Re: New features added into C23 standard

From	Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups	comp.lang.c
Subject	Re: New features added into C23 standard
Date	2022-07-31 16:55 -0700
Organization	None to speak of
Message-ID	<87a68ox2fd.fsf@nosuchdomain.example.com> (permalink)
References	(3 earlier) <87mtcwzwt9.fsf@nosuchdomain.example.com> <a21a0f85-d913-4a8c-a20b-17606a125bc9n@googlegroups.com> <88293f89-5f19-4fb3-844b-20e3a063b65cn@googlegroups.com> <tc5h2p$83f9$1@dont-email.me> <GStFK.601320$J0r9.21383@fx11.iad>

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Richard Damon <Richard@Damon-Family.org> writes:
> On 7/31/22 5:12 AM, David Brown wrote:
>> It is also not uncommon in microcontrollers for address zero to be a 
>> useful address - and then it is user code that needs access.  But it
>> is rarely an issue in practice.  The most common situation is having
>> it as part of the flash for code, and it is typically part of the
>> interrupt vectors or reset vector.  You would only want to read it
>> for something like a CRC check of the flash, and if you are
>> concerned about the compiler handling a pointer to address zero in
>> an unhelpful manner, you can use a pointer-to-volatile.  The other
>> cases I have seen are having memory mapped peripherals there - and
>> again, you always use volatile accesses.
>
> And for machine specific case like this, the fact that derefencing a
> NULL pointer is "just" Undefined Behavior, that can be Implementaton 
> Defined to do what is wanted, as opposed to some how PROHIBITED.
>
> *(char *0) = 0;
>
> Is a fully legal statement that must be translated.

No, it can be rejected at compile time.  See the definition of
"undefined behavior":

    NOTE  Possible undefined behavior ranges from ignoring the
    situation completely with unpredictable results, to behaving
    during translation or program execution in a documented manner
    characteristic of the environment (with or without the issuance
    of a diagnostic message), to terminating a translation or
    execution (with the issuance of a diagnostic message).

> Yes, when you execute it, you get undefined behavior, but that might
> be just simply defined to write to location 0.

Certainly an implementation *can* do that, and it's what I'd want and
expect on an implementation where location 0 is usable.

Having location 0 be usable doesn't preclude using all-bits-zero to
represent a null pointer.  It means you can't distinguish between a
valid pointer to location 0 and a null pointer, but in practice that's
not likely to be a huge problem; it just requires some care by the
programmer.  Similarly, (time_t)-1 is an error value when returned by
the time() function, but it can also be a valid time, typically
1969-12-31 23:59:59 UTC.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for Philips
void Void(void) { Void(); } /* The recursive call of the void */

