Struggling with strings in c - issues will null terminator

I don't understand how there is uninitialised variable access in my code and stack buffer overflow even though the two strings are the same when debugging. When i add a null terminator to str2 it also does not fix the issue. When I use fgets the null terminator issue persists. How can i address this? And could someone explain why this is happening and what to look for when doing similar problems.

Note: code is checking if a string is a palindrome

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

#define MAX 4096

int main(void) {
    // Handle taking in input
    char str[MAX];
    int len = 0;
    while (scanf("%c", &str[len]) != EOF) {
        len++;
    }
    char str_rev[MAX];
    int j = 0;
    for (int i = len-1; i >= 0; i--) {
        str_rev[j] = str[i];
        j++;
    }
    
    if (strcmp(str, str_rev) == 0) {
        printf("String is a palindrome\n");
    } else {
        printf("String is not a palindrome\n");
    }

    return 0;
}

Error: stack buffer overflow.

Values when execution stopped:

j = 7
len = 7
str = "racecar", <4089 uninitialized values>
str_rev = "racecar", <4089 uninitialized values>
str_rev[j] = <uninitialized value>

Solution

To use strcmp both arrays must be C-strings. So, both must have the nul-terminator at the end.

You have to add

str[len] = 0;
str_rev[len] = 0;

before the call to strcmp.

complete example: https://godbolt.org/z/GGKKdPoPc

As a side note: Your code has a problem when someone enters ABA<enter>, as it will also read the Enter, so str_rev will be "\nABA".