# Process lifecycle: signals over IPC

**Status: increments 1–4 built** (2026-07-12): claim release on death, exit
reasons, published exit events, and signals + one-shot timers + the service
harness are all in — the interface below is as-built. The primitives underneath
predate this design ([process-management.md](process-management.md):
spawn, the supervision link, kill, child-exit notifications); this document designs
the layer above them — the standard vocabulary a danos process speaks about its own
life, and the stable `process` interface that carries it. Nothing here is
device- or driver-specific: a driver, the VFS, and a user application all stop,
reload, and die the same way. The device manager is simply this design's first
serious customer ([device-manager.md](../device-driver-development/device-manager.md)).

**"POSIX" in this document means the concepts, never the letter of the standard.**
danos borrows the ideas and the hard-won lessons (what SIGTERM *means*, why SIGPIPE
was a mistake) without inheriting the mechanism, the API, or the names. The naming
rule is danos's own and it is strict: plain words that communicate intent
(`terminate`, `reload`, `exited`) and the IPC vocabulary the system already speaks
(`bind`, `subscribe`, `publish`, `endpoint`) — never `SIG*`, never a second word for
a concept that already has one. Literal POSIX arrives later and lives elsewhere: the
`std.os.danos` seam that makes danos a Zig target, and eventually a **musl-based C
layer** on the same native surface (see [zig-self-hosting.md](../zig-self-hosting.md)) —
musl's syscall surface retargeted at danos system calls and IPC protocols (files onto
the VFS protocol, `sigaction`/`wait` onto this lifecycle, sockets onto whatever
networking becomes). Ported programs see POSIX; the system underneath never does.

## Why a standard vocabulary

A supervisor can only manage processes it has never heard of if "please exit" means
the same thing to all of them. That is the one thing POSIX signals got deeply right:
`SIGTERM` means the same thing to nginx and to a five-line script, which is why
process supervision on Unix (init systems, container runtimes) is possible at all.
danos wants that property from day one, because supervision-and-restart is the
system's core motivation ([resilience.md](resilience.md)).

What POSIX got wrong — for a system like this — is the **delivery mechanism**:
asynchronous control-flow hijack. A Unix handler runs on a stolen stack at an
arbitrary instruction boundary, which is why the async-signal-safe function list
exists, why `errno` must be saved, and why the canonical signal bug is a SIGTERM
handler innocently calling `printf` mid-`malloc`. That entire bug class comes from
the mechanism, not the vocabulary, and none of it is worth importing.

A microkernel already has the right channel: **a signal is a message.** QNX delivers
POSIX signals over its message passing; seL4 has notification objects; Erlang turned
"death is a message to whoever linked" into a reliability philosophy. danos has
already done it once without naming it: a child's death arrives as a notification
badge on the supervisor's endpoint — the microkernel's SIGCHLD, the IRQ-as-IPC
pattern reused. Signals are the same pattern reused a third time.

## The mechanism

- **`signal_bind(endpoint)`** — a process nominates the endpoint its signals arrive
  on, exactly as `irq_bind` nominates where a device's interrupts land. The runtime
  does this at startup for any program that opts in.
- **`process_signal(id, signal)`** — posts the signal as an asynchronous
  notification to the target's bound endpoint: badge = `notify_badge_bit |
  notify_signal_bit | pending signals`. Non-blocking for the sender, always.
  Signals address the *process*: `id` may name any member of a threaded process
  and resolves to its leader — whose endpoint the harness binds — with authority
  mirroring `process_kill` ([shared-fate-plan.md](shared-fate-plan.md)).
- **Pending signals coalesce** in a per-process bitmask while the target has no
  signal endpoint bound, and the whole mask arrives as one notification at bind —
  POSIX's own semantics for non-realtime signals (two pending SIGTERMs are one
  SIGTERM). Once bound, each `process_signal` flushes the mask straight into the
  endpoint's notification ring, so a busy receiver drains separate posts as
  separate notifications — harmless, because the badge is a set of bits, never a
  count. The bitmask *is* the design: signals carry no payload. Anything with a
  payload is a protocol message.
- **Authority**: the supervisor may signal its children — the same link that is
  already the kill authority. A process may signal itself. Anything broader waits
  for transferable process handles.
- **No binding, no problem**: a process that never calls `signal_bind` is not
  broken — its signals pend unread and only `process_kill` works on it. Simple
  programs stay simple; the vocabulary is opt-in, the kill authority is not.

