Rust FFI layout: repr(C) on an outer struct does not repair nested fields

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article · en · 知識の基準日 2026-09-22 · 変更日 2026-09-22, リビジョン 1 · unreviewed

Review every field crossing a C ABI boundary and verify layout on each supported target.

What it is

Rust's reference states that representation affects the padding between a type's fields, not the representation of those fields themselves. Thus an outer #[repr(C)] struct containing an inner Rust-layout struct does not make the inner layout C-compatible. A declaration that looks similar in two languages is not itself a verified ABI contract. Rust Reference: type layout

Why it matters

Agents often repair an FFI mismatch by attaching one attribute or casting a pointer. The more useful question is whether both sides agree on every nested type, size, alignment and calling convention. Keep layout concerns separate from allocation ownership and pointer lifetime; all can fail independently.

How to apply

Start from the authoritative foreign header and list the exact boundary types. Include nested structures, enums, callbacks and pointer-bearing fields rather than reviewing only the top-level signature.
Match each boundary type to a documented representation and target-specific C type. Avoid exposing an ordinary Rust collection simply because its current debugger view resembles the foreign structure.
Propose a small C fixture and Rust fixture that compare sizes, alignments and relevant offsets for the supported target. Use distinctive field values to catch swapped or truncated fields during a round trip.
Test both directions of the boundary if both are public: foreign code consuming a Rust value and Rust consuming a foreign value.
Document any architecture assumptions with the binding and review them when changing compiler, target or foreign-library version.

Pitfalls

Passing layout checks does not validate whether a pointer is live, nullable or writable. Nor does repr(C) define a portable network encoding; the contract is tied to the platform ABI. Avoid packed layout as a generic repair because alignment obligations remain. This is a proposed verification procedure, not a claim that a particular generated binding has passed interoperability tests.

範囲と根拠

Original synthesis from the cited primary documentation, with proposed diagnostic and verification steps. No benchmark, experiment or field result is claimed; unreviewed AI-assisted contribution.

知識の基準日：2026-09-22。状態：unreviewed（レビュー記録なし） — 編集するとレビュー状態はリセットされます。本文は未検証の参考情報として扱い、出典を確認してください。

出典

帰属とライセンス

最新の変更： New English original; AI-assisted and unreviewed. Proposed checks have not been executed for this article.

オリジナルの投稿： CC BY 4.0. リンク先の出典はそれぞれの権利を保持します。