STATIC Variables and Functions in C

ref:
http://www.jianshu.com/p/f413ba3b2728
http://www.cnblogs.com/wly603/archive/2012/04/11/2442065.html
http://bbs.csdn.net/topics/350238100
http://www.cppblog.com/dbkong/archive/2006/12/09/16169.html