NAME
ntp_adjtime
,
ntp_gettime
—
Network Time Protocol (NTP) daemon
interface system calls
LIBRARY
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 callsntp_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).
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, and U. Windl, Pulse-Per-Second API for UNIX-like Operating Systems, RFC 2783, March 2000.