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Quelle atomic.rs
Sprache: unbekannt
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use core::borrow::{Borrow, BorrowMut};
use core::cmp;
use core::fmt;
use core::marker::PhantomData;
use core::mem::{self, MaybeUninit};
use core::ops::{Deref, DerefMut};
use core::slice;
use core::sync::atomic::Ordering;
use crate::alloc::alloc;
use crate::alloc::boxed::Box;
use crate::guard::Guard;
use crate::primitive::sync::atomic::AtomicUsize;
use crossbeam_utils::atomic::AtomicConsume;
/// Given ordering for the success case in a compare-exchange operation, returns the stronges t
/// appropriate ordering for the failure case.
#[inline]
fn strongest_failure_ordering(ord: Ordering) -> Ordering {
use self::Ordering::*;
match ord {
Relaxed | Release => Relaxed,
Acquire | AcqRel => Acquire,
_ => SeqCst,
}
}
/// The error returned on failed compare-and-set operation.
// TODO: remove in the next major version.
#[deprecated(note = "Use `CompareExchangeError` instead")]
pub type CompareAndSetError<'g, T, P> = CompareExchangeError<'g, T, P>;
/// The error returned on failed compare-and-swap operation.
pub struct CompareExchangeError<'g, T: ?Sized + Pointable, P: Pointer<T>> {
/// The value in the atomic pointer at the time of the failed operation.
pub current: Shared<'g, T>,
/// The new value, which the operation failed to store.
pub new: P,
}
impl<T, P: Pointer<T> + fmt::Debug> fmt::Debug for CompareExchangeError<'_, T, P> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("CompareExchangeError")
.field("current", &self.current)
.field("new", &self.new)
.finish()
}
}
/// Memory orderings for compare-and-set operations.
///
/// A compare-and-set operation can have different memory orderings depending on whether it
/// succeeds or fails. This trait generalizes different ways of specifying memory orderings.
///
/// The two ways of specifying orderings for compare-and-set are:
///
/// 1. Just one `Ordering` for the success case. In case of failure, the strongest appropriate
/// ordering is chosen.
/// 2. A pair of `Ordering`s. The first one is for the success case, while the second one is
/// for the failure case.
// TODO: remove in the next major version.
#[deprecated(
note = "`compare_and_set` and `compare_and_set_weak` that use this trait are deprecated, \
use `compare_exchange` or `compare_exchange_weak instead`"
)]
pub trait CompareAndSetOrdering {
/// The ordering of the operation when it succeeds.
fn success(&self) -> Ordering;
/// The ordering of the operation when it fails.
///
/// The failure ordering can't be `Release` or `AcqRel` and must be equivalent or weaker than
/// the success ordering.
fn failure(&self) -> Ordering;
}
#[allow(deprecated)]
impl CompareAndSetOrdering for Ordering {
#[inline]
fn success(&self) -> Ordering {
*self
}
#[inline]
fn failure(&self) -> Ordering {
strongest_failure_ordering(*self)
}
}
#[allow(deprecated)]
impl CompareAndSetOrdering for (Ordering, Ordering) {
#[inline]
fn success(&self) -> Ordering {
self.0
}
#[inline]
fn failure(&self) -> Ordering {
self.1
}
}
/// Returns a bitmask containing the unused least significant bits of an aligned pointer to `T`.
#[inline]
fn low_bits<T: ?Sized + Pointable>() -> usize {
(1 << T::ALIGN.trailing_zeros()) - 1
}
/// Panics if the pointer is not properly unaligned.
#[inline]
fn ensure_aligned<T: ?Sized + Pointable>(raw: usize) {
assert_eq!(raw & low_bits::<T>(), 0, "unaligned pointer");
}
/// Given a tagged pointer `data`, returns the same pointer, but tagged with `tag`.
///
/// `tag` is truncated to fit into the unused bits of the pointer to `T`.
#[inline]
fn compose_tag<T: ?Sized + Pointable>(data: usize, tag: usize) -> usize {
(data & !low_bits::<T>()) | (tag & low_bits::<T>())
}
/// Decomposes a tagged pointer `data` into the pointer and the tag.
#[inline]
fn decompose_tag<T: ?Sized + Pointable>(data: usize) -> (usize, usize) {
(data & !low_bits::<T>(), data & low_bits::<T>())
}
/// Types that are pointed to by a single word.
///
/// In concurrent programming, it is necessary to represent an object within a word because atomic
/// operations (e.g., reads, writes, read-modify-writes) support only single words. This trait
/// qualifies such types that are pointed to by a single word.
///
/// The trait generalizes `Box<T>` for a sized type `T`. In a box, an object of type `T` is
/// allocated in heap and it is owned by a single-word pointer. This trait is also implemented for
/// `[MaybeUninit<T>]` by storing its size along with its elements and pointing to the pair of array
/// size and elements.
///
/// Pointers to `Pointable` types can be stored in [`Atomic`], [`Owned`], and [`Shared`]. In
/// particular, Crossbeam supports dynamically sized slices as follows.
///
/// ```
/// use std::mem::MaybeUninit;
/// use crossbeam_epoch::Owned;
///
/// let o = Owned::<[MaybeUninit<i32>]>::init(10); // allocating [i32; 10]
/// ```
pub trait Pointable {
/// The alignment of pointer.
const ALIGN: usize;
/// The type for initializers.
type Init;
/// Initializes a with the given initializer.
///
/// # Safety
///
/// The result should be a multiple of `ALIGN`.
unsafe fn init(init: Self::Init) -> usize;
/// Dereferences the given pointer.
///
/// # Safety
///
/// - The given `ptr` should have been initialized with [`Pointable::init`].
/// - `ptr` should not have yet been dropped by [`Pointable::drop`].
/// - `ptr` should not be mutably dereferenced by [`Pointable::deref_mut`] concurrently.
unsafe fn deref<'a>(ptr: usize) -> &'a Self;
/// Mutably dereferences the given pointer.
///
/// # Safety
///
/// - The given `ptr` should have been initialized with [`Pointable::init`].
/// - `ptr` should not have yet been dropped by [`Pointable::drop`].
/// - `ptr` should not be dereferenced by [`Pointable::deref`] or [`Pointable::deref_mut`]
/// concurrently.
unsafe fn deref_mut<'a>(ptr: usize) -> &'a mut Self;
/// Drops the object pointed to by the given pointer.
///
/// # Safety
///
/// - The given `ptr` should have been initialized with [`Pointable::init`].
