1TIMERFD_CREATE(2)          Linux Programmer's Manual         TIMERFD_CREATE(2)
2
3
4

NAME

6       timerfd_create,  timerfd_settime,  timerfd_gettime - timers that notify
7       via file descriptors
8

SYNOPSIS

10       #include <sys/timerfd.h>
11
12       int timerfd_create(int clockid, int flags);
13
14       int timerfd_settime(int fd, int flags,
15                           const struct itimerspec *new_value,
16                           struct itimerspec *old_value);
17       int timerfd_gettime(int fd, struct itimerspec *curr_value);
18

DESCRIPTION

20       These system calls create and operate on a timer  that  delivers  timer
21       expiration notifications via a file descriptor.  They provide an alter‐
22       native to the use of setitimer(2) or timer_create(2), with  the  advan‐
23       tage  that  the file descriptor may be monitored by select(2), poll(2),
24       and epoll(7).
25
26       The use of these  three  system  calls  is  analogous  to  the  use  of
27       timer_create(2),  timer_settime(2), and timer_gettime(2).  (There is no
28       analog of timer_getoverrun(2), since that functionality is provided  by
29       read(2), as described below.)
30
31   timerfd_create()
32       timerfd_create()  creates  a  new  timer object, and returns a file de‐
33       scriptor that refers to that timer.  The clockid argument specifies the
34       clock  that  is used to mark the progress of the timer, and must be one
35       of the following:
36
37       CLOCK_REALTIME
38              A settable system-wide real-time clock.
39
40       CLOCK_MONOTONIC
41              A nonsettable monotonically increasing clock that measures  time
42              from some unspecified point in the past that does not change af‐
43              ter system startup.
44
45       CLOCK_BOOTTIME (Since Linux 3.15)
46              Like CLOCK_MONOTONIC, this is a monotonically increasing  clock.
47              However,  whereas the CLOCK_MONOTONIC clock does not measure the
48              time while a system is suspended, the CLOCK_BOOTTIME clock  does
49              include  the time during which the system is suspended.  This is
50              useful  for  applications  that  need   to   be   suspend-aware.
51              CLOCK_REALTIME is not suitable for such applications, since that
52              clock is affected by discontinuous changes to the system clock.
53
54       CLOCK_REALTIME_ALARM (since Linux 3.11)
55              This clock is like CLOCK_REALTIME, but will wake the  system  if
56              it  is suspended.  The caller must have the CAP_WAKE_ALARM capa‐
57              bility in order to set a timer against this clock.
58
59       CLOCK_BOOTTIME_ALARM (since Linux 3.11)
60              This clock is like CLOCK_BOOTTIME, but will wake the  system  if
61              it  is suspended.  The caller must have the CAP_WAKE_ALARM capa‐
62              bility in order to set a timer against this clock.
63
64       See clock_getres(2) for some further details on the above clocks.
65
66       The current value of each  of  these  clocks  can  be  retrieved  using
67       clock_gettime(2).
68
69       Starting with Linux 2.6.27, the following values may be bitwise ORed in
70       flags to change the behavior of timerfd_create():
71
72       TFD_NONBLOCK  Set the O_NONBLOCK file status flag on the open file  de‐
73                     scription  (see  open(2)) referred to by the new file de‐
74                     scriptor.  Using this flag saves extra calls to  fcntl(2)
75                     to achieve the same result.
76
77       TFD_CLOEXEC   Set  the  close-on-exec (FD_CLOEXEC) flag on the new file
78                     descriptor.  See the description of the O_CLOEXEC flag in
79                     open(2) for reasons why this may be useful.
80
81       In  Linux  versions up to and including 2.6.26, flags must be specified
82       as zero.
83
84   timerfd_settime()
85       timerfd_settime() arms (starts) or disarms (stops) the  timer  referred
86       to by the file descriptor fd.
87
88       The  new_value  argument  specifies the initial expiration and interval
89       for the timer.  The itimerspec structure used for  this  argument  con‐
90       tains  two  fields,  each of which is in turn a structure of type time‐
91       spec:
92
93           struct timespec {
94               time_t tv_sec;                /* Seconds */
95               long   tv_nsec;               /* Nanoseconds */
96           };
97
98           struct itimerspec {
99               struct timespec it_interval;  /* Interval for periodic timer */
100               struct timespec it_value;     /* Initial expiration */
101           };
102
103       new_value.it_value specifies the initial expiration of  the  timer,  in
104       seconds and nanoseconds.  Setting either field of new_value.it_value to
105       a  nonzero  value   arms   the   timer.    Setting   both   fields   of
106       new_value.it_value to zero disarms the timer.
107
108       Setting  one  or both fields of new_value.it_interval to nonzero values
109       specifies the period, in seconds and nanoseconds,  for  repeated  timer
110       expirations   after   the   initial  expiration.   If  both  fields  of
111       new_value.it_interval are zero, the timer expires  just  once,  at  the
112       time specified by new_value.it_value.
113
114       By  default,  the initial expiration time specified in new_value is in‐
115       terpreted relative to the current time on the timer's clock at the time
116       of  the call (i.e., new_value.it_value specifies a time relative to the
117       current value of the clock specified by clockid).  An absolute  timeout
118       can be selected via the flags argument.
119
120       The flags argument is a bit mask that can include the following values:
121
122       TFD_TIMER_ABSTIME
123              Interpret new_value.it_value as an absolute value on the timer's
124              clock.  The timer will expire when  the  value  of  the  timer's
125              clock reaches the value specified in new_value.it_value.
