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 /// Returns `Err` if the padded size would overflow.
218 /// This is equivalent to adding the result of `padding_needed_for`
219 /// to the layout's current size.
220 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
222 pub fn pad_to_align(&self) -> Result<Layout, LayoutErr> {
223 let pad = self.padding_needed_for(self.align());
224 let new_size = self.size().checked_add(pad)
225 .ok_or(LayoutErr { private: () })?;
227 Layout::from_size_align(new_size, self.align())
230 /// Creates a layout describing the record for `n` instances of
231 /// `self`, with a suitable amount of padding between each to
232 /// ensure that each instance is given its requested size and
233 /// alignment. On success, returns `(k, offs)` where `k` is the
234 /// layout of the array and `offs` is the distance between the start
235 /// of each element in the array.
237 /// On arithmetic overflow, returns `LayoutErr`.
238 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
240 pub fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutErr> {
241 let padded_size = self.size().checked_add(self.padding_needed_for(self.align()))
242 .ok_or(LayoutErr { private: () })?;
243 let alloc_size = padded_size.checked_mul(n)
244 .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 result 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)
274 .ok_or(LayoutErr { private: () })?;
275 let new_size = offset.checked_add(next.size())
276 .ok_or(LayoutErr { private: () })?;
278 let layout = Layout::from_size_align(new_size, new_align)?;
282 /// Creates a layout describing the record for `n` instances of
283 /// `self`, with no padding between each instance.
285 /// Note that, unlike `repeat`, `repeat_packed` does not guarantee
286 /// that the repeated instances of `self` will be properly
287 /// aligned, even if a given instance of `self` is properly
288 /// aligned. In other words, if the layout returned by
289 /// `repeat_packed` is used to allocate an array, it is not
290 /// guaranteed that all elements in the array will be properly
293 /// On arithmetic overflow, returns `LayoutErr`.
294 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
296 pub fn repeat_packed(&self, n: usize) -> Result<Self, LayoutErr> {
297 let size = self.size().checked_mul(n).ok_or(LayoutErr { private: () })?;
298 Layout::from_size_align(size, self.align())
301 /// Creates a layout describing the record for `self` followed by
302 /// `next` with no additional padding between the two. Since no
303 /// padding is inserted, the alignment of `next` is irrelevant,
304 /// and is not incorporated *at all* into the resulting layout.
306 /// On arithmetic overflow, returns `LayoutErr`.
307 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
309 pub fn extend_packed(&self, next: Self) -> Result<Self, LayoutErr> {
310 let new_size = self.size().checked_add(next.size())
311 .ok_or(LayoutErr { private: () })?;
312 let layout = Layout::from_size_align(new_size, self.align())?;
316 /// Creates a layout describing the record for a `[T; n]`.
318 /// On arithmetic overflow, returns `LayoutErr`.
319 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
321 pub fn array<T>(n: usize) -> Result<Self, LayoutErr> {
325 debug_assert!(offs == mem::size_of::<T>());
331 /// The parameters given to `Layout::from_size_align`
332 /// or some other `Layout` constructor
333 /// do not satisfy its documented constraints.
334 #[stable(feature = "alloc_layout", since = "1.28.0")]
335 #[derive(Clone, PartialEq, Eq, Debug)]
336 pub struct LayoutErr {
340 // (we need this for downstream impl of trait Error)
341 #[stable(feature = "alloc_layout", since = "1.28.0")]
342 impl fmt::Display for LayoutErr {
343 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
344 f.write_str("invalid parameters to Layout::from_size_align")
348 /// The `AllocErr` error indicates an allocation failure
349 /// that may be due to resource exhaustion or to
350 /// something wrong when combining the given input arguments with this
352 #[unstable(feature = "allocator_api", issue = "32838")]
353 #[derive(Clone, PartialEq, Eq, Debug)]
356 // (we need this for downstream impl of trait Error)
357 #[unstable(feature = "allocator_api", issue = "32838")]
358 impl fmt::Display for AllocErr {
359 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
360 f.write_str("memory allocation failed")
364 /// The `CannotReallocInPlace` error is used when [`grow_in_place`] or
365 /// [`shrink_in_place`] were unable to reuse the given memory block for
366 /// a requested layout.
