2 use crate::cmp::Ordering::{self, Equal, Greater, Less};
5 use crate::slice::{self, SliceIndex};
7 impl<T: ?Sized> *const T {
8 /// Returns `true` if the pointer is null.
10 /// Note that unsized types have many possible null pointers, as only the
11 /// raw data pointer is considered, not their length, vtable, etc.
12 /// Therefore, two pointers that are null may still not compare equal to
15 /// ## Behavior during const evaluation
17 /// When this function is used during const evaluation, it may return `false` for pointers
18 /// that turn out to be null at runtime. Specifically, when a pointer to some memory
19 /// is offset beyond its bounds in such a way that the resulting pointer is null,
20 /// the function will still return `false`. There is no way for CTFE to know
21 /// the absolute position of that memory, so we cannot tell if the pointer is
29 /// let s: &str = "Follow the rabbit";
30 /// let ptr: *const u8 = s.as_ptr();
31 /// assert!(!ptr.is_null());
33 #[stable(feature = "rust1", since = "1.0.0")]
34 #[rustc_const_unstable(feature = "const_ptr_is_null", issue = "74939")]
36 pub const fn is_null(self) -> bool {
37 // Compare via a cast to a thin pointer, so fat pointers are only
38 // considering their "data" part for null-ness.
39 (self as *const u8).guaranteed_eq(null())
42 /// Casts to a pointer of another type.
43 #[stable(feature = "ptr_cast", since = "1.38.0")]
44 #[rustc_const_stable(feature = "const_ptr_cast", since = "1.38.0")]
46 pub const fn cast<U>(self) -> *const U {
50 /// Use the pointer value in a new pointer of another type.
52 /// In case `val` is a (fat) pointer to an unsized type, this operation
53 /// will ignore the pointer part, whereas for (thin) pointers to sized
54 /// types, this has the same effect as a simple cast.
56 /// The resulting pointer will have provenance of `self`, i.e., for a fat
57 /// pointer, this operation is semantically the same as creating a new
58 /// fat pointer with the data pointer value of `self` but the metadata of
63 /// This function is primarily useful for allowing byte-wise pointer
64 /// arithmetic on potentially fat pointers:
67 /// #![feature(set_ptr_value)]
68 /// # use core::fmt::Debug;
69 /// let arr: [i32; 3] = [1, 2, 3];
70 /// let mut ptr = arr.as_ptr() as *const dyn Debug;
71 /// let thin = ptr as *const u8;
73 /// ptr = thin.add(8).with_metadata_of(ptr);
74 /// # assert_eq!(*(ptr as *const i32), 3);
75 /// println!("{:?}", &*ptr); // will print "3"
78 #[unstable(feature = "set_ptr_value", issue = "75091")]
79 #[must_use = "returns a new pointer rather than modifying its argument"]
81 pub fn with_metadata_of<U>(self, mut val: *const U) -> *const U
85 let target = &mut val as *mut *const U as *mut *const u8;
86 // SAFETY: In case of a thin pointer, this operations is identical
87 // to a simple assignment. In case of a fat pointer, with the current
88 // fat pointer layout implementation, the first field of such a
89 // pointer is always the data pointer, which is likewise assigned.
90 unsafe { *target = self as *const u8 };
94 /// Changes constness without changing the type.
96 /// This is a bit safer than `as` because it wouldn't silently change the type if the code is
98 #[unstable(feature = "ptr_const_cast", issue = "92675")]
99 #[rustc_const_unstable(feature = "ptr_const_cast", issue = "92675")]
100 pub const fn as_mut(self) -> *mut T {
104 /// Casts a pointer to its raw bits.
106 /// This is equivalent to `as usize`, but is more specific to enhance readability.
107 /// The inverse method is [`from_bits`](#method.from_bits).
109 /// In particular, `*p as usize` and `p as usize` will both compile for
110 /// pointers to numeric types but do very different things, so using this
111 /// helps emphasize that reading the bits was intentional.
116 /// #![feature(ptr_to_from_bits)]
117 /// let array = [13, 42];
118 /// let p0: *const i32 = &array[0];
119 /// assert_eq!(<*const _>::from_bits(p0.to_bits()), p0);
120 /// let p1: *const i32 = &array[1];
121 /// assert_eq!(p1.to_bits() - p0.to_bits(), 4);
123 #[unstable(feature = "ptr_to_from_bits", issue = "91126")]
124 pub fn to_bits(self) -> usize
131 /// Creates a pointer from its raw bits.
133 /// This is equivalent to `as *const T`, but is more specific to enhance readability.
134 /// The inverse method is [`to_bits`](#method.to_bits).
139 /// #![feature(ptr_to_from_bits)]
140 /// use std::ptr::NonNull;
141 /// let dangling: *const u8 = NonNull::dangling().as_ptr();
142 /// assert_eq!(<*const u8>::from_bits(1), dangling);
144 #[unstable(feature = "ptr_to_from_bits", issue = "91126")]
145 pub fn from_bits(bits: usize) -> Self
152 /// Gets the "address" portion of the pointer.
154 /// This is similar to `self as usize`, which semantically discards *provenance* and
155 /// *address-space* information. However, unlike `self as usize`, casting the returned address
156 /// back to a pointer yields [`invalid`][], which is undefined behavior to dereference. To
157 /// properly restore the lost information and obtain a dereferencable pointer, use
158 /// [`with_addr`][pointer::with_addr] or [`map_addr`][pointer::map_addr].
