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From: "Randy Brukardt" <randy@rrsoftware.com>
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 17:32:12 -0500
Organization: JSA Research & Innovation
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Xref: reader02.eternal-september.org comp.lang.ada:52302
Date: 2018-05-11T17:32:12-05:00
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"Dan'l Miller" <optikos@verizon.net> wrote in message 
news:3c2c787e-227e-4066-b397-47b23bad3c43@googlegroups.com...
...
>> Normally a network programmer wouldn't do that.
>
>My point is normally a network programmer does exactly what you
>describe ..in C.., not Ada, because the engineer years or decades ago
>established that C's macros and pointer arithmetic can do what Ada's
>record representation clauses cannot, and C's macros and pointer
>arithmetic can do what is disdained as uncouth to be performed by
> Ada's various accesses: byte manipulations.

A pretty silly programmer, if you ask me. An experienced Ada programmer 
wouldn't use macros and pointer arithmetic because the job can be trivially 
done with Unchecked_Conversion. (Jeff Carter showed how in another message). 
I've done so several times in my career, and it works fine and even isn't 
expensive if implemented correctly. (Unfortunately, GNAT doesn't take 
advantage of the permissions to share, so it does a lot more operations than 
necessary - or at least it did a few years back.)

Using a representation clause to swap bytes might slightly simplify some 
code, but it is hugely expensive to (non-GNAT) implementations, prone to 
misuse, and it is very rarely used, so the cost-benefit ratio is out of 
whack.

                                      Randy.