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    use crate::mem;
    use crate::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);
        }
        v
    }

    #[cfg(all(
        unix,
        not(target_os = "macos"),
        not(target_os = "ios"),
        not(target_os = "openbsd"),
        not(target_os = "freebsd"),
        not(target_os = "netbsd"),
        not(target_os = "fuchsia"),
        not(target_os = "redox")
    ))]
    mod imp {
        use crate::fs::File;
        use crate::io::Read;

        #[cfg(any(target_os = "linux", target_os = "android"))]
        fn getrandom(buf: &mut [u8]) -> libc::ssize_t {
            // A weak symbol allows interposition, e.g. for perf measurements that want to
            // disable randomness for consistency. Otherwise, we'll try a raw syscall.
            // (`getrandom` was added in glibc 2.25, musl 1.1.20, android API level 28)
            syscall! {
                fn getrandom(
                    buffer: *mut libc::c_void,
                    length: libc::size_t,
                    flags: libc::c_uint
                ) -> libc::ssize_t
            }

            unsafe { getrandom(buf.as_mut_ptr().cast(), 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 crate::sync::atomic::{AtomicBool, Ordering};
            use crate::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 || err == libc::EPERM {
                        // Fall back to reading /dev/urandom if `getrandom` is not
                        // supported on the current kernel.
                        //
                        // Also fall back in case it is disabled by something like
                        // seccomp or inside of virtual machines.
                        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 = "macos")]
    mod imp {
        use crate::fs::File;
        use crate::io::Read;
        use crate::sys::os::errno;
        use libc::{c_int, c_void, size_t};

        fn getentropy_fill_bytes(v: &mut [u8]) -> bool {
            weak!(fn getentropy(*mut c_void, size_t) -> c_int);

            getentropy
                .get()
                .map(|f| {
                    // getentropy(2) permits a maximum buffer size of 256 bytes
                    for s in v.chunks_mut(256) {
                        let ret = unsafe { f(s.as_mut_ptr() as *mut c_void, s.len()) };
                        if ret == -1 {
                            panic!("unexpected getentropy error: {}", errno());
                        }
                    }
                    true
                })
                .unwrap_or(false)
        }

        pub fn fill_bytes(v: &mut [u8]) {
            if getentropy_fill_bytes(v) {
                return;
            }

            // for older macos which doesn't support getentropy
            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 crate::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());
                }
            }
        }
    }

    // On iOS and MacOS `SecRandomCopyBytes` calls `CCRandomCopyBytes` with
    // `kCCRandomDefault`. `CCRandomCopyBytes` manages a CSPRNG which is seeded
    // from `/dev/random` and which runs on its own thread accessed via GCD.
    // This seems needlessly heavyweight for the purposes of generating two u64s
    // once per thread in `hashmap_random_keys`. Therefore `SecRandomCopyBytes` is
    // only used on iOS where direct access to `/dev/urandom` is blocked by the
    // sandbox.
    #[cfg(target_os = "ios")]
    mod imp {
        use crate::io;
        use crate::ptr;
        use libc::{c_int, size_t};

        enum SecRandom {}

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

        extern "C" {
            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(any(target_os = "freebsd", target_os = "netbsd"))]
    mod imp {
        use crate::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 "C" {
            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()) }
        }
    }

    #[cfg(target_os = "redox")]
    mod imp {
        use crate::fs::File;
        use crate::io::Read;

        pub fn fill_bytes(v: &mut [u8]) {
            // Open rand:, read from it, and close it again.
            let mut file = File::open("rand:").expect("failed to open rand:");
            file.read_exact(v).expect("failed to read rand:")
        }
    }