glib/docs/reference/glib/tmpl/messages.sgml

<!-- ##### SECTION Title ##### -->
Message Logging

<!-- ##### SECTION Short_Description ##### -->
versatile support for logging messages with different levels of importance.

<!-- ##### SECTION Long_Description ##### -->
<para>
These functions provide support for logging error messages or messages
used for debugging.
</para>

<para>
There are several built-in levels of messages, defined in #GLogLevelFlags.
These can be extended with user-defined levels.
</para>

<!-- ##### SECTION See_Also ##### -->
<para>

</para>

<!-- ##### MACRO G_LOG_DOMAIN ##### -->
<para>
Defines the log domain.
For applications, this is typically left as the default %NULL (or "") domain.
Libraries should define this so that any messages which they log can
be differentiated from messages from other libraries and application code.
But be careful not to define it in any public header files.
</para>
<para>
For example, GTK+ uses this in its Makefile.am:
</para>
<informalexample><programlisting>
INCLUDES = -DG_LOG_DOMAIN=\"Gtk\"
</programlisting></informalexample>



<!-- ##### MACRO G_LOG_FATAL_MASK ##### -->
<para>
GLib log levels that are considered fatal by default.
</para>



<!-- ##### MACRO G_LOG_LEVEL_USER_SHIFT ##### -->
<para>
Log level shift offset for user defined log levels (0-7 are used by GLib).
</para>



<!-- ##### USER_FUNCTION GLogFunc ##### -->
<para>
Specifies the prototype of log handler functions.
</para>

@log_domain: the log domain of the message.
@log_level: the log level of the message (including the fatal and recursion
flags).
@message: the message to process.
@user_data: user data, set in g_log_set_handler().


<!-- ##### ENUM GLogLevelFlags ##### -->
<para>
Flags specifying the level of log messages.
</para>

@G_LOG_FLAG_RECURSION: 
@G_LOG_FLAG_FATAL: 
@G_LOG_LEVEL_ERROR: 
@G_LOG_LEVEL_CRITICAL: 
@G_LOG_LEVEL_WARNING: 
@G_LOG_LEVEL_MESSAGE: 
@G_LOG_LEVEL_INFO: 
@G_LOG_LEVEL_DEBUG: 
@G_LOG_LEVEL_MASK: 

<!-- ##### FUNCTION g_log ##### -->
<para>
Logs an error or debugging message.
If the log level has been set as fatal, the <function>abort()</function>
function is called to terminate the program.
</para>

@log_domain: the log domain, usually #G_LOG_DOMAIN.
@log_level: the log level, either from #GLogLevelFlags or a user-defined level.
@format: the message format. See the <function>printf()</function>
documentation.
@Varargs: the parameters to insert into the format string.


<!-- ##### FUNCTION g_logv ##### -->
<para>
Logs an error or debugging message.
If the log level has been set as fatal, the <function>abort()</function>
function is called to terminate the program.
</para>

@log_domain: the log domain.
@log_level: the log level.
@format: the message format. See the <function>printf()</function>
documentation.
@args: the parameters to insert into the format string.


<!-- ##### MACRO g_message ##### -->
<para>
A convenience function/macro to log a normal message.
</para>

@...: the parameters to insert into the format string.
<!-- # Unused Parameters # -->
@format: the message format. See the <function>printf()</function>
documentation.
@args...: the parameters to insert into the format string.


<!-- ##### MACRO g_warning ##### -->
<para>
A convenience function/macro to log a warning message.
</para>

@...: format string, followed by parameters to insert into the format string (as with <function>printf()</function>)


<!-- ##### MACRO g_critical ##### -->
<para>
Logs a "critical warning" (#G_LOG_LEVEL_CRITICAL). It's more or less
application-defined what constitutes a critical vs. a regular
warning. You could call g_log_set_always_fatal() to make critical
warnings exit the program, then use g_critical() for fatal errors, for
example.
</para>

@...: format string, followed by parameters to insert into the format string (as with <function>printf()</function>)


<!-- ##### MACRO g_error ##### -->
<para>
A convenience function/macro to log an error message.
Error messages are always fatal, resulting in a call to
<function>abort()</function> to terminate the application.
This function will result in a core dump; don't use it for errors you
expect. Using this function indicates a bug in your program, i.e. an
assertion failure.
</para>

@...: the parameters to insert into the format string.
<!-- # Unused Parameters # -->
@format: the message format. See the <function>printf()</function>
documentation.
@args...: the parameters to insert into the format string.


