Io used for a simple ioctl that sends nothing but the type and number, and receives back nothing but an (integer) retval.
IoR used for an ioctl that reads data from the device driver. The driver will be allowed to return sizeof(data_type) bytes to the user.
IoRW a combination of IoR and IoW. That is, data is both written to the driver and then read back from the driver by the client.
IoW used for an ioctl that writes data to the device driver.
Ioctl simplified ioct call