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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

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.

Answer 2

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 3

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 4

You can use shutdown(Sock, SHUT_RDWR) call from main thread to come out of waiting select call which will also exit your another thread before the timeout so you don't need to wait till timeout expires.

cheers. :)