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Message-ID: <6907976.QsGR0pgVzX@linux1.krischik.com>
From: Martin Krischik <martin@krischik.com>
Subject: Re: Interfacing to C
Newsgroups: comp.lang.ada
Reply-To: martin@krischik.com
Date: Tue, 08 Feb 2005 20:27:06 +0100
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Xref: g2news1.google.com comp.lang.ada:8197
Date: 2005-02-08T20:27:06+01:00
List-Id: <comp.lang.ada>

Garry wrote:

> I'm just starting in ADA.  Can anyone explain why I have to define a
> dummy procedure in my hsdbg package to get my code to work?

Because you misunderstood something. Of corse you don't need DUMMY.
 
> Error without dummy procedure:
> hsdbg.ads:4:09: package "HSDBG" does not allow a body
> hsdbg.ads:4:09: remove incorrect body in file "hsdbg.adb"
> gnatmake: "hsdbg.ads" compilation error
> error: "hsdbg.adb" must be recompiled ("hsdbg.ads" has been modified)
> gnatlink: Failed to open binder output
> 
> Code follows:
> /* Start of marktime.c */
> #include <stdio.h>
> 
> void marktime(char  str[255])
> {
>    printf("marktime: str='%s'\n",str);
> }
> /* End of marktime.c */
> 
> 
> 
> -- Start of hsdbg.ads
> --
> with Interfaces.C;
> with Interfaces.C.Strings;
> 
> package HSDBG is
> --  Dummy procedure?
>       type DUMB is digits 5 range 0.0 .. 1.0E10;
>       procedure DUMMY (B : DUMB);

As Georg pointed out you need:

  pragma Elaborate_Body;

or prehaps:

  procedure marktime(Variable : String);

that would export the Ada version of marktime(Variable : String);

> --  End Dummy procedure
>       procedure marktime(Variable : Interfaces.C.Strings.chars_ptr);
>       pragma Import(C, marktime, "marktime");
> end HSDBG;
> -- End of hsdbg.ads
> 
> 
> 
> -- Start of hsdbg.adb
> --
> with Ada.Text_IO; use Ada.Text_IO;
> with Interfaces.C.Strings; use Interfaces.C.Strings;
> --
> --  Define package body
> --
> package body HSDBG is
> --
> --
> --  dummy procedure?
>   procedure DUMMY(B : DUMB) is
>   begin
>      put("In Dummy");
>   end DUMMY;
> --  end dummy procedure
> 
>    procedure marktime(Variable : String) is
> 
>       procedure marktime(Variable : chars_ptr);
>       pragma Import(C, marktime, "marktime");

Well you don't need to repeat the C declarartion here. You should decide for
one and delete the other. Which one is design decision - you have to take
that yourself

>       Variable_In_C_Format : chars_ptr := New_String(Variable);
> 
> 
>    begin
>       marktime(Variable_In_C_Format);
>       Free(Variable_In_C_Format);
>    end marktime;
> 
> 
>  end HSDBG;
> -- End of hsdbg.adb
> 
> 
> 
> 
> -- Start of tst.adb
> with Ada.Text_IO; use Ada.Text_IO;
> with Interfaces.C.Strings; use Interfaces.C.Strings;
> with hsdbg; use hsdbg;
> 
> procedure tst is
> 
>   C_Format : chars_ptr := New_String("This is a string in ADA that is
> written by the marktime C procedure");
> 
> begin
>    Put_Line ("test started");
>    marktime(C_Format);
> 
> end tst;
> -- End of tst.adb

Well it look all very confusing here. It looks like you have two test codes
and two C binding.

One by which the pragma Import shows in the body of hsdbg - one where is
shows in the spec of hsdbg.

One where you test with a test procedure one where you test with a package
body.

In both cases both designs are possible and have distict advantages and
disadvantges. If you have a tutor then talk with him/her and then make a
decision.

Martin
-- 
mailto://krischik@users.sourceforge.net
Ada programming at: http://ada.krischik.com