1//! Thread local storage
2
3#![unstable(feature = "thread_local_internals", issue = "none")]
4
5use crate::cell::{Cell, RefCell};
6use crate::error::Error;
7use crate::fmt;
8
9/// A thread local storage (TLS) key which owns its contents.
10///
11/// This key uses the fastest possible implementation available to it for the
12/// target platform. It is instantiated with the [`thread_local!`] macro and the
13/// primary method is the [`with`] method, though there are helpers to make
14/// working with [`Cell`] types easier.
15///
16/// The [`with`] method yields a reference to the contained value which cannot
17/// outlive the current thread or escape the given closure.
18///
19/// [`thread_local!`]: crate::thread_local
20///
21/// # Initialization and Destruction
22///
23/// Initialization is dynamically performed on the first call to a setter (e.g.
24/// [`with`]) within a thread, and values that implement [`Drop`] get
25/// destructed when a thread exits. Some platform-specific caveats apply, which
26/// are explained below.
27/// Note that, should the destructor panics, the whole process will be [aborted].
28///
29/// A `LocalKey`'s initializer cannot recursively depend on itself. Using a
30/// `LocalKey` in this way may cause panics, aborts or infinite recursion on
31/// the first call to `with`.
32///
33/// [aborted]: crate::process::abort
34///
35/// # Single-thread Synchronization
36///
37/// Though there is no potential race with other threads, it is still possible to
38/// obtain multiple references to the thread-local data in different places on
39/// the call stack. For this reason, only shared (`&T`) references may be obtained.
40///
41/// To allow obtaining an exclusive mutable reference (`&mut T`), typically a
42/// [`Cell`] or [`RefCell`] is used (see the [`std::cell`] for more information
43/// on how exactly this works). To make this easier there are specialized
44/// implementations for [`LocalKey<Cell<T>>`] and [`LocalKey<RefCell<T>>`].
45///
46/// [`std::cell`]: `crate::cell`
47/// [`LocalKey<Cell<T>>`]: struct.LocalKey.html#impl-LocalKey<Cell<T>>
48/// [`LocalKey<RefCell<T>>`]: struct.LocalKey.html#impl-LocalKey<RefCell<T>>
49///
50///
51/// # Examples
52///
53/// ```
54/// use std::cell::Cell;
55/// use std::thread;
56///
57/// // explicit `const {}` block enables more efficient initialization
58/// thread_local!(static FOO: Cell<u32> = const { Cell::new(1) });
59///
60/// assert_eq!(FOO.get(), 1);
61/// FOO.set(2);
62///
63/// // each thread starts out with the initial value of 1
64/// let t = thread::spawn(move || {
65/// assert_eq!(FOO.get(), 1);
66/// FOO.set(3);
67/// });
68///
69/// // wait for the thread to complete and bail out on panic
70/// t.join().unwrap();
71///
72/// // we retain our original value of 2 despite the child thread
73/// assert_eq!(FOO.get(), 2);
74/// ```
75///
76/// # Platform-specific behavior
77///
78/// Note that a "best effort" is made to ensure that destructors for types
79/// stored in thread local storage are run, but not all platforms can guarantee
80/// that destructors will be run for all types in thread local storage. For
81/// example, there are a number of known caveats where destructors are not run:
82///
83/// 1. On Unix systems when pthread-based TLS is being used, destructors will
84/// not be run for TLS values on the main thread when it exits. Note that the
85/// application will exit immediately after the main thread exits as well.
86/// 2. On all platforms it's possible for TLS to re-initialize other TLS slots
87/// during destruction. Some platforms ensure that this cannot happen
88/// infinitely by preventing re-initialization of any slot that has been
89/// destroyed, but not all platforms have this guard. Those platforms that do
90/// not guard typically have a synthetic limit after which point no more
91/// destructors are run.
92/// 3. When the process exits on Windows systems, TLS destructors may only be
93/// run on the thread that causes the process to exit. This is because the
94/// other threads may be forcibly terminated.
