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       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  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_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 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 capability 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 capability 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 open file description (see
                     open(2)) referred to by the new file  descriptor.   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
       itimerspec  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

              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   undergoes   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 ECANCELED.

       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
       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

   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—that is, 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)  fails  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_REALTIME_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 multiplexing 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 expirations.  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 checkpoint/restore.  This operation  is  available  only  if  the
                     kernel was configured with the CONFIG_CHECKPOINT_RESTORE option.

              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 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.


       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 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)


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