adjtimex, ntp_adjtime — tune kernel clock
#include <sys/timex.h>
int
adjtimex( |
struct timex *buf); |
int
ntp_adjtime( |
struct timex *buf); |
Linux uses David L. Mills' 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_OFFSETSet 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_FREQUENCYSet 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_MAXERRORSet maximum time error from buf.maxerror.
ADJ_ESTERRORSet estimated time error from buf.esterror.
ADJ_STATUSSet clock status bits from buf.status. A
description of these bits is provided below.
ADJ_TIMECONSTSet PLL time constant from buf.constant. If the
STA_NANO status flag (see
below) is clear, the kernel adds 4 to this value.
ADJ_SETOFFSET (since Linux
2.6.29)Add buf.time to the current
time. If buf.status includes the
ADJ_NANO flag, then
buf.time.tv_usec is
interpreted as a nanosecond value; otherwise it is
interpreted as microseconds.
ADJ_MICRO (since Linux
2.6.36)Select microsecond resolution.
ADJ_NANO (since Linux
2.6.36)Select nanosecond resolution. Only one of
ADJ_MICRO and
ADJ_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 with ADJ_TIMECONST, since the latter mode
also employs the buf.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_TICKSet 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_SINGLESHOTOld-fashioned adjtime(): (gradually) adjust time by
value specified in buf.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
earlier ADJ_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 via
ADJ_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.
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 for ADJ_OFFSET_SINGLESHOT.
MOD_CLKB is the
synonym for ADJ_TICK.
The is no synonym for ADJ_OFFSET_SS_READ, which is not
described in the KAPI.
On success, adjtimex() and
ntp_adjtime() return the clock
state; that is, one of the following values:
TIME_OKClock synchronized, no leap second adjustment pending.
TIME_INSIndicates that a leap second will be added at the end of the UTC day.
TIME_DELIndicates that a leap second will be deleted at the end of the UTC day.
TIME_OOPInsertion of a leap second is in progress.
TIME_WAITA leap-second insertion or deletion has been
completed. This value will be returned until the next
ADJ_STATUS operation
clears the STA_INS and
STA_DEL flags.
TIME_ERRORThe system clock is not synchronized to a reliable server. This value is returned when any of the following holds true:
Either
STA_UNSYNCorSTA_CLOCKERRis set.
STA_PPSSIGNALis clear and eitherSTA_PPSFREQorSTA_PPSTIMEis set.
STA_PPSTIMEandSTA_PPSJITTERare both set.
STA_PPSFREQis set and eitherSTA_PPSWANDERorSTA_PPSJITTERis set.
The symbolic name TIME_BAD is a synonym for
TIME_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.
buf does not
point to writable memory.
An attempt was made to set buf.freq to a value
outside the range (−33554432, +33554432).
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).
An attempt was made to set buf.status to a value
other than those listed above.
An attempt was made to set buf.tick to a value
outside the range 900000/HZ to 1100000/HZ, where HZ is the system timer interrupt
frequency.
buf.modes
is neither 0 nor ADJ_OFFSET_SS_READ, and the caller
does not have sufficient privilege. Under Linux, the
CAP_SYS_TIME capability
is required.
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.
For an explanation of the terms used in this section, see attributes(7).
| Interface | Attribute | Value |
ntp_adjtime() |
Thread safety | MT-Safe |
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(3).
This page is part of release 4.07 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'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> |