1 // Copyright 2012-2014 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 #![stable(feature = "rust1", since = "1.0.0")]
13 //! Thread-safe reference-counting pointers.
15 //! See the [`Arc<T>`][arc] documentation for more details.
17 //! [arc]: struct.Arc.html
21 use core::sync::atomic;
22 use core::sync::atomic::Ordering::{Acquire, Relaxed, Release, SeqCst};
25 use core::cmp::Ordering;
26 use core::intrinsics::abort;
28 use core::mem::uninitialized;
30 use core::ops::CoerceUnsized;
31 use core::ptr::{self, Shared};
32 use core::marker::Unsize;
33 use core::hash::{Hash, Hasher};
34 use core::{isize, usize};
35 use core::convert::From;
37 use heap::{Heap, Alloc, Layout};
39 /// A soft limit on the amount of references that may be made to an `Arc`.
41 /// Going above this limit will abort your program (although not
42 /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
43 const MAX_REFCOUNT: usize = (isize::MAX) as usize;
45 /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
46 /// Reference Counted'.
48 /// The type `Arc<T>` provides shared ownership of a value of type `T`,
49 /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
50 /// a new pointer to the same value in the heap. When the last `Arc`
51 /// pointer to a given value is destroyed, the pointed-to value is
54 /// Shared references in Rust disallow mutation by default, and `Arc` is no
55 /// exception. If you need to mutate through an `Arc`, use [`Mutex`][mutex],
56 /// [`RwLock`][rwlock], or one of the [`Atomic`][atomic] types.
60 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
61 /// counting This means that it is thread-safe. The disadvantage is that
62 /// atomic operations are more expensive than ordinary memory accesses. If you
63 /// are not sharing reference-counted values between threads, consider using
64 /// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
65 /// compiler will catch any attempt to send an [`Rc<T>`] between threads.
66 /// However, a library might choose `Arc<T>` in order to give library consumers
69 /// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
70 /// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
71 /// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
72 /// first: after all, isn't the point of `Arc<T>` thread safety? The key is
73 /// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
74 /// data, but it doesn't add thread safety to its data. Consider
75 /// `Arc<RefCell<T>>`. `RefCell<T>` isn't [`Sync`], and if `Arc<T>` was always
76 /// [`Send`], `Arc<RefCell<T>>` would be as well. But then we'd have a problem:
77 /// `RefCell<T>` is not thread safe; it keeps track of the borrowing count using
78 /// non-atomic operations.
80 /// In the end, this means that you may need to pair `Arc<T>` with some sort of
81 /// `std::sync` type, usually `Mutex<T>`.
83 /// ## Breaking cycles with `Weak`
85 /// The [`downgrade`][downgrade] method can be used to create a non-owning
86 /// [`Weak`][weak] pointer. A [`Weak`][weak] pointer can be [`upgrade`][upgrade]d
87 /// to an `Arc`, but this will return [`None`] if the value has already been
90 /// A cycle between `Arc` pointers will never be deallocated. For this reason,
91 /// [`Weak`][weak] is used to break cycles. For example, a tree could have
92 /// strong `Arc` pointers from parent nodes to children, and [`Weak`][weak]
93 /// pointers from children back to their parents.
95 /// # Cloning references
97 /// Creating a new reference from an existing reference counted pointer is done using the
98 /// `Clone` trait implemented for [`Arc<T>`][`arc`] and [`Weak<T>`][`weak`].
101 /// use std::sync::Arc;
102 /// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
103 /// // The two syntaxes below are equivalent.
104 /// let a = foo.clone();
105 /// let b = Arc::clone(&foo);
106 /// // a and b both point to the same memory location as foo.
109 /// The `Arc::clone(&from)` syntax is the most idiomatic because it conveys more explicitly
110 /// the meaning of the code. In the example above, this syntax makes it easier to see that
111 /// this code is creating a new reference rather than copying the whole content of foo.
113 /// ## `Deref` behavior
115 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
116 /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
117 /// clashes with `T`'s methods, the methods of `Arc<T>` itself are [associated
118 /// functions][assoc], called using function-like syntax:
121 /// use std::sync::Arc;
122 /// let my_arc = Arc::new(());
124 /// Arc::downgrade(&my_arc);
127 /// [`Weak<T>`][weak] does not auto-dereference to `T`, because the value may have
128 /// already been destroyed.
