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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 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 *curr_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(3), 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(3),  timer_settime(3), and timer_gettime(3).  (There is no
       analog of timer_gettoverrun(3), 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
       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(3).

       The flags argument is reserved for future use.   As  at  Linux  2.6.25,
       this argument 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_interval 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_inter-

       The  curr_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() 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, i.e., 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 expi-
              ration, 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

              The file descriptor also supports the other file-descriptor mul-
              tiplexing 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 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-
              TIME; or flags is invalid.

       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

       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.

       EINVAL fd  is  not  a  valid timerfd file descriptor.  new_value is not
              properly initialized (one of the tv_nsec falls outside the range
              zero to 999,999,999).

       These system calls are available on Linux since kernel 2.6.25.  Library
       support is provided by in 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 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
           [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
           [type control-C to terminate the program]

       #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(3), timer_gettime(3), timer_settime(3), epoll(7), time(7)

       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

Linux                             2008-02-22                 TIMERFD_CREATE(2)

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