--- recv_io([klass[, mode]])  ruby 1.7 feature
    եǥץޤ

    klass  nil ξ硢եǥץ Fixnum Ȥ
    ֤ޤ

    klass  nil Ǥʤ硢
    klass.for_fd(fd[, mode]) ƤФ졢֤ͤޤ
    klass ά줿 IO ꤵ줿ΤȤߤʤ졢
    IO.for_fd(fd[, mode]) ƤФޤ

