C scanf format string warning

Question

I'm working on Euler #3 (http://projecteuler.net/problem=3). I think I've got the logic right, but I'm getting an error when trying to use scanf (and printf) with a long. I'm current trying to use %li and this is the error I'm getting:

euler3.c: In function ‘main’:
euler3.c:30: warning: format ‘%li’ expects type ‘long int **’, but argument 2 has type ‘long int’
euler3.c:30: warning: format ‘%li’ expects type ‘long int *’, but argument 2 has type ‘long int’

I understand the error, but for the life of me I can't find the solution. Here's my code if it's needed.

#include <stdio.h>

long greatestPrime(long num)
{
        int i;

        for(i = 2; i <= num; i++)
        {
                if(num%i == 0)
                {
                        num = num/i;
                        i--;
                }
        }

        return num;
}

int main(int argc, char *argv[])
{
        unsigned long greatest;

        printf("Enter number to find prime factor: ");
        scanf("%li",greatest);

        printf("%li",greatestPrime(greatest));

        return 0;
}

Answer 1

scanf is looking for a pointer to a long integer (long int *), not a long integer, so you need to pass the address of greatest by using the & operator:

scanf("%li", &greatest);

As another answer shows as well, you need to use %lu, as you're using an unsigned long int:

scanf("%lu", &greatest);

Answer 2

Use %lu format, as it represents an unsigned long int instead of just a long int

scanf("%lu",&greatest);