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24 Threads

Functions for using GDK in multi-threaded programs

24.1 Overview

For thread safety, GDK relies on the thread primitives in GLib, and on the thread-safe GLib main loop.

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 <g-hash-table> from multiple threads.

GTK+ is "thread aware" but not thread safe &#x2014; 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.

Unfortunately the above holds with the X11 backend only. With the Win32 backend, GDK calls should not be attempted from multiple threads at all.

You must call g-thread-init and gdk-threads-init before executing any other GTK+ or GDK functions in a threaded GTK+ program.

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.)

In particular, this means, if you are writing widgets that might be used in threaded programs, you must surround timeouts and idle functions in this matter.

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.

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.)

A minimal main program for a threaded GTK+ application looks like:

     main (int argc, char *argv[])
       GtkWidget *window;
       g_thread_init (NULL);
       gdk_threads_init ();
       gdk_threads_enter ();
       gtk_init (&argc, &argv);
       window = create_window ();
       gtk_widget_show (window);
       gtk_main ();
       gdk_threads_leave ();
       return 0;

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.

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

      * Filename:      gtk-thread.c
      * Version:       0.99.1
      * Copyright:     Copyright (C) 1999, Erik Mouw
      * Author:        Erik Mouw <>
      * Description:   GTK threads example.
      * Created at:    Sun Oct 17 21:27:09 1999
      * Modified by:   Erik Mouw <>
      * 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.
     &#x0023;include <stdio.h>
     &#x0023;include <stdlib.h>
     &#x0023;include <unistd.h>
     &#x0023;include <time.h>
     &#x0023;include <gtk/gtk.h>
     &#x0023;include <glib.h>
     &#x0023;include <pthread.h>
     &#x0023;define YES_IT_IS    (1)
     &#x0023;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 */
           /* do we have to say something? */
           say_something = (yes_or_no != data->what);
     	  /* 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!");
     	    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(&argc, &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 (&yes_tid, NULL, argument_thread, &yes_args);
       no_args.label = label;
       no_args.what = NO_IT_IS_NOT;
       pthread_create (&no_tid, NULL, argument_thread, &no_args);
       /* enter the GTK main loop */
       gtk_main ();
       gdk_threads_leave ();
       return 0;

24.2 Usage

— Function: gdk-threads-init

Initializes GDK so that it can be used from multiple threads in conjunction with gdk-threads-enter and gdk-threads-leave. g-thread-init must be called previous to this function.

This call must be made before any use of the main loop from GTK+; to be safe, call it before gtk-init.

— Function: gdk-threads-enter

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.

— Function: gdk-threads-leave

Leaves a critical region begun with gdk-threads-enter.