C 双循环链表删除

Question

I'm having a problem deleting from a circular doubly linked list. I have tried various different methods mentioned here, but they all cause some sort of error. Here, I'm getting an error saying "double free detected in tcache 2"

void delete_list(Node *node)
{
        Node *pt;
        while(node != NULL){
                pt = node;
                node = node->next;
                free(pt);
        }
}

Answer 1

I'm having a problem deleting from a circular doubly linked list

A circular doubly linked list does not have a node with the data member next that is equal to NULL . The tail node in its data member next has the address of the head node.

So if your list is not empty then this function

void delete_list(Node *node)
{
        Node *pt;
        while(node != NULL){
                pt = node;
                node = node->next;
                free(pt);
        }
}

invokes undefined behavior after it moves from the tail node to the head node that it tries to free twice.

The function can look for example the following way (without testing)

void delete_list( Node **head )
{
    if ( *head )
    {
        Node *current = ( *head )->next;
     
        while ( current != *head )
        {
            Mode *tmp = current;
            current = current->next;
            free( tmp );
        }

        free( *head );
        *head = NULL;
    }
}

If in main you have a declaration like

Node *head = NULL;
//...

then the function can be called like

delete_list( &head );

Pay attention to that the function does not deal with the pointer to the tail node if it is declared separately apart from the pointer to the head node.

In this case you need to introduce one more structure as for example

struct CircularList
{
    Node *head;
    Node *tail;
};

that indeed defines a list. And call the (modified) function for an object of this type.

Answer 2

free'ing a node in a doubly linked list does NOT remove that node from the list.

The code must correct the 'next' field of the prior node in the list and correct the 'prev' field of next node in the list so those fields skip the node to be deleted, Then the node to be deleted can be passed to free()

Answer 3

void delete_list(Node **head){
        Node *current, *garbage;

        if (*head==NULL) return; else current=(*head)->next;

        while(current != *head){
                garbage=current;
                current = current->next;
                free(garbage);
        }

        free(current);
        *head=NULL;
}