Create child process with custom environment using boost

Question

The documentation boost doesn't provide any example for creating a child process with a custom environment using process::child(...).
An example is given with process::system(...) but the function system has less possible operations (such as pipes or waitpid) so I would like to have a full example using process::child if possible.

Answer 1

in system.hpp, system_impl, which supports custom env, is implemented in terms of child,

template<typename IoService, typename ...Args>
inline int system_impl(
        std::true_type, /*needs ios*/
        std::true_type, /*has io_context*/
        Args && ...args)
{
    IoService & ios = ::boost::process::detail::get_io_context_var(args...);

    system_impl_success_check check;

    std::atomic_bool exited{false};

    child c(std::forward<Args>(args)...,
            check,
            ::boost::process::on_exit(
                [&](int, const std::error_code&)
                {
                    ios.post([&]{exited.store(true);});
                }));
    if (!c.valid() || !check.succeeded)
        return -1;

    while (!exited.load())
        ios.poll();

    return c.exit_code();
}

So the call to system from the docs:

bp::system("stuff", bp::env["VALUE_1"]="foo", bp::env["VALUE_2"]+={"bar1", "bar2"});

which calls:

template<typename ...Args>
inline int system(Args && ...args)
{
    typedef typename ::boost::process::detail::needs_io_context<Args...>::type
            need_ios;
    typedef typename ::boost::process::detail::has_io_context<Args...>::type
            has_ios;
    return ::boost::process::detail::system_impl<boost::asio::io_context>(
            need_ios(), has_ios(),
            std::forward<Args>(args)...);
}

translates into this function, so you should be able to do the same.