This server follows the age-old Berkeley select() server style. This allows it to handle many connections within a single thread, at the cost of roughly twice as much code compared to the multithreaded server. For low numbers of connections, this server is also less efficient, because of all the setup select() requires. (Note that this code could be optimized a bit by keeping separate pristine copies of the fd_sets to reduce setup time.)

select() has one big advantage, though: it's portable to virtually all flavors of Unix. It also scales a bit better than threads, simply because threads cause context switches, which eventually becomes much more significant than the overhead in setting up fd_sets.

The select()-based server has several similarities to the threaded server code. The only real difference is that all the I/O in this version is handled in the AcceptConnections() function, rather than in a bunch of concurrent threads. In fact, AcceptConnection() is pretty much the main function in this version, whereas in the threaded server, it just accepts connections and passes them off to handler threads.

Notice that we have to keep a list of clients and an I/O buffer for each client in this version. That's the price of non-synchronous I/O: you have to keep a lot of state around so you can juggle between many connections within a single thread.

Also notice that we do not call WSAGetLastError() in this program, but instead call getsockopt() asking for the SO_ERROR value. With select(), several sockets can go into an error state at once, so the "last" error is not necessarily the error value we're interested in. We have to find the last error that occured on a particular socket instead.

Building the Program

The only module you will need to compile this program, aside from the common files listed on the main examples page, is select-server.cpp. The comment at the top of the file gives complete compilation instructions; alternately, you can use the common Makefile.