126
127       TFD_TIMER_CANCEL_ON_SET
128              If  this  flag is specified along with TFD_TIMER_ABSTIME and the
129              clock for this timer is CLOCK_REALTIME or  CLOCK_REALTIME_ALARM,
130              then mark this timer as cancelable if the real-time clock under‐
131              goes a discontinuous change (settimeofday(2),  clock_settime(2),
132              or  similar).   When  such  changes  occur,  a current or future
133              read(2) from the file descriptor will fail with the error  ECAN‐
134              CELED.
135
136       If  the  old_value  argument is not NULL, then the itimerspec structure
137       that it points to is used to return the setting of the timer  that  was
138       current  at  the  time of the call; see the description of timerfd_get‐
139       time() following.
140
141   timerfd_gettime()
142       timerfd_gettime() returns, in curr_value, an itimerspec structure  that
143       contains  the  current setting of the timer referred to by the file de‐
144       scriptor fd.
145
146       The it_value field returns the amount of time until the timer will next
147       expire.   If  both fields of this structure are zero, then the timer is
148       currently disarmed.  This field always contains a relative  value,  re‐
149       gardless  of whether the TFD_TIMER_ABSTIME flag was specified when set‐
150       ting the timer.
151
152       The it_interval field returns the  interval  of  the  timer.   If  both
153       fields of this structure are zero, then the timer is set to expire just
154       once, at the time specified by curr_value.it_value.
155
156   Operating on a timer file descriptor
157       The file descriptor returned by timerfd_create() supports the following
158       additional operations:
159
160       read(2)
161              If  the  timer  has  already expired one or more times since its
162              settings were last modified using  timerfd_settime(),  or  since
163              the  last  successful  read(2), then the buffer given to read(2)
164              returns an unsigned 8-byte  integer  (uint64_t)  containing  the
165              number  of  expirations that have occurred.  (The returned value
166              is in host byte order—that is, the native byte order  for  inte‐
167              gers on the host machine.)
168
169              If  no  timer  expirations  have  occurred  at  the  time of the
170              read(2), then the call either blocks until the next timer  expi‐
171              ration,  or  fails  with the error EAGAIN if the file descriptor
172              has been made nonblocking (via the use of the  fcntl(2)  F_SETFL
173              operation to set the O_NONBLOCK flag).
174
175              A  read(2)  fails  with the error EINVAL if the size of the sup‐
176              plied buffer is less than 8 bytes.
177
178              If the associated clock is either CLOCK_REALTIME or  CLOCK_REAL‐
179              TIME_ALARM,  the  timer is absolute (TFD_TIMER_ABSTIME), and the
180              flag  TFD_TIMER_CANCEL_ON_SET   was   specified   when   calling
181              timerfd_settime(),  then  read(2) fails with the error ECANCELED
182              if the real-time clock undergoes a discontinuous change.   (This
183              allows  the  reading  application to discover such discontinuous
184              changes to the clock.)
185
186              If the associated clock is either CLOCK_REALTIME or  CLOCK_REAL‐
187              TIME_ALARM,  the  timer is absolute (TFD_TIMER_ABSTIME), and the
188              flag TFD_TIMER_CANCEL_ON_SET  was  not  specified  when  calling
189              timerfd_settime(),  then  a discontinuous negative change to the
190              clock (e.g., clock_settime(2)) may cause read(2) to unblock, but
191              return  a  value of 0 (i.e., no bytes read), if the clock change
192              occurs after the time expired, but before  the  read(2)  on  the
193              file descriptor.
194
195       poll(2), select(2) (and similar)
196              The file descriptor is readable (the select(2) readfds argument;
197              the poll(2) POLLIN flag) if one or more timer  expirations  have
198              occurred.
199
200              The file descriptor also supports the other file-descriptor mul‐
201              tiplexing APIs: pselect(2), ppoll(2), and epoll(7).
202
203       ioctl(2)
204              The following timerfd-specific command is supported:
205
206              TFD_IOC_SET_TICKS (since Linux 3.17)
207                     Adjust the number of  timer  expirations  that  have  oc‐
208                     curred.   The  argument  is a pointer to a nonzero 8-byte
209                     integer (uint64_t*) containing the new number of  expira‐
210                     tions.   Once  the number is set, any waiter on the timer
211                     is woken up.  The only purpose of this command is to  re‐
212                     store  the  expirations for the purpose of checkpoint/re‐
213                     store.  This operation is available only  if  the  kernel
214                     was configured with the CONFIG_CHECKPOINT_RESTORE option.
215
216       close(2)
217              When  the  file  descriptor  is  no longer required it should be
218              closed.  When all file  descriptors  associated  with  the  same
219              timer object have been closed, the timer is disarmed and its re‐
220              sources are freed by the kernel.
221
222   fork(2) semantics
223       After a fork(2), the child inherits a copy of the file descriptor  cre‐
224       ated  by  timerfd_create().  The file descriptor refers to the same un‐
225       derlying timer object as the corresponding file descriptor in the  par‐
226       ent,  and  read(2)s  in the child will return information about expira‐
227       tions of the timer.
228
229   execve(2) semantics
230       A file descriptor created by timerfd_create() is preserved  across  ex‐
231       ecve(2),  and  continues to generate timer expirations if the timer was
232       armed.
233

