This week's code snippet, Factorial in C, is brought to you by Subete and the Sample Programs repo.
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <errno.h>
#include <limits.h>
int uint64_overflow(long, long);
int main (int argc, char *argv[])
{
long n;
char *endptr;
uint64_t result = 1;
long i;
/* Correct nmber of args? */
if (argc != 2) {
printf("Usage: please input a non-negative integer\n");
exit(0);
}
/* Convert argument to number */
errno = 0;
n = strtol(argv[1], &endptr, 10);
if ((errno == ERANGE && (n == LONG_MAX || n == LONG_MIN))
|| (errno != 0 && n == 0)) {
perror("strtol");
exit(1);
}
if (endptr == argv[1] ||
!(*endptr == '\0' || *endptr == '\n') ||
n < 0) {
fprintf(stderr, "Usage: please input a non-negative integer\n");
exit(1);
}
/* For non-zero numbers, compute factorial */
if (n) {
for (i = 1; i <= n; i++) {
if (uint64_overflow(result, i)) {
printf("Results overflowed\n");
exit(1);
}
result *= i;
}
}
/* Priunt result */
printf("%ld\n", result);
return 0;
}
/*
* Detects overflow from 64-bit multiplication.
*
* Returns 1 if an overflow occurs, 0 if not
*/
int uint64_overflow(long a, long b) {
uint64_t x;
uint64_t b_64;
x = a * b;
b_64 = b;
return (a != 0 && x / a != b_64) ? 1 : 0;
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