Running GLib Applications

Running GLib Applications

Environment variables

The runtime behaviour of GLib applications can be influenced by a number of environment variables.

Standard variables
GLib reads standard environment variables like LANG, PATH, HOME, TMPDIR, TZ and LOGNAME.
XDG directories
GLib consults the environment variables XDG_DATA_HOME, XDG_DATA_DIRS, XDG_CONFIG_HOME, XDG_CONFIG_DIRS, XDG_CACHE_HOME and XDG_RUNTIME_DIR for the various XDG directories. For more information, see the XDG basedir specification.
This environment variable can be set to a comma-separated list of character set names. GLib assumes that filenames are encoded in the first character set from that list rather than in UTF-8. The special token “locale” can be used to specify the character set for the current locale.
If this environment variable is set, GLib assumes that filenames are in the locale encoding rather than in UTF-8. G_FILENAME_ENCODING takes priority over G_BROKEN_FILENAMES.
A list of log levels for which messages should be prefixed by the program name and PID of the application. The default is to prefix everything except G_LOG_LEVEL_MESSAGE and G_LOG_LEVEL_INFO. The possible values are error, warning, critical, message, info and debug. You can also use the special values all and help. This environment variable only affects the default log handler, g_log_default_handler().
A space-separated list of log domains for which informational and debug messages should be printed. By default, these messages are not printed. You can also use the special value all. This environment variable only affects the default log handler, g_log_default_handler().
This environment variable can be set to a list of debug options, which cause GLib to print out different types of debugging information.
  • fatal-warnings: Causes GLib to abort the program at the first call to g_warning() or g_critical(). Use of this flag is not recommended except when debugging.
  • fatal-criticals: Causes GLib to abort the program at the first call to g_critical(). This flag can be useful during debugging and testing.
  • gc-friendly: Newly allocated memory that isn’t directly initialized, as well as memory being freed will be reset to 0. The point here is to allow memory checkers and similar programs that use Boehm GC alike algorithms to produce more accurate results.
  • resident-modules: All modules loaded by GModule will be made resident. This can be useful for tracking memory leaks in modules which are later unloaded; but it can also hide bugs where code is accessed after the module would have normally been unloaded.
  • bind-now-modules: All modules loaded by GModule will bind their symbols at load time, even when the code uses G_MODULE_BIND_LAZY.

The special value all can be used to turn on all debug options. The special value help can be used to print all available options.

This environment variable allows reconfiguration of the GSlice memory allocator.
  • always-malloc: This will cause all slices allocated through g_slice_alloc() and released by g_slice_free1() to be actually allocated via direct calls to g_malloc() and g_free(). This is most useful for memory checkers and similar programs that use Boehm GC alike algorithms to produce more accurate results. It can also be in conjunction with debugging features of the system’s malloc() implementation such as glibc’s MALLOC_CHECK_=2 to debug erroneous slice allocation code, although debug-blocks is usually a better suited debugging tool.
  • debug-blocks: Using this option (present since GLib 2.13) engages extra code which performs sanity checks on the released memory slices. Invalid slice addresses or slice sizes will be reported and lead to a program halt. This option is for debugging scenarios. In particular, client packages sporting their own test suite should always enable this option when running tests. Global slice validation is ensured by storing size and address information for each allocated chunk, and maintaining a global hash table of that data. That way, multi-thread scalability is given up, and memory consumption is increased. However, the resulting code usually performs acceptably well, possibly better than with comparable memory checking carried out using external tools.

An example of a memory corruption scenario that cannot be reproduced with G_SLICE=always-malloc, but will be caught by G_SLICE=debug-blocks is as follows:

 void *slist = g_slist_alloc (); /* void* gives up type-safety */
 g_list_free (slist);            /* corruption: sizeof (GSList) != sizeof (GList) */

The special value all can be used to turn on all options. The special value help can be used to print all available options.

