The Main Event Loop

## The Main Event Loop The Main Event Loop — manages all available sources of events |

## Synopsis

    #include <glib.h>

                        GMainLoop;
    GMainLoop*          g_main_loop_new                     (GMainContext *context,
                                                             gboolean is_running);
    GMainLoop*          g_main_loop_ref                     (GMainLoop *loop);
    void                g_main_loop_unref                   (GMainLoop *loop);
    void                g_main_loop_run                     (GMainLoop *loop);
    void                g_main_loop_quit                    (GMainLoop *loop);
    gboolean            g_main_loop_is_running              (GMainLoop *loop);
    GMainContext*       g_main_loop_get_context             (GMainLoop *loop);
    #define             g_main_new                          (is_running)
    #define             g_main_destroy                      (loop)
    #define             g_main_run                          (loop)
    #define             g_main_quit                         (loop)
    #define             g_main_is_running                   (loop)

    #define             G_PRIORITY_HIGH
    #define             G_PRIORITY_DEFAULT
    #define             G_PRIORITY_HIGH_IDLE
    #define             G_PRIORITY_DEFAULT_IDLE
    #define             G_PRIORITY_LOW

                        GMainContext;
    GMainContext*       g_main_context_new                  (void);
    GMainContext*       g_main_context_ref                  (GMainContext *context);
    void                g_main_context_unref                (GMainContext *context);
    GMainContext*       g_main_context_default              (void);
    gboolean            g_main_context_iteration            (GMainContext *context,
                                                             gboolean may_block);
    #define             g_main_iteration                    (may_block)
    gboolean            g_main_context_pending              (GMainContext *context);
    #define             g_main_pending                      ()
    GSource*            g_main_context_find_source_by_id    (GMainContext *context,
                                                             guint source_id);
    GSource*            g_main_context_find_source_by_user_data
                                                            (GMainContext *context,
                                                             gpointer user_data);
    GSource*            g_main_context_find_source_by_funcs_user_data
                                                            (GMainContext *context,
                                                             GSourceFuncs *funcs,
                                                             gpointer user_data);
    void                g_main_context_wakeup               (GMainContext *context);
    gboolean            g_main_context_acquire              (GMainContext *context);
    void                g_main_context_release              (GMainContext *context);
    gboolean            g_main_context_is_owner             (GMainContext *context);
    gboolean            g_main_context_wait                 (GMainContext *context,
                                                             GCond *cond,
                                                             GMutex *mutex);
    gboolean            g_main_context_prepare              (GMainContext *context,
                                                             gint *priority);
    gint                g_main_context_query                (GMainContext *context,
                                                             gint max_priority,
                                                             gint *timeout_,
                                                             GPollFD *fds,
                                                             gint n_fds);
    gint                g_main_context_check                (GMainContext *context,
                                                             gint max_priority,
                                                             GPollFD *fds,
                                                             gint n_fds);
    void                g_main_context_dispatch             (GMainContext *context);
    void                g_main_context_set_poll_func        (GMainContext *context,
                                                             GPollFunc func);
    GPollFunc           g_main_context_get_poll_func        (GMainContext *context);
    gint                (*GPollFunc)                        (GPollFD *ufds,
                                                             guint nfsd,
                                                             gint timeout_);
    void                g_main_context_add_poll             (GMainContext *context,
                                                             GPollFD *fd,
                                                             gint priority);
    void                g_main_context_remove_poll          (GMainContext *context,
                                                             GPollFD *fd);
    gint                g_main_depth                        (void);
    GSource*            g_main_current_source               (void);
    #define             g_main_set_poll_func                (func)

    GSource*            g_timeout_source_new                (guint interval);
    GSource*            g_timeout_source_new_seconds        (guint interval);
    guint               g_timeout_add                       (guint interval,
                                                             GSourceFunc function,
                                                             gpointer data);
    guint               g_timeout_add_full                  (gint priority,
                                                             guint interval,
                                                             GSourceFunc function,
                                                             gpointer data,
                                                             GDestroyNotify notify);
    guint               g_timeout_add_seconds               (guint interval,
                                                             GSourceFunc function,
                                                             gpointer data);
    guint               g_timeout_add_seconds_full          (gint priority,
                                                             guint interval,
                                                             GSourceFunc function,
                                                             gpointer data,
                                                             GDestroyNotify notify);

    GSource*            g_idle_source_new                   (void);
    guint               g_idle_add                          (GSourceFunc function,
                                                             gpointer data);
    guint               g_idle_add_full                     (gint priority,
                                                             GSourceFunc function,
                                                             gpointer data,
                                                             GDestroyNotify notify);
    gboolean            g_idle_remove_by_data               (gpointer data);

    typedef             GPid;
    void                (*GChildWatchFunc)                  (GPid pid,
                                                             gint status,
                                                             gpointer data);
    GSource*            g_child_watch_source_new            (GPid pid);
    guint               g_child_watch_add                   (GPid pid,
                                                             GChildWatchFunc function,
                                                             gpointer data);
    guint               g_child_watch_add_full              (gint priority,
                                                             GPid pid,
                                                             GChildWatchFunc function,
                                                             gpointer data,
                                                             GDestroyNotify notify);

                        GPollFD;

