al_get_new_display_adapter - Allegro 5 API

       Gets  the video adapter index where new displays will be created by the calling thread, if previously set
       with al_set_new_display_adapter(3alleg5).  Otherwise returns ALLEGRO_DEFAULT_DISPLAY_ADAPTER.

       al_get_new_display_adapter - Allegro 5 API

al_set_new_display_adapter(3alleg5)

Allegro reference manual                                                     al_get_new_display_adapter(3alleg5)

              #include <allegro5/allegro.h>

              int al_get_new_display_adapter(void)