1 //! Memory allocation APIs
3 // ignore-tidy-undocumented-unsafe
5 #![stable(feature = "alloc_module", since = "1.28.0")]
11 use crate::ptr
::{self, NonNull}
;
12 use crate::num
::NonZeroUsize
;
14 /// Represents the combination of a starting address and
15 /// a total capacity of the returned block.
16 #[unstable(feature = "allocator_api", issue = "32838")]
18 pub struct Excess(pub NonNull
<u8>, pub usize);
20 fn size_align
<T
>() -> (usize, usize) {
21 (mem
::size_of
::<T
>(), mem
::align_of
::<T
>())
24 /// Layout of a block of memory.
26 /// An instance of `Layout` describes a particular layout of memory.
27 /// You build a `Layout` up as an input to give to an allocator.
29 /// All layouts have an associated non-negative size and a
30 /// power-of-two alignment.
32 /// (Note however that layouts are *not* required to have positive
33 /// size, even though many allocators require that all memory
34 /// requests have positive size. A caller to the `Alloc::alloc`
35 /// method must either ensure that conditions like this are met, or
36 /// use specific allocators with looser requirements.)
37 #[stable(feature = "alloc_layout", since = "1.28.0")]
38 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
39 #[lang = "alloc_layout"]
41 // size of the requested block of memory, measured in bytes.
44 // alignment of the requested block of memory, measured in bytes.
45 // we ensure that this is always a power-of-two, because API's
46 // like `posix_memalign` require it and it is a reasonable
47 // constraint to impose on Layout constructors.
49 // (However, we do not analogously require `align >= sizeof(void*)`,
50 // even though that is *also* a requirement of `posix_memalign`.)
55 /// Constructs a `Layout` from a given `size` and `align`,
56 /// or returns `LayoutErr` if any of the following conditions
59 /// * `align` must not be zero,
61 /// * `align` must be a power of two,
63 /// * `size`, when rounded up to the nearest multiple of `align`,
64 /// must not overflow (i.e., the rounded value must be less than
66 #[stable(feature = "alloc_layout", since = "1.28.0")]
68 pub fn from_size_align(size
: usize, align
: usize) -> Result
<Self, LayoutErr
> {
69 if !align
.is_power_of_two() {
70 return Err(LayoutErr { private: () }
);
73 // (power-of-two implies align != 0.)
75 // Rounded up size is:
76 // size_rounded_up = (size + align - 1) & !(align - 1);
78 // We know from above that align != 0. If adding (align - 1)
79 // does not overflow, then rounding up will be fine.
81 // Conversely, &-masking with !(align - 1) will subtract off
82 // only low-order-bits. Thus if overflow occurs with the sum,
83 // the &-mask cannot subtract enough to undo that overflow.
85 // Above implies that checking for summation overflow is both
86 // necessary and sufficient.
87 if size
> usize::MAX
- (align
- 1) {
88 return Err(LayoutErr { private: () }
);
92 Ok(Layout
::from_size_align_unchecked(size
, align
))
96 /// Creates a layout, bypassing all checks.
100 /// This function is unsafe as it does not verify the preconditions from
101 /// [`Layout::from_size_align`](#method.from_size_align).
102 #[stable(feature = "alloc_layout", since = "1.28.0")]
103 #[cfg_attr(not(bootstrap), rustc_const_stable(feature = "alloc_layout", since = "1.28.0"))]
105 pub const unsafe fn from_size_align_unchecked(size
: usize, align
: usize) -> Self {
106 Layout { size_: size, align_: NonZeroUsize::new_unchecked(align) }
109 /// The minimum size in bytes for a memory block of this layout.
110 #[stable(feature = "alloc_layout", since = "1.28.0")]
112 pub fn size(&self) -> usize { self.size_ }
114 /// The minimum byte alignment for a memory block of this layout.
115 #[stable(feature = "alloc_layout", since = "1.28.0")]
117 pub fn align(&self) -> usize { self.align_.get() }
119 /// Constructs a `Layout` suitable for holding a value of type `T`.
120 #[stable(feature = "alloc_layout", since = "1.28.0")]
122 pub fn new
<T
>() -> Self {
123 let (size
, align
) = size_align
::<T
>();
124 // Note that the align is guaranteed by rustc to be a power of two and
125 // the size+align combo is guaranteed to fit in our address space. As a
126 // result use the unchecked constructor here to avoid inserting code
127 // that panics if it isn't optimized well enough.
128 debug_assert
!(Layout
::from_size_align(size
, align
).is_ok());
130 Layout
::from_size_align_unchecked(size
, align
)
134 /// Produces layout describing a record that could be used to
135 /// allocate backing structure for `T` (which could be a trait
136 /// or other unsized type like a slice).
137 #[stable(feature = "alloc_layout", since = "1.28.0")]
139 pub fn for_value
<T
: ?Sized
>(t
: &T
) -> Self {
140 let (size
, align
) = (mem
::size_of_val(t
), mem
::align_of_val(t
));
141 // See rationale in `new` for why this is using an unsafe variant below
142 debug_assert
!(Layout
::from_size_align(size
, align
).is_ok());
144 Layout
::from_size_align_unchecked(size
, align
)
148 /// Creates a layout describing the record that can hold a value
149 /// of the same layout as `self`, but that also is aligned to
150 /// alignment `align` (measured in bytes).
