Getting Started with QNX Neutrino - QNX Software Systems

© 2009, QNX Software Systems GmbH & Co. KG. Using timers
Signal, pulse, or thread?
What kind of timer?
• CLOCK_MONOTONIC
For now, we’ll ignore CLOCK_SOFTTIME and CLOCK_MONOTONIC but we will
come back to them in the “Other clock sources” section, below.
The second parameter is a pointer to a struct sigevent data structure. This data
structure is used to inform the kernel about what kind of event the timer should deliver
whenever it “fires.” We discussed how to fill in the struct sigevent above in the
discussion of signals versus pulses versus thread creation.
So, you’d call timer_create() with CLOCK_REALTIME and a pointer to your struct
sigevent data structure, and the kernel would create a timer object for you (which
gets returned in the last argument). This timer object is just a small integer that acts as
an index into the kernel’s timer tables; think of it as a “handle.”
At this point, nothing else is going to happen. You’ve only just created the timer; you
haven’t triggered it yet.
Having created the timer, you now have to decide what kind of timer it is. This is done
by a combination of arguments to timer_settime(), the function used to actually start
the timer:
#include
int
timer_settime (timer_t timerid,
int flags,
struct itimerspec *value,
struct itimerspec *oldvalue);
The timerid argument is the value that you got back from the timer_create() function
call — you can create a bunch of timers, and then call timer_settime() on them
individually to set and start them at your convenience.
The flags argument is where you specify absolute versus relative.
If you pass the constant TIMER_ABSTIME, then it’s absolute, pretty much as you’d
expect. You then pass the actual date and time when you want the timer to go off.
If you pass a zero, then the timer is considered relative to the current time.
Let’s look at how you specify the times. Here are key portions of two data structures
(in ):
struct timespec {
long tv_sec,
tv_nsec;
};
struct itimerspec {
struct timespec it_value,
it_interval;
};
April 30, 2009 Chapter 3 • Clocks, Timers, and Getting a Kick Every So Often 147