160 /// If using those APIs is not possible because there is no way to preserve a pointer with the
161 /// required provenance, use [`expose_addr`][pointer::expose_addr] and
162 /// [`from_exposed_addr`][from_exposed_addr] instead. However, note that this makes
163 /// your code less portable and less amenable to tools that check for compliance with the Rust
166 /// On most platforms this will produce a value with the same bytes as the original
167 /// pointer, because all the bytes are dedicated to describing the address.
168 /// Platforms which need to store additional information in the pointer may
169 /// perform a change of representation to produce a value containing only the address
170 /// portion of the pointer. What that means is up to the platform to define.
172 /// This API and its claimed semantics are part of the Strict Provenance experiment, and as such
173 /// might change in the future (including possibly weakening this so it becomes wholly
174 /// equivalent to `self as usize`). See the [module documentation][crate::ptr] for details.
177 #[unstable(feature = "strict_provenance", issue = "95228")]
178 pub fn addr(self) -> usize
182 // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
186 /// Gets the "address" portion of the pointer, and 'exposes' the "provenance" part for future
187 /// use in [`from_exposed_addr`][].
189 /// This is equivalent to `self as usize`, which semantically discards *provenance* and
190 /// *address-space* information. Furthermore, this (like the `as` cast) has the implicit
191 /// side-effect of marking the provenance as 'exposed', so on platforms that support it you can
192 /// later call [`from_exposed_addr`][] to reconstitute the original pointer including its
193 /// provenance. (Reconstructing address space information, if required, is your responsibility.)
195 /// Using this method means that code is *not* following Strict Provenance rules. Supporting
196 /// [`from_exposed_addr`][] complicates specification and reasoning and may not be supported by
197 /// tools that help you to stay conformant with the Rust memory model, so it is recommended to
198 /// use [`addr`][pointer::addr] wherever possible.
200 /// On most platforms this will produce a value with the same bytes as the original pointer,
201 /// because all the bytes are dedicated to describing the address. Platforms which need to store
202 /// additional information in the pointer may not support this operation, since the 'expose'
203 /// side-effect which is required for [`from_exposed_addr`][] to work is typically not
206 /// This API and its claimed semantics are part of the Strict Provenance experiment, see the
207 /// [module documentation][crate::ptr] for details.
209 /// [`from_exposed_addr`]: from_exposed_addr
212 #[unstable(feature = "strict_provenance", issue = "95228")]
213 pub fn expose_addr(self) -> usize
217 // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
221 /// Creates a new pointer with the given address.
223 /// This performs the same operation as an `addr as ptr` cast, but copies
224 /// the *address-space* and *provenance* of `self` to the new pointer.
225 /// This allows us to dynamically preserve and propagate this important
226 /// information in a way that is otherwise impossible with a unary cast.
228 /// This is equivalent to using [`wrapping_offset`][pointer::wrapping_offset] to offset
229 /// `self` to the given address, and therefore has all the same capabilities and restrictions.
231 /// This API and its claimed semantics are part of the Strict Provenance experiment,
232 /// see the [module documentation][crate::ptr] for details.
235 #[unstable(feature = "strict_provenance", issue = "95228")]
236 pub fn with_addr(self, addr: usize) -> Self
240 // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
242 // In the mean-time, this operation is defined to be "as if" it was
243 // a wrapping_offset, so we can emulate it as such. This should properly
244 // restore pointer provenance even under today's compiler.
245 let self_addr = self.addr() as isize;
246 let dest_addr = addr as isize;
247 let offset = dest_addr.wrapping_sub(self_addr);
249 // This is the canonical desugarring of this operation
250 self.cast::<u8>().wrapping_offset(offset).cast::<T>()
253 /// Creates a new pointer by mapping `self`'s address to a new one.
255 /// This is a convenience for [`with_addr`][pointer::with_addr], see that method for details.
257 /// This API and its claimed semantics are part of the Strict Provenance experiment,
258 /// see the [module documentation][crate::ptr] for details.
261 #[unstable(feature = "strict_provenance", issue = "95228")]
262 pub fn map_addr(self, f: impl FnOnce(usize) -> usize) -> Self
266 self.with_addr(f(self.addr()))
269 /// Decompose a (possibly wide) pointer into its address and metadata components.
271 /// The pointer can be later reconstructed with [`from_raw_parts`].
272 #[unstable(feature = "ptr_metadata", issue = "81513")]
273 #[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
275 pub const fn to_raw_parts(self) -> (*const (), <T as super::Pointee>::Metadata) {
276 (self.cast(), metadata(self))
279 /// Returns `None` if the pointer is null, or else returns a shared reference to
280 /// the value wrapped in `Some`. If the value may be uninitialized, [`as_uninit_ref`]
281 /// must be used instead.
283 /// [`as_uninit_ref`]: #method.as_uninit_ref
287 /// When calling this method, you have to ensure that *either* the pointer is null *or*
288 /// all of the following is true:
290 /// * The pointer must be properly aligned.
292 /// * It must be "dereferenceable" in the sense defined in [the module documentation].
294 /// * The pointer must point to an initialized instance of `T`.
296 /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
297 /// arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
298 /// In particular, while this reference exists, the memory the pointer points to must
299 /// not get mutated (except inside `UnsafeCell`).
301 /// This applies even if the result of this method is unused!
302 /// (The part about being initialized is not yet fully decided, but until
303 /// it is, the only safe approach is to ensure that they are indeed initialized.)
305 /// [the module documentation]: crate::ptr#safety
312 /// let ptr: *const u8 = &10u8 as *const u8;
315 /// if let Some(val_back) = ptr.as_ref() {
316 /// println!("We got back the value: {val_back}!");
321 /// # Null-unchecked version
323 /// If you are sure the pointer can never be null and are looking for some kind of
324 /// `as_ref_unchecked` that returns the `&T` instead of `Option<&T>`, know that you can
325 /// dereference the pointer directly.
