library/core/src/alloc/mod.rs

   1 //! Memory allocation APIs
   2
   3 #![stable(feature = "alloc_module", since = "1.28.0")]
   4
   5 mod global;
   6 mod layout;
   7
   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")]
  13 #[deprecated(
  14     since = "1.52.0",
  15     note = "Name does not follow std convention, use LayoutError",
  16     suggestion = "LayoutError"
  17 )]
  18 #[allow(deprecated, deprecated_in_future)]
  19 pub use self::layout::LayoutErr;
  20
  21 #[stable(feature = "alloc_layout_error", since = "1.50.0")]
  22 pub use self::layout::LayoutError;
  23
  24 use crate::fmt;
  25 use crate::ptr::{self, NonNull};
  26
  27 /// The `AllocError` error indicates an allocation failure
  28 /// that may be due to resource exhaustion or to
  29 /// something wrong when combining the given input arguments with this
  30 /// allocator.
  31 #[unstable(feature = "allocator_api", issue = "32838")]
  32 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
  33 pub struct AllocError;
  34
  35 // (we need this for downstream impl of trait Error)
  36 #[unstable(feature = "allocator_api", issue = "32838")]
  37 impl fmt::Display for AllocError {
  38     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
  39         f.write_str("memory allocation failed")
  40     }
  41 }
  42
  43 /// An implementation of `Allocator` can allocate, grow, shrink, and deallocate arbitrary blocks of
  44 /// data described via [`Layout`][].
  45 ///
  46 /// `Allocator` is designed to be implemented on ZSTs, references, or smart pointers because having
  47 /// an allocator like `MyAlloc([u8; N])` cannot be moved, without updating the pointers to the
  48 /// allocated memory.
  49 ///
  50 /// Unlike [`GlobalAlloc`][], zero-sized allocations are allowed in `Allocator`. If an underlying
  51 /// allocator does not support this (like jemalloc) or return a null pointer (such as
  52 /// `libc::malloc`), this must be caught by the implementation.
  53 ///
  54 /// ### Currently allocated memory
  55 ///
  56 /// Some of the methods require that a memory block be *currently allocated* via an allocator. This
  57 /// means that:
  58 ///
  59 /// * the starting address for that memory block was previously returned by [`allocate`], [`grow`], or
  60 ///   [`shrink`], and
  61 ///
  62 /// * the memory block has not been subsequently deallocated, where blocks are either deallocated
  63 ///   directly by being passed to [`deallocate`] or were changed by being passed to [`grow`] or
  64 ///   [`shrink`] that returns `Ok`. If `grow` or `shrink` have returned `Err`, the passed pointer
  65 ///   remains valid.
  66 ///
  67 /// [`allocate`]: Allocator::allocate
  68 /// [`grow`]: Allocator::grow
  69 /// [`shrink`]: Allocator::shrink
  70 /// [`deallocate`]: Allocator::deallocate
  71 ///
  72 /// ### Memory fitting
  73 ///
  74 /// Some of the methods require that a layout *fit* a memory block. What it means for a layout to
  75 /// "fit" a memory block means (or equivalently, for a memory block to "fit" a layout) is that the
  76 /// following conditions must hold:
  77 ///
  78 /// * The block must be allocated with the same alignment as [`layout.align()`], and
  79 ///
  80 /// * The provided [`layout.size()`] must fall in the range `min ..= max`, where:
  81 ///   - `min` is the size of the layout most recently used to allocate the block, and
  82 ///   - `max` is the latest actual size returned from [`allocate`], [`grow`], or [`shrink`].
  83 ///
  84 /// [`layout.align()`]: Layout::align
  85 /// [`layout.size()`]: Layout::size
  86 ///
  87 /// # Safety
  88 ///
  89 /// * Memory blocks returned from an allocator must point to valid memory and retain their validity
  90 ///   until the instance and all of its clones are dropped,
  91 ///
  92 /// * cloning or moving the allocator must not invalidate memory blocks returned from this
  93 ///   allocator. A cloned allocator must behave like the same allocator, and
  94 ///
  95 /// * any pointer to a memory block which is [*currently allocated*] may be passed to any other
  96 ///   method of the allocator.
  97 ///
  98 /// [*currently allocated*]: #currently-allocated-memory
  99 #[unstable(feature = "allocator_api", issue = "32838")]
 100 pub unsafe trait Allocator {
 101     /// Attempts to allocate a block of memory.
 102     ///
 103     /// On success, returns a [`NonNull<[u8]>`][NonNull] meeting the size and alignment guarantees of `layout`.
 104     ///
 105     /// The returned block may have a larger size than specified by `layout.size()`, and may or may
 106     /// not have its contents initialized.
