timers/demo_timerfd.c

This is timers/demo_timerfd.c (Listing 23-8, page 510), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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/* 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);
}

 

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