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    // Copyright 2019 Developers of the Rand project.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.
    #![allow(dead_code)]
    use crate::Error;
    use core::{
        num::NonZeroU32,
        ptr::NonNull,
        sync::atomic::{fence, AtomicPtr, Ordering},
    };
    use libc::c_void;

    cfg_if! {
        if #[cfg(any(target_os = "netbsd", target_os = "openbsd", target_os = "android"))] {
            use libc::__errno as errno_location;
        } else if #[cfg(any(target_os = "linux", target_os = "emscripten", target_os = "redox"))] {
            use libc::__errno_location as errno_location;
        } else if #[cfg(any(target_os = "solaris", target_os = "illumos"))] {
            use libc::___errno as errno_location;
        } else if #[cfg(any(target_os = "macos", target_os = "freebsd"))] {
            use libc::__error as errno_location;
        } else if #[cfg(target_os = "haiku")] {
            use libc::_errnop as errno_location;
        } else if #[cfg(all(target_os = "horizon", target_arch = "arm"))] {
            extern "C" {
                // Not provided by libc: https://github.com/rust-lang/libc/issues/1995
                fn __errno() -> *mut libc::c_int;
            }
            use __errno as errno_location;
        }
    }

    cfg_if! {
        if #[cfg(target_os = "vxworks")] {
            use libc::errnoGet as get_errno;
        } else if #[cfg(target_os = "dragonfly")] {
            // Until rust-lang/rust#29594 is stable, we cannot get the errno value
            // on DragonFlyBSD. So we just return an out-of-range errno.
            unsafe fn get_errno() -> libc::c_int { -1 }
        } else {
            unsafe fn get_errno() -> libc::c_int { *errno_location() }
        }
    }

    pub fn last_os_error() -> Error {
        let errno = unsafe { get_errno() };
        if errno > 0 {
            Error::from(NonZeroU32::new(errno as u32).unwrap())
        } else {
            Error::ERRNO_NOT_POSITIVE
        }
    }

    // Fill a buffer by repeatedly invoking a system call. The `sys_fill` function:
    //   - should return -1 and set errno on failure
    //   - should return the number of bytes written on success
    pub fn sys_fill_exact(
        mut buf: &mut [u8],
        sys_fill: impl Fn(&mut [u8]) -> libc::ssize_t,
    ) -> Result<(), Error> {
        while !buf.is_empty() {
            let res = sys_fill(buf);
            if res < 0 {
                let err = last_os_error();
                // We should try again if the call was interrupted.
                if err.raw_os_error() != Some(libc::EINTR) {
                    return Err(err);
                }
            } else {
                // We don't check for EOF (ret = 0) as the data we are reading
                // should be an infinite stream of random bytes.
                buf = &mut buf[(res as usize)..];
            }
        }
        Ok(())
    }

    // A "weak" binding to a C function that may or may not be present at runtime.
    // Used for supporting newer OS features while still building on older systems.
    // Based off of the DlsymWeak struct in libstd:
    // https://github.com/rust-lang/rust/blob/1.61.0/library/std/src/sys/unix/weak.rs#L84
    // except that the caller must manually cast self.ptr() to a function pointer.
    pub struct Weak {
        name: &'static str,
        addr: AtomicPtr<c_void>,
    }

    impl Weak {
        // A non-null pointer value which indicates we are uninitialized. This
        // constant should ideally not be a valid address of a function pointer.
        // However, if by chance libc::dlsym does return UNINIT, there will not
        // be undefined behavior. libc::dlsym will just be called each time ptr()
        // is called. This would be inefficient, but correct.
        // TODO: Replace with core::ptr::invalid_mut(1) when that is stable.
        const UNINIT: *mut c_void = 1 as *mut c_void;

        // Construct a binding to a C function with a given name. This function is
        // unsafe because `name` _must_ be null terminated.
        pub const unsafe fn new(name: &'static str) -> Self {
            Self {
                name,
                addr: AtomicPtr::new(Self::UNINIT),
            }
        }

        // Return the address of a function if present at runtime. Otherwise,
        // return None. Multiple callers can call ptr() concurrently. It will
        // always return _some_ value returned by libc::dlsym. However, the
        // dlsym function may be called multiple times.
        pub fn ptr(&self) -> Option<NonNull<c_void>> {
            // Despite having only a single atomic variable (self.addr), we still
            // cannot always use Ordering::Relaxed, as we need to make sure a
            // successful call to dlsym() is "ordered before" any data read through
            // the returned pointer (which occurs when the function is called).
            // Our implementation mirrors that of the one in libstd, meaning that
            // the use of non-Relaxed operations is probably unnecessary.
            match self.addr.load(Ordering::Relaxed) {
                Self::UNINIT => {
                    let symbol = self.name.as_ptr() as *const _;
                    let addr = unsafe { libc::dlsym(libc::RTLD_DEFAULT, symbol) };
                    // Synchronizes with the Acquire fence below
                    self.addr.store(addr, Ordering::Release);
                    NonNull::new(addr)
                }
                addr => {
                    let func = NonNull::new(addr)?;
                    fence(Ordering::Acquire);
                    Some(func)
                }
            }
        }
    }

    cfg_if! {
        if #[cfg(any(target_os = "linux", target_os = "emscripten"))] {
            use libc::open64 as open;
        } else {
            use libc::open;
        }
    }

    // SAFETY: path must be null terminated, FD must be manually closed.
    pub unsafe fn open_readonly(path: &str) -> Result<libc::c_int, Error> {
        debug_assert_eq!(path.as_bytes().last(), Some(&0));
        loop {
            let fd = open(path.as_ptr() as *const _, libc::O_RDONLY | libc::O_CLOEXEC);
            if fd >= 0 {
                return Ok(fd);
            }
            let err = last_os_error();
            // We should try again if open() was interrupted.
            if err.raw_os_error() != Some(libc::EINTR) {
                return Err(err);
            }
        }
    }