1 //! Memory allocation APIs
3 // ignore-tidy-undocumented-unsafe
5 #![stable(feature = "alloc_module", since = "1.28.0")]
10 use crate::num::NonZeroUsize;
11 use crate::ptr::{self, NonNull};
14 /// Represents the combination of a starting address and
15 /// a total capacity of the returned block.
16 #[unstable(feature = "allocator_api", issue = "32838")]
18 pub struct Excess(pub NonNull<u8>, pub usize);
20 const fn size_align<T>() -> (usize, usize) {
21 (mem::size_of::<T>(), mem::align_of::<T>())
24 /// Layout of a block of memory.
26 /// An instance of `Layout` describes a particular layout of memory.
27 /// You build a `Layout` up as an input to give to an allocator.
29 /// All layouts have an associated non-negative size and a
30 /// power-of-two alignment.
32 /// (Note however that layouts are *not* required to have positive
33 /// size, even though many allocators require that all memory
34 /// requests have positive size. A caller to the `Alloc::alloc`
35 /// method must either ensure that conditions like this are met, or
36 /// use specific allocators with looser requirements.)
37 #[stable(feature = "alloc_layout", since = "1.28.0")]
38 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
39 #[lang = "alloc_layout"]
41 // size of the requested block of memory, measured in bytes.
44 // alignment of the requested block of memory, measured in bytes.
45 // we ensure that this is always a power-of-two, because API's
46 // like `posix_memalign` require it and it is a reasonable
47 // constraint to impose on Layout constructors.
49 // (However, we do not analogously require `align >= sizeof(void*)`,
50 // even though that is *also* a requirement of `posix_memalign`.)
55 /// Constructs a `Layout` from a given `size` and `align`,
56 /// or returns `LayoutErr` if any of the following conditions
59 /// * `align` must not be zero,
61 /// * `align` must be a power of two,
63 /// * `size`, when rounded up to the nearest multiple of `align`,
64 /// must not overflow (i.e., the rounded value must be less than
66 #[stable(feature = "alloc_layout", since = "1.28.0")]
67 #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
69 pub const fn from_size_align(size: usize, align: usize) -> Result<Self, LayoutErr> {
70 if !align.is_power_of_two() {
71 return Err(LayoutErr { private: () });
74 // (power-of-two implies align != 0.)
76 // Rounded up size is:
77 // size_rounded_up = (size + align - 1) & !(align - 1);
79 // We know from above that align != 0. If adding (align - 1)
80 // does not overflow, then rounding up will be fine.
82 // Conversely, &-masking with !(align - 1) will subtract off
83 // only low-order-bits. Thus if overflow occurs with the sum,
84 // the &-mask cannot subtract enough to undo that overflow.
86 // Above implies that checking for summation overflow is both
87 // necessary and sufficient.
88 if size > usize::MAX - (align - 1) {
89 return Err(LayoutErr { private: () });
92 unsafe { Ok(Layout::from_size_align_unchecked(size, align)) }
95 /// Creates a layout, bypassing all checks.
99 /// This function is unsafe as it does not verify the preconditions from
100 /// [`Layout::from_size_align`](#method.from_size_align).
101 #[stable(feature = "alloc_layout", since = "1.28.0")]
102 #[rustc_const_stable(feature = "alloc_layout", since = "1.28.0")]
104 pub const unsafe fn from_size_align_unchecked(size: usize, align: usize) -> Self {
105 Layout { size_: size, align_: NonZeroUsize::new_unchecked(align) }
108 /// The minimum size in bytes for a memory block of this layout.
109 #[stable(feature = "alloc_layout", since = "1.28.0")]
110 #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
112 pub const fn size(&self) -> usize {
116 /// The minimum byte alignment for a memory block of this layout.
117 #[stable(feature = "alloc_layout", since = "1.28.0")]
118 #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
120 pub const fn align(&self) -> usize {
124 /// Constructs a `Layout` suitable for holding a value of type `T`.
125 #[stable(feature = "alloc_layout", since = "1.28.0")]
126 #[rustc_const_stable(feature = "alloc_layout_const_new", since = "1.42.0")]
128 pub const fn new<T>() -> Self {
129 let (size, align) = size_align::<T>();
130 // Note that the align is guaranteed by rustc to be a power of two and
131 // the size+align combo is guaranteed to fit in our address space. As a
132 // result use the unchecked constructor here to avoid inserting code
133 // that panics if it isn't optimized well enough.
134 unsafe { Layout::from_size_align_unchecked(size, align) }
137 /// Produces layout describing a record that could be used to
138 /// allocate backing structure for `T` (which could be a trait
139 /// or other unsized type like a slice).
140 #[stable(feature = "alloc_layout", since = "1.28.0")]
142 pub fn for_value<T: ?Sized>(t: &T) -> Self {
143 let (size, align) = (mem::size_of_val(t), mem::align_of_val(t));
144 // See rationale in `new` for why this is using an unsafe variant below
145 debug_assert!(Layout::from_size_align(size, align).is_ok());
146 unsafe { Layout::from_size_align_unchecked(size, align) }
149 /// Creates a layout describing the record that can hold a value
150 /// of the same layout as `self`, but that also is aligned to
151 /// alignment `align` (measured in bytes).
