Surprising segfault in C code - c

Question

I cannot understand how this code is causing a segfault when I run it, can anyone help me understand what is going on??
#include <stdio.h>
unsigned long long factorial(unsigned long long x, unsigned long long amt)
{
if (x == 1ULL) return amt;
else return factorial(x-1ULL, amt*x);
}
int main(int argc, char *argv[])
{
for (unsigned long long i = 0; i < 10ULL ;i++) {
printf("%llu\n", factorial(i, 1ULL));
}
}

Answer 1

First, the segfaults are not necessarily caused be invalid pointer dereference. In this case, it is actually caused by infinite recursion and eventual running out of stack space. Why? The essential requirement of a recursion function is it has to finish and terminate the recursion at some state, if you look carefully your code, in function factorial, if x is 0, then the recursion will become endless and eventually crash your program. You can fix this by change the terminate condition to:
if (x <= 1ULL) return amt;

Answer 2

In the orginal code :
if (x == 1ULL) return amt;
is meant to be the exit condition for this recursive function factorial. However when a value of zero is passed to the function and given that the type of x is unsigned long long, first recursive call to the function factorial with the parameter x-1ULL would set the value of x to very large value (18446744073709551615 is what I got here). Successive recursive calls to factorial will gradually deplete the stack space allocated for the program to the point where you get a segmentation fault.
You should have been doing this:
#include <stdio.h>
unsigned long long factorial(unsigned long long x, unsigned long long amt)
{
if (x == 0ULL) // Changed the exit condition, see Reference [1]
return amt; // Bear in mind that the initial value for amt you passed is 1
amt*=x; // See Reference [2]
return factorial(x-1ULL, amt);
}
int main(int argc, char *argv[]) {
for (unsigned long long i = 0; i < 10ULL ;i++) {
printf("%llu\n", factorial(i, 1ULL));
}
}
References
The idea of the factorial (in simple terms) is used to compute the number of permutations (combinations) of arranging a set of n numbers. It can be said that an empty set can only be ordered one way, so 0! = 1. Check this.
I feel this is more readable than factorial(x-1ULL, amt*x)
Note
The ULL suffixes are redundant here and may be removed altogether.