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NAME
adjtimex, clock_adjtime, ntp_adjtime - tune kernel clock
SYNOPSIS
#include <sys/timex.h>
int adjtimex(struct timex *buf);
int clock_adjtime(clockid_t clk_id, struct timex *buf);
int ntp_adjtime(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 (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.
RETURN VALUE
On success, adjtimex() and ntp_adjtime() return the clock state; that is, one of the following values:
TIME_OK Clock synchronized, no leap second adjustment pending.
TIME_INS Indicates that a leap second will be added at the end of the UTC day.
TIME_DEL Indicates that a leap second will be deleted at the end of the UTC day.
TIME_OOP Insertion of a leap second is in progress.
TIME_WAIT A leap-second insertion or deletion has been completed. This value will be returned until
the next ADJ_STATUS operation clears the STA_INS and STA_DEL flags.
TIME_ERROR The system clock is not synchronized to a reliable server. This value is returned when any
of the following holds true:
* Either STA_UNSYNC or STA_CLOCKERR is set.
* STA_PPSSIGNAL is clear and either STA_PPSFREQ or STA_PPSTIME is set.
* STA_PPSTIME and STA_PPSJITTER are both set.
* STA_PPSFREQ is set and either STA_PPSWANDER or STA_PPSJITTER is set.
The symbolic name TIME_BAD is a synonym for TIME_ERROR, provided for backward compatibility.
Note that starting with Linux 3.4, the call operates asynchronously and the return value usually will not
reflect a state change caused by the call itself.
On failure, these calls return -1 and set errno.
ERRORS
EFAULT buf does not point to writable memory.
EINVAL (kernels before Linux 2.6.26)
An attempt was made to set buf.freq to a value outside the range (-33554432, +33554432).
EINVAL (kernels before Linux 2.6.26)
An attempt was made to set buf.offset to a value outside the permitted range. In kernels before
Linux 2.0, the permitted range was (-131072, +131072). From Linux 2.0 onwards, the permitted
range was (-512000, +512000).
EINVAL An attempt was made to set buf.status to a value other than those listed above.
EINVAL The clk_id given to clock_adjtime() is invalid for one of two reasons. Either the System-V style
hard-coded positive clock ID value is out of range, or the dynamic clk_id does not refer to a
valid instance of a clock object. See clock_gettime(2) for a discussion of dynamic clocks.
EINVAL An attempt was made to set buf.tick to a value outside the range 900000/HZ to 1100000/HZ, where HZ
is the system timer interrupt frequency.
ENODEV The hot-pluggable device (like USB for example) represented by a dynamic clk_id has disappeared
after its character device was opened. See clock_gettime(2) for a discussion of dynamic clocks.
EOPNOTSUPP
The given clk_id does not support adjustment.
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.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌──────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├──────────────┼───────────────┼─────────┤
│ntp_adjtime() │ Thread safety │ MT-Safe │
└──────────────┴───────────────┴─────────┘
CONFORMING TO
None of these interfaces is described in POSIX.1
adjtimex() and clock_adjtime() are Linux-specific and should not be used in programs intended to be
portable.
The preferred API for the NTP daemon is ntp_adjtime().
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.
The leap-second processing triggered by STA_INS and STA_DEL is done by the kernel in timer context.
Thus, it will take one tick into the second for the leap second to be inserted or deleted.
SEE ALSO
clock_gettime(2), clock_settime(2), settimeofday(2), adjtime(3), ntp_gettime(3), capabilities(7),
time(7), adjtimex(8), hwclock(8)
NTP "Kernel Application Program Interface" ⟨http://www.slac.stanford.edu/comp/unix/package/rtems/src/
ssrlApps/ntpNanoclock/api.htm⟩
COLOPHON
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https://www.kernel.org/doc/man-pages/.