#define #ifdef #undef #endif

#include 
int main()
{
	printf("%sn",__TIME__);
	printf("%dn",__LINE__);
	printf("%sn",__DATE__);
	printf("%sn",__TIME__);
}

#include 
#define SWAP(T,x,y) {T t=x;x=y;y=t;}
int main()
{
	int a=1,b=2;
	double c=1.1,d =2.2;
	SWAP(int, a,b);
	SWAP(double,c,d);
	return 0;
}

#include 
#define Pi 3.14
#define S(r) Pi*(r)*(r)

int main()
{	
	printf("%g",S(2));
	return 0;	
}

#include 
#define MAX(x,y) ((x)>(y)?(x):(y))//remember the braces
 
int main()
{	
	int a =1,b=2;
	int c =3,d =4;
	printf("%dn",MAX(a,b)+MAX(c,d));
	return 0;	
}

#include 
#define STR(x) puts(#x)

int main()
{	
	STR(hello);//puts("hello")
	return 0;	
}

#include 
#define P(XXXXX) print##XXXXX()//##replace the string
void printhello(){
	printf("hello n");
	}
void printworld(){
	printf("world n");
	}
int main()
{
	P(hello);//printhello()
	P(world);//printworld()
	return 0;
}

C语言#if、##ifdef、#ifndef的用法详解，C语言条件编译详解

宏定义来实现条件编译（#define #undef #ifdef）_麒麒川的博客-CSDN博客_宏定义条件编译

#include 

#ifndef varX
    #define varX 1
    int x =10000;
#endif

#ifdef varX
    #undef varX//cancel define varX
#endif

#ifndef varX
	#define varX 2
	//int X =10086;//cause conflict
#endif

int main()
{
	printf("%d n",x);
    printf("%d n",varX);
	return 0;
}