328 /// let ptr: *const u8 = &10u8 as *const u8;
331 /// let val_back = &*ptr;
332 /// println!("We got back the value: {val_back}!");
335 #[stable(feature = "ptr_as_ref", since = "1.9.0")]
336 #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
338 pub const unsafe fn as_ref<'a>(self) -> Option<&'a T> {
339 // SAFETY: the caller must guarantee that `self` is valid
340 // for a reference if it isn't null.
341 if self.is_null() { None } else { unsafe { Some(&*self) } }
344 /// Returns `None` if the pointer is null, or else returns a shared reference to
345 /// the value wrapped in `Some`. In contrast to [`as_ref`], this does not require
346 /// that the value has to be initialized.
348 /// [`as_ref`]: #method.as_ref
352 /// When calling this method, you have to ensure that *either* the pointer is null *or*
353 /// all of the following is true:
355 /// * The pointer must be properly aligned.
357 /// * It must be "dereferenceable" in the sense defined in [the module documentation].
359 /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
360 /// arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
361 /// In particular, while this reference exists, the memory the pointer points to must
362 /// not get mutated (except inside `UnsafeCell`).
364 /// This applies even if the result of this method is unused!
366 /// [the module documentation]: crate::ptr#safety
373 /// #![feature(ptr_as_uninit)]
375 /// let ptr: *const u8 = &10u8 as *const u8;
378 /// if let Some(val_back) = ptr.as_uninit_ref() {
379 /// println!("We got back the value: {}!", val_back.assume_init());
384 #[unstable(feature = "ptr_as_uninit", issue = "75402")]
385 #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
386 pub const unsafe fn as_uninit_ref<'a>(self) -> Option<&'a MaybeUninit<T>>
390 // SAFETY: the caller must guarantee that `self` meets all the
391 // requirements for a reference.
392 if self.is_null() { None } else { Some(unsafe { &*(self as *const MaybeUninit<T>) }) }
395 /// Calculates the offset from a pointer.
397 /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
398 /// offset of `3 * size_of::<T>()` bytes.
402 /// If any of the following conditions are violated, the result is Undefined
405 /// * Both the starting and resulting pointer must be either in bounds or one
406 /// byte past the end of the same [allocated object].
408 /// * The computed offset, **in bytes**, cannot overflow an `isize`.
410 /// * The offset being in bounds cannot rely on "wrapping around" the address
411 /// space. That is, the infinite-precision sum, **in bytes** must fit in a usize.
413 /// The compiler and standard library generally tries to ensure allocations
414 /// never reach a size where an offset is a concern. For instance, `Vec`
415 /// and `Box` ensure they never allocate more than `isize::MAX` bytes, so
416 /// `vec.as_ptr().add(vec.len())` is always safe.
418 /// Most platforms fundamentally can't even construct such an allocation.
419 /// For instance, no known 64-bit platform can ever serve a request
420 /// for 2<sup>63</sup> bytes due to page-table limitations or splitting the address space.
421 /// However, some 32-bit and 16-bit platforms may successfully serve a request for
422 /// more than `isize::MAX` bytes with things like Physical Address
423 /// Extension. As such, memory acquired directly from allocators or memory
424 /// mapped files *may* be too large to handle with this function.
426 /// Consider using [`wrapping_offset`] instead if these constraints are
427 /// difficult to satisfy. The only advantage of this method is that it
428 /// enables more aggressive compiler optimizations.
430 /// [`wrapping_offset`]: #method.wrapping_offset
431 /// [allocated object]: crate::ptr#allocated-object
438 /// let s: &str = "123";
439 /// let ptr: *const u8 = s.as_ptr();
442 /// println!("{}", *ptr.offset(1) as char);
443 /// println!("{}", *ptr.offset(2) as char);
446 #[stable(feature = "rust1", since = "1.0.0")]
447 #[must_use = "returns a new pointer rather than modifying its argument"]
448 #[rustc_const_stable(feature = "const_ptr_offset", since = "1.61.0")]
450 pub const unsafe fn offset(self, count: isize) -> *const T
454 // SAFETY: the caller must uphold the safety contract for `offset`.
455 unsafe { intrinsics::offset(self, count) }
458 /// Calculates the offset from a pointer using wrapping arithmetic.
460 /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
461 /// offset of `3 * size_of::<T>()` bytes.
465 /// This operation itself is always safe, but using the resulting pointer is not.
467 /// The resulting pointer "remembers" the [allocated object] that `self` points to; it must not
468 /// be used to read or write other allocated objects.
470 /// In other words, `let z = x.wrapping_offset((y as isize) - (x as isize))` does *not* make `z`
471 /// the same as `y` even if we assume `T` has size `1` and there is no overflow: `z` is still
472 /// attached to the object `x` is attached to, and dereferencing it is Undefined Behavior unless
473 /// `x` and `y` point into the same allocated object.
475 /// Compared to [`offset`], this method basically delays the requirement of staying within the
476 /// same allocated object: [`offset`] is immediate Undefined Behavior when crossing object
477 /// boundaries; `wrapping_offset` produces a pointer but still leads to Undefined Behavior if a
478 /// pointer is dereferenced when it is out-of-bounds of the object it is attached to. [`offset`]
479 /// can be optimized better and is thus preferable in performance-sensitive code.
481 /// The delayed check only considers the value of the pointer that was dereferenced, not the
482 /// intermediate values used during the computation of the final result. For example,
483 /// `x.wrapping_offset(o).wrapping_offset(o.wrapping_neg())` is always the same as `x`. In other
484 /// words, leaving the allocated object and then re-entering it later is permitted.