95///
96/// ## Synchronization in thread-local destructors
97///
98/// On Windows, synchronization operations (such as [`JoinHandle::join`]) in
99/// thread local destructors are prone to deadlocks and so should be avoided.
100/// This is because the [loader lock] is held while a destructor is run. The
101/// lock is acquired whenever a thread starts or exits or when a DLL is loaded
102/// or unloaded. Therefore these events are blocked for as long as a thread
103/// local destructor is running.
104///
105/// [loader lock]: https://docs.microsoft.com/en-us/windows/win32/dlls/dynamic-link-library-best-practices
106/// [`JoinHandle::join`]: crate::thread::JoinHandle::join
107/// [`with`]: LocalKey::with
108#[cfg_attr(not(test), rustc_diagnostic_item = "LocalKey")]
109#[stable(feature = "rust1", since = "1.0.0")]
110pub struct LocalKey<T: 'static> {
111 // This outer `LocalKey<T>` type is what's going to be stored in statics,
112 // but actual data inside will sometimes be tagged with #[thread_local].
113 // It's not valid for a true static to reference a #[thread_local] static,
114 // so we get around that by exposing an accessor through a layer of function
115 // indirection (this thunk).
116 //
117 // Note that the thunk is itself unsafe because the returned lifetime of the
118 // slot where data lives, `'static`, is not actually valid. The lifetime
119 // here is actually slightly shorter than the currently running thread!
120 //
121 // Although this is an extra layer of indirection, it should in theory be
122 // trivially devirtualizable by LLVM because the value of `inner` never
123 // changes and the constant should be readonly within a crate. This mainly
124 // only runs into problems when TLS statics are exported across crates.
125 inner: fn(Option<&mut Option<T>>) -> *const T,
126}
127
128#[stable(feature = "std_debug", since = "1.16.0")]
129impl<T: 'static> fmt::Debug for LocalKey<T> {
130 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
131 f.debug_struct(name:"LocalKey").finish_non_exhaustive()
132 }
133}
134
135/// Declare a new thread local storage key of type [`std::thread::LocalKey`].
136///
137/// # Syntax
138///
139/// The macro wraps any number of static declarations and makes them thread local.
140/// Publicity and attributes for each static are allowed. Example:
141///
142/// ```
143/// use std::cell::{Cell, RefCell};
144///
145/// thread_local! {
146/// pub static FOO: Cell<u32> = const { Cell::new(1) };
147///
148/// static BAR: RefCell<Vec<f32>> = RefCell::new(vec![1.0, 2.0]);
149/// }
150///
151/// assert_eq!(FOO.get(), 1);
152/// BAR.with_borrow(|v| assert_eq!(v[1], 2.0));
153/// ```
154///
155/// Note that only shared references (`&T`) to the inner data may be obtained, so a
156/// type such as [`Cell`] or [`RefCell`] is typically used to allow mutating access.
157///
158/// This macro supports a special `const {}` syntax that can be used
159/// when the initialization expression can be evaluated as a constant.
160/// This can enable a more efficient thread local implementation that
161/// can avoid lazy initialization. For types that do not
162/// [need to be dropped][crate::mem::needs_drop], this can enable an
163/// even more efficient implementation that does not need to
164/// track any additional state.
165///
166/// ```
167/// use std::cell::RefCell;
168///
169/// thread_local! {
170/// pub static FOO: RefCell<Vec<u32>> = const { RefCell::new(Vec::new()) };
171/// }
172///
173/// FOO.with_borrow(|v| assert_eq!(v.len(), 0));
174/// ```
175///
176/// See [`LocalKey` documentation][`std::thread::LocalKey`] for more
177/// information.
