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 *dereferenceable*: 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::{self, is_aligned_and_not_null, is_nonoverlapping};
76 use crate::mem::{self, MaybeUninit};
78 #[stable(feature = "rust1", since = "1.0.0")]
80 pub use crate::intrinsics::copy_nonoverlapping;
82 #[stable(feature = "rust1", since = "1.0.0")]
84 pub use crate::intrinsics::copy;
86 #[stable(feature = "rust1", since = "1.0.0")]
88 pub use crate::intrinsics::write_bytes;
91 #[stable(feature = "nonnull", since = "1.25.0")]
92 pub use non_null::NonNull;
95 #[unstable(feature = "ptr_internals", issue = "none")]
96 pub use unique::Unique;
101 /// Executes the destructor (if any) of the pointed-to value.
103 /// This is semantically equivalent to calling [`ptr::read`] and discarding
104 /// the result, but has the following advantages:
106 /// * It is *required* to use `drop_in_place` to drop unsized types like
107 /// trait objects, because they can't be read out onto the stack and
108 /// dropped normally.
110 /// * It is friendlier to the optimizer to do this over [`ptr::read`] when
111 /// dropping manually allocated memory (e.g., when writing Box/Rc/Vec),
112 /// as the compiler doesn't need to prove that it's sound to elide the
115 /// Unaligned values cannot be dropped in place, they must be copied to an aligned
116 /// location first using [`ptr::read_unaligned`].
118 /// [`ptr::read`]: ../ptr/fn.read.html
119 /// [`ptr::read_unaligned`]: ../ptr/fn.read_unaligned.html
123 /// Behavior is undefined if any of the following conditions are violated:
125 /// * `to_drop` must be [valid] for both reads and writes.
127 /// * `to_drop` must be properly aligned.
129 /// * The value `to_drop` points to must be valid for dropping, which may mean it must uphold
130 /// additional invariants - this is type-dependent.
132 /// Additionally, if `T` is not [`Copy`], using the pointed-to value after
133 /// calling `drop_in_place` can cause undefined behavior. Note that `*to_drop =
134 /// foo` counts as a use because it will cause the value to be dropped
135 /// again. [`write`] can be used to overwrite data without causing it to be
138 /// Note that even if `T` has size `0`, the pointer must be non-NULL and properly aligned.
140 /// [valid]: ../ptr/index.html#safety
141 /// [`Copy`]: ../marker/trait.Copy.html
142 /// [`write`]: ../ptr/fn.write.html
146 /// Manually remove the last item from a vector:
152 /// let last = Rc::new(1);
153 /// let weak = Rc::downgrade(&last);
155 /// let mut v = vec![Rc::new(0), last];
158 /// // Get a raw pointer to the last element in `v`.
159 /// let ptr = &mut v[1] as *mut _;
160 /// // Shorten `v` to prevent the last item from being dropped. We do that first,
161 /// // to prevent issues if the `drop_in_place` below panics.
163 /// // Without a call `drop_in_place`, the last item would never be dropped,
164 /// // and the memory it manages would be leaked.
165 /// ptr::drop_in_place(ptr);
168 /// assert_eq!(v, &[0.into()]);
170 /// // Ensure that the last item was dropped.
171 /// assert!(weak.upgrade().is_none());
174 /// Notice that the compiler performs this copy automatically when dropping packed structs,
175 /// i.e., you do not usually have to worry about such issues unless you call `drop_in_place`
177 #[stable(feature = "drop_in_place", since = "1.8.0")]
178 #[lang = "drop_in_place"]
179 #[allow(unconditional_recursion)]
180 pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
181 // Code here does not matter - this is replaced by the
182 // real drop glue by the compiler.
183 drop_in_place(to_drop)
186 /// Creates a null raw pointer.
193 /// let p: *const i32 = ptr::null();
194 /// assert!(p.is_null());
197 #[stable(feature = "rust1", since = "1.0.0")]
199 #[rustc_const_stable(feature = "const_ptr_null", since = "1.32.0")]
200 pub const fn null<T>() -> *const T {
204 /// Creates a null mutable raw pointer.
211 /// let p: *mut i32 = ptr::null_mut();
212 /// assert!(p.is_null());
215 #[stable(feature = "rust1", since = "1.0.0")]
217 #[rustc_const_stable(feature = "const_ptr_null", since = "1.32.0")]
218 pub const fn null_mut<T>() -> *mut T {
223 pub(crate) union Repr<T> {
224 pub(crate) rust: *const [T],
226 pub(crate) raw: FatPtr<T>,
230 pub(crate) struct FatPtr<T> {
232 pub(crate) len: usize,
235 /// Forms a raw slice from a pointer and a length.
237 /// The `len` argument is the number of **elements**, not the number of bytes.
239 /// This function is safe, but actually using the return value is unsafe.
240 /// See the documentation of [`from_raw_parts`] for slice safety requirements.
242 /// [`from_raw_parts`]: ../../std/slice/fn.from_raw_parts.html
249 /// // create a slice pointer when starting out with a pointer to the first element
250 /// let x = [5, 6, 7];
251 /// let ptr = x.as_ptr();
252 /// let slice = ptr::slice_from_raw_parts(ptr, 3);
253 /// assert_eq!(unsafe { &*slice }[2], 7);
256 #[stable(feature = "slice_from_raw_parts", since = "1.42.0")]
257 #[rustc_const_unstable(feature = "const_slice_from_raw_parts", issue = "67456")]
258 pub const fn slice_from_raw_parts<T>(data: *const T, len: usize) -> *const [T] {
259 unsafe { Repr { raw: FatPtr { data, len } }.rust }
262 /// Performs the same functionality as [`slice_from_raw_parts`], except that a
263 /// raw mutable slice is returned, as opposed to a raw immutable slice.
