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 disambiguate 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 lifetime (of 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 (e.g. "data" vs "code" in WASM).
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. In other words, pointer-integer-pointer
174 //! roundtrips are not possible (in the sense that the resulting pointer is not dereferencable).
176 //! The key insight to making this model *at all* viable is the [`with_addr`][] method:
179 //! /// Creates a new pointer with the given address.
181 //! /// This performs the same operation as an `addr as ptr` cast, but copies
182 //! /// the *address-space* and *provenance* of `self` to the new pointer.
183 //! /// This allows us to dynamically preserve and propagate this important
184 //! /// information in a way that is otherwise impossible with a unary cast.
186 //! /// This is equivalent to using `wrapping_offset` to offset `self` to the
187 //! /// given address, and therefore has all the same capabilities and restrictions.
188 //! pub fn with_addr(self, addr: usize) -> Self;
191 //! So you're still able to drop down to the address representation and do whatever
192 //! clever bit tricks you want *as long as* you're able to keep around a pointer
193 //! into the allocation you care about that can "reconstitute" the other parts of the pointer.
194 //! Usually this is very easy, because you only are taking a pointer, messing with the address,
195 //! and then immediately converting back to a pointer. To make this use case more ergonomic,
196 //! we provide the [`map_addr`][] method.
198 //! To help make it clear that code is "following" Strict Provenance semantics, we also provide an
199 //! [`addr`][] method which promises that the returned address is not part of a
200 //! pointer-usize-pointer roundtrip. In the future we may provide a lint for pointer<->integer
201 //! casts to help you audit if your code conforms to strict provenance.
204 //! ## Using Strict Provenance
206 //! Most code needs no changes to conform to strict provenance, as the only really concerning
207 //! operation that *wasn't* obviously already Undefined Behaviour is casts from usize to a
208 //! pointer. For code which *does* cast a usize to a pointer, the scope of the change depends
209 //! on exactly what you're doing.
211 //! In general you just need to make sure that if you want to convert a usize address to a
212 //! pointer and then use that pointer to read/write memory, you need to keep around a pointer
213 //! that has sufficient provenance to perform that read/write itself. In this way all of your
214 //! casts from an address to a pointer are essentially just applying offsets/indexing.
216 //! This is generally trivial to do for simple cases like tagged pointers *as long as you
217 //! represent the tagged pointer as an actual pointer and not a usize*. For instance:
220 //! #![feature(strict_provenance)]
223 //! // A flag we want to pack into our pointer
224 //! static HAS_DATA: usize = 0x1;
225 //! static FLAG_MASK: usize = !HAS_DATA;
227 //! // Our value, which must have enough alignment to have spare least-significant-bits.
228 //! let my_precious_data: u32 = 17;
229 //! assert!(core::mem::align_of::<u32>() > 1);
231 //! // Create a tagged pointer
232 //! let ptr = &my_precious_data as *const u32;
233 //! let tagged = ptr.map_addr(|addr| addr | HAS_DATA);
235 //! // Check the flag:
236 //! if tagged.addr() & HAS_DATA != 0 {
237 //! // Untag and read the pointer
238 //! let data = *tagged.map_addr(|addr| addr & FLAG_MASK);
239 //! assert_eq!(data, 17);
246 //! (Yes, if you've been using AtomicUsize for pointers in concurrent datastructures, you should
247 //! be using AtomicPtr instead. If that messes up the way you atomically manipulate pointers,
248 //! we would like to know why, and what needs to be done to fix it.)
250 //! Something more complicated and just generally *evil* like an XOR-List requires more significant
251 //! changes like allocating all nodes in a pre-allocated Vec or Arena and using a pointer
252 //! to the whole allocation to reconstitute the XORed addresses.
254 //! Situations where a valid pointer *must* be created from just an address, such as baremetal code
255 //! accessing a memory-mapped interface at a fixed address, are an open question on how to support.
256 //! These situations *will* still be allowed, but we might require some kind of "I know what I'm
257 //! doing" annotation to explain the situation to the compiler. It's also possible they need no
258 //! special attention at all, because they're generally accessing memory outside the scope of
259 //! "the abstract machine", or already using "I know what I'm doing" annotations like "volatile".
261 //! Under [Strict Provenance] it is Undefined Behaviour to:
263 //! * Access memory through a pointer that does not have provenance over that memory.
265 //! * [`offset`] a pointer to or from an address it doesn't have provenance over.
266 //! This means it's always UB to offset a pointer derived from something deallocated,
267 //! even if the offset is 0. Note that a pointer "one past the end" of its provenance
268 //! is not actually outside its provenance, it just has 0 bytes it can load/store.
270 //! But it *is* still sound to:
272 //! * Create an invalid pointer from just an address (see [`ptr::invalid`][]). This can
273 //! be used for sentinel values like `null` *or* to represent a tagged pointer that will
274 //! never be dereferencable. In general, it is always sound for an integer to pretend
275 //! to be a pointer "for fun" as long as you don't use operations on it which require
276 //! it to be valid (offset, read, write, etc).
278 //! * Forge an allocation of size zero at any sufficiently aligned non-null address.
279 //! i.e. the usual "ZSTs are fake, do what you want" rules apply *but* this only applies
280 //! for actual forgery (integers cast to pointers). If you borrow some struct's field
281 //! that *happens* to be zero-sized, the resulting pointer will have provenance tied to
282 //! that allocation and it will still get invalidated if the allocation gets deallocated.
283 //! In the future we may introduce an API to make such a forged allocation explicit.
285 //! * [`wrapping_offset`][] a pointer outside its provenance. This includes invalid pointers
286 //! which have "no" provenance. Unfortunately there may be practical limits on this for a
287 //! particular platform, and it's an open question as to how to specify this (if at all).
288 //! Notably, [CHERI][] relies on a compression scheme that can't handle a
289 //! pointer getting offset "too far" out of bounds. If this happens, the address
290 //! returned by `addr` will be the value you expect, but the provenance will get invalidated
291 //! and using it to read/write will fault. The details of this are architecture-specific
292 //! and based on alignment, but the buffer on either side of the pointer's range is pretty
293 //! generous (think kilobytes, not bytes).
295 //! * Compare arbitrary pointers by address. Addresses *are* just integers and so there is
296 //! always a coherent answer, even if the pointers are invalid or from different
297 //! address-spaces/provenances. Of course, comparing addresses from different address-spaces
298 //! is generally going to be *meaningless*, but so is comparing Kilograms to Meters, and Rust
299 //! doesn't prevent that either. Similarly, if you get "lucky" and notice that a pointer
300 //! one-past-the-end is the "same" address as the start of an unrelated allocation, anything
301 //! you do with that fact is *probably* going to be gibberish. The scope of that gibberish
302 //! is kept under control by the fact that the two pointers *still* aren't allowed to access
303 //! the other's allocation (bytes), because they still have different provenance.
305 //! * Perform pointer tagging tricks. This falls out of [`wrapping_offset`] but is worth
306 //! mentioning in more detail because of the limitations of [CHERI][]. Low-bit tagging
307 //! is very robust, and often doesn't even go out of bounds because types ensure
308 //! size >= align (and over-aligning actually gives CHERI more flexibility). Anything
309 //! more complex than this rapidly enters "extremely platform-specific" territory as
310 //! certain things may or may not be allowed based on specific supported operations.
311 //! For instance, ARM explicitly supports high-bit tagging, and so CHERI on ARM inherits
312 //! that and should support it.
314 //! ## Pointer-usize-pointer roundtrips and 'exposed' provenance
316 //! **This section is *non-normative* and is part of the [Strict Provenance] experiment.**
318 //! As discussed above, pointer-usize-pointer roundtrips are not possible under [Strict Provenance].
319 //! However, there exists legacy Rust code that is full of such roundtrips, and legacy platform APIs
320 //! regularly assume that `usize` can capture all the information that makes up a pointer. There
321 //! also might be code that cannot be ported to Strict Provenance (which is something we would [like
322 //! to hear about][Strict Provenance]).
324 //! For situations like this, there is a fallback plan, a way to 'opt out' of Strict Provenance.
325 //! However, note that this makes your code a lot harder to specify, and the code will not work
326 //! (well) with tools like [Miri] and [CHERI].
328 //! This fallback plan is provided by the [`expose_addr`] and [`from_exposed_addr`] methods (which
329 //! are equivalent to `as` casts between pointers and integers). [`expose_addr`] is a lot like
330 //! [`addr`], but additionally adds the provenance of the pointer to a global list of 'exposed'
331 //! provenances. (This list is purely conceptual, it exists for the purpose of specifying Rust but
332 //! is not materialized in actual executions, except in tools like [Miri].) [`from_exposed_addr`]
333 //! can be used to construct a pointer with one of these previously 'exposed' provenances.
