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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: int a = a
Date: Thu, 20 Mar 2025 03:20:05 -0700
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Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
> Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
> [how to indicate a variable not being used is okay]
> [some quoted text rearranged]
>
>> Unless I'm missing something, `(void)x` also has undefined beahvior
>> if x is uninitialized,
>
> Right.  Using (void)&x is better.

I'm not convinced -- and it's far less idiomatic.  I don't think
I've ever seen (void)&x in code, and if I did I'd wonder what the
author's intent was.

(void)x is a common idiom for hinting to the compiler that it
doesn't need to complain about x being unused.  (void)&x doesn't
tell the compiler that the *value* of x is used.  I'm not sure how
much difference that makes.

Even with (void)x and/or (void)&x, a compiler *could* still warn
about x being unused, or about the programmer's use of an ugly font.

>> though it's very likely to do nothing in practice.
>
> Unless x is volatile qualified, in which there must be an access
> to x in the generated code.
>
>> The behavior [of int a = a;] is undefined.  In C11 and later
>> (N1570 6.3.2.1p2):
>>
>>     Except when [...] an lvalue that does not have array type is
>>     converted to the value stored in the designated object (and is
>>     no longer an lvalue); this is called lvalue conversion.
>>     [...]
>>     If the lvalue designates an object of automatic storage
>>     duration that could have been declared with the register
>>     storage class (never had its address taken), and that object
>>     is uninitialized (not declared with an initializer and no
>>     assignment to it has been performed prior to use), the
>>     behavior is undefined.
>
>> Long digression follows.
>>
>> The "could have been declared with the register storage class"
>> seems quite odd.  And in fact it is quite odd.
>
> I don't have the same reaction.  The point of this phrase is that
> undefined behavior occurs only for variables that don't have
> their address taken.  The phrase used describes that nicely.
> Any questions related to "registerness" can be ignored, because
> 'register' in C really has nothing to do with hardware registers,
> despite the name.

DR 338 is explicitly motivated by an IA-64 feature that applies only to
CPU registers.  An object whose address is taken can't be stored (only)
in a register, so it can't have a NaT representation.

The phrase used is "could have been declared with register storage class
(never had its address taken)".  Surely "never had its address taken"
would have been clear enough if CPU registers weren't a big part of the
motivation.

[SNIP]

>> https://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_338.htm
>> 
>> So the "could have been declared with the register storage class"
>> wording was added in C11 specifically to cater to the IA64.  This
>> change would have been superfluous in C90, where the behavior was
>> undefined anyway, but is a semantically significant change between
>> C99 and C11.  (If some future CPU has something like NaT that can
>> be stored in memory, the wording might need to be updated yet again.)
>>
>> My takeaway is that if it requires this much research to determine
>> whether accessing the value of an uninitialized object has undefined
>> behavior (in which circumstances and which edition of the standard),
>> I'll just avoid doing so altogether.  I'll initialize objects
>> when they're defined whenever practical.  If it's not practical
>> for some reason, I won't initialize it with some dummy value; I'll
>> leave it uninitialized so the compiler has a chance to warn me if
>> I accidentally use it before assigning a value to it.
>
> I think you are overthinking the question.  In cases where it's
> important to give an initial value to a variable, and can be done
> so at the point of its declaration, use an initializer;  otherwise
> don't.

My overthinking led me to essentially the same conclusion, so I don't
see the problem.  And I also found it to be an interesting exploration
of how certain aspects of the C standard have evolved over time.

>         We don't have to read several different C standards, or
> even only one, to reach that conclusion.

No, but we do have to read one or more C standards to counter an
argument that `int a = a;` is well defined.

>                                           If someone wants to know
> exactly which border cases are safe and which cases are not, then
> reading the relevant version(s) of the C standard is needed, but
> in most situations it isn't.  It's important for the C standard to
> be precise about what it prescribes, but as far as initialization
> goes it's easy to write code that doesn't need that level of
> detail.  Compiler writers need to know such things;  in the
> particular case of when and where to initialize, most developers
> don't.

Most developers don't read this newsgroup.

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