                        GSource;
    void                (*GSourceDummyMarshal)              (void);
                        GSourceFuncs;
                        GSourceCallbackFuncs;
    GSource*            g_source_new                        (GSourceFuncs *source_funcs,
                                                             guint struct_size);
    GSource*            g_source_ref                        (GSource *source);
    void                g_source_unref                      (GSource *source);
    void                g_source_set_funcs                  (GSource *source,
                                                             GSourceFuncs *funcs);
    guint               g_source_attach                     (GSource *source,
                                                             GMainContext *context);
    void                g_source_destroy                    (GSource *source);
    gboolean            g_source_is_destroyed               (GSource *source);
    void                g_source_set_priority               (GSource *source,
                                                             gint priority);
    gint                g_source_get_priority               (GSource *source);
    void                g_source_set_can_recurse            (GSource *source,
                                                             gboolean can_recurse);
    gboolean            g_source_get_can_recurse            (GSource *source);
    guint               g_source_get_id                     (GSource *source);
    GMainContext*       g_source_get_context                (GSource *source);
    void                g_source_set_callback               (GSource *source,
                                                             GSourceFunc func,
                                                             gpointer data,
                                                             GDestroyNotify notify);
    gboolean            (*GSourceFunc)                      (gpointer data);
    void                g_source_set_callback_indirect      (GSource *source,
                                                             gpointer callback_data,
                                                             GSourceCallbackFuncs *callback_funcs);
    void                g_source_add_poll                   (GSource *source,
                                                             GPollFD *fd);
    void                g_source_remove_poll                (GSource *source,
                                                             GPollFD *fd);
    void                g_source_get_current_time           (GSource *source,
                                                             GTimeVal *timeval);
    gboolean            g_source_remove                     (guint tag);
    gboolean            g_source_remove_by_funcs_user_data  (GSourceFuncs *funcs,
                                                             gpointer user_data);
    gboolean            g_source_remove_by_user_data        (gpointer user_data);

## Description

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.

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.

GTK+ contains wrappers of some of these functions, e.g. `gtk_main()`, `gtk_main_quit()` and `gtk_events_pending()`.

### Creating new sources types

One of the unusual features of the GTK+ main loop 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 types.

New source types basically interact with 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()`.

The operation of these functions can best be seen in terms of a state diagram, as shown in Figure 1, “States of a Main Context”.

Figure 1. States of a Main Context

## Details

### GMainLoop

    typedef struct _GMainLoop GMainLoop;

The GMainLoop struct is an opaque data type representing the main event loop of a GLib or GTK+ application.

### g_main_loop_new ()

    GMainLoop*          g_main_loop_new                     (GMainContext *context,
                                                             gboolean is_running);

Creates a new GMainLoop structure.

`context` : | a GMainContext (if `NULL`, the default context will be used).
`is_running` : | set to `TRUE` to indicate that the loop is running. This is not very important since calling `g_main_loop_run()` will set this to `TRUE` anyway.
Returns : | a new GMainLoop.

### g_main_loop_ref ()

    GMainLoop*          g_main_loop_ref                     (GMainLoop *loop);

Increases the reference count on a GMainLoop object by one.

`loop` : | a GMainLoop
Returns : | `loop`

### g_main_loop_unref ()

    void                g_main_loop_unref                   (GMainLoop *loop);

Decreases the reference count on a GMainLoop object by one. If the result is zero, free the loop and free all associated memory.

`loop` : | a GMainLoop

### g_main_loop_run ()

    void                g_main_loop_run                     (GMainLoop *loop);

Runs a main loop until `g_main_loop_quit()` is called on the loop. If this is called for the thread of the loop's GMainContext, it will process events from the loop, otherwise it will simply wait.

`loop` : | a GMainLoop

### g_main_loop_quit ()

    void                g_main_loop_quit                    (GMainLoop *loop);

Stops a GMainLoop from running. Any calls to `g_main_loop_run()` for the loop will return.

`loop` : | a GMainLoop

### g_main_loop_is_running ()

    gboolean            g_main_loop_is_running              (GMainLoop *loop);

Checks to see if the main loop is currently being run via `g_main_loop_run()`.

`loop` : | a GMainLoop.
Returns : | `TRUE` if the mainloop is currently being run.

### g_main_loop_get_context ()

    GMainContext*       g_main_loop_get_context             (GMainLoop *loop);

Returns the GMainContext of `loop`.

`loop` : | a GMainLoop.
Returns : | the GMainContext of `loop`

### g_main_new()

    #define             g_main_new(is_running)

### Warning

`g_main_new` has been deprecated since version 2.2 and should not be used in newly-written code. Use `g_main_loop_new()` instead.

Creates a new GMainLoop for the default main loop.

`is_running` : | set to `TRUE` to indicate that the loop is running. This is not very important since calling `g_main_run()` will set this to `TRUE` anyway.
Returns : | a new GMainLoop.

### g_main_destroy()

    #define             g_main_destroy(loop)

### Warning

`g_main_destroy` has been deprecated since version 2.2 and should not be used in newly-written code. Use `g_main_loop_unref()` instead.

Frees the memory allocated for the GMainLoop.

`loop` : | a GMainLoop.

### g_main_run()

    #define             g_main_run(loop)

### Warning

`g_main_run` has been deprecated since version 2.2 and should not be used in newly-written code. Use `g_main_loop_run()` instead.

Runs a main loop until it stops running.

`loop` : | a GMainLoop.

### g_main_quit()

    #define             g_main_quit(loop)

### Warning

`g_main_quit` has been deprecated since version 2.2 and should not be used in newly-written code. Use `g_main_loop_quit()` instead.

Stops the GMainLoop. If `g_main_run()` was called to run the GMainLoop, it will now return.

`loop` : | a GMainLoop.

### g_main_is_running()

    #define             g_main_is_running(loop)

### Warning

`g_main_is_running` has been deprecated since version 2.2 and should not be used in newly-written code. USe `g_main_loop_is_running()` instead.

Checks if the main loop is running.

`loop` : | a GMainLoop.
Returns : | `TRUE` if the main loop is running.

### G_PRIORITY_HIGH

    #define G_PRIORITY_HIGH            -100

Use this for high priority event sources. It is not used within GLib or GTK+.

### G_PRIORITY_DEFAULT

    #define G_PRIORITY_DEFAULT          0

Use this for default priority event sources. In GLib this priority is used when adding timeout functions with `g_timeout_add()`. In GDK this priority is used for events from the X server.

### G_PRIORITY_HIGH_IDLE

    #define G_PRIORITY_HIGH_IDLE        100

Use this for high priority idle functions. GTK+ uses G_PRIORITY_HIGH_IDLE + 10 for resizing operations, and G_PRIORITY_HIGH_IDLE + 20 for redrawing operations. (This is done to ensure that any pending resizes are processed before any pending redraws, so that widgets are not redrawn twice unnecessarily.)