152 /// If `self` already meets the prescribed alignment, then returns
155 /// Note that this method does not add any padding to the overall
156 /// size, regardless of whether the returned layout has a different
157 /// alignment. In other words, if `K` has size 16, `K.align_to(32)`
158 /// will *still* have size 16.
160 /// Returns an error if the combination of `self.size()` and the given
161 /// `align` violates the conditions listed in
162 /// [`Layout::from_size_align`](#method.from_size_align).
163 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
165 pub fn align_to(&self, align
: usize) -> Result
<Self, LayoutErr
> {
166 Layout
::from_size_align(self.size(), cmp
::max(self.align(), align
))
169 /// Returns the amount of padding we must insert after `self`
170 /// to ensure that the following address will satisfy `align`
171 /// (measured in bytes).
173 /// e.g., if `self.size()` is 9, then `self.padding_needed_for(4)`
174 /// returns 3, because that is the minimum number of bytes of
175 /// padding required to get a 4-aligned address (assuming that the
176 /// corresponding memory block starts at a 4-aligned address).
178 /// The return value of this function has no meaning if `align` is
179 /// not a power-of-two.
181 /// Note that the utility of the returned value requires `align`
182 /// to be less than or equal to the alignment of the starting
183 /// address for the whole allocated block of memory. One way to
184 /// satisfy this constraint is to ensure `align <= self.align()`.
185 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
187 pub fn padding_needed_for(&self, align
: usize) -> usize {
188 let len
= self.size();
190 // Rounded up value is:
191 // len_rounded_up = (len + align - 1) & !(align - 1);
192 // and then we return the padding difference: `len_rounded_up - len`.
194 // We use modular arithmetic throughout:
196 // 1. align is guaranteed to be > 0, so align - 1 is always
199 // 2. `len + align - 1` can overflow by at most `align - 1`,
200 // so the &-mask with `!(align - 1)` will ensure that in the
201 // case of overflow, `len_rounded_up` will itself be 0.
202 // Thus the returned padding, when added to `len`, yields 0,
203 // which trivially satisfies the alignment `align`.
205 // (Of course, attempts to allocate blocks of memory whose
206 // size and padding overflow in the above manner should cause
207 // the allocator to yield an error anyway.)
209 let len_rounded_up
= len
.wrapping_add(align
).wrapping_sub(1)
210 & !align
.wrapping_sub(1);
211 len_rounded_up
.wrapping_sub(len
)
214 /// Creates a layout by rounding the size of this layout up to a multiple
215 /// of the layout's alignment.
217 /// This is equivalent to adding the result of `padding_needed_for`
218 /// to the layout's current size.
219 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
221 pub fn pad_to_align(&self) -> Layout
{
222 let pad
= self.padding_needed_for(self.align());
223 // This cannot overflow. Quoting from the invariant of Layout:
224 // > `size`, when rounded up to the nearest multiple of `align`,
225 // > must not overflow (i.e., the rounded value must be less than
227 let new_size
= self.size() + pad
;
229 Layout
::from_size_align(new_size
, self.align()).unwrap()
232 /// Creates a layout describing the record for `n` instances of
233 /// `self`, with a suitable amount of padding between each to
234 /// ensure that each instance is given its requested size and
235 /// alignment. On success, returns `(k, offs)` where `k` is the
236 /// layout of the array and `offs` is the distance between the start
237 /// of each element in the array.
239 /// On arithmetic overflow, returns `LayoutErr`.
240 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
242 pub fn repeat(&self, n
: usize) -> Result
<(Self, usize), LayoutErr
> {
243 // Warning, removing the checked_add here led to segfaults in #67174. Further
244 // analysis in #69225 seems to indicate that this is an LTO-related
245 // miscompilation, so #67174 might be able to be reapplied in the future.
246 let padded_size
= self
248 .checked_add(self.padding_needed_for(self.align()))
249 .ok_or(LayoutErr { private: () }
)?
;
250 let alloc_size
= padded_size
.checked_mul(n
).ok_or(LayoutErr { private: () }
)?
;
253 // self.align is already known to be valid and alloc_size has been
255 Ok((Layout
::from_size_align_unchecked(alloc_size
, self.align()), padded_size
))
259 /// Creates a layout describing the record for `self` followed by
260 /// `next`, including any necessary padding to ensure that `next`
261 /// will be properly aligned. Note that the resulting layout will
262 /// satisfy the alignment properties of both `self` and `next`.
264 /// The resulting layout will be the same as that of a C struct containing
265 /// two fields with the layouts of `self` and `next`, in that order.
267 /// Returns `Some((k, offset))`, where `k` is layout of the concatenated
268 /// record and `offset` is the relative location, in bytes, of the
269 /// start of the `next` embedded within the concatenated record
270 /// (assuming that the record itself starts at offset 0).
272 /// On arithmetic overflow, returns `LayoutErr`.
273 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
275 pub fn extend(&self, next
: Self) -> Result
<(Self, usize), LayoutErr
> {
276 let new_align
= cmp
::max(self.align(), next
.align());
277 let pad
= self.padding_needed_for(next
.align());
279 let offset
= self.size().checked_add(pad
)
280 .ok_or(LayoutErr { private: () }
)?
;
281 let new_size
= offset
.checked_add(next
.size())
282 .ok_or(LayoutErr { private: () }
)?
;
284 let layout
= Layout
::from_size_align(new_size
, new_align
)?
;
288 /// Creates a layout describing the record for `n` instances of
289 /// `self`, with no padding between each instance.
