Home Man
Today's Posts

Linux & Unix Commands - Search Man Pages
Man Page or Keyword Search:
Select Section of Man Page:
Select Man Page Repository:

Linux 2.6 - man page for clock (linux section 8)

HWCLOCK(8)									       HWCLOCK(8)

       hwclock - query and set the hardware clock (RTC)

       hwclock [functions] [options]

       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 systematic drift (where the clock consistently gains or  loses  time  at	a
       certain rate if left to run).

       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.

       --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.

	      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.

	      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.

	      Print the kernel's Hardware Clock epoch value to standard output.  This is the num-
	      ber  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.

	      Set the kernel's Hardware Clock epoch value to the value specified by  the  --epoch
	      option.  See the --getepoch option for details.

       -v, --version
	      Print the version of hwclock on Standard Output.

	      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.

	      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

	      Predict what the RTC will read at time given by the --date option based on the adj-
	      time file. This is useful for example if you need to set an RTC wakeup time to dis-
	      tant future and want to account for the RTC drift.

       The following options apply to most functions.

       -u, --utc

	      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  speci-
	      fied 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.

	      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.

	      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.

	      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

	      The rtc device driver was new in Linux Release 2.

	      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 cal-
	      ibrated 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, which-
	      ever 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).


	      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 automat-
	      ically. 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.

	      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.

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  pro-
       gram 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 preci-

       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 inte-
       grated 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 preci-

       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 mis-
       led -- 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  time-
       stamps 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_dst-
       time 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 hang-
       ing).  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  hard-

       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 pro-
       vides 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.

The Adjust Function
       The Hardware Clock is usually not very accurate.  However, much of its inaccuracy is  com-
       pletely 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 infor-
       mation.	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 sec-
       onds.  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

       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 Univer-
       sal 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

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 sophisti-
       cated  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 "sta-
       tus".  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 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.


       /etc/adjtime /usr/share/zoneinfo/ /dev/rtc /dev/rtc0 /dev/port /dev/tty1 /proc/cpuinfo

       adjtimex(8),   date(1),	  gettimeofday(2),    settimeofday(2),	  crontab(1),	 tzset(3)
       /etc/init.d/hwclock.sh, /usr/share/doc/util-linux/README.Debian.hwclock

       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.

       The hwclock command is part of the util-linux package and is available from ftp://ftp.ker-

					  06 August 2008			       HWCLOCK(8)

All times are GMT -4. The time now is 05:10 AM.

Unix & Linux Forums Content Copyrightę1993-2018. All Rights Reserved.
Show Password