### G_PRIORITY_DEFAULT_IDLE

    #define G_PRIORITY_DEFAULT_IDLE     200

Use this for default priority idle functions. In GLib this priority is used when adding idle functions with `g_idle_add()`.

### G_PRIORITY_LOW

    #define G_PRIORITY_LOW	            300

Use this for very low priority background tasks. It is not used within GLib or GTK+.

### GMainContext

    typedef struct _GMainContext GMainContext;

The GMainContext struct is an opaque data type representing a set of sources to be handled in a main loop.

### g_main_context_new ()

    GMainContext*       g_main_context_new                  (void);

Creates a new GMainContext strcuture

Returns : | the new GMainContext

### g_main_context_ref ()

    GMainContext*       g_main_context_ref                  (GMainContext *context);

Increases the reference count on a GMainContext object by one.

`context` : | a GMainContext
Returns : | the `context` that was passed in (since 2.6)

### g_main_context_unref ()

    void                g_main_context_unref                (GMainContext *context);

Decreases the reference count on a GMainContext object by one. If the result is zero, free the context and free all associated memory.

`context` : | a GMainContext

### g_main_context_default ()

    GMainContext*       g_main_context_default              (void);

Returns the default main context. This is the main context used for main loop functions when a main loop is not explicitly specified.

Returns : | the default main context.

### g_main_context_iteration ()

    gboolean            g_main_context_iteration            (GMainContext *context,
                                                             gboolean may_block);

Runs a single iteration for the given main loop. This involves checking to see if any event sources are ready to be processed, then if no events sources are ready and `may_block` is `TRUE`, waiting for a source to become ready, then dispatching the highest priority events sources that are ready. Note that even when `may_block` is `TRUE`, it is still possible for `g_main_context_iteration()` to return `FALSE`, since the the wait may be interrupted for other reasons than an event source becoming ready.

`context` : | a GMainContext (if `NULL`, the default context will be used)
`may_block` : | whether the call may block.
Returns : | `TRUE` if events were dispatched.

### g_main_iteration()

    #define             g_main_iteration(may_block)

### Warning

`g_main_iteration` has been deprecated since version 2.2 and should not be used in newly-written code. Use `g_main_context_iteration()` instead.

Runs a single iteration for the default GMainContext.

`may_block` : | set to `TRUE` if it should block (i.e. wait) until an event source becomes ready. It will return after an event source has been processed. If set to `FALSE` it will return immediately if no event source is ready to be processed.
Returns : | `TRUE` if more events are pending.

### g_main_context_pending ()

    gboolean            g_main_context_pending              (GMainContext *context);

Checks if any sources have pending events for the given context.

`context` : | a GMainContext (if `NULL`, the default context will be used)
Returns : | `TRUE` if events are pending.

### g_main_pending()

    #define             g_main_pending()

### Warning

`g_main_pending` has been deprecated since version 2.2 and should not be used in newly-written code. Use `g_main_context_pending()` instead.

Checks if any events are pending for the default GMainContext (i.e. ready to be processed).

Returns : | `TRUE` if any events are pending.

### g_main_context_find_source_by_id ()

    GSource*            g_main_context_find_source_by_id    (GMainContext *context,
                                                             guint source_id);

Finds a GSource given a pair of context and ID.

`context` : | a GMainContext (if `NULL`, the default context will be used)
`source_id` : | the source ID, as returned by `g_source_get_id()`.
Returns : | the GSource if found, otherwise, `NULL`

### g_main_context_find_source_by_user_data ()

    GSource*            g_main_context_find_source_by_user_data
                                                            (GMainContext *context,
                                                             gpointer user_data);

Finds a source with the given user data for the callback. If multiple sources exist with the same user data, the first one found will be returned.

`context` : | a GMainContext
`user_data` : | the user_data for the callback.
Returns : | the source, if one was found, otherwise `NULL`

### g_main_context_find_source_by_funcs_user_data ()

    GSource*            g_main_context_find_source_by_funcs_user_data
                                                            (GMainContext *context,
                                                             GSourceFuncs *funcs,
                                                             gpointer user_data);

Finds a source with the given source functions and user data. If multiple sources exist with the same source function and user data, the first one found will be returned.

`context` : | a GMainContext (if `NULL`, the default context will be used).
`funcs` : | the `source_funcs` passed to `g_source_new()`.
`user_data` : | the user data from the callback.
Returns : | the source, if one was found, otherwise `NULL`

### g_main_context_wakeup ()

    void                g_main_context_wakeup               (GMainContext *context);

If `context` is currently waiting in a `poll()`, interrupt the `poll()`, and continue the iteration process.

`context` : | a GMainContext

### g_main_context_acquire ()

    gboolean            g_main_context_acquire              (GMainContext *context);

Tries to become the owner of the specified context. If some other context is the owner of the context, returns `FALSE` immediately. Ownership is properly recursive: the owner can require ownership again and will release ownership when `g_main_context_release()` is called as many times as `g_main_context_acquire()`.

You must be the owner of a context before you can call `g_main_context_prepare()`, `g_main_context_query()`, `g_main_context_check()`, `g_main_context_dispatch()`.

`context` : | a GMainContext
Returns : | `TRUE` if the operation succeeded, and this thread is now the owner of `context`.

### g_main_context_release ()

    void                g_main_context_release              (GMainContext *context);

Releases ownership of a context previously acquired by this thread with `g_main_context_acquire()`. If the context was acquired multiple times, the only release ownership when `g_main_context_release()` is called as many times as it was acquired.

`context` : | a GMainContext

### g_main_context_is_owner ()

    gboolean            g_main_context_is_owner             (GMainContext *context);

Determines whether this thread holds the (recursive) ownership of this GMaincontext. This is useful to know before waiting on another thread that may be blocking to get ownership of `context`.

`context` : | a GMainContext
Returns : | `TRUE` if current thread is owner of `context`.

Since 2.10

### g_main_context_wait ()

    gboolean            g_main_context_wait                 (GMainContext *context,
                                                             GCond *cond,
                                                             GMutex *mutex);

Tries to become the owner of the specified context, as with `g_main_context_acquire()`. But if another thread is the owner, atomically drop `mutex` and wait on `cond` until that owner releases ownership or until `cond` is signaled, then try again (once) to become the owner.

