Yes, _that_ is not to be allowed. I think you could have LLVM know about `vfork(2)` and treat that as an error -- heck, clang already knows how to do that, does it not?

That's just one example and the problem isn't just checking this one case, but any generalization of it. It can't be a fully static check because you could achieve similar things with longjmp or exceptions (both of which Fil-C supports).

And then there are similar issues with heap accesses and safepoints. The vfork child has to particular in safepoints except that it cannot quite (you can't use pthread_mutex/pthread_cond primitives in the vfork child to synchronize with threads in the vfork parent).

Anyway, I'm thought about this a lot, and I could keep coming at you with reasons why it's hard. It's not like I haven't implemented vfork(2) out of some abstract fear.

Can we make the C library mutex lock and condvar primitives work on the vfork() child? It's practically like a thread...

That might be the way to go but my best ideas for how to make vfork work sidestep all of that.

It's just a lot of work. It basically means that the whole Fil-C runtime has to be aware of this alternate mode where we're the vfork child and we have to play by different rules.

Anyway, long story short:

- Yeah I know vfork(2) is important. It's just not the most important thing on my plate. The constant time crypto situation we were talking about in the other thread is definitely more important, for example!

- I know how to do vfork(2) but every version of it that I've come up with is a ton of work and carries the risk of introducing really wacky stability bugs and safety bugs.

So, vfork is just a high risk, high cost, medium reward, medium urgency kind of thing. That probably means it'll be a while before I implement it.

Understood, and I agree that constant-time assembly-coded crypto is much more important. Moreover, if you make `posix_spawn()` fast that's enough, and you can probably do that by proxying it from the victim program's C library to Fil-C's.

Another option is to make `posix_spawn(3)` in the program's C library work with Fil-C to let Fil-C implement it w/o instrumentation in its C library. This would work for me.