where uptime utility gets its info

[m4rtin]

If I run uptime utility in FreeBSD I can see the uptime of my system. However, where is this information collected from? Are there different places where FreeBSD writes it's information when it boots up? :stud

[vivek]

w gets it data from /proc and w is also uptime, see source code for yourself /usr/src/usr.bin/w/w.c

[SirDice]

The beauty of FreeBSD is that you can have a look at the source for all binaries that make up the base OS.

The answer is in the source of w (uptime is symlinked to w):
http://www.freebsd.org/cgi/cvsweb.cgi/src/usr.bin/w/w.c?rev=1.62


See clock_gettime

[SirDice]

w gets it data from /proc

It doesn't, /proc isn't loaded by default so all the base OS tools don't use it.

[m4rtin]

ok, I see. As I understand, clock_gettime is the system call, which asks for how long system has been up:

/*
* Print how long system has been up.
*/
if (clock_gettime(CLOCK_MONOTONIC, &tp) != -1) {
uptime = tp.tv_sec;
if (uptime > 60)
uptime += 30;
days = uptime / 86400;
uptime %= 86400;
hrs = uptime / 3600;
uptime %= 3600;
mins = uptime / 60;
secs = uptime % 60;
(void)printf(" up");
if (days > 0)
(void)printf(" %d day%s,", days, days > 1 ? "s" : "");
if (hrs > 0 && mins > 0)
(void)printf(" %2d:%02d,", hrs, mins);
else if (hrs > 0)
(void)printf(" %d hr%s,", hrs, hrs > 1 ? "s" : "");
else if (mins > 0)
(void)printf(" %d min%s,", mins, mins > 1 ? "s" : "");
else
(void)printf(" %d sec%s,", secs, secs > 1 ? "s" : "");
}



It looks like there isn't a particular file where uptime is written in. In the CLOCK_GETTIME(2) there is said:

CLOCK_UPTIME which starts at zero when the kernel boots
and increments monotonically in SI seconds while the machine is running

So basically there is some sort of uptime_daemon working in the background, which counts seconds and starts from zero if the device is rebooted?:)

[SirDice]

You will have to view the source for libc, in particular what the clock_gettime call does.

[expl]

You get uptime the same way you get normal real time/date, by calling SYS_CLOCK_GETTIME syscall just with flag CLOCK_UPTIME (defined in time.h). There is no 'daemon' running and calculating uptime, kernel just knows time it booted and gets uptime from its clock.