153 /// If `self` already meets the prescribed alignment, then returns
156 /// Note that this method does not add any padding to the overall
157 /// size, regardless of whether the returned layout has a different
158 /// alignment. In other words, if `K` has size 16, `K.align_to(32)`
159 /// will *still* have size 16.
161 /// Returns an error if the combination of `self.size()` and the given
162 /// `align` violates the conditions listed in
163 /// [`Layout::from_size_align`](#method.from_size_align).
164 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
166 pub fn align_to(&self, align: usize) -> Result<Self, LayoutErr> {
167 Layout::from_size_align(self.size(), cmp::max(self.align(), align))
170 /// Returns the amount of padding we must insert after `self`
171 /// to ensure that the following address will satisfy `align`
172 /// (measured in bytes).
174 /// e.g., if `self.size()` is 9, then `self.padding_needed_for(4)`
175 /// returns 3, because that is the minimum number of bytes of
176 /// padding required to get a 4-aligned address (assuming that the
177 /// corresponding memory block starts at a 4-aligned address).
179 /// The return value of this function has no meaning if `align` is
180 /// not a power-of-two.
182 /// Note that the utility of the returned value requires `align`
183 /// to be less than or equal to the alignment of the starting
184 /// address for the whole allocated block of memory. One way to
185 /// satisfy this constraint is to ensure `align <= self.align()`.
186 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
187 #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
189 pub const fn padding_needed_for(&self, align: usize) -> usize {
190 let len = self.size();
192 // Rounded up value is:
193 // len_rounded_up = (len + align - 1) & !(align - 1);
194 // and then we return the padding difference: `len_rounded_up - len`.
196 // We use modular arithmetic throughout:
198 // 1. align is guaranteed to be > 0, so align - 1 is always
201 // 2. `len + align - 1` can overflow by at most `align - 1`,
202 // so the &-mask with `!(align - 1)` will ensure that in the
203 // case of overflow, `len_rounded_up` will itself be 0.
204 // Thus the returned padding, when added to `len`, yields 0,
205 // which trivially satisfies the alignment `align`.
207 // (Of course, attempts to allocate blocks of memory whose
208 // size and padding overflow in the above manner should cause
209 // the allocator to yield an error anyway.)
211 let len_rounded_up = len.wrapping_add(align).wrapping_sub(1) & !align.wrapping_sub(1);
212 len_rounded_up.wrapping_sub(len)
215 /// Creates a layout by rounding the size of this layout up to a multiple
216 /// of the layout's alignment.
218 /// This is equivalent to adding the result of `padding_needed_for`
219 /// to the layout's current size.
220 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
222 pub fn pad_to_align(&self) -> Layout {
223 let pad = self.padding_needed_for(self.align());
224 // This cannot overflow. Quoting from the invariant of Layout:
225 // > `size`, when rounded up to the nearest multiple of `align`,
226 // > must not overflow (i.e., the rounded value must be less than
228 let new_size = self.size() + pad;
230 Layout::from_size_align(new_size, self.align()).unwrap()
233 /// Creates a layout describing the record for `n` instances of
234 /// `self`, with a suitable amount of padding between each to
235 /// ensure that each instance is given its requested size and
236 /// alignment. On success, returns `(k, offs)` where `k` is the
237 /// layout of the array and `offs` is the distance between the start
238 /// of each element in the array.
240 /// On arithmetic overflow, returns `LayoutErr`.
241 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
243 pub fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutErr> {
244 // This cannot overflow. Quoting from the invariant of Layout:
245 // > `size`, when rounded up to the nearest multiple of `align`,
246 // > must not overflow (i.e., the rounded value must be less than
248 let padded_size = self.size() + self.padding_needed_for(self.align());
249 let alloc_size = padded_size.checked_mul(n).ok_or(LayoutErr { private: () })?;
252 // self.align is already known to be valid and alloc_size has been
254 Ok((Layout::from_size_align_unchecked(alloc_size, self.align()), padded_size))
258 /// Creates a layout describing the record for `self` followed by
259 /// `next`, including any necessary padding to ensure that `next`
260 /// will be properly aligned. Note that the resulting layout will
261 /// satisfy the alignment properties of both `self` and `next`.
263 /// The resulting layout will be the same as that of a C struct containing
264 /// two fields with the layouts of `self` and `next`, in that order.
266 /// Returns `Some((k, offset))`, where `k` is layout of the concatenated
267 /// record and `offset` is the relative location, in bytes, of the
268 /// start of the `next` embedded within the concatenated record
269 /// (assuming that the record itself starts at offset 0).
271 /// On arithmetic overflow, returns `LayoutErr`.
272 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
274 pub fn extend(&self, next: Self) -> Result<(Self, usize), LayoutErr> {
275 let new_align = cmp::max(self.align(), next.align());
276 let pad = self.padding_needed_for(next.align());
278 let offset = self.size().checked_add(pad).ok_or(LayoutErr { private: () })?;
279 let new_size = offset.checked_add(next.size()).ok_or(LayoutErr { private: () })?;
281 let layout = Layout::from_size_align(new_size, new_align)?;
285 /// Creates a layout describing the record for `n` instances of
286 /// `self`, with no padding between each instance.
