<refentry id="glib-Message-Logging">
<refmeta>
<refentrytitle>Message Logging</refentrytitle>
<manvolnum>3</manvolnum>
<refmiscinfo>GLIB Library</refmiscinfo>
</refmeta>
<refnamediv>
<refname>Message Logging</refname><refpurpose>versatile support for logging messages with different levels of importance.</refpurpose>
</refnamediv>
<refsynopsisdiv><title>Synopsis</title>
<synopsis>
#include <glib.h>
#define <link linkend="G-LOG-DOMAIN-CAPS">G_LOG_DOMAIN</link>
#define <link linkend="G-LOG-FATAL-MASK-CAPS">G_LOG_FATAL_MASK</link>
#define <link linkend="G-LOG-LEVEL-USER-SHIFT-CAPS">G_LOG_LEVEL_USER_SHIFT</link>
<link linkend="void">void</link> (<link linkend="GLogFunc">*GLogFunc</link>) (const <link linkend="gchar">gchar</link> *log_domain,
<link linkend="GLogLevelFlags">GLogLevelFlags</link> log_level,
const <link linkend="gchar">gchar</link> *message,
<link linkend="gpointer">gpointer</link> user_data);
enum <link linkend="GLogLevelFlags">GLogLevelFlags</link>;
<link linkend="void">void</link> <link linkend="g-log">g_log</link> (const <link linkend="gchar">gchar</link> *log_domain,
<link linkend="GLogLevelFlags">GLogLevelFlags</link> log_level,
const <link linkend="gchar">gchar</link> *format,
...);
<link linkend="void">void</link> <link linkend="g-logv">g_logv</link> (const <link linkend="gchar">gchar</link> *log_domain,
<link linkend="GLogLevelFlags">GLogLevelFlags</link> log_level,
const <link linkend="gchar">gchar</link> *format,
<link linkend="va-list">va_list</link> args);
#define <link linkend="g-message">g_message</link> (...)
#define <link linkend="g-warning">g_warning</link> (...)
#define <link linkend="g-critical">g_critical</link> (...)
#define <link linkend="g-error">g_error</link> (...)
<link linkend="guint">guint</link> <link linkend="g-log-set-handler">g_log_set_handler</link> (const <link linkend="gchar">gchar</link> *log_domain,
<link linkend="GLogLevelFlags">GLogLevelFlags</link> log_levels,
<link linkend="GLogFunc">GLogFunc</link> log_func,
<link linkend="gpointer">gpointer</link> user_data);
<link linkend="void">void</link> <link linkend="g-log-remove-handler">g_log_remove_handler</link> (const <link linkend="gchar">gchar</link> *log_domain,
<link linkend="guint">guint</link> handler_id);
<link linkend="GLogLevelFlags">GLogLevelFlags</link> <link linkend="g-log-set-always-fatal">g_log_set_always_fatal</link> (<link linkend="GLogLevelFlags">GLogLevelFlags</link> fatal_mask);
<link linkend="GLogLevelFlags">GLogLevelFlags</link> <link linkend="g-log-set-fatal-mask">g_log_set_fatal_mask</link> (const <link linkend="gchar">gchar</link> *log_domain,
<link linkend="GLogLevelFlags">GLogLevelFlags</link> fatal_mask);
<link linkend="void">void</link> <link linkend="g-log-default-handler">g_log_default_handler</link> (const <link linkend="gchar">gchar</link> *log_domain,
<link linkend="GLogLevelFlags">GLogLevelFlags</link> log_level,
const <link linkend="gchar">gchar</link> *message,
<link linkend="gpointer">gpointer</link> unused_data);
</synopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<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 <link linkend="GLogLevelFlags"><type>GLogLevelFlags</type></link>.
These can be extended with user-defined levels.
</para>
</refsect1>
<refsect1>
<title>Details</title>
<refsect2>
<title><anchor id="G-LOG-DOMAIN-CAPS"/>G_LOG_DOMAIN</title>
<indexterm><primary>G_LOG_DOMAIN</primary></indexterm><programlisting>#define G_LOG_DOMAIN ((gchar*) 0)
</programlisting>
<para>
Defines the log domain.
