al_set_shader_sampler - Allegro 5 API

       Sets  a texture sampler uniform and texture unit of the current target bitmap’s shader.  The given bitmap
       must be a video bitmap.

       Different samplers should use different units.  The bitmap passed to Allegro’s drawing functions uses the
       0th unit, so if you’re planning on using the al_tex variable in your pixel  shader  as  well  as  another
       sampler, set the other sampler to use a unit different from 0.  With the primitives addon, it is possible
       to  free  up  the 0th unit by passing NULL as the texture argument to the relevant drawing functions.  In
       this case, you may set a sampler to use the 0th unit and thus not use  al_tex  (the  al_use_tex  variable
       will be set to false).

       Returns true on success.  Otherwise returns false, e.g. if the uniform by that name does not exist in the
       shader.

       al_set_shader_sampler - Allegro 5 API

al_use_shader(3alleg5)

Allegro reference manual                                                          al_set_shader_sampler(3alleg5)

       5.1.0

              #include <allegro5/allegro.h>

              bool al_set_shader_sampler(const char *name,
                 ALLEGRO_BITMAP *bitmap, int unit)