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// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your // option. This file may not be copied, modified, or distributed // except according to those terms. #![stable(feature = "rust1", since = "1.0.0")] //! Thread-safe reference-counting pointers. //! //! See the [`Arc<T>`][arc] documentation for more details. //! //! [arc]: struct.Arc.html use boxed::Box; use core::sync::atomic; use core::sync::atomic::Ordering::{Acquire, Relaxed, Release, SeqCst}; use core::borrow; use core::fmt; use core::cmp::Ordering; use core::mem::{align_of_val, size_of_val}; use core::intrinsics::abort; use core::mem; use core::mem::uninitialized; use core::ops::Deref; use core::ops::CoerceUnsized; use core::ptr::{self, Shared}; use core::marker::Unsize; use core::hash::{Hash, Hasher}; use core::{isize, usize}; use core::convert::From; use heap::deallocate; /// A soft limit on the amount of references that may be made to an `Arc`. /// /// Going above this limit will abort your program (although not /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references. const MAX_REFCOUNT: usize = (isize::MAX) as usize; /// A thread-safe reference-counting pointer. /// /// The type `Arc<T>` provides shared ownership of a value of type `T`, /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces /// a new pointer to the same value in the heap. When the last `Arc` /// pointer to a given value is destroyed, the pointed-to value is /// also destroyed. /// /// Shared references in Rust disallow mutation by default, and `Arc` is no /// exception. If you need to mutate through an `Arc`, use [`Mutex`][mutex], /// [`RwLock`][rwlock], or one of the [`Atomic`][atomic] types. /// /// `Arc` uses atomic operations for reference counting, so `Arc`s can be /// sent between threads. In other words, `Arc<T>` implements [`Send`] /// as long as `T` implements [`Send`] and [`Sync`][sync]. The disadvantage is /// that atomic operations are more expensive than ordinary memory accesses. /// If you are not sharing reference-counted values between threads, consider /// using [`rc::Rc`][`Rc`] for lower overhead. [`Rc`] is a safe default, because /// the compiler will catch any attempt to send an [`Rc`] between threads. /// However, a library might choose `Arc` in order to give library consumers /// more flexibility. /// /// The [`downgrade`][downgrade] method can be used to create a non-owning /// [`Weak`][weak] pointer. A [`Weak`][weak] pointer can be [`upgrade`][upgrade]d /// to an `Arc`, but this will return [`None`] if the value has already been /// dropped. /// /// A cycle between `Arc` pointers will never be deallocated. For this reason, /// [`Weak`][weak] is used to break cycles. For example, a tree could have /// strong `Arc` pointers from parent nodes to children, and [`Weak`][weak] /// pointers from children back to their parents. /// /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait), /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name /// clashes with `T`'s methods, the methods of `Arc<T>` itself are [associated /// functions][assoc], called using function-like syntax: /// /// ``` /// use std::sync::Arc; /// let my_arc = Arc::new(()); /// /// Arc::downgrade(&my_arc); /// ``` /// /// [`Weak<T>`][weak] does not auto-dereference to `T`, because the value may have /// already been destroyed. /// /// [arc]: struct.Arc.html /// [weak]: struct.Weak.html /// [`Rc`]: ../../std/rc/struct.Rc.html /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone /// [mutex]: ../../std/sync/struct.Mutex.html /// [rwlock]: ../../std/sync/struct.RwLock.html /// [atomic]: ../../std/sync/atomic/index.html /// [`Send`]: ../../std/marker/trait.Send.html /// [sync]: ../../std/marker/trait.Sync.html /// [deref]: ../../std/ops/trait.Deref.html /// [downgrade]: struct.Arc.html#method.downgrade /// [upgrade]: struct.Weak.html#method.upgrade /// [`None`]: ../../std/option/enum.Option.html#variant.None /// [assoc]: ../../book/method-syntax.html#associated-functions /// /// # Examples /// /// Sharing some immutable data between threads: /// // Note that we **do not** run these tests here. The windows builders get super // unhappy if a thread outlives the main thread and then exits at the same time // (something deadlocks) so we just avoid this entirely by not running these // tests. /// ```no_run /// use std::sync::Arc; /// use std::thread; /// /// let five = Arc::new(5); /// /// for _ in 0..10 { /// let five = five.clone(); /// /// thread::spawn(move || { /// println!("{:?}", five); /// }); /// } /// ``` /// /// Sharing a mutable [`AtomicUsize`]: /// /// [`AtomicUsize`]: ../../std/sync/atomic/struct.AtomicUsize.html /// /// ```no_run /// use std::sync::Arc; /// use std::sync::atomic::{AtomicUsize, Ordering}; /// use std::thread; /// /// let val = Arc::new(AtomicUsize::new(5)); /// /// for _ in 0..10 { /// let val = val.clone(); /// /// thread::spawn(move || { /// let v = val.fetch_add(1, Ordering::SeqCst); /// println!