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::mem::{align_of_val, size_of_val};
27 use core::intrinsics::abort;
29 use core::mem::uninitialized;
31 use core::ops::CoerceUnsized;
32 use core::ptr::{self, Shared};
33 use core::marker::Unsize;
34 use core::hash::{Hash, Hasher};
35 use core::{isize, usize};
36 use core::convert::From;
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.
47 /// The type `Arc<T>` provides shared ownership of a value of type `T`,
48 /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
49 /// a new pointer to the same value in the heap. When the last `Arc`
50 /// pointer to a given value is destroyed, the pointed-to value is
53 /// Shared references in Rust disallow mutation by default, and `Arc` is no
54 /// exception. If you need to mutate through an `Arc`, use [`Mutex`][mutex],
55 /// [`RwLock`][rwlock], or one of the [`Atomic`][atomic] types.
57 /// `Arc` uses atomic operations for reference counting, so `Arc`s can be
58 /// sent between threads. In other words, `Arc<T>` implements [`Send`]
59 /// as long as `T` implements [`Send`] and [`Sync`][sync]. The disadvantage is
60 /// that atomic operations are more expensive than ordinary memory accesses.
61 /// If you are not sharing reference-counted values between threads, consider
62 /// using [`rc::Rc`][`Rc`] for lower overhead. [`Rc`] is a safe default, because
63 /// the compiler will catch any attempt to send an [`Rc`] between threads.
64 /// However, a library might choose `Arc` in order to give library consumers
67 /// The [`downgrade`][downgrade] method can be used to create a non-owning
68 /// [`Weak`][weak] pointer. A [`Weak`][weak] pointer can be [`upgrade`][upgrade]d
69 /// to an `Arc`, but this will return [`None`] if the value has already been
72 /// A cycle between `Arc` pointers will never be deallocated. For this reason,
73 /// [`Weak`][weak] is used to break cycles. For example, a tree could have
74 /// strong `Arc` pointers from parent nodes to children, and [`Weak`][weak]
75 /// pointers from children back to their parents.
77 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
78 /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
79 /// clashes with `T`'s methods, the methods of `Arc<T>` itself are [associated
80 /// functions][assoc], called using function-like syntax:
83 /// use std::sync::Arc;
84 /// let my_arc = Arc::new(());
86 /// Arc::downgrade(&my_arc);
89 /// [`Weak<T>`][weak] does not auto-dereference to `T`, because the value may have
90 /// already been destroyed.
92 /// [arc]: struct.Arc.html
93 /// [weak]: struct.Weak.html
94 /// [`Rc`]: ../../std/rc/struct.Rc.html
95 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
96 /// [mutex]: ../../std/sync/struct.Mutex.html
97 /// [rwlock]: ../../std/sync/struct.RwLock.html
98 /// [atomic]: ../../std/sync/atomic/index.html
99 /// [`Send`]: ../../std/marker/trait.Send.html
100 /// [sync]: ../../std/marker/trait.Sync.html
101 /// [deref]: ../../std/ops/trait.Deref.html
102 /// [downgrade]: struct.Arc.html#method.downgrade
103 /// [upgrade]: struct.Weak.html#method.upgrade
104 /// [`None`]: ../../std/option/enum.Option.html#variant.None
105 /// [assoc]: ../../book/first-edition/method-syntax.html#associated-functions
109 /// Sharing some immutable data between threads:
111 // Note that we **do not** run these tests here. The windows builders get super
112 // unhappy if a thread outlives the main thread and then exits at the same time
113 // (something deadlocks) so we just avoid this entirely by not running these
116 /// use std::sync::Arc;
119 /// let five = Arc::new(5);
122 /// let five = five.clone();
124 /// thread::spawn(move || {
125 /// println!("{:?}", five);
130 /// Sharing a mutable [`AtomicUsize`]:
132 /// [`AtomicUsize`]: ../../std/sync/atomic/struct.AtomicUsize.html
135 /// use std::sync::Arc;
136 /// use std::sync::atomic::{AtomicUsize, Ordering};
139 /// let val = Arc::new(AtomicUsize::new(5));
142 /// let val = val.clone();
144 /// thread::spawn(move || {
145 /// let v = val.fetch_add(1, Ordering::SeqCst);
146 /// println!("{:?}", v);
151 /// See the [`rc` documentation][rc_examples] for more examples of reference
152 /// counting in general.
154 /// [rc_examples]: ../../std/rc/index.html#examples
155 #[stable(feature = "rust1", since = "1.0.0")]
156 pub struct Arc<T: ?Sized> {
157 ptr: Shared<ArcInner<T>>,
160 #[stable(feature = "rust1", since = "1.0.0")]
161 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
162 #[stable(feature = "rust1", since = "1.0.0")]
163 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
165 #[unstable(feature = "coerce_unsized", issue = "27732")]
166 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
168 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
169 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
170 /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
172 /// Since a `Weak` reference does not count towards ownership, it will not
173 /// prevent the inner value from being dropped, and `Weak` itself makes no
174 /// guarantees about the value still being present and may return [`None`]
175 /// when [`upgrade`]d.
