Exchanging strings (PChar) between a Freepascal compiled DLL and a Delphi compiled EXE

Question

After a lot of experimentations, I found a way to exchange PChar from a FreePascal compiled DLL with a Delphi compiled EXE. I\'m in charge of both the DLL and EXE source cod

Answer 1

The rule of thumb for allocating/deallocating memory in EXE and DLL is: the module that allocates memory is responsible for deallocation the same memory. In your example it is DLL who allocates and deallocates memory, so it is correct.

There is one exception from the general rule. Both Delphi and the latest Free Pascal versions implement WideStrings as BSTR - a string data type that is used by COM. WideStrings are allocated and freed by Windows, not by Delphi (or Free Pascal) memory manager. So there is no need to use PWideChar to pass WideString arguments between EXE and DLL - WideStrings can be passed directly.

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Updated:

I have tested the following code:

Free Pascal (version 2.2.4) DLL:

library TestLib1;

{$mode objfpc}{$H+}

{$IFDEF WINDOWS}{$R TestLib1.rc}{$ENDIF}

procedure GetAStringProc(var S: WideString); stdcall;
begin
  S:= '12345';
end;

function GetAStringFunc: WideString; stdcall;
begin
  Result:= '12345';
end;

exports
  GetAStringProc, GetAStringFunc;

begin
end.

Delphi 2009 EXE (main form):

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    Button1: TButton;
    Button2: TButton;
    procedure FormCreate(Sender: TObject);
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
  private
    S: WideString;
  end;

var
  Form1: TForm1;

type
  TGetAStringProc = procedure(var S: WideString); stdcall;
  TGetAStringFunc = function: WideString; stdcall;

var
  GetAStringProc: TGetAStringProc;
  GetAStringFunc: TGetAStringFunc;

implementation

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
begin
    GetAStringProc(S);
    Caption:= S;
    S:= '';
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
    S:= GetAStringFunc;
    Caption:= S;
    S:= '';
end;

procedure TForm1.FormCreate(Sender: TObject);
var
  LibHandle: THandle;

begin
  LibHandle:= LoadLibrary('TestLib1.dll');
  if LibHandle <> 0 then begin
    @GetAStringProc:= GetProcAddress(LibHandle, 'GetAStringProc');
    @GetAStringFunc:= GetProcAddress(LibHandle, 'GetAStringFunc');
  end;
  if not Assigned(GetAStringProc) or not Assigned(GetAStringFunc) then
      ShowMessage('Error!');
end;

end.

It appeared that the procedure GetAStringProc works fine while the function GetAStringFunc leads to access violation. Delphi and Free Pascal seems to return the string-type result differently.