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 alter-
       native to the use of setitimer(2) or timer_create(2), with  the  advan-
       tage  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 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 capa-
              bility 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 capa-
              bility in order to set a timer against this clock.

       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 new open  file
                     description.   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 con-
       tains two fields, each of which is in turn a structure  of  type  time-

           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 under-
              goes  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 ECAN-

       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 following

              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 expi-
              ration, 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  sup-
              plied buffer is less than 8 bytes.

              If  the associated clock is either CLOCK_REALTIME or CLOCK_REAL-
              TIME_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.)

       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 mul-
              tiplexing 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  expira-
                     tions.   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_RESTORE

              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 cre-
       ated 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 expira-
       tions 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

       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 argument is neither CLOCK_MONOTONIC nor CLOCK_REAL-

       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.

       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:

       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.

       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 pro-
       gram 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 <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: %llu; total=%llu\n",
                       (unsigned long long) exp,
                       (unsigned long long) 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 4.15 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                             2017-09-15                 TIMERFD_CREATE(2)
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