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.

This page shows the "distribution" or "book" version of the file (why are there two versions?), or the differences between the two versions. You can switch between the views using the tabs below.

In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.


Download timers/demo_timerfd.c

  Cover of The Linux Programming Interface

Function list (Bold in this list means a function is not static)

/* 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"
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)

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

    if (clock_gettime(CLOCK_MONOTONIC, &start) == -1)

    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))

        totalExp += numExp;

        if (clock_gettime(CLOCK_MONOTONIC, &now) == -1)

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

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



Download timers/demo_timerfd.c

Note that, in most cases, the programs rendered in these web pages are not free standing: you'll typically also need a few other source files (mostly in the lib/ subdirectory) as well. Generally, it's easier to just download the entire source tarball and build the programs with make(1). By hovering your mouse over the various hyperlinked include files and function calls above, you can see which other source files this file depends on.

Valid XHTML 1.1