sync::Weak::{as,from,into}_raw
Methods on the Weak to access it as raw pointer to the data.
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@ -13,7 +13,7 @@ use core::borrow;
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use core::fmt;
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use core::cmp::{self, Ordering};
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use core::intrinsics::abort;
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use core::mem::{self, align_of_val, size_of_val};
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use core::mem::{self, align_of, align_of_val, size_of_val};
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use core::ops::{Deref, Receiver, CoerceUnsized, DispatchFromDyn};
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use core::pin::Pin;
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use core::ptr::{self, NonNull};
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@ -397,11 +397,7 @@ impl<T: ?Sized> Arc<T> {
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/// ```
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#[stable(feature = "rc_raw", since = "1.17.0")]
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pub unsafe fn from_raw(ptr: *const T) -> Self {
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// Align the unsized value to the end of the ArcInner.
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// Because it is ?Sized, it will always be the last field in memory.
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let align = align_of_val(&*ptr);
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let layout = Layout::new::<ArcInner<()>>();
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let offset = (layout.size() + layout.padding_needed_for(align)) as isize;
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let offset = data_offset(ptr);
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// Reverse the offset to find the original ArcInner.
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let fake_ptr = ptr as *mut ArcInner<T>;
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@ -1071,6 +1067,144 @@ impl<T> Weak<T> {
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ptr: NonNull::new(usize::MAX as *mut ArcInner<T>).expect("MAX is not 0"),
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}
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}
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/// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
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///
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/// It is up to the caller to ensure that the object is still alive when accessing it through
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/// the pointer.
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///
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/// The pointer may be [`null`] or be dangling in case the object has already been destroyed.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(weak_into_raw)]
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///
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/// use std::sync::{Arc, Weak};
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/// use std::ptr;
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///
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/// let strong = Arc::new(42);
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/// let weak = Arc::downgrade(&strong);
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/// // Both point to the same object
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/// assert!(ptr::eq(&*strong, Weak::as_raw(&weak)));
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/// // The strong here keeps it alive, so we can still access the object.
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/// assert_eq!(42, unsafe { *Weak::as_raw(&weak) });
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///
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/// drop(strong);
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/// // But not any more. We can do Weak::as_raw(&weak), but accessing the pointer would lead to
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/// // undefined behaviour.
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/// // assert_eq!(42, unsafe { *Weak::as_raw(&weak) });
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/// ```
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///
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/// [`null`]: ../../std/ptr/fn.null.html
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#[unstable(feature = "weak_into_raw", issue = "60728")]
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pub fn as_raw(this: &Self) -> *const T {
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match this.inner() {
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None => ptr::null(),
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Some(inner) => {
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let offset = data_offset_sized::<T>();
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let ptr = inner as *const ArcInner<T>;
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// Note: while the pointer we create may already point to dropped value, the
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// allocation still lives (it must hold the weak point as long as we are alive).
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// Therefore, the offset is OK to do, it won't get out of the allocation.
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let ptr = unsafe { (ptr as *const u8).offset(offset) };
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ptr as *const T
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}
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}
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}
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/// Consumes the `Weak<T>` and turns it into a raw pointer.
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///
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/// This converts the weak pointer into a raw pointer, preserving the original weak count. It
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/// can be turned back into the `Weak<T>` with [`from_raw`].
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///
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/// The same restrictions of accessing the target of the pointer as with
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/// [`as_raw`] apply.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(weak_into_raw)]
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///
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/// use std::sync::{Arc, Weak};
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///
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/// let strong = Arc::new(42);
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/// let weak = Arc::downgrade(&strong);
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/// let raw = Weak::into_raw(weak);
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///
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/// assert_eq!(1, Arc::weak_count(&strong));
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/// assert_eq!(42, unsafe { *raw });
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///
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/// drop(unsafe { Weak::from_raw(raw) });
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/// assert_eq!(0, Arc::weak_count(&strong));
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/// ```
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///
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/// [`from_raw`]: struct.Weak.html#method.from_raw
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/// [`as_raw`]: struct.Weak.html#method.as_raw
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#[unstable(feature = "weak_into_raw", issue = "60728")]
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pub fn into_raw(this: Self) -> *const T {
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let result = Self::as_raw(&this);
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mem::forget(this);
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result
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}
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/// Converts a raw pointer previously created by [`into_raw`] back into
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/// `Weak<T>`.
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///
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/// This can be used to safely get a strong reference (by calling [`upgrade`]
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/// later) or to deallocate the weak count by dropping the `Weak<T>`.
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///
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/// It takes ownership of one weak count. In case a [`null`] is passed, a dangling [`Weak`] is
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/// returned.
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///
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/// # Safety
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///
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/// The pointer must represent one valid weak count. In other words, it must point to `T` which
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/// is or *was* managed by an [`Arc`] and the weak count of that [`Arc`] must not have reached
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/// 0. It is allowed for the strong count to be 0.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(weak_into_raw)]
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///
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/// use std::sync::{Arc, Weak};
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///
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/// let strong = Arc::new(42);
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///
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/// let raw_1 = Weak::into_raw(Arc::downgrade(&strong));
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/// let raw_2 = Weak::into_raw(Arc::downgrade(&strong));
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///
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/// assert_eq!(2, Arc::weak_count(&strong));
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///
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/// assert_eq!(42, *Weak::upgrade(&unsafe { Weak::from_raw(raw_1) }).unwrap());
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/// assert_eq!(1, Arc::weak_count(&strong));
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///
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/// drop(strong);
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///
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/// // Decrement the last weak count.
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/// assert!(Weak::upgrade(&unsafe { Weak::from_raw(raw_2) }).is_none());
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/// ```
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///
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/// [`null`]: ../../std/ptr/fn.null.html
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/// [`into_raw`]: struct.Weak.html#method.into_raw
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/// [`upgrade`]: struct.Weak.html#method.upgrade
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/// [`Weak`]: struct.Weak.html
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/// [`Arc`]: struct.Arc.html
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#[unstable(feature = "weak_into_raw", issue = "60728")]
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pub unsafe fn from_raw(ptr: *const T) -> Self {
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if ptr.is_null() {
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Self::new()
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} else {
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// See Arc::from_raw for details
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let offset = data_offset(ptr);
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let fake_ptr = ptr as *mut ArcInner<T>;
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let ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
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Weak {
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ptr: NonNull::new(ptr).expect("Invalid pointer passed to from_raw"),
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}
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}
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}
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}
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impl<T: ?Sized> Weak<T> {
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@ -2150,3 +2284,21 @@ impl<T: ?Sized> AsRef<T> for Arc<T> {
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#[stable(feature = "pin", since = "1.33.0")]
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impl<T: ?Sized> Unpin for Arc<T> { }
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/// Computes the offset of the data field within ArcInner.
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unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
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// Align the unsized value to the end of the ArcInner.
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// Because it is ?Sized, it will always be the last field in memory.
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let align = align_of_val(&*ptr);
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let layout = Layout::new::<ArcInner<()>>();
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(layout.size() + layout.padding_needed_for(align)) as isize
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}
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/// Computes the offset of the data field within ArcInner.
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///
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/// Unlike [`data_offset`], this doesn't need the pointer, but it works only on `T: Sized`.
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fn data_offset_sized<T>() -> isize {
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let align = align_of::<T>();
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let layout = Layout::new::<ArcInner<()>>();
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(layout.size() + layout.padding_needed_for(align)) as isize
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}
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