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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: Fri, 11 May 2018 09:02:06 +0100
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Xref: reader02.eternal-september.org comp.lang.ada:52250
Date: 2018-05-11T09:02:06+01:00
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Paul Rubin <no.email@nospam.invalid> writes:

> "Dan'l Miller" <optikos@verizon.net> writes:
>> Btw, bitfields in C and in Erlang are •the• roughly analogous language
>> feature corresponding to representation clauses in Ada
[...]
> That is, if you use a hypothetical endianness representation clause to
> declare something like "type bigint32 is int32 big_endian", then a
> record containing "x, y : bigint32" will actually require x and y to
> be stored in memory in big-endian byte order, and will show up that
> way if you dump the memory region to disk.
>
> That particularly means if the hardware happens to be little-endian
> and you want to add x and y together with a 32-bit "add" machine
> instruction, you are requiring the compiler to generate code that
> reads x and y from memory and performs byte swaps before doing the
> addition.

One would hope that the design review would catch this mistake.

> I've never seen a network program do it like that though.  Everyone
> just makes sure that network integers are converted to the host format
> before being stored in memory, and converted back when sent over the
> network.

Which is exactly what a competent Ada team would do, too. No different
from ensuring that the units are sensible: just because the network data
is in feet and pounds is no reason for the system to work internally in
other than metric units.