1 // Copyright 2015 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! Memory allocation APIs
13 #![stable(feature = "alloc_module", since = "1.28.0")]
19 use ptr::{self, NonNull};
20 use num::NonZeroUsize;
22 /// Represents the combination of a starting address and
23 /// a total capacity of the returned block.
24 #[unstable(feature = "allocator_api", issue = "32838")]
26 pub struct Excess(pub NonNull<u8>, pub usize);
28 fn size_align<T>() -> (usize, usize) {
29 (mem::size_of::<T>(), mem::align_of::<T>())
32 /// Layout of a block of memory.
34 /// An instance of `Layout` describes a particular layout of memory.
35 /// You build a `Layout` up as an input to give to an allocator.
37 /// All layouts have an associated non-negative size and a
38 /// power-of-two alignment.
40 /// (Note however that layouts are *not* required to have positive
41 /// size, even though many allocators require that all memory
42 /// requests have positive size. A caller to the `Alloc::alloc`
43 /// method must either ensure that conditions like this are met, or
44 /// use specific allocators with looser requirements.)
45 #[stable(feature = "alloc_layout", since = "1.28.0")]
46 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
47 #[lang = "alloc_layout"]
49 // size of the requested block of memory, measured in bytes.
52 // alignment of the requested block of memory, measured in bytes.
53 // we ensure that this is always a power-of-two, because API's
54 // like `posix_memalign` require it and it is a reasonable
55 // constraint to impose on Layout constructors.
57 // (However, we do not analogously require `align >= sizeof(void*)`,
58 // even though that is *also* a requirement of `posix_memalign`.)
63 /// Constructs a `Layout` from a given `size` and `align`,
64 /// or returns `LayoutErr` if either of the following conditions
67 /// * `align` must not be zero,
69 /// * `align` must be a power of two,
71 /// * `size`, when rounded up to the nearest multiple of `align`,
72 /// must not overflow (i.e. the rounded value must be less than
74 #[stable(feature = "alloc_layout", since = "1.28.0")]
76 pub fn from_size_align(size: usize, align: usize) -> Result<Self, LayoutErr> {
77 if !align.is_power_of_two() {
78 return Err(LayoutErr { private: () });
81 // (power-of-two implies align != 0.)
83 // Rounded up size is:
84 // size_rounded_up = (size + align - 1) & !(align - 1);
86 // We know from above that align != 0. If adding (align - 1)
87 // does not overflow, then rounding up will be fine.
89 // Conversely, &-masking with !(align - 1) will subtract off
90 // only low-order-bits. Thus if overflow occurs with the sum,
91 // the &-mask cannot subtract enough to undo that overflow.
93 // Above implies that checking for summation overflow is both
94 // necessary and sufficient.
95 if size > usize::MAX - (align - 1) {
96 return Err(LayoutErr { private: () });
100 Ok(Layout::from_size_align_unchecked(size, align))
104 /// Creates a layout, bypassing all checks.
108 /// This function is unsafe as it does not verify the preconditions from
109 /// [`Layout::from_size_align`](#method.from_size_align).
110 #[stable(feature = "alloc_layout", since = "1.28.0")]
112 pub unsafe fn from_size_align_unchecked(size: usize, align: usize) -> Self {
113 Layout { size_: size, align_: NonZeroUsize::new_unchecked(align) }
116 /// The minimum size in bytes for a memory block of this layout.
117 #[stable(feature = "alloc_layout", since = "1.28.0")]
119 pub fn size(&self) -> usize { self.size_ }
121 /// The minimum byte alignment for a memory block of this layout.
122 #[stable(feature = "alloc_layout", since = "1.28.0")]
124 pub fn align(&self) -> usize { self.align_.get() }
126 /// Constructs a `Layout` suitable for holding a value of type `T`.
127 #[stable(feature = "alloc_layout", since = "1.28.0")]
129 pub fn new<T>() -> Self {
130 let (size, align) = size_align::<T>();
131 // Note that the align is guaranteed by rustc to be a power of two and
132 // the size+align combo is guaranteed to fit in our address space. As a
133 // result use the unchecked constructor here to avoid inserting code
134 // that panics if it isn't optimized well enough.
135 debug_assert!(Layout::from_size_align(size, align).is_ok());
137 Layout::from_size_align_unchecked(size, align)
141 /// Produces layout describing a record that could be used to
142 /// allocate backing structure for `T` (which could be a trait
143 /// or other unsized type like a slice).
144 #[stable(feature = "alloc_layout", since = "1.28.0")]
146 pub fn for_value<T: ?Sized>(t: &T) -> Self {
147 let (size, align) = (mem::size_of_val(t), mem::align_of_val(t));
148 // See rationale in `new` for why this us using an unsafe variant below
149 debug_assert!(Layout::from_size_align(size, align).is_ok());
151 Layout::from_size_align_unchecked(size, align)
155 /// Creates a layout describing the record that can hold a value
156 /// of the same layout as `self`, but that also is aligned to
157 /// alignment `align` (measured in bytes).
159 /// If `self` already meets the prescribed alignment, then returns
162 /// Note that this method does not add any padding to the overall
163 /// size, regardless of whether the returned layout has a different
164 /// alignment. In other words, if `K` has size 16, `K.align_to(32)`
165 /// will *still* have size 16.
