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Re: Opinion sought: Is this strictly conforming C?

From Manfred <noname@invalid.add>
Newsgroups comp.lang.c
Subject Re: Opinion sought: Is this strictly conforming C?
Date 2018-02-16 20:05 +0100
Organization Aioe.org NNTP Server
Message-ID <p67a0r$hl$1@gioia.aioe.org> (permalink)
References <e054cddb-6cb6-46d3-805a-db0733421bed@googlegroups.com> <p65ibn$par$1@dont-email.me>

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On 2/16/2018 4:15 AM, James Kuyper wrote:
> On 02/15/2018 08:20 PM, lee@ansync.com wrote:
>> I am involved in a debate on StackOverflow about whether this code,
>> which accesses the same chunk of memory as both a 1D and 2D array, is
>> strictly conforming, well-defined C with no undefined behavior:
>>
>> ----
>>
>> #include <stdio.h>
>>
>> int a1[100];
>> int (*a2)[10] = (int (*)[10])a1;
> 
> In this context, a1 is implicitly converted into a pointer to the first
> element of a1. Here's what the standard has to say about the conversion
> of a pointer to int into a pointer to int[10]:
> "If the resulting pointer is not correctly aligned 68) for the
> referenced type, the behavior is undefined. Otherwise, when converted
> back again, the result shall compare equal to the original pointer. When
> a pointer to an object is converted to a pointer to a character type,
> the result points to the lowest addressed byte of the object. Successive
> increments of the result, up to the size of the object, yield pointers
> to the remaining bytes of the object." (6.3.2.3p7).
> 
> It's permissible for an object of type 'int[10]' to have stricter
> alignment requirements than an object of type int[100], but it's not at
> all likely. As a practical matter, we can ignore that issue - but as far
> as the standard is concerned, it is a possible problem. Using C2011,
> it's possible to check for that possibility using the _Alignof() feature.
> 
> However, there's a much more important problem, and one which is so
> subtle that it might be hard to notice. It's about what the standard
> doesn't say, which is much harder to notice than what it does say. The
> ONLY thing the standard says about the result of the conversion is that
> it can be reversed.

I understand what you are saying, but I am not sure that this is the 
only thing.
My doubt arises primarily from the fact that according to such a strict 
interpretation, any converted pointer would be substantially unusable.

Also, I think that it is intentional that the 'special case' of pointer 
to char is defined within the same clause as for generic pointers, and 
together with the requirement of correct alignment.

I think (but this should be more formally substantiated) that the 
committee intended to give a definition of 'definedness' of the 
resulting pointer by ensuring that the representation of the referenced 
object is valid:

- First, the alignment requirement of the resulting pointer is enforced.

- Then, self-consistency of the pointer value is enforced (when 
converted back it will compare equal)

- Then, it is IMHO relevant here that note 68) recalls that the concept 
of "correctly aligned" is transitive, so that in order to access the 
object representation one can assume something like:
- a1: pointer to int
- a2: pointer to int[10]
- a3: pointer to char
are all correctly aligned for the type of interest (int[10] in this case)

- Finally, the conversion rules for pointer to char ensure that:
   a) the converted pointer points to the first byte of the object
   b) the byte sequence that constitutes the object representation can 
be accessed byte-by-byte through a converted pointer to char which will 
keep the byte ordering of the object representation.

Since the object representation is /defined/ as a byte sequence (6.2.6.1 
p2), and "Two values (other than NaNs) with the same object 
representation compare equal" (6.2.6.1 p4), I would understand that:


/If/ a1 (pointer to int) actually points to (has a value of the address 
of) an object representation (sequence of bytes) of an int[10] object,
/Then/ a1 and a2 both point to (have the same value of the address of) 
the object representation of the int[10] object (and thus a2 can be used 
to access the int[10] object).

Obviously this only holds if a1 points to an int[10], i.e. if the 
pointer to be converted actually points to an object of the converted 
type, which is under the responsibility of the programmer.
This is why the standard cannot specify anything more than a 
byte-by-byte guarantee on the sequence of bytes that can be accessed by 
a converted pointer.



