al_rebase_path - Allegro 5 API

       Concatenate  two path structures, modifying the second path structure.  If tail is an absolute path, this
       function does nothing.  Otherwise, the drive and path components in head are inserted  at  the  start  of
       tail.

       For  example,  if  head  is  “/anchor/”  and  tail  is  “data/file.ext”, then after the call tail becomes
       “/anchor/data/file.ext”.

       al_rebase_path - Allegro 5 API

al_join_paths(3alleg5)

Allegro reference manual                                                                 al_rebase_path(3alleg5)

              #include <allegro5/allegro.h>

              bool al_rebase_path(const ALLEGRO_PATH *head, ALLEGRO_PATH *tail)