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Re: Union test for endianess

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"christian.bau" <christian.bau@cbau.wanadoo.co.uk> writes:
> On Jun 17, 2:32 pm, James Kuyper <jameskuy...@verizon.net> wrote:
>> Neither C nor C++ use the term legal. It contains no syntax errors, it
>> has no a constraint violations, no diagnostics are required, and the
>> behavior is not undefined, according to the rules of either language. In
>> C++ it qualifies as "well-formed code". The closest comparable term in C
>> is "strictly conforming", but it doesn't qualify for that: it produces
>> different results on different platforms, which is the whole point of
>> this particular program, but such platform dependence is prohibited for
>> strictly conforming programs.
>
> Is there actually any guarantee in the C Standard that there is such a
> thing as "byte order"? I thought with 32 bit ints stored in 32 bits
> worth of bytes there would be 32! (32 factorial) possible bit orders?

C99 6.2.6.1 requires a "pure binary notation" for unsigned bit-fields
and objects of type unsigned char, with a footnote:

    A positional representation for integers that uses the binary
    digits 0 and 1, in which the values represented by successive
    bits are additive, begin with 1, and are multiplied by successive
    integral powers of 2,  except perhaps the bit with the highest
    position.

The words "positional" and "successive" imply to me that only two
bit orders are permitted for unsigned char.  (Or perhaps just one;
it's not at all clear that there's even any meaning to the positions
of the bits beyond the values they represent.)

6.2.6.2, discussing the representation of unsigned integer types,
again uses the phrase "pure binary notation".

Each integer type is required to have the same values for its value
bits as the corresponding bits in the corresponding unsigned type.

Though it's not 100% clear what "successive" means.  I supppose
it could just mean traversing the bits in order of the values they
represent, which isn't necessarily the same as either the order of
the bits in the constituent bytes or the physical order (if that's
even meaningful).

I think that either it permits 32 factorial bit orders for a 32-bit
integer, or it forbids PDP-11 middle-endian order (and I seriously
doubt that the latter was intended.)

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Nokia
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Union test for endianess Bhasker Penta <bskdsp@gmail.com> - 2011-06-16 20:16 -0700
  Re: Union test for endianess Ian Collins <ian-news@hotmail.com> - 2011-06-17 15:36 +1200
    Re: Union test for endianess Willem <willem@toad.stack.nl> - 2011-06-17 14:13 +0000
      Re: Union test for endianess Joe Wright <joewwright@comcast.net> - 2011-06-17 14:00 -0400
        Re: Union test for endianess Jorgen Grahn <grahn+nntp@snipabacken.se> - 2011-06-19 19:25 +0000
  Re: Union test for endianess China Blue Angels <chine.bleu@yahoo.com> - 2011-06-16 20:54 -0700
    Re: Union test for endianess Willem <willem@toad.stack.nl> - 2011-06-17 14:14 +0000
      Re: Union test for endianess James Kuyper <jameskuyper@verizon.net> - 2011-06-17 10:28 -0400
        Re: Union test for endianess Keith Thompson <kst-u@mib.org> - 2011-06-17 10:54 -0700
      Re: Union test for endianess Shao Miller <sha0.miller@gmail.com> - 2011-06-17 10:35 -0500
      Re: Union test for endianess "J. J. Farrell" <jjf@bcs.org.uk> - 2011-06-18 01:50 +0100
  Re: Union test for endianess China Blue Angels <chine.bleu@yahoo.com> - 2011-06-16 21:00 -0700
    Re: Union test for endianess Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-06-17 12:09 +0100
  Re: Union test for endianess Shao Miller <sha0.miller@gmail.com> - 2011-06-17 00:50 -0500
    Re: Union test for endianess Bhasker Penta <bskdsp@gmail.com> - 2011-06-16 23:17 -0700
      Re: Union test for endianess Shao Miller <sha0.miller@gmail.com> - 2011-06-17 03:01 -0500
        Re: Union test for endianess Shao Miller <sha0.miller@gmail.com> - 2011-06-17 13:33 -0500
          Re: Union test for endianess James Kuyper <jameskuyper@verizon.net> - 2011-06-18 09:18 -0400
          Re: Union test for endianess Shao Miller <sha0.miller@gmail.com> - 2011-06-19 17:22 -0500
          Re: Union test for endianess Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-19 16:24 -0700
        Re: Union test for endianess Bhasker Penta <bskdsp@gmail.com> - 2011-06-18 01:08 -0700
        Re: Union test for endianess Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-19 15:52 -0700
  Re: Union test for endianess James Kuyper <jameskuyper@verizon.net> - 2011-06-17 09:32 -0400
    Re: Union test for endianess James Kuyper <jameskuyper@verizon.net> - 2011-06-17 09:52 -0400
      Re: Union test for endianess Bhasker Penta <bskdsp@gmail.com> - 2011-06-17 07:13 -0700
        Re: Union test for endianess James Kuyper <jameskuyper@verizon.net> - 2011-06-17 10:31 -0400
    Re: Union test for endianess "christian.bau" <christian.bau@cbau.wanadoo.co.uk> - 2011-06-17 15:57 -0700
      Re: Union test for endianess Keith Thompson <kst-u@mib.org> - 2011-06-17 16:42 -0700
        Re: Union test for endianess James Waldby <not@valid.invalid> - 2011-06-18 02:53 +0000
          Re: Union test for endianess James Kuyper <jameskuyper@verizon.net> - 2011-06-17 23:59 -0400
            Re: Union test for endianess James Waldby <not@valid.invalid> - 2011-06-18 14:45 +0000
              Re: Union test for endianess James Kuyper <jameskuyper@verizon.net> - 2011-06-18 11:08 -0400
              Re: Union test for endianess cri@tiac.net (Richard Harter) - 2011-06-18 19:47 +0000
                Re: Union test for endianess James Waldby <not@valid.invalid> - 2011-06-18 23:15 +0000
                Re: Union test for endianess James Kuyper <jameskuyper@verizon.net> - 2011-06-18 22:42 -0400
              Re: Union test for endianess Jens <jens.gustedt@gmail.com> - 2011-06-19 07:41 -0700
              Re: Union test for endianess Roberto Waltman <usenet@rwaltman.com> - 2011-06-22 11:25 -0400
                Re: Union test for endianess James Kuyper <jameskuyper@verizon.net> - 2011-06-22 11:40 -0400
                Re: Union test for endianess James Waldby <not@valid.invalid> - 2011-06-22 21:42 +0000
      Re: Union test for endianess James Kuyper <jameskuyper@verizon.net> - 2011-06-17 21:18 -0400
  Re: Union test for endianess falk@rahul.net (Edward A. Falk) - 2011-06-17 20:50 +0000
  Re: Union test for endianess "Morris Keesan" <mkeesan@post.harvard.edu> - 2011-06-18 10:42 -0400
    Re: Union test for endianess Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-06-18 22:55 +0100
      Re: Union test for endianess "Morris Keesan" <mkeesan@post.harvard.edu> - 2011-06-19 21:14 -0400
  Re: Union test for endianess Jens <jens.gustedt@gmail.com> - 2011-06-19 07:46 -0700
    Re: Union test for endianess Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-19 16:14 -0700
    Re: Union test for endianess Shao Miller <sha0.miller@gmail.com> - 2011-06-19 20:24 -0500
      Re: Union test for endianess Jens <jens.gustedt@gmail.com> - 2011-06-20 00:31 -0700
        Re: Union test for endianess Shao Miller <sha0.miller@gmail.com> - 2011-06-20 04:33 -0500
        Re: Union test for endianess Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-20 12:22 -0700

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