334 //! [`from_exposed_addr`] takes only `addr: usize` as arguments, so unlike in [`with_addr`] there is
335 //! no indication of what the correct provenance for the returned pointer is -- and that is exactly
336 //! what makes pointer-usize-pointer roundtrips so tricky to rigorously specify! There is no
337 //! algorithm that decides which provenance will be used. You can think of this as "guessing" the
338 //! right provenance, and the guess will be "maximally in your favor", in the sense that if there is
339 //! any way to avoid undefined behavior, then that is the guess that will be taken. However, if
340 //! there is *no* previously 'exposed' provenance that justifies the way the returned pointer will
341 //! be used, the program has undefined behavior.
343 //! Using [`expose_addr`] or [`from_exposed_addr`] (or the equivalent `as` casts) means that code is
344 //! *not* following Strict Provenance rules. The goal of the Strict Provenance experiment is to
345 //! determine whether it is possible to use Rust without [`expose_addr`] and [`from_exposed_addr`].
346 //! If this is successful, it would be a major win for avoiding specification complexity and to
347 //! facilitate adoption of tools like [CHERI] and [Miri] that can be a big help in increasing the
348 //! confidence in (unsafe) Rust code.
350 //! [aliasing]: ../../nomicon/aliasing.html
351 //! [book]: ../../book/ch19-01-unsafe-rust.html#dereferencing-a-raw-pointer
352 //! [ub]: ../../reference/behavior-considered-undefined.html
353 //! [zst]: ../../nomicon/exotic-sizes.html#zero-sized-types-zsts
354 //! [atomic operations]: crate::sync::atomic
355 //! [`offset`]: pointer::offset
356 //! [`wrapping_offset`]: pointer::wrapping_offset
357 //! [`with_addr`]: pointer::with_addr
358 //! [`map_addr`]: pointer::map_addr
359 //! [`addr`]: pointer::addr
360 //! [`ptr::invalid`]: core::ptr::invalid
361 //! [`expose_addr`]: pointer::expose_addr
362 //! [`from_exposed_addr`]: from_exposed_addr
363 //! [Miri]: https://github.com/rust-lang/miri
364 //! [CHERI]: https://www.cl.cam.ac.uk/research/security/ctsrd/cheri/
365 //! [Strict Provenance]: https://github.com/rust-lang/rust/issues/95228
366 //! [Stacked Borrows]: https://plv.mpi-sws.org/rustbelt/stacked-borrows/
368 #![stable(feature = "rust1", since = "1.0.0")]
370 use crate::cmp::Ordering;
373 use crate::intrinsics::{
374 self, assert_unsafe_precondition, is_aligned_and_not_null, is_nonoverlapping,
377 use crate::mem::{self, MaybeUninit};
379 #[stable(feature = "rust1", since = "1.0.0")]
381 pub use crate::intrinsics::copy_nonoverlapping;
383 #[stable(feature = "rust1", since = "1.0.0")]
385 pub use crate::intrinsics::copy;
387 #[stable(feature = "rust1", since = "1.0.0")]
389 pub use crate::intrinsics::write_bytes;
392 pub(crate) use metadata::PtrRepr;
393 #[unstable(feature = "ptr_metadata", issue = "81513")]
394 pub use metadata::{from_raw_parts, from_raw_parts_mut, metadata, DynMetadata, Pointee, Thin};
397 #[stable(feature = "nonnull", since = "1.25.0")]
398 pub use non_null::NonNull;
401 #[unstable(feature = "ptr_internals", issue = "none")]
402 pub use unique::Unique;
407 /// Executes the destructor (if any) of the pointed-to value.
409 /// This is semantically equivalent to calling [`ptr::read`] and discarding
410 /// the result, but has the following advantages:
412 /// * It is *required* to use `drop_in_place` to drop unsized types like
413 /// trait objects, because they can't be read out onto the stack and
414 /// dropped normally.
416 /// * It is friendlier to the optimizer to do this over [`ptr::read`] when
417 /// dropping manually allocated memory (e.g., in the implementations of
418 /// `Box`/`Rc`/`Vec`), as the compiler doesn't need to prove that it's
419 /// sound to elide the copy.
421 /// * It can be used to drop [pinned] data when `T` is not `repr(packed)`
422 /// (pinned data must not be moved before it is dropped).
424 /// Unaligned values cannot be dropped in place, they must be copied to an aligned
425 /// location first using [`ptr::read_unaligned`]. For packed structs, this move is
426 /// done automatically by the compiler. This means the fields of packed structs
427 /// are not dropped in-place.
429 /// [`ptr::read`]: self::read
430 /// [`ptr::read_unaligned`]: self::read_unaligned
431 /// [pinned]: crate::pin
435 /// Behavior is undefined if any of the following conditions are violated:
437 /// * `to_drop` must be [valid] for both reads and writes.
439 /// * `to_drop` must be properly aligned.
441 /// * The value `to_drop` points to must be valid for dropping, which may mean it must uphold
442 /// additional invariants - this is type-dependent.
444 /// Additionally, if `T` is not [`Copy`], using the pointed-to value after
445 /// calling `drop_in_place` can cause undefined behavior. Note that `*to_drop =
446 /// foo` counts as a use because it will cause the value to be dropped
447 /// again. [`write()`] can be used to overwrite data without causing it to be
450 /// Note that even if `T` has size `0`, the pointer must be non-null and properly aligned.
452 /// [valid]: self#safety
456 /// Manually remove the last item from a vector:
462 /// let last = Rc::new(1);
463 /// let weak = Rc::downgrade(&last);
465 /// let mut v = vec![Rc::new(0), last];
468 /// // Get a raw pointer to the last element in `v`.
469 /// let ptr = &mut v[1] as *mut _;
470 /// // Shorten `v` to prevent the last item from being dropped. We do that first,
471 /// // to prevent issues if the `drop_in_place` below panics.
473 /// // Without a call `drop_in_place`, the last item would never be dropped,
474 /// // and the memory it manages would be leaked.
475 /// ptr::drop_in_place(ptr);
478 /// assert_eq!(v, &[0.into()]);
480 /// // Ensure that the last item was dropped.
481 /// assert!(weak.upgrade().is_none());
483 #[stable(feature = "drop_in_place", since = "1.8.0")]
484 #[lang = "drop_in_place"]
485 #[allow(unconditional_recursion)]
486 pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
487 // Code here does not matter - this is replaced by the
488 // real drop glue by the compiler.
490 // SAFETY: see comment above
491 unsafe { drop_in_place(to_drop) }
494 /// Creates a null raw pointer.
501 /// let p: *const i32 = ptr::null();
502 /// assert!(p.is_null());
506 #[stable(feature = "rust1", since = "1.0.0")]
508 #[rustc_const_stable(feature = "const_ptr_null", since = "1.24.0")]
509 #[rustc_diagnostic_item = "ptr_null"]
510 pub const fn null<T>() -> *const T {
514 /// Creates a null mutable raw pointer.
521 /// let p: *mut i32 = ptr::null_mut();
522 /// assert!(p.is_null());
526 #[stable(feature = "rust1", since = "1.0.0")]
528 #[rustc_const_stable(feature = "const_ptr_null", since = "1.24.0")]
529 #[rustc_diagnostic_item = "ptr_null_mut"]
530 pub const fn null_mut<T>() -> *mut T {
534 /// Creates an invalid pointer with the given address.
536 /// This is *currently* equivalent to `addr as *const T` but it expresses the intended semantic
537 /// more clearly, and may become important under future memory models.
539 /// The module's top-level documentation discusses the precise meaning of an "invalid"
540 /// pointer but essentially this expresses that the pointer is not associated
541 /// with any actual allocation and is little more than a usize address in disguise.
543 /// This pointer will have no provenance associated with it and is therefore
544 /// UB to read/write/offset. This mostly exists to facilitate things
545 /// like ptr::null and NonNull::dangling which make invalid pointers.
547 /// (Standard "Zero-Sized-Types get to cheat and lie" caveats apply, although it
548 /// may be desirable to give them their own API just to make that 100% clear.)
550 /// This API and its claimed semantics are part of the Strict Provenance experiment,
551 /// see the [module documentation][crate::ptr] for details.
554 #[rustc_const_stable(feature = "stable_things_using_strict_provenance", since = "1.61.0")]
555 #[unstable(feature = "strict_provenance", issue = "95228")]
556 pub const fn invalid<T>(addr: usize) -> *const T {
557 // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
561 /// Creates an invalid mutable pointer with the given address.
563 /// This is *currently* equivalent to `addr as *mut T` but it expresses the intended semantic
564 /// more clearly, and may become important under future memory models.
