1CLOCK_GETRES(2) Linux Programmer's Manual CLOCK_GETRES(2)
2
6 clock_getres, clock_gettime, clock_settime - clock and time functions
7
9 #include <time.h>
10 int clock_getres(clockid_t clockid, struct timespec *res);
12 int clock_gettime(clockid_t clockid, struct timespec *tp);
14 int clock_settime(clockid_t clockid, const struct timespec *tp);
16 Link with -lrt (only for glibc versions before 2.17).
18 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
20 clock_getres(), clock_gettime(), clock_settime():
22 _POSIX_C_SOURCE >= 199309L
23
25 The function clock_getres() finds the resolution (precision) of the
26 specified clock clockid, and, if res is non-NULL, stores it in the
27 struct timespec pointed to by res. The resolution of clocks depends on
28 the implementation and cannot be configured by a particular process.
29 If the time value pointed to by the argument tp of clock_settime() is
30 not a multiple of res, then it is truncated to a multiple of res.
31 The functions clock_gettime() and clock_settime() retrieve and set the
33 time of the specified clock clockid.
34 The res and tp arguments are timespec structures, as specified in
36 <time.h>:
37 struct timespec {
39 time_t tv_sec; /* seconds */
40 long tv_nsec; /* nanoseconds */
41 };
42 The clockid argument is the identifier of the particular clock on which
44 to act. A clock may be system-wide and hence visible for all pro‐
45 cesses, or per-process if it measures time only within a single
46 process.
47 All implementations support the system-wide real-time clock, which is
49 identified by CLOCK_REALTIME. Its time represents seconds and nanosec‐
50 onds since the Epoch. When its time is changed, timers for a relative
51 interval are unaffected, but timers for an absolute point in time are
52 affected.
53 More clocks may be implemented. The interpretation of the correspond‐
55 ing time values and the effect on timers is unspecified.
56 Sufficiently recent versions of glibc and the Linux kernel support the
58 following clocks:
59 CLOCK_REALTIME
61 A settable system-wide clock that measures real (i.e., wall-
62 clock) time. Setting this clock requires appropriate privi‐
63 leges. This clock is affected by discontinuous jumps in the
64 system time (e.g., if the system administrator manually changes
65 the clock), and by the incremental adjustments performed by adj‐
66 time(3) and NTP.
67 CLOCK_REALTIME_ALARM (since Linux 3.0; Linux-specific)
69 Like CLOCK_REALTIME, but not settable. See timer_create(2) for
70 further details.
71 CLOCK_REALTIME_COARSE (since Linux 2.6.32; Linux-specific)
73 A faster but less precise version of CLOCK_REALTIME. This clock
74 is not settable. Use when you need very fast, but not fine-
75 grained timestamps. Requires per-architecture support, and
76 probably also architecture support for this flag in the vdso(7).
77 CLOCK_TAI (since Linux 3.10; Linux-specific)
79 A nonsettable system-wide clock derived from wall-clock time but
80 ignoring leap seconds. This clock does not experience disconti‐
81 nuities and backwards jumps caused by NTP inserting leap seconds
82 as CLOCK_REALTIME does.
83 The acronym TAI refers to International Atomic Time.
85 CLOCK_MONOTONIC
87 A nonsettable system-wide clock that represents monotonic time
88 since—as described by POSIX—"some unspecified point in the
89 past". On Linux, that point corresponds to the number of sec‐
90 onds that the system has been running since it was booted.
91 The CLOCK_MONOTONIC clock is not affected by discontinuous jumps
93 in the system time (e.g., if the system administrator manually
94 changes the clock), but is affected by the incremental adjust‐
95 ments performed by adjtime(3) and NTP. This clock does not
96 count time that the system is suspended. All CLOCK_MONOTONIC
97 variants guarantee that the time returned by consecutive calls
98 will not go backwards, but successive calls may—depending on the
99 architecture—return identical (not-increased) time values.
100 CLOCK_MONOTONIC_COARSE (since Linux 2.6.32; Linux-specific)
102 A faster but less precise version of CLOCK_MONOTONIC. Use when
103 you need very fast, but not fine-grained timestamps. Requires
104 per-architecture support, and probably also architecture support
105 for this flag in the vdso(7).
106 CLOCK_MONOTONIC_RAW (since Linux 2.6.28; Linux-specific)
108 Similar to CLOCK_MONOTONIC, but provides access to a raw hard‐
109 ware-based time that is not subject to NTP adjustments or the
110 incremental adjustments performed by adjtime(3). This clock
111 does not count time that the system is suspended.
112 CLOCK_BOOTTIME (since Linux 2.6.39; Linux-specific)
114 A nonsettable system-wide clock that is identical to CLOCK_MONO‐
115 TONIC, except that it also includes any time that the system is
116 suspended. This allows applications to get a suspend-aware
117 monotonic clock without having to deal with the complications of
118 CLOCK_REALTIME, which may have discontinuities if the time is
119 changed using settimeofday(2) or similar.
