1 // Copyright 2014-2015 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 //! Thread local storage
13 #![unstable(feature = "thread_local_internals", issue = "0")]
19 /// A thread local storage key which owns its contents.
21 /// This key uses the fastest possible implementation available to it for the
22 /// target platform. It is instantiated with the `thread_local!` macro and the
23 /// primary method is the `with` method.
25 /// The `with` method yields a reference to the contained value which cannot be
26 /// sent across threads or escape the given closure.
28 /// # Initialization and Destruction
30 /// Initialization is dynamically performed on the first call to `with()`
31 /// within a thread, and values that implement `Drop` get destructed when a
32 /// thread exits. Some caveats apply, which are explained below.
37 /// use std::cell::RefCell;
40 /// thread_local!(static FOO: RefCell<u32> = RefCell::new(1));
43 /// assert_eq!(*f.borrow(), 1);
44 /// *f.borrow_mut() = 2;
47 /// // each thread starts out with the initial value of 1
48 /// thread::spawn(move|| {
50 /// assert_eq!(*f.borrow(), 1);
51 /// *f.borrow_mut() = 3;
55 /// // we retain our original value of 2 despite the child thread
57 /// assert_eq!(*f.borrow(), 2);
61 /// # Platform-specific behavior
63 /// Note that a "best effort" is made to ensure that destructors for types
64 /// stored in thread local storage are run, but not all platforms can guarantee
65 /// that destructors will be run for all types in thread local storage. For
66 /// example, there are a number of known caveats where destructors are not run:
68 /// 1. On Unix systems when pthread-based TLS is being used, destructors will
69 /// not be run for TLS values on the main thread when it exits. Note that the
70 /// application will exit immediately after the main thread exits as well.
71 /// 2. On all platforms it's possible for TLS to re-initialize other TLS slots
72 /// during destruction. Some platforms ensure that this cannot happen
73 /// infinitely by preventing re-initialization of any slot that has been
74 /// destroyed, but not all platforms have this guard. Those platforms that do
75 /// not guard typically have a synthetic limit after which point no more
76 /// destructors are run.
77 /// 3. On OSX, initializing TLS during destruction of other TLS slots can
78 /// sometimes cancel *all* destructors for the current thread, whether or not
79 /// the slots have already had their destructors run or not.
80 #[stable(feature = "rust1", since = "1.0.0")]
81 pub struct LocalKey<T: 'static> {
82 // This outer `LocalKey<T>` type is what's going to be stored in statics,
83 // but actual data inside will sometimes be tagged with #[thread_local].
84 // It's not valid for a true static to reference a #[thread_local] static,
85 // so we get around that by exposing an accessor through a layer of function
86 // indirection (this thunk).
88 // Note that the thunk is itself unsafe because the returned lifetime of the
89 // slot where data lives, `'static`, is not actually valid. The lifetime
90 // here is actually `'thread`!
92 // Although this is an extra layer of indirection, it should in theory be
93 // trivially devirtualizable by LLVM because the value of `inner` never
94 // changes and the constant should be readonly within a crate. This mainly
95 // only runs into problems when TLS statics are exported across crates.
96 inner: fn() -> Option<&'static UnsafeCell<Option<T>>>,
98 // initialization routine to invoke to create a value
102 #[stable(feature = "std_debug", since = "1.16.0")]
103 impl<T: 'static> fmt::Debug for LocalKey<T> {
104 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
105 f.pad("LocalKey { .. }")
109 /// Declare a new thread local storage key of type `std::thread::LocalKey`.
113 /// The macro wraps any number of static declarations and makes them thread local.
