1ADJTIMEX(2) Linux Programmer's Manual ADJTIMEX(2)
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6 adjtimex, clock_adjtime, ntp_adjtime - tune kernel clock
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9 #include <sys/timex.h>
10 int adjtimex(struct timex *buf);
12 int clock_adjtime(clockid_t clk_id, struct timex *buf);
14 int ntp_adjtime(struct timex *buf);
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18 Linux uses David L. Mills' clock adjustment algorithm (see RFC 5905).
19 The system call adjtimex() reads and optionally sets adjustment parame‐
20 ters for this algorithm. It takes a pointer to a timex structure, up‐
21 dates kernel parameters from (selected) field values, and returns the
22 same structure updated with the current kernel values. This structure
23 is declared as follows:
24 struct timex {
26 int modes; /* Mode selector */
27 long offset; /* Time offset; nanoseconds, if STA_NANO
28 status flag is set, otherwise
29 microseconds */
30 long freq; /* Frequency offset; see NOTES for units */
31 long maxerror; /* Maximum error (microseconds) */
32 long esterror; /* Estimated error (microseconds) */
33 int status; /* Clock command/status */
34 long constant; /* PLL (phase-locked loop) time constant */
35 long precision; /* Clock precision
36 (microseconds, read-only) */
37 long tolerance; /* Clock frequency tolerance (read-only);
38 see NOTES for units */
39 struct timeval time;
40 /* Current time (read-only, except for
41 ADJ_SETOFFSET); upon return, time.tv_usec
42 contains nanoseconds, if STA_NANO status
43 flag is set, otherwise microseconds */
44 long tick; /* Microseconds between clock ticks */
45 long ppsfreq; /* PPS (pulse per second) frequency
46 (read-only); see NOTES for units */
47 long jitter; /* PPS jitter (read-only); nanoseconds, if
48 STA_NANO status flag is set, otherwise
49 microseconds */
50 int shift; /* PPS interval duration
51 (seconds, read-only) */
52 long stabil; /* PPS stability (read-only);
53 see NOTES for units */
54 long jitcnt; /* PPS count of jitter limit exceeded
55 events (read-only) */
56 long calcnt; /* PPS count of calibration intervals
57 (read-only) */
58 long errcnt; /* PPS count of calibration errors
59 (read-only) */
60 long stbcnt; /* PPS count of stability limit exceeded
61 events (read-only) */
62 int tai; /* TAI offset, as set by previous ADJ_TAI
63 operation (seconds, read-only,
64 since Linux 2.6.26) */
65 /* Further padding bytes to allow for future expansion */
66 };
67 The modes field determines which parameters, if any, to set. (As de‐
69 scribed later in this page, the constants used for ntp_adjtime() are
70 equivalent but differently named.) It is a bit mask containing a bit‐
71 wise-or combination of zero or more of the following bits:
72 ADJ_OFFSET
74 Set time offset from buf.offset. Since Linux 2.6.26, the sup‐
75 plied value is clamped to the range (-0.5s, +0.5s). In older
76 kernels, an EINVAL error occurs if the supplied value is ou