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

NAME
       timerfd_create,	timerfd_settime,  timerfd_gettime - timers that notify
       via file descriptors

SYNOPSIS
       #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);

DESCRIPTION
       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()
       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:

       CLOCK_REALTIME
	      A settable system-wide real-time clock.

       CLOCK_MONOTONIC
	      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
       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 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()
       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‐
       spec:

	   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:

       TFD_TIMER_ABSTIME
	      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.

       TFD_TIMER_CANCEL_ON_SET
	      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‐
	      CELED.

       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()
       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
       operations:

       read(2)
	      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
	      occurred.

	      The file descriptor also supports the other file-descriptor mul‐
	      tiplexing APIs: pselect(2), ppoll(2), and epoll(7).

       ioctl(2)
	      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
		     option.

       close(2)
	      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
       armed.

RETURN VALUE
       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.

ERRORS
       timerfd_create() can fail with the following errors:

       EINVAL The  clockid argument is neither CLOCK_MONOTONIC nor CLOCK_REAL‐
	      TIME;

       EINVAL flags is invalid; or, in	Linux  2.6.26  or  earlier,  flags  is
	      nonzero.

       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
	      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.

VERSIONS
       These system calls are available on Linux since kernel 2.6.25.  Library
       support is provided by glibc since version 2.8.

CONFORMING TO
       These system calls are Linux-specific.

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

EXAMPLE
       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
       print_elapsed_time(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)
		   handle_error("clock_gettime");
	   }

	   if (clock_gettime(CLOCK_MONOTONIC, &curr) == -1)
	       handle_error("clock_gettime");

	   secs = curr.tv_sec - start.tv_sec;
	   nsecs = curr.tv_nsec - start.tv_nsec;
	   if (nsecs < 0) {
	       secs--;
	       nsecs += 1000000000;
	   }
	   printf("%d.%03d: ", secs, (nsecs + 500000) / 1000000);
       }

       int
       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",
		       argv[0]);
	       exit(EXIT_FAILURE);
	   }

	   if (clock_gettime(CLOCK_REALTIME, &now) == -1)
	       handle_error("clock_gettime");

	   /* 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)
	       handle_error("timerfd_create");

	   if (timerfd_settime(fd, TFD_TIMER_ABSTIME, &new_value, NULL) == -1)
	       handle_error("timerfd_settime");

	   print_elapsed_time();
	   printf("timer started\n");

	   for (tot_exp = 0; tot_exp < max_exp;) {
	       s = read(fd, &exp, sizeof(uint64_t));
	       if (s != sizeof(uint64_t))
		   handle_error("read");

	       tot_exp += exp;
	       print_elapsed_time();
	       printf("read: %llu; total=%llu\n",
		       (unsigned long long) exp,
		       (unsigned long long) tot_exp);
	   }

	   exit(EXIT_SUCCESS);
       }

SEE ALSO
       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)

COLOPHON
       This page is part of release 4.14 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
       https://www.kernel.org/doc/man-pages/.

Linux				  2017-09-15		     TIMERFD_CREATE(2)
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