Testing function with pointer parameter C

I started learning C this week and here is the first difficulty I'm encountering with pointers.

I would like to test this function:

void    ft_ultimate_div_mod(int *a, int *b)
    {
        int *tmp;

        if (b != 0)
        {
            *tmp = *a % *b;
            *a = *a / *b;
            *b = *tmp;
        } 
}

But I can't figure out the main using pointers. My program is not printing anything with:

int     main(void)
{
    int *a;
    int *b;

    *a = 5;
    *b = 3;

    ft_ultimate_div_mod(a, b);
    printf("%d", *a);
    printf("%d", *b);

    return (0);

}

What can I do? Thx!