c++ - boost::asio signal_set 处理程序仅在捕获到第一个信号后执行，并忽略相同类型的连续信号

Question

我有一个程序，我想通过发送 SIGINT 将一些数据写入文件而不是立即退出来停止它。但是，如果程序的用户再次发送 SIGINT，则程序应立即退出并忘记将数据写入文件。

出于可移植性的原因，我想为此目的使用 boost::asio。

我最初的(简化的)方法(见下文)没有奏效。这是不可能的还是我遗漏了什么？

处理程序似乎只被调用一次(打印出消息)并且程序总是在循环达到最大迭代次数时停止。

void handler(
const boost::system::error_code& error,
int signal_number) {
    if (!error) {
        static bool first = true;
        if(first) {
            std::cout << " A signal(SIGINT) occurred." << std::endl;
            // do something like writing data to a file
            first = false;
        }
        else {
            std::cout << " A signal(SIGINT) occurred, exiting...." << std::endl;
            exit(0);
        }
    }
}

int main() {
    // Construct a signal set registered for process termination.
    boost::asio::io_service io;
    boost::asio::signal_set signals(io, SIGINT);

    // Start an asynchronous wait for one of the signals to occur.
    signals.async_wait(handler);
    io.run();
    size_t i;

    for(i=0;i<std::numeric_limits<size_t>::max();++i){
        // time stepping loop, do some computations
    }

    std::cout << i << std::endl;
    return 0;
}

Answer 1

当您处理第一个事件时，您不会在服务对象上发布任何新工作，因此它会退出。

这意味着然后(在 ioservice 退出后)紧密循环开始。这可能不是您所期望的。

1. 如果你想再次监听 SIGINT，你必须等待处理程序再次设置信号:

   #include <boost/asio.hpp>
   #include <boost/asio/signal_set.hpp>
   #include <boost/bind.hpp>
   #include <boost/atomic.hpp>
   #include <iostream>
   
   void handler(boost::asio::signal_set& this_, boost::system::error_code error, int signal_number) {
       if (!error) {
           static boost::atomic_bool first(true);
           if(first) {
               // do something like writing data to a file
               std::cout << " A signal(SIGINT) occurred." << std::endl;
               first = false;
   
               this_.async_wait(boost::bind(handler, boost::ref(this_), _1, _2));
           }
           else {
               std::cout << " A second signal(SIGINT) occurred, exiting...." << std::endl;
               exit(1);
           }
       }
   }
   
   int main() {
       // Construct a signal set registered for process termination.
       boost::asio::io_service io;
       boost::asio::signal_set signals(io, SIGINT);
   
       // Start an asynchronous wait for one of the signals to occur.
       signals.async_wait(boost::bind(handler, boost::ref(signals), _1, _2));
       io.run();
       return 2;
   }
   

   如您所见，我将 signal_set& 引用绑定(bind)到处理程序，以便能够在接收到第一个信号后对其进行 async_wait。此外，原则上，我将 first 设为原子(尽管在多个线程上运行 io_service 之前这不是必需的)。

2. 您真的希望在后台运行 io_service 吗？在这种情况下，让它看起来像这样:

   signals.async_wait(boost::bind(handler, boost::ref(signals), _1, _2));
   
   boost::thread(boost::bind(&boost::asio::io_service::run, boost::ref(io))).detach();
   
   while (true)
   {
       std::cout << "Some work on the main thread...\n";
       boost::this_thread::sleep_for(boost::chrono::seconds(1));
   }
   

   典型输出:

   Some work on the main thread...
   Some work on the main thread...
   Some work on the main thread...
   ^CSome work on the main thread...
    A signal(SIGINT) occurred.
   Some work on the main thread...
   Some work on the main thread...
   ^CSome work on the main thread...
    A second signal(SIGINT) occurred, exiting....