mruby-ioのつくりかた ref: http://qiita.com/akiray03/items/d6f29db921ff680bc699

file0.rb

class IO
  def write(string)
    str = string.is_a?(String) ? string : string.to_s
    return str.size unless str.size > 0

    len = syswrite(str)
    if str.size == len
      @pos += len
      return len
    end

    raise IOError
  end
end

file1.rb

class IO
  def _read_buf
    return @buf if @buf && @buf.size > 0
    @buf = sysread(BUF_SIZE)
  end
end