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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: AI12-0218:  What is the portable representation clause for
 processing IETF packets on little-endian machines?
Date: Thu, 10 May 2018 20:28:49 +0100
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Date: 2018-05-10T20:28:49+01:00
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"Dan'l Miller" <optikos@verizon.net> writes:

> so the ••representation clause•• in AI12-0218 (on only GNAT for now)
> is the only game in Adatown

I don't have a reference, but there was a cunning plan once involving
the fact that Boolean'Pos (False) is 0, and Boolean'Pos (True) is 1. So
you could write expressions involving

   BE : constant :=
      Boolean'Pos (System.Default_Bit_Order = System.High_Order_First);

& likewise for LE, and you could then write representation components as

   (le-value * LE + be-value * BE)

(something like that, anyway). Ugh.

But you still have to byte-swap.

The worst case I came across was where the remote system was LE, the
network was LE (there's British for you), we were BE, and the interface
device - in a spirit of helpfulness - did byte-swapping for you assuming
that the whole message consisted of 16-bit values (because the message
header did, and that was the only bit they needed to deal with; they
were on a BE device). Bad luck if one of the message fields was 32-bits.

It is a pain in the neck, and something that newer programmers found
hard to get their heads round, but it is only needed at the edges of the
system (letting byte-swapping inside the system proper is going to be
very expensive).