头件的中的#ifndef，这是一个很关键的东西。比如你有两个C文件，这两个C文件都include了同一个头文件。而编译时，这两个C文件要一同编译成一个可运行文件，于是问题来了-The first pieces of# Ifndef, this is a very key things. For example, you have two C file, the two C files include the same file size. And compile-time, the two C