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 either 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")]
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")]
111 pub fn size(&self) -> usize { self.size_ }
113 /// The minimum byte alignment for a memory block of this layout.
114 #[stable(feature = "alloc_layout", since = "1.28.0")]
116 pub fn align(&self) -> usize { self.align_.get() }
118 /// Constructs a `Layout` suitable for holding a value of type `T`.
119 #[stable(feature = "alloc_layout", since = "1.28.0")]
121 pub fn new<T>() -> Self {
122 let (size, align) = size_align::<T>();
123 // Note that the align is guaranteed by rustc to be a power of two and
124 // the size+align combo is guaranteed to fit in our address space. As a
125 // result use the unchecked constructor here to avoid inserting code
126 // that panics if it isn't optimized well enough.
127 debug_assert!(Layout::from_size_align(size, align).is_ok());
129 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 us using an unsafe variant below
141 debug_assert!(Layout::from_size_align(size, align).is_ok());
143 Layout::from_size_align_unchecked(size, align)
147 /// Creates a layout describing the record that can hold a value
148 /// of the same layout as `self`, but that also is aligned to
149 /// alignment `align` (measured in bytes).
151 /// If `self` already meets the prescribed alignment, then returns
154 /// Note that this method does not add any padding to the overall
155 /// size, regardless of whether the returned layout has a different
156 /// alignment. In other words, if `K` has size 16, `K.align_to(32)`
157 /// will *still* have size 16.
159 /// Returns an error if the combination of `self.size()` and the given
160 /// `align` violates the conditions listed in
161 /// [`Layout::from_size_align`](#method.from_size_align).
162 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
164 pub fn align_to(&self, align: usize) -> Result<Self, LayoutErr> {
165 Layout::from_size_align(self.size(), cmp::max(self.align(), align))
168 /// Returns the amount of padding we must insert after `self`
169 /// to ensure that the following address will satisfy `align`
170 /// (measured in bytes).
172 /// e.g., if `self.size()` is 9, then `self.padding_needed_for(4)`
173 /// returns 3, because that is the minimum number of bytes of
174 /// padding required to get a 4-aligned address (assuming that the
175 /// corresponding memory block starts at a 4-aligned address).
177 /// The return value of this function has no meaning if `align` is
178 /// not a power-of-two.
180 /// Note that the utility of the returned value requires `align`
181 /// to be less than or equal to the alignment of the starting
182 /// address for the whole allocated block of memory. One way to
183 /// satisfy this constraint is to ensure `align <= self.align()`.
184 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
186 pub fn padding_needed_for(&self, align: usize) -> usize {
187 let len = self.size();
189 // Rounded up value is:
190 // len_rounded_up = (len + align - 1) & !(align - 1);
191 // and then we return the padding difference: `len_rounded_up - len`.
193 // We use modular arithmetic throughout:
195 // 1. align is guaranteed to be > 0, so align - 1 is always
198 // 2. `len + align - 1` can overflow by at most `align - 1`,
199 // so the &-mask wth `!(align - 1)` will ensure that in the
200 // case of overflow, `len_rounded_up` will itself be 0.
201 // Thus the returned padding, when added to `len`, yields 0,
202 // which trivially satisfies the alignment `align`.
204 // (Of course, attempts to allocate blocks of memory whose
205 // size and padding overflow in the above manner should cause
206 // the allocator to yield an error anyway.)
208 let len_rounded_up = len.wrapping_add(align).wrapping_sub(1)
209 & !align.wrapping_sub(1);
210 len_rounded_up.wrapping_sub(len)
213 /// Creates a layout by rounding the size of this layout up to a multiple
214 /// of the layout's alignment.
216 /// This is equivalent to adding the result of `padding_needed_for`
217 /// to the layout's current size.
218 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
220 pub fn pad_to_align(&self) -> Layout {
221 let pad = self.padding_needed_for(self.align());
222 // This cannot overflow. Quoting from the invariant of Layout:
223 // > `size`, when rounded up to the nearest multiple of `align`,
224 // > must not overflow (i.e., the rounded value must be less than
226 let new_size = self.size() + pad;
228 Layout::from_size_align(new_size, self.align()).unwrap()
231 /// Creates a layout describing the record for `n` instances of
232 /// `self`, with a suitable amount of padding between each to
233 /// ensure that each instance is given its requested size and
234 /// alignment. On success, returns `(k, offs)` where `k` is the
235 /// layout of the array and `offs` is the distance between the start
236 /// of each element in the array.
238 /// On arithmetic overflow, returns `LayoutErr`.