/// - `ptr` should not have yet been dropped by [`Pointable::drop`].
/// - `ptr` should not be dereferenced by [`Pointable::deref`] or [`Pointable::deref_mut`]
/// concurrently.
unsafe fn drop(ptr: usize);
}
impl<T> Pointable for T {
const ALIGN: usize = mem::align_of::<T>();
type Init = T;
unsafe fn init(init: Self::Init) -> usize {
Box::into_raw(Box::new(init)) as usize
}
unsafe fn deref<'a>(ptr: usize) -> &'a Self {
&*(ptr as *const T)
}
unsafe fn deref_mut<'a>(ptr: usize) -> &'a mut Self {
&mut *(ptr as *mut T)
}
unsafe fn drop(ptr: usize) {
drop(Box::from_raw(ptr as *mut T));
}
}
/// Array with size.
///
/// # Memory layout
///
/// An array consisting of size and elements:
///
/// ```text
/// elements
/// |
/// |
/// ------------------------------------
/// | size | 0 | 1 | 2 | 3 | 4 | 5 | 6 |
/// ------------------------------------
/// ```
///
/// Its memory layout is different from that of `Box<[T]>` in that size is in the allocation (not
/// along with pointer as in `Box<[T]>`).
///
/// Elements are not present in the type, but they will be in the allocation.
/// ```
///
// TODO(@jeehoonkang): once we bump the minimum required Rust version to 1.44 or newer, use
// [`alloc::alloc::Layout::extend`] instead.
#[repr(C)]
struct Array<T> {
/// The number of elements (not the number of bytes).
len: usize,
elements: [MaybeUninit<T>; 0],
}
impl<T> Pointable for [MaybeUninit<T>] {
const ALIGN: usize = mem::align_of::<Array<T>>();
type Init = usize;
unsafe fn init(len: Self::Init) -> usize {
let size = mem::size_of::<Array<T>>() + mem::size_of::<MaybeUninit<T>>() * len;
let align = mem::align_of::<Array<T>>();
let layout = alloc::Layout::from_size_align(size, align).unwrap();
let ptr = alloc::alloc(layout).cast::<Array<T>>();
if ptr.is_null() {
alloc::handle_alloc_error(layout);
}
(*ptr).len = len;
ptr as usize
}
unsafe fn deref<'a>(ptr: usize) -> &'a Self {
let array = &*(ptr as *const Array<T>);
slice::from_raw_parts(array.elements.as_ptr() as *const _, array.len)
}
unsafe fn deref_mut<'a>(ptr: usize) -> &'a mut Self {
let array = &*(ptr as *mut Array<T>);
slice::from_raw_parts_mut(array.elements.as_ptr() as *mut _, array.len)
}
unsafe fn drop(ptr: usize) {
let array = &*(ptr as *mut Array<T>);
let size = mem::size_of::<Array<T>>() + mem::size_of::<MaybeUninit<T>>() * array.len;
let align = mem::align_of::<Array<T>>();
let layout = alloc::Layout::from_size_align(size, align).unwrap();
alloc::dealloc(ptr as *mut u8, layout);
}
}
/// An atomic pointer that can be safely shared between threads.
///
/// The pointer must be properly aligned. Since it is aligned, a tag can be stored into the unused
/// least significant bits of the address. For example, the tag for a pointer to a sized type `T`
/// should be less than `(1 << mem::align_of::<T>().trailing_zeros())`.
///
/// Any method that loads the pointer must be passed a reference to a [`Guard`].
///
/// Crossbeam supports dynamically sized types. See [`Pointable`] for details.
pub struct Atomic<T: ?Sized + Pointable> {
data: AtomicUsize,
_marker: PhantomData<*mut T>,
}
unsafe impl<T: ?Sized + Pointable + Send + Sync> Send for Atomic<T> {}
unsafe impl<T: ?Sized + Pointable + Send + Sync> Sync for Atomic<T> {}
impl<T> Atomic<T> {
/// Allocates `value` on the heap and returns a new atomic pointer pointing to it.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::Atomic;
///
/// let a = Atomic::new(1234);
/// # unsafe { drop(a.into_owned()); } // avoid leak
/// ```
pub fn new(init: T) -> Atomic<T> {
Self::init(init)
}
}
impl<T: ?Sized + Pointable> Atomic<T> {
/// Allocates `value` on the heap and returns a new atomic pointer pointing to it.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::Atomic;
///
/// let a = Atomic::<i32>::init(1234);
/// # unsafe { drop(a.into_owned()); } // avoid leak
/// ```
pub fn init(init: T::Init) -> Atomic<T> {
Self::from(Owned::init(init))
}
/// Returns a new atomic pointer pointing to the tagged pointer `data`.
fn from_usize(data: usize) -> Self {
Self {
data: AtomicUsize::new(data),
_marker: PhantomData,
}
}
/// Returns a new null atomic pointer.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::Atomic;
///
/// let a = Atomic::<i32>::null();
/// ```
#[cfg(all(not(crossbeam_no_const_fn_trait_bound), not(crossbeam_loom)))]
pub const fn null() -> Atomic<T> {
Self {
data: AtomicUsize::new(0),
_marker: PhantomData,
}
}
/// Returns a new null atomic pointer.
#[cfg(not(all(not(crossbeam_no_const_fn_trait_bound), not(crossbeam_loom))))]
pub fn null() -> Atomic<T> {
Self {
data: AtomicUsize::new(0),
_marker: PhantomData,
}
}
/// Loads a `Shared` from the atomic pointer.
///
/// This method takes an [`Ordering`] argument which describes the memory ordering of this
/// operation.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::new(1234);
/// let guard = &epoch::pin();
/// let p = a.load(SeqCst, guard);
/// # unsafe { drop(a.into_owned()); } // avoid leak
/// ```
pub fn load<'g>(&self, ord: Ordering, _: &'g Guard) -> Shared<'g, T> {
unsafe { Shared::from_usize(self.data.load(ord)) }
}
/// Loads a `Shared` from the atomic pointer using a "consume" memory ordering.
///
/// This is similar to the "acquire" ordering, except that an ordering is
/// only guaranteed with operations that "depend on" the result of the load.
/// However consume loads are usually much faster than acquire loads on
/// architectures with a weak memory model since they don't require memory
/// fence instructions.
