Ubuntu Manpage: rtc - real-time clock

NAME

       rtc - real-time clock

SYNOPSIS

       #include <linux/rtc.h>

       int ioctl(fd, RTC_request, param);

DESCRIPTION

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.

FILES

       /dev/rtc, /dev/rtc0, /dev/rtc1, etc.
              RTC special character device files.

       /proc/driver/rtc
              status of the (first) RTC.

NOTES

       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.

COLOPHON