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


       Standard C library (libc, -lc)


       #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 *_Nullable 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 clock_gettime(2).

       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

       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 is described in itimerspec(3type).

       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

              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() 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

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

              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 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 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 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 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 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.


       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 nonzero.

       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.

       EPERM  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:

       EBADF  fd is not a valid file descriptor.

       EFAULT new_value, old_value, or curr_value is not a valid 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 since Linux 2.6.25.  Library support  is  provided  since
       glibc 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():

       (1)  The timer  has  been  started  (timerfd_settime())  with  the  TFD_TIMER_ABSTIME  and
            TFD_TIMER_CANCEL_ON_SET flags;

       (2)  A   discontinuous   change  (e.g.,  settimeofday(2))  is  subsequently  made  to  the
            CLOCK_REALTIME clock; and

       (3)  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 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 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 <err.h>
       #include <inttypes.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <sys/timerfd.h>
       #include <time.h>
       #include <unistd.h>

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

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

           if (clock_gettime(CLOCK_MONOTONIC, &curr) == -1)
               err(EXIT_FAILURE, "clock_gettime");

           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[])
           int                fd;
           ssize_t            s;
           uint64_t           exp, tot_exp, max_exp;
           struct timespec    now;
           struct itimerspec  new_value;

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

           if (clock_gettime(CLOCK_REALTIME, &now) == -1)
               err(EXIT_FAILURE, "clock_gettime");

           /* 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)
               err(EXIT_FAILURE, "timerfd_create");

           if (timerfd_settime(fd, TFD_TIMER_ABSTIME, &new_value, NULL) == -1)
               err(EXIT_FAILURE, "timerfd_settime");

           printf("timer started\n");

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

               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), timespec(3), epoll(7), time(7)