Interoperability with C

Rust has full support for linking object files with a C calling convention. Similarly, you can export Rust functions and call them from C.

You can do it by hand if you want:

unsafe extern "C" {
    safe fn abs(x: i32) -> i32;
}

fn main() {
    let x = -42;
    let abs_x = abs(x);
    println!("{x}, {abs_x}");
}

We already saw this in the Safe FFI Wrapper exercise.

This assumes full knowledge of the target platform. Not recommended for production.

We will look at better options next.

The "C" part of the extern block tells Rust that abs can be called using the C ABI (application binary interface).
The safe fn abs part tells Rust that abs is a safe function. By default, extern functions are unsafe, but since abs(x) can’t trigger undefined behavior with any x, we can declare it safe.

Comprehensive Rust 🦀