///
/// The exact definition of "depend on" is a bit vague, but it works as you
/// would expect in practice since a lot of software, especially the Linux
/// kernel, rely on this behavior.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic};
///
/// let a = Atomic::new(1234);
/// let guard = &epoch::pin();
/// let p = a.load_consume(guard);
/// # unsafe { drop(a.into_owned()); } // avoid leak
/// ```
pub fn load_consume<'g>(&self, _: &'g Guard) -> Shared<'g, T> {
unsafe { Shared::from_usize(self.data.load_consume()) }
}
/// Stores a `Shared` or `Owned` pointer into the atomic pointer.
///
/// This method takes an [`Ordering`] argument which describes the memory ordering of this
/// operation.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{Atomic, Owned, Shared};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::new(1234);
/// # unsafe { drop(a.load(SeqCst, &crossbeam_epoch::pin()).into_owned()); } // avoid leak
/// a.store(Shared::null(), SeqCst);
/// a.store(Owned::new(1234), SeqCst);
/// # unsafe { drop(a.into_owned()); } // avoid leak
/// ```
pub fn store<P: Pointer<T>>(&self, new: P, ord: Ordering) {
self.data.store(new.into_usize(), ord);
}
/// Stores a `Shared` or `Owned` pointer into the atomic pointer, returning the previous
/// `Shared`.
///
/// This method takes an [`Ordering`] argument which describes the memory ordering of this
/// operation.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic, Shared};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::new(1234);
/// let guard = &epoch::pin();
/// let p = a.swap(Shared::null(), SeqCst, guard);
/// # unsafe { drop(p.into_owned()); } // avoid leak
/// ```
pub fn swap<'g, P: Pointer<T>>(&self, new: P, ord: Ordering, _: &'g Guard) -> Shared<'g, T> {
unsafe { Shared::from_usize(self.data.swap(new.into_usize(), ord)) }
}
/// Stores the pointer `new` (either `Shared` or `Owned`) into the atomic pointer if the current
/// value is the same as `current`. The tag is also taken into account, so two pointers to the
/// same object, but with different tags, will not be considered equal.
///
/// The return value is a result indicating whether the new pointer was written. On success the
/// pointer that was written is returned. On failure the actual current value and `new` are
/// returned.
///
/// This method takes two `Ordering` arguments to describe the memory
/// ordering of this operation. `success` describes the required ordering for the
/// read-modify-write operation that takes place if the comparison with `current` succeeds.
/// `failure` describes the required ordering for the load operation that takes place when
/// the comparison fails. Using `Acquire` as success ordering makes the store part
/// of this operation `Relaxed`, and using `Release` makes the successful load
/// `Relaxed`. The failure ordering can only be `SeqCst`, `Acquire` or `Relaxed`
/// and must be equivalent to or weaker than the success ordering.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic, Owned, Shared};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::new(1234);
///
/// let guard = &epoch::pin();
/// let curr = a.load(SeqCst, guard);
/// let res1 = a.compare_exchange(curr, Shared::null(), SeqCst, SeqCst, guard);
/// let res2 = a.compare_exchange(curr, Owned::new(5678), SeqCst, SeqCst, guard);
/// # unsafe { drop(curr.into_owned()); } // avoid leak
/// ```
pub fn compare_exchange<'g, P>(
&self,
current: Shared<'_, T>,
new: P,
success: Ordering,
failure: Ordering,
_: &'g Guard,
) -> Result<Shared<'g, T>, CompareExchangeError<'g, T, P>>
where
P: Pointer<T>,
{
let new = new.into_usize();
self.data
.compare_exchange(current.into_usize(), new, success, failure)
.map(|_| unsafe { Shared::from_usize(new) })
.map_err(|current| unsafe {
CompareExchangeError {
current: Shared::from_usize(current),
new: P::from_usize(new),
}
})
}
/// Stores the pointer `new` (either `Shared` or `Owned`) into the atomic pointer if the current
/// value is the same as `current`. The tag is also taken into account, so two pointers to the
/// same object, but with different tags, will not be considered equal.
///
/// Unlike [`compare_exchange`], this method is allowed to spuriously fail even when comparison
/// succeeds, which can result in more efficient code on some platforms. The return value is a
/// result indicating whether the new pointer was written. On success the pointer that was
/// written is returned. On failure the actual current value and `new` are returned.
///
/// This method takes two `Ordering` arguments to describe the memory
/// ordering of this operation. `success` describes the required ordering for the
/// read-modify-write operation that takes place if the comparison with `current` succeeds.
/// `failure` describes the required ordering for the load operation that takes place when
/// the comparison fails. Using `Acquire` as success ordering makes the store part
/// of this operation `Relaxed`, and using `Release` makes the successful load
/// `Relaxed`. The failure ordering can only be `SeqCst`, `Acquire` or `Relaxed`
/// and must be equivalent to or weaker than the success ordering.
///
/// [`compare_exchange`]: Atomic::compare_exchange
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic, Owned, Shared};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::new(1234);
/// let guard = &epoch::pin();
///
/// let mut new = Owned::new(5678);
/// let mut ptr = a.load(SeqCst, guard);
/// # unsafe { drop(a.load(SeqCst, guard).into_owned()); } // avoid leak
/// loop {
/// match a.compare_exchange_weak(ptr, new, SeqCst, SeqCst, guard) {
/// Ok(p) => {
/// ptr = p;
/// break;
/// }
/// Err(err) => {
/// ptr = err.current;
/// new = err.new;
/// }
/// }
/// }
///
/// let mut curr = a.load(SeqCst, guard);
/// loop {
/// match a.compare_exchange_weak(curr, Shared::null(), SeqCst, SeqCst, guard) {
/// Ok(_) => break,
/// Err(err) => curr = err.current,
/// }
/// }
/// # unsafe { drop(curr.into_owned()); } // avoid leak
/// ```
pub fn compare_exchange_weak<'g, P>(
&self,
current: Shared<'_, T>,
new: P,
success: Ordering,
failure: Ordering,
_: &'g Guard,
) -> Result<Shared<'g, T>, CompareExchangeError<'g, T, P>>
where
P: Pointer<T>,
{
let new = new.into_usize();
self.data
.compare_exchange_weak(current.into_usize(), new, success, failure)
.map(|_| unsafe { Shared::from_usize(new) })
.map_err(|current| unsafe {
CompareExchangeError {
current: Shared::from_usize(current),
new: P::from_usize(new),
}
})
}
/// Fetches the pointer, and then applies a function to it that returns a new value.
