1 // Copyright 2014 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.
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.
11 //! OS-based thread local storage
13 //! This module provides an implementation of OS-based thread local storage,
14 //! using the native OS-provided facilities (think `TlsAlloc` or
15 //! `pthread_setspecific`). The interface of this differs from the other types
16 //! of thread-local-storage provided in this crate in that OS-based TLS can only
19 //! This module also provides two flavors of TLS. One is intended for static
20 //! initialization, and does not contain a `Drop` implementation to deallocate
21 //! the OS-TLS key. The other is a type which does implement `Drop` and hence
22 //! has a safe interface.
26 //! This module should likely not be used directly unless other primitives are
27 //! being built on. types such as `thread_local::spawn::Key` are likely much
28 //! more useful in practice than this OS-based version which likely requires
29 //! unsafe code to interoperate with.
33 //! Using a dynamically allocated TLS key. Note that this key can be shared
34 //! among many threads via an `Arc`.
36 //! ```ignore (cannot-doctest-private-modules)
37 //! let key = Key::new(None);
38 //! assert!(key.get().is_null());
39 //! key.set(1 as *mut u8);
40 //! assert!(!key.get().is_null());
42 //! drop(key); // deallocate this TLS slot.
45 //! Sometimes a statically allocated key is either required or easier to work
48 //! ```ignore (cannot-doctest-private-modules)
49 //! static KEY: StaticKey = INIT;
52 //! assert!(KEY.get().is_null());
53 //! KEY.set(1 as *mut u8);
57 #![allow(non_camel_case_types)]
58 #![unstable(feature = "thread_local_internals", issue = "0")]
59 #![allow(dead_code)] // sys isn't exported yet
62 use sync::atomic::{self, AtomicUsize, Ordering};
63 use sys::thread_local as imp;
64 use sys_common::mutex::Mutex;
66 /// A type for TLS keys that are statically allocated.
68 /// This type is entirely `unsafe` to use as it does not protect against
69 /// use-after-deallocation or use-during-deallocation.
71 /// The actual OS-TLS key is lazily allocated when this is used for the first
72 /// time. The key is also deallocated when the Rust runtime exits or `destroy`
73 /// is called, whichever comes first.
77 /// ```ignore (cannot-doctest-private-modules)
78 /// use tls::os::{StaticKey, INIT};
80 /// static KEY: StaticKey = INIT;
83 /// assert!(KEY.get().is_null());
84 /// KEY.set(1 as *mut u8);
87 pub struct StaticKey {
88 /// Inner static TLS key (internals).
90 /// Destructor for the TLS value.
92 /// See `Key::new` for information about when the destructor runs and how
94 dtor: Option<unsafe extern fn(*mut u8)>,
97 /// A type for a safely managed OS-based TLS slot.
99 /// This type allocates an OS TLS key when it is initialized and will deallocate
100 /// the key when it falls out of scope. When compared with `StaticKey`, this
101 /// type is entirely safe to use.
103 /// Implementations will likely, however, contain unsafe code as this type only
104 /// operates on `*mut u8`, a raw pointer.
108 /// ```ignore (cannot-doctest-private-modules)
109 /// use tls::os::Key;
111 /// let key = Key::new(None);
112 /// assert!(key.get().is_null());
113 /// key.set(1 as *mut u8);
114 /// assert!(!key.get().is_null());
116 /// drop(key); // deallocate this TLS slot.
122 /// Constant initialization value for static TLS keys.
124 /// This value specifies no destructor by default.
125 pub const INIT: StaticKey = StaticKey::new(None);
128 pub const fn new(dtor: Option<unsafe extern fn(*mut u8)>) -> StaticKey {
130 key: atomic::AtomicUsize::new(0),
135 /// Gets the value associated with this TLS key
137 /// This will lazily allocate a TLS key from the OS if one has not already
140 pub unsafe fn get(&self) -> *mut u8 { imp::get(self.key()) }
142 /// Sets this TLS key to a new value.
144 /// This will lazily allocate a TLS key from the OS if one has not already
147 pub unsafe fn set(&self, val: *mut u8) { imp::set(self.key(), val) }
150 unsafe fn key(&self) -> imp::Key {
151 match self.key.load(Ordering::Relaxed) {
152 0 => self.lazy_init() as imp::Key,
157 unsafe fn lazy_init(&self) -> usize {
158 // Currently the Windows implementation of TLS is pretty hairy, and
159 // it greatly simplifies creation if we just synchronize everything.
