man/dispatch_object.3

.\" Copyright (c) 2008-2010 Apple Inc. All rights reserved.
.Dd May 1, 2009
.Dt dispatch_object 3
.Os Darwin
.Sh NAME
.Nm dispatch_object
.Nd General manipulation of dispatch objects
.Sh SYNOPSIS
.Fd #include <dispatch/dispatch.h>
.Ft void
.Fo dispatch_retain
.Fa "dispatch_object_t object"
.Fc
.Ft void
.Fo dispatch_release
.Fa "dispatch_object_t object"
.Fc
.Ft void
.Fo dispatch_suspend
.Fa "dispatch_object_t object"
.Fc
.Ft void
.Fo dispatch_resume
.Fa "dispatch_object_t object"
.Fc
.Ft "void *"
.Fo dispatch_get_context
.Fa "dispatch_object_t object"
.Fc
.Ft void
.Fo dispatch_set_context
.Fa "dispatch_object_t object"
.Fa "void *context"
.Fc
.Ft void
.Fo dispatch_set_finalizer_f
.Fa "dispatch_object_t object"
.Fa "dispatch_function_t finalizer"
.Fc
.Sh DESCRIPTION
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).
.Sh MEMORY MANGEMENT
Objects returned by creation functions in the dispatch framework may be
uniformly retained and released with the functions
.Fn dispatch_retain
and
.Fn dispatch_release
respectively.
.Pp
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.
.Sh SUSPENSION
The invocation of blocks on dispatch queues or dispatch sources may be suspended
or resumed with the functions
.Fn dispatch_suspend
and
.Fn 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.
.Pp
.Em 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
.Fn dispatch_suspend
and
.Fn 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.
.Sh CONTEXT POINTERS
Dispatch queues and sources support supplemental context pointers. The value of
the context pointer may be retrieved and updated with
.Fn dispatch_get_context
and
.Fn dispatch_set_context
respectively.
The
.Fn 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.
.Pp
The result of getting or setting the context of an object that is not a
dispatch queue or a dispatch source is undefined.
.Sh SEE ALSO
.Xr dispatch 3 ,
.Xr dispatch_group_create 3 ,
.Xr dispatch_queue_create 3 ,
.Xr dispatch_semaphore_create 3 ,
.Xr dispatch_source_create 3 ,
.Xr dispatch_set_target_queue 3