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.
12 * Concurrency-enabled mechanisms for sharing mutable and/or immutable state
17 use core::clone::Clone;
18 use core::kinds::{Share, Send};
19 use core::mem::{min_align_of, size_of, drop};
21 use core::ops::{Drop, Deref};
22 use core::option::{Some, None, Option};
24 use core::ptr::RawPtr;
27 /// An atomically reference counted wrapper for shared state.
31 /// In this example, a large vector of floats is shared between several tasks.
32 /// With simple pipes, without `Arc`, a copy would have to be made for each
36 /// extern crate sync;
41 /// let numbers = Vec::from_fn(100, |i| i as f32);
42 /// let shared_numbers = Arc::new(numbers);
44 /// for _ in range(0, 10) {
45 /// let child_numbers = shared_numbers.clone();
48 /// let local_numbers = child_numbers.as_slice();
50 /// // Work with the local numbers
55 #[unsafe_no_drop_flag]
57 // FIXME #12808: strange name to try to avoid interfering with
58 // field accesses of the contained type via Deref
59 _ptr: *mut ArcInner<T>,
62 /// A weak pointer to an `Arc`.
64 /// Weak pointers will not keep the data inside of the `Arc` alive, and can be
65 /// used to break cycles between `Arc` pointers.
66 #[unsafe_no_drop_flag]
68 // FIXME #12808: strange name to try to avoid interfering with
69 // field accesses of the contained type via Deref
70 _ptr: *mut ArcInner<T>,
74 strong: atomics::AtomicUint,
75 weak: atomics::AtomicUint,
79 impl<T: Share + Send> Arc<T> {
80 /// Create an atomically reference counted wrapper.
82 pub fn new(data: T) -> Arc<T> {
83 // Start the weak pointer count as 1 which is the weak pointer that's
84 // held by all the strong pointers (kinda), see std/rc.rs for more info
85 let x = box ArcInner {
86 strong: atomics::AtomicUint::new(1),
87 weak: atomics::AtomicUint::new(1),
90 Arc { _ptr: unsafe { mem::transmute(x) } }
94 fn inner<'a>(&'a self) -> &'a ArcInner<T> {
95 // This unsafety is ok because while this arc is alive we're guaranteed
96 // that the inner pointer is valid. Furthermore, we know that the
97 // `ArcInner` structure itself is `Share` because the inner data is
98 // `Share` as well, so we're ok loaning out an immutable pointer to
100 unsafe { &*self._ptr }
103 /// Downgrades a strong pointer to a weak pointer
105 /// Weak pointers will not keep the data alive. Once all strong references
106 /// to the underlying data have been dropped, the data itself will be
108 pub fn downgrade(&self) -> Weak<T> {
109 // See the clone() impl for why this is relaxed
110 self.inner().weak.fetch_add(1, atomics::Relaxed);
111 Weak { _ptr: self._ptr }
115 impl<T: Share + Send> Clone for Arc<T> {
116 /// Duplicate an atomically reference counted wrapper.
118 /// The resulting two `Arc` objects will point to the same underlying data
119 /// object. However, one of the `Arc` objects can be sent to another task,
120 /// allowing them to share the underlying data.
122 fn clone(&self) -> Arc<T> {
123 // Using a relaxed ordering is alright here, as knowledge of the
124 // original reference prevents other threads from erroneously deleting
127 // As explained in the [Boost documentation][1], Increasing the
128 // reference counter can always be done with memory_order_relaxed: New
129 // references to an object can only be formed from an existing
130 // reference, and passing an existing reference from one thread to
131 // another must already provide any required synchronization.
133 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
134 self.inner().strong.fetch_add(1, atomics::Relaxed);
135 Arc { _ptr: self._ptr }
139 impl<T: Send + Share> Deref<T> for Arc<T> {
141 fn deref<'a>(&'a self) -> &'a T {
146 impl<T: Send + Share + Clone> Arc<T> {
147 /// Acquires a mutable pointer to the inner contents by guaranteeing that
148 /// the reference count is one (no sharing is possible).
