.\" Copyright (c) 2008-2012 Apple Inc. All rights reserved.
.Dd May 1, 2008
.Dt dispatch_queue_create 3
.Os Darwin
.Sh NAME
.Nm dispatch_queue_create ,
.Nm dispatch_queue_get_label ,
.Nm dispatch_get_current_queue ,
.Nm dispatch_get_global_queue ,
.Nm dispatch_get_main_queue ,
.Nm dispatch_main ,
.Nm dispatch_set_target_queue
.Nd where blocks are scheduled for execution
.Sh SYNOPSIS
.Fd #include <dispatch/dispatch.h>
.Ft dispatch_queue_t
.Fo dispatch_queue_create
.Fa "const char *label" "dispatch_queue_attr_t attr"
.Fc
.Ft "const char *"
.Fo dispatch_queue_get_label
.Fa "dispatch_queue_t queue"
.Fc
.Ft dispatch_queue_t
.Fo dispatch_get_global_queue
.Fa "long priority"
.Fa "unsigned long flags"
.Fc
.Ft dispatch_queue_t
.Fo dispatch_get_main_queue
.Fa void
.Fc
.Ft void
.Fo dispatch_main
.Fa void
.Fc
.Ft void
.Fo dispatch_set_target_queue
.Fa "dispatch_object_t object"
.Fa "dispatch_queue_t target"
.Fc
.Sh DESCRIPTION
Queues are the fundamental mechanism for scheduling blocks for execution within
the
.Xr dispatch 3
framework.
.Pp
All blocks submitted to dispatch queues are dequeued in FIFO order.
Queues created with the
.Dv DISPATCH_QUEUE_SERIAL
attribute wait for the previously 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 execution and
blocks submitted to independent queues may execute concurrently.
.Pp
Queues created with the
.Dv DISPATCH_QUEUE_CONCURRENT
attribute may execute dequeued blocks concurrently and support barrier blocks
submitted with the dispatch barrier API.
.Sh CREATION
Queues are created with the
.Fn dispatch_queue_create
function. Queues, like all dispatch objects, are reference counted and newly
created queues have a reference count of one.
.Pp
The optional
.Fa 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:
.Pp
.Bd -literal -offset indent
my_queue = dispatch_queue_create("com.example.subsystem.taskXYZ", NULL);
.Ed
.Pp
The
.Fa attr
argument specifies the type of queue to create and must be either
.Dv DISPATCH_QUEUE_SERIAL
or
.Dv DISPATCH_QUEUE_CONCURRENT .
.Pp
The
.Fn dispatch_queue_get_label
function returns the label provided when the given
.Fa queue
was created (or an empty C string if no label was provided at creation).
Passing the constant
.Dv DISPATCH_CURRENT_QUEUE_LABEL
to
.Fn dispatch_queue_get_label
returns the label of the current queue.
.Sh SUSPENSION
Queues may be temporarily suspended and resumed with the functions
.Fn dispatch_suspend
and
.Fn dispatch_resume
respectively. Suspension is checked prior to block execution and is
.Em not
preemptive.
.Sh MAIN QUEUE
The dispatch framework provides a default serial queue for the application to
use. This queue is accessed via the
.Fn dispatch_get_main_queue
function.
.Pp
Programs must call
.Fn dispatch_main
at the end of
.Fn main
in order to process blocks submitted to the main queue. (See the
.Sx COMPATIBILITY
section for exceptions.) The
.Fn dispatch_main
function never returns.
.Sh GLOBAL CONCURRENT QUEUES
Unlike the main queue or queues allocated with
.Fn 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:
.Bl -bullet -compact -offset indent
.It
DISPATCH_QUEUE_PRIORITY_HIGH
.It
DISPATCH_QUEUE_PRIORITY_DEFAULT
.It
DISPATCH_QUEUE_PRIORITY_LOW
.It
DISPATCH_QUEUE_PRIORITY_BACKGROUND
.El
.Pp
The priority of a global concurrent queue controls the scheduling priority 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 scheduled. Additionally, items on
the background priority global queue will execute on threads with background
state as described in
.Xr setpriority 2
(i.e.\& disk I/O is throttled and the thread's scheduling priority is set to
lowest value).
.Pp
Use the
.Fn dispatch_get_global_queue
function to obtain the global queue of given priority. The
.Fa flags
argument is reserved for future use and must be zero. Passing any value other
than zero may result in a NULL return value.
.Sh TARGET QUEUE
The
.Fn 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.
.Pp
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).
