Rust Linking With C Library Functions

#[link(name = "c")]
extern "C" {
    fn scanf(format: *const u8, ...);
}

fn main() {
    let mut a = ;
    let mut b = ;
    unsafe {
        scanf("%d%d\0".as_ptr(), &mut a, &mut b);
    }
    println!("{}", a + b);
}