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.
169 /// Panics if the combination of `self.size()` and the given `align`
170 /// violates the conditions listed in
171 /// [`Layout::from_size_align`](#method.from_size_align).
172 #[unstable(feature = "allocator_api", issue = "32838")]
174 pub fn align_to(&self, align: usize) -> Self {
175 Layout::from_size_align(self.size(), cmp::max(self.align(), align)).unwrap()
178 /// Returns the amount of padding we must insert after `self`
179 /// to ensure that the following address will satisfy `align`
180 /// (measured in bytes).
182 /// E.g. if `self.size()` is 9, then `self.padding_needed_for(4)`
183 /// returns 3, because that is the minimum number of bytes of
184 /// padding required to get a 4-aligned address (assuming that the
185 /// corresponding memory block starts at a 4-aligned address).
187 /// The return value of this function has no meaning if `align` is
188 /// not a power-of-two.
190 /// Note that the utility of the returned value requires `align`
191 /// to be less than or equal to the alignment of the starting
192 /// address for the whole allocated block of memory. One way to
193 /// satisfy this constraint is to ensure `align <= self.align()`.
194 #[unstable(feature = "allocator_api", issue = "32838")]
196 pub fn padding_needed_for(&self, align: usize) -> usize {
197 let len = self.size();
199 // Rounded up value is:
200 // len_rounded_up = (len + align - 1) & !(align - 1);
201 // and then we return the padding difference: `len_rounded_up - len`.
203 // We use modular arithmetic throughout:
205 // 1. align is guaranteed to be > 0, so align - 1 is always
208 // 2. `len + align - 1` can overflow by at most `align - 1`,
209 // so the &-mask wth `!(align - 1)` will ensure that in the
210 // case of overflow, `len_rounded_up` will itself be 0.
211 // Thus the returned padding, when added to `len`, yields 0,
212 // which trivially satisfies the alignment `align`.
214 // (Of course, attempts to allocate blocks of memory whose
215 // size and padding overflow in the above manner should cause
216 // the allocator to yield an error anyway.)
218 let len_rounded_up = len.wrapping_add(align).wrapping_sub(1)
219 & !align.wrapping_sub(1);
220 len_rounded_up.wrapping_sub(len)
223 /// Creates a layout describing the record for `n` instances of
224 /// `self`, with a suitable amount of padding between each to
225 /// ensure that each instance is given its requested size and
226 /// alignment. On success, returns `(k, offs)` where `k` is the
227 /// layout of the array and `offs` is the distance between the start
228 /// of each element in the array.
230 /// On arithmetic overflow, returns `LayoutErr`.
231 #[unstable(feature = "allocator_api", issue = "32838")]
233 pub fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutErr> {
234 let padded_size = self.size().checked_add(self.padding_needed_for(self.align()))
235 .ok_or(LayoutErr { private: () })?;
236 let alloc_size = padded_size.checked_mul(n)
237 .ok_or(LayoutErr { private: () })?;
240 // self.align is already known to be valid and alloc_size has been
242 Ok((Layout::from_size_align_unchecked(alloc_size, self.align()), padded_size))
246 /// Creates a layout describing the record for `self` followed by
247 /// `next`, including any necessary padding to ensure that `next`
248 /// will be properly aligned. Note that the result layout will
249 /// satisfy the alignment properties of both `self` and `next`.
251 /// The resulting layout will be the same as that of a C struct containing
252 /// two fields with the layouts of `self` and `next`, in that order.
254 /// Returns `Some((k, offset))`, where `k` is layout of the concatenated
255 /// record and `offset` is the relative location, in bytes, of the
256 /// start of the `next` embedded within the concatenated record
257 /// (assuming that the record itself starts at offset 0).
259 /// On arithmetic overflow, returns `LayoutErr`.
260 #[unstable(feature = "allocator_api", issue = "32838")]
262 pub fn extend(&self, next: Self) -> Result<(Self, usize), LayoutErr> {
263 let new_align = cmp::max(self.align(), next.align());
264 let pad = self.padding_needed_for(next.align());
266 let offset = self.size().checked_add(pad)
267 .ok_or(LayoutErr { private: () })?;
268 let new_size = offset.checked_add(next.size())
269 .ok_or(LayoutErr { private: () })?;
271 let layout = Layout::from_size_align(new_size, new_align)?;
275 /// Creates a layout describing the record for `n` instances of
276 /// `self`, with no padding between each instance.
