dispatch_queue_create(3) BSD Library Functions Manual dispatch_queue_create(3)NAME
dispatch_queue_create, dispatch_queue_get_label,
dispatch_get_current_queue, dispatch_get_global_queue,
dispatch_get_main_queue, dispatch_main, dispatch_set_target_queue — where
blocks are scheduled for execution
SYNOPSIS
#include <dispatch/dispatch.h>
dispatch_queue_t
dispatch_queue_create(const char *label, dispatch_queue_attr_t attr);
const char *
dispatch_queue_get_label(dispatch_queue_t queue);
dispatch_queue_t
dispatch_get_global_queue(long priority, unsigned long flags);
dispatch_queue_t
dispatch_get_main_queue(void);
void
dispatch_main(void);
void
dispatch_set_target_queue(dispatch_object_t object,
dispatch_queue_t target);
DESCRIPTION
Queues are the fundamental mechanism for scheduling blocks for execution
within the dispatch(3) framework.
All blocks submitted to dispatch queues are dequeued in FIFO order.
Queues created with the DISPATCH_QUEUE_SERIAL attribute wait for the pre‐
viously dequeued block to complete before dequeuing the next block. A
queue with this FIFO completion behavior is usually simply described as a
"serial queue." All memory writes performed by a block dispatched to a
serial queue are guaranteed to be visible to subsequent blocks dispatched
to the same queue. Queues are not bound to any specific thread of execu‐
tion and blocks submitted to independent queues may execute concurrently.
Queues created with the DISPATCH_QUEUE_CONCURRENT attribute may execute
dequeued blocks concurrently and support barrier blocks submitted with
the dispatch barrier API.
CREATION
Queues are created with the dispatch_queue_create() function. Queues,
like all dispatch objects, are reference counted and newly created queues
have a reference count of one.
The optional label argument is used to describe the purpose of the queue
and is useful during debugging and performance analysis. If a label is
provided, it is copied. By convention, clients should pass a reverse DNS
style label. For example:
my_queue = dispatch_queue_create("com.example.subsystem.taskXYZ", NULL);
The attr argument specifies the type of queue to create and must be
either DISPATCH_QUEUE_SERIAL or DISPATCH_QUEUE_CONCURRENT.
The dispatch_queue_get_label() function returns the label provided when
the given queue was created (or an empty C string if no label was pro‐
vided at creation). Passing the constant DISPATCH_CURRENT_QUEUE_LABEL to
dispatch_queue_get_label() returns the label of the current queue.
SUSPENSION
Queues may be temporarily suspended and resumed with the functions
dispatch_suspend() and dispatch_resume() respectively. Suspension is
checked prior to block execution and is not preemptive.
MAIN QUEUE
The dispatch framework provides a default serial queue for the applica‐
tion to use. This queue is accessed via the dispatch_get_main_queue()
function.
Programs must call dispatch_main() at the end of main() in order to
process blocks submitted to the main queue. (See the COMPATIBILITY sec‐
tion for exceptions.) The dispatch_main() function never returns.
GLOBAL CONCURRENT QUEUES
Unlike the main queue or queues allocated with dispatch_queue_create(),
the global concurrent queues schedule blocks as soon as threads become
available (non-FIFO completion order). Four global concurrent queues are
provided, representing the following priority bands:
· DISPATCH_QUEUE_PRIORITY_HIGH
· DISPATCH_QUEUE_PRIORITY_DEFAULT
· DISPATCH_QUEUE_PRIORITY_LOW
· DISPATCH_QUEUE_PRIORITY_BACKGROUND
The priority of a global concurrent queue controls the scheduling prior‐
ity of the threads created by the system to invoke the blocks submitted
to that queue. Global queues with lower priority will be scheduled for
execution after all global queues with higher priority have been sched‐
uled. Additionally, items on the background priority global queue will
execute on threads with background state as described in setpriority(2)
(i.e. disk I/O is throttled and the thread's scheduling priority is set
to lowest value).
Use the dispatch_get_global_queue() function to obtain the global queue
of given priority. The flags argument is reserved for future use and must
be zero. Passing any value other than zero may result in a NULL return
value.
TARGET QUEUE
The dispatch_set_target_queue() function updates the target queue of the
given dispatch object. The target queue of an object is responsible for
processing the object.
The new target queue is retained by the given object before the previous
target queue is released. The new target queue setting will take effect
between block executions on the object, but not in the middle of any
existing block executions (non-preemptive).
The default target queue of all dispatch objects created by the applica‐
tion is the default priority global concurrent queue. To reset an
object's target queue to the default, pass the
DISPATCH_TARGET_QUEUE_DEFAULT constant to dispatch_set_target_queue().
The priority of a dispatch queue is inherited from its target queue. In
order to change the priority of a queue created with
dispatch_queue_create(), use the dispatch_get_global_queue() function to
obtain a target queue of the desired priority.
Blocks submitted to a serial queue whose target queue is another serial
queue will not be invoked concurrently with blocks submitted to the tar‐
get queue or to any other queue with that same target queue.
The target queue of a dispatch source specifies where its event handler
and cancellation handler blocks will be submitted. See
dispatch_source_create(3) for more information about dispatch sources.
