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.