hwclock
query or set the hardware clock (RTC)
see also :
date - crontab
Synopsis
hwclock
[function] [option...]
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examples
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description
hwclock
is a tool for accessing the Hardware Clock. You can display
the current time, set the Hardware Clock to a specified
time, set the Hardware Clock from the System Time, or set
the System Time from the Hardware Clock.
You can also
run hwclock periodically to add or subtract time from
the Hardware Clock to compensate for systematic drift (where
the clock consistently loses or gains time at a certain rate
when left to run).
options
The first two
options apply to just a few specific functions, the others
apply to most functions.
--date=date_string
You need this option if you
specify the --set or
--predict functions, otherwise it is
ignored. It specifies the time to which to set the Hardware
Clock, or the time for which to predict the Hardware Clock
reading. The value of this option is an argument to the
date(1) program. For example:
hwclock
--set --date="2011-08-14 16:45:05"
The argument
must be in local time, even if you keep your Hardware Clock
in Coordinated Universal time. See the
--utc option.
--epoch=year
Specifies the year which is the
beginning of the Hardware Clock’s epoch, that is the
number of years into AD to which a zero value in the
Hardware Clock’s year counter refers. It is used
together with the --setepoch option to
set the kernel’s idea of the epoch of the Hardware
Clock, or otherwise to specify the epoch for use with direct
ISA access.
For example, on
a Digital Unix machine:
hwclock
--setepoch --epoch=1952
-u, --utc
--localtime
Indicates that the Hardware
Clock is kept in Coordinated Universal Time or local time,
respectively. It is your choice whether to keep your clock
in UTC or local time, but nothing in the clock tells which
you’ve chosen. So this option is how you give that
information to hwclock.
If you specify
the wrong one of these options (or specify neither and take
a wrong default), both setting and querying of the Hardware
Clock will be messed up.
If you specify
neither --utc nor
--localtime, the default is whichever was
specified the last time hwclock was used to set the
clock (i.e. hwclock was successfully run with the
--set, --systohc, or
--adjust options), as recorded in the
adjtime file. If the adjtime file doesn’t exist, the
default is UTC time.
--noadjfile
Disables the facilities
provided by /etc/adjtime. hwclock will not
read nor write to that file with this option. Either
--utc or --localtime
must be specified when using this option.
--adjfile=filename
Overrides the default
/etc/adjtime.
-f, --rtc=filename
Overrides the default /dev file
name, which is /dev/rtc on many platforms but may be
/dev/rtc0, /dev/rtc1, and so on.
--directisa
This option is meaningful only
on an ISA machine or an Alpha (which implements enough of
ISA to be, roughly speaking, an ISA machine for
hwclock’s purposes). For other machines, it has
no effect. This option tells hwclock to use explicit
I/O instructions to access the Hardware Clock. Without this
option, hwclock will try to use the /dev/rtc device
(which it assumes to be driven by the RTC device driver). If
it is unable to open the device (for reading), it will use
the explicit I/O instructions anyway.
--badyear
Indicates that the Hardware
Clock is incapable of storing years outside the range
1994-1999. There is a problem in some BIOSes (almost all
Award BIOSes made between 4/26/94 and 5/31/95) wherein they
are unable to deal with years after 1999. If one attempts to
set the year-of-century value to something less than 94 (or
95 in some cases), the value that actually gets set is 94
(or 95). Thus, if you have one of these machines,
hwclock cannot set the year after 1999 and cannot use
the value of the clock as the true time in the normal
way.
To compensate
for this (without your getting a BIOS update, which would
definitely be preferable), always use
--badyear if you have one of these
machines. When hwclock knows it’s working with
a brain-damaged clock, it ignores the year part of the
Hardware Clock value and instead tries to guess the year
based on the last calibrated date in the adjtime file, by
assuming that that date is within the past year. For this to
work, you had better do a hwclock --set
or hwclock --systohc at least once a
year!
Though
hwclock ignores the year value when it reads the
Hardware Clock, it sets the year value when it sets the
clock. It sets it to 1995, 1996, 1997, or 1998, whichever
one has the same position in the leap year cycle as the true
year. That way, the Hardware Clock inserts leap days where
they belong. Again, if you let the Hardware Clock run for
more than a year without setting it, this scheme could be
defeated and you could end up losing a day.
hwclock
warns you that you probably need
--badyear whenever it finds your Hardware
Clock set to 1994 or 1995.
--srm
This option is equivalent to
--epoch=1900 and is used to specify the
most common epoch on Alphas with SRM console.
