1 //! Memory allocation APIs
3 // ignore-tidy-undocumented-unsafe
5 #![stable(feature = "alloc_module", since = "1.28.0")]
11 use crate::ptr::{self, NonNull};
12 use crate::num::NonZeroUsize;
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 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")]
68 pub fn from_size_align(size: usize, align: usize) -> Result<Self, LayoutErr> {
69 if !align.is_power_of_two() {
70 return Err(LayoutErr { private: () });
73 // (power-of-two implies align != 0.)
75 // Rounded up size is:
76 // size_rounded_up = (size + align - 1) & !(align - 1);
78 // We know from above that align != 0. If adding (align - 1)
79 // does not overflow, then rounding up will be fine.
81 // Conversely, &-masking with !(align - 1) will subtract off
82 // only low-order-bits. Thus if overflow occurs with the sum,
83 // the &-mask cannot subtract enough to undo that overflow.
85 // Above implies that checking for summation overflow is both
86 // necessary and sufficient.
87 if size > usize::MAX - (align - 1) {
88 return Err(LayoutErr { private: () });
92 Ok(Layout::from_size_align_unchecked(size, align))
96 /// Creates a layout, bypassing all checks.
100 /// This function is unsafe as it does not verify the preconditions from
101 /// [`Layout::from_size_align`](#method.from_size_align).
102 #[stable(feature = "alloc_layout", since = "1.28.0")]
103 #[rustc_const_stable(feature = "alloc_layout", since = "1.28.0")]
105 pub const unsafe fn from_size_align_unchecked(size: usize, align: usize) -> Self {
106 Layout { size_: size, align_: NonZeroUsize::new_unchecked(align) }
109 /// The minimum size in bytes for a memory block of this layout.
110 #[stable(feature = "alloc_layout", since = "1.28.0")]
112 pub fn size(&self) -> usize { self.size_ }
114 /// The minimum byte alignment for a memory block of this layout.
115 #[stable(feature = "alloc_layout", since = "1.28.0")]
117 pub fn align(&self) -> usize { self.align_.get() }
119 /// Constructs a `Layout` suitable for holding a value of type `T`.
120 #[stable(feature = "alloc_layout", since = "1.28.0")]
122 pub fn new<T>() -> Self {
123 let (size, align) = size_align::<T>();
124 // Note that the align is guaranteed by rustc to be a power of two and
125 // the size+align combo is guaranteed to fit in our address space. As a
126 // result use the unchecked constructor here to avoid inserting code
127 // that panics if it isn't optimized well enough.
128 debug_assert!(Layout::from_size_align(size, align).is_ok());
130 Layout::from_size_align_unchecked(size, align)
134 /// Produces layout describing a record that could be used to
135 /// allocate backing structure for `T` (which could be a trait
136 /// or other unsized type like a slice).
137 #[stable(feature = "alloc_layout", since = "1.28.0")]
139 pub fn for_value<T: ?Sized>(t: &T) -> Self {
140 let (size, align) = (mem::size_of_val(t), mem::align_of_val(t));
141 // See rationale in `new` for why this is using an unsafe variant below
142 debug_assert!(Layout::from_size_align(size, align).is_ok());
144 Layout::from_size_align_unchecked(size, align)
148 /// Creates a layout describing the record that can hold a value
149 /// of the same layout as `self`, but that also is aligned to
150 /// alignment `align` (measured in bytes).
152 /// If `self` already meets the prescribed alignment, then returns
155 /// Note that this method does not add any padding to the overall
156 /// size, regardless of whether the returned layout has a different
157 /// alignment. In other words, if `K` has size 16, `K.align_to(32)`
158 /// will *still* have size 16.
160 /// Returns an error if the combination of `self.size()` and the given
161 /// `align` violates the conditions listed in
162 /// [`Layout::from_size_align`](#method.from_size_align).
163 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
165 pub fn align_to(&self, align: usize) -> Result<Self, LayoutErr> {
166 Layout::from_size_align(self.size(), cmp::max(self.align(), align))
169 /// Returns the amount of padding we must insert after `self`
170 /// to ensure that the following address will satisfy `align`
171 /// (measured in bytes).
173 /// e.g., if `self.size()` is 9, then `self.padding_needed_for(4)`
174 /// returns 3, because that is the minimum number of bytes of
175 /// padding required to get a 4-aligned address (assuming that the
176 /// corresponding memory block starts at a 4-aligned address).
178 /// The return value of this function has no meaning if `align` is
179 /// not a power-of-two.
181 /// Note that the utility of the returned value requires `align`
182 /// to be less than or equal to the alignment of the starting
183 /// address for the whole allocated block of memory. One way to
184 /// satisfy this constraint is to ensure `align <= self.align()`.
185 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
187 pub fn padding_needed_for(&self, align: usize) -> usize {
188 let len = self.size();
190 // Rounded up value is:
191 // len_rounded_up = (len + align - 1) & !(align - 1);
192 // and then we return the padding difference: `len_rounded_up - len`.