486 /// [`offset`]: #method.offset
487 /// [allocated object]: crate::ptr#allocated-object
494 /// // Iterate using a raw pointer in increments of two elements
495 /// let data = [1u8, 2, 3, 4, 5];
496 /// let mut ptr: *const u8 = data.as_ptr();
498 /// let end_rounded_up = ptr.wrapping_offset(6);
500 /// // This loop prints "1, 3, 5, "
501 /// while ptr != end_rounded_up {
503 /// print!("{}, ", *ptr);
505 /// ptr = ptr.wrapping_offset(step);
508 #[stable(feature = "ptr_wrapping_offset", since = "1.16.0")]
509 #[must_use = "returns a new pointer rather than modifying its argument"]
510 #[rustc_const_stable(feature = "const_ptr_offset", since = "1.61.0")]
512 pub const fn wrapping_offset(self, count: isize) -> *const T
516 // SAFETY: the `arith_offset` intrinsic has no prerequisites to be called.
517 unsafe { intrinsics::arith_offset(self, count) }
520 /// Calculates the distance between two pointers. The returned value is in
521 /// units of T: the distance in bytes is divided by `mem::size_of::<T>()`.
523 /// This function is the inverse of [`offset`].
525 /// [`offset`]: #method.offset
529 /// If any of the following conditions are violated, the result is Undefined
532 /// * Both the starting and other pointer must be either in bounds or one
533 /// byte past the end of the same [allocated object].
535 /// * Both pointers must be *derived from* a pointer to the same object.
536 /// (See below for an example.)
538 /// * The distance between the pointers, in bytes, must be an exact multiple
539 /// of the size of `T`.
541 /// * The distance between the pointers, **in bytes**, cannot overflow an `isize`.
543 /// * The distance being in bounds cannot rely on "wrapping around" the address space.
545 /// Rust types are never larger than `isize::MAX` and Rust allocations never wrap around the
546 /// address space, so two pointers within some value of any Rust type `T` will always satisfy
547 /// the last two conditions. The standard library also generally ensures that allocations
548 /// never reach a size where an offset is a concern. For instance, `Vec` and `Box` ensure they
549 /// never allocate more than `isize::MAX` bytes, so `ptr_into_vec.offset_from(vec.as_ptr())`
550 /// always satisfies the last two conditions.
552 /// Most platforms fundamentally can't even construct such a large allocation.
553 /// For instance, no known 64-bit platform can ever serve a request
554 /// for 2<sup>63</sup> bytes due to page-table limitations or splitting the address space.
555 /// However, some 32-bit and 16-bit platforms may successfully serve a request for
556 /// more than `isize::MAX` bytes with things like Physical Address
557 /// Extension. As such, memory acquired directly from allocators or memory
558 /// mapped files *may* be too large to handle with this function.
559 /// (Note that [`offset`] and [`add`] also have a similar limitation and hence cannot be used on
560 /// such large allocations either.)
562 /// [`add`]: #method.add
563 /// [allocated object]: crate::ptr#allocated-object
567 /// This function panics if `T` is a Zero-Sized Type ("ZST").
575 /// let ptr1: *const i32 = &a[1];
576 /// let ptr2: *const i32 = &a[3];
578 /// assert_eq!(ptr2.offset_from(ptr1), 2);
579 /// assert_eq!(ptr1.offset_from(ptr2), -2);
580 /// assert_eq!(ptr1.offset(2), ptr2);
581 /// assert_eq!(ptr2.offset(-2), ptr1);
585 /// *Incorrect* usage:
588 /// let ptr1 = Box::into_raw(Box::new(0u8)) as *const u8;
589 /// let ptr2 = Box::into_raw(Box::new(1u8)) as *const u8;
590 /// let diff = (ptr2 as isize).wrapping_sub(ptr1 as isize);
591 /// // Make ptr2_other an "alias" of ptr2, but derived from ptr1.
592 /// let ptr2_other = (ptr1 as *const u8).wrapping_offset(diff);
593 /// assert_eq!(ptr2 as usize, ptr2_other as usize);
594 /// // Since ptr2_other and ptr2 are derived from pointers to different objects,
595 /// // computing their offset is undefined behavior, even though
596 /// // they point to the same address!
598 /// let zero = ptr2_other.offset_from(ptr2); // Undefined Behavior
601 #[stable(feature = "ptr_offset_from", since = "1.47.0")]
602 #[rustc_const_unstable(feature = "const_ptr_offset_from", issue = "92980")]
604 pub const unsafe fn offset_from(self, origin: *const T) -> isize
608 let pointee_size = mem::size_of::<T>();
609 assert!(0 < pointee_size && pointee_size <= isize::MAX as usize);
610 // SAFETY: the caller must uphold the safety contract for `ptr_offset_from`.
611 unsafe { intrinsics::ptr_offset_from(self, origin) }
614 /// Returns whether two pointers are guaranteed to be equal.
616 /// At runtime this function behaves like `self == other`.
617 /// However, in some contexts (e.g., compile-time evaluation),
618 /// it is not always possible to determine equality of two pointers, so this function may
619 /// spuriously return `false` for pointers that later actually turn out to be equal.
620 /// But when it returns `true`, the pointers are guaranteed to be equal.
622 /// This function is the mirror of [`guaranteed_ne`], but not its inverse. There are pointer
623 /// comparisons for which both functions return `false`.
625 /// [`guaranteed_ne`]: #method.guaranteed_ne
627 /// The return value may change depending on the compiler version and unsafe code must not
628 /// rely on the result of this function for soundness. It is suggested to only use this function
629 /// for performance optimizations where spurious `false` return values by this function do not
630 /// affect the outcome, but just the performance.
