Waiting On A Condition Variable

To wait on a condition variable, we use a combination of a mutex and a condition variable, and the "pthread_cond_wait" or 
"pthread_cond_timedwait" function. Lets see the waiting code, and then try to figure out what the heck this mutex is doing here.

 /* assume 'mutex' is an initialized mutes.         */
 /* assume 'cond' is an initialized cond. variable. */
 /* must lock the mutex first. */
 if (pthread_mutex_lock(&mutex) != 0) {
  /* handle error... */
 }
 /* now wait on the condition */
 /* mutex will be unlocked while we wait. */
 if (pthread_cond_wait(&cond, &mutex) == 0) {
   /* condition was signaled. mutex is locked again. */
   /* do your stuff... */
 }
 /* finally, unlock the mutex. */
 if (pthread_mutex_unlock(&mutex) != 0) {
   /* handle error... */
 }