C语言条件编译用途：

1.判断宏是否被定义；

2.判断宏是否未被定义；

3.选择性执行

#include <stdio.h>




#define DEBUG 10
#define HIGH 2

int main() {
	int value = 10;

//1.用于判断宏是否被定义
#ifdef DEBUG
	printf("1.DEBUG defined\r\n");
#else
	printf("1.DEBUG not defined\r\n");
#endif 

//2.判断宏是否未被定义
#ifndef DEBUG
	prinf("2.DEBUG not defined\r\n");
#else
	printf("2.DEBUG defined\r\n");
#endif

//3.选择性执行代码
#if DEBUG==10
	printf("xiangdeng\r\n");
#elif DEBUG>10
	printf("dayu10\r\n");
#else
	printf("xiaoyu10\r\n");
#endif


#if HIGH
	printf("success\r\n");
#else
	printf("not success\r\n");
#endif

}