The target queue of a dispatch I/O channel specifies the priority of the
global queue where its I/O operations are executed. See
dispatch_io_create(3) for more information about dispatch I/O channels.
For all other dispatch object types, the only function of the target
queue is to determine where an object's finalizer function is invoked.
The result of passing the main queue or a global concurrent queue as the
first argument of dispatch_set_target_queue() is undefined.
Directly or indirectly setting the target queue of a dispatch queue to
itself is undefined.
DEPRECATED FUNCTIONS
The following functions are deprecated and will be removed in a future
release:
dispatch_queue_t dispatch_get_current_queue(void);
dispatch_get_current_queue() always returns a valid queue. When called
from within a block submitted to a dispatch queue, that queue will be
returned. If this function is called from the main thread before
dispatch_main() is called, then the result of dispatch_get_main_queue()
is returned. In all other cases, the default target queue will be
returned.
The use of dispatch_get_current_queue() is strongly discouraged except
for debugging and logging purposes. Code must not make any assumptions
about the queue returned, unless it is one of the global queues or a
queue the code has itself created. The returned queue may have arbitrary
policies that may surprise code that tries to schedule work with the
queue. The list of policies includes, but is not limited to, queue width
(i.e. serial vs. concurrent), scheduling priority, security credential
or filesystem configuration. This function is deprecated and will be
removed in a future release.
It is equally unsafe for code to assume that synchronous execution onto a
queue is safe from deadlock if that queue is not the one returned by
dispatch_get_current_queue().
The result of dispatch_get_main_queue() may or may not equal the result
of dispatch_get_current_queue() when called on the main thread. Comparing
the two is not a valid way to test whether code is executing on the main
thread. Foundation/AppKit programs should use [NSThread isMainThread].
POSIX programs may use pthread_main_np(3).
dispatch_get_current_queue() may return a queue owned by a different sub‐
system which has already had all external references to it released.
While such a queue will continue to exist until all blocks submitted to
it have completed, attempting to retain it is forbidden and will trigger
an assertion. If Objective-C Automatic Reference Counting is enabled, any
use of the object returned by dispatch_get_current_queue() will cause
retain calls to be automatically generated, so the use of
dispatch_get_current_queue() for any reason in code built with ARC is
particularly strongly discouraged.
COMPATIBILITY
Cocoa applications need not call dispatch_main(). Blocks submitted to
the main queue will be executed as part of the "common modes" of the
application's main NSRunLoop or CFRunLoop. However, blocks submitted to
the main queue in applications using dispatch_main() are not guaranteed
to execute on the main thread.
The dispatch framework is a pure C level API. As a result, it does not
catch exceptions generated by higher level languages such as Objective-C
or C++. Applications MUST catch all exceptions before returning from a
block submitted to a dispatch queue; otherwise the process will be termi‐
nated with an uncaught exception.
The dispatch framework manages the relationship between dispatch queues
and threads of execution. As a result, applications MUST NOT delete or
mutate objects that they did not create. The following interfaces MUST
NOT be called by blocks submitted to a dispatch queue:
· pthread_cancel()
· pthread_detach()
· pthread_join()
· pthread_kill()
· pthread_exit()
Applications MAY call the following interfaces from a block submitted to
a dispatch queue if and only if they restore the thread to its original
state before returning:
· pthread_setcancelstate()
· pthread_setcanceltype()
· pthread_setschedparam()
· pthread_sigmask()
· pthread_setugid_np()
· pthread_chdir()
· pthread_fchdir()
Applications MUST NOT rely on the following interfaces returning pre‐
dictable results between invocations of blocks submitted to a dispatch
queue:
· pthread_self()
· pthread_getschedparam()
· pthread_get_stacksize_np()
· pthread_get_stackaddr_np()
· pthread_mach_thread_np()
· pthread_from_mach_thread_np()
While the result of pthread_self() may change between invocations of
blocks, the value will not change during the execution of any single
block. Because the underlying thread may change beteween block invoca‐
tions on a single queue, using per-thread data as an out-of-band return
value is error prone. In other words, the result of calling
pthread_setspecific() and pthread_getspecific() is well defined within a
signle block, but not across multiple blocks. Also, one cannot make any
assumptions about when the destructor passed to pthread_key_create() is
called. The destructor may be called between the invocation of blocks on
the same queue, or during the idle state of a process.
The following example code correctly handles per-thread return values:
__block int r;
__block int e;
dispatch_sync(queue, ^{
r = kill(1, 0);
// Copy the per-thread return value to the callee thread
e = errno;
});
printf("kill(1,0) returned %d and errno %d0, r, e);
Note that in the above example errno is a per-thread variable and must be
copied out explicitly as the block may be invoked on different thread of
execution than the caller. Another example of per-thread data that would
need to be copied is the use of getpwnam() instead of getpwnam_r().
As an optimization, dispatch_sync() invokes the block on the current
thread when possible. In this case, the thread specific data such as
errno may persist from the block until back to the caller. Great care
should be taken not to accidentally rely on this side-effect.
SEE ALSOdispatch(3), dispatch_async(3), dispatch_object(3),
dispatch_source_create(3)Darwin May 1, 2008 Darwin
