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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.

xaxxon
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zack evein
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2 Answers2

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The last answer and comment are quite old but I can confirm that boost::process::child with environment parameter works using Boost version 1.65 under Ubuntu 18.04. The documentation about that is quite thin, so I had to find out by myself:

std::string command = "/usr/bin/something";
ipstream pipe_stream;

// Get current env
auto env = boost::this_process::environment();
// Add something
env["CHINESE_FOOD"] = "GOOD";
// Change something
env["CHINESE_FOOD"] = "GREAT";
// Delete something
env["CHINESE_FOOD"].clear();

boost::process::child childProc(command, env, std_out > pipe_stream);

And of course, if environment is not needed, it will inherit from parent process automatically

std::string command = "/usr/bin/something";
ipstream pipe_stream;

boost::process::child childProc(command, std_out > pipe_stream);
Serge Grondin
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  • For some reason on Linux (CentOS 7) passing a boost::this_process::environment object to boost::process::child constructor also changes the environment of the parent process. Is this intended? – Ivan_a_bit_Ukrainivan Jan 20 '22 at 12:24
  • You are right, according to Boost documentation my last exemple will modify the current process environment. The correct way here would have been: ` // Get current env auto env = boost::this_process::environment(); // Copy it into an environment separate to the one of this process boost::process::environment envChild = env; // Add something envChild["CHINESE_FOOD"] = "GOOD"; // Change something envChild["CHINESE_FOOD"] = "GREAT"; // Delete something envChild["CHINESE_FOOD"].clear(); boost::process::child childProc(command, envChild, std_out > pipe_stream);` – Serge Grondin Jan 21 '22 at 13:18
  • You can also declare an empty environment instead of taking the one from the process `boost::process::environment env;` – Serge Grondin Jan 21 '22 at 13:29
  • ahh... Yes! My mistake was that I had wrong assumption about `boost::process::native_environment` type. I had to copy it to `boost::process::environment` and further modify it. `boost::process::native_environment` is a copyable accessor object! – Ivan_a_bit_Ukrainivan Jan 28 '22 at 10:55
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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.

xaxxon
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