--- read([length])
--- read([length[, buf]])       ruby 1.7 feature

    length Хɤ߹ǡʸ֤ޤ
    length ά줿ˤϡEOF ޤǤ
    ƤΥǡɤ߹ߤޤ

    IO  EOF ãƤ nil ֤ޤ

    ǡɤ߹ߤ˼Ԥ㳰 Errno::EXXX ȯ
    length ξ硢㳰 ArgumentError ȯޤ

    ruby 1.7 feature: Ȥʸꤹȡɤ߹
    ǡ򤽤ʸ󥪥֥Ȥ˾񤭤֤ޤꤷʸ
    󥪥֥Ȥ餫 length ĹΰǤС;פʥ
    γƤԤޤ󡣻ꤷʸĹ length 
    ۤʤ硢ʸϰö length Ĺ˳ĥ(뤤Ͻ̾)
    ޤ(ơºݤɤ߹ǡΥˤʤޤ)

    ꤷ read θƤӽФǥǡǤä
    (read nil ֤)buf ϶ʸˤʤ
    

        buf = "x" * 20;
        io = File.open("/dev/null")
        p io.read(10,buf)
        p buf
        => nil
           ""

