adjtimex.2 - Online Linux Manual Page

adjtimex - Online Linux Manual PageSection : 2
Updated : 2023-02-10
Source : Linux man-pages 6.03

NAMEadjtimex, clock_adjtime, ntp_adjtime − tune kernel clock

LIBRARYStandard C library (libc, −lc)

SYNOPSIS#include <sys/timex.h>int adjtimex(struct timex *buf);int clock_adjtime(clockid_t clk_id, struct timex *buf);int ntp_adjtime(struct timex *buf);

DESCRIPTIONLinux uses David L. Mills' clock adjustment algorithm (see RFC 5905). 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 (selected) field values, and returns the same structure updated with the current kernel values. This structure is declared as follows: struct timex { int modes; /* Mode selector */ long offset; /* Time offset; nanoseconds, if STA_NANO status flag is set, otherwise microseconds */ long freq; /* Frequency offset; see NOTES for units */ long maxerror; /* Maximum error (microseconds) */ long esterror; /* Estimated error (microseconds) */ int status; /* Clock command/status */ long constant; /* PLL (phase−locked loop) time constant */ long precision; /* Clock precision (microseconds, read−only) */ long tolerance; /* Clock frequency tolerance (read−only); see NOTES for units */ struct timeval time; /* Current time (read−only, except for ADJ_SETOFFSET); upon return, time.tv_usec contains nanoseconds, if STA_NANO status flag is set, otherwise microseconds */ long tick; /* Microseconds between clock ticks */ long ppsfreq; /* PPS (pulse per second) frequency (read−only); see NOTES for units */ long jitter; /* PPS jitter (read−only); nanoseconds, if STA_NANO status flag is set, otherwise microseconds */ int shift; /* PPS interval duration (seconds, read−only) */ long stabil; /* PPS stability (read−only); see NOTES for units */ long jitcnt; /* PPS count of jitter limit exceeded events (read−only) */ long calcnt; /* PPS count of calibration intervals (read−only) */ long errcnt; /* PPS count of calibration errors (read−only) */ long stbcnt; /* PPS count of stability limit exceeded events (read−only) */ int tai; /* TAI offset, as set by previous ADJ_TAI operation (seconds, read−only, since Linux 2.6.26) */ /* Further padding bytes to allow for future expansion */ };The modes field determines which parameters, if any, to set. (As described later in this page, the constants used for ntp_adjtime() are equivalent but differently named.) It is a bit mask containing a bitwise-or combination of zero or more of the following bits: ADJ_OFFSET Set time offset from buf.offset. Since Linux 2.6.26, the supplied value is clamped to the range (−0.5s, +0.5s). In older kernels, an EINVAL error occurs if the supplied value is out of range. ADJ_FREQUENCY Set frequency offset from buf.freq. Since Linux 2.6.26, the supplied value is clamped to the range (−32768000, +32768000). In older kernels, an EINVAL error occurs if the supplied value is out of range. ADJ_MAXERROR Set maximum time error from buf.maxerror. ADJ_ESTERROR Set estimated time error from buf.esterror. ADJ_STATUS Set clock status bits from buf.status. A description of these bits is provided below. ADJ_TIMECONST Set PLL time constant from buf.constant. If the STA_NANO status flag (see below) is clear, the kernel adds 4 to this value. ADJ_SETOFFSET (since Linux 2.6.39) Add buf.time to the current time. If buf.status includes the ADJ_NANO flag, then buf.time.tv_usec is interpreted as a nanosecond value; otherwise it is interpreted as microseconds. The value of buf.time is the sum of its two fields, but the field buf.time.tv_usec must always be nonnegative. The following example shows how to normalize a timeval with nanosecond resolution. while (buf.time.tv_usec < 0) { buf.time.tv_sec −= 1; buf.time.tv_usec += 1000000000; }ADJ_MICRO (since Linux 2.6.26) Select microsecond resolution. ADJ_NANO (since Linux 2.6.26) Select nanosecond resolution. Only one of ADJ_MICRO and ADJ_NANO should be specified. ADJ_TAI (since Linux 2.6.26) Set TAI (Atomic International Time) offset from buf.constant. ADJ_TAI should not be used in conjunction with ADJ_TIMECONST, since the latter mode also employs the buf.constant field. For a complete explanation of TAI and the difference between TAI and UTC, see BIPM ADJ_TICK Set tick value from buf.tick. Alternatively, modes can be specified as either of the following (multibit mask) values, in which case other bits should not be specified in modes: ADJ_OFFSET_SINGLESHOT Old-fashioned adjtime(3): (gradually) adjust time by value specified in buf.offset, which specifies an adjustment in microseconds. ADJ_OFFSET_SS_READ (functional since Linux 2.6.28) Return (in buf.offset) the remaining amount of time to be adjusted after an earlier ADJ_OFFSET_SINGLESHOT operation. This feature was added in Linux 2.6.24, but did not work correctly until Linux 2.6.28. Ordinary users are restricted to a value of either 0 or ADJ_OFFSET_SS_READ for modes. Only the superuser may set any parameters. The buf.status field is a bit mask that is used to set and/or retrieve status bits associated with the NTP implementation. Some bits in the mask are both readable and settable, while others are read-only. STA_PLL (read-write) Enable phase-locked loop (PLL) updates via ADJ_OFFSET. STA_PPSFREQ (read-write) Enable PPS (pulse-per-second) frequency discipline. STA_PPSTIME (read-write) Enable PPS time discipline. STA_FLL (read-write) Select frequency-locked loop (FLL) mode. STA_INS (read-write) Insert a leap second after the last second of the UTC day, thus extending the last minute of the day by one second. Leap-second insertion will occur each day, so long as this flag remains set. STA_DEL (read-write) Delete a leap second at the last second of the UTC day. Leap second deletion will occur each day, so long as this flag remains set. STA_UNSYNC (read-write) Clock unsynchronized. STA_FREQHOLD (read-write) Hold frequency. Normally adjustments made via ADJ_OFFSET result in dampened frequency adjustments also being made. So a single call corrects the current offset, but as offsets in the same direction are made repeatedly, the small frequency adjustments will accumulate to fix the long-term skew. This flag prevents the small frequency adjustment from being made when correcting for an ADJ_OFFSET value. STA_PPSSIGNAL (read-only) A valid PPS (pulse-per-second) signal is present. STA_PPSJITTER (read-only) PPS signal jitter exceeded. STA_PPSWANDER (read-only) PPS signal wander exceeded. STA_PPSERROR (read-only) PPS signal calibration error. STA_CLOCKERR (read-only) Clock hardware fault. STA_NANO (read-only; since Linux 2.6.26) Resolution (0 = microsecond, 1 = nanoseconds). Set via ADJ_NANO, cleared via ADJ_MICRO. STA_MODE (since Linux 2.6.26) Mode (0 = Phase Locked Loop, 1 = Frequency Locked Loop). STA_CLK (read-only; since Linux 2.6.26) Clock source (0 = A, 1 = B); currently unused. Attempts to set read-only status bits are silently ignored.

