GNOME Documentation Library

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

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.

g_main_loop_ref ()

GMainLoop*          g_main_loop_ref                     (GMainLoop *loop);

Increases the reference count on a GMainLoop object by one.

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.

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.

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.

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().

g_main_loop_get_context ()

GMainContext*       g_main_loop_get_context             (GMainLoop *loop);

Returns the GMainContext of loop.

g_main_new()

#define             g_main_new(is_running)

Creates a new GMainLoop for the default main loop.

g_main_destroy()

#define             g_main_destroy(loop)

Frees the memory allocated for the GMainLoop.

g_main_run()

#define             g_main_run(loop)

Runs a main loop until it stops running.

g_main_quit()

#define             g_main_quit(loop)

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

g_main_is_running()

#define             g_main_is_running(loop)

Checks 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.

g_main_context_ref ()

GMainContext*       g_main_context_ref                  (GMainContext *context);

Increases the reference count on a GMainContext object by one.

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.

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.

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.

g_main_iteration()

#define             g_main_iteration(may_block)

Runs a single iteration for the default GMainContext.

g_main_context_pending ()

gboolean            g_main_context_pending              (GMainContext *context);

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

g_main_pending()

#define             g_main_pending()

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

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.

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.

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.

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.

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().

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.

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.

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.

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().

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.

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.

g_main_context_dispatch ()

void                g_main_context_dispatch             (GMainContext *context);

Dispatches all pending sources.

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.

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().

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.

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.

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.

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:

g_main_current_source ()

GSource*            g_main_current_source               (void);

Returns the currently firing source for this thread.

Since 2.12

g_main_set_poll_func()

#define             g_main_set_poll_func(func)

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

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.

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.

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.

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).

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().

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().

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.

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.

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.