</section>
<section class="Sh">
<h1 class="Sh" id="RETURN_VALUES"><a class="permalink" href="#RETURN_VALUES">RETURN
  VALUES</a></h1>
<p class="Pp">Upon successful completion, a pointer to an extra field is
    returned, or <code class="Dv">NULL</code> if there is no extra field with
    that <var class="Ar">extra_field_index</var> for the file with index
    <var class="Ar">index</var>. In case of an error,
    <code class="Dv">NULL</code> is returned and the error code in
    <var class="Ar">archive</var>