boost::asio，为啥我的套接字在调用 async_receive_from 后立即运行回调函数？

Posted 2023-02-21

【中文标题】boost::asio，为啥我的套接字在调用 async_receive_from 后立即运行回调函数？

【英文标题】：boost::asio, why does my socket immediately run callback function after calling async_receive_from?boost::asio，为什么我的套接字在调用 async_receive_from 后立即运行回调函数？

【发布时间】：2016-03-26 06:13:58

【问题描述】：

我正在尝试使用 3 次握手建立异步连接。在代码的底部有一个 main 方法，它接受一个参数来确定它是作为客户端还是服务器。

如果它充当服务器，它会创建一个套接字并等待它接收到数据，然后调用回调函数multiplex 来确定如何处理它接收到的数据。

如果它充当客户端，我还将创建一个异步等待接收数据的套接字，但它也会使用 create_connection 方法通过 udp 向服务器发送一个同步数据包。

在 Socket 的构造函数中，我执行方法 start_receive，它应该在 udp 套接字上调用 async_receive_from。问题是它立即使用 0.0.0.0:0 的端点调用回调函数multiplex，而不是仅仅等待接收数据。

#include <iostream>
#include <string>
#include <boost/asio.hpp>
#include <boost/shared_ptr.hpp>
#include <boost/cstdint.hpp>
#include <boost/function.hpp>
#include <boost/bind.hpp>
#include <unordered_map>
#include "packed_message.h"
#include "segment.pb.h"
#include "rtp.hpp"

#define DEBUG true
#define BUFFER_SIZE 10000

using boost::asio::ip::udp;
typedef boost::shared_ptr<rtp::Segment> SegmentPtr;
typedef std::vector<uint8_t> data_buffer;

// constructor for socket
rtp::Socket::Socket(boost::asio::io_service& io_service_, std::string source_ip, std::string source_port): 
    io_service_(io_service_),
    socket_(io_service_, udp::endpoint(boost::asio::ip::address::from_string(source_ip), std::stoi(source_port))),
    source_port(source_port),
    source_ip(source_ip)

    // accept incoming rtp segments
    std::cout << rtp::get_endpoint_str(socket_.local_endpoint()) << std::endl; 
    start_receive();


/**
 *  Accept incoming rtp segments
 */
void rtp::Socket::start_receive()

    data_buffer tmp_buf;
    socket_.async_receive_from(boost::asio::buffer(tmp_buf), remote_endpoint_, 
        boost::bind(&rtp::Socket::multiplex, this,
            tmp_buf,
            boost::asio::placeholders::error, 
            boost::asio::placeholders::bytes_transferred));



void rtp::Socket::multiplex(data_buffer& tmp_buf,
    const boost::system::error_code& error,
    std::size_t /*bytes_transferred*/)

    std::string identifier = rtp::get_endpoint_str(remote_endpoint_);

    if (connections.count(identifier) == 0 )
    
        boost::shared_ptr<Connection> connection(new Connection(remote_endpoint_));
        connections.insert(rtp::get_endpoint_str(remote_endpoint_), connection);
        std::cout << rtp::get_endpoint_str(remote_endpoint_) << std::endl;
        connection_establishment(tmp_buf, connections.at(identifier));


    
    else if(!(connections.at(identifier)->is_valid())) // connection not in list of connections
    
        connection_establishment(tmp_buf, connections.at(identifier));




boost::shared_ptr<rtp::Connection> rtp::Socket::create_connection(std::string ip, std::string port)

    udp::resolver resolver_(io_service_);
    udp::resolver::query query_(ip, port);
    udp::endpoint remote_endpoint_ = *resolver_.resolve(query_);
    boost::shared_ptr<Connection> connection(new Connection(remote_endpoint_));

    connections.insert(rtp::get_endpoint_str(remote_endpoint_), connection);

