Contract First Ffi

Contract-First FFI

Purpose

Make native APIs boring to call from other languages. The binding is not a second implementation; it is a strict projection of the public contract.

Contract Workflow

1. Start from the canonical native API.

   • Identify the public header or IDL.
   • Treat it as the source of truth for structs, enums, function signatures, ownership, and lifecycle.
   • Remove duplicate local declarations where possible.

2. Match layouts explicitly.

   • For every struct crossing the boundary, verify field order, width, signedness, alignment, and array length.
   • Add native static_assert(sizeof(...)) and offsetof(...) checks when possible.
   • In dynamic languages, keep struct declarations adjacent to comments naming the native equivalent.

3. Make symbols required unless the product supports mixed versions.

   • If the library and binding ship together, bind symbols directly and fail at import/init time when missing.
   • Reserve hasattr, dlsym optionality, and feature probes for genuinely optional runtime capabilities.
   • Delete “older build” guards when creating a clean version line.

4. Keep ownership explicit.

   • Document who allocates and frees buffers, handles, strings, images, GPU resources, and borrowed pointers.
   • Make borrowed-handle APIs say how long the owner must keep the object alive.
   • Avoid returning raw pointers without a matching unmap/free/close function.

5. Test the boundary, not just internals.

   • Import the binding.
   • Create and destroy the native object.
   • Call at least one function that exercises each important struct.
   • Run one smoke path through the same binding used by real clients.

Binding Discipline

• Keep binding names close to native names unless idiomatic wrappers add value.
• Wrap errors consistently: native status -> binding exception or result type.
• Never silently ignore failed optional symbol binding unless absence is a supported capability.
• Avoid per-call marshaling for large arrays; pass contiguous buffers and pointers.
• Keep high-frequency data paths zero-copy or single-copy by design.

Native API Design

Prefer:

• Opaque handles for resources.
• Plain old data structs for descriptors.
• int/enum status codes plus get_last_error.
• Explicit create/destroy, map/unmap, enable/fetch.
• Separate capability queries for hardware/runtime features.

Avoid:

• ABI-visible C++ types.
• Caller-visible ownership ambiguity.
• Structs that change size without version strategy.
• Global mutable state hidden behind per-instance APIs.
• Binding-only behavior not present in native API.

Version Policy

Decide before adding compatibility code:

• Version-locked: binding and library are built together. Missing symbols are fatal. Delete stale guards.
• Version-tolerant: binding supports several deployed library versions. Feature probes are explicit, documented, and tested.
• Plugin ecosystem: negotiate ABI version at load time before binding symbols.

Do not mix these policies accidentally.

Review Checklist

• Public header and binding signatures agree.
• All struct sizes and offsets are verified or manually audited.
• Optional paths are truly optional.
• Errors propagate with actionable text.
• Build scripts install/update bindings after native rebuilds.
• Smoke test uses the installed binding, not an in-tree import accident.