 107     ///
 108     /// # Errors
 109     ///
 110     /// Returning `Err` indicates that either memory is exhausted or `layout` does not meet
 111     /// allocator's size or alignment constraints.
 112     ///
 113     /// Implementations are encouraged to return `Err` on memory exhaustion rather than panicking or
 114     /// aborting, but this is not a strict requirement. (Specifically: it is *legal* to implement
 115     /// this trait atop an underlying native allocation library that aborts on memory exhaustion.)
 116     ///
 117     /// Clients wishing to abort computation in response to an allocation error are encouraged to
 118     /// call the [`handle_alloc_error`] function, rather than directly invoking `panic!` or similar.
 119     ///
 120     /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
 121     fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError>;
 122
 123     /// Behaves like `allocate`, but also ensures that the returned memory is zero-initialized.
 124     ///
 125     /// # Errors
 126     ///
 127     /// Returning `Err` indicates that either memory is exhausted or `layout` does not meet
 128     /// allocator's size or alignment constraints.
 129     ///
 130     /// Implementations are encouraged to return `Err` on memory exhaustion rather than panicking or
 131     /// aborting, but this is not a strict requirement. (Specifically: it is *legal* to implement
 132     /// this trait atop an underlying native allocation library that aborts on memory exhaustion.)
 133     ///
 134     /// Clients wishing to abort computation in response to an allocation error are encouraged to
 135     /// call the [`handle_alloc_error`] function, rather than directly invoking `panic!` or similar.
 136     ///
 137     /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
 138     fn allocate_zeroed(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
 139         let ptr = self.allocate(layout)?;
 140         // SAFETY: `alloc` returns a valid memory block
 141         unsafe { ptr.as_non_null_ptr().as_ptr().write_bytes(0, ptr.len()) }
 142         Ok(ptr)
 143     }
 144
 145     /// Deallocates the memory referenced by `ptr`.
 146     ///
 147     /// # Safety
 148     ///
 149     /// * `ptr` must denote a block of memory [*currently allocated*] via this allocator, and
 150     /// * `layout` must [*fit*] that block of memory.
 151     ///
 152     /// [*currently allocated*]: #currently-allocated-memory
 153     /// [*fit*]: #memory-fitting
 154     unsafe fn deallocate(&self, ptr: NonNull<u8>, layout: Layout);
 155
 156     /// Attempts to extend the memory block.
 157     ///
 158     /// Returns a new [`NonNull<[u8]>`][NonNull] containing a pointer and the actual size of the allocated
 159     /// memory. The pointer is suitable for holding data described by `new_layout`. To accomplish
 160     /// this, the allocator may extend the allocation referenced by `ptr` to fit the new layout.
 161     ///
 162     /// If this returns `Ok`, then ownership of the memory block referenced by `ptr` has been
 163     /// transferred to this allocator. The memory may or may not have been freed, and should be
 164     /// considered unusable.
 165     ///
 166     /// If this method returns `Err`, then ownership of the memory block has not been transferred to
 167     /// this allocator, and the contents of the memory block are unaltered.
 168     ///
 169     /// # Safety
 170     ///
 171     /// * `ptr` must denote a block of memory [*currently allocated*] via this allocator.
 172     /// * `old_layout` must [*fit*] that block of memory (The `new_layout` argument need not fit it.).
 173     /// * `new_layout.size()` must be greater than or equal to `old_layout.size()`.
 174     ///
 175     /// Note that `new_layout.align()` need not be the same as `old_layout.align()`.
 176     ///
 177     /// [*currently allocated*]: #currently-allocated-memory
 178     /// [*fit*]: #memory-fitting
 179     ///
 180     /// # Errors
 181     ///
 182     /// Returns `Err` if the new layout does not meet the allocator's size and alignment
 183     /// constraints of the allocator, or if growing otherwise fails.
 184     ///
 185     /// Implementations are encouraged to return `Err` on memory exhaustion rather than panicking or
 186     /// aborting, but this is not a strict requirement. (Specifically: it is *legal* to implement
 187     /// this trait atop an underlying native allocation library that aborts on memory exhaustion.)
 188     ///
 189     /// Clients wishing to abort computation in response to an allocation error are encouraged to
 190     /// call the [`handle_alloc_error`] function, rather than directly invoking `panic!` or similar.
