NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/std/c/46

Internet Message Format | 1996-08-06 | 1KB

Path: teal.csn.net!not-for-mail
From: [e-mail] (Thad Smith)
Newsgroups: comp.std.c
Subject: Re: Undefined result vs. int's holding undefined values.
Date: 7 Jan 1996 15:41:47 -0700
Organization: T3 Systems
Message-ID: <[e-mail]>
References: <4ck70b$[e-mail]> <4ckms5$[e-mail]>
 <4cmg0s$[e-mail]>
Reply-To: [e-mail]
NNTP-Posting-Host: 199.117.27.22

In article <4cmg0s$[e-mail]>,
[e-mail] (Paul Eggert) wrote:
>This reminds me of a similar bug I found a long time ago when porting
>the Modula-3 runtime, which contained code that acted something like this:
>
>    int sum_overflow (int x, int y) {
>        return (x + y < x) != (y < 0);
>    }
>
>The C Standard does not guarantee that the above function works,
>since integer overflow leads to undefined behavior,
>but when I found that the function did not work with whatever old version
>of GCC I was using at the time, I reported it as a bug to the GCC maintainers
>and got a fix from them in a few days.
>
>Regardless of what the C Standard says, it should be obvious that it's
>crucial to have integer overflow checking working properly in an
>application that needs it.

I agree, but it is possible to rewrite the function so that it doesn't
invoke undefined behavior:

  #include <limits.h>
  int sum_overflow (int x, int y) {
      return x > 0? (y > INT_MAX - x) : (y < INT_MIN - x);
  }

Thad