Please take a look at the library's class constructor to see how to use the session struct. Based on your posted code your missing some parameters.
// set community name set/get sizes
session->setSize = strlen(session->setCommName);
session->getSize = strlen(session->getCommName);
//
// validate get/set community name sizes
if ( session->setSize > SNMP_MAX_NAME_LEN || session->getSize > SNMP_MAX_NAME_LEN ) {
return SNMP_API_STAT_NAME_TOO_BIG;
}
//
// set session property
_session = session;
//
// validate session port number
if ( session->port == NULL || session->port == 0 ) session->port = SNMP_DEFAULT_PORT;
//
// init UDP socket
_socket.initUDP(session->port);
//
return SNMP_API_STAT_SUCCESS;