rtc <root
RTC(4)                              Linux Programmer's Manual                              RTC(4)

NAME
       rtc - real-time clock

SYNOPSIS
       #include 

       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  time‐
       stamps  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 ini‐
       tialized, 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 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 sys‐
       tem from a low power state such as Suspend-to-RAM (STR, called S3 in ACPI systems), Hiber‐
       nation  (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 struc‐
              ture 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 capa‐
              bility) 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 inter‐
              rupt  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 sys‐
              tems 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  a unsigned long * or a 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;  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 4.04 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 http://www.kernel.org/doc/man-pages/.

Linux                                       2010-02-25                                     RTC(4)

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