`context` : | a GMainContext
`cond` : | a condition variable
`mutex` : | a mutex, currently held
Returns : | `TRUE` if the operation succeeded, and this thread is now the owner of `context`.

### g_main_context_prepare ()

    gboolean            g_main_context_prepare              (GMainContext *context,
                                                             gint *priority);

Prepares to poll sources within a main loop. The resulting information for polling is determined by calling `g_main_context_query()`.

`context` : | a GMainContext
`priority` : | location to store priority of highest priority source already ready.
Returns : | `TRUE` if some source is ready to be dispatched prior to polling.

### g_main_context_query ()

    gint                g_main_context_query                (GMainContext *context,
                                                             gint max_priority,
                                                             gint *timeout_,
                                                             GPollFD *fds,
                                                             gint n_fds);

Determines information necessary to poll this main loop.

`context` : | a GMainContext
`max_priority` : | maximum priority source to check
`timeout_` : | location to store timeout to be used in polling
`fds` : | location to store GPollFD records that need to be polled.
`n_fds` : | length of `fds`.
Returns : | the number of records actually stored in `fds`, or, if more than `n_fds` records need to be stored, the number of records that need to be stored.

### g_main_context_check ()

    gint                g_main_context_check                (GMainContext *context,
                                                             gint max_priority,
                                                             GPollFD *fds,
                                                             gint n_fds);

Passes the results of polling back to the main loop.

`context` : | a GMainContext
`max_priority` : | the maximum numerical priority of sources to check
`fds` : | array of GPollFD's that was passed to the last call to `g_main_context_query()`
`n_fds` : | return value of `g_main_context_query()`
Returns : | `TRUE` if some sources are ready to be dispatched.

### g_main_context_dispatch ()

    void                g_main_context_dispatch             (GMainContext *context);

Dispatches all pending sources.

`context` : | a GMainContext

### g_main_context_set_poll_func ()

    void                g_main_context_set_poll_func        (GMainContext *context,
                                                             GPollFunc func);

Sets the function to use to handle polling of file descriptors. It will be used instead of the `poll()` system call (or GLib's replacement function, which is used where `poll()` isn't available).

This function could possibly be used to integrate the GLib event loop with an external event loop.

`context` : | a GMainContext
`func` : | the function to call to poll all file descriptors

### g_main_context_get_poll_func ()

    GPollFunc           g_main_context_get_poll_func        (GMainContext *context);

Gets the poll function set by `g_main_context_set_poll_func()`.

`context` : | a GMainContext
Returns : | the poll function

### GPollFunc ()

    gint                (*GPollFunc)                        (GPollFD *ufds,
                                                             guint nfsd,
                                                             gint timeout_);

Specifies the type of function passed to `g_main_context_set_poll_func()`. The semantics of the function should match those of the ``poll()`` system call.

`ufds` : | an array of GPollFD elements.
`nfsd` : | the number of elements in `ufds`.
`timeout_` : | the maximum time to wait for an event of the file descriptors. A negative value indicates an infinite timeout.
Returns : | the number of GPollFD elements which have events or errors reported, or -1 if an error occurred.

### g_main_context_add_poll ()

    void                g_main_context_add_poll             (GMainContext *context,
                                                             GPollFD *fd,
                                                             gint priority);

Adds a file descriptor to the set of file descriptors polled for this context. This will very seldomly be used directly. Instead a typical event source will use `g_source_add_poll()` instead.

`context` : | a GMainContext (or `NULL` for the default context)
`fd` : | a GPollFD structure holding information about a file descriptor to watch.
`priority` : | the priority for this file descriptor which should be the same as the priority used for `g_source_attach()` to ensure that the file descriptor is polled whenever the results may be needed.

### g_main_context_remove_poll ()

    void                g_main_context_remove_poll          (GMainContext *context,
                                                             GPollFD *fd);

Removes file descriptor from the set of file descriptors to be polled for a particular context.

`context` : | a GMainContext
`fd` : | a GPollFD descriptor previously added with `g_main_context_add_poll()`

### g_main_depth ()

    gint                g_main_depth                        (void);

Returns the depth of the stack of calls to `g_main_context_dispatch()` on any GMainContext in the current thread. That is, when called from the toplevel, it gives 0. When called from within a callback from `g_main_context_iteration()` (or `g_main_loop_run()`, etc.) it returns 1. When called from within a callback to a recursive call to `g_main_context_iterate()`, it returns 2. And so forth.

This function is useful in a situation like the following: Imagine an extremely simple "garbage collected" system.

    static GList *free_list;

    gpointer
    allocate_memory (gsize size)
    {
      gpointer result = g_malloc (size);
      free_list = g_list_prepend (free_list, result);
      return result;
    }

    void
    free_allocated_memory (void)
    {
      GList *l;
      for (l = free_list; l; l = l->next);
        g_free (l->data);
      g_list_free (free_list);
      free_list = NULL;
     }

    [...]

    while (TRUE);
     {
       g_main_context_iteration (NULL, TRUE);
       free_allocated_memory();
      }

This works from an application, however, if you want to do the same thing from a library, it gets more difficult, since you no longer control the main loop. You might think you can simply use an idle function to make the call to `free_allocated_memory()`, but that doesn't work, since the idle function could be called from a recursive callback. This can be fixed by using `g_main_depth()`

    gpointer
    allocate_memory (gsize size)
    {
      FreeListBlock *block = g_new (FreeListBlock, 1);
      block->mem = g_malloc (size);
      block->depth = g_main_depth ();
      free_list = g_list_prepend (free_list, block);
      return block->mem;
    }

    void
    free_allocated_memory (void)
    {
      GList *l;

      int depth = g_main_depth ();
      for (l = free_list; l; );
        {
          GList *next = l->next;
          FreeListBlock *block = l->data;
          if (block->depth > depth)
            {
              g_free (block->mem);
              g_free (block);
              free_list = g_list_delete_link (free_list, l);
            }

          l = next;
        }
      }

There is a temptation to use `g_main_depth()` to solve problems with reentrancy. For instance, while waiting for data to be received from the network in response to a menu item, the menu item might be selected again. It might seem that one could make the menu item's callback return immediately and do nothing if `g_main_depth()` returns a value greater than 1. However, this should be avoided since the user then sees selecting the menu item do nothing. Furthermore, you'll find yourself adding these checks all over your code, since there are doubtless many, many things that the user could do. Instead, you can use the following techniques:

1. Use `gtk_widget_set_sensitive()` or modal dialogs to prevent the user from interacting with elements while the main loop is recursing.

