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RTC(4) Linux Programmer's Manual 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 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-
     rupts

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:

     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 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
            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.)

     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.

     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  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).

     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  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.

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  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.