adjtimex(2)



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 parame-
       ters  for this algorithm.  It takes a pointer to a timex structure, up-
       dates 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  de-
       scribed  later  in  this page, the constants used for ntp_adjtime() are
       equivalent but differently named.)  It is a bit mask containing a  bit-
       wise-or combination of zero or more of the following bits:

       ADJ_OFFSET
              Set  time  offset from buf.offset.  Since Linux 2.6.26, the sup-
              plied 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  follow-
              ing  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  speci-
       fied in modes:

       ADJ_OFFSET_SINGLESHOT
              Old-fashioned adjtime(3): (gradually) adjust time by value spec-
              ified in buf.offset, which specifies an adjustment in  microsec-
              onds.

       ADJ_OFFSET_SS_READ (functional since Linux 2.6.28)
              Return  (in  buf.offset)  the remaining amount of time to be ad-
              justed 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_OFF-
       SET_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  re-
              mains 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 di-
              rection 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  par-
       ticular clock on which to act.

   ntp_adjtime ()
       The ntp_adjtime() library function (described in the NTP "Kernel Appli-
       cation 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 oper-
                   ation 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_PP-
                      STIME is set.

                   *  STA_PPSTIME and STA_PPSJITTER are both set.

                   *  STA_PPSFREQ  is  set and either STA_PPSWANDER or STA_PP-
                      SJITTER 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  per-
              mitted 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 at-
       tributes(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 val-
       ues.

       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 sec-
       ond 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)

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