man2/adjtimex.2

   1 .\" Copyright (c) 1995 Michael Chastain (mec@shell.portal.com), 15 April 1995.
   2 .\" and Copyright (C) 2014 Michael Kerrisk <mtk.manpages@gmail.com>
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  25 .\" Modified 1997-01-31 by Eric S. Raymond <esr@thyrsus.com>
  26 .\" Modified 1997-07-30 by Paul Slootman <paul@wurtel.demon.nl>
  27 .\" Modified 2004-05-27 by Michael Kerrisk <mtk.manpages@gmail.com>
  28 .\"
  29 .TH ADJTIMEX 2 2014-12-31 "Linux" "Linux Programmer's Manual"
  30 .SH NAME
  31 adjtimex, ntp_adjtime \- tune kernel clock
  32 .SH SYNOPSIS
  33 .nf
  34 .B #include <sys/timex.h>
  35
  36 .BI "int adjtimex(struct timex *" "buf" );
  37
  38 .BI "int ntp_adjtime(struct timex *" buf );
  39 .fi
  40 .SH DESCRIPTION
  41 Linux uses David L. Mills' clock adjustment algorithm (see RFC\ 5905).
  42 The system call
  43 .BR adjtimex ()
  44 reads and optionally sets adjustment parameters for this algorithm.
  45 It takes a pointer to a
  46 .I timex
  47 structure, updates kernel parameters from (selected) field values,
  48 and returns the same structure updated with the current kernel values.
  49 This structure is declared as follows:
  50 .PP
  51 .in +4n
  52 .nf
  53 struct timex {
  54     int  modes;      /* Mode selector */
  55     long offset;     /* Time offset; nanoseconds, if STA_NANO
  56                         status flag is set, otherwise microseconds */
  57     long freq;       /* Frequency offset, in units of 2^-16 ppm
  58                         (parts per million, see NOTES below) */
  59     long maxerror;   /* Maximum error (microseconds) */
  60     long esterror;   /* Estimated error (microseconds) */
  61     int  status;     /* Clock command/status */
  62     long constant;   /* PLL (phase-locked loop) time constant */
  63     long precision;  /* Clock precision (microseconds, read-only) */
  64     long tolerance;  /* Clock frequency tolerance (ppm, read-only) */
  65     struct timeval time;
  66                      /* Current time (read-only, except for
  67                         ADJ_SETOFFSET); upon return, time.tv_usec
  68                         contains nanoseconds, if STA_NANO status
  69                         flag is set, otherwise microseconds */
  70     long tick;       /* Microseconds between clock ticks */
  71     long ppsfreq;    /* PPS (pulse per second) frequency (in units
  72                         of 2^-16 ppm\-\-see NOTES, read-only) */
  73     long jitter;     /* PPS jitter (read-only); nanoseconds, if
  74                         STA_NANO status flag is set, otherwise
  75                         microseconds */
  76     int  shift;      /* PPS interval duration (seconds, read-only) */
  77     long stabil;     /* PPS stability (2^-16 ppm\-\-see NOTES,
  78                         read-only) */
  79     long jitcnt;     /* PPS count of jitter limit exceeded
  80                         events (read-only) */
  81     long calcnt;     /* PPS count of calibration intervals
  82                         (read-only) */
  83     long errcnt;     /* PPS count of calibration errors
  84                         (read-only) */
  85     long stbcnt;     /* PPS count of stability limit exceeded
  86                         events (read-only) */
  87     int tai;         /* TAI offset, as set by previous ADJ_TAI
  88                         operation (seconds, read-only,
  89                         since Linux 2.6.26) */
  90     /* Further padding bytes to allow for future expansion */
  91 };
  92 .fi
  93 .in
  94 .PP
  95 The
  96 .I modes
  97 field determines which parameters, if any, to set.
  98 (As described later in this page,
  99 the constants used for
 100 .BR ntp_adjtime ()
 101 are equivalent but differently named.)
 102 It is a bit mask containing a
 103 .RI bitwise- or
 104 combination of zero or more of the following bits:
 105 .TP
 106 .BR ADJ_OFFSET
 107 Set time offset from
 108 .IR buf.offset .