194 // We use modular arithmetic throughout:
196 // 1. align is guaranteed to be > 0, so align - 1 is always
199 // 2. `len + align - 1` can overflow by at most `align - 1`,
200 // so the &-mask with `!(align - 1)` will ensure that in the
201 // case of overflow, `len_rounded_up` will itself be 0.
202 // Thus the returned padding, when added to `len`, yields 0,
203 // which trivially satisfies the alignment `align`.
205 // (Of course, attempts to allocate blocks of memory whose
206 // size and padding overflow in the above manner should cause
207 // the allocator to yield an error anyway.)
209 let len_rounded_up = len.wrapping_add(align).wrapping_sub(1)
210 & !align.wrapping_sub(1);
211 len_rounded_up.wrapping_sub(len)
214 /// Creates a layout by rounding the size of this layout up to a multiple
215 /// of the layout's alignment.
217 /// This is equivalent to adding the result of `padding_needed_for`
218 /// to the layout's current size.
219 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
221 pub fn pad_to_align(&self) -> Layout {
222 let pad = self.padding_needed_for(self.align());
223 // This cannot overflow. Quoting from the invariant of Layout:
224 // > `size`, when rounded up to the nearest multiple of `align`,
225 // > must not overflow (i.e., the rounded value must be less than
227 let new_size = self.size() + pad;
229 Layout::from_size_align(new_size, self.align()).unwrap()
232 /// Creates a layout describing the record for `n` instances of
233 /// `self`, with a suitable amount of padding between each to
234 /// ensure that each instance is given its requested size and
235 /// alignment. On success, returns `(k, offs)` where `k` is the
236 /// layout of the array and `offs` is the distance between the start
237 /// of each element in the array.
239 /// On arithmetic overflow, returns `LayoutErr`.
240 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
242 pub fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutErr> {
243 // This cannot overflow. Quoting from the invariant of Layout:
244 // > `size`, when rounded up to the nearest multiple of `align`,
245 // > must not overflow (i.e., the rounded value must be less than
247 let padded_size = self.size() + self.padding_needed_for(self.align());
248 let alloc_size = padded_size.checked_mul(n)
249 .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)
279 .ok_or(LayoutErr { private: () })?;
280 let new_size = offset.checked_add(next.size())
281 .ok_or(LayoutErr { private: () })?;
283 let layout = Layout::from_size_align(new_size, new_align)?;
287 /// Creates a layout describing the record for `n` instances of
288 /// `self`, with no padding between each instance.
290 /// Note that, unlike `repeat`, `repeat_packed` does not guarantee
291 /// that the repeated instances of `self` will be properly
292 /// aligned, even if a given instance of `self` is properly
293 /// aligned. In other words, if the layout returned by
294 /// `repeat_packed` is used to allocate an array, it is not
295 /// guaranteed that all elements in the array will be properly
298 /// On arithmetic overflow, returns `LayoutErr`.
299 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
301 pub fn repeat_packed(&self, n: usize) -> Result<Self, LayoutErr> {
302 let size = self.size().checked_mul(n).ok_or(LayoutErr { private: () })?;
303 Layout::from_size_align(size, self.align())
306 /// Creates a layout describing the record for `self` followed by
307 /// `next` with no additional padding between the two. Since no
308 /// padding is inserted, the alignment of `next` is irrelevant,
309 /// and is not incorporated *at all* into the resulting layout.
311 /// On arithmetic overflow, returns `LayoutErr`.
312 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
314 pub fn extend_packed(&self, next: Self) -> Result<Self, LayoutErr> {
315 let new_size = self.size().checked_add(next.size())
316 .ok_or(LayoutErr { private: () })?;
317 Layout::from_size_align(new_size, self.align())
320 /// Creates a layout describing the record for a `[T; n]`.
322 /// On arithmetic overflow, returns `LayoutErr`.
323 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
325 pub fn array<T>(n: usize) -> Result<Self, LayoutErr> {
329 debug_assert!(offs == mem::size_of::<T>());
335 /// The parameters given to `Layout::from_size_align`
336 /// or some other `Layout` constructor
337 /// do not satisfy its documented constraints.
338 #[stable(feature = "alloc_layout", since = "1.28.0")]
339 #[derive(Clone, PartialEq, Eq, Debug)]
340 pub struct LayoutErr {
344 // (we need this for downstream impl of trait Error)
345 #[stable(feature = "alloc_layout", since = "1.28.0")]
346 impl fmt::Display for LayoutErr {
347 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
348 f.write_str("invalid parameters to Layout::from_size_align")
352 /// The `AllocErr` error indicates an allocation failure
353 /// that may be due to resource exhaustion or to
354 /// something wrong when combining the given input arguments with this
356 #[unstable(feature = "allocator_api", issue = "32838")]
357 #[derive(Clone, PartialEq, Eq, Debug)]
360 // (we need this for downstream impl of trait Error)
361 #[unstable(feature = "allocator_api", issue = "32838")]
362 impl fmt::Display for AllocErr {
363 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
364 f.write_str("memory allocation failed")
368 /// The `CannotReallocInPlace` error is used when [`grow_in_place`] or
369 /// [`shrink_in_place`] were unable to reuse the given memory block for
370 /// a requested layout.