239 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
241 pub fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutErr> {
242 let padded_size = self.size().checked_add(self.padding_needed_for(self.align()))
243 .ok_or(LayoutErr { private: () })?;
244 let alloc_size = padded_size.checked_mul(n)
245 .ok_or(LayoutErr { private: () })?;
248 // self.align is already known to be valid and alloc_size has been
250 Ok((Layout::from_size_align_unchecked(alloc_size, self.align()), padded_size))
254 /// Creates a layout describing the record for `self` followed by
255 /// `next`, including any necessary padding to ensure that `next`
256 /// will be properly aligned. Note that the result layout will
257 /// satisfy the alignment properties of both `self` and `next`.
259 /// The resulting layout will be the same as that of a C struct containing
260 /// two fields with the layouts of `self` and `next`, in that order.
262 /// Returns `Some((k, offset))`, where `k` is layout of the concatenated
263 /// record and `offset` is the relative location, in bytes, of the
264 /// start of the `next` embedded within the concatenated record
265 /// (assuming that the record itself starts at offset 0).
267 /// On arithmetic overflow, returns `LayoutErr`.
268 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
270 pub fn extend(&self, next: Self) -> Result<(Self, usize), LayoutErr> {
271 let new_align = cmp::max(self.align(), next.align());
272 let pad = self.padding_needed_for(next.align());
274 let offset = self.size().checked_add(pad)
275 .ok_or(LayoutErr { private: () })?;
276 let new_size = offset.checked_add(next.size())
277 .ok_or(LayoutErr { private: () })?;
279 let layout = Layout::from_size_align(new_size, new_align)?;
283 /// Creates a layout describing the record for `n` instances of
284 /// `self`, with no padding between each instance.
286 /// Note that, unlike `repeat`, `repeat_packed` does not guarantee
287 /// that the repeated instances of `self` will be properly
288 /// aligned, even if a given instance of `self` is properly
289 /// aligned. In other words, if the layout returned by
290 /// `repeat_packed` is used to allocate an array, it is not
291 /// guaranteed that all elements in the array will be properly
294 /// On arithmetic overflow, returns `LayoutErr`.
295 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
297 pub fn repeat_packed(&self, n: usize) -> Result<Self, LayoutErr> {
298 let size = self.size().checked_mul(n).ok_or(LayoutErr { private: () })?;
299 Layout::from_size_align(size, self.align())
302 /// Creates a layout describing the record for `self` followed by
303 /// `next` with no additional padding between the two. Since no
304 /// padding is inserted, the alignment of `next` is irrelevant,
305 /// and is not incorporated *at all* into the resulting layout.
307 /// On arithmetic overflow, returns `LayoutErr`.
308 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
310 pub fn extend_packed(&self, next: Self) -> Result<Self, LayoutErr> {
311 let new_size = self.size().checked_add(next.size())
312 .ok_or(LayoutErr { private: () })?;
313 let layout = 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> {
326 debug_assert!(offs == mem::size_of::<T>());
332 /// The parameters given to `Layout::from_size_align`
333 /// or some other `Layout` constructor
334 /// do not satisfy its documented constraints.
335 #[stable(feature = "alloc_layout", since = "1.28.0")]
336 #[derive(Clone, PartialEq, Eq, Debug)]
337 pub struct LayoutErr {
341 // (we need this for downstream impl of trait Error)
342 #[stable(feature = "alloc_layout", since = "1.28.0")]
343 impl fmt::Display for LayoutErr {
344 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
345 f.write_str("invalid parameters to Layout::from_size_align")
349 /// The `AllocErr` error indicates an allocation failure
350 /// that may be due to resource exhaustion or to
351 /// something wrong when combining the given input arguments with this
353 #[unstable(feature = "allocator_api", issue = "32838")]
354 #[derive(Clone, PartialEq, Eq, Debug)]
357 // (we need this for downstream impl of trait Error)
358 #[unstable(feature = "allocator_api", issue = "32838")]
359 impl fmt::Display for AllocErr {
360 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
361 f.write_str("memory allocation failed")
365 /// The `CannotReallocInPlace` error is used when [`grow_in_place`] or
366 /// [`shrink_in_place`] were unable to reuse the given memory block for
367 /// a requested layout.
369 /// [`grow_in_place`]: ./trait.Alloc.html#method.grow_in_place
370 /// [`shrink_in_place`]: ./trait.Alloc.html#method.shrink_in_place
371 #[unstable(feature = "allocator_api", issue = "32838")]
372 #[derive(Clone, PartialEq, Eq, Debug)]
373 pub struct CannotReallocInPlace;
375 #[unstable(feature = "allocator_api", issue = "32838")]
376 impl CannotReallocInPlace {
377 pub fn description(&self) -> &str {
378 "cannot reallocate allocator's memory in place"
382 // (we need this for downstream impl of trait Error)
383 #[unstable(feature = "allocator_api", issue = "32838")]
384 impl fmt::Display for CannotReallocInPlace {
385 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
386 write!(f, "{}", self.description())
390 /// A memory allocator that can be registered as the standard library’s default
391 /// though the `#[global_allocator]` attributes.
