span class="identifier">bytes_transferred</span>   <span class="comment">// Number of bytes sent from the</span>
                                    <span class="comment">// buffers. If an error occurred,</span>
                                    <span class="comment">// this will be less than the buffer_size.</span>
<span class="special">);</span>
</pre>
                  <p>
                    Regardless of whether the asynchronous operation completes immediately
                    or not, the handler 