TL;DR: gdk_threads_add_idle() / gdk_threads_add_timeout()

GTK+ is not thread safe1. As is described in GDK 3 reference manual:

GLib is completely thread safe… GTK+, however, is not thread safe. You should only use GTK+ and GDK from the thread gtk_init() and gtk_main() were called on. This is usually referred to as the “main thread”.

Signals on GTK+ and GDK types, as well as non-signal callbacks, are emitted in the main thread.

Consider the following case:

  • We wait for an event from user (e.g. a button “clicked” by the user)
  • We then have a task that may take long time to finish (e.g. network request, expensive computation)
  • We want to do the above asynchronously (so we do not block the main UI thread, because signal callbacks are executed in the UI thread (“main thread”))
  • Meanwhile, we want to show user a pulsing progress bar because we have no idea how long the procedure will last

We need:

  • A thread to perform the task itself (“task thread”)
  • A thread to update UI elements
  • A communication method for the task thread to tell the UI thread about the result

As GTK+ is not thread safe, we cannot update UI elements on a thread other than the “main thread”, or race condition will occur, the program will crash. Thus, we need a way to put our UI update function in the “main thread”.

To achieve this, use:

gdk_threads_add_idle(ui_update_func, user_data);

… to spawn the UI updating thread, where

  • ui_update_func is the function name of our UI update code
  • user_data is the extra argument to be passed to ui_update_func

In this way, ui_update_func will be executed “whenever there are no higher priority events pending”2 (e.g. no signals are emitting), so in the UI update code we can (busy-)wait for the task thread to finish, receive the result, and display it on the UI. Note that ui_update_func must not block (e.g. if the GLib Asynchronous Queue is used, use g_async_queue_try_pop instead of g_async_queue_pop), otherwise the whole application will freeze, since the event loop is single-threaded.

Another similar way to achieve asynchronous UI update is:

/* 1000 in millisecond, so here ui_update_func will be run every 1 second */
gdk_threads_add_timeout(1000, ui_update_func, user_data);

This allows us to manipulate widgets (e.g. call gtk_progress_bar_pulse) from time to time safely in the UI update function, because it is run in the “main thread” with a time interval. I think this is most useful for pulsing progress bars because this seems to be the only widget that requires manual advancing!

As you can see, these are utilities provided by GDK, not GTK nor GLib. There are, however, GLib “equivalents”:

g_idle_add (GSourceFunc function,
            gpointer data);

g_timeout_add (guint interval,
               GSourceFunc function,
               gpointer data);

Previously, I use these, and it works. However, as is described in the GDK 3 developer manual, using them is not recommended:

You should use gdk_threads_add_idle() and gdk_threads_add_timeout() instead of g_idle_add() and g_timeout_add() since libraries not under your control might be using the deprecated GDK locking mechanism. If you are sure that none of the code in your application and libraries use the deprecated gdk_threads_enter() or gdk_threads_leave() methods, then you can safely use g_idle_add() and g_timeout_add().

(It looks like my program is so modern, wow)

Another issue here is that we need a mechanism for the two threads to communicate with each other, so that:

  • When the task is finished, the UI thread can receive the result and display it on the UI
  • When the task fails, the UI thread can display an error message on time

The Asynchronous Queue functionality provided by GLib seems to be the best solution. With asynchronous queue created and passed to the two threads described above, the task thread can pass result or error through the queue, and the UI thread can check the queue from time to time in order to display the result or error to user.


  • 2021-03-03:
    • gdk_threads_add_idle_full should be gdk_threads_add_idle in the first code block.
    • add a notice of not blocking ui_update_func.