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#include <time.h> int clock_settime(clockid_t clock_id, const struct timespec *tp);
int clock_gettime(clockid_t clock_id, struct timespec *tp);
int clock_getres(clockid_t clock_id, struct timespec *res);
The clock_settime() function sets the specified clock, clock_id, to the value specified by tp. Time values that are between two consecutive non-negative integer multiples of the resolution of the specified clock are truncated down to the smaller multiple of the resolution.
The clock_gettime() function returns the current value tp for the specified clock, clock_id.
The resolution of any clock can be obtained by calling clock_getres(). Clock resolutions are system-dependent and cannot be set by a process. If the argument res is not NULL, the resolution of the specified clock is stored in the location pointed to by res. If res is NULL, the clock resolution is not returned. If the time argument of clock_settime() is not a multiple of res, then the value is truncated to a multiple of res.
A clock may be systemwide (that is, visible to all processes) or per-process (measuring time that is meaningful only within a process).
A clock_id of CLOCK_REALTIME is defined in <time.h>. This clock represents the realtime clock for the system. For this clock, the values returned by clock_gettime() and specified by clock_settime() represent the amount of time (in seconds and nanoseconds) since the Epoch. Additional clocks may also be supported. The interpretation of time values for these clocks is unspecified.
A clock_id of CLOCK_HIGHRES represents the non-adjustable, high-resolution clock for the system. For this clock, the value returned by clock_gettime(3C) represents the amount of time (in seconds and nanoseconds) since some arbitrary time in the past; it is not correlated in any way to the time of day, and thus is not subject to resetting or drifting by way of adjtime(2), ntp_adjtime(2), settimeofday(3C), or clock_settime(). The time source for this clock is the same as that for gethrtime(3C).
Additional clocks may also be supported. The interpretation of time values for these clocks is unspecified.
Upon successful completion, 0 is returned. Otherwise, -1 is returned and errno is set to indicate the error.
The clock_settime(), clock_gettime() and clock_getres() functions will fail if:
EINVAL
The clock_id argument does not specify a known clock.
ENOSYS
The functions clock_settime(), clock_gettime(), and clock_getres() are not supported by this implementation.
The clock_settime() function will fail if:
EINVAL
The tp argument to clock_settime() is outside the range for the given clock ID; or the tp argument specified a nanosecond value less than zero or greater than or equal to 1000 million.
The clock_settime() function may fail if:
EPERM
The requesting process does not have the appropriate privilege to set the specified clock.
See attributes(5) for descriptions of the following attributes:
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
| Interface Stability | Committed |
| MT-Level | clock_gettime() is Async-Signal-Safe |
| Standard | See standards(5). |
time(2), ctime(3C), gethrtime(3C), time.h(3HEAD), timer_gettime(3C), attributes(5), standards(5)