al_set_new_display_refresh_rate - Allegro 5 API

       Sets  the  refresh  rate to use when creating new displays on the calling thread.  If the refresh rate is
       not available, al_create_display(3alleg5) will fail.  A list of modes with refresh  rates  can  be  found
       with al_get_num_display_modes(3alleg5) and al_get_display_mode(3alleg5).

       The default setting is zero (don’t care).

       al_set_new_display_refresh_rate - Allegro 5 API

al_get_new_display_refresh_rate(3alleg5)

Allegro reference manual                                                al_set_new_display_refresh_rate(3alleg5)

              #include <allegro5/allegro.h>

              void al_set_new_display_refresh_rate(int refresh_rate)