Getting Started with QNX Neutrino - QNX Software Systems

Using timers
© 2009, QNX Software Systems GmbH & Co. KG.
There are two members in struct itimerspec:
it_value
it_interval
the one-shot value
the reload value
The it_value specifies either how long from now the timer should go off (in the case of
a relative timer), or when the timer should go off (in the case of an absolute timer).
Once the timer fires, the it_interval value specifies a relative value to reload the timer
with so that it can trigger again. Note that specifying a value of zero for the it_interval
makes it into a one-shot timer. You might expect that to create a “pure” periodic timer,
you’d just set the it_interval to the reload value, and set it_value to zero.
Unfortunately, the last part of that statement is false — setting the it_value to zero
disables the timer. If you want to create a pure periodic timer, set it_value equal to
it_interval and create the timer as a relative timer. This will fire once (for the it_value
delay) and then keep reloading with the it_interval delay.
Both the it_value and it_interval members are actually structures of type struct
timespec, another POSIX thing. The structure lets you specify sub-second
resolutions. The first member, tv_sec, is the number of seconds; the second member,
tv_nsec, is the number of nanoseconds in the current second. (What this means is that
you should never set tv_nsec past the value 1 billion — this would imply more than a
one-second offset.)
Here are some examples:
it_value.tv_sec = 5;
it_value.tv_nsec = 500000000;
it_interval.tv_sec = 0;
it_interval.tv_nsec = 0;
This creates a one-shot timer that goes off in 5.5 seconds. (We got the “.5” because of
the 500,000,000 nanoseconds value.)
We’re assuming that this is used as a relative timer, because if it weren’t, then that time
would have elapsed long ago (5.5 seconds past January 1, 1970, 00:00 GMT).
Here’s another example:
it_value.tv_sec = 987654321;
it_value.tv_nsec = 0;
it_interval.tv_sec = 0;
it_interval.tv_nsec = 0;
This creates a one-shot timer that goes off Thursday, April 19, 2001 at 00:25:21 EDT.
(There are a bunch of functions that help you convert between the human-readable date
and the “number of seconds since January 1, 1970, 00:00:00 GMT” representation.
Take a look in the C library at time(), asctime(), ctime(), mktime(), strftime(), etc.)
For this example, we’re assuming that it’s an absolute timer, because of the huge
number of seconds that we’d be waiting if it were relative (987654321 seconds is
about 31.3 years).
148 Chapter 3 • Clocks, Timers, and Getting a Kick Every So Often April 30, 2009