Name

adjtimex — tune kernel clock

Synopsis

#define _BSD_SOURCE       /* See feature_test_macros(7) */
#include <sys/timex.h>
int adjtimex( struct timex *buf);
 

DESCRIPTION

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.

RETURN VALUE

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.

ERRORS

EFAULT

buf does not point to writable memory.

EINVAL

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.

EPERM

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.

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.

CONFORMING TO

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.

SEE ALSO

settimeofday(2), adjtime(3), capabilities(7), time(7), adjtimex(8)

COLOPHON

This page is part of release 4.00 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>

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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>