https://blog.wongcw.com/2020/06/04/signal%e5%bc%95%e5%85%a5%e4%ba%ba%e8%87%89%e6%a8%a1%e7%b3%8a%e5%8a%9f%e8%83%bd%ef%bc%9a%e5%8f%af%e9%9a%b1%e8%97%8f%e5%9c%96%e7%89%87%e4%b8%ad%e4%ba%ba%e8%87%89%e5%92%8c%e5%85%b6%e4%bb%96%e4%bf%a1/