C++ UDP Server io_context running in thread exits before work can start

Question

I'm new to C++ but so far most of the asio stuff has made sense. I am however stuggling to get my UDPServer working.

My question is possibly similar to: Trying to write UDP server class, io_context doesn't block

I think my UDPServer stops before work can be given to its io_context. However, I am issuing work to the context before calling io_context.run() so I don't understand why.

Of course, I am not entirely sure if I am even on the right track with the above statement and would appreciate some guidance. Here is my class:

template<typename message_T>
    class UDPServer
    {
    public:
        UDPServer(uint16_t port)
            : m_socket(m_asioContext, asio::ip::udp::endpoint(asio::ip::udp::v4(), port))
        {
            m_port = port;
        }

        virtual ~UDPServer()
        {
            Stop();
        }



    public:

        // Starts the server!
        bool Start()
        {
            try
            {
                // Issue a task to the asio context
                WaitForMessages();

                m_threadContext = std::thread([this]() { m_asioContext.run(); });
            }
            catch (std::exception& e)
            {
                // Something prohibited the server from listening
                std::cerr << "[SERVER @ PORT " << m_port << "] Exception: " << e.what() << "\n";
                return false;
            }

            std::cout << "[SERVER @ PORT " << m_port << "] Started!\n";
            return true;
        }

        // Stops the server!
        void Stop()
        {
            // Request the context to close
            m_asioContext.stop();

            // Tidy up the context thread
            if (m_threadContext.joinable()) m_threadContext.join();

            // Inform someone, anybody, if they care...
            std::cout << "[SERVER @ PORT " << m_port << "] Stopped!\n";
        }


        void WaitForMessages()
        {

            m_socket.async_receive_from(asio::buffer(vBuffer.data(), vBuffer.size()), m_endpoint,
                [this](std::error_code ec, std::size_t length)
                {
                    if (!ec)
                    {
                        
                        std::cout << "[SERVER @ PORT " << m_port << "] Got " << length << " bytes \n Data: " << vBuffer.data() << "\n" << "Address: " << m_endpoint.address() << " Port: " << m_endpoint.port() << "\n" << "Data: " << m_endpoint.data() << "\n";

                    }
                    else
                    {
                        std::cerr << "[SERVER @ PORT " << m_port << "] Exception: " << ec.message() << "\n";
                        return;
                    }

                    WaitForMessages();
                }
            );
        }

        void Send(message_T& msg, const asio::ip::udp::endpoint& ep)
        {
            asio::post(m_asioContext,
                [this, msg, ep]()
                {
                    // If the queue has a message in it, then we must
                    // assume that it is in the process of asynchronously being written.
                    
                    bool bWritingMessage = !m_messagesOut.empty();
                    m_messagesOut.push_back(msg);
                    if (!bWritingMessage)
                    {
                        WriteMessage(ep);
                    }
                }
            );
        }

    private:

        void WriteMessage(const asio::ip::udp::endpoint& ep)
        {

            m_socket.async_send_to(asio::buffer(&m_messagesOut.front(), sizeof(message_T)), ep,
                [this, ep](std::error_code ec, std::size_t length)
                {

                    if (!ec)
                    {

                        m_messagesOut.pop_front();

                        // If the queue is not empty, there are more messages to send, so
                        // make this happen by issuing the task to send the next header.
                        if (!m_messagesOut.empty())
                        {
                            WriteMessage(ep);
                        }
                        
                    }
                    else
                    {
                        std::cout << "[SERVER @ PORT " << m_port << "] Write Header Fail.\n";
                        m_socket.close();
                    }
                });
        }

        void ReadMessage()
        {
        }

    private:
        
        uint16_t m_port = 0;
        asio::ip::udp::endpoint m_endpoint;
        std::vector<char> vBuffer = std::vector<char>(21);


    protected:
        TSQueue<message_T> m_messagesIn;
        TSQueue<message_T> m_messagesOut;
        Message<message_T> m_tempMessageBuf;
        asio::io_context m_asioContext;
        std::thread m_threadContext;
        asio::ip::udp::socket m_socket;
    };
}

Code is invoked in the main function for now:

enum class TestMsg {
    Ping,
    Join,
    Leave
};

int main() {
    Message<TestMsg> msg; // Message is a pretty basic struct that I'm not using yet. When I was, I was only receiving the first 4 bytes - which led me down this path of investigation
    msg.id = TestMsg::Join;
    msg << "hello";

    UDPServer<Message<TestMsg>> server(60000);
}

When invoked the Server immediately exits before it gets chance to print "[SERVER] Started"

I'll try adding the work guard as the link post describes but I would still like to understand why the io_context is not being primed with work quick enough.