372 /// [`grow_in_place`]: ./trait.Alloc.html#method.grow_in_place
373 /// [`shrink_in_place`]: ./trait.Alloc.html#method.shrink_in_place
374 #[unstable(feature = "allocator_api", issue = "32838")]
375 #[derive(Clone, PartialEq, Eq, Debug)]
376 pub struct CannotReallocInPlace;
378 #[unstable(feature = "allocator_api", issue = "32838")]
379 impl CannotReallocInPlace {
380 pub fn description(&self) -> &str {
381 "cannot reallocate allocator's memory in place"
385 // (we need this for downstream impl of trait Error)
386 #[unstable(feature = "allocator_api", issue = "32838")]
387 impl fmt::Display for CannotReallocInPlace {
388 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
389 write!(f, "{}", self.description())
393 /// A memory allocator that can be registered as the standard library’s default
394 /// through the `#[global_allocator]` attribute.
396 /// Some of the methods require that a memory block be *currently
397 /// allocated* via an allocator. This means that:
399 /// * the starting address for that memory block was previously
400 /// returned by a previous call to an allocation method
401 /// such as `alloc`, and
403 /// * the memory block has not been subsequently deallocated, where
404 /// blocks are deallocated either by being passed to a deallocation
405 /// method such as `dealloc` or by being
406 /// passed to a reallocation method that returns a non-null pointer.
412 /// use std::alloc::{GlobalAlloc, Layout, alloc};
413 /// use std::ptr::null_mut;
415 /// struct MyAllocator;
417 /// unsafe impl GlobalAlloc for MyAllocator {
418 /// unsafe fn alloc(&self, _layout: Layout) -> *mut u8 { null_mut() }
419 /// unsafe fn dealloc(&self, _ptr: *mut u8, _layout: Layout) {}
422 /// #[global_allocator]
423 /// static A: MyAllocator = MyAllocator;
427 /// assert!(alloc(Layout::new::<u32>()).is_null())
434 /// The `GlobalAlloc` trait is an `unsafe` trait for a number of reasons, and
435 /// implementors must ensure that they adhere to these contracts:
437 /// * It's undefined behavior if global allocators unwind. This restriction may
438 /// be lifted in the future, but currently a panic from any of these
439 /// functions may lead to memory unsafety.
441 /// * `Layout` queries and calculations in general must be correct. Callers of
442 /// this trait are allowed to rely on the contracts defined on each method,
443 /// and implementors must ensure such contracts remain true.
444 #[stable(feature = "global_alloc", since = "1.28.0")]
445 pub unsafe trait GlobalAlloc {
446 /// Allocate memory as described by the given `layout`.
448 /// Returns a pointer to newly-allocated memory,
449 /// or null to indicate allocation failure.
453 /// This function is unsafe because undefined behavior can result
454 /// if the caller does not ensure that `layout` has non-zero size.
456 /// (Extension subtraits might provide more specific bounds on
457 /// behavior, e.g., guarantee a sentinel address or a null pointer
458 /// in response to a zero-size allocation request.)
460 /// The allocated block of memory may or may not be initialized.
464 /// Returning a null pointer indicates that either memory is exhausted
465 /// or `layout` does not meet this allocator's size or alignment constraints.
467 /// Implementations are encouraged to return null on memory
468 /// exhaustion rather than aborting, but this is not
469 /// a strict requirement. (Specifically: it is *legal* to
470 /// implement this trait atop an underlying native allocation
471 /// library that aborts on memory exhaustion.)
473 /// Clients wishing to abort computation in response to an
474 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
475 /// rather than directly invoking `panic!` or similar.
477 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
478 #[stable(feature = "global_alloc", since = "1.28.0")]
479 unsafe fn alloc(&self, layout: Layout) -> *mut u8;
481 /// Deallocate the block of memory at the given `ptr` pointer with the given `layout`.
485 /// This function is unsafe because undefined behavior can result
486 /// if the caller does not ensure all of the following:
488 /// * `ptr` must denote a block of memory currently allocated via
491 /// * `layout` must be the same layout that was used
492 /// to allocate that block of memory,
493 #[stable(feature = "global_alloc", since = "1.28.0")]
494 unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout);
496 /// Behaves like `alloc`, but also ensures that the contents
497 /// are set to zero before being returned.
501 /// This function is unsafe for the same reasons that `alloc` is.
502 /// However the allocated block of memory is guaranteed to be initialized.
