It seems to me that in the interface file unix.mli (from the directory
otherlibs/unix of the Objective Caml release 1.03), one line should be
amended. Instead of 

(*** Basic file input/output *)

type file_descr

there should be:

(*** Basic file input/output *)

type file_descr = int 

This allowed me to avoid some problems when using file descriptors 
(I recently signalled my problem on the mailing list).

File descriptors are integers indeed. On the other hand, there is 
a correct definition of this type (ie as above) in the main file unix.ml,
so everything should be ok (or I am wrong?). Unfortunately without that 
small amendment in the library files, I couldn't compile my program
without an error: "This expression has type int but is here used with 
type Unix.file_descr"

cheers,

Pawel