Thread

New features added into C23 standard Thiago Adams <thiago.adams@gmail.com> - 2022-07-22 12:32 -0700
  Re: New features added into C23 standard Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-07-22 23:15 +0100
  Re: New features added into C23 standard Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-07-23 01:31 -0700
    Re: New features added into C23 standard gazelle@shell.xmission.com (Kenny McCormack) - 2022-07-23 10:10 +0000
  Re: New features added into C23 standard Thiago Adams <thiago.adams@gmail.com> - 2022-07-25 13:49 -0700
    Re: New features added into C23 standard Thiago Adams <thiago.adams@gmail.com> - 2022-07-25 13:57 -0700
      Re: New features added into C23 standard Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-07-25 14:50 -0700
        Re: New features added into C23 standard Thiago Adams <thiago.adams@gmail.com> - 2022-07-25 15:51 -0700
          Re: New features added into C23 standard Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-07-25 18:04 -0700
            Re: New features added into C23 standard Thiago Adams <thiago.adams@gmail.com> - 2022-07-25 19:23 -0700
              Re: New features added into C23 standard Opus <ifonly@youknew.org> - 2022-07-26 05:13 +0200
                Re: New features added into C23 standard Richard Damon <Richard@Damon-Family.org> - 2022-07-25 23:36 -0400
                Re: New features added into C23 standard Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-07-25 21:52 -0700
                Re: New features added into C23 standard Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-07-26 11:44 -0700
              Re: New features added into C23 standard Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-07-26 11:39 -0700
          Re: New features added into C23 standard David Brown <david.brown@hesbynett.no> - 2022-07-30 14:42 +0200
            Re: New features added into C23 standard scott@slp53.sl.home (Scott Lurndal) - 2022-07-30 15:18 +0000
              Re: New features added into C23 standard Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-07-30 09:13 -0700
            Re: New features added into C23 standard Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-07-30 17:31 +0100
              Re: New features added into C23 standard Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-07-30 16:46 -0700
            Re: New features added into C23 standard Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-07-30 14:36 -0700
              Re: New features added into C23 standard Richard Damon <Richard@Damon-Family.org> - 2022-07-30 18:15 -0400
                Re: New features added into C23 standard Kaz Kylheku <480-992-1380@kylheku.com> - 2022-07-30 23:28 +0000
                Re: New features added into C23 standard Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-07-31 16:48 -0700
                Re: New features added into C23 standard Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-07-31 22:42 -0700
                Re: New features added into C23 standard Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-07-31 23:25 -0700
                Re: New features added into C23 standard Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-08-01 00:10 -0700
                Re: New features added into C23 standard David Brown <david.brown@hesbynett.no> - 2022-08-01 09:55 +0200
                Re: New features added into C23 standard Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-08-01 20:17 +0100
              Re: New features added into C23 standard Kaz Kylheku <480-992-1380@kylheku.com> - 2022-07-30 23:21 +0000
        Re: New features added into C23 standard Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-07-25 22:59 -0700
          Re: New features added into C23 standard Öö Tiib <ootiib@hot.ee> - 2022-07-26 00:48 -0700
            Re: New features added into C23 standard Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-07-26 02:08 -0700
              Re: New features added into C23 standard Richard Damon <Richard@Damon-Family.org> - 2022-07-26 06:59 -0400
              Re: New features added into C23 standard Öö Tiib <ootiib@hot.ee> - 2022-07-26 04:02 -0700
                Re: New features added into C23 standard Vir Campestris <vir.campestris@invalid.invalid> - 2022-07-29 21:18 +0100
                Re: New features added into C23 standard Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-07-29 16:55 -0700
              Re: New features added into C23 standard Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-07-26 11:51 -0700
            Re: New features added into C23 standard David Brown <david.brown@hesbynett.no> - 2022-07-31 11:12 +0200
              Re: New features added into C23 standard Richard Damon <Richard@Damon-Family.org> - 2022-07-31 07:40 -0400
                Re: New features added into C23 standard Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-07-31 17:40 +0100
                Re: New features added into C23 standard Richard Damon <Richard@Damon-Family.org> - 2022-07-31 12:56 -0400
                Re: New features added into C23 standard Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-07-31 19:28 +0100
                Re: New features added into C23 standard Richard Damon <Richard@Damon-Family.org> - 2022-07-31 14:52 -0400
                Re: New features added into C23 standard Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-07-31 21:35 +0100
                Re: New features added into C23 standard Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-08-15 23:24 -0700
                Re: New features added into C23 standard Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-07-31 16:57 -0700
                Re: New features added into C23 standard Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-07-31 16:55 -0700
                Re: New features added into C23 standard Richard Damon <Richard@Damon-Family.org> - 2022-07-31 20:09 -0400
                Re: New features added into C23 standard Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-08-15 22:52 -0700
                Re: New features added into C23 standard David Brown <david.brown@hesbynett.no> - 2022-08-16 09:21 +0200
                Re: New features added into C23 standard Philipp Klaus Krause <pkk@spth.de> - 2022-08-16 09:32 +0200
                Re: New features added into C23 standard David Brown <david.brown@hesbynett.no> - 2022-08-16 11:06 +0200
                Re: New features added into C23 standard Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-08-15 23:17 -0700
                Re: New features added into C23 standard Richard Damon <Richard@Damon-Family.org> - 2022-08-17 20:11 -0400
                Re: New features added into C23 standard Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-08-17 20:08 -0700
                Re: New features added into C23 standard Richard Damon <Richard@Damon-Family.org> - 2022-08-17 23:19 -0400
                Re: New features added into C23 standard Kaz Kylheku <480-992-1380@kylheku.com> - 2022-08-18 07:03 +0000
                Re: New features added into C23 standard Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-08-23 09:43 -0700
                Re: New features added into C23 standard Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-08-23 11:02 -0700
                Re: New features added into C23 standard Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-08-23 13:40 -0700
          Re: New features added into C23 standard Richard Damon <Richard@Damon-Family.org> - 2022-07-26 07:04 -0400
            Re: New features added into C23 standard Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-07-26 05:24 -0700
              Re: New features added into C23 standard Richard Damon <Richard@Damon-Family.org> - 2022-07-26 22:30 -0400
                Re: New features added into C23 standard Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-07-29 00:02 +0100
                Re: New features added into C23 standard Richard Damon <Richard@Damon-Family.org> - 2022-07-28 21:20 -0400
                Re: New features added into C23 standard William Ahern <william@25thandClement.com> - 2022-07-28 19:36 -0700
                Re: New features added into C23 standard scott@slp53.sl.home (Scott Lurndal) - 2022-07-29 16:39 +0000
                Re: New features added into C23 standard Richard Damon <Richard@Damon-Family.org> - 2022-07-29 18:55 -0400
                Re: New features added into C23 standard Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-07-29 15:17 +0100
          Re: New features added into C23 standard scott@slp53.sl.home (Scott Lurndal) - 2022-07-26 13:27 +0000
          Re: New features added into C23 standard Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-07-26 11:47 -0700
            Re: New features added into C23 standard Kaz Kylheku <480-992-1380@kylheku.com> - 2022-07-26 20:27 +0000
              Re: New features added into C23 standard Philipp Klaus Krause <pkk@spth.de> - 2022-08-16 08:42 +0200
  Re: New features added into C23 standard Bonita Montero <Bonita.Montero@gmail.com> - 2022-07-26 05:37 +0200
    Re: New features added into C23 standard Lynn McGuire <lynnmcguire5@gmail.com> - 2022-07-28 16:47 -0500
      Re: New features added into C23 standard Bonita Montero <Bonita.Montero@gmail.com> - 2022-07-29 07:14 +0200
        Re: New features added into C23 standard Vir Campestris <vir.campestris@invalid.invalid> - 2022-07-29 21:21 +0100
  Re: New features added into C23 standard Grant Mulholland <grantlmul@gmail.com> - 2022-08-13 22:21 -0700
    Re: New features added into C23 standard Lynn McGuire <lynnmcguire5@gmail.com> - 2022-08-15 21:21 -0500
      Re: New features added into C23 standard Bonita Montero <Bonita.Montero@gmail.com> - 2022-08-16 08:39 +0200
    Re: New features added into C23 standard Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-08-15 22:54 -0700
    Re: New features added into C23 standard Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-08-18 12:03 -0700
      Re: New features added into C23 standard "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-08-18 12:56 -0700
      Re: New features added into C23 standard William Ahern <william@25thandClement.com> - 2022-08-18 16:07 -0700
        Re: New features added into C23 standard Thiago Adams <thiago.adams@gmail.com> - 2022-08-18 18:47 -0700
          Re: New features added into C23 standard Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-08-18 20:23 -0700

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