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

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

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

    using 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;
        }

        asio::io_context io_context;

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_context);
        udp::endpoint receiver_endpoint =
          *resolver.resolve(udp::v4(), argv[1], "daytime").begin();

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.

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

        std::array<char, 1> send_buf  = {{ 0 }};
        socket.send_to(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().

        std::array<char, 128> recv_buf;
        udp::endpoint sender_endpoint;
        size_t len = socket.receive_from(
            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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