gistfile1.rb

require 'ffi'

module Foo
  extend FFI::Library
  ffi_lib 'ruby-1.9.1'
  attach_function :rb_intern, [:string], :long
  attach_function :rb_obj_id, [:long], :long
  attach_function :rb_funcall2, [:long, :long, :int, :pointer], :long
end

def id2ref(id)
  # TODO: only works for non-special objects
  return id ^ 1
end

def ref(obj)
  return id2ref(obj.__id__)
end

obj = Object
ptr = FFI::MemoryPointer.new(:pointer, 0)
ret = Foo.rb_funcall2(ref(obj), Foo.rb_intern("to_s"), 0, ptr)
p ret
objid = Foo.rb_obj_id(ret)
p objid
s = ObjectSpace._id2ref(objid) # TODO: not sure why this line isn't doing the job...
p s