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Re: check integer overflow with GCC 4.5.2

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sailer <xiangdong.ji@gmail.com> writes:
[...]
> I rewrite the c code to the follows and checking overflow works with
> GCC 4.5.2 when enabling -O3,
> is it safe enough? thanks.

No.

> #include <stdio.h>
> #include <limits.h>
>
> void
> ovf(int a, int b, int *v, unsigned int *r)
> {
>         *r = a * b;

If the mathematical result of multiplying a and b is outside
the range INT_MIN .. INT_MAX, then the behavior of the above is
undefined.  Nothing you do *after* performing the multiplication can
change that.  For example, a conforming implementation could cause
your program to terminate as soon as the multiplication is attempted.

>         if (b == 0 || (a = *r/b && *r <= INT_MAX))
>                 *v = 0;
>         else
>                 *v = 1;
> }
>
> int main ()
> {
>         int words;
>         int bytes;
>         int overflow;
>
>         ovf(words, sizeof(int), &bytes, &overflow);

And you're passing an uninitialized value to ovf().

And I *think* you've got the 3rd and 4th arguments reversed.
It's hard to tell because you called the parameters "v" and "r"
rather than something legible.

>         if (overflow)
>                 printf("overflow\n");
>
>         return 0;
> }

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Thread

check integer overflow with GCC 4.5.2 sailer <xiangdong.ji@gmail.com> - 2011-11-23 20:25 -0800
  Re: check integer overflow with GCC 4.5.2 pete <pfiland@mindspring.com> - 2011-11-24 00:47 -0500
    Re: check integer overflow with GCC 4.5.2 sailer <xiangdong.ji@gmail.com> - 2011-11-23 22:29 -0800
      Re: check integer overflow with GCC 4.5.2 "Philip Lantz" <prl@canterey.us> - 2011-11-24 01:23 -0800
        Re: check integer overflow with GCC 4.5.2 sailer <xiangdong.ji@gmail.com> - 2011-11-27 23:14 -0800
          Re: check integer overflow with GCC 4.5.2 James Kuyper <jameskuyper@verizon.net> - 2011-11-28 07:04 -0500
          Re: check integer overflow with GCC 4.5.2 Noob <root@127.0.0.1> - 2011-11-29 14:12 +0100
            Re: check integer overflow with GCC 4.5.2 Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-11-29 16:16 +0200
            Re: check integer overflow with GCC 4.5.2 88888 Dihedral <dihedral88888@googlemail.com> - 2011-11-29 06:28 -0800
  Re: check integer overflow with GCC 4.5.2 Robert Wessel <robertwessel2@yahoo.com> - 2011-11-24 00:28 -0600
  Re: check integer overflow with GCC 4.5.2 Vincenzo Mercuri <vincenzo.mercuri@yahoo.it> - 2011-11-24 09:46 +0100
    Re: check integer overflow with GCC 4.5.2 sailer <xiangdong.ji@gmail.com> - 2011-11-24 01:08 -0800
      Re: check integer overflow with GCC 4.5.2 "Philip Lantz" <prl@canterey.us> - 2011-11-24 01:26 -0800
        Re: check integer overflow with GCC 4.5.2 sailer <xiangdong.ji@gmail.com> - 2011-11-27 23:09 -0800
    Re: check integer overflow with GCC 4.5.2 "io_x" <a@b.c.invalid> - 2011-11-24 14:19 +0100
  Re: check integer overflow with GCC 4.5.2 gwowen <gwowen@gmail.com> - 2011-11-24 09:10 -0800
  Re: check integer overflow with GCC 4.5.2 sailer <xiangdong.ji@gmail.com> - 2011-11-27 23:21 -0800
    Re: check integer overflow with GCC 4.5.2 Keith Thompson <kst-u@mib.org> - 2011-11-28 16:31 -0800
      Re: check integer overflow with GCC 4.5.2 Ian Collins <ian-news@hotmail.com> - 2011-11-29 13:53 +1300
  Re: check integer overflow with GCC 4.5.2 sailer <xiangdong.ji@gmail.com> - 2011-11-27 23:34 -0800
    Re: check integer overflow with GCC 4.5.2 pete <pfiland@mindspring.com> - 2011-11-28 18:59 -0500
      Re: check integer overflow with GCC 4.5.2 sailer <xiangdong.ji@gmail.com> - 2011-11-29 19:44 -0800
        Re: check integer overflow with GCC 4.5.2 James Kuyper <jameskuyper@verizon.net> - 2011-11-30 11:08 -0500
          Re: check integer overflow with GCC 4.5.2 Keith Thompson <kst-u@mib.org> - 2011-11-30 09:01 -0800
  Re: check integer overflow with GCC 4.5.2 sailer <xiangdong.ji@gmail.com> - 2011-11-29 19:46 -0800

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