("{:?}", v); /// }); /// } /// ``` /// /// See the [`rc` documentation][rc_examples] for more examples of reference /// counting in general. /// /// [rc_examples]: ../../std/rc/index.html#examples #[stable(feature = "rust1", since = "1.0.0")] pub struct Arc<T: ?Sized> { ptr: Shared<ArcInner<T>>, } #[stable(feature = "rust1", since = "1.0.0")] unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {} #[stable(feature = "rust1", since = "1.0.0")] unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {} #[unstable(feature = "coerce_unsized", issue = "27732")] impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {} /// A weak version of [`Arc`][arc]. /// /// `Weak` pointers do not count towards determining if the inner value /// should be dropped. /// /// The typical way to obtain a `Weak` pointer is to call /// [`Arc::downgrade`][downgrade]. /// /// See the [`Arc`][arc] documentation for more details. /// /// [arc]: struct.Arc.html /// [downgrade]: struct.Arc.html#method.downgrade #[stable(feature = "arc_weak", since = "1.4.0")] pub struct Weak<T: ?Sized> { ptr: Shared<ArcInner<T>>, } #[stable(feature = "arc_weak", since = "1.4.0")] unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {} #[stable(feature = "arc_weak", since = "1.4.0")] unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {} #[unstable(feature = "coerce_unsized", issue = "27732")] impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {} #[stable(feature = "arc_weak", since = "1.4.0")] impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { write!(f, "(Weak)") } } struct ArcInner<T: ?Sized> { strong: atomic::AtomicUsize, // the value usize::MAX acts as a sentinel for temporarily "locking" the // ability to upgrade weak pointers or downgrade strong ones; this is used // to avoid races in `make_mut` and `get_mut`. weak: atomic::AtomicUsize, data: T, } unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {} unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {} impl<T> Arc<T> { /// Constructs a new `Arc<T>`. /// /// # Examples /// /// ``` /// use std::sync::Arc; /// /// let five = Arc::new(5); /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn new(data: T) -> Arc<T> { // Start the weak pointer count as 1 which is the weak pointer that's // held by all the strong pointers (kinda), see std/rc.rs for more info let x: Box<_> = box ArcInner { strong: atomic::AtomicUsize::new(1), weak: atomic::AtomicUsize::new(1), data: data, }; Arc { ptr: unsafe { Shared::new(Box::into_raw(x)) } } } /// Returns the contained value, if the `Arc` has exactly one strong reference. /// /// Otherwise, an [`Err`][result] is returned with the same `Arc` that was /// passed in. /// /// This will succeed even if there are outstanding weak references. /// /// [result]: ../../std/result/enum.Result.html /// /// # Examples /// /// ``` /// use std::sync::Arc; /// /// let x = Arc::new(3); /// assert_eq!(Arc::try_unwrap(x), Ok(3)); /// /// let x = Arc::new(4); /// let _y = x.clone(); /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4); /// ``` #[inline] #[stable(feature = "arc_unique", since = "1.4.0")] pub fn try_unwrap(this: Self) -> Result<T, Self> { // See `drop` for why all these atomics are like this if this.inner().strong.compare_exchange(1, 0, Release, Relaxed).is_err() { return Err(this); } atomic::fence(Acquire); unsafe { let ptr = *this.ptr; let elem = ptr::read(&(*ptr).data); // Make a weak pointer to clean up the implicit strong-weak reference let _weak = Weak { ptr: this.ptr }; mem::forget(this); Ok(elem) } } /// Consumes the `Arc`, returning the wrapped pointer. /// /// To avoid a memory leak the pointer must be converted back to an `Arc` using /// [`Arc::from_raw`][from_raw]. /// /// [from_raw]: struct.Arc.html#method.from_raw /// /// # Examples /// /// ``` /// use std::sync::Arc; /// /// let x = Arc::new(10); /// let x_ptr = Arc::into_raw(x); /// assert_eq!(unsafe { *x_ptr }, 10); /// ``` #[stable(feature = "rc_raw", since = "1.17.0")] pub fn into_raw(this: Self) -> *const T { let ptr = unsafe { &(**this.ptr).data as *const _ }; mem::forget(this); ptr } /// Constructs an `Arc` from a raw pointer. /// /// The raw pointer must have been previously returned by a call to a /// [`Arc::into_raw`][into_raw]. /// /// This function is unsafe because improper use may lead to memory problems. For example, a /// double-free may occur if the function is called twice on the same raw pointer. /// /// [into_raw]: struct.Arc.html#method.into_raw /// /// # Examples /// /// ``` /// use std::sync::Arc; /// /// let x = Arc::new(10); /// let x_ptr = Arc::into_raw(x); /// /// unsafe { /// // Convert back to an `Arc` to prevent leak. /// let x = Arc::from_raw(x_ptr); /// assert_eq!