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 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

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.

Note that sources that have already been dispatched when g_main_loop_quit() is called will still be executed.

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 structure.

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. Otherwise, if may_block is FALSE sources are not waited to become ready, only those highest priority events sources will be dispatched (if any), that are ready at this given moment without further waiting.

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 thread 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 ownership will be released only 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.

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.

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.

GSourceFunc closure_callback;

GSourceDummyMarshal closure_marshal;


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.