boost asio udp socket async_receive_from不会调用处理程序

Question

我想创建一个自治线程,专门用于使用boost库(asio)从UDP套接字接收数据.该线程应该是由UDP套接字接收的一些数据触发的无限循环.在我的应用程序中,我需要使用异步接收操作.

如果我使用同步函数receive_from,一切都按预期工作.

但是,如果我使用async_receive_from,则永远不会调用处理程序.由于我使用信号量来检测是否已收到某些数据,因此程序会锁定并且永远不会触发循环.

我已经验证(使用网络分析器)发送方设备正确地在UDP套接字上发送数据.

我已在以下代码中隔离了该问题.

#include <boost\array.hpp>
#include <boost\asio.hpp>
#include <boost\thread.hpp>
#include <boost\interprocess\sync\interprocess_semaphore.hpp>

#include <iostream>

typedef boost::interprocess::interprocess_semaphore Semaphore;

using namespace boost::asio::ip;

class ReceiveUDP
{
public:

    boost::thread*  m_pThread;

    boost::asio::io_service         m_io_service;
    udp::endpoint                   m_local_endpoint;
    udp::endpoint                   m_sender_endpoint;

    udp::socket                     m_socket;

    size_t      m_read_bytes;
    Semaphore   m_receive_semaphore;

    ReceiveUDP() :
        m_socket(m_io_service),
        m_local_endpoint(boost::asio::ip::address::from_string("192.168.0.254"), 11),
        m_sender_endpoint(boost::asio::ip::address::from_string("192.168.0.11"), 5550),
        m_receive_semaphore(0)
    {
        Start();
    }

    void Start()
    {
        m_pThread = new boost::thread(&ReceiveUDP::_ThreadFunction, this);
    }

    void _HandleReceiveFrom(
        const boost::system::error_code& error,
        size_t                                  received_bytes)
    {
        m_receive_semaphore.post();

        m_read_bytes = received_bytes;
    }

    void _ThreadFunction()
    {
        try
        {
            boost::array<char, 100> recv_buf;

            m_socket.open(udp::v4());
            m_socket.bind(m_local_endpoint);
            m_io_service.run();

            while (1)
            {
#if 1 // THIS WORKS

                m_read_bytes = m_socket.receive_from(
                    boost::asio::buffer(recv_buf), m_sender_endpoint);

#else // THIS DOESN'T WORK

                m_socket.async_receive_from(
                    boost::asio::buffer(recv_buf),
                    m_sender_endpoint,
                    boost::bind(&ReceiveUDP::_HandleReceiveFrom, this,
                    boost::asio::placeholders::error,
                    boost::asio::placeholders::bytes_transferred));

                /* The program locks on this wait since _HandleReceiveFrom
                is never called. */
                m_receive_semaphore.wait();

#endif

                std::cout.write(recv_buf.data(), m_read_bytes);
            }

            m_socket.close();
        }
        catch (std::exception& e)
        {
            std::cerr << e.what() << std::endl;
        }
    }
};

void main()
{
    ReceiveUDP  receive_thread;

    receive_thread.m_pThread->join();
}

信号量上的timed_wait是首选,但是出于调试目的,我使用了阻塞等待,如上面的代码所示.

我错过了什么？我的错误在哪里？

Answer 1

你的电话io_service.run()正在退出,因为没有工作io_service要做.然后代码进入while循环并调用m_socket.async_receive_from.此时,io_service它没有运行ergo它从不读取数据并调用您的处理程序.

你需要在调用io_service run之前安排工作:

即:

// Configure io service
ReceiveUDP  receiver;

m_socket.open(udp::v4());
m_socket.bind(m_local_endpoint);
m_socket.async_receive_from(
    boost::asio::buffer(recv_buf),
    m_sender_endpoint,
    boost::bind(&ReceiveUDP::_HandleReceiveFrom, receiver,
    boost::asio::placeholders::error,
    boost::asio::placeholders::bytes_transferred));

处理函数将执行以下操作:

// start the io service
void HandleReceiveFrom(
    const boost::system::error_code& error,
    size_t received_bytes)
{
    m_receive_semaphore.post();

    // schedule the next asynchronous read
    m_socket.async_receive_from(
        boost::asio::buffer(recv_buf),
        m_sender_endpoint,
        boost::bind(&ReceiveUDP::_HandleReceiveFrom, receiver,
        boost::asio::placeholders::error,
        boost::asio::placeholders::bytes_transferred));

    m_read_bytes = received_bytes;
}

然后你的线程只是等待信号量:

while (1)
{
    m_receive_semaphore.wait();
    std::cout.write(recv_buf.data(), m_read_bytes);
}

笔记:

1. 你真的需要这个额外的线程吗？处理程序是完全异步的,boost :: asio可用于管理线程池(请参阅:think-async)
2. 请不要使用下划线,然后使用国会大厦字母表示变量/功能名称.它们是保留的.