clock_adjtime()The clock_adjtime() system call (added in Linux 2.6.39) behaves like adjtimex() but takes an additional clk_id argument to specify the particular clock on which to act.

ntp_adjtime()The ntp_adjtime() library function (described in the NTP "Kernel Application Program API", KAPI) is a more portable interface for performing the same task as adjtimex(). Other than the following points, it is identical to adjtimex(): • The constants used in modes are prefixed with "MOD_" rather than "ADJ_", and have the same suffixes (thus, MOD_OFFSET, MOD_FREQUENCY, and so on), other than the exceptions noted in the following points. • MOD_CLKA is the synonym for ADJ_OFFSET_SINGLESHOT. • MOD_CLKB is the synonym for ADJ_TICK. • The is no synonym for ADJ_OFFSET_SS_READ, which is not described in the KAPI.

RETURN VALUEOn success, adjtimex() and ntp_adjtime() return the clock state; that is, one of the following values: TIME_OK Clock synchronized, no leap second adjustment pending. TIME_INS Indicates that a leap second will be added at the end of the UTC day. TIME_DEL Indicates that a leap second will be deleted at the end of the UTC day. TIME_OOP Insertion of a leap second is in progress. TIME_WAIT A leap-second insertion or deletion has been completed. This value will be returned until the next ADJ_STATUS operation clears the STA_INS and STA_DEL flags. TIME_ERROR The system clock is not synchronized to a reliable server. This value is returned when any of the following holds true: • Either STA_UNSYNC or STA_CLOCKERR is set. • STA_PPSSIGNAL is clear and either STA_PPSFREQ or STA_PPSTIME is set. • STA_PPSTIME and STA_PPSJITTER are both set. • STA_PPSFREQ is set and either STA_PPSWANDER or STA_PPSJITTER is set. The symbolic name TIME_BAD is a synonym for TIME_ERROR, provided for backward compatibility. Note that starting with Linux 3.4, the call operates asynchronously and the return value usually will not reflect a state change caused by the call itself. On failure, these calls return −1 and set errno to indicate the error.

ERRORSEFAULT buf does not point to writable memory. EINVAL (before Linux 2.6.26) An attempt was made to set buf.freq to a value outside the range (−33554432, +33554432). EINVAL (before Linux 2.6.26) An attempt was made to set buf.offset to a value outside the permitted range. Before Linux 2.0, the permitted range was (−131072, +131072). From Linux 2.0 onwards, the permitted range was (−512000, +512000). EINVAL An attempt was made to set buf.status to a value other than those listed above. EINVAL The clk_id given to clock_adjtime() is invalid for one of two reasons. Either the System-V style hard-coded positive clock ID value is out of range, or the dynamic clk_id does not refer to a valid instance of a clock object. See clock_gettime(2) for a discussion of dynamic clocks. EINVAL An attempt was made to set buf.tick to a value outside the range 900000/HZ to 1100000/HZ, where HZ is the system timer interrupt frequency. ENODEV The hot-pluggable device (like USB for example) represented by a dynamic clk_id has disappeared after its character device was opened. See clock_gettime(2) for a discussion of dynamic clocks. EOPNOTSUPP The given clk_id does not support adjustment. EPERM buf.modes is neither 0 nor ADJ_OFFSET_SS_READ, and the caller does not have sufficient privilege. Under Linux, the CAP_SYS_TIME capability is required.

ATTRIBUTESFor an explanation of the terms used in this section, see attributes(7). InterfaceAttributeValue ntp_adjtime() Thread safetyMT-Safe

STANDARDSNone of these interfaces is described in POSIX.1 adjtimex() and clock_adjtime() are Linux-specific and should not be used in programs intended to be portable. The preferred API for the NTP daemon is ntp_adjtime().

NOTESIn struct timex, freq, ppsfreq, and stabil are ppm (parts per million) with a 16-bit fractional part, which means that a value of 1 in one of those fields actually means 2^-16 ppm, and 2^16=65536 is 1 ppm. This is the case for both input values (in the case of freq) and output values. The leap-second processing triggered by STA_INS and STA_DEL is done by the kernel in timer context. Thus, it will take one tick into the second for the leap second to be inserted or deleted.

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