How To Use Condition Variable

Question

The Linux Programming Interface book has a piece of code (producer/consumer) to show how condition variable works:

static pthread_mutex_t mtx = PTHREAD_MUTEX_INITIALIZER;
static pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

static int avail = 0;

while (TRUE) {
    s = pthread_mutex_lock(&mtx);

    while (avail == 0) {   /* Wait for something to consume */
       s = pthread_cond_wait(&cond, &mtx);
    }

    while (avail > 0) {   /* Consume all available units */ 
        avail--;
    }

    s = pthread_mutex_unlock(&mtx);
}

Why we use pthread_mutex_lock in while? why we don't use it in an if?

Monocoque

Answer 1

Because pthread_cond_signal() is allowed to wake up more than one thread waiting on the condition variable. Therefore you must double-check the condition once you wake up, because some other thread might have woken up and changed it ahead of you.

If you know you have just one thread waiting, and you are sure nobody in the future will ever modify code elsewhere in the program to add another thread waiting, then you can use if. But you never know that for sure, so always use while.

[update]

As ninjalj points out in a comment, my answer is incomplete for failing to mention "spurious wakeups". For example, the POSIX standard makes it clear that if the waiting thread receives a signal (e.g. via kill()), pthread_cond_wait() can return 0 even if no other thread signaled the condition variable. The standard is ambiguous (in my view) as to whether the waiting thread can be woken up for no reason at all... But the bottom line is: Always use while, not if.

Sigil

Additionally, there can be spurious wakeups, so always use while. – Assuasive

What do you mean by: spurious wakeups – Monocoque

@Majid Azimi the condition variable can be signalled for no apparent reason, not just because someone called pthread_cond_signal/broadcast – Biodegradable

A minimal implementation could even implement pthread_cond_wait as return 0; and let the caller spin... ;-) Of course this would be extremely ugly. – Phocaea

I thought multiple threads being woken by pthread_cond_signal was the definition of "spurious wakeup". Can any of you cite chapter and verse of the POSIX spec that says pthread_cond_wait() can return even if no thread signals the condition variable? – Sigil

You might be right that that's what posix says. Though at least historically that was not the case in thread implementation(see e.g. here for more discussions ). Though - you wouldn't really care nor be able to determin if the spurious wakeup was due to someone calling pthread_cond_broadast or something else. – Biodegradable

One mistake in my "minimal implementation": it would at least need to unlock and relock the mutex to allow other threads the opportunity to obtain it. – Phocaea