Check if a string is a palindrome in C programming language.

To determine if a string is a palindrome, follow these steps:

1. Define two pointers, one pointing to the beginning of the string and one pointing to the end of the string.
2. For each pair of pointers pointing to characters, compare if they are equal. If they are, move both pointers towards the center by one position; if they are not, then the string is not a palindrome.
3. Repeat step 2 until the two pointers meet or cross each other.
4. If the two pointers meet, it means the string is a palindrome; if the two pointers cross, it means the string is not a palindrome.

Here is an example code:

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

int isPalindrome(char* str) {
    int start = 0;
    int end = strlen(str) - 1;
    
    while(start < end) {
        if(str[start] != str[end]) {
            return 0;   // 不是回文串
        }
        
        start++;
        end--;
    }
    
    return 1;   // 是回文串
}

int main() {
    char str[100];
    printf("请输入一个字符串：");
    scanf("%s", str);

    if(isPalindrome(str)) {
        printf("%s 是回文串\n", str);
    } else {
        printf("%s 不是回文串\n", str);
    }
    
    return 0;
}

The above code determines if a string is a palindrome by comparing its first and last characters. During the comparison, if any unequal characters are found, it can immediately return 0, indicating it is not a palindrome. The final result is output based on the return value of the function.