clinuxmultithreadingpthreads

How to join a thread that is hanging on blocking IO?

I have a thread running in the background that is reading events from an input device in a blocking fashion, now when I exit the application I want to clean up the thread properly, but I can't just run a pthread_join() because the thread would never exit due to the blocking IO.

How do I properly solve that situation? Should I send a pthread_kill(theard, SIGIO) or a pthread_kill(theard, SIGALRM) to break the block? Is either of that even the right signal? Or is there another way to solve this situation and let that child thread exit the blocking read?

Currently a bit puzzled since none of my googling turned up a solution.

This is on Linux and using pthreads.

Edit: I played around a bit with SIGIO and SIGALRM, when I don't install a signal handler they break the blocking IO up, but give a message on the console ("I/O possible") but when I install a signal handler, to avoid that message, they no longer break the blocking IO, so the thread doesn't terminate. So I am kind of back to step one.

Solution

  • Old question which could very well get a new answer as things have evolved and a new technology is now available to better handle signals in threads.

    Since Linux kernel 2.6.22, the system offers a new function called signalfd() which can be used to open a file descriptor for a given set of Unix signals (outside of those that outright kill a process.)

    // defined a set of signals
sigset_t set;
sigemptyset(&set);
sigaddset(&set, SIGUSR1);
// ... you can add more than one ...

// prevent the default signal behavior (very important)
sigprocmask(SIG_BLOCK, &set, nullptr);

// open a file descriptor using that set of Unix signals
f_socket = signalfd(-1, &set, SFD_NONBLOCK | SFD_CLOEXEC);

    Now you can use the poll() or select() functions to listen to the signal along the more usual file descriptor (socket, file on disk, etc.) you were listening on.

    The NONBLOCK is important if you want a loop that can check signals and other file descriptors over and over again (i.e. it is also important on your other file descriptors).

    I have such an implementation that works with (1) timers, (2) sockets, (3) pipes, (4) Unix signals, (5) regular files. Actually, really any file descriptor plus timers.

    https://github.com/m2osw/eventdispatcher/blob/main/eventdispatcher/signal.cpp (the eventdispatcher library includes all the above, the link is specifically about the signalfd() connection implementation)

    You may also be interested by libraries such as libevent