Re: Padding involved

From	James Kuyper <jameskuyper@verizon.net>
Newsgroups	comp.lang.c
Subject	Re: Padding involved
Date	2014-03-31 17:53 -0400
Organization	Self
Message-ID	<5339E3CE.4030403@verizon.net> (permalink)
References	(14 earlier) <lh5mmu$7oq$1@dont-email.me> <lh6fjv$lva$1@dont-email.me> <lh71r5$shv$1@dont-email.me> <lha8ck$dng$1@dont-email.me> <lhcm73$i1n$1@dont-email.me>

Show all headers | View raw

On 03/31/2014 05:20 PM, Stephen Sprunk wrote:
> On 30-Mar-14 18:12, James Kuyper wrote:
>> On 03/29/2014 02:02 PM, Stephen Sprunk wrote:
...
>>> It seems obvious that, for array indexing to work, a struct's alignment
>>> must be at least as strict as any of its members, but I can't find the
>>> specific text in N1570 that says so.
>>
>> There is no text that says so directly. It's something that can be
>> derived from what the standard says about arrays, the offsetof() macro
>> (which implies that the offset is a constant depending only upon the
>> struct definition, rather than being dependent upon which instance of
>> the struct is being referred to), and alignment requirements.
> 
> That seems sufficient, but wouldn't it be simpler for them to just say
> so explicitly if that was the intent?

Yes, and I wouldn't object to adding such text, at least as a footnote.

However, realize that there's a wide variety of other interesting
mathematical results that can be derived from those same facts. For
instance, in C2011, the requirement was added that alignment
requirements had to be powers of 2, which matches what essentially all
known implementations of C actually do, or have ever actually done.
However, prior to that change, a conforming implementation could have
had alignment requirements for short and long that were 3 bytes and 5
bytes, respectively. From C99, it could be proven, using the same set of
facts mentioned above, that any struct or union containing both a short
and a long must have a alignment requirement when using that
implementation, which is an integer multiple of both 3 and 5, and
therefore must be a multiple of 15.

Should the C99 standard have explicitly said something that would lead
to that conclusion more directly, perhaps by expressing the relationship
in terms of LCM (least common multiple)? I know from personal experience
that very few people were aware of that fact.  More than a few people
argued with me about this when I brought it up. I think this was mainly
because most of them had the "power of 2" rule already engraved in their
brains, long before it was officially adopted as a requirement. When a
and b are non-negative integer powers of two, LCM(a,b) == MAX(a,b),
which is what most people actually used when thinking about such issues.

How do you decide which technically redundant facts need to be
explicitly spelled out, and which ones should be left for readers to
derive for themselves?