 109 Since Linux 2.6.26,
 110 .\" commit 074b3b87941c99bc0ce35385b5817924b1ed0c23
 111 the supplied value is clamped to the range (\-0.5s, +0.5s).
 112 In older kernels, an
 113 .B EINVAL
 114 error occurs if the supplied value is out of range.
 115 .TP
 116 .BR ADJ_FREQUENCY
 117 Set frequency offset from
 118 .IR buf.freq .
 119 Since Linux 2.6.26,
 120 .\" commit 074b3b87941c99bc0ce35385b5817924b1ed0c23
 121 the supplied value is clamped to the range (\-32768000, +32768000).
 122 In older kernels, an
 123 .B EINVAL
 124 error occurs if the supplied value is out of range.
 125 .TP
 126 .BR ADJ_MAXERROR
 127 Set maximum time error from
 128 .IR buf.maxerror .
 129 .TP
 130 .BR ADJ_ESTERROR
 131 Set estimated time error from
 132 .IR buf.esterror .
 133 .TP
 134 .BR ADJ_STATUS
 135 Set clock status bits from
 136 .IR buf.status .
 137 A description of these bits is provided below.
 138 .TP
 139 .BR ADJ_TIMECONST
 140 Set PLL time constant from
 141 .IR buf.constant .
 142 If the
 143 .B STA_NANO
 144 status flag (see below) is clear, the kernel adds 4 to this value.
 145 .TP
 146 .BR ADJ_SETOFFSET " (since Linux 2.6.29)"
 147 .\" commit 094aa1881fdc1b8889b442eb3511b31f3ec2b762
 148 .\" Author: Richard Cochran <richardcochran@gmail.com>
 149 Add
 150 .I buf.time
 151 to the current time.
 152 If
 153 .I buf.status
 154 includes the
 155 .B ADJ_NANO
 156 flag, then
 157 .I buf.time.tv_usec
 158 is interpreted as a nanosecond value;
 159 otherwise it is interpreted as microseconds.
 160 .TP
 161 .BR ADJ_MICRO " (since Linux 2.6.36)"
 162 .\" commit eea83d896e318bda54be2d2770d2c5d6668d11db
 163 .\" Author: Roman Zippel <zippel@linux-m68k.org>
 164 Select microsecond resolution.
 165 .TP
 166 .BR ADJ_NANO " (since Linux 2.6.36)"
 167 .\" commit eea83d896e318bda54be2d2770d2c5d6668d11db
 168 .\" Author: Roman Zippel <zippel@linux-m68k.org>
 169 Select nanosecond resolution.
 170 Only one of
 171 .BR ADJ_MICRO
 172 and
 173 .BR ADJ_NANO
 174 should be specified.
 175 .TP
 176 .BR ADJ_TAI " (since Linux 2.6.26)"
 177 .\" commit 153b5d054ac2d98ea0d86504884326b6777f683d
 178 Set TAI (Atomic International Time) offset from
 179 .IR buf.constant .
 180
 181 .BR ADJ_TAI
 182 should not be used in conjunction with
 183 .BR ADJ_TIMECONST ,
 184 since the latter mode also employs the
 185 .IR buf.constant
 186 field.
 187
 188 For a complete explanation of TAI
 189 and the difference between TAI and UTC, see
 190 .UR http://www.bipm.org/en/bipm/tai/tai.html
 191 .I BIPM
 192 .UE
 193 .TP
 194 .BR ADJ_TICK
 195 Set tick value from
 196 .IR buf.tick .
 197 .PP
 198 Alternatively,
 199 .I modes
 200 can be specified as either of the following (multibit mask) values,
 201 in which case other bits should not be specified in
 202 .IR modes :
 203 .\" In general, the other bits are ignored, but ADJ_OFFSET_SINGLESHOT 0x8001
 204 .\" ORed with ADJ_NANO (0x2000) gives 0xa0001 == ADJ_OFFSET_SS_READ!!
 205 .TP
 206 .BR ADJ_OFFSET_SINGLESHOT
 207 .\" In user space, ADJ_OFFSET_SINGLESHOT is 0x8001
 208 .\" In kernel space it is 0x0001, and must be ANDed with ADJ_ADJTIME (0x8000)
 209 Old-fashioned
 210 .BR adjtime ():
 211 (gradually) adjust time by value specified in
 212 .IR buf.offset ,
 213 which specifies an adjustment in microseconds.
