Communicating with many readers (smart card)

SCardListReaders returns a string of unknown length. It depends on what readers you have attached (and potentially how many). You could guess how long the string is likely to be and pass a pointer to the start of the storage area. However if that string length was exceeded then you start writing over other variables or even your stack, which would produce interesting results!!

The first call (passing NULL for the storage area) says "Don't write any data, just tell me how many characters long the string would have been.". You can then allocate a character buffer that long (plus one for the null terminator) and pass the pointer to that buffer in the second call. It's a common mechanism for returning a data set of unknown length.

At the lowest level you communicate with SmartCards using packets called APDUs (See http://en.wikipedia.org/wiki/Smart_card_application_protocol_data_unit). Most cards will initially only recognise a small subset of commands until you establish secure communications with them, usually using a standard APDU. The command SCardTransmit is used to send the APDU. Of course you have to implement the higher protocol at the PC end yourself if you are going at such a low level. There are libraries / drivers etc that will implement the higher level protocols for you. At the other end the higher level protocol is either implemented in the reader or the card itself. If you just want to talk to the card in terms of APDUs then most (if not all smartcard readers) will implement the ISO/IEC 7816-4 standard. If you want to talk to a smartcard at a higher level then you will need a reader that understands the protocol or a smartcard that understands the protocol. If you have a reader that talks to the card using something other than ISO/IEC 7816-4 then you will need obviously need a reader that will talk to cards using that standard. I am not really familiar with Mifare Plus or the higher level functions of the smartcard services, so I can not advise on those. Hope that helps. In summary you need to find out what protocols you need to talk to the card and reader and then find readers / cards / software / driver that will allow you to talk to them using those protocols.