566 /// The module's top-level documentation discusses the precise meaning of an "invalid"
567 /// pointer but essentially this expresses that the pointer is not associated
568 /// with any actual allocation and is little more than a usize address in disguise.
570 /// This pointer will have no provenance associated with it and is therefore
571 /// UB to read/write/offset. This mostly exists to facilitate things
572 /// like ptr::null and NonNull::dangling which make invalid pointers.
574 /// (Standard "Zero-Sized-Types get to cheat and lie" caveats apply, although it
575 /// may be desirable to give them their own API just to make that 100% clear.)
577 /// This API and its claimed semantics are part of the Strict Provenance experiment,
578 /// see the [module documentation][crate::ptr] for details.
581 #[rustc_const_stable(feature = "stable_things_using_strict_provenance", since = "1.61.0")]
582 #[unstable(feature = "strict_provenance", issue = "95228")]
583 pub const fn invalid_mut<T>(addr: usize) -> *mut T {
584 // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
588 /// Convert an address back to a pointer, picking up a previously 'exposed' provenance.
590 /// This is equivalent to `addr as *const T`. The provenance of the returned pointer is that of *any*
591 /// pointer that was previously passed to [`expose_addr`][pointer::expose_addr] or a `ptr as usize`
592 /// cast. If there is no previously 'exposed' provenance that justifies the way this pointer will be
593 /// used, the program has undefined behavior. Note that there is no algorithm that decides which
594 /// provenance will be used. You can think of this as "guessing" the right provenance, and the guess
595 /// will be "maximally in your favor", in the sense that if there is any way to avoid undefined
596 /// behavior, then that is the guess that will be taken.
598 /// On platforms with multiple address spaces, it is your responsibility to ensure that the
599 /// address makes sense in the address space that this pointer will be used with.
601 /// Using this method means that code is *not* following strict provenance rules. "Guessing" a
602 /// suitable provenance complicates specification and reasoning and may not be supported by
603 /// tools that help you to stay conformant with the Rust memory model, so it is recommended to
604 /// use [`with_addr`][pointer::with_addr] wherever possible.
606 /// On most platforms this will produce a value with the same bytes as the address. Platforms
607 /// which need to store additional information in a pointer may not support this operation,
608 /// since it is generally not possible to actually *compute* which provenance the returned
609 /// pointer has to pick up.
611 /// This API and its claimed semantics are part of the Strict Provenance experiment, see the
612 /// [module documentation][crate::ptr] for details.
615 #[unstable(feature = "strict_provenance", issue = "95228")]
616 pub fn from_exposed_addr<T>(addr: usize) -> *const T
620 // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
624 /// Convert an address back to a mutable pointer, picking up a previously 'exposed' provenance.
626 /// This is equivalent to `addr as *mut T`. The provenance of the returned pointer is that of *any*
627 /// pointer that was previously passed to [`expose_addr`][pointer::expose_addr] or a `ptr as usize`
628 /// cast. If there is no previously 'exposed' provenance that justifies the way this pointer will be
629 /// used, the program has undefined behavior. Note that there is no algorithm that decides which
630 /// provenance will be used. You can think of this as "guessing" the right provenance, and the guess
631 /// will be "maximally in your favor", in the sense that if there is any way to avoid undefined
632 /// behavior, then that is the guess that will be taken.
634 /// On platforms with multiple address spaces, it is your responsibility to ensure that the
635 /// address makes sense in the address space that this pointer will be used with.
637 /// Using this method means that code is *not* following strict provenance rules. "Guessing" a
638 /// suitable provenance complicates specification and reasoning and may not be supported by
639 /// tools that help you to stay conformant with the Rust memory model, so it is recommended to
640 /// use [`with_addr`][pointer::with_addr] wherever possible.
642 /// On most platforms this will produce a value with the same bytes as the address. Platforms
643 /// which need to store additional information in a pointer may not support this operation,
644 /// since it is generally not possible to actually *compute* which provenance the returned
645 /// pointer has to pick up.
647 /// This API and its claimed semantics are part of the Strict Provenance experiment, see the
648 /// [module documentation][crate::ptr] for details.
651 #[unstable(feature = "strict_provenance", issue = "95228")]
652 pub fn from_exposed_addr_mut<T>(addr: usize) -> *mut T
656 // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
660 /// Forms a raw slice from a pointer and a length.
662 /// The `len` argument is the number of **elements**, not the number of bytes.
664 /// This function is safe, but actually using the return value is unsafe.
665 /// See the documentation of [`slice::from_raw_parts`] for slice safety requirements.
667 /// [`slice::from_raw_parts`]: crate::slice::from_raw_parts
674 /// // create a slice pointer when starting out with a pointer to the first element
675 /// let x = [5, 6, 7];
676 /// let raw_pointer = x.as_ptr();
677 /// let slice = ptr::slice_from_raw_parts(raw_pointer, 3);
678 /// assert_eq!(unsafe { &*slice }[2], 7);
681 #[stable(feature = "slice_from_raw_parts", since = "1.42.0")]
682 #[rustc_const_unstable(feature = "const_slice_from_raw_parts", issue = "67456")]
683 pub const fn slice_from_raw_parts<T>(data: *const T, len: usize) -> *const [T] {
684 from_raw_parts(data.cast(), len)
687 /// Performs the same functionality as [`slice_from_raw_parts`], except that a
688 /// raw mutable slice is returned, as opposed to a raw immutable slice.
690 /// See the documentation of [`slice_from_raw_parts`] for more details.
692 /// This function is safe, but actually using the return value is unsafe.
693 /// See the documentation of [`slice::from_raw_parts_mut`] for slice safety requirements.
695 /// [`slice::from_raw_parts_mut`]: crate::slice::from_raw_parts_mut
702 /// let x = &mut [5, 6, 7];
703 /// let raw_pointer = x.as_mut_ptr();
704 /// let slice = ptr::slice_from_raw_parts_mut(raw_pointer, 3);
707 /// (*slice)[2] = 99; // assign a value at an index in the slice
710 /// assert_eq!(unsafe { &*slice }[2], 99);
713 #[stable(feature = "slice_from_raw_parts", since = "1.42.0")]
714 #[rustc_const_unstable(feature = "const_slice_from_raw_parts", issue = "67456")]
715 pub const fn slice_from_raw_parts_mut<T>(data: *mut T, len: usize) -> *mut [T] {
716 from_raw_parts_mut(data.cast(), len)
719 /// Swaps the values at two mutable locations of the same type, without
720 /// deinitializing either.
722 /// But for the following two exceptions, this function is semantically
723 /// equivalent to [`mem::swap`]:
725 /// * It operates on raw pointers instead of references. When references are
726 /// available, [`mem::swap`] should be preferred.
728 /// * The two pointed-to values may overlap. If the values do overlap, then the
729 /// overlapping region of memory from `x` will be used. This is demonstrated
730 /// in the second example below.
734 /// Behavior is undefined if any of the following conditions are violated:
736 /// * Both `x` and `y` must be [valid] for both reads and writes.
738 /// * Both `x` and `y` must be properly aligned.
740 /// Note that even if `T` has size `0`, the pointers must be non-null and properly aligned.
742 /// [valid]: self#safety
746 /// Swapping two non-overlapping regions:
751 /// let mut array = [0, 1, 2, 3];
753 /// let (x, y) = array.split_at_mut(2);
754 /// let x = x.as_mut_ptr().cast::<[u32; 2]>(); // this is `array[0..2]`
755 /// let y = y.as_mut_ptr().cast::<[u32; 2]>(); // this is `array[2..4]`
759 /// assert_eq!([2, 3, 0, 1], array);
763 /// Swapping two overlapping regions:
768 /// let mut array: [i32; 4] = [0, 1, 2, 3];
770 /// let array_ptr: *mut i32 = array.as_mut_ptr();
772 /// let x = array_ptr as *mut [i32; 3]; // this is `array[0..3]`
773 /// let y = unsafe { array_ptr.add(1) } as *mut [i32; 3]; // this is `array[1..4]`
777 /// // The indices `1..3` of the slice overlap between `x` and `y`.
778 /// // Reasonable results would be for to them be `[2, 3]`, so that indices `0..3` are
779 /// // `[1, 2, 3]` (matching `y` before the `swap`); or for them to be `[0, 1]`
780 /// // so that indices `1..4` are `[0, 1, 2]` (matching `x` before the `swap`).
781 /// // This implementation is defined to make the latter choice.
782 /// assert_eq!([1, 0, 1, 2], array);
786 #[stable(feature = "rust1", since = "1.0.0")]
787 #[rustc_const_unstable(feature = "const_swap", issue = "83163")]
788 pub const unsafe fn swap<T>(x: *mut T, y: *mut T) {
789 // Give ourselves some scratch space to work with.
