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 macOS, 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.
81 /// [`with`]: ../../std/thread/struct.LocalKey.html#method.with
82 /// [`thread_local!`]: ../../std/macro.thread_local.html
83 /// [`Drop`]: ../../std/ops/trait.Drop.html
84 #[stable(feature = "rust1", since = "1.0.0")]
85 pub struct LocalKey<T: 'static> {
86 // This outer `LocalKey<T>` type is what's going to be stored in statics,
87 // but actual data inside will sometimes be tagged with #[thread_local].
88 // It's not valid for a true static to reference a #[thread_local] static,
89 // so we get around that by exposing an accessor through a layer of function
90 // indirection (this thunk).
92 // Note that the thunk is itself unsafe because the returned lifetime of the
93 // slot where data lives, `'static`, is not actually valid. The lifetime
94 // here is actually `'thread`!
96 // Although this is an extra layer of indirection, it should in theory be
97 // trivially devirtualizable by LLVM because the value of `inner` never
98 // changes and the constant should be readonly within a crate. This mainly
99 // only runs into problems when TLS statics are exported across crates.
100 inner: fn() -> Option<&'static UnsafeCell<Option<T>>>,
102 // initialization routine to invoke to create a value
106 #[stable(feature = "std_debug", since = "1.16.0")]
107 impl<T: 'static> fmt::Debug for LocalKey<T> {
108 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
109 f.pad("LocalKey { .. }")
113 /// Declare a new thread local storage key of type [`std::thread::LocalKey`].
117 /// The macro wraps any number of static declarations and makes them thread local.
118 /// Each static may be public or private, and attributes are allowed. Example:
121 /// use std::cell::RefCell;
123 /// pub static FOO: RefCell<u32> = RefCell::new(1);
126 /// static BAR: RefCell<f32> = RefCell::new(1.0);
131 /// See [LocalKey documentation][`std::thread::LocalKey`] for more
134 /// [`std::thread::LocalKey`]: ../std/thread/struct.LocalKey.html
136 #[stable(feature = "rust1", since = "1.0.0")]
137 #[allow_internal_unstable]
138 macro_rules! thread_local {
139 // rule 0: empty (base case for the recursion)
142 // rule 1: process multiple declarations where the first one is private
143 ($(#[$attr:meta])* static $name:ident: $t:ty = $init:expr; $($rest:tt)*) => (
144 thread_local!($(#[$attr])* static $name: $t = $init); // go to rule 2
145 thread_local!($($rest)*);
148 // rule 2: handle a single private declaration
149 ($(#[$attr:meta])* static $name:ident: $t:ty = $init:expr) => (
150 $(#[$attr])* static $name: $crate::thread::LocalKey<$t> =
151 __thread_local_inner!($t, $init);
154 // rule 3: handle multiple declarations where the first one is public
155 ($(#[$attr:meta])* pub static $name:ident: $t:ty = $init:expr; $($rest:tt)*) => (
156 thread_local!($(#[$attr])* pub static $name: $t = $init); // go to rule 4
157 thread_local!($($rest)*);
160 // rule 4: handle a single public declaration
161 ($(#[$attr:meta])* pub static $name:ident: $t:ty = $init:expr) => (
162 $(#[$attr])* pub static $name: $crate::thread::LocalKey<$t> =
163 __thread_local_inner!($t, $init);
168 #[unstable(feature = "thread_local_internals",
169 reason = "should not be necessary",
172 #[allow_internal_unstable]
173 macro_rules! __thread_local_inner {
174 ($t:ty, $init:expr) => {{
175 fn __init() -> $t { $init }
177 fn __getit() -> $crate::option::Option<
178 &'static $crate::cell::UnsafeCell<
179 $crate::option::Option<$t>>>
182 #[cfg(target_thread_local)]
183 static __KEY: $crate::thread::__FastLocalKeyInner<$t> =
184 $crate::thread::__FastLocalKeyInner::new();
186 #[cfg(not(target_thread_local))]
187 static __KEY: $crate::thread::__OsLocalKeyInner<$t> =
188 $crate::thread::__OsLocalKeyInner::new();
193 $crate::thread::LocalKey::new(__getit, __init)
197 /// Indicator of the state of a thread local storage key.
