iphone - Why dispatch_sync( ) call on main queue is blocking the main queue?

Question

I know this is not a strong question but I have to clear my mind on this concept.

I have defined myBlock as follows.

void(^myBlock)(void) = ^{
   for(int i = 0;i < 10 ; i++)
   {
       NSLog(@"%d and current queue = %@",i,[NSThread currentThread]);
   } 
};

Now In viewDidLoad method when I uses the dispatch_sync() method independently on main queue then the main queue gets blocked.

Here is the Sample.

- (void)viewDidLoad
 {
    [super viewDidLoad];
    dispatch_queue_t queue = dispatch_get_main_queue();
    dispatch_sync(queue,myBlock);
 }

But But, when I use the same dispatch_sync() function on main thread Inside a block of dispatch_async() function which is fired on concurrent queue then the main thread does not blocked.

Here is the sample.

- (void)viewDidLoad
 {
    [super viewDidLoad];
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

    dispatch_async(queue,^{  

        dispatch_sync(dispatch_get_main_queue(),myBlock);
    });
 }

I am not clear why this is happening? Why main thread blocked when calling dispatch_sync() independently?

Answer 1

When using dispatch_sync on a serial queue (like the main queue) the current thread has to wait until the dispatched code is executed.

A dead lock occurs when a block is dispatched synchronously on from a serial queue to the same queue.