506 /// Returning a null pointer indicates that either memory is exhausted
507 /// or `layout` does not meet allocator's size or alignment constraints,
508 /// just as in `alloc`.
510 /// Clients wishing to abort computation in response to an
511 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
512 /// rather than directly invoking `panic!` or similar.
514 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
515 #[stable(feature = "global_alloc", since = "1.28.0")]
516 unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
517 let size = layout.size();
518 let ptr = self.alloc(layout);
520 ptr::write_bytes(ptr, 0, size);
525 /// Shrink or grow a block of memory to the given `new_size`.
526 /// The block is described by the given `ptr` pointer and `layout`.
528 /// If this returns a non-null pointer, then ownership of the memory block
529 /// referenced by `ptr` has been transferred to this allocator.
530 /// The memory may or may not have been deallocated,
531 /// and should be considered unusable (unless of course it was
532 /// transferred back to the caller again via the return value of
535 /// If this method returns null, then ownership of the memory
536 /// block has not been transferred to this allocator, and the
537 /// contents of the memory block are unaltered.
541 /// This function is unsafe because undefined behavior can result
542 /// if the caller does not ensure all of the following:
544 /// * `ptr` must be currently allocated via this allocator,
546 /// * `layout` must be the same layout that was used
547 /// to allocate that block of memory,
549 /// * `new_size` must be greater than zero.
551 /// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
552 /// must not overflow (i.e., the rounded value must be less than `usize::MAX`).
554 /// (Extension subtraits might provide more specific bounds on
555 /// behavior, e.g., guarantee a sentinel address or a null pointer
556 /// in response to a zero-size allocation request.)
560 /// Returns null if the new layout does not meet the size
561 /// and alignment constraints of the allocator, or if reallocation
564 /// Implementations are encouraged to return null on memory
565 /// exhaustion rather than panicking or aborting, but this is not
566 /// a strict requirement. (Specifically: it is *legal* to
567 /// implement this trait atop an underlying native allocation
568 /// library that aborts on memory exhaustion.)
570 /// Clients wishing to abort computation in response to a
571 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
572 /// rather than directly invoking `panic!` or similar.
574 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
575 #[stable(feature = "global_alloc", since = "1.28.0")]
576 unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
577 let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
578 let new_ptr = self.alloc(new_layout);
579 if !new_ptr.is_null() {
580 ptr::copy_nonoverlapping(
583 cmp::min(layout.size(), new_size),
585 self.dealloc(ptr, layout);
591 /// An implementation of `Alloc` can allocate, reallocate, and
592 /// deallocate arbitrary blocks of data described via `Layout`.
594 /// Some of the methods require that a memory block be *currently
595 /// allocated* via an allocator. This means that:
597 /// * the starting address for that memory block was previously
598 /// returned by a previous call to an allocation method (`alloc`,
599 /// `alloc_zeroed`, `alloc_excess`, `alloc_one`, `alloc_array`) or
600 /// reallocation method (`realloc`, `realloc_excess`, or
601 /// `realloc_array`), and
603 /// * the memory block has not been subsequently deallocated, where
604 /// blocks are deallocated either by being passed to a deallocation
605 /// method (`dealloc`, `dealloc_one`, `dealloc_array`) or by being
606 /// passed to a reallocation method (see above) that returns `Ok`.
608 /// A note regarding zero-sized types and zero-sized layouts: many
609 /// methods in the `Alloc` trait state that allocation requests
610 /// must be non-zero size, or else undefined behavior can result.
612 /// * However, some higher-level allocation methods (`alloc_one`,
613 /// `alloc_array`) are well-defined on zero-sized types and can
614 /// optionally support them: it is left up to the implementor
615 /// whether to return `Err`, or to return `Ok` with some pointer.
617 /// * If an `Alloc` implementation chooses to return `Ok` in this
618 /// case (i.e., the pointer denotes a zero-sized inaccessible block)
619 /// then that returned pointer must be considered "currently
620 /// allocated". On such an allocator, *all* methods that take
621 /// currently-allocated pointers as inputs must accept these
622 /// zero-sized pointers, *without* causing undefined behavior.
624 /// * In other words, if a zero-sized pointer can flow out of an
625 /// allocator, then that allocator must likewise accept that pointer
626 /// flowing back into its deallocation and reallocation methods.
628 /// Some of the methods require that a layout *fit* a memory block.
629 /// What it means for a layout to "fit" a memory block means (or
630 /// equivalently, for a memory block to "fit" a layout) is that the
631 /// following two conditions must hold:
633 /// 1. The block's starting address must be aligned to `layout.align()`.
635 /// 2. The block's size must fall in the range `[use_min, use_max]`, where:
637 /// * `use_min` is `self.usable_size(layout).0`, and
639 /// * `use_max` is the capacity that was (or would have been)
640 /// returned when (if) the block was allocated via a call to
641 /// `alloc_excess` or `realloc_excess`.
