Is it safe to call CFRunLoopStop from another thread?

Question

Hi all,

The Mac build of my (mainly POSIX) application spawns a child thread that calls CFRunLoopRun() to do an event loop (to get network configuration change events from MacOS).

When it's time to pack things up and go away, the main thread calls CFRunLoopStop() on the child thread's run-loop, at which point CFRunLoopRun() returns in the child thread, the child thread exits, and the main thread (which was blocking waiting for the child thread to exit) can continue.

This appears to work, but my question is: is this a safe/recommended way to do it? In particular, is calling CFRunLoopStop() from another thread liable to cause a race condition? Apple's documentation is silent on the subject, as far as I can tell.

If calling CFRunLoopStop() from the main thread is not the solution, what is a good solution? I know I could have the child thread call CFRunLoopRunInMode() and wake up every so often to check a boolean or something, but I'd prefer not to have the child thread do any polling if I can avoid it.

Answer 1

In particular, is calling CFRunLoopStop() from another thread [safe]?

Here's what Run Loop Management says:

The functions in Core Foundation are generally thread-safe and can be called from any thread.

So maybe CFRunLoopStop is safe. But I do worry about their use of the word “generally”. My rule is: If Apple doesn't say it's safe, you should assume it's not.

To err on the safe side, you might consider creating a run loop source, adding that to your run loop, and signaling that source when it's time to end the thread. That same document includes an example of a custom run loop source.

Answer 2

In the case of CFRunLoopStop - if it could only be called safely on the current run loop, then it would not be necessary to pass it a parameter indicating which run loop to stop.

The presence of the parameter is a strong indication that its ok to use it to stop run loops other than the current run loop.