1 //! Manually manage memory through raw pointers.
3 //! *[See also the pointer primitive types](../../std/primitive.pointer.html).*
7 //! Many functions in this module take raw pointers as arguments and read from
8 //! or write to them. For this to be safe, these pointers must be *valid*.
9 //! Whether a pointer is valid depends on the operation it is used for
10 //! (read or write), and the extent of the memory that is accessed (i.e.,
11 //! how many bytes are read/written). Most functions use `*mut T` and `*const T`
12 //! to access only a single value, in which case the documentation omits the size
13 //! and implicitly assumes it to be `size_of::<T>()` bytes.
15 //! The precise rules for validity are not determined yet. The guarantees that are
16 //! provided at this point are very minimal:
18 //! * A [null] pointer is *never* valid, not even for accesses of [size zero][zst].
19 //! * All pointers (except for the null pointer) are valid for all operations of
21 //! * For a pointer to be valid, it is necessary, but not always sufficient, that the pointer
22 //! be *dereferencable*: the memory range of the given size starting at the pointer must all be
23 //! within the bounds of a single allocated object. Note that in Rust,
24 //! every (stack-allocated) variable is considered a separate allocated object.
25 //! * All accesses performed by functions in this module are *non-atomic* in the sense
26 //! of [atomic operations] used to synchronize between threads. This means it is
27 //! undefined behavior to perform two concurrent accesses to the same location from different
28 //! threads unless both accesses only read from memory. Notice that this explicitly
29 //! includes [`read_volatile`] and [`write_volatile`]: Volatile accesses cannot
30 //! be used for inter-thread synchronization.
31 //! * The result of casting a reference to a pointer is valid for as long as the
32 //! underlying object is live and no reference (just raw pointers) is used to
33 //! access the same memory.
35 //! These axioms, along with careful use of [`offset`] for pointer arithmetic,
36 //! are enough to correctly implement many useful things in unsafe code. Stronger guarantees
37 //! will be provided eventually, as the [aliasing] rules are being determined. For more
38 //! information, see the [book] as well as the section in the reference devoted
39 //! to [undefined behavior][ub].
43 //! Valid raw pointers as defined above are not necessarily properly aligned (where
44 //! "proper" alignment is defined by the pointee type, i.e., `*const T` must be
45 //! aligned to `mem::align_of::<T>()`). However, most functions require their
46 //! arguments to be properly aligned, and will explicitly state
47 //! this requirement in their documentation. Notable exceptions to this are
48 //! [`read_unaligned`] and [`write_unaligned`].
50 //! When a function requires proper alignment, it does so even if the access
51 //! has size 0, i.e., even if memory is not actually touched. Consider using
52 //! [`NonNull::dangling`] in such cases.
54 //! [aliasing]: ../../nomicon/aliasing.html
55 //! [book]: ../../book/ch19-01-unsafe-rust.html#dereferencing-a-raw-pointer
56 //! [ub]: ../../reference/behavior-considered-undefined.html
57 //! [null]: ./fn.null.html
58 //! [zst]: ../../nomicon/exotic-sizes.html#zero-sized-types-zsts
59 //! [atomic operations]: ../../std/sync/atomic/index.html
60 //! [`copy`]: ../../std/ptr/fn.copy.html
61 //! [`offset`]: ../../std/primitive.pointer.html#method.offset
62 //! [`read_unaligned`]: ./fn.read_unaligned.html
63 //! [`write_unaligned`]: ./fn.write_unaligned.html
64 //! [`read_volatile`]: ./fn.read_volatile.html
65 //! [`write_volatile`]: ./fn.write_volatile.html
66 //! [`NonNull::dangling`]: ./struct.NonNull.html#method.dangling
68 // ignore-tidy-undocumented-unsafe
70 #![stable(feature = "rust1", since = "1.0.0")]
72 use crate::cmp::Ordering;
75 use crate::intrinsics;
76 use crate::mem::{self, MaybeUninit};
78 #[stable(feature = "rust1", since = "1.0.0")]
79 pub use crate::intrinsics::copy_nonoverlapping;
81 #[stable(feature = "rust1", since = "1.0.0")]
82 pub use crate::intrinsics::copy;
84 #[stable(feature = "rust1", since = "1.0.0")]
85 pub use crate::intrinsics::write_bytes;
88 #[stable(feature = "nonnull", since = "1.25.0")]
89 pub use non_null::NonNull;
92 #[unstable(feature = "ptr_internals", issue = "none")]
93 pub use unique::Unique;
98 /// Executes the destructor (if any) of the pointed-to value.
100 /// This is semantically equivalent to calling [`ptr::read`] and discarding
101 /// the result, but has the following advantages:
103 /// * It is *required* to use `drop_in_place` to drop unsized types like
104 /// trait objects, because they can't be read out onto the stack and
105 /// dropped normally.
107 /// * It is friendlier to the optimizer to do this over [`ptr::read`] when
108 /// dropping manually allocated memory (e.g., when writing Box/Rc/Vec),
109 /// as the compiler doesn't need to prove that it's sound to elide the
112 /// Unaligned values cannot be dropped in place, they must be copied to an aligned
113 /// location first using [`ptr::read_unaligned`].
115 /// [`ptr::read`]: ../ptr/fn.read.html
116 /// [`ptr::read_unaligned`]: ../ptr/fn.read_unaligned.html
120 /// Behavior is undefined if any of the following conditions are violated:
122 /// * `to_drop` must be [valid] for reads.
124 /// * `to_drop` must be properly aligned.
126 /// Additionally, if `T` is not [`Copy`], using the pointed-to value after
127 /// calling `drop_in_place` can cause undefined behavior. Note that `*to_drop =
128 /// foo` counts as a use because it will cause the value to be dropped
129 /// again. [`write`] can be used to overwrite data without causing it to be
132 /// Note that even if `T` has size `0`, the pointer must be non-NULL and properly aligned.
