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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: int a = a (Was: Bart's Language)
Date: Thu, 20 Mar 2025 02:02:39 -0700
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scott@slp53.sl.home (Scott Lurndal) writes:

> David Brown <david.brown@hesbynett.no> writes:
>
>> On 18/03/2025 19:36, Janis Papanagnou wrote:
>>
>>> On 18.03.2025 19:04, Kenny McCormack wrote:
>>>
>>>> In article <vrc75b$2r4lt$1@dont-email.me>,
>>>> David Brown  <david.brown@hesbynett.no> wrote:
>>>> ...
>>>>
>>>>>> gcc won't warn until you say '-Wextra', and then only for:
>>>>>>
>>>>>>     int a = a + 1;
>>>>>
>>>>> People would not normally write "int a = a;".  It is used as a
>>>>> common idiom meaning "I know it is not clear to the compiler
>>>>> that the variable is always initialised before use, but /I/ know
>>>>> it is - so disable the use-without-initialisation warnings for
>>>>> this variable".
>>>
>>> Wow! - It would never have occurred to me that "int a = a;" being
>>> considered an idiom, let alone a "common idiom".
>>
>> It is certainly an idiom, and certainly viewed by gcc as a way to
>> avoid an "uninitialized" warning (unless "-Winit-self" is also
>> enabled), and it is an idiom I have seen documented in at least one
>> other compiler (though I can't remember which - I've read many
>> compiler manuals over the decades).
>>
>> But judging from the posts here, it may not be a "common" idiom.
>>
>> (And I am not suggesting it is a /good/ idiom.  It's not one I use
>> myself, and I have "-Winit-self" in my list of standard warning
>> flags because it is conceivable that I write "int a = a;" in error.
>> But there are lots of idioms and common practices in C that I don't
>> like personally.)
>>
>>
>> As far as I understand it (and I hope to be corrected if I am
>> wrong), "int a = a;" is not undefined behaviour as long as the
>> implementation does not have trap values for "int".  It simply
>> leaves "a" as an unspecified value - just like "int a;" does.  Thus
>> it is not in any way "worse" than "int a;" as far as C semantics
>> are concerned.  Any difference is a matter of implementation - and
>> the usual implementation effect is to disable "not initialised"
>> warnings.
>>
>> It is in much the same category as "(void) x;", which is an idiom
>> for skipping an "unused variable" or "unused parameter" warning.
>
> I would disagree with this last statement.  (void)x is genuinely
> useful and has no ill side effects.   'int a = a;' is exactly
> the opposite - not useful and has potenial bad side effects.

I concur except that I recommend using (void)&x rather than (void)x,
because (void)&x is safer.  If x is volatile qualified, for example,
the expression (void)x actually does something, and must not be
compiled away, whereas (void)&x does not have these properties.