(*x, 10); /// /// // Further calls to `Arc::from_raw(x_ptr)` would be memory unsafe. /// } /// /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling! /// ``` #[stable(feature = "rc_raw", since = "1.17.0")] pub unsafe fn from_raw(ptr: *const T) -> Self { // To find the corresponding pointer to the `ArcInner` we need to subtract the offset of the // `data` field from the pointer. let ptr = (ptr as *const u8).offset(-offset_of!(ArcInner<T>, data)); Arc { ptr: Shared::new(ptr as *const _), } } } impl<T: ?Sized> Arc<T> { /// Creates a new [`Weak`][weak] pointer to this value. /// /// [weak]: struct.Weak.html /// /// # Examples /// /// ``` /// use std::sync::Arc; /// /// let five = Arc::new(5); /// /// let weak_five = Arc::downgrade(&five); /// ``` #[stable(feature = "arc_weak", since = "1.4.0")] pub fn downgrade(this: &Self) -> Weak<T> { // This Relaxed is OK because we're checking the value in the CAS // below. let mut cur = this.inner().weak.load(Relaxed); loop { // check if the weak counter is currently "locked"; if so, spin. if cur == usize::MAX { cur = this.inner().weak.load(Relaxed); continue; } // NOTE: this code currently ignores the possibility of overflow // into usize::MAX; in general both Rc and Arc need to be adjusted // to deal with overflow. // Unlike with Clone(), we need this to be an Acquire read to // synchronize with the write coming from `is_unique`, so that the // events prior to that write happen before this read. match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) { Ok(_) => return Weak { ptr: this.ptr }, Err(old) => cur = old, } } } /// Gets the number of [`Weak`][weak] pointers to this value. /// /// [weak]: struct.Weak.html /// /// # Safety /// /// This method by itself is safe, but using it correctly requires extra care. /// Another thread can change the weak count at any time, /// including potentially between calling this method and acting on the result. /// /// # Examples /// /// ``` /// use std::sync::Arc; /// /// let five = Arc::new(5); /// let _weak_five = Arc::downgrade(&five); /// /// // This assertion is deterministic because we haven't shared /// // the `Arc` or `Weak` between threads. /// assert_eq!(1, Arc::weak_count(&five)); /// ``` #[inline] #[stable(feature = "arc_counts", since = "1.15.0")] pub fn weak_count(this: &Self) -> usize { this.inner().weak.load(SeqCst) - 1 } /// Gets the number of strong (`Arc`) pointers to this value. /// /// # Safety /// /// This method by itself is safe, but using it correctly requires extra care. /// Another thread can change the strong count at any time, /// including potentially between calling this method and acting on the result. /// /// # Examples /// /// ``` /// use std::sync::Arc; /// /// let five = Arc::new(5); /// let _also_five = five.clone(); /// /// // This assertion is deterministic because we haven't shared /// // the `Arc` between threads. /// assert_eq!(2, Arc::strong_count(&five)); /// ``` #[inline] #[stable(feature = "arc_counts", since = "1.15.0")] pub fn strong_count(this: &Self) -> usize { this.inner().strong.load(SeqCst) } #[inline] fn inner(&self) -> &ArcInner<T> { // This unsafety is ok because while this arc is alive we're guaranteed // that the inner pointer is valid. Furthermore, we know that the // `ArcInner` structure itself is `Sync` because the inner data is // `Sync` as well, so we're ok loaning out an immutable pointer to these // contents. unsafe { &**self.ptr } } // Non-inlined part of `drop`. #[inline(never)] unsafe fn drop_slow(&mut self) { let ptr = self.ptr.as_mut_ptr(); // Destroy the data at this time, even though we may not free the box // allocation itself (there may still be weak pointers lying around). ptr::drop_in_place(&mut (*ptr).data); if self.inner().weak.fetch_sub(1, Release) == 1 { atomic::fence(Acquire); deallocate(ptr as *mut u8, size_of_val(&*ptr), align_of_val(&*ptr)) } } #[inline] #[stable(feature = "ptr_eq", since = "1.17.0")] /// Returns true if the two `Arc`s point to the same value (not /// just values that compare as equal). /// /// # Examples /// /// ``` /// use std::sync::Arc; /// /// let five = Arc::new(5); /// let same_five = five.clone(); /// let other_five = Arc::new(5); /// /// assert!(Arc::ptr_eq(&five, &same_five)); /// assert!