Rust by Example

Foreign Function Interface

Rust provides a Foreign Function Interface (FFI) to C libraries. Foreign functions must be declared inside an extern block annotated with a #[link] attribute containing the name of the foreign library.

use std::fmt;

// this extern block links to the libm library

#[link(name = "m")]

extern {

// this is a foreign function

// that computes the square root of a single precision complex number

fn csqrtf(z: Complex) -> Complex;

fn ccosf(z: Complex) -> Complex;

}

// Since calling foreign functions is considered unsafe,

// it's common to write safe wrappers around them.

fn cos(z: Complex) -> Complex {

unsafe { ccosf(z) }

}

fn main() {

// z = -1 + 0i

let z = Complex { re: -1., im: 0. };

// calling a foreign function is an unsafe operation

let z_sqrt = unsafe { csqrtf(z) };

println!("the square root of {:?} is {:?}", z, z_sqrt);

// calling safe API wrapped around unsafe operation

println!("cos({:?}) = {:?}", z, cos(z));

}

// Minimal implementation of single precision complex numbers

#[repr(C)]

#[derive(Clone, Copy)]

struct Complex {

re: f32,

im: f32,

}

impl fmt::Debug for Complex {

fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {

if self.im < 0. {

write!(f, "{}-{}i", self.re, -self.im)

} else {

write!(f, "{}+{}i", self.re, self.im)

}

}

}