790 // We do not have to worry about drops: `MaybeUninit` does nothing when dropped.
791 let mut tmp = MaybeUninit::<T>::uninit();
794 // SAFETY: the caller must guarantee that `x` and `y` are
795 // valid for writes and properly aligned. `tmp` cannot be
796 // overlapping either `x` or `y` because `tmp` was just allocated
797 // on the stack as a separate allocated object.
799 copy_nonoverlapping(x, tmp.as_mut_ptr(), 1);
800 copy(y, x, 1); // `x` and `y` may overlap
801 copy_nonoverlapping(tmp.as_ptr(), y, 1);
805 /// Swaps `count * size_of::<T>()` bytes between the two regions of memory
806 /// beginning at `x` and `y`. The two regions must *not* overlap.
810 /// Behavior is undefined if any of the following conditions are violated:
812 /// * Both `x` and `y` must be [valid] for both reads and writes of `count *
813 /// size_of::<T>()` bytes.
815 /// * Both `x` and `y` must be properly aligned.
817 /// * The region of memory beginning at `x` with a size of `count *
818 /// size_of::<T>()` bytes must *not* overlap with the region of memory
819 /// beginning at `y` with the same size.
821 /// Note that even if the effectively copied size (`count * size_of::<T>()`) is `0`,
822 /// the pointers must be non-null and properly aligned.
824 /// [valid]: self#safety
833 /// let mut x = [1, 2, 3, 4];
834 /// let mut y = [7, 8, 9];
837 /// ptr::swap_nonoverlapping(x.as_mut_ptr(), y.as_mut_ptr(), 2);
840 /// assert_eq!(x, [7, 8, 3, 4]);
841 /// assert_eq!(y, [1, 2, 9]);
844 #[stable(feature = "swap_nonoverlapping", since = "1.27.0")]
845 #[rustc_const_unstable(feature = "const_swap", issue = "83163")]
846 pub const unsafe fn swap_nonoverlapping<T>(x: *mut T, y: *mut T, count: usize) {
848 macro_rules! attempt_swap_as_chunks {
850 if mem::align_of::<T>() >= mem::align_of::<$ChunkTy>()
851 && mem::size_of::<T>() % mem::size_of::<$ChunkTy>() == 0
853 let x: *mut MaybeUninit<$ChunkTy> = x.cast();
854 let y: *mut MaybeUninit<$ChunkTy> = y.cast();
855 let count = count * (mem::size_of::<T>() / mem::size_of::<$ChunkTy>());
856 // SAFETY: these are the same bytes that the caller promised were
857 // ok, just typed as `MaybeUninit<ChunkTy>`s instead of as `T`s.
858 // The `if` condition above ensures that we're not violating
859 // alignment requirements, and that the division is exact so
860 // that we don't lose any bytes off the end.
861 return unsafe { swap_nonoverlapping_simple(x, y, count) };
866 // SAFETY: the caller must guarantee that `x` and `y` are
867 // valid for writes and properly aligned.
869 assert_unsafe_precondition!(
870 is_aligned_and_not_null(x)
871 && is_aligned_and_not_null(y)
872 && is_nonoverlapping(x, y, count)
876 // NOTE(scottmcm) Miri is disabled here as reading in smaller units is a
877 // pessimization for it. Also, if the type contains any unaligned pointers,
878 // copying those over multiple reads is difficult to support.
881 // Split up the slice into small power-of-two-sized chunks that LLVM is able
882 // to vectorize (unless it's a special type with more-than-pointer alignment,
883 // because we don't want to pessimize things like slices of SIMD vectors.)
884 if mem::align_of::<T>() <= mem::size_of::<usize>()
885 && (!mem::size_of::<T>().is_power_of_two()
886 || mem::size_of::<T>() > mem::size_of::<usize>() * 2)
888 attempt_swap_as_chunks!(usize);
889 attempt_swap_as_chunks!(u8);
893 // SAFETY: Same preconditions as this function
894 unsafe { swap_nonoverlapping_simple(x, y, count) }
897 /// Same behaviour and safety conditions as [`swap_nonoverlapping`]
899 /// LLVM can vectorize this (at least it can for the power-of-two-sized types
900 /// `swap_nonoverlapping` tries to use) so no need to manually SIMD it.
902 #[rustc_const_unstable(feature = "const_swap", issue = "83163")]
903 const unsafe fn swap_nonoverlapping_simple<T>(x: *mut T, y: *mut T, count: usize) {
907 // SAFETY: By precondition, `i` is in-bounds because it's below `n`
908 unsafe { &mut *x.add(i) };
910 // SAFETY: By precondition, `i` is in-bounds because it's below `n`
911 // and it's distinct from `x` since the ranges are non-overlapping
912 unsafe { &mut *y.add(i) };
913 mem::swap_simple(x, y);
919 /// Moves `src` into the pointed `dst`, returning the previous `dst` value.
921 /// Neither value is dropped.
923 /// This function is semantically equivalent to [`mem::replace`] except that it
924 /// operates on raw pointers instead of references. When references are
925 /// available, [`mem::replace`] should be preferred.
929 /// Behavior is undefined if any of the following conditions are violated:
931 /// * `dst` must be [valid] for both reads and writes.
933 /// * `dst` must be properly aligned.
935 /// * `dst` must point to a properly initialized value of type `T`.
937 /// Note that even if `T` has size `0`, the pointer must be non-null and properly aligned.
939 /// [valid]: self#safety
946 /// let mut rust = vec!['b', 'u', 's', 't'];
948 /// // `mem::replace` would have the same effect without requiring the unsafe
951 /// ptr::replace(&mut rust[0], 'r')
954 /// assert_eq!(b, 'b');
955 /// assert_eq!(rust, &['r', 'u', 's', 't']);
958 #[stable(feature = "rust1", since = "1.0.0")]
959 #[rustc_const_unstable(feature = "const_replace", issue = "83164")]
960 pub const unsafe fn replace<T>(dst: *mut T, mut src: T) -> T {
961 // SAFETY: the caller must guarantee that `dst` is valid to be
962 // cast to a mutable reference (valid for writes, aligned, initialized),
963 // and cannot overlap `src` since `dst` must point to a distinct
966 assert_unsafe_precondition!(is_aligned_and_not_null(dst));
967 mem::swap(&mut *dst, &mut src); // cannot overlap
972 /// Reads the value from `src` without moving it. This leaves the
973 /// memory in `src` unchanged.
977 /// Behavior is undefined if any of the following conditions are violated:
979 /// * `src` must be [valid] for reads.
981 /// * `src` must be properly aligned. Use [`read_unaligned`] if this is not the
984 /// * `src` must point to a properly initialized value of type `T`.
986 /// Note that even if `T` has size `0`, the pointer must be non-null and properly aligned.
994 /// let y = &x as *const i32;
997 /// assert_eq!(std::ptr::read(y), 12);
1001 /// Manually implement [`mem::swap`]:
1006 /// fn swap<T>(a: &mut T, b: &mut T) {
1008 /// // Create a bitwise copy of the value at `a` in `tmp`.
1009 /// let tmp = ptr::read(a);
1011 /// // Exiting at this point (either by explicitly returning or by
1012 /// // calling a function which panics) would cause the value in `tmp` to
1013 /// // be dropped while the same value is still referenced by `a`. This
1014 /// // could trigger undefined behavior if `T` is not `Copy`.
1016 /// // Create a bitwise copy of the value at `b` in `a`.
1017 /// // This is safe because mutable references cannot alias.
1018 /// ptr::copy_nonoverlapping(b, a, 1);
1020 /// // As above, exiting here could trigger undefined behavior because
1021 /// // the same value is referenced by `a` and `b`.
1023 /// // Move `tmp` into `b`.
1024 /// ptr::write(b, tmp);
1026 /// // `tmp` has been moved (`write` takes ownership of its second argument),
1027 /// // so nothing is dropped implicitly here.
1031 /// let mut foo = "foo".to_owned();
1032 /// let mut bar = "bar".to_owned();
1034 /// swap(&mut foo, &mut bar);
1036 /// assert_eq!(foo, "bar");
1037 /// assert_eq!(bar, "foo");
1040 /// ## Ownership of the Returned Value
1042 /// `read` creates a bitwise copy of `T`, regardless of whether `T` is [`Copy`].
1043 /// If `T` is not [`Copy`], using both the returned value and the value at
1044 /// `*src` can violate memory safety. Note that assigning to `*src` counts as a
1045 /// use because it will attempt to drop the value at `*src`.
1047 /// [`write()`] can be used to overwrite data without causing it to be dropped.
1052 /// let mut s = String::from("foo");
1054 /// // `s2` now points to the same underlying memory as `s`.
