# Unsafe Checker

> Use when reviewing unsafe code or writing FFI. Keywords: unsafe, raw pointer, FFI, extern, transmute, *mut, *const, union, #[repr(C)], libc, std::ffi, MaybeUninit, NonNull, PhantomData, Send, Sync, SAFETY comment, soundness, undefined behavior, UB, how to call C functions, safe wrapper for unsafe code, when is unsafe necessary, memory layout, bindgen, cbindgen, CString, CStr, invariant, 安全抽象, 裸指针, 外部函数接口, 内存布局, 不安全代码, FFI 绑定, 未定义行为

- Skill: `majiayu000/unsafe-checker` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add majiayu000/unsafe-checker`
- Raw SKILL.md: https://api.skillmd.com/api/skills/majiayu000/unsafe-checker/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Security
- Author: majiayu000 (https://skillmd.com/u/majiayu000)
- Updated: 2026-09-09
- Page: https://skillmd.com/skills/majiayu000/unsafe-checker

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# Unsafe Rust Checker

## When Unsafe is Valid

| Use Case | Example |
|----------|---------|
| FFI | Calling C functions |
| Low-level abstractions | Implementing `Vec`, `Arc` |
| Performance | Measured bottleneck with safe alternative too slow |

**NOT valid:** Escaping borrow checker without understanding why.

## Required Documentation

```rust
// SAFETY: <why this is safe>
unsafe { ... }

/// # Safety
/// <caller requirements>
pub unsafe fn dangerous() { ... }
```

## Quick Reference

| Operation | Safety Requirements |
|-----------|---------------------|
| `*ptr` deref | Valid, aligned, initialized |
| `&*ptr` | + No aliasing violations |
| `transmute` | Same size, valid bit pattern |
| `extern "C"` | Correct signature, ABI |
| `static mut` | Synchronization guaranteed |
| `impl Send/Sync` | Actually thread-safe |

## Common Errors

| Error | Fix |
|-------|-----|
| Null pointer deref | Check for null before deref |
| Use after free | Ensure lifetime validity |
| Data race | Add proper synchronization |
| Alignment violation | Use `#[repr(C)]`, check alignment |
| Invalid bit pattern | Use `MaybeUninit` |
| Missing SAFETY comment | Add `// SAFETY:` |

## Deprecated → Better

| Deprecated | Use Instead |
|------------|-------------|
| `mem::uninitialized()` | `MaybeUninit<T>` |
| `mem::zeroed()` for refs | `MaybeUninit<T>` |
| Raw pointer arithmetic | `NonNull<T>`, `ptr::add` |
| `CString::new().unwrap().as_ptr()` | Store `CString` first |
| `static mut` | `AtomicT` or `Mutex` |
| Manual extern | `bindgen` |

## FFI Crates

| Direction | Crate |
|-----------|-------|
| C → Rust | bindgen |
| Rust → C | cbindgen |
| Python | PyO3 |
| Node.js | napi-rs |

Claude knows unsafe Rust. Focus on SAFETY comments and soundness.