Does Linux RTC alarm use relative or absolute time? - linux

Question

I'm trying to configure RTC alarm on a Linux device. I've used an example from the RTC documentation:
int retval
struct rtc_time rtc_tm;
/* .... */
/* Read the RTC time/date */
retval = ioctl(fd, RTC_RD_TIME, &rtc_tm);
if (retval == -1) {
exit(errno);
}
/* Set the alarm to 5 sec in the future, and check for rollover */
rtc_tm.tm_sec += 5;
if (rtc_tm.tm_sec >= 60) {
rtc_tm.tm_sec %= 60;
rtc_tm.tm_min++;
}
if (rtc_tm.tm_min == 60) {
rtc_tm.tm_min = 0;
rtc_tm.tm_hour++;
}
if (rtc_tm.tm_hour == 24)
rtc_tm.tm_hour = 0;
retval = ioctl(fd, RTC_ALM_SET, &rtc_tm);
if (retval == -1) {
exit(errno);
}
This code snippet uses absolute time (from the epoch start) and it did not work for me. I thought this was due to a bug in hardware, but after some seemingly random time the alarm did fire. The only other piece of documentation that I've managed to find was a comment in rtc.cc:
case RTC_ALM_SET: /* Store a time into the alarm */
{
/*
* This expects a struct rtc_time. Writing 0xff means
* "don't care" or "match all". Only the tm_hour,
* tm_min and tm_sec are used.
*/
The fact that only hours, minutes and second are used suggests that time is relative to the moment when ioctl was called.
Should time passed to ioctl(fd, RTC_ALM_SET, &rtc_tm) be relative or absolute?

Answer 1

The RTC alarm works off absolute time, in other words if you want the alarm to go off in 5 minutes then you should read the current time and add 5 minutes to the current time and use the result to set the alarm time.
Here is a snip of text from a TI RTC chip doc: (http://www.ti.com/lit/ds/symlink/bq3285ld.pdf)
During each update cycle, the RTC compares the day-of-the-month, hours, minutes, and seconds bytes with the four corresponding alarm bytes. If a match of all bytes is found, the alarm interrupt event flag bit, AF in register C, is set to 1. If the alarm event is enabled, an interrupt request is generated.
I believe this to be pretty standard across RTCs out there...