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Old 01-10-2001, 08:15 PM
xrayted xrayted is offline
Junior Member
Join Date: Jan 2001
Posts: 1
Default passing by reference to a DLL call.


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();
resp : integer;
msg : PChar;

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


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

Any suggestions?

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Old 01-10-2001, 10:49 PM
PoseFant PoseFant is offline
Senior Member
Join Date: Jan 2001
Posts: 1,002
Default RE: passing by reference to a DLL call.

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 ...

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

Try with and without the @ ...

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

Right after the function :


Good Luck ...


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