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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: you think rust may outthrone c?
Date: Fri, 21 Jul 2023 02:03:51 -0700
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Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
[...]
> The problem is that (referring to N1570, and I don't see any relevant
> changes in the latest public C23 draft, N3096), Annex J says gives
>
>     The value of an object with automatic storage duration is used
>     while it is indeterminate (6.2.4, 6.7.9, 6.8).
>
> as an example of undefined behavior.
>
> If that is accurate, then this program:
>
> #include <limits.h>
> int main(void) {
>     int foo;
>     if (sizeof (int) == 4 &&
>         CHAR_BIT == 8 &&
>         INT_MAX == 2147483647 &&
>         INT_MIN == -INT_MAX-1)
>     {
>         int bar = foo;
>     }
> }
>
> has undefined behavior on an implementation where the condition is true.
> (Of course that UB is likely to manifest as quietly doing nothing.)
>
> Annex J is non-normative, and depends on normative statements
> elsewhere in the standard.  Following the section numbers given in
> Annex J and searching for occurrences of "indeterminate value",
> I have so far been unable to confirm that the statement in Annex
> J is correct.
>
> Either Annex J is incorrect, or accessing an uninitalized
> automatic object is UB even if the object cannot possibly hold a
> trap representation.  (I'd be satisfied either way; I'd like the
> standard to be internally consistent.)
>
> Perhaps someone else can find something in the standard that I've
> missed.

I've posted about this to comp.std.c (subject "Does reading an
uninitialized object have undefined behavior?") and emailed the C23
editors.  (It's likely too late to make any significant changes to C23.)

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Will write code for food.
void Void(void) { Void(); } /* The recursive call of the void */