Langue: en

Version: 266500 (debian - 07/07/09)

Section: 2 (Appels système)

BSD mandoc


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


Lb libc


In sys/timex.h Ft int Fn ntp_adjtime struct timex * Ft int Fn ntp_gettime struct ntptimeval *


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

The Fn 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 Fn ntp_adjtime are used by Fn hardclock to adjust the phase and frequency of the phase- or frequency-lock loop (PLL resp. FLL) which controls the system clock.

The Fn 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.

Fn 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 Fn ntp_adjtime :

Fa modes
Defines what settings should be changed with the current Fn ntp_adjtime call (write-only). Bitwise OR of the following:
set time offset
set frequency offset
set maximum time error
set estimated time error
set clock status bits
set PLL time constant
set clock A
set clock B
Fa offset
Time offset (in microseconds), used by the PLL/FLL to adjust the system time in small increments (read-write).
Fa freq
Frequency offset (scaled ppm) (read-write).
Fa maxerror
Maximum error (in microseconds). Initialized by an Fn ntp_adjtime call, and increased by the kernel once each second to reflect the maximum error bound growth (read-write).
Fa esterror
Estimated error (in microseconds). Set and read by Fn ntp_adjtime , but unused by the kernel (read-write).
Fa status
System clock status bits (read-write). Bitwise OR of the following:
Enable PLL updates (read-write).
Enable PPS freq discipline (read-write).
Enable PPS time discipline (read-write).
Select frequency-lock mode (read-write).
Insert leap (read-write).
Delete leap (read-write).
Clock unsynchronized (read-write).
Hold frequency (read-write).
PPS signal present (read-only).
PPS signal jitter exceeded (read-only).
PPS signal wander exceeded (read-only).
PPS signal calibration error (read-only).
Clock hardware fault (read-only).
Fa constant
PLL time constant, determines the bandwidth, or ``stiffness'' of the PLL (read-write).
Fa 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).
Fa 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).
Fa ppsfreq
PPS frequency offset produced by the frequency median filter (scaled ppm) (read-only).
Fa jitter
PPS jitter measured by the time median filter in microseconds (read-only).
Fa shift
Logarithm to base 2 of the interval duration in seconds (PPS, read-only).
Fa stabil
PPS stability (scaled ppm); dispersion (wander) measured by the frequency median filter (read-only).
Fa jitcnt
Number of seconds that have been discarded because the jitter measured by the time median filter exceeded the limit MAXTIME (PPS, read-only).
Fa calcnt
Count of calibration intervals (PPS, read-only).
Fa 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).
Fa stbcnt
Number of calibration intervals that have been discarded because the frequency wander exceeded the limit MAXFREQ /4 (PPS, read-only).

After the Fn ntp_adjtime call, the struct timex structure contains the current values of the corresponding variables.

Fn 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:

Fa time
Current time (read-only).
Fa maxerror
Maximum error in microseconds (read-only).
Fa esterror
Estimated error in microseconds (read-only).


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

Possible states of the clock are:

Everything okay, no leap second warning.
``insert leap second'' warning. At the end of the day, a leap second will be inserted after 23:59:59.
``delete leap second'' warning. At the end of the day, second 23:59:59 will be skipped.
Leap second in progress.
Leap second has occurred within the last few seconds..
Clock not synchronized.


The Fn ntp_adjtime system call may return Er EPERM if the caller does not have sufficient permissions.


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


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