.\" rtc.4
.\" Copyright 2002 Urs Thuermann (urs@isnogud.escape.de)
.\"
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.\" modify it under the terms of the GNU General Public License as
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.\" $Id: rtc.4,v 1.4 2005/12/05 17:19:49 urs Exp $
.\"
.\" 2006-02-08 Various additions by mtk
.\"
.TH RTC 4 "2005-12-05" "Linux" "Linux Programmer's Manual"
.SH NAME
rtc \- real-time clock
.SH SYNOPSIS
#include <linux/rtc.h>
.SH DESCRIPTION
This is the driver for the real-time clock (RTC).  

Most computers have a built-in hardware clock, usually called the
real-time clock.  
This clock is normally battery powered so
that it keeps the time even while the computer is switched off.  
It represents the current time as year, month, day of month, hour,
minute, and second.

The RTC is a chip that maintains the time and date and is able to
generate interrupts at specified times.  This chip typically used to
be a Motorola MC146818, a Dallas DS12887, or similar,
but today it is usually implemented in the mainboard's chipset.

The RTC should not be confused with the system time which is an
independent, interrupt-driven software clock maintained by the kernel.
The software clock is maintained by an interrupt routine that
typically has a frequency of 100, 250, or 1000 Hz.
The software clock counts seconds and microsecond since the POSIX
Epoch, i.e., Jan 1, 1970, 0:00 UTC.
This clock does not involve any special hardware.

The RTC can be read and set with
.BR hwclock (8).

The RTC is almost never used by the Linux kernel.  Instead,
the kernel uses the software clock time for
.BR time (2),
.BR gettimeofday (2),
timestamps on files, etc.
However, at boot time the kernel initializes its software clock by 
reading the RTC.

Besides counting the date and time, the RTC can also generate
interrupts
.IP *
on every clock update (i.e. once per second);
.IP *
at periodic intervals with a frequency that can be set to
any power-of-2 multiple in the range 2 Hz to 8192 Hz;
.IP *
on reaching a previously specified alarm time.
.PP
Each of these interrupt sources can be enabled or disabled separately.

The
.B /dev/rtc
device can be opened only once simultaneously and it is read-only.  On
.BR read (2)
and
.BR select (2)
the calling process is blocked until the next interrupt from the RTC
is received.
Following the interrupt, the process can read a long integer, of which
the least significant byte contains the type of interrupt that occurred,
while the remaining 3 bytes contain the number of interrupts since the
last
.BR read (2).

The following 
.BR ioctl (2)
operations are provided:
.TP
.B RTC_RD_TIME
Returns the RTC time in the following structure:
.PP
.RS
.in +0.5i
.nf
struct rtc_time {
    int tm_sec;
    int tm_min;
    int tm_hour;
    int tm_mday;
    int tm_mon;
    int tm_year;
    int tm_wday;     /* unused */
    int tm_yday;     /* unused */
    int tm_isdst;    /* unused */
};
.fi
.in -0.5i
.RE
.IP
The fields in this structure have the same meaning and ranges as for the
.I tm
structure described in
.BR gmtime (3).
A pointer to this structure should be passed as the third
.BR ioctl ()
argument.
.TP
.B RTC_SET_TIME
Sets the RTC time to the time specified by the 
.I rtc_time
structure pointed to by the third 
.BR ioctl ()
argument.
To set the
RTC time the process must be privileged (i.e., have the
.B CAP_SYS_TIME
capability).
.TP
.BR RTC_ALM_READ ", " RTC_ALM_SET
Read and set the alarm time.  
The third \fBioctl\fP() argument is a pointer to an
.I rtc_time 
structure.  
Only the 
.IR tm_sec ,
.IR tm_min ,
and
.IR tm_hour
fields of this structure are used.
.TP
.BR RTC_IRQP_READ ", " RTC_IRQP_SET
Read and set the frequency for periodic interrupts.  
The third \fBioctl\fP() argument is a
.I "long\ *"
or a 
.IR long , 
respectively.
The value is the frequency in interrupts per second.  
The set of allowable frequencies is the multiples of two 
in the range 2 to 8192.
Only a privileged process (i.e., one having the
.B CAP_SYS_RESOURCE
capability) can set frequencies above the value specified in
.IR /proc/sys/dev/rtc/max-user-freq .
(This file contains the value 64 by default.)
.TP
.BR RTC_AIE_ON ", " RTC_AIE_OFF
Enable or disable the alarm interrupt.
The third \fBioctl\fP() argument is ignored.
.TP
.BR RTC_UIE_ON ", " RTC_UIE_OFF
Enable or disable the interrupt on every clock update.  
The third \fBioctl\fP() argument is ignored.
.TP
.BR RTC_PIE_ON ", " RTC_PIE_OFF
Enable or disable the periodic interrupt.  
The third \fBioctl\fP() argument is ignored.
Only a privileged process (i.e., one having the
.B CAP_SYS_RESOURCE
capability) can enable the periodic interrupt if the frequency is 
currently set above the value specified in
.IR /proc/sys/dev/rtc/max-user-freq .
.TP
.BR RTC_EPOCH_READ ", " RTC_EPOCH_SET
The RTC encodes the year in an 8-bit register which is either
interpreted as an 8-bit binary number or as a BCD number.  
In both cases,
the number is interpreted relative to the RTC Epoch.  
The RTC Epoch is
initialized to 1900 on most systems but on Alpha and Mips it might
also be initialized to 1952, 1980, or 2000, depending on the value of
RTC register for the year.  
These operations can be used to read or to set the RTC Epoch, respectively.
To set the RTC Epoch the process must be privileged (i.e., have the
.B CAP_SYS_TIME
capability).
.SH FILES
.IR /dev/rtc : 
the RTC special character device file.

.IR /proc/driver/rtc : 
status of the RTC.
.SH NOTES
When the kernel's system time is synchronized with an external
reference using
.BR adjtimex (2)
it will update the RTC periodically every 11 minutes.  To
do so, the kernel has to briefly turn off periodic interrupts;
this might affect programs using the RTC.

The RTC Epoch has nothing to do with the POSIX Epoch which is only
used for the system clock.

If the year according to the RTC Epoch and the RTC's year register is
less than 1970 it is assumed to be 100 years later, i.e. between 2000
and 2069.
.SH "SEE ALSO"
.BR hwclock (8),
.BR date (1),
.BR time (2),
.BR stime (2),
.BR gettimeofday (2),
.BR settimeofday (2),
.BR adjtimex (2),
.BR gmtime (3),
.BR time (7),
/usr/src/linux/Documentation/rtc.txt