Question

in mine project i have a Windows application and a dll. I have wrote dll so:

library MyDLL;

uses
  System.SysUtils,
  System.Classes;

{$R *.res}

function Prova: string; export;
begin
  result := 'prova';
end;

exports Prova;

begin
end.

and in main program i have called routine so:

unit FrmMain;

interface

uses
 // declaration uses // 

function Prova: string; external 'MyDLL.dll';

type    
 // declaration type //

implementation

begin
  ...
  TAdvEdit1.Text := Prova;  // [1] // 
  ...
end;

end.

When i compile all project not is reported error, and status report SUCCESS, but application not start.
If i remove the line [1] then it works correctly. In general, application not start when i call the function Prova.
What i can solve this problem? Thanks very much.

Answer 1

Your program and your DLL have separate memory manager. As a general rule memory allocated from DLL should not be used inside your application (the opposite is also true).
Where the allocation comes from? In Delphi “string” is managed type i.e. when you assign some text to a string variable (in your case result := ‘prova’), Delphi (behind the scene) allocates memory for that string using DLL’s memory manager. Then, for example, if you assign other text value within your main application, the reallocation uses application’s memory manager which is bad i.e. app’s MM is touching memory that it hasn’t allocated itself.

To solve this issue you have to include “SimpleShareMem” (Delphi >= 2010 IIRC?) unit as the first unit of your USES clause in the application (.dpr file) AND in the DLL:

library MyDLL;

uses
  SimpleShareMem, // **MUST BE THE FIRST UNIT**
  System.SysUtils,
  System.Classes;
...


program YourApp;

uses
  SimpleShareMem, // **MUST BE THE FIRST UNIT**
  // declaration uses // 

An example of this approach can be found in “Start > Programs > Embarcadero RAD Studio > Samples > Delphi > RTL > SimpleShareMem“

You can also use PCHAR to transfers strings between DLL and the APP.