177 /// A `Weak` pointer is useful for keeping a temporary reference to the value
178 /// within [`Arc`] without extending its lifetime. It is also used to prevent
179 /// circular references between [`Arc`] pointers, since mutual owning references
180 /// would never allow either [`Arc`] to be dropped. For example, a tree could
181 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
182 /// pointers from children back to their parents.
184 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
186 /// [`Arc`]: struct.Arc.html
187 /// [`Arc::downgrade`]: struct.Arc.html#method.downgrade
188 /// [`upgrade`]: struct.Weak.html#method.upgrade
189 /// [`Option`]: ../../std/option/enum.Option.html
190 /// [`None`]: ../../std/option/enum.Option.html#variant.None
191 #[stable(feature = "arc_weak", since = "1.4.0")]
192 pub struct Weak<T: ?Sized> {
193 ptr: Shared<ArcInner<T>>,
196 #[stable(feature = "arc_weak", since = "1.4.0")]
197 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
198 #[stable(feature = "arc_weak", since = "1.4.0")]
199 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
201 #[unstable(feature = "coerce_unsized", issue = "27732")]
202 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
204 #[stable(feature = "arc_weak", since = "1.4.0")]
205 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
206 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
211 struct ArcInner<T: ?Sized> {
212 strong: atomic::AtomicUsize,
214 // the value usize::MAX acts as a sentinel for temporarily "locking" the
215 // ability to upgrade weak pointers or downgrade strong ones; this is used
216 // to avoid races in `make_mut` and `get_mut`.
217 weak: atomic::AtomicUsize,
222 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
223 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
226 /// Constructs a new `Arc<T>`.
231 /// use std::sync::Arc;
233 /// let five = Arc::new(5);
236 #[stable(feature = "rust1", since = "1.0.0")]
237 pub fn new(data: T) -> Arc<T> {
238 // Start the weak pointer count as 1 which is the weak pointer that's
239 // held by all the strong pointers (kinda), see std/rc.rs for more info
240 let x: Box<_> = box ArcInner {
241 strong: atomic::AtomicUsize::new(1),
242 weak: atomic::AtomicUsize::new(1),
245 Arc { ptr: unsafe { Shared::new(Box::into_raw(x)) } }
248 /// Returns the contained value, if the `Arc` has exactly one strong reference.
250 /// Otherwise, an [`Err`][result] is returned with the same `Arc` that was
253 /// This will succeed even if there are outstanding weak references.
255 /// [result]: ../../std/result/enum.Result.html
260 /// use std::sync::Arc;
262 /// let x = Arc::new(3);
263 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
265 /// let x = Arc::new(4);
266 /// let _y = x.clone();
267 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
270 #[stable(feature = "arc_unique", since = "1.4.0")]
271 pub fn try_unwrap(this: Self) -> Result<T, Self> {
272 // See `drop` for why all these atomics are like this
273 if this.inner().strong.compare_exchange(1, 0, Release, Relaxed).is_err() {
277 atomic::fence(Acquire);
281 let elem = ptr::read(&(*ptr).data);
283 // Make a weak pointer to clean up the implicit strong-weak reference
284 let _weak = Weak { ptr: this.ptr };
291 /// Consumes the `Arc`, returning the wrapped pointer.
293 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
294 /// [`Arc::from_raw`][from_raw].
296 /// [from_raw]: struct.Arc.html#method.from_raw
301 /// use std::sync::Arc;
303 /// let x = Arc::new(10);
304 /// let x_ptr = Arc::into_raw(x);
305 /// assert_eq!(unsafe { *x_ptr }, 10);
307 #[stable(feature = "rc_raw", since = "1.17.0")]
308 pub fn into_raw(this: Self) -> *const T {
309 let ptr = unsafe { &(**this.ptr).data as *const _ };
314 /// Constructs an `Arc` from a raw pointer.
316 /// The raw pointer must have been previously returned by a call to a
317 /// [`Arc::into_raw`][into_raw].
319 /// This function is unsafe because improper use may lead to memory problems. For example, a
320 /// double-free may occur if the function is called twice on the same raw pointer.
322 /// [into_raw]: struct.Arc.html#method.into_raw
327 /// use std::sync::Arc;
329 /// let x = Arc::new(10);
330 /// let x_ptr = Arc::into_raw(x);
333 /// // Convert back to an `Arc` to prevent leak.
334 /// let x = Arc::from_raw(x_ptr);
335 /// assert_eq!(*x, 10);
337 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory unsafe.
340 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
342 #[stable(feature = "rc_raw", since = "1.17.0")]
343 pub unsafe fn from_raw(ptr: *const T) -> Self {
344 // To find the corresponding pointer to the `ArcInner` we need to subtract the offset of the
345 // `data` field from the pointer.