198 #[unstable(feature = "thread_local_state",
199 reason = "state querying was recently added",
201 #[derive(Debug, Eq, PartialEq, Copy, Clone)]
202 pub enum LocalKeyState {
203 /// All keys are in this state whenever a thread starts. Keys will
204 /// transition to the `Valid` state once the first call to [`with`] happens
205 /// and the initialization expression succeeds.
207 /// Keys in the `Uninitialized` state will yield a reference to the closure
208 /// passed to [`with`] so long as the initialization routine does not panic.
210 /// [`with`]: ../../std/thread/struct.LocalKey.html#method.with
213 /// Once a key has been accessed successfully, it will enter the `Valid`
214 /// state. Keys in the `Valid` state will remain so until the thread exits,
215 /// at which point the destructor will be run and the key will enter the
216 /// `Destroyed` state.
218 /// Keys in the `Valid` state will be guaranteed to yield a reference to the
219 /// closure passed to [`with`].
221 /// [`with`]: ../../std/thread/struct.LocalKey.html#method.with
224 /// When a thread exits, the destructors for keys will be run (if
225 /// necessary). While a destructor is running, and possibly after a
226 /// destructor has run, a key is in the `Destroyed` state.
228 /// Keys in the `Destroyed` states will trigger a panic when accessed via
231 /// [`with`]: ../../std/thread/struct.LocalKey.html#method.with
235 impl<T: 'static> LocalKey<T> {
237 #[unstable(feature = "thread_local_internals",
238 reason = "recently added to create a key",
240 pub const fn new(inner: fn() -> Option<&'static UnsafeCell<Option<T>>>,
241 init: fn() -> T) -> LocalKey<T> {
248 /// Acquires a reference to the value in this TLS key.
250 /// This will lazily initialize the value if this thread has not referenced
255 /// This function will `panic!()` if the key currently has its
256 /// destructor running, and it **may** panic if the destructor has
257 /// previously been run for this thread.
258 #[stable(feature = "rust1", since = "1.0.0")]
259 pub fn with<F, R>(&'static self, f: F) -> R
260 where F: FnOnce(&T) -> R {
262 let slot = (self.inner)();
263 let slot = slot.expect("cannot access a TLS value during or \
264 after it is destroyed");
265 f(match *slot.get() {
266 Some(ref inner) => inner,
267 None => self.init(slot),
272 unsafe fn init(&self, slot: &UnsafeCell<Option<T>>) -> &T {
273 // Execute the initialization up front, *then* move it into our slot,
274 // just in case initialization fails.
275 let value = (self.init)();
276 let ptr = slot.get();
278 // note that this can in theory just be `*ptr = Some(value)`, but due to
279 // the compiler will currently codegen that pattern with something like:
281 // ptr::drop_in_place(ptr)
282 // ptr::write(ptr, Some(value))
284 // Due to this pattern it's possible for the destructor of the value in
285 // `ptr` (e.g. if this is being recursively initialized) to re-access
286 // TLS, in which case there will be a `&` and `&mut` pointer to the same
287 // value (an aliasing violation). To avoid setting the "I'm running a
288 // destructor" flag we just use `mem::replace` which should sequence the
289 // operations a little differently and make this safe to call.
290 mem::replace(&mut *ptr, Some(value));
292 (*ptr).as_ref().unwrap()
295 /// Query the current state of this key.
297 /// A key is initially in the `Uninitialized` state whenever a thread
298 /// starts. It will remain in this state up until the first call to [`with`]
299 /// within a thread has run the initialization expression successfully.
301 /// Once the initialization expression succeeds, the key transitions to the
302 /// `Valid` state which will guarantee that future calls to [`with`] will
303 /// succeed within the thread.
305 /// When a thread exits, each key will be destroyed in turn, and as keys are
306 /// destroyed they will enter the `Destroyed` state just before the
307 /// destructor starts to run. Keys may remain in the `Destroyed` state after
308 /// destruction has completed. Keys without destructors (e.g. with types
309 /// that are [`Copy`]), may never enter the `Destroyed` state.