368 /// [`grow_in_place`]: ./trait.Alloc.html#method.grow_in_place
369 /// [`shrink_in_place`]: ./trait.Alloc.html#method.shrink_in_place
370 #[unstable(feature = "allocator_api", issue = "32838")]
371 #[derive(Clone, PartialEq, Eq, Debug)]
372 pub struct CannotReallocInPlace;
374 #[unstable(feature = "allocator_api", issue = "32838")]
375 impl CannotReallocInPlace {
376 pub fn description(&self) -> &str {
377 "cannot reallocate allocator's memory in place"
381 // (we need this for downstream impl of trait Error)
382 #[unstable(feature = "allocator_api", issue = "32838")]
383 impl fmt::Display for CannotReallocInPlace {
384 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
385 write!(f, "{}", self.description())
389 /// A memory allocator that can be registered as the standard library’s default
390 /// though the `#[global_allocator]` attributes.
392 /// Some of the methods require that a memory block be *currently
393 /// allocated* via an allocator. This means that:
395 /// * the starting address for that memory block was previously
396 /// returned by a previous call to an allocation method
397 /// such as `alloc`, and
399 /// * the memory block has not been subsequently deallocated, where
400 /// blocks are deallocated either by being passed to a deallocation
401 /// method such as `dealloc` or by being
402 /// passed to a reallocation method that returns a non-null pointer.
408 /// use std::alloc::{GlobalAlloc, Layout, alloc};
409 /// use std::ptr::null_mut;
411 /// struct MyAllocator;
413 /// unsafe impl GlobalAlloc for MyAllocator {
414 /// unsafe fn alloc(&self, _layout: Layout) -> *mut u8 { null_mut() }
415 /// unsafe fn dealloc(&self, _ptr: *mut u8, _layout: Layout) {}
418 /// #[global_allocator]
419 /// static A: MyAllocator = MyAllocator;
423 /// assert!(alloc(Layout::new::<u32>()).is_null())
430 /// The `GlobalAlloc` trait is an `unsafe` trait for a number of reasons, and
431 /// implementors must ensure that they adhere to these contracts:
433 /// * It's undefined behavior if global allocators unwind. This restriction may
434 /// be lifted in the future, but currently a panic from any of these
435 /// functions may lead to memory unsafety.
437 /// * `Layout` queries and calculations in general must be correct. Callers of
438 /// this trait are allowed to rely on the contracts defined on each method,
439 /// and implementors must ensure such contracts remain true.
440 #[stable(feature = "global_alloc", since = "1.28.0")]
441 pub unsafe trait GlobalAlloc {
442 /// Allocate memory as described by the given `layout`.
444 /// Returns a pointer to newly-allocated memory,
445 /// or null to indicate allocation failure.
449 /// This function is unsafe because undefined behavior can result
450 /// if the caller does not ensure that `layout` has non-zero size.
452 /// (Extension subtraits might provide more specific bounds on
453 /// behavior, e.g., guarantee a sentinel address or a null pointer
454 /// in response to a zero-size allocation request.)
456 /// The allocated block of memory may or may not be initialized.
460 /// Returning a null pointer indicates that either memory is exhausted
461 /// or `layout` does not meet allocator's size or alignment constraints.
463 /// Implementations are encouraged to return null on memory
464 /// exhaustion rather than aborting, but this is not
465 /// a strict requirement. (Specifically: it is *legal* to
466 /// implement this trait atop an underlying native allocation
467 /// library that aborts on memory exhaustion.)
469 /// Clients wishing to abort computation in response to an
470 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
471 /// rather than directly invoking `panic!` or similar.
473 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
474 #[stable(feature = "global_alloc", since = "1.28.0")]
475 unsafe fn alloc(&self, layout: Layout) -> *mut u8;
477 /// Deallocate the block of memory at the given `ptr` pointer with the given `layout`.
