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 clk_id, struct timespec *res);
12 int clock_gettime(clockid_t clk_id, struct timespec *tp);
14 int clock_settime(clockid_t clk_id, 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 clk_id, 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 clk_id.
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 clk_id 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 System-wide clock that measures real (i.e., wall-clock) time.
62 Setting this clock requires appropriate privileges. This clock
63 is affected by discontinuous jumps in the system time (e.g., if
64 the system administrator manually changes the clock), and by the
65 incremental adjustments performed by adjtime(3) and NTP.
66 CLOCK_REALTIME_COARSE (since Linux 2.6.32; Linux-specific)
68 A faster but less precise version of CLOCK_REALTIME. Use when
69 you need very fast, but not fine-grained timestamps. Requires
70 per-architecture support, and probably also architecture support
71 for this flag in the vdso(7).
72 CLOCK_MONOTONIC
74 Clock that cannot be set and represents monotonic time since—as
75 described by POSIX—"some unspecified point in the past". On
76 Linux, that point corresponds to the number of seconds that the
77 system has been running since it was booted.
78 The CLOCK_MONOTONIC clock is not affected by discontinuous jumps
80 in the system time (e.g., if the system administrator manually
81 changes the clock), but is affected by the incremental adjust‐
82 ments performed by adjtime(3) and NTP. This clock does not
83 count time that the system is suspended.
84 CLOCK_MONOTONIC_COARSE (since Linux 2.6.32; Linux-specific)
86 A faster but less precise version of CLOCK_MONOTONIC. Use when
87 you need very fast, but not fine-grained timestamps. Requires
88 per-architecture support, and probably also architecture support
89 for this flag in the vdso(7).
90 CLOCK_MONOTONIC_RAW (since Linux 2.6.28; Linux-specific)
92 Similar to CLOCK_MONOTONIC, but provides access to a raw hard‐
93 ware-based time that is not subject to NTP adjustments or the
94 incremental adjustments performed by adjtime(3). This clock
95 does not count time that the system is suspended.
96 CLOCK_BOOTTIME (since Linux 2.6.39; Linux-specific)
98 Identical to CLOCK_MONOTONIC, except it also includes any time
99 that the system is suspended. This allows applications to get a
100 suspend-aware monotonic clock without having to deal with the
101 complications of CLOCK_REALTIME, which may have discontinuities
102 if the time is changed using settimeofday(2) or similar.
103 CLOCK_PROCESS_CPUTIME_ID (since Linux 2.6.12)
105 Per-process CPU-time clock (measures CPU time consumed by all
106 threads in the process).
107 CLOCK_THREAD_CPUTIME_ID (since Linux 2.6.12)
109 Thread-specific CPU-time clock.
110
112 clock_gettime(), clock_settime(), and clock_getres() return 0 for suc‐
113 cess, or -1 for failure (in which case errno is set appropriately).
114
116 EFAULT tp points outside the accessible address space.
117 EINVAL The clk_id specified is not supported on this system.
119 EINVAL (clock_settime()): tp.tv_sec is negative or tp.tv_nsec is out‐
121 side the range [0..999,999,999].
122 EINVAL (since Linux 4.3)
124 A call to clock_settime() with a clk_id of CLOCK_REALTIME
125 attempted to set the time to a value less than the current value
126 of the CLOCK_MONOTONIC clock.
127 EPERM clock_settime() does not have permission to set the clock indi‐
129 cated.
130
132 These system calls first appeared in Linux 2.6.
133
135 For an explanation of the terms used in this section, see
136 attributes(7).
137 ┌─────────────────────────────────┬───────────────┬─────────┐
139 │Interface │ Attribute │ Value │
140 ├─────────────────────────────────┼───────────────┼─────────┤
141 │clock_getres(), clock_gettime(), │ Thread safety │ MT-Safe │
142 │clock_settime() │ │ │
143 └─────────────────────────────────┴───────────────┴─────────┘
144
146 POSIX.1-2001, POSIX.1-2008, SUSv2.
147
149 On POSIX systems on which these functions are available, the symbol
150 _POSIX_TIMERS is defined in <unistd.h> to a value greater than 0. The
151 symbols _POSIX_MONOTONIC_CLOCK, _POSIX_CPUTIME, _POSIX_THREAD_CPUTIME
152 indicate that CLOCK_MONOTONIC, CLOCK_PROCESS_CPUTIME_ID,
153 CLOCK_THREAD_CPUTIME_ID are available. (See also sysconf(3).)
154
156 POSIX.1 specifies the following:
157 Setting the value of the CLOCK_REALTIME clock via clock_set‐
159 time() shall have no effect on threads that are blocked waiting
160 for a relative time service based upon this clock, including the
161 nanosleep() function; nor on the expiration of relative timers
162 based upon this clock. Consequently, these time services shall
163 expire when the requested relative interval elapses, indepen‐
164 dently of the new or old value of the clock.
165 C library/kernel differences
167 On some architectures, an implementation of clock_gettime() is provided
168 in the vdso(7).
169 Historical note for SMP systems
171 Before Linux added kernel support for CLOCK_PROCESS_CPUTIME_ID and
172 CLOCK_THREAD_CPUTIME_ID, glibc implemented these clocks on many plat‐
173 forms using timer registers from the CPUs (TSC on i386, AR.ITC on Ita‐
174 nium). These registers may differ between CPUs and as a consequence
175 these clocks may return bogus results if a process is migrated to
176 another CPU.
177 If the CPUs in an SMP system have different clock sources, then there
179 is no way to maintain a correlation between the timer registers since
180 each CPU will run at a slightly different frequency. If that is the
181 case, then clock_getcpuclockid(0) will return ENOENT to signify this
182 condition. The two clocks will then be useful only if it can be
183 ensured that a process stays on a certain CPU.
184 The processors in an SMP system do not start all at exactly the same
186 time and therefore the timer registers are typically running at an off‐
187 set. Some architectures include code that attempts to limit these off‐
188 sets on bootup. However, the code cannot guarantee to accurately tune
189 the offsets. Glibc contains no provisions to deal with these offsets
190 (unlike the Linux Kernel). Typically these offsets are small and
191 therefore the effects may be negligible in most cases.
192 Since glibc 2.4, the wrapper functions for the system calls described
194 in this page avoid the abovementioned problems by employing the kernel
195 implementation of CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID,
196 on systems that provide such an implementation (i.e., Linux 2.6.12 and
197 later).
198
200 According to POSIX.1-2001, a process with "appropriate privileges" may
201 set the CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID clocks
202 using clock_settime(). On Linux, these clocks are not settable (i.e.,
203 no process has "appropriate privileges").
204
206 date(1), gettimeofday(2), settimeofday(2), time(2), adjtime(3),
207 clock_getcpuclockid(3), ctime(3), ftime(3), pthread_getcpuclockid(3),
208 sysconf(3), time(7), vdso(7), hwclock(8)
209
211 This page is part of release 5.02 of the Linux man-pages project. A
212 description of the project, information about reporting bugs, and the
213 latest version of this page, can be found at
214 https://www.kernel.org/doc/man-pages/.
215 2019-03-06 CLOCK_GETRES(2)