278 /// Note that, unlike `repeat`, `repeat_packed` does not guarantee
279 /// that the repeated instances of `self` will be properly
280 /// aligned, even if a given instance of `self` is properly
281 /// aligned. In other words, if the layout returned by
282 /// `repeat_packed` is used to allocate an array, it is not
283 /// guaranteed that all elements in the array will be properly
286 /// On arithmetic overflow, returns `LayoutErr`.
287 #[unstable(feature = "allocator_api", issue = "32838")]
289 pub fn repeat_packed(&self, n: usize) -> Result<Self, LayoutErr> {
290 let size = self.size().checked_mul(n).ok_or(LayoutErr { private: () })?;
291 Layout::from_size_align(size, self.align())
294 /// Creates a layout describing the record for `self` followed by
295 /// `next` with no additional padding between the two. Since no
296 /// padding is inserted, the alignment of `next` is irrelevant,
297 /// and is not incorporated *at all* into the resulting layout.
299 /// Returns `(k, offset)`, where `k` is layout of the concatenated
300 /// record and `offset` is the relative location, in bytes, of the
301 /// start of the `next` embedded within the concatenated record
302 /// (assuming that the record itself starts at offset 0).
304 /// (The `offset` is always the same as `self.size()`; we use this
305 /// signature out of convenience in matching the signature of
308 /// On arithmetic overflow, returns `LayoutErr`.
309 #[unstable(feature = "allocator_api", issue = "32838")]
311 pub fn extend_packed(&self, next: Self) -> Result<(Self, usize), LayoutErr> {
312 let new_size = self.size().checked_add(next.size())
313 .ok_or(LayoutErr { private: () })?;
314 let layout = Layout::from_size_align(new_size, self.align())?;
315 Ok((layout, self.size()))
318 /// Creates a layout describing the record for a `[T; n]`.
320 /// On arithmetic overflow, returns `LayoutErr`.
321 #[unstable(feature = "allocator_api", issue = "32838")]
323 pub fn array<T>(n: usize) -> Result<Self, LayoutErr> {
327 debug_assert!(offs == mem::size_of::<T>());
333 /// The parameters given to `Layout::from_size_align`
334 /// or some other `Layout` constructor
335 /// do not satisfy its documented constraints.
336 #[stable(feature = "alloc_layout", since = "1.28.0")]
337 #[derive(Clone, PartialEq, Eq, Debug)]
338 pub struct LayoutErr {
342 // (we need this for downstream impl of trait Error)
343 #[stable(feature = "alloc_layout", since = "1.28.0")]
344 impl fmt::Display for LayoutErr {
345 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
346 f.write_str("invalid parameters to Layout::from_size_align")
350 /// The `AllocErr` error indicates an allocation failure
351 /// that may be due to resource exhaustion or to
352 /// something wrong when combining the given input arguments with this
354 #[unstable(feature = "allocator_api", issue = "32838")]
355 #[derive(Clone, PartialEq, Eq, Debug)]
358 // (we need this for downstream impl of trait Error)
359 #[unstable(feature = "allocator_api", issue = "32838")]
360 impl fmt::Display for AllocErr {
361 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
362 f.write_str("memory allocation failed")
366 /// The `CannotReallocInPlace` error is used when `grow_in_place` or
367 /// `shrink_in_place` were unable to reuse the given memory block for
368 /// a requested layout.
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 /// though the `#[global_allocator]` attributes.
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 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 allocated 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 /// Shink 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
530 /// If this method returns null, then ownership of the memory
531 /// block has not been transferred to this allocator, and the
532 /// contents of the memory block are unaltered.
536 /// This function is unsafe because undefined behavior can result
537 /// if the caller does not ensure all of the following:
539 /// * `ptr` must be currently allocated via this allocator,
541 /// * `layout` must be the same layout that was used
542 /// to allocated that block of memory,
544 /// * `new_size` must be greater than zero.
546 /// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
547 /// must not overflow (i.e. the rounded value must be less than `usize::MAX`).
549 /// (Extension subtraits might provide more specific bounds on
550 /// behavior, e.g. guarantee a sentinel address or a null pointer
551 /// in response to a zero-size allocation request.)
555 /// Returns null if the new layout does not meet the size
556 /// and alignment constraints of the allocator, or if reallocation
559 /// Implementations are encouraged to return null on memory
560 /// exhaustion rather than panicking or aborting, but this is not
561 /// a strict requirement. (Specifically: it is *legal* to
562 /// implement this trait atop an underlying native allocation
563 /// library that aborts on memory exhaustion.)
