1 //! Memory allocation APIs
3 // ignore-tidy-undocumented-unsafe
5 #![stable(feature = "alloc_module", since = "1.28.0")]
10 use crate::num::NonZeroUsize;
11 use crate::ptr::{self, NonNull};
14 /// Represents the combination of a starting address and
15 /// a total capacity of the returned block.
16 #[unstable(feature = "allocator_api", issue = "32838")]
18 pub struct Excess(pub NonNull<u8>, pub usize);
20 const fn size_align<T>() -> (usize, usize) {
21 (mem::size_of::<T>(), mem::align_of::<T>())
24 /// Layout of a block of memory.
26 /// An instance of `Layout` describes a particular layout of memory.
27 /// You build a `Layout` up as an input to give to an allocator.
29 /// All layouts have an associated non-negative size and a
30 /// power-of-two alignment.
32 /// (Note however that layouts are *not* required to have positive
33 /// size, even though many allocators require that all memory
34 /// requests have positive size. A caller to the `AllocRef::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")]
67 #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
69 pub const fn from_size_align(size: usize, align: usize) -> Result<Self, LayoutErr> {
70 if !align.is_power_of_two() {
71 return Err(LayoutErr { private: () });
74 // (power-of-two implies align != 0.)
76 // Rounded up size is:
77 // size_rounded_up = (size + align - 1) & !(align - 1);
79 // We know from above that align != 0. If adding (align - 1)
80 // does not overflow, then rounding up will be fine.
82 // Conversely, &-masking with !(align - 1) will subtract off
83 // only low-order-bits. Thus if overflow occurs with the sum,
84 // the &-mask cannot subtract enough to undo that overflow.
86 // Above implies that checking for summation overflow is both
87 // necessary and sufficient.
88 if size > usize::MAX - (align - 1) {
89 return Err(LayoutErr { private: () });
92 unsafe { Ok(Layout::from_size_align_unchecked(size, align)) }
95 /// Creates a layout, bypassing all checks.
99 /// This function is unsafe as it does not verify the preconditions from
100 /// [`Layout::from_size_align`](#method.from_size_align).
101 #[stable(feature = "alloc_layout", since = "1.28.0")]
102 #[rustc_const_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")]
110 #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
112 pub const fn size(&self) -> usize {
116 /// The minimum byte alignment for a memory block of this layout.
117 #[stable(feature = "alloc_layout", since = "1.28.0")]
118 #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
120 pub const fn align(&self) -> usize {
124 /// Constructs a `Layout` suitable for holding a value of type `T`.
125 #[stable(feature = "alloc_layout", since = "1.28.0")]
126 #[rustc_const_stable(feature = "alloc_layout_const_new", since = "1.42.0")]
128 pub const fn new<T>() -> Self {
129 let (size, align) = size_align::<T>();
130 // Note that the align is guaranteed by rustc to be a power of two and
131 // the size+align combo is guaranteed to fit in our address space. As a
132 // result use the unchecked constructor here to avoid inserting code
133 // that panics if it isn't optimized well enough.
134 unsafe { Layout::from_size_align_unchecked(size, align) }
137 /// Produces layout describing a record that could be used to
138 /// allocate backing structure for `T` (which could be a trait
139 /// or other unsized type like a slice).
140 #[stable(feature = "alloc_layout", since = "1.28.0")]
142 pub fn for_value<T: ?Sized>(t: &T) -> Self {
143 let (size, align) = (mem::size_of_val(t), mem::align_of_val(t));
144 // See rationale in `new` for why this is using an unsafe variant below
145 debug_assert!(Layout::from_size_align(size, align).is_ok());
146 unsafe { Layout::from_size_align_unchecked(size, align) }
149 /// Creates a layout describing the record that can hold a value
150 /// of the same layout as `self`, but that also is aligned to
151 /// alignment `align` (measured in bytes).
