Programming Embedded Systems in C and C++

Das Blinkenlights, Revisited 97

Timer::cancel(void)

{

//

// Remove the timer from the timer list.

//

if (state == Active)

{

timerList.remove(this);

}

//

// Reset the timer's state.

//

state = Idle;

} /* cancel() */

Of course, there is also a destructor for the Timer class, though I won’t show the

code here. Suffice it to say that it just checks to see if the software timer is active

and, if so, removes it from the timer list. This prevents a periodic timer that has

gone out of scope from remaining in the timer list indefinitely and any pointers to

the “dead” timer from remaining in the system.

For completeness, it might be nice to add a public method, perhaps called poll,

that allows users of the Timer class to test the state of a software timer without

blocking. In the interest of space, I have left this out of my implementation, but it

would be easy to add such a routine. It need only return the current value of the

comparison state == Done. However, in order to do this, some technique would

need to be devised to restart periodic timers for which waitfor is never called.

Another potential feature of the Timer class is asynchronous callbacks. In other

words, why not allow the creator of a software timer to attach a function to it. This

function could then be called automatically—via timerList.tick—each time that

timer expires. As you read the next section, be sure to think about how different ...