Before you can schedule work on a queue, you need to get ahold of a queue to do the work on. You can get the main queue with
dispatch_queue_t dispatch_get_main_queue (void);
You can schedule activity on the main thread by using this queue. This is a serial queue, so work will issue and complete in FIFO order.
dispatch_queue_t dispatch_get_global_queue (long priority, unsigned long flags);
This gives you one of the global concurrent queues, whose priority is one of the DISPATCH_QUEUE_PRIORITY_LOW, DEFAULT, or HIGH priority constants. Pass 0UL for flags; it is ignored now and is an expansion point for the future. The priority refers to which queue gets its blocks scheduled. The default queue’s blocks only get run on threads ...