boost::process async IO example doesn't work?

Question

The following program:

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

namespace bp = boost::process;

int main() {
    boost::asio::io_service ios;
    std::vector<char> buf(4096);

    bp::async_pipe ap(ios);

    bp::child c("/bin/ls", bp::std_out > ap);

    boost::asio::async_read(ap, boost::asio::buffer(buf),
            [](const boost::system::error_code &ec, std::size_t size){});

    ios.run();
    int result = c.exit_code();
    std::cout << result << std::endl;
}

outputs 383. I would expect it to output 0.

This is very nearly a copy-and-paste of the example from:

https://www.boost.org/doc/libs/1_71_0/doc/html/boost_process/tutorial.html#boost_process.tutorial.async_io

Shouldn't you have a `c.wait()` before retrieving the exit_code? — m88, Dec 03 '19 at 13:07
@m88: See the example in the docs. It doesn't have one. Is the example wrong? — Andrew Tomazos, Dec 03 '19 at 14:11
What platform/boost version is this? 1.71? It worked with Boost 1.66 for me. ISTR some changes dealing with native process wait status codes vs. exit-codes (see https://stackoverflow.com/questions/57732569/cant-get-segmentation-fault-exit-code-from-boost-child-process/57733210#57733210). Perhaps something regressed there. — sehe, Dec 03 '19 at 18:10
Here's a live demo on Coliru (boost version 1.66) http://coliru.stacked-crooked.com/a/8ffa1b882204a9cc — sehe, Dec 03 '19 at 18:10

score 3 · Answer 1 · answered Dec 03 '19 at 18:26

There may be a few issues here. Firstly, I think the comment in the documentation...

Passing an instance of boost::asio::io_service to the launching function automatically cause it to wait asynchronously for the exit, so no call of wait is needed

...refers to the example after the code you've shown. Specifically...

boost::asio::io_service ios;
std::vector<char> buf(4096);

bp::child c(bp::search_path("g++"), "main.cpp", bp::std_out > boost::asio::buffer(buf), ios);

ios.run();
int result = c.exit_code();

Where the io_service is passed by reference to the child ctor.

The comment is also slightly misleading. While it's true that the subsequent call to ios.run() does wait asynchronously for the exit it also appears (boost 1.71.0) that the exit code is not fixed up as one might hope. The exit code is stored within the child class as...

std::shared_ptr<std::atomic<int>> _exit_status;

From a quick scan of the source code it seems _exit_status->store(...) is only invoked from the following members...

boost::process::child::running
boost::process::child::wait
boost::process::child::wait_until

So, even though the process has exited (assuming all went well) when ios.run() returns one or more of running, wait or wait_until must be called to make the exit code available.

As commented elsewhere by @sehe this looks like it's possibly a regression. If I can find a bug report I'll update this. In the meantime the workaround is to simply call c.wait() before c.exit_code().

Remind me to no accept pull requests as easily. This is in deed a bug, since the whole wait handling will only occur when you pass in `on_exit`. It filters for that, if no handler for the asynchronous event is there it gets ignored. https://github.com/klemens-morgenstern/boost-process/blob/develop/include/boost/process/detail/posix/io_context_ref.hpp#L90 — Klemens Morgenstern, Dec 08 '19 at 17:14
@KlemensMorgenstern: I opened github issue here with link to this question: https://github.com/boostorg/process/issues/112 — Andrew Tomazos, Dec 09 '19 at 17:54

boost::process async IO example doesn't work?

1 Answers1

Linked