GMainContext

GMainContext is a core concept in GLib that represents a set of event sources (GSource objects) to be dispatched by a main loop. It provides the mechanism GLib uses for event-driven programming and for coordinating work across threads. A main loop runs within a GMainContext, processing ready sources by invoking their callbacks.

Contexts are thread-aware. Each thread can have its own thread-default GMainContext, and multiple contexts can be created with g_main_context_new for private use. Context ownership is controllable via g_main_context_acquire and g_main_context_release, which determine which thread may dispatch sources for a given context. GMainContext objects are reference-counted, so they can be shared and kept alive across different parts of an application using g_main_context_ref and g_main_context_unref.

A GMainLoop is associated with a GMainContext. When you create a loop with g_main_loop_new, you provide the context to use; g_main_loop_run starts processing sources in that context, and g_main_context_dispatch executes ready callbacks. Functions like g_main_context_pending help check for pending sources, while g_main_context_invoke schedules a function to run in the context’s thread. Additionally, sources can be attached to a specific context, allowing inter-thread signaling and coordinated scheduling across contexts.

In practice, GMainContext underpins GTK, GNOME apps, and many network services. It enables flexible, thread-aware event handling by isolating work in separate contexts while still permitting cross-context callbacks and synchronization when needed.