178///
179/// [`std::thread::LocalKey`]: crate::thread::LocalKey
180#[macro_export]
181#[stable(feature = "rust1", since = "1.0.0")]
182#[cfg_attr(not(test), rustc_diagnostic_item = "thread_local_macro")]
183#[allow_internal_unstable(thread_local_internals)]
184macro_rules! thread_local {
185 // empty (base case for the recursion)
186 () => {};
187
188 ($(#[$attr:meta])* $vis:vis static $name:ident: $t:ty = const $init:block; $($rest:tt)*) => (
189 $crate::thread::local_impl::thread_local_inner!($(#[$attr])* $vis $name, $t, const $init);
190 $crate::thread_local!($($rest)*);
191 );
192
193 ($(#[$attr:meta])* $vis:vis static $name:ident: $t:ty = const $init:block) => (
194 $crate::thread::local_impl::thread_local_inner!($(#[$attr])* $vis $name, $t, const $init);
195 );
196
197 // process multiple declarations
198 ($(#[$attr:meta])* $vis:vis static $name:ident: $t:ty = $init:expr; $($rest:tt)*) => (
199 $crate::thread::local_impl::thread_local_inner!($(#[$attr])* $vis $name, $t, $init);
200 $crate::thread_local!($($rest)*);
201 );
202
203 // handle a single declaration
204 ($(#[$attr:meta])* $vis:vis static $name:ident: $t:ty = $init:expr) => (
205 $crate::thread::local_impl::thread_local_inner!($(#[$attr])* $vis $name, $t, $init);
206 );
207}
208
209/// An error returned by [`LocalKey::try_with`](struct.LocalKey.html#method.try_with).
210#[stable(feature = "thread_local_try_with", since = "1.26.0")]
211#[non_exhaustive]
212#[derive(Clone, Copy, Eq, PartialEq)]
213pub struct AccessError;
214
215#[stable(feature = "thread_local_try_with", since = "1.26.0")]
216impl fmt::Debug for AccessError {
217 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
218 f.debug_struct(name:"AccessError").finish()
219 }
220}
221
222#[stable(feature = "thread_local_try_with", since = "1.26.0")]
223impl fmt::Display for AccessError {
224 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
225 fmt::Display::fmt(self:"already destroyed", f)
226 }
227}
228
229#[stable(feature = "thread_local_try_with", since = "1.26.0")]
230impl Error for AccessError {}
231
232// This ensures the panicking code is outlined from `with` for `LocalKey`.
233#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
234#[track_caller]
235#[cold]
236fn panic_access_error(err: AccessError) -> ! {
237 panic!("cannot access a Thread Local Storage value during or after destruction: {err:?}")
238}
239
240impl<T: 'static> LocalKey<T> {
241 #[doc(hidden)]
242 #[unstable(
243 feature = "thread_local_internals",
244 reason = "recently added to create a key",
245 issue = "none"
246 )]
247 pub const unsafe fn new(inner: fn(Option<&mut Option<T>>) -> *const T) -> LocalKey<T> {
248 LocalKey { inner }
249 }
250
251 /// Acquires a reference to the value in this TLS key.
252 ///
253 /// This will lazily initialize the value if this thread has not referenced
254 /// this key yet.
255 ///
256 /// # Panics
257 ///
258 /// This function will `panic!()` if the key currently has its
259 /// destructor running, and it **may** panic if the destructor has
260 /// previously been run for this thread.
261 ///
262 /// # Examples
263 ///
264 /// ```
265 /// thread_local! {
266 /// pub static STATIC: String = String::from("I am");
267 /// }
268 ///
269 /// assert_eq!(
270 /// STATIC.with(|original_value| format!("{original_value} initialized")),
271 /// "I am initialized",
272 /// );
273 /// ```
274 #[stable(feature = "rust1", since = "1.0.0")]
275 pub fn with<F, R>(&'static self, f: F) -> R
276 where
277 F: FnOnce(&T) -> R,
278 {
279 match self.try_with(f) {
280 Ok(r) => r,
281 Err(err) => panic_access_error(err),
282 }
283 }
284
285 /// Acquires a reference to the value in this TLS key.
