Re: Padding involved

From	Kaz Kylheku <kaz@kylheku.com>
Newsgroups	comp.lang.c
Subject	Re: Padding involved
Date	2014-03-28 19:54 +0000
Organization	Aioe.org NNTP Server
Message-ID	<20140328124409.987@kylheku.com> (permalink)
References	(5 earlier) <lnr46dv8ol.fsf@nuthaus.mib.org> <6c398cac-bf10-4213-b147-4a01ac23568f@googlegroups.com> <20140307165348.597@kylheku.com> <ejiaj9l37s39s39mnrf9rrrshaqr5mbb0k@4ax.com> <lh4g05$uu7$1@speranza.aioe.org>

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On 2014-03-28, glen herrmannsfeldt <gah@ugcs.caltech.edu> wrote:
> David Thompson <dave.thompson2@verizon.net> wrote:
>
> (snip)
>
>> No, the reverse. The offset must divide the size, or the size must be
>> divisible by the offset. E.g. struct { long a; float b; } if both long
>> and float are 4 bytes (as is common, though not universal and not
>> required by the Standard) then the struct has size at least 8 but is
>> unlikely to have alignment more than 4.
>
> (snip)
>
>> Note that alignment can be less than size, most commonly on systems
>> that can align everything to 1, but I've used a system where int is 4
>> bytes and aligned to 2. Alignment cannot be more than size.
>
> As I understand it, for some systems an alignment greater than
> size is necessary for optimal use.

C does not support this. C compilers can support extra alignment for efficient
access in the way local variables are laid out and perhaps struct members.

No such thing will be supported for arrays and pointers.

If a greater alignment than size is required for correctness, then misaligned
access for pointers and arrays must be implemented.

E.g. if a short is two bytes, but must be aligned on a four-byte boundary, then
code generates for array indexing and pointer dereferncing has to somehow
handle the accesses at odd indices. Perhaps by rounding down to an address
divisible by four, loading a four byte word, and then shifting down
the half-word.

The guys who designed C were no strangers to machines that didn't provide
access to certain small types such as characters.

In fact, the B language, predecessor to C, handled strings similarly to C:
characters were packed into arrays of cells, which had to be unpacked and
re-packed by routines.

   http://cm.bell-labs.com/who/dmr/chist.html

  "[B's] character-handling mechanisms, inherited with few changes from BCPL,
   were clumsy: using library procedures to spread packed strings into
   individual cells and then repack, or to access and replace individual
   characters, began to feel awkward, even silly, on a byte-oriented machine. "

So at that point Ritchie went for an addressable character type.
That can basically be seen as the point of departure at which the design
of C shifted toward "every type, down to the character/byte, is accessible at
an address that is no more strictly aligned than a multiple of its size".

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

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