ntp_adjtime

ADJTIMEX(2)                Linux Programmer's Manual               ADJTIMEX(2)



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.htmlADJ_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
       This page is part of release 5.07 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/.




Linux                             2020-06-09                       ADJTIMEX(2)