CLOCK_GETTIME(2)

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

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

DESCRIPTION
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 the following predefined values:

CLOCK_REALTIME
    time that increments as a wall clock should

CLOCK_PROCESS_CPUTIME_ID
    time that increments when the CPU is running in user or kernel mode on behalf of the calling process

CLOCK_THREAD_CPUTIME_ID
    time that increments when the CPU is running in user or kernel mode on behalf of the calling thread

CLOCK_MONOTONIC
    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

CLOCK_BOOTTIME
    time whose absolute value is the time that has elapsed since the system was booted

CLOCK_UPTIME
    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.

RETURN VALUES

ERRORS
clock_gettime(), clock_settime(), and clock_getres() will fail if:

[EINVAL]
    clock_id is not a valid value.

[EFAULT]
    The tp argument address referenced invalid memory.

In addition, clock_settime() may return the following errors:

[EPERM]
    A user other than the superuser attempted to set the time.

[EINVAL]
    clock_id specifies a clock that isn't settable, tp specifies a nanosecond value less than zero or greater than or equal to 1000 million, or a value outside the range of the specified clock.

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

STANDARDS
The clock_getres(), clock_gettime(), and clock_settime() functions conform to p1003.1-2008.

The CLOCK_BOOTTIME and CLOCK_UPTIME clocks are extensions to that.

HISTORY
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. The CLOCK_BOOTTIME clock first appeared in Linux 2.6.39 and was added to OpenBSD in OpenBSD 6.3.