ntp_adjtime

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

NAME
       adjtimex, ntp_adjtime - tune kernel clock

SYNOPSIS
       #include <sys/timex.h>

       int adjtimex(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,
       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  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.

       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(): (gradually) adjust time by value speci-
              fied  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
              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_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
              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.

   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 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.

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_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 per-
              mitted range was (-512000, +512000).

       EINVAL An attempt was made to set buf.status  to  a  value  other  than
              those listed above.

       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.

       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
       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().

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 second
       for the leap second to be inserted or deleted.

SEE ALSO
       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 4.15 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                             2016-10-08                       ADJTIMEX(2)
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