Large number factorial

#include <iostream>

#define log(x) std::cout << x ;
#define MAX 50000

int multiply(int x, int res[], int res_size);

uint64_t factorial(uint64_t a)
{
    uint64_t factorial = 1;
    while (a)
    {
        factorial *= a--;
    }
    return factorial;
}

void factorial(int n)
{
    int res[MAX];
    res[0] = 1;
    int res_size = 1;

    for (int x = 2; x <= n ; x++)
    {
        res_size = multiply(x, res, res_size);
    }
    for (int i = res_size - 1; i >= 0; i--)
    {
        log(res[i]);
    }
}

int multiply(int x, int res[], int res_size)
{
    int carry = 0;
    for (int i = 0; i < res_size; i++)
    {
        int prod = res[i] * x + carry;
        res[i] = prod % 10;
        carry = prod / 10;

    }
    while (carry)
    {
        res[res_size] = carry % 10;
        carry = carry / 10;
        res_size++;
    }
    return res_size;
}

int main()
{

    factorial(300);

    std::cin.get();

}