From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-0.3 required=5.0 tests=BAYES_00,
	REPLYTO_WITHOUT_TO_CC autolearn=no autolearn_force=no version=3.4.4
Path: 
 buffer2.nntp.dca1.giganews.com!border2.nntp.dca1.giganews.com!border1.nntp.dca1.giganews.com!nntp.giganews.com!usenet.blueworldhosting.com!feeder01.blueworldhosting.com!feeder.erje.net!1.eu.feeder.erje.net!news.fcku.it!aioe.org!.POSTED!not-for-mail
From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Returning a string from C
Date: Thu, 23 Jul 2015 09:17:54 +0200
Organization: cbb software GmbH
Message-ID: <svrr8jeuj5gn$.16vmrvatwo7mk$.dlg@40tude.net>
References: <27cd219d-fff7-4c2a-90c7-ec10a917b4cb@googlegroups.com>
Reply-To: mailbox@dmitry-kazakov.de
NNTP-Posting-Host: 5ZS5s7Q3rAN7MBP/UXLPIQ.user.speranza.aioe.org
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
X-Complaints-To: abuse@aioe.org
User-Agent: 40tude_Dialog/2.0.15.1
X-Notice: Filtered by postfilter v. 0.8.2
Xref: number.nntp.giganews.com comp.lang.ada:194310
Date: 2015-07-23T09:17:54+02:00
List-Id: <comp.lang.ada>

On Wed, 22 Jul 2015 19:20:34 -0700 (PDT), NiGHTS wrote:

> I am having some difficulty with returning a string from a C function.
> 
> Say I have the following C function:
> 
>     void Return_Hello (char *inout_Test_String ) {
>         strcpy( inout_Test_String, "Hello World" );
>     }

[...]

> What am I doing wrong here?

When you pass a string to C, use To_C from Interfaces.C that returns
char_array

 When you receive a string from C, use Chars_Ptr and Value from
Interfaces.C.Strings. E.g.

   function Foo (Text : String) return String is
      function Internal (S : char_array) return Chars_Ptr;
      pragma Import (C, Internal, "c_stuff");
      Result : Chars_Ptr;
   begin
      Result := Internal (To_C (Text));
      if Result /= Null_Ptr then
         return Value (Result);
      else
         return "";
      end if;
   end Foo;

When C side allocates a new string and your side is responsible for
deallocating it, use Free on Chars_Ptr from Interfaces.C.Strings.

If you want in-place string modified by C, pass char_array, maximum size,
actual length to C.

Never ever use strcpy, always strncpy.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de