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CLOCK_GETTIME(2)	       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 a value obtained  from  clock_getcpuclockid(3)
     or pthread_getcpuclockid(3) as well as the following values:

     CLOCK_REALTIME
     CLOCK_REALTIME_PRECISE
     CLOCK_REALTIME_FAST
	     Increments as a wall clock should.
     CLOCK_MONOTONIC
     CLOCK_MONOTONIC_PRECISE
     CLOCK_MONOTONIC_FAST
	     Increments in SI seconds.
     CLOCK_UPTIME
     CLOCK_UPTIME_PRECISE
     CLOCK_UPTIME_FAST
	     Starts  at  zero when the kernel boots and increments monotonically
	     in SI seconds while the machine is running.
     CLOCK_VIRTUAL
	     Increments only when the CPU is running in user mode on  behalf  of
	     the calling process.
     CLOCK_PROF
	     Increments when the CPU is running in user or kernel mode.
     CLOCK_SECOND
	     Returns  the  current second without performing a full time counter
	     query, using an in-kernel cached value of the current second.
     CLOCK_PROCESS_CPUTIME_ID
	     Returns the execution time of the calling process.
     CLOCK_THREAD_CPUTIME_ID
	     Returns the execution time of the calling thread.

     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 or timespec argument was not a valid value.

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

SEE ALSO
     date(1),  adjtime(2),  clock_getcpuclockid(3),  ctime(3),	 pthread_getcpu-
     clockid(3), timed(8)

STANDARDS
     The  clock_gettime(), clock_settime(), and clock_getres() system calls con-
     form   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 in-
     terface.

FreeBSD 11.1			 March 20, 2017 		CLOCK_GETTIME(2)
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