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")]
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 is 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 with `!(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 // This cannot overflow. Quoting from the invariant of Layout:
243 // > `size`, when rounded up to the nearest multiple of `align`,
244 // > must not overflow (i.e., the rounded value must be less than
246 let padded_size = self.size() + self.padding_needed_for(self.align());
247 let alloc_size = padded_size.checked_mul(n)
248 .ok_or(LayoutErr { private: () })?;
251 // self.align is already known to be valid and alloc_size has been
253 Ok((Layout::from_size_align_unchecked(alloc_size, self.align()), padded_size))
257 /// Creates a layout describing the record for `self` followed by
258 /// `next`, including any necessary padding to ensure that `next`
259 /// will be properly aligned. Note that the resulting layout will
260 /// satisfy the alignment properties of both `self` and `next`.
262 /// The resulting layout will be the same as that of a C struct containing
263 /// two fields with the layouts of `self` and `next`, in that order.
265 /// Returns `Some((k, offset))`, where `k` is layout of the concatenated
266 /// record and `offset` is the relative location, in bytes, of the
267 /// start of the `next` embedded within the concatenated record
268 /// (assuming that the record itself starts at offset 0).
270 /// On arithmetic overflow, returns `LayoutErr`.
271 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
273 pub fn extend(&self, next: Self) -> Result<(Self, usize), LayoutErr> {
274 let new_align = cmp::max(self.align(), next.align());
275 let pad = self.padding_needed_for(next.align());
277 let offset = self.size().checked_add(pad)
278 .ok_or(LayoutErr { private: () })?;
279 let new_size = offset.checked_add(next.size())
280 .ok_or(LayoutErr { private: () })?;
282 let layout = Layout::from_size_align(new_size, new_align)?;
286 /// Creates a layout describing the record for `n` instances of
287 /// `self`, with no padding between each instance.
289 /// Note that, unlike `repeat`, `repeat_packed` does not guarantee
290 /// that the repeated instances of `self` will be properly
291 /// aligned, even if a given instance of `self` is properly
292 /// aligned. In other words, if the layout returned by
293 /// `repeat_packed` is used to allocate an array, it is not
294 /// guaranteed that all elements in the array will be properly
297 /// On arithmetic overflow, returns `LayoutErr`.
298 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
300 pub fn repeat_packed(&self, n: usize) -> Result<Self, LayoutErr> {
301 let size = self.size().checked_mul(n).ok_or(LayoutErr { private: () })?;
302 Layout::from_size_align(size, self.align())
305 /// Creates a layout describing the record for `self` followed by
306 /// `next` with no additional padding between the two. Since no
307 /// padding is inserted, the alignment of `next` is irrelevant,
308 /// and is not incorporated *at all* into the resulting layout.
310 /// On arithmetic overflow, returns `LayoutErr`.
311 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
313 pub fn extend_packed(&self, next: Self) -> Result<Self, LayoutErr> {
314 let new_size = self.size().checked_add(next.size())
315 .ok_or(LayoutErr { private: () })?;
316 Layout::from_size_align(new_size, self.align())
319 /// Creates a layout describing the record for a `[T; n]`.
321 /// On arithmetic overflow, returns `LayoutErr`.
322 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
324 pub fn array<T>(n: usize) -> Result<Self, LayoutErr> {
328 debug_assert!(offs == mem::size_of::<T>());
334 /// The parameters given to `Layout::from_size_align`
335 /// or some other `Layout` constructor
336 /// do not satisfy its documented constraints.
337 #[stable(feature = "alloc_layout", since = "1.28.0")]
338 #[derive(Clone, PartialEq, Eq, Debug)]
339 pub struct LayoutErr {
343 // (we need this for downstream impl of trait Error)
344 #[stable(feature = "alloc_layout", since = "1.28.0")]
345 impl fmt::Display for LayoutErr {
346 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
347 f.write_str("invalid parameters to Layout::from_size_align")
351 /// The `AllocErr` error indicates an allocation failure
352 /// that may be due to resource exhaustion or to
353 /// something wrong when combining the given input arguments with this
355 #[unstable(feature = "allocator_api", issue = "32838")]
356 #[derive(Clone, PartialEq, Eq, Debug)]
359 // (we need this for downstream impl of trait Error)
360 #[unstable(feature = "allocator_api", issue = "32838")]
361 impl fmt::Display for AllocErr {
362 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
363 f.write_str("memory allocation failed")
367 /// The `CannotReallocInPlace` error is used when [`grow_in_place`] or
368 /// [`shrink_in_place`] were unable to reuse the given memory block for
369 /// a requested layout.
