1 //! Manually manage memory through raw pointers.
3 //! *[See also the pointer primitive types](pointer).*
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 //! * For a pointer to be valid, it is necessary, but not always sufficient, that the pointer
20 //! be *dereferenceable*: the memory range of the given size starting at the pointer must all be
21 //! within the bounds of a single allocated object. Note that in Rust,
22 //! every (stack-allocated) variable is considered a separate allocated object.
23 //! * Even for operations of [size zero][zst], the pointer must not be pointing to deallocated
24 //! memory, i.e., deallocation makes pointers invalid even for zero-sized operations. However,
25 //! casting any non-zero integer *literal* to a pointer is valid for zero-sized accesses, even if
26 //! some memory happens to exist at that address and gets deallocated. This corresponds to writing
27 //! your own allocator: allocating zero-sized objects is not very hard. The canonical way to
28 //! obtain a pointer that is valid for zero-sized accesses is [`NonNull::dangling`].
29 //! * All accesses performed by functions in this module are *non-atomic* in the sense
30 //! of [atomic operations] used to synchronize between threads. This means it is
31 //! undefined behavior to perform two concurrent accesses to the same location from different
32 //! threads unless both accesses only read from memory. Notice that this explicitly
33 //! includes [`read_volatile`] and [`write_volatile`]: Volatile accesses cannot
34 //! be used for inter-thread synchronization.
35 //! * The result of casting a reference to a pointer is valid for as long as the
36 //! underlying object is live and no reference (just raw pointers) is used to
37 //! access the same memory.
39 //! These axioms, along with careful use of [`offset`] for pointer arithmetic,
40 //! are enough to correctly implement many useful things in unsafe code. Stronger guarantees
41 //! will be provided eventually, as the [aliasing] rules are being determined. For more
42 //! information, see the [book] as well as the section in the reference devoted
43 //! to [undefined behavior][ub].
47 //! Valid raw pointers as defined above are not necessarily properly aligned (where
48 //! "proper" alignment is defined by the pointee type, i.e., `*const T` must be
49 //! aligned to `mem::align_of::<T>()`). However, most functions require their
50 //! arguments to be properly aligned, and will explicitly state
51 //! this requirement in their documentation. Notable exceptions to this are
52 //! [`read_unaligned`] and [`write_unaligned`].
54 //! When a function requires proper alignment, it does so even if the access
55 //! has size 0, i.e., even if memory is not actually touched. Consider using
56 //! [`NonNull::dangling`] in such cases.
58 //! ## Allocated object
60 //! For several operations, such as [`offset`] or field projections (`expr.field`), the notion of an
61 //! "allocated object" becomes relevant. An allocated object is a contiguous region of memory.
62 //! Common examples of allocated objects include stack-allocated variables (each variable is a
63 //! separate allocated object), heap allocations (each allocation created by the global allocator is
64 //! a separate allocated object), and `static` variables.
67 //! # Strict Provenance
69 //! **The following text is non-normative, insufficiently formal, and is an extremely strict
70 //! interpretation of provenance. It's ok if your code doesn't strictly conform to it.**
72 //! [Strict Provenance][] is an experimental set of APIs that help tools that try
73 //! to validate the memory-safety of your program's execution. Notably this includes [miri][]
74 //! and [CHERI][], which can detect when you access out of bounds memory or otherwise violate
75 //! Rust's memory model.
77 //! Provenance must exist in some form for any programming
78 //! language compiled for modern computer architectures, but specifying a model for provenance
79 //! in a way that is useful to both compilers and programmers is an ongoing challenge.
80 //! The [Strict Provenance][] experiment seeks to explore the question: *what if we just said you
81 //! couldn't do all the nasty operations that make provenance so messy?*
83 //! What APIs would have to be removed? What APIs would have to be added? How much would code
84 //! have to change, and is it worse or better now? Would any patterns become truly inexpressible?
85 //! Could we carve out special exceptions for those patterns? Should we?
87 //! A secondary goal of this project is to see if we can disamiguate the many functions of
88 //! pointer<->integer casts enough for the definition of `usize` to be loosened so that it
89 //! isn't *pointer*-sized but address-space/offset/allocation-sized (we'll probably continue
90 //! to conflate these notions). This would potentially make it possible to more efficiently
91 //! target platforms where pointers are larger than offsets, such as CHERI and maybe some
92 //! segmented architecures.
96 //! **This section is *non-normative* and is part of the [Strict Provenance][] experiment.**
98 //! Pointers are not *simply* an "integer" or "address". For instance, it's uncontroversial
99 //! to say that a Use After Free is clearly Undefined Behaviour, even if you "get lucky"
100 //! and the freed memory gets reallocated before your read/write (in fact this is the
101 //! worst-case scenario, UAFs would be much less concerning if this didn't happen!).
102 //! To rationalize this claim, pointers need to somehow be *more* than just their addresses:
103 //! they must have provenance.
105 //! When an allocation is created, that allocation has a unique Original Pointer. For alloc
106 //! APIs this is literally the pointer the call returns, and for local variables and statics,
107 //! this is the name of the variable/static. This is mildly overloading the term "pointer"
108 //! for the sake of brevity/exposition.
110 //! The Original Pointer for an allocation is guaranteed to have unique access to the entire
111 //! allocation and *only* that allocation. In this sense, an allocation can be thought of
112 //! as a "sandbox" that cannot be broken into or out of. *Provenance* is the permission
113 //! to access an allocation's sandbox and has both a *spatial* and *temporal* component:
115 //! * Spatial: A range of bytes that the pointer is allowed to access.
116 //! * Temporal: The allocation that access to these bytes is tied to.
118 //! Spatial provenance makes sure you don't go beyond your sandbox, while temporal provenance
119 //! makes sure that you can't "get lucky" after your permission to access some memory
120 //! has been revoked (either through deallocations or borrows expiring).
122 //! Provenance is implicitly shared with all pointers transitively derived from
123 //! The Original Pointer through operations like [`offset`], borrowing, and pointer casts.
124 //! Some operations may *shrink* the derived provenance, limiting how much memory it can
125 //! access or how long it's valid for (i.e. borrowing a subfield and subslicing).
127 //! Shrinking provenance cannot be undone: even if you "know" there is a larger allocation, you
128 //! can't derive a pointer with a larger provenance. Similarly, you cannot "recombine"
129 //! two contiguous provenances back into one (i.e. with a `fn merge(&[T], &[T]) -> &[T]`).
131 //! A reference to a value always has provenance over exactly the memory that field occupies.
132 //! A reference to a slice always has provenance over exactly the range that slice describes.
134 //! If an allocation is deallocated, all pointers with provenance to that allocation become
135 //! invalidated, and effectively lose their provenance.
137 //! The strict provenance experiment is mostly only interested in exploring stricter *spatial*
138 //! provenance. In this sense it can be thought of as a subset of the more ambitious and
139 //! formal [Stacked Borrows][] research project, which is what tools like [miri][] are based on.
140 //! In particular, Stacked Borrows is necessary to properly describe what borrows are allowed
141 //! to do and when they become invalidated. This necessarily involves much more complex
142 //! *temporal* reasoning than simply identifying allocations. Adjusting APIs and code
143 //! for the strict provenance experiment will also greatly help Stacked Borrows.
146 //! ## Pointer Vs Addresses
148 //! **This section is *non-normative* and is part of the [Strict Provenance][] experiment.**
150 //! One of the largest historical issues with trying to define provenance is that programmers
151 //! freely convert between pointers and integers. Once you allow for this, it generally becomes
152 //! impossible to accurately track and preserve provenance information, and you need to appeal
153 //! to very complex and unreliable heuristics. But of course, converting between pointers and
154 //! integers is very useful, so what can we do?
156 //! Also did you know WASM is actually a "Harvard Architecture"? As in function pointers are
157 //! handled completely differently from data pointers? And we kind of just shipped Rust on WASM
158 //! without really addressing the fact that we let you freely convert between function pointers
159 //! and data pointers, because it mostly Just Works? Let's just put that on the "pointer casts
160 //! are dubious" pile.
162 //! Strict Provenance attempts to square these circles by decoupling Rust's traditional conflation
163 //! of pointers and `usize` (and `isize`), and defining a pointer to semantically contain the
164 //! following information:
166 //! * The **address-space** it is part of.
167 //! * The **address** it points to, which can be represented by a `usize`.
168 //! * The **provenance** it has, defining the memory it has permission to access.
170 //! Under Strict Provenance, a usize *cannot* accurately represent a pointer, and converting from
171 //! a pointer to a usize is generally an operation which *only* extracts the address. It is
172 //! therefore *impossible* to construct a valid pointer from a usize because there is no way
173 //! to restore the address-space and provenance.
175 //! The key insight to making this model *at all* viable is the [`with_addr`][] method:
178 //! /// Creates a new pointer with the given address.
180 //! /// This performs the same operation as an `addr as ptr` cast, but copies
181 //! /// the *address-space* and *provenance* of `self` to the new pointer.
