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[Delphi] Exporting function (and using it)

 
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h4c0r-BG
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PostPosted: Mon Mar 16, 2009 1:27 pm    Post subject: [Delphi] Exporting function (and using it) Reply with quote
I can't believe i have problems with such a silly thing. Who knows what i am missing ...

Anyway here is what i have:

DLL:

Code:
library my_lib;

uses
  Windows;

function cool_func():string;
var
good:string;
begin
good:='this function is good';
result := good;
end;

exports
cool_func;

begin
end.


EXECUTABLE:

Code:

............

function cool_func:string;
external 'my_lib.dll' name 'cool_func';

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
begin
showmessage(cool_func);
end;

end.


So far - so good!
I click button1 and get the string "this function is good" in a showmessage.
After i click [OK] in that "showmessage" i get the following error:



How can i fix that? What i am doing wrong?
(my function must be able to be called from another languages/applications too without errors)
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iNoobHacker
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PostPosted: Mon Mar 16, 2009 2:11 pm    Post subject: Reply with quote
Code:
result := good;

"good" isn't even a string, I'm suprised it compiled...
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samuri25404
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PostPosted: Mon Mar 16, 2009 2:27 pm    Post subject: Reply with quote
I recall something about exporting functions which return strings in Delphi DLLs, where you have to add Borland's string library to the first of your list of uses or something like that.

You ought to check it out.
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Tsabo
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PostPosted: Mon Mar 16, 2009 3:00 pm    Post subject: Reply with quote
Read the comment that was created by the DLL project wizard.
Code:
{ Important note about DLL memory management: ShareMem must be the
  first unit in your library's USES clause AND your project's (select
  Project-View Source) USES clause if your DLL exports any procedures or
  functions that pass strings as parameters or function results. This
  applies to all strings passed to and from your DLL--even those that
  are nested in records and classes. ShareMem is the interface unit to
  the BORLNDMM.DLL shared memory manager, which must be deployed along
  with your DLL. To avoid using BORLNDMM.DLL, pass string information
  using PChar or ShortString parameters. }


This would be a workaround if you didn't want to use ShareMem would be:
Code:
library my_lib;

uses
  Windows;

var
  HoldingString: string;

function cool_func: PChar; stdcall;
begin
  HoldingString := 'this function is good';
  Result := PChar(HoldingString);
end;

exports
  cool_func;
begin
end.


With that code you could then do this:
Code:
function cool_func: PChar; stdcall; external 'my_lib.dll' name 'cool_func';

implementation

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
begin
  ShowMessage(cool_func);
end;


All of the supplied code is completely untested.

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h4c0r-BG
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PostPosted: Mon Mar 16, 2009 3:51 pm    Post subject: Reply with quote
Thank you Tsabo, the problem was solved with conversion from string to pchar. Smile
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