Original bug ID: 6010
Reporter: @glondu
Assigned to: @xavierleroy
Status: closed (set by @xavierleroy on 2015-12-11T18:18:51Z)
Resolution: fixed
Priority: normal
Severity: major
Version: 4.00.1
Category: otherlibs

Bug description

This bug has been originally reported in Debian by Drake Wilson at:

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=707127

Big_int.extract_big_int does not handle negative numbers properly.

This function seems to assume that unrepresented prefix regions of
bits in a Big_int.big_int are always zero, but this is not so for
negative numbers. The documentation explicitly states that a two's
complement representation is used for those, but see the transcript
with comments inline.

Steps to reproduce

% ocaml nums.cma
Objective Caml version 3.12.1

open Big_int;;

let mu = minus_big_int unit_big_int;;

val mu : Big_int.big_int =

let s offset count = string_of_big_int (extract_big_int mu offset count);;

val s : int -> int -> string =

s 0 16;;

• : string = "65535"

s 16 16;;

• : string = "65535"

s 32 16;;

• : string = "65535"

s 48 16;;

• : string = "65535"
  (* Okay so far... *)

s 64 16;;

• : string = "0"
  (* Oops! *)

s 56 16;;

• : string = "255"
  (* Function seems to think this is a 2^64-1. *)

eq_big_int (big_int_of_int 255) (extract_big_int mu 56 16);;

• : bool = true
  (* It's not just a problem with the stringifier. *)

eq_big_int (big_int_of_int (-1)) mu;;

• : bool = true
  (* Nor is it a problem with the earlier negation. *)

Additional information

Same behaviour with OCaml version 4.00.1.