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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: Constants and undefined behavior
Date: Sat, 06 Jun 2026 18:06:37 -0700
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Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:

> cross@spitfire.i.gajendra.net (Dan Cross) writes:
>
>> In article <865x3yd21n.fsf@linuxsc.com>,
>> Tim Rentsch  <tr.17687@z991.linuxsc.com> wrote:
>>
>>> cross@spitfire.i.gajendra.net (Dan Cross) writes:
>>>
>>>> In article <86ik81cfk5.fsf_-_@linuxsc.com>,
>>>> Tim Rentsch  <tr.17687@z991.linuxsc.com> wrote:
>
> [...]
>
>>>>> There's an important distinction to make here.  Consider this
>>>>> program:
>>>>>
>>>>>    #include <limits.h>
>>>>>
>>>>>    int
>>>>>    foo(){
>>>>>        int zero = (INT_MAX+1)*0;
>>>>>        return  zero;
>>>>>    }
>>>>>
>>>>>    int
>>>>>    main(){
>>>>>        return  0;
>>>>>    }
>>>>>
>>>>> This program does not transgress the bounds of undefined behavior.
>>>
>>> To clarify, the comments in my posting were meant to be read as
>>> saying the given text is the entire program, and that it is strictly
>>> conforming with respect to conforming hosted implementations.
>>> (Incidentally, given the rules for freestanding implementations, I'm
>>> not sure that it is even possible for any program to be strictly
>>> conforming with respect to conforming freestanding implementations.
>>> In any case my statements were meant only in the context of hosted
>>> implementations.)

[...]

> foo() has undefined behavior if it's called, so replacing its
> body with trapping code is valid.

Right.

> But (I'm reasonably sure that)
> an implementation cannot reject a program just because it can't
> prove that it has no undefined behavior during execution.  [...]

Right.

>> In your example, `foo` clearly exhibits UB;  I think your
>> argument is whether that has a realized effect or not, since the
>> UB is not invoked.  I'm saying that in general a compiler cannot
>> possibly know that when it compiles `foo`, and is free to assume
>> the worst.
>
> foo() exhibits UB if and only if it's called during execution.

Right.