153 /// If `self` already meets the prescribed alignment, then returns
156 /// Note that this method does not add any padding to the overall
157 /// size, regardless of whether the returned layout has a different
158 /// alignment. In other words, if `K` has size 16, `K.align_to(32)`
159 /// will *still* have size 16.
161 /// Returns an error if the combination of `self.size()` and the given
162 /// `align` violates the conditions listed in
163 /// [`Layout::from_size_align`](#method.from_size_align).
164 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
166 pub fn align_to(&self, align: usize) -> Result<Self, LayoutErr> {
167 Layout::from_size_align(self.size(), cmp::max(self.align(), align))
170 /// Returns the amount of padding we must insert after `self`
171 /// to ensure that the following address will satisfy `align`
172 /// (measured in bytes).
174 /// e.g., if `self.size()` is 9, then `self.padding_needed_for(4)`
175 /// returns 3, because that is the minimum number of bytes of
176 /// padding required to get a 4-aligned address (assuming that the
177 /// corresponding memory block starts at a 4-aligned address).
179 /// The return value of this function has no meaning if `align` is
180 /// not a power-of-two.
182 /// Note that the utility of the returned value requires `align`
183 /// to be less than or equal to the alignment of the starting
184 /// address for the whole allocated block of memory. One way to
185 /// satisfy this constraint is to ensure `align <= self.align()`.
186 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
187 #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
189 pub const fn padding_needed_for(&self, align: usize) -> usize {
190 let len = self.size();
192 // Rounded up value is:
193 // len_rounded_up = (len + align - 1) & !(align - 1);
194 // and then we return the padding difference: `len_rounded_up - len`.
196 // We use modular arithmetic throughout:
198 // 1. align is guaranteed to be > 0, so align - 1 is always
201 // 2. `len + align - 1` can overflow by at most `align - 1`,
202 // so the &-mask with `!(align - 1)` will ensure that in the
203 // case of overflow, `len_rounded_up` will itself be 0.
204 // Thus the returned padding, when added to `len`, yields 0,
205 // which trivially satisfies the alignment `align`.
207 // (Of course, attempts to allocate blocks of memory whose
208 // size and padding overflow in the above manner should cause
209 // the allocator to yield an error anyway.)
211 let len_rounded_up = len.wrapping_add(align).wrapping_sub(1) & !align.wrapping_sub(1);
212 len_rounded_up.wrapping_sub(len)
215 /// Creates a layout by rounding the size of this layout up to a multiple
216 /// of the layout's alignment.
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) -> Layout {
223 let pad = self.padding_needed_for(self.align());
224 // This cannot overflow. Quoting from the invariant of Layout:
225 // > `size`, when rounded up to the nearest multiple of `align`,
226 // > must not overflow (i.e., the rounded value must be less than
228 let new_size = self.size() + pad;
230 Layout::from_size_align(new_size, self.align()).unwrap()
233 /// Creates a layout describing the record for `n` instances of
234 /// `self`, with a suitable amount of padding between each to
235 /// ensure that each instance is given its requested size and
236 /// alignment. On success, returns `(k, offs)` where `k` is the
237 /// layout of the array and `offs` is the distance between the start
238 /// of each element in the array.
240 /// On arithmetic overflow, returns `LayoutErr`.
241 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
243 pub fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutErr> {
244 // This cannot overflow. Quoting from the invariant of Layout:
245 // > `size`, when rounded up to the nearest multiple of `align`,
246 // > must not overflow (i.e., the rounded value must be less than
248 let padded_size = self.size() + self.padding_needed_for(self.align());
249 let alloc_size = padded_size.checked_mul(n).ok_or(LayoutErr { private: () })?;
252 // self.align is already known to be valid and alloc_size has been
254 Ok((Layout::from_size_align_unchecked(alloc_size, self.align()), padded_size))
258 /// Creates a layout describing the record for `self` followed by
259 /// `next`, including any necessary padding to ensure that `next`
260 /// will be properly aligned. Note that the resulting layout will
261 /// satisfy the alignment properties of both `self` and `next`.
