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")]
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: () });
91 unsafe { Ok(Layout::from_size_align_unchecked(size, align)) }
94 /// Creates a layout, bypassing all checks.
98 /// This function is unsafe as it does not verify the preconditions from
99 /// [`Layout::from_size_align`](#method.from_size_align).
100 #[stable(feature = "alloc_layout", since = "1.28.0")]
101 #[rustc_const_stable(feature = "alloc_layout", since = "1.28.0")]
103 pub const unsafe fn from_size_align_unchecked(size: usize, align: usize) -> Self {
104 Layout { size_: size, align_: NonZeroUsize::new_unchecked(align) }
107 /// The minimum size in bytes for a memory block of this layout.
108 #[stable(feature = "alloc_layout", since = "1.28.0")]
110 pub fn size(&self) -> usize {
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 {
121 /// Constructs a `Layout` suitable for holding a value of type `T`.
122 #[stable(feature = "alloc_layout", since = "1.28.0")]
124 pub const fn new<T>() -> Self {
125 let (size, align) = size_align::<T>();
126 // Note that the align is guaranteed by rustc to be a power of two and
127 // the size+align combo is guaranteed to fit in our address space. As a
128 // result use the unchecked constructor here to avoid inserting code
129 // that panics if it isn't optimized well enough.
130 unsafe { Layout::from_size_align_unchecked(size, align) }
133 /// Produces layout describing a record that could be used to
134 /// allocate backing structure for `T` (which could be a trait
135 /// or other unsized type like a slice).
136 #[stable(feature = "alloc_layout", since = "1.28.0")]
138 pub fn for_value<T: ?Sized>(t: &T) -> Self {
139 let (size, align) = (mem::size_of_val(t), mem::align_of_val(t));
140 // See rationale in `new` for why this is using an unsafe variant below
141 debug_assert!(Layout::from_size_align(size, align).is_ok());
142 unsafe { Layout::from_size_align_unchecked(size, align) }
145 /// Creates a layout describing the record that can hold a value
146 /// of the same layout as `self`, but that also is aligned to
147 /// alignment `align` (measured in bytes).
149 /// If `self` already meets the prescribed alignment, then returns
152 /// Note that this method does not add any padding to the overall
153 /// size, regardless of whether the returned layout has a different
154 /// alignment. In other words, if `K` has size 16, `K.align_to(32)`
155 /// will *still* have size 16.
157 /// Returns an error if the combination of `self.size()` and the given
158 /// `align` violates the conditions listed in
159 /// [`Layout::from_size_align`](#method.from_size_align).
160 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
162 pub fn align_to(&self, align: usize) -> Result<Self, LayoutErr> {
163 Layout::from_size_align(self.size(), cmp::max(self.align(), align))
166 /// Returns the amount of padding we must insert after `self`
167 /// to ensure that the following address will satisfy `align`
168 /// (measured in bytes).
170 /// e.g., if `self.size()` is 9, then `self.padding_needed_for(4)`
171 /// returns 3, because that is the minimum number of bytes of
172 /// padding required to get a 4-aligned address (assuming that the
173 /// corresponding memory block starts at a 4-aligned address).
175 /// The return value of this function has no meaning if `align` is
176 /// not a power-of-two.
178 /// Note that the utility of the returned value requires `align`
179 /// to be less than or equal to the alignment of the starting
180 /// address for the whole allocated block of memory. One way to
181 /// satisfy this constraint is to ensure `align <= self.align()`.
182 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
184 pub fn padding_needed_for(&self, align: usize) -> usize {
185 let len = self.size();
187 // Rounded up value is:
188 // len_rounded_up = (len + align - 1) & !(align - 1);
189 // and then we return the padding difference: `len_rounded_up - len`.
191 // We use modular arithmetic throughout:
193 // 1. align is guaranteed to be > 0, so align - 1 is always
196 // 2. `len + align - 1` can overflow by at most `align - 1`,
197 // so the &-mask with `!(align - 1)` will ensure that in the
198 // case of overflow, `len_rounded_up` will itself be 0.
199 // Thus the returned padding, when added to `len`, yields 0,
200 // which trivially satisfies the alignment `align`.
202 // (Of course, attempts to allocate blocks of memory whose
203 // size and padding overflow in the above manner should cause
204 // the allocator to yield an error anyway.)
206 let len_rounded_up = len.wrapping_add(align).wrapping_sub(1) & !align.wrapping_sub(1);
207 len_rounded_up.wrapping_sub(len)
210 /// Creates a layout by rounding the size of this layout up to a multiple
211 /// of the layout's alignment.
213 /// This is equivalent to adding the result of `padding_needed_for`
214 /// to the layout's current size.