(!Arc::ptr_eq(&five, &other_five)); /// ``` pub fn ptr_eq(this: &Self, other: &Self) -> bool { let this_ptr: *const ArcInner<T> = *this.ptr; let other_ptr: *const ArcInner<T> = *other.ptr; this_ptr == other_ptr } } #[stable(feature = "rust1", since = "1.0.0")] impl<T: ?Sized> Clone for Arc<T> { /// Makes a clone of the `Arc` pointer. /// /// This creates another pointer to the same inner value, increasing the /// strong reference count. /// /// # Examples /// /// ``` /// use std::sync::Arc; /// /// let five = Arc::new(5); /// /// five.clone(); /// ``` #[inline] fn clone(&self) -> Arc<T> { // Using a relaxed ordering is alright here, as knowledge of the // original reference prevents other threads from erroneously deleting // the object. // // As explained in the [Boost documentation][1], Increasing the // reference counter can always be done with memory_order_relaxed: New // references to an object can only be formed from an existing // reference, and passing an existing reference from one thread to // another must already provide any required synchronization. // // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html) let old_size = self.inner().strong.fetch_add(1, Relaxed); // However we need to guard against massive refcounts in case someone // is `mem::forget`ing Arcs. If we don't do this the count can overflow // and users will use-after free. We racily saturate to `isize::MAX` on // the assumption that there aren't ~2 billion threads incrementing // the reference count at once. This branch will never be taken in // any realistic program. // // We abort because such a program is incredibly degenerate, and we // don't care to support it. if old_size > MAX_REFCOUNT { unsafe { abort(); } } Arc { ptr: self.ptr } } } #[stable(feature = "rust1", since = "1.0.0")] impl<T: ?Sized> Deref for Arc<T> { type Target = T; #[inline] fn deref(&self) -> &T { &self.inner().data } } impl<T: Clone> Arc<T> { /// Makes a mutable reference into the given `Arc`. /// /// If there are other `Arc` or [`Weak`][weak] pointers to the same value, /// then `make_mut` will invoke [`clone`][clone] on the inner value to /// ensure unique ownership. This is also referred to as clone-on-write. /// /// See also [`get_mut`][get_mut], which will fail rather than cloning. /// /// [weak]: struct.Weak.html /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone /// [get_mut]: struct.Arc.html#method.get_mut /// /// # Examples /// /// ``` /// use std::sync::Arc; /// /// let mut data = Arc::new(5); /// /// *Arc::make_mut(&mut data) += 1; // Won't clone anything /// let mut other_data = data.clone(); // Won't clone inner data /// *Arc::make_mut(&mut data) += 1; // Clones inner data /// *Arc::make_mut(&mut data) += 1; // Won't clone anything /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything /// /// // Now `data` and `other_data` point to different values. /// assert_eq!(*data, 8); /// assert_eq!(*other_data, 12); /// ``` #[inline] #[stable(feature = "arc_unique", since = "1.4.0")] pub fn make_mut(this: &mut Self) -> &mut T { // Note that we hold both a strong reference and a weak reference. // Thus, releasing our strong reference only will not, by itself, cause // the memory to be deallocated. // // Use Acquire to ensure that we see any writes to `weak` that happen // before release writes (i.e., decrements) to `strong`. Since we hold a // weak count, there's no chance the ArcInner itself could be // deallocated. if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() { // Another strong pointer exists; clone *this = Arc::new((**this).clone()); } else if this.inner().weak.load(Relaxed) != 1 { // Relaxed suffices in the above because this is fundamentally an // optimization: we are always racing with weak pointers being // dropped. Worst case, we end up allocated a new Arc unnecessarily. // We removed the last strong ref, but there are additional weak // refs remaining. We'll move the contents to a new Arc, and // invalidate the other weak refs. // Note that it is not possible for the read of `weak` to yield // usize::MAX (i.e., locked), since the weak count can only be // locked by a thread with a strong reference. // Materialize our own implicit weak pointer, so that it can clean // up the ArcInner as needed. let weak = Weak { ptr: this.ptr }; // mark the data itself as already deallocated unsafe { // there is no data race in the implicit write caused by `read` // here (due to zeroing) because data is no longer accessed by // other threads (due to there being no more strong refs at this // point). let mut swap = Arc::new(ptr::read(&(**weak.ptr).data)); mem::swap(this, &mut swap); mem::forget(swap); } } else { // We were the sole reference of either kind; bump back up the // strong ref count. this.inner().strong.store(1, Release); } // As with `get_mut()`, the unsafety is ok because our reference was // either unique to begin with, or became one upon cloning the contents. unsafe { let inner = &mut *this.ptr.as_mut_ptr(); &mut inner.data } } } impl<T: ?Sized> Arc<T> { /// Returns a mutable reference to the inner value, if there are /// no other `Arc` or [`Weak`][weak] pointers to the same value. /// /// Returns [`None`][option] otherwise, because it is not safe to /// mutate a shared value. /// /// See also [`make_mut`][make_mut], which will [`clone`][clone] /// the inner value when it's shared. /// /// [weak]: struct.Weak.html /// [option]: ../../std/option/enum.Option.html /// [make_mut]: struct.Arc.html#method.make_mut /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone /// /// # Examples /// /// ``` /// use std::sync::Arc; /// /// let mut x = Arc::new(3); /// *Arc::get_mut(&mut x).unwrap() = 4; /// assert_eq!