C언어 단순연결리스트 관련 질문입니다.

#include <stdio.h> 
#include <stdlib.h>
 
typedef int element;
typedef struct ListNode
{
	element data;
	struct ListNode *link;
} ListNode;
 
typedef struct {
	ListNode *head;
	int length;
} ListType;
 
ListNode *create_node(int data, ListNode *link)
{
	ListNode *new_node = (ListNode*)malloc(sizeof(ListNode));
	if (new_node == NULL)
	{
		puts("메모리 할당 에러");
		exit(1);
	}
 
	new_node->data = data;
	new_node->link = NULL;
 
	return new_node;
}
 
void insert_node(ListNode **phead, ListNode *p, ListNode *new_node)
{
	if (*phead == NULL)
	{
		new_node->link = NULL;
		*phead = new_node;
	}
	else if (p == NULL)
	{
		new_node->link = *phead;
		*phead = new_node;
	}
	else
	{
		new_node->link = p->link;
		p->link = new_node;
	}
}
 
 
ListNode *get_node_at(ListType *list, int pos)
{
	int i;
 
	ListNode *tmp_node = list->head;
 
	if (pos < 0)
		return NULL;
 
	for (i = 0; i < pos; i++)
		tmp_node = tmp_node->link;
 
	return tmp_node;
}
 
void add(ListType *list, int pos, element data)
{
	ListNode *p;
 
	if ((pos >= 0) && (pos <= list->length)) {
 
		ListNode *node = (ListNode *)malloc(sizeof(ListNode));
 
		if (node == NULL)
			printf("메모리 할당에러");
 
		node->data = data;
		p = get_node_at(list, pos - 1);
		insert_node(&(list->head), p, node);
		list->length++;
	}
}
 
int remove_node(ListNode **phead, ListNode *p, ListNode *removed)
{
	if (p == NULL)
		*phead = (*phead)->link;
	else
		p->link = removed->link;
	free(removed);
	return 0;
}
 
void display(ListNode *head)
{
	ListNode *p = head;
 
	while (p != NULL) {
		printf("%d->", p->data);
		p = p->link;
	}
	printf("\n");
}
 
int main(void)
{
	ListType *list2 = NULL;
 
	int i;
 
	for (i = 10; i > 0; i--)
		insert_node(&list2->head, NULL, create_node(i, NULL));
	display(list2->head);
 
	for (i = 0; i < 10; i++)
		remove_node(&list2->head, NULL, list2->head);
	display(list2->head);
 
	add(list2, NULL, 1);
	display(list2->head);
}