134 /// [valid]: ../ptr/index.html#safety
135 /// [`Copy`]: ../marker/trait.Copy.html
136 /// [`write`]: ../ptr/fn.write.html
140 /// Manually remove the last item from a vector:
146 /// let last = Rc::new(1);
147 /// let weak = Rc::downgrade(&last);
149 /// let mut v = vec![Rc::new(0), last];
152 /// // Get a raw pointer to the last element in `v`.
153 /// let ptr = &mut v[1] as *mut _;
154 /// // Shorten `v` to prevent the last item from being dropped. We do that first,
155 /// // to prevent issues if the `drop_in_place` below panics.
157 /// // Without a call `drop_in_place`, the last item would never be dropped,
158 /// // and the memory it manages would be leaked.
159 /// ptr::drop_in_place(ptr);
162 /// assert_eq!(v, &[0.into()]);
164 /// // Ensure that the last item was dropped.
165 /// assert!(weak.upgrade().is_none());
168 /// Notice that the compiler performs this copy automatically when dropping packed structs,
169 /// i.e., you do not usually have to worry about such issues unless you call `drop_in_place`
171 #[stable(feature = "drop_in_place", since = "1.8.0")]
172 #[lang = "drop_in_place"]
173 #[allow(unconditional_recursion)]
174 pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
175 // Code here does not matter - this is replaced by the
176 // real drop glue by the compiler.
177 drop_in_place(to_drop)
180 /// Creates a null raw pointer.
187 /// let p: *const i32 = ptr::null();
188 /// assert!(p.is_null());
191 #[stable(feature = "rust1", since = "1.0.0")]
193 #[rustc_const_stable(feature = "const_ptr_null", since = "1.32.0")]
194 pub const fn null<T>() -> *const T {
198 /// Creates a null mutable raw pointer.
205 /// let p: *mut i32 = ptr::null_mut();
206 /// assert!(p.is_null());
209 #[stable(feature = "rust1", since = "1.0.0")]
211 #[rustc_const_stable(feature = "const_ptr_null", since = "1.32.0")]
212 pub const fn null_mut<T>() -> *mut T {
217 pub(crate) union Repr<T> {
218 pub(crate) rust: *const [T],
220 pub(crate) raw: FatPtr<T>,
224 pub(crate) struct FatPtr<T> {
226 pub(crate) len: usize,
229 /// Forms a raw slice from a pointer and a length.
231 /// The `len` argument is the number of **elements**, not the number of bytes.
233 /// This function is safe, but actually using the return value is unsafe.
234 /// See the documentation of [`from_raw_parts`] for slice safety requirements.
236 /// [`from_raw_parts`]: ../../std/slice/fn.from_raw_parts.html
241 /// #![feature(slice_from_raw_parts)]
244 /// // create a slice pointer when starting out with a pointer to the first element
245 /// let mut x = [5, 6, 7];
246 /// let ptr = &mut x[0] as *mut _;
247 /// let slice = ptr::slice_from_raw_parts_mut(ptr, 3);
248 /// assert_eq!(unsafe { &*slice }[2], 7);
251 #[unstable(feature = "slice_from_raw_parts", reason = "recently added", issue = "36925")]
252 #[rustc_const_unstable(feature = "const_slice_from_raw_parts", issue = "67456")]
253 pub const fn slice_from_raw_parts<T>(data: *const T, len: usize) -> *const [T] {
254 unsafe { Repr { raw: FatPtr { data, len } }.rust }
257 /// Performs the same functionality as [`slice_from_raw_parts`], except that a
258 /// raw mutable slice is returned, as opposed to a raw immutable slice.
260 /// See the documentation of [`slice_from_raw_parts`] for more details.
262 /// This function is safe, but actually using the return value is unsafe.
263 /// See the documentation of [`from_raw_parts_mut`] for slice safety requirements.
265 /// [`slice_from_raw_parts`]: fn.slice_from_raw_parts.html
266 /// [`from_raw_parts_mut`]: ../../std/slice/fn.from_raw_parts_mut.html
268 #[unstable(feature = "slice_from_raw_parts", reason = "recently added", issue = "36925")]
269 #[rustc_const_unstable(feature = "const_slice_from_raw_parts", issue = "67456")]
270 pub const fn slice_from_raw_parts_mut<T>(data: *mut T, len: usize) -> *mut [T] {
271 unsafe { Repr { raw: FatPtr { data, len } }.rust_mut }
274 /// Swaps the values at two mutable locations of the same type, without
275 /// deinitializing either.
277 /// But for the following two exceptions, this function is semantically
278 /// equivalent to [`mem::swap`]:
280 /// * It operates on raw pointers instead of references. When references are
281 /// available, [`mem::swap`] should be preferred.
283 /// * The two pointed-to values may overlap. If the values do overlap, then the
284 /// overlapping region of memory from `x` will be used. This is demonstrated
285 /// in the second example below.
287 /// [`mem::swap`]: ../mem/fn.swap.html
291 /// Behavior is undefined if any of the following conditions are violated:
293 /// * Both `x` and `y` must be [valid] for reads and writes.
295 /// * Both `x` and `y` must be properly aligned.
297 /// Note that even if `T` has size `0`, the pointers must be non-NULL and properly aligned.
299 /// [valid]: ../ptr/index.html#safety
303 /// Swapping two non-overlapping regions:
308 /// let mut array = [0, 1, 2, 3];
310 /// let x = array[0..].as_mut_ptr() as *mut [u32; 2]; // this is `array[0..2]`
311 /// let y = array[2..].as_mut_ptr() as *mut [u32; 2]; // this is `array[2..4]`
315 /// assert_eq!([2, 3, 0, 1], array);
319 /// Swapping two overlapping regions:
324 /// let mut array = [0, 1, 2, 3];
326 /// let x = array[0..].as_mut_ptr() as *mut [u32; 3]; // this is `array[0..3]`
327 /// let y = array[1..].as_mut_ptr() as *mut [u32; 3]; // this is `array[1..4]`
331 /// // The indices `1..3` of the slice overlap between `x` and `y`.
