al_use_shader - Allegro 5 API

       Uses  the shader for subsequent drawing operations on the current target bitmap.  Pass NULL to stop using
       any shader on the current target bitmap.

       Returns true on success.  Otherwise returns false, e.g. because  the  shader  is  incompatible  with  the
       target bitmap.

       al_use_shader - Allegro 5 API

al_destroy_shader(3alleg5),         al_set_shader_sampler(3alleg5),        al_set_shader_matrix(3alleg5),
       al_set_shader_int(3alleg5),          al_set_shader_float(3alleg5),           al_set_shader_bool(3alleg5),
       al_set_shader_int_vector(3alleg5), al_set_shader_float_vector(3alleg5), al_get_current_shader(3alleg5)

Allegro reference manual                                                                  al_use_shader(3alleg5)

       5.1.6

              #include <allegro5/allegro.h>

              bool al_use_shader(ALLEGRO_SHADER *shader)