291 /// Note that, unlike `repeat`, `repeat_packed` does not guarantee
292 /// that the repeated instances of `self` will be properly
293 /// aligned, even if a given instance of `self` is properly
294 /// aligned. In other words, if the layout returned by
295 /// `repeat_packed` is used to allocate an array, it is not
296 /// guaranteed that all elements in the array will be properly
299 /// On arithmetic overflow, returns `LayoutErr`.
300 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
302 pub fn repeat_packed(&self, n
: usize) -> Result
<Self, LayoutErr
> {
303 let size
= self.size().checked_mul(n
).ok_or(LayoutErr { private: () }
)?
;
304 Layout
::from_size_align(size
, self.align())
307 /// Creates a layout describing the record for `self` followed by
308 /// `next` with no additional padding between the two. Since no
309 /// padding is inserted, the alignment of `next` is irrelevant,
310 /// and is not incorporated *at all* into the resulting layout.
312 /// On arithmetic overflow, returns `LayoutErr`.
313 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
315 pub fn extend_packed(&self, next
: Self) -> Result
<Self, LayoutErr
> {
316 let new_size
= self.size().checked_add(next
.size())
317 .ok_or(LayoutErr { private: () }
)?
;
318 Layout
::from_size_align(new_size
, self.align())
321 /// Creates a layout describing the record for a `[T; n]`.
323 /// On arithmetic overflow, returns `LayoutErr`.
324 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
326 pub fn array
<T
>(n
: usize) -> Result
<Self, LayoutErr
> {
330 debug_assert
!(offs
== mem
::size_of
::<T
>());
336 /// The parameters given to `Layout::from_size_align`
337 /// or some other `Layout` constructor
338 /// do not satisfy its documented constraints.
339 #[stable(feature = "alloc_layout", since = "1.28.0")]
340 #[derive(Clone, PartialEq, Eq, Debug)]
341 pub struct LayoutErr
{
345 // (we need this for downstream impl of trait Error)
346 #[stable(feature = "alloc_layout", since = "1.28.0")]
347 impl fmt
::Display
for LayoutErr
{
348 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
349 f
.write_str("invalid parameters to Layout::from_size_align")
353 /// The `AllocErr` error indicates an allocation failure
354 /// that may be due to resource exhaustion or to
355 /// something wrong when combining the given input arguments with this
357 #[unstable(feature = "allocator_api", issue = "32838")]
358 #[derive(Clone, PartialEq, Eq, Debug)]
361 // (we need this for downstream impl of trait Error)
362 #[unstable(feature = "allocator_api", issue = "32838")]
363 impl fmt
::Display
for AllocErr
{
364 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
365 f
.write_str("memory allocation failed")
369 /// The `CannotReallocInPlace` error is used when [`grow_in_place`] or
370 /// [`shrink_in_place`] were unable to reuse the given memory block for
371 /// a requested layout.
373 /// [`grow_in_place`]: ./trait.Alloc.html#method.grow_in_place
374 /// [`shrink_in_place`]: ./trait.Alloc.html#method.shrink_in_place
375 #[unstable(feature = "allocator_api", issue = "32838")]
376 #[derive(Clone, PartialEq, Eq, Debug)]
377 pub struct CannotReallocInPlace
;
379 #[unstable(feature = "allocator_api", issue = "32838")]
380 impl CannotReallocInPlace
{
381 pub fn description(&self) -> &str {
382 "cannot reallocate allocator's memory in place"
386 // (we need this for downstream impl of trait Error)
387 #[unstable(feature = "allocator_api", issue = "32838")]
388 impl fmt
::Display
for CannotReallocInPlace
{
389 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
390 write
!(f
, "{}", self.description())
394 /// A memory allocator that can be registered as the standard library’s default
395 /// through the `#[global_allocator]` attribute.
397 /// Some of the methods require that a memory block be *currently
398 /// allocated* via an allocator. This means that:
400 /// * the starting address for that memory block was previously
401 /// returned by a previous call to an allocation method
402 /// such as `alloc`, and
404 /// * the memory block has not been subsequently deallocated, where
405 /// blocks are deallocated either by being passed to a deallocation
406 /// method such as `dealloc` or by being
407 /// passed to a reallocation method that returns a non-null pointer.
413 /// use std::alloc::{GlobalAlloc, Layout, alloc};
414 /// use std::ptr::null_mut;
416 /// struct MyAllocator;
418 /// unsafe impl GlobalAlloc for MyAllocator {
419 /// unsafe fn alloc(&self, _layout: Layout) -> *mut u8 { null_mut() }
420 /// unsafe fn dealloc(&self, _ptr: *mut u8, _layout: Layout) {}
423 /// #[global_allocator]
424 /// static A: MyAllocator = MyAllocator;
428 /// assert!(alloc(Layout::new::<u32>()).is_null())
435 /// The `GlobalAlloc` trait is an `unsafe` trait for a number of reasons, and
436 /// implementors must ensure that they adhere to these contracts:
438 /// * It's undefined behavior if global allocators unwind. This restriction may
439 /// be lifted in the future, but currently a panic from any of these
440 /// functions may lead to memory unsafety.
442 /// * `Layout` queries and calculations in general must be correct. Callers of
443 /// this trait are allowed to rely on the contracts defined on each method,
444 /// and implementors must ensure such contracts remain true.
445 #[stable(feature = "global_alloc", since = "1.28.0")]
446 pub unsafe trait GlobalAlloc
{
447 /// Allocate memory as described by the given `layout`.
