adjtimex - tune kernel clock


#include <sys/time> 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


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
  1. define ADJ_OFFSET 0x0001 /* time offset */
  2. define ADJ_FREQUENCY 0x0002 /* frequency offset */
  3. define ADJ_MAXERROR 0x0004 /* maximum time error */
  4. define ADJ_ESTERROR 0x0008 /* estimated time error */
  5. define ADJ_STATUS 0x0010 /* clock status */
  6. define ADJ_TIMECONST 0x0020 /* pll time constant */
  7. define ADJ_TICK 0x4000 /* tick value */
  8. 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
  1. define TIME_OK 0 /* clock synchronized */
  2. define TIME_INS 1 /* insert leap second */
  3. define TIME_DEL 2 /* delete leap second */
  4. define TIME_OOP 3 /* leap second in progress */
  5. define TIME_WAIT 4 /* leap second has occurred */
  6. define TIME_BAD 5 /* clock not synchronized */

On failure, adjtimex returns -1 and sets errno.


buf does not point to writable memory.
buf.mode is non-zero and the user is not super-user.
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.


adjtimex is Linux specific and should not be used in programs intended to be portable. There is a similar but less general call adjtime in SVr4.



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