TIME(9)                FreeBSD Kernel Developer's Manual               TIME(9)

NAME

     boottime, time_second, time_uptime - system time variables

SYNOPSIS

     #include <sys/time.h>

     extern struct timeval boottime;
     extern time_t time_second;
     extern time_t time_uptime;

DESCRIPTION

     The boottime variable holds the estimated system boot time.  This time is
     initially set when the system boots, either from the RTC, or from a time
     estimated from the system's root filesystem.  When the current system
     time is set, stepped by ntpd(8), or a new time is read from the RTC as
     the system resumes, boottime is recomputed as new_time - uptime.  The
     sysctl(8) kern.boottime returns this value.

     The time_second variable is the system's "wall time" clock to the second.

     The time_uptime variable is the number of seconds since boot.

     The bintime(9), getbintime(9), microtime(9), getmicrotime(9),
     nanotime(9), and getnanotime(9) functions can be used to get the current
     time more accurately and in an atomic manner.  Similarly, the
     binuptime(9), getbinuptime(9), microuptime(9), getmicrouptime(9),
     nanouptime(9), and getnanouptime(9) functions can be used to get the time
     elapse since boot more accurately and in an atomic manner.  The boottime
     variable may be read and written without special precautions.  It is
     adjusted when the phase of the system time changes.

SEE ALSO

     clock_settime(2), ntp_adjtime(2), settimeofday(2), bintime(9),
     binuptime(9), getbintime(9), getbinuptime(9), getmicrotime(9),
     getmicrouptime(9), getnanotime(9), getnanouptime(9), microtime(9),
     microuptime(9), nanotime(9), nanouptime(9)

     Poul-Henning Kamp, "Timecounters: Efficient and precise timekeeping in
     SMP kernels", Proceedings of EuroBSDCon 2002, Amsterdam,
     /usr/share/doc/papers/timecounter.ascii.gz.

     Marshall Kirk McKusick and George V. Neville-Neil, The Design and
     Implementation of the FreeBSD Operating System, Addison-Wesley,
     57-61,65-66, July 2004.

FreeBSD 15.1-STABLE-HBSD          May 4, 2021                          TIME(9)