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Date: Wed, 11 May 2005 08:19:55 -0500
From: "Marc A. Criley" <mcNOSPAM@mckae.com>
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Newsgroups: comp.lang.ada
Subject: Re: Sending Variable Length Messages with GNAT.Sockets
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Date: 2005-05-11T08:19:55-05:00
List-Id: <comp.lang.ada>

Simon Wright wrote:

> If the C side says
> 
>   struct rec {
>     char flag;
>     int count;
>   };
> 
> there will typically be 3 bytes of padding between flag & count, for 8
> bytes on the wire.
> 
> If the Ada side says
> 
>   type Rec is record
>      Flag : Interfaces.Unsigned_8;
>      Count : Interfaces.Integer_32;
>   end record;
>   pragma Convention (C, Rec);
> 
> the 'Write will output *5* bytes; but an unchecked conversion to
> Stream_Element_Array (1 .. 8) will succeed & do The Right Thing
> (endianness permitting).

This is why trying to use Ada.Streams "out-of-the-box" to communicate 
between Ada applications and those written in other languages is 
problematic at best.  You pretty much have to write your own stream I/O 
routines for each type to be transferred to get the data into an 
interoperable layout, especially for structured types like those in 
Simon's example.

I've pretty much given up on doing that, only using streams for 
intra-system Ada-to-Ada communication, and doing Unchecked_Conversions 
to Stream_Element_Array and using GNAT.Sockets for all external comms.

Marc A. Criley
www.mckae.com