title: What is external linkage and internal linkage?
body: <p>I want to understand the external linkage and internal linkage and their difference. </p>
  
  <p>I also want to know the meaning of</p>
  
  <blockquote>
    <p><code>const</code> variables internally link by default unless otherwise declared as <code>extern</code>.</p>
  </blockquote>