Difference in storage of memory in string and an integer array in C after using sprintf function

Question

Could you please explain me memory allocation in C for strings and integer array after using sprintf function?

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

int main() {
    char str[10];
    long long int i = 0, n = 7564368987643389, l;
    sprintf(str, "%lld", n);  // made it to a string 
    printf("%c%c", str[11], str[12]);
}

In the above code, string size is 10 inclusive of null character. How come we access 11 and 12 elements in it? The program prints 43

Answer 1

Here

sprintf(str,"%lld",n);

n i.e 7564368987643389 converted into character buffer & stored into str. It looks like

str[0]     str[2]   str[4]    str[6]     str[8]   str[10]   str[12] ..
--------------------------------------------------------------------------------------
| 7  | 5  | 6  | 4  | 3  | 6  | 8  | 9  | 8  | 7  | 6  | 4  | 3  | 3  | 8  | 9  | \0 |
--------------------------------------------------------------------------------------
str   str[1]    str[3]    str[5]    str[7]    str[9]    str[11] ..

As you can see str[11] is 4 and str[12] is 3. Hence the printf() statement below prints:

printf("%c %c",str[11],str[12]);

4 3

But since you have declared str of 10 characters and in sprintf() you are trying to store more than 10 characters, it causes undefined behavior.