How pass a "null" as a parameter of anonymous access type

How pass a "null" as a parameter of anonymous access type

[Freddy]

Hello !

I want call a C-binding procedure which is defined as "procedure p (a :
access X; b : access Y);"
Both a or b may be a null pointer, but Ada say "null not allowed for
anonymous access type". 
Any idea what to do ?

How pass a "null" as a parameter of anonymous access type

[tmoran]

> Both a or b may be a null pointer, but Ada say "null not allowed for
> anonymous access type".
> Any idea what to do ?
  Make them non-anonymous access types.
> I want call a C-binding procedure which is defined as "procedure p (a :
> access X; b : access Y);"
    Instead use
  type a1 is access X;
  type b1 is access Y;
    procedure p(a : a1; b : b1);
A variable of type a1 or b1 can be null.

Re: How pass a "null" as a parameter of anonymous access type

[Thierry Lelegard]

[-- Attachment #1: Type: text/plain, Size: 846 bytes --]

> I want call a C-binding procedure which is defined as "procedure p (a :
> access X; b : access Y);"
> Both a or b may be a null pointer, but Ada say "null not allowed for
> anonymous access type".
> Any idea what to do ?

A hack (it works with GNAT, but may not with others, not defined by the
language):

    Null_Access : aliased X; -- don't access it !!
    for Null_Access'Address use System.Null_Address;

    ....

    P (Null_Access'Access, ...);

-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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Re: How pass a "null" as a parameter of anonymous access type

[Freddy]

As I stated, this is a C-binding (a library package we've got from
elsewhere) and I'm not allowed to modify it.

Re: How pass a "null" as a parameter of anonymous access type

[Freddy]

Thanks. I've tried it, but at runtime I always get a constraint_error
(GNAT seems to check the passing of "null" values).

Re: How pass a "null" as a parameter of anonymous access type

[Marc A. Criley]

Freddy wrote:
> 
> Hello !
> 
> I want call a C-binding procedure which is defined as "procedure p (a :
> access X; b : access Y);"
> Both a or b may be a null pointer, but Ada say "null not allowed for
> anonymous access type".
> Any idea what to do ?

Here's another approach, which worked with JGNAT both at compile-time
and run-time:

  type X_Access is access all X;
  type Y_Access is access all Y;

  Null_X : constant X_Access := null;
  Null_Y : constant Y_Access := null;

  ...

  P(Null_X'access, Null_Y'access);
  

Even if this works (which it shouldn't), you should submit a bug report
to the providers of the binding stating that the C routine can accept a
null value, which their binding does not support.

Marc A. Criley
Senior Staff Engineer
Quadrus Corporation
www.quadruscorp.com

Re: How pass a

[Ted Dennison]

In article <3ABE63F7.3E088F9D@t-online.de>, Freddy says...
>
>As I stated, this is a C-binding (a library package we've got from
>elsewhere) and I'm not allowed to modify it.

Perhaps, but nothing's stopping you from making your own "pragma interface"'d
routine based on theirs, but with the proper arguments.

---
T.E.D.    homepage   - http://www.telepath.com/dennison/Ted/TED.html
          home email - mailto:dennison@telepath.com

Re: How pass a "null" as a parameter of anonymous access type

[Mark Lundquist]

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.

But it seems like any Ada binding to C that uses access parameters for
things that you are supposed to be able to pass a null value is in error,
plain and simple!

Why can't you write your own corrected version of the subprogram
declaration, write your own "pragma Import (C, ..." for it, and use the
corrected version instead of the erroneous one?  Then you're not actually
"changing" it, just not using it... and instead using something else that is
very similar... :-)

Mark Lundquist
Rational Software

Re: How pass a "null" as a parameter of anonymous access type

[Mark Lundquist]

Marc A. Criley <mcqada@earthlink.net> wrote in message
news:3ABF3A34.D961881E@earthlink.net...
> Freddy wrote:
> >
> > Hello !
> >
> > I want call a C-binding procedure which is defined as "procedure p (a :
> > access X; b : access Y);"
> > Both a or b may be a null pointer, but Ada say "null not allowed for
> > anonymous access type".
> > Any idea what to do ?
>
> Here's another approach, which worked with JGNAT both at compile-time
> and run-time:
>
>   type X_Access is access all X;
>   type Y_Access is access all Y;
>
>   Null_X : constant X_Access := null;
>   Null_Y : constant Y_Access := null;
>
>   ...
>
>   P(Null_X'access, Null_Y'access);

Hokey Smokes, Bullwinkle! :-)

>
>
> Even if this works (which it shouldn't),

No kidding... :-)

> you should submit a bug report
> to the providers of the binding

And to your Ada vendor...

