i need help finishing my C code

#include "bigint_dllist.h"

NODE *new_node(char data) {

NODE *np = (NODE*) malloc(sizeof(NODE));

if (np == NULL) {

printf("malloc fails");

return NULL;

}

np->data = data;

np->prev = NULL;

np->next = NULL;

return np;

}

void display_forward(NODE *np) {

if (np == NULL)

return;

NODE *ptr = np;

while (ptr != NULL) {

printf("%d", ptr->data);

ptr = ptr->next;

}

}

void display_backward(NODE *np) {

if (np == NULL)

return 0;

NODE *ptr = np;

while (ptr != NULL) {

printf("%d ", ptr->data);

ptr = ptr->prev;

}

}

void insert_start(NODE **startp, NODE **endp, NODE *new_np) {

// this inserts a new node at the start the of doubly linked list.

}

void insert_end(NODE **startp, NODE **endp, NODE *new_np) {

// this inserts a new node at the end of the doubly linked list.

}

void delete_start(NODE **startp, NODE **endp) {

// this deletes the first node of the doubly linked list.

}

void delete_end(NODE **startp, NODE **endp) {

// this deletes the last node of the doubly linked list.

}

void clean(NODE **startp, NODE **endp) {

// this cleans the doubly linked list.

}