167 /// Returns an error if the combination of `self.size()` and the given
168 /// `align` violates the conditions listed in
169 /// [`Layout::from_size_align`](#method.from_size_align).
170 #[unstable(feature = "allocator_api", issue = "32838")]
172 pub fn align_to(&self, align: usize) -> Result<Self, LayoutErr> {
173 Layout::from_size_align(self.size(), cmp::max(self.align(), align))
176 /// Returns the amount of padding we must insert after `self`
177 /// to ensure that the following address will satisfy `align`
178 /// (measured in bytes).
180 /// E.g. if `self.size()` is 9, then `self.padding_needed_for(4)`
181 /// returns 3, because that is the minimum number of bytes of
182 /// padding required to get a 4-aligned address (assuming that the
183 /// corresponding memory block starts at a 4-aligned address).
185 /// The return value of this function has no meaning if `align` is
186 /// not a power-of-two.
188 /// Note that the utility of the returned value requires `align`
189 /// to be less than or equal to the alignment of the starting
190 /// address for the whole allocated block of memory. One way to
191 /// satisfy this constraint is to ensure `align <= self.align()`.
192 #[unstable(feature = "allocator_api", issue = "32838")]
194 pub fn padding_needed_for(&self, align: usize) -> usize {
195 let len = self.size();
197 // Rounded up value is:
198 // len_rounded_up = (len + align - 1) & !(align - 1);
199 // and then we return the padding difference: `len_rounded_up - len`.
201 // We use modular arithmetic throughout:
203 // 1. align is guaranteed to be > 0, so align - 1 is always
206 // 2. `len + align - 1` can overflow by at most `align - 1`,
207 // so the &-mask wth `!(align - 1)` will ensure that in the
208 // case of overflow, `len_rounded_up` will itself be 0.
209 // Thus the returned padding, when added to `len`, yields 0,
210 // which trivially satisfies the alignment `align`.
212 // (Of course, attempts to allocate blocks of memory whose
213 // size and padding overflow in the above manner should cause
214 // the allocator to yield an error anyway.)
216 let len_rounded_up = len.wrapping_add(align).wrapping_sub(1)
217 & !align.wrapping_sub(1);
218 len_rounded_up.wrapping_sub(len)
221 /// Creates a layout describing the record for `n` instances of
222 /// `self`, with a suitable amount of padding between each to
223 /// ensure that each instance is given its requested size and
224 /// alignment. On success, returns `(k, offs)` where `k` is the
225 /// layout of the array and `offs` is the distance between the start
226 /// of each element in the array.
228 /// On arithmetic overflow, returns `LayoutErr`.
229 #[unstable(feature = "allocator_api", issue = "32838")]
231 pub fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutErr> {
232 let padded_size = self.size().checked_add(self.padding_needed_for(self.align()))
233 .ok_or(LayoutErr { private: () })?;
234 let alloc_size = padded_size.checked_mul(n)
235 .ok_or(LayoutErr { private: () })?;
238 // self.align is already known to be valid and alloc_size has been
240 Ok((Layout::from_size_align_unchecked(alloc_size, self.align()), padded_size))
244 /// Creates a layout describing the record for `self` followed by
245 /// `next`, including any necessary padding to ensure that `next`
246 /// will be properly aligned. Note that the result layout will
247 /// satisfy the alignment properties of both `self` and `next`.
249 /// The resulting layout will be the same as that of a C struct containing
250 /// two fields with the layouts of `self` and `next`, in that order.
252 /// Returns `Some((k, offset))`, where `k` is layout of the concatenated
253 /// record and `offset` is the relative location, in bytes, of the
254 /// start of the `next` embedded within the concatenated record
255 /// (assuming that the record itself starts at offset 0).
257 /// On arithmetic overflow, returns `LayoutErr`.
258 #[unstable(feature = "allocator_api", issue = "32838")]
260 pub fn extend(&self, next: Self) -> Result<(Self, usize), LayoutErr> {
261 let new_align = cmp::max(self.align(), next.align());
262 let pad = self.padding_needed_for(next.align());
264 let offset = self.size().checked_add(pad)
265 .ok_or(LayoutErr { private: () })?;
266 let new_size = offset.checked_add(next.size())
267 .ok_or(LayoutErr { private: () })?;
269 let layout = Layout::from_size_align(new_size, new_align)?;
273 /// Creates a layout describing the record for `n` instances of
274 /// `self`, with no padding between each instance.
276 /// Note that, unlike `repeat`, `repeat_packed` does not guarantee
277 /// that the repeated instances of `self` will be properly
278 /// aligned, even if a given instance of `self` is properly
279 /// aligned. In other words, if the layout returned by
280 /// `repeat_packed` is used to allocate an array, it is not
281 /// guaranteed that all elements in the array will be properly
284 /// On arithmetic overflow, returns `LayoutErr`.
