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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 *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 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  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)  will	fail  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_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) will fail with the error ECAN-
	      CELED if the real-time clock undergoes a	discontinuous  change.
	      (This allows the reading application to discover such discontin-
	      uous 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 4.10 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				  2016-12-12		     TIMERFD_CREATE(2)