Executing code at an interval, with polling

Question

I am try to implement a periodic event in C++, on an embedded device with limited library functions available. I cannot use sleep or other delays since it stop the current execution.

For eg.

do
{
    if(something)
    {
       //Do something
    }

    if(something)
    {
       //Do something
    }

    if(every 5 minutes)
    {
       //Do something only once after 5 minutes
    }
}
while(true)

I am not sure how to go about this. Can you help me with this since I need to implement this in this do while loop? I would have used threads, but on this device that's not really possible, so I'm looking for another way.

Answer 1

Well, of course, the advice to spawn off a separate thread (that's what I would do) or use alarm() given in the comments is good advice, but may not be possible on your device. However, focusing on your current approach, generally the idea is, pseudo-code:

last action time = current time

while (true) {
   do things.
   if current time - last action time >= 5 minutes then
       perform action
       last action time = current time
       // or last action time += 5 minutes if you want to mitigate drift
   end
}

So now you just need to pick your favorite way to get the current time. On a desktop platform you could use e.g. time(), gettimeofday(), GetTickCount() on Windows, etc. Some embedded platforms may provide those as well. However, this could be problematic:

Since you mentioned in a comment that you are working on an embedded device, "your favorite way to get the current time" will vary depending on the platform, you'll just have to check the device docs. If you're lucky, your MCU may provide a way to query the CPU counter, and the frequency you usually know already from your setup. But it may not.

If it does not, another alternative is if your device provides an on-board timer. In this case, you may be able to configure it and respond to periodic interrupts, in that case your two options are generally:

1. Do the action in the timer interrupt handler - but only do this if it's short, and you take proper precautions for interrupt-safety.
2. Set a volatile flag in the timer interrupt handler, check that flag in the main loop and execute the action + reset the flag there if it is set. This is generally a much simpler approach.

Doing it that way is architecturally similar to using alarm(). However, there are certainly embedded platforms that implement time() and/or gettimeofday() as well. But those are your choices.

Answer 2

This sounds like you're looking for two different capabilities; one, to dispatch a command without disrupting execution; two, to wait for a certain amount of time. For the first, you're looking for asynchronous programming. For the second, I recommend Boost::deadline_timer. To combine the two, look at Boost::io_service, which will allow you to register asynchronous callbacks with or without the use of a timer.