TIMERFD_CREATE(2)           Linux Programmer's Manual          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 delivers 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 functionality 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 progress of the timer, and must be one of
       the following:

              A settable system-wide real-time clock.

              A nonsettable monotonically increasing clock that measures 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 increasing 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 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 clocks.

       The current value of each of these clocks can be retrieved using

       Starting with Linux 2.6.27, the following values may be bitwise ORed in
       flags to change the behavior of timerfd_create():

       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

       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 */

           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.  Setting both fields of new_value.it_value
       to zero disarms the timer.

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

       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 timeout
       can be selected via the flags argument.

       The flags argument is a bit mask that can include the following 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 cancelable 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 ECANCELED.

       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()

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

   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 number of
              expirations that have occurred.  (The returned value is in host
              byte order—that is, the native byte order for integers on the host

              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 flag).

              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 discontinuous 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) 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

       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

              The file descriptor also supports the other file-descriptor
              multiplexing APIs: pselect(2), ppoll(2), and epoll(7).

              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 number 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 corresponding 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 expirations if the timer was

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

       timerfd_settime() and timerfd_gettime() return 0 on success; on error
       they return -1, and set errno to indicate the error.

       timerfd_create() can fail with the following errors:

       EINVAL The clockid is not valid.

       EINVAL flags is invalid; or, in Linux 2.6.26 or earlier, flags is

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

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

       ENODEV Could not mount (internal) anonymous inode device.

       ENOMEM There was insufficient kernel memory to create the timer.

              caller did not have the CAP_WAKE_ALARM capability.

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

       EBADF  fd is not a valid file descriptor.

       EFAULT new_value, old_value, or curr_value is not valid a pointer.

       EINVAL fd is not a valid timerfd file descriptor.

       timerfd_settime() can also fail with the following errors:

              See NOTES.

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

       EINVAL flags is invalid.

       These system calls are available on Linux since kernel 2.6.25.  Library
       support is provided by glibc since version 2.8.

       These system calls are Linux-specific.

       Suppose the following scenario for CLOCK_REALTIME or CLOCK_REALTIME_ALARM
       timer that was created with timerfd_create():

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

       (b) A discontinuous change (e.g., settimeofday(2)) is subsequently 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 descriptor).

       In this case the following occurs:

       • The timerfd_settime() returns -1 with errno set to ECANCELED.  (This
         enables the caller to know that the previous timer was affected by a
         discontinuous change to the clock.)

       • The timer is successfully rearmed with the settings provided 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 behaviour.)

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

       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 seconds for the initial expiration of
       the timer.  The second argument specifies the interval for the timer, in
       seconds.  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 program:

           $ a.out 3 1 100
           0.000: timer started
           3.000: read: 1; total=1
           4.000: read: 1; total=2
           ^Z                  # 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
           ^C                  # 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;
           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))

               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 5.13 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

Linux                              2021-03-22                  TIMERFD_CREATE(2)