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NTP_ADJTIME(2)		    BSD System Calls Manual		NTP_ADJTIME(2)

NAME
     ntp_adjtime, ntp_gettime — Network Time Protocol (NTP) daemon interface
     system calls

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <sys/timex.h>

     int
     ntp_adjtime(struct timex *);

     int
     ntp_gettime(struct ntptimeval *);

DESCRIPTION
     The two system calls ntp_adjtime() and ntp_gettime() are the kernel
     interface to the Network Time Protocol (NTP) daemon ntpd(8).

     The ntp_adjtime() function is used by the NTP daemon to adjust the system
     clock to an externally derived time.  The time offset and related vari‐
     ables which are set by ntp_adjtime() are used by hardclock() to adjust
     the phase and frequency of the phase- or frequency-lock loop (PLL resp.
     FLL) which controls the system clock.

     The ntp_gettime() function provides the time, maximum error (sync dis‐
     tance) and estimated error (dispersion) to client user application pro‐
     grams.

     In the following, all variables that refer PPS are only relevant if the
     PPS_SYNC option is enabled in the kernel.

     ntp_adjtime() has as argument a struct timex * of the following form:

     struct timex {
	     unsigned int modes;     /* clock mode bits (wo) */
	     long offset;	     /* time offset (us) (rw) */
	     long freq;		     /* frequency offset (scaled ppm) (rw) */
	     long maxerror;	     /* maximum error (us) (rw) */
	     long esterror;	     /* estimated error (us) (rw) */
	     int status;	     /* clock status bits (rw) */
	     long constant;	     /* pll time constant (rw) */
	     long precision;	     /* clock precision (us) (ro) */
	     long tolerance;	     /* clock frequency tolerance (scaled
				      * ppm) (ro) */
	     /*
	      * The following read-only structure members are implemented
	      * only if the PPS signal discipline is configured in the
	      * kernel.
	      */
	     long ppsfreq;	     /* pps frequency (scaled ppm) (ro) */
	     long jitter;	     /* pps jitter (us) (ro) */
	     int shift;		     /* interval duration (s) (shift) (ro) */
	     long stabil;	     /* pps stability (scaled ppm) (ro) */
	     long jitcnt;	     /* jitter limit exceeded (ro) */
	     long calcnt;	     /* calibration intervals (ro) */
	     long errcnt;	     /* calibration errors (ro) */
	     long stbcnt;	     /* stability limit exceeded (ro) */
     };

