说明：  CRC 校验源码分析，第一次接触还真难以理解。这些东西不想在这里讲，随便找一下都是一大把。我想根据源代码来分析会比较好懂一些。
(CRC-source analysis, the first contact really hard to understand. These things do not want to speak of here, what kid is a lot. I wish to source code analysis to better understand some.)