.\" Copyright (c) 1995 Michael Chastain (mec@shell.portal.com), 15 April 1995. .\" and Copyright (C) 2014, 2016 Michael Kerrisk .\" .\" %%%LICENSE_START(GPLv2+_DOC_FULL) .\" This is free documentation; you can redistribute it and/or .\" modify it under the terms of the GNU General Public License as .\" published by the Free Software Foundation; either version 2 of .\" the License, or (at your option) any later version. .\" .\" The GNU General Public License's references to "object code" .\" and "executables" are to be interpreted as the output of any .\" document formatting or typesetting system, including .\" intermediate and printed output. .\" .\" This manual is distributed in the hope that it will be useful, .\" but WITHOUT ANY WARRANTY; without even the implied warranty of .\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the .\" GNU General Public License for more details. .\" .\" You should have received a copy of the GNU General Public .\" License along with this manual; if not, see .\" . .\" %%%LICENSE_END .\" .\" Modified 1997-01-31 by Eric S. Raymond .\" Modified 1997-07-30 by Paul Slootman .\" Modified 2004-05-27 by Michael Kerrisk .\" .TH ADJTIMEX 2 2020-06-09 "Linux" "Linux Programmer's Manual" .SH NAME adjtimex, clock_adjtime, ntp_adjtime \- tune kernel clock .SH SYNOPSIS .nf .B #include .PP .BI "int adjtimex(struct timex *" "buf" ); .PP .BI "int clock_adjtime(clockid_t " clk_id, " struct timex *" "buf" ); .PP .BI "int ntp_adjtime(struct timex *" buf ); .fi .SH DESCRIPTION Linux uses David L.\& Mills' clock adjustment algorithm (see RFC\ 5905). The system call .BR adjtimex () reads and optionally sets adjustment parameters for this algorithm. It takes a pointer to a .I 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: .PP .in +4n .EX 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 */ }; .EE .in .PP The .I modes field determines which parameters, if any, to set. (As described later in this page, the constants used for .BR ntp_adjtime () are equivalent but differently named.) It is a bit mask containing a .RI bitwise- or combination of zero or more of the following bits: .TP .BR ADJ_OFFSET Set time offset from .IR buf.offset . Since Linux 2.6.26, .\" commit 074b3b87941c99bc0ce35385b5817924b1ed0c23 the supplied value is clamped to the range (\-0.5s, +0.5s). In older kernels, an .B EINVAL error occurs if the supplied value is out of range. .TP .BR ADJ_FREQUENCY Set frequency offset from .IR buf.freq . Since Linux 2.6.26, .\" commit 074b3b87941c99bc0ce35385b5817924b1ed0c23 the supplied value is clamped to the range (\-32768000, +32768000). In older kernels, an .B EINVAL error occurs if the supplied value is out of range. .TP .BR ADJ_MAXERROR Set maximum time error from .IR buf.maxerror . .TP .BR ADJ_ESTERROR Set estimated time error from .IR buf.esterror . .TP .BR ADJ_STATUS Set clock status bits from .IR buf.status . A description of these bits is provided below. .TP .BR ADJ_TIMECONST Set PLL time constant from .IR buf.constant . If the .B STA_NANO status flag (see below) is clear, the kernel adds 4 to this value. .TP .BR ADJ_SETOFFSET " (since Linux 2.6.39)" .\" commit 094aa1881fdc1b8889b442eb3511b31f3ec2b762 .\" Author: Richard Cochran Add .I buf.time to the current time. If .I buf.status includes the .B ADJ_NANO flag, then .I buf.time.tv_usec is interpreted as a nanosecond value; otherwise it is interpreted as microseconds. .IP The value of .I buf.time is the sum of its two fields, but the field .I buf.time.tv_usec must always be nonnegative. The following example shows how to normalize a .I timeval with nanosecond resolution. .IP .in +4n .EX while (buf.time.tv_usec < 0) { buf.time.tv_sec -= 1; buf.time.tv_usec += 1000000000; } .EE .in .TP .BR ADJ_MICRO " (since Linux 2.6.26)" .\" commit eea83d896e318bda54be2d2770d2c5d6668d11db .\" Author: Roman Zippel Select microsecond resolution. .TP .BR ADJ_NANO " (since Linux 2.6.26)" .\" commit eea83d896e318bda54be2d2770d2c5d6668d11db .\" Author: Roman Zippel Select nanosecond resolution. Only one of .BR ADJ_MICRO and .BR ADJ_NANO should be specified. .TP .BR ADJ_TAI " (since Linux 2.6.26)" .\" commit 153b5d054ac2d98ea0d86504884326b6777f683d Set TAI (Atomic International Time) offset from .IR buf.constant . .IP .BR ADJ_TAI should not be used in conjunction with .BR ADJ_TIMECONST , since the latter mode also employs the .IR buf.constant field. .IP For a complete explanation of TAI and the difference between TAI and UTC, see .UR http://www.bipm.org/en/bipm/tai/tai.html .I BIPM .UE .TP .BR ADJ_TICK Set tick value from .IR buf.tick . .PP Alternatively, .I modes can be specified as either of the following (multibit mask) values, in which case other bits should not be specified in .IR modes : .