     The members of this struct have the following meanings when used as argu‐
     ment for ntp_adjtime():
     modes	Defines what settings should be changed with the current
		ntp_adjtime() call (write-only).  Bitwise OR of the following:
		      MOD_OFFSET     set time offset
		      MOD_FREQUENCY  set frequency offset
		      MOD_MAXERROR   set maximum time error
		      MOD_ESTERROR   set estimated time error
		      MOD_STATUS     set clock status bits
		      MOD_TIMECONST  set PLL time constant
		      MOD_CLKA	     set clock A
		      MOD_CLKB	     set clock B
     offset	Time offset (in microseconds), used by the PLL/FLL to adjust
		the system time in small increments (read-write).
     freq	Frequency offset (scaled ppm) (read-write).
     maxerror	Maximum error (in microseconds).  Initialized by an
		ntp_adjtime() call, and increased by the kernel once each sec‐
		ond to reflect the maximum error bound growth (read-write).
     esterror	Estimated error (in microseconds).  Set and read by
		ntp_adjtime(), but unused by the kernel (read-write).
     status	System clock status bits (read-write).	Bitwise OR of the fol‐
		lowing:
		      STA_PLL	     Enable PLL updates (read-write).
		      STA_PPSFREQ    Enable PPS freq discipline (read-write).
		      STA_PPSTIME    Enable PPS time discipline (read-write).
		      STA_FLL	     Select frequency-lock mode (read-write).
		      STA_INS	     Insert leap (read-write).
		      STA_DEL	     Delete leap (read-write).
		      STA_UNSYNC     Clock unsynchronized (read-write).
		      STA_FREQHOLD   Hold frequency (read-write).
		      STA_PPSSIGNAL  PPS signal present (read-only).
		      STA_PPSJITTER  PPS signal jitter exceeded (read-only).
		      STA_PPSWANDER  PPS signal wander exceeded (read-only).
		      STA_PPSERROR   PPS signal calibration error (read-only).
		      STA_CLOCKERR   Clock hardware fault (read-only).
     constant	PLL time constant, determines the bandwidth, or “stiffness”,
		of the PLL (read-write).
     precision	Clock precision (in microseconds).  In most cases the same as
		the kernel tick variable (see hz(9)).  If a precision clock
		counter or external time-keeping signal is available, it could
		be much lower (and depend on the state of the signal) (read-
		only).
     tolerance	Maximum frequency error, or tolerance of the CPU clock oscil‐
		lator (scaled ppm).  Ordinarily a property of the architec‐
		ture, but could change under the influence of external time-
		keeping signals (read-only).
     ppsfreq	PPS frequency offset produced by the frequency median filter
		(scaled ppm) (read-only).
     jitter	PPS jitter measured by the time median filter in microseconds
		(read-only).
     shift	Logarithm to base 2 of the interval duration in seconds (PPS,
		read-only).
     stabil	PPS stability (scaled ppm); dispersion (wander) measured by
		the frequency median filter (read-only).
     jitcnt	Number of seconds that have been discarded because the jitter
		measured by the time median filter exceeded the limit MAXTIME
		(PPS, read-only).
     calcnt	Count of calibration intervals (PPS, read-only).
     errcnt	Number of calibration intervals that have been discarded
		because the wander exceeded the limit MAXFREQ or where the
		calibration interval jitter exceeded two ticks (PPS, read-
		only).
     stbcnt	Number of calibration intervals that have been discarded
		because the frequency wander exceeded the limit MAXFREQ/4
		(PPS, read-only).
     After the ntp_adjtime() call, the struct timex * structure contains the
     current values of the corresponding variables.

     ntp_gettime() has as argument a struct ntptimeval * with the following
     members:

     struct ntptimeval {
	     struct timeval time;    /* current time (ro) */
	     long maxerror;	     /* maximum error (us) (ro) */
	     long esterror;	     /* estimated error (us) (ro) */
     };

     These have the following meaning:
     time	Current time (read-only).
     maxerror	Maximum error in microseconds (read-only).
     esterror	Estimated error in microseconds (read-only).

RETURN VALUES
     ntp_adjtime() and ntp_gettime() return the current state of the clock on
     success, or any of the errors of copyin(9) and copyout(9).	 ntp_adjtime()
     may additionally return EPERM if the user calling ntp_adjtime() does not
     have sufficient permissions.

     Possible states of the clock are:
	   TIME_OK     Everything okay, no leap second warning.
	   TIME_INS    “insert leap second” warning.  At the end of the day, a
		       leap second will be inserted after 23:59:59.
	   TIME_DEL    “delete leap second” warning.  At the end of the day,
		       second 23:59:59 will be skipped.
	   TIME_OOP    Leap second in progress.
	   TIME_WAIT   Leap second has occurred within the last few seconds..
	   TIME_ERROR  Clock not synchronized.

ERRORS
     The ntp_adjtime() system call may return EPERM if the caller does not
     have sufficient permissions.

SEE ALSO
     options(4), ntpd(8), hardclock(9), hz(9)

     http://www.bipm.fr/enus/5_Scientific/c_time/time_1.html

     http://www.boulder.nist.gov/timefreq/general/faq.htm

     ftp://time.nist.gov/pub/leap-seconds.list

BUGS
     Take note that this API is extremely complex and stateful.	 Users should
     not attempt modification without first reviewing the ntpd(8) sources in
     depth.

BSD				 July 13, 2005				   BSD
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