adjtimex(2) tune kernel clock


#define _BSD_SOURCE /* See feature_test_macros(7) */
#include <sys/timex.h>
int adjtimex(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 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:

Set time offset from buf.offset.
Set frequency offset from buf.freq.
Set maximum time error from buf.maxerror.
Set estimated time error from buf.esterror.
Set clock status from buf.status.
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

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.
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.

Enable phase-locked loop (PLL) updates (read-write) via ADJ_OFFSET.
Enable PPS freq discipline (read-write).
Enable PPS time discipline (read-write).
Select frequency-locked loop (FLL) mode (read-write).
Insert leap second (read-write).
Delete leap second (read-write).
Clock unsynchronized (read-write).
Hold frequency (read-write).
PPS signal present (read-only).
PPS signal jitter exceeded (read-only).
PPS signal wander exceeded (read-only).
PPS signal calibration error (read-only).
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.


On success, adjtimex() returns the clock state; that is, one of the following values:
Clock synchronized.
Insert leap second.
Delete leap second.
Leap second in progress.
Leap second has occurred.
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.


buf does not point to writable memory.
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.
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.


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.


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.


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