Because delivery is a message into the process's own event loop, there is no
async-signal-safe list in danos: a handler is ordinary code running at a point the
process chose. The bug class is gone by construction, not by discipline.

## The vocabulary: POSIX.1-1990, sorted honestly

The full 1990 set, and what each becomes. Two intrinsically problematic cases get a
defense below the table.

| POSIX.1-1990 | danos disposition | Notes |
|---|---|---|
| SIGTERM | signal `terminate` | finish up and exit; the supervisor's polite half |
| SIGHUP | signal `reload` | re-read configuration / re-scan |
| SIGINT | signal `interrupt` | interactive interrupt; meaningful once a console can send it, in the vocabulary now so numbering is stable |
| SIGQUIT | signal `quit` | as SIGINT, without the core-dump baggage |
| SIGALRM | signal `alarm` | timer expiry as a message; the Unix SIGALRM+`longjmp` timeout hacks are impossible here. In the vocabulary, unbuilt: no consumer yet, and when one appears it is runtime sugar over the existing timer — zero kernel work |
| SIGUSR1, SIGUSR2 | signals `user_1`, `user_2` | service-defined |
| SIGCHLD | **already exists** — the exit notification | the badge carries the child id, dodging the classic coalescing bug (Unix code must loop `waitpid`) |
| SIGKILL | `process_kill` — kernel mechanism | its definition is "cannot be handled"; it was never really a signal |
| SIGABRT | exit reason `aborted` | synchronous self-termination is an exit, not an event; recorded for any nonzero exit code |
| SIGSEGV, SIGILL, SIGFPE | exit reasons, **never delivered** | see below |
| SIGPIPE | **an error return**, not a signal | see below |
| SIGSTOP, SIGTSTP, SIGTTIN, SIGTTOU, SIGCONT | deferred | job control needs terminals, sessions, and process groups; stop/continue is scheduler territory |

**The fault signals (SIGSEGV, SIGILL, SIGFPE) are intrinsically wrong for messages.**
They are *synchronous* — raised at a specific faulting instruction, not "sometime
soon". A message cannot be delivered to a process whose next instruction re-faults;
it never reaches its event loop to read it. POSIX only makes fault handlers "work"
via the async hijack (run the handler *instead of* the instruction), and even there,
returning from a SIGSEGV handler without curing the cause is undefined behavior.
danos's architecture already has the better answer: fault → the kernel kills the
process ([resilience.md](resilience.md) step 2, built) → the supervisor reads the
reason → restart. Recovery is restart, not a handler. This is also truer to the 1990
standard than handling is: the standard's default action for all three was
"terminate the process".

**SIGPIPE deserves special contempt.** Its default kills a process that writes to a
closed pipe — which is why "the whole server died because one client disconnected"
is roughly every network daemon's first production bug, and why every mature codebase
contains the same fix: ignore SIGPIPE, handle the `EPIPE` error return. danos made
the right choice natively already — a reply owed to a dead peer fails with `-EPEER`.
Errors from operations are error returns from those operations. The posix layer can
synthesize SIGPIPE for ported code that expects it.

### Statements, not questions

A signal and a protocol message both travel over IPC — the difference is the
**contract**, not the transport. danos IPC has two primitives, both already in
daily use: the **asynchronous notification** (a badge — bits that coalesce into a
pending mask; the sender never blocks; no payload, *no reply path*; how IRQs and
exit events arrive) and the **synchronous call** (a rendezvous — payload both
ways, the caller waits for the reply; how VFS requests work). A signal is the
first kind: a *statement*. `terminate` wants no reply — the exit notification is
its acknowledgement.