371 /// [`grow_in_place`]: ./trait.Alloc.html#method.grow_in_place
372 /// [`shrink_in_place`]: ./trait.Alloc.html#method.shrink_in_place
373 #[unstable(feature = "allocator_api", issue = "32838")]
374 #[derive(Clone, PartialEq, Eq, Debug)]
375 pub struct CannotReallocInPlace;
377 #[unstable(feature = "allocator_api", issue = "32838")]
378 impl CannotReallocInPlace {
379 pub fn description(&self) -> &str {
380 "cannot reallocate allocator's memory in place"
384 // (we need this for downstream impl of trait Error)
385 #[unstable(feature = "allocator_api", issue = "32838")]
386 impl fmt::Display for CannotReallocInPlace {
387 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
388 write!(f, "{}", self.description())
392 /// A memory allocator that can be registered as the standard library’s default
393 /// through the `#[global_allocator]` attribute.
395 /// Some of the methods require that a memory block be *currently
396 /// allocated* via an allocator. This means that:
398 /// * the starting address for that memory block was previously
399 /// returned by a previous call to an allocation method
400 /// such as `alloc`, and
402 /// * the memory block has not been subsequently deallocated, where
403 /// blocks are deallocated either by being passed to a deallocation
404 /// method such as `dealloc` or by being
405 /// passed to a reallocation method that returns a non-null pointer.
411 /// use std::alloc::{GlobalAlloc, Layout, alloc};
412 /// use std::ptr::null_mut;
414 /// struct MyAllocator;
416 /// unsafe impl GlobalAlloc for MyAllocator {
417 /// unsafe fn alloc(&self, _layout: Layout) -> *mut u8 { null_mut() }
418 /// unsafe fn dealloc(&self, _ptr: *mut u8, _layout: Layout) {}
421 /// #[global_allocator]
422 /// static A: MyAllocator = MyAllocator;
426 /// assert!(alloc(Layout::new::<u32>()).is_null())
433 /// The `GlobalAlloc` trait is an `unsafe` trait for a number of reasons, and
434 /// implementors must ensure that they adhere to these contracts:
436 /// * It's undefined behavior if global allocators unwind. This restriction may
437 /// be lifted in the future, but currently a panic from any of these
438 /// functions may lead to memory unsafety.
440 /// * `Layout` queries and calculations in general must be correct. Callers of
441 /// this trait are allowed to rely on the contracts defined on each method,
442 /// and implementors must ensure such contracts remain true.
443 #[stable(feature = "global_alloc", since = "1.28.0")]
444 pub unsafe trait GlobalAlloc {
445 /// Allocate memory as described by the given `layout`.
447 /// Returns a pointer to newly-allocated memory,
448 /// or null to indicate allocation failure.
452 /// This function is unsafe because undefined behavior can result
453 /// if the caller does not ensure that `layout` has non-zero size.
455 /// (Extension subtraits might provide more specific bounds on
456 /// behavior, e.g., guarantee a sentinel address or a null pointer
457 /// in response to a zero-size allocation request.)
459 /// The allocated block of memory may or may not be initialized.
463 /// Returning a null pointer indicates that either memory is exhausted
464 /// or `layout` does not meet this allocator's size or alignment constraints.
466 /// Implementations are encouraged to return null on memory
467 /// exhaustion rather than aborting, but this is not
468 /// a strict requirement. (Specifically: it is *legal* to
469 /// implement this trait atop an underlying native allocation
470 /// library that aborts on memory exhaustion.)
472 /// Clients wishing to abort computation in response to an
473 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
474 /// rather than directly invoking `panic!` or similar.
476 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
477 #[stable(feature = "global_alloc", since = "1.28.0")]
478 unsafe fn alloc(&self, layout: Layout) -> *mut u8;
480 /// Deallocate the block of memory at the given `ptr` pointer with the given `layout`.
