Segmentation fault issue while reversing linked list in c

Question

#include <stdio.h>
#include <stdlib.h>

struct node {
    int data;
    struct node *next;
};

struct node *insert(struct node *link, int data) {
    if (link == NULL) {
        link = (struct node *)malloc(sizeof(struct node));
        link->data = data;
        link->next = NULL;
    } else {
        struct node *newlink = (struct node *)malloc(sizeof(struct node));

        newlink->data = data;
        newlink->next = link;
        link = newlink;
    }
    return link;
}

void reverse(struct node *link) {
    int i, j = 0;
    int arr1[100], arr2[100];
    struct node *current;
    int count = 0;

    current = link;

    while (current != NULL) {
        arr1[i] = current->data;
        i = i + 1;
        count = count + 1;
        current = current->next;
    }

    printf("\n");
    i = 0;
    j = 0;

    for (i = count - 1; i >= 0; i--) {
        arr2[j] = arr1[i];
        j = j + 1;
    }

    printf("The elements in the linked list are: ");

    for (i = 0; i < count; i++) {
        printf("%d ", arr1[i]);
    }

    printf("The elements in the reversed linked list are: ");

    for (j = 0; j < count; i++) {
        printf("%d ", arr2[j]);
    }
}

void print(struct node *link) {
    struct node *temp = link;

    printf("The elements in the linked list are: ");

    while (temp != NULL) {
        printf("%d ", temp->data);
        temp = temp->next;
    }
}

void main() {
    int value;

    printf("Enter the value:\n");
    scanf("%d", &value);
    struct node *link = NULL;

    link = insert(link, value);
    char ans[3] = "yes";

    while (ans[0] == 'y') {
        printf("Do you want to add another node? Type Yes/No\n");
        scanf("%s", ans);

        if (ans[0] == 'y') {
            printf("Enter the value:\n");
            scanf("%d", &value);
            link = insert(link, value);
        } else {
            reverse(link);
        }
    }
}

Here is the code I wrote to reverse a single linked list in C. I seem to try different combinations of the program but while doing it by array method, I am unable to get rid of the segmentation fault, and hence it doesn't give output.

Answer 1

There are some problems in your code:

• i is uninitialized when used in the while loop in function reverse , causing undefined behavior which could explain the segmentation fault .

• j is not modified in the loop at the end of the reverse function, causing an infinite loop:

   for (j = 0; j < count; i++) { printf("%d ", arr2[j]); } 

• you are not reversing the list, you just print the list contents in reverse order, and assume its length is at most 100 . This is probably not what you are expected to do.

• in function main , the array ans should be made larger to accommodate at least the word yes , and you should prevent scanf() from storing more characters into it than would fit. Also reorganize the code to avoid duplication:

   int main(void) { struct node *link = NULL; for (;;) { char ans[80]; int value; printf("Enter the value:\\n"); if (scanf("%d", &value) != 1) break; link = insert(link, value); printf("Do you want to add another node? Type Yes/No\\n"); if (scanf("%79s", ans) != 1 || ans[0] != 'y') { break; } } reverse(link); return 0; } 

Most of the above problems would have been spotted immediately by increasing the compiler warning level (for example gcc -Wall -Werror or clang -Weverything -Werror ).

Here is a simpler version that reads numbers and allocates the list in the same order as you do, inserting each new element before the previous one, then reverses the list and finally prints it. As expected, the list is printed in the order of entry.

#include <stdio.h>
#include <stdlib.h>

struct node {
    int data;
    struct node *next;
};

struct node *insert(struct node *head, int data) {
    struct node *newlink = malloc(sizeof(*newlink));
    newlink->data = data;
    newlink->next = head;
    return newlink;
}

struct node *reverse(struct node *link) {
    struct node *prev = NULL;
    while (link) {
        struct node *temp = link->next;
        link->next = prev;
        prev = link;
        link = temp;
    }
    return prev;
}

void print(struct node *link) {
    printf("The elements in the linked list are: ");

    for (struct node *n = link; n; n = n->next) {
        printf("%d ", n->data);
    }
    printf("\n");
}

int main(void) {
    struct node *link = NULL;
    int value;

    printf("Enter the values, end the list with 0:\n");
    while (scanf("%d", &value) == 1 && value != 0) {
        link = insert(link, value);
    }
    link = reverse(link);
    print(link);
    return 0;
}