- Qt 6.9
- Qt Core
- <QtLogging> - Qt Logging Types
- Qt 6.9.3 Reference Documentation

# <QtLogging> - Qt Logging Types

The <QtLogging> header file defines Qt logging types, functions and macros. More...

Header: | `#include <QtLogging>`

## Types

| QtMessageHandler
enum | QtMsgType { QtDebugMsg, QtInfoMsg, QtWarningMsg, QtCriticalMsg, QtFatalMsg }

## Functions

QString | qFormatLogMessage(QtMsgType type, const QMessageLogContext &context, const QString &str)
QtMessageHandler | qInstallMessageHandler(QtMessageHandler handler)
void | qSetMessagePattern(const QString &pattern)

## Macros

| qCritical(const char *format, ...)
| qDebug(const char *format, ...)
| qFatal(const char *format, ...)
| qInfo(const char *format, ...)
| qWarning(const char *format, ...)

## Detailed Description

The <QtLogging> header file contains several types, functions and macros for logging.

The QtMsgType enum identifies the various messages that can be generated and sent to a Qt message handler; QtMessageHandler is a type definition for a pointer to a function with the signature `void myMessageHandler(QtMsgType, const QMessageLogContext &, const char *)`. qInstallMessageHandler() function can be used to install the given QtMessageHandler. QMessageLogContext class contains the line, file, and function the message was logged at. This information is created by the QMessageLogger class.

<QtLogging> also contains functions that generate messages from the given string argument: qDebug(), qInfo(), qWarning(), qCritical(), and qFatal(). These functions call the message handler with the given message.

Example:

     if (!driver()->isOpen() || driver()->isOpenError()) {
         qWarning("QSqlQuery::exec: database not open");
         return false;
     }

See also QLoggingCategory.

## Type Documentation

### QtMessageHandler

This is a typedef for a pointer to a function with the following signature:

     void myMessageHandler(QtMsgType, const QMessageLogContext &, const QString &);

See also QtMsgType and qInstallMessageHandler().

### enum QtMsgType

This enum describes the messages that can be sent to a message handler (QtMessageHandler). You can use the enum to identify and associate the various message types with the appropriate actions. Its values are, in order of increasing severity:

Constant | Value | Description
--- | --- | ---
`QtDebugMsg` | `0` | A message generated by the qDebug() function.
`QtInfoMsg` | `4` | A message generated by the qInfo() function.
`QtWarningMsg` | `1` | A message generated by the qWarning() function.
`QtCriticalMsg` | `2` | A message generated by the qCritical() function.
`QtFatalMsg` | `3` | A message generated by the qFatal() function.

See also QtMessageHandler, qInstallMessageHandler(), and QLoggingCategory.

## Function Documentation

### QString qFormatLogMessage(QtMsgType type, const QMessageLogContext &context, const QString &str)

Generates a formatted string out of the type, context, str arguments.

qFormatLogMessage returns a QString that is formatted according to the current message pattern. It can be used by custom message handlers to format output similar to Qt's default message handler.

The function is thread-safe.

See also qInstallMessageHandler() and qSetMessagePattern().

### QtMessageHandler qInstallMessageHandler(QtMessageHandler handler)

Installs a Qt message handler. Returns a pointer to the previously installed message handler.

A message handler is a function that prints out debug, info, warning, critical, and fatal messages from Qt's logging infrastructure. By default, Qt uses a standard message handler that formats and prints messages to different sinks specific to the operating system and Qt configuration. Installing your own message handler allows you to assume full control, and for instance log messages to the file system.

Note that Qt supports logging categories for grouping related messages in semantic categories. You can use these to enable or disable logging per category and message type. As the filtering for logging categories is done even before a message is created, messages for disabled types and categories will not reach the message handler.

A message handler needs to be reentrant. That is, it might be called from different threads, in parallel. Therefore, writes to common sinks (like a database, or a file) often need to be synchronized.

Qt allows to enrich logging messages with further meta-information by calling qSetMessagePattern(), or setting the `QT_MESSAGE_PATTERN` environment variable. To keep this formatting, a custom message handler can use qFormatLogMessage().

Try to keep the code in the message handler itself minimal, as expensive operations might block the application. Also, to avoid recursion, any logging messages generated in the message handler itself will be ignored.

The message handler should always return. For fatal messages, the application aborts immediately after handling that message.

Only one message handler can be installed at a time, for the whole application. If there was a previous custom message handler installed, the function will return a pointer to it. This handler can then be later reinstalled by another call to the method. Also, calling `qInstallMessageHandler(nullptr)` will restore the default message handler.

