<!-- ##### 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 aglobal 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 executingany other GTK+ or GDK functions in a threaded GTK+ program.</para><para>Idles, timeouts, and input functions are executed outsideof the main GTK+ lock. So, if you need to call GTK+inside of such a callback, you must surround the callbackwith a gdk_threads_enter()/gdk_threads_leave() pair.(However, signals are still executed within the mainGTK+ 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 otherthan 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 threadis that GDK always automatically flushes pending commandswhen it runs out of incoming events to process and hasto sleep while waiting for more events.)</para><para>A minimal main program for a threaded GTK+ applicationlooks like:</para><informalexample><programlisting role="C">intmain (int argc, char *argv[]){  GtkWidget *window;  g_thread_init (NULL);  gdk_threads_init (<!-- -->);  gdk_threads_enter (<!-- -->);  gtk_init (&amp;argc, &amp;argv);  window = create_window (<!-- -->);  gtk_widget_show (window);  gtk_main (<!-- -->);  gdk_threads_leave (<!-- -->);  return 0;}</programlisting></informalexample><para>Callbacks require a bit of attention. Callbacks from GTK+ signalsare 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 callgdk_threads_enter(), but within the other types of callbacks, youdo.</para><para>Erik Mouw contributed the following code example toillustrate 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 (<!-- -->);  gdk_threads_enter (<!-- -->);  /* 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 */  gtk_main (<!-- -->);  gdk_threads_leave (<!-- -->);  return 0;}</programlisting></informalexample><!-- ##### SECTION See_Also ##### --><para></para><!-- ##### SECTION Stability_Level ##### --><!-- ##### 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 acritial section. The macro expands to a no-op if #G_THREADS_ENABLEDhas 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 sectionin 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 forgdk_threads_enter()/gdk_threads_leave(). This variable should not beused directly &mdash; consider it private.</para><!-- ##### FUNCTION gdk_threads_set_lock_functions ##### --><para></para>@enter_fn: @leave_fn: