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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 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.

       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 BIPMhttp://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(): (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.

   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 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 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 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).

       ┌──────────────┬───────────────┬─────────┐
       │InterfaceAttributeValue   │
       ├──────────────┼───────────────┼─────────┤
       │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 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

       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 4.16 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/.