DESCRIPTION

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.