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1 .\" Copyright (c) 2003 Nick Clifford (zaf@nrc.co.nz), Jan 25, 2003
2 .\" Copyright (c) 2003 Andries Brouwer (aeb@cwi.nl), Aug 24, 2003
3 .\" Copyright (c) 2020 Michael Kerrisk <mtk.manpages@gmail.com>
4 .\"
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10 .\" Permission is granted to copy and distribute modified versions of this
11 .\" manual under the conditions for verbatim copying, provided that the
12 .\" entire resulting derived work is distributed under the terms of a
13 .\" permission notice identical to this one.
14 .\"
15 .\" Since the Linux kernel and libraries are constantly changing, this
16 .\" manual page may be incorrect or out-of-date. The author(s) assume no
17 .\" responsibility for errors or omissions, or for damages resulting from
18 .\" the use of the information contained herein. The author(s) may not
19 .\" have taken the same level of care in the production of this manual,
20 .\" which is licensed free of charge, as they might when working
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23 .\" Formatted or processed versions of this manual, if unaccompanied by
24 .\" the source, must acknowledge the copyright and authors of this work.
25 .\" %%%LICENSE_END
26 .\"
27 .\" 2003-08-23 Martin Schulze <joey@infodrom.org> improvements
28 .\" 2003-08-24 aeb, large parts rewritten
29 .\" 2004-08-06 Christoph Lameter <clameter@sgi.com>, SMP note
30 .\"
31 .TH CLOCK_GETRES 2 2021-03-22 "" "Linux Programmer's Manual"
32 .SH NAME
33 clock_getres, clock_gettime, clock_settime \- clock and time functions
34 .SH SYNOPSIS
35 .nf
36 .B #include <time.h>
37 .PP
38 .BI "int clock_getres(clockid_t " clockid ", struct timespec *" res );
39 .PP
40 .BI "int clock_gettime(clockid_t " clockid ", struct timespec *" tp );
41 .BI "int clock_settime(clockid_t " clockid ", const struct timespec *" tp );
42 .fi
43 .PP
44 Link with \fI\-lrt\fP (only for glibc versions before 2.17).
45 .PP
46 .RS -4
47 Feature Test Macro Requirements for glibc (see
48 .BR feature_test_macros (7)):
49 .RE
50 .PP
51 .BR clock_getres (),
52 .BR clock_gettime (),
53 .BR clock_settime ():
54 .nf
55 _POSIX_C_SOURCE >= 199309L
56 .fi
57 .SH DESCRIPTION
58 The function
59 .BR clock_getres ()
60 finds the resolution (precision) of the specified clock
61 .IR clockid ,
62 and, if
63 .I res
64 is non-NULL, stores it in the \fIstruct timespec\fP pointed to by
65 .IR res .
66 The resolution of clocks depends on the implementation and cannot be
67 configured by a particular process.
68 If the time value pointed to by the argument
69 .I tp
70 of
71 .BR clock_settime ()
72 is not a multiple of
73 .IR res ,
74 then it is truncated to a multiple of
75 .IR res .
76 .PP
77 The functions
78 .BR clock_gettime ()
79 and
80 .BR clock_settime ()
81 retrieve and set the time of the specified clock
82 .IR clockid .
83 .PP
84 The
85 .I res
86 and
87 .I tp
88 arguments are
89 .I timespec
90 structures, as specified in
91 .IR <time.h> :
92 .PP
93 .in +4n
94 .EX
95 struct timespec {
96 time_t tv_sec; /* seconds */
97 long tv_nsec; /* nanoseconds */
98 };
99 .EE
100 .in
101 .PP
102 The
103 .I clockid
104 argument is the identifier of the particular clock on which to act.
105 A clock may be system-wide and hence visible for all processes, or
106 per-process if it measures time only within a single process.
107 .PP
108 All implementations support the system-wide real-time clock,
109 which is identified by
110 .BR CLOCK_REALTIME .
111 Its time represents seconds and nanoseconds since the Epoch.
112 When its time is changed, timers for a relative interval are
113 unaffected, but timers for an absolute point in time are affected.
