Question

我正在尝试使用boost.process使用类似的东西来编写和读取孩子的stdio：

boost::asio::io_service writeService, readService;
bp::async_pipe in{writeService};
bp::async_pipe out{readService};

bp::child process(CompressCmd.c_str(), bp::std_in < in, bp::std_out > out);
Buffer src;
src.reserve(4 * 1024 * 1024);
integer_type read = 0;
//std::atomic_int64_t totalWrite{0};
integer_type totalWrite = 0;
while (callback(CallbackActions::NeedMoreInput, src, read)) {
    in.async_write_some(
        boost::asio::buffer(src.data(), read),
        [](const boost::system::error_code &e, std::size_t) { });
    // written data is not important, that's why using same buffer
    out.async_read_some(boost::asio::buffer(src.data(), src.capacity()),
                        [&](const boost::system::error_code &e,
                           std::size_t byte_transferred) { totalWrite += byte_transferred; });
}
writeService.run();
in.close();
readService.run();

所有读写操作都被认定为成功，但totalWrite的值完全不正确f.e报告29356032，实际值应该在50000000左右
我注意到程序正在中途终止，在readService.run（）之后使用process.wait（）冻结子，使用原子int产生相同的行为

现在我实际上只需要知道实际上有多少数据，这就是为什么我使用相同的缓冲区

Answer 1

此模式：
```
while (callback(CallbackActions::NeedMoreInput, src, read)) {
    in.async_write_some(...);
    out.async_read_some(...);
}
```
很可能被误导（异步操作总是立即返回，因此您只需继续添加更多异步操作而不给他们机会运行）。
同样误导的是，您为管道提供了单独的服务，但是您完全以排除方式运行它们，因此在writeService完成之前不会运行任何读取操作。
atomic类型被误导，因为多个线程无法访问
你想做什么？您保留一个大缓冲区但从不向其中放入任何数据（reserve！= resize）。因此，你只能希望什么都不写。

更具讽刺意味的是，你正在完全相同的位置读取相同的缓冲区。但是，当你知道src.capacity()时，会立即Undefined Behaviour¹，因为你传递src.size()==0。

即使没有这个错误，你怎么能“同时”从内存中完全相同的字节进行读写，仍然知道预期的结果是什么？
您没有将自己的io_service传递给Boost进程

工作演示

这是一份工作样本。当然，我不得不猜测你真正想做什么。

我选择让程序将自己的源（main.cpp）发送到stdin，并迭代读取stdout，记录total_received个字节。然后打印退出代码和总数。

作为一个make-shift压缩器，我使用'/usr/bin/xxd'因为它可用，甚至可以打印到std::cout进行调试。

Live On Coliru //在Coliru上遇到麻烦

#include <boost/asio.hpp>
#include <boost/process.hpp>
#include <boost/process/async.hpp>
#include <iostream>
std::vector<char> read_file(std::string const&);

namespace bp = boost::process;
using boost::system::error_code;

using Loop = boost::function<void()>;
using Buffer = std::array<char, 4*1024>;

int main() {
    boost::asio::io_service svc;

    std::string const CompressCmd = "/usr/bin/xxd";

    bp::async_pipe in{svc}, out{svc};
    bp::child process(CompressCmd, bp::std_in < in, bp::std_out > out, svc);

    auto data = read_file("main.cpp");

    Loop read_loop, write_loop;

    Buffer recv_buffer;
    std::size_t total_received = 0;
    read_loop = [&read_loop, &out, &recv_buffer, &total_received] {
        out.async_read_some(boost::asio::buffer(recv_buffer),
            [&](error_code ec, size_t transferred) {
                std::cout << "ReadLoop: " << ec.message() << " got " << transferred << " bytes\n";
                total_received += transferred; 
                if (!ec)
                    read_loop(); // continue reading
            });
    };

    boost::asio::async_write(in, boost::asio::buffer(data),
        [&](error_code ec, size_t transferred) {
            std::cout << "WriteLoop: " << ec.message() << " done, " << transferred << " bytes\n";
            in.close(ec);
            std::cout << "WriteLoop: closed pipe (" << ec.message() << ")\n";
        }); // async

    read_loop(); // async

    svc.run(); // Await all async operations

    std::cout << "Process exitcode " << process.exit_code() << ", total_received=" << total_received << "\n";
}

#include <fstream>
#include <iterator>
std::vector<char> read_file(std::string const& fname) {
    std::ifstream ifs(fname);
    return {std::istreambuf_iterator<char>(ifs), {}};
}

打印

WriteLoop: Success done, 1787 bytes
WriteLoop: closed pipe (Success)
ReadLoop: Success got 4096 bytes
ReadLoop: Success got 3515 bytes
ReadLoop: End of file got 0 bytes
Process exitcode 0, total_received=7611

