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        adjtimex - tune kernel clock


        #include <sys/timex.h>
        int adjtimex(struct timex *buf);


        Linux  uses  David L. Mills’ clock adjustment algorithm (see RFC 1305).
        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  field  values,  and  returns  the  same
        structure  with  current  kernel values.  This structure is declared as
               struct timex {
                   int modes;           /* mode selector */
                   long offset;         /* time offset (usec) */
                   long freq;           /* frequency offset (scaled ppm) */
                   long maxerror;       /* maximum error (usec) */
                   long esterror;       /* estimated error (usec) */
                   int status;          /* clock command/status */
                   long constant;       /* pll time constant */
                   long precision;      /* clock precision (usec) (read only) */
                   long tolerance;      /* clock frequency tolerance (ppm)
                                           (read only) */
                   struct timeval time; /* current time (read only) */
                   long tick;           /* usecs between clock ticks */
        The modes field determines which parameters, if any, to  set.   It  may
        contain a bitwise-or combination of zero or more of the following bits:
               #define ADJ_OFFSET            0x0001 /* time offset */
               #define ADJ_FREQUENCY         0x0002 /* frequency offset */
               #define ADJ_MAXERROR          0x0004 /* maximum time error */
               #define ADJ_ESTERROR          0x0008 /* estimated time error */
               #define ADJ_STATUS            0x0010 /* clock status */
               #define ADJ_TIMECONST         0x0020 /* pll time constant */
               #define ADJ_TICK              0x4000 /* tick value */
               #define ADJ_OFFSET_SINGLESHOT 0x8001 /* old-fashioned adjtime() */
        Ordinary users are restricted to a zero value for mode.  Only the supe‐
        ruser may set any parameters.
        On success, adjtimex() returns the clock state:
               #define TIME_OK   0 /* clock synchronized */
               #define TIME_INS  1 /* insert leap second */
               #define TIME_DEL  2 /* delete leap second */
               #define TIME_OOP  3 /* leap second in progress */
               #define TIME_WAIT 4 /* leap second has occurred */
               #define TIME_BAD  5 /* clock not synchronized */
        On failure, adjtimex() returns -1 and sets errno.


        EFAULT buf does not point to writable memory.
        EINVAL An  attempt  is  made  to  set buf.offset to a value outside the
               range -131071 to +131071, or to set buf.status to a value  other
               than  those  listed above, or 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.mode  is  non-zero  and  the caller does not have sufficient
               privilege.  Under Linux the CAP_SYS_TIME capability is required.
        adjtimex()  is  Linux  specific  and  should  not  be  used in programs
adjtime(3) for a more portable, but  less
        flexible, method of adjusting the system clock.


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