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From: Martin Ward <martin@gkc.org.uk>
Newsgroups: comp.compilers
Subject: Undefined behaviour, was: for or against equality
Date: Fri, 7 Jan 2022 14:02:50 +0000
Organization: Compilers Central
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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