182 //! /// This allows us to dynamically preserve and propagate this important
183 //! /// information in a way that is otherwise impossible with a unary cast.
185 //! /// This is equivalent to using `wrapping_offset` to offset `self` to the
186 //! /// given address, and therefore has all the same capabilities and restrictions.
187 //! pub fn with_addr(self, addr: usize) -> Self;
190 //! So you're still able to drop down to the address representation and do whatever
191 //! clever bit tricks you want *as long as* you're able to keep around a pointer
192 //! into the allocation you care about that can "reconstitute" the other parts of the pointer.
193 //! Usually this is very easy, because you only are taking a pointer, messing with the address,
194 //! and then immediately converting back to a pointer. To make this use case more ergonomic,
195 //! we provide the [`map_addr`][] method.
197 //! To help make it clear that code is "following" Strict Provenance semantics, we also
198 //! provide an [`addr`][] method which is currently equivalent to `ptr as usize`. In the
199 //! future we may provide a lint for pointer<->integer casts to help you audit if your
200 //! code conforms to strict provenance.
203 //! ## Using Strict Provenance
205 //! Most code needs no changes to conform to strict provenance, as the only really concerning
206 //! operation that *wasn't* obviously already Undefined Behaviour is casts from usize to a
207 //! pointer. For code which *does* cast a usize to a pointer, the scope of the change depends
208 //! on exactly what you're doing.
210 //! In general you just need to make sure that if you want to convert a usize address to a
211 //! pointer and then use that pointer to read/write memory, you need to keep around a pointer
212 //! that has sufficient provenance to perform that read/write itself. In this way all of your
213 //! casts from an address to a pointer are essentially just applying offsets/indexing.
215 //! This is generally trivial to do for simple cases like tagged pointers *as long as you
216 //! represent the tagged pointer as an actual pointer and not a usize*. For instance:
219 //! #![feature(strict_provenance)]
222 //! // A flag we want to pack into our pointer
223 //! static HAS_DATA: usize = 0x1;
224 //! static FLAG_MASK: usize = !HAS_DATA;
226 //! // Our value, which must have enough alignment to have spare least-significant-bits.
227 //! let my_precious_data: u32 = 17;
228 //! assert!(core::mem::align_of::<u32>() > 1);
230 //! // Create a tagged pointer
231 //! let ptr = &my_precious_data as *const u32;
232 //! let tagged = ptr.map_addr(|addr| addr | HAS_DATA);
234 //! // Check the flag:
235 //! if tagged.addr() & HAS_DATA != 0 {
236 //! // Untag and read the pointer
237 //! let data = *tagged.map_addr(|addr| addr & FLAG_MASK);
238 //! assert_eq!(data, 17);
245 //! (Yes, if you've been using AtomicUsize for pointers in concurrent datastructures, you should
246 //! be using AtomicPtr instead. If that messes up the way you atomically manipulate pointers,
247 //! we would like to know why, and what needs to be done to fix it.)
249 //! Something more complicated and just generally *evil* like a XOR-List requires more significant
250 //! changes like allocating all nodes in a pre-allocated Vec or Arena and using a pointer
251 //! to the whole allocation to reconstitute the XORed addresses.
253 //! Situations where a valid pointer *must* be created from just an address, such as baremetal code
254 //! accessing a memory-mapped interface at a fixed address, are an open question on how to support.
255 //! These situations *will* still be allowed, but we might require some kind of "I know what I'm
256 //! doing" annotation to explain the situation to the compiler. It's also possible they need no
257 //! special attention at all, because they're generally accessing memory outside the scope of
258 //! "the abstract machine", or already using "I know what I'm doing" annotations like "volatile".
260 //! Under [Strict Provenance] is is Undefined Behaviour to:
262 //! * Access memory through a pointer that does not have provenance over that memory.
264 //! * [`offset`] a pointer to or from an address it doesn't have provenance over.
265 //! This means it's always UB to offset a pointer derived from something deallocated,
266 //! even if the offset is 0. Note that a pointer "one past the end" of its provenance
267 //! is not actually outside its provenance, it just has 0 bytes it can load/store.
269 //! But it *is* still sound to:
271 //! * Create an invalid pointer from just an address (see [`ptr::invalid`][]). This can
272 //! be used for sentinel values like `null` *or* to represent a tagged pointer that will
273 //! never be dereferencable. In general, it is always sound for an integer to pretend
274 //! to be a pointer "for fun" as long as you don't use operations on it which require
275 //! it to be valid (offset, read, write, etc).
277 //! * Forge an allocation of size zero at any sufficiently aligned non-null address.
278 //! i.e. the usual "ZSTs are fake, do what you want" rules apply *but* this only applies
279 //! for actual forgery (integers cast to pointers). If you borrow some struct's field
280 //! that *happens* to be zero-sized, the resulting pointer will have provenance tied to
281 //! that allocation and it will still get invalidated if the allocation gets deallocated.
282 //! In the future we may introduce an API to make such a forged allocation explicit.
284 //! * [`wrapping_offset`][] a pointer outside its provenance. This includes invalid pointers
285 //! which have "no" provenance. Unfortunately there may be practical limits on this for a
286 //! particular platform, and it's an open question as to how to specify this (if at all).
287 //! Notably, [CHERI][] relies on a compression scheme that can't handle a
288 //! pointer getting offset "too far" out of bounds. If this happens, the address
289 //! returned by `addr` will be the value you expect, but the provenance will get invalidated
290 //! and using it to read/write will fault. The details of this are architecture-specific
291 //! and based on alignment, but the buffer on either side of the pointer's range is pretty
292 //! generous (think kilobytes, not bytes).
294 //! * Compare arbitrary pointers by address. Addresses *are* just integers and so there is
295 //! always a coherent answer, even if the pointers are invalid or from different
296 //! address-spaces/provenances. Of course, comparing addresses from different address-spaces
297 //! is generally going to be *meaningless*, but so is comparing Kilograms to Meters, and Rust
298 //! doesn't prevent that either. Similarly, if you get "lucky" and notice that a pointer
299 //! one-past-the-end is the "same" address as the start of an unrelated allocation, anything
300 //! you do with that fact is *probably* going to be gibberish. The scope of that gibberish
301 //! is kept under control by the fact that the two pointers *still* aren't allowed to access
302 //! the other's allocation (bytes), because they still have different provenance.
304 //! * Perform pointer tagging tricks. This falls out of [`wrapping_offset`] but is worth
305 //! mentioning in more detail because of the limitations of [CHERI][]. Low-bit tagging
306 //! is very robust, and often doesn't even go out of bounds because types ensure
307 //! size >= align (and over-aligning actually gives CHERI more flexibility). Anything
308 //! more complex than this rapidly enters "extremely platform-specific" territory as
309 //! certain things may or may not be allowed based on specific supported operations.
310 //! For instance, ARM explicitly supports high-bit tagging, and so CHERI on ARM inherits
311 //! that and should support it.
314 //! [aliasing]: ../../nomicon/aliasing.html
315 //! [book]: ../../book/ch19-01-unsafe-rust.html#dereferencing-a-raw-pointer
316 //! [ub]: ../../reference/behavior-considered-undefined.html
317 //! [zst]: ../../nomicon/exotic-sizes.html#zero-sized-types-zsts
318 //! [atomic operations]: crate::sync::atomic
319 //! [`offset`]: pointer::offset
320 //! [`wrapping_offset`]: pointer::offset
321 //! [`with_addr`]: pointer::with_addr
322 //! [`map_addr`]: pointer::map_addr
323 //! [`addr`]: pointer::addr
324 //! [`ptr::invalid`]: core::ptr::invalid
325 //! [miri]: https://github.com/rust-lang/miri
326 //! [CHERI]: https://www.cl.cam.ac.uk/research/security/ctsrd/cheri/
327 //! [Strict Provenance]: https://github.com/rust-lang/rust/issues/95228
328 //! [Stacked Borrows]: https://plv.mpi-sws.org/rustbelt/stacked-borrows/
330 #![stable(feature = "rust1", since = "1.0.0")]
332 use crate::cmp::Ordering;
335 use crate::intrinsics::{self, abort, is_aligned_and_not_null};
336 use crate::mem::{self, MaybeUninit};
338 #[stable(feature = "rust1", since = "1.0.0")]
340 pub use crate::intrinsics::copy_nonoverlapping;
342 #[stable(feature = "rust1", since = "1.0.0")]
344 pub use crate::intrinsics::copy;
346 #[stable(feature = "rust1", since = "1.0.0")]
348 pub use crate::intrinsics::write_bytes;
351 pub(crate) use metadata::PtrRepr;
352 #[unstable(feature = "ptr_metadata", issue = "81513")]
353 pub use metadata::{from_raw_parts, from_raw_parts_mut, metadata, DynMetadata, Pointee, Thin};
356 #[stable(feature = "nonnull", since = "1.25.0")]
357 pub use non_null::NonNull;
360 #[unstable(feature = "ptr_internals", issue = "none")]
361 pub use unique::Unique;
366 /// Executes the destructor (if any) of the pointed-to value.