286 ///
287 /// This will lazily initialize the value if this thread has not referenced
288 /// this key yet. If the key has been destroyed (which may happen if this is called
289 /// in a destructor), this function will return an [`AccessError`].
290 ///
291 /// # Panics
292 ///
293 /// This function will still `panic!()` if the key is uninitialized and the
294 /// key's initializer panics.
295 ///
296 /// # Examples
297 ///
298 /// ```
299 /// thread_local! {
300 /// pub static STATIC: String = String::from("I am");
301 /// }
302 ///
303 /// assert_eq!(
304 /// STATIC.try_with(|original_value| format!("{original_value} initialized")),
305 /// Ok(String::from("I am initialized")),
306 /// );
307 /// ```
308 #[stable(feature = "thread_local_try_with", since = "1.26.0")]
309 #[inline]
310 pub fn try_with<F, R>(&'static self, f: F) -> Result<R, AccessError>
311 where
312 F: FnOnce(&T) -> R,
313 {
314 let thread_local = unsafe { (self.inner)(None).as_ref().ok_or(AccessError)? };
315 Ok(f(thread_local))
316 }
317
318 /// Acquires a reference to the value in this TLS key, initializing it with
319 /// `init` if it wasn't already initialized on this thread.
320 ///
321 /// If `init` was used to initialize the thread local variable, `None` is
322 /// passed as the first argument to `f`. If it was already initialized,
323 /// `Some(init)` is passed to `f`.
324 ///
325 /// # Panics
326 ///
327 /// This function will panic if the key currently has its destructor
328 /// running, and it **may** panic if the destructor has previously been run
329 /// for this thread.
330 fn initialize_with<F, R>(&'static self, init: T, f: F) -> R
331 where
332 F: FnOnce(Option<T>, &T) -> R,
333 {
334 let mut init = Some(init);
335
336 let reference = unsafe {
337 match (self.inner)(Some(&mut init)).as_ref() {
338 Some(r) => r,
339 None => panic_access_error(AccessError),
340 }
341 };
342
343 f(init, reference)
344 }
345}
346
347impl<T: 'static> LocalKey<Cell<T>> {
348 /// Sets or initializes the contained value.
349 ///
350 /// Unlike the other methods, this will *not* run the lazy initializer of
351 /// the thread local. Instead, it will be directly initialized with the
352 /// given value if it wasn't initialized yet.
353 ///
354 /// # Panics
355 ///
356 /// Panics if the key currently has its destructor running,
357 /// and it **may** panic if the destructor has previously been run for this thread.
358 ///
359 /// # Examples
360 ///
361 /// ```
362 /// use std::cell::Cell;
363 ///
364 /// thread_local! {
365 /// static X: Cell<i32> = panic!("!");
366 /// }
367 ///
368 /// // Calling X.get() here would result in a panic.
369 ///
370 /// X.set(123); // But X.set() is fine, as it skips the initializer above.
371 ///
372 /// assert_eq!(X.get(), 123);
373 /// ```
374 #[stable(feature = "local_key_cell_methods", since = "1.73.0")]
375 pub fn set(&'static self, value: T) {
376 self.initialize_with(Cell::new(value), |value, cell| {
377 if let Some(value) = value {
378 // The cell was already initialized, so `value` wasn't used to
379 // initialize it. So we overwrite the current value with the
380 // new one instead.
381 cell.set(value.into_inner());
382 }
383 });
384 }
385
386 /// Returns a copy of the contained value.
387 ///
388 /// This will lazily initialize the value if this thread has not referenced
389 /// this key yet.
390 ///
391 /// # Panics
392 ///
393 /// Panics if the key currently has its destructor running,
394 /// and it **may** panic if the destructor has previously been run for this thread.
395 ///
396 /// # Examples
397 ///
398 /// ```
399 /// use std::cell::Cell;
400 ///
401 /// thread_local! {
402 /// static X: Cell<i32> = const { Cell::new(1) };
403 /// }
404 ///
405 /// assert_eq!(X.get(), 1);
406 /// ```
407 #[stable(feature = "local_key_cell_methods", since = "1.73.0")]
408 pub fn get(&'static self) -> T
409 where
410 T: Copy,
411 {
412 self.with(Cell::get)
413 }
414
415 /// Takes the contained value, leaving `Default::default()` in its place.
