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RTC(4) Linux Programmer's Manual RTC(4)
rtc - real-time clock
#include <linux/rtc.h>
int ioctl(fd, RTC_request, param);
This is the interface to drivers for real-time clocks (RTCs).
Most computers have one or more hardware clocks which record the current "wall
clock" time. These are called "Real Time Clocks" (RTCs). One of these
usually has battery backup power so that it tracks the time even while the
computer is turned off. RTCs often provide alarms and other interrupts.
All i386 PCs, and ACPI-based systems, have an RTC that is compatible with the
Motorola MC146818 chip on the original PC/AT. Today such an RTC is usually
integrated into the mainboard's chipset (south bridge), and uses a replaceable
coin-sized backup battery.
Non-PC systems, such as embedded systems built around system-on-chip
processors, use other implementations. They usually won't offer the same
functionality as the RTC from a PC/AT.
RTCs should not be confused with the system clock, which is a software clock
maintained by the kernel and used to implement gettimeofday(2) and time(2), as
well as setting timestamps on files, etc. The system clock reports seconds
and microseconds since a start point, defined to be the POSIX Epoch:
1970-01-01 00:00:00 +0000 (UTC). (One common implementation counts timer
interrupts, once per "jiffy", at a frequency of 100, 250, or 1000 Hz.) That
is, it is supposed to report wall clock time, which RTCs also do.
A key difference between an RTC and the system clock is that RTCs run even
when the system is in a low power state (including "off"), and the system
clock can't. Until it is initialized, the system clock can only report time
since system boot ... not since the POSIX Epoch. So at boot time, and after
resuming from a system low power state, the system clock will often be set to
the current wall clock time using an RTC. Systems without an RTC need to set
the system clock using another clock, maybe across the network or by entering
that data manually.
RTCs can be read and written with hwclock(8), or directly with the ioctl
requests listed below.
Besides tracking the date and time, many RTCs can also generate interrupts
* on every clock update (i.e., once per second);
* at periodic intervals with a frequency that can be set to any power-of-2
multiple in the range 2 Hz to 8192 Hz;
* on reaching a previously specified alarm time.
Each of those interrupt sources can be enabled or disabled separately. On
many systems, the alarm interrupt can be configured as a system wakeup event,
which can resume the system from a low power state such as Suspend-to-RAM
(STR, called S3 in ACPI systems), Hibernation (called S4 in ACPI systems), or
even "off" (called S5 in ACPI systems). On some systems, the battery backed
RTC can't issue interrupts, but another one can.
The /dev/rtc (or /dev/rtc0, /dev/rtc1, etc.) device can be opened only once
(until it is closed) and it is read-only. On read(2) and select(2) the
calling process is blocked until the next interrupt from that RTC is received.
Following the interrupt, the process can read a long integer, of which the
least significant byte contains a bit mask encoding the types of interrupt
that occurred, while the remaining 3 bytes contain the number of interrupts
since the last read(2).
The following ioctl(2) requests are defined on file descriptors connected to
RTC devices:
RTC_RD_TIME
Returns this RTC's time in the following structure:
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 */
};
The fields in this structure have the same meaning and ranges as for
the tm structure described in gmtime(3). A pointer to this structure
should be passed as the third ioctl(2) argument.
RTC_SET_TIME
Sets this RTC's time to the time specified by the rtc_time structure
pointed to by the third ioctl(2) argument. To set the RTC's time the
process must be privileged (i.e., have the CAP_SYS_TIME capability).
RTC_ALM_READ, RTC_ALM_SET
Read and set the alarm time, for RTCs that support alarms. The alarm
interrupt must be separately enabled or disabled using the RTC_AIE_ON,
RTC_AIE_OFF requests. The third ioctl(2) argument is a pointer to an
rtc_time structure. Only the tm_sec, tm_min, and tm_hour fields of
this structure are used.
