C to Rust Migration Guide

This skill provides a structured approach for rewriting or interfacing with legacy C code from Rust.

Migration Strategy: The "Strangler Fig" Pattern

Don't rewrite everything at once. Replace C modules one by one, keeping the C interface intact during the transition.

•Identify a module: Pick a small, self-contained C file (e.g., utils.c).
•Write Rust equivalent: Reimplement the logic in Rust.
•Create C-compatible API: Expose extern "C" functions from Rust that match the original C header.
•Update Build System: Link the new Rust object/library instead of the old C object.
•Verify: Run existing C tests against the new Rust implementation.

1. Interfacing with C (FFI)

Use bindgen to automatically generate Rust bindings for C headers. Avoid manual extern "C" blocks for complex headers.

bash

bindgen wrapper.h -o bindings.rs

Safety First

•Pointers are unsafe: Wrap raw pointers in safe abstractions as soon as possible.
•Strings: Convert *const c_char to CStr (borrowed) or CString (owned).
•Structs: Ensure #[repr(C)] layout compatibility for structs shared across the boundary.

2. Wrapping Unsafe C in Safe Rust

Pattern: The Handle Wrapper If C uses opaque pointers (e.g., Context*), wrap them in a Rust struct that manages lifetime.

rust

struct SafeContext {
    raw: *mut sys::Context,
}

impl Drop for SafeContext {
    fn drop(&mut self) {
        unsafe { sys::context_free(self.raw) };
    }
}

3. Common Translations

C Concept	Rust Equivalent	Notes
`void*` context	`Box<T>` / `*mut c_void`	Use `Box::into_raw` and `Box::from_raw`
callbacks	`extern "C" fn` + user_data	Pass a closure/trait object as `user_data`
`malloc`/`free`	`Vec<T>`, `Box<T>`, `String`	Rust manages memory automatically
`#define` constant	`const` or `enum`	Use `bitflags!` for bitmasks
`goto error;`	`Result<T, E>` + `?` operator	Idiomatic error handling

Tools

•c2rust: Automatic translation tool (produces unsafe Rust). Good for initial porting of algorithms, but requires heavy refactoring.
•bindgen: Generates FFI bindings.
•cc: Build dependency for compiling C code within build.rs.