481 /// This function is unsafe because undefined behavior can result
482 /// if the caller does not ensure all of the following:
484 /// * `ptr` must denote a block of memory currently allocated via
487 /// * `layout` must be the same layout that was used
488 /// to allocate that block of memory,
489 #[stable(feature = "global_alloc", since = "1.28.0")]
490 unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout);
492 /// Behaves like `alloc`, but also ensures that the contents
493 /// are set to zero before being returned.
497 /// This function is unsafe for the same reasons that `alloc` is.
498 /// However the allocated block of memory is guaranteed to be initialized.
502 /// Returning a null pointer indicates that either memory is exhausted
503 /// or `layout` does not meet allocator's size or alignment constraints,
504 /// just as in `alloc`.
506 /// Clients wishing to abort computation in response to an
507 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
508 /// rather than directly invoking `panic!` or similar.
510 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
511 #[stable(feature = "global_alloc", since = "1.28.0")]
512 unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
513 let size = layout.size();
514 let ptr = self.alloc(layout);
516 ptr::write_bytes(ptr, 0, size);
521 /// Shrink or grow a block of memory to the given `new_size`.
522 /// The block is described by the given `ptr` pointer and `layout`.
524 /// If this returns a non-null pointer, then ownership of the memory block
525 /// referenced by `ptr` has been transferred to this allocator.
526 /// The memory may or may not have been deallocated,
527 /// and should be considered unusable (unless of course it was
528 /// transferred back to the caller again via the return value of
531 /// If this method returns null, then ownership of the memory
532 /// block has not been transferred to this allocator, and the
533 /// contents of the memory block are unaltered.
537 /// This function is unsafe because undefined behavior can result
538 /// if the caller does not ensure all of the following:
540 /// * `ptr` must be currently allocated via this allocator,
542 /// * `layout` must be the same layout that was used
543 /// to allocate that block of memory,
545 /// * `new_size` must be greater than zero.
547 /// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
548 /// must not overflow (i.e., the rounded value must be less than `usize::MAX`).
550 /// (Extension subtraits might provide more specific bounds on
551 /// behavior, e.g., guarantee a sentinel address or a null pointer
552 /// in response to a zero-size allocation request.)
556 /// Returns null if the new layout does not meet the size
557 /// and alignment constraints of the allocator, or if reallocation
560 /// Implementations are encouraged to return null on memory
561 /// exhaustion rather than panicking or aborting, but this is not
562 /// a strict requirement. (Specifically: it is *legal* to
563 /// implement this trait atop an underlying native allocation
564 /// library that aborts on memory exhaustion.)
566 /// Clients wishing to abort computation in response to a
567 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
568 /// rather than directly invoking `panic!` or similar.
570 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
571 #[stable(feature = "global_alloc", since = "1.28.0")]
572 unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
573 let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
574 let new_ptr = self.alloc(new_layout);
575 if !new_ptr.is_null() {
576 ptr::copy_nonoverlapping(
579 cmp::min(layout.size(), new_size),
581 self.dealloc(ptr, layout);
587 /// An implementation of `Alloc` can allocate, reallocate, and
588 /// deallocate arbitrary blocks of data described via `Layout`.
590 /// Some of the methods require that a memory block be *currently
591 /// allocated* via an allocator. This means that:
593 /// * the starting address for that memory block was previously
594 /// returned by a previous call to an allocation method (`alloc`,
595 /// `alloc_zeroed`, `alloc_excess`, `alloc_one`, `alloc_array`) or
596 /// reallocation method (`realloc`, `realloc_excess`, or
597 /// `realloc_array`), and
599 /// * the memory block has not been subsequently deallocated, where
600 /// blocks are deallocated either by being passed to a deallocation
601 /// method (`dealloc`, `dealloc_one`, `dealloc_array`) or by being
602 /// passed to a reallocation method (see above) that returns `Ok`.
604 /// A note regarding zero-sized types and zero-sized layouts: many
605 /// methods in the `Alloc` trait state that allocation requests
606 /// must be non-zero size, or else undefined behavior can result.
608 /// * However, some higher-level allocation methods (`alloc_one`,
609 /// `alloc_array`) are well-defined on zero-sized types and can
610 /// optionally support them: it is left up to the implementor
611 /// whether to return `Err`, or to return `Ok` with some pointer.
