title: What does "implicit declaration of function" mean?
body: <pre><code>#include &lt;stdio.h&gt;
  
  int main()
  {
      int a = 4;
      int b = 3;
      addNumbers(a, b);
  }
  
  int addNumbers(int a, int b)
  {
      return a + b;
  }
  </code></pre>
  
  <p>Why does this not compile, I get a message saying implicit declaration of function <code>addNumbers()</code>?</p>