565 /// Clients wishing to abort computation in response to a
566 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
567 /// rather than directly invoking `panic!` or similar.
569 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
570 #[stable(feature = "global_alloc", since = "1.28.0")]
571 unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
572 let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
573 let new_ptr = self.alloc(new_layout);
574 if !new_ptr.is_null() {
575 ptr::copy_nonoverlapping(
578 cmp::min(layout.size(), new_size),
580 self.dealloc(ptr, layout);
586 /// An implementation of `Alloc` can allocate, reallocate, and
587 /// deallocate arbitrary blocks of data described via `Layout`.
589 /// Some of the methods require that a memory block be *currently
590 /// allocated* via an allocator. This means that:
592 /// * the starting address for that memory block was previously
593 /// returned by a previous call to an allocation method (`alloc`,
594 /// `alloc_zeroed`, `alloc_excess`, `alloc_one`, `alloc_array`) or
595 /// reallocation method (`realloc`, `realloc_excess`, or
596 /// `realloc_array`), and
598 /// * the memory block has not been subsequently deallocated, where
599 /// blocks are deallocated either by being passed to a deallocation
600 /// method (`dealloc`, `dealloc_one`, `dealloc_array`) or by being
601 /// passed to a reallocation method (see above) that returns `Ok`.
603 /// A note regarding zero-sized types and zero-sized layouts: many
604 /// methods in the `Alloc` trait state that allocation requests
605 /// must be non-zero size, or else undefined behavior can result.
607 /// * However, some higher-level allocation methods (`alloc_one`,
608 /// `alloc_array`) are well-defined on zero-sized types and can
609 /// optionally support them: it is left up to the implementor
610 /// whether to return `Err`, or to return `Ok` with some pointer.
612 /// * If an `Alloc` implementation chooses to return `Ok` in this
613 /// case (i.e. the pointer denotes a zero-sized inaccessible block)
614 /// then that returned pointer must be considered "currently
615 /// allocated". On such an allocator, *all* methods that take
616 /// currently-allocated pointers as inputs must accept these
617 /// zero-sized pointers, *without* causing undefined behavior.
619 /// * In other words, if a zero-sized pointer can flow out of an
620 /// allocator, then that allocator must likewise accept that pointer
621 /// flowing back into its deallocation and reallocation methods.
623 /// Some of the methods require that a layout *fit* a memory block.
624 /// What it means for a layout to "fit" a memory block means (or
625 /// equivalently, for a memory block to "fit" a layout) is that the
626 /// following two conditions must hold:
628 /// 1. The block's starting address must be aligned to `layout.align()`.
630 /// 2. The block's size must fall in the range `[use_min, use_max]`, where:
632 /// * `use_min` is `self.usable_size(layout).0`, and
634 /// * `use_max` is the capacity that was (or would have been)
635 /// returned when (if) the block was allocated via a call to
636 /// `alloc_excess` or `realloc_excess`.
640 /// * the size of the layout most recently used to allocate the block
641 /// is guaranteed to be in the range `[use_min, use_max]`, and
643 /// * a lower-bound on `use_max` can be safely approximated by a call to
646 /// * if a layout `k` fits a memory block (denoted by `ptr`)
647 /// currently allocated via an allocator `a`, then it is legal to
648 /// use that layout to deallocate it, i.e. `a.dealloc(ptr, k);`.
652 /// The `Alloc` trait is an `unsafe` trait for a number of reasons, and
653 /// implementors must ensure that they adhere to these contracts:
655 /// * Pointers returned from allocation functions must point to valid memory and
656 /// retain their validity until at least the instance of `Alloc` is dropped
659 /// * `Layout` queries and calculations in general must be correct. Callers of
660 /// this trait are allowed to rely on the contracts defined on each method,
661 /// and implementors must ensure such contracts remain true.
663 /// Note that this list may get tweaked over time as clarifications are made in
665 #[unstable(feature = "allocator_api", issue = "32838")]
666 pub unsafe trait Alloc {
668 // (Note: some existing allocators have unspecified but well-defined
669 // behavior in response to a zero size allocation request ;
670 // e.g. in C, `malloc` of 0 will either return a null pointer or a
671 // unique pointer, but will not have arbitrary undefined
673 // However in jemalloc for example,
674 // `mallocx(0)` is documented as undefined behavior.)
