c언어, 네트워크 프로그래밍에서 여러 가지의 메세지를 받을 때 생기는 문제에 대해서 질문있습니다.

typedef struct _dgt_auth_req_msg
{
	UB2 msg_type;
	SCHAR db_user[33];
	SCHAR password[33];
	SCHAR program_name[33];
	SCHAR db_name[33];
}dgt_auth_req_msg; // msg type 1
 
typedef struct _dgt_auth_res_msg
{
	UB2 msg_type;
	UB2 rtn_len;
	SCHAR rtn_msg[257];
}dgt_auth_res_msg; // msg type 2
 
typedef struct _dgt_sql_req_msg
{
	UB2 msg_type;
	UB4 sql_len;
	SCHAR sql_text[1025];
	UB2 sql_type;
}dgt_sql_req_msg; // msg type 3
 
typedef struct _dgt_sql_res_msg
{
	UB2 msg_type;
	UB4 rtn_len;
	SCHAR rtn_data[1025];
	UB2 sql_type;
}dgt_sql_res_msg; // msg type 4

#define UB2 unsigned short
#define UB4 unsigned int
#define SCHAR signed char
 
#define auth_req_msg 1
#define auth_res_msg 2
#define sql_req_msg 3
#define sql_res_msg 4
#define close_req_msg 5

void CNetwork::gateway_Select()
{
	int nIdx;
	while(true)
	{
		cpy_reads = reads;
		timeout.tv_sec = 5;
		timeout.tv_usec = 5000;
 
		if((fd_num = select(fd_max + 1, &cpy_reads, 0, 0, &timeout)) == -1)
		{
			cout << "gateway_Select() select error" << endl;
			break;
		}
 
		if(fd_num == 0)
			continue;
 
		for(nIdx = 0; nIdx < fd_max + 1; nIdx++)
		{
			if(FD_ISSET(nIdx, &cpy_reads))
			{
				if(nIdx == nSock) // request connection.
				{
					adr_sz = sizeof(clnt_addr);
					nSock = socket(PF_INET, SOCK_STREAM, 0);
					nClntSock = accept(nSock, (struct sockaddr *)&clnt_addr, &adr_sz);
					FD_SET(nClntSock, &reads);
 
					if(fd_max < nClntSock)
					{
						fd_max = nClntSock;
						cout << "connected client.." << endl;
					}
				}
				else
				{
 
					int nCnt = 0;
					str_len = recv(nIdx, buf + nCnt, BUFSIZ, 0);
					if(str_len == -1)
						cout << "gateway_Select() recv error.." << endl;
 
					nCnt += str_len;
 
					switch()
					{
						case auth_req_msg: // 1
							nPacketSize = 134;
							break;
						case auth_res_msg: // 2
							nPacketSize = 261;
							break;
						case sql_req_msg: // 3
							nPacketSize = 1033;
							break;
						case sql_res_msg: // 4
							nPacketSize = 1033;
							break;
						case close_req_msg: // 5
							nPacketSize = 2;
							break;
						case close_res_msg: // 6
							nPacketSize = 261;
							break;
						default :
							break;
					}
 
					if(nCnt < nPacketSize)
					{
						while(nCnt < nPacketSize)
						{
							str_len = recv(nIdx, buf + nCnt, BUFSIZ, 0);
							if(str_len == -1)
								cout <<"gateway_Select() recv 2st error" << endl;
 
							nCnt += str_len;
						}
 
						;
					}
				}
 
				gateway_MsgService(nIdx, buf);
			}
		}
	}
}