http://tetzzang.com/unixlinux-c-%ed%95%98%ec%9c%84-%eb%94%94%eb%a0%89%ed%86%a0%eb%a6%ac-%ec%b6%9c%eb%a0%a5%ed%95%98%eb%8a%94-%ed%94%84%eb%a1%9c%ea%b7%b8%eb%9e%a8/