265 /// See the documentation of [`slice_from_raw_parts`] for more details.
267 /// This function is safe, but actually using the return value is unsafe.
268 /// See the documentation of [`from_raw_parts_mut`] for slice safety requirements.
270 /// [`slice_from_raw_parts`]: fn.slice_from_raw_parts.html
271 /// [`from_raw_parts_mut`]: ../../std/slice/fn.from_raw_parts_mut.html
273 #[stable(feature = "slice_from_raw_parts", since = "1.42.0")]
274 #[rustc_const_unstable(feature = "const_slice_from_raw_parts", issue = "67456")]
275 pub const fn slice_from_raw_parts_mut<T>(data: *mut T, len: usize) -> *mut [T] {
276 unsafe { Repr { raw: FatPtr { data, len } }.rust_mut }
279 /// Swaps the values at two mutable locations of the same type, without
280 /// deinitializing either.
282 /// But for the following two exceptions, this function is semantically
283 /// equivalent to [`mem::swap`]:
285 /// * It operates on raw pointers instead of references. When references are
286 /// available, [`mem::swap`] should be preferred.
288 /// * The two pointed-to values may overlap. If the values do overlap, then the
289 /// overlapping region of memory from `x` will be used. This is demonstrated
290 /// in the second example below.
292 /// [`mem::swap`]: ../mem/fn.swap.html
296 /// Behavior is undefined if any of the following conditions are violated:
298 /// * Both `x` and `y` must be [valid] for both reads and writes.
300 /// * Both `x` and `y` must be properly aligned.
302 /// Note that even if `T` has size `0`, the pointers must be non-NULL and properly aligned.
304 /// [valid]: ../ptr/index.html#safety
308 /// Swapping two non-overlapping regions:
313 /// let mut array = [0, 1, 2, 3];
315 /// let x = array[0..].as_mut_ptr() as *mut [u32; 2]; // this is `array[0..2]`
316 /// let y = array[2..].as_mut_ptr() as *mut [u32; 2]; // this is `array[2..4]`
320 /// assert_eq!([2, 3, 0, 1], array);
324 /// Swapping two overlapping regions:
329 /// let mut array = [0, 1, 2, 3];
331 /// let x = array[0..].as_mut_ptr() as *mut [u32; 3]; // this is `array[0..3]`
332 /// let y = array[1..].as_mut_ptr() as *mut [u32; 3]; // this is `array[1..4]`
336 /// // The indices `1..3` of the slice overlap between `x` and `y`.
337 /// // Reasonable results would be for to them be `[2, 3]`, so that indices `0..3` are
338 /// // `[1, 2, 3]` (matching `y` before the `swap`); or for them to be `[0, 1]`
339 /// // so that indices `1..4` are `[0, 1, 2]` (matching `x` before the `swap`).
340 /// // This implementation is defined to make the latter choice.
341 /// assert_eq!([1, 0, 1, 2], array);
345 #[stable(feature = "rust1", since = "1.0.0")]
346 pub unsafe fn swap<T>(x: *mut T, y: *mut T) {
347 // Give ourselves some scratch space to work with.
348 // We do not have to worry about drops: `MaybeUninit` does nothing when dropped.
349 let mut tmp = MaybeUninit::<T>::uninit();
352 copy_nonoverlapping(x, tmp.as_mut_ptr(), 1);
353 copy(y, x, 1); // `x` and `y` may overlap
354 copy_nonoverlapping(tmp.as_ptr(), y, 1);
357 /// Swaps `count * size_of::<T>()` bytes between the two regions of memory
358 /// beginning at `x` and `y`. The two regions must *not* overlap.
362 /// Behavior is undefined if any of the following conditions are violated:
364 /// * Both `x` and `y` must be [valid] for both reads and writes of `count *
365 /// size_of::<T>()` bytes.
367 /// * Both `x` and `y` must be properly aligned.
369 /// * The region of memory beginning at `x` with a size of `count *
370 /// size_of::<T>()` bytes must *not* overlap with the region of memory
371 /// beginning at `y` with the same size.
373 /// Note that even if the effectively copied size (`count * size_of::<T>()`) is `0`,
374 /// the pointers must be non-NULL and properly aligned.
376 /// [valid]: ../ptr/index.html#safety
385 /// let mut x = [1, 2, 3, 4];
386 /// let mut y = [7, 8, 9];
389 /// ptr::swap_nonoverlapping(x.as_mut_ptr(), y.as_mut_ptr(), 2);
392 /// assert_eq!(x, [7, 8, 3, 4]);
393 /// assert_eq!(y, [1, 2, 9]);
396 #[stable(feature = "swap_nonoverlapping", since = "1.27.0")]
397 pub unsafe fn swap_nonoverlapping<T>(x: *mut T, y: *mut T, count: usize) {
398 debug_assert!(is_aligned_and_not_null(x), "attempt to swap unaligned or null pointer");
399 debug_assert!(is_aligned_and_not_null(y), "attempt to swap unaligned or null pointer");
400 debug_assert!(is_nonoverlapping(x, y, count), "attempt to swap overlapping memory");
402 let x = x as *mut u8;
403 let y = y as *mut u8;
404 let len = mem::size_of::<T>() * count;
405 swap_nonoverlapping_bytes(x, y, len)
409 pub(crate) unsafe fn swap_nonoverlapping_one<T>(x: *mut T, y: *mut T) {
410 // For types smaller than the block optimization below,
411 // just swap directly to avoid pessimizing codegen.