114 /// Each static may be public or private, and attributes are allowed. Example:
117 /// use std::cell::RefCell;
119 /// pub static FOO: RefCell<u32> = RefCell::new(1);
122 /// static BAR: RefCell<f32> = RefCell::new(1.0);
127 /// See [LocalKey documentation](thread/struct.LocalKey.html) for more
130 #[stable(feature = "rust1", since = "1.0.0")]
131 #[allow_internal_unstable]
132 macro_rules! thread_local {
133 // rule 0: empty (base case for the recursion)
136 // rule 1: process multiple declarations where the first one is private
137 ($(#[$attr:meta])* static $name:ident: $t:ty = $init:expr; $($rest:tt)*) => (
138 thread_local!($(#[$attr])* static $name: $t = $init); // go to rule 2
139 thread_local!($($rest)*);
142 // rule 2: handle a single private declaration
143 ($(#[$attr:meta])* static $name:ident: $t:ty = $init:expr) => (
144 $(#[$attr])* static $name: $crate::thread::LocalKey<$t> =
145 __thread_local_inner!($t, $init);
148 // rule 3: handle multiple declarations where the first one is public
149 ($(#[$attr:meta])* pub static $name:ident: $t:ty = $init:expr; $($rest:tt)*) => (
150 thread_local!($(#[$attr])* pub static $name: $t = $init); // go to rule 4
151 thread_local!($($rest)*);
154 // rule 4: handle a single public declaration
155 ($(#[$attr:meta])* pub static $name:ident: $t:ty = $init:expr) => (
156 $(#[$attr])* pub static $name: $crate::thread::LocalKey<$t> =
157 __thread_local_inner!($t, $init);
162 #[unstable(feature = "thread_local_internals",
163 reason = "should not be necessary",
166 #[allow_internal_unstable]
167 macro_rules! __thread_local_inner {
168 ($t:ty, $init:expr) => {{
169 fn __init() -> $t { $init }
171 fn __getit() -> $crate::option::Option<
172 &'static $crate::cell::UnsafeCell<
173 $crate::option::Option<$t>>>
176 #[cfg(target_thread_local)]
177 static __KEY: $crate::thread::__FastLocalKeyInner<$t> =
178 $crate::thread::__FastLocalKeyInner::new();
180 #[cfg(not(target_thread_local))]
181 static __KEY: $crate::thread::__OsLocalKeyInner<$t> =
182 $crate::thread::__OsLocalKeyInner::new();
187 $crate::thread::LocalKey::new(__getit, __init)
191 /// Indicator of the state of a thread local storage key.
192 #[unstable(feature = "thread_local_state",
193 reason = "state querying was recently added",
195 #[derive(Debug, Eq, PartialEq, Copy, Clone)]
196 pub enum LocalKeyState {
197 /// All keys are in this state whenever a thread starts. Keys will
198 /// transition to the `Valid` state once the first call to `with` happens
199 /// and the initialization expression succeeds.
201 /// Keys in the `Uninitialized` state will yield a reference to the closure
202 /// passed to `with` so long as the initialization routine does not panic.
205 /// Once a key has been accessed successfully, it will enter the `Valid`
206 /// state. Keys in the `Valid` state will remain so until the thread exits,
207 /// at which point the destructor will be run and the key will enter the
208 /// `Destroyed` state.
210 /// Keys in the `Valid` state will be guaranteed to yield a reference to the
211 /// closure passed to `with`.
214 /// When a thread exits, the destructors for keys will be run (if
215 /// necessary). While a destructor is running, and possibly after a
216 /// destructor has run, a key is in the `Destroyed` state.
218 /// Keys in the `Destroyed` states will trigger a panic when accessed via
223 impl<T: 'static> LocalKey<T> {
225 #[unstable(feature = "thread_local_internals",
226 reason = "recently added to create a key",
228 pub const fn new(inner: fn() -> Option<&'static UnsafeCell<Option<T>>>,
229 init: fn() -> T) -> LocalKey<T> {
236 /// Acquires a reference to the value in this TLS key.
238 /// This will lazily initialize the value if this thread has not referenced
243 /// This function will `panic!()` if the key currently has its
244 /// destructor running, and it **may** panic if the destructor has
245 /// previously been run for this thread.
