Divisor script in C

#include <stdio.h>

// C script for calculating the divisors of a number.
// It turns out calculating the square root was not required -- we can determine if we hit it ourselves. Go to the 28th line to see how.

// I wrote this script because I wanted a script that is faster than the Python one.
// Indeed, computing the divisors of 9007199254740992 was about 9x faster (23642ms on Python vs. 2641ms on C).

int main(int argc, char** argv) {
    if (argc < 2) { // for some reason, argc is 1 if no arguments are specified.
        printf("Specify the number whose divisors you want to take as the first argument.");
    }
    unsigned long long num = 0;
    // The below "for" loop converts a string into an unsigned long long.
    // Say you input "123".
    // The loop:
    // - takes the character "1" (value 49), subtracts 48, gets 1.
    // - multiplies the number by 10 and adds 1 (result = 1).
    // - takes the character "2" (value 50), subtracts 48, gets 2.
    // - multiplies the number by 10 and adds 2 (result = 12).
    // - takes the character "3" (value 51), subtracts 48, gets 3.
    // - multiplies the number by 10 and adds 3 (result = 123).
    for (int i = 0; argv[1][i]; ++i) {
        num = argv[1][i]-48+num*10;
    }
    // i is an unsigned long because the square root of an unsigned long long will always be an unsigned long.
    for (unsigned long i = 1; num/i>=i; // This is the part that stops at the square root of the number.
                                        // When evaluating a non-square number:
                                        // 23 / 1 = 23
                                        // 23 / 2 = 11.5
                                        // 23 / 3 = 7.(6)
                                        // 23 / 4 = 5.75
                                        // 23 / 5 = 4.6
                                        // When evaluating a square number:
                                        // 25 / 1 = 25
                                        // 25 / 2 = 12.5
                                        // 25 / 3 = 8.(3)
                                        // 25 / 4 = 6.25
                                        // 25 / 5 = 5
                                        // We stop once the result, rounded down, is less than or equal to the divisor (the opposite of it being greater than the divisor).
                                        // In both examples, the square root of the number, rounded down, is the result of the final division rounded down.
                                        ++i) {
        if (num % i == 0) {
            printf("%lu\n",i);
            if (i * i != num) { // To avoid getting a duplicate result.
                printf("%llu\n",num/i);
            }
        }
    }
}