Re: Is this code considered to transgress the signed overflow purism?

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.lang.c
Subject	Re: Is this code considered to transgress the signed overflow purism?
Date	2017-04-29 15:20 +0200
Organization	A noiseless patient Spider
Message-ID	<oe23oh$kaj$1@dont-email.me> (permalink)
References	(8 earlier) <w1JMA.24039$MR.21855@fx35.am4> <10c71913-2871-4474-b05c-804bfdb7074d@googlegroups.com> <XXZMA.508605$fL.438836@fx24.am4> <87a86z4gwa.fsf@bsb.me.uk> <M3%MA.48408$mr5.21062@fx36.am4>

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On 29/04/17 13:47, bartc wrote:
> On 29/04/2017 12:01, Ben Bacarisse wrote:
>> bartc <bc@freeuk.com> writes:
>> <snip>
>>> So (32-bit) unsigned A and B are both 1000000, A*B is 3,567,587,328
>>> rather than 1,000,000,000,000, but no problem because it's modular
>>> arithmetic, and its behaviour is well defined. Wrong, but at least
>>> it's not UB. It will be predictably wrong on all systems.
>>
>> Why is it wrong?  I like languages like Haskell where 10^6 * 10^6 ==
>> 10^12 but that comes at a price I doubt you'd consider acceptable for
>> C.  Slightly cheaper (though still unacceptable to most C programmers)
>> would be to insist that all such operations raise an exception.
>> The cheapest would be to have made unsigned overflow undefined too, but,
>> again, I doubt you'd want *more* UB in C.  The current definition of
>> unsigned arithmetic is cheap to implement, easy to understand and very
>> useful.  That's not what I'd call "wrong".
>>
>
> It's wrong because it gives the wrong answer, and is likely an error in
> the program if it was intended to be mathematically correct.

What would, in your opinion, be the "right" answer - given the 
constraint that the value has to be stored in a 32-bit integer (signed 
or unsigned - you pick) ?

Overflows are always going to be "wrong" in some way for some uses - 
there is no possibility of making them entirely correct.  You cannot 
store 10^12 in a 32-bit value.  So if you, like in Haskell or Python, 
extend the value to hold 10^12 for correct mathematics, you are now 
"wrong" regarding the size of the object and its efficiency in coding.

If you are designing a programming language, you make decisions about 
how these are handled - and different languages take different approaches.

In C, unsigned arithmetic is done as modulo - so the result of 
multiplying two 32-bit unsigned values 1000000 /is/ 3,567,587,328.  It 
is not wrong, it is correct - mathematically correct.  It is just a 
different mathematical operation that you might first expect.

For signed arithmetic, C takes the stance that the many different ways 
of handling it are wrong in too many cases, and inefficient to implement 
in at least some target architectures - so it makes it undefined behaviour.

>
> My point is that the emphasis seems to be on avoiding undefined
> behaviour rather than getting correct behaviour.
>
> I would call that erroneous result above 'overflow', but C suggests that
> that is not possible. The answer is still wrong however if A and B
> represent, say, width and height of some rectangle, and you want to work
> out its area.
>
> (Solutions needn't be expensive when a wider type is available, eg. 64
> bits when A and B are 32.)
>