484 /// This function is unsafe because undefined behavior can result
485 /// if the caller does not ensure all of the following:
487 /// * `ptr` must denote a block of memory currently allocated via
490 /// * `layout` must be the same layout that was used
491 /// to allocate that block of memory,
492 #[stable(feature = "global_alloc", since = "1.28.0")]
493 unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout);
495 /// Behaves like `alloc`, but also ensures that the contents
496 /// are set to zero before being returned.
500 /// This function is unsafe for the same reasons that `alloc` is.
501 /// However the allocated block of memory is guaranteed to be initialized.
505 /// Returning a null pointer indicates that either memory is exhausted
506 /// or `layout` does not meet allocator's size or alignment constraints,
507 /// just as in `alloc`.
509 /// Clients wishing to abort computation in response to an
510 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
511 /// rather than directly invoking `panic!` or similar.
513 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
514 #[stable(feature = "global_alloc", since = "1.28.0")]
515 unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
516 let size = layout.size();
517 let ptr = self.alloc(layout);
519 ptr::write_bytes(ptr, 0, size);
524 /// Shrink or grow a block of memory to the given `new_size`.
525 /// The block is described by the given `ptr` pointer and `layout`.
527 /// If this returns a non-null pointer, then ownership of the memory block
528 /// referenced by `ptr` has been transferred to this allocator.
529 /// The memory may or may not have been deallocated,
530 /// and should be considered unusable (unless of course it was
531 /// transferred back to the caller again via the return value of
534 /// If this method returns null, then ownership of the memory
535 /// block has not been transferred to this allocator, and the
536 /// contents of the memory block are unaltered.
540 /// This function is unsafe because undefined behavior can result
541 /// if the caller does not ensure all of the following:
543 /// * `ptr` must be currently allocated via this allocator,
545 /// * `layout` must be the same layout that was used
546 /// to allocate that block of memory,
548 /// * `new_size` must be greater than zero.
550 /// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
551 /// must not overflow (i.e., the rounded value must be less than `usize::MAX`).
553 /// (Extension subtraits might provide more specific bounds on
554 /// behavior, e.g., guarantee a sentinel address or a null pointer
555 /// in response to a zero-size allocation request.)
559 /// Returns null if the new layout does not meet the size
560 /// and alignment constraints of the allocator, or if reallocation
563 /// Implementations are encouraged to return null on memory
564 /// exhaustion rather than panicking or aborting, but this is not
565 /// a strict requirement. (Specifically: it is *legal* to
566 /// implement this trait atop an underlying native allocation
567 /// library that aborts on memory exhaustion.)
569 /// Clients wishing to abort computation in response to a
570 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
571 /// rather than directly invoking `panic!` or similar.
573 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
574 #[stable(feature = "global_alloc", since = "1.28.0")]
575 unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
576 let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
577 let new_ptr = self.alloc(new_layout);
578 if !new_ptr.is_null() {
579 ptr::copy_nonoverlapping(
582 cmp::min(layout.size(), new_size),
584 self.dealloc(ptr, layout);
590 /// An implementation of `Alloc` can allocate, reallocate, and
591 /// deallocate arbitrary blocks of data described via `Layout`.
593 /// Some of the methods require that a memory block be *currently
594 /// allocated* via an allocator. This means that:
596 /// * the starting address for that memory block was previously
597 /// returned by a previous call to an allocation method (`alloc`,
598 /// `alloc_zeroed`, `alloc_excess`, `alloc_one`, `alloc_array`) or
599 /// reallocation method (`realloc`, `realloc_excess`, or
600 /// `realloc_array`), and
602 /// * the memory block has not been subsequently deallocated, where
603 /// blocks are deallocated either by being passed to a deallocation
604 /// method (`dealloc`, `dealloc_one`, `dealloc_array`) or by being
605 /// passed to a reallocation method (see above) that returns `Ok`.
607 /// A note regarding zero-sized types and zero-sized layouts: many
608 /// methods in the `Alloc` trait state that allocation requests
609 /// must be non-zero size, or else undefined behavior can result.
