QT windowsの下でただ1つの実例を実行することを許可します

int main(int argc, char *argv[])
{
     QApplication a(argc, argv);
     QSharedMemory *shareMem = new QSharedMemory(QString("SingleInstanceIdentify"));

     /* if the sharedmemory has not been created, it returns false, otherwise true.
      * But if the application exit unexpectedly, the sharedmemory will not detach.
      * So, we try twice.
      */
     volatile short i = 2;
     while (i--)
     {
         if (shareMem->attach(QSharedMemory::ReadOnly)) /* no need to lock, bcs it's read only */
         {
             shareMem->detach();
         }
     }

     if (shareMem->create(1)) 


    {
         MainWindow w;
         w.show();
         a.exec();
         
         if (shareMem->isAttached())
             shareMem->detach();
         delete shareMem;
     }

     return 0;
}