613 /// * If an `Alloc` implementation chooses to return `Ok` in this
614 /// case (i.e., the pointer denotes a zero-sized inaccessible block)
615 /// then that returned pointer must be considered "currently
616 /// allocated". On such an allocator, *all* methods that take
617 /// currently-allocated pointers as inputs must accept these
618 /// zero-sized pointers, *without* causing undefined behavior.
620 /// * In other words, if a zero-sized pointer can flow out of an
621 /// allocator, then that allocator must likewise accept that pointer
622 /// flowing back into its deallocation and reallocation methods.
624 /// Some of the methods require that a layout *fit* a memory block.
625 /// What it means for a layout to "fit" a memory block means (or
626 /// equivalently, for a memory block to "fit" a layout) is that the
627 /// following two conditions must hold:
629 /// 1. The block's starting address must be aligned to `layout.align()`.
631 /// 2. The block's size must fall in the range `[use_min, use_max]`, where:
633 /// * `use_min` is `self.usable_size(layout).0`, and
635 /// * `use_max` is the capacity that was (or would have been)
636 /// returned when (if) the block was allocated via a call to
637 /// `alloc_excess` or `realloc_excess`.
641 /// * the size of the layout most recently used to allocate the block
642 /// is guaranteed to be in the range `[use_min, use_max]`, and
644 /// * a lower-bound on `use_max` can be safely approximated by a call to
647 /// * if a layout `k` fits a memory block (denoted by `ptr`)
648 /// currently allocated via an allocator `a`, then it is legal to
649 /// use that layout to deallocate it, i.e., `a.dealloc(ptr, k);`.
653 /// The `Alloc` trait is an `unsafe` trait for a number of reasons, and
654 /// implementors must ensure that they adhere to these contracts:
656 /// * Pointers returned from allocation functions must point to valid memory and
657 /// retain their validity until at least the instance of `Alloc` is dropped
660 /// * `Layout` queries and calculations in general must be correct. Callers of
661 /// this trait are allowed to rely on the contracts defined on each method,
662 /// and implementors must ensure such contracts remain true.
664 /// Note that this list may get tweaked over time as clarifications are made in
666 #[unstable(feature = "allocator_api", issue = "32838")]
667 pub unsafe trait Alloc {
669 // (Note: some existing allocators have unspecified but well-defined
670 // behavior in response to a zero size allocation request ;
671 // e.g., in C, `malloc` of 0 will either return a null pointer or a
672 // unique pointer, but will not have arbitrary undefined
674 // However in jemalloc for example,
675 // `mallocx(0)` is documented as undefined behavior.)
677 /// Returns a pointer meeting the size and alignment guarantees of
680 /// If this method returns an `Ok(addr)`, then the `addr` returned
681 /// will be non-null address pointing to a block of storage
682 /// suitable for holding an instance of `layout`.
684 /// The returned block of storage may or may not have its contents
685 /// initialized. (Extension subtraits might restrict this
686 /// behavior, e.g., to ensure initialization to particular sets of
691 /// This function is unsafe because undefined behavior can result
692 /// if the caller does not ensure that `layout` has non-zero size.
694 /// (Extension subtraits might provide more specific bounds on
695 /// behavior, e.g., guarantee a sentinel address or a null pointer
696 /// in response to a zero-size allocation request.)
700 /// Returning `Err` indicates that either memory is exhausted or
701 /// `layout` does not meet allocator's size or alignment
704 /// Implementations are encouraged to return `Err` on memory
705 /// exhaustion rather than panicking or aborting, but this is not
706 /// a strict requirement. (Specifically: it is *legal* to
707 /// implement this trait atop an underlying native allocation
708 /// library that aborts on memory exhaustion.)
710 /// Clients wishing to abort computation in response to an
711 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
712 /// rather than directly invoking `panic!` or similar.
714 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
715 unsafe fn alloc(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr>;
717 /// Deallocate the memory referenced by `ptr`.
721 /// This function is unsafe because undefined behavior can result
722 /// if the caller does not ensure all of the following:
724 /// * `ptr` must denote a block of memory currently allocated via
727 /// * `layout` must *fit* that block of memory,
729 /// * In addition to fitting the block of memory `layout`, the
730 /// alignment of the `layout` must match the alignment used
731 /// to allocate that block of memory.