285 #[unstable(feature = "allocator_api", issue = "32838")]
287 pub fn repeat_packed(&self, n: usize) -> Result<Self, LayoutErr> {
288 let size = self.size().checked_mul(n).ok_or(LayoutErr { private: () })?;
289 Layout::from_size_align(size, self.align())
292 /// Creates a layout describing the record for `self` followed by
293 /// `next` with no additional padding between the two. Since no
294 /// padding is inserted, the alignment of `next` is irrelevant,
295 /// and is not incorporated *at all* into the resulting layout.
297 /// On arithmetic overflow, returns `LayoutErr`.
298 #[unstable(feature = "allocator_api", issue = "32838")]
300 pub fn extend_packed(&self, next: Self) -> Result<Self, LayoutErr> {
301 let new_size = self.size().checked_add(next.size())
302 .ok_or(LayoutErr { private: () })?;
303 let layout = Layout::from_size_align(new_size, self.align())?;
307 /// Creates a layout describing the record for a `[T; n]`.
309 /// On arithmetic overflow, returns `LayoutErr`.
310 #[unstable(feature = "allocator_api", issue = "32838")]
312 pub fn array<T>(n: usize) -> Result<Self, LayoutErr> {
316 debug_assert!(offs == mem::size_of::<T>());
322 /// The parameters given to `Layout::from_size_align`
323 /// or some other `Layout` constructor
324 /// do not satisfy its documented constraints.
325 #[stable(feature = "alloc_layout", since = "1.28.0")]
326 #[derive(Clone, PartialEq, Eq, Debug)]
327 pub struct LayoutErr {
331 // (we need this for downstream impl of trait Error)
332 #[stable(feature = "alloc_layout", since = "1.28.0")]
333 impl fmt::Display for LayoutErr {
334 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
335 f.write_str("invalid parameters to Layout::from_size_align")
339 /// The `AllocErr` error indicates an allocation failure
340 /// that may be due to resource exhaustion or to
341 /// something wrong when combining the given input arguments with this
343 #[unstable(feature = "allocator_api", issue = "32838")]
344 #[derive(Clone, PartialEq, Eq, Debug)]
347 // (we need this for downstream impl of trait Error)
348 #[unstable(feature = "allocator_api", issue = "32838")]
349 impl fmt::Display for AllocErr {
350 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
351 f.write_str("memory allocation failed")
355 /// The `CannotReallocInPlace` error is used when `grow_in_place` or
356 /// `shrink_in_place` were unable to reuse the given memory block for
357 /// a requested layout.
358 #[unstable(feature = "allocator_api", issue = "32838")]
359 #[derive(Clone, PartialEq, Eq, Debug)]
360 pub struct CannotReallocInPlace;
362 #[unstable(feature = "allocator_api", issue = "32838")]
363 impl CannotReallocInPlace {
364 pub fn description(&self) -> &str {
365 "cannot reallocate allocator's memory in place"
369 // (we need this for downstream impl of trait Error)
370 #[unstable(feature = "allocator_api", issue = "32838")]
371 impl fmt::Display for CannotReallocInPlace {
372 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
373 write!(f, "{}", self.description())
377 /// A memory allocator that can be registered as the standard library’s default
378 /// though the `#[global_allocator]` attributes.
380 /// Some of the methods require that a memory block be *currently
381 /// allocated* via an allocator. This means that:
383 /// * the starting address for that memory block was previously
384 /// returned by a previous call to an allocation method
385 /// such as `alloc`, and
387 /// * the memory block has not been subsequently deallocated, where
388 /// blocks are deallocated either by being passed to a deallocation
389 /// method such as `dealloc` or by being
390 /// passed to a reallocation method that returns a non-null pointer.
396 /// use std::alloc::{GlobalAlloc, Layout, alloc};
397 /// use std::ptr::null_mut;
399 /// struct MyAllocator;
401 /// unsafe impl GlobalAlloc for MyAllocator {
402 /// unsafe fn alloc(&self, _layout: Layout) -> *mut u8 { null_mut() }
403 /// unsafe fn dealloc(&self, _ptr: *mut u8, _layout: Layout) {}
406 /// #[global_allocator]
407 /// static A: MyAllocator = MyAllocator;
411 /// assert!(alloc(Layout::new::<u32>()).is_null())
418 /// The `GlobalAlloc` trait is an `unsafe` trait for a number of reasons, and
419 /// implementors must ensure that they adhere to these contracts:
421 /// * It's undefined behavior if global allocators unwind. This restriction may
422 /// be lifted in the future, but currently a panic from any of these
423 /// functions may lead to memory unsafety.
425 /// * `Layout` queries and calculations in general must be correct. Callers of
426 /// this trait are allowed to rely on the contracts defined on each method,
427 /// and implementors must ensure such contracts remain true.
428 #[stable(feature = "global_alloc", since = "1.28.0")]
429 pub unsafe trait GlobalAlloc {
430 /// Allocate memory as described by the given `layout`.
432 /// Returns a pointer to newly-allocated memory,
433 /// or null to indicate allocation failure.
437 /// This function is unsafe because undefined behavior can result
438 /// if the caller does not ensure that `layout` has non-zero size.
440 /// (Extension subtraits might provide more specific bounds on
441 /// behavior, e.g. guarantee a sentinel address or a null pointer
442 /// in response to a zero-size allocation request.)
