未发送async_send数据

Question

[disclaimer] I am new to boost.

Looking into boost::asio and tried to create a simple asynchronous TCP server with the following functionality:

1. Listen for connections on port 13
2. When connected, receive data
3. If data received == time, then return current datetime, else return a predefined string ("Something else was requested")

The problem: Although, I accept the connection and receive the data, when transmitting data using async_send, although I receive no error and the value of bytes_transferred is correct, I receive empty data on the client side.

If I try to transmit the data from within handle_accept (instead of handle_read), this works fine.

The implementation: I worked on the boost asio tutorial found here : Instantiate a tcp_server object, that basically initiates the acceptor and starts listening. as shown below:

int main()
{
    try
    {
        boost::asio::io_service io_service;
        tcp_server server(io_service);
        io_service.run();
    }
    catch (std::exception& e)
    {
        std::cerr << e.what() << std::endl;
    }

    return 0;
}

and in tcp_server:

class tcp_server
{
public:
    tcp_server(boost::asio::io_service& io_service)
        : acceptor_(io_service, tcp::endpoint(tcp::v4(), 13))
    {
        start_accept();
    }

private:
    void start_accept()
    {
        using std::cout;
        tcp_connection::pointer new_connection =
            tcp_connection::create(acceptor_.get_io_service());

        acceptor_.async_accept(new_connection->socket(),
            boost::bind(&tcp_server::handle_accept, this, new_connection,
                boost::asio::placeholders::error));
        cout << "Done";
    }
    ...
}

Once a connection is accepted, I am handling it as shown below:

void handle_accept(tcp_connection::pointer new_connection,
        const boost::system::error_code& error)
{
    if (!error)
    {
        new_connection->start();
    }

    start_accept();
}

Below is the tcp_connection::start() method:

void start()
{

    boost::asio::async_read(socket_, boost::asio::buffer(inputBuffer_),
        boost::bind(&tcp_connection::handle_read, shared_from_this(),
            boost::asio::placeholders::error,
            boost::asio::placeholders::bytes_transferred));

    /* the snippet below works here - but not in handle_read 
    outputBuffer_ = make_daytime_string();

    boost::asio::async_write(socket_, boost::asio::buffer(outputBuffer_),
        boost::bind(&tcp_connection::handle_write, shared_from_this(),
            boost::asio::placeholders::error,
            boost::asio::placeholders::bytes_transferred));*/
}

and in handle_read :

void handle_read(const boost::system::error_code& error, size_t bytes_transferred)
{
    outputBuffer_ = make_daytime_string();
    if (strcmp(inputBuffer_, "time"))
    {
        /*this does not work - correct bytes_transferred but nothing shown on receiving end */
        boost::asio::async_write(socket_, boost::asio::buffer(outputBuffer_),
            boost::bind(&tcp_connection::handle_write, shared_from_this(),
                boost::asio::placeholders::error,
                boost::asio::placeholders::bytes_transferred)); 
    }
    else
    {
        outputBuffer_ = "Something else was requested";//, 128);
        boost::asio::async_write(socket_, boost::asio::buffer(outputBuffer_),
            boost::bind(&tcp_connection::handle_write, shared_from_this(),
                boost::asio::placeholders::error,
                boost::asio::placeholders::bytes_transferred));
    }
}

The handle_write is shown below:

void handle_write(const boost::system::error_code& error,
    size_t bytes_transferred)
{
    if (!error)
    {
        std::cout << "Bytes transferred: " << bytes_transferred;
        std::cout << "Message sent: " << outputBuffer_;
    }
    else
    {
        std::cout << "Error in writing: " << error.message();
    }
}

Note the following regarding handle_write ( and this is the really strange thing ):

• There is no error
• The bytes_transferred variable has the correct value
• outputBuffer_ has the correct value (as set in handle_read)

Nevertheless, the package received at the client side ( Packet Sender ) is empty (as far as data is concerned).

The complete code is shared here .

Answer 1

Complete test program (c++14). Note the handling of asynchronous buffering when responding to a receive - there may be a send already in progress.

#include <boost/asio.hpp>
#include <thread>
#include <future>
#include <vector>
#include <array>
#include <memory>
#include <mutex>
#include <condition_variable>
#include <iterator>
#include <iostream>

namespace asio = boost::asio;

asio::io_service        server_service;
asio::io_service::work  server_work{server_service};

bool listening = false;
std::condition_variable cv_listening;
std::mutex              management_mutex;

auto const shared_query = asio::ip::tcp::resolver::query(asio::ip::tcp::v4(), "localhost", "8082");

void client()
try
{
    asio::io_service      client_service;
    asio::ip::tcp::socket socket(client_service);

    auto lock = std::unique_lock<std::mutex>(management_mutex);
    cv_listening.wait(lock, [] { return listening; });
    lock.unlock();

    asio::ip::tcp::resolver resolver(client_service);
    asio::connect(socket, resolver.resolve(shared_query));
    auto s = std::string("time\ntime\ntime\n");
    asio::write(socket, asio::buffer(s));
    socket.shutdown(asio::ip::tcp::socket::shutdown_send);

    asio::streambuf sb;
    boost::system::error_code sink;
    asio::read(socket, sb, sink);
    std::cout << std::addressof(sb);
    socket.close();
    server_service.stop();
}
catch(const boost::system::system_error& se)
{
    std::cerr << "client: " << se.code().message() << std::endl;
}

struct connection
    : std::enable_shared_from_this<connection>
{
    connection(asio::io_service& ios)
        : strand_(ios)
    {

