C 预处理器的作用
以下内容可使用 yum info cpp 获得，摘录如下：
The C preprocessor provides four separate functionalities: the inclusion of header files (files of declarations that can be substituted into your program); macro expansion (you can define macros,and the C preprocessor will replace the macros with their definitions throughout the program); conditional compilation (using special preprocessing directives, you can include or exclude parts of the program according to various conditions); and line control (if you use a program to combine or rearrange source files into an intermediate file which is then compiled, you can use line control to inform the compiler about where each source line originated).