passing by reference to a DLL call.

[xrayted]

Hello,

I have a dll that has a function as follows:

int process( char *toBeProcessed, char *returnMessage );

I want to call this from within Delphi and have done the following:

function process( toBeProcessed : PChar; var message : PChar
):Integer; external 'mydll.dll';


procedure callTheDllFunction();
var
resp : integer;
msg : PChar;
begin

resp := process( 'whatiwanttoprocess', msg );

....
doSomethingWithMsg(msg);
...
end;

I was hoping that msg would contain the return message but all I get
is an 'invalid varient type conversion'

Any suggestions?

regards

[PoseFant]

I think you shold be more specific when you explain your problem ... I can see that you are switching between Char and PChar several times, and this may affect on the memory handling routines.

As far as I can see you hav a "C/C++" declared function :

INT Process(
Char * ToBeProcessed,
Char * ReturnMessage
);

As far as I know (I'm not a C/C++ expert) I think you need a reference pointer to the ReturnMessage Variable ...

INT Process(
Char * ToBeProcessed,
[out] Char * ReturnMessage
);

Ok, lets say it is declared this way ... )

What to declare in Delphi ...

Function Process(ToBeProcessed : PChar; Var ReturnMessage : PChar) : LongInt; StdCall; External 'YourDLL.dll';

As you can see we have a little var reference before the returning parameter, and a StdCall convention ...

The Conventions directs the parameter handling to functions, and perhaps this should help you out ... )

Another directive is CDecl, also used for C/C++ typical functions...

Ok ... so far so good ... I hope ... )


How to call it within your application ...

Var
Resp : Integer;
Begin
Resp := Process(@ToBeProcessed, @Msg);
End;

Try with and without the @ ...

To get the error result form your function call ...

Right after the function :

ShowMessage(SysErrorMessage(GetLastError));


Good Luck ...


Regards


/Filip