调用是system函数
还有这个
http://zhidao.baidu.com/question/249018805.html?fr=ala&word=%E6%94%B9%E7%94%A8%E6%88%B7%E5%90%8Ddos&device=mobile&ssid=0&from=844b&uid=0&pu=usm%400%2Csz%401328_1001%2Cta%40pad_2_4.1_3_534&bd_page_type=1&baiduid=C624BA9662485CA333FBA4DFA1819B38&tj=zhidao_1_0_10_title