Manpage of ADJTIMEX

ADJTIMEX

Section: Linux Programmer's Manual (2)
Updated: 2016-03-15
Index
 

NAME

adjtimex, ntp_adjtime - tune kernel clock  

SYNOPSIS

#include <sys/timex.h>int adjtimex(struct timex *buf);int ntp_adjtime(struct timex *buf);
 

DESCRIPTION

Linux 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 timexstructure, 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 modesfield 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-orcombination 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 EINVALerror 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 EINVALerror 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_NANOstatus flag (see below) is clear, the kernel adds 4 to this value.
ADJ_SETOFFSET (since Linux 2.6.29)
Add buf.timeto the current time. If buf.statusincludes the ADJ_NANOflag, then buf.time.tv_usecis interpreted as a nanosecond value; otherwise it is interpreted as microseconds.
ADJ_MICRO (since Linux 2.6.36)
Select microsecond resolution.
ADJ_NANO (since Linux 2.6.36)
Select nanosecond resolution. Only one of ADJ_MICROand ADJ_NANOshould be specified.
ADJ_TAI (since Linux 2.6.26)
Set TAI (Atomic International Time) offset from buf.constant.

ADJ_TAIshould not be used in conjunction with ADJ_TIMECONST, since the latter mode also employs the buf.constantfield.

For a complete explanation of TAI and the difference between TAI and UTC, see BIPM

ADJ_TICK
Set tick value from buf.tick.

Alternatively, modescan 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(): (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_SINGLESHOToperation. 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_READfor modes. Only the superuser may set any parameters.

The buf.statusfield 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_OFFSETresult 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_OFFSETvalue.

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 statusbits are silently ignored.  

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 adjtime():
*
The constants used in modesare 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_CLKAis the synonym for ADJ_OFFSET_SINGLESHOT.
*
MOD_CLKBis the synonym for ADJ_TICK.
*
The is no synonym for ADJ_OFFSET_SS_READ, which is not described in the KAPI.
 

RETURN VALUE

On 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_STATUSoperation clears the STA_INSand STA_DELflags.
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_UNSYNCor STA_CLOCKERRis set.
*
STA_PPSSIGNALis clear and either STA_PPSFREQor STA_PPSTIMEis set.
*
STA_PPSTIMEand STA_PPSJITTERare both set.
*
STA_PPSFREQis set and either STA_PPSWANDERor STA_PPSJITTERis set.
The symbolic name TIME_BADis 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.  

ERRORS

EFAULT
bufdoes not point to writable memory.
EINVAL (kernels before Linux 2.6.26)
An attempt was made to set buf.freqto a value outside the range (-33554432, +33554432).
EINVAL (kernels before Linux 2.6.26)
An attempt was made to set buf.offsetto a value outside the permitted range. In kernels 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.statusto a value other than those listed above.
EINVAL
An attempt was made to set buf.tickto a value outside the range 900000/HZto 1100000/HZ, where HZis the system timer interrupt frequency.
EPERM
buf.modesis neither 0 nor ADJ_OFFSET_SS_READ, and the caller does not have sufficient privilege. Under Linux, the CAP_SYS_TIMEcapability is required.
 

NOTES

In struct timex, freq, ppsfreq, and stabilare 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_INSand STA_DELis 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.  

ATTRIBUTES

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

CONFORMING TO

Neither of these interfaces is described in POSIX.1

adjtimex() is Linux-specific and should not be used in programs intended to be portable.

The preferred API for the NTP daemon is ntp_adjtime(3).  

SEE ALSO

settimeofday(2), adjtime(3), ntp_gettime(3), capabilities(7), time(7), adjtimex(8)

NTP "Kernel Application Program Interface"


 

Index

NAME
SYNOPSIS
DESCRIPTION
ntp_adjtime ()
RETURN VALUE
ERRORS
NOTES
ATTRIBUTES
CONFORMING TO
SEE ALSO

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Time: 22:27:42 GMT, June 20, 2016