346 let ptr = (ptr as *const u8).offset(-offset_of!(ArcInner<T>, data));
348 ptr: Shared::new(ptr as *const _),
353 impl<T: ?Sized> Arc<T> {
354 /// Creates a new [`Weak`][weak] pointer to this value.
356 /// [weak]: struct.Weak.html
361 /// use std::sync::Arc;
363 /// let five = Arc::new(5);
365 /// let weak_five = Arc::downgrade(&five);
367 #[stable(feature = "arc_weak", since = "1.4.0")]
368 pub fn downgrade(this: &Self) -> Weak<T> {
369 // This Relaxed is OK because we're checking the value in the CAS
371 let mut cur = this.inner().weak.load(Relaxed);
374 // check if the weak counter is currently "locked"; if so, spin.
375 if cur == usize::MAX {
376 cur = this.inner().weak.load(Relaxed);
380 // NOTE: this code currently ignores the possibility of overflow
381 // into usize::MAX; in general both Rc and Arc need to be adjusted
382 // to deal with overflow.
384 // Unlike with Clone(), we need this to be an Acquire read to
385 // synchronize with the write coming from `is_unique`, so that the
386 // events prior to that write happen before this read.
387 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
388 Ok(_) => return Weak { ptr: this.ptr },
389 Err(old) => cur = old,
394 /// Gets the number of [`Weak`][weak] pointers to this value.
396 /// [weak]: struct.Weak.html
400 /// This method by itself is safe, but using it correctly requires extra care.
401 /// Another thread can change the weak count at any time,
402 /// including potentially between calling this method and acting on the result.
407 /// use std::sync::Arc;
409 /// let five = Arc::new(5);
410 /// let _weak_five = Arc::downgrade(&five);
412 /// // This assertion is deterministic because we haven't shared
413 /// // the `Arc` or `Weak` between threads.
414 /// assert_eq!(1, Arc::weak_count(&five));
417 #[stable(feature = "arc_counts", since = "1.15.0")]
418 pub fn weak_count(this: &Self) -> usize {
419 this.inner().weak.load(SeqCst) - 1
422 /// Gets the number of strong (`Arc`) pointers to this value.
426 /// This method by itself is safe, but using it correctly requires extra care.
427 /// Another thread can change the strong count at any time,
428 /// including potentially between calling this method and acting on the result.
433 /// use std::sync::Arc;
435 /// let five = Arc::new(5);
436 /// let _also_five = five.clone();
438 /// // This assertion is deterministic because we haven't shared
439 /// // the `Arc` between threads.
440 /// assert_eq!(2, Arc::strong_count(&five));
443 #[stable(feature = "arc_counts", since = "1.15.0")]
444 pub fn strong_count(this: &Self) -> usize {
445 this.inner().strong.load(SeqCst)
449 fn inner(&self) -> &ArcInner<T> {
450 // This unsafety is ok because while this arc is alive we're guaranteed
451 // that the inner pointer is valid. Furthermore, we know that the
452 // `ArcInner` structure itself is `Sync` because the inner data is
453 // `Sync` as well, so we're ok loaning out an immutable pointer to these
455 unsafe { &**self.ptr }
458 // Non-inlined part of `drop`.
460 unsafe fn drop_slow(&mut self) {
461 let ptr = self.ptr.as_mut_ptr();
463 // Destroy the data at this time, even though we may not free the box
464 // allocation itself (there may still be weak pointers lying around).
465 ptr::drop_in_place(&mut (*ptr).data);
467 if self.inner().weak.fetch_sub(1, Release) == 1 {
468 atomic::fence(Acquire);
469 deallocate(ptr as *mut u8, size_of_val(&*ptr), align_of_val(&*ptr))
474 #[stable(feature = "ptr_eq", since = "1.17.0")]
475 /// Returns true if the two `Arc`s point to the same value (not
476 /// just values that compare as equal).
481 /// use std::sync::Arc;
483 /// let five = Arc::new(5);
484 /// let same_five = five.clone();
485 /// let other_five = Arc::new(5);
487 /// assert!(Arc::ptr_eq(&five, &same_five));
488 /// assert!(!Arc::ptr_eq(&five, &other_five));
490 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
491 let this_ptr: *const ArcInner<T> = *this.ptr;
492 let other_ptr: *const ArcInner<T> = *other.ptr;
493 this_ptr == other_ptr
497 #[stable(feature = "rust1", since = "1.0.0")]
498 impl<T: ?Sized> Clone for Arc<T> {
499 /// Makes a clone of the `Arc` pointer.
501 /// This creates another pointer to the same inner value, increasing the
502 /// strong reference count.
507 /// use std::sync::Arc;
509 /// let five = Arc::new(5);
514 fn clone(&self) -> Arc<T> {
515 // Using a relaxed ordering is alright here, as knowledge of the
516 // original reference prevents other threads from erroneously deleting
519 // As explained in the [Boost documentation][1], Increasing the
520 // reference counter can always be done with memory_order_relaxed: New
521 // references to an object can only be formed from an existing
522 // reference, and passing an existing reference from one thread to
523 // another must already provide any required synchronization.
