Sharing a global variable between C and OCaml

I was asked today if it is possible to share a global variable between C and OCaml. This was my first response:

OCaml cannot just directly access C globals. At best you'd need to have a function that returns the address of the C global, _and_ the C global would need to be in a form that OCaml code could understand although that is pretty easy to arrange.

In a followup, I gave some example code:

------------------------------------------------------------ test_c.c
/* Variable shared between C and OCaml. */

#include <caml/mlvalues.h>
                                                                                
/* On the OCaml side, this will be made to look like a structure
 * containing a single int (well, provided you don't look *too*
 * closely at it). 
 */
value shared_var = Val_int (0); 

value
get_struct_addr (value unitv)
{
  /* Return the address of the 'structure'. */
  return ((value) &shared_var);
}

/* Increment the shared variable. */
value
increment_it (value unitv)
{
  int i = Int_val (shared_var);
  i++;
  shared_var = Val_int (i);
  return (Val_unit);
}
----------------------------------------------------------------------

------------------------------------------------------------ test.ml
(* Variable shared between C and OCaml. *)

type var = {
  shared_var : int;
}

external get_struct_addr : unit -> var = "get_struct_addr" "noalloc"
external increment_it : unit -> unit = "increment_it" "noalloc"

let var = get_struct_addr () ;;

while true do
  Printf.printf "value of the variable now is %d\n%!" var.shared_var;
  increment_it ();
  (* OCaml isn't expecting that increment_it modifies a variable, so
   * there is no guarantee that we will see the changed value next
   * time around.
   *)
  Unix.sleep 1;
done
----------------------------------------------------------------------

$ gcc -I /usr/lib/ocaml -c test_c.c
$ ocamlopt -c test.ml
$ ocamlopt unix.cmxa test_c.o test.cmx -o test
$ ./test
value of the variable now is 0
value of the variable now is 1
value of the variable now is 2
value of the variable now is 3
value of the variable now is 4
[etc.]

Before you use this code, be aware that it's a real hack.