645 /// * the size of the layout most recently used to allocate the block
646 /// is guaranteed to be in the range `[use_min, use_max]`, and
648 /// * a lower-bound on `use_max` can be safely approximated by a call to
651 /// * if a layout `k` fits a memory block (denoted by `ptr`)
652 /// currently allocated via an allocator `a`, then it is legal to
653 /// use that layout to deallocate it, i.e., `a.dealloc(ptr, k);`.
657 /// The `Alloc` trait is an `unsafe` trait for a number of reasons, and
658 /// implementors must ensure that they adhere to these contracts:
660 /// * Pointers returned from allocation functions must point to valid memory and
661 /// retain their validity until at least the instance of `Alloc` is dropped
664 /// * `Layout` queries and calculations in general must be correct. Callers of
665 /// this trait are allowed to rely on the contracts defined on each method,
666 /// and implementors must ensure such contracts remain true.
668 /// Note that this list may get tweaked over time as clarifications are made in
670 #[unstable(feature = "allocator_api", issue = "32838")]
671 pub unsafe trait Alloc {
673 // (Note: some existing allocators have unspecified but well-defined
674 // behavior in response to a zero size allocation request ;
675 // e.g., in C, `malloc` of 0 will either return a null pointer or a
676 // unique pointer, but will not have arbitrary undefined
678 // However in jemalloc for example,
679 // `mallocx(0)` is documented as undefined behavior.)
681 /// Returns a pointer meeting the size and alignment guarantees of
684 /// If this method returns an `Ok(addr)`, then the `addr` returned
685 /// will be non-null address pointing to a block of storage
686 /// suitable for holding an instance of `layout`.
688 /// The returned block of storage may or may not have its contents
689 /// initialized. (Extension subtraits might restrict this
690 /// behavior, e.g., to ensure initialization to particular sets of
695 /// This function is unsafe because undefined behavior can result
696 /// if the caller does not ensure that `layout` has non-zero size.
698 /// (Extension subtraits might provide more specific bounds on
699 /// behavior, e.g., guarantee a sentinel address or a null pointer
700 /// in response to a zero-size allocation request.)
704 /// Returning `Err` indicates that either memory is exhausted or
705 /// `layout` does not meet allocator's size or alignment
708 /// Implementations are encouraged to return `Err` on memory
709 /// exhaustion rather than panicking or aborting, but this is not
710 /// a strict requirement. (Specifically: it is *legal* to
711 /// implement this trait atop an underlying native allocation
712 /// library that aborts on memory exhaustion.)
714 /// Clients wishing to abort computation in response to an
715 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
716 /// rather than directly invoking `panic!` or similar.
718 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
719 unsafe fn alloc(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr>;
721 /// Deallocate the memory referenced by `ptr`.
725 /// This function is unsafe because undefined behavior can result
726 /// if the caller does not ensure all of the following:
728 /// * `ptr` must denote a block of memory currently allocated via
731 /// * `layout` must *fit* that block of memory,
733 /// * In addition to fitting the block of memory `layout`, the
734 /// alignment of the `layout` must match the alignment used
735 /// to allocate that block of memory.
736 unsafe fn dealloc(&mut self, ptr: NonNull<u8>, layout: Layout);
738 // == ALLOCATOR-SPECIFIC QUANTITIES AND LIMITS ==
741 /// Returns bounds on the guaranteed usable size of a successful
742 /// allocation created with the specified `layout`.
744 /// In particular, if one has a memory block allocated via a given
745 /// allocator `a` and layout `k` where `a.usable_size(k)` returns
746 /// `(l, u)`, then one can pass that block to `a.dealloc()` with a
747 /// layout in the size range [l, u].
749 /// (All implementors of `usable_size` must ensure that
750 /// `l <= k.size() <= u`)
752 /// Both the lower- and upper-bounds (`l` and `u` respectively)
753 /// are provided, because an allocator based on size classes could
754 /// misbehave if one attempts to deallocate a block without
755 /// providing a correct value for its size (i.e., one within the
758 /// Clients who wish to make use of excess capacity are encouraged
759 /// to use the `alloc_excess` and `realloc_excess` instead, as
760 /// this method is constrained to report conservative values that
761 /// serve as valid bounds for *all possible* allocation method
764 /// However, for clients that do not wish to track the capacity
765 /// returned by `alloc_excess` locally, this method is likely to
766 /// produce useful results.
768 fn usable_size(&self, layout: &Layout) -> (usize, usize) {
769 (layout.size(), layout.size())
772 // == METHODS FOR MEMORY REUSE ==
773 // realloc. alloc_excess, realloc_excess
775 /// Returns a pointer suitable for holding data described by
776 /// a new layout with `layout`’s alignment and a size given
777 /// by `new_size`. To
778 /// accomplish this, this may extend or shrink the allocation
779 /// referenced by `ptr` to fit the new layout.