Thread

Padding involved anish singh <anish198519851985@gmail.com> - 2014-03-07 13:13 -0800
  Re: Padding involved jacob navia <jacob@spamsink.net> - 2014-03-07 22:17 +0100
  Re: Padding involved Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-03-07 16:26 -0500
    Re: Padding involved anish singh <anish198519851985@gmail.com> - 2014-03-07 13:49 -0800
      Re: Padding involved Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-03-07 18:11 -0500
        Re: Padding involved anish kumar <yesanishhere@gmail.com> - 2014-03-07 15:19 -0800
          Re: Padding involved Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2014-03-07 16:26 -0700
            Re: Padding involved anish kumar <yesanishhere@gmail.com> - 2014-03-07 15:41 -0800
              Re: Padding involved James Kuyper <jameskuyper@verizon.net> - 2014-03-07 18:50 -0500
            Re: Padding involved glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-03-08 00:10 +0000
              Re: Padding involved Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2014-03-08 09:27 -0700
          Re: Padding involved Keith Thompson <kst-u@mib.org> - 2014-03-07 16:24 -0800
            Re: Padding involved anish kumar <yesanishhere@gmail.com> - 2014-03-07 16:45 -0800
              Re: Padding involved Kaz Kylheku <kaz@kylheku.com> - 2014-03-08 01:03 +0000
                Re: Padding involved David Thompson <dave.thompson2@verizon.net> - 2014-03-28 06:11 -0400
                Re: Padding involved glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-03-28 18:45 +0000
                Re: Padding involved James Kuyper <jameskuyper@verizon.net> - 2014-03-28 15:23 -0400
                Re: Padding involved Keith Thompson <kst-u@mib.org> - 2014-03-28 12:40 -0700
                Re: Padding involved glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-03-28 21:19 +0000
                Re: Padding involved Stephen Sprunk <stephen@sprunk.org> - 2014-03-28 17:07 -0500
                Re: Padding involved Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-03-28 18:25 -0400
                Re: Padding involved Stephen Sprunk <stephen@sprunk.org> - 2014-03-28 21:15 -0500
                Re: Padding involved Keith Thompson <kst-u@mib.org> - 2014-03-29 01:03 -0700
                Re: Padding involved Tim Rentsch <txr@alumni.caltech.edu> - 2014-03-29 14:23 -0700
                Re: Padding involved glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-03-30 01:43 +0000
                Re: Padding involved James Kuyper <jameskuyper@verizon.net> - 2014-03-28 21:32 -0400
                Re: Padding involved Richard Damon <Richard@Damon-Family.org> - 2014-03-28 21:59 -0400
                Re: Padding involved Stephen Sprunk <stephen@sprunk.org> - 2014-03-29 00:46 -0500
                Re: Padding involved James Kuyper <jameskuyper@verizon.net> - 2014-03-29 08:51 -0400
                Re: Padding involved Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-03-29 08:57 -0400
                Re: Padding involved Stephen Sprunk <stephen@sprunk.org> - 2014-03-30 18:53 -0500
                Re: Padding involved Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-03-30 20:25 -0400
                Re: Padding involved Stephen Sprunk <stephen@sprunk.org> - 2014-03-31 11:34 -0500
                Re: Padding involved Keith Thompson <kst-u@mib.org> - 2014-03-31 11:27 -0700
                Re: Padding involved Keith Thompson <kst-u@mib.org> - 2014-03-31 11:36 -0700
                Re: Padding involved Kaz Kylheku <kaz@kylheku.com> - 2014-03-31 19:14 +0000
                Re: Padding involved Stephen Sprunk <stephen@sprunk.org> - 2014-03-31 15:43 -0500
                Re: Padding involved Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2014-04-01 03:12 +0300
                Re: Padding involved Stephen Sprunk <stephen@sprunk.org> - 2014-04-03 00:45 -0500
                Re: Padding involved Stephen Sprunk <stephen@sprunk.org> - 2014-03-29 13:02 -0500
                Re: Padding involved Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-03-29 14:44 -0400
                Re: Padding involved glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-03-29 19:49 +0000
                Re: Padding involved James Kuyper <jameskuyper@verizon.net> - 2014-03-30 19:12 -0400
                Re: Padding involved Stephen Sprunk <stephen@sprunk.org> - 2014-03-31 16:20 -0500
                Re: Padding involved James Kuyper <jameskuyper@verizon.net> - 2014-03-31 17:53 -0400
                Re: Padding involved Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2014-04-01 03:22 +0300
                Re: Padding involved Stephen Sprunk <stephen@sprunk.org> - 2014-04-04 14:36 -0500
                Re: Padding involved James Kuyper <jameskuyper@verizon.net> - 2014-04-04 17:33 -0400
                Re: Padding involved Tim Rentsch <txr@alumni.caltech.edu> - 2014-04-14 13:46 -0700
                Re: Padding involved glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-14 22:00 +0000
                Re: Padding involved Keith Thompson <kst-u@mib.org> - 2014-04-14 15:37 -0700
                Re: Padding involved Tim Rentsch <txr@alumni.caltech.edu> - 2014-04-17 09:11 -0700
                Re: Padding involved Tim Rentsch <txr@alumni.caltech.edu> - 2014-04-14 13:25 -0700
                Re: Padding involved Keith Thompson <kst-u@mib.org> - 2014-03-29 00:57 -0700
                Re: Padding involved James Kuyper <jameskuyper@verizon.net> - 2014-03-29 08:57 -0400
                Re: Padding involved Keith Thompson <kst-u@mib.org> - 2014-03-29 14:18 -0700
                Re: Padding involved Tim Rentsch <txr@alumni.caltech.edu> - 2014-03-30 12:02 -0700
                Re: Padding involved Keith Thompson <kst-u@mib.org> - 2014-03-30 19:35 -0700
                Re: Padding involved Tim Rentsch <txr@alumni.caltech.edu> - 2014-04-14 12:54 -0700
                Re: Padding involved Tim Rentsch <txr@alumni.caltech.edu> - 2014-03-29 14:44 -0700
                Re: Padding involved glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-03-29 03:50 +0000
                Re: Padding involved James Kuyper <jameskuyper@verizon.net> - 2014-03-29 09:02 -0400
                Re: Padding involved Stephen Sprunk <stephen@sprunk.org> - 2014-03-29 13:19 -0500
                Re: Padding involved James Kuyper <jameskuyper@verizon.net> - 2014-03-28 21:21 -0400
                Re: Padding involved Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-03-28 15:27 -0400
                Re: Padding involved Kaz Kylheku <kaz@kylheku.com> - 2014-03-28 19:54 +0000
                Re: Padding involved Stephen Sprunk <stephen@sprunk.org> - 2014-03-28 15:02 -0500
          Re: Padding involved Kaz Kylheku <kaz@kylheku.com> - 2014-03-08 00:43 +0000
          Re: Padding involved Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-03-07 20:20 -0500
    Re: Padding involved anish singh <anish198519851985@gmail.com> - 2014-03-07 13:49 -0800
  Re: Padding involved James Kuyper <jameskuyper@verizon.net> - 2014-03-07 16:46 -0500
  Re: Padding involved "BartC" <bc@freeuk.com> - 2014-03-07 21:59 +0000
    Re: Padding involved anish kumar <yesanishhere@gmail.com> - 2014-03-07 14:34 -0800
      Re: Padding involved "BartC" <bc@freeuk.com> - 2014-03-07 23:59 +0000
    Re: Padding involved Keith Thompson <kst-u@mib.org> - 2014-03-07 15:15 -0800
      Re: Padding involved "BartC" <bc@freeuk.com> - 2014-03-08 10:08 +0000
        Re: Padding involved Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-03-08 12:39 +0000
        Re: Padding involved Keith Thompson <kst-u@mib.org> - 2014-03-08 14:03 -0800
  Re: Padding involved Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-03-08 09:01 -0800

csiph-web