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Groups > comp.lang.c > #21920

Re: Signed mod unsigned

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On 06/06/2012 06:15 PM, Edward Rutherford wrote:
> Johannes Bauer wrote:
> 
>> Hi group,
>>
>> today I ran into something I honestly did not expect. Consider this
>> snippet:
>>
>> #include <stdio.h>
>>
>> int main() {
>> 	int i1, i2;
>> 	unsigned int u2;
>>
>> 	i1 = -1;
>>
>> 	i2 = 30;
>> 	u2 = 30;
>>
>> 	printf("%d\n", i1 % i2);
>> 	printf("%d\n", i1 % u2);
>> 	return 0;
>> }
>>
>> I expected to find the output "-1 / -1". Instead the result I got was
>> "-1 / 15". Apparently when getting the remainder of a signed by a
>> unsigned int, the signed is silently promoted to unsigned in twos
>> complement and then the operation is performed, i.e.
>>
>> 0xffffffff % 30 == 15
>>
>> I would have expected at least a warning since I find this highly
>> unintuitive -- my expectation would be that the unsigned becomes a
>> (truncated) signed and a warning. Instead gcc does not emit a warning
>> (-Wall -Wextra) and silently performs above operation.
>>
>> Is this really in accordance with the C standard or is gcc doing
>> something weird here?
> 
> This looks like a compiler bug. I would report it to a Gcc mailing list.

I explained in my response why I believe that this behavior is not
merely allowed, but mandated, by the C standard. Do you disagree? If so,
on what grounds?

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Signed mod unsigned Johannes Bauer <dfnsonfsduifb@gmx.de> - 2012-06-06 15:55 +0200
  Re: Signed mod unsigned James Kuyper <jameskuyper@verizon.net> - 2012-06-06 10:41 -0400
    Re: Signed mod unsigned Johannes Bauer <dfnsonfsduifb@gmx.de> - 2012-06-07 12:12 +0200
      Re: Signed mod unsigned James Kuyper <jameskuyper@verizon.net> - 2012-06-07 06:44 -0400
        Re: Signed mod unsigned Johannes Bauer <dfnsonfsduifb@gmx.de> - 2012-06-07 13:31 +0200
  Re: Signed mod unsigned gwowen <gwowen@gmail.com> - 2012-06-06 07:53 -0700
    Re: Signed mod unsigned Johannes Bauer <dfnsonfsduifb@gmx.de> - 2012-06-07 12:15 +0200
      Re: Signed mod unsigned "BartC" <bc@freeuk.com> - 2012-06-07 12:29 +0100
        Re: Signed mod unsigned Johannes Bauer <dfnsonfsduifb@gmx.de> - 2012-06-07 13:43 +0200
        Re: Signed mod unsigned Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-06-07 07:46 -0400
          Re: Signed mod unsigned Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-06-07 20:58 +0100
            Re: Signed mod unsigned Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-06-07 19:58 -0400
            Re: Signed mod unsigned Tim Rentsch <txr@alumni.caltech.edu> - 2012-06-08 02:20 -0700
              Re: Signed mod unsigned Johannes Bauer <dfnsonfsduifb@gmx.de> - 2012-06-08 11:33 +0200
                Re: Signed mod unsigned Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-06-08 12:30 +0100
                Re: Signed mod unsigned Tim Rentsch <txr@alumni.caltech.edu> - 2012-06-08 11:12 -0700
                Re: Signed mod unsigned Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-06-08 20:37 +0100
                Re: Signed mod unsigned Keith Thompson <kst-u@mib.org> - 2012-06-08 13:44 -0700
                Re: Signed mod unsigned Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-06-08 22:22 +0100
                Re: Signed mod unsigned Johannes Bauer <dfnsonfsduifb@gmx.de> - 2012-06-10 10:56 +0200
                Re: Signed mod unsigned Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-06-10 14:40 +0100
                Re: Signed mod unsigned Keith Thompson <kst-u@mib.org> - 2012-06-11 15:49 -0700
                Re: Signed mod unsigned Tim Rentsch <txr@alumni.caltech.edu> - 2012-06-24 09:32 -0700
                Re: Signed mod unsigned Tim Rentsch <txr@alumni.caltech.edu> - 2012-06-08 11:05 -0700
              Re: Signed mod unsigned Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-06-08 12:06 +0100
                Re: Signed mod unsigned Tim Rentsch <txr@alumni.caltech.edu> - 2012-06-08 11:32 -0700
  Re: Signed mod unsigned Edward Rutherford <edward.p.rutherford79@REMOVETHIS.gmail.com> - 2012-06-06 22:15 +0000
    Re: Signed mod unsigned James Kuyper <jameskuyper@verizon.net> - 2012-06-06 18:29 -0400
      Re: Signed mod unsigned Edward Rutherford <edward.p.rutherford79@REMOVETHIS.gmail.com> - 2012-06-07 20:44 +0000
        Re: Signed mod unsigned James Kuyper <jameskuyper@verizon.net> - 2012-06-07 17:08 -0400

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