adjtimex(2) — Linux manual page


adjtimex(2)                System Calls Manual               adjtimex(2)

NAME         top

       adjtimex, clock_adjtime, ntp_adjtime - tune kernel clock

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #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         top

       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:

              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 provided 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

              The value of buf.time is the sum of its two fields, but
              the field buf.time.tv_usec must always be nonnegative.
              The following 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

              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
              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(3): (gradually) adjust time by value
              specified in buf.offset, which specifies an adjustment in

       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

       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.

   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 particular clock on which to act.

   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 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         top

       On success, adjtimex() and ntp_adjtime() return the clock state;
       that is, one of the following values:

              Clock synchronized, no leap second adjustment pending.

              Indicates that a leap second will be added at the end of
              the UTC day.

              Indicates that a leap second will be deleted at the end of
              the UTC day.

              Insertion of a leap second is in progress.

              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.

              The system clock is not synchronized to a reliable server.
              This value is returned when any of the following holds

              •  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 to indicate the

ERRORS         top

       EFAULT buf does not point to writable memory.

       EINVAL (before Linux 2.6.26)
              An attempt was made to set buf.freq to a value outside the
              range (-33554432, +33554432).

       EINVAL (before Linux 2.6.26)
              An attempt was made to set buf.offset to a value outside
              the permitted range.  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 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

       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.

              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         top

       For an explanation of the terms used in this section, see
       │ Interface                           Attribute     Value   │
       │ ntp_adjtime()                       │ Thread safety │ MT-Safe │

STANDARDS         top


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

NOTES         top

       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         top

       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" 
       ⟨ man-pages (unreleased)     (date)                      adjtimex(2)

Pages that refer to this page: clock_getres(2)gettimeofday(2)syscalls(2)adjtime(3)clockid_t(3type)ntp_gettime(3)rtc(4)systemd.exec(5)capabilities(7)time(7)hwclock(8)