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;
}

See the full source listing

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Next: Daytime.5 - A synchronous UDP daytime server