The C to Rust Migration Book

Foreign Function Interfaces

We kick off with the fundamentals of FFI: how C and Rust communicate at the binary level through the C ABI, symbol tables, and the linker. You'll learn why C became the universal standard for language interoperability and write your first extern "C" blocks by hand. You'll understand exactly what you're promising the compiler when you declare a binding — and what happens when you get the signature wrong.

Types and Data Across the Boundary

This module covers how to safely move primitive values, strings, structs, and collections across the language boundary while preserving clear ownership semantics. You'll use bindgen to automate the generation of Rust bindings from C headers, and cbindgen to generate C headers from your Rust code, and learn how to configure both for a real codebase with non-trivial types.

FFI-Safe Types

Raw pointers and untyped primitives are the biggest source of bugs at the FFI boundary. This module introduces safer alternatives: NonNull, Option, newtype wrappers, and enum-based discriminants that carry meaning instead of just values. You'll learn to design types that make invalid states unrepresentable, even when talking to C code that has no type safety of its own.

Validation and Narrowing

Every FFI function is a trust boundary. C callers can pass null pointers, out-of-range values, or invalid UTF-8 — and Rust has no way to know at compile time. This module builds a discipline around validating and narrowing inputs at the boundary before they reach safe Rust, so that garbage data from C can never propagate into your business logic.

Error Handling and API Design

Rust's Result and ? stop at extern "C". This module covers how to convert Rust's error types into C-compatible results: error code enums, out-parameters, and how to surface meaningful failure information without leaking Rust internals. You'll also learn to expose coarse operations rather than fine-grained setters and keep your API surface intentionally small.

Production-Quality FFI

The final module ties everything together. You'll document FFI functions using Rust's /// # Safety convention with numbered invariants, so every unsafe contract is explicit and surfaces as C doc comments in the generated headers via cbindgen. We cover build integration, automated checks, and the mindset of treating your FFI surface as an API you'll maintain forever.