Question

I have written my own linked list implementation in C, and it works fine for storing values, but whenever I try to run through the list I get a segmentation fault. I’m not sure why the issue arises, and I spent a decent amount of time trying to run through the program myself but to no avail. Could you guys help me find the reason for the error? Here is my code:

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

// A double linkd list node
struct list
{
    struct list * prev;
    int data;
    struct list * next;
};
typedef struct list node;
node *head = NULL;
node *tail = NULL;

// Adds an item to the end of the list
void append(int item);
// Adds an item at the given index
int insert(int item, int index);
// Removes and returns an item at the specified index
int remove_node(int index);
// Gets an item at the specified index
int get(int index);
// Returns the node at the specified index
node* get_node(int idex);


// Adds an item to the end of the list
void append(int item)
{
    // If no items have been added to the list yet   
    if(head == NULL)
    {
        head = (node*) malloc(sizeof(node));
        head->prev = NULL;
        head->data = item;
        head->next = NULL;
        tail = head;
        printf("Successfully added the head\n");
    }
    // If items have previously been added to the list
    else
    {
        node *current = (node*) malloc(sizeof(node));
        printf("Successfully allocated memory for the next item\n");
        current->prev = tail;
        printf("Assigned current previous link to tail\n");
        current->data = item;
        printf("Assignd current's data value: %d\n", item);
        current->next = NULL;
        printf("Assigned current's next value to NULL\n");
        tail = current;
        printf("Assigned tail to current\n\n\n");
    }
}


// Adds an item at the given index
// Returns an int. Nonzero means there was an error
int insert(int item, int index)
{
    node *current, *new;
    current = head;

    for( ; index > 0; index--)
    {
        current = current->next;
    }

    // Make a new node and properly connect it
    new = (node*) malloc(sizeof(node));
    new->prev = current->prev;
    new->data = item;
    new->next = current;
    current->prev = new;

    return 0;
}


// Removes and returns an item at the specified index
int remove_node(int index)
{
    int ret;
    node *current = head;

    for( ; index > 0; index--)
    {
        current = current->next;
    }
    ret = current->data;

    // Connect the nodes on both sides of current
    (current->prev)->next = current->next;
    (current->next)->prev = current->prev;

    free(current);
    return ret;
}


// Gets an item at the specified index
int get(int index)
{
    return (get_node(index))->data;
}

// Returns the node at the specified index
node* get_node(int index)
{
    node *current = head;
    for( ; index > 0; index--)
    {
        current = current->next;
    }
    return current;
}


int main(void)
{
    int i;
    for(i = 0; i < 10; i++)
    {
        append(i);
    }

    node *current = head;
    for(i = 0; i < 10; i++)
    {
        printf("%d\n", current->data);
        current = current->next;
    }

    return 0;
}

Answer 1

Your append() function is incorrect – it also needs to set tail->next = current; (before changing tail).

Your insert() function similarly needs to set current->prev->next = new; if current->prev is not NULL; otherwise it needs to set head = new;. It also needs to handle current being NULL.

Your remove_node() function needs to handle the cases of current->prev and/or current->next being NULL (it should update head and tail respectively in those cases).

It would also be a good idea if your functions that take an index take care not to run off the end of the list, if the index given is out of bounds.