120 CLOCK_BOOTTIME_ALARM (since Linux 3.0; Linux-specific)
122 Like CLOCK_BOOTTIME. See timer_create(2) for further details.
123 CLOCK_PROCESS_CPUTIME_ID (since Linux 2.6.12)
125 This is a clock that measures CPU time consumed by this process
126 (i.e., CPU time consumed by all threads in the process). On
127 Linux, this clock is not settable.
128 CLOCK_THREAD_CPUTIME_ID (since Linux 2.6.12)
130 This is a clock that measures CPU time consumed by this thread.
131 On Linux, this clock is not settable.
132 Linux also implements dynamic clock instances as described below.
134 Dynamic clocks
136 In addition to the hard-coded System-V style clock IDs described above,
137 Linux also supports POSIX clock operations on certain character
138 devices. Such devices are called "dynamic" clocks, and are supported
139 since Linux 2.6.39.
140 Using the appropriate macros, open file descriptors may be converted
142 into clock IDs and passed to clock_gettime(), clock_settime(), and
143 clock_adjtime(2). The following example shows how to convert a file
144 descriptor into a dynamic clock ID.
145 #define CLOCKFD 3
147 #define FD_TO_CLOCKID(fd) ((~(clockid_t) (fd) << 3) | CLOCKFD)
148 #define CLOCKID_TO_FD(clk) ((unsigned int) ~((clk) >> 3))
149 struct timeval tv;
151 clockid_t clkid;
152 int fd;
153 fd = open("/dev/ptp0", O_RDWR);
155 clkid = FD_TO_CLOCKID(fd);
156 clock_gettime(clkid, &tv);
157
159 clock_gettime(), clock_settime(), and clock_getres() return 0 for suc‐
160 cess, or -1 for failure (in which case errno is set appropriately).
161
163 EFAULT tp points outside the accessible address space.
164 EINVAL The clockid specified is invalid for one of two reasons. Either
166 the System-V style hard coded positive value is out of range, or
167 the dynamic clock ID does not refer to a valid instance of a
168 clock object.
169 EINVAL (clock_settime()): tp.tv_sec is negative or tp.tv_nsec is out‐
171 side the range [0..999,999,999].
172 EINVAL The clockid specified in a call to clock_settime() is not a set‐
174 table clock.
175 ENOTSUP
177 The operation is not supported by the dynamic POSIX clock device
178 specified.
179 EINVAL (since Linux 4.3)
181 A call to clock_settime() with a clockid of CLOCK_REALTIME
182 attempted to set the time to a value less than the current value
183 of the CLOCK_MONOTONIC clock.
184 ENODEV The hot-pluggable device (like USB for example) represented by a
186 dynamic clk_id has disappeared after its character device was
187 opened.
188 EPERM clock_settime() does not have permission to set the clock indi‐
190 cated.
191 EACCES clock_settime() does not have write permission for the dynamic
193 POSIX clock device indicated.
194
196 These system calls first appeared in Linux 2.6.
197
199 For an explanation of the terms used in this section, see
200 attributes(7).
201 ┌─────────────────────────────────┬───────────────┬─────────┐
203 │Interface │ Attribute │ Value │
204 ├─────────────────────────────────┼───────────────┼─────────┤
205 │clock_getres(), clock_gettime(), │ Thread safety │ MT-Safe │
206 │clock_settime() │ │ │
207 └─────────────────────────────────┴───────────────┴─────────┘
208
210 POSIX.1-2001, POSIX.1-2008, SUSv2.
211 On POSIX systems on which these functions are available, the symbol
213 _POSIX_TIMERS is defined in <unistd.h> to a value greater than 0. The
214 symbols _POSIX_MONOTONIC_CLOCK, _POSIX_CPUTIME, _POSIX_THREAD_CPUTIME
215 indicate that CLOCK_MONOTONIC, CLOCK_PROCESS_CPUTIME_ID,
216 CLOCK_THREAD_CPUTIME_ID are available. (See also sysconf(3).)
217
219 POSIX.1 specifies the following:
220 Setting the value of the CLOCK_REALTIME clock via clock_set‐
222 time() shall have no effect on threads that are blocked waiting
223 for a relative time service based upon this clock, including the
224 nanosleep() function; nor on the expiration of relative timers
225 based upon this clock. Consequently, these time services shall
226 expire when the requested relative interval elapses, indepen‐
227 dently of the new or old value of the clock.
228 According to POSIX.1-2001, a process with "appropriate privileges" may
230 set the CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID clocks
231 using clock_settime(). On Linux, these clocks are not settable (i.e.,
232 no process has "appropriate privileges").
