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 non-zero 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)