线程访问外部数据

NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
   initData();
    
    __block int sum = 0;
    
   // Create a semaphore with 0 resource
    __block dispatch_semaphore_t sem = dispatch_semaphore_create(0);
   __blockdispatch_semaphore_t taskSem =dispatch_semaphore_create(0);
    
   // create dispatch semaphore
    dispatch_queue_t queue = dispatch_queue_create("StudyBlocks",NULL);
    
    dispatch_block_t task1 = ^(void) {
        int s = 0;
        for (int i = 0; i < Length; i++)
            s += data;
        sum = s;
        
       NSLog(@" >> after add: %d", sum);
        
       dispatch_semaphore_signal(taskSem);
    };