332 /// // Reasonable results would be for to them be `[2, 3]`, so that indices `0..3` are
333 /// // `[1, 2, 3]` (matching `y` before the `swap`); or for them to be `[0, 1]`
334 /// // so that indices `1..4` are `[0, 1, 2]` (matching `x` before the `swap`).
335 /// // This implementation is defined to make the latter choice.
336 /// assert_eq!([1, 0, 1, 2], array);
340 #[stable(feature = "rust1", since = "1.0.0")]
341 pub unsafe fn swap<T>(x: *mut T, y: *mut T) {
342 // Give ourselves some scratch space to work with.
343 // We do not have to worry about drops: `MaybeUninit` does nothing when dropped.
344 let mut tmp = MaybeUninit::<T>::uninit();
347 copy_nonoverlapping(x, tmp.as_mut_ptr(), 1);
348 copy(y, x, 1); // `x` and `y` may overlap
349 copy_nonoverlapping(tmp.as_ptr(), y, 1);
352 /// Swaps `count * size_of::<T>()` bytes between the two regions of memory
353 /// beginning at `x` and `y`. The two regions must *not* overlap.
357 /// Behavior is undefined if any of the following conditions are violated:
359 /// * Both `x` and `y` must be [valid] for reads and writes of `count *
360 /// size_of::<T>()` bytes.
362 /// * Both `x` and `y` must be properly aligned.
364 /// * The region of memory beginning at `x` with a size of `count *
365 /// size_of::<T>()` bytes must *not* overlap with the region of memory
366 /// beginning at `y` with the same size.
368 /// Note that even if the effectively copied size (`count * size_of::<T>()`) is `0`,
369 /// the pointers must be non-NULL and properly aligned.
371 /// [valid]: ../ptr/index.html#safety
380 /// let mut x = [1, 2, 3, 4];
381 /// let mut y = [7, 8, 9];
384 /// ptr::swap_nonoverlapping(x.as_mut_ptr(), y.as_mut_ptr(), 2);
387 /// assert_eq!(x, [7, 8, 3, 4]);
388 /// assert_eq!(y, [1, 2, 9]);
391 #[stable(feature = "swap_nonoverlapping", since = "1.27.0")]
392 pub unsafe fn swap_nonoverlapping<T>(x: *mut T, y: *mut T, count: usize) {
393 let x = x as *mut u8;
394 let y = y as *mut u8;
395 let len = mem::size_of::<T>() * count;
396 swap_nonoverlapping_bytes(x, y, len)
400 pub(crate) unsafe fn swap_nonoverlapping_one<T>(x: *mut T, y: *mut T) {
401 // For types smaller than the block optimization below,
402 // just swap directly to avoid pessimizing codegen.
403 if mem::size_of::<T>() < 32 {
405 copy_nonoverlapping(y, x, 1);
408 swap_nonoverlapping(x, y, 1);
413 unsafe fn swap_nonoverlapping_bytes(x: *mut u8, y: *mut u8, len: usize) {
414 // The approach here is to utilize simd to swap x & y efficiently. Testing reveals
415 // that swapping either 32 bytes or 64 bytes at a time is most efficient for Intel
416 // Haswell E processors. LLVM is more able to optimize if we give a struct a
417 // #[repr(simd)], even if we don't actually use this struct directly.
419 // FIXME repr(simd) broken on emscripten and redox
420 #[cfg_attr(not(any(target_os = "emscripten", target_os = "redox")), repr(simd))]
421 struct Block(u64, u64, u64, u64);
422 struct UnalignedBlock(u64, u64, u64, u64);
424 let block_size = mem::size_of::<Block>();
426 // Loop through x & y, copying them `Block` at a time
427 // The optimizer should unroll the loop fully for most types
428 // N.B. We can't use a for loop as the `range` impl calls `mem::swap` recursively
430 while i + block_size <= len {
431 // Create some uninitialized memory as scratch space
432 // Declaring `t` here avoids aligning the stack when this loop is unused
433 let mut t = mem::MaybeUninit::<Block>::uninit();
434 let t = t.as_mut_ptr() as *mut u8;
438 // Swap a block of bytes of x & y, using t as a temporary buffer
439 // This should be optimized into efficient SIMD operations where available
440 copy_nonoverlapping(x, t, block_size);
441 copy_nonoverlapping(y, x, block_size);
442 copy_nonoverlapping(t, y, block_size);
447 // Swap any remaining bytes
448 let mut t = mem::MaybeUninit::<UnalignedBlock>::uninit();
451 let t = t.as_mut_ptr() as *mut u8;
455 copy_nonoverlapping(x, t, rem);
456 copy_nonoverlapping(y, x, rem);
457 copy_nonoverlapping(t, y, rem);
461 /// Moves `src` into the pointed `dst`, returning the previous `dst` value.
463 /// Neither value is dropped.
465 /// This function is semantically equivalent to [`mem::replace`] except that it
466 /// operates on raw pointers instead of references. When references are
467 /// available, [`mem::replace`] should be preferred.
469 /// [`mem::replace`]: ../mem/fn.replace.html
473 /// Behavior is undefined if any of the following conditions are violated:
475 /// * `dst` must be [valid] for writes.
477 /// * `dst` must be properly aligned.
479 /// Note that even if `T` has size `0`, the pointer must be non-NULL and properly aligned.
