gtk/docs/reference/gdk/tmpl/threads.sgml

<!-- ##### SECTION Title ##### -->
Threads

<!-- ##### SECTION Short_Description ##### -->
Functions for using GDK in multi-threaded programs

<!-- ##### SECTION Long_Description ##### -->
<para>
For thread safety, GDK relies on the thread primitives in GLib, 
and on the thread-safe GLib main loop.
</para>
<para>
GLib is completely thread safe (all global data is automatically 
locked), but individual data structure instances are not automatically 
locked for performance reasons. So e.g. you must coordinate 
accesses to the same #GHashTable from multiple threads.
</para>
<para>
GTK+ is "thread aware" but not thread safe &mdash; it provides a
global lock controlled by gdk_threads_enter()/gdk_threads_leave()
which protects all use of GTK+. That is, only one thread can use GTK+ 
at any given time.
</para>
<para>
You must call g_thread_init() and gdk_threads_init() before executing
any other GTK+ or GDK functions in a threaded GTK+ program.
</para>
<para>
Idles, timeouts, and input functions are executed outside
of the main GTK+ lock. So, if you need to call GTK+
inside of such a callback, you must surround the callback
with a gdk_threads_enter()/gdk_threads_leave() pair.
(However, signals are still executed within the main
GTK+ lock.)
</para>
<para>
In particular, this means, if you are writing widgets that might 
be used in threaded programs, you <emphasis>must</emphasis> surround 
timeouts and idle functions in this matter.
</para>
<para>
As always, you must also surround any calls to GTK+ not made within 
a signal handler with a gdk_threads_enter()/gdk_threads_leave() pair.
</para>

<para>
Before calling gdk_threads_leave() from a thread other
than your main thread, you probably want to call gdk_flush()
to send all pending commands to the windowing system.
(The reason you don't need to do this from the main thread
is that GDK always automatically flushes pending commands
when it runs out of incoming events to process and has
to sleep while waiting for more events.)
</para>

<para>A minimal main program for a threaded GTK+ application
looks like:</para>

<informalexample>
<programlisting role="C">
int
main (int argc, char *argv[])
{
  GtkWidget *window;

  g_thread_init (NULL);
  gdk_threads_init (<!-- -->);
  gtk_init (&amp;argc, &amp;argv);

  window = create_window (<!-- -->);
  gtk_widget_show (window);

  gdk_threads_enter (<!-- -->);
  gtk_main (<!-- -->);
  gdk_threads_leave (<!-- -->);

  return 0;
}
</programlisting>
</informalexample>

<para>
Callbacks require a bit of attention. Callbacks from GTK+ signals
are made within the GTK+ lock. However callbacks from GLib (timeouts,
IO callbacks, and idle functions) are made outside of the GTK+
lock. So, within a signal handler you do not need to call
gdk_threads_enter(), but within the other types of callbacks, you
do.
</para>

<para>Erik Mouw contributed the following code example to
illustrate how to use threads within GTK+ programs.
</para>

<informalexample>
<programlisting role="C">
/*-------------------------------------------------------------------------
 * Filename:      gtk-thread.c
 * Version:       0.99.1
 * Copyright:     Copyright (C) 1999, Erik Mouw
 * Author:        Erik Mouw &lt;J.A.K.Mouw@its.tudelft.nl&gt;
 * Description:   GTK threads example. 
 * Created at:    Sun Oct 17 21:27:09 1999
 * Modified by:   Erik Mouw &lt;J.A.K.Mouw@its.tudelft.nl&gt;
 * Modified at:   Sun Oct 24 17:21:41 1999
 *-----------------------------------------------------------------------*/
/*
 * Compile with:
 *
 * cc -o gtk-thread gtk-thread.c `gtk-config --cflags --libs gthread`
 *
 * Thanks to Sebastian Wilhelmi and Owen Taylor for pointing out some
 * bugs.
 *
 */

&num;include &lt;stdio.h&gt;
&num;include &lt;stdlib.h&gt;
&num;include &lt;unistd.h&gt;
&num;include &lt;time.h&gt;
&num;include &lt;gtk/gtk.h&gt;
&num;include &lt;glib.h&gt;
&num;include &lt;pthread.h&gt;

&num;define YES_IT_IS    (1)
&num;define NO_IT_IS_NOT (0)

typedef struct 
{
  GtkWidget *label;
  int what;
} yes_or_no_args;

