1clock_getres(2) System Calls Manual clock_getres(2)
2
6 clock_getres, clock_gettime, clock_settime - clock and time functions
7
9 Standard C library (libc, -lc), since glibc 2.17
10 Before glibc 2.17, Real-time library (librt, -lrt)
12
14 #include <time.h>
15 int clock_getres(clockid_t clockid, struct timespec *_Nullable res);
17 int clock_gettime(clockid_t clockid, struct timespec *tp);
19 int clock_settime(clockid_t clockid, const struct timespec *tp);
20 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
22 clock_getres(), clock_gettime(), clock_settime():
24 _POSIX_C_SOURCE >= 199309L
25
27 The function clock_getres() finds the resolution (precision) of the
28 specified clock clockid, and, if res is non-NULL, stores it in the
29 struct timespec pointed to by res. The resolution of clocks depends on
30 the implementation and cannot be configured by a particular process.
31 If the time value pointed to by the argument tp of clock_settime() is
32 not a multiple of res, then it is truncated to a multiple of res.
33 The functions clock_gettime() and clock_settime() retrieve and set the
35 time of the specified clock clockid.
36 The res and tp arguments are timespec(3) structures.
38 The clockid argument is the identifier of the particular clock on which
40 to act. A clock may be system-wide and hence visible for all pro‐
41 cesses, or per-process if it measures time only within a single
42 process.
43 All implementations support the system-wide real-time clock, which is
45 identified by CLOCK_REALTIME. Its time represents seconds and nanosec‐
46 onds since the Epoch. When its time is changed, timers for a relative
47 interval are unaffected, but timers for an absolute point in time are
48 affected.
49 More clocks may be implemented. The interpretation of the correspond‐
51 ing time values and the effect on timers is unspecified.
52 Sufficiently recent versions of glibc and the Linux kernel support the
54 following clocks:
55 CLOCK_REALTIME
57 A settable system-wide clock that measures real (i.e., wall-
58 clock) time. Setting this clock requires appropriate privi‐
59 leges. This clock is affected by discontinuous jumps in the
60 system time (e.g., if the system administrator manually changes
61 the clock), and by the incremental adjustments performed by adj‐
62 time(3) and NTP.
63 CLOCK_REALTIME_ALARM (since Linux 3.0; Linux-specific)
65 Like CLOCK_REALTIME, but not settable. See timer_create(2) for
66 further details.
67 CLOCK_REALTIME_COARSE (since Linux 2.6.32; Linux-specific)
69 A faster but less precise version of CLOCK_REALTIME. This clock
70 is not settable. Use when you need very fast, but not fine-
71 grained timestamps. Requires per-architecture support, and
72 probably also architecture support for this flag in the vdso(7).
73 CLOCK_TAI (since Linux 3.10; Linux-specific)
75 A nonsettable system-wide clock derived from wall-clock time but
76 ignoring leap seconds. This clock does not experience disconti‐
77 nuities and backwards jumps caused by NTP inserting leap seconds
78 as CLOCK_REALTIME does.
79 The acronym TAI refers to International Atomic Time.
81 CLOCK_MONOTONIC
83 A nonsettable system-wide clock that represents monotonic time
84 since—as described by POSIX—"some unspecified point in the
85 past". On Linux, that point corresponds to the number of sec‐
86 onds that the system has been running since it was booted.
87 The CLOCK_MONOTONIC clock is not affected by discontinuous jumps
89 in the system time (e.g., if the system administrator manually
90 changes the clock), but is affected by the incremental adjust‐
91 ments performed by adjtime(3) and NTP. This clock does not
92 count time that the system is suspended. All CLOCK_MONOTONIC
93 variants guarantee that the time returned by consecutive calls
94 will not go backwards, but successive calls may—depending on the
95 architecture—return identical (not-increased) time values.
96 CLOCK_MONOTONIC_COARSE (since Linux 2.6.32; Linux-specific)
98 A faster but less precise version of CLOCK_MONOTONIC. Use when
99 you need very fast, but not fine-grained timestamps. Requires
100 per-architecture support, and probably also architecture support
101 for this flag in the vdso(7).
102 CLOCK_MONOTONIC_RAW (since Linux 2.6.28; Linux-specific)
104 Similar to CLOCK_MONOTONIC, but provides access to a raw hard‐
105 ware-based time that is not subject to NTP adjustments or the
106 incremental adjustments performed by adjtime(3). This clock
107 does not count time that the system is suspended.
