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CLOCK_GETTIME(2)	  FreeBSD System Calls Manual	      CLOCK_GETTIME(2)

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

LIBRARY
     Standard C	Library	(libc, -lc)

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() system calls allow	the calling
     process to	retrieve or set	the value used by a clock which	is specified
     by	clock_id.

     The clock_id argument can be one of the following values: CLOCK_REALTIME,
     CLOCK_REALTIME_PRECISE, CLOCK_REALTIME_FAST for time that increments as a
     wall clock	should;	CLOCK_MONOTONIC, CLOCK_MONOTONIC_PRECISE,
     CLOCK_MONOTONIC_FAST which	increments in SI seconds; CLOCK_UPTIME,
     CLOCK_UPTIME_PRECISE, CLOCK_UPTIME_FAST which starts at zero when the
     kernel boots and increments monotonically in SI seconds while the machine
     is	running; CLOCK_VIRTUAL for time	that increments	only when the CPU is
     running in	user mode on behalf of the calling process; CLOCK_PROF for
     time that increments when the CPU is running in user or kernel mode; or
     CLOCK_SECOND which	returns	the current second without performing a	full
     time counter query, using in-kernel cached	value of current second.

     The clock IDs CLOCK_REALTIME_FAST,	CLOCK_MONOTONIC_FAST,
     CLOCK_UPTIME_FAST are analogs of corresponding IDs	without	_FAST suffix
     but do not	perform	a full time counter query, so their accuracy is	one
     timer tick.  Similarly, CLOCK_REALTIME_PRECISE, CLOCK_MONOTONIC_PRECISE,
     CLOCK_UPTIME_PRECISE are used to get the most exact value as possible, at
     the expense of execution time.

     The structure pointed to by tp is defined in <sys/timespec.h> as:

     struct timespec {
	     time_t  tv_sec;	     /*	seconds	*/
	     long    tv_nsec;	     /*	and nanoseconds	*/
     };

     Only the super-user may set the time of day, using	only CLOCK_REALTIME.
     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
     super-user	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()
     system call.  This	value is placed	in a (non-NULL)	*tp.

RETURN VALUES
     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.

ERRORS
     The following error codes may be set in errno:

     [EINVAL]		The clock_id argument was not a	valid value.

     [EFAULT]		The *tp	argument address referenced invalid memory.

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

SEE ALSO
     date(1), adjtime(2), ctime(3), timed(8)

STANDARDS
     The clock_gettime(), clock_settime(), and clock_getres() system calls
     conform to	IEEE Std 1003.1b-1993 (``POSIX.1'').  The clock	IDs
     CLOCK_REALTIME_FAST, CLOCK_REALTIME_PRECISE, CLOCK_MONOTONIC_FAST,
     CLOCK_MONOTONIC_PRECISE, CLOCK_UPTIME, CLOCK_UPTIME_FAST,
     CLOCK_UPTIME_PRECISE, CLOCK_SECOND	are FreeBSD extensions to the POSIX
     interface.

FreeBSD	10.1		       December	29, 2009		  FreeBSD 10.1

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | STANDARDS

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