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           #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 be
           either CLOCK_REALTIME or CLOCK_MONOTONIC.  CLOCK_REALTIME is a settable
           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.
           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_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_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
                  tiplexing 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 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 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.
           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;
           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",

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