611 /// * However, some higher-level allocation methods (`alloc_one`,
612 /// `alloc_array`) are well-defined on zero-sized types and can
613 /// optionally support them: it is left up to the implementor
614 /// whether to return `Err`, or to return `Ok` with some pointer.
616 /// * If an `Alloc` implementation chooses to return `Ok` in this
617 /// case (i.e., the pointer denotes a zero-sized inaccessible block)
618 /// then that returned pointer must be considered "currently
619 /// allocated". On such an allocator, *all* methods that take
620 /// currently-allocated pointers as inputs must accept these
621 /// zero-sized pointers, *without* causing undefined behavior.
623 /// * In other words, if a zero-sized pointer can flow out of an
624 /// allocator, then that allocator must likewise accept that pointer
625 /// flowing back into its deallocation and reallocation methods.
627 /// Some of the methods require that a layout *fit* a memory block.
628 /// What it means for a layout to "fit" a memory block means (or
629 /// equivalently, for a memory block to "fit" a layout) is that the
630 /// following two conditions must hold:
632 /// 1. The block's starting address must be aligned to `layout.align()`.
634 /// 2. The block's size must fall in the range `[use_min, use_max]`, where:
636 /// * `use_min` is `self.usable_size(layout).0`, and
638 /// * `use_max` is the capacity that was (or would have been)
639 /// returned when (if) the block was allocated via a call to
640 /// `alloc_excess` or `realloc_excess`.
644 /// * the size of the layout most recently used to allocate the block
645 /// is guaranteed to be in the range `[use_min, use_max]`, and
647 /// * a lower-bound on `use_max` can be safely approximated by a call to
650 /// * if a layout `k` fits a memory block (denoted by `ptr`)
651 /// currently allocated via an allocator `a`, then it is legal to
652 /// use that layout to deallocate it, i.e., `a.dealloc(ptr, k);`.
656 /// The `Alloc` trait is an `unsafe` trait for a number of reasons, and
657 /// implementors must ensure that they adhere to these contracts:
659 /// * Pointers returned from allocation functions must point to valid memory and
660 /// retain their validity until at least the instance of `Alloc` is dropped
663 /// * `Layout` queries and calculations in general must be correct. Callers of
664 /// this trait are allowed to rely on the contracts defined on each method,
665 /// and implementors must ensure such contracts remain true.
667 /// Note that this list may get tweaked over time as clarifications are made in
669 #[unstable(feature = "allocator_api", issue = "32838")]
670 pub unsafe trait Alloc {
672 // (Note: some existing allocators have unspecified but well-defined
673 // behavior in response to a zero size allocation request ;
674 // e.g., in C, `malloc` of 0 will either return a null pointer or a
675 // unique pointer, but will not have arbitrary undefined
677 // However in jemalloc for example,
678 // `mallocx(0)` is documented as undefined behavior.)
680 /// Returns a pointer meeting the size and alignment guarantees of
683 /// If this method returns an `Ok(addr)`, then the `addr` returned
684 /// will be non-null address pointing to a block of storage
685 /// suitable for holding an instance of `layout`.
687 /// The returned block of storage may or may not have its contents
688 /// initialized. (Extension subtraits might restrict this
689 /// behavior, e.g., to ensure initialization to particular sets of
694 /// This function is unsafe because undefined behavior can result
695 /// if the caller does not ensure that `layout` has non-zero size.
697 /// (Extension subtraits might provide more specific bounds on
698 /// behavior, e.g., guarantee a sentinel address or a null pointer
699 /// in response to a zero-size allocation request.)
703 /// Returning `Err` indicates that either memory is exhausted or
704 /// `layout` does not meet allocator's size or alignment
707 /// Implementations are encouraged to return `Err` on memory
708 /// exhaustion rather than panicking or aborting, but this is not
709 /// a strict requirement. (Specifically: it is *legal* to
710 /// implement this trait atop an underlying native allocation
711 /// library that aborts on memory exhaustion.)
713 /// Clients wishing to abort computation in response to an
714 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
715 /// rather than directly invoking `panic!` or similar.
717 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
718 unsafe fn alloc(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr>;
720 /// Deallocate the memory referenced by `ptr`.
