Fibonacci number program crashes with exit code 139 - c

Question

I am testing edge cases in my Fibonacci number program. It runs fine if the input value for first variable is 999999 or below. It crashes when the value is anything bigger. I am already using long int somewhere, really no clue what am I doing wrong.
#include <stdio.h>
#include <stdlib.h>
long ft(long num, int m) {
long arr[num];
int i;
int j = 0;
i = 3;
arr[0] = 0;
arr[1] = 1;
arr[2] = 1;
while (i <= num && j != 1) {
arr[i] = 0;
if ((arr[i - 1] + arr[i - 2]) < m) {
arr[i] = arr[i - 1] + arr[i - 2];
} else {
arr[i] = (arr[i - 1] + arr[i - 2]) % m;
}
if (arr[i] == 1 && arr[i - 1] == 1 && arr[i - 2] == 0)
j = 1;
i++;
}
if (i < num)
return arr[num % (i - 3)];
else
return arr[i - 1];
}
int main() {
long n = 0;
int m = 0;
scanf("%ld %d", &n, &m);
printf("%ld", ft(n, m));
return 0;
}

Answer 1

That's too much stack. Please allocate arr with malloc() and please check for allocation failures. Don't forget to free arr.
long *arr = malloc(sizeof(long) * (size_t)(num + 1));
if (!arr) return -1; // Error
//...
long r;
if (i < num) r= arr[num%(i-3)];
else r= arr[i-1];
free(arr);
return r;

Answer 2

Your program computes Fibonacci numbers modulo a given number.
The reason it crashes for values above 999999 is you allocate a variable length array of long with automatic storage (aka on the stack), that exceeds the available space in this segment (4 or 8 MB). Your program has undefined behavior and causes a stack overflow.
You should instead allocate this array from the heap with malloc or calloc and free it before leaving the function.
Note also these other problems:
arr should have a length of num + 1 because you store the result in arr[num].
the result is incorrect for num == 0 and for m == 1.
the function has undefined behavior for m == 0.
negative values of m and num should return an error result such as -1.
the test for cycle detection can be simplified.
Here is a modified version:
#include <stdio.h>
#include <stdlib.h>
long ft(long num, int m) {
long *arr;
int i;
long res;
if (m < 0 || num < 0 || (arr = calloc(sizeof(*arr), num + 1)) == NULL)
return -1;
if (m == 1)
return 0;
arr[0] = 0;
arr[1] = 1;
arr[2] = 1;
res = arr[num];
for (i = 3; i <= num; i++) {
res = arr[i - 1] + arr[i - 2];
if (res >= m)
res -= m;
arr[i] = res;
if (res == 0 && arr[i - 1] == 1) { // handle cycle
res = arr[num % i];
break;
}
}
free(arr);
return res;
}
int main(int argc, char *argv[]) {
long n, n2;
int m = 0;
char *p;
if (argc > 1) { // command line arguments: start[..end] [mod]
n = n2 = strtol(argv[0], &p, 0);
if (*p == '.' && p[1] == '.')
n2 = strtol(p + 2, NULL, 0);
if (argc > 2)
m = strtol(argv[0], NULL, 0);
} else {
if (scanf("%ld %d", &n, &m) != 2)
return 1;
n2 = n;
}
while (n <= n2)
printf("%ld\n", ft(n++, m));
return 0;
}