288 /// Note that, unlike `repeat`, `repeat_packed` does not guarantee
289 /// that the repeated instances of `self` will be properly
290 /// aligned, even if a given instance of `self` is properly
291 /// aligned. In other words, if the layout returned by
292 /// `repeat_packed` is used to allocate an array, it is not
293 /// guaranteed that all elements in the array will be properly
296 /// On arithmetic overflow, returns `LayoutErr`.
297 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
299 pub fn repeat_packed(&self, n: usize) -> Result<Self, LayoutErr> {
300 let size = self.size().checked_mul(n).ok_or(LayoutErr { private: () })?;
301 Layout::from_size_align(size, self.align())
304 /// Creates a layout describing the record for `self` followed by
305 /// `next` with no additional padding between the two. Since no
306 /// padding is inserted, the alignment of `next` is irrelevant,
307 /// and is not incorporated *at all* into the resulting layout.
309 /// On arithmetic overflow, returns `LayoutErr`.
310 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
312 pub fn extend_packed(&self, next: Self) -> Result<Self, LayoutErr> {
313 let new_size = self.size().checked_add(next.size()).ok_or(LayoutErr { private: () })?;
314 Layout::from_size_align(new_size, self.align())
317 /// Creates a layout describing the record for a `[T; n]`.
319 /// On arithmetic overflow, returns `LayoutErr`.
320 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
322 pub fn array<T>(n: usize) -> Result<Self, LayoutErr> {
323 Layout::new::<T>().repeat(n).map(|(k, offs)| {
324 debug_assert!(offs == mem::size_of::<T>());
330 /// The parameters given to `Layout::from_size_align`
331 /// or some other `Layout` constructor
332 /// do not satisfy its documented constraints.
333 #[stable(feature = "alloc_layout", since = "1.28.0")]
334 #[derive(Clone, PartialEq, Eq, Debug)]
335 pub struct LayoutErr {
339 // (we need this for downstream impl of trait Error)
340 #[stable(feature = "alloc_layout", since = "1.28.0")]
341 impl fmt::Display for LayoutErr {
342 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
343 f.write_str("invalid parameters to Layout::from_size_align")
347 /// The `AllocErr` error indicates an allocation failure
348 /// that may be due to resource exhaustion or to
349 /// something wrong when combining the given input arguments with this
351 #[unstable(feature = "allocator_api", issue = "32838")]
352 #[derive(Clone, PartialEq, Eq, Debug)]
355 // (we need this for downstream impl of trait Error)
356 #[unstable(feature = "allocator_api", issue = "32838")]
357 impl fmt::Display for AllocErr {
358 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
359 f.write_str("memory allocation failed")
363 /// The `CannotReallocInPlace` error is used when [`grow_in_place`] or
364 /// [`shrink_in_place`] were unable to reuse the given memory block for
365 /// a requested layout.
367 /// [`grow_in_place`]: ./trait.Alloc.html#method.grow_in_place
368 /// [`shrink_in_place`]: ./trait.Alloc.html#method.shrink_in_place
369 #[unstable(feature = "allocator_api", issue = "32838")]
370 #[derive(Clone, PartialEq, Eq, Debug)]
371 pub struct CannotReallocInPlace;
373 #[unstable(feature = "allocator_api", issue = "32838")]
374 impl CannotReallocInPlace {
375 pub fn description(&self) -> &str {
376 "cannot reallocate allocator's memory in place"
380 // (we need this for downstream impl of trait Error)
381 #[unstable(feature = "allocator_api", issue = "32838")]
382 impl fmt::Display for CannotReallocInPlace {
383 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
384 write!(f, "{}", self.description())
388 /// A memory allocator that can be registered as the standard library’s default
389 /// through the `#[global_allocator]` attribute.
391 /// Some of the methods require that a memory block be *currently
392 /// allocated* via an allocator. This means that:
394 /// * the starting address for that memory block was previously
395 /// returned by a previous call to an allocation method
396 /// such as `alloc`, and
398 /// * the memory block has not been subsequently deallocated, where
399 /// blocks are deallocated either by being passed to a deallocation
400 /// method such as `dealloc` or by being
401 /// passed to a reallocation method that returns a non-null pointer.
407 /// use std::alloc::{GlobalAlloc, Layout, alloc};
408 /// use std::ptr::null_mut;
410 /// struct MyAllocator;
412 /// unsafe impl GlobalAlloc for MyAllocator {
413 /// unsafe fn alloc(&self, _layout: Layout) -> *mut u8 { null_mut() }
414 /// unsafe fn dealloc(&self, _ptr: *mut u8, _layout: Layout) {}
417 /// #[global_allocator]
418 /// static A: MyAllocator = MyAllocator;
422 /// assert!(alloc(Layout::new::<u32>()).is_null())
429 /// The `GlobalAlloc` trait is an `unsafe` trait for a number of reasons, and
430 /// implementors must ensure that they adhere to these contracts:
432 /// * It's undefined behavior if global allocators unwind. This restriction may
433 /// be lifted in the future, but currently a panic from any of these
434 /// functions may lead to memory unsafety.
436 /// * `Layout` queries and calculations in general must be correct. Callers of
437 /// this trait are allowed to rely on the contracts defined on each method,
438 /// and implementors must ensure such contracts remain true.
