8:hwclock
From Linux Man Pages
hwclock - query and set the hardware clock (RTC)
Contents |
SYNOPSIS
hwclock -r or hwclock --show
hwclock -w or hwclock --systohc
hwclock -s or hwclock --hctosys
hwclock -a or hwclock --adjust
hwclock -v or hwclock --version
hwclock --set --date=newdate
hwclock --getepoch
hwclock --setepoch --epoch=year
other options:
[-u|--utc] --localtime --noadjfile --directisa --test [-D|--debug]
and arcane options for DEC Alpha:
[-A|--arc] [-J|--jensen] [-S|--srm] [-F|--funky-toy]
Minimum unique abbreviations of all options are acceptable.
Also, -h asks for a help message.
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 to the System Time, and set the System Time from the Hardware Clock.
You can also run hwclock periodically to insert or remove time from the Hardware Clock to compensate for system-
atic drift (where the clock consistently gains or loses time at a certain rate if left to run).
OPTIONS
You need exactly one of the following options to tell hwclock what function to perform:
--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.
--set Set the Hardware Clock to the time given by the --date option.
--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 ker-
nel's timezone value is set to DST_NONE. (For details on what this field used to mean, see settimeof-
day(2).)
This is a good option to use in one of the system startup scripts.
--systohc
Set the Hardware Clock to the current System 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 Counter 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.
--version
Print the version of hwclock on Standard Output.
--date=date_string
You need this option if you specify the --set option. Otherwise, it is ignored. This specifies the time
to which to set the Hardware Clock. The value of this option is an argument to the date(1) program. For
example,
hwclock --set --date="9/22/96 16:45:05"
The argument is 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. I.e. 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
The following options apply to most functions.
--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
local 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.
--directisa
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 read), it will use the explicit I/O instructions anyway.
The rtc device driver was new in Linux Release 2.
--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 automati-
cally. Output of `hwclock --debug' and `cat /proc/cpuinfo' may be of interest.)
--jensen means you are running on a Jensen model.
--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.
NOTES
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.
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 preci-
sion.
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 time-
zone: 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).)
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).
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 avail-
able 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, 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.
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 cali-
brated, 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 periodi-
cally 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.
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 ker-
nel 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.
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 appropri-
ate.
Note that this section is only relevant when you are using the "direct ISA" method of accessing the Hardware
Clock.
ENVIRONMENT VARIABLES
TZ
FILES
/etc/adjtime /usr/share/zoneinfo/ (/usr/lib/zoneinfo on old systems) /dev/rtc /dev/port /dev/tty1 /proc/cpuinfo
RELATED
adjtimex(8), date(1), gettimeofday(2), settimeofday(2), crontab(1), tzset(3)