Cocoa Lock that does not use cpu power

Question

Hi, I need a lock in cocoa that does not use one cpu when I try to lock it and it is locked somewhere else. Something that is implemented in the kernel scheduler.

Answer 1

Do you mean "lock" as in a mutex between threads, or a mutex between processes, or a mutex between disparate resources on a network, or...?

If it's between threads, you use NSLock. If it's between processes, then you can use POSIX named semaphores.

Answer 2

It sounds like you're trying to find a lock that's not a spin lock. EVERY lock must use some CPU, or else it couldn't function. :-)

NSLock is the most obvious in Cocoa. It has a simple -lock, -unlock interface and uses pthread mutexes in its implementation. There are a number of more sophisticated locks in Cocoa for more specific needs: NSRecursiveLock, NSCondition, NSDistributedLock, etc.

There is also the @synchronized directive which is even simpler to use but has some additional overhead to it.

GCD also has a counted semaphore object if you're looking for something like that.

Answer 3

My recommendation is that, instead of locks, you look at using NSOperations and an NSOperationQueue where you -setMaxConcurrentOperationCount: to 1 to access the shared resource. By using a single-wide operation queue, you can guarantee that only one thing at a time will make use of a resource, while still allowing for multiple threads to do so.

This avoids the need for locks, and since everything is done in user space, can provide much better performance. I've replaced almost all of my locking around shared resources with this technique, and have been very pleased with the results.

Answer 4

If you really want kernel locks and know what you are doing, you can use

<libkern/OSAtomic.h>

Be sure to always use the "barrier" variants. These are faster and much more dangerous than posix locks. If you can target 10.6 with new code, then GCD is a great way to go. There is a great podcast on using the kernel synchronization primitives at: http://www.mac-developer-network.com/shows/podcasts/lnc/lnc032/