/// Returns a `Result` of `Ok(previous_value)` if the function returned `Some`, else `Err(_)`.
///
/// Note that the given function may be called multiple times if the value has been changed by
/// other threads in the meantime, as long as the function returns `Some(_)`, but the function
/// will have been applied only once to the stored value.
///
/// `fetch_update` takes two [`Ordering`] arguments to describe the memory
/// ordering of this operation. The first describes the required ordering for
/// when the operation finally succeeds while the second describes the
/// required ordering for loads. These correspond to the success and failure
/// orderings of [`Atomic::compare_exchange`] respectively.
///
/// Using [`Acquire`] as success ordering makes the store part of this
/// operation [`Relaxed`], and using [`Release`] makes the final successful
/// load [`Relaxed`]. The (failed) load ordering can only be [`SeqCst`],
/// [`Acquire`] or [`Relaxed`] and must be equivalent to or weaker than the
/// success ordering.
///
/// [`Relaxed`]: Ordering::Relaxed
/// [`Acquire`]: Ordering::Acquire
/// [`Release`]: Ordering::Release
/// [`SeqCst`]: Ordering::SeqCst
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::new(1234);
/// let guard = &epoch::pin();
///
/// let res1 = a.fetch_update(SeqCst, SeqCst, guard, |x| Some(x.with_tag(1)));
/// assert!(res1.is_ok());
///
/// let res2 = a.fetch_update(SeqCst, SeqCst, guard, |x| None);
/// assert!(res2.is_err());
/// # unsafe { drop(a.into_owned()); } // avoid leak
/// ```
pub fn fetch_update<'g, F>(
&self,
set_order: Ordering,
fail_order: Ordering,
guard: &'g Guard,
mut func: F,
) -> Result<Shared<'g, T>, Shared<'g, T>>
where
F: FnMut(Shared<'g, T>) -> Option<Shared<'g, T>>,
{
let mut prev = self.load(fail_order, guard);
while let Some(next) = func(prev) {
match self.compare_exchange_weak(prev, next, set_order, fail_order, guard) {
Ok(shared) => return Ok(shared),
Err(next_prev) => prev = next_prev.current,
}
}
Err(prev)
}
/// Stores the pointer `new` (either `Shared` or `Owned`) into the atomic pointer if the current
/// value is the same as `current`. The tag is also taken into account, so two pointers to the
/// same object, but with different tags, will not be considered equal.
///
/// The return value is a result indicating whether the new pointer was written. On success the
/// pointer that was written is returned. On failure the actual current value and `new` are
/// returned.
///
/// This method takes a [`CompareAndSetOrdering`] argument which describes the memory
/// ordering of this operation.
///
/// # Migrating to `compare_exchange`
///
/// `compare_and_set` is equivalent to `compare_exchange` with the following mapping for
/// memory orderings:
///
/// Original | Success | Failure
/// -------- | ------- | -------
/// Relaxed | Relaxed | Relaxed
/// Acquire | Acquire | Acquire
/// Release | Release | Relaxed
/// AcqRel | AcqRel | Acquire
/// SeqCst | SeqCst | SeqCst
///
/// # Examples
///
/// ```
/// # #![allow(deprecated)]
/// use crossbeam_epoch::{self as epoch, Atomic, Owned, Shared};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::new(1234);
///
/// let guard = &epoch::pin();
/// let curr = a.load(SeqCst, guard);
/// let res1 = a.compare_and_set(curr, Shared::null(), SeqCst, guard);
/// let res2 = a.compare_and_set(curr, Owned::new(5678), SeqCst, guard);
/// # unsafe { drop(curr.into_owned()); } // avoid leak
/// ```
// TODO: remove in the next major version.
#[allow(deprecated)]
#[deprecated(note = "Use `compare_exchange` instead")]
pub fn compare_and_set<'g, O, P>(
&self,
current: Shared<'_, T>,
new: P,
ord: O,
guard: &'g Guard,
) -> Result<Shared<'g, T>, CompareAndSetError<'g, T, P>>
where
O: CompareAndSetOrdering,
P: Pointer<T>,
{
self.compare_exchange(current, new, ord.success(), ord.failure(), guard)
}
/// Stores the pointer `new` (either `Shared` or `Owned`) into the atomic pointer if the current
/// value is the same as `current`. The tag is also taken into account, so two pointers to the
/// same object, but with different tags, will not be considered equal.
///
/// Unlike [`compare_and_set`], this method is allowed to spuriously fail even when comparison
/// succeeds, which can result in more efficient code on some platforms. The return value is a
/// result indicating whether the new pointer was written. On success the pointer that was
/// written is returned. On failure the actual current value and `new` are returned.
///
/// This method takes a [`CompareAndSetOrdering`] argument which describes the memory
/// ordering of this operation.
///
/// [`compare_and_set`]: Atomic::compare_and_set
///
/// # Migrating to `compare_exchange_weak`
///
/// `compare_and_set_weak` is equivalent to `compare_exchange_weak` with the following mapping for
/// memory orderings:
///
/// Original | Success | Failure
/// -------- | ------- | -------
/// Relaxed | Relaxed | Relaxed
/// Acquire | Acquire | Acquire
/// Release | Release | Relaxed
/// AcqRel | AcqRel | Acquire
/// SeqCst | SeqCst | SeqCst
///
/// # Examples
///
/// ```
/// # #![allow(deprecated)]
/// use crossbeam_epoch::{self as epoch, Atomic, Owned, Shared};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::new(1234);
/// let guard = &epoch::pin();
///
/// let mut new = Owned::new(5678);
/// let mut ptr = a.load(SeqCst, guard);
/// # unsafe { drop(a.load(SeqCst, guard).into_owned()); } // avoid leak
/// loop {
/// match a.compare_and_set_weak(ptr, new, SeqCst, guard) {
/// Ok(p) => {
/// ptr = p;
/// break;
/// }
/// Err(err) => {
/// ptr = err.current;
/// new = err.new;
/// }
/// }
/// }
///
/// let mut curr = a.load(SeqCst, guard);
/// loop {
/// match a.compare_and_set_weak(curr, Shared::null(), SeqCst, guard) {
/// Ok(_) => break,
/// Err(err) => curr = err.current,
/// }
/// }
/// # unsafe { drop(curr.into_owned()); } // avoid leak
/// ```
// TODO: remove in the next major version.
