Rust 1.29.2

# FFI

There is a new edition of the book and this is an old link.

> Sometimes, your Rust code may need to interact with code written in another language. To do this, Rust has a keyword, `extern`, that facilitates creating and using a Foreign Function Interface (FFI).

    extern "C" {
        fn abs(input: i32) -> i32;
    }

    fn main() {
        unsafe {
            println!("Absolute value of -3 according to C: {}", abs(-3));
        }
    }Run

Here are the relevant sections in the new and old books:

- In the second edition: Ch 19.01 — Unsafe Rust, section `extern` functions
- In the first edition: Ch 4.9 — FFI