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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
       parameters  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 follows:

           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
       superuser 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  nonzero  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
       intended to be portable.  See adjtime(3) for a more portable, but  less
       flexible, method of adjusting the system clock.


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


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