중첩 구조체의 연결리스트

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
 
typedef struct _user
{
    char user_name[20];
    struct _user *user_next;
}user;
user *waituser_head, *waituser_tail;
 
typedef struct _channel
{
    int user_count;
    char channel_name[20];
    struct _channel *channel_next;
    user *user_head, *user_tail;
}channel;
channel *channel_head, *channel_tail;
 
void init_user(void) //waituser를 위한 초기화 
{
    waituser_head=(user*)malloc(sizeof(user));
    waituser_tail=(user*)malloc(sizeof(user));
    waituser_head->user_next=waituser_tail;
    waituser_tail->user_next=waituser_tail;
}
void init_channel_user(void) //channel안의 user에 대한 초기화 
{
    channel_head=(channel*)malloc(sizeof(channel));
 channel->user_head=(user*)malloc(sizeof(user));
    channel_tail=(channel*)malloc(sizeof(channel));
 channel->user_tail=(user*)malloc(sizeof(user));
 channel_head->channel_next=channel_tail;
 channel_tail->channel_next=channel_tail;
 channel_head->user_head->user_next=channel_head->user_tail;
 channel_tail->user_tail->user_next=channel_tail->user_tail;
}
 
void insert()
{
 char buf[20];
 user *t;
 t=(user*)malloc(sizeof(user));
 
 printf("Input name: ");
 fgets(buf,20,stdin);
 buf[(strlen(buf)-1)]='\0';
 printf("-----> 빈방에 입장하셨음\n");
 
 t->user_next=waituser_head->user_next;
 waituser_head->user_next=t;
}
 
void check_channel(char *s)
{
 channel *p;
 p=channel_head->channel_next;
 while(strcmp(s,p->channel_name) !=0 && p!=channel_tail)
 {
  p=p->channel_next;
 }
 if(p!=channel_tail)
 {
  printf("이미존재하는방입니다\n");
  join_channel();
 }
}
 
void join_channel()
{
 int a;
 char buf[20];
 char user_buf[20];
 printf("들어갈 채널: ");
 fgets(buf,20,stdin);
 buf[(strlen(buf)-1)]='\0';
 check_channel(buf);
 printf("누가들어갈것인가? ");
 fgets(user_buf,20,stdin);
 user_buf[(strlen(user_buf)-1)]='\0';
 
 channel *t;
 t=(channel*)malloc(sizeof(channel));
 strcpy(t->channel_name,buf);
 t->channel_next=channel_head->channel_next;
 channel_head->channel_next=t;
 
 user *i, *j;
 channel->user *p;
 
 i=waituser_head;
 j=i->user_next;
 while(strcmp(user_buf,j->waituser_name) !=0 && j!=waituser_tail)
 {
  i=i->user_next;
  j=j->user_next;
 }
 if(j==waituser_tail)
  return 0;
 i->user_next=j->user_next;
 free(j);
 
 p=(user*)malloc(sizeof(user));
 strcpy(p->user_name,user_buf);
 p->user_next=channel->user_head->user_next;
 channel->user_head->user_next=p;
}