130 /// [arc]: struct.Arc.html
131 /// [weak]: struct.Weak.html
132 /// [`Rc<T>`]: ../../std/rc/struct.Rc.html
133 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
134 /// [mutex]: ../../std/sync/struct.Mutex.html
135 /// [rwlock]: ../../std/sync/struct.RwLock.html
136 /// [atomic]: ../../std/sync/atomic/index.html
137 /// [`Send`]: ../../std/marker/trait.Send.html
138 /// [`Sync`]: ../../std/marker/trait.Sync.html
139 /// [deref]: ../../std/ops/trait.Deref.html
140 /// [downgrade]: struct.Arc.html#method.downgrade
141 /// [upgrade]: struct.Weak.html#method.upgrade
142 /// [`None`]: ../../std/option/enum.Option.html#variant.None
143 /// [assoc]: ../../book/first-edition/method-syntax.html#associated-functions
147 /// Sharing some immutable data between threads:
149 // Note that we **do not** run these tests here. The windows builders get super
150 // unhappy if a thread outlives the main thread and then exits at the same time
151 // (something deadlocks) so we just avoid this entirely by not running these
154 /// use std::sync::Arc;
157 /// let five = Arc::new(5);
160 /// let five = Arc::clone(&five);
162 /// thread::spawn(move || {
163 /// println!("{:?}", five);
168 /// Sharing a mutable [`AtomicUsize`]:
170 /// [`AtomicUsize`]: ../../std/sync/atomic/struct.AtomicUsize.html
173 /// use std::sync::Arc;
174 /// use std::sync::atomic::{AtomicUsize, Ordering};
177 /// let val = Arc::new(AtomicUsize::new(5));
180 /// let val = Arc::clone(&val);
182 /// thread::spawn(move || {
183 /// let v = val.fetch_add(1, Ordering::SeqCst);
184 /// println!("{:?}", v);
189 /// See the [`rc` documentation][rc_examples] for more examples of reference
190 /// counting in general.
192 /// [rc_examples]: ../../std/rc/index.html#examples
193 #[stable(feature = "rust1", since = "1.0.0")]
194 pub struct Arc<T: ?Sized> {
195 ptr: Shared<ArcInner<T>>,
198 #[stable(feature = "rust1", since = "1.0.0")]
199 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
200 #[stable(feature = "rust1", since = "1.0.0")]
201 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
203 #[unstable(feature = "coerce_unsized", issue = "27732")]
204 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
206 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
207 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
208 /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
210 /// Since a `Weak` reference does not count towards ownership, it will not
211 /// prevent the inner value from being dropped, and `Weak` itself makes no
212 /// guarantees about the value still being present and may return [`None`]
213 /// when [`upgrade`]d.
215 /// A `Weak` pointer is useful for keeping a temporary reference to the value
216 /// within [`Arc`] without extending its lifetime. It is also used to prevent
217 /// circular references between [`Arc`] pointers, since mutual owning references
218 /// would never allow either [`Arc`] to be dropped. For example, a tree could
219 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
220 /// pointers from children back to their parents.
222 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
224 /// [`Arc`]: struct.Arc.html
225 /// [`Arc::downgrade`]: struct.Arc.html#method.downgrade
226 /// [`upgrade`]: struct.Weak.html#method.upgrade
227 /// [`Option`]: ../../std/option/enum.Option.html
228 /// [`None`]: ../../std/option/enum.Option.html#variant.None
229 #[stable(feature = "arc_weak", since = "1.4.0")]
230 pub struct Weak<T: ?Sized> {
231 ptr: Shared<ArcInner<T>>,
234 #[stable(feature = "arc_weak", since = "1.4.0")]
235 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
236 #[stable(feature = "arc_weak", since = "1.4.0")]
237 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
239 #[unstable(feature = "coerce_unsized", issue = "27732")]
240 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
242 #[stable(feature = "arc_weak", since = "1.4.0")]
243 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
244 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
249 struct ArcInner<T: ?Sized> {
250 strong: atomic::AtomicUsize,
252 // the value usize::MAX acts as a sentinel for temporarily "locking" the
253 // ability to upgrade weak pointers or downgrade strong ones; this is used
254 // to avoid races in `make_mut` and `get_mut`.
255 weak: atomic::AtomicUsize,
260 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
261 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
264 /// Constructs a new `Arc<T>`.
269 /// use std::sync::Arc;
271 /// let five = Arc::new(5);
274 #[stable(feature = "rust1", since = "1.0.0")]
275 pub fn new(data: T) -> Arc<T> {
276 // Start the weak pointer count as 1 which is the weak pointer that's
277 // held by all the strong pointers (kinda), see std/rc.rs for more info
278 let x: Box<_> = box ArcInner {
279 strong: atomic::AtomicUsize::new(1),
280 weak: atomic::AtomicUsize::new(1),
283 Arc { ptr: unsafe { Shared::new(Box::into_raw(x)) } }
286 /// Returns the contained value, if the `Arc` has exactly one strong reference.
288 /// Otherwise, an [`Err`][result] is returned with the same `Arc` that was
291 /// This will succeed even if there are outstanding weak references.
293 /// [result]: ../../std/result/enum.Result.html
298 /// use std::sync::Arc;
300 /// let x = Arc::new(3);
301 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
303 /// let x = Arc::new(4);
304 /// let _y = Arc::clone(&x);
305 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
308 #[stable(feature = "arc_unique", since = "1.4.0")]
309 pub fn try_unwrap(this: Self) -> Result<T, Self> {
310 // See `drop` for why all these atomics are like this
311 if this.inner().strong.compare_exchange(1, 0, Release, Relaxed).is_err() {
315 atomic::fence(Acquire);
318 let elem = ptr::read(&this.ptr.as_ref().data);
320 // Make a weak pointer to clean up the implicit strong-weak reference
321 let _weak = Weak { ptr: this.ptr };
328 /// Consumes the `Arc`, returning the wrapped pointer.
330 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
331 /// [`Arc::from_raw`][from_raw].
