> I am writing a program that encodes data in 32 bit ints.  Ideally I would
> like to have 32 bit unsigned ints, but OCaml only has 31 bit signed ints.

It also has 32 bits signed ints (and 64 bits).

> I have a large binary file with the data that I will mmap with the
> Bigarray.Array1.map_file function.
>
> When I test the following code I am puzzled by the fact that Ocaml doesn't
> give me back the int in the same format as I wrote it.
>
> Why doesn't OCaml read back data in the same format?

because `output_binary_int' uses a big-endian encoding (cf. the reference
manual) whereas Bigarray.map_file uses the native endianess (probably
little-endian in your case).

Don't use different methods to write your data and read it back !

-- 
  Olivier