Section (2) adjtimex
Name
adjtimex, ntp_adjtime — tune kernel clock
Synopsis
#include <sys/timex.h>
int
adjtimex( |
struct timex *buf) ; |
int
ntp_adjtime( |
struct timex *buf) ; |
DESCRIPTION
Linux uses David L. Mills_zsingle_quotesz_ clock adjustment algorithm (see
RFC 5905). The system call adjtimex
() reads and optionally sets
adjustment parameters for this algorithm. It takes a pointer
to a timex structure, updates
kernel parameters from (selected) field values, and returns
the same structure updated with the current kernel values.
This structure is declared as follows:
struct timex { int modes
; /* Mode selector */long offset
; /* Time offset; nanoseconds, if STA_NANO
status flag is set, otherwise
microseconds */long freq
; /* Frequency offset; see NOTES for units */long maxerror
; /* Maximum error (microseconds) */long esterror
; /* Estimated error (microseconds) */int status
; /* Clock command/status */long constant
; /* PLL (phase-locked loop) time constant */long precision
; /* Clock precision
(microseconds, read-only) */long tolerance
; /* Clock frequency tolerance (read-only);
see NOTES for units */struct timeval time
; /* Current time (read-only, except for
ADJ_SETOFFSET); upon return, time.tv_usec
contains nanoseconds, if STA_NANO status
flag is set, otherwise microseconds */long tick
; /* Microseconds between clock ticks */long ppsfreq
; /* PPS (pulse per second) frequency
(read-only); see NOTES for units */long jitter
; /* PPS jitter (read-only); nanoseconds, if
STA_NANO status flag is set, otherwise
microseconds */int shift
; /* PPS interval duration
(seconds, read-only) */long stabil
; /* PPS stability (read-only);
see NOTES for units */long jitcnt
; /* PPS count of jitter limit exceeded
events (read-only) */long calcnt
; /* PPS count of calibration intervals
(read-only) */long errcnt
; /* PPS count of calibration errors
(read-only) */long stbcnt
; /* PPS count of stability limit exceeded
events (read-only) */int tai
; /* TAI offset, as set by previous ADJ_TAI
operation (seconds, read-only,
since Linux 2.6.26) */ /* Further padding bytes to allow for future expansion */};
The modes
field
determines which parameters, if any, to set. (As described
later in this page, the constants used for ntp_adjtime
() are equivalent but
differently named.) It is a bit mask containing a
bitwise-or
combination of zero or more of the following bits:
ADJ_OFFSET
-
Set time offset from
buf.offset
. Since Linux 2.6.26, the supplied value is clamped to the range (−0.5s, +0.5s). In older kernels, an EINVAL error occurs if the supplied value is out of range. ADJ_FREQUENCY
-
Set frequency offset from
buf.freq
. Since Linux 2.6.26, the supplied value is clamped to the range (−32768000, +32768000). In older kernels, an EINVAL error occurs if the supplied value is out of range. ADJ_MAXERROR
-
Set maximum time error from
buf.maxerror
. ADJ_ESTERROR
-
Set estimated time error from
buf.esterror
. ADJ_STATUS
-
Set clock status bits from
buf.status
. A description of these bits is provided below. ADJ_TIMECONST
-
Set PLL time constant from
buf.constant
. If theSTA_NANO
status flag (see below) is clear, the kernel adds 4 to this value. ADJ_SETOFFSET
(since Linux 2.6.39)-
Add
buf.time
to the current time. Ifbuf.status
includes theADJ_NANO
flag, thenbuf.time.tv_usec
is interpreted as a nanosecond value; otherwise it is interpreted as microseconds. ADJ_MICRO
(since Linux 2.6.26)-
Select microsecond resolution.
ADJ_NANO
(since Linux 2.6.26)-
Select nanosecond resolution. Only one of
ADJ_MICRO
andADJ_NANO
should be specified. ADJ_TAI
(since Linux 2.6.26)-
Set TAI (Atomic International Time) offset from
buf.constant
.ADJ_TAI
should not be used in conjunction withADJ_TIMECONST
, since the latter mode also employs thebuf.constant
field.For a complete explanation of TAI and the difference between TAI and UTC, see
BIPM
http://www.bipm.org/en/bipm/tai/tai.html ADJ_TICK
-
Set tick value from
buf.tick
.
