The Main Event Loop

The main event loop manages all the available sources of events for GLib and GTK applications. These events can come from any number of different types of sources such as file descriptors (plain files, pipes or sockets) and timeouts. New types of event sources can also be added using g_source_attach().

To allow multiple independent sets of sources to be handled in different threads, each source is associated with a GMainContext. A GMainContext can only be running in a single thread, but sources can be added to it and removed from it from other threads. All functions which operate on a GMainContext or a built-in GSource are thread-safe.

Each event source is assigned a priority. The default priority, G_PRIORITY_DEFAULT, is 0. Values less than 0 denote higher priorities. Values greater than 0 denote lower priorities. Events from high priority sources are always processed before events from lower priority sources.

Idle functions can also be added, and assigned a priority. These will be run whenever no events with a higher priority are ready to be processed.

The GMainLoop data type represents a main event loop. A GMainLoop is created with g_main_loop_new(). After adding the initial event sources, g_main_loop_run() is called. This continuously checks for new events from each of the event sources and dispatches them. Finally, the processing of an event from one of the sources leads to a call to g_main_loop_quit() to exit the main loop, and g_main_loop_run() returns.

It is possible to create new instances of GMainLoop recursively. This is often used in GTK applications when showing modal dialog boxes. Note that event sources are associated with a particular GMainContext, and will be checked and dispatched for all main loops associated with that GMainContext.

Libraries may contain wrappers of some of these functions, e.g. gtk_main(), gtk_main_quit() and gtk_events_pending().

Creating new source types

One of the unusual features of the GMainLoop functionality is that new types of event source can be created and used in addition to the builtin type of event source. A new event source type is used for handling GDK events. A new source type is created by “deriving” from the GSource structure. The derived type of source is represented by a structure that has the GSource structure as a first element, and other elements specific to the new source type. To create an instance of the new source type, call g_source_new() passing in the size of the derived structure and a table of functions. These GSourceFuncs determine the behavior of the new source type.

New source types basically interact with the main context in two ways. Their prepare function in GSourceFuncs can set a timeout to determine the maximum amount of time that the main loop will sleep before checking the source again. In addition, or as well, the source can add file descriptors to the set that the main context checks using g_source_add_poll().

Customizing the main loop iteration

Single iterations of a GMainContext can be run with g_main_context_iteration(). In some cases, more detailed control of exactly how the details of the main loop work is desired, for instance, when integrating the GMainLoop with an external main loop. In such cases, you can call the component functions of g_main_context_iteration() directly. These functions are g_main_context_prepare(), g_main_context_query(), g_main_context_check() and g_main_context_dispatch().

State of a Main Context

The operation of these functions can best be seen in terms of a state diagram, as shown in this image.

On UNIX, the GLib mainloop is incompatible with fork(). Any program using the mainloop must either exec() or exit() from the child without returning to the mainloop.

Memory management of sources

There are two options for memory management of the user data passed to a GSource to be passed to its callback on invocation. This data is provided in calls to g_timeout_add(), g_timeout_add_full(), g_idle_add(), etc. and more generally, using g_source_set_callback(). This data is typically an object which ‘owns’ the timeout or idle callback, such as a widget or a network protocol implementation. In many cases, it is an error for the callback to be invoked after this owning object has been destroyed, as that results in use of freed memory.

The first, and preferred, option is to store the source ID returned by functions such as g_timeout_add() or g_source_attach(), and explicitly remove that source from the main context using g_source_remove() when the owning object is finalized. This ensures that the callback can only be invoked while the object is still alive.

The second option is to hold a strong reference to the object in the callback, and to release it in the callback’s GDestroyNotify. This ensures that the object is kept alive until after the source is finalized, which is guaranteed to be after it is invoked for the final time. The GDestroyNotify is another callback passed to the ‘full’ variants of GSource functions (for example, g_timeout_add_full()). It is called when the source is finalized, and is designed for releasing references like this.

One important caveat of this second approach is that it will keep the object alive indefinitely if the main loop is stopped before the GSource is invoked, which may be undesirable.

Tutorial

GMainContext is complicated, and can be particularly intimidating for developers new to working with GLib. Unfortunately, improper use of GMainContext often results in bugs that may be difficult to debug. The Main Contexts tutorial provides valuable guidance for developers working with GMainContext and is highly recommended reading. In particular, the section Using GMainContext in a Library documents several pitfalls that library authors should avoid.