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_current_queue(void);
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. By default, queues created with dispatch_queue_create() wait for the previously dequeued block to complete before dequeuing the next block. This FIFO completion behavior is sometimes simply described as a "serial queue." Queues are not bound to any specific thread of execution and blocks submitted to independent queues may execute concurrently. 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. By convention, clients should pass a reverse DNS style label. If a label is provided, it is copied. If a label is not provided, then dispatch_queue_get_label() returns an empty C string. For example:

    my_queue = dispatch_queue_create("com.example.subsystem.taskXYZ", NULL);

The attr argument is reserved for future use and must be NULL.

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 application to use. This queue is accessed via dispatch_get_main_queue(). Programs must call dispatch_main() at the end of main() in order to process blocks submitted to the main queue. (See the compatibility section for exceptions.)

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). The global concurrent queues represent three priority bands:
•   DISPATCH_QUEUE_PRIORITY_HIGH
•   DISPATCH_QUEUE_PRIORITY_DEFAULT
•   DISPATCH_QUEUE_PRIORITY_LOW

Blocks submitted to the high priority global queue will be invoked before those submitted to the default or low priority global queues. Blocks submitted to the low priority global queue will only be invoked if no blocks are pending on the default or high priority queues.

RETURN VALUES
The dispatch_queue_create() function returns NULL on failure.

The dispatch_queue_get_label() function always returns a valid C string. An empty C string is returned if the label was NULL creation time.

The dispatch_get_main_queue() function returns the default main queue.

The dispatch_get_current_queue() function 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. Otherwise, the result of dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0) will be returned in all other cases.

The dispatch_main() function never returns.

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. Currently only dispatch queues and dispatch sources are supported by this function. The result of using dispatch_set_target_queue() with any other dispatch object type is undefined.

The new target queue is retained by the given object before the previous target queue is released. The new target queue will take effect between block executions, but not in the middle of any existing block executions (non-preemptive).

The priority of a dispatch queue is inherited by 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. 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.

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 result of passing the main queue or a global concurrent queue to 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.

CAVEATS
Code cannot make any assumptions about the queue returned by dispatch_get_current_queue(). 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. Therefore, dispatch_get_current_queue() MUST only be used for identity tests or debugging.

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 internal data structures of the dispatch framework will be left in an inconsistent state.

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 predictable 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 invocations 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 %d, 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 ALSO
dispatch(3), dispatch_async(3), dispatch_object(3), dispatch_source_create(3)