Manual Pages  — NTP_ADJTIME

#include <sys/timex.h>

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 variables 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 distance) and estimated error (dispersion) to client user application programs.

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 argument 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 second 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 following:
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 oscillator (scaled ppm). Ordinarily a property of the architecture, 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).

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.

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

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