416 ///
417 /// This will lazily initialize the value if this thread has not referenced
418 /// this key yet.
419 ///
420 /// # Panics
421 ///
422 /// Panics if the key currently has its destructor running,
423 /// and it **may** panic if the destructor has previously been run for this thread.
424 ///
425 /// # Examples
426 ///
427 /// ```
428 /// use std::cell::Cell;
429 ///
430 /// thread_local! {
431 /// static X: Cell<Option<i32>> = const { Cell::new(Some(1)) };
432 /// }
433 ///
434 /// assert_eq!(X.take(), Some(1));
435 /// assert_eq!(X.take(), None);
436 /// ```
437 #[stable(feature = "local_key_cell_methods", since = "1.73.0")]
438 pub fn take(&'static self) -> T
439 where
440 T: Default,
441 {
442 self.with(Cell::take)
443 }
444
445 /// Replaces the contained value, returning the old value.
446 ///
447 /// This will lazily initialize the value if this thread has not referenced
448 /// this key yet.
449 ///
450 /// # Panics
451 ///
452 /// Panics if the key currently has its destructor running,
453 /// and it **may** panic if the destructor has previously been run for this thread.
454 ///
455 /// # Examples
456 ///
457 /// ```
458 /// use std::cell::Cell;
459 ///
460 /// thread_local! {
461 /// static X: Cell<i32> = const { Cell::new(1) };
462 /// }
463 ///
464 /// assert_eq!(X.replace(2), 1);
465 /// assert_eq!(X.replace(3), 2);
466 /// ```
467 #[stable(feature = "local_key_cell_methods", since = "1.73.0")]
468 #[rustc_confusables("swap")]
469 pub fn replace(&'static self, value: T) -> T {
470 self.with(|cell| cell.replace(value))
471 }
472}
473
474impl<T: 'static> LocalKey<RefCell<T>> {
475 /// Acquires a reference to the contained value.
476 ///
477 /// This will lazily initialize the value if this thread has not referenced
478 /// this key yet.
479 ///
480 /// # Panics
481 ///
482 /// Panics if the value is currently mutably borrowed.
483 ///
484 /// Panics if the key currently has its destructor running,
485 /// and it **may** panic if the destructor has previously been run for this thread.
486 ///
487 /// # Examples
488 ///
489 /// ```
490 /// use std::cell::RefCell;
491 ///
492 /// thread_local! {
493 /// static X: RefCell<Vec<i32>> = RefCell::new(Vec::new());
494 /// }
495 ///
496 /// X.with_borrow(|v| assert!(v.is_empty()));
497 /// ```
498 #[stable(feature = "local_key_cell_methods", since = "1.73.0")]
499 pub fn with_borrow<F, R>(&'static self, f: F) -> R
500 where
501 F: FnOnce(&T) -> R,
502 {
503 self.with(|cell| f(&cell.borrow()))
504 }
505
506 /// Acquires a mutable reference to the contained value.
507 ///
508 /// This will lazily initialize the value if this thread has not referenced
509 /// this key yet.
510 ///
511 /// # Panics
512 ///
513 /// Panics if the value is currently borrowed.
514 ///
515 /// Panics if the key currently has its destructor running,
516 /// and it **may** panic if the destructor has previously been run for this thread.
517 ///
518 /// # Examples
519 ///
520 /// ```
521 /// use std::cell::RefCell;
522 ///
523 /// thread_local! {
524 /// static X: RefCell<Vec<i32>> = RefCell::new(Vec::new());
525 /// }
526 ///
527 /// X.with_borrow_mut(|v| v.push(1));
528 ///
529 /// X.with_borrow(|v| assert_eq!(*v, vec![1]));
530 /// ```
531 #[stable(feature = "local_key_cell_methods", since = "1.73.0")]
532 pub fn with_borrow_mut<F, R>(&'static self, f: F) -> R
533 where
534 F: FnOnce(&mut T) -> R,
535 {
536 self.with(|cell| f(&mut cell.borrow_mut()))
537 }
538
539 /// Sets or initializes the contained value.