\" In general, the other bits are ignored, but ADJ_OFFSET_SINGLESHOT 0x8001 .\" ORed with ADJ_NANO (0x2000) gives 0xa0001 == ADJ_OFFSET_SS_READ!! .TP .BR ADJ_OFFSET_SINGLESHOT .\" In user space, ADJ_OFFSET_SINGLESHOT is 0x8001 .\" In kernel space it is 0x0001, and must be ANDed with ADJ_ADJTIME (0x8000) Old-fashioned .BR adjtime (3): (gradually) adjust time by value specified in .IR buf.offset , which specifies an adjustment in microseconds. .TP .BR ADJ_OFFSET_SS_READ " (functional since Linux 2.6.28)" .\" In user space, ADJ_OFFSET_SS_READ is 0xa001 .\" In kernel space there is ADJ_OFFSET_READONLY (0x2000) anded with .\" ADJ_ADJTIME (0x8000) and ADJ_OFFSET_SINGLESHOT (0x0001) to give 0xa001) Return (in .IR buf.offset ) the remaining amount of time to be adjusted after an earlier .BR ADJ_OFFSET_SINGLESHOT operation. This feature was added in Linux 2.6.24, .\" commit 52bfb36050c8529d9031d2c2513b281a360922ec but did not work correctly .\" commit 916c7a855174e3b53d182b97a26b2e27a29726a1 until Linux 2.6.28. .PP Ordinary users are restricted to a value of either 0 or .B ADJ_OFFSET_SS_READ for .IR modes . Only the superuser may set any parameters. .PP The .I 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. .TP .BR STA_PLL " (read-write)" Enable phase-locked loop (PLL) updates via .BR ADJ_OFFSET . .TP .BR STA_PPSFREQ " (read-write)" Enable PPS (pulse-per-second) frequency discipline. .TP .BR STA_PPSTIME " (read-write)" Enable PPS time discipline. .TP .BR STA_FLL " (read-write)" Select frequency-locked loop (FLL) mode. .TP .BR 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. .\" John Stultz; .\" Usually this is written as extending the day by one second, .\" which is represented as: .\" 23:59:59 .\" 23:59:60 .\" 00:00:00 .\" .\" But since posix cannot represent 23:59:60, we repeat the last second: .\" 23:59:59 + TIME_INS .\" 23:59:59 + TIME_OOP .\" 00:00:00 + TIME_WAIT .\" .TP .BR STA_DEL " (read-write)" Delete a leap second at the last second of the UTC day. .\" John Stultz: .\" Similarly the progression here is: .\" 23:59:57 + TIME_DEL .\" 23:59:58 + TIME_DEL .\" 00:00:00 + TIME_WAIT Leap second deletion will occur each day, so long as this flag remains set. .\" FIXME Does there need to be a statement that it is nonsensical to set .\" to set both STA_INS and STA_DEL? .TP .BR STA_UNSYNC " (read-write)" Clock unsynchronized. .TP .BR STA_FREQHOLD " (read-write)" Hold frequency. .\" Following text from John Stultz: Normally adjustments made via .B 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. .IP This flag prevents the small frequency adjustment from being made when correcting for an .B ADJ_OFFSET value. .\" According to the Kernel Application Program Interface document, .\" STA_FREQHOLD is not used by the NTP version 4 daemon .TP .BR STA_PPSSIGNAL " (read-only)" A valid PPS (pulse-per-second) signal is present. .TP .BR STA_PPSJITTER " (read-only)" PPS signal jitter exceeded. .TP .BR STA_PPSWANDER " (read-only)" PPS signal wander exceeded. .TP .BR STA_PPSERROR " (read-only)" PPS signal calibration error. .TP .BR STA_CLOCKERR " (read-only)" Clock hardware fault. .\" Not set in current kernel (4.5), but checked in a few places .TP .BR STA_NANO " (read-only; since Linux 2.6.26)" .\" commit eea83d896e318bda54be2d2770d2c5d6668d11db .\" Author: Roman Zippel Resolution (0 = microsecond, 1 = nanoseconds). Set via .BR ADJ_NANO , cleared via .BR ADJ_MICRO . .TP .BR STA_MODE " (since Linux 2.6.26)" .\" commit eea83d896e318bda54be2d2770d2c5d6668d11db .\" Author: Roman Zippel Mode (0 = Phase Locked Loop, 1 = Frequency Locked Loop). .TP .BR STA_CLK " (read-only; since Linux 2.6.26)" .\" commit eea83d896e318bda54be2d2770d2c5d6668d11db .\" Author: Roman Zippel Clock source (0 = A, 1 = B); currently unused. .PP Attempts to set read-only .I status bits are silently ignored. .