TIMERFD_CREATE(2)         (2020-08-13)          TIMERFD_CREATE(2)

          timerfd_create, timerfd_settime, timerfd_gettime - timers
          that notify via file descriptors

          #include <sys/timerfd.h>

          int timerfd_create(int clockid, int flags);

          int timerfd_settime(int fd, int flags,
                              const struct itimerspec *new_value,
                              struct itimerspec *old_value);

          int timerfd_gettime(int fd, struct itimerspec *curr_value);

          These system calls create and operate on a timer that deliv-
          ers timer expiration notifications via a file descriptor.
          They provide an alternative to the use of setitimer(2) or
          timer_create(2), with the advantage that the file descriptor
          may be monitored by select(2), poll(2), and epoll(7).

          The use of these three system calls is analogous to the use
          of timer_create(2), timer_settime(2), and timer_gettime(2).
          (There is no analog of timer_getoverrun(2), since that func-
          tionality is provided by read(2), as described below.)

          timerfd_create() creates a new timer object, and returns a
          file descriptor that refers to that timer.  The clockid
          argument specifies the clock that is used to mark the pro-
          gress of the timer, and must be one of the following:

               A settable system-wide real-time clock.

               A nonsettable monotonically increasing clock that mea-
               sures time from some unspecified point in the past that
               does not change after system startup.

          CLOCK_BOOTTIME (Since Linux 3.15)
               Like CLOCK_MONOTONIC, this is a monotonically increas-
               ing clock.  However, whereas the CLOCK_MONOTONIC clock
               does not measure the time while a system is suspended,
               the CLOCK_BOOTTIME clock does include the time during
               which the system is suspended.  This is useful for
               applications that need to be suspend-aware.
               CLOCK_REALTIME is not suitable for such applications,
               since that clock is affected by discontinuous changes

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               to the system clock.

          CLOCK_REALTIME_ALARM (since Linux 3.11)
               This clock is like CLOCK_REALTIME, but will wake the
               system if it is suspended.  The caller must have the
               CAP_WAKE_ALARM capability in order to set a timer
               against this clock.

          CLOCK_BOOTTIME_ALARM (since Linux 3.11)
               This clock is like CLOCK_BOOTTIME, but will wake the
               system if it is suspended.  The caller must have the
               CAP_WAKE_ALARM capability in order to set a timer
               against this clock.

          See clock_getres(2) for some further details on the above

          The current value of each of these clocks can be retrieved
          using clock_gettime(2).

          Starting with Linux 2.6.27, the following values may be bit-
          wise ORed in flags to change the behavior of

          TFD_NONBLOCK  Set the O_NONBLOCK file status flag on the
                        open file description (see open(2)) referred
                        to by the new file descriptor.  Using this
                        flag saves extra calls to fcntl(2) to achieve
                        the same result.

          TFD_CLOEXEC   Set the close-on-exec (FD_CLOEXEC) flag on the
                        new file descriptor.  See the description of
                        the O_CLOEXEC flag in open(2) for reasons why
                        this may be useful.

          In Linux versions up to and including 2.6.26, flags must be
          specified as zero.

          timerfd_settime() arms (starts) or disarms (stops) the timer
          referred to by the file descriptor fd.

          The new_value argument specifies the initial expiration and
          interval for the timer.  The itimerspec structure used for
          this argument contains two fields, each of which is in turn
          a structure of type timespec:

              struct timespec {
                  time_t tv_sec;                /* Seconds */
                  long   tv_nsec;               /* Nanoseconds */

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              struct itimerspec {
                  struct timespec it_interval;  /* Interval for periodic timer */
                  struct timespec it_value;     /* Initial expiration */

          new_value.it_value specifies the initial expiration of the
          timer, in seconds and nanoseconds.  Setting either field of
          new_value.it_value to a nonzero value arms the timer.  Set-
          ting both fields of new_value.it_value to zero disarms the

          Setting one or both fields of new_value.it_interval to
          nonzero values specifies the period, in seconds and nanosec-
          onds, for repeated timer expirations after the initial expi-
          ration.  If both fields of new_value.it_interval are zero,
          the timer expires just once, at the time specified by

          By default, the initial expiration time specified in
          new_value is interpreted relative to the current time on the
          timer's clock at the time of the call (i.e.,
          new_value.it_value specifies a time relative to the current
          value of the clock specified by clockid). An absolute time-
          out can be selected via the flags argument.

          The flags argument is a bit mask that can include the fol-
          lowing values:

               Interpret new_value.it_value as an absolute value on
               the timer's clock.  The timer will expire when the
               value of the timer's clock reaches the value specified
               in new_value.it_value.

