使用 BOOST 进程在单独的线程中读取子进程标准输出

Question

我有一个主程序，它使用 boost 进程库来生成一个打印的子进程

Hello World !

每 5 秒在其标准输出上。

我想在主进程中读取/监视子进程的标准输出，当它可用时，以及在主程序中执行其他操作。

我已经尝试了boost asynchronous IO (http://www.boost.org/doc/libs/1_66_0/doc/html/boost_process/tutorial.html) 的示例，但所有这些似乎都会阻塞主程序，直到子进程退出。

我们是否需要在单独的线程中读取孩子的标准输出？有人可以提供一个例子，主程序可以同时做其他事情，而不是阻止来自孩子的标准输出吗？

Answer 1

我已经尝试了 boost 异步 IO (http://www.boost.org/doc/libs/1_66_0/doc/html/boost_process/tutorial.html) 的示例，但所有这些似乎都会阻塞主程序，直到子进程退出。

再看一遍。 Asynchronous I/O 下的所有示例应该可以帮助您选择适合您的方法。

我们是否需要在单独的线程中读取孩子的标准输出？有人可以提供一个例子，主程序可以同时做其他事情，而不是阻止来自孩子的标准输出吗？

不，你不需要。虽然您可以并且根据您想要达到的目标，这可能是最简单的事情。

同步

你没有告诉我们你想做什么，所以我们假设你只想打印输出：

Live On Coliru：

bp::child c("/bin/bash", std::vector<std::string> { "-c", "for a in {1..10}; do sleep 2; echo 'Hello World !'; done" });
c.wait();

这是同步的，所以你不能同时工作

使用阅读器线程

就像这样：

Live On Coliru：

#include <boost/process.hpp>
#include <boost/process/async.hpp>
#include <iostream>

namespace bp = boost::process;

int main() {
    bp::ipstream output;
    std::thread reader([&output] {
        std::string line;
        while (std::getline(output, line))
            std::cout << "Received: '" << line << "'" << std::endl;
    });

    bp::child c("/bin/bash",
        std::vector<std::string> { "-c", "for a in {1..10}; do sleep 2; echo 'Hello World ('$a')!'; done" },
        bp::std_out > output);

    while (c.running()) {
        std::this_thread::sleep_for(std::chrono::milliseconds(2793));
        std::cout << "(main thread working)" << std::endl;
    }

    std::cout << "(done)" << std::endl;
    c.wait();

    output.pipe().close();
    reader.join();
}

打印（Live On Coliru）：

Received: 'Hello World (1)!'
(main thread working)
Received: 'Hello World (2)!'
(main thread working)
Received: 'Hello World (3)!'
Received: 'Hello World (4)!'
(main thread working)
Received: 'Hello World (5)!'
(main thread working)
Received: 'Hello World (6)!'
(main thread working)
Received: 'Hello World (7)!'
Received: 'Hello World (8)!'
(main thread working)
Received: 'Hello World (9)!'
(main thread working)
Received: 'Hello World (10)!'
(main thread working)
(done)

异步 IO

不使用线程（嗯，只是主线程），可能看起来像：

Live On Coliru

#include <boost/process.hpp>
#include <boost/process/async.hpp>
#include <boost/asio/high_resolution_timer.hpp>
#include <iostream>
#include <iomanip>

namespace bp = boost::process;

struct OtherWork {
    using clock = std::chrono::high_resolution_clock;

    OtherWork(boost::asio::io_context& io) : timer(io) { }

    void start() {
        timer.expires_at(clock::time_point::max());
        loop();
    }

    void stop() {
        timer.expires_at(clock::time_point::min());
    }

  private:
    void loop() {
        if (timer.expires_at() == clock::time_point::min()) {
            std::cout << "(done)" << std::endl;
            return;
        }

        timer.expires_from_now(std::chrono::milliseconds(2793));
        timer.async_wait([=](boost::system::error_code ec) {
            if (!ec) {
                std::cout << "(other work in progress)" << std::endl;
                start();
            } else {
                std::cout << "(" << ec.message() << ")" << std::endl;
            }
        });
    }

    boost::asio::high_resolution_timer timer;
};

int main() {
    boost::asio::io_context io;
    bp::async_pipe output(io);

    OtherWork mainwork{io};

    bp::child c("/bin/bash", std::vector<std::string> { "-c", "for a in {1..10}; do sleep 2; echo 'Hello World ('$a')!'; done" },
            bp::std_out > output, io, bp::on_exit([&mainwork,&output](auto...) {
                    output.close();
                    mainwork.stop();
                }));

    std::function<void()> readloop = [&,buffer=std::array<char, 32>{}]() mutable {
        output.async_read_some(bp::buffer(buffer), [&](boost::system::error_code ec, size_t transferred) {
                if (transferred) {
                    std::cout << "Received: '";
                    while (transferred && buffer[transferred-1] == '\n') // strip newline(s)
                        --transferred;
                    std::cout.write(buffer.data(), transferred);
                    std::cout << "'" << std::endl;
                }

                if (ec)
                    std::cout << "Output pipe: " << ec.message() << std::endl;
                else
                    readloop();
            });
    };

    mainwork.start();
    readloop();
    io.run();
}

打印Live On Coliru

Received: 'Hello World (1)!'
(other work in progress)
Received: 'Hello World (2)!'
(other work in progress)
Received: 'Hello World (3)!'
Received: 'Hello World (4)!'
(other work in progress)
Received: 'Hello World (5)!'
(other work in progress)
Received: 'Hello World (6)!'
(other work in progress)
Received: 'Hello World (7)!'
Received: 'Hello World (8)!'
(other work in progress)
Received: 'Hello World (9)!'
(other work in progress)
Received: 'Hello World (10)!'
Output pipe: End of file
Child exited with code=0(Success)
(Operation canceled)