Tutorial

Calling Delphi DLL from MS Visual C++

First, you need to declare all the functions exported by your Delphi DLL with the keywords export; stdcall;

Secondly, your VC++ header file should declare all the functions as of type __declspec(dllexport) __stdcall (double leading underscores, and should enclose the function prototypes within extern "C" { ... } . (You can also use __declspec(dllimport) instead...) For example:

extern "C"

{

  int __declspec(dllexport) __stdcall plusone(int);

}

Thirdly, in VC++ the compiler does insist on decorating __stcall function names, so your Delphi DLL has to export these functions accordingly. The way to do it is to modify the Delphi 2.0 .DPR file for your DLL by modifying the names of all functions as they appear in the exports clause. For example, if you are exporting a function plusone(intval : Integer) you need to include it in the .DPR file's exports clause as:

  plusone name 'plusone@4'

The number following the @ symbol is the total length in bytes of all the function's arguments. The simplest way to find out the correct values is to try to link your VC++ program and inspect the "unresolved external" linker errors that result.

FINALLY, you can easily create an import library using the LIB utility supplied with VC++. To do it, you need to manually (!!) create a .DEF file for your DLL, with an exports clause listing the names and/or ordinals of all the functions the DLL exports. The format of the .DEF file is simple:

library MYLIB

description 'My very own DLL'

exports

   plusone@4

Then you execute LIB from the DOS/Win95 command line, supplying your .DEF file as a command-line parameter. For example, LIB /DEF:MYDLL.DEF . You tell VC++ about your .LIB file in the Build|Settings|Linker dialog.

Get Hardware ID Extractor DLL

It works in Visual C, C Builder, Delphi, Visual Basic, etc. Details it here.

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Sample code follows:

*******

MYDLLMU.PAS

unit MyDLLMU;

interface

function plusone(val : Integer) : Integer; export; stdcall;

procedure ChangeString(AString : PChar); export; stdcall;

implementation

uses

  Dialogs,

  SysUtils;

function plusone(val : Integer) : Integer;

begin

  Result := val + 1;

end;

procedure ChangeString(AString : PChar);

begin

  if AString = 'Hello' then

  StrPCopy(AString, 'World');

end;

end.

***********

MYDLL.DPR

library mydll;

{ Important note about DLL memory management: ShareMem must be the first unit in your library's USES clause AND your project's (select View-Project 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 DELPHIMM.DLL shared memory manager, which must be deployed along with your DLL. To avoid using DELPHIMM.DLL, pass string information using PChar or ShortString parameters. }

uses

  SysUtils,

  Classes,

  MyDLLMU in 'MyDLLMU.pas';

exports

  plusone name 'plusone@4',

  ChangeString name 'ChangeString@4';

begin

end.

***************

MYDLL.DEF

; -----------------------------------------------------------------

; File name: MYDLL.DEF

; -----------------------------------------------------------------

LIBRARY MYDLL

DESCRIPTION 'Test Delphi DLL, static loading into VC++ app'

EXPORTS

  plusone@4

************** DLLTSTADlg.H

// DLLTSTADlg.h : header file

//

#define USELIB

#ifdef USELIB

extern "C" {

  int __declspec(dllimport) __stdcall plusone(int);

}

#endif //USELIB

// CDLLTSTADlg dialog

class CDLLTSTADlg : public CDialog

{

// Construction

public:

  CDLLTSTADlg(CWnd* pParent = NULL); // standard constructor

  ~CDLLTSTADlg();

// Dialog Data

  //{{AFX_DATA(CDLLTSTADlg)

  enum { IDD = IDD_DLLTSTA_DIALOG };

  CString m_sVal;

  CString m_sStr;

  //}}AFX_DATA

  // ClassWizard generated virtual function overrides

  //{{AFX_VIRTUAL(CDLLTSTADlg)

  protected:

  virtual void DoDataExchange(CDataExchange* pDX); // DDX/DDV support

  //}}AFX_VIRTUAL

// Implementation

protected:

#ifndef USELIB

  HINSTANCE hMyDLL;

  FARPROC lpfnplusone;

  typedef int (*pIIFUNC)(int);

  pIIFUNC plusone;

#endif //USELIB

  HICON m_hIcon;

  // Generated message map functions

  //{{AFX_MSG(CDLLTSTADlg)

  virtual BOOL OnInitDialog();

  afx_msg void OnPaint();

  afx_msg HCURSOR OnQueryDragIcon();

  afx_msg void OnBtnplusone();

  afx_msg void OnBtnplusoneClick();

  afx_msg void OnBtndostringClick();