368 /// This is semantically equivalent to calling [`ptr::read`] and discarding
369 /// the result, but has the following advantages:
371 /// * It is *required* to use `drop_in_place` to drop unsized types like
372 /// trait objects, because they can't be read out onto the stack and
373 /// dropped normally.
375 /// * It is friendlier to the optimizer to do this over [`ptr::read`] when
376 /// dropping manually allocated memory (e.g., in the implementations of
377 /// `Box`/`Rc`/`Vec`), as the compiler doesn't need to prove that it's
378 /// sound to elide the copy.
380 /// * It can be used to drop [pinned] data when `T` is not `repr(packed)`
381 /// (pinned data must not be moved before it is dropped).
383 /// Unaligned values cannot be dropped in place, they must be copied to an aligned
384 /// location first using [`ptr::read_unaligned`]. For packed structs, this move is
385 /// done automatically by the compiler. This means the fields of packed structs
386 /// are not dropped in-place.
388 /// [`ptr::read`]: self::read
389 /// [`ptr::read_unaligned`]: self::read_unaligned
390 /// [pinned]: crate::pin
394 /// Behavior is undefined if any of the following conditions are violated:
396 /// * `to_drop` must be [valid] for both reads and writes.
398 /// * `to_drop` must be properly aligned.
400 /// * The value `to_drop` points to must be valid for dropping, which may mean it must uphold
401 /// additional invariants - this is type-dependent.
403 /// Additionally, if `T` is not [`Copy`], using the pointed-to value after
404 /// calling `drop_in_place` can cause undefined behavior. Note that `*to_drop =
405 /// foo` counts as a use because it will cause the value to be dropped
406 /// again. [`write()`] can be used to overwrite data without causing it to be
409 /// Note that even if `T` has size `0`, the pointer must be non-null and properly aligned.
411 /// [valid]: self#safety
415 /// Manually remove the last item from a vector:
421 /// let last = Rc::new(1);
422 /// let weak = Rc::downgrade(&last);
424 /// let mut v = vec![Rc::new(0), last];
427 /// // Get a raw pointer to the last element in `v`.
428 /// let ptr = &mut v[1] as *mut _;
429 /// // Shorten `v` to prevent the last item from being dropped. We do that first,
430 /// // to prevent issues if the `drop_in_place` below panics.
432 /// // Without a call `drop_in_place`, the last item would never be dropped,
433 /// // and the memory it manages would be leaked.
434 /// ptr::drop_in_place(ptr);
437 /// assert_eq!(v, &[0.into()]);
439 /// // Ensure that the last item was dropped.
440 /// assert!(weak.upgrade().is_none());
442 #[stable(feature = "drop_in_place", since = "1.8.0")]
443 #[lang = "drop_in_place"]
444 #[allow(unconditional_recursion)]
445 pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
446 // Code here does not matter - this is replaced by the
447 // real drop glue by the compiler.
449 // SAFETY: see comment above
450 unsafe { drop_in_place(to_drop) }
453 /// Creates a null raw pointer.
460 /// let p: *const i32 = ptr::null();
461 /// assert!(p.is_null());
465 #[stable(feature = "rust1", since = "1.0.0")]
467 #[rustc_const_stable(feature = "const_ptr_null", since = "1.24.0")]
468 #[rustc_diagnostic_item = "ptr_null"]
469 pub const fn null<T>() -> *const T {
473 /// Creates a null mutable raw pointer.
480 /// let p: *mut i32 = ptr::null_mut();
481 /// assert!(p.is_null());
485 #[stable(feature = "rust1", since = "1.0.0")]
487 #[rustc_const_stable(feature = "const_ptr_null", since = "1.24.0")]
488 #[rustc_diagnostic_item = "ptr_null_mut"]
489 pub const fn null_mut<T>() -> *mut T {
493 /// Creates an invalid pointer with the given address.
495 /// This is *currently* equivalent to `addr as *const T` but it expresses the intended semantic
496 /// more clearly, and may become important under future memory models.
498 /// The module's top-level documentation discusses the precise meaning of an "invalid"
499 /// pointer but essentially this expresses that the pointer is not associated
500 /// with any actual allocation and is little more than a usize address in disguise.
502 /// This pointer will have no provenance associated with it and is therefore
503 /// UB to read/write/offset. This mostly exists to facilitate things
504 /// like ptr::null and NonNull::dangling which make invalid pointers.
506 /// (Standard "Zero-Sized-Types get to cheat and lie" caveats apply, although it
507 /// may be desirable to give them their own API just to make that 100% clear.)
509 /// This API and its claimed semantics are part of the Strict Provenance experiment,
510 /// see the [module documentation][crate::ptr] for details.
513 #[rustc_const_stable(feature = "strict_provenance", since = "1.61.0")]
514 #[unstable(feature = "strict_provenance", issue = "95228")]
515 pub const fn invalid<T>(addr: usize) -> *const T {
516 // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
520 /// Creates an invalid mutable pointer with the given address.
522 /// This is *currently* equivalent to `addr as *mut T` but it expresses the intended semantic
523 /// more clearly, and may become important under future memory models.
525 /// The module's top-level documentation discusses the precise meaning of an "invalid"
526 /// pointer but essentially this expresses that the pointer is not associated
527 /// with any actual allocation and is little more than a usize address in disguise.
529 /// This pointer will have no provenance associated with it and is therefore
530 /// UB to read/write/offset. This mostly exists to facilitate things
531 /// like ptr::null and NonNull::dangling which make invalid pointers.
533 /// (Standard "Zero-Sized-Types get to cheat and lie" caveats apply, although it
534 /// may be desirable to give them their own API just to make that 100% clear.)
536 /// This API and its claimed semantics are part of the Strict Provenance experiment,
537 /// see the [module documentation][crate::ptr] for details.
540 #[rustc_const_stable(feature = "strict_provenance", since = "1.61.0")]
541 #[unstable(feature = "strict_provenance", issue = "95228")]
542 pub const fn invalid_mut<T>(addr: usize) -> *mut T {
543 // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
547 /// Forms a raw slice from a pointer and a length.
549 /// The `len` argument is the number of **elements**, not the number of bytes.
551 /// This function is safe, but actually using the return value is unsafe.
552 /// See the documentation of [`slice::from_raw_parts`] for slice safety requirements.
554 /// [`slice::from_raw_parts`]: crate::slice::from_raw_parts
561 /// // create a slice pointer when starting out with a pointer to the first element
562 /// let x = [5, 6, 7];
563 /// let raw_pointer = x.as_ptr();
564 /// let slice = ptr::slice_from_raw_parts(raw_pointer, 3);
565 /// assert_eq!(unsafe { &*slice }[2], 7);
568 #[stable(feature = "slice_from_raw_parts", since = "1.42.0")]
569 #[rustc_const_unstable(feature = "const_slice_from_raw_parts", issue = "67456")]
570 pub const fn slice_from_raw_parts<T>(data: *const T, len: usize) -> *const [T] {
571 from_raw_parts(data.cast(), len)
574 /// Performs the same functionality as [`slice_from_raw_parts`], except that a
575 /// raw mutable slice is returned, as opposed to a raw immutable slice.
577 /// See the documentation of [`slice_from_raw_parts`] for more details.
579 /// This function is safe, but actually using the return value is unsafe.
580 /// See the documentation of [`slice::from_raw_parts_mut`] for slice safety requirements.
582 /// [`slice::from_raw_parts_mut`]: crate::slice::from_raw_parts_mut
589 /// let x = &mut [5, 6, 7];
590 /// let raw_pointer = x.as_mut_ptr();
591 /// let slice = ptr::slice_from_raw_parts_mut(raw_pointer, 3);
594 /// (*slice)[2] = 99; // assign a value at an index in the slice
597 /// assert_eq!(unsafe { &*slice }[2], 99);
600 #[stable(feature = "slice_from_raw_parts", since = "1.42.0")]
601 #[rustc_const_unstable(feature = "const_slice_from_raw_parts", issue = "67456")]
602 pub const fn slice_from_raw_parts_mut<T>(data: *mut T, len: usize) -> *mut [T] {
603 from_raw_parts_mut(data.cast(), len)
606 /// Swaps the values at two mutable locations of the same type, without
607 /// deinitializing either.
609 /// But for the following two exceptions, this function is semantically
610 /// equivalent to [`mem::swap`]:
612 /// * It operates on raw pointers instead of references. When references are
613 /// available, [`mem::swap`] should be preferred.
615 /// * The two pointed-to values may overlap. If the values do overlap, then the
616 /// overlapping region of memory from `x` will be used. This is demonstrated
617 /// in the second example below.
621 /// Behavior is undefined if any of the following conditions are violated:
623 /// * Both `x` and `y` must be [valid] for both reads and writes.
