1 //! Memory allocation APIs
3 #![stable(feature = "alloc_module", since = "1.28.0")]
8 #[stable(feature = "global_alloc", since = "1.28.0")]
9 pub use self::global::GlobalAlloc;
10 #[stable(feature = "alloc_layout", since = "1.28.0")]
11 pub use self::layout::Layout;
12 #[stable(feature = "alloc_layout", since = "1.28.0")]
15 note = "Name does not follow std convention, use LayoutError",
16 suggestion = "LayoutError"
18 #[allow(deprecated, deprecated_in_future)]
19 pub use self::layout::LayoutErr;
21 #[stable(feature = "alloc_layout_error", since = "1.50.0")]
22 pub use self::layout::LayoutError;
24 use crate::error::Error;
26 use crate::ptr::{self, NonNull};
28 /// The `AllocError` error indicates an allocation failure
29 /// that may be due to resource exhaustion or to
30 /// something wrong when combining the given input arguments with this
32 #[unstable(feature = "allocator_api", issue = "32838")]
33 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
34 pub struct AllocError;
37 feature = "allocator_api",
38 reason = "the precise API and guarantees it provides may be tweaked.",
41 impl Error for AllocError {}
43 // (we need this for downstream impl of trait Error)
44 #[unstable(feature = "allocator_api", issue = "32838")]
45 impl fmt::Display for AllocError {
46 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
47 f.write_str("memory allocation failed")
51 /// An implementation of `Allocator` can allocate, grow, shrink, and deallocate arbitrary blocks of
52 /// data described via [`Layout`][].
54 /// `Allocator` is designed to be implemented on ZSTs, references, or smart pointers because having
55 /// an allocator like `MyAlloc([u8; N])` cannot be moved, without updating the pointers to the
58 /// Unlike [`GlobalAlloc`][], zero-sized allocations are allowed in `Allocator`. If an underlying
59 /// allocator does not support this (like jemalloc) or return a null pointer (such as
60 /// `libc::malloc`), this must be caught by the implementation.
62 /// ### Currently allocated memory
64 /// Some of the methods require that a memory block be *currently allocated* via an allocator. This
67 /// * the starting address for that memory block was previously returned by [`allocate`], [`grow`], or
70 /// * the memory block has not been subsequently deallocated, where blocks are either deallocated
71 /// directly by being passed to [`deallocate`] or were changed by being passed to [`grow`] or
72 /// [`shrink`] that returns `Ok`. If `grow` or `shrink` have returned `Err`, the passed pointer
75 /// [`allocate`]: Allocator::allocate
76 /// [`grow`]: Allocator::grow
77 /// [`shrink`]: Allocator::shrink
78 /// [`deallocate`]: Allocator::deallocate
80 /// ### Memory fitting
82 /// Some of the methods require that a layout *fit* a memory block. What it means for a layout to
83 /// "fit" a memory block means (or equivalently, for a memory block to "fit" a layout) is that the
84 /// following conditions must hold:
86 /// * The block must be allocated with the same alignment as [`layout.align()`], and
88 /// * The provided [`layout.size()`] must fall in the range `min ..= max`, where:
89 /// - `min` is the size of the layout most recently used to allocate the block, and
90 /// - `max` is the latest actual size returned from [`allocate`], [`grow`], or [`shrink`].
92 /// [`layout.align()`]: Layout::align
93 /// [`layout.size()`]: Layout::size
97 /// * Memory blocks returned from an allocator must point to valid memory and retain their validity
98 /// until the instance and all of its clones are dropped,
100 /// * cloning or moving the allocator must not invalidate memory blocks returned from this
101 /// allocator. A cloned allocator must behave like the same allocator, and
103 /// * any pointer to a memory block which is [*currently allocated*] may be passed to any other
104 /// method of the allocator.
106 /// [*currently allocated*]: #currently-allocated-memory
107 #[unstable(feature = "allocator_api", issue = "32838")]
109 pub unsafe trait Allocator {
110 /// Attempts to allocate a block of memory.
112 /// On success, returns a [`NonNull<[u8]>`][NonNull] meeting the size and alignment guarantees of `layout`.
114 /// The returned block may have a larger size than specified by `layout.size()`, and may or may
115 /// not have its contents initialized.
119 /// Returning `Err` indicates that either memory is exhausted or `layout` does not meet
120 /// allocator's size or alignment constraints.
122 /// Implementations are encouraged to return `Err` on memory exhaustion rather than panicking or
123 /// aborting, but this is not a strict requirement. (Specifically: it is *legal* to implement
124 /// this trait atop an underlying native allocation library that aborts on memory exhaustion.)
126 /// Clients wishing to abort computation in response to an allocation error are encouraged to
127 /// call the [`handle_alloc_error`] function, rather than directly invoking `panic!` or similar.
129 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
130 fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError>;
132 /// Behaves like `allocate`, but also ensures that the returned memory is zero-initialized.
136 /// Returning `Err` indicates that either memory is exhausted or `layout` does not meet
137 /// allocator's size or alignment constraints.
