How to Use Condition Variables
               by Gene Cooperman (gene@ccs.neu.edu)
	       Copyright (c) 2021 -- all rights reserved

  Simple pattern for condition variables:
      A. Acquire resource
           pthread_mutex_lock(&guard_mutex);
           // Update some guard variables (some state variables)
           //   For example, update the number of threads that are waiting.
           while (safety_condition(guard_vars) == False) {
             pthread_cond_wait(&cond, &guard_mutex);
           }
           // Update some guard variables (some state variables)
           //   For example, update number of threads that are using resource
           //   and decrement the number of threads that are waiting.
           pthread_mutex_unlock(&guard_mutex);
      B. Use resource
           (If we acquired a read resource, we should be guaranteed
            that any writers are waiting.  If we acquired a write
            resource, we should be guaranteed that any readers _and_
            other writers are waiting.
      C. Release resource
           pthread_mutex_lock(&guard_mutex);
           // This can be replaced by pthread_cond_signal for optimizing
           //   the performance, but pthread_cond_broadcast()
           //   is always safe.
           pthread_cond_broadcast(&cond);
           // Update some guard variables (some state variables)
           //   For example, update number of threads that are using resource.
           pthread_mutex_unlock(&guard_mutex);

    COMMENT:  We can never deadlock when using guard variables in a
              critical section for a mutex.  So, the only danger is:
              a. deadlock: if we have bad logic in the safety_condition()
                   or in updating the guard/state variables:
                   For example, if a writer is waiting, but the only
                   readers in the system are also waiting.
              b. safety: if we have bad logic in the safety_condition()
                   or in updating the guard/state variables:
                   For example, if two writers are able to acquire the
                   write resource at the same time.

COMMENTS IN DEPTH:

  Variations that are more specialized -- for greater performance:
       [ Mostly in class, we care about correctness only.  The concepts
         here are only for greater performance.  Unless specified in
         an assignment, you don't need to use these optimizations. ]
       Declare two condition variables:  cond_readers and cond_writers
       Now, if you want to wake up the writers first (writer preference),
         just do:
            pthread_cond_broadcast(&cond_writers);
            pthread_cond_broadcast(&cond_readers);
         or maybe for even greater performance:
            // Add 'num_writers_waiting' to your state variables, and:
            if (num_writers_waiting > 0) {
              // We only need to wake up one writer right now.
              pthread_cond_signal(&cond_writers);
            } else {
              pthread_cond_broadcast(&cond_readers);
            }

  IMPORTANT TRAP:  A condition variable is not a semaphore!
      Don't try to use a condition variable where what you really want
      is a semaphore.  It's tempting to write:
        SEM_WAIT:
          pthread_mutex_lock(guard_vars);
          pthread_cond_wait(&cond, &guard_mutex);
          pthread_mutex_unlock(guard_vars);
        SEM_POST:
          pthread_mutex_lock(guard_vars);
          pthread_cond_signal(&cond);
          pthread_mutex_unlock(guard_vars);
      Either of two bad things can happen here.
        (a) spurious wakeup: the system sends a random signal (e.g., SIGWINCH)
              to the waiter.  The waiter wakes up without any post.
        (b) lost wakeup: the poster posts first.  Then the waiter begins
              to wait.  It never saw the post.

      Either directly use sem_wait() and sem_post() (preferred) or implement
      your own semaphore using condition variables:  (Yuck!)
        SEM_WAIT:
          pthread_mutex_lock(guard_vars);
          count--;
          while (count < 0) {
            pthread_cond_wait(&cond, &guard_mutex);
          }
          pthread_mutex_unlock(guard_vars);
        SEM_POST:
          pthread_mutex_lock(guard_vars);
            count++;
            pthread_cond_signal(&cond);
          pthread_mutex_unlock(guard_vars);