Getting Started with QNX Neutrino - QNX Software Systems

© 2009, QNX Software Systems GmbH & Co. KG. Advanced topics
CLOCK_MONOTONIC
As you can imagine, there could be a problem if we try to implement a mutex that
times out in 30 seconds. Let’s go through the steps. At 11:22:00 (Neutrino time) we
decide that we’re going to try and lock a mutex, but we only want to block for a
maximum of 30 seconds. Since the pthread_mutex_timedlock() function takes an
absolute time, we perform a calculation: we add 30 seconds to the current time, giving
us 11:22:30. If we follow the example above, we would wake up at 11:22:30, which
means that we would have only locked the mutex for 25 seconds, instead of the full 30.
The POSIX people thought about this, and the solution they came up with was to make
the pthread_mutex_timedlock() function be based on CLOCK_MONOTONIC instead
of CLOCK_REALTIME. This is built in to the pthread_mutex_timedlock() function and
isn’t something that you can change.
They way CLOCK_MONOTONIC works is that its timebase is never adjusted. The
impact of that is that regardless of what time it is in the real world, if you base a timer
in CLOCK_MONOTONIC and add 30 seconds to it (and then do whatever adjustments
you want to the time), the timer will expire in 30 elapsed seconds.
The clock source CLOCK_MONOTONIC has the following characteristics:
• always increasing count
• based on real time
• starts at zero
The important thing about the clock starting at zero is that this is a different “epoch”
(or “base”) than CLOCK_REALTIME’s epoch of Jan 1 1970, 00:00:00 GMT. So, even
though both clocks run at the same rate, their values are not interchangeable.
So what does CLOCK_SOFTTIME do?
If we wanted to sort our clock sources by “hardness” we’d have the following
ordering. You can think of CLOCK_MONOTONIC as being a freight train — it doesn’t
stop for anyone. Next on the list is CLOCK_REALTIME, because it can be pushed
around a bit (as we saw with the time adjustment). Finally, we have
CLOCK_SOFTTIME, which we can push around a lot.
The main use of CLOCK_SOFTTIME is for things that are “soft” — things that aren’t
going to cause a critical failure if they don’t get done. CLOCK_SOFTTIME is “active”
only when the CPU is running. (Yes, this does sound obvious :-) but wait!) When the
CPU is powered down due to Power Management detecting that nothing is going to
happen for a little while, CLOCK_SOFTTIME gets powered down as well!
Here’s a timing chart showing the three clock sources:
April 30, 2009 Chapter 3 • Clocks, Timers, and Getting a Kick Every So Often 161