timerfd_gettime(2) — Linux manual page


TIMERFD_CREATE(2)         Linux Programmer's Manual        TIMERFD_CREATE(2)

NAME         top

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

SYNOPSIS         top

       #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);

DESCRIPTION         top

       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 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

           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 in‐
       terpreted 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 val‐

              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

              If this flag is specified along with TFD_TIMER_ABSTIME and the
              clock for this timer is CLOCK_REALTIME or CLOCK_REAL‐
              TIME_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_get‐
       time() following.

       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 follow‐
       ing 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 inte‐
              gers 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 ex‐
              piration, or fails with the error EAGAIN if the file descrip‐
              tor 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 sup‐
              plied buffer is less than 8 bytes.

              If the associated clock is either CLOCK_REALTIME or CLOCK_RE‐
              ALTIME_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 discon‐
              tinuous changes to the clock.)

              If the associated clock is either CLOCK_REALTIME or CLOCK_RE‐
              ALTIME_ALARM, the timer is absolute (TFD_TIMER_ABSTIME), and
              the flag TFD_TIMER_CANCEL_ON_SET was not specified when call‐
              ing timerfd_settime(), then a discontinuous negative change to
              the clock (e.g., clock_settime(2)) may cause read(2) to un‐
              block, 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 argu‐
              ment; the poll(2) POLLIN flag) if one or more timer expira‐
              tions have occurred.

              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 oc‐
                     curred.  The argument is a pointer to a nonzero 8-byte
                     integer (uint64_t*) containing the new number of expi‐
                     rations.  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 check‐
                     point/restore.  This operation is available only if the
                     kernel was configured with the CONFIG_CHECKPOINT_RE‐
                     STORE 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 expi‐
       rations of the timer.

   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 armed.

RETURN VALUE         top

       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.

ERRORS         top

       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 reached.

       ENODEV Could not mount (internal) anonymous inode device.

       ENOMEM There was insufficient kernel memory to create the timer.

              the 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.

VERSIONS         top

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

CONFORMING TO         top

       These system calls are Linux-specific.

NOTES         top

       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.)

BUGS         top

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

EXAMPLES         top

       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);


SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.09 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                            2020-08-13                TIMERFD_CREATE(2)

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