Character Array and Null character

Question

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

int main()
{
    int i;
    char str[4];

    scanf("%s",str);
    printf("%s",str);
}

input scan output scan

Here I declare an array of 4 characters. I used '%s' that is used for strings. I am not able to understand how can we input 4 char elements and get correct answer when one space should be utilized for the NULL character. The input should only work with up to 3 elements.

see https://stackoverflow.com/a/12019987/12139819 . Long story short: it's UB, avoid `scanf`, use `fgets` — vtronko, Apr 03 '20 at 11:14

score 4 · Accepted Answer · answered Apr 03 '20 at 11:15

scanf() does not check its arguments. You could even enter more than 4 characters and scanf() would happily overwrite the memory area that comes after your array. After that, your program might crash or all kinds of funny things might happen. This is called a buffer overflow and it is a common cause of vulnerabilities in software.

score 2 · Answer 2 · answered Apr 03 '20 at 11:44

as mentioned when you take more than 3 character as input ,and extra chars and \0 will be written outside of array memory(after it) and over write memory which doesn't belong to array.which will cause undefined behavior.

but you can use these to prevent buffer overflow from happening:

scanf("%3s",str);

or

fgets(str, sizeof str, stdin)

Character Array and Null character

2 Answers2