timers/demo_timerfd.c

/* demo_timerfd.c

   Demonstrate the use of the timerfd API, which creates timers whose
   expirations can be read via a file descriptor.

   This program is Linux-specific. The timerfd API is supported since kernel
   2.6.25. Library support is provided since glibc 2.8.
*/
#include <sys/timerfd.h>
#include <time.h>
#include <stdint.h>                     /* Definition of uint64_t */
#include "itimerspec_from_str.h"        /* Declares itimerspecFromStr() */
#include "tlpi_hdr.h"

int
main(int argc, char *argv[])
{
    struct itimerspec ts;
    struct timespec start, now;
    int maxExp, fd, secs, nanosecs;
    uint64_t numExp, totalExp;
    ssize_t s;

    if (argc < 2 || strcmp(argv[1], "--help") == 0)
        usageErr("%s secs[/nsecs][:int-secs[/int-nsecs]] [max-exp]\n", argv[0]);

    itimerspecFromStr(argv[1], &ts);
    maxExp = (argc > 2) ? getInt(argv[2], GN_GT_0, "max-exp") : 1;

    fd = timerfd_create(CLOCK_REALTIME, 0);
    if (fd == -1)
        errExit("timerfd_create");

    if (timerfd_settime(fd, 0, &ts, NULL) == -1)
        errExit("timerfd_settime");

    if (clock_gettime(CLOCK_MONOTONIC, &start) == -1)
        errExit("clock_gettime");

    for (totalExp = 0; totalExp < maxExp;) {

        /* Read number of expirations on the timer, and then display
           time elapsed since timer was started, followed by number
           of expirations read and total expirations so far. */

        s = read(fd, &numExp, sizeof(uint64_t));
        if (s != sizeof(uint64_t))
            errExit("read");

        totalExp += numExp;

        if (clock_gettime(CLOCK_MONOTONIC, &now) == -1)
            errExit("clock_gettime");

        secs = now.tv_sec - start.tv_sec;
        nanosecs = now.tv_nsec - start.tv_nsec;
        if (nanosecs < 0) {
            secs--;
            nanosecs += 1000000000;
        }

        printf("%d.%03d: expirations read: %llu; total=%llu\n",
                secs, (nanosecs + 500000) / 1000000,
                (unsigned long long) numExp, (unsigned long long) totalExp);
    }

    exit(EXIT_SUCCESS);
}