rg/onlinepubs/9699919799/functions/free.html">free(3)</a>'d, and becomes
    invalid when <var class="Ar">archive</var> is closed. If
    <var class="Ar">idp</var> is not <code class="Dv">NULL</code>, the integer
    to which it points will be set to the ID (two-byte signature) of the
    selected extra field. If <var class="Ar">lenp</var> is not
    <code class="Dv">NULL</code>, the integer to which it points will be set to
    the length of the extra field. Generally speaking,
    <var class="Ar">lenp</va