writeonce/docs/plan/oop-vm/00-wob-format.md

The .wob format v1 — normative reference

Copied verbatim from the normative section of docs/superpowers/plans/2026-08-01-wob-format-and-vm-core.md (plan 1 of the approved spec 2026-08-01-oop-compiler-vm-design.md). The machine-readable twin is runtime/src/wob.h — constants there and prose here must never disagree. The test-side assembler runtime/test/wob_build.c is a second, independent encoding; builder/loader disagreements surface as test failures.

All integers little-endian; offsets are absolute file offsets.

Header (44 bytes): magic "WOB1", version 1, then offset/count u32 pairs for the constant pool, class table, interface section, and method table, then a u32 entry-method index (all-ones = none).

Constant pool — sequential entries: one tag byte; tag 0 = i64 follows; tag 1 = text (u32 length + bytes, no NUL).

Class table — per class: name constant index, flags u32 (bit0 = instances are @gc), field count, then one kind byte per field padded to a 4-byte boundary. Field kinds: 0 SCALAR, 1 OWNED, 2 GCREF, 3 TEXT, 4 MULTI, 5 MAP. Runtime object layout: 16-byte header then one 8-byte slot per field, in declaration order.

Interface section — per interface: name constant index, method count. Global slot ids are assigned sequentially across interfaces in declaration order. Then a vtable row count and rows: class id, interface id, one method index per interface method.

Method table — per method: name constant index, class id (all-ones = free fn), arg count u8, register count u8, reserved u16, code length in bytes (multiple of 4), the u32 instructions, a line table (count + ascending pc→line pairs), and a drop table (count + ascending entries of pc, owned-register bitmask u64, gc-register bitmask u64). Drop-table lookup = last entry with pc ≤ current pc; no entry means nothing live.

Instructions — fixed 32-bit, Lua-style fields: opcode byte, A byte, then either B and C bytes or a 16-bit Bx (signed jumps encode as Bx − 32768).

op	name	semantics (in words)
0	NOP	nothing
1	LOADK A Bx	register A = constant Bx (int inline; text = pointer to interned const string)
2	MOVE A B	copy register; for owned values this IS the move — compiler guarantees the source is dead
3–7	ADD/SUB/MUL/DIV/NEG	i64 arithmetic, two's-complement wrapping (no signed-overflow UB); DIV traps on zero divisor and on INT64_MIN ÷ −1
8	CONCAT A B C	new owned text from two texts
9–12	EQ/LT/LE/EQS	i64 compares and text-content equality, result 0/1
13–14	JMP / JZ	relative jump (JZ when register A is zero)
15	CALL A Bx	call method Bx; callee's register window starts at caller base + A (register-window overlap, Lua-style); args sit at A, A+1, …; return value lands back in slot A
16	ICALL A Bx	interface call by global slot id Bx; receiver in A; vtable lookup by the receiver's class
17–18	RET A / RET0	return value from register A (or zero), pop frame
19	NEW A Bx	new zeroed instance of class Bx
20–21	GETF / SETF	field read/write with runtime null/native/bounds checks (trap T_BOUNDS); overwriting a non-scalar field does NOT auto-drop the old value — the compiler emits the drop
22	DROP A	recursively drop the owned value in A per its class drop plan, null the register
23–26	BORROW_S/BORROW_X/RELEASE_S/RELEASE_X	borrow-word ops on the object in A; violation traps T_BORROW
27–28	RC_INC / RC_DEC	refcount ops on the @gc object in A
29	BUILTIN A B C	register A = builtin C applied to args starting at register B (fixed arity per builtin; multi_new/map_new carry kind immediates in B instead)
30	DB_STUB	trap T_DB "engine not linked" (spec: SQL-layer statements in milestone 1)
31	TRAP Bx	explicit trap with code Bx

Builtins: now (ms), print (text), print_int, words (whitespace token count), multi_new/multi_push/multi_get/count/latest, map_new/map_set/map_get/map_has.