732 unsafe fn dealloc(&mut self, ptr: NonNull<u8>, layout: Layout);
734 // == ALLOCATOR-SPECIFIC QUANTITIES AND LIMITS ==
737 /// Returns bounds on the guaranteed usable size of a successful
738 /// allocation created with the specified `layout`.
740 /// In particular, if one has a memory block allocated via a given
741 /// allocator `a` and layout `k` where `a.usable_size(k)` returns
742 /// `(l, u)`, then one can pass that block to `a.dealloc()` with a
743 /// layout in the size range [l, u].
745 /// (All implementors of `usable_size` must ensure that
746 /// `l <= k.size() <= u`)
748 /// Both the lower- and upper-bounds (`l` and `u` respectively)
749 /// are provided, because an allocator based on size classes could
750 /// misbehave if one attempts to deallocate a block without
751 /// providing a correct value for its size (i.e., one within the
754 /// Clients who wish to make use of excess capacity are encouraged
755 /// to use the `alloc_excess` and `realloc_excess` instead, as
756 /// this method is constrained to report conservative values that
757 /// serve as valid bounds for *all possible* allocation method
760 /// However, for clients that do not wish to track the capacity
761 /// returned by `alloc_excess` locally, this method is likely to
762 /// produce useful results.
764 fn usable_size(&self, layout: &Layout) -> (usize, usize) {
765 (layout.size(), layout.size())
768 // == METHODS FOR MEMORY REUSE ==
769 // realloc. alloc_excess, realloc_excess
771 /// Returns a pointer suitable for holding data described by
772 /// a new layout with `layout`’s alignment and a size given
773 /// by `new_size`. To
774 /// accomplish this, this may extend or shrink the allocation
775 /// referenced by `ptr` to fit the new layout.
777 /// If this returns `Ok`, then ownership of the memory block
778 /// referenced by `ptr` has been transferred to this
779 /// allocator. The memory may or may not have been freed, and
780 /// should be considered unusable (unless of course it was
781 /// transferred back to the caller again via the return value of
784 /// If this method returns `Err`, then ownership of the memory
785 /// block has not been transferred to this allocator, and the
786 /// contents of the memory block are unaltered.
790 /// This function is unsafe because undefined behavior can result
791 /// if the caller does not ensure all of the following:
793 /// * `ptr` must be currently allocated via this allocator,
795 /// * `layout` must *fit* the `ptr` (see above). (The `new_size`
796 /// argument need not fit it.)
798 /// * `new_size` must be greater than zero.
800 /// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
801 /// must not overflow (i.e., the rounded value must be less than `usize::MAX`).
803 /// (Extension subtraits might provide more specific bounds on
804 /// behavior, e.g., guarantee a sentinel address or a null pointer
805 /// in response to a zero-size allocation request.)
809 /// Returns `Err` only if the new layout
810 /// does not meet the allocator's size
811 /// and alignment constraints of the allocator, or if reallocation
814 /// Implementations are encouraged to return `Err` on memory
815 /// exhaustion rather than panicking or aborting, but this is not
816 /// a strict requirement. (Specifically: it is *legal* to
817 /// implement this trait atop an underlying native allocation
818 /// library that aborts on memory exhaustion.)
820 /// Clients wishing to abort computation in response to a
821 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
822 /// rather than directly invoking `panic!` or similar.
824 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
825 unsafe fn realloc(&mut self,
828 new_size: usize) -> Result<NonNull<u8>, AllocErr> {
829 let old_size = layout.size();
831 if new_size >= old_size {
832 if let Ok(()) = self.grow_in_place(ptr, layout, new_size) {
835 } else if new_size < old_size {
836 if let Ok(()) = self.shrink_in_place(ptr, layout, new_size) {
841 // otherwise, fall back on alloc + copy + dealloc.
842 let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
843 let result = self.alloc(new_layout);
844 if let Ok(new_ptr) = result {
845 ptr::copy_nonoverlapping(ptr.as_ptr(),
847 cmp::min(old_size, new_size));
848 self.dealloc(ptr, layout);
853 /// Behaves like `alloc`, but also ensures that the contents
854 /// are set to zero before being returned.
