Using array to represent a big Number and its multiplication

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We need to print factorials of numbers up-to 100.
I am successful to do that with doing using an array to represent the digit where each element is a single digit.
But, we can improve by storing multiple digits at same element using datatypes like long long and unsigned long long.
So, I tried implementing that

ull digits[160/8+1];
ull size = 0;
void multiply (int n) {
    ull index = 0;
    ull t = n;
    ull carry = 0;
    while(carry != 0 || index < size) {
        ull x = digits[index] * n + carry;
        // cout << index << " " << x << " " << carry <<  "\n";
        digits[index] = (x % (ull)1e8);
        
        carry = x / 1e8;
        index ++;
    }
    size = max(size, index);
}

Here we store 8 digits in single element of array.
But it gives wrong answer with some numbers like
21
OUTPUT:
51090942171709440000
REAL:
51090942171709440000
[21, 22, 38, 41, etc.]
It’s always some problem with zero.
Is my logic correct?