rtc(4) — Linux manual page


RTC(4)                    Linux Programmer's 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 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

       *  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

   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) argu‐

              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 us‐
              ing 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 un‐
              signed 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_RE‐
              SOURCE capability) can set frequencies above the value speci‐
              fied 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 sup‐
              port 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

              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 inter‐
              rupt, 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 man‐
              aged 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 struc‐
              ture 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

       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

COLOPHON         top

       This page is part of release 5.09 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

Linux                            2017-09-15                           RTC(4)

Pages that refer to this page: time(7)