2. Avoid main loop recursion in situations where you can't handle arbitrary callbacks. Instead, structure your code so that you simply return to the main loop and then get called again when there is more work to do.

Returns : | The main loop recursion level in the current thread

### g_main_current_source ()

    GSource*            g_main_current_source               (void);

Returns the currently firing source for this thread.

Returns : | The currently firing source or `NULL`.

Since 2.12

### g_main_set_poll_func()

    #define             g_main_set_poll_func(func)

### Warning

`g_main_set_poll_func` has been deprecated since version 2.2 and should not be used in newly-written code. Use `g_main_context_set_poll_func()` instead.

Sets the function to use for the handle polling of file descriptors for the default main context.

`func` : | the function to call to poll all file descriptors.

### g_timeout_source_new ()

    GSource*            g_timeout_source_new                (guint interval);

Creates a new timeout source.

The source will not initially be associated with any GMainContext and must be added to one with `g_source_attach()` before it will be executed.

`interval` : | the timeout interval in milliseconds.
Returns : | the newly-created timeout source

### g_timeout_source_new_seconds ()

    GSource*            g_timeout_source_new_seconds        (guint interval);

Creates a new timeout source.

The source will not initially be associated with any GMainContext and must be added to one with `g_source_attach()` before it will be executed.

The scheduling granularity/accuracy of this timeout source will be in seconds.

`interval` : | the timeout interval in seconds
Returns : | the newly-created timeout source

Since 2.14

### g_timeout_add ()

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

Sets a function to be called at regular intervals, with the default priority, G_PRIORITY_DEFAULT. The function is called repeatedly until it returns `FALSE`, at which point the timeout is automatically destroyed and the function will not be called again. The first call to the function will be at the end of the first `interval`.

Note that timeout functions may be delayed, due to the processing of other event sources. Thus they should not be relied on for precise timing. After each call to the timeout function, the time of the next timeout is recalculated based on the current time and the given interval (it does not try to 'catch up' time lost in delays).

If you want to have a timer in the "seconds" range and do not care about the exact time of the first call of the timer, use the `g_timeout_add_seconds()` function; this function allows for more optimizations and more efficient system power usage.

`interval` : | the time between calls to the function, in milliseconds (1/1000ths of a second)
`function` : | function to call
`data` : | data to pass to `function`
Returns : | the ID (greater than 0) of the event source.

### g_timeout_add_full ()

    guint               g_timeout_add_full                  (gint priority,
                                                             guint interval,
                                                             GSourceFunc function,
                                                             gpointer data,
                                                             GDestroyNotify notify);

Sets a function to be called at regular intervals, with the given priority. The function is called repeatedly until it returns `FALSE`, at which point the timeout is automatically destroyed and the function will not be called again. The `notify` function is called when the timeout is destroyed. The first call to the function will be at the end of the first `interval`.

Note that timeout functions may be delayed, due to the processing of other event sources. Thus they should not be relied on for precise timing. After each call to the timeout function, the time of the next timeout is recalculated based on the current time and the given interval (it does not try to 'catch up' time lost in delays).

`priority` : | the priority of the timeout source. Typically this will be in the range between G_PRIORITY_DEFAULT and G_PRIORITY_HIGH.
`interval` : | the time between calls to the function, in milliseconds (1/1000ths of a second)
`function` : | function to call
`data` : | data to pass to `function`
`notify` : | function to call when the timeout is removed, or `NULL`
Returns : | the ID (greater than 0) of the event source.

### g_timeout_add_seconds ()

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

Sets a function to be called at regular intervals with the default priority, G_PRIORITY_DEFAULT. The function is called repeatedly until it returns `FALSE`, at which point the timeout is automatically destroyed and the function will not be called again.

See `g_timeout_add_seconds_full()` for the differences between `g_timeout_add()` and `g_timeout_add_seconds()`.

`interval` : | the time between calls to the function, in seconds
`function` : | function to call
`data` : | data to pass to `function`
Returns : | the ID (greater than 0) of the event source.

Since 2.14

### g_timeout_add_seconds_full ()

    guint               g_timeout_add_seconds_full          (gint priority,
                                                             guint interval,
                                                             GSourceFunc function,
                                                             gpointer data,
                                                             GDestroyNotify notify);

Sets a function to be called at regular intervals, with `priority`. The function is called repeatedly until it returns `FALSE`, at which point the timeout is automatically destroyed and the function will not be called again.

Unlike `g_timeout_add()`, this function operates at whole second granularity. The initial starting point of the timer is determined by the implementation and the implementation is expected to group multiple timers together so that they fire all at the same time. To allow this grouping, the `interval` to the first timer is rounded and can deviate up to one second from the specified interval. Subsequent timer iterations will generally run at the specified interval.

Note that timeout functions may be delayed, due to the processing of other event sources. Thus they should not be relied on for precise timing. After each call to the timeout function, the time of the next timeout is recalculated based on the current time and the given `interval`

If you want timing more precise than whole seconds, use `g_timeout_add()` instead.

The grouping of timers to fire at the same time results in a more power and CPU efficient behavior so if your timer is in multiples of seconds and you don't require the first timer exactly one second from now, the use of `g_timeout_add_seconds()` is preferred over `g_timeout_add()`.

`priority` : | the priority of the timeout source. Typically this will be in the range between G_PRIORITY_DEFAULT and G_PRIORITY_HIGH.
`interval` : | the time between calls to the function, in seconds
`function` : | function to call
`data` : | data to pass to `function`
`notify` : | function to call when the timeout is removed, or `NULL`
Returns : | the ID (greater than 0) of the event source.