215 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
217 pub fn pad_to_align(&self) -> Layout {
218 let pad = self.padding_needed_for(self.align());
219 // This cannot overflow. Quoting from the invariant of Layout:
220 // > `size`, when rounded up to the nearest multiple of `align`,
221 // > must not overflow (i.e., the rounded value must be less than
223 let new_size = self.size() + pad;
225 Layout::from_size_align(new_size, self.align()).unwrap()
228 /// Creates a layout describing the record for `n` instances of
229 /// `self`, with a suitable amount of padding between each to
230 /// ensure that each instance is given its requested size and
231 /// alignment. On success, returns `(k, offs)` where `k` is the
232 /// layout of the array and `offs` is the distance between the start
233 /// of each element in the array.
235 /// On arithmetic overflow, returns `LayoutErr`.
236 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
238 pub fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutErr> {
239 // This cannot overflow. Quoting from the invariant of Layout:
240 // > `size`, when rounded up to the nearest multiple of `align`,
241 // > must not overflow (i.e., the rounded value must be less than
243 let padded_size = self.size() + self.padding_needed_for(self.align());
244 let alloc_size = padded_size.checked_mul(n).ok_or(LayoutErr { private: () })?;
247 // self.align is already known to be valid and alloc_size has been
249 Ok((Layout::from_size_align_unchecked(alloc_size, self.align()), padded_size))
253 /// Creates a layout describing the record for `self` followed by
254 /// `next`, including any necessary padding to ensure that `next`
255 /// will be properly aligned. Note that the resulting layout will
256 /// satisfy the alignment properties of both `self` and `next`.
258 /// The resulting layout will be the same as that of a C struct containing
259 /// two fields with the layouts of `self` and `next`, in that order.
261 /// Returns `Some((k, offset))`, where `k` is layout of the concatenated
262 /// record and `offset` is the relative location, in bytes, of the
263 /// start of the `next` embedded within the concatenated record
264 /// (assuming that the record itself starts at offset 0).
266 /// On arithmetic overflow, returns `LayoutErr`.
267 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
269 pub fn extend(&self, next: Self) -> Result<(Self, usize), LayoutErr> {
270 let new_align = cmp::max(self.align(), next.align());
271 let pad = self.padding_needed_for(next.align());
273 let offset = self.size().checked_add(pad).ok_or(LayoutErr { private: () })?;
274 let new_size = offset.checked_add(next.size()).ok_or(LayoutErr { private: () })?;
276 let layout = Layout::from_size_align(new_size, new_align)?;
280 /// Creates a layout describing the record for `n` instances of
281 /// `self`, with no padding between each instance.
283 /// Note that, unlike `repeat`, `repeat_packed` does not guarantee
284 /// that the repeated instances of `self` will be properly
285 /// aligned, even if a given instance of `self` is properly
286 /// aligned. In other words, if the layout returned by
287 /// `repeat_packed` is used to allocate an array, it is not
288 /// guaranteed that all elements in the array will be properly
291 /// On arithmetic overflow, returns `LayoutErr`.
292 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
294 pub fn repeat_packed(&self, n: usize) -> Result<Self, LayoutErr> {
295 let size = self.size().checked_mul(n).ok_or(LayoutErr { private: () })?;
296 Layout::from_size_align(size, self.align())
299 /// Creates a layout describing the record for `self` followed by
300 /// `next` with no additional padding between the two. Since no
301 /// padding is inserted, the alignment of `next` is irrelevant,
302 /// and is not incorporated *at all* into the resulting layout.
304 /// On arithmetic overflow, returns `LayoutErr`.
305 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
307 pub fn extend_packed(&self, next: Self) -> Result<Self, LayoutErr> {
308 let new_size = self.size().checked_add(next.size()).ok_or(LayoutErr { private: () })?;
309 Layout::from_size_align(new_size, self.align())
312 /// Creates a layout describing the record for a `[T; n]`.
314 /// On arithmetic overflow, returns `LayoutErr`.
315 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
317 pub fn array<T>(n: usize) -> Result<Self, LayoutErr> {
318 Layout::new::<T>().repeat(n).map(|(k, offs)| {
319 debug_assert!(offs == mem::size_of::<T>());
325 /// The parameters given to `Layout::from_size_align`
326 /// or some other `Layout` constructor
327 /// do not satisfy its documented constraints.
328 #[stable(feature = "alloc_layout", since = "1.28.0")]
329 #[derive(Clone, PartialEq, Eq, Debug)]
330 pub struct LayoutErr {
334 // (we need this for downstream impl of trait Error)
335 #[stable(feature = "alloc_layout", since = "1.28.0")]
336 impl fmt::Display for LayoutErr {
337 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
338 f.write_str("invalid parameters to Layout::from_size_align")
342 /// The `AllocErr` error indicates an allocation failure
343 /// that may be due to resource exhaustion or to
344 /// something wrong when combining the given input arguments with this
346 #[unstable(feature = "allocator_api", issue = "32838")]
347 #[derive(Clone, PartialEq, Eq, Debug)]
350 // (we need this for downstream impl of trait Error)
351 #[unstable(feature = "allocator_api", issue = "32838")]
352 impl fmt::Display for AllocErr {
353 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
354 f.write_str("memory allocation failed")
358 /// The `CannotReallocInPlace` error is used when [`grow_in_place`] or
359 /// [`shrink_in_place`] were unable to reuse the given memory block for
360 /// a requested layout.
