c++ - 更好地理解 boost 的聊天客户端示例

Question

首先，the code

//
// chat_client.cpp
// ~~~~~~~~~~~~~~~
//
// Copyright (c) 2003-2010 Christopher M. Kohlhoff (chris at kohlhoff dot com)
//
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
//

#include <cstdlib>
#include <deque>
#include <iostream>
#include <boost/bind.hpp>
#include <boost/asio.hpp>
#include <boost/thread.hpp>
#include "chat_message.hpp"

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

typedef std::deque<chat_message> chat_message_queue;

class chat_client
{
public:
  chat_client(boost::asio::io_service& io_service,
      tcp::resolver::iterator endpoint_iterator)
    : io_service_(io_service),
      socket_(io_service)
  {
    tcp::endpoint endpoint = *endpoint_iterator;
    socket_.async_connect(endpoint,
        boost::bind(&chat_client::handle_connect, this,
          boost::asio::placeholders::error, ++endpoint_iterator));
  }

  void write(const chat_message& msg)
  {
    io_service_.post(boost::bind(&chat_client::do_write, this, msg));
  }

  void close()
  {
    io_service_.post(boost::bind(&chat_client::do_close, this));
  }

private:

  void handle_connect(const boost::system::error_code& error,
      tcp::resolver::iterator endpoint_iterator)
  {
    if (!error)
    {
      boost::asio::async_read(socket_,
          boost::asio::buffer(read_msg_.data(), chat_message::header_length),
          boost::bind(&chat_client::handle_read_header, this,
            boost::asio::placeholders::error));
    }
    else if (endpoint_iterator != tcp::resolver::iterator())
    {
      socket_.close();
      tcp::endpoint endpoint = *endpoint_iterator;
      socket_.async_connect(endpoint,
          boost::bind(&chat_client::handle_connect, this,
            boost::asio::placeholders::error, ++endpoint_iterator));
    }
  }

  void handle_read_header(const boost::system::error_code& error)
  {
    if (!error && read_msg_.decode_header())
    {
      boost::asio::async_read(socket_,
          boost::asio::buffer(read_msg_.body(), read_msg_.body_length()),
          boost::bind(&chat_client::handle_read_body, this,
            boost::asio::placeholders::error));
    }
    else
    {
      do_close();
    }
  }

  void handle_read_body(const boost::system::error_code& error)
  {
    if (!error)
    {
      std::cout.write(read_msg_.body(), read_msg_.body_length());
      std::cout << "\n";
      boost::asio::async_read(socket_,
          boost::asio::buffer(read_msg_.data(), chat_message::header_length),
          boost::bind(&chat_client::handle_read_header, this,
            boost::asio::placeholders::error));
    }
    else
    {
      do_close();
    }
  }

  void do_write(chat_message msg)
  {
    bool write_in_progress = !write_msgs_.empty();
    write_msgs_.push_back(msg);
    if (!write_in_progress)
    {
      boost::asio::async_write(socket_,
          boost::asio::buffer(write_msgs_.front().data(),
            write_msgs_.front().length()),
          boost::bind(&chat_client::handle_write, this,
            boost::asio::placeholders::error));
    }
  }

  void handle_write(const boost::system::error_code& error)
  {
    if (!error)
    {
      write_msgs_.pop_front();
      if (!write_msgs_.empty())
      {
        boost::asio::async_write(socket_,
            boost::asio::buffer(write_msgs_.front().data(),
              write_msgs_.front().length()),
            boost::bind(&chat_client::handle_write, this,
              boost::asio::placeholders::error));
      }
    }
    else
    {
      do_close();
    }
  }

  void do_close()
  {
    socket_.close();
  }

private:
  boost::asio::io_service& io_service_;
  tcp::socket socket_;
  chat_message read_msg_;
  chat_message_queue write_msgs_;
};

int main(int argc, char* argv[])
{
  try
  {
    if (argc != 3)
    {
      std::cerr << "Usage: chat_client <host> <port>\n";
      return 1;
    }

    boost::asio::io_service io_service;

    tcp::resolver resolver(io_service);
    tcp::resolver::query query(argv[1], argv[2]);
    tcp::resolver::iterator iterator = resolver.resolve(query);

    chat_client c(io_service, iterator);

    boost::thread t(boost::bind(&boost::asio::io_service::run, &io_service));

    char line[chat_message::max_body_length + 1];
    while (std::cin.getline(line, chat_message::max_body_length + 1))
    {
      using namespace std; // For strlen and memcpy.
      chat_message msg;
      msg.body_length(strlen(line));
      memcpy(msg.body(), line, msg.body_length());
      msg.encode_header();
      c.write(msg);
    }

    c.close();
    t.join();
  }
  catch (std::exception& e)
  {
    std::cerr << "Exception: " << e.what() << "\n";
  }

  return 0;
}

现在我不明白他们为什么要在 io_service 中使用 write 函数来发布调用？所以它是线程安全的并且 no1 会同时使用套接字？这是否确保不会发生 2Xasync_write？并且 async_write 和 async_read 永远不会一起使用？我是否必须确保 asynch_write 和 async_read 不会同时发生？或者同时做这两件事是否安全？顺便说一句，如果我想让代码运行得更快，我知道我可以让 2 个(或更多)线程来执行 io_service::run() 所以在那种情况下我是否必须使用互斥锁来确保我上面写的事情不会发生吗？

Answer 1

io_service::post 从 chat_client::write 公共(public)方法中使用，因为 async_write 是 composed operation ，并且应用程序需要确保在流完成之前不会对流执行其他操作。

This operation is implemented in terms of zero or more calls to the stream's async_write_some function, and is known as a composed operation. The program must ensure that the stream performs no other write operations (such as async_write, the stream's async_write_some function, or any other composed operations that perform writes) until this operation completes.

真正的工作是在 chat_client::do_write 中完成的，其中使用了传出消息队列。

编辑:

在聊天客户端示例中，只有一个线程调用io_service::run，因此处理程序中没有线程安全问题。如果您有多个调用 io_service::run 的线程，您应该调查链而不是互斥体，如我在 your previous question 中所述。 .