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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Using representation clauses in networking software
Newsgroups: comp.lang.ada
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Date: Sun, 15 Aug 2010 16:32:47 +0200
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Date: 2010-08-15T16:32:45+02:00
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On Sun, 15 Aug 2010 15:44:34 +0200, Yannick Duch�ne (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.

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

> Just to try to meet your request (something else later) : I do not know a  
> way to have a method which would automatically know the bit order of its  
> platform,

Bit order is useless if bits are not directly addressable.

> except perhaps receiving a standard data, like 16#1234#  
> (0x1234), and then check if it looks like 16#3412# or like 16#1234#.

This would determine byte ordering. Bit ordering is when you serialize a
sequence of bits {0, 0, 0, 0, 0, 0, 0, 1} and get 2#1000_0000# (little
endian) 2#0000_0001# (big endian).

Most types of hardware have opaque bytes compatible with the machine, e.g.
UARTs. If the hardware is not, then representation clause would not be my
choice anyway. I would recode bytes or whatever units immediately as
obtained from the device:

   type Strange_Octet is mod 2**8;
   Decode_Strange_Octet : array (Strange_Octet) of Unsigned_8 :=
      (  2#0000_0001# => 2#0000_1000#,
         2#0000_0010# => 2#0000_0100#,
         2#0000_0011# => 2#0000_1100#,
         ...
      );

> Or else you may suggest it for a future Ada revision (via this newsgroup  
> or else ada-auth.org ).

One could introduce enumeration representation clauses for modular types,
e.g.

   type Strange_Octet is mod 2**8;
   for Strange_Octet use (2#0000_0001# => 2#0000_1000#, ...);

Better would be to allow user-defined integer literals, so that the
programmer could define its own type.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de