781 /// If this returns `Ok`, then ownership of the memory block
782 /// referenced by `ptr` has been transferred to this
783 /// allocator. The memory may or may not have been freed, and
784 /// should be considered unusable (unless of course it was
785 /// transferred back to the caller again via the return value of
788 /// If this method returns `Err`, then ownership of the memory
789 /// block has not been transferred to this allocator, and the
790 /// contents of the memory block are unaltered.
794 /// This function is unsafe because undefined behavior can result
795 /// if the caller does not ensure all of the following:
797 /// * `ptr` must be currently allocated via this allocator,
799 /// * `layout` must *fit* the `ptr` (see above). (The `new_size`
800 /// argument need not fit it.)
802 /// * `new_size` must be greater than zero.
804 /// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
805 /// must not overflow (i.e., the rounded value must be less than `usize::MAX`).
807 /// (Extension subtraits might provide more specific bounds on
808 /// behavior, e.g., guarantee a sentinel address or a null pointer
809 /// in response to a zero-size allocation request.)
813 /// Returns `Err` only if the new layout
814 /// does not meet the allocator's size
815 /// and alignment constraints of the allocator, or if reallocation
818 /// Implementations are encouraged to return `Err` on memory
819 /// exhaustion rather than panicking or aborting, but this is not
820 /// a strict requirement. (Specifically: it is *legal* to
821 /// implement this trait atop an underlying native allocation
822 /// library that aborts on memory exhaustion.)
824 /// Clients wishing to abort computation in response to a
825 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
826 /// rather than directly invoking `panic!` or similar.
828 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
829 unsafe fn realloc(&mut self,
832 new_size: usize) -> Result<NonNull<u8>, AllocErr> {
833 let old_size = layout.size();
835 if new_size >= old_size {
836 if let Ok(()) = self.grow_in_place(ptr, layout, new_size) {
839 } else if new_size < old_size {
840 if let Ok(()) = self.shrink_in_place(ptr, layout, new_size) {
845 // otherwise, fall back on alloc + copy + dealloc.
846 let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
847 let result = self.alloc(new_layout);
848 if let Ok(new_ptr) = result {
849 ptr::copy_nonoverlapping(ptr.as_ptr(),
851 cmp::min(old_size, new_size));
852 self.dealloc(ptr, layout);
857 /// Behaves like `alloc`, but also ensures that the contents
858 /// are set to zero before being returned.
862 /// This function is unsafe for the same reasons that `alloc` is.
866 /// Returning `Err` indicates that either memory is exhausted or
867 /// `layout` does not meet allocator's size or alignment
868 /// constraints, just as in `alloc`.
870 /// Clients wishing to abort computation in response to an
871 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
872 /// rather than directly invoking `panic!` or similar.
874 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
875 unsafe fn alloc_zeroed(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr> {
876 let size = layout.size();
877 let p = self.alloc(layout);
879 ptr::write_bytes(p.as_ptr(), 0, size);
884 /// Behaves like `alloc`, but also returns the whole size of
885 /// the returned block. For some `layout` inputs, like arrays, this
886 /// may include extra storage usable for additional data.
890 /// This function is unsafe for the same reasons that `alloc` is.
894 /// Returning `Err` indicates that either memory is exhausted or
895 /// `layout` does not meet allocator's size or alignment
896 /// constraints, just as in `alloc`.
898 /// Clients wishing to abort computation in response to an
899 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
900 /// rather than directly invoking `panic!` or similar.
902 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
903 unsafe fn alloc_excess(&mut self, layout: Layout) -> Result<Excess, AllocErr> {
904 let usable_size = self.usable_size(&layout);
905 self.alloc(layout).map(|p| Excess(p, usable_size.1))
908 /// Behaves like `realloc`, but also returns the whole size of
909 /// the returned block. For some `layout` inputs, like arrays, this
910 /// may include extra storage usable for additional data.
914 /// This function is unsafe for the same reasons that `realloc` is.
918 /// Returning `Err` indicates that either memory is exhausted or
919 /// `layout` does not meet allocator's size or alignment
920 /// constraints, just as in `realloc`.
922 /// Clients wishing to abort computation in response to a
923 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
924 /// rather than directly invoking `panic!` or similar.
926 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
927 unsafe fn realloc_excess(&mut self,
930 new_size: usize) -> Result<Excess, AllocErr> {
931 let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
932 let usable_size = self.usable_size(&new_layout);
933 self.realloc(ptr, layout, new_size)
934 .map(|p| Excess(p, usable_size.1))
937 /// Attempts to extend the allocation referenced by `ptr` to fit `new_size`.
939 /// If this returns `Ok`, then the allocator has asserted that the
940 /// memory block referenced by `ptr` now fits `new_size`, and thus can
941 /// be used to carry data of a layout of that size and same alignment as
942 /// `layout`. (The allocator is allowed to
943 /// expend effort to accomplish this, such as extending the memory block to
944 /// include successor blocks, or virtual memory tricks.)