631 /// The consequences of using this method to make runtime and compile-time code behave
632 /// differently have not been explored. This method should not be used to introduce such
633 /// differences, and it should also not be stabilized before we have a better understanding
635 #[unstable(feature = "const_raw_ptr_comparison", issue = "53020")]
636 #[rustc_const_unstable(feature = "const_raw_ptr_comparison", issue = "53020")]
638 pub const fn guaranteed_eq(self, other: *const T) -> bool
642 intrinsics::ptr_guaranteed_eq(self, other)
645 /// Returns whether two pointers are guaranteed to be unequal.
647 /// At runtime this function behaves like `self != other`.
648 /// However, in some contexts (e.g., compile-time evaluation),
649 /// it is not always possible to determine the inequality of two pointers, so this function may
650 /// spuriously return `false` for pointers that later actually turn out to be unequal.
651 /// But when it returns `true`, the pointers are guaranteed to be unequal.
653 /// This function is the mirror of [`guaranteed_eq`], but not its inverse. There are pointer
654 /// comparisons for which both functions return `false`.
656 /// [`guaranteed_eq`]: #method.guaranteed_eq
658 /// The return value may change depending on the compiler version and unsafe code must not
659 /// rely on the result of this function for soundness. It is suggested to only use this function
660 /// for performance optimizations where spurious `false` return values by this function do not
661 /// affect the outcome, but just the performance.
662 /// The consequences of using this method to make runtime and compile-time code behave
663 /// differently have not been explored. This method should not be used to introduce such
664 /// differences, and it should also not be stabilized before we have a better understanding
666 #[unstable(feature = "const_raw_ptr_comparison", issue = "53020")]
667 #[rustc_const_unstable(feature = "const_raw_ptr_comparison", issue = "53020")]
669 pub const fn guaranteed_ne(self, other: *const T) -> bool
673 intrinsics::ptr_guaranteed_ne(self, other)
676 /// Calculates the offset from a pointer (convenience for `.offset(count as isize)`).
678 /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
679 /// offset of `3 * size_of::<T>()` bytes.
683 /// If any of the following conditions are violated, the result is Undefined
686 /// * Both the starting and resulting pointer must be either in bounds or one
687 /// byte past the end of the same [allocated object].
689 /// * The computed offset, **in bytes**, cannot overflow an `isize`.
691 /// * The offset being in bounds cannot rely on "wrapping around" the address
692 /// space. That is, the infinite-precision sum must fit in a `usize`.
694 /// The compiler and standard library generally tries to ensure allocations
695 /// never reach a size where an offset is a concern. For instance, `Vec`
696 /// and `Box` ensure they never allocate more than `isize::MAX` bytes, so
697 /// `vec.as_ptr().add(vec.len())` is always safe.
699 /// Most platforms fundamentally can't even construct such an allocation.
700 /// For instance, no known 64-bit platform can ever serve a request
701 /// for 2<sup>63</sup> bytes due to page-table limitations or splitting the address space.
702 /// However, some 32-bit and 16-bit platforms may successfully serve a request for
703 /// more than `isize::MAX` bytes with things like Physical Address
704 /// Extension. As such, memory acquired directly from allocators or memory
705 /// mapped files *may* be too large to handle with this function.
707 /// Consider using [`wrapping_add`] instead if these constraints are
708 /// difficult to satisfy. The only advantage of this method is that it
709 /// enables more aggressive compiler optimizations.
711 /// [`wrapping_add`]: #method.wrapping_add
712 /// [allocated object]: crate::ptr#allocated-object
719 /// let s: &str = "123";
720 /// let ptr: *const u8 = s.as_ptr();
723 /// println!("{}", *ptr.add(1) as char);
724 /// println!("{}", *ptr.add(2) as char);
727 #[stable(feature = "pointer_methods", since = "1.26.0")]
728 #[must_use = "returns a new pointer rather than modifying its argument"]
729 #[rustc_const_stable(feature = "const_ptr_offset", since = "1.61.0")]
731 pub const unsafe fn add(self, count: usize) -> Self
735 // SAFETY: the caller must uphold the safety contract for `offset`.
736 unsafe { self.offset(count as isize) }
739 /// Calculates the offset from a pointer (convenience for
740 /// `.offset((count as isize).wrapping_neg())`).
742 /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
743 /// offset of `3 * size_of::<T>()` bytes.
747 /// If any of the following conditions are violated, the result is Undefined
750 /// * Both the starting and resulting pointer must be either in bounds or one
751 /// byte past the end of the same [allocated object].
753 /// * The computed offset cannot exceed `isize::MAX` **bytes**.
755 /// * The offset being in bounds cannot rely on "wrapping around" the address
756 /// space. That is, the infinite-precision sum must fit in a usize.
758 /// The compiler and standard library generally tries to ensure allocations
759 /// never reach a size where an offset is a concern. For instance, `Vec`
760 /// and `Box` ensure they never allocate more than `isize::MAX` bytes, so
761 /// `vec.as_ptr().add(vec.len()).sub(vec.len())` is always safe.
763 /// Most platforms fundamentally can't even construct such an allocation.
764 /// For instance, no known 64-bit platform can ever serve a request
765 /// for 2<sup>63</sup> bytes due to page-table limitations or splitting the address space.
766 /// However, some 32-bit and 16-bit platforms may successfully serve a request for
767 /// more than `isize::MAX` bytes with things like Physical Address
768 /// Extension. As such, memory acquired directly from allocators or memory
769 /// mapped files *may* be too large to handle with this function.
