boost::asio + std::future - Access violation after closing socket

Question

I am writing a simple tcp client to send and receive single lines of text. The asynchronous operations are handled by std::future in order to faciliate blocking queries with

Answer 1

recvmsg is receiving into a buffer (streambuf) that was freed after throwing the exception in TCPClient::sendMessage (line 105, end of scope).

You forgot to cancel the asynchronous operation (async_read_until) started in line 97. Fix it:

else {
    socket->cancel(); // ADDED
    std::cout << "socket points to " << std::addressof(*socket) << std::endl;
    throw std::runtime_error("timeout");
}

Or even, just

    socket.reset(); // ADDED

Same goes for other timeout paths.

Answer 2

The other answer addresses what went wrong.

On a higher level, though, you're using futures, just to immediately await their return.

It struck me that this is actually not asynchrony at all, and you should be able to do:

• without threading, and joining
• without .stop()
• without work and work.reset()
• without a explicit constructor or destructor
• without the unique_ptr<socket> and the lifetime management that came with it
• without the future<>, and the .get() and future_status checking that come with it

All in all, you can do a lot simpler, e.g. using a simple helper function like this:

class TCPClient {
public:
    void        disconnect();
    void        connect(const std::string& address, const std::string& port);
    std::string sendMessage(const std::string& msg);

private:
    using error_code = boost::system::error_code;

    template<typename AllowTime> void await_operation(AllowTime const& deadline_or_duration) {
        using namespace boost::asio;

        ioservice.reset();
        {
            high_resolution_timer tm(ioservice, deadline_or_duration);
            tm.async_wait([this](error_code ec) { if (ec != error::operation_aborted) socket.cancel(); });
            ioservice.run_one();
        }
        ioservice.run();
    }

    boost::asio::io_service      ioservice { };
    boost::asio::ip::tcp::socket socket { ioservice };
};

E.g. connect(...) used to be:

socket.reset(new ip::tcp::socket(ioservice));

ip::tcp::resolver::query query(address, port);
std::future<ip::tcp::resolver::iterator> conn_result = async_connect(*socket, ip::tcp::resolver(ioservice).resolve(query), use_future);

if (conn_result.wait_for(std::chrono::seconds(6)) != std::future_status::timeout) {
    conn_result.get(); // throws boost::system::system_error if the operation fails
}
else {
    socket->cancel();
    // throw timeout_error("Timeout");
    throw std::runtime_error("timeout");
}

It now becomes:

async_connect(socket, 
        ip::tcp::resolver(ioservice).resolve({address, port}),
        [&](error_code ec, ip::tcp::resolver::iterator it) { if (ec) throw std::runtime_error(ec.message()); });

await_operation(std::chrono::seconds(6));

Like wise, sendMessage becomes:

streambuf response;
async_read_until(socket, response, '\n', [&](error_code ec, size_t bytes_read) {
        if (ec) throw std::runtime_error(ec.message());
        std::cout << "read " << bytes_read << " bytes" << std::endl;
    });

await_operation(std::chrono::system_clock::now() + std::chrono::seconds(4));

return {std::istreambuf_iterator<char>(&response), {}};

Note these are significantly simpler. Note, also, that correct exception messages are now thrown, depending on the cause of the failures.

Full Demo

Live On Coliru

#ifndef __TCPCLIENT_H__
#define __TCPCLIENT_H__

#include <boost/asio.hpp>
#include <boost/asio/high_resolution_timer.hpp>
#include <iostream>

class TCPClient {
public:
    void        disconnect();
    void        connect(const std::string& address, const std::string& port);
    std::string sendMessage(const std::string& msg);

private:
    using error_code = boost::system::error_code;

    template<typename AllowTime> void await_operation(AllowTime const& deadline_or_duration) {
        using namespace boost::asio;

        ioservice.reset();
        {
            high_resolution_timer tm(ioservice, deadline_or_duration);
            tm.async_wait([this](error_code ec) { if (ec != error::operation_aborted) socket.cancel(); });
            ioservice.run_one();
        }
        ioservice.run();
    }

    boost::asio::io_service      ioservice { };
    boost::asio::ip::tcp::socket socket { ioservice };
};


inline void TCPClient::connect(const std::string& address, const std::string& port) {
    using namespace boost::asio;

    async_connect(socket, 
            ip::tcp::resolver(ioservice).resolve({address, port}),
            [&](error_code ec, ip::tcp::resolver::iterator it) { if (ec) throw std::runtime_error(ec.message()); });

    await_operation(std::chrono::seconds(6));
}

inline void TCPClient::disconnect() {
    using namespace boost::asio;

    if (socket.is_open()) {
        try {
            socket.shutdown(ip::tcp::socket::shutdown_both);
            socket.close();
        }
        catch (const boost::system::system_error& e) {
            // ignore
            std::cerr << "ignored error " << e.what() << std::endl;
        }
    }
}

inline std::string TCPClient::sendMessage(const std::string& msg) {
    using namespace boost::asio;

    streambuf response;
    async_read_until(socket, response, '\n', [&](error_code ec, size_t bytes_read) {
            if (ec) throw std::runtime_error(ec.message());
            std::cout << "read " << bytes_read << " bytes" << std::endl;
        });

    await_operation(std::chrono::system_clock::now() + std::chrono::seconds(4));

    return {std::istreambuf_iterator<char>(&response), {}};
}
#endif

#include <iostream>

//#include "TCPClient.hpp"

int main(/*int argc, char* argv[]*/) {
    TCPClient client;
    try {
        client.connect("127.0.0.1", "27015");
        std::cout << "Response: " << client.sendMessage("Hello!") << std::endl;
    }
    catch (const boost::system::system_error& e) {
        std::cerr << e.what() << std::endl;
    }
    catch (const std::exception& e) {
        std::cerr << e.what() << std::endl;
    }
}

BONUS

If you want even more convenience, have a generalized callback handler that just raises the exception:

struct raise {
    template <typename... A> void operator()(error_code ec, A...) const {
        if (ec) throw std::runtime_error(ec.message()); 
    }
};

Now, the bodies become even simpler in absense of lambdas:

inline void TCPClient::connect(const std::string& address, const std::string& port) {
    async_connect(socket, ip::tcp::resolver(ioservice).resolve({address, port}), raise());
    await_operation(std::chrono::seconds(6));
}

inline std::string TCPClient::sendMessage(const std::string& msg) {
    streambuf response;
    async_read_until(socket, response, '\n', raise());
    await_operation(std::chrono::system_clock::now() + std::chrono::seconds(4));

    return {std::istreambuf_iterator<char>(&response), {}};
}

See the adapted demo: Live On Coliru too