#[allow(deprecated)]
#[deprecated(note = "Use `compare_exchange_weak` instead")]
pub fn compare_and_set_weak<'g, O, P>(
&self,
current: Shared<'_, T>,
new: P,
ord: O,
guard: &'g Guard,
) -> Result<Shared<'g, T>, CompareAndSetError<'g, T, P>>
where
O: CompareAndSetOrdering,
P: Pointer<T>,
{
self.compare_exchange_weak(current, new, ord.success(), ord.failure(), guard)
}
/// Bitwise "and" with the current tag.
///
/// Performs a bitwise "and" operation on the current tag and the argument `val`, and sets the
/// new tag to the result. Returns the previous pointer.
///
/// This method takes an [`Ordering`] argument which describes the memory ordering of this
/// operation.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic, Shared};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::<i32>::from(Shared::null().with_tag(3));
/// let guard = &epoch::pin();
/// assert_eq!(a.fetch_and(2, SeqCst, guard).tag(), 3);
/// assert_eq!(a.load(SeqCst, guard).tag(), 2);
/// ```
pub fn fetch_and<'g>(&self, val: usize, ord: Ordering, _: &'g Guard) -> Shared<'g, T> {
unsafe { Shared::from_usize(self.data.fetch_and(val | !low_bits::<T>(), ord)) }
}
/// Bitwise "or" with the current tag.
///
/// Performs a bitwise "or" operation on the current tag and the argument `val`, and sets the
/// new tag to the result. Returns the previous pointer.
///
/// This method takes an [`Ordering`] argument which describes the memory ordering of this
/// operation.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic, Shared};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::<i32>::from(Shared::null().with_tag(1));
/// let guard = &epoch::pin();
/// assert_eq!(a.fetch_or(2, SeqCst, guard).tag(), 1);
/// assert_eq!(a.load(SeqCst, guard).tag(), 3);
/// ```
pub fn fetch_or<'g>(&self, val: usize, ord: Ordering, _: &'g Guard) -> Shared<'g, T> {
unsafe { Shared::from_usize(self.data.fetch_or(val & low_bits::<T>(), ord)) }
}
/// Bitwise "xor" with the current tag.
///
/// Performs a bitwise "xor" operation on the current tag and the argument `val`, and sets the
/// new tag to the result. Returns the previous pointer.
///
/// This method takes an [`Ordering`] argument which describes the memory ordering of this
/// operation.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic, Shared};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::<i32>::from(Shared::null().with_tag(1));
/// let guard = &epoch::pin();
/// assert_eq!(a.fetch_xor(3, SeqCst, guard).tag(), 1);
/// assert_eq!(a.load(SeqCst, guard).tag(), 2);
/// ```
pub fn fetch_xor<'g>(&self, val: usize, ord: Ordering, _: &'g Guard) -> Shared<'g, T> {
unsafe { Shared::from_usize(self.data.fetch_xor(val & low_bits::<T>(), ord)) }
}
/// Takes ownership of the pointee.
///
/// This consumes the atomic and converts it into [`Owned`]. As [`Atomic`] doesn't have a
/// destructor and doesn't drop the pointee while [`Owned`] does, this is suitable for
/// destructors of data structures.
///
/// # Panics
///
/// Panics if this pointer is null, but only in debug mode.
///
/// # Safety
///
/// This method may be called only if the pointer is valid and nobody else is holding a
/// reference to the same object.
///
/// # Examples
///
/// ```rust
/// # use std::mem;
/// # use crossbeam_epoch::Atomic;
/// struct DataStructure {
/// ptr: Atomic<usize>,
/// }
///
/// impl Drop for DataStructure {
/// fn drop(&mut self) {
/// // By now the DataStructure lives only in our thread and we are sure we don't hold
/// // any Shared or & to it ourselves.
/// unsafe {
/// drop(mem::replace(&mut self.ptr, Atomic::null()).into_owned());
/// }
/// }
/// }
/// ```
pub unsafe fn into_owned(self) -> Owned<T> {
#[cfg(crossbeam_loom)]
{
// FIXME: loom does not yet support into_inner, so we use unsync_load for now,
// which should have the same synchronization properties:
// https://github.com/tokio-rs/loom/issues/117
Owned::from_usize(self.data.unsync_load())
}
#[cfg(not(crossbeam_loom))]
{
Owned::from_usize(self.data.into_inner())
}
}
/// Takes ownership of the pointee if it is non-null.
///
/// This consumes the atomic and converts it into [`Owned`]. As [`Atomic`] doesn't have a
/// destructor and doesn't drop the pointee while [`Owned`] does, this is suitable for
/// destructors of data structures.
///
/// # Safety
///
/// This method may be called only if the pointer is valid and nobody else is holding a
/// reference to the same object, or the pointer is null.
///
/// # Examples
///
/// ```rust
/// # use std::mem;
/// # use crossbeam_epoch::Atomic;
/// struct DataStructure {
/// ptr: Atomic<usize>,
/// }
///
/// impl Drop for DataStructure {
/// fn drop(&mut self) {
/// // By now the DataStructure lives only in our thread and we are sure we don't hold
/// // any Shared or & to it ourselves, but it may be null, so we have to be careful.
/// let old = mem::replace(&mut self.ptr, Atomic::null());
/// unsafe {
/// if let Some(x) = old.try_into_owned() {
/// drop(x)
/// }
/// }
/// }
/// }
/// ```
pub unsafe fn try_into_owned(self) -> Option<Owned<T>> {
// FIXME: See self.into_owned()
#[cfg(crossbeam_loom)]
let data = self.data.unsync_load();
#[cfg(not(crossbeam_loom))]
let data = self.data.into_inner();
if decompose_tag::<T>(data).0 == 0 {
None
} else {
Some(Owned::from_usize(data))
}
}
}
impl<T: ?Sized + Pointable> fmt::Debug for Atomic<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
let data = self.data.load(Ordering::SeqCst);
let (raw, tag) = decompose_tag::<T>(data);
f.debug_struct("Atomic")
.field("raw", &raw)
.field("tag", &tag)
.finish()
}
}
impl<T: ?Sized + Pointable> fmt::Pointer for Atomic<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
let data = self.data.load(Ordering::SeqCst);
let (raw, _) = decompose_tag::<T>(data);
fmt::Pointer::fmt(&(unsafe { T::deref(raw) as *const _ }), f)
}
}
impl<T: ?Sized + Pointable> Clone for Atomic<T> {
/// Returns a copy of the atomic value.