771 /// Consider using [`wrapping_sub`] instead if these constraints are
772 /// difficult to satisfy. The only advantage of this method is that it
773 /// enables more aggressive compiler optimizations.
775 /// [`wrapping_sub`]: #method.wrapping_sub
776 /// [allocated object]: crate::ptr#allocated-object
783 /// let s: &str = "123";
786 /// let end: *const u8 = s.as_ptr().add(3);
787 /// println!("{}", *end.sub(1) as char);
788 /// println!("{}", *end.sub(2) as char);
791 #[stable(feature = "pointer_methods", since = "1.26.0")]
792 #[must_use = "returns a new pointer rather than modifying its argument"]
793 #[rustc_const_stable(feature = "const_ptr_offset", since = "1.61.0")]
795 pub const unsafe fn sub(self, count: usize) -> Self
799 // SAFETY: the caller must uphold the safety contract for `offset`.
800 unsafe { self.offset((count as isize).wrapping_neg()) }
803 /// Calculates the offset from a pointer using wrapping arithmetic.
804 /// (convenience for `.wrapping_offset(count as isize)`)
806 /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
807 /// offset of `3 * size_of::<T>()` bytes.
811 /// This operation itself is always safe, but using the resulting pointer is not.
813 /// The resulting pointer "remembers" the [allocated object] that `self` points to; it must not
814 /// be used to read or write other allocated objects.
816 /// In other words, `let z = x.wrapping_add((y as usize) - (x as usize))` does *not* make `z`
817 /// the same as `y` even if we assume `T` has size `1` and there is no overflow: `z` is still
818 /// attached to the object `x` is attached to, and dereferencing it is Undefined Behavior unless
819 /// `x` and `y` point into the same allocated object.
821 /// Compared to [`add`], this method basically delays the requirement of staying within the
822 /// same allocated object: [`add`] is immediate Undefined Behavior when crossing object
823 /// boundaries; `wrapping_add` produces a pointer but still leads to Undefined Behavior if a
824 /// pointer is dereferenced when it is out-of-bounds of the object it is attached to. [`add`]
825 /// can be optimized better and is thus preferable in performance-sensitive code.
827 /// The delayed check only considers the value of the pointer that was dereferenced, not the
828 /// intermediate values used during the computation of the final result. For example,
829 /// `x.wrapping_add(o).wrapping_sub(o)` is always the same as `x`. In other words, leaving the
830 /// allocated object and then re-entering it later is permitted.
832 /// [`add`]: #method.add
833 /// [allocated object]: crate::ptr#allocated-object
840 /// // Iterate using a raw pointer in increments of two elements
841 /// let data = [1u8, 2, 3, 4, 5];
842 /// let mut ptr: *const u8 = data.as_ptr();
844 /// let end_rounded_up = ptr.wrapping_add(6);
846 /// // This loop prints "1, 3, 5, "
847 /// while ptr != end_rounded_up {
849 /// print!("{}, ", *ptr);
851 /// ptr = ptr.wrapping_add(step);
854 #[stable(feature = "pointer_methods", since = "1.26.0")]
855 #[must_use = "returns a new pointer rather than modifying its argument"]
856 #[rustc_const_stable(feature = "const_ptr_offset", since = "1.61.0")]
858 pub const fn wrapping_add(self, count: usize) -> Self
862 self.wrapping_offset(count as isize)
865 /// Calculates the offset from a pointer using wrapping arithmetic.
866 /// (convenience for `.wrapping_offset((count as isize).wrapping_neg())`)
868 /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
869 /// offset of `3 * size_of::<T>()` bytes.
873 /// This operation itself is always safe, but using the resulting pointer is not.
875 /// The resulting pointer "remembers" the [allocated object] that `self` points to; it must not
876 /// be used to read or write other allocated objects.
878 /// In other words, `let z = x.wrapping_sub((x as usize) - (y as usize))` does *not* make `z`
879 /// the same as `y` even if we assume `T` has size `1` and there is no overflow: `z` is still
880 /// attached to the object `x` is attached to, and dereferencing it is Undefined Behavior unless
881 /// `x` and `y` point into the same allocated object.
883 /// Compared to [`sub`], this method basically delays the requirement of staying within the
884 /// same allocated object: [`sub`] is immediate Undefined Behavior when crossing object
885 /// boundaries; `wrapping_sub` produces a pointer but still leads to Undefined Behavior if a
886 /// pointer is dereferenced when it is out-of-bounds of the object it is attached to. [`sub`]
887 /// can be optimized better and is thus preferable in performance-sensitive code.
889 /// The delayed check only considers the value of the pointer that was dereferenced, not the
890 /// intermediate values used during the computation of the final result. For example,
891 /// `x.wrapping_add(o).wrapping_sub(o)` is always the same as `x`. In other words, leaving the
892 /// allocated object and then re-entering it later is permitted.
894 /// [`sub`]: #method.sub
895 /// [allocated object]: crate::ptr#allocated-object
902 /// // Iterate using a raw pointer in increments of two elements (backwards)
903 /// let data = [1u8, 2, 3, 4, 5];
904 /// let mut ptr: *const u8 = data.as_ptr();
905 /// let start_rounded_down = ptr.wrapping_sub(2);
906 /// ptr = ptr.wrapping_add(4);
908 /// // This loop prints "5, 3, 1, "
909 /// while ptr != start_rounded_down {
911 /// print!("{}, ", *ptr);
913 /// ptr = ptr.wrapping_sub(step);
916 #[stable(feature = "pointer_methods", since = "1.26.0")]
917 #[must_use = "returns a new pointer rather than modifying its argument"]
918 #[rustc_const_stable(feature = "const_ptr_offset", since = "1.61.0")]
920 pub const fn wrapping_sub(self, count: usize) -> Self
924 self.wrapping_offset((count as isize).wrapping_neg())
927 /// Reads the value from `self` without moving it. This leaves the
928 /// memory in `self` unchanged.
