c++ - 使用 boost::asio 时是否需要实现阻塞？

Question

我的问题是，如果我在多线程上运行io_service::run()，是否需要对这些异步函数实现阻塞？

例子:

int i = 0;
int j = 0;

void test_timer(boost::system::error_code ec)
{
    //I need to lock up here ?
    if (i++ == 10)
    {
        j = i * 10;
    }
    timer.expires_at(timer.expires_at() + boost::posix_time::milliseconds(500));
    timer.async_wait(&test_timer);
}

void threadMain()
{
    io_service.run();
}

int main()
{
    boost::thread_group workers;
    timer.async_wait(&test_timer);

    for (int i = 0; i < 5; i++){
        workers.create_thread(&threadMain);
    }

    io_service.run();
    workers.join_all();
    return 0;
}

Answer 1

async的定义是非阻塞的。

如果你想问“我是否必须同步访问来自不同线程的共享对象”——这个问题是不相关的，答案取决于为你共享的对象记录的线程安全。

对于 Asio，基本上(粗略总结)您需要将并发访问(并发，如:来自多个线程)同步到所有类型，除了boost::asio::io_context ¹,².

你的样本

您的示例使用多个线程运行 io 服务，这意味着处理程序在这些线程中的任何一个上运行。这意味着您实际上是在共享全局变量，而且它们确实需要保护。

但是 因为您的应用程序逻辑(异步调用链)规定只有一个操作永远挂起，而共享计时器对象上的下一个异步操作是总是从该链中调度，访问逻辑上全部来自单个线程(称为隐式链。参见Why do I need strand per connection when using boost::asio?

最简单的方法:

逻辑链

Live On Coliru

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

boost::asio::io_service io_service;
boost::asio::deadline_timer timer { io_service };

struct state_t {
    int i = 0;
    int j = 0;
} state;

void test_timer(boost::system::error_code ec)
{
    if (ec != boost::asio::error::operation_aborted) {
        {
            if (state.i++ == 10) {
                state.j = state.i * 10;
                if (state.j > 100)
                    return; // stop after 5 seconds
            }
        }
        timer.expires_at(timer.expires_at() + boost::posix_time::milliseconds(50));
        timer.async_wait(&test_timer);
    }
}

int main()
{
    boost::thread_group workers;
    timer.expires_from_now(boost::posix_time::milliseconds(50));
    timer.async_wait(&test_timer);

    for (int i = 0; i < 5; i++){
        workers.create_thread([] { io_service.run(); });
    }

    workers.join_all();
    std::cout << "i = " << state.i << std::endl;
    std::cout << "j = " << state.j << std::endl;
}

Note I removed the io_service::run() from the main thread as it is redundant with the join() (unless you really wanted 6 threads running the handlers, not 5).

打印

i = 11
j = 110

警告

这里潜伏着一个陷阱。比方说，您不想像我一样以固定数量退出，但想停下来，您可能会想这样做:

timer.cancel();

来自 main .那是不合法的，因为deadline_timer对象不是线程安全的。你需要要么

• 使用全局 atomic_bool发出终止请求信号
• 发布timer.cancel()在与定时器异步链相同的 strand 上。但是，只有一个显式链，因此如果不更改代码以使用显式链，您将无法做到这一点。

更多计时器

让我们通过使用两个计时器来使事情复杂化，这两个计时器具有它们自己的隐式链。这意味着对定时器实例的访问仍然不需要同步，但是对 i 的访问和 j 确实需要。

Note In this demo I use synchronized_value<> for elegance. You can write similar logic manually using mutex and lock_guard.

Live On Coliru

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

boost::asio::io_service io_service;

struct state {
    int i = 0;
    int j = 0;
};

boost::synchronized_value<state> shared_state;

struct TimerChain {
    boost::asio::deadline_timer _timer;

    TimerChain() : _timer{io_service} {
        _timer.expires_from_now(boost::posix_time::milliseconds(50));
        resume();
    }

    void resume() {
        _timer.async_wait(boost::bind(&TimerChain::test_timer, this, _1));
    };

    void test_timer(boost::system::error_code ec)
    {
        if (ec != boost::asio::error::operation_aborted) {
            {
                auto state = shared_state.synchronize();
                if (state->i++ == 10) {
                    state->j = state->i * 10;
                }
                if (state->j > 100) return; // stop after some iterations
            }
            _timer.expires_at(_timer.expires_at() + boost::posix_time::milliseconds(50));
            resume();
        }
    }
};

int main()
{
    boost::thread_group workers;
    TimerChain timer1;
    TimerChain timer2;

    for (int i = 0; i < 5; i++){
        workers.create_thread([] { io_service.run(); });
    }

    workers.join_all();
    auto state = shared_state.synchronize();
    std::cout << "i = " << state->i << std::endl;
    std::cout << "j = " << state->j << std::endl;
}

打印

i = 12
j = 110

添加显式链

现在添加它们非常简单:

struct TimerChain {
    boost::asio::io_service::strand _strand;
    boost::asio::deadline_timer _timer;

    TimerChain() : _strand{io_service}, _timer{io_service} {
        _timer.expires_from_now(boost::posix_time::milliseconds(50));
        resume();
    }

    void resume() {
        _timer.async_wait(_strand.wrap(boost::bind(&TimerChain::test_timer, this, _1)));
    };

    void stop() { // thread safe
        _strand.post([this] { _timer.cancel(); });
    }

    // ...

Live On Coliru

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

boost::asio::io_service io_service;

struct state {
    int i = 0;
    int j = 0;
};

boost::synchronized_value<state> shared_state;

struct TimerChain {
    boost::asio::io_service::strand _strand;
    boost::asio::deadline_timer _timer;

    TimerChain() : _strand{io_service}, _timer{io_service} {
        _timer.expires_from_now(boost::posix_time::milliseconds(50));
        resume();
    }

    void resume() {
        _timer.async_wait(_strand.wrap(boost::bind(&TimerChain::test_timer, this, _1)));
    };

    void stop() { // thread safe
        _strand.post([this] { _timer.cancel(); });
    }

    void test_timer(boost::system::error_code ec)
    {
        if (ec != boost::asio::error::operation_aborted) {
            {
                auto state = shared_state.synchronize();
                if (state->i++ == 10) {
                    state->j = state->i * 10;
                }
            }
            // continue indefinitely
            _timer.expires_at(_timer.expires_at() + boost::posix_time::milliseconds(50));
            resume();
        }
    }
};

int main()
{
    boost::thread_group workers;
    TimerChain timer1;
    TimerChain timer2;

    for (int i = 0; i < 5; i++){
        workers.create_thread([] { io_service.run(); });
    }

    boost::this_thread::sleep_for(boost::chrono::seconds(10));
    timer1.stop();
    timer2.stop();

    workers.join_all();

    auto state = shared_state.synchronize();
    std::cout << "i = " << state->i << std::endl;
    std::cout << "j = " << state->j << std::endl;
}

打印

i = 400
j = 110

---

¹(或使用旧名称 boost::asio::io_service)

² 在这方面，生命周期突变不被视为成员操作(即使对于线程安全对象，您也必须手动同步共享对象的构造/销毁)