311 /// Keys in the `Uninitialized` state can be accessed so long as the
312 /// initialization does not panic. Keys in the `Valid` state are guaranteed
313 /// to be able to be accessed. Keys in the `Destroyed` state will panic on
314 /// any call to [`with`].
316 /// [`with`]: ../../std/thread/struct.LocalKey.html#method.with
317 /// [`Copy`]: ../../std/marker/trait.Copy.html
318 #[unstable(feature = "thread_local_state",
319 reason = "state querying was recently added",
321 pub fn state(&'static self) -> LocalKeyState {
323 match (self.inner)() {
326 Some(..) => LocalKeyState::Valid,
327 None => LocalKeyState::Uninitialized,
330 None => LocalKeyState::Destroyed,
337 #[cfg(target_thread_local)]
339 use cell::{Cell, UnsafeCell};
343 use sys::fast_thread_local::{register_dtor, requires_move_before_drop};
346 inner: UnsafeCell<Option<T>>,
348 // Metadata to keep track of the state of the destructor. Remember that
349 // these variables are thread-local, not global.
350 dtor_registered: Cell<bool>,
351 dtor_running: Cell<bool>,
354 impl<T> fmt::Debug for Key<T> {
355 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
360 unsafe impl<T> ::marker::Sync for Key<T> { }
363 pub const fn new() -> Key<T> {
365 inner: UnsafeCell::new(None),
366 dtor_registered: Cell::new(false),
367 dtor_running: Cell::new(false)
371 pub fn get(&'static self) -> Option<&'static UnsafeCell<Option<T>>> {
373 if mem::needs_drop::<T>() && self.dtor_running.get() {
376 self.register_dtor();
381 unsafe fn register_dtor(&self) {
382 if !mem::needs_drop::<T>() || self.dtor_registered.get() {
386 register_dtor(self as *const _ as *mut u8,
388 self.dtor_registered.set(true);
392 unsafe extern fn destroy_value<T>(ptr: *mut u8) {
393 let ptr = ptr as *mut Key<T>;
394 // Right before we run the user destructor be sure to flag the
395 // destructor as running for this thread so calls to `get` will return
397 (*ptr).dtor_running.set(true);
399 // Some implementations may require us to move the value before we drop
400 // it as it could get re-initialized in-place during destruction.
402 // Hence, we use `ptr::read` on those platforms (to move to a "safe"
403 // location) instead of drop_in_place.
404 if requires_move_before_drop() {
405 ptr::read((*ptr).inner.get());
407 ptr::drop_in_place((*ptr).inner.get());
414 use cell::{Cell, UnsafeCell};
418 use sys_common::thread_local::StaticKey as OsStaticKey;
421 // OS-TLS key that we'll use to key off.
423 marker: marker::PhantomData<Cell<T>>,
426 impl<T> fmt::Debug for Key<T> {
427 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
432 unsafe impl<T> ::marker::Sync for Key<T> { }
434 struct Value<T: 'static> {
435 key: &'static Key<T>,
436 value: UnsafeCell<Option<T>>,
439 impl<T: 'static> Key<T> {
440 pub const fn new() -> Key<T> {
442 os: OsStaticKey::new(Some(destroy_value::<T>)),
443 marker: marker::PhantomData
447 pub fn get(&'static self) -> Option<&'static UnsafeCell<Option<T>>> {
449 let ptr = self.os.get() as *mut Value<T>;
451 if ptr as usize == 1 {
454 return Some(&(*ptr).value);
457 // If the lookup returned null, we haven't initialized our own
458 // local copy, so do that now.
459 let ptr: Box<Value<T>> = box Value {
461 value: UnsafeCell::new(None),
463 let ptr = Box::into_raw(ptr);
464 self.os.set(ptr as *mut u8);
470 unsafe extern fn destroy_value<T: 'static>(ptr: *mut u8) {
471 // The OS TLS ensures that this key contains a NULL value when this
472 // destructor starts to run. We set it back to a sentinel value of 1 to
473 // ensure that any future calls to `get` for this thread will return
476 // Note that to prevent an infinite loop we reset it back to null right
477 // before we return from the destructor ourselves.