  //}}AFX_MSG

  DECLARE_MESSAGE_MAP()

};

************

DLLTSTADlg.CPP

// DLLTSTADlg.cpp : implementation file

//

#include "stdafx.h"

#include "DLLTSTA.h"

#include "DLLTSTADlg.h"

#ifdef _DEBUG

#define new DEBUG_NEW

#undef THIS_FILE

static char THIS_FILE[] = __FILE__;

#endif

extern CDLLTSTAApp theApp;

// CDLLTSTADlg dialog

CDLLTSTADlg::CDLLTSTADlg(CWnd* pParent /*=NULL*/)

  : CDialog(CDLLTSTADlg::IDD, pParent)

{

  //{{AFX_DATA_INIT(CDLLTSTADlg)

  m_sVal = _T("1");

  m_sStr = _T("Hello");

  //}}AFX_DATA_INIT

  // Note that LoadIcon does not require a subsequent DestroyIcon in Win32

  m_hIcon = AfxGetApp()->LoadIcon(IDR_MAINFRAME);

#ifndef USELIB

  hMyDLL = LoadLibrary("C:\\delpwork\\MYDLL.DLL");

  if(hMyDLL == NULL)

  PostQuitMessage(1);

  lpfnplusone = GetProcAddress(HMODULE(hMyDLL), "_plusone");

  if(lpfnplusone == NULL)

  PostQuitMessage(2);

  plusone = pIIFUNC(lpfnplusone);

#endif //USELIB

}

CDLLTSTADlg::~CDLLTSTADlg()

{

#ifndef USELIB

  if (hMyDLL != NULL)

  FreeLibrary(hMyDLL);

#endif //USELIB

}

void CDLLTSTADlg::DoDataExchange(CDataExchange* pDX)

{

  CDialog::DoDataExchange(pDX);

  //{{AFX_DATA_MAP(CDLLTSTADlg)

  DDX_Text(pDX, IDC_LBLINT, m_sVal);

  DDX_Text(pDX, IDC_LBLSTRING, m_sStr);

  //}}AFX_DATA_MAP

}

BEGIN_MESSAGE_MAP(CDLLTSTADlg, CDialog)

  //{{AFX_MSG_MAP(CDLLTSTADlg)

  ON_WM_PAINT()

  ON_WM_QUERYDRAGICON()

  ON_BN_CLICKED(IDC_BTNPLUSONE, OnBtnplusoneClick)

  ON_BN_CLICKED(IDC_BTNDOSTRING, OnBtndostringClick)

  //}}AFX_MSG_MAP

END_MESSAGE_MAP()

// CDLLTSTADlg message handlers

BOOL CDLLTSTADlg::OnInitDialog()

{

  CDialog::OnInitDialog();

  // Set the icon for this dialog. The framework does this automatically

  // when the application's main window is not a dialog

  SetIcon(m_hIcon, TRUE); // Set big icon

  SetIcon(m_hIcon, FALSE); // Set small icon

  // TODO: Add extra initialization here

  return TRUE; // return TRUE unless you set the focus to a control

}

// If you add a minimize button to your dialog, you will need the code below

// to draw the icon. For MFC applications using the document/view model,

// this is automatically done for you by the framework.

void CDLLTSTADlg::OnPaint()

{

  if (IsIconic())

  {

  CPaintDC dc(this); // device context for painting

  SendMessage(WM_ICONERASEBKGND, (WPARAM) dc.GetSafeHdc(), 0);

  // Center icon in client rectangle

  int cxIcon = GetSystemMetrics(SM_CXICON);

  int cyIcon = GetSystemMetrics(SM_CYICON);

  CRect rect;

  GetClientRect(&rect);

  int x = (rect.Width() - cxIcon + 1) / 2;

  int y = (rect.Height() - cyIcon + 1) / 2;

  // Draw the icon

  dc.DrawIcon(x, y, m_hIcon);

  }

  else

  {

  CDialog::OnPaint();

  }

}

// The system calls this to obtain the cursor to display while the user drags

// the minimized window.

HCURSOR CDLLTSTADlg::OnQueryDragIcon()

{

  return (HCURSOR) m_hIcon;

}

void CDLLTSTADlg::OnBtnplusoneClick()

{

  int iTemp;

  char sTemp[10];

  iTemp = atoi(m_sVal);

  iTemp = plusone(iTemp);

  m_sVal = itoa(iTemp, sTemp, 10);

  UpdateData(FALSE);

}

void CDLLTSTADlg::OnBtndostringClick()

{

  UpdateData(FALSE);

}

To call our Hardware ID Extractor DLL from Visual C you might want to do something like this.