modbus TCP-IP로 데이터 읽기/쓰기 위한 데이터 패킷 형식 문의

int main(int argc, char **argv)
{
	struct sockaddr_in_serveraddr;
	int server_sockfd;
	int client_len;
	char but[MAXLINE];
 
	if ((server_sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1)
	{
		perror("error : ");
		return 1;
	}
 
	server_sockfd = socket(AF_INET, SOCK_STREAM, 0);
	serveraddr.sin_family = AF_INET;
	serveraddr.sin_addr.s_addr = inet_addr("192.168.0.111");
	serveraddr.sin_port = htons(3500);
 
	client_len = sizeof(serveraddr);
 
	if (connect(server_sockfd, (struct sockaddr *) &serveraddr, client_len) == -1)
	{
		perror("connect error : ");
		return 1;
	}
 
	memset(buf, 0x00, MAXLINE);
	read(0, buf, MAXLINE);
	if (write(server_sockfd, buf, MAXLINE) <= 0)
	{
		perror("write error : ");
		return 1;
	}
 
	memset(buf, 0x00, MAXLINE);
 
	if(read(server_sockfd, buf, MAXLINE) <=0)
	{
		perror("read error :");
		return 1;
	}
	close(server_sockfd);
 
	printf("read : %s", buf);
	return 0;
}