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 de-
       scriptor 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 af-
              ter 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 open file  de-
                     scription  (see  open(2)) referred to by the new file de-
                     scriptor.  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 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 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 de-
       scriptor 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,  re-
       gardless  of whether the TFD_TIMER_ABSTIME flag was specified when set-
       ting 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 oc-
                     curred.  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 re-
                     store the expirations for the purpose  of  checkpoint/re-
                     store.   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 re-
              sources 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  un-
       derlying  timer object as the corresponding file descriptor in the par-
       ent, 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 ex-
       ecve(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  non-

       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 er-

       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  ar-
       gument  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 5.05 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                             2019-03-06                 TIMERFD_CREATE(2)
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