ORA-12504 error when connecting to Oracle 11G DB using OCI and instantclient-basic-nt-11.2.0.3.0.zip

Question

Dear Friends,
I am using OCI and instant_client (11.2.0.3.0)  in my C++ program (on Windows 7) to connect remotely to 11G database.
While doing a connection using server name as well as servername:port/service_name as connection strings, I am getting error of ORA-12504 TNS:listener was not given the SERVICE_NAME in CONNECT_DATA.
According to instant client FAQ & white paper at http://www.oracle.com/technetwork/database/features/instant-client/index.html, no tnsnames.ora is required for instant client. Then why could I be getting this error?
I ensured that only the instant client directory is in Path environment variable. There was a ODBC driver installed on this same host. Could that be modifying the behavior of instant client?
Kindly guide.
Many thanks in advance for your help and time.
Best Regards,
- ganesh

Answer 1

>>what is the exact connect string you are using?
For connection, I am using the plain hostname on which the DB server is running.
>>are you calling OCIServerAttach before OCISessionBegin?
I am not using OCISessionBegin. For connecting to DB server, I am using the following sequence of OCI calls (in this same order):
1) g_OCIEnvNlsCreate
2) g_OCIHandleAlloc //-- error handle: OCI_HTYPE_ERROR
3) g_OCIHandleAlloc // --- sevice context: OCI_HTYPE_SVCCTX
4) Setting driver name and application name for the session:
    OCISession* usersession = 0;
    ret = g_OCIAttrGet(m_pSvcCtx, OCI_HTYPE_SVCCTX, &usersession, NULL,
                             OCI_ATTR_SESSION, m_pErrHdl);
    if (ret == OCI_SUCCESS)
        ret = g_OCIAttrSet(usersession, OCI_HTYPE_SESSION, (void*)"OCI_DRV", 2,OCI_ATTR_DRIVER_NAME, m_pErrHdl);
        ret = g_OCIAttrSet(usersession, OCI_HTYPE_SESSION, (void*)"ClientApplication", <ub4>(strlen("DataReconScanning")), OCI_ATTR_CLIENT_IDENTIFIER, m_pErrHdl);
5) g_OCILogon() using the user, password and host name, as below:
g_OCILogon(m_pDbCon, m_pErrHdl, &m_pSvcCtx,
                               (text*)user.c_str(), static_cast<ub4>(user.length()),
                               (text*)pass.c_str(), static_cast<ub4>(pass.length()),
                               (text*)host.c_str(), static_cast<ub4>(host.length()));
Thanks.