if a device name is not supplied (that is, devicename is null), the device object created by iocreatedevice will not (and cannot) have a discretionary access control list (dacl) associated with it.
如果没有提供这个参数,即设备名为空的话,那么创建的设备对象将不能任意访问控制列表(dacl)。
devicename should be the same as the device name defined in the corresponding device driver program.