A health probe is the second kind: a *question*, worthless without its answer —
and the answer's absence within a deadline is the very thing being measured.
Asked as a signal it has no reply channel (a coalescing bit can't carry an answer,
and the authority rule forbids a child signalling its supervisor back); asked as a
call, the timeout-is-the-diagnosis semantics come free. So there is no `health`
signal. Liveness is the common **`ping`**: a reserved request every harness-run
service answers automatically on its main endpoint — still free for the service
author, still one obvious way — and a supervisor's probe is a `ping` call with a
deadline.

## The two iron rules

1. **Cleanup is the kernel's job.** A process can die with no warning — fault,
   kill, power. Correctness must never depend on a `terminate` handler running. On
   any death the kernel releases the address space, IPC handles, IRQ bindings,
   owed replies, and **device, I/O-port, and interrupt claims and MSI vectors** —
   the last of these was once the known gap in
   [process-management.md](process-management.md), closed by increment 1
   (`releaseTaskResourcesLocked`, on every death path). A signal handler is
   for *graceful* work — flushing, deregistering, saving — never for *necessary*
   work.
2. **Kill is not a signal, and exit reasons are load-bearing.** The standard stop
   sequence is *terminate → deadline → `process_kill`*; the unhandleable kill stays
   a kernel mechanism. And a supervisor deciding whether to restart must know *how*
   the child died: clean exit (meant to — don't restart), fault (restart with
   backoff), killed (the supervisor did it). The exit notification carries only the
   id; the reason is recorded before the notification posts and read with the
   supervisor-gated `process_exit_reason` query. Restart policy cannot be
   written without it.

## Who learns of a death

A death has three audiences, and conflating them is how systems end up with either
zombie state or privileged snooping:

1. **The supervisor** — gets the exit notification on the endpoint it gave at spawn
   (built), then reads the `ExitReason` with the `process_exit_reason` query
   (increment 2). The supervisor is the only
   audience that needs the *reason*, because it is the only one deciding whether to
   restart.
2. **The peer owed a reply** — already built: a client that dies mid-request fails
   the server's reply with `-EPEER`; a server that dies fails its waiting clients
   the same way. This covers the *synchronous* case only.
3. **The subscribers** — the new piece, and it is the input service's
   publish/subscribe shape ([input.md](../device-driver-development/input.md)) applied to exits. A stateful
   service accumulates per-client state across many requests: a filesystem server
   (FAT today) holds a dead client's open file handles, the input service holds
   its subscriptions, a future network stack holds its sockets. None of these
   are the client's supervisor, and none learn anything from a failed reply if
   the client simply never calls again.
   So the kernel **publishes every exit** to whoever subscribed:
   `process_subscribe(endpoint)` adds a subscriber, and each death posts a
   notification to every subscriber (badge = `notify_exit_bit | process id` — the
   same encoding supervisors already decode, the IRQ-as-IPC pattern once more). The
   subscriber filters for ids it holds state for and releases what the dead client
   held. Correlating is free of bookkeeping: an IPC sender's badge already *is* its
   task id (`ipc.Received`), so the id a service has been keying client
   state by all along is the id the exit event carries.

   Subscription, not broadcast-to-everyone: only processes that asked receive
   events, the kernel keeps a bounded subscriber table (sixteen — a normal boot
   already fields six, since this is what *every* provider with per-client state
   releases on), and delivery is the same non-blocking coalescing notification as
   everything else — a dying process never waits on its mourners.
   A service does not usually write the sweep itself: the shared service harness
   subscribes for it and drops a dead task's event subscriptions
   (`service.Subscribers`), and a provider adds its own handler only for state the
   harness knows nothing about — open files, layers, device tokens.
   Subscribing is ungated, like `process_enumerate`: what is
   running (and dying) is not a secret between cooperating processes. Subscribers
   do not receive the exit reason — the filesystem server does not care *why*
   the client died.

