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_OFFSET
Set 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_FREQUENCY
Set 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_MAXERROR
Set maximum time error from buf.maxerror.
ADJ_ESTERROR
Set estimated time error from buf.esterror.
ADJ_STATUS
Set clock status bits from buf.status. A description of
these bits is provided below.
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 2.6.39)
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.
The value of buf.time is the sum of its two fields, but
the field buf.time.tv_usec must always be nonnegative.
The following example shows how to normalize a timeval
with nanosecond resolution.
while (buf.time.tv_usec < 0) {
buf.time.tv_sec -= 1;
buf.time.tv_usec += 1000000000;
}
ADJ_MICRO (since Linux 2.6.26)
Select microsecond resolution.
ADJ_NANO (since Linux 2.6.26)
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(3): (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.
clock_adjtime ()
The clock_adjtime() system call (added in Linux 2.6.39) behaves
like adjtimex() but takes an additional clk_id argument to
specify the particular clock on which to act.
ntp_adjtime ()
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 adjtimex():
* 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.