114 .PP
115 More clocks may be implemented.
116 The interpretation of the
117 corresponding time values and the effect on timers is unspecified.
118 .PP
119 Sufficiently recent versions of glibc and the Linux kernel
120 support the following clocks:
121 .TP
122 .B CLOCK_REALTIME
123 A settable system-wide clock that measures real (i.e., wall-clock) time.
124 Setting this clock requires appropriate privileges.
125 This clock is affected by discontinuous jumps in the system time
126 (e.g., if the system administrator manually changes the clock),
127 and by the incremental adjustments performed by
128 .BR adjtime (3)
129 and NTP.
130 .TP
131 .BR CLOCK_REALTIME_ALARM " (since Linux 3.0; Linux-specific)"
132 Like
133 .BR CLOCK_REALTIME ,
134 but not settable.
135 See
136 .BR timer_create (2)
137 for further details.
138 .TP
139 .BR CLOCK_REALTIME_COARSE " (since Linux 2.6.32; Linux-specific)"
140 .\" Added in commit da15cfdae03351c689736f8d142618592e3cebc3
141 A faster but less precise version of
142 .BR CLOCK_REALTIME .
143 This clock is not settable.
144 Use when you need very fast, but not fine-grained timestamps.
145 Requires per-architecture support,
146 and probably also architecture support for this flag in the
147 .BR vdso (7).
148 .TP
149 .BR CLOCK_TAI " (since Linux 3.10; Linux-specific)"
150 .\" commit 1ff3c9677bff7e468e0c487d0ffefe4e901d33f4
151 A nonsettable system-wide clock derived from wall-clock time
152 but ignoring leap seconds.
153 This clock does
154 not experience discontinuities and backwards jumps caused by NTP
155 inserting leap seconds as
156 .BR CLOCK_REALTIME
157 does.
158 .IP
159 The acronym TAI refers to International Atomic Time.
160 .TP
161 .B CLOCK_MONOTONIC
162 A nonsettable system-wide clock that
163 represents monotonic time since\(emas described
164 by POSIX\(em"some unspecified point in the past".
165 On Linux, that point corresponds to the number of seconds that the system
166 has been running since it was booted.
167 .IP
168 The
169 .B CLOCK_MONOTONIC
170 clock is not affected by discontinuous jumps in the system time
171 (e.g., if the system administrator manually changes the clock),
172 but is affected by the incremental adjustments performed by
173 .BR adjtime (3)
174 and NTP.
175 This clock does not count time that the system is suspended.
176 All
177 .B CLOCK_MONOTONIC
178 variants guarantee that the time returned by consecutive calls will not go
179 backwards, but successive calls may\(emdepending on the architecture\(emreturn
180 identical (not-increased) time values.
181 .TP
182 .BR CLOCK_MONOTONIC_COARSE " (since Linux 2.6.32; Linux-specific)"
183 .\" Added in commit da15cfdae03351c689736f8d142618592e3cebc3
184 A faster but less precise version of
185 .BR CLOCK_MONOTONIC .
186 Use when you need very fast, but not fine-grained timestamps.
187 Requires per-architecture support,
188 and probably also architecture support for this flag in the
189 .BR vdso (7).
190 .TP
191 .BR CLOCK_MONOTONIC_RAW " (since Linux 2.6.28; Linux-specific)"
192 .\" Added in commit 2d42244ae71d6c7b0884b5664cf2eda30fb2ae68, John Stultz
193 Similar to
194 .BR CLOCK_MONOTONIC ,
195 but provides access to a raw hardware-based time
196 that is not subject to NTP adjustments or
197 the incremental adjustments performed by
198 .BR adjtime (3).
199 This clock does not count time that the system is suspended.
200 .TP
201 .BR CLOCK_BOOTTIME " (since Linux 2.6.39; Linux-specific)"
202 .\" commit 7fdd7f89006dd5a4c702fa0ce0c272345fa44ae0
203 .\" commit 70a08cca1227dc31c784ec930099a4417a06e7d0
204 A nonsettable system-wide clock that is identical to
205 .BR CLOCK_MONOTONIC ,
206 except that it also includes any time that the system is suspended.
