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Annotated edit history of hwclock(8) version 2, including all changes. View license author blame.
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2 IanMcDonald 1 <verbatim>
1 perry 2 NAME
2 IanMcDonald 3 hwclock - query and set the hardware clock (RTC)
4
1 perry 5 SYNOPSIS
2 IanMcDonald 6 hwclock -r or hwclock --show
7 hwclock -w or hwclock --systohc
8 hwclock -s or hwclock --hctosys
9 hwclock -a or hwclock --adjust
10 hwclock -v or hwclock --version
11 hwclock --set --date=newdate
12 hwclock --getepoch
13 hwclock --setepoch --epoch=year
1 perry 14
2 IanMcDonald 15 other options:
1 perry 16
2 IanMcDonald 17 [-u|--utc] --localtime --noadjfile --directisa --test [-D|--debug]
18 --rtc=filename
1 perry 19
2 IanMcDonald 20 and arcane options for DEC Alpha:
1 perry 21
2 IanMcDonald 22 [-A|--arc] [-J|--jensen] [-S|--srm] [-F|--funky-toy]
1 perry 23
2 IanMcDonald 24 Minimum unique abbreviations of all options are acceptable.
1 perry 25
2 IanMcDonald 26 Also, -h asks for a help message.
1 perry 27
28
2 IanMcDonald 29 DESCRIPTION
30 hwclock is a tool for accessing the Hardware Clock. You can display
31 the current time, set the Hardware Clock to a specified time, set the
32 Hardware Clock to the System Time, and set the System Time from the
33 Hardware Clock.
1 perry 34
2 IanMcDonald 35 You can also run hwclock periodically to insert or remove time from the
36 Hardware Clock to compensate for systematic drift (where the clock con‐
37 sistently gains or loses time at a certain rate if left to run).
1 perry 38
39
2 IanMcDonald 40 OPTIONS
41 You need exactly one of the following options to tell hwclock what
42 function to perform:
1 perry 43
44
2 IanMcDonald 45 --show Read the Hardware Clock and print the time on Standard Output.
46 The time shown is always in local time, even if you keep your
47 Hardware Clock in Coordinated Universal Time. See the --utc
48 option.
1 perry 49
50
2 IanMcDonald 51 --set Set the Hardware Clock to the time given by the --date option.
1 perry 52
2 IanMcDonald 53 --hctosys
54 Set the System Time from the Hardware Clock.
1 perry 55
2 IanMcDonald 56 Also set the kernel’s timezone value to the local timezone as
57 indicated by the TZ environment variable and/or /usr/share/zone‐
58 info, as tzset(3) would interpret them. The obsolete tz_dsttime
59 field of the kernel’s timezone value is set to DST_NONE. (For
60 details on what this field used to mean, see settimeofday(2).)
1 perry 61
2 IanMcDonald 62 This is a good option to use in one of the system startup
63 scripts.
1 perry 64
2 IanMcDonald 65 --systohc
66 Set the Hardware Clock to the current System Time.
1 perry 67
2 IanMcDonald 68 --adjust
69 Add or subtract time from the Hardware Clock to account for sys‐
70 tematic drift since the last time the clock was set or adjusted.
71 See discussion below.
1 perry 72
2 IanMcDonald 73 --getepoch
74 Print the kernel’s Hardware Clock epoch value to standard out‐
75 put. This is the number of years into AD to which a zero year
76 value in the Hardware Clock refers. For example, if you are
77 using the convention that the year counter in your Hardware
78 Clock contains the number of full years since 1952, then the
79 kernel’s Hardware Counter epoch value must be 1952.
1 perry 80
2 IanMcDonald 81 This epoch value is used whenever hwclock reads or sets the
82 Hardware Clock.
1 perry 83
2 IanMcDonald 84 --setepoch
85 Set the kernel’s Hardware Clock epoch value to the value speci‐
86 fied by the --epoch option. See the --getepoch option for
87 details.
