The libc crate is 'feature gated' and can only be accessed on nightly Rust versions until it is considered stable.

#![feature(libc)]
extern crate libc;
use libc::pid_t;

#[link(name = "c")]
extern {
    fn getpid() -> pid_t;
}

fn main() {
    let x = unsafe { getpid() };
    println!("Process PID is {}", x);
}