150 /// This is also referred to as a copy-on-write operation because the inner
151 /// data is cloned if the reference count is greater than one.
154 pub fn make_unique<'a>(&'a mut self) -> &'a mut T {
155 // Note that we hold a strong reference, which also counts as
156 // a weak reference, so we only clone if there is an
157 // additional reference of either kind.
158 if self.inner().strong.load(atomics::SeqCst) != 1 ||
159 self.inner().weak.load(atomics::SeqCst) != 1 {
160 *self = Arc::new(self.deref().clone())
162 // This unsafety is ok because we're guaranteed that the pointer
163 // returned is the *only* pointer that will ever be returned to T. Our
164 // reference count is guaranteed to be 1 at this point, and we required
165 // the Arc itself to be `mut`, so we're returning the only possible
166 // reference to the inner data.
167 let inner = unsafe { &mut *self._ptr };
173 impl<T: Share + Send> Drop for Arc<T> {
175 // This structure has #[unsafe_no_drop_flag], so this drop glue may run
176 // more than once (but it is guaranteed to be zeroed after the first if
177 // it's run more than once)
178 if self._ptr.is_null() { return }
180 // Because `fetch_sub` is already atomic, we do not need to synchronize
181 // with other threads unless we are going to delete the object. This
182 // same logic applies to the below `fetch_sub` to the `weak` count.
183 if self.inner().strong.fetch_sub(1, atomics::Release) != 1 { return }
185 // This fence is needed to prevent reordering of use of the data and
186 // deletion of the data. Because it is marked `Release`, the
187 // decreasing of the reference count synchronizes with this `Acquire`
188 // fence. This means that use of the data happens before decreasing
189 // the refernce count, which happens before this fence, which
190 // happens before the deletion of the data.
192 // As explained in the [Boost documentation][1],
194 // It is important to enforce any possible access to the object in
195 // one thread (through an existing reference) to *happen before*
196 // deleting the object in a different thread. This is achieved by a
197 // "release" operation after dropping a reference (any access to the
198 // object through this reference must obviously happened before),
199 // and an "acquire" operation before deleting the object.
201 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
202 atomics::fence(atomics::Acquire);
204 // Destroy the data at this time, even though we may not free the box
205 // allocation itself (there may still be weak pointers lying around).
206 unsafe { drop(ptr::read(&self.inner().data)); }
208 if self.inner().weak.fetch_sub(1, atomics::Release) == 1 {
209 atomics::fence(atomics::Acquire);
210 unsafe { deallocate(self._ptr as *mut u8, size_of::<ArcInner<T>>(),
211 min_align_of::<ArcInner<T>>()) }
216 impl<T: Share + Send> Weak<T> {
217 /// Attempts to upgrade this weak reference to a strong reference.
219 /// This method will fail to upgrade this reference if the strong reference
220 /// count has already reached 0, but if there are still other active strong
221 /// references this function will return a new strong reference to the data
222 pub fn upgrade(&self) -> Option<Arc<T>> {
223 // We use a CAS loop to increment the strong count instead of a
224 // fetch_add because once the count hits 0 is must never be above 0.