333 /// [from_raw]: struct.Arc.html#method.from_raw
338 /// use std::sync::Arc;
340 /// let x = Arc::new(10);
341 /// let x_ptr = Arc::into_raw(x);
342 /// assert_eq!(unsafe { *x_ptr }, 10);
344 #[stable(feature = "rc_raw", since = "1.17.0")]
345 pub fn into_raw(this: Self) -> *const T {
346 let ptr: *const T = &*this;
351 /// Constructs an `Arc` from a raw pointer.
353 /// The raw pointer must have been previously returned by a call to a
354 /// [`Arc::into_raw`][into_raw].
356 /// This function is unsafe because improper use may lead to memory problems. For example, a
357 /// double-free may occur if the function is called twice on the same raw pointer.
359 /// [into_raw]: struct.Arc.html#method.into_raw
364 /// use std::sync::Arc;
366 /// let x = Arc::new(10);
367 /// let x_ptr = Arc::into_raw(x);
370 /// // Convert back to an `Arc` to prevent leak.
371 /// let x = Arc::from_raw(x_ptr);
372 /// assert_eq!(*x, 10);
374 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory unsafe.
377 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
379 #[stable(feature = "rc_raw", since = "1.17.0")]
380 pub unsafe fn from_raw(ptr: *const T) -> Self {
381 // To find the corresponding pointer to the `ArcInner` we need to subtract the offset of the
382 // `data` field from the pointer.
383 let ptr = (ptr as *const u8).offset(-offset_of!(ArcInner<T>, data));
385 ptr: Shared::new(ptr as *mut u8 as *mut _),
390 impl<T: ?Sized> Arc<T> {
391 /// Creates a new [`Weak`][weak] pointer to this value.
393 /// [weak]: struct.Weak.html
398 /// use std::sync::Arc;
400 /// let five = Arc::new(5);
402 /// let weak_five = Arc::downgrade(&five);
404 #[stable(feature = "arc_weak", since = "1.4.0")]
405 pub fn downgrade(this: &Self) -> Weak<T> {
406 // This Relaxed is OK because we're checking the value in the CAS
408 let mut cur = this.inner().weak.load(Relaxed);
411 // check if the weak counter is currently "locked"; if so, spin.
412 if cur == usize::MAX {
413 cur = this.inner().weak.load(Relaxed);
417 // NOTE: this code currently ignores the possibility of overflow
418 // into usize::MAX; in general both Rc and Arc need to be adjusted
419 // to deal with overflow.
421 // Unlike with Clone(), we need this to be an Acquire read to
422 // synchronize with the write coming from `is_unique`, so that the
423 // events prior to that write happen before this read.
424 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
425 Ok(_) => return Weak { ptr: this.ptr },
426 Err(old) => cur = old,
431 /// Gets the number of [`Weak`][weak] pointers to this value.
433 /// [weak]: struct.Weak.html
437 /// This method by itself is safe, but using it correctly requires extra care.
438 /// Another thread can change the weak count at any time,
439 /// including potentially between calling this method and acting on the result.
444 /// use std::sync::Arc;
446 /// let five = Arc::new(5);
447 /// let _weak_five = Arc::downgrade(&five);
449 /// // This assertion is deterministic because we haven't shared
450 /// // the `Arc` or `Weak` between threads.
451 /// assert_eq!(1, Arc::weak_count(&five));
454 #[stable(feature = "arc_counts", since = "1.15.0")]
455 pub fn weak_count(this: &Self) -> usize {
456 this.inner().weak.load(SeqCst) - 1
459 /// Gets the number of strong (`Arc`) pointers to this value.
463 /// This method by itself is safe, but using it correctly requires extra care.
464 /// Another thread can change the strong count at any time,
465 /// including potentially between calling this method and acting on the result.
470 /// use std::sync::Arc;
472 /// let five = Arc::new(5);
473 /// let _also_five = Arc::clone(&five);
475 /// // This assertion is deterministic because we haven't shared
476 /// // the `Arc` between threads.
477 /// assert_eq!(2, Arc::strong_count(&five));
480 #[stable(feature = "arc_counts", since = "1.15.0")]
481 pub fn strong_count(this: &Self) -> usize {
482 this.inner().strong.load(SeqCst)
486 fn inner(&self) -> &ArcInner<T> {
487 // This unsafety is ok because while this arc is alive we're guaranteed
488 // that the inner pointer is valid. Furthermore, we know that the
489 // `ArcInner` structure itself is `Sync` because the inner data is
490 // `Sync` as well, so we're ok loaning out an immutable pointer to these
492 unsafe { self.ptr.as_ref() }
495 // Non-inlined part of `drop`.
497 unsafe fn drop_slow(&mut self) {
498 let ptr = self.ptr.as_ptr();
500 // Destroy the data at this time, even though we may not free the box
501 // allocation itself (there may still be weak pointers lying around).
502 ptr::drop_in_place(&mut self.ptr.as_mut().data);
504 if self.inner().weak.fetch_sub(1, Release) == 1 {
505 atomic::fence(Acquire);
506 Heap.dealloc(ptr as *mut u8, Layout::for_value(&*ptr))
511 #[stable(feature = "ptr_eq", since = "1.17.0")]
512 /// Returns true if the two `Arc`s point to the same value (not
513 /// just values that compare as equal).