263 /// The resulting layout will be the same as that of a C struct containing
264 /// two fields with the layouts of `self` and `next`, in that order.
266 /// Returns `Some((k, offset))`, where `k` is layout of the concatenated
267 /// record and `offset` is the relative location, in bytes, of the
268 /// start of the `next` embedded within the concatenated record
269 /// (assuming that the record itself starts at offset 0).
271 /// On arithmetic overflow, returns `LayoutErr`.
272 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
274 pub fn extend(&self, next: Self) -> Result<(Self, usize), LayoutErr> {
275 let new_align = cmp::max(self.align(), next.align());
276 let pad = self.padding_needed_for(next.align());
278 let offset = self.size().checked_add(pad).ok_or(LayoutErr { private: () })?;
279 let new_size = offset.checked_add(next.size()).ok_or(LayoutErr { private: () })?;
281 let layout = Layout::from_size_align(new_size, new_align)?;
285 /// Creates a layout describing the record for `n` instances of
286 /// `self`, with no padding between each instance.
288 /// Note that, unlike `repeat`, `repeat_packed` does not guarantee
289 /// that the repeated instances of `self` will be properly
290 /// aligned, even if a given instance of `self` is properly
291 /// aligned. In other words, if the layout returned by
292 /// `repeat_packed` is used to allocate an array, it is not
293 /// guaranteed that all elements in the array will be properly
296 /// On arithmetic overflow, returns `LayoutErr`.
297 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
299 pub fn repeat_packed(&self, n: usize) -> Result<Self, LayoutErr> {
300 let size = self.size().checked_mul(n).ok_or(LayoutErr { private: () })?;
301 Layout::from_size_align(size, self.align())
304 /// Creates a layout describing the record for `self` followed by
305 /// `next` with no additional padding between the two. Since no
306 /// padding is inserted, the alignment of `next` is irrelevant,
307 /// and is not incorporated *at all* into the resulting layout.
309 /// On arithmetic overflow, returns `LayoutErr`.
310 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
312 pub fn extend_packed(&self, next: Self) -> Result<Self, LayoutErr> {
313 let new_size = self.size().checked_add(next.size()).ok_or(LayoutErr { private: () })?;
314 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> {
323 Layout::new::<T>().repeat(n).map(|(k, offs)| {
324 debug_assert!(offs == mem::size_of::<T>());
330 /// The parameters given to `Layout::from_size_align`
331 /// or some other `Layout` constructor
332 /// do not satisfy its documented constraints.
333 #[stable(feature = "alloc_layout", since = "1.28.0")]
334 #[derive(Clone, PartialEq, Eq, Debug)]
335 pub struct LayoutErr {
339 // (we need this for downstream impl of trait Error)
340 #[stable(feature = "alloc_layout", since = "1.28.0")]
341 impl fmt::Display for LayoutErr {
342 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
343 f.write_str("invalid parameters to Layout::from_size_align")
347 /// The `AllocErr` error indicates an allocation failure
348 /// that may be due to resource exhaustion or to
349 /// something wrong when combining the given input arguments with this
351 #[unstable(feature = "allocator_api", issue = "32838")]
352 #[derive(Clone, PartialEq, Eq, Debug)]
355 // (we need this for downstream impl of trait Error)
356 #[unstable(feature = "allocator_api", issue = "32838")]
357 impl fmt::Display for AllocErr {
358 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
359 f.write_str("memory allocation failed")
363 /// The `CannotReallocInPlace` error is used when [`grow_in_place`] or
364 /// [`shrink_in_place`] were unable to reuse the given memory block for
365 /// a requested layout.
