breaking out from socket select

Question

I have a loop which basically calls this every few seconds (after the timeout):

 while(true){

    if(finished)
       return;

    switch(select(FD_SETSIZE, &readfds, 0, 0, &tv)){
        case SOCKET_ERROR : report bad stuff etc; return;
        default : break;
    }

    // do stuff with the incoming connection
 }

So basically for every few seconds (which is specified by tv), it reactivates the listening.

This is run on thread B (not a main thread). There are times when I want to end this acceptor loop immediately from thread A (main thread), but seems like I have to wait until the time interval finishes..

Is there a way to disrupt the select function from another thread so thread B can quit instantly?

Answer 1

Yes, you create a connected pair of sockets. Then thread B writes to one side of socket and thread A adds the other side socket to select. So once B writes to socket A exits select, do not forget to read this byte from socket.

This is the most standard and common way to interrupt selects.

Notes:

Under Unix, use socketpair to create a pair of sockets, under windows it is little bit tricky but googling for Windows socketpair would give you samples of code.

Answer 2

Can't you just make the timeout sufficiently short (like 10ms or so?).

These "just create a dummy connection"-type solution seem sort of hacked. I personally think that if an application is well designed, concurrent tasks never have to be interrupted forcefully, the just has worker check often enough (this is also a reason why boost.threads do not have a terminate function).

Edit Made this answer CV. It is bad, but it might help other to understand why it is bad, which is explained in the comments.

Answer 3

The easiest way is probably to use pipe(2) to create a pipe and add the read end to readfds. When the other thread wants to interrupt the select() just write a byte to it, then consume it afterward.