Since 2.14

### g_idle_source_new ()

    GSource*            g_idle_source_new                   (void);

Creates a new idle source.

The source will not initially be associated with any GMainContext and must be added to one with `g_source_attach()` before it will be executed. Note that the default priority for idle sources is `G_PRIORITY_DEFAULT_IDLE`, as compared to other sources which have a default priority of `G_PRIORITY_DEFAULT`.

Returns : | the newly-created idle source

### g_idle_add ()

    guint               g_idle_add                          (GSourceFunc function,
                                                             gpointer data);

Adds a function to be called whenever there are no higher priority events pending to the default main loop. The function is given the default idle priority, G_PRIORITY_DEFAULT_IDLE. If the function returns `FALSE` it is automatically removed from the list of event sources and will not be called again.

`function` : | function to call
`data` : | data to pass to `function`.
Returns : | the ID (greater than 0) of the event source.

### g_idle_add_full ()

    guint               g_idle_add_full                     (gint priority,
                                                             GSourceFunc function,
                                                             gpointer data,
                                                             GDestroyNotify notify);

Adds a function to be called whenever there are no higher priority events pending. If the function returns `FALSE` it is automatically removed from the list of event sources and will not be called again.

`priority` : | the priority of the idle source. Typically this will be in the range btweeen G_PRIORITY_DEFAULT_IDLE and G_PRIORITY_HIGH_IDLE.
`function` : | function to call
`data` : | data to pass to `function`
`notify` : | function to call when the idle is removed, or `NULL`
Returns : | the ID (greater than 0) of the event source.

### g_idle_remove_by_data ()

    gboolean            g_idle_remove_by_data               (gpointer data);

Removes the idle function with the given data.

`data` : | the data for the idle source's callback.
Returns : | `TRUE` if an idle source was found and removed.

### GPid

    typedef int GPid;

A type which is used to hold a process identification. On Unix, processes are identified by a process id (an integer), while Windows uses process handles (which are pointers).

### GChildWatchFunc ()

    void                (*GChildWatchFunc)                  (GPid pid,
                                                             gint status,
                                                             gpointer data);

The type of functions to be called when a child exists.

`pid` : | the process id of the child process
`status` : | Status information about the child process, see waitpid(2) for more information about this field
`data` : | user data passed to `g_child_watch_add()`

### g_child_watch_source_new ()

    GSource*            g_child_watch_source_new            (GPid pid);

Creates a new child_watch source.

The source will not initially be associated with any GMainContext and must be added to one with `g_source_attach()` before it will be executed.

Note that child watch sources can only be used in conjunction with `g_spawn...` when the `G_SPAWN_DO_NOT_REAP_CHILD` flag is used.

Note that on platforms where GPid must be explicitly closed (see `g_spawn_close_pid()`) `pid` must not be closed while the source is still active. Typically, you will want to call `g_spawn_close_pid()` in the callback function for the source.

Note further that using `g_child_watch_source_new()` is not compatible with calling `waitpid(-1)` in the application. Calling `waitpid()` for individual pids will still work fine.

`pid` : | process id of a child process to watch. On Windows, a HANDLE for the process to watch (which actually doesn't have to be a child).
Returns : | the newly-created child watch source

Since 2.4

### g_child_watch_add ()

    guint               g_child_watch_add                   (GPid pid,
                                                             GChildWatchFunc function,
                                                             gpointer data);

Sets a function to be called when the child indicated by `pid` exits, at a default priority, G_PRIORITY_DEFAULT.

If you obtain `pid` from `g_spawn_async()` or `g_spawn_async_with_pipes()` you will need to pass G_SPAWN_DO_NOT_REAP_CHILD as flag to the spawn function for the child watching to work.

Note that on platforms where GPid must be explicitly closed (see `g_spawn_close_pid()`) `pid` must not be closed while the source is still active. Typically, you will want to call `g_spawn_close_pid()` in the callback function for the source.

GLib supports only a single callback per process id.

`pid` : | process id of a child process to watch
`function` : | function to call
`data` : | data to pass to `function`
Returns : | the ID (greater than 0) of the event source.

Since 2.4

### g_child_watch_add_full ()

    guint               g_child_watch_add_full              (gint priority,
                                                             GPid pid,
                                                             GChildWatchFunc function,
                                                             gpointer data,
                                                             GDestroyNotify notify);

Sets a function to be called when the child indicated by `pid` exits, at the priority `priority`.

If you obtain `pid` from `g_spawn_async()` or `g_spawn_async_with_pipes()` you will need to pass G_SPAWN_DO_NOT_REAP_CHILD as flag to the spawn function for the child watching to work.

Note that on platforms where GPid must be explicitly closed (see `g_spawn_close_pid()`) `pid` must not be closed while the source is still active. Typically, you will want to call `g_spawn_close_pid()` in the callback function for the source.

GLib supports only a single callback per process id.

`priority` : | the priority of the idle source. Typically this will be in the range between G_PRIORITY_DEFAULT_IDLE and G_PRIORITY_HIGH_IDLE.
`pid` : | process id of a child process to watch
`function` : | function to call
`data` : | data to pass to `function`
`notify` : | function to call when the idle is removed, or `NULL`
Returns : | the ID (greater than 0) of the event source.

Since 2.4

### GPollFD

    typedef struct {
    #if defined (G_OS_WIN32) && GLIB_SIZEOF_VOID_P == 8
      gint64	fd;
    #else
      gint		fd;
    #endif
      gushort 	events;
      gushort 	revents;
    } GPollFD;

gint fd; | the file descriptor to poll (or a HANDLE on Win32 platforms).
gushort events; | a bitwise combination of flags from GIOCondition, specifying which events should be polled for. Typically for reading from a file descriptor you would use `G_IO_IN` | `G_IO_HUP` | `G_IO_ERR`, and for writing you would use `G_IO_OUT` | `G_IO_ERR`.
gushort revents; | a bitwise combination of flags from GIOCondition, returned from the ``poll()`` function to indicate which events occurred.

