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

From	Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups	comp.lang.c
Subject	Re: Is this code considered to transgress the signed overflow purism?
Date	2017-04-29 15:18 +0100
Organization	A noiseless patient Spider
Message-ID	<87tw572t77.fsf@bsb.me.uk> (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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bartc <bc@freeuk.com> writes:

> 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.

That's not an interesting kind of wrong -- it depends on what the intent
was (you didn't say) and on a bunch of definitions.  The interesting
meaning (which was the point I thought you were making) is that some of
C's arithmetic choices were the wrong ones.

> 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.)

But that was not available when C was being defined, and by the time 64
bit types where mandated, too much code was written with the old
semantics.  And, of course, that does not solve the problem but simply
pushes it to another type.  Provided you mean "it's not what I want
nowadays" then I accept that C's choices are "wrong".  But if you mean
more that that, what should the rules have been (around 1980) to avoid
people on Usenet declaring it wrong 35 years later?

-- 
Ben.

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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