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From: Florian Weimer <fw@deneb.enyo.de>
Newsgroups: comp.lang.ada
Subject: Re: AdaYY wish list from current projects
Date: 23 Feb 2001 16:04:21 +0100
Organization: Enyo's not your organization
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References: <96ulad$r9e$1@belenus.iks-jena.de> <87u25nnfer.fsf@deneb.enyo.de>
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Date: 2001-02-23T16:04:21+01:00
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lutz@iks-jena.de (Lutz Donnerhacke) writes:

> Yep. I talk about values crossing a storage_element boundary and you
> talk about invalid values.

Oh, sorry.

I still have to see some code which shows that record representation
clauses are practicall even if the components occupy multiple storage
elements. ;-)

> 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?

> >> 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.