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 /// use std::sync::Arc;
39 /// let numbers = Vec::from_fn(100, |i| i as f32);
40 /// let shared_numbers = Arc::new(numbers);
42 /// for _ in range(0, 10) {
43 /// let child_numbers = shared_numbers.clone();
46 /// let local_numbers = child_numbers.as_slice();
48 /// // Work with the local numbers
53 #[unsafe_no_drop_flag]
55 // FIXME #12808: strange name to try to avoid interfering with
56 // field accesses of the contained type via Deref
57 _ptr: *mut ArcInner<T>,
60 /// A weak pointer to an `Arc`.
62 /// Weak pointers will not keep the data inside of the `Arc` alive, and can be
63 /// used to break cycles between `Arc` pointers.
64 #[unsafe_no_drop_flag]
66 // FIXME #12808: strange name to try to avoid interfering with
67 // field accesses of the contained type via Deref
68 _ptr: *mut ArcInner<T>,
72 strong: atomics::AtomicUint,
73 weak: atomics::AtomicUint,
77 impl<T: Share + Send> Arc<T> {
78 /// Create an atomically reference counted wrapper.
80 pub fn new(data: T) -> Arc<T> {
81 // Start the weak pointer count as 1 which is the weak pointer that's
82 // held by all the strong pointers (kinda), see std/rc.rs for more info
83 let x = box ArcInner {
84 strong: atomics::AtomicUint::new(1),
85 weak: atomics::AtomicUint::new(1),
88 Arc { _ptr: unsafe { mem::transmute(x) } }
92 fn inner<'a>(&'a self) -> &'a ArcInner<T> {
93 // This unsafety is ok because while this arc is alive we're guaranteed
94 // that the inner pointer is valid. Furthermore, we know that the
95 // `ArcInner` structure itself is `Share` because the inner data is
96 // `Share` as well, so we're ok loaning out an immutable pointer to
98 unsafe { &*self._ptr }
101 /// Downgrades a strong pointer to a weak pointer
103 /// Weak pointers will not keep the data alive. Once all strong references
104 /// to the underlying data have been dropped, the data itself will be
106 pub fn downgrade(&self) -> Weak<T> {
107 // See the clone() impl for why this is relaxed
108 self.inner().weak.fetch_add(1, atomics::Relaxed);
109 Weak { _ptr: self._ptr }
113 impl<T: Share + Send> Clone for Arc<T> {
114 /// Duplicate an atomically reference counted wrapper.
116 /// The resulting two `Arc` objects will point to the same underlying data
117 /// object. However, one of the `Arc` objects can be sent to another task,
118 /// allowing them to share the underlying data.
120 fn clone(&self) -> Arc<T> {
121 // Using a relaxed ordering is alright here, as knowledge of the
122 // original reference prevents other threads from erroneously deleting
125 // As explained in the [Boost documentation][1], Increasing the
126 // reference counter can always be done with memory_order_relaxed: New
127 // references to an object can only be formed from an existing
128 // reference, and passing an existing reference from one thread to
129 // another must already provide any required synchronization.
131 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
132 self.inner().strong.fetch_add(1, atomics::Relaxed);
133 Arc { _ptr: self._ptr }
137 impl<T: Send + Share> Deref<T> for Arc<T> {
139 fn deref<'a>(&'a self) -> &'a T {
144 impl<T: Send + Share + Clone> Arc<T> {
145 /// Acquires a mutable pointer to the inner contents by guaranteeing that
146 /// the reference count is one (no sharing is possible).
148 /// This is also referred to as a copy-on-write operation because the inner
149 /// data is cloned if the reference count is greater than one.
152 pub fn make_unique<'a>(&'a mut self) -> &'a mut T {
153 // Note that we hold a strong reference, which also counts as
154 // a weak reference, so we only clone if there is an
155 // additional reference of either kind.
156 if self.inner().strong.load(atomics::SeqCst) != 1 ||
157 self.inner().weak.load(atomics::SeqCst) != 1 {
158 *self = Arc::new(self.deref().clone())
160 // This unsafety is ok because we're guaranteed that the pointer
161 // returned is the *only* pointer that will ever be returned to T. Our
162 // reference count is guaranteed to be 1 at this point, and we required
163 // the Arc itself to be `mut`, so we're returning the only possible
164 // reference to the inner data.
165 let inner = unsafe { &mut *self._ptr };
171 impl<T: Share + Send> Drop for Arc<T> {
173 // This structure has #[unsafe_no_drop_flag], so this drop glue may run
174 // more than once (but it is guaranteed to be zeroed after the first if
175 // it's run more than once)
176 if self._ptr.is_null() { return }
178 // Because `fetch_sub` is already atomic, we do not need to synchronize
179 // with other threads unless we are going to delete the object. This
180 // same logic applies to the below `fetch_sub` to the `weak` count.
181 if self.inner().strong.fetch_sub(1, atomics::Release) != 1 { return }
183 // This fence is needed to prevent reordering of use of the data and
184 // deletion of the data. Because it is marked `Release`, the
185 // decreasing of the reference count synchronizes with this `Acquire`
186 // fence. This means that use of the data happens before decreasing
187 // the refernce count, which happens before this fence, which
188 // happens before the deletion of the data.
190 // As explained in the [Boost documentation][1],
192 // It is important to enforce any possible access to the object in
193 // one thread (through an existing reference) to *happen before*
194 // deleting the object in a different thread. This is achieved by a
195 // "release" operation after dropping a reference (any access to the
196 // object through this reference must obviously happened before),
197 // and an "acquire" operation before deleting the object.
199 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
200 atomics::fence(atomics::Acquire);
202 // Destroy the data at this time, even though we may not free the box
203 // allocation itself (there may still be weak pointers lying around).
