Getting Started with QNX Neutrino - QNX Software Systems

Clocks and timers
© 2009, QNX Software Systems GmbH & Co. KG.
Notification schemes
The implementation in the kernel is still based on the same principle as the delay timer
that we used in our first example. The kernel takes the absolute time (if you specified
it that way) and stores it. In the clock ISR, the stored time is compared against the
time of day in the usual manner.
However, instead of your thread being removed from the running queue when you call
the kernel, your thread continues to run. When the time of day reaches the stored time,
the kernel notifies your thread that the designated time has been reached.
How do you receive a timeout notification? With the delay timer, you received
notification by virtue of being made READY again.
With periodic and one-shot timers, you have a choice:
• send a pulse
• send a signal
• create a thread
How to fill in the struct sigevent
We’ve talked about pulses in the Message Passing chapter; signals are a standard
UNIX-style mechanism, and we’ll see the thread creation notification type shortly.
Let’s take a quick look at how you fill in the struct sigevent structure.
Regardless of the notification scheme you choose, you’ll need to fill in a struct
sigevent structure:
struct sigevent {
int
union {
int
int
int
void
};
union sigval
sigev_notify;
sigev_signo;
sigev_coid;
sigev_id;
(*sigev_notify_function) (union sigval);
sigev_value;
};
union {
struct {
short sigev_code;
short sigev_priority;
};
pthread_attr_t *sigev_notify_attributes;
};
142 Chapter 3 • Clocks, Timers, and Getting a Kick Every So Often April 30, 2009