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    use mem;
    use slice;

    pub fn hashmap_random_keys() -> (u64, u64) {
        let mut v = (0, 0);
        unsafe {
            let view = slice::from_raw_parts_mut(&mut v as *mut _ as *mut u8,
                                                 mem::size_of_val(&v));
            imp::fill_bytes(view);
        }
        return v
    }

    #[cfg(all(unix,
              not(target_os = "ios"),
              not(target_os = "openbsd"),
              not(target_os = "freebsd"),
              not(target_os = "fuchsia")))]
    mod imp {
        use fs::File;
        use io::Read;
        #[cfg(any(target_os = "linux", target_os = "android"))]
        use libc;

        #[cfg(any(target_os = "linux", target_os = "android"))]
        fn getrandom(buf: &mut [u8]) -> libc::c_long {
            unsafe {
                libc::syscall(libc::SYS_getrandom, buf.as_mut_ptr(), buf.len(), libc::GRND_NONBLOCK)
            }
        }

        #[cfg(not(any(target_os = "linux", target_os = "android")))]
        fn getrandom_fill_bytes(_buf: &mut [u8]) -> bool { false }

        #[cfg(any(target_os = "linux", target_os = "android"))]
        fn getrandom_fill_bytes(v: &mut [u8]) -> bool {
            use sync::atomic::{AtomicBool, Ordering};
            use sys::os::errno;

            static GETRANDOM_UNAVAILABLE: AtomicBool = AtomicBool::new(false);
            if GETRANDOM_UNAVAILABLE.load(Ordering::Relaxed) {
                return false;
            }

            let mut read = 0;
            while read < v.len() {
                let result = getrandom(&mut v[read..]);
                if result == -1 {
                    let err = errno() as libc::c_int;
                    if err == libc::EINTR {
                        continue;
                    } else if err == libc::ENOSYS {
                        GETRANDOM_UNAVAILABLE.store(true, Ordering::Relaxed);
                        return false;
                    } else if err == libc::EAGAIN {
                        return false;
                    } else {
                        panic!("unexpected getrandom error: {}", err);
                    }
                } else {
                    read += result as usize;
                }
            }
            true
        }

        pub fn fill_bytes(v: &mut [u8]) {
            // getrandom_fill_bytes here can fail if getrandom() returns EAGAIN,
            // meaning it would have blocked because the non-blocking pool (urandom)
            // has not initialized in the kernel yet due to a lack of entropy. The
            // fallback we do here is to avoid blocking applications which could
            // depend on this call without ever knowing they do and don't have a
            // work around. The PRNG of /dev/urandom will still be used but over a
            // possibly predictable entropy pool.
            if getrandom_fill_bytes(v) {
                return;
            }

            // getrandom failed because it is permanently or temporarily (because
            // of missing entropy) unavailable. Open /dev/urandom, read from it,
            // and close it again.
            let mut file = File::open("/dev/urandom").expect("failed to open /dev/urandom");
            file.read_exact(v).expect("failed to read /dev/urandom")
        }
    }

    #[cfg(target_os = "openbsd")]
    mod imp {
        use libc;
        use sys::os::errno;

        pub fn fill_bytes(v: &mut [u8]) {
            // getentropy(2) permits a maximum buffer size of 256 bytes
            for s in v.chunks_mut(256) {
                let ret = unsafe {
                    libc::getentropy(s.as_mut_ptr() as *mut libc::c_void, s.len())
                };
                if ret == -1 {
                    panic!("unexpected getentropy error: {}", errno());
                }
            }
        }
    }

    #[cfg(target_os = "ios")]
    mod imp {
        use io;
        use libc::{c_int, size_t};
        use ptr;

        enum SecRandom {}

        #[allow(non_upper_case_globals)]
        const kSecRandomDefault: *const SecRandom = ptr::null();

        extern {
            fn SecRandomCopyBytes(rnd: *const SecRandom,
                                  count: size_t,
                                  bytes: *mut u8) -> c_int;
        }

        pub fn fill_bytes(v: &mut [u8]) {
            let ret = unsafe {
                SecRandomCopyBytes(kSecRandomDefault,
                                   v.len(),
                                   v.as_mut_ptr())
            };
            if ret == -1 {
                panic!("couldn't generate random bytes: {}",
                       io::Error::last_os_error());
            }
        }
    }

    #[cfg(target_os = "freebsd")]
    mod imp {
        use libc;
        use ptr;

        pub fn fill_bytes(v: &mut [u8]) {
            let mib = [libc::CTL_KERN, libc::KERN_ARND];
            // kern.arandom permits a maximum buffer size of 256 bytes
            for s in v.chunks_mut(256) {
                let mut s_len = s.len();
                let ret = unsafe {
                    libc::sysctl(mib.as_ptr(), mib.len() as libc::c_uint,
                                 s.as_mut_ptr() as *mut _, &mut s_len,
                                 ptr::null(), 0)
                };
                if ret == -1 || s_len != s.len() {
                    panic!("kern.arandom sysctl failed! (returned {}, s.len() {}, oldlenp {})",
                           ret, s.len(), s_len);
                }
            }
        }
    }

    #[cfg(target_os = "fuchsia")]
    mod imp {
        #[link(name = "zircon")]
        extern {
            fn zx_cprng_draw(buffer: *mut u8, len: usize);
        }

        pub fn fill_bytes(v: &mut [u8]) {
            unsafe { zx_cprng_draw(v.as_mut_ptr(), v.len()) }
        }
    }

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