boost process running() and exit_code() thread saf

问题:

I am using boost::process::child and boost::process::async_pipe to start an application and read asynchronously (through the means of boost::asio) everything that app outputs on screen whenever this happens.

I want to check also if the application is alive by using child::running() method; if not running I'd like to read the exit code using child::exit_code.

This is very useful ESPECIALLY as it is a way to be notified about an application crashing or exiting unexpectedly (I could not find a better way); when the app exits the callback is called with boost::system::error_code set.

Do you know if I can use these two methods inside the callback called by async_pipe::async_read_some ?

In general the much more simple question would be if child::running() and child::exit_code() are thread safe (in both Windows and Linux).

 namespace bp = boost::process;

 char my_buffer[1024];

 boost::asio::io_service io;
 bp::async_pipe in_pipe(io);

 void handle_pipe_read(const boost::system::error_code &ec, std::size_t bytes_transferred);

 void schedule_read()  {
     in_pipe.async_read_some(
         boost::asio::buffer(my_buffer),
         boost::bind(&handle_pipe_read, 
                  boost::asio::placeholders::error,
                  boost::asio::placeholders::bytes_transferred));
 }

 void handle_pipe_read(
    const boost::system::error_code &ec, 
    std::size_t bytes_transferred
    )
 {
     // Q: Is this call possible? 'handle_pipe_read' can run in any thread
     if(c->running())
        std::cout << "I am alive" << std::endl;
     else
        std::cout << "EXIT CODE:" << c->exit_code() << std::endl;

     if(ec) return; //app probably exit

     // Do something with buffer and re-schedule

     schedule_read();
 }

 int main() {
   bp::child c("my_program_url", bp::std_out > in_pipe);
   any_c = &c;

   schedule_read();
   io.run();    
 }

回答1:

Since you only run the io_service::run() on the main thread, all completion handlers also run there. There's no threading.

Remember to pass the io_service to the child, and use the on_exit handler:

Live On Coliru

#include <boost/process.hpp>
//#include <boost/asio.hpp>
#include <boost/bind.hpp>
#include <system_error>
#include <utility>
#include <iostream>

namespace bp = boost::process;

char my_buffer[1024];

boost::asio::io_service io;
bp::async_pipe in_pipe(io);

void handle_pipe_read(const boost::system::error_code &ec, std::size_t bytes_transferred);

void schedule_read() {
    in_pipe.async_read_some(
        boost::asio::buffer(my_buffer),
        boost::bind(&handle_pipe_read, _1, _2));
}

void handle_pipe_read(const boost::system::error_code &ec, std::size_t bytes_transferred) {
    if (ec)
        return; // app probably exit

    // Do something with buffer and re-schedule
    std::cout.write(my_buffer, bytes_transferred);

    if (in_pipe.is_open())
        schedule_read();
}

int main() {
    bp::child c("/bin/ls", bp::std_out > in_pipe,
            bp::on_exit([](int code, std::error_code ec) {
                std::cout << "Child exited (" << code << "): " << ec.message() << std::endl;
                in_pipe.close();
            }), io);

    schedule_read();
    io.run();

    std::cout << "Service done (" << c.exit_code() << ")" << std::endl;
}

Prints:

a.out
main.cpp
Child exited (0): Success
Service done (0)

回答2:

The only solution that worked for me is the following

1. schedule a completion read
2. always check calling child::running()
3. on error_code set don't reschedule

When the pipe gets broken (because of a crash) the completion handler for a read has the boost::error_code argument set to true. Despite this I've seen cases where child::running() is false also when boost::error_code is NOT set.