title: Linux command "kill" can send signal to threads
body: <p>I created 3 thread using  <code>pthread_create</code> utility.  I could send signal to  thread using<br />
  <code>kill -SIGUSR1 thread-id</code>.</p>
  <p>I got thread id  using  <code>ps -eLF</code> command ( LWP field ).</p>
  <p>I am wondering:</p>
  <ol>
  <li><p>Why we need kill?</p>
  </li>
  <li><p>How can I use kill command to send a signal to thread group id.?  How do I find the group id ?</p>
  </li>
  </ol>