481 /// [valid]: ../ptr/index.html#safety
488 /// let mut rust = vec!['b', 'u', 's', 't'];
490 /// // `mem::replace` would have the same effect without requiring the unsafe
493 /// ptr::replace(&mut rust[0], 'r')
496 /// assert_eq!(b, 'b');
497 /// assert_eq!(rust, &['r', 'u', 's', 't']);
500 #[stable(feature = "rust1", since = "1.0.0")]
501 pub unsafe fn replace<T>(dst: *mut T, mut src: T) -> T {
502 mem::swap(&mut *dst, &mut src); // cannot overlap
506 /// Reads the value from `src` without moving it. This leaves the
507 /// memory in `src` unchanged.
511 /// Behavior is undefined if any of the following conditions are violated:
513 /// * `src` must be [valid] for reads.
515 /// * `src` must be properly aligned. Use [`read_unaligned`] if this is not the
518 /// Note that even if `T` has size `0`, the pointer must be non-NULL and properly aligned.
526 /// let y = &x as *const i32;
529 /// assert_eq!(std::ptr::read(y), 12);
533 /// Manually implement [`mem::swap`]:
538 /// fn swap<T>(a: &mut T, b: &mut T) {
540 /// // Create a bitwise copy of the value at `a` in `tmp`.
541 /// let tmp = ptr::read(a);
543 /// // Exiting at this point (either by explicitly returning or by
544 /// // calling a function which panics) would cause the value in `tmp` to
545 /// // be dropped while the same value is still referenced by `a`. This
546 /// // could trigger undefined behavior if `T` is not `Copy`.
548 /// // Create a bitwise copy of the value at `b` in `a`.
549 /// // This is safe because mutable references cannot alias.
550 /// ptr::copy_nonoverlapping(b, a, 1);
552 /// // As above, exiting here could trigger undefined behavior because
553 /// // the same value is referenced by `a` and `b`.
555 /// // Move `tmp` into `b`.
556 /// ptr::write(b, tmp);
558 /// // `tmp` has been moved (`write` takes ownership of its second argument),
559 /// // so nothing is dropped implicitly here.
563 /// let mut foo = "foo".to_owned();
564 /// let mut bar = "bar".to_owned();
566 /// swap(&mut foo, &mut bar);
568 /// assert_eq!(foo, "bar");
569 /// assert_eq!(bar, "foo");
572 /// ## Ownership of the Returned Value
574 /// `read` creates a bitwise copy of `T`, regardless of whether `T` is [`Copy`].
575 /// If `T` is not [`Copy`], using both the returned value and the value at
576 /// `*src` can violate memory safety. Note that assigning to `*src` counts as a
577 /// use because it will attempt to drop the value at `*src`.
579 /// [`write`] can be used to overwrite data without causing it to be dropped.
584 /// let mut s = String::from("foo");
586 /// // `s2` now points to the same underlying memory as `s`.
587 /// let mut s2: String = ptr::read(&s);
589 /// assert_eq!(s2, "foo");
591 /// // Assigning to `s2` causes its original value to be dropped. Beyond
592 /// // this point, `s` must no longer be used, as the underlying memory has
594 /// s2 = String::default();
595 /// assert_eq!(s2, "");
597 /// // Assigning to `s` would cause the old value to be dropped again,
598 /// // resulting in undefined behavior.
599 /// // s = String::from("bar"); // ERROR
601 /// // `ptr::write` can be used to overwrite a value without dropping it.
602 /// ptr::write(&mut s, String::from("bar"));
605 /// assert_eq!(s, "bar");
608 /// [`mem::swap`]: ../mem/fn.swap.html
609 /// [valid]: ../ptr/index.html#safety
610 /// [`Copy`]: ../marker/trait.Copy.html
611 /// [`read_unaligned`]: ./fn.read_unaligned.html
612 /// [`write`]: ./fn.write.html
614 #[stable(feature = "rust1", since = "1.0.0")]
615 pub unsafe fn read<T>(src: *const T) -> T {
616 let mut tmp = MaybeUninit::<T>::uninit();
617 copy_nonoverlapping(src, tmp.as_mut_ptr(), 1);
621 /// Reads the value from `src` without moving it. This leaves the
622 /// memory in `src` unchanged.
624 /// Unlike [`read`], `read_unaligned` works with unaligned pointers.
628 /// Behavior is undefined if any of the following conditions are violated:
630 /// * `src` must be [valid] for reads.
632 /// Like [`read`], `read_unaligned` creates a bitwise copy of `T`, regardless of
633 /// whether `T` is [`Copy`]. If `T` is not [`Copy`], using both the returned
634 /// value and the value at `*src` can [violate memory safety][read-ownership].
636 /// Note that even if `T` has size `0`, the pointer must be non-NULL.
638 /// [`Copy`]: ../marker/trait.Copy.html
639 /// [`read`]: ./fn.read.html
640 /// [`write_unaligned`]: ./fn.write_unaligned.html
641 /// [read-ownership]: ./fn.read.html#ownership-of-the-returned-value
642 /// [valid]: ../ptr/index.html#safety
644 /// ## On `packed` structs
646 /// It is currently impossible to create raw pointers to unaligned fields
647 /// of a packed struct.
649 /// Attempting to create a raw pointer to an `unaligned` struct field with
650 /// an expression such as `&packed.unaligned as *const FieldType` creates an
651 /// intermediate unaligned reference before converting that to a raw pointer.
652 /// That this reference is temporary and immediately cast is inconsequential
653 /// as the compiler always expects references to be properly aligned.
654 /// As a result, using `&packed.unaligned as *const FieldType` causes immediate
655 /// *undefined behavior* in your program.
657 /// An example of what not to do and how this relates to `read_unaligned` is:
660 /// #[repr(packed, C)]
666 /// let packed = Packed {
668 /// unaligned: 0x01020304,
672 /// // Here we attempt to take the address of a 32-bit integer which is not aligned.
