Can't get segmentation fault exit code from boost child process

I am trying to get the exit code of a child process (using boost::process and boost::asio) when that child process is killed due to a segmentation violation or divide be zero or any other kill signal. The exit code and error code always return with 0 and success.

I am running this on CentOS 7 using g++ 4.8.5 and boost 1.66

If I run the same code with a child process that simply returns a non-zero exit code it successfully returns that exit code.

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

namespace bp = boost::process;
using namespace std;

int main (int argc, char** argv)
{
   string exe = "./crashes";

   vector<string> data;
   boost::asio::io_service ios;

   int exit_code;
   error_code ec;
   future<string> ostr;

   bp::child c(exe,
               (bp::std_out & bp::std_err) > ostr,
               ios,
               bp::on_exit=[&exit_code, &ec](int exit, const error_code& ecin)
                                             {exit_code = exit; ec = ecin;});

   ios.run();

   cout << "Exit Code = " << exit_code << endl;
   cout << "Error Code = " << ec.message() << endl;
   cout << "child stdin & stderr:\n";
   cout << ostr.get() << endl;
   return exit_code;
}

and the crashes code

int main (int argc, char** argv)
{
   int* y = 0;
   int c = *y;
}

The results show a 0 exit code and Success error_code

Exit Code = 0
Error Code = Success
child stdin & stderr:

running the crashes executable alone returns an exit code of 139

bash-4.2$ ./crashes 
Segmentation fault (core dumped)
bash-4.2$ echo $?
139

Solution

The details of process termination and exit codes are platform dependent.

Boost process papers over the differences in the default interface: your on_exit handler is called with the result of boost::process::detail::posix::eval_exit_status() of the exit status, which means:

inline int eval_exit_status(int code)
{
    if (WIFEXITED(code))
    {
        return WEXITSTATUS(code);
    }
    else if (WIFSIGNALED(code))
    {
        return WTERMSIG(code);
    }
    else
    {
        return code;
    }
}

So, you get "exit-code 11" meaning segfault... If you want to actually know, you can look at native_exit_code()

bp::on_exit = [&result, &c](int /*ignored*/, const std::error_code &ec) {
    auto exit_status = c.native_exit_code();
    result.exit_code = boost::make_optional(WIFEXITED(exit_status), WEXITSTATUS(exit_status));
    result.signal    = boost::make_optional(WIFSIGNALED(exit_status), WTERMSIG(exit_status));
    result.ec = ec;
}

Now this assumes some changes to the result variables. Full listing:

Listing

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

namespace bp = boost::process;

int main(int argc, char**) {
    std::string exe = argc>1? "./ltua" : "./crashes";

    boost::asio::io_service ios;

    struct {
        boost::optional<int> exit_code;
        boost::optional<int> signal;
        std::error_code ec{};
    } result;

    std::future<std::string> ostr;

    bp::group g;
    bp::child c(exe, g, (bp::std_out & bp::std_err) > ostr, ios,
        bp::on_exit = [&result, &c](int /*ignored*/, const std::error_code &ec) {
            auto exit_status = c.native_exit_code();
            result.exit_code = boost::make_optional(WIFEXITED(exit_status), WEXITSTATUS(exit_status));
            result.signal    = boost::make_optional(WIFSIGNALED(exit_status), WTERMSIG(exit_status));
            result.ec = ec;
        });

    //g.wait();
    ios.run();

    if (result.exit_code) {
        std::cout << "Exited with " << *result.exit_code << std::endl;
    }
    if (result.signal) {
        std::cout << "Signaled with sginal #" << *result.signal << ", aka " << ::strsignal(*result.signal) <<  std::endl;
    }

    std::cout << "Error Code = " << result.ec.message() << std::endl;
    std::cout << "child stdin & stderr:\n";
    std::cout << ostr.get() << std::endl;
    return result.exit_code? *result.exit_code : 255;
}

Output

When run with ltua.cpp:

#include <iostream>
int main() {
    std::cout << "so  long"   << std::end;
    std::cerr << "and thanks" << std::end;
    std::cout << "for all"    << std::end;
    std::cerr << "the fish"   << std::end;
    return 42;
}

Prints

Exited with 42
Error Code = Success
child stdin & stderr:
so  long
and thanks
for all
the fish

And with crashes.cpp:

int main() {
    int *y = 0;
    int c = *y;
}

Prints

Signaled with sginal #11, aka Segmentation fault
Error Code = Success
child stdin & stderr: