title: Set and Oldset in sigprocmask()
body: <p>I haven't completely understood, how to use <code>sigprocmask()</code>. Particularly, how the <code>set</code> and <code>oldset</code> and its syntax work and how to use them.</p>
  
  <pre><code>int sigprocmask(int how, const sigset_t *set, sigset_t *oldset);
  </code></pre>
  
  <p>Please explain with an example, to block, say SIGUSR1 for a few seconds and then unblock and handle it.
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