139 /// Implementations are encouraged to return `Err` on memory exhaustion rather than panicking or
140 /// aborting, but this is not a strict requirement. (Specifically: it is *legal* to implement
141 /// this trait atop an underlying native allocation library that aborts on memory exhaustion.)
143 /// Clients wishing to abort computation in response to an allocation error are encouraged to
144 /// call the [`handle_alloc_error`] function, rather than directly invoking `panic!` or similar.
146 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
147 fn allocate_zeroed(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
148 let ptr = self.allocate(layout)?;
149 // SAFETY: `alloc` returns a valid memory block
150 unsafe { ptr.as_non_null_ptr().as_ptr().write_bytes(0, ptr.len()) }
154 /// Deallocates the memory referenced by `ptr`.
158 /// * `ptr` must denote a block of memory [*currently allocated*] via this allocator, and
159 /// * `layout` must [*fit*] that block of memory.
161 /// [*currently allocated*]: #currently-allocated-memory
162 /// [*fit*]: #memory-fitting
163 unsafe fn deallocate(&self, ptr: NonNull<u8>, layout: Layout);
165 /// Attempts to extend the memory block.
167 /// Returns a new [`NonNull<[u8]>`][NonNull] containing a pointer and the actual size of the allocated
168 /// memory. The pointer is suitable for holding data described by `new_layout`. To accomplish
169 /// this, the allocator may extend the allocation referenced by `ptr` to fit the new layout.
171 /// If this returns `Ok`, then ownership of the memory block referenced by `ptr` has been
172 /// transferred to this allocator. The memory may or may not have been freed, and should be
173 /// considered unusable.
175 /// If this method returns `Err`, then ownership of the memory block has not been transferred to
176 /// this allocator, and the contents of the memory block are unaltered.
180 /// * `ptr` must denote a block of memory [*currently allocated*] via this allocator.
181 /// * `old_layout` must [*fit*] that block of memory (The `new_layout` argument need not fit it.).
182 /// * `new_layout.size()` must be greater than or equal to `old_layout.size()`.
184 /// Note that `new_layout.align()` need not be the same as `old_layout.align()`.
186 /// [*currently allocated*]: #currently-allocated-memory
187 /// [*fit*]: #memory-fitting
191 /// Returns `Err` if the new layout does not meet the allocator's size and alignment
192 /// constraints of the allocator, or if growing otherwise fails.
194 /// Implementations are encouraged to return `Err` on memory exhaustion rather than panicking or
195 /// aborting, but this is not a strict requirement. (Specifically: it is *legal* to implement
196 /// this trait atop an underlying native allocation library that aborts on memory exhaustion.)
198 /// Clients wishing to abort computation in response to an allocation error are encouraged to
199 /// call the [`handle_alloc_error`] function, rather than directly invoking `panic!` or similar.
201 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
207 ) -> Result<NonNull<[u8]>, AllocError> {
209 new_layout.size() >= old_layout.size(),
210 "`new_layout.size()` must be greater than or equal to `old_layout.size()`"
213 let new_ptr = self.allocate(new_layout)?;
215 // SAFETY: because `new_layout.size()` must be greater than or equal to
216 // `old_layout.size()`, both the old and new memory allocation are valid for reads and
217 // writes for `old_layout.size()` bytes. Also, because the old allocation wasn't yet
218 // deallocated, it cannot overlap `new_ptr`. Thus, the call to `copy_nonoverlapping` is
219 // safe. The safety contract for `dealloc` must be upheld by the caller.
221 ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_mut_ptr(), old_layout.size());
222 self.deallocate(ptr, old_layout);
228 /// Behaves like `grow`, but also ensures that the new contents are set to zero before being
231 /// The memory block will contain the following contents after a successful call to
233 /// * Bytes `0..old_layout.size()` are preserved from the original allocation.
234 /// * Bytes `old_layout.size()..old_size` will either be preserved or zeroed, depending on
235 /// the allocator implementation. `old_size` refers to the size of the memory block prior
236 /// to the `grow_zeroed` call, which may be larger than the size that was originally
237 /// requested when it was allocated.
238 /// * Bytes `old_size..new_size` are zeroed. `new_size` refers to the size of the memory
239 /// block returned by the `grow_zeroed` call.
243 /// * `ptr` must denote a block of memory [*currently allocated*] via this allocator.
244 /// * `old_layout` must [*fit*] that block of memory (The `new_layout` argument need not fit it.).
245 /// * `new_layout.size()` must be greater than or equal to `old_layout.size()`.
247 /// Note that `new_layout.align()` need not be the same as `old_layout.align()`.
249 /// [*currently allocated*]: #currently-allocated-memory
250 /// [*fit*]: #memory-fitting
254 /// Returns `Err` if the new layout does not meet the allocator's size and alignment
255 /// constraints of the allocator, or if growing otherwise fails.
257 /// Implementations are encouraged to return `Err` on memory exhaustion rather than panicking or
258 /// aborting, but this is not a strict requirement. (Specifically: it is *legal* to implement
259 /// this trait atop an underlying native allocation library that aborts on memory exhaustion.)