225 let inner = self.inner();
227 let n = inner.strong.load(atomics::SeqCst);
228 if n == 0 { return None }
229 let old = inner.strong.compare_and_swap(n, n + 1, atomics::SeqCst);
230 if old == n { return Some(Arc { _ptr: self._ptr }) }
235 fn inner<'a>(&'a self) -> &'a ArcInner<T> {
236 // See comments above for why this is "safe"
237 unsafe { &*self._ptr }
241 impl<T: Share + Send> Clone for Weak<T> {
243 fn clone(&self) -> Weak<T> {
244 // See comments in Arc::clone() for why this is relaxed
245 self.inner().weak.fetch_add(1, atomics::Relaxed);
246 Weak { _ptr: self._ptr }
251 impl<T: Share + Send> Drop for Weak<T> {
253 // see comments above for why this check is here
254 if self._ptr.is_null() { return }
256 // If we find out that we were the last weak pointer, then its time to
257 // deallocate the data entirely. See the discussion in Arc::drop() about
258 // the memory orderings
259 if self.inner().weak.fetch_sub(1, atomics::Release) == 1 {
260 atomics::fence(atomics::Acquire);
261 unsafe { deallocate(self._ptr as *mut u8, size_of::<ArcInner<T>>(),
262 min_align_of::<ArcInner<T>>()) }
268 #[allow(experimental)]
270 use std::clone::Clone;
271 use std::comm::channel;
274 use std::option::{Option, Some, None};
275 use std::sync::atomics;
278 use super::{Arc, Weak};
281 struct Canary(*mut atomics::AtomicUint);
289 (*c).fetch_add(1, atomics::SeqCst);
297 fn manually_share_arc() {
298 let v = vec!(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
299 let arc_v = Arc::new(v);
301 let (tx, rx) = channel();
304 let arc_v: Arc<Vec<int>> = rx.recv();
305 assert_eq!(*arc_v.get(3), 4);
308 tx.send(arc_v.clone());
310 assert_eq!(*arc_v.get(2), 3);
311 assert_eq!(*arc_v.get(4), 5);
313 info!("{:?}", arc_v);
317 fn test_cowarc_clone_make_unique() {
318 let mut cow0 = Arc::new(75u);
319 let mut cow1 = cow0.clone();
320 let mut cow2 = cow1.clone();
322 assert!(75 == *cow0.make_unique());
323 assert!(75 == *cow1.make_unique());
324 assert!(75 == *cow2.make_unique());
326 *cow0.make_unique() += 1;
327 *cow1.make_unique() += 2;
328 *cow2.make_unique() += 3;
330 assert!(76 == *cow0);
331 assert!(77 == *cow1);
332 assert!(78 == *cow2);
334 // none should point to the same backing memory
335 assert!(*cow0 != *cow1);
336 assert!(*cow0 != *cow2);
337 assert!(*cow1 != *cow2);
341 fn test_cowarc_clone_unique2() {
342 let mut cow0 = Arc::new(75u);
343 let cow1 = cow0.clone();
344 let cow2 = cow1.clone();
346 assert!(75 == *cow0);
347 assert!(75 == *cow1);
348 assert!(75 == *cow2);
350 *cow0.make_unique() += 1;
352 assert!(76 == *cow0);
353 assert!(75 == *cow1);
354 assert!(75 == *cow2);
356 // cow1 and cow2 should share the same contents
357 // cow0 should have a unique reference
358 assert!(*cow0 != *cow1);
359 assert!(*cow0 != *cow2);
360 assert!(*cow1 == *cow2);
364 fn test_cowarc_clone_weak() {
365 let mut cow0 = Arc::new(75u);
366 let cow1_weak = cow0.downgrade();
368 assert!(75 == *cow0);
369 assert!(75 == *cow1_weak.upgrade().unwrap());
371 *cow0.make_unique() += 1;
373 assert!(76 == *cow0);
374 assert!(cow1_weak.upgrade().is_none());
380 let y = x.downgrade();
381 assert!(y.upgrade().is_some());
387 let y = x.downgrade();
389 assert!(y.upgrade().is_none());
393 fn weak_self_cyclic() {
395 x: Mutex<Option<Weak<Cycle>>>
398 let a = Arc::new(Cycle { x: Mutex::new(None) });
399 let b = a.clone().downgrade();
400 *a.x.lock() = Some(b);
402 // hopefully we don't double-free (or leak)...
407 let mut canary = atomics::AtomicUint::new(0);
408 let x = Arc::new(Canary(&mut canary as *mut atomics::AtomicUint));
410 assert!(canary.load(atomics::Acquire) == 1);
415 let mut canary = atomics::AtomicUint::new(0);
416 let arc = Arc::new(Canary(&mut canary as *mut atomics::AtomicUint));
417 let arc_weak = arc.downgrade();
418 assert!(canary.load(atomics::Acquire) == 0);
420 assert!(canary.load(atomics::Acquire) == 1);