--arc
This option is equivalent to
--epoch=1980 and is used to specify the
most common epoch on Alphas with ARC console (but Ruffians
have epoch 1900).
--jensen
--funky-toy
These two options specify what
kind of Alpha machine you have. They are invalid if you
don’t have an Alpha and are usually unnecessary if you
do, because hwclock should be able to determine by
itself what it’s running on, at least when
/proc is mounted. (If you find you need one of these
options to make hwclock work, contact the maintainer
to see if the program can be improved to detect your system
automatically. Output of ’hwclock --debug’ and
’cat /proc/cpuinfo’ may be of interest.)
Option
--jensen means you are running on a
Jensen model. And --funky-toy means
that on your machine one has to use the UF bit instead of
the UIP bit in the Hardware Clock to detect a time
transition. "Toy" in the option name refers to the
Time Of Year facility of the machine.
--test
Do everything except actually updating the Hardware
Clock or anything else. This is useful, especially in
conjunction with --debug, in learning
about hwclock.
--debug
Display a lot of information
about what hwclock is doing internally. Some of its
function is complex and this output can help you understand
how the program works.
availability
The hwclock command is part of the util-linux package and is
available from ftp://ftp.kernel.org/pub/linux/utils/util-linux/.
automatic hardware clock synchronization by the kernel
You should be aware of another way that the Hardware Clock is
kept synchronized in some systems. The Linux kernel has a mode
wherein it copies the System Time to the Hardware Clock every 11
minutes. This is a good mode to use when you are using something
sophisticated like ntp to keep your System Time synchronized.
(ntp is a way to keep your System Time synchronized either to a
time server somewhere on the network or to a radio clock hooked
up to your system. See RFC 1305).
This mode (we’ll call it "11 minute mode") is off until something
turns it on. The ntp daemon xntpd is one thing that turns it on.
You can turn it off by running anything, including hwclock
--hctosys, that sets the System Time the old fashioned way.
To see if it is on or off, use the command adjtimex
--print and look at the value of "status". If the "64" bit of
this number (expressed in binary) equal to 0, 11 minute mode is
on. Otherwise, it is off.
If your system runs with 11 minute mode on, don’t use hwclock
--adjust or hwclock --hctosys. You’ll just make a
mess. It is acceptable to use a hwclock --hctosys at
startup time to get a reasonable System Time until your system is
able to set the System Time from the external source and start 11
minute mode.
clocks in a linux system
There are two main clocks in a Linux system:
The Hardware Clock: This is a clock that runs
independently of any control program running in the CPU and even
when the machine is powered off.
On an ISA system, this clock is specified as part of the ISA
standard. The control program can read or set this clock to a
whole second, but the control program can also detect the edges
of the 1 second clock ticks, so the clock actually has virtually
infinite precision.
This clock is commonly called the hardware clock, the real time
clock, the RTC, the BIOS clock, and the CMOS clock. Hardware
Clock, in its capitalized form, was coined for use by
hwclock because all of the other names are inappropriate
to the point of being misleading.
So for example, some non-ISA systems have a few real time clocks
with only one of them having its own power domain. A very low
power external I2C or SPI clock chip might be used with a backup
battery as the hardware clock to initialize a more functional
integrated real-time clock which is used for most other purposes.
The System Time: This is the time kept by a clock inside
the Linux kernel and driven by a timer interrupt. (On an ISA
machine, the timer interrupt is part of the ISA standard). It has
meaning only while Linux is running on the machine. The System
Time is the number of seconds since 00:00:00 January 1, 1970 UTC
(or more succinctly, the number of seconds since 1969). The
System Time is not an integer, though. It has virtually infinite
precision.
The System Time is the time that matters. The Hardware Clock’s
basic purpose in a Linux system is to keep time when Linux is not
running. You initialize the System Time to the time from the
Hardware Clock when Linux starts up, and then never use the
Hardware Clock again. Note that in DOS, for which ISA was
designed, the Hardware Clock is the only real time clock.
It is important that the System Time not have any discontinuities
such as would happen if you used the date(1L) program to
set it while the system is running. You can, however, do whatever
you want to the Hardware Clock while the system is running, and
the next time Linux starts up, it will do so with the adjusted
time from the Hardware Clock. You can also use the program
adjtimex(8) to smoothly adjust the System Time while the
system runs.