439 #[stable(feature = "global_alloc", since = "1.28.0")]
440 pub unsafe trait GlobalAlloc {
441 /// Allocate memory as described by the given `layout`.
443 /// Returns a pointer to newly-allocated memory,
444 /// or null to indicate allocation failure.
448 /// This function is unsafe because undefined behavior can result
449 /// if the caller does not ensure that `layout` has non-zero size.
451 /// (Extension subtraits might provide more specific bounds on
452 /// behavior, e.g., guarantee a sentinel address or a null pointer
453 /// in response to a zero-size allocation request.)
455 /// The allocated block of memory may or may not be initialized.
459 /// Returning a null pointer indicates that either memory is exhausted
460 /// or `layout` does not meet this allocator's size or alignment constraints.
462 /// Implementations are encouraged to return null on memory
463 /// exhaustion rather than aborting, but this is not
464 /// a strict requirement. (Specifically: it is *legal* to
465 /// implement this trait atop an underlying native allocation
466 /// library that aborts on memory exhaustion.)
468 /// Clients wishing to abort computation in response to an
469 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
470 /// rather than directly invoking `panic!` or similar.
472 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
473 #[stable(feature = "global_alloc", since = "1.28.0")]
474 unsafe fn alloc(&self, layout: Layout) -> *mut u8;
476 /// Deallocate the block of memory at the given `ptr` pointer with the given `layout`.
480 /// This function is unsafe because undefined behavior can result
481 /// if the caller does not ensure all of the following:
483 /// * `ptr` must denote a block of memory currently allocated via
486 /// * `layout` must be the same layout that was used
487 /// to allocate that block of memory,
488 #[stable(feature = "global_alloc", since = "1.28.0")]
489 unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout);
491 /// Behaves like `alloc`, but also ensures that the contents
492 /// are set to zero before being returned.
496 /// This function is unsafe for the same reasons that `alloc` is.
497 /// However the allocated block of memory is guaranteed to be initialized.
501 /// Returning a null pointer indicates that either memory is exhausted
502 /// or `layout` does not meet allocator's size or alignment constraints,
503 /// just as in `alloc`.
505 /// Clients wishing to abort computation in response to an
506 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
507 /// rather than directly invoking `panic!` or similar.
509 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
510 #[stable(feature = "global_alloc", since = "1.28.0")]
511 unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
512 let size = layout.size();
513 let ptr = self.alloc(layout);
515 ptr::write_bytes(ptr, 0, size);
520 /// Shrink or grow a block of memory to the given `new_size`.
521 /// The block is described by the given `ptr` pointer and `layout`.
523 /// If this returns a non-null pointer, then ownership of the memory block
524 /// referenced by `ptr` has been transferred to this allocator.
525 /// The memory may or may not have been deallocated,
526 /// and should be considered unusable (unless of course it was
527 /// transferred back to the caller again via the return value of
530 /// If this method returns null, then ownership of the memory
531 /// block has not been transferred to this allocator, and the
532 /// contents of the memory block are unaltered.
536 /// This function is unsafe because undefined behavior can result
537 /// if the caller does not ensure all of the following:
539 /// * `ptr` must be currently allocated via this allocator,
541 /// * `layout` must be the same layout that was used
542 /// to allocate that block of memory,
544 /// * `new_size` must be greater than zero.
546 /// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
547 /// must not overflow (i.e., the rounded value must be less than `usize::MAX`).
549 /// (Extension subtraits might provide more specific bounds on
550 /// behavior, e.g., guarantee a sentinel address or a null pointer
551 /// in response to a zero-size allocation request.)
555 /// Returns null if the new layout does not meet the size
556 /// and alignment constraints of the allocator, or if reallocation
559 /// Implementations are encouraged to return null on memory
560 /// exhaustion rather than panicking or aborting, but this is not
561 /// a strict requirement. (Specifically: it is *legal* to
562 /// implement this trait atop an underlying native allocation
563 /// library that aborts on memory exhaustion.)
565 /// Clients wishing to abort computation in response to a
566 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
567 /// rather than directly invoking `panic!` or similar.
569 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
570 #[stable(feature = "global_alloc", since = "1.28.0")]
571 unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
572 let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
573 let new_ptr = self.alloc(new_layout);
574 if !new_ptr.is_null() {
575 ptr::copy_nonoverlapping(ptr, new_ptr, cmp::min(layout.size(), new_size));
576 self.dealloc(ptr, layout);
582 /// An implementation of `Alloc` can allocate, reallocate, and
583 /// deallocate arbitrary blocks of data described via `Layout`.
585 /// Some of the methods require that a memory block be *currently
586 /// allocated* via an allocator. This means that:
588 /// * the starting address for that memory block was previously
589 /// returned by a previous call to an allocation method (`alloc`,
590 /// `alloc_zeroed`, `alloc_excess`, `alloc_one`, `alloc_array`) or
591 /// reallocation method (`realloc`, `realloc_excess`, or
592 /// `realloc_array`), and
594 /// * the memory block has not been subsequently deallocated, where
595 /// blocks are deallocated either by being passed to a deallocation
596 /// method (`dealloc`, `dealloc_one`, `dealloc_array`) or by being
597 /// passed to a reallocation method (see above) that returns `Ok`.