<!-- ##### FUNCTION g_log_set_handler ##### -->
<para>
Sets the log handler for a domain and a set of log levels.
To handle fatal and recursive messages the @log_levels parameter
must be combined with the #G_LOG_FLAG_FATAL and #G_LOG_FLAG_RECURSION bit flags.
</para>
<para>
Note that since the #G_LOG_LEVEL_ERROR log level is always fatal, if you want
to set a handler for this log level you must combine it with #G_LOG_FLAG_FATAL.
</para>

<example>
<title>Adding a log handler for all warning messages in the default (application) domain</title>
<programlisting>
  g_log_set_handler (NULL, G_LOG_LEVEL_WARNING | G_LOG_FLAG_FATAL
                     | G_LOG_FLAG_RECURSION, my_log_handler, NULL);
</programlisting>
</example>

<example>
<title>Adding a log handler for all critical messages from GTK+</title>
<programlisting>
  g_log_set_handler ("Gtk", G_LOG_LEVEL_CRITICAL | G_LOG_FLAG_FATAL
                     | G_LOG_FLAG_RECURSION, my_log_handler, NULL);
</programlisting>
</example>

<example>
<title>Adding a log handler for <emphasis>all</emphasis> messages from GLib</title>
<programlisting>
  g_log_set_handler ("GLib", G_LOG_LEVEL_MASK | G_LOG_FLAG_FATAL
                     | G_LOG_FLAG_RECURSION, my_log_handler, NULL);
</programlisting>
</example>

@log_domain: the log domain, or %NULL for the default "" application domain.
@log_levels: the log levels to apply the log handler for. To handle fatal
and recursive messages as well, combine the log levels with the
#G_LOG_FLAG_FATAL and #G_LOG_FLAG_RECURSION bit flags.
@log_func: the log handler function.
@user_data: data passed to the log handler.
@Returns: the id of the new handler.


<!-- ##### FUNCTION g_log_remove_handler ##### -->
<para>
Removes the log handler.
</para>

@log_domain: the log domain.
@handler_id: the id of the handler, which was returned in g_log_set_handler().


<!-- ##### FUNCTION g_log_set_always_fatal ##### -->
<para>
Sets the message levels which are always fatal, in any log domain.
When a message with any of these levels is logged the program terminates.
You can only set the levels defined by GLib to be fatal.
%G_LOG_LEVEL_ERROR is always fatal.
</para>

@fatal_mask: the mask containing bits set for each level of error which is
to be fatal.
@Returns: the old fatal mask.


<!-- ##### FUNCTION g_log_set_fatal_mask ##### -->
<para>
Sets the log levels which are fatal in the given domain.
%G_LOG_LEVEL_ERROR is always fatal.
</para>

@log_domain: the log domain.
@fatal_mask: the new fatal mask.
@Returns: the old fatal mask for the log domain.


<!-- ##### FUNCTION g_log_default_handler ##### -->
<para>
The default log handler.
This is used if no log handler has been set for the particular log domain
and log level combination. It outputs the message to stderr or stdout
and if the log level is fatal it calls <function>abort()</function>.
</para>
<para>
stderr is used for levels %G_LOG_LEVEL_ERROR, %G_LOG_LEVEL_CRITICAL, and
%G_LOG_LEVEL_WARNING. stdout is used for the rest.
</para>

@log_domain: the log domain of the message.
@log_level: the level of the message.
@message: the message.
@unused_data: data passed from g_log() which is unused.