Thread

Is this code considered to transgress the signed overflow purism? joel.rees@gmail.com - 2017-04-27 03:13 -0700
  Re: Is this code considered to transgress the signed overflow purism? joel.rees@gmail.com - 2017-04-27 03:41 -0700
  Re: Is this code considered to transgress the signed overflow purism? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-04-27 11:59 +0100
    Re: Is this code considered to transgress the signed overflow purism? joel.rees@gmail.com - 2017-04-27 14:35 -0700
      Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-04-27 15:13 -0700
        Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-28 00:58 +0200
          Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-04-27 16:44 -0700
            Re: Is this code considered to transgress the signed overflow purism? joel.rees@gmail.com - 2017-04-27 20:04 -0700
            Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-28 08:53 +0200
              Re: Is this code considered to transgress the signed overflow purism? supercat@casperkitty.com - 2017-04-28 08:23 -0700
                Re: Is this code considered to transgress the signed overflow purism? Manfred <noname@invalid.add> - 2017-04-28 18:05 +0200
                Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-04-28 09:22 -0700
                Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-04-28 12:24 -0700
                Re: Is this code considered to transgress the signed overflow purism? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-04-29 00:03 +0100
                Re: Is this code considered to transgress the signed overflow purism? supercat@casperkitty.com - 2017-04-28 16:37 -0700
                Re: Is this code considered to transgress the signed overflow purism? David Kleinecke <dkleinecke@gmail.com> - 2017-04-28 17:09 -0700
                Re: Is this code considered to transgress the signed overflow purism? supercat@casperkitty.com - 2017-04-28 17:34 -0700
                Re: Is this code considered to transgress the signed overflow purism? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-04-29 01:43 +0100
                Re: Is this code considered to transgress the signed overflow purism? David Kleinecke <dkleinecke@gmail.com> - 2017-04-28 21:21 -0700
                Re: Is this code considered to transgress the signed overflow purism? joel.rees@gmail.com - 2017-04-29 02:04 -0700
                Re: Is this code considered to transgress the signed overflow purism? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-04-29 11:46 +0100
                Re: Is this code considered to transgress the signed overflow purism? Robert Wessel <robertwessel2@yahoo.com> - 2017-04-30 12:05 -0500
                Re: Is this code considered to transgress the signed overflow purism? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-04-29 11:33 +0100
                Re: Is this code considered to transgress the signed overflow purism? David Kleinecke <dkleinecke@gmail.com> - 2017-04-29 08:26 -0700
                Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-29 17:58 +0200
                Re: Is this code considered to transgress the signed overflow purism? David Kleinecke <dkleinecke@gmail.com> - 2017-04-29 09:13 -0700
                Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-30 12:55 +0200
                Re: Is this code considered to transgress the signed overflow purism? David Kleinecke <dkleinecke@gmail.com> - 2017-04-30 09:17 -0700
                Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-30 20:35 +0200
                Re: Is this code considered to transgress the signed overflow purism? David Kleinecke <dkleinecke@gmail.com> - 2017-04-30 15:39 -0700
                Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-05-01 16:21 +0200
                Re: Is this code considered to transgress the signed overflow purism? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2017-05-02 06:35 -0700
                Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-05-02 16:29 +0200
                Re: Is this code considered to transgress the signed overflow purism? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-04-29 20:58 +0100
                Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-28 18:43 +0200
            Re: Is this code considered to transgress the signed overflow purism? bartc <bc@freeuk.com> - 2017-04-28 14:09 +0100
              Re: Is this code considered to transgress the signed overflow purism? scott@slp53.sl.home (Scott Lurndal) - 2017-04-28 14:23 +0000
                Re: Is this code considered to transgress the signed overflow purism? bartc <bc@freeuk.com> - 2017-04-28 16:17 +0100
                Re: Is this code considered to transgress the signed overflow purism? scott@slp53.sl.home (Scott Lurndal) - 2017-04-28 15:55 +0000
                Re: Is this code considered to transgress the signed overflow purism? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2017-04-28 19:11 -0700
                Re: Is this code considered to transgress the signed overflow purism? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-04-29 11:23 +0100
                Re: Is this code considered to transgress the signed overflow purism? bartc <bc@freeuk.com> - 2017-04-29 11:31 +0100
