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

     boottime, mono_time, runtime, time -- system time variables

     extern struct timeval boottime;
     extern volatile struct timeval mono_time;
     extern struct timeval runtime;
     extern volatile struct timeval time;

     The time variable is the system's ``wall time'' clock.  It is set at boot
     by inittodr(9), and is updated by the settimeofday(2) system call and by
     periodic clock interrupts.

     The boottime variable holds the system boot time.  It is set from time at
     system boot, and is updated when the system time is adjusted with

     The runtime variable holds the time that the system switched to the cur-
     rent process.  It is set after each context switch, and is updated when
     the system time is adjusted with settimeofday(2).  Because runtime is
     used for system accounting, it is set with the high-resolution
     microtime(9) function, rather than being copied from time.

     The mono_time variable is a monotonically increasing system clock.  It is
     set from time at boot, and is updated by the periodic timer interrupt.
     (It is not updated by settimeofday(2).)

     All of these variables contain times expressed in seconds and microsec-
     onds since midnight (0 hour), January 1, 1970.

     Clock interrupts should be blocked when reading or writing time or
     mono_time, because those variables are updated by hardclock(9).  Boottime
     and runtime may be read and written without special precautions.

     settimeofday(2), hardclock(9), hz(9), inittodr(9), microtime(9)

     Runtime should not be updated when the system time is changed with
     settimeofday(2), because it is used for resource accounting.

     The notion of having a single runtime variable obviously won't be appro-
     priate in multiprocessor systems.

BSD                              May 31, 2007                              BSD