Creating new source types

One of the unusual features of the GMainLoop 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 type.

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().

State of a Main Context

The operation of these functions can best be seen in terms of a state diagram, as shown in this image.

On UNIX, the GLib mainloop is incompatible with fork(). Any program using the mainloop must either exec() or exit() from the child without returning to the mainloop.

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 th default main context.

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_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 global default main context. This is the main context used for main loop functions when a main loop is not explicitly specified, and corresponds to the "main" main loop. See also g_main_context_get_thread_default().

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 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.

It is a programmer error to attempt to lookup a non-existent source.

More specifically: source IDs can be reissued after a source has been destroyed and therefore it is never valid to use this function with a source ID which may have already been removed. An example is when scheduling an idle to run in another thread with g_idle_add(): the idle may already have run and been removed by the time this function is called on its (now invalid) source ID. This source ID may have been reissued, leading to the operation being performed against the wrong source.

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 blocking in g_main_context_iteration() waiting for a source to become ready, cause it to stop blocking and return. Otherwise, cause the next invocation of g_main_context_iteration() to return without blocking.

This API is useful for low-level control over GMainContext; for example, integrating it with main loop implementations such as GMainLoop.

Another related use for this function is when implementing a main loop with a termination condition, computed from multiple threads:

1
2
3
4
5
6

#define NUM_TASKS 10
static volatile gint tasks_remaining = NUM_TASKS;
...
 
while (g_atomic_int_get (&tasks_remaining) != 0)
  g_main_context_iteration (NULL, TRUE);

Then in a thread:

1
2
3
4

perform_work();

if (g_atomic_int_dec_and_test (&tasks_remaining))
  g_main_context_wakeup (NULL);

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().

You must have successfully acquired the context with g_main_context_acquire() before you may call this function.

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.

You must have successfully acquired the context with g_main_context_acquire() before you may call this function.

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.

You must have successfully acquired the context with g_main_context_acquire() before you may call this function.

g_main_context_dispatch ()

void
g_main_context_dispatch (GMainContext *context);

Dispatches all pending sources.

You must have successfully acquired the context with g_main_context_acquire() before you may call this function.

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 seldom be used directly. Instead a typical event source will use g_source_add_unix_fd() 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_iteration(), it returns 2. And so forth.

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

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27

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()

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30

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_main_context_invoke ()

void
g_main_context_invoke (GMainContext *context,
                       GSourceFunc function,
                       gpointer data);

Invokes a function in such a way that context is owned during the invocation of function .

If context is NULL then the global default main context — as returned by g_main_context_default() — is used.

If context is owned by the current thread, function is called directly. Otherwise, if context is the thread-default main context of the current thread and g_main_context_acquire() succeeds, then function is called and g_main_context_release() is called afterwards.

In any other case, an idle source is created to call function and that source is attached to context (presumably to be run in another thread). The idle source is attached with G_PRIORITY_DEFAULT priority. If you want a different priority, use g_main_context_invoke_full().

Note that, as with normal idle functions, function should probably return FALSE. If it returns TRUE, it will be continuously run in a loop (and may prevent this call from returning).

Since 2.28

g_main_context_invoke_full ()

void
g_main_context_invoke_full (GMainContext *context,
                            gint priority,
                            GSourceFunc function,
                            gpointer data,
                            GDestroyNotify notify);

Invokes a function in such a way that context is owned during the invocation of function .

This function is the same as g_main_context_invoke() except that it lets you specify the priority incase function ends up being scheduled as an idle and also lets you give a GDestroyNotify for data .

notify should not assume that it is called from any particular thread or with any particular context acquired.

Since 2.28

g_main_context_get_thread_default ()

GMainContext *
g_main_context_get_thread_default (void);

Gets the thread-default GMainContext for this thread. Asynchronous operations that want to be able to be run in contexts other than the default one should call this method or g_main_context_ref_thread_default() to get a GMainContext to add their GSources to. (Note that even in single-threaded programs applications may sometimes want to temporarily push a non-default context, so it is not safe to assume that this will always return NULL if you are running in the default thread.)

If you need to hold a reference on the context, use g_main_context_ref_thread_default() instead.

Since 2.22

g_main_context_ref_thread_default ()

GMainContext *
g_main_context_ref_thread_default (void);

Gets the thread-default GMainContext for this thread, as with g_main_context_get_thread_default(), but also adds a reference to it with g_main_context_ref(). In addition, unlike g_main_context_get_thread_default(), if the thread-default context is the global default context, this will return that GMainContext (with a ref added to it) rather than returning NULL.

