关于boost.asio中，一个官方异步UDP通信的例子所产生的疑问

槐荫飞龙 2014-12-31 08:54:34

有个问题是在不明白，特来请教大家。当调用：
socket_.async_receive_from(
boost::asio::buffer(data_, max_length), sender_endpoint_,
boost::bind(&server::handle_receive_from, this,
boost::asio::placeholders::error,
boost::asio::placeholders::bytes_transferred));
函数应该会立即返回，只有接收完成handle_receive_from()才会得到回调。
现在的问题是：async_receive_from（）中传入的缓冲区data_和发送者的send_endpointer_都是当前类的成员变量，由于async_receive_from()是异步执行的，如果在async_receive_from()和handle_receive_from()之间的这个时候，有新的客户端发包进来，data_和send_endpointer_就有可能被修改啊，那么在handle_receive_from()被回调时，使用的data_和sender_endpointer_就有可能不是期望的值了。
附上源代码：
//
// async_udp_echo_server.cpp
// ~~~~~~~~~~~~~~~~~~~~~~~~~
//
// Copyright (c) 2003-2014 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 <iostream>
#include <boost/bind.hpp>
#include <boost/asio.hpp>

using boost::asio::ip::udp;

class server
{
public:
server(boost::asio::io_service& io_service, short port)
: socket_(io_service, udp::endpoint(udp::v4(), port))
{
socket_.async_receive_from(
boost::asio::buffer(data_, max_length), sender_endpoint_,
boost::bind(&server::handle_receive_from, this,
boost::asio::placeholders::error,
boost::asio::placeholders::bytes_transferred));
}

void handle_receive_from(const boost::system::error_code& error,
size_t bytes_recvd)
{
if (!error && bytes_recvd > 0)
{
socket_.async_send_to(
boost::asio::buffer(data_, bytes_recvd), sender_endpoint_,
boost::bind(&server::handle_send_to, this,
boost::asio::placeholders::error,
boost::asio::placeholders::bytes_transferred));
}
else
{
socket_.async_receive_from(
boost::asio::buffer(data_, max_length), sender_endpoint_,
boost::bind(&server::handle_receive_from, this,
boost::asio::placeholders::error,
boost::asio::placeholders::bytes_transferred));
}
}

void handle_send_to(const boost::system::error_code& /*error*/,
size_t /*bytes_sent*/)
{
socket_.async_receive_from(
boost::asio::buffer(data_, max_length), sender_endpoint_,
boost::bind(&server::handle_receive_from, this,
boost::asio::placeholders::error,
boost::asio::placeholders::bytes_transferred));
}

private:
udp::socket socket_;
udp::endpoint sender_endpoint_;
enum { max_length = 1024 };
char data_[max_length];
};

int main(int argc, char* argv[])
{
try
{
if (argc != 2)
{
std::cerr << "Usage: async_udp_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;
}

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