# Module Netsys_signal

    module Netsys_signal: sig .. end

Signal handler framework

This module defines a simple framework for setting signal handlers. When two modules want to set the handler for the same signal, the framework decides in which order the handlers are executed.

The module also defines an empty handler list for `Sys.sigpipe`, so these signals are ignored by the program. This empty list can be extended, however.

Win32: Only `Sys.sigint` handlers can effectively be registered. Registrations for other signal types are accepted but ignored.

    val register_handler : ?library:string ->
           ?priority:int ->
           ?keep_default:bool ->
           name:string -> signal:int -> callback:(int -> unit) -> unit -> unit

This function registers a handler called `name` for signal number `signal`. The handler function is `callback`. The int argument of the callback is the signal number.

By default, the handler is an application handler. If `library` is set, the handler is for this library. The name passed as `library` is the findlib name of the library.

By registering another handler for the same `library`, `name`, and `signal`, the old handler is overridden.

When several handlers are defined for the same signal, all handlers are executed that are defined for the signal (when the signal happens). The order of execution is given by `priority`. The handler functions are executed in ascending priority order. If the priority number is equal for two handlers, the order is undefined.

The priority defaults to 0 for library handlers, and to 100 for application handlers. Libraries should only use values from 0 to 99, and applications only from 100 to 199.

If all handlers for a certain signal set `keep_default`, then there will be a special action after all signal handlers have been executed. The special action emulates the default behavior for the signal. For now, there is only a simple emulation: If the signal terminates the process, the process is immediately exited with code 126. If the default behaviour is "no-op", nothing happens. We don't try (yet) to do better (emulate core-dumps, emulate the right process status) because this is difficult in the general case.

The handler definition takes place immediately.

Any exceptions occuring during the execution of a handler are caught and ignored.

    val register_exclusive_handler : name:string -> signal:int -> install:(unit -> unit) -> unit -> unit

An exclusive handler for a signal is the only handler for the signal. If it is tried to register another handler when there is already an exclusive handler, the second registration fails. Also, an exclusive handler cannot be registered when there is already a normal handler for the signal. It is, however, possible to replace the registered exclusive handler by another exclusive handler for the same signal.

An exclusive handler is installed by running the `install` function, which can e.g. call `Sys.set_signal` to define the handler. Other methods (e.g. use some C wrapper) are also possible. It is assumed that `install` overrides any existing handler.

    val restore_management : int -> unit

`restore_management signo`: Restores signal handling management for `signo` as defined by the handler list for this signal. Calling `restore_management` makes sense when the signal handler has been overridden with `Sys.set_signal`, but at some point this module is again responsible for managing the signal handling for this signal.

    val keep_away_from : int -> unit

`keep_away_from signo`: This signal `signo` is added to the "keep away list". This means that this module will never try to change the signal behavior again for `signo`. Even `restore_management` will not restore the signal handling again. This function should only by called by applications wishing to do the signal handling all themselves.

This function does not have any effect on the already installed handlers. It is nevertheless useful for applications calling `Sys.set_signal` directly to ensure that `Netsys_signal` will never again try to override the handler.

    type action = [ `Callback of int -> unit | `Install of unit -> unit ]

``Callback` is used for normal handlers, and ``Install` for exclusive handlers.

    type entry = {

`` | `sig_number :`int`;`
`` | `sig_library :`string option`;`
`` | `sig_priority :`int`;`
`` | `sig_keep_default :`bool`;`
`` | `sig_name :`string`;`
`` | `sig_action :`action`;`

}

    val list : unit -> entry list

Returns the list of signal handlers

    val keep_away_list : unit -> int list

Returns the "keep away list".

    val init : unit -> unit

Call this function to ensure that this module is initialized. It is also possible to call any other function. After initialization the Sigpipe handler is set.

# Debugging

    module Debug: sig .. end