boost::asio 取消或关闭在 async_handle 上不起作用

Question

boost::asio::ip::udp:socket 不能cancel() async_handle，也不能close()。

这是测试程序：

{
    boost::asio::io_context io_core;
    boost::asio::ip::udp::socket udp_socket1(io_core, udp::v4());
    char buff[200];
    boost::asio::ip::udp::endpoint endpoint1(boost::asio::ip::address::from_string("127.0.0.1"), 9999);

    cout << udp_socket1.is_open() << endl;
    udp_socket1.async_send_to(boost::asio::buffer(buff, 200), endpoint1, [&udp_socket1](const boost::system::error_code& ec, size_t wlen) {
        cout << udp_socket1.is_open() << endl;
        //assert(0);
    });
    cout << udp_socket1.is_open() << endl;
    udp_socket1.close();
    udp_socket1.async_send_to(boost::asio::buffer(buff, 200), endpoint1, [&udp_socket1](const boost::system::error_code& ec, size_t wlen) {
        cout << udp_socket1.is_open() << endl;
        //assert(0);
    });
    cout << udp_socket1.is_open() << endl;

    udp_socket1.close();
    io_core.run();
    system("pause");
}

使用 boost 1.67.0、WIN10、vs2017 BOOST_ASIO_DISABLE_IOCP 和 BOOST_ASIO_CANCELIO 根据documentation定义

我是不是没抓住重点？

如果我是正确的，如何解决这个问题？

Answer 1

异步操作不会立即完成。您的代码看起来像您期望的那样。

将您的代码简化为：

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#include <boost/asio.hpp>
#include <iostream>

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

int main() {
    std::cout << std::boolalpha;

    boost::asio::io_context io;
    boost::asio::ip::udp::socket s {io, udp::v4()};
    boost::asio::ip::udp::endpoint const ep {{}, 9999};

    auto trace = [&s](char const* caption) { std::cout << caption << s.is_open() << std::endl; };
    auto handler = [=](boost::system::error_code, size_t) { trace("handler: "); };

    trace("main #1: ");

    char buff[200] = {};
    s.async_send_to(boost::asio::buffer(buff), ep, handler);

    trace("main #2: ");
    s.close();

    s.async_send_to(boost::asio::buffer(buff), ep, handler);

    trace("main #3: ");
    s.close();

    io.run();
}

处理程序只在io.run() 之后运行是有道理的；显然，套接字在第一次关闭之前只是“打开”的：

main #1: true
main #2: true
main #3: false
handler: false
handler: false

这完全符合预期。所以，要么你应该处理错误：

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auto trace = [&s](char const* caption) { std::cout << caption << (s.is_open()?"open":"closed") << std::endl; };
auto handler = [=](boost::system::error_code ec, size_t) { 
    trace(("handler(" + ec.message() + "): ").c_str()); 
};

改为打印：

main #1: open
main #2: open
main #3: closed
handler(Success): closed
handler(Bad file descriptor): closed

注意，也许令人惊讶的是，第一次发送操作仍然成功。与我的预期相反，这表明发送实际上是在 async_send_to 调用时启动的，但完成延迟到 io.run() 之后（仍然显示套接字已关闭）。

也许你根本不想要异步操作：

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#include <boost/asio.hpp>
#include <iostream>

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

int main() {
    std::cout << std::boolalpha;

    boost::asio::io_context io;
    boost::asio::ip::udp::socket s {io, udp::v4()};
    boost::asio::ip::udp::endpoint const ep {{}, 9999};

    auto trace = [&s](char const* caption) { std::cout << caption << s.is_open() << std::endl; };

    trace("main #1: ");

    char buff[200] = {};
    try {
        /*size_t wlen =*/ s.send_to(boost::asio::buffer(buff), ep);
        trace("sent #1: ");
    } catch(boost::system::system_error const& e) {
        std::cout << "Send #1 failed: " << e.code().message() << std::endl;
    }

    trace("main #2: ");
    s.close();

    try {
        /*size_t wlen =*/ s.send_to(boost::asio::buffer(buff), ep);
        trace("sent #2: ");
    } catch(boost::system::system_error const& e) {
        std::cout << "Send #2 failed: " << e.code().message() << std::endl;
    }

    trace("main #3: ");
    s.close();

    io.run();
}

打印

main #1: true
sent #1: true
main #2: true
Send #2 failed: Bad file descriptor
main #3: false