RETURN VALUE

235       On success, timerfd_create() returns a new file descriptor.  On  error,
236       -1 is returned and errno is set to indicate the error.
237
238       timerfd_settime()  and  timerfd_gettime() return 0 on success; on error
239       they return -1, and set errno to indicate the error.
240

ERRORS

242       timerfd_create() can fail with the following errors:
243
244       EINVAL The clockid is not valid.
245
246       EINVAL flags is invalid; or, in Linux 2.6.26 or earlier, flags is  non‐
247              zero.
248
249       EMFILE The per-process limit on the number of open file descriptors has
250              been reached.
251
252       ENFILE The system-wide limit on the total number of open files has been
253              reached.
254
255       ENODEV Could not mount (internal) anonymous inode device.
256
257       ENOMEM There was insufficient kernel memory to create the timer.
258
259       EPERM  clockid was CLOCK_REALTIME_ALARM or CLOCK_BOOTTIME_ALARM but the
260              caller did not have the CAP_WAKE_ALARM capability.
261
262       timerfd_settime() and timerfd_gettime() can fail with the following er‐
263       rors:
264
265       EBADF  fd is not a valid file descriptor.
266
267       EFAULT new_value, old_value, or curr_value is not valid a pointer.
268
269       EINVAL fd is not a valid timerfd file descriptor.
270
271       timerfd_settime() can also fail with the following errors:
272
273       ECANCELED
274              See NOTES.
275
276       EINVAL new_value  is not properly initialized (one of the tv_nsec falls
277              outside the range zero to 999,999,999).
278
279       EINVAL flags is invalid.
280

VERSIONS

282       These system calls are available on Linux since kernel 2.6.25.  Library
283       support is provided by glibc since version 2.8.
284

CONFORMING TO

286       These system calls are Linux-specific.
287

NOTES

289       Suppose  the  following  scenario  for  CLOCK_REALTIME  or  CLOCK_REAL‐
290       TIME_ALARM timer that was created with timerfd_create():
291
292       (a) The  timer  has   been   started   (timerfd_settime())   with   the
293           TFD_TIMER_ABSTIME and TFD_TIMER_CANCEL_ON_SET flags;
294
295       (b) A discontinuous change (e.g., settimeofday(2)) is subsequently made
296           to the CLOCK_REALTIME clock; and
297
298       (c) the caller once more calls timerfd_settime()  to  rearm  the  timer
299           (without first doing a read(2) on the file descriptor).
300
301       In this case the following occurs:
302
303       • The  timerfd_settime() returns -1 with errno set to ECANCELED.  (This
304         enables the caller to know that the previous timer was affected by  a
305         discontinuous change to the clock.)
306
307       • The  timer  is successfully rearmed with the settings provided in the
308         second timerfd_settime() call.  (This was probably an  implementation
309         accident, but won't be fixed now, in case there are applications that
310         depend on this behaviour.)
311