Null_X is not aliased, so Null_X'Access is illegal.

Make it aliased, then you can take the 'Access... but then, Null_X'Access
should *not* yield a null value (the fact that Null_X itself *has* a null
value is irrelevant, the value of Null_X makes no difference).
Null_X'Access by definition yields an access value that denotes the object
Null_X (and a null value does not denote anything... that's why it's called
"null" :-).

And even if there is some compiler bug that would cause Null_X'Access to
yield a null value, that should cause Constraint_Error to be raised when
it's passed to P (it doesn't matter where the null value came from).

For that cr@p to compile *and* run would require an incredible conjunction
of bugs in the Ada implementation.

-- mark

Re: How pass a "null" as a parameter of anonymous access type

[Marc A. Criley]

Mark Lundquist wrote:
> 
   <snips>
> 
> Null_X is not aliased, so Null_X'Access is illegal.

Mea culpa, I mistyped.  Yes, it is not Null_X'Access that was used,
merely Null_X.

> 
> Make it aliased, then you can take the 'Access... but then, Null_X'Access
> should *not* yield a null value (the fact that Null_X itself *has* a null
> value is irrelevant, the value of Null_X makes no difference).
> Null_X'Access by definition yields an access value that denotes the object
> Null_X (and a null value does not denote anything... that's why it's called
> "null" :-).
> 
> And even if there is some compiler bug that would cause Null_X'Access to
> yield a null value, that should cause Constraint_Error to be raised when
> it's passed to P (it doesn't matter where the null value came from).
> 

Well, not necessarily...  I ran into this same problem when using JGNAT
and the bindings that product produces for interfacing to the JDK.  As I
mentioned, the way I got around this was by using a (correctly formed)
constant null pointer.  That suggestion was made in response to a
posting I made in this group about 3 weeks ago.  There was some
follow-up discussion, and Tucker Taft chimed in with the reason that
this approach was allowable with JGNAT:

From: Tucker Taft (stt@averstar.com)
 Subject: Re: JGNAT and Null references 
 Newsgroups: comp.lang.ada
 Date: 2001-03-07 08:04:05 PST 

Randy Brukardt wrote:
> 
> Marc A. Criley wrote in message <3AA151BF.C9193700@earthlink.net>...
> >Uh...it did work.  Cleared up the problem just fine.  That it works
> >doesn't totally sit well with me, since my purist mental picture of a
> >"constant" involves only a value, and not storage.  Nonetheless,
> >
> >   Null_Icon : constant Javax.swing.TabbedPanel.ref := null;
> >
> >was accepted by the AddTab procedure call.
> 
> But does it run? The RM  4.6(49) says that an access parameter cannot be
> null, and must raise Constraint_Error. If it does run, you're using a
> compiler bug (or "feature"); your code won't be portable...

There was discussion of avoiding the null check when interfacing
with code from some other language (e.g. C or Java).  Given that the
rules for making Java-convention calls are not standardized at
this point, so long as there is an explicit (or implicit?) Java
convention on the routines, JGNAT can do what it wants at this point.

> 
>         Randy. 
-- 
-Tucker Taft   stt@avercom.net   http://www.averstar.com/~stt/

Re: How pass a "null" as a parameter of anonymous access type

[Alfred Hilscher]

"Marc A. Criley" wrote:
> 
> Mark Lundquist wrote:
> >
>    <snips>
> >
> > Null_X is not aliased, so Null_X'Access is illegal.
> 
> Mea culpa, I mistyped.  Yes, it is not Null_X'Access that was used,
> merely Null_X.


I tried this, and GNAT (for Windows) raises CONSTRAINT_ERROR, too.

Re: How pass a "null" as a parameter of anonymous access type

[Alfred Hilscher]

Mark Lundquist wrote:
> 
> 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.

Does that prgma supress checks for the entire package or can it switched
on/off by statement ?

Re: How pass a "null" as a parameter of anonymous access type

[Thierry Lelegard]

[-- Attachment #1: Type: text/plain, Size: 1582 bytes --]

> 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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