dispatch_object(3)					   BSD Library Functions Manual 					dispatch_object(3)

NAME

     dispatch_object -- General manipulation of dispatch objects

SYNOPSIS

     #include <dispatch/dispatch.h>

     void
     dispatch_retain(dispatch_object_t object);

     void
     dispatch_release(dispatch_object_t object);

     void
     dispatch_suspend(dispatch_object_t object);

     void
     dispatch_resume(dispatch_object_t object);

     void *
     dispatch_get_context(dispatch_object_t object);

     void
     dispatch_set_context(dispatch_object_t object, void *context);

     void
     dispatch_set_finalizer_f(dispatch_object_t object, dispatch_function_t finalizer);

DESCRIPTION

     Dispatch objects share functions for coordinating memory management, suspension, cancellation and context pointers.

MEMORY MANGEMENT

     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.

   INTEGRATION WITH OBJECTIVE-C
	   When  building  with  an Objective-C or Objective-C++ compiler, dispatch objects are declared as Objective-C types. This results in the
	   following differences compared to building as plain C/C++:

	   -   if Objective-C Automated Reference Counting is enabled, dispatch objects are memory managed by the Objective-C runtime and explicit
	       calls to the dispatch_retain() and dispatch_release() functions will produce build errors.

	       Note:  when ARC is enabled, care needs to be taken with dispatch API returning an interior pointer that is only valid as long as an
	       associated object has not been released. If that object is held in a variable with automatic storage, it may need to  be  annotated
	       with  the objc_precise_lifetime attribute, or stored in a __strong instance variable instead, to ensure that the object is not pre-
	       maturely released. The functions returning interior pointers are dispatch_data_create_map(3) and dispatch_data_apply(3).

	   -   the Blocks runtime automatically retains and releases dispatch objects captured by blocks upon  Block_copy()  and  Block_release(),
	       e.g. as performed during asynchronous execution of a block via dispatch_async(3).

	       Note:  retain cycles may be encountered if dispatch source objects are captured by their handler blocks; these cycles can be broken
	       by declaring the captured object __weak or by calling dispatch_source_cancel(3) to cause its handler blocks to be released  explic-
	       itly.

	   -   dispatch objects can be added directly to Cocoa collections, and their lifetime is tracked by the Objective-C static analyzer.

	   Integration	of  dispatch objects with Objective-C requires targeting Mac OS X 10.8 or later, and is disabled when building with Objec-
	   tive-C Garbage Collection or for the legacy Objective-C runtime. It can also be disabled manually by using compiler options	to  define
	   the OS_OBJECT_USE_OBJC preprocessor macro to 0.

     Important: When building with a plain C/C++ compiler or when integration with Objective-C is disabled, dispatch objects are not automatically
     retained and released when captured by a block. Therefore, when a dispatch object is captured by a block that will be executed asyn-
     chronously, the object must be manually retained and released:

	   dispatch_retain(object);
	   dispatch_async(queue, ^{
		   do_something_with_object(object);
		   dispatch_release(object);
	   });

SUSPENSION

     The invocation of blocks on dispatch queues or dispatch sources may be suspended or resumed with the functions dispatch_suspend() and
     dispatch_resume() respectively. Other dispatch objects do not support suspension.

     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.
     Suspending an object causes it to be retained and resuming an object causes it to be released. 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.

CONTEXT POINTERS

     Dispatch objects 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 finalizer will be run on the object's target queue.

SEE ALSO

     dispatch(3), dispatch_async(3), dispatch_group_create(3), dispatch_queue_create(3), dispatch_semaphore_create(3),
     dispatch_set_target_queue(3), dispatch_source_cancel(3), dispatch_source_create(3)

Darwin								   March 1, 2012							    Darwin