724 /// This function is unsafe because undefined behavior can result
725 /// if the caller does not ensure all of the following:
727 /// * `ptr` must denote a block of memory currently allocated via
730 /// * `layout` must *fit* that block of memory,
732 /// * In addition to fitting the block of memory `layout`, the
733 /// alignment of the `layout` must match the alignment used
734 /// to allocate that block of memory.
735 unsafe fn dealloc(&mut self, ptr: NonNull<u8>, layout: Layout);
737 // == ALLOCATOR-SPECIFIC QUANTITIES AND LIMITS ==
740 /// Returns bounds on the guaranteed usable size of a successful
741 /// allocation created with the specified `layout`.
743 /// In particular, if one has a memory block allocated via a given
744 /// allocator `a` and layout `k` where `a.usable_size(k)` returns
745 /// `(l, u)`, then one can pass that block to `a.dealloc()` with a
746 /// layout in the size range [l, u].
748 /// (All implementors of `usable_size` must ensure that
749 /// `l <= k.size() <= u`)
751 /// Both the lower- and upper-bounds (`l` and `u` respectively)
752 /// are provided, because an allocator based on size classes could
753 /// misbehave if one attempts to deallocate a block without
754 /// providing a correct value for its size (i.e., one within the
757 /// Clients who wish to make use of excess capacity are encouraged
758 /// to use the `alloc_excess` and `realloc_excess` instead, as
759 /// this method is constrained to report conservative values that
760 /// serve as valid bounds for *all possible* allocation method
763 /// However, for clients that do not wish to track the capacity
764 /// returned by `alloc_excess` locally, this method is likely to
765 /// produce useful results.
767 fn usable_size(&self, layout: &Layout) -> (usize, usize) {
768 (layout.size(), layout.size())
771 // == METHODS FOR MEMORY REUSE ==
772 // realloc. alloc_excess, realloc_excess
774 /// Returns a pointer suitable for holding data described by
775 /// a new layout with `layout`’s alignment and a size given
776 /// by `new_size`. To
777 /// accomplish this, this may extend or shrink the allocation
778 /// referenced by `ptr` to fit the new layout.
780 /// If this returns `Ok`, then ownership of the memory block
781 /// referenced by `ptr` has been transferred to this
782 /// allocator. The memory may or may not have been freed, and
783 /// should be considered unusable (unless of course it was
784 /// transferred back to the caller again via the return value of
787 /// If this method returns `Err`, then ownership of the memory
788 /// block has not been transferred to this allocator, and the
789 /// contents of the memory block are unaltered.
793 /// This function is unsafe because undefined behavior can result
794 /// if the caller does not ensure all of the following:
796 /// * `ptr` must be currently allocated via this allocator,
798 /// * `layout` must *fit* the `ptr` (see above). (The `new_size`
799 /// argument need not fit it.)
801 /// * `new_size` must be greater than zero.
803 /// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
804 /// must not overflow (i.e., the rounded value must be less than `usize::MAX`).
806 /// (Extension subtraits might provide more specific bounds on
807 /// behavior, e.g., guarantee a sentinel address or a null pointer
808 /// in response to a zero-size allocation request.)
812 /// Returns `Err` only if the new layout
813 /// does not meet the allocator's size
814 /// and alignment constraints of the allocator, or if reallocation
817 /// Implementations are encouraged to return `Err` on memory
818 /// exhaustion rather than panicking or aborting, but this is not
819 /// a strict requirement. (Specifically: it is *legal* to
820 /// implement this trait atop an underlying native allocation
821 /// library that aborts on memory exhaustion.)
823 /// Clients wishing to abort computation in response to a
824 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
825 /// rather than directly invoking `panic!` or similar.
827 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
828 unsafe fn realloc(&mut self,
831 new_size: usize) -> Result<NonNull<u8>, AllocErr> {
832 let old_size = layout.size();
834 if new_size >= old_size {
835 if let Ok(()) = self.grow_in_place(ptr, layout, new_size) {
838 } else if new_size < old_size {
839 if let Ok(()) = self.shrink_in_place(ptr, layout, new_size) {
844 // otherwise, fall back on alloc + copy + dealloc.
