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)

Darwin                                             May 1, 2009                                 dispatch_apply(3)