1055 /// let mut s2: String = ptr::read(&s);
1057 /// assert_eq!(s2, "foo");
1059 /// // Assigning to `s2` causes its original value to be dropped. Beyond
1060 /// // this point, `s` must no longer be used, as the underlying memory has
1062 /// s2 = String::default();
1063 /// assert_eq!(s2, "");
1065 /// // Assigning to `s` would cause the old value to be dropped again,
1066 /// // resulting in undefined behavior.
1067 /// // s = String::from("bar"); // ERROR
1069 /// // `ptr::write` can be used to overwrite a value without dropping it.
1070 /// ptr::write(&mut s, String::from("bar"));
1073 /// assert_eq!(s, "bar");
1076 /// [valid]: self#safety
1078 #[stable(feature = "rust1", since = "1.0.0")]
1079 #[rustc_const_unstable(feature = "const_ptr_read", issue = "80377")]
1080 pub const unsafe fn read<T>(src: *const T) -> T {
1081 // We are calling the intrinsics directly to avoid function calls in the generated code
1082 // as `intrinsics::copy_nonoverlapping` is a wrapper function.
1083 extern "rust-intrinsic" {
1084 #[rustc_const_unstable(feature = "const_intrinsic_copy", issue = "80697")]
1085 fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize);
1088 let mut tmp = MaybeUninit::<T>::uninit();
1089 // SAFETY: the caller must guarantee that `src` is valid for reads.
1090 // `src` cannot overlap `tmp` because `tmp` was just allocated on
1091 // the stack as a separate allocated object.
1093 // Also, since we just wrote a valid value into `tmp`, it is guaranteed
1094 // to be properly initialized.
1096 copy_nonoverlapping(src, tmp.as_mut_ptr(), 1);
1101 /// Reads the value from `src` without moving it. This leaves the
1102 /// memory in `src` unchanged.
1104 /// Unlike [`read`], `read_unaligned` works with unaligned pointers.
1108 /// Behavior is undefined if any of the following conditions are violated:
1110 /// * `src` must be [valid] for reads.
1112 /// * `src` must point to a properly initialized value of type `T`.
1114 /// Like [`read`], `read_unaligned` creates a bitwise copy of `T`, regardless of
1115 /// whether `T` is [`Copy`]. If `T` is not [`Copy`], using both the returned
1116 /// value and the value at `*src` can [violate memory safety][read-ownership].
1118 /// Note that even if `T` has size `0`, the pointer must be non-null.
1120 /// [read-ownership]: read#ownership-of-the-returned-value
1121 /// [valid]: self#safety
1123 /// ## On `packed` structs
1125 /// Attempting to create a raw pointer to an `unaligned` struct field with
1126 /// an expression such as `&packed.unaligned as *const FieldType` creates an
1127 /// intermediate unaligned reference before converting that to a raw pointer.
1128 /// That this reference is temporary and immediately cast is inconsequential
1129 /// as the compiler always expects references to be properly aligned.
1130 /// As a result, using `&packed.unaligned as *const FieldType` causes immediate
1131 /// *undefined behavior* in your program.
1133 /// Instead you must use the [`ptr::addr_of!`](addr_of) macro to
1134 /// create the pointer. You may use that returned pointer together with this
1137 /// An example of what not to do and how this relates to `read_unaligned` is:
1140 /// #[repr(packed, C)]
1146 /// let packed = Packed {
1148 /// unaligned: 0x01020304,
1151 /// // Take the address of a 32-bit integer which is not aligned.
1152 /// // In contrast to `&packed.unaligned as *const _`, this has no undefined behavior.
1153 /// let unaligned = std::ptr::addr_of!(packed.unaligned);
1155 /// let v = unsafe { std::ptr::read_unaligned(unaligned) };
1156 /// assert_eq!(v, 0x01020304);
1159 /// Accessing unaligned fields directly with e.g. `packed.unaligned` is safe however.
1163 /// Read a usize value from a byte buffer:
1168 /// fn read_usize(x: &[u8]) -> usize {
1169 /// assert!(x.len() >= mem::size_of::<usize>());
1171 /// let ptr = x.as_ptr() as *const usize;
1173 /// unsafe { ptr.read_unaligned() }
1177 #[stable(feature = "ptr_unaligned", since = "1.17.0")]
1178 #[rustc_const_unstable(feature = "const_ptr_read", issue = "80377")]
1179 pub const unsafe fn read_unaligned<T>(src: *const T) -> T {
1180 let mut tmp = MaybeUninit::<T>::uninit();
1181 // SAFETY: the caller must guarantee that `src` is valid for reads.
1182 // `src` cannot overlap `tmp` because `tmp` was just allocated on
1183 // the stack as a separate allocated object.
1185 // Also, since we just wrote a valid value into `tmp`, it is guaranteed
1186 // to be properly initialized.
1188 copy_nonoverlapping(src as *const u8, tmp.as_mut_ptr() as *mut u8, mem::size_of::<T>());
1193 /// Overwrites a memory location with the given value without reading or
1194 /// dropping the old value.
1196 /// `write` does not drop the contents of `dst`. This is safe, but it could leak
1197 /// allocations or resources, so care should be taken not to overwrite an object
1198 /// that should be dropped.
1200 /// Additionally, it does not drop `src`. Semantically, `src` is moved into the
1201 /// location pointed to by `dst`.
1203 /// This is appropriate for initializing uninitialized memory, or overwriting
1204 /// memory that has previously been [`read`] from.
1208 /// Behavior is undefined if any of the following conditions are violated:
1210 /// * `dst` must be [valid] for writes.
1212 /// * `dst` must be properly aligned. Use [`write_unaligned`] if this is not the
1215 /// Note that even if `T` has size `0`, the pointer must be non-null and properly aligned.
1217 /// [valid]: self#safety
1225 /// let y = &mut x as *mut i32;
1229 /// std::ptr::write(y, z);
1230 /// assert_eq!(std::ptr::read(y), 12);
1234 /// Manually implement [`mem::swap`]:
1239 /// fn swap<T>(a: &mut T, b: &mut T) {
1241 /// // Create a bitwise copy of the value at `a` in `tmp`.
1242 /// let tmp = ptr::read(a);
1244 /// // Exiting at this point (either by explicitly returning or by
1245 /// // calling a function which panics) would cause the value in `tmp` to
1246 /// // be dropped while the same value is still referenced by `a`. This
1247 /// // could trigger undefined behavior if `T` is not `Copy`.
1249 /// // Create a bitwise copy of the value at `b` in `a`.
1250 /// // This is safe because mutable references cannot alias.
1251 /// ptr::copy_nonoverlapping(b, a, 1);
1253 /// // As above, exiting here could trigger undefined behavior because
1254 /// // the same value is referenced by `a` and `b`.
1256 /// // Move `tmp` into `b`.
1257 /// ptr::write(b, tmp);
1259 /// // `tmp` has been moved (`write` takes ownership of its second argument),
1260 /// // so nothing is dropped implicitly here.
1264 /// let mut foo = "foo".to_owned();
1265 /// let mut bar = "bar".to_owned();
1267 /// swap(&mut foo, &mut bar);
1269 /// assert_eq!(foo, "bar");
1270 /// assert_eq!(bar, "foo");
1273 #[stable(feature = "rust1", since = "1.0.0")]
1274 #[rustc_const_unstable(feature = "const_ptr_write", issue = "86302")]
1275 pub const unsafe fn write<T>(dst: *mut T, src: T) {
1276 // We are calling the intrinsics directly to avoid function calls in the generated code
1277 // as `intrinsics::copy_nonoverlapping` is a wrapper function.
1278 extern "rust-intrinsic" {
1279 #[rustc_const_unstable(feature = "const_intrinsic_copy", issue = "80697")]
1280 fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize);
1283 // SAFETY: the caller must guarantee that `dst` is valid for writes.
1284 // `dst` cannot overlap `src` because the caller has mutable access
1285 // to `dst` while `src` is owned by this function.
1287 copy_nonoverlapping(&src as *const T, dst, 1);
1288 intrinsics::forget(src);
1292 /// Overwrites a memory location with the given value without reading or
1293 /// dropping the old value.
1295 /// Unlike [`write()`], the pointer may be unaligned.
1297 /// `write_unaligned` does not drop the contents of `dst`. This is safe, but it
1298 /// could leak allocations or resources, so care should be taken not to overwrite
1299 /// an object that should be dropped.
1301 /// Additionally, it does not drop `src`. Semantically, `src` is moved into the
1302 /// location pointed to by `dst`.
1304 /// This is appropriate for initializing uninitialized memory, or overwriting
1305 /// memory that has previously been read with [`read_unaligned`].