845 let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
846 let result = self.alloc(new_layout);
847 if let Ok(new_ptr) = result {
848 ptr::copy_nonoverlapping(ptr.as_ptr(),
850 cmp::min(old_size, new_size));
851 self.dealloc(ptr, layout);
856 /// Behaves like `alloc`, but also ensures that the contents
857 /// are set to zero before being returned.
861 /// This function is unsafe for the same reasons that `alloc` is.
865 /// Returning `Err` indicates that either memory is exhausted or
866 /// `layout` does not meet allocator's size or alignment
867 /// constraints, just as in `alloc`.
869 /// Clients wishing to abort computation in response to an
870 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
871 /// rather than directly invoking `panic!` or similar.
873 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
874 unsafe fn alloc_zeroed(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr> {
875 let size = layout.size();
876 let p = self.alloc(layout);
878 ptr::write_bytes(p.as_ptr(), 0, size);
883 /// Behaves like `alloc`, but also returns the whole size of
884 /// the returned block. For some `layout` inputs, like arrays, this
885 /// may include extra storage usable for additional data.
889 /// This function is unsafe for the same reasons that `alloc` is.
893 /// Returning `Err` indicates that either memory is exhausted or
894 /// `layout` does not meet allocator's size or alignment
895 /// constraints, just as in `alloc`.
897 /// Clients wishing to abort computation in response to an
898 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
899 /// rather than directly invoking `panic!` or similar.
901 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
902 unsafe fn alloc_excess(&mut self, layout: Layout) -> Result<Excess, AllocErr> {
903 let usable_size = self.usable_size(&layout);
904 self.alloc(layout).map(|p| Excess(p, usable_size.1))
907 /// Behaves like `realloc`, but also returns the whole size of
908 /// the returned block. For some `layout` inputs, like arrays, this
909 /// may include extra storage usable for additional data.
913 /// This function is unsafe for the same reasons that `realloc` is.
917 /// Returning `Err` indicates that either memory is exhausted or
918 /// `layout` does not meet allocator's size or alignment
919 /// constraints, just as in `realloc`.
921 /// Clients wishing to abort computation in response to a
922 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
923 /// rather than directly invoking `panic!` or similar.
925 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
926 unsafe fn realloc_excess(&mut self,
929 new_size: usize) -> Result<Excess, AllocErr> {
930 let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
931 let usable_size = self.usable_size(&new_layout);
932 self.realloc(ptr, layout, new_size)
933 .map(|p| Excess(p, usable_size.1))
936 /// Attempts to extend the allocation referenced by `ptr` to fit `new_size`.
938 /// If this returns `Ok`, then the allocator has asserted that the
939 /// memory block referenced by `ptr` now fits `new_size`, and thus can
940 /// be used to carry data of a layout of that size and same alignment as
941 /// `layout`. (The allocator is allowed to
942 /// expend effort to accomplish this, such as extending the memory block to
943 /// include successor blocks, or virtual memory tricks.)
945 /// Regardless of what this method returns, ownership of the
946 /// memory block referenced by `ptr` has not been transferred, and
947 /// the contents of the memory block are unaltered.
951 /// This function is unsafe because undefined behavior can result
952 /// if the caller does not ensure all of the following:
954 /// * `ptr` must be currently allocated via this allocator,
956 /// * `layout` must *fit* the `ptr` (see above); note the
957 /// `new_size` argument need not fit it,
959 /// * `new_size` must not be less than `layout.size()`,
963 /// Returns `Err(CannotReallocInPlace)` when the allocator is
964 /// unable to assert that the memory block referenced by `ptr`
965 /// could fit `layout`.
967 /// Note that one cannot pass `CannotReallocInPlace` to the `handle_alloc_error`
968 /// function; clients are expected either to be able to recover from
969 /// `grow_in_place` failures without aborting, or to fall back on
970 /// another reallocation method before resorting to an abort.
971 unsafe fn grow_in_place(&mut self,
974 new_size: usize) -> Result<(), CannotReallocInPlace> {
975 let _ = ptr; // this default implementation doesn't care about the actual address.
