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From: lutz@iks-jena.de (Lutz Donnerhacke)
Newsgroups: comp.lang.ada
Subject: Re: AdaYY wish list from current projects
Date: 23 Feb 2001 15:15:35 GMT
Organization: IKS GmbH Jena
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Date: 2001-02-23T15:15:35+00:00
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* Florian Weimer wrote:
>I still have to see some code which shows that record representation
>clauses are practicall even if the components occupy multiple storage
>elements. ;-)

Try to map an ARP packet. Have fun.
Try to map an Pascal String with a 16bit length field.

>lutz@iks-jena.de (Lutz Donnerhacke) writes:
>> procedure t is
>>    type pstring(len : Integer) is record
>>       data : String (1 .. len);
>>    end record;
>>    pragma Pack (pstring); --  record representation clause not possible
>>    
>>    ex : pstring;  --  Won't work;
>>    for ex'Address use 16#00000#;
>> begin
>>    Put_Line (Integer'Image (ex.len) & " => " & ex.data);
>> end t;
>
>Perhaps you want to use a constrained integer type in conjunction with
>a record discriminant with a default expression?

  ex : pstring(x); for ex'Address use 16#00000#;
will compile, but ex.len is always x and not correlated to (16#00000#).

>>>> The problem escaped from a 'multiprotocol router'. Please try to
>>>> map a IPv4 header incl. payload and IP-header options.
>>>
>>>Ah.  Well, obviously, you can't use a single record in this case.
>>
>> So I'm stuck to implement gather-scatter from the beginning. ;-/
>
>Why that?  You can still have the records in consecutive storage
>locations, but you have to do some manual address calculations.

And than store those results in some meta data records called 'Packet.IP.v4'.
http://www.iks-jena.de/mitarb/lutz/ada/net/