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
75 some unspecified starting point. This clock is not affected by
76 discontinuous jumps in the system time (e.g., if the system
77 administrator manually changes the clock), but is affected by
78 the incremental adjustments performed by adjtime(3) and NTP.
79 CLOCK_MONOTONIC_COARSE (since Linux 2.6.32; Linux-specific)
81 A faster but less precise version of CLOCK_MONOTONIC. Use when
82 you need very fast, but not fine-grained timestamps. Requires
83 per-architecture support, and probably also architecture support
84 for this flag in the vdso(7).
85 CLOCK_MONOTONIC_RAW (since Linux 2.6.28; Linux-specific)
87 Similar to CLOCK_MONOTONIC, but provides access to a raw hard‐
88 ware-based time that is not subject to NTP adjustments or the
89 incremental adjustments performed by adjtime(3).
90 CLOCK_BOOTTIME (since Linux 2.6.39; Linux-specific)
92 Identical to CLOCK_MONOTONIC, except it also includes any time
93 that the system is suspended. This allows applications to get a
94 suspend-aware monotonic clock without having to deal with the
95 complications of CLOCK_REALTIME, which may have discontinuities
96 if the time is changed using settimeofday(2) or similar.
97 CLOCK_PROCESS_CPUTIME_ID (since Linux 2.6.12)
99 Per-process CPU-time clock (measures CPU time consumed by all
100 threads in the process).
101 CLOCK_THREAD_CPUTIME_ID (since Linux 2.6.12)
103 Thread-specific CPU-time clock.
104
106 clock_gettime(), clock_settime(), and clock_getres() return 0 for suc‐
107 cess, or -1 for failure (in which case errno is set appropriately).
108
110 EFAULT tp points outside the accessible address space.
111 EINVAL The clk_id specified is not supported on this system.
113 EPERM clock_settime() does not have permission to set the clock indi‐
115 cated.
116
118 These system calls first appeared in Linux 2.6.
119
121 For an explanation of the terms used in this section, see
122 attributes(7).
123 ┌─────────────────────────────────┬───────────────┬─────────┐
125 │Interface │ Attribute │ Value │
126 ├─────────────────────────────────┼───────────────┼─────────┤
127 │clock_getres(), clock_gettime(), │ Thread safety │ MT-Safe │
128 │clock_settime() │ │ │
129 └─────────────────────────────────┴───────────────┴─────────┘
130
132 POSIX.1-2001, POSIX.1-2008, SUSv2.
133
135 On POSIX systems on which these functions are available, the symbol
136 _POSIX_TIMERS is defined in <unistd.h> to a value greater than 0. The
137 symbols _POSIX_MONOTONIC_CLOCK, _POSIX_CPUTIME, _POSIX_THREAD_CPUTIME
138 indicate that CLOCK_MONOTONIC, CLOCK_PROCESS_CPUTIME_ID,
139 CLOCK_THREAD_CPUTIME_ID are available. (See also sysconf(3).)
140
142 POSIX.1 specifies the following:
143 Setting the value of the CLOCK_REALTIME clock via clock_set‐
145 time() shall have no effect on threads that are blocked waiting
146 for a relative time service based upon this clock, including the
147 nanosleep() function; nor on the expiration of relative timers
148 based upon this clock. Consequently, these time services shall
149 expire when the requested relative interval elapses, indepen‐
150 dently of the new or old value of the clock.
151 C library/kernel differences
153 On some architectures, an implementation of clock_gettime() is provided
154 in the vdso(7).
155 Historical note for SMP systems
157 Before Linux added kernel support for CLOCK_PROCESS_CPUTIME_ID and
158 CLOCK_THREAD_CPUTIME_ID, glibc implemented these clocks on many plat‐
159 forms using timer registers from the CPUs (TSC on i386, AR.ITC on Ita‐
160 nium). These registers may differ between CPUs and as a consequence
161 these clocks may return bogus results if a process is migrated to
162 another CPU.
163 If the CPUs in an SMP system have different clock sources, then there
165 is no way to maintain a correlation between the timer registers since
166 each CPU will run at a slightly different frequency. If that is the
167 case, then clock_getcpuclockid(0) will return ENOENT to signify this
168 condition. The two clocks will then be useful only if it can be
169 ensured that a process stays on a certain CPU.
170 The processors in an SMP system do not start all at exactly the same
172 time and therefore the timer registers are typically running at an off‐
173 set. Some architectures include code that attempts to limit these off‐
174 sets on bootup. However, the code cannot guarantee to accurately tune
175 the offsets. Glibc contains no provisions to deal with these offsets
176 (unlike the Linux Kernel). Typically these offsets are small and
177 therefore the effects may be negligible in most cases.
178 Since glibc 2.4, the wrapper functions for the system calls described
180 in this page avoid the abovementioned problems by employing the kernel
181 implementation of CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID,
182 on systems that provide such an implementation (i.e., Linux 2.6.12 and
183 later).
184
186 According to POSIX.1-2001, a process with "appropriate privileges" may
187 set the CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID clocks
188 using clock_settime(). On Linux, these clocks are not settable (i.e.,
189 no process has "appropriate privileges").
190
192 date(1), gettimeofday(2), settimeofday(2), time(2), adjtime(3),
193 clock_getcpuclockid(3), ctime(3), ftime(3), pthread_getcpuclockid(3),
194 sysconf(3), time(7), vdso(7), hwclock(8)
195
197 This page is part of release 4.15 of the Linux man-pages project. A
198 description of the project, information about reporting bugs, and the
199 latest version of this page, can be found at
200 https://www.kernel.org/doc/man-pages/.
201 2017-09-15 CLOCK_GETRES(2)