976 debug_assert!(new_size >= layout.size());
977 let (_l, u) = self.usable_size(&layout);
978 // _l <= layout.size() [guaranteed by usable_size()]
979 // layout.size() <= new_layout.size() [required by this method]
983 Err(CannotReallocInPlace)
987 /// Attempts to shrink the allocation referenced by `ptr` to fit `new_size`.
989 /// If this returns `Ok`, then the allocator has asserted that the
990 /// memory block referenced by `ptr` now fits `new_size`, and
991 /// thus can only be used to carry data of that smaller
992 /// layout. (The allocator is allowed to take advantage of this,
993 /// carving off portions of the block for reuse elsewhere.) The
994 /// truncated contents of the block within the smaller layout are
995 /// unaltered, and ownership of block has not been transferred.
997 /// If this returns `Err`, then the memory block is considered to
998 /// still represent the original (larger) `layout`. None of the
999 /// block has been carved off for reuse elsewhere, ownership of
1000 /// the memory block has not been transferred, and the contents of
1001 /// the memory block are unaltered.
1005 /// This function is unsafe because undefined behavior can result
1006 /// if the caller does not ensure all of the following:
1008 /// * `ptr` must be currently allocated via this allocator,
1010 /// * `layout` must *fit* the `ptr` (see above); note the
1011 /// `new_size` argument need not fit it,
1013 /// * `new_size` must not be greater than `layout.size()`
1014 /// (and must be greater than zero),
1018 /// Returns `Err(CannotReallocInPlace)` when the allocator is
1019 /// unable to assert that the memory block referenced by `ptr`
1020 /// could fit `layout`.
1022 /// Note that one cannot pass `CannotReallocInPlace` to the `handle_alloc_error`
1023 /// function; clients are expected either to be able to recover from
1024 /// `shrink_in_place` failures without aborting, or to fall back
1025 /// on another reallocation method before resorting to an abort.
1026 unsafe fn shrink_in_place(&mut self,
1029 new_size: usize) -> Result<(), CannotReallocInPlace> {
1030 let _ = ptr; // this default implementation doesn't care about the actual address.
1031 debug_assert!(new_size <= layout.size());
1032 let (l, _u) = self.usable_size(&layout);
1033 // layout.size() <= _u [guaranteed by usable_size()]
1034 // new_layout.size() <= layout.size() [required by this method]
1038 Err(CannotReallocInPlace)
1043 // == COMMON USAGE PATTERNS ==
1044 // alloc_one, dealloc_one, alloc_array, realloc_array. dealloc_array
1046 /// Allocates a block suitable for holding an instance of `T`.
1048 /// Captures a common usage pattern for allocators.
1050 /// The returned block is suitable for passing to the
1051 /// `realloc`/`dealloc` methods of this allocator.
1053 /// Note to implementors: If this returns `Ok(ptr)`, then `ptr`
1054 /// must be considered "currently allocated" and must be
1055 /// acceptable input to methods such as `realloc` or `dealloc`,
1056 /// *even if* `T` is a zero-sized type. In other words, if your
1057 /// `Alloc` implementation overrides this method in a manner
1058 /// that can return a zero-sized `ptr`, then all reallocation and
1059 /// deallocation methods need to be similarly overridden to accept
1060 /// such values as input.
1064 /// Returning `Err` indicates that either memory is exhausted or
1065 /// `T` does not meet allocator's size or alignment constraints.
1067 /// For zero-sized `T`, may return either of `Ok` or `Err`, but
1068 /// will *not* yield undefined behavior.
1070 /// Clients wishing to abort computation in response to an
1071 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
1072 /// rather than directly invoking `panic!` or similar.
1074 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1075 fn alloc_one<T>(&mut self) -> Result<NonNull<T>, AllocErr>
1078 let k = Layout::new::<T>();
1080 unsafe { self.alloc(k).map(|p| p.cast()) }
1086 /// Deallocates a block suitable for holding an instance of `T`.
1088 /// The given block must have been produced by this allocator,
1089 /// and must be suitable for storing a `T` (in terms of alignment
1090 /// as well as minimum and maximum size); otherwise yields
1091 /// undefined behavior.
1093 /// Captures a common usage pattern for allocators.
1097 /// This function is unsafe because undefined behavior can result
1098 /// if the caller does not ensure both:
1100 /// * `ptr` must denote a block of memory currently allocated via this allocator
1102 /// * the layout of `T` must *fit* that block of memory.