  Since int[10] is not a character type, that clause
> does doesn't tell you anything else that you can do with the resulting
> pointer. Note, in particular, that it doesn't bother telling you what,
> if anything, the resulting pointer points at. What happens when you do
> anything to such a pointer other than converting it back to it's
> original type is undefined behavior, by "omission of any explicit
> definition of the behavior" (4p2).
> 
> This interpretation of the words of the standard tends to be
> controversial, primarily because code like this will, in fact, generally
> work as you may have mistakenly thought it was guaranteed to work. That
> fact can make it very hard for people to accept that it is not, in fact,
> guaranteed to work.

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Thread

Opinion sought: Is this strictly conforming C? lee@ansync.com - 2018-02-15 17:20 -0800
  Re: Opinion sought: Is this strictly conforming C? James Kuyper <jameskuyper@verizon.net> - 2018-02-15 22:15 -0500
    Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-16 08:44 -0800
      Re: Opinion sought: Is this strictly conforming C? jameskuyper@verizon.net - 2018-02-16 11:47 -0800
        Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-16 13:00 -0800
      Re: Opinion sought: Is this strictly conforming C? Tim Rentsch <txr@alumni.caltech.edu> - 2018-02-19 12:25 -0800
        Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-19 13:21 -0800
          Re: Opinion sought: Is this strictly conforming C? Randy Howard <rhoward.mx@EverybodyUsesIt.com> - 2018-02-19 15:26 -0600
          Re: Opinion sought: Is this strictly conforming C? Keith Thompson <kst-u@mib.org> - 2018-02-19 13:59 -0800
            Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-19 14:38 -0800
              Re: Opinion sought: Is this strictly conforming C? Keith Thompson <kst-u@mib.org> - 2018-02-19 15:21 -0800
                Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-19 15:59 -0800
                Re: Opinion sought: Is this strictly conforming C? Keith Thompson <kst-u@mib.org> - 2018-02-19 17:11 -0800
                Re: Opinion sought: Is this strictly conforming C? David Brown <david.brown@hesbynett.no> - 2018-02-20 09:17 +0100
                Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-20 07:58 -0800
                Re: Opinion sought: Is this strictly conforming C? David Brown <david.brown@hesbynett.no> - 2018-02-20 18:21 +0100
                Re: Opinion sought: Is this strictly conforming C? Keith Thompson <kst-u@mib.org> - 2018-02-20 09:25 -0800
                Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-20 11:52 -0800
                Re: Opinion sought: Is this strictly conforming C? Richard Damon <Richard@Damon-Family.org> - 2018-02-20 22:27 -0500
                Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-20 23:07 -0800
                Re: Opinion sought: Is this strictly conforming C? Manfred <noname@invalid.add> - 2018-02-21 14:07 +0100
                Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-21 09:02 -0800
                Re: Opinion sought: Is this strictly conforming C? Keith Thompson <kst-u@mib.org> - 2018-02-21 09:50 -0800
                Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-21 12:46 -0800
                Re: Opinion sought: Is this strictly conforming C? Keith Thompson <kst-u@mib.org> - 2018-02-21 13:09 -0800
                Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-21 15:15 -0800
                Re: Opinion sought: Is this strictly conforming C? Keith Thompson <kst-u@mib.org> - 2018-02-21 15:51 -0800
                Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-21 17:06 -0800
                Re: Opinion sought: Is this strictly conforming C? Keith Thompson <kst-u@mib.org> - 2018-02-21 17:46 -0800
                Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-22 00:18 -0800
                Re: Opinion sought: Is this strictly conforming C? Tim Rentsch <txr@alumni.caltech.edu> - 2018-02-21 22:14 -0800
                Re: Opinion sought: Is this strictly conforming C? Tim Rentsch <txr@alumni.caltech.edu> - 2018-02-21 11:39 -0800
                Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-21 14:59 -0800
                Re: Opinion sought: Is this strictly conforming C? Richard Damon <Richard@Damon-Family.org> - 2018-02-21 22:14 -0500
                Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-22 00:50 -0800
                Re: Opinion sought: Is this strictly conforming C? Richard Damon <Richard@Damon-Family.org> - 2018-02-22 07:22 -0500
                Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-22 12:40 -0800
                Re: Opinion sought: Is this strictly conforming C? Keith Thompson <kst-u@mib.org> - 2018-02-22 13:00 -0800
                Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-22 13:48 -0800
                Re: Opinion sought: Is this strictly conforming C? scott@slp53.sl.home (Scott Lurndal) - 2018-02-22 22:14 +0000
