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From: "David C. Hoos, Sr." <david.c.hoos.sr@ada95.com>
Subject: Re: Ada/Motif problem...
Date: 1996/08/28
Message-ID: <01bb94d3$613fc4c0$328371a5@dhoossr.iquest.com>#1/1
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distribution: world
references: <4vupg6$ljf@goanna.cs.rmit.edu.au>
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organization: DBH Enterprises, Inc.
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newsgroups: comp.lang.ada
Date: 1996-08-28T00:00:00+00:00
List-Id: <comp.lang.ada>


Hi Dale,
It appears that your problem is passing an Ada string where an XtPointer is
required.  This confusion may result from the fact that in C a literal
string is stored as a null-terminated static string in a memory location
determined by the compiler/linker, and that the value of that literal
string expression is its address.  Further, if you are compiling the C
without strict ANSI constraints, that pointer to char will be accepted as a
valid XtPointer!
I haven't worked with these X11Ada bindings yet, but I would suggest the
string "Hello World" & Ascii.NUL be assigned to an object declared as
aliased -- e.g.,
Hello_World : aliased constant STRING := "Hello World" & Ascii.NUL;

Then I would pass the value Hello_World'ACCESS as the parameter.

The reason I am suggesting the explicit NUL termination for the string, is
that that's what the X11 library, written in C expects.  There may be some
fancier way with some of the packages in the Ada95 annexes, but I haven't
learned enough about them yet.

Hope this helps

-- 
David C. Hoos, Sr.,
http://www.dbhwww.com
http://www.ada95.com

Dale Stanbrough <dale@goanna.cs.rmit.EDU.AU> wrote in article
<4vupg6$ljf@goanna.cs.rmit.edu.au>...
> Hi,
> 
> One of our students has run into problems writing a Motif routine.
> The callback data is to be a string as follows..
> 
> 
> Xt.Intrinsic.XtAddCallback(
>                                  pb,
>                                  Motif.xmstrdefs.XmNactivateCallback,
>                                  hello_dialog_pkg.popup'access,
>                                  "Hello World");
>                                  
> 
> I've tried using unchecked_conversion from an access to all string
> to XtPointer, but Gnat complains (quite rightly) about these pointer
> types being of different sizes.
> 
> Any inspiration on how to pass Ada strings as callback data in Motif?
> 
> Here's the subprogram declaration...
>                                  
> procedure XtAddCallback(
>                 widget       : Xt.Intrinsic.Widget;
>                 callback_name: X.Strings.const_charp;
>                 callback     : XtCallbackProc;
>                 closure      : XtPointer);
> 
> Dale
>