dispatch_sync() always execute block in main thread

Question

Is there any difference between if dispatch_sync is called in 3 different queue like

1.

 dispatch_sync(dispatch_get_main_queue(),^(void){
      NSLo         


        

Answer 1

See dispatch_sync documentation, which notes

As an optimization, this function invokes the block on the current thread when possible.

If you dispatch something synchronously, since the thread must wait for the dispatched code to complete, anyway, it will frequently run that code on the current thread. So if dispatched synchronously from the main thread, it will run on main thread. If dispatched synchronously from a background thread, it will run on that background thread.

As noted by ipmcc, a well-known exception is when a background thread dispatches something synchronously to the main thread. As the libdispatch source says:

It's preferred to execute synchronous blocks on the current thread due to thread-local side effects, garbage collection, etc. However, blocks submitted to the main thread MUST be run on the main thread.