207 This allows applications to get a suspend-aware monotonic clock
208 without having to deal with the complications of
209 .BR CLOCK_REALTIME ,
210 which may have discontinuities if the time is changed using
211 .BR settimeofday (2)
212 or similar.
213 .TP
214 .BR CLOCK_BOOTTIME_ALARM " (since Linux 3.0; Linux-specific)"
215 Like
216 .BR CLOCK_BOOTTIME .
217 See
218 .BR timer_create (2)
219 for further details.
220 .TP
221 .BR CLOCK_PROCESS_CPUTIME_ID " (since Linux 2.6.12)"
222 This is a clock that measures CPU time consumed by this process
223 (i.e., CPU time consumed by all threads in the process).
224 On Linux, this clock is not settable.
225 .TP
226 .BR CLOCK_THREAD_CPUTIME_ID " (since Linux 2.6.12)"
227 This is a clock that measures CPU time consumed by this thread.
228 On Linux, this clock is not settable.
229 .PP
230 Linux also implements dynamic clock instances as described below.
231 .SS Dynamic clocks
232 In addition to the hard-coded System-V style clock IDs described above,
233 Linux also supports
234 POSIX clock operations on certain character devices.
235 Such devices are
236 called "dynamic" clocks, and are supported since Linux 2.6.39.
237 .PP
238 Using the appropriate macros, open file
239 descriptors may be converted into clock IDs and passed to
240 .BR clock_gettime (),
241 .BR clock_settime (),
242 and
243 .BR clock_adjtime (2).
244 The following example shows how to convert a file descriptor into a
245 dynamic clock ID.
246 .PP
247 .in +4n
248 .EX
249 #define CLOCKFD 3
250 #define FD_TO_CLOCKID(fd) ((\(ti(clockid_t) (fd) << 3) | CLOCKFD)
251 #define CLOCKID_TO_FD(clk) ((unsigned int) \(ti((clk) >> 3))
253 struct timespec ts;
254 clockid_t clkid;
255 int fd;
257 fd = open("/dev/ptp0", O_RDWR);
258 clkid = FD_TO_CLOCKID(fd);
259 clock_gettime(clkid, &ts);
260 .EE
261 .in
262 .SH RETURN VALUE
263 .BR clock_gettime (),
264 .BR clock_settime (),
265 and
266 .BR clock_getres ()
267 return 0 for success.
268 On error, \-1 is returned and
269 .I errno
270 is set to indicate the error.
271 .SH ERRORS
272 .TP
273 .B EACCES
274 .BR clock_settime ()
275 does not have write permission for the dynamic POSIX
276 clock device indicated.
277 .TP
278 .B EFAULT
279 .I tp
280 points outside the accessible address space.
281 .TP
282 .B EINVAL
283 The
284 .I clockid
285 specified is invalid for one of two reasons.
286 Either the System-V style
287 hard coded positive value is out of range, or the dynamic clock ID
288 does not refer to a valid instance of a clock object.
289 .\" Linux also gives this error on attempts to set CLOCK_PROCESS_CPUTIME_ID
290 .\" and CLOCK_THREAD_CPUTIME_ID, when probably the proper error should be
291 .\" EPERM.
292 .TP
293 .B EINVAL
294 .RB ( clock_settime ()):
295 .I tp.tv_sec
296 is negative or
297 .I tp.tv_nsec
298 is outside the range [0..999,999,999].
299 .TP
300 .B EINVAL
301 The
302 .I clockid
303 specified in a call to
304 .BR clock_settime ()
305 is not a settable clock.
306 .TP
307 .BR EINVAL " (since Linux 4.3)"
308 .\" commit e1d7ba8735551ed79c7a0463a042353574b96da3
309 A call to
310 .BR clock_settime ()
311 with a
312 .I clockid
313 of
314 .B CLOCK_REALTIME
315 attempted to set the time to a value less than
316 the current value of the
317 .B CLOCK_MONOTONIC
318 clock.