233 C library/kernel differences
235 On some architectures, an implementation of clock_gettime() is provided
236 in the vdso(7).
237 Historical note for SMP systems
239 Before Linux added kernel support for CLOCK_PROCESS_CPUTIME_ID and
240 CLOCK_THREAD_CPUTIME_ID, glibc implemented these clocks on many plat‐
241 forms using timer registers from the CPUs (TSC on i386, AR.ITC on Ita‐
242 nium). These registers may differ between CPUs and as a consequence
243 these clocks may return bogus results if a process is migrated to
244 another CPU.
245 If the CPUs in an SMP system have different clock sources, then there
247 is no way to maintain a correlation between the timer registers since
248 each CPU will run at a slightly different frequency. If that is the
249 case, then clock_getcpuclockid(0) will return ENOENT to signify this
250 condition. The two clocks will then be useful only if it can be
251 ensured that a process stays on a certain CPU.
252 The processors in an SMP system do not start all at exactly the same
254 time and therefore the timer registers are typically running at an off‐
255 set. Some architectures include code that attempts to limit these off‐
256 sets on bootup. However, the code cannot guarantee to accurately tune
257 the offsets. Glibc contains no provisions to deal with these offsets
258 (unlike the Linux Kernel). Typically these offsets are small and
259 therefore the effects may be negligible in most cases.
260 Since glibc 2.4, the wrapper functions for the system calls described
262 in this page avoid the abovementioned problems by employing the kernel
263 implementation of CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID,
264 on systems that provide such an implementation (i.e., Linux 2.6.12 and
265 later).
266
268 The program below demonstrates the use of clock_gettime() and
269 clock_getres() with various clocks. This is an example of what we
270 might see when running the program:
271 $ ./clock_times x
273 CLOCK_REALTIME : 1585985459.446 (18356 days + 7h 30m 59s)
274 resolution: 0.000000001
275 CLOCK_TAI : 1585985496.447 (18356 days + 7h 31m 36s)
276 resolution: 0.000000001
277 CLOCK_MONOTONIC: 52395.722 (14h 33m 15s)
278 resolution: 0.000000001
279 CLOCK_BOOTTIME : 72691.019 (20h 11m 31s)
280 resolution: 0.000000001
281 Program source
283 /* clock_times.c
285 Licensed under GNU General Public License v2 or later.
287 */
288 #define _XOPEN_SOURCE 600
289 #include <time.h>
290 #include <stdio.h>
291 #include <stdlib.h>
292 #include <stdbool.h>
293 #include <unistd.h>
294 #define SECS_IN_DAY (24 * 60 * 60)
296 static void
298 displayClock(clockid_t clock, char *name, bool showRes)
299 {
300 struct timespec ts;
301 if (clock_gettime(clock, &ts) == -1) {
303 perror("clock_gettime");
304 exit(EXIT_FAILURE);
305 }
306 printf("%-15s: %10ld.%03ld (", name,
308 (long) ts.tv_sec, ts.tv_nsec / 1000000);
309 long days = ts.tv_sec / SECS_IN_DAY;
311 if (days > 0)
312 printf("%ld days + ", days);
313 printf("%2ldh %2ldm %2lds", (ts.tv_sec % SECS_IN_DAY) / 3600,
315 (ts.tv_sec % 3600) / 60, ts.tv_sec % 60);
316 printf(")\n");
317 if (clock_getres(clock, &ts) == -1) {
319 perror("clock_getres");
320 exit(EXIT_FAILURE);
321 }
322 if (showRes)
324 printf(" resolution: %10ld.%09ld\n",
325 (long) ts.tv_sec, ts.tv_nsec);
326 }
327 int
329 main(int argc, char *argv[])
330 {
331 bool showRes = argc > 1;
332 displayClock(CLOCK_REALTIME, "CLOCK_REALTIME", showRes);
334 #ifdef CLOCK_TAI
335 displayClock(CLOCK_TAI, "CLOCK_TAI", showRes);
336 #endif
337 displayClock(CLOCK_MONOTONIC, "CLOCK_MONOTONIC", showRes);
338 #ifdef CLOCK_BOOTTIME
339 displayClock(CLOCK_BOOTTIME, "CLOCK_BOOTTIME", showRes);
340 #endif
341 exit(EXIT_SUCCESS);
342 }
343
345 date(1), gettimeofday(2), settimeofday(2), time(2), adjtime(3),
346 clock_getcpuclockid(3), ctime(3), ftime(3), pthread_getcpuclockid(3),
347 sysconf(3), time(7), time_namespaces(7), vdso(7), hwclock(8)
348
350 This page is part of release 5.07 of the Linux man-pages project. A
351 description of the project, information about reporting bugs, and the
352 latest version of this page, can be found at
353 https://www.kernel.org/doc/man-pages/.
354 2020-04-11 CLOCK_GETRES(2)