On the server side, as soon as you receive data from a client, you should call send() to respond. You do not need to check on readyness to write (i.e., you do not need to wait until FD_ISSET for writing). The only time you need to check on readyness to write is if you call send() and send() returns an indication that the socket is not ready to write.

In addition, your code assumes that all data (including a terminating NULL) will be received in a single call to recv(), and further assumes that all data will be sent in a single call to send(). Both are bad assumptions. TCP is a stream-based protocol, not a message-based protocol, so when using TCP, you must implement some sort of serialization when sending and receiving bytes.

Mike