al_rebase_path man page
al_rebase_path — Allegro 5 API
#include <allegro5/allegro.h> bool al_rebase_path(const ALLEGRO_PATH *head, ALLEGRO_PATH *tail)
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".