599 /// A note regarding zero-sized types and zero-sized layouts: many
600 /// methods in the `Alloc` trait state that allocation requests
601 /// must be non-zero size, or else undefined behavior can result.
603 /// * However, some higher-level allocation methods (`alloc_one`,
604 /// `alloc_array`) are well-defined on zero-sized types and can
605 /// optionally support them: it is left up to the implementor
606 /// whether to return `Err`, or to return `Ok` with some pointer.
608 /// * If an `Alloc` implementation chooses to return `Ok` in this
609 /// case (i.e., the pointer denotes a zero-sized inaccessible block)
610 /// then that returned pointer must be considered "currently
611 /// allocated". On such an allocator, *all* methods that take
612 /// currently-allocated pointers as inputs must accept these
613 /// zero-sized pointers, *without* causing undefined behavior.
615 /// * In other words, if a zero-sized pointer can flow out of an
616 /// allocator, then that allocator must likewise accept that pointer
617 /// flowing back into its deallocation and reallocation methods.
619 /// Some of the methods require that a layout *fit* a memory block.
620 /// What it means for a layout to "fit" a memory block means (or
621 /// equivalently, for a memory block to "fit" a layout) is that the
622 /// following two conditions must hold:
624 /// 1. The block's starting address must be aligned to `layout.align()`.
626 /// 2. The block's size must fall in the range `[use_min, use_max]`, where:
628 /// * `use_min` is `self.usable_size(layout).0`, and
630 /// * `use_max` is the capacity that was (or would have been)
631 /// returned when (if) the block was allocated via a call to
632 /// `alloc_excess` or `realloc_excess`.
636 /// * the size of the layout most recently used to allocate the block
637 /// is guaranteed to be in the range `[use_min, use_max]`, and
639 /// * a lower-bound on `use_max` can be safely approximated by a call to
642 /// * if a layout `k` fits a memory block (denoted by `ptr`)
643 /// currently allocated via an allocator `a`, then it is legal to
644 /// use that layout to deallocate it, i.e., `a.dealloc(ptr, k);`.
648 /// The `Alloc` trait is an `unsafe` trait for a number of reasons, and
649 /// implementors must ensure that they adhere to these contracts:
651 /// * Pointers returned from allocation functions must point to valid memory and
652 /// retain their validity until at least the instance of `Alloc` is dropped
655 /// * `Layout` queries and calculations in general must be correct. Callers of
656 /// this trait are allowed to rely on the contracts defined on each method,
657 /// and implementors must ensure such contracts remain true.
659 /// Note that this list may get tweaked over time as clarifications are made in
661 #[unstable(feature = "allocator_api", issue = "32838")]
662 pub unsafe trait Alloc {
663 // (Note: some existing allocators have unspecified but well-defined
664 // behavior in response to a zero size allocation request ;
665 // e.g., in C, `malloc` of 0 will either return a null pointer or a
666 // unique pointer, but will not have arbitrary undefined
668 // However in jemalloc for example,
669 // `mallocx(0)` is documented as undefined behavior.)
671 /// Returns a pointer meeting the size and alignment guarantees of
674 /// If this method returns an `Ok(addr)`, then the `addr` returned
675 /// will be non-null address pointing to a block of storage
676 /// suitable for holding an instance of `layout`.
678 /// The returned block of storage may or may not have its contents
679 /// initialized. (Extension subtraits might restrict this
680 /// behavior, e.g., to ensure initialization to particular sets of
685 /// This function is unsafe because undefined behavior can result
686 /// if the caller does not ensure that `layout` has non-zero size.
688 /// (Extension subtraits might provide more specific bounds on
689 /// behavior, e.g., guarantee a sentinel address or a null pointer
690 /// in response to a zero-size allocation request.)
694 /// Returning `Err` indicates that either memory is exhausted or
695 /// `layout` does not meet allocator's size or alignment
698 /// Implementations are encouraged to return `Err` on memory
699 /// exhaustion rather than panicking or aborting, but this is not
700 /// a strict requirement. (Specifically: it is *legal* to
701 /// implement this trait atop an underlying native allocation
702 /// library that aborts on memory exhaustion.)
704 /// Clients wishing to abort computation in response to an
705 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
706 /// rather than directly invoking `panic!` or similar.
708 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
709 unsafe fn alloc(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr>;
711 /// Deallocate the memory referenced by `ptr`.
715 /// This function is unsafe because undefined behavior can result
716 /// if the caller does not ensure all of the following:
718 /// * `ptr` must denote a block of memory currently allocated via
721 /// * `layout` must *fit* that block of memory,
723 /// * In addition to fitting the block of memory `layout`, the
724 /// alignment of the `layout` must match the alignment used
725 /// to allocate that block of memory.
726 unsafe fn dealloc(&mut self, ptr: NonNull<u8>, layout: Layout);
728 // == ALLOCATOR-SPECIFIC QUANTITIES AND LIMITS ==
731 /// Returns bounds on the guaranteed usable size of a successful
732 /// allocation created with the specified `layout`.
734 /// In particular, if one has a memory block allocated via a given
735 /// allocator `a` and layout `k` where `a.usable_size(k)` returns
736 /// `(l, u)`, then one can pass that block to `a.dealloc()` with a
737 /// layout in the size range [l, u].