625 /// * Both `x` and `y` must be properly aligned.
627 /// Note that even if `T` has size `0`, the pointers must be non-null and properly aligned.
629 /// [valid]: self#safety
633 /// Swapping two non-overlapping regions:
638 /// let mut array = [0, 1, 2, 3];
640 /// let x = array[0..].as_mut_ptr() as *mut [u32; 2]; // this is `array[0..2]`
641 /// let y = array[2..].as_mut_ptr() as *mut [u32; 2]; // this is `array[2..4]`
645 /// assert_eq!([2, 3, 0, 1], array);
649 /// Swapping two overlapping regions:
654 /// let mut array: [i32; 4] = [0, 1, 2, 3];
656 /// let array_ptr: *mut i32 = array.as_mut_ptr();
658 /// let x = array_ptr as *mut [i32; 3]; // this is `array[0..3]`
659 /// let y = unsafe { array_ptr.add(1) } as *mut [i32; 3]; // this is `array[1..4]`
663 /// // The indices `1..3` of the slice overlap between `x` and `y`.
664 /// // Reasonable results would be for to them be `[2, 3]`, so that indices `0..3` are
665 /// // `[1, 2, 3]` (matching `y` before the `swap`); or for them to be `[0, 1]`
666 /// // so that indices `1..4` are `[0, 1, 2]` (matching `x` before the `swap`).
667 /// // This implementation is defined to make the latter choice.
668 /// assert_eq!([1, 0, 1, 2], array);
672 #[stable(feature = "rust1", since = "1.0.0")]
673 #[rustc_const_unstable(feature = "const_swap", issue = "83163")]
674 pub const unsafe fn swap<T>(x: *mut T, y: *mut T) {
675 // Give ourselves some scratch space to work with.
676 // We do not have to worry about drops: `MaybeUninit` does nothing when dropped.
677 let mut tmp = MaybeUninit::<T>::uninit();
680 // SAFETY: the caller must guarantee that `x` and `y` are
681 // valid for writes and properly aligned. `tmp` cannot be
682 // overlapping either `x` or `y` because `tmp` was just allocated
683 // on the stack as a separate allocated object.
685 copy_nonoverlapping(x, tmp.as_mut_ptr(), 1);
686 copy(y, x, 1); // `x` and `y` may overlap
687 copy_nonoverlapping(tmp.as_ptr(), y, 1);
691 /// Swaps `count * size_of::<T>()` bytes between the two regions of memory
692 /// beginning at `x` and `y`. The two regions must *not* overlap.
696 /// Behavior is undefined if any of the following conditions are violated:
698 /// * Both `x` and `y` must be [valid] for both reads and writes of `count *
699 /// size_of::<T>()` bytes.
701 /// * Both `x` and `y` must be properly aligned.
703 /// * The region of memory beginning at `x` with a size of `count *
704 /// size_of::<T>()` bytes must *not* overlap with the region of memory
705 /// beginning at `y` with the same size.
707 /// Note that even if the effectively copied size (`count * size_of::<T>()`) is `0`,
708 /// the pointers must be non-null and properly aligned.
710 /// [valid]: self#safety
719 /// let mut x = [1, 2, 3, 4];
720 /// let mut y = [7, 8, 9];
723 /// ptr::swap_nonoverlapping(x.as_mut_ptr(), y.as_mut_ptr(), 2);
726 /// assert_eq!(x, [7, 8, 3, 4]);
727 /// assert_eq!(y, [1, 2, 9]);
730 #[stable(feature = "swap_nonoverlapping", since = "1.27.0")]
731 #[rustc_const_unstable(feature = "const_swap", issue = "83163")]
732 pub const unsafe fn swap_nonoverlapping<T>(x: *mut T, y: *mut T, count: usize) {
734 macro_rules! attempt_swap_as_chunks {
736 if mem::align_of::<T>() >= mem::align_of::<$ChunkTy>()
737 && mem::size_of::<T>() % mem::size_of::<$ChunkTy>() == 0
739 let x: *mut MaybeUninit<$ChunkTy> = x.cast();
740 let y: *mut MaybeUninit<$ChunkTy> = y.cast();
741 let count = count * (mem::size_of::<T>() / mem::size_of::<$ChunkTy>());
742 // SAFETY: these are the same bytes that the caller promised were
743 // ok, just typed as `MaybeUninit<ChunkTy>`s instead of as `T`s.
744 // The `if` condition above ensures that we're not violating
745 // alignment requirements, and that the division is exact so
746 // that we don't lose any bytes off the end.
747 return unsafe { swap_nonoverlapping_simple(x, y, count) };
752 // NOTE(scottmcm) MIRI is disabled here as reading in smaller units is a
753 // pessimization for it. Also, if the type contains any unaligned pointers,
754 // copying those over multiple reads is difficult to support.
757 // Split up the slice into small power-of-two-sized chunks that LLVM is able
758 // to vectorize (unless it's a special type with more-than-pointer alignment,
759 // because we don't want to pessimize things like slices of SIMD vectors.)
760 if mem::align_of::<T>() <= mem::size_of::<usize>()
761 && (!mem::size_of::<T>().is_power_of_two()
762 || mem::size_of::<T>() > mem::size_of::<usize>() * 2)
764 attempt_swap_as_chunks!(usize);
765 attempt_swap_as_chunks!(u8);
769 // SAFETY: Same preconditions as this function
770 unsafe { swap_nonoverlapping_simple(x, y, count) }
773 /// Same behaviour and safety conditions as [`swap_nonoverlapping`]
775 /// LLVM can vectorize this (at least it can for the power-of-two-sized types
776 /// `swap_nonoverlapping` tries to use) so no need to manually SIMD it.
778 #[rustc_const_unstable(feature = "const_swap", issue = "83163")]
779 const unsafe fn swap_nonoverlapping_simple<T>(x: *mut T, y: *mut T, count: usize) {
783 // SAFETY: By precondition, `i` is in-bounds because it's below `n`
784 unsafe { &mut *x.add(i) };
786 // SAFETY: By precondition, `i` is in-bounds because it's below `n`
787 // and it's distinct from `x` since the ranges are non-overlapping
788 unsafe { &mut *y.add(i) };
789 mem::swap_simple(x, y);
795 /// Moves `src` into the pointed `dst`, returning the previous `dst` value.
797 /// Neither value is dropped.
799 /// This function is semantically equivalent to [`mem::replace`] except that it
800 /// operates on raw pointers instead of references. When references are
801 /// available, [`mem::replace`] should be preferred.
805 /// Behavior is undefined if any of the following conditions are violated:
807 /// * `dst` must be [valid] for both reads and writes.
809 /// * `dst` must be properly aligned.
811 /// * `dst` must point to a properly initialized value of type `T`.
813 /// Note that even if `T` has size `0`, the pointer must be non-null and properly aligned.
815 /// [valid]: self#safety
822 /// let mut rust = vec!['b', 'u', 's', 't'];
824 /// // `mem::replace` would have the same effect without requiring the unsafe
827 /// ptr::replace(&mut rust[0], 'r')
830 /// assert_eq!(b, 'b');
831 /// assert_eq!(rust, &['r', 'u', 's', 't']);
834 #[stable(feature = "rust1", since = "1.0.0")]
835 #[rustc_const_unstable(feature = "const_replace", issue = "83164")]
836 pub const unsafe fn replace<T>(dst: *mut T, mut src: T) -> T {
837 // SAFETY: the caller must guarantee that `dst` is valid to be
838 // cast to a mutable reference (valid for writes, aligned, initialized),
839 // and cannot overlap `src` since `dst` must point to a distinct
842 mem::swap(&mut *dst, &mut src); // cannot overlap
847 /// Reads the value from `src` without moving it. This leaves the
848 /// memory in `src` unchanged.
852 /// Behavior is undefined if any of the following conditions are violated:
854 /// * `src` must be [valid] for reads.
856 /// * `src` must be properly aligned. Use [`read_unaligned`] if this is not the
859 /// * `src` must point to a properly initialized value of type `T`.
861 /// Note that even if `T` has size `0`, the pointer must be non-null and properly aligned.
869 /// let y = &x as *const i32;
872 /// assert_eq!(std::ptr::read(y), 12);
876 /// Manually implement [`mem::swap`]:
881 /// fn swap<T>(a: &mut T, b: &mut T) {
883 /// // Create a bitwise copy of the value at `a` in `tmp`.
884 /// let tmp = ptr::read(a);
886 /// // Exiting at this point (either by explicitly returning or by
887 /// // calling a function which panics) would cause the value in `tmp` to
888 /// // be dropped while the same value is still referenced by `a`. This
889 /// // could trigger undefined behavior if `T` is not `Copy`.
891 /// // Create a bitwise copy of the value at `b` in `a`.
892 /// // This is safe because mutable references cannot alias.
893 /// ptr::copy_nonoverlapping(b, a, 1);
895 /// // As above, exiting here could trigger undefined behavior because
896 /// // the same value is referenced by `a` and `b`.
