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Re: check integer overflow with GCC 4.5.2

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On Thu, 24 Nov 2011 01:08:28, sailer wrote:

> On Nov 24, 4:46 pm, Vincenzo Mercuri wrote:
>> sailer ha scritto:> Hi,
>>
>> > I have the following code which was used to check whether  the result
>> > of (a * b)  is larger than the maximum integer value.
>>
>> > ovf(int a, int b, int *r, int *v)
>> > {
>> >          *r = a * b;
>> >          if (b == 0 || a == *r / b)
>> >                  *v = 0;
>> >          else
>> >                  *v = 1;
>> > }
>>
>> [...]
>>
>> Since you are concerned with "signed int"s you should check against
>> both INT_MAX and INT_MIN. I just wrote the following function  
>> "overflow()"
>> (sorry for the poor choice of its name), that should do the trick
>> (it takes the two factors and returns 1 if an overflow will occur, 0 if
>> not):
>>
>> #include <stdio.h>
>> #include <limits.h>
>>
>> int overflow(int a, int b)
>> {
>>    if( b > 0 ) {
>>      if( (a > 0 && b <= INT_MAX / a) ||
>>          (a <= 0 && a >= INT_MIN / b) )
>>        return 0;
>>      else return 1;
>>    } else { /* b <= 0 */
>>      if( (a > 0 && b >= INT_MIN / a) ||
>>          (a <= 0 && (b == 0 || a >= INT_MAX / b)) )
>>        return 0;
>>      else return 1;
>>    }
>>
>> }
>>
>> void result(int a, int b)
>> {
>>    printf("\na = %d, b = %d\n", a, b), overflow(a, b) ?
>>    printf("[overflow detected]\n") :
>>    printf("[result = %d]\n", a * b);
>>
>> }
>>
>> int main(void)
>> {
>>    printf("INT_MAX = %d, INT_MIN = %d\n", INT_MAX, INT_MIN);
>>    result(INT_MAX, INT_MAX);
>>    result(INT_MIN, INT_MIN);
>>    result(INT_MIN, INT_MAX);
>>    result(4000000, -80000);
>>    result(20000, 20000);
>>    result(0, 4);
>>    result(3, 0);
>>    result(0, 0);
>>    return 0;
>>
>> }
>
> Thanks all for your help, currently I'm using "__attribute__
> ((noinline))" to bypass the issue.

I doubt that this avoids the undefined behavior in ovf, so you still
may find that it does not work reliably.

Philip

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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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