Trap codes: DIV0, BORROW, STACK, OOM, DB, BOUNDS, KEY, EXPLICIT.

Single-binary trailer (woc build, plan 3 Task 6)

This section is not part of the .wob format above — .wob v1 is unchanged. It documents the wrapper a deployable executable carries: woc build <dir> -o app makes app by copying the wovm runtime binary and appending the compiled .wob image plus a small fixed-size trailer. wovm's own startup (runtime/src/main.c) looks for this trailer in its own executable (/proc/self/exe) before falling back to the classic wovm file.wob argv contract, so the result runs standalone with no separate .wob file. Writer: compiler/bin/main.ml. Reader: runtime/src/main.c's load_self_embedded. Append-based only, deliberately — no ELF section manipulation.

Layout — the trailer is the fixed last 20 bytes of the file, all integers little-endian, found by seeking from the end (no scanning):

byte offset from EOF   size   field
  -20                   8     payload_off  -- absolute file offset where the embedded .wob image starts
  -12                   8     payload_len  -- length in bytes of the embedded .wob image
   -4                   4     magic        -- 0x31544257 ("WBT1" read as LE u32, mirrors WOB_MAGIC's "WOB1")

[ wovm runtime bytes (payload_off bytes) ][ .wob image (payload_len bytes) ][ trailer: payload_off | payload_len | magic ]
^ byte 0                                  ^ byte payload_off                ^ byte payload_off+payload_len == file_size-20
                                                                                                              file_size ^

Reader algorithm (load_self_embedded): open /proc/self/exe; if the file is shorter than 20 bytes, or its last 4 bytes don't equal the magic, there is no trailer — fall back to the argv .wob path unchanged. If the magic matches, payload_off and payload_len are validated to account for every trailing byte exactly (payload_off + payload_len == file_size - 20, checked via a bounds-safe subtraction so a corrupt/huge value can't wrap the arithmetic and slip past); any mismatch is reported as a clear "corrupt trailer" error (exit 2) rather than a crash or silent misbehavior. On success, the executable is mmap'd and wo_load_buf parses the embedded region exactly as wo_load_file parses a standalone .wob today — argv is never consulted.

Runtime location (writer side): --runtime <path> wins when given; otherwise the default is runtime/wovm resolved relative to the current working directory (the same repo-root-relative assumption every other just/build-tooling entry point in this repo already makes). A missing runtime binary is a build-time error naming the recipe: make -C runtime wovm. woc build never invokes or inspects the runtime binary beyond reading its bytes — it does not need to be executable as run by woc, only as run by whoever runs the produced artifact.

Edge cases decided for woc build (each implemented deliberately, not left to fall out accidentally):

• Output path already exists: overwritten, but atomically — the new binary is assembled in a temp file (<out>.woc-build.tmp, freshly created with mode 0755 each time so a stale temp file's permissions can never leak through) next to -o, then renamed over it. A failed build (bad compile, missing runtime, disk-full mid-write) never clobbers a previously-working binary with a partial one.
• A directory with no main: unlike --emit (where a .wob with no entry method is a legitimate, already-specified artifact), build's entire purpose is something runnable, so a clean compile with no zero-argument free fn named main is a build-time error, no output written — not deferred to wovm's own "module has no entry method" message at run time. Detected by reading the compiled image's own entry field (WOB_OFF_ENTRY, offset 40) rather than plumbing a new return value through the emitter.
• --runtime itself already carries a trailer (rebuilding from a previously-built single binary): its embedded payload is stripped before copying — the writer recognizes its own trailer on the input runtime binary the same way the C reader does, and keeps only the pristine runtime prefix (payload_off bytes). This makes woc build ... --runtime already-built-app -o new-app produce a binary byte-identical in size to building fresh from runtime/wovm directly, instead of chaining stale payloads and bloating on every rebuild. Any input that doesn't unambiguously look like our own trailer (wrong magic, or offsets that don't exactly account for every trailing byte) is left untouched and copied as-is — the safe default when it's not certain.