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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
       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 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() 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.

       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_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).

       The old_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
              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

              The file descriptor  also  supports  the  other  file-descriptor
              multiplexing 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
       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


       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

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

       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.

       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.


       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);



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


       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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