518 /// use std::sync::Arc;
520 /// let five = Arc::new(5);
521 /// let same_five = Arc::clone(&five);
522 /// let other_five = Arc::new(5);
524 /// assert!(Arc::ptr_eq(&five, &same_five));
525 /// assert!(!Arc::ptr_eq(&five, &other_five));
527 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
528 this.ptr.as_ptr() == other.ptr.as_ptr()
532 #[stable(feature = "rust1", since = "1.0.0")]
533 impl<T: ?Sized> Clone for Arc<T> {
534 /// Makes a clone of the `Arc` pointer.
536 /// This creates another pointer to the same inner value, increasing the
537 /// strong reference count.
542 /// use std::sync::Arc;
544 /// let five = Arc::new(5);
546 /// Arc::clone(&five);
549 fn clone(&self) -> Arc<T> {
550 // Using a relaxed ordering is alright here, as knowledge of the
551 // original reference prevents other threads from erroneously deleting
554 // As explained in the [Boost documentation][1], Increasing the
555 // reference counter can always be done with memory_order_relaxed: New
556 // references to an object can only be formed from an existing
557 // reference, and passing an existing reference from one thread to
558 // another must already provide any required synchronization.
560 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
561 let old_size = self.inner().strong.fetch_add(1, Relaxed);
563 // However we need to guard against massive refcounts in case someone
564 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
565 // and users will use-after free. We racily saturate to `isize::MAX` on
566 // the assumption that there aren't ~2 billion threads incrementing
567 // the reference count at once. This branch will never be taken in
568 // any realistic program.
570 // We abort because such a program is incredibly degenerate, and we
571 // don't care to support it.
572 if old_size > MAX_REFCOUNT {
578 Arc { ptr: self.ptr }
582 #[stable(feature = "rust1", since = "1.0.0")]
583 impl<T: ?Sized> Deref for Arc<T> {
587 fn deref(&self) -> &T {
592 impl<T: Clone> Arc<T> {
593 /// Makes a mutable reference into the given `Arc`.
595 /// If there are other `Arc` or [`Weak`][weak] pointers to the same value,
596 /// then `make_mut` will invoke [`clone`][clone] on the inner value to
597 /// ensure unique ownership. This is also referred to as clone-on-write.
599 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
601 /// [weak]: struct.Weak.html
602 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
603 /// [get_mut]: struct.Arc.html#method.get_mut
608 /// use std::sync::Arc;
610 /// let mut data = Arc::new(5);
612 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
613 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
614 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
615 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
616 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
618 /// // Now `data` and `other_data` point to different values.
619 /// assert_eq!(*data, 8);
620 /// assert_eq!(*other_data, 12);
623 #[stable(feature = "arc_unique", since = "1.4.0")]
624 pub fn make_mut(this: &mut Self) -> &mut T {
625 // Note that we hold both a strong reference and a weak reference.
626 // Thus, releasing our strong reference only will not, by itself, cause
627 // the memory to be deallocated.
629 // Use Acquire to ensure that we see any writes to `weak` that happen
630 // before release writes (i.e., decrements) to `strong`. Since we hold a
631 // weak count, there's no chance the ArcInner itself could be
633 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
634 // Another strong pointer exists; clone
635 *this = Arc::new((**this).clone());
636 } else if this.inner().weak.load(Relaxed) != 1 {
637 // Relaxed suffices in the above because this is fundamentally an
638 // optimization: we are always racing with weak pointers being
639 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
641 // We removed the last strong ref, but there are additional weak
642 // refs remaining. We'll move the contents to a new Arc, and
643 // invalidate the other weak refs.
645 // Note that it is not possible for the read of `weak` to yield
646 // usize::MAX (i.e., locked), since the weak count can only be
647 // locked by a thread with a strong reference.
649 // Materialize our own implicit weak pointer, so that it can clean
650 // up the ArcInner as needed.
651 let weak = Weak { ptr: this.ptr };
653 // mark the data itself as already deallocated
655 // there is no data race in the implicit write caused by `read`
656 // here (due to zeroing) because data is no longer accessed by
657 // other threads (due to there being no more strong refs at this
659 let mut swap = Arc::new(ptr::read(&weak.ptr.as_ref().data));
660 mem::swap(this, &mut swap);
664 // We were the sole reference of either kind; bump back up the
666 this.inner().strong.store(1, Release);
669 // As with `get_mut()`, the unsafety is ok because our reference was
670 // either unique to begin with, or became one upon cloning the contents.
672 &mut this.ptr.as_mut().data
677 impl<T: ?Sized> Arc<T> {
678 /// Returns a mutable reference to the inner value, if there are
679 /// no other `Arc` or [`Weak`][weak] pointers to the same value.
681 /// Returns [`None`][option] otherwise, because it is not safe to
682 /// mutate a shared value.
684 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
685 /// the inner value when it's shared.
687 /// [weak]: struct.Weak.html
688 /// [option]: ../../std/option/enum.Option.html
689 /// [make_mut]: struct.Arc.html#method.make_mut
690 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
695 /// use std::sync::Arc;
697 /// let mut x = Arc::new(3);
698 /// *Arc::get_mut(&mut x).unwrap() = 4;
699 /// assert_eq!(*x, 4);
701 /// let _y = Arc::clone(&x);
702 /// assert!(Arc::get_mut(&mut x).is_none());
705 #[stable(feature = "arc_unique", since = "1.4.0")]
706 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
707 if this.is_unique() {
708 // This unsafety is ok because we're guaranteed that the pointer
709 // returned is the *only* pointer that will ever be returned to T. Our
710 // reference count is guaranteed to be 1 at this point, and we required
711 // the Arc itself to be `mut`, so we're returning the only possible
712 // reference to the inner data.
