title: Linux IOCTL commands
body: <p>Trying to implement IOCTL commands, and just encountered as below:</p>
  
  <p>If <code>_IO</code> is for both read and write</p>
  
  <p>than why should I consider <code>_IOR</code> and <code>_IOW</code></p>
  
  <p>any clue please ?</p>