Since 2.32

g_main_context_push_thread_default ()

void
g_main_context_push_thread_default (GMainContext *context);

Acquires context and sets it as the thread-default context for the current thread. This will cause certain asynchronous operations (such as most gio-based I/O) which are started in this thread to run under context and deliver their results to its main loop, rather than running under the global default context in the main thread. Note that calling this function changes the context returned by g_main_context_get_thread_default(), not the one returned by g_main_context_default(), so it does not affect the context used by functions like g_idle_add().

Normally you would call this function shortly after creating a new thread, passing it a GMainContext which will be run by a GMainLoop in that thread, to set a new default context for all async operations in that thread. (In this case, you don't need to ever call g_main_context_pop_thread_default().) In some cases however, you may want to schedule a single operation in a non-default context, or temporarily use a non-default context in the main thread. In that case, you can wrap the call to the asynchronous operation inside a g_main_context_push_thread_default() / g_main_context_pop_thread_default() pair, but it is up to you to ensure that no other asynchronous operations accidentally get started while the non-default context is active.

Beware that libraries that predate this function may not correctly handle being used from a thread with a thread-default context. Eg, see g_file_supports_thread_contexts().

Since 2.22

g_main_context_pop_thread_default ()

void
g_main_context_pop_thread_default (GMainContext *context);

Pops context off the thread-default context stack (verifying that it was on the top of the stack).

Since 2.22

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.

The interval given is in terms of monotonic time, not wall clock time. See g_get_monotonic_time().

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.

The interval given in terms of monotonic time, not wall clock time. See g_get_monotonic_time().

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.

This internally creates a main loop source using g_timeout_source_new() and attaches it to the main loop context using g_source_attach(). You can do these steps manually if you need greater control.

The interval given is in terms of monotonic time, not wall clock time. See g_get_monotonic_time().

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).

This internally creates a main loop source using g_timeout_source_new() and attaches it to the main loop context using g_source_attach(). You can do these steps manually if you need greater control.

The interval given in terms of monotonic time, not wall clock time. See g_get_monotonic_time().

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.

This internally creates a main loop source using g_timeout_source_new_seconds() and attaches it to the main loop context using g_source_attach(). You can do these steps manually if you need greater control. Also see g_timeout_add_seconds_full().

Note that the first call of the timer may not be precise for timeouts of one second. If you need finer precision and have such a timeout, you may want to use g_timeout_add() instead.

The interval given is in terms of monotonic time, not wall clock time. See g_get_monotonic_time().

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().

This internally creates a main loop source using g_timeout_source_new_seconds() and attaches it to the main loop context using g_source_attach(). You can do these steps manually if you need greater control.

The interval given is in terms of monotonic time, not wall clock time. See g_get_monotonic_time().

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.

This internally creates a main loop source using g_idle_source_new() and attaches it to the main loop context using g_source_attach(). You can do these steps manually if you need greater control.

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.

This internally creates a main loop source using g_idle_source_new() and attaches it to the main loop context using g_source_attach(). You can do these steps manually if you need greater control.

g_idle_remove_by_data ()

gboolean
g_idle_remove_by_data (gpointer data);

Removes the idle function with the given data.

GChildWatchFunc ()

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

Prototype of a GChildWatchSource callback, called when a child process has exited. To interpret status , see the documentation for g_spawn_check_exit_status().

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 with a nonpositive first argument in the application. Calling waitpid() for individual pids will still work fine.

Similarly, on POSIX platforms, the pid passed to this function must be greater than 0 (i.e. this function must wait for a specific child, and cannot wait for one of many children by using a nonpositive argument).

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.

This internally creates a main loop source using g_child_watch_source_new() and attaches it to the main loop context using g_source_attach(). You can do these steps manually if you need greater control.

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.

In many programs, you will want to call g_spawn_check_exit_status() in the callback to determine whether or not the child exited successfully.

Also, 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 should invoke g_spawn_close_pid() in the callback function for the source.

GLib supports only a single callback per process id.

This internally creates a main loop source using g_child_watch_source_new() and attaches it to the main loop context using g_source_attach(). You can do these steps manually if you need greater control.

Since 2.4

g_poll ()

gint
g_poll (GPollFD *fds,
        guint nfds,
        gint timeout);

Polls fds , as with the poll() system call, but portably. (On systems that don't have poll(), it is emulated using select().) This is used internally by GMainContext, but it can be called directly if you need to block until a file descriptor is ready, but don't want to run the full main loop.

Each element of fds is a GPollFD describing a single file descriptor to poll. The fd field indicates the file descriptor, and the events field indicates the events to poll for. On return, the revents fields will be filled with the events that actually occurred.

