How to use dispatch_queue_set_specific() and dispatch_get_specific()

Question

I'm having a hard time finding good examples on how to use these functions.

static void * kQueue1Key = "key1";
static void * kQueue2Key = "key2";

dispatch_queue_t queue1 = dispatch_queue_create("com.company.queue1", DISPATCH_QUEUE_SERIAL);
dispatch_queue_t queue2 = dispatch_queue_create("com.company.queue2", DISPATCH_QUEUE_SERIAL);

dispatch_queue_set_specific(queue1, kQueue1Key, (void *)kQueue1Key, NULL);
dispatch_queue_set_specific(queue2, kQueue2Key, (void *)kQueue2Key, NULL);

dispatch_sync(queue1, ^{
    if(dispatch_get_specific(kQueue1Key))
    {
        NSLog(@"I'm expecting this line to run (A)");

        dispatch_sync(queue2, ^{

            NSLog(@"I'm expecting this line to run (B)");

            if(dispatch_get_specific(kQueue2Key))
            {
                if(dispatch_get_specific(kQueue1Key))
                {
                    NSLog(@"I'm expecting this line to run (C)");
                }
                else
                {
                    [NSException raise:NSInternalInconsistencyException format:@"Should not end up here (C)"];
                }
            }
            else
            {
                [NSException raise:NSInternalInconsistencyException format:@"Should not end up here (B)"];
            }
        });
    }
    else
    {
        [NSException raise:NSInternalInconsistencyException format:@"Should not end up here (A)"];
    }
});

Result

I'm expecting this line to run (A)
I'm expecting this line to run (B)
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Should not end up here (C)'

Is it expected behavior? If I were to dispatch_sync to queue1 since I'm not on the queue I would deadlock. What am I missing?

Answer 1

Oh here, it popped into my head why you're getting what you're getting. Notes in line:

dispatch_sync(queue1, ^{

When you get to this point, the "current queue" is queue1

    if(dispatch_get_specific(kQueue1Key))

You're asking the current queue for the value it has for kQueue1Key, you set that earlier, so it gives it back to you.

    {
        NSLog(@"I'm expecting this line to run (A)");

        dispatch_sync(queue2, ^{

When you get to this point, the "current queue" is now queue2

            NSLog(@"I'm expecting this line to run (B)");

            if(dispatch_get_specific(kQueue2Key))

You're asking the current queue for the value it has for kQueue2Key, you set that earlier, so it gives it back to you.

            {

                if(dispatch_get_specific(kQueue1Key))

You're now asking the current queue for the value it has for kQueue1Key. Since the current queue is queue2 and you never set a value with kQueue1Key on queue2 you get back NULL.

                {
                    NSLog(@"I'm expecting this line to run (C)");
                }
                else
                {
                    [NSException raise:NSInternalInconsistencyException format:@"Should not end up here (C)"];
                }

The misunderstanding here is that dispatch_get_specific doesn't traverse the stack of nested queues, it traverses the queue targeting lineage. For instance, if you did this instead,

static void * kQueue1Key = (void*)"key1";
static void * kQueue2Key = (void*)"key2";

dispatch_queue_t queue1 = dispatch_queue_create("com.company.queue1", DISPATCH_QUEUE_SERIAL);
dispatch_queue_t queue2 = dispatch_queue_create("com.company.queue2", DISPATCH_QUEUE_SERIAL);

dispatch_queue_set_specific(queue1, kQueue1Key, (void *)kQueue1Key, NULL);
dispatch_queue_set_specific(queue2, kQueue2Key, (void *)kQueue2Key, NULL);

// Set Queue2 to target Queue1
dispatch_set_target_queue(queue2, queue1);

dispatch_sync(queue2, ^{

    if(dispatch_get_specific(kQueue1Key))
    {
        NSLog(@"I'm expecting this line to run (A)");
    }
    else
    {
        [NSException raise:NSInternalInconsistencyException format:@"Should not end up here (C)"];
    }

    if(dispatch_get_specific(kQueue2Key))
    {
        NSLog(@"I'm expecting this line to run (B)");
    }
    else
    {
        [NSException raise:NSInternalInconsistencyException format:@"Should not end up here (C)"];
    }
});

...the targeting relationship is the one that gets traversed, not the stack relationship. It would be nice if there were something that traversed the stack relationship, but I'm not aware of anything (that you wouldn't have to implement yourself).