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Linux 2.6 - man page for timerfd_gettime (linux section 2)

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 notifica-
       tions 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_set-
       time(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 be either CLOCK_REALTIME or CLOCK_MONOTONIC.  CLOCK_REALTIME is a set-
       table  system-wide  clock.  CLOCK_MONOTONIC is a nonsettable clock that is not affected by
       discontinuous changes in the system clock (e.g., manual changes to system time).  The cur-
       rent 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

       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  nanosec-
       onds.  Setting either field of new_value.it_value to a nonzero value arms the timer.  Set-
       ting 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 expi-
       ration.	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_value 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).

       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_gettime() following.

       timerfd_gettime()  returns,  in curr_value, an itimerspec structure that contains the cur-
       rent 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

   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 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 expiration, 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.

       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: pse-
	      lect(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_cre-
       ate().  The file descriptor refers to the same underlying timer object as the  correspond-
       ing 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 nonzero.

       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  pro-
       vided by glibc since version 2.8.

       These system calls are Linux-specific.

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

       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  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);


       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 3.55 of the Linux man-pages project.  A description of the
       project,    and	  information	 about	  reporting    bugs,	can    be    found     at

Linux					    2011-09-14				TIMERFD_CREATE(2)

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