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.



buf does not point to writable memory.


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.


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), adjtimex(8)


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−pages/.

  Copyright (c) 1995 Michael Chastain (, 15 April 1995.

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Modified 1997-01-31 by Eric S. Raymond <>
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Modified 2004-05-27 by Michael Kerrisk <>