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

       timerfd_create,  timerfd_settime,  timerfd_gettime  -  timers  that notify via file

       #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_getover-
       run(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 either CLOCK_REALTIME or CLOCK_MONO-
       TONIC.  CLOCK_REALTIME is a settable system-wide clock.  CLOCK_MONOTONIC is a  non-
       settable  clock  that  is not affected by discontinuous changes in the system clock
       (e.g., manual changes to system time).  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 new open file description.
                     Using  this  flag  saves  extra calls to fcntl(2) to achieve the same

       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 itimer 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 non-zero  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 non-zero 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.

       The flags argument is either 0, to start a  relative  timer  (new_value.it_interval
       specifies  a time relative to the current value of the clock specified by clockid),
       or TFD_TIMER_ABSTIME, to start an absolute timer (new_value.it_value  specifies  an
       absolute  time  for  the clock specified by clockid; that is, the timer will expire
       when the value of that clock reaches the value specified in new_value.it_value).

       The old_value argument returns a structure containing the setting of the timer that
       was  current at the time of the call; see the description of timerfd_gettime() fol-

       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 speci-
       fied by curr_value.it_value.

   Operating on a timer file descriptor
       The file descriptor returned by timerfd_create() supports the following 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, i.e., 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 expiration, or fails with the error
              EAGAIN if the file descriptor has been made non-blocking (via the use of the
              fcntl(2) F_SETFL operation to set the O_NONBLOCK flag).

              A read(2) will fail with the error EINVAL if the size of the supplied buffer
              is less than 8 bytes.

       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).

              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 argument is neither CLOCK_MONOTONIC nor CLOCK_REALTIME;

       EINVAL flags is invalid; or, in Linux 2.6.26 or earlier, flags is non-zero.

       EMFILE The per-process limit 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.

       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 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.

       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 num-
       ber of seconds for the initial expiration of the timer.  The second argument speci-
       fies the interval for the timer, in seconds.  The third argument specifies the num-
       ber 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 <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);


       Currently, timerfd_create() supports fewer types of clock IDs than timer_create(2).

       eventfd(2), poll(2),  read(2),  select(2),  setitimer(2),  signalfd(2),  timer_cre-
       ate(2), timer_gettime(2), timer_settime(2), epoll(7), time(7)

       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the project, and information about reporting bugs, can be found at  http://www.ker-

Linux                             2009-03-10                 TIMERFD_CREATE(2)

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