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There can be only one...

, 1 Apr 2002 CPOL
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Limiting an application to a single instance.

Introduction

When reading Joseph Newcomer's article about this matter, I thought there must be a way to do this simpler. So based on his excellent work the past few years I've spent all my time devoted on this theorem. In the mean time I never saw any daylight and I've lost all my friends (Just kidding' Wink | ;) ...

But after this long research and keeping it secret for too long I finally bring out the goods just to make sure such an important discovery like this doesn't fade away in history... uhum...

So here we go:

  1. We need a registered message that our application will respond to. Make this one global in your application.
    static const UINT WM_CHECK_ITS_ME = RegisterWindowMessage("MY_UNIQUE_GUID");
  2. Now we create a mutex somewhere at the start of your program:
    HANDLE hMutexOneInstance = CreateMutex( NULL, FALSE, _T("MY_UNIQUE_GUID"));
  3. Check the last error to see what happened:
    bool AlreadyRunning = ( GetLastError() == ERROR_ALREADY_EXISTS || 
                               GetLastError() == ERROR_ACCESS_DENIED);
  4. Then we send out a message to all top level windows to make sure it pops up and becomes the active window. The trick here is to use HWND_BROADCAST, which is a little bit undocumented.
    if ( AlreadyRunning )
    {    
     DWORD result;
     LRESULT ok = SendMessageTimeout(HWND_BROADCAST,
                                           WM_CHECK_ITS_ME,
                                           0, 0, 
                                           SMTO_BLOCK |
                                           SMTO_ABORTIFHUNG,
                                           200,
                                           &result);
     return FALSE; // Here we quit this application
     // or exit(0);
     // or PostQuitMessage() ... etc...
    }
  5. The already running application needs to respond the registered window message, so we add a message handler to the application.

    If you are using MFC edit your messagemap:
    ON_REGISTERED_MESSAGE( WM_CHECK_ITS_ME, OnCheckItsMe )
    or WTL:
    MESSAGE_HANDLER(WM_CHECK_ITS_ME, OnCheckItsMe)
    or plain Windows API:
    LRESULT CALLBACK MyWindowProc(HWND hwnd, UINT msg, WPARAM wp, LPARAM lp)
    {
      if (msg == WM_CHECK_ITS_ME)
      {
        ...
        return 0;
      }
      ....
      return ...;
    }
  6. Now in the handler for this message we just make sure this application becomes the foreground and active one.
    LRESULT CMainFrame::OnCheckItsMe(...)
    {
    // If we are using Chris Maunder's CSystemTray it's handy
    
     m_SystemTray.MaximiseFromTray(this->m_hWnd); 
     
    // or else you can call all sorts of restore/maximise functions:
    
     - ShowWindow(SW_RESTORE);
     - SetForegroundWindow();
     - SetWindowPlacement(m_MyWindowPlacement);
    
     return WM_CHECK_ITS_ME;
    }
  7. That should work!

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Niek Albers
Web Developer
Netherlands Netherlands
Niek is the founder and programmer of DaanSystems.com and is working on many projects all the time. He makes a living by doing contractwork for others.

Comments and Discussions

 
Questionwait for exit of previous app? Pinmemberhobnob25-Apr-13 23:49 
QuestionSome needed extensions PinmemberKarstenK4-Jan-07 22:51 
GeneralUsing event object will be simpler... Pinmemberopsoft-op5-Dec-05 23:59 
GeneralStill the same old problem! PinmemberDaniel Lohmann19-Jul-02 7:07 
GeneralRe: Still the same old problem! Pinsussandy_kanaka17-Aug-02 17:23 
GeneralRe: Still the same old problem! PinmemberDaniel Lohmann17-Aug-02 21:52 
GeneralAccess violation PinmemberCapsuleDweller29-May-02 8:33 
GeneralRe: Access violation PinmemberAnonymous29-May-02 8:32 
GeneralRe: Access violation PinmemberCarl Ferreira11-Jul-02 5:05 
GeneralRe: Access violation PinmemberJan Deinhard5-Sep-02 3:27 
GeneralRe: Access violation Pinmemberxxxyyyzzz22-Dec-02 19:02 
QuestionWhere to register the message?? Pinmembercycosi9-Apr-02 5:17 
AnswerRe: Where to register the message?? PinmemberTrapper23-Apr-02 7:05 
GeneralOK but not perfect PinmemberPhilippe Lhoste8-Apr-02 3:47 
GeneralRe: OK but not perfect PinmemberAndreas Saurwein14-May-02 21:26 
QuestionIs this way good? PinmemberDaniel Madden8-Apr-02 0:53 
AnswerRe: Is this way good? PinmemberPhilippe Lhoste8-Apr-02 3:31 
GeneralRe: Is this way good? PinmemberDaniel Madden8-Apr-02 8:23 
GeneralThanks! PinmemberAnonymous2-Apr-02 11:16 
GeneralDialogs PinmemberAtlantys2-Apr-02 2:35 
GeneralRe: Dialogs PinmemberNiek Albers2-Apr-02 3:11 
GeneralScary PinadminChris Maunder2-Apr-02 0:23 
GeneralRe: Scary PinmemberJosh Knox4-Apr-02 6:00 

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