676 /// Returns a pointer meeting the size and alignment guarantees of
679 /// If this method returns an `Ok(addr)`, then the `addr` returned
680 /// will be non-null address pointing to a block of storage
681 /// suitable for holding an instance of `layout`.
683 /// The returned block of storage may or may not have its contents
684 /// initialized. (Extension subtraits might restrict this
685 /// behavior, e.g. to ensure initialization to particular sets of
690 /// This function is unsafe because undefined behavior can result
691 /// if the caller does not ensure that `layout` has non-zero size.
693 /// (Extension subtraits might provide more specific bounds on
694 /// behavior, e.g. guarantee a sentinel address or a null pointer
695 /// in response to a zero-size allocation request.)
699 /// Returning `Err` indicates that either memory is exhausted or
700 /// `layout` does not meet allocator's size or alignment
703 /// Implementations are encouraged to return `Err` on memory
704 /// exhaustion rather than panicking or aborting, but this is not
705 /// a strict requirement. (Specifically: it is *legal* to
706 /// implement this trait atop an underlying native allocation
707 /// library that aborts on memory exhaustion.)
709 /// Clients wishing to abort computation in response to an
710 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
711 /// rather than directly invoking `panic!` or similar.
713 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
714 unsafe fn alloc(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr>;
716 /// Deallocate the memory referenced by `ptr`.
720 /// This function is unsafe because undefined behavior can result
721 /// if the caller does not ensure all of the following:
723 /// * `ptr` must denote a block of memory currently allocated via
726 /// * `layout` must *fit* that block of memory,
728 /// * In addition to fitting the block of memory `layout`, the
729 /// alignment of the `layout` must match the alignment used
730 /// to allocate that block of memory.
731 unsafe fn dealloc(&mut self, ptr: NonNull<u8>, layout: Layout);
733 // == ALLOCATOR-SPECIFIC QUANTITIES AND LIMITS ==
736 /// Returns bounds on the guaranteed usable size of a successful
737 /// allocation created with the specified `layout`.
739 /// In particular, if one has a memory block allocated via a given
740 /// allocator `a` and layout `k` where `a.usable_size(k)` returns
741 /// `(l, u)`, then one can pass that block to `a.dealloc()` with a
742 /// layout in the size range [l, u].
744 /// (All implementors of `usable_size` must ensure that
745 /// `l <= k.size() <= u`)
747 /// Both the lower- and upper-bounds (`l` and `u` respectively)
748 /// are provided, because an allocator based on size classes could
749 /// misbehave if one attempts to deallocate a block without
750 /// providing a correct value for its size (i.e., one within the
753 /// Clients who wish to make use of excess capacity are encouraged
754 /// to use the `alloc_excess` and `realloc_excess` instead, as
755 /// this method is constrained to report conservative values that
756 /// serve as valid bounds for *all possible* allocation method
759 /// However, for clients that do not wish to track the capacity
760 /// returned by `alloc_excess` locally, this method is likely to
761 /// produce useful results.
763 fn usable_size(&self, layout: &Layout) -> (usize, usize) {
764 (layout.size(), layout.size())
767 // == METHODS FOR MEMORY REUSE ==
768 // realloc. alloc_excess, realloc_excess
770 /// Returns a pointer suitable for holding data described by
771 /// a new layout with `layout`’s alginment and a size given
772 /// by `new_size`. To
773 /// accomplish this, this may extend or shrink the allocation
774 /// referenced by `ptr` to fit the new layout.
776 /// If this returns `Ok`, then ownership of the memory block
777 /// referenced by `ptr` has been transferred to this
778 /// allocator. The memory may or may not have been freed, and
779 /// should be considered unusable (unless of course it was
780 /// transferred back to the caller again via the return value of
783 /// If this method returns `Err`, then ownership of the memory
784 /// block has not been transferred to this allocator, and the
785 /// contents of the memory block are unaltered.
789 /// This function is unsafe because undefined behavior can result
790 /// if the caller does not ensure all of the following:
792 /// * `ptr` must be currently allocated via this allocator,
794 /// * `layout` must *fit* the `ptr` (see above). (The `new_size`
795 /// argument need not fit it.)
797 /// * `new_size` must be greater than zero.
799 /// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
800 /// must not overflow (i.e. the rounded value must be less than `usize::MAX`).
