From: "Peter Below (TeamB)" <100113.1101@compuXXserve.com>
Subject: Re: passing strings
Date: 24 Jun 1999 00:00:00 GMT
Message-ID: <VA.000031f1.0106a688@noname>
Content-Transfer-Encoding: 8bit
References: <3772243A.2D3A@kmc-controls.com>
Content-Type: text/plain; charset=iso-8859-1
Organization: TeamB
Mime-Version: 1.0
Reply-To: 100113.1101@compuXXserve.com
Newsgroups: borland.public.delphi.objectpascal

> I have finally been able to pass a string TO my BCB DLL routine to a
> calling app in Delphi with the following code :
>
> (BCB)
> extern "C" __declspec( dllexport ) __stdcall void test( char* );
> void __stdcall test( char* InputBuffer )

> (Delphi)
> procedure test( InputBuffer : String );
>         stdcall; external 'MyDLL.dll';

No, you do this wrong, sorry. A Delphi String (AnsiString) is NOT
equivalent to a C char*. It happens to work if the string is only read by
the C side since AnsiStrings are always zero-terminated (but the #0 at end
does not define the length, there is a length dword at negative offsets to
the string pointer) and a AnsiString variable is a pointer that points at
the first character, but will break if the routine tries to write to the
character buffer since that not only circumvents the reference counting
mechanism but also has the potential to write beyond the end of buffer.

Build your function like the API does for functions like
GetWindowsDirectory: you pass them a buffer pointer (PChar) and the size of
the buffer, so the routine can guard against overwriting.

 procedure Test( inputbuffer: PChar; buffersize: Integer ); stdcall;
external ....

You use this like you would an API function from delphi:

 Var
   S: String;
 Begin
   SetLength( S, 512 );
   Test( PChar(S), Length(S));
   SetLength( S, StrLen( PChar(S));

The Test routine is required to zero-terminate its result if it is shorted
than 512 characters. It can leave off the #0 if it copies exactly 512
characters since there is an extra #0 at S[513] (not a legal index!) added
by the Delphi string management.

Peter Below (TeamB)  100113.1101@compuserve.com)
No e-mail responses, please, unless explicitly requested!