A Linux kernel maintains a concept of a local timezone for the
system. But don’t be misled -- almost nobody cares what timezone
the kernel thinks it is in. Instead, programs that care about the
timezone (perhaps because they want to display a local time for
you) almost always use a more traditional method of determining
the timezone: They use the TZ environment variable and/or the
/usr/share/zoneinfo directory, as explained in the man
page for tzset(3). However, some programs and fringe parts
of the Linux kernel such as filesystems use the kernel timezone
value. An example is the vfat filesystem. If the kernel timezone
value is wrong, the vfat filesystem will report and set the wrong
timestamps on files.
hwclock sets the kernel timezone to the value indicated by
TZ and/or /usr/share/zoneinfo when you set the System Time
using the --hctosys option.
The timezone value actually consists of two parts: 1) a field
tz_minuteswest indicating how many minutes local time (not
adjusted for DST) lags behind UTC, and 2) a field tz_dsttime
indicating the type of Daylight Savings Time (DST) convention
that is in effect in the locality at the present time. This
second field is not used under Linux and is always zero. (See
also settimeofday(2).)
environment variables
TZ
files
/etc/adjtime /usr/share/zoneinfo/ /dev/rtc /dev/rtc0 /dev/port
/dev/tty1 /proc/cpuinfo
functions
You need exactly one of the following options to tell
hwclock what function to perform:
-r, --show
Read the Hardware Clock and print the time on standard output.
The time shown is always in local time, even if you keep your
Hardware Clock in Coordinated Universal Time. See the
--utc option. Showing the Hardware Clock time is the
default when no function is specified.
--set
Set the Hardware Clock to the time given by the --date
option.
-s, --hctosys
Set the System Time from the Hardware Clock.
Also set the kernel’s timezone value to the local timezone as
indicated by the TZ environment variable and/or
/usr/share/zoneinfo, as tzset(3) would interpret
them. The obsolete tz_dsttime field of the kernel’s timezone
value is set to DST_NONE. (For details on what this field used to
mean, see settimeofday(2).)
This is a good option to use in one of the system startup
scripts.
-w, --systohc
Set the Hardware Clock to the current System Time.
--systz
Reset the System Time based on the current timezone.
Also set the kernel’s timezone value to the local timezone as
indicated by the TZ environment variable and/or
/usr/share/zoneinfo, as tzset(3) would interpret
them. The obsolete tz_dsttime field of the kernel’s timezone
value is set to DST_NONE. (For details on what this field used to
mean, see settimeofday(2).)
This is an alternate option to --hctosys that does not
read the hardware clock, and may be used in system startup
scripts for recent 2.6 kernels where you know the System Time
contains the Hardware Clock time.
--adjust
Add or subtract time from the Hardware Clock to account for
systematic drift since the last time the clock was set or
adjusted. See discussion below.
--getepoch
Print the kernel’s Hardware Clock epoch value to standard output.
This is the number of years into AD to which a zero year value in
the Hardware Clock refers. For example, if you are using the
convention that the year counter in your Hardware Clock contains
the number of full years since 1952, then the kernel’s Hardware
Clock epoch value must be 1952.
This epoch value is used whenever hwclock reads or sets
the Hardware Clock.
--setepoch
Set the kernel’s Hardware Clock epoch value to the value
specified by the --epoch option. See the --getepoch
option for details.
--predict
Predict what the RTC will read at time given by the --date
option based on the adjtime file. This is useful for example if
you need to set an RTC wakeup time to distant future and want to
account for the RTC drift.
-h, --help
Display a help text and exit.
-V, --version
Display the version of hwclock and exit.
how hwclock accesses the hardware clock
hwclock uses many different ways to get and set Hardware
Clock values. The most normal way is to do I/O to the device
special file /dev/rtc, which is presumed to be driven by the rtc
device driver. However, this method is not always available. For
one thing, the rtc driver is a relatively recent addition to
Linux. Older systems don’t have it. Also, though there are
versions of the rtc driver that work on DEC Alphas, there appear
to be plenty of Alphas on which the rtc driver does not work (a
common symptom is hwclock hanging). Moreover, recent Linux
systems have more generic support for RTCs, even systems that
have more than one, so you might need to override the default by
specifying /dev/rtc0 or /dev/rtc1 instead.
On older systems, the method of accessing the Hardware Clock
depends on the system hardware.
On an ISA system, hwclock can directly access the "CMOS
memory" registers that constitute the clock, by doing I/O to
Ports 0x70 and 0x71. It does this with actual I/O instructions
and consequently can only do it if running with superuser
effective userid. (In the case of a Jensen Alpha, there is no way
for hwclock to execute those I/O instructions, and so it
uses instead the /dev/port device special file, which provides
almost as low-level an interface to the I/O subsystem).