739 /// (All implementors of `usable_size` must ensure that
740 /// `l <= k.size() <= u`)
742 /// Both the lower- and upper-bounds (`l` and `u` respectively)
743 /// are provided, because an allocator based on size classes could
744 /// misbehave if one attempts to deallocate a block without
745 /// providing a correct value for its size (i.e., one within the
748 /// Clients who wish to make use of excess capacity are encouraged
749 /// to use the `alloc_excess` and `realloc_excess` instead, as
750 /// this method is constrained to report conservative values that
751 /// serve as valid bounds for *all possible* allocation method
754 /// However, for clients that do not wish to track the capacity
755 /// returned by `alloc_excess` locally, this method is likely to
756 /// produce useful results.
758 fn usable_size(&self, layout: &Layout) -> (usize, usize) {
759 (layout.size(), layout.size())
762 // == METHODS FOR MEMORY REUSE ==
763 // realloc. alloc_excess, realloc_excess
765 /// Returns a pointer suitable for holding data described by
766 /// a new layout with `layout`’s alignment and a size given
767 /// by `new_size`. To
768 /// accomplish this, this may extend or shrink the allocation
769 /// referenced by `ptr` to fit the new layout.
771 /// If this returns `Ok`, then ownership of the memory block
772 /// referenced by `ptr` has been transferred to this
773 /// allocator. The memory may or may not have been freed, and
774 /// should be considered unusable (unless of course it was
775 /// transferred back to the caller again via the return value of
778 /// If this method returns `Err`, then ownership of the memory
779 /// block has not been transferred to this allocator, and the
780 /// contents of the memory block are unaltered.
784 /// This function is unsafe because undefined behavior can result
785 /// if the caller does not ensure all of the following:
787 /// * `ptr` must be currently allocated via this allocator,
789 /// * `layout` must *fit* the `ptr` (see above). (The `new_size`
790 /// argument need not fit it.)
792 /// * `new_size` must be greater than zero.
794 /// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
795 /// must not overflow (i.e., the rounded value must be less than `usize::MAX`).
797 /// (Extension subtraits might provide more specific bounds on
798 /// behavior, e.g., guarantee a sentinel address or a null pointer
799 /// in response to a zero-size allocation request.)
803 /// Returns `Err` only if the new layout
804 /// does not meet the allocator's size
805 /// and alignment constraints of the allocator, or if reallocation
808 /// Implementations are encouraged to return `Err` on memory
809 /// exhaustion rather than panicking or aborting, but this is not
810 /// a strict requirement. (Specifically: it is *legal* to
811 /// implement this trait atop an underlying native allocation
812 /// library that aborts on memory exhaustion.)
814 /// Clients wishing to abort computation in response to a
815 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
816 /// rather than directly invoking `panic!` or similar.
818 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
824 ) -> Result<NonNull<u8>, AllocErr> {
825 let old_size = layout.size();
827 if new_size >= old_size {
828 if let Ok(()) = self.grow_in_place(ptr, layout, new_size) {
831 } else if new_size < old_size {
832 if let Ok(()) = self.shrink_in_place(ptr, layout, new_size) {
837 // otherwise, fall back on alloc + copy + dealloc.
838 let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
839 let result = self.alloc(new_layout);
840 if let Ok(new_ptr) = result {
841 ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_ptr(), cmp::min(old_size, new_size));
842 self.dealloc(ptr, layout);
847 /// Behaves like `alloc`, but also ensures that the contents
848 /// are set to zero before being returned.
852 /// This function is unsafe for the same reasons that `alloc` is.
856 /// Returning `Err` indicates that either memory is exhausted or
857 /// `layout` does not meet allocator's size or alignment
858 /// constraints, just as in `alloc`.
860 /// Clients wishing to abort computation in response to an
861 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
862 /// rather than directly invoking `panic!` or similar.
864 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
865 unsafe fn alloc_zeroed(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr> {
866 let size = layout.size();
867 let p = self.alloc(layout);
869 ptr::write_bytes(p.as_ptr(), 0, size);
874 /// Behaves like `alloc`, but also returns the whole size of
875 /// the returned block. For some `layout` inputs, like arrays, this
876 /// may include extra storage usable for additional data.
880 /// This function is unsafe for the same reasons that `alloc` is.
884 /// Returning `Err` indicates that either memory is exhausted or
885 /// `layout` does not meet allocator's size or alignment
886 /// constraints, just as in `alloc`.
888 /// Clients wishing to abort computation in response to an
889 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
890 /// rather than directly invoking `panic!` or similar.
892 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
893 unsafe fn alloc_excess(&mut self, layout: Layout) -> Result<Excess, AllocErr> {
894 let usable_size = self.usable_size(&layout);
895 self.alloc(layout).map(|p| Excess(p, usable_size.1))
898 /// Behaves like `realloc`, but also returns the whole size of
899 /// the returned block. For some `layout` inputs, like arrays, this
900 /// may include extra storage usable for additional data.
904 /// This function is unsafe for the same reasons that `realloc` is.
908 /// Returning `Err` indicates that either memory is exhausted or
909 /// `layout` does not meet allocator's size or alignment
910 /// constraints, just as in `realloc`.
