C/C++ Add socket to fd_set when select is pending

Question

I'm working on a windows/unix multithreaded server application network layer with Berkeley sockets and stumbled upon a problem:

• I have one thread waiting at select with given fd sets.
• From another thread, I need to add a socket to one of the fd sets.
• As the select is currently pending, it cannot be done and leads to a starvation.

Possible solution is to add a timeout to the select. I have seen that on sites addressing networking with select (dated 15 years back).

The question is:
Are there any other solutions? Waiting for timeout still leads to some level of starvation and takes CPU time from the select-waiter thread. I thought it would be possible to redesign the application but adding sockets is also done from threads that select-waiter thread has (and most definitely should have) no idea about, so the condition cannot be avoided.

If not, what sort of timeout should be chosen to achieve best performance / service quality?

Also note that I do realize that it would be better idea to use more advanced API (iocp, kqueue, ...) or a lib that would do it for me, but that is not an option for me at the given point.

Thanks

Answer 1

Create an additional socket pair and add one of these sockets to every select . To interrupt a running select , send a message to it via the other socket.

On the Unix side only, one can send any signal (eg SIGUSR1 ) to the waiting thread with pthread_kill . select with then return a negative value, and errno will be set to EINTR . But there is nothing like that on the Windows side.