解释，简化

请注意，我们在没有循环的情况下完成所有的写作。这是因为boost::asio::async_write是一个组合操作（它隐藏了循环）。

同样，如果你能“负担得起”将整个收到的数据存储在内存中，你可以使用boost::asio::streambuf并使用类似的组合操作来简化：

Live On Coliru //在Coliru上遇到麻烦

#include <boost/asio.hpp>
#include <boost/process.hpp>
#include <boost/process/async.hpp>
#include <iostream>
std::vector<char> read_file(std::string const&);

namespace bp = boost::process;
using boost::system::error_code;

int main() {
    boost::asio::io_service svc;

    std::string const CompressCmd = "/usr/bin/xxd";

    bp::async_pipe in{svc}, out{svc};
    bp::child process(CompressCmd, bp::std_in < in, bp::std_out > out, svc);

    auto data = read_file("main.cpp");

    boost::asio::streambuf recv_buffer;
    boost::asio::async_read(out, recv_buffer,
            [&](error_code ec, size_t transferred) {
                std::cout << "ReadLoop: " << ec.message() << " got " << transferred << " bytes\n";
            });

    boost::asio::async_write(in, boost::asio::buffer(data),
        [&](error_code ec, size_t transferred) {
            std::cout << "WriteLoop: " << ec.message() << " done, " << transferred << " bytes\n";
            in.close(ec);
            std::cout << "WriteLoop: closed pipe (" << ec.message() << ")\n";
        }); // async

    svc.run(); // Await all async operations

    std::cout << "Process exitcode " << process.exit_code() << ", total_received=" << recv_buffer.size() << "\n";
}

#include <fstream>
#include <iterator>
std::vector<char> read_file(std::string const& fname) {
    std::ifstream ifs(fname);
    return {std::istreambuf_iterator<char>(ifs), {}};
}

相反，如果你在发送之前无法承担内存中的所有数据，你可以创建一个循环来逐块发送输入

两个异步循环，带延迟

让我们这样做，并通过在编写每个块之前延迟一秒来使其更有趣。您期望看到的是由于延迟而发生的交替读/写：

Live On Coliru //在Coliru上运行

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

namespace bp = boost::process;
using boost::system::error_code;
using namespace std::chrono_literals;

using Loop = boost::function<void()>;
using Buffer = std::array<char, 500>;

int main() {
    boost::asio::io_service svc;
    auto on_exit = [](int code, std::error_code ec) {
            std::cout << "Exited " << code << " (" << ec.message() << ")\n";
        };

    std::string const CompressCmd = "/usr/bin/xxd";

    bp::async_pipe in{svc}, out{svc};
    bp::child process(CompressCmd, bp::std_in < in, bp::std_out > out, svc, bp::on_exit(on_exit));

    Loop read_loop, write_loop;

    Buffer recv_buffer;
    std::size_t total_received = 0;
    read_loop = [&read_loop, &out, &recv_buffer, &total_received] {
        out.async_read_some(boost::asio::buffer(recv_buffer),
            [&](error_code ec, size_t transferred) {
                std::cout << "ReadLoop: " << ec.message() << " got " << transferred << " bytes\n";
                total_received += transferred; 
                if (!ec)
                    read_loop(); // continue reading
            });
    };

    std::ifstream ifs("main.cpp");
    std::size_t total_written = 0;
    Buffer send_buffer;
    boost::asio::high_resolution_timer send_delay(svc);
    write_loop = [&write_loop, &in, &ifs, &send_buffer, &total_written, &send_delay] {
        if (!ifs.good())
        {
            error_code ec;
            in.close(ec);
            std::cout << "WriteLoop: closed stdin (" << ec.message() << ")\n";
            return;
        }
        ifs.read(send_buffer.data(), send_buffer.size());

        boost::asio::async_write(in, boost::asio::buffer(send_buffer.data(), ifs.gcount()),
            [&](error_code ec, size_t transferred) {
                std::cout << "WriteLoop: " << ec.message() << " sent " << transferred << " bytes\n";
                total_written += transferred; 
                if (!ec) {
                    send_delay.expires_from_now(1s);
                    send_delay.async_wait([&write_loop](error_code ec) {
                        std::cout << "WriteLoop: send delay " << ec.message() << "\n";
                        if (!ec) write_loop(); // continue writing
                    });
                }
            });
    };

    read_loop(); // async
    write_loop(); // async

    svc.run(); // Await all async operations

    std::cout << "Process exitcode " << process.exit_code() << ", total_received=" << total_received << "\n";
}

打印

WriteLoop: Success sent 500 bytes
WriteLoop: send delay Success
WriteLoop: Success sent 500 bytes
ReadLoop: Success got 500 bytes
ReadLoop: Success got 500 bytes
ReadLoop: Success got 500 bytes
ReadLoop: Success got 500 bytes
ReadLoop: Success got 500 bytes
ReadLoop: Success got 500 bytes
ReadLoop: Success got 500 bytes
ReadLoop: Success got 500 bytes
ReadLoop: Success got 96 bytes
WriteLoop: send delay Success
WriteLoop: Success sent 500 bytes
WriteLoop: send delay Success
WriteLoop: Success sent 500 bytes
ReadLoop: Success got 500 bytes
ReadLoop: Success got 500 bytes
ReadLoop: Success got 500 bytes
ReadLoop: Success got 500 bytes
ReadLoop: Success got 500 bytes
ReadLoop: Success got 500 bytes
ReadLoop: Success got 500 bytes
ReadLoop: Success got 500 bytes
ReadLoop: Success got 96 bytes
WriteLoop: send delay Success
WriteLoop: Success sent 500 bytes
WriteLoop: send delay Success
WriteLoop: Success sent 134 bytes
WriteLoop: send delay Success
WriteLoop: closed stdin (Success)
ReadLoop: Success got 500 bytes
ReadLoop: Success got 500 bytes
ReadLoop: Success got 500 bytes
ReadLoop: Success got 500 bytes
ReadLoop: Success got 500 bytes
ReadLoop: Success got 22 bytes
Exited 0 (Success)
ReadLoop: End of file got 0 bytes
Process exitcode 0, total_received=11214

¹或许只是未指明，我现在不想发现差异

使用boost.process同时读取和写入child的stdio

1 个答案:

工作演示

解释，简化

两个异步循环，带延迟