858 /// This function is unsafe for the same reasons that `alloc` is.
862 /// Returning `Err` indicates that either memory is exhausted or
863 /// `layout` does not meet allocator's size or alignment
864 /// constraints, just as in `alloc`.
866 /// Clients wishing to abort computation in response to an
867 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
868 /// rather than directly invoking `panic!` or similar.
870 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
871 unsafe fn alloc_zeroed(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr> {
872 let size = layout.size();
873 let p = self.alloc(layout);
875 ptr::write_bytes(p.as_ptr(), 0, size);
880 /// Behaves like `alloc`, but also returns the whole size of
881 /// the returned block. For some `layout` inputs, like arrays, this
882 /// may include extra storage usable for additional data.
886 /// This function is unsafe for the same reasons that `alloc` is.
890 /// Returning `Err` indicates that either memory is exhausted or
891 /// `layout` does not meet allocator's size or alignment
892 /// constraints, just as in `alloc`.
894 /// Clients wishing to abort computation in response to an
895 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
896 /// rather than directly invoking `panic!` or similar.
898 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
899 unsafe fn alloc_excess(&mut self, layout: Layout) -> Result<Excess, AllocErr> {
900 let usable_size = self.usable_size(&layout);
901 self.alloc(layout).map(|p| Excess(p, usable_size.1))
904 /// Behaves like `realloc`, but also returns the whole size of
905 /// the returned block. For some `layout` inputs, like arrays, this
906 /// may include extra storage usable for additional data.
910 /// This function is unsafe for the same reasons that `realloc` is.
914 /// Returning `Err` indicates that either memory is exhausted or
915 /// `layout` does not meet allocator's size or alignment
916 /// constraints, just as in `realloc`.
918 /// Clients wishing to abort computation in response to a
919 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
920 /// rather than directly invoking `panic!` or similar.
922 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
923 unsafe fn realloc_excess(&mut self,
926 new_size: usize) -> Result<Excess, AllocErr> {
927 let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
928 let usable_size = self.usable_size(&new_layout);
929 self.realloc(ptr, layout, new_size)
930 .map(|p| Excess(p, usable_size.1))
933 /// Attempts to extend the allocation referenced by `ptr` to fit `new_size`.
935 /// If this returns `Ok`, then the allocator has asserted that the
936 /// memory block referenced by `ptr` now fits `new_size`, and thus can
937 /// be used to carry data of a layout of that size and same alignment as
938 /// `layout`. (The allocator is allowed to
939 /// expend effort to accomplish this, such as extending the memory block to
940 /// include successor blocks, or virtual memory tricks.)
942 /// Regardless of what this method returns, ownership of the
943 /// memory block referenced by `ptr` has not been transferred, and
944 /// the contents of the memory block are unaltered.
948 /// This function is unsafe because undefined behavior can result
949 /// if the caller does not ensure all of the following:
951 /// * `ptr` must be currently allocated via this allocator,
953 /// * `layout` must *fit* the `ptr` (see above); note the
954 /// `new_size` argument need not fit it,
956 /// * `new_size` must not be less than `layout.size()`,
960 /// Returns `Err(CannotReallocInPlace)` when the allocator is
961 /// unable to assert that the memory block referenced by `ptr`
962 /// could fit `layout`.
964 /// Note that one cannot pass `CannotReallocInPlace` to the `handle_alloc_error`
965 /// function; clients are expected either to be able to recover from
966 /// `grow_in_place` failures without aborting, or to fall back on
967 /// another reallocation method before resorting to an abort.
968 unsafe fn grow_in_place(&mut self,
971 new_size: usize) -> Result<(), CannotReallocInPlace> {
972 let _ = ptr; // this default implementation doesn't care about the actual address.
973 debug_assert!(new_size >= layout.size());
974 let (_l, u) = self.usable_size(&layout);
975 // _l <= layout.size() [guaranteed by usable_size()]
976 // layout.size() <= new_layout.size() [required by this method]
980 Err(CannotReallocInPlace)
984 /// Attempts to shrink the allocation referenced by `ptr` to fit `new_size`.
