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\n", 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\n", j++);
    	} while (j < j_stop);
    });
    size_t i;
    for (i = count - (count % STRIDE); i < count; i++) {
    	printf("%zu\n", i);
    }

FUNDAMENTALS
Conceptually, dispatch_apply() is a convenient wrapper around dispatch_async() and a semaphore to wait for completion. In practice, the dispatch 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)