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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Sending Variable Length Messages with GNAT.Sockets
Date: Sat, 14 May 2005 12:12:43 +0100
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Date: 2005-05-14T12:12:43+01:00
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Jacob Sparre Andersen <sparre@nbi.dk> writes:

> Simon Wright wrote:
>
>> On our side there's quite a lot of generated code to cope with
>> mapping Booleans to packed arrays of bits, driven by UML tags in the
>> models, but once the data is in the 32-bit form we do indeed use
>> Streams.
>
> Why not use representation clauses for doing those mappings?  (or is
> the generated code actually mostly generated representation clauses?)

Because rep clauses are endian-dependent. I know that there are
techniques for writing bit offsets in an endian-independent way, but
if humans are involved it's often easier (for both writing and
reviewing) to have an Unsigned_32 and do shifts and masks in the good
old C way (IME).

I have used the

  in the spec
     declare un-represented record

  in the body
     read raw bytes
     if big-endian then
        declare
           big-endian represented derived record
           unchecked conversion from raw bytes to represented record
        begin
           convert raw bytes to represented record
           return conversion from represented record to un-represented record
        end
     else
        the same for the little-endian representation

technique, but it's a lot of work!