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Re: struct alignment

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On 18-Jan-12 11:03, Jorgen Grahn wrote:
> On Thu, 2012-01-12, Keith Thompson wrote:
>> Ian Collins <ian-news@hotmail.com> writes:
>>> On 01/12/12 07:56 PM, Dr Nick wrote:
>>>> Keith Thompson<kst-u@mib.org>  writes:
>>>>> The compiler will (in fact, it *must*) insert whatever padding is
>>>>> necessary so that all the members can be accesssed correctly.  It may or
>>>>> may not insert more padding than it strictly needs to.
>>>>>
>>>>> The language standard guarantees that the first declared member is at
>>>>> offset 0, and that all other members are at increasing offsets in the
>>>>> order in which they're declared (the compiler can't rearrange
>>>>> members).
>>>>
>>>> Does any compiler let you do this?  I have done things where the order
>>>> of a least some elements (usually the first or the last) matters, but
>>>> it's rare.
>>>
>>> Let you do what?  I didn't see anything optional in Keith's reply.
>>
>> Permit reordering of struct members, I presume.  Plenty of C
>> compilers have plenty of options that break standard compliance.
> 
> But is it really breaking compliance?
> 
>   struct Foo {
>       int  a;
>       char b;
>       int  c;
>       char d;
>   };
> 
> If the compiler lays these out as a-c-b-d, is that really something
> which is observable using correct C code?

Yes, using offsetof().  And there are plenty of cases where this
actually matters in real-world code.

> I seem to recall (but I can be wrong) that the expression
> &foo.b < &foo.d is not guaranteed to be meaningful, unlike e.g.
> comparisons inside an array.

It's guaranteed to be meaningful.

N1256, 6.5.8.5:
"If the objects pointed to are members of the same aggregate object,
pointers to structure members declared later compare greater than
pointers to members declared earlier in the structure, ..."

Reordering structure members is therefore a constraint violation, and
any compiler that does it is non-compliant.

S

-- 
Stephen Sprunk         "God does not play dice."  --Albert Einstein
CCIE #3723         "God is an inveterate gambler, and He throws the
K5SSS        dice at every possible opportunity." --Stephen Hawking

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struct alignment "Mark" <mark_cruzNOTFORSPAM@hotmail.com> - 2012-01-11 13:31 -0500
  Re: struct alignment Kleuske <kleuske@nowhere.net> - 2012-01-11 19:23 +0000
  Re: struct alignment "Scott Fluhrer" <sfluhrer@ix.netcom.com> - 2012-01-11 16:58 -0500
  Re: struct alignment Ralf Damaschke <rwspam@gmx.de> - 2012-01-11 22:08 +0000
  Re: struct alignment Keith Thompson <kst-u@mib.org> - 2012-01-11 14:42 -0800
    Re: struct alignment Dr Nick <3-nospam@temporary-address.org.uk> - 2012-01-12 06:56 +0000
      Re: struct alignment Ian Collins <ian-news@hotmail.com> - 2012-01-12 20:42 +1300
        Re: struct alignment Keith Thompson <kst-u@mib.org> - 2012-01-12 02:20 -0800
          Re: struct alignment Jorgen Grahn <grahn+nntp@snipabacken.se> - 2012-01-18 17:03 +0000
            Re: struct alignment Kaz Kylheku <kaz@kylheku.com> - 2012-01-18 17:21 +0000
              Re: struct alignment jgk@panix.com (Joe keane) - 2012-01-19 21:42 +0000
                Re: struct alignment blp@cs.stanford.edu (Ben Pfaff) - 2012-01-19 13:57 -0800
            Re: struct alignment Stephen Sprunk <stephen@sprunk.org> - 2012-01-18 11:57 -0600
              Re: struct alignment Keith Thompson <kst-u@mib.org> - 2012-01-18 14:28 -0800
            Re: struct alignment Robert Wessel <robertwessel2@yahoo.com> - 2012-01-18 13:10 -0600
              Re: struct alignment Jorgen Grahn <grahn+nntp@snipabacken.se> - 2012-01-19 11:34 +0000
                Re: struct alignment Robert Wessel <robertwessel2@yahoo.com> - 2012-01-19 14:50 -0600
                Re: struct alignment Jorgen Grahn <grahn+nntp@snipabacken.se> - 2012-01-21 11:49 +0000

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