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Calling vars from a DLL

Posted on 2003-12-06
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Last Modified: 2010-04-05
Hello, I would Export Vars from a DLL and call them in my programm, example I would export S:String from a Dll and call that in another programm. Could you help me there??
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Question by:k4hvd88
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4 Comments
 
LVL 7

Expert Comment

by:sftweng
ID: 9888397
It would be much better practice to export a Function call from the DLL that returns a value of the expected type (e.g., in this case, a string), or to make it a property of a class.

The history of software is strewn with defects caused by the use of "global" shared variables. That's why encapsulation is such a valuable concept.
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Accepted Solution

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gmayo earned 180 total points
ID: 9888439
An example (DLL code first):

library Project2;

uses
  SysUtils,
  Classes;

{$R *.res}

var
      MyInt : integer = 999;
      MyStr : string = 'Hello, world!';

function GetMyInt : integer; stdcall;
begin
      Result := MyInt;
end;

procedure SetMyInt(v : integer); stdcall;
begin
      MyInt := v;
end;

function GetMyStr : PChar; stdcall;
begin
      Result := PChar(MyStr);
end;

procedure SetMyStr(s : PChar); stdcall;
begin
      MyStr := s;
end;

exports
      GetMyInt,
      SetMyInt,
      GetMyStr,
      SetMyStr;

begin
end.


...and the code to call it:

program Project1;

uses
  Dialogs, SysUtils;

{$R *.res}

const
      MyDLL = 'Project2.dll';

function GetMyInt : integer; stdcall; external MyDLL;
function GetMyStr : PChar; stdcall; external MyDLL;
procedure SetMyInt(v : integer); stdcall; external MyDLL;
procedure SetMyStr(s : PChar); stdcall; external MyDLL;

begin
      ShowMessage(GetMyStr);
      ShowMessage(IntToStr(GetMyInt));
      SetMyStr('Goodbye');
      SetMyInt(123);
      ShowMessage(GetMyStr);
      ShowMessage(IntToStr(GetMyInt));
end.


Geoff M.
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Author Comment

by:k4hvd88
ID: 9888542
is there any method to call vars directly?
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LVL 8

Expert Comment

by:gmayo
ID: 9888572
I believe not. From Delphi help:

"Global variables declared in a shared library cannot be imported by a Delphi application.

A library can be used by several applications at once, but each application has a copy of the library in its own process space with its own set of global variables. For multiple libraries--or multiple instances of a library--to share memory, they must use memory-mapped files. Refer to the your system documentation for further information."

Geoff M.
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