912 /// Clients wishing to abort computation in response to a
913 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
914 /// rather than directly invoking `panic!` or similar.
916 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
917 unsafe fn realloc_excess(
922 ) -> Result<Excess, AllocErr> {
923 let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
924 let usable_size = self.usable_size(&new_layout);
925 self.realloc(ptr, layout, new_size).map(|p| Excess(p, usable_size.1))
928 /// Attempts to extend the allocation referenced by `ptr` to fit `new_size`.
930 /// If this returns `Ok`, then the allocator has asserted that the
931 /// memory block referenced by `ptr` now fits `new_size`, and thus can
932 /// be used to carry data of a layout of that size and same alignment as
933 /// `layout`. (The allocator is allowed to
934 /// expend effort to accomplish this, such as extending the memory block to
935 /// include successor blocks, or virtual memory tricks.)
937 /// Regardless of what this method returns, ownership of the
938 /// memory block referenced by `ptr` has not been transferred, and
939 /// the contents of the memory block are unaltered.
943 /// This function is unsafe because undefined behavior can result
944 /// if the caller does not ensure all of the following:
946 /// * `ptr` must be currently allocated via this allocator,
948 /// * `layout` must *fit* the `ptr` (see above); note the
949 /// `new_size` argument need not fit it,
951 /// * `new_size` must not be less than `layout.size()`,
955 /// Returns `Err(CannotReallocInPlace)` when the allocator is
956 /// unable to assert that the memory block referenced by `ptr`
957 /// could fit `layout`.
959 /// Note that one cannot pass `CannotReallocInPlace` to the `handle_alloc_error`
960 /// function; clients are expected either to be able to recover from
961 /// `grow_in_place` failures without aborting, or to fall back on
962 /// another reallocation method before resorting to an abort.
963 unsafe fn grow_in_place(
968 ) -> Result<(), CannotReallocInPlace> {
969 let _ = ptr; // this default implementation doesn't care about the actual address.
970 debug_assert!(new_size >= layout.size());
971 let (_l, u) = self.usable_size(&layout);
972 // _l <= layout.size() [guaranteed by usable_size()]
973 // layout.size() <= new_layout.size() [required by this method]
974 if new_size <= u { Ok(()) } else { Err(CannotReallocInPlace) }
977 /// Attempts to shrink the allocation referenced by `ptr` to fit `new_size`.
979 /// If this returns `Ok`, then the allocator has asserted that the
980 /// memory block referenced by `ptr` now fits `new_size`, and
981 /// thus can only be used to carry data of that smaller
982 /// layout. (The allocator is allowed to take advantage of this,
983 /// carving off portions of the block for reuse elsewhere.) The
984 /// truncated contents of the block within the smaller layout are
985 /// unaltered, and ownership of block has not been transferred.
987 /// If this returns `Err`, then the memory block is considered to
988 /// still represent the original (larger) `layout`. None of the
989 /// block has been carved off for reuse elsewhere, ownership of
990 /// the memory block has not been transferred, and the contents of
991 /// the memory block are unaltered.
995 /// This function is unsafe because undefined behavior can result
996 /// if the caller does not ensure all of the following:
998 /// * `ptr` must be currently allocated via this allocator,
1000 /// * `layout` must *fit* the `ptr` (see above); note the
1001 /// `new_size` argument need not fit it,
1003 /// * `new_size` must not be greater than `layout.size()`
1004 /// (and must be greater than zero),
1008 /// Returns `Err(CannotReallocInPlace)` when the allocator is
1009 /// unable to assert that the memory block referenced by `ptr`
1010 /// could fit `layout`.
1012 /// Note that one cannot pass `CannotReallocInPlace` to the `handle_alloc_error`
1013 /// function; clients are expected either to be able to recover from
1014 /// `shrink_in_place` failures without aborting, or to fall back
1015 /// on another reallocation method before resorting to an abort.
1016 unsafe fn shrink_in_place(
1021 ) -> Result<(), CannotReallocInPlace> {
1022 let _ = ptr; // this default implementation doesn't care about the actual address.
1023 debug_assert!(new_size <= layout.size());
1024 let (l, _u) = self.usable_size(&layout);
1025 // layout.size() <= _u [guaranteed by usable_size()]
1026 // new_layout.size() <= layout.size() [required by this method]
1027 if l <= new_size { Ok(()) } else { Err(CannotReallocInPlace) }
1030 // == COMMON USAGE PATTERNS ==
1031 // alloc_one, dealloc_one, alloc_array, realloc_array. dealloc_array
1033 /// Allocates a block suitable for holding an instance of `T`.
1035 /// Captures a common usage pattern for allocators.
1037 /// The returned block is suitable for passing to the
1038 /// `realloc`/`dealloc` methods of this allocator.
1040 /// Note to implementors: If this returns `Ok(ptr)`, then `ptr`
1041 /// must be considered "currently allocated" and must be
1042 /// acceptable input to methods such as `realloc` or `dealloc`,
1043 /// *even if* `T` is a zero-sized type. In other words, if your
1044 /// `Alloc` implementation overrides this method in a manner
1045 /// that can return a zero-sized `ptr`, then all reallocation and
1046 /// deallocation methods need to be similarly overridden to accept
1047 /// such values as input.