(*x, 4); /// /// let _y = x.clone(); /// assert!(Arc::get_mut(&mut x).is_none()); /// ``` #[inline] #[stable(feature = "arc_unique", since = "1.4.0")] pub fn get_mut(this: &mut Self) -> Option<&mut T> { if this.is_unique() { // This unsafety is ok because we're guaranteed that the pointer // returned is the *only* pointer that will ever be returned to T. Our // reference count is guaranteed to be 1 at this point, and we required // the Arc itself to be `mut`, so we're returning the only possible // reference to the inner data. unsafe { let inner = &mut *this.ptr.as_mut_ptr(); Some(&mut inner.data) } } else { None } } /// Determine whether this is the unique reference (including weak refs) to /// the underlying data. /// /// Note that this requires locking the weak ref count. fn is_unique(&mut self) -> bool { // lock the weak pointer count if we appear to be the sole weak pointer // holder. // // The acquire label here ensures a happens-before relationship with any // writes to `strong` prior to decrements of the `weak` count (via drop, // which uses Release). if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() { // Due to the previous acquire read, this will observe any writes to // `strong` that were due to upgrading weak pointers; only strong // clones remain, which require that the strong count is > 1 anyway. let unique = self.inner().strong.load(Relaxed) == 1; // The release write here synchronizes with a read in `downgrade`, // effectively preventing the above read of `strong` from happening // after the write. self.inner().weak.store(1, Release); // release the lock unique } else { false } } } #[stable(feature = "rust1", since = "1.0.0")] unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> { /// Drops the `Arc`. /// /// This will decrement the strong reference count. If the strong reference /// count reaches zero then the only other references (if any) are /// [`Weak`][weak], so we `drop` the inner value. /// /// [weak]: struct.Weak.html /// /// # Examples /// /// ``` /// use std::sync::Arc; /// /// struct Foo; /// /// impl Drop for Foo { /// fn drop(&mut self) { /// println!("dropped!"); /// } /// } /// /// let foo = Arc::new(Foo); /// let foo2 = foo.clone(); /// /// drop(foo); // Doesn't print anything /// drop(foo2); // Prints "dropped!" /// ``` #[inline] fn drop(&mut self) { // Because `fetch_sub` is already atomic, we do not need to synchronize // with other threads unless we are going to delete the object. This // same logic applies to the below `fetch_sub` to the `weak` count. if self.inner().strong.fetch_sub(1, Release) != 1 { return; } // This fence is needed to prevent reordering of use of the data and // deletion of the data. Because it is marked `Release`, the decreasing // of the reference count synchronizes with this `Acquire` fence. This // means that use of the data happens before decreasing the reference // count, which happens before this fence, which happens before the // deletion of the data. // // As explained in the [Boost documentation][1], // // > It is important to enforce any possible access to the object in one // > thread (through an existing reference) to *happen before* deleting // > the object in a different thread. This is achieved by a "release" // > operation after dropping a reference (any access to the object // > through this reference must obviously happened before), and an // > "acquire" operation before deleting the object. // // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html) atomic::fence(Acquire); unsafe { self.drop_slow(); } } } impl<T> Weak<T> { /// Constructs a new `Weak<T>`, without an accompanying instance of `T`. /// /// This allocates memory for `T`, but does not initialize it. Calling /// [`upgrade`][upgrade] on the return value always gives /// [`None`][option]. /// /// [upgrade]: struct.Weak.html#method.upgrade /// [option]: ../../std/option/enum.Option.html /// /// # Examples /// /// ``` /// use std::sync::Weak; /// /// let empty: Weak<i64> = Weak::new(); /// assert!