246 #[stable(feature = "rust1", since = "1.0.0")]
247 pub fn with<F, R>(&'static self, f: F) -> R
248 where F: FnOnce(&T) -> R {
250 let slot = (self.inner)();
251 let slot = slot.expect("cannot access a TLS value during or \
252 after it is destroyed");
253 f(match *slot.get() {
254 Some(ref inner) => inner,
255 None => self.init(slot),
260 unsafe fn init(&self, slot: &UnsafeCell<Option<T>>) -> &T {
261 // Execute the initialization up front, *then* move it into our slot,
262 // just in case initialization fails.
263 let value = (self.init)();
264 let ptr = slot.get();
266 // note that this can in theory just be `*ptr = Some(value)`, but due to
267 // the compiler will currently codegen that pattern with something like:
269 // ptr::drop_in_place(ptr)
270 // ptr::write(ptr, Some(value))
272 // Due to this pattern it's possible for the destructor of the value in
273 // `ptr` (e.g. if this is being recursively initialized) to re-access
274 // TLS, in which case there will be a `&` and `&mut` pointer to the same
275 // value (an aliasing violation). To avoid setting the "I'm running a
276 // destructor" flag we just use `mem::replace` which should sequence the
277 // operations a little differently and make this safe to call.
278 mem::replace(&mut *ptr, Some(value));
280 (*ptr).as_ref().unwrap()
283 /// Query the current state of this key.
285 /// A key is initially in the `Uninitialized` state whenever a thread
286 /// starts. It will remain in this state up until the first call to `with`
287 /// within a thread has run the initialization expression successfully.
289 /// Once the initialization expression succeeds, the key transitions to the
290 /// `Valid` state which will guarantee that future calls to `with` will
291 /// succeed within the thread.
293 /// When a thread exits, each key will be destroyed in turn, and as keys are
294 /// destroyed they will enter the `Destroyed` state just before the
295 /// destructor starts to run. Keys may remain in the `Destroyed` state after
296 /// destruction has completed. Keys without destructors (e.g. with types
297 /// that are `Copy`), may never enter the `Destroyed` state.
299 /// Keys in the `Uninitialized` state can be accessed so long as the
300 /// initialization does not panic. Keys in the `Valid` state are guaranteed
301 /// to be able to be accessed. Keys in the `Destroyed` state will panic on
302 /// any call to `with`.
303 #[unstable(feature = "thread_local_state",
304 reason = "state querying was recently added",
306 pub fn state(&'static self) -> LocalKeyState {
308 match (self.inner)() {
311 Some(..) => LocalKeyState::Valid,
312 None => LocalKeyState::Uninitialized,
315 None => LocalKeyState::Destroyed,
323 use cell::{Cell, UnsafeCell};
327 use sys_common::thread_local::StaticKey as OsStaticKey;
330 // OS-TLS key that we'll use to key off.
332 marker: marker::PhantomData<Cell<T>>,
335 impl<T> fmt::Debug for Key<T> {
336 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
341 unsafe impl<T> ::marker::Sync for Key<T> { }
343 struct Value<T: 'static> {
344 key: &'static Key<T>,
345 value: UnsafeCell<Option<T>>,
348 impl<T: 'static> Key<T> {
349 pub const fn new() -> Key<T> {
351 os: OsStaticKey::new(Some(destroy_value::<T>)),
352 marker: marker::PhantomData
356 pub fn get(&'static self) -> Option<&'static UnsafeCell<Option<T>>> {
358 let ptr = self.os.get() as *mut Value<T>;
360 if ptr as usize == 1 {
363 return Some(&(*ptr).value);
366 // If the lookup returned null, we haven't initialized our own local
367 // copy, so do that now.
368 let ptr: Box<Value<T>> = box Value {
370 value: UnsafeCell::new(None),
372 let ptr = Box::into_raw(ptr);
373 self.os.set(ptr as *mut u8);
379 pub unsafe extern fn destroy_value<T: 'static>(ptr: *mut u8) {
380 // The OS TLS ensures that this key contains a NULL value when this
381 // destructor starts to run. We set it back to a sentinel value of 1 to
382 // ensure that any future calls to `get` for this thread will return
385 // Note that to prevent an infinite loop we reset it back to null right
386 // before we return from the destructor ourselves.
