Interop with C

This section will take you through a practical example of how to integrate Rust code into an existing C codebase.

What you learn here is fundamental to all interop with Rust since the C ABI (Application Binary Interface) is the only way to communicate with Rust functions from foreign languages. All tools and frameworks that offer integration with other programming languages build on the same techniques we use here.

The tools we'll use

On the Rust side of the fence, we have the FFI (Foreign Function Interface). We are provided a few keywords and a handful of library utilities to expose functions to the outside world (through the C ABI).

We will start by making a minimal Rust library, exposing a function over the FFI, and manually writing the matching C declarations to be able to call our function.

Further on, we will explore how we can use the cbindgen tool to automatically generate function declarations and types for our C code.

Finally, we'll show how we can make function calls in the other direction, from C to Rust, using Rust bindgen to keep the Rust declarations in sync.

Other helpful resources

• The FFI chapter of the Rustonomicon offers some technical details on FFI and a few practical examples (focusing mainly on calling C from Rust).
• The Rust FFI Omnibus has a handful of snippets demonstrating how to pass data across the language boundary.
• cbindgen has a User Guide, but it doesn't really show how to use the tool, focusing mainly on configuration options.