Deleting the head of doubly linked list - doubly-linked-list

Question

Sorry for such a low quality question but I have no idea why the following code doesn't work.
list points the head of the node.
void DeleteHeadNode(Node** list, Node * node)
{
if ((*list) == node) {
Node * next = node->nextnode;
next->prevnode = NULL;
free(*list);
}
}

Answer 1

The usage of Node** is used as alias. If passed the address of a list variable, one may alter it, by altering the Node* value.
In this way one set the passed list variable to null or to the second node.
The following is a usage pattern.
void DeleteNode(Node** list, int data)
{
while (*list != NULL && (*list)->data != data) {
list = &(node->nextnode);
}
if (*list != NULL) {
Node* found = *list;
*list = found->nextnode;
if (found->nextnode != NULL) {
found->nextnode->prevnode = *list;
}
free(found);
}
}
Usage:
Node* list;
...
DeleteNode(&list, ...);
A previous node (prevnode, a double linked list) is not really necessary.
The while loop is of interest: there the alias list is set from the list head to the address of a nextnode field. Hence a subsequent change of *list either changes the passed list variable or some nextnode field.
void SortedInsertNode(Node** list, int data)
{
Node* node = (Node*) malloc(sizeof(Node));
node->data = data;
node->prevnode = NULL;
node->nextnode = NULL;
while (*list != NULL && data >= (*list)->data) {
node->prevnode = *list;
list = &(node->nextnode);
}
node->nextnode = *list;
*list = node;
if (node->nextnode != NULL) {
node->nextnode->prevnode = *list;
}
}
Deleting the head:
void DeleteHead(Node** list)
{
if (*list != null) {
Node* dead = *list;
*list = (*list)->nextnode;
(*list)->prevnede = NULL;
free(dead);
}
}

Answer 2

I'm guessing "doesn't work" means list is no longer valid after your function - which will be true.
You should have
void DeleteHeadNode(Node** list, Node* node)
{
if ((node != NULL) && (*list == node)) // check node being deleted
{
Node* next = node->nextnode;
if (next != NULL) // ensure next exists
{
next->prevnode = NULL;
free(*list);
list = &next; // update pointer to head of list
}
else // no next, list is now becoming empty
{
free(*list);
list = NULL; // update pointer to head of list
}
}
}