| rtc(4) | Device Drivers 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 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.
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.
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 interrupts
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).
The following ioctl(2) requests are defined on file descriptors connected to RTC devices:
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 */
};
struct rtc_wkalrm {
unsigned char enabled;
unsigned char pending;
struct rtc_time time;
};
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; 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
| 2023-10-31 | Linux man-pages 6.7 |