Alternatively, modes
can be specified as
either of the following (multibit mask) values, in which case
other bits should not be specified in modes
:
ADJ_OFFSET_SINGLESHOT
-
Old-fashioned
adjtime
(): (gradually) adjust time by value specified inbuf.offset
, which specifies an adjustment in microseconds. ADJ_OFFSET_SS_READ
(functional since Linux 2.6.28)-
Return (in
buf.offset
) the remaining amount of time to be adjusted after an earlierADJ_OFFSET_SINGLESHOT
operation. This feature was added in Linux 2.6.24, but did not work correctly until Linux 2.6.28.
Ordinary users are restricted to a value of either 0 or
ADJ_OFFSET_SS_READ
for
modes
. Only the
superuser may set any parameters.
The buf.status
field is a bit mask that is used to set and/or retrieve
status bits associated with the NTP implementation. Some bits
in the mask are both readable and settable, while others are
read-only.
STA_PLL
(read-write)-
Enable phase-locked loop (PLL) updates via
ADJ_OFFSET
. STA_PPSFREQ
(read-write)-
Enable PPS (pulse-per-second) frequency discipline.
STA_PPSTIME
(read-write)-
Enable PPS time discipline.
STA_FLL
(read-write)-
Select frequency-locked loop (FLL) mode.
STA_INS
(read-write)-
Insert a leap second after the last second of the UTC day, thus extending the last minute of the day by one second. Leap-second insertion will occur each day, so long as this flag remains set.
STA_DEL
(read-write)-
Delete a leap second at the last second of the UTC day. Leap second deletion will occur each day, so long as this flag remains set.
STA_UNSYNC
(read-write)-
Clock unsynchronized.
STA_FREQHOLD
(read-write)-
Hold frequency. Normally adjustments made via
ADJ_OFFSET
result in dampened frequency adjustments also being made. So a single call corrects the current offset, but as offsets in the same direction are made repeatedly, the small frequency adjustments will accumulate to fix the long-term skew.This flag prevents the small frequency adjustment from being made when correcting for an
ADJ_OFFSET
value. STA_PPSSIGNAL
(read-only)-
A valid PPS (pulse-per-second) signal is present.
STA_PPSJITTER
(read-only)-
PPS signal jitter exceeded.
STA_PPSWANDER
(read-only)-
PPS signal wander exceeded.
STA_PPSERROR
(read-only)-
PPS signal calibration error.
STA_CLOCKERR
(read-only)-
Clock hardware fault.
STA_NANO
(read-only; since Linux 2.6.26)-
Resolution (0 = microsecond, 1 = nanoseconds). Set via
ADJ_NANO
, cleared viaADJ_MICRO
. STA_MODE
(since Linux 2.6.26)-
Mode (0 = Phase Locked Loop, 1 = Frequency Locked Loop).
STA_CLK
(read-only; since Linux 2.6.26)-
Clock source (0 = A, 1 = B); currently unused.
Attempts to set read-only status
bits are silently
ignored.
ntp_adjtime ()
The ntp_adjtime
() library
function (described in the NTP Kernel Application Program
API, KAPI) is a more portable interface for performing the
same task as adjtimex
().
Other than the following points, it is identical to
adjtime
():
-
The constants used in
modes
are prefixed with MOD_ rather than ADJ_, and have the same suffixes (thus,MOD_OFFSET
,MOD_FREQUENCY
, and so on), other than the exceptions noted in the following points. -
MOD_CLKA
is the synonym forADJ_OFFSET_SINGLESHOT
. -
MOD_CLKB
is the synonym forADJ_TICK
. -
The is no synonym for
ADJ_OFFSET_SS_READ
, which is not described in the KAPI.