RTC_IRQP_READ, RTC_IRQP_SET
Read and set the frequency for periodic interrupts, for RTCs that
support periodic interrupts. The periodic interrupt must be separately
enabled or disabled using the RTC_PIE_ON, RTC_PIE_OFF requests. The
third ioctl(2) argument is an unsigned long * or an unsigned 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
CAP_SYS_RESOURCE capability) can set frequencies above the value
specified in /proc/sys/dev/rtc/max-user-freq. (This file contains the
value 64 by default.)
RTC_AIE_ON, RTC_AIE_OFF
Enable or disable the alarm interrupt, for RTCs that support alarms.
The third ioctl(2) argument is ignored.
RTC_UIE_ON, RTC_UIE_OFF
Enable or disable the interrupt on every clock update, for RTCs that
support this once-per-second interrupt. The third ioctl(2) argument is
ignored.
RTC_PIE_ON, RTC_PIE_OFF
Enable or disable the periodic interrupt, for RTCs that support these
periodic interrupts. The third ioctl(2) argument is ignored. Only a
privileged process (i.e., one having the CAP_SYS_RESOURCE capability)
can enable the periodic interrupt if the frequency is currently set
above the value specified in /proc/sys/dev/rtc/max-user-freq.
RTC_EPOCH_READ, RTC_EPOCH_SET
Many RTCs encode 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 this RTC's Epoch. The
RTC's 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 an RTC register for the year. With some RTCs, these
operations can be used to read or to set the RTC's Epoch, respectively.
The third ioctl(2) argument is a unsigned long * or a unsigned long,
respectively, and the value returned (or assigned) is the Epoch. To
set the RTC's Epoch the process must be privileged (i.e., have the
CAP_SYS_TIME capability).
RTC_WKALM_RD, RTC_WKALM_SET
Some RTCs support a more powerful alarm interface, using these ioctls
to read or write the RTC's alarm time (respectively) with this
structure:
struct rtc_wkalrm {
unsigned char enabled;
unsigned char pending;
struct rtc_time time;
};
The enabled flag is used to enable or disable the alarm interrupt, or
to read its current status; when using these calls, RTC_AIE_ON and
RTC_AIE_OFF are not used. The pending flag is used by RTC_WKALM_RD to
report a pending interrupt (so it's mostly useless on Linux, except
when talking to the RTC managed by EFI firmware). The time field is as
used with RTC_ALM_READ and RTC_ALM_SET except that the tm_mday, tm_mon,
and tm_year fields are also valid. A pointer to this structure should
be passed as the third ioctl(2) argument.
/dev/rtc, /dev/rtc0, /dev/rtc1, etc: RTC special character device files.
/proc/driver/rtc: status of the (first) RTC.
When the kernel's system time is synchronized with an external reference using
adjtimex(2) it will update a designated RTC periodically every 11 minutes. To
do so, the kernel has to briefly turn off periodic interrupts; this might
affect programs using that RTC.
An RTC's Epoch has nothing to do with the POSIX Epoch which is only used for
the system clock.
If the year according to the RTC's Epoch and the year register is less than
1970 it is assumed to be 100 years later, that is, between 2000 and 2069.
Some RTCs support "wildcard" values in alarm fields, to support scenarios like
periodic alarms at fifteen minutes after every hour, or on the first day of
each month. Such usage is nonportable; portable user space code only expects
a single alarm interrupt, and will either disable or reinitialize the alarm
after receiving it.
Some RTCs support periodic interrupts with periods that are multiples of a
second rather than fractions of a second; multiple alarms; programmable output
clock signals; nonvolatile memory; and other hardware capabilities that are
not currently exposed by this API.
date(1), adjtimex(2), gettimeofday(2), settimeofday(2), stime(2), time(2),
gmtime(3), time(7), hwclock(8), /usr/src/linux/Documentation/rtc.txt
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at http://www.kernel.org/doc/man-pages/.
Linux 2010-02-25 RTC(4)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface