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Message Output and Debugging Functions

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functions to output messages and help debug applications

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<para>
These functions provide support for outputting messages.
</para>
<para>
The <function>g_return</function> family of macros (g_return_if_fail(), 
g_return_val_if_fail(), g_return_if_reached(), g_return_val_if_reached())
should only be used for programming errors, a typical use case is
checking for invalid parameters at the beginning of a public function.
They should not be used if you just mean "if (error) return", they
should only be used if you mean "if (bug in program) return".
The program behavior is generally considered undefined after one of these 
checks fails. They are not intended for normal control flow, only to
give a perhaps-helpful warning before giving up. 
</para>

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<para>

</para>

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<!-- ##### FUNCTION g_print ##### -->
<para>
Outputs a formatted message via the print handler.
The default print handler simply outputs the message to stdout.
</para>
<para>
g_print() should not be used from within libraries for debugging messages,
since it may be redirected by applications to special purpose message
windows or even files.
Instead, libraries should use g_log(), or the convenience functions
g_message(), g_warning() and g_error().
</para>

@format: the message format. See the printf() documentation.
@Varargs: the parameters to insert into the format string.


<!-- ##### FUNCTION g_set_print_handler ##### -->
<para>
Sets the print handler.
Any messages passed to g_print() will be output via the new handler.
The default handler simply outputs the message to stdout.
By providing your own handler you can redirect the output, to a GTK+
widget or a log file for example.
</para>

@func: the new print handler.
@Returns: the old print handler.


<!-- ##### USER_FUNCTION GPrintFunc ##### -->
<para>
Specifies the type of the print handler functions.
These are called with the complete formatted string to output.
</para>

@string: the message to be output.


<!-- ##### FUNCTION g_printerr ##### -->
<para>
Outputs a formatted message via the error message handler.
The default handler simply outputs the message to stderr.
</para>
<para>
g_printerr() should not be used from within libraries. Instead g_log() should
be used, or the convenience functions g_message(), g_warning() and g_error().
</para>

@format: the message format. See the printf() documentation.
@Varargs: the parameters to insert into the format string.


<!-- ##### FUNCTION g_set_printerr_handler ##### -->
<para>
Sets the handler for printing error messages.
Any messages passed to g_printerr() will be output via the new handler.
The default handler simply outputs the message to stderr.
By providing your own handler you can redirect the output, to a GTK+
widget or a log file for example.
</para>

@func: the new error message handler.
@Returns: the old error message handler.


<!-- ##### MACRO g_return_if_fail ##### -->
<para>
Returns from the current function if the expression is not true.
If the expression evaluates to %FALSE, a critical message is logged and
the function returns. This can only be used in functions which do not return
a value.
</para>

@expr: the expression to check.


<!-- ##### MACRO g_return_val_if_fail ##### -->
<para>
Returns from the current function, returning the value @val, if the expression
is not true.
If the expression evaluates to %FALSE, a critical message is logged and
@val is returned.
</para>

@expr: the expression to check.
@val: the value to return from the current function if the expression is not
true.


<!-- ##### MACRO g_return_if_reached ##### -->
<para>
Logs a critical message and returns from the current function. 
This can only be used in functions which do not return a value.
</para>


<!-- ##### MACRO g_return_val_if_reached ##### -->
<para>
Logs a critical message and returns @val. 
</para>

@val: the value to return from the current function.


<!-- ##### MACRO g_warn_if_fail ##### -->
<para>
Logs a warning if the expression is not true.
</para>

@expr: the expression to check
@Since: 2.16


<!-- ##### MACRO g_warn_if_reached ##### -->
<para>
Logs a critical warning.
</para>

@Since: 2.16


<!-- ##### FUNCTION g_on_error_query ##### -->
<para>
Prompts the user with <computeroutput>[E]xit, [H]alt, show [S]tack trace or [P]roceed</computeroutput>.
This function is intended to be used for debugging use only. The following
example shows how it can be used together with the g_log() functions.
</para>
<informalexample><programlisting>
&num;include &lt;glib.h&gt;

static void 
log_handler (const gchar   *log_domain,
	     GLogLevelFlags log_level,
	     const gchar   *message,
	     gpointer       user_data)
{
  g_log_default_handler (log_domain, log_level, message, user_data);

  g_on_error_query (MY_PROGRAM_NAME);
}

int main (int argc, char *argv[])
{
  g_log_set_handler (MY_LOG_DOMAIN,
		     G_LOG_LEVEL_WARNING | 
                     G_LOG_LEVEL_ERROR | 
                     G_LOG_LEVEL_CRITICAL,
		     log_handler,
		     NULL);

 /* ... */  
</programlisting></informalexample>
<para>
If [E]xit is selected, the application terminates with a call to
<function>_exit(0)</function>.
</para>
<para>
If [H]alt is selected, the application enters an infinite loop.
The infinite loop can only be stopped by killing the application,
or by setting #glib_on_error_halt to %FALSE (possibly via a debugger).
</para>
<para>
If [S]tack trace is selected, g_on_error_stack_trace() is called. This
invokes <command>gdb</command>, which attaches to the current process and shows a stack trace.
The prompt is then shown again.
</para>
<para>
If [P]roceed is selected, the function returns.
</para>
<para>
This function may cause different actions on non-UNIX platforms.
</para>

@prg_name: the program name, needed by <command>gdb</command> for the [S]tack trace option.
If @prg_name is %NULL, g_get_prgname() is called to get the program name
(which will work correctly if gdk_init() or gtk_init() has been called).


<!-- ##### FUNCTION g_on_error_stack_trace ##### -->
<para>
Invokes <command>gdb</command>, which attaches to the current process and shows a stack trace.
Called by g_on_error_query() when the [S]tack trace option is selected.
</para>
<para>
This function may cause different actions on non-UNIX platforms.
</para>

@prg_name: the program name, needed by <command>gdb</command> for the [S]tack trace option.
If @prg_name is %NULL, g_get_prgname() is called to get the program name
(which will work correctly if gdk_init() or gtk_init() has been called).


<!-- ##### MACRO G_BREAKPOINT ##### -->
<para>
Inserts a breakpoint instruction into the code.  On x86 and alpha systems
this is implemented as a soft interrupt and on other architectures it raises
a %SIGTRAP signal.
</para>