    PackedMessage<rtp::Segment> m_packed_segment(SegmentPtr(new rtp::Segment()));
    boost::shared_ptr<data_buffer> message(new data_buffer);
    SegmentPtr ackseg(new rtp::Segment());
    ackseg->set_ack(true);
    PackedMessage<rtp::Segment> initialack(ackseg);
    initialack.pack(*message);



    socket_.async_send_to(boost::asio::buffer(*message), remote_endpoint_,
      boost::bind(&rtp::Socket::handle_send, this, message,
        boost::asio::placeholders::error,
        boost::asio::placeholders::bytes_transferred));

    return connection;



void rtp::Socket::connection_establishment(data_buffer& m_readbuf, boost::shared_ptr<Connection> connection)

    int buffer_position(0);
    PackedMessage<rtp::Segment> m_packed_segment(boost::shared_ptr<rtp::Segment>(new rtp::Segment()));
    boost::shared_ptr<data_buffer> message(new data_buffer);

    unsigned msg_len = m_packed_segment.decode_header(m_readbuf, buffer_position);
    buffer_position += HEADER_SIZE;
    m_packed_segment.unpack(m_readbuf, msg_len, buffer_position);
    buffer_position += msg_len;

    SegmentPtr synackseg = m_packed_segment.get_msg();
    if (synackseg->syn() && synackseg->ack())
    
        SegmentPtr ackseg(new rtp::Segment());
        ackseg->set_ack(true);
        PackedMessage<rtp::Segment> finalack(ackseg);
        finalack.pack(*message);

        connection->set_valid(true);

        socket_.async_send_to(boost::asio::buffer(*message), remote_endpoint_,
            boost::bind(&rtp::Socket::handle_send, this,
                message,
                boost::asio::placeholders::error,
                boost::asio::placeholders::bytes_transferred));

    
    else if (synackseg->syn() )
    
        SegmentPtr synackseg(new rtp::Segment());
        synackseg->set_ack(true);
        synackseg->set_syn(true);
        PackedMessage<rtp::Segment> synack(synackseg);
        synack.pack(*message);

        socket_.async_send_to(boost::asio::buffer(*message), remote_endpoint_,
            boost::bind(&rtp::Socket::handle_send, this,
                message,
                boost::asio::placeholders::error,
                boost::asio::placeholders::bytes_transferred));


    
    else if (synackseg->ack())
    
        connection->set_valid(true);
    

    start_receive();


void rtp::Socket::handle_send(boost::shared_ptr<data_buffer> /*message*/,
            const boost::system::error_code& /*error*/, 
            std::size_t /*bytes_transferred*/)



/**
 *  Get remote peer ip and port in string "<ip>:<port>"
 */
std::string rtp::get_endpoint_str(udp::endpoint remote_endpoint_)

    std::string ip = remote_endpoint_.address().to_string();
    std::string port = std::to_string(remote_endpoint_.port());
    return ip + ":" + port;



rtp::Connection::Connection(udp::endpoint remote_endpoint_):
    remote_endpoint_(remote_endpoint_),
    dest_ip(remote_endpoint_.address().to_string()),
    dest_port(std::to_string(remote_endpoint_.port())),
    valid(false)



bool rtp::Connection::is_valid()

    return valid;


void rtp::Connection::set_valid(bool val)

    if(DEBUG) std::cerr << "Connection Created" << std::endl;
    valid = val;



int main(int argc, char* argv[])

    if (argc == 1)
    
        std::cerr << "Not enough args" << std::endl;
        return 1;
    
    boost::asio::io_service io_service_;

    if (std::string(argv[1]) == u8"server")
    
        rtp::Socket socket(io_service_, u8"127.0.0.1", u8"4545");
    
    else if (std::string(argv[1]) == u8"client")
    
        rtp::Socket socket(io_service_, u8"127.0.0.1", u8"4546");
        socket.create_connection(u8"127.0.0.1", u8"4545");

    
    io_service_.run();
    return 0;

【参考方案1】：

问题很可能是以下几行：

if (std::string(argv[1]) == u8"server")

    rtp::Socket socket(io_service_, u8"127.0.0.1", u8"4545");

else if (std::string(argv[1]) == u8"client")

    rtp::Socket socket(io_service_, u8"127.0.0.1", u8"4546");
    socket.create_connection(u8"127.0.0.1", u8"4545");

在每个if 语句体中，您声明了一个变量socket，但它仅在if 语句体的范围内是局部的。一旦该范围结束，变量将被破坏并且不再存在。

您看到的行为可能与该问题有关，即您在这些范围内创建的对象被破坏。

您需要创建一个生命周期比任何内部范围都长的套接字，并且将持续到io_service_.run() 返回。

【讨论】：

感谢您的帮助。这就是造成问题的原因。