802 /// (Extension subtraits might provide more specific bounds on
803 /// behavior, e.g. guarantee a sentinel address or a null pointer
804 /// in response to a zero-size allocation request.)
808 /// Returns `Err` only if the new layout
809 /// does not meet the allocator's size
810 /// and alignment constraints of the allocator, or if reallocation
813 /// Implementations are encouraged to return `Err` on memory
814 /// exhaustion rather than panicking or aborting, but this is not
815 /// a strict requirement. (Specifically: it is *legal* to
816 /// implement this trait atop an underlying native allocation
817 /// library that aborts on memory exhaustion.)
819 /// Clients wishing to abort computation in response to a
820 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
821 /// rather than directly invoking `panic!` or similar.
823 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
824 unsafe fn realloc(&mut self,
827 new_size: usize) -> 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.clone(), new_size) {
834 } else if new_size < old_size {
835 if let Ok(()) = self.shrink_in_place(ptr, layout.clone(), 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(),
846 cmp::min(old_size, new_size));
847 self.dealloc(ptr, layout);
852 /// Behaves like `alloc`, but also ensures that the contents
853 /// are set to zero before being returned.
857 /// This function is unsafe for the same reasons that `alloc` is.
861 /// Returning `Err` indicates that either memory is exhausted or
862 /// `layout` does not meet allocator's size or alignment
863 /// constraints, just as in `alloc`.
865 /// Clients wishing to abort computation in response to an
866 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
867 /// rather than directly invoking `panic!` or similar.
869 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
870 unsafe fn alloc_zeroed(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr> {
871 let size = layout.size();
872 let p = self.alloc(layout);
874 ptr::write_bytes(p.as_ptr(), 0, size);
879 /// Behaves like `alloc`, but also returns the whole size of
880 /// the returned block. For some `layout` inputs, like arrays, this
881 /// may include extra storage usable for additional data.
885 /// This function is unsafe for the same reasons that `alloc` is.
889 /// Returning `Err` indicates that either memory is exhausted or
890 /// `layout` does not meet allocator's size or alignment
891 /// constraints, just as in `alloc`.
893 /// Clients wishing to abort computation in response to an
894 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
895 /// rather than directly invoking `panic!` or similar.
897 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
898 unsafe fn alloc_excess(&mut self, layout: Layout) -> Result<Excess, AllocErr> {
899 let usable_size = self.usable_size(&layout);
900 self.alloc(layout).map(|p| Excess(p, usable_size.1))
903 /// Behaves like `realloc`, but also returns the whole size of
904 /// the returned block. For some `layout` inputs, like arrays, this
905 /// may include extra storage usable for additional data.
909 /// This function is unsafe for the same reasons that `realloc` is.
913 /// Returning `Err` indicates that either memory is exhausted or
914 /// `layout` does not meet allocator's size or alignment
915 /// constraints, just as in `realloc`.
917 /// Clients wishing to abort computation in response to a
918 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
919 /// rather than directly invoking `panic!` or similar.
921 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
922 unsafe fn realloc_excess(&mut self,
925 new_size: usize) -> 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)
929 .map(|p| Excess(p, usable_size.1))
932 /// Attempts to extend the allocation referenced by `ptr` to fit `new_size`.
934 /// If this returns `Ok`, then the allocator has asserted that the
935 /// memory block referenced by `ptr` now fits `new_size`, and thus can
936 /// be used to carry data of a layout of that size and same alignment as
937 /// `layout`. (The allocator is allowed to
938 /// expend effort to accomplish this, such as extending the memory block to
939 /// include successor blocks, or virtual memory tricks.)
941 /// Regardless of what this method returns, ownership of the
942 /// memory block referenced by `ptr` has not been transferred, and
943 /// the contents of the memory block are unaltered.
947 /// This function is unsafe because undefined behavior can result
948 /// if the caller does not ensure all of the following:
950 /// * `ptr` must be currently allocated via this allocator,
952 /// * `layout` must *fit* the `ptr` (see above); note the
953 /// `new_size` argument need not fit it,
955 /// * `new_size` must not be less than `layout.size()`,
959 /// Returns `Err(CannotReallocInPlace)` when the allocator is
960 /// unable to assert that the memory block referenced by `ptr`
961 /// could fit `layout`.
963 /// Note that one cannot pass `CannotReallocInPlace` to the `handle_alloc_error`
964 /// function; clients are expected either to be able to recover from
965 /// `grow_in_place` failures without aborting, or to fall back on
966 /// another reallocation method before resorting to an abort.