1051 /// Returning `Err` indicates that either memory is exhausted or
1052 /// `T` does not meet allocator's size or alignment constraints.
1054 /// For zero-sized `T`, may return either of `Ok` or `Err`, but
1055 /// will *not* yield undefined behavior.
1057 /// Clients wishing to abort computation in response to an
1058 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
1059 /// rather than directly invoking `panic!` or similar.
1061 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1062 fn alloc_one<T>(&mut self) -> Result<NonNull<T>, AllocErr>
1066 let k = Layout::new::<T>();
1067 if k.size() > 0 { unsafe { self.alloc(k).map(|p| p.cast()) } } else { Err(AllocErr) }
1070 /// Deallocates a block suitable for holding an instance of `T`.
1072 /// The given block must have been produced by this allocator,
1073 /// and must be suitable for storing a `T` (in terms of alignment
1074 /// as well as minimum and maximum size); otherwise yields
1075 /// undefined behavior.
1077 /// Captures a common usage pattern for allocators.
1081 /// This function is unsafe because undefined behavior can result
1082 /// if the caller does not ensure both:
1084 /// * `ptr` must denote a block of memory currently allocated via this allocator
1086 /// * the layout of `T` must *fit* that block of memory.
1087 unsafe fn dealloc_one<T>(&mut self, ptr: NonNull<T>)
1091 let k = Layout::new::<T>();
1093 self.dealloc(ptr.cast(), k);
1097 /// Allocates a block suitable for holding `n` instances of `T`.
1099 /// Captures a common usage pattern for allocators.
1101 /// The returned block is suitable for passing to the
1102 /// `realloc`/`dealloc` methods of this allocator.
1104 /// Note to implementors: If this returns `Ok(ptr)`, then `ptr`
1105 /// must be considered "currently allocated" and must be
1106 /// acceptable input to methods such as `realloc` or `dealloc`,
1107 /// *even if* `T` is a zero-sized type. In other words, if your
1108 /// `Alloc` implementation overrides this method in a manner
1109 /// that can return a zero-sized `ptr`, then all reallocation and
1110 /// deallocation methods need to be similarly overridden to accept
1111 /// such values as input.
1115 /// Returning `Err` indicates that either memory is exhausted or
1116 /// `[T; n]` does not meet allocator's size or alignment
1119 /// For zero-sized `T` or `n == 0`, may return either of `Ok` or
1120 /// `Err`, but will *not* yield undefined behavior.
1122 /// Always returns `Err` on arithmetic overflow.
1124 /// Clients wishing to abort computation in response to an
1125 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
1126 /// rather than directly invoking `panic!` or similar.
1128 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1129 fn alloc_array<T>(&mut self, n: usize) -> Result<NonNull<T>, AllocErr>
1133 match Layout::array::<T>(n) {
1134 Ok(layout) if layout.size() > 0 => unsafe { self.alloc(layout).map(|p| p.cast()) },
1139 /// Reallocates a block previously suitable for holding `n_old`
1140 /// instances of `T`, returning a block suitable for holding
1141 /// `n_new` instances of `T`.
1143 /// Captures a common usage pattern for allocators.
1145 /// The returned block is suitable for passing to the
1146 /// `realloc`/`dealloc` methods of this allocator.
1150 /// This function is unsafe because undefined behavior can result
1151 /// if the caller does not ensure all of the following:
1153 /// * `ptr` must be currently allocated via this allocator,
1155 /// * the layout of `[T; n_old]` must *fit* that block of memory.
1159 /// Returning `Err` indicates that either memory is exhausted or
1160 /// `[T; n_new]` does not meet allocator's size or alignment
1163 /// For zero-sized `T` or `n_new == 0`, may return either of `Ok` or
1164 /// `Err`, but will *not* yield undefined behavior.
1166 /// Always returns `Err` on arithmetic overflow.
1168 /// Clients wishing to abort computation in response to a
1169 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
1170 /// rather than directly invoking `panic!` or similar.
1172 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1173 unsafe fn realloc_array<T>(
1178 ) -> Result<NonNull<T>, AllocErr>
1182 match (Layout::array::<T>(n_old), Layout::array::<T>(n_new)) {
1183 (Ok(k_old), Ok(k_new)) if k_old.size() > 0 && k_new.size() > 0 => {
1184 debug_assert!(k_old.align() == k_new.align());
1185 self.realloc(ptr.cast(), k_old, k_new.size()).map(NonNull::cast)
1191 /// Deallocates a block suitable for holding `n` instances of `T`.
1193 /// Captures a common usage pattern for allocators.
1197 /// This function is unsafe because undefined behavior can result
1198 /// if the caller does not ensure both:
1200 /// * `ptr` must denote a block of memory currently allocated via this allocator
1202 /// * the layout of `[T; n]` must *fit* that block of memory.
1206 /// Returning `Err` indicates that either `[T; n]` or the given
1207 /// memory block does not meet allocator's size or alignment
1210 /// Always returns `Err` on arithmetic overflow.
1211 unsafe fn dealloc_array<T>(&mut self, ptr: NonNull<T>, n: usize) -> Result<(), AllocErr>
1215 match Layout::array::<T>(n) {
1216 Ok(k) if k.size() > 0 => Ok(self.dealloc(ptr.cast(), k)),