GLib Reference Manual
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# Message Output and Debugging Functions

## Name

Message Output and Debugging Functions -- functions to output messages and help debug applications.

## Synopsis

    #include <glib.h>

    void        g_print                         (const gchar *format,
                                                 ...);
    GPrintFunc  g_set_print_handler             (GPrintFunc func);
    void        (*GPrintFunc)                   (const gchar *string);

    void        g_printerr                      (const gchar *format,
                                                 ...);
    GPrintFunc  g_set_printerr_handler          (GPrintFunc func);

    #define     g_return_if_fail                (expr)
    #define     g_return_val_if_fail            (expr,val)
    #define     g_return_if_reached             ()
    #define     g_return_val_if_reached         (val)

    #define     g_assert                        (expr)
    #define     g_assert_not_reached            ()

    void        g_on_error_query                (const gchar *prg_name);
    void        g_on_error_stack_trace          (const gchar *prg_name);

    #define     G_BREAKPOINT                    ()

## Description

These functions provide support for outputting messages.

## Details

### g_print ()

    void        g_print                         (const gchar *format,
                                                 ...);

Outputs a formatted message via the print handler. The default print handler simply outputs the message to stdout.

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().

`format` : | the message format. See the `printf()` documentation.
`...` : | the parameters to insert into the format string.

### g_set_print_handler ()

    GPrintFunc  g_set_print_handler             (GPrintFunc func);

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.

`func` : | the new print handler.
Returns : | the old print handler.

### GPrintFunc ()

    void        (*GPrintFunc)                   (const gchar *string);

Specifies the type of the print handler functions. These are called with the complete formatted string to output.

`string` : | the message to be output.

### g_printerr ()

    void        g_printerr                      (const gchar *format,
                                                 ...);

Outputs a formatted message via the error message handler. The default handler simply outputs the message to stderr.

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().

`format` : | the message format. See the `printf()` documentation.
`...` : | the parameters to insert into the format string.

### g_set_printerr_handler ()

    GPrintFunc  g_set_printerr_handler          (GPrintFunc func);

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.

`func` : | the new error message handler.
Returns : | the old error message handler.

### g_return_if_fail()

    #define     g_return_if_fail(expr)

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.

`expr` : | the expression to check.

### g_return_val_if_fail()

    #define     g_return_val_if_fail(expr,val)

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.

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

### g_return_if_reached()

    #define     g_return_if_reached()

Logs a critical message and returns from the current function. This can only be used in functions which do not return a value.

### g_return_val_if_reached()

    #define     g_return_val_if_reached(val)

Logs a critical message and returns `val`.

`val` : | the value to return from the current function.

### g_assert()

    #define     g_assert(expr)

Debugging macro to terminate the application if the assertion fails. If the assertion fails (i.e. the expression is not true), an error message is logged and the application is terminated.

The macro can be turned off in final releases of code by defining G_DISABLE_ASSERT when compiling the application.

`expr` : | the expression to check.

### g_assert_not_reached()

    #define     g_assert_not_reached()

Debugging macro to terminate the application if it is ever reached. If it is reached, an error message is logged and the application is terminated.

The macro can be turned off in final releases of code by defining G_DISABLE_ASSERT when compiling the application.

### g_on_error_query ()

    void        g_on_error_query                (const gchar *prg_name);

Prompts the user with `[E]xit, [H]alt, show [S]tack trace or [P]roceed`. This function is intended to be used for debugging use only. FIXME: How do you set it up?

If [E]xit is selected, the application terminates with a call to `_exit(0)`.

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).

If [S]tack trace is selected, g_on_error_stack_trace() is called. This invokes gdb, which attaches to the current process and shows a stack trace. The prompt is then shown again.

If [P]roceed is selected, the function returns.

This function may cause different actions on non-UNIX platforms.

`prg_name` : | the program name, needed by gdb 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).

### g_on_error_stack_trace ()

    void        g_on_error_stack_trace          (const gchar *prg_name);

Invokes gdb, 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.

This function may cause different actions on non-UNIX platforms.

`prg_name` : | the program name, needed by gdb 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).

### G_BREAKPOINT()

    #define     G_BREAKPOINT()

Inserts a breakpoint instruction into the code (on x86 machines only).

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Error Reporting | Message Logging