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ADJTIMEX(2)                Linux Programmer's ManualADJTIMEX(2)NAME       adjtimex, ntp_adjtime - tune kernel clockSYNOPSIS       #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, 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.       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  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.   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_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 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 at-tributes(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  in-       tended 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 ALSOsettimeofday(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 5.05 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                             2019-03-06ADJTIMEX(2)