Foreign-function interface guidance for connecting Rust code to Skia's C and C++ graphics library. It defines naming, pointer safety, resource cleanup, and type conversions.

Use it when adding or reviewing graphics bindings, exported C functions, opaque pointers, null checks, or Rust-to-Skia type mappings.

It reduces crashes and memory errors when code crosses between Rust and C or C++.

ffi — Cursor rule for Cursor by quinnjr: Foreign-function interface guidance for connecting Rust code to Skia's…

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods

npx agentmods add rules/quinnjr/skia-rs/ffi

Clone the repo

git clone --depth 1 https://github.com/quinnjr/skia-rs

Made for: Cursor.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

README.md

[![agentmods](https://agentmods.dev/badge/rules/quinnjr/skia-rs/ffi.svg)](https://agentmods.dev/rules/quinnjr/skia-rs/ffi)

Your own site

<a href="https://agentmods.dev/rules/quinnjr/skia-rs/ffi"><img src="https://agentmods.dev/badge/rules/quinnjr/skia-rs/ffi.svg" alt="Measured on agentmods" height="20"></a>

Per session 0 Nothing until a file matches its globs; then the whole rule loads.

When invoked 525 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. Scan, not verified.

Origin original No closer match found in the catalogue.

Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

Model	Per session	Once invoked
Fable 5.1	$0.00000	$0.00525
Opus 5	$0.00000	$0.00262
Sonnet 5	$0.00000	$0.00105
Haiku 4.5	$0.00000	$0.00052

Measured 4d ago against content hash a1ad0ac4e653, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-05, from the pricing page.

Security

Grade A, and why

ffi scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 4d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

.cursor/rules/ffi.mdc · 70 lines

What it actually says

FFI Guidelines

C API Design

All FFI functions in skia-rs-ffi
Use opaque pointers for complex types
Follow Skia's C API naming: sk_{type}_{method}
Always check for null pointers

// skia-rs-ffi/src/lib.rs

/// Create a new paint object.
///
/// # Safety
/// Returns a valid pointer that must be freed with `sk_paint_delete`.
#[no_mangle]
pub unsafe extern "C" fn sk_paint_new() -> *mut Paint {
    Box::into_raw(Box::new(Paint::new()))
}

/// Delete a paint object.
///
/// # Safety
/// `paint` must be a valid pointer returned by `sk_paint_new`.
#[no_mangle]
pub unsafe extern "C" fn sk_paint_delete(paint: *mut Paint) {
    if !paint.is_null() {
        drop(Box::from_raw(paint));
    }
}

/// Set the paint color.
///
/// # Safety
/// `paint` must be a valid pointer.
#[no_mangle]
pub unsafe extern "C" fn sk_paint_set_color(paint: *mut Paint, color: u32) {
    if let Some(p) = paint.as_mut() {
        p.set_color32(Color(color));
    }
}

Type Mappings

Skia C++	Rust	C FFI
`SkScalar`	`Scalar` (f32)	`float`
`SkPoint`	`Point`	`sk_point_t`
`SkRect`	`Rect`	`sk_rect_t`
`SkColor`	`Color`	`uint32_t`
`SkMatrix`	`Matrix`	`sk_matrix_t`
`SkPath*`	`*mut Path`	`sk_path_t*`
`SkPaint*`	`*mut Paint`	`sk_paint_t*`
`SkCanvas*`	`*mut Canvas`	`sk_canvas_t*`

Safety Requirements

All FFI functions must be marked unsafe
Document safety requirements in doc comments
Check for null pointers before dereferencing
Use #[repr(C)] for all FFI-visible structs

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

4d ago First seen · 70 lines · 0 tokens per session scan A a1ad0ac4e653

Subscribe to this mod's changes

ffi is a cursor rule published in the GitHub repository quinnjr/skia-rs (33 stars, last pushed 16d ago), licensed Apache-2.0. It costs nothing until one of its globs matches a file; then it loads 525 tokens. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-09-01.

C and C++Languages Rust

ffi

Getting it into your agent

Wrote this? Show the measurements

What it costs to keep this loaded

Grade A, and why

Nothing flagged

What it actually says

FFI Guidelines

C API Design

Type Mappings

Safety Requirements

What this file has done since we first saw it

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