Unit 8 — Safe conversion design

Objectives

State the policy for integers, floats, atoms, binaries, lists, and maps, and point at the test that locks it.

Why

A NIF that accepts every term will create atoms without bound or treat a bitstring as bytes. Both are OTP footguns.

Model

enif_get_* fails  ->  ArgumentError     (bad argument)
value is in range but the operation is not  ->  {:error, reason}
literal atom only ->  the error reason

Theory

Integers: the width is the enif_get_long / enif_get_ulong / enif_get_int / enif_get_uint that the function calls. Just outside the 64-bit range is a bignum and becomes badarg. Negative-to-unsigned is badarg.

Floats: enif_get_double does not accept an integer. This OTP will not build a NaN with 0.0 / 0.0. It raises ArithmeticError. The NIF's n != n branch is defensive and was not fed a term.

Atoms: echo_atom compares against five literals. :other is {:error, :unknown_atom}. The error atom is also a literal.

Lists: enif_get_list_length fails on an improper list. Length above 64 is :too_long. A nested list is :bad_element (depth 1). The walk is a loop.

Maps: missing n is :missing. Extra keys are dropped. That is the unknown-field policy, and the test checks %{n: 7, extra: 1}.

Lab

mix test test/term_test.exs

Expected

The integer edges, the overflow tuple, the atom vocabulary, the sub-binary copy, and the improper list all pass.

Code

native/term.mojo. fits_int64 is the same overflow idea as poc.mojo: the check itself stays inside c_long.

Failure

Returning enif_make_atom(env, user_bytes) would let a caller create atoms. The code does not take that path. If a new error reason is needed, add a literal and a test.

Exercise

What does copy_binary(<<1::7>>) return, and why?

Answer

{:error, :not_binary}. enif_inspect_binary rejects a bitstring that is not a whole number of bytes. The NIF does not pad it.