 191     ///
 192     /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
 193     unsafe fn grow(
 194         &self,
 195         ptr: NonNull<u8>,
 196         old_layout: Layout,
 197         new_layout: Layout,
 198     ) -> Result<NonNull<[u8]>, AllocError> {
 199         debug_assert!(
 200             new_layout.size() >= old_layout.size(),
 201             "`new_layout.size()` must be greater than or equal to `old_layout.size()`"
 202         );
 203
 204         let new_ptr = self.allocate(new_layout)?;
 205
 206         // SAFETY: because `new_layout.size()` must be greater than or equal to
 207         // `old_layout.size()`, both the old and new memory allocation are valid for reads and
 208         // writes for `old_layout.size()` bytes. Also, because the old allocation wasn't yet
 209         // deallocated, it cannot overlap `new_ptr`. Thus, the call to `copy_nonoverlapping` is
 210         // safe. The safety contract for `dealloc` must be upheld by the caller.
 211         unsafe {
 212             ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_mut_ptr(), old_layout.size());
 213             self.deallocate(ptr, old_layout);
 214         }
 215
 216         Ok(new_ptr)
 217     }
 218
 219     /// Behaves like `grow`, but also ensures that the new contents are set to zero before being
 220     /// returned.
 221     ///
 222     /// The memory block will contain the following contents after a successful call to
 223     /// `grow_zeroed`:
 224     ///   * Bytes `0..old_layout.size()` are preserved from the original allocation.
 225     ///   * Bytes `old_layout.size()..old_size` will either be preserved or zeroed, depending on
 226     ///     the allocator implementation. `old_size` refers to the size of the memory block prior
 227     ///     to the `grow_zeroed` call, which may be larger than the size that was originally
 228     ///     requested when it was allocated.
 229     ///   * Bytes `old_size..new_size` are zeroed. `new_size` refers to the size of the memory
 230     ///     block returned by the `grow_zeroed` call.
 231     ///
 232     /// # Safety
 233     ///
 234     /// * `ptr` must denote a block of memory [*currently allocated*] via this allocator.
 235     /// * `old_layout` must [*fit*] that block of memory (The `new_layout` argument need not fit it.).
 236     /// * `new_layout.size()` must be greater than or equal to `old_layout.size()`.
 237     ///
 238     /// Note that `new_layout.align()` need not be the same as `old_layout.align()`.
 239     ///
 240     /// [*currently allocated*]: #currently-allocated-memory
 241     /// [*fit*]: #memory-fitting
 242     ///
 243     /// # Errors
 244     ///
 245     /// Returns `Err` if the new layout does not meet the allocator's size and alignment
 246     /// constraints of the allocator, or if growing otherwise fails.
 247     ///
 248     /// Implementations are encouraged to return `Err` on memory exhaustion rather than panicking or
 249     /// aborting, but this is not a strict requirement. (Specifically: it is *legal* to implement
 250     /// this trait atop an underlying native allocation library that aborts on memory exhaustion.)
 251     ///
 252     /// Clients wishing to abort computation in response to an allocation error are encouraged to
 253     /// call the [`handle_alloc_error`] function, rather than directly invoking `panic!` or similar.
 254     ///
 255     /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
 256     unsafe fn grow_zeroed(
 257         &self,
 258         ptr: NonNull<u8>,
 259         old_layout: Layout,
 260         new_layout: Layout,
 261     ) -> Result<NonNull<[u8]>, AllocError> {
 262         debug_assert!(
 263             new_layout.size() >= old_layout.size(),
 264             "`new_layout.size()` must be greater than or equal to `old_layout.size()`"
 265         );
 266
 267         let new_ptr = self.allocate_zeroed(new_layout)?;
 268
 269         // SAFETY: because `new_layout.size()` must be greater than or equal to
 270         // `old_layout.size()`, both the old and new memory allocation are valid for reads and
 271         // writes for `old_layout.size()` bytes. Also, because the old allocation wasn't yet
 272         // deallocated, it cannot overlap `new_ptr`. Thus, the call to `copy_nonoverlapping` is
 273         // safe. The safety contract for `dealloc` must be upheld by the caller.
 274         unsafe {
 275             ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_mut_ptr(), old_layout.size());
 276             self.deallocate(ptr, old_layout);
 277         }
 278
 279         Ok(new_ptr)
 280     }
 281
 282     /// Attempts to shrink the memory block.
 283     ///
 284     /// Returns a new [`NonNull<[u8]>`][NonNull] containing a pointer and the actual size of the allocated
 285     /// memory. The pointer is suitable for holding data described by `new_layout`. To accomplish
 286     /// this, the allocator may shrink the allocation referenced by `ptr` to fit the new layout.
 287     ///
 288     /// If this returns `Ok`, then ownership of the memory block referenced by `ptr` has been
 289     /// transferred to this allocator. The memory may or may not have been freed, and should be
 290     /// considered unusable.
 291     ///
 292     /// If this method returns `Err`, then ownership of the memory block has not been transferred to
 293     /// this allocator, and the contents of the memory block are unaltered.