BUGS

313       Currently, timerfd_create() supports fewer  types  of  clock  IDs  than
314       timer_create(2).
315

EXAMPLES

317       The  following  program creates a timer and then monitors its progress.
318       The program accepts up to three command-line arguments.  The first  ar‐
319       gument  specifies  the  number of seconds for the initial expiration of
320       the timer.  The second argument specifies the interval for  the  timer,
321       in  seconds.  The third argument specifies the number of times the pro‐
322       gram should allow the timer to expire before terminating.   The  second
323       and third command-line arguments are optional.
324
325       The following shell session demonstrates the use of the program:
326
327           $ a.out 3 1 100
328           0.000: timer started
329           3.000: read: 1; total=1
330           4.000: read: 1; total=2
331           ^Z                  # type control-Z to suspend the program
332           [1]+  Stopped                 ./timerfd3_demo 3 1 100
333           $ fg                # Resume execution after a few seconds
334           a.out 3 1 100
335           9.660: read: 5; total=7
336           10.000: read: 1; total=8
337           11.000: read: 1; total=9
338           ^C                  # type control-C to suspend the program
339
340   Program source
341
342       #include <sys/timerfd.h>
343       #include <time.h>
344       #include <unistd.h>
345       #include <inttypes.h>      /* Definition of PRIu64 */
346       #include <stdlib.h>
347       #include <stdio.h>
348       #include <stdint.h>        /* Definition of uint64_t */
349
350       #define handle_error(msg) \
351               do { perror(msg); exit(EXIT_FAILURE); } while (0)
352
353       static void
354       print_elapsed_time(void)
355       {
356           static struct timespec start;
357           struct timespec curr;
358           static int first_call = 1;
359           int secs, nsecs;
360
361           if (first_call) {
362               first_call = 0;
363               if (clock_gettime(CLOCK_MONOTONIC, &start) == -1)
364                   handle_error("clock_gettime");
365           }
366
367           if (clock_gettime(CLOCK_MONOTONIC, &curr) == -1)
368               handle_error("clock_gettime");
369
370           secs = curr.tv_sec - start.tv_sec;
371           nsecs = curr.tv_nsec - start.tv_nsec;
372           if (nsecs < 0) {
373               secs--;
374               nsecs += 1000000000;
375           }
376           printf("%d.%03d: ", secs, (nsecs + 500000) / 1000000);
377       }
378
379       int
380       main(int argc, char *argv[])
381       {
382           struct itimerspec new_value;
383           int max_exp, fd;
384           struct timespec now;
385           uint64_t exp, tot_exp;
386           ssize_t s;
387
388           if ((argc != 2) && (argc != 4)) {
389               fprintf(stderr, "%s init-secs [interval-secs max-exp]\n",
390                       argv[0]);
391               exit(EXIT_FAILURE);
392           }
393
394           if (clock_gettime(CLOCK_REALTIME, &now) == -1)
395               handle_error("clock_gettime");
396
397           /* Create a CLOCK_REALTIME absolute timer with initial
398              expiration and interval as specified in command line. */
399
400           new_value.it_value.tv_sec = now.tv_sec + atoi(argv[1]);
401           new_value.it_value.tv_nsec = now.tv_nsec;
402           if (argc == 2) {
403               new_value.it_interval.tv_sec = 0;
404               max_exp = 1;
405           } else {
406               new_value.it_interval.tv_sec = atoi(argv[2]);
407               max_exp = atoi(argv[3]);
408           }
409           new_value.it_interval.tv_nsec = 0;
410
411           fd = timerfd_create(CLOCK_REALTIME, 0);
412           if (fd == -1)
413               handle_error("timerfd_create");
414
415           if (timerfd_settime(fd, TFD_TIMER_ABSTIME, &new_value, NULL) == -1)
416               handle_error("timerfd_settime");
417
418           print_elapsed_time();
419           printf("timer started\n");
420
421           for (tot_exp = 0; tot_exp < max_exp;) {
422               s = read(fd, &exp, sizeof(uint64_t));
423               if (s != sizeof(uint64_t))
424                   handle_error("read");
425
426               tot_exp += exp;
427               print_elapsed_time();
428               printf("read: %" PRIu64 "; total=%" PRIu64 "\n", exp, tot_exp);
429           }
430
431           exit(EXIT_SUCCESS);
432       }
433

SEE ALSO

435       eventfd(2),  poll(2),  read(2),  select(2),  setitimer(2), signalfd(2),
436       timer_create(2), timer_gettime(2), timer_settime(2), epoll(7), time(7)
437

COLOPHON

439       This page is part of release 5.12 of the Linux  man-pages  project.   A
440       description  of  the project, information about reporting bugs, and the
441       latest    version    of    this    page,    can     be     found     at
442       https://www.kernel.org/doc/man-pages/.
443
444
445
446Linux                             2021-03-22                 TIMERFD_CREATE(2)
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