525 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
526 let old_size = self.inner().strong.fetch_add(1, Relaxed);
528 // However we need to guard against massive refcounts in case someone
529 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
530 // and users will use-after free. We racily saturate to `isize::MAX` on
531 // the assumption that there aren't ~2 billion threads incrementing
532 // the reference count at once. This branch will never be taken in
533 // any realistic program.
535 // We abort because such a program is incredibly degenerate, and we
536 // don't care to support it.
537 if old_size > MAX_REFCOUNT {
543 Arc { ptr: self.ptr }
547 #[stable(feature = "rust1", since = "1.0.0")]
548 impl<T: ?Sized> Deref for Arc<T> {
552 fn deref(&self) -> &T {
557 impl<T: Clone> Arc<T> {
558 /// Makes a mutable reference into the given `Arc`.
560 /// If there are other `Arc` or [`Weak`][weak] pointers to the same value,
561 /// then `make_mut` will invoke [`clone`][clone] on the inner value to
562 /// ensure unique ownership. This is also referred to as clone-on-write.
564 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
566 /// [weak]: struct.Weak.html
567 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
568 /// [get_mut]: struct.Arc.html#method.get_mut
573 /// use std::sync::Arc;
575 /// let mut data = Arc::new(5);
577 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
578 /// let mut other_data = data.clone(); // Won't clone inner data
579 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
580 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
581 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
583 /// // Now `data` and `other_data` point to different values.
584 /// assert_eq!(*data, 8);
585 /// assert_eq!(*other_data, 12);
588 #[stable(feature = "arc_unique", since = "1.4.0")]
589 pub fn make_mut(this: &mut Self) -> &mut T {
590 // Note that we hold both a strong reference and a weak reference.
591 // Thus, releasing our strong reference only will not, by itself, cause
592 // the memory to be deallocated.
594 // Use Acquire to ensure that we see any writes to `weak` that happen
595 // before release writes (i.e., decrements) to `strong`. Since we hold a
596 // weak count, there's no chance the ArcInner itself could be
598 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
599 // Another strong pointer exists; clone
600 *this = Arc::new((**this).clone());
601 } else if this.inner().weak.load(Relaxed) != 1 {
602 // Relaxed suffices in the above because this is fundamentally an
603 // optimization: we are always racing with weak pointers being
604 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
606 // We removed the last strong ref, but there are additional weak
607 // refs remaining. We'll move the contents to a new Arc, and
608 // invalidate the other weak refs.
610 // Note that it is not possible for the read of `weak` to yield
611 // usize::MAX (i.e., locked), since the weak count can only be
612 // locked by a thread with a strong reference.
614 // Materialize our own implicit weak pointer, so that it can clean
615 // up the ArcInner as needed.
616 let weak = Weak { ptr: this.ptr };
618 // mark the data itself as already deallocated
620 // there is no data race in the implicit write caused by `read`
621 // here (due to zeroing) because data is no longer accessed by
622 // other threads (due to there being no more strong refs at this
624 let mut swap = Arc::new(ptr::read(&(**weak.ptr).data));
625 mem::swap(this, &mut swap);
629 // We were the sole reference of either kind; bump back up the
631 this.inner().strong.store(1, Release);
634 // As with `get_mut()`, the unsafety is ok because our reference was
635 // either unique to begin with, or became one upon cloning the contents.
637 let inner = &mut *this.ptr.as_mut_ptr();
643 impl<T: ?Sized> Arc<T> {
644 /// Returns a mutable reference to the inner value, if there are
645 /// no other `Arc` or [`Weak`][weak] pointers to the same value.
647 /// Returns [`None`][option] otherwise, because it is not safe to
648 /// mutate a shared value.
650 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
651 /// the inner value when it's shared.
653 /// [weak]: struct.Weak.html
654 /// [option]: ../../std/option/enum.Option.html
655 /// [make_mut]: struct.Arc.html#method.make_mut
656 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
661 /// use std::sync::Arc;
663 /// let mut x = Arc::new(3);
664 /// *Arc::get_mut(&mut x).unwrap() = 4;
665 /// assert_eq!(*x, 4);
667 /// let _y = x.clone();
668 /// assert!(Arc::get_mut(&mut x).is_none());
671 #[stable(feature = "arc_unique", since = "1.4.0")]
672 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
673 if this.is_unique() {
674 // This unsafety is ok because we're guaranteed that the pointer
675 // returned is the *only* pointer that will ever be returned to T. Our
676 // reference count is guaranteed to be 1 at this point, and we required
677 // the Arc itself to be `mut`, so we're returning the only possible
678 // reference to the inner data.
680 let inner = &mut *this.ptr.as_mut_ptr();
681 Some(&mut inner.data)
688 /// Determine whether this is the unique reference (including weak refs) to
689 /// the underlying data.
691 /// Note that this requires locking the weak ref count.
