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

       adjtimex, ntp_adjtime - tune kernel clock

       #include <sys/timex.h>

       int adjtimex(struct timex *buf);

       int ntp_adjtime(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 (selected)
       field values, and returns the same structure updated with the current  kernel  val-
       ues.  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:

              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.

              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.

              Set maximum time error from buf.maxerror.

              Set estimated time error from buf.esterror.

              Set clock status bits from buf.status.  A description of these bits is  pro-
              vided below.

              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

       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 lat-
              ter 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.

       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  dele-
              tion 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  cur-
              rent  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  cor-
              recting 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  Pro-
       gram  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.

       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.

       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,

       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,

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

       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.

       For an explanation of the terms used in this section, see attributes(7).

       Neither of these interfaces is described in POSIX.1

       adjtimex()  is  Linux-specific  and  should  not be used in programs intended to be

       The preferred API for the NTP daemon is ntp_adjtime(3).

       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 actu-
       ally 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.

       settimeofday(2), adjtime(3), capabilities(7), time(7)

       NTP "Kernel Application Program Interface"

Linux                             2016-10-08                       ADJTIMEX(2)

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