# 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)
            }
        }
    }