362 /// [`grow_in_place`]: ./trait.Alloc.html#method.grow_in_place
363 /// [`shrink_in_place`]: ./trait.Alloc.html#method.shrink_in_place
364 #[unstable(feature = "allocator_api", issue = "32838")]
365 #[derive(Clone, PartialEq, Eq, Debug)]
366 pub struct CannotReallocInPlace;
368 #[unstable(feature = "allocator_api", issue = "32838")]
369 impl CannotReallocInPlace {
370 pub fn description(&self) -> &str {
371 "cannot reallocate allocator's memory in place"
375 // (we need this for downstream impl of trait Error)
376 #[unstable(feature = "allocator_api", issue = "32838")]
377 impl fmt::Display for CannotReallocInPlace {
378 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
379 write!(f, "{}", self.description())
383 /// A memory allocator that can be registered as the standard library’s default
384 /// through the `#[global_allocator]` attribute.
386 /// Some of the methods require that a memory block be *currently
387 /// allocated* via an allocator. This means that:
389 /// * the starting address for that memory block was previously
390 /// returned by a previous call to an allocation method
391 /// such as `alloc`, and
393 /// * the memory block has not been subsequently deallocated, where
394 /// blocks are deallocated either by being passed to a deallocation
395 /// method such as `dealloc` or by being
396 /// passed to a reallocation method that returns a non-null pointer.
402 /// use std::alloc::{GlobalAlloc, Layout, alloc};
403 /// use std::ptr::null_mut;
405 /// struct MyAllocator;
407 /// unsafe impl GlobalAlloc for MyAllocator {
408 /// unsafe fn alloc(&self, _layout: Layout) -> *mut u8 { null_mut() }
409 /// unsafe fn dealloc(&self, _ptr: *mut u8, _layout: Layout) {}
412 /// #[global_allocator]
413 /// static A: MyAllocator = MyAllocator;
417 /// assert!(alloc(Layout::new::<u32>()).is_null())
424 /// The `GlobalAlloc` trait is an `unsafe` trait for a number of reasons, and
425 /// implementors must ensure that they adhere to these contracts:
427 /// * It's undefined behavior if global allocators unwind. This restriction may
428 /// be lifted in the future, but currently a panic from any of these
429 /// functions may lead to memory unsafety.
431 /// * `Layout` queries and calculations in general must be correct. Callers of
432 /// this trait are allowed to rely on the contracts defined on each method,
433 /// and implementors must ensure such contracts remain true.
434 #[stable(feature = "global_alloc", since = "1.28.0")]
435 pub unsafe trait GlobalAlloc {
436 /// Allocate memory as described by the given `layout`.
438 /// Returns a pointer to newly-allocated memory,
439 /// or null to indicate allocation failure.
443 /// This function is unsafe because undefined behavior can result
444 /// if the caller does not ensure that `layout` has non-zero size.
446 /// (Extension subtraits might provide more specific bounds on
447 /// behavior, e.g., guarantee a sentinel address or a null pointer
448 /// in response to a zero-size allocation request.)
450 /// The allocated block of memory may or may not be initialized.
454 /// Returning a null pointer indicates that either memory is exhausted
455 /// or `layout` does not meet this allocator's size or alignment constraints.
457 /// Implementations are encouraged to return null on memory
458 /// exhaustion rather than aborting, but this is not
459 /// a strict requirement. (Specifically: it is *legal* to
460 /// implement this trait atop an underlying native allocation
461 /// library that aborts on memory exhaustion.)
463 /// Clients wishing to abort computation in response to an
464 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
465 /// rather than directly invoking `panic!` or similar.
467 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
468 #[stable(feature = "global_alloc", since = "1.28.0")]
469 unsafe fn alloc(&self, layout: Layout) -> *mut u8;
471 /// Deallocate the block of memory at the given `ptr` pointer with the given `layout`.
475 /// This function is unsafe because undefined behavior can result
476 /// if the caller does not ensure all of the following:
478 /// * `ptr` must denote a block of memory currently allocated via
481 /// * `layout` must be the same layout that was used
482 /// to allocate that block of memory,
483 #[stable(feature = "global_alloc", since = "1.28.0")]
484 unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout);
486 /// Behaves like `alloc`, but also ensures that the contents
487 /// are set to zero before being returned.
491 /// This function is unsafe for the same reasons that `alloc` is.
492 /// However the allocated block of memory is guaranteed to be initialized.