 214 .TP
 215 .BR ADJ_OFFSET_SS_READ " (functional since Linux 2.6.28)"
 216 .\" In user space, ADJ_OFFSET_SS_READ is 0xa001
 217 .\" In kernel space there is ADJ_OFFSET_READONLY (0x2000) anded with
 218 .\" ADJ_ADJTIME (0x8000) and ADJ_OFFSET_SINGLESHOT (0x0001) to give 0xa001)
 219 Return (in
 220 .IR buf.offset )
 221 the remaining amount of time to be adjusted after an earlier
 222 .BR ADJ_OFFSET_SINGLESHOT
 223 operation.
 224 This feature was added in Linux 2.6.24,
 225 .\" commit 52bfb36050c8529d9031d2c2513b281a360922ec
 226 but did not work correctly
 227 .\" commit 916c7a855174e3b53d182b97a26b2e27a29726a1
 228 until Linux 2.6.28.
 229 .PP
 230 Ordinary users are restricted to a value of either 0 or
 231 .B ADJ_OFFSET_SS_READ
 232 for
 233 .IR modes .
 234 Only the superuser may set any parameters.
 235
 236 The
 237 .I buf.status
 238 field is a bit mask that is used to set and/or retrieve status
 239 bits associated with the NTP implementation.
 240 Some bits in the mask are both readable and settable,
 241 while others are read-only.
 242 .TP
 243 .BR STA_PLL " (read-write)"
 244 Enable phase-locked loop (PLL) updates via
 245 .BR ADJ_OFFSET .
 246 .TP
 247 .BR STA_PPSFREQ " (read-write)"
 248 Enable PPS (pulse-per-second) frequency discipline.
 249 .TP
 250 .BR STA_PPSTIME " (read-write)"
 251 Enable PPS time discipline.
 252 .TP
 253 .BR STA_FLL " (read-write)"
 254 Select frequency-locked loop (FLL) mode.
 255 .TP
 256 .BR STA_INS " (read-write)"
 257 Insert a leap second after the last second of the UTC day,
 258 thus extending the last minute of the day by one second.
 259 Leap-second insertion will occur each day, so long as this flag remains set.
 260 .\" John Stultz;
 261 .\"     Usually this is written as extending the day by one second,
 262 .\"     which is represented as:
 263 .\"        23:59:59
 264 .\"        23:59:60
 265 .\"        00:00:00
 266 .\"
 267 .\"     But since posix cannot represent 23:59:60, we repeat the last second:
 268 .\"        23:59:59 + TIME_INS
 269 .\"        23:59:59 + TIME_OOP
 270 .\"        00:00:00 + TIME_WAIT
 271 .\"
 272 .TP
 273 .BR STA_DEL " (read-write)"
 274 Delete a leap second at the last second of the UTC day.
 275 .\" John Stultz:
 276 .\"     Similarly the progression here is:
 277 .\"        23:59:57 + TIME_DEL
 278 .\"        23:59:58 + TIME_DEL
 279 .\"        00:00:00 + TIME_WAIT
 280 Leap second deletion will occur each day, so long as this flag
 281 remains set.
 282 .\" FIXME Does there need to be a statement that it is nonsensical to set
 283 .\"     to set both STA_INS and STA_DEL?
 284 .TP
 285 .BR STA_UNSYNC " (read-write)"
 286 Clock unsynchronized.
 287 .TP
 288 .BR STA_FREQHOLD " (read-write)"
 289 Hold frequency.
 290 .\" Following text from John Stultz:
 291 Normally adjustments made via
 292 .B ADJ_OFFSET
 293 result in dampened frequency adjustments also being made.
 294 So a single call corrects the current offset,
 295 but as offsets in the same direction are made repeatedly,
 296 the small frequency adjustments will accumulate to fix the long-term skew.
 297
 298 This flag prevents the small frequency adjustment from being made
 299 when correcting for an
 300 .B ADJ_OFFSET
 301 value.