412 if mem::size_of::<T>() < 32 {
414 copy_nonoverlapping(y, x, 1);
417 swap_nonoverlapping(x, y, 1);
422 unsafe fn swap_nonoverlapping_bytes(x: *mut u8, y: *mut u8, len: usize) {
423 // The approach here is to utilize simd to swap x & y efficiently. Testing reveals
424 // that swapping either 32 bytes or 64 bytes at a time is most efficient for Intel
425 // Haswell E processors. LLVM is more able to optimize if we give a struct a
426 // #[repr(simd)], even if we don't actually use this struct directly.
428 // FIXME repr(simd) broken on emscripten and redox
429 #[cfg_attr(not(any(target_os = "emscripten", target_os = "redox")), repr(simd))]
430 struct Block(u64, u64, u64, u64);
431 struct UnalignedBlock(u64, u64, u64, u64);
433 let block_size = mem::size_of::<Block>();
435 // Loop through x & y, copying them `Block` at a time
436 // The optimizer should unroll the loop fully for most types
437 // N.B. We can't use a for loop as the `range` impl calls `mem::swap` recursively
439 while i + block_size <= len {
440 // Create some uninitialized memory as scratch space
441 // Declaring `t` here avoids aligning the stack when this loop is unused
442 let mut t = mem::MaybeUninit::<Block>::uninit();
443 let t = t.as_mut_ptr() as *mut u8;
447 // Swap a block of bytes of x & y, using t as a temporary buffer
448 // This should be optimized into efficient SIMD operations where available
449 copy_nonoverlapping(x, t, block_size);
450 copy_nonoverlapping(y, x, block_size);
451 copy_nonoverlapping(t, y, block_size);
456 // Swap any remaining bytes
457 let mut t = mem::MaybeUninit::<UnalignedBlock>::uninit();
460 let t = t.as_mut_ptr() as *mut u8;
464 copy_nonoverlapping(x, t, rem);
465 copy_nonoverlapping(y, x, rem);
466 copy_nonoverlapping(t, y, rem);
470 /// Moves `src` into the pointed `dst`, returning the previous `dst` value.
472 /// Neither value is dropped.
474 /// This function is semantically equivalent to [`mem::replace`] except that it
475 /// operates on raw pointers instead of references. When references are
476 /// available, [`mem::replace`] should be preferred.
478 /// [`mem::replace`]: ../mem/fn.replace.html
482 /// Behavior is undefined if any of the following conditions are violated:
484 /// * `dst` must be [valid] for both reads and writes.
486 /// * `dst` must be properly aligned.
488 /// * `dst` must point to a properly initialized value of type `T`.
490 /// Note that even if `T` has size `0`, the pointer must be non-NULL and properly aligned.
492 /// [valid]: ../ptr/index.html#safety
499 /// let mut rust = vec!['b', 'u', 's', 't'];
501 /// // `mem::replace` would have the same effect without requiring the unsafe
504 /// ptr::replace(&mut rust[0], 'r')
507 /// assert_eq!(b, 'b');
508 /// assert_eq!(rust, &['r', 'u', 's', 't']);
511 #[stable(feature = "rust1", since = "1.0.0")]
512 pub unsafe fn replace<T>(dst: *mut T, mut src: T) -> T {
513 mem::swap(&mut *dst, &mut src); // cannot overlap
517 /// Reads the value from `src` without moving it. This leaves the
518 /// memory in `src` unchanged.
522 /// Behavior is undefined if any of the following conditions are violated:
524 /// * `src` must be [valid] for reads.
526 /// * `src` must be properly aligned. Use [`read_unaligned`] if this is not the
529 /// * `src` must point to a properly initialized value of type `T`.
531 /// Note that even if `T` has size `0`, the pointer must be non-NULL and properly aligned.
539 /// let y = &x as *const i32;
542 /// assert_eq!(std::ptr::read(y), 12);
546 /// Manually implement [`mem::swap`]:
551 /// fn swap<T>(a: &mut T, b: &mut T) {
553 /// // Create a bitwise copy of the value at `a` in `tmp`.
554 /// let tmp = ptr::read(a);
556 /// // Exiting at this point (either by explicitly returning or by
557 /// // calling a function which panics) would cause the value in `tmp` to
558 /// // be dropped while the same value is still referenced by `a`. This
559 /// // could trigger undefined behavior if `T` is not `Copy`.
561 /// // Create a bitwise copy of the value at `b` in `a`.
562 /// // This is safe because mutable references cannot alias.
563 /// ptr::copy_nonoverlapping(b, a, 1);
565 /// // As above, exiting here could trigger undefined behavior because
566 /// // the same value is referenced by `a` and `b`.
568 /// // Move `tmp` into `b`.
569 /// ptr::write(b, tmp);
571 /// // `tmp` has been moved (`write` takes ownership of its second argument),
572 /// // so nothing is dropped implicitly here.
576 /// let mut foo = "foo".to_owned();
577 /// let mut bar = "bar".to_owned();
579 /// swap(&mut foo, &mut bar);
581 /// assert_eq!(foo, "bar");
582 /// assert_eq!(bar, "foo");
585 /// ## Ownership of the Returned Value
587 /// `read` creates a bitwise copy of `T`, regardless of whether `T` is [`Copy`].
