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### Daytime.4 - A synchronous UDP daytime client

This tutorial program shows how to use asio to implement a client application with UDP.

    #include <iostream>
    #include <boost/array.hpp>
    #include <boost/asio.hpp>

    using boost::asio::ip::udp;

The start of the application is essentially the same as for the TCP daytime client.

    int main(int argc, char* argv[])
    {
      try
      {
        if (argc != 2)
        {
          std::cerr << "Usage: client <host>" << std::endl;
          return 1;
        }

        boost::asio::io_service io_service;

We use an ip::udp::resolver object to find the correct remote endpoint to use based on the host and service names. The query is restricted to return only IPv4 endpoints by the ip::udp::v4() argument.

        udp::resolver resolver(io_service);
        udp::resolver::query query(udp::v4(), argv[1], "daytime");

The ip::udp::resolver::resolve() function is guaranteed to return at least one endpoint in the list if it does not fail. This means it is safe to dereference the return value directly.

        udp::endpoint receiver_endpoint = *resolver.resolve(query);

Since UDP is datagram-oriented, we will not be using a stream socket. Create an ip::udp::socket and initiate contact with the remote endpoint.

        udp::socket socket(io_service);
        socket.open(udp::v4());

        boost::array<char, 1> send_buf  = {{ 0 }};
        socket.send_to(boost::asio::buffer(send_buf), receiver_endpoint);

Now we need to be ready to accept whatever the server sends back to us. The endpoint on our side that receives the server's response will be initialised by ip::udp::socket::receive_from().

        boost::array<char, 128> recv_buf;
        udp::endpoint sender_endpoint;
        size_t len = socket.receive_from(
            boost::asio::buffer(recv_buf), sender_endpoint);

        std::cout.write(recv_buf.data(), len);
      }

Finally, handle any exceptions that may have been thrown.

      catch (std::exception& e)
      {
        std::cerr << e.what() << std::endl;
      }

      return 0;
    }

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