393 /// Some of the methods require that a memory block be *currently
394 /// allocated* via an allocator. This means that:
396 /// * the starting address for that memory block was previously
397 /// returned by a previous call to an allocation method
398 /// such as `alloc`, and
400 /// * the memory block has not been subsequently deallocated, where
401 /// blocks are deallocated either by being passed to a deallocation
402 /// method such as `dealloc` or by being
403 /// passed to a reallocation method that returns a non-null pointer.
409 /// use std::alloc::{GlobalAlloc, Layout, alloc};
410 /// use std::ptr::null_mut;
412 /// struct MyAllocator;
414 /// unsafe impl GlobalAlloc for MyAllocator {
415 /// unsafe fn alloc(&self, _layout: Layout) -> *mut u8 { null_mut() }
416 /// unsafe fn dealloc(&self, _ptr: *mut u8, _layout: Layout) {}
419 /// #[global_allocator]
420 /// static A: MyAllocator = MyAllocator;
424 /// assert!(alloc(Layout::new::<u32>()).is_null())
431 /// The `GlobalAlloc` trait is an `unsafe` trait for a number of reasons, and
432 /// implementors must ensure that they adhere to these contracts:
434 /// * It's undefined behavior if global allocators unwind. This restriction may
435 /// be lifted in the future, but currently a panic from any of these
436 /// functions may lead to memory unsafety.
438 /// * `Layout` queries and calculations in general must be correct. Callers of
439 /// this trait are allowed to rely on the contracts defined on each method,
440 /// and implementors must ensure such contracts remain true.
441 #[stable(feature = "global_alloc", since = "1.28.0")]
442 pub unsafe trait GlobalAlloc {
443 /// Allocate memory as described by the given `layout`.
445 /// Returns a pointer to newly-allocated memory,
446 /// or null to indicate allocation failure.
450 /// This function is unsafe because undefined behavior can result
451 /// if the caller does not ensure that `layout` has non-zero size.
453 /// (Extension subtraits might provide more specific bounds on
454 /// behavior, e.g., guarantee a sentinel address or a null pointer
455 /// in response to a zero-size allocation request.)
457 /// The allocated block of memory may or may not be initialized.
461 /// Returning a null pointer indicates that either memory is exhausted
462 /// or `layout` does not meet allocator's size or alignment constraints.
464 /// Implementations are encouraged to return null on memory
465 /// exhaustion rather than aborting, but this is not
466 /// a strict requirement. (Specifically: it is *legal* to
467 /// implement this trait atop an underlying native allocation
468 /// library that aborts on memory exhaustion.)
470 /// Clients wishing to abort computation in response to an
471 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
472 /// rather than directly invoking `panic!` or similar.
474 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
475 #[stable(feature = "global_alloc", since = "1.28.0")]
476 unsafe fn alloc(&self, layout: Layout) -> *mut u8;
478 /// Deallocate the block of memory at the given `ptr` pointer with the given `layout`.
482 /// This function is unsafe because undefined behavior can result
483 /// if the caller does not ensure all of the following:
485 /// * `ptr` must denote a block of memory currently allocated via
488 /// * `layout` must be the same layout that was used
489 /// to allocate that block of memory,
490 #[stable(feature = "global_alloc", since = "1.28.0")]
491 unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout);
493 /// Behaves like `alloc`, but also ensures that the contents
494 /// are set to zero before being returned.
498 /// This function is unsafe for the same reasons that `alloc` is.
499 /// However the allocated block of memory is guaranteed to be initialized.
503 /// Returning a null pointer indicates that either memory is exhausted
504 /// or `layout` does not meet allocator's size or alignment constraints,
505 /// just as in `alloc`.
507 /// Clients wishing to abort computation in response to an
508 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
509 /// rather than directly invoking `panic!` or similar.
511 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
512 #[stable(feature = "global_alloc", since = "1.28.0")]
513 unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
514 let size = layout.size();
515 let ptr = self.alloc(layout);
517 ptr::write_bytes(ptr, 0, size);
522 /// Shrink or grow a block of memory to the given `new_size`.
523 /// The block is described by the given `ptr` pointer and `layout`.
525 /// If this returns a non-null pointer, then ownership of the memory block
526 /// referenced by `ptr` has been transferred to this allocator.
527 /// The memory may or may not have been deallocated,
528 /// and should be considered unusable (unless of course it was
529 /// transferred back to the caller again via the return value of
532 /// If this method returns null, then ownership of the memory
533 /// block has not been transferred to this allocator, and the
534 /// contents of the memory block are unaltered.
