al_signal_cond - Allegro 5 API

       Unblock at least one thread waiting on a condition variable.

       Generally  you  should  use  al_broadcast_cond(3alleg5) but al_signal_cond(3alleg5) may be more efficient
       when it’s applicable.

       al_signal_cond - Allegro 5 API

al_broadcast_cond(3alleg5).

Allegro reference manual                                                                 al_signal_cond(3alleg5)

              #include <allegro5/allegro.h>

              void al_signal_cond(ALLEGRO_COND *cond)