Netx Duo Question - NX_TCP_SOCKET and BSD fd_sets Functionality

Hi,

I hope this is the correct place to ask this question.

I'm looking at porting an existing application across to Duo, this app uses WEBSOCKETS/TLS/TCP within a BSD socket and makes use of the BSD fd_sets mechanism to detect receive data, ability to send data and loss of connection, sending multiple messages over a single socket.

Using the Duo API's I need to use the NX_TCP_SOCKET based APIs, to gain WEBSOCKETS/TLS functionality but so far, I haven't spotted a bsd_fd_set equivalent mechanism for this socket type. Is there a similar feature available? 

The examples I've found don't persist a socket across multiple messages so don't need that feature. Any help/information on this appreciated.

Many Thanks

Wayne