Specify port no to connect to Informix Database through ODBC

Hi,
I'm developing a socket program to connect to Informix database
through the ODBC. In here i called my socket program as "tap" . My tap
will listen for data from unix through port 1070. After checking of
the data, tap will then insert the data to the informix database. I
know we can point the tap to one specified port in Informix database
through the services file. My question is how should i know to which
port my tap program send out to the informix database or should i ask
how should i specify one port to connect to the informix and lock that
port from other process to use it?For current moment, my tap will
randomly pick one port to connect to informix database.

My coding for connect to Informix looks like below;
// Initialize ODBC variables
// NOTE : Make sure these variables is intialized before used.
henv = SQL_NULL_HENV;
hdbc = SQL_NULL_HDBC;
hstmt = hOrderStmt = hTradeStmt = SQL_NULL_HSTMT;

SQLAllocEnv(&henv);
SQLAllocConnect(henv, &hdbc);

char szRetMsg[500];
memset(szRetMsg, '\0', sizeof(szRetMsg));

if (SQLSetConnectOption(hdbc,SQL_TXN_ISOLATION,SQL_TXN_READ_UNCOMMITTED)
!= SQL_SUCCESS) {
SQLErrorMessageBox(henv,hdbc,hstmt,"Connect",(LPSTR)&szRetMsg);
::MessageBox(NULL,szRetMsg,"Tap32",MB_ICONSTOP|MB_TASKMODAL);
PostQuitMessage(0);
return FALSE;
}

if (SQLConnect(hdbc, szDSN, SQL_NTS, szUserID, SQL_NTS, szPassword,
SQL_NTS) != SQL_SUCCESS)
{
SQLErrorMessageBox(henv,hdbc,hstmt,"Connect",(LPSTR)&szRetMsg);
::MessageBox(NULL,szRetMsg,"Tap32",MB_ICONSTOP|MB_TASKMODAL);
PostQuitMessage(0);
return FALSE;
}

I really appreciate if someone can help me out. Thank you very much.