rtc(4) — Linux manual page


rtc(4)                  Kernel Interfaces Manual                  rtc(4)

NAME         top

       rtc - real-time clock

SYNOPSIS         top

       #include <linux/rtc.h>

       int ioctl(fd, RTC_request, param);

DESCRIPTION         top

       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

       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

       •  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:

              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.

              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).

              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.

              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.)

              Enable or disable the alarm interrupt, for RTCs that
              support alarms.  The third ioctl(2) argument is ignored.

              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.

              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.

              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).

              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         top

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

              status of the (first) RTC.

NOTES         top

       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.

SEE ALSO         top

       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

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