108 CLOCK_BOOTTIME (since Linux 2.6.39; Linux-specific)
110 A nonsettable system-wide clock that is identical to CLOCK_MONO‐
111 TONIC, except that it also includes any time that the system is
112 suspended. This allows applications to get a suspend-aware
113 monotonic clock without having to deal with the complications of
114 CLOCK_REALTIME, which may have discontinuities if the time is
115 changed using settimeofday(2) or similar.
116 CLOCK_BOOTTIME_ALARM (since Linux 3.0; Linux-specific)
118 Like CLOCK_BOOTTIME. See timer_create(2) for further details.
119 CLOCK_PROCESS_CPUTIME_ID (since Linux 2.6.12)
121 This is a clock that measures CPU time consumed by this process
122 (i.e., CPU time consumed by all threads in the process). On
123 Linux, this clock is not settable.
124 CLOCK_THREAD_CPUTIME_ID (since Linux 2.6.12)
126 This is a clock that measures CPU time consumed by this thread.
127 On Linux, this clock is not settable.
128 Linux also implements dynamic clock instances as described below.
130 Dynamic clocks
132 In addition to the hard-coded System-V style clock IDs described above,
133 Linux also supports POSIX clock operations on certain character de‐
134 vices. Such devices are called "dynamic" clocks, and are supported
135 since Linux 2.6.39.
136 Using the appropriate macros, open file descriptors may be converted
138 into clock IDs and passed to clock_gettime(), clock_settime(), and
139 clock_adjtime(2). The following example shows how to convert a file
140 descriptor into a dynamic clock ID.
141 #define CLOCKFD 3
143 #define FD_TO_CLOCKID(fd) ((~(clockid_t) (fd) << 3) | CLOCKFD)
144 #define CLOCKID_TO_FD(clk) ((unsigned int) ~((clk) >> 3))
145 struct timespec ts;
147 clockid_t clkid;
148 int fd;
149 fd = open("/dev/ptp0", O_RDWR);
151 clkid = FD_TO_CLOCKID(fd);
152 clock_gettime(clkid, &ts);
153
155 clock_gettime(), clock_settime(), and clock_getres() return 0 for suc‐
156 cess. On error, -1 is returned and errno is set to indicate the error.
157
159 EACCES clock_settime() does not have write permission for the dynamic
160 POSIX clock device indicated.
161 EFAULT tp points outside the accessible address space.
163 EINVAL The clockid specified is invalid for one of two reasons. Either
165 the System-V style hard coded positive value is out of range, or
166 the dynamic clock ID does not refer to a valid instance of a
167 clock object.
168 EINVAL (clock_settime()): tp.tv_sec is negative or tp.tv_nsec is out‐
170 side the range [0, 999,999,999].
171 EINVAL The clockid specified in a call to clock_settime() is not a set‐
173 table clock.
174 EINVAL (since Linux 4.3)
176 A call to clock_settime() with a clockid of CLOCK_REALTIME at‐
177 tempted to set the time to a value less than the current value
178 of the CLOCK_MONOTONIC clock.
179 ENODEV The hot-pluggable device (like USB for example) represented by a
181 dynamic clk_id has disappeared after its character device was
182 opened.
183 ENOTSUP
185 The operation is not supported by the dynamic POSIX clock device
186 specified.
187 EPERM clock_settime() does not have permission to set the clock indi‐
189 cated.
190
192 For an explanation of the terms used in this section, see at‐
193 tributes(7).
194 ┌────────────────────────────────────────────┬───────────────┬─────────┐
196 │Interface │ Attribute │ Value │
197 ├────────────────────────────────────────────┼───────────────┼─────────┤
198 │clock_getres(), clock_gettime(), │ Thread safety │ MT-Safe │
199 │clock_settime() │ │ │
200 └────────────────────────────────────────────┴───────────────┴─────────┘
201
203 POSIX.1 specifies the following:
204 Setting the value of the CLOCK_REALTIME clock via
206 clock_settime() shall have no effect on threads that are blocked
207 waiting for a relative time service based upon this clock,
208 including the nanosleep() function; nor on the expiration of
209 relative timers based upon this clock. Consequently, these time
210 services shall expire when the requested relative interval
211 elapses, independently of the new or old value of the clock.