               If this flag is specified along with TFD_TIMER_ABSTIME
               and the clock for this timer is CLOCK_REALTIME or
               CLOCK_REALTIME_ALARM, then mark this timer as cance-
               lable if the real-time clock undergoes a discontinuous
               change (settimeofday(2), clock_settime(2), or similar).
               When such changes occur, a current or future read(2)
               from the file descriptor will fail with the error

          If the old_value argument is not NULL, then the itimerspec
          structure that it points to is used to return the setting of
          the timer that was current at the time of the call; see the
          description of timerfd_gettime() following.

          timerfd_gettime() returns, in curr_value, an itimerspec
          structure that contains the current setting of the timer

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          referred to by the file descriptor fd.

          The it_value field returns the amount of time until the
          timer will next expire.  If both fields of this structure
          are zero, then the timer is currently disarmed.  This field
          always contains a relative value, regardless of whether the
          TFD_TIMER_ABSTIME flag was specified when setting the timer.

          The it_interval field returns the interval of the timer.  If
          both fields of this structure are zero, then the timer is
          set to expire just once, at the time specified by

        Operating on a timer file descriptor
          The file descriptor returned by timerfd_create() supports
          the following additional operations:

               If the timer has already expired one or more times
               since its settings were last modified using
               timerfd_settime(), or since the last successful
               read(2), then the buffer given to read(2) returns an
               unsigned 8-byte integer (uint64_t) containing the num-
               ber of expirations that have occurred.  (The returned
               value is in host byte order-that is, the native byte
               order for integers on the host machine.)

               If no timer expirations have occurred at the time of
               the read(2), then the call either blocks until the next
               timer expiration, or fails with the error EAGAIN if the
               file descriptor has been made nonblocking (via the use
               of the fcntl(2) F_SETFL operation to set the O_NONBLOCK

               A read(2) fails with the error EINVAL if the size of
               the supplied buffer is less than 8 bytes.

               If the associated clock is either CLOCK_REALTIME or
               CLOCK_REALTIME_ALARM, the timer is absolute
               (TFD_TIMER_ABSTIME), and the flag
               TFD_TIMER_CANCEL_ON_SET was specified when calling
               timerfd_settime(), then read(2) fails with the error
               ECANCELED if the real-time clock undergoes a discontin-
               uous change.  (This allows the reading application to
               discover such discontinuous changes to the clock.)

               If the associated clock is either CLOCK_REALTIME or
               CLOCK_REALTIME_ALARM, the timer is absolute
               (TFD_TIMER_ABSTIME), and the flag
               TFD_TIMER_CANCEL_ON_SET was not specified when calling
               timerfd_settime(), then a discontinuous negative change
               to the clock (e.g., clock_settime(2)) may cause read(2)

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               to unblock, but return a value of 0 (i.e., no bytes
               read), if the clock change occurs after the time
               expired, but before the read(2) on the file descriptor.

          poll(2), select(2) (and similar)
               The file descriptor is readable (the select(2) readfds
               argument; the poll(2) POLLIN flag) if one or more timer
               expirations have occurred.

               The file descriptor also supports the other file-
               descriptor multiplexing APIs: pselect(2), ppoll(2), and

               The following timerfd-specific command is supported:

               TFD_IOC_SET_TICKS (since Linux 3.17)
                    Adjust the number of timer expirations that have
                    occurred.  The argument is a pointer to a nonzero
                    8-byte integer (uint64_t*) containing the new num-
                    ber of expirations.  Once the number is set, any
                    waiter on the timer is woken up.  The only purpose
                    of this command is to restore the expirations for
                    the purpose of checkpoint/restore.  This operation
                    is available only if the kernel was configured
                    with the CONFIG_CHECKPOINT_RESTORE option.

               When the file descriptor is no longer required it
               should be closed.  When all file descriptors associated
               with the same timer object have been closed, the timer
               is disarmed and its resources are freed by the kernel.

        fork(2) semantics
          After a fork(2), the child inherits a copy of the file
          descriptor created by timerfd_create().  The file descriptor
          refers to the same underlying timer object as the corre-
          sponding file descriptor in the parent, and read(2)s in the
          child will return information about expirations of the

        execve(2) semantics
          A file descriptor created by timerfd_create() is preserved
          across execve(2), and continues to generate timer expira-
          tions if the timer was armed.

          On success, timerfd_create() returns a new file descriptor.
          On error, -1 is returned and errno is set to indicate the

          timerfd_settime() and timerfd_gettime() return 0 on success;

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          on error they return -1, and set errno to indicate the

          timerfd_create() can fail with the following errors:

               The clockid is not valid.

               flags is invalid; or, in Linux 2.6.26 or earlier, flags
               is nonzero.

               The per-process limit on the number of open file
               descriptors has been reached.

               The system-wide limit on the total number of open files
               has been reached.

               Could not mount (internal) anonymous inode device.