(empty.upgrade().is_none()); /// ``` #[stable(feature = "downgraded_weak", since = "1.10.0")] pub fn new() -> Weak<T> { unsafe { Weak { ptr: Shared::new(Box::into_raw(box ArcInner { strong: atomic::AtomicUsize::new(0), weak: atomic::AtomicUsize::new(1), data: uninitialized(), })), } } } } impl<T: ?Sized> Weak<T> { /// Upgrades the `Weak` pointer to an [`Arc`][arc], if possible. /// /// Returns [`None`][option] if the strong count has reached zero and the /// inner value was destroyed. /// /// [arc]: struct.Arc.html /// [option]: ../../std/option/enum.Option.html /// /// # Examples /// /// ``` /// use std::sync::Arc; /// /// let five = Arc::new(5); /// /// let weak_five = Arc::downgrade(&five); /// /// let strong_five: Option<Arc<_>> = weak_five.upgrade(); /// assert!(strong_five.is_some()); /// /// // Destroy all strong pointers. /// drop(strong_five); /// drop(five); /// /// assert!(weak_five.upgrade().is_none()); /// ``` #[stable(feature = "arc_weak", since = "1.4.0")] pub fn upgrade(&self) -> Option<Arc<T>> { // We use a CAS loop to increment the strong count instead of a // fetch_add because once the count hits 0 it must never be above 0. let inner = self.inner(); // Relaxed load because any write of 0 that we can observe // leaves the field in a permanently zero state (so a // "stale" read of 0 is fine), and any other value is // confirmed via the CAS below. let mut n = inner.strong.load(Relaxed); loop { if n == 0 { return None; } // See comments in `Arc::clone` for why we do this (for `mem::forget`). if n > MAX_REFCOUNT { unsafe { abort(); } } // Relaxed is valid for the same reason it is on Arc's Clone impl match inner.strong.compare_exchange_weak(n, n + 1, Relaxed, Relaxed) { Ok(_) => return Some(Arc { ptr: self.ptr }), Err(old) => n = old, } } } #[inline] fn inner(&self) -> &ArcInner<T> { // See comments above for why this is "safe" unsafe { &**self.ptr } } } #[stable(feature = "arc_weak", since = "1.4.0")] impl<T: ?Sized> Clone for Weak<T> { /// Makes a clone of the `Weak` pointer. /// /// This creates another pointer to the same inner value, increasing the /// weak reference count. /// /// # Examples /// /// ``` /// use std::sync::Arc; /// /// let weak_five = Arc::downgrade(&Arc::new(5)); /// /// weak_five.clone(); /// ``` #[inline] fn clone(&self) -> Weak<T> { // See comments in Arc::clone() for why this is relaxed. This can use a // fetch_add (ignoring the lock) because the weak count is only locked // where are *no other* weak pointers in existence. (So we can't be // running this code in that case). let old_size = self.inner().weak.fetch_add(1, Relaxed); // See comments in Arc::clone() for why we do this (for mem::forget). if old_size > MAX_REFCOUNT { unsafe { abort(); } } return Weak { ptr: self.ptr }; } } #[stable(feature = "downgraded_weak", since = "1.10.0")] impl<T> Default for Weak<T> { /// Constructs a new `Weak<T>`, without an accompanying instance of `T`. /// /// This allocates memory for `T`, but does not initialize it. Calling /// [`upgrade`][upgrade] on the return value always gives /// [`None`][option]. /// /// [upgrade]: struct.Weak.html#method.upgrade /// [option]: ../../std/option/enum.Option.html /// /// # Examples /// /// ``` /// use std::sync::Weak; /// /// let empty: Weak<i64> = Default::default(); /// assert!(empty.upgrade().is_none()); /// ``` fn default() -> Weak<T> { Weak::new() } } #[stable(feature = "arc_weak", since = "1.4.0")] impl<T: ?Sized> Drop for Weak<T> { /// Drops the `Weak` pointer. /// /// This will decrement the weak reference count. /// /// # Examples /// /// ``` /// use std::sync::Arc; /// /// struct Foo; /// /// impl Drop for Foo { /// fn drop(&mut self) { /// println!("dropped!"); /// } /// } /// /// let foo = Arc::new(Foo); /// let weak_foo = Arc::downgrade(&foo); /// let other_weak_foo = weak_foo.clone(); /// /// drop(weak_foo); // Doesn't print anything /// drop(foo); // Prints "dropped!" /// /// assert!(other_weak_foo.upgrade().is_none()); /// ``` fn drop(&mut self) { let ptr = *self.ptr; // If we find out that we were the last weak pointer, then its time to // deallocate the data entirely. See the discussion in Arc::drop() about // the memory orderings // // It's not necessary to check for the locked state here, because the // weak count can only be locked if there was precisely one weak ref, // meaning that drop could only subsequently run ON that remaining weak // ref, which can only happen after the lock is released. if self.inner().weak.fetch_sub(1, Release) == 1 { atomic::fence(Acquire); unsafe { deallocate(ptr as *mut u8, size_of_val(&*ptr), align_of_val(&*ptr)) } } } } #[stable(feature = "rust1", since = "1.0.0")] impl<T: ?Sized + PartialEq> PartialEq for Arc<T> { /// Equality for two `Arc`s. /// /// Two `Arc`s are equal if their inner values are equal. /// /// # Examples /// /// ``` /// use std::sync::Arc; /// /// let five = Arc::new(5); /// /// assert!(five == Arc::new(5)); /// ``` fn eq(&self, other: &Arc<T>) -> bool { *(*self) == *(*other) } /// Inequality for two `Arc`s. /// /// Two `Arc`s are unequal if their inner values are unequal. /// /// # Examples /// /// ``` /// use std::sync::Arc; /// /// let five = Arc::new(5); /// /// assert!