986 /// If this returns `Ok`, then the allocator has asserted that the
987 /// memory block referenced by `ptr` now fits `new_size`, and
988 /// thus can only be used to carry data of that smaller
989 /// layout. (The allocator is allowed to take advantage of this,
990 /// carving off portions of the block for reuse elsewhere.) The
991 /// truncated contents of the block within the smaller layout are
992 /// unaltered, and ownership of block has not been transferred.
994 /// If this returns `Err`, then the memory block is considered to
995 /// still represent the original (larger) `layout`. None of the
996 /// block has been carved off for reuse elsewhere, ownership of
997 /// the memory block has not been transferred, and the contents of
998 /// the memory block are unaltered.
1002 /// This function is unsafe because undefined behavior can result
1003 /// if the caller does not ensure all of the following:
1005 /// * `ptr` must be currently allocated via this allocator,
1007 /// * `layout` must *fit* the `ptr` (see above); note the
1008 /// `new_size` argument need not fit it,
1010 /// * `new_size` must not be greater than `layout.size()`
1011 /// (and must be greater than zero),
1015 /// Returns `Err(CannotReallocInPlace)` when the allocator is
1016 /// unable to assert that the memory block referenced by `ptr`
1017 /// could fit `layout`.
1019 /// Note that one cannot pass `CannotReallocInPlace` to the `handle_alloc_error`
1020 /// function; clients are expected either to be able to recover from
1021 /// `shrink_in_place` failures without aborting, or to fall back
1022 /// on another reallocation method before resorting to an abort.
1023 unsafe fn shrink_in_place(&mut self,
1026 new_size: usize) -> Result<(), CannotReallocInPlace> {
1027 let _ = ptr; // this default implementation doesn't care about the actual address.
1028 debug_assert!(new_size <= layout.size());
1029 let (l, _u) = self.usable_size(&layout);
1030 // layout.size() <= _u [guaranteed by usable_size()]
1031 // new_layout.size() <= layout.size() [required by this method]
1035 Err(CannotReallocInPlace)
1040 // == COMMON USAGE PATTERNS ==
1041 // alloc_one, dealloc_one, alloc_array, realloc_array. dealloc_array
1043 /// Allocates a block suitable for holding an instance of `T`.
1045 /// Captures a common usage pattern for allocators.
1047 /// The returned block is suitable for passing to the
1048 /// `alloc`/`realloc` methods of this allocator.
1050 /// Note to implementors: If this returns `Ok(ptr)`, then `ptr`
1051 /// must be considered "currently allocated" and must be
1052 /// acceptable input to methods such as `realloc` or `dealloc`,
1053 /// *even if* `T` is a zero-sized type. In other words, if your
1054 /// `Alloc` implementation overrides this method in a manner
1055 /// that can return a zero-sized `ptr`, then all reallocation and
1056 /// deallocation methods need to be similarly overridden to accept
1057 /// such values as input.
1061 /// Returning `Err` indicates that either memory is exhausted or
1062 /// `T` does not meet allocator's size or alignment constraints.
1064 /// For zero-sized `T`, may return either of `Ok` or `Err`, but
1065 /// will *not* yield undefined behavior.
1067 /// Clients wishing to abort computation in response to an
1068 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
1069 /// rather than directly invoking `panic!` or similar.
1071 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1072 fn alloc_one<T>(&mut self) -> Result<NonNull<T>, AllocErr>
1075 let k = Layout::new::<T>();
1077 unsafe { self.alloc(k).map(|p| p.cast()) }
1083 /// Deallocates a block suitable for holding an instance of `T`.
1085 /// The given block must have been produced by this allocator,
1086 /// and must be suitable for storing a `T` (in terms of alignment
1087 /// as well as minimum and maximum size); otherwise yields
1088 /// undefined behavior.
1090 /// Captures a common usage pattern for allocators.
1094 /// This function is unsafe because undefined behavior can result
1095 /// if the caller does not ensure both:
1097 /// * `ptr` must denote a block of memory currently allocated via this allocator
1099 /// * the layout of `T` must *fit* that block of memory.
