Question

I’m getting curious about using dispatch_after and dispatch_semaphore_t together to execute some tasks and wait some time between each thread.

I use the following code:

dispatch_time_t time = dispatch_time(DISPATCH_TIME_NOW, 3ull * NSEC_PER_SEC);

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);

dispatch_after(time, queue, ^{

    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

    NSLog(@"blk01");

    dispatch_semaphore_signal(semaphore);

});

dispatch_after(time, queue, ^{

    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

    NSLog(@"blk02");

    dispatch_semaphore_signal(semaphore);

});

dispatch_after(time, queue, ^{

    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

    NSLog(@"blk03");

    dispatch_semaphore_signal(semaphore);

});

I’m getting the execution of threads after 3 seconds, not with the expected delay.

What am I doing wrong? Is there any other way to execute threads in dispatch_semaphore_t with delay time between each thread execution?

Answer 1

If you want to execute three blocks serially (not concurrently), and you want to pause between blocks, there are simpler ways than using a semaphore. The simplest is to just create a serial queue and put a sleep (or usleep) at the beginning or end of each block:

dispatch_queue_t queue = dispatch_queue_create("my queue", 0);

dispatch_async(queue, ^{
    sleep(3);
    NSLog(@"blk01");
});

dispatch_async(queue, ^{
    sleep(3);
    NSLog(@"blk02");
});

dispatch_async(queue, ^{
    sleep(3);
    NSLog(@"blk03");
});

dispatch_release(queue);

Putting sleep in your blocks is very simple, but ties up a thread. That probably doesn’t matter unless you’re creating a lot of queues and sleeping on many of them.

If you tell us why you think you need a semaphore, perhaps we can offer you a better alternative or advice on how to use it properly.