496 /// Returning a null pointer indicates that either memory is exhausted
497 /// or `layout` does not meet allocator's size or alignment constraints,
498 /// just as in `alloc`.
500 /// Clients wishing to abort computation in response to an
501 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
502 /// rather than directly invoking `panic!` or similar.
504 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
505 #[stable(feature = "global_alloc", since = "1.28.0")]
506 unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
507 let size = layout.size();
508 let ptr = self.alloc(layout);
510 ptr::write_bytes(ptr, 0, size);
515 /// Shrink or grow a block of memory to the given `new_size`.
516 /// The block is described by the given `ptr` pointer and `layout`.
518 /// If this returns a non-null pointer, then ownership of the memory block
519 /// referenced by `ptr` has been transferred to this allocator.
520 /// The memory may or may not have been deallocated,
521 /// and should be considered unusable (unless of course it was
522 /// transferred back to the caller again via the return value of
525 /// If this method returns null, then ownership of the memory
526 /// block has not been transferred to this allocator, and the
527 /// contents of the memory block are unaltered.
531 /// This function is unsafe because undefined behavior can result
532 /// if the caller does not ensure all of the following:
534 /// * `ptr` must be currently allocated via this allocator,
536 /// * `layout` must be the same layout that was used
537 /// to allocate that block of memory,
539 /// * `new_size` must be greater than zero.
541 /// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
542 /// must not overflow (i.e., the rounded value must be less than `usize::MAX`).
544 /// (Extension subtraits might provide more specific bounds on
545 /// behavior, e.g., guarantee a sentinel address or a null pointer
546 /// in response to a zero-size allocation request.)
550 /// Returns null if the new layout does not meet the size
551 /// and alignment constraints of the allocator, or if reallocation
554 /// Implementations are encouraged to return null on memory
555 /// exhaustion rather than panicking or aborting, but this is not
556 /// a strict requirement. (Specifically: it is *legal* to
557 /// implement this trait atop an underlying native allocation
558 /// library that aborts on memory exhaustion.)
560 /// Clients wishing to abort computation in response to a
561 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
562 /// rather than directly invoking `panic!` or similar.
564 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
565 #[stable(feature = "global_alloc", since = "1.28.0")]
566 unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
567 let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
568 let new_ptr = self.alloc(new_layout);
569 if !new_ptr.is_null() {
570 ptr::copy_nonoverlapping(ptr, new_ptr, cmp::min(layout.size(), new_size));
571 self.dealloc(ptr, layout);
577 /// An implementation of `Alloc` can allocate, reallocate, and
578 /// deallocate arbitrary blocks of data described via `Layout`.
580 /// Some of the methods require that a memory block be *currently
581 /// allocated* via an allocator. This means that:
583 /// * the starting address for that memory block was previously
584 /// returned by a previous call to an allocation method (`alloc`,
585 /// `alloc_zeroed`, `alloc_excess`, `alloc_one`, `alloc_array`) or
586 /// reallocation method (`realloc`, `realloc_excess`, or
587 /// `realloc_array`), and
589 /// * the memory block has not been subsequently deallocated, where
590 /// blocks are deallocated either by being passed to a deallocation
591 /// method (`dealloc`, `dealloc_one`, `dealloc_array`) or by being
592 /// passed to a reallocation method (see above) that returns `Ok`.
594 /// A note regarding zero-sized types and zero-sized layouts: many
595 /// methods in the `Alloc` trait state that allocation requests
596 /// must be non-zero size, or else undefined behavior can result.
598 /// * However, some higher-level allocation methods (`alloc_one`,
599 /// `alloc_array`) are well-defined on zero-sized types and can
600 /// optionally support them: it is left up to the implementor
601 /// whether to return `Err`, or to return `Ok` with some pointer.
603 /// * If an `Alloc` implementation chooses to return `Ok` in this
604 /// case (i.e., the pointer denotes a zero-sized inaccessible block)
605 /// then that returned pointer must be considered "currently
606 /// allocated". On such an allocator, *all* methods that take
607 /// currently-allocated pointers as inputs must accept these
608 /// zero-sized pointers, *without* causing undefined behavior.
610 /// * In other words, if a zero-sized pointer can flow out of an
611 /// allocator, then that allocator must likewise accept that pointer
612 /// flowing back into its deallocation and reallocation methods.
614 /// Some of the methods require that a layout *fit* a memory block.
615 /// What it means for a layout to "fit" a memory block means (or
616 /// equivalently, for a memory block to "fit" a layout) is that the
617 /// following two conditions must hold:
619 /// 1. The block's starting address must be aligned to `layout.align()`.
621 /// 2. The block's size must fall in the range `[use_min, use_max]`, where:
623 /// * `use_min` is `self.usable_size(layout).0`, and
625 /// * `use_max` is the capacity that was (or would have been)
626 /// returned when (if) the block was allocated via a call to
627 /// `alloc_excess` or `realloc_excess`.
