Date: Thu, 19 Aug 2010 13:09:46 +0300 From: phil hefferan <wdef200@gmail.com> To: freebsd-hackers@freebsd.org Subject: Reading rtc on FreeBSD Message-ID: <AANLkTinu-7wxfHU3OweQ3rM9P6DvR10gTxNYys8OEpjq@mail.gmail.com>
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I have C code for Linux that, among other things, caches the difference between the rtc and system time, so that the program can detect if the system time has changed more than a threshold between runs. I want to port this code to FreeBSD/Mac. I'm trying to clarify the relationship between rtc and system time on FreeBSD and how the rtc is read. Linux has the utility hwclock which reads /dev/rtc and anyway getting the cmos clock time directly from this interface can be done in a few lines of code. I've been looking around for how to read the cmos/rtc on FreeBSD. There is no hwclock utility in FreeBSD that I can read sources for to see how it is done. http://www.wraith.sf.ca.us/ntp/not-hwclock.c.txt is supposed to be C code to read the software time and then set the cmos clock to match. This code implies that, on FreeBSD, gettimeofday reads the software time and settimeofday sets the cmos clock. I read here http://www.mail-archive.com/freebsd-hardware@freebsd.org/msg03414.html that settimeofday in fact sets both rtc and system time together. On Linux both gettimeofday and settimeofday apply to the software time only. BUT the source to adjkerntz.c for FreeBSD seems to say that gettimeofday reads the CMOS clock not the system time: /* get local CMOS clock and possible kernel offset */ if (gettimeofday(&tv, &tz)) { syslog(LOG_ERR, "gettimeofday: %m"); return 1; } Which is it? Does gettimeofday read the cmos clock/rtc on FreeBSD? If not, how do I read the battery-backed clock on FreeBSD?
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