boottime, mono_time, runtime, time -- system time variables
extern struct timeval boottime;
extern volatile struct timeval mono_time;
extern struct timeval runtime;
extern 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 current
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 microseconds
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(). The
gettime(9) function can be used to read the time variable in an atomic
manner. There is no equivalent function for accessing mono_time. The
boottime and runtime variables may be read and written without special
It should be noted that due to the differences in how the time values
returned by gettime(9) and microtime(9) are updated, comparing the
results of the two routines may result in a time value that appears to go
backwards. This can be avoided by consistently using one function or the
other for use in any given context.
settimeofday(2), gettime(9), inittodr(9), microtime(9)
The notion of having a single runtime variable obviously would not be
appropriate in multiprocessor systems.
FreeBSD 5.2.1 March 22, 1997 FreeBSD 5.2.1 [ Back ]