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From: Stephen Leake <stephen_leake@stephe-leake.org>
Newsgroups: comp.lang.ada
Subject: Re: Using representation clauses in networking software
References: <8739ugqfeb.fsf@mid.deneb.enyo.de> <op.vhhe4kqfule2fv@garhos>
 	<43fkp7an4c5m$.3db3n6adym42.dlg@40tude.net>
Date: Mon, 16 Aug 2010 06:57:46 -0400
Message-ID: <82eidyvn85.fsf@stephe-leake.org>
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Date: 2010-08-16T06:57:46-04:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> On Sun, 15 Aug 2010 15:44:34 +0200, Yannick Duchêne (Hibou57) wrote:
>
> (I agree with what you wrote. I am programming a lot of communication
> stuff, but never used representation clauses to handle endianness.)
>
>> I do not know neither any representation clause for Byte Ordering. 
>
> Byte ordering is what S'Bit_Ordering is, when bytes are addressable.

Not exactly.

> IMO, Bit_Ordering was an unfortunate choice. The attribute name suggests
> ordering of bits in some machine storage unit, which it is not.

Bit_ordering specifies how to interpret the bit numbers in Ada record
representation clauses.

The restriction of 'Bit_Order only being valid for the target endianness
was removed in Ada 2005.

It is useful for representing record layouts that may be used in
systems with different byte orders.

See http://www.ada-auth.org/cgi-bin/cvsweb.cgi/ais/ai-00133.txt?rev=1.17
for a good discussion. The concept of 'machine scalars' is important.
Only part of this discussion made it into the ARM or AARM, so they are
hard to understand.

Still, S'Bit_ordering does _not_ solve the inter-machine byte endianness
problem. It could be part of a solution.

-- 
-- Stephe