# FFI Design

> Expert skill for designing and implementing foreign function interfaces to native code

- Skill: `a5c-ai/ffi-design` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add a5c-ai/ffi-design`
- Raw SKILL.md: https://api.skillmd.com/api/skills/a5c-ai/ffi-design/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: a5c-ai (https://skillmd.com/u/a5c-ai)
- Updated: 2026-09-09
- Page: https://skillmd.com/skills/a5c-ai/ffi-design

---


# FFI Design Skill

## Overview

Expert skill for designing and implementing foreign function interfaces to native code.

## Capabilities

- Design FFI declaration syntax
- Implement type marshaling between languages
- Handle C calling conventions (cdecl, stdcall, fastcall)
- Implement callback support (native calling managed)
- Handle string encoding conversions
- Implement struct layout matching (padding, alignment)
- Design memory ownership transfer rules
- Support dynamic library loading

## Target Processes

- ffi-implementation.js
- interpreter-implementation.js
- bytecode-vm-implementation.js
- code-generation-llvm.js

## Dependencies

- libffi
- Platform ABI documentation

## Usage Guidelines

1. **Safety**: Design safe wrappers around unsafe FFI calls
2. **Marshaling**: Document type marshaling rules explicitly
3. **Ownership**: Make memory ownership clear at FFI boundaries
4. **Callbacks**: Handle callback lifetimes carefully
5. **Strings**: Handle string encoding conversions correctly (UTF-8, UTF-16)

## Output Schema

```json
{
  "type": "object",
  "properties": {
    "callingConventions": {
      "type": "array",
      "items": { "type": "string" }
    },
    "marshalingRules": {
      "type": "array",
      "items": {
        "type": "object",
        "properties": {
          "sourceType": { "type": "string" },
          "targetType": { "type": "string" }
        }
      }
    },
    "callbackSupport": { "type": "boolean" },
    "generatedFiles": {
      "type": "array",
      "items": { "type": "string" }
    }
  }
}
```