(five != Arc::new(6)); /// ``` fn ne(&self, other: &Arc<T>) -> bool { *(*self) != *(*other) } } #[stable(feature = "rust1", since = "1.0.0")] impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> { /// Partial comparison for two `Arc`s. /// /// The two are compared by calling `partial_cmp()` on their inner values. /// /// # Examples /// /// ``` /// use std::sync::Arc; /// use std::cmp::Ordering; /// /// let five = Arc::new(5); /// /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6))); /// ``` fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> { (**self).partial_cmp(&**other) } /// Less-than comparison for two `Arc`s. /// /// The two are compared by calling `<` on their inner values. /// /// # Examples /// /// ``` /// use std::sync::Arc; /// /// let five = Arc::new(5); /// /// assert!(five < Arc::new(6)); /// ``` fn lt(&self, other: &Arc<T>) -> bool { *(*self) < *(*other) } /// 'Less than or equal to' comparison for two `Arc`s. /// /// The two are compared by calling `<=` on their inner values. /// /// # Examples /// /// ``` /// use std::sync::Arc; /// /// let five = Arc::new(5); /// /// assert!(five <= Arc::new(5)); /// ``` fn le(&self, other: &Arc<T>) -> bool { *(*self) <= *(*other) } /// Greater-than comparison for two `Arc`s. /// /// The two are compared by calling `>` on their inner values. /// /// # Examples /// /// ``` /// use std::sync::Arc; /// /// let five = Arc::new(5); /// /// assert!(five > Arc::new(4)); /// ``` fn gt(&self, other: &Arc<T>) -> bool { *(*self) > *(*other) } /// 'Greater than or equal to' comparison for two `Arc`s. /// /// The two are compared by calling `>=` on their inner values. /// /// # Examples /// /// ``` /// use std::sync::Arc; /// /// let five = Arc::new(5); /// /// assert!(five >= Arc::new(5)); /// ``` fn ge(&self, other: &Arc<T>) -> bool { *(*self) >= *(*other) } } #[stable(feature = "rust1", since = "1.0.0")] impl<T: ?Sized + Ord> Ord for Arc<T> { /// Comparison for two `Arc`s. /// /// The two are compared by calling `cmp()` on their inner values. /// /// # Examples /// /// ``` /// use std::sync::Arc; /// use std::cmp::Ordering; /// /// let five = Arc::new(5); /// /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6))); /// ``` fn cmp(&self, other: &Arc<T>) -> Ordering { (**self).cmp(&**other) } } #[stable(feature = "rust1", since = "1.0.0")] impl<T: ?Sized + Eq> Eq for Arc<T> {} #[stable(feature = "rust1", since = "1.0.0")] impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { fmt::Display::fmt(&**self, f) } } #[stable(feature = "rust1", since = "1.0.0")] impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { fmt::Debug::fmt(&**self, f) } } #[stable(feature = "rust1", since = "1.0.0")] impl<T: ?Sized> fmt::Pointer for Arc<T> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { fmt::Pointer::fmt(&*self.ptr, f) } } #[stable(feature = "rust1", since = "1.0.0")] impl<T: Default> Default for Arc<T> { /// Creates a new `Arc<T>`, with the `Default` value for `T`. /// /// # Examples /// /// ``` /// use std::sync::Arc; /// /// let x: Arc<i32> = Default::default(); /// assert_eq!(*x, 0); /// ``` fn default() -> Arc<T> { Arc::new(Default::default()) } } #[stable(feature = "rust1", since = "1.0.0")] impl<T: ?Sized + Hash> Hash for Arc<T> { fn hash<H: Hasher>(&self, state: &mut H) { (**self).hash(state) } } #[stable(feature = "from_for_ptrs", since = "1.6.0")] impl<T> From<T> for Arc<T> { fn from(t: T) -> Self { Arc::new(t) } } #[cfg(test)] mod tests { use std::clone::Clone; use std::sync::mpsc::channel; use std::mem::drop; use std::ops::Drop; use std::option::Option; use std::option::Option::{None, Some}; use std::sync::atomic; use std::sync::atomic::Ordering::{Acquire, SeqCst}; use std::thread; use std::vec::Vec; use super::{Arc, Weak}; use std::sync::Mutex; use std::convert::From; struct Canary(*mut atomic::AtomicUsize); impl Drop for Canary { fn drop(&mut self) { unsafe { match *self { Canary(c) => { (*c).fetch_add(1, SeqCst); } } } } } #[test] #[cfg_attr(target_os = "emscripten", ignore)] fn manually_share_arc() { let v = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10]; let arc_v = Arc::new(v); let (tx, rx) = channel(); let _t = thread::spawn(move || { let arc_v: Arc<Vec<i32>> = rx.recv().unwrap(); assert_eq!((*arc_v)[3], 4); }); tx.send(arc_v.clone()).unwrap(); assert_eq!((*arc_v)[2], 3); assert_eq!((*arc_v)[4], 5); } #[test] fn test_arc_get_mut() { let mut x = Arc::new(3); *Arc::get_mut(&mut x).unwrap() = 4; assert_eq!(*x, 4); let y = x.clone(); assert!(Arc::get_mut(&mut x).is_none()); drop(y); assert!(Arc::get_mut(&mut x).is_some()); let _w = Arc::downgrade(&x); assert!(Arc::get_mut(&mut x).is_none()); } #[test] fn try_unwrap() { let x = Arc::new(3); assert_eq!