On POSIX systems, the file descriptors in fds can be any sort of file descriptor, but the situation is much more complicated on Windows. If you need to use g_poll() in code that has to run on Windows, the easiest solution is to construct all of your GPollFDs with g_io_channel_win32_make_pollfd().

Since 2.20

GSourceDummyMarshal ()

void
(*GSourceDummyMarshal) (void);

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

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.

g_source_ref ()

GSource *
g_source_ref (GSource *source);

Increases the reference count on a source by one.

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.

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.

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. Remove it by calling g_source_destroy().

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. It is safe to call this on sources which have already been removed from their context.

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.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29

static gboolean 
idle_callback (gpointer data)
{
  SomeWidget *self = data;
   
  GDK_THREADS_ENTER ();
  // do stuff with self
  self->idle_id = 0;
  GDK_THREADS_LEAVE ();
   
  return G_SOURCE_REMOVE;
}
 
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.

1
2
3
4
5
6
7
8
9
10
11
12
13
14

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;
}

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.

A child source always has the same priority as its parent. It is not permitted to change the priority of a source once it has been added as a child of another source.

g_source_get_priority ()

gint
g_source_get_priority (GSource *source);

Gets the priority of a 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.

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().

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().

g_source_get_name ()

const char *
g_source_get_name (GSource *source);

Gets a name for the source, used in debugging and profiling. The name may be NULL if it has never been set with g_source_set_name().

Since 2.26

g_source_set_name ()

void
g_source_set_name (GSource *source,
                   const char *name);

Sets a name for the source, used in debugging and profiling. The name defaults to NULL.

The source name should describe in a human-readable way what the source does. For example, "X11 event queue" or "GTK+ repaint idle handler" or whatever it is.

It is permitted to call this function multiple times, but is not recommended due to the potential performance impact. For example, one could change the name in the "check" function of a GSourceFuncs to include details like the event type in the source name.

Use caution if changing the name while another thread may be accessing it with g_source_get_name(); that function does not copy the value, and changing the value will free it while the other thread may be attempting to use it.

Since 2.26

g_source_set_name_by_id ()

void
g_source_set_name_by_id (guint tag,
                         const char *name);

Sets the name of a source using its ID.

This is a convenience utility to set source names from the return value of g_idle_add(), g_timeout_add(), etc.

It is a programmer error to attempt to set the name of a non-existent source.

More specifically: source IDs can be reissued after a source has been destroyed and therefore it is never valid to use this function with a source ID which may have already been removed. An example is when scheduling an idle to run in another thread with g_idle_add(): the idle may already have run and been removed by the time this function is called on its (now invalid) source ID. This source ID may have been reissued, leading to the operation being performed against the wrong source.

Since 2.26

g_source_get_context ()

GMainContext *
g_source_get_context (GSource *source);

Gets the GMainContext with which the source is associated.

You can call this on a source that has been destroyed, provided that the GMainContext it was attached to still exists (in which case it will return that GMainContext). In particular, you can always call this function on the source returned from g_main_current_source(). But calling this function on a source whose GMainContext has been destroyed is an error.

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.

GSourceFunc ()

gboolean
(*GSourceFunc) (gpointer user_data);

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

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 .

g_source_set_ready_time ()

void
g_source_set_ready_time (GSource *source,
                         gint64 ready_time);

Sets a GSource to be dispatched when the given monotonic time is reached (or passed). If the monotonic time is in the past (as it always will be if ready_time is 0) then the source will be dispatched immediately.

If ready_time is -1 then the source is never woken up on the basis of the passage of time.

Dispatching the source does not reset the ready time. You should do so yourself, from the source dispatch function.

Note that if you have a pair of sources where the ready time of one suggests that it will be delivered first but the priority for the other suggests that it would be delivered first, and the ready time for both sources is reached during the same main context iteration then the order of dispatch is undefined.

This API is only intended to be used by implementations of GSource. Do not call this API on a GSource that you did not create.

Since 2.36

g_source_get_ready_time ()

gint64
g_source_get_ready_time (GSource *source);

Gets the "ready time" of source , as set by g_source_set_ready_time().

Any time before the current monotonic time (including 0) is an indication that the source will fire immediately.

g_source_add_unix_fd ()

gpointer
g_source_add_unix_fd (GSource *source,
                      gint fd,
                      GIOCondition events);

Monitors fd for the IO events in events .

The tag returned by this function can be used to remove or modify the monitoring of the fd using g_source_remove_unix_fd() or g_source_modify_unix_fd().

It is not necessary to remove the fd before destroying the source; it will be cleaned up automatically.

