1. Introduction
2. What is a 'multiple instance'
3. The correct solution
4. The basic problem
5. Why FindWindow doesn't work
6. Send Message: Race Conditions
7. Shared Variable: A different problem
8. Generalizing the solution for NT
9. The source code
10. Summary

Comments

1. harpreetBamrah
   13 Jul 2005 at 11:38

   I am making a single instance appilcation and have followed all your suggestions . And i am really thankful to you for writing such an informative article.But i have a problem. It is my requirement that user can close my running application from the command prompt.To impliment this i am sending a message (WM_COPYDATA) from new instance which tries to launch but since it finds that an instance is already running it passes the cmdLine parameter from new instance to already existing instance and if the cmdLine parameter is /exit the application is closed. Everything works Like a charm.But problem is this. If i keep the exe in the folder named similar to the window name of my application  then the second instance is not able to find the first instance and thus the application does not exit when it should have exited. If i change the name of the folder in which the exe is residing then everything works fine.
   I just want to understand why is it happening and would love to have your views on this one.

2. vivek.kumbhojkar
   17 May 2004 at 23:59
   The article is realy good Even other tips and artcles by Joseph M. Newcomer are excellent for those who want to master vc++
3. Mark Malyj
   01 Dec 2003 at 11:36

   Brilliant article!! Here it is again, boiled down to cookbook code for your MyApp.cpp file.
   Many thanks to Joseph Newcomer & Daniel Lohmann. // Mark Malyj
   

   
   //Avoiding Multiple Instances of an Application
   

   
   //Step 1 of 5.
   //Include exclusion.h at the top of app.cpp.
   

   include "exclusion.h"

   
   

   
   //Step 2 of 5.
   //Define UNIQUE_GUID after the includes
   //Run Visual Studio tool GUIDGEN.EXE to generate a worldwide-unique GUID for this app;
   //choose option 4: registry format, Copy, and define here (DON'T USE this SAMPLE GUID!):
   

   define UNIQUE_GUID _T("{088FA840-B10D-11D3-BC36-006067709674}")

   
   

   
   //Step 3 of 5.
   //Use a shared variable between all instances of the application.
   //This can be done by creating a shared data segment.
   //Place this code block following all includes in the App .cpp file
   

   pragma comment(linker, "/SECTION:.shr,RWS")

   
   

   pragma data_seg(".shr")

   
   HWND g_hWnd = NULL;
   

   pragma data_seg()

   
   

   
   //Step 4 of 5.
   //Place this code block at the top of App::InitInstance
   bool AlreadyRunning;
   HANDLE hMutexOneInstance = ::CreateMutex( NULL, TRUE,
      //Choose UNIQUETODESKTOP, UNIQUETOSYSTEM,
      //UNIQUETOSESSION, or UNIQUETOTRUSTEE here:
      createExclusionName(UNIQUEGUID, UNIQUETODESKTOP));
   AlreadyRunning = (GetLastError() == ERRORALREADYEXISTS);
   if (hMutexOneInstance != NULL)
   {
      ::ReleaseMutex(hMutexOneInstance);
   }
   if ( AlreadyRunning )
   { /* kill this */
      HWND hOther = ghWnd;
      if (hOther != NULL)
      { /* pop up /
          :: SetForegroundWindow(hOther);
          if (IsIconic(hOther))
          { / restore /
              :: ShowWindow(hOther, SW_RESTORE);
          } / restore /
   } / pop up /
      return FALSE; // terminates the creation
   } / kill this */
   // ... continue with InitInstance
   

   
   //Step 5 of 5.
   //After Creating the Main Frame, get the handle and store it on the shared variable below.
   //Place this code block at the bottom of App::InitInstance, after the mainframe has been created or loaded
   ghWnd = mpMainWnd->m_hWnd;
   

   
   //Remember to add exlusion.h, exclusion.cpp to your project (see the original article)!!
   

4. Dan Randall
   19 Apr 2002 at 14:54

   I found this information useful for an implementation that did not involve "Avoiding Multiple Instances of an Application," but did involve use of an EnumWindows() callback loop which was freezing up before all of the windows were enumerated.  I was able to find a window that was blocking a call to GetWindowText() (it was busy with an I/O operation).  Once I discontinued using GetWindowText(), the enumeration of all windows succeeded.  I was then able to find the handle to a specific password prompt (launched by a third-party application) and automatically set the password (as well as post an IDOK message to make the window disapear).

5. Developer Fusion Bot
   01 Jan 1999 at 00:00

   This thread is for discussions of Avoiding Multiple Instances of an Application.