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    //! Support for "weak linkage" to symbols on Unix
    //!
    //! Some I/O operations we do in libstd require newer versions of OSes but we
    //! need to maintain binary compatibility with older releases for now. In order
    //! to use the new functionality when available we use this module for
    //! detection.
    //!
    //! One option to use here is weak linkage, but that is unfortunately only
    //! really workable on Linux. Hence, use dlsym to get the symbol value at
    //! runtime. This is also done for compatibility with older versions of glibc,
    //! and to avoid creating dependencies on GLIBC_PRIVATE symbols. It assumes that
    //! we've been dynamically linked to the library the symbol comes from, but that
    //! is currently always the case for things like libpthread/libc.
    //!
    //! A long time ago this used weak linkage for the __pthread_get_minstack
    //! symbol, but that caused Debian to detect an unnecessarily strict versioned
    //! dependency on libc6 (#23628).

    // There are a variety of `#[cfg]`s controlling which targets are involved in
    // each instance of `weak!` and `syscall!`. Rather than trying to unify all of
    // that, we'll just allow that some unix targets don't use this module at all.
    #![allow(dead_code, unused_macros)]

    use crate::ffi::CStr;
    use crate::marker;
    use crate::mem;
    use crate::sync::atomic::{self, AtomicUsize, Ordering};

    macro_rules! weak {
        (fn $name:ident($($t:ty),*) -> $ret:ty) => (
            static $name: crate::sys::weak::Weak<unsafe extern fn($($t),*) -> $ret> =
                crate::sys::weak::Weak::new(concat!(stringify!($name), '\0'));
        )
    }

    pub struct Weak<F> {
        name: &'static str,
        addr: AtomicUsize,
        _marker: marker::PhantomData<F>,
    }

    impl<F> Weak<F> {
        pub const fn new(name: &'static str) -> Weak<F> {
            Weak { name, addr: AtomicUsize::new(1), _marker: marker::PhantomData }
        }

        pub fn get(&self) -> Option<F> {
            assert_eq!(mem::size_of::<F>(), mem::size_of::<usize>());
            unsafe {
                // Relaxed is fine here because we fence before reading through the
                // pointer (see the comment below).
                match self.addr.load(Ordering::Relaxed) {
                    1 => self.initialize(),
                    0 => None,
                    addr => {
                        let func = mem::transmute_copy::<usize, F>(&addr);
                        // The caller is presumably going to read through this value
                        // (by calling the function we've dlsymed). This means we'd
                        // need to have loaded it with at least C11's consume
                        // ordering in order to be guaranteed that the data we read
                        // from the pointer isn't from before the pointer was
                        // stored. Rust has no equivalent to memory_order_consume,
                        // so we use an acquire fence (sorry, ARM).
                        //
                        // Now, in practice this likely isn't needed even on CPUs
                        // where relaxed and consume mean different things. The
                        // symbols we're loading are probably present (or not) at
                        // init, and even if they aren't the runtime dynamic loader
                        // is extremely likely have sufficient barriers internally
                        // (possibly implicitly, for example the ones provided by
                        // invoking `mprotect`).
                        //
                        // That said, none of that's *guaranteed*, and so we fence.
                        atomic::fence(Ordering::Acquire);
                        Some(func)
                    }
                }
            }
        }

        // Cold because it should only happen during first-time initalization.
        #[cold]
        unsafe fn initialize(&self) -> Option<F> {
            let val = fetch(self.name);
            // This synchronizes with the acquire fence in `get`.
            self.addr.store(val, Ordering::Release);

            match val {
                0 => None,
                addr => Some(mem::transmute_copy::<usize, F>(&addr)),
            }
        }
    }

    unsafe fn fetch(name: &str) -> usize {
        let name = match CStr::from_bytes_with_nul(name.as_bytes()) {
            Ok(cstr) => cstr,
            Err(..) => return 0,
        };
        libc::dlsym(libc::RTLD_DEFAULT, name.as_ptr()) as usize
    }

    #[cfg(not(any(target_os = "linux", target_os = "android")))]
    macro_rules! syscall {
        (fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
            unsafe fn $name($($arg_name: $t),*) -> $ret {
                use super::os;

                weak! { fn $name($($t),*) -> $ret }

                if let Some(fun) = $name.get() {
                    fun($($arg_name),*)
                } else {
                    os::set_errno(libc::ENOSYS);
                    -1
                }
            }
        )
    }

    #[cfg(any(target_os = "linux", target_os = "android"))]
    macro_rules! syscall {
        (fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
            unsafe fn $name($($arg_name:$t),*) -> $ret {
                // This looks like a hack, but concat_idents only accepts idents
                // (not paths).
                use libc::*;

                weak! { fn $name($($t),*) -> $ret }

                // Use a weak symbol from libc when possible, allowing `LD_PRELOAD`
                // interposition, but if it's not found just use a raw syscall.
                if let Some(fun) = $name.get() {
                    fun($($arg_name),*)
                } else {
                    syscall(
                        concat_idents!(SYS_, $name),
                        $($arg_name),*
                    ) as $ret
                }
            }
        )
    }