367 /// [`grow_in_place`]: ./trait.AllocRef.html#method.grow_in_place
368 /// [`shrink_in_place`]: ./trait.AllocRef.html#method.shrink_in_place
369 #[unstable(feature = "allocator_api", issue = "32838")]
370 #[derive(Clone, PartialEq, Eq, Debug)]
371 pub struct CannotReallocInPlace;
373 #[unstable(feature = "allocator_api", issue = "32838")]
374 impl CannotReallocInPlace {
375 pub fn description(&self) -> &str {
376 "cannot reallocate allocator's memory in place"
380 // (we need this for downstream impl of trait Error)
381 #[unstable(feature = "allocator_api", issue = "32838")]
382 impl fmt::Display for CannotReallocInPlace {
383 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
384 write!(f, "{}", self.description())
388 /// A memory allocator that can be registered as the standard library’s default
389 /// through the `#[global_allocator]` attribute.
391 /// Some of the methods require that a memory block be *currently
392 /// allocated* via an allocator. This means that:
394 /// * the starting address for that memory block was previously
395 /// returned by a previous call to an allocation method
396 /// such as `alloc`, and
398 /// * the memory block has not been subsequently deallocated, where
399 /// blocks are deallocated either by being passed to a deallocation
400 /// method such as `dealloc` or by being
401 /// passed to a reallocation method that returns a non-null pointer.
407 /// use std::alloc::{GlobalAlloc, Layout, alloc};
408 /// use std::ptr::null_mut;
410 /// struct MyAllocator;
412 /// unsafe impl GlobalAlloc for MyAllocator {
413 /// unsafe fn alloc(&self, _layout: Layout) -> *mut u8 { null_mut() }
414 /// unsafe fn dealloc(&self, _ptr: *mut u8, _layout: Layout) {}
417 /// #[global_allocator]
418 /// static A: MyAllocator = MyAllocator;
422 /// assert!(alloc(Layout::new::<u32>()).is_null())
429 /// The `GlobalAlloc` trait is an `unsafe` trait for a number of reasons, and
430 /// implementors must ensure that they adhere to these contracts:
432 /// * It's undefined behavior if global allocators unwind. This restriction may
433 /// be lifted in the future, but currently a panic from any of these
434 /// functions may lead to memory unsafety.
436 /// * `Layout` queries and calculations in general must be correct. Callers of
437 /// this trait are allowed to rely on the contracts defined on each method,
438 /// and implementors must ensure such contracts remain true.
439 #[stable(feature = "global_alloc", since = "1.28.0")]
440 pub unsafe trait GlobalAlloc {
441 /// Allocate memory as described by the given `layout`.
443 /// Returns a pointer to newly-allocated memory,
444 /// or null to indicate allocation failure.
448 /// This function is unsafe because undefined behavior can result
449 /// if the caller does not ensure that `layout` has non-zero size.
451 /// (Extension subtraits might provide more specific bounds on
452 /// behavior, e.g., guarantee a sentinel address or a null pointer
453 /// in response to a zero-size allocation request.)
455 /// The allocated block of memory may or may not be initialized.
459 /// Returning a null pointer indicates that either memory is exhausted
460 /// or `layout` does not meet this allocator's size or alignment constraints.
462 /// Implementations are encouraged to return null on memory
463 /// exhaustion rather than aborting, but this is not
464 /// a strict requirement. (Specifically: it is *legal* to
465 /// implement this trait atop an underlying native allocation
466 /// library that aborts on memory exhaustion.)
468 /// Clients wishing to abort computation in response to an
469 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
470 /// rather than directly invoking `panic!` or similar.
472 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
473 #[stable(feature = "global_alloc", since = "1.28.0")]
474 unsafe fn alloc(&self, layout: Layout) -> *mut u8;
476 /// Deallocate the block of memory at the given `ptr` pointer with the given `layout`.
480 /// This function is unsafe because undefined behavior can result
481 /// if the caller does not ensure all of the following:
483 /// * `ptr` must denote a block of memory currently allocated via
486 /// * `layout` must be the same layout that was used
487 /// to allocate that block of memory,
488 #[stable(feature = "global_alloc", since = "1.28.0")]
489 unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout);
491 /// Behaves like `alloc`, but also ensures that the contents
492 /// are set to zero before being returned.
496 /// This function is unsafe for the same reasons that `alloc` is.
497 /// However the allocated block of memory is guaranteed to be initialized.