1 perry 88
2 IanMcDonald 89 --version
90 Print the version of hwclock on Standard Output.
1 perry 91
2 IanMcDonald 92 --date=date_string
93 You need this option if you specify the --set option. Other‐
94 wise, it is ignored. This specifies the time to which to set
95 the Hardware Clock. The value of this option is an argument to
96 the date(1) program. For example,
1 perry 97
2 IanMcDonald 98 hwclock --set --date="9/22/96 16:45:05"
1 perry 99
2 IanMcDonald 100 The argument is in local time, even if you keep your Hardware
101 Clock in Coordinated Universal time. See the --utc option.
1 perry 102
103
2 IanMcDonald 104 --epoch=year
105 Specifies the year which is the beginning of the Hardware
106 Clock’s epoch. I.e. the number of years into AD to which a zero
107 value in the Hardware Clock’s year counter refers. It is used
108 together with the --setepoch option to set the kernel’s idea of
109 the epoch of the Hardware Clock, or otherwise to specify the
110 epoch for use with direct ISA access.
1 perry 111
2 IanMcDonald 112 For example, on a Digital Unix machine:
1 perry 113
2 IanMcDonald 114 hwclock --setepoch --epoch=1952
1 perry 115
116
2 IanMcDonald 117 The following options apply to most functions.
1 perry 118
2 IanMcDonald 119 --utc
1 perry 120
2 IanMcDonald 121 --localtime
122 Indicates that the Hardware Clock is kept in Coordinated Univer‐
123 sal Time or local time, respectively. It is your choice whether
124 to keep your clock in UTC or local time, but nothing in the
125 clock tells which you’ve chosen. So this option is how you give
126 that information to hwclock.
1 perry 127
2 IanMcDonald 128 If you specify the wrong one of these options (or specify nei‐
129 ther and take a wrong default), both setting and querying of the
130 Hardware Clock will be messed up.
1 perry 131
2 IanMcDonald 132 If you specify neither --utc nor --localtime , the default is
133 whichever was specified the last time hwclock was used to set
134 the clock (i.e. hwclock was successfully run with the --set ,
135 --systohc , or --adjust options), as recorded in the adjtime
136 file. If the adjtime file doesn’t exist, the default is local
137 time.
1 perry 138
139
2 IanMcDonald 140 --noadjfile
141 disables the facilities provided by /etc/adjtime. hwclock will
142 not read nor write to that file with this option. Either --utc
143 or --localtime must be specified when using this option.
1 perry 144
145
2 IanMcDonald 146 --rtc=filename
147 overrides the default /dev file name, which is /dev/rtc on many
148 platforms but may be /dev/rtc0, /dev/rtc1, and so on.
1 perry 149
150
2 IanMcDonald 151 --directisa
152 is meaningful only on an ISA machine or an Alpha (which imple‐
153 ments enough of ISA to be, roughly speaking, an ISA machine for
154 hwclock’s purposes). For other machines, it has no effect.
155 This option tells hwclock to use explicit I/O instructions to
156 access the Hardware Clock. Without this option, hwclock will
157 try to use the /dev/rtc device (which it assumes to be driven by
158 the rtc device driver). If it is unable to open the device (for
159 read), it will use the explicit I/O instructions anyway.
1 perry 160
2 IanMcDonald 161 The rtc device driver was new in Linux Release 2.
1 perry 162
2 IanMcDonald 163 --badyear
164 Indicates that the Hardware Clock is incapable of storing years
165 outside the range 1994-1999. There is a problem in some BIOSes
166 (almost all Award BIOSes made between 4/26/94 and 5/31/95)
167 wherein they are unable to deal with years after 1999. If one
168 attempts to set the year-of-century value to something less than
169 94 (or 95 in some cases), the value that actually gets set is 94
170 (or 95). Thus, if you have one of these machines, hwclock can‐
171 not set the year after 1999 and cannot use the value of the
172 clock as the true time in the normal way.
