title: what's the meaning of 'I' in S_IRUSR
body: <p>S_IRUSR is a macro constant in sys/stat.h of posix. it stands for user read permission bit.</p>
  
  <p>the prefix S_ may stand for 'status of'
  the RUSR maybe Read of User.
  but what's the meaning of 'I'?</p>