261 /// Clients wishing to abort computation in response to an allocation error are encouraged to
262 /// call the [`handle_alloc_error`] function, rather than directly invoking `panic!` or similar.
264 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
265 unsafe fn grow_zeroed(
270 ) -> Result<NonNull<[u8]>, AllocError> {
272 new_layout.size() >= old_layout.size(),
273 "`new_layout.size()` must be greater than or equal to `old_layout.size()`"
276 let new_ptr = self.allocate_zeroed(new_layout)?;
278 // SAFETY: because `new_layout.size()` must be greater than or equal to
279 // `old_layout.size()`, both the old and new memory allocation are valid for reads and
280 // writes for `old_layout.size()` bytes. Also, because the old allocation wasn't yet
281 // deallocated, it cannot overlap `new_ptr`. Thus, the call to `copy_nonoverlapping` is
282 // safe. The safety contract for `dealloc` must be upheld by the caller.
284 ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_mut_ptr(), old_layout.size());
285 self.deallocate(ptr, old_layout);
291 /// Attempts to shrink the memory block.
293 /// Returns a new [`NonNull<[u8]>`][NonNull] containing a pointer and the actual size of the allocated
294 /// memory. The pointer is suitable for holding data described by `new_layout`. To accomplish
295 /// this, the allocator may shrink the allocation referenced by `ptr` to fit the new layout.
297 /// If this returns `Ok`, then ownership of the memory block referenced by `ptr` has been
298 /// transferred to this allocator. The memory may or may not have been freed, and should be
299 /// considered unusable.
301 /// If this method returns `Err`, then ownership of the memory block has not been transferred to
302 /// this allocator, and the contents of the memory block are unaltered.
306 /// * `ptr` must denote a block of memory [*currently allocated*] via this allocator.
307 /// * `old_layout` must [*fit*] that block of memory (The `new_layout` argument need not fit it.).
308 /// * `new_layout.size()` must be smaller than or equal to `old_layout.size()`.
310 /// Note that `new_layout.align()` need not be the same as `old_layout.align()`.
312 /// [*currently allocated*]: #currently-allocated-memory
313 /// [*fit*]: #memory-fitting
317 /// Returns `Err` if the new layout does not meet the allocator's size and alignment
318 /// constraints of the allocator, or if shrinking otherwise fails.
320 /// Implementations are encouraged to return `Err` on memory exhaustion rather than panicking or
321 /// aborting, but this is not a strict requirement. (Specifically: it is *legal* to implement
322 /// this trait atop an underlying native allocation library that aborts on memory exhaustion.)
324 /// Clients wishing to abort computation in response to an allocation error are encouraged to
325 /// call the [`handle_alloc_error`] function, rather than directly invoking `panic!` or similar.
327 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
333 ) -> Result<NonNull<[u8]>, AllocError> {
335 new_layout.size() <= old_layout.size(),
336 "`new_layout.size()` must be smaller than or equal to `old_layout.size()`"
339 let new_ptr = self.allocate(new_layout)?;
341 // SAFETY: because `new_layout.size()` must be lower than or equal to
342 // `old_layout.size()`, both the old and new memory allocation are valid for reads and
343 // writes for `new_layout.size()` bytes. Also, because the old allocation wasn't yet
344 // deallocated, it cannot overlap `new_ptr`. Thus, the call to `copy_nonoverlapping` is
345 // safe. The safety contract for `dealloc` must be upheld by the caller.
347 ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_mut_ptr(), new_layout.size());
348 self.deallocate(ptr, old_layout);
354 /// Creates a "by reference" adapter for this instance of `Allocator`.
356 /// The returned adapter also implements `Allocator` and will simply borrow this.
358 fn by_ref(&self) -> &Self
366 #[unstable(feature = "allocator_api", issue = "32838")]
367 unsafe impl<A> Allocator for &A
369 A: Allocator + ?Sized,
372 fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
373 (**self).allocate(layout)
377 fn allocate_zeroed(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
378 (**self).allocate_zeroed(layout)
382 unsafe fn deallocate(&self, ptr: NonNull<u8>, layout: Layout) {
383 // SAFETY: the safety contract must be upheld by the caller
384 unsafe { (**self).deallocate(ptr, layout) }
393 ) -> Result<NonNull<[u8]>, AllocError> {
394 // SAFETY: the safety contract must be upheld by the caller
395 unsafe { (**self).grow(ptr, old_layout, new_layout) }
399 unsafe fn grow_zeroed(
404 ) -> Result<NonNull<[u8]>, AllocError> {
405 // SAFETY: the safety contract must be upheld by the caller
406 unsafe { (**self).grow_zeroed(ptr, old_layout, new_layout) }
415 ) -> Result<NonNull<[u8]>, AllocError> {
416 // SAFETY: the safety contract must be upheld by the caller
417 unsafe { (**self).shrink(ptr, old_layout, new_layout) }