967 unsafe fn grow_in_place(&mut self,
970 new_size: usize) -> Result<(), CannotReallocInPlace> {
971 let _ = ptr; // this default implementation doesn't care about the actual address.
972 debug_assert!(new_size >= layout.size());
973 let (_l, u) = self.usable_size(&layout);
974 // _l <= layout.size() [guaranteed by usable_size()]
975 // layout.size() <= new_layout.size() [required by this method]
979 Err(CannotReallocInPlace)
983 /// Attempts to shrink the allocation referenced by `ptr` to fit `new_size`.
985 /// If this returns `Ok`, then the allocator has asserted that the
986 /// memory block referenced by `ptr` now fits `new_size`, and
987 /// thus can only be used to carry data of that smaller
988 /// layout. (The allocator is allowed to take advantage of this,
989 /// carving off portions of the block for reuse elsewhere.) The
990 /// truncated contents of the block within the smaller layout are
991 /// unaltered, and ownership of block has not been transferred.
993 /// If this returns `Err`, then the memory block is considered to
994 /// still represent the original (larger) `layout`. None of the
995 /// block has been carved off for reuse elsewhere, ownership of
996 /// the memory block has not been transferred, and the contents of
997 /// the memory block are unaltered.
1001 /// This function is unsafe because undefined behavior can result
1002 /// if the caller does not ensure all of the following:
1004 /// * `ptr` must be currently allocated via this allocator,
1006 /// * `layout` must *fit* the `ptr` (see above); note the
1007 /// `new_size` argument need not fit it,
1009 /// * `new_size` must not be greater than `layout.size()`
1010 /// (and must be greater than zero),
1014 /// Returns `Err(CannotReallocInPlace)` when the allocator is
1015 /// unable to assert that the memory block referenced by `ptr`
1016 /// could fit `layout`.
1018 /// Note that one cannot pass `CannotReallocInPlace` to the `handle_alloc_error`
1019 /// function; clients are expected either to be able to recover from
1020 /// `shrink_in_place` failures without aborting, or to fall back
1021 /// on another reallocation method before resorting to an abort.
1022 unsafe fn shrink_in_place(&mut self,
1025 new_size: usize) -> Result<(), CannotReallocInPlace> {
1026 let _ = ptr; // this default implementation doesn't care about the actual address.
1027 debug_assert!(new_size <= layout.size());
1028 let (l, _u) = self.usable_size(&layout);
1029 // layout.size() <= _u [guaranteed by usable_size()]
1030 // new_layout.size() <= layout.size() [required by this method]
1034 Err(CannotReallocInPlace)
1039 // == COMMON USAGE PATTERNS ==
1040 // alloc_one, dealloc_one, alloc_array, realloc_array. dealloc_array
1042 /// Allocates a block suitable for holding an instance of `T`.
1044 /// Captures a common usage pattern for allocators.
1046 /// The returned block is suitable for passing to the
1047 /// `alloc`/`realloc` methods of this allocator.
1049 /// Note to implementors: If this returns `Ok(ptr)`, then `ptr`
1050 /// must be considered "currently allocated" and must be
1051 /// acceptable input to methods such as `realloc` or `dealloc`,
1052 /// *even if* `T` is a zero-sized type. In other words, if your
1053 /// `Alloc` implementation overrides this method in a manner
1054 /// that can return a zero-sized `ptr`, then all reallocation and
1055 /// deallocation methods need to be similarly overridden to accept
1056 /// such values as input.
1060 /// Returning `Err` indicates that either memory is exhausted or
1061 /// `T` does not meet allocator's size or alignment constraints.
1063 /// For zero-sized `T`, may return either of `Ok` or `Err`, but
1064 /// will *not* yield undefined behavior.
1066 /// Clients wishing to abort computation in response to an
1067 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
1068 /// rather than directly invoking `panic!` or similar.
1070 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1071 fn alloc_one<T>(&mut self) -> Result<NonNull<T>, AllocErr>
1074 let k = Layout::new::<T>();
1076 unsafe { self.alloc(k).map(|p| p.cast()) }
1082 /// Deallocates a block suitable for holding an instance of `T`.
1084 /// The given block must have been produced by this allocator,
1085 /// and must be suitable for storing a `T` (in terms of alignment
1086 /// as well as minimum and maximum size); otherwise yields
1087 /// undefined behavior.
