segmentation fault

[codedinC]

I have a C program which creates a doubly linked list, by having nodes store a single int as data, a pointer to the previous node, and a pointer to the next node. The problem is that when I run the program I receive a segmentation fault when I try to print the data in the head node. Through debugging I have noticed that the head node does not stay constant, it advances itself to the next node as new nodes are added.

Here is the function in which I add a new node to the end of the list.

void addnode(item *curr, item *head, item *tail)
{
    if(head == NULL)
    {
        curr->prev = NULL;
        head = curr;
    }//end if
    else
    {
        tail->next = curr;
        curr->prev = tail;
    }//end else
    curr->next = NULL; //set the current nodes  next to NULL
    tail = curr; //new tail is the current node
    printf("head: %d\n", head->val);
    printf("curr: %d\n", curr->val);
}//end addnode

Any suggestions?

[Ganon11]

When do you initialize head, tail, or curr? Outside the function? In particular, I don't think tail is being properly allocated, as you assume that any list with a head present already has a tail - yet you also assume that, if no head is present, you only need to set the head.

Basically, for a list of 1 element, that element is both the head and the tail. Your code should reflect this.

[mattmao]

Hi.

Keep it simple man. Your could divide this problem into two steps. First, create a head sentinel as well as a tail sentinel, two nodes that would always be there. The second step would be just inserting nodes in the middle, which is some sort like this: break the chain, add a new node, relink the chain.

I found it is a good approach to this adding a node functionality, anyway, you don't need to consider the different cases anymore, even there is no node before, you just "insert" your first node between the head and the tail sentinels.