Question

The typical pattern I’ve seen for using pthread_cond_wait is:

pthread_mutex_lock(&lock);
  while (!test)
    pthread_cond_wait(&condition, &lock);
pthread_mutex_unlock(&lock);

Why can’t an if statement be used instead of a while loop.

pthread_mutex_lock(&lock);
  if (!test)
    pthread_cond_wait(&condition, &lock);
pthread_mutex_unlock(&lock);

Answer 1

http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_cond_timedwait.html

When using condition variables there is always a Boolean predicate involving shared variables associated with each condition wait that is true if the thread should proceed. Spurious wakeups from the pthread_cond_timedwait() or pthread_cond_wait() functions may occur. Since the return from pthread_cond_timedwait() or pthread_cond_wait() does not imply anything about the value of this predicate, the predicate should be re-evaluated upon such return.

The while loop is a very standard way of re-evaluating the predicate, as required by POSIX.