714 Some(&mut this.ptr.as_mut().data)
721 /// Determine whether this is the unique reference (including weak refs) to
722 /// the underlying data.
724 /// Note that this requires locking the weak ref count.
725 fn is_unique(&mut self) -> bool {
726 // lock the weak pointer count if we appear to be the sole weak pointer
729 // The acquire label here ensures a happens-before relationship with any
730 // writes to `strong` prior to decrements of the `weak` count (via drop,
731 // which uses Release).
732 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
733 // Due to the previous acquire read, this will observe any writes to
734 // `strong` that were due to upgrading weak pointers; only strong
735 // clones remain, which require that the strong count is > 1 anyway.
736 let unique = self.inner().strong.load(Relaxed) == 1;
738 // The release write here synchronizes with a read in `downgrade`,
739 // effectively preventing the above read of `strong` from happening
741 self.inner().weak.store(1, Release); // release the lock
749 #[stable(feature = "rust1", since = "1.0.0")]
750 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
753 /// This will decrement the strong reference count. If the strong reference
754 /// count reaches zero then the only other references (if any) are
755 /// [`Weak`][weak], so we `drop` the inner value.
757 /// [weak]: struct.Weak.html
762 /// use std::sync::Arc;
766 /// impl Drop for Foo {
767 /// fn drop(&mut self) {
768 /// println!("dropped!");
772 /// let foo = Arc::new(Foo);
773 /// let foo2 = Arc::clone(&foo);
775 /// drop(foo); // Doesn't print anything
776 /// drop(foo2); // Prints "dropped!"
780 // Because `fetch_sub` is already atomic, we do not need to synchronize
781 // with other threads unless we are going to delete the object. This
782 // same logic applies to the below `fetch_sub` to the `weak` count.
783 if self.inner().strong.fetch_sub(1, Release) != 1 {
787 // This fence is needed to prevent reordering of use of the data and
788 // deletion of the data. Because it is marked `Release`, the decreasing
789 // of the reference count synchronizes with this `Acquire` fence. This
790 // means that use of the data happens before decreasing the reference
791 // count, which happens before this fence, which happens before the
792 // deletion of the data.
794 // As explained in the [Boost documentation][1],
796 // > It is important to enforce any possible access to the object in one
797 // > thread (through an existing reference) to *happen before* deleting
798 // > the object in a different thread. This is achieved by a "release"
799 // > operation after dropping a reference (any access to the object
800 // > through this reference must obviously happened before), and an
801 // > "acquire" operation before deleting the object.
803 // In particular, while the contents of an Arc are usually immutable, it's
804 // possible to have interior writes to something like a Mutex<T>. Since a
805 // Mutex is not acquired when it is deleted, we can't rely on its
806 // synchronization logic to make writes in thread A visible to a destructor
807 // running in thread B.
809 // Also note that the Acquire fence here could probably be replaced with an
810 // Acquire load, which could improve performance in highly-contended
811 // situations. See [2].
813 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
814 // [2]: (https://github.com/rust-lang/rust/pull/41714)
815 atomic::fence(Acquire);
824 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
825 /// it. Calling [`upgrade`] on the return value always gives [`None`].
827 /// [`upgrade`]: struct.Weak.html#method.upgrade
828 /// [`None`]: ../../std/option/enum.Option.html#variant.None
833 /// use std::sync::Weak;
835 /// let empty: Weak<i64> = Weak::new();
836 /// assert!(empty.upgrade().is_none());
838 #[stable(feature = "downgraded_weak", since = "1.10.0")]
839 pub fn new() -> Weak<T> {
842 ptr: Shared::new(Box::into_raw(box ArcInner {
843 strong: atomic::AtomicUsize::new(0),
844 weak: atomic::AtomicUsize::new(1),
845 data: uninitialized(),
852 impl<T: ?Sized> Weak<T> {
853 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], extending
854 /// the lifetime of the value if successful.
856 /// Returns [`None`] if the value has since been dropped.
858 /// [`Arc`]: struct.Arc.html
859 /// [`None`]: ../../std/option/enum.Option.html#variant.None
864 /// use std::sync::Arc;
866 /// let five = Arc::new(5);
868 /// let weak_five = Arc::downgrade(&five);
870 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
871 /// assert!(strong_five.is_some());
873 /// // Destroy all strong pointers.
874 /// drop(strong_five);
877 /// assert!(weak_five.upgrade().is_none());
879 #[stable(feature = "arc_weak", since = "1.4.0")]
880 pub fn upgrade(&self) -> Option<Arc<T>> {
881 // We use a CAS loop to increment the strong count instead of a
882 // fetch_add because once the count hits 0 it must never be above 0.
883 let inner = self.inner();
885 // Relaxed load because any write of 0 that we can observe
886 // leaves the field in a permanently zero state (so a
887 // "stale" read of 0 is fine), and any other value is
888 // confirmed via the CAS below.
