al_get_default_shader_source - Allegro 5 API

       Returns  a  string containing the source code to Allegro’s default vertex or pixel shader appropriate for
       the passed platform.  The ALLEGRO_SHADER_AUTO value means GLSL is used if OpenGL is being used  otherwise
       HLSL.   ALLEGRO_SHADER_AUTO  requires  that  there  is a current display set on the calling thread.  This
       function can return NULL if Allegro was built without support for shaders of the selected platform.

       al_get_default_shader_source - Allegro 5 API

al_attach_shader_source(3alleg5)

Allegro reference manual                                                   al_get_default_shader_source(3alleg5)

       5.1.6

              #include <allegro5/allegro.h>

              char const *al_get_default_shader_source(ALLEGRO_SHADER_PLATFORM platform,
                 ALLEGRO_SHADER_TYPE type)