title: What do the identifiers PID, PPID, SID, PGID, UID, EUID mean?
body: <p>I was assigned to</p>
  
  <blockquote>
    <p>Write a C program that prints the following identifiers PID, PPID, SID, PGID, UID, EUID.</p>
  </blockquote>
  
  <p>The assignment then went on to ask</p>
  
  <blockquote>
    <p>What represents each identifier?</p>
  </blockquote>
  
  <p>I have completed the program, but have not found adequate explanations for the identifiers.</p>
  
  <ul>
  <li>What do they really represent?</li>
  <li>Especially what is the purpose of the SID and PGID identifiers?</li>
  </ul>