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, sus-
pension, 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 inter-
nally by the system.
INTEGRATION WITH OBJECTIVE-C
When building with an Objective-C or Objective-C++ compiler, dis-
patch 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, dis-
patch 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 prematurely 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 execu-
tion 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 explicitly.
- dispatch objects can be added directly to Cocoa collections,
and their lifetime is tracked by the Objective-C static ana-
lyzer.
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 dis-
patch object is captured by a block that will be executed asynchronously,
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 execut-
ing 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. Sus-
pending 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 releas-
ing 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 asyn-
chronously 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
Mac OS X 10.8 - Generated Sat Aug 25 18:06:53 CDT 2012