1RTC(4) Linux Programmer's Manual RTC(4)
2
6 rtc - real-time clock
7
9 #include <linux/rtc.h>
10 int ioctl(fd, RTC_request, param);
12
14 This is the interface to drivers for real-time clocks (RTCs).
15 Most computers have one or more hardware clocks which record the cur‐
17 rent "wall clock" time. These are called "Real Time Clocks" (RTCs).
18 One of these usually has battery backup power so that it tracks the
19 time even while the computer is turned off. RTCs often provide alarms
20 and other interrupts.
21 All i386 PCs, and ACPI-based systems, have an RTC that is compatible
23 with the Motorola MC146818 chip on the original PC/AT. Today such an
24 RTC is usually integrated into the mainboard's chipset (south bridge),
25 and uses a replaceable coin-sized backup battery.
26 Non-PC systems, such as embedded systems built around system-on-chip
28 processors, use other implementations. They usually won't offer the
29 same functionality as the RTC from a PC/AT.
30 RTC vs system clock
32 RTCs should not be confused with the system clock, which is a software
33 clock maintained by the kernel and used to implement gettimeofday(2)
34 and time(2), as well as setting timestamps on files, and so on. The
35 system clock reports seconds and microseconds since a start point, de‐
36 fined to be the POSIX Epoch: 1970-01-01 00:00:00 +0000 (UTC). (One
37 common implementation counts timer interrupts, once per "jiffy", at a
38 frequency of 100, 250, or 1000 Hz.) That is, it is supposed to report
39 wall clock time, which RTCs also do.
40 A key difference between an RTC and the system clock is that RTCs run
42 even when the system is in a low power state (including "off"), and the
43 system clock can't. Until it is initialized, the system clock can only
44 report time since system boot ... not since the POSIX Epoch. So at
45 boot time, and after resuming from a system low power state, the system
46 clock will often be set to the current wall clock time using an RTC.
47 Systems without an RTC need to set the system clock using another
48 clock, maybe across the network or by entering that data manually.
49 RTC functionality
51 RTCs can be read and written with hwclock(8), or directly with the
52 ioctl(2) requests listed below.
53 Besides tracking the date and time, many RTCs can also generate inter‐
55 rupts
56 * on every clock update (i.e., once per second);
58 * at periodic intervals with a frequency that can be set to any power-
60 of-2 multiple in the range 2 Hz to 8192 Hz;
61 * on reaching a previously specified alarm time.
63 Each of those interrupt sources can be enabled or disabled separately.
65 On many systems, the alarm interrupt can be configured as a system
66 wakeup event, which can resume the system from a low power state such
67 as Suspend-to-RAM (STR, called S3 in ACPI systems), Hibernation (called
68 S4 in ACPI systems), or even "off" (called S5 in ACPI systems). On
69 some systems, the battery backed RTC can't issue interrupts, but an‐
70 other one can.
71 The /dev/rtc (or /dev/rtc0, /dev/rtc1, etc.) device can be opened only
73 once (until it is closed) and it is read-only. On read(2) and se‐
74 lect(2) the calling process is blocked until the next interrupt from
75 that RTC is received. Following the interrupt, the process can read a
76 long integer, of which the least significant byte contains a bit mask
77 encoding the types of interrupt that occurred, while the remaining 3
78 bytes contain the number of interrupts since the last read(2).
79 ioctl(2) interface
81 The following ioctl(2) requests are defined on file descriptors con‐
82 nected to RTC devices:
83 RTC_RD_TIME
85 Returns this RTC's time in the following structure:
86 struct rtc_time {
88 int tm_sec;
89 int tm_min;
90 int tm_hour;
91 int tm_mday;
92 int tm_mon;
93 int tm_year;
94 int tm_wday; /* unused */
95 int tm_yday; /* unused */
96 int tm_isdst; /* unused */
97 };
98 The fields in this structure have the same meaning and ranges as
100 for the tm structure described in gmtime(3). A pointer to this
101 structure should be passed as the third ioctl(2) argument.
102 RTC_SET_TIME
104 Sets this RTC's time to the time specified by the rtc_time
105 structure pointed to by the third ioctl(2) argument. To set the
106 RTC's time the process must be privileged (i.e., have the
107 CAP_SYS_TIME capability).
108 RTC_ALM_READ, RTC_ALM_SET
110 Read and set the alarm time, for RTCs that support alarms. The
111 alarm interrupt must be separately enabled or disabled using the
112 RTC_AIE_ON, RTC_AIE_OFF requests. The third ioctl(2) argument
113 is a pointer to an rtc_time structure. Only the tm_sec, tm_min,
114 and tm_hour fields of this structure are used.
