Select being blocking is normal, non-blocking IO does not depend on select being non-blocking.

Select returns the list of non-blocking sockets that have data read/write pending.

select() is just a means to multiplex network I/O. It can be either blocking or non-blocking, depending on your application architecture. I have worked on both kinds.

BTW, the sockets themselves don't even have to be non-blocking.

Also, I'd like to stress that the concepts or blocking/non-blocking I/O and synchronous/asynchronous I/O are really orthogonal. You can do synchronous networking with non-blocking sockets and vice versa.

Or to put it another way: depending on the timeout value, select() can achieve blocking or non-blocking I/O on several sockets simultaneously. In both cases, the sockets themselves can be either blocking or non-blocking.

There is no practical difference between blocking on select() for multiple sockets or blocking on recv() for a single socket - in both cases the thread can't do anything else.