This is a really poor method of accessing the clock, for all the
reasons that user space programs are generally not supposed to do
direct I/O and disable interrupts. Hwclock provides it because it
is the only method available on ISA and Alpha systems which don’t
have working rtc device drivers available.
On an m68k system, hwclock can access the clock via the
console driver, via the device special file /dev/tty1.
hwclock tries to use /dev/rtc. If it is compiled for a
kernel that doesn’t have that function or it is unable to open
/dev/rtc (or the alternative special file you’ve defined on the
command line) hwclock will fall back to another method, if
available. On an ISA or Alpha machine, you can force
hwclock to use the direct manipulation of the CMOS
registers without even trying /dev/rtc by specifying the
--directisa option.
isa hardware clock century value
There is some sort of standard that defines CMOS memory Byte 50
on an ISA machine as an indicator of what century it is.
hwclock does not use or set that byte because there are
some machines that don’t define the byte that way, and it really
isn’t necessary anyway, since the year-of-century does a good job
of implying which century it is.
If you have a bona fide use for a CMOS century byte, contact the
hwclock maintainer; an option may be appropriate.
Note that this section is only relevant when you are using the
"direct ISA" method of accessing the Hardware Clock. ACPI
provides a standard way to access century values, when they are
supported by the hardware.
notes
the adjust function
The Hardware Clock is usually not very accurate. However, much of
its inaccuracy is completely predictable - it gains or loses the
same amount of time every day. This is called systematic drift.
hwclock’s "adjust" function lets you make systematic
corrections to correct the systematic drift.
It works like this: hwclock keeps a file,
/etc/adjtime, that keeps some historical information. This
is called the adjtime file.
Suppose you start with no adjtime file. You issue a hwclock
--set command to set the Hardware Clock to the true current
time. Hwclock creates the adjtime file and records in it
the current time as the last time the clock was calibrated. 5
days later, the clock has gained 10 seconds, so you issue another
hwclock --set command to set it back 10 seconds.
Hwclock updates the adjtime file to show the current time
as the last time the clock was calibrated, and records 2 seconds
per day as the systematic drift rate. 24 hours go by, and then
you issue a hwclock --adjust command. Hwclock
consults the adjtime file and sees that the clock gains 2 seconds
per day when left alone and that it has been left alone for
exactly one day. So it subtracts 2 seconds from the Hardware
Clock. It then records the current time as the last time the
clock was adjusted. Another 24 hours goes by and you issue
another hwclock --adjust. Hwclock does the same
thing: subtracts 2 seconds and updates the adjtime file with the
current time as the last time the clock was adjusted.
Every time you calibrate (set) the clock (using --set or
--systohc), hwclock recalculates the systematic
drift rate based on how long it has been since the last
calibration, how long it has been since the last adjustment, what
drift rate was assumed in any intervening adjustments, and the
amount by which the clock is presently off.
A small amount of error creeps in any time hwclock sets
the clock, so it refrains from making an adjustment that would be
less than 1 second. Later on, when you request an adjustment
again, the accumulated drift will be more than a second and
hwclock will do the adjustment then.
It is good to do a hwclock --adjust just before the
hwclock --hctosys at system startup time, and maybe
periodically while the system is running via cron.
The adjtime file, while named for its historical purpose of
controlling adjustments only, actually contains other information
for use by hwclock in remembering information from one invocation
to the next.
The format of the adjtime file is, in ASCII:
Line 1: 3 numbers, separated by blanks: 1) systematic drift rate
in seconds per day, floating point decimal; 2) Resulting number
of seconds since 1969 UTC of most recent adjustment or
calibration, decimal integer; 3) zero (for compatibility with
clock(8)) as a decimal integer.
Line 2: 1 number: Resulting number of seconds since 1969 UTC of
most recent calibration. Zero if there has been no calibration
yet or it is known that any previous calibration is moot (for
example, because the Hardware Clock has been found, since that
calibration, not to contain a valid time). This is a decimal
integer.
Line 3: "UTC" or "LOCAL". Tells whether the Hardware Clock is set
to Coordinated Universal Time or local time. You can always
override this value with options on the hwclock command
line.
You can use an adjtime file that was previously used with the
clock(8) program with hwclock.
see also
adjtimex,
date , gettimeofday,
settimeofday, crontab , tzset
/etc/init.d/hwclock.sh,
/usr/share/doc/util-linux/README.Debian.hwclock
authors
Written by
Bryan Henderson, September 1996 (bryanh@giraffe-data.com),
based on work done on the clock program by Charles
Hedrick, Rob Hooft, and Harald Koenig. See the source code
for complete history and credits.