Using fmod() in C always results in zero (0)

I'm trying to split a 16 digits number into digits. Using unsigned long int and % doesn't work because this type of variable only supports numbers smaller than 4.294.967.295. So I adapted the code and used fmod instead of %, but now the only result I get is zero.

#include <stdio.h>
#include <stdlib.h>
#include <locale.h>
#include <math.h>

int main() {
    setlocale(LC_ALL, "");

    double num, digit;
    int i;

    printf("Enter the 16-digit number: ");
    scanf("%lf", &num);

    for (i = 1; num > 0; i++) {
        printf("%d - ", i);
        digit = fmod(num, 10.0);
        printf("%.2lf   ", digit);

        num /= 10.0;
        printf("%.2f\n", num);
    }
}

What am I doing wrong?

fmod is not the right tool to handle large integers, not is the double type if you wish to have exacts digits. Type unsigned long int might indeed not be large enough for numbers as large as 10¹⁶, depending on the target architecture, especially legacy systems where the long type only has 32 bits even in 64-bit mode.

You should use the type unsigned long long int, which is guaranteed to have at least 64 value bits, more than enough for your large integers:

#include <stdio.h>
#include <stdlib.h>

int main() {
    unsigned long long num;
    int i, digit;

    printf("Enter the 16-digit number: ");
    if (scanf("%llu", &num) == 1) {
        for (i = 1; num > 0; i++) {
            printf("%d - ", i);
            digit = num % 10;
            printf("%d   ", digit);
            num /= 10;
            printf("%llu\n", num);
        }
    }
    return 0;
}