Portably know the name of the filesystem root directory

[Florian Kaufmann]

I am programming an emacs lisp function which searches a file from a
specific point in the directory tree towards the root. But how can I
portably dedect when root is reached? The name of the root is "/" on
unixes, is probably something like "[A-Z]:" (a regex) on DOS, and
probably even something else on an apple OS. This is what I have so
far, but I fear it only works on Unixes.

(defun mks-file-to-sandbox(file)
  "Returns the sandbox file (.pj) the given FILE is a member of."
  (let* ((dir (file-name-directory file))
         sandboxes)
    (while (not (setq sandboxes (directory-files dir t ".*\\.pj$" t)))
      (when (string= dir "/")
        (error "no sandbox found"))
      (setq dir (file-name-directory dir)))
    (when (< (length sandboxes) 1)
      (message "found multiple sandboxes, choosing the first: %S"
sandboxes))
    (car sandboxes)))