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C++ Equivalent of SpinWait.SpinUntil

November 30th, 2019 No comments

I like to use threads in my code, but the problem with threads is that sometimes you need to wait for them to finish.

In c++ it is very easy to do.

All the code above does is, start a thread, call the function my_function and passes the argument 1.
The t.join(); tells the main thread to wait until the thread is finished.

Now the problem is that t.join() could hang forever … and I would never know about it.

What I needed was something like join_until( 1000) where we either wait for the thread to finish or wait for 1000ms. What I needed was something similar to c# SpinUntil

In very simple terms all SpinUntil does is check a condition, (very often), and if it is true then return (true), but if a timeout happens then we return (false).
So in my case I would wait for the thread to complete, (yes I know, I know, you can’t easily tell if a std::thread is complete or not.

Enter Wait.SpinUntil …

Basically we will

  1. Start a new thread
  2. Check if the given condition is true
    1. If the condition is true, return true
    2. If the condition is false … continue.
  3. Wait for a couple of nanoseconds for other threads to do their bit
  4. Look how long we have been spinning
    1. If less than milliseconds go back to number 2
    2. If more than milliseconds, return false.

The code

This is a very simplified version of the code, you can find a working solution on my github page

So now the code above will wait for a couple of ms while running in the background and if the given condition is true we will get out and return true, otherwise we will return false after a while.

Possible issues

  • For one thing, the condition() function could hang, it is up to the caller to make sure this never happens.
  • The condition() function could take a rather long time to execute … it is up to the caller to make sure this never happens

In other words, don’t stuff things up in your condition() function.

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