If this environment variable is set to ‘2.0’, the outdated pseudo-random number seeding and generation algorithms from GLib 2.0 are used instead of the newer, better ones. You should only set this variable if you have sequences of numbers that were generated with Glib 2.0 that you need to reproduce exactly.
Allows to specify a nonstandard location for the charset.aliases file that is used by the character set conversion routines. The default location is the libdir specified at compilation time.
Allows to specify a nonstandard location for the timezone data files that are used by the GDateTime API. The default location is under /usr/share/zoneinfo. For more information, also look at the tzset manual page.
If set to a non-zero value, this environment variable enables diagnostic messages, like deprecation messages for GObject properties and signals.
When running on Windows, if set to a non-empty string, GLib will try to interpret the contents of this environment variable as a command line to a debugger, and run it if the process crashes. The debugger command line should contain %p and %e substitution tokens, which GLib will replace with the process ID of the crashing process and a handle to an event that the debugger should signal to let GLib know that the debugger successfully attached to the process. If %e is absent, or if the debugger is not able to signal events, GLib will resume execution after 60 seconds. If %p is absent, the debugger won’t know which process to attach to, and GLib will also resume execution after 60 seconds. Additionally, even if G_DEBUGGER is not set, GLib would still try to print basic exception information (code and address) into stderr. By default the debugger gets a new console allocated for it. Set the G_DEBUGGER_OLD_CONSOLE environment variable to any non-empty string to make the debugger inherit the console of the crashing process. Normally this is only used by the GLib testsuite. The exception handler is written with the aim of making it as simple as possible, to minimize the risk of it invoking buggy functions or running buggy code, which would result in exceptions being raised recursively. Because of that it lacks most of the amenities that one would expect of GLib. Namely, it does not support Unicode, so it is highly advisable to only use ASCII characters in G_DEBUGGER. See also G_VEH_CATCH.
Catching some exceptions can break the program, since Windows will sometimes use exceptions for execution flow control and other purposes other than signalling a crash. The G_VEH_CATCH environment variable augments Vectored Exception Handling on Windows (see G_DEBUGGER), allowing GLib to catch more exceptions. Set this variable to a comma-separated list of hexadecimal exception codes that should additionally be caught. By default GLib will only catch Access Violation, Stack Overflow and Illegal Instruction exceptions.


A number of interfaces in GLib depend on the current locale in which an application is running. Therefore, most GLib-using applications should call setlocale (LC_ALL, "") to set up the current locale.

On Windows, in a C program there are several locale concepts that not necessarily are synchronized. On one hand, there is the system default ANSI code-page, which determines what encoding is used for file names handled by the C library’s functions and the Win32 API. (We are talking about the “narrow” functions here that take character pointers, not the “wide” ones.)

On the other hand, there is the C library’s current locale. The character set (code-page) used by that is not necessarily the same as the system default ANSI code-page. Strings in this character set are returned by functions like strftime().

Debugging with GDB

GLib ships with a set of Python macros for the GDB debugger. These includes pretty printers for lists, hashtables and GObject types. It also has a backtrace filter that makes backtraces with signal emissions easier to read.

To use this you need a version of GDB that supports Python scripting; anything from 7.0 should be fine. You then need to install GLib in the same prefix as GDB so that the Python GDB autoloaded files get installed in the right place for GDB to pick up.

General pretty printing should just happen without having to do anything special. To get the signal emission filtered backtrace you must use the “new-backtrace” command instead of the standard one.

There is also a new command called gforeach that can be used to apply a command on each item in a list. E.g. you can do

gforeach i in some_list_variable: print *(GtkWidget *)l

Which would print the contents of each widget in a list of widgets.


SystemTap is a dynamic whole-system analysis toolkit. GLib ships with a file*.stp which defines a set of probe points, which you can hook into with custom SystemTap scripts. See the files*.stp,*.stp and*.stp which are in your shared SystemTap scripts directory.

Memory statistics

g_mem_profile() will output a summary g_malloc() memory usage, if memory profiling has been enabled by calling:

g_mem_set_vtable (glib_mem_profiler_table);

upon startup.

If GLib has been configured with full debugging support, then g_slice_debug_tree_statistics() can be called in a debugger to output details about the memory usage of the slice allocator.