889 let mut n = inner.strong.load(Relaxed);
896 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
897 if n > MAX_REFCOUNT {
903 // Relaxed is valid for the same reason it is on Arc's Clone impl
904 match inner.strong.compare_exchange_weak(n, n + 1, Relaxed, Relaxed) {
905 Ok(_) => return Some(Arc { ptr: self.ptr }),
912 fn inner(&self) -> &ArcInner<T> {
913 // See comments above for why this is "safe"
914 unsafe { self.ptr.as_ref() }
918 #[stable(feature = "arc_weak", since = "1.4.0")]
919 impl<T: ?Sized> Clone for Weak<T> {
920 /// Makes a clone of the `Weak` pointer that points to the same value.
925 /// use std::sync::{Arc, Weak};
927 /// let weak_five = Arc::downgrade(&Arc::new(5));
929 /// Weak::clone(&weak_five);
932 fn clone(&self) -> Weak<T> {
933 // See comments in Arc::clone() for why this is relaxed. This can use a
934 // fetch_add (ignoring the lock) because the weak count is only locked
935 // where are *no other* weak pointers in existence. (So we can't be
936 // running this code in that case).
937 let old_size = self.inner().weak.fetch_add(1, Relaxed);
939 // See comments in Arc::clone() for why we do this (for mem::forget).
940 if old_size > MAX_REFCOUNT {
946 return Weak { ptr: self.ptr };
950 #[stable(feature = "downgraded_weak", since = "1.10.0")]
951 impl<T> Default for Weak<T> {
952 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
953 /// it. Calling [`upgrade`] on the return value always gives [`None`].
955 /// [`upgrade`]: struct.Weak.html#method.upgrade
956 /// [`None`]: ../../std/option/enum.Option.html#variant.None
961 /// use std::sync::Weak;
963 /// let empty: Weak<i64> = Default::default();
964 /// assert!(empty.upgrade().is_none());
966 fn default() -> Weak<T> {
971 #[stable(feature = "arc_weak", since = "1.4.0")]
972 impl<T: ?Sized> Drop for Weak<T> {
973 /// Drops the `Weak` pointer.
978 /// use std::sync::{Arc, Weak};
982 /// impl Drop for Foo {
983 /// fn drop(&mut self) {
984 /// println!("dropped!");
988 /// let foo = Arc::new(Foo);
989 /// let weak_foo = Arc::downgrade(&foo);
990 /// let other_weak_foo = Weak::clone(&weak_foo);
992 /// drop(weak_foo); // Doesn't print anything
993 /// drop(foo); // Prints "dropped!"
995 /// assert!(other_weak_foo.upgrade().is_none());
998 let ptr = self.ptr.as_ptr();
1000 // If we find out that we were the last weak pointer, then its time to
1001 // deallocate the data entirely. See the discussion in Arc::drop() about
1002 // the memory orderings
1004 // It's not necessary to check for the locked state here, because the
1005 // weak count can only be locked if there was precisely one weak ref,
1006 // meaning that drop could only subsequently run ON that remaining weak
1007 // ref, which can only happen after the lock is released.
1008 if self.inner().weak.fetch_sub(1, Release) == 1 {
1009 atomic::fence(Acquire);
1011 Heap.dealloc(ptr as *mut u8, Layout::for_value(&*ptr))
1017 #[stable(feature = "rust1", since = "1.0.0")]
1018 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
1019 /// Equality for two `Arc`s.
1021 /// Two `Arc`s are equal if their inner values are equal.
1026 /// use std::sync::Arc;
1028 /// let five = Arc::new(5);
1030 /// assert!(five == Arc::new(5));
1032 fn eq(&self, other: &Arc<T>) -> bool {
1033 *(*self) == *(*other)
1036 /// Inequality for two `Arc`s.
1038 /// Two `Arc`s are unequal if their inner values are unequal.
1043 /// use std::sync::Arc;
1045 /// let five = Arc::new(5);
1047 /// assert!(five != Arc::new(6));
1049 fn ne(&self, other: &Arc<T>) -> bool {
1050 *(*self) != *(*other)
1053 #[stable(feature = "rust1", since = "1.0.0")]
1054 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
1055 /// Partial comparison for two `Arc`s.
1057 /// The two are compared by calling `partial_cmp()` on their inner values.
1062 /// use std::sync::Arc;
1063 /// use std::cmp::Ordering;
1065 /// let five = Arc::new(5);
1067 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
1069 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
1070 (**self).partial_cmp(&**other)
1073 /// Less-than comparison for two `Arc`s.
1075 /// The two are compared by calling `<` on their inner values.
1080 /// use std::sync::Arc;
1082 /// let five = Arc::new(5);
1084 /// assert!(five < Arc::new(6));
1086 fn lt(&self, other: &Arc<T>) -> bool {
1087 *(*self) < *(*other)
1090 /// 'Less than or equal to' comparison for two `Arc`s.
1092 /// The two are compared by calling `<=` on their inner values.
1097 /// use std::sync::Arc;
1099 /// let five = Arc::new(5);
1101 /// assert!(five <= Arc::new(5));
1103 fn le(&self, other: &Arc<T>) -> bool {
1104 *(*self) <= *(*other)
1107 /// Greater-than comparison for two `Arc`s.
1109 /// The two are compared by calling `>` on their inner values.
