Waiting for a variable

[Shadowwolf]

I am working on a program that should be able to monitor another process. I do not have the code of this other process and thus cannot modify the code. This process changes a variable after it is done with a specific task. I need to detect this change so my own program can execute a function after this task is done.

I have already managed to get the actual variable, so it could be treated as if it were located in my own program. So this case could be simplified to:

volatile bool task_finished = false;

void task_finished();

int main()
{
   //Wait for the variable to change
   ...
   //Now execute a function
   task_finished();
   return 0;
}

Now I know it would be possible to use a loop to busy-wait for the variable to change, like this:

while(!task_finished)
    ;

But this would waste a lot of CPU time, so I'm looking for a less CPU-intensive solution. Is is possible under Windows to wait for this variable to change without having to modify the external process in any way?

[Moschops]

If you don't mind not acting on the changed variable instantly, you could add a sleep in the while loop.

[Shadowwolf]

Acting on the changed variable instantly is quite important actually, sorry for not adding that to the original post. So sleeping in the loop won't solve the problem.

[Moschops]

Instant is physically impossible. How much time is permitted?

[Shadowwolf]

Since the variable can change at most once in the 500ms, a little less than that seems to be the longest possible time that waiting is permitted.

[helios]

A spinlock seems like a sensible solution to me in this situation, if responding quickly is important.

[Shadowwolf]

Then it looks like that will be the final solution. Thank you all for all your help.