Limiting Applications to a Single Instance

Here is a very simple example of how to implement a "One Instance" Application. If the application is yet running and the user tries to launch another instance, the running app is activated and line command parameter parsed to the runnin app.

The following code is added to the CWinApp-derived class in the 'InitInstance()' function, after the main window has been initialized but not displayed (m_uUserMessage is a UINT member variable of the same class)

// Define a user message indicating that another instance of
// the app is trying to run:

// Test for the presence of another instance from an
// existing main window whose handle has previously
// be written to the Registry
CString strHand=GetProfileString("Control","MainWndHwnd","0");
HWND hWnd;

// Test existing window
if( IsWindow(hWnd) )	// There is an instance running
 // Copy to the registry the command line data, as
 // for example a file name

 // Sent user message to the running app

 // Push running app to the foreground

 // Exit current instance
 return FALSE;

// No instance running, so save to Registry
// the handle of the current app main window

A 'PreTranslateMessage()' function is surcharged in the CWinApp-derived class ('COneInstanceAppApp', in the present example):
BOOL COneInstanceAppApp::PreTranslateMessage(MSG* pMsg) 
 if( pMsg->message == m_uUserMessage )
  // Retrive relevant command line info
  CString file=GetProfileString("Control","CmdLineFile","");

  if( !file.IsEmpty() )
   // Do what you want with this info!
  return TRUE; // User message has been handled
 return CWinApp::PreTranslateMessage(pMsg);


Download source - 15 Kb


  • What I was searching for...

    Posted by drnb on 12/07/2004 06:50pm

    This article answers the question I was just asking myself. It is simple, and understandable in first reading. However, as one of the comments corretly mention, the author has forgotten the important step of cleaning the registy. If you don't do that, you are unable to make the program run again!!!

  • Multiple Instances

    Posted by Legacy on 08/04/2002 12:00am

    Originally posted by: JOSE MENDOZA

    How we can to limit that one progran can be unning by a determined number of instances. This example limits to one instance of the exe. But if I want to let the program to run 2 or 3 (a number determined) times simultaneously.

  • mutex isn't possible?

    Posted by Legacy on 06/27/2001 12:00am

    Originally posted by: Renato Czar Tome

    Why don't you try to use mutex instead?

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