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<div class="titlepage"><div><div><h3 class="title">
<a name="asio.tutorial.tutdaytime6"></a><a class="link" href="tutdaytime6.html" title="Daytime.6 - An asynchronous UDP daytime server">Daytime.6 - An asynchronous
      UDP daytime server</a>
</h3></div></div></div>
<h5>
<a name="asio.tutorial.tutdaytime6.h0"></a>
        <span><a name="asio.tutorial.tutdaytime6.the_main___function"></a></span><a class="link" href="tutdaytime6.html#asio.tutorial.tutdaytime6.the_main___function">The
        main() function</a>
      </h5>
<pre class="programlisting">int main()
{
  try
  {
</pre>
<p>
        Create a server object to accept incoming client requests, and run the <a class="link" href="../reference/io_context.html" title="io_context">io_context</a> object.
      </p>
<pre class="programlisting">    asio::io_context io_context;
    udp_server server(io_context);
    io_context.run();
  }
  catch (std::exception&amp; e)
  {
    std::cerr &lt;&lt; e.what() &lt;&lt; std::endl;
  }

  return 0;
}
</pre>
<h5>
<a name="asio.tutorial.tutdaytime6.h1"></a>
        <span><a name="asio.tutorial.tutdaytime6.the_udp_server_class"></a></span><a class="link" href="tutdaytime6.html#asio.tutorial.tutdaytime6.the_udp_server_class">The
        udp_server class</a>
      </h5>
<pre class="programlisting">class udp_server
{
public:
</pre>
<p>
        The constructor initialises a socket to listen on UDP port 13.
      </p>
<pre class="programlisting">  udp_server(asio::io_context&amp; io_context)
    : socket_(io_context, udp::endpoint(udp::v4(), 13))
  {
    start_receive();
  }

private:
  void start_receive()
  {
</pre>
<p>
        The function <a class="link" href="../reference/basic_datagram_socket/async_receive_from.html" title="basic_datagram_socket::async_receive_from">ip::udp::socket::async_receive_from()</a>
        will cause the application to listen in the background for a new request.
        When such a request is received, the <a class="link" href="../reference/io_context.html" title="io_context">io_context</a>
        object will invoke the <code class="computeroutput">handle_receive()</code> function with two arguments:
        a value of type <a class="link" href="../reference/error_code.html" title="error_code">error_code</a>
        indicating whether the operation succeeded or failed, and a <code class="computeroutput">size_t</code>
        value <code class="computeroutput">bytes_transferred</code> specifying the number of bytes received.
      </p>
<pre class="programlisting">    socket_.async_receive_from(
        asio::buffer(recv_buffer_), remote_endpoint_,
        boost::bind(&amp;udp_server::handle_receive, this,
          asio::placeholders::error,
          asio::placeholders::bytes_transferred));
  }
</pre>
<p>
        The function <code class="computeroutput">handle_receive()</code> will service the client request.
      </p>
<pre class="programlisting">  void handle_receive(const asio::error_code&amp; error,
      std::size_t /*bytes_transferred*/)
  {
</pre>
<p>
        The <code class="computeroutput">error</code> parameter contains the result of the asynchronous
        operation. Since we only provide the 1-byte <code class="computeroutput">recv_buffer_</code> to
        contain the client's request, the <a class="link" href="../reference/io_context.html" title="io_context">io_context</a>
        object would return an error if the client sent anything larger. We can ignore
        such an error if it comes up.
      </p>
<pre class="programlisting">    if (!error)
    {
</pre>
<p>
        Determine what we are going to send.
      </p>
<pre class="programlisting">      boost::shared_ptr&lt;std::string&gt; message(
          new std::string(make_daytime_string()));
</pre>
<p>
        We now call <a class="link" href="../reference/basic_datagram_socket/async_send_to.html" title="basic_datagram_socket::async_send_to">ip::udp::socket::async_send_to()</a>
        to serve the data to the client.
      </p>
<pre class="programlisting">      socket_.async_send_to(asio::buffer(*message), remote_endpoint_,
          boost::bind(&amp;udp_server::handle_send, this, message,
            asio::placeholders::error,
            asio::placeholders::bytes_transferred));
</pre>
<p>
        When initiating the asynchronous operation, and if using <a class="link" href="boost_bind.html" title="boost::bind">boost::bind</a>
        , you must specify only the arguments that match the handler's parameter
        list. In this program, both of the argument placeholders (asio::placeholders::error
        and asio::placeholders::bytes_transferred) could potentially have been removed.
      </p>
<p>
        Start listening for the next client request.
      </p>
<pre class="programlisting">      start_receive();
</pre>
<p>
        Any further actions for this client request are now the responsibility of
        <code class="computeroutput">handle_send()</code>.
      </p>
<pre class="programlisting">    }
  }
</pre>
<p>
        The function <code class="computeroutput">handle_send()</code> is invoked after the service request
        has been completed.
      </p>
<pre class="programlisting">  void handle_send(boost::shared_ptr&lt;std::string&gt; /*message*/,
      const asio::error_code&amp; /*error*/,
      std::size_t /*bytes_transferred*/)
  {
  }

  udp::socket socket_;
  udp::endpoint remote_endpoint_;
  boost::array&lt;char, 1&gt; recv_buffer_;
};
</pre>
<p>
        See the <a class="link" href="tutdaytime6/src.html" title="Source listing for Daytime.6">full source listing</a>
      </p>
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        Return to the <a class="link" href="../tutorial.html" title="Tutorial">tutorial index</a>
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<p>
        Previous: <a class="link" href="tutdaytime5.html" title="Daytime.5 - A synchronous UDP daytime server">Daytime.5 - A synchronous
        UDP daytime server</a>
      </p>
<p>
        Next: <a class="link" href="tutdaytime7.html" title="Daytime.7 - A combined TCP/UDP asynchronous server">Daytime.7 - A combined TCP/UDP
        asynchronous server</a>
      </p>
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      Kohlhoff<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
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