obj_hackery.ml

let _ =
  (*
   * Construct a string comprising two words (and a header, which
   * we shall ignore). The bytes are laid out like this:
   *
   *   char:   a  b  c  d   \0 \0 \0 03
   *   hex:   61 62 63 64 . 00 00 00 03
   *)
  let str = "abcd" in

  (*
   * Now we mutate the last word of the string, adding a character.
   *
   * Not all bit sequences can be achieved using an Ocaml int,
   * since they are represented by 2n+1. Instead, we get the bits
   * from an int32.
   *
   * We want to lengthen the string by 1, so we change the 03 to
   * an 02. We also set the first byte to our desired character value
   * ('e' : hex 65). Other bytes are zeroes.
   *
   * Since x86 is a wrong-endian machine, the first byte in a
   * word is the low bits: thus the int32 literal is written in
   * reverse.
   *
   * Ta da, we've extended a string.
   *)
  Obj.set_field (Obj.repr str) 1
    (Obj.field (Obj.repr 0x02_00_00_65l) 1);
  Gc.minor ();
  str;;