                Re: Is this code considered to transgress the signed overflow purism? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-04-29 12:01 +0100
                Re: Is this code considered to transgress the signed overflow purism? bartc <bc@freeuk.com> - 2017-04-29 12:47 +0100
                Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-29 15:20 +0200
                Re: Is this code considered to transgress the signed overflow purism? bartc <bc@freeuk.com> - 2017-04-29 15:22 +0100
                Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-29 16:52 +0200
                Re: Is this code considered to transgress the signed overflow purism? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-04-29 15:18 +0100
                Re: Is this code considered to transgress the signed overflow purism? Jean-Marc Bourguet <jm@bourguet.org> - 2017-04-29 14:25 +0200
                Re: Is this code considered to transgress the signed overflow purism? Jerry Stuckle <jstucklex@attglobal.net> - 2017-04-29 08:05 -0400
                Re: Is this code considered to transgress the signed overflow purism? Gareth Owen <gwowen@gmail.com> - 2017-04-29 13:34 +0100
                Re: Is this code considered to transgress the signed overflow purism? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2017-04-29 05:55 -0700
                Re: Is this code considered to transgress the signed overflow purism? GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-04-29 15:11 +0200
                Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-29 15:24 +0200
                Re: Is this code considered to transgress the signed overflow purism? Gareth Owen <gwowen@gmail.com> - 2017-04-29 14:57 +0100
                Re: Is this code considered to transgress the signed overflow purism? asetofsymbols@gmail.com - 2017-05-03 12:17 -0700
                Re: Is this code considered to transgress the signed overflow purism? supercat@casperkitty.com - 2017-05-03 16:36 -0700
                Re: Is this code considered to transgress the signed overflow purism? asetofsymbols@gmail.com - 2017-05-03 11:26 -0700
                Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-05-03 21:40 +0200
                Re: Is this code considered to transgress the signed overflow purism? asetofsymbols@gmail.com - 2017-05-03 23:46 -0700
              Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-04-28 08:55 -0700
        Re: Is this code considered to transgress the signed overflow purism? joel.rees@gmail.com - 2017-04-27 20:20 -0700
          Re: Is this code considered to transgress the signed overflow purism? joel.rees@gmail.com - 2017-04-27 20:26 -0700
          Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-28 10:19 +0200
            Re: Is this code considered to transgress the signed overflow purism? James Kuyper <jameskuyper@verizon.net> - 2017-04-28 09:57 -0400
              Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-28 17:06 +0200
            Re: Is this code considered to transgress the signed overflow purism? supercat@casperkitty.com - 2017-04-28 08:38 -0700
              Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-28 18:51 +0200
            Re: Is this code considered to transgress the signed overflow purism? supercat@casperkitty.com - 2017-04-29 12:43 -0700
              Re: Is this code considered to transgress the signed overflow purism? Tim Rentsch <txr@alumni.caltech.edu> - 2017-04-30 14:02 -0700
                Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-04-30 14:25 -0700
                Re: Is this code considered to transgress the signed overflow purism? Tim Rentsch <txr@alumni.caltech.edu> - 2017-04-30 15:01 -0700
          Re: Is this code considered to transgress the signed overflow purism? James Kuyper <jameskuyper@verizon.net> - 2017-04-28 09:48 -0400
          Re: Is this code considered to transgress the signed overflow purism? supercat@casperkitty.com - 2017-04-28 08:07 -0700
            Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-28 18:56 +0200
              Re: Is this code considered to transgress the signed overflow purism? supercat@casperkitty.com - 2017-04-28 11:30 -0700
          Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-04-28 08:35 -0700
      Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-28 00:54 +0200
        Re: Is this code considered to transgress the signed overflow purism? joel.rees@gmail.com - 2017-04-27 21:05 -0700
          Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-28 10:58 +0200
            Re: Is this code considered to transgress the signed overflow purism? bartc <bc@freeuk.com> - 2017-04-28 10:50 +0100
          Re: Is this code considered to transgress the signed overflow purism? James Kuyper <jameskuyper@verizon.net> - 2017-04-28 10:01 -0400
            Re: Is this code considered to transgress the signed overflow purism? Tim Rentsch <txr@alumni.caltech.edu> - 2017-05-02 05:06 -0700
      Re: Is this code considered to transgress the signed overflow purism? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-04-28 23:57 +0100
      Re: Is this code considered to transgress the signed overflow purism? Tim Rentsch <txr@alumni.caltech.edu> - 2017-04-30 12:28 -0700
        Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-04-30 12:41 -0700
