How to cleanly interrupt a thread blocking on a recv call?

Question

I have a multithreaded server written in C, with each client thread looking something like this:

ssize_t n;
struct request request;

// Main loop: receive re         


        

Answer 1

To interrupt the thread, make the socket non-blocking (set O_NONBLOCK using fcntl) and then signal the thread with pthread_kill. This way, recv will fail with either EINTR if it was sleeping, or EAGAIN or EWOULDBLOCK if it wasn’t (also maybe if SA_RESTART is in effect, didn’t check). Note that the socket doesn’t need to, and actually should not, be non-blocking before that. (And of course the signal needs to be handled; empty handler is sufficient).

To be sure to catch the stop-signal but not anything else, use a flag; there are things that may go wrong. For example, recv may fail with EINTR on some spurious signal. Or it may succeed if there was some data available, effectively ignoring the stop request.

And what not to do:

1. Don’t use pthread_kill alone or with any plain check. It may arrive right before issuing the recv syscall, too early to interrupt it but after all the checks.

2. Don’t close the socket. That may not even work, and as @R.. pointer out, is dangerous as the socket file descriptor may be reused between close and recv (unless you’re sure nothing opens file descriptors).