Here is an example of a message handler that logs to a local file before calling the default handler:

     #include <QApplication>
     #include <stdio.h>
     #include <stdlib.h>

     QtMessageHandler originalHandler = nullptr;

     void logToFile(QtMsgType type, const QMessageLogContext &context, const QString &msg)
     {
         QString message = qFormatLogMessage(type, context, msg);
         static FILE *f = fopen("log.txt", "a");
         fprintf(f, "%s\n", qPrintable(message));
         fflush(f);

         if (originalHandler)
             *originalHandler(type, context, msg);
     }

     int main(int argc, char **argv)
     {
         originalHandler = qInstallMessageHandler(logToFile);
         QApplication app(argc, argv);
         ...
         return app.exec();
     }

Note that the C++ standard guarantees that `static FILE *f` is initialized in a thread-safe way. We can also expect `fprintf()` and `fflush()` to be thread-safe, so no further synchronization is necessary.

See also QtMessageHandler, QtMsgType, qDebug(), qInfo(), qWarning(), qCritical(), qFatal(), Debugging Techniques, and qFormatLogMessage().

### void qSetMessagePattern(const QString &pattern)

Changes the output of the default message handler.

Allows to tweak the output of qDebug(), qInfo(), qWarning(), qCritical(), and qFatal(). The category logging output of qCDebug(), qCInfo(), qCWarning(), and qCCritical() is formatted, too.

Following placeholders are supported:

Placeholder | Description
--- | ---
`%{appname}` | QCoreApplication::applicationName()
`%{category}` | Logging category
`%{file}` | Path to source file
`%{function}` | Function
`%{line}` | Line in source file
`%{message}` | The actual message
`%{pid}` | QCoreApplication::applicationPid()
`%{threadid}` | The system-wide ID of current thread (if it can be obtained)
`%{qthreadptr}` | A pointer to the current QThread (result of QThread::currentThread())
`%{type}` | "debug", "warning", "critical" or "fatal"
`%{time process}` | time of the message, in seconds since the process started (the token "process" is literal)
`%{time boot}` | the time of the message, in seconds since the system boot if that can be determined (the token "boot" is literal). If the time since boot could not be obtained, the output is indeterminate (see QElapsedTimer::msecsSinceReference()).
`%{time [format]}` | system time when the message occurred, formatted by passing the `format` to QDateTime::toString(). If the format is not specified, the format of Qt::ISODate is used.

`%{backtrace [depth=N] [separator="..."]}`

A backtrace with the number of frames specified by the optional `depth` parameter (defaults to 5), and separated by the optional `separator` parameter (defaults to "|").

This expansion is available only on some platforms:

- platforms using glibc;
- platforms shipping C++23's `<stacktrace>` header (requires compiling Qt in C++23 mode).

Depending on the platform, there are some restrictions on the function names printed by this expansion.

On some platforms, names are only known for exported functions. If you want to see the name of every function in your application, make sure your application is compiled and linked with `-rdynamic`, or an equivalent of it.

When reading backtraces, take into account that frames might be missing due to inlining or tail call optimization.

You can also use conditionals on the type of the message using `%{if-debug}`, `%{if-info}` `%{if-warning}`, `%{if-critical}` or `%{if-fatal}` followed by an `%{endif}`. What is inside the `%{if-*}` and `%{endif}` will only be printed if the type matches.

Finally, text inside `%{if-category}` ... `%{endif}` is only printed if the category is not the default one.

Example:

         QT_MESSAGE_PATTERN="[%{time yyyyMMdd h:mm:ss.zzz ttt} %{if-debug}D%{endif}%{if-info}I%{endif}%{if-warning}W%{endif}%{if-critical}C%{endif}%{if-fatal}F%{endif}] %{file}:%{line} - %{message}"

The default pattern is `%{if-category}%{category}: %{endif}%{message}`.

Note: On Android, the default pattern is `%{message}` because the category is used as tag since Android logcat has a dedicated field for the logging categories, see Android Logging. If a custom pattern including the category is used, QCoreApplication::applicationName() is used as tag.

The pattern can also be changed at runtime by setting the QT_MESSAGE_PATTERN environment variable; if both qSetMessagePattern() is called and QT_MESSAGE_PATTERN is set, the environment variable takes precedence.

Note: The information for the placeholders `category`, `file`, `function` and `line` is only recorded in debug builds. Alternatively, `QT_MESSAGELOGCONTEXT` can be defined explicitly. For more information refer to the QMessageLogContext documentation.

Note: The message pattern only applies to unstructured logging, such as the default `stderr` output. Structured logging such as systemd will record the message as is, along with as much structured information as can be captured.

Custom message handlers can use qFormatLogMessage() to take pattern into account.

See also qInstallMessageHandler(), Debugging Techniques, QLoggingCategory, and QMessageLogContext.

## Macro Documentation

### qCritical(const char *format, ...)

Logs critical message format to the central message handler. format can contain format specifiers that are replaced by values specificed in additional arguments.

