cfg_if::cfg_if! {
    if #[cfg(target_os = "linux")] {
        /// pthread_t is a pointer on some platforms,
        /// so we wrap it in this to impl Send + Sync.
        #[derive(Clone, Copy)]
        #[repr(transparent)]
        struct PThread(libc::pthread_t);
        // Safety: pthread_t is safe to send between threads
        unsafe impl Send for PThread {}
        // Safety: pthread_t is safe to share between threads
        unsafe impl Sync for PThread {}
        /// Mitigation for <https://github.com/rust-lang/rust/issues/126600>
        ///
        /// On glibc, `libc::exit` has been observed to not always be thread-safe.
        /// It is currently unclear whether that is a glibc bug or allowed by the standard.
        /// To mitigate this problem, we ensure that only one
        /// Rust thread calls `libc::exit` (or returns from `main`) by calling this function before
        /// calling `libc::exit` (or returning from `main`).
        ///
        /// Technically, this is not enough to ensure soundness, since other code directly calling
        /// `libc::exit` will still race with this.
        ///
        /// *This function does not itself call `libc::exit`.* This is so it can also be used
        /// to guard returning from `main`.
        ///
        /// This function will return only the first time it is called in a process.
        ///
        /// * If it is called again on the same thread as the first call, it will abort.
        /// * If it is called again on a different thread, it will wait in a loop
        ///   (waiting for the process to exit).
        #[cfg_attr(any(test, doctest), allow(dead_code))]
        pub(crate) fn unique_thread_exit() {
            let this_thread_id = unsafe { libc::pthread_self() };
            use crate::sync::{Mutex, PoisonError};
            static EXITING_THREAD_ID: Mutex<Option<PThread>> = Mutex::new(None);
            let mut exiting_thread_id =
                EXITING_THREAD_ID.lock().unwrap_or_else(PoisonError::into_inner);
            match *exiting_thread_id {
                None => {
                    // This is the first thread to call `unique_thread_exit`,
                    // and this is the first time it is called.
                    // Set EXITING_THREAD_ID to this thread's ID and return.
                    *exiting_thread_id = Some(PThread(this_thread_id));
                },
                Some(exiting_thread_id) if exiting_thread_id.0 == this_thread_id => {
                    // This is the first thread to call `unique_thread_exit`,
                    // but this is the second time it is called.
                    // Abort the process.
                    core::panicking::panic_nounwind("std::process::exit called re-entrantly")
                }
                Some(_) => {
                    // This is not the first thread to call `unique_thread_exit`.
                    // Pause until the process exits.
                    drop(exiting_thread_id);
                    loop {
                        // Safety: libc::pause is safe to call.
                        unsafe { libc::pause(); }
                    }
                }
            }
        }
    } else {
        /// Mitigation for <https://github.com/rust-lang/rust/issues/126600>
        ///
        /// Mitigation is ***NOT*** implemented on this platform, either because this platform
        /// is not affected, or because mitigation is not yet implemented for this platform.
        #[cfg_attr(any(test, doctest), allow(dead_code))]
        pub(crate) fn unique_thread_exit() {
            // Mitigation not required on platforms where `exit` is thread-safe.
        }
    }
}