Are there any well-behaved POSIX interval timers?

Question

Inspired by the last leap second, I\'ve been exploring timing (specifically, interval timers) using POSIX calls.

POSIX provides several ways to set up timers, but th

Answer 1

kqueue and kevent can be utilized for this purpose. OSX 10.6 and FreeBSD 8.1 add support for EVFILT_USER, which we can use to wake up the event loop from another thread.

Note that if you use this to implement your own condition and timedwait, you do not need locks in order to avoid race conditions, contrary to this excellent answer, because you cannot "miss" an event on the queue.

Sources:

• FreeBSD man page
• OS X man page
• kqueue tutorial
• libevent source code

Example Code

Compile with clang -o test -std=c99 test.c

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

// arbitrary number used for the identifier property
const int NOTIFY_IDENT = 1337;

static int kq;

static void diep(const char *s) {
   perror(s);
   exit(EXIT_FAILURE);
}

static void *run_thread(void *arg) {
    struct kevent kev;
    struct kevent out_kev;
    memset(&kev, 0, sizeof(kev));
    kev.ident = NOTIFY_IDENT;
    kev.filter = EVFILT_USER;
    kev.flags = EV_ADD | EV_CLEAR;

    struct timespec timeout;
    timeout.tv_sec = 3;
    timeout.tv_nsec = 0;

    fprintf(stderr, "thread sleep\n");

    if (kevent(kq, &kev, 1, &out_kev, 1, &timeout) == -1)
        diep("kevent: waiting");

    fprintf(stderr, "thread wakeup\n");

    return NULL;
}

int main(int argc, char **argv) {
    // create a new kernel event queue
    kq = kqueue();
    if (kq == -1)
        diep("kqueue()");


    fprintf(stderr, "spawn thread\n");
    pthread_t thread;
    if (pthread_create(&thread, NULL, run_thread, NULL))
        diep("pthread_create");

    if (argc > 1) {
        fprintf(stderr, "sleep for 1 second\n");
        sleep(1);
        fprintf(stderr, "wake up thread\n");

        struct kevent kev;
        struct timespec timeout = { 0, 0 };

        memset(&kev, 0, sizeof(kev));
        kev.ident = NOTIFY_IDENT;
        kev.filter = EVFILT_USER;
        kev.fflags = NOTE_TRIGGER;

        if (kevent(kq, &kev, 1, NULL, 0, &timeout) == -1)
            diep("kevent: triggering");
    } else {
        fprintf(stderr, "not waking up thread, pass --wakeup to wake up thread\n");
    }

    pthread_join(thread, NULL);
    close(kq);
    return EXIT_SUCCESS;
}

Output

$ time ./test
spawn thread
not waking up thread, pass --wakeup to wake up thread
thread sleep
thread wakeup

real    0m3.010s
user    0m0.001s
sys 0m0.002s

$ time ./test --wakeup
spawn thread
sleep for 1 second
thread sleep
wake up thread
thread wakeup

real    0m1.010s
user    0m0.002s
sys 0m0.002s