Why is a while loop needed around pthread wait conditions?

Question

I\'m learning pthread and wait conditions. As far as I can tell a typical waiting thread is like this:

pthread_mutex_lock(&m);
while(!condition)
     pth         


        

Answer 1

The real reason you should put pthread_cond_wait in a while loop is not because of spurious wakeup. Even if your condition variable did not have spurious wakeup, you would still need the loop to catch a common type of error. Why? Consider what can happen if multiple threads wait on the same condition:

Thread 1                         Thread 2           Thread 3
check condition (fails)
(in cond_wait) unlock mutex
(in cond_wait) wait
                                 lock mutex
                                 set condition
                                 signal condvar
                                 unlock mutex
                                                    lock mutex
                                                    check condition (succeeds)
                                                    do stuff
                                                    unset condition
                                                    unlock mutex
(in cond_wait) wake up
(in cond_wait) lock mutex
<thread is awake, but condition
is unset>

The problem here is that the thread must release the mutex before waiting, potentially allowing another thread to 'steal' whatever that thread was waiting for. Unless it is guaranteed that only one thread can wait on that condition, it is incorrect to assume that the condition is valid when a thread wakes up.

Answer 2

Suppose you don't check the condition. Then usually you can't avoid the following bad thing happening (at least, you can't avoid it in one line of code):

 Sender                             Receiver
locks mutex
sets condition
signals condvar, but nothing 
  is waiting so has no effect
releases mutex
                                    locks mutex
                                    waits. Forever.

Of course your second code example could avoid this by doing:

pthread_mutex_lock(&m);
if (!condition) pthread_cond_wait(&cond, &m);
// Thread stuff here
pthread_mutex_unlock(&m);

Then it would certainly be the case that if there is only ever at most one receiver, and if cond_signal were the only thing that could wake it up, then it would only ever wake up when the condition was set and hence would not need a loop. nos covers why the second "if" isn't true.