1 perry 173
2 IanMcDonald 174 To compensate for this (without your getting a BIOS update,
175 which would definitely be preferable), always use --badyear if
176 you have one of these machines. When hwclock knows it’s working
177 with a brain-damaged clock, it ignores the year part of the
178 Hardware Clock value and instead tries to guess the year based
179 on the last calibrated date in the adjtime file, by assuming
180 that that date is within the past year. For this to work, you
181 had better do a hwclock --set or hwclock --systohc at least once
182 a year!
1 perry 183
2 IanMcDonald 184 Though hwclock ignores the year value when it reads the Hardware
185 Clock, it sets the year value when it sets the clock. It sets
186 it to 1995, 1996, 1997, or 1998, whichever one has the same
187 position in the leap year cycle as the true year. That way, the
188 Hardware Clock inserts leap days where they belong. Again, if
189 you let the Hardware Clock run for more than a year without set‐
190 ting it, this scheme could be defeated and you could end up los‐
191 ing a day.
1 perry 192
2 IanMcDonald 193 hwclock warns you that you probably need --badyear whenever it
194 finds your Hardware Clock set to 1994 or 1995.
1 perry 195
196
2 IanMcDonald 197 --srm This option is equivalent to --epoch=1900 and is used to specify
198 the most common epoch on Alphas with SRM console.
1 perry 199
2 IanMcDonald 200 --arc This option is equivalent to --epoch=1980 and is used to specify
201 the most common epoch on Alphas with ARC console (but Ruffians
202 have epoch 1900).
1 perry 203
2 IanMcDonald 204 --jensen
1 perry 205
2 IanMcDonald 206 --funky-toy
207 These two options specify what kind of Alpha machine you have.
208 They are invalid if you don’t have an Alpha and are usually
209 unnecessary if you do, because hwclock should be able to deter‐
210 mine by itself what it’s running on, at least when /proc is
211 mounted. (If you find you need one of these options to make
212 hwclock work, contact the maintainer to see if the program can
213 be improved to detect your system automatically. Output of
214 ‘hwclock --debug’ and ‘cat /proc/cpuinfo’ may be of interest.)
1 perry 215
2 IanMcDonald 216 --jensen means you are running on a Jensen model.
1 perry 217
2 IanMcDonald 218 --funky-toy means that on your machine, one has to use the UF
219 bit instead of the UIP bit in the Hardware Clock to detect a
220 time transition. "Toy" in the option name refers to the Time Of
221 Year facility of the machine.
1 perry 222
223
224
2 IanMcDonald 225 --test Do everything except actually updating the Hardware Clock or
226 anything else. This is useful, especially in conjunction with
227 --debug, in learning about hwclock.
1 perry 228
2 IanMcDonald 229 --debug
230 Display a lot of information about what hwclock is doing inter‐
231 nally. Some of its function is complex and this output can help
232 you understand how the program works.
1 perry 233
234
235
2 IanMcDonald 236 NOTES
237 Clocks in a Linux System
238 There are two main clocks in a Linux system:
1 perry 239
2 IanMcDonald 240 The Hardware Clock: This is a clock that runs independently of any con‐
241 trol program running in the CPU and even when the machine is powered
242 off.
1 perry 243
2 IanMcDonald 244 On an ISA system, this clock is specified as part of the ISA standard.
245 The control program can read or set this clock to a whole second, but
246 the control program can also detect the edges of the 1 second clock
247 ticks, so the clock actually has virtually infinite precision.
1 perry 248
2 IanMcDonald 249 This clock is commonly called the hardware clock, the real time clock,
250 the RTC, the BIOS clock, and the CMOS clock. Hardware Clock, in its
251 capitalized form, was coined for use by hwclock because all of the
252 other names are inappropriate to the point of being misleading.
1 perry 253
2 IanMcDonald 254 So for example, some non-ISA systems have a few real time clocks with
255 only one of them having its own power domain. A very low power exter‐
256 nal I2C or SPI clock chip might be used with a backup battery as the
257 hardware clock to initialize a more functional integrated real-time
258 clock which is used for most other purposes.
