using dispatch_sync in Grand Central Dispatch

Question

Can anyone explain with really clear use cases what the purpose of dispatch_sync in GCD is for? I can\'t understand where and why I would have to u

Answer 1

dispatch_sync is semantically equivalent to a traditional mutex lock.

dispatch_sync(queue, ^{
    //access shared resource
});

works the same as

pthread_mutex_lock(&lock);
//access shared resource
pthread_mutex_unlock(&lock);