 302 .\" According to the Kernel Application Program Interface document,
 303 .\" STA_FREQHOLD is not used by the NTP version 4 daemon
 304 .TP
 305 .BR STA_PPSSIGNAL " (read-only)"
 306 A valid PPS (pulse-per-second) signal is present.
 307 .TP
 308 .BR STA_PPSJITTER " (read-only)"
 309 PPS signal jitter exceeded.
 310 .TP
 311 .BR STA_PPSWANDER " (read-only)"
 312 PPS signal wander exceeded.
 313 .TP
 314 .BR STA_PPSERROR " (read-only)"
 315 PPS signal calibration error.
 316 .TP
 317 .BR STA_CLOCKERR " (read-only)"
 318 Clock hardware fault.
 319 .\" Not set in current kernel (4.5), but checked in a few places
 320 .TP
 321 .BR STA_NANO " (read-only; since Linux 2.6.26)"
 322 .\" commit eea83d896e318bda54be2d2770d2c5d6668d11db
 323 .\" Author: Roman Zippel <zippel@linux-m68k.org>
 324 Resolution (0 = microsecond, 1 = nanoseconds).
 325 Set via
 326 .BR ADJ_NANO ,
 327 cleared via
 328 .BR ADJ_MICRO .
 329 .TP
 330 .BR STA_MODE " (since Linux 2.6.26)"
 331 .\" commit eea83d896e318bda54be2d2770d2c5d6668d11db
 332 .\" Author: Roman Zippel <zippel@linux-m68k.org>
 333 Mode (0 = Phase Locked Loop, 1 = Frequency Locked Loop).
 334 .TP
 335 .BR STA_CLK " (read-only; since Linux 2.6.26)"
 336 .\" commit eea83d896e318bda54be2d2770d2c5d6668d11db
 337 .\" Author: Roman Zippel <zippel@linux-m68k.org>
 338 Clock source (0 = A, 1 = B); currently unused.
 339 .PP
 340 Attempts to set read-only
 341 .I status
 342 bits are silently ignored.
 343 .\"
 344 .SS ntp_adjtime ()
 345 The
 346 .BR ntp_adjtime ()
 347 library function
 348 (described in the NTP "Kernel Application Program API", KAPI)
 349 is a more portable interface for performing the same task as
 350 .BR adjtimex ().
 351 Other than the following points, it is identical to
 352 .BR adjtime ():
 353 .IP * 3
 354 The constants used in
 355 .I modes
 356 are prefixed with "MOD_" rather than "ADJ_", and have the same suffixes (thus,
 357 .BR MOD_OFFSET ,
 358 .BR MOD_FREQUENCY ,
 359 and so on), other than the exceptions noted in the following points.
 360 .IP *
 361 .BR MOD_CLKA
 362 is the synonym for
 363 .BR ADJ_OFFSET_SINGLESHOT .
 364 .IP *
 365 .BR MOD_CLKB
 366 is the synonym for
 367 .BR ADJ_TICK .
 368 .IP *
 369 The is no synonym for
 370 .BR ADJ_OFFSET_SS_READ ,
 371 which is not described in the KAPI.
 372 .SH RETURN VALUE
 373 On success,
 374 .BR adjtimex ()
 375 and
 376 .BR ntp_adjtime ()
 377 return the clock state; that is, one of the following values:
 378 .TP 12
 379 .BR TIME_OK
 380 Clock synchronized, no leap second adjustment pending.
 381 .TP
 382 .BR TIME_INS
 383 Indicates that a leap second will be added at the end of the UTC day.
 384 .TP
 385 .BR TIME_DEL
 386 Indicates that a leap second will be deleted at the end of the UTC day.
 387 .TP
 388 .BR TIME_OOP
 389 Insertion of a leap second is in progress.
 390 .TP
 391 .BR TIME_WAIT
 392 A leap-second insertion or deletion has been completed.
 393 This value will be returned until the next
 394 .BR ADJ_STATUS
 395 operation clears the
 396 .B STA_INS
 397 and
 398 .B STA_DEL
 399 flags.
 400 .TP
 401 .BR TIME_ERROR
 402 The system clock is not synchronized to a reliable server.
 403 This value is returned when any of the following holds true:
 404 .RS
 405 .IP * 3
 406 Either
 407 .B STA_UNSYNC
 408 or
 409 .B STA_CLOCKERR
 410 is set.
