pset1 credit; buffer overflow when converting to char array

Question

My initial idea for the credit problem was to convert the long from get_long into a char array, in order to obtain something iterable to get each digit for Luhn's algo.

I recognize that this could be achieved using modulo instead without having to convert the long.

However, I am wondering why this code does not result in a buffer overflow.

#include <cs50.h>
#include <stdio.h>

#define DIGIT 4     /* max digit length for any credit card */

int main (void) {
    // prompt user for number
    long n = get_long("Number: ");

    // verify Luhn's checksum algo

    // convert long to string for enumerating char array
    char str[DIGIT];
    sprintf(str, "%lu", n);
    int len = snprintf(NULL, 0, "%s", str);
    printf("%d", len);
    printf("%s", str);
}

Originally, the DIGIT constant was set to 16, but I was testing and now am confused as to why sprintf does not result in a buffer overflow when I input a number with more than 4 digits, ie

Number: 12345
5
12345

Answer 1

A buffer overflow can be silent. It sometimes leads to a segfault, and sometimes overwrites important data. Other times it's benign, existing in code for years without causing problems. Try declaring another string which is initialized in the declaration. This prints 'abc' for strB to start, but shows the buffer overflow after str is assigned.

#include <cs50.h>
#include <stdio.h>

#define DIGIT 4     /* max digit length for any credit card */

int main (void) {
    // prompt user for number
    long n = get_long("Number: ");

    // verify Luhn's checksum algo

    // convert long to string for enumerating char array
    char strB[DIGIT] = "abc\0";
    char str[DIGIT];
    printf("%s\n", strB);
    sprintf(str, "%lu", n);
    int len = snprintf(NULL, 0, "%s", str);
    printf("%d\n", len);
    printf("%s\n", str);
    printf("%s\n", strB);
}