debian man page for dispatch_apply

dispatch_apply(3)					   BSD Library Functions Manual 					 dispatch_apply(3)

NAME
     dispatch_apply -- schedule blocks for iterative execution

SYNOPSIS
     #include <dispatch/dispatch.h>

     void
     dispatch_apply(size_t iterations, dispatch_queue_t queue, void (^block)(size_t));

     void
     dispatch_apply_f(size_t iterations, dispatch_queue_t queue, void *context, void (*function)(void *, size_t));

DESCRIPTION
     The dispatch_apply() function provides data-level concurrency through a "for (;;)" loop like primitive:

     dispatch_queue_t the_queue = dispatch_get_concurrent_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT);
     size_t iterations = 10;

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

     dispatch_apply(iterations, the_queue, ^(size_t idx) {
	     printf("%zu
", idx);
     });

     Like a "for (;;)" loop, the dispatch_apply() function is synchronous.  If asynchronous behavior is desired, please wrap the call to
     dispatch_apply() with a call to dispatch_async() against another queue.

     Sometimes, when the block passed to 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):

     #define STRIDE  3

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

     size_t i;
     for (i = count - (count % STRIDE); i < count; i++) {
	     printf("%zu
", i);
     }

FUNDAMENTALS
     Conceptually, dispatch_apply() is a convenient wrapper around dispatch_async() and a semaphore to wait for completion.  In practice, the dis-
     patch library optimizes this function.

     The dispatch_apply() function is a wrapper around dispatch_apply_f().

SEE ALSO
     dispatch(3), dispatch_async(3), dispatch_queue_create(3), dispatch_semaphore_create(3)

Darwin								    May 1, 2009 							    Darwin