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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 }