NAME

       adjtimex - tune kernel clock

SYNOPSIS

       #include <sys/timex.h>

       int adjtimex(struct timex *buf);

DESCRIPTION

       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.

RETURN VALUE

       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.

ERRORS

       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.

CONFORMING TO

       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.

SEE ALSO

       settimeofday(2), adjtime(3), capabilities(7), time(7), adjtimex(8)

COLOPHON

       This page is part of release 3.54 of the Linux man-pages project.  A  description  of  the
       project,     and    information    about    reporting    bugs,    can    be    found    at
       http://www.kernel.org/doc/man-pages/.