               There was insufficient kernel memory to create the

               clockid was CLOCK_REALTIME_ALARM or
               CLOCK_BOOTTIME_ALARM but the caller did not have the
               CAP_WAKE_ALARM capability.

          timerfd_settime() and timerfd_gettime() can fail with the
          following errors:

               fd is not a valid file descriptor.

               new_value, old_value, or curr_value is not valid a

               fd is not a valid timerfd file descriptor.

          timerfd_settime() can also fail with the following errors:

               See NOTES.


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               new_value is not properly initialized (one of the
               tv_nsec falls outside the range zero to 999,999,999).

               flags is invalid.

          These system calls are available on Linux since kernel
          2.6.25.  Library support is provided by glibc since version

          These system calls are Linux-specific.

          Suppose the following scenario for CLOCK_REALTIME or
          CLOCK_REALTIME_ALARM timer that was created with

          (a) The timer has been started (timerfd_settime()) with the

          (b) A discontinuous change (e.g., settimeofday(2)) is subse-
              quently made to the CLOCK_REALTIME clock; and

          (c) the caller once more calls timerfd_settime() to rearm
              the timer (without first doing a read(2) on the file

          In this case the following occurs:

          +o The timerfd_settime() returns -1 with errno set to
            ECANCELED.  (This enables the caller to know that the pre-
            vious timer was affected by a discontinuous change to the

          +o The timer is successfully rearmed with the settings pro-
            vided in the second timerfd_settime() call.  (This was
            probably an implementation accident, but won't be fixed
            now, in case there are applications that depend on this

          Currently, timerfd_create() supports fewer types of clock
          IDs than timer_create(2).

          The following program creates a timer and then monitors its
          progress.  The program accepts up to three command-line
          arguments.  The first argument specifies the number of sec-
          onds for the initial expiration of the timer.  The second
          argument specifies the interval for the timer, in seconds.

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          The third argument specifies the number of times the program
          should allow the timer to expire before terminating.  The
          second and third command-line arguments are optional.

          The following shell session demonstrates the use of the pro-

              $ a.out 3 1 100
              0.000: timer started
              3.000: read: 1; total=1
              4.000: read: 1; total=2
              haZ                  # type control-Z to suspend the program
              [1]+  Stopped                 ./timerfd3_demo 3 1 100
              $ fg                # Resume execution after a few seconds
              a.out 3 1 100
              9.660: read: 5; total=7
              10.000: read: 1; total=8
              11.000: read: 1; total=9
              haC                  # type control-C to suspend the program

        Program source

          #include <sys/timerfd.h>
          #include <time.h>
          #include <unistd.h>
          #include <inttypes.h>      /* Definition of PRIu64 */
          #include <stdlib.h>
          #include <stdio.h>
          #include <stdint.h>        /* Definition of uint64_t */

          #define handle_error(msg) \
                  do { perror(msg); exit(EXIT_FAILURE); } while (0)

          static void
              static struct timespec start;
              struct timespec curr;
              static int first_call = 1;
              int secs, nsecs;

              if (first_call) {
                  first_call = 0;
                  if (clock_gettime(CLOCK_MONOTONIC, &start) == -1)

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

              secs = curr.tv_sec - start.tv_sec;
              nsecs = curr.tv_nsec - start.tv_nsec;

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              if (nsecs < 0) {
                  nsecs += 1000000000;
              printf("%d.%03d: ", secs, (nsecs + 500000) / 1000000);

          main(int argc, char *argv[])
              struct itimerspec new_value;
              int max_exp, fd;
              struct timespec now;
              uint64_t exp, tot_exp;
              ssize_t s;

              if ((argc != 2) && (argc != 4)) {
                  fprintf(stderr, "%s init-secs [interval-secs max-exp]\n",

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

              /* Create a CLOCK_REALTIME absolute timer with initial
                 expiration and interval as specified in command line */

              new_value.it_value.tv_sec = now.tv_sec + atoi(argv[1]);
              new_value.it_value.tv_nsec = now.tv_nsec;
              if (argc == 2) {
                  new_value.it_interval.tv_sec = 0;
                  max_exp = 1;
              } else {
                  new_value.it_interval.tv_sec = atoi(argv[2]);
                  max_exp = atoi(argv[3]);
              new_value.it_interval.tv_nsec = 0;

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

              if (timerfd_settime(fd, TFD_TIMER_ABSTIME, &new_value, NULL) == -1)

              printf("timer started\n");

              for (tot_exp = 0; tot_exp < max_exp;) {
                  s = read(fd, &exp, sizeof(uint64_t));
                  if (s != sizeof(uint64_t))

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                  tot_exp += exp;
                  printf("read: %" PRIu64 "; total=%" PRIu64 "\n", exp, tot_exp);


          eventfd(2), poll(2), read(2), select(2), setitimer(2),
          signalfd(2), timer_create(2), timer_gettime(2),
          timer_settime(2), epoll(7), time(7)

          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at https://www.kernel.org/doc/man-pages/.

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