(** [valid_pointer m b ofs] returns [true] if the address [b, ofs]
  is nonempty in [m] and [false] if it is empty. *)
Definition valid_pointer (m: mem) (b: block) (ofs: Z): bool :=
  perm_dec m b ofs Cur Nonempty.

Theorem valid_pointer_valid_access: forall m b ofs, valid_pointer m b ofs = true <-> valid_access m Mint8unsigned b ofs Nonempty.