title: function works ok w/o "const" flag. importance of "const"?
body: <p>The function below uses <code>const char *s1</code></p>
  
  <p>What the function does is perhaps not important. It returns 1 if the string
  contains characters other than allowed characters. It returns 0 if it
  doesn't.</p>
  
  <pre><code>int TEST (const char *s1);
  
  int TEST (const char *s1) {
      char * s2= "o123";
      return s1[ strspn(s1, s2) ] != '\0';
  } 
  </code></pre>
  
  <p>The function seems to work just fine if I were to remove <code>const</code> portion from it.</p>
  
  <p>Why do I need <code>const</code> terminology in that function, if it seems to work fine without it?  What is the importance of it? What are the implications of</p>
  
  <ul>
  <li>using?</li>
  <li>not using?</li>
  </ul>