538 /// This function is unsafe because undefined behavior can result
539 /// if the caller does not ensure all of the following:
541 /// * `ptr` must be currently allocated via this allocator,
543 /// * `layout` must be the same layout that was used
544 /// to allocate that block of memory,
546 /// * `new_size` must be greater than zero.
548 /// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
549 /// must not overflow (i.e., the rounded value must be less than `usize::MAX`).
551 /// (Extension subtraits might provide more specific bounds on
552 /// behavior, e.g., guarantee a sentinel address or a null pointer
553 /// in response to a zero-size allocation request.)
557 /// Returns null if the new layout does not meet the size
558 /// and alignment constraints of the allocator, or if reallocation
561 /// Implementations are encouraged to return null on memory
562 /// exhaustion rather than panicking or aborting, but this is not
563 /// a strict requirement. (Specifically: it is *legal* to
564 /// implement this trait atop an underlying native allocation
565 /// library that aborts on memory exhaustion.)
567 /// Clients wishing to abort computation in response to a
568 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
569 /// rather than directly invoking `panic!` or similar.
571 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
572 #[stable(feature = "global_alloc", since = "1.28.0")]
573 unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
574 let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
575 let new_ptr = self.alloc(new_layout);
576 if !new_ptr.is_null() {
577 ptr::copy_nonoverlapping(
580 cmp::min(layout.size(), new_size),
582 self.dealloc(ptr, layout);
588 /// An implementation of `Alloc` can allocate, reallocate, and
589 /// deallocate arbitrary blocks of data described via `Layout`.
591 /// Some of the methods require that a memory block be *currently
592 /// allocated* via an allocator. This means that:
594 /// * the starting address for that memory block was previously
595 /// returned by a previous call to an allocation method (`alloc`,
596 /// `alloc_zeroed`, `alloc_excess`, `alloc_one`, `alloc_array`) or
597 /// reallocation method (`realloc`, `realloc_excess`, or
598 /// `realloc_array`), and
600 /// * the memory block has not been subsequently deallocated, where
601 /// blocks are deallocated either by being passed to a deallocation
602 /// method (`dealloc`, `dealloc_one`, `dealloc_array`) or by being
603 /// passed to a reallocation method (see above) that returns `Ok`.
605 /// A note regarding zero-sized types and zero-sized layouts: many
606 /// methods in the `Alloc` trait state that allocation requests
607 /// must be non-zero size, or else undefined behavior can result.
609 /// * However, some higher-level allocation methods (`alloc_one`,
610 /// `alloc_array`) are well-defined on zero-sized types and can
611 /// optionally support them: it is left up to the implementor
612 /// whether to return `Err`, or to return `Ok` with some pointer.
614 /// * If an `Alloc` implementation chooses to return `Ok` in this
615 /// case (i.e., the pointer denotes a zero-sized inaccessible block)
616 /// then that returned pointer must be considered "currently
617 /// allocated". On such an allocator, *all* methods that take
618 /// currently-allocated pointers as inputs must accept these
619 /// zero-sized pointers, *without* causing undefined behavior.
621 /// * In other words, if a zero-sized pointer can flow out of an
622 /// allocator, then that allocator must likewise accept that pointer
623 /// flowing back into its deallocation and reallocation methods.
625 /// Some of the methods require that a layout *fit* a memory block.
626 /// What it means for a layout to "fit" a memory block means (or
627 /// equivalently, for a memory block to "fit" a layout) is that the
628 /// following two conditions must hold:
630 /// 1. The block's starting address must be aligned to `layout.align()`.
632 /// 2. The block's size must fall in the range `[use_min, use_max]`, where:
634 /// * `use_min` is `self.usable_size(layout).0`, and
636 /// * `use_max` is the capacity that was (or would have been)
637 /// returned when (if) the block was allocated via a call to
638 /// `alloc_excess` or `realloc_excess`.
642 /// * the size of the layout most recently used to allocate the block
643 /// is guaranteed to be in the range `[use_min, use_max]`, and
645 /// * a lower-bound on `use_max` can be safely approximated by a call to
648 /// * if a layout `k` fits a memory block (denoted by `ptr`)
649 /// currently allocated via an allocator `a`, then it is legal to
650 /// use that layout to deallocate it, i.e., `a.dealloc(ptr, k);`.
654 /// The `Alloc` trait is an `unsafe` trait for a number of reasons, and
655 /// implementors must ensure that they adhere to these contracts:
657 /// * Pointers returned from allocation functions must point to valid memory and
658 /// retain their validity until at least the instance of `Alloc` is dropped
661 /// * `Layout` queries and calculations in general must be correct. Callers of
662 /// this trait are allowed to rely on the contracts defined on each method,
663 /// and implementors must ensure such contracts remain true.
