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dispatch_object(3) Library Functions Manual dispatch_object(3)


General manipulation of dispatch objects


#include <dispatch/dispatch.h>

dispatch_retain(dispatch_object_t object);

dispatch_release(dispatch_object_t object);

dispatch_suspend(dispatch_object_t object);

dispatch_resume(dispatch_object_t object);

void *
dispatch_get_context(dispatch_object_t object);

dispatch_set_context(dispatch_object_t object, void *context);

dispatch_set_finalizer_f(dispatch_object_t object, dispatch_function_t finalizer);


Dispatch objects share functions for coordinating memory management, suspension, cancellation and context pointers. While all dispatch objects are retainable, not all objects support suspension, context pointers or finalizers (currently only queues and sources support these additional interfaces).


Objects returned by creation functions in the dispatch framework may be uniformly retained and released with the functions dispatch_retain() and dispatch_release() respectively.

The dispatch framework does not guarantee that any given client has the last or only reference to a given object. Objects may be retained internally by the system.


The invocation of blocks on dispatch queues or dispatch sources may be suspended or resumed with the functions dispatch_suspend() and dispatch_resume() respectively. The dispatch framework always checks the suspension status before executing a block, but such changes never affect a block during execution (non-preemptive). Therefore the suspension of an object is asynchronous, unless it is performed from the context of the target queue for the given object. The result of suspending or resuming an object that is not a dispatch queue or a dispatch source is undefined.

Important: suspension applies to all aspects of the dispatch object life cycle, including the finalizer function and cancellation handler. Therefore it is important to balance calls to dispatch_suspend() and dispatch_resume() such that the dispatch object is fully resumed when the last reference is released. The result of releasing all references to a dispatch object while in a suspended state is undefined.


Dispatch queues and sources support supplemental context pointers. The value of the context pointer may be retrieved and updated with dispatch_get_context() and dispatch_set_context() respectively. The dispatch_set_finalizer_f() specifies an optional per-object finalizer function that is invoked asynchronously if the context pointer is not NULL when the last reference to the object is released. This gives the application an opportunity to free the context data associated with the object.

The result of getting or setting the context of an object that is not a dispatch queue or a dispatch source is undefined.


dispatch(3), dispatch_group_create(3), dispatch_queue_create(3), dispatch_semaphore_create(3), dispatch_source_create(3)
May 1, 2009 Darwin