dispatch_group_create - Man Page

group blocks submitted to queues


#include <dispatch/dispatch.h>


dispatch_group_enter(dispatch_group_t group);

dispatch_group_leave(dispatch_group_t group);

dispatch_group_wait(dispatch_group_t group, dispatch_time_t timeout);

dispatch_group_notify(dispatch_group_t group, dispatch_queue_t queue, void (^block)(void));

dispatch_group_notify_f(dispatch_group_t group, dispatch_queue_t queue, void *context, void (*function)(void *));

dispatch_group_async(dispatch_group_t group, dispatch_queue_t queue, void (^block)(void));

dispatch_group_async_f(dispatch_group_t group, dispatch_queue_t queue, void *context, void (*function)(void *));


A dispatch group is an association of one or more blocks submitted to dispatch queues for asynchronous invocation. Applications may use dispatch groups to wait for the completion of blocks associated with the group.

The dispatch_group_create() function returns a new and empty dispatch group.

The dispatch_group_enter() and dispatch_group_leave() functions update the number of blocks running within a group.

The dispatch_group_wait() function waits until all blocks associated with the group have completed, or until the specified timeout has elapsed. If the group becomes empty within the specified amount of time, the function will return zero indicating success. Otherwise, a non-zero return code will be returned. When DISPATCH_TIME_FOREVER is passed as the timeout, calls to this function will wait an unlimited amount of time until the group becomes empty and the return value is always zero.

The dispatch_group_notify() function provides asynchronous notification of the completion of the blocks associated with the group by submitting the block to the specified queue once all blocks associated with the group have completed. The system holds a reference to the dispatch group while an asynchronous notification is pending, therefore it is valid to release the group after setting a notification block. The group will be empty at the time the notification block is submitted to the target queue. The group may either be released with dispatch_release() or reused for additional operations.

The dispatch_group_async() convenience function behaves like so:

dispatch_group_async(dispatch_group_t group, dispatch_queue_t queue, dispatch_block_t block)
	dispatch_async(queue, ^{

Return Value

The dispatch_group_create() function returns NULL on failure and non-NULL on success.

The dispatch_group_wait() function returns zero upon success and non-zero after the timeout expires. If the timeout is DISPATCH_TIME_FOREVER, then dispatch_group_wait() waits forever and always returns zero.

Memory Model

Dispatch groups are retained and released via calls to dispatch_retain() and dispatch_release().


The dispatch_group_async() and dispatch_group_notify() functions are wrappers around dispatch_group_async_f() and dispatch_group_notify_f() respectively.


In order to ensure deterministic behavior, it is recommended to call dispatch_group_wait() only once all blocks have been submitted to the group. If it is later determined that new blocks should be run, it is recommended not to reuse an already-running group, but to create a new group.

dispatch_group_wait() returns as soon as there are exactly zero enqueued or running blocks associated with a group (more precisely, as soon as every dispatch_group_enter() call has been balanced by a dispatch_group_leave() call). If one thread waits for a group while another thread submits new blocks to the group, then the count of associated blocks might momentarily reach zero before all blocks have been submitted. If this happens, dispatch_group_wait() will return too early: some blocks associated with the group have finished, but some have not yet been submitted or run.

However, as a special case, a block associated with a group may submit new blocks associated with its own group. In this case, the behavior is deterministic: a waiting thread will not wake up until the newly submitted blocks have also finished.

All of the foregoing also applies to dispath_group_notify() as well, with "block to be submitted" substituted for "waiting thread".

See Also

dispatch(3), dispatch_async(3), dispatch_object(3), dispatch_queue_create(3), dispatch_semaphore_create(3), dispatch_time(3)

Referenced By

dispatch(3), dispatch_object(3), dispatch_time(3).

May 1, 2009