444 /// The allocated block of memory may or may not be initialized.
448 /// Returning a null pointer indicates that either memory is exhausted
449 /// or `layout` does not meet allocator's size or alignment constraints.
451 /// Implementations are encouraged to return null on memory
452 /// exhaustion rather than aborting, but this is not
453 /// a strict requirement. (Specifically: it is *legal* to
454 /// implement this trait atop an underlying native allocation
455 /// library that aborts on memory exhaustion.)
457 /// Clients wishing to abort computation in response to an
458 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
459 /// rather than directly invoking `panic!` or similar.
461 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
462 #[stable(feature = "global_alloc", since = "1.28.0")]
463 unsafe fn alloc(&self, layout: Layout) -> *mut u8;
465 /// Deallocate the block of memory at the given `ptr` pointer with the given `layout`.
469 /// This function is unsafe because undefined behavior can result
470 /// if the caller does not ensure all of the following:
472 /// * `ptr` must denote a block of memory currently allocated via
475 /// * `layout` must be the same layout that was used
476 /// to allocated that block of memory,
477 #[stable(feature = "global_alloc", since = "1.28.0")]
478 unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout);
480 /// Behaves like `alloc`, but also ensures that the contents
481 /// are set to zero before being returned.
485 /// This function is unsafe for the same reasons that `alloc` is.
486 /// However the allocated block of memory is guaranteed to be initialized.
490 /// Returning a null pointer indicates that either memory is exhausted
491 /// or `layout` does not meet allocator's size or alignment constraints,
492 /// just as in `alloc`.
494 /// Clients wishing to abort computation in response to an
495 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
496 /// rather than directly invoking `panic!` or similar.
498 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
499 #[stable(feature = "global_alloc", since = "1.28.0")]
500 unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
501 let size = layout.size();
502 let ptr = self.alloc(layout);
504 ptr::write_bytes(ptr, 0, size);
509 /// Shink or grow a block of memory to the given `new_size`.
510 /// The block is described by the given `ptr` pointer and `layout`.
512 /// If this returns a non-null pointer, then ownership of the memory block
513 /// referenced by `ptr` has been transferred to this allocator.
514 /// The memory may or may not have been deallocated,
515 /// and should be considered unusable (unless of course it was
516 /// transferred back to the caller again via the return value of
519 /// If this method returns null, then ownership of the memory
520 /// block has not been transferred to this allocator, and the
521 /// contents of the memory block are unaltered.
525 /// This function is unsafe because undefined behavior can result
526 /// if the caller does not ensure all of the following:
528 /// * `ptr` must be currently allocated via this allocator,
530 /// * `layout` must be the same layout that was used
531 /// to allocated that block of memory,
533 /// * `new_size` must be greater than zero.
535 /// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
536 /// must not overflow (i.e. the rounded value must be less than `usize::MAX`).
538 /// (Extension subtraits might provide more specific bounds on
539 /// behavior, e.g. guarantee a sentinel address or a null pointer
540 /// in response to a zero-size allocation request.)
544 /// Returns null if the new layout does not meet the size
545 /// and alignment constraints of the allocator, or if reallocation
548 /// Implementations are encouraged to return null on memory
549 /// exhaustion rather than panicking or aborting, but this is not
550 /// a strict requirement. (Specifically: it is *legal* to
551 /// implement this trait atop an underlying native allocation
552 /// library that aborts on memory exhaustion.)
554 /// Clients wishing to abort computation in response to a
555 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
556 /// rather than directly invoking `panic!` or similar.
558 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
559 #[stable(feature = "global_alloc", since = "1.28.0")]
560 unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
561 let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
562 let new_ptr = self.alloc(new_layout);
563 if !new_ptr.is_null() {
564 ptr::copy_nonoverlapping(
567 cmp::min(layout.size(), new_size),
569 self.dealloc(ptr, layout);
575 /// An implementation of `Alloc` can allocate, reallocate, and
576 /// deallocate arbitrary blocks of data described via `Layout`.
578 /// Some of the methods require that a memory block be *currently
579 /// allocated* via an allocator. This means that:
581 /// * the starting address for that memory block was previously
582 /// returned by a previous call to an allocation method (`alloc`,
583 /// `alloc_zeroed`, `alloc_excess`, `alloc_one`, `alloc_array`) or
584 /// reallocation method (`realloc`, `realloc_excess`, or
585 /// `realloc_array`), and
587 /// * the memory block has not been subsequently deallocated, where
588 /// blocks are deallocated either by being passed to a deallocation
589 /// method (`dealloc`, `dealloc_one`, `dealloc_array`) or by being
590 /// passed to a reallocation method (see above) that returns `Ok`.
592 /// A note regarding zero-sized types and zero-sized layouts: many
593 /// methods in the `Alloc` trait state that allocation requests
594 /// must be non-zero size, or else undefined behavior can result.
596 /// * However, some higher-level allocation methods (`alloc_one`,
597 /// `alloc_array`) are well-defined on zero-sized types and can
598 /// optionally support them: it is left up to the implementor
599 /// whether to return `Err`, or to return `Ok` with some pointer.