This API is only intended to be used by implementations of GSource. Do not call this API on a GSource that you did not create.

As the name suggests, this function is not available on Windows.

Since 2.36

g_source_remove_unix_fd ()

void
g_source_remove_unix_fd (GSource *source,
                         gpointer tag);

Reverses the effect of a previous call to g_source_add_unix_fd().

You only need to call this if you want to remove an fd from being watched while keeping the same source around. In the normal case you will just want to destroy the source.

This API is only intended to be used by implementations of GSource. Do not call this API on a GSource that you did not create.

As the name suggests, this function is not available on Windows.

Since 2.36

g_source_modify_unix_fd ()

void
g_source_modify_unix_fd (GSource *source,
                         gpointer tag,
                         GIOCondition new_events);

Updates the event mask to watch for the fd identified by tag .

tag is the tag returned from g_source_add_unix_fd().

If you want to remove a fd, don't set its event mask to zero. Instead, call g_source_remove_unix_fd().

This API is only intended to be used by implementations of GSource. Do not call this API on a GSource that you did not create.

As the name suggests, this function is not available on Windows.

Since 2.36

g_source_query_unix_fd ()

GIOCondition
g_source_query_unix_fd (GSource *source,
                        gpointer tag);

Queries the events reported for the fd corresponding to tag on source during the last poll.

The return value of this function is only defined when the function is called from the check or dispatch functions for source .

This API is only intended to be used by implementations of GSource. Do not call this API on a GSource that you did not create.

As the name suggests, this function is not available on Windows.

Since 2.36

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.

This API is only intended to be used by implementations of GSource. Do not call this API on a GSource that you did not create.

Using this API forces the linear scanning of event sources on each main loop iteration. Newly-written event sources should try to use g_source_add_unix_fd() instead of this API.

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.

This API is only intended to be used by implementations of GSource. Do not call this API on a GSource that you did not create.

g_source_add_child_source ()

void
g_source_add_child_source (GSource *source,
                           GSource *child_source);

Adds child_source to source as a "polled" source; when source is added to a GMainContext, child_source will be automatically added with the same priority, when child_source is triggered, it will cause source to dispatch (in addition to calling its own callback), and when source is destroyed, it will destroy child_source as well. (source will also still be dispatched if its own prepare/check functions indicate that it is ready.)

If you don't need child_source to do anything on its own when it triggers, you can call g_source_set_dummy_callback() on it to set a callback that does nothing (except return TRUE if appropriate).

source will hold a reference on child_source while child_source is attached to it.

This API is only intended to be used by implementations of GSource. Do not call this API on a GSource that you did not create.

Since 2.28

g_source_remove_child_source ()

void
g_source_remove_child_source (GSource *source,
                              GSource *child_source);

Detaches child_source from source and destroys it.

This API is only intended to be used by implementations of GSource. Do not call this API on a GSource that you did not create.

Since 2.28

g_source_get_time ()

gint64
g_source_get_time (GSource *source);

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

The time here is the system monotonic time, if available, or some other reasonable alternative otherwise. See g_get_monotonic_time().

Since 2.28

g_source_get_current_time ()

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

This function ignores source and is otherwise the same as g_get_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(). You must use g_source_destroy() for sources added to a non-default main context.

It is a programmer error to attempt to remove a non-existent source.

More specifically: source IDs can be reissued after a source has been destroyed and therefore it is never valid to use this function with a source ID which may have already been removed. An example is when scheduling an idle to run in another thread with g_idle_add(): the idle may already have run and been removed by the time this function is called on its (now invalid) source ID. This source ID may have been reissued, leading to the operation being performed against the wrong source.

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.

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.

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_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+.

G_SOURCE_CONTINUE

#define G_SOURCE_CONTINUE       TRUE

Use this macro as the return value of a GSourceFunc to leave the GSource in the main loop.

Since 2.32

G_SOURCE_REMOVE

#define G_SOURCE_REMOVE         FALSE

Use this macro as the return value of a GSourceFunc to remove the GSource from the main loop.

Since 2.32

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.

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).

GPid is used in GLib only for descendant processes spawned with the g_spawn functions.

struct GPollFD

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

Represents a file descriptor, which events to poll for, and which events occurred.

G_POLLFD_FORMAT

#define G_POLLFD_FORMAT "%#I64x"

A format specifier that can be used in printf()-style format strings when printing the fd member of a GPollFD.

struct GSource

struct GSource {
};

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

struct GSourceFuncs

struct GSourceFuncs {
  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 */
};

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.

struct GSourceCallbackFuncs

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

The GSourceCallbackFuncs struct contains functions for managing callback objects.