588 /// If `T` is not [`Copy`], using both the returned value and the value at
589 /// `*src` can violate memory safety. Note that assigning to `*src` counts as a
590 /// use because it will attempt to drop the value at `*src`.
592 /// [`write`] can be used to overwrite data without causing it to be dropped.
597 /// let mut s = String::from("foo");
599 /// // `s2` now points to the same underlying memory as `s`.
600 /// let mut s2: String = ptr::read(&s);
602 /// assert_eq!(s2, "foo");
604 /// // Assigning to `s2` causes its original value to be dropped. Beyond
605 /// // this point, `s` must no longer be used, as the underlying memory has
607 /// s2 = String::default();
608 /// assert_eq!(s2, "");
610 /// // Assigning to `s` would cause the old value to be dropped again,
611 /// // resulting in undefined behavior.
612 /// // s = String::from("bar"); // ERROR
614 /// // `ptr::write` can be used to overwrite a value without dropping it.
615 /// ptr::write(&mut s, String::from("bar"));
618 /// assert_eq!(s, "bar");
621 /// [`mem::swap`]: ../mem/fn.swap.html
622 /// [valid]: ../ptr/index.html#safety
623 /// [`Copy`]: ../marker/trait.Copy.html
624 /// [`read_unaligned`]: ./fn.read_unaligned.html
625 /// [`write`]: ./fn.write.html
627 #[stable(feature = "rust1", since = "1.0.0")]
628 pub unsafe fn read<T>(src: *const T) -> T {
629 // `copy_nonoverlapping` takes care of debug_assert.
630 let mut tmp = MaybeUninit::<T>::uninit();
631 copy_nonoverlapping(src, tmp.as_mut_ptr(), 1);
635 /// Reads the value from `src` without moving it. This leaves the
636 /// memory in `src` unchanged.
638 /// Unlike [`read`], `read_unaligned` works with unaligned pointers.
642 /// Behavior is undefined if any of the following conditions are violated:
644 /// * `src` must be [valid] for reads.
646 /// * `src` must point to a properly initialized value of type `T`.
648 /// Like [`read`], `read_unaligned` creates a bitwise copy of `T`, regardless of
649 /// whether `T` is [`Copy`]. If `T` is not [`Copy`], using both the returned
650 /// value and the value at `*src` can [violate memory safety][read-ownership].
652 /// Note that even if `T` has size `0`, the pointer must be non-NULL.
654 /// [`Copy`]: ../marker/trait.Copy.html
655 /// [`read`]: ./fn.read.html
656 /// [`write_unaligned`]: ./fn.write_unaligned.html
657 /// [read-ownership]: ./fn.read.html#ownership-of-the-returned-value
658 /// [valid]: ../ptr/index.html#safety
660 /// ## On `packed` structs
662 /// It is currently impossible to create raw pointers to unaligned fields
663 /// of a packed struct.
665 /// Attempting to create a raw pointer to an `unaligned` struct field with
666 /// an expression such as `&packed.unaligned as *const FieldType` creates an
667 /// intermediate unaligned reference before converting that to a raw pointer.
668 /// That this reference is temporary and immediately cast is inconsequential
669 /// as the compiler always expects references to be properly aligned.
670 /// As a result, using `&packed.unaligned as *const FieldType` causes immediate
671 /// *undefined behavior* in your program.
673 /// An example of what not to do and how this relates to `read_unaligned` is:
676 /// #[repr(packed, C)]
682 /// let packed = Packed {
684 /// unaligned: 0x01020304,
688 /// // Here we attempt to take the address of a 32-bit integer which is not aligned.
690 /// // A temporary unaligned reference is created here which results in
691 /// // undefined behavior regardless of whether the reference is used or not.
692 /// &packed.unaligned
693 /// // Casting to a raw pointer doesn't help; the mistake already happened.
696 /// let v = std::ptr::read_unaligned(unaligned);
702 /// Accessing unaligned fields directly with e.g. `packed.unaligned` is safe however.
703 // FIXME: Update docs based on outcome of RFC #2582 and friends.
707 /// Read an usize value from a byte buffer:
712 /// fn read_usize(x: &[u8]) -> usize {
713 /// assert!(x.len() >= mem::size_of::<usize>());
715 /// let ptr = x.as_ptr() as *const usize;
717 /// unsafe { ptr.read_unaligned() }
721 #[stable(feature = "ptr_unaligned", since = "1.17.0")]
722 pub unsafe fn read_unaligned<T>(src: *const T) -> T {
723 // `copy_nonoverlapping` takes care of debug_assert.
724 let mut tmp = MaybeUninit::<T>::uninit();
725 copy_nonoverlapping(src as *const u8, tmp.as_mut_ptr() as *mut u8, mem::size_of::<T>());
729 /// Overwrites a memory location with the given value without reading or
730 /// dropping the old value.
732 /// `write` does not drop the contents of `dst`. This is safe, but it could leak
733 /// allocations or resources, so care should be taken not to overwrite an object
734 /// that should be dropped.
736 /// Additionally, it does not drop `src`. Semantically, `src` is moved into the
737 /// location pointed to by `dst`.
739 /// This is appropriate for initializing uninitialized memory, or overwriting
740 /// memory that has previously been [`read`] from.
742 /// [`read`]: ./fn.read.html
746 /// Behavior is undefined if any of the following conditions are violated:
748 /// * `dst` must be [valid] for writes.
750 /// * `dst` must be properly aligned. Use [`write_unaligned`] if this is not the
753 /// Note that even if `T` has size `0`, the pointer must be non-NULL and properly aligned.
