Re: int a = a (Was: Bart's Language)

From	Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups	comp.lang.c
Subject	Re: int a = a (Was: Bart's Language)
Date	2025-03-20 02:02 -0700
Organization	A noiseless patient Spider
Message-ID	<864izooz0w.fsf@linuxsc.com> (permalink)
References	(3 earlier) <vrc75b$2r4lt$1@dont-email.me> <vrccjb$b3m6$1@news.xmission.com> <vrcef2$33076$1@dont-email.me> <vrelvn$12ddq$1@dont-email.me> <aqCCP.590465$SZca.348513@fx13.iad>

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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.

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Bart's Language bart <bc@freeuk.com> - 2025-03-17 23:51 +0000
  Re: Bart's Language antispam@fricas.org (Waldek Hebisch) - 2025-03-18 12:17 +0000
    Re: Bart's Language bart <bc@freeuk.com> - 2025-03-18 13:54 +0000
      Re: Bart's Language antispam@fricas.org (Waldek Hebisch) - 2025-03-18 15:10 +0000
        Re: Bart's Language bart <bc@freeuk.com> - 2025-03-18 15:45 +0000
          Re: Bart's Language David Brown <david.brown@hesbynett.no> - 2025-03-18 17:31 +0100
            int a = a (Was: Bart's Language) gazelle@shell.xmission.com (Kenny McCormack) - 2025-03-18 18:04 +0000
              Re: int a = a (Was: Bart's Language) Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-03-18 19:36 +0100
                Re: int a = a (Was: Bart's Language) Kaz Kylheku <643-408-1753@kylheku.com> - 2025-03-18 19:11 +0000
                Re: int a = a (Was: Bart's Language) David Brown <david.brown@hesbynett.no> - 2025-03-19 15:56 +0100
                Re: int a = a (Was: Bart's Language) scott@slp53.sl.home (Scott Lurndal) - 2025-03-19 16:38 +0000
                Re: int a = a (Was: Bart's Language) "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-03-19 14:29 -0700
                Re: int a = a (Was: Bart's Language) David Brown <david.brown@hesbynett.no> - 2025-03-20 09:39 +0100
                Re: int a = a (Was: Bart's Language) bart <bc@freeuk.com> - 2025-03-20 11:59 +0000
                Re: int a = a (Was: Bart's Language) David Brown <david.brown@hesbynett.no> - 2025-03-20 15:46 +0100
                Re: int a = a (Was: Bart's Language) wij <wyniijj5@gmail.com> - 2025-03-20 23:13 +0800
                Re: int a = a (Was: Bart's Language) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-03-20 02:02 -0700
                Re: int a = a (Was: Bart's Language) David Brown <david.brown@hesbynett.no> - 2025-03-20 15:57 +0100
                Re: int a = a (Was: Bart's Language) Kaz Kylheku <643-408-1753@kylheku.com> - 2025-03-19 17:07 +0000
                Re: int a = a Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-03-19 13:34 -0700
                Re: int a = a Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-03-20 02:54 -0700
                Re: int a = a Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-03-20 03:20 -0700
                Re: int a = a David Brown <david.brown@hesbynett.no> - 2025-03-20 16:22 +0100
                Re: int a = a Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-03-20 12:46 -0700
                Re: int a = a David Brown <david.brown@hesbynett.no> - 2025-03-21 10:44 +0100
                Re: int a = a Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-03-21 12:23 -0700
                Re: int a = a David Brown <david.brown@hesbynett.no> - 2025-03-21 21:46 +0100
                Re: int a = a Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-03-22 13:59 -0700
                Re: int a = a Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-03-22 15:37 -0700
                Re: int a = a Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-04-28 09:39 -0700
                Re: int a = a Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-04-29 13:12 -0700
                Re: int a = a Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-04-29 13:34 -0700
                Re: int a = a David Brown <david.brown@hesbynett.no> - 2025-03-20 15:42 +0100
              Re: int a = a (Was: Bart's Language) scott@slp53.sl.home (Scott Lurndal) - 2025-03-18 19:37 +0000
              Re: int a = a (Was: Bart's Language) David Brown <david.brown@hesbynett.no> - 2025-03-18 20:51 +0100
                Re: int a = a (Was: Bart's Language) Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-03-18 23:27 +0100
                Re: int a = a (Was: Bart's Language) David Brown <david.brown@hesbynett.no> - 2025-03-19 11:40 +0100
              Re: int a = a (Was: Bart's Language) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-03-18 23:52 -0700
                Re: int a = a Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-03-19 01:55 -0700
                Re: int a = a Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-04-27 13:41 -0700
                Re: int a = a (Was: Bart's Language) David Brown <david.brown@hesbynett.no> - 2025-03-19 11:43 +0100
                Re: int a = a (Was: Bart's Language) Rosario19 <Ros@invalid.invalid> - 2025-03-19 13:23 +0100
                Re: int a = a (Was: Bart's Language) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-03-20 01:32 -0700
          Re: Bart's Language antispam@fricas.org (Waldek Hebisch) - 2025-03-20 22:55 +0000
            Re: Bart's Language Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-03-20 16:22 -0700
              Re: Bart's Language antispam@fricas.org (Waldek Hebisch) - 2025-03-22 14:37 +0000
                Re: Bart's Language James Kuyper <jameskuyper@alumni.caltech.edu> - 2025-03-22 11:41 -0400
                Re: Bart's Language antispam@fricas.org (Waldek Hebisch) - 2025-03-22 16:52 +0000
                Re: Bart's Language James Kuyper <jameskuyper@alumni.caltech.edu> - 2025-03-22 20:12 -0400
                By definition... (Was: Bart's Language) gazelle@shell.xmission.com (Kenny McCormack) - 2025-03-23 17:20 +0000
              Re: Bart's Language Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-04-27 11:53 -0700
                Re: Bart's Language Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-04-27 14:29 -0700
                Re: Bart's Language Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-01-06 14:04 -0800
                Re: Bart's Language Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-01-06 17:12 -0800
                Re: Bart's Language Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-03-06 09:04 -0800
        Re: Bart's Language bart <bc@freeuk.com> - 2025-03-18 22:19 +0000
          Re: Bart's Language antispam@fricas.org (Waldek Hebisch) - 2025-03-20 22:38 +0000
            Re: Bart's Language Kaz Kylheku <643-408-1753@kylheku.com> - 2025-03-20 23:45 +0000
              Re: Bart's Language bart <bc@freeuk.com> - 2025-03-21 00:56 +0000
                Re: Bart's Language Kaz Kylheku <643-408-1753@kylheku.com> - 2025-03-21 17:47 +0000
                Re: Bart's Language Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-03-22 07:12 -0700
            Re: Bart's Language bart <bc@freeuk.com> - 2025-03-21 00:33 +0000

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