/*
 * Program to compute factorial using recursion,
 * with check for overflow.
 *
 * (Exits the program on overflow -- not good in a larger 
 * program but okay for purposes of illustration.)
 */
#include <stdio.h>
#include <limits.h> /* has INT_MAX */
#include <stdlib.h> /* has exit() */

int factorial(int n);

int main(void) {

	int n;
	printf("enter n\n");
	if (scanf("%d", &n) != 1) {
		printf("not number\n");
		return 1;
	}
	if (n < 0) {
		printf("negative\n");
		return 1;
	}
	printf("factorial(%d) is %d\n", n, factorial(n));

	return 0;
}

int factorial(int n) {
	if (n <= 0)
		return 1;
	else {
		/* can't test n * factorial(n-1) > INT_MAX directly */
		if (factorial(n-1) > (INT_MAX / n)) {
			printf("overflow!\n");
			exit(1);
		}
		return n * factorial(n-1);
	}
}