          Re: Is this code considered to transgress the signed overflow purism? bartc <bc@freeuk.com> - 2017-04-30 21:46 +0100
            Re: Is this code considered to transgress the signed overflow purism? Thiago Adams <thiago.adams@gmail.com> - 2017-05-01 05:41 -0700
              Re: Is this code considered to transgress the signed overflow purism? bartc <bc@freeuk.com> - 2017-05-01 13:57 +0100
            Re: Is this code considered to transgress the signed overflow purism? Tim Rentsch <txr@alumni.caltech.edu> - 2017-05-02 05:30 -0700
              Re: Is this code considered to transgress the signed overflow purism? bartc <bc@freeuk.com> - 2017-05-02 16:13 +0100
                Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-05-02 09:12 -0700
                Re: Is this code considered to transgress the signed overflow purism? bartc <bc@freeuk.com> - 2017-05-02 18:15 +0100
                Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-05-02 11:26 -0700
                Re: Is this code considered to transgress the signed overflow purism? bartc <bc@freeuk.com> - 2017-05-02 21:39 +0100
                Re: Is this code considered to transgress the signed overflow purism? supercat@casperkitty.com - 2017-05-02 13:53 -0700
                Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-05-02 14:10 -0700
                Re: Is this code considered to transgress the signed overflow purism? bartc <bc@freeuk.com> - 2017-05-02 22:36 +0100
                Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-05-02 15:27 -0700
                Re: Is this code considered to transgress the signed overflow purism? Robert Wessel <robertwessel2@yahoo.com> - 2017-05-03 00:45 -0500
                Re: Is this code considered to transgress the signed overflow purism? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-05-03 11:57 +0100
                Re: Is this code considered to transgress the signed overflow purism? Robert Wessel <robertwessel2@yahoo.com> - 2017-05-03 13:34 -0500
                Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-05-03 12:05 -0700
                Re: Is this code considered to transgress the signed overflow purism? Tim Rentsch <txr@alumni.caltech.edu> - 2017-05-05 02:58 -0700
                Re: Is this code considered to transgress the signed overflow purism? Thiago Adams <thiago.adams@gmail.com> - 2017-05-03 05:26 -0700
                Re: Is this code considered to transgress the signed overflow purism? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2017-05-03 05:54 -0700
                Re: Is this code considered to transgress the signed overflow purism? Thiago Adams <thiago.adams@gmail.com> - 2017-05-03 06:49 -0700
                Re: Is this code considered to transgress the signed overflow purism? Thiago Adams <thiago.adams@gmail.com> - 2017-05-03 06:51 -0700
                Re: Is this code considered to transgress the signed overflow purism? jadill33@gmail.com - 2017-05-03 09:03 -0700
                Re: Is this code considered to transgress the signed overflow purism? supercat@casperkitty.com - 2017-05-02 14:47 -0700
                Re: Is this code considered to transgress the signed overflow purism? joel.rees@gmail.com - 2017-05-03 17:45 -0700
          Re: Is this code considered to transgress the signed overflow purism? Tim Rentsch <txr@alumni.caltech.edu> - 2017-04-30 14:42 -0700
        Re: Is this code considered to transgress the signed overflow purism? joel.rees@gmail.com - 2017-04-30 23:57 -0700
          Re: Is this code considered to transgress the signed overflow purism? Tim Rentsch <txr@alumni.caltech.edu> - 2017-05-02 04:32 -0700
            Re: Is this code considered to transgress the signed overflow purism? joel.rees@gmail.com - 2017-05-03 18:20 -0700
          Re: Is this code considered to transgress the signed overflow purism? luser droog <luser.droog@gmail.com> - 2017-05-08 23:12 -0700
            Re: Is this code considered to transgress the signed overflow purism? joel.rees@gmail.com - 2017-05-09 19:54 -0700
              Re: Is this code considered to transgress the signed overflow purism? luser droog <luser.droog@gmail.com> - 2017-05-09 21:21 -0700
  Re: Is this code considered to transgress the signed overflow purism? James Kuyper <jameskuyper@verizon.net> - 2017-04-27 10:46 -0400
    Re: Is this code considered to transgress the signed overflow purism? supercat@casperkitty.com - 2017-04-27 08:37 -0700
      Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-04-27 09:38 -0700
        Re: Is this code considered to transgress the signed overflow purism? supercat@casperkitty.com - 2017-04-27 10:17 -0700
          Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-04-27 12:10 -0700
        Re: Is this code considered to transgress the signed overflow purism? Tim Rentsch <txr@alumni.caltech.edu> - 2017-04-30 13:50 -0700
          Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-04-30 14:11 -0700
            Re: Is this code considered to transgress the signed overflow purism? Tim Rentsch <txr@alumni.caltech.edu> - 2017-04-30 14:44 -0700
            Re: Is this code considered to transgress the signed overflow purism? joel.rees@gmail.com - 2017-04-30 22:05 -0700

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