The tricky part is ensuring that the signal handler code is async-signal-safe (which pretty much boils down to "ensure you're not acquiring any locks and be careful about reentrant code"), but at least that only has to be verified for a self-contained small function.
Is there anything similar to signals on Windows?
[1] https://learn.microsoft.com/en-us/windows/win32/api/processt...
The special APC is nicer because the OS is then aware of what you’re doing— it will perform the user-mode stack changes while transitioning back to user-mode and handle cleanup once the APC queue is drained.
The older API is less like signals and more like cooperative scheduling in that it waits for the target thread to be in an "alertable" state before it runs (the thread executes a sleep or a wait for something)
I wasn’t implying that APCs were new, I was implying that the ability to enqueue special (as opposed to normal) APCs from user-mode is new. And of course, that has always been possible from kernel-mode with NT.