755 /// [valid]: ../ptr/index.html#safety
756 /// [`write_unaligned`]: ./fn.write_unaligned.html
764 /// let y = &mut x as *mut i32;
768 /// std::ptr::write(y, z);
769 /// assert_eq!(std::ptr::read(y), 12);
773 /// Manually implement [`mem::swap`]:
778 /// fn swap<T>(a: &mut T, b: &mut T) {
780 /// // Create a bitwise copy of the value at `a` in `tmp`.
781 /// let tmp = ptr::read(a);
783 /// // Exiting at this point (either by explicitly returning or by
784 /// // calling a function which panics) would cause the value in `tmp` to
785 /// // be dropped while the same value is still referenced by `a`. This
786 /// // could trigger undefined behavior if `T` is not `Copy`.
788 /// // Create a bitwise copy of the value at `b` in `a`.
789 /// // This is safe because mutable references cannot alias.
790 /// ptr::copy_nonoverlapping(b, a, 1);
792 /// // As above, exiting here could trigger undefined behavior because
793 /// // the same value is referenced by `a` and `b`.
795 /// // Move `tmp` into `b`.
796 /// ptr::write(b, tmp);
798 /// // `tmp` has been moved (`write` takes ownership of its second argument),
799 /// // so nothing is dropped implicitly here.
803 /// let mut foo = "foo".to_owned();
804 /// let mut bar = "bar".to_owned();
806 /// swap(&mut foo, &mut bar);
808 /// assert_eq!(foo, "bar");
809 /// assert_eq!(bar, "foo");
812 /// [`mem::swap`]: ../mem/fn.swap.html
814 #[stable(feature = "rust1", since = "1.0.0")]
815 pub unsafe fn write<T>(dst: *mut T, src: T) {
816 debug_assert!(is_aligned_and_not_null(dst), "attempt to write to unaligned or null pointer");
817 intrinsics::move_val_init(&mut *dst, src)
820 /// Overwrites a memory location with the given value without reading or
821 /// dropping the old value.
823 /// Unlike [`write`], the pointer may be unaligned.
825 /// `write_unaligned` does not drop the contents of `dst`. This is safe, but it
826 /// could leak allocations or resources, so care should be taken not to overwrite
827 /// an object that should be dropped.
829 /// Additionally, it does not drop `src`. Semantically, `src` is moved into the
830 /// location pointed to by `dst`.
832 /// This is appropriate for initializing uninitialized memory, or overwriting
833 /// memory that has previously been read with [`read_unaligned`].
835 /// [`write`]: ./fn.write.html
836 /// [`read_unaligned`]: ./fn.read_unaligned.html
840 /// Behavior is undefined if any of the following conditions are violated:
842 /// * `dst` must be [valid] for writes.
844 /// Note that even if `T` has size `0`, the pointer must be non-NULL.
846 /// [valid]: ../ptr/index.html#safety
848 /// ## On `packed` structs
850 /// It is currently impossible to create raw pointers to unaligned fields
851 /// of a packed struct.
853 /// Attempting to create a raw pointer to an `unaligned` struct field with
854 /// an expression such as `&packed.unaligned as *const FieldType` creates an
855 /// intermediate unaligned reference before converting that to a raw pointer.
856 /// That this reference is temporary and immediately cast is inconsequential
857 /// as the compiler always expects references to be properly aligned.
858 /// As a result, using `&packed.unaligned as *const FieldType` causes immediate
859 /// *undefined behavior* in your program.
861 /// An example of what not to do and how this relates to `write_unaligned` is:
864 /// #[repr(packed, C)]
870 /// let v = 0x01020304;
871 /// let mut packed: Packed = unsafe { std::mem::zeroed() };
874 /// // Here we attempt to take the address of a 32-bit integer which is not aligned.
876 /// // A temporary unaligned reference is created here which results in
877 /// // undefined behavior regardless of whether the reference is used or not.
878 /// &mut packed.unaligned
879 /// // Casting to a raw pointer doesn't help; the mistake already happened.
882 /// std::ptr::write_unaligned(unaligned, v);
888 /// Accessing unaligned fields directly with e.g. `packed.unaligned` is safe however.
889 // FIXME: Update docs based on outcome of RFC #2582 and friends.
893 /// Write an usize value to a byte buffer:
898 /// fn write_usize(x: &mut [u8], val: usize) {
899 /// assert!(x.len() >= mem::size_of::<usize>());
901 /// let ptr = x.as_mut_ptr() as *mut usize;
903 /// unsafe { ptr.write_unaligned(val) }
907 #[stable(feature = "ptr_unaligned", since = "1.17.0")]
908 pub unsafe fn write_unaligned<T>(dst: *mut T, src: T) {
909 // `copy_nonoverlapping` takes care of debug_assert.
910 copy_nonoverlapping(&src as *const T as *const u8, dst as *mut u8, mem::size_of::<T>());
914 /// Performs a volatile read of the value from `src` without moving it. This
915 /// leaves the memory in `src` unchanged.
917 /// Volatile operations are intended to act on I/O memory, and are guaranteed
918 /// to not be elided or reordered by the compiler across other volatile
921 /// [`write_volatile`]: ./fn.write_volatile.html
925 /// Rust does not currently have a rigorously and formally defined memory model,
926 /// so the precise semantics of what "volatile" means here is subject to change
927 /// over time. That being said, the semantics will almost always end up pretty
928 /// similar to [C11's definition of volatile][c11].
