程序代码：

#include <stdio.h>
#define fac(n,sum) while(n>1) { --n;sum *=n; }

int main (void) {
        int n,sum;
        scanf("%i",&n);
        sum=n;
        fac(n,sum);
        printf("%i\n",sum);

        return 0;

}

程序代码：

#include <stdio.h>
#define fac(n, product)     \
                            while(n > 1) {     \
                                product *= n--;     \
                            }

int main(void) {
    int n, product = 1;
    scanf("%d", &n);
    fac(n, product );
    printf("product : %d\n", product);
    return 0;
}