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PODoes Linux RTC alarm use relative or absolute time?
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copied!<p>I'm trying to configure RTC alarm on a Linux device. I've used an example from the <a href="https://www.kernel.org/doc/Documentation/rtc.txt" rel="nofollow">RTC documentation</a>:</p> <pre><code> 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); } </code></pre> <p>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 <a href="http://tomoyo.sourceforge.jp/cgi-bin/lxr/source/drivers/char/rtc.c#L491" rel="nofollow">rtc.cc</a>:</p> <pre><code> 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. */ </code></pre> <p>The fact that only hours, minutes and second are used suggests that time is relative to the moment when ioctl was called.</p> <p>Should time passed to ioctl(fd, RTC_ALM_SET, &rtc_tm) be relative or absolute?</p>

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