gist.rb

# a daemon method that blocks until the grandchild is ready
#
# sending the pid up a pipe (grandchild -> child -> parent)
#
  def daemon(options = {}, &block)
  # setup
  #
    raise(ArgumentError, 'no block!') unless block

    chdir = options[:chdir] || options['chdir'] || '.'
    umask = options[:umask] || options['umask'] || 0

  # setup pipes we'll use for IPC
  #
    ra, wa = IO.pipe
    rb, wb = IO.pipe

    pid = fork
    parent = !!pid
    child = !parent

  # in the parent we wait for the child to send back the grandchild's pid
  #
    if parent
      at_exit{ exit! }
      wa.close
      r = ra
      rb.close
      w = wb
      pid = r.gets
      w.puts(pid)
      return Integer(pid.strip)
    end

  # in the child start a grandchild.  handshake the pid with the parent.
  #
    if child
      ra.close
      w = wa
      wb.close
      r = rb

      open("/dev/null", "r+") do |fd|
        STDIN.reopen(fd)
        STDOUT.reopen(fd)
        STDERR.reopen(fd)
      end

      Process::setsid rescue nil

      pid =
        fork do
          Dir::chdir(chdir)
          File::umask(umask)
          $DAEMON = true
          block.call()
          exit!
        end

      w.puts(pid)
      r.gets
      exit!
    end
  end


if $0 == __FILE__
  pid =
    daemon do
      'this code runs in a grandchild...'
    end

 STDERR.puts "parent says #{ pid }..."
end