The Mask Replaces the Choice
The Mask Replaces the Choice
Seed: Plan 9 process model (rfork).
Central fact: rfork replaces fork/exec/threads with a single system call parameterized by an explicit bitmask (RFPROC, RFMEM, RFFDG, RFNAMEG, and others), letting the caller choose exactly which resources — memory, file descriptors, namespace — the child shares or copies, rather than assuming a fixed sharing model per mechanism.
Unix asks a process to choose: fork a whole copy of yourself, or exec and become someone new, and thread libraries bolt a third option on from outside as a compromise. Plan 9 refuses to choose in advance. It gives every process one call, rfork, and a bitmask, and lets the caller decide at the moment of birth exactly which parts of the old self the new self will keep.
Share the memory or copy it. Share the file descriptors or start fresh. Inherit the namespace or cut loose from it entirely. Nothing is assumed; everything is named. A thread and a process and a sandboxed child are not three different mechanisms needing three different calls — they are the same call, answering a different mask.