The Fork Forgets What You Do Not Name
The Fork Forgets What You Do Not Name
Seed: Plan 9 process model (rfork).
Central fact: rfork replaces the fixed vocabulary of fork, thread, and exec with a single call parameterized by a bitmask — a fork forgets whatever you do not explicitly ask it to keep.
Unix hands a programmer a menu of nouns: a process is one thing, a thread is another, and the difference between them is baked into which system call you reach for. Plan 9 asks a different question at the moment of splitting in two — not "what kind of child is this" but "what, specifically, should this child still share with its parent." RFPROC decides whether a new process exists at all, versus a mere new control flow in the old one. RFMEM decides whether the address space is shared or copied. RFNOMNT and RFNAME decide whether the child inherits the parent's namespace or starts building its own. RFFDG decides whether it gets its own file descriptor table. Each bit is a door left open or shut at the instant of the fork, and nothing is inherited that the mask did not name.
This is why Plan 9 needed no separate thread library bolted onto its process model: a "thread" is simply a process forked with RFMEM set and RFPROC unset in the old style, or with both set for a fully independent sibling — the same primitive, differently masked, rather than a second abstraction living beside the first.
What a process shares with its origin is never assumed. It is declared, once, in a word made of switches, at the exact moment the one path becomes two.