For applications, this is typically left as the default <literal>NULL</literal> (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></refsect2>
<refsect2>
<title><anchor id="G-LOG-FATAL-MASK-CAPS"/>G_LOG_FATAL_MASK</title>
<indexterm><primary>G_LOG_FATAL_MASK</primary></indexterm><programlisting>#define G_LOG_FATAL_MASK (G_LOG_FLAG_RECURSION | G_LOG_LEVEL_ERROR)
</programlisting>
<para>
GLib log levels that are considered fatal by default.
</para></refsect2>
<refsect2>
<title><anchor id="G-LOG-LEVEL-USER-SHIFT-CAPS"/>G_LOG_LEVEL_USER_SHIFT</title>
<indexterm><primary>G_LOG_LEVEL_USER_SHIFT</primary></indexterm><programlisting>#define G_LOG_LEVEL_USER_SHIFT (8)
</programlisting>
<para>
Log level shift offset for user defined log levels (0-7 are used by GLib).
</para></refsect2>
<refsect2>
<title><anchor id="GLogFunc"/>GLogFunc ()</title>
<indexterm><primary>GLogFunc</primary></indexterm><programlisting><link linkend="void">void</link> (*GLogFunc) (const <link linkend="gchar">gchar</link> *log_domain,
<link linkend="GLogLevelFlags">GLogLevelFlags</link> log_level,
const <link linkend="gchar">gchar</link> *message,
<link linkend="gpointer">gpointer</link> user_data);</programlisting>
<para>
Specifies the prototype of log handler functions.
</para><variablelist role="params">
<varlistentry><term><parameter>log_domain</parameter> :</term>
<listitem><simpara>the log domain of the message.
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>log_level</parameter> :</term>
<listitem><simpara>the log level of the message (including the fatal and recursion
flags).
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>message</parameter> :</term>
<listitem><simpara>the message to process.
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>user_data</parameter> :</term>
<listitem><simpara>user data, set in <link linkend="g-log-set-handler"><function>g_log_set_handler()</function></link>.
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="GLogLevelFlags"/>enum GLogLevelFlags</title>
<indexterm><primary>GLogLevelFlags</primary></indexterm><programlisting>typedef enum
{
/* log flags */
G_LOG_FLAG_RECURSION = 1 << 0,
G_LOG_FLAG_FATAL = 1 << 1,
/* GLib log levels */
G_LOG_LEVEL_ERROR = 1 << 2, /* always fatal */
G_LOG_LEVEL_CRITICAL = 1 << 3,
G_LOG_LEVEL_WARNING = 1 << 4,
G_LOG_LEVEL_MESSAGE = 1 << 5,
G_LOG_LEVEL_INFO = 1 << 6,
G_LOG_LEVEL_DEBUG = 1 << 7,
G_LOG_LEVEL_MASK = ~(G_LOG_FLAG_RECURSION | G_LOG_FLAG_FATAL)
} GLogLevelFlags;
</programlisting>
<para>
Flags specifying the level of log messages.
</para></refsect2>
<refsect2>
<title><anchor id="g-log"/>g_log ()</title>
<indexterm><primary>g_log</primary></indexterm><programlisting><link linkend="void">void</link> g_log (const <link linkend="gchar">gchar</link> *log_domain,
<link linkend="GLogLevelFlags">GLogLevelFlags</link> log_level,
const <link linkend="gchar">gchar</link> *format,
...);</programlisting>
<para>
Logs an error or debugging message.
If the log level has been set as fatal, the <function><link linkend="abort"><function>abort()</function></link></function>
function is called to terminate the program.
</para><variablelist role="params">
<varlistentry><term><parameter>log_domain</parameter> :</term>
<listitem><simpara>the log domain, usually <link linkend="G-LOG-DOMAIN-CAPS"><type>G_LOG_DOMAIN</type></link>.
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>log_level</parameter> :</term>
<listitem><simpara>the log level, either from <link linkend="GLogLevelFlags"><type>GLogLevelFlags</type></link> or a user-defined level.
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>format</parameter> :</term>
<listitem><simpara>the message format. See the <function><link linkend="printf"><function>printf()</function></link></function>
documentation.
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>...</parameter> :</term>
<listitem><simpara>the parameters to insert into the format string.
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-logv"/>g_logv ()</title>
<indexterm><primary>g_logv</primary></indexterm><programlisting><link linkend="void">void</link> g_logv (const <link linkend="gchar">gchar</link> *log_domain,
<link linkend="GLogLevelFlags">GLogLevelFlags</link> log_level,
const <link linkend="gchar">gchar</link> *format,
<link linkend="va-list">va_list</link> args);</programlisting>
<para>
Logs an error or debugging message.