1089 /// Captures a common usage pattern for allocators.
1093 /// This function is unsafe because undefined behavior can result
1094 /// if the caller does not ensure both:
1096 /// * `ptr` must denote a block of memory currently allocated via this allocator
1098 /// * the layout of `T` must *fit* that block of memory.
1099 unsafe fn dealloc_one<T>(&mut self, ptr: NonNull<T>)
1102 let k = Layout::new::<T>();
1104 self.dealloc(ptr.cast(), k);
1108 /// Allocates a block suitable for holding `n` instances of `T`.
1110 /// Captures a common usage pattern for allocators.
1112 /// The returned block is suitable for passing to the
1113 /// `alloc`/`realloc` methods of this allocator.
1115 /// Note to implementors: If this returns `Ok(ptr)`, then `ptr`
1116 /// must be considered "currently allocated" and must be
1117 /// acceptable input to methods such as `realloc` or `dealloc`,
1118 /// *even if* `T` is a zero-sized type. In other words, if your
1119 /// `Alloc` implementation overrides this method in a manner
1120 /// that can return a zero-sized `ptr`, then all reallocation and
1121 /// deallocation methods need to be similarly overridden to accept
1122 /// such values as input.
1126 /// Returning `Err` indicates that either memory is exhausted or
1127 /// `[T; n]` does not meet allocator's size or alignment
1130 /// For zero-sized `T` or `n == 0`, may return either of `Ok` or
1131 /// `Err`, but will *not* yield undefined behavior.
1133 /// Always returns `Err` on arithmetic overflow.
1135 /// Clients wishing to abort computation in response to an
1136 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
1137 /// rather than directly invoking `panic!` or similar.
1139 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1140 fn alloc_array<T>(&mut self, n: usize) -> Result<NonNull<T>, AllocErr>
1143 match Layout::array::<T>(n) {
1144 Ok(ref layout) if layout.size() > 0 => {
1146 self.alloc(layout.clone()).map(|p| p.cast())
1153 /// Reallocates a block previously suitable for holding `n_old`
1154 /// instances of `T`, returning a block suitable for holding
1155 /// `n_new` instances of `T`.
1157 /// Captures a common usage pattern for allocators.
1159 /// The returned block is suitable for passing to the
1160 /// `alloc`/`realloc` methods of this allocator.
1164 /// This function is unsafe because undefined behavior can result
1165 /// if the caller does not ensure all of the following:
1167 /// * `ptr` must be currently allocated via this allocator,
1169 /// * the layout of `[T; n_old]` must *fit* that block of memory.
1173 /// Returning `Err` indicates that either memory is exhausted or
1174 /// `[T; n_new]` does not meet allocator's size or alignment
1177 /// For zero-sized `T` or `n_new == 0`, may return either of `Ok` or
1178 /// `Err`, but will *not* yield undefined behavior.
1180 /// Always returns `Err` on arithmetic overflow.
1182 /// Clients wishing to abort computation in response to a
1183 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
1184 /// rather than directly invoking `panic!` or similar.
1186 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1187 unsafe fn realloc_array<T>(&mut self,
1190 n_new: usize) -> Result<NonNull<T>, AllocErr>
1193 match (Layout::array::<T>(n_old), Layout::array::<T>(n_new)) {
1194 (Ok(ref k_old), Ok(ref k_new)) if k_old.size() > 0 && k_new.size() > 0 => {
1195 debug_assert!(k_old.align() == k_new.align());
1196 self.realloc(ptr.cast(), k_old.clone(), k_new.size()).map(NonNull::cast)
1204 /// Deallocates a block suitable for holding `n` instances of `T`.
1206 /// Captures a common usage pattern for allocators.
1210 /// This function is unsafe because undefined behavior can result
1211 /// if the caller does not ensure both:
1213 /// * `ptr` must denote a block of memory currently allocated via this allocator
1215 /// * the layout of `[T; n]` must *fit* that block of memory.
1219 /// Returning `Err` indicates that either `[T; n]` or the given
1220 /// memory block does not meet allocator's size or alignment
1223 /// Always returns `Err` on arithmetic overflow.
1224 unsafe fn dealloc_array<T>(&mut self, ptr: NonNull<T>, n: usize) -> Result<(), AllocErr>
1227 match Layout::array::<T>(n) {
1228 Ok(ref k) if k.size() > 0 => {
1229 Ok(self.dealloc(ptr.cast(), k.clone()))