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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\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 perfor-
     mance.  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_async(3), dispatch_semaphore_create(3), dispatch_queue_create(3)

Darwin                                           May 1, 2009                                          Darwin