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From: mheaney@on2.com (Matthew Heaney)
Newsgroups: comp.lang.ada
Subject: Re: Importing C structs?
Date: 31 Jul 2003 09:59:33 -0700
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Ching Bon Lam <c.lam_REMOVE_THIS@student.utwente.nl> wrote in message news:<Xns93C98A38494B8cblamstudentutwenten@194.109.133.20>...
> "Matthew Heaney" <matthewjheaney@earthlink.net> wrote in
> news:1V1Wa.299$jp.55@newsread4.news.pas.earthlink.net: 
> 
> > This is probably overkill.  For an opaque type all you need to do is:
> > 
> > type DB_Type is limited null record;
> > type DB_Access is access all DB_Type;
> > for DB_Access'Storage_Size use 0;
> > pragma Convention (C, DB_Access);
> 
> overkill? i don't think so.. opaque_structure_def is defined in 
> Interfaces.C.Extensions (i-cexten.ads in gnat) as:
> 
> subtype opaque_structure_def is System.Address;
> 
> just a memory address. That's the same as DB *. Since the api (well, most 
> api written in C) only use DB * and not DB, you don't need another access 
> type. that's why 

A memory address is not the same thing as DB*.  A "memory address" has
type void*.  A void* can point to anything.  A DB* can not.

You are free of course to use System.Address everywhere in Ada, just
as you are free to use void* everywhere in C, but you lose strong
typing that way.  There is a reason why the C API used type DB* and
not type void*.

So I stand by my assertion that type opaque_structure_def is not the
correct choice for a binding to dbopen().