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How to handle endianness and binary string conversion for 32 bits integers (Int32)?
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Date: 2005-06-17 (18:10)
From: David MENTRE <dmentre@l...>
Subject: Re: [Caml-list] How to handle endianness and binary string conversion for 32 bits integers (Int32)?

"Nicolas Cannasse" <warplayer@free.fr> writes:

> Extlib IO module have some code about that,
> See http://ocaml-lib.sourceforge.net/doc/IO.html 

I've look at this code but, from a quick look, it seems to handle only
OCaml int.

For what is worth, here is my own weel (here under public domain

open Int32

\section{32 bits integer to/from string conversion}

Function [[be_of_int32]] converts an [[Int32]] into its big
endian binary representation.

let be_of_int32 n =
  let byte_mask = of_int 0xff in
  let char_of_int32 x = Char.chr (to_int x) in
  let d0 = char_of_int32 (logand n byte_mask) in
  let d1 = char_of_int32 (logand (shift_right_logical n 8) byte_mask) in
  let d2 = char_of_int32 (logand (shift_right_logical n 16) byte_mask) in
  let d3 = char_of_int32 (logand (shift_right_logical n 24) byte_mask) in
  let big_endian = String.make 4 d3 in
  big_endian.[1] <- d2;
  big_endian.[2] <- d1;
  big_endian.[3] <- d0;

Function [[int32_of_be]] converts an big endian binary
representation of a 32 bits integer into an [[Int32]].

let int32_of_be be =
  if String.length be <> 4 then 
   raise (Invalid_argument "int32_from_big_endian");
  let d3 = of_int (Char.code be.[3])
  and d2 = of_int (Char.code be.[2]) 
  and d1 = of_int (Char.code be.[1]) 
  and d0 = of_int (Char.code be.[0]) in
  (logor (shift_left d0 24) 
     (logor (shift_left d1 16) 
        (logor (shift_left d2 8) d3)))

\section{Automatic tests}

let _ =
  if Config.do_autotests then begin
    Printf.printf "  timestamp autotests...";
    assert(int32_of_be "\001\002\003\004" = of_string "0x01020304");
    assert(be_of_int32 (of_string "0x01020304") = "\001\002\003\004");
    assert(int32_of_be "\255\254\253\252" = of_string "0xfffefdfc");
    assert(be_of_int32 (of_string "0xfffefdfc") = "\255\254\253\252");
    Printf.printf "done\n"

pub  1024D/A3AD7A2A 2004-10-03 David MENTRE <dmentre@linux-france.org>
 5996 CC46 4612 9CA4 3562  D7AC 6C67 9E96 A3AD 7A2A