title: How to determine if the user is known or not using pam_authenticate?
body: <p>I am authenticating a user and I want to determine if the user is known (a valid existing username) or unknown.</p>
  
  <pre><code>int ret = pam_authenticate(pamh, PAM_DISALLOW_NULL_AUTHTOK);
  </code></pre>
  
  <p>Now even if the username is unknown, <code>pam_authenticate</code> returns <code>PAM_AUTH_ERR</code> instead of (expected) <code>PAM_USER_UNKNOWN</code>.</p>
  
  <p>How to find if the username is known or not?</p>