946 /// Regardless of what this method returns, ownership of the
947 /// memory block referenced by `ptr` has not been transferred, and
948 /// the contents of the memory block are unaltered.
952 /// This function is unsafe because undefined behavior can result
953 /// if the caller does not ensure all of the following:
955 /// * `ptr` must be currently allocated via this allocator,
957 /// * `layout` must *fit* the `ptr` (see above); note the
958 /// `new_size` argument need not fit it,
960 /// * `new_size` must not be less than `layout.size()`,
964 /// Returns `Err(CannotReallocInPlace)` when the allocator is
965 /// unable to assert that the memory block referenced by `ptr`
966 /// could fit `layout`.
968 /// Note that one cannot pass `CannotReallocInPlace` to the `handle_alloc_error`
969 /// function; clients are expected either to be able to recover from
970 /// `grow_in_place` failures without aborting, or to fall back on
971 /// another reallocation method before resorting to an abort.
972 unsafe fn grow_in_place(&mut self,
975 new_size: usize) -> Result<(), CannotReallocInPlace> {
976 let _ = ptr; // this default implementation doesn't care about the actual address.
977 debug_assert!(new_size >= layout.size());
978 let (_l, u) = self.usable_size(&layout);
979 // _l <= layout.size() [guaranteed by usable_size()]
980 // layout.size() <= new_layout.size() [required by this method]
984 Err(CannotReallocInPlace)
988 /// Attempts to shrink the allocation referenced by `ptr` to fit `new_size`.
990 /// If this returns `Ok`, then the allocator has asserted that the
991 /// memory block referenced by `ptr` now fits `new_size`, and
992 /// thus can only be used to carry data of that smaller
993 /// layout. (The allocator is allowed to take advantage of this,
994 /// carving off portions of the block for reuse elsewhere.) The
995 /// truncated contents of the block within the smaller layout are
996 /// unaltered, and ownership of block has not been transferred.
998 /// If this returns `Err`, then the memory block is considered to
999 /// still represent the original (larger) `layout`. None of the
1000 /// block has been carved off for reuse elsewhere, ownership of
1001 /// the memory block has not been transferred, and the contents of
1002 /// the memory block are unaltered.
1006 /// This function is unsafe because undefined behavior can result
1007 /// if the caller does not ensure all of the following:
1009 /// * `ptr` must be currently allocated via this allocator,
1011 /// * `layout` must *fit* the `ptr` (see above); note the
1012 /// `new_size` argument need not fit it,
1014 /// * `new_size` must not be greater than `layout.size()`
1015 /// (and must be greater than zero),
1019 /// Returns `Err(CannotReallocInPlace)` when the allocator is
1020 /// unable to assert that the memory block referenced by `ptr`
1021 /// could fit `layout`.
1023 /// Note that one cannot pass `CannotReallocInPlace` to the `handle_alloc_error`
1024 /// function; clients are expected either to be able to recover from
1025 /// `shrink_in_place` failures without aborting, or to fall back
1026 /// on another reallocation method before resorting to an abort.
1027 unsafe fn shrink_in_place(&mut self,
1030 new_size: usize) -> Result<(), CannotReallocInPlace> {
1031 let _ = ptr; // this default implementation doesn't care about the actual address.
1032 debug_assert!(new_size <= layout.size());
1033 let (l, _u) = self.usable_size(&layout);
1034 // layout.size() <= _u [guaranteed by usable_size()]
1035 // new_layout.size() <= layout.size() [required by this method]
1039 Err(CannotReallocInPlace)
1044 // == COMMON USAGE PATTERNS ==
1045 // alloc_one, dealloc_one, alloc_array, realloc_array. dealloc_array
1047 /// Allocates a block suitable for holding an instance of `T`.
1049 /// Captures a common usage pattern for allocators.
1051 /// The returned block is suitable for passing to the
1052 /// `realloc`/`dealloc` methods of this allocator.
1054 /// Note to implementors: If this returns `Ok(ptr)`, then `ptr`
1055 /// must be considered "currently allocated" and must be
1056 /// acceptable input to methods such as `realloc` or `dealloc`,
1057 /// *even if* `T` is a zero-sized type. In other words, if your
1058 /// `Alloc` implementation overrides this method in a manner
1059 /// that can return a zero-sized `ptr`, then all reallocation and
1060 /// deallocation methods need to be similarly overridden to accept
1061 /// such values as input.
1065 /// Returning `Err` indicates that either memory is exhausted or
1066 /// `T` does not meet allocator's size or alignment constraints.
1068 /// For zero-sized `T`, may return either of `Ok` or `Err`, but
1069 /// will *not* yield undefined behavior.
1071 /// Clients wishing to abort computation in response to an
1072 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
1073 /// rather than directly invoking `panic!` or similar.
