glib/docs/reference/glib/running.sgml

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<refentry id="glib-running" revision="17 Jan 2002">
<refmeta>
<refentrytitle>Running GLib Applications</refentrytitle>
<manvolnum>3</manvolnum>
<refmiscinfo>GLib Library</refmiscinfo>
</refmeta>

<refnamediv>
<refname>Running GLib Applications</refname>
<refpurpose>
How to run and debug your GLib application
</refpurpose>
</refnamediv>

<refsect1>
<title>Running and debugging GLib Applications</title>

<refsect2>
<title>Environment variables</title>

<para> 
GLib inspects a few of environment variables in addition to standard
variables like <envar>LANG</envar>, <envar>PATH</envar> or <envar>HOME</envar>. 
</para>

<formalpara id="G_FILENAME_ENCODING">
  <title><envar>G_FILENAME_ENCODING</envar></title>

  <para>
    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.
  </para>
</formalpara>

<formalpara id="G_BROKEN_FILENAMES">
  <title><envar>G_BROKEN_FILENAMES</envar></title>

  <para>
    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. 
  </para>
</formalpara>

<formalpara id="G_MESSAGES_PREFIXED">
  <title><envar>G_MESSAGES_PREFIXED</envar></title>

  <para>
    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 <literal>G_LOG_LEVEL_MESSAGE</literal> and <literal>G_LOG_LEVEL_INFO</literal>. 
  </para>
</formalpara>

<formalpara id="G_DEBUG">
  <title><envar>G_DEBUG</envar></title>
  <para>
    If GLib has been configured with <option>--enable-debug=yes</option>,
    this variable can be set to a list of debug options, which cause GLib
    to print out different types of debugging information.
    <variablelist>
      <varlistentry>
        <term>fatal_warnings</term>
        <listitem><para>Causes GLib to abort the program at the first call
           to <link linkend="g-warning">g_warning</link>() or
	   <link linkend="g-critical">g_critical</link>(). This option is 
           special in that it doesn't require GLib to be configured with 
           debugging support.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>fatal_criticals</term>
        <listitem><para>Causes GLib to abort the program at the first call
           to <link linkend="g-critical">g_critical</link>(). This option is 
           special in that it doesn't require GLib to be configured with 
           debugging support.</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>gc-friendly</term>
	<listitem>
		<para>
		  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 bohem GC alike
		  algorithms to produce more accurate results.
		  This option is special in that it doesn't require GLib to be
		  configured with debugging support.
		</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>resident-modules</term>
	<listitem>
	  <para>
	    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.
	    This option is special in that it doesn't require GLib to be
 	    configured with debugging support.
	  </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>bind-now-modules</term>
	<listitem>
	  <para>
	    All modules loaded by GModule will bind their symbols at load time, even
	    when the code uses %G_MODULE_BIND_LAZY.
	    This option is special in that it doesn't require GLib to be
 	    configured with debugging support.
	  </para>
        </listitem>
      </varlistentry>
    </variablelist>
    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.  
  </para>
</formalpara>

<formalpara id="G_SLICE">
	<title><envar>G_SLICE</envar></title>
	<para>
	  This environment variable allows reconfiguration of the GSlice
	  memory allocator.
	  <variablelist>
	    <varlistentry>
	      <term>always-malloc</term>
	      <listitem>
		<para>
		  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 Bohem 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, allthough <literal>debug-blocks</literal>
		  usually is a better suited debugging tool.
		</para>
	      </listitem>
	    </varlistentry>
	    <varlistentry>
	      <term>debug-blocks</term>
	      <listitem>
		<para>
		  Using this option (present since GLib-2.13) engages extra code
		  which performs sanity checks on the released memory slices.
		  Invalid slice adresses 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
		  <emphasis>always enable this option when running tests</emphasis>.
		  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 <literal>G_SLICE=always-malloc</literal>, but will be caught
		  by <literal>G_SLICE=debug-blocks</literal> is as follows:
		  <programlisting>
		    void *slist = g_slist_alloc(); /* void* gives up type-safety */
		    g_list_free (slist);           /* corruption: sizeof (GSList) != sizeof (GList) */
		  </programlisting>
		</para>
	      </listitem>
	    </varlistentry>
	  </variablelist>
          The special value all can be used to turn on all options. 
          The special value help can be used to print all available options.  
	</para>
</formalpara>

<formalpara id="G_RANDOM_VERSION">
  <title><envar>G_RANDOM_VERSION</envar></title>

  <para>
    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 new better ones. Use the GLib-2.0
    algorithms only if you have sequences of numbers generated with
    Glib-2.0 that you need to reproduce exactly.  
  </para>
</formalpara> 

<formalpara id="LIBCHARSET_ALIAS_DIR">
  <title><envar>LIBCHARSET_ALIAS_DIR</envar></title>

  <para>
    Allows to specify a nonstandard location for the 
    <filename>charset.aliases</filename> file that is used by the
    character set conversion routines. The default location is the 
    <replaceable>libdir</replaceable> specified at compilation time.
  </para>
</formalpara> 

</refsect2>

<refsect2 id="setlocale">
<title>Locale</title>

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

<para>
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.)
</para>

<para>
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 <function>strftime()</function>.
</para>

</refsect2>

<refsect2>
<title>Traps and traces</title>

<para>
<indexterm><primary>g_trap_free_size</primary></indexterm>
<indexterm><primary>g_trap_realloc_size</primary></indexterm>
<indexterm><primary>g_trap_malloc_size</primary></indexterm>
Some code portions contain trap variables that can be set during debugging 
time if GLib has been configured with <option>--enable-debug=yes</option>. 
Such traps lead to immediate code halts to examine the current program state 
and backtrace.
</para>

<para>
Currently, the following trap variables exist:
<programlisting>
static volatile gulong g_trap_free_size;
static volatile gulong g_trap_realloc_size;
static volatile gulong g_trap_malloc_size;
</programlisting>
If set to a size > 0, <link linkend="g-free">g_free</link>(), 
<link linkend="g-realloc">g_realloc</link>() and 
<link linkend="g-malloc">g_malloc</link>() will be intercepted if the size 
matches the size of the corresponding memory block. This will only work with 
<literal>g_mem_set_vtable (glib_mem_profiler_table)</literal> upon startup 
though, because memory profiling is required to match on the memory block sizes.
</para>
<para>
Note that many modern debuggers support conditional breakpoints, which achieve
pretty much the same. E.g. in gdb, you can do
<programlisting>
break g_malloc
condition 1 n_bytes == 20
</programlisting>
to break only on g_malloc() calls where the size of the allocated memory block
is 20. 
</para>
</refsect2>

<refsect2>
<title>Gdb debugging macros</title>

<para>
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.
</para>

<para>
To use this you need a recent enough gdb that supports python scripting. Gdb 7.0
should be recent enough, but branches of the "archer" gdb tree as used in Fedora 11
and Fedora 12 should work too. 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.
</para>

<para>
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.
</para>

<para>
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
<programlisting>
gforeach i in some_list_variable: print *(GtkWidget *)l
</programlisting>
Which would print the contents of each widget in a list of widgets.
</para>

</refsect2>

<refsect2>
<title>Memory statistics</title>

<para>
g_mem_profile() will output a summary g_malloc() memory usage, if memory
profiling has been enabled by calling 
<literal>g_mem_set_vtable (glib_mem_profiler_table)</literal> upon startup.
</para>

<para>
If GLib has been configured with <option>--enable-debug=yes</option>,
then g_slice_debug_tree_statistics() can be called in a debugger to 
output details about the memory usage of the slice allocator.
</para>

</refsect2>
</refsect1>
</refentry>