RETURN VALUE
On success, adjtimex
() and
ntp_adjtime
() return the clock
state; that is, one of the following values:
TIME_OK
-
Clock synchronized, no leap second adjustment pending.
TIME_INS
-
Indicates that a leap second will be added at the end of the UTC day.
TIME_DEL
-
Indicates that a leap second will be deleted at the end of the UTC day.
TIME_OOP
-
Insertion of a leap second is in progress.
TIME_WAIT
-
A leap-second insertion or deletion has been completed. This value will be returned until the next
ADJ_STATUS
operation clears theSTA_INS
andSTA_DEL
flags. TIME_ERROR
-
The system clock is not synchronized to a reliable server. This value is returned when any of the following holds true:
-
Either
STA_UNSYNC
orSTA_CLOCKERR
is set. -
STA_PPSSIGNAL
is clear and eitherSTA_PPSFREQ
orSTA_PPSTIME
is set. -
STA_PPSTIME
andSTA_PPSJITTER
are both set. -
STA_PPSFREQ
is set and eitherSTA_PPSWANDER
orSTA_PPSJITTER
is set.
The symbolic name
TIME_BAD
is a synonym forTIME_ERROR
, provided for backward compatibility. -
Note that starting with Linux 3.4, the call operates asynchronously and the return value usually will not reflect a state change caused by the call itself.
On failure, these calls return −1 and set
errno
.
ERRORS
- EFAULT
-
buf
does not point to writable memory. - EINVAL (kernels before Linux 2.6.26)
-
An attempt was made to set
buf.freq
to a value outside the range (−33554432, +33554432). - EINVAL (kernels before Linux 2.6.26)
-
An attempt was made to set
buf.offset
to a value outside the permitted range. In kernels before Linux 2.0, the permitted range was (−131072, +131072). From Linux 2.0 onwards, the permitted range was (−512000, +512000). - EINVAL
-
An attempt was made to set
buf.status
to a value other than those listed above. - EINVAL
-
An attempt was made to set
buf.tick
to a value outside the range 900000/HZ
to 1100000/HZ
, whereHZ
is the system timer interrupt frequency. - EPERM
-
buf.modes
is neither 0 norADJ_OFFSET_SS_READ
, and the caller does not have sufficient privilege. Under Linux, theCAP_SYS_TIME
capability is required.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
Interface | Attribute | Value |
ntp_adjtime () |
Thread safety | MT-Safe |
CONFORMING TO
Neither of these interfaces is described in POSIX.1
adjtimex
() is Linux-specific
and should not be used in programs intended to be
portable.
The preferred API for the NTP daemon is ntp_adjtime
().
NOTES
In struct timex,
freq
, ppsfreq
, and stabil
are ppm (parts per
million) with a 16-bit fractional part, which means that a
value of 1 in one of those fields actually means 2^-16 ppm,
and 2^16=65536 is 1 ppm. This is the case for both input
values (in the case of freq
) and output values.
The leap-second processing triggered by STA_INS
and STA_DEL
is done by the kernel in timer
context. Thus, it will take one tick into the second for the
leap second to be inserted or deleted.
SEE ALSO
settimeofday(2), adjtime(3), ntp_gettime(3), capabilities(7), time(7), adjtimex(8), hwclock(8)
COLOPHON
This page is part of release 5.04 of the Linux man-pages
project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
Copyright (c) 1995 Michael Chastain (mecshell.portal.com), 15 April 1995. and Copyright (C) 2014, 2016 Michael Kerrisk <mtk.manpagesgmail.com> %%%LICENSE_START(GPLv2+_DOC_FULL) This is free documentation; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. The GNU General Public License_zsingle_quotesz_s references to object code and executables are to be interpreted as the output of any document formatting or typesetting system, including intermediate and printed output. This manual is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this manual; if not, see <http://www.gnu.org/licenses/>. %%%LICENSE_END Modified 1997-01-31 by Eric S. Raymond <esrthyrsus.com> Modified 1997-07-30 by Paul Slootman <paulwurtel.demon.nl> Modified 2004-05-27 by Michael Kerrisk <mtk.manpagesgmail.com> |