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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: int a = a
Date: Sat, 22 Mar 2025 15:37:48 -0700
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Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
> Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
>> David Brown <david.brown@hesbynett.no> writes:
>>> [...]I believe it would be much simpler and clearer if attempting
>>> to read an uninitialised and unassigned local variable were
>>> undefined behaviour in every case.
>>
>> I probably agree (I haven't given it all that much thought), but
>> the committee made a specific decision between C90 and C99 to say
>> that reading an uninitialized automatic object is *not* undefined
>> behavior.  I'm don't know why they did that (though, all else
>> being equal, reducing the number of instances of undefined
>> behavior is a good thing), but reversing that decision for this
>> one issue is not something they decided to do.
>
> Your description of what was done is wrong.  It is still the case in
> C99 that trying to access an uninitialized object is undefined
> behavior, at least potentially, except for accesses using a type
> that either is a character type or has no trap representations (and
> all types other than unsigned char may have trap representations,
> depending on the implementation).  A statement like
>
>     int a = a;
>
> may still be given a warning as potential undefined behavior, even
> in C99.

I had already mentioned that distinction earlier in the thread.

[...]

> The mistake is thinking that UB for uninitialized access was
> removed in C99.  It wasn't.  Narrowed, yes;  removed, no.

Acknowledged.

[...]

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