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int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    QSharedMemory sharedMemory("appname");
    if (!(sharedMemory.create(1) && sharedMemory.error() != QSharedMemory::AlreadyExists)) {
        exit(1);
    }
    ....
}

 
bool MyApp::StopSecondInstance(const char * appTitle)
{
    mMutex = ::CreateMutex(NULL, TRUE, mCaption);
    if (GetLastError() != ERROR_ALREADY_EXISTS) return false;

// now returned value is a handle to other creator thus it should not be deleted, set it 0 
    mMutex = 0; 

// activate previous instance
    HWND wnd = FindWindow(mFrameClass, appTitle);
    if (wnd) {
		if (IsIconic(wnd)) ShowWindow(wnd, SW_RESTORE);
		SetForegroundWindow(wnd);
    }
    return true;
}