631 /// * the size of the layout most recently used to allocate the block
632 /// is guaranteed to be in the range `[use_min, use_max]`, and
634 /// * a lower-bound on `use_max` can be safely approximated by a call to
637 /// * if a layout `k` fits a memory block (denoted by `ptr`)
638 /// currently allocated via an allocator `a`, then it is legal to
639 /// use that layout to deallocate it, i.e., `a.dealloc(ptr, k);`.
643 /// The `Alloc` trait is an `unsafe` trait for a number of reasons, and
644 /// implementors must ensure that they adhere to these contracts:
646 /// * Pointers returned from allocation functions must point to valid memory and
647 /// retain their validity until at least the instance of `Alloc` is dropped
650 /// * `Layout` queries and calculations in general must be correct. Callers of
651 /// this trait are allowed to rely on the contracts defined on each method,
652 /// and implementors must ensure such contracts remain true.
654 /// Note that this list may get tweaked over time as clarifications are made in
656 #[unstable(feature = "allocator_api", issue = "32838")]
657 pub unsafe trait Alloc {
658 // (Note: some existing allocators have unspecified but well-defined
659 // behavior in response to a zero size allocation request ;
660 // e.g., in C, `malloc` of 0 will either return a null pointer or a
661 // unique pointer, but will not have arbitrary undefined
663 // However in jemalloc for example,
664 // `mallocx(0)` is documented as undefined behavior.)
666 /// Returns a pointer meeting the size and alignment guarantees of
669 /// If this method returns an `Ok(addr)`, then the `addr` returned
670 /// will be non-null address pointing to a block of storage
671 /// suitable for holding an instance of `layout`.
673 /// The returned block of storage may or may not have its contents
674 /// initialized. (Extension subtraits might restrict this
675 /// behavior, e.g., to ensure initialization to particular sets of
680 /// This function is unsafe because undefined behavior can result
681 /// if the caller does not ensure that `layout` has non-zero size.
683 /// (Extension subtraits might provide more specific bounds on
684 /// behavior, e.g., guarantee a sentinel address or a null pointer
685 /// in response to a zero-size allocation request.)
689 /// Returning `Err` indicates that either memory is exhausted or
690 /// `layout` does not meet allocator's size or alignment
693 /// Implementations are encouraged to return `Err` on memory
694 /// exhaustion rather than panicking or aborting, but this is not
695 /// a strict requirement. (Specifically: it is *legal* to
696 /// implement this trait atop an underlying native allocation
697 /// library that aborts on memory exhaustion.)
699 /// Clients wishing to abort computation in response to an
700 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
701 /// rather than directly invoking `panic!` or similar.
703 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
704 unsafe fn alloc(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr>;
706 /// Deallocate the memory referenced by `ptr`.
710 /// This function is unsafe because undefined behavior can result
711 /// if the caller does not ensure all of the following:
713 /// * `ptr` must denote a block of memory currently allocated via
716 /// * `layout` must *fit* that block of memory,
718 /// * In addition to fitting the block of memory `layout`, the
719 /// alignment of the `layout` must match the alignment used
720 /// to allocate that block of memory.
721 unsafe fn dealloc(&mut self, ptr: NonNull<u8>, layout: Layout);
723 // == ALLOCATOR-SPECIFIC QUANTITIES AND LIMITS ==
726 /// Returns bounds on the guaranteed usable size of a successful
727 /// allocation created with the specified `layout`.
729 /// In particular, if one has a memory block allocated via a given
730 /// allocator `a` and layout `k` where `a.usable_size(k)` returns
731 /// `(l, u)`, then one can pass that block to `a.dealloc()` with a
732 /// layout in the size range [l, u].
734 /// (All implementors of `usable_size` must ensure that
735 /// `l <= k.size() <= u`)
737 /// Both the lower- and upper-bounds (`l` and `u` respectively)
738 /// are provided, because an allocator based on size classes could
739 /// misbehave if one attempts to deallocate a block without
740 /// providing a correct value for its size (i.e., one within the
743 /// Clients who wish to make use of excess capacity are encouraged
744 /// to use the `alloc_excess` and `realloc_excess` instead, as
745 /// this method is constrained to report conservative values that
746 /// serve as valid bounds for *all possible* allocation method
749 /// However, for clients that do not wish to track the capacity
750 /// returned by `alloc_excess` locally, this method is likely to
751 /// produce useful results.
753 fn usable_size(&self, layout: &Layout) -> (usize, usize) {
754 (layout.size(), layout.size())
757 // == METHODS FOR MEMORY REUSE ==
758 // realloc. alloc_excess, realloc_excess
760 /// Returns a pointer suitable for holding data described by
761 /// a new layout with `layout`’s alignment and a size given
762 /// by `new_size`. To
763 /// accomplish this, this may extend or shrink the allocation
764 /// referenced by `ptr` to fit the new layout.
