1CLOCK_GETRES(2) Linux Programmer's Manual CLOCK_GETRES(2)
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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.
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 real-time clock. Setting this clock requires appro‐
62 priate privileges.
63 CLOCK_MONOTONIC
65 Clock that cannot be set and represents monotonic time since
66 some unspecified starting point.
67 CLOCK_MONOTONIC_RAW (since Linux 2.6.28; Linux-specific)
69 Similar to CLOCK_MONOTONIC, but provides access to a raw hard‐
70 ware-based time that is not subject to NTP adjustments.
71 CLOCK_PROCESS_CPUTIME_ID
73 High-resolution per-process timer from the CPU.
74 CLOCK_THREAD_CPUTIME_ID
76 Thread-specific CPU-time clock.
77
79 clock_gettime(), clock_settime() and clock_getres() return 0 for suc‐
80 cess, or -1 for failure (in which case errno is set appropriately).
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83 EFAULT tp points outside the accessible address space.
84 EINVAL The clk_id specified is not supported on this system.
86 EPERM clock_settime() does not have permission to set the clock indi‐
88 cated.
89
91 SUSv2, POSIX.1-2001.
92
94 On POSIX systems on which these functions are available, the symbol
95 _POSIX_TIMERS is defined in <unistd.h> to a value greater than 0. The
96 symbols _POSIX_MONOTONIC_CLOCK, _POSIX_CPUTIME, _POSIX_THREAD_CPUTIME
97 indicate that CLOCK_MONOTONIC, CLOCK_PROCESS_CPUTIME_ID,
98 CLOCK_THREAD_CPUTIME_ID are available. (See also sysconf(3).)
99
101 Note for SMP systems
102 The CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID clocks are
103 realized on many platforms using timers from the CPUs (TSC on i386,
104 AR.ITC on Itanium). These registers may differ between CPUs and as a
105 consequence these clocks may return bogus results if a process is
106 migrated to another CPU.
107 If the CPUs in an SMP system have different clock sources then there is
109 no way to maintain a correlation between the timer registers since each
110 CPU will run at a slightly different frequency. If that is the case
111 then clock_getcpuclockid(0) will return ENOENT to signify this condi‐
112 tion. The two clocks will then only be useful if it can be ensured
113 that a process stays on a certain CPU.
114 The processors in an SMP system do not start all at exactly the same
116 time and therefore the timer registers are typically running at an off‐
117 set. Some architectures include code that attempts to limit these off‐
118 sets on bootup. However, the code cannot guarantee to accurately tune
119 the offsets. Glibc contains no provisions to deal with these offsets
120 (unlike the Linux Kernel). Typically these offsets are small and
121 therefore the effects may be negligible in most cases.
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124 According to POSIX.1-2001, a process with "appropriate privileges" may
125 set the CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID clocks
126 using clock_settime(). On Linux, these clocks are not settable (i.e.,
127 no process has "appropriate privileges").
128
130 date(1), adjtimex(2), gettimeofday(2), settimeofday(2), time(2),
131 clock_getcpuclockid(3), ctime(3), ftime(3), pthread_getcpuclockid(3),
132 sysconf(3), time(7)
133
135 This page is part of release 3.25 of the Linux man-pages project. A
136 description of the project, and information about reporting bugs, can
137 be found at http://www.kernel.org/doc/man-pages/.
138 2010-02-03 CLOCK_GETRES(2)