boottime(9) [freebsd man page]

TIME(9) 						   BSD 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 system boot time.

     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.

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.

BSD
								September 17, 2004							       BSD

MICROTIME(9)						   BSD Kernel Developer's Manual					      MICROTIME(9)

NAME

     bintime, getbintime, microtime, getmicrotime, nanotime, getnanotime -- get the current time

SYNOPSIS

     #include <sys/time.h>

     void
     bintime(struct bintime *bt);

     void
     getbintime(struct bintime *bt);

     void
     microtime(struct timeval *tv);

     void
     getmicrotime(struct timeval *tv);

     void
     nanotime(struct timespec *ts);

     void
     getnanotime(struct timespec *tsp);

DESCRIPTION

     The bintime() and getbintime() functions store the system time as a struct bintime at the addresses specified by bt.  The microtime() and
     getmicrotime() functions perform the same utility, but record the time as a struct timeval instead.  Similarly the nanotime() and
     getnanotime() functions store the time as a struct timespec.  The structures are described in timeval(3).

     The bintime(), microtime(), and nanotime() functions always query the timecounter to return the current time as precisely as possible.
     Whereas getbintime(), getmicrotime(), and getnanotime() functions are abstractions which return a less precise, but faster to obtain, time.

     The intent of the getbintime(), getmicrotime(), and getnanotime() functions is to enforce the user's preference for timer accuracy versus
     execution time.  They should be used where a precision of 1/HZ (e.g., 10 msec on a 100HZ machine, see hz(9)) is acceptable or where perfor-
     mance is priority.

     The system realtime clock is guaranteed to be monotonically increasing at all times.  As such, all calls to these functions are guaranteed to
     return a system time greater than or equal to the system time returned in any previous calls.  Comparable functions exist to retrieve the
     time elapsed since boot; see microuptime(9).

SEE ALSO

     settimeofday(2), bintime_add(9), inittodr(9), time_second(9), tvtohz(9)

CODE REFERENCES

     The implementation of the microtime() family of functions is in sys/kern/kern_tc.c as a part of the timecounter(9) framework.

     The implementation of the time counter sources used by the timecounter(9) is machine dependent, hence its location in the source code tree
     varies from architecture to architecture.

AUTHORS

     This manual page was written by Jeremy Cooper and
     Kelly Yancey <kbyanc@posi.net>.

BUGS

     Despite the guarantee that the system realtime clock will always be monotonically increasing, it is always possible for the system clock to
     be manually reset by the system administrator to any date.

BSD
								   June 8, 2010 							       BSD