665 /// Note that this list may get tweaked over time as clarifications are made in
667 #[unstable(feature = "allocator_api", issue = "32838")]
668 pub unsafe trait Alloc {
670 // (Note: some existing allocators have unspecified but well-defined
671 // behavior in response to a zero size allocation request ;
672 // e.g., in C, `malloc` of 0 will either return a null pointer or a
673 // unique pointer, but will not have arbitrary undefined
675 // However in jemalloc for example,
676 // `mallocx(0)` is documented as undefined behavior.)
678 /// Returns a pointer meeting the size and alignment guarantees of
681 /// If this method returns an `Ok(addr)`, then the `addr` returned
682 /// will be non-null address pointing to a block of storage
683 /// suitable for holding an instance of `layout`.
685 /// The returned block of storage may or may not have its contents
686 /// initialized. (Extension subtraits might restrict this
687 /// behavior, e.g., to ensure initialization to particular sets of
692 /// This function is unsafe because undefined behavior can result
693 /// if the caller does not ensure that `layout` has non-zero size.
695 /// (Extension subtraits might provide more specific bounds on
696 /// behavior, e.g., guarantee a sentinel address or a null pointer
697 /// in response to a zero-size allocation request.)
701 /// Returning `Err` indicates that either memory is exhausted or
702 /// `layout` does not meet allocator's size or alignment
705 /// Implementations are encouraged to return `Err` on memory
706 /// exhaustion rather than panicking or aborting, but this is not
707 /// a strict requirement. (Specifically: it is *legal* to
708 /// implement this trait atop an underlying native allocation
709 /// library that aborts on memory exhaustion.)
711 /// Clients wishing to abort computation in response to an
712 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
713 /// rather than directly invoking `panic!` or similar.
715 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
716 unsafe fn alloc(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr>;
718 /// Deallocate the memory referenced by `ptr`.
722 /// This function is unsafe because undefined behavior can result
723 /// if the caller does not ensure all of the following:
725 /// * `ptr` must denote a block of memory currently allocated via
728 /// * `layout` must *fit* that block of memory,
730 /// * In addition to fitting the block of memory `layout`, the
731 /// alignment of the `layout` must match the alignment used
732 /// to allocate that block of memory.
733 unsafe fn dealloc(&mut self, ptr: NonNull<u8>, layout: Layout);
735 // == ALLOCATOR-SPECIFIC QUANTITIES AND LIMITS ==
738 /// Returns bounds on the guaranteed usable size of a successful
739 /// allocation created with the specified `layout`.
741 /// In particular, if one has a memory block allocated via a given
742 /// allocator `a` and layout `k` where `a.usable_size(k)` returns
743 /// `(l, u)`, then one can pass that block to `a.dealloc()` with a
744 /// layout in the size range [l, u].
746 /// (All implementors of `usable_size` must ensure that
747 /// `l <= k.size() <= u`)
749 /// Both the lower- and upper-bounds (`l` and `u` respectively)
750 /// are provided, because an allocator based on size classes could
751 /// misbehave if one attempts to deallocate a block without
752 /// providing a correct value for its size (i.e., one within the
755 /// Clients who wish to make use of excess capacity are encouraged
756 /// to use the `alloc_excess` and `realloc_excess` instead, as
757 /// this method is constrained to report conservative values that
758 /// serve as valid bounds for *all possible* allocation method
761 /// However, for clients that do not wish to track the capacity
762 /// returned by `alloc_excess` locally, this method is likely to
763 /// produce useful results.
765 fn usable_size(&self, layout: &Layout) -> (usize, usize) {
766 (layout.size(), layout.size())
769 // == METHODS FOR MEMORY REUSE ==
770 // realloc. alloc_excess, realloc_excess
772 /// Returns a pointer suitable for holding data described by
773 /// a new layout with `layout`’s alignment and a size given
774 /// by `new_size`. To
775 /// accomplish this, this may extend or shrink the allocation
776 /// referenced by `ptr` to fit the new layout.
778 /// If this returns `Ok`, then ownership of the memory block
779 /// referenced by `ptr` has been transferred to this
780 /// allocator. The memory may or may not have been freed, and
781 /// should be considered unusable (unless of course it was
782 /// transferred back to the caller again via the return value of
785 /// If this method returns `Err`, then ownership of the memory
786 /// block has not been transferred to this allocator, and the
787 /// contents of the memory block are unaltered.
