When to use pthread condition variables?

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Condition variables should be used as a place to wait and be notified. They are not the condition itself and they are not events. The condition is contained in the surrounding programming logic. The typical usage pattern of condition variables is

// safely examine the condition, prevent other threads from
// altering it
pthread_mutex_lock (&lock);
while ( SOME-CONDITION is false)
    pthread_cond_wait (&cond, &lock);

// Do whatever you need to do when condition becomes true
do_stuff();
pthread_mutex_unlock (&lock);

On the other hand, a thread, signaling the condition variable,
typically looks like

// ensure we have exclusive access to whathever comprises the condition
pthread_mutex_lock (&lock);

ALTER-CONDITION

// Wakeup at least one of the threads that are waiting on the condition (if any)
pthread_cond_signal (&cond);

// allow others to proceed
pthread_mutex_unlock (&lock)

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