[MarkSweep]

Not OP, but the whole business of PID 1 having to reap orphan PIDs seems like something the kernel should have to do. Is there a good reason for when a process exits that not other process is waiting on, that a user-mode PID 1 process has to observe that exit?

[yjftsjthsd-h]

My understanding is that reaping children is a normal thing for most processes to do, and it's only orphans that fall through to PID 1, at which point it's easier to deal with it there rather than need to do anything special in ring zero.

[amluto]

Reaping children is "normal" in a universe where processes have numeric ids that can't be reused for unrelated processes until some handshake occurs that frees the id for reuse.

If you take anything resembling a fresh look at this concept, it's absurd. Imagine if every open file had a systemwide unique id, and one specific process owned that id and would continue to own it until it released it.

Reasonable designs use weak references that don't have values that can be compared across processes. These are usually called "handles" or "file descriptors", and they don't have this problem at all. Nothing reaps sockets, for example, and nothing needs to.