--- IO.new(fd[, mode])
--- IO.for_fd(fd[, mode]) ruby 1.7 feature

    ץѤߤΥեǥץ fd Ф뿷 
    IO ֥Ȥ֤ޤIO ֥Ȥ
    ˼Ԥ㳰 Errno::EXXX ȯޤ

    mode ˤϡFile.open Ʊ IO Υ⡼ɤ
    ꤷޤ(ʸΤ)
    mode Υǥեͤ "r" Ǥ

    ruby 1.8 feature:
    mode λϴ open,
    File.open ƱǤ(Ĥޤꡢ1.8  File::RDONLY 
    ʤɤ()ǥ⡼ɤǤޤ)

