unsigned long long type printing in hexadecimal format

You can use the same ll size modifier for %x, thus:

#include <stdio.h>

int main() {
    unsigned long long x = 123456789012345ULL;
    printf("%llx\n", x);
    return 0;
}

The full range of conversion and formatting specifiers is in a great table here:

• printf documentation on cppeference.com