[C语言]变量的声明和定义有什么区别

参考：

https://www.cnblogs.com/LeeCookies/p/12663366.html

https://blog.csdn.net/littledouble/article/details/46340137

https://blog.csdn.net/hxg130435477/article/details/6775182

https://blog.csdn.net/liuchunjie11/article/details/80333224