161 // Additionally a 0-index of a tls key hasn't been seen on windows, so
162 // we just simplify the whole branch.
163 if imp::requires_synchronized_create() {
164 static INIT_LOCK: Mutex = Mutex::new();
166 let mut key = self.key.load(Ordering::SeqCst);
168 key = imp::create(self.dtor) as usize;
169 self.key.store(key, Ordering::SeqCst);
176 // POSIX allows the key created here to be 0, but the compare_and_swap
177 // below relies on using 0 as a sentinel value to check who won the
178 // race to set the shared TLS key. As far as I know, there is no
179 // guaranteed value that cannot be returned as a posix_key_create key,
180 // so there is no value we can initialize the inner key with to
181 // prove that it has not yet been set. As such, we'll continue using a
182 // value of 0, but with some gyrations to make sure we have a non-0
183 // value returned from the creation routine.
184 // FIXME: this is clearly a hack, and should be cleaned up.
185 let key1 = imp::create(self.dtor);
186 let key = if key1 != 0 {
189 let key2 = imp::create(self.dtor);
194 match self.key.compare_and_swap(0, key as usize, Ordering::SeqCst) {
195 // The CAS succeeded, so we've created the actual key
197 // If someone beat us to the punch, use their key instead
198 n => { imp::destroy(key); n }
204 /// Creates a new managed OS TLS key.
206 /// This key will be deallocated when the key falls out of scope.
208 /// The argument provided is an optionally-specified destructor for the
209 /// value of this TLS key. When a thread exits and the value for this key
210 /// is non-null the destructor will be invoked. The TLS value will be reset
211 /// to null before the destructor is invoked.
213 /// Note that the destructor will not be run when the `Key` goes out of
216 pub fn new(dtor: Option<unsafe extern fn(*mut u8)>) -> Key {
217 Key { key: unsafe { imp::create(dtor) } }
220 /// See StaticKey::get
222 pub fn get(&self) -> *mut u8 {
223 unsafe { imp::get(self.key) }
226 /// See StaticKey::set
228 pub fn set(&self, val: *mut u8) {
229 unsafe { imp::set(self.key, val) }
235 // Right now Windows doesn't support TLS key destruction, but this also
236 // isn't used anywhere other than tests, so just leak the TLS key.
237 // unsafe { imp::destroy(self.key) }
241 pub unsafe fn register_dtor_fallback(t: *mut u8,
242 dtor: unsafe extern fn(*mut u8)) {
243 // The fallback implementation uses a vanilla OS-based TLS key to track
244 // the list of destructors that need to be run for this thread. The key
245 // then has its own destructor which runs all the other destructors.
247 // The destructor for DTORS is a little special in that it has a `while`
248 // loop to continuously drain the list of registered destructors. It
249 // *should* be the case that this loop always terminates because we
250 // provide the guarantee that a TLS key cannot be set after it is
251 // flagged for destruction.
253 static DTORS: StaticKey = StaticKey::new(Some(run_dtors));
254 type List = Vec<(*mut u8, unsafe extern fn(*mut u8))>;
255 if DTORS.get().is_null() {
256 let v: Box<List> = box Vec::new();
257 DTORS.set(Box::into_raw(v) as *mut u8);
259 let list: &mut List = &mut *(DTORS.get() as *mut List);
260 list.push((t, dtor));
262 unsafe extern fn run_dtors(mut ptr: *mut u8) {
263 while !ptr.is_null() {
264 let list: Box<List> = Box::from_raw(ptr as *mut List);
265 for &(ptr, dtor) in list.iter() {
269 DTORS.set(ptr::null_mut());
276 use super::{Key, StaticKey};
278 fn assert_sync<T: Sync>() {}
279 fn assert_send<T: Send>() {}
283 assert_sync::<Key>();
284 assert_send::<Key>();
286 let k1 = Key::new(None);
287 let k2 = Key::new(None);
288 assert!(k1.get().is_null());
289 assert!(k2.get().is_null());
292 assert_eq!(k1.get() as usize, 1);
293 assert_eq!(k2.get() as usize, 2);
298 static K1: StaticKey = StaticKey::new(None);
299 static K2: StaticKey = StaticKey::new(None);
302 assert!(K1.get().is_null());
303 assert!(K2.get().is_null());
306 assert_eq!(K1.get() as usize, 1);
307 assert_eq!(K2.get() as usize, 2);