1114 /// use std::sync::Arc;
1116 /// let five = Arc::new(5);
1118 /// assert!(five > Arc::new(4));
1120 fn gt(&self, other: &Arc<T>) -> bool {
1121 *(*self) > *(*other)
1124 /// 'Greater than or equal to' comparison for two `Arc`s.
1126 /// The two are compared by calling `>=` on their inner values.
1131 /// use std::sync::Arc;
1133 /// let five = Arc::new(5);
1135 /// assert!(five >= Arc::new(5));
1137 fn ge(&self, other: &Arc<T>) -> bool {
1138 *(*self) >= *(*other)
1141 #[stable(feature = "rust1", since = "1.0.0")]
1142 impl<T: ?Sized + Ord> Ord for Arc<T> {
1143 /// Comparison for two `Arc`s.
1145 /// The two are compared by calling `cmp()` on their inner values.
1150 /// use std::sync::Arc;
1151 /// use std::cmp::Ordering;
1153 /// let five = Arc::new(5);
1155 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
1157 fn cmp(&self, other: &Arc<T>) -> Ordering {
1158 (**self).cmp(&**other)
1161 #[stable(feature = "rust1", since = "1.0.0")]
1162 impl<T: ?Sized + Eq> Eq for Arc<T> {}
1164 #[stable(feature = "rust1", since = "1.0.0")]
1165 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
1166 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1167 fmt::Display::fmt(&**self, f)
1171 #[stable(feature = "rust1", since = "1.0.0")]
1172 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
1173 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1174 fmt::Debug::fmt(&**self, f)
1178 #[stable(feature = "rust1", since = "1.0.0")]
1179 impl<T: ?Sized> fmt::Pointer for Arc<T> {
1180 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1181 fmt::Pointer::fmt(&self.ptr, f)
1185 #[stable(feature = "rust1", since = "1.0.0")]
1186 impl<T: Default> Default for Arc<T> {
1187 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
1192 /// use std::sync::Arc;
1194 /// let x: Arc<i32> = Default::default();
1195 /// assert_eq!(*x, 0);
1197 fn default() -> Arc<T> {
1198 Arc::new(Default::default())
1202 #[stable(feature = "rust1", since = "1.0.0")]
1203 impl<T: ?Sized + Hash> Hash for Arc<T> {
1204 fn hash<H: Hasher>(&self, state: &mut H) {
1205 (**self).hash(state)
1209 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1210 impl<T> From<T> for Arc<T> {
1211 fn from(t: T) -> Self {
1218 use std::clone::Clone;
1219 use std::sync::mpsc::channel;
1222 use std::option::Option;
1223 use std::option::Option::{None, Some};
1224 use std::sync::atomic;
1225 use std::sync::atomic::Ordering::{Acquire, SeqCst};
1227 use std::sync::Mutex;
1228 use std::convert::From;
1230 use super::{Arc, Weak};
1233 struct Canary(*mut atomic::AtomicUsize);
1235 impl Drop for Canary {
1236 fn drop(&mut self) {
1240 (*c).fetch_add(1, SeqCst);
1248 #[cfg_attr(target_os = "emscripten", ignore)]
1249 fn manually_share_arc() {
1250 let v = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
1251 let arc_v = Arc::new(v);
1253 let (tx, rx) = channel();
1255 let _t = thread::spawn(move || {
1256 let arc_v: Arc<Vec<i32>> = rx.recv().unwrap();
1257 assert_eq!((*arc_v)[3], 4);
1260 tx.send(arc_v.clone()).unwrap();
1262 assert_eq!((*arc_v)[2], 3);
1263 assert_eq!((*arc_v)[4], 5);
1267 fn test_arc_get_mut() {
1268 let mut x = Arc::new(3);
1269 *Arc::get_mut(&mut x).unwrap() = 4;
1272 assert!(Arc::get_mut(&mut x).is_none());
1274 assert!(Arc::get_mut(&mut x).is_some());
1275 let _w = Arc::downgrade(&x);
1276 assert!(Arc::get_mut(&mut x).is_none());
1281 let x = Arc::new(3);
1282 assert_eq!(Arc::try_unwrap(x), Ok(3));
1283 let x = Arc::new(4);
1285 assert_eq!(Arc::try_unwrap(x), Err(Arc::new(4)));
1286 let x = Arc::new(5);
1287 let _w = Arc::downgrade(&x);
1288 assert_eq!(Arc::try_unwrap(x), Ok(5));
1292 fn into_from_raw() {
1293 let x = Arc::new(box "hello");
1296 let x_ptr = Arc::into_raw(x);
1299 assert_eq!(**x_ptr, "hello");
1301 let x = Arc::from_raw(x_ptr);
1302 assert_eq!(**x, "hello");
1304 assert_eq!(Arc::try_unwrap(x).map(|x| *x), Ok("hello"));
1309 fn test_cowarc_clone_make_mut() {
1310 let mut cow0 = Arc::new(75);
1311 let mut cow1 = cow0.clone();
1312 let mut cow2 = cow1.clone();
1314 assert!(75 == *Arc::make_mut(&mut cow0));
1315 assert!(75 == *Arc::make_mut(&mut cow1));
