gistfile1.cpp

#include <iostream>
#include <boost/asio.hpp>
#include <boost/system/system_error.hpp>

#include <unistd.h>

namespace asio = boost::asio;

template<typename Lambda>
void fork_with_pipe(asio::io_service& io, Lambda&& on_line_read) {
  int fd[2], &rd = fd[0], &wr = fd[1];
  if (pipe(fd) == -1)
    throw boost::system::system_error(
        errno, boost::system::system_category(), "pipe");
  pid_t pid = fork();
  switch (pid) {
  case -1:
    throw boost::system::system_error(
        errno, boost::system::system_category(), "fork");
  case 0:
    close(rd);
    dup2(wr, STDOUT_FILENO);
    dup2(wr, STDERR_FILENO);
    for (int fd = getdtablesize() - 1; fd >= 3; fd--)
        close(fd);
    execlp("cat", "cat", nullptr);
    throw boost::system::system_error(
        errno, boost::system::system_category(), "exec");
  default:
    close(wr);
    struct stream_aux {
      asio::posix::stream_descriptor stream;
      asio::streambuf buf;
      using HandlerType = void (boost::system::error_code, std::size_t size);
      std::function<HandlerType> handler;

      stream_aux(asio::io_service& io, int fd)
        : stream(io, fd) {}
    };
    std::shared_ptr<stream_aux> aux(std::make_shared<stream_aux>(io, rd));
    aux->handler = [=](boost::system::error_code ec, std::size_t size) {
          std::ostringstream ss;
          ss << &aux->buf;
          on_line_read(ss.str());
          asio::async_read_until(aux->stream, aux->buf, '\n', aux->handler);
        };
    asio::async_read_until(aux->stream, aux->buf, '\n', aux->handler);
    break;
  }
}

int main() {
  asio::io_service io;

  fork_with_pipe(io, [](std::string const& line) {
        std::cout << "from child process: " << line << std::flush;
      });

  io.run();
}