Question

I’ve been working with pthreads a fair bit recently and there’s one little thing I still don’t quite get. I know that condition variables are designed to wait for a specific condition to come true (or be ‘signalled’). My question is, how does this differ at all from normal mutexes?

From what I understand, aren’t condition variables just a mutex with additional logic to unlock another mutex (and lock it again) when the condition becomes true?

Psuedocode example:

mutex mymutex;
condvar mycond;
int somevalue = 0;

onethread()
{
    lock(mymutex);

    while(somevalue == 0)
        cond_wait(mycond, mymutex);

    if(somevalue == 0xdeadbeef)
        some_func()

    unlock(mymutex);
}

otherthread()
{
    lock(mymutex);

    somevalue = 0xdeadbeef;

    cond_signal(mycond);

    unlock(mymutex);
}

So cond_wait in this example unlocks mymutex, and then waits for mycond to be signalled.

If this is so, aren’t condition variables just mutexes with extra magic? Or do I have a misunderstanding of the fundamental basics of mutexes and condition variables?

Answer 1

The simple answer is that you might want to wake more than one thread from the condition variables, but mutex allows only one thread execute the guarded block.