.Pp
The default target queue of all dispatch objects created by the application is
the default priority global concurrent queue. To reset an object's target queue
to the default, pass the
.Dv DISPATCH_TARGET_QUEUE_DEFAULT
constant to
.Fn dispatch_set_target_queue .
.Pp
The priority of a dispatch queue is inherited from its target queue.
In order to change the priority of a queue created with
.Fn dispatch_queue_create ,
use the
.Fn dispatch_get_global_queue
function to obtain a target queue of the desired priority.
.Pp
Blocks submitted to a serial queue whose target queue is another serial queue
will not be invoked concurrently with blocks submitted to the target queue or
to any other queue with that same target queue.
.Pp
The target queue of a dispatch source specifies where its event handler and
cancellation handler blocks will be submitted. See
.Xr dispatch_source_create 3
for more information about dispatch sources.
.Pp
The target queue of a dispatch I/O channel specifies the priority of the global
queue where its I/O operations are executed. See
.Xr dispatch_io_create 3
for more information about dispatch I/O channels.
.Pp
For all other dispatch object types, the only function of the target queue is
to determine where an object's finalizer function is invoked.
.Pp
The result of passing the main queue or a global concurrent queue as the first
argument of
.Fn dispatch_set_target_queue
is undefined.
.Pp
Directly or indirectly setting the target queue of a dispatch queue to itself is
undefined.
.Sh DEPRECATED FUNCTIONS
The following functions are deprecated and will be removed in a future release:
.Bl -item
.It
.Ft dispatch_queue_t
.Fn dispatch_get_current_queue void ;
.El
.Pp
.Fn 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
.Fn dispatch_main
is called, then the result of
.Fn dispatch_get_main_queue
is returned. In all other cases, the default target queue will be returned.
.Pp
The use of
.Fn 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.
.Pp
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
.Fn dispatch_get_current_queue .
.Pp
The result of
.Fn dispatch_get_main_queue
may or may not equal the result of
.Fn 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
.Xr pthread_main_np 3 .
.Pp
.Fn dispatch_get_current_queue
may return a queue owned by a different subsystem 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
.Fn dispatch_get_current_queue
will cause retain calls to be automatically generated, so the use of
.Fn dispatch_get_current_queue
for any reason in code built with ARC is particularly strongly discouraged.
.Sh COMPATIBILITY
Cocoa applications need not call
.Fn 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
.Fn dispatch_main
are not guaranteed to execute on the main thread.
.Pp
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
.Em MUST
catch all exceptions before returning from a block submitted to a dispatch
queue; otherwise the process will be terminated with an uncaught exception.
.Pp
The dispatch framework manages the relationship between dispatch queues and
threads of execution. As a result, applications
.Em MUST NOT
delete or mutate objects that they did not create. The following interfaces
.Em MUST NOT
be called by blocks submitted to a dispatch queue:
.Bl -bullet -offset indent
.It
.Fn pthread_cancel
.It
.Fn pthread_detach
.It
.Fn pthread_join
.It
.Fn pthread_kill
.It
.Fn pthread_exit
.El
.Pp
Applications
.Em 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:
.Bl -bullet -offset indent
.It
.Fn pthread_setcancelstate
.It
.Fn pthread_setcanceltype
.It
.Fn pthread_setschedparam
.It
.Fn pthread_sigmask
.It
.Fn pthread_setugid_np
.It
.Fn pthread_chdir
.It
.Fn pthread_fchdir
.El
.Pp
Applications
.Em MUST NOT
rely on the following interfaces returning predictable results between
invocations of blocks submitted to a dispatch queue:
.Bl -bullet -offset indent
.It
.Fn pthread_self
.It
.Fn pthread_getschedparam
.It
.Fn pthread_get_stacksize_np
.It
.Fn pthread_get_stackaddr_np
.It
.Fn pthread_mach_thread_np
.It
.Fn pthread_from_mach_thread_np
.El
.Pp
While the result of
.Fn 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
.Fn pthread_setspecific
and
.Fn 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
.Fn 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.
.Pp
The following example code correctly handles per-thread return values:
.Bd -literal -offset indent
__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\n", r, e);
.Ed
.Pp
Note that in the above example
.Va 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
.Fn getpwnam
instead of
.Fn getpwnam_r .
.Pp
As an optimization,
.Fn dispatch_sync
invokes the block on the current thread when possible. In this case, the thread
specific data such as
.Va errno
may persist from the block until back to the caller. Great care should be taken
not to accidentally rely on this side-effect.
.Pp
.Sh SEE ALSO
.Xr dispatch 3 ,
.Xr dispatch_async 3 ,
.Xr dispatch_object 3 ,
.Xr dispatch_source_create 3