若x是一个宏参量，那么#x可以把参数转换为相应的字符串
example:
#include<stdio.h>
#define PSQR (x) printf("the square of " #x " is %d.\n",((x)*(x)))
int main()
{
int y = 5;
PSQR (y);
PSQR(2+4);
return 0;
}
result:
the square of y is 25.
the square of 2+4 is 64.
