adjtimex (2) manual page

Table of Contents


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 */
0x0002 /* frequency offset */
0x0004 /* maximum time error */
0x0008 /* estimated time error */
#define ADJ_STATUS
0x0010 /* clock status */
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.


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.

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)

Table of Contents