 411 .IP *
 412 .B STA_PPSSIGNAL
 413 is clear and either
 414 .B STA_PPSFREQ
 415 or
 416 .B STA_PPSTIME
 417 is set.
 418 .IP *
 419 .B STA_PPSTIME
 420 and
 421 .B STA_PPSJITTER
 422 are both set.
 423 .IP *
 424 .B STA_PPSFREQ
 425 is set and either
 426 .B STA_PPSWANDER
 427 or
 428 .B STA_PPSJITTER
 429 is set.
 430 .RE
 431 .IP
 432 The symbolic name
 433 .B TIME_BAD
 434 is a synonym for
 435 .BR TIME_ERROR ,
 436 provided for backward compatibility.
 437 .PP
 438 Note that starting with Linux 3.4,
 439 .\" commit 6b43ae8a619d17c4935c3320d2ef9e92bdeed05d changed to asynchronous
 440 .\"  operation, so we can no longer rely on the return code.
 441 the call operates asynchronously and the return value usually will
 442 not reflect a state change caused by the call itself.
 443 .PP
 444 On failure, these calls return \-1 and set
 445 .IR errno .
 446 .SH ERRORS
 447 .TP
 448 .B EFAULT
 449 .I buf
 450 does not point to writable memory.
 451 .TP
 452 .BR EINVAL " (kernels before Linux 2.6.26)"
 453 An attempt was made to set
 454 .I buf.freq
 455 to a value outside the range (\-33554432, +33554432).
 456 .\" From a quck glance, it appears there was no clamping or range check
 457 .\" for buf.freq in kernels before 2.0
 458 .TP
 459 .BR EINVAL " (kernels before Linux 2.6.26)"
 460 An attempt was made to set
 461 .I buf.offset
 462 to a value outside the permitted range.
 463 In kernels before Linux 2.0, the permitted range was (\-131072, +131072).
 464 From Linux 2.0 onwards, the permitted range was (\-512000, +512000).
 465 .TP
 466 .B EINVAL
 467 An attempt was made to set
 468 .I buf.status
 469 to a value other than those listed above.
 470 .TP
 471 .B EINVAL
 472 An attempt was made to set
 473 .I buf.tick
 474 to a value outside the range
 475 .RB 900000/ HZ
 476 to
 477 .RB 1100000/ HZ ,
 478 where
 479 .B HZ
 480 is the system timer interrupt frequency.
 481 .TP
 482 .B EPERM
 483 .I buf.modes
 484 is neither 0 nor
 485 .BR ADJ_OFFSET_SS_READ ,
 486 and the caller does not have sufficient privilege.
 487 Under Linux, the
 488 .B CAP_SYS_TIME
 489 capability is required.
 490 .SH NOTES
 491 In struct
 492 .IR timex ,
 493 .IR freq ,
 494 .IR ppsfreq ,
 495 and
 496 .I stabil
 497 are ppm (parts per million) with a 16-bit fractional part,
 498 which means that a value of 1 in one of those fields
 499 actually means 2^-16 ppm, and 2^16=65536 is 1 ppm.
 500 This is the case for both input values (in the case of
 501 .IR freq )
 502 and output values.
 503
 504 The leap-second processing triggered vy
 505 .B STA_INS
 506 and
 507 .B STA_DEL
 508 is done by the kernel in timer context
 509 Thus, it will take one tick into the second
 510 for the leap second to be inserted or deleted.
 511 .SH CONFORMING TO
 512 Neither of these interfaces is described in POSIX.1
 513
 514 .BR adjtimex ()
 515 is Linux-specific and should not be used in programs
 516 intended to be portable.
 517
 518 The preferred API for NTP daemon is
 519 .BR ntp_adjtime (3),
 520 which is described in the NTP KAPI documentation.
 521 .SH SEE ALSO
 522 .BR settimeofday (2),
 523 .BR adjtime (3),
 524 .\" .BR ntp_gettime (3),
 525 .BR capabilities (7),
 526 .BR time (7),
 527 .BR adjtimex (8)
 528
 529 .ad l
 530 .UR http://www.slac.stanford.edu/comp/unix/\:package/\:rtems/\:src/\:ssrlApps/\:ntpNanoclock/\:api.htm
 531 NTP "Kernel Application Program Interface"
 532 .UE