How to use extern crate for FFI

You do not use extern crate for FFI in modern Rust; you use extern blocks to declare foreign functions and #[link] attributes to specify the library. Add the library to your Cargo.toml dependencies or use a system library, then declare the functions in an extern block and call them like normal Rust functions.

#[link(name = "m")]
extern "C" {
    fn sin(x: f64) -> f64;
}

fn main() {
    let result = unsafe { sin(0.0) };
    println!("Result: {}", result);
}

For C++ or other languages, ensure you link against the correct library and use the appropriate ABI (e.g., "C" for C, "C++" for C++).

FFI lets Rust talk to code written in other languages like C or C++. You declare the external functions in an extern block and link to the library they live in. Think of it like plugging a foreign device into your computer; you need the right adapter (the extern block) and the right cable (the library link) to make it work.