766 /// If this returns `Ok`, then ownership of the memory block
767 /// referenced by `ptr` has been transferred to this
768 /// allocator. The memory may or may not have been freed, and
769 /// should be considered unusable (unless of course it was
770 /// transferred back to the caller again via the return value of
773 /// If this method returns `Err`, then ownership of the memory
774 /// block has not been transferred to this allocator, and the
775 /// contents of the memory block are unaltered.
779 /// This function is unsafe because undefined behavior can result
780 /// if the caller does not ensure all of the following:
782 /// * `ptr` must be currently allocated via this allocator,
784 /// * `layout` must *fit* the `ptr` (see above). (The `new_size`
785 /// argument need not fit it.)
787 /// * `new_size` must be greater than zero.
789 /// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
790 /// must not overflow (i.e., the rounded value must be less than `usize::MAX`).
792 /// (Extension subtraits might provide more specific bounds on
793 /// behavior, e.g., guarantee a sentinel address or a null pointer
794 /// in response to a zero-size allocation request.)
798 /// Returns `Err` only if the new layout
799 /// does not meet the allocator's size
800 /// and alignment constraints of the allocator, or if reallocation
803 /// Implementations are encouraged to return `Err` on memory
804 /// exhaustion rather than panicking or aborting, but this is not
805 /// a strict requirement. (Specifically: it is *legal* to
806 /// implement this trait atop an underlying native allocation
807 /// library that aborts on memory exhaustion.)
809 /// Clients wishing to abort computation in response to a
810 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
811 /// rather than directly invoking `panic!` or similar.
813 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
819 ) -> Result<NonNull<u8>, AllocErr> {
820 let old_size = layout.size();
822 if new_size >= old_size {
823 if let Ok(()) = self.grow_in_place(ptr, layout, new_size) {
826 } else if new_size < old_size {
827 if let Ok(()) = self.shrink_in_place(ptr, layout, new_size) {
832 // otherwise, fall back on alloc + copy + dealloc.
833 let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
834 let result = self.alloc(new_layout);
835 if let Ok(new_ptr) = result {
836 ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_ptr(), cmp::min(old_size, new_size));
837 self.dealloc(ptr, layout);
842 /// Behaves like `alloc`, but also ensures that the contents
843 /// are set to zero before being returned.
847 /// This function is unsafe for the same reasons that `alloc` is.
851 /// Returning `Err` indicates that either memory is exhausted or
852 /// `layout` does not meet allocator's size or alignment
853 /// constraints, just as in `alloc`.
855 /// Clients wishing to abort computation in response to an
856 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
857 /// rather than directly invoking `panic!` or similar.
859 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
860 unsafe fn alloc_zeroed(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr> {
861 let size = layout.size();
862 let p = self.alloc(layout);
864 ptr::write_bytes(p.as_ptr(), 0, size);
869 /// Behaves like `alloc`, but also returns the whole size of
870 /// the returned block. For some `layout` inputs, like arrays, this
871 /// may include extra storage usable for additional data.
875 /// This function is unsafe for the same reasons that `alloc` is.
879 /// Returning `Err` indicates that either memory is exhausted or
880 /// `layout` does not meet allocator's size or alignment
881 /// constraints, just as in `alloc`.
883 /// Clients wishing to abort computation in response to an
884 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
885 /// rather than directly invoking `panic!` or similar.
887 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
888 unsafe fn alloc_excess(&mut self, layout: Layout) -> Result<Excess, AllocErr> {
889 let usable_size = self.usable_size(&layout);
890 self.alloc(layout).map(|p| Excess(p, usable_size.1))
893 /// Behaves like `realloc`, but also returns the whole size of
894 /// the returned block. For some `layout` inputs, like arrays, this
895 /// may include extra storage usable for additional data.
899 /// This function is unsafe for the same reasons that `realloc` is.
903 /// Returning `Err` indicates that either memory is exhausted or
904 /// `layout` does not meet allocator's size or alignment
905 /// constraints, just as in `realloc`.
907 /// Clients wishing to abort computation in response to a
908 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
909 /// rather than directly invoking `panic!` or similar.
911 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
912 unsafe fn realloc_excess(
917 ) -> Result<Excess, AllocErr> {
918 let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
919 let usable_size = self.usable_size(&new_layout);
920 self.realloc(ptr, layout, new_size).map(|p| Excess(p, usable_size.1))
923 /// Attempts to extend the allocation referenced by `ptr` to fit `new_size`.
925 /// If this returns `Ok`, then the allocator has asserted that the
926 /// memory block referenced by `ptr` now fits `new_size`, and thus can
927 /// be used to carry data of a layout of that size and same alignment as
928 /// `layout`. (The allocator is allowed to
929 /// expend effort to accomplish this, such as extending the memory block to
930 /// include successor blocks, or virtual memory tricks.)
