with the program "clock", while \-h asks for a help message.
.SH DESCRIPTION
-.I hwclock
+.B 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.
.PP
You can also run
-.I hwclock
+.B 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).
.SH OPTIONS
You need exactly one of the following options to tell
-.I hwclock
+.B hwclock
what function to perform:
.PP
.TP
.TP
.B \-\-version
Print the version of
-.I hwclock
+.B hwclock
on Standard Output.
.br
You need the following option if you specify
.B \-\-date=date_string
Specifies the time to which to set the Hardware Clock. The value of this
option is an argument to the
-.I date(1)
+.BR date (1)
program. For example,
.sp
.I hwclock --set --date="9/22/96 16:45:05"
.sp
The argument is in local time, even if you keep your Hardware Clock in
Coordinated Universal time. See the
-.I \-\-utc
+.B \-\-utc
option.
.TP
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
-.IR hwclock .
+.BR 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
nor
.B \-\-localtime
, the default is whichever was specified the last time
-.I hwclock
+.B hwclock
was used to set the clock (i.e. hwclock was successfully run with the
.B \-\-set
,
.B \-\-directisa
is meaningful only on an ISA machine or an Alpha (which implements enough
of ISA to be, roughly speaking, an ISA machine for
-.IR hwclock 's
+.BR hwclock 's
purposes). For other machines, it has no effect. This option tells
-.I hwclock
+.B hwclock
to use explicit I/O instructions to access the Hardware Clock.
Without this option,
-.I hwclock
+.B 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.
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,
-.I hwclock
+.B hwclock
cannot set the year after 1999 and cannot use the value of the clock as
the true time in the normal way.
definitely be preferable), always use
.B \-\-badyear
if you have one of these machines. When
-.I hwclock
+.B 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
at least once a year!
Though
-.I hwclock
+.B 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
year without setting it, this scheme could be defeated and you could
end up losing a day.
-.I hwclock
+.B hwclock
warns you that you probably need
.B \-\-badyear
whenever it finds your Hardware Clock set to 1994 or 1995.
.TP
.B \-\-funky\-toy
These options all tell
-.I hwclock
+.B hwclock
what kind of Alpha machine you have. They
are invalid if you don't have an Alpha and shouldn't be necessary if you
do, because
-.I hwclock
+.B hwclock
should be able to determine by itself what it's
running on, at least when
.I /proc
is mounted. These options make it possible for
-.I hwclock
+.B hwclock
to work even when
its environment does not conform to its expectations and thus it cannot
accurately determine what sort of system it is running on. If you think
.B \-\-debug
option to see what conclusions the program is
reaching and how. If you find you need one of these options to make
-.I hwclock
+.B hwclock
work, contact the
-.I hwclock
+.B hwclock
maintainer to see if the program can be improved to detect your system
automatically.
else. This is useful, especially in conjunction with
.B \-\-debug,
in learning about
-.I hwclock.
+.B hwclock.
.TP
.B \-\-debug
Display a lot of information about what
-.I hwclock
+.B hwclock
is doing internally. Some of its function is complex and this output
can help you understand how the program works.
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
-.I hwclock
+.B hwclock
because all of the other names are inappropriate to the point of being
misleading.
.PP
kernel timezone value is wrong, the vfat filesystem will report and
set the wrong timestamps on files.
.PP
-.I hwclock
+.B hwclock
sets the kernel timezone to the value indicated by TZ and/or
/usr/local/timezone when you set the System Time using the
.B \-\-hctosys
.SH How hwclock Accesses the Hardware Clock
.PP
-.I hwclock
+.B 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,
the system hardware.
.PP
On an ISA system,
-.I hwclock
+.B 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
-.I hwclock
+.B 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).
.PP
On an m68k system,
-.I hwclock
+.B hwclock
can access the clock via the console driver, via the device special
file /dev/tty1.
.PP
-.I hwclock
+.B 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,
-.I hwclock
+.B hwclock
will fall back to another method, if available. On an ISA or Alpha
machine, you can force
-.I hwclock
+.B hwclock
to use the direct manipulation of the CMOS registers without even trying
.I /dev/rtc
by specifying the \-\-directisa option.
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.
-.IR hwclock 's
+.BR hwclock 's
"adjust" function lets you make systematic corrections to correct the
systematic drift.
.PP
It works like this:
-.I hwclock
+.B hwclock
keeps a file,
.I /etc/adjtime,
that keeps some historical information. This is called the adjtime file.
Suppose you start with no adjtime file. You issue a
.I hwclock \-\-set
command to set the Hardware Clock to the true current time.
-.I Hwclock
+.B 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
+5 days later, the clock has gained 10 seconds, so you issue another
.I hwclock \-\-set
command to set it back 10 seconds.
-.I Hwclock
+.B 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
.I hwclock \-\-adjust
command.
-.I Hwclock
+.B 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
.I hwclock \-\-adjust.
-.I Hwclock
+.B 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.
.PP
or
.I \-\-systohc
),
-.I hwclock
+.B 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.
.PP
A small amount of error creeps in any time
-.I hwclock
+.B 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
-.I hwclock
+.B hwclock
will do the adjustment then.
.PP
It is good to do a
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
-.IR clock )
+.BR clock (8))
as a decimal integer.
.PP
Line 2: 1 number: Resulting number of seconds since 1969 UTC of most
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
-.I hwclock
+.B hwclock
command line.
.PP
You can use an adjtime file that was previously used with the
-.I clock
+.BR clock (8)
program with
-.I hwclock.
+.B hwclock.
.SH "Automatic Hardware Clock Synchronization By the Kernel"
There is some sort of standard that defines CMOS memory Byte 50 on an ISA
machine as an indicator of what century it is.
-.I hwclock
+.B 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
-.I hwclock
+.B hwclock
maintainer; an option may be appropriate.
Note that this section is only relevant when you are using the "direct
projects / thirdparty / util-linux.git / blobdiff