| //! 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). |
| |
| use crate::ffi::CStr; |
| use crate::marker; |
| use crate::mem; |
| use crate::sync::atomic::{AtomicUsize, Ordering}; |
| |
| 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 { |
| if self.addr.load(Ordering::SeqCst) == 1 { |
| self.addr.store(fetch(self.name), Ordering::SeqCst); |
| } |
| match self.addr.load(Ordering::SeqCst) { |
| 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, |
| }; |
| assert!(false, "FIXME: fetch"); |
| libc::dlsym(libc::RTLD_DEFAULT, name.as_ptr()) as usize |
| } |