If the log level has been set as fatal, the <function><link linkend="abort"><function>abort()</function></link></function>
function is called to terminate the program.
</para><variablelist role="params">
<varlistentry><term><parameter>log_domain</parameter> :</term>
<listitem><simpara>the log domain.
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>log_level</parameter> :</term>
<listitem><simpara>the log level.
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>format</parameter> :</term>
<listitem><simpara>the message format. See the <function><link linkend="printf"><function>printf()</function></link></function>
documentation.
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>args</parameter> :</term>
<listitem><simpara>the parameters to insert into the format string.
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-message"/>g_message()</title>
<indexterm><primary>g_message</primary></indexterm><programlisting>#define g_message(...)</programlisting>
<para>
A convenience function/macro to log a normal message.
</para><variablelist role="params">
<varlistentry><term><parameter>...</parameter> :</term>
<listitem><simpara>format string, followed by parameters to insert into the format string (as with <function><link linkend="printf"><function>printf()</function></link></function>)
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-warning"/>g_warning()</title>
<indexterm><primary>g_warning</primary></indexterm><programlisting>#define g_warning(...)</programlisting>
<para>
A convenience function/macro to log a warning message.
</para><variablelist role="params">
<varlistentry><term><parameter>...</parameter> :</term>
<listitem><simpara>format string, followed by parameters to insert into the format string (as with <function><link linkend="printf"><function>printf()</function></link></function>)
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-critical"/>g_critical()</title>
<indexterm><primary>g_critical</primary></indexterm><programlisting>#define g_critical(...)</programlisting>
<para>
Logs a "critical warning" (<link linkend="G-LOG-LEVEL-CRITICAL-CAPS"><type>G_LOG_LEVEL_CRITICAL</type></link>). It's more or less
application-defined what constitutes a critical vs. a regular
warning. You could call <link linkend="g-log-set-always-fatal"><function>g_log_set_always_fatal()</function></link> to make critical
warnings exit the program, then use <link linkend="g-critical"><function>g_critical()</function></link> for fatal errors, for
example.
</para><variablelist role="params">
<varlistentry><term><parameter>...</parameter> :</term>
<listitem><simpara>format string, followed by parameters to insert into the format string (as with <function><link linkend="printf"><function>printf()</function></link></function>)
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-error"/>g_error()</title>
<indexterm><primary>g_error</primary></indexterm><programlisting>#define g_error(...)</programlisting>
<para>
A convenience function/macro to log an error message.
Error messages are always fatal, resulting in a call to
<function><link linkend="abort"><function>abort()</function></link></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><variablelist role="params">
<varlistentry><term><parameter>...</parameter> :</term>
<listitem><simpara>the parameters to insert into the format string.
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-log-set-handler"/>g_log_set_handler ()</title>
<indexterm><primary>g_log_set_handler</primary></indexterm><programlisting><link linkend="guint">guint</link> g_log_set_handler (const <link linkend="gchar">gchar</link> *log_domain,
<link linkend="GLogLevelFlags">GLogLevelFlags</link> log_levels,
<link linkend="GLogFunc">GLogFunc</link> log_func,
<link linkend="gpointer">gpointer</link> user_data);</programlisting>
<para>
Sets the log handler for a domain and a set of log levels.
To handle fatal and recursive messages the <parameter>log_levels</parameter> parameter
must be combined with the <link linkend="G-LOG-FLAG-FATAL-CAPS"><type>G_LOG_FLAG_FATAL</type></link> and <link linkend="G-LOG-FLAG-RECURSION-CAPS"><type>G_LOG_FLAG_RECURSION</type></link> bit flags.
</para>
<para>
Note that since the <link linkend="G-LOG-LEVEL-ERROR-CAPS"><type>G_LOG_LEVEL_ERROR</type></link> log level is always fatal, if you want
to set a handler for this log level you must combine it with <link linkend="G-LOG-FLAG-FATAL-CAPS"><type>G_LOG_FLAG_FATAL</type></link>.
</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><variablelist role="params">
<varlistentry><term><parameter>log_domain</parameter> :</term>
<listitem><simpara>the log domain, or <literal>NULL</literal> for the default "" application domain.
