equivalent signature:
                </p>
<pre class="programlisting"><span class="keyword">void</span>
<span class="identifier">callback</span><span class="special">(</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">uint64_t</span> <span class="identifier">size</span><span class="special">,</span>         <span class="comment">// Size of the chunk, zero for the last chunk</span>
    <span class="identifier">string_view</span> <span class="identifier">ex