692 fn is_unique(&mut self) -> bool {
693 // lock the weak pointer count if we appear to be the sole weak pointer
696 // The acquire label here ensures a happens-before relationship with any
697 // writes to `strong` prior to decrements of the `weak` count (via drop,
698 // which uses Release).
699 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
700 // Due to the previous acquire read, this will observe any writes to
701 // `strong` that were due to upgrading weak pointers; only strong
702 // clones remain, which require that the strong count is > 1 anyway.
703 let unique = self.inner().strong.load(Relaxed) == 1;
705 // The release write here synchronizes with a read in `downgrade`,
706 // effectively preventing the above read of `strong` from happening
708 self.inner().weak.store(1, Release); // release the lock
716 #[stable(feature = "rust1", since = "1.0.0")]
717 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
720 /// This will decrement the strong reference count. If the strong reference
721 /// count reaches zero then the only other references (if any) are
722 /// [`Weak`][weak], so we `drop` the inner value.
724 /// [weak]: struct.Weak.html
729 /// use std::sync::Arc;
733 /// impl Drop for Foo {
734 /// fn drop(&mut self) {
735 /// println!("dropped!");
739 /// let foo = Arc::new(Foo);
740 /// let foo2 = foo.clone();
742 /// drop(foo); // Doesn't print anything
743 /// drop(foo2); // Prints "dropped!"
747 // Because `fetch_sub` is already atomic, we do not need to synchronize
748 // with other threads unless we are going to delete the object. This
749 // same logic applies to the below `fetch_sub` to the `weak` count.
750 if self.inner().strong.fetch_sub(1, Release) != 1 {
754 // This fence is needed to prevent reordering of use of the data and
755 // deletion of the data. Because it is marked `Release`, the decreasing
756 // of the reference count synchronizes with this `Acquire` fence. This
757 // means that use of the data happens before decreasing the reference
758 // count, which happens before this fence, which happens before the
759 // deletion of the data.
761 // As explained in the [Boost documentation][1],
763 // > It is important to enforce any possible access to the object in one
764 // > thread (through an existing reference) to *happen before* deleting
765 // > the object in a different thread. This is achieved by a "release"
766 // > operation after dropping a reference (any access to the object
767 // > through this reference must obviously happened before), and an
768 // > "acquire" operation before deleting the object.
770 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
771 atomic::fence(Acquire);
780 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
781 /// it. Calling [`upgrade`] on the return value always gives [`None`].
783 /// [`upgrade`]: struct.Weak.html#method.upgrade
784 /// [`None`]: ../../std/option/enum.Option.html#variant.None
789 /// use std::sync::Weak;
791 /// let empty: Weak<i64> = Weak::new();
792 /// assert!(empty.upgrade().is_none());
794 #[stable(feature = "downgraded_weak", since = "1.10.0")]
795 pub fn new() -> Weak<T> {
798 ptr: Shared::new(Box::into_raw(box ArcInner {
799 strong: atomic::AtomicUsize::new(0),
800 weak: atomic::AtomicUsize::new(1),
801 data: uninitialized(),
808 impl<T: ?Sized> Weak<T> {
809 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], extending
810 /// the lifetime of the value if successful.
812 /// Returns [`None`] if the value has since been dropped.
814 /// [`Arc`]: struct.Arc.html
815 /// [`None`]: ../../std/option/enum.Option.html#variant.None
820 /// use std::sync::Arc;
822 /// let five = Arc::new(5);
824 /// let weak_five = Arc::downgrade(&five);
826 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
827 /// assert!(strong_five.is_some());
829 /// // Destroy all strong pointers.
830 /// drop(strong_five);
833 /// assert!(weak_five.upgrade().is_none());
835 #[stable(feature = "arc_weak", since = "1.4.0")]
836 pub fn upgrade(&self) -> Option<Arc<T>> {
837 // We use a CAS loop to increment the strong count instead of a
838 // fetch_add because once the count hits 0 it must never be above 0.
839 let inner = self.inner();
841 // Relaxed load because any write of 0 that we can observe
842 // leaves the field in a permanently zero state (so a
843 // "stale" read of 0 is fine), and any other value is
844 // confirmed via the CAS below.
845 let mut n = inner.strong.load(Relaxed);
852 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
853 if n > MAX_REFCOUNT {
859 // Relaxed is valid for the same reason it is on Arc's Clone impl
860 match inner.strong.compare_exchange_weak(n, n + 1, Relaxed, Relaxed) {
861 Ok(_) => return Some(Arc { ptr: self.ptr }),
868 fn inner(&self) -> &ArcInner<T> {
869 // See comments above for why this is "safe"
870 unsafe { &**self.ptr }
874 #[stable(feature = "arc_weak", since = "1.4.0")]
875 impl<T: ?Sized> Clone for Weak<T> {
876 /// Makes a clone of the `Weak` pointer that points to the same value.
881 /// use std::sync::Arc;
883 /// let weak_five = Arc::downgrade(&Arc::new(5));
885 /// weak_five.clone();
888 fn clone(&self) -> Weak<T> {
889 // See comments in Arc::clone() for why this is relaxed. This can use a
890 // fetch_add (ignoring the lock) because the weak count is only locked
891 // where are *no other* weak pointers in existence. (So we can't be
892 // running this code in that case).