791 /// This function is unsafe because undefined behavior can result
792 /// if the caller does not ensure all of the following:
794 /// * `ptr` must be currently allocated via this allocator,
796 /// * `layout` must *fit* the `ptr` (see above). (The `new_size`
797 /// argument need not fit it.)
799 /// * `new_size` must be greater than zero.
801 /// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
802 /// must not overflow (i.e., the rounded value must be less than `usize::MAX`).
804 /// (Extension subtraits might provide more specific bounds on
805 /// behavior, e.g., guarantee a sentinel address or a null pointer
806 /// in response to a zero-size allocation request.)
810 /// Returns `Err` only if the new layout
811 /// does not meet the allocator's size
812 /// and alignment constraints of the allocator, or if reallocation
815 /// Implementations are encouraged to return `Err` on memory
816 /// exhaustion rather than panicking or aborting, but this is not
817 /// a strict requirement. (Specifically: it is *legal* to
818 /// implement this trait atop an underlying native allocation
819 /// library that aborts on memory exhaustion.)
821 /// Clients wishing to abort computation in response to a
822 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
823 /// rather than directly invoking `panic!` or similar.
825 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
826 unsafe fn realloc(&mut self,
829 new_size: usize) -> Result<NonNull<u8>, AllocErr> {
830 let old_size = layout.size();
832 if new_size >= old_size {
833 if let Ok(()) = self.grow_in_place(ptr, layout, new_size) {
836 } else if new_size < old_size {
837 if let Ok(()) = self.shrink_in_place(ptr, layout, new_size) {
842 // otherwise, fall back on alloc + copy + dealloc.
843 let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
844 let result = self.alloc(new_layout);
845 if let Ok(new_ptr) = result {
846 ptr::copy_nonoverlapping(ptr.as_ptr(),
848 cmp::min(old_size, new_size));
849 self.dealloc(ptr, layout);
854 /// Behaves like `alloc`, but also ensures that the contents
855 /// are set to zero before being returned.
859 /// This function is unsafe for the same reasons that `alloc` is.
863 /// Returning `Err` indicates that either memory is exhausted or
864 /// `layout` does not meet allocator's size or alignment
865 /// constraints, just as in `alloc`.
867 /// Clients wishing to abort computation in response to an
868 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
869 /// rather than directly invoking `panic!` or similar.
871 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
872 unsafe fn alloc_zeroed(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr> {
873 let size = layout.size();
874 let p = self.alloc(layout);
876 ptr::write_bytes(p.as_ptr(), 0, size);
881 /// Behaves like `alloc`, but also returns the whole size of
882 /// the returned block. For some `layout` inputs, like arrays, this
883 /// may include extra storage usable for additional data.
887 /// This function is unsafe for the same reasons that `alloc` is.
891 /// Returning `Err` indicates that either memory is exhausted or
892 /// `layout` does not meet allocator's size or alignment
893 /// constraints, just as in `alloc`.
895 /// Clients wishing to abort computation in response to an
896 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
897 /// rather than directly invoking `panic!` or similar.
899 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
900 unsafe fn alloc_excess(&mut self, layout: Layout) -> Result<Excess, AllocErr> {
901 let usable_size = self.usable_size(&layout);
902 self.alloc(layout).map(|p| Excess(p, usable_size.1))
905 /// Behaves like `realloc`, but also returns the whole size of
906 /// the returned block. For some `layout` inputs, like arrays, this
907 /// may include extra storage usable for additional data.
911 /// This function is unsafe for the same reasons that `realloc` is.
915 /// Returning `Err` indicates that either memory is exhausted or
916 /// `layout` does not meet allocator's size or alignment
917 /// constraints, just as in `realloc`.
919 /// Clients wishing to abort computation in response to a
920 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
921 /// rather than directly invoking `panic!` or similar.
923 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
924 unsafe fn realloc_excess(&mut self,
927 new_size: usize) -> Result<Excess, AllocErr> {
928 let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
929 let usable_size = self.usable_size(&new_layout);
930 self.realloc(ptr, layout, new_size)
931 .map(|p| Excess(p, usable_size.1))
934 /// Attempts to extend the allocation referenced by `ptr` to fit `new_size`.
936 /// If this returns `Ok`, then the allocator has asserted that the
937 /// memory block referenced by `ptr` now fits `new_size`, and thus can
938 /// be used to carry data of a layout of that size and same alignment as
939 /// `layout`. (The allocator is allowed to
940 /// expend effort to accomplish this, such as extending the memory block to
941 /// include successor blocks, or virtual memory tricks.)
943 /// Regardless of what this method returns, ownership of the
944 /// memory block referenced by `ptr` has not been transferred, and
945 /// the contents of the memory block are unaltered.