///
/// Note that a `Relaxed` load is used here. If you need synchronization, use it with other
/// atomics or fences.
fn clone(&self) -> Self {
let data = self.data.load(Ordering::Relaxed);
Atomic::from_usize(data)
}
}
impl<T: ?Sized + Pointable> Default for Atomic<T> {
fn default() -> Self {
Atomic::null()
}
}
impl<T: ?Sized + Pointable> From<Owned<T>> for Atomic<T> {
/// Returns a new atomic pointer pointing to `owned`.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{Atomic, Owned};
///
/// let a = Atomic::<i32>::from(Owned::new(1234));
/// # unsafe { drop(a.into_owned()); } // avoid leak
/// ```
fn from(owned: Owned<T>) -> Self {
let data = owned.data;
mem::forget(owned);
Self::from_usize(data)
}
}
impl<T> From<Box<T>> for Atomic<T> {
fn from(b: Box<T>) -> Self {
Self::from(Owned::from(b))
}
}
impl<T> From<T> for Atomic<T> {
fn from(t: T) -> Self {
Self::new(t)
}
}
impl<'g, T: ?Sized + Pointable> From<Shared<'g, T>> for Atomic<T> {
/// Returns a new atomic pointer pointing to `ptr`.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{Atomic, Shared};
///
/// let a = Atomic::<i32>::from(Shared::<i32>::null());
/// ```
fn from(ptr: Shared<'g, T>) -> Self {
Self::from_usize(ptr.data)
}
}
impl<T> From<*const T> for Atomic<T> {
/// Returns a new atomic pointer pointing to `raw`.
///
/// # Examples
///
/// ```
/// use std::ptr;
/// use crossbeam_epoch::Atomic;
///
/// let a = Atomic::<i32>::from(ptr::null::<i32>());
/// ```
fn from(raw: *const T) -> Self {
Self::from_usize(raw as usize)
}
}
/// A trait for either `Owned` or `Shared` pointers.
pub trait Pointer<T: ?Sized + Pointable> {
/// Returns the machine representation of the pointer.
fn into_usize(self) -> usize;
/// Returns a new pointer pointing to the tagged pointer `data`.
///
/// # Safety
///
/// The given `data` should have been created by `Pointer::into_usize()`, and one `data` should
/// not be converted back by `Pointer::from_usize()` multiple times.
unsafe fn from_usize(data: usize) -> Self;
}
/// An owned heap-allocated object.
///
/// This type is very similar to `Box<T>`.
///
/// The pointer must be properly aligned. Since it is aligned, a tag can be stored into the unused
/// least significant bits of the address.
pub struct Owned<T: ?Sized + Pointable> {
data: usize,
_marker: PhantomData<Box<T>>,
}
impl<T: ?Sized + Pointable> Pointer<T> for Owned<T> {
#[inline]
fn into_usize(self) -> usize {
let data = self.data;
mem::forget(self);
data
}
/// Returns a new pointer pointing to the tagged pointer `data`.
///
/// # Panics
///
/// Panics if the data is zero in debug mode.
#[inline]
unsafe fn from_usize(data: usize) -> Self {
debug_assert!(data != 0, "converting zero into `Owned`");
Owned {
data,
_marker: PhantomData,
}
}
}
impl<T> Owned<T> {
/// Returns a new owned pointer pointing to `raw`.
///
/// This function is unsafe because improper use may lead to memory problems. Argument `raw`
/// must be a valid pointer. Also, a double-free may occur if the function is called twice on
/// the same raw pointer.
///
/// # Panics
///
/// Panics if `raw` is not properly aligned.
///
/// # Safety
///
/// The given `raw` should have been derived from `Owned`, and one `raw` should not be converted
/// back by `Owned::from_raw()` multiple times.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::Owned;
///
/// let o = unsafe { Owned::from_raw(Box::into_raw(Box::new(1234))) };
/// ```
pub unsafe fn from_raw(raw: *mut T) -> Owned<T> {
let raw = raw as usize;
ensure_aligned::<T>(raw);
Self::from_usize(raw)
}
/// Converts the owned pointer into a `Box`.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::Owned;
///
/// let o = Owned::new(1234);
/// let b: Box<i32> = o.into_box();
/// assert_eq!(*b, 1234);
/// ```
pub fn into_box(self) -> Box<T> {
let (raw, _) = decompose_tag::<T>(self.data);
mem::forget(self);
unsafe { Box::from_raw(raw as *mut _) }
}
/// Allocates `value` on the heap and returns a new owned pointer pointing to it.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::Owned;
///
/// let o = Owned::new(1234);
/// ```
pub fn new(init: T) -> Owned<T> {
Self::init(init)
}
}
impl<T: ?Sized + Pointable> Owned<T> {
/// Allocates `value` on the heap and returns a new owned pointer pointing to it.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::Owned;
///
/// let o = Owned::<i32>::init(1234);
/// ```
pub fn init(init: T::Init) -> Owned<T> {
unsafe { Self::from_usize(T::init(init)) }
}
/// Converts the owned pointer into a [`Shared`].
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Owned};
///
/// let o = Owned::new(1234);
/// let guard = &epoch::pin();
/// let p = o.into_shared(guard);
/// # unsafe { drop(p.into_owned()); } // avoid leak
/// ```
#[allow(clippy::needless_lifetimes)]
pub fn into_shared<'g>(self, _: &'g Guard) -> Shared<'g, T> {
unsafe { Shared::from_usize(self.into_usize()) }
}
/// Returns the tag stored within the pointer.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::Owned;
///
/// assert_eq!(Owned::new(1234).tag(), 0);
/// ```
pub fn tag(&self) -> usize {
let (_, tag) = decompose_tag::<T>(self.data);
tag
}
/// Returns the same pointer, but tagged with `tag`. `tag` is truncated to be fit into the
/// unused bits of the pointer to `T`.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::Owned;
///
/// let o = Owned::new(0u64);
/// assert_eq!(o.tag(), 0);
/// let o = o.with_tag(2);
/// assert_eq!(o.tag(), 2);
/// ```
pub fn with_tag(self, tag: usize) -> Owned<T> {
let data = self.into_usize();
unsafe { Self::from_usize(compose_tag::<T>(data, tag)) }
}
}
impl<T: ?Sized + Pointable> Drop for Owned<T> {
fn drop(&mut self) {
let (raw, _) = decompose_tag::<T>(self.data);
unsafe {
T::drop(raw);
}
}
}
impl<T: ?Sized + Pointable> fmt::Debug for Owned<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
let (raw, tag) = decompose_tag::<T>(self.data);
f.debug_struct("Owned")
.field("raw", &raw)
.field("tag", &tag)
.finish()
}
}
impl<T: Clone> Clone for Owned<T> {
fn clone(&self) -> Self {
Owned::new((**self).clone()).with_tag(self.tag())
}
}
impl<T: ?Sized + Pointable> Deref for Owned<T> {
type Target = T;
fn deref(&self) -> &T {
let (raw, _) = decompose_tag::<T>(self.data);
unsafe { T::deref(raw) }
}
}
impl<T: ?Sized + Pointable> DerefMut for Owned<T> {
fn deref_mut(&mut self) -> &mut T {
let (raw, _) = decompose_tag::<T>(self.data);
unsafe { T::deref_mut(raw) }
}
}
impl<T> From<T> for Owned<T> {
fn from(t: T) -> Self {
Owned::new(t)
}
}
impl<T> From<Box<T>> for Owned<T> {
/// Returns a new owned pointer pointing to `b`.
