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.


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.


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)


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