This is the service-side mirror of iron rule 1: **a service must never depend on
its clients cleaning up after themselves.** Handle release on client death is the
service's job, triggered by the published exit event — never by a courtesy
"closing now" message that a crashed client will never send.

## The stable interface: `process`

`process` already owns what a process receives at birth (`Init`, the
argv contract). It grows to own the other end of life.

**The runtime is the stable interface; the numbers are not.** danos applications do
not make system calls — they call the runtime library, and the system-call numbers,
notification bits, and signal bit positions beneath it are a **private kernel ↔
runtime contract** that may change at any time (settled 2026-07-12). This is why
the runtime exists. Today kernel and runtime ship from one tree in one image, so
"stability" is simply building them together. When driver binaries start shipping
as separately-versioned applications — the whole point of the restart design — the
binary's embedded runtime version becomes compatibility metadata (the same idea as
the protocol version in the device manager's `hello`), and the kernel refuses what
it cannot serve. Signals therefore need no reserved numbering scheme: the enum
below is vocabulary, not ABI.

```zig
/// The signal vocabulary. The value is the bit position in the pending mask — a
/// private kernel/runtime detail, free to change while they ship together.
pub const Signal = enum(u5) {
    terminate = 0, // SIGTERM: finish up and exit
    reload = 1, // SIGHUP: re-read configuration
    interrupt = 2, // SIGINT
    quit = 3, // SIGQUIT
    alarm = 4, // SIGALRM
    user_1 = 5, // SIGUSR1
    user_2 = 6, // SIGUSR2
};

/// A decoded pending mask: the coalesced set of signals a notification delivered.
pub const SignalSet = struct {
    pending: u32,
    pub fn has(set: SignalSet, signal: Signal) bool { ... }
};

/// Nominate `endpoint` as this process's signal endpoint (signal_bind). The
/// runtime's service harness calls this; a bare program may call it directly and
/// fold signals into its own replyWait loop.
pub fn bindSignals(endpoint: usize) bool { ... }

/// Decode a received badge into signals, or null if the badge is not a signal
/// notification (mirrors ipc.Received.isChildExit).
pub fn signalsFrom(badge: u64) ?SignalSet { ... }

/// Send `signal` to process `id`. Supervisor-gated, like kill; non-blocking.
pub fn sendSignal(id: u32, signal: Signal) bool { ... }

/// The standard stop sequence: terminate, wait up to `deadline_ms` for the exit
/// notification on `exit_endpoint` (the endpoint the child was spawned with),
/// then process_kill. The one call a supervisor needs.
pub fn stop(id: u32, deadline_ms: u64, exit_endpoint: usize) void { ... }

/// Subscribe `endpoint` to published exit events (process_subscribe). Every
/// process death posts an asynchronous notification: badge = notify_exit_bit |
/// process id — the same encoding a supervisor's exit notification uses, decoded
/// by the same ipc.Received helpers. For stateful services: release what the dead
/// client held (file handles, subscriptions, sockets). Ungated, like
/// process_enumerate.
pub fn subscribeExits(endpoint: usize) bool { ... }

/// How a process ended — queried after the exit notification (the kernel records
/// it first, so the two never race). What restart policy reads. (Built in M17.2.)
pub const ExitReason = enum(u8) {
    exited, // returned from main / clean exit
    aborted, // deliberate failure exit — any nonzero exit code (SIGABRT's ghost)
    segmentation_fault, // SIGSEGV's ghost
    illegal_instruction, // SIGILL's ghost
    arithmetic_fault, // SIGFPE's ghost
    protection_fault, // general protection fault
    fault, // any other CPU exception
    killed, // process_kill
};
```

Two deliberate absences. There is no `mask`/`block` API — a process that is not
ready for a signal simply has not waited on its endpoint yet; the pending mask *is*
the blocked set. And there is no per-signal handler registration at this layer —
dispatch is the process's own `switch` over `SignalSet`, or the service harness's
callbacks (`on_terminate`, `on_reload`) for programs that want defaults.