(Arc::try_unwrap(x), Ok(3)); let x = Arc::new(4); let _y = x.clone(); assert_eq!(Arc::try_unwrap(x), Err(Arc::new(4))); let x = Arc::new(5); let _w = Arc::downgrade(&x); assert_eq!(Arc::try_unwrap(x), Ok(5)); } #[test] fn into_from_raw() { let x = Arc::new(box "hello"); let y = x.clone(); let x_ptr = Arc::into_raw(x); drop(y); unsafe { assert_eq!(**x_ptr, "hello"); let x = Arc::from_raw(x_ptr); assert_eq!(**x, "hello"); assert_eq!(Arc::try_unwrap(x).map(|x| *x), Ok("hello")); } } #[test] fn test_cowarc_clone_make_mut() { let mut cow0 = Arc::new(75); let mut cow1 = cow0.clone(); let mut cow2 = cow1.clone(); assert!(75 == *Arc::make_mut(&mut cow0)); assert!(75 == *Arc::make_mut(&mut cow1)); assert!(75 == *Arc::make_mut(&mut cow2)); *Arc::make_mut(&mut cow0) += 1; *Arc::make_mut(&mut cow1) += 2; *Arc::make_mut(&mut cow2) += 3; assert!(76 == *cow0); assert!(77 == *cow1); assert!(78 == *cow2); // none should point to the same backing memory assert!(*cow0 != *cow1); assert!(*cow0 != *cow2); assert!(*cow1 != *cow2); } #[test] fn test_cowarc_clone_unique2() { let mut cow0 = Arc::new(75); let cow1 = cow0.clone(); let cow2 = cow1.clone(); assert!(75 == *cow0); assert!(75 == *cow1); assert!(75 == *cow2); *Arc::make_mut(&mut cow0) += 1; assert!(76 == *cow0); assert!(75 == *cow1); assert!(75 == *cow2); // cow1 and cow2 should share the same contents // cow0 should have a unique reference assert!(*cow0 != *cow1); assert!(*cow0 != *cow2); assert!(*cow1 == *cow2); } #[test] fn test_cowarc_clone_weak() { let mut cow0 = Arc::new(75); let cow1_weak = Arc::downgrade(&cow0); assert!(75 == *cow0); assert!(75 == *cow1_weak.upgrade().unwrap()); *Arc::make_mut(&mut cow0) += 1; assert!(76 == *cow0); assert!(cow1_weak.upgrade().is_none()); } #[test] fn test_live() { let x = Arc::new(5); let y = Arc::downgrade(&x); assert!(y.upgrade().is_some()); } #[test] fn test_dead() { let x = Arc::new(5); let y = Arc::downgrade(&x); drop(x); assert!(y.upgrade().is_none()); } #[test] fn weak_self_cyclic() { struct Cycle { x: Mutex<Option<Weak<Cycle>>>, } let a = Arc::new(Cycle { x: Mutex::new(None) }); let b = Arc::downgrade(&a.clone()); *a.x.lock().unwrap() = Some(b); // hopefully we don't double-free (or leak)... } #[test] fn drop_arc() { let mut canary = atomic::AtomicUsize::new(0); let x = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize)); drop(x); assert!(canary.load(Acquire) == 1); } #[test] fn drop_arc_weak() { let mut canary = atomic::AtomicUsize::new(0); let arc = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize)); let arc_weak = Arc::downgrade(&arc); assert!(canary.load(Acquire) == 0); drop(arc); assert!(canary.load(Acquire) == 1); drop(arc_weak); } #[test] fn test_strong_count() { let a = Arc::new(0); assert!(Arc::strong_count(&a) == 1); let w = Arc::downgrade(&a); assert!(Arc::strong_count(&a) == 1); let b = w.upgrade().expect(""); assert!(Arc::strong_count(&b) == 2); assert!(Arc::strong_count(&a) == 2); drop(w); drop(a); assert!(Arc::strong_count(&b) == 1); let c = b.clone(); assert!(Arc::strong_count(&b) == 2); assert!(Arc::strong_count(&c) == 2); } #[test] fn test_weak_count() { let a = Arc::new(0); assert!(Arc::strong_count(&a) == 1); assert!(Arc::weak_count(&a) == 0); let w = Arc::downgrade(&a); assert!(Arc::strong_count(&a) == 1); assert!(Arc::weak_count(&a) == 1); let x = w.clone(); assert!(Arc::weak_count(&a) == 2); drop(w); drop(x); assert!(Arc::strong_count(&a) == 1); assert!(Arc::weak_count(&a) == 0); let c = a.clone(); assert!(Arc::strong_count(&a) == 2); assert!(Arc::weak_count(&a) == 0); let d = Arc::downgrade(&c); assert!(Arc::weak_count(&c) == 1); assert!(Arc::strong_count(&c) == 2); drop(a); drop(c); drop(d); } #[test] fn show_arc() { let a = Arc::new(5); assert_eq!(format!("{:?}", a), "5"); } // Make sure deriving works with Arc<T> #[derive(Eq, Ord, PartialEq, PartialOrd, Clone, Debug, Default)] struct Foo { inner: Arc<i32>, } #[test] fn test_unsized() { let x: Arc<[i32]> = Arc::new([1, 2, 3]); assert_eq!(format!("{:?}", x), "[1, 2, 3]"); let y = Arc::downgrade(&x.clone()); drop(x); assert!(y.upgrade().is_none()); } #[test] fn test_from_owned() { let foo = 123; let foo_arc = Arc::from(foo); assert!(123 == *foo_arc); } #[test] fn test_new_weak() { let foo: Weak<usize> = Weak::new(); assert!(foo.upgrade().is_none()); } #[test] fn test_ptr_eq() { let five = Arc::new(5); let same_five = five.clone(); let other_five = Arc::new(5); assert!(Arc::ptr_eq(&five, &same_five)); assert!(!Arc::ptr_eq(&five, &other_five)); } } #[stable(feature = "rust1", since = "1.0.0")] impl<T: ?Sized> borrow::Borrow<T> for Arc<T> { fn borrow(&self) -> &T { &**self } } #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")] impl<T: ?Sized> AsRef<T> for Arc<T> { fn as_ref(&self) -> &T { &**self } }