不断获取简单C程序的运行时错误

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 (eg: 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;
}