1309 /// Behavior is undefined if any of the following conditions are violated:
1311 /// * `dst` must be [valid] for writes.
1313 /// Note that even if `T` has size `0`, the pointer must be non-null.
1315 /// [valid]: self#safety
1317 /// ## On `packed` structs
1319 /// Attempting to create a raw pointer to an `unaligned` struct field with
1320 /// an expression such as `&packed.unaligned as *const FieldType` creates an
1321 /// intermediate unaligned reference before converting that to a raw pointer.
1322 /// That this reference is temporary and immediately cast is inconsequential
1323 /// as the compiler always expects references to be properly aligned.
1324 /// As a result, using `&packed.unaligned as *const FieldType` causes immediate
1325 /// *undefined behavior* in your program.
1327 /// Instead you must use the [`ptr::addr_of_mut!`](addr_of_mut)
1328 /// macro to create the pointer. You may use that returned pointer together with
1331 /// An example of how to do it and how this relates to `write_unaligned` is:
1334 /// #[repr(packed, C)]
1340 /// let mut packed: Packed = unsafe { std::mem::zeroed() };
1342 /// // Take the address of a 32-bit integer which is not aligned.
1343 /// // In contrast to `&packed.unaligned as *mut _`, this has no undefined behavior.
1344 /// let unaligned = std::ptr::addr_of_mut!(packed.unaligned);
1346 /// unsafe { std::ptr::write_unaligned(unaligned, 42) };
1348 /// assert_eq!({packed.unaligned}, 42); // `{...}` forces copying the field instead of creating a reference.
1351 /// Accessing unaligned fields directly with e.g. `packed.unaligned` is safe however
1352 /// (as can be seen in the `assert_eq!` above).
1356 /// Write a usize value to a byte buffer:
1361 /// fn write_usize(x: &mut [u8], val: usize) {
1362 /// assert!(x.len() >= mem::size_of::<usize>());
1364 /// let ptr = x.as_mut_ptr() as *mut usize;
1366 /// unsafe { ptr.write_unaligned(val) }
1370 #[stable(feature = "ptr_unaligned", since = "1.17.0")]
1371 #[rustc_const_unstable(feature = "const_ptr_write", issue = "86302")]
1372 pub const unsafe fn write_unaligned<T>(dst: *mut T, src: T) {
1373 // SAFETY: the caller must guarantee that `dst` is valid for writes.
1374 // `dst` cannot overlap `src` because the caller has mutable access
1375 // to `dst` while `src` is owned by this function.
1377 copy_nonoverlapping(&src as *const T as *const u8, dst as *mut u8, mem::size_of::<T>());
1378 // We are calling the intrinsic directly to avoid function calls in the generated code.
1379 intrinsics::forget(src);
1383 /// Performs a volatile read of the value from `src` without moving it. This
1384 /// leaves the memory in `src` unchanged.
1386 /// Volatile operations are intended to act on I/O memory, and are guaranteed
1387 /// to not be elided or reordered by the compiler across other volatile
1392 /// Rust does not currently have a rigorously and formally defined memory model,
1393 /// so the precise semantics of what "volatile" means here is subject to change
1394 /// over time. That being said, the semantics will almost always end up pretty
1395 /// similar to [C11's definition of volatile][c11].
1397 /// The compiler shouldn't change the relative order or number of volatile
1398 /// memory operations. However, volatile memory operations on zero-sized types
1399 /// (e.g., if a zero-sized type is passed to `read_volatile`) are noops
1400 /// and may be ignored.
1402 /// [c11]: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
1406 /// Behavior is undefined if any of the following conditions are violated:
1408 /// * `src` must be [valid] for reads.
1410 /// * `src` must be properly aligned.
1412 /// * `src` must point to a properly initialized value of type `T`.
1414 /// Like [`read`], `read_volatile` creates a bitwise copy of `T`, regardless of
1415 /// whether `T` is [`Copy`]. If `T` is not [`Copy`], using both the returned
1416 /// value and the value at `*src` can [violate memory safety][read-ownership].
1417 /// However, storing non-[`Copy`] types in volatile memory is almost certainly
1420 /// Note that even if `T` has size `0`, the pointer must be non-null and properly aligned.
1422 /// [valid]: self#safety
1423 /// [read-ownership]: read#ownership-of-the-returned-value
1425 /// Just like in C, whether an operation is volatile has no bearing whatsoever
1426 /// on questions involving concurrent access from multiple threads. Volatile
1427 /// accesses behave exactly like non-atomic accesses in that regard. In particular,
1428 /// a race between a `read_volatile` and any write operation to the same location
1429 /// is undefined behavior.
1437 /// let y = &x as *const i32;
1440 /// assert_eq!(std::ptr::read_volatile(y), 12);
1444 #[stable(feature = "volatile", since = "1.9.0")]
1445 pub unsafe fn read_volatile<T>(src: *const T) -> T {
1446 // SAFETY: the caller must uphold the safety contract for `volatile_load`.
1448 assert_unsafe_precondition!(is_aligned_and_not_null(src));
1449 intrinsics::volatile_load(src)
1453 /// Performs a volatile write of a memory location with the given value without
1454 /// reading or dropping the old value.
1456 /// Volatile operations are intended to act on I/O memory, and are guaranteed
1457 /// to not be elided or reordered by the compiler across other volatile
1460 /// `write_volatile` does not drop the contents of `dst`. This is safe, but it
1461 /// could leak allocations or resources, so care should be taken not to overwrite
1462 /// an object that should be dropped.
1464 /// Additionally, it does not drop `src`. Semantically, `src` is moved into the
1465 /// location pointed to by `dst`.
1469 /// Rust does not currently have a rigorously and formally defined memory model,
1470 /// so the precise semantics of what "volatile" means here is subject to change
1471 /// over time. That being said, the semantics will almost always end up pretty
1472 /// similar to [C11's definition of volatile][c11].
1474 /// The compiler shouldn't change the relative order or number of volatile
1475 /// memory operations. However, volatile memory operations on zero-sized types
1476 /// (e.g., if a zero-sized type is passed to `write_volatile`) are noops
1477 /// and may be ignored.
1479 /// [c11]: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
1483 /// Behavior is undefined if any of the following conditions are violated:
1485 /// * `dst` must be [valid] for writes.
1487 /// * `dst` must be properly aligned.
1489 /// Note that even if `T` has size `0`, the pointer must be non-null and properly aligned.
1491 /// [valid]: self#safety
1493 /// Just like in C, whether an operation is volatile has no bearing whatsoever
1494 /// on questions involving concurrent access from multiple threads. Volatile
1495 /// accesses behave exactly like non-atomic accesses in that regard. In particular,
1496 /// a race between a `write_volatile` and any other operation (reading or writing)
1497 /// on the same location is undefined behavior.
1505 /// let y = &mut x as *mut i32;
1509 /// std::ptr::write_volatile(y, z);
1510 /// assert_eq!(std::ptr::read_volatile(y), 12);
1514 #[stable(feature = "volatile", since = "1.9.0")]
1515 pub unsafe fn write_volatile<T>(dst: *mut T, src: T) {
1516 // SAFETY: the caller must uphold the safety contract for `volatile_store`.
1518 assert_unsafe_precondition!(is_aligned_and_not_null(dst));
1519 intrinsics::volatile_store(dst, src);
1523 /// Align pointer `p`.
1525 /// Calculate offset (in terms of elements of `stride` stride) that has to be applied
1526 /// to pointer `p` so that pointer `p` would get aligned to `a`.
1528 /// Note: This implementation has been carefully tailored to not panic. It is UB for this to panic.
1529 /// The only real change that can be made here is change of `INV_TABLE_MOD_16` and associated
1532 /// If we ever decide to make it possible to call the intrinsic with `a` that is not a
1533 /// power-of-two, it will probably be more prudent to just change to a naive implementation rather
1534 /// than trying to adapt this to accommodate that change.
1536 /// Any questions go to @nagisa.
1537 #[lang = "align_offset"]
1538 pub(crate) unsafe fn align_offset<T: Sized>(p: *const T, a: usize) -> usize {
1539 // FIXME(#75598): Direct use of these intrinsics improves codegen significantly at opt-level <=
1540 // 1, where the method versions of these operations are not inlined.
1542 unchecked_shl, unchecked_shr, unchecked_sub, wrapping_add, wrapping_mul, wrapping_sub,
1545 let addr = p.addr();
1547 /// Calculate multiplicative modular inverse of `x` modulo `m`.
1549 /// This implementation is tailored for `align_offset` and has following preconditions:
1551 /// * `m` is a power-of-two;
1552 /// * `x < m`; (if `x ≥ m`, pass in `x % m` instead)
1554 /// Implementation of this function shall not panic. Ever.
