Pointer return by a function in C - c

Question

Following is the code for making a linked list which has 2 pointers. Linked list is getting created (printed) and all pointers (prev + next) are fine. But when I call the function "copay" and assign its value(pointer) to "duplicate", I am getting segmentation fault but if I use only "copay" and don't assign it to any other variable then there is no issue.
typedef struct node {
int data;
struct node *next;
struct node *prev;
} node;
void insert(node **head, int data) {
node *new = (struct node *)malloc(sizeof(node));
new->data = data;
new->next = NULL;
node *temp = *head;
if (!(temp)) {
*head = new;
new->prev = NULL;
// printf("\n return : %d",data);
return;
}
while (temp->next)
temp = temp->next;
temp->next = new;
new->prev = temp;
// printf("\n return : %d",data);
}
void print(node **head) {
node *temp = *head;
printf("\n");
while (temp) {
printf(" %d ->", temp->data);
temp = temp->next;
}
printf(" NULL\n");
}
node *copay(node **head) {
node *temp = *head;
return temp;
}
int main() {
node *head;
insert(&head, 1);
insert(&head, 3);
insert(&head, 5);
insert(&head, 7);
insert(&head, 9);
(head)->prev = (head)->next->next;
(head)->next->next->prev = (head)->next->next->next->next;
(head)->next->next->next->next->prev = (head)->next;
print(&head);
node *duplicate = copay(&head);
// print(&duplicate);
}

Answer 1

There is a very simple problem in function main():
node *head;
head is defined but not initialized. You must initialize it to NULL for insert() to function properly, otherwise you have undefined behaviour. Incidentally, it is confusing to name insert a function that actually appends a node to a list. Change this line to:
node *head = NULL;
I do not understand you you are trying to achieve with these lines:
(head)->prev = (head)->next->next;
(head)->next->next->prev = (head)->next->next->next->next;
(head)->next->next->next->next->prev = (head)->next;
The rest looks fine to me.

Answer 2

copay() is actually fine while it is running. The problem happens to its caller(here it is main function) after copay() exits. copay() returns a pointer to a node, but the problem is that the local node, temp, is allocated only while copay() is running. When copay() exits, all of its locals are deallocated. So the caller is left with a pointer to a deallocated node.
I would refer you to go through Section 2 of Pointers and Memory from Stanford CS Education Library which is on Local Memory.
Source: Pointers and Memory