932 /// Regardless of what this method returns, ownership of the
933 /// memory block referenced by `ptr` has not been transferred, and
934 /// the contents of the memory block are unaltered.
938 /// This function is unsafe because undefined behavior can result
939 /// if the caller does not ensure all of the following:
941 /// * `ptr` must be currently allocated via this allocator,
943 /// * `layout` must *fit* the `ptr` (see above); note the
944 /// `new_size` argument need not fit it,
946 /// * `new_size` must not be less than `layout.size()`,
950 /// Returns `Err(CannotReallocInPlace)` when the allocator is
951 /// unable to assert that the memory block referenced by `ptr`
952 /// could fit `layout`.
954 /// Note that one cannot pass `CannotReallocInPlace` to the `handle_alloc_error`
955 /// function; clients are expected either to be able to recover from
956 /// `grow_in_place` failures without aborting, or to fall back on
957 /// another reallocation method before resorting to an abort.
958 unsafe fn grow_in_place(
963 ) -> Result<(), CannotReallocInPlace> {
964 let _ = ptr; // this default implementation doesn't care about the actual address.
965 debug_assert!(new_size >= layout.size());
966 let (_l, u) = self.usable_size(&layout);
967 // _l <= layout.size() [guaranteed by usable_size()]
968 // layout.size() <= new_layout.size() [required by this method]
969 if new_size <= u { Ok(()) } else { Err(CannotReallocInPlace) }
972 /// Attempts to shrink the allocation referenced by `ptr` to fit `new_size`.
974 /// If this returns `Ok`, then the allocator has asserted that the
975 /// memory block referenced by `ptr` now fits `new_size`, and
976 /// thus can only be used to carry data of that smaller
977 /// layout. (The allocator is allowed to take advantage of this,
978 /// carving off portions of the block for reuse elsewhere.) The
979 /// truncated contents of the block within the smaller layout are
980 /// unaltered, and ownership of block has not been transferred.
982 /// If this returns `Err`, then the memory block is considered to
983 /// still represent the original (larger) `layout`. None of the
984 /// block has been carved off for reuse elsewhere, ownership of
985 /// the memory block has not been transferred, and the contents of
986 /// the memory block are unaltered.
990 /// This function is unsafe because undefined behavior can result
991 /// if the caller does not ensure all of the following:
993 /// * `ptr` must be currently allocated via this allocator,
995 /// * `layout` must *fit* the `ptr` (see above); note the
996 /// `new_size` argument need not fit it,
998 /// * `new_size` must not be greater than `layout.size()`
999 /// (and must be greater than zero),
1003 /// Returns `Err(CannotReallocInPlace)` when the allocator is
1004 /// unable to assert that the memory block referenced by `ptr`
1005 /// could fit `layout`.
1007 /// Note that one cannot pass `CannotReallocInPlace` to the `handle_alloc_error`
1008 /// function; clients are expected either to be able to recover from
1009 /// `shrink_in_place` failures without aborting, or to fall back
1010 /// on another reallocation method before resorting to an abort.
1011 unsafe fn shrink_in_place(
1016 ) -> Result<(), CannotReallocInPlace> {
1017 let _ = ptr; // this default implementation doesn't care about the actual address.
1018 debug_assert!(new_size <= layout.size());
1019 let (l, _u) = self.usable_size(&layout);
1020 // layout.size() <= _u [guaranteed by usable_size()]
1021 // new_layout.size() <= layout.size() [required by this method]
1022 if l <= new_size { Ok(()) } else { Err(CannotReallocInPlace) }
1025 // == COMMON USAGE PATTERNS ==
1026 // alloc_one, dealloc_one, alloc_array, realloc_array. dealloc_array
1028 /// Allocates a block suitable for holding an instance of `T`.
1030 /// Captures a common usage pattern for allocators.
1032 /// The returned block is suitable for passing to the
1033 /// `realloc`/`dealloc` methods of this allocator.
1035 /// Note to implementors: If this returns `Ok(ptr)`, then `ptr`
1036 /// must be considered "currently allocated" and must be
1037 /// acceptable input to methods such as `realloc` or `dealloc`,
1038 /// *even if* `T` is a zero-sized type. In other words, if your
1039 /// `Alloc` implementation overrides this method in a manner
1040 /// that can return a zero-sized `ptr`, then all reallocation and
1041 /// deallocation methods need to be similarly overridden to accept
1042 /// such values as input.
1046 /// Returning `Err` indicates that either memory is exhausted or
1047 /// `T` does not meet allocator's size or alignment constraints.
1049 /// For zero-sized `T`, may return either of `Ok` or `Err`, but
1050 /// will *not* yield undefined behavior.
1052 /// Clients wishing to abort computation in response to an
1053 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
1054 /// rather than directly invoking `panic!` or similar.