601 /// * If an `Alloc` implementation chooses to return `Ok` in this
602 /// case (i.e. the pointer denotes a zero-sized inaccessible block)
603 /// then that returned pointer must be considered "currently
604 /// allocated". On such an allocator, *all* methods that take
605 /// currently-allocated pointers as inputs must accept these
606 /// zero-sized pointers, *without* causing undefined behavior.
608 /// * In other words, if a zero-sized pointer can flow out of an
609 /// allocator, then that allocator must likewise accept that pointer
610 /// flowing back into its deallocation and reallocation methods.
612 /// Some of the methods require that a layout *fit* a memory block.
613 /// What it means for a layout to "fit" a memory block means (or
614 /// equivalently, for a memory block to "fit" a layout) is that the
615 /// following two conditions must hold:
617 /// 1. The block's starting address must be aligned to `layout.align()`.
619 /// 2. The block's size must fall in the range `[use_min, use_max]`, where:
621 /// * `use_min` is `self.usable_size(layout).0`, and
623 /// * `use_max` is the capacity that was (or would have been)
624 /// returned when (if) the block was allocated via a call to
625 /// `alloc_excess` or `realloc_excess`.
629 /// * the size of the layout most recently used to allocate the block
630 /// is guaranteed to be in the range `[use_min, use_max]`, and
632 /// * a lower-bound on `use_max` can be safely approximated by a call to
635 /// * if a layout `k` fits a memory block (denoted by `ptr`)
636 /// currently allocated via an allocator `a`, then it is legal to
637 /// use that layout to deallocate it, i.e. `a.dealloc(ptr, k);`.
641 /// The `Alloc` trait is an `unsafe` trait for a number of reasons, and
642 /// implementors must ensure that they adhere to these contracts:
644 /// * Pointers returned from allocation functions must point to valid memory and
645 /// retain their validity until at least the instance of `Alloc` is dropped
648 /// * `Layout` queries and calculations in general must be correct. Callers of
649 /// this trait are allowed to rely on the contracts defined on each method,
650 /// and implementors must ensure such contracts remain true.
652 /// Note that this list may get tweaked over time as clarifications are made in
654 #[unstable(feature = "allocator_api", issue = "32838")]
655 pub unsafe trait Alloc {
657 // (Note: some existing allocators have unspecified but well-defined
658 // behavior in response to a zero size allocation request ;
659 // e.g. in C, `malloc` of 0 will either return a null pointer or a
660 // unique pointer, but will not have arbitrary undefined
662 // However in jemalloc for example,
663 // `mallocx(0)` is documented as undefined behavior.)
665 /// Returns a pointer meeting the size and alignment guarantees of
668 /// If this method returns an `Ok(addr)`, then the `addr` returned
669 /// will be non-null address pointing to a block of storage
670 /// suitable for holding an instance of `layout`.
672 /// The returned block of storage may or may not have its contents
673 /// initialized. (Extension subtraits might restrict this
674 /// behavior, e.g. to ensure initialization to particular sets of
679 /// This function is unsafe because undefined behavior can result
680 /// if the caller does not ensure that `layout` has non-zero size.
682 /// (Extension subtraits might provide more specific bounds on
683 /// behavior, e.g. guarantee a sentinel address or a null pointer
684 /// in response to a zero-size allocation request.)
688 /// Returning `Err` indicates that either memory is exhausted or
689 /// `layout` does not meet allocator's size or alignment
692 /// Implementations are encouraged to return `Err` on memory
693 /// exhaustion rather than panicking or aborting, but this is not
694 /// a strict requirement. (Specifically: it is *legal* to
695 /// implement this trait atop an underlying native allocation
696 /// library that aborts on memory exhaustion.)
698 /// Clients wishing to abort computation in response to an
699 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
700 /// rather than directly invoking `panic!` or similar.
702 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
703 unsafe fn alloc(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr>;
705 /// Deallocate the memory referenced by `ptr`.
709 /// This function is unsafe because undefined behavior can result
710 /// if the caller does not ensure all of the following:
712 /// * `ptr` must denote a block of memory currently allocated via
715 /// * `layout` must *fit* that block of memory,
717 /// * In addition to fitting the block of memory `layout`, the
718 /// alignment of the `layout` must match the alignment used
719 /// to allocate that block of memory.
720 unsafe fn dealloc(&mut self, ptr: NonNull<u8>, layout: Layout);
722 // == ALLOCATOR-SPECIFIC QUANTITIES AND LIMITS ==
725 /// Returns bounds on the guaranteed usable size of a successful
726 /// allocation created with the specified `layout`.
728 /// In particular, if one has a memory block allocated via a given
729 /// allocator `a` and layout `k` where `a.usable_size(k)` returns
730 /// `(l, u)`, then one can pass that block to `a.dealloc()` with a
731 /// layout in the size range [l, u].
733 /// (All implementors of `usable_size` must ensure that
734 /// `l <= k.size() <= u`)
736 /// Both the lower- and upper-bounds (`l` and `u` respectively)
737 /// are provided, because an allocator based on size classes could
738 /// misbehave if one attempts to deallocate a block without
739 /// providing a correct value for its size (i.e., one within the
742 /// Clients who wish to make use of excess capacity are encouraged
743 /// to use the `alloc_excess` and `realloc_excess` instead, as
744 /// this method is constrained to report conservative values that
745 /// serve as valid bounds for *all possible* allocation method
748 /// However, for clients that do not wish to track the capacity
749 /// returned by `alloc_excess` locally, this method is likely to
750 /// produce useful results.