930 /// See [`ptr::read`] for safety concerns and examples.
932 /// [`ptr::read`]: crate::ptr::read()
933 #[stable(feature = "pointer_methods", since = "1.26.0")]
934 #[rustc_const_unstable(feature = "const_ptr_read", issue = "80377")]
936 pub const unsafe fn read(self) -> T
940 // SAFETY: the caller must uphold the safety contract for `read`.
941 unsafe { read(self) }
944 /// Performs a volatile read of the value from `self` without moving it. This
945 /// leaves the memory in `self` unchanged.
947 /// Volatile operations are intended to act on I/O memory, and are guaranteed
948 /// to not be elided or reordered by the compiler across other volatile
951 /// See [`ptr::read_volatile`] for safety concerns and examples.
953 /// [`ptr::read_volatile`]: crate::ptr::read_volatile()
954 #[stable(feature = "pointer_methods", since = "1.26.0")]
956 pub unsafe fn read_volatile(self) -> T
960 // SAFETY: the caller must uphold the safety contract for `read_volatile`.
961 unsafe { read_volatile(self) }
964 /// Reads the value from `self` without moving it. This leaves the
965 /// memory in `self` unchanged.
967 /// Unlike `read`, the pointer may be unaligned.
969 /// See [`ptr::read_unaligned`] for safety concerns and examples.
971 /// [`ptr::read_unaligned`]: crate::ptr::read_unaligned()
972 #[stable(feature = "pointer_methods", since = "1.26.0")]
973 #[rustc_const_unstable(feature = "const_ptr_read", issue = "80377")]
975 pub const unsafe fn read_unaligned(self) -> T
979 // SAFETY: the caller must uphold the safety contract for `read_unaligned`.
980 unsafe { read_unaligned(self) }
983 /// Copies `count * size_of<T>` bytes from `self` to `dest`. The source
984 /// and destination may overlap.
986 /// NOTE: this has the *same* argument order as [`ptr::copy`].
988 /// See [`ptr::copy`] for safety concerns and examples.
990 /// [`ptr::copy`]: crate::ptr::copy()
991 #[rustc_const_unstable(feature = "const_intrinsic_copy", issue = "80697")]
992 #[stable(feature = "pointer_methods", since = "1.26.0")]
994 pub const unsafe fn copy_to(self, dest: *mut T, count: usize)
998 // SAFETY: the caller must uphold the safety contract for `copy`.
999 unsafe { copy(self, dest, count) }
1002 /// Copies `count * size_of<T>` bytes from `self` to `dest`. The source
1003 /// and destination may *not* overlap.
1005 /// NOTE: this has the *same* argument order as [`ptr::copy_nonoverlapping`].
1007 /// See [`ptr::copy_nonoverlapping`] for safety concerns and examples.
1009 /// [`ptr::copy_nonoverlapping`]: crate::ptr::copy_nonoverlapping()
1010 #[rustc_const_unstable(feature = "const_intrinsic_copy", issue = "80697")]
1011 #[stable(feature = "pointer_methods", since = "1.26.0")]
1013 pub const unsafe fn copy_to_nonoverlapping(self, dest: *mut T, count: usize)
1017 // SAFETY: the caller must uphold the safety contract for `copy_nonoverlapping`.
1018 unsafe { copy_nonoverlapping(self, dest, count) }
1021 /// Computes the offset that needs to be applied to the pointer in order to make it aligned to
1024 /// If it is not possible to align the pointer, the implementation returns
1025 /// `usize::MAX`. It is permissible for the implementation to *always*
1026 /// return `usize::MAX`. Only your algorithm's performance can depend
1027 /// on getting a usable offset here, not its correctness.
1029 /// The offset is expressed in number of `T` elements, and not bytes. The value returned can be
1030 /// used with the `wrapping_add` method.
1032 /// There are no guarantees whatsoever that offsetting the pointer will not overflow or go
1033 /// beyond the allocation that the pointer points into. It is up to the caller to ensure that
1034 /// the returned offset is correct in all terms other than alignment.
1038 /// The function panics if `align` is not a power-of-two.
1042 /// Accessing adjacent `u8` as `u16`
1045 /// # fn foo(n: usize) {
1046 /// # use std::mem::align_of;
1048 /// let x = [5u8, 6u8, 7u8, 8u8, 9u8];
1049 /// let ptr = x.as_ptr().add(n) as *const u8;
1050 /// let offset = ptr.align_offset(align_of::<u16>());
1051 /// if offset < x.len() - n - 1 {
1052 /// let u16_ptr = ptr.add(offset) as *const u16;
1053 /// assert_ne!(*u16_ptr, 500);
1055 /// // while the pointer can be aligned via `offset`, it would point
1056 /// // outside the allocation
1060 #[stable(feature = "align_offset", since = "1.36.0")]
1061 #[rustc_const_unstable(feature = "const_align_offset", issue = "90962")]
1062 pub const fn align_offset(self, align: usize) -> usize
1066 if !align.is_power_of_two() {
1067 panic!("align_offset: align is not a power-of-two");
1070 fn rt_impl<T>(p: *const T, align: usize) -> usize {
1071 // SAFETY: `align` has been checked to be a power of 2 above
1072 unsafe { align_offset(p, align) }
1075 const fn ctfe_impl<T>(_: *const T, _: usize) -> usize {
1080 // It is permisseble for `align_offset` to always return `usize::MAX`,
1081 // algorithm correctness can not depend on `align_offset` returning non-max values.
