def do_at_exit(str1)
  at_exit { print str1 }
end
at_exit { puts "cruel world" }
do_at_exit("goodbye ")
exit
#=> goodbye cruel world

MRuby::Build.new do |conf|
  # by mgem
  conf.gem :mgem => 'mruby-at_exit'
  # by github
  conf.gem :github => 'ksss/mruby-at_exit'
end