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Re: C11 6.3.2.3p7 Referenced Type

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On 1/4/2013 23:45, Wojtek Lerch wrote:
> On 04/01/2013 2:08 AM, Shao Miller wrote:
> ...
>
>>    "A pointer to an object type may be converted to a pointer to a
>> different object type. If the resulting pointer is not correctly
>> aligned for the referenced type, the behavior is undefined.
>
> What I don't like about this wording (both old and new) is that it talks
> about the alignment of the *resulting* pointer, even though the standard
> does not generally specify what the result of the conversion is (other
> than that it's a pointer that can be converted back; a few exceptions
> include conversions to char*, between pointers to a struct and to the
> type of its first member, etc.).  Would it be wrong to say, instead,
> that the behaviour is undefined if the *original* pointer, the one
> before the conversion, is not correctly aligned for the type referenced
> by the type of the result?  Or to say, separately, that if the original
> pointer satisfies the alignment requirements of some type, then the
> converted pointer satisfies them too?
>
> Or maybe even that the converted pointer points at the same byte in
> memory than the original pointer did (using some more formal wording)?
> I've seen claims that this follows from some other requirements of the
> standard (C99 at the time), but I never managed to find them.
>

Agreed.  That was part of the discussion I had yesterday or so...  My 
interpretation is that it means:

   "A pointer to an object type may be converted to a pointer to a 
different object type.  If the resulting pointer does not address 
storage that is correctly aligned for the referenced type of the 
converted pointer type, the behavior is undefined. ..."

C99 was a touch clearer, in my opinion, because it's not intuitive (to 
me, at least) that the _result_'s alignment requirement should have 
anything at all to do with "the pointed-to type", at least in part 
because it's at least sometimes an rvalue (whose alignment requirement 
doesn't seem intuitive to care about).  So I make the leap from:

   "A pointer to an object or incomplete type may be converted to a 
pointer to a different object or incomplete type. If the resulting 
pointer is not correctly aligned57) for the pointed-to type, the 
behavior is undefined. ..."

to:

   "A pointer to an object or incomplete type may be converted to a 
pointer to a different object or incomplete type. If the resulting 
pointer does not address storage that is correctly aligned57) for the 
pointed-to type of the converted pointer type, the behavior is 
undefined. ..."

A bit of a boggle, to be sure.

- Shao Miller

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C11 6.3.2.3p7 Referenced Type Shao Miller <sha0.miller@gmail.com> - 2013-01-04 02:08 -0500
  Re: C11 6.3.2.3p7 Referenced Type Keith Thompson <kst-u@mib.org> - 2013-01-04 08:09 -0800
    Re: C11 6.3.2.3p7 Referenced Type Shao Miller <sha0.miller@gmail.com> - 2013-01-04 12:25 -0500
    Re: C11 6.3.2.3p7 Referenced Type lawrence.jones@siemens.com - 2013-01-04 13:47 -0500
      Re: C11 6.3.2.3p7 Referenced Type Shao Miller <sha0.miller@gmail.com> - 2013-01-04 14:56 -0500
        Re: C11 6.3.2.3p7 Referenced Type lawrence.jones@siemens.com - 2013-01-04 17:10 -0500
          Re: C11 6.3.2.3p7 Referenced Type Shao Miller <sha0.miller@gmail.com> - 2013-01-04 20:34 -0500
  Re: C11 6.3.2.3p7 Referenced Type Wojtek Lerch <wojtek_l@yahoo.ca> - 2013-01-04 23:45 -0500
    Re: C11 6.3.2.3p7 Referenced Type Shao Miller <sha0.miller@gmail.com> - 2013-01-05 02:07 -0500
      Re: C11 6.3.2.3p7 Referenced Type Shao Miller <sha0.miller@gmail.com> - 2013-01-05 02:16 -0500
    Re: C11 6.3.2.3p7 Referenced Type Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-06 17:25 -0800
      Re: C11 6.3.2.3p7 Referenced Type Wojtek Lerch <wojtek_l@yahoo.ca> - 2013-01-06 22:13 -0500
        Re: C11 6.3.2.3p7 Referenced Type Shao Miller <sha0.miller@gmail.com> - 2013-01-06 23:30 -0500
          Re: C11 6.3.2.3p7 Referenced Type Wojtek Lerch <wojtek_l@yahoo.ca> - 2013-01-07 00:07 -0500
            Re: C11 6.3.2.3p7 Referenced Type Shao Miller <sha0.miller@gmail.com> - 2013-01-07 01:29 -0500
              Re: C11 6.3.2.3p7 Referenced Type Shao Miller <sha0.miller@gmail.com> - 2013-01-07 02:18 -0500
              Re: C11 6.3.2.3p7 Referenced Type Wojtek Lerch <wojtek_l@yahoo.ca> - 2013-01-10 17:45 -0500
          Re: C11 6.3.2.3p7 Referenced Type Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-06 22:30 -0800
            Re: C11 6.3.2.3p7 Referenced Type Shao Miller <sha0.miller@gmail.com> - 2013-01-07 01:58 -0500
              Re: C11 6.3.2.3p7 Referenced Type Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-06 23:16 -0800
        Re: C11 6.3.2.3p7 Referenced Type Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-06 22:02 -0800

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