1316 assert!(75 == *Arc::make_mut(&mut cow2));
1318 *Arc::make_mut(&mut cow0) += 1;
1319 *Arc::make_mut(&mut cow1) += 2;
1320 *Arc::make_mut(&mut cow2) += 3;
1322 assert!(76 == *cow0);
1323 assert!(77 == *cow1);
1324 assert!(78 == *cow2);
1326 // none should point to the same backing memory
1327 assert!(*cow0 != *cow1);
1328 assert!(*cow0 != *cow2);
1329 assert!(*cow1 != *cow2);
1333 fn test_cowarc_clone_unique2() {
1334 let mut cow0 = Arc::new(75);
1335 let cow1 = cow0.clone();
1336 let cow2 = cow1.clone();
1338 assert!(75 == *cow0);
1339 assert!(75 == *cow1);
1340 assert!(75 == *cow2);
1342 *Arc::make_mut(&mut cow0) += 1;
1343 assert!(76 == *cow0);
1344 assert!(75 == *cow1);
1345 assert!(75 == *cow2);
1347 // cow1 and cow2 should share the same contents
1348 // cow0 should have a unique reference
1349 assert!(*cow0 != *cow1);
1350 assert!(*cow0 != *cow2);
1351 assert!(*cow1 == *cow2);
1355 fn test_cowarc_clone_weak() {
1356 let mut cow0 = Arc::new(75);
1357 let cow1_weak = Arc::downgrade(&cow0);
1359 assert!(75 == *cow0);
1360 assert!(75 == *cow1_weak.upgrade().unwrap());
1362 *Arc::make_mut(&mut cow0) += 1;
1364 assert!(76 == *cow0);
1365 assert!(cow1_weak.upgrade().is_none());
1370 let x = Arc::new(5);
1371 let y = Arc::downgrade(&x);
1372 assert!(y.upgrade().is_some());
1377 let x = Arc::new(5);
1378 let y = Arc::downgrade(&x);
1380 assert!(y.upgrade().is_none());
1384 fn weak_self_cyclic() {
1386 x: Mutex<Option<Weak<Cycle>>>,
1389 let a = Arc::new(Cycle { x: Mutex::new(None) });
1390 let b = Arc::downgrade(&a.clone());
1391 *a.x.lock().unwrap() = Some(b);
1393 // hopefully we don't double-free (or leak)...
1398 let mut canary = atomic::AtomicUsize::new(0);
1399 let x = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1401 assert!(canary.load(Acquire) == 1);
1405 fn drop_arc_weak() {
1406 let mut canary = atomic::AtomicUsize::new(0);
1407 let arc = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1408 let arc_weak = Arc::downgrade(&arc);
1409 assert!(canary.load(Acquire) == 0);
1411 assert!(canary.load(Acquire) == 1);
1416 fn test_strong_count() {
1417 let a = Arc::new(0);
1418 assert!(Arc::strong_count(&a) == 1);
1419 let w = Arc::downgrade(&a);
1420 assert!(Arc::strong_count(&a) == 1);
1421 let b = w.upgrade().expect("");
1422 assert!(Arc::strong_count(&b) == 2);
1423 assert!(Arc::strong_count(&a) == 2);
1426 assert!(Arc::strong_count(&b) == 1);
1428 assert!(Arc::strong_count(&b) == 2);
1429 assert!(Arc::strong_count(&c) == 2);
1433 fn test_weak_count() {
1434 let a = Arc::new(0);
1435 assert!(Arc::strong_count(&a) == 1);
1436 assert!(Arc::weak_count(&a) == 0);
1437 let w = Arc::downgrade(&a);
1438 assert!(Arc::strong_count(&a) == 1);
1439 assert!(Arc::weak_count(&a) == 1);
1441 assert!(Arc::weak_count(&a) == 2);
1444 assert!(Arc::strong_count(&a) == 1);
1445 assert!(Arc::weak_count(&a) == 0);
1447 assert!(Arc::strong_count(&a) == 2);
1448 assert!(Arc::weak_count(&a) == 0);
1449 let d = Arc::downgrade(&c);
1450 assert!(Arc::weak_count(&c) == 1);
1451 assert!(Arc::strong_count(&c) == 2);
1460 let a = Arc::new(5);
1461 assert_eq!(format!("{:?}", a), "5");
1464 // Make sure deriving works with Arc<T>
1465 #[derive(Eq, Ord, PartialEq, PartialOrd, Clone, Debug, Default)]
1472 let x: Arc<[i32]> = Arc::new([1, 2, 3]);
1473 assert_eq!(format!("{:?}", x), "[1, 2, 3]");
1474 let y = Arc::downgrade(&x.clone());
1476 assert!(y.upgrade().is_none());
1480 fn test_from_owned() {
1482 let foo_arc = Arc::from(foo);
1483 assert!(123 == *foo_arc);
1487 fn test_new_weak() {
1488 let foo: Weak<usize> = Weak::new();
1489 assert!(foo.upgrade().is_none());
1494 let five = Arc::new(5);
1495 let same_five = five.clone();
1496 let other_five = Arc::new(5);
1498 assert!(Arc::ptr_eq(&five, &same_five));
1499 assert!(!Arc::ptr_eq(&five, &other_five));
1503 #[stable(feature = "rust1", since = "1.0.0")]
1504 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
1505 fn borrow(&self) -> &T {
1510 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1511 impl<T: ?Sized> AsRef<T> for Arc<T> {
1512 fn as_ref(&self) -> &T {