### GSource

    typedef struct {
    } GSource;

The GSource struct is an opaque data type representing an event source.

### GSourceDummyMarshal ()

    void                (*GSourceDummyMarshal)              (void);

This is just a placeholder for GClosureMarshal, which cannot be used here for dependency reasons.

### GSourceFuncs

    typedef struct {
      gboolean (*prepare)  (GSource    *source,
    			gint       *timeout_);
      gboolean (*check)    (GSource    *source);
      gboolean (*dispatch) (GSource    *source,
    			GSourceFunc callback,
    			gpointer    user_data);
      void     (*finalize) (GSource    *source); /* Can be NULL */

      /* For use by g_source_set_closure */
      GSourceFunc     closure_callback;
      GSourceDummyMarshal closure_marshal; /* Really is of type GClosureMarshal */
    } GSourceFuncs;

The GSourceFuncs struct contains a table of functions used to handle event sources in a generic manner.

prepare | Called before all the file descriptors are polled. If the source can determine that it is ready here (without waiting for the results of the ``poll()`` call) it should return `TRUE`. It can also return a `timeout_` value which should be the maximum timeout (in milliseconds) which should be passed to the ``poll()`` call. The actual timeout used will be -1 if all sources returned -1, or it will be the minimum of all the `timeout_` values returned which were >= 0.
check | Called after all the file descriptors are polled. The source should return `TRUE` if it is ready to be dispatched. Note that some time may have passed since the previous prepare function was called, so the source should be checked again here.
dispatch | Called to dispatch the event source, after it has returned `TRUE` in either its `prepare` or its `check` function. The `dispatch` function is passed in a callback function and data. The callback function may be `NULL` if the source was never connected to a callback using `g_source_set_callback()`. The `dispatch` function should call the callback function with `user_data` and whatever additional parameters are needed for this type of event source.
finalize | Called when the source is finalized.

For idle sources, the prepare and check functions always return `TRUE` to indicate that the source is always ready to be processed. The prepare function also returns a timeout value of 0 to ensure that the ``poll()`` call doesn't block (since that would be time wasted which could have been spent running the idle function).

For timeout sources, the prepare and check functions both return `TRUE` if the timeout interval has expired. The prepare function also returns a timeout value to ensure that the ``poll()`` call doesn't block too long and miss the next timeout.

For file descriptor sources, the prepare function typically returns `FALSE`, since it must wait until ``poll()`` has been called before it knows whether any events need to be processed. It sets the returned timeout to -1 to indicate that it doesn't mind how long the ``poll()`` call blocks. In the check function, it tests the results of the ``poll()`` call to see if the required condition has been met, and returns `TRUE` if so.

### GSourceCallbackFuncs

    typedef struct {
      void (*ref)   (gpointer     cb_data);
      void (*unref) (gpointer     cb_data);
      void (*get)   (gpointer     cb_data,
    		 GSource     *source,
    		 GSourceFunc *func,
    		 gpointer    *data);
    } GSourceCallbackFuncs;

The GSourceCallbackFuncs struct contains functions for managing callback objects.

`ref` () | Called when a reference is added to the callback object.
`unref` () | Called when a reference to the callback object is dropped.
`get` () | Called to extract the callback function and data from the callback object.

### g_source_new ()

    GSource*            g_source_new                        (GSourceFuncs *source_funcs,
                                                             guint struct_size);

Creates a new GSource structure. The size is specified to allow creating structures derived from GSource that contain additional data. The size passed in must be at least `sizeof (GSource)`.

The source will not initially be associated with any GMainContext and must be added to one with `g_source_attach()` before it will be executed.

`source_funcs` : | structure containing functions that implement the sources behavior.
`struct_size` : | size of the GSource structure to create.
Returns : | the newly-created GSource.

### g_source_ref ()

    GSource*            g_source_ref                        (GSource *source);

Increases the reference count on a source by one.

`source` : | a GSource
Returns : | `source`

### g_source_unref ()

    void                g_source_unref                      (GSource *source);

Decreases the reference count of a source by one. If the resulting reference count is zero the source and associated memory will be destroyed.

`source` : | a GSource

### g_source_set_funcs ()

    void                g_source_set_funcs                  (GSource *source,
                                                             GSourceFuncs *funcs);

Sets the source functions (can be used to override default implementations) of an unattached source.

`source` : | a GSource
`funcs` : | the new GSourceFuncs

Since 2.12

### g_source_attach ()

    guint               g_source_attach                     (GSource *source,
                                                             GMainContext *context);

Adds a GSource to a `context` so that it will be executed within that context.

`source` : | a GSource
`context` : | a GMainContext (if `NULL`, the default context will be used)
Returns : | the ID (greater than 0) for the source within the GMainContext.

### g_source_destroy ()

    void                g_source_destroy                    (GSource *source);

Removes a source from its GMainContext, if any, and mark it as destroyed. The source cannot be subsequently added to another context.

`source` : | a GSource

### g_source_is_destroyed ()

    gboolean            g_source_is_destroyed               (GSource *source);

Returns whether `source` has been destroyed.

This is important when you operate upon your objects from within idle handlers, but may have freed the object before the dispatch of your idle handler.

    static gboolean
    idle_callback (gpointer data)
    {
      SomeWidget *self = data;

      GDK_THREADS_ENTER ();
      /* do stuff with self */
      self->idle_id = 0;
      GDK_THREADS_LEAVE ();

      return FALSE;
    }

    static void
    some_widget_do_stuff_later (SomeWidget *self)
    {
      self->idle_id = g_idle_add (idle_callback, self);
    }

    static void
    some_widget_finalize (GObject *object)
    {
      SomeWidget *self = SOME_WIDGET (object);

      if (self->idle_id)
        g_source_remove (self->idle_id);

      G_OBJECT_CLASS (parent_class)->finalize (object);
    }

This will fail in a multi-threaded application if the widget is destroyed before the idle handler fires due to the use after free in the callback. A solution, to this particular problem, is to check to if the source has already been destroy within the callback.

    static gboolean
    idle_callback (gpointer data)
    {
      SomeWidget *self = data;

      GDK_THREADS_ENTER ();
      if (!g_source_is_destroyed (g_main_current_source ()))
        {
          /* do stuff with self */
        }
      GDK_THREADS_LEAVE ();

      return FALSE;
    }

`source` : | a GSource
Returns : | `TRUE` if the source has been destroyed

Since 2.12

### g_source_set_priority ()

    void                g_source_set_priority               (GSource *source,
                                                             gint priority);

Sets the priority of a source. While the main loop is being run, a source will be dispatched if it is ready to be dispatched and no sources at a higher (numerically smaller) priority are ready to be dispatched.