752 fn usable_size(&self, layout: &Layout) -> (usize, usize) {
753 (layout.size(), layout.size())
756 // == METHODS FOR MEMORY REUSE ==
757 // realloc. alloc_excess, realloc_excess
759 /// Returns a pointer suitable for holding data described by
760 /// a new layout with `layout`’s alginment and a size given
761 /// by `new_size`. To
762 /// accomplish this, this may extend or shrink the allocation
763 /// referenced by `ptr` to fit the new layout.
765 /// If this returns `Ok`, then ownership of the memory block
766 /// referenced by `ptr` has been transferred to this
767 /// allocator. The memory may or may not have been freed, and
768 /// should be considered unusable (unless of course it was
769 /// transferred back to the caller again via the return value of
772 /// If this method returns `Err`, then ownership of the memory
773 /// block has not been transferred to this allocator, and the
774 /// contents of the memory block are unaltered.
778 /// This function is unsafe because undefined behavior can result
779 /// if the caller does not ensure all of the following:
781 /// * `ptr` must be currently allocated via this allocator,
783 /// * `layout` must *fit* the `ptr` (see above). (The `new_size`
784 /// argument need not fit it.)
786 /// * `new_size` must be greater than zero.
788 /// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
789 /// must not overflow (i.e. the rounded value must be less than `usize::MAX`).
791 /// (Extension subtraits might provide more specific bounds on
792 /// behavior, e.g. guarantee a sentinel address or a null pointer
793 /// in response to a zero-size allocation request.)
797 /// Returns `Err` only if the new layout
798 /// does not meet the allocator's size
799 /// and alignment constraints of the allocator, or if reallocation
802 /// Implementations are encouraged to return `Err` on memory
803 /// exhaustion rather than panicking or aborting, but this is not
804 /// a strict requirement. (Specifically: it is *legal* to
805 /// implement this trait atop an underlying native allocation
806 /// library that aborts on memory exhaustion.)
808 /// Clients wishing to abort computation in response to a
809 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
810 /// rather than directly invoking `panic!` or similar.
812 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
813 unsafe fn realloc(&mut self,
816 new_size: usize) -> Result<NonNull<u8>, AllocErr> {
817 let old_size = layout.size();
819 if new_size >= old_size {
820 if let Ok(()) = self.grow_in_place(ptr, layout.clone(), new_size) {
823 } else if new_size < old_size {
824 if let Ok(()) = self.shrink_in_place(ptr, layout.clone(), new_size) {
829 // otherwise, fall back on alloc + copy + dealloc.
830 let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
831 let result = self.alloc(new_layout);
832 if let Ok(new_ptr) = result {
833 ptr::copy_nonoverlapping(ptr.as_ptr(),
835 cmp::min(old_size, new_size));
836 self.dealloc(ptr, layout);
841 /// Behaves like `alloc`, but also ensures that the contents
842 /// are set to zero before being returned.
846 /// This function is unsafe for the same reasons that `alloc` is.
850 /// Returning `Err` indicates that either memory is exhausted or
851 /// `layout` does not meet allocator's size or alignment
852 /// constraints, just as in `alloc`.
854 /// Clients wishing to abort computation in response to an
855 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
856 /// rather than directly invoking `panic!` or similar.
858 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
859 unsafe fn alloc_zeroed(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr> {
860 let size = layout.size();
861 let p = self.alloc(layout);
863 ptr::write_bytes(p.as_ptr(), 0, size);
868 /// Behaves like `alloc`, but also returns the whole size of
869 /// the returned block. For some `layout` inputs, like arrays, this
870 /// may include extra storage usable for additional data.
874 /// This function is unsafe for the same reasons that `alloc` is.
878 /// Returning `Err` indicates that either memory is exhausted or
879 /// `layout` does not meet allocator's size or alignment
880 /// constraints, just as in `alloc`.
882 /// Clients wishing to abort computation in response to an
883 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
884 /// rather than directly invoking `panic!` or similar.
886 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
887 unsafe fn alloc_excess(&mut self, layout: Layout) -> Result<Excess, AllocErr> {
888 let usable_size = self.usable_size(&layout);
889 self.alloc(layout).map(|p| Excess(p, usable_size.1))
892 /// Behaves like `realloc`, but also returns the whole size of
893 /// the returned block. For some `layout` inputs, like arrays, this
894 /// may include extra storage usable for additional data.
898 /// This function is unsafe for the same reasons that `realloc` is.
902 /// Returning `Err` indicates that either memory is exhausted or
903 /// `layout` does not meet allocator's size or alignment
904 /// constraints, just as in `realloc`.
906 /// Clients wishing to abort computation in response to a
907 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
908 /// rather than directly invoking `panic!` or similar.
