Asynchronous read/write operations

Unlike classical UNIX socket programming, boost.asio has battery-included asynchronous read/write abilities. Still use basic_stream_socket as an example, and one pair of the implementations is like this:

template <typename ConstBufferSequence, typename WriteHandler>
  BOOST_ASIO_INITFN_RESULT_TYPE(WriteHandler,
      void (boost::system::error_code, std::size_t))
  async_send(const ConstBufferSequence& buffers,
      BOOST_ASIO_MOVE_ARG(WriteHandler) handler)
{
    .......
}
template <typename MutableBufferSequence, typename ReadHandler>
  BOOST_ASIO_INITFN_RESULT_TYPE(ReadHandler,
      void (boost::system::error_code, std::size_t))
  async_receive(const MutableBufferSequence& buffers,
      BOOST_ASIO_MOVE_ARG(ReadHandler) handler)
{
    .......
}

Since async_send and async_receive functions will return immediately, and not block current thread, you should pass a callback function as the parameter which receives the result of read/write operations:

void handler(
    const boost::system::error_code& error, // Result of operation.
    std::size_t bytes_transferred           // Number of bytes processed.
)

There is a simple client/server example. Below is client code:

#include <boost/asio.hpp>
#include <functional>
#include <iostream>
#include <memory>

void callback(
        const boost::system::error_code& error,
        std::size_t bytes_transferred,
        std::shared_ptr<boost::asio::ip::tcp::socket> socket,
        std::string str)
{
    if (error)
    {
        std::cout << error.message() << '\n';
    }
    else if (bytes_transferred == str.length())
    {
        std::cout << "Message is sent successfully!" << '\n';
    }
    else
    {
        socket->async_send(
                boost::asio::buffer(str.c_str() + bytes_transferred, str.length() - bytes_transferred),
                std::bind(callback, std::placeholders::_1, std::placeholders::_2, socket, str));
    }
}


int main()
{
    try
    {
        boost::asio::io_context io_context;

        boost::asio::ip::tcp::endpoint endpoint{
                boost::asio::ip::make_address("192.168.35.145"),
                3303};

        std::shared_ptr<boost::asio::ip::tcp::socket> socket{new boost::asio::ip::tcp::socket{io_context}};
        socket->connect(endpoint);

        std::cout << "Connect to " << endpoint << " successfully!\n";

        std::string str{"Hello world!"};
        socket->async_send(
                boost::asio::buffer(str),
                std::bind(callback, std::placeholders::_1, std::placeholders::_2, socket, str));
        socket->get_executor().context().run();
    }
    catch (std::exception& e)
    {
        std::cerr << e.what() << '\n';
        return -1;
    }

    return 0;
}

Let's go through the code:

(1) Since socket object is non-copyable (please refer socket), socket is created as an shared pointer:

......
std::shared_ptr<boost::asio::ip::tcp::socket> socket{new boost::asio::ip::tcp::socket{io_context}};
......

(2) Because the callback only has two parameters, it needs to use std::bind to pass additional parameters:

......
std::bind(callback, std::placeholders::_1, std::placeholders::_2, socket, str)
......

(3) async_send does not guarantee all the bytes are sent (boost::asio::async_write returns either all bytes are sent successfully or an error occurs), so needs to reissue async_send in callback:

......
if (error)
{
    ......
}
else if (bytes_transferred == str.length())
{
    ......
}
else
{
    socket->async_send(......);
}

(4) io_context.run function will block until all work has finished and there are no more handlers to be dispatched, or until the io_context has been stopped:

socket->get_executor().context().run();

If there is no io_context.run function, the program will exit immediately.

Check the server code who uses async_receive:

#include <ctime>
#include <functional>
#include <iostream>
#include <string>
#include <boost/asio.hpp>

void callback(
        const boost::system::error_code& error,
        std::size_t,
        char recv_str[]) {
    if (error)
    {
        std::cout << error.message() << '\n';
    }
    else
    {
        std::cout << recv_str << '\n';
    }
}

int main()
{
    try
    {
        boost::asio::io_context io_context;

        boost::asio::ip::tcp::acceptor acceptor(
                                        io_context,
                                        boost::asio::ip::tcp::endpoint(boost::asio::ip::tcp::v4(), 3303));

        for (;;)
        {
            boost::asio::ip::tcp::socket socket(io_context);
            acceptor.accept(socket);

            char recv_str[1024] = {};
            socket.async_receive(
                    boost::asio::buffer(recv_str),
                    std::bind(callback, std::placeholders::_1, std::placeholders::_2, recv_str));
            socket.get_executor().context().run();
            socket.get_executor().context().restart();
        }
    }
    catch (std::exception& e)
    {
        std::cerr << e.what() << std::endl;
    }

    return 0;
}

There are two caveats you need to pay attention to:

(1) Just for demo purpose: for every client, the callback is called only once;
(2) io_context.restart must be called to invoke another io_context.run.

Correspondingly, you can also check how to use boost::asio::async_read.

Build and run programs. Client outputs following:

$ ./client
Connect to 192.168.35.145:3303 successfully!
Message is sent successfully!

Server outputs following:

$ ./server
Hello world!