478 let ptr = Box::from_raw(ptr as *mut Value<T>);
480 key.os.set(1 as *mut u8);
482 key.os.set(ptr::null_mut());
486 #[cfg(all(test, not(target_os = "emscripten")))]
488 use sync::mpsc::{channel, Sender};
489 use cell::{Cell, UnsafeCell};
490 use super::LocalKeyState;
493 struct Foo(Sender<()>);
497 let Foo(ref s) = *self;
504 thread_local!(static FOO: Cell<i32> = Cell::new(1));
507 assert_eq!(f.get(), 1);
510 let (tx, rx) = channel();
511 let _t = thread::spawn(move|| {
513 assert_eq!(f.get(), 1);
515 tx.send(()).unwrap();
520 assert_eq!(f.get(), 2);
529 assert!(FOO.state() == LocalKeyState::Destroyed);
533 assert!(FOO.state() == LocalKeyState::Uninitialized);
536 thread_local!(static FOO: Foo = foo());
539 assert!(FOO.state() == LocalKeyState::Uninitialized);
541 assert!(FOO.state() == LocalKeyState::Valid);
543 assert!(FOO.state() == LocalKeyState::Valid);
544 }).join().ok().unwrap();
549 thread_local!(static FOO: UnsafeCell<Option<Foo>> = UnsafeCell::new(None));
551 let (tx, rx) = channel();
552 let _t = thread::spawn(move|| unsafe {
553 let mut tx = Some(tx);
555 *f.get() = Some(Foo(tx.take().unwrap()));
565 thread_local!(static K1: UnsafeCell<Option<S1>> = UnsafeCell::new(None));
566 thread_local!(static K2: UnsafeCell<Option<S2>> = UnsafeCell::new(None));
567 static mut HITS: u32 = 0;
573 if K2.state() == LocalKeyState::Destroyed {
577 K2.with(|s| *s.get() = Some(S2));
589 assert!(K1.state() != LocalKeyState::Destroyed);
591 K1.with(|s| *s.get() = Some(S1));
596 thread::spawn(move|| {
598 }).join().ok().unwrap();
602 fn self_referential() {
604 thread_local!(static K1: UnsafeCell<Option<S1>> = UnsafeCell::new(None));
608 assert!(K1.state() == LocalKeyState::Destroyed);
612 thread::spawn(move|| unsafe {
613 K1.with(|s| *s.get() = Some(S1));
614 }).join().ok().unwrap();
617 // Note that this test will deadlock if TLS destructors aren't run (this
618 // requires the destructor to be run to pass the test). macOS has a known bug
619 // where dtors-in-dtors may cancel other destructors, so we just ignore this
622 #[cfg_attr(target_os = "macos", ignore)]
623 fn dtors_in_dtors_in_dtors() {
624 struct S1(Sender<()>);
625 thread_local!(static K1: UnsafeCell<Option<S1>> = UnsafeCell::new(None));
626 thread_local!(static K2: UnsafeCell<Option<Foo>> = UnsafeCell::new(None));
630 let S1(ref tx) = *self;
632 if K2.state() != LocalKeyState::Destroyed {
633 K2.with(|s| *s.get() = Some(Foo(tx.clone())));
639 let (tx, rx) = channel();
640 let _t = thread::spawn(move|| unsafe {
641 let mut tx = Some(tx);
642 K1.with(|s| *s.get() = Some(S1(tx.take().unwrap())));
651 use collections::HashMap;
655 fn square(i: i32) -> i32 { i * i }
656 thread_local!(static FOO: i32 = square(3));
665 fn map() -> RefCell<HashMap<i32, i32>> {
666 let mut m = HashMap::new();
670 thread_local!(static FOO: RefCell<HashMap<i32, i32>> = map());
673 assert_eq!(map.borrow()[&1], 2);
679 thread_local!(static FOO: RefCell<Vec<u32>> = RefCell::new(vec![1, 2, 3]));
682 assert_eq!(vec.borrow().len(), 3);
683 vec.borrow_mut().push(4);
684 assert_eq!(vec.borrow()[3], 4);