Access Violation Exception in Visual Studio

#include <stdio.h>
#include <string.h>

int string_length(char* str) {
    char* ptr = str;

    int length = 0;
    while (*(ptr++) != '\0')
        length++;
    return length;
}


char comma(char* str) {
    static char result[100];
    int length;
    int index = 0;

    length = string_length(str);

    for (int i = 0; i < length; i++) {
        if ((length - i) % 3 == 0 && i != 0)
            result[index++] = ',';
        result[index++] = *(str + i); // = str[i]
    }
    result[index] = '\0';

    return result;


}




int main(void)
{
    char str[100];

    printf ("Enter an integer: ");
    scanf_s("%s", str, sizeof(str));

    printf("%s\n", comma(str));

    return 0;
}