449 /// Returns a pointer to newly-allocated memory,
450 /// or null to indicate allocation failure.
454 /// This function is unsafe because undefined behavior can result
455 /// if the caller does not ensure that `layout` has non-zero size.
457 /// (Extension subtraits might provide more specific bounds on
458 /// behavior, e.g., guarantee a sentinel address or a null pointer
459 /// in response to a zero-size allocation request.)
461 /// The allocated block of memory may or may not be initialized.
465 /// Returning a null pointer indicates that either memory is exhausted
466 /// or `layout` does not meet this allocator's size or alignment constraints.
468 /// Implementations are encouraged to return null on memory
469 /// exhaustion rather than aborting, but this is not
470 /// a strict requirement. (Specifically: it is *legal* to
471 /// implement this trait atop an underlying native allocation
472 /// library that aborts on memory exhaustion.)
474 /// Clients wishing to abort computation in response to an
475 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
476 /// rather than directly invoking `panic!` or similar.
478 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
479 #[stable(feature = "global_alloc", since = "1.28.0")]
480 unsafe fn alloc(&self, layout
: Layout
) -> *mut u8;
482 /// Deallocate the block of memory at the given `ptr` pointer with the given `layout`.
486 /// This function is unsafe because undefined behavior can result
487 /// if the caller does not ensure all of the following:
489 /// * `ptr` must denote a block of memory currently allocated via
492 /// * `layout` must be the same layout that was used
493 /// to allocate that block of memory,
494 #[stable(feature = "global_alloc", since = "1.28.0")]
495 unsafe fn dealloc(&self, ptr
: *mut u8, layout
: Layout
);
497 /// Behaves like `alloc`, but also ensures that the contents
498 /// are set to zero before being returned.
502 /// This function is unsafe for the same reasons that `alloc` is.
503 /// However the allocated block of memory is guaranteed to be initialized.
507 /// Returning a null pointer indicates that either memory is exhausted
508 /// or `layout` does not meet allocator's size or alignment constraints,
509 /// just as in `alloc`.
511 /// Clients wishing to abort computation in response to an
512 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
513 /// rather than directly invoking `panic!` or similar.
515 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
516 #[stable(feature = "global_alloc", since = "1.28.0")]
517 unsafe fn alloc_zeroed(&self, layout
: Layout
) -> *mut u8 {
518 let size
= layout
.size();
519 let ptr
= self.alloc(layout
);
521 ptr
::write_bytes(ptr
, 0, size
);
526 /// Shrink or grow a block of memory to the given `new_size`.
527 /// The block is described by the given `ptr` pointer and `layout`.
529 /// If this returns a non-null pointer, then ownership of the memory block
530 /// referenced by `ptr` has been transferred to this allocator.
531 /// The memory may or may not have been deallocated,
532 /// and should be considered unusable (unless of course it was
533 /// transferred back to the caller again via the return value of
536 /// If this method returns null, then ownership of the memory
537 /// block has not been transferred to this allocator, and the
538 /// contents of the memory block are unaltered.
542 /// This function is unsafe because undefined behavior can result
543 /// if the caller does not ensure all of the following:
545 /// * `ptr` must be currently allocated via this allocator,
547 /// * `layout` must be the same layout that was used
548 /// to allocate that block of memory,
550 /// * `new_size` must be greater than zero.
552 /// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
553 /// must not overflow (i.e., the rounded value must be less than `usize::MAX`).
555 /// (Extension subtraits might provide more specific bounds on
556 /// behavior, e.g., guarantee a sentinel address or a null pointer
557 /// in response to a zero-size allocation request.)
561 /// Returns null if the new layout does not meet the size
562 /// and alignment constraints of the allocator, or if reallocation
565 /// Implementations are encouraged to return null on memory
566 /// exhaustion rather than panicking or aborting, but this is not
567 /// a strict requirement. (Specifically: it is *legal* to
568 /// implement this trait atop an underlying native allocation
569 /// library that aborts on memory exhaustion.)
571 /// Clients wishing to abort computation in response to a
572 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
573 /// rather than directly invoking `panic!` or similar.
575 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
576 #[stable(feature = "global_alloc", since = "1.28.0")]
577 unsafe fn realloc(&self, ptr
: *mut u8, layout
: Layout
, new_size
: usize) -> *mut u8 {
578 let new_layout
= Layout
::from_size_align_unchecked(new_size
, layout
.align());
579 let new_ptr
= self.alloc(new_layout
);
580 if !new_ptr
.is_null() {
581 ptr
::copy_nonoverlapping(
584 cmp
::min(layout
.size(), new_size
),
586 self.dealloc(ptr
, layout
);
592 /// An implementation of `Alloc` can allocate, reallocate, and
593 /// deallocate arbitrary blocks of data described via `Layout`.
595 /// Some of the methods require that a memory block be *currently
596 /// allocated* via an allocator. This means that:
598 /// * the starting address for that memory block was previously
599 /// returned by a previous call to an allocation method (`alloc`,
600 /// `alloc_zeroed`, `alloc_excess`, `alloc_one`, `alloc_array`) or
601 /// reallocation method (`realloc`, `realloc_excess`, or
602 /// `realloc_array`), and
604 /// * the memory block has not been subsequently deallocated, where
605 /// blocks are deallocated either by being passed to a deallocation
606 /// method (`dealloc`, `dealloc_one`, `dealloc_array`) or by being
607 /// passed to a reallocation method (see above) that returns `Ok`.
