ADJTIMEX

Man Pages


NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
NOTES
CONFORMING TO
SEE ALSO
COLOPHON

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

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

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