674 /// // A temporary unaligned reference is created here which results in
675 /// // undefined behavior regardless of whether the reference is used or not.
676 /// &packed.unaligned
677 /// // Casting to a raw pointer doesn't help; the mistake already happened.
680 /// let v = std::ptr::read_unaligned(unaligned);
686 /// Accessing unaligned fields directly with e.g. `packed.unaligned` is safe however.
687 // FIXME: Update docs based on outcome of RFC #2582 and friends.
691 /// Read an usize value from a byte buffer:
696 /// fn read_usize(x: &[u8]) -> usize {
697 /// assert!(x.len() >= mem::size_of::<usize>());
699 /// let ptr = x.as_ptr() as *const usize;
701 /// unsafe { ptr.read_unaligned() }
705 #[stable(feature = "ptr_unaligned", since = "1.17.0")]
706 pub unsafe fn read_unaligned<T>(src: *const T) -> T {
707 let mut tmp = MaybeUninit::<T>::uninit();
708 copy_nonoverlapping(src as *const u8, tmp.as_mut_ptr() as *mut u8, mem::size_of::<T>());
712 /// Overwrites a memory location with the given value without reading or
713 /// dropping the old value.
715 /// `write` does not drop the contents of `dst`. This is safe, but it could leak
716 /// allocations or resources, so care should be taken not to overwrite an object
717 /// that should be dropped.
719 /// Additionally, it does not drop `src`. Semantically, `src` is moved into the
720 /// location pointed to by `dst`.
722 /// This is appropriate for initializing uninitialized memory, or overwriting
723 /// memory that has previously been [`read`] from.
725 /// [`read`]: ./fn.read.html
729 /// Behavior is undefined if any of the following conditions are violated:
731 /// * `dst` must be [valid] for writes.
733 /// * `dst` must be properly aligned. Use [`write_unaligned`] if this is not the
736 /// Note that even if `T` has size `0`, the pointer must be non-NULL and properly aligned.
738 /// [valid]: ../ptr/index.html#safety
739 /// [`write_unaligned`]: ./fn.write_unaligned.html
747 /// let y = &mut x as *mut i32;
751 /// std::ptr::write(y, z);
752 /// assert_eq!(std::ptr::read(y), 12);
756 /// Manually implement [`mem::swap`]:
761 /// fn swap<T>(a: &mut T, b: &mut T) {
763 /// // Create a bitwise copy of the value at `a` in `tmp`.
764 /// let tmp = ptr::read(a);
766 /// // Exiting at this point (either by explicitly returning or by
767 /// // calling a function which panics) would cause the value in `tmp` to
768 /// // be dropped while the same value is still referenced by `a`. This
769 /// // could trigger undefined behavior if `T` is not `Copy`.
771 /// // Create a bitwise copy of the value at `b` in `a`.
772 /// // This is safe because mutable references cannot alias.
773 /// ptr::copy_nonoverlapping(b, a, 1);
775 /// // As above, exiting here could trigger undefined behavior because
776 /// // the same value is referenced by `a` and `b`.
778 /// // Move `tmp` into `b`.
779 /// ptr::write(b, tmp);
781 /// // `tmp` has been moved (`write` takes ownership of its second argument),
782 /// // so nothing is dropped implicitly here.
786 /// let mut foo = "foo".to_owned();
787 /// let mut bar = "bar".to_owned();
789 /// swap(&mut foo, &mut bar);
791 /// assert_eq!(foo, "bar");
792 /// assert_eq!(bar, "foo");
795 /// [`mem::swap`]: ../mem/fn.swap.html
797 #[stable(feature = "rust1", since = "1.0.0")]
798 pub unsafe fn write<T>(dst: *mut T, src: T) {
799 intrinsics::move_val_init(&mut *dst, src)
802 /// Overwrites a memory location with the given value without reading or
803 /// dropping the old value.
805 /// Unlike [`write`], the pointer may be unaligned.
807 /// `write_unaligned` does not drop the contents of `dst`. This is safe, but it
808 /// could leak allocations or resources, so care should be taken not to overwrite
809 /// an object that should be dropped.
811 /// Additionally, it does not drop `src`. Semantically, `src` is moved into the
812 /// location pointed to by `dst`.
814 /// This is appropriate for initializing uninitialized memory, or overwriting
815 /// memory that has previously been read with [`read_unaligned`].
817 /// [`write`]: ./fn.write.html
818 /// [`read_unaligned`]: ./fn.read_unaligned.html
822 /// Behavior is undefined if any of the following conditions are violated:
824 /// * `dst` must be [valid] for writes.
826 /// Note that even if `T` has size `0`, the pointer must be non-NULL.
828 /// [valid]: ../ptr/index.html#safety
830 /// ## On `packed` structs
832 /// It is currently impossible to create raw pointers to unaligned fields
833 /// of a packed struct.
835 /// Attempting to create a raw pointer to an `unaligned` struct field with
836 /// an expression such as `&packed.unaligned as *const FieldType` creates an
837 /// intermediate unaligned reference before converting that to a raw pointer.
838 /// That this reference is temporary and immediately cast is inconsequential
839 /// as the compiler always expects references to be properly aligned.
840 /// As a result, using `&packed.unaligned as *const FieldType` causes immediate
841 /// *undefined behavior* in your program.
843 /// An example of what not to do and how this relates to `write_unaligned` is:
846 /// #[repr(packed, C)]
852 /// let v = 0x01020304;
853 /// let mut packed: Packed = unsafe { std::mem::zeroed() };
856 /// // Here we attempt to take the address of a 32-bit integer which is not aligned.
858 /// // A temporary unaligned reference is created here which results in
859 /// // undefined behavior regardless of whether the reference is used or not.