319 .TP
320 .B ENODEV
321 The hot-pluggable device (like USB for example) represented by a
322 dynamic
323 .I clk_id
324 has disappeared after its character device was opened.
325 .TP
326 .B ENOTSUP
327 The operation is not supported by the dynamic POSIX clock device
328 specified.
329 .TP
330 .B EPERM
331 .BR clock_settime ()
332 does not have permission to set the clock indicated.
333 .SH VERSIONS
334 These system calls first appeared in Linux 2.6.
335 .SH ATTRIBUTES
336 For an explanation of the terms used in this section, see
337 .BR attributes (7).
338 .ad l
339 .nh
340 .TS
341 allbox;
342 lbx lb lb
343 l l l.
344 Interface Attribute Value
345 T{
346 .BR clock_getres (),
347 .BR clock_gettime (),
348 .BR clock_settime ()
349 T} Thread safety MT-Safe
350 .TE
351 .hy
352 .ad
353 .sp 1
354 .SH CONFORMING TO
355 POSIX.1-2001, POSIX.1-2008, SUSv2.
356 .PP
357 On POSIX systems on which these functions are available, the symbol
358 .B _POSIX_TIMERS
359 is defined in \fI<unistd.h>\fP to a value greater than 0.
360 The symbols
361 .BR _POSIX_MONOTONIC_CLOCK ,
362 .BR _POSIX_CPUTIME ,
363 .B _POSIX_THREAD_CPUTIME
364 indicate that
365 .BR CLOCK_MONOTONIC ,
366 .BR CLOCK_PROCESS_CPUTIME_ID ,
367 .B CLOCK_THREAD_CPUTIME_ID
368 are available.
369 (See also
370 .BR sysconf (3).)
371 .SH NOTES
372 POSIX.1 specifies the following:
373 .RS
374 .PP
375 Setting the value of the
376 .B CLOCK_REALTIME
377 clock via
378 .BR clock_settime ()
379 shall have no effect on threads that are blocked waiting for a relative time
380 service based upon this clock, including the
381 .BR nanosleep ()
382 function; nor on the expiration of relative timers based upon this clock.
383 Consequently, these time services shall expire when the requested relative
384 interval elapses, independently of the new or old value of the clock.
385 .RE
386 .PP
387 According to POSIX.1-2001, a process with "appropriate privileges" may set the
388 .B CLOCK_PROCESS_CPUTIME_ID
389 and
390 .B CLOCK_THREAD_CPUTIME_ID
391 clocks using
392 .BR clock_settime ().
393 On Linux, these clocks are not settable
394 (i.e., no process has "appropriate privileges").
395 .\" See http://bugzilla.kernel.org/show_bug.cgi?id=11972
396 .\"
397 .SS C library/kernel differences
398 On some architectures, an implementation of
399 .BR clock_gettime ()
400 is provided in the
401 .BR vdso (7).
402 .\"
403 .SS Historical note for SMP systems
404 Before Linux added kernel support for
405 .B CLOCK_PROCESS_CPUTIME_ID
406 and
407 .BR CLOCK_THREAD_CPUTIME_ID ,
408 glibc implemented these clocks on many platforms using timer
409 registers from the CPUs
410 (TSC on i386, AR.ITC on Itanium).
411 These registers may differ between CPUs and as a consequence
412 these clocks may return
413 .B bogus results
414 if a process is migrated to another CPU.
415 .PP
416 If the CPUs in an SMP system have different clock sources, then
417 there is no way to maintain a correlation between the timer registers since
418 each CPU will run at a slightly different frequency.
419 If that is the case, then
420 .I clock_getcpuclockid(0)
421 will return
422 .B ENOENT
423 to signify this condition.
424 The two clocks will then be useful only if it
425 can be ensured that a process stays on a certain CPU.
426 .PP
427 The processors in an SMP system do not start all at exactly the same
428 time and therefore the timer registers are typically running at an offset.
429 Some architectures include code that attempts to limit these offsets on bootup.
430 However, the code cannot guarantee to accurately tune the offsets.
431 Glibc contains no provisions to deal with these offsets (unlike the Linux
432 Kernel).
433 Typically these offsets are small and therefore the effects may be
434 negligible in most cases.
