ADJTIMEX(2)

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 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)