Example:

     void load(const QString &fileName)
     {
         QFile file(fileName);
         if (!file.exists())
             qCritical("File '%s' does not exist!", qUtf8Printable(fileName));
     }

format can contain format specifiers like `%s` for UTF-8 strings, or `%i` for integers. This is similar to how the C `printf()` function works. For more details on the formatting, see QString::asprintf().

For more convenience and further type support, you can also use QDebug::qCritical(), which follows the streaming paradigm (similar to `std::cout` or `std::cerr`).

To suppress the output at runtime, you can define logging rules or register a custom filter.

For debugging purposes, it is sometimes convenient to let the program abort for critical messages. This allows you to inspect the core dump, or attach a debugger - see also qFatal(). To enable this, set the environment variable `QT_FATAL_CRITICALS` to a number `n`. The program terminates then for the n-th critical message. That is, if the environment variable is set to 1, it will terminate on the first call; if it contains the value 10, it will exit on the 10th call. Any non-numeric value in the environment variable is equivalent to 1.

Note: This macro is thread-safe.

See also QDebug::qCritical, qCCritical(), qDebug(), qInfo(), qWarning(), qFatal(), qInstallMessageHandler(), and Debugging Techniques.

### qDebug(const char *format, ...)

Logs debug message format to the central message handler. format can contain format specifiers that are replaced by values specificed in additional arguments.

Example:

     qDebug("Items in list: %d", myList.size());

format can contain format specifiers like `%s` for UTF-8 strings, or `%i` for integers. This is similar to how the C `printf()` function works. For more details on the formatting, see QString::asprintf().

For more convenience and further type support, you can also use QDebug::qDebug(), which follows the streaming paradigm (similar to `std::cout` or `std::cerr`).

This function does nothing if `QT_NO_DEBUG_OUTPUT` was defined during compilation.

To suppress the output at runtime, install your own message handler with qInstallMessageHandler().

Note: This macro is thread-safe.

See also QDebug::qDebug(), qCDebug(), qInfo(), qWarning(), qCritical(), qFatal(), qInstallMessageHandler(), and Debugging Techniques.

### qFatal(const char *format, ...)

Logs fatal message format to the central message handler. format can contain format specifiers that are replaced by values specificed in additional arguments.

Example:

     int divide(int a, int b)
     {
         if (b == 0)                                // program error
             qFatal("divide: cannot divide by zero");
         return a / b;
     }

If you are using the default message handler this function will abort to create a core dump. On Windows, for debug builds, this function will report a _CRT_ERROR enabling you to connect a debugger to the application.

To suppress the output at runtime, install your own message handler with qInstallMessageHandler().

See also qCFatal(), qDebug(), qInfo(), qWarning(), qCritical(), qInstallMessageHandler(), and Debugging Techniques.

### qInfo(const char *format, ...)

Logs informational message format to the central message handler. format can contain format specifiers that are replaced by values specificed in additional arguments.

Example:

     qInfo("Items in list: %d", myList.size());

format can contain format specifiers like `%s` for UTF-8 strings, or `%i` for integers. This is similar to how the C `printf()` function works. For more details on the formatting, see QString::asprintf().

For more convenience and further type support, you can also use QDebug::qInfo(), which follows the streaming paradigm (similar to `std::cout` or `std::cerr`).

This function does nothing if `QT_NO_INFO_OUTPUT` was defined during compilation.

To suppress the output at runtime, install your own message handler using qInstallMessageHandler().

Note: This macro is thread-safe.

See also QDebug::qInfo(), qCInfo(), qDebug(), qWarning(), qCritical(), qFatal(), qInstallMessageHandler(), and Debugging Techniques.

### qWarning(const char *format, ...)

Logs warning message format to the central message handler. format can contain format specifiers that are replaced by values specificed in additional arguments.

Example:

     void f(int c)
     {
         if (c > 200)
             qWarning("f: bad argument, c == %d", c);
     }

format can contain format specifiers like `%s` for UTF-8 strings, or `%i` for integers. This is similar to how the C `printf()` function works. For more details on the formatting, see QString::asprintf().

For more convenience and further type support, you can also use QDebug::qWarning(), which follows the streaming paradigm (similar to `std::cout` or `std::cerr`).

This function does nothing if `QT_NO_WARNING_OUTPUT` was defined during compilation. To suppress the output at runtime, you can set logging rules or register a custom filter.

For debugging purposes, it is sometimes convenient to let the program abort for warning messages. This allows you to inspect the core dump, or attach a debugger - see also qFatal(). To enable this, set the environment variable `QT_FATAL_WARNINGS` to a number `n`. The program terminates then for the n-th warning. That is, if the environment variable is set to 1, it will terminate on the first call; if it contains the value 10, it will exit on the 10th call. Any non-numeric value in the environment variable is equivalent to 1.

Note: This macro is thread-safe.

See also QDebug::qWarning(), qCWarning(), qDebug(), qInfo(), qCritical(), qFatal(), qInstallMessageHandler(), and Debugging Techniques.