Returns the numeric ID for a particular source. The ID of a source is a positive integer which is unique within a particular main loop context. The reverse mapping from ID to source is done by g_main_context_find_source_by_id().
You can only call this function while the source is associated to a
GMainContext instance; calling this function before
g_source_destroy() yields undefined behavior. The ID returned
is unique within the
GMainContext instance passed to g_source_attach().