///
/// # Panics
///
/// Panics if the pointer (the `Box`) is not properly aligned.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::Owned;
///
/// let o = unsafe { Owned::from_raw(Box::into_raw(Box::new(1234))) };
/// ```
fn from(b: Box<T>) -> Self {
unsafe { Self::from_raw(Box::into_raw(b)) }
}
}
impl<T: ?Sized + Pointable> Borrow<T> for Owned<T> {
fn borrow(&self) -> &T {
self.deref()
}
}
impl<T: ?Sized + Pointable> BorrowMut<T> for Owned<T> {
fn borrow_mut(&mut self) -> &mut T {
self.deref_mut()
}
}
impl<T: ?Sized + Pointable> AsRef<T> for Owned<T> {
fn as_ref(&self) -> &T {
self.deref()
}
}
impl<T: ?Sized + Pointable> AsMut<T> for Owned<T> {
fn as_mut(&mut self) -> &mut T {
self.deref_mut()
}
}
/// A pointer to an object protected by the epoch GC.
///
/// The pointer is valid for use only during the lifetime `'g`.
///
/// The pointer must be properly aligned. Since it is aligned, a tag can be stored into the unused
/// least significant bits of the address.
pub struct Shared<'g, T: 'g + ?Sized + Pointable> {
data: usize,
_marker: PhantomData<(&'g (), *const T)>,
}
impl<T: ?Sized + Pointable> Clone for Shared<'_, T> {
fn clone(&self) -> Self {
Self {
data: self.data,
_marker: PhantomData,
}
}
}
impl<T: ?Sized + Pointable> Copy for Shared<'_, T> {}
impl<T: ?Sized + Pointable> Pointer<T> for Shared<'_, T> {
#[inline]
fn into_usize(self) -> usize {
self.data
}
#[inline]
unsafe fn from_usize(data: usize) -> Self {
Shared {
data,
_marker: PhantomData,
}
}
}
impl<'g, T> Shared<'g, T> {
/// Converts the pointer to a raw pointer (without the tag).
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic, Owned};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let o = Owned::new(1234);
/// let raw = &*o as *const _;
/// let a = Atomic::from(o);
///
/// let guard = &epoch::pin();
/// let p = a.load(SeqCst, guard);
/// assert_eq!(p.as_raw(), raw);
/// # unsafe { drop(a.into_owned()); } // avoid leak
/// ```
pub fn as_raw(&self) -> *const T {
let (raw, _) = decompose_tag::<T>(self.data);
raw as *const _
}
}
impl<'g, T: ?Sized + Pointable> Shared<'g, T> {
/// Returns a new null pointer.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::Shared;
///
/// let p = Shared::<i32>::null();
/// assert!(p.is_null());
/// ```
pub fn null() -> Shared<'g, T> {
Shared {
data: 0,
_marker: PhantomData,
}
}
/// Returns `true` if the pointer is null.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic, Owned};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::null();
/// let guard = &epoch::pin();
/// assert!(a.load(SeqCst, guard).is_null());
/// a.store(Owned::new(1234), SeqCst);
/// assert!(!a.load(SeqCst, guard).is_null());
/// # unsafe { drop(a.into_owned()); } // avoid leak
/// ```
pub fn is_null(&self) -> bool {
let (raw, _) = decompose_tag::<T>(self.data);
raw == 0
}
/// Dereferences the pointer.
///
/// Returns a reference to the pointee that is valid during the lifetime `'g`.
///
/// # Safety
///
/// Dereferencing a pointer is unsafe because it could be pointing to invalid memory.
///
/// Another concern is the possibility of data races due to lack of proper synchronization.
/// For example, consider the following scenario:
///
/// 1. A thread creates a new object: `a.store(Owned::new(10), Relaxed)`
/// 2. Another thread reads it: `*a.load(Relaxed, guard).as_ref().unwrap()`
///
/// The problem is that relaxed orderings don't synchronize initialization of the object with
/// the read from the second thread. This is a data race. A possible solution would be to use
/// `Release` and `Acquire` orderings.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::new(1234);
/// let guard = &epoch::pin();
/// let p = a.load(SeqCst, guard);
/// unsafe {
/// assert_eq!(p.deref(), &1234);
/// }
/// # unsafe { drop(a.into_owned()); } // avoid leak
/// ```
pub unsafe fn deref(&self) -> &'g T {
let (raw, _) = decompose_tag::<T>(self.data);
T::deref(raw)
}
/// Dereferences the pointer.
///
/// Returns a mutable reference to the pointee that is valid during the lifetime `'g`.
///
/// # Safety
///
/// * There is no guarantee that there are no more threads attempting to read/write from/to the
/// actual object at the same time.
///
/// The user must know that there are no concurrent accesses towards the object itself.
///
/// * Other than the above, all safety concerns of `deref()` applies here.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::new(vec![1, 2, 3, 4]);
/// let guard = &epoch::pin();
///
/// let mut p = a.load(SeqCst, guard);
/// unsafe {
/// assert!(!p.is_null());
/// let b = p.deref_mut();
/// assert_eq!(b, &vec![1, 2, 3, 4]);
/// b.push(5);
/// assert_eq!(b, &vec![1, 2, 3, 4, 5]);
/// }
///
/// let p = a.load(SeqCst, guard);
/// unsafe {
/// assert_eq!(p.deref(), &vec![1, 2, 3, 4, 5]);
/// }
/// # unsafe { drop(a.into_owned()); } // avoid leak
/// ```
pub unsafe fn deref_mut(&mut self) -> &'g mut T {
let (raw, _) = decompose_tag::<T>(self.data);
T::deref_mut(raw)
}
/// Converts the pointer to a reference.