949 /// This function is unsafe because undefined behavior can result
950 /// if the caller does not ensure all of the following:
952 /// * `ptr` must be currently allocated via this allocator,
954 /// * `layout` must *fit* the `ptr` (see above); note the
955 /// `new_size` argument need not fit it,
957 /// * `new_size` must not be less than `layout.size()`,
961 /// Returns `Err(CannotReallocInPlace)` when the allocator is
962 /// unable to assert that the memory block referenced by `ptr`
963 /// could fit `layout`.
965 /// Note that one cannot pass `CannotReallocInPlace` to the `handle_alloc_error`
966 /// function; clients are expected either to be able to recover from
967 /// `grow_in_place` failures without aborting, or to fall back on
968 /// another reallocation method before resorting to an abort.
969 unsafe fn grow_in_place(&mut self,
972 new_size: usize) -> Result<(), CannotReallocInPlace> {
973 let _ = ptr; // this default implementation doesn't care about the actual address.
974 debug_assert!(new_size >= layout.size());
975 let (_l, u) = self.usable_size(&layout);
976 // _l <= layout.size() [guaranteed by usable_size()]
977 // layout.size() <= new_layout.size() [required by this method]
981 Err(CannotReallocInPlace)
985 /// Attempts to shrink the allocation referenced by `ptr` to fit `new_size`.
987 /// If this returns `Ok`, then the allocator has asserted that the
988 /// memory block referenced by `ptr` now fits `new_size`, and
989 /// thus can only be used to carry data of that smaller
990 /// layout. (The allocator is allowed to take advantage of this,
991 /// carving off portions of the block for reuse elsewhere.) The
992 /// truncated contents of the block within the smaller layout are
993 /// unaltered, and ownership of block has not been transferred.
995 /// If this returns `Err`, then the memory block is considered to
996 /// still represent the original (larger) `layout`. None of the
997 /// block has been carved off for reuse elsewhere, ownership of
998 /// the memory block has not been transferred, and the contents of
999 /// the memory block are unaltered.
1003 /// This function is unsafe because undefined behavior can result
1004 /// if the caller does not ensure all of the following:
1006 /// * `ptr` must be currently allocated via this allocator,
1008 /// * `layout` must *fit* the `ptr` (see above); note the
1009 /// `new_size` argument need not fit it,
1011 /// * `new_size` must not be greater than `layout.size()`
1012 /// (and must be greater than zero),
1016 /// Returns `Err(CannotReallocInPlace)` when the allocator is
1017 /// unable to assert that the memory block referenced by `ptr`
1018 /// could fit `layout`.
1020 /// Note that one cannot pass `CannotReallocInPlace` to the `handle_alloc_error`
1021 /// function; clients are expected either to be able to recover from
1022 /// `shrink_in_place` failures without aborting, or to fall back
1023 /// on another reallocation method before resorting to an abort.
1024 unsafe fn shrink_in_place(&mut self,
1027 new_size: usize) -> Result<(), CannotReallocInPlace> {
1028 let _ = ptr; // this default implementation doesn't care about the actual address.
1029 debug_assert!(new_size <= layout.size());
1030 let (l, _u) = self.usable_size(&layout);
1031 // layout.size() <= _u [guaranteed by usable_size()]
1032 // new_layout.size() <= layout.size() [required by this method]
1036 Err(CannotReallocInPlace)
1041 // == COMMON USAGE PATTERNS ==
1042 // alloc_one, dealloc_one, alloc_array, realloc_array. dealloc_array
1044 /// Allocates a block suitable for holding an instance of `T`.
1046 /// Captures a common usage pattern for allocators.
1048 /// The returned block is suitable for passing to the
1049 /// `alloc`/`realloc` methods of this allocator.
1051 /// Note to implementors: If this returns `Ok(ptr)`, then `ptr`
1052 /// must be considered "currently allocated" and must be
1053 /// acceptable input to methods such as `realloc` or `dealloc`,
1054 /// *even if* `T` is a zero-sized type. In other words, if your
1055 /// `Alloc` implementation overrides this method in a manner
1056 /// that can return a zero-sized `ptr`, then all reallocation and
1057 /// deallocation methods need to be similarly overridden to accept
1058 /// such values as input.
1062 /// Returning `Err` indicates that either memory is exhausted or
1063 /// `T` does not meet allocator's size or alignment constraints.
1065 /// For zero-sized `T`, may return either of `Ok` or `Err`, but
1066 /// will *not* yield undefined behavior.
1068 /// Clients wishing to abort computation in response to an
1069 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
1070 /// rather than directly invoking `panic!` or similar.
1072 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1073 fn alloc_one<T>(&mut self) -> Result<NonNull<T>, AllocErr>
1076 let k = Layout::new::<T>();
1078 unsafe { self.alloc(k).map(|p| p.cast()) }
1084 /// Deallocates a block suitable for holding an instance of `T`.
