Obj.magic: not even once.

gistfile1.ml

(*
 * Uses Obj.magic at 'a -> int, once; the rest is safe.  When compiled with
 * ocamlopt, should fail with a segmentation fault or bus error at some point,
 * details depending on CPU.
 *)
open Printf;;
let () = 
  let x = [|0|] in
  let xi : int = Obj.magic x in
  printf "xi = %d\n%!" xi;
  printf "xi lor 0 = %d\n%!" (xi lor 0);
  printf "xi - (xi lor 0) = %d\n%!" (xi - (xi lor 0));
  (printf "[|0|].(xi - (xi lor 0)) = %!";
   printf "0x%x\n%!" ([|0|].(xi - (xi lor 0))));
  printf "(let a = [|0|] in a.(xi - (xi lor 0)) <- -1; Array.length a) = 0x%x\n%!"
    (let a = [|0|] in a.(xi - (xi lor 0)) <- -1; Array.length a);
  (printf "(let a = [|0|] in a.(xi - (xi lor 0)) <- -1; a.(pred (Array.length a))) = %!";
   printf "0x%x\n" (let a = [|0|] in a.(xi - (xi lor 0)) <- -1; a.(pred (Array.length a))))

Alternative last expression:

let a = [|0|] in a.(xi - (xi lor 0)) <- min_int; Gc.minor (); Array.length a