al_wait_for_event - Allegro 5 API

       Wait  until  the  event  queue  specified is non-empty.  If ret_event is not NULL, the first event in the
       queue will be copied into ret_event and removed from the queue.  If ret_event is NULL the first event  is
       left at the head of the queue.

       al_wait_for_event - Allegro 5 API

ALLEGRO_EVENT(3alleg5),        al_wait_for_event_timed(3alleg5),        al_wait_for_event_until(3alleg5),
       al_get_next_event(3alleg5)

Allegro reference manual                                                              al_wait_for_event(3alleg5)

              #include <allegro5/allegro.h>

              void al_wait_for_event(ALLEGRO_EVENT_QUEUE *queue, ALLEGRO_EVENT *ret_event)