`source` : | a GSource
`priority` : | the new priority.

### g_source_get_priority ()

    gint                g_source_get_priority               (GSource *source);

Gets the priority of a source.

`source` : | a GSource
Returns : | the priority of the source

### g_source_set_can_recurse ()

    void                g_source_set_can_recurse            (GSource *source,
                                                             gboolean can_recurse);

Sets whether a source can be called recursively. If `can_recurse` is `TRUE`, then while the source is being dispatched then this source will be processed normally. Otherwise, all processing of this source is blocked until the dispatch function returns.

`source` : | a GSource
`can_recurse` : | whether recursion is allowed for this source

### g_source_get_can_recurse ()

    gboolean            g_source_get_can_recurse            (GSource *source);

Checks whether a source is allowed to be called recursively. see `g_source_set_can_recurse()`.

`source` : | a GSource
Returns : | whether recursion is allowed.

### g_source_get_id ()

    guint               g_source_get_id                     (GSource *source);

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()`.

`source` : | a GSource
Returns : | the ID (greater than 0) for the source

### g_source_get_context ()

    GMainContext*       g_source_get_context                (GSource *source);

Gets the GMainContext with which the source is associated. Calling this function on a destroyed source is an error.

`source` : | a GSource
Returns : | the GMainContext with which the source is associated, or `NULL` if the context has not yet been added to a source.

### g_source_set_callback ()

    void                g_source_set_callback               (GSource *source,
                                                             GSourceFunc func,
                                                             gpointer data,
                                                             GDestroyNotify notify);

Sets the callback function for a source. The callback for a source is called from the source's dispatch function.

The exact type of `func` depends on the type of source; ie. you should not count on `func` being called with `data` as its first parameter.

Typically, you won't use this function. Instead use functions specific to the type of source you are using.

`source` : | the source
`func` : | a callback function
`data` : | the data to pass to callback function
`notify` : | a function to call when `data` is no longer in use, or `NULL`.

### GSourceFunc ()

    gboolean            (*GSourceFunc)                      (gpointer data);

Specifies the type of function passed to `g_timeout_add()`, `g_timeout_add_full()`, `g_idle_add()`, and `g_idle_add_full()`.

`data` : | data passed to the function, set when the source was created with one of the above functions.
Returns : | it should return `FALSE` if the source should be removed.

### g_source_set_callback_indirect ()

    void                g_source_set_callback_indirect      (GSource *source,
                                                             gpointer callback_data,
                                                             GSourceCallbackFuncs *callback_funcs);

Sets the callback function storing the data as a refcounted callback "object". This is used internally. Note that calling `g_source_set_callback_indirect()` assumes an initial reference count on `callback_data`, and thus `callback_funcs->unref` will eventually be called once more than `callback_funcs->ref`.

`source` : | the source
`callback_data` : | pointer to callback data "object"
`callback_funcs` : | functions for reference counting `callback_data` and getting the callback and data

### g_source_add_poll ()

    void                g_source_add_poll                   (GSource *source,
                                                             GPollFD *fd);

Adds a file descriptor to the set of file descriptors polled for this source. This is usually combined with `g_source_new()` to add an event source. The event source's check function will typically test the `revents` field in the GPollFD struct and return `TRUE` if events need to be processed.

`source` : | a GSource
`fd` : | a GPollFD structure holding information about a file descriptor to watch.

### g_source_remove_poll ()

    void                g_source_remove_poll                (GSource *source,
                                                             GPollFD *fd);

Removes a file descriptor from the set of file descriptors polled for this source.

`source` : | a GSource
`fd` : | a GPollFD structure previously passed to `g_source_add_poll()`.

### g_source_get_current_time ()

    void                g_source_get_current_time           (GSource *source,
                                                             GTimeVal *timeval);

Gets the "current time" to be used when checking this source. The advantage of calling this function over calling `g_get_current_time()` directly is that when checking multiple sources, GLib can cache a single value instead of having to repeatedly get the system time.

`source` : | a GSource
`timeval` : | GTimeVal structure in which to store current time.

### g_source_remove ()

    gboolean            g_source_remove                     (guint tag);

Removes the source with the given id from the default main context. The id of a GSource is given by `g_source_get_id()`, or will be returned by the functions `g_source_attach()`, `g_idle_add()`, `g_idle_add_full()`, `g_timeout_add()`, `g_timeout_add_full()`, `g_child_watch_add()`, `g_child_watch_add_full()`, `g_io_add_watch()`, and `g_io_add_watch_full()`.

See also `g_source_destroy()`.

`tag` : | the ID of the source to remove.
Returns : | `TRUE` if the source was found and removed.

### g_source_remove_by_funcs_user_data ()

    gboolean            g_source_remove_by_funcs_user_data  (GSourceFuncs *funcs,
                                                             gpointer user_data);

Removes a source from the default main loop context given the source functions and user data. If multiple sources exist with the same source functions and user data, only one will be destroyed.

`funcs` : | The `source_funcs` passed to `g_source_new()`
`user_data` : | the user data for the callback
Returns : | `TRUE` if a source was found and removed.

### g_source_remove_by_user_data ()

    gboolean            g_source_remove_by_user_data        (gpointer user_data);

Removes a source from the default main loop context given the user data for the callback. If multiple sources exist with the same user data, only one will be destroyed.

`user_data` : | the user_data for the callback.
Returns : | `TRUE` if a source was found and removed.