860 /// &mut packed.unaligned
861 /// // Casting to a raw pointer doesn't help; the mistake already happened.
864 /// std::ptr::write_unaligned(unaligned, v);
870 /// Accessing unaligned fields directly with e.g. `packed.unaligned` is safe however.
871 // FIXME: Update docs based on outcome of RFC #2582 and friends.
875 /// Write an usize value to a byte buffer:
880 /// fn write_usize(x: &mut [u8], val: usize) {
881 /// assert!(x.len() >= mem::size_of::<usize>());
883 /// let ptr = x.as_mut_ptr() as *mut usize;
885 /// unsafe { ptr.write_unaligned(val) }
889 #[stable(feature = "ptr_unaligned", since = "1.17.0")]
890 pub unsafe fn write_unaligned<T>(dst: *mut T, src: T) {
891 copy_nonoverlapping(&src as *const T as *const u8, dst as *mut u8, mem::size_of::<T>());
895 /// Performs a volatile read of the value from `src` without moving it. This
896 /// leaves the memory in `src` unchanged.
898 /// Volatile operations are intended to act on I/O memory, and are guaranteed
899 /// to not be elided or reordered by the compiler across other volatile
902 /// [`write_volatile`]: ./fn.write_volatile.html
906 /// Rust does not currently have a rigorously and formally defined memory model,
907 /// so the precise semantics of what "volatile" means here is subject to change
908 /// over time. That being said, the semantics will almost always end up pretty
909 /// similar to [C11's definition of volatile][c11].
911 /// The compiler shouldn't change the relative order or number of volatile
912 /// memory operations. However, volatile memory operations on zero-sized types
913 /// (e.g., if a zero-sized type is passed to `read_volatile`) are noops
914 /// and may be ignored.
916 /// [c11]: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
920 /// Behavior is undefined if any of the following conditions are violated:
922 /// * `src` must be [valid] for reads.
924 /// * `src` must be properly aligned.
926 /// Like [`read`], `read_volatile` creates a bitwise copy of `T`, regardless of
927 /// whether `T` is [`Copy`]. If `T` is not [`Copy`], using both the returned
928 /// value and the value at `*src` can [violate memory safety][read-ownership].
929 /// However, storing non-[`Copy`] types in volatile memory is almost certainly
932 /// Note that even if `T` has size `0`, the pointer must be non-NULL and properly aligned.
934 /// [valid]: ../ptr/index.html#safety
935 /// [`Copy`]: ../marker/trait.Copy.html
936 /// [`read`]: ./fn.read.html
937 /// [read-ownership]: ./fn.read.html#ownership-of-the-returned-value
939 /// Just like in C, whether an operation is volatile has no bearing whatsoever
940 /// on questions involving concurrent access from multiple threads. Volatile
941 /// accesses behave exactly like non-atomic accesses in that regard. In particular,
942 /// a race between a `read_volatile` and any write operation to the same location
943 /// is undefined behavior.
951 /// let y = &x as *const i32;
954 /// assert_eq!(std::ptr::read_volatile(y), 12);
958 #[stable(feature = "volatile", since = "1.9.0")]
959 pub unsafe fn read_volatile<T>(src: *const T) -> T {
960 intrinsics::volatile_load(src)
963 /// Performs a volatile write of a memory location with the given value without
964 /// reading or dropping the old value.
966 /// Volatile operations are intended to act on I/O memory, and are guaranteed
967 /// to not be elided or reordered by the compiler across other volatile
970 /// `write_volatile` does not drop the contents of `dst`. This is safe, but it
971 /// could leak allocations or resources, so care should be taken not to overwrite
972 /// an object that should be dropped.
974 /// Additionally, it does not drop `src`. Semantically, `src` is moved into the
975 /// location pointed to by `dst`.
977 /// [`read_volatile`]: ./fn.read_volatile.html
981 /// Rust does not currently have a rigorously and formally defined memory model,
982 /// so the precise semantics of what "volatile" means here is subject to change
983 /// over time. That being said, the semantics will almost always end up pretty
984 /// similar to [C11's definition of volatile][c11].
986 /// The compiler shouldn't change the relative order or number of volatile
987 /// memory operations. However, volatile memory operations on zero-sized types
988 /// (e.g., if a zero-sized type is passed to `write_volatile`) are noops
989 /// and may be ignored.
991 /// [c11]: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
995 /// Behavior is undefined if any of the following conditions are violated:
997 /// * `dst` must be [valid] for writes.
999 /// * `dst` must be properly aligned.
1001 /// Note that even if `T` has size `0`, the pointer must be non-NULL and properly aligned.
1003 /// [valid]: ../ptr/index.html#safety
1005 /// Just like in C, whether an operation is volatile has no bearing whatsoever
1006 /// on questions involving concurrent access from multiple threads. Volatile
1007 /// accesses behave exactly like non-atomic accesses in that regard. In particular,
1008 /// a race between a `write_volatile` and any other operation (reading or writing)
1009 /// on the same location is undefined behavior.
1017 /// let y = &mut x as *mut i32;
1021 /// std::ptr::write_volatile(y, z);
1022 /// assert_eq!(std::ptr::read_volatile(y), 12);
1026 #[stable(feature = "volatile", since = "1.9.0")]
1027 pub unsafe fn write_volatile<T>(dst: *mut T, src: T) {
1028 intrinsics::volatile_store(dst, src);
1031 /// Align pointer `p`.
1033 /// Calculate offset (in terms of elements of `stride` stride) that has to be applied
1034 /// to pointer `p` so that pointer `p` would get aligned to `a`.
1036 /// Note: This implementation has been carefully tailored to not panic. It is UB for this to panic.
