RTC(4)                     Linux Programmer's 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 cur-
       rent "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 requests listed below.

       Besides  tracking the date and time, many RTCs can also generate inter-

       *  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 con-
       nected 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

              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)  argu-
              ment  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  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, etc.
              RTC special character device files.

              status of the (first) RTC.

       When the kernel's system time is synchronized with an  external  refer-
       ence  using  adjtimex(2)  it  will update a designated RTC periodically
       every 11 minutes.  To do so, the kernel has to briefly turn  off  peri-
       odic 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 scenar-
       ios 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 dis-
       able 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; pro-
       grammable 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

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       description  of  the project, information about reporting bugs, and the
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Linux                             2017-09-15                            RTC(4)
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