adjtimex — tune kernel clock
#define _BSD_SOURCE /* See feature_test_macros(7) */ #include <sys/timex.h>
int
adjtimex( |
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 field values, and returns the same
structure with 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, in units of 2^-16 ppm
(parts per million, see NOTES below) */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 (ppm, read-only) */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 (in units
of 2^-16 ppm\-\-see NOTES, read-only) */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 (2^-16 ppm\-\-see NOTES,
read-only) */long jitcnt
; /* PPS jitter limit exceeded (read-only) */long calcnt
; /* PPS calibration intervals (read-only) */long errcnt
; /* PPS calibration errors (read-only) */long stbcnt
; /* PPS stability limit exceeded (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. 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
.
ADJ_FREQUENCY
Set frequency offset from buf.freq
.
ADJ_MAXERROR
Set maximum time error from buf.maxerror
.
ADJ_ESTERROR
Set estimated time error from buf.esterror
.
ADJ_STATUS
Set clock status from buf.status
.
ADJ_TIMECONST
Set 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_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 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
Enable phase-locked loop (PLL) updates (read-write)
via ADJ_OFFSET
.
STA_PPSFREQ
Enable PPS freq discipline (read-write).
STA_PPSTIME
Enable PPS time discipline (read-write).
STA_FLL
Select frequency-locked loop (FLL) mode (read-write).
STA_INS
Insert leap second (read-write).
STA_DEL
Delete leap second (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).
STA_NANO
(since Linux
2.6.26)Resolution (0 = microsecond, 1 = nanoseconds;
read-only). Set via ADJ_NANO
, cleared via ADJ_MICRO
.
STA_MODE
(since Linux
2.6.26)Mode (0 = Phase Locked Loop, 1 = Frequency Locked Loop; read-only).
STA_CLK
(since Linux
2.6.26)Clock source (0 = A, 1 = B; read-only).
Attempts to set read-only status
bits are silently
ignored.
On success, adjtimex
()
returns the clock state; that is, one of the following
values:
TIME_OK
Clock synchronized.
TIME_INS
Insert leap second.
TIME_DEL
Delete leap second.
TIME_OOP
Leap second in progress.
TIME_WAIT
Leap second has occurred.
TIME_ERROR
Clock not synchronized. The symbolic name
TIME_BAD
is a synonym for
TIME_ERROR
, provided for
backward compatibility.
On failure, adjtimex
()
returns −1 and sets errno
.
buf
does not
point to writable memory.
An attempt was made to set buf.offset
to a value
outside the range −131071 to +131071, or to set
buf.status
to
a value other than those listed above, or 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.
adjtimex
() is Linux-specific
and should not be used in programs intended to be portable.
See adjtime(3) for a more
portable, but less flexible, method of adjusting the system
clock.
This page is part of release 3.83 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
http://www.kernel.org/doc/man−pages/.
Copyright (c) 1995 Michael Chastain (mecshell.portal.com), 15 April 1995. and Copyright (C) 2014 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> |