Getting Started with QNX Neutrino - QNX Software Systems

Using timers
© 2009, QNX Software Systems GmbH & Co. KG.
Signal notification
Using timers
Creating a timer
Suppose that, on the other hand, you’re performing some kind of work, but don’t want
that work to go on forever. For example, you may be waiting for some function call to
return, but you can’t predict how long it takes.
In this case, using a signal as the notification scheme, with perhaps a signal handler, is
a good choice (another choice we’ll discuss later is to use kernel timeouts; see
_NTO_CHF_UNBLOCK in the Message Passing chapter as well). In the “Using
timers” section below, we’ll see a sample that uses signals.
Alternatively, a signal with sigwait() is cheaper than creating a channel to receive a
pulse on, if you’re not going to be receiving messages in your application anyway.
Having looked at all this wonderful theory, let’s turn our attention to some specific
code samples to see what you can do with timers.
To work with a timer, you must:
1 Create the timer object.
2 Decide how you wish to be notified (signal, pulse, or thread creation), and create
the notification structure (the struct sigevent).
3 Decide what kind of timer you wish to have (relative versus absolute, and
one-shot versus periodic).
4 Start it.
Let’s look at these in order.
The first step is to create the timer with timer_create():
#include
#include
int
timer_create (clockid_t clock_id,
struct sigevent *event,
timer_t *timerid);
The clock_id argument tells the timer_create() function which time base you’re
creating this timer for. This is a POSIX thing — POSIX says that on different
platforms you can have multiple time bases, but that every platform must support at
least the CLOCK_REALTIME time base. Under Neutrino, there are three time bases to
choose from:
• CLOCK_REALTIME
• CLOCK_SOFTTIME
146 Chapter 3 • Clocks, Timers, and Getting a Kick Every So Often April 30, 2009