C++ udp boost client doesn't receive udp messages - c++

Question

When I run:
nc -ul <ip address> <port>
I receive messages from the udp server. However when I try to reproduce the equivalent using the below C++ program with boost it just freezes on receive_from:
#include <iostream>
#include <boost/array.hpp>
#include <boost/asio.hpp>
using boost::asio::ip::udp;
int main(int argc, char *argv[]) {
try {
if (argc != 3) {
std::cerr << "Usage: client <host> <port>" << std::endl;
return 1;
}
boost::asio::io_context ioContext;
udp::resolver resolver(ioContext);
udp::resolver::query query(udp::v4(), argv[1], argv[2]);
udp::endpoint endpoint = *resolver.resolve(query);
udp::socket socket(ioContext);
socket.open(udp::v4());
// boost::array<char, 1> sendBuf = {0};
// socket.send_to(boost::asio::buffer(sendBuf), endpoint);
char letter = 0;
udp::endpoint sender_endpoint;
while(socket.receive_from(boost::asio::buffer(&letter, 1), endpoint)) {
std::cout << letter;
}
}
catch (std::exception &e) {
std::cerr << e.what() << std::endl;
}
return 0;
}
Additionally I'm just modeling the above code off of the boost udp client code in their tutorial seen here:
//
// client.cpp
// ~~~~~~~~~~
//
// Copyright (c) 2003-2020 Christopher M. Kohlhoff (chris at kohlhoff dot com)
//
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
//
#include <iostream>
#include <boost/array.hpp>
#include <boost/asio.hpp>
using boost::asio::ip::udp;
int main(int argc, char* argv[])
{
try
{
if (argc != 2)
{
std::cerr << "Usage: client <host>" << std::endl;
return 1;
}
boost::asio::io_context io_context;
udp::resolver resolver(io_context);
udp::endpoint receiver_endpoint =
*resolver.resolve(udp::v4(), argv[1], "daytime").begin();
udp::socket socket(io_context);
socket.open(udp::v4());
boost::array<char, 1> send_buf = {{ 0 }};
socket.send_to(boost::asio::buffer(send_buf), receiver_endpoint);
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);
}
catch (std::exception& e)
{
std::cerr << e.what() << std::endl;
}
return 0;
}
I have a suspicion that the tutorial code just doesn't work for my case? Not sure what's wrong or what's the difference. What's the right way to make a standard udp echo client?

Answer 1

I tested the following code successfully receives message. udp::resolver in your code is not necessary since you're not sending any message, but receiving.
When opening a socket in your case, you should designate a port number in which UDP is expecting data. This can be done by passing a second argument to udp::socket. The second example in your post, however, does not require to do so, because it is reusing its endpoint - sender_endpoint.
receive_from() takes its second argument to store sender's endpoint. You should not pass your peer's endpoint, but a variable to keep where the message came from.
int main(int argc, char* argv[]) {
try {
if (argc != 3) {
std::cerr << "Usage: client <host> <port>" << std::endl;
return 1;
}
boost::asio::io_context ioContext;
udp::socket socket(ioContext, udp::endpoint(udp::v4(), 6078)); // designated port
char letter = 0;
udp::endpoint ep_sender;
while (socket.receive_from(boost::asio::buffer(&letter, 1), ep_sender)) {
std::cout << letter;
//std::cout << ep_sender.address().to_string() << endl; // sender's address.
}
return 0;
}
catch (std::exception& e) {
std::cerr << e.what() << std::endl;
}
}