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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 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).
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.
Debian July 13, 2005 NTP_ADJTIME(2)