Grand Central Dispatch async vs sync [duplicate]

The first answer isn't quite complete, unfortunately. Yes, sync will block and async will not, however there are additional semantics to take into account. Calling dispatch_sync() will also cause your code to wait until each and every pending item on that queue has finished executing, also making it a synchronization point for said work. dispatch_async() will simply submit the work to the queue and return immediately, after which it will be executed "at some point" and you need to track completion of that work in some other way (usually by nesting one dispatch_async inside another dispatch_async - see the man page for example).

sync means the function WILL BLOCK the current thread until it has completed, async means it will be handled in the background and the function WILL NOT BLOCK the current thread.

If you want serial execution of blocks check out the creation of a serial dispatch queue

From the man page:

FUNDAMENTALS

Conceptually, dispatch_sync() is a convenient wrapper around dispatch_async() with the addition of a semaphore to wait for completion of the block, and a wrapper around the block to signal its completion.

See dispatch_semaphore_create(3) for more information about dispatch semaphores. The actual implementation of the dispatch_sync() function may be optimized and differ from the above description.