501 /// Returning a null pointer indicates that either memory is exhausted
502 /// or `layout` does not meet allocator's size or alignment constraints,
503 /// just as in `alloc`.
505 /// Clients wishing to abort computation in response to an
506 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
507 /// rather than directly invoking `panic!` or similar.
509 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
510 #[stable(feature = "global_alloc", since = "1.28.0")]
511 unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
512 let size = layout.size();
513 let ptr = self.alloc(layout);
515 ptr::write_bytes(ptr, 0, size);
520 /// Shrink or grow a block of memory to the given `new_size`.
521 /// The block is described by the given `ptr` pointer and `layout`.
523 /// If this returns a non-null pointer, then ownership of the memory block
524 /// referenced by `ptr` has been transferred to this allocator.
525 /// The memory may or may not have been deallocated,
526 /// and should be considered unusable (unless of course it was
527 /// transferred back to the caller again via the return value of
528 /// this method). The new memory block is allocated with `layout`, but
529 /// with the `size` updated to `new_size`.
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(ptr, new_ptr, cmp::min(layout.size(), new_size));
577 self.dealloc(ptr, layout);
583 /// An implementation of `AllocRef` can allocate, reallocate, and
584 /// deallocate arbitrary blocks of data described via `Layout`.
586 /// `AllocRef` is designed to be implemented on ZSTs, references, or
587 /// smart pointers because having an allocator like `MyAlloc([u8; N])`
588 /// cannot be moved, without updating the pointers to the allocated
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`) or reallocation method
597 /// (`realloc`, `realloc_excess`), 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 `AllocRef` trait state that allocation requests
606 /// must be non-zero size, or else undefined behavior can result.
608 /// * If an `AllocRef` implementation chooses to return `Ok` in this
609 /// case (i.e., the pointer denotes a zero-sized inaccessible block)
610 /// then that returned pointer must be considered "currently
611 /// allocated". On such an allocator, *all* methods that take
612 /// currently-allocated pointers as inputs must accept these
613 /// zero-sized pointers, *without* causing undefined behavior.
615 /// * In other words, if a zero-sized pointer can flow out of an
616 /// allocator, then that allocator must likewise accept that pointer
617 /// flowing back into its deallocation and reallocation methods.
619 /// Some of the methods require that a layout *fit* a memory block.
620 /// What it means for a layout to "fit" a memory block means (or
621 /// equivalently, for a memory block to "fit" a layout) is that the
622 /// following two conditions must hold:
624 /// 1. The block's starting address must be aligned to `layout.align()`.
626 /// 2. The block's size must fall in the range `[use_min, use_max]`, where:
628 /// * `use_min` is `self.usable_size(layout).0`, and
630 /// * `use_max` is the capacity that was (or would have been)
631 /// returned when (if) the block was allocated via a call to
632 /// `alloc_excess` or `realloc_excess`.
636 /// * the size of the layout most recently used to allocate the block
637 /// is guaranteed to be in the range `[use_min, use_max]`, and
639 /// * a lower-bound on `use_max` can be safely approximated by a call to
642 /// * if a layout `k` fits a memory block (denoted by `ptr`)
643 /// currently allocated via an allocator `a`, then it is legal to
644 /// use that layout to deallocate it, i.e., `a.dealloc(ptr, k);`.
648 /// The `AllocRef` trait is an `unsafe` trait for a number of reasons, and
649 /// implementors must ensure that they adhere to these contracts:
651 /// * Pointers returned from allocation functions must point to valid memory and
652 /// retain their validity until at least one instance of `AllocRef` is dropped
655 /// * Cloning or moving the allocator must not invalidate pointers returned
656 /// from this allocator. Cloning must return a reference to the same allocator.
658 /// * `Layout` queries and calculations in general must be correct. Callers of
659 /// this trait are allowed to rely on the contracts defined on each method,
660 /// and implementors must ensure such contracts remain true.