609 /// A note regarding zero-sized types and zero-sized layouts: many
610 /// methods in the `Alloc` trait state that allocation requests
611 /// must be non-zero size, or else undefined behavior can result.
613 /// * However, some higher-level allocation methods (`alloc_one`,
614 /// `alloc_array`) are well-defined on zero-sized types and can
615 /// optionally support them: it is left up to the implementor
616 /// whether to return `Err`, or to return `Ok` with some pointer.
618 /// * If an `Alloc` implementation chooses to return `Ok` in this
619 /// case (i.e., the pointer denotes a zero-sized inaccessible block)
620 /// then that returned pointer must be considered "currently
621 /// allocated". On such an allocator, *all* methods that take
622 /// currently-allocated pointers as inputs must accept these
623 /// zero-sized pointers, *without* causing undefined behavior.
625 /// * In other words, if a zero-sized pointer can flow out of an
626 /// allocator, then that allocator must likewise accept that pointer
627 /// flowing back into its deallocation and reallocation methods.
629 /// Some of the methods require that a layout *fit* a memory block.
630 /// What it means for a layout to "fit" a memory block means (or
631 /// equivalently, for a memory block to "fit" a layout) is that the
632 /// following two conditions must hold:
634 /// 1. The block's starting address must be aligned to `layout.align()`.
636 /// 2. The block's size must fall in the range `[use_min, use_max]`, where:
638 /// * `use_min` is `self.usable_size(layout).0`, and
640 /// * `use_max` is the capacity that was (or would have been)
641 /// returned when (if) the block was allocated via a call to
642 /// `alloc_excess` or `realloc_excess`.
646 /// * the size of the layout most recently used to allocate the block
647 /// is guaranteed to be in the range `[use_min, use_max]`, and
649 /// * a lower-bound on `use_max` can be safely approximated by a call to
652 /// * if a layout `k` fits a memory block (denoted by `ptr`)
653 /// currently allocated via an allocator `a`, then it is legal to
654 /// use that layout to deallocate it, i.e., `a.dealloc(ptr, k);`.
658 /// The `Alloc` trait is an `unsafe` trait for a number of reasons, and
659 /// implementors must ensure that they adhere to these contracts:
661 /// * Pointers returned from allocation functions must point to valid memory and
662 /// retain their validity until at least the instance of `Alloc` is dropped
665 /// * `Layout` queries and calculations in general must be correct. Callers of
666 /// this trait are allowed to rely on the contracts defined on each method,
667 /// and implementors must ensure such contracts remain true.
669 /// Note that this list may get tweaked over time as clarifications are made in
671 #[unstable(feature = "allocator_api", issue = "32838")]
672 pub unsafe trait Alloc
{
674 // (Note: some existing allocators have unspecified but well-defined
675 // behavior in response to a zero size allocation request ;
676 // e.g., in C, `malloc` of 0 will either return a null pointer or a
677 // unique pointer, but will not have arbitrary undefined
679 // However in jemalloc for example,
680 // `mallocx(0)` is documented as undefined behavior.)
682 /// Returns a pointer meeting the size and alignment guarantees of
685 /// If this method returns an `Ok(addr)`, then the `addr` returned
686 /// will be non-null address pointing to a block of storage
687 /// suitable for holding an instance of `layout`.
689 /// The returned block of storage may or may not have its contents
690 /// initialized. (Extension subtraits might restrict this
691 /// behavior, e.g., to ensure initialization to particular sets of
696 /// This function is unsafe because undefined behavior can result
697 /// if the caller does not ensure that `layout` has non-zero size.
699 /// (Extension subtraits might provide more specific bounds on
700 /// behavior, e.g., guarantee a sentinel address or a null pointer
701 /// in response to a zero-size allocation request.)
705 /// Returning `Err` indicates that either memory is exhausted or
706 /// `layout` does not meet allocator's size or alignment
709 /// Implementations are encouraged to return `Err` on memory
710 /// exhaustion rather than panicking or aborting, but this is not
711 /// a strict requirement. (Specifically: it is *legal* to
712 /// implement this trait atop an underlying native allocation
713 /// library that aborts on memory exhaustion.)
715 /// Clients wishing to abort computation in response to an
716 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
717 /// rather than directly invoking `panic!` or similar.
719 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
720 unsafe fn alloc(&mut self, layout
: Layout
) -> Result
<NonNull
<u8>, AllocErr
>;
722 /// Deallocate the memory referenced by `ptr`.
726 /// This function is unsafe because undefined behavior can result
727 /// if the caller does not ensure all of the following:
729 /// * `ptr` must denote a block of memory currently allocated via
732 /// * `layout` must *fit* that block of memory,
734 /// * In addition to fitting the block of memory `layout`, the
735 /// alignment of the `layout` must match the alignment used
736 /// to allocate that block of memory.
737 unsafe fn dealloc(&mut self, ptr
: NonNull
<u8>, layout
: Layout
);
739 // == ALLOCATOR-SPECIFIC QUANTITIES AND LIMITS ==
742 /// Returns bounds on the guaranteed usable size of a successful
743 /// allocation created with the specified `layout`.
745 /// In particular, if one has a memory block allocated via a given
746 /// allocator `a` and layout `k` where `a.usable_size(k)` returns
747 /// `(l, u)`, then one can pass that block to `a.dealloc()` with a
748 /// layout in the size range [l, u].