204 unsafe { drop(ptr::read(&self.inner().data)); }
206 if self.inner().weak.fetch_sub(1, atomics::Release) == 1 {
207 atomics::fence(atomics::Acquire);
208 unsafe { deallocate(self._ptr as *mut u8, size_of::<ArcInner<T>>(),
209 min_align_of::<ArcInner<T>>()) }
214 impl<T: Share + Send> Weak<T> {
215 /// Attempts to upgrade this weak reference to a strong reference.
217 /// This method will fail to upgrade this reference if the strong reference
218 /// count has already reached 0, but if there are still other active strong
219 /// references this function will return a new strong reference to the data
220 pub fn upgrade(&self) -> Option<Arc<T>> {
221 // We use a CAS loop to increment the strong count instead of a
222 // fetch_add because once the count hits 0 is must never be above 0.
223 let inner = self.inner();
225 let n = inner.strong.load(atomics::SeqCst);
226 if n == 0 { return None }
227 let old = inner.strong.compare_and_swap(n, n + 1, atomics::SeqCst);
228 if old == n { return Some(Arc { _ptr: self._ptr }) }
233 fn inner<'a>(&'a self) -> &'a ArcInner<T> {
234 // See comments above for why this is "safe"
235 unsafe { &*self._ptr }
239 impl<T: Share + Send> Clone for Weak<T> {
241 fn clone(&self) -> Weak<T> {
242 // See comments in Arc::clone() for why this is relaxed
243 self.inner().weak.fetch_add(1, atomics::Relaxed);
244 Weak { _ptr: self._ptr }
249 impl<T: Share + Send> Drop for Weak<T> {
251 // see comments above for why this check is here
252 if self._ptr.is_null() { return }
254 // If we find out that we were the last weak pointer, then its time to
255 // deallocate the data entirely. See the discussion in Arc::drop() about
256 // the memory orderings
257 if self.inner().weak.fetch_sub(1, atomics::Release) == 1 {
258 atomics::fence(atomics::Acquire);
259 unsafe { deallocate(self._ptr as *mut u8, size_of::<ArcInner<T>>(),
260 min_align_of::<ArcInner<T>>()) }
266 #[allow(experimental)]
268 use std::clone::Clone;
269 use std::comm::channel;
272 use std::option::{Option, Some, None};
273 use std::sync::atomics;
276 use super::{Arc, Weak};
277 use std::sync::Mutex;
279 struct Canary(*mut atomics::AtomicUint);
287 (*c).fetch_add(1, atomics::SeqCst);
295 fn manually_share_arc() {
296 let v = vec!(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
297 let arc_v = Arc::new(v);
299 let (tx, rx) = channel();
302 let arc_v: Arc<Vec<int>> = rx.recv();
303 assert_eq!(*arc_v.get(3), 4);
306 tx.send(arc_v.clone());
308 assert_eq!(*arc_v.get(2), 3);
309 assert_eq!(*arc_v.get(4), 5);
311 info!("{:?}", arc_v);
315 fn test_cowarc_clone_make_unique() {
316 let mut cow0 = Arc::new(75u);
317 let mut cow1 = cow0.clone();
318 let mut cow2 = cow1.clone();
320 assert!(75 == *cow0.make_unique());
321 assert!(75 == *cow1.make_unique());
322 assert!(75 == *cow2.make_unique());
324 *cow0.make_unique() += 1;
325 *cow1.make_unique() += 2;
326 *cow2.make_unique() += 3;
328 assert!(76 == *cow0);
329 assert!(77 == *cow1);
330 assert!(78 == *cow2);
332 // none should point to the same backing memory
333 assert!(*cow0 != *cow1);
334 assert!(*cow0 != *cow2);
335 assert!(*cow1 != *cow2);
339 fn test_cowarc_clone_unique2() {
340 let mut cow0 = Arc::new(75u);
341 let cow1 = cow0.clone();
342 let cow2 = cow1.clone();
344 assert!(75 == *cow0);
345 assert!(75 == *cow1);
346 assert!(75 == *cow2);
348 *cow0.make_unique() += 1;
350 assert!(76 == *cow0);
351 assert!(75 == *cow1);
352 assert!(75 == *cow2);
354 // cow1 and cow2 should share the same contents
355 // cow0 should have a unique reference
356 assert!(*cow0 != *cow1);
357 assert!(*cow0 != *cow2);
358 assert!(*cow1 == *cow2);
362 fn test_cowarc_clone_weak() {
363 let mut cow0 = Arc::new(75u);
364 let cow1_weak = cow0.downgrade();
366 assert!(75 == *cow0);
367 assert!(75 == *cow1_weak.upgrade().unwrap());
369 *cow0.make_unique() += 1;
371 assert!(76 == *cow0);
372 assert!(cow1_weak.upgrade().is_none());
378 let y = x.downgrade();
379 assert!(y.upgrade().is_some());
385 let y = x.downgrade();
387 assert!(y.upgrade().is_none());
391 fn weak_self_cyclic() {
393 x: Mutex<Option<Weak<Cycle>>>
396 let a = Arc::new(Cycle { x: Mutex::new(None) });
397 let b = a.clone().downgrade();
398 *a.x.lock() = Some(b);
400 // hopefully we don't double-free (or leak)...
405 let mut canary = atomics::AtomicUint::new(0);
406 let x = Arc::new(Canary(&mut canary as *mut atomics::AtomicUint));
408 assert!(canary.load(atomics::Acquire) == 1);
413 let mut canary = atomics::AtomicUint::new(0);
414 let arc = Arc::new(Canary(&mut canary as *mut atomics::AtomicUint));
415 let arc_weak = arc.downgrade();
416 assert!(canary.load(atomics::Acquire) == 0);
418 assert!(canary.load(atomics::Acquire) == 1);