898 /// // Move `tmp` into `b`.
899 /// ptr::write(b, tmp);
901 /// // `tmp` has been moved (`write` takes ownership of its second argument),
902 /// // so nothing is dropped implicitly here.
906 /// let mut foo = "foo".to_owned();
907 /// let mut bar = "bar".to_owned();
909 /// swap(&mut foo, &mut bar);
911 /// assert_eq!(foo, "bar");
912 /// assert_eq!(bar, "foo");
915 /// ## Ownership of the Returned Value
917 /// `read` creates a bitwise copy of `T`, regardless of whether `T` is [`Copy`].
918 /// If `T` is not [`Copy`], using both the returned value and the value at
919 /// `*src` can violate memory safety. Note that assigning to `*src` counts as a
920 /// use because it will attempt to drop the value at `*src`.
922 /// [`write()`] can be used to overwrite data without causing it to be dropped.
927 /// let mut s = String::from("foo");
929 /// // `s2` now points to the same underlying memory as `s`.
930 /// let mut s2: String = ptr::read(&s);
932 /// assert_eq!(s2, "foo");
934 /// // Assigning to `s2` causes its original value to be dropped. Beyond
935 /// // this point, `s` must no longer be used, as the underlying memory has
937 /// s2 = String::default();
938 /// assert_eq!(s2, "");
940 /// // Assigning to `s` would cause the old value to be dropped again,
941 /// // resulting in undefined behavior.
942 /// // s = String::from("bar"); // ERROR
944 /// // `ptr::write` can be used to overwrite a value without dropping it.
945 /// ptr::write(&mut s, String::from("bar"));
948 /// assert_eq!(s, "bar");
951 /// [valid]: self#safety
953 #[stable(feature = "rust1", since = "1.0.0")]
954 #[rustc_const_unstable(feature = "const_ptr_read", issue = "80377")]
955 pub const unsafe fn read<T>(src: *const T) -> T {
956 // We are calling the intrinsics directly to avoid function calls in the generated code
957 // as `intrinsics::copy_nonoverlapping` is a wrapper function.
958 extern "rust-intrinsic" {
959 #[rustc_const_unstable(feature = "const_intrinsic_copy", issue = "80697")]
960 fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize);
963 let mut tmp = MaybeUninit::<T>::uninit();
964 // SAFETY: the caller must guarantee that `src` is valid for reads.
965 // `src` cannot overlap `tmp` because `tmp` was just allocated on
966 // the stack as a separate allocated object.
968 // Also, since we just wrote a valid value into `tmp`, it is guaranteed
969 // to be properly initialized.
971 copy_nonoverlapping(src, tmp.as_mut_ptr(), 1);
976 /// Reads the value from `src` without moving it. This leaves the
977 /// memory in `src` unchanged.
979 /// Unlike [`read`], `read_unaligned` works with unaligned pointers.
983 /// Behavior is undefined if any of the following conditions are violated:
985 /// * `src` must be [valid] for reads.
987 /// * `src` must point to a properly initialized value of type `T`.
989 /// Like [`read`], `read_unaligned` creates a bitwise copy of `T`, regardless of
990 /// whether `T` is [`Copy`]. If `T` is not [`Copy`], using both the returned
991 /// value and the value at `*src` can [violate memory safety][read-ownership].
993 /// Note that even if `T` has size `0`, the pointer must be non-null.
995 /// [read-ownership]: read#ownership-of-the-returned-value
996 /// [valid]: self#safety
998 /// ## On `packed` structs
1000 /// Attempting to create a raw pointer to an `unaligned` struct field with
1001 /// an expression such as `&packed.unaligned as *const FieldType` creates an
1002 /// intermediate unaligned reference before converting that to a raw pointer.
1003 /// That this reference is temporary and immediately cast is inconsequential
1004 /// as the compiler always expects references to be properly aligned.
1005 /// As a result, using `&packed.unaligned as *const FieldType` causes immediate
1006 /// *undefined behavior* in your program.
1008 /// Instead you must use the [`ptr::addr_of!`](addr_of) macro to
1009 /// create the pointer. You may use that returned pointer together with this
1012 /// An example of what not to do and how this relates to `read_unaligned` is:
1015 /// #[repr(packed, C)]
1021 /// let packed = Packed {
1023 /// unaligned: 0x01020304,
1026 /// // Take the address of a 32-bit integer which is not aligned.
1027 /// // In contrast to `&packed.unaligned as *const _`, this has no undefined behavior.
1028 /// let unaligned = std::ptr::addr_of!(packed.unaligned);
1030 /// let v = unsafe { std::ptr::read_unaligned(unaligned) };
1031 /// assert_eq!(v, 0x01020304);
1034 /// Accessing unaligned fields directly with e.g. `packed.unaligned` is safe however.
1038 /// Read a usize value from a byte buffer:
1043 /// fn read_usize(x: &[u8]) -> usize {
1044 /// assert!(x.len() >= mem::size_of::<usize>());
1046 /// let ptr = x.as_ptr() as *const usize;
1048 /// unsafe { ptr.read_unaligned() }
1052 #[stable(feature = "ptr_unaligned", since = "1.17.0")]
1053 #[rustc_const_unstable(feature = "const_ptr_read", issue = "80377")]
1054 pub const unsafe fn read_unaligned<T>(src: *const T) -> T {
1055 let mut tmp = MaybeUninit::<T>::uninit();
1056 // SAFETY: the caller must guarantee that `src` is valid for reads.
1057 // `src` cannot overlap `tmp` because `tmp` was just allocated on
1058 // the stack as a separate allocated object.
1060 // Also, since we just wrote a valid value into `tmp`, it is guaranteed
1061 // to be properly initialized.
1063 copy_nonoverlapping(src as *const u8, tmp.as_mut_ptr() as *mut u8, mem::size_of::<T>());
1068 /// Overwrites a memory location with the given value without reading or
1069 /// dropping the old value.
1071 /// `write` does not drop the contents of `dst`. This is safe, but it could leak
1072 /// allocations or resources, so care should be taken not to overwrite an object
1073 /// that should be dropped.
1075 /// Additionally, it does not drop `src`. Semantically, `src` is moved into the
1076 /// location pointed to by `dst`.
1078 /// This is appropriate for initializing uninitialized memory, or overwriting
1079 /// memory that has previously been [`read`] from.
1083 /// Behavior is undefined if any of the following conditions are violated:
1085 /// * `dst` must be [valid] for writes.
1087 /// * `dst` must be properly aligned. Use [`write_unaligned`] if this is not the
1090 /// Note that even if `T` has size `0`, the pointer must be non-null and properly aligned.
1092 /// [valid]: self#safety
1100 /// let y = &mut x as *mut i32;
1104 /// std::ptr::write(y, z);
1105 /// assert_eq!(std::ptr::read(y), 12);
1109 /// Manually implement [`mem::swap`]:
1114 /// fn swap<T>(a: &mut T, b: &mut T) {
1116 /// // Create a bitwise copy of the value at `a` in `tmp`.
1117 /// let tmp = ptr::read(a);
1119 /// // Exiting at this point (either by explicitly returning or by
1120 /// // calling a function which panics) would cause the value in `tmp` to
1121 /// // be dropped while the same value is still referenced by `a`. This
1122 /// // could trigger undefined behavior if `T` is not `Copy`.
1124 /// // Create a bitwise copy of the value at `b` in `a`.
1125 /// // This is safe because mutable references cannot alias.
1126 /// ptr::copy_nonoverlapping(b, a, 1);
1128 /// // As above, exiting here could trigger undefined behavior because
1129 /// // the same value is referenced by `a` and `b`.
1131 /// // Move `tmp` into `b`.
1132 /// ptr::write(b, tmp);
1134 /// // `tmp` has been moved (`write` takes ownership of its second argument),
1135 /// // so nothing is dropped implicitly here.
1139 /// let mut foo = "foo".to_owned();
1140 /// let mut bar = "bar".to_owned();
1142 /// swap(&mut foo, &mut bar);
1144 /// assert_eq!(foo, "bar");
1145 /// assert_eq!(bar, "foo");
1148 #[stable(feature = "rust1", since = "1.0.0")]
1149 #[rustc_const_unstable(feature = "const_ptr_write", issue = "86302")]
1150 pub const unsafe fn write<T>(dst: *mut T, src: T) {
1151 // We are calling the intrinsics directly to avoid function calls in the generated code
1152 // as `intrinsics::copy_nonoverlapping` is a wrapper function.
1153 extern "rust-intrinsic" {
1154 #[rustc_const_unstable(feature = "const_intrinsic_copy", issue = "80697")]
1155 fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize);
1158 // SAFETY: the caller must guarantee that `dst` is valid for writes.
1159 // `dst` cannot overlap `src` because the caller has mutable access
1160 // to `dst` while `src` is owned by this function.