1056 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1057 fn alloc_one<T>(&mut self) -> Result<NonNull<T>, AllocErr>
1061 let k = Layout::new::<T>();
1062 if k.size() > 0 { unsafe { self.alloc(k).map(|p| p.cast()) } } else { Err(AllocErr) }
1065 /// Deallocates a block suitable for holding an instance of `T`.
1067 /// The given block must have been produced by this allocator,
1068 /// and must be suitable for storing a `T` (in terms of alignment
1069 /// as well as minimum and maximum size); otherwise yields
1070 /// undefined behavior.
1072 /// Captures a common usage pattern for allocators.
1076 /// This function is unsafe because undefined behavior can result
1077 /// if the caller does not ensure both:
1079 /// * `ptr` must denote a block of memory currently allocated via this allocator
1081 /// * the layout of `T` must *fit* that block of memory.
1082 unsafe fn dealloc_one<T>(&mut self, ptr: NonNull<T>)
1086 let k = Layout::new::<T>();
1088 self.dealloc(ptr.cast(), k);
1092 /// Allocates a block suitable for holding `n` instances of `T`.
1094 /// Captures a common usage pattern for allocators.
1096 /// The returned block is suitable for passing to the
1097 /// `realloc`/`dealloc` methods of this allocator.
1099 /// Note to implementors: If this returns `Ok(ptr)`, then `ptr`
1100 /// must be considered "currently allocated" and must be
1101 /// acceptable input to methods such as `realloc` or `dealloc`,
1102 /// *even if* `T` is a zero-sized type. In other words, if your
1103 /// `Alloc` implementation overrides this method in a manner
1104 /// that can return a zero-sized `ptr`, then all reallocation and
1105 /// deallocation methods need to be similarly overridden to accept
1106 /// such values as input.
1110 /// Returning `Err` indicates that either memory is exhausted or
1111 /// `[T; n]` does not meet allocator's size or alignment
1114 /// For zero-sized `T` or `n == 0`, may return either of `Ok` or
1115 /// `Err`, but will *not* yield undefined behavior.
1117 /// Always returns `Err` on arithmetic overflow.
1119 /// Clients wishing to abort computation in response to an
1120 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
1121 /// rather than directly invoking `panic!` or similar.
1123 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1124 fn alloc_array<T>(&mut self, n: usize) -> Result<NonNull<T>, AllocErr>
1128 match Layout::array::<T>(n) {
1129 Ok(layout) if layout.size() > 0 => unsafe { self.alloc(layout).map(|p| p.cast()) },
1134 /// Reallocates a block previously suitable for holding `n_old`
1135 /// instances of `T`, returning a block suitable for holding
1136 /// `n_new` instances of `T`.
1138 /// Captures a common usage pattern for allocators.
1140 /// The returned block is suitable for passing to the
1141 /// `realloc`/`dealloc` methods of this allocator.
1145 /// This function is unsafe because undefined behavior can result
1146 /// if the caller does not ensure all of the following:
1148 /// * `ptr` must be currently allocated via this allocator,
1150 /// * the layout of `[T; n_old]` must *fit* that block of memory.
1154 /// Returning `Err` indicates that either memory is exhausted or
1155 /// `[T; n_new]` does not meet allocator's size or alignment
1158 /// For zero-sized `T` or `n_new == 0`, may return either of `Ok` or
1159 /// `Err`, but will *not* yield undefined behavior.
1161 /// Always returns `Err` on arithmetic overflow.
1163 /// Clients wishing to abort computation in response to a
1164 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
1165 /// rather than directly invoking `panic!` or similar.
1167 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1168 unsafe fn realloc_array<T>(
1173 ) -> Result<NonNull<T>, AllocErr>
1177 match (Layout::array::<T>(n_old), Layout::array::<T>(n_new)) {
1178 (Ok(k_old), Ok(k_new)) if k_old.size() > 0 && k_new.size() > 0 => {
1179 debug_assert!(k_old.align() == k_new.align());
1180 self.realloc(ptr.cast(), k_old, k_new.size()).map(NonNull::cast)
1186 /// Deallocates a block suitable for holding `n` instances of `T`.
1188 /// Captures a common usage pattern for allocators.
1192 /// This function is unsafe because undefined behavior can result
1193 /// if the caller does not ensure both:
1195 /// * `ptr` must denote a block of memory currently allocated via this allocator
1197 /// * the layout of `[T; n]` must *fit* that block of memory.
1201 /// Returning `Err` indicates that either `[T; n]` or the given
1202 /// memory block does not meet allocator's size or alignment
1205 /// Always returns `Err` on arithmetic overflow.
1206 unsafe fn dealloc_array<T>(&mut self, ptr: NonNull<T>, n: usize) -> Result<(), AllocErr>
1210 match Layout::array::<T>(n) {
1211 Ok(k) if k.size() > 0 => Ok(self.dealloc(ptr.cast(), k)),