930 /// The compiler shouldn't change the relative order or number of volatile
931 /// memory operations. However, volatile memory operations on zero-sized types
932 /// (e.g., if a zero-sized type is passed to `read_volatile`) are noops
933 /// and may be ignored.
935 /// [c11]: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
939 /// Behavior is undefined if any of the following conditions are violated:
941 /// * `src` must be [valid] for reads.
943 /// * `src` must be properly aligned.
945 /// * `src` must point to a properly initialized value of type `T`.
947 /// Like [`read`], `read_volatile` creates a bitwise copy of `T`, regardless of
948 /// whether `T` is [`Copy`]. If `T` is not [`Copy`], using both the returned
949 /// value and the value at `*src` can [violate memory safety][read-ownership].
950 /// However, storing non-[`Copy`] types in volatile memory is almost certainly
953 /// Note that even if `T` has size `0`, the pointer must be non-NULL and properly aligned.
955 /// [valid]: ../ptr/index.html#safety
956 /// [`Copy`]: ../marker/trait.Copy.html
957 /// [`read`]: ./fn.read.html
958 /// [read-ownership]: ./fn.read.html#ownership-of-the-returned-value
960 /// Just like in C, whether an operation is volatile has no bearing whatsoever
961 /// on questions involving concurrent access from multiple threads. Volatile
962 /// accesses behave exactly like non-atomic accesses in that regard. In particular,
963 /// a race between a `read_volatile` and any write operation to the same location
964 /// is undefined behavior.
972 /// let y = &x as *const i32;
975 /// assert_eq!(std::ptr::read_volatile(y), 12);
979 #[stable(feature = "volatile", since = "1.9.0")]
980 pub unsafe fn read_volatile<T>(src: *const T) -> T {
981 debug_assert!(is_aligned_and_not_null(src), "attempt to read from unaligned or null pointer");
982 intrinsics::volatile_load(src)
985 /// Performs a volatile write of a memory location with the given value without
986 /// reading or dropping the old value.
988 /// Volatile operations are intended to act on I/O memory, and are guaranteed
989 /// to not be elided or reordered by the compiler across other volatile
992 /// `write_volatile` does not drop the contents of `dst`. This is safe, but it
993 /// could leak allocations or resources, so care should be taken not to overwrite
994 /// an object that should be dropped.
996 /// Additionally, it does not drop `src`. Semantically, `src` is moved into the
997 /// location pointed to by `dst`.
999 /// [`read_volatile`]: ./fn.read_volatile.html
1003 /// Rust does not currently have a rigorously and formally defined memory model,
1004 /// so the precise semantics of what "volatile" means here is subject to change
1005 /// over time. That being said, the semantics will almost always end up pretty
1006 /// similar to [C11's definition of volatile][c11].
1008 /// The compiler shouldn't change the relative order or number of volatile
1009 /// memory operations. However, volatile memory operations on zero-sized types
1010 /// (e.g., if a zero-sized type is passed to `write_volatile`) are noops
1011 /// and may be ignored.
1013 /// [c11]: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
1017 /// Behavior is undefined if any of the following conditions are violated:
1019 /// * `dst` must be [valid] for writes.
1021 /// * `dst` must be properly aligned.
1023 /// Note that even if `T` has size `0`, the pointer must be non-NULL and properly aligned.
1025 /// [valid]: ../ptr/index.html#safety
1027 /// Just like in C, whether an operation is volatile has no bearing whatsoever
1028 /// on questions involving concurrent access from multiple threads. Volatile
1029 /// accesses behave exactly like non-atomic accesses in that regard. In particular,
1030 /// a race between a `write_volatile` and any other operation (reading or writing)
1031 /// on the same location is undefined behavior.
1039 /// let y = &mut x as *mut i32;
1043 /// std::ptr::write_volatile(y, z);
1044 /// assert_eq!(std::ptr::read_volatile(y), 12);
1048 #[stable(feature = "volatile", since = "1.9.0")]
1049 pub unsafe fn write_volatile<T>(dst: *mut T, src: T) {
1050 debug_assert!(is_aligned_and_not_null(dst), "attempt to write to unaligned or null pointer");
1051 intrinsics::volatile_store(dst, src);
1054 /// Align pointer `p`.
1056 /// Calculate offset (in terms of elements of `stride` stride) that has to be applied
1057 /// to pointer `p` so that pointer `p` would get aligned to `a`.
1059 /// Note: This implementation has been carefully tailored to not panic. It is UB for this to panic.
1060 /// The only real change that can be made here is change of `INV_TABLE_MOD_16` and associated
1063 /// If we ever decide to make it possible to call the intrinsic with `a` that is not a
1064 /// power-of-two, it will probably be more prudent to just change to a naive implementation rather
1065 /// than trying to adapt this to accommodate that change.
1067 /// Any questions go to @nagisa.
1068 #[lang = "align_offset"]
1069 pub(crate) unsafe fn align_offset<T: Sized>(p: *const T, a: usize) -> usize {
1070 /// Calculate multiplicative modular inverse of `x` modulo `m`.
1072 /// This implementation is tailored for align_offset and has following preconditions:
1074 /// * `m` is a power-of-two;
1075 /// * `x < m`; (if `x ≥ m`, pass in `x % m` instead)
1077 /// Implementation of this function shall not panic. Ever.
1079 fn mod_inv(x: usize, m: usize) -> usize {
1080 /// Multiplicative modular inverse table modulo 2⁴ = 16.
