D1 — Pin this Mac, OTP 28.3.1, Elixir 1.20.2, Mojo 1.1.0
Linux was not available when this pin was chosen. The newest installed OTP/Elixir pair was already 28.3.1 and 1.20.2. Stable mojo==1.1.0 emitted --emit shared-lib, so nightly was not used. Consequences: every constant in nif_init is for NIF 2.17. Linux x86-64 later matched those sizes (E18). Other OTP minors stay untested.
D2 — Call enif_get_long and enif_make_long
SIZEOF_LONG is 8, so enif_get_int64 and enif_make_int64 are macros. beam.smp exports the long symbols. Calling the macro name would not link. The Windows function-pointer table is a different ABI and is not filled in on Unix.
D3 — Link with -undefined dynamic_lookup
mojo build accepts -Xlinker. Darwin will not bind undefined NIF symbols to the BEAM without flat lookup. OwnedDLHandle() on the current process was the backup and was not required once this link succeeded.
D4 — calloc the entry and the function table
Mojo 1.1.0 rejects global var. A stack ErlNifEntry would dangle after nif_init returns. calloc is libc, called from Mojo, and it is not a C source file in the link. Unload does not free the blocks. That leak is accepted for Stage 0 and called out in chapter 07.
D5 — NULL callbacks are Optional[Pointer], not optional functions
Optional of a thin function measured 16 bytes and would break the entry layout. Optional[Pointer[NoneType, MutUntrackedOrigin]] measured 8 bytes, and None measured as eight zero bytes, which is C NULL.
D6 — Do not call initialize_runtime in add
The integer path passed without it. A separate NIF called it twice and then returned 7, so the function can run inside nif_init. Leaving it out of add keeps the Mojo thread pool from being part of the success claim.
D7 — Ship libpoc.so, not libpoc.dylib
This OTP's load_nif appends .so on Darwin. The Mach-O type can still be a dylib; the filename is what the loader concatenates.
D8 — Diagnostic C never joins the NIF link
experiments/abi/diagnostic/layout.c exists to ask clang for offsetof. scripts/build-native.sh compiles only native/poc.mojo. The feasibility verdict does not treat that C file as a shim.
D10 — Linux CI uses --unresolved-symbols=ignore-all
.github/workflows/linux.yml runs the CPU NIF on ubuntu-24.04. Darwin keeps -undefined dynamic_lookup. Linux first used --unresolved-symbols=ignore-in-shared-libs. Actions run 39365275751 linked with /usr/bin/ld and failed: undefined reference to enif_make_long, because that flag still reports undefined symbols from the Mojo archive. The Linux line is --unresolved-symbols=ignore-all plus -z undefs. Run 39451722783 is the green log (E18, E19). The workflow does not rent a GPU and does not change the macOS PASS.
D11 — Generate the ABI, and do not free the entry
tools/abi/generate.py reads clang's AST and a layout probe. The probe is not a mojo build input. nif_init still callocs the entry and the function table. OTP keeps that pointer, and a second load_nif is rejected, so those two blocks are bounded by one load per module per OS process. Mojoler.Own frees its private type box in unload with free, the pair of calloc. Upgrade returns 1. That callback was not invoked by a test.
D12 — Do not bundle the Mojo runtime
The Darwin .so records an rpath into the venv that built it. Redistribution of libKGENCompilerRTShared was not established, so a release prerequisite is a Mojo install that satisfies that rpath. CPU apps do not install MAX. The copied consumer release on this Mac printed 42 with the original venv still present.
D13 — Declarations are Elixir data
tools/dsl/decls.json is the author-facing model. tools/dsl/generate.py emits the wrapper, nif_init, and the Elixir stub. User functions stay in native/examples/. A schedule other than normal, dirty_cpu, or dirty_io fails the generator. A borrow combined with a dirty schedule fails it too. The generator does not eval the JSON as code.
D9 — Checked overflow is ArgumentError
Elixir integers are unbounded. The NIF's range is signed 64-bit. Sums outside that range, and terms enif_get_long rejects, become badarg. They do not wrap. The comparison is written so the check itself stays inside c_long.