1037 /// The only real change that can be made here is change of `INV_TABLE_MOD_16` and associated
1040 /// If we ever decide to make it possible to call the intrinsic with `a` that is not a
1041 /// power-of-two, it will probably be more prudent to just change to a naive implementation rather
1042 /// than trying to adapt this to accommodate that change.
1044 /// Any questions go to @nagisa.
1045 #[lang = "align_offset"]
1046 pub(crate) unsafe fn align_offset<T: Sized>(p: *const T, a: usize) -> usize {
1047 /// Calculate multiplicative modular inverse of `x` modulo `m`.
1049 /// This implementation is tailored for align_offset and has following preconditions:
1051 /// * `m` is a power-of-two;
1052 /// * `x < m`; (if `x ≥ m`, pass in `x % m` instead)
1054 /// Implementation of this function shall not panic. Ever.
1056 fn mod_inv(x: usize, m: usize) -> usize {
1057 /// Multiplicative modular inverse table modulo 2⁴ = 16.
1059 /// Note, that this table does not contain values where inverse does not exist (i.e., for
1060 /// `0⁻¹ mod 16`, `2⁻¹ mod 16`, etc.)
1061 const INV_TABLE_MOD_16: [u8; 8] = [1, 11, 13, 7, 9, 3, 5, 15];
1062 /// Modulo for which the `INV_TABLE_MOD_16` is intended.
1063 const INV_TABLE_MOD: usize = 16;
1065 const INV_TABLE_MOD_SQUARED: usize = INV_TABLE_MOD * INV_TABLE_MOD;
1067 let table_inverse = INV_TABLE_MOD_16[(x & (INV_TABLE_MOD - 1)) >> 1] as usize;
1068 if m <= INV_TABLE_MOD {
1069 table_inverse & (m - 1)
1071 // We iterate "up" using the following formula:
1073 // $$ xy ≡ 1 (mod 2ⁿ) → xy (2 - xy) ≡ 1 (mod 2²ⁿ) $$
1075 // until 2²ⁿ ≥ m. Then we can reduce to our desired `m` by taking the result `mod m`.
1076 let mut inverse = table_inverse;
1077 let mut going_mod = INV_TABLE_MOD_SQUARED;
1079 // y = y * (2 - xy) mod n
1081 // Note, that we use wrapping operations here intentionally – the original formula
1082 // uses e.g., subtraction `mod n`. It is entirely fine to do them `mod
1083 // usize::max_value()` instead, because we take the result `mod n` at the end
1085 inverse = inverse.wrapping_mul(2usize.wrapping_sub(x.wrapping_mul(inverse)))
1088 return inverse & (m - 1);
1090 going_mod = going_mod.wrapping_mul(going_mod);
1095 let stride = mem::size_of::<T>();
1096 let a_minus_one = a.wrapping_sub(1);
1097 let pmoda = p as usize & a_minus_one;
1100 // Already aligned. Yay!
1105 return if stride == 0 {
1106 // If the pointer is not aligned, and the element is zero-sized, then no amount of
1107 // elements will ever align the pointer.
1110 a.wrapping_sub(pmoda)
1114 let smoda = stride & a_minus_one;
1115 // a is power-of-two so cannot be 0. stride = 0 is handled above.
1116 let gcdpow = intrinsics::cttz_nonzero(stride).min(intrinsics::cttz_nonzero(a));
1117 let gcd = 1usize << gcdpow;
1119 if p as usize & (gcd - 1) == 0 {
1120 // This branch solves for the following linear congruence equation:
1122 // $$ p + so ≡ 0 mod a $$
1124 // $p$ here is the pointer value, $s$ – stride of `T`, $o$ offset in `T`s, and $a$ – the
1125 // requested alignment.
1128 // o = (a - (p mod a))/g * ((s/g)⁻¹ mod a)
1130 // The first term is “the relative alignment of p to a”, the second term is “how does
1131 // incrementing p by s bytes change the relative alignment of p”. Division by `g` is
1132 // necessary to make this equation well formed if $a$ and $s$ are not co-prime.
1134 // Furthermore, the result produced by this solution is not “minimal”, so it is necessary
1135 // to take the result $o mod lcm(s, a)$. We can replace $lcm(s, a)$ with just a $a / g$.
1136 let j = a.wrapping_sub(pmoda) >> gcdpow;
1137 let k = smoda >> gcdpow;
1138 return intrinsics::unchecked_rem(j.wrapping_mul(mod_inv(k, a)), a >> gcdpow);
1141 // Cannot be aligned at all.
1145 /// Compares raw pointers for equality.
1147 /// This is the same as using the `==` operator, but less generic:
1148 /// the arguments have to be `*const T` raw pointers,
1149 /// not anything that implements `PartialEq`.
1151 /// This can be used to compare `&T` references (which coerce to `*const T` implicitly)
1152 /// by their address rather than comparing the values they point to
1153 /// (which is what the `PartialEq for &T` implementation does).
1161 /// let other_five = 5;
1162 /// let five_ref = &five;
1163 /// let same_five_ref = &five;
1164 /// let other_five_ref = &other_five;
1166 /// assert!(five_ref == same_five_ref);
1167 /// assert!(ptr::eq(five_ref, same_five_ref));
1169 /// assert!(five_ref == other_five_ref);
1170 /// assert!(!ptr::eq(five_ref, other_five_ref));
1173 /// Slices are also compared by their length (fat pointers):
1176 /// let a = [1, 2, 3];
1177 /// assert!(std::ptr::eq(&a[..3], &a[..3]));
1178 /// assert!(!std::ptr::eq(&a[..2], &a[..3]));
1179 /// assert!(!std::ptr::eq(&a[0..2], &a[1..3]));
1182 /// Traits are also compared by their implementation:
1185 /// #[repr(transparent)]
1186 /// struct Wrapper { member: i32 }
1189 /// impl Trait for Wrapper {}
1190 /// impl Trait for i32 {}
1192 /// let wrapper = Wrapper { member: 10 };
1194 /// // Pointers have equal addresses.