    }

    void run()
    {
        asio::async_read_until(socket_, buffer_, "\n",
                               strand_.wrap([self = shared_from_this()](auto const&ec, auto size)
        {
            if (size == 0 )
            {
                // error condition
                boost::system::error_code sink;
                self->socket_.shutdown(asio::ip::tcp::socket::shutdown_receive, sink);
            }
            else {
                self->buffer_.commit(size);
                std::istream is(std::addressof(self->buffer_));
                std::string str;
                while (std::getline(is, str))
                {
                    if (str == "time") {
                        self->queue_send("eight o clock");
                    }
                }
                self->run();
            }
        }));
    }

    void queue_send(std::string s)
    {
        assert(strand_.running_in_this_thread());
        s += '\n';
        send_buffers_pending_.push_back(std::move(s));
        nudge_send();
    }

    void nudge_send()
    {
        assert(strand_.running_in_this_thread());
        if (send_buffers_sending_.empty() and not send_buffers_pending_.empty())
        {
            std::swap(send_buffers_pending_, send_buffers_sending_);
            std::vector<asio::const_buffers_1> send_buffers;
            send_buffers.reserve(send_buffers_sending_.size());
            std::transform(send_buffers_sending_.begin(), send_buffers_sending_.end(),
                           std::back_inserter(send_buffers),
            [](auto&& str) {
                return asio::buffer(str);
            });
            asio::async_write(socket_, send_buffers,
                              strand_.wrap([self = shared_from_this()](auto const& ec, auto size)
            {
                // should check for errors here...
                self->send_buffers_sending_.clear();
                self->nudge_send();
            }));
        }
    }

    asio::io_service::strand strand_;
    asio::ip::tcp::socket    socket_{strand_.get_io_service()};
    asio::streambuf          buffer_;

    std::vector<std::string> send_buffers_pending_;
    std::vector<std::string> send_buffers_sending_;
};

void begin_accepting(asio::ip::tcp::acceptor& acceptor)
{
    auto candidate = std::make_shared<connection>(acceptor.get_io_service());
    acceptor.async_accept(candidate->socket_, [candidate, &acceptor](auto const& ec)
    {
        if (not ec) {
            candidate->run();
            begin_accepting(acceptor);
        }
    });
}

void server()
try
{
    asio::ip::tcp::acceptor acceptor(server_service);
    asio::ip::tcp::resolver resolver(server_service);

    auto first = resolver.resolve(shared_query);
    acceptor.open(first->endpoint().protocol());
    acceptor.bind(first->endpoint());

    acceptor.listen();

    begin_accepting(acceptor);

    auto lock = std::unique_lock<std::mutex>(management_mutex);
    listening = true;
    lock.unlock();
    cv_listening.notify_all();


    server_service.run();

}
catch(const boost::system::system_error& se)
{
    std::cerr << "server: " << se.code().message() << std::endl;
}

int main()
{

    using future_type = std::future<void>;

    auto stuff = std::array<future_type, 2> {{std::async(std::launch::async, client),
                                                 std::async(std::launch::async, server)}};

    for (auto& f : stuff) f.wait();

}

Answer 2

There are multiple issues in this code. Some of them may be responsible for your problem:

• TCP has no definition of packets, so there's no guarantee that you will ever receive time at once in handle_read. You need a statemachine for that and to respect the bytes_transferred info. If you only have received a part of the message you need to continue at the correct offset. Or you can use asio utility functions, like reading exactly a length of bytes or reading a line.
• In addition the last point, you shouldn't really compare the received data with strcmp . That will only work if the remote also sends a null terminator over the connection - does it?
• You don't check whether an error happend, although that might manifest itself in other errors.
• You are possibly issueing multiple concurrent async writes if you receive multiple data fragments in a shart timespan. This is not valid in asio.
• More important, you mutate the send buffer ( outputBuffer_ ) while the send is in progress. This will pretty much lead to undefined behavior. asio might try to write a piece of memory which is no longer valid.

Answer 3

I have solved the problem with the collective help of the comments provided in the question. The behavior that I was experiencing was because of the functionality of async_read . More specifically in the boost asio documentation it reads:

This function is used to asynchronously read a certain number of bytes of data from a stream. The function call always returns immediately. The asynchronous operation will continue until one of the following conditions is true:

• The supplied buffers are full. That is, the bytes transferred is equal to the sum of the buffer sizes.
• An error occurred.

The inputBuffer_ I was using to read the input, was a 128 char array. The client I was using, would only transfer the real data (without padding), and therefore the async_read would not return until the connection was closed by the client (or 128 bytes of data were transferred). When the connection was closed by the client there was no way to send back the requested data. This is also the reason that it was working with @Arunmu's simple python tcp client (because he was sending 128 bytes of data always).

To fix the issues, I made the following changes (the full working code is supplied here for reference):

• In tcp_connection::start : I am now using async_read_until to read the incoming data (and use \\n as a delimiter). The input is stored in a boost::asio::streambuf . async_read is guaranteed to return once the delimiter has been found, or an error has occurred. So there is no chance to issue multiple async_write concurrently.
• In handle_read : I have included error checking, which made it much simpler to debug.