On Apple platforms the simple set of guidelines would look something like: * Can you use dispatch_queue? Great, use it. * If not, can you use pthread_mutex? N

On Apple platforms the simple set of guidelines would look something like: * Can you use dispatch_queue? Great, use it. * If not, can you use pthread_mutex? Note that this can be configured to be unfair if desired. If so, use it. * If not (e.g. you need actual semaphore behavior), use dispatch_semaphore. os_unfair_lock is more of an optimization when speed is a must. It's a replacement for the old OSSpinLock, but AFAICT it basically acts like a normal mutex under contention, so I'm not actually sure why pthread_mutex would be preferred over it, except the documentation says to prefer pthread or libdispatch as they're "higher level" primitives. Also of interest is dispatch_group_t, which is basically an inverted semaphore (it counts active tasks, and lets you wait until all tasks have completed). Under the hood it's actually a semaphore that's initialized with LONG_MAX as the value and some custom logic (including atomics) around waiting.