540 ///
541 /// Unlike the other methods, this will *not* run the lazy initializer of
542 /// the thread local. Instead, it will be directly initialized with the
543 /// given value if it wasn't initialized yet.
544 ///
545 /// # Panics
546 ///
547 /// Panics if the value is currently borrowed.
548 ///
549 /// Panics if the key currently has its destructor running,
550 /// and it **may** panic if the destructor has previously been run for this thread.
551 ///
552 /// # Examples
553 ///
554 /// ```
555 /// use std::cell::RefCell;
556 ///
557 /// thread_local! {
558 /// static X: RefCell<Vec<i32>> = panic!("!");
559 /// }
560 ///
561 /// // Calling X.with() here would result in a panic.
562 ///
563 /// X.set(vec![1, 2, 3]); // But X.set() is fine, as it skips the initializer above.
564 ///
565 /// X.with_borrow(|v| assert_eq!(*v, vec![1, 2, 3]));
566 /// ```
567 #[stable(feature = "local_key_cell_methods", since = "1.73.0")]
568 pub fn set(&'static self, value: T) {
569 self.initialize_with(RefCell::new(value), |value, cell| {
570 if let Some(value) = value {
571 // The cell was already initialized, so `value` wasn't used to
572 // initialize it. So we overwrite the current value with the
573 // new one instead.
574 *cell.borrow_mut() = value.into_inner();
575 }
576 });
577 }
578
579 /// Takes the contained value, leaving `Default::default()` in its place.
580 ///
581 /// This will lazily initialize the value if this thread has not referenced
582 /// this key yet.
583 ///
584 /// # Panics
585 ///
586 /// Panics if the value is currently borrowed.
587 ///
588 /// Panics if the key currently has its destructor running,
589 /// and it **may** panic if the destructor has previously been run for this thread.
590 ///
591 /// # Examples
592 ///
593 /// ```
594 /// use std::cell::RefCell;
595 ///
596 /// thread_local! {
597 /// static X: RefCell<Vec<i32>> = RefCell::new(Vec::new());
598 /// }
599 ///
600 /// X.with_borrow_mut(|v| v.push(1));
601 ///
602 /// let a = X.take();
603 ///
604 /// assert_eq!(a, vec![1]);
605 ///
606 /// X.with_borrow(|v| assert!(v.is_empty()));
607 /// ```
608 #[stable(feature = "local_key_cell_methods", since = "1.73.0")]
609 pub fn take(&'static self) -> T
610 where
611 T: Default,
612 {
613 self.with(RefCell::take)
614 }
615
616 /// Replaces the contained value, returning the old value.
617 ///
618 /// # Panics
619 ///
620 /// Panics if the value is currently borrowed.
621 ///
622 /// Panics if the key currently has its destructor running,
623 /// and it **may** panic if the destructor has previously been run for this thread.
624 ///
625 /// # Examples
626 ///
627 /// ```
628 /// use std::cell::RefCell;
629 ///
630 /// thread_local! {
631 /// static X: RefCell<Vec<i32>> = RefCell::new(Vec::new());
632 /// }
633 ///
634 /// let prev = X.replace(vec![1, 2, 3]);
635 /// assert!(prev.is_empty());
636 ///
637 /// X.with_borrow(|v| assert_eq!(*v, vec![1, 2, 3]));
638 /// ```
639 #[stable(feature = "local_key_cell_methods", since = "1.73.0")]
640 #[rustc_confusables("swap")]
641 pub fn replace(&'static self, value: T) -> T {
642 self.with(|cell| cell.replace(value))
643 }
644}
645

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