662 /// Note that this list may get tweaked over time as clarifications are made in
664 #[unstable(feature = "allocator_api", issue = "32838")]
665 pub unsafe trait AllocRef {
666 // (Note: some existing allocators have unspecified but well-defined
667 // behavior in response to a zero size allocation request ;
668 // e.g., in C, `malloc` of 0 will either return a null pointer or a
669 // unique pointer, but will not have arbitrary undefined
671 // However in jemalloc for example,
672 // `mallocx(0)` is documented as undefined behavior.)
674 /// Returns a pointer meeting the size and alignment guarantees of
677 /// If this method returns an `Ok(addr)`, then the `addr` returned
678 /// will be non-null address pointing to a block of storage
679 /// suitable for holding an instance of `layout`.
681 /// The returned block of storage may or may not have its contents
682 /// initialized. (Extension subtraits might restrict this
683 /// behavior, e.g., to ensure initialization to particular sets of
688 /// This function is unsafe because undefined behavior can result
689 /// if the caller does not ensure that `layout` has non-zero size.
691 /// (Extension subtraits might provide more specific bounds on
692 /// behavior, e.g., guarantee a sentinel address or a null pointer
693 /// in response to a zero-size allocation request.)
697 /// Returning `Err` indicates that either memory is exhausted or
698 /// `layout` does not meet allocator's size or alignment
701 /// Implementations are encouraged to return `Err` on memory
702 /// exhaustion rather than panicking or aborting, but this is not
703 /// a strict requirement. (Specifically: it is *legal* to
704 /// implement this trait atop an underlying native allocation
705 /// library that aborts on memory exhaustion.)
707 /// Clients wishing to abort computation in response to an
708 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
709 /// rather than directly invoking `panic!` or similar.
711 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
712 unsafe fn alloc(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr>;
714 /// Deallocate the memory referenced by `ptr`.
718 /// This function is unsafe because undefined behavior can result
719 /// if the caller does not ensure all of the following:
721 /// * `ptr` must denote a block of memory currently allocated via
724 /// * `layout` must *fit* that block of memory,
726 /// * In addition to fitting the block of memory `layout`, the
727 /// alignment of the `layout` must match the alignment used
728 /// to allocate that block of memory.
729 unsafe fn dealloc(&mut self, ptr: NonNull<u8>, layout: Layout);
731 // == ALLOCATOR-SPECIFIC QUANTITIES AND LIMITS ==
734 /// Returns bounds on the guaranteed usable size of a successful
735 /// allocation created with the specified `layout`.
737 /// In particular, if one has a memory block allocated via a given
738 /// allocator `a` and layout `k` where `a.usable_size(k)` returns
739 /// `(l, u)`, then one can pass that block to `a.dealloc()` with a
740 /// layout in the size range [l, u].
742 /// (All implementors of `usable_size` must ensure that
743 /// `l <= k.size() <= u`)
745 /// Both the lower- and upper-bounds (`l` and `u` respectively)
746 /// are provided, because an allocator based on size classes could
747 /// misbehave if one attempts to deallocate a block without
748 /// providing a correct value for its size (i.e., one within the
751 /// Clients who wish to make use of excess capacity are encouraged
752 /// to use the `alloc_excess` and `realloc_excess` instead, as
753 /// this method is constrained to report conservative values that
754 /// serve as valid bounds for *all possible* allocation method
757 /// However, for clients that do not wish to track the capacity
758 /// returned by `alloc_excess` locally, this method is likely to
759 /// produce useful results.
761 fn usable_size(&self, layout: &Layout) -> (usize, usize) {
762 (layout.size(), layout.size())
765 // == METHODS FOR MEMORY REUSE ==
766 // realloc. alloc_excess, realloc_excess
768 /// Returns a pointer suitable for holding data described by
769 /// a new layout with `layout`’s alignment and a size given
770 /// by `new_size`. To
771 /// accomplish this, this may extend or shrink the allocation
772 /// referenced by `ptr` to fit the new layout.