1103 unsafe fn dealloc_one<T>(&mut self, ptr: NonNull<T>)
1106 let k = Layout::new::<T>();
1108 self.dealloc(ptr.cast(), k);
1112 /// Allocates a block suitable for holding `n` instances of `T`.
1114 /// Captures a common usage pattern for allocators.
1116 /// The returned block is suitable for passing to the
1117 /// `realloc`/`dealloc` methods of this allocator.
1119 /// Note to implementors: If this returns `Ok(ptr)`, then `ptr`
1120 /// must be considered "currently allocated" and must be
1121 /// acceptable input to methods such as `realloc` or `dealloc`,
1122 /// *even if* `T` is a zero-sized type. In other words, if your
1123 /// `Alloc` implementation overrides this method in a manner
1124 /// that can return a zero-sized `ptr`, then all reallocation and
1125 /// deallocation methods need to be similarly overridden to accept
1126 /// such values as input.
1130 /// Returning `Err` indicates that either memory is exhausted or
1131 /// `[T; n]` does not meet allocator's size or alignment
1134 /// For zero-sized `T` or `n == 0`, may return either of `Ok` or
1135 /// `Err`, but will *not* yield undefined behavior.
1137 /// Always returns `Err` on arithmetic overflow.
1139 /// Clients wishing to abort computation in response to an
1140 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
1141 /// rather than directly invoking `panic!` or similar.
1143 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1144 fn alloc_array<T>(&mut self, n: usize) -> Result<NonNull<T>, AllocErr>
1147 match Layout::array::<T>(n) {
1148 Ok(layout) if layout.size() > 0 => {
1150 self.alloc(layout).map(|p| p.cast())
1157 /// Reallocates a block previously suitable for holding `n_old`
1158 /// instances of `T`, returning a block suitable for holding
1159 /// `n_new` instances of `T`.
1161 /// Captures a common usage pattern for allocators.
1163 /// The returned block is suitable for passing to the
1164 /// `realloc`/`dealloc` methods of this allocator.
1168 /// This function is unsafe because undefined behavior can result
1169 /// if the caller does not ensure all of the following:
1171 /// * `ptr` must be currently allocated via this allocator,
1173 /// * the layout of `[T; n_old]` must *fit* that block of memory.
1177 /// Returning `Err` indicates that either memory is exhausted or
1178 /// `[T; n_new]` does not meet allocator's size or alignment
1181 /// For zero-sized `T` or `n_new == 0`, may return either of `Ok` or
1182 /// `Err`, but will *not* yield undefined behavior.
1184 /// Always returns `Err` on arithmetic overflow.
1186 /// Clients wishing to abort computation in response to a
1187 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
1188 /// rather than directly invoking `panic!` or similar.
1190 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1191 unsafe fn realloc_array<T>(&mut self,
1194 n_new: usize) -> Result<NonNull<T>, AllocErr>
1197 match (Layout::array::<T>(n_old), Layout::array::<T>(n_new)) {
1198 (Ok(k_old), Ok(k_new)) if k_old.size() > 0 && k_new.size() > 0 => {
1199 debug_assert!(k_old.align() == k_new.align());
1200 self.realloc(ptr.cast(), k_old, k_new.size()).map(NonNull::cast)
1208 /// Deallocates a block suitable for holding `n` instances of `T`.
1210 /// Captures a common usage pattern for allocators.
1214 /// This function is unsafe because undefined behavior can result
1215 /// if the caller does not ensure both:
1217 /// * `ptr` must denote a block of memory currently allocated via this allocator
1219 /// * the layout of `[T; n]` must *fit* that block of memory.
1223 /// Returning `Err` indicates that either `[T; n]` or the given
1224 /// memory block does not meet allocator's size or alignment
1227 /// Always returns `Err` on arithmetic overflow.
1228 unsafe fn dealloc_array<T>(&mut self, ptr: NonNull<T>, n: usize) -> Result<(), AllocErr>
1231 match Layout::array::<T>(n) {
1232 Ok(k) if k.size() > 0 => {
1233 Ok(self.dealloc(ptr.cast(), k))