1556 unsafe fn mod_inv(x: usize, m: usize) -> usize {
1557 /// Multiplicative modular inverse table modulo 2⁴ = 16.
1559 /// Note, that this table does not contain values where inverse does not exist (i.e., for
1560 /// `0⁻¹ mod 16`, `2⁻¹ mod 16`, etc.)
1561 const INV_TABLE_MOD_16: [u8; 8] = [1, 11, 13, 7, 9, 3, 5, 15];
1562 /// Modulo for which the `INV_TABLE_MOD_16` is intended.
1563 const INV_TABLE_MOD: usize = 16;
1565 const INV_TABLE_MOD_SQUARED: usize = INV_TABLE_MOD * INV_TABLE_MOD;
1567 let table_inverse = INV_TABLE_MOD_16[(x & (INV_TABLE_MOD - 1)) >> 1] as usize;
1568 // SAFETY: `m` is required to be a power-of-two, hence non-zero.
1569 let m_minus_one = unsafe { unchecked_sub(m, 1) };
1570 if m <= INV_TABLE_MOD {
1571 table_inverse & m_minus_one
1573 // We iterate "up" using the following formula:
1575 // $$ xy ≡ 1 (mod 2ⁿ) → xy (2 - xy) ≡ 1 (mod 2²ⁿ) $$
1577 // until 2²ⁿ ≥ m. Then we can reduce to our desired `m` by taking the result `mod m`.
1578 let mut inverse = table_inverse;
1579 let mut going_mod = INV_TABLE_MOD_SQUARED;
1581 // y = y * (2 - xy) mod n
1583 // Note, that we use wrapping operations here intentionally – the original formula
1584 // uses e.g., subtraction `mod n`. It is entirely fine to do them `mod
1585 // usize::MAX` instead, because we take the result `mod n` at the end
1587 inverse = wrapping_mul(inverse, wrapping_sub(2usize, wrapping_mul(x, inverse)));
1589 return inverse & m_minus_one;
1591 going_mod = wrapping_mul(going_mod, going_mod);
1596 let stride = mem::size_of::<T>();
1597 // SAFETY: `a` is a power-of-two, therefore non-zero.
1598 let a_minus_one = unsafe { unchecked_sub(a, 1) };
1600 // `stride == 1` case can be computed more simply through `-p (mod a)`, but doing so
1601 // inhibits LLVM's ability to select instructions like `lea`. Instead we compute
1603 // round_up_to_next_alignment(p, a) - p
1605 // which distributes operations around the load-bearing, but pessimizing `and` sufficiently
1606 // for LLVM to be able to utilize the various optimizations it knows about.
1607 return wrapping_sub(wrapping_add(addr, a_minus_one) & wrapping_sub(0, a), addr);
1610 let pmoda = addr & a_minus_one;
1612 // Already aligned. Yay!
1614 } else if stride == 0 {
1615 // If the pointer is not aligned, and the element is zero-sized, then no amount of
1616 // elements will ever align the pointer.
1620 let smoda = stride & a_minus_one;
1621 // SAFETY: a is power-of-two hence non-zero. stride == 0 case is handled above.
1622 let gcdpow = unsafe { intrinsics::cttz_nonzero(stride).min(intrinsics::cttz_nonzero(a)) };
1623 // SAFETY: gcdpow has an upper-bound that’s at most the number of bits in a usize.
1624 let gcd = unsafe { unchecked_shl(1usize, gcdpow) };
1626 // SAFETY: gcd is always greater or equal to 1.
1627 if addr & unsafe { unchecked_sub(gcd, 1) } == 0 {
1628 // This branch solves for the following linear congruence equation:
1630 // ` p + so = 0 mod a `
1632 // `p` here is the pointer value, `s` - stride of `T`, `o` offset in `T`s, and `a` - the
1633 // requested alignment.
1635 // With `g = gcd(a, s)`, and the above condition asserting that `p` is also divisible by
1636 // `g`, we can denote `a' = a/g`, `s' = s/g`, `p' = p/g`, then this becomes equivalent to:
1638 // ` p' + s'o = 0 mod a' `
1639 // ` o = (a' - (p' mod a')) * (s'^-1 mod a') `
1641 // The first term is "the relative alignment of `p` to `a`" (divided by the `g`), the second
1642 // term is "how does incrementing `p` by `s` bytes change the relative alignment of `p`" (again
1644 // Division by `g` is necessary to make the inverse well formed if `a` and `s` are not
1647 // Furthermore, the result produced by this solution is not "minimal", so it is necessary
1648 // to take the result `o mod lcm(s, a)`. We can replace `lcm(s, a)` with just a `a'`.
1650 // SAFETY: `gcdpow` has an upper-bound not greater than the number of trailing 0-bits in
1652 let a2 = unsafe { unchecked_shr(a, gcdpow) };
1653 // SAFETY: `a2` is non-zero. Shifting `a` by `gcdpow` cannot shift out any of the set bits
1654 // in `a` (of which it has exactly one).
1655 let a2minus1 = unsafe { unchecked_sub(a2, 1) };
1656 // SAFETY: `gcdpow` has an upper-bound not greater than the number of trailing 0-bits in
1658 let s2 = unsafe { unchecked_shr(smoda, gcdpow) };
1659 // SAFETY: `gcdpow` has an upper-bound not greater than the number of trailing 0-bits in
1660 // `a`. Furthermore, the subtraction cannot overflow, because `a2 = a >> gcdpow` will
1661 // always be strictly greater than `(p % a) >> gcdpow`.
1662 let minusp2 = unsafe { unchecked_sub(a2, unchecked_shr(pmoda, gcdpow)) };
1663 // SAFETY: `a2` is a power-of-two, as proven above. `s2` is strictly less than `a2`
1664 // because `(s % a) >> gcdpow` is strictly less than `a >> gcdpow`.
1665 return wrapping_mul(minusp2, unsafe { mod_inv(s2, a2) }) & a2minus1;
1668 // Cannot be aligned at all.
1672 /// Compares raw pointers for equality.
1674 /// This is the same as using the `==` operator, but less generic:
1675 /// the arguments have to be `*const T` raw pointers,
1676 /// not anything that implements `PartialEq`.
1678 /// This can be used to compare `&T` references (which coerce to `*const T` implicitly)
1679 /// by their address rather than comparing the values they point to
1680 /// (which is what the `PartialEq for &T` implementation does).
1688 /// let other_five = 5;
1689 /// let five_ref = &five;
1690 /// let same_five_ref = &five;
1691 /// let other_five_ref = &other_five;
1693 /// assert!(five_ref == same_five_ref);
1694 /// assert!(ptr::eq(five_ref, same_five_ref));
1696 /// assert!(five_ref == other_five_ref);
1697 /// assert!(!ptr::eq(five_ref, other_five_ref));
1700 /// Slices are also compared by their length (fat pointers):
1703 /// let a = [1, 2, 3];
1704 /// assert!(std::ptr::eq(&a[..3], &a[..3]));
1705 /// assert!(!std::ptr::eq(&a[..2], &a[..3]));
1706 /// assert!(!std::ptr::eq(&a[0..2], &a[1..3]));
1709 /// Traits are also compared by their implementation:
1712 /// #[repr(transparent)]
1713 /// struct Wrapper { member: i32 }
1716 /// impl Trait for Wrapper {}
1717 /// impl Trait for i32 {}
1719 /// let wrapper = Wrapper { member: 10 };
1721 /// // Pointers have equal addresses.
1722 /// assert!(std::ptr::eq(
1723 /// &wrapper as *const Wrapper as *const u8,
1724 /// &wrapper.member as *const i32 as *const u8
1727 /// // Objects have equal addresses, but `Trait` has different implementations.
1728 /// assert!(!std::ptr::eq(
1729 /// &wrapper as &dyn Trait,
1730 /// &wrapper.member as &dyn Trait,
1732 /// assert!(!std::ptr::eq(
1733 /// &wrapper as &dyn Trait as *const dyn Trait,
1734 /// &wrapper.member as &dyn Trait as *const dyn Trait,
1737 /// // Converting the reference to a `*const u8` compares by address.
1738 /// assert!(std::ptr::eq(
1739 /// &wrapper as &dyn Trait as *const dyn Trait as *const u8,
1740 /// &wrapper.member as &dyn Trait as *const dyn Trait as *const u8,
1743 #[stable(feature = "ptr_eq", since = "1.17.0")]
1745 pub fn eq<T: ?Sized>(a: *const T, b: *const T) -> bool {
1749 /// Hash a raw pointer.
1751 /// This can be used to hash a `&T` reference (which coerces to `*const T` implicitly)
1752 /// by its address rather than the value it points to
1753 /// (which is what the `Hash for &T` implementation does).