1 perry 259
2 IanMcDonald 260 The System Time: This is the time kept by a clock inside the Linux ker‐
261 nel and driven by a timer interrupt. (On an ISA machine, the timer
262 interrupt is part of the ISA standard). It has meaning only while
263 Linux is running on the machine. The System Time is the number of sec‐
264 onds since 00:00:00 January 1, 1970 UTC (or more succinctly, the number
265 of seconds since 1969). The System Time is not an integer, though. It
266 has virtually infinite precision.
1 perry 267
2 IanMcDonald 268 The System Time is the time that matters. The Hardware Clock’s basic
269 purpose in a Linux system is to keep time when Linux is not running.
270 You initialize the System Time to the time from the Hardware Clock when
271 Linux starts up, and then never use the Hardware Clock again. Note
272 that in DOS, for which ISA was designed, the Hardware Clock is the only
273 real time clock.
1 perry 274
2 IanMcDonald 275 It is important that the System Time not have any discontinuities such
276 as would happen if you used the date(1L) program to set it while the
277 system is running. You can, however, do whatever you want to the Hard‐
278 ware Clock while the system is running, and the next time Linux starts
279 up, it will do so with the adjusted time from the Hardware Clock. You
280 can also use the program adjtimex(8) to smoothly adjust the System Time
281 while the system runs.
1 perry 282
2 IanMcDonald 283 A Linux kernel maintains a concept of a local timezone for the system.
284 But don’t be misled -- almost nobody cares what timezone the kernel
285 thinks it is in. Instead, programs that care about the timezone (per‐
286 haps because they want to display a local time for you) almost always
287 use a more traditional method of determining the timezone: They use the
288 TZ environment variable and/or the /usr/share/zoneinfo directory, as
289 explained in the man page for tzset(3). However, some programs and
290 fringe parts of the Linux kernel such as filesystems use the kernel
291 timezone value. An example is the vfat filesystem. If the kernel
292 timezone value is wrong, the vfat filesystem will report and set the
293 wrong timestamps on files.
1 perry 294
2 IanMcDonald 295 hwclock sets the kernel timezone to the value indicated by TZ and/or
296 /usr/share/zoneinfo when you set the System Time using the --hctosys
297 option.
1 perry 298
2 IanMcDonald 299 The timezone value actually consists of two parts: 1) a field tz_min‐
300 uteswest indicating how many minutes local time (not adjusted for DST)
301 lags behind UTC, and 2) a field tz_dsttime indicating the type of Day‐
302 light Savings Time (DST) convention that is in effect in the locality
303 at the present time. This second field is not used under Linux and is
304 always zero. (See also settimeofday(2).)
1 perry 305
306
2 IanMcDonald 307 How hwclock Accesses the Hardware Clock
308 hwclock Uses many different ways to get and set Hardware Clock values.
309 The most normal way is to do I/O to the device special file /dev/rtc,
310 which is presumed to be driven by the rtc device driver. However, this
311 method is not always available. For one thing, the rtc driver is a
312 relatively recent addition to Linux. Older systems don’t have it.
313 Also, though there are versions of the rtc driver that work on DEC
314 Alphas, there appear to be plenty of Alphas on which the rtc driver
315 does not work (a common symptom is hwclock hanging). Moreover, recent
316 Linux systems have more generic support for RTCs, even systems that
317 have more than one, so you might need to override the default by speci‐
318 fying /dev/rtc0 or /dev/rtc1 instead.
1 perry 319
2 IanMcDonald 320 On older systems, the method of accessing the Hardware Clock depends on
321 the system hardware.
1 perry 322
2 IanMcDonald 323 On an ISA system, hwclock can directly access the "CMOS memory" regis‐
324 ters that constitute the clock, by doing I/O to Ports 0x70 and 0x71.
325 It does this with actual I/O instructions and consequently can only do
326 it if running with superuser effective userid. (In the case of a
327 Jensen Alpha, there is no way for hwclock to execute those I/O
328 instructions, and so it uses instead the /dev/port device special file,
329 which provides almost as low-level an interface to the I/O subsystem).
