examples/Dispatch Samples/nWide.c

   1 /*
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  43
  44 /*
  45  *  nWide.c
  46  *  Samples project
  47  *
  48  *  Created by Mensch on 5/1/09.
  49  *  Copyright 2009 Apple, Inc. All rights reserved.
  50  *
  51  */
  52
  53 #include <stdio.h>
  54 #include <unistd.h>
  55 #include <stdlib.h>
  56 #include <sys/errno.h>
  57 #include <assert.h>
  58 #include <dispatch/dispatch.h>
  59 #include <mach/mach_time.h>
  60 #import <libkern/OSAtomic.h>
  61 #include <string.h>
  62
  63
  64 /*
  65  *      Demonstrate using dispatch_semaphore to create a concurrent queue that
  66  *      allows only a fixed number of blocks to be in flight at any given time
  67  */
  68
  69 int main (int argc, const char * argv[]) {
  70         dispatch_group_t        mg = dispatch_group_create();
  71         dispatch_semaphore_t ds;
  72         __block int numRunning = 0;
  73         int     qWidth = 5;
  74         int numWorkBlocks = 100;
  75
  76         if (argc >= 2) {
  77                 qWidth = atoi(argv[1]); // use the command 1st line parameter as the queue width
  78                 if (qWidth==0) qWidth==1; // protect against bad values
  79         }
  80
  81         if (argc >=3) {
  82                 numWorkBlocks = atoi(argv[2]);  // use the 2nd command line parameter as the queue width
  83                 if (numWorkBlocks==0) numWorkBlocks==1; // protect against bad values
  84         }
  85
  86         printf("Starting dispatch semaphore test to simulate a %d wide dispatch queue\n", qWidth );
  87         ds = dispatch_semaphore_create(qWidth);
  88
  89         int i;
  90         for (i=0; i<numWorkBlocks; i++) {
  91                 // synchronize the whole shebang every 25 work units...
  92                 if (i % 25 == 24) {
  93                         dispatch_group_async(mg,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0), ^{
  94                                 int x;
  95                                 // wait for all pending work units to finish up...
  96                                 for (int x=0; x<qWidth; x++) dispatch_semaphore_wait(ds, DISPATCH_TIME_FOREVER);
  97                                 // do the thing that is critical here
  98                                 printf("doing something critical...while %d work units are running \n",numRunning);
  99                                 // and let work continue unimpeeded
 100                                 for (int x=0; x<qWidth; x++) dispatch_semaphore_signal(ds);
 101                         });
 102                 } else {
 103                         // schedule the next block waiting when there are qWidth blocks running
 104                         dispatch_semaphore_wait(ds, DISPATCH_TIME_FOREVER);
 105                         dispatch_group_async(mg,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0), ^{
 106                                 OSAtomicIncrement32( &numRunning );
 107                                 usleep(random() % 10000);       // simulate some random amount of work
 108                                 printf("Value of i is %d  Number of blocks in flight %d\n",i, numRunning);
 109                                 // tell the loop it's time to schedule the next block if there is one
 110                                 OSAtomicDecrement32( &numRunning );
 111                                 dispatch_semaphore_signal(ds);
 112                         });
 113                 }
 114         }
 115
 116         dispatch_group_notify(mg, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
 117                 printf("And we are done!\n");
 118                 dispatch_release(mg);
 119                 dispatch_release(ds);
 120                 exit(0);
 121         });
 122
 123         dispatch_main();
 124     return 0;
 125 }
 126
 127