Locking mutex in one thread and unlocking it in the other

Question

Will this code be correct and portable?

void* aThread(void*)
{
    while(conditionA)
    {
        pthread_mutex_lock(mutex1);
        //do something
        pthread_mutex_unlock(mutex2);
    }
}

void* bThread(void*)
{
    while(conditionB)
    {
        pthread_mutex_lock(mutex2);
        //do something
        pthread_mutex_unlock(mutex1);
    }
}

In the actual application I have three threads - two for adding values to an array and one for reading them. And I need the third thread to display the contents of the array right after one of the other threads adds a new item.

Answer 1

It is not. If thread A gets to mutex_unlock(2) before thread B got to mutex_lock(2), you are facing undefined behavior. You must not unlock another thread's mutex either.

The pthread_mutex_lock Open Group Base Specification says so:

If the mutex type is PTHREAD_MUTEX_NORMAL [...] If a thread attempts to unlock a mutex that it has not locked or a mutex which is unlocked, undefined behavior results.