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NAME | SYNOPSIS | DESCRIPTION | FILES | NOTES | SEE ALSO | COLOPHON |
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rtc(4) Kernel Interfaces 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.
RTC vs system clock
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, and so on. 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.
RTC functionality
RTCs can be read and written with hwclock(8), or directly with
the ioctl(2) 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).
ioctl(2) interface
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 an
unsigned long * or an 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
... 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
used only 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 expects only 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)
Documentation/rtc.txt in the Linux kernel source tree
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
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⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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Linux man-pages 6.9.1 2024-05-02 rtc(4)
Pages that refer to this page: time(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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