1 perry 330
2 IanMcDonald 331 This is a really poor method of accessing the clock, for all the rea‐
332 sons that user space programs are generally not supposed to do direct
333 I/O and disable interrupts. Hwclock provides it because it is the only
334 method available on ISA and Alpha systems which don’t have working rtc
335 device drivers available.
1 perry 336
337
2 IanMcDonald 338 On an m68k system, hwclock can access the clock via the console driver,
339 via the device special file /dev/tty1.
1 perry 340
2 IanMcDonald 341 hwclock tries to use /dev/rtc. If it is compiled for a kernel that
342 doesn’t have that function or it is unable to open /dev/rtc (or the
343 alternative special file you’ve defined on the command line) hwclock
344 will fall back to another method, if available. On an ISA or Alpha
345 machine, you can force hwclock to use the direct manipulation of the
346 CMOS registers without even trying /dev/rtc by specifying the --direc‐
347 tisa option.
1 perry 348
349
350
2 IanMcDonald 351 The Adjust Function
352 The Hardware Clock is usually not very accurate. However, much of its
353 inaccuracy is completely predictable - it gains or loses the same
354 amount of time every day. This is called systematic drift. hwclock’s
355 "adjust" function lets you make systematic corrections to correct the
356 systematic drift.
1 perry 357
2 IanMcDonald 358 It works like this: hwclock keeps a file, /etc/adjtime, that keeps some
359 historical information. This is called the adjtime file.
1 perry 360
2 IanMcDonald 361 Suppose you start with no adjtime file. You issue a hwclock --set com‐
362 mand to set the Hardware Clock to the true current time. Hwclock cre‐
363 ates the adjtime file and records in it the current time as the last
364 time the clock was calibrated. 5 days later, the clock has gained 10
365 seconds, so you issue another hwclock --set command to set it back 10
366 seconds. Hwclock updates the adjtime file to show the current time as
367 the last time the clock was calibrated, and records 2 seconds per day
368 as the systematic drift rate. 24 hours go by, and then you issue a
369 hwclock --adjust command. Hwclock consults the adjtime file and sees
370 that the clock gains 2 seconds per day when left alone and that it has
371 been left alone for exactly one day. So it subtracts 2 seconds from
372 the Hardware Clock. It then records the current time as the last time
373 the clock was adjusted. Another 24 hours goes by and you issue another
374 hwclock --adjust. Hwclock does the same thing: subtracts 2 seconds and
375 updates the adjtime file with the current time as the last time the
376 clock was adjusted.
1 perry 377
2 IanMcDonald 378 Every time you calibrate (set) the clock (using --set or --systohc ),
379 hwclock recalculates the systematic drift rate based on how long it has
380 been since the last calibration, how long it has been since the last
381 adjustment, what drift rate was assumed in any intervening adjustments,
382 and the amount by which the clock is presently off.
1 perry 383
2 IanMcDonald 384 A small amount of error creeps in any time hwclock sets the clock, so
385 it refrains from making an adjustment that would be less than 1 second.
386 Later on, when you request an adjustment again, the accumulated drift
387 will be more than a second and hwclock will do the adjustment then.
1 perry 388
2 IanMcDonald 389 It is good to do a hwclock --adjust just before the hwclock --hctosys
390 at system startup time, and maybe periodically while the system is run‐
391 ning via cron.
1 perry 392
2 IanMcDonald 393 The adjtime file, while named for its historical purpose of controlling
394 adjustments only, actually contains other information for use by
395 hwclock in remembering information from one invocation to the next.
1 perry 396
2 IanMcDonald 397 The format of the adjtime file is, in ASCII:
1 perry 398
2 IanMcDonald 399 Line 1: 3 numbers, separated by blanks: 1) systematic drift rate in
400 seconds per day, floating point decimal; 2) Resulting number of seconds
401 since 1969 UTC of most recent adjustment or calibration, decimal inte‐
402 ger; 3) zero (for compatibility with clock(8)) as a decimal integer.
1 perry 403
2 IanMcDonald 404 Line 2: 1 number: Resulting number of seconds since 1969 UTC of most
405 recent calibration. Zero if there has been no calibration yet or it is
406 known that any previous calibration is moot (for example, because the
407 Hardware Clock has been found, since that calibration, not to contain a
408 valid time). This is a decimal integer.