115 RTC_IRQP_READ, RTC_IRQP_SET
117 Read and set the frequency for periodic interrupts, for RTCs
118 that support periodic interrupts. The periodic interrupt must
119 be separately enabled or disabled using the RTC_PIE_ON,
120 RTC_PIE_OFF requests. The third ioctl(2) argument is an un‐
121 signed long * or an unsigned long, respectively. The value is
122 the frequency in interrupts per second. The set of allowable
123 frequencies is the multiples of two in the range 2 to 8192.
124 Only a privileged process (i.e., one having the CAP_SYS_RESOURCE
125 capability) can set frequencies above the value specified in
126 /proc/sys/dev/rtc/max-user-freq. (This file contains the value
127 64 by default.)
128 RTC_AIE_ON, RTC_AIE_OFF
130 Enable or disable the alarm interrupt, for RTCs that support
131 alarms. The third ioctl(2) argument is ignored.
132 RTC_UIE_ON, RTC_UIE_OFF
134 Enable or disable the interrupt on every clock update, for RTCs
135 that support this once-per-second interrupt. The third ioctl(2)
136 argument is ignored.
137 RTC_PIE_ON, RTC_PIE_OFF
139 Enable or disable the periodic interrupt, for RTCs that support
140 these periodic interrupts. The third ioctl(2) argument is ig‐
141 nored. Only a privileged process (i.e., one having the
142 CAP_SYS_RESOURCE capability) can enable the periodic interrupt
143 if the frequency is currently set above the value specified in
144 /proc/sys/dev/rtc/max-user-freq.
145 RTC_EPOCH_READ, RTC_EPOCH_SET
147 Many RTCs encode the year in an 8-bit register which is either
148 interpreted as an 8-bit binary number or as a BCD number. In
149 both cases, the number is interpreted relative to this RTC's
150 Epoch. The RTC's Epoch is initialized to 1900 on most systems
151 but on Alpha and MIPS it might also be initialized to 1952,
152 1980, or 2000, depending on the value of an RTC register for the
153 year. With some RTCs, these operations can be used to read or
154 to set the RTC's Epoch, respectively. The third ioctl(2) argu‐
155 ment is an unsigned long * or an unsigned long, respectively,
156 and the value returned (or assigned) is the Epoch. To set the
157 RTC's Epoch the process must be privileged (i.e., have the
158 CAP_SYS_TIME capability).
159 RTC_WKALM_RD, RTC_WKALM_SET
161 Some RTCs support a more powerful alarm interface, using these
162 ioctls to read or write the RTC's alarm time (respectively) with
163 this structure:
164 struct rtc_wkalrm {
166 unsigned char enabled;
167 unsigned char pending;
168 struct rtc_time time;
169 };
170 The enabled flag is used to enable or disable the alarm inter‐
172 rupt, or to read its current status; when using these calls,
173 RTC_AIE_ON and RTC_AIE_OFF are not used. The pending flag is
174 used by RTC_WKALM_RD to report a pending interrupt (so it's
175 mostly useless on Linux, except when talking to the RTC managed
176 by EFI firmware). The time field is as used with RTC_ALM_READ
177 and RTC_ALM_SET except that the tm_mday, tm_mon, and tm_year
178 fields are also valid. A pointer to this structure should be
179 passed as the third ioctl(2) argument.
180
182 /dev/rtc, /dev/rtc0, /dev/rtc1, etc.
183 RTC special character device files.
184 /proc/driver/rtc
186 status of the (first) RTC.
187
189 When the kernel's system time is synchronized with an external refer‐
190 ence using adjtimex(2) it will update a designated RTC periodically ev‐
191 ery 11 minutes. To do so, the kernel has to briefly turn off periodic
192 interrupts; this might affect programs using that RTC.
193 An RTC's Epoch has nothing to do with the POSIX Epoch which is used
195 only for the system clock.
196 If the year according to the RTC's Epoch and the year register is less
198 than 1970 it is assumed to be 100 years later, that is, between 2000
199 and 2069.
200 Some RTCs support "wildcard" values in alarm fields, to support scenar‐
202 ios like periodic alarms at fifteen minutes after every hour, or on the
203 first day of each month. Such usage is nonportable; portable user-
204 space code expects only a single alarm interrupt, and will either dis‐
205 able or reinitialize the alarm after receiving it.
206 Some RTCs support periodic interrupts with periods that are multiples
208 of a second rather than fractions of a second; multiple alarms; pro‐
209 grammable output clock signals; nonvolatile memory; and other hardware
210 capabilities that are not currently exposed by this API.
211
213 date(1), adjtimex(2), gettimeofday(2), settimeofday(2), stime(2),
214 time(2), gmtime(3), time(7), hwclock(8)
215 Documentation/rtc.txt in the Linux kernel source tree
217
219 This page is part of release 5.13 of the Linux man-pages project. A
220 description of the project, information about reporting bugs, and the
221 latest version of this page, can be found at
222 https://www.kernel.org/doc/man-pages/.
223Linux 2021-03-22 RTC(4)