1075 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1076 fn alloc_one<T>(&mut self) -> Result<NonNull<T>, AllocErr>
1079 let k = Layout::new::<T>();
1081 unsafe { self.alloc(k).map(|p| p.cast()) }
1087 /// Deallocates a block suitable for holding an instance of `T`.
1089 /// The given block must have been produced by this allocator,
1090 /// and must be suitable for storing a `T` (in terms of alignment
1091 /// as well as minimum and maximum size); otherwise yields
1092 /// undefined behavior.
1094 /// Captures a common usage pattern for allocators.
1098 /// This function is unsafe because undefined behavior can result
1099 /// if the caller does not ensure both:
1101 /// * `ptr` must denote a block of memory currently allocated via this allocator
1103 /// * the layout of `T` must *fit* that block of memory.
1104 unsafe fn dealloc_one<T>(&mut self, ptr: NonNull<T>)
1107 let k = Layout::new::<T>();
1109 self.dealloc(ptr.cast(), k);
1113 /// Allocates a block suitable for holding `n` instances of `T`.
1115 /// Captures a common usage pattern for allocators.
1117 /// The returned block is suitable for passing to the
1118 /// `realloc`/`dealloc` methods of this allocator.
1120 /// Note to implementors: If this returns `Ok(ptr)`, then `ptr`
1121 /// must be considered "currently allocated" and must be
1122 /// acceptable input to methods such as `realloc` or `dealloc`,
1123 /// *even if* `T` is a zero-sized type. In other words, if your
1124 /// `Alloc` implementation overrides this method in a manner
1125 /// that can return a zero-sized `ptr`, then all reallocation and
1126 /// deallocation methods need to be similarly overridden to accept
1127 /// such values as input.
1131 /// Returning `Err` indicates that either memory is exhausted or
1132 /// `[T; n]` does not meet allocator's size or alignment
1135 /// For zero-sized `T` or `n == 0`, may return either of `Ok` or
1136 /// `Err`, but will *not* yield undefined behavior.
1138 /// Always returns `Err` on arithmetic overflow.
1140 /// Clients wishing to abort computation in response to an
1141 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
1142 /// rather than directly invoking `panic!` or similar.
1144 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1145 fn alloc_array<T>(&mut self, n: usize) -> Result<NonNull<T>, AllocErr>
1148 match Layout::array::<T>(n) {
1149 Ok(layout) if layout.size() > 0 => {
1151 self.alloc(layout).map(|p| p.cast())
1158 /// Reallocates a block previously suitable for holding `n_old`
1159 /// instances of `T`, returning a block suitable for holding
1160 /// `n_new` instances of `T`.
1162 /// Captures a common usage pattern for allocators.
1164 /// The returned block is suitable for passing to the
1165 /// `realloc`/`dealloc` methods of this allocator.
1169 /// This function is unsafe because undefined behavior can result
1170 /// if the caller does not ensure all of the following:
1172 /// * `ptr` must be currently allocated via this allocator,
1174 /// * the layout of `[T; n_old]` must *fit* that block of memory.
1178 /// Returning `Err` indicates that either memory is exhausted or
1179 /// `[T; n_new]` does not meet allocator's size or alignment
1182 /// For zero-sized `T` or `n_new == 0`, may return either of `Ok` or
1183 /// `Err`, but will *not* yield undefined behavior.
1185 /// Always returns `Err` on arithmetic overflow.
1187 /// Clients wishing to abort computation in response to a
1188 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
1189 /// rather than directly invoking `panic!` or similar.
1191 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1192 unsafe fn realloc_array<T>(&mut self,
1195 n_new: usize) -> Result<NonNull<T>, AllocErr>
1198 match (Layout::array::<T>(n_old), Layout::array::<T>(n_new)) {
1199 (Ok(k_old), Ok(k_new)) if k_old.size() > 0 && k_new.size() > 0 => {
1200 debug_assert!(k_old.align() == k_new.align());
1201 self.realloc(ptr.cast(), k_old, k_new.size()).map(NonNull::cast)
1209 /// Deallocates a block suitable for holding `n` instances of `T`.
1211 /// Captures a common usage pattern for allocators.
1215 /// This function is unsafe because undefined behavior can result
1216 /// if the caller does not ensure both:
1218 /// * `ptr` must denote a block of memory currently allocated via this allocator
1220 /// * the layout of `[T; n]` must *fit* that block of memory.
1224 /// Returning `Err` indicates that either `[T; n]` or the given
1225 /// memory block does not meet allocator's size or alignment
1228 /// Always returns `Err` on arithmetic overflow.
1229 unsafe fn dealloc_array<T>(&mut self, ptr: NonNull<T>, n: usize) -> Result<(), AllocErr>
1232 match Layout::array::<T>(n) {
1233 Ok(k) if k.size() > 0 => {
1234 Ok(self.dealloc(ptr.cast(), k))