where to unlock mutex in pthread?

Question

Is it a good practice to lock a mutex from the main thread, and release from another thread?

Or should I make sure a thread will do it all in one? ie: lock, and unlock

Answer 1

http://www.manpagez.com/man/3/pthread_mutex_unlock/

(also from the POSIX spec site: http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutex_lock.html )

If the current thread holds the lock on mutex, then the pthread_mutex_unlock() function unlocks mutex.

Calling pthread_mutex_unlock() with a mutex that the calling thread does not hold will result in undefined behavior.

Answer 2

A mutex can only be unlocked by the same thread that locked it. A program that violates this rule has undefined behavior and is not portable or stable; it may seem to work at times and fail horribly at other times, when compiled on a slightly different system, during a different phase of the moon, or after you upgrade.

If you really need this sort of behavior (locking by one thread and unlocking by another), a semaphore may meet your needs. Semaphores do not have owners, and any thread may call sem_post or sem_wait at basically any time.

Answer 3

It is bad practice to lock in one thread and unlock in another thread as this will require the two threads to communicate with each other. A thread should perform its own locking and unlocking.

Answer 4

It's never good practice to lock from one thread and unlock from another. The name says it all -- mutual exclusion. A thread that takes it holds it until done.