1 perry 409
2 IanMcDonald 410 Line 3: "UTC" or "LOCAL". Tells whether the Hardware Clock is set to
411 Coordinated Universal Time or local time. You can always override this
412 value with options on the hwclock command line.
1 perry 413
2 IanMcDonald 414 You can use an adjtime file that was previously used with the clock(8)
415 program with hwclock.
1 perry 416
417
418
2 IanMcDonald 419 Automatic Hardware Clock Synchronization By the Kernel
420 You should be aware of another way that the Hardware Clock is kept syn‐
421 chronized in some systems. The Linux kernel has a mode wherein it
422 copies the System Time to the Hardware Clock every 11 minutes. This is
423 a good mode to use when you are using something sophisticated like ntp
424 to keep your System Time synchronized. (ntp is a way to keep your Sys‐
425 tem Time synchronized either to a time server somewhere on the network
426 or to a radio clock hooked up to your system. See RFC 1305).
1 perry 427
2 IanMcDonald 428 This mode (we’ll call it "11 minute mode") is off until something turns
429 it on. The ntp daemon xntpd is one thing that turns it on. You can
430 turn it off by running anything, including hwclock --hctosys, that sets
431 the System Time the old fashioned way.
1 perry 432
2 IanMcDonald 433 To see if it is on or off, use the command adjtimex --print and look at
434 the value of "status". If the "64" bit of this number (expressed in
435 binary) equal to 0, 11 minute mode is on. Otherwise, it is off.
1 perry 436
2 IanMcDonald 437 If your system runs with 11 minute mode on, don’t use hwclock --adjust
438 or hwclock --hctosys. You’ll just make a mess. It is acceptable to
439 use a hwclock --hctosys at startup time to get a reasonable System Time
440 until your system is able to set the System Time from the external
441 source and start 11 minute mode.
1 perry 442
443
444
2 IanMcDonald 445 ISA Hardware Clock Century value
446 There is some sort of standard that defines CMOS memory Byte 50 on an
447 ISA machine as an indicator of what century it is. hwclock does not
448 use or set that byte because there are some machines that don’t define
449 the byte that way, and it really isn’t necessary anyway, since the
450 year-of-century does a good job of implying which century it is.
1 perry 451
2 IanMcDonald 452 If you have a bona fide use for a CMOS century byte, contact the
453 hwclock maintainer; an option may be appropriate.
1 perry 454
2 IanMcDonald 455 Note that this section is only relevant when you are using the "direct
456 ISA" method of accessing the Hardware Clock. ACPI provides a standard
457 way to access century values, when they are supported by the hardware.
1 perry 458
459
2 IanMcDonald 460 ENVIRONMENT VARIABLES
461 TZ
1 perry 462
463
2 IanMcDonald 464 FILES
465 /etc/adjtime /usr/share/zoneinfo/ /dev/rtc /dev/rtc0 /dev/port
466 /dev/tty1 /proc/cpuinfo
1 perry 467
468
2 IanMcDonald 469 SEE ALSO
470 </verbatim>
471 <pre>
472 [adjtimex(8)], [date(1)], [gettimeofday(2)], [settimeofday(2)], [crontab(1)],
473 [tzset(3)] /etc/init.d/hwclock.sh, /usr/share/doc/util-
474 linux/README.Debian.hwclock
475 </pre>
476 <verbatim>
1 perry 477
2 IanMcDonald 478 AUTHORS
479 Written by Bryan Henderson, September 1996 (bryanh@giraffe-data.com),
480 based on work done on the clock program by Charles Hedrick, Rob Hooft,
481 and Harald Koenig. See the source code for complete history and cred‐
482 its.
1 perry 483
484
2 IanMcDonald 485 AVAILABILITY
486 The hwclock command is part of the util-linux-ng package and is avail‐
487 able from ftp://ftp.kernel.org/pub/linux/utils/util-linux-ng/.
488 </verbatim>
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