387 let ptr = Box::from_raw(ptr as *mut Value<T>);
389 key.os.set(1 as *mut u8);
391 key.os.set(ptr::null_mut());
395 #[cfg(all(test, not(target_os = "emscripten")))]
397 use sync::mpsc::{channel, Sender};
398 use cell::{Cell, UnsafeCell};
399 use super::LocalKeyState;
402 struct Foo(Sender<()>);
406 let Foo(ref s) = *self;
413 thread_local!(static FOO: Cell<i32> = Cell::new(1));
416 assert_eq!(f.get(), 1);
419 let (tx, rx) = channel();
420 let _t = thread::spawn(move|| {
422 assert_eq!(f.get(), 1);
424 tx.send(()).unwrap();
429 assert_eq!(f.get(), 2);
438 assert!(FOO.state() == LocalKeyState::Destroyed);
442 assert!(FOO.state() == LocalKeyState::Uninitialized);
445 thread_local!(static FOO: Foo = foo());
448 assert!(FOO.state() == LocalKeyState::Uninitialized);
450 assert!(FOO.state() == LocalKeyState::Valid);
452 assert!(FOO.state() == LocalKeyState::Valid);
453 }).join().ok().unwrap();
458 thread_local!(static FOO: UnsafeCell<Option<Foo>> = UnsafeCell::new(None));
460 let (tx, rx) = channel();
461 let _t = thread::spawn(move|| unsafe {
462 let mut tx = Some(tx);
464 *f.get() = Some(Foo(tx.take().unwrap()));
474 thread_local!(static K1: UnsafeCell<Option<S1>> = UnsafeCell::new(None));
475 thread_local!(static K2: UnsafeCell<Option<S2>> = UnsafeCell::new(None));
476 static mut HITS: u32 = 0;
482 if K2.state() == LocalKeyState::Destroyed {
486 K2.with(|s| *s.get() = Some(S2));
498 assert!(K1.state() != LocalKeyState::Destroyed);
500 K1.with(|s| *s.get() = Some(S1));
505 thread::spawn(move|| {
507 }).join().ok().unwrap();
511 fn self_referential() {
513 thread_local!(static K1: UnsafeCell<Option<S1>> = UnsafeCell::new(None));
517 assert!(K1.state() == LocalKeyState::Destroyed);
521 thread::spawn(move|| unsafe {
522 K1.with(|s| *s.get() = Some(S1));
523 }).join().ok().unwrap();
526 // Note that this test will deadlock if TLS destructors aren't run (this
527 // requires the destructor to be run to pass the test). OSX has a known bug
528 // where dtors-in-dtors may cancel other destructors, so we just ignore this
531 #[cfg_attr(target_os = "macos", ignore)]
532 fn dtors_in_dtors_in_dtors() {
533 struct S1(Sender<()>);
534 thread_local!(static K1: UnsafeCell<Option<S1>> = UnsafeCell::new(None));
535 thread_local!(static K2: UnsafeCell<Option<Foo>> = UnsafeCell::new(None));
539 let S1(ref tx) = *self;
541 if K2.state() != LocalKeyState::Destroyed {
542 K2.with(|s| *s.get() = Some(Foo(tx.clone())));
548 let (tx, rx) = channel();
549 let _t = thread::spawn(move|| unsafe {
550 let mut tx = Some(tx);
551 K1.with(|s| *s.get() = Some(S1(tx.take().unwrap())));
560 use collections::HashMap;
564 fn square(i: i32) -> i32 { i * i }
565 thread_local!(static FOO: i32 = square(3));
574 fn map() -> RefCell<HashMap<i32, i32>> {
575 let mut m = HashMap::new();
579 thread_local!(static FOO: RefCell<HashMap<i32, i32>> = map());
582 assert_eq!(map.borrow()[&1], 2);
588 thread_local!(static FOO: RefCell<Vec<u32>> = RefCell::new(vec![1, 2, 3]));
591 assert_eq!(vec.borrow().len(), 3);
592 vec.borrow_mut().push(4);
593 assert_eq!(vec.borrow()[3], 4);