Using boost to create multiple child processes to be run asynchronously

Question

How do I spawn multiple child processes that should run asynchronously? Do I use a vector of child processes? I'm afraid the code below doesn't do what I'm trying to accomplish. Any other suggestions for a different way to pass arguments to the child process is also welcome!

#include <iostream>
#include <string>
#include <sstream>
#include "boost/process.hpp"

int main()
{
    namespace bp = boost::process;

    std::string child_process_name = "child_process";

    std::cout << "main_process: before spawning" << std::endl;

    int num_processes = 5;
    for (int i = 0; i < num_processes; ++i)
    {
        std::cout << "main_process: spawning child " << i << std::endl;
        std::stringstream ss;
        ss << i;
        std::string is;
        ss >> is;
        bp::child c(child_process_name, std::vector<std::string> {is});
        c.join();
    }

    std::cout << "main_process: waiting for children" << std::endl;

    std::cout << "main_process: finished" << std::endl;

    return 0;
}

I think this one could be helpful https://stackoverflow.com/a/61369535/85371 — sehe, Jun 26 '20 at 22:03

score 2 · Accepted Answer · answered Jun 26 '20 at 20:36

you are waiting for each process before starting the next one . instead you should spawn them all then wait for them outside the loop . here is the edited code :

#include <iostream>
#include <string>
#include <sstream>
#include "boost/process.hpp"

int main()
{
    namespace bp = boost::process;

    std::string child_process_name = "child_process";

    std::cout << "main_process: before spawning" << std::endl;

    constexpr int num_processes = 5;
    std::vector<bp::child> childs;
    childs.reserve(num_processes);

    for (int i = 0; i < num_processes; ++i)
    {
        std::cout << "main_process: spawning child " << i << std::endl;
        std::stringstream ss;
        ss << i;
        std::string is;
        ss >> is;
        childs.emplace_back(bp::child(child_process_name, std::vector<std::string> {is}));
    }

    std::cout << "main_process: waiting for children" << std::endl;
    
    for (auto& ch : childs)
        ch.join();
    
    std::cout << "main_process: finished" << std::endl;

    return 0;
}

Thanks! I was creating the vector as `std::vector children(num_processes);` and then assigning its elements as `children[i] = bp::child(child_process_name, std::vector {is});`, which was wrong. — Bruno, Jun 26 '20 at 20:47

divisionbell · Answer 2 · 2020-06-26T20:33:49.547

1

How about adding each process (ie. bp::child ) to a vector. Then in a separate loop that iterates over the vector, make your call to join(). Don't call join() within that loop you have there, or things will just run sequentially.

edited Jun 26 '20 at 20:33

answered Jun 26 '20 at 20:25

divisionbell

11
4

I tried that, but I was doing something wrong. See my comment on @dev65's answer. – Bruno Jun 26 '20 at 20:47

Using boost to create multiple child processes to be run asynchronously

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