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 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);
15 Link with -lrt (only for glibc versions before 2.17).
17 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
19 clock_getres(), clock_gettime(), clock_settime():
21 _POSIX_C_SOURCE >= 199309L
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24 The function clock_getres() finds the resolution (precision) of the
25 specified clock clockid, and, if res is non-NULL, stores it in the
26 struct timespec pointed to by res. The resolution of clocks depends on
27 the implementation and cannot be configured by a particular process.
28 If the time value pointed to by the argument tp of clock_settime() is
29 not a multiple of res, then it is truncated to a multiple of res.
30 The functions clock_gettime() and clock_settime() retrieve and set the
32 time of the specified clock clockid.
33 The res and tp arguments are timespec structures, as specified in
35 <time.h>:
36 struct timespec {
38 time_t tv_sec; /* seconds */
39 long tv_nsec; /* nanoseconds */
40 };
41 The clockid argument is the identifier of the particular clock on which
43 to act. A clock may be system-wide and hence visible for all pro‐
44 cesses, or per-process if it measures time only within a single
45 process.
46 All implementations support the system-wide real-time clock, which is
48 identified by CLOCK_REALTIME. Its time represents seconds and nanosec‐
49 onds since the Epoch. When its time is changed, timers for a relative
50 interval are unaffected, but timers for an absolute point in time are
51 affected.
52 More clocks may be implemented. The interpretation of the correspond‐
54 ing time values and the effect on timers is unspecified.
55 Sufficiently recent versions of glibc and the Linux kernel support the
57 following clocks:
58 CLOCK_REALTIME
60 A settable system-wide clock that measures real (i.e., wall-
61 clock) time. Setting this clock requires appropriate privi‐
62 leges. This clock is affected by discontinuous jumps in the
63 system time (e.g., if the system administrator manually changes
64 the clock), and by the incremental adjustments performed by adj‐
65 time(3) and NTP.
66 CLOCK_REALTIME_ALARM (since Linux 3.0; Linux-specific)
68 Like CLOCK_REALTIME, but not settable. See timer_create(2) for
69 further details.
70 CLOCK_REALTIME_