 294     ///
 295     /// # Safety
 296     ///
 297     /// * `ptr` must denote a block of memory [*currently allocated*] via this allocator.
 298     /// * `old_layout` must [*fit*] that block of memory (The `new_layout` argument need not fit it.).
 299     /// * `new_layout.size()` must be smaller than or equal to `old_layout.size()`.
 300     ///
 301     /// Note that `new_layout.align()` need not be the same as `old_layout.align()`.
 302     ///
 303     /// [*currently allocated*]: #currently-allocated-memory
 304     /// [*fit*]: #memory-fitting
 305     ///
 306     /// # Errors
 307     ///
 308     /// Returns `Err` if the new layout does not meet the allocator's size and alignment
 309     /// constraints of the allocator, or if shrinking otherwise fails.
 310     ///
 311     /// Implementations are encouraged to return `Err` on memory exhaustion rather than panicking or
 312     /// aborting, but this is not a strict requirement. (Specifically: it is *legal* to implement
 313     /// this trait atop an underlying native allocation library that aborts on memory exhaustion.)
 314     ///
 315     /// Clients wishing to abort computation in response to an allocation error are encouraged to
 316     /// call the [`handle_alloc_error`] function, rather than directly invoking `panic!` or similar.
 317     ///
 318     /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
 319     unsafe fn shrink(
 320         &self,
 321         ptr: NonNull<u8>,
 322         old_layout: Layout,
 323         new_layout: Layout,
 324     ) -> Result<NonNull<[u8]>, AllocError> {
 325         debug_assert!(
 326             new_layout.size() <= old_layout.size(),
 327             "`new_layout.size()` must be smaller than or equal to `old_layout.size()`"
 328         );
 329
 330         let new_ptr = self.allocate(new_layout)?;
 331
 332         // SAFETY: because `new_layout.size()` must be lower than or equal to
 333         // `old_layout.size()`, both the old and new memory allocation are valid for reads and
 334         // writes for `new_layout.size()` bytes. Also, because the old allocation wasn't yet
 335         // deallocated, it cannot overlap `new_ptr`. Thus, the call to `copy_nonoverlapping` is
 336         // safe. The safety contract for `dealloc` must be upheld by the caller.
 337         unsafe {
 338             ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_mut_ptr(), new_layout.size());
 339             self.deallocate(ptr, old_layout);
 340         }
 341
 342         Ok(new_ptr)
 343     }
 344
 345     /// Creates a "by reference" adapter for this instance of `Allocator`.
 346     ///
 347     /// The returned adapter also implements `Allocator` and will simply borrow this.
 348     #[inline(always)]
 349     fn by_ref(&self) -> &Self
 350     where
 351         Self: Sized,
 352     {
 353         self
 354     }
 355 }
 356
 357 #[unstable(feature = "allocator_api", issue = "32838")]
 358 unsafe impl<A> Allocator for &A
 359 where
 360     A: Allocator + ?Sized,
 361 {
 362     #[inline]
 363     fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
 364         (**self).allocate(layout)
 365     }
 366
 367     #[inline]
 368     fn allocate_zeroed(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
 369         (**self).allocate_zeroed(layout)
 370     }
 371
 372     #[inline]
 373     unsafe fn deallocate(&self, ptr: NonNull<u8>, layout: Layout) {
 374         // SAFETY: the safety contract must be upheld by the caller
 375         unsafe { (**self).deallocate(ptr, layout) }
 376     }
 377
 378     #[inline]
 379     unsafe fn grow(
 380         &self,
 381         ptr: NonNull<u8>,
 382         old_layout: Layout,
 383         new_layout: Layout,
 384     ) -> Result<NonNull<[u8]>, AllocError> {
 385         // SAFETY: the safety contract must be upheld by the caller
 386         unsafe { (**self).grow(ptr, old_layout, new_layout) }
 387     }
 388
 389     #[inline]
 390     unsafe fn grow_zeroed(
 391         &self,
 392         ptr: NonNull<u8>,
 393         old_layout: Layout,
 394         new_layout: Layout,
 395     ) -> Result<NonNull<[u8]>, AllocError> {
 396         // SAFETY: the safety contract must be upheld by the caller
 397         unsafe { (**self).grow_zeroed(ptr, old_layout, new_layout) }
 398     }
 399
 400     #[inline]
 401     unsafe fn shrink(
 402         &self,
 403         ptr: NonNull<u8>,
 404         old_layout: Layout,
 405         new_layout: Layout,
 406     ) -> Result<NonNull<[u8]>, AllocError> {
 407         // SAFETY: the safety contract must be upheld by the caller
 408         unsafe { (**self).shrink(ptr, old_layout, new_layout) }
 409     }
 410 }