893 let old_size = self.inner().weak.fetch_add(1, Relaxed);
895 // See comments in Arc::clone() for why we do this (for mem::forget).
896 if old_size > MAX_REFCOUNT {
902 return Weak { ptr: self.ptr };
906 #[stable(feature = "downgraded_weak", since = "1.10.0")]
907 impl<T> Default for Weak<T> {
908 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
909 /// it. Calling [`upgrade`] on the return value always gives [`None`].
911 /// [`upgrade`]: struct.Weak.html#method.upgrade
912 /// [`None`]: ../../std/option/enum.Option.html#variant.None
917 /// use std::sync::Weak;
919 /// let empty: Weak<i64> = Default::default();
920 /// assert!(empty.upgrade().is_none());
922 fn default() -> Weak<T> {
927 #[stable(feature = "arc_weak", since = "1.4.0")]
928 impl<T: ?Sized> Drop for Weak<T> {
929 /// Drops the `Weak` pointer.
934 /// use std::sync::Arc;
938 /// impl Drop for Foo {
939 /// fn drop(&mut self) {
940 /// println!("dropped!");
944 /// let foo = Arc::new(Foo);
945 /// let weak_foo = Arc::downgrade(&foo);
946 /// let other_weak_foo = weak_foo.clone();
948 /// drop(weak_foo); // Doesn't print anything
949 /// drop(foo); // Prints "dropped!"
951 /// assert!(other_weak_foo.upgrade().is_none());
956 // If we find out that we were the last weak pointer, then its time to
957 // deallocate the data entirely. See the discussion in Arc::drop() about
958 // the memory orderings
960 // It's not necessary to check for the locked state here, because the
961 // weak count can only be locked if there was precisely one weak ref,
962 // meaning that drop could only subsequently run ON that remaining weak
963 // ref, which can only happen after the lock is released.
964 if self.inner().weak.fetch_sub(1, Release) == 1 {
965 atomic::fence(Acquire);
966 unsafe { deallocate(ptr as *mut u8, size_of_val(&*ptr), align_of_val(&*ptr)) }
971 #[stable(feature = "rust1", since = "1.0.0")]
972 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
973 /// Equality for two `Arc`s.
975 /// Two `Arc`s are equal if their inner values are equal.
980 /// use std::sync::Arc;
982 /// let five = Arc::new(5);
984 /// assert!(five == Arc::new(5));
986 fn eq(&self, other: &Arc<T>) -> bool {
987 *(*self) == *(*other)
990 /// Inequality for two `Arc`s.
992 /// Two `Arc`s are unequal if their inner values are unequal.
997 /// use std::sync::Arc;
999 /// let five = Arc::new(5);
1001 /// assert!(five != Arc::new(6));
1003 fn ne(&self, other: &Arc<T>) -> bool {
1004 *(*self) != *(*other)
1007 #[stable(feature = "rust1", since = "1.0.0")]
1008 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
1009 /// Partial comparison for two `Arc`s.
1011 /// The two are compared by calling `partial_cmp()` on their inner values.
1016 /// use std::sync::Arc;
1017 /// use std::cmp::Ordering;
1019 /// let five = Arc::new(5);
1021 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
1023 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
1024 (**self).partial_cmp(&**other)
1027 /// Less-than comparison for two `Arc`s.
1029 /// The two are compared by calling `<` on their inner values.
1034 /// use std::sync::Arc;
1036 /// let five = Arc::new(5);
1038 /// assert!(five < Arc::new(6));
1040 fn lt(&self, other: &Arc<T>) -> bool {
1041 *(*self) < *(*other)
1044 /// 'Less than or equal to' comparison for two `Arc`s.
1046 /// The two are compared by calling `<=` on their inner values.
1051 /// use std::sync::Arc;
1053 /// let five = Arc::new(5);
1055 /// assert!(five <= Arc::new(5));
1057 fn le(&self, other: &Arc<T>) -> bool {
1058 *(*self) <= *(*other)
1061 /// Greater-than comparison for two `Arc`s.
1063 /// The two are compared by calling `>` on their inner values.
1068 /// use std::sync::Arc;
1070 /// let five = Arc::new(5);
1072 /// assert!(five > Arc::new(4));
1074 fn gt(&self, other: &Arc<T>) -> bool {
1075 *(*self) > *(*other)
1078 /// 'Greater than or equal to' comparison for two `Arc`s.
1080 /// The two are compared by calling `>=` on their inner values.
1085 /// use std::sync::Arc;
1087 /// let five = Arc::new(5);
1089 /// assert!(five >= Arc::new(5));
1091 fn ge(&self, other: &Arc<T>) -> bool {
1092 *(*self) >= *(*other)
1095 #[stable(feature = "rust1", since = "1.0.0")]
1096 impl<T: ?Sized + Ord> Ord for Arc<T> {
1097 /// Comparison for two `Arc`s.