910 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
911 unsafe fn realloc_excess(&mut self,
914 new_size: usize) -> Result<Excess, AllocErr> {
915 let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
916 let usable_size = self.usable_size(&new_layout);
917 self.realloc(ptr, layout, new_size)
918 .map(|p| Excess(p, usable_size.1))
921 /// Attempts to extend the allocation referenced by `ptr` to fit `new_size`.
923 /// If this returns `Ok`, then the allocator has asserted that the
924 /// memory block referenced by `ptr` now fits `new_size`, and thus can
925 /// be used to carry data of a layout of that size and same alignment as
926 /// `layout`. (The allocator is allowed to
927 /// expend effort to accomplish this, such as extending the memory block to
928 /// include successor blocks, or virtual memory tricks.)
930 /// Regardless of what this method returns, ownership of the
931 /// memory block referenced by `ptr` has not been transferred, and
932 /// the contents of the memory block are unaltered.
936 /// This function is unsafe because undefined behavior can result
937 /// if the caller does not ensure all of the following:
939 /// * `ptr` must be currently allocated via this allocator,
941 /// * `layout` must *fit* the `ptr` (see above); note the
942 /// `new_size` argument need not fit it,
944 /// * `new_size` must not be less than `layout.size()`,
948 /// Returns `Err(CannotReallocInPlace)` when the allocator is
949 /// unable to assert that the memory block referenced by `ptr`
950 /// could fit `layout`.
952 /// Note that one cannot pass `CannotReallocInPlace` to the `handle_alloc_error`
953 /// function; clients are expected either to be able to recover from
954 /// `grow_in_place` failures without aborting, or to fall back on
955 /// another reallocation method before resorting to an abort.
956 unsafe fn grow_in_place(&mut self,
959 new_size: usize) -> Result<(), CannotReallocInPlace> {
960 let _ = ptr; // this default implementation doesn't care about the actual address.
961 debug_assert!(new_size >= layout.size());
962 let (_l, u) = self.usable_size(&layout);
963 // _l <= layout.size() [guaranteed by usable_size()]
964 // layout.size() <= new_layout.size() [required by this method]
968 Err(CannotReallocInPlace)
972 /// Attempts to shrink the allocation referenced by `ptr` to fit `new_size`.
974 /// If this returns `Ok`, then the allocator has asserted that the
975 /// memory block referenced by `ptr` now fits `new_size`, and
976 /// thus can only be used to carry data of that smaller
977 /// layout. (The allocator is allowed to take advantage of this,
978 /// carving off portions of the block for reuse elsewhere.) The
979 /// truncated contents of the block within the smaller layout are
980 /// unaltered, and ownership of block has not been transferred.
982 /// If this returns `Err`, then the memory block is considered to
983 /// still represent the original (larger) `layout`. None of the
984 /// block has been carved off for reuse elsewhere, ownership of
985 /// the memory block has not been transferred, and the contents of
986 /// the memory block are unaltered.
990 /// This function is unsafe because undefined behavior can result
991 /// if the caller does not ensure all of the following:
993 /// * `ptr` must be currently allocated via this allocator,
995 /// * `layout` must *fit* the `ptr` (see above); note the
996 /// `new_size` argument need not fit it,
998 /// * `new_size` must not be greater than `layout.size()`
999 /// (and must be greater than zero),
1003 /// Returns `Err(CannotReallocInPlace)` when the allocator is
1004 /// unable to assert that the memory block referenced by `ptr`
1005 /// could fit `layout`.
1007 /// Note that one cannot pass `CannotReallocInPlace` to the `handle_alloc_error`
1008 /// function; clients are expected either to be able to recover from
1009 /// `shrink_in_place` failures without aborting, or to fall back
1010 /// on another reallocation method before resorting to an abort.
1011 unsafe fn shrink_in_place(&mut self,
1014 new_size: usize) -> Result<(), CannotReallocInPlace> {
1015 let _ = ptr; // this default implementation doesn't care about the actual address.
1016 debug_assert!(new_size <= layout.size());
1017 let (l, _u) = self.usable_size(&layout);
1018 // layout.size() <= _u [guaranteed by usable_size()]
1019 // new_layout.size() <= layout.size() [required by this method]
1023 Err(CannotReallocInPlace)
1028 // == COMMON USAGE PATTERNS ==
1029 // alloc_one, dealloc_one, alloc_array, realloc_array. dealloc_array
1031 /// Allocates a block suitable for holding an instance of `T`.
1033 /// Captures a common usage pattern for allocators.
1035 /// The returned block is suitable for passing to the
1036 /// `alloc`/`realloc` methods of this allocator.
1038 /// Note to implementors: If this returns `Ok(ptr)`, then `ptr`
1039 /// must be considered "currently allocated" and must be
1040 /// acceptable input to methods such as `realloc` or `dealloc`,
1041 /// *even if* `T` is a zero-sized type. In other words, if your
1042 /// `Alloc` implementation overrides this method in a manner
1043 /// that can return a zero-sized `ptr`, then all reallocation and
1044 /// deallocation methods need to be similarly overridden to accept
1045 /// such values as input.
1049 /// Returning `Err` indicates that either memory is exhausted or
1050 /// `T` does not meet allocator's size or alignment constraints.