                Re: Opinion sought: Is this strictly conforming C? Tim Rentsch <txr@alumni.caltech.edu> - 2018-02-21 23:05 -0800
          Re: Opinion sought: Is this strictly conforming C? Tim Rentsch <txr@alumni.caltech.edu> - 2018-02-21 23:39 -0800
            Re: Opinion sought: Is this strictly conforming C? Keith Thompson <kst-u@mib.org> - 2018-02-22 10:39 -0800
              Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-22 13:20 -0800
              Re: Opinion sought: Is this strictly conforming C? asetofsymbols@gmail.com - 2018-02-23 00:30 -0800
    Re: Opinion sought: Is this strictly conforming C? Manfred <noname@invalid.add> - 2018-02-16 20:05 +0100
      Re: Opinion sought: Is this strictly conforming C? lee@ansync.com - 2018-02-16 11:14 -0800
      Re: Opinion sought: Is this strictly conforming C? Tim Rentsch <txr@alumni.caltech.edu> - 2018-02-19 11:30 -0800
      Re: Opinion sought: Is this strictly conforming C? Steve Carroll <frelwizzen@gmail.com> - 2018-03-01 09:01 -0800
  Re: Opinion sought: Is this strictly conforming C? Tim Rentsch <txr@alumni.caltech.edu> - 2018-02-19 11:08 -0800
    Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-19 17:16 -0800
      Re: Opinion sought: Is this strictly conforming C? Tim Rentsch <txr@alumni.caltech.edu> - 2018-02-21 23:16 -0800
        Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-22 00:25 -0800
          Re: Opinion sought: Is this strictly conforming C? Richard Damon <Richard@Damon-Family.org> - 2018-02-22 07:41 -0500
          Re: Opinion sought: Is this strictly conforming C? Tim Rentsch <txr@alumni.caltech.edu> - 2018-02-24 13:24 -0800
            Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-25 12:07 -0800
              Re: Opinion sought: Is this strictly conforming C? Richard Damon <Richard@Damon-Family.org> - 2018-02-25 18:34 -0500
                Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-26 08:21 -0800
                Re: Opinion sought: Is this strictly conforming C? Richard Damon <Richard@Damon-Family.org> - 2018-02-26 22:22 -0500
                Re: Opinion sought: Is this strictly conforming C? David Brown <david.brown@hesbynett.no> - 2018-02-27 09:16 +0100
                Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-27 08:48 -0800
                Re: Opinion sought: Is this strictly conforming C? David Brown <david.brown@hesbynett.no> - 2018-02-28 10:24 +0100
                Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-28 09:03 -0800
                Re: Opinion sought: Is this strictly conforming C? David Brown <david.brown@hesbynett.no> - 2018-02-28 22:31 +0100
                Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-28 15:19 -0800
                Re: Opinion sought: Is this strictly conforming C? David Brown <david.brown@hesbynett.no> - 2018-03-01 12:37 +0100
                Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-03-01 11:03 -0800
                Re: Opinion sought: Is this strictly conforming C? Keith Thompson <kst-u@mib.org> - 2018-03-01 11:22 -0800
                Re: Opinion sought: Is this strictly conforming C? Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-01 11:48 -0800
                Re: Opinion sought: Is this strictly conforming C? Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-01 15:11 -0800
                Re: Opinion sought: Is this strictly conforming C? Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-01 16:14 -0800
                Re: Opinion sought: Is this strictly conforming C? Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-01 19:53 -0800
                Re: Opinion sought: Is this strictly conforming C? Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-01 21:27 -0800
                Re: Opinion sought: Is this strictly conforming C? Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-02 00:52 -0800
                Re: Opinion sought: Is this strictly conforming C? Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-02 01:54 -0800
                Re: Opinion sought: Is this strictly conforming C? Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-03 06:11 -0800
                Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-03-01 12:31 -0800
                Re: Opinion sought: Is this strictly conforming C? Keith Thompson <kst-u@mib.org> - 2018-03-01 13:15 -0800
                Re: Opinion sought: Is this strictly conforming C? Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-01 14:55 -0800
                Re: Opinion sought: Is this strictly conforming C? Tim Rentsch <txr@alumni.caltech.edu> - 2018-03-03 02:19 -0800
                Re: Opinion sought: Is this strictly conforming C? Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-03 03:03 -0800
              Re: Opinion sought: Is this strictly conforming C? Keith Thompson <kst-u@mib.org> - 2018-02-25 15:36 -0800
                Re: Opinion sought: Is this strictly conforming C? supercat@casperkitty.com - 2018-02-26 10:17 -0800

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