al_get_display_mode - Allegro 5 API

       Retrieves   a   fullscreen   mode.    Display  parameters  should  not  be  changed  between  a  call  of
       al_get_num_display_modes(3alleg5) and al_get_display_mode(3alleg5).  index must  be  between  0  and  the
       number  returned  from  al_get_num_display_modes-1.   mode  must  be  an  allocated  ALLEGRO_DISPLAY_MODE
       structure.  This function will return NULL on failure, and the mode  parameter  that  was  passed  in  on
       success.

       al_get_display_mode - Allegro 5 API

ALLEGRO_DISPLAY_MODE(3alleg5), al_get_num_display_modes(3alleg5)

Allegro reference manual                                                            al_get_display_mode(3alleg5)

              #include <allegro5/allegro.h>

              ALLEGRO_DISPLAY_MODE *al_get_display_mode(int index, ALLEGRO_DISPLAY_MODE *mode)