1195 /// assert!(std::ptr::eq(
1196 /// &wrapper as *const Wrapper as *const u8,
1197 /// &wrapper.member as *const i32 as *const u8
1200 /// // Objects have equal addresses, but `Trait` has different implementations.
1201 /// assert!(!std::ptr::eq(
1202 /// &wrapper as &dyn Trait,
1203 /// &wrapper.member as &dyn Trait,
1205 /// assert!(!std::ptr::eq(
1206 /// &wrapper as &dyn Trait as *const dyn Trait,
1207 /// &wrapper.member as &dyn Trait as *const dyn Trait,
1210 /// // Converting the reference to a `*const u8` compares by address.
1211 /// assert!(std::ptr::eq(
1212 /// &wrapper as &dyn Trait as *const dyn Trait as *const u8,
1213 /// &wrapper.member as &dyn Trait as *const dyn Trait as *const u8,
1216 #[stable(feature = "ptr_eq", since = "1.17.0")]
1218 pub fn eq<T: ?Sized>(a: *const T, b: *const T) -> bool {
1222 /// Hash a raw pointer.
1224 /// This can be used to hash a `&T` reference (which coerces to `*const T` implicitly)
1225 /// by its address rather than the value it points to
1226 /// (which is what the `Hash for &T` implementation does).
1231 /// use std::collections::hash_map::DefaultHasher;
1232 /// use std::hash::{Hash, Hasher};
1236 /// let five_ref = &five;
1238 /// let mut hasher = DefaultHasher::new();
1239 /// ptr::hash(five_ref, &mut hasher);
1240 /// let actual = hasher.finish();
1242 /// let mut hasher = DefaultHasher::new();
1243 /// (five_ref as *const i32).hash(&mut hasher);
1244 /// let expected = hasher.finish();
1246 /// assert_eq!(actual, expected);
1248 #[stable(feature = "ptr_hash", since = "1.35.0")]
1249 pub fn hash<T: ?Sized, S: hash::Hasher>(hashee: *const T, into: &mut S) {
1250 use crate::hash::Hash;
1254 // Impls for function pointers
1255 macro_rules! fnptr_impls_safety_abi {
1256 ($FnTy: ty, $($Arg: ident),*) => {
1257 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1258 impl<Ret, $($Arg),*> PartialEq for $FnTy {
1260 fn eq(&self, other: &Self) -> bool {
1261 *self as usize == *other as usize
1265 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1266 impl<Ret, $($Arg),*> Eq for $FnTy {}
1268 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1269 impl<Ret, $($Arg),*> PartialOrd for $FnTy {
1271 fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
1272 (*self as usize).partial_cmp(&(*other as usize))
1276 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1277 impl<Ret, $($Arg),*> Ord for $FnTy {
1279 fn cmp(&self, other: &Self) -> Ordering {
1280 (*self as usize).cmp(&(*other as usize))
1284 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1285 impl<Ret, $($Arg),*> hash::Hash for $FnTy {
1286 fn hash<HH: hash::Hasher>(&self, state: &mut HH) {
1287 state.write_usize(*self as usize)
1291 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1292 impl<Ret, $($Arg),*> fmt::Pointer for $FnTy {
1293 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1294 fmt::Pointer::fmt(&(*self as *const ()), f)
1298 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1299 impl<Ret, $($Arg),*> fmt::Debug for $FnTy {
1300 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1301 fmt::Pointer::fmt(&(*self as *const ()), f)
1307 macro_rules! fnptr_impls_args {
1308 ($($Arg: ident),+) => {
1309 fnptr_impls_safety_abi! { extern "Rust" fn($($Arg),+) -> Ret, $($Arg),+ }
1310 fnptr_impls_safety_abi! { extern "C" fn($($Arg),+) -> Ret, $($Arg),+ }
1311 fnptr_impls_safety_abi! { extern "C" fn($($Arg),+ , ...) -> Ret, $($Arg),+ }
1312 fnptr_impls_safety_abi! { unsafe extern "Rust" fn($($Arg),+) -> Ret, $($Arg),+ }
1313 fnptr_impls_safety_abi! { unsafe extern "C" fn($($Arg),+) -> Ret, $($Arg),+ }
1314 fnptr_impls_safety_abi! { unsafe extern "C" fn($($Arg),+ , ...) -> Ret, $($Arg),+ }
1317 // No variadic functions with 0 parameters
1318 fnptr_impls_safety_abi! { extern "Rust" fn() -> Ret, }
1319 fnptr_impls_safety_abi! { extern "C" fn() -> Ret, }
1320 fnptr_impls_safety_abi! { unsafe extern "Rust" fn() -> Ret, }
1321 fnptr_impls_safety_abi! { unsafe extern "C" fn() -> Ret, }
1325 fnptr_impls_args! {}
1326 fnptr_impls_args! { A }
1327 fnptr_impls_args! { A, B }
1328 fnptr_impls_args! { A, B, C }
1329 fnptr_impls_args! { A, B, C, D }
1330 fnptr_impls_args! { A, B, C, D, E }
1331 fnptr_impls_args! { A, B, C, D, E, F }
1332 fnptr_impls_args! { A, B, C, D, E, F, G }
1333 fnptr_impls_args! { A, B, C, D, E, F, G, H }
1334 fnptr_impls_args! { A, B, C, D, E, F, G, H, I }
1335 fnptr_impls_args! { A, B, C, D, E, F, G, H, I, J }
1336 fnptr_impls_args! { A, B, C, D, E, F, G, H, I, J, K }
1337 fnptr_impls_args! { A, B, C, D, E, F, G, H, I, J, K, L }