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Right, but libdispatch doesn't have a mutex. It has semaphores and queues. So if you're trying to use libdispatch and you don't want the closure-based aspect of

Right, but libdispatch doesn't have a mutex. It has semaphores and queues. So if you're trying to use libdispatch and you don't want the closure-based aspect of queues, you might be tempted to use a semaphore instead. Don't do that, use os_unfair_lock or pthread_mutex (or a higher-level construct like NSLock) instead.

asveikau 7y ago
I was hazy on remembering details for my comment above, but I now realize fully that I do have experience with semaphores on Darwin. It went like this: First I tried POSIX semaphores. They seemed poorly done on Darwin. [Maybe they needed sem_open() rather than sem_init()? But unnamed ones are way more useful!] Then I tried Mach semaphores. Then I saw a recommendation for libdispatch semaphores to avoid too many syscalls. Given that Linux does the syscall-avoiding trick by default for standard-conforming semaphores, it seems to me like on Darwin you can't go too far wrong always using libdispatch. But I guess you miss that priority inversion workaround you were talking about.
- lilyball 7y ago
  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.
  - asveikau 7y ago
    Yeah, obviously if I needed a mutex I would use a mutex, I don't have that particular confusion and would expect pthread mutex to be well worn on the platform. (And I noticed Darwin has some helpful pthread_mutex_attr values.) I am talking only about the case where you really want a semaphore.

asveikau 7y ago

I was hazy on remembering details for my comment above, but I now realize fully that I do have experience with semaphores on Darwin. It went like this: First I tried POSIX semaphores. They seemed poorly done on Darwin. [Maybe they needed sem_open() rather than sem_init()? But unnamed ones are way more useful!] Then I tried Mach semaphores. Then I saw a recommendation for libdispatch semaphores to avoid too many syscalls. Given that Linux does the syscall-avoiding trick by default for standard-conforming semaphores, it seems to me like on Darwin you can't go too far wrong always using libdispatch. But I guess you miss that priority inversion workaround you were talking about.

lilyball 7y ago
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.
- asveikau 7y ago
  Yeah, obviously if I needed a mutex I would use a mutex, I don't have that particular confusion and would expect pthread mutex to be well worn on the platform. (And I noticed Darwin has some helpful pthread_mutex_attr values.) I am talking only about the case where you really want a semaphore.