Unit 4 — ABI and layout

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Objectives

Compare one clang measurement with one Mojo measurement, field by field.

Why

RegisterPassable means the compiler will not secretly insert a pointer. It does not mean the field order matches erl_nif.h. The diff does.

Model

clang offsetof(ErlNifFunc, fptr)  ==  reflect field fptr in Mojo
Optional[Pointer] None            ==  eight zero bytes
thin function                     ==  eight-byte code address

Theory

On both measured LP64 targets, ErlNifFunc is 32 bytes and ErlNifEntry is 96. ErlNifBinary is 40. The C field __spare__[2] is spare_0 and spare_1 in Mojo. The layout diff keeps spare_0 (the offsetof of __spare__) and drops spare_1 before comparing, because C has one array field and Mojo has two slots.

Nullable callbacks in the entry are Optional[Pointer[NoneType, MutUntrackedOrigin]]. The code address of a thin function is the bits stored in the function value. Pointer(to=callback) is the stack slot. prim.callback_opt reads those bits.

enif_open_resource_type's generated wrapper takes a generic origin inside the function type, and a concrete dtor does not convert to it. native/own.mojo calls the symbol with external_call and the concrete dtor type. That is a recorded hole in the wrapper, not a second layout.

Lab

scripts/check-abi.sh

Expected

layout ok and dirty flags ok. A mismatch prints a unified diff and exits 1.

Code

tools/abi/print_layout.mojo uses std.reflection for the offsets. The C probe is a heredoc inside scripts/check-abi.sh. Neither file is passed to mojo build of a NIF.

Failure

Editing otp_nif.mojo by hand makes --check fail. Regenerate with python3 tools/abi/generate.py. If the header hash changed because OTP moved, the generator's pin file should fail first.

Exercise

Why is one generated file valid on both Darwin and the recorded Linux runner?

Answer

E7 and E18 measured the same sizes and offsets. The generator still refuses to label a host probe as a different triple. The Linux file was not regenerated on Ubuntu for the later stages; that rerun is PENDING.