Read boost::asio UDP Broadcast Response

Question

I am attempting to use boost::asio to implement a simple device discovery protocol. Basically I want to send a broadcast message (port 9000) with 2 byte payload. Then read the response from the device (assuming currently it exists). In wireshark I can see the broadcast is been sent and that the device is responding. However, in my example code I get that the bytes returned is 0 in the UDP read, not 30 bytes of data.

No.  Time      Source         Destination         Protocol  Length
1    0.00000   192.168.0.20   255.255.255.255     UDP       44        52271 -> 9000  Len = 2
2    0.00200   192.168.0.21   192.168.0.20        UDP       72        9000  -> 52271 Len = 30

Should I be reading from a different endpoint than broadcastEndpoint? How do I get the end point?

I am new to asio and trying to teach my self, but I cannot work what I have done wrong.

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

class udp_find {
public:
    udp_find(boost::asio::io_context& service, unsigned int port)
    : broadcastEndpoint_(boost::asio::ip::address_v4::broadcast(), port),
      socket_(service)
    {
        socket_.open(boost::asio::ip::udp::v4());
        socket_.set_option(boost::asio::ip::udp::socket::reuse_address(true));
        socket_.set_option(boost::asio::socket_base::broadcast(true));

        boost::array<unsigned int, 2> data = {255, 255};
        socket_.async_send_to(
                          boost::asio::buffer(data, 2), broadcastEndpoint_,
                              boost::bind(&udp_find::handle_send, this,
                                          boost::asio::placeholders::error,
                                          boost::asio::placeholders::bytes_transferred));
}

void handle_receive(const boost::system::error_code& error,
                    std::size_t bytes_transferred)
{
    std::cout << "Received Data" << bytes_transferred << std::endl;
}

void handle_send(const boost::system::error_code& error, std::size_t bytes_transferred)
{
    std::cout << "Sent Data"  << bytes_transferred << std::endl;

    socket_.async_receive_from(
                               boost::asio::buffer(buffer_), broadcastEndpoint_,
                               boost::bind(&udp_find::handle_receive, this,
                                           boost::asio::placeholders::error,
                                               boost::asio::placeholders::bytes_transferred));
    }

private:
    boost::asio::ip::udp::socket socket_;
    boost::array<char, 128> buffer_;
    boost::asio::ip::udp::endpoint broadcastEndpoint_;
};

int main()
{
    boost::asio::io_context service;
    udp_find(service, 9000);
    service.run();
}

Answer 1

Your first problem is Udefined Behaviour.

You start asynchronous operations on a temporary object of type udp_find. The object is destructed immediately after construction, so it doesn't exist anymore even before you start any of the async work (service.run()).

That is easily fixed by making udp_find a local variable instead of a temporary:

udp_find op(service, 9000);

Now sending works for me. You will want to test that receiving works as well. In my netstat output it appears that the UDP socket is bound to an ephemeral port. Sending a datagram to that port makes the test succeed for me.

You might want to actually bind/connect to the broadcast address before receiving (the endpoint& parameter to async_receive_from is not for that, I think it is an output parameter).