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Re: Signed mod unsigned

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On 06/06/2012 09:55 AM, 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?

In most binary expressions (modulus expressions are no exception -
6.5.5p3), the operands are subject to what is called the usual
arithmetic conversions (6.3.1.8) before the operation is performed.
Those conversions include the integer promotions, which leave both
operands unchanged in this case. The relevant rule is that when one
promoted type is unsigned, and the other is signed, if the unsigned type
has an integer conversion rank greater than or equal to that of the
signed type, then the operand of the signed type is converted to the
unsigned type. "unsigned int" is required to have the same integer
conversion rank as "int" (6.3.1.1p1);

So the behavior you describe is not only allowed, but required, by the C
standard. There's some minor details about your description that should
be modified: this is not an example of what the C standard calls a
promotion, and it has nothing to do with 2's complement. The C standard
also allows 1's complement and sign-magnitude representations
(6.2.6.2p2), and the conversion from a signed value to an unsigned type
follows the same rules regardless of the representation used for the
signed type. A value 1 greater than the maximum value representable in
the unsigned type is either repeatedly added to, or repeatedly subtract
from, the value until the result is representable in the unsigned type
(6.3.1.3p2).

The machine instructions needed to obtain this result can be quite
different, depending upon which representation is used for the signed
type. However, the result of the conversion should be exactly the same
for all three representations, it depends only upon the initial value
represented and maximum representable value of the unsigned type.

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Thread

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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