Undefined behaviour, was: for or against equality

From	Martin Ward <martin@gkc.org.uk>
Newsgroups	comp.compilers
Subject	Undefined behaviour, was: for or against equality
Date	2022-01-07 14:02 +0000
Organization	Compilers Central
Message-ID	<22-01-028@comp.compilers> (permalink)
References	<17d70d74-1cf1-cc41-6b38-c0b307aeb35a@gkc.org.uk> <22-01-016@comp.compilers> <22-01-018@comp.compilers>

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On 06/01/2022 08:11, David Brown wrote:
> The trick is to memorize the/defined/  behaviours, and stick to them.

Isn't the set of defined behaviours bigger than the set
of undefined behaviours? How do you know what is defined
if you don't know what is undefined?

For example, a = b + c is precisely defined in C and C++ for
floating point variables, but the result can be "undefined behaviour"
for ordinary 32 bit signed integer values.

If you want to stick to defined behaviours then you need
to add extra code. For example, CERT recommends:

   if (((si_b > 0) && (si_a > (INT_MAX - si_b))) ||
       ((si_b < 0) && (si_a < (INT_MIN - si_b)))) {
     /* Handle error */
   } else {
     sum = si_a + si_b;
   }

--
			Martin

Dr Martin Ward | Email: martin@gkc.org.uk | http://www.gkc.org.uk
G.K.Chesterton site: http://www.gkc.org.uk/gkc | Erdos number: 4

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Re: for or against equality, was Why are ambiguous grammars usually a bad idea? Martin Ward <martin@gkc.org.uk> - 2022-01-05 10:25 +0000
  Re: for or against equality, was Why are ambiguous grammars usually a bad idea? David Brown <david.brown@hesbynett.no> - 2022-01-06 09:11 +0100
    Re: what is defined, was for or against equality Thomas Koenig <tkoenig@netcologne.de> - 2022-01-06 16:43 +0000
      Re: what is defined, was for or against equality David Brown <david.brown@hesbynett.no> - 2022-01-07 12:06 +0100
      Re: what is defined, was for or against equality Spiros Bousbouras <spibou@gmail.com> - 2022-01-07 13:21 +0000
        Re: what is defined, was for or against equality Thomas Koenig <tkoenig@netcologne.de> - 2022-01-08 09:31 +0000
          Re: what is defined, was for or against equality Spiros Bousbouras <spibou@gmail.com> - 2022-01-08 22:28 +0000
            Re: what is defined, was for or against equality Thomas Koenig <tkoenig@netcologne.de> - 2022-01-09 00:09 +0000
              Re: what is defined, was for or against equality Spiros Bousbouras <spibou@gmail.com> - 2022-01-09 21:30 +0000
          Re: what is defined, was for or against equality David Brown <david.brown@hesbynett.no> - 2022-01-09 23:00 +0100
            Re: what is defined, was for or against equality Thomas Koenig <tkoenig@netcologne.de> - 2022-01-10 12:04 +0000
              Re: what is defined, was for or against equality David Brown <david.brown@hesbynett.no> - 2022-01-11 18:16 +0100
                Re: what is defined, was for or against equality Kaz Kylheku <480-992-1380@kylheku.com> - 2022-01-11 19:19 +0000
                Re: what is defined, was for or against equality gah4 <gah4@u.washington.edu> - 2022-01-11 14:18 -0800
                Re: what is defined, was for or against equality Thomas Koenig <tkoenig@netcologne.de> - 2022-01-12 19:02 +0000
                Re: what is defined, was for or against equality David Brown <david.brown@hesbynett.no> - 2022-01-13 08:24 +0100
                Re: what is defined, was for or against equality Thomas Koenig <tkoenig@netcologne.de> - 2022-01-13 11:17 +0000
          Re: what is defined, was for or against equality gah4 <gah4@u.washington.edu> - 2022-01-10 16:58 -0800
    Re: for or against equality, was Why are ambiguous grammars usually a bad idea? Robert Prins <robert@prino.org> - 2022-01-06 19:07 +0000
    Undefined behaviour, was: for or against equality Martin Ward <martin@gkc.org.uk> - 2022-01-07 14:02 +0000
      Re: Undefined behaviour, was: for or against equality Spiros Bousbouras <spibou@gmail.com> - 2022-01-08 03:41 +0000
    Re: Undefined behaviour, was: for or against equality David Brown <david.brown@hesbynett.no> - 2022-01-07 15:56 +0100
      Re: Undefined behaviour, was: for or against equality anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2022-01-08 17:52 +0000
        Re: Undefined behaviour, was: for or against equality David Brown <david.brown@hesbynett.no> - 2022-01-09 23:53 +0100
        Re: Undefined behaviour, was: for or against equality Kaz Kylheku <480-992-1380@kylheku.com> - 2022-01-11 16:55 +0000
          Re: Undefined behaviour, was: for or against equality George Neuner <gneuner2@comcast.net> - 2022-01-11 22:01 -0500

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