Write a program in C language to check that a string entered by the user is Palindrome or not.

To check whether a string entered by the user is a palindrome or not in C language, follow these steps:

1. Prompt the user to enter a string.
2. Read the string input from the user and store it in a character array.
3. Define two pointers, one pointing to the beginning of the string (let's call it start) and the other pointing to the end of the string (let's call it end).
4. Compare characters at start and end positions iteratively until start becomes greater than or equal to end.
5. If at any point the characters at start and end positions are not equal, the string is not a palindrome. Otherwise, continue comparing characters until the pointers meet.
6. If all characters match, the string is a palindrome.

Here's the C code implementing the above logic:

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

int main() {
    char str[100];
    int start, end, isPalindrome = 1; // Assume palindrome initially

    printf("Enter a string: ");
    fgets(str, sizeof(str), stdin);

    // Remove newline character at the end of the input string
    str[strcspn(str, "\n")] = '\0';

    // Convert the string to lowercase for case-insensitive comparison
    for (int i = 0; str[i]; i++) {
        str[i] = tolower(str[i]);
    }

    // Initialize start and end pointers
    start = 0;
    end = strlen(str) - 1;

    // Compare characters at start and end positions
    while (start < end) {
        if (str[start] != str[end]) {
            isPalindrome = 0; // Not a palindrome
            break;
        }
        start++;
        end--;
    }

    if (isPalindrome) {
        printf("The string is a palindrome.\n");
    } else {
        printf("The string is not a palindrome.\n");
    }

    return 0;
}

This program prompts the user to enter a string, reads the input string, and then checks whether it is a palindrome or not. It performs case-insensitive comparison by converting the string to lowercase. Finally, it prints the result indicating whether the string is a palindrome or not.