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adjtimex, ntp_adjtime

tune kernel clock

#include <sys/timex.h>
int adjtimex(struct timex *buf);
int ntp_adjtime(struct timex *buf);

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:

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.
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.
Set maximum time error from buf.maxerror.
Set estimated time error from buf.esterror.
Set clock status bits from buf.status. A description of these bits is provided below.
Set PLL time constant from buf.constant. If the STA_NANO status flag (see below) is clear, the kernel adds 4 to this value.
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.
Select microsecond resolution.
Select nanosecond resolution. Only one of ADJ_MICRO and ADJ_NANO should be specified.
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
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:

Old-fashioned adjtime(): (gradually) adjust time by value specified in buf.offset, which specifies an adjustment in microseconds.
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.

Enable phase-locked loop (PLL) updates via ADJ_OFFSET.
Enable PPS (pulse-per-second) frequency discipline.
Enable PPS time discipline.
Select frequency-locked loop (FLL) mode.
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.
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.
Clock unsynchronized.
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.
A valid PPS (pulse-per-second) signal is present.
PPS signal jitter exceeded.
PPS signal wander exceeded.
PPS signal calibration error.
Clock hardware fault.
Resolution (0 = microsecond, 1 = nanoseconds). Set via ADJ_NANO, cleared via ADJ_MICRO.
Mode (0 = Phase Locked Loop, 1 = Frequency Locked Loop).
Clock source (0 = A, 1 = B); currently unused.

Attempts to set read-only status bits are silently ignored.

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 adjtime():

  • 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.

On success, adjtimex() and ntp_adjtime() return the clock state; that is, one of the following values:

Clock synchronized, no leap second adjustment pending.
Indicates that a leap second will be added at the end of the UTC day.
Indicates that a leap second will be deleted at the end of the UTC day.
Insertion of a leap second is in progress.
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.
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.

buf does not point to writable memory.
An attempt was made to set buf.freq to a value outside the range (-33554432, +33554432).
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).
An attempt was made to set buf.status to a value other than those listed above.
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.
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.

For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
ntp_adjtime () Thread safety MT-Safe

Neither of these interfaces is described in POSIX.1

adjtimex() is Linux-specific and should not be used in programs intended to be portable.

The preferred API for the NTP daemon is ntp_adjtime().

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.

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

NTP "Kernel Application Program Interface"

This page is part of release 5.05 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 https://www.kernel.org/doc/man-pages/.