Answer 1

Update (Now I also read the question not just the code) While in WaitForMessages you do start listening by calling the m_socket.async_receive_from function, as it is async, that function will return/unblock as soon as it has setup the listening. So as long as you don't actually have a client sending you something, you server has nothing do to. Only when it has received something the callback will be called, by a thread calling io_context::run. So you need the work guard so that your thread running run won't unblock right after start, but will block as long as the work guard is there. Usually it is also combined with a try/while pattern if an exception gets thrown in a handler and you still want to move on with your server.

Also in the code you posted, you never actually call UDPServer::Start!

---

This was my first idea of an answer:

This is normal behavior of ASIO. The io_context::run function will return as soon as it has no work to do.

So to change the behaviour of the run function to block you have to use a boost::asio::executor_work_guard<boost::asio::io_context::executor_type> i.e. a so called work guard. Construct that object with a reference to your io_context and hold it i.e. don't let it destruct as long as you want to let the server run, i.e. do not want to let io_context::run return when there is not work.

So given

    boost::asio::io_context io_context_;
    boost::asio::executor_work_guard<boost::asio::io_context::executor_type> work_guard_;

you then could call

      work_guard_{boost::asio::make_work_guard(io_context_)},

const auto thread_count{std::max<unsigned>(std::thread::hardware_concurrency(), 1)};

std::generate_n(std::back_inserter(this->io_run_threads_),
                thread_count,
                [this]() {
                    return std::thread{io_run_loop,
                                       std::ref(this->io_context_), std::ref(this->error_handler_)};
                });

void io_run_loop(boost::asio::io_context &context,
                                    const std::function<void(std::exception &)> &error_handler) {
    while (true) {
        try {
            context.run();
            break;
        } catch (std::exception &e) {
            error_handler(e);
        }
    }

}

And then for server shutdown:

work_guard_.reset();
io_context_.stop();
std::for_each(this->io_run_threads_.begin(), this->io_run_threads_.end(), [](auto &thread) {
    if (thread.joinable()) thread.join();
});

For a more graceful shutdown you can omit the stop call and rather close all sockets before.

Answer 2

Looks like you forgot to call server.Start();. Moreover, you will want to make the main thread wait for some amount of time, otherwise the destructor of Server will immediately cause Stop() to be called:

int main()
{
    Message<TestMsg> msg;
    msg.id = TestMsg::Join;
    msg << "hello";

    UDPServer<Message<TestMsg>> server(60000);
    server.Start();

    std::this_thread::sleep_for(30s);
}

Issues

1. There is a conceptual problem with the Send API. It takes an endpoint on each call, but it only uses the one that starts the write call chain! This means that if you do

    srv.Send(msg1, {mymachine, 60001});
    srv.Send(msg1, {otherserver, 5517});
   

   It is likely they both get sent to mymachine:60001.

2. How you treat the buffer received. Just using .data() blindly assumes that the data is NUL-terminated. Don't do that:

   std::string const data(vBuffer.data(), length);
   

   Also, you seem to have at some time been confused about data and printed m_endpoint.data() - your princess is in another castle.

   In reality you probably want ways to extract the typed data. I'm leaving that as beyond the scope of this question for today.

3. Regardless you should clear the buffer before reuse, because you might be seeing old data in subsequent reads.

   vBuffer.assign(vBuffer.size(), '\0');
   

4. This is most likely undefined behaviour:

   asio::buffer(&m_messagesOut.front(), sizeof(message_T)), ep,
   

   This is only valid if message_T is trivial and standard-layout ("POD" - Plain Old Data). The presence of operator<< strongly suggests that is not the case.

   Instead, build a (sequence of) buffer(s) hat represents the message as raw bytes, e.g.

   auto& msg = m_messagesOut.front();
   msg.length = msg.body.size();
   
   m_socket.async_send_to(
       std::vector<asio::const_buffer>{
           asio::buffer(&msg.id, sizeof(msg.id)),
           asio::buffer(&msg.length, sizeof(msg.length)),
           asio::buffer(msg.body),
       },
   
   // ...
   

5. Thread safe queues seem to be overkill since you have a single service thread; that is an implicit "strand" so you can post to it to have single-threaded semantics.