1162 copy_nonoverlapping(&src as *const T, dst, 1);
1163 intrinsics::forget(src);
1167 /// Overwrites a memory location with the given value without reading or
1168 /// dropping the old value.
1170 /// Unlike [`write()`], the pointer may be unaligned.
1172 /// `write_unaligned` does not drop the contents of `dst`. This is safe, but it
1173 /// could leak allocations or resources, so care should be taken not to overwrite
1174 /// an object that should be dropped.
1176 /// Additionally, it does not drop `src`. Semantically, `src` is moved into the
1177 /// location pointed to by `dst`.
1179 /// This is appropriate for initializing uninitialized memory, or overwriting
1180 /// memory that has previously been read with [`read_unaligned`].
1184 /// Behavior is undefined if any of the following conditions are violated:
1186 /// * `dst` must be [valid] for writes.
1188 /// Note that even if `T` has size `0`, the pointer must be non-null.
1190 /// [valid]: self#safety
1192 /// ## On `packed` structs
1194 /// Attempting to create a raw pointer to an `unaligned` struct field with
1195 /// an expression such as `&packed.unaligned as *const FieldType` creates an
1196 /// intermediate unaligned reference before converting that to a raw pointer.
1197 /// That this reference is temporary and immediately cast is inconsequential
1198 /// as the compiler always expects references to be properly aligned.
1199 /// As a result, using `&packed.unaligned as *const FieldType` causes immediate
1200 /// *undefined behavior* in your program.
1202 /// Instead you must use the [`ptr::addr_of_mut!`](addr_of_mut)
1203 /// macro to create the pointer. You may use that returned pointer together with
1206 /// An example of how to do it and how this relates to `write_unaligned` is:
1209 /// #[repr(packed, C)]
1215 /// let mut packed: Packed = unsafe { std::mem::zeroed() };
1217 /// // Take the address of a 32-bit integer which is not aligned.
1218 /// // In contrast to `&packed.unaligned as *mut _`, this has no undefined behavior.
1219 /// let unaligned = std::ptr::addr_of_mut!(packed.unaligned);
1221 /// unsafe { std::ptr::write_unaligned(unaligned, 42) };
1223 /// assert_eq!({packed.unaligned}, 42); // `{...}` forces copying the field instead of creating a reference.
1226 /// Accessing unaligned fields directly with e.g. `packed.unaligned` is safe however
1227 /// (as can be seen in the `assert_eq!` above).
1231 /// Write a usize value to a byte buffer:
1236 /// fn write_usize(x: &mut [u8], val: usize) {
1237 /// assert!(x.len() >= mem::size_of::<usize>());
1239 /// let ptr = x.as_mut_ptr() as *mut usize;
1241 /// unsafe { ptr.write_unaligned(val) }
1245 #[stable(feature = "ptr_unaligned", since = "1.17.0")]
1246 #[rustc_const_unstable(feature = "const_ptr_write", issue = "86302")]
1247 pub const unsafe fn write_unaligned<T>(dst: *mut T, src: T) {
1248 // SAFETY: the caller must guarantee that `dst` is valid for writes.
1249 // `dst` cannot overlap `src` because the caller has mutable access
1250 // to `dst` while `src` is owned by this function.
1252 copy_nonoverlapping(&src as *const T as *const u8, dst as *mut u8, mem::size_of::<T>());
1253 // We are calling the intrinsic directly to avoid function calls in the generated code.
1254 intrinsics::forget(src);
1258 /// Performs a volatile read of the value from `src` without moving it. This
1259 /// leaves the memory in `src` unchanged.
1261 /// Volatile operations are intended to act on I/O memory, and are guaranteed
1262 /// to not be elided or reordered by the compiler across other volatile
1267 /// Rust does not currently have a rigorously and formally defined memory model,
1268 /// so the precise semantics of what "volatile" means here is subject to change
1269 /// over time. That being said, the semantics will almost always end up pretty
1270 /// similar to [C11's definition of volatile][c11].
1272 /// The compiler shouldn't change the relative order or number of volatile
1273 /// memory operations. However, volatile memory operations on zero-sized types
1274 /// (e.g., if a zero-sized type is passed to `read_volatile`) are noops
1275 /// and may be ignored.
1277 /// [c11]: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
1281 /// Behavior is undefined if any of the following conditions are violated:
1283 /// * `src` must be [valid] for reads.
1285 /// * `src` must be properly aligned.
1287 /// * `src` must point to a properly initialized value of type `T`.
1289 /// Like [`read`], `read_volatile` creates a bitwise copy of `T`, regardless of
1290 /// whether `T` is [`Copy`]. If `T` is not [`Copy`], using both the returned
1291 /// value and the value at `*src` can [violate memory safety][read-ownership].
1292 /// However, storing non-[`Copy`] types in volatile memory is almost certainly
1295 /// Note that even if `T` has size `0`, the pointer must be non-null and properly aligned.
1297 /// [valid]: self#safety
1298 /// [read-ownership]: read#ownership-of-the-returned-value
1300 /// Just like in C, whether an operation is volatile has no bearing whatsoever
1301 /// on questions involving concurrent access from multiple threads. Volatile
1302 /// accesses behave exactly like non-atomic accesses in that regard. In particular,
1303 /// a race between a `read_volatile` and any write operation to the same location
1304 /// is undefined behavior.
1312 /// let y = &x as *const i32;
1315 /// assert_eq!(std::ptr::read_volatile(y), 12);
1319 #[stable(feature = "volatile", since = "1.9.0")]
1320 pub unsafe fn read_volatile<T>(src: *const T) -> T {
1321 if cfg!(debug_assertions) && !is_aligned_and_not_null(src) {
1322 // Not panicking to keep codegen impact smaller.
1325 // SAFETY: the caller must uphold the safety contract for `volatile_load`.
1326 unsafe { intrinsics::volatile_load(src) }
1329 /// Performs a volatile write of a memory location with the given value without
1330 /// reading or dropping the old value.
1332 /// Volatile operations are intended to act on I/O memory, and are guaranteed
1333 /// to not be elided or reordered by the compiler across other volatile
1336 /// `write_volatile` does not drop the contents of `dst`. This is safe, but it
1337 /// could leak allocations or resources, so care should be taken not to overwrite
1338 /// an object that should be dropped.
1340 /// Additionally, it does not drop `src`. Semantically, `src` is moved into the
1341 /// location pointed to by `dst`.
1345 /// Rust does not currently have a rigorously and formally defined memory model,
1346 /// so the precise semantics of what "volatile" means here is subject to change
1347 /// over time. That being said, the semantics will almost always end up pretty
1348 /// similar to [C11's definition of volatile][c11].
1350 /// The compiler shouldn't change the relative order or number of volatile
1351 /// memory operations. However, volatile memory operations on zero-sized types
1352 /// (e.g., if a zero-sized type is passed to `write_volatile`) are noops
1353 /// and may be ignored.
1355 /// [c11]: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
1359 /// Behavior is undefined if any of the following conditions are violated:
1361 /// * `dst` must be [valid] for writes.
1363 /// * `dst` must be properly aligned.
1365 /// Note that even if `T` has size `0`, the pointer must be non-null and properly aligned.
1367 /// [valid]: self#safety
1369 /// Just like in C, whether an operation is volatile has no bearing whatsoever
1370 /// on questions involving concurrent access from multiple threads. Volatile
1371 /// accesses behave exactly like non-atomic accesses in that regard. In particular,
1372 /// a race between a `write_volatile` and any other operation (reading or writing)
1373 /// on the same location is undefined behavior.
1381 /// let y = &mut x as *mut i32;
1385 /// std::ptr::write_volatile(y, z);
1386 /// assert_eq!(std::ptr::read_volatile(y), 12);
1390 #[stable(feature = "volatile", since = "1.9.0")]
1391 pub unsafe fn write_volatile<T>(dst: *mut T, src: T) {
1392 if cfg!(debug_assertions) && !is_aligned_and_not_null(dst) {
1393 // Not panicking to keep codegen impact smaller.
1396 // SAFETY: the caller must uphold the safety contract for `volatile_store`.
1398 intrinsics::volatile_store(dst, src);
1402 /// Align pointer `p`.
1404 /// Calculate offset (in terms of elements of `stride` stride) that has to be applied
1405 /// to pointer `p` so that pointer `p` would get aligned to `a`.
1407 /// Note: This implementation has been carefully tailored to not panic. It is UB for this to panic.
1408 /// The only real change that can be made here is change of `INV_TABLE_MOD_16` and associated
1411 /// If we ever decide to make it possible to call the intrinsic with `a` that is not a
1412 /// power-of-two, it will probably be more prudent to just change to a naive implementation rather
1413 /// than trying to adapt this to accommodate that change.
1415 /// Any questions go to @nagisa.
1416 #[lang = "align_offset"]
1417 pub(crate) unsafe fn align_offset<T: Sized>(p: *const T, a: usize) -> usize {
1418 // FIXME(#75598): Direct use of these intrinsics improves codegen significantly at opt-level <=
1419 // 1, where the method versions of these operations are not inlined.
