HOWTO: Limit 32-bit Applications to One Instance Using C++

• Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1, 4.2, 5.0, 6.0

SUMMARY

This article shows how to limit an application to one instance. It does not rely on any creation of windows, so the method used in this article could be used for about any 32-bit application that is developed in C++. This includes console applications, WinCE applications, dialog box based applications, applications without a graphical user interface and many others.

MORE INFORMATION

The method used in this article is the one described in MSDN under the WinMain topic. It uses the CreateMutex() function to create a named mutex that can be checked across processes. Instead of duplicating the same code for every application that intends to be used as a single instance, the needed code is in a C++ wrapper class that can be reused across each application.

In order to use this functionality, you can use the following steps:

1. Create a new header file with the name LimitSingleInstance.h and add it to your project.



2. Copy the following code into the LimitSingleInstance.h file and save the file.




#ifndef LimitSingleInstance_H
#define LimitSingleInstance_H

#include <windows.h> 

//this code is from Q243953 in case you lose the article and wonder
//where this code came from...
class CLimitSingleInstance
{
protected:
  DWORD  m_dwLastError;
  HANDLE m_hMutex;

public:
  CLimitSingleInstance(TCHAR *strMutexName)
  {
    //be sure to use a name that is unique for this application otherwise
    //two apps may think they are the same if they are using same name for
    //3rd parm to CreateMutex
    m_hMutex = CreateMutex(NULL, FALSE, strMutexName); //do early
    m_dwLastError = GetLastError(); //save for use later...
  }
   
  ~CLimitSingleInstance() 
  {
    if (m_hMutex)  //don't forget to close handles...
    {
       CloseHandle(m_hMutex); //do as late as possible
       m_hMutex = NULL; //good habit to be in
    }
  }

  BOOL IsAnotherInstanceRunning() 
  {
    return (ERROR_ALREADY_EXISTS == m_dwLastError);
  }
};
#endif 

3. #include the LimitSingleInstance.h file where the entry point of the program is located. If this is to be used in an MFC Application, it is the file where the InitInstance() function for the application is located. In a Win32 SDK application, it is where the WinMain() function is located. In a console application, it is where the main() function is located.




#include "LimitSingleInstance.H" 

4. Create a global instance of the CLimitSingleInstance class before the entry point function. If this is being used in a MFC application, create the instance before the InitInstance() function.



5. Pass a unique name to the constructor of the global CLimitSingleInstance instance. It is recommended to use a unique name so another application that may be using this article will not conflict when doing the duplicate checking. An easy way to get a unique name that no one else will have is to use the GUIDGEN tool. You can access the tool by typing GUIDGEN from the Start button and then select Run in Windows. If for some reason you do not have the tool, the tool is provided as a sample in MSDN. Type in GUIDGEN in the MSDN index to find it. Be sure to use the Registry Format option in the GUIDGEN tool.




#include "LimitSingleInstance.H"

// The one and only CLimitSingleInstance object
// Change what is passed to constructor. GUIDGEN Tool may be of help.
CLimitSingleInstance g_SingleInstanceObj(TEXT("{719967F0-DCC6-49b5-9C61-DE91175C3187}")); 

6. In your entry point function, call the IsAnotherInstanceRunning() method on the global instance of the CLimitSingleInstance class and check the return value. If the function returns TRUE, return from the entry point function. Otherwise, continue execution as normal.
   
   In an MFC Application, you could do something like the following:

   
   #include "LimitSingleInstance.H"
   
   // The one and only CLimitSingleInstance object
   CLimitSingleInstance g_SingleInstanceObj(TEXT("{05CA3573-B449-4e0b-83F5-7FD612E378E9}"));
   
   BOOL CSingleInstDlg5App::InitInstance()
   {
       if (g_SingleInstanceObj.IsAnotherInstanceRunning())
          return FALSE; 
   
   //rest of code
   } 

   
   
   In a Console Application, you could do something like the following:

   
   #include "LimitSingleInstance.H"
   
   // The one and only CLimitSingleInstance object
   CLimitSingleInstance g_SingleInstanceObj(TEXT("{9DA0BEED-7248-450a-B27C-C0409BDC377D}"));
   
   int main(int argc, char* argv[])
   {
       if (g_SingleInstanceObj.IsAnotherInstanceRunning())
          return 0;
   //rest of code
   } 

   
   
   In a Win32 SDK Application, you could do something like the following:

   
   #include "LimitSingleInstance.H"
   
   // The one and only CLimitSingleInstance object
   CLimitSingleInstance g_SingleInstanceObj(TEXT("{2194ABA1-BFFA-4e6b-8C26-D191BB16F9E6}"));
   
   int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int cmdShow)
   {
       if (g_SingleInstanceObj.IsAnotherInstanceRunning())
          return FALSE; 
   //rest of code
   } 

REFERENCES

For additional information, please see the following article(s) in the Microsoft Knowledge Base:

Q141752 Limiting 32-Bit MFC SDI/MDI Applications to a Single Instance

Q238100 Limiting 32-bit MFC SDI Applications to a Single Instance in WinCE

Additional query words: one time Chapter 12 Advanced Windows HINSTANCE hPrevInstance single

Keywords : kbMFC KbUIDesign kbVC400 kbVC500 kbVC600 kbDSupport kbGrpMFCATL
Version : winnt:4.0,4.1,4.2,5.0,6.0
Platform : winnt
Issue type : kbhowto