title: How do I use extern to share variables between source files?
body: <p>I know that global variables in C sometimes have the <code>extern</code> keyword. What is an <code>extern</code> variable? What is the declaration like? What is its scope?</p>
  
  <p>This is related to sharing variables across source files, but how does that work precisely? Where do I use <code>extern</code>?</p>