al_clear_depth_buffer - Allegro 5 API

       Clear  the  depth buffer (confined by the clipping rectangle) to the given value.  A depth buffer is only
       available if it was requested with al_set_new_display_option(3alleg5) and the requirement could be met by
       the al_create_display(3alleg5) call creating the current display.  Operations involving the depth  buffer
       are also affected by al_set_render_state(3alleg5).

       For  example,  if  ALLEGRO_DEPTH_FUNCTION  is  set  to  ALLEGRO_RENDER_LESS  then depth buffer value of 1
       represents infinite distance, and thus is a good value to use when clearing the depth buffer.

       al_clear_depth_buffer - Allegro 5 API

al_clear_to_color(3alleg5),       al_set_clipping_rectangle(3alleg5),       al_set_render_state(3alleg5),
       al_set_new_display_option(3alleg5)

Allegro reference manual                                                          al_clear_depth_buffer(3alleg5)

       5.1.2

              #include <allegro5/allegro.h>

              void al_clear_depth_buffer(float z)