Calling dispatch_sync from a concurrent queue - does it block entirely?

Question

Let's say I hypothetically call a dispatch_sync from a concurrent queue - does it block the entire queue or just that thread of execution?

Answer 1

dispatch_sync will block the caller thread until execution completes, a concurrent queue has multiple threads so it will only block one of those on that queue, the other threads will still execute.

Here is what Apple says about this:

Submits a block to a dispatch queue for synchronous execution. Unlike dispatch_async, this function does not return until the block has finished. Calling this function and targeting the current queue results in deadlock.

Unlike with dispatch_async, no retain is performed on the target queue. Because calls to this function are synchronous, it "borrows" the reference of the caller. Moreover, no Block_copy is performed on the block.

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

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