1052 /// For zero-sized `T`, may return either of `Ok` or `Err`, but
1053 /// will *not* yield undefined behavior.
1055 /// Clients wishing to abort computation in response to an
1056 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
1057 /// rather than directly invoking `panic!` or similar.
1059 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1060 fn alloc_one<T>(&mut self) -> Result<NonNull<T>, AllocErr>
1063 let k = Layout::new::<T>();
1065 unsafe { self.alloc(k).map(|p| p.cast()) }
1071 /// Deallocates a block suitable for holding an instance of `T`.
1073 /// The given block must have been produced by this allocator,
1074 /// and must be suitable for storing a `T` (in terms of alignment
1075 /// as well as minimum and maximum size); otherwise yields
1076 /// undefined behavior.
1078 /// Captures a common usage pattern for allocators.
1082 /// This function is unsafe because undefined behavior can result
1083 /// if the caller does not ensure both:
1085 /// * `ptr` must denote a block of memory currently allocated via this allocator
1087 /// * the layout of `T` must *fit* that block of memory.
1088 unsafe fn dealloc_one<T>(&mut self, ptr: NonNull<T>)
1091 let k = Layout::new::<T>();
1093 self.dealloc(ptr.cast(), k);
1097 /// Allocates a block suitable for holding `n` instances of `T`.
1099 /// Captures a common usage pattern for allocators.
1101 /// The returned block is suitable for passing to the
1102 /// `alloc`/`realloc` methods of this allocator.
1104 /// Note to implementors: If this returns `Ok(ptr)`, then `ptr`
1105 /// must be considered "currently allocated" and must be
1106 /// acceptable input to methods such as `realloc` or `dealloc`,
1107 /// *even if* `T` is a zero-sized type. In other words, if your
1108 /// `Alloc` implementation overrides this method in a manner
1109 /// that can return a zero-sized `ptr`, then all reallocation and
1110 /// deallocation methods need to be similarly overridden to accept
1111 /// such values as input.
1115 /// Returning `Err` indicates that either memory is exhausted or
1116 /// `[T; n]` does not meet allocator's size or alignment
1119 /// For zero-sized `T` or `n == 0`, may return either of `Ok` or
1120 /// `Err`, but will *not* yield undefined behavior.
1122 /// Always returns `Err` on arithmetic overflow.
1124 /// Clients wishing to abort computation in response to an
1125 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
1126 /// rather than directly invoking `panic!` or similar.
1128 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1129 fn alloc_array<T>(&mut self, n: usize) -> Result<NonNull<T>, AllocErr>
1132 match Layout::array::<T>(n) {
1133 Ok(ref layout) if layout.size() > 0 => {
1135 self.alloc(layout.clone()).map(|p| p.cast())
1142 /// Reallocates a block previously suitable for holding `n_old`
1143 /// instances of `T`, returning a block suitable for holding
1144 /// `n_new` instances of `T`.
1146 /// Captures a common usage pattern for allocators.
1148 /// The returned block is suitable for passing to the
1149 /// `alloc`/`realloc` methods of this allocator.
1153 /// This function is unsafe because undefined behavior can result
1154 /// if the caller does not ensure all of the following:
1156 /// * `ptr` must be currently allocated via this allocator,
1158 /// * the layout of `[T; n_old]` must *fit* that block of memory.
1162 /// Returning `Err` indicates that either memory is exhausted or
1163 /// `[T; n_new]` does not meet allocator's size or alignment
1166 /// For zero-sized `T` or `n_new == 0`, may return either of `Ok` or
1167 /// `Err`, but will *not* yield undefined behavior.
1169 /// Always returns `Err` on arithmetic overflow.
1171 /// Clients wishing to abort computation in response to a
1172 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
1173 /// rather than directly invoking `panic!` or similar.
1175 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1176 unsafe fn realloc_array<T>(&mut self,
1179 n_new: usize) -> Result<NonNull<T>, AllocErr>
1182 match (Layout::array::<T>(n_old), Layout::array::<T>(n_new)) {
1183 (Ok(ref k_old), Ok(ref k_new)) if k_old.size() > 0 && k_new.size() > 0 => {
1184 debug_assert!(k_old.align() == k_new.align());
1185 self.realloc(ptr.cast(), k_old.clone(), k_new.size()).map(NonNull::cast)
1193 /// Deallocates a block suitable for holding `n` instances of `T`.
1195 /// Captures a common usage pattern for allocators.
1199 /// This function is unsafe because undefined behavior can result
1200 /// if the caller does not ensure both:
1202 /// * `ptr` must denote a block of memory currently allocated via this allocator
1204 /// * the layout of `[T; n]` must *fit* that block of memory.
1208 /// Returning `Err` indicates that either `[T; n]` or the given
1209 /// memory block does not meet allocator's size or alignment
1212 /// Always returns `Err` on arithmetic overflow.
1213 unsafe fn dealloc_array<T>(&mut self, ptr: NonNull<T>, n: usize) -> Result<(), AllocErr>
1216 match Layout::array::<T>(n) {
1217 Ok(ref k) if k.size() > 0 => {
1218 Ok(self.dealloc(ptr.cast(), k.clone()))