\" .SS clock_adjtime () The .BR clock_adjtime () system call (added in Linux 2.6.39) behaves like .BR adjtimex () but takes an additional .IR clk_id argument to specify the particular clock on which to act. .SS ntp_adjtime () The .BR ntp_adjtime () library function (described in the NTP "Kernel Application Program API", KAPI) is a more portable interface for performing the same task as .BR adjtimex (). Other than the following points, it is identical to .BR adjtimex (): .IP * 3 The constants used in .I modes are prefixed with "MOD_" rather than "ADJ_", and have the same suffixes (thus, .BR MOD_OFFSET , .BR MOD_FREQUENCY , and so on), other than the exceptions noted in the following points. .IP * .BR MOD_CLKA is the synonym for .BR ADJ_OFFSET_SINGLESHOT . .IP * .BR MOD_CLKB is the synonym for .BR ADJ_TICK . .IP * The is no synonym for .BR ADJ_OFFSET_SS_READ , which is not described in the KAPI. .SH RETURN VALUE On success, .BR adjtimex () and .BR ntp_adjtime () return the clock state; that is, one of the following values: .TP 12 .BR TIME_OK Clock synchronized, no leap second adjustment pending. .TP .BR TIME_INS Indicates that a leap second will be added at the end of the UTC day. .TP .BR TIME_DEL Indicates that a leap second will be deleted at the end of the UTC day. .TP .BR TIME_OOP Insertion of a leap second is in progress. .TP .BR TIME_WAIT A leap-second insertion or deletion has been completed. This value will be returned until the next .BR ADJ_STATUS operation clears the .B STA_INS and .B STA_DEL flags. .TP .BR TIME_ERROR The system clock is not synchronized to a reliable server. This value is returned when any of the following holds true: .RS .IP * 3 Either .B STA_UNSYNC or .B STA_CLOCKERR is set. .IP * .B STA_PPSSIGNAL is clear and either .B STA_PPSFREQ or .B STA_PPSTIME is set. .IP * .B STA_PPSTIME and .B STA_PPSJITTER are both set. .IP * .B STA_PPSFREQ is set and either .B STA_PPSWANDER or .B STA_PPSJITTER is set. .RE .IP The symbolic name .B TIME_BAD is a synonym for .BR TIME_ERROR , provided for backward compatibility. .PP Note that starting with Linux 3.4, .\" commit 6b43ae8a619d17c4935c3320d2ef9e92bdeed05d changed to asynchronous .\" operation, so we can no longer rely on the return code. the call operates asynchronously and the return value usually will not reflect a state change caused by the call itself. .PP On failure, these calls return \-1 and set .IR errno . .SH ERRORS .TP .B EFAULT .I buf does not point to writable memory. .TP .BR EINVAL " (kernels before Linux 2.6.26)" An attempt was made to set .I buf.freq to a value outside the range (\-33554432, +33554432). .\" From a quick glance, it appears there was no clamping or range check .\" for buf.freq in kernels before 2.0 .TP .BR EINVAL " (kernels before Linux 2.6.26)" An attempt was made to set .I buf.offset to 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). .TP .B EINVAL An attempt was made to set .I buf.status to a value other than those listed above. .TP .B EINVAL The .I clk_id given to .BR 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 .I clk_id does not refer to a valid instance of a clock object. See .BR clock_gettime (2) for a discussion of dynamic clocks. .TP .B EINVAL An attempt was made to set .I buf.tick to a value outside the range .RB 900000/ HZ to .RB 1100000/ HZ , where .B HZ is the system timer interrupt frequency. .TP .B ENODEV The hot-pluggable device (like USB for example) represented by a dynamic .I clk_id has disappeared after its character device was opened. See .BR clock_gettime (2) for a discussion of dynamic clocks. .TP .B EOPNOTSUPP The given .I clk_id does not support adjustment. .TP .B EPERM .I buf.modes is neither 0 nor .BR ADJ_OFFSET_SS_READ , and the caller does not have sufficient privilege. Under Linux, the .B CAP_SYS_TIME capability is required. .SH ATTRIBUTES For an explanation of the terms used in this section, see .BR attributes (7). .TS allbox; lb lb lb l l l. Interface Attribute Value T{ .BR ntp_adjtime () T} Thread safety MT-Safe .TE .SH CONFORMING TO None of these interfaces is described in POSIX.1 .PP .BR adjtimex () and .BR clock_adjtime () are Linux-specific and should not be used in programs intended to be portable. .PP The preferred API for the NTP daemon is .BR ntp_adjtime (). .SH NOTES In struct .IR timex , .IR freq , .IR ppsfreq , and .I 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 .IR freq ) and output values. .PP The leap-second processing triggered by .B STA_INS and .B 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. .SH SEE ALSO .BR clock_gettime (2), .BR clock_settime (2), .BR settimeofday (2), .BR adjtime (3), .BR ntp_gettime (3), .BR capabilities (7), .BR time (7), .BR adjtimex (8), .BR hwclock (8) .PP .ad l .UR http://www.slac.stanford.edu/comp/unix/\:package/\:rtems/\:src/\:ssrlApps/\:ntpNanoclock/\:api.htm NTP "Kernel Application Program Interface" .UE .SH COLOPHON This page is part of release 5.10 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%https://www.kernel.org/doc/man\-pages/.