750 /// (All implementors of `usable_size` must ensure that
751 /// `l <= k.size() <= u`)
753 /// Both the lower- and upper-bounds (`l` and `u` respectively)
754 /// are provided, because an allocator based on size classes could
755 /// misbehave if one attempts to deallocate a block without
756 /// providing a correct value for its size (i.e., one within the
759 /// Clients who wish to make use of excess capacity are encouraged
760 /// to use the `alloc_excess` and `realloc_excess` instead, as
761 /// this method is constrained to report conservative values that
762 /// serve as valid bounds for *all possible* allocation method
765 /// However, for clients that do not wish to track the capacity
766 /// returned by `alloc_excess` locally, this method is likely to
767 /// produce useful results.
769 fn usable_size(&self, layout
: &Layout
) -> (usize, usize) {
770 (layout
.size(), layout
.size())
773 // == METHODS FOR MEMORY REUSE ==
774 // realloc. alloc_excess, realloc_excess
776 /// Returns a pointer suitable for holding data described by
777 /// a new layout with `layout`’s alignment and a size given
778 /// by `new_size`. To
779 /// accomplish this, this may extend or shrink the allocation
780 /// referenced by `ptr` to fit the new layout.
782 /// If this returns `Ok`, then ownership of the memory block
783 /// referenced by `ptr` has been transferred to this
784 /// allocator. The memory may or may not have been freed, and
785 /// should be considered unusable (unless of course it was
786 /// transferred back to the caller again via the return value of
789 /// If this method returns `Err`, then ownership of the memory
790 /// block has not been transferred to this allocator, and the
791 /// contents of the memory block are unaltered.
795 /// This function is unsafe because undefined behavior can result
796 /// if the caller does not ensure all of the following:
798 /// * `ptr` must be currently allocated via this allocator,
800 /// * `layout` must *fit* the `ptr` (see above). (The `new_size`
801 /// argument need not fit it.)
803 /// * `new_size` must be greater than zero.
805 /// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
806 /// must not overflow (i.e., the rounded value must be less than `usize::MAX`).
808 /// (Extension subtraits might provide more specific bounds on
809 /// behavior, e.g., guarantee a sentinel address or a null pointer
810 /// in response to a zero-size allocation request.)
814 /// Returns `Err` only if the new layout
815 /// does not meet the allocator's size
816 /// and alignment constraints of the allocator, or if reallocation
819 /// Implementations are encouraged to return `Err` on memory
820 /// exhaustion rather than panicking or aborting, but this is not
821 /// a strict requirement. (Specifically: it is *legal* to
822 /// implement this trait atop an underlying native allocation
823 /// library that aborts on memory exhaustion.)
825 /// Clients wishing to abort computation in response to a
826 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
827 /// rather than directly invoking `panic!` or similar.
829 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
830 unsafe fn realloc(&mut self,
833 new_size
: usize) -> Result
<NonNull
<u8>, AllocErr
> {
834 let old_size
= layout
.size();
836 if new_size
>= old_size
{
837 if let Ok(()) = self.grow_in_place(ptr
, layout
, new_size
) {
840 } else if new_size
< old_size
{
841 if let Ok(()) = self.shrink_in_place(ptr
, layout
, new_size
) {
846 // otherwise, fall back on alloc + copy + dealloc.
847 let new_layout
= Layout
::from_size_align_unchecked(new_size
, layout
.align());
848 let result
= self.alloc(new_layout
);
849 if let Ok(new_ptr
) = result
{
850 ptr
::copy_nonoverlapping(ptr
.as_ptr(),
852 cmp
::min(old_size
, new_size
));
853 self.dealloc(ptr
, layout
);
858 /// Behaves like `alloc`, but also ensures that the contents
859 /// are set to zero before being returned.
863 /// This function is unsafe for the same reasons that `alloc` is.
867 /// Returning `Err` indicates that either memory is exhausted or
868 /// `layout` does not meet allocator's size or alignment
869 /// constraints, just as in `alloc`.
871 /// Clients wishing to abort computation in response to an
872 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
873 /// rather than directly invoking `panic!` or similar.
875 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
876 unsafe fn alloc_zeroed(&mut self, layout
: Layout
) -> Result
<NonNull
<u8>, AllocErr
> {
877 let size
= layout
.size();
878 let p
= self.alloc(layout
);
880 ptr
::write_bytes(p
.as_ptr(), 0, size
);
885 /// Behaves like `alloc`, but also returns the whole size of
886 /// the returned block. For some `layout` inputs, like arrays, this
887 /// may include extra storage usable for additional data.
891 /// This function is unsafe for the same reasons that `alloc` is.
895 /// Returning `Err` indicates that either memory is exhausted or
896 /// `layout` does not meet allocator's size or alignment
897 /// constraints, just as in `alloc`.
899 /// Clients wishing to abort computation in response to an
900 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
901 /// rather than directly invoking `panic!` or similar.
903 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
904 unsafe fn alloc_excess(&mut self, layout
: Layout
) -> Result
<Excess
, AllocErr
> {
905 let usable_size
= self.usable_size(&layout
);
906 self.alloc(layout
).map(|p
| Excess(p
, usable_size
.1))
909 /// Behaves like `realloc`, but also returns the whole size of
910 /// the returned block. For some `layout` inputs, like arrays, this
911 /// may include extra storage usable for additional data.
915 /// This function is unsafe for the same reasons that `realloc` is.
919 /// Returning `Err` indicates that either memory is exhausted or
920 /// `layout` does not meet allocator's size or alignment
921 /// constraints, just as in `realloc`.
