NTP_ADJTIME(2)

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

LIBRARY
libc

SYNOPSIS
#include <sys/time.h>
#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 ntp_adjtime() system call is available only for the super user. If the system call fails, the ntp_adjtime() function in the standard C library will try to use the clockctl(4) device if present, thus making it possible for the NTP daemon to run as a non-privileged user. If clockctl(4) is not present, ntp_adjtime() returns EPERM.

The time offset and related variables which are set by ntp_adjtime() are used by hardclock(9) 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 (see options(4)) 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 timespec time;	/* current time (ro) */
    	long maxerror;		/* maximum error (us) (ro) */
    	long esterror;		/* estimated error (us) (ro) */
    	/* the following are placeholders for now */
    	long tai;		/* TAI offset */
    	int time_state;		/* time status */
    };

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.
TIME_DEL
    “delete leap second” warning.
TIME_OOP
    Leap second in progress.
TIME_WAIT
    Leap second has occurred.
TIME_ERROR
    Clock not synchronized.

SEE ALSO
options(4), ntpd(8), hardclock(9), hz(9) J. Mogul, D. Mills, J. Brittenson, J. Stone, U. Windl, Pulse-Per-Second API for UNIX-like Operating Systems, RFC 2783, March 2000.