al_get_path_drive man page
al_get_path_drive — Allegro 5 API
#include <allegro5/allegro.h> const char *al_get_path_drive(const ALLEGRO_PATH *path)
Return the drive letter on a path, or the empty string if there is none.
The "drive letter" is only used on Windows, and is usually a string like "c:", but may be something like "\\Computer Name" in the case of UNC (Uniform Naming Convention) syntax.