Here's a few adaptations to make it work so far (except the exercise-for-the-reader pointed out):

Live On Coliru

#include <boost/asio.hpp>
#include <iostream>
#include <deque>
#include <sstream>

// Library facilities
namespace asio = boost::asio;
using asio::ip::udp;
using boost::system::error_code;
using namespace std::chrono_literals;

/////////////////////////////////
// mock ups:
template <typename message_T> struct Message {
    message_T   id;
    uint16_t    length; // automatically filled on send, UDP packets are < 64k
    std::string body;

    template <typename T> friend Message& operator<<(Message& m, T const& v)
    {
        std::ostringstream oss;
        oss << v;
        m.body += oss.str();
        //m.body += '\0'; // suggestion for easier message extraction

        return m;
    }
};

// Thread-safety can be replaced with the implicit strand of a single service
// thread
template <typename T> using TSQueue = std::deque<T>;
// end mock ups
/////////////////////////////////

template <typename message_T> class UDPServer {
  public:
    UDPServer(uint16_t port)
        : m_socket(m_asioContext, udp::endpoint(udp::v4(), port))
    {
        m_port = port;
    }

    virtual ~UDPServer() { Stop(); }

  public:
    // Starts the server!
    bool Start()
    {
        if (m_threadContext.joinable() && !m_asioContext.stopped())
            return false;

        try {
            // Issue a task to the asio context
            WaitForMessages();

            m_threadContext = std::thread([this]() { m_asioContext.run(); });
        } catch (std::exception const& e) {
            // Something prohibited the server from listening
            std::cerr << "[SERVER @ PORT " << m_port
                      << "] Exception: " << e.what() << "\n";
            return false;
        }
        std::cout << "[SERVER @ PORT " << m_port << "] Started!\n";
        return true;
    }

    // Stops the server!
    void Stop()
    {
        // Tell the context to stop processing
        m_asioContext.stop();

        // Tidy up the context thread
        if (m_threadContext.joinable())
            m_threadContext.join();

        // Inform someone, anybody, if they care...
        std::cout << "[SERVER @ PORT " << m_port << "] Stopped!\n";

        m_asioContext
            .reset(); // required in case you want to reuse this Server object
    }

    void Send(message_T& msg, const udp::endpoint& ep)
    {
        asio::post(m_asioContext, [this, msg, ep]() {
            // If the queue has a message in it, then we must
            // assume that it is in the process of asynchronously being written.

            bool bWritingMessage = !m_messagesOut.empty();
            m_messagesOut.push_back(msg);
            if (!bWritingMessage) {
                WriteMessage(ep);
            }
        });
    }

  private:
    void WaitForMessages() // assumed to be on-strand
    {
        vBuffer.assign(vBuffer.size(), '\0');
        m_socket.async_receive_from(
            asio::buffer(vBuffer.data(), vBuffer.size()), m_endpoint,
            [this](std::error_code ec, std::size_t length) {
                if (!ec) {
                    std::string const data(vBuffer.data(), length);

                    std::cout << "[SERVER @ PORT " << m_port << "] Got "
                              << length << " bytes \n Data: " << data << "\n"
                              << "Address: " << m_endpoint.address()
                              << " Port: " << m_endpoint.port() << "\n"
                              << std::endl;
                } else {
                    std::cerr << "[SERVER @ PORT " << m_port
                              << "] Exception: " << ec.message() << "\n";
                    return;
                }

                WaitForMessages();
            });
    }

    void WriteMessage(const udp::endpoint& ep)
    {
        auto& msg = m_messagesOut.front();
        msg.length = msg.body.size();

        m_socket.async_send_to(
            std::vector<asio::const_buffer>{
                asio::buffer(&msg.id, sizeof(msg.id)),
                asio::buffer(&msg.length, sizeof(msg.length)),
                asio::buffer(msg.body),
            },
            ep, [this, ep](std::error_code ec, std::size_t length) {
                if (!ec) {
                    m_messagesOut.pop_front();

                    // If the queue is not empty, there are more messages to
                    // send, so make this happen by issuing the task to send the
                    // next header.
                    if (!m_messagesOut.empty()) {
                        WriteMessage(ep);
                    }

                } else {
                    std::cout << "[SERVER @ PORT " << m_port
                              << "] Write Header Fail.\n";
                    m_socket.close();
                }
            });
    }

  private:
    uint16_t          m_port = 0;
    udp::endpoint     m_endpoint;
    std::vector<char> vBuffer = std::vector<char>(21);

  protected:
    TSQueue<message_T> m_messagesIn;
    TSQueue<message_T> m_messagesOut;
    Message<message_T> m_tempMessageBuf;

    asio::io_context m_asioContext;
    std::thread      m_threadContext;
    udp::socket      m_socket;
};

enum class TestMsg {
    Ping,
    Join,
    Leave
};

int main()
{
    UDPServer<Message<TestMsg>> server(60'000);
    if (server.Start()) {
        std::this_thread::sleep_for(3s);

        {
            Message<TestMsg> msg;
            msg.id = TestMsg::Join;
            msg << "hello PI equals " << M_PI  << " in this world";

            server.Send(msg, {{}, 60'001});
        }

        std::this_thread::sleep_for(27s);
    }
}

For some reason netcat doesn't work with UDP on Coliru, so here's a "live" demo:

You can see our netcat client messages arriving. You can see the message Sent to 60001 arriving in the tcpdump output.