212 According to POSIX.1-2001, a process with "appropriate privileges" may
214 set the CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID clocks
215 using clock_settime(). On Linux, these clocks are not settable (i.e.,
216 no process has "appropriate privileges").
217 C library/kernel differences
219 On some architectures, an implementation of clock_gettime() is provided
220 in the vdso(7).
221
223 POSIX.1-2008.
224
226 POSIX.1-2001, SUSv2. Linux 2.6.
227 On POSIX systems on which these functions are available, the symbol
229 _POSIX_TIMERS is defined in <unistd.h> to a value greater than 0. The
230 symbols _POSIX_MONOTONIC_CLOCK, _POSIX_CPUTIME, _POSIX_THREAD_CPUTIME
231 indicate that CLOCK_MONOTONIC, CLOCK_PROCESS_CPUTIME_ID,
232 CLOCK_THREAD_CPUTIME_ID are available. (See also sysconf(3).)
233 POSIX.1-2008 makes these APIs mandatory.
234 Historical note for SMP systems
236 Before Linux added kernel support for CLOCK_PROCESS_CPUTIME_ID and
237 CLOCK_THREAD_CPUTIME_ID, glibc implemented these clocks on many
238 platforms using timer registers from the CPUs (TSC on i386, AR.ITC on
239 Itanium). These registers may differ between CPUs and as a consequence
240 these clocks may return bogus results if a process is migrated to
241 another CPU.
242 If the CPUs in an SMP system have different clock sources, then there
244 is no way to maintain a correlation between the timer registers since
245 each CPU will run at a slightly different frequency. If that is the
246 case, then clock_getcpuclockid(0) will return ENOENT to signify this
247 condition. The two clocks will then be useful only if it can be
248 ensured that a process stays on a certain CPU.
249 The processors in an SMP system do not start all at exactly the same
251 time and therefore the timer registers are typically running at an
252 offset. Some architectures include code that attempts to limit these
253 offsets on bootup. However, the code cannot guarantee to accurately
254 tune the offsets. glibc contains no provisions to deal with these
255 offsets (unlike the Linux Kernel). Typically these offsets are small
256 and therefore the effects may be negligible in most cases.
257 Since glibc 2.4, the wrapper functions for the system calls described
259 in this page avoid the abovementioned problems by employing the kernel
260 implementation of CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID,
261 on systems that provide such an implementation (i.e., Linux 2.6.12 and
262 later).
263
265 The program below demonstrates the use of clock_gettime() and
266 clock_getres() with various clocks. This is an example of what we
267 might see when running the program:
268 $ ./clock_times x
270 CLOCK_REALTIME : 1585985459.446 (18356 days + 7h 30m 59s)
271 resolution: 0.000000001
272 CLOCK_TAI : 1585985496.447 (18356 days + 7h 31m 36s)
273 resolution: 0.000000001
274 CLOCK_MONOTONIC: 52395.722 (14h 33m 15s)
275 resolution: 0.000000001
276 CLOCK_BOOTTIME : 72691.019 (20h 11m 31s)
277 resolution: 0.000000001
278 Program source
280 /* clock_times.c
282 Licensed under GNU General Public License v2 or later.
284 */
285 #define _XOPEN_SOURCE 600
286 #include <stdbool.h>
287 #include <stdint.h>
288 #include <stdio.h>
289 #include <stdlib.h>
290 #include <time.h>
291 #define SECS_IN_DAY (24 * 60 * 60)
293 static void
295 displayClock(clockid_t clock, const char *name, bool showRes)
296 {
297 long days;
298 struct timespec ts;
299 if (clock_gettime(clock, &ts) == -1) {
301 perror("clock_gettime");
302 exit(EXIT_FAILURE);
303 }
304 printf("%-15s: %10jd.%03ld (", name,
306 (intmax_t) ts.tv_sec, ts.tv_nsec / 1000000);
307 days = ts.tv_sec / SECS_IN_DAY;
309 if (days > 0)
310 printf("%ld days + ", days);
311 printf("%2dh %2dm %2ds",
313 (int) (ts.tv_sec % SECS_IN_DAY) / 3600,
314 (int) (ts.tv_sec % 3600) / 60,
315 (int) 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: %10jd.%09ld\n",
325 (intmax_t) 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), timespec(3), time(7), time_namespaces(7), vdso(7), hw‐
348 clock(8)
349Linux man-pages 6.05 2023-07-20 clock_getres(2)