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>log_levels</parameter> :</term>
<listitem><simpara>the log levels to apply the log handler for. To handle fatal
and recursive messages as well, combine the log levels with the
<link linkend="G-LOG-FLAG-FATAL-CAPS"><type>G_LOG_FLAG_FATAL</type></link> and <link linkend="G-LOG-FLAG-RECURSION-CAPS"><type>G_LOG_FLAG_RECURSION</type></link> bit flags.
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>log_func</parameter> :</term>
<listitem><simpara>the log handler function.
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>user_data</parameter> :</term>
<listitem><simpara>data passed to the log handler.
</simpara></listitem></varlistentry>
<varlistentry><term><emphasis>Returns</emphasis> :</term><listitem><simpara>the id of the new handler.
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-log-remove-handler"/>g_log_remove_handler ()</title>
<indexterm><primary>g_log_remove_handler</primary></indexterm><programlisting><link linkend="void">void</link> g_log_remove_handler (const <link linkend="gchar">gchar</link> *log_domain,
<link linkend="guint">guint</link> handler_id);</programlisting>
<para>
Removes the log handler.
</para><variablelist role="params">
<varlistentry><term><parameter>log_domain</parameter> :</term>
<listitem><simpara>the log domain.
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>handler_id</parameter> :</term>
<listitem><simpara>the id of the handler, which was returned in <link linkend="g-log-set-handler"><function>g_log_set_handler()</function></link>.
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-log-set-always-fatal"/>g_log_set_always_fatal ()</title>
<indexterm><primary>g_log_set_always_fatal</primary></indexterm><programlisting><link linkend="GLogLevelFlags">GLogLevelFlags</link> g_log_set_always_fatal (<link linkend="GLogLevelFlags">GLogLevelFlags</link> fatal_mask);</programlisting>
<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.
<literal>G_LOG_LEVEL_ERROR</literal> is always fatal.
</para><variablelist role="params">
<varlistentry><term><parameter>fatal_mask</parameter> :</term>
<listitem><simpara>the mask containing bits set for each level of error which is
to be fatal.
</simpara></listitem></varlistentry>
<varlistentry><term><emphasis>Returns</emphasis> :</term><listitem><simpara>the old fatal mask.
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-log-set-fatal-mask"/>g_log_set_fatal_mask ()</title>
<indexterm><primary>g_log_set_fatal_mask</primary></indexterm><programlisting><link linkend="GLogLevelFlags">GLogLevelFlags</link> g_log_set_fatal_mask (const <link linkend="gchar">gchar</link> *log_domain,
<link linkend="GLogLevelFlags">GLogLevelFlags</link> fatal_mask);</programlisting>
<para>
Sets the log levels which are fatal in the given domain.
<literal>G_LOG_LEVEL_ERROR</literal> is always fatal.
</para><variablelist role="params">
<varlistentry><term><parameter>log_domain</parameter> :</term>
<listitem><simpara>the log domain.
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>fatal_mask</parameter> :</term>
<listitem><simpara>the new fatal mask.
</simpara></listitem></varlistentry>
<varlistentry><term><emphasis>Returns</emphasis> :</term><listitem><simpara>the old fatal mask for the log domain.
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-log-default-handler"/>g_log_default_handler ()</title>
<indexterm><primary>g_log_default_handler</primary></indexterm><programlisting><link linkend="void">void</link> g_log_default_handler (const <link linkend="gchar">gchar</link> *log_domain,
<link linkend="GLogLevelFlags">GLogLevelFlags</link> log_level,
const <link linkend="gchar">gchar</link> *message,
<link linkend="gpointer">gpointer</link> unused_data);</programlisting>
<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><link linkend="abort"><function>abort()</function></link></function>.
</para>
<para>
stderr is used for levels <literal>G_LOG_LEVEL_ERROR</literal>, <literal>G_LOG_LEVEL_CRITICAL</literal>, and
<literal>G_LOG_LEVEL_WARNING</literal>. stdout is used for the rest.
</para><variablelist role="params">
<varlistentry><term><parameter>log_domain</parameter> :</term>
<listitem><simpara>the log domain of the message.
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>log_level</parameter> :</term>
<listitem><simpara>the level of the message.
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>message</parameter> :</term>
<listitem><simpara>the message.
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>unused_data</parameter> :</term>
<listitem><simpara>data passed from <link linkend="g-log"><function>g_log()</function></link> which is unused.
</simpara></listitem></varlistentry>
</variablelist></refsect2>
</refsect1>
</refentry>