Keep getting runtime error for simple C program - c

Question

I have to write a program that checks whether the string is a palindrome or not.
A palindrome is a sequence which is the same forwards as backwards.
For example, kayak is a palindrome, canoe is not a palindrome, hannah is a palindrome, etc.
Here is my code so far:
#include <stdio.h>
#include <string.h>
#define MAX_CHAR 4096
int main(void) {
printf("Enter a string: ");
char line[MAX_CHAR] = {0};
fgets(line, MAX_CHAR, stdin);
int length = strlen(line) - 1;
int i = 0;
int j = length - 1;
char line2[length];
while (i < length){
if (j >= 0){
line2[i] = line[j];
}
i++;
j--;
}
if (strcmp(line, line2) != 0){
printf("String is not a palindrome\n");
} else if (strcmp(line, line2) == 0) {
printf("String is a palindrome\n");
}
return 0;
}
This works for non-palindromes but each time I test with a palindrome, I am getting a runtime error as shown in the image.
How do I solve this?

Answer 1

The issue you are facing is caused by the fact that you do not add a null terminator to line2 (and the array also doesn't have enough space for one), therefore line2 is not a null-terminated byte string.
Passing a pointer to anything other than a null-terminated byte string to strcmp invokes undefined behavior
The simplest way to fix your code is to make the following changes:
/* Don't subtract 1 from `strlen`, otherwise you don't copy the entire string in your loop */
int length = strlen(line);
/* Unchanged */
int i = 0;
int j = length - 1;
/* increase size of array by 1 to have space for null-terminator */
char line2[length + 1];
/* Loop is unchanged */
while (i < length){
if (j >= 0){
line2[i] = line[j];
}
i++;
j--;
}
/* Add null-terminator to have a valid byte string */
line2[length] = '\0';
Please also note that there are simpler ways to achieve the palindrome check, but this answer only focuses on why your code runs into the runtime error.
EDIT: As pointed out in the comments fgets also stores the newline character inside the array. In order for your palindrome check to work correctly you need to adjust your code (e.g.: removing the newline from line before creating line2 and copying the characters)

Answer 2

Easier way:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int palindrome(const char *word)
{
size_t len = strlen(word);
const char *end = word + len -1;
len >>= 1;
while(len--)
{
if(*word++ != *end--)
return 1; //not
}
return 0; //yes;
}
int main()
{
printf("%s\n", palindrome("kayak") ? "No" : "Yes");
}

Answer 3

You have wrong bounds checking, and also fgets reads newline which you have to delete. This is the only compete answer which also check if string entered is not empty:
#include <string.h>
#define MAX_CHAR 4096
int main(void) {
printf("Enter a string: ");
char line[MAX_CHAR] = {0};
fgets(line, MAX_CHAR, stdin);
//remove new line
int length = strlen(line);
if (length > 0) {
line[length-1] = '\0';
}
//update length
length = strlen(line);
if (!length) {
printf("String is empty\n");
return 1;
}
int i = 0;
int j = length - 1;
char line2[length+1];
while (i < length){
line2[i++] = line[j--];
}
//Add 0 char
line2[i] = '\0';
if (strcmp(line, line2) != 0){
printf("String is not a palindrome\n");
} else if (strcmp(line, line2) == 0) {
printf("String is a palindrome\n");
}
return 0;
}