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From: Thierry Lelegard <thierry.lelegard@canal-plus.fr>
Newsgroups: comp.lang.ada
Subject: Re: How pass a "null" as a parameter of anonymous access type
Date: Tue, 27 Mar 2001 18:53:32 +0200
Organization: CANAL+ Technologies
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Date: 2001-03-27T18:53:32+02:00
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> Freddy <Alfred.Hilscher.X@t-online.de> wrote in message
> news:3ABE6443.4119AB52@t-online.de...
> > Thanks. I've tried it, but at runtime I always get a constraint_error
> > (GNAT seems to check the passing of "null" values).
> 
> Yes -- as any correct Ada compiler must do!
> 
> The value "null" just does not exist for the type of the parameter.
> 
> Now, you might be able use "pragma Suppress (Access_Check)" to defeat this.

I don't agree. I don't use the value "null". I use a variable which
pretends to be at address zero.

The following example works with GNAT 3.15w (20010305) on Sun Solaris.
But, again, this is a hack, unsupported non-standard Ada.

$ cat tnull.adb
with System;

procedure Tnull is

    Null_Access : aliased Integer;
    for Null_Access'Address use System.Null_Address;

    procedure Tnull_C (X : access Integer);
    pragma Import (C, Tnull_C, "tnull_c");

begin
    Tnull_C (Null_Access'Access);
end Tnull;
$ 
$ cat tnull_c.c 
#include <stdio.h>

void tnull_c (int *x)
{
    printf ("x add = %08X\n", x);
}
$
$ gcc -c tnull_c.c
$ gnatmake tnull -largs tnull_c.o
gcc -c tnull.adb
gnatbind -x tnull.ali
gnatlink tnull_c.o tnull.ali
$ tnull
x add = 00000000
$ 

-Thierry
____________________________________________________________________________

Thierry Lelegard, "The Jazzing Troll", Email: thierry.lelegard@canal-plus.fr
CANAL+ Technologies, 34 place Raoul Dautry, 75516 Paris Cedex 15, France
Tel: +33 1 71 71 54 30   Fax: +33 1 71 71 52 08   Mobile: +33 6 03 00 65 75
____________________________________________________________________________
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