c 宏的定义

#include <stdio.h>
#include <conio.h>

#define  VAL  40

#ifdef   VAL
	#undef  VAL
#endif

#define VAL 50      

int main ()
{
	clrscr();

	printf( "\n Value = %d",  VAL );   

	getch();
	return 0;
}

输出

Value = 50