How to bind the local endpoint to a socket?

问题

I have the following code where I'm trying to send a packet to a specific client by changing the endpoint of the socket manually and I'm not sure how to configure the socket to use the async_write function to send the packet. Lines 44-47 are used to try solving the problem within the boost async server example (modified).

#include <cstdlib>
#include <iostream>
#include <boost/bind.hpp>
#include <boost/asio.hpp>

using boost::asio::ip::tcp;

class session
{
public:
  session(boost::asio::io_service& io_service)
    : socket_(io_service)
  {
  }

  tcp::socket& socket()
  {
    return socket_;
  }

  void start()
  {
      std::cout<<"start(): "<<std::endl;
          socket_.async_read_some(boost::asio::buffer(data_, max_length),
          boost::bind(&session::handle_read, this,
          boost::asio::placeholders::error,
          boost::asio::placeholders::bytes_transferred));
  }
void read_handler(const boost::system::error_code& ec, std::size_t bytes_transferred);


private:
  void handle_read(const boost::system::error_code& error,
      size_t bytes_transferred)
  {
      std::cout<<"handle read: bytes_transferred"<<bytes_transferred<<std::endl;
    if (!error)
    {
          data_ = {'0','1','2','3','4','5','6','7','8','9'};
          /*read_handler(error, bytes_transferred);
          boost::asio::async_write(socket_,boost::asio::buffer(data_, bytes_transferred),
                boost::bind(&session::handle_write, this,
                            boost::asio::placeholders::error));*/
          boost::system::error_code ec;
          boost::asio::ip::tcp::endpoint endpoint = socket_.remote_endpoint(ec);
          std::cout<<"ip address: "<<endpoint.address()<<std::endl;
          std::cout<<"port: "<<endpoint.port()<<std::endl;
          //how to get the socket to send data to specific port, ip-address e.g. changing the ip-address and port by editing it?
          async_write(socket_, boost::asio::buffer(data_, max_length), boost::bind(&session::writeHandler, this, boost::asio::placeholders::error, boost::asio::placeholders::bytes_transferred));
    }
    else
    {
      delete this;
    }
  }
  void writeHandler(const boost::system::error_code& errorCode, size_t bytesTransferred)
  {
        std::cout << "DEBUG> Transfered " << bytesTransferred << " bytes to " << socket_.remote_endpoint().address().to_string() << std::endl;
  }
  void handle_write(const boost::system::error_code& error)
  {
      std::cout<<"handle write: "<<std::endl;

    if (!error)
    {
            std::cout<<"before: "<<data_<<std::endl;

            socket_.async_read_some(boost::asio::buffer(data_, max_length),
            boost::bind(&session::handle_read, this,
            boost::asio::placeholders::error,
            boost::asio::placeholders::bytes_transferred));

            std::cout<<"after: "<<data_<<std::endl;
    }
    else
    {
      delete this;
    }
  }

  tcp::socket socket_;
  enum { max_length = 1024 };
  char data_[max_length];
};
void session::read_handler(const boost::system::error_code& ec, std::size_t bytes_transferred)
{
      std::cout<<bytes_transferred<<std::endl;
}

class server
{
public:
  server(boost::asio::io_service& io_service, short port)
    : io_service_(io_service),
      acceptor_(io_service, tcp::endpoint(tcp::v4(), port))
  {
    start_accept();
  }


private:
  void start_accept()
  {
    session* new_session = new session(io_service_);
    acceptor_.async_accept(new_session->socket(),
        boost::bind(&server::handle_accept, this, new_session,
          boost::asio::placeholders::error));
  }

  void handle_accept(session* new_session,
      const boost::system::error_code& error)
  {
    if (!error)
    {
      new_session->start();
    }
    else
    {
      delete new_session;
    }

    start_accept();
  }

  boost::asio::io_service& io_service_;
  tcp::acceptor acceptor_;
};

int main()
{
  try
  {
    boost::asio::io_service io_service;
    server s(io_service, 4000);

    io_service.run();
  }
  catch (std::exception& e)
  {
    std::cerr << "Exception: " << e.what() << "\n";
  }

  return 0;
}

回答1:

It looks like you are trying to apply some concepts of UDP to a TCP stream.

TCP needs to establish a connection with a 3-way handshake. For that reason, you can't send data to another client simply by modifying the ip remote endpoint address of a socket.

A TCP server listens for incoming connections, accepts them and at the end of this process has a valid TCP socket where you can write data. Each client has it's own socket. Also note that a TCP socket provides a data stream (not packets/datagrams).

On the other hand, UDP does not have the concept of a connection. You can send datagrams to multiple endpoints using the same UDP socket (boost::asio::ip::udp::socket::send_to)

来源：https://stackoverflow.com/questions/16016174/how-to-bind-the-local-endpoint-to-a-socket