1083 // As such the behaviour can't change after replacing `align_offset` with `usize::MAX`, only performance can.
1084 unsafe { intrinsics::const_eval_select((self, align), ctfe_impl, rt_impl) }
1088 impl<T> *const [T] {
1089 /// Returns the length of a raw slice.
1091 /// The returned value is the number of **elements**, not the number of bytes.
1093 /// This function is safe, even when the raw slice cannot be cast to a slice
1094 /// reference because the pointer is null or unaligned.
1099 /// #![feature(slice_ptr_len)]
1103 /// let slice: *const [i8] = ptr::slice_from_raw_parts(ptr::null(), 3);
1104 /// assert_eq!(slice.len(), 3);
1107 #[unstable(feature = "slice_ptr_len", issue = "71146")]
1108 #[rustc_const_unstable(feature = "const_slice_ptr_len", issue = "71146")]
1109 pub const fn len(self) -> usize {
1113 /// Returns a raw pointer to the slice's buffer.
1115 /// This is equivalent to casting `self` to `*const T`, but more type-safe.
1120 /// #![feature(slice_ptr_get)]
1123 /// let slice: *const [i8] = ptr::slice_from_raw_parts(ptr::null(), 3);
1124 /// assert_eq!(slice.as_ptr(), ptr::null());
1127 #[unstable(feature = "slice_ptr_get", issue = "74265")]
1128 #[rustc_const_unstable(feature = "slice_ptr_get", issue = "74265")]
1129 pub const fn as_ptr(self) -> *const T {
1133 /// Returns a raw pointer to an element or subslice, without doing bounds
1136 /// Calling this method with an out-of-bounds index or when `self` is not dereferenceable
1137 /// is *[undefined behavior]* even if the resulting pointer is not used.
1139 /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
1144 /// #![feature(slice_ptr_get)]
1146 /// let x = &[1, 2, 4] as *const [i32];
1149 /// assert_eq!(x.get_unchecked(1), x.as_ptr().add(1));
1152 #[unstable(feature = "slice_ptr_get", issue = "74265")]
1153 #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
1155 pub const unsafe fn get_unchecked<I>(self, index: I) -> *const I::Output
1157 I: ~const SliceIndex<[T]>,
1159 // SAFETY: the caller ensures that `self` is dereferenceable and `index` in-bounds.
1160 unsafe { index.get_unchecked(self) }
1163 /// Returns `None` if the pointer is null, or else returns a shared slice to
1164 /// the value wrapped in `Some`. In contrast to [`as_ref`], this does not require
1165 /// that the value has to be initialized.
1167 /// [`as_ref`]: #method.as_ref
1171 /// When calling this method, you have to ensure that *either* the pointer is null *or*
1172 /// all of the following is true:
1174 /// * The pointer must be [valid] for reads for `ptr.len() * mem::size_of::<T>()` many bytes,
1175 /// and it must be properly aligned. This means in particular:
1177 /// * The entire memory range of this slice must be contained within a single [allocated object]!
1178 /// Slices can never span across multiple allocated objects.
1180 /// * The pointer must be aligned even for zero-length slices. One
1181 /// reason for this is that enum layout optimizations may rely on references
1182 /// (including slices of any length) being aligned and non-null to distinguish
1183 /// them from other data. You can obtain a pointer that is usable as `data`
1184 /// for zero-length slices using [`NonNull::dangling()`].
1186 /// * The total size `ptr.len() * mem::size_of::<T>()` of the slice must be no larger than `isize::MAX`.
1187 /// See the safety documentation of [`pointer::offset`].
1189 /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
1190 /// arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
1191 /// In particular, while this reference exists, the memory the pointer points to must
1192 /// not get mutated (except inside `UnsafeCell`).
1194 /// This applies even if the result of this method is unused!
1196 /// See also [`slice::from_raw_parts`][].
1198 /// [valid]: crate::ptr#safety
1199 /// [allocated object]: crate::ptr#allocated-object
1201 #[unstable(feature = "ptr_as_uninit", issue = "75402")]
1202 #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
1203 pub const unsafe fn as_uninit_slice<'a>(self) -> Option<&'a [MaybeUninit<T>]> {
1207 // SAFETY: the caller must uphold the safety contract for `as_uninit_slice`.
1208 Some(unsafe { slice::from_raw_parts(self as *const MaybeUninit<T>, self.len()) })
1213 // Equality for pointers
1214 #[stable(feature = "rust1", since = "1.0.0")]
1215 impl<T: ?Sized> PartialEq for *const T {
1217 fn eq(&self, other: &*const T) -> bool {
1222 #[stable(feature = "rust1", since = "1.0.0")]
1223 impl<T: ?Sized> Eq for *const T {}
1225 // Comparison for pointers
1226 #[stable(feature = "rust1", since = "1.0.0")]
1227 impl<T: ?Sized> Ord for *const T {
1229 fn cmp(&self, other: &*const T) -> Ordering {
1232 } else if self == other {
1240 #[stable(feature = "rust1", since = "1.0.0")]
1241 impl<T: ?Sized> PartialOrd for *const T {
1243 fn partial_cmp(&self, other: &*const T) -> Option<Ordering> {
1244 Some(self.cmp(other))
1248 fn lt(&self, other: &*const T) -> bool {
1253 fn le(&self, other: &*const T) -> bool {
1258 fn gt(&self, other: &*const T) -> bool {
1263 fn ge(&self, other: &*const T) -> bool {