RTC(4)                    (2017-09-15)                     RTC(4)

     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 imple-
          ment gettimeofday(2) and time(2), as well as setting times-
          tamps 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 sup-
          posed 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.

     Page 1                        Linux             (printed 5/24/22)

     RTC(4)                    (2017-09-15)                     RTC(4)

        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 gen-
          erate 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.  Follow-
          ing the interrupt, the process can read a long integer, of
          which the least significant byte contains a bit mask encod-
          ing the types of interrupt that occurred, while the remain-
          ing 3 bytes contain the number of interrupts since the last
          read(2).

        ioctl(2) interface
          The following ioctl(2) requests are defined on file descrip-
          tors 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 */
                   };

     Page 2                        Linux             (printed 5/24/22)

     RTC(4)                    (2017-09-15)                     RTC(4)

               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 frequen-
               cies 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.

     Page 3                        Linux             (printed 5/24/22)

     RTC(4)                    (2017-09-15)                     RTC(4)

          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 ini-
               tialized 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) argu-
               ment.

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

               status of the (first) RTC.

     NOTES
          When the kernel's system time is synchronized with an exter-
          nal 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.

     Page 4                        Linux             (printed 5/24/22)

     RTC(4)                    (2017-09-15)                     RTC(4)

          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 regis-
          ter 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 sup-
          port 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 sin-
          gle 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; non-
          volatile memory; and other hardware capabilities that are
          not currently exposed by this API.

     SEE ALSO
          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

     COLOPHON
          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at https://www.kernel.org/doc/man-pages/.

     Page 5                        Linux             (printed 5/24/22)