1、in.h:
An implementation of the TCP/IP protocol suite for the LINUX
operating system. INET is implemented using the BSD Socket
interface as the means of communication with the user level.
2、unisted
macro-宏
3、inet.h
定义了一组用于网络地址的字符串形式和二进制形式之间的转换的函数，函数的形式为inet_xxx()
4、usr/include
linux库所在的文件夹，能用#inlcude<>包含的，基本都在这里