1100 unsafe fn dealloc_one<T>(&mut self, ptr: NonNull<T>)
1103 let k = Layout::new::<T>();
1105 self.dealloc(ptr.cast(), k);
1109 /// Allocates a block suitable for holding `n` instances of `T`.
1111 /// Captures a common usage pattern for allocators.
1113 /// The returned block is suitable for passing to the
1114 /// `alloc`/`realloc` methods of this allocator.
1116 /// Note to implementors: If this returns `Ok(ptr)`, then `ptr`
1117 /// must be considered "currently allocated" and must be
1118 /// acceptable input to methods such as `realloc` or `dealloc`,
1119 /// *even if* `T` is a zero-sized type. In other words, if your
1120 /// `Alloc` implementation overrides this method in a manner
1121 /// that can return a zero-sized `ptr`, then all reallocation and
1122 /// deallocation methods need to be similarly overridden to accept
1123 /// such values as input.
1127 /// Returning `Err` indicates that either memory is exhausted or
1128 /// `[T; n]` does not meet allocator's size or alignment
1131 /// For zero-sized `T` or `n == 0`, may return either of `Ok` or
1132 /// `Err`, but will *not* yield undefined behavior.
1134 /// Always returns `Err` on arithmetic overflow.
1136 /// Clients wishing to abort computation in response to an
1137 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
1138 /// rather than directly invoking `panic!` or similar.
1140 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1141 fn alloc_array<T>(&mut self, n: usize) -> Result<NonNull<T>, AllocErr>
1144 match Layout::array::<T>(n) {
1145 Ok(ref layout) if layout.size() > 0 => {
1147 self.alloc(layout.clone()).map(|p| p.cast())
1154 /// Reallocates a block previously suitable for holding `n_old`
1155 /// instances of `T`, returning a block suitable for holding
1156 /// `n_new` instances of `T`.
1158 /// Captures a common usage pattern for allocators.
1160 /// The returned block is suitable for passing to the
1161 /// `alloc`/`realloc` methods of this allocator.
1165 /// This function is unsafe because undefined behavior can result
1166 /// if the caller does not ensure all of the following:
1168 /// * `ptr` must be currently allocated via this allocator,
1170 /// * the layout of `[T; n_old]` must *fit* that block of memory.
1174 /// Returning `Err` indicates that either memory is exhausted or
1175 /// `[T; n_new]` does not meet allocator's size or alignment
1178 /// For zero-sized `T` or `n_new == 0`, may return either of `Ok` or
1179 /// `Err`, but will *not* yield undefined behavior.
1181 /// Always returns `Err` on arithmetic overflow.
1183 /// Clients wishing to abort computation in response to a
1184 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
1185 /// rather than directly invoking `panic!` or similar.
1187 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1188 unsafe fn realloc_array<T>(&mut self,
1191 n_new: usize) -> Result<NonNull<T>, AllocErr>
1194 match (Layout::array::<T>(n_old), Layout::array::<T>(n_new)) {
1195 (Ok(ref k_old), Ok(ref k_new)) if k_old.size() > 0 && k_new.size() > 0 => {
1196 debug_assert!(k_old.align() == k_new.align());
1197 self.realloc(ptr.cast(), k_old.clone(), k_new.size()).map(NonNull::cast)
1205 /// Deallocates a block suitable for holding `n` instances of `T`.
1207 /// Captures a common usage pattern for allocators.
1211 /// This function is unsafe because undefined behavior can result
1212 /// if the caller does not ensure both:
1214 /// * `ptr` must denote a block of memory currently allocated via this allocator
1216 /// * the layout of `[T; n]` must *fit* that block of memory.
1220 /// Returning `Err` indicates that either `[T; n]` or the given
1221 /// memory block does not meet allocator's size or alignment
1224 /// Always returns `Err` on arithmetic overflow.
1225 unsafe fn dealloc_array<T>(&mut self, ptr: NonNull<T>, n: usize) -> Result<(), AllocErr>
1228 match Layout::array::<T>(n) {
1229 Ok(ref k) if k.size() > 0 => {
1230 Ok(self.dealloc(ptr.cast(), k.clone()))