1082 /// Note, that this table does not contain values where inverse does not exist (i.e., for
1083 /// `0⁻¹ mod 16`, `2⁻¹ mod 16`, etc.)
1084 const INV_TABLE_MOD_16: [u8; 8] = [1, 11, 13, 7, 9, 3, 5, 15];
1085 /// Modulo for which the `INV_TABLE_MOD_16` is intended.
1086 const INV_TABLE_MOD: usize = 16;
1088 const INV_TABLE_MOD_SQUARED: usize = INV_TABLE_MOD * INV_TABLE_MOD;
1090 let table_inverse = INV_TABLE_MOD_16[(x & (INV_TABLE_MOD - 1)) >> 1] as usize;
1091 if m <= INV_TABLE_MOD {
1092 table_inverse & (m - 1)
1094 // We iterate "up" using the following formula:
1096 // $$ xy ≡ 1 (mod 2ⁿ) → xy (2 - xy) ≡ 1 (mod 2²ⁿ) $$
1098 // until 2²ⁿ ≥ m. Then we can reduce to our desired `m` by taking the result `mod m`.
1099 let mut inverse = table_inverse;
1100 let mut going_mod = INV_TABLE_MOD_SQUARED;
1102 // y = y * (2 - xy) mod n
1104 // Note, that we use wrapping operations here intentionally – the original formula
1105 // uses e.g., subtraction `mod n`. It is entirely fine to do them `mod
1106 // usize::max_value()` instead, because we take the result `mod n` at the end
1108 inverse = inverse.wrapping_mul(2usize.wrapping_sub(x.wrapping_mul(inverse)));
1110 return inverse & (m - 1);
1112 going_mod = going_mod.wrapping_mul(going_mod);
1117 let stride = mem::size_of::<T>();
1118 let a_minus_one = a.wrapping_sub(1);
1119 let pmoda = p as usize & a_minus_one;
1122 // Already aligned. Yay!
1127 return if stride == 0 {
1128 // If the pointer is not aligned, and the element is zero-sized, then no amount of
1129 // elements will ever align the pointer.
1132 a.wrapping_sub(pmoda)
1136 let smoda = stride & a_minus_one;
1137 // a is power-of-two so cannot be 0. stride = 0 is handled above.
1138 let gcdpow = intrinsics::cttz_nonzero(stride).min(intrinsics::cttz_nonzero(a));
1139 let gcd = 1usize << gcdpow;
1141 if p as usize & (gcd.wrapping_sub(1)) == 0 {
1142 // This branch solves for the following linear congruence equation:
1144 // ` p + so = 0 mod a `
1146 // `p` here is the pointer value, `s` - stride of `T`, `o` offset in `T`s, and `a` - the
1147 // requested alignment.
1149 // With `g = gcd(a, s)`, and the above asserting that `p` is also divisible by `g`, we can
1150 // denote `a' = a/g`, `s' = s/g`, `p' = p/g`, then this becomes equivalent to:
1152 // ` p' + s'o = 0 mod a' `
1153 // ` o = (a' - (p' mod a')) * (s'^-1 mod a') `
1155 // The first term is "the relative alignment of `p` to `a`" (divided by the `g`), the second
1156 // term is "how does incrementing `p` by `s` bytes change the relative alignment of `p`" (again
1158 // Division by `g` is necessary to make the inverse well formed if `a` and `s` are not
1161 // Furthermore, the result produced by this solution is not "minimal", so it is necessary
1162 // to take the result `o mod lcm(s, a)`. We can replace `lcm(s, a)` with just a `a'`.
1163 let a2 = a >> gcdpow;
1164 let a2minus1 = a2.wrapping_sub(1);
1165 let s2 = smoda >> gcdpow;
1166 let minusp2 = a2.wrapping_sub(pmoda >> gcdpow);
1167 return (minusp2.wrapping_mul(mod_inv(s2, a2))) & a2minus1;
1170 // Cannot be aligned at all.
1174 /// Compares raw pointers for equality.
1176 /// This is the same as using the `==` operator, but less generic:
1177 /// the arguments have to be `*const T` raw pointers,
1178 /// not anything that implements `PartialEq`.
1180 /// This can be used to compare `&T` references (which coerce to `*const T` implicitly)
1181 /// by their address rather than comparing the values they point to
1182 /// (which is what the `PartialEq for &T` implementation does).
1190 /// let other_five = 5;
1191 /// let five_ref = &five;
1192 /// let same_five_ref = &five;
1193 /// let other_five_ref = &other_five;
1195 /// assert!(five_ref == same_five_ref);
1196 /// assert!(ptr::eq(five_ref, same_five_ref));
1198 /// assert!(five_ref == other_five_ref);
1199 /// assert!(!ptr::eq(five_ref, other_five_ref));
1202 /// Slices are also compared by their length (fat pointers):
1205 /// let a = [1, 2, 3];
1206 /// assert!(std::ptr::eq(&a[..3], &a[..3]));
1207 /// assert!(!std::ptr::eq(&a[..2], &a[..3]));
1208 /// assert!(!std::ptr::eq(&a[0..2], &a[1..3]));
1211 /// Traits are also compared by their implementation:
1214 /// #[repr(transparent)]
1215 /// struct Wrapper { member: i32 }
1218 /// impl Trait for Wrapper {}
1219 /// impl Trait for i32 {}
1221 /// let wrapper = Wrapper { member: 10 };
1223 /// // Pointers have equal addresses.