774 /// If this returns `Ok`, then ownership of the memory block
775 /// referenced by `ptr` has been transferred to this
776 /// allocator. The memory may or may not have been freed, and
777 /// should be considered unusable (unless of course it was
778 /// transferred back to the caller again via the return value of
781 /// If this method returns `Err`, then ownership of the memory
782 /// block has not been transferred to this allocator, and the
783 /// contents of the memory block are unaltered.
787 /// This function is unsafe because undefined behavior can result
788 /// if the caller does not ensure all of the following:
790 /// * `ptr` must be currently allocated via this allocator,
792 /// * `layout` must *fit* the `ptr` (see above). (The `new_size`
793 /// argument need not fit it.)
795 /// * `new_size` must be greater than zero.
797 /// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
798 /// must not overflow (i.e., the rounded value must be less than `usize::MAX`).
800 /// (Extension subtraits might provide more specific bounds on
801 /// behavior, e.g., guarantee a sentinel address or a null pointer
802 /// in response to a zero-size allocation request.)
806 /// Returns `Err` only if the new layout
807 /// does not meet the allocator's size
808 /// and alignment constraints of the allocator, or if reallocation
811 /// Implementations are encouraged to return `Err` on memory
812 /// exhaustion rather than panicking or aborting, but this is not
813 /// a strict requirement. (Specifically: it is *legal* to
814 /// implement this trait atop an underlying native allocation
815 /// library that aborts on memory exhaustion.)
817 /// Clients wishing to abort computation in response to a
818 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
819 /// rather than directly invoking `panic!` or similar.
821 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
827 ) -> Result<NonNull<u8>, AllocErr> {
828 let old_size = layout.size();
830 if new_size >= old_size {
831 if let Ok(()) = self.grow_in_place(ptr, layout, new_size) {
834 } else if new_size < old_size {
835 if let Ok(()) = self.shrink_in_place(ptr, layout, new_size) {
840 // otherwise, fall back on alloc + copy + dealloc.
841 let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
842 let result = self.alloc(new_layout);
843 if let Ok(new_ptr) = result {
844 ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_ptr(), cmp::min(old_size, new_size));
845 self.dealloc(ptr, layout);
850 /// Behaves like `alloc`, but also ensures that the contents
851 /// are set to zero before being returned.
855 /// This function is unsafe for the same reasons that `alloc` is.
859 /// Returning `Err` indicates that either memory is exhausted or
860 /// `layout` does not meet allocator's size or alignment
861 /// constraints, just as in `alloc`.
863 /// Clients wishing to abort computation in response to an
864 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
865 /// rather than directly invoking `panic!` or similar.
867 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
868 unsafe fn alloc_zeroed(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr> {
869 let size = layout.size();
870 let p = self.alloc(layout);
872 ptr::write_bytes(p.as_ptr(), 0, size);
877 /// Behaves like `alloc`, but also returns the whole size of
878 /// the returned block. For some `layout` inputs, like arrays, this
879 /// may include extra storage usable for additional data.
883 /// This function is unsafe for the same reasons that `alloc` is.
887 /// Returning `Err` indicates that either memory is exhausted or
888 /// `layout` does not meet allocator's size or alignment
889 /// constraints, just as in `alloc`.
891 /// Clients wishing to abort computation in response to an
892 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
893 /// rather than directly invoking `panic!` or similar.
895 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
896 unsafe fn alloc_excess(&mut self, layout: Layout) -> Result<Excess, AllocErr> {
897 let usable_size = self.usable_size(&layout);
898 self.alloc(layout).map(|p| Excess(p, usable_size.1))
901 /// Behaves like `realloc`, but also returns the whole size of
902 /// the returned block. For some `layout` inputs, like arrays, this
903 /// may include extra storage usable for additional data.
907 /// This function is unsafe for the same reasons that `realloc` is.
911 /// Returning `Err` indicates that either memory is exhausted or
912 /// `layout` does not meet allocator's size or alignment
913 /// constraints, just as in `realloc`.
915 /// Clients wishing to abort computation in response to a
916 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
917 /// rather than directly invoking `panic!` or similar.