1099 /// The two are compared by calling `cmp()` on their inner values.
1104 /// use std::sync::Arc;
1105 /// use std::cmp::Ordering;
1107 /// let five = Arc::new(5);
1109 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
1111 fn cmp(&self, other: &Arc<T>) -> Ordering {
1112 (**self).cmp(&**other)
1115 #[stable(feature = "rust1", since = "1.0.0")]
1116 impl<T: ?Sized + Eq> Eq for Arc<T> {}
1118 #[stable(feature = "rust1", since = "1.0.0")]
1119 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
1120 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1121 fmt::Display::fmt(&**self, f)
1125 #[stable(feature = "rust1", since = "1.0.0")]
1126 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
1127 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1128 fmt::Debug::fmt(&**self, f)
1132 #[stable(feature = "rust1", since = "1.0.0")]
1133 impl<T: ?Sized> fmt::Pointer for Arc<T> {
1134 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1135 fmt::Pointer::fmt(&*self.ptr, f)
1139 #[stable(feature = "rust1", since = "1.0.0")]
1140 impl<T: Default> Default for Arc<T> {
1141 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
1146 /// use std::sync::Arc;
1148 /// let x: Arc<i32> = Default::default();
1149 /// assert_eq!(*x, 0);
1151 fn default() -> Arc<T> {
1152 Arc::new(Default::default())
1156 #[stable(feature = "rust1", since = "1.0.0")]
1157 impl<T: ?Sized + Hash> Hash for Arc<T> {
1158 fn hash<H: Hasher>(&self, state: &mut H) {
1159 (**self).hash(state)
1163 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1164 impl<T> From<T> for Arc<T> {
1165 fn from(t: T) -> Self {
1172 use std::clone::Clone;
1173 use std::sync::mpsc::channel;
1176 use std::option::Option;
1177 use std::option::Option::{None, Some};
1178 use std::sync::atomic;
1179 use std::sync::atomic::Ordering::{Acquire, SeqCst};
1182 use super::{Arc, Weak};
1183 use std::sync::Mutex;
1184 use std::convert::From;
1186 struct Canary(*mut atomic::AtomicUsize);
1188 impl Drop for Canary {
1189 fn drop(&mut self) {
1193 (*c).fetch_add(1, SeqCst);
1201 #[cfg_attr(target_os = "emscripten", ignore)]
1202 fn manually_share_arc() {
1203 let v = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
1204 let arc_v = Arc::new(v);
1206 let (tx, rx) = channel();
1208 let _t = thread::spawn(move || {
1209 let arc_v: Arc<Vec<i32>> = rx.recv().unwrap();
1210 assert_eq!((*arc_v)[3], 4);
1213 tx.send(arc_v.clone()).unwrap();
1215 assert_eq!((*arc_v)[2], 3);
1216 assert_eq!((*arc_v)[4], 5);
1220 fn test_arc_get_mut() {
1221 let mut x = Arc::new(3);
1222 *Arc::get_mut(&mut x).unwrap() = 4;
1225 assert!(Arc::get_mut(&mut x).is_none());
1227 assert!(Arc::get_mut(&mut x).is_some());
1228 let _w = Arc::downgrade(&x);
1229 assert!(Arc::get_mut(&mut x).is_none());
1234 let x = Arc::new(3);
1235 assert_eq!(Arc::try_unwrap(x), Ok(3));
1236 let x = Arc::new(4);
1238 assert_eq!(Arc::try_unwrap(x), Err(Arc::new(4)));
1239 let x = Arc::new(5);
1240 let _w = Arc::downgrade(&x);
1241 assert_eq!(Arc::try_unwrap(x), Ok(5));
1245 fn into_from_raw() {
1246 let x = Arc::new(box "hello");
1249 let x_ptr = Arc::into_raw(x);
1252 assert_eq!(**x_ptr, "hello");
1254 let x = Arc::from_raw(x_ptr);
1255 assert_eq!(**x, "hello");
1257 assert_eq!(Arc::try_unwrap(x).map(|x| *x), Ok("hello"));
1262 fn test_cowarc_clone_make_mut() {
1263 let mut cow0 = Arc::new(75);
1264 let mut cow1 = cow0.clone();
1265 let mut cow2 = cow1.clone();
1267 assert!(75 == *Arc::make_mut(&mut cow0));
1268 assert!(75 == *Arc::make_mut(&mut cow1));
1269 assert!(75 == *Arc::make_mut(&mut cow2));
1271 *Arc::make_mut(&mut cow0) += 1;
1272 *Arc::make_mut(&mut cow1) += 2;
1273 *Arc::make_mut(&mut cow2) += 3;
1275 assert!(76 == *cow0);
1276 assert!(77 == *cow1);
1277 assert!(78 == *cow2);
1279 // none should point to the same backing memory
