[ILUG] [OT] C time_elapsed function

[Kenn Humborg]

> kevin lyda wrote:
> 
> > On Tue, Nov 06, 2001 at 06:47:03PM +0000, Padraig Brady wrote:
> > 
> >>Feck that. It would be easier to write a simple kernel module
> >>to export /proc/seconds_since_boot
> >>
> > 
> > again, what is the problem you're trying to solve?
> > 
> > you seem to want a number that increments.  a thread could supply that.
> > if it needs to cross process instances, you could use a sequence file.
> 
> Yes within process is trivial.
> Across process a sequence file will not do as I need the correct
> number of seconds since boot, not just a unique monotonically
> increasing number (I'm writing a scheduling app).
> doze had GetTickCount in 3.1!

... but GetTickCount() wrapped after 49.7 days.

Anyway, if you can guarantee that you will read the
32-bit jiffies counter at least once every 497 days,
then simply watch for the new reading being less
that the old and increment another 32-bit count.

Note that, (at least in earlier versions of linux
development kernels and maybe even now), Linus often
preloads jiffies with a very high value (rather than
zero) to force jiffy wraparound to happen much sooner,
to force other kernel developers and driver writers
to deal with wraparound properly.

So you probably can't trust the raw jiffies value anyway,
only the delta between two readings.  Which is not much good
if you need the "correct number of seconds since boot".
Would the "correct number of seconds since the start of your
scheduling app" be enough?

Later,
Kenn