
Node: GPL3174750
Node: Tips3212524
Node: Coding Conventions3213982
Ref: Coding Conventions-Footnote-13223151
Node: Key Binding Conventions3223252
Node: Programming Tips3226343
Node: Compilation Tips3230662
Node: Warning Tips3232635
Node: Documentation Tips3234265
Ref: Docstring hyperlinks3238881
Node: Comment Tips3245982
Node: Library Headers3248806
Node: GNU Emacs Internals3256023
Node: Building Emacs3257010
Node: Pure Storage3262717
Node: Garbage Collection3265693
Node: Stack-allocated Objects3278758
Node: Memory Usage3280287
Node: C Dialect3281925
Node: Writing Emacs Primitives3282536
Ref: Defining Lisp variables in C3289922
Node: Object Internals3293428
Node: Buffer Internals3297106
Node: Window Internals3306325
Node: Process Internals3313436
Node: C Integer Types3316250
Node: Standard Errors3320853
Node: Standard Keymaps3327706
Node: Standard Hooks3332970
Node: Index3339375

End Tag Table


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