1086 /// The given block must have been produced by this allocator,
1087 /// and must be suitable for storing a `T` (in terms of alignment
1088 /// as well as minimum and maximum size); otherwise yields
1089 /// undefined behavior.
1091 /// Captures a common usage pattern for allocators.
1095 /// This function is unsafe because undefined behavior can result
1096 /// if the caller does not ensure both:
1098 /// * `ptr` must denote a block of memory currently allocated via this allocator
1100 /// * the layout of `T` must *fit* that block of memory.
1101 unsafe fn dealloc_one<T>(&mut self, ptr: NonNull<T>)
1104 let k = Layout::new::<T>();
1106 self.dealloc(ptr.cast(), k);
1110 /// Allocates a block suitable for holding `n` instances of `T`.
1112 /// Captures a common usage pattern for allocators.
1114 /// The returned block is suitable for passing to the
1115 /// `alloc`/`realloc` methods of this allocator.
1117 /// Note to implementors: If this returns `Ok(ptr)`, then `ptr`
1118 /// must be considered "currently allocated" and must be
1119 /// acceptable input to methods such as `realloc` or `dealloc`,
1120 /// *even if* `T` is a zero-sized type. In other words, if your
1121 /// `Alloc` implementation overrides this method in a manner
1122 /// that can return a zero-sized `ptr`, then all reallocation and
1123 /// deallocation methods need to be similarly overridden to accept
1124 /// such values as input.
1128 /// Returning `Err` indicates that either memory is exhausted or
1129 /// `[T; n]` does not meet allocator's size or alignment
1132 /// For zero-sized `T` or `n == 0`, may return either of `Ok` or
1133 /// `Err`, but will *not* yield undefined behavior.
1135 /// Always returns `Err` on arithmetic overflow.
1137 /// Clients wishing to abort computation in response to an
1138 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
1139 /// rather than directly invoking `panic!` or similar.
1141 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1142 fn alloc_array<T>(&mut self, n: usize) -> Result<NonNull<T>, AllocErr>
1145 match Layout::array::<T>(n) {
1146 Ok(ref layout) if layout.size() > 0 => {
1148 self.alloc(layout.clone()).map(|p| p.cast())
1155 /// Reallocates a block previously suitable for holding `n_old`
1156 /// instances of `T`, returning a block suitable for holding
1157 /// `n_new` instances of `T`.
1159 /// Captures a common usage pattern for allocators.
1161 /// The returned block is suitable for passing to the
1162 /// `alloc`/`realloc` methods of this allocator.
1166 /// This function is unsafe because undefined behavior can result
1167 /// if the caller does not ensure all of the following:
1169 /// * `ptr` must be currently allocated via this allocator,
1171 /// * the layout of `[T; n_old]` must *fit* that block of memory.
1175 /// Returning `Err` indicates that either memory is exhausted or
1176 /// `[T; n_new]` does not meet allocator's size or alignment
1179 /// For zero-sized `T` or `n_new == 0`, may return either of `Ok` or
1180 /// `Err`, but will *not* yield undefined behavior.
1182 /// Always returns `Err` on arithmetic overflow.
1184 /// Clients wishing to abort computation in response to a
1185 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
1186 /// rather than directly invoking `panic!` or similar.
1188 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1189 unsafe fn realloc_array<T>(&mut self,
1192 n_new: usize) -> Result<NonNull<T>, AllocErr>
1195 match (Layout::array::<T>(n_old), Layout::array::<T>(n_new)) {
1196 (Ok(ref k_old), Ok(ref k_new)) if k_old.size() > 0 && k_new.size() > 0 => {
1197 debug_assert!(k_old.align() == k_new.align());
1198 self.realloc(ptr.cast(), k_old.clone(), k_new.size()).map(NonNull::cast)
1206 /// Deallocates a block suitable for holding `n` instances of `T`.
1208 /// Captures a common usage pattern for allocators.
1212 /// This function is unsafe because undefined behavior can result
1213 /// if the caller does not ensure both:
1215 /// * `ptr` must denote a block of memory currently allocated via this allocator
1217 /// * the layout of `[T; n]` must *fit* that block of memory.
1221 /// Returning `Err` indicates that either `[T; n]` or the given
1222 /// memory block does not meet allocator's size or alignment
1225 /// Always returns `Err` on arithmetic overflow.
1226 unsafe fn dealloc_array<T>(&mut self, ptr: NonNull<T>, n: usize) -> Result<(), AllocErr>
1229 match Layout::array::<T>(n) {
1230 Ok(ref k) if k.size() > 0 => {
1231 Ok(self.dealloc(ptr.cast(), k.clone()))