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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
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