1758 /// use std::collections::hash_map::DefaultHasher;
1759 /// use std::hash::{Hash, Hasher};
1763 /// let five_ref = &five;
1765 /// let mut hasher = DefaultHasher::new();
1766 /// ptr::hash(five_ref, &mut hasher);
1767 /// let actual = hasher.finish();
1769 /// let mut hasher = DefaultHasher::new();
1770 /// (five_ref as *const i32).hash(&mut hasher);
1771 /// let expected = hasher.finish();
1773 /// assert_eq!(actual, expected);
1775 #[stable(feature = "ptr_hash", since = "1.35.0")]
1776 pub fn hash<T: ?Sized, S: hash::Hasher>(hashee: *const T, into: &mut S) {
1777 use crate::hash::Hash;
1781 // FIXME(strict_provenance_magic): function pointers have buggy codegen that
1782 // necessitates casting to a usize to get the backend to do the right thing.
1783 // for now I will break AVR to silence *a billion* lints. We should probably
1784 // have a proper "opaque function pointer type" to handle this kind of thing.
1786 // Impls for function pointers
1787 macro_rules! fnptr_impls_safety_abi {
1788 ($FnTy: ty, $($Arg: ident),*) => {
1789 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1790 impl<Ret, $($Arg),*> PartialEq for $FnTy {
1792 fn eq(&self, other: &Self) -> bool {
1793 *self as usize == *other as usize
1797 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1798 impl<Ret, $($Arg),*> Eq for $FnTy {}
1800 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1801 impl<Ret, $($Arg),*> PartialOrd for $FnTy {
1803 fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
1804 (*self as usize).partial_cmp(&(*other as usize))
1808 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1809 impl<Ret, $($Arg),*> Ord for $FnTy {
1811 fn cmp(&self, other: &Self) -> Ordering {
1812 (*self as usize).cmp(&(*other as usize))
1816 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1817 impl<Ret, $($Arg),*> hash::Hash for $FnTy {
1818 fn hash<HH: hash::Hasher>(&self, state: &mut HH) {
1819 state.write_usize(*self as usize)
1823 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1824 impl<Ret, $($Arg),*> fmt::Pointer for $FnTy {
1825 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1826 // HACK: The intermediate cast as usize is required for AVR
1827 // so that the address space of the source function pointer
1828 // is preserved in the final function pointer.
1830 // https://github.com/avr-rust/rust/issues/143
1831 fmt::Pointer::fmt(&(*self as usize as *const ()), f)
1835 #[stable(feature = "fnptr_impls", since = "1.4.0")]
1836 impl<Ret, $($Arg),*> fmt::Debug for $FnTy {
1837 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1838 // HACK: The intermediate cast as usize is required for AVR
1839 // so that the address space of the source function pointer
1840 // is preserved in the final function pointer.
1842 // https://github.com/avr-rust/rust/issues/143
1843 fmt::Pointer::fmt(&(*self as usize as *const ()), f)
1849 macro_rules! fnptr_impls_args {
1850 ($($Arg: ident),+) => {
1851 fnptr_impls_safety_abi! { extern "Rust" fn($($Arg),+) -> Ret, $($Arg),+ }
1852 fnptr_impls_safety_abi! { extern "C" fn($($Arg),+) -> Ret, $($Arg),+ }
1853 fnptr_impls_safety_abi! { extern "C" fn($($Arg),+ , ...) -> Ret, $($Arg),+ }
1854 fnptr_impls_safety_abi! { unsafe extern "Rust" fn($($Arg),+) -> Ret, $($Arg),+ }
1855 fnptr_impls_safety_abi! { unsafe extern "C" fn($($Arg),+) -> Ret, $($Arg),+ }
1856 fnptr_impls_safety_abi! { unsafe extern "C" fn($($Arg),+ , ...) -> Ret, $($Arg),+ }
1859 // No variadic functions with 0 parameters
1860 fnptr_impls_safety_abi! { extern "Rust" fn() -> Ret, }
1861 fnptr_impls_safety_abi! { extern "C" fn() -> Ret, }
1862 fnptr_impls_safety_abi! { unsafe extern "Rust" fn() -> Ret, }
1863 fnptr_impls_safety_abi! { unsafe extern "C" fn() -> Ret, }
1867 fnptr_impls_args! {}
1868 fnptr_impls_args! { A }
1869 fnptr_impls_args! { A, B }
1870 fnptr_impls_args! { A, B, C }
1871 fnptr_impls_args! { A, B, C, D }
1872 fnptr_impls_args! { A, B, C, D, E }
1873 fnptr_impls_args! { A, B, C, D, E, F }
1874 fnptr_impls_args! { A, B, C, D, E, F, G }
1875 fnptr_impls_args! { A, B, C, D, E, F, G, H }
1876 fnptr_impls_args! { A, B, C, D, E, F, G, H, I }
1877 fnptr_impls_args! { A, B, C, D, E, F, G, H, I, J }
1878 fnptr_impls_args! { A, B, C, D, E, F, G, H, I, J, K }
1879 fnptr_impls_args! { A, B, C, D, E, F, G, H, I, J, K, L }
1881 /// Create a `const` raw pointer to a place, without creating an intermediate reference.
1883 /// Creating a reference with `&`/`&mut` is only allowed if the pointer is properly aligned
1884 /// and points to initialized data. For cases where those requirements do not hold,
1885 /// raw pointers should be used instead. However, `&expr as *const _` creates a reference
1886 /// before casting it to a raw pointer, and that reference is subject to the same rules
1887 /// as all other references. This macro can create a raw pointer *without* creating
1888 /// a reference first.
1890 /// Note, however, that the `expr` in `addr_of!(expr)` is still subject to all
1891 /// the usual rules. In particular, `addr_of!(*ptr::null())` is Undefined
1892 /// Behavior because it dereferences a null pointer.
1905 /// let packed = Packed { f1: 1, f2: 2 };
1906 /// // `&packed.f2` would create an unaligned reference, and thus be Undefined Behavior!
1907 /// let raw_f2 = ptr::addr_of!(packed.f2);
1908 /// assert_eq!(unsafe { raw_f2.read_unaligned() }, 2);
1911 /// See [`addr_of_mut`] for how to create a pointer to unininitialized data.
1912 /// Doing that with `addr_of` would not make much sense since one could only
1913 /// read the data, and that would be Undefined Behavior.
1914 #[stable(feature = "raw_ref_macros", since = "1.51.0")]
1915 #[rustc_macro_transparency = "semitransparent"]
1916 #[allow_internal_unstable(raw_ref_op)]
1917 pub macro addr_of($place:expr) {
1921 /// Create a `mut` raw pointer to a place, without creating an intermediate reference.
1923 /// Creating a reference with `&`/`&mut` is only allowed if the pointer is properly aligned
1924 /// and points to initialized data. For cases where those requirements do not hold,
1925 /// raw pointers should be used instead. However, `&mut expr as *mut _` creates a reference
1926 /// before casting it to a raw pointer, and that reference is subject to the same rules
1927 /// as all other references. This macro can create a raw pointer *without* creating
1928 /// a reference first.
1930 /// Note, however, that the `expr` in `addr_of_mut!(expr)` is still subject to all
1931 /// the usual rules. In particular, `addr_of_mut!(*ptr::null_mut())` is Undefined
1932 /// Behavior because it dereferences a null pointer.
1936 /// **Creating a pointer to unaligned data:**
1947 /// let mut packed = Packed { f1: 1, f2: 2 };
1948 /// // `&mut packed.f2` would create an unaligned reference, and thus be Undefined Behavior!
1949 /// let raw_f2 = ptr::addr_of_mut!(packed.f2);
1950 /// unsafe { raw_f2.write_unaligned(42); }
1951 /// assert_eq!({packed.f2}, 42); // `{...}` forces copying the field instead of creating a reference.
1954 /// **Creating a pointer to uninitialized data:**
1957 /// use std::{ptr, mem::MaybeUninit};
1963 /// let mut uninit = MaybeUninit::<Demo>::uninit();
1964 /// // `&uninit.as_mut().field` would create a reference to an uninitialized `bool`,
1965 /// // and thus be Undefined Behavior!
1966 /// let f1_ptr = unsafe { ptr::addr_of_mut!((*uninit.as_mut_ptr()).field) };
1967 /// unsafe { f1_ptr.write(true); }
1968 /// let init = unsafe { uninit.assume_init() };
1970 #[stable(feature = "raw_ref_macros", since = "1.51.0")]
1971 #[rustc_macro_transparency = "semitransparent"]
1972 #[allow_internal_unstable(raw_ref_op)]
1973 pub macro addr_of_mut($place:expr) {