预处理指令

// The include directive instructs the preprocessor to paste the text of the given file into the current file.
// 粘贴指定文件的内容
#include

// 定义宏PI(注意行末没有分号)
#define PI 3.1415926
// 取消定义PI
#undef PI

条件编译(Conditional Compilation)

// 检查xxx是否已被定义为宏,如果是,则到#endif为止的内容都包含进来
#ifdef xxx
 ...
#endif

// 检查xxx是否没有被宏定义
// One common use of '#ifndef' is to include code only the first time a header file is included.
#ifndef xxx
 ...
#endif

// 如果condition为true,则includes the code until the closing #endif.
// 如果不是true,则不包含
#if condition
 ...
#endif

The '#endif' directive closes off the following directives: #if, #ifdef, or #ifndef.
In other words, '#endif' always matches the nearest '#ifdef' (or '#ifndef', or '#if').

在程序中使用宏来控制日志是否打印

//如果要启用debug,编译的时候指定-D__DEBUG__
#ifdef __DEBUG__
  //这种形式最少需要传两个参数
  #define DEBUG(format,...) printf(__FILE__",Line:%d,"format"\n", __LINE__, ##__VA_ARGS__) 
#else
  #define DEBUG(format,...)
#endif

#define RED_COLOR "\033[31m"
#define RESET_COLOR "\033[m"
//使用
DEBUG(RED_COLOR"Program Start."RESET_COLOR,"");

https://www.techonthenet.com/c_language/directives/endif.php
https://gcc.gnu.org/onlinedocs/cpp/Ifdef.html
https://en.cppreference.com/w/c/preprocessor/replace
https://en.cppreference.com/w/c/language/translation_phases
https://engineering.purdue.edu/ece264/22sp/hw/HW06
https://stackoverflow.com/questions/17091746/too-many-arguments-for-format-wformat-extra-args
https://litux.nl/mirror/cinanutshell/0596006977/cinanut-CHP-14-SECT-2.html
https://www.cnblogs.com/cosail/articles/3672145.html