///
/// Returns `None` if the pointer is null, or else a reference to the object wrapped in `Some`.
///
/// # Safety
///
/// Dereferencing a pointer is unsafe because it could be pointing to invalid memory.
///
/// Another concern is the possibility of data races due to lack of proper synchronization.
/// For example, consider the following scenario:
///
/// 1. A thread creates a new object: `a.store(Owned::new(10), Relaxed)`
/// 2. Another thread reads it: `*a.load(Relaxed, guard).as_ref().unwrap()`
///
/// The problem is that relaxed orderings don't synchronize initialization of the object with
/// the read from the second thread. This is a data race. A possible solution would be to use
/// `Release` and `Acquire` orderings.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::new(1234);
/// let guard = &epoch::pin();
/// let p = a.load(SeqCst, guard);
/// unsafe {
/// assert_eq!(p.as_ref(), Some(&1234));
/// }
/// # unsafe { drop(a.into_owned()); } // avoid leak
/// ```
pub unsafe fn as_ref(&self) -> Option<&'g T> {
let (raw, _) = decompose_tag::<T>(self.data);
if raw == 0 {
None
} else {
Some(T::deref(raw))
}
}
/// Takes ownership of the pointee.
///
/// # Panics
///
/// Panics if this pointer is null, but only in debug mode.
///
/// # Safety
///
/// This method may be called only if the pointer is valid and nobody else is holding a
/// reference to the same object.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::new(1234);
/// unsafe {
/// let guard = &epoch::unprotected();
/// let p = a.load(SeqCst, guard);
/// drop(p.into_owned());
/// }
/// ```
pub unsafe fn into_owned(self) -> Owned<T> {
debug_assert!(!self.is_null(), "converting a null `Shared` into `Owned`");
Owned::from_usize(self.data)
}
/// Takes ownership of the pointee if it is not null.
///
/// # Safety
///
/// This method may be called only if the pointer is valid and nobody else is holding a
/// reference to the same object, or if the pointer is null.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::new(1234);
/// unsafe {
/// let guard = &epoch::unprotected();
/// let p = a.load(SeqCst, guard);
/// if let Some(x) = p.try_into_owned() {
/// drop(x);
/// }
/// }
/// ```
pub unsafe fn try_into_owned(self) -> Option<Owned<T>> {
if self.is_null() {
None
} else {
Some(Owned::from_usize(self.data))
}
}
/// Returns the tag stored within the pointer.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic, Owned};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::<u64>::from(Owned::new(0u64).with_tag(2));
/// let guard = &epoch::pin();
/// let p = a.load(SeqCst, guard);
/// assert_eq!(p.tag(), 2);
/// # unsafe { drop(a.into_owned()); } // avoid leak
/// ```
pub fn tag(&self) -> usize {
let (_, tag) = decompose_tag::<T>(self.data);
tag
}
/// Returns the same pointer, but tagged with `tag`. `tag` is truncated to be fit into the
/// unused bits of the pointer to `T`.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::new(0u64);
/// let guard = &epoch::pin();
/// let p1 = a.load(SeqCst, guard);
/// let p2 = p1.with_tag(2);
///
/// assert_eq!(p1.tag(), 0);
/// assert_eq!(p2.tag(), 2);
/// assert_eq!(p1.as_raw(), p2.as_raw());
/// # unsafe { drop(a.into_owned()); } // avoid leak
/// ```
pub fn with_tag(&self, tag: usize) -> Shared<'g, T> {
unsafe { Self::from_usize(compose_tag::<T>(self.data, tag)) }
}
}
impl<T> From<*const T> for Shared<'_, T> {
/// Returns a new pointer pointing to `raw`.
///
/// # Panics
///
/// Panics if `raw` is not properly aligned.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::Shared;
///
/// let p = Shared::from(Box::into_raw(Box::new(1234)) as *const _);
/// assert!(!p.is_null());
/// # unsafe { drop(p.into_owned()); } // avoid leak
/// ```
fn from(raw: *const T) -> Self {
let raw = raw as usize;
ensure_aligned::<T>(raw);
unsafe { Self::from_usize(raw) }
}
}
impl<'g, T: ?Sized + Pointable> PartialEq<Shared<'g, T>> for Shared<'g, T> {
fn eq(&self, other: &Self) -> bool {
self.data == other.data
}
}
impl<T: ?Sized + Pointable> Eq for Shared<'_, T> {}
impl<'g, T: ?Sized + Pointable> PartialOrd<Shared<'g, T>> for Shared<'g, T> {
fn partial_cmp(&self, other: &Self) -> Option<cmp::Ordering> {
self.data.partial_cmp(&other.data)
}
}
impl<T: ?Sized + Pointable> Ord for Shared<'_, T> {
fn cmp(&self, other: &Self) -> cmp::Ordering {
self.data.cmp(&other.data)
}
}
impl<T: ?Sized + Pointable> fmt::Debug for Shared<'_, T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
let (raw, tag) = decompose_tag::<T>(self.data);
f.debug_struct("Shared")
.field("raw", &raw)
.field("tag", &tag)
.finish()
}
}
impl<T: ?Sized + Pointable> fmt::Pointer for Shared<'_, T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Pointer::fmt(&(unsafe { self.deref() as *const _ }), f)
}
}
impl<T: ?Sized + Pointable> Default for Shared<'_, T> {
fn default() -> Self {
Shared::null()
}
}
#[cfg(all(test, not(crossbeam_loom)))]
mod tests {
use super::{Owned, Shared};
use std::mem::MaybeUninit;
#[test]
fn valid_tag_i8() {
Shared::<i8>::null().with_tag(0);
}
#[test]
fn valid_tag_i64() {
Shared::<i64>::null().with_tag(7);
}
#[rustversion::since(1.61)]
#[test]
fn const_atomic_null() {
use super::Atomic;
static _U: Atomic<u8> = Atomic::<u8>::null();
}
#[test]
fn array_init() {
let owned = Owned::<[MaybeUninit<usize>]>::init(10);
let arr: &[MaybeUninit<usize>] = &owned;
assert_eq!(arr.len(), 10);
}
}
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2026-04-04
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