al_wait_cond - Allegro 5 API

       On  entering  this  function,  mutex  must be locked by the calling thread.  The function will atomically
       release mutex and block on cond.  The function will return when cond is “signalled”, acquiring  the  lock
       on the mutex in the process.

       Example of proper use:

              al_lock_mutex(mutex);
              while (something_not_true) {
                 al_wait_cond(cond, mutex);
              }
              do_something();
              al_unlock_mutex(mutex);

       The  mutex  should be locked before checking the condition, and should be rechecked al_wait_cond(3alleg5)
       returns.  al_wait_cond(3alleg5) can return for other reasons than the condition becoming  true  (e.g. the
       process  was signalled).  If multiple threads are blocked on the condition variable, the condition may no
       longer be true by the time the second and later threads are unblocked.  Remember not to unlock the  mutex
       prematurely.

       al_wait_cond - Allegro 5 API

al_wait_cond_until(3alleg5), al_broadcast_cond(3alleg5), al_signal_cond(3alleg5).

Allegro reference manual                                                                   al_wait_cond(3alleg5)

              #include <allegro5/allegro.h>

              void al_wait_cond(ALLEGRO_COND *cond, ALLEGRO_MUTEX *mutex)