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clock_gettime, clock_settime, clock_getres
get/set/calibrate date and time

#include <time.h>
clock_gettime(clockid_t clock_id, struct timespec *tp);
clock_settime(clockid_t clock_id, const struct timespec *tp);
clock_getres(clockid_t clock_id, struct timespec *tp);

The clock_gettime() and clock_settime() functions allow the calling process to retrieve or set the value used by a clock which is specified by clock_id.
clock_id can be a value from clock_getcpuclockid(3) or pthread_getcpuclockid(3) or one of five predefined values:
time that increments as a wall clock should
time that increments when the CPU is running in user or kernel mode on behalf of the calling process
time that increments when the CPU is running in user or kernel mode on behalf of the calling thread
time that increments as a wall clock should but whose absolute value is meaningless and cannot jump, providing accurate realtime interval measurement, even across suspend and resume
time whose absolute value is the time the system has been running and not suspended, providing accurate uptime measurement, both absolute and interval
The structure pointed to by tp is defined in <sys/time.h> as:
struct timespec { 
	time_t	tv_sec;		/* seconds */ 
	long	tv_nsec;	/* and nanoseconds */ 
Only the CLOCK_REALTIME clock can be set, and only the superuser may do so. If the system securelevel is greater than 1 (see init(8)), the time may only be advanced. This limitation is imposed to prevent a malicious superuser from setting arbitrary time stamps on files. The system time can still be adjusted backwards using the adjtime(2) system call even when the system is secure.
The resolution (granularity) of a clock is returned by the clock_getres() call. This value is placed in a (non-null) *tp.

Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

clock_gettime(), clock_settime(), and clock_getres() will fail if:
clock_id is not a valid value.
The tp argument address referenced invalid memory.
In addition, clock_settime() may return the following errors:
A user other than the superuser attempted to set the time.
clock_id specifies a clock that isn't settable, tp specifies a nanosecond value less than zero or greater than 1000 million, or a value outside the range of the specified clock.

date(1), adjtime(2), getitimer(2), gettimeofday(2), clock_getcpuclockid(3), ctime(3), pthread_getcpuclockid(3)

The clock_getres(), clock_gettime(), and clock_settime() functions conform to IEEE Std 1003.1-2008 (“POSIX.1”).
The CLOCK_UPTIME clock is an extension to that.

The CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID clocks appeared in OpenBSD 5.4. The CLOCK_UPTIME clock first appeared in FreeBSD 7.0 and was added to OpenBSD in OpenBSD 5.5.
December 10, 2014 OpenBSD-5.8