버퍼를 공유메모리로 설정하고 접근할때 질문있습니다.

#ifndef QUEUE_H
#define QUEUE_H
 
typedef struct tagNode
{
	char data[1400];
} Node;
 
typedef struct tagCircularQueue
{
	int capacity;  
  int front;
	int rear;
	Node* nodes;
} CircularQueue;
 
void CreateQueue(CircularQueue** queue, int capacity);
void DestroyQueue(CircularQueue* queue);
void Enqueue(CircularQueue* queue, char data[1400]);
char *Dequeue(CircularQueue* queue);
int GetSize(CircularQueue* queue);
int IsEmpty(CircularQueue* queue);
int IsFull(CircularQueue* queue);
#endif

#include "queue.h"
#include <stdio.h>
#include <stdlib.h>
#include <sys/shm.h>
#include <sys/ipc.h>
#include <unistd.h>
#include <fcntl.h>
#include <string.h>
 
#define SHARED_MEMORY_KEY 9876
 
int shmid;
void *shm_addr = (void *)0;
Node *data;
 
void CreateQueue(CircularQueue** queue, int capacity)
{
	*queue = (CircularQueue*)malloc(sizeof(CircularQueue));
 
	(*queue)->nodes = (Node*)malloc(sizeof(Node)* (capacity + 1));
 
	(*queue)->capacity = capacity;
	(*queue)->front = 0;
	(*queue)->rear = 0;
 
  shmid = shmget((key_t)SHARED_MEMORY_KEY, sizeof(CircularQueue), 0666|IPC_CREAT);
 
  if(shmid == -1)
  {
    perror("shmget failed : ");
    exit(0);
  }
 
  printf("shmid = %d\n", shmid);
 
  shm_addr = shmat(shmid, (void *)0, 0);
  if(shm_addr == (void *)-1)
  {
    perror("shmat failed : ");
    exit(0);
  }
  data = (Node *)shm_addr;
}
 
void DestroyQueue(CircularQueue* queue)
{
  if(shmdt(shmid) == -1)
  {
    perror("shmdt failed : ");
    exit(0);
  }
 
  free(queue->nodes);
	free(queue);
}
 
void Enqueue(CircularQueue* queue, char data[1400])
{
	int position = 0;
 
	if (queue->rear == queue->capacity + 1)
	{	
		queue->rear = 0;
		position = 0;
	}
	else
	  position = queue->rear++;
 
 	strcpy(queue->nodes[position].data, data);
}
 
char *Dequeue(CircularQueue* queue)
{
	int position = queue->front;
 
  shmat(shmid, (void *)0, 0);
  if (position == queue->capacity)
		queue->front = 0;
	else
		queue->front++; 
 
 	return queue->nodes[position].data;
}
 
 
int GetSize(CircularQueue* queue)
{
 
	if (queue->front <= queue->rear)
		return queue->rear - queue->front;
	else
		return (queue->capacity + 1 - queue->front) + queue->rear;
}
 
int IsEmpty(CircularQueue* queue)
{
	return (queue->front == queue->rear);
}
 
 
int IsFull(CircularQueue* queue)
{
	if (queue->front < queue->rear)
		return (queue->rear - queue->front) == queue->capacity;
	else
		return (queue->rear + 1) == queue->front;
}

#include "queue.h"
#include <stdio.h>
#include <stdlib.h>
 
int main(void)
{
   CircularQueue* queue;
   CreateQueue(&queue, 100);
 
   Enqueue(queue, "abcd");
   Enqueue(queue, "dcba");
 
   return 0;
}

#include "queue.h"
#include <stdio.h>
 
int main(void)
{
   CircularQueue* queue;
   CreateQueue(&queue, 100);
 
   printf("data : ");
   printf("%s\n", Dequeue(queue));
   printf("%s\n", Dequeue(queue));