G_LOCK_DEFINE_STATIC (yes_or_no);
static volatile int yes_or_no = YES_IT_IS;

void destroy (GtkWidget *widget, gpointer data)
{
  gtk_main_quit (<!-- -->);
}

void *argument_thread (void *args)
{
  yes_or_no_args *data = (yes_or_no_args *)args;
  gboolean say_something;

  for (;;)
    {
      /* sleep a while */
      sleep(rand(<!-- -->) / (RAND_MAX / 3) + 1);

      /* lock the yes_or_no_variable */
      G_LOCK(yes_or_no);

      /* do we have to say something? */
      say_something = (yes_or_no != data->what);

      if(say_something)
	{
	  /* set the variable */
	  yes_or_no = data->what;
	}

      /* Unlock the yes_or_no variable */
      G_UNLOCK (yes_or_no);

      if (say_something)
	{
	  /* get GTK thread lock */
	  gdk_threads_enter (<!-- -->);

	  /* set label text */
	  if(data->what == YES_IT_IS)
	    gtk_label_set_text (GTK_LABEL (data->label), "O yes, it is!");
	  else
	    gtk_label_set_text (GTK_LABEL (data->label), "O no, it isn't!");

	  /* release GTK thread lock */
	  gdk_threads_leave (<!-- -->);
	}
    }

  return NULL;
}

int main (int argc, char *argv[])
{
  GtkWidget *window;
  GtkWidget *label;
  yes_or_no_args yes_args, no_args;
  pthread_t no_tid, yes_tid;

  /* init threads */
  g_thread_init (NULL);
  gdk_threads_init (<!-- -->);

  /* init gtk */
  gtk_init(&amp;argc, &amp;argv);

  /* init random number generator */
  srand ((unsigned int) time (NULL));

  /* create a window */
  window = gtk_window_new (GTK_WINDOW_TOPLEVEL);

  gtk_signal_connect (GTK_OBJECT (window), "destroy",
		      GTK_SIGNAL_FUNC (destroy), NULL);

  gtk_container_set_border_width (GTK_CONTAINER (window), 10);

  /* create a label */
  label = gtk_label_new ("And now for something completely different ...");
  gtk_container_add (GTK_CONTAINER (window), label);
  
  /* show everything */
  gtk_widget_show (label);
  gtk_widget_show (window);

  /* create the threads */
  yes_args.label = label;
  yes_args.what = YES_IT_IS;
  pthread_create (&amp;yes_tid, NULL, argument_thread, &amp;yes_args);

  no_args.label = label;
  no_args.what = NO_IT_IS_NOT;
  pthread_create (&amp;no_tid, NULL, argument_thread, &amp;no_args);

  /* enter the GTK main loop */
  gdk_threads_enter (<!-- -->);
  gtk_main (<!-- -->);
  gdk_threads_leave (<!-- -->);

  return 0;
}
</programlisting>
</informalexample>

<!-- ##### SECTION See_Also ##### -->
<para>

</para>

<!-- ##### MACRO GDK_THREADS_ENTER ##### -->
<para>
This macro marks the beginning of a critical section in which GDK and GTK+
functions can be called.  Only one thread at a time can be in such a
critial section. The macro expands to a no-op if #G_THREADS_ENABLED
has not been defined. Typically gdk_threads_enter() should be used 
instead of this macro.
</para>



<!-- ##### MACRO GDK_THREADS_LEAVE ##### -->
<para>
This macro marks the end of a critical section 
begun with #GDK_THREADS_ENTER.
</para>



<!-- ##### FUNCTION gdk_threads_init ##### -->
<para>

</para>



<!-- ##### FUNCTION gdk_threads_enter ##### -->
<para>
This macro marks the beginning of a critical section
in which GDK and GTK+ functions can be called.
Only one thread at a time can be in such a critial 
section.
</para>



<!-- ##### FUNCTION gdk_threads_leave ##### -->
<para>
Leaves a critical region begun with gdk_threads_enter(). 
</para>



<!-- ##### VARIABLE gdk_threads_mutex ##### -->
<para>
The #GMutex used to implement the critical region for
gdk_threads_enter()/gdk_threads_leave(). This variable should not be
used directly &mdash; consider it private.
</para>