923 /// Clients wishing to abort computation in response to a
924 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
925 /// rather than directly invoking `panic!` or similar.
927 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
928 unsafe fn realloc_excess(&mut self,
931 new_size
: usize) -> Result
<Excess
, AllocErr
> {
932 let new_layout
= Layout
::from_size_align_unchecked(new_size
, layout
.align());
933 let usable_size
= self.usable_size(&new_layout
);
934 self.realloc(ptr
, layout
, new_size
)
935 .map(|p
| Excess(p
, usable_size
.1))
938 /// Attempts to extend the allocation referenced by `ptr` to fit `new_size`.
940 /// If this returns `Ok`, then the allocator has asserted that the
941 /// memory block referenced by `ptr` now fits `new_size`, and thus can
942 /// be used to carry data of a layout of that size and same alignment as
943 /// `layout`. (The allocator is allowed to
944 /// expend effort to accomplish this, such as extending the memory block to
945 /// include successor blocks, or virtual memory tricks.)
947 /// Regardless of what this method returns, ownership of the
948 /// memory block referenced by `ptr` has not been transferred, and
949 /// the contents of the memory block are unaltered.
953 /// This function is unsafe because undefined behavior can result
954 /// if the caller does not ensure all of the following:
956 /// * `ptr` must be currently allocated via this allocator,
958 /// * `layout` must *fit* the `ptr` (see above); note the
959 /// `new_size` argument need not fit it,
961 /// * `new_size` must not be less than `layout.size()`,
965 /// Returns `Err(CannotReallocInPlace)` when the allocator is
966 /// unable to assert that the memory block referenced by `ptr`
967 /// could fit `layout`.
969 /// Note that one cannot pass `CannotReallocInPlace` to the `handle_alloc_error`
970 /// function; clients are expected either to be able to recover from
971 /// `grow_in_place` failures without aborting, or to fall back on
972 /// another reallocation method before resorting to an abort.
973 unsafe fn grow_in_place(&mut self,
976 new_size
: usize) -> Result
<(), CannotReallocInPlace
> {
977 let _
= ptr
; // this default implementation doesn't care about the actual address.
978 debug_assert
!(new_size
>= layout
.size());
979 let (_l
, u
) = self.usable_size(&layout
);
980 // _l <= layout.size() [guaranteed by usable_size()]
981 // layout.size() <= new_layout.size() [required by this method]
985 Err(CannotReallocInPlace
)
989 /// Attempts to shrink the allocation referenced by `ptr` to fit `new_size`.
991 /// If this returns `Ok`, then the allocator has asserted that the
992 /// memory block referenced by `ptr` now fits `new_size`, and
993 /// thus can only be used to carry data of that smaller
994 /// layout. (The allocator is allowed to take advantage of this,
995 /// carving off portions of the block for reuse elsewhere.) The
996 /// truncated contents of the block within the smaller layout are
997 /// unaltered, and ownership of block has not been transferred.
999 /// If this returns `Err`, then the memory block is considered to
1000 /// still represent the original (larger) `layout`. None of the
1001 /// block has been carved off for reuse elsewhere, ownership of
1002 /// the memory block has not been transferred, and the contents of
1003 /// the memory block are unaltered.
1007 /// This function is unsafe because undefined behavior can result
1008 /// if the caller does not ensure all of the following:
1010 /// * `ptr` must be currently allocated via this allocator,
1012 /// * `layout` must *fit* the `ptr` (see above); note the
1013 /// `new_size` argument need not fit it,
1015 /// * `new_size` must not be greater than `layout.size()`
1016 /// (and must be greater than zero),
1020 /// Returns `Err(CannotReallocInPlace)` when the allocator is
1021 /// unable to assert that the memory block referenced by `ptr`
1022 /// could fit `layout`.
1024 /// Note that one cannot pass `CannotReallocInPlace` to the `handle_alloc_error`
1025 /// function; clients are expected either to be able to recover from
1026 /// `shrink_in_place` failures without aborting, or to fall back
1027 /// on another reallocation method before resorting to an abort.
1028 unsafe fn shrink_in_place(&mut self,
1031 new_size
: usize) -> Result
<(), CannotReallocInPlace
> {
1032 let _
= ptr
; // this default implementation doesn't care about the actual address.
1033 debug_assert
!(new_size
<= layout
.size());
1034 let (l
, _u
) = self.usable_size(&layout
);
1035 // layout.size() <= _u [guaranteed by usable_size()]
1036 // new_layout.size() <= layout.size() [required by this method]
1040 Err(CannotReallocInPlace
)
1045 // == COMMON USAGE PATTERNS ==
1046 // alloc_one, dealloc_one, alloc_array, realloc_array. dealloc_array
1048 /// Allocates a block suitable for holding an instance of `T`.
1050 /// Captures a common usage pattern for allocators.
1052 /// The returned block is suitable for passing to the
1053 /// `realloc`/`dealloc` methods of this allocator.
1055 /// Note to implementors: If this returns `Ok(ptr)`, then `ptr`
1056 /// must be considered "currently allocated" and must be
1057 /// acceptable input to methods such as `realloc` or `dealloc`,
1058 /// *even if* `T` is a zero-sized type. In other words, if your
1059 /// `Alloc` implementation overrides this method in a manner
1060 /// that can return a zero-sized `ptr`, then all reallocation and
1061 /// deallocation methods need to be similarly overridden to accept
1062 /// such values as input.