### The service harness

`service` owns the `replyWait` loop and folds every event source — signals,
child exits, protocol messages — into callbacks, with the vocabulary's defaults:
it also owns the **subscriber side** of any protocol that declares events
(`service.Subscribers`: the table, the reserved `subscribe`/`unsubscribe` verbs,
the fan-out, and the sweep on a subscriber's published exit), so every event
stream in the system behaves identically.
`terminate` returns from the loop (clean exit), the common `ping` is answered automatically,
`reload` is ignored unless overridden. One loop, no locking, nothing reentrant. A
service author writes domain logic; the lifecycle contract is satisfied by the
harness. A process that bypasses the harness and ignores its signals meets the
deadline-then-kill escalation — you cannot force a process to implement an
interface, but you can make compliance free and non-compliance fatal.

### The musl layer later

The POSIX C layer is a **musl port**: musl's arch/syscall layer retargeted so that
what musl believes are kernel syscalls become danos runtime calls and IPC — `open`
and `read` onto the VFS protocol, `kill`/`sigaction`/`waitpid` onto this document's
vocabulary, `exit` onto the runtime's exit path. `sigaction` handlers registered
through it are invoked by the runtime's loop when the signal message arrives —
synchronous underneath, async-looking to ported code, delivered at wait boundaries
the way most Unix programs already experience signals (at syscalls). No stack hijack
ever happens, `SA_RESTART` semantics come free because nothing was interrupted, and
SIGPIPE can be synthesized from `-EPEER` for the programs that expect it. C programs
get POSIX; danos-native programs never pay for it.

## Increments

1. **Kernel: release device/port/IRQ claims and MSI vectors on death** — the
   cleanup half of iron rule 1, and the prerequisite for any restart story. Test:
   kill a claiming driver, spawn it again, the claim succeeds.
2. **Exit reason in the death notification** (`ExitReason` above).
3. **Exit events**: `process_subscribe` in the kernel (bounded subscriber table,
   publishes on every death), `process.subscribeExits`; the userspace VFS
   router was the first subscriber — releasing a dead client's handles was its
   proof test — and the FAT server inherited the role when the router moved into
   the kernel (clients now hold the filesystem server's node ids directly).
4. **Signals**: `signal_bind` + `process_signal` + the pending mask in the kernel;
   `process` grows the interface above; the service harness handles
   `terminate` and answers the common `ping`; `stop()` for supervisors.

[device-manager.md](../device-driver-development/device-manager.md) builds directly on all four.

## Settled questions (2026-07-12)

- **Signal numbering is not ABI**: the runtime is the stable interface; the numbers
  beneath it are a private kernel ↔ runtime contract (see "The stable interface").
- **Liveness is a `ping` call, not a signal**: signals are statements, questions
  are synchronous calls (see "Statements, not questions"). A service wanting *deep*
  health ("can I reach my hardware?") defines its own protocol message on top.
- **Process handles: deferred.** Pids + the supervisor gate cover everything
  planned; transferable handles (Fuchsia-style, delegating signalling without
  delegating kill) wait for the capability table to grow types beyond endpoints.
- **`alarm`: in the vocabulary, unbuilt.** No consumer yet; when one appears it is
  runtime sugar over the existing timer (arm a timer that posts your own signal) —
  zero kernel work, so deferring costs nothing.
- **Subscription granularity: all exits**, subscriber-side filtering — one
  subscription per service, a bounded kernel table. Per-id subscriptions only if
  event volume ever matters (hundreds of processes, not before).
- **Client identity across the exit boundary: no convention needed** — an IPC
  sender's badge already is its task id (see "Who learns of a death").