src/libstd/sys/unix/weak.rs

   1 // Copyright 2016 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! Support for "weak linkage" to symbols on Unix
  12 //!
  13 //! Some I/O operations we do in libstd require newer versions of OSes but we
  14 //! need to maintain binary compatibility with older releases for now. In order
  15 //! to use the new functionality when available we use this module for
  16 //! detection.
  17 //!
  18 //! One option to use here is weak linkage, but that is unfortunately only
  19 //! really workable on Linux. Hence, use dlsym to get the symbol value at
  20 //! runtime. This is also done for compatibility with older versions of glibc,
  21 //! and to avoid creating dependencies on GLIBC_PRIVATE symbols. It assumes that
  22 //! we've been dynamically linked to the library the symbol comes from, but that
  23 //! is currently always the case for things like libpthread/libc.
  24 //!
  25 //! A long time ago this used weak linkage for the __pthread_get_minstack
  26 //! symbol, but that caused Debian to detect an unnecessarily strict versioned
  27 //! dependency on libc6 (#23628).
  28
  29 use libc;
  30
  31 use ffi::CString;
  32 use marker;
  33 use mem;
  34 use sync::atomic::{AtomicUsize, Ordering};
  35
  36 macro_rules! weak {
  37     (fn $name:ident($($t:ty),*) -> $ret:ty) => (
  38         static $name: ::sys::weak::Weak<unsafe extern fn($($t),*) -> $ret> =
  39             ::sys::weak::Weak::new(stringify!($name));
  40     )
  41 }
  42
  43 pub struct Weak<F> {
  44     name: &'static str,
  45     addr: AtomicUsize,
  46     _marker: marker::PhantomData<F>,
  47 }
  48
  49 impl<F> Weak<F> {
  50     pub const fn new(name: &'static str) -> Weak<F> {
  51         Weak {
  52             name: name,
  53             addr: AtomicUsize::new(1),
  54             _marker: marker::PhantomData,
  55         }
  56     }
  57
  58     pub fn get(&self) -> Option<&F> {
  59         assert_eq!(mem::size_of::<F>(), mem::size_of::<usize>());
  60         unsafe {
  61             if self.addr.load(Ordering::SeqCst) == 1 {
  62                 self.addr.store(fetch(self.name), Ordering::SeqCst);
  63             }
  64             if self.addr.load(Ordering::SeqCst) == 0 {
  65                 None
  66             } else {
  67                 mem::transmute::<&AtomicUsize, Option<&F>>(&self.addr)
  68             }
  69         }
  70     }
  71 }
  72
  73 unsafe fn fetch(name: &str) -> usize {
  74     let name = match CString::new(name) {
  75         Ok(cstr) => cstr,
  76         Err(..) => return 0,
  77     };
  78     libc::dlsym(libc::RTLD_DEFAULT, name.as_ptr()) as usize
  79 }