man/dispatch_apply.3

   1 .\" Copyright (c) 2008-2009 Apple Inc. All rights reserved.
   2 .Dd May 1, 2009
   3 .Dt dispatch_apply 3
   4 .Os Darwin
   5 .Sh NAME
   6 .Nm dispatch_apply
   7 .Nd schedule blocks for iterative execution
   8 .Sh SYNOPSIS
   9 .Fd #include <dispatch/dispatch.h>
  10 .Ft void
  11 .Fo dispatch_apply
  12 .Fa "size_t iterations" "dispatch_queue_t queue" "void (^block)(size_t)"
  13 .Fc
  14 .Ft void
  15 .Fo dispatch_apply_f
  16 .Fa "size_t iterations" "dispatch_queue_t queue" "void *context" "void (*function)(void *, size_t)"
  17 .Fc
  18 .Sh DESCRIPTION
  19 The
  20 .Fn dispatch_apply
  21 function provides data-level concurrency through a "for (;;)" loop like primitive:
  22 .Bd -literal
  23 dispatch_queue_t the_queue = dispatch_get_concurrent_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT);
  24 size_t iterations = 10;
  25
  26 // 'idx' is zero indexed, just like:
  27 // for (idx = 0; idx < iterations; idx++)
  28
  29 dispatch_apply(iterations, the_queue, ^(size_t idx) {
  30         printf("%zu\\n", idx);
  31 });
  32 .Ed
  33 .Pp
  34 Like a "for (;;)" loop, the
  35 .Fn dispatch_apply
  36 function is synchronous.
  37 If asynchronous behavior is desired, please wrap the call to
  38 .Fn dispatch_apply
  39 with a call to
  40 .Fn dispatch_async
  41 against another queue.
  42 .Pp
  43 Sometimes, when the block passed to
  44 .Fn dispatch_apply
  45 is simple, the use of striding can tune performance.
  46 Calculating the optimal stride is best left to experimentation.
  47 Start with a stride of one and work upwards until the desired performance is
  48 achieved (perhaps using a power of two search):
  49 .Bd -literal
  50 #define STRIDE  3
  51
  52 dispatch_apply(count / STRIDE, queue, ^(size_t idx) {
  53         size_t j = idx * STRIDE;
  54         size_t j_stop = j + STRIDE;
  55         do {
  56                 printf("%zu\\n", j++);
  57         } while (j < j_stop);
  58 });
  59
  60 size_t i;
  61 for (i = count - (count % STRIDE); i < count; i++) {
  62         printf("%zu\\n", i);
  63 }
  64 .Ed
  65 .Sh FUNDAMENTALS
  66 Conceptually,
  67 .Fn dispatch_apply
  68 is a convenient wrapper around
  69 .Fn dispatch_async
  70 and a semaphore to wait for completion.
  71 In practice, the dispatch library optimizes this function.
  72 .Pp
  73 The
  74 .Fn dispatch_apply
  75 function is a wrapper around
  76 .Fn dispatch_apply_f .
  77 .Sh SEE ALSO
  78 .Xr dispatch_async 3 ,
  79 .Xr dispatch_semaphore_create 3 ,
  80 .Xr dispatch_queue_create 3