dispatch_queue_t main = dispatch_get_main_queue();
dispatch_sync(main, ^
                  {
                      [[NSNotificationCenter defaultCenter] postNotificationName:kNotificationAuthenticationSuccess object:nil userInfo:ret];
                  });

dispatch_queue_t main = dispatch_get_main_queue();
dispatch_block_t block = ^
              {
                  [[NSNotificationCenter defaultCenter] postNotificationName:kNotificationAuthenticationSuccess object:nil userInfo:ret];
              };
if (dispatch_get_current_queue() == main)
  block(); // execute the block directly, as main is already the current active queue
else
  dispatch_sync(main, block); // ask the main queue, which is not the current queue, to execute the block, and wait for it to be executed before continuing