링크드 리스트에 관한 질문입니다.

코드는 다음과 같습니다.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

struct theater {
	int row;
	int sit;

	struct theater *prev;
	struct theater *next;
};

struct theater *first_theater;

void theater_select();
void theater_insert();
int theater_total();
void main(void)
{
	
	int sub_menu=-1;
	first_theater=NULL;

	while ( sub_menu !=0 ){
		
		printf("1. 상영관 정보 출력\n");
		printf("2. 상영관 추가\n");
		printf("3. 상영관 정보 수정\n");
		printf("4. 상영관 삭제\n");
		printf("0. 상위메뉴\n");


		printf("\n>");
		
		scanf("%d",&sub_menu);

		
		switch(sub_menu){
			case 1:
				theater_select();
				break;
			case 2:
				theater_insert();
				break;
		}
		
	}

}

void theater_select()
{
	int i=0;
	
	struct theater *current = first_theater;

	i = theater_total();
	while(current)
	{
		printf("%d 관 열수 : %d 좌석수 :%d\n",i,current->row,current->sit);
		current  = current->next;
	}
	if(i==0)
		printf("\n없다..\n");
	
	

}
void theater_insert(void)
{
	int i=0;
	int s,r;

	struct theater *res;

	struct theater *tmp = first_theater;

	
	printf("%d관 정보 입력\n",theater_total()+1);
	printf("열 수: ");
	scanf("%d",&r);
	printf("\n좌석 수: ");
	scanf("%d",&s);
	
	printf("\n입력 내용이 정확합니까(yes : 1 , no : 2)?");
	scanf("%d",&i);
	if(i==2)
		return;
	
	res = malloc(sizeof(*res));
	res->row=r;
	res->sit=s;

	if(first_theater == NULL) {
		first_theater = res;
		res->next = NULL;
		res->prev = NULL;

	} else {
		tmp = first_theater;
		
		while(tmp){
		
			tmp = tmp->next;
		}
		if(tmp == NULL) {
			printf("sdfsfds\n");
			return;
		}
		tmp->next = res;
		res->next = NULL;
		res->prev = tmp;
	}

	
}

int theater_total()
{
	int i=0; 
	struct theater *current = first_theater;

	while(current){
		i++;
		current=current->next;
	}
	return i;
}