435 .PP
436 Since glibc 2.4,
437 the wrapper functions for the system calls described in this page avoid
438 the abovementioned problems by employing the kernel implementation of
439 .B CLOCK_PROCESS_CPUTIME_ID
440 and
441 .BR CLOCK_THREAD_CPUTIME_ID ,
442 on systems that provide such an implementation
443 (i.e., Linux 2.6.12 and later).
444 .SH EXAMPLES
445 The program below demonstrates the use of
446 .BR clock_gettime ()
447 and
448 .BR clock_getres ()
449 with various clocks.
450 This is an example of what we might see when running the program:
451 .PP
452 .in +4n
453 .EX
454 $ \fB./clock_times x\fP
455 CLOCK_REALTIME : 1585985459.446 (18356 days + 7h 30m 59s)
456 resolution: 0.000000001
457 CLOCK_TAI : 1585985496.447 (18356 days + 7h 31m 36s)
458 resolution: 0.000000001
459 CLOCK_MONOTONIC: 52395.722 (14h 33m 15s)
460 resolution: 0.000000001
461 CLOCK_BOOTTIME : 72691.019 (20h 11m 31s)
462 resolution: 0.000000001
463 .EE
464 .in
465 .SS Program source
466 \&
467 .EX
468 /* clock_times.c
470 Licensed under GNU General Public License v2 or later.
471 */
472 #define _XOPEN_SOURCE 600
473 #include <time.h>
474 #include <stdint.h>
475 #include <stdio.h>
476 #include <stdlib.h>
477 #include <stdbool.h>
478 #include <unistd.h>
480 #define SECS_IN_DAY (24 * 60 * 60)
482 static void
483 displayClock(clockid_t clock, const char *name, bool showRes)
484 {
485 struct timespec ts;
487 if (clock_gettime(clock, &ts) == \-1) {
488 perror("clock_gettime");
489 exit(EXIT_FAILURE);
490 }
492 printf("%\-15s: %10jd.%03ld (", name,
493 (intmax_t) ts.tv_sec, ts.tv_nsec / 1000000);
495 long days = ts.tv_sec / SECS_IN_DAY;
496 if (days > 0)
497 printf("%ld days + ", days);
499 printf("%2dh %2dm %2ds",
500 (int) (ts.tv_sec % SECS_IN_DAY) / 3600,
501 (int) (ts.tv_sec % 3600) / 60,
502 (int) ts.tv_sec % 60);
503 printf(")\en");
505 if (clock_getres(clock, &ts) == \-1) {
506 perror("clock_getres");
507 exit(EXIT_FAILURE);
508 }
510 if (showRes)
511 printf(" resolution: %10jd.%09ld\en",
512 (intmax_t) ts.tv_sec, ts.tv_nsec);
513 }
515 int
516 main(int argc, char *argv[])
517 {
518 bool showRes = argc > 1;
520 displayClock(CLOCK_REALTIME, "CLOCK_REALTIME", showRes);
521 #ifdef CLOCK_TAI
522 displayClock(CLOCK_TAI, "CLOCK_TAI", showRes);
523 #endif
524 displayClock(CLOCK_MONOTONIC, "CLOCK_MONOTONIC", showRes);
525 #ifdef CLOCK_BOOTTIME
526 displayClock(CLOCK_BOOTTIME, "CLOCK_BOOTTIME", showRes);
527 #endif
528 exit(EXIT_SUCCESS);
529 }
530 .EE
531 .SH SEE ALSO
532 .BR date (1),
533 .BR gettimeofday (2),
534 .BR settimeofday (2),
535 .BR time (2),
536 .BR adjtime (3),
537 .BR clock_getcpuclockid (3),
538 .BR ctime (3),
539 .BR ftime (3),
540 .BR pthread_getcpuclockid (3),
541 .BR sysconf (3),
542 .BR time (7),
543 .BR time_namespaces (7),
544 .BR vdso (7),
545 .BR hwclock (8)