1066 /// Returning `Err` indicates that either memory is exhausted or
1067 /// `T` does not meet allocator's size or alignment constraints.
1069 /// For zero-sized `T`, may return either of `Ok` or `Err`, but
1070 /// will *not* yield undefined behavior.
1072 /// Clients wishing to abort computation in response to an
1073 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
1074 /// rather than directly invoking `panic!` or similar.
1076 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1077 fn alloc_one
<T
>(&mut self) -> Result
<NonNull
<T
>, AllocErr
>
1080 let k
= Layout
::new
::<T
>();
1082 unsafe { self.alloc(k).map(|p| p.cast()) }
1088 /// Deallocates a block suitable for holding an instance of `T`.
1090 /// The given block must have been produced by this allocator,
1091 /// and must be suitable for storing a `T` (in terms of alignment
1092 /// as well as minimum and maximum size); otherwise yields
1093 /// undefined behavior.
1095 /// Captures a common usage pattern for allocators.
1099 /// This function is unsafe because undefined behavior can result
1100 /// if the caller does not ensure both:
1102 /// * `ptr` must denote a block of memory currently allocated via this allocator
1104 /// * the layout of `T` must *fit* that block of memory.
1105 unsafe fn dealloc_one
<T
>(&mut self, ptr
: NonNull
<T
>)
1108 let k
= Layout
::new
::<T
>();
1110 self.dealloc(ptr
.cast(), k
);
1114 /// Allocates a block suitable for holding `n` instances of `T`.
1116 /// Captures a common usage pattern for allocators.
1118 /// The returned block is suitable for passing to the
1119 /// `realloc`/`dealloc` methods of this allocator.
1121 /// Note to implementors: If this returns `Ok(ptr)`, then `ptr`
1122 /// must be considered "currently allocated" and must be
1123 /// acceptable input to methods such as `realloc` or `dealloc`,
1124 /// *even if* `T` is a zero-sized type. In other words, if your
1125 /// `Alloc` implementation overrides this method in a manner
1126 /// that can return a zero-sized `ptr`, then all reallocation and
1127 /// deallocation methods need to be similarly overridden to accept
1128 /// such values as input.
1132 /// Returning `Err` indicates that either memory is exhausted or
1133 /// `[T; n]` does not meet allocator's size or alignment
1136 /// For zero-sized `T` or `n == 0`, may return either of `Ok` or
1137 /// `Err`, but will *not* yield undefined behavior.
1139 /// Always returns `Err` on arithmetic overflow.
1141 /// Clients wishing to abort computation in response to an
1142 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
1143 /// rather than directly invoking `panic!` or similar.
1145 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1146 fn alloc_array
<T
>(&mut self, n
: usize) -> Result
<NonNull
<T
>, AllocErr
>
1149 match Layout
::array
::<T
>(n
) {
1150 Ok(layout
) if layout
.size() > 0 => {
1152 self.alloc(layout
).map(|p
| p
.cast())
1159 /// Reallocates a block previously suitable for holding `n_old`
1160 /// instances of `T`, returning a block suitable for holding
1161 /// `n_new` instances of `T`.
1163 /// Captures a common usage pattern for allocators.
1165 /// The returned block is suitable for passing to the
1166 /// `realloc`/`dealloc` methods of this allocator.
1170 /// This function is unsafe because undefined behavior can result
1171 /// if the caller does not ensure all of the following:
1173 /// * `ptr` must be currently allocated via this allocator,
1175 /// * the layout of `[T; n_old]` must *fit* that block of memory.
1179 /// Returning `Err` indicates that either memory is exhausted or
1180 /// `[T; n_new]` does not meet allocator's size or alignment
1183 /// For zero-sized `T` or `n_new == 0`, may return either of `Ok` or
1184 /// `Err`, but will *not* yield undefined behavior.
1186 /// Always returns `Err` on arithmetic overflow.
1188 /// Clients wishing to abort computation in response to a
1189 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
1190 /// rather than directly invoking `panic!` or similar.
1192 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
1193 unsafe fn realloc_array
<T
>(&mut self,
1196 n_new
: usize) -> Result
<NonNull
<T
>, AllocErr
>
1199 match (Layout
::array
::<T
>(n_old
), Layout
::array
::<T
>(n_new
)) {
1200 (Ok(k_old
), Ok(k_new
)) if k_old
.size() > 0 && k_new
.size() > 0 => {
1201 debug_assert
!(k_old
.align() == k_new
.align());
1202 self.realloc(ptr
.cast(), k_old
, k_new
.size()).map(NonNull
::cast
)
1210 /// Deallocates a block suitable for holding `n` instances of `T`.
1212 /// Captures a common usage pattern for allocators.
1216 /// This function is unsafe because undefined behavior can result
1217 /// if the caller does not ensure both:
1219 /// * `ptr` must denote a block of memory currently allocated via this allocator
1221 /// * the layout of `[T; n]` must *fit* that block of memory.
1225 /// Returning `Err` indicates that either `[T; n]` or the given
1226 /// memory block does not meet allocator's size or alignment
1229 /// Always returns `Err` on arithmetic overflow.
1230 unsafe fn dealloc_array
<T
>(&mut self, ptr
: NonNull
<T
>, n
: usize) -> Result
<(), AllocErr
>
1233 match Layout
::array
::<T
>(n
) {
1234 Ok(k
) if k
.size() > 0 => {
1235 Ok(self.dealloc(ptr
.cast(), k
))