Question

I need to set a mutex before to make an asynchronous request, and then unlock the mutex in the callback of this request that is on another thread.

Apple documentation say:

Warning: The NSLock class uses POSIX
threads to implement its locking
behavior. When sending an unlock
message to an NSLock object, you must
be sure that message is sent from the
same thread that sent the initial lock
message. Unlocking a lock from a
different thread can result in
undefined behavior.

How can I avoid this “undefined behaviour” and make it work as expected?

Answer 1

Better yet; use an NSOperationQueue or a GCD queue as your synchronization primitive.

Locks are expensive and semaphores are, more or less, a lock with a counter.

Queue based coding is far more efficient, especially when using the built in queuing mechanisms.