1421 unchecked_shl, unchecked_shr, unchecked_sub, wrapping_add, wrapping_mul, wrapping_sub,
1424 let addr = p.addr();
1426 /// Calculate multiplicative modular inverse of `x` modulo `m`.
1428 /// This implementation is tailored for `align_offset` and has following preconditions:
1430 /// * `m` is a power-of-two;
1431 /// * `x < m`; (if `x ≥ m`, pass in `x % m` instead)
1433 /// Implementation of this function shall not panic. Ever.
1435 unsafe fn mod_inv(x: usize, m: usize) -> usize {
1436 /// Multiplicative modular inverse table modulo 2⁴ = 16.
1438 /// Note, that this table does not contain values where inverse does not exist (i.e., for
1439 /// `0⁻¹ mod 16`, `2⁻¹ mod 16`, etc.)
1440 const INV_TABLE_MOD_16: [u8; 8] = [1, 11, 13, 7, 9, 3, 5, 15];
1441 /// Modulo for which the `INV_TABLE_MOD_16` is intended.
1442 const INV_TABLE_MOD: usize = 16;
1444 const INV_TABLE_MOD_SQUARED: usize = INV_TABLE_MOD * INV_TABLE_MOD;
1446 let table_inverse = INV_TABLE_MOD_16[(x & (INV_TABLE_MOD - 1)) >> 1] as usize;
1447 // SAFETY: `m` is required to be a power-of-two, hence non-zero.
1448 let m_minus_one = unsafe { unchecked_sub(m, 1) };
1449 if m <= INV_TABLE_MOD {
1450 table_inverse & m_minus_one
1452 // We iterate "up" using the following formula:
1454 // $$ xy ≡ 1 (mod 2ⁿ) → xy (2 - xy) ≡ 1 (mod 2²ⁿ) $$
1456 // until 2²ⁿ ≥ m. Then we can reduce to our desired `m` by taking the result `mod m`.
1457 let mut inverse = table_inverse;
1458 let mut going_mod = INV_TABLE_MOD_SQUARED;
1460 // y = y * (2 - xy) mod n
1462 // Note, that we use wrapping operations here intentionally – the original formula
1463 // uses e.g., subtraction `mod n`. It is entirely fine to do them `mod
1464 // usize::MAX` instead, because we take the result `mod n` at the end
1466 inverse = wrapping_mul(inverse, wrapping_sub(2usize, wrapping_mul(x, inverse)));
1468 return inverse & m_minus_one;
1470 going_mod = wrapping_mul(going_mod, going_mod);
1475 let stride = mem::size_of::<T>();
1476 // SAFETY: `a` is a power-of-two, therefore non-zero.
1477 let a_minus_one = unsafe { unchecked_sub(a, 1) };
1479 // `stride == 1` case can be computed more simply through `-p (mod a)`, but doing so
1480 // inhibits LLVM's ability to select instructions like `lea`. Instead we compute
1482 // round_up_to_next_alignment(p, a) - p
1484 // which distributes operations around the load-bearing, but pessimizing `and` sufficiently
1485 // for LLVM to be able to utilize the various optimizations it knows about.
1486 return wrapping_sub(wrapping_add(addr, a_minus_one) & wrapping_sub(0, a), addr);
1489 let pmoda = addr & a_minus_one;
1491 // Already aligned. Yay!
1493 } else if stride == 0 {
1494 // If the pointer is not aligned, and the element is zero-sized, then no amount of
1495 // elements will ever align the pointer.
1499 let smoda = stride & a_minus_one;
1500 // SAFETY: a is power-of-two hence non-zero. stride == 0 case is handled above.
1501 let gcdpow = unsafe { intrinsics::cttz_nonzero(stride).min(intrinsics::cttz_nonzero(a)) };
1502 // SAFETY: gcdpow has an upper-bound that’s at most the number of bits in a usize.
1503 let gcd = unsafe { unchecked_shl(1usize, gcdpow) };
1505 // SAFETY: gcd is always greater or equal to 1.
1506 if addr & unsafe { unchecked_sub(gcd, 1) } == 0 {
1507 // This branch solves for the following linear congruence equation:
1509 // ` p + so = 0 mod a `
1511 // `p` here is the pointer value, `s` - stride of `T`, `o` offset in `T`s, and `a` - the
1512 // requested alignment.
1514 // With `g = gcd(a, s)`, and the above condition asserting that `p` is also divisible by
1515 // `g`, we can denote `a' = a/g`, `s' = s/g`, `p' = p/g`, then this becomes equivalent to:
1517 // ` p' + s'o = 0 mod a' `
1518 // ` o = (a' - (p' mod a')) * (s'^-1 mod a') `
1520 // The first term is "the relative alignment of `p` to `a`" (divided by the `g`), the second
1521 // term is "how does incrementing `p` by `s` bytes change the relative alignment of `p`" (again
1523 // Division by `g` is necessary to make the inverse well formed if `a` and `s` are not
1526 // Furthermore, the result produced by this solution is not "minimal", so it is necessary
1527 // to take the result `o mod lcm(s, a)`. We can replace `lcm(s, a)` with just a `a'`.
1529 // SAFETY: `gcdpow` has an upper-bound not greater than the number of trailing 0-bits in
1531 let a2 = unsafe { unchecked_shr(a, gcdpow) };
1532 // SAFETY: `a2` is non-zero. Shifting `a` by `gcdpow` cannot shift out any of the set bits
1533 // in `a` (of which it has exactly one).
1534 let a2minus1 = unsafe { unchecked_sub(a2, 1) };
1535 // SAFETY: `gcdpow` has an upper-bound not greater than the number of trailing 0-bits in
1537 let s2 = unsafe { unchecked_shr(smoda, gcdpow) };
1538 // SAFETY: `gcdpow` has an upper-bound not greater than the number of trailing 0-bits in
1539 // `a`. Furthermore, the subtraction cannot overflow, because `a2 = a >> gcdpow` will
1540 // always be strictly greater than `(p % a) >> gcdpow`.
1541 let minusp2 = unsafe { unchecked_sub(a2, unchecked_shr(pmoda, gcdpow)) };
1542 // SAFETY: `a2` is a power-of-two, as proven above. `s2` is strictly less than `a2`
1543 // because `(s % a) >> gcdpow` is strictly less than `a >> gcdpow`.
1544 return wrapping_mul(minusp2, unsafe { mod_inv(s2, a2) }) & a2minus1;
1547 // Cannot be aligned at all.
1551 /// Compares raw pointers for equality.
1553 /// This is the same as using the `==` operator, but less generic:
1554 /// the arguments have to be `*const T` raw pointers,
1555 /// not anything that implements `PartialEq`.
1557 /// This can be used to compare `&T` references (which coerce to `*const T` implicitly)
1558 /// by their address rather than comparing the values they point to
1559 /// (which is what the `PartialEq for &T` implementation does).
1567 /// let other_five = 5;
1568 /// let five_ref = &five;
1569 /// let same_five_ref = &five;
1570 /// let other_five_ref = &other_five;
1572 /// assert!(five_ref == same_five_ref);
1573 /// assert!(ptr::eq(five_ref, same_five_ref));
1575 /// assert!(five_ref == other_five_ref);
1576 /// assert!(!ptr::eq(five_ref, other_five_ref));
1579 /// Slices are also compared by their length (fat pointers):
1582 /// let a = [1, 2, 3];
1583 /// assert!(std::ptr::eq(&a[..3], &a[..3]));
1584 /// assert!(!std::ptr::eq(&a[..2], &a[..3]));
1585 /// assert!(!std::ptr::eq(&a[0..2], &a[1..3]));
1588 /// Traits are also compared by their implementation:
1591 /// #[repr(transparent)]
1592 /// struct Wrapper { member: i32 }
1595 /// impl Trait for Wrapper {}
1596 /// impl Trait for i32 {}
1598 /// let wrapper = Wrapper { member: 10 };
1600 /// // Pointers have equal addresses.
1601 /// assert!(std::ptr::eq(
1602 /// &wrapper as *const Wrapper as *const u8,
1603 /// &wrapper.member as *const i32 as *const u8
1606 /// // Objects have equal addresses, but `Trait` has different implementations.
1607 /// assert!(!std::ptr::eq(
1608 /// &wrapper as &dyn Trait,
1609 /// &wrapper.member as &dyn Trait,
1611 /// assert!(!std::ptr::eq(
1612 /// &wrapper as &dyn Trait as *const dyn Trait,
1613 /// &wrapper.member as &dyn Trait as *const dyn Trait,
1616 /// // Converting the reference to a `*const u8` compares by address.
1617 /// assert!(std::ptr::eq(
1618 /// &wrapper as &dyn Trait as *const dyn Trait as *const u8,
1619 /// &wrapper.member as &dyn Trait as *const dyn Trait as *const u8,
1622 #[stable(feature = "ptr_eq", since = "1.17.0")]
1624 pub fn eq<T: ?Sized>(a: *const T, b: *const T) -> bool {
1628 /// Hash a raw pointer.