1280 assert!(*cow0 != *cow1);
1281 assert!(*cow0 != *cow2);
1282 assert!(*cow1 != *cow2);
1286 fn test_cowarc_clone_unique2() {
1287 let mut cow0 = Arc::new(75);
1288 let cow1 = cow0.clone();
1289 let cow2 = cow1.clone();
1291 assert!(75 == *cow0);
1292 assert!(75 == *cow1);
1293 assert!(75 == *cow2);
1295 *Arc::make_mut(&mut cow0) += 1;
1296 assert!(76 == *cow0);
1297 assert!(75 == *cow1);
1298 assert!(75 == *cow2);
1300 // cow1 and cow2 should share the same contents
1301 // cow0 should have a unique reference
1302 assert!(*cow0 != *cow1);
1303 assert!(*cow0 != *cow2);
1304 assert!(*cow1 == *cow2);
1308 fn test_cowarc_clone_weak() {
1309 let mut cow0 = Arc::new(75);
1310 let cow1_weak = Arc::downgrade(&cow0);
1312 assert!(75 == *cow0);
1313 assert!(75 == *cow1_weak.upgrade().unwrap());
1315 *Arc::make_mut(&mut cow0) += 1;
1317 assert!(76 == *cow0);
1318 assert!(cow1_weak.upgrade().is_none());
1323 let x = Arc::new(5);
1324 let y = Arc::downgrade(&x);
1325 assert!(y.upgrade().is_some());
1330 let x = Arc::new(5);
1331 let y = Arc::downgrade(&x);
1333 assert!(y.upgrade().is_none());
1337 fn weak_self_cyclic() {
1339 x: Mutex<Option<Weak<Cycle>>>,
1342 let a = Arc::new(Cycle { x: Mutex::new(None) });
1343 let b = Arc::downgrade(&a.clone());
1344 *a.x.lock().unwrap() = Some(b);
1346 // hopefully we don't double-free (or leak)...
1351 let mut canary = atomic::AtomicUsize::new(0);
1352 let x = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1354 assert!(canary.load(Acquire) == 1);
1358 fn drop_arc_weak() {
1359 let mut canary = atomic::AtomicUsize::new(0);
1360 let arc = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1361 let arc_weak = Arc::downgrade(&arc);
1362 assert!(canary.load(Acquire) == 0);
1364 assert!(canary.load(Acquire) == 1);
1369 fn test_strong_count() {
1370 let a = Arc::new(0);
1371 assert!(Arc::strong_count(&a) == 1);
1372 let w = Arc::downgrade(&a);
1373 assert!(Arc::strong_count(&a) == 1);
1374 let b = w.upgrade().expect("");
1375 assert!(Arc::strong_count(&b) == 2);
1376 assert!(Arc::strong_count(&a) == 2);
1379 assert!(Arc::strong_count(&b) == 1);
1381 assert!(Arc::strong_count(&b) == 2);
1382 assert!(Arc::strong_count(&c) == 2);
1386 fn test_weak_count() {
1387 let a = Arc::new(0);
1388 assert!(Arc::strong_count(&a) == 1);
1389 assert!(Arc::weak_count(&a) == 0);
1390 let w = Arc::downgrade(&a);
1391 assert!(Arc::strong_count(&a) == 1);
1392 assert!(Arc::weak_count(&a) == 1);
1394 assert!(Arc::weak_count(&a) == 2);
1397 assert!(Arc::strong_count(&a) == 1);
1398 assert!(Arc::weak_count(&a) == 0);
1400 assert!(Arc::strong_count(&a) == 2);
1401 assert!(Arc::weak_count(&a) == 0);
1402 let d = Arc::downgrade(&c);
1403 assert!(Arc::weak_count(&c) == 1);
1404 assert!(Arc::strong_count(&c) == 2);
1413 let a = Arc::new(5);
1414 assert_eq!(format!("{:?}", a), "5");
1417 // Make sure deriving works with Arc<T>
1418 #[derive(Eq, Ord, PartialEq, PartialOrd, Clone, Debug, Default)]
1425 let x: Arc<[i32]> = Arc::new([1, 2, 3]);
1426 assert_eq!(format!("{:?}", x), "[1, 2, 3]");
1427 let y = Arc::downgrade(&x.clone());
1429 assert!(y.upgrade().is_none());
1433 fn test_from_owned() {
1435 let foo_arc = Arc::from(foo);
1436 assert!(123 == *foo_arc);
1440 fn test_new_weak() {
1441 let foo: Weak<usize> = Weak::new();
1442 assert!(foo.upgrade().is_none());
1447 let five = Arc::new(5);
1448 let same_five = five.clone();
1449 let other_five = Arc::new(5);
1451 assert!(Arc::ptr_eq(&five, &same_five));
1452 assert!(!Arc::ptr_eq(&five, &other_five));
1456 #[stable(feature = "rust1", since = "1.0.0")]
1457 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
1458 fn borrow(&self) -> &T {
1463 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1464 impl<T: ?Sized> AsRef<T> for Arc<T> {
1465 fn as_ref(&self) -> &T {