boost::asio::async_read_until problem

Question

Hi again,

I'm modify the boost asio echo example to use async_read_until to read the input word by word. Even though I am using async_read_until all the data sent seems to be read from the socket. Could someone please advise:

   #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<<"starting"<<std::endl;
  boost::asio::async_read_until(socket_, buffer, ' ',
        boost::bind(&session::handle_read, this,
          boost::asio::placeholders::error,
          boost::asio::placeholders::bytes_transferred));
  }

  void handle_read(const boost::system::error_code& error,
      size_t bytes_transferred)
  {

 std::ostringstream ss;
 ss<<&buffer;
 std::string s = ss.str();
 std::cout<<s<<std::endl;

 if (!error)
    {
      boost::asio::async_write(socket_,
          boost::asio::buffer(s),
          boost::bind(&session::handle_write, this,
            boost::asio::placeholders::error));
    }
    else
    {
      delete this;
    }
  }

  void handle_write(const boost::system::error_code& error)
  {
 std::cout<<"handling write"<<std::endl;
    if (!error)
    {
    }
    else
    {
      delete this;
    }
  }

private:
  tcp::socket socket_;
  boost::asio::streambuf buffer;
};

class server
{
public:
  server(boost::asio::io_service& io_service, short port)
    : io_service_(io_service),
      acceptor_(io_service, tcp::endpoint(tcp::v4(), port))
  {
    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();
      new_session = new session(io_service_);
      acceptor_.async_accept(new_session->socket(),
          boost::bind(&server::handle_accept, this, new_session,
            boost::asio::placeholders::error));
    }
    else
    {
      delete new_session;
    }
  }

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

int main(int argc, char* argv[])
{
  try
  {
    if (argc != 2)
    {
      std::cerr << "Usage: async_tcp_echo_server <port>\n";
      return 1;
    }

    boost::asio::io_service io_service;

    using namespace std; // For atoi.
    server s(io_service, atoi(argv[1]));

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

  return 0;
}

Thanks!

Answer 1

Read the description of async_read_until carefully. It says:

After a successful async_read_until operation, the streambuf may contain additional data beyond the delimiter.

What this means in your case is that inside handle_read(), you should only access the first bytes_transferred bytes from the buffer. As of now, your bytes_transferred parameter is unused.