1224 /// assert!(std::ptr::eq(
1225 /// &wrapper as *const Wrapper as *const u8,
1226 /// &wrapper.member as *const i32 as *const u8
1229 /// // Objects have equal addresses, but `Trait` has different implementations.
1230 /// assert!(!std::ptr::eq(
1231 /// &wrapper as &dyn Trait,
1232 /// &wrapper.member as &dyn Trait,
1234 /// assert!(!std::ptr::eq(
1235 /// &wrapper as &dyn Trait as *const dyn Trait,
1236 /// &wrapper.member as &dyn Trait as *const dyn Trait,
1239 /// // Converting the reference to a `*const u8` compares by address.
1240 /// assert!(std::ptr::eq(
1241 /// &wrapper as &dyn Trait as *const dyn Trait as *const u8,
1242 /// &wrapper.member as &dyn Trait as *const dyn Trait as *const u8,
1245 #[stable(feature = "ptr_eq", since = "1.17.0")]
1247 pub fn eq<T: ?Sized>(a: *const T, b: *const T) -> bool {
1251 /// Hash a raw pointer.
1253 /// This can be used to hash a `&T` reference (which coerces to `*const T` implicitly)
1254 /// by its address rather than the value it points to
1255 /// (which is what the `Hash for &T` implementation does).
1260 /// use std::collections::hash_map::DefaultHasher;
1261 /// use std::hash::{Hash, Hasher};
1265 /// let five_ref = &five;
1267 /// let mut hasher = DefaultHasher::new();
1268 /// ptr::hash(five_ref, &mut hasher);
1269 /// let actual = hasher.finish();
1271 /// let mut hasher = DefaultHasher::new();
1272 /// (five_ref as *const i32).hash(&mut hasher);
1273 /// let expected = hasher.finish();
1275 /// assert_eq!(actual, expected);
1277 #[stable(feature = "ptr_hash", since = "1.35.0")]
1278 pub fn hash<T: ?Sized, S: hash::Hasher>(hashee: *const T, into: &mut S) {
1279 use crate::hash::Hash;
1283 // Impls for function pointers
1284 macro_rules! fnptr_impls_safety_abi {
1285 ($FnTy: ty, $($Arg: ident),*) => {
1286 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1287 impl<Ret, $($Arg),*> PartialEq for $FnTy {
1289 fn eq(&self, other: &Self) -> bool {
1290 *self as usize == *other as usize
1294 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1295 impl<Ret, $($Arg),*> Eq for $FnTy {}
1297 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1298 impl<Ret, $($Arg),*> PartialOrd for $FnTy {
1300 fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
1301 (*self as usize).partial_cmp(&(*other as usize))
1305 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1306 impl<Ret, $($Arg),*> Ord for $FnTy {
1308 fn cmp(&self, other: &Self) -> Ordering {
1309 (*self as usize).cmp(&(*other as usize))
1313 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1314 impl<Ret, $($Arg),*> hash::Hash for $FnTy {
1315 fn hash<HH: hash::Hasher>(&self, state: &mut HH) {
1316 state.write_usize(*self as usize)
1320 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1321 impl<Ret, $($Arg),*> fmt::Pointer for $FnTy {
1322 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1323 fmt::Pointer::fmt(&(*self as *const ()), f)
1327 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1328 impl<Ret, $($Arg),*> fmt::Debug for $FnTy {
1329 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1330 fmt::Pointer::fmt(&(*self as *const ()), f)
1336 macro_rules! fnptr_impls_args {
1337 ($($Arg: ident),+) => {
1338 fnptr_impls_safety_abi! { extern "Rust" fn($($Arg),+) -> Ret, $($Arg),+ }
1339 fnptr_impls_safety_abi! { extern "C" fn($($Arg),+) -> Ret, $($Arg),+ }
1340 fnptr_impls_safety_abi! { extern "C" fn($($Arg),+ , ...) -> Ret, $($Arg),+ }
1341 fnptr_impls_safety_abi! { unsafe extern "Rust" fn($($Arg),+) -> Ret, $($Arg),+ }
1342 fnptr_impls_safety_abi! { unsafe extern "C" fn($($Arg),+) -> Ret, $($Arg),+ }
1343 fnptr_impls_safety_abi! { unsafe extern "C" fn($($Arg),+ , ...) -> Ret, $($Arg),+ }
1346 // No variadic functions with 0 parameters
1347 fnptr_impls_safety_abi! { extern "Rust" fn() -> Ret, }
1348 fnptr_impls_safety_abi! { extern "C" fn() -> Ret, }
1349 fnptr_impls_safety_abi! { unsafe extern "Rust" fn() -> Ret, }
1350 fnptr_impls_safety_abi! { unsafe extern "C" fn() -> Ret, }
1354 fnptr_impls_args! {}
1355 fnptr_impls_args! { A }
1356 fnptr_impls_args! { A, B }
1357 fnptr_impls_args! { A, B, C }
1358 fnptr_impls_args! { A, B, C, D }
1359 fnptr_impls_args! { A, B, C, D, E }
1360 fnptr_impls_args! { A, B, C, D, E, F }
1361 fnptr_impls_args! { A, B, C, D, E, F, G }
1362 fnptr_impls_args! { A, B, C, D, E, F, G, H }
1363 fnptr_impls_args! { A, B, C, D, E, F, G, H, I }
1364 fnptr_impls_args! { A, B, C, D, E, F, G, H, I, J }
1365 fnptr_impls_args! { A, B, C, D, E, F, G, H, I, J, K }
1366 fnptr_impls_args! { A, B, C, D, E, F, G, H, I, J, K, L }