1630 /// This can be used to hash a `&T` reference (which coerces to `*const T` implicitly)
1631 /// by its address rather than the value it points to
1632 /// (which is what the `Hash for &T` implementation does).
1637 /// use std::collections::hash_map::DefaultHasher;
1638 /// use std::hash::{Hash, Hasher};
1642 /// let five_ref = &five;
1644 /// let mut hasher = DefaultHasher::new();
1645 /// ptr::hash(five_ref, &mut hasher);
1646 /// let actual = hasher.finish();
1648 /// let mut hasher = DefaultHasher::new();
1649 /// (five_ref as *const i32).hash(&mut hasher);
1650 /// let expected = hasher.finish();
1652 /// assert_eq!(actual, expected);
1654 #[stable(feature = "ptr_hash", since = "1.35.0")]
1655 pub fn hash<T: ?Sized, S: hash::Hasher>(hashee: *const T, into: &mut S) {
1656 use crate::hash::Hash;
1660 // FIXME(strict_provenance_magic): function pointers have buggy codegen that
1661 // necessitates casting to a usize to get the backend to do the right thing.
1662 // for now I will break AVR to silence *a billion* lints. We should probably
1663 // have a proper "opaque function pointer type" to handle this kind of thing.
1665 // Impls for function pointers
1666 macro_rules! fnptr_impls_safety_abi {
1667 ($FnTy: ty, $($Arg: ident),*) => {
1668 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1669 impl<Ret, $($Arg),*> PartialEq for $FnTy {
1671 fn eq(&self, other: &Self) -> bool {
1672 *self as usize == *other as usize
1676 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1677 impl<Ret, $($Arg),*> Eq for $FnTy {}
1679 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1680 impl<Ret, $($Arg),*> PartialOrd for $FnTy {
1682 fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
1683 (*self as usize).partial_cmp(&(*other as usize))
1687 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1688 impl<Ret, $($Arg),*> Ord for $FnTy {
1690 fn cmp(&self, other: &Self) -> Ordering {
1691 (*self as usize).cmp(&(*other as usize))
1695 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1696 impl<Ret, $($Arg),*> hash::Hash for $FnTy {
1697 fn hash<HH: hash::Hasher>(&self, state: &mut HH) {
1698 state.write_usize(*self as usize)
1702 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1703 impl<Ret, $($Arg),*> fmt::Pointer for $FnTy {
1704 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1705 // HACK: The intermediate cast as usize is required for AVR
1706 // so that the address space of the source function pointer
1707 // is preserved in the final function pointer.
1709 // https://github.com/avr-rust/rust/issues/143
1710 fmt::Pointer::fmt(&(*self as usize as *const ()), f)
1714 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1715 impl<Ret, $($Arg),*> fmt::Debug for $FnTy {
1716 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1717 // HACK: The intermediate cast as usize is required for AVR
1718 // so that the address space of the source function pointer
1719 // is preserved in the final function pointer.
1721 // https://github.com/avr-rust/rust/issues/143
1722 fmt::Pointer::fmt(&(*self as usize as *const ()), f)
1728 macro_rules! fnptr_impls_args {
1729 ($($Arg: ident),+) => {
1730 fnptr_impls_safety_abi! { extern "Rust" fn($($Arg),+) -> Ret, $($Arg),+ }
1731 fnptr_impls_safety_abi! { extern "C" fn($($Arg),+) -> Ret, $($Arg),+ }
1732 fnptr_impls_safety_abi! { extern "C" fn($($Arg),+ , ...) -> Ret, $($Arg),+ }
1733 fnptr_impls_safety_abi! { unsafe extern "Rust" fn($($Arg),+) -> Ret, $($Arg),+ }
1734 fnptr_impls_safety_abi! { unsafe extern "C" fn($($Arg),+) -> Ret, $($Arg),+ }
1735 fnptr_impls_safety_abi! { unsafe extern "C" fn($($Arg),+ , ...) -> Ret, $($Arg),+ }
1738 // No variadic functions with 0 parameters
1739 fnptr_impls_safety_abi! { extern "Rust" fn() -> Ret, }
1740 fnptr_impls_safety_abi! { extern "C" fn() -> Ret, }
1741 fnptr_impls_safety_abi! { unsafe extern "Rust" fn() -> Ret, }
1742 fnptr_impls_safety_abi! { unsafe extern "C" fn() -> Ret, }
1746 fnptr_impls_args! {}
1747 fnptr_impls_args! { A }
1748 fnptr_impls_args! { A, B }
1749 fnptr_impls_args! { A, B, C }
1750 fnptr_impls_args! { A, B, C, D }
1751 fnptr_impls_args! { A, B, C, D, E }
1752 fnptr_impls_args! { A, B, C, D, E, F }
1753 fnptr_impls_args! { A, B, C, D, E, F, G }
1754 fnptr_impls_args! { A, B, C, D, E, F, G, H }
1755 fnptr_impls_args! { A, B, C, D, E, F, G, H, I }
1756 fnptr_impls_args! { A, B, C, D, E, F, G, H, I, J }
1757 fnptr_impls_args! { A, B, C, D, E, F, G, H, I, J, K }
1758 fnptr_impls_args! { A, B, C, D, E, F, G, H, I, J, K, L }
1760 /// Create a `const` raw pointer to a place, without creating an intermediate reference.
1762 /// Creating a reference with `&`/`&mut` is only allowed if the pointer is properly aligned
1763 /// and points to initialized data. For cases where those requirements do not hold,
1764 /// raw pointers should be used instead. However, `&expr as *const _` creates a reference
1765 /// before casting it to a raw pointer, and that reference is subject to the same rules
1766 /// as all other references. This macro can create a raw pointer *without* creating
1767 /// a reference first.
1769 /// Note, however, that the `expr` in `addr_of!(expr)` is still subject to all
1770 /// the usual rules. In particular, `addr_of!(*ptr::null())` is Undefined
1771 /// Behavior because it dereferences a null pointer.
1784 /// let packed = Packed { f1: 1, f2: 2 };
1785 /// // `&packed.f2` would create an unaligned reference, and thus be Undefined Behavior!
1786 /// let raw_f2 = ptr::addr_of!(packed.f2);
1787 /// assert_eq!(unsafe { raw_f2.read_unaligned() }, 2);
1790 /// See [`addr_of_mut`] for how to create a pointer to unininitialized data.
1791 /// Doing that with `addr_of` would not make much sense since one could only
1792 /// read the data, and that would be Undefined Behavior.
1793 #[stable(feature = "raw_ref_macros", since = "1.51.0")]
1794 #[rustc_macro_transparency = "semitransparent"]
1795 #[allow_internal_unstable(raw_ref_op)]
1796 pub macro addr_of($place:expr) {
1800 /// Create a `mut` raw pointer to a place, without creating an intermediate reference.
1802 /// Creating a reference with `&`/`&mut` is only allowed if the pointer is properly aligned
1803 /// and points to initialized data. For cases where those requirements do not hold,
1804 /// raw pointers should be used instead. However, `&mut expr as *mut _` creates a reference
1805 /// before casting it to a raw pointer, and that reference is subject to the same rules
1806 /// as all other references. This macro can create a raw pointer *without* creating
1807 /// a reference first.
1809 /// Note, however, that the `expr` in `addr_of_mut!(expr)` is still subject to all
1810 /// the usual rules. In particular, `addr_of_mut!(*ptr::null_mut())` is Undefined
1811 /// Behavior because it dereferences a null pointer.
1815 /// **Creating a pointer to unaligned data:**
1826 /// let mut packed = Packed { f1: 1, f2: 2 };
1827 /// // `&mut packed.f2` would create an unaligned reference, and thus be Undefined Behavior!
1828 /// let raw_f2 = ptr::addr_of_mut!(packed.f2);
1829 /// unsafe { raw_f2.write_unaligned(42); }
1830 /// assert_eq!({packed.f2}, 42); // `{...}` forces copying the field instead of creating a reference.
1833 /// **Creating a pointer to uninitialized data:**
1836 /// use std::{ptr, mem::MaybeUninit};
1842 /// let mut uninit = MaybeUninit::<Demo>::uninit();
1843 /// // `&uninit.as_mut().field` would create a reference to an uninitialized `bool`,
1844 /// // and thus be Undefined Behavior!
1845 /// let f1_ptr = unsafe { ptr::addr_of_mut!((*uninit.as_mut_ptr()).field) };
1846 /// unsafe { f1_ptr.write(true); }
1847 /// let init = unsafe { uninit.assume_init() };
1849 #[stable(feature = "raw_ref_macros", since = "1.51.0")]
1850 #[rustc_macro_transparency = "semitransparent"]
1851 #[allow_internal_unstable(raw_ref_op)]
1852 pub macro addr_of_mut($place:expr) {