.\" Copyright (c) 2008-2017 Apple Inc. All rights reserved.
.Dd May 1, 2009
.Dt dispatch_apply 3
.Os Darwin
.Sh NAME
.Nm dispatch_apply
.Nd schedule blocks for iterative execution
.Sh SYNOPSIS
.Fd #include <dispatch/dispatch.h>
.Ft void
.Fo dispatch_apply
.Fa "size_t iterations" "dispatch_queue_t queue" "void (^block)(size_t)"
.Fc
.Ft void
.Fo dispatch_apply_f
.Fa "size_t iterations" "dispatch_queue_t queue" "void *context" "void (*function)(void *, size_t)"
.Fc
.Sh DESCRIPTION
The
.Fn dispatch_apply
function provides data-level concurrency through a "for (;;)" loop like primitive:
.Bd -literal
size_t iterations = 10;

// 'idx' is zero indexed, just like:
// for (idx = 0; idx < iterations; idx++)

dispatch_apply(iterations, DISPATCH_APPLY_AUTO, ^(size_t idx) {
	printf("%zu\\n", idx);
});
.Ed
.Pp
Although any queue can be used, it is strongly recommended to use 
.Vt DISPATCH_APPLY_AUTO
as the 
.Vt queue 
argument to both
.Fn dispatch_apply
and
.Fn dispatch_apply_f , 
as shown in the example above, since this allows the system to automatically use worker threads
that match the configuration of the current thread as closely as possible.
No assumptions should be made about which global concurrent queue will be used.
.Pp
Like a "for (;;)" loop, the
.Fn dispatch_apply
function is synchronous.
If asynchronous behavior is desired, wrap the call to
.Fn dispatch_apply
with a call to
.Fn dispatch_async
against another queue.
.Pp
Sometimes, when the block passed to
.Fn dispatch_apply
is simple, the use of striding can tune performance.
Calculating the optimal stride is best left to experimentation.
Start with a stride of one and work upwards until the desired performance is
achieved (perhaps using a power of two search):
.Bd -literal
#define STRIDE 3

dispatch_apply(count / STRIDE, DISPATCH_APPLY_AUTO, ^(size_t idx) {
	size_t j = idx * STRIDE;
	size_t j_stop = j + STRIDE;
	do {
		printf("%zu\\n", j++);
	} while (j < j_stop);
});

size_t i;
for (i = count - (count % STRIDE); i < count; i++) {
	printf("%zu\\n", i);
}
.Ed
.Sh IMPLIED REFERENCES
Synchronous functions within the dispatch framework hold an implied reference
on the target queue. In other words, the synchronous function borrows the
reference of the calling function (this is valid because the calling function
is blocked waiting for the result of the synchronous function, and therefore
cannot modify the reference count of the target queue until after the
synchronous function has returned).
.Pp
This is in contrast to asynchronous functions which must retain both the block
and target queue for the duration of the asynchronous operation (as the calling
function may immediately release its interest in these objects).
.Sh FUNDAMENTALS
.Fn dispatch_apply
and
.Fn dispatch_apply_f
attempt to quickly create enough worker threads to efficiently iterate work in parallel.
By contrast, a loop that passes work items individually to
.Fn dispatch_async
or
.Fn dispatch_async_f
will incur more overhead and does not express the desired parallel execution semantics to
the system, so may not create an optimal number of worker threads for a parallel workload.
For this reason, prefer to use 
.Fn dispatch_apply
or
.Fn dispatch_apply_f
when parallel execution is important.
.Pp
The
.Fn dispatch_apply
function is a wrapper around
.Fn dispatch_apply_f .
.Sh CAVEATS
Unlike
.Fn dispatch_async ,
a block submitted to
.Fn dispatch_apply
is expected to be either independent or dependent
.Em only
on work already performed in lower-indexed invocations of the block. If
the block's index dependency is non-linear, it is recommended to
use a for-loop around invocations of
.Fn dispatch_async .
.Sh SEE ALSO
.Xr dispatch 3 ,
.Xr dispatch_async 3 ,
.Xr dispatch_queue_create 3