MFC Application: How can I limit my application to one instance?

[Andreas Masur]

Q: How can I limit my application to one instance?

A: You need to create a named mutex semaphore when you start your application. When the second one starts it tries to get access to the mutex but will fail...

// app.h
class CYourApp : public CWinApp
{
  ...

private:
  HANDLE hMutex;
};

// app.cpp
BOOL CYourApp::InitInstance()
{
  // Create mutex
  hMutex = ::CreateMutex(NULL, TRUE, "GlobalMutex");

  switch(::GetLastError())
  {
    case ERROR_SUCCESS:
      // Mutex created successfully. There is no instance running
      break;

    case ERROR_ALREADY_EXISTS:
      // Mutex already exists so there is a running instance of our app.
      return FALSE;

    default:
      // Failed to create mutex by unknown reason
      return FALSE;
  }
}