919 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
920 unsafe fn realloc_excess(
925 ) -> Result<Excess, AllocErr> {
926 let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
927 let usable_size = self.usable_size(&new_layout);
928 self.realloc(ptr, layout, new_size).map(|p| Excess(p, usable_size.1))
931 /// Attempts to extend the allocation referenced by `ptr` to fit `new_size`.
933 /// If this returns `Ok`, then the allocator has asserted that the
934 /// memory block referenced by `ptr` now fits `new_size`, and thus can
935 /// be used to carry data of a layout of that size and same alignment as
936 /// `layout`. (The allocator is allowed to
937 /// expend effort to accomplish this, such as extending the memory block to
938 /// include successor blocks, or virtual memory tricks.)
940 /// Regardless of what this method returns, ownership of the
941 /// memory block referenced by `ptr` has not been transferred, and
942 /// the contents of the memory block are unaltered.
946 /// This function is unsafe because undefined behavior can result
947 /// if the caller does not ensure all of the following:
949 /// * `ptr` must be currently allocated via this allocator,
951 /// * `layout` must *fit* the `ptr` (see above); note the
952 /// `new_size` argument need not fit it,
954 /// * `new_size` must not be less than `layout.size()`,
958 /// Returns `Err(CannotReallocInPlace)` when the allocator is
959 /// unable to assert that the memory block referenced by `ptr`
960 /// could fit `layout`.
962 /// Note that one cannot pass `CannotReallocInPlace` to the `handle_alloc_error`
963 /// function; clients are expected either to be able to recover from
964 /// `grow_in_place` failures without aborting, or to fall back on
965 /// another reallocation method before resorting to an abort.
966 unsafe fn grow_in_place(
971 ) -> 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]
977 if new_size <= u { Ok(()) } else { Err(CannotReallocInPlace) }
980 /// Attempts to shrink the allocation referenced by `ptr` to fit `new_size`.
982 /// If this returns `Ok`, then the allocator has asserted that the
983 /// memory block referenced by `ptr` now fits `new_size`, and
984 /// thus can only be used to carry data of that smaller
985 /// layout. (The allocator is allowed to take advantage of this,
986 /// carving off portions of the block for reuse elsewhere.) The
987 /// truncated contents of the block within the smaller layout are
988 /// unaltered, and ownership of block has not been transferred.
990 /// If this returns `Err`, then the memory block is considered to
991 /// still represent the original (larger) `layout`. None of the
992 /// block has been carved off for reuse elsewhere, ownership of
993 /// the memory block has not been transferred, and the contents of
994 /// the memory block are unaltered.
998 /// This function is unsafe because undefined behavior can result
999 /// if the caller does not ensure all of the following:
1001 /// * `ptr` must be currently allocated via this allocator,
1003 /// * `layout` must *fit* the `ptr` (see above); note the
1004 /// `new_size` argument need not fit it,
1006 /// * `new_size` must not be greater than `layout.size()`
1007 /// (and must be greater than zero),
1011 /// Returns `Err(CannotReallocInPlace)` when the allocator is
1012 /// unable to assert that the memory block referenced by `ptr`
1013 /// could fit `layout`.
1015 /// Note that one cannot pass `CannotReallocInPlace` to the `handle_alloc_error`
1016 /// function; clients are expected either to be able to recover from
1017 /// `shrink_in_place` failures without aborting, or to fall back
1018 /// on another reallocation method before resorting to an abort.
1019 unsafe fn shrink_in_place(
1024 ) -> Result<(), CannotReallocInPlace> {
1025 let _